Chemical vapor deposition manifold

A manifold for use in a chemical vapor deposition reactor, optimized for providing effective deposition on a substrate of a specific diameter. The manifold has upstream and downstream faces and is of substantially circular shape, with a central region of the downstream face being perforated by a plurality of upstream-directed bores. The central region is substantially larger than a circle of the specific wafer diameter for which the reactor is optimized. A centrally located plurality of the bores are through-bores or holes to the upstream face of the manifold that define a gas flow path from an upstream gas source to the wafer.

BACKGROUND 
This invention relates to the field of chemical vapor deposition (CVD), and 
more particularly to CVD reactors of the type featuring a gas inlet 
manifold or face plate in close facing relationship to a substrate upon 
which deposition is desired. 
CVD, in its many forms, is a process which may used to apply a thin layer 
or film of material to a substrate, typically a semiconductor wafer, by 
introduction of reactant gasses in the presence of applied heat. The wafer 
is typically held on a flat susceptor or other heater element. Common 
deposited materials include silicon nitride, silicon oxide, silicon 
oxynitride and/or tungsten silicide. Some reactors utilize a plasma of the 
reactant gasses in a plasma enhanced CVD process (PECVD). Common means of 
forming the plasma used in PECVD reactors are radio frequency (RF) 
excitation. In common constructions, both PECVD and basic CVD reactors 
feature a flat plate distribution manifold or "face plate" in an operative 
position in close facing relationship to the face of the substrate onto 
which the film is to be deposited. As is shown in a most schematic form in 
FIG. 1, the manifold 10 typically has flat upstream and downstream faces 
12 and 14 with a central array of holes 16 extending therebetween and 
covering an area corresponding to the plan area of the substrate 18. 
Reactant gas 20 flows through the holes to the substrate. In RF reactors, 
the manifold serves as an RF electrode. In some CVD reactors, one manifold 
configuration involves a hole array wherein the holes are formed as narrow 
straight bores arranged along the circumferences of circles of 
incrementally greater radius starting in the center of the face plate and 
spanning a region from the center to the a radial location approximately 
the same as the radius of the substrate. In such a configuration, the 
majority of the surface area of the upstream and downstream faces of the 
manifold is left intact and unaffected by the holes (FIG. 2). 
In one particular design for a manifold used in an RF reactor, shown in 
U.S. Pat. No. 4,854,263, issued Aug. 8, 1989, the disclosure of which is 
incorporated herein by reference, the straight bores are replaced with 
holes having an outlet diameter greater than their inlet diameter so as to 
increase the dissociation and reactivity of reactant gas passing through 
the manifold. To maximize hole density, the holes are arranged in an 
hexagonal closed packed array. 
During the CVD process, deposition occurs not simply upon the target face 
of the wafer but, especially, on any hot surface such as a heater 19 or a 
susceptor. The growth of such unwanted deposit layers can cause any of a 
number of problems. To keep unwanted deposits under control, a reactive 
plasma cleaning may be performed, often at predetermined intervals during 
a process run, such as after every n.sup.th wafer is processed. Typical 
cleaning processes include the introduction of a cleaning gas and the 
application of heat. As with deposition, the heat may come from a number 
of sources. Typical cleaning gases are compounds of fluorine such as 
chlorine trifluoride (ClF.sub.3), or perfluorocompounds such as nitrogen 
trifluoride (NF.sub.3). 
Although an inherent purpose of cleaning is to maintain the performance of 
the reactor, a correlation has been observed between reactor cleaning and 
degradation in the consistency and the uniformity of deposition. In 
particular, it is believed that periodic cleaning of the reactor with a 
gas such as ClF.sub.3 may result in the formation of aluminum fluoride 
(AF) deposits on the manifold. Specifically, the gas flows through the 
manifold along the same path as reactant gas 20 of FIG. 1, and then comes 
into contact with the hot heater 19 (there typically being no wafer 18 in 
the reactor during cleaning). With the heater formed of a substance such 
as aluminum nitride (AlN), fluorine radicals are believed to react with 
the heater to produce AlF.sub.3, which joins the radial outward flow 22 of 
gas in the space between the heater and manifold. The flow 22 shown in 
FIG. 1 is highly generalized and schematic. An actual flow may involve a 
variety of flow structures which can provide gas transport to and between 
the surfaces of the heater (when the wafer is removed) and face plate. 
Because the manifold is kept at a relatively lower temperature than the 
heater, the AlF.sub.3 molecules are believed to preferentially deposit on 
the manifold. These deposits are believed to increase the emissivity of 
the manifold and, thereby, to reduce the temperature of the wafers in 
subsequent processing. This has led to observed reductions in film 
thickness and resistivity in wafers processed after cleaning. 
