US20080023143A1 - Capacitively coupled plasma reactor with magnetic plasma control - Google Patents

Abstract

A plasma reactor includes a vacuum enclosure including a side wall and a ceiling defining a vacuum chamber, and a workpiece support within the chamber and facing the ceiling for supporting a planar workpiece, the workpiece support and the ceiling together defining a processing region between the workpiece support and the ceiling. Process gas inlets furnish a process gas into the chamber. A plasma source power electrode is connected to an RF power generator for capacitively coupling plasma source power into the chamber for maintaining a plasma within the chamber. The reactor further includes at least a first overhead solenoidal electromagnet adjacent the ceiling, the overhead solenoidal electromagnet, the ceiling, the side wall and the workpiece support being located along a common axis of symmetry.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 10/841,116, filed May 7, 2004 entitled CAPACITIVELY COUPLED PLASMA REACTOR WITH MAGNETIC PLASMA CONTROL, by Daniel Hoffman, et al., which is a divisional of U.S. patent application Ser. No. 10/192,271, filed Jul. 9, 2002 entitled CAPACITIVELY COUPLED PLASMA REACTOR WITH MAGNETIC PLASMA CONTROL, by Daniel Hoffman, et al., issued as U.S. Pat. No. 6,853,141 on Feb. 8, 2005, which claims priority of U.S. Provisional Application Ser. No. 60/638,194, filed May 22, 2002 entitled CAPACITIVELY COUPLED PLASMA REACTOR WITH MAGNETIC PLASMA CONTROL by Daniel Hoffman, et al. This application is also a continuation-in-part of U.S. patent application Ser. No. 11/105,307, filed Apr. 12, 2005 entitled MERIE PLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMA WITH ARCING SUPPRESSION, by Daniel Hoffman, et al., issued as U.S. Pat. No. 7,186,943 on Mar. 6, 2007, which is a continuation of U.S. patent application Ser. No. 10/007,367, filed Oct. 22, 2001 entitled MERIE PLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMA WITH ARCING SUPPRESSION, by Daniel Hoffman, et al., issued as U.S. Pat. No. 6,894,245 on May 17, 2005, which is a continuation-in-part of U.S. patent application Ser. No. 09/527,342, filed Mar. 17, 2000 entitled PLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMA by Daniel Hoffman, et al., issued as U.S. Pat. No. 6,528,751 on Mar. 4, 2003, all of which are assigned to the present assignee.
BACKGROUND
• Capacitively coupled plasma reactors are used in fabricating semiconductor microelectronic structures with high aspect ratios. Such structures typically have narrow, deep openings through one or more thin films formed on a semiconductor substrate. Capacitively coupled plasma reactors are used in various types of processes in fabricating such devices, including dielectric etch processes, metal etch processes, chemical vapor deposition and others. Such reactors are also employed in fabricating photolithographic masks and in fabricating semiconductor flat panel displays. Such applications depend upon plasma ions to enhance or enable desired processes. The density of the plasma ions over the surface of the semiconductor workpiece affects the process parameters, and is particularly critical in the fabrication of high aspect ratio microelectronic structures. In fact, a problem in fabricating high aspect ratio microelectronic integrated circuits is that non-uniformities in the plasma ion density across the workpiece surface can lead to process failure due to non-uniform etch rates or deposition rates.
• A typical capacitively coupled reactor has a wafer support pedestal in the reactor chamber and a ceiling overlying the wafer support. The ceiling may include a gas distribution plate that sprays process gas into the chamber. An RF power source is applied across the wafer support and ceiling or wall to strike and maintain a plasma over the wafer support. The chamber is generally cylindrical, while the ceiling and wafer support are circular and coaxial with the cylindrical chamber to enhance uniform processing. Nevertheless, such reactors have non-uniform plasma density distributions. Typically, the radial density distribution of plasma ions is high over the center of the wafer support and low near the periphery, a significant problem. Various approaches are used to control the plasma ion density distribution so as to improve process uniformity across the wafer or workpiece surface, and at least partially overcome this problem.
• One such approach is to provide a set of magnetic coils spaced circumferentially around the side of the reactor chamber, the coils all facing the center of the chamber. A relatively low frequency sinusoidal current is supplied to each coil, the sinusoidal currents in adjacent coils being offset in phase so as to produce a slowly rotating magnetic field over the wafer support. This feature tends to improve the radial distribution of plasma ion density over the wafer support. Where this approach is employed in reactive ion etching, it is called magnetically enhanced reactive ion etching (MERIE). This approach has certain limitations. In particular, the strength of the magnetic field may need to be limited in order to avoid device damage to microelectronic structures on the semiconductor workpiece associated with the strength of the magnetic field. The strength must also be limited to avoid chamber arcing associated with the rate of change of magnetic field strength. As a result, the total MERIE magnetic field may need to be substantially reduced and therefore may face substantial limitations in plasma ion density uniformity control.
• Another approach is called configurable magnetic fields (CMF) and employs the same circumferentially spaced coils referred to above. But, in CMF the coils are operated so as to impose a magnetic field that extends across the plane of the workpiece support, from one side to the other. In addition, the magnetic field rotates about the axis of the wafer support, to produce a time-averaged magnetic field that is radial. This is all accomplished, in the case of a reactor having four side-by-side coils, by furnishing one D.C. current to one pair of adjacent coils and a different (or opposite) D.C. current to the opposite pair of adjacent coils. The coils are switched to rotate this pattern so that the magnetic field rotates, as mentioned above. This approach is vulnerable to chamber or wafer arcing problems due to the abrupt switching of the CMF magnetic fields, and therefore the magnetic field strength must be limited. As a result, in some applications the magnetic field cannot be sufficient to compensate for plasma ion density non-uniformities produced by the reactor.
• Thus, what is needed is a way of compensating for plasma ion density distribution non-uniformities more efficiently (so that the magnetic field strength can be less) and with less (or with no) time fluctuations in the magnetic field.
SUMMARY OF THE INVENTION
• A plasma reactor includes a vacuum enclosure including a side wall and a ceiling defining a vacuum chamber, and a workpiece support within the chamber and facing the ceiling for supporting a planar workpiece, the workpiece support and the ceiling together defining a processing region between the workpiece support and the ceiling. Process gas inlets furnish a process gas into the chamber. A plasma source power electrode is connected to an RF power generator for capacitively coupling plasma source power into the chamber for maintaining a plasma within the chamber. The reactor further includes at least a first overhead solenoidal electromagnet adjacent the ceiling, the overhead solenoidal electromagnet, the ceiling, the side wall and the workpiece support being located along a common axis of symmetry. A current source is connected to the first solenoidal electromagnet and furnishes a first electric current in the first solenoidal electromagnet whereby to generate within the chamber a magnetic field which is a function of the first electric current, the first electric current having a value such that the magnetic field increases uniformity of plasma ion density radial distribution about the axis of symmetry near a surface of the workpiece support.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIGS. 1A, 1B and1C illustrate a plasma reactor with an overhead VHF electrode and overhead coils for controlling plasma ion uniformity.
• FIG. 2 illustrates an exemplary apparatus for controlling the overhead coils ofFIG. 1.
• FIGS. 3A and 3B are graphical representations of a magnetic field of the overhead coils ofFIG. 1 andFIG. 3C is a spatial representation of the same field.
• FIGS. 4A, 4B,4C and4D are graphs of the etch rate (vertical axis) on the wafer surface as a function of radial location (horizontal axis) for various modes of operation of the reactor ofFIG. 1.
• FIGS. 5A, 5B,5C and5D are graphs of the etch rate (vertical axis) on the wafer surface as a function of radial location (horizontal axis) for further modes of operation of the reactor ofFIG. 1.
• FIG. 6 is a graph depicting etch rate as a function of magnetic field.
• FIGS. 7 and 8 illustrate the reactor ofFIG. 1A with MERIE magnets.
• FIG. 9 depicts a method of operating the reactor ofFIG. 1A.
• FIG. 10 is a graph illustrating a comparative example of magnetic pressure and ion or electron density as functions of radial location on the wafer surface in the reactor ofFIG. 1A.
• FIG. 11 is a graph depicting etch rate non-uniformity as a function of coil current.
• FIG. 12 illustrates radial ion distribution at zero coil current in the example ofFIG. 11.
• FIGS. 13A and 13B compare measured and predicted etch rate distributions at a coil current of about 11 amperes in the example ofFIG. 11.
• FIGS. 14A and 14B compare measured and predicted etch rate distributions at a coil current of about 35 amperes in the example ofFIG. 11.
• FIG. 15 depicts a further method of operating the reactor ofFIG. 1A.
• FIG. 16 illustrates a magnetic field distribution obtained in a reactor corresponding toFIG. 1A.
• FIG. 17 depicts the gradient of the square of the magnetic field ofFIG. 16 in the wafer plane.
• FIG. 18 illustrates another magnetic field distribution obtained in a reactor corresponding toFIG. 1A.
• FIG. 19 depicts the gradient of the square of the magnetic field ofFIG. 18 in the wafer plane.
• FIG. 20 illustrates a yet further magnetic field distribution obtained in a reactor corresponding toFIG. 1A.
• FIG. 21 depicts the gradient of the square of the magnetic field ofFIG. 20 in the wafer plane.
• FIG. 22 depicts yet another method of operating the reactor ofFIG. 1A.
• FIG. 23 illustrates an exemplary microcontroller operation for controlling the reactor ofFIG. 1A.
• FIG. 24 illustrates a plasma reactor including features contained in the reactor ofFIG. 1A.
• FIG. 25 illustrates another plasma reactor including features contained in the reactor ofFIG. 1A.
• FIGS. 26, 27,28,29A and29B illustrate a gas distribution plate for the reactors ofFIGS. 1A, 24 and25.
• FIGS. 30 and 31 illustrate thermal control features in gas distribution plate like that ofFIG. 26.
• FIGS. 32 and 33 illustrate a gas distribution plate corresponding toFIG. 26 having dual zone gas flow control.
• FIG. 34 illustrates a plasma reactor corresponding toFIG. 1A having the dual zone gas distribution plate.
• FIGS. 35 and 36 illustrate exemplary dual zone gas flow controllers.
• FIG. 37 illustrates a plasma reactor corresponding toFIG. 34 having three overhead coils for controlling plasma ion distribution.
• FIGS. 38 and 39 depict different gas injection hole patterns in the gas distribution plate ofFIG. 26 for producing center low or center high gas flow distributions, respectively.
• FIGS. 40, 41,42 and43 illustrate different arrangements of overhead coils for controlling plasma ion distribution.
• FIGS. 44 and 45 illustrate a plasma reactor corresponding toFIG. 1A in which the overhead coils are replaced by upper and lower magnetic coils above and below the reactor chamber to produce a cusp-shaped magnetic field best seen inFIG. 45.
• FIG. 46 illustrates how the upper and lower coils ofFIG. 44 can be replaced by configurable magnetic field (CMF) coils operated in such a manner as to produce the cusp-shaped magnetic field ofFIG. 45.
• FIGS. 47A-47D illustrate a mode of operation of the CMF coils ofFIG. 46 to produce a desired magnetic field configuration.
• FIGS. 48, 49 and50 illustrate an annular apertured plate in the reactor ofFIG. 1A for preventing plasma ions from entering the reactor's pumping annulus.
• FIG. 51 illustrates a rectangular version of the reactor ofFIG. 1A for processing rectangularly shaped workpieces.
• FIG. 52 illustrates a reactor corresponding toFIG. 1A having a retractable workpiece support pedestal.
DETAILED DESCRIPTION
