Multi-ledge substrate support for a thermal processing chamber

A substrate support, for example an edge ring, includes an upper ledge for supporting a first substrate, such as a semiconductor wafer, during a first process, and a lower ledge contiguous with the upper ledge for supporting a second substrate during a second process for cleaning the substrate support. A method of processing substrates supported by the edge ring in a thermal process chamber is also disclosed.

BACKGROUND 
This invention relates to a substrate support for a thermal processing 
chamber. 
In many semiconductor device manufacturing processes, the required high 
levels of device performance, yield, and process repeatability can only be 
achieved if the temperature of the substrate (e.g., a semiconductor wafer) 
is tightly monitored and controlled during processing of the substrate. 
Rapid thermal processing (RTP), for example, is used for several different 
fabrication processes, including rapid thermal annealing (RTA), rapid 
thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), 
rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). 
In some semiconductor processing systems, such as susceptorless systems, 
the substrate is supported around its perimeter with an edge ring. Some 
fabrication processes, such as RTCVD processes, can lead to the formation 
of deposits, such as silicon, on the edge ring as well as on the 
semiconductor substrate. Over time, the deposits accumulate on the edge 
ring. Moreover, due to variations in the placement of substrates within 
the edge ring pocket, the accumulated deposits may not be uniform across 
the surface of the substrate. Such uneven deposits may introduce undesired 
features on the backside of the substrate and adversely impact subsequent 
lithography or other processing steps, for example, if the substrates do 
not rest flat within the pocket of the edge ring. Therefore, the edge ring 
must be cleaned periodically so that the substrates rest properly on the 
edge ring and so that thermal interactions between the substrates and the 
edge ring do not adversely affect operation of the system. 
One technique for cleaning the edge ring involves the use of a cleaning 
gas. To bring the edge ring to a temperature at which the cleaning gas is 
chemically effective, a surrogate substrate the same size as the 
semiconductor substrate can be used. During the cleaning process, the 
surrogate substrate is supported by the edge ring in the same manner as 
the semiconductor substrate. Such a cleaning procedure, however, can 
result in the surrogate substrate covering portions of the edge ring which 
have deposits thereon and which, therefore, should be cleaned prior to 
using the edge ring to process additional semiconductor substrates. 
SUMMARY 
In general, in one aspect, a substrate support includes an upper ledge for 
supporting a first substrate, such as a semiconductor wafer, during a 
first process and a lower ledge contiguous with the upper ledge for 
supporting a second substrate during a second process for cleaning the 
substrate support. The substrate support can be, for example, an edge ring 
disposed in a thermal processing chamber. 
Various implementations include one or more of the following features. The 
upper ledge and lower ledge can be annular-shaped, with the lower ledge 
having an outer diameter smaller than an outer diameter of the upper 
ledge. The upper and lower ledges can have substantially flat upper 
surfaces. In some implementations, the lower ledge has a radial width 
approximately the same as a radial width of the upper ledge. For example, 
the ledges can have respective radial widths of approximately 0.2 inches. 
In general, however, the dimensions of the edge ring, including the radial 
widths of the ledges, depend on the particular substrates and processing 
systems with which the edge ring is to be used. 
The substrate support can include an outer portion contiguous with and 
extending radially outward from the upper ledge. An upstanding structure 
can connect the outer portion and the upper ledge so that the upstanding 
structure retains the first substrate on the upper ledge during the first 
process. Similarly, the substrate support can include another upstanding 
structure connecting the upper and lower ledges to retain the second 
substrate on the lower ledge during the cleaning process. 
In some implementations, the substrate support includes, for example, 
silicon carbide or other materials containing silicon. 
In another aspect, a method of processing substrates in a thermal process 
chamber includes providing within the chamber an edge ring having a lower 
ledge contiguous with an upper ledge, wherein the lower ledge has an outer 
diameter smaller than an outer diameter of the upper ledge. The method 
also includes supporting a first substrate on the upper ledge of the edge 
ring. 
In some implementations, a process gas is provided in the chamber. The 
method can include heating the chamber and spinning the edge ring and the 
first substrate about a central axis during heating. 
The first substrate can be removed from the chamber, and a second substrate 
can be supported on the lower ledge of the edge ring. A cleaning agent can 
be provided in the chamber while the second substrate is supported by the 
lower ledge. The cleaning agent can include a gas, such as HCl, Cl.sub.2 
or ClF.sub.3. In general, the cleaning agent can be selected to remove 
deposits formed on either ledge depending on the location of a substrate 
supported by the edge ring. Such deposits may occur, for example, during 
RTCVD processing. 
