US20060172542A1

Movatterモバイル変換

Info

Publication number: US20060172542A1
Application number: US11/046,135
Authority: US
Inventors: Kallol Bera; Daniel Hoffman; Yan Ye; Michael Kutney; Douglas Buchberger
Original assignee: Applied Materials Inc
Current assignee: Applied Materials Inc
Priority date: 2005-01-28
Filing date: 2005-01-28
Publication date: 2006-08-03
Also published as: CN1812681A; KR20060087432A; JP4713352B2; US7674353B2; EP1686611A1; TW200627501A; CN101008072A; TWI333225B; SG124401A1; JP3123883U; CN1812681B; JP2006270054A; TWM301400U; EP1686611B1; US20060193102A1; EP1686611B9; CN101008072B; KR20060087474A; KR100900595B1; DE602006002282D1

Abstract

The embodiments of the present invention generally relate to a method and an apparatus to confine a plasma within a processing region in a plasma processing chamber. The apparatus may include an annular ring with a gap distance with the chamber wall at between about 0.8 inch to about 1.5 inch. In addition to the annular plasma confinement ring, the plasma can also be confined by reducing a voltage supplied to the top electrode by a voltage ratio during plasma processing and supplying the remaining voltage supplied to the top electrode at a negative phase at the substrate support and the substrate, if the substrate is present during processing. The voltage ratio can be adjusted by changing the impedances of the substrate support and the dielectric seal surrounding the top electrode. Lowering top electrode voltage by a voltage ratio and supplying the remaining voltage supplied to the top electrode at a negative phase at the substrate support reduce the amount of plasma got attracted to the grounded chamber walls and thus improves plasma confinement. This method of plasma confinement is called impedance confinement. Plasma confinement can be improved by using either the described annular ring, the impedance confinement scheme or a combination of both.

Description

FIELD OF THE INVENTION

The embodiments of the present invention generally relate to method and apparatus to confine plasma and to enhance flow conductance in plasma processing reactors.

BACKGROUND OF THE INVENTION

Plasma processing of semiconductor wafers in the manufacture of microelectronic integrated circuits is used in dielectric etching, metal etching, chemical vapor deposition and other processes. In semiconductor substrate processing, the trend towards increasingly smaller feature sizes and line-widths has placed a premium on the ability to mask, etch, and deposit material on a semiconductor substrate, with greater precision.

Typically, etching is accomplished by applying radio frequency (RF) power to a working gas supplied to a low pressure processing region over a substrate supported by a support member. The resulting electric field creates a reaction zone in the processing region that excites the working gas into a plasma. The support member is biased to attract ions within the plasma towards the substrate supported thereon. Ions migrate towards a boundary layer of the plasma adjacent to the substrate and accelerate upon leaving the boundary layer. The accelerated ions produce the energy required to remove, or etch, the material from the surface of the substrate. As the accelerated ions can etch other components within the processing chamber, it is important that the plasma be confined to the processing region above the substrate.

Unconfined plasmas cause etch-byproduct (typically polymer) deposition on the chamber walls and could also etch the chamber walls. Etch-byproduct deposition on the chamber walls could cause the process to drift. The etched materials from the chamber walls could contaminate the substrate by re-deposition and/or could create particles for the chamber. In addition, unconfined plasmas could also cause etch-byproduct deposition in the downstream areas. The accumulated etch-byproduct can flake off and result in particles. To reduce the particle issues caused by the deposition of etch-byproduct in the downstream areas, additional downstream clean is needed, which could reduce process throughput and increase processing cost.

Confined plasmas could reduce chamber contamination, chamber cleaning and improve process repeatability (or reduce process drift). Plasma confinement devices, such as slotted plasma confinement ring (described below), have been developed to confine plasma. Certain front end of line (FEOL) applications, such as contact etch and high aspect ratio trench etch, require relatively low process pressure (e.g. ≦30 mTorr) under relatively high total gas flow rate (e.g. between about 900 sccm to about 1500 sccm). Plasma confinement devices, such as a slotted plasma confinement ring, could cause flow resistance for the gas flow to the downstream and results in pressure in the plasma chamber that is not low enough (e.g. ≦30 mTorr) for the FEOL applications described.

Therefore, there is a need for an improved method and apparatus that not only confine plasma within a processing region inside the plasma chamber but also enhance flow conductance.

