The invention relates to a polygonal planar multipath component, a system containing the polygonal planar multipath component and a using method thereof, which are filed by divisional application, wherein the filing date of the original application is 10/9/1996, and the application number is 96112704. X.
Semiconductor Integrated Circuits (ICs) are manufactured through a series of processes, many of which involve the use of gaseous materials. These processing treatments include: etching, diffusion, chemical vapor deposition, ion implantation, sputtering, and rapid thermal processing. In these processes, contact is made between the semiconductor substrate and the gas-phase material. As a result of the best performance of integrated circuit devices, it is generally believed that it is necessary to achieve parts per billion (ppb) and lower levels of all impurities in the gases contacting the semiconductor substrate to minimize product loss. Among the impurities that may be present, moisture is the most difficult to eliminate and can have a deleterious effect on many semiconductor manufacturing processes.
In nitrogen, argon and hydrogen environments, there are several analytical techniques that can provide the necessary sensitivity for measuring moisture and other impurities. Among these, pressure ionization mass spectrometry (APIMS) is most notable, however, APIMS is not suitable for many reactive gases. However, this method is in SiH4The use of (II) has been described (see article Y. Mitsui et al, IES 40 th technical annual meeting, proceedings P, 246-254, Chicago 1994, IES1994), but it is difficult to be applied to practice. There is a need for highly sensitive technologies that are suitable for use with a variety of gases. Fourier transform Infrared Spectroscopy (FT-IR) is the technique currently selected by many gas analysis laboratories for reactive gases. Unfortunately, FT-IR does not appear to provide the 1ppb sensitivity required by the semiconductor manufacturing industry. Furthermore, the spectral resolution achievable with FT-IR is limited and it is more difficult to detect impurities in the presence of other infrared absorbing molecules. It is also difficult to design a very compact FT-IR.
Tunable Diode Laser Absorption Spectroscopy (TDLAS) is a highly adaptable and sensitive technology that is widely used in environmental monitoring, spectroscopy, chemical kinetics, and the like. TDLAS is suitable for construction of miniature sensors because its components can be made small. With constant pressure and concentration, as the optical path length of light through the sample increases, the sensitivity of absorption spectroscopy to detect various gas phase molecular species also increases. The intensity of the illumination reaching the detector is given by Beer's law:
I=I0·e-αLCPwherein I0Is the intensity of the incident light, α is the absorbance, L is the length of light passing through the sample, C is the concentration of impurities in the sample (volumetric measurement), and P is the total pressure of the sample. For the case of small absorption, the amount of light absorbed is given by:
I-I0to increase L, the source and detector are located far apart, but this is often difficult to achieve. It is therefore common to "fold" the optical path and reflect the light back and forth multiple times in the gas sample using mirrors.
The element of White100 is the most well known design of a folded optical path element, as shown in FIG. 1. In a generally cylindrical gas sample cell, a single curved mirror 101 is mounted at one end and a pair of curved mirrors 102 are mounted at the opposite end. The Herriott design 200 shown in FIG. 2 is often preferred for TDLAS. This design, typically in a cylindrical gas sample cell 202, uses two curved mirrors 201 arranged at opposite ends. Furthermore, "folded path elements" exist, in which the light does not pass repeatedly through the same gas space, but rather a long single path sample element is folded on itself to achieve a small geometry, with mirrors in the element to transmit the light around the fold. Finally, simple multi-path arrangements are also often employed, for example, a simple mirror, which is a single mirror that returns light to a detector mounted adjacent to the light source; and pairs of parallel flat mirrors that direct light onto one mirror at a small angle and reflect back and forth between the two mirrors until the light reaches the end of one mirror or the end of the other mirror, such as described in U.S. patent No. 3,524,066.
