COLD CRUCIBLE SEMICONDUCTOR DEPOSITION PROCESS AND APPARATUS

Abstract: Method and apparatus for the production of high purity semiconductors, such as polycrystalline silicon. An initial charge of solid semiconductor (30) is inductively heated in a cold crucible (16) by application of high-frequency electromagnetic field produced by electric coil (26), such heat being sufficient to melt solid semiconductor (30) and to partially levitate the molten semiconductor (32) so that there is no contact between the molten semiconductor and the walls of the cold crucible. The base of the molten column (32) formed is supported on the unmelted semiconductor (30) so that the melt is not contaminated by contact with any foreign material. Vaporized semiconductor is then brought through conduit (22) into contact with the exposed surfaces of the molten, partially levitated semiconductor (32) in the cold crucible (16), and a portion of the vaporized semiconductor is absorbed into the molten semiconductor (32) thereby adding to its mass. Relative translational motion between the molten partially levitated semiconductor (32) and cold crucible (16) provides for the continuous withdrawal and solidification of semiconductor (32), preferably at a rate substantially equal to the rate at which vaporized semiconductor is added to the molten mass through conduit (22).

Description: Description COLD CRUCIBLE SEMICONDUCTOR DEPOSITION PROCESS AND APPARATUS Technical Field This invention is in the field of materials and more particularly relates to the manufacture of high purity semiconductors. Background Art As semiconductor devices have become more and more sophisticated, there has been a concomitant in¬ crease in the stringency of the requirements for con¬ trol of the purity levels in the semiconductor materials, such as silicon. In many of these devices, for example, it has become necessary to control impurity levels to within the order of a few parts per trillion. Additionally, dislocation densities and lifetime specifications must be even more rigorously controlled to attain suitable devices with acceptable production yields. The problem of producing highly pure silicon is aggravated because molten silicon is a particularly outstanding solvent for most materials. It is known, for example, that molten silicon will attack a quartz reactor vessel and will become contaminated with oxygen beyond the level which can be tolerated in some semi¬ conductor device applications. Because of the strong demand for highly pure silicon coupled with the in¬ ability to use most conventional production techniques, there has been a tremendous amount of. research directed to finding suitable methods of producing highly puri¬ fied silicon in commercial quantities. One method which has been widely employed to produce high purity silicon involves the thermal de¬ composition of a purified silicon compound, such as trichlorosilane, followed by deposition in a reaction vessel onto an-electrically heated high purity silicon filament. The filament must be maintained, of course, below the melting point of silicon. This process is not without problems. For example, the silicon deposition rate is limited because the sili- ' con filament must be maintained at a temperature below the melting point of silicon, which is approximately 1440°C. Additionally, this process is usually carried out within a quartz reaction vessel and the vaporized reactants often react with the inner walls of the reactor producing oxygen contamination of the silicon deposit. Although double-walled, water-cooled quartz reaction chambers have been developed to overcome this, such reactors are inherently fragile and are often thermally inefficient. In addition, power leads enter- ing the reaction chamber must be carefully selected to prevent reaction with the feed vapor or by-products. Furthermore, silicon deposition in this process is in¬ herently a batch operation which does not lend itself to modifications for continuous deposition. After formation of a silicon rod by this process, the rod is typically removed from the reactor and subjected to float-zoning operations to purify it. Such operations not only enhance the possibility of introducing contami¬ nants, but are also time-consuming and costly. Although not widely practiced commercially, many other techniques for producing high purity silicon have OM been proposed which involve deposition of vaporized silicon into a molten portion of a solid silicon element which is typically inductively heated to produce the molten portion. Such techniques are described, for example, in ϋ, S. Patent Nos. 2,993,762; 3,069,244; 2,964,396; 3,055,741; 3,095,279; 3,036,892; and 2,999,737. All of the above methods have been attempts to avoid contact between molten silicon and any part of the reaction vessel due to severe contamination prob¬ lems which invariably have resulted from such contact. However, another patented method is described in U. S. 3,078,150, issued to Raymond, where there is contact between an annular water-cooled partition employed very much like a die in the production of a rod of single crystal silicon. The Raymond method