Additionally, the altered emissivity is believed to be associated with a 
lack of deposition uniformity across the surface of each individual wafer 
processed in the reactor. In particular, a reduced resistivity has been 
observed near the wafer perimeter relative to resistivity near the wafer 
center. 
It is believed that deposits on the portion 24 of the manifold immediately 
opposite the wafer, i.e. in the central circular region of the manifold of 
the same radius as the wafer, have significant effect on the wafer and 
process. However, deposits on the portion 25 of the manifold immediately 
radially beyond this region also have significant effect, especially upon 
peripheral portions of the wafer. In particular, deposits on the portion 
of the manifold immediately beyond the perimeter of the wafer may be 
involved in the lack of uniformity of deposition on a given wafer. The 
peripheral portion of the wafer is still in sufficient proximity to the 
portion 25 of the manifold immediately laterally beyond the wafer so that 
an emissivity increase along this portion 25 of the manifold will decrease 
wafer temperature near the wafer periphery. 
Accordingly, it is desirable to alleviate or reduce the separate and 
combined effects of deposits in both the central and peripheral regions of 
the manifold. 
SUMMARY OF THE INVENTION 
In general, in one aspect, the invention is directed to a manifold for use 
in a chemical vapor deposition reactor, optimized for providing effective 
deposition on a substrate of a specific diameter. The manifold has 
upstream and downstream faces and is of substantially circular shape, with 
a central region of the downstream face being perforated by a plurality of 
upstream-directed bores. The region is substantially larger than a circle 
of the specific wafer diameter for which the reactor is optimized. A 
centrally located plurality of the bores are through-bores or holes to the 
upstream face of the manifold that define a gas flow path from an upstream 
gas source to the wafer downstream. 
Implementations of the invention may include one or more of the following 
features. A remaining plurality of the bores may have no through-flow of 
gas and may reduce the surface area of the downstream face of the manifold 
so as to reduce changes in the manifold's emissivity caused by operation 
of the reactor. This remaining plurality of bores may be blind bores 
terminating without reaching the upstream face of the manifold. The 
centrally located plurality of bores, or all the bores, may be 
substantially hexagonally close packed and may have cross-sectional areas 
at their outlets larger than cross-sectional areas at a region between 
their inlets and outlets. 
In another aspect, the invention is directed to a manifold having a central 
plurality of holes which bound an upstream to downstream gas flow path. 
Each hole has an inlet at the upstream face of the manifold and an outlet 
at the downstream face of the manifold. Substantially each hole has an 
intermediate region between its inlet and outlet, which intermediate 
region has a smaller cross-sectional area than the cross-sectional areas 
at the inlet and outlet. 
Implementations of the invention may include one or more of the following 
features. The intermediate region of each hole may be of uniform circular 
section. Each hole may have a downstream region of uniform circular 
section at the outlet. Each hole may have an upstream region of uniform 
circular section at the inlet. Each hole may have a conical transition 
region between the downstream region and the intermediate region and/or a 
conical transition region between the upstream region and the intermediate 
region. The cross-sectional area of each hole at the inlet may be smaller 
than the cross-sectional area at the outlet. The holes may be arranged in 
a hexagonal packed array, and may be arrayed over an array radius of 
between about 105% and 120% of the radius of the wafer, or, more 
particularly of approximately 115%. 
In yet another aspect, the invention relates to a manifold having at least 
a central plurality of channels extending between upstream and downstream 
faces and bounding a gas flow path through the channels. Each channel is 
of non-uniform co-axial circular section. A first region of each channel 
immediately adjacent the downstream face is of uniform section and joins 
an upstream second region of smaller diameter than that of the first 
region. 
Implementations of the invention may include one or more of the following. 
The second region may be of conical form and may join an upstream third 
region of uniform section having a diameter less than the diameter of the 
first region. The central plurality of channels may be in a hexagonal 
packed array extending over a central circular region of the manifold. The 
second region may be of uniform section and may extend to the upstream 
face. The second region may join an upstream third region of conical form 
of greater diameter than the diameter of the second region. 
In yet another aspect, the invention relates to a manifold formed as a 
unitary metal plate having upstream and downstream faces and a generally 
flat central section. The manifold has at least a central plurality of 
channels which bound an upstream to downstream gas flow path, each such 
channel having an inlet at the upstream face of the manifold and an outlet 
at the downstream face of the manifold. A plurality of blind bores extend 
upstream from the downstream face of the manifold and terminate prior to 
reaching the upstream face of the manifold. 
