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DESCRIPTION
1. Technical Field
The present invention is directed to an improved reactor for performing an
ultra-high vacuum chemical vapor deposition (UHV/CVD) process.
2. Background of the Invention
In co-pending application Ser. No. 906,854, filed Sep. 12, 1986, assigned
to the same assignee as the present application, and incorporated herein
by reference, an ultra-high vacuum chemical vapor deposition (UHV/CVD)
process is described. This process achieves epitaxial deposition of
silicon layers at lower temperatures and pressures then were previously
used. The use of lower temperatures is directed at the problem of dopant
diffusion, wherein dopants from the substrate on which the epitaxial layer
is deposited move into the epitaxial layer and vice versa, which movement
tends to increase with temperature. High quality epitaxial layers having
relatively low defect densities are provided by the process of the prior
invention.
Some difficulties which have been associated with the practice of this
process have been caused by the fact that the prior art reactor has
required the use of two relatively large ultra-high vacuum (UHV) seals,
which are disposed at respective ends of the quartz reactor tube. At these
ends, the quartz tube is joined to metal components, and it therefore has
been necessary to use either graded quartz to metal seals or seals made of
VITON material. Both such options present problems, since graded quartz to
metal seals of the relatively large circumference required (approximately
30") are extraordinarily expensive as well as being somewhat unreliable,
while VITON seals of circumferences this large tend to significantly
degrade the ultra-high vacuum which is necessary to carry out the process,
as the leak rate of such seals is a function of their circumference.
Additionally, the use of inflexible large circumference seals at opposite
ends of the reactor tube requires that the respective ends of the
apparatus be perfectly aligned, which is difficult to accomplish in
practice. Finally, the use of such seals makes it difficult to maintain
the apparatus, as replacement of the reactor tube requires the removal of
all pumping and valving hardware located behind it, which procedure takes
approximately three hours.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide a UHV/CVD reactor
which uses a more inexpensive seal arrangement than has heretofore been
used.
It is a further object of the invention to provide a UHV/CVD reactor which
uses a more reliable seal arrangement than has heretofore been used.
It is still a further object of the invention to provide a UHV/CVD reactor
wherein the respective ends do not have to be perfectly aligned.
It is still a further object of the invention to provide a UHV/CVD reactor
in which the reactor tube can be withdrawn and/or replaced in a small
fraction of the time that was previously required.
In accordance with the invention the above objects are accomplished by
providing a UHV/CVD reactor wherein the pump hardware and load station are
at the same end of the apparatus. An ultra-high vacuum seal similar to the
type which was previously used is required at this end, but the other end
of the reactor tube, which is the gas inlet end, has an opening of
substantially reduced diameter, so that the seal at this end will have a
much smaller circumference, and will not present the problems which are
associated with the larger circumference seals. A metal tube communicates
with the reactor tube at the pump/load station end, and the pump is
connected to this tube by a tee section for drawing gas which is inputted
at the gas inlet end through the reactor tube.
The use of a small diameter seal at the gas inlet end, which can be a
graded seal, Viton rubber seal, or other type, substantially reduces the
cost of the apparatus. Additionally, since such seals of small
circumference do not have a substantial leak rate, the vacuum in the
reactor tube is not degraded. Since there is only one large diameter,
inflexible seal in the apparatus of the invention instead of two, as in
the prior apparatus, the ends of the reactor do not have to be perfectly
aligned. Finally, since all UHV pumps and associated connections are
eliminated at one end of the apparatus, the process tube may be withdrawn
at that end by only disconnecting a single, small joint. This operation is
a great savings over the procedure associated with the prior apparatus,
which required the removal of all pumping and valving hardware located
behind the process tube, so that the time which is required to replace the
process tube drops from roughly three hours to ten minutes. Additionally,
the new replacement procedure is a more reliable operation by virtue of
having to break fewer large diameter UHV seals.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better appreciated by referring to the accompanying
drawings, wherein:
FIG. 1 is a schematic representation of the prior art apparatus.
FIG. 2 is a schematic representation of the apparatus of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a UHV/CVD reactor generally as described in
above-mentioned U.S. appl. Ser. No. 906,854 is shown.