involves thermal decomposition of a silicon hydride by bringing it into contact with a molten zone at one end of a single crystal seed with the unmelted portion of seed being shielded from the hydride. A quartz reactor is employed which is divided into an upper and a lower chamber by the coaxially arranged hollow annular partition which is preferably formed of silver, and is kept cool by circu¬ lating water or other cooling fluid therethrough. The upper end of the seed crystal is melted by directly coupl¬ ing it to high frequency alternating currents applied through an annular coil surrounding the upper portion of the seed crystal. A solid liquid interface is main¬ tained in the lower chamber. Molten silicon contacts the inner walls of a central hole in the water-cooled partition. - m ΪEA lP- ■ OMPI The Raymond method is particularly directed to the production of single crystal silicon and is limited to the production of relatively small quantities since the diameter of the hole in the annular partition must be kept relatively small to allow proper support of the molten silicon column. Although the patentees state that the inner walls of the annular partition are not wetted by molten silicon, it is likely that some molten silicon would contact these walls and become contaminated to a degree which is intolerable for many semiconductor device applications. Although much effort has been directed to producing high purity silicon by methods which eliminated the use of a crucible, there have been some efforts devoted to employing a more recently developed form of crucible known as a cold crucible. In general, cold crucibles are formed from a plurality of tubes which form a cage. Cooling fluid is circulated through the tubes to keep their surfaces cold and an electric induction coil is located outside of the cooling tubes to apply a high- frequency electromagnetic field to solid material located within the tubes. Such cold crucibles were originally employed in a crystal-growing technique known as the skull-melting technique because a very thin skull of solid material formed at the inner surface of the cooling tubes and all molten material was contained within this skull. Contamination was avoided because only the skull, formed from the same material as the molten mass, contacted the tubes. Cold crucibles typical of those employed in the skull-melting process are described in the following U. S. Patents: 3,461,215 to J. Rebo x; 3,582,528 to Veale et al.; 3,824,302 and 3,984,524 to Alexandrov et al.; 4,049,384 to enckus et al.; and in British Patent No. 1,373,888 to Alexandrov et al. Initially, . OM such cold crucibles were employed to produce refractory oxides, such as hafnia, zirconia, sapphire and ruby. Rummel discloses in U. S. 3,759,670 the use of a cold crucible in much the same way but to form a molten mass of silicon from which a crystal of silicon can be pulled employing a seed crystal by the Czochralski method or any similar technique. Other researchers attempting to employ cold crucibles for the production of highly pure silicon have noted an interesting levitation phenomenon which is achieved if the current density resulting from the applied Rf field is maintained within certain ranges. Thus, Sterling et al. describe and claim in U. S. 3,520,980 a cold crucible which is sufficient for maintaining a molten mass of silicon in a levitated state which can be employed in a Gzochralzki crystal pulling technique. Similarly, Reboux in French Patent Application 74954 de¬ scribes the inductive heating of silicon in a cold crucible followed by pulling of a monocrystal of sili- con from the molten mass. Sterling et al. in ϋ. 'S. 3,069,241 disclose a method for producing silicon in a cold crucible wherein an initial charge consisting of a silicon slug is introduced into a cold crucible, preheated to reduce its resistance and inductively heated by high-frequency alternating current to form a molten mass in the cold crucible. Silane gas is then bubbled through the molten silane from the bottom to build up the molten mass. When the molten mass reaches a certain level, the bubbling silane gas is discontinued and the crystal may be pulled by the Czochralski or other crystal pulling techniques. Silane gas is then used to regenerate the molten mass so that another crystal can be pulled. OMPI /». WIPO - * The technique described ' in the Sterling et al. '241 patent is inherently a batch operation. It also appears that this method involves additional problems, such as plugging of the silane inlet pipe, since this pipe is located at the base of the cold crucible where freezing occurs. Despite the vast amount of research which has been carried on to conceive and develop a method for produc¬ ing high purity silicon on a commercial scale, very few of the attempts have resulted in processes which have reached commercial production. Even those few which have reached such a stage suffer from numerable disadvantages which must be overcome and which invaria¬ bly result in relatively high costs for high purity silicon. Disclosure of the Invention The invention relates to a method and apparatus for producing high