Exhaust baffle for uniform gas flow pattern

An arc shaped exhaust baffle having two ends that define a gap therebetween that is not directly accessible by the exhaust plenum, that provides improved process gas flow distribution in a semiconductor process chamber, has a plurality of apertures formed therethrough and distributed about a baffle circumference. The apertures communicate with an underlying exhaust plenum to generate a uniform gas flow across the substrate surface during substrate processing. The exhaust baffle has at least one offset passage formed therethrough that may be adapted to draw process gas across a substrate surface at an acute angle relative thereto from the gap, and thereby provide an exhaust pattern that influences the process gas flow about the entire circumference of the substrate. An alternative embodiment of the invention provides a exhaust baffle having two baffle sections in facing abutment with one another that define a plurality of apertures and at least one offset passage for conducting process gas from the process chamber from the gap.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention relates to semiconductor process chambers. More particularly, 
the invention relates to establishing optimum gas flow patterns in 
semiconductor process chambers. 
2. Description of the Prior Art 
A critical goal when processing semiconductor substrates in a semiconductor 
process chamber is to deposit a uniform film of a desired material on the 
substrate surface. However, known substrate deposition processes do not 
produce optimal film uniformity, for example as indicated by the film 
refractive index, which is a measure of the uniformity of the thickness 
and composition of the film. Lack of film uniformity results in a 
processed substrate on which there is a variation across the substrate in 
the electrical properties of the individual devices fabricated thereon. It 
is thought that lack of uniformity in process gas flow patterns across the 
substrate surface during substrate processing may be one cause of poor 
film uniformity. 
FIG. 1 is a partially sectioned side elevation view of a prior art 
semiconductor process chamber that is used for such processes as chemical 
vapor deposition ("CVD"). 
As shown in FIG. 1, a semiconductor substrate 28 is loaded into a process 
chamber 20 through an opening 21 by a robot blade 15. A susceptor 30 is 
adapted to both support and heat the substrate, and to function as an 
electrode to which an RF voltage is applied to excite a plasma during 
substrate processing. The susceptor is supported by a central susceptor 
hub 36, while a susceptor support arm 31 coupled to a susceptor lift 
mechanism (not shown) supports the susceptor hub. 
During substrate loading, the blade 15 inserts a substrate through the 
chamber opening 21 and positions the substrate directly above the 
susceptor 30. Two or more lift fingers 39 are positioned below a substrate 
transfer location. The lift fingers are supported by a lift finger support 
ring 40 and lift finger support mechanism 41. Once the substrate is 
properly positioned by the blade within the process chamber, the lift 
fingers 39 are raised to lift the substrate 28 from the robot blade 15. 
The blade is then retracted, and the chamber is sealed and pumped down 
with a vacuum pump to a desired processing pressure. 
Once the substrate is positioned on the lift fingers, the susceptor is 
raised and brought into contact with the substrate. The susceptor then 
lifts the substrate along a process chamber central axis, as indicated by 
the numeric designator 25, until the substrate 28 is positioned at a 
substrate processing location. 
During substrate processing, a process gas is pumped into the process 
chamber through a port 56 to an apertured gas distribution plate 55, also 
referred to as a shower head, that directs the process gas toward the 
substrate. The process gas is subjected to a strong electrical field that 
creates a plasma above the substrate, and produces a gas-phase chemical 
reaction which enhances substrate film deposition. 
A series of arrows, some of which are identified by the numeric designator 
37, show process gas flow within the process chamber. As shown in FIGS. 1 
and 2, the process gas flows across the substrate surface, through 
apertures 61 formed in an exhaust baffle 60, and into an exhaust plenum 23 
that partially surrounds the susceptor. The exhaust plenum 23 serves to 
minimize changes in exhaust pressure within the process chamber, and 
operates in conjunction with the apertured exhaust baffle to control gas 
extraction from the process chamber in a manner that distributes gas flow 
evenly across the substrate surface. The gas is then drawn from the 
exhaust plenum 23 into a vacuum conduit 24 by a vacuum pump (not shown). A 
close fit is provided between the adjacent edges 35 of the susceptor and 
the exhaust baffle to prevent process gas from flowing therebetween. 
