APPARATUS FOR GROWING CRYSTALS

Abstract: A crystal growing apparatus includes a crucible (4), a feed pipe (10) and a pulling mechanism. The crucible serves to melt a crystalline material. The feed pipe serves to cause the crystalline material to fall into the crucible to charge the crucible with the crystalline material. The pulling mechanism serves to pull a single-crystal from the molten material in the crucible. A deceleration part (14,15) is provided on the feed pipe for lowering the falling speed of the material into the crucible.

Description: Field of the Invention The present invention pertains to an apparatus for growing crystals such as high purity silicon single-crystals. Prior Art In the manufacture of silicon single-crystals by Czochralski technique, the contact area between a silicon melt and a quartz crucible varies as the quantity of the melt decreases due to the pulling of the crystals, and hence the quantity of oxygen dissolved from the crucible varies. In recent years, however, sever standards have been adopted as to both oxygen and dopant concentrations of silicon single-crystals for use as substrates for semiconductor devices. For this reason, it has been only a part of the ingot that can be utilized as the semiconductor devices, deteriorating the yield of material. Proposed in order to obviate the above problem is an apparatus of the type which keeps the quantity of the melt constant by continuously adding granulated silicon powder material to a crucible to compensate the amount of the pulled crystal (Examined Japanese Patent Application Publication No. 57-40119). In such apparatus, however, the surface of the melt is ruffled upon the falling of the material to develop a disturbance which is transmitted to the crystallization front of the single-crystal ingot, so that crystal defects such as dislocation are caused. To cope with this problem, a cylindrical partition wall has been disposed between the single-crystal ingot and the portion into which the material falls (United States Patent No. 2,892,739 and Non-Examined Japanese Patent Application Nos. 57-183392 and 61-36197). However, there still remains a disturbance which is transmitted to the crystallization front through apertures formed to communicate the outer and inner sides of the partition wall. Besides, the provision of such partition wall requires the use of a crucible greater in size since the space between the partition wall and the crucible as well as the space between the crucible and the crystal ingot has to be rendered sufficiently large. As a result, a large quantity of silicon material is required to ensure a sufficient depth of the silicon melt, resulting in an increase of costs as compared with the apparatus without the partition wall. In addition, the above apparatuses have further disadvantages that the silicon crystals are liable to deposit on the surface of the partition wall, so that the conditions on crystal growth may vary. On a certain occasion, the growing of single-crystals may be even hindered. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved apparatus for growing crystals which can prevent the ruffling and disturbance of the surface of the melt without providing the partition wall. According to the present invention, there is provided an improved apparatus for melting a crystalline material and growing a single-crystal from the molten material, the apparatus comprising a crucible for melting the crystalline material, a feed pipe for causing the crystalline material to fall into the crucible to charge the crucible with the crystalline material, and pulling means for pulling the single-crystal from the molten material in the crucible, characterized by deceleration means mounted on the feed pipe for lowering the falling speed of the crystalline material into the crucible. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of an apparatus in accordance with the present invention; Fig. 2 is a transverse sectional view of a feed pipe of the apparatus of Fig. 1; Fig. 3 is a longitudinal sectional view of the feed pipe of the apparatus of Fig. 1; Figs. 4 to 7 are cross-sectional views showing modifications of the feed pipe of the apparatus of Fig. 1, respectively; Fig. 8 is a view similar to Fig. 1, but showing a further modified apparatus of the invention; Fig. 9 is a cross-sectional view showing a feed pipe of the apparatus of Fig. 8; Figs. 10 to 12 are cross-sectional views showing modifications of the feed pipe of Fig. 9; and Fig. 13 is a cross-sectional view of a part of a still further modified apparatus of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Referring to Figs. 1 through 3 of the accompanying drawings, there is illustrated an