In addition, the chamber can be heated while the second substrate is 
supported by the lower ledge to expose the upper ledge to the cleaning 
gas. 
In some implementations, the method also includes spinning the edge ring 
and the second substrate during heating. 
The second substrate can include silicon carbide, graphite, or another 
material that is substantially impervious to the cleaning agent. 
The second substrate can be removed from the chamber, and another substrate 
can be supported by the cleaned upper ledge of the edge ring. The third 
substrate, which can be another semiconductor wafer, for example, then can 
be processed in the chamber with more uniform contact with the edge ring. 
Various implementations include one or more of the following advantages. An 
edge ring or other substrate support with upper and lower ledges can allow 
surfaces of the edge ring to be cleaned more thoroughly. By providing one 
ledge to support a semiconductor wafer, for example, during thermal 
processing, and another ledge to support a surrogate substrate during a 
cleaning process, surfaces of the edge ring that support and contact the 
semiconductor wafer can be cleaned more easily and more uniformly to 
remove deposits that may have formed on the edge ring surface. Such 
deposits can form, for example, during RTCVD processing of the 
semiconductor wafers. Removal of the deposits from the edge ring surface 
can improve the thermal contact between the edge ring and the 
semiconductor wafer or other substrate. The improved thermal contact can 
lead to a more uniform temperature across the semiconductor wafer surface 
because the contact between the edge ring and the entire perimeter of the 
wafer is more uniform. The improved temperature control can provide higher 
levels of device performance, yield, and process repeatability. In 
addition, a more through and uniform removal of the deposits allows the 
semiconductor substrates to rest flatter on the edge ring, which can lead 
to better yield and repeatability with respect to subsequent lithographic 
or other processing steps. 
Additional features and advantages will be readily apparent from the 
following detailed description, drawings and claims.

DETAILED DESCRIPTION 
FIGS. 1 and 2 illustrate a rapid thermal processing (RTP) system including 
a processing chamber 100 for processing a disk-shaped silicon substrate 
106. Various features of the RTP system are described in further detail in 
co-pending U.S. patent application Ser. No. 08/641,477, now U.S. Pat. No. 
5,755,511 entitled "Method and Apparatus for Measuring Substrate 
Temperatures", filed on May 1, 1996, which is incorporated herein by 
reference. 
The substrate 106 is mounted inside the chamber on a substrate support 
structure 108 and is heated by a heating element 110 located directly 
above the substrate. The heating element 110, which can include tungsten 
(W) halogen lamps 111, generates radiation 112 which enters the processing 
chamber 100 through a water-cooled quartz window assembly 114 disposed 
above the substrate. The lamps 111 can be arranged in multiple zones which 
are grouped together in several control groups. A temperature control 
algorithm is used to control lamps and thereby to control the temperature. 
Beneath substrate 106 is a reflector 102 which is mounted on a 
water-cooled, stainless steel base 116. The reflector 102 can be made of 
aluminum and has a highly reflective surface coating. The underside of 
substrate 106 and the top of reflector 102 form a reflecting cavity 118 
for enhancing the effective emissivity of the substrate, thereby improving 
the accuracy of temperature measurement. 
The temperatures at localized regions 109 of the substrate 106 are measured 
by a plurality of temperature probes 126 and pyrometers 128. The 
temperature probes 126, which can include fiber-optic probes, are 
distributed at varying distances from the center of the substrate 106. 
During thermal processing, the support structure 108 is rotated, for 
example, at about 90 revolutions per minute. Thus, each probe samples the 
temperature profile of a corresponding annular ring area on the substrate. 
The support structure which rotates the substrate includes an edge ring 
134 which contacts the substrate around the substrate's outer perimeter, 
thereby leaving all of the underside of the substrate exposed except for a 
small annular region near the substrate's edge. To minimize the thermal 
discontinuities that may occur at the edge of the substrate 106 during 
processing, the edge ring 134 can be made of the same, or similar, 
material as the substrate, for example, silicon carbide coated with 
silicon or an oxide of silicon. When the support structure 108 is rotated, 
the edge ring 134 and substrate 106 spin about a central axis as the 
chamber is heated. 
