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Description  |
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DESCRIPTION
1. Field of the Invention
This invention relates to low temperature, low pressure chemical vapor
deposition techniques for the formation of silicon rich tungsten silicide
using as a source of silicon higher order silanes, such as disilane
(Si.sub.2 H.sub.6) and trisilane (Si.sub.3 H.sub.8).
2. Background Art
In the microelectronics industry, refractory metal silicide films are
becomming increasingly more prevalent. For example, in MOSFET technology,
these metal silicide films are used to reduce the polysilicon gate
electrode resistance in the silicon gate technology by depositing the
films over the underlying polysilicon layer. These metal silicide films
possess high conductivity, the ability to withstand high processing
temperatures, oxidizability for passivation, ease of patterning by dry
etching techniques, compatibility with processing chemicals, and adequate
adhesion and surface morphology. In particular, tungsten silicide is a
very desirable material for use as a conducting layer in devices due to
its low resistivity as well as its processing capability. This material
has been used as a contact material as well as an interconnect material
between devices on a circuit, and might in the future find a use as a
metallic emitter.
The fabrication of refractory metal silicides has been developed over the
years using many different processes. For example, the metal and silicon
can be coevaporated at proper evaporation rates and subsequently subjected
to high temperature annealing to form the metal silicides. There are
difficulties with this technique, however, such as the lack of adhesion
which often results between the overlying metal silicide layer and the
underlying silicon layer. Some of these adhesion difficulties are caused
by the lack of composition control during coevaporation. Further, this
technique does not have adequate throughout capabilities for use in
manufacturing lines.
Another technique for providing for metal silicide films is that shown in
U.S. Pat. No. 4,218,291. In this reference, a transition metal target is
sputtered to yield metal atoms while a silicon hydride, such as silane or
disilane, is also present as a gas. The silicon hydride is decomposed by a
plasma produced during sputtering in an inert gas atmosphere. The
sputtered metal atoms react with the gaseous reactive species to produce
the metal silicide in the gaseous state, which then deposits on the
substrate. A subsequent heat treatment in a nonoxidizing atmosphere is
used to reduce the resistivity of the metal silicide.
Plasma driven processes often lead to contaminated films, since whatever is
in the plasma is often incorporated into the deposited film. Further,
large amounts of hydrogen are often introduced into the silicide films,
and pinholes can also be produced in these films. For these reasons, there
is a tendency to try to avoid plasma processes when depositing silicon or
metal silicides. This is particularly true when contacts are to be made to
silicon surfaces by metal silicides, as the plasma can damage the silicon
prior to formation of the metal silicides thereon.
Another technique used to form metal silicide films is low pressure
chemical vapor deposition (LPCVD) as described in the following two
references:
D. L. Brors et al "Solid State Technology", BP. 183-186, April 1983
K. C. Saraswat et al, IEEE Transactions on Electron Devices, Vol. ED-30,
No. 11, pp. 1497-1505, November 1983.
In particular, tungsten silicide has been made by this technique, in which
the source gases are silane and tungsten hexafluoride (WF.sub.6). A
commonly used apparatus for this process is the Genus Tool, provided by
the Genus Corporation. This is essentially a cold wall/hot susceptor
reactor in which these species are pyrolyzed under LPCVD conditions P=200
mTorr, and T=425.degree. C. The gas flows used in this apparatus are about
20 sccm (Standard Cubic Centimeters, a known mass quantity at known
temperatures and pressures) WF.sub.6 and 1000 sccm SiH.sub.4. The
stoichiometry of the as-deposited tungsten silicide is required to be in
the range of WSi.sub.2.2-2.5, Si rich compared to the desired WSi.sub.2
phase that is achieved after annealing of the as-deposited film. This is
required to avoid the formation of voids and cracks when the silicide is
annealed, as well as to avoid delamination of the layer.
The incorporation of silicon from the silane source is highly inefficient
as is apparent from the very high flow rate required for silane in this
process. These high flow rates lead to significant hazards associated with
the process, since silane can be explosive and dangerous. Further,
disposing of large volumes of silane can create a major safety hazard,
since conventional scrubbers that neutralize WF.sub.6 do not always
dissociate the silane gas. This effluent, if neither adequately diluted or
reacted to products, will burn or explode on contact with air. Further,
inefficient source utilization results in high costs due to large silane
consumption.