SUMMARY OF THE INVENTION

The embodiments of the present invention generally relate to a method and an apparatus to confine plasma and to enhance flow conductance in plasma processing reactors. In one embodiment, an apparatus configured to confine a plasma within a substrate processing region during processing a substrate in a plasma processing chamber comprises a substrate support having one or more dielectric layers, an annular ring surrounding the top portion of the substrate support, wherein there is a gap between the annular ring and process chamber walls having a gap width from about 0.8 inch to about 1.5 inch, and a dielectric seal placed between a top electrode and a process chamber body, wherein impedances of the top electrode, the dielectric seal, the substrate along with the substrate support, and plasma reduce a voltage supplied to the top electrode by a voltage ratio and supply the remaining voltage supplied to the top electrode at a negative phase at the substrate and the substrate support during plasma processing.

In another embodiment, an apparatus configured to confine a plasma within a processing region in a plasma processing chamber comprises an annular ring surrounding the top portion of a substrate support, wherein there is a gap between the annular ring and process chamber walls with gap width equaling to or greater than about 0.8 inch and not greater than 1.5 inch.

In another embodiment, an apparatus configured to confine a plasma within a substrate processing region during processing a substrate in a plasma processing chamber comprises a substrate support having one or more dielectric layers, a dielectric seal surrounding a top electrode, wherein impedances of the top electrode, the dielectric seal, the substrate along with the substrate support, and plasma reduce a voltage supplied to the top electrode by a voltage ratio and supply the remaining voltage supplied to the top electrode at a negative phase at the substrate and the substrate support during plasma processing.

In another embodiment, a method of confining a plasma within a substrate processing region during substrate processing in a plasma processing chamber comprises placing a substrate on a substrate support in a plasma processing chamber with a top electrode, an annular ring surrounding the top portion of the substrate support with a gap between the annular ring and process chamber walls having a gap width from about 0.8 inch to about 1.5 inch, flowing process gas(es) into the plasma chamber, and creating a plasma in the plasma process chamber.

In another embodiment, a method of confining a plasma within a substrate processing region during substrate processing in a plasma processing chamber comprises placing a substrate on a substrate support in a plasma processing chamber having a top electrode, a dielectric seal surrounding the top electrode, an annular ring surrounding the top portion of the substrate support with a gap between the annular ring and process chamber walls having a gap width from about 0.8 inch to about 1.5 inch, flowing process gas(es) into the plasma chamber, and creating a plasma in the plasma process chamber by supplying a voltage ratio of the voltage supplied to the top electrode and supplying the remaining voltage supplied to the top electrode at a negative phase at the substrate and the substrate support.

In yet another embodiment, a method of confining a plasma within a substrate processing region during substrate processing in a plasma processing chamber comprises placing a substrate on a substrate support in a plasma processing chamber with a top electrode, and a dielectric seal surrounding the top electrode, flowing process gas(es) into the plasma chamber, and creating a plasma in the plasma process chamber by supplying a voltage at a voltage ratio of the voltage supplied to the top electrode and supplying the remaining voltage supplied to the top electrode at a negative phase at the substrate and the substrate support.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1A shows the process flow of processing a substrate in a plasma chamber.

FIG. 1B shows a schematic drawing of a plasma processing

FIG. 2 (Prior Art) shows a schematic drawing of a slotted plasma confinement ring.

FIG. 3A shows a schematic drawing of a plasma processing chamber with one embodiment of an annular plasma confinement ring in the process chamber.

FIG. 3B shows a schematic drawing of a plasma processing chamber with another embodiment of an annular plasma confinement ring in the process chamber.

FIG. 3C shows the simulated results of plasma density ratio and chamber pressure as a function of the gap width.

FIG. 3D shows the simulated result of plasma density in the plasma processing chamber when the gap width between the annular ring and the chamber walls is 0.5 inch.

FIG. 3E shows the simulated result of plasma density in the plasma processing chamber when the gap width between the annular ring and the chamber walls is 3 inches.

FIG. 4A shows the voltage between the top electrode and the grounded cathode when the voltage ratio is 1 (or source voltage fully supplied at top electrode).

FIG. 4B shows the voltage between the top electrode and the grounded chamber wall when the voltage ratio is 1 (or source voltage fully supplied at top electrode).

FIG. 4C shows the voltage between the top electrode and the cathode when the voltage ratio is 0.5 (or half of source voltage is supplied at top electrode).

FIG. 4D shows the voltage between the top electrode and the grounded chamber wall when the voltage ratio is 0.5 (or half of source voltage is supplied at top electrode).

FIG. 5A shows the simulated plasma density ratio as a function of voltage ratio.

FIG. 5B shows the simulated result of plasma density in the plasma processing chamber when the gap width between the annular ring and the chamber walls is 1.5 inch and the voltage ratio is 1.

FIG. 5C shows the simulated result of plasma density in the plasma processing chamber when the gap width between the annular ring and the chamber walls is 1.5 inch and the voltage ratio is 0.5.

