Chemical vapor deposition method and apparatus

A chemical vapor deposition apparatus particularly useful for forming uniformly thick epitaxial layers of III-V semiconductor compounds on a plurality of substrates comprises a susceptor for supporting the substrates contained within a housing, gas inlet and outlet ports including a plurality of gas inlet ports spaced around the periphery of the housing and wherein the susceptor is provided with a plurality of helical flights extending from top to bottom which control the gas flow pattern.

TECHNICAL FIELD 
This invention relates to semiconductor fabrication, and more particularly, 
to an improved method and apparatus for chemically vapor depositing 
materials in a chemical vapor deposition reactor. 
BACKGROUND OF THE INVENTION 
Chemical vapor deposition (CVD) is a dominant process in the semiconductor 
industry for growing thin solid films on substrates. Crucial to the 
success of a CVD process is the design of the reactor. A typical reactor 
design consists of a quartz jar and a susceptor. The substrates are placed 
on the susceptor which is maintained at a constant temperature by either 
RF induction or radiant heating. Reactant gases are introduced into a flow 
chamber formed by a quartz jar and the susceptor. The shape of the flow 
chamber is designed to deliver reactants to the wafers efficiently to 
yield uniform deposition. The performance of the reactor is measured in 
terms of growth uniformity, throughput and chemical consumption. 
The demand for improved performance of CVD reactor systems and processes 
has increased rapidly with the advent of very large scale integrated 
circuits which necessitates the growth of films with highly uniform 
thickness and composition to meet the decrease in feature size. Further, 
new devices for fiber optic and high speed digital applications demand new 
capabilities in growing epitaxial layers by CVD of different materials. 
The need for improved CVD reactors and processes is more critical for the 
growth of electronic materials such as the III-V or II-VI semiconductor 
compounds which have been found to be very difficult to deposit uniformly 
using existing reactors developed for CVD of silicon. This difficulty is 
due to the different rate-controlling mechanism of the chemical reactions: 
the growth rate for III-V or II-VI epitaxial layers is generally 
controlled by a mass transfer process (commonly called 
diffusion-controlled process) while the growth rate of silicon epitaxial 
layers is controlled by the reaction kinetics at the surface of the 
silicon wafer substrates. A suitable reactor for a diffusion-controlled 
system is generally more difficult to design since it is strongly 
dependent on the flow geometry which influences the mass transfer process. 
Recent reports indicate that even for the relatively simple epitaxial 
deposition of gallium arsenide, good uniformity is difficult to achieve. 
Consequently, good deposition of such material has been accomplished 
potentially in single wafer reactors in which the chemical flow over the 
one wafer can be fully controlled and thoroughly mixed. However, for 
commercial production one must be able to simultaneously and uniformly 
deposit layers on a multiplicity of substrates. Also, epitaxial deposition 
of ternary or quaternary layers is even more difficult due to the increase 
in the number of chemical reactants and the added parasitic deposition on 
the reactor walls. Hence, improved apparatus design is required to obtain 
better thickness uniformity, with high throughput and processing yields, 
especially in the case of diffusion-controlled CVD systems. 
SUMMARY OF THE INVENTION 
An apparatus for chemical vapor deposition particularly useful for the 
deposition of epitaxial semiconductor layers or films on a semiconductor 
substrate comprises a susceptor confined within a housing to shield it 
from the external environment and forming a deposition chamber, a 
plurality of gas inlets and a gas exhaust orifice, means for heating the 
susceptor and means for affecting rotation of the susceptor, wherein the 
susceptor includes a plurality of helical flights extending from at or 
near the end of the susceptor closest to the inlet port to at or near tne 
end of the susceptor closest to the exhaust orifice.

