1. Field of the Invention
The present invention pertains to apparatuses and processes for growing crystals of semiconductor materials for use in making integrated circuit devices and solar cells, for example. More particularly, the invention relates to such apparatuses and processes which operate in accordance with a Czochralski technique making use of a multi-wall type quartz crucible.
2. Prior Art
In a batch type apparatus for the growing of silicon single-crystal ingots, the quantity of a silicon melt contained in a quartz crucible decreases as the crystal grows. As a result, the quality is not uniform longitudinally within the crystalline ingot obtained by the apparatus of this type. More specifically, the oxygen concentration in the crystal depends upon the quantity of oxygen dissolved in the silicon melt, thereby varying as the silicon melt in the quartz crucible varies. In addition, since the quantity of the melt varies, the thermal distribution in the furnace fluctuates and the convection in the melt varies, so that the crystallization front varies. Further, dopant such as phosphorus, boron and antimony is generally added to the melt to control the electric conductivity of the single-crystal. However, insomuch as the segregation constant of such impurities is not identical to 1, the dopant concentration in the crystal differs as the crystal grows.
Thus, the quality of the single-crystal ingot varies in its longitudinal direction, and therefore it has been only a part of the ingot that has a desired quality. Additionally, the productivity has been low in such batch process.
The approach to resolve the above disadvantages is the use of a continuous type apparatus hitherto proposed wherein the single-crystal ingot is grown while the quartz crucible is being charged with a material. U.S. Pat. No. 2,892,739 or Japanese Patent Application A-Publication No. 61-36197 describes one such apparatus which is the simplest in structure and makes use of the combination of a double crucible and the continuous charging of the material in the form of powders, lumps or granules.
The above continuous type apparatus has, however, the following problems. First, although the temperature of the silicon melt is above 1,420.degree. C., the quartz of which the crucible is made begins to get softened at about 1,100.degree. C. As a result, although the outer crucible supported by a graphite susceptor is not deformed from its original shape so greatly, the inner crucible is susceptible to a large deformation since it is not supported sufficiently, so that the growing of the single-crystal is adversely affected.
Specifically, a great quantity of heat has to be applied to the crucible in an initial stage of the pulling cycle in order to melt the material fed in the crucible, and the temperature in the furnace reaches the maximum at that time. Therefore, the deformation of the inner crucible becomes the greatest at that time. FIGS. 1 to 4 of the accompanying drawings schematically illustrate conventional apparatuses each of which comprises a quartz double crucible assembly 100 housed in a susceptor 102 and comprising inner and outer crucibles 100a and 100b, and a resistance heater 104 disposed so as to surround the crucible assembly 100. The double crucible assembly 100 is filled with a charge of silicon material 106, as shown in FIGS. 1 and 3, and the charge is heated by the heater 104 until it is melted thoroughly. Each inner crucible 100a, however, is subjected to deformation as shown in FIGS. 2 and 4 as the silicon material is melted.
Secondly, since quartz has a high heat-insulating effect, the temperature is lower at the inner side of the inner crucible than at the outer side thereof. This may be advantageous during the growth of crystals since the silicon material introduced between the inner and outer crucibles can be melted efficiently, but disadvantageous when melting the silicon material initially fed in the crucible since the inner crucible itself prevents the heat generated by the heater from being transferred to its interior, so that the efficiency of the melting is lowered. If the quantity of heat applied to the material should be increased to reduce the time required for the melting, the inner crucible would be subjected to an excessive deformation.
Furthermore, in Japanese Patent Application A-Publication No. 55-47300, there has been proposed an apparatus as shown in FIG. 5 which comprises the inner crucible 100a fixedly secured to a fixed portion 108 through a support 110. Such apparatus, however, has the disadvantages that the inner crucible 100a cannot be rotated together with the outer crucible 100b. For growing a single-crystal ingot, the crucible has to be generally rotated at a speed of 5 rpm to 10 rpm. Without such rotation, the formation of single-crystals may be difficult.
In conjunction with the use of the double crucible, Japanese Patent Application A-Publication No. 58-204895 discloses an apparatus which comprises a mechanism for causing the inner crucible to rotate and move upwardly and downwardly. The apparatus, however, has the drawbacks that it in itself is large in size and intricate in construction, thereby being high in cost.