One source of undesirable nonuniformities in the gas flow pattern is 
chamber hardware which interrupts the symmetry and continuity of the 
exhaust plenum 23. For example, the substrate insertion opening 21 is 
located at the same elevation in the process chamber as the exhaust plenum 
23, such that the continuity of the exhaust plenum is interrupted at the 
opening. Additionally, both the lift finger support mechanism 41 and the 
susceptor lift mechanism are located adjacent to the central core of the 
processing chamber at the same elevation in the process chamber as the 
exhaust plenum 23. Thus, the continuity of the exhaust plenum 23 is also 
interrupted to provide space for the lift finger support mechanism 41 and 
the susceptor lift mechanism. As a result, the exhaust plenum is typically 
arc shaped, having two ends that define a gap therebetween, such that the 
exhaust plenum does not completely surround the substrate, and such that 
it cannot draw process gas from the process chamber uniformly about the 
entire circumference of the substrate. 
The prior art exhaust baffle 60, shown in FIG. 2, has a six-sided outer 
edge 64, and includes a circular ring portion 63 that has a plurality of 
apertures formed therethrough to allow process gas to flow to the exhaust 
plenum below. Two sets of apertures 66, 68 are positioned opposite each 
other on the exhaust baffle ring. The apertures are typically positioned 
equidistant from the center of the processing chamber. A third set of 
apertures 67 is positioned intermediate the other two sets of apertures on 
one side of the exhaust baffle ring. 
The dashed arrows, identified in FIG. 2 by the numeric designator 69, show 
an expected gas flow path across the substrate surface to the apertures 
near the edge of the exhaust baffle 60 for gas introduced into the process 
chamber near the center of the gas distribution plate. As shown in the 
figure, process gas flow across the substrate surface to the apertures 66, 
67, 68 leaves several arc sections of the substrate with little process 
gas cross flow, and only minimal exposure to gas molecules passing from 
adjacent areas. Although the entire substrate surface has some exposure to 
process gas, because the gas is pumped directly to the substrate surface 
from the gas distribution plate 55, the failure to distribute the process 
gas flow uniformly across the substrate surface is a significant cause of 
nonuniform film deposition on the substrate surface. 
SUMMARY OF THE INVENTION 
The invention improves the uniformity of flow of process gases across the 
surface of a semiconductor substrate during processing in a process 
chamber. The invention provides an exhaust baffle in which at least one of 
the apertures formed therethrough is adapted to provide an offset opening 
into the process chamber that extends the effective range of the exhaust 
plenum beyond its boundaries, and thereby provides a uniform process gas 
exhaust pattern that influences the process gas flow about the entire 
circumference of the substrate during substrate processing. 
A preferred embodiment of the invention provides a compound exhaust baffle 
having two plate sections in facing abutment with one another that define 
at least one offset passage for conducting process gas from the process 
chamber from a region that is not directly accessible by the underlying 
exhaust plenum.

DETAILED DESCRIPTION OF THE INVENTION 
A typical process chamber, for example as is shown in FIG. 1, includes an 
exhaust baffle 60 surrounding a workpiece, such as semiconductor substrate 
28. A plan view of an improved exhaust baffle 60a according to the 
invention is shown in FIG. 3. The exhaust baffle is shown positioned in a 
process chamber and superimposed over an exhaust plenum 23. The gas flow 
pattern across the improved exhaust baffle, indicated by the lines 
identified by the numeric designator 88, shows a substantially uniform 
distribution of process gas across the region of the substrate surface, in 
contrast to that of the prior art exhaust baffle, as shown in FIG. 2. The 
improved process gas flow pattern is achieved by the novel placement of 
apertures 76, 77, 78, and the inclusion of two offset apertures in the 
form of passages 81 and 82 that extend the exhaust plenum to an otherwise 
unavailable portion of the exhaust baffle surface, and thereby provide a 
uniform exhaust pattern that influences the process gas flow about the 
entire circumference of the substrate during substrate processing. 