U.S. patent No. 5,173,749 also describes a non-planar multipath component for spectral measurement of a gas sample. This element comprises a cylindrical measuring tube divided into 12 30 ° angular segments, each segment containing a non-conductive mirror for reflecting the light beam. An entrance and an exit for the light beam are arranged at opposite ends of the element. Within the measuring cell, the light beam is reflected by each of the divided portions, automatically repeating itself with the light beam pattern as it passes upwardly between the entrance and exit of the cell.
The White and Herriott multipass cell designs are well suited for analyzing cylindrical sample cells, but they require curved mirrors of equal size in both directions perpendicular to the optical path, which is generally inconvenient and expensive to manufacture. The design of the element proposed in U.S. patent No. 5,173, 749 avoids the problem of minimizing the height of the element as the beam path extends from one end of the element to the opposite end. In view of these characteristics, the prior art element design is not suitable for use in many semiconductor processing equipment designs, such as those in which gas is to be exhausted from a vacuum chamber that is part of a semiconductor processing apparatus, a vacuum pump is connected to the vacuum chamber to allow the gas in the vacuum chamber to be exhausted through a large diameter outlet, and improved sensors are desired when multiple optical paths are to be used to detect the gas molecules exhausted from the vacuum chamber. In such an arrangement it is desirable to be able to mount the multi-path element between the vacuum chamber and the pump with the displacement of the pump carriage being minimised as much as possible, and therefore the multi-path element should have the smallest possible dimensions of the layers in the direction of gas flow through the element. The requirement for small displacement of the vacuum pump is because other equipment is typically located close to the vacuum pump, and there is little space available to move the pump without redesigning the entire processing equipment. Such a multi-path element is well suited to use of TDLAS to obtain a compact measurement system.
To meet the needs of the semiconductor processing industry and overcome the deficiencies of the prior art, it is an object of the present invention to provide a new element for use in absorption spectrometry that accurately measures gas phase molecular impurities in a sample in situ without redesigning existing semiconductor processing equipment.
It is another object of the present invention to provide an absorption spectroscopy system employing the elements of the present invention for detecting various gas phase molecular species in a sample.
It is a further object of the present invention to provide a semiconductor processing apparatus including an absorption spectroscopy system for detecting gaseous molecular impurities in a sample to at least ppb level.
It is a further object of the present invention to provide an absorption spectrometry method using the element of the present invention for detecting gas phase impurities.
Other objects and related aspects of the present invention will become apparent to those skilled in the art after a review of the specification, drawings and appended claims.
According to a first aspect of the present invention, there is provided a novel polygonal planar multipass element for use in absorption spectrometry. The element comprises a sample region defined by a plurality of thin walls. Each of the thin walls has a light reflecting surface facing the sample area, and each of the thin walls is connected to at least one other of the thin walls so as to form a substantially polygonal shape in cross section. At least one side of the polygon has a light entrance/exit opening, each entrance/exit opening comprising a light transmissive window having a surface facing the sample area. Each window is configured such that the element is enclosed in a circumferential direction. The element has a central axis which is parallel to the thin-walled light-reflecting surface and the surface of each light-transmitting window facing the sample area. The polygonal planar multipath component of the present invention makes it possible to accurately detect various gaseous molecular species in a sample in situ.
According to a second aspect of the present invention, there is provided a system for detecting various gaseous molecular species in a sample. This system includes a polygonal planar multipath component as described above with reference to the first aspect of the invention. The system of the present invention further comprises a light source for directing a light beam through at least one light transmission window into the component; a primary detector is included for measuring the beam entering the element through the at least one transmission window. A sample gas flow passes through the sample region in a direction parallel to the central axis of the element.
According to a third aspect of the present invention, a semiconductor processing apparatus is provided. The apparatus includes a vacuum chamber connected to a vacuum pump for evacuating the vacuum chamber; systems of the invention are also included, see the second aspect of the invention.
A fourth aspect of the invention is a method of detecting gas phase impurities using the sample elements, systems and processing devices of the invention.
