Chemical vapor deposition of epitaxial silicon

A single chamber continuous chemical vapor deposition (CVD) reactor is described for depositing continuously on flat substrates, for example, epitaxial layers of semiconductor materials. The single chamber reactor is formed into three separate zones by baffles or tubes carrying chemical source material and a carrier gas in one gas stream and hydrogen gas in the other stream without interaction while the wafers are heated to deposition temperature. Diffusion of the two gas streams on heated wafers effects the epitaxial deposition in the intermediate zone and the wafers are cooled in the final zone by coolant gases. A CVD reactor for batch processing is also described embodying the deposition principles of the continuous reactor.

This invention relates to chemical vapor deposition (CVD) and particularly 
to a continuous chemical vapor deposition process and apparatus. 
BACKGROUND OF THE INVENTION 
CVD reactors in the semiconductor processing field serve to deposit a layer 
of material on the surface of a substrate formed of a material such as 
silicon. One form of such reactors deposit a hetero-or homoepitaxial layer 
of monocrystalline silicon on a wafer of monocrystalline silicon. The 
layer can be provided with a dopant to convert the layer to an N-type 
semiconductor with, for example, phosphorus or a P-type semiconductor with 
boron. A conventional reactor using chemical vapor as the agent to deposit 
the epitaxial layer is termed a chemical vapor deposition (CVD) reactor. 
Several steps are needed to achieve the final deposition layer, each step 
being at a different temperature than the other steps. For this reason 
separate compartments or sections of the reactor are needed to provide the 
different temperature conditions of the several steps of the process. 
Accordingly, seals are used to isolate the chamber from the external 
ambient during each step of the process. 
Many CVD reactor chambers have been proposed using fluid cooled elastomer 
seals to achieve the desired isolation of the chambers. While such seals 
have proved adequate for low or moderate temperature polycrystalline CVD 
deposition of silicon, silicon oxide, or silicon nitride, they have not 
been successful in use for high temperature production of silicon 
epitaxial devices. 
Several continuous vapor deposition reactors have been proposed. For 
example, see U.S. Pat. No. 4,048,955 entitled CONTINUOUS CHEMICAL VAPOR 
DEPOSITION REACTOR BY R. N. Anderson, issued on Sept. 20, 1977. This 
patent describes a reactor having a plurality of chambers that are 
separated from each other along the path of movement of wafers passing 
therethrough by the use of elastomer seals to achieve the desired 
isolation of one chamber section from the other. The quartz reactor 
chambers of this patent are joined to one another by multi-ported fluid 
cooled, flanged junctions sealed with silicon rubber. The junctions 
include directed gas streams that are used to separate the atmosphere in 
the various adjacent CVD deposition chambers. The difficulty with the use 
of this type of reactor is that silicon wafers, for example, must pass 
through the fluid cooled junctions at high temperatures, that is, 
temperatures greater than 1120.degree. C., the minimum temperature 
required for conventional silicon epitaxial growth. The wafers are then 
exposed to the cold gases exiting from the junctions onto the wafers. The 
wafers as they pass through the cold junctions cannot be sustained at the 
required heat by infrared lamps. The operation of the isolation junctions 
require very small passage clearances for the wafers and their carriers, 
such clearances being in the order of 1 mm. Using hydrogen carrier gas 
which is typically used for epitaxial deposition and the practical 
materials used in such reactor construction, it appears that the junctions 
will either be overheated or that the silicon wafers would be under-cooled 
as they pass through each of the isolation junctions. Another disadvantage 
of the system of the Anderson U.S. Pat. No. 4,048,955 is that it requires 
infrared heating. Induction or resistance heating cannot be used without 
interfering with the correct use of the reactor junctions. 
See, also, U.S. Pat. No. 3,672,948 entitled METHOD FOR DIFFUSION LIMITED 
MASS TRANSPORT by R. A. Foehring, et al. issued on June 27, 1972. This 
patent discloses a continuous reactor having a single chamber through 
which a carrier for substrates is passed during the deposition process 
steps. The various gases used during the process are passed into the 
chamber across the movement direction of the wafers. It seems that the gas 
flow across the wafer movement produces inherently non-uniform deposits, 
as seen from the growth rate curves of FIG. 5 of the patent, since the 
material in the gas is depleted across the wafers. Moreover, a system such 
as that of the patent, using the perforated or porous members, can easily 
clog and cause frequent shut-downs. In addition, such porous members, 
being formed of quartz, are very fragile. If such members are formed of 
metal, then the chemical source gas containing chlorine can react with the 
metal and develop contaminants to harm the substrates or wafers being 
processed. 