The improved exhaust baffle 60a is a compound structure having a top 
portion 71 and a bottom portion 72 (please see FIG. 5). The exhaust baffle 
includes a six-sided border section 74 which acts as a perimeter flange 
around the circular bottom portion 72. Although the invention is described 
herein in connection with a six-sided process chamber, it will be readily 
understood by those skilled in the art that the invention is readily 
adapted for use with any process chamber configuration. Additionally, 
although a compound exhaust baffle is described herein, the exhaust baffle 
is readily fabricated by known techniques as any of a unitary or a 
compound structure. 
FIG. 4 is a partially sectioned perspective view of an exhaust baffle 
according to the invention. In the figure, process gas flow is shown by a 
series of arrows, as identified by the numeric designator 89. Thus, 
process gas enters the chamber from a gas inlet plate 55 spaced above the 
substrate 28 (also see FIG. 1). The gas flows across the surface of a 
substrate 28, and then through the apertures 76, 77, 78 and passages 81, 
82 that are formed at the periphery of the circular ring portion 73 of the 
exhaust baffle 60a, and then to the exhaust plenum 23. Although not 
apparent from the exploded view in FIG. 4, FIGS. 1 and 7 show that the 
exhaust baffle 60a mounts directly on the exhaust plenum 23. 
As shown in FIGS. 4-7, and especially in FIG. 5, compound exhaust baffle 
60a includes a bottom portion 72 and a top portion 71. A bottom surface of 
the exhaust baffle bottom portion 72 abuts an upper exhaust plenum surface 
26 to provide a substantially gas-tight seal therebetween. The baffle 
bottom portion 72 includes three sets of vacuum apertures 76, 77 and 78 
formed therethrough and spaced in groups that are located approximately 
90.degree. apart. 
The three sets of vacuum apertures 76, 77 and 78 are positioned directly 
above, and open directly into, the exhaust plenum. One set of vacuum 
apertures 77 is located intermediate the other two sets of vacuum 
apertures. As shown in FIGS. 1 and 4, the vacuum conduit 24 to a vacuum 
pump (not shown) is located at an intermediate portion of the exhaust 
plenum. Each aperture in the intermediate set of apertures is therefore 
situated proximate to the vacuum connection, while the other two sets of 
apertures are spaced from the vacuum connection. It is therefore 
preferable to space each of the intermediate apertures as far apart as is 
possible to prevent the intermediate apertures from excessively 
influencing the process gas flow, which otherwise would cause a negative 
pressure gradient in the adjacent region of the chamber. 
Additionally, any or all of the apertures may be formed through the exhaust 
baffle such that the inner surface of the aperture, i.e. that portion of 
the exhaust baffle that defines the aperture, is oriented at an acute 
angle relative to the exhaust baffle surface, rather than at a right angle 
relative to the exhaust baffle surface as described above, such that the 
exhaust pressure exerted by the aperture is at least partially directed 
across the substrate surface. In this embodiment of the invention, forming 
the aperture with a more acute angle relative to the exhaust baffle 
surface, i.e. with an opening directed more toward the surface of the 
exhaust baffle, imparts more directionality across the surface of the 
exhaust baffle to the pressure exerted by the aperture, while forming the 
aperture with a less acute angle relative to the exhaust baffle surface, 
i.e. with an opening directed less toward the surface of the exhaust 
baffle, imparts less directionality across the surface of the exhaust 
baffle to the pressure exerted by the aperture. In this way, each aperture 
may be formed at a selected angle to exert optimum influence on process 
gas distribution across the substrate surface, thereby improving gas flow 
uniformity and, accordingly, process uniformity. Although the figures do 
not specifically show the angled apertures of this embodiment of the 
invention, it will be appreciated that the formation of such angled 
apertures is within the general level of skill in the art. For example, 
such apertures may be formed by boring a hole through the surface of the 
exhaust baffle on a drill press on which the workpiece table has been 
inclined. 
