Compact process chamber for improved process uniformity

A semiconductor processing chamber, capable of withstanding low pressures while transmitting radiant energy, is provided in a lightweight, compact design. The inner surface of the window is preferably substantially flat and parallel to the wafer to be processed. The window is thin in a center portion and thicker in a surrounding peripheral portion. The thickness increases in the radially outward direction, defined between the flat inner surface and a concave outer surface. Deposition uniformity is improved by employing multiple outlet ports for distributing gas laterally in a short length, enabling a compact, symmetrical geometry. Preferably, a quadra-flow system of gas distribution is used, whereby the chamber contains one inlet port and three outlet ports distributed approximately at 90 degrees around a cylindrical side wall defining the chamber space.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to process chambers for chemical vapor deposition 
and other processing of semiconductor wafers and the like. More 
particularly, the present invention relates to cold wall process chambers 
capable of withstanding stresses associated with high temperature, low 
pressure processes and having improved temperature uniformity and gas flow 
characteristics. 
2. Description of the Related Art 
Process chambers for thermally processing semiconductor wafers such as 
silicon can desirably be made of quartz (vitreous silica) or similar 
materials which are substantially transparent to radiant energy. Reactors 
incorporating radiant heat lamps and reaction or process chambers with 
transparent walls are known in the industry as "cold wall" reactors. Thus, 
radiant heat lamps may be positioned adjacent the exterior of the chamber 
and a wafer being processed in the chamber can be heated to elevated 
temperatures without having the chamber walls heated to the same level. 
Quartz is also desirable because it can withstand very high temperatures, 
and because it is inert, i.e., does not react with the various processing 
gases typically used in semiconductor processing. 
Conventional quartz windows used in semiconductor processing chambers 
generally employ either a flat or outwardly curved configuration. Flat 
windows are more commonly used when the pressure on the inside of the 
chamber is substantially the same as the pressure on the outside of the 
chamber. Flat windows have the advantage of providing a uniform height 
between the wafer and the inside surface of the window to provide for 
uniform cross-sections along the flow path of process gases in chemical 
vapor deposition (CVD), and hence a more laminar flow. Flatwall chambers 
may also be used when the external pressure outside the chamber differs 
significantly from the internal pressure within the chamber. However, in 
such a chamber the windows must be very thick to resist the stresses on 
the chamber. Thick flatwall chambers unfortunately require additional 
material and thus add weight to the reactor. 
Cold wall chamber designs must also account for thermal effects. In 
general, the wall temperature during thermal processing should be confined 
to a very small window. If the temperature gets too high, processing gases 
can react with one another at the wall (e.g. chemical vapor deposition 
occurs on the chamber walls). Too low a temperature can cause condensation 
of constituent gases. In either case, clouding of the walls can cause 
absorption of radiant heat, leading to cracking and catastrophic failure. 
A typical cold wall processing chamber contains a susceptor for supporting 
the wafer to be processed. This susceptor is often made of a heat 
absorbing material, which causes the center of the chamber to run 
extremely hot. When the windows of the chamber are made thick to handle 
high or low pressure applications, the quartz windows absorb more heat 
from the inside of the chamber. Additionally, a greater amount of radiant 
heat is absorbed when passing through thicker transparent walls. Moreover, 
hotter inner surfaces tend to expand more rapidly than the outer surfaces 
due to thermal expansion, thereby causing the window to crack. 
Forced air cooling is typically applied to the outside of the windows to 
keep the chamber walls cool during processing. But thick, more massive 
windows retain more heat, such that forced air cooling is less effective 
for thick windows. The high temperature at the inner surface of the 
windows therefore results in chemical deposition on this surface. In 
addition, it is difficult to direct an appropriate amount of cooling air 
to a specific location without affecting an adjacent location. Thus, it is 
difficult to control wall temperature in a desired location to minimize 
the occurrence of localized depositions. 
For applications in which the pressure within a quartz chamber is to be 
reduced much lower than the surrounding ambient pressure, the strength of 
the chamber walls becomes important. Dome-shaped chambers have been 
disclosed, for example, in U.S. Pat. Nos. 5,085,887 and 5,108,792. U.S. 
Pat. No. 5,085,887 discloses a chamber which includes an upper wall having 
a convex outer surface and a concave inner surface. A greatly thickened 
peripheral flange is provided to radially confine the upper wall, causing 
the wall to bow outward due to thermal expansion, helping to resist the 
exterior ambient pressure in vacuum applications. The chamber requires a 
complex mechanism for clamping the thickened exterior flanges of the upper 
and lower chamber walls. 
A lenticular chamber has been described in a pending application entitled 
PROCESS CHAMBER WITH INNER SUPPORT, Ser. No. 08/637,616, filed Apr. 25, 
1996, the disclosure of which is incorporated by reference. This chamber 
has thin upper and lower curved walls having a convex exterior surface and 
a concave interior surface in the lateral dimension, with constant 
longitudinal cross-sections (longitude being defined by the axis internal 
of gas flow). These walls are joined at their side edges by side rails, 
thus giving the chamber a generally flattened or ellipsoidal 
cross-section. The chamber upper and lower walls are generally rectangular 
in shape, such that a wafer disposed within the chamber is located farther 
from the upstream and downstream ends than from the lateral side rails. 
The rectangular shape of the lenticular chambers is advantageous in keeping 
elastomeric O-rings located at the longitudinal ends of the chamber 
farther away from the center of the chamber where the wafer is located. 
These O-rings have a tendency to heat up, and therefore, if located too 
close to the susceptor/wafer combination at the center of the chamber, 
they will become difficult to cool and may burn more easily due to 
exposure to high temperatures. Moreover, a rectangular shape evenly 
distributes gas flow through the chamber. By providing a longer 
longitudinal distance for gas to flow over the wafer to be processed, the 
gas can spread out in the chamber before reaching the wafer, thereby 
allowing a more uniform deposition. 