Implementations of the invention may include one or more of the following. 
The central plurality of channels may be arrayed in a central circular 
portion of the manifold and the plurality of bores may be spread over a 
peripheral portion of the manifold radially beyond the central circular 
portion. 
In yet another aspect, the invention relates to a manifold having upstream 
and downstream faces with at least a central plurality of channels 
extending therebetween and bounding a gas flow path through the channels. 
The manifold has means for reducing aluminum fluoride deposits on the 
downstream face of the manifold, which reducing means are located at least 
in part at a radial distance from a central axis of the manifold greater 
than a radius of a substrate on which a film will be deposited. 
An advantage of the invention is the reduction or alleviation of the 
effects of deposits on the downstream face of the manifold. This is 
achieved by removing material from the downstream manifold face so as to 
reduce the proportion of the downstream face left intact. The invention 
enhances deposition uniformity between sequential wafers processed in a 
CVD reactor and enhances deposition uniformity across the surface area of 
individual wafers being processed. Embodiments of the invention may be 
specifically useful as retrofits for existing CVD reactors or may be 
integrated within the original reactor design. 
The details of one or more embodiments of the invention are set forth in 
the accompanying drawings and the description below. Other features, 
objects, and advantages of the invention will be apparent from the 
description and drawings, and from the claims.

Like reference numbers and designations in the various drawings indicate 
like elements. 
DETAILED DESCRIPTION 
As shown in FIGS. 3-6, a manifold 30 according to the present invention has 
a substantially circular shape with a cylindrical perimeter 40 at a radius 
R.sub.1 from a central axis 100. The manifold has a planar downstream face 
32 and a substantially planar upstream face 34 having a longitudinal 
structural lip 36 at its perimeter (FIG. 6). Adjacent the perimeter, a 
plurality of counterbored mounting holes 38, at a radius R.sub.2 from the 
central longitudinal axis 100 of the manifold, extend from the downstream 
face 32 to the upstream face 34. An array of holes or through-bores 50 
extend between the upstream and downstream faces, each having an inlet at 
the upstream face and an outlet at the downstream face. The through-bores 
are in a hexagonally close packed array of packing separation S (the 
distance between the centers of adjacent bores). The array is centered 
about the longitudinal axis 100 of the manifold and extends so that the 
centers of the farthest through-bores 50 are at a radius R.sub.3 from the 
central axis. Of all the through-bores at the periphery of the array, the 
closest to the axis 100 has a center at a radius R.sub.4 from the axis. 
The lip 36 begins at a radius R.sub.5 from the central axis 100, slightly 
beyond the perimeter of the array of through-bores 50 (FIG. 5). 
As shown in FIG. 4, from upstream to downstream, each through-bore 50 has 
an upstream region 52 of uniform circular section at the inlet, connecting 
to a first conical transition region 54 which in turn connects to an 
intermediate region 56 of uniform circular section. The intermediate 
region 56, in turn, connects to a second conical transition region 58 
which finally connects to a downstream region 60, at the outlet, of 
uniform circular section. 
In an exemplary construction, the upstream region has a diameter D.sub.1 
equal to about 0.110 inches, the intermediate region has a diameter 
D.sub.2 equal to about 0.016 inches and the downstream region has a 
diameter D.sub.3 equal to about 0.213 inches. The cone angles .alpha. and 
.beta. of the first and second conical transition regions, respectively, 
are both about 120.degree.. In an exemplary embodiment, configured for use 
with one particular type of reactor, but more importantly, configured for 
use with a 200 millimeter diameter wafer, the thickness T.sub.1 (FIG. 6) 
of the manifold between upstream and downstream faces is about 0.400 
inches along the central region of the manifold, and the thickness T.sub.2 
along the lip 36 is equal to about 0.800 inches. The intermediate region 
has a length L.sub.2 and the combined downstream region and second 
transition region have a length L.sub.1. The length L.sub.1 is about 0.20 
inches and the length L.sub.2 is about 0.08 inches. The radius R.sub.1 is 
about 5.450 inches, the radius R.sub.2 is about 4.925 inches, the radius 
R.sub.3 is about 4.532 inches, the radius R.sub.4 is about 4.322 inches, 
and the radius R.sub.5 is about 4.675 inches. In the exemplary embodiment, 
the array may contain 1,574 through-bores 50. 
The selection of D.sub.2 and the packing separation S, which in the 
exemplary construction can be approximately 0.218 inches, is made largely 
to achieve desired gas flow characteristics and their selection may be a 
matter of optimization in the ordinary course of manifold design. With 
these dimensions substantially determined, the diameter D.sub.3 is chosen 
to be as large as possible without leaving too fragile a web 62 of 
material between the downstream cylindrical sections 60 of adjacent bores. 