The baffle bottom portion 72 also includes two passages 81/81a, 82/82a 
(also shown in FIGS. 5 and 6) that open into the exhaust plenum 23. A 
first baffle bottom passage 81 is shown connected to a related aperture 
81a, while a second baffle bottom passage 82 is connected to a related 
aperture 82a. The passages 81, 82 are positioned about the exhaust baffle 
at a location that is substantially intermediate the two sets of vacuum 
apertures 76 and 78, and that is substantially opposite the third set of 
apertures 77. The passages 81, 82 should have smooth surfaces to prevent 
and/or minimize the deposition of process residues and byproducts within 
the passages; and to allow efficient cleaning of the passages, for example 
when the process chamber is cleaned. Additionally, the openings of one or 
both of the passages 81 and 82 may be oriented at an acute angle relative 
to the exhaust baffle and substrate surfaces, as described above, rather 
than at a right angle relative to these surfaces, such that the exhaust 
pressure exerted by the passage is at least partially directed across the 
substrate surface to exert optimum influence on process gas distribution 
across the substrate surface. 
A "C" shaped exhaust baffle top portion 71 (also shown in FIGS. 5 and 6) 
includes a plurality of apertures formed therethrough that correspond to 
the three sets of apertures 76, 77, 78, which are formed in the baffle 
bottom portion. The baffle top portion 71 also includes a pair of passages 
83, 84 that extend to the ends 85, 86 of the baffle top portion. The shape 
of the passages 83,84 is substantially identical to that of the baffle 
bottom portion passages 81, 82, except that the baffle bottom portion 
passages 81, 82 are preferably configured to extend beyond the ends 85 and 
86 of the "C" shaped baffle top portion 71. 
An outline of the process gas duct 94 formed by the top and bottom plate 
passages is shown in FIG. 6. It will be appreciated that the baffle top 
portion need not be "C" shaped, but may be formed in any shape that is 
appropriate for the application to which the invention is put. 
Additionally, the top and bottom baffle portions may be formed from 
multiple piece assemblies. Finally, more than one discrete upper or lower 
baffle portion may be provided. 
A port 87 may be included in the exhaust baffle to allow an operator to 
view the plasma, for example through a process chamber view port 22 (FIG. 
1). A flange 92 may also be formed in the exhaust baffle to minimize the 
gap 35 between the susceptor and the exhaust baffle, and thereby reduce 
the tendency of process gas to flow therebetween and into the exhaust 
plenum. 
The exemplary exhaust baffle includes a bottom portion having an inside 
diameter of about 25.4 cm (10") and an outside diameter of about 32.5 cm 
(12.7") The depth of the passage 81 in the baffle bottom portion is about 
0.5 cm (0.200"), and the sides and bottom of the passage formed therein 
have radii of about 0.32 cm (0.125"); while the depth of the passage 83 in 
the baffle top portion is approximately 0.13 cm (0.050"), and the sides 
and top of the passage formed therein have radii of about 0.05 cm 
(0.020"). The passages 81, 82, 83, 84 should have straight sides and 
rounded corners. The diameters of the bottom portion apertures 81a, 82a 
are about 1.3 cm (0.50") and exactly match the width of the passages 81, 
82 at the point where the apertures and the passages coincide. The baffle 
bottom portion passages 81, 82 flare somewhat as they extend from the 
apertures 81a, 82a until they terminate as a wide, shallow slot having an 
open end that may be configured to face partially toward the substrate 
surface. The diameter of the apertures 76, 78, 77 is about 0.95 cm 
(0.375"). 
The baffle top and bottom portions may be held together in registered 
contact with screws, nut and bolts, adhesives, or by welding. The baffle 
top and bottom portions may be fabricated from aluminum, Kovar.RTM., or 
any other process compatible materials by casting, stamping, molding, or 
machining, as appropriate. 
