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FIELD OF THE INVENTION
This invention relates to the deposition of epitaxial silicon layers, and
more particularly to a method and apparatus for depositing epitaxial
silicon layers on a plurality of substrates at low temperatures and low
pressures. Device quality layers are produced which can be doped in-situ.
BACKGROUND ART
It is well recognized that the production of thin epitaxial films of
silicon having abrupt and arbitrary dopant profiles is vital in device and
circuit fabrication, and particularly in applications such as scaled-down
bipolar and CMOS VLSI circuits and processes. In particular, such thin
epitaxial layers are useful in order to reduce the dimensions of high
performance integrated circuitry. However, the fabrication of such thin
epitaxial films is not possible owing to physical phenomena implicit in
materials preparatory techniques heretofore known, as described by H.
Ogirima et al, J. Electrochemical Soc., 124, 903 (1977). Specifically, the
thickness of a device layer deposited by silicon epitaxy has been fixed at
values greater than the diffusion length of dopants out of the substrate
on which the epitaxial layer is deposited. These dimensions can be on the
order of a micron under typical high temperature processing conditions
(T.gtoreq.1000.degree. C.).
In more detail, prior techniques for depositing epitaxial silicon, as for
instance the techniques described by G. R. Srinivasan, J. Cryst. Growth
70, 201 (1984) require high processing temperatures. At these high
temperatures dopants in the substrate on which the epitaxial layer is
deposited can move into the epitaxial layer either by evaporation and
redeposition from the gas phase (autodoping), or can move out from the
substrate by solid-state diffusion. Still further, dopants intended to be
introduced into the epitaxial silicon layer can move in that layer and can
diffuse into the substrate. All of the prior art processes for providing
epitaxial layers (except for single wafer physical vapor deposition
methods such as molecular beam epitaxy) operate at sufficiently high
temperatures that dopant redistribution can occur. Because of this, the
thickness of the deposited epitaxial layer must be fixed at values greater
than the diffusion length of dopants out of the substrate, which in turn
means that the ultimate size of a device produced in the epitaxial layer
cannot be reduced below this dimension.
Over the past decade, the deposition of homoepitaxial silicon films for
technical applications has been performed in essentially the same manner.
Typically, the process takes place at temperatures in excess of
1000.degree. C. (or involves a high temperature cycle to clean wafers
prior to deposition), using a cold wall/hot susceptor deposition apparatus
of the type described by G. R. Srinivasan in Solid-State Technology, 24,
101 (1981). Advances in this technique have reduced autodoping by a
lowering of processing pressures, a factor which has allowed continued use
of the process.
However, the fabrication of very thin epitaxial layers having abrupt
transitions (several atomic widths) in dopant concentration between
adjacent single crystal layers cannot be achieved by such prior art
techniques.
A low temperature process will be required which is sufficient to produce
device quality, reproducible epitaxial films having the necessary thinness
for device miniaturization. Several classes of such techniques presently
being developed are described by G. R. Srinivasan and B. S. Meyerson in
the Electrochemical Society Softbound Proceedings Series, Pennington, N.J.
(1985).
Various types of low pressure chemical vapor deposition (LPCVD) processing
techniques are known in the art, but these are used to produce
polycrystalline and amorphous silicon. Typical process pressures used in
such techniques are in the range of 200-1000 mTorr. The source gas used in
these reactors is typically silane, with a carrier gas such as hydrogen.
However, hydrogen carrier gas has a certain contamination level of H.sub.2
O (typically in excess of 1 part per million (ppm)) it reaches the process
environment and, for this reason, processing is generally performed in an
atmosphere containing partial pressures of .gtoreq.10.sup.-4 Torr water
vapor and oxygen. The effect of water vapor and oxygen must be taken into
account in order to provide epitaxial silicon, since the crystallographic
perfection of the initial silicon surface upon which epitaxy is to take
place is the determining factor in the quality of the resultant epitaxial
layer. Systematic investigations have been done in the past to determine
the optimum cleaning procedure for a silicon surface prior to its
insertion into the deposition apparatus. For example, reference is made to
F. Hottier et al, J. Cryst. Growth, 61, 245 (1983) for an analysis of the
procedures. Additionally, the quality of the environment into which the
substrates are introduced is important. Ghidini and F. W. Smith, J.
Electrochemical Soc. 109, 1300 (1982) and ibid 131, 2924 (1984) have
conducted basic surface investigations of the Si/H.sub.2 O/SiO.sub.2 and
the Si/O.sub.2 /SiO.sub.2 equilibrium systems to determine the equilibrium
conditions in which both oxygen and water vapor background are such that
silicon is effectively etched by these species in order to favor the
maintenance of an oxide-free silicon surface.