FIG. 5D shows the simulated result of power deposition in the plasma processing chamber when the gap width between the annular ring and the chamber walls is 1.5 inch and the voltage ratio is 1.

FIG. 5E shows the simulated result of power deposition in the plasma processing chamber when the gap width between the annular ring and the chamber walls is 1.5 inch and the voltage ratio is 0.5.

FIG. 6 shows a circuit drawing between the top electrode, the cathode and the chamber walls.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings in the figures are all schematic and not to scale.

DETAILED DESCRIPTION

The process of processing a substrate in a plasma process chamber is shown inFIG. 1A. The process starts atstep201 by placing a substrate in a plasma process chamber. Next atstep202, process gas(es) is flown into the plasma process chamber. Then atstep203, a plasma is created in the plasma process chamber. Atstep204, the substrate is processed in the plasma process chamber. The processing conducted in the plasma process chamber could be deposition, etching or plasma-treatment. The concept of the invention applies to any types of plasma processing.

FIG. 1B illustrates an example of a plasma reactor, such as the Enabler® etch system manufactured by Applied Materials, Inc., of Santa Clara, Calif., that includes areactor chamber100, which may include liners to protect the walls, with a substrate support (or pedestal)105 at the bottom of the chamber supporting asemiconductor wafer110. Thechamber100 is bounded at the top by a disc shapedoverhead aluminum electrode125 supported at a predetermined gap length above thewafer110 on groundedchamber body127 by a dielectric (quartz)seal130. Apower generator150 applies very high frequency (VHF) power to theelectrode125. VHF is typically between about 30 MHz to about 300 MHz and is one of the RF bands, which range from about 10 kHz to about 10 GHz. In one embodiment, the VHF source power frequency is 162 MHz for a 300 mm wafer diameter. VHF 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 andcoaxial cable162 or theVHF power generator150. The chamber body is connected to the VHF return (VHF ground) of theVHF generator150. Bias power is applied to the wafer by a bias powerRF signal generator200 coupled through a conventionalimpedance match circuit210 to thewafer support105. The power level of thebias generator200 controls the ion energy near the wafer surface. The bias power (typically at 13.56 MHz) is typically used to control ion energy, while the VHF source power is applied to the overhead electrode to govern plasma density. Avacuum pump system111 evacuates thechamber100 through aplenum112.

Thesubstrate support105 includes ametal pedestal layer5505 supporting alower insulation layer5510, an electricallyconductive mesh layer5515 overlying thelower insulation layer5510 and a thintop insulation layer5520 covering theconductive mesh layer5515. The semiconductor workpiece orwafer110 is placed on top of thetop insulation layer5520. Thesubstrate support105 and thewafer110 form a cathode during substrate processing. If thewafer110 is not present, thesubstrate support105 is the cathode during plasma processing. The electricallyconductive mesh layer5515 and themetal pedestal layer5505 may be formed of materials such as molybdenum and aluminum respectively. The insulation layers5510 and5520 may be formed of materials such as aluminum nitride or alumina. Theconductive mesh layer5515 supplies the RF bias voltage to control ion bombardment energy at the surface of thewafer110. Theconductive mesh5515 also can be used for electrostatically chucking and de-chucking thewafer110, and in such a case can be connected to a chucking voltage source in the well-known fashion. Theconductive mesh5515 therefore is not necessarily grounded and can have, alternately, a floating electric potential or a fixed D.C. potential in accordance with conventional chucking and de-chucking operations. Thewafer support105, in particular themetal pedestal layer5505, typically (but not necessarily) is connected to ground, and forms part of a return path for VHF power radiated by theoverhead electrode125.

In order to improve the uniformity of impedance across the substrate support, a dielectriccylindrical sleeve5550 is designed to surround theRF conductor5525. The axial length and the dielectric constant of the material constituting thesleeve5550 determine the feed point impedance presented by theRF conductor5525 to the VHF power. By adjusting the axial length and the dielectric constant of the material constituting thesleeve5550, a more uniform radial distribution of impedance can be attained, for more uniform capacitive coupling of VHF source power.

A terminatingconductor165 at thefar end135aof thestub135 shorts the inner and

outer conductors

140,145 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 can provide 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 inlets172 in or adjacent theoverhead electrode125 while themetallic coolant line173 feeds coolant passages orjackets174 within theoverhead electrode125.

As described earlier, unconfined plasmas cause etch-byproduct (typically polymer) deposition on the chamber walls and could also etch the chamber walls. Etch-byproduct deposition on the chamber walls could cause the process to drift. The etched materials from the chamber walls could contaminate the substrate by re-deposition and/or could create particles for the chamber. In addition, unconfined plasmas could also reach the downstream areas of the processing zone and cause etch-byproduct, which is typically polymer, deposition in the downstream areas. The etch-byproduct deposited in the downstream areas is difficult to clean. The accumulated etch-byproduct can flake off and result in particles. To reduce the particle issues and cleaning time, a slotted confinement ring50 (seeFIG. 2 prior art) placed around thewafer110 and between theoverhead electrode125 andsubstrate support105 had been previously proposed.