• The plasma ion density distribution exhibited by a particular plasma reactor is a function of chamber pressure, gas mixture and diffusion, and source power radiation pattern. In the present invention, this distribution is magnetically altered to approximate a selected or ideal distribution that has been predetermined to improve process uniformity. The magnetically altered or corrected plasma ion density distribution is such that process uniformity across the surface of the wafer or workpiece is improved. For this purpose, the magnetically corrected plasma distribution may be non-uniform or it may be uniform, depending upon the needs determined by the user. We have discovered that the efficiency with which an average magnetic field strength exerts pressure on a plasma to change its distribution to a desired one can be improved. This surprising result can be achieved in accordance with this discovery by increasing the radial component of the gradient of the magnetic field. The radial direction is understood to be about the axis of symmetry of the cylindrical chamber. Thus, what is needed is a magnetic field configuration which has a large radial gradient and a small field strength in other directions. Such a magnetic field is cusp-shaped with its axis of symmetry coinciding with the axis of the cylindrical reactor chamber. One way of producing a cusp-shaped magnetic field is to provide coils above and below the cylindrical chamber and run D.C. currents through these coils in opposite directions.
• Depending upon the chamber design, it may be impractical to provide a coil below the wafer pedestal, and therefore in a first case, a top coil suffices for these purposes. In addition, what is needed is for the cusp-shaped magnetic field to be configurable or adjustable for accurate control or alteration of a plasma ion distribution inherent in a given plasma reactor chamber (the “ambient” plasma ion distribution). Since the plasma ion distribution provided in different capacitively coupled reactors can vary widely, such adjustability may be essential in some cases. The radial component of the magnetic field gradient is chosen to apply the magnetic pressure required to alter the ambient distribution to the desired distribution. For example, if the desired distribution is a uniform distribution, then the applied magnetic field is selected to counteract the non-uniformity in the radial distribution of plasma ion density exhibited by the reactor in the absence of the magnetic field. In this case, for example, if the reactor tends to have a center-high distribution of plasma ion density, then the magnetic field gradient is chosen to sustain the plasma density over the center of the wafer support pedestal and enhance it near the periphery to achieve uniformity.
• Such adjustability of the cusp-shaped magnetic field is achieved in accordance with our discovery by providing at least a second overhead coil of a different (e.g., smaller) diameter than the first coil. The D.C. currents in the respective coils are independently adjustable so as to permit configuration of the cusp-shaped magnetic field in a highly flexible manner to alter virtually any ambient plasma ion distribution to approximate some desired plasma ion distribution. This choice of field configuration can be designed to modify center-high or center-low plasma ion density distributions.
• One advantage that can be realized is two-fold, in that the cusp-shaped magnetic field has a large radial gradient relative to the magnetic field strength (as noted above) and therefore is highly efficient in exerting corrective pressure on the plasma; but, since the magnetic field is constant over time, there is far less tendency to produce arcing, and therefore a somewhat stronger magnetic field may be employed for even greater corrective capacity when required. As will be described later in this specification, this feature can be quite helpful at higher chamber pressures.
• FIG. 1A illustrates a capacitively coupled plasma reactor capable of providing an adjustable cusp-shaped magnetic field. The reactor ofFIG. 1A includes acylindrical side wall5, aceiling10 that is a gas distribution plate, and awafer support pedestal15 that holds asemiconductor workpiece20. Theceiling10 or gas distribution plate may be conductive so as to enable it to serve as an anode or it may have an anode attached to it. Theceiling10 or gas distribution plate is typically made of aluminum and has an internal gas manifold and gas injection orifices in its interior surface that face into the chamber. Aprocess gas supply25 furnishes process gas to thegas distribution plate10. Avacuum pump30 controls the pressure inside the reactor chamber. Plasma source power for igniting and maintaining a plasma inside the reactor chamber is produced by anRF generator40 connected through animpedance match circuit45 to thewafer support pedestal15 so that the wafer support pedestal serves as an RF electrode. The anode (which may be theceiling10 formed of a conductor material) is connected to RF ground so that is serves as the counter electrode. Such a reactor tends to have a very non-uniform plasma ion density distribution, which is typically center-high.
• FIG. 1B illustrates a feature in which theceiling10, rather than being connected directly to ground as inFIG. 1A, is connected through an RF impedance match element11 (shown only schematically) to aVHF signal generator12 that furnishes the plasma source power. In this case, theRF generator40 merely controls the RF bias on the semiconductor wafer orworkpiece20. (The RFimpedance match element11 may be a fixed tuning element such as for example a coaxial tuning stub or a strip line circuit.) Such a feature is discussed in greater detail in a later portion of this specification.
• In order to control distribution of plasma ion density, a set of inductive coils are provided above theceiling10. In the case ofFIG. 1A, the set of coils includes aninner coil60 and anouter coil65 which are coaxial with the cylindrical chamber and each constitutes single winding of a conductor. While thewindings60,65 are illustrated inFIG. 1A as being single turns, they may each consist of plural turns arranged vertically, for example as shown inFIG. 1B. Or, as shown inFIG. 1C, thewindings60,65 may extend both vertically and horizontally. In the case ofFIG. 1A, theinner coil60 is located farther above theceiling10 than theouter coil65. However, in other cases this arrangement may be reversed, or the twocoils60,65 may be at the same height above theceiling10.
• In the case ofFIGS. 1A and 1B, acontroller90 determines the magnitude and polarity of currents flowing to the respectiveoverhead coils60,65 by controlling respective independent D.C. current supplies70,75 that are connected to respective ones of thecoils60,65. Referring now toFIG. 2, a case is illustrated in which thecontroller90 governs the D.C. currents to thecoils60,65 from a D.C.current supply76 that furnished current through thecontroller90, thecontroller90 being connected to respective ones of thecoils60,65. In either case, thecontroller90 is capable of causing D.C. currents of different polarities and magnitudes to flow in different ones of thecoils60,65. In the case ofFIG. 2, thecontroller90 includes a pair ofpotentiometers82a,82bthat adjust the D.C. current applied to therespective coils60,65 and a pair of gangedswitches84a,84bthat independently determine the polarity of the D.C. current applied to each of thecoils60,65. A programmable device such as amicroprocessor91 can be included in thecontroller90 in order to intelligently govern thepotentiometers82a,82band the ganged switches84a,84b.
• The arrangement of the twocoils60,65 illustrated inFIGS. 1A, 1B and1C, in which theinner coil60 is placed at a greater height above theceiling10 than theouter coil65, provides certain advantages. Specifically, the radial component of the magnetic field gradient provided by either coil is, at least roughly, proportional to the radius of the coil and inversely proportional to the axial displacement from the coil. Thus, the inner andouter coils60,65 will perform different roles because of their different sizes and displacements: Theouter coil65 will dominate across the entire surface of thewafer20 because of its greater radius and closer proximity to thewafer20, while theinner coil60 will have its greatest effect near the wafer center and can be regarded as a trim coil for finer adjustments or sculpting of the magnetic field. Other arrangements may be possible for realizing such differential control by different coils which are of different radii and placed at different displacements from the plasma. As will be described later in this specification with reference to certain working examples, different changes to the ambient plasma ion density distribution are obtained by selecting not only different magnitudes of the currents flowing in the respective overhead coils (60,65) but also by selecting different polarities or directions of current flow for the different overhead coils.
• FIG. 3A illustrates the radial (solid line) and azimuthal (dashed line) components of the magnetic field produced by theinner coil60 as a function of radial position on thewafer20, in the case ofFIG. 1A.FIG. 3B illustrates the radial (solid line) and azimuthal (dashed line) components of the magnetic field produced by theouter coil65 as a function of radial position on thewafer20. The data illustrated inFIGS. 3A and 3B were obtained in an implementation in which thewafer20 was 300 mm in diameter, theinner coil60 was 12 inches in diameter and placed about 10 inches above the plasma, and theouter coil65 was 22 inches in diameter and placed about 6 inches above the plasma.FIG. 3C is a simplified diagram of the half-cusp shaped magnetic field line pattern produced by the inner and outeroverhead coils60,65.
• Thecontroller90 ofFIG. 2 can change the currents applied to therespective coils60,65 in order to adjust the magnetic field at the wafer surface and thereby change the spatial distribution of plasma ion density. What will now be illustrated are the effects of different magnetic fields applied by different ones of thecoils60,65, in order to illustrate how profoundly thecontroller90 can affect and improve plasma ion distribution in the chamber by changing these magnetic fields. In the following examples, the spatial distribution of the etch rate across the wafer surface rather than the plasma ion distribution is measured directly. The etch rate distribution changes directly with changes in the plasma ion distribution and therefore changes in one are reflected by changes in the other.
• FIGS. 4A, 4B,4C and4D illustrate the beneficial effects realized using theinner coil60 only at a low chamber pressure (30 mT).FIG. 4A illustrates measured etch rate (vertical Z axis) as a function of location (horizontal X and Y axes) on the surface of thewafer20.FIG. 4A thus illustrates the spatial distribution of the etch rate in the plane of the wafer surface. The center-high non-uniformity of the etch rate distribution is clearly seen inFIG. 4A.FIG. 4A corresponds to the case in which no magnetic field is applied, and therefore illustrates a non-uniform etch rate distribution that is inherent in the reactor and needs correction. The etch rate has a standard deviation of 5.7% in this case. In FIGS.4A-D and FIGS.5A-D, the magnetic field strength will be described as the axial field near the center of the wafer although it is to be understood that the radial field is the one that works on the radial distribution of plasma ion density to improve uniformity. The axial field is chosen in this description because it is more readily measured. The radial field at the edge of the wafer typically is about one third the axial field at this location.
• FIG. 4B illustrates how the etch rate distribution changes when theinner coil60 has been energized to generate a magnetic field of 9 Gauss. The non-uniformity decreases to a standard deviation of 4.7%.
• InFIG. 4C the magnetic field of theinner coil60 has been increased to 18 Gauss, and it can be seen that the peak at the center has been greatly diminished, with the result that the etch rate standard deviation across the wafer is reduced to 2.1%.
• InFIG. 4D the magnetic field of theinner coil60 has been further increased to 27 Gauss, so that the center high pattern ofFIG. 4A has been nearly inverted to a center low pattern. The standard deviation of the etch rate across the wafer surface in the case ofFIG. 4D was 5.0%.
• FIGS. 5A, 5B,5C and5D illustrate the beneficial effects of using both thecoils60,65 at higher chamber pressures (200 mT).FIG. 5A corresponds toFIG. 4A and depicts the center-high etch rate non-uniformity of the reactor uncorrected by a magnetic field. In this case, the standard deviation of the etch rate across the wafer surface was 5.2%.
• InFIG. 5B, theouter coil65 has been energized to produce a 22 Gauss magnetic field, which decreases somewhat the center peak in the etch rate distribution. In this case, the etch rate standard deviation has been decreased to 3.5%.
• InFIG. 5C, bothcoils60,65 are energized to produce a 24 Gauss magnetic field. The result seen inFIG. 5C is that the center peak in the etch rate distribution has been significantly decreased, while the etch rate near the periphery has increased. The overall effect is a more uniform etch rate distribution with a low standard deviation of 3.2%.
• InFIG. 5D, both coils are energized to produce a 40 Guass magnetic field, producing an over-correction, so that the etch rate distribution across the wafer surface has been transformed to a center-low distribution. The etch rate standard deviation in this latter case has risen slightly (relative to the case ofFIG. 5C) to 3.5%.