In prior epitaxial silicon processing conducted at p.gtoreq.10 Torr.,
partial pressures greater than about 10.sup.-4 Torr water vapor and oxygen
were present. An oxide free silicon substrate surface is obtained in such
a system only if the deposition temperatures remain above 1025.degree. C.,
in accordance with the data of Ghidini and Smith, described in the
referenced articles hereinabove. In these prior systems, source purity
requirements are quite stringent in order to be able to operate at process
temperatures as low as 1025.degree. C., which is in itself a high
temperature.
The present invention is an apparatus and process for achieving device
quality epitaxial silicon films without the heretofore mentioned problems,
and in particular is a technique for high density batch processing of
multiple wafers to provide epitaxial silicon films thereon. In applicant's
technique, the temperatures and pressures utilized are much less than
those previously utilized and are such that the process is nonequilibrium
in nature, i.e., growth kinetics rather than equilibrium thermodynamics
govern the deposition process. A hot wall, isothermal CVD apparatus is
used in which essentially no homogeneous (gas phase) pyrolysis of the
source takes place in the residence time (less than 1 second)/gas
temperature regime where the process is operated. Instead, heterogeneous
chemistry, where reactions at the surface of the substrate occur, are
important.
As will become more apparent later, the present apparatus and process
provide the following results and features, not heretofore reported in the
literature or elsewhere:
1. In-situ doped CVD silicon epilayers at temperatures .ltoreq.800.degree.
C.
2. Use of ultrahigh vacuum (UHV) in combination with a CVD apparatus,
thermally driven
3. Use of a hot wall, isothermal system for Si epitaxy at temperatures less
than about 800.degree. C., with high throughput
4. Deposition of epitaxial silicon layers at low temperatures where the
epitaxial layers are equilivant or superior in electrical characteristics
to all epitaxial layers heretofore made
5. The use of deposition temperatures as low as approximately 550.degree.
C. to provide single crystal Si epitaxial layers having low defect
densities, without the use of any external energy (lasers, RF plasmas, . .
.)
6. A process and apparatus for batch fabrication of single crystal,
epitaxial Si layers on a plurality of substrates wherein an isotropic
source gas bath is produced in a thermally driven CVD apparatus.
Accordingly, it is a primary object of the present invention to provide a
method and apparatus for epitaxial deposition of silicon layers in a batch
process.
It is another object of this invention to provide an apparatus and method
for enabling low temperature epitaxy of silicon layers.
It is another object of this invention to provide a CVD apparatus including
means for providing ultrahigh vacuum therein, wherein the total system
base pressure is less than about 10.sup.-8 Torr.
It is another object of this invention to provide a method and apparatus
for low pressure, low temperature fabrication of epitaxial, single crystal
silicon layers.
It is another object of this invention to provide a method and apparatus
for producing an isotropic gas bath from a silicon source during thermally
driven chemical vapor deposition.
It is yet another object of this invention to provide a method and
apparatus for gas phase vapor deposition of single crystal silicon in a
thermally driven process.
It is another object of this invention to provide a method and apparatus
for growing single crystal silicon epitaxially upon a plurality of
substrates at temperatures less than 800.degree. C.
It is another object of this invention to provide a method and apparatus
for hot wall isothermal epitaxy of silicon layers from a gas source of
silicon, said epitaxial layers being grown on multiple substrates at
temperatures less than about 800.degree. C.
It is another object of this invention to provide a method and apparatus
for chemical vapor deposition of epitaxial silicon layers via a thermally
driven process.
It is another object of this invention to provide a method and apparatus
for uniform epitaxial deposition of single crystal silicon onto multiple
substrates via a low temperature, low pressure thermally driven chemical
vapor deposition process.
It is another object of this invention to provide an apparatus and method
for avoiding gas phase depletion in a hot wall thermally driven CVD
process for depositing single crystal epitaxial silicon layers.
It is another object of the present invention to provide an improved
thermal technique for epitaxially depositing silicon at temperatures less
than about 800.degree. C.
It is another object of this invention to provide a method and apparatus
for thermally driven chemical vapor deposition of epitaxial silicon
wherein the source gases have reduced purity requirements.
It is another object of this invention to provide a method and apparatus
for thermally driven chemical vapor deposition of single crystal silicon
films upon multiple substrates wherein these films exhibit significantly
reduced impurity contamination.
It is another object of this invention to provide a method and apparatus
for thermally driven chemical vapor deposition of uniform single crystal
silicon films upon multiple substrates.