Prior art continuous chemical vapor deposition reactors are not known to be 
used in production for silicon epitaxial devices. The reason for this, 
seemingly, is that single crystal epitaxial devices grown in such reactors 
cannot tolerate any leakage of the atmosphere, external from the reactor, 
or any out-gassing of the elastomer seals which might result from 
deterioration due to prolonged use at excessive temperatures. Amorphous 
and polycrystalline growth of silicon, silicon oxide and silicon nitride 
can be accomplished in the temperature range of approximately 350.degree. 
to 900.degree. C. In contrast, silicon epitaxial temperatures commonly 
used in production are in the range 1050.degree. to 1250.degree. C. Except 
for very special applications using silane at 960.degree.-1000.degree. C. 
the preferred minimum temperature for silicon epitaxial growth is 
1120.degree. C. to 1140.degree. C. using other silicon halide gases as a 
source material which is sometimes termed a "gaseous phase material," or a 
"chemical source gas stream." 
It is clear there is a need in the art for an improved CVD reactor for 
epitaxial growth of silicon useful in solid state device structures. 
SUMMARY OF THE INVENTION 
A continuous CVD reactor is formed of an elongated open chamber having 
input and output sections. Two separated gas streams comprising 
respectively first a chemical source gas stream, such as silane, with a 
carrier gas, such as hydrogen, and second a stream of hydrogen are 
provided in laminar flow longitudinally along the chamber. Substrates, 
such as silicon wafers, are moved continuously in serial fashion through 
the chamber and adjacent the carrier gas in the first zone of the chamber. 
The substrates are heated while being kept separated from the source gas 
in the first zone of the chamber. The two gas streams are arranged to meet 
in the deposition zone to effect a reaction by the diffusion of the two 
streams on the heated wafers whereby the material from the one stream is 
epitaxially deposited on the substrates. Thereafter, the substrates are 
continued along the chamber into a third zone within which they are 
allowed to cool while being exposed to a longitudinally flowing gas in a 
direction opposite to the movement of the substrates. 
In another aspect of the invention the two separated gas streams are 
combined after the wafers are heated in a conventional CVD batch process 
reactor.

Reactor tube 10 as shown in FIG. 1 is formed of an elongated outer tube 16, 
rectangular in cross section, defining a reactor chamber 11, a first inner 
rectangular tube 18 and a second inner rectangular tube 20 both in 
telescopic relationship with tube 16 and having their respective 
longitudinal axes parallel to the longitudinal axis of tube 16. The tubes 
16, 18 and 20 are formed of quartz or vicor. Tube 16 is typically about 15 
feet (4.5 meters) long while tubes 18 and 20 are each typically 4.5 feet 
(1.37 meters). The respective heights of the tubes above the substrates 
are about 1.5 to 2 inches (3.8 to 5 cm) as indicated by height b for tube 
16 while tube 18 is approximately half that height, namely b/2 and tube 20 
is about 1/4 of that height, namely b/4. The respective widths of the tube 
16, 18 and 20 are about 18 inches (45.7 cm) wide along the direction into 
the paper as seen in FIG. 1. 
Input section 12 and output section 14 are formed of metal advantageously 
since they can be and are operated at relatively low temperatures 
according to the invention as compared to the higher operating 
temperatures within the reactor tube 10. Input section 12 is formed of an 
input section tube 22 which is rectangular in cross section, mating with 
the outer tube 16. Tube 22 is suitably attached to a tube input end wall 
26 formed of metal having an aperture 26a for receiving a tube 34 and an 
aperture 26b arranged to receive wafers 32 into chamber 11 of the tube 10 
through a tube 18 in register with aperture 26b. Tube 18 within tube 16 
thus defines two passageways for two respective gas streams, as will be 
further explained. 
A ramp or rail 30 is suitably positioned in the reactor structure to 
support the movement of susceptors (not shown) carrying substrates such as 
wafers 32 from a supply location not shown through the input section 12 
into the chamber 11 and thence outward from the output section 14 for the 
processing. 
A suitable means, such as a wafer susceptor, is positioned on the ramp 30 
and moved by suitable means at a selected rate through the reactor. 
Suitable wafer moving means are shown in the above-identified U.S. Pat. 
