Method for producing articles of high-purity synthetic quartz glass

A method is described according to which articles of quartz glass, particrly quartz crucibles for use in crucible pulling according to Czochralski of high-purity synthetic quartz glass can be produced. High-purity silicon tetrachloride is hydrolized with water. The hydrolysis product is separated, dried, shaped, sintered and superficially fused.

BACKGROUND OF THE INVENTION 
The invention relates to a method for producing articles of high-purity 
synthetic quartz glass, especially quartz crucibles for use in the 
crucible pulling of silicon crystals according to Czochralski. 
High quality quartz glass is an indispensible material in semiconductor 
technology in producing and processing materials such as germanium, indium 
phosphide, gallium arsenide and particularly silicon, because of its 
purity, temperature stability and chemical resistance. Thus, for instance, 
the precipitation of very high-purity polycrystalline silicon by 
decomposition of trichlorosilane on heated substrates is often carried out 
in quartz bells (see, for instance, DE-AS No. 31 07 421). Also for 
high-temperature processes, such as oxidation, to which silicon wafers are 
subjected in the course of being processed into semiconductor components, 
reaction chambers of quartz glass have proven themselves. 
The highest requirements are demanded of quartz glass if used as the 
melting crucible material in the crucible pulling of silicon crystals 
according to Czochralski. Because of the direct contact between the 
aggressive silicon melt and the wall of the quartz crucible, the danger of 
contamination of the melt and of softening of the crucible wall is 
particularly great. For this reason, the resistance of the pulled silicon 
crystals can be increased only to a certain limit if quartz crucibles are 
used which are made of natural quartz, i.e., quartz crystals or quartz 
sand because of the large content of the doping substances boron, 
phosphorus and aluminum in the crucible material. Crystals with higher 
resistance, on the other hand, must be made by the more expensive 
float-zone-pulling method. Crucibles of natural quartz are therefore 
coated with the purer synthetic quartz according to DE-AS No. 29 28 089. 
However, the coating process must be carried out in a cumbersome manner 
under continuous rotation of the workpiece. There is the further danger 
that parts of the synthetic inner coating may burst off the outer wall of 
natural quartz. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a method which 
simplifies the production of articles of high-purity synthetic quartz 
glass. 
This object is attained according to the invention by a method which is 
characterized by the following steps: 
(a) hydrolysis of high-purity silicon tetrachloride with high-purity water; 
(b) separation and drying of the hydrolysis product; 
(c) shaping of the dried product; 
(d) sintering of the shaped product; and 
(e) superficial fusing of the sintered product. 
In principle, this method is suitable for manufacturing the most varied 
articles of high-purity synthetic quartz glass, for instance, reaction 
tubes, quartz bells, quartz boats or the like. In particular, however, 
quartz crucibles are produced, preferably for use in crucible pulling of 
silicon crystals according to Czochralski. By means of such high-purity 
crucibles, resistance values can also be obtained in crucible-pulled 
single crystals which could otherwise only be achieved by float-zone 
pulling. Thereby, crystals with diameters and an oxygen content made 
possible only by the crucible pulling method, but which have resistance 
values corresponding to zone-pulled material, can be produced in a 
cost-effective manner. Such an increased oxygen content cannot be achieved 
in zone-pulled silicon crystals even though it is required for many 
applications, for instance, because of its gettering action. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now in detail to the method embodying the present invention, 
high-purity silicon tetrachloride and high-purity water serve as the 
starting point. For both substances, a multiplicity of different 
production possibilities is known. Silicon tetrachloride is obtained, 
e.g., advantageously in the production of polycrystalline silicon by 
decomposition of trichlorosilane as a by-product in large quantities with 
very high purity. Water can, e.g., be brought to a suitable degree of 
purity by desalination and reverse osmosis. In principle, an approximately 
equal degree of purity should be present in both components. 
The hydrolysis can be carried out in the temperature range from room 
temperature to boiling temperature. The silicon tetrachloride can be 
introduced expediently in liquid form into an excess of water. Preferably, 
however, water is added to an excess of liquid silicon tetrachloride 
because then, and end product is obtained that has a particularly low 
content of OH groups and is distinguished by surprisingly high softening 
resistance at high temperatures. This property is often poorly developed 
in customary synthetic quartz glasses (see in this connection, for 
instance, Encyclopedia of Chemical Technology, Vol. 18, pages 85 and 86 
(1969), Second Edition, John Wiley and Sons, Inc.). 
The hydrolysis reaction is advantageously carried out in a protective gas, 
e.g., nitrogen or argon. It is also advantageous to stir the reaction 
mixture, for instance, by means of a vane stirrer. As reaction vessels 
suitable for reasons of purity, are especially vessels of quartz glass or 
polytetrafluoroethylene (Teflon). 
After the hydrolysis reaction is completed, the hydrolysis product is 
separated from the remaining liquid phase, e.g., by filtration or 
centrifuging. In addition, particularly for products which were obtained 
by the addition of water, preferably in a deficiency to the silicon 
tetrachloride presented, also evaporation by mere heating, optionally in a 
vacuum, can be considered. A temperature range of 100.degree. to 
200.degree. C. has been found suitable for this purpose. By a further 
increase of the temperature to about 800.degree. to 1000.degree. C., the 
powdered raw product obtained can then be dried further, preferably until 
approximate weight constancy is achieved. 
The thus pre-treated raw product is subsequently subjected to a shaping 
step, for instance, by molding it into the desired shape. As a rule, the 
product obtained by the shaping step has already a strength such that it 
can be taken from the mold for further processing. The blank which then 
has already substantially the final form, is subsequently sintered for 
further strengthening at temperatures of 1100.degree. to 1300.degree. C. 
This can be done, for instance, in the known ovens customary for sintering 
operations. Isostatic hot-pressing has been found to be a particularly 
advantageous shaping method, particularly at temperatures of about 
800.degree. to about 1200.degree. C. and pressures of 500 to 2500 bar, 
since here, the shaping and sintering steps are combined in one operation. 
However, isostatic hot-pressing can optionally also be followed by an 
additional sintering process. 
In the last process step, the sintered article is finally fused 
superficially and specifically, as a rule, at least at that surface which 
is subjected to special stresses with respect, for instance, to 
resistivity, chemical stability, density and the like. In the case of 
quartz crucibles this is generally the inside wall. The fusing is 
advantageously accomplished by the action of a hydrogen-oxygen flame, 
although in principle other heat sources such as arc heaters, are also 
suitable for this purpose. In general, it is sufficient if the fused layer 
has a thickness of about 20 to 40% of the total wall thickness. 
By the method according to the invention, crucibles for use in the crucible 
pulling process according to Czochralski can be made to particular 
advantage. The advantage is, on the one hand, the purity of the crucible 
material which is higher than natural quartz. On the other hand, such 
crucibles also have high heat stability, since the synthetic quartz glass 
according to the invention is surprisingly superior to conventional 
synthetic material as far as softening resistance is concerned.