While these lenticular chambers present a good design for low pressure 
applications, scaling this design to larger sizes presents difficulties. A 
lenticular chamber designed to accommodate a 200 mm wafer has a length of 
about 600 mm, a width of about 325 mm, and a chamber height of about 115 
mm. To increase the chamber size for a 300 mm wafer, while maintaining 
relatively the same rectangular proportions, the chamber would have to 
have a length of about 900 mm and a width of about 488 mm. Such a chamber 
is big and heavy, and more difficult to fabricate, requiring special 
cranes and lifting devices. The increased footprint also decreases the 
amount of clean room space available. Furthermore, the larger size makes 
the chamber more difficult to clean. 
Lenticularly-shaped chambers could also be improved to favor a more uniform 
deposition of material. In such chambers, the quartz wall disposed over 
the wafer to be processed is curved, creating a greater chamber volume 
above the center of the wafer than over the lateral edges, such that 
uniform deposition is difficult to achieve. 
Deposition uniformity is affected by the gas flow profile produced over the 
wafer, both in lenticular and other types of chambers. There have been 
attempts to control the gas flow profile in parallel across the wafer to 
be processed, to create a more uniform deposition. For example U.S. Pat. 
No. 5,221,556 discloses a system in which the apertures of the gas inlet 
manifold are varied in size to allow relatively more gas through a 
particular section, typically the center section. U.S. Pat. No. 5,269,847 
includes valves for adjustment of pairs of gas flows merging into a number 
of independent streams distributed laterally upstream of the wafer to be 
processed. This system emphasizes the importance of channeling the various 
gas flows separately until just before the wafer leading edge in order to 
prevent premature mixing and enable greater control over the flow and 
concentration profiles of reactant and carrier gases across the wafer. 
Despite recent advancements, a need still exists for a processing chamber 
with an improved design. Preferably, such a chamber should exhibit uniform 
deposition. At the same time, the chamber should be lightweight and 
compact, but still able to withstand pressure differentials and high 
temperatures, particularly for wafers 300 mm and larger. Furthermore, this 
chamber should be made lightweight and strong without subjecting the 
chamber to depositions or cracking due to thermal effects. 
SUMMARY OF THE INVENTION 
A semiconductor processing chamber is provided for use at either low or 
ambient pressures with a compact size which runs cleaner and produces a 
more uniform deposition profile than the chambers of the prior art. The 
inner surface of the window is preferably substantially flat and parallel 
to the wafer to be processed, creating a uniform space above the wafer to 
lead to a more even deposition of material. The window is thin in a center 
portion and increases in thickness as determined by an outer surface 
having a substantially concave shape. Deposition uniformity is improved by 
employing multiple outlet ports for distributing gas throughout the 
chamber. Preferably, the reactor employs a multiple-port system of gas 
distribution. In the disclosed embodiment, the chamber contains one inlet 
port and three outlet ports distributed approximately at 90 degrees around 
a cylindrical side wall defining the chamber space. 
In accordance with one aspect of the present invention, a single substrate 
thermal processing chamber is provided with a first wall and a second 
wall. The first wall is substantially transparent to radiant heat, having 
a center portion which is thinner than a peripheral portion surrounding 
the center portion. The second wall similarly includes a thin center 
portion. A side wall connects the first and second walls to define a 
chamber space surrounded by the walls. A substrate support structure is 
positioned within the chamber space. 
In accordance with another aspect of the present invention, a chamber is 
provided with a window which allows transmission of radiant heat to a 
substrate supported within the chamber. The window has a center portion 
and a thicker surrounding peripheral portion. 
In accordance with another aspect of the present invention, a reduced 
pressure chamber for processing a semiconductor wafer is disclosed. This 
chamber includes a window allowing transmission of radiant heat 
therethrough to a wafer to be processed. The window has inner and outer 
surfaces for facing the wafer and a radiant heat source, respectively, 
during processing. The outer surface includes a concavely shaped section. 
In accordance with another aspect of the present invention, a cold wall 
thermal processing reactor is provided. The reactor includes at least one 
radiant heat lamp, a substrate support structure, and a window disposed 
between the radiant heat lamp and the substrate support structure. The 
window has a center portion and a peripheral portion, both of which allow 
transmission of radiant heat from the lamp to the substrate support 
structure. The center portion is thinner than the peripheral portion. 
In accordance with another aspect of the present invention, a semiconductor 
processing chamber is provided. The chamber walls define a deposition 
chamber, and a substrate support is positioned within the chamber for 
horizontally supporting a single semiconductor substrate. A gas inlet is 
disposed in the walls of the chamber for producing gas flow into the 
chamber. At least two gas outlets are disposed in the walls of the chamber 
for exhausting gas flow from the chamber. 
In accordance with another aspect of the present invention, a gas system 
for processing semiconductor wafers is provided. The system includes a 
chamber having an upstream end and a downstream end, with an inlet port 
located at the upstream end of the chamber for releasing processing gases 
into the chamber. A primary outlet port is located at the downstream end 
of the chamber for removing processing gases from the chamber to produce 
gas flow in a longitudinal direction across the wafer. A pair of secondary 
outlet ports is located in a sidewall connecting the upstream and 
downstream ends for removing processing gases from the chamber. 
In the preferred embodiment, the secondary outlet ports are positioned and 
configured to minimize gas recirculation within the chamber. Such 
recirculation can cause process non-uniformity and chamber coatings which 
affect the overall cleanliness of current state-of-the-art systems. 
In accordance with another aspect of the present invention, a cold wall 
processing reactor is provided with a window between a plurality of 
radiant heat lamps and a susceptor designed to hold a semiconductor wafer. 
