DE1128413B - Process for the production of decomposition-free single-crystal silicon by crucible-free zone melting - Google Patents

Description

GERMAN

PATENT OFFICE

KL.12i 38

INTERNAT.KL. C Ol b

S 71415 IVa / 12 i

REGISTRATION DATE: NOVEMBER 25, 1960

NOTICE THE REGISTRATION ANDOUTPUTE EDITORIAL: 26. A P R I L 1962

It is already known to convert polycrystalline silicon rods into single crystals with the aid of seed crystals to transform by creating a melting zone, optionally several times, from the end to which the seed crystal is attached, can migrate to the other end of the silicon rod. The silicon rod is mostly clamped vertically in two brackets. Often this becomes a bracket rotated around the rod axis during zone melting, creating a symmetrical growth of the solidifying material is guaranteed.

Single crystals, which are produced in this way by crucible-free zone melting, mostly show after numerous dislocations, twinning and the like. They can be made visible by suitable etching are and appear as so-called etch pits on the surface of the sanded and brightly etched Crystal. Since these dislocations are harmful - they extend the life of the Minority carriers in silicon undesirably degrade and lead to the manufacture of semiconductor devices after the alloying process leads to an uneven alloy attack at the beginning of the Procedural - one tries to reduce it.

The proposal has already been made (German Auslegeschrift 1094 710) for crucible-free zone melting to melt a seed crystal with a significantly smaller cross section than the semiconductor rod. In this way it can be achieved that the axial temperature gradient of the semiconductor rod runs much flatter to the seed crystal, which leads to a reduction in the dislocations, and that noticeably fewer dislocations grow from the seed crystal into the semiconductor rod.

A further proposal has already been made (German Auslegeschrift 1079 593) for crucible-free zone melting of semiconductor material with multiple passes through the melting zone before the last pass the melting zone a narrowing of the semiconductor rod in the immediate vicinity of the melting point of the To make the seed crystal. These measures also lead to a reduction in the dislocations and thus increasing the service life of the minority carriers in the semiconductor material. The production however, completely dislocation-free silicon cannot be achieved in this way.

The invention relates to a method for producing dislocation-free single-crystal silicon by crucible-free zone melting with multiple passage of the melting zone through a vertical one standing silicon rod held at its ends, method of manufacture

of non-decomposition single crystal

Silicon through crucible-free zone melting

Applicant:
Siemens-Schuckertwerke Aktiengesellschaft,

Berlin and Erlangen,
Erlangen, Werner-von-Siemens-Str. 50

Dr. rer. nat. Wolfgang Keller, Pretzfeld,

and Dr. Günther Ziegler, Erlangen,

have been named as inventors

at one end of which a single-crystalline seed crystal is attached. According to the invention, the method is characterized in that a seed crystal with a significantly smaller cross section than the silicon rod is melted, that all passages of the melting zone begin in the seed crystal and that the migration speed of the melting zone in the seed crystal between 7 and 15 mm per Minute is chosen so that the silicon cross-section is constricted at the transition point from the seed crystal to the silicon rod by temporarily moving the rod ends apart at a speed greater than 25 mm per minute, so that from this constriction point the speed of the melting zone is steadily reduced until the full cross-section of the silicon rod is reached and that finally the melting zone is passed through the silicon rod at a speed of less than 7 mm per minute.
The invention is to be explained in more detail using an exemplary embodiment. In the drawing, a silicon rod 2 is shown in which there is a molten zone 3, which is through a suitable

209 577/377

Heater, ζ. B. an induction coil 4> can be generated. The induction coil 4 is operated at a high frequency of, for. B. 4 MHz and can conveniently be designed as a flat coil. This has the advantage that the melting zone can be kept particularly short. The induction coil 4 can with the help of a. Transport device, e.g. B. a spindle that moves a slide can be guided over the entire length of the rod. ^r - - *

At the lower end of the silicon rod 2 is a Fused seed crystal, which has a significantly smaller cross section than the silicon rod 2. Of the Silicon rod 2 can, for example, have a diameter of 12 mm and seed crystal 5 such from 3 to 5 mm. Both the upper end of the silicon rod 2 and the lower end of the seed crystal 5 are clamped in brackets that are moved against each other in the axial direction of the rod can.