purity-semiconductors, such as silicon. The apparatus comprises a controlled-atmosphere chamber with a cold crucible located therein. Means for applying a high-frequency electromagnetic field to semiconductor contained in the cold crucible are provided together with means for introducing vaporous semiconductor into the controlled-atmosphere chamber. Means for exhausting vaporous species from the chamber, means for cooling a portion of molten semiconductor contained within the cold crucible, and means for withdrawing solidified semiconductor product are also provided. The method comprises placing a charge of solid semiconductor in the cold crucible contained within the controlled-atmosphere chamber. The controlled-atmosphere OMP cha ber may be flushed with a protective atmosphere, such as argon, and the initial charge may be preheated to improve electrical coupling. Subsequently, the elec¬ tric coil positioned around the cold crucible is ener- gized to apply ' a high-frequency electromagnetic field which is sufficient to melt the initial charge of semi¬ conductor contained within the reaction zone of the cold crucible -and to partially levitate the molten semiconductor so that it does not contact the walls of the cold crucible. A vapor stream which contains the semiconductor to be produced in elemental form is intro¬ duced into the controlled-atmosphere chamber and thereby contacts exposed surfaces of the molten mass of semiconductor contained in a partially levitated state in the reaction zone of the cold crucible. At least a portion of the vaporous semiconductor is ab¬ sorbed by the molten semiconductor which adds to the • total molten mass present. Relative translational movement between the molten semiconductor mass and cold crucible is provided which causes a portion of the molten mass to be withdrawn from the heated reaction zone of the cold crucible after which it solidifies as solid semiconductor product. The method and apparatus for producing high purity semiconductors described herein has many significant advantages. Among these is the fact that the deposition rate of semiconductor is not temperature limited, as is the case with the heated filament technique for producing silicon. In fact, molten silicon contained within the cold crucible employed in this process can be superheated to provide even further acceleration of the process, if desired. The partial levitation achieved in the cold crucible has the major advantage of preventing silicon contami¬ nation due to contact with the crucible walls coupled with its effect of providing maximum surface area for contact between vaporized semiconductor compound and molten semiconductor compound. Thus, partial levitation operates to eliminate 1 the major drawback of all prior processes involving crucibles other than cold crucibles while maximizing surface area available for contact with silicon vapor which increases production capacity. Another major advantage is that the method de¬ scribed herein be run continuously, or quasi-continuously by extracting solidified silicon from the bottom of the cold crucible at a rate equal to the rate of deposi¬ tion of vaporized semiconductor feed. Finally, the diameter of the molten semiconductor mass, and there¬ fore the resultant solidified product, is limited only by the inner dimensions of the cold crucible structure and the power of the induction heater. This offers far greater latitude in producing large volumes of material than other techniques as the heated silicon filament technique. Brief Description of the Drawing The Figure is a schematic illustration of an apparatus for producing high purity semiconductor ma¬ terial according to the cold crucible deposition process described herein. Best Mode of Carrying Out the Invention The invention can be further described by refer¬ ence to the Figure. OM A controlled-atmosphere chamber is provided which may be formed from a fluid-cooled reaction chamber jacket 10 fabricated from stainless steel, quartz or similar high-temperature materials. Chamber 10 can be cooled by water or other cooling fluid which enters at a cooling fluid inlet 12, circulates through the reaction chamber jacket 10, and exits from cooling fluid outlet 14-. Jacket 10 is cooled in this manner to mini¬ mize deposition of vaporous feed materials or by-products on its inner surfaces. Cold crucible 16 is contained within reactor chamber 10. It can be formed from a plurality of ver¬ tically oriented annular tubes 18 arranged to form a cage. Typically, tubes 18 are connected to one or more manifolds 20 which supply water or other cooling fluid to tubes 18 or collect such fluids therefrom. Although cold crucible 16 can have a variety of con¬ figurations, a ' particularly convenient configuration is one similar to the cold crucible described in Wenckus et al., ϋ. S. 4,049,384, the teachings of which are hereby incorporated by reference. Vaporous feed materials, as well as carrier and flushing gases, enter reaction chamber through inlet 22. Vaporous feed materials not consumed during operation, as well as vaporous by-products, can be exhausted through the exhaust outlet 24. Electric coil 26 surrounds cold