The edge ring 134 rests on a rotatable tubular quartz cylinder 136 that is 
coated with silicon to render it opaque in the frequency range of 
pyrometers 128. The silicon coating on the quartz cylinder acts as a 
baffle to block out radiation from external sources that might disturb the 
temperature measurements. The bottom of the quartz cylinder is held by an 
annular upper bearing race 142 which rests on a plurality of ball bearings 
138 that are, in turn, held within a stationary, annular, lower bearing 
race 140. 
During processing, a process gas is introduced into the space between the 
substrate 106 and the water-cooled quartz window assembly 114 through an 
inlet port 101. Gases are exhausted through an exhaust port 105 coupled to 
a vacuum pump (not shown). 
An optional purge ring 107 is fitted into the chamber body and surrounds 
the quartz cylinder 136. The purge ring 107 has an internal annular cavity 
which opens to a region above the upper bearing race 142. The internal 
cavity is connected to a regulated purge gas supply through a passageway 
103. During process steps that include providing a process gas into the 
upper portion of the chamber 100, a flow of purge gas enters the chamber 
through the purge ring 107. 
Referring to FIGS. 3-4, the edge ring 134 has an outer portion 152 
supported from underneath by the quartz cylinder 136; a central portion 
150 forming a substantially flat first, or upper, annular ledge; and an 
inner portion 148 forming a substantially flat second, or lower, annular 
ledge. 
A 200 mm substrate has a diameter of approximately 8 inches and a thickness 
of approximately 0.03 inches (0.775 mm). The outer diameter of the upper 
ledge 150 is slightly larger than the nominal diameter of the 
semiconductor substrate 106. The transition between the upper ledge 150 
and the outer portion 152 forms an inward facing, upstanding surface 154 
that keeps the semiconductor substrate 106 centered on the edge ring 134 
during processing. The upper ledge 150 thus supports the semiconductor 
substrate 106, and the upstanding surface 154 and the upper ledge form a 
pocket for the substrate (FIG. 5). In some implementations, the upstanding 
surface 154 forms substantially right angles with the upper and lower 
ledges 150, 148. However, in other implementations, the surface 154 can be 
inclined between the lower ledge 148 and the upper ledge 150. 
The outer portion 152 of the edge ring 134 in the illustrated 
implementation has a substantially flat upper surface 156 that is at the 
same elevation as the top of the upstanding surface 154 to allow smooth 
flow of process gases across the surface. The support structure 108 also 
is designed to create a light seal between the edge ring 134 and the 
quartz cylinder 136. The bottom of the edge ring 134, near its outer edge 
160, forms an annular-shaped shoulder 162 which has an inside diameter 
that is slightly larger than the outside diameter of the quartz cylinder 
136, so that it fits over the cylinder forming a light seal. 
The transition between the lower ledge 148 and the upper ledge 150 forms a 
second inward facing, upstanding surface 158. The lower ledge 148 supports 
a surrogate substrate 170 (FIG. 6). The upstanding surface 158 and the 
lower ledge 150 thus form a lower pocket for the surrogate substrate and 
keep the surrogate substrate centered in the lower ledge 148 during an 
edge ring cleaning process. The diameter of the surrogate substrate 170 is 
smaller than the diameter of the semiconductor substrate 106. Accordingly, 
the outer diameter of the lower ledge 148 can be made smaller than the 
outer diameter of the upper ledge 150. 
In one exemplary implementation for an 8-inch (200 mm) substrate 106, the 
edge ring can have an outer diameter (d) of approximately 9.3 inches. The 
outer diameter D.sub.1 of the lower ledge 148 can be approximately 7.6 
inches (190 mm). The radial width (w.sub.1) of the upper ledge 150 can be 
approximately 0.2 inches (5 mm) such that the outer diameter D.sub.2 of 
the upper ledge is approximately 8 inches (200 mm). Thus, the outer 
diameter D.sub.1 of the lower ledge is smaller than the outer diameter 
D.sub.2 of the upper ledge. 
In general, the dimensions of the upstanding surface 154 and the ledge 150 
are designed to be sufficiently high and wide, respectively, to make sure 
that the substrate 106 does not slip off the ledge 152 when the support 
structure 108 and substrate spin during substrate processing. The radial 
width (w.sub.1) of the upper ledge 150 also should be selected to ensure 
that if a substrate 106 is positioned slightly off-center on the ledge 
150, a gap will not be formed between the ledge and one side of the 
substrate. Such a gap could allow light to leak into the cavity 118. Other 
dimensions may suitable for particular applications. 