Nos. 4,043,955 and 3,672,948. A tube 36 is provided through the upper wall 
of tube 22 and is provided with a flanged portion 36a extending 
longitudinally in the reactor and spaced above the upper surface of the 
substrates 32 to allow a passage of a carrier gas such as pure hydrogen as 
indicated by arrow 36b. A tube 38 is provided in the upper wall of tube 22 
to exhaust gases, such as nitrogen and hydrogen, as shown by arrow 38a. A 
flanged tube 40 is also provided in the upper wall of tube 22 to carry a 
purge gas such as nitrogen, into the system as indicated by the arrows 40a 
and 40b. Tube 34 carries the gaseous phase material such as 
trichlorosilane plus hydrogen as a carrier directly into the chamber 11 as 
by arrow 34a, as will be further explained. 
The output section 14, also formed of metal, includes an output section 
tube 24 of similar construction as input section tube 22 and is provided 
with a series of supply and exhaust tubes similar to the tubes for input 
section 12. Thus, tube 134 provides an exhaust as indicated by arrow 134a 
of the gases from the chamber 11 comprising hydrochloric acid and 
trichlorosilane that has not been deposited on the wafers. Tube 136 
provides an input for the hydrogen needed to purge the exiting wafers 32 
and flanged tube 140 provides an inlet for the pure nitrogen also used to 
isolate the wafers 32 from the ambient as they exit from the deposition 
chamber 11. Tube 138 carries the gases from the output section. 
A metal wall 28 closing the end of metal tube 24 is connected by a suitable 
seal means 29 to the open end of quartz tube 16. Seal means 29 may, for 
example, consist of annular flanges extending respectively from the ends 
of quartz tube 16 and metal tube 24. An elastomer seal member, such as 
silicon rubber, is positioned between the flange to effect a seal of the 
mating flanges when compressed. A bolted clamping means is also provided 
in such seal means to compress the flanges together, thereby connecting 
and sealing the tubes 16 and 24 together. A suitable seal means is 
illustrated in FIG. 4, to be described. Other convenient ways of 
connecting and sealing a quartz tube to a metal tube may be used if 
desired. 
Tube 134 is mounted to an aperture 28b in wall 28. Chamber tube 20 is 
mounted in register to an aperture 28a in wall 28 by another seal means 
21, similar to seal means 29. Tube 20 thus provides an exit passageway 
from chamber 11 for the wafers 32. Similarly, the opposite metal end wall 
26 is connected and sealed to the input end of quartz tube 16 by a similar 
seal means 29a. It should be appreciated that the seals of elastomer that 
can be used will have a relatively low temperature for melting and 
decomposition at about 250.degree. C. According to the invention the 
temperature at these tubular end portions of the reactor is low enough to 
be able to utilize such a relatively simple seal. 
While the input and output sections 12 and 14 are arranged as shown in FIG. 
1, other arrangements can be used to provide isolation of the chamber 11 
from the ambient as the wafers 32 are moved therethrough. Thus, the input 
and output sections 12 and 14 can be constructed so as to make use of 
standard gas isolation practices using gas seals as illustrated, for 
example, in the above-identified U.S. Pat. No. 4,043,955 or designed to 
feed the wafers by a suitable cartridge feeding mechanism of conventional 
form from a sealed and purged compartment into the reactor. 
In operation the reactor of the invention provides, for example, a single 
epitaxial layer for an integrated circuit device or one layer for a thin 
film epitaxial solar cell, using, for example, trichlorosilane as the 
chemical source gas which is known to have a conversion efficiency of 
about 80% resulting in 20% of the material being exhausted from the 
system. A preferred form of a chemical source gas for epitaxial deposition 
purposes is dichlorosilane (SiH.sub.2 Cl.sub.2) since this form of gaseous 
phase material can approach or go to a complete reaction within the 
chamber 11. A mixture of chemical source gas and hydrogen is passed 
through tube 34 to provide a gas stream of chemical source gas as 
represented by velocity profile 43 and arrow 44. Pure hydrogen, serving as 
the purge gas, is applied to the system via tube 36 passing both 
rearwardly and forwardly through the aperture 26b represented by arrow 46 
and into the interior of tube 18 developing a velocity profile 47 of 
hydrogen. The streams of source gas and hydrogen gas are respectively 
passed through the two passages in the chamber 11 spaced apart by the 
upper wall of the first inner tube 18. The respective heights of the two 
passageways are about equal and define or constrain the stream to be 
substantially laminar with very little turbulence. 
The respective heights, actually the cross-sectional areas, of the upper 
and lower passageways in the first zone of the chamber, namely, the wafer 
heat-up zone 48, are selected according to design requirements. In the 
example illustrated, the heights and areas are equal for convenience. 
Conventional laminar flow design equations are used, in the practice of 
the invention, to determine the parameters of longitudinal length and 
height to establish substantially laminar flow of both gas streams. See 
"An Analysis of the Gas-Flow Dynamics in a Horizontal CVD Reactor" by S. 
