Process for the epitaxial production of semiconductor stock material

In order, in the epitaxial production of semiconductor products and of articles provided with a layer, to be able to make the junction between the layers applied to the substrates atomically sharp, it is important to be able to change the gas mixture, to be introduced into a pulsed reactor or MBE reactor, rapidly, accurately and without losses in respect of quantity and of composition. To this purpose, each of the gases to be introduced into the reactor is conveyed to a separate gas pipette and thereafter the content of the gas pipette is cyclically passed, by means of a pressure differential, into the pulse reactor, with the composition of the mixture being changed per one or more cycles.

The invention relates firstly to a process for the epitaxial production of 
semiconductor products or of articles provided with a layer, wherein a 
number of metered gases are passed into a pulsed reactor, in which reactor 
a decomposition of molecules and deposition of atoms takes place. 
CVD materials (CVD=chemical vapour deposition), produced by deposition of 
elements of gas molecules on heated substrates, have been used 
increasingly in the last 25 years. The uniformity and reproducibility of 
the deposit in this process is of great importance. In the fabrication of 
semi-conductor products, such as transistors, chips, lasers and LEDs, 
layers of semiconductors of various compositions and/or various doping 
levels are deposited on one another epitaxially (ie. with a continuous 
crystal lattice arrangement). Usually, the above-mentioned CVD technique 
is used for this purpose. A conventional CVD reactor operates in 
accordance with the principle that a gas mixture is passed continuously 
through the reactor, in the course of which gas molecules are decomposed 
on heated substrates and certain atoms then deposit on the substrates in 
such a way as to become inserted in the crystal lattice. 
A shortcoming of the use of a continuous flow reactor is that the gas flux 
flowing along the row of substrates becomes so exhausted that a layer 
deposited on the last substrate is noticeably thinner than the layer 
deposited on the first substrate. It will be clear that this difference is 
unacceptable in achieving good reproducibility. 
It is known to counteract this depletion effect by making the gas stream so 
rich in semiconductor component that only a small fraction thereof is 
decomposed on the substrates while the remainder of the gas leaves the 
reactor unconsumed. It is also possible to increase the velocity of the 
gas to high levels; in that case, too, the majority of the active gas 
component is not consumed. In such cases, it is necessary to strike a 
compromise between uniformity of the layer thickness and degree of 
utilization of the gas mixture. Some compounds (for example organometallic 
compounds, in particular of metals of group III of the periodic system) 
are so expensive that a high degree of utilization is of major commercial 
importance. Use of a continuous flow reactor is in that case economically 
unattractive. Another shortcoming of a continuous flow reactor is that 
switching over from one component of the gas mixture to another is 
accompanied by a certain smudging as a result of diffusion and convection 
in the gas flow within the reactor and of gases temporarily being left 
behind in dead spaces of valves. In order to be able to achieve the 
frequently desired great sharpness of the junction it is here again 
necessary to choose an excessively high flow of gas. 
The abovementioned compromise between uniformity of layer thickness and 
degree of utilization of the gas mixture does not matter when using a 
pulsed reactor, i.e. a reactor in which the gas mixture can be metered 
cyclically be means of valves at the inlet and outlet. The reactive gas 
mixtures injected pulsewise into the reaction chamber and is pumped out 
when it has fully reacted. During the reaction the gas no longer flows 
along the substrates and a uniform layer thickness can be achieved without 
having to handle the gas uneconomically. 
In order to be able to make the junction between the layers, deposited on 
the substrates, atomically sharp it is important to be able to change the 
composition of the gas per cycle. The usual method in conventional 
reactors for regulating the composition of the gas mixture is by means of 
mass flow controllers (MFCs) for each of the components. These gas flow 
controllers suffer from the shortcoming that it is difficult to measure 
out a precise quantity of gas. For this, valves would be necessary, but 
the inertia of these results in an excessive inaccuracy. In practicing, 
the setting of a gas flow controller is left unchanged during an epitaxial 
run and a junction is brought about by switching-in various flows of gas 
with the aid of valves; each flow of gas can either be passed through the 
reactor, or made to bypass the reactor, by means of a three-way stop cock. 
The bypassing of the reactor again causes loss. 