From a processing point of view there are other problems associated with
this CVD process. Ideally, in a two component CVD process, it is desirable
to adjust the stoichiometry of the resulting films by adjusting the ratios
of the input gases. However, this process operates in a temperature regime
where the silane gas is quite stable, and does not decompose to form a
film on its own. The presence of WF.sub.6 is required for silane
decomposition to occur. Thus, although film growth rates vary linearly in
WF.sub.6 gas input, they are essentially independent of SiH.sub.4. This
makes it difficult to adjust the film stoichiometry, since a change in
WF.sub.6 input alters both the silicon and tungsten growth rate, while a
change in silane input has little or no effect.
In order to overcome these problems, it has been discovered that higher
order silanes, such as disilanes and trisilanes, can be used in low
pressure CVD processes in a manner to provide films without defect
contamination and without the attendant safety problems. Less stringent
requirements are placed on the pumps used in the apparatus and less
contaminants are introduced into the deposited films. Further, the
possibility of gas phase nucleation is reduced.
The use of higher order silanes has not been taught or suggested by the
prior art for the formation of metal silicides, and specifically for the
formation of tungsten silicide in a thermally driven CVD apparatus. It is
acknowledged that, however, in other plasma driven or laser driven
processes, both silane and disilane have been used. For example, reference
is made to U.S. Pat. Nos. 4,363,828 and 4,495,218. In these references,
metal silicides are not formed, it being the intent of these references to
form amorphous silicon or insulating films. When plasmas or laser light
are used to dissociate source gases, the large amount of available power
enables one to use different types of source gases. For example, silane or
disilane can be used, since the energy inputs in plasma or laser driven
systems are very high, being several orders of magnitude (approximately
200.times.) larger than those found in thermal CVD systems. Thus, in
plasma or laser driven systems, the input energy is tuned to the source
gas in order to decompose the source gas. In contrast with this, however,
the energy available from thermal CVD systems is so small that the choice
of source gas is not as extensive. As an example, U.S. Pat. Nos. 4,283,439
and 4,359,490 describe the formation of metal silicide films using only
silane gas CVD processes.
In the temperature and pressure ranges used in the present invention,
disilane will not decompose by itself to grow a tungsten silicide film.
Some type of cooperative phenomenon is present wherein the presence of
WF.sub.6 causes a cooperative interaction between WF.sub.6 and disilane at
the gas/substrate interface in order to produce the metal silicide film.
Thus, absent this knowledge there is no reason to be led to use a higher
order silane.
Still further, disilane would normally be considered to nucleate more
rapidly in the gas phase than would silane. The presence of gas phase
nucleation in low pressure CVD processes is harmful, resulting in the
production of pinholes and defects in the deposited film. However, the
applicants have discovered that the likelihood of gas phase nucleation is
greater when silane is used than when disilane is used, probably because
the significantly reduced gas flows for disilane mean that significantly
reduced quantities of disilane are required in comparison with silane.
Thus, although one would be lead away from using disilane or another
higher order silane in this process, applicants have discovered that in
fact the likelihood of gas phase nucleation is reduced when disilane or
trisilane is used in place of silane.
Since the disilane flow rates can be so significantly reduced with respect
to the silane flow rates, while still providing silicon-rich as-deposited
films, the complexity of the processing equipment is significantly
reduced. Further, the likelihood of carrying contaminants in a very large
gas flow is also reduced, and the tungsten silicide films deposited by
this technique are superior to those deposited when silane is used as a
silicon source. This also is a safer process, since the amount of disilane
that is present is so significantly reduced. The cooperative effect
between WF.sub.6 and the higher order silane appears to be enhanced,
leading to silicon-rich tungsten silicide films without the necessity for
very high disilane gas flows.
Accordingly, it is a primary object of this invention to provide an
improved LPCVD process for producing tungsten silicide films.
It is another object of this invention to provide LPCVD processes for
producing tungsten silicide films at low pressures and temperatures using
higher order silanes.
It is another object of this invention to provide an improved process for
forming tungsten silicide, which process has increased safety.
It is another object of this invention to provide a low pressure CVD
process for producing tungsten silicide films in which the film quality is
superior to that used by previously known techniques.
BRIEF SUMMARY OF THE INVENTION
In the practice of this invention, a low pressure thermally and chemically
driven CVD process is used to form either tungsten silicide, using higher
order silanes WF.sub.6 and source gases. The as-deposited tungsten
silicide films are silicon-rich, the stoichiometry being in a range of W
Si.sub.2.2-2.5.
The reactor system is a low pressure, low temperature, cold wall system in
which the source gases (disilane or trisilane and tungsten hexafluoride)
are introduced into a chamber. These gases react in the chamber and
deposit a uniformly thick tungsten silicide film on the substrate. The
source gases are injected toward the susceptor at ambient temperature. Due
to the low total pressures, little or no heating of the gas sources occurs
prior to impact at the substrate on the heated susceptor. The
heterogeneous (gas/susceptor interface) reactions lead to film formation.