It is desirable that D.sub.1 be larger than D.sub.2 so as to present less 
restriction to gas flow through the channel. However, D.sub.2 would 
preferably be somewhat less than D.sub.3 (leaving a thicker web 64 between 
the upstream cylindrical sections 52) so as to provide a modicum of 
strength and rigidity to the plate. 
Thus as is shown schematically in FIG. 7, the array covers an area of the 
manifold including: a) a central circular area 80 having a perimeter 84 of 
the same radius as the wafer; and b) a generally annular perimeter region 
82 between the central area's perimeter 84 and the array's perimeter 86, 
shown in FIG. 3. 
Precise characterization of the size of the array of through-bores is 
difficult. For example, assuming that a characteristic radius is sought, 
is such radius the distance (from the central axis) of the farthest hole 
in the array or of the closest perimeter hole, which may be at a somewhat 
lesser radius? Also, once the particular hole is chosen, is the 
characteristic radius to the center of the hole or to its perimeter? As 
can be seen from the discussion below, the relative differences among 
choosing any of these formulas for a particular array are small. The 
respective radii (as measured from axis 100) of the farthest and closest 
through bores at the periphery of the array are R.sub.3 =4.532 inches and 
R.sub.4 =4.322 inches. It can thus be seen that the array is substantially 
larger than the 200 millimeter wafer for which the manifold is optimized. 
Although nominally described as "8 inch" wafers, 200 millimeter wafers are 
more accurately 7.87 inches in diameter or 3.937 inches in radius. If, 
rather than the by radii R.sub.3 or R.sub.4, the size of the array were 
described relative to the perimeter of the downstream section 60 of the 
through-bores at radii R.sub.3 and R.sub.4 respectively, then 0.1065 (half 
of D.sub.3) is added to R.sub.3 and R.sub.4 to determine the perimeter of 
the array as measured by the material removed from the downstream surface 
32 of the manifold. This yields measurements of 4.64 inches (R.sub.3 
+1/2D.sub.3) and 4.43 inches (R.sub.4 +1/2D.sub.3), respectively. Thus, by 
any reasonable measurement the radius of the array is substantially larger 
than the wafer radius. By way of example, the radius (and thus diameter) 
at the perimeter 86 may preferably be at least about 105% of the radius 
(or diameter) at perimeter 84. A preferable range would be between 
approximately 110% and 120%, with the latter being constrained largely by 
the total size of the manifold and the effort required to make the holes. 
As shown in FIG. 8, in one embodiment the bores in the peripheral annular 
portion 82 of the array, at a radial location beyond the radius of the 
wafer, are formed as blind bores 150 extending upstream from the 
downstream face 32 of the manifold but terminating before reaching the 
upstream face 34 of the manifold. There is no through flow of gas within a 
blind bore. Rather, the blind bores serve to reduce the intact surface of 
the downstream face 32 of the manifold. As such, this reduces changes in 
the manifold's emissivity otherwise caused by aluminum fluoride deposits 
which would deposit on the downstream face in the peripheral portion 82, 
thereby affecting the temperature of the wafer during deposition 
(especially near the periphery of the wafer). The blind bores may be 
formed in the same hexagonal close packed array as are the through-bores 
in the central region 80 of the face plate. 
The use of blind bores or, more generally, bores through which there is no 
through-flow of gas, has a number of potential applications. As is shown 
in FIG. 8, in the peripheral annular portion 82 of the manifold, 
through-bores may not be particularly relevant during the deposition 
process but become relevant during the cleaning process as described 
above. There may be situations where either: a) it is impractical to have 
gas flow through the annular portion 82, such as with a manifold made as a 
retrofit for an existing reactor which simply does not have an appropriate 
interface to allow gas flow over an area larger than the area of the 
wafer; or b) the gas flow would be possible but may not be desired, such 
as simply for economizing the use of reactant gas or for more complex 
process-related purposes. In the former situations, it may be irrelevant 
whether the bores in annular portion 82 are through-bores or blind bores, 
for example in situations where if they were through-bores they would be 
blocked. In certain retrofit situations it may be desirable to have the 
blind bores extending in an array substantially all the way to the 
manifold perimeter 40. For example, if the overall diameter of the 
faceplate is such that there is significant effect on wafer temperature 
from deposits very close to the manifold perimeter, the bores (blind or 
through) may be extended substantially all the way to the manifold 
perimeter. 
