Bonded silicon carbide parts in a plasma reactor

A plasma reactor, for example, for processing a semiconductor wafer, in which parts of the chamber are formed of multiple pieces of silicon carbide that have been bonded together. The bonding may be performed by diffusion bonding or by using a bonding agent such as polyimide. These silicon carbide parts typically face and define a plasma region. Preferably, the surface facing the plasma is coated with a silicon carbide film, such as that deposited by chemical vapor deposition, which is more resistant to erosion by the plasma. Advantageously, the different parts are formed with different electrical resistivities consistent with forming an advantageous plasma.

FIELD OF THE INVENTION 
The invention relates generally to plasma reactors and their operation. In 
particular, the invention relates to the plasma etching of semiconductors, 
and more particularly to oxide etching. 
BACKGROUND ART 
Many of the critical steps in the fabrication of advanced semiconductor 
integrated circuits involve processing in a plasma reactor. These steps 
include etching, chemical vapor deposition (CVD), and physical vapor 
deposition (PVD). In all these processes, a processing gas flows into the 
processing chamber, and an electric field excites the gas into a plasma. 
Particularly in etching and to a lesser extent in CVD, the excited 
processing gas is very reactive (which is why it is excited into a plasma) 
and reacts not only with the wafer but also with chamber parts that are 
exposed to the plasma. As a result, many of the parts in a plasma reactor 
facing the plasma have presented materials problems. If the plasma 
significantly reacts with the chamber part, a number of problems may 
occur. The processing chemistry involving the wafer may be disturbed by 
the side reactions with the material of the chamber. Some ceramics, such 
as quartz, are preferentially etched along grain boundaries, and the 
preferential etching of the inter-granular portions liberates grains from 
the ceramics. As a result, extraneous particles are formed which settle on 
the wafer and may drastically reduce the yield of operable chips. 
Long-term exposure of the chamber part to the plasma may erode the part to 
such an extent that it fails in its mechanical or electrical function. 
A newer generation of plasma reactors is being developed and commercialized 
which can be characterized as high-density plasma (HDP) reactors. By 
various means, the ion density of the plasma is increased to levels 
significantly above prior generations of commercial plasma reactors. The 
higher density of plasma not only accelerates the processing, but in 
several applications is required for effectively processing the 
increasingly smaller features of semiconductor integrated circuits. 
However, the high-density plasmas have increased the severity of the 
problems associated with the chamber parts, and prior materials solutions 
have been shown to be insufficient. 
Furthermore, the mechanisms for creating the high-density plasma often 
require a complexity of chamber design not previously required in 
commercial semiconductor fabrication equipment. 
An example of an advanced plasma etching reactor, particularly useful for 
oxide etching, is illustrated in the cross-sectional view of FIG. 1. This 
reactor is simplified from one disclosed by Collins et al. in U.S. Pat. 
application, Ser. No. 08/648,254, filed May 13, 1996. A wafer 10 is 
supported on a pedestal 12 facing a processing space 14 within a vacuum 
chamber. An annular pumping channel 16 formed in a chamber base 18 is 
pumped by an unillustrated vacuum system. An insulating ring 20 
electrically isolates the pedestal 12 from the chamber base 18. A slit 
valve opening 21 in the chamber base 18 and an associated but 
unillustrated slit valve allow the wafer 10 to be transferred into and out 
of the vacuum chamber. The top of the vacuum chamber is formed by a dome 
22 composed of silicon, and a silicon ring 24 surrounds the wafer 10. One 
or more unillustrated gas ports supply, for oxide etching, a 
fluorine-based etching gas. 
The etching gas is excited into a high-density plasma primarily by RF power 
inductively coupled into the chamber through two concentric helical coils 
26, 28 extending above a flat roof 29 of the dome 22. An RF power splitter 
30 splits RF power from a source RF power supply 32, for example operating 
at 2 MHz, between the two coils 26, 28 so as to tailor the RF magnetic 
field induced within the plasma inside the chamber. A bias RF power supply 
34 connected to the pedestal 12 provides a controllable DC self-bias in 
the plasma sheath adjacent to the wafer 10 for controlling the etching 
kinetics. The silicon dome 22 is electrically grounded to provide a 
grounding plane for the chamber. 