The window has an inner surface facing the susceptor and an outer surface 
facing the heat lamps. The window includes a first portion, which is 
relatively close to a center axis of the susceptor, and second portion, 
which is farther from a center axis of the susceptor. The minimum 
thickness of the first portion is smaller than the minimum thickness in 
the second portion. 
In accordance with another aspect of the present invention, a method is 
provided for producing a uniform gas flow across a semiconductor wafer 
being processed in a reaction chamber. This method includes tuning the gas 
flow out of the chamber through a plurality of outlet ports.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although the chamber described herein is applicable to batch processing 
systems, it is of particular utility in single wafer processing systems. 
In particular, the chamber 10 described herein is applicable to processing 
of a single 300 mm silicon wafer at a reduced pressure of about 20 to 60 
Torr. It should be recognized, however, that the principles of the present 
invention are applicable to other size wafers processed at different 
pressures and temperatures. The skilled artisan may also find application 
for the principles disclosed herein to both cold wall and hot wall 
reactors. Furthermore, advantages of the chamber described herein are 
applicable to several types of processing, including thermal annealing, 
deposition, etching, lithography, diffusion, and implantation. 
Preferred Chamber 
FIGS. 1 and 2 show a semiconductor processing reactor 8, which includes a 
reaction or process chamber 10, constructed in accordance with a preferred 
embodiment. The chamber 10 has an upper wall 12 and a lower wall 14 
defining a chamber space 16 between the two. The upper and lower walls 12, 
14 are connected by a side wall 18 surrounding the chamber space 16. A 
flange 20 further surrounds the side wall 18. 
As described in further detail below, a system of multiple gas ports is 
provided in the chamber. The illustrated embodiment shows an inlet port 
22, a primary or main outlet port 24, a first side outlet port 26, and a 
second side outlet port 28 (shown in FIG. 3), provided through the flange 
20 and side wall 18 to allow entry and exhaust of processing gases to and 
from chamber space 16. The inlet 22 is also sized to allow entry and 
removal of a semiconductor wafer 56. The primary direction of gas flow 
defines a longitudinal direction, extending from an upstream to a 
downstream end, where the upstream end corresponds to the location of the 
gas inlet port 22, and the downstream end corresponds to the location of 
the main gas outlet port 24, positioned opposite the inlet port 22. A 
lateral direction is oriented perpendicular to the longitudinal direction. 
The illustrated side ports 26 and 28 are located in the side wall 18, 
opposite each other and approximately 90 degrees from the inlet and outlet 
ports 22 and 24, respectively. Thus, in the illustrated embodiment, the 
lateral direction extends from the first side outlet port 26 to the second 
side outlet port 28. 
Preferably, at least portions of the upper wall 12 and the lower wall 14 
are transparent to radiant heat energy, and preferably comprise quartz. 
The transparent portions of the walls 12, 14 through which radiant energy 
actually passes during processing can be referred to as "windows." 
Although quartz is the preferred material for the upper and lower windows, 
other materials having similar desirable characteristics may be 
substituted. Some of these desirable characteristics include a high 
melting point, the ability to withstand large and rapid temperature 
changes, chemical inertness and high transparency to radiant heat. 
As shown in FIG. 2, the chamber forms part of a reactor 8. A plurality of 
radiant heat sources are supported outside the chamber 10, to provide heat 
energy to the chamber 10 without appreciable absorption by the quartz 
chamber walls. While the preferred embodiments are described in the 
context of a "cold wall" CVD reactor for processing semiconductor wafers, 
it will be understood that the processing methods described herein will 
have utility in conjunction with other heating/cooling systems, such as 
those employing inductive or resistive heating. 
The illustrated radiant heat sources comprise an upper heating assembly of 
elongated tube-type radiant elements 89. The upper heating elements 89 are 
preferably disposed in spaced-apart parallel relationship and also 
substantially parallel with the process gas flow path through the 
underlying reaction chamber 10 as described below. A lower heating 
assembly comprises similar elongated tube-type radiant heating elements 90 
below the reaction chamber 10, preferably oriented transverse to the upper 
heating elements 89. 
Desirably, a portion of the radiant heat is diffusely reflected into the 
chamber 10 by rough specular plates (not shown) above and below the upper 
and lower lamps 89, 90, respectively, while some of lamps 89, 90 are 
backed by curved reflectors (not shown) to direct concentrated heat. 
Additionally, a plurality of spot lamps 91 supply concentrated heat to the 
underside of the wafer support structure (described below), to counteract 
a heat sink effect created by support structures extending through the 
bottom of the reaction chamber 10 to the relatively colder environs. 
Each of the elongated tube type heating elements 89, 90 is preferably a 
high intensity tungsten filament lamp having a transparent quartz envelope 
containing a halogen gas, such as iodine. Such lamps produce full-spectrum 
radiant heat energy transmitted through the walls of the reaction chamber 
10 without appreciable absorption. As is known in the art of semiconductor 
processing equipment, the power of the various lamps 89, 90, 91 can be 
controlled independently or in grouped zones in response to temperature 
sensors. 
As shown in FIG. 3, the flange 20 defining the horizontal perimeter of the 
chamber 10 has a generally rectangular shape with chamfered comers. The 
flange 20 may also be square, circular, or any other shape that will 
accommodate a compact design for processing a wafer in a small space. The 
flange 20 is preferably made of 316L stainless steel, and is water cooled 
during processing in any suitable manner, as will be readily appreciated 
by one of skill in the art. As shown in FIG. 2, O-rings 126 are provided 
between the side wall 18 and flange 20. The side wall 18 is preferably 
made of opaque quartz. Opaque quartz is preferred for the side wall to 
reflect radiant energy away from the metal flange 20 and back to the 
chamber, to protect the flange 20 and the O-rings 126 from extreme 
temperatures. 