The first object of the present invention is to avoid the abovementioned 
shortcomings and to provide a method of producing semiconductor products 
and articles provided with a layer, by preparing a gas mixture which can 
be regulated very accurately in respect of quantity and composition, with 
it being possible to change the composition very rapidly without losses. 
According to the invention, the process is characterized in that each of 
the said gases is fed to a separate gas pipette and thereafter the content 
of the gas pipette is passed cyclically, by pressure differential, into 
the pulsed reactor, with the composition of the mixture being changed per 
one or more cycles. 
The same mixing method with the aid of gas pipettes which are discharged by 
pressure differential, in which the composition of the mixture can be 
changed per one or more cycles, can also be used in other processes. 
Within the semiconductor field, attention is drawn to the use of the 
mixing method according to the invention in MBE (molecular beam epitaxy) 
reactors, for epitaxial growing-on of semiconductors. These reactors 
operate on the principle that molecular beams are caused to impinge on a 
heated substrate in a space which is kept at a very low pressure 
(&lt;10.sup.-6 mbar). 
It will be clear that at constant input pressure the control range of the 
gas pipettes is determined by the minimum and maximum volume thereof, but 
that the control range can be increased by adjusting the input pressure. 
The input pressure is thus a reserve degree of freedom. For organometallic 
components which are introduced by saturating a carrier gas--such as 
H.sub.2 --in a thermostatically controlled liquid container with the 
vapour of the volatile MO compounds, this amounts to regulating the 
temperature of the MO thermostats. Thanks to the extra degree of freedom, 
the control range of the gas pipettes is effectively much greater than 
would appear from the volume control range. 
A characteristic of the method described is that there is a relatively 
large pressure differential between the input side and the output side. As 
a result, it is possible to achieve pipetting by mere opening and closing 
of valves. Where there is little or no pressure differential, moving 
suction devices would be necessary in order to produce gas mixtures with 
the desired precision. For this, mechanically driven suction devices and 
valves are needed. A shortcoming thereof is that "dead spaces" are 
introduced. 
Preferably, the component gases are mixed in a conjoint mixing chamber 
before they are passed into the reactor. 
It is to be noted that there is known, from Swiss Pat. No. 357,206, a 
process and device for metering a gas mixture wherein, for the purpose of 
allowing the component gases of the mixture to pass from a number of 
auxiliary vessels to a mixing chamber, use is made of a solenoid valve in 
the feed line of each component gas into a metering vessel, and a solenoid 
valve in the outlet line from the metering vessel into the mixing chamber. 
The magnetic coils of the solenoid valves are activated in accordance with 
a particular adjustable frequency. This means that the magnetic valves are 
repeatedly opened and closed in accordance with a particular frequency and 
that each gas is passed, in short, pulses, in a quasiconstant flow into 
the mixing chamber. There is no mention of changing the composition of the 
gas mixture per cycle. Furthermore, the pressure differential between the 
input side and the output side is too low to permit pipetting exclusively 
by opening and closing of valves. 
A device for the epitaxial production of semiconductor products or of 
articles provided with a layer is characterized, according to the 
invention, by a number of gas pipettes into each of which there opens a 
feed line and an outlet line for a gas, which outlet lines run to a 
conjoint chamber with a shut-off valve being mounted in each feedline and 
each outlet line, and there are provided means for lowering the pressure 
in the reactor as well as control means for cyclically opening and closing 
the said valves in such a way that the composition of the gas mixture in 
the reactor can be changed per one or more cycles. 
The abovementioned reserve degree of freedom, obtained by changing the 
input pressure, could be brought into effect by also mounting an 
adjustable reducing valve in each of the feed lines to the pipettes. 
A rapid change in the volume of the gas pipettes can be achieved if they 
consist of a length of the feed line, wherein a cylinder, in which a 
plunger can be moved, is attached laterally. With the aid of a fast 
stepping motor and the microprocessor control this plunger can be adjusted 
rapidly between two cycles. This emphasises the flexibility of the gas 
mixing device. 
Preferably, a shut-off valve is located at both ends of the said lengths of 
feed line. 
In addition to being used for the production of semiconductor products, 
chemical vapour deposition is also used extensively for applying hard 
and/or abrasion-resistant layers of metal (for example, on certain machine 
components and lathe bits), the assembly of multilayer structures for 
optical applications (anti-reflection coatings or transmission filters), 
the application of layers having special chemical or electrical 
properties, superconductors etc. The mixing method and the device can be 
useful for all these applications. 