The objects and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments, given in conjunction with the accompanying drawings. In the drawings, like elements are denoted by like numerals. In the following figures:
Referring to the drawings, FIGS. 3A and 3B are transverse and longitudinal cross-sectional views, respectively, of a preferred polygonal planar multipass element 300 according to the present invention, the element 300 being for use in absorption spectrometry. In this embodiment, sample region 301 is defined by a plurality of thin walls 302, thin walls 302 having a light reflective surface facing sample region 301. The light reflecting surface is a well polished metal. Since it is desirable that these surfaces be highly reflective, the surfaces may be coated with one or more layers of reflective material, such as gold, other metal layers, or with a highly reflective non-conductive coating. In addition, one or more heaters may also be provided to heat the light reflecting surface in order to improve the reflection performance, minimizing the detrimental effects of deposits formed on the light reflecting surface.
Each thin wall 302 of the element is connected to at least one other thin wall such that it substantially forms a polygon in cross-section. The polygon is preferably a regular polygon and may have any number of thin walls. Octagon is a preferred polygonal geometry. At least one side of the polygon has a light entry/exit port 303 from which a light beam enters and/or exits the element. The entrance/exit port 303 includes a light transmission window 304 having a surface facing the sample area 301. The light beam is incident on the sample area of the cell through the light transmission window, and exits the cell through the light transmission window. Thus, the light beams can enter or exit the element through the same window or different windows, and the windows can be located on the same side or different sides of the element. Suitable light transmitting materials for the window will be familiar to those skilled in the art.
The light transmission window 304 encloses the elements in the circumferential direction. To form a seal between the window and the element, a closure device such as an O-ring 305, or other suitable structure, may be used. Sealing the element in this way makes it possible to measure gas samples at low pressures, for example in a vacuum state. An additional cover layer may be provided on the surface of the light transmission window 304 opposite the surface facing the sample area to reflect a portion of the light beam 306. As will be explained below, subtracting the signal caused by the reflected beam portion from the signal caused by the transmitted beam portion results in a more accurate absorption measurement. Among commercially available covering materials, a metal covering is preferred. The element also has a central axis 307 which is parallel to the thin-walled light-reflecting surface and the surface of the light-transmitting window 304 facing the sample region 301.
The light reflecting surface of each thin wall 302 is preferably planar, since the design of the element of the present invention does not require the use of curved mirrors as in the prior art elements, and therefore the element of the present invention is less expensive to manufacture than the prior art structures. Furthermore, the thin wall is arranged such that the light beam entering the cell remains in the same plane inside the cell, so that the cell can be made arbitrarily small both in the direction parallel to the plane of propagation of the light and in the direction perpendicular thereto, depending on the diameter of the light beam and the geometrical constraints imposed by the window through which the light beam enters and exits the vacuum chamber. This feature of the invention makes it particularly suitable for use in existing semiconductor processing equipment.
The walls are preferably arranged so that a beam of light entering the element is reflected from one wall light reflecting surface to another wall light reflecting surface, so that the beam of light is reflected from each wall at least once before it exits the element through the entrance/exit opening. The dwell time and the effective path length of the beam in the sample area of the element are thus extended.
In fig. 3B, the height h of the element, measured in a direction parallel to the central axis of the element, is approximately in the range of 1 to 5 cm. The diameter d of the element, measured perpendicular to the central axis of the element, may be slightly larger than the inlet of the vacuum pump, preferably in the range of about 5 to 40 cm. The element is of an open design and the sample area surrounded by the thin wall and the light transmission window preferably extends over the entire height of the element. This configuration allows the sample to pass through the cell in a direction parallel to the central axis of the cell along the station, which makes the cell particularly suitable for in situ measurements in semiconductor processing equipment.