A number, four in the illustration, of rings 36, 38, 40, 42 composed of a 
ceramic that is preferably thermally conductive but electrically highly 
resistive are located on top of the dome roof 29. The two coils 26, 28 are 
wrapped around two rings 42, 36. A number of radiant lamps 44 are placed 
in remaining annular channels formed between the rings 36, 38, 40, 42 and 
control the temperature of the silicon dome 22. Unillustrated radiant 
heating means also control the temperature of the silicon ring 24 around 
the wafer 10. 
When the chamber is being used to etch a layer of silicon dioxide in the 
wafer 10 and the etching process must be highly selective against etching 
underlying silicon or polysilicon, the preferred etching gas is a 
fluorocarbon, such as CF.sub.4, and the silicon dome 22 and ring 24 are 
used to scavenge fluorine from the plasma of the etching gas, that is, to 
remove fluorine, leaving a carbon-rich plasma. As a result, a polymer 
having a low fluorine content forms on non-oxygen parts of the wafer 10, 
specifically silicon or silicon nitride parts once the silicon dioxide 
covering them has been etched away. The low-fluorine polymer protects the 
non-oxygen parts from etching, thus producing a high etch selectivity. The 
radiant lamps 44 and other temperature control elements control the 
temperature of the scavenging dome 22 and ring 24 since the scavenging 
process is sensitive to temperature. 
One of the difficulties with the silicon dome 22 of FIG. 1 is the 
requirement that it both act as a grounding plane and also pass an RF 
magnetic field from the coils 26, 28 into the chamber. A good grounding 
plane requires a high electrical conductivity while an RF wall should have 
a low electrical conductivity. These conflicting requirements are 
addressed by striking a balance between the bulk electrical conductivity 
and the RF skin depth relative to the thickness of the window. Typical 
resistivities for bulk pieces providing sufficient mechanical strength for 
a vacuum chamber are in the range of 30 to 200.OMEGA.-cm. Although 
acceptable results have been achieved, it is desired to better address 
both requirements. Other difficulties with the silicon dome 22 are that 
large pieces of silicon of high electrical resistivity are expensive and 
that it is difficult to control the reproducibility of the ingots from 
which the pieces are formed. Furthermore, large silicon pieces are prone 
to chipping and cracking. 
Lu et al. in U.S. Pat. application, Ser. No. 08/687,740, filed Jul. 26, 
1996, incorporated herein by reference, disclose a dome of a robust, 
cost-effective material which independently addresses the requirements of 
the grounding plane and of the RF window. As illustrated for a crown dome 
50 in the cross-sectional view of FIG. 2, the dome 50 includes a base part 
52 of sintered, preferably hot pressed, silicon carbide and a thin film 54 
of silicon carbide extending on the side of the base part 52 exposed to 
the plasma within the chamber. As Lu et al. explain, the silicon carbide 
is preferably stoichiometric or nearly so, but its composition may extend 
through the range of 40 to 60 atomic % of both silicon and carbon. The SiC 
thin film 54 is preferably formed by chemical vapor deposition (CVD) 
although other deposition and formation techniques are possible, as is 
explained by Lu et al. Hirai et al. review CVD formation of silicon 
carbide in "Silicon carbide prepared by chemical vapor deposition," 
Silicon Carbide Ceramics - 1: Fundamental and Solid Reaction, eds. Somiya 
et al., (Elsevier, 1991), pp. 77-98. However, the SiC film may be formed 
by other well known processes. Silicon carbide is also known to operate as 
a fluorine scavenger. The base body part 52 includes a number of circular 
rings 60, 62, 64, 66 providing annular channels 68 as well as one central 
hole 70 for the two coils 26, 28 and the radiant heaters 44. 