apparatus for growing crystals in accordance with the present invention. The apparatus comprises a furnace 1, a heat insulating shell 2 housed in the furnace, a heater 3 disposed in the heat insulating shell, a graphite susceptor 4 or support bowl mounted on a shaft 5 and a quartz crucible 6 accommodated in and supported by the susceptor. The shaft 5 is drivingly connected to a drive mechanism, now shown, which permits the vertical movement and axial rotation of the shaft 5 during operation. A crystal pulling mechanism, which comprises a pulling wire 7 retaining a seed crystal S at its lower end, and a drive mechanism, not shown, for pulling and rotating the wire, is also disposed above the crucible 6. The structure as described above is similar to that of the prior art apparatus, and the feature of the apparatus in accordance with the invention resides in the special design of a material feed pipe as designated at 10. The feed pipe 10, which has a cross-section of a circular shape and is made of quartz, is fixedly secured to and supported by the furnace 1 with its portion at 11 passing through the furnace wall. The pipe is connected at its one end to a material supply mechanism, not shown, and is bent at 12 within the furnace 1 so as to be inclined downwardly toward the interior of the crucible 6. The other end of the feed pipe 10 is positioned adjacent to the inner peripheral surface of the crucible 6 so as to be disposed slightly above the surface of a melt at Y. The inclined portion as at 13 is provided with deceleration means for reducing the falling speed of the material through the pipe. More specifically, a plurality of upper and lower baffle plates 14, 15 of quartz are fixedly secured to the inner peripheral wall of the inclined portion 13 so as to be alternately disposed in longitudinally equally spaced relation to each other. Each upper baffle plate 14, which is mounted on a downwardly facing surface of the inner peripheral wall of the pipe, is inclined downwardly toward its upwardly facing surface, while each lower baffle plate 15, which is mounted on the upwardly facing surface, is inclined downwardly toward the downwardly facing surface. Each baffle plate 14, 15 defines a gap or space 16 between its lower end and the inner wall of the pipe. The minimum width T of the space 16 is set so that the granulated powder material can easily pass therethrough, and that the falling speed of the material can be appropriately reduced. Further, the inclination angle of the baffle plate 14, 15 is set by taking the inclination angle of the pipe into consideration so that the material should not remain there and that the falling speed of the material is decreased below a prescribed value. In the crystal growing apparatus as described above, the material falling through the feed pipe 10 is caused to collide with the baffle plates 14 and 15 to lose its kinetic energy, and besides its path is bent complicatedly. As a result, the falling speed of the material into the melt Y can be reduced, and the ruffling of the melt Y can be rendered sufficiently small. Accordingly, the crystallization front of the single-crystal ingot becomes less susceptible to disturbance which may cause crystal dislocation, so that single-crystals of a high quality having no defects can be manufactured at a high yield. Besides, since a smaller crucible can be employed as compared with the prior art apparatus having a partition wall, the residual quantity of the material can be reduced substantially, so that the yield is further increased. The disadvantage that the crystal may adhere to the partition wall can as well be obviated. Further, since the residence time of the material within the feed pipe 10 is prolonged, the material can be preheated by the atmosphere in the furnace 1. Consequently, the temperature variation caused due to the supply of the material can be reduced, and the unnecessary recrystallization of the melt can be prevented. Further, in the foregoing, the material to be added may be in the form of flakes or chips other than granules. Fig. 4 depicts a modified apparatus in accordance with the present invention which differs from the previous apparatus only in that the lower end of the feed pipe 10 is branched off to provide a breeches pipe portion 20. The breeches pipe portion, each pipe portion of which is inclined with respect to the remaining portion, is disposed in such a manner that the pipe portions are directed circumferentially of the crucible 6. With this construction, the falling speed of the material can be further reduced since the material collides with the breeaches pipe portion. In addition, inasmuch as the material is scattered circumferentially of the crucible 