FIG. 2 (prior art) illustrates a perspective view of a slottedconfinement ring50 that has been previously proposed to confine plasma. The details of the slottedconfinement ring50 are further described in commonly assigned U.S. patent application Ser. No. 10/418,996, entitled “Apparatus And Method To Confine Plasma And Reduce Flow Resistance In A Plasma Reactor, filed Apr. 17, 2003, which is incorporated herein by reference. The slots in theconfinement ring50 allow the process gas mixture to pass through and reduce the flow resistance across thechamber100. Theconfinement ring50 includes abaffle55 and a base58 coupled to a bottom portion of thebaffle55. Thebase58 is generally configured to provide electrical grounding and mechanical strength for theconfinement ring50. Thebaffle55 defines anopening70 at its top portion. Theopening70 is configured to receive the showerhead of thegas distribution plate125 so that gases flowing the showerhead will be confined within theprocessing region72 inside thebaffle55. Thebaffle55 further includes a plurality ofslots57 and a plurality offingers59, disposed around thewafer110. Neutrals in the plasma are configured to pass through theslots57 into theplenum112.

Although the slottedconfinement ring50 provides good plasma confinement and theslots57 in theconfinement ring50 reduce flow resistance across thechamber100 low enough for most applications, for some FEOL applications, the flow resistance is too high. As described earlier, for front end of line (FEOL) applications, such as contact etch and high aspect ratio trench etch, that requires relatively low process pressure (e.g. ≦30 mTorr) and high total gas flow rate (e.g. between about 900 sccm to about 1500 sccm). The flow resistance created by the slotted confinement ring could make the chamber pressure rise above the required low pressure range for these applications. Therefore, there is a need to design a confinement ring that not only confines plasma but also further reduces flow resistance.

Since plasma density is relatively low near the wall, an annular ring placed around thesubstrate110 with a distance (or gap) from theinner chamber wall128 could possibly have the same level of plasma confinement as the slotted confinement ring design, and yet decreases the flow resistance. The distance (or gap) between the edge of the annular ring and theinner chamber wall128 can not be too large. If the gap distance is larger than the plasma sheath thickness near the chamber wall, it could increase the amount of plasma being drawn away from the reaction zone above the wafer and toward the chamber wall and downstream, which makes the plasma less confined. The distance (or gap) between the edge of the annular ring and theinner chamber wall128 cannot be too small either, since the flow resistance, which affects the chamber pressure, would increase to an unacceptable level. Therefore, an annular plasma confinement ring, placed around thesubstrate110 with a suitable distance from theinner chamber wall128, is proposed to meet the requirement of good plasma confinement and low flow resistance.

FIG. 3A shows a schematic drawing of an embodiment of the processing chamber with an annularplasma confinement ring115. Theannular ring115 could be made of conductive materials, such as silicon carbide (SiC) or aluminum (Al). Theannular ring115 surrounds thewafer110. Theannular ring115 is coupled to the groundedchamber body127 and is electrically separated from thesubstrate support105 by a dielectric (quartz)ring120, which is needed to prevent the conductiveannular ring115 from touching thesubstrate110 andconductive mesh layer5515 to prevent eliminating the effect of bias power. The lowest point of thedielectric ring120 should be below the lowest point of theconductive mesh layer5515. The top surface of theannular ring115 should be at about the same surface plane as thesubstrate110 to allowsubstrate110 to be placed properly on thesubstrate support105 and to minimize flow re-circulation. The top surface of thedielectric ring120 could be at the same height as the top surface ofsubstrate110 and the top surface of theannular ring115, as shown in the embodiment inFIG. 3A. The top surface of thedielectric ring120 could also be slightly lower than the top surface ofsubstrate110 and the top surface of theannular ring115, as shown in another embodiment inFIG. 3B. In the embodiment shown inFIG. 3B, the plasma confinementannular ring115 is place on top of thedielectric ring120.

Theannular ring115 is away from theinner chamber wall128 at agap width117. Thethickness119 of the top section of theannular ring115 should not be too thick, since the flow resistance would increase with increasingthickness119. In one embodiment, thethickness119 is in the range between about ⅛ inch to about ¼ inch. Thecorner118 of the annular ring is used to provide the annular ring mechanical strength, since the top section withthickness119 is limited in its thickness and mechanical strength. Structures other than thecorner118 that can provide mechanical strength can also be used.