As mentioned above, this reactor is used to perform an epitaxial deposition
process at lower temperatures and pressures than had previously been used.
Thus, the operating deposition temperature of the process performed by the
reactor shown in FIG. 1 is less than about 800.degree. C. while the
operating total pressure is generally less than about 200 mTorr.
Additionally, the apparatus is operated such that the partial pressures of
all contaminants in the process tube are maintained less than 10.sup.-8
Torr at all times, that is both prior to and during deposition onto the
substrates.
Referring to FIG. 1, the apparatus is comprised of hot wall UHV reactor
chamber 2 which is made of quartz, and load chamber 4, which is made of a
metal such as stainless steel. The reactor chamber and load chamber are
separated by main valve 3, which serves as the isolation valve between the
chambers. Pumping means 6 and 8 are also provided for the reactor and load
chambers respectively, and as initial process steps, the load chamber is
pumped down to a pressure of 10.sup.-7 Torr, while the reactor chamber is
pumped down to 10.sup.-8 Torr.
After the performance of other process steps, the quartz substrate boat 10,
which contains wafers 12, is transferred from the load chamber to the
reactor by transfer means 13. Silicon containing gas is admitted through
gas inlet tube 14 and is pumped through the reactor in the direction
shown. Epitaxial deposition onto all of the wafers then occurs, and the
pumping system 6 is maintained at all times, the operating pressure within
the chamber 2 being determined by the amount and flow of the gas species
in the chamber. The thickness of the epitaxial layer produced depends upon
the growth rate and the time of deposition, which is generally controlled
by the temperature in the deposition reactor, or to a lesser degree by the
reactant pressure.
Seal 16, at the right end of the apparatus in the Figure, seals the quartz
reactor tube and the metal isolation valve 3, while seal 18, at the left
end of the apparatus seals the quartz reactor tube and the metal pump
tubing 19. Since an ultra-high vacuum background pressure of contaminants
is maintained in reactor tube 2 at all times during the process, seals 16
and 18 must be of the UHV type. In general, two types of UHV seals are
available, but both suffer from disadvantages at the relatively large
diameter required, which is about ten inches for the apparatus shown in
FIG. 1.
The first type of seal is of the "graded" type, wherein a graded
borosilicate glass member connects to the quartz at one side and to the
metal on the other. This type of seal is exceedingly expensive in the
required diameter, and additionally is somewhat unreliable. Further, it is
inflexible, and if one such seal is used at each end of the reactor tube,
great difficulties are encountered in attaining the perfect alignment
which is then necessary in assembling the apparatus.
The second type of seal which may be used is of the VITON O-ring type, as
depicted by seal 18 in FIG. 1. In such a seal, the leak rate is dependent
on the circumference, and at the required circumference of more than about
30 inches, it is found that the vacuum becomes significantly degraded.
Additionally, with both types of seals, when it is necessary to withdraw
the reactor tube, all of the pumping and valving hardware which is
represented by pumping means 6 in FIG. 1, must be removed, which is a
major job taking about three hours. It is noted that in terms of actual
hardware, pumping means 6 may be comprised of two pumps and several
valves.
The foregoing problems are obviated by the unique structure of the present
invention, and an illustrative embodiment thereof is shown in FIG. 2,
wherein like components are identified with the same numerals as in FIG.
1.
Referring to FIG. 2, it is seen that the geometry of the system is "single
ended", in that both the reactor chamber pump and the load station are
located at the same end of the system. Thus, it is seen that reactor
chamber 2' and load chamber 4' both communicate with conduit 20, which
typically is made of stainless steel. An optional valve 3' separates the
reactor chamber 2' from conduit 20, while another valve 22 is intermediate
the conduit 20 and the load chamber 4'. Additionally, there can be a
quartz to metal seal or a water cooled VITON seal 16' between the reactor
chamber 2' and the valve 3', while there is a metal to metal seal 21
between the conduit 20 and the valve 22.
As can be seen by referring to the figure, the reactor chamber 2' has a
substantially reduced diameter at the left end in the figure, and is
terminated in a substantially reduced diameter cylindrical portion 24.
This portion is joined to a metal gas inlet tube 26 of similarly small
diameter by a quartz to metal seal 18'. For example, gas inlet tube 26 may
be made of a section of flexible conduit.