• Comparing the results obtained in the low pressure tests ofFIGS. 4A-4D with the high pressure tests ofFIGS. 5A-5D, it is seen that the higher chamber pressure requires a much greater magnetic field to achieve a similar correction to etch rate non-uniform distribution. For example, at 30 mT an optimum correction was obtained using only theinner coil60 at 18 Gauss, whereas at 300 mT a magnetic field of 24 Gauss using bothcoils60,65 was required to achieve an optimum correction.
• FIG. 6 shows that the magnetic fields of the overhead coils greatly affect the uniformity of plasma ion density or etch rate distribution, but do not greatly affect etch rate itself. This is an advantage because, while it is desirable to improve uniformity of etch rate distribution, it is preferable to not change the etch rate chosen for a particular semiconductor process. InFIG. 6, the diamond symbols depict measured etch rate (left-hand vertical axis) as a function of magnetic field (horizontal axis), while the square symbols depict standard deviation (non-uniformity) of the etch rate (right-hand vertical scale) as a function of the magnetic field. The change in non-uniformity over the illustrated range is about one order of magnitude, the change in etch rate is only about 25%.
• Theoverhead coil inductors60,65 ofFIGS. 1A, 1B and1C may be used with a conventional MERIE reactor.FIGS. 7 and 8 illustrate an case corresponding toFIG. 1A with the additional feature of fourconventional MERIE electromagnets92,94,96,98 and an MERIEcurrent controller99. Thecurrent controller99 provides A.C. currents to therespective MERIE electromagnets92,94,96,98. The respective currents are of the same low frequency but have their phases offset by 90 degrees so as to produce a slowly rotating magnetic field within the chamber in the conventional way.
• Controlling Plasma Distribution with the Overhead Coils:
• In accordance with a method of the invention, plasma ion density distribution across the wafer surface that is inherent in a particular reactor is tailored in a particular way by selecting a particular the magnetic field produced by the overhead coils60,65. For example, the plasma distribution may be tailored to produce a more uniform etch rate distribution across the wafer surface. This tailoring is accomplished, for example, by programming thecontroller90 to select optimum polarities and amplitudes of the D.C. current flow in the overhead coils. While the present example concerns a reactor with only two concentric overhead coils (i.e., thecoils60 and65), the method can be carried out with more than two coils, and may provide more accurate results with a greater number of overhead coils. The magnetic field is tailored by thecontroller90 to change the plasma ion density distribution across the wafer surface, which in turn affects the etch rate distribution.
• A first step is to measure the etch rate distribution across the wafer surface in the absence of any corrective magnetic field from the overhead coils60,65. A next step is to determine a change in the plasma ion density distribution that renders the etch rate distribution more uniform. A final step is to determine a magnetic field that would produce the desired change in plasma ion density distribution. Given this magnetic field, the magnitudes and directions of the currents in the overhead coils60,65 necessary to produce such a field can be computed from well-known static magnetic field equations.
• We have found a way of computing, from the magnetic field, pressure exerted by the magnetic field of the overhead coils60,65 on the plasma (the so-called “magnetic pressure”). This will be discussed below. The magnetic pressure on the plasma produces a change in plasma ion density distribution. This change in plasma ion density distribution produces a proportional change in etch rate distribution across the wafer surface, which can be directly observed. The plasma ion density distribution across the wafer surface and the etch rate distribution are therefore at least roughly related by a factor of proportionality.
• Initially, the spatial distribution of the etch rate across the wafer surface is measured prior to the application of magnetic fields from the overhead coils60,65. From this, a desired change in etch rate distribution (to achieve a uniform distribution) can be determined. Next, the spatial distribution of the magnetic field produced by eachoverhead coil60,65 as a function of location within the chamber and current flow in the coil is determined analytically from the geometry of each coil. Then, by applying a known set of currents to the coils and then measuring the resulting change in etch rate distribution across the wafer surface, a linear scale factor can be deduced that relates the vector sum of the magnetic fields from all the coils at the wafer surface to the change in etch rate distribution at the wafer surface. (This scale factor is generally a function of neutral pressure in the plasma and is operative up to about 500 mT chamber pressure.) Therefore, given a desired change or correction in etch rate distribution (to achieve better uniformity), the necessary magnetic fields can be found (in a manner described later in this specification), and the corresponding coil currents can be inferred therefrom using the magnetic field spatial distribution function previously determined analytically.
• The desired correction to the non-uniformity in etch rate distribution can be established in a variety of ways. For example, the 2-dimensional etch rate distribution across the wafer surface can be subtracted from a uniform or average etch rate to produce a “difference” distribution. The non-uniformities in etch rate distribution to be corrected in this method are the result of various factors in the reactor chamber, including non-uniform application of the capacitively coupled source power, non-uniform process gas distribution as well as non-uniform plasma ion density distribution. In the foregoing method, the non-uniformities are corrected by changing the plasma ion density distribution by magnetic pressure.
• The following method can also be employed to establish a “corrected” plasma distribution that is non-uniform in some desired way. In this case, the correction to be made is the difference between the “uncorrected” or ambient plasma ion density distribution and the desired distribution (that is itself non-uniform). Thus, the method is useful for making the plasma density distribution either more uniform or of a particular selected density distribution pattern that is not necessarily uniform.
• A series of steps for carrying out the foregoing method will now be described with reference toFIG. 9.
• The first step (block910 ofFIG. 9) is to analytically determine, for each one of the overhead coils60,65, the expression for the magnetic field at the wafer surface as a function of current flow in the coil and radial location on the wafer surface. Using cylindrical coordinates, this expression may be written, for the i^thcoil, as B_i(r, z=wafer, I_i). It is determined from the Biot-Savart law in a very straight-forward manner.
• The next step (block920 ofFIG. 9) is carried out with no current flowing in the overhead coils60,65. In this step, the spatial distribution of plasma ion density across the wafer surface is measured. This spatial distribution may be written as n(r, z=wafer). In this step, the plasma ion density distribution can be measured indirectly by measuring the etch rate distribution across the surface of a test wafer. The skilled worker can readily infer the plasma ion density distribution from the etch rate distribution.
• Next, in the step ofblock930, a correction, c(r), to the measured plasma ion density spatial distribution function n(r, z=wafer) measured in the previous step is determined. The correction c(r) may be defined in any number of appropriate ways. For example, it may be defined as the maximum value n(r, z=wafer)_maxminus n(r, z=wafer). In this way, adding c(r) to n(r, z=wafer) produces a “corrected” distribution with a uniform amplitude equal to n(r)_max. Of course, the correction function c(r) may be defined differently to produce a different uniform amplitude. Or, as briefly noted above, if the desired distribution is non-uniform, then the correction is the difference between the desired distribution and n(r, z=wafer).
• The next step (block940) is to select a “test” current I_ifor each of the overhead coils60,65 and apply that current to the appropriate coil and measure the resulting plasma ion distribution, which may be written n(r, z=wafer)_test. The change in ion distribution Δn(r) is obtained by subtracting the ion distributions measured with and without the magnetic field:
  Δn(r)≈n(r,z=wafer)−n(r,z=wafer)_test
• The next step (block950) is to compute a scale factor S relating the pressure gradient exerted by the magnetic field (i.e., the magnetic pressure) to the change in ion distribution Δn(r). This computation is performed by dividing the magnetic pressure gradient by Δn(r). The magnetic pressure gradient of the magnetic field B(r, z=wafer, I_i) of the i^thcoil is computed individually for each of the coils in accordance with the magneto-hydrodynamics equation:
  ∇_rP≈−∇_r[B(r,z=wafer,I_i)²/2μ₀]
• where the subscript r denotes radial component. The results thus obtained for each coil individually are then summed together. Therefore, the total magnetic pressure gradient is:
  −∇_r{Σ_i[B(r,z=wafer,I_i)²/2μ₀]}
  Therefore, the scale factor S is:
  S={−∇_r{Σ_i[B(r,z=wafer,I_i)²/2μ₀]}}/Δn(r)
• This division operation may be carried out at different values of r and the results averaged to obtain S in scalar form. Otherwise, the scale factor S will be a function of r and used in the appropriate manner.
• The scale factor S found in the step ofblock950 is a link between the coil currents I_ithat determine the magnetic pressure and a resulting change in ion distribution. Specifically, given a set of coil currents I_i, a corresponding change in ion distribution n(r) can be computed by multiplying the magnetic pressure determined from the set of I_iby the scale factor S:
  Δn(r)={−∇_r{Σ_i[B(r,z=wafer,I_i)²/2μ₀]}}/S
• This fact provides the basis for the following step (block960) in which a computer (such as the microprocessor91) uses the foregoing equation to search for a set of coil currents I_ithat produces the best approximation to previously specified or desired change in plasma ion density distribution, Δn(r). In this case, the desired change is equal to the correction function c(r) computed in the step ofblock930. In other words, the computer searches for a set of coil currents I_ithat satisfies the following condition:
  {−∇_r{Σ_i[B(r,z=wafer,I_i)²/2μ₀]}}=c(r)S
• This search may be carried out by well-known optimization techniques involving, for example, the method of steepest descents. Such techniques are readily carried out by the worker skilled in this field and need not be described here.
• The magnitudes and polarities of the set of coil currents I_idiscovered by the search are then sent to thecontroller90, which in turn applies these currents to therespective coils60,65.
• FIG. 10 compares magnetic pressure (solid line) with the measured change in plasma ion distribution (dotted line) as a function of radial position at the wafer surface. As discussed above, the magnetic pressure is the gradient of the square of the magnetic fields of the overhead coils.FIG. 10 indicates that there is good correlation between magnetic pressure and change in ion density distribution.
• The application of such a method is illustrated inFIGS. 11-14.FIG. 11 illustrates how non-uniformity or the standard deviation (vertical axis) in the etch rate spatial distribution at the wafer surface varied with coil current in one of the overhead coils. At zero coil current, the standard deviation was about 12%, and the ion distribution was center-high as shown inFIG. 12.
• The minimum non-uniformity at about 3% was achieved at a coil current of about 17 amperes. This represents an improvement by about a factor of four (i.e., 12% to 3% standard deviation in the etch rate distribution). The actual or measured etch rate distribution was as shown inFIG. 13A, while the etch rate distribution predicted using the techniques ofFIG. 9 was as shown inFIG. 13B.
• At the high coil current of 35 amperes, the etch rate distribution standard deviation was about 14%. The measured etch rate spatial distribution was as shown inFIG. 14A while the predicted distribution was as shown inFIG. 14B.
• Referring again toFIG. 13A, the most uniform ion distribution obtained is certainly not flat and in fact has “bowl” shape, being concave near the periphery and convex near the center. It is possible that with a greater number of independent overhead coils (e.g., three or more), the optimization of currents may be carried out with greater resolution and better uniformity in results. Therefore, the invention is not limited to the cases having only two coils. The invention may be implemented with varying results using less than or more than two overhead coils.
• The same method may be applied in order to control plasma ion density distribution or etch rate distribution at the ceiling surface. Such an approach may be useful during chamber cleaning operations, for example.FIG. 15 illustrates a version of the method ofFIG. 9 in which uniformity of the spatial distribution of ion density (or, etch rate) is optimized. The steps ofFIG. 15, namely blocks910′,920′,930′,940′,950′ and960′ are the same as the steps ofFIG. 9, namely blocks910,920,930,940,950 and960, except that they are carried out for the ceiling plane rather than the wafer plane:
• The first step (block910′ ofFIG. 15) is to analytically determine, for each one of the overhead coils60,65, the expression for the magnetic field at the ceiling surface as a function of current flow in the coil and radial location on the wafer surface. Using cylindrical coordinates, this expression may be written, for the i^thcoil, as B_i(r, z=ceiling, I_i). It is determined from simple static magnetic field equations and is a function not only of coil current I_iand radial location r on the ceiling surface but also of certain constants such as the radius of the coil and the distance, z=ceiling, between the coil and the ceiling interior surface.
• The next step (block920′ ofFIG. 15) is carried out with no current flowing in the overhead coils60,65. In this step, the spatial distribution of plasma ion density across the ceiling surface is measured. This spatial distribution may be written as n(r, z=ceiling). In this step, the plasma ion density distribution can be measured by a conventional probe or other indirect techniques.
• Next, in the step ofblock930′, a correction, c′(r), to the measured plasma ion density spatial distribution function n(r, z=ceiling) measured in the previous step is determined. (It should be noted that the prime notation ′ is employed here to distinguish the computations ofFIG. 15 from those ofFIG. 9 described above, and does not connote a derivative as used herein.) The correction c′(r) may be defined in any number of appropriate ways. For example, it may be defined as the maximum value n(r, z=ceiling)_maxminus n(r, z=ceiling). In this way, adding c′(r) to n(r, z=ceiling) produces a “corrected” distribution with a uniform amplitude equal to n(r)_max. Of course, the correction function c′(r) may be defined differently to produce a different uniform amplitude. Also, if a particular non-uniform distribution is desired, then the correction is the difference between the uncorrected or ambient plasma distribution n(r, z=ceiling) and the desired non-uniform distribution. Thus, the method can be employed to establish either a desired plasma ion distribution having a particular non-uniform pattern or to establish a uniform plasma ion density distribution.
• The next step (block940′) is to select a “test” current I_ifor each of the overhead coils60,65 and apply that current to the appropriate coil and measure the resulting plasma ion distribution, which may be written n(r, z=ceiling)_test. The change in ion distribution Δn(r) is obtained by subtracting the ion distributions measured with and without the magnetic field:
  Δn′(r)=n(r,z=ceiling)−n(r,z=ceiling)_test
• The next step (block950′) is to compute a scale factor S′ relating the pressure gradient exerted by the magnetic field (i.e., the magnetic pressure) to the change in ion distribution Δn′(r). This computation is performed by dividing the magnetic pressure gradient by Δn′(r). The magnetic pressure gradient of the magnetic field B(r, z=ceiling, I_i) of the i^thcoil is computed individually for each of the coils in accordance with the magneto-hydrodynamics equation:
  ∇_rP=−∇_r[B(r,z=ceiling,I_i)²/2μ₀]
  where the subscript r denotes radial component. The results thus obtained for each coil individually are then summed together. Therefore, the total magnetic pressure gradient is:
  −∇_r{_—i[B(r,z=wafer,I_i)²/2μ₀]}
  Therefore, the scale factor S is:
  S′={−∇_r{Σ_i[B(r,z=wafer,I_i)²/2μ₀]}}/Δn′(r)
• The scale factor S′ found in the step ofblock950′ is a link between the coil currents I_ithat determine the magnetic pressure and a resulting change in ion distribution. Specifically, given a set of coil currents I_i, a corresponding change in ion distribution n′(r) can be computed by multiplying the magnetic pressure determined from the set of I_iby the scale factor S′:
  Δn′(r)={−∇_r{Σ_i[B(r,z=wafer,I_i)²/2μ₀]}}/S′
• This fact provides the basis for the following step (block960′) in which a computer (such as the microprocessor91) uses the foregoing equation to search for a set of coil currents I_ithat produces the best approximation to previously specified or desired change in plasma ion density distribution, Δn′(r). In this case, the desired change is equal to the correction function c′(r) computed in the step ofblock930′. In other words, the computer searches for a set of coil currents I_ithat satisfies the following condition:
  {−∇_r{Σ_i[B(r,z=wafer,I_i)²/2μ₀]}}=c′(r)S′
• This search may be carried out by well-known optimization techniques involving, for example, the method of steepest descents. Such techniques are readily carried out by the worker skilled in this field and need not be described here.
• The magnitudes and polarities of the set of coil currents I_idiscovered by the search are then sent to thecontroller90, which in turn applies these currents to therespective coils60,65.
• With only a single overhead coil, the apparatus can be used to optimize plasma ion distribution uniformity at either the wafer or the ceiling but not both simultaneously. With at least two overhead coils (e.g., the overhead coils60 and65), plasma ion distribution uniformity can be at least approximately optimized at both the wafer and the ceiling simultaneously.
• Steering Plasma with the Overhead Coils:
• We have discovered that the coil currents I_imay be selected in such a manner as to steer the plasma toward the ceiling and/or side walls or to steer it to the wafer surface. The coil currents I_imay also be selected to improve uniformity of plasma density distribution at the ceiling surface in a manner similar to the method ofFIG. 9. As a result, the plasma may be concentrated during processing on the wafer, and then during cleaning may be concentrated on the ceiling and/or side walls. By thus concentrating the plasma at the ceiling, cleaning time may be reduced.
• In one example, the plasma was steered to the side wall of the chamber by thecontroller90 applying a current of −17.5 amperes to theinner coil60 and a current of +12.5 amperes to theouter coil65.FIG. 16 illustrates a radial portion of the chamber interior extending along the horizontal axis from zero radius to the periphery of the chamber and extending along the vertical axis from the wafer surface to the ceiling. The small arrows inFIG. 16 indicate the magnitude and direction of the magnetic field at various locations in the chamber when the plasma is steered to the side wall of the chamber by thecontroller90 applying a current of −17.5 amperes to theinner coil60 and a current of +12.5 amperes to theouter coil65.FIG. 17 illustrates the corresponding gradient of the square of the magnetic field at the wafer surface as a function of radial position.
• In another example, the plasma was steered to the roof of the chamber by thecontroller90 applying a current of −12.5 amperes to theinner coil60 and a current of +5 amperes to theouter coil65.FIG. 18 illustrates a radial portion of the chamber interior extending along the horizontal axis from zero radius to the periphery of the chamber and extending along the vertical axis from the wafer surface to the ceiling. The small arrows inFIG. 18 indicate the magnitude and direction of the magnetic field at various locations in the chamber when the plasma is steered to the side wall of the chamber by thecontroller90 applying a current of −12.5 amperes to theinner coil60 and a current of +5 amperes to theouter coil65.FIG. 19 illustrates the corresponding gradient of the square of the magnetic field at the wafer surface as a function of radial position.
• In a further example, plasma was steered along field lines extending from the center of the ceiling to the side wall by thecontroller90 applying a current of −25 amperes to theinner coil60 and a current of +2.75 to theouter coil65.FIG. 20 illustrates a radial portion of the chamber interior extending along the horizontal axis from zero radius to the periphery of the chamber and extending along the vertical axis from the wafer surface to the ceiling. The small arrows inFIG. 20 indicate the magnitude and direction of the magnetic field at various locations in the chamber when the plasma is steered to the side wall of the chamber by thecontroller90 applying a current of −25 amperes to theinner coil60 and a current of +2.5 amperes to theouter coil65.FIG. 21 illustrates the corresponding gradient of the square of the magnetic field at the wafer surface as a function of radial position.
• FIG. 17 shows that a high positive magnetic pressure on the plasma is exerted near the edge of the chamber when the plasma is steered to the edge.FIG. 19 shows that a low magnetic pressure on the plasma is exerted near the edge of the chamber when the plasma is directed to the edge of the ceiling.FIG. 21 shows that a high negative pressure is present near the chamber edge when the field lines extend from the ceiling to the edge.
• Thus, the currents in the overhead coils60,65 may be chosen to direct the plasma to various locations in the chamber that may require cleaning, such as the ceiling and the side wall. Or, the plasma may be concentrated more near the wafer. In order to steer the plasma to either the wafer or the ceiling, or to apportion the plasma between the wafer and the ceiling in accordance with some steering ratio SR, a method such as that illustrated inFIG. 22 may be carried out.
• Referring now toFIG. 22, the first step (block2210 ofFIG. 22) is to define an analytical model of the magnetic field inside the chamber as a function of all coil currents in the overhead coils (e.g., the pair ofcoils60,65). This is readily accomplished using static magnetic field equations by a worker skilled in this field, and need not be described here. The magnetic field is the sum of the individual magnetic fields from each of the coils. Each individual magnetic field is a function of the diameter of the respective coil, the location of each coil, the current flow in the coil and the location in the chamber. Thus, the magnetic field produced by the i^thcoil may be written as:
  B(x,y,z,I_i)
  so that the total magnetic field is:
  Σ_i{B(x,y,z,I_i)}
• The next step (block2220) is to select a set of magnetic fields that fulfill a set of desired process conditions. For example, to steer plasma to the ceiling, a magnetic field is selected that produces a magnetic pressure on the plasma that pushes the plasma toward the ceiling, as illustrated in the example ofFIG. 18. To steer the plasma toward the side wall, a magnetic field is chosen that produces a magnetic pressure on the plasma that pushes the plasma toward the periphery, as illustrated inFIG. 16.
• For each magnetic field defined in the step ofblock2220 above that fulfills a particular condition, a computer searches the model defined in the step ofblock2210 for a set of coil currents that produce the desired magnetic field. This is the next step ofblock2230. Each set of currents found in the step ofblock2230 is stored along with the name of the corresponding condition in a memory location associated with the corresponding process condition (block2240 ofFIG. 22). Whenever a particular process condition is selected (e.g., steering the plasma to the ceiling), then themicroprocessor91 fetches the set of current values from the corresponding memory location (block2250) and causes the corresponding currents to be applied to the appropriate coils (block2260).
• FIG. 23 shows how themicroprocessor91 may be programmed to respond to user inputs. A determination is first made whether the processing includes etching of the wafer surface (block2310 and whether the process includes cleaning (etching) the ceiling (block2320). If only the wafer is to be etched, then the plasma is steered to the wafer (block2330) and the plasma distribution uniformity at the wafer surface is optimized (block2350) using the method ofFIG. 9. If the wafer is to etched while the ceiling is to cleaned at the same time, then the plasma density is apportioned between the ceiling and the wafer (block2360) and plasma density uniformity is optimized at the wafer surface as inFIG. 9 and at the ceiling as inFIG. 15 (block2370). If only the ceiling is to be cleaned, then the plasma is steered to the ceiling (block2380) and plasma density uniformity at the ceiling is optimized (block2390).
• Use with VHF Overhead Electrode:
• FIG. 24 illustrates how the inner andouter coils60,65 may be combined with a capacitively coupled reactor that has an overhead electrode connected to a VHF plasma source power generator through a fixed tuning stub. Such a reactor is described in U.S. patent application Ser. No. 10/028,922 filed Dec. 19, 2001 by Daniel Hoffman et al. entitled “Plasma Reactor with Overhead RF Electrode Tuned to the Plasma” and assigned to the present assignee, the disclosure of which is incorporated herein by reference.