It is a further object of this invention to provide a method and apparatus
for in-situ doping of silicon epitaxial layers to levels above theoretical
limits set by solid solubility during chemical vapor deposition of these
layers.
DISCLOSURE OF THE INVENTION
High quality homoepitaxial silicon layers are deposited on multiple
substrates through use of ultrahigh vacuum CVD techniques. In contrast
with LPCVD techniques, the present technique requires base (ultimate)
pressures of less than 10.sup.-8 Torr. and works in temperature and
pressure ranges wherein nonequilibrium processes (growth kinetics)
dominate the formation of the silicon epitaxial layers. The heterogeneous
reactions occurring at the substrates surface are the primary determinants
of epitaxial deposition, homogeneous gas phase reactions being, by design,
essentially absent in this technique.
The apparatus is a hot-wall, multi-wafer CVD system having means for
providing base pressures (ultimate vacuum) less than about 10.sup.-8 Torr.
This is a flow system in which the source gases are injected at one end
and high speed pumps operate at the other end, there being a load lock to
eliminate contamination upon loading of the substrates prior to
deposition. The system typically operates in a molecular flow regime where
the total operating pressure of the Si source gas (such as silane) is less
than several hundred mTorr. during deposition. Deposition temperatures are
in the range of about 550.degree. C. to 800.degree. C., the preferred
range being 550.degree. C. to about 750.degree. C. While the preferred
total operating pressure of silane during deposition is in the range
10.sup.-2 -10.sup.-4 Torr, silane pressures up to several hundred militorr
may be possible, if very low growth temperatures (500.degree.-650.degree.
C.) are used. The technique is more tolerant of the Si source gas pressure
as the deposition temperature decreases.
In contrast with prior CVD systems, the present technique utilizes base
pressures several orders of magnitude (a factor of 10.sup.3 -10.sup.6)
below that previously used as well as very low operating pressures and
temperatures during deposition, in order to provide a new regime for the
epitaxial growth of silicon films. In-situ doping is possible by
introducing dopant source gases along with the silicon source gas, or
after epitaxial deposition and while vacuum conditions are maintained.
Suitable choices for the silicon source gas include silane and higher
order silanes, as well as other species. Flow rates are not critical.
These and other objects, features, and advantages will be apparent from the
following more particular description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a UHV/CVD system suitable for the
practice of the present invention.
FIG. 2 is a plot of the homogeneous pyrolysis rate constants K.sub.1 as a
function of operating pressure in a process utilizing silane pyrolysis at
800.degree. C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention is an apparatus and process for epitaxially depositing
single crystal silicon layers of a desired thickness on a substrate, and
more particularly to such a process in which single crystal silicon layers
can be epitaxially deposited on a multitude of substrates. Silicon thin
films of high crystallographic perfection are deposited on the substrates
and can be in-situ doped to any desired level.
Ultra high vacuum (UHV) is used in combination with a CVD system in order
to provide initial base pressures less than 10.sup.-8 Torr. After the
substrates (silicon) are loaded into the system, deposition proceeds in a
manner to prevent the formation of oxide films on the silicon substrates.
The total operating pressure of the Si gas source during silicon epitaxy
is generally several orders of magnitude less than that heretofore used,
creating an isothermal gas bath for deposition onto the substrates and
substantially preventing gas phase depletion of the silicon source gas.
This allows batch processing to occur. Since the deposition temperature is
less than about 800.degree. C., dopant redistribution is avoided, and the
problems of autodoping and solid state diffusion described hereinabove are
substantially eliminated.
The term "Si gas source" refers to the gas species that contains Si, for
example SiH.sub.4. It is recognized that the input gas in the CVD reactor
can include other non-Si-containing gases, such as H.sub.2 and H.sub.e.
More specifically, the "Si gas source" or "Si-containing source gas" is
that gas which decomposes in the reactor to provide Si for epitaxial
deposition onto the substrates.
UHV/CVD Apparatus (FIG. 1)
FIG. 1 schematically illustrates an ultrahigh vacuum CVD deposition system
capable of depositing epitaxial silicon layers on a plurality of
substrates. The apparatus is comprised of an UHV section 10 (deposition
chamber, or reactor) and a load chamber section 12 used to load substrates
14 into the UHV section 10. Main valve 16 serves as an isolation valve
between UHV section 10 and the load chamber 12.
The function of loading chamber 12 is to provide a low pressure chamber
from which the substrates 14 are transferred into the UHV section 10 after
the load chamber 12 has been pumped to a pressure below 10.sup.-6 Torr.