Berkman, V. S. Ban and N. Goldsmith, published as Chapter 7 in the text 
entitled "Heteroepitaxial Semiconductors for Electronic Devices" edited by 
G. W. Cullen and C. C. Wang, 1978 by Springer-Berlag, New York, for 
conventional for laminar flow equations. In order to minimize or at least 
reduce the turbulence of the respective gas streams defined by velocity 
profiles 43 and 46, the forward momentum of the two streams must be 
balanced, that is they must be substantially equal. This can be achieved 
by providing suitable input pressures of the respective gas streams 
through tubes 34 and 36 and to provide the proper geometry of the 
passageways. 
In the alternative, or, indeed, as a supplement of the source gas means, a 
low momentum injection system, for example a transverse tube not shown 
with a suitable number of small holes or apertures, can be used to 
introduce the chemical source gas and the carrier gas into the upper 
passageway of the tube 10. Such an arrangement will serve to reduce or 
minimize turbulent flow of the gas. 
By maintaining the separation of the two gas streams before they are 
brought together for deposition. The wafers 32 are heated continuously as 
the two separated gas streams progress through the tube 10. A suitable 
heating mechanism 56, such as an RF coil, resistance coil or infrared (IR) 
lamps, is placed around or near the tube 16 in zone 48 to provide heat to 
the wafers 32. A suitable susceptor, not shown, is provided to respond to 
the heating mechanism to develop the heat to heat the wafers in a manner 
well known in the art. As the wafers 32 are moved longitudinally into the 
chamber 11, they are heated as they progress from right to left in what 
shall be termed the heat up zone 48. The rate of movement of the wafers 32 
throughout the entire system and particularly in the heat up zone 48 is 
selected to allow the wafers to arrive into the chamber 11 at a relatively 
low temperature of about 300.degree. C. and thence to progress through the 
tube 18 for heating to exit at the left end into what is termed the 
deposition zone 50 at a temperature of about 1100.degree. to 1160.degree. 
C. The wafers 32, it will be noticed, are entering into the deposition 
zone 50 in the environment of a carrier gas of pure hydrogen. As the 
wafers 32 move into the deposition zone 50 they are exposed to the 
reactive chemicals derived from the chemical source gas stream that 
diffused downwardly from the stream 44. 
The two streams of gas combine as indicated above at the end of the tube 18 
to provide a total flow Q.sub.T. The forward velocity of the reactive gas 
molecules are caused to travel a horizontal distance X=Vt (as shown in 
FIG. 1) while they are diffusing downwardly towards the wafer 32 by the 
relationship y=Dt, where V is the velocity of the gas stream, t is time 
and D is the diffusion coefficient. One can determine analytically where 
each reactive gas particle will be on the surface of the wafers 32. In 
general, the theoretical relationship of what is known as the mass 
transport equation can be determined from the literature and particularly 
the theory of gas flow. See the above-cited Chapter 7 by Berkman, et al. 
for a description of the theoretical and practical aspects of CVD 
reactors. From the fundamental equations of mass transport one can 
determine the gas flow for the system according to the present invention 
as follows: 
##EQU1## 
wherein G (L.sub.1) is the growth rate in micrometers per minute along the 
direction of length L.sub.1 as shown in FIG. 1, that is, through the 
distance L.sub.1 where X starts at 0 and approaches L.sub.1 and wherein 
L.sub.1 =b.sup.2 V.sub.T /4D.sub.T where b is the height above the wafers 
in the chamber 11 and and D.sub.T is as defined hereinabove; .eta. is the 
thermodynamic chemical efficiency of the reactive gas which is typically 
0.8; D.sub.T is the diffusion coefficient of the gas in H.sub.2 at the 
average gas temperature (T.sub.A); P.sub.O is the input partial pressure 
of the chemical source material species (trichlorosilane, e.g.); T.sub.A 
is the average gas stream temperature in the deposition zone 50; b is the 
height of tube 16; V.sub.T is the velocity of the gas streams at the 
average temperature (T.sub.A), and X is the distance in deposition zone 
50. 
Similarly, the equation to determine the mass transport in the deposition 
zone defined by the distance L.sub.2 of zone 50 is represented as follows: 
##EQU2## 
wherein X is 0 at the full distance L.sub.1 and P.sub.L1 equals P.sub.O 
exp. 0.334 .eta. and is thus the partial pressure of the gas in zone 50 
along distance L.sub.1. According to equations (1) and (2) one can 
determine the amount of material that will be uniformly deposited in zone 
50 on the wafers 32. 