crucible 16 and is used to apply a high-frequency electromagnetic field to semiconductor contained therein. A wide range of frequencies and powers are satisfactory. For a 3-inch diameter cold crucible, for example, frequen¬ cies in the range of 200-250 KHz and applied powers of 25-50 KW are satisfactory. It should be noted that significant superheating cannot only be tolerated, but OMPI can be preferable in some cases. Those skilled in the art will be able to ascertain appropriate operat¬ ing parameters, such as frequency and applied power levels, using no more than routine experimentation. Pedestal " 28 is provided at the lower end of reac¬ tion chamber 10 to support a rod of solid semiconductor 30. Pedestal 28 may be moved up and down to move the semiconductor material longitudinally with respect to cold crucible 16. The shaft of pedestal 28 has a pressur tight seal with reaction chamber 10 so that the pressure within chamber 10 can be elevated or.reduced without leaks around the pedestal shaft. The apparatus is used as- follows. A solid charge of semiconductor material 30, such as a rod of poly- crystalline silicon, is placed on retractable pedestal 28. Pedestal 28 is then elevated to its upper most position. At this point, the reactor is flushed with a protective atmosphere such as argon, hydrogen, or a combination of both. After the reactor has been flushed, solid semiconductor 30 is preheated to reduce its electrical resistance and permit and/or increase the efficiency of Rf coupling. This can be done by employing a radiant heater or by other known means. After preheating, coil 26 is energized and an Rf energy field is applied to heat solid semiconductor 30. As heating takes place, the portion of semiconductor 30 contained in the reaction zone of cold crucible 16, which is that portion surrounded by the Rf coil, is heated above its melting point and becomes molten se i- conductor 32. As is known, molten semiconductors such as silicon do not contact the cold crucible walls if an appropriate field is applied, and thus, are referred to herein as being "partially levitated. " They are not totally levitated, of course, because their bottom portion rests on the solid semiconductor 30. The Rf input is adjusted to stabilize this partially levitated condition. When this state exists, it can be seen that there is no portion of the molten semiconductor 32 which contacts the walls of cold crucible 16, and the only portion of molten semiconductor 32 contacting any other surface is the bottom surface which rests upon the rod of solid semiconductor 30. Thus, there is no opportunity for contamination of molten semiconductor 32. When partial levitation of molten semiconductor 32 has been achieved, vaporous feed is introduced through inlet 22 to the reaction chamber. Vaporous feed suitable for producing silicon would be a gaseous silicon com¬ pound, such as SiH., Sil, or SiBr.. Any silicon compound is suitable as long as it is vaporous or vaporizable, capable of being purified to the degree required in the final semiconductor material, and as long as its breakdown products don't serve as impurities in the silicon being produced. Of course, carrier and/or diluent gases which don't interfere with the purity of the semiconductor being formed can be employed with the vaporous silicon feed. • When the vaporous feed, such as SiH. gas, enters the reaction chamber and contacts hot molten silicon, it undergoes thermal cracking into pure silicon and vaporous by-products. Pure silicon vapor is absorbed by the molten silicon thereby adding to the mass of pure molten silicon. The method can be carried on continuously by lower¬ ing support pedestal 28 thereby extracting molten semi- conductor from cold crucible 16 whereupon it solidifies as part of solid semiconductor 30. Solid semiconductor 30 can be withdrawn from cold crucible" 16 and the reaction chamber, if desirable, at a rate of approximately equal to the rate of deposition of vaporous semiconductor feed into molten semiconductor 32. In a production operation, sensors could be used to monitor the height of the molten semiconductor column and to transmit a signal electronically to lower support pedestal 28. Gaseous by-products of the operation, as well as excess vaporous feed riot absorbed into the molten semi¬ conductor, can be exhausted through exhaust outlet 24. The pressure within reaction- chambe 10 can be maintained at atmosphereic pressure, or in the alternative can be elevated or reduced. It may be desirable in some cases, for example, to run the process at elevated pressure to obtain more throughput for a given reaction chamber size. On the other hand, reduced pressures may also be desirable under some circumstances, particu¬ larly if it is desirable to minimize the formation of a boundary layer of gas around the surface of the molten semiconductor. In cases where elevated or reduced pressures are employed, appropriate pressure-tight- seals should be employed at locations where elements extend through the walls of. reaction chamber 10 to prevent leaks out of or into reaction chamber 10. Industrial Applicability This invention has industrial applicability in the production of high purity semiconductors. Equivalents It will be clear to those skilled in the art, of course, that the solid semiconductor formed need not be in circular rod form but could have any shape desired. Similarly, it will be clear that although the process has been described in terms of the production of silicon. -£ 3REA OMPI other semiconductors, such as germanium, could be produced by the same basic techniques. Those skilled in the art will also recognize other equivalents to the specific elements of the apparatus, starting materials, techniques, etc., and such, equivalents are intended to be covered by the following claims. OMPI

