Source: https://patents.justia.com/patent/5904778
Timestamp: 2020-02-23 11:21:19
Document Index: 480293250

Matched Legal Cases: ['artz               2', 'art 74', 'art 76', 'art 74', 'art 76', 'art 76', 'art 74', 'art.\n2', 'art.\n12', 'art.\n19']

US Patent for Silicon carbide composite article particularly useful for plasma reactors Patent (Patent # 5,904,778 issued May 18, 1999) - Justia Patents Search
Justia Patents By Creating Electric Field (e.g., Gas Activation, Plasma, Etc.)US Patent for Silicon carbide composite article particularly useful for plasma reactors Patent (Patent # 5,904,778)
Jul 26, 1996 - Applied Materials, Inc.
However, the temperature control of quartz parts is difficult. Quartz is both an electrical and thermal insulator. Its coefficient of thermal conductivity is less than 1 W/m.multidot.K, a relatively low value, so that it is difficult to closely control the temperature of the entire surface of a quartz piece exposed to the plasma. Furthermore, quartz has a chemical composition closely resembling that of the silicon oxide layer being etched in the semiconductor oxide etch reactor. Thus, one must assure that the quartz part is being operated in a deposition or slow etch mode rather than a strong etch mode while the wafer of similar composition is being strongly etched. If not, the quartz parts will have short lifetimes and hence impose a high cost of replacing consumable parts, both in terms of parts cost and machine down time. Furthermore, the quartz, although it is a silicate material providing some silicon scavenging, also produces a relatively uncontrollable amount of CO and CO.sub.2 from the reaction between, for example, CF.sub.4 and SiO.sub.2. The resultant carbon monoxide and dioxide are particular problems for selectivity to photoresist. Also, quartz is a ceramically formed material and typically includes large amounts of non-silicate components, which become contaminants in the fluorocarbon etching environment. Even further, the etching of the quartz can undercut surface portions to the extent that particles of quartz are separated from the reactor elements and fall onto the wafer as fatal particles. Oxide etching is particularly critical against particles since the etching produces interfaces between two electrically conducting parts, one of which may be semiconducting and any particle falling on the interface before deposition of the subsequent layer can seriously affect the electrical characteristics of the junction across the interface.
Collins et al. have suggested in European Patent Application 601,468-A1 and in U.S. patent application, Ser. No. 08/597,577, filed Feb. 2, 1996 that an inductive coil be placed in back of the silicon roof 26. Further, in the latter, Collins et al. have suggested that other parts of the chamber, including the side walls 22 in front of the RF coils 24 be formed of silicon, either in its crystalline or polysilicon structure. The silicon composition provides some scavenging functions and also avoids contamination by quartz or other ceramics. However, silicon is a semiconductor, not a dielectric. As described by Collins et al., the silicon of the proper doping and thickness can advantageously also be electrically biased, either DC or RF, even while, in a preferred usage, electromagnetic radiation is being propagated therethrough. However, silicon in such uses presents many compromises and disadvantages. First, silicon in such large dimensions is not readily available at reasonable prices, particularly in times of shortage of polysilicon. Secondly, silicon, although affording relatively high structural strength, is prone to fracture from local micro-defects arising from its growth in the form of polysilicon and its subsequent machining. Thirdly, semiconducting silicon (bandgap of about 1.2 eV) affords an uneasy compromise between structural strength and electromagnetic transparency. Electromagnetic radiation can only penetrate a semiconductor or other conductor to the extent of a skin depth which can be expressed as ##EQU1## where f is the frequency of the electromagnetic radiation in hertz, .mu..sub.0 is the magnetic permeability in H/m, and .rho. is the bulk DC plasma resistivity of the semiconductor in ohm-m. The penetration of electromagnetic radiation through a conductive sheet is generally an exponential function of the sheet thickness z having the general form to first order of where surface effects are disregarded. These relationships show that the transparency of a layer depends on both the material resistivity and the frequency of the electromagnetic radiation. The functional dependence (2) shows that for a thickness of no more than the skin depth, the resistive absorption is less than 64%; for a thickness of no more than one-third the skin depth, the absorption is less than 29%; and for a thickness of no more than one-tenth the skin depth, the absorption is less than 10%.