This effect will be better understood by reference to FIGS. 5 and 6 which 
demonstrate respectively the growth profile in a system of the prior art, 
such as described in the previously-mentioned U.S. Pat. No. 4,048,955 and 
the growth profile according to the present invention. FIG. 5 shows the 
growth profile 70 plotting the growth rate or deposition rate in 
micrometers/sec against the distance X in centimeters over the surface of 
the substrate or wafer receiving the deposition material. Such a plot 
would apply to any deposition system of the prior art whether in a 
stationary or moving wafer environment. It is to be appreciated that the 
relative movement of the gas stream over the surface of the wafer is what 
effects the deposition rate across the wafer. If the wafer is moving 
relatively to the gas stream then the rates of deposition are modified 
accordingly. The distance X represents in a system such as described in 
U.S. Pat. No. 4,048,955, the growth rate profile along the longitudinal 
axis of the chamber with the wafers moving in the same direction. The 
growth rate profile would also be applicable to a system in which the 
distance X is transverse the movement along the longitudinal axis of the 
chamber as illustrated in the above-mentioned U.S. Pat. No. 3,672,948. 
Moreover, it should be understood the principle of the invention is 
applicable to conventional CVD batch reactors in which the substrates or 
wafers are stationary. 
The significant aspect of the prior art concept of deposition from a 
chemical source gas is the very high deposition rates that occur in the 
region identified in FIG. 5 as region 72. For this profile the prior art 
places the substrates or wafers such that the depositions occur in region 
74 in order to make more uniform the deposited material. It will be 
appreciated that using this growth profile in practice is very inefficient 
and difficult to control for uniformity. 
The growth profile 76 illustrated in FIG. 6 shows the growth of the 
deposited material in the chamber 11 according to the present invention in 
which the deposition starts at X=0 with about 10% of the maximum growth 
rate (G MAX) and increases to a maximum at a distance approximately at 
which X=L.sub.1 (as shown in FIG. 1) and therefor reduces gradually to a 
small value depending on the thermodynamic properties of the chemicals at 
the distance wherein X=L.sub.2. In practice, the profile 76 is adjusted 
such that the ratio of the maximum growth rate (G MAX) to the average 
growth rate (G AVER) is about 2 using equations (1) and (2). However, it 
should be understood that the profile 76 can be adjusted according to 
equations (1) and (2) to have different rates of deposition. Thus, the 
downstream portion in the vicinity where X=L.sub.2 can be arranged to have 
a larger growth rate with the sacrifice of efficiency. Advantageously such 
a growth rate is useful in conventional CVD reaction for batch processing 
of wafers. 
Thus, according to the principles of the invention, the wafers 32 see the 
same growth rate profile 76 (FIG. 6) they are uniformly deposited with an 
epitaxial layer of silicon derived from the gaseous phase material from 
the diffusion reaction of gas streams 44 and 46. This occurs in a 
continuous reactor in the deposition zone 50 between the tubes 18 and 20. 
The wafers then are passed into the tube 20 through which a counter flow 
of hydrogen from tube 136 is passed. This hydrogen gas is somewhat 
turbulent. The wafers will cool down to a temperature of about 300.degree. 
C. Turbulence from the counter flow gas stream identified by the arrows 60 
and 62 will be exhausted through the tube 134. Even though this flow is 
turbulent, the effects that would be considered to be intolerable in this 
zone of the process are tolerable according to this invention because the 
reactive gases will have been substantially depleted due to the epitaxial 
deposition in zone 50. The wafers 32 are then removed from the ramp 30 
carrying the substrates and deployed for other steps of the semiconductor 
process that are to be performed on the wafers. 
It should be understood that the velocity of the gas stream in a continuous 
reactor is greater than the movement of the wafers. In a typical design, 
the wafers will be moved at about 0.5 feet/min at the process temperature. 
That is, one wafer will pass through zone 50 of the reactor chamber 10 
(FIG. 1) at about 0.5 feet/min. The velocity of the respective gas streams 
44 and 46 is typically about 40 feet/min. Accordingly, a particular wafer 
will, upon passing through zone 50, receive a deposition layer of material 
that will be substantially uniform and moreover, each wafer will have 
substantially the same thickness as every other wafer preceding and 
following it since the thickness of the layer is determined by integrating 
the growth profile curve 76 (FIG. 6) over the distance each wafer moves. 