Preferably, the device is also provided with a bypass line with shut-off 
valve, in order to be able to arrange for gases to bypass the reactor. 
The valves must be constructed that they must be able to switch rapidly at 
least a million times without losing their hermetic seal in the closed 
state. Rocker valves can meet this requirement, especially if they possess 
a rocker axle which at one end is provided with a valve body and in the 
middle section has a hinge axle which on both sides is clamped between two 
O-rings. 
The control of the valves must also not interfere with the switching speed 
and the reliability. Each of the rocker valves can also be controllable 
by an electromagnet, with a microprocessor or programme switch being 
attached to the device in order to activate the electromagnets, and thus 
operate the valves, in accordance with a selected programme. The rocker 
valves can also be driven electropneumatically. This drive method is 
slower but more powerful and may in certain cases be preferable to 
electromagnetic driving.

The device shown diagrammatically in FIG. 1 comprises a chemical vapour 
deposition reactor 1 with a shut-off valve 2 in the gas feed line and a 
shut-off valve 3 in the gas outlet line. A vacuum pump 4 is also located 
in the outlet line. A bypass line 5 with valve 6 bypasses the reactor. 
Upstream of the reactor 1 there is a mixing chamber 7 into which--in the 
example shown--there open seven lines 8. Each of the lines is connected to 
a gas pipette 9 which can be filled with gas via a feed line 10. A 
shut-off valve, 11 and 12 respectively, is located both on the feed side 
and on the outlet side of the pipettes. 
The reactor is a pulsed reactor, i.e. a reactor which does not operate 
continuously but in cycles. It is very important that the composition of 
the gas mixture to be fed to the reactor can be changed very rapidly and 
per cycle. The gas components, which must be mixed in the mixing chamber 
before being fed into the pulsed reactor 1, are fed via the lines 10, with 
valves 11 open and valves 12 closed, under pressure into the gas pipettes. 
The pressure is set by means of reducing valves which are not shown. In 
each of the gas pipettes, to be discussed in more detail, a preset volume 
of gas can be taken up and, after closing the particular valve 11 and 
opening the particular valve 12, be discharged to the mixing chamber 7 by 
pressure differential (for example from 1 bar to 1 mbar). On emptying all 
the filled pipettes the pressure, with valve 2 closed, should increase 
from, for example, about 1 mbar to, for example, about 40 mbar. The 
pressure differential is brought about by the pump 4. An additional 
possibility for setting the quantity of gas to be fed to the mixing 
chamber is provided by changing the input pressure, that is to say, by 
adjusting the particle reducing valve. The control range of the gas 
pipettes 9 is thereby increased substantially. Furthermore, use of the 
mixing chamber is not essential. The gases can flow directly via the lines 
8 into the reactor. 
The most important advantage of the mixing device described is that in the 
cyclic mixing of the feed gases, changes in the composition can be 
implemented per cycle. By one cycle there is understood the filling of the 
pipettes by opening of the valves 11 with the valves 12 closed, and 
filling of the mixing chamber by closing the valves 11 and opening the 
valves 12. The content of the mixing chamber is fed into the reactor via 
the valve 2, while the valves 3 and 6 are closed. The filling of the 
pipettes 9 with a fresh charge of gas, and the filling of the reactor with 
the mixture formed prior thereto, should take place simultaneously, in 
other words, the decomposition and deposition process in the reactor and 
the pumping out of the reactor should take place during the cycle time of 
the mixing process. The reactor contains substrates in the shape of thin 
crystalline wafers 13 which are brought to a high temperature. The gas 
contained in the reactor is decomposed or cracked at the high temperature 
of the substrates, after which atoms settle on the substrate and combine 
with the crystal lattice. So-called doping elements are also fed in this 
way in a metered manner into the reactor. By switching over rapidly 
between different doping elements it is, for example, possible to produce 
np junctions and the like. 
Since, during cracking and growth of the crystal lattice, the gas does not 
flow along the substrates, the grown-on layers on the substrates will all 
be of even thickness, which leads to good reproducibility. Furthermore, 
the gas fed in can be consumed to a substantial degree. 
However, the most important advantage is that as a consequence of the new 
mixing device the composition of the mixture can be changed per cycle. The 
junctions between the layers on the substrates can be atomically sharp. 
