Low temperature, high pressure silicon deposition method

A method of producing amorphous silicon layers on a substrate by chemical vapor deposition at elevated pressures of at least about 25 Torr whereby deposition occurs at practicable rates. A substrate is loaded in a vacuum chamber, the temperature adjusted to obtain an amorphous silicon deposit of predetermined microcrystalline density, and the silicon precursor gases fed to the chamber to a preselected high pressure. Doped amorphous silicon films also can be deposited at high deposition rates. The above amorphous silicon films have a low density of nucleation sites; thus when the films are annealed, polycrystalline films having large crystal grains are produced.

This invention relates to a process for depositing undoped or doped silicon 
at high growth rates. More particularly, this invention relates to a 
process for depositing doped or undoped silicon in a single wafer chamber 
at practicable deposition rates. 
BACKGROUND OF THE INVENTION 
Conventional prior art doped or undoped silicon deposition has been carried 
out in accordance with a low pressure chemical vapor deposition process 
(LPCVD). A silicon precursor gas, such as silane, disilane, silicon 
tetrachloride and the like, which can also include a dopant gas such as 
phosphine, diborane or arsine, is fed to a chamber containing a substrate 
on which the silicon layer is to be deposited. The substrate is heated to 
deposition temperature and the gases fed to the chamber where they are 
decomposed, whereupon silicon deposits on the surface of the substrate. 
These systems are typically run at pressures of about 200 to 400 millitorr; 
thus the low pressure designation. However, at these pressures the silicon 
deposition rate is very low, on the order of about 100 angstroms per 
minute for undoped silicon and only about 30-50 angstroms per minute for 
doped silicon. The prior art processes have compensated for the low 
deposition rates by loading a plurality, e.g., up to about 100, of 
substrates at a time in a chamber to be processed. 
In the prior art LPCVD system illustrated in FIG. 1, a chamber 10 includes 
a boat 11 carrying a plurality of silicon wafers 12. A gas feed from a gas 
source 13 is controlled by a flow controller 14 and enters the chamber 10 
from gas inlet port 15. The gas feed is maintained across the wafers 12 in 
the direction of the arrows. The low pressure in the chamber 10 is 
maintained by an exhaust system 16. Because the concentration of the feed 
gases can decrease as they flow toward the exhaust system 16, the chamber 
also includes three separately controlled heater elements 17 that provide 
temperature variations in the chamber 10 to compensate for the variation 
of concentrations of reactant gases within the chamber 10. 
FIG. 2 illustrates another prior art LPCVD batch-type silicon deposition 
chamber. In this chamber, a plurality of wafers 21 are stacked vertically 
and the reactant gases are injected through a plurality of holes 23 in a 
gas injector 22. The gas injector 22 is located between two rows of wafers 
21. The low pressure in this chamber again enables sufficient uniformity 
of deposition that the deposition can be performed in a batch type 
process. 
While careful adjustment of the gas pressures and temperatures can deposit 
smooth, uniform silicon films onto a substrate in this manner, the 
disadvantage is that if anything goes wrong during the deposition, e.g., a 
power outage, impurities in the feed gases or the like, a large batch of 
wafers is damaged and rendered useless. 
Further, more modern semiconductor processing equipment employs multiple 
chambers for the multiple process steps of preparing devices onto 
substrates such as silicon, gallium arsenide and the like. Several 
processes are sequentially performed on a single wafer at one time in a 
series of interconnected chambers, all under vacuum. This eliminates the 
need to ramp pressures up and down between process steps which is both 
costly and exposes the substrates to contaminants in the ambient. 
However, since the deposition of silicon in a LPCVD process is slow, the 
time required for depositing a layer of silicon onto one wafer at a time 
is unduly long and adds greatly to the costs of producing devices. 
Further, the silicon deposition step would be a bottleneck in multiple 
stage process equipment. 
Still further, thin film transistors made from amorphous silicon are of 
increasing interest to the semiconductor industry. Amorphous silicon films 
used to make thin film transistors desirably contain a minimum number of 
nucleation sites for grain growth during subsequent annealing steps. This 
is because when making transistors from amorphous silicon, it is desired 
that the deposited silicon film imitate crystalline silicon as much as 
possible; the presence of fewer nucleation sites in the deposited 
amorphous silicon results in fewer and larger crystal grains in the 
annealed silicon films, since the grains grow during annealing until they 
meet an adjacent crystal grain. Thus the fewer the number of nucleation 
sites present, the larger the grains become during subsequent annealing. 