For a wafer 56 having a diameter of about 300 mm processed at a pressure of 
about 20 torr, the chamber 10 has a length in the longitudinal direction, 
as measured from the ends of flange 20, preferably of about 15 to 30 
inches, and more preferably about 23 inches. The width of flange 20 in the 
lateral direction is preferably about 15 to 30 inches, and more preferably 
about 22 inches. The height of flange 20 is preferably between about 3 to 
6 inches, and more preferably, about 4 inches. The inner diameter of the 
flange 20, corresponding to the outer diameter of side wall 18, is 
preferably about 14 to 22 inches, and more preferably, about 18 inches. 
The side wall 18 has an inner diameter preferably of about 12 to 21 
inches, and more preferably, about 16.75 inches. The height of side wall 
18 is preferably about 2 to 5 inches, and more preferably, about 3.25 
inches. 
The preferred embodiment thus enables processing a single 300 mm wafer with 
a small reactor 8 footprint. In the illustrated embodiment, the outer 
dimensions of the flange 20 define a footprint of less than about 3,300 
cm.sup.2, whereas the process chamber area, defined by the inner 
boundaries of the side wall 18, is less than about 1,500 cm.sup.2. Despite 
the compact size and design which avoids recirculation, the novel chamber 
design enables reduced or low pressure processing, preferably at less than 
about 100Torr, and more preferably less than about 60 Torr. 
Wafer Support Structure 
As shown in FIG. 2, the chamber 10 is divided into an upper section 30 and 
a lower section 32 by a wafer support structure which comprises a 
generally flat circular susceptor or wafer holder 36, which supports the 
wafer 56. The susceptor 36 is located approximately in the center of the 
chamber 10, and is surrounded by a ring 38, sometimes referred to as a 
temperature compensation ring or a slip ring. The slip ring 38 can be used 
to house thermocouples 39, as illustrated, and also to absorb radiant heat 
during high temperature processing. This compensates for a tendency toward 
greater heat loss at the edges of wafer 56, a phenomenon which is known to 
occur due to a greater concentration of surface area for a given volume 
near such edges. By minimizing edge losses and the attending radial 
temperature non-uniformities across the wafer 56, the slip ring 38 reduces 
the risk of wafer crystallographic slip or other consequences of 
non-uniform temperatures during processing. 
As shown in FIG. 2, the susceptor 36 and slip ring 38 are positioned on a 
plane just below the inlet and outlet ports 22, 24, 26 and 28. A 
sacrificial or divider plate 34 is provided surrounding the ring 38, 
preferably comprising quartz. This plate 34 serves to confine gas flow to 
the upper section 30, thereby producing better laminar flow. The 
sacrificial plate 34 has an inner diameter which closely conforms with the 
outer diameter of the ring 38, and an outer shape which conforms with and 
desirably abuts against the side wall 18. In this manner, the side wall 18 
is protected from devitrification from repeated heating of the reaction 
chamber 10. This enables the sacrificial plate 34 to be replaced when it 
devitrifies from repeated heat cycles, while preserving the more expensive 
side wall 18. The plate 34 is preferably supported by ledges formed in the 
side wall 18, and is preferably located in the same plane as the susceptor 
36. The ring 38, in turn, is supported by brackets 84 extending from the 
plate 34. Alternatively, the slip ring could be supported by a stand 
resting on the lower chamber wall, or by ledges extending inwardly from 
the chamber side walls. 
The susceptor 36 is supported by a spider 40 having three arms extending 
radially outward from a central hub and having upwardly extending 
projections on the ends of the arms engaging the susceptor. The susceptor 
36 may also be provided with one or more recesses (not shown) on its lower 
surface for receiving the ends of the projections to facilitate centrally 
positioning the susceptor and forming a coupling with the spider 40 for 
rotating the susceptor. The spider 40 is mounted on a shaft 42 which 
extends through the chamber lower wall 14 and also extends through a tube 
44 integrally attached to and depending from the lower chamber wall 14. 
The shaft 42 is connected to a drive motor (not shown) for rotating the 
shaft 42, the spider 40, and the susceptor 36. Details of a similar 
arrangement together with a drive mechanism may be seen in U.S. Pat. No. 
4,821,674, the disclosure of which is incorporated herein by reference. 
The susceptor 36 and preferably the slip ring 38 are made from a material 
which can withstand high temperature processing. Desirably, this material 
is one that does not devitrify, is a good absorber of radiant energy, is a 
reasonably good thermal conductor, has good resistance to thermal shock, 
and is durable and compatible with the various materials and chemicals 
used in processing. The illustrated susceptor 36 and slip ring 38 comprise 
silicon carbide. Other potential materials include boron nitride, silicon 
nitride, silicon dioxide, aluminum nitride, aluminum oxide, combinations 
or compounds of these materials, pyrolytic graphite and other similar high 
temperature ceramic compounds. 
Design of the Upper and Lower Chamber Walls 
As shown in FIG. 4, the upper and lower walls 12 and 14 are preferably 
substantially circular in shape. While in other arrangements, walls of 
other shapes may also be used, such as rectangles and squares, the round 
shape advantageously reduces the material costs, weight, and footprint of 
the reactor 8. Preferably, the wafer 56, as shown in FIG. 3, is 
substantially circular in shape and is disposed directly below upper wall 
12 and above lower wall 14, such that the wafer 56, the upper wall 12, and 
the lower wall 14 all share a central axis 92, as shown in FIGS. 2 and 4. 
Referring to FIG. 2, in the preferred embodiment, the upper wall 12 of 
chamber 10 has an inner surface 46 which is substantially planar or flat. 
The planar inner surface 46 of upper wall 12 is substantially parallel to 
the wafer 56, producing a substantially uniform distance from the upper 
wall 12 to the wafer 56 and to the divider plate 34. In a center portion 
48 of the upper wall 12, an outer surface 50 of chamber 10 is also planar 
to produce a planar center portion 48 of substantially uniform thickness. 