In the following, the invention will be more fully described in an example. 
It should, however, be understood limitation. 
EXAMPLE 
Into a 2-liter Teflon flask, 1000 ml of silicon tetrachloride produced in 
the silicon preparation process by decomposition of trichlorosilane are 
introduced at room temperature and reacted gradually with 250 ml of water 
purified by reverse osmosis, corresponding to about 20% less than the 
stoichiometrically required amount, while flushing with nitrogen and 
simultaneous vigorous stirring. In the course of the reaction, the mixture 
heats up to the boiling temperature of the silicon tetrachloride. After 
the end of the water addition and completion of the reaction, the 
temperature is raised to 150.degree. C. in order to free the SiO.sub.2 
powder produced of adhering silicon tetrachloride. In this manner, 450 g 
of x-ray amorphous raw SiO.sub.2 are obtained; by gradual heating to 
950.degree. C., the product loses another 50 g in weight. With the 
remaining powder, a crucible form is filled and a crucible blank (diameter 
about 120 mm, height about 110 mm, wall thickness about 4 mm) is prepared 
by isostatic hot-pressing at 900.degree. C. and 1000 bar. This blank is 
additionally sintered for 2 hours at 1200.degree. C. and, finally, fused 
at its inside surface to 30% of the total wall thickness in a 
hydrogen-oxygen flame. 
From the crucible so obtained, a silicon single crystal of 54 mm diameter 
and 30 cm length is pulled by crucible pulling according to Czochralski. 
The doping substance concentrations determined at the end of the rod by 
low-temperature IR measurements are 5.times.10.sup.12 atoms/cm.sup.3 P and 
6.times.10.sup.12 atoms/cm.sup.3 B; the aluminum content is below the 
detection limit. 
After the pulling process is completed, sticking or adhesion of the quartz 
crucible to the surrounding graphite crucible cannot be found. 
From such crucibles, silicon single crystals can be pulled, the doping 
substance content of which correspond to zone-pulled material and the 
oxygen content of which correspond to crucible-pulled material. 
While only one embodiment and example of the present invention have been 
described, it is obvious that many changes and modifications may be made 
thereunto, without departing from the spirit and scope of the invention.