Resistivity  Skin Depth
(ohm-cm)     (mm)
0.1          1.13
0.3          1.95
1            3.56
3            6.17
10           11.3
30           19.5
100          35.6
Silicon carbide is well known as a susceptor material for RF induction heating of a chamber or wafer support within the chamber. Its thermal conductivity is in the range of 100 to 200 W/m.multidot.K, vastly superior to quartz. An RF coil wrapped around the chamber induces eddy currents in a highly conductive silicon carbide part to thereby heat it to high temperatures, such as are required for thermal CVD, as disclosed by Ban in U.S. Pat. No. 4,401,689. Induction heating is to be contrasted with inductive coupling of RF power into a plasma reactor chamber. Plasma reaction chambers are usually operated at much lower temperatures, and the RF energy should be coupled into the plasma and not into chamber parts. Hence, any silicon carbide parts used in a reaction chamber, at least in the vicinity of the coils, should have relatively high electrical resistivity, for example, above 10.sup.4 ohm-cm.
Silicon carbide composites are well known in which a bulk piece of silicon carbide is coated with a thin film of silicon carbide using a chemical vapor deposition (CVD) or similar process. Such composites include resistive heaters, as disclosed by Ito et al. in U.S. Pat. No. 4,810,526, and rugged mirrors, as disclosed by Hotate et al. in U.S. Pat. No. 5,448,418. Matsumoto et al. in U.S. Pat. No. 4,999,288 discloses using a silicon carbide composite as a diffusion tube for heat treating semiconductor wafers at about 1200.degree. C. According to Matsumoto et al., a 500 .mu.m-thick silicon carbide film is CVD deposited on the interior of a reaction sintered silicon carbide tube, and the film has a low concentration of iron impurities, although this level is defined as 5 parts per million (ppm). If desired, an interfacial region in the silicon carbide tube can be depleted of silicon. Electrical resistivity is immaterial in most diffusion tubes.
The chemical vapor deposition of silicon carbide is relatively well known, particularly in fields outside of semiconductor fabrication equipment. See, for example, U.S. Pat. No. 4,810,526 to Ito et al. and U.S. Pat. No. 5,448,418 to Hotate et al. Chemical vapor deposition involves the growth upon a substrate of a film from a gaseous precursor through a reaction which is activated, by example, thermally or by a plasma. Protective SiC carbide coatings are well known having thicknesses of 100 .mu.m or less, but the CVD films of 1 mm or more envisioned in many of the embodiments of the inventions are believed to be novel. At the present time, several manufacturers supply free-standing CVD silicon carbide for which a relatively thick layer of at 1 to 10 mm is CVD deposited and then the underlying substrate of graphite is etched away.
An important parameter for the silicon carbide or other material to be used in a plasma etching reactor, particularly one being used for oxide etching, is the etching rate of the material. A number of test coupons of various materials were prepared and placed at two positions in the reactor of FIG. 1. The first position 30 was at the center of a silicon wafer 14 supported on the pedestal 15. The second position 32 was on top of a base plate 34 of the chamber which is located outside and below the area of the wafer. The reactor was operated as an HDP oxide etcher with an etching gas of C.sub.4 F.sub.8 and an argon carrier gas.
Mfgr.   Material
Type         Base Plate
A       SiC     CVD          0.5    1.0
A       SiC     CVD          0.4    1.2
B       SiC     CVD          0.4    3.2
C       SiC     hot-pressed  0.6    3.2
C       SiC     hot-pressed  0.8    3.0
D       SiC     sintered     1.0    6.8
E       Si      Czochralski  1.4    4.6
E       Si      cast         1.3    10.2
E       quartz               2.1    2.4
All of these same samples were inspected by microscope at about a 1000.times.magnification prior to etching (including a pre-etch polishing) and after 30 hours of etching. The general conclusions follow. The CVD silicon carbide shows better surface morphology than sintered SiC, silicon, or quartz, and, as TABLE 2 shows, its etching rate is the lowest of these materials. The silicon shows reasonably good morphology, and its etching rate is acceptable although higher than that of CVD SiC. Sintered silicon carbide shows poor surface morphology and exhibits a non-uniform etching pattern, which may create particles and impact contamination and process control.