This ensures that no contaminants are introduced into the UHV section 10
when samples are transferred therefrom load chamber 12 in order to grow
epitaxial silicon layers on the substrate 14.
In more detail, loading chamber 12 is comprised of a stainless steel tube
18 having attached thereto means 20 for evacuating chamber 12 to a desired
vacuum level. Means 20 is generally a pumping means comprised of a turbo
pump 22, an Al.sub.2 O.sub.3 trap 24, a rotary pump (not shown) and a
valve 26. Pumping means 20 is used to evacuate the loading chamber to a
low pressure (approximately 10.sup.-7 Torr) prior to transfer of the
substrates 14 into the UHV section 10. Also attached to load chamber 12 is
a transfer means 28 for transferring the substrates 14 from the load
chamber into the UHV section. Transfer means 28 is used to push the quartz
boat 30, on which the substrates 14 are located, into UHV section 10.
The pumping means 20 is comprised of a turbo molecular pump 22 of a type
that is commercially well known. The trap 24 is also commercially
available and is an alumina trap that prevents contamination of the turbo
pump 22. Its function is to minimize contamination of turbo pump 22 by
hyrdocarbons which may be introduced from the rotary pump (not shown). The
rotary pump is a mechanical pump which is also well known, as for instance
those manufactured by Leybold-Heraeus Co.
Pumping means 20 is used to evacuate the loading chamber prior to movement
of the substrate-loaded quartz boat 30 from loading chamber 12 into UHV
section 10. This loading operation is done after the desired ultimate
vacuum is produced in section 10, and is achieved by the vacuum transfer
apparatus 28, which can be any type well known in the art. For example, a
magnetic transfer rod, a bellows system, etc. cia be used. The function of
the transfer apparatus is to move the substrate boat 30 into the UHV
section 10 without breaking the vacuums in load chamber 12 and UHV section
10.
The UHV section (also termed a reactor, or deposition chamber) is generally
comprised of a hot-wall furnace in which chemical vapor deposition onto
the substrates occurs, instrumentation (mass spectrometer) for determining
the impurity levels in the furnace, and pumping means for creating an
ultimate vacuum at least 10.sup.-8 Torr in the furnace. In more detail, a
standard hot wall furnace 32 is comprised of a quartz tube 34, while the
rest 18 of the tube is comprised of stainless steel. The RF coils 36 are
not required, and are used only for cleaning the quartz tube. Coils 36
produce a hydrogen plasma in order to scrub the interior of quartz tube 34
before its first use. Coils 36 are not required for the practice of this
invention, and are not employed except after tube replacement.
Pumping means 38 is used to establish an ultimate vacuum at least 10.sup.-8
Torr in the tube 34, and is comprised of a main process turbo pump 40, and
a rotary pump and Roots blower (not shown). Turbo pump 40 is connected to
the deposition tube 34 via valve 42, and is also connected to the tube 44
(leading to the rotary pump and Roots blower) by the valve 46. Tube 44 is
connected to the deposition tube 34 via valve 48. Pump 40 is also a turbo
molecular pump of a type well known in the art, and can be similar to the
turbo pump 22 previously described. The rotary pump and Roots blower are
high speed mechanical pumps known in the art and provided by several
manufacturers, including Leybold-Heraeus Co. A pressure control valve 50
is located in tube 44, and is used to slow the pumping speed only during
the initial pumpdown of the UHV section following repairs. Valve 48 and
control valve 50 are never opened at any time during actual system use.
Adjustment of valve 50 allows one to start the evacuation of tube 34
slowly in order to achieve the proper vacuum therein without drawing
contaminants into tube 34 after, for example, guartz tube replacement.
In operation, turbo pump 40 is turned on first, followed by the Roots
blower and the rotary pump. Valve 46 is opened most of the time, as is
valve 42, to allow pump-down of the UHV section 10. Valve 48 is primarily
closed, and is opened only for repair of the apparatus. For example, if it
is desired to change the furnace tube, valve 48 will be opened only to
pump down to vacuum after changing the furnace tube. During the normal
pumping operation to evacuate tube 34 to a pressure at least as small as
10.sup.-8, valve 48 is closed.
A mass spectrometer 52 is connected to tube 34 via the valve 54, and is
used to test the impurity levels within tube 34 in order to insure the
continued absence of impurities therein. By opening valve 54 and examining
the contents of tube 34, the amount of impurities such as oxygen, carbon,
and water vapor can be examined. After this analysis, valve 54 is closed.
