Method of deposition of silicon in fine crystalline form

Method of deposition of silicon in fine crystalline form upon a substrate from a silicon-containing reaction gas which includes, at a set mole ratio of the reaction gas and throughput selected for the deposition process, setting the deposition rate-determining temperature of the substrate, at the beginning of the deposition, at a temperature at least equal to optimal temperature for deposition of silicon thereon and, in accordance with consequent increase in thickness of the silicon deposited on the substrate, gradually reducing the temperature of the substrate while maintaining at a minimal value the other parameters determining the rate of deposition.

The invention relates to a method of depositing silicon in fine crystalline 
form, more particularly, on a substrate preferably heated by direct 
passage of current therethrough, from an appropriate reaction gas 
especially formed of a silicon-halogen compound and hydrogen. 
It has been known heretofore, in methods of deposition from the gaseous 
phase, to vary the parameters determining the deposition at the beginning 
or also during the deposition or precipitation process. It is furthermore 
known from U.S. Pat. No. 3,120,451 to operate, at the beginning of the 
deposition or precipitation process, with a lower throughput or flow rate 
of the flowing gas mixture, and gradually to increase the throughput or 
flow rate. The objective of this measure is to increase the purity of the 
silicon by preventing undesired boron precipitation or deposition. 
As is well known, boron has a disadvantageous property or characteristic of 
being unable to be displaced appreciably during zone melting because the 
distribution coefficient thereof is nearly at 1, and of vaporizing into 
the vacuum only in very small quantities. Consequently, an effort is made 
to permit as little boron as possible to penetrate into the semiconductor 
material during the production thereof. The precipitation or deposition of 
boron is therefore prevented by operating with a smaller throughput or 
flow rate of the reaction gas mixture at the beginning of the 
precipitation or deposition process and, then, gradually increasing the 
throughput or flow rate. 
The invention of the instant application, on the other hand, is based on a 
different effect. It has been found that during the production of 
polycrystalline silicon rods, a coarse crystalline growth occurs, at 
times, leading to the occurrence of considerable crystal lattice faults 
during subsequent production of monocrystalline rods from these polyrods 
in a crucible-free floating zone melting process. 
For this purpose, it has been proposed heretofore in German Published 
Non-Prosecuted Patent Application No. DE-OS 27 27 305 to decrease or 
increase the mole ratio in the reaction gas as well as the precipitation 
or deposition temperature and the gas throughput or flow rate starting 
with a high mole ratio, a high gas throughput or flow rate and optimal 
precipitation or deposition temperature during the precipitation or 
deposition in accordance with a predetermined program. 
In such a method, the mole ratio is advantageously set at 0.5 at the 
beginning of the precipitation or deposition and the optimal precipitation 
or deposition temperature to 1100.degree. C. whereas, during the 
precipitation or deposition, a gas throughput or flow rate ranging from 
3,000 to 15,000 liters per hour (l/h) is used. The gas throughput or flow 
rate, as is generally known, is the quantity of reaction gas flowing along 
the heated substrate per unit time. According to one exemplifying 
embodiment of the invention in this application, the mole ratio is lowered 
to 0.2 after precipitation or deposition for about 10 minutes at a high 
mole ratio. 
It is accordingly an object of the invention to provide a method of 
deposition of silicon in fine crystalline form which avoids the 
disadvantages of the heretofore known as well as the heretofore proposed 
methods of this general type mentioned hereinbefore. 
With the foregoing and other objects in view, there is provided in 
accordance with the invention, a method of deposition of silicon in fine 
crystalline form upon a substrate from a silicon-containing reaction gas 
which comprises, at a set mole ratio of the reaction gas and throughput 
selected for the deposition process, setting the deposition 
rate-determining temperature of the substrate, at the beginning of the 
deposition, at a temperature at least equal to optimal temperature for 
deposition of silicon thereon and, in accordance with consequent increase 
in thickness of the silicon deposited on the substrate, gradually reducing 
the temperature to a minimal value of the substrate while maintaining the 
other parameters determining the rate of deposition. 
In accordance with another mode of the method invention, the temperature at 
the beginning of the deposition is optimal deposition temperature and is 
about 1100.degree. C. 
In accordance with a further mode of the method invention, the the 
temperature at the beginning of the deposition is above optimal 
temperature and is about 1150.degree. C. 
In accordance with an added mode of the method invention, minimal substrate 
temperature during the deposition is between about 1000.degree. C. to 
1050.degree. C. 
In accordance with an additional mode of the invention, the method 
comprises maintaining the throughput of the silicon-containing reaction 
gas constant during the deposition. 
In accordance with yet another mode of the invention, the method comprises 
increasing throughput of the silicon-containing reaction gas with 
increasing thickness of the deposited silicon. 
In accordance with yet a further mode of the method invention, the 
silicon-containing reaction gas is formed of a silicon-halogen compound 
and hydrogen. 
In accordance with yet an added mode of the invention, the method includes 
passing an electric current directly through the substrate so as to heat 
the substrate. 
In accordance with an additional mode of the method invention, the 
substrate is in the form of a silicon rod, and the increase in thickness 
of the silicon deposited on the substrate is an increase in diameter of 
the silicon rod. 
In accordance with yet another mode of the method invention, the gradual 
reduction in the temperature of the substrate is continuous. 
In accordance with a concomitant mode of the method invention, the gradual 
reduction in the temperature of the substrate is stepwise. 
The idea upon which the invention of the instant application is based is 
that, in the production of polycrystalline silicon rods, a good 
silicon-halogen yield, on the one hand, and a good fine crystalline 
surface of the silicon, on the other hand, is obtained. Normally, both are 
not simultaneously attainable i.e. a better yield results, at the same 
time, in a correspondingly poorer silicon surface. 
Heretofore, a good surface i.e. a fine crystalline surface, has been 
obtained only at the expense of the silicon-halogen yield and, in fact, 
either by decreasing the precipitation or deposition temperature or by 
increasing the silicon-halogen throughput or flow rate i.e. a high 
throughput or flow rate and/or a high mole ratio. 
As indicated by the invention of the instant application, an improvement of 
the surface quality is achievable with respect to fine crystalline 
structure, however, without considerably impairing the silicon yield, if a 
constant temperature is not employed during the entire precipitation or 
deposition period but, rather, in accordance with the invention of the 
instant application, the temperature is gradually reduced so as to form 
new crystal nuclei or seed crystals. 
Although the invention is illustrated and described herein as embodied in 
method of deposition of silicon in fine crystalline form, it is 
nevertheless not intended to be limited to the details shown, since 
various modifications and structural changes may be made therein without 
departing from the spirit of the invention and within the scope and range 
of equivalents of the claims.