A silicon carbide composite was fabricated in which a bulk body of sintered silicon carbide was formed having a sharp corner. The bulk body was then covered with a thick layer of up to 6 mm of silicon carbide deposited by CVD. The specimen was sectioned and inspected in both a scanning electron microscope (SEM) and in an optical microscope. The SEM micrograph is shown in FIG. 3. Several observations can be made. The sintered silicon carbide forms a matrix of generally round particles having sizes of approximately 10 .mu.m and less. Silicon carbide can be formed to significant thicknesses by CVD. The CVD silicon carbide forms a dense structure having a crystalline orientation extending generally perpendicularly to the underlying sintered silicon carbide substrate. The CVD silicon carbide easily coats a corner having a radius of less than 20 .mu.m, and the CVD silicon carbide exhibits the above crystalline orientation on both the horizontally and vertically extending faces around the corner. This latter results demonstrates that a layer of CVD SiC can be applied to complex shapes, such as clamping rings, collars, focus rings, etc.
A surface film of CVD SiC produces fewer particles in a plasma etch reactor than sintered SiC and many fewer than quartz. As shown by the etching data of TABLE 2, CVD SiC etches less than the other materials. More importantly, as is clear from the micrograph, CVD SiC does not have the granular structure of sintered SiC. When sintered SiC is etched, it is likely that the grain boundaries etch more quickly than the grains themselves. In some cases, the etch dynamics cause the grains to be etched free of the matrix and thus to be liberated as fairly large particles within the reactor, which are prone to fall upon the wafer being etched. Modem integrated circuits can fail if a single particle of size larger than approximately 0.2 .mu.m falls upon it during fabrication. In contrast, the crystalline nature of CVD SiC causes it to be more uniformly etched with the etched particles being of atomic or molecular sizes. Surface profiles were measured on many of the etched samples of TABLE 2. The CVD SiC showed much smoother surface after etching than sintered SiC, quartz, and even silicon. A smoother etch indicates less propensity for particles being liberated.
The silicon carbide composite of the invention offers several advantages for high-temperature operation. It offers very good resistance to thermal shock at temperatures at least as great as 300 or 500.degree. C. It excels even with respect to homogeneous silicon. For the large structures allowed by the invention, silicon is available now as polysilicon, which has a tendency to chip and flake under wide temperature cycling. Silicon is a semiconductive material with a moderately narrow bandgap. Extremely pure silicon is required to achieve the desired high resistivities of greater than 10 .OMEGA.-cm and preferably up to 500 .OMEGA.-cm. Any variation of the impurity or doping levels or of the temperature of operation will cause significant variations in the high silicon resistivity. On the other hand, silicon carbide is a wide-bandgap semiconductor, high resistivities are easily achieved at moderate impurity levels, and further the temperature dependence of the resultant electrical conductivity is not nearly as steep as in silicon. The high-resistivity sintered silicon carbide we have tested has maintained a resistivity of greater than 10.sup.4 .OMEGA.-cm and up to 10.sup.5 .OMEGA.-cm at 300.degree. C. The thickness of the CVD silicon carbide film is so thin that the thermal dependence of its electrical characteristics should not present a problem.
The temperature dependence is shown in trace 44 in the graph of FIG. 4 for the DC resistivity of low-resistivity CVD silicon carbide from one commercial source. Advanced oxide etchers are being designed to operate at temperatures well below 200.degree. C., preferably around 150.degree. C., a good range for resistivity as indicated by the dashed lines. In FIG. 5 are shown trace 45 for a high-resistivity sintered silicon carbide, and traces 46, 48 respectively for high-resistivity and low-resistivity CVD SiC. Traces 46, 48 show that for the preferred operating temperature of 150.degree. C. resistivities of 10.sup.5 to 10.sup.6 .OMEGA.-cm are readily available in both sintered and CVD SiC. Trace 48 shows that moderately conductive CVD SiC is also available. Although low-resistivity sintered SiC is not shown, it is appreciated that its resistivity can be lowered by appropriate doping or other measures, as is well known by the commercial suppliers of such materials.