In order to better understand the impact of thegap width117 to the effectiveness of plasma confinement and the chamber pressure, chamber plasma density and pressure simulations have been conducted for the annular ring design and the slotted ring design for comparison. For chamber pressure simulation, computation fluid dynamics (CFD) software CFD-ACE+ by ESI group of France is used. CFD-ACE+ is a general, partial differential equation (PDE) solver for a broad range of physics disciplines including: flow, heat transfer, stress/deformation, chemical kinetics, electrochemistry, and others. It solves them in multidimensional (0D to 3D), steady and transient form. CFD-ACE+ is used for complex multiphysics and multidisciplinary applications. For the current study, the “Flow” module of the software is used. Pressure simulation by using the “Flow” module of CFD-ACE+ simulator matches experimental results quite well. Table 1 shows comparison of simulation and experimental results for a reactor described inFIG. 1B with a slotted plasma confinement ring described inFIG. 2. In Table 1, the pump pressure refers to the pressure set value forpump111 ofFIG. 1B. The chamber inner radius is 27 cm and the distance between thewafer110 and the lower surface of thetop electrode125 is 3.2 cm. The chamber pressure data are collected at 6.8 cm away from the wafer center and right above the wafer. The below-ring pressure data are collected right beneath the slotted confinement ring. The results show a good match between the simulated and experimental results. The results also show that the slotted confinement ring has relatively high flow resistance and increase the pressure inside the reaction chamber significantly above the pressure set value.

TABLE 1


Experimental and simulated chamber pressure
and below-ring pressure comparison.

	“Set”	Measured	Simulated	Measured	Simulated
Gas	Pump	Chamber	Chamber	Below-Ring	Below-Ring
Flow	Pressure	Pressure	Pressure	Pressure	Pressure
(sccm)	(mTorr)	(mTorr)	(mTorr)	(mTorr)	(mTorr)

2000	40	55.6	58.8	40.2	43.5
900	10	21.5	25.0	11.6	14.5
900	40	46.5	49.3	40.2	41.6

The chamber plasma density simulation uses the hybrid plasma equipment model (HPEM), developed by the Department of Electrical and Computer Engineering of University of Illinois at Urbana-Champaign, Urbana, Ill. The HPEM is a comprehensive modeling platform for low pressure (<10's Torr) plasma processing reactors. Details about plasma density simulation by this simulator can be found in an article, titled “Argon Metastable Densities In Radio Frequency Ar, Ar/O₂and Ar/CF₄Electrical Discharges”, published in pages 2805-2813 of Journal of Applied Physics, volume 82 (6), 1997. The plasma simulator is widely used in the semiconductor equipment industry. Our experience shows that plasma simulation of process parameter variation by HPEM matches the process results quite well.

For the annular ring design, the simulation includesgap width117 from 0.5 inch to 3 inch. The process condition simulated resembles the contact etch and deep trench etch mentioned previously. High gas flow rate of 1500 sccm is used to simulate high gas flow rate. The process gas only includes O₂, instead of including other types of process gases, such as C₄F₆and argon (Ar), to simplify the simulation. For plasma confinement study that compares degree of plasma confinement as a function of thegap width117, using only O₂gas in simulation could provide learning of the impact of thegas distance117 on plasma confinement. The top electrode power (or source power) simulated is 1.85 KW and the gas temperature is 80° C. The total source power is 1.85 kW. The top electrode voltage (or source voltage), V_s, is typically between about 100 to about 200 volts. 175 volts of V_shas been used in the simulation. The radius of the substrate (or wafer) is 15 cm (or 6 inch) and the spacing between the top electrode to the substrate is 3.2 cm (or 1.25 inch). The radius ofinner chamber wall128 is 27 cm (or 10.6 inch). The width of thedielectric ring120 is 2.2 cm (or 0.87 inch) and the width of the annularplasma confinement ring115 simulated varies between 8.5 cm (or 3.3 inch) to 2.2 cm (or 0.9 inch). The spacing between theannular confinement ring115 with theinner chamber wall128 simulated varies between 1.3 cm (or 0.5 inch) to 7.6 cm (or 3.0 inch).

FIG. 3C shows plasma simulation results for the plasma chamber described in FIGS.1 with anannular ring115 described inFIG. 3A. In a low pressure plasma chamber, pressure and plasma density are not completely uniform across the entire chamber. The pressure is typically higher near the center of the wafer, lower near the wafer edge, and reaches the pump pressure set point at the pump. The pressure data inFIG. 3B are pressure at intersection of the chamber wall and the wafer top surface plane, or location “P” inFIG. 3A. In order to quantify the degree of confinement level, a plasma density ratio is defined as the ratio of maximum plasma density belowline116, which is extended along right below the top section of theannular ring115, to the maximum plasma density in the process chamber, which occurs in the volume between the wafer surface and theoverhead aluminum electrode125. The lower the plasma density ratio, the better the plasma confinement ring has performed in confining plasma.