Referring now to the FIGURE of the drawing, there is shown therein a device 
for depositing or precipitating polycrystalline silicon, made up of a 
metallic base plate 1 and, seated thereon, a hood 2 formed, for example, 
of quartz. Gas-tight electrodes 4 which are electrically insulated from 
one another extend through the metallic base plate 1 and are connected to 
respective ends of a rod-shaped substrate bent into the shape of a U, and 
formed of highly pure silicon. A nozzle 5 extends through the metallic 
base plate into the interior of the reaction chamber defined by the hood 2 
and serves to supply fresh gas; according to the illustrated embodiment, 
the nozzle 5 is surrounded concentrically by an exhaust tube 6 for spent 
reaction gas mixture. The rod-shaped substrate 3 is heated by a power 
supply 7. 
A reservoir 11 for hydrogen is provided for heating the reaction gas. The 
hydrogen flowing from the tank 11 passes through a flowmeter 12 and 
through an evaporator 13 which is filled with liquid silicochloroform, the 
evaporator 13 having an outlet which leads to the supply nozzle 5 in the 
reaction vessel. To adjust the temperature in the evaporator 13, the 
latter is housed in a thermostat 14. The amount of silicochloroform 
entrained in the hydrogen gas i.e. the mole ratio between silicochloroform 
and hydrogen, can be adjusted thereby. 
The conventional methods of this general type are normally performed at 
constant temperature, the gas throughput or flow rate being increased with 
increasing diameter of the silicon rod. This soon results, however, in a 
very unduly heavy precipitation or deposition of silicon on the 
quartz-glass reactor, which contributes to impairment or deterioration 
thereof. 
An increase in the gas throughput or flow rate is thereby sharply 
restricted, and passage can then be effected with good quality results yet 
only at constant throughput or flow rate. 
According to the invention of the instant application, the silicon 
deposition or precipitation is begun with thin rods at high temperature of 
1100.degree. to 1150.degree. C., and then the temperature is permitted to 
go down slowly with increasing rod diameter, for example, to 1000.degree. 
to 1050.degree. C. for a rod diameter of 50 mm. With a constant throughput 
or flow rate of the reaction gas, the silicon yield from the 
silicochloroform which is used is preserved in spite of the very low 
temperature because the silicon surface and the heat content of the 
reactor increase. Moreover, an increase in the gas throughput or flow rate 
and in the silicochloroform throughput or flow rate is possible without 
any impermissible or undue increase in the silicon precipitation or 
deposition on the quartz bell jar. Due to the very low temperature toward 
the end of the deposition process, the fine crystallinity of the surface 
is decidedly improved. 
In comparison with the hereinaforementioned previously proposed method, the 
method according to the invention is more easily performed so that it is 
especially suited for mass production i.e. parallel operation of a great 
number of reactors for especially thick rods of, for example, 5" diameter 
and more. As an additional effect, which should not be underestimated, 
energy conservation of from 25 to 30% resulted from the method carried out 
in accordance with the invention. 
The precipitations or depositions which also cause difficulties in 
maintaining cleanliness of the observation window especially for 
pyrometric measurements of the substrate temperature could virtually be 
suppressed entirely.