1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing silicon single crystals by the Czochralski method.
2. Description of the Prior Art
The manufacturing method of silicon single crystals by the Czochralski method has heretofore been in use and it has become substantially a complete technique.
In accordance with this technique, as is well known in the art, after a molten silicon material has been charged into a quartz crucible, a seed crystal is brought into contact with the surface of the molten material and simultaneously the need crystal is slowly rotated and pulled from the melt, thereby causing the contact surface to solidify and growing a crystal to obtain a cylindrical single crystal.
At this time, in order to produce a silicon single crystal as a p-type or N-type semiconductor in dependence on the purpose, a suitable amount of a dopant such as boron, antimony or phosphorous is introduced into the molten material. However, the introduction of such dopant into a silicon single crystal is not uniform so that lower the portion of the silicon single crystal, greater is the concentration of the dopant.
Also, in addition to the dopant introduced intentionally into the silicon single crystal as mentioned above, impurities introduced unavoidably during the manufacture, such as, oxygen and carbon are present in considerable amounts. In other words, since the characteristics and the yield of the semiconductor can be improved by the oxygen introduced into the silicon single crystal and thus it is desirable that the oxygen is contained uniformly within the silicon single crystal from top to bottom, generally the lower the portion of the crystal, gets the lower the oxygen concentration becomes.
This is because of the fact that the molten material in the crucible is decreased, as a silicon single crystal is pulled, and the concentration of the dopant is increased and the concentration of oxygen is decreased in the molten material in the crucible. As a result, the dopant present in the pulled and grown silicon single crytal is gradually increased and the oxygen is decreased, thus giving rise to a problem that the quality of the produced silicon single crystal varies along the pulling direction.
Where the specification relating to the components are severe, there is the possibility of the yield of usable wafers being decreased to less than 50% due to such maldistributions of the dopant and oxygen.
As an effective means of overcoming these deficiencies, there have been known methods of continuously or intermittently feeding a silicon starting material so as to maintain constant the liquid level of the molten material. Included among the methods of pulling a silicon single crystal while continuously or intermittently feeding the silicon starting material in such a manner are the inventions disclosed for example in Japanese Laid-Open Patents No. 56-84397 and No. 56-164097.
The former invention relates to a method of manufacturing single crystals by inserting into the molten or liquid starting material in a crucible starting material ingots in the form of single crystals pulled from a molten material of the same composition as the molten starting material and having the same form as the desired single crystal to be grown at a constant rate.
On the other hand, the latter invention relates to a single crystal pull apparatus equipped with a molten material feeder whereby a powdered sample feed tube is externally inserted into an insulating tube and a powdered sample is temporarily stored and melted in the forward end of the powdered sample feed tube thereby intermittently supplying the molted material into a crucible, and these methods have not been put in practical use due to their technical difficulties encountered.
Then, recently the manufacture of high-quality granular polycrystalline silicon has been made possible and it has been considered comparatively easy to feed such granular silicon continuously at a constant rate into the molten starting material as will be seen from Japanese Laid-Open Patent No. 58-172289. However, due to the fact that the falling of the granular silicon onto the surface of the molten starting material causes the solidification to start from the granular silicon, as a principle, it is impossible to use this method to continuously feed granular silicon and grow a single crystal. The solidification starts from the fallen granular silicon for the reasons enumerated as follows.
(a) The melt temperature at the time of pulling a single crystal is just above the melting point of silicon as will be seen from the above-mentioned principle.
(b) The specific gravity of silicon is smaller in solid form than in liquid form and therefore granular silicon floats on the surface of the molten material.
(c) The emissivity of silicon is greater in solid form than in liquid form.
In other words, the granular silicon floats on the surface of the molten silicon of the temperature just above its freezing point so that the heat is rapidly dissipated as radiant heat from the granular silicon and the solidification progresses around the floating granular silicon. In addition, the ripples caused by the falling of the granular silicon gives rise to a problem.
On the other hand, the inventions disclosed in Japanese Laid-Open Patents No. 56-88896 and No. 58-36997 are known in the field of oxide semiconductors. In accordance with these inventions, the diameter of a crystal to be pulled is small so that a small metal crucible of the double type can be used and the double crucible can be heated directly by induction heating, thereby preventing any solidification of the melt between the crucibles. In the case of silicon single crystals, however, it is impossible to use a metal crucible due to the large diameter of single crystals to be pulled, the increased cost and the occurrence of contamination, and therefore a high-purity quartz crucible is generally used. As a result, such induction heating method cannot be used for the manufacture of silicon single crystals.
On the other hand, the invention disclosed in Japanese Laid-Open Patent No. 58-130195 employs a quartz crucible of the double structure type and at glance it appears to have immunity for the solidification in the starting material melting portion. However, as pointed out (at lines 12 to 16 of "Problems that the Invention is to Solve, p. 2, Japanese Laid-Open Patent No. 62-241889), the problem of solidification at the portion in contact with the molten material surface in the inner crucible has not been solved as yet.