The invention in practice utilizes preferably a chemical source gas such as 
dichlorosilane to provide the silicon material for deposition on the 
wafers. The gas as provided through tube 34 (FIG. 1) is typically mixed 
with hydrogen at a ratio of 50:1 of hydrogen to dichlorosilane. The ratio 
is adjusted as known in the art according to the desired rate and 
equations (1) and (2). The hydrogen in this gas stream thus serves as a 
carrier gas for the chemical source material. Hydrogen provided to the 
system via pipe 36 serves alone as an inert gas blanket in a continuous 
reactor to inhibit back flow of the chemical source gas from chamber 11 
through tube 18 into the input section 12. Moreover, the hydrogen input 
serves as a viscous seal to isolate the chamber 11 from the input and 
output environment. Accordingly, the structure of the input and output 
sections can be formed of metal, because the temperatures in these 
sections are relatively low, about 200 to 300 C. as indicated in FIG. 3A, 
to be described. 
In order to isolate the input section from the room environment and 
moreover to isolate the hydrogen, nitrogen is provided through pipe 40 and 
exited through pipe 38. Pipe 38 carries both the exhaust hydrogen as well 
as nitrogen. The exit section 14 also provides a viscous seal to isolate 
chamber 11 from the ambient by a similar supply of hydrogen and nitrogen 
as described above. 
The reactor just described is a single zone or a single chamber hot 
reaction system which is capable of providing continuous deposition of 
thin film epitaxial layers for solar cells or I.C.'s. However, the reactor 
according to FIG. 1 does not provide for any etching of the wafers in 
situ. The the deposition zone 50 can be relatively short, that is about 
4-5 feet in length for high throughputs with modest epitaxial growth 
rates. Typically, a reactor zone 50 is approximately 5 feet long with a 
width of about 14 inches. Such a system can provide epitaxial layers with 
an average growth rate of 2 .mu.m/minute, allowing for a production of 
300, 4 inch square solar cell wafers/hour. 
For systems in which it is desired to provide etching of the wafers prior 
to deposition, modification of the reactor of FIG. 1 is needed as 
illustrated in FIG. 2A. FIG. 2A illustrates the structure required for 
such a system. The structure of FIG. 2A replaces the input or right side 
of the structure shown in FIG. 1. End isolation zones such as zones 54 and 
56 are omitted from the schematic of FIG. 2A. Such isolation zones however 
an provided to the input (right) arm output (left) sections of the 
structures of FIG. 2A. In order to provide an etching zone that is 
isolated from the deposition zone 50 of the reactor, a stacked tube design 
is used. The stacked tube 64 comprises a quartz T-joint 66 having a metal 
cap 68 connected thereto with an elastomer seal 70. Through the cap 68 is 
passed a tube 72 through which the chemical source gas is passed as 
indicated by arrows 74 and 76. A tube 78 is coupled into the chamber 110 
(corresponding to chamber 11 of FIG. 1) for carrying hydrogen, as 
indicated by arrows 80 and 82. A tube 180, similar to tube 18 of FIG. 1, 
is used to maintain a separation of the chemical source gas flow 76 and 
the hydrogen gas flow 82 through the wafer heat-up zone 48a in a manner as 
described above with respect to FIG. 1. A hollow transition member 83 
having stepped surfaces 83a and 83b and an aperture 91 (see in perspective 
in FIG. 2B) is positioned within tube 64 such as to provide two 
passageways for hydrogen as indicated by arrows 80 and 82 (tube 78) 86 and 
88 (tube 84). The hydrogen gas serves both as a purging gas and a viscous 
seal to aid in the exhaust of the etchant gas and isolate the etching 
zone, as will be described. The split arrows 88 it will be noticed 
indicate that the hydrogen gas passes both forwardly and rearwardly to 
provide a viscous seal. 
The T-joint 66 is connected to a quartz tube 162 which in turn extends to 
an end section such as end section 12 of FIG. 1. Within the tube 162 is 
provided a tube 90 with an end wall 93 having an aperture 92 in register 
with opening 92 for passing the wafers into the reaction chamber 110 
through tube 180. A pressure-butt seal 95 seals wall 93 to member 83. 
In telescopic relationship within tube 90 are another pair of telescoping 
tubes 94 and 96. Tube 96 is provided with a stream of heated hydrogen 
aiding to heat up the wafers 32 passing therethrough. Above tube 96 is 
passed a stream of an etchant gas of, for example, hydrogen and hydrogen 
chloride as indicated by arrow 98. This etching gas stream 98 is kept 
separate from the wafers 32 in tube 96 while the wafers are heated in zone 
104 by a suitable heating means 102, such as an RF coil. The wafers are 
heated in this heat-up zone 104 to a temperature of about 
1200.degree.-1220.degree. C. At position 106, at the end of tube 96, the 
gas flow 98 mixes with the hydrogen gas as shown by arrows 108 to etch the 
wafers 32 in the etch zone 117. As the wafers 32 pass out through the left 
end of tube 94, into isolation zone 111, they pass through the aperture 92 
into tube 180. In this zone 111, the purging gas 88 is passed over the 
wafers and exhaust rearwardly in the channel 112 between tubes 90 and 94 
as shown by arrow 13. 