6, the ruffling of the melt Y can be further prevented. Fig. 5 shows another modified apparatus in accordance with the present invention which differs from the first embodiment shown in Figs. 1 to 3 in that the feed pipe 10 is elongated transversely so as to have an elliptical cross section. Although not illustrated, the pipe could as well be formed to have a rectangular cross section. In this embodiment, the deceleration effect can be further improved, and the residence time can be further prolonged to enhance the heating effect. Although in the above embodiments, the baffle plates 14 and 15 are provided as the deceleration means, the upwardly and downwardly facing portions of the wall of the feed pipe 10 may be alternately recessed to provide a plurality of protrusions 21 serving as the deceleration means as shown in Fig. 6. Further, as illustrated in Fig. 7, the feed pipe may be formed so as to meander spirally to provide the deceleration means. The feed pipes of these two embodiments can be manufactured at a reduced cost. Figs. 8 and 9 depict a further modified apparatus in accordance with the present invention which differs from the embodiment of Fig. 4 only in that the breeches pipe portion is horizontally formed to define a tee pipe portion 22. The horizontal length and diameter of the tee pipe portion 22 are set so that the material falling through the feed pipe 10 is held at the tee portion for a desired length of time and then is caused to fall into the melt Y. In this embodiment, the material is caused to fall into the melt after having been held at the tee portion 22 for a prescribed length of time. As a result, the material undergoes a radiant heat for a longer period of time, and besides thermal conduction is caused through the inner bottom surface of the tee pipe portion 22. Accordingly, the material can be markedly heated before it falls into the melt. Therefore, the temperature difference between the material and the melt can be rendered small, so that the temperature fluctuation of the melt due to the addition of the material can be effectively eliminated. Consequently, the material newly added to the melt can be certainly dissolved and the solidification of the melt can be surely prevented, so that the development of crystal defects due to the temperature fluctuation in the crystallization front can be prevented. In order to clarify the effect of this embodiment, the temperature rise of a silicon material particle due to the heat radiation will be hereinafter calculated. Suppose the particle size D and emmisivity ε to be 1 mm and 0.45, respectively. Other parameters of the particle will be set forth as follows. Surface area: S = 4π(D/2)² = 3.142 × 10⁻⁶ (m²) Volume: V = 4/3π(D/2)³ = 5.24 × 10⁻¹⁰ (m³) Mass: M = γV = 1.22 × 10⁻⁶ (kg) Heat capacity: C = MC p = 8.3 × 10⁻⁴ (J/°C) Radiant energy transferred to the particle for a period of t seconds: Q = 4.88Sε((1273/100)⁴ × ((T + 273)/100)⁴ × 10³ × 4.187 × 1/60²t(J) wherein T (°C) denotes the temperature of the particle while the temperature of the atmosphere of Ar gas is set to be 1,000°C. Increase of temperature of the particle: ΔT = Q/C(°C) Thus, the change of the increase of the particle temperature with the passage of time will be as follows. It takes 0.32 seconds when the silicon particle is caused to fall 50 cm in a free condition, and hence only about 100°C increases according to the above formula. Accordingly, it is clear that the temperature of the particle can be markedly increased by prolonging the falling period of time by several seconds. In addition, although in the above calculation, the radiation heat transfer is only taken into consideration, thermal conduction is in fact caused by the high temperature atmospheric gas in the feed pipe 10 as well as by the contact of the material with the inner peripheral surface of the feed pipe. Accordingly, the temperature would be still more increased by the prolongation of the residence time. Fig. 10 shows a further modified apparatus according to the present invention which differs from the previous embodiment shown in Figs. 8 and 9 only in that the tee pipe portion is replaced by a bent portion 23 of a prescribed length slighty sloping downwardly and extending circumferentially of the crucible 6. Such bent portion 23 serves to cause the material falling through the feed pipe to move slowly therealong. With this arrangement, the residence time can be easily regulated by modifying the inclination angle or length of the bent portion 23. Besides, since the bent portion 23 is disposed near to the melt, the material moving therealong undergoes a great radiant heat. Fig. 11 shows a further modification in which the lower end of the feed pipe 