DETAILED DESCRIPTION 
While the novel reactor set forth herein is particularly useful for 
diffusion-controlled reactions, it should be understood that it is also 
useful for and should even perform better for less critical systems, 
namely, partially or totally kinetic-controlled reactions such as the 
deposition of silicon. Further, the novel reactor is also particularly 
suitable for the simultaneous deposition of a multiplicity of wafers or 
substrates onto which a film is to be deposited. The reactor uses a unique 
flow pattern of gas to deliver reactant species to the substrates so as to 
give rise to the proper cross and back mixing necessary for uniform 
deposition without using high chemical flow rate. It also makes possible a 
larger number of wafers to be deposited per run while maintaining 
uniformity of tnickness of the deposit from wafer to wafer and across a 
wafer as well as a relatively low flow rate. 
Various configurations of typical prior art reactors for chemical vapor 
deposition are shown in FIGS. 1A-1D. As can be seen, these typical prior 
art reactors do not employ helical flights nor other of the preferred 
features of the current invention as hereinafter described. The figures 
show typical gas flow patterns of these reactors. Such flow leads to 
thickness uniformity problems of the deposit. 
A simplified version of an embodiment of the novel apparatus showing the 
concept of the invention is found with reference to FIGS. 2 and 3. In 
accordance with the apparatus as shown in FIG. 2, the reactor 10 comprises 
a housing such as a quartz bell jar 11 having a reactant gas inlet port 12 
at the top of the bell jar 11 and a reactant gas outlet port 13 at the 
bottom of the bell jar 11. Alternatively, of course, one can provide a 
plurality of inlet ports for individual reactant gases which may be 
employed. When the reaction is to be run at low pressures, the reactor 
pressure is pumped down via the outlet port 13. Within the bell jar 11 is 
a susceptor 14, typically graphite, having spaced recesses 15 in which the 
substrates or wafers 16 to be coated are held. The spaced recesses 15 are 
arranged so that a plurality of such recesses 15 is separated by a 
plurality of spaced helical flights 17 extending from an upper portion 18 
of the susceptor 14 above the upper most recess 15 to the lower portion 19 
of the susceptor 14 below the lower most recess, the susceptor is 
preferably cylindrical or conical. Means, e.g., an axial center shaft 20, 
extending from the bottom of the susceptor 14 is provided to rotate the 
susceptor 14 around its vertical axis. The shaft 20 is coupled to a motor 
(not shown) to rotate the susceptor 14 within the bell jar 11. The 
apparatus also includes means for heating the susceptor 21, e.g., an RF 
coil, which surrounds the outside of the bell jar 11 adjacent to the 
susceptor 14. Alternatively, of course, one may use other heating means 
such as an infrared heating source or a resistance heater. It has also 
been found to be preferred to have a plurality of vertically spaced 
reaction gas injection ports 22 extending through and around the side of 
the bell jar 11 so as to enable additional reactant gases to be injected 
in the spaces between the helical flights 17. 
Referring to FIG. 3, the gas flow achieved in the vicinity of the wafer 
using an apparatus as described is shown. As can be seen, the flow pattern 
achieved results in a uniform mixing of the reactant gases which in turn 
results in the deposition of films having essentially uniform thickness 
with only moderate gas flow rates. 
FIG. 4 indicates the importance of the use of reactant gas injection ports 
through the bell jar. The figure is a plot representing reactant gas 
concentration as a function of distance along the helix due to gas 
supplied via the upper inlet port 12 (C.sub.no injection) and gas supplied 
via the injection ports 22 (C.sub.injection). The combination of these 
concentrations (C.sub.total/2) represents the combined concentration of 
reactant gases along the helix. As can be seen C.sub.total/2 is constant. 
It should be noted that the graph is a theoretical representation and has 
not been plotted from actual data. It is provided as a means for better 
understanding the benefit derived from the use of side injection ports 22 
with the helical flights 17. 
It should be understood that a commercial reactor may include other 
features such as a concentric dual bell jar assembly to insure cleanliness 
and help prevent the escape of gas; means for raising and lowering the 
susceptor and/or bell jars for ease of loading; and means, e.g., valves, 
for controlling gas flow rates. As an example of the use of the apparatus, 
typical conditions calculated for the growth of an epitaxial GaAs layer on 
an Si wafer are: reactor pressure, 40-760 torr; wafer (and susceptor) 
temperature, 600.degree. C.-700.degree. C.; total reactant gas flow rate, 
10-20 l/min.; epitaxial growth rate, 2-3 .mu.m/hr.; and thickness 
uniformity, .+-.3%.