The center portion 48 preferably is circular in shape, but other shapes may 
be used as well. The center portion 48 is preferably centered along the 
same central axis 92 as the wafer 56 and the upper and lower walls 12 and 
14. Extending outward from the flat center portion 48, the upper wall has 
a varying thickness. Preferably, the thickness increases outwardly in a 
peripheral portion or outer ring 52 surrounding the center portion 48 
until it reaches the edge 54 of the upper wall 12. At the edge 54 the 
upper wall 12 has a constant thickness. The center portion 48 is 
preferably smaller in diameter than the largest wafer for which the 
chamber 10 is designed (300 mm in the illustrated embodiment), such that 
the peripheral portion 52 having a greater thickness than the center 
portion is disposed at least partly above the wafer 56 to be processed. 
This arrangement enables thermal advantages for the window while allowing 
structural support for high or low pressure applications. It will be 
understood, of course, that the same chamber 10 can be utilized to process 
smaller wafers. 
The thickness of the upper wall 12 increases from the perimeter of the thin 
center portion 48 to the outer edge 54 in thicker peripheral portion 52, 
as shown in FIG. 2. This increasing thickness preferably gives the upper 
wall 12 a concave-outward curvature on the outer surface 50 in peripheral 
portion 52. Unwanted stresses are introduced in curved walls with varying 
radii and thus, a circular wall with a regular or constant curvature is 
desirable. In the illustrated embodiment, the thickness of the upper wall 
12 in the peripheral portion 52 is defined between the planar inner 
surface 46 and the outer surface 50 which conforms to a toroidal shape. In 
other arrangements, however, the thickness may increase in the peripheral 
portion in accordance with different configurations. For instance, the 
thickness of the upper wall 12 may gradually increase linearly or by 
steps. Other configurations include inner or outer surfaces conforming to 
spherical, toroidal, elliptical, parabolic, and hyperbolic shapes. 
Preferably, for a 300 mm (about 11.8 inch) processing chamber, the upper 
quartz wall 12 has a diameter of about 14 to 22 inches, and more 
preferably about 18 inches. At the flat center portion 48, the wall 12 
preferably has a thickness of about 0.12 inch to 0.35 inch, more 
preferably about 0.2 inch to 0.3 inches, and is about 0.25 inch in the 
illustrated embodiment. The diameter of flat center portion 48 is 
preferably about 6 to 12 inches, and more preferably about 9 inches. The 
edge 54 has a constant thickness of about 1 to 2 inches, and more 
preferably, about 1.4 inches, extending over a radial length of about 0.5 
to 1.5 inches, more preferably about 0.87 inches. The inner surface 46 of 
upper wall 12 is preferably about 0.5 to 2 inches above the wafer 56 to be 
processed, more preferably about 1 inch. The increase in thickness of the 
upper wall 12 in peripheral portion 52 is determined by the outer surface 
50 having a radius of curvature in the vertical dimension of about 5 to 40 
inches, more preferably about 8 inches. 
As shown in FIG. 2, the lower wall 14 also has a substantially flat or 
planar inner surface 58 but, unlike the upper wall 12, has a continuously 
curved concave outer surface 60. This gives the lower wall a center 
portion 62 which is thinner than a peripheral portion 64. The lower wall 
14 has a varying thickness which gradually increases towards an outer edge 
66, as preferably determined by the outer surface 60 which has a concave 
shape with a substantially constant radius of curvature. For the chamber 
10 shown in FIG. 2, designed for processing a 300 mm wafer, this radius of 
curvature is preferably about 20 to 60 inches, more preferably about 40 
inches. At the edge 66 the lower wall 14 has a constant thickness. The 
lower quartz wall 14 preferably has a diameter of about 14 to 22 inches, 
and more preferably about 18 inches. At the center portion 62 of the wall 
14, the minimum thickness can be as described above for the upper wall 12. 
At the edge 66, the lower wall preferably has a constant thickness of 
about 0.5 to 1.5 inches, more preferably about 0.88 inches. 
The shape of the upper and lower walls 12 and 14, each with an inner 
surface which is substantially planar and an outer surface with at least a 
section which is concavely curved, is preferably manufactured by grinding 
away a center portion of a thick, flat quartz plate. The quartz side wall 
18 has a generally cylindrical shape and can be made by cutting a 
cylindrical tube made of quartz or similar material. Other methods for 
manufacturing the quartz parts (or other suitable materials) will be 
readily apparent to those skilled in the art. 
Although the chamber 10 shown in FIGS. 1-4 has been described with respect 
to a certain preferred configuration and certain dimensions, the invention 
is not intended to be limited to this embodiment. In other arrangements, 
the upper wall may be shaped as described for the illustrated lower wall 
14, and similarly, the lower wall may be shaped as described for the 
illustrated upper wall 12. Thus, the outer surfaces of either or both the 
upper and lower walls can be continuously concave in shape, and either or 
both can have a flattened or planar center portion. Furthermore, the upper 
and lower walls may simply comprise a center portion which is thinner than 
a surrounding peripheral portion. Moreover, the lower wall may simply be 
substantially flat and uniformly thick throughout, while the upper wall 
has varying thickness. Varying thickness in the upper or lower walls can 
be determined by a variety of shapes and curvatures, including, but not 
limited to, spherical, toroidal, elliptical, parabolic, and hyperbolic 
configurations. Various other combinations of upper wall and lower wall 
configurations will be readily apparent to the skilled artisan, in light 
of the present disclosure. 
Process Chamber Assembly 
Assembly of the process chamber is shown in an exploded perspective view in 
FIG. 4. More particularly, FIG. 4 shows assembly of the upper wall 12, 
sacrificial plate 34, side wall 18, flange 20, susceptor 36, slip ring 38, 
lower wall 14, and tube 44. The flange 20 is comprised of an inlet flange 
108 and an outlet flange 110, which are joined around the side wall 18. 