Claims: Claims 1. A method for the continuous production of a semiconductor in a cold crucible reactor con¬ tained within a controlled-atmosphere chamber and having a coil therearound for applying a high frequency electromagnetic field to a reac¬ tion zone within said cold crucible, comprising: a. introducing an initial charge of solid semiconductor into the • cold crucible; b. energizing said coil to apply a high-frequency electromagnetic field sufficient to melt the initial charge of semiconductor within the reaction zone of the cold crucible and to partially levitate the molten semiconductor so that it does not contact the walls of said cold crucible; c. introducing a vapor stream containing a source- of semiconductor into the controlled-atmosphere chamber whereby at least a portion of vaporous semi¬ conductor contacts the molten mass within the reaction zone and is ab- sorbed therein, and, d. providing relative translational movement between molten semiconductor and the cold crucible whereby a portion of said molten mass is continuously - withdrawn from the heated reaction zone causing it to solidify. -15- 2. A method of Claim 1 wherein said semiconductor comprises silicon. 3. A method of Claim 2 wherein said vapor stream comprises a vaporous silicon compound. 4. A method of Claim 3 wherein said vaporous silicon compound is selected from the group con¬ sisting of SiH 4 , SiHCl,, Sil 4 and SiBr,. 5. A method of Claim 4 wherein said relative trans- lational movement is adjusted to cause withdrawal of solidified silicon at a rate substantially equal to the rate at which vaporous silicon is absorbed within molten silicon contained within the reaction zone of the cold crucible. 6. A method of Claim 5 wherein said relative translational movement is provided by lowering said molten silicon-while maintaining the cold crucible stationary. . • 7. A method of Claim 6 wherein a carrier gas is employed with said vaporous silicon compound. 8. In the production of silicon wherein molten silicon is inductively heated and maintained within a cold crucible, vaporous silicon is added to said molten silicon and solidified silicon is withdrawn from the cold crucible, the improve- ment comprising the steps of: OMPI a. applying a high-frequency electromagnetic field to molten silicon within the cold crucible which is sufficient to partially levitate said molten silicon so that it does not contact the walls of said cold crucible; b. contacting exposed surfaces of said molten silicon with vaporous elemental silicon; and, c. withdrawing a portion of said molten silicon from the cold crucible whereby it solidifies. 9. Apparatus for producing solid semiconductor, comprising, in combination: a. a controlled-atmosphere chamber; b. a cold crucible located within said controlled-atmosphere chamber; c. means for applying a high-frequency electromagnetic-field to semiconductor contained within said cold crucible to main¬ tain said semiconductor in a molten and partially levitated state; d. means for introducing vaporous semiconductor into said controlled-atmosphere chamber whereby said vaporous semiconductor compound contacts exposed surfaces of said molten, partially levitated semiconductor; e. means for exhausting vaporous species from said controlled-atmosphere chamber; f. means for cooling a portion of said molten semiconductor below its freezing point to thereby solidify it; and, g. means for withdrawing solidified semi¬ conductor from said cold crucible. OMPI -17- 10. Apparatus of Claim 9 wherein said controlled- atmosphere chamber comprises a high-temperature, high-pressure furnace. 11. Apparatus of Claim 10 wherein said apparatus additionally includes means for introducing a carrier gas with said vaporous semiconductor into said high-temperature, high-pressure furnace. 12. Apparatus of Claim 11 wherein said means for applying a high-frequency electromagnetic field comprises an Rf coil surrounding said cold crucible. 13. Apparatus of Claim 12 wherein said means for cooling and said means for withdrawing comprise means for providing relative translational move¬ ment between molten silicon contained within said cold crucible and said cold crucible. 14. Apparatus of Claim 13 wherein said means for providing relative translational movement com¬ prise a movable platform located at the bottom of said cold crucible. OMPI

Inventors: WENCKUS J, WILSON P
Applicants: CERES CORP
Application year: 1980
Main classification: H05B6
IPC codes: C01B33, C22B41, C30B11, C30B23, H05B6