Doping elements can also be unusually accurately introduced into the 
crystal lattice. The rapid change of composition is, for example, of 
importance is GaAs/GaxAl1-xAS layer structures or layer structures of 
quaternary III-V compounds. Using the mixing device described, the 
composition of the gas mixture can be changed in the following manners: 
(a) Where a particular component has to be admixed, or stopped as from a 
particular cycle, the particular pipette can be caused to participate, or 
no longer allowed to participate, from that cycle onward. 
(b) Where the quantity of a particular component is to be changed as from a 
particular cycle, the setting of the particular gas pipette can be 
changed. 
(c) Where the quantity of a particular component has to be varied between 
two fixed percentages, the component can be fed to two pipettes, with 
switching between the pipettes. This last arrangement can for example be 
used for GaxAl1-xAs multiplayer structures, where x has two values 
alternately. 
(d) Where the control range of the gas pipettes has to be increased, the 
setting of the reducing valves in the lines 10 can be changed. In the case 
of organo-metallic components which are introduced by saturating a carrier 
gas, such as H.sub.2, in a thermostatically controlled liquid vessel, with 
the vapor of MO (=metal organic) compounds, this amounts to regulating the 
temperature of the MO thermostats. As already mentioned, this provides an 
extra degree of freedom for the setting of the quantity of gas. 
FIG. 2 shows a cross-section through a practical embodiment of the mixing 
device. Corresponding components are marked with the same reference 
numerals. In each case, the combination of a length 10a of the feed line 
10, a valve 11 and a gas pipette 9 is mounted in a pipette block 14. For 
example, seven such blocks are located in a circle around the inlet side 
of the externally octagonal mixing chamber 7. The remaining eighth side of 
the mixing chamber is located a block 15 in which are mounted two tubes 5 
and 16 which open into the mixing chamber 7. The line 5 is the previously 
mentioned bypass line which is in direct communication, via the connection 
17, with a vacuum pump. The line 16 is connected to line 18, which leads 
to the pulsed reactor. 
The said eight blocks are fixed by means of locking bolts 19 against the 
eight sides of the mixing chamber 7. 
Each of the rocker valves 2,3,6,11 and 12 (see FIG. 3) comprises a rocker 
axle 21 which at its inner end is provided with a valve body 22 and at its 
outer ends is connected, via a knuckle joint, to an operating rod 23. The 
hinge axle 24 of the rocker axle is clamped between two O-rings 25. The 
rocker axle can rock, from its middle position, only through an angle of 
about 5.degree. to one side and 5.degree. to the other side. As a result 
of the low rocking stroke, the deformation of the O-rings is slight. The 
operating reliability is unusually high. 
In the closed position, the valve body 22 cooperates with a sealing ring 26 
which is fixed to one end of the tube which is to be sealed. In 
cross-section this ring has, for example, the shape of a star or cross and 
is known under the name of Quadring. 
The operating rod 23 of each rocker valve is connected to a separate 
electromagnet 27 (see FIG. 2). In the case of the embodiment shown, with 
seven pipette blocks 14 and one block 15, there are sixteen rocker valves 
and accordingly sixteen electromagnets 27. It will be clear that the 
number of pipette blocks can be chosen at will. Electropneumatic devices 
or piezoelectric actuators can be used in place of electromagnets. 
The operation of the rocker valves via the electromagnet 27 and the rods 23 
is achieved by means of a microprocessor or--in the case of a simple 
embodiment--by means of a programme switch (sequence controller). 
The programme is so chosen that the structure of the epitaxial 
semoconductor products exhibits the desired multilayer growth with 
atomically sharp junctions. 
The gas pipette shown comprises, in a length 10a of the line 10, 
transversely entering stainless steel cylinders 28 in which a plunger 29 
can be caused to travel. The position of the plunger is determined by a 
micrometer screw 30 with adjustment knob 31. In a particular embodiment, 
the volume of the pipettes can be regulated from 0.1 to 2 cm.sup.3, and 
the volume of the mixing chamber is 40 cm.sup.3 and the volume of the 
reactor is 1,500 cm.sup.3. Starting with a completely evacuated reactor, 
the pressure of the mixture is reduced, at each flowthrough stage, by a 
factor of 40 (the working pressure in the reactor is about 1 mbar). This 
means that a residual fraction remains in the pipettes and in the mixing 
chamber. Depending on the question of whether the next cycle is again to 
have the same composition or a different composition, the residual 
fraction can simply be left behind, or be pumped out (with valves 12 and 6 
open and valves 2 and 3 closed). Further, when emptying the mixing chamber 
into the reactor, there is the choice between leaving the valve 12 open or 
keeping it closed. This choice will be determined by considerations of 
flow dynamics and of the required precision in composition. The pipette 
settings must be corrected in accordance with the valve programmes. 