In practice, the chamber 10 can be easily assembled and disassembled on 
site. For example, the inlet flange 108 can be first mounted to the frame 
of a reactor cabinet. The quartz side wall 18 is then slid into the inlet 
flange 108 with the elongated slot 68 aligned with the aperture 70 within 
the flange 108, thus defining the inlet port 22 (see FIGS. 2 and 3). As 
shown, the side wall 18 includes an annular recess to receive an annular 
ring extending inwardly from the inlet flange 108. 
The outlet flange 110 can then be fitted onto the side wall 18 in a similar 
fashion, until the outlet flange 110 contacts the inlet flange 108 and can 
be bolted or otherwise fastened together. The lower wall 14, susceptor 
support (not shown) and susceptor 36 can then be installed in sequence, 
and the appropriate joints sealed against process gas leakage. The quartz 
ring 34 sits on an annular ledge provide on the inner surface of the side 
wall 18, just below the slots 68, 72, 76, 80. The ring 34, which includes 
brackets 84 best seen in FIG. 2, supports the slip ring 38 surrounding the 
susceptor 36. The upper wall 12 is then fitted onto the side wall 18, 
supported by the outer edge 54 of the wall 12. Exhaust manifolds (not 
shown) and valves can then be installed to receive effluent gases through 
the ports 24, 26, 28. 
The chamber 10 is thus field assembleable, thereby eliminating the need for 
expensive welded chambers. Due to the field assembleable feature, the 
chamber geometry can be changed easily. Thus, different upper and lower 
walls 12 and 14 may be provided to the chamber to produce different 
thermal effects and other desired characteristics for the chamber 10. 
Moreover, the susceptor 36 and sacrificial plate 34 can be quickly and 
easily replaced by removal of the upper wall 12. 
Inlets and Outlets for Improved Gas Flow 
The preferred chamber 10 also provides improved gas flow distribution by 
employing multiple outlet ports. These ports are distributed around the 
chamber 10 to spread out the flow of processing gases. Preferably, at 
least some of these outlet ports include variably openable valves to tune 
the flow out of the chamber 10. Moreover, the outlet ports are preferably 
symmetrically distributed in the chamber to facilitate uniform, laminar 
flow and reduce recirculations. 
The illustrated chamber 10 of the preferred embodiment includes four gas 
ports (one inlet 22 and three outlets 24, 26, 28). As shown in FIG. 3, a 
wafer 56 (shown in phantom) is disposed horizontally in the center of 
chamber 10. The gas inlet port 22 is provided at the upstream end of the 
chamber 10. This port 22 allows entry into the chamber 10 of reaction 
gases as well as the wafer 56. At the downstream end, opposite the inlet 
port 22, the primary or main outlet port 24 is provided for exhaust of the 
processing gases. The side ports 26 and 28 are located at the same 
vertical level as the inlet port 22 and the main outlet port 24. Thus, all 
four gas ports are positioned at or about a common horizontal plane. 
Preferably, the side outlet ports 26 and 28 are located approximately 90 
degrees from the ports 22 and 24 along the substantially cylindrical side 
wall 18 of chamber 10, and approximately 180 degrees apart from each 
other. Accordingly, a line joining the side ports 26 and 28 is 
approximately transverse to the longitudinal primary flow of processing 
gases. 
Apertures for the inlet and outlet ports are machined into the side wall 18 
of the preferred quartz materials. Inlet port 22 is defined by a 
horizontal elongated slot 68 machined into the side wall 18, which mates 
with an aperture 70 in flange 20. The slot 68 allows wafer insertion. The 
slot also permits the introduction of processing gases after an isolation 
valve (not shown) between the slot 68 and a wafer handling chamber (not 
shown) has been closed. Similarly, at the three outlet ports 24, 26 and 
28, corresponding horizontal slots 72, 76 and 80 are machined into the 
side wall 18, mating with apertures 74, 78 and 82, respectively, in the 
flange 20 when the chamber 10 is assembled. Alternatively, the quartz side 
wall can be molded with apertures. The outlet ports allow exhaust of 
process gas from the chamber 10, as well as the application of a vacuum to 
the chamber. 
Referring now to FIG. 2, a gas injector 112 is positioned upstream of the 
process chamber 10 and includes a plurality of reactant gas flow needle 
valves 114 for controlling the flow of process gases into the chamber 
through multiple ports. The gas injector may be of a type described in a 
pending application entitled PROCESS CHAMBER WITH INNER SUPPORT, Ser. No. 
08/637,616, filed Apr. 25, 1996, the disclosure of which is incorporated 
by reference. Gases are metered through the injector 112 in a downward 
direction indicated by arrow 116, and thereafter passes through the inlet 
port 22 leading into the upper section 30 of the process chamber 10. Gas 
flow outside the chamber through the port 22 is prevented by a gate valve 
or door (not shown). The gate valve is open only during wafer transfer, 
during which process gases are not flowing. While the illustrated 
embodiment of FIG. 2 shows the injector 112 extending through the flange 
20, it will be understood that, in other arrangements, injectors can be 
arranged in a separate outside flange which interfaces with the gate 
valve. 
The gas flow into the chamber is indicated by the arrow 94. The gas flow 
moves generally longitudinally across the wafer 56, primarily due to a 
decreasing pressure gradient along the chamber in the direction of a 
vacuum source (not shown) downstream at the outlet port 24 for low 
pressure applications. It will be understood by one of skill in the art, 
however, that gas flow can also be maintained in the absence of vacuum 
pump. At the outlet port 24, an exhaust apparatus 120 is provided for 
receiving the exhaust gases. 
As shown in FIG. 2, at the main outlet port 24, the gas flow communicates 
with an exhaust conduit 122 according to arrow 106, leading to an exhaust 
manifold 124 which is attached to a suitable source of vacuum. 