When making transistors in such thin films, the larger the grain size, the 
greater the chance that any one transistor will be built upon a single 
grain or single crystal. 
Thus a process that would improve the throughput of silicon deposition onto 
single substrates in a multiple stage reactor, and reduce the number of 
nucleation sites in amorphous silicon thin films, would be highly 
desirable. 
SUMMARY OF THE INVENTION 
We have found that the rate of deposition of doped and undoped amorphous 
silicon can be greatly increased by increasing the pressure within a 
single substrate CVD chamber. Deposition rates up to about 3000 angstroms 
per minute can be achieved at a gas pressure of over about 25 Torr. 
By proper choice of deposition temperature, the deposited films are 
amorphous silicon layers that can vary in microcrystalline density. 
Amorphous silicon layers can be deposited according to the invention that 
are useful in the manufacture of thin film transistors. By depositing 
silicon at low temperatures, from about 500.degree. C. and up to about 
600.degree. C., and at high pressures of about 25 Torr or higher, 
amorphous silicon films having very few nucleation sites are obtained. 
Nucleation of these films by heating at about 600.degree. C. for extended 
periods produce a polycrystalline silicon film having large crystal 
grains. This is advantageous because, at least locally, large grain size 
resembles a crystalline silicon film.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, a single substrate is loaded into 
a chamber of a multiple chamber reactor, the temperature of the substrate 
is adjusted to the desired deposition temperature, and deposition gases 
are fed to the chamber at the desired high pressure for the desired period 
to deposit amorphous silicon to a predetermined thickness. 
In order to increase the deposition rate of amorphous silicon, the pressure 
in the chamber must be maintained at from at least about 25 Torr, and 
preferably from about 25 to about 150 Torr. Deposition rates of up to 
about 3000 angstroms per minute of silicon can be achieved at these 
pressures, even at comparatively low temperatures. At lower pressures and 
temperatures the deposition rate drops, so these parameters are adjusted 
in accordance with the invention to obtain a commercially practicable rate 
of deposition of amorphous silicon. In single wafer deposition chambers 
for silicon wafers, above about 350 Torr pressure, a significant amount of 
deposition will occur on the walls and other fixtures of the reaction 
chamber and particulates that can contaminate the substrates can also be 
formed, which is undesirable. The unexpectedly large increase in 
deposition rate in accordance with the invention enables single substrate 
processing at time periods that remain competitive with multiple substrate 
batch processing. Since deposits of amorphous silicon for thin film 
transistors require only thin films of 1000 angstroms or less, even if the 
deposition rate is fairly low, the time required for the deposition 
remains satisfactory. 
This process is economical to deposit amorphous silicon layers. Amorphous 
silicon thin films can be deposited using silane, disilane, silicon 
tetrachloride, silicon trichloride, silicon dichloride and the like as the 
silicon precursur gas, generally together with a carrier gas such as 
nitrogen or hydrogen. The exact crystallographic nature of the silicon 
deposited depends upon the temperature of deposition. For example, at low 
deposition temperatures of about 500.degree.-630.degree. C., the silicon 
deposited is mostly amorphous. At somewhat higher temperatures of about 
630.degree.-670.degree. C. a mixture of amorphous and polysilicon will be 
obtained. At still higher temperatures of about 670.degree.-700.degree. C. 
the deposited silicon will be polycrystalline silicon. 
The present process will be further described by reference to FIG. 3. FIG. 
3 illustrates a single substrate reactor 31 in which either doped or 
undoped silicon layers can be deposited at commercially attractive rates. 
This reactor has a top wall 32, side walls 33 and a bottom wall 34 that 
define a reaction chamber 30 into which a single substrate 35, such as a 
wafer, can be loaded. The substrate 35 is mounted on a pedestal 36 that is 
rotated by a motor 37 to provide a time averaged environment for the 
substrate that is cylindrically symmetric. The substrate 35 is heated by 
light from high intensity lamps 38 and 39. The top wall 32 and the bottom 
wall 34 should be substantially transparent to light to enable the light 
from the lamps 38 and 39 to enter the chamber 30. Quartz is a particularly 
useful choice for the top and bottom walls 32 and 34 because it is 
transparent to light at visible and uv frequencies, it is a relatively 
high strength material that can support a large pressure difference across 
these walls, and it has a low rate of outgassing. 