The exact mechanisms for controlling resistivity are not totally clear, but the resistivity is known to depend on the electrical dopant, such as boron, and its doping and impurity concentrations and upon the morphology and crystalline orientation, including grain size and boundary effects. Silicon carbide can form in two distinctly different crystalline phases, the .alpha. and .beta. phases, and the semiconductor bandgaps of these two phases are different. Hot-pressed silicon carbide usually forms in the hexagonal a-phase while CVD silicon carbide tends to form in the cubic .beta.-phase. Doping impurities ultimately determine the electrical resistivities of both forms. However, we have established several suppliers who can dependably deliver silicon carbide with widely differing but reproducible resistivities at room temperature for both sintered and CVD SiC, as shown in TABLE 3.
High .rho.    10.sup.6 to >10.sup.9
10.sup.6 -10.sup.7
Low .rho.     1-10      1-50
It is thus seen that silicon carbide can be obtained with low resistivities below 100 .OMEGA.-cm and with high resistivities above 10.sup.5 and even 10.sup.6 .OMEGA..multidot.cm. These ranges should be compared to silicon for which obtaining consistent and controllable resistivities above 30 .OMEGA..multidot.cm is difficult. On the other hand, silicon can be doped to be highly conductive. These differences arise because silicon carbide can be characterized as a wide bandgap material, the bandgap being much larger than that of silicon.
(50-200.degree. C.)
CVD SiC     Bulk     POSSIBLE
Coating     SiC      APPLICATIONS
Low         Low      Electrode, RF power, Hot
Ring, Focus Ring,
Nozzles, DC Pickup
High        Low      Collar, Floating Ring, Focus
Low         High     Roof/RF, Wall
High        High     Collar, Floating Roof
High resistivity in the bulk and low resistivity in the film are desirable in the case of the previously described planar RF coil in back of a composite SiC counter electrode or wall inside the helical RF coil. The data in TABLE 1 shows that structurally thick SiC members can still pass 2 MHz electromagnetic radiation if their resistivity is about 10 .OMEGA.-cm and above. The table also shows that films of 3 mm and less can pass the 2 MHz electromagnetic radiation while still having a sheet resistance of about 1 ohm per square or less, adequate for a grounding plane or lower-frequency electrode inside an RF coil. That is, the sintered bulk SiC is made highly resistive so as to freely pass the RF magnetic field with no eddy currents while the CVD SiC thin film is made relatively highly conductive to serve as an electrode as well as be made thinner than an RF skin depth.
A specific example is illustrated in partially sectioned perspective view of FIG. 8 showing a truncated conical dome 70 having an RF inductive coil 72 wrapped around its outside. Such a structure can form a plasma reactor chamber having several advantages over the right cylindrical chamber of FIG. 1. The conical dome 70 itself does not form part of the present invention, but was invented by another entity. However, the invention can be advantageously applied to it. The conical dome 70 is formed of a high-resistivity sintered bulk part 74 and a low-resistivity film part 76 that covers the interior of the conical dome 70, is wrapped around its bottom, and extends to a tab 78 on the outside to be electrically contacted to ground or to other electrical biasing source. The bulk part 74 has such a high resistivity as to not significantly perturb the induction field from the coil 72. The film part 76 may have a resistivity of 1-10 ohm-cm at 150.degree. C. and a thickness of 5 mm so as to substantially pass the induction field while still providing sufficient electrical conductivity as to allow the film part 76 to present a grounding plane to the plasma within the conical dome 70.
A composite SiC dome, similar to that of FIG. 9, was fabricated and electrically characterized. The sintered bulk part 74 had a thickness of between 3/8" and 3/4" (10 to 20 mm) and had an electrical resistivity of 10.sup.9 .OMEGA.-cm at room temperature. The CVD film 74 had a thickness of 2.5 mm. A four-probe measurement determined the electrical resistivity on a free-standing coupon from the same run as the coating on the sintered part. The temperature dependence of the measured resistivity is shown in the graph of FIG. 11. The sintered bulk part as a result introduces substantially no loss for RF energy being coupled through it. The film part has a resistivity which, according to TABLE 1, produces a skin depth substantially greater than the film thickness. Hence, the film part is substantially transparent to electromagnetic radiation, but the film can still ground current from the electrode.