The dashedline301 inFIG. 3C shows the 35.3 mTorr chamber pressure for the slotted confinement ring design. Dashedline302 inFIG. 3C shows the 0.004 plasma density ratio obtained for the slotted confinement ring design. The 35.3 mTorr chamber pressure and 0.004 plasma density ratio are both obtained from simulation results. Since slotted ring design does not vary thegap width117, they dashed

lines

301 and302 are horizontal lines.Curve311 shows chamber pressure as a function ofgap width117, whilecurve312 shows plasma density ratio as a function ofgap width117. For annular ring design at 0.5 inch gap width, the chamber pressure is found to be 35.8 mTorr, which is higher than the slotted confinement ring design, and the plasma density ratio is 0.00013, which is lower than the slotted confinement ring design. Although the lower plasma density ratio is desirable, the higher chamber pressure is not. When thegap width117 is increased to 1 inch, the chamber pressure reduces to 27.9 mTorr, which is lower than the slotted ring design and lower than the low pressure requirement of <30 mTorr for front end process, and the plasma density ratio is 0.002, which is still lower than the slotted ring design. When thegap width117 is increased to 1.5 inch, the chamber pressure further reduces to 26.2 mTorr, and the plasma density ratio is 0.023, which is higher than the slotted ring design but is still relatively low. As thegap width117 increases beyond 1.5 inch, the effect of thewider gap width117 in lowering the chamber pressure is reduced; however, the plasma density ratio continues to increases.

Table 2 shows comparison of simulation results for a reactor described inFIG. 1B with a slotted plasma confinement ring described inFIG. 2 and an annular plasma confinement ring described inFIG. 3A. The gap distance between the annular ring to thechamber walls128 is 1 inch. In Table 2, the pump pressure refers to the pressure set value forpump111 ofFIG. 1B. The chamber inner radius is 27 cm and the distance between thewafer110 and the lower surface of thetop electrode125 is 3.2 cm. The chamber pressure data are collected at 6.8 cm away from the wafer center and right above the wafer. The below-ring pressure data are collected right beneath the slotted confinement ring or the annular ring. The results show that the chamber pressure is higher for the slotted plasma confinement ring than the annular plasma confinement ring. In addition, the pressure difference between the chamber and below the confinement ring is higher for the slotted ring (ΔP=15.3 mTorr) than the annular ring (ΔP=9.4 mTorr).

TABLE 2


Comparison of simulated chamber pressure and below-ring
pressure for slotted confinement ring and annular ring
with 1 inch gap distance from the chamber walls.

		Chamber	Chamber	Below-Ring	Below-Ring
	“Set”	Pressure	Pressure	Pressure	Pressure
Gas	Pump	(mTorr)	(mTorr)	(mTorr)	(mTorr)
Flow	Pressure	Slotted	Annular	Slotted	Annular
(sccm)	(mTorr)	Ring	Ring	Ring	Ring

2000	40	58.8	54.1	43.5	44.7

FIG. 3D shows the simulation result of plasma density in the process chamber when thegap width117 is 0.5 inch, wherein the plasma density ratio is 0.00013. The X-axis is the distance from the center of the process chamber and the Y-axis the distance from 3.9 cm below the top surface of thesubstrate support105. The results show that the plasma is relatively confined within the region above the substrate. Unfortunately, the chamber pressure is 35.8 mTorr, which is higher than the spec of ≦30 mTorr.FIG. 3E shows the simulation result of plasma density in the process chamber when thegap width117 is 3 inch, wherein the plasma density ratio is 0.12. The results show that there is a significant plasma loss to the reactor downstream.

The simulation results inFIG. 3C show that as thegap width117 increases, the resistance to the flow decreases, hence the wafer pressure decreases. While, with increase ingap width117, more plasma penetrates downstream the confinement ring, hence, the plasma density ratio increases. In order to keep the chamber pressure ≦30 mTorr, thegap width117 should be equal to or greater than about 0.8 inch, according to simulation results inFIG. 3B. However, thegap width117 cannot be too large, sincelarge gap width117 results in higher plasma loss to the downstream. As described earlier, as thegap width117 increases beyond 1.5 inch, the effect of thewider gap width117 in lowering the chamber pressure is not significant; however, the plasma density ratio continues to increases. The plasma density ratio atgap width117 of 1.5 inch is 0.023, which is reasonably low. Therefore, thegap width117 should be kept below 1.5 inch.