10 is formed into a closed semispherical shape, and a number of apertures 24 are formed therethrough. Fig. 12 shows a modified apparatus in which the lower end of the feed pipe is formed into an expanded spherical shape and a small through hole 25 is formed in its lower face. In the embodiments shown in Figs. 8 to 12, the tee pipe portion, bent portion, semispherical portion and spherical portion respectively serve as means for holding the material in the feed pipe for a prescribed length of time. Obviously many other modifications and variations of the present invention are possible in the light of the above teachings. Each embodiment may be practiced alone or in combination with any one of the other embodiments. Besides, as shown in Fig. 13, a crucible with a partition wall 6a may be employed to make the prevention of the disturbance of the melt Y more perfect. Anyway, it is to be understood that within the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: 1. An apparatus for melting a crystalline material and growing a single-crystal from the molten material, said apparatus comprising a crucible (6) for melting the crystalline material, a feed pipe (10) for causing the crystalline material to fall into said crucible (6) to charge said crucible (6) with the crystalline material, and pulling means (7) for pulling the single-crystal from the molten material in said crucible (6), characterized by deceleration means (14, 15, 21) mounted on said feed pipe (10) for lowering the falling speed of the material into said crucible (6). 2. A crystal growing apparatus as defined in claim 1, wherein said deceleration means comprises a plurality of baffle members (14, 15) mounted on an inner peripheral wall of said feed pipe (10). 3. A crystal growing apparatus as defined in claim 2, wherein said feed pipe (10) has an inclined portion (13) sloping downwardly toward the interior of said crucible (6) and a lower end positioned adjacent to an inner peripheral surface of said crucible (6) so as to be disposed, in use slightly above the surface of said molten material in said crucible (6), said baffle members (14, 15) being comprised of upper and lower baffle plates fixedly secured to the inner peripheral wall of said inclined portion (13) so as to be alternately disposed in longitudinally spaced relation to each other. 4. A crystal growing apparatus as defined in claim 3, wherein said feed pipe (10) has a generally circular cross-section. 5. A crystal growing apparatus as defined in claim 3, wherein at least said inclined portion (13) of said feed pipe (10) has a generally elliptical cross-section. 6. A crystal growing apparatus as defined in claim 3, wherein at least said inclined portion (13) of said feed pipe (10) has a generally rectangular cross-section. 7. A crystal growing apparatus as defined in claim 3, wherein said lower end of said feed pipe (10) is branched off to provide a breeches pipe portion (20). 8. A crystal growing apparatus as defined in claim 1, wherein said feed pipe (10) has a plurality of portions spaced longitudinally thereof and recessed to provide a plurality of protrusions (21) protruding inwardly thereof and serving as said deceleration means. 9. A crystal growing apparatus as defined in claim 1, wherein said feed pipe (10) is formed so as to meander spirally to provide said deceleration means. 10. A crystal growing apparatus as defined in claim 1, further comprising means (22, 23) for holding said crystalline material in said feed pipe (10) for a prescribed length of time, said holding means (22, 23) being formed on said feed pipe (10). 11. A crystal growing apparatus as defined in claim 10, wherein said feed pipe (10) has a tee pipe portion (22) formed at its lower end and serving as said holding means. 12. A crystal growing apparatus as defined in claim 10, wherein the lower end of said feed pipe (10) is bent to provide a bent portion (23) slightly sloping downwardly and extending circumferentially of said crucible, said bent portion (23) serving as said holding means. 13. A crystal growing apparatus as defined in claim 10, wherein said feed pipe (10) has a closed lower end having at least one aperture formed therethrough, said closed end serving as said holding means. 14. A crystal growing apparatus as defined in claim 13, wherein said closed lower end of said feed pipe (10) is of a generally semispherical shape. 15. A crystal growing apparatus as defined in claim 13, wherein said closed end of said feed pipe (10) is of a generally spherical shape.

Inventors: ARAI, YOSHIAKI, KIDA, MICHIO, ONO, NAOKI, SAHIRA, KENSHO
Applicants: MITSUBISHI MATERIALS CORPORATION
Application year: 1988
Main classification: C30B15
IPC codes: C30B15, B01J4