The provision of a load chamber means 12 and means 38 to provide an
ultrahigh vacuum in the deposition tube of a CVD apparatus has not
heretofore been done in the art, and is essential to the deposition of
epitaxial silicon layers of high quality onto the substrates 14. For this
deposition, the source gases and any dopant gases are injected into tube
34 via the gas source inlet 56.
The major steps in the deposition process are the following:
1. The UHV section is pumped to a pressure which is less than about
10.sup.-8 Torr total pressure.
2. The substrates 14 are transferred from load chamber 12 to the UHV
section 10, for epitaxial deposition thereon.
3. The desired operating temperature and pressure are established.
4. The source gas is introduced through inlet 56 in order to create an
isothermal gas bath for uniform epitaxial deposition of silicon layers
onto the substrates 14.
In the practice of this invention, the operating deposition temperature is
less than about 800.degree. C., and the operating total pressure is
generally less than about 200 mTorr. Further, the apparatus is operated
such that the partial pressures of all contaminants in UHV section 10 are
maintained less than 10.sup.-8 Torr at all times, that is, prior to
deposition and during deposition onto the substrates 14. No other
apparatus or process heretofore known in the art accomplishes this.
While the basic operational steps of the invention have been described in
previous paragraphs, the following will provide additional detail of a
representative total deposition process. This process is as follows:
1. The pumping apparatus 38 is employed to create a base total pressure
less than about 10.sup.-8 Torr. in UHV section 10. During this time, the
main isolation valve 16 is closed, isolating reactor section 10 from load
chamber 12.
2. The quartz substrate boat 30 is placed into the loading chamber 12, and
is baked at approximately 100.degree. C. while the loading chamber 12 is
being pumped to a pressure of approximately 10.sup.-7 Torr, using the
pumping apparatus 20.
3. Hydrogen gas is injected into section 10 via inlet 56 and the
temperature therein is set at about 650.degree. C. The introduction of
hydrogen into this section raises the total pressure to about 250 mTorr.
4. The main isolation valve 16 is then opened so that the flow of hydrogen
through inlet 56 splits, about half of the flow going into section 10
while the other half goes into load chamber 12. This prevents cross
contamination of impurities from load chamber 12 into the deposition
section 10.
5. The quartz substrate boat 30 is then transferred from load chamber 12 to
deposition chamber 10, and the main isolation valve 16 is closed. The
substrates 14 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 550.degree. C. to about
800.degree. C.
6. The hydrogen flow is stopped and the silicon gas source is activated to
introduce a gaseous species containing silicon into the deposition chamber
10. If the epitaxial silicon layers are to be doped in-situ, a
dopant-containing gas species can also be introduced via inlet 56.
7. Epitaxial deposition onto all of the substrates then occurs. The pumping
system 38 is maintained at all times, the operating pressure within
deposition chamber 10 being determined by the amount and flow of the gas
species in chamber 10. The thickness of the epitaxial layers so produced
depends upon the growth rate 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.
It had previously been noted that the present apparatus differs from a low
pressure CVD apparatus (LPCVD) in that the present apparatus can be used
to deposit epitaxial silicon films, while LPCVD systems have always been
used to deposit polycrystalline or amorphous silicon, or insulating layers
such as silicon nitride. It is recognized that conventional LPCVD systems
have been used at low operating temperatures and pressures, such as
temperatures less than 800.degree. C., and operating pressures down to
several hundred mTorr. However, such prior LPCVD systems are not provided
with means for producing the base pressures (ultimate vacuum) used by the
present applicant. In conventional LPCVD systems, the base pressures are
approximately 10.sup.-3 Torr, a factor of 10.sup.6 above those employed
herein, and such that the partial pressures of contaminants in those
conventional reactors during deposition will be whatever the base pressure
is. Consequently, such prior art systems cannot be used to grow single
crystal silicon, and certainly not single crystal silicon of adequate
purity and low defect concentration so as to be useful in device
fabrication.
In contrast with prior LPCVD systems, the present deposition apparatus is
operated such that the partial pressures of all contaminants are
maintained at pressures less than 10.sup.-8 Torr at all times, that is,
prior to deposition and during deposition. These pressure differentials
are about 6 orders of magnitude less than those used in conventional LPCVD
systems.
In order to maintain a clean interface on which to epitaxially deposit
silicon films, the UHV section of the apparatus, on initial systems
startup, is pumped and baked until reaching a base pressure in the
10.sup.-9 Torr range. Once at this base pressure, the system is kept under
vacuum. Sample introduction is carried out employing, for example, a
magnetically coupled load lock. As noted, the UHV section of the apparatus
is a hot wall isothermal environment with the substrates being mounted
coaxially within the furnace tube. The mass spectrometer is mounted
internal to the UHV section of the apparatus and allows rapid in-situ
diagnosis of the system. It is employed to insure operation within a
stringent vacuum criterion.
