Method for the deposition of pure semiconductor material

The invention relates to a method and device for making pure semiconductor aterial by thermal decomposition of compounds of the semiconductor material on suitable carrier elements. The quantity and quality of the obtained semiconductor material is increased in accordance with the invention by introducing the compounds to be decomposed into the reactor chamber in at least a partially liquid state through a nozzle having a plurality of discharge openings.

The present invention relates to a method for the deposition of pure 
semiconductor material, in particular, silicon, by thermal decomposition 
of a compound of the semiconductor material, on the surface of a heated 
carrier element, which carrier element is heated by applying an electrical 
current thereto, so as to heat the same to the decomposition temperature 
of the corresponding decomposable compound in a gas-tight, closed reactor. 
The invention also relates to a nozzle used in the inventive process for 
feeding the decomposable compound. 
When making pure semiconductor material, for example, polycrystalline 
silicon from, e.g., trichlorosilane, the deposition speed substantially 
depends on the temperature, the reactor pressure and the amount of 
trichlorosilane which is processed. At a given pressure and at a given 
temperature, an increase of the deposition rate may only be obtained by 
increasing the quantity of the trichlorosilane which is being processed. 
With the usual trichlorosilane-hydrogen mixtures having a trichlorosilane 
proportion of 5 to 10 Vol.%, such an increase is no longer possible, since 
an increased quantity results in a correspondingly increased flow speed, 
as a result of which the dwell time of the decomposable compound for a 
quantitative deposition on the heated carrier element would be too short. 
On the other hand, substantially higher concentrations, for example, 50 
Vol.% trichlorosilane in the mixture, result in problems during the gas 
feeding and for making the mixture itself. 
Moreover, very expensive saturators are required, wherein the hydrogen 
charges itself with the corresponding quantity of the gaseous 
trichlorosilane. On the other hand, all gas feeding lines must be heated 
up to and into the reactor, so as to prevent the trichlorosilane from 
condensating out. A further problem is that with an increase in the 
quantity of the decomposable compound and an increase in the quantity of 
the semiconductor material to be deposited, the diameter of the 
polycrystalline silicon rods which are in the reactor, grows faster in the 
upper portion of the reactor than in the lower portion of the reactor. In 
addition, the surface characteristics of such polycrystalline rods is 
disadvantageously changed due to bubble or groove formations. In 
particular, when using such rods for making monocrystals in accordance 
with the zone-drawing method, extended operating times are required and 
increased grinding losses are experienced. 
It is therefore an object of the present invention to make polycrystalline 
semiconductor material with a site-independent, uniform thickness and 
surface quality, by increasing the charge of the decomposable compound of 
the desired semiconductor material. 
This object of the invention is obtained by a method which is characterized 
in that the corresponding decomposable compound is at least partially fed 
into the reactor in a liquid state. 
Basically, in addition to the corresponding compound which forms the 
semiconductor material due to decomposition, hydrogen is introduced into 
the reactor vessel. When making silicon, in addition to the 
trichlorosilane, enough hydrogen is preferably introduced into the 
reactor, such that, after the evaporation of the trichlorosilane in the 
reactor, at least 40 Vol.% of hydrogen remains present in the reactor. In 
order to assure a uniform growth of the rods, the trichlorosilane quantity 
which is introduced into the reaction vessel during the deposition 
process, is continuously increased in the same quantity and relationship 
with respect to the hydrogen. 
In accordance with a preferred embodiment of the invention, the 
decomposable compound of the semiconductor material to be deposited, (for 
example, trichlorosilane, when making silicon), is preheated in a heat 
exchanger outside of the reactor by means of the hot gases which discharge 
from the reactor. Thereby, a partial evaporation takes place and this 
partially evaporated trichlorosilane is fed into the reactor. For a better 
distribution of the still liquid trichlorosilane, the hydrogen is admixed 
therewith before being introduced into the reactor. The liquid 
trichlorosilane which is sprayed into the reactor is immediately 
evaporated by the high heat emission of the glowing silicon rods in the 
reactor. 