Reactant gases flow from a gas input port 310 and across the wafer 35 to an 
exhaust port 311. The gas input port 310 is connected to a gas manifold 
that provides one or a mixture of gases to enter via a plurality of pipes 
into this slot. The locations of the input ends of these pipes, the gas 
concentrations and/or flow rate through each of these pipes are selected 
to produce reactant gas flow and concentration profiles that optimize 
processing uniformity. Although the rotation of the substrate and thermal 
gradients caused by the heat from lamps 38 and 39 can significantly affect 
the flow profile of gases in the reaction chamber 30, the dominant shape 
of the flow profile is laminar flow from the gas input port and across the 
substrate to the exhaust port 311. 
In a typical process producing a silicon layer on a silicon wafer, a 
pressure of 80 Torr in a vacuum chamber was maintained by feeding hydrogen 
at about 10 liters per minute into the chamber and adding about 525 sccm 
of silane after the temperature of the wafer reached 650.degree. C. Under 
these conditions a mixture of about 50:50 polycrystalline and amorphous 
silicon was deposited at a rate of 2000 angstroms per minute. 
At a higher wafer temperature of about 690.degree. C. using about 250 sccm 
of silane, the deposited silicon was polycrystalline silicon. 
Phosphorus doped polycrystalline silicon was deposited onto a wafer in the 
chamber of FIG. 3 by feeding a mixture of 525 sccm of silane and 300 sccm 
of 1% phosphine in hydrogen at a temperature of 650.degree. C. The 
resultant silicon layer contained about 1.5.times.10.sup.21 cm.sup.-3 of 
phosphorus and was deposited at a rate of about 1500 angstroms per minute. 
Low temperature deposition of amorphous silicon at practicable deposition 
rates can also be obtained by the present high pressure deposition 
process. 
FIG. 4 is a graph illustrating deposition rates at pressures of both 100 
Torr and 600 Torr at varying temperature using a gas flow of silane and 
nitrogen as the carrier gas. This graph confirms that the lower the 
temperature, the lower the deposition rate. However, even at very low 
temperatures of about 520.degree.-560.degree. C., deposition rates of 
several hundred angstroms per minute can be achieved if the pressure is 
high enough (600 Torr). A preferred temperature range is from about 
520.degree.-590.degree. C. 
FIG. 5 is a graph illustrating the change in deposition rate with pressure 
using silane and either hydrogen or nitrogen as the carrier gas at a 
deposition temperature of 560.degree. C. It is apparent that the 
deposition rate increases at increased pressures. The use of nitrogen 
carrier gas instead of hydrogen is also advantageous as far as deposition 
rate is concerned. 
FIG. 6 is a graph illustrating microcrystalline density variation with 
temperature of deposition of amorphous silicon films. There is a maximum 
number of microcrystals at a deposition temperature of about 570.degree. 
C.; thus to deposit an amorphous silicon film having a low 
microcrystalline density, either a lower or higher deposition temperature 
range should be employed. The use of a lower temperature range, such as 
520.degree.-560.degree. C., reduces the microcrystalline density of the 
amorphous silicon films, but the deposition rate is quite low. Deposition 
at a higher temperature range of about 580.degree.-590.degree. C. produces 
an amorphous silicon film with low, but somewhat higher microcrystalline 
density, but a faster deposition rate. In any event, by adjusting the 
temperature and pressure of deposition, the microcrystalline density and 
deposition rate can be chosen to optimize the amorphous silicon films for 
particular end uses. 
The above amorphous silicon films, having a low density of nucleation 
sites, when annealed at crystallization temperatures of about 600.degree. 
C. and higher, produce polycrystalline films having large grain size. 
Doped amorphous silicon layers can also be produced by CVD, but generally 
at lower rates of deposition. Further, the prior art LPCVD deposition 
processes performed in the apparatus of FIGS. 1 and 2 produce films of 
nonuniform thickness. The feed gases are adjusted to admix an appropriate 
amount of dopant gas in addition to the silicon precursor gas. For 
example, small amounts of phosphine can be added to produce 
phosphorus-doped silicon; small amounts of diborane can be added to 
produce boron-doped silicon; and small amounts of arsine can be added to 
produce arsenic-doped silicon. 
Although the invention has been described with reference to particular 
pressures, temperature and reaction chamber type, one skilled in the art 
will recognize that other pressures, temperatures, gas feedstocks and 
deposition chambers can be substituted and are meant to be included 
herein. The invention is only meant to be limited by the appended claims.