Fig. 4A and 4B depict transverse and longitudinal cross-sectional views, respectively, of a system 408 of the present invention. This system is used to detect various gas phase molecular forms in a sample by absorption spectrometry. The inventive element can be used in any absorption spectroscopy technique, preferably in Tunable Diode Laser Absorption Spectroscopy (TDLAS). This system includes element 401, as described above, and can be seen in fig. 3A and 3B. The system further comprises a light source 409, preferably a diode laser, for directing a light beam 406 through the light transmission window 404 into the component 401. To measure the light beam 406 exiting the cell through the light transmission window 404, the system 408 further comprises a primary detector 410, which may be a photodiode.
Any molecular impurity of interest can be detected, taking into account only the appropriate light source available. For example, water vapor, nitrogen oxide, carbon monoxide, methane; or other hydrocarbons, may be detected by measuring the attenuation of light from a diode laser source. The diode laser light source emits light having a wavelength characteristic of impurities.
The spectral range of the light emitted by the laser source is the spectral range in which the molecule of interest absorbs most strongly, which results in an improved measurement sensitivity. In particular, light sources emitting at wavelengths greater than 2 μm are preferred because many of the molecular impurities of interest have strong absorption bands in this range.
Any suitable wavelength tunable light source may be used. Of the currently available light sources, diode laser light sources are preferred because of their narrow linewidth (less than about 10)-3cm-1) And a relatively high intensity in the emitted wavelength (about 0.1 to a few milliwatts).
Examples of diode lasers include the lead-salt type (Pb-salt) and the gallium arsenide type (GaAs). Lead salt type lasers are required to operate at low temperatures and emit infrared light (i.e., wavelengths greater than 3 μm); the GaAs diode laser can work at a temperature close to room temperature, and the emitted light is close to the infrared range (0.8-2 μm).
Recently, diode lasers have been described that incorporate Sb (or other paired III-V compounds such as AsP) into gallium arsenide (see "Mid-isolated wavelengths en-hand trace gas sensing," R.Martinelli, Laser Focus World, March1996, p.77). These diodes emit light at wavelengths greater than 2 μm when operated at-87.8 ℃. Although such low temperatures are inconvenient, they are advantageous over the low temperatures required for lead salt lasers (below-170 ℃). Similar Lasers operating at 4 μm and 12 ℃ have also been reported (see Lasers and Optronics, March 1996). The diode laser type described above is preferably operated at a temperature of at least-40 c. Temperature control to achieve the above temperatures using thermoelectric cooling devices can reduce the complexity of these light sources over systems using low temperature diodes. In order to make the use of these lasers satisfactory, it is important to improve their optical properties on the current level. For example, a single type diode (i.e., a diode that emits a single wavelength at a fixed temperature and excitation current, with at least a 40dB intensity attenuation of emission at other wavelengths) should be utilized.
Suitable light sources that can be used in the present invention are not limited to the diode lasers described above, for example other types of lasers, such as fiber lasers, quantum cascade lasers, of similar dimensions and adjustable by simple electronic means are foreseen, the application of which is also contemplated when these lasers are commercially available.
The system may further comprise at least one first mirror 411 for reflecting the light beam 406, the light beam 406 coming from the light source 409 through the light transmission window 404 into the element 401; at least one second mirror 415, 417 may also be included for reflecting the beam from the element towards the primary detector 410. The mirror 411 is preferably curved in order to collimate the beam because the light from the diode laser light source is diffuse. Likewise, the mirror 417 is preferably curved in order to focus the parallel beam on the detector 410. Optionally, a second detector 412, which may also be a photodiode, is provided in the system for measuring a portion of the light beam 413 reflected from the light transmission window 404; optionally, means may be provided in the system for subtracting the reference signal from the measurement of the main detector, and an operational amplifier may be used as the means for subtracting the reference signal, the structure of such an operational amplifier being described in the literature (see, for example, Moore, j.h. et al, "Building scientific apparatus", Addison Wesley, London, 1983).