Alternatively, condensation at a downstream scrubber can assist 
flow-through of the exhausted gases. 
Referring to FIG. 3, gas flow through the outlet ports can be adjusted or 
tuned through the use of valves (not shown). Preferably, the valves are of 
a type which can be selectively opened and closed to produce a pressure 
gradient from the inlet port 22 to these outlet ports, and thereby 
regulate gas flow. Any suitable adjustable valve means, such as 
bellows-type, ball valves, butterfly valves, etc., can be employed for 
this purpose, as will be understood by the skilled artisan. All three 
exhaust ports 24, 26, 28 are plumbed to a common port from the valves. 
In the preferred embodiment, variably adjustable bellows-type valves are 
disposed at the side outlet ports 26 and 28, but not at the main outlet 
port 24. Thus, gas flows primarily longitudinally through the chamber, as 
indicated in FIG. 2 by a gas inlet flow arrow 94 and a gas outlet flow 
arrow 106. In addition, secondary gas flow branches laterally off the 
primary gas flow path, as shown by arrows 96 and 98. By tuning valves at 
these side outlet ports 26, 28, the gas flow can be spread or narrowed 
within the chamber 10 to provide a better distribution of gases and a more 
even process treatment. In another embodiment, plates with fixed orifices, 
optimaly sized for a desired gas flow pattern, are use in place of 
adjustable valves at the secondary exhaust ports 26, 28. 
Processing with this chamber 10 is now described in the context of an 
exemplary chemical vapor deposition (CVD) process. It will be understood, 
however, that the preferred chamber will also be advantageous for other 
non-CVD processing. Initially, process gases enter the chamber 10 at an 
ambient, non-reacting temperature and are heated to a temperature suitable 
for deposition as they pass over the susceptor and wafer. The radiant heat 
lamps 89, 90, 91 heat the wafer 56, either directly or by way of the 
susceptor and the slip ring. The surrounding slip ring 38 is a high heat 
absorbency material, allowing the ring to preheat the reactant gas stream 
before it reaches the leading edge of the susceptor, and subsequently, the 
leading edge of the wafer. The preheated process gases thus readily react 
at the wafer surface, which is at the desired reaction temperature. 
As shown in FIG. 2, purge gas is supplied upward through the hollow tube 44 
surrounding the shaft 42. The purge gas enters the lower portion 32 as 
indicated by arrows 100. The purge gas inhibits process gases from flowing 
around the edges of the susceptor 36 into the lower section 32, thus 
inhibiting unwanted deposition of particulates underneath the susceptor 
36. The bulk of this purge gas exits through the lower longitudinal 
aperture 102 near the outlet port 24 (beneath the plate 34), as indicated 
by arrow 104. The purge gas then mixes with the spent reaction gas and 
continues downward through the exhaust conduit 122. Purge gas ports may 
also be provided below the inlet or other outlet ports provided in the 
side wall 18. 
In the illustrated embodiment, for processing a 300 mm wafer, the inlet 
port 22 preferably has a length of about 12 to 15 inches, and more 
preferably about 13.25 inches, and a height preferably of about 0.5 to 1.5 
inches, and more preferably, about 1 inch. The main outlet port 24 
preferably has a length of about 12 to 15 inches, and more preferably, 
about 13 inches, and a height preferably of about 0.2 to 1 inch, and more 
preferably, 1.5 inch. The side outlet ports 26 and 28 each preferably have 
a length of about 2 to 10 inches, and more preferably, about 5.5 inches, 
and a height preferably of about 0.2 to 1 inch, and more preferably, about 
0.5 inch. 
While the invention discussed herein describes a preferred embodiment with 
one inlet port and three outlet ports located approximately 90 degrees 
apart, it should be recognized that a gas flow system utilizing fewer or 
additional ports may be employed without deviating from the essential 
design of the invention. For example, a "main" gas outlet need not be 
provided at the downstream end of the chamber. Even for reactors with the 
disclosed quadra flow structure, the main gas outlet need not handle the 
majority of the exhaust flow. Chambers with two, four or more outlet ports 
can be utilized, these ports located at various locations within the 
chamber to control gas flow. Furthermore, any or all of the outlet ports 
can be provided with valves or plates with orifices of a fixed size for 
tuning the distribution of gas flow within the chamber 10. 
Reduced Pressure Applications 
The chamber 10 designed as described above offers several advantages over 
previously known processing chambers. First, the chamber utilizes a 
substantially thin wall configuration while still being able to withstand 
compressive stresses due to low pressure processing. Finite element 
analysis shows that a chamber having a thin wall disposed over a wafer to 
be processed, with an increasing thickness towards an outer edge, will 
have sufficient strength for reduced pressure processing. A quartz window 
having a substantially planar inner surface and a substantially concave 
outer surface has been found to be a particularly strong design. 
Preferably, the chamber can withstand pressure differentials of greater 
than 0.5 atm., and more preferably about 1 atm. 
In the illustrated preferred embodiment, the upper chamber wall 12 has a 
uniformly thick center portion 48, and a peripheral portion 52 with 
greater thickness than in the center portion. This peripheral portion 52 
preferably is disposed at least partly above the wafer 56 in order to 
provide strength to chamber 10 near the center of the chamber. For 
example, the ratio of the center portion 48 to the diameter of the wafer 
56 can be about 0.75. Similarly, the lower wall 14 has a planar inner 
surface 58 and a concave outer surface 60 to give the wall 14 an 
increasing thickness from a center portion 62 to an outer edge 66, thereby 
giving the chamber 10 sufficient strength in the lower wall to withstand 
compressive stresses due to low pressure processing. 