Assume that the silicon rod 2 has such a purity that it is practically non-conductive is. In order to create a melting zone, the rod must therefore preheated at least at one point become that it becomes slightly conductive. The heating coil can then be used for further heating done by high frequency. The preheating can be carried out by means of radiation or z. B-, in such a way that at the upper clamping point of the silicon rod 2, a conductive material, e.g. B. molybdenum is attached, which is then with the help of Induction coil 4 can be heated and the neighboring semiconductor material is heated by conduction. The entire process is carried out again under protective gas in a high vacuum chamber.

From the preheated point, an annealing zone is through the silicon rod 2 down to the Attachment point of the seed crystal 5 out. The silicon rod and the seed crystal are melted here 5 melted. The transition between the thick silicon rod 2 and the is advantageous thin seed crystal designed into a flat cone.

After the seed crystal has melted, the melting zone then runs through the entire silicon rod led to its upper end, an annealing zone led to the seed crystal and this several times repeated. Here, the speed of the heating coil during the upward movement (melting zone) for example 3 mm per minute and in the downward movement (annealing zone) 200 mm per Minute.

The following procedure is used for the last passage through the melting zone. After the glow zone has arrived at the seed crystal, the coil movement is stopped. After arising the melting zone is the heating coil 4 at a speed greater than 7 mm per minute, z. B. 10 mm per minute, guided upwards. At the same time, at the point 6 at which the seed crystal 5 in passes the flat cone, the rod ends at a speed of 25 mm per minute or more apart until the diameter of the silicon is reduced to about 2 mm at this point. Thereafter the movement of the rod ends is switched off. You can optionally a few millimeters above add a second taper to the first taper in the same way.

After the end of the movement of the rod ends, the speed of the heating coil is steadily reduced. This steady reduction should extend approximately over the length of the cone. In full Cross-section of the silicon rod - in the present example therefore with a diameter of 12 mm - is then a speed of the heating coil of less than 7 mm per minute, for example 4 mm per minute, achieved. This speed is maintained until the top of the rod. If in known Put one of the rod holders, for example the lower one, in rotation during the zone melting » is offset, this rotation must be vibration-free. Otherwise it will last during this one The passage of the melting zone is switched off in order to exclude possible vibrations. Those made by this process Silicon single crystals were completely free of dislocations.

An interpretation of the improvement in crystal quality resulting from the process described can be found in the following considerations: First of all, growing from a seed crystal with a small cross-section, significantly fewer dislocations than from a seed crystal with a stronger one Cross-section into the resulting single crystal. A further reduction then brings the Reduction in cross-section in the last pass through the melting zone. The reduction in dislocations by reducing the temperature gradient as a result of these two measures also plays a role a role.

In addition, the dislocations present in the seed crystal are probably due to the high speed of migration can no longer follow the melting zone in the seed crystal and are, so to speak, "suspended" will. On the other hand, the emergence of new dislocations in the thicker part of the silicon rod becomes prevented among other things by a lower speed of the movement of the melting zone. In this The relationship is the steady decrease in speed in the transition area between the both different cross-sections are very important.

Claims (2)

PATENT CLAIMS: 1. A process for the production of dislocation-free single-crystal silicon by crucible-free zone melting with multiple passages of the melting zone through a vertical silicon rod held at its ends, at one end of which a single-crystal seed crystal is attached, characterized in that a seed crystal with a substantially smaller cross-section than the silicon rod is melted so that all passages of the melting zone begin in the seed crystal and that the migration speed of the melting zone in the seed crystal is selected between 7 and 15 mm per minute during the last passage of the melting zone, so that the silicon cross-section at the transition point from the seed crystal to the silicon rod is temporarily moved apart the rod ends is constricted at a speed greater than 25 mm per minute, that from this constriction point until the full cross section of the silicon rod is reached, the speed of the melting zone is steadily ver is reduced and 5 6 that finally the melting zone through the induction coil designed as a flat coil Silicon rod is produced at a speed small coil. than 7 mm per minute is pulled through. 3. The method according to claim 1, characterized 2. The method according to claim 1, characterized in that the seed crystal to the lower indicates that the melting zone is melted with hooves 5 end of the silicon rod . For this purpose, 1 sheet of drawings ι 209 577/377 4.62