1. A plasma collar for being fit onto a peripheral recess in a pedestal for supporting a substrate within a plasma reaction chamber, comprising:
a lower part comprising sintered silicon carbide; and
an upper part comprising silicon carbide formed over said lower part.
2. The plasma collar of claim 1, wherein said upper and lower parts are formed as annuli extending about an axis and arranged along said axis.
4. A plasma reactor, comprising:
a plasma reaction chamber having a plasma selectively generated within said chamber; and
a composite part placed within said chamber having a sintered silicon carbide portion overlaid with a deposited silicon carbide film facing said plasma, wherein said deposited film has a metal impurity concentration of less than 100 parts per billion.
5. The plasma reactor as recited in claim 4
wherein said sintered silicon carbide portion and said deposited film have electrical resistivities differing by a predetermined amount.
6. The plasma reactor as recited in claim 5, wherein said resistivity of said sintered silicon carbide portion is higher than said resistivity of said deposited film.
a plasma reaction chamber having a plasma selectively generated within said chamber;
a pedestal for supporting a substrate; and
a composite part placed within said chamber having a sintered silicon carbide portion overlaid with a deposited silicon carbide film facing said plasma, wherein said composite part is formed as a ring disposed at an upper and outer periphery of said pedestal.
8. The plasma reactor as recited in claim 7, wherein said ring is fittable into a peripheral recess of said pedestal.
9. The plasma reactor as recited in claim 7, wherein both said sintered silicon carbide portion and said deposited film have resistivities of greater than 10.sup.6 ohm-cm at 150.degree. C.
a composite part placed within said chamber having a sintered silicon carbide portion overlaid with a deposited silicon carbide film facing said plasma, wherein said film has a thickness of at least 3 mm.
a wall of said reaction chamber comprising a bulk part comprising sintered silicon carbide and a film part comprising silicon carbide formed over said bulk part on a side of said wall facing an interior of said reaction chamber; and
a source of electromagnetic radiation positioned on a side of said wall opposite said interior of said reaction chamber;
wherein said bulk part is electrically insulative such that a skin depth of said electromagnetic radiation in said bulk part is greater than a thickness of said bulk part.
12. The reaction chamber of claim 11, wherein said film part is sufficiently electrically insulative that a skin depth of said electromagnetic radiation, in said skin part, is greater than a thickness of said film part.
19. An RF plasma reactor, comprising:
an inductive antenna positioned outside a wall of said chamber; and
a window in said chamber wall to admit the inductive field of said antenna into the interior of said chamber;
wherein said window is a composite comprising a sintered silicon carbide layer overlaid with a deposited silicon carbide film facing said interior of said chamber.
20. The plasma reactor as in claim 19, wherein said window is also an electrode.
33. An RF plasma reactor, comprising:
a pedestal electrode for supporting a workpiece to be processed within the chamber; and
a counter electrode facing said pedestal electrode in spaced relationship thereto, said counter electrode being a composite comprising a sintered silicon carbide layer and a deposited silicon carbide film, said film facing said pedestal electrode;
wherein said electrodes are adapted to accept RF power so as to capacitively couple energy into a plasma within said reaction chamber.
34. The RF plasma reactor of claim 33, wherein a roof of said chamber comprises a metal, and wherein said counter electrode is attached thereto.
4810526 March 7, 1989 Ito et al.
4999228 March 12, 1991 Matsumoto et al.
5448418 September 5, 1995 Hotate et al.
5578129 November 26, 1996 Moriya
552491-A1 July 1993 EPX
0552490 September 1993 EPX
601468-A1 June 1994 EPX
60-200519 October 1985 JPX
61-245558 October 1986 JPX
63-138737 December 1986 JPX
03201322 September 1991 JPX
06317894 November 1994 JPX
8-17745 January 1996 JPX
Inventors: Hao A Lu (San Mateo, CA), Nianci Han (Sunnyvale, CA), Gerald Z Yin (Cupertino, CA), Robert W Wu (Pleasanton, CA)
Application Number: 8/687,740
Current U.S. Class: By Creating Electric Field (e.g., Gas Activation, Plasma, Etc.) (118/723R); 156/345; Gas Or Vapor Form Agent Condensed Or Absorbed On Work (134/11); 315/11121; 204/29802; 204/29831
International Classification: C23C16/00;