As noted previously, a partial pressure of less than 10.sup.-8 Torr H.sub.2
O must be maintained in order to achieve an oxide-free surface on the
substrate wafers, for deposition at 800.degree. C. A somewhat less
stringent requirement of partial pressure is required for oxygen, in order
to maintain an oxide free substrate surface. For oxygen, a partial
pressure of approximately 10.sup.-7 Torr must be maintained at 800.degree.
C. These are minimum design criteria, and in the practice of this
invention initial base pressures less than about 10.sup.-8 Torr are used.
After reaching this base pressure, the system is then scrubbed using a
hydrogen plasma afterglow produced by the RF coils at the upstream end of
the furnace. This step is performed only on system startup after
maintenance, and is not critical. It is never performed as part of the
water cleaning process, as it would cause an extra ordinarily high defect
density in any deposited film. Once returned to base pressure, the system
is kept under vacuum using the turbo molecular pump, Roots blower, and
rotary pump system. The substrates are then introduced by employing the
magnetically coupled load lock. Samples are allowed to equilibrate under a
hydrogen flow prior to deposition, a flow of 600 sccm (standard cubic
centimeters of a minimum known mass quantity at a known temperature and
pressure), for five minutes being typical. After this, film deposition is
initiated by adjusting the growth temperature to be less than 800.degree.
C. where the Si gas source operating pressure is reduced to a pressure
less than about 200 mTorr. For the case of silicon epitaxy using silane
(SiH.sub.4) as a silicon source gas, the flow rate of silane can be, for
example, 2.sup.-10 sccm. A hydrogen carrier gas can also be introduced at
a flow rate of 20 sccm.
In the preceding example, the total silane operating pressure of the system
is less than about 200 mTorr, and preferrably in the range 10.sup.-2
10.sup.-4 Torr. Depending upon the deposition temperature, and the time of
deposition, epitaxial silicon layers of varying thickness will be
produced. For example, a deposition temperature of 550.degree. C. will
provide epitaxial growth at a rate of approximately 5 angstroms/min. As
the deposition temperature increases, the growth rate will also increase.
For example, growth rates can range from 50-150 angstroms/min. for
temperatures of 750.degree.-850.degree. C. Uniformity across any substrate
was found to be within .+-.2.degree. overall and wafer-to-wafer variation
was small, typically within a few percent for all wafers mounted in the
furnace.
The excellent uniformity which is obtained is directly attributable to the
low pressure chemical kinetic limit in which this process operates. At the
low silane pressures used during deposition, silane homogeneous pyrolysis
rates are limited by the availability of other molecules to impact with
for collisional excitation. A consequence is that in this limit chemical
kinetic rate constants become linear in total system pressure. Essentially
no homogeneous pyrolysis of the silicon source gas takes place in the gas
residence time and temperature regime where the process operates. As will
be described in more detail later, this unexpected result is in direct
contrast to the predicted high rate of decomposition of silane in the gas
phase (which is predicted from published literature).
As noted previously, it is difficult to achieve better than one part per
million O.sub.2 or H.sub.2 O impurity levels in a gaseous source at a
point of use, and for this reason operating temperatures in prior systems
have been typically greater than 1025.degree. C. However, in the present
process where very low Si source gas operating pressures (less than about
2 mTorr) are typically used, gaseous sources containing as high as 10
parts per million H.sub.2 O can be used without exceeding the limiting
partial pressure for stable oxide formation on the substrate surfaces. To
emphasize this point, the present system can be compared to conventional
systems not using these very low pressures. For example, if a conventional
hot wall reactor is operated in a conventional mode where it is pumped
from room atmosphere under a H.sub.2 purge at 10 Torr base pressure, the
residual water vapor partial pressure will be essentially equal to 1 ppm
of the system pressure, which is about 10.sup.-5 Torr H.sub.2 O after a
one hour pump down cycle. This exceeds the UHV operational level of
H.sub.2 O used in the present invention by more than 4 orders of
magnitude, and silicon layers grown in such an environment will remain
polycrystalline even above 850.degree. C.
As noted previously, deposition onto multiple wafers can occur with
excellent uniformity (.+-.1-2%) of deposition, both within a wafer
substrate and from wafer to wafer. This result is remarkable, as it allows
batch processing in direct contrast to the results indicated by the
published literature. As an example, a model published by H. H. Lee in J.