A principal advantage of the composite structure is that the bulk, SiC base 
part 52 can be made fairly resistive so as to not ground out the RF 
electromagnetic field which the inductive coils couple from one side of 
the roof to the other. At the same time, the SiC thin film 54 can be made 
of low-resistivity material that allows effective low-frequency biasing of 
the thin film 54 by an unillustrated electrical connection and hence of 
the entire roof. Although such a low resistivity in a bulk part would 
ground out the electromagnetic field, the thin film 54 may be made thin 
enough that its thickness is comparable to or less than the RF skin depth, 
thus not grounding out the electromagnetic field. Examples of thicknesses 
and resistivities of the roof of the sintered base 52 and of the film 54 
are tabulated in TABLE 
TABLE 1 
______________________________________ 
RESISTIVITY 
RF COUPLING 
T (.OMEGA.-cm) 
EFFICIENCY 
______________________________________ 
Base &gt;10.sup.5 Low RF Loss 
(18-23 mm) 
CVD 20-40 Better than Si 
(4 mm) 
______________________________________ 
The highly resistive base produces negligible RF loss while the moderately 
conductive but thin CVD film provides 20 to 25% higher RF coupling 
efficiency than a comparable silicon roof Further advantages of the 
composite structure are that silicon carbide is a reasonably good thermal 
conductor and that the two SiC elements 52, 54 have virtually the same 
coefficients of thermal expansion, as tabulated in TABLE 2 for a number of 
temperatures 
TABLE 2 
______________________________________ 
Coefficient of Thermal 
Temperature Expansion (10.sup.-6 /.degree. C.) 
(.degree. C.) Base CVD Film 
______________________________________ 
100 3.5 3.7 
200 4.0 4.0 
300 4.2 4.3 
______________________________________ 
The closeness of thermal expansion between the sintered base and the CVD 
film reduces thermal shock. Also, a silicon carbide CVD film 54, which is 
the only silicon carbide exposed to the processing space, is a very clean 
material, producing few particles, even in the corrosive fluorine plasma 
environment of oxide etching. On the other hand, bulk sintered silicon 
carbide is relatively inexpensive and can be made in even larger sizes for 
future 300 mm-wafer applications. Finally, sintered silicon carbide 
exhibits good mechanical characteristics. 
Nonetheless, the composite crown dome 50 of FIG. 2 has some disadvantages. 
The crown dome 50 is relatively large, about 40 cm in diameter and 12 cm 
in height, and is complexly shaped. Such shapes of silicon carbide can be 
preformed into near net shape (NNS) by hot pressing, a form of sintering. 
An example of a NNS base 80 is illustrated in cross section in FIG. 3. In 
this case, the NNS base 80 is integrally formed with a base plate 82, a 
base ring 84, and an undefined base roof 86 to be formed later into the 
rings for the inductive coils and radiant lamps. After the NNS base 80 is 
formed by sintering, it is machined into final shape, and thereafter the 
CVD film 54 is deposited to form the crown dome 50 of FIG. 2. The NNS part 
may assume a more or less complex shape depending on the tradeoff between 
the amount of machining and the length of the sintering time and cool down 
required for complex sintered parts. Sintering is a high-temperature 
and/or high-pressure process, and the size and shape of the crown dome 
requires a long sintering process including cool down. Furthermore, a 
single defect at any point in the sintering can ruin an entire dome, thus 
presenting a large financial risk. 
Another problem arises from the fact that the film thickness, of the order 
of a few millimeters, still must balance grounding resistance and RF skin 
depth. As a result, the grounding resistance is designed to be only barely 
adequate. However, during long term processing, the CVD thin film 54 is 
partially eroded, thus increasing the grounding resistance. Any such 
variation in grounding resistance introduces a process variation that is 
best avoided in a reactor intended for continuous production. 
SUMMARY OF THE INVENTION 
The invention may be summarized as a silicon carbide body, particularly 
useful as a wall or other part for a plasma reactor. The body is formed by 
bonding together multiple sub-parts of sintered or hot pressed silicon 
carbide. 
Advantageously for plasma processing, the multiple sintered parts may have 
differing electrical resistivities. The differing resistivities are 
usefully applied to chamber walls facing the plasma. For example, the part 
nearer the plasma can be of low electrical resistance and used as an 
electrical grounding plane or other electrode capacitively coupling energy 
into the plasma while the part further away from the plasma is of high 
electrical resistance and presents less resistive loss for an 
electromagnetic field coupling energy into the chamber, such as from an RF 
inductive coil. 