In order to obtain silicon rods with a high uniform diameter, it is 
necessary to generate a uniform trichlorosilane concentration throughout 
the total reactor. This is achieved in accordance with the invention in 
that the decomposable semiconductor compound is introduced into the 
reactor in two or more partial streams. 
Other objects and features of the present invention will become apparent 
from the following detailed description considered in connection with the 
accompanying drawings, which disclose several embodiments of the 
invention. It is to be understood, however, that the drawings are designed 
for the purpose of illustration only and not as a definition of the limits 
of the invention.

Turning now in detail to the drawings, the deposition of the gas-liquid 
mixture which is introduced into the reactor is essentially carried out 
with a nozzle, as shown in FIG. 2. Suitable materials for the nozzle are, 
e.g., silver, silver-plated steel or refined steel. The nozzle essentially 
consists of a tubular element 1 having an upper portion 2, which is 
provided with a funnel-like recess 3, the opening angle .alpha. of which 
is about 30.degree. to 120.degree. and preferably 40.degree. to 
80.degree.. An insert 4 is screwed into the tubular element 1. Insert 4 is 
provided with a central bore 5, having a diameter between 2 to 11 mm, and 
preferably, about 5 to 10 mm. Preferably, bore 5 of insert 4 is also 
upwardly flared and, if such is the case, defines an opening angle .beta. 
of about 8.degree. to 15.degree.. 
The radial distance between upper portion 2 and the "inserted" insert 4 
forms an annular slot, having a width of about 0.4 to 3 mm (preferably 0.7 
to 1.2 mm). The width of this slot can be easily adjusted before mounting 
the insert 4, by inserting corresponding distance discs 6. Supply lines 7 
and 8 are provided which run from the central bore of the nozzle into the 
annular slot. In the present case, they consist of four cylindrical supply 
lines, which are disposed horizontally in a radially, spaced-apart manner 
at a distance of 90.degree. from each other, or four bores in insert 4, 
through which the gas-liquid mixture penetrates into a free space 9, which 
is provided for technical flow reasons, and then into the annular slot. 
FIG. 1 illustrates the basic deposition device for carrying out the method 
of the invention; this deposition device generally corresponds to the one 
disclosed in German Patent Application P No. 28 54 707, with the exception 
of the nozzle and the preheating unit for the decomposable semiconductor 
compound. This deposition device is particularly well suited because the 
inventive method is carried out at a pressure in the reactor of about 1 to 
16 bar and, preferably, about 4 to 8 bar. 
This deposition device is composed of a silver-plated base plate 10 and a 
bell consisting of a steel inner hood 11, plated on the inside with silver 
and a covering outer hood 12 made of steel. In base plate 10, on which the 
bell is fastened by a gas-tight, flanged connection, hollow spaces are 
provided throughout its entire body, which are connected to a cooling 
water system by means of an inlet pipe 13 and an outlet pipe 14. The 
intermediary space defined between inner hood 11 and outer hood 12 is 
cooled during the deposition process by pumping cooling water 
therethrough, the water entering through pipe 15 and discharging through 
pipe 16. 
Thin rods 17 which serve as deposition carriers and which are, for example, 
disposed in a U-shaped manner, are held at the two free ends by electrodes 
18 and 19. In order to prevent deposition upon the electrodes themselves, 
hollow electrode holders or retainers 20 and 21 are provided, which are 
also cooled with cooling water. The cooling water flows into the cavities 
of the electrode holders 20 and 21 through the inlet pipes 22 and 23, and 
discharges through the outlet pipes 24 and 25, respectively. The 
electrodes are conductively coupled by electrical contacts 26 and 27 with 
a power source (not shown). 
The liquid decomposable semiconductor compound, for example, 
trichlorosilane in the case of silicon deposition, is fed through a heat 
exchanger 29 by a feed line 28. Due to the hot exhaust gases which are 
discharged through the discharge pipe 30, which exits from the bottom 
plate of the reactor and through inlet 31 into heat exchanger 29, the 
trichlorosilane which flows through line 28 is not only heated but is also 
partially evaporated. The hydrogen which is pumped through line 32 is also 
heated before the exhaust gases discharge from the heat exchanger at 
outlet 33. The exhaust gases are then fed to a condensation-distilling 
device (not shown) for recovering non-converted trichlorosilane. 