Cryst. Growth 69, 82 (1984) indicates that as much as 98% of the silane in
a CVD reactor will be converted by homogeneous (gas phase) reactions to
the reactive radical silylene (SiH.sub.2) by the following reaction
SiH.sub.4 .fwdarw.SiH.sub.2 +H.sub.2. (1)
Either rapid depletion of silane or its homogeneous pyrolysis to form a
highly reactive intermediate such as silylene would lead to severe film
inhomogeneity, and would completely preclude batch processing so that
uniform deposition onto multiple substrates would not occur. However,
neither effect occurs in the present invention where gas phase pyrolysis
of the silicon source gas is prevented and batch processing of up to
hundreds of wafers can be achieved with excellent uniformity of
deposition. This result is also an enormous advance over the previously
reported MBE technique where only a line of sight, single wafer deposition
is possible. (If very small wafers are placed on a flat holder, it is
possible to deposit on up to 4 wafers in sequence in MBE).
In the present invention, no evidence was found of the classical "bullseye
effect" wherein the growth rate for epitaxy at the center of a substrate
is significantly diminished with respect to the growth rate at the edge of
the substrate. In this invention, heterogeneous (surface) modes of silicon
source pyrolysis account for film growth, which is due to the
extraordinarily low processing pressures employed. At these extremes of
pressure, gas phase chemical kinetics cannot be described by simple
thermally activated rate constants. Instead, the rate constants must be
corrected to allow for the low pressure limits used in the present
invention where these rate constants are first order in total system
pressure. These corrections are done using the technique developed by
Rice, Rampsburger, Kassel, and Marcus in the reference P. J. Robinson and
K. A. Holbrook, Unimolecular Reactions, published by Wiley-Interscience,
New York, 1972. When the proper correction is applied for the low pressure
operating regime of the present invention, the results indicate a
reduction in rate constants by more than 2 orders of magnitude from the
values described in the literature.
FIG. 2 illustrates the difference in results when the proper correction is
applied to the thermally activated rate constants that apply to a
particular chemical vapor deposition process. In more detail, FIG. 2 plots
the chemical kinetic rate constant K.sub.1 (sec.sup.-1) as a function of
system pressure for the pyrolysis of silane at 800.degree. C. The top
curve indicates the uncorrected data based on the original silane
homogeneous pyrolysis data published by J. H. Purnell and R. Walsh in the
Proceedings of the Royal Society, 293, 543 (1966). The bottom curve in
FIG. 2 contains the corrected data using the aforementioned techniques
(RRKM) to allow for the low pressure limit where the rate constants become
first order in total system pressure. The proper corrections are made for
the pressures utilized in the present invention, the corrections being
described by B. S. Meyerson et al in the Proceeding of the Symposium on
Low Temperature Processing for VLSI, Oct. 14-18, 1985 (Electrochemical
Society, Pennington, N.J. 1985). When the correction is made, the results
show a reduction in rate constants by more than 2 orders of magnitude. As
a consequence, less than 1 part per thousand conversion of silane to
silylene will occur by the homogeneous pathway. This is consistent with
laboratory film growth observations and is a fundamentional reason for the
success of this UHV/CVD technique.
As the epitaxial process temperature is decreased, the growth of an
interfacial oxide prior to deposition as well as the incorporation of
contaminants into the growing film are favored. As is known in the art,
biased plasma bombardment (using the RF coils to produce the plasma) can
be used to enhance low temperature epilayer growth by scouring the initial
growth interface. However, high levels of contamination (carbon, oxygen)
can occur in the deposited layers. In addition, even at the upper extreme
of 800.degree. C., films made by this technique are reported to contain
typically 10.sup.10 defects/cm.sup.2, a number vastly greater than the
technically useful standard of .ltoreq.500/cm.sup.2. Although enhancement
techniques which optimize the initial growth interface do aid in achieving
low temperature epilayers, they directly preclude the use of high density
wafer packing, and it is still mandatory to adhere to the stringent vacuum
criteria described hereinabove (initial pressures less than or equal to
10.sup.-8 Torr), if epilayers of high purity and high crystallographic
quality are to be achieved at the deposition temperatures used in the
present process.