The plasma chamber part can be further enhanced by coating its surface 
facing the interior of the plasma chamber with a silicon carbide film, 
such as produced by chemical vapor deposition, thereby reducing particle 
generation while providing yet an additional electrical biasing plane. 
Advantageously, the resistivities of the CVD silicon carbide film and the 
innermost silicon carbide part have nearly the same resistivities so that 
erosion of the film during prolonged plasma process does not significantly 
change the effective lateral resistance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention includes silicon carbide members advantageously used as walls 
in plasma reactors. One such part is formed by bonding together multiple 
sintered silicon carbide parts. Of particular advantage for plasma 
processing is the additional feature that the different parts may have 
significantly different electrical resistivities. 
A first embodiment of the invention is a composite silicon carbide crown 
dome. It is formed from multiple hot-pressed cylindrically shaped silicon 
parts shown in the orthographic view of FIG. 4, specifically four roof 
rings 90, 92, 94, 96, a base plate 98, and a base ring 100. Although 
illustrated together, at this stage all the parts are mechanically 
separate from each other. All the parts are formed of hot pressed silicon 
carbide. Hot pressing is a form of sintering in which high pressure is 
applied during the sintering step. See, for example, Yamada et al., 
"Properties and applications of silicon carbide ceramics," Silicon Carbide 
Ceramics - 1: Fundamental and Solid Reaction, ibid., p. 18. 
In a single additional hot press process, the parts 90 through 100 are 
diffusion bonded together to form the composite near-net-shape (NSS) base 
102 illustrated in cross section in FIG. 5. The parts 90 through 100 have 
substantially the same composition, with the possible exception of doping 
elements used to control resistivity, as will be discussed later. However, 
definable bonding planes 103, 104 exist between the parts. Depending upon 
the bonding process, the bonding planes 103, 104 may be only diffuse 
boundaries following diffusion bonding or may represent a bonding agent, 
for example, an adhesive used in thermoplastic bonding. 
Subsequent to bonding into a single larger part, the composite NNS base 102 
is machined into its final shape, as illustrated in the cross-sectional 
view of FIG. 6, and the silicon carbide CVD coating 54 is then deposited, 
thereby forming a doubly composite crown dome 106. It is doubly composite 
because not only are there separate sintered and CVD silicon carbide 
parts, but the sintered silicon carbide body is formed of multiple bonded 
pieces. 
The composite crown dome has the advantage that the parts from which it is 
formed can be more simply and quickly sintered since the parts are smaller 
and have more regular shape. The amount of machining of the NNS part can 
be reduced. Also, for moderately sized parts, multiple pieces can be hot 
pressed at one time in a large oven, thus reducing furnace time. 
A particular advantage of the composite base is that the different silicon 
carbide bulk pieces may be made to have different resistivities. Examples 
of the resistivities of the parts in the conventional singly composite 
crown dome 50 of FIG. 2 and the doubly composite crown dome 106 of FIG. 6 
are presented in TABLE 
TABLE 3 
______________________________________ 
RESISTIVITY (.OMEGA.-cm) 
Singly Doubly 
T Composite 
Composite 
______________________________________ 
Roof Rings &gt;10.sup.5 
&gt;10.sup.5 
Base Plate &gt;10.sup.5 
100-200 
Base Ring &gt;10.sup.5 
100-200 
or &gt;10.sup.5 
CVD 20-40 20-40 
______________________________________ 
The roof rings 90, 92, 94, 96 are made highly resistive to as much as 
possible eliminate RF loss in a portion of the chamber being used 
primarily for heat sinking. The resistivity of the CVD film 54 is made low 
to provide low grounding resistance. One approach for achieving 
controllably low resistivity in a CVD film involves nitrogen doping during 
deposition. For example, the volumetric flow rate of N.sub.2 into the CVD 
chamber is 3 to 10% of that of the remainder of the CVD precursor gases. 
The resistivities of the base plate 98 and base ring 100 are chosen as a 
compromise between not creating excessive RF loss while aiding in the 
grounding resistance (actually electrical conductance) otherwise performed 
by the CVD film 54. Thereby, as the CVD film 54 erodes during processing, 
the grounding resistance does not dramatically increase. 