Theoretically, with no through-flow of gas, the interior of a blind bore 
may be subject to some level of aluminum fluoride deposits. The effect of 
such deposits is believed to be reduced (relative to deposits on the flat 
surface of the downstream face) because the blind bore will tend to behave 
somewhat as a black hole, especially if it is relatively narrow and deep, 
thus causing less of an emissivity increase and less of a temperature 
reduction at the wafer. 
In a further embodiment shown in FIG. 9, relative to the through-bore 50 of 
FIG. 4, the upstream region and its associated transition region and/or 
the downstream region and its associated transition region may be combined 
such as in a bore 250. Bore 250 has conical upstream and downstream 
regions 252 and 260, respectively. 
In another embodiment shown in FIG. 10, the upstream region and 
intermediate region of bore 350 are joined as a single region 356 of 
uniform circular section. The downstream region 360 and its associated 
conical region 358 may be otherwise the same as regions 60 and 58 of the 
embodiment of FIG. 4. 
In yet another embodiment shown in FIG. 11, relative to the bore 350 of the 
embodiment in FIG. 10, the conical transition region 358 has been replaced 
in bore 450 with a planar transition region 458. In another embodiment in 
FIG. 12, both conical transition regions have been replaced by planar 
transition regions 554 and 558 in bore 550. Such a planar region may 
replace a conical transition region of the bore 50 of FIG. 4 if desired. 
As shown in FIG. 13, a manifold 610, according to the invention, may be 
used in a CVD reactor 600 of any appropriate type. The reactor includes a 
gas source/delivery system 601 for delivering reactant gas and/or cleaning 
gas to the reactor chamber 602. The chamber has a body 603 which is 
additionally connected to a vacuum pump 604 for evacuating the chamber. A 
lamp module 605, having lamps 606 may provide light through a glass window 
607 in the chamber base to heat the susceptor 619, which is shown in a 
lowered wafer loading/unloading position. The susceptor is held by 
raisable and lowerable supports 608 to allow it to be raised to a cleaning 
position 619A and processing position 619B. An exemplary gas flow path 622 
is shown for introducing gas into the chamber through the manifold, 
however alternate paths with additional branches are possible. 
Summarizing the problem, the use of cleaning gases containing fluorine has 
been observed to be associated with degradation in the consistency and 
uniformity of deposition on substrates in CVD reactors. During cleaning, 
the flow of the cleaning gas is generally longitudinally through the holes 
whereupon it is diverted radially outward between the downstream face of 
the face plate and the heater, the flow from the radially inboard holes 
joining that from more radially outboard holes as the flow proceeds by the 
latter. The combined flow includes not simply the cleaning gas but also 
byproducts such as deposited material removed from the heater and the 
aluminum fluoride in question. As this contaminated flow flows alongside 
the downstream face of the manifold, wherever there is intact surface (the 
areas between the holes and the area radially outboard of the perimeter of 
the array of holes), the aluminum fluoride may be deposited, raising the 
emissivity of the manifold wherever such deposits occur. When subsequent 
processes are run, the increased emissivity of those areas will tend to 
lower the temperature of those portions of the wafer within immediate 
sight of the areas (relative to a faceplate without deposits). 
In a variety of embodiments and by a variety of methods, the invention 
reduces and minimizes the opportunity for the formation of aluminum 
fluoride deposits and ameliorates their effect. This is achieved by 
removing material from the downstream face of the manifold in a central 
region aligned with a wafer in the reactor and/or in a region radially 
therebeyond. In the central region, process considerations determine the 
desired flow of gas through the manifold and the flow of gas through the 
manifold is largely controlled by the number of holes and their minimal 
cross-sectional areas. Thus, by maximizing the size of each hole's outlet 
at the downstream face of the manifold and providing the hole with an 
intermediate region between the inlet and outlet, which intermediate 
region has a cross-sectional area smaller then the outlet, a desired gas 
flow can be achieved while reducing the effect of deposits on the 
downstream face of the manifold. The invention may thus be used to both 
increase the uniformity of deposition within individual wafers and to 
increase consistency of deposition on subsequent batches of wafers 
processed in the reactor. The manifold of the present invention may be 
provided as a retrofit to an existing reactor or incorporated into the 
design of a new reactor. 
A number of embodiments of the present invention have been described. 
Nevertheless, it will be understood that various modifications may be made 
without departing from the spirit and scope of the invention. For example, 
it is easily understood that certain structural features of the manifold 
will be influenced or determined by the interface presented in the reactor 
where the manifold is to be used. Specific dimensions may be varied 
depending on the specific intended application and the scale of the 
reactor being used. Accordingly, other embodiments are within the scope of 
the following claims.