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 diagram of an apparatus suitable for carrying out the
present invention.
FIG. 2 is a plot of growth rate and unannealed film resistivity versus
disilane flow rate for the deposition of tungsten silicide films by the
present invention.
FIG. 3 is a plot of growth rate and unannealed film resistivity versus
WF.sub.6 flow rate for the deposition of tungsten silicide films.
FIG. 4 is a plot of growth rate and unannealed film resistivity versus
substrate temperature for the deposition of tungsten silicide films.
FIG. 5 is a plot of comparative stoichiometry data (Si/W ratio) for
tungsten silicide films produced from silane (dataset A) and from disilane
(dataset B).
FIG. 6 is a plot of film stoichiometry (Si/W ratio) as a function of silane
flow for tungsten silicide films deposited by a CVD process.
FIG. 7 is a plot of annealed film resistivity versus film stoichiometry
(Si/W ratio) for tungsten silicide films deposited using silane (dataset
A) and disilane (dataset B).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the process of this invention, low pressure CVD is used to deposit
tungsten silicide films which are silicon-rich in an as-deposited state.
As a source gas of silicon, a higher order silane is used, such as
disilane or trisilane.
In this invention, a cold wall/hot susceptor reactor system is used, where
the source of W is WF.sub.6. The silicon source is generally either
disilane (Si.sub.2 H.sub.6)ortrisilane(Si.sub.3 H.sub.8. Even higher order
silanes can be used but the advantage over disilane and trisilane is
minimal. The total pressure in the system is in the range of about 0.05-1
Torr, while the susceptor temperature is in the range of approximately
room temperature to less than 400.degree. C. A preferred susceptor
temperature is 200.degree.-300.degree. C. and a preferred pressure range
is 50-500 mTorr. The flow rate of WF.sub.6 is less than about 25 sccm,
while the flow rate of the higher order silane is generally less than
about 500 sccm. WF.sub.6 flow rates of about 5-15 sccm are preferred,
while the preferred flow rate for Si.sub.2 H.sub.6 is 100-200 sccm. The
preferred flow rate of Si.sub.3 H.sub.8 is 50-150 sccm.
An apparatus for carrying out the present invention is shown in FIG. 1.
This is a schematic of the apparatus sold by Genus Inc. as the Genus 8301
System. It is a low pressure, low temperature, cold wall, chemical vapor
deposition system particularly designed for the deposition of tungsten
silicide films on substrates such as silicon or doped silicon wafers. This
system consists of two cabinets, a process cabinet schematically shown in
FIG. 1, and an electronics cabinet, (not shown), and an RF generator and
remotely located vane pump/oil filtration assembly.
Referring in more detail now to FIG. 1, the process cabinet 10 consists of
the process chamber 12, a pumping system, a gas distribution system, a
substrate/turret heater assembly, an automatic wafer handling assembly, a
differential seal pump, blower, and control electronics. Wafers 14 are
loaded in the process chamber 12 by an automatically operated arm. The
wafers 14 are mounted on the turret 16 which rotates at one revolution per
minute. Turret 16 is heated by quartz lamps, there being an infrared
sensor assembly 18 for sensing the turret temperature. The
gases--disilane, trisilane, etc. and tungsten hexafluoride WF.sub.6, react
in process chamber 12 and deposit uniform tungsten silicide films on the
wafers 14. This system can achieve a base pressure of less than 10 mT with
the help of a blower 20 and a vane pump 22. Blower 20 and pump 22 are
connected between the exhause port 24 and the process chamber 12, being
connected to the funnel valve 26 and a high vacuum valve 28.
A differential seal pump maintains the vacuum integrity between the
atmosphere and the inner chamber pressure. Water cooling is used to keep
the chamber walls cold. The cold walls prevent gas phase nucleation and
deposition on the walls, and thus allow the thermally driven deposition to
take place on only the surface of the wafers 14. A RF generator (not
shown) is used to clean the process chamber 12, using a NF.sub.3 plasma.
The system can be automatically or manually operated using the controls of
the aforementioned cabinet. The control module of the cabinet consists of
a touch panel CRT and a floppy disk drive, and communicates with the user
via menu-driven displays. Software programs containing the process
information such as temperature, pressure, and gas flows are the process
recipes provided by the manufacturer. These recipes are stored on a disk.
In the automode, the system loads the wafer, processes and unloads without
any break in the operation while, in the manual mode, loading, process and
unload operations can be individually carried out.