It is possible to further reduce the change of grounding resistance of the 
part over the lifetime of the crown dome by designing the base plate, base 
ring, and CVD film to all have the same resistivity, 20 to 40.OMEGA.-cm 
following the general example of TABLE 3. As noted in TABLE 1, the 
thickness of the base plate 98 is substantially greater than that of the 
CVD film 54 so that the CVD film 54 could be nearly completely eroded away 
with no substantial effect on processing. It is still preferred to include 
the CVD film 54, even though it has the same resistivity as the underlying 
bulk parts 98, 100, because a CVD film of silicon carbide is much cleaner 
than sintered silicon carbide and thus reduces the production of particles 
and the resultant contamination compared to sintered silicon carbide. If 
the underlying base plate 98 provides adequately low grounding resistance, 
then the resistivity of the CVD film can be increased somewhat as long as 
the resistance perpendicular to the plane of the film does not 
substantially increase the net effective lateral grounding resistance 
through both the film and the base plate. Films are distinguished from the 
sintered parts in that they were never free standing bodies prior to being 
formed on the substrate. Films are either deposited, usually by CVD or by 
thermal spraying, or are formed by chemical reaction with the underlying 
substrate. 
Other resistivity combinations are possible. In some plasma processing, 
relatively little RF power is grounded through the base ring 100 but the 
RF magnetic field can sink much power there. In this situation, it may be 
preferable to assign a high electrical resistivity to the base ring 100 
and rely only upon the CVD film 54 for the grounding current on the inside 
top of the crown dome. This high resistivity also reduces loss in the 
circulating RF electrical field. 
It is thus seen that the parts may be formed with substantially different 
resistivities differing by at least a factor of ten, or they may be formed 
with nearly equal resistivities differing by no more than a factor of 
four. Even the intermediate range of moderately different resistivities 
differing by between four and ten is useful when the trade offs are close. 
The design of the crown dome can be further improved by forming many of the 
parts of FIG. 4 from multiple, vertically separate sub-parts. As 
illustrated in the cross-sectional view of FIG. 7, a composite base plate 
110 is formed from multiple disks 112, 114, 116, here illustrated as three 
but four disks may be preferred. Similarly, as illustrated in FIG. 8, a 
composite base ring 120 is formed from multiple tubular annuli 122, 124, 
126. The disks and annuli all are formed of hot-pressed silicon carbide. 
Although not illustrated, the roof rings may be similarly vertically 
segmented. All the parts in the crown dome may be bonded together in a 
single operation, or the sub-parts having a similar shape may be first 
bonded into a larger part, which is thereafter bonded into the dome. 
The vertical segmentation offers several advantages. The sub-parts which 
are formed by sintering are substantially thinner than the parts which 
they thereafter form. The smaller sub-parts can be much more quickly 
sintered and cooled down since the mulitiplicity of parts is not an 
overriding concern for relatively small parts being fabricated in large 
ovens. Again, a defect in one sub-part does not affect any other 
sub-parts. Further, the electrical resistivities may be varied between the 
sub-parts. For example, the lowest disk 116 of the base plate 110 may have 
a relatively low resistivity close to that of the CVD film to reduce the 
grounding resistance while the upper two disks 112, 114 have very high 
resistivity to reduce the inductively coupled RF loss. 
Pieces of sintered silicon carbide can be joined by a number of methods, as 
is discussed by Iseki in "Joining of SiC ceramics," Silicon Carbide 
Ceramics - 1: Fundamental and Solid Reaction, ibid., pp. 239-263. The two 
methods tried with the invention are diffusion bonding and polyimide 
adhesive. For diffusion bonding, the already hot pressed silicon carbide 
pieces are preferably bonded together by another hot pressing in which two 
or more pieces are bonded together in a hot, isostatic press process. This 
diffusion bonding process is very similar to hot pressing used for 
sintering except the time, pressure, and temperature may be different to 
avoid possible grain growth of the SiC. Undesirable grain growth changes 
the material characteristics of the silicon carbide. 
Although the invention is particularly advantageous to oxide etching of 
semiconductor integrated circuits in a plasma reactor, it may be applied 
to other forms of etching and even of CVD. Indeed, the invention is also 
advantageous to plasma reactors for processing other types of workpieces.