• Referring toFIG. 24, a plasma reactor includes areactor chamber100 with awafer support105 at the bottom of the chamber supporting asemiconductor wafer110. A process kit may include, in an exemplary implementation, a conductive orsemi-conductive ring115 supported by adielectric ring120 on a groundedchamber body127. Thechamber100 is bounded at the top by a disc shaped overheadconductive electrode125 supported at a gap length above thewafer110 on groundedchamber body127 by a dielectric seal. In one implementation, thewafer support105 is movable in the vertical direction so that the gap length may change. In other implementations, the gap length may be a fixed predetermined length. Theoverhead electrode125 may be a metal (e.g., aluminum) which may be covered with a semi-metal material (e.g., Si or SiC) on its interior surface, or it may be itself a semi-metal material. AnRF generator150 applies RF power to theelectrode125. RF power from thegenerator150 is coupled through acoaxial cable162 matched to thegenerator150 and into acoaxial stub135 connected to theelectrode125. Thestub135 has a characteristic impedance, has a resonance frequency, and provides an impedance match between theelectrode125 and thecoaxial cable162 or the output of theRF power generator150, as will be more fully described below. The chamber body is connected to the RF return (RF ground) of theRF generator150. The RF path from theoverhead electrode125 to RF ground is affected by the capacitance of thedielectric seal120 and by the capacitance of thedielectric seal130. Thewafer support105, thewafer110 and the process kit conductive orsemiconductive ring115 provide the primary RF return path for RF power applied to theelectrode125.
• As in the case ofFIG. 1A, theinner coil60 is less than half the diameter of theouter coil65 and is in a plane farther away from the chamber than theouter coil65. Theouter coil65 is located at or close to the plane of the top of theelectrode125, while theinner coil60 is located well above theelectrode125. As in the case ofFIG. 1A, the D.C. currents in thecoils60,65 are controlled by theplasma steering controller90 governing thecurrent supplies70,75 of thecoils60,65.
• The capacitance of theoverhead electrode assembly126, including theelectrode125, theprocess kit115,120 and thedielectric seal130 measured with respect to RF return or ground was, in one exemplary case, 180 pico farads. The electrode assembly capacitance is affected by the electrode area, the gap length (distance between wafer support and overhead electrode), and by factors affecting stray capacitances, especially the dielectric values of theseal130 and of thedielectric ring120, which in turn are affected by the dielectric constants and thicknesses of the materials employed. More generally, the capacitance of the electrode assembly126 (an unsigned number or scalar) is equal or nearly equal in magnitude to the negative capacitance of the plasma (a complex number) at a particular source power frequency, plasma density and operating pressure, as will be discussed below.
• Many of the factors influencing the foregoing relationship are in great part predetermined due to the realities of the plasma process requirements needed to be performed by the reactor, the size of the wafer, and the requirement that the processing be carried out uniformly over the wafer. Thus, the plasma capacitance is a function of the plasma density and the source power frequency, while the electrode capacitance is a function of the wafer support-to-electrode gap (height), electrode diameter, and dielectric values of the insulators of the assembly. Plasma density, operating pressure, gap, and electrode diameter must satisfy the requirements of the plasma process to be performed by the reactor. In particular, the ion density must be within a certain range. For example, silicon and dielectric plasma etch processes generally require the plasma ion density to be within the range of 10⁹to 10¹²ions/cc. The wafer electrode gap provides an optimum plasma ion distribution uniformity for 8 inch wafers, for example, if the gap is about 2 inches. The electrode diameter is preferably at least as great as, if not greater than the diameter of the wafer. Operating pressures similarly have practical ranges for typical etch and other plasma processes.
• But it has been found that other factors remain which can be selected to achieve the above preferred relationship, particularly choice of source frequency and choice of capacitances for theoverhead electrode assembly126. Within the foregoing dimensional constraints imposed on the electrode and the constraints (e.g., density range) imposed on the plasma, the electrode capacitance can be matched to the magnitude of the negative capacitance of the plasma if the source power frequency is selected to be a VHF frequency, and if the dielectric values of the insulator components ofelectrode assembly126 are selected properly. Such selection can achieve a match or near match between source power frequency and plasma-electrode resonance frequency.
• Accordingly in one exemplary case, for an 8-inch wafer the overhead electrode diameter is approximately 11 inches, the gap is about 2 inches, the plasma density and operating pressure is typical for etch processes as above-stated, the VHF source power frequency is 210 MHz (although other VHF frequencies could be equally effective), and the source power frequency, the plasma electrode resonance frequency and the stub resonance frequency are all matched or nearly matched.
• More particularly, these three frequencies are slightly offset from one another, with the source power frequency being 210 MHz, the electrode-plasma resonant frequency being approximately 200 MHz, and the stub frequency being about 220 MHz, in order to achieve a de-tuning effect which advantageously reduces the system Q. Such a reduction in system Q renders the reactor performance less susceptible to changes in conditions inside the chamber, so that the entire process is much more stable and can be carried out over a far wider process window.
• A currently preferred mode has chamber and pedestal diameters suitable for accommodating a 12 inch diameter wafer, a wafer-to-ceiling gap of about 1.25 inch and an VHF source power frequency of 162 MHz (rather than the 210 MHz referred to above).
• Thecoaxial stub135 is a specially configured design which further contributes to the overall system stability, its wide process window capabilities, as well as many other valuable advantages. It includes an innercylindrical conductor140 and an outer concentriccylindrical conductor145. An insulator147 (denoted by cross-hatching inFIG. 24), having a relative dielectric constant of 1 for example, fills the space between the inner andouter conductors140,145. The inner andouter conductors140,145 may be formed, for example, of nickel-coated aluminum. In an exemplary case, theouter conductor145 has a diameter of about 4 inches and theinner conductor140 has a diameter of about 1.5 inches. The stub characteristic impedance is determined by the radii of the inner andouter conductors140,145 and the dielectric constant of theinsulator147. Thestub135 of the case described above has a characteristic impedance of 65Ω. More generally, the stub characteristic impedance exceeds the source power output impedance by about 20%-40% and preferably by about 30%. Thestub135 has an axial length of about 29 inches (a half wavelength at 220 MHz) in order to have a resonance in the vicinity of 220 MHz to generally match while being slightly offset from the VHF source power frequency of 210 MHz.
• Atap160 is provided at a particular point along the axial length of thestub135 for applying RF power from theRF generator150 to thestub135, as will be discussed below. TheRF power terminal150band theRF return terminal150aof thegenerator150 are connected at thetap160 on thestub135 to the inner and outercoaxial stub conductors140,145, respectively. These connections are made via a generator-to-stub coaxial cable162 having a characteristic impedance that matches the output impedance of the generator150 (typically, 50Ω) in the well-known manner. A terminatingconductor165 at thefar end135aof thestub135 shorts the inner andouter conductors140,145 together, so that thestub135 is shorted at itsfar end135a. At thenear end135b(the unshorted end) of thestub135, theouter conductor145 is connected to the chamber body via an annular conductive housing orsupport175, while theinner conductor140 is connected to the center ofelectrode125 via a conductive cylinder orsupport176. Adielectric ring180 is held between and separates theconductive cylinder176 and theelectrode125.
• Theinner conductor140 provides a conduit for utilities such as process gases and coolant. The principal advantage of this feature is that, unlike typical plasma reactors, thegas line170 and thecoolant line173 do not cross large electrical potential differences. They therefore may be constructed of metal, a less expensive and more reliable material for such a purpose. Themetallic gas line170 feedsgas outlets172 in or adjacent theoverhead electrode125 while themetallic coolant line173 feeds coolant passages orjackets174 within theoverhead electrode125.
• An active and resonant impedance transformation is thereby provided by this specially configured stub match between theRF generator150, and theoverhead electrode assembly126 and processing plasma load, minimizing reflected power and providing a very wide impedance match space accommodating wide changes in load impedance. Consequently, wide process windows and process flexibility is provided, along with previously unobtainable efficiency in use of power, all while minimizing or avoiding the need for typical impedance match apparatus. As noted above, the stub resonance frequency is also offset from ideal match to further enhance overall system Q, system stability and process windows and multi-process capability.
• Matching the Electrode-Plasma Resonance Frequency and the VHF Source Power Frequency:
• As outlined above, a principal feature is to configure theoverhead electrode assembly126 for resonance with the plasma at the electrode-plasma resonant frequency and for the matching (or the near match of) the source power frequency and the electrode-plasma frequency. Theelectrode assembly126 has a predominantly capacitive reactance while the plasma reactance is a complex function of frequency, plasma density and other parameters. (As will be described below in greater detail, a plasma is analyzed in terms of a reactance which is a complex function involving imaginary terms and generally corresponds to a negative capacitance.) The electrode-plasma resonant frequency is determined by the reactances of theelectrode assembly126 and of the plasma (in analogy with the resonant frequency of a capacitor/inductor resonant circuit being determined by the reactances of the capacitor and the inductor). Thus the electrode-plasma resonant frequency may not necessarily be the source power frequency, depending as it does upon the plasma density. The problem, therefore, is to find a source power frequency at which the plasma reactance is such that the electrode-plasma resonant frequency is equal or nearly equal to the source power frequency, given the constraints of practical confinement to a particular range of plasma density and electrode dimensions. The problem is even more difficult, because the plasma density (which affects the plasma reactance) and the electrode dimensions (which affect electrode capacitance) must meet certain process constraints. Specifically, for dielectric and conductor plasma etch processes, the plasma density should be within the range of 10⁹-10¹²ions/cc, which is a constraint on the plasma reactance. Moreover, a more uniform plasma ion density distribution for processing 8-inch diameter wafers for example, is realized by a wafer-to-electrode gap or height of about 2 inches and an electrode diameter on the order of the wafer diameter, or greater, which is a constraint on the electrode capacitance. On the other hand, a different gap may be utilized for a 12-inch diameter wafer.
• Accordingly, by matching (or nearly matching) the electrode capacitance to the magnitude of the negative capacitance of the plasma, the electrode-plasma resonant frequency and the source power frequency are at least nearly matched. For the general conductor and dielectric etch process conditions enumerated above (i.e., plasma density between 10⁹-10¹²ions/cc, a 2-inch gap and an electrode diameter on the order of roughly 11 inches), the match is possible if the source power frequency is a VHF frequency. Other conditions (e.g., different wafer diameters, different plasma densities, etc.) may dictate a different frequency range to realize such a match in carrying out this feature of the reactor. As will be detailed below, under favored plasma processing conditions for processing 8-inch wafers in several principal applications including dielectric and metal plasma etching and chemical vapor deposition, the plasma capacitance in one typical working example having plasma densities as set forth above was between −50 and −400 pico farads. In an exemplary case the capacitance of theoverhead electrode assembly126 was matched to the magnitude of this negative plasma capacitance by using an electrode diameter of 11 inches, a gap length (electrode to pedestal spacing) of approximately 2 inches, choosing a dielectric material forseal130 having a dielectric constant of 9, and a thickness of the order of one inch, and a dielectric material for thering120 having a dielectric constant of 4 and thickness of the order of 10 mm.