Also, in accordance with this invention a silicon starting material feed pipe is inserted between the inner and outer crucibles so that the supply of the starting material is effected by the feed pipe immersed in the molted material on the outer side of the inner crucible. Thus, with such feeding method, the silicon starting material is not melted instantaneously at the molten material surface and, although heated to an elevated temperature, the starting material in solid form is accumulated as such within the feed pipe. Once the accumulation takes place, there is a problem that sintering is caused among the particulate material and between the material and the feed pipe inner wall, thus making impossible the subsequent supply of the material. For these reasons, this invention has not been put in practice as yet.
The invention disclosed in Japanese Laid-Open Patent No. 63-95195 is an improvement on the above-mentioned invention (Japanese Laid-Open No. 58-130195). This invention is constructed so that the interior of a crucible is divided into a crystal growing section and a starting material melting section by a ring-shaped partition and the growing of a crystal is effected while charging granular starting material into the starting material melting section. The improvement on the invention of Japanese Laid-Open Patent No. 58-130195 resides in that a second heater of the annular shape is arranged at the bottom portion of the crucible so as to prevent solidification of the charged starting material and facilitate its melting. However, the problem of solidification starting from the contacting portion with the molten material surface on the inner side of the partition has not been fully solved even in accordance with this invention as yet.
Those silimar to the above-mentioned inventions (Japanese Laid-Open Patents No. 58-130195 and No. 63-93195) include the inventions disclosed in Japanese Laid-Open Utility Model No. 59-141578 and Japanese Laid-Open Patent No. 62-241889 and the invention introduced by the paper in Ann. Rev. Mater. Sci, 1987, Vol. 17, P. 273-279. The first invention (Japanese Laid-Open Utility Model No. 59-141578) shows that a ring-shaped object is floated within the molten material or melt. In this invention, however, there is a convection of the melt between the single crystal pull section and the granular starting material feed section below the floating ring and the temperature on the outer side of the floating ring attains as a principle a temperature just above the melting point of silicon which is substantially equal to that of the single crystal pull section. Thus, the basic problem of the solidification proceeding from the granular silicon floating on the melt surface has not been solved at all. Also, no solution has been made for the problem of the progress of solidification from the floating ring as pointed out in the specification of the second invention (Japanese Laid-Open Patent Specification No. 62-241889, "Problems that the Invention is to solve", lines 12-16), and only the problem of ripples has been solved.
On the other hand, the second invention (Japanese Laid-Open Patent No. 62-241889) shows the provision of a vertical trough along the outer side of a crucible so as to feed silicon starting material into the crucible through the holes formed through the crucible. However, the volume of the material melting section of the vertical trough is so small that if silicon starting material having a very high latent heat of fusion is fed continuously, it eventually becomes impossible to melt the material any longer. In addition, the holes are close to the molten material surface so that the melts of different concentrations are entrained on the convection to move straight to the single crystal interface, thus tending to cause concentration variations and thereby impeding the growth of a high-quality crystal. In addition, the invention requires the processing of a quartz crucible requiring a very high processing cost, thus increasing the cost.
Also, the invention reviewed on the paper (Ann. Rev. Mater. Sci, 1987, vol. 17, P. 273-279) employs a double crucible and partition rings of the fixed type and floating type, respectively, and the problem of solidification from the partition rings remain still unsolved.
On the other hand, the invention disclosed in the Japanese Laid-Open Patent No. 61-36197 shows that a crucible is divided by a partition ring and a heat insulation cover is disposed only above a peripheral starting material melting section to increase the temperature of the melting section and thereby to promote the melting of the starting material. However, this invention is merely intended for heat insulation of the starting material melting section and there is no solution for the problem that during the pulling of a silicon single crystal the inner side of the partition ring is cooled and thus the solidification starts from the partition ring.
The following difficulties will be encountered if a single crystal is pulled while continuously and directly feeding granular silicon into a crucible.
(1) While the temperature of a melt attains a value which is considerably close to the melting point of silicon during the pulling of a silicon single crystal, if, in this condition, granular silicon of a temperature close to the room temperature is fed continuously to the outer side of the partition ring, the granular silicon is no longer melted so that the granular silicon in solid form floats as such on the surface of the melt and the melt solidifies and grows with the granular silicon as a nucleus.
(2) Where the granular silicon melting section and the single crystal pull section are separated, due to the so-called fin effect in the heat transfer and the higher emissivity than the silicon melt, solidification tends to occur from this partition portion so that once the solidification occurs, the solidified material continues to grow and the growth of a sound silicon single crystal is impeded.