To further improve the plasma confinement, the concept of lowering the top electrode voltage to reduce voltage drop between thetop electrode125 andchamber walls128 has been investigated. Typically, the source power is mainly supplied through the top electrode at a source voltage, V_s. If the top electrode voltage is lower to a fraction, f, of the source voltage at fV_sand the cathode, which is formed by thesubstrate support105 and thewafer110 during substrate processing, maintains a voltage of −(1-f)V_s, the voltage difference between thetop electrode 125 and the cathode, which is formed by thesubstrate support105 and thewafer110 during substrate processing, is kept at the same voltage value, V_s, but the voltage difference between thetop electrode125 and the groundedchamber walls128 will be lowered to fV_s. Lower voltage difference between thetop electrode125 and theground chamber walls128 would reduce the amount of plasma being drawn to thechamber walls128. The way to supply the source power at a lower top electrode voltage, fV_s, and to maintain the cathode at a negative phase from the top electrode at −(1-f)V_sis by adjusting the impedance of chamber components associated with thetop electrode125, the cathode, which is formed by thesubstrate support105 and thewafer110 during substrate processing, andchamber walls128. When thewafer110 is not present in the chamber during processing, thesubstrate support105 forms the cathode. Details of how to adjust the impedance of the chamber components to lower the top electrode voltage will be described below.

FIG. 4A shows the relative voltage values of top electrode125 (or source) and cathode (substrate support105 along with thewafer110 during substrate processing), which is grounded.FIG. 4B shows the relative voltage values oftop electrode125 andchamber walls128, which is grounded. The X axes in both figures represent the spaces between thetop electrode125 and the cathode, which is formed by thesubstrate support105 and thesubstrate110, or innersurfaces chamber walls128. The distances of X-axes are not to scale. The top electrode voltage oscillates between +V_sand −V_s, while cathode and chamber walls stay at 0 (ground). The bulk of the plasma has a voltage that is higher than the top electrode by V_o, which is much smaller than V_s. Curve401 represents the voltage between thetop electrode125 and cathode, which is formed by thesubstrate support105 and thewafer110 during substrate processing, when the top electrode voltage is at +V_s. Thevoltage difference411 between thetop electrode125 and the cathode when the top electrode voltage is at +V_sequals to V_s. Dashedcurve402 represents the voltage between the source and the cathode when the source voltage is at −V_s. Thevoltage difference412 between thetop electrode125 and the cathode when thetop electrode125 voltage is at −V_sequals to −V_s.

By tuning impedance of thesubstrate support105 and the impedance of thedielectric seal130, which will be described below in more depth, the source voltage supplied to the top electrode can be reduced to a fraction of the total source voltage, such as half (V_s/2), while the cathode voltage is maintained at a negative phase of the top electrode to make up the difference, such as −V_s/2. The plasma process does not change, since the voltage difference between the source and cathode is still V_sor −V_s.FIG. 4C shows the relative values oftop electrode125 and the cathode (not grounded). The top electrode voltage oscillates between +V_s/2 and −V_s/2, while cathode voltage oscillates between −V_s/2 and V_s/2 correspondingly.Curve405 represents the voltage value between the electrode and cathode when the top electrode voltage is at +V_s/2. Thevoltage difference415 between thetop electrode125 and cathode, which is formed by thesubstrate support105 and thewafer110, when thetop electrode125 voltage is at +V_s/2 equals to V_s. Dashedcurve406 represents the voltage between thetop electrode125 and the cathode when the source voltage is at −V_s/2. Thevoltage difference416 between thetop electrode125 and the cathode when the source voltage is at −V_s/2 equals to −V_s.

In addition to lower voltage difference, the amount of power that could be lost due to un-confined plasma is also reduced to ¼. Theequation 1 below shows the relationship between P (power) and voltage difference between the top electrode to the chamber walls when the top electrode voltage is V_s.
P∝(V_s)²=V_s² (1)

Theequation 2 below shows the relationship between P (power) and voltage difference between the top electrode to the chamber walls when the top electrode voltage is only V_s/2.
P∝(V_s/2)²=V_s²/4 (2)
By lowering the top electrode voltage to half, the power available to lose to the chamber wall is lowered to a quarter of the original value.