The T-joint 66 isolates the etch zone 117 from the reaction chamber 110. 
The joint 64 is designed so as not to cause a problem with the heating or 
passage of hot silicon wafers. Except for the cap 68 with the appropriate 
elastomer seals 70, the apparatus as shown in FIG. 2A is constructed of 
quartz. 
The exhaust channel 112 is kept at a slightly negative pressure with 
respect to the other zones such as Heat-up zone 104 and the heat up zone 
48a. Accordingly cross-leakage of compatible gases will be reduced and any 
that leak will be drawn along with the purging gas 88 into the exhaust 
channel 112. 
In practice, the silicon wafers are heated in hydrogen gas only within the 
tube portion 96 while the reactive etch gas stream enters the etch zone 
117 as by arrow 98. The HCl gas etches the wafers 32 inside tube 94 in the 
etch zone 117 and the reaction products exhaust along the channel 112. The 
respective tubes 90, 94 and 96 are suitably sealed from one another at 
their respective cold-end junctions in the entrance zone 54 as illustrated 
in FIG. 1. 
Tube 180 similar in telescopic arrangement slides over member 83 in a 
crossfitting relationship that is sufficient to prevent any harmful 
cross-leakage of gases between tube 180 and portions external to tube 180 
within heat-up zone 48a. A nominal amount of cross-leakage however can be 
tolerated in this vicinity because the wafers 32 are in the process of 
being allowed to cool from the higher temperature in etch zone 117 and, 
moreover, because there are no cold gases blowing on the wafers 32. In 
other words, the purging gases 88 can be allowed to be preheated at a high 
temperature because there are no low temperature limited seals in zone 
111. Moreover, heating of the wafers in the heat-up zone 48a or the 
susceptors associated with the wafers may advantageous be heated from the 
bottom surface alone of the reactor. 
After the wafers are passed through the heat up zone 48a the diffusion of 
the gases after passing through tube 72 and 80 cause the deposition in the 
deposition zone 50a of the epitaxial layers on the heated wafers in the 
manner described hereinabove for FIG. 1. 
A suitable baffle 116 is provided to restrict the nominal diffusion of 
gases around entrance paths such as between the outside surface of tube 
90, that is, the channel 115, and the inside surface of the main reactor 
tube 160. 
FIG. 3A illustrates typical temperatures at respective portions of the 
reaction chamber 10 (shown in FIG. 1). The wafers 32 are passed through 
the input section 12 at an initial temperature of 200.degree. C. and 
heated by the hydrogen gases to about 300.degree. C. As the wafers are 
passed through heat up zone 48 they are heated to a temperature in the 
range of 940.degree. to 1250.degree. C. in order to prepare the wafers to 
receive the particular chemical source material selected for the process. 
In the preferred form of the invention using dichlorosilane the 
temperature range at the input to the deposition zone 50 should be about 
1080.degree. to 1150.degree. C. Using silicon tetrachloride the 
temperature should be about 1160.degree. to 1250.degree. C., for 
trichlorosilane the temperature should be about 1100.degree. to 
1200.degree. C. and for silane the temperature should be about 940.degree. 
to 1050.degree. C. As the wafers are passed through zone 50, the material 
from the reacting gases are deposited on the surface in a manner as 
described hereinabove. The wafers are then passed into the cool down zone 
52 wherein the temperature of the wafer drops to about 300.degree. C. as 
they exit the zone 52 into the exit zone 14. The schematic in FIG. 3A 
illustrates alternate ways of feeding the wafers into the reaction chamber 
11 of FIG. 1. The arrows 82 illustrate a system similar to that shown in 
FIG. 1 by which the wafers are supplied longitudinally of the system and 
exit as shown by arrows 84 also longitudinally of the system. 
In an alternative embodiment, the wafers 32 are carried vertically into the 
input section 12 as indicated by arrows 86 and then are moved 
longitudinally into the chamber 11, exiting vertically downwardly as 
indicated by arrow 88. A suitable wafer carrier 90 (illustrated in FIG. 
3B) is formed of a series of horizontal plates 92 coupled to each other by 
a U-bracket 94 passing through apertures 96. A moving chain 98 of 
molybdenum is attached to the brackets 94 to effect the movement of the 
carriers 90 through the reactor. 