• The combination ofelectrode assembly126 and the plasma resonates at an electrode-plasma resonant frequency that at least nearly matches the source power frequency applied to theelectrode125, assuming a matching of their capacitances as just described. We have discovered that for favored etch plasma processing recipes, environments and plasmas, this electrode-plasma resonant frequency and the source power frequency can be matched or nearly matched at VHF frequencies; and that it is highly advantageous that such a frequency match or near-match be implemented. In an exemplary case, the electrode-plasma resonance frequency corresponding to the foregoing values of plasma negative capacitance is approximately 200 MHz, as will be detailed below. The source power frequency is 210 MHz, a near-match in which the source power frequency is offset slightly above the electrode-plasma resonance frequency in order to realize other advantages to be discussed below.
• The plasma capacitance is a function of among other things, plasma electron density. This is related to plasma ion density, which needs, in order to provide good plasma processing conditions, to be kept in a range generally 10⁹to 10¹²ions/cc. This density, together with the source power frequency and other parameters, determines the plasma negative capacitance, the selection of which is therefore constrained by the need to optimize plasma processing conditions, as will be further detailed below. But the overhead electrode assembly capacitance is affected by many physical factors, e.g. gap length (spacing betweenelectrode125 and the wafer); the area ofelectrode125; the range of the dielectric loss tangent for thedielectric seal130; the choice of dielectric constant of thedielectric seal130 betweenelectrode125 and groundedchamber body127; the choice of dielectric constant for the process kitdielectric seal130; and the thickness of thedielectric seals130 and120 and the thickness and dielectric constant of thering180. This permits some adjustment of the electrode assembly capacitance through choices made among these and other physical factors affecting the overhead electrode capacitance. We have found that the range of this adjustment is sufficient to achieve the necessary degree of matching of the overhead electrode assembly capacitance to the magnitude of the negative plasma capacitance. In particular, the dielectric materials and dimensions for theseal130 andring120 are chosen to provide the desired dielectric constants and resulting dielectric values. Matching the electrode capacitance and the plasma capacitance can then be achieved despite the fact that some of the same physical factors influencing electrode capacitance, particularly gap length, will be dictated or limited by the following practicalities: the need to handle larger diameter wafers; to do so with good uniformity of distribution of plasma ion density over the full diameter of the wafer; and to have good control of ion density vs. ion energy.
• Given the foregoing range for the plasma capacitance and the matching overhead electrode capacitance, the electrode-plasma resonance frequency was approximately 200 MHz for a source power frequency of 210 MHz.
• A great advantage of choosing the capacitance of theelectrode assembly126 in this manner, and then matching the resultant electrode-plasma resonant frequency and the source power frequency, is that resonance of the electrode and plasma near the source power frequency provides a wider impedance match and wider process window, and consequently much greater immunity to changes in process conditions, and therefore greater performance stability. The entire processing system is rendered less sensitive to variations in operating conditions, e.g., shifts in plasma impedance, and therefore more reliable along with a greater range of process applicability. As will be discussed later in the specification, this advantage is further enhanced by the small offset between the electrode-plasma resonant frequency and the source power frequency.
• FIG. 25 illustrate how the inner andouter coils60,65 may be combined with a capacitively coupled reactor that has an overhead electrode connected to a VHF plasma source power generator through a fixed tuning stub, and has MERIE electromagnets around its periphery. Such a reactor is described in U.S. patent application Ser. No. 10/028,922 filed Dec. 19, 2001 by Daniel Hoffman et al. entitled “Plasma Reactor with Overhead RF Electrode Tuned to the Plasma” and assigned to the present assignee, the disclosure of which is incorporated herein by reference.
• Referring toFIG. 25, a VHF capacitively coupled plasma reactor includes the following elements found in the reactor ofFIG. 1A: areactor chamber100 with awafer support105 at the bottom of the chamber supporting asemiconductor wafer110. A process kit in the illustrated case consists of a semi-conductive orconductive ring115 supported by adielectric ring120 on the groundedchamber body127. Thechamber100 is bounded at the top by a disc shapedoverhead aluminum electrode125 supported at a predetermined gap length above thewafer110 on groundedchamber body127 by adielectric seal130. Theoverhead electrode125 also may be a metal (e.g., aluminum) which may be covered with a semi-metal material (e.g., Si or SiC) on its interior surface, or it may be itself a semi-metal material. AnRF generator150 applies RF power to theelectrode125. RF power from thegenerator150 is coupled through acoaxial cable162 matched to thegenerator150 and into acoaxial stub135 connected to theelectrode125. Thestub135 has a characteristic impedance, resonance frequency, and provides an impedance match between theelectrode125 and thecoaxial cable162/RF power generator150, as will be more fully described below. The chamber body is connected to the RF return (RF ground) of theRF generator150. The RF path from theoverhead electrode125 to RF ground is affected by the capacitance of the process kitdielectric ring120 and thedielectric seal130. Thewafer support105, thewafer110 and the process kit semiconductive (or conductive)ring115 provide the primary RF return path for RF power applied to theelectrode125.
• As in the case ofFIG. 1A, theinner coil60 is less than half the diameter of theouter coil65 and is in a plane farther away from the chamber than theouter coil65. Theouter coil65 is located at or close to the plane of the top of theelectrode125, while theinner coil60 is located well above theelectrode125. As in the case ofFIG. 1, the D.C. currents in thecoils60,65 are controlled by theplasma steering controller90 governing thecurrent supplies70,75 of thecoils60,65.
• The improvement in plasma density distribution uniformity is achieved by the introduction of a set of MERIE electromagnets902 spaced equally about the periphery of the wafer support pedestal and outside of the reactor chamber (like those shown inFIGS. 7 and 8). These MERIE magnets are adapted to produce a magnetic field that slowly rotates about the axis of symmetry of the cylindrical chamber generally across the surface of the wafer support pedestal. In one case this feature is realized by the MERIE magnets902 having electromagnet windings wound about respective axes tangent to the circumference of the wafer support pedestal. In this case, an MERIEcurrent controller904 controls the individual current to each MERIE magnet. A circulating magnetic field is generated in the plane of the workpiece support by thecontroller904 providing individual AC currents to each of the individual magnet windings of the same frequency but offset in phase by 90 degrees (or by 360 degrees divided by the number of MERIE magnets). In an alternative case, the feature of a rotating magnetic field is realized by a support frame1020 (dashed line) supporting all of the MERIE magnets that is rotated about the axis of symmetry by a rotor1025 (dashed line). In this alternative case, the MERIE magnets are permanent magnets.
• A second array of MERIE magnets906 (shown in dashed line) equally spaced about the workpiece or wafer support pedestal but in a higher plane than the first set of MERIE magnets902 may be provided as well. Both sets of magnets lie in respective planes that are near the plane of the workpiece support.
• Thecontroller904 applies a low frequency (0.5-10 Hz) AC current to each of theelectromagnets902,906, the phases of the currents applied to neighboring magnets being offset as described above by 90 degrees. The result is a magnetic field that rotates about the axis of symmetry of the workpiece support at the low frequency of the AC current. The magnetic field causes the plasma to be drawn toward the magnetic field near the workpiece surface and to circulate with the field. This stirs the plasma so that its density distribution becomes more uniform. As a result, reactor performance is significantly improved because more uniform etch results are obtained across the entire surface of the wafer.
• Combination Overhead Electrode and Gas Distribution Plate:
• It is desirable to feed the process gas from the overhead ceiling to improve uniformity of gas distribution within the chamber. For this purpose, theoverhead electrode125 in the cases ofFIGS. 24 and 25 can be a gas distribution showerhead, and therefore has a large number of gas injection ports orsmall holes300 in its bottom surface facing theworkpiece support105. In an exemplary case, theholes300 were between 0.01 and 0.03 inch in diameter and their centers were uniformly spaced apart by about ⅜ inch.
• The overhead electrode/gas distribution plate125 (hereinafter referred to as the gas distribution plate125) has improved resistance to arcing. This is due to the introduction of an arc suppression feature that excludes process gas and/or plasma from the center of each opening orhole300. This arc suppressing feature is a set of center pieces ordisks302 in the centers of theholes300 supported at the ends of respective cylindrical fingers orthin rods303 as shown in the cross-sectional view ofFIG. 26 and the enlarged cross-sectional view ofFIG. 27. Arcing within a typical gas distribution plate tends to occur near the center of the gas injection holes. Therefore, placing thecenter pieces302 at the center of eachhole300 prevents process gas from reaching the center of eachhole300 and therefore reduces the occurrence of arcing. As shown in the plan view ofFIG. 28, introduction of thecenter pieces302 in theholes300 transforms the otherwise circular openings orholes300 into annular openings.
• Referring toFIG. 29A, thegas distribution plate125 with improved arc suppression constitutes acover1402 and abase1404. Thebase1404 is adiscoid plate1406 with the gas injection openings formed therethrough surrounded by anannular wall1408 having aninterior shoulder1410. Thecover1402 is also a discoid plate. Thedisks302 are the end sections of thecylindrical fingers303 attached to and extending downwardly from the bottom surface of thecover1402. The outer edge of thecover1402 rests on theshoulder1410 of the base1404 to form a gas manifold1414 (FIG. 26) between thecover1402 and thebase1404. Process gas flows into the manifold1414 from agas inlet1416 in the center of thecover1402.
• The portions of thegas distribution plate125 that contact process gas or plasma in the chamber can be formed of a metal such as aluminum coated with a semiconductor processing compatible material such as silicon carbide. In this example, all surfaces of the gas distribution plate, with the exception of the top surface of thecover1402, are covered with asilicon carbide coating1502 as indicated in the enlarged partial cross-sectional view ofFIG. 29B. As shown inFIG. 30, the aluminum top surface of thecover1402 is in contact with a temperature-controlledmember1520 that may be water-cooled bywater jackets1522 with coolant circulated by aheat exchanger1524, so that the thermally conductive aluminum material of thegas distribution plate125 has a controlled temperature. Alternatively, as shown inFIG. 31, the water jackets may be within thegas distribution plate125.
• However, in order for thesilicon carbide coating1502 to have the same controlled temperature, there must be a thermally conductive bond between the silicon carbide coating and the aluminum. Otherwise, the temperature of the silicon carbide coating could fluctuate uncontrollably. In order to achieve good thermal conductivity between the aluminum material of thegas distribution plate125 and the silicon carbide coating, apolymer bonding layer1504 is formed between the aluminum gas distribution plate and thesilicon carbide coating1502, as shown inFIG. 29A.FIG. 29A shows that thepolymer bonding layer1504 is between thesilicon carbide coating1502 and thealuminum base1404. The polymer bonding layer provides good thermal conductivity between the aluminum and thesilicon carbide coating1502, so that the temperature of thecoating1502 is controlled by theheat exchanger1524.