Thus, in the operation of the reactor, gas is fed to gas inlet tube 26, and
is pumped through the reactor tube by pump means 6', while wafers on which
substances in the gas are to be deposited are fed to the reactor tube from
load chamber 4' through conduit 20. Pump means 6' may include a turbo
pump, mechanical pump, and roots blower.
Thus, it is seen that in the arrangement of FIG. 2, only a single large
diameter quartz to metal seal 16' or water cooled VITON seal is required,
while the other quartz to metal seal 18' is of much smaller diameter.
In this regard, while the reactor tube 2' is about ten inches in diameter
through most of its length, at end portion 24, the diameter can be reduced
to about 1/2 inch. Thus, the ratio of the circumference of seal 18' in
FIG. 1 to the circumference of seal 18' in FIG. 2 is about 20:1. In this
case, seal 18' can be any of a variety of different types, since as
previously explained the problems associated with large circumference
quartz to metal seals are not present with seals of smaller circumference.
Thus, for example, the seal 18' can be a small seal of the graded type or
of VITON, or of the MINI-CONFLAT type.
It can thus be appreciated that the reactor of the present invention is
considerably less expensive to build than the prior art reactor, and has
less of a vacuum leakage problem. Further, it is no longer a requirement
that the two ends of the system be perfectly aligned. Thus, the reactor
tube can be aligned with its mate at the load end of the system and a
flexible steel conduit can be used for the gas inlet tube 26. Finally,
withdrawing the process tube is a simple operation which involves
disconnecting the seal 18', rather than removal of all of the pumping and
valving hardware behind the process tube as in the prior art system, and
the replacement time drops from about three hours to ten minutes. The
replacement process is a far more reliable operation by virtue of having
to replace fewer UHV seals.
A representative deposition process utilizing the reactor of the invention
would be performed as follows:
1. The pumping apparatus 6' is employed to create a base total pressure
less than about 10.sup.-8 Torr in UHV section 2'. During this time, the
valve 22 is closed, thus isolating the load station, while valve 3' is
open.
2. The quartz substrate boat is placed into the load chamber 4', and is
baked at approximately 100.degree. C. while the load chamber is being
pumped to a pressure of approximately 10.sup.-7 Torr.
3. Hydrogen gas is injected into section 2' via inlet 26, and the
temperature therein is set at about 550.degree. C. The introduction of
hydrogen into this section raises the total pressure to about 250 mTorr.
4. Valve 22 is then opened, and the quartz substrate carrier is transferred
from load chamber 4' to the reactor chamber, and the valve 22 is then
closed. The substrates are then baked for about 5 minutes in a hydrogen
atmosphere, the baking temperature being whatever temperature is to be the
deposition temperature. This will generally be from about 450.degree. C.
to about 800.degree. C.
5. The hydrogen flow is stopped and the silicon gas source is activated to
introduce a gaseous species containing silicon into the deposition chamber
2'. If the epitaxial silicon layers are to be doped in-situ, a
dopant-containing gas species can also be introduced via inlet 26.
6. Epitaxial deposition onto all of the substrates then occurs. The pumping
system 6' is maintained at all times, the operating pressure within
deposition chamber 2' being determined by the amount and flow of the gas
species in the chamber. The thickness of the epitaxial layers so produced
depends upon the growth rae and the time of deposition, which are in turn
generally controlled by the temperature in the deposition reactor, or to a
lesser degree by the reactant pressure.
There thus has been provided a simple, inexpensive, and effective
arrangement for performing UHV/CVD processes. Further, the arrangement of
the invention in which flow is inverted in unique to UHV/CVD systems.
Thus, in prior epitaxy systems, where flow is laminar, system geometries
cannot be inverted while still retaining function. Further, particles
entrained in the laminar flowing gas would result in ruining the sealing
surfaces of the valve separating the load chamber from the reactor
chamber. This is obviated in UHV/CVD processes wherein operations under
laminar flow is bypassed such that particulates are not carried in the gas
stream.
It should be appreciated that while the invention has been disclosed in
connection with a specific embodiment for purposes of illustration, it is
to be limited only by the claims which are appended hereto and equivalents
.
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