In the following discussion, the gaseous silicon source is disilane
(Si.sub.2 H.sub.6), while the metal source is tungsten hexaflouride
(WF.sub.6). These gases form a stable mixture at ambient temperature and
no spontaneous reaction occurs. The possible hetergeneous (gas/susceptor
interface) reactions that lead to tungsten silicide film formation are as
follows:
WF.sub.6 (g)+*.fwdarw.W(a)+6F(a) [1]
Si.sub.2 H.sub.6 (g)+*.fwdarw.Si.sub.2 H.sub.6 (a).fwdarw.Si.sub.x H.sub.y
(a)+Si.sub.2-x (a)+(6-y)H(a) [2]
Si(a)+4F(a).fwdarw.SiF.sub.4 (g) [3]
H(a)+F(a).fwdarw.HF(g) [4]
The asterisk (*) used in these equations refers to active adsorption sites
where gases can stick to the surface of the wafers 14. The designation (a)
refers to adsorbed species while the designation (g) refers to a gaseous
species. Reaction 1 has been observed by Auger (AES) and Photoemission
Spectroscopy (PES), where WF.sub.6 was seen in UHV to adsorb on the bare
Si (100) wafer, and then transfer its fluorine onto the silicon surface
after this initial step. The necessity for the presence of active (bare)
adsorption sites has been established, as the WF.sub.6 adsorption process
is self passivating when full surface coverage by W and F is attained.
Beyond this initial step in the growth process, the remainder of the
mechanism is somewhat speculative, as both the kinetic rate data and the
details of silane surface decomposition pathways are known. Thus, several
different explanations may be invoked to explain the enhanced
incorporation of silicon from disilane in the present process.
From the frame work of the reactions (1)-(4), it is expected that enhanced
Si incorporation will result if the adsorption/dissociation reaction (2)
for the silicon bearing species is enhanced for the case of disilane
versus silane. Alternatively, the unspecified fragments produced upon the
disassociative adsorption of disilane on the hot substrate may more
efficiently remove flourine and hydrogen from the surface, enhancing the
availability of free adsorption site for further film formation to take
place. Arriving at a unique model as to the Si incorporation enhancement
mechanism requires a more detailed investigation based upon a study of the
surface reaction kinetics of this complex system. Apart from considering
heterogeneous chemistry, homogeneous (gas phase) chemistry can participate
in this process, dependent upon the exact nature of the species ejected
from the reaction surface. Prediction of exactly what rate controlling
silicon incorporation step has been directly enhanced by the use of
disilane was not fully determined.
As will be seen further with reference to FIGS. 2-5, even at the lowest
disilane flows that could be maintained, the resulting tungsten silicide
films were still silicon-rich. When it was attempted to lower disilane
flows below 200 sccm, a significant reduction in film deposition rates
occurred. In order to further reduce disilane flows and still maintain
adequate growth rates, the injection of hydrogen can be employed. This
will enhance the tungsten component of the film content by directly
enhancing deposition of tungsten via the hydrogen reduction of the
WF.sub.6 source. As this growth chemistry is a cooperative phenomena,
where no deposition is seen if either one of the reactants is absent,
accelerating the pyrolysis of WF.sub.6 by the addition of hydrogen may in
fact enhance disilane pyrolysis as well.
Another advantage of the use of disilane is its lower vapor pressure
relative to silane, approximately 30 psig. at ambient temperature, which
means that the mass quantity and tank pressure for the storage of disilane
are not related. Also, the low delivery pressure of disilane, though more
than adequate for use with standard flow control devices, is sufficiently
low that special high pressure valving is not required, even in large gram
quantity installations. The order ot magnitude reduction of gram quantity
requirements for the present process using disilane rather than silane
will considerably ease the hazards associated with disilane. Further,
films have been grown which are rich in silicon even at minimum disilane
flows.
FIGS. 2-7 relate to the deposition of tungsten silicide films and in
particular show the effects of varying parameters such as disilane flow
rate, WF.sub.6 flow rate, substrate temperature, deposition temperature,
silane flow rate, and the Si/W ratio. These parameters have been varied to
study the growth rate, the as-deposited resistivity, the Si/W ratio, and
the annealed resistivity of tungsten silicide films. From this data, it is
apparent that the deposition temperature and WF.sub.6 flow are the
critical parameters in determining film properties, while other factors
weigh far less heavily. To evaluate the quality of disilane prepared
samples, based on the data in some of these figures, a standard set of
growth conditions was selected, which will be indicated for each of these
figures.
In FIG. 2, the growth rate and resistivity of as-deposited samples of
tungsten silicide are plotted against the disilane flow rate. The standard
conditions are P=200 mTorr, WF.sub.6 flow rate=10 sccm, and T=360.degree.