The reflected light does not indicate what is absorbed by the molecule of interest in the sample area, and provides a reference signal for this purpose. By subtracting the reference signal from the signal of the light passing through the element, which is measured by the main detector, variations in the light source can be compensated. This also allows for molecules in the vacuum chamber 407 of the system to cause enhanced sensitivity to signal changes. Although the "dual beam" technique is well known for subtraction of a reference beam, it generally requires a dedicated beam splitting device, i.e. an optical element that merely splits the beam. According to the invention, the entrance window of the vacuum chamber can provide this function without any additional elements, and the ratio of transmitted light and reflected light at this window can be controlled by applying a suitable cover layer to the window.
The novel system described above allows for in situ detection of molecular gas contaminants, such as water vapor, and in this regard, FIG. 6 illustrates the absorption peak height of light as a function of water vapor pressure. Wherein the light is emitted by a diode laser with a wavelength of about 1.38 μm; the water vapor pressure was at 27.8 ℃. These data were collected using second harmonic spectroscopy. This technique is discussed in the following co-pending applications: serial No. 08/711, 646, filed on even date, attorney docket No. 016499-203; serial No. 08/711, 781, attorney docket No. 016499-206, filed on even date herewith.
The system of the invention has particular applicability to detecting various molecular forms in gas exhausted from a vacuum chamber, inIn this case, the element may be disposed between the vacuum chamber and the vacuum pump system. The system is applicable to a variety of materials, for example, the vacuum chamber may include certain reactive or non-reactive (inert) gas species, which may be in a plasma or non-plasma state. Examples of reactive gases suitable for use in the system of the present invention include SiH4HCl and Cl2Provided that the level of moisture content is less than 1000 ppm. Any inert gas, e.g. O2，N2Ar, and H2Can be used in the system of the present invention, and when the system of the present invention is used in a plasma environment, the system is preferably mounted about 6 inches from the plasma region in order to minimize the formation of deposits on the surfaces of the window and other components.
Since the detection system described above with reference to fig. 4A and 4B can be adapted for use in a plasma or non-plasma environment, as well as in the presence of inert or reactive gases, the system is particularly suitable for use in semiconductor processing equipment for monitoring various gaseous molecular forms, such as water vapor. The semiconductor processing device can monitor gas phase molecular impurities in situ in real time by combining the adoption of the detector. And since the component height h and diameter d (see fig. 3A, 3B) can be made arbitrarily small, as previously mentioned, the application of the component to a semiconductor processing apparatus will not have a detrimental effect on the apparatus or require expensive replacement costs in retrofitting.
An example of such a structure is shown in fig. 5. Although the structure of the semiconductor processing apparatus is shown only generally in fig. 5, the skilled artisan will appreciate that the system can be readily adapted to virtually any semiconductor processing apparatus that employs a vacuum system. Examples of such devices are etching, diffusion, chemical vapor deposition, ion implantation, sputtering, rapid thermal processing, and the like.
The apparatus 515 shown in fig. 5 comprises a vacuum chamber 516 inside which a semiconductor substrate 517 is placed on a substrate holder 518. Gas inlet 519 is used to deliver one or more gases into vacuum chamber 516, which is evacuated through an exhaust 520 in the chamber. The vacuum chamber 516 is connected to a vacuum chamber 521 directly or through a vacuum line, a pump exhaust line 522 is connected to the pump 521, and the pump exhaust line 522 may be connected to other pumps or to a scrubber (not shown). Examples of vacuum pumps that may be employed are mechanical rotary and booster diffusion pumps, cryogenic air pumps, sorption pumps, and turbo-molecular pumps. The system 508 for detecting gas phase molecules is described in detail above with reference to fig. 4A and 4B. Although the vacuum pump 521 and the system for detecting gas molecules 508 are shown disposed below the vacuum chamber 516, it will be understood by those skilled in the art that they may be disposed in other orientations.
Although the invention has been described in detail with reference to specific embodiments, the following facts will be apparent to those skilled in the art: i.e. various changes or modifications may be made, and equivalents employed, without departing from the scope of the appended claims.