Lowering top electrode voltage by a voltage ratio and supplying the remaining voltage supplied to the top electrode at a negative phase at the substrate support reduce the amount of plasma got attracted to the grounded chamber walls and thus improves plasma confinement. This method of plasma confinement is called impedance confinement. The fraction of total source voltage used in the discussion above is ½; however, other fraction value can also be used and could also improve plasma confinement. The fraction of source voltage supplied at the top electrode can also be defined as “voltage ratio”.FIG. 5A is a graph of plasma density simulation result of voltage ratios of 1, 0.75, 0.5 and 0.25. The pressure at the pump entry of the simulation process is 10 mTorr and the total source power is 1.85 kW. The spacing between theannular confinement ring115 with the inner chamber wall simulated is 1.5 inch (or 3.8 cm).Curve501 shows that as the voltage ratio decreases from 1, the plasma density ratio is reduced. The plasma density ratio of 0.001 is lowest when the voltage ratio is at 0.5. However, plasma density ratio of 0.003 when the voltage ratio is at 0.25 and plasma density ratio of 0.008 when the voltage ratio is at 0.75 are both lower than the plasma density ratio when the voltage ratio is 1.

FIG. 5B shows the simulation result of plasma density of 0.023 in the process chamber when the voltage ratio is 1 (or source voltage is completely supplied at top electrode). The simulation results show significant amount of plasma are outside the region above the substrate.FIG. 5C shows the simulation result when the voltage ratio is reduced to 0.5. The results show that plasma is mostly confined near the region above the substrate surface. Referring back toFIG. 3B, with gap width of 1.5 in, the pressure of the chamber can be maintained at about 26.2 mTorr, which is below 30 mTorr as targeted. According toFIG. 5A, to achieve the same plasma confinement results as the slotted confinement ring, which achieves plasma density ratio of 0.004, the voltage ratio can be operated between about 0.2 to about 0.6. However, when plasma density ratio is ≦0.01, the plasma confinement is considered quite reasonable. Therefore, the voltage ratio could be operated between about 0.1 to about 0.75, according to simulation results inFIG. 5A.

The combined usage of annular plasma confinement ring and impedance confinement achieves good plasma confinement and lower chamber pressure as desired for the front end processes with a wide process window. The annularring gap width117 could be between about 0.8 inch to about 1.5 inch and the voltage ratio for impedance confinement could be between about 0.1 to about 0.75 and preferably between about 0.2 to about 0.6.

In addition to plasma confinement improvement, lowering the voltage ratio also reduces the power loss outside the process region.FIG. 5D shows the simulation results of power deposition, which is defined as power per volume or power density, in the process chamber when the voltage ratio is maintained at 1. The results show significant power deposition outside the process region, which is above the substrate surface or the region within 15 cm from the center of the reactor. In contrast,FIG. 5E shows the power deposition of the process chamber when the voltage ratio is 0.5. The power loss outside the process region is much reduced, compared toFIG. 5D.

FIG. 6 is a simplified schematic diagram representing the impedance components of thereactor 100 ofFIG. 1, showing theoverhead electrode125, which has an impedance Z₁. Theelectrode125 is connected to thedielectric seal130, which acts like a capacitor and has an impedance Z₆.

The cathode is formed by thesubstrate support105, which has

dielectric layers

5520 and5510, and thewafer110 during substrate processing, and the cathode has an impedance Z₅. If thewafer110 is not present during processing, thesubstrate support105 is the cathode. In addition to theoverhead electrode125 impedance Z₁and cathode impedance Z₅, the bulk plasma also has impedance Z₃. In addition, there is an anode plasma sheath represented by an equivalent capacitor with impedance Z₂in series between the electrode impedance Z₁and the bulk plasma impedance Z₃. Furthermore, a cathode plasma sheath is represented by an equivalent capacitor with impedance Z₄in series between the bulk plasma impedance Z₃and the cathode impedance Z₅.

Equation 1 shows the relationship between impedance (Z), resistance (R) and capacitance reactance (X_c). “j” inequation 1 is an imaginary number.
Z=R−jX_c (1)
Equation 2 shows the relationship between the capacitance reactance (X_c) and capacitance C.
X_c=1/(2πf C) (2)
where f is the frequency of the source power and C is the capacitance.

FIG. 6 shows that thetop electrode125, anode plasma sheath, plasma, cathode plasma sheath, and cathode are in serial and these impedance components are in parallel with thedielectric seal130.Equation 3 shows the total impedance, Z_total.
Z_total=Z₁+1/(1/(Z₂+Z₃+Z₄+Z₅)+1/Z₆) (3)

Since the top electrode is typically made of conductive material, its impedance Z₁is mainly made of the resistance of the top electrode. Z₂, Z₃and Z₄are affected by the plasma. However, impedance Z5 and Z6 can be adjusted by changing the thicknesses and dielectric constants of the dielectric layers of thesubstrate support105, and thedielectric seal130. The magnitude of the cathode impedance can be affected the cathode capacitance. Z5 and Z6 can be adjusted to allow supplying thetop electrode125 at a fraction of conventional source voltage, fV_s, and maintaining the cathode at a voltage of negative phase from the top electrode, −(1-f)V_s.

While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.