C. While there is some change in growth rate and unannealed resistivity,
the effect of disilane flow rate is not greatly significant.
In FIG. 3, the growth rate and unannealed resistivity of tungsten silicide
films are plotted against the WF.sub.6 flow rate. As is apparent, this
flow rate has a more significant effect on the growth rate and the
as-deposited film resistivity. As the WF.sub.6 flow rate increases, the
growth rate increases while the resistivity decreases. In this data, the
disilane flow rate was 210 sccm., while the pressure and substrate
temperature were 200 mTorr and 300.degree. C., respectively.
FIG. 4 plots the growth rate and unannealed resistivity of tungsten
silicide films versus substrate temperature, for P=200 mTorr, disilane
flow rate=210 sccm., and WF.sub.6 flow rate=10 sccm. As is apparent, as
the substrate temperature increases, the growth rate increases as does the
as-deposited resistivity. As mentioned previously, the substrate
temperature is a more critical parameter affecting growth rate and film
properties.
FIG. 5 plots the Si/W ratio as determined by Rutherford backscattering
(RBS) as a function of deposition temperature for both silane and
disilane. Dataset A is for films produced using silane where the SiH.sub.4
flow rate is 1000 sccm., while dataset B is for films produced using
disilane flowing at a rate of 210 sccm. Although silane flows in this
experiment exceeded disilane flows by a factor of approximately 5,
disilane process samples were consistently silicon-rich compared to silane
process samples. If a common substrate temperature 360.degree. C. is
chosen, using the data for W/Si stoichiometry versus silane flow as shown
in FIG. 6, in excess of 2.0 liters/minute silane flow would be required to
achieve the as-deposited silicon content (Si/W=2.95) that is reached using
only 0.21 liters/min. disilane.
FIG. 6 is a plot of Si/W ratio as determined by RBS, as a function of
disilane flow rate for conditions P=200 mTorr, WF.sub.6 flow rate=10
sccm., and the substrate temperature=360.degree. C. As is apparent, very
high flow rates of silane are required to begin to increase the Si/W ratio
by significant amounts.
FIG. 7 plots the tungsten silicide resistivity of annealed samples as a
function of film stoichiometry (Si/W ratio) as determined by RBS. Samples
were prepared using both silane (dataset A) and disilane (dataset B). When
plotted on the same axis, films of like stoichiometry, whether prepared
from silane or disilane, show essentially the same resistivities. Thus,
the source of silicon content can be changed, and only the final film
stoichiometry determines film properties. Upon examination by
cross-sectional transmission electron microscopy (TEM) both sets of films
were found to be conformal, with vertical side wall coverage 80% that of
the base when deposited over a 3000 angstrom high oxide step. X-ray data
from the annealed disilane produced film showed there to be the expected
WSi.sub.2 phase upon annealing, while prior to annealing no x-ray
structure was observed. Resistivity mapping of the samples showed a
typical sample uniformity (3.sigma.) of better than .+-.2% within the
sample.
Thus, disilane can be used as a source of silicon in a low pressure, low
temperature CVD process for depositing tungsten silicide. Films prepared
by this technique have properties comparable to those prepared from a
silane source, but with an order of magnitude reduction in silicon source
flow requirements in order to achieve films of comparable silicon content.
Additionally, the present process produces films of the correct
stoichiometry, resistivity, uniformity and grain structure at temperatures
of only 250.degree. C., well below the growth temperature required by
conventional silane-based processing where the temperature is greater than
370.degree. C. Further silicon source flow reduction may be possible by
the use of hydrogen injection, as mentioned previously.
What has been described is a new process for producing tungsten silicide in
a silicon-rich as-deposited state. This process is a low temperature, low
pressure CVD process in which the source of silicon is disilane,
trisilane, or an even higher order silane. The flow rates of these higher
order silanes are significantly less than the flow rates of silane used in
prior CVD processes. There is a cooperative phenomenon that occurs between
the metal source gas and the higher order silane at these temperatures and
pressures since, without both gas sources being present, no deposition
will occur on the substrate in this cold wall, hot substrate apparatus. As
another feature, the present process produces films of the correct
stoichiometry, resistivity, uniformity, and grain structure at
temperatures of only about 250.degree. C., which is well below the growth
temperature required by conventional silane-based processing where the
substrate temperature is greater than about 370.degree. C.
Although the invention has been described with respect to specific
embodiments thereof, it will be apparent to those of skill in the art that
variations may be made therein without departing from the spirit and scope
of the present invention. However, the specific ranges of temperature and
pressure are restricted to the ranges cited herein, while the higher order
silane flow and metal source gas flow can be varied somewhat.
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