• FIGS. 32, 33 and34 illustrate how thegas distribution plate125 ofFIG. 29A can be modified to provide dual zone gas flow control. Such a feature can be employed to help correct an etch rate or deposition rate spatial distribution that is either center high or center low by selecting a process gas distribution that is complementary. Specifically, an annular partition orwall1602 divides thegas manifold1414 into acenter manifold1414aand anouter manifold1414b. In addition to thecenter gas feed1416 that feeds thecenter manifold1414a, anothergas feed1418 between the center and periphery of thegas distribution plate125 feeds theouter manifold1414b. Adual zone controller1610 apportions gas flow from aprocess gas supply1612 between the inner and outer gas feeds1416,1418.FIG. 35 illustrates one implementation of thevalve1610 in which an articulatingvane1618 controls the relative amount of gas flow to the inner andouter manifolds1414a,1414bof the gas distribution plate. Anintelligent flow controller1640 governs the position of thevane1618. In another implementation illustrated inFIG. 36, a pair ofvalves1651,1652 perform individual gas flow control for respective radial zones of the chamber.
• FIG. 37 illustrates an case in which thegas distribution plate125 has three gas flow zones, the manifold1414 being separated by inner and outerannular partitions1604,1606 into threemanifolds1414a,1414band1414c. Three respective gas feeds1416,1418,1420 provide gas flow to therespective manifolds1414a, b, c.
• While various cases have been described above in this specification as having a pair ofoverhead coils60,65,FIG. 37 shows that there can be more than two overhead coils. In fact, the case ofFIG. 37 is illustrated as having three concentric overhead coils or coils60,64 and65. By increasing the number of independently controlled overhead coils, it is felt the resolution with which processing non-uniformities are corrected is increased.
• The multiple zone gas distribution plates ofFIGS. 34 and 37 enjoy the advantage of flexible control over gas apportionment between inner and outer processing zones of the workpiece. However, another way of customizing gas flow is to do so permanently by providing different gas injection hole sizes at different radii of thegas distribution plate125. For example, if the reactor tends to exhibit a spatial etch rate distribution that is center high, then less gas would be supplied near the center and more at the periphery of the chamber by using smaller gas injection holes300 at the center and larger ones near the periphery. Such a gas distribution plate is illustrated in plan view inFIG. 38. For a center low etch distribution, the opposite hole arrangement would be employed as illustrated inFIG. 39.
• Plasma Steering in the Reactor ofFIG. 9:
• Plasma steering as described above with reference toFIGS. 11-14 was performed in the case ofFIG. 9. A magnetic field pointing to the side wall was produced by applying a current of −13 amperes to theinner coil60 and a current of +1.4 amperes to theouter coil65. A magnetic field pointing toward the periphery of the ceiling orelectrode125 was produced by applying a current of −13 amperes to theinner coil60 and a current of +5.2 amperes to theouter coil65. A dense magnetic field at the side wall was produced by applying a current of −13 amperes to theinner coil60 and a current of +9.2 amperes to theouter coil65. We found that the etch rate of chamber surfaces during cleaning were improved by as much as 40% by applying a magnetic field pointing toward the periphery of the ceiling orelectrode125 in the manner described above.
• Coil Configurations:
• While the foregoing cases have been described with reference to the inner andouter coils60,65, a greater number of coils may be employed. For example, the case ofFIG. 40 has fiveoverhead coils4060,4062,4064,4066,4068, each with its own current separately controlled by thecontroller90. Thecoils4060,4062,4064,4066,4068 may be at the same height above the ceiling125 (as inFIG. 40) or at different heights.FIG. 41 illustrates an case in which the overhead coils60,65 are at the same height. InFIG. 41, the windings in eachcoil60,65 are stacked in both vertical and radial directions.FIGS. 42 and 43 illustrate different cases in which thecoils60,65 have windings extending in the vertical direction and in the radial direction.
• As discussed previously in this specification with reference toFIG. 1A, magnetic pressure on the plasma for correcting non-uniform distribution is proportional to the radial component of the gradient of the square of the magnetic field. Thus, the most efficient approach is to employ a magnetic field having a large radial gradient, such as a cusp-shaped magnetic field. As further discussed above, the greater efficiency of the cusp-shaped magnetic field reduces the required strength of the magnetic field for a given amount of magnetic pressure, thereby reducing or eliminating device damage associated with high magnetic fields.FIG. 44 illustrates an case in which a fully cusp-shaped magnetic field is produced by a pair ofcoils4420,4440 located above and below the chamber, respectively. Current flow in the top andbottom coils4420,4440 is clockwise and counter-clockwise, respectively.FIG. 45 is a simplified illustration of the magnetic field line pattern of the fully cusp-shaped magnetic field produced by the pair ofcoils4420,4440.
• FIG. 46 illustrates an case in which the fourelectromagnets4610,4620,4630,4640 of aconventional MERIE reactor4650 are employed to generate the fully cusp-shaped magnetic field ofFIG. 45. Acurrent controller4660 controlling the currents in each of theelectromagnets4610,4620,4630,4640 is programmed to apply D.C. currents flowing in the same (e.g., clockwise) direction in all theelectromagnets4610,4620,4630,4640, as indicated by the arrows inFIG. 46. In this way the D.C. currents in thetop conductors4610a,4620a,4630a,4640aform a clockwise current loop, the D.C. currents in thebottom conductors4610b,4620b,4630b,4640bform a counter-clockwise current loop, while at each corner of the array the currents in the vertical conductors of adjacent electromagnets (e.g., the pair ofvertical conductors4620cand4630d) cancel the magnetic fields of one another at the wafer surface. The net effect is to produce clockwise and counter-clockwise current loops at the top and bottom of the chamber, respectively, analogous to the case ofFIG. 44, with the same resulting fully cusp-shaped magnetic field illustrated inFIG. 45. The reactor ofFIG. 46 is operated in any one of three modes:
• magnetic pressure mode, in which the cusp-shaped field is produced;
• sine wave mode, in which four sine wave currents are applied in quadrature to the fourelectromagnets4610,4620,4630,4640 to produce a slowly rotating magnetic field over the wafer surface;
• configurable magnetic field (CMF) mode, in which the fourelectromagnets4610,4620,4630,4640 are grouped into to opposing sets of adjacent pairs, one pair having one D.C. current and the opposite pair having the opposite D.C. current, to produce generally straight magnetic field lines extending across the wafer surface in a diagonal direction relative to the orientation of the fourelectromagnets4610,4620,4630,4640. This grouping is rotated by switching the currents so that the magnetic field rotates through four diagonal orientations. A time sequence of these orientations are illustrated inFIGS. 47A, 47B,47C and47D.
• InFIG. 47A, theelectromagnets4610,4620 have a positive D.C. current flow while theelectromagnets4630,4640 have negative D.C. current flow, and the resulting average magnetic field direction is generally from the upper left corner to the lower right corner of the drawing. InFIG. 47B, the groupings have been switched so that theelectromagnets4620,4630 have the positive current flow while theelectromagnets4640,4610 have the negative current flow, and the average magnetic field has rotated clockwise by 90 degrees.FIGS. 47C and 47D complete the cycle. The strength of the magnetic field lines is determined by the magnitude difference in the positive and negative D.C. currents thus applied, and may be adjusted by programming thecontroller4650 as desired.
• The method ofFIG. 9 may be employed in the CMF mode to accurately select the D.C. currents of the fourelectromagnets4610,4620,4630,4640 to produce the best correction for non-uniform etch rate or plasma ion density distribution. In applying the method ofFIG. 9 to the CMF mode of FIGS.47A-D, the coils of each of the electromagnets orcoils4610,4620,4630,4640 are substituted for the overhead coils60,65, and all steps ofFIG. 9 are performed in accordance with that substitution. The only difference is that the calculation of the magnetic field from each coil is computed as an average over the four time periods corresponding to FIGS.47A-D.
• FIG. 48 illustrates a reactor including aspecial grating4810 inserted over the pumping annulus. Thegrating4810 is formed of a semiconductive material such as silicon carbide or of a conductive material such as aluminum and hasopenings4820 for permitting gas to be evacuated from the chamber through the pumping annulus. Thespecial grating4810 excludes plasma from the pumping annulus, providing needed protection and process control. For this purpose, the distance across the interior of eachopening4820 in the radial plane is no greater than twice the plasma sheath thickness. In this way it very difficult if not impossible for a plasma to penetrate through thegrating4810. This reduces or eliminates plasma interaction with chamber surfaces within the pumping annulus.
• FIGS. 49 and 50 illustrate an integrally formedremovable chamber liner4910 that incorporates the plasma-confininggrating4810 ofFIG. 48. Theliner4910 covers the portions of the chamber that are radially outside of the region underlying theelectrode125 and overlying thewafer110. Thus, theliner4910 includes an upperhorizontal section4920 covering an outer periphery of the chamber ceiling, avertical section4930 covering the chamber side wall and a lowerhorizontal section4940 that includes the plasma-confininggrating4810 and covers the pumping annulus as well as an annular surface adjacent thewafer110. In one case, each of thesections4920,4930,4940 are formed together as a monolithicsilicon carbide piece4950. Theliner4910 further includes analuminum base4960 underlying the lowerhorizontal section4940 of thesilicon carbide piece4950 and is bonded thereto. Thealuminum base4960 includes a pair of downwardly extendingannular rails4962,4964 that are relatively long and thin and provide good electrical conductivity to grounded structural elements of the chamber below thewafer support pedestal105.
• The reactor can havetemperature control elements4972,4974 in thermal contact with the downwardly extendingannular rails4962,4964 as well as atemperature control element4976 in thermal contact with thevertical side section4930. Each of thethermal control elements4972,4974,4976 can include cooling apparatus including coolant passages and heating apparatus including an electric heater. It can be desirable to maintain theliner4910 at a sufficiently high temperature (e.g., as high as 120 degrees F.) to minimize or prevent deposition of polymer or fluorocarbon compounds on interior surfaces of theliner4910.
• Theliner4910 enhances process stability because it provides a good ground return path. This is due to the fact that the electric potential is uniform along the interior surface of the silicon carbide piece4950 (including the interior-facing surfaces of the upperhorizontal section4920, thevertical section4930 and the lower horizontal section4940). As a result, theliner4910 provides a uniform RF return path at all of its interior-facing surfaces for power delivered either from theoverhead electrode125 or from thewafer pedestal105. One advantage is that as plasma fluctuations move the RF return current distribution to concentrate at different parts of the interior surface of theliner4910, the impedance presented to that current remains fairly constant. This feature promotes process stability.
• FIG. 51 illustrates a modification of the case ofFIG. 7 in which theoverhead solenoids60,65 define a square pattern symmetrical with the square pattern of theMERIE magnets92,94,96,98, and is particularly suited for uniform processing of a square semiconductor ordielectric workpiece4910, such as a photolithographic mask.
• FIG. 52 illustrates a version of the reactor ofFIG. 24 in which thewafer support pedestal105 may be moved up and down. In addition to the twooverhead coils60,65 for controlling plasma ion radial distribution, there is abottom coil5210 below the plane of thewafer support pedestal105. In addition, there is anouter coil5220 at the periphery of the chamber. The outeroverhead coil65 and thebottom coil5210 can have opposing D.C. currents to form a full cusp magnetic field within the chamber.
• While the overhead coils60,65 have been described in combination with reactor having an overhead ceiling that serves as both an overhead source power electrode and as a gas distribution plate, the ceiling may be of the type that is not a gas distribution plate, with process gases being introduced in another conventional fashion (e.g., through the side wall). Moreover, thecoils60,65 may be employed in a reactor in which source power is not capacitively coupled by a ceiling electrode. Also, the impedance match element for the overhead electrode has been described as being a fixed element such as a coaxial tuning stub. However, the impedance match element may be any suitable or conventional impedance match device such as a conventional dynamic impedance match circuit.
• While the invention has been described in detail by specific reference to preferred cases, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.