The radial width (w.sub.2) of the lower ledge 148 and the height of the 
upstanding surface 158 can be approximately the same as the radial width 
(w.sub.1) of the upper ledge 150 and the height of the surface 154, 
respectively. In general, the dimensions of the upstanding surface 158 and 
the ledge 148 are designed to be sufficiently high and wide, respectively, 
to make sure that the surrogate substrate 170 does not slip off the ledge 
148 when the support structure 108 and substrate spin during the cleaning 
process. 
The foregoing dimensions are suitable for one implementation of the edge 
ring 134 in certain processing chambers, such as the RTP Centura.TM. or 
the RTP Centura XE.TM., manufactured by Applied Materials, Inc. Other 
dimensions may be suitable for wafers of different sizes, for example, a 
6-inch (150 mm) or a 12-inch (300 mm) semiconductor wafer, or for wafer 
processing systems different from the RTP system described above. Thus, 
for example, the dimensions of an edge ring suitable for use with 300 mm 
substrates can be scaled appropriately. 
The illustrated implementation of FIGS. 3-6 can be formed by grinding a 
disk of chemical vapor deposited (CVD) silicon carbide with a diamond 
grinding head. Exterior corners can be fully rounded and interior corners 
can be rounded to a radius of at least approximately 0.01 inches (0.25 mm) 
to reduce mechanical stresses in the edge ring 134. The entire edge ring 
134 can be coated with approximately 0.004 inches (0.1 mm) of poly-silicon 
on each side. In various implementations, layers of different or 
additional materials also can be added to the edge ring. 
A method of using the edge ring 134 is described with reference to FIG. 7. 
With the edge ring 134 disposed in the chamber 100, a semiconductor 
substrate 106 is placed on the upper ledge 150 of the edge ring for 
processing (step 200). The semiconductor substrate 106 then can be 
processed in the chamber 100 as described generally above (step 202). Such 
processing can include an RTCVD or similar process during which a process 
gas is provided to the chamber. After processing, the substrate 106 is 
lifted from the edge ring 134 and removed from the chamber 100 (step 204). 
One or more semiconductor substrates 106 can be processed in this manner. 
At a subsequent time, with the edge ring 134 disposed in the chamber 100, a 
surrogate substrate 170 is placed on the lower ledge 148 of the edge ring 
(step 206). The surrogate substrate 170 can be made, for example, of 
silicon carbide (SiC), graphite, or another material that is substantially 
impervious to the gases that are used during the cleaning process. With 
the surrogate substrate 170 supported by the lower ledge 148, the chamber 
100 is heated to an elevated temperature, for example, approximately 
1100.degree. C. (step 208). A cleaning agent, such as hydrochloric acid 
(HCl), is allowed to flow into the chamber 100 through the port 101 (step 
210), and the support structure 108 is caused to spin so that the edge 
ring 134 and the surrogate substrate 170 spin about the central axis of 
the chamber (step 212). As the cleaning or etching agent flows through the 
chamber 100, the silicon or other deposits that may have formed on the 
edge ring 134 during semiconductor substrate processing are removed from 
the edge ring surfaces 156, 154, 164 by chemical reactions with the 
cleaning or etching agent. The cleaning process can be continued for 
several minutes. In general, the duration of the etching or other cleaning 
process will depend on the particular etching agent used, the length of 
time since the last cleaning, and the chamber temperature, as well as 
other factors. The resulting gases are removed from the chamber 100 
through the exhaust port 105. 
In various implementations, other gases, such as chlorine (Cl.sub.2) or 
chlorine tri-fluoride (ClF.sub.3), can be used as the etching agent to 
clean the edge ring and remove undesired materials that formed on the edge 
ring surfaces during semiconductor processing. In some situations, the 
etching agent flows into the chamber 100 through dedicated passageways 
(not shown) rather than through the inlet port 101. Also, a purge gas can 
flow through the passageway 103 during the cleaning process. 
Once the cleaning process is completed, the surrogate substrate 170 is 
lifted off the lower ledge 148 of the edge ring and removed from the 
chamber 100 (step 214). Additional semiconductor substrates 106 then can 
be processed in the chamber 100 as described above (step 216). 
Other implementations are within the scope of the following claims.