It is known in the art to utilize in-situ HCl etching of the substrates
prior to deposition. However, the surface obtained upon HCl etching at low
temperatures quickly becomes faceted, with feature size and number density
growing with time. Along with this difficulty, higher chlorosilanes are
formed as a consequence of the etching chemistry, rapidly degrading the
system vacuum quality. The appearance of facets on the silicon surface
during HCl etching have been associated in the literature with the
presence of oxygen and water contamination in the gaseous source. Although
such problems may be resolved or reduced by the point-of-use purification
of the HCl source, the generation of vacuum incompatible byproducts of the
etching reaction will not be resolved. Thus, HCl etching can be
eliminated, and reliance placed solely on the reduction of oxygen and
water background levels in the furnace environment (i.e., the use of very
low base pressures) in order to obtain adequately clean silicon substrate
surfaces for epitaxial deposition.
As noted previously, the silicon source can be silane, higher order silanes
such as disilane (Si.sub.2 H.sub.6, as well as other gaseous sources of
silicon such as Si.sub.2 Cl.sub.6, SiCl.sub.4, SiH.sub.2 Cl.sub.2, etc.).
Whereas prior reactors would not work properly with certain chemistries,
the present technique and apparatus are not so limited, and can be used
with any gaseous silicon source. In the examples to follow, particularly
at the lower end of the temperature range, silane is the preferred source
for silicon epitaxy onto a plurality of substrates. The use of chlorine
based sources is less desirable, leading to the production of higher
chlorisilanes which adversely interfaces with the desired pressure level
in the reactor, and leads to chlorine contamination.
EXAMPLE 1
Epitaxial silicon layers were produced upon silicon substrates using the
apparatus schematically shown in FIG. 1. Prior to its initial use, the
entire UHV section of the apparatus is subjected to a thorough bakeout as
well as to hydrogen plasma scouring, until an ultimate vacuum in the
10.sup.-9 Torr range was achieved. In-situ mass spectrometry showed
hydrogen to be in the greatest abundance, with water vapor and oxygen
present at less than 10.sup.-10 Torr partial pressures. No trace of carbon
or parent hydrocarbon species was detected.
The substrates were 3.25 inch diameter Si[100], boron doped to 10.sup.16
cm.sup.-3. These substrates were employed to investigate epitaxial film
morphology, while several depositions were performed upon 10.sup.19
cm.sup.-3 B-doped wafers to investigate autodoping. All wafers were
subjected to a 45 second, 1.0% HF dip, and then mounted in a 35-wafer
carrier placed coaxially with the tube centerline. The wafer carrier was
subjected to a 30 minute bake at 100.degree. C. in the load chamber prior
to its transfer into the UHV section. A five minute purge with 600 sccm
hydrogen was then performed, followed by the deposition cycle. Typical
growth conditions were T.gtoreq.550.ltoreq.T.ltoreq.750.degree. C.
P=1.0-2.0 mTorr, with a gas source of 10 sccm SiH.sub.4. Deposition was
carried out in 30 minute cycles, forming 3000 angstrom epitaxial layers
for T=750.degree. C. Processing was performed using only the turbopump
system to avoid hydrocarbon backstreaming.
Rutherford backscattering (RBS) channeling studies showed spectra obtained
from the epitaxial layers produced here to be identical to those observed
from a bulk crystal, with the channeled backscattering intensity found to
be 4.0% that of a random spectra. Plane view transmission electron
microscopy (TEM) was employed to yield quantitative data for crystalline
perfection, and showed essentially defect-free (.ltoreq.10.sup.3
defects/cm.sup.2) material for T.gtoreq.550.degree. C. High contrast
optical microscopy revealed a native defect density of the order 10.sup.3
cm.sup.-2 in as-prepared 800.degree. C. material. The defects are
pinholes, extending to the substrate, in which no material deposition had
occurred. The density of these pits in the layers ws independent of
deposition temperature; however, the use of dust-free sample preparation
stations and transfer apparatus resulted in an order of magnitude
reduction in their density. The nucleation mechanism of these defects is
being investigated but they are obviously related to operation in a
standard, non-cleanroom laboratory environment. Secco etching failed to
reveal any hidden crystallographic defects in these layers.
A typical secondary ion mass spectrometry (SIMS) result for an 800.degree.
C. run showed both carbon and oxygen in the film at the background levels
of the SIMS instrument. The extent of dopant outdiffusion from the p.sup.+
substrate during a full 120 min, 800.degree. C. deposition cycle is
minimal and this is a worse case. The dopant level falls below 10.sup.15
cm.sup.-3 B in the first 1000 angstroms of the epitaxial layer. Spreading
resistance measurements verify this, and active carrier levels of only
10.sup.14 cm.sup.-3 are found in the undoped epilayers.
EXAMPLE 2
The system of FIG. 1 was subjected to the same pre-deposition steps
described with respect to example 1. Addi | | |