Claims (21)

1. A wafer processing apparatus, comprising:

a housing defining a process chamber

a wafer support configured to support a wafer within the chamber during processing;

a first process gas inlet;

a second process gas inlet;

a gas distribution system, comprising:

a center circular gas disperser configured to receive a process gas from the first process gas inlet and to distribute the process gas into the chamber over the wafer through a first plurality of injection ports; and

an outer annular gas disperser centered around the center gas disperser configured to receive the process gas from the second process gas inlet and to distribute the process gas into the chamber over the wafer through a second plurality of injection ports.

2. The apparatus ofclaim 1 wherein the first and second plurality of injection ports are annular.

3. The apparatus ofclaim 1 wherein the first and second plurality of injection ports are circular holes, wherein the holes have diameters ranging between 0.01 and 0.03 inches.

4. The apparatus ofclaim 1 wherein the gas distribution system comprises an annular wall that separates the center gas disperser and the outer gas disperser.

5. The apparatus ofclaim 1 further comprising:

a first gas flow controller coupled to the first process gas inlet, wherein the first gas flow controller can be independently controlled to adjust the amount of the process gas flowing into the center gas disperser;

a second gas flow controller coupled to the second process gas inlet; and

wherein the first and the second gas flow controllers can be independently controlled to adjust the amount of the process gas flowing into the center circular gas disperser relative to the amount of process gas flowing into the outer annular gas disperser.

6. The apparatus ofclaim 5 wherein the first gas flow controller comprises a first valve and wherein the second gas flow controller comprises a second valve.

7. The apparatus ofclaim 5 further comprising a dual zone controller coupled to the first gas flow controller and the second gas flow controller, the dual zone controller configured to adjust flow through the first gas flow controller and through the second gas flow controller.

8. A wafer processing apparatus, comprising:

a housing defining a processing chamber, the housing coupled to an RF ground;

a substrate support located in a chamber configured to support a wafer during processing;

first and second process gas inlets configured to deliver a process gas into the chamber;

a gas distribution system comprising a circular gas disperser having a circular center gas dispersing region fluidly coupled to the first process gas inlet and an annular gas dispersing region surrounding the center region and fluidly coupled to the second process gas inlet, wherein the center gas dispersing region comprises a first plurality of gas injection holes configured to introduce the process gas into the chamber above a wafer supported on the substrate support and the annular gas dispersing region comprises a second plurality of gas injection holes configured to introduce the process gas into the chamber annularly to the center gas dispersion region above the wafer; and

an RF generator coupled to an impedance match circuit used to provide RF power to the wafer support, wherein the impedance match circuit is coupled to the wafer support and wherein the RF generator is coupled to the RF ground.

9. The apparatus ofclaim 8 wherein the first and second plurality of gas injections holes are annular.

10. The apparatus ofclaim 8 wherein the first and second plurality of injection holes have diameters ranging between 0.01 and 0.03 inches.

11. The apparatus ofclaim 8 wherein the gas distribution system comprises an annular wall that forms a boundary separating the center gas disperser and the outer gas disperser.

12. The apparatus ofclaim 8 wherein the first plurality of gas injection holes are configured to introduce the process gas into a center portion of a wafer supported on the substrate support and the second plurality of gas injection holes are configured to introduce the process gas into the chamber above an outer peripheral portion of the wafer.

13. The apparatus ofclaim 8 further comprising:

a first gas flow controller coupled to the first process gas inlet, wherein the first gas flow controller can be independently controlled to adjust the amount of the process gas flowing into the center gas disperser; and

a second gas flow controller coupled to the second process gas inlet, wherein the second gas flow controller can be independently controlled to adjust the amount of the process gas flowing into the outer annular gas disperser.

14. The apparatus ofclaim 13 further comprising a dual zone controller coupled to the first gas flow controller and the second gas flow controller, the dual zone controller configured to adjust flow through the first gas flow controller and through the second gas flow controller.

15. The apparatus ofclaim 8 further comprising an annular pumping channel below and surrounding the wafer support coupled to an exhaust line.

16. A wafer processing apparatus, comprising:

a vacuum chamber configured to support a plasma;

a process gas inlet configured to deliver a process gas used for the plasma into the vacuum chamber;

a gas disperser coupled to the process gas inlet, comprising:

a base having a plurality of injection ports formed throughout, surrounded by an annular wall having an interior shoulder;

a cover having a top surface, a bottom surface and a plurality of fingers, the plurality of fingers attached to the bottom surface and extending downwardly from the bottom surface, the top surface coupled to the process gas inlet; and

wherein the fingers extend into the injection ports of the base to form a plurality of annular ports in the base for the process gas to flow from the gas disperser to a processing region.

17. The apparatus ofclaim 16 wherein the plurality of injection ports are circular holes, wherein the holes have diameters ranging between 0.01 and 0.03 inches.

18. The apparatus ofclaim 17 further comprising an annular wall positioned between the base and the cover forming a center gas disperser and an outer gas disperser.

19. The apparatus ofclaim 18 further comprising a second process gas inlet coupled to the top surface of the cover and positioned to be over the outer gas disperser.

20. The apparatus ofclaim 19 further comprising:

a first gas flow controller coupled to the process gas inlet, wherein the first gas flow controller can be independently controlled to adjust the amount of the process gas flowing into the center gas disperser; and

a second gas flow controller coupled to the second process gas inlet, wherein the second gas flow controller can be independently controlled to adjust the amount of the process gas flowing into the outer gas disperser.

21. The apparatus ofclaim 19 further comprising a dual zone controller coupled to the first gas flow controller and the second gas flow controller, the dual zone controller configured to adjust flow through the first gas flow controller and through the second gas flow controller.

Images