The gas or vapours (metallo-organic compounds) to be fed to the gas 
pipettes can be divided into compounds of elements, built into the crystal 
lattice, of group III of the periodic system, compounds of elements, built 
into the crystal lattice, of group V of the periodic system and doping 
constituents which, added in small quantities, determine the p-type or 
n-type conductivity and the charge carrier concentration. The table which 
follows shows the usual substances, followed by their function (III 
element, V element, n-type dopant or p-type dopant) and their physical 
state (gas in a cylinder, or metallo-organic constituent in a liquid 
vessel). 
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TMG trimethylgallium 
III (Ga) MO 
TMA trimethylaluminium 
III (Al) MO 
TMSb trimethylantimony 
V (Sb) MO 
TEIn trimethylindium III (In) MO 
AsH3 arsine V (As) gas 
PH3 phosphine V (P) gas 
DEZ diethyl zinc p-type dopant 
(Zn) MO 
H2Se hydrogen selenide 
n-type dopant 
(Se) gas 
SiH4 silane n-type dopant 
(Si) gas 
CP2Mg dicyclopentadienyl-Mg 
p-type dopant 
(Mg) MO 
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The above table relates exclusively to the growth of III-V semiconductors 
such as GaAs, AlxGa1-xAs, GaxIn1-x, AsyP1-y etc. In addition there are of 
course also the II-VI semiconductors such as CdTe, HgTe, CdS, ZnS, ZnSe, 
ZnO, CdO, CdxHg1-x, TeySe1-y etc. These can also be produced effectively 
in the abovementioned system. The sources used in that case are, for 
example: 
______________________________________ 
DMC dimethylcadium 
II (Cd) MO 
DEC diethylcadmium 
II (Cd) MO 
Hg mercury II (Hg) vapour 
H2Se or H2S VI (Se, Si) 
gas 
DET diethyltellurium 
VI (Te) MO 
DMZ dimethylzinc II (Zn) MO 
DEZ diethylzinc II (Zn) MO 
C4H4Se thiophene VI (Se) org. 
C4H8Se selenophene VI (Se) org. 
TEA triethylaluminium 
n-type dopant 
(Al) MO 
TEG triethylgallium 
n-type dopant 
(Ga) MO 
NH3 ammonia p-type dopant 
(N) gas 
______________________________________ 
It will be clear that the pulsed reactor, being a universally usable CVD 
reactor, can also be employed for Si epitaxy. At the present time, 90% of 
all semiconductor products are silicon products. As advantages there may 
be mentioned: uniformity of layer thickness and composition, sharpness of 
junctions and ability to be scaled up to larger reactors containing 
numerous substrates. 
The following data concerning the pipette volumes and other parameters may 
be given for a typical growth run. 
Arsine pipette 1 cm.sup.3 : trimethylgallium pipette 1 cm.sup.3, gas 
mixture passing through the trimethylgallium pipette: 90% H.sub.2, 10% 
trimethylgallium (trimethylgallium thermostat at 0.degree. C.), substrate 
temperature 630.degree. C. Cycle duration 1.5 seconds, comprising 1,200 
msec. growth time and 300 msec. pumping time; growth duration 1 hour, i.e. 
a total of 2,400 cycles. Working pressure in the reactor approx 1 mbar, 
which during the cycle varies somewhat through heating up and reaction. 
After completion, the layer thickness of the GaAs is 4.5 m, this layer 
thickness being epitaxically grown onto the GaAs substrate. 
It will be clear that numerous variations are possible within the scope of 
the invention. The mixing chamber 7 is not essential, and the gas could be 
fed via the lines 8 directly into the reactor 13. It is an essential 
feature of the invention that per cycle it is possible to form a mixture 
of which both the composition and the amount can be regulated accurately. 
No gas is lost. A relatively high pressure differential between the feed 
side and the outlet side is essential for the operation.