Thermal Effects 
During conventional processing of a semiconductor wafer in a cold wall 
reactor, the walls of a chamber are substantially transparent to radiant 
heat emitted from the lamps 89, 90, 91, such that radiation will pass 
through the walls without significant absorption. This radiation then 
heats the wafer and susceptor disposed inside the chamber. Once heated, 
the wafer and susceptor will re-radiate energy toward the walls. Because 
this re-radiated energy is of lower frequency than the IR energy coming 
directly from the lamps, the quartz windows tend to absorb this 
re-radiated energy. The thicker the quartz material, the more heat that is 
retained in the walls. Heating of the walls causes thermal expansion 
forces which can cause cracking or failure of the walls, particularly 
where different sections of a wall are heated to different degrees. 
Furthermore, inadequate control over the temperature of the walls can lead 
to deposition of reactant gases on the inner surfaces of the walls. 
When processing a semiconductor wafer in a chamber with upper and lower 
walls at high temperatures, the most intense heat is re-radiated from the 
susceptor and wafer toward the center portions of the upper and lower 
walls closest to the wafer, while less intense heat is directed toward the 
peripheral portions of the upper and lower walls. The design of chamber 10 
of the present invention accounts for this heat distribution in order to 
control the temperature of the walls by providing the upper and lower 
walls 12 and 14 with a thinner center portion and a thicker peripheral 
portion. Because thinner walls do not retain heat as much as thicker 
walls, varying the thickness of the walls according to the intensity of 
heat emitted from the susceptor and wafer can help to control the 
temperature of the upper and lower walls. Therefore, the upper and lower 
walls are made thinner in their center where greater heat is directed, and 
thicker in their periphery where less heat is directed. 
The varying thickness of the upper and lower walls enables effective 
cooling by forced air or liquid cooling. During high temperature 
processing of a wafer 56, the chamber 10 is provided with cooling air 
across the outer surfaces 50 and 60 of upper and lower walls 12 and 14, 
respectively, to cool the inner surfaces 48 and 58. Because heat 
re-radiated from the wafer and susceptor is more intense at the center 
portions of the upper and lower walls, these center portions are 
advantageously made thinner so that the air has a larger effect on cooling 
the walls. Toward the peripheral portions of the upper and lower walls, by 
contrast, less heat is re-radiated from the wafer and susceptor, and the 
walls require less cooling from the air and therefore can be made thicker. 
It will be understood that quartz walls can also be provided with 
throughbores running parallel to the wall surfaces, through which cooling 
liquid can be circulated. 
Thus, the chamber 10 of the present invention is preferably designed to 
have upper and lower walls of varying thickness, where the thicknesses of 
the walls is determined to provide control over the temperature of the 
inner surfaces of the walls. This temperature control is of particular 
utility in reducing chamber wall deposits during chemical vapor deposition 
(CVD). The walls of chamber 10 would be kept in the correct temperature 
range to prevent coatings deposited on the inner walls. If the inner 
surface of the chamber walls gets too hot, deposition can occur by 
decomposition of reactant gases. For example, silane can decompose and 
deposit silicon on hot chamber surfaces. If cooled too much, process gas 
condensation can contaminate the inner wall. For example, silane, 
trichlorosilane or other silicon source gases can condense upon overly 
cooled chamber surfaces. Controlling the wall temperature by varying the 
wall thickness thereby reduces deposition of reactant gases onto the walls 
and thereby lowers particulate generation in the chamber. The illustrated 
chamber thus has a longer service life and greater throughput, since the 
chamber does not have to be cleaned as frequently. 
Furthermore, by varying the thickness of the upper and lower walls in order 
to control wall temperature, these walls can be provided with a controlled 
temperature both across their inner surfaces and between the inner 
surfaces and outer surfaces of the walls. Moreover, a substantially 
uniform temperature across the inner surfaces reduces the occurrence of 
localized depositions which can affect the distribution of heat within the 
chamber, and ultimately affect the uniformity of wafer treatment (e.g., 
thickness of deposition). 
Deposition Uniformity and Compact Design 
The present invention improves deposition uniformity for low pressure 
processing of a semiconductor wafer in a chamber having thin walls by the 
design of the chamber walls. The upper and lower walls 12 and 14 are 
designed to have inner surfaces 46 and 58, respectively, which are 
substantially planar. This creates a uniform cross-sectional area over the 
wafer 16. The uniform chamber height provided over the wafer leads to a 
more laminar gas flow, which in turn leads to more uniform deposition of 
reactant material onto the wafer. Moreover, the flat inner surfaces 46, 58 
enable confined chamber volumes and faster gas flow rates, leading to 
faster deposition and greater throughput. 
Deposition uniformity is further improved by the multiple outlet port gas 
system as described above and in particular, the illustrated quadra gas 
flow system. By providing side gas outlets 26 and 28, in addition to main 
gas outlet 24, gas distribution within the chamber can be controlled to 
provide gas flow toward the sides of the chamber where the side outlet 
ports are located, as well as in a generally downstream direction toward 
the main outlet port. By tuning adjustable valves in the outlet ports 26 
and 28, laminar gas flow can be provided. 
Furthermore, by improving gas flow in this manner, the chamber 10 can be 
made more compact than previously designed chambers because the reactant 
gases do not require additional space to spread out toward the sides of 
the chamber before reaching the wafer. The gas flow configuration, by 
providing multiple gas outlets in the lateral walls of the chamber, 
provides deposition towards the sides of the wafer without the need for a 
larger chamber. The chamber 10 can also be made more compact by making the 
side wall 18 out of opaque quartz in order to protect the O-rings 126 
found between the side wall 18 and flange 20. The opaque quartz reflects 
more heat back to the chamber than would transparent quartz. Therefore, 
the O-rings can be placed closer to the susceptor and wafer without 
increasing the risk that the O-rings will heat too much or burn. 
Although this invention has been described in terms of certain preferred 
embodiments, other embodiments that are apparent to those of ordinary 
skill in the art are also within the scope of this invention. Accordingly, 
the scope of the invention is intended to be defined by the claims that 
follow.