FIG. 7 is a partially sectioned, perspective view of an exhaust baffle and 
exhaust plenum showing process gas flow across a substrate surface and 
into offset exhaust baffle apertures formed according to the invention. 
During substrate processing, process gas flows from the perforated gas 
inlet plate 55 toward the substrate 28 in the general direction shown in 
FIG. 4 by arrows identified by numeric designator 89. As shown in FIGS. 4 
and 7, process gas is drawn in the exhaust plenum 23 surrounding the 
substrate 28 through the three sets of apertures 76, 77, 78, and through 
the passages 81 and 82 (as best seen in FIG. 7 with reference to the 
arrows identified by numeric designator 100). The arrangement of the three 
sets of apertures and the two passages distributes exhaust pressure 
uniformly about the circumference of the substrate, and thereby effect 
uniform distribution of gas flow within the process chamber. 
The novel exhaust baffle described herein allows uniform gas flow across 
the substrate surface during substrate processing, resulting in greatly 
improved uniformity in deposited film composition and thickness. The 
refractive index of the deposited film, which is expressed as the ratio of 
the velocity of light in free space to the velocity of light in a given 
medium, i.e. the film, and which is a standard measure of the uniformity 
of the film composition and thickness, was used to evaluate the 
improvement in deposited film composition uniformity achieved when using a 
exhaust baffle according to the invention herein described. 
FIG. 8 is a graph showing substrate refractive index for several substrates 
in which a first line 101 plots refractive index for substrates that have 
been processed in accordance with the prior art, and in which a second 
line 102 plots refractive index for substrates that have been processed in 
accordance with the invention. The refractive index of a deposited film 
was measured at five points on a substrate, i.e. at the center of the 
substrate and at four other locations equally spaced from the center of 
the substrate approximately 1.5 cm from the substrate edge, and which form 
the corners of a square centered on the substrate. The measurements 
obtained were subjected to a calculation in which a maximum refractive 
index value was subtracted from a minimum refractive index value to 
produce a maximum-minus-minimum value for each substrate 
To obtain the values shown on FIG. 8, 140 substrates each were processed 
using a prior art exhaust baffle and an exhaust baffle according the 
invention herein described. Measurements made for every tenth substrate 
were plotted. The upper curve 101 plots the difference in refractive 
indexes found when using the prior art exhaust baffle shown in FIG. 3. The 
lower curve 102 plots the same measurements when using an exhaust baffle 
according to the invention, as shown in FIG. 4. It can be seen in the 
figure that the exhaust baffle according to the invention is produces a 
deposited film having a lower refractive index in which variance from 
substrate to substrate is less. Thus, there is both a substantial 
improvement in the uniformity of process gas flow across the substrate 
surface, thereby improving the uniformity of deposited film composition 
and thickness across the surface of each individual substrate; and there 
is less variance in refractive index from substrate to substrate. 
Accordingly, the invention is instrumental in producing a substrate on 
which the individual devices across the substrate, from the substrate edge 
to the substrate center, exhibit substantially the same electrical 
performance. 
Deposition process conditions for substrates from which these measurements 
were taken include the following: a 200 mm diameter substrate; having a 
susceptor temperature of 400.degree. C., and a substrate temperature of 
approximately 376.degree. C.; a maximum process time of 56 seconds; a 
chamber pressure maintained at 4.2 Torr; and process gas constituents and 
flow of SiH.sub.4 -270 sccm, N.sub.2 -4000 sccm and NH.sub.3 120 sccm. 
After several process cycles, e.g. 5-10, the process chamber is cleaned 
using a mixture of CF.sub.4 at a flow of 2200 sccm and N.sub.2 O flowing 
at 733 sccm. 
Although the invention is described herein with reference to the preferred 
embodiment, one skilled in the art will readily appreciate that other 
applications may be substituted for those set forth herein without 
departing from the spirit and scope of the present invention. Accordingly, 
the invention should only be limited by the claims included below.