Reference is now made to FIG. 4 which illustrates in partially exploded 
sections an epitaxial reactor test chamber 200 according to the invention. 
Reactor 200 provides an experimental test chamber for experimentally 
determining various parameters of the system that would be needed to 
design a full scale manufacturing reactor of continuous or fixed batch 
form. Reactor 200 comprises a conventional horizontal epitaxial reactor 
tube 202 formed of quartz. It may be 6 inches (15 cm) wide, 2 inches (5.1 
cm) high and 82 inches (208 cm) long. A flange 204 is formed on or affixed 
to one end of the tube 202 and has an aperture 206 to receive a horizontal 
flat tube 208. A tube 210 serving to supply hydrogen into the reactor is 
passed through an closure end plate 212. In addition, a tube 214 for 
providing the chemical source gases is provided in another portion of the 
plate 212 in a position to be spacially separated from the tube 210 and 
thereby allow the respective gases to be passed into the chamber in 
spacially separated paths longitudinally of the reactor. 
Exhaust pipe 216 is provided at the distal end of the tube 202 for 
exhausting the gases therefrom. A tube 218 is provided for passing 
hydrogen into the chamber for the purpose of purging the wafers after 
deposition. A horizontal flat tube 220 is positioned at the distal end of 
the tube serving as a separation means similar to tube 20 of FIG. 1. Tube 
208 serves to separate the two gas streams in a manner similar to that of 
tube 18 of FIG. 1 to provide the growth profile shown in FIG. 6. The 
substrates, not shown, to be deposited for test purposes are suitably 
placed into the chamber on a susceptor to simulate reactor 10 (FIG. 1) in 
a stationary mode. The wafers are first heated by suitable means, not 
shown, while hydrogen is passed through tubes 210 and 214 to a temperature 
in each of the respective zones 222, 224 and 226 to simulate the 
temperature gradient as, indicated, for example, in FIG. 3A. After the 
wafers are heated to deposition temperature, in the deposition zone 224 
source gas is added to hydrogen through tube 214. Deposition occurs as a 
result of the diffusion of the gases after passing through tubes 210 and 
214 causing the material to be deposited on the heated substrates to the 
desired thickness. The flow of source gas is stopped, and hydrogen is used 
alone to purge the chemicals from the chamber 202 through tubes 210, 214 
and 218 from the chamber 202 via tube 216. The wafers are then allowed to 
cool for measurements. 
Measurements of deposition rates on the wafers responsive to predetermined 
gas flows using equations (1) and (2) described above, may be used to 
provide data to determine the design criteria needed for a large scale 
reactor. 
The CVD reactor may be designed to deposit material on moving wafers as in 
FIG. 1 or on non-moving wafers in a fixed batch system. FIG. 4 illustrates 
the principle of this invention as it would be applied to a conventional 
CVD batch reactor. For such an application, the tube 220 will not be used 
since it serves only to determine experimentally the parameters needed for 
a continuous mode CVD reactor. In a batch process the wafers are placed in 
a predetermined location in zone 224. After heating the wafers to 
epitaxial deposition temperature chemical source gas is added to the 
hydrogen gas through tube 214 and the two streams through tubes 214 and 
210 are passed into the chamber and kept separated in laminar flow by the 
upper wall of tube 208. At the end of the tube the two streams join with 
little turbulence and diffuse to deposit the material on the heated 
wafers. The separated streams thus develop a growth profile such as shown 
in FIG. 6. 
The invention is particularly useful in CVD processes for making solar 
cells and integrated circuit (IC) wafers. Since solar cells typically need 
layers 20 .mu.m thick, the deposition zone 50 should be 4-5 feet (1.22 to 
1.52 meters) long. However, since IC's need thinner layers, in the range 
of 1-15 .mu.m, the deposition zone 50 can be shorter. For a 3 .mu.m layer, 
for example, zone 50 need be only two feet (0.6 meters) long. 
While the invention has been described in terms of epitaxial silicon growth 
in a CVD reactor, it will be appreciated that any process for depositing 
one material on a substrate in a CVD reactor may be used in the practice 
of this invention. For example, silicon oxide deposited on a silicon 
substrate using silane gas plus an oxygen containing gas will serve as the 
source gas. Other examples will include depositing silicon nitride, 
polycrystalline or amorphous silicon on suitable substrates. 
Moreover, while the chamber 10 is described as being rectangular, for low 
pressure or vacuum CVD system, chamber 10 is advantageously oval or 
cylindrical for structural strength.