Before the liquid and partially evaporated trichlorosilane is introduced 
into the nozzle arrangement 34 (shown in detail in FIG. 1), the preheated 
hydrogen from feed line 32 is admixed therewith in inlet pipe 35 of bottom 
plate 10 of the reactor. A quartz window 36, which is also water cooled, 
permits one to observe the progress of the deposition process in the 
reactor. At the apex or tip of the bell, an opening 37 is provided which 
can be closed off by means of cooling pot 38, fastened by a flanged 
connection. This cooling pot 38 is open at its top and may be charged with 
a cooling agent, for example, water, by means of an inlet pipe 39. The 
water flows out through an overflow pipe 40. This cooling pot 38 and a 
disc 41, which serves as a seal, may be removed at the start of the 
deposition process, so as to lower a heating finger into the reactor for 
preheating the rods, for example. 
Due to the invention nozzle, the trichlorosilane-hydrogen mixture is 
separated into two streams-namely, a conical-like, partial stream which is 
responsible for the required turbulence in the lower half of the reactor, 
and a vertical, partial stream, which has the same effect in the upper 
reactor half. On account of these turbulences throughout the total 
reactor, semiconductor pieces or rods are generated with smooth surfaces 
while, simultaneously, a supply of fresh trichlorosilane in both reactor 
halves achieves the desired improvement of uniform rod diameter. Due to 
the preheating of the liquid semiconductor compounds, for example, 
trichlorosilane in the case of silicon, by means of the reactor gases 
which discharge from the reactor, a considerable energy saving is 
obtained. 
The inventive method permits deposition speeds of up to 4 mm/hr at a 
deposition temperature of 1100.degree. C. and a reactor superpressure of 4 
bar. 
EXAMPLE 
In a deposition device of the aforementioned type having a height of 260 cm 
and a diameter of 120 cm, wherein the intermediary space between the 
silver-plated inner hood and the outer steel hood has a radial width of 
about 2.5 cm, eight undoped thin rods with a specific resistance of about 
5000 Ohm cm, a diameter of 0.7 cm and 200 cm in length, were coupled 
together in pairs in a U-shaped arrangement with a bridge made from the 
same material, and were retained in the associated water-cooled 
electrodes. Thereby, four thin pairs of rods were grouped symmetrically 
around the longitudinal axis of the reactor. Subsequently, a heating 
finger was introduced into the reactor, air was expelled from the reactor 
by the introduction of argon gas and the rods were heated to a red glow, 
i.e., to 600.degree. C. in about one hour. About one-half hour before the 
end of this heating period, a voltage was applied to the thin rods. After 
reaching the ignition temperature of the rods, the heating finger was 
removed from the bell and the opening was closed by coupling the cooling 
pot by its flange connection. While the thin rods were heated by the 
electrodes to the required deposition temperature of about 1100.degree. 
C., a trichlorosilane-hydrogen mixture was divided into two partial 
streams by the nozzle shown in FIG. 1, and fed into the reactor with a 
superpressure of 5 bar. The acute angle .alpha. of the funnel-like recess 
of the nozzle was 60.degree. and the radial width of the annular slot was 
0.8 mm. The diameter of the central bore in the nozzle insert was 5 mm and 
it flared upwardly at an acute angle .beta. of 11.degree. to a diameter of 
7 mm at its discharge end. The quantity used initially was 30 kg/hr of 
trichlorosilane, preheated in the heat exchanger with 20 m.sup.3 /hr 
hydrogen, also preheated in the heat exchanger. The quantity was uniformly 
increased toward the end of the deposition process to about 470 kg 
trichlorosilane/hr and 100 m.sup.3 hydrogen/hr. 
After 100 hours, the deposition was interrupted. The total weight of the 
deposited silicon was 472 kg and the rod diameter was 127 mm. The average 
deposition rate was 1.2 mm/hr. 
While several embodiments of the present invention have been shown and 
described, it will be obvious to those persons of ordinary skill in the 
art, that many changes and modifications may be made thereunto, without 
departing from the spirit and scope of the invention.