GB2059405A - Method of and apparatus for producing liquid silicon - Google Patents

Abstract

Hydrogen and a hydrogenated silane in the gaseous state are mixed, preferably with a source of a small amount of oxygen, in a heated chamber (11) producing the liquid silicon, which is kept saturated with oxygen, with exhaust gases bubbling out of the melt under a baffle (26). The liquid silicon may be used in making high purity vitreous silica or in making castings of silicon. In making castings, the liquid silicon is accumulated in a second chamber (30) and is periodically drawn from the second chamber into a third chamber (46) which contains a mould (40) for the casting. <IMAGE>

Description

SPECIFICATION Method of and apparatus for producing liquid silicon This invention relates to a method of and apparatus for producing liquid silicon, particularly liquid silicon of high purity suitable for use in the production of vitreous silica as described in my copending U.S. Patent Application Serial No. 684 108, filed May 7, 1976, and entitled "Method and Apparatus for Making Vitreous Silica", and for making castings of silicon. The cast silicon may be for example in the form of billets for crystal pullers, in the form of tubes and boats for use in semi-conductor processing equipment, or in the form of infrared transmission windows.

The subject of vitreous silica is discussed extensively in the "Encylopedia of Chemical Technology", 2nd Ed., Vol. 18, by Kirk-Othmer at pages 73-105. Various types of silica glasses are discussed in the article "Properties and Structure of Vitreous Silica" by R. Bruckner in "Journal of Non Crystalline Solids", 5 (1970), 123-175. This article identifies four types of silica glasses by the manner in which they are produced and also refers to a fifth type produced in a plasma flame.

The silicon produced by prior art methods and apparatus suffers from various disadvantages, primarily that of purity.

The present invention in one aspect provides a method of producing liquid silicon, comprising maintaining a pool of liquid silicon in a first chamber having an inner wall of silica, continuously mixing hydrogen and at least one halogenated silane in the gaseous state directly over the pool of liquid silicon, heating the hydrogen and halogenated silane to react to produce additional liquid silicon in the pool, maintaining the pool of liquid silicon substantially saturated with oxygen, and continuously withdrawing liquid silicon from the pool.

The invention in another aspect provides apparatus for producing liquid silicon, comprising a first chamber for holding liquid silicon, means for heating the first chamber, means for directing streams of hydrogen and at least one gaseous halogenated silane into the first chamber, and means for removing exhaust gases from the first chamber.

The present invention provides a method and apparatus whereby there may be produced liquid silicon and silicon castings the purity of which is limited only by the purity of the starting materials.

In a preferred embodiment of the invention, streams of hydrogen and halogenated silane in the gaseous state are mixed in a heated chamber to produce liquid silicon; exhaust gases are bubbled out of the melt under a baffie, and the chamber is preferably lined with silicon dioxide.

The liquid silicon may be drawn from the first chamber into a second chamber wherein liquid silicon is accumulated for the purpose of making castings, with the liquid silicon being drawn from the second chamber into a mould at periodic intervals. Alternatively, the liquid silicon may be utilized in making a high purity vitreous silica as described in the above-mentioned U.S. Patent Application Serial No.684 108.

The invention will be further described, by way of example only, with reference to the accompanying drawing, which is a sectional view of an apparatus for producing and casting liquid silicon.

The apparatus shown in the drawing includes a source 10 of liquid silicon having a preferred form as shown. A chamber 11 has an outer wall 12 of a refractory metal such as tungsten or molybdenum and an inner lining 13 of fused quartz. The container may be made by fabricating fused quartz in the desired configuration and then plasma spraying the refractory metal outer layer thereon.

A pool of liquid silicon 1 5 is produced by mixing a stream of hydrogen supplied through a tube 1 6 and a stream of trichlorosilane supplied through a tube 1 7. The silicon is heated above its melting point of 1 6850K, for example by means of an induction heating coil 20 positioned about the chamber 11. If desired, silicon tetrachloride can be used in combination with or in place of the trichlorosilane. Other halogenated silanes may be used, but most are more expensive and/or more difficult to handle. The two named are the only ones presently known to be available in commercial quantities.

The incoming gases in the tubes 1 6, 1 7 are heated by another induction heating coil 21, with the tube 1 6 preferably having a tungsten inner lining 22 and a fused quartz outer layer 23. The excess gases from the reaction which forms the liquid silicon bubble outward around a baffle 26 and are removed as exhaust gases through an outlet line 27.

A second chamber 30 is positioned below the chamber 11 and preferably is made in the same manner as the upper chamber 11 with an outer wall 31 of a refractory metal and an inner lining 32 of fused quartz. The chamber 30 may be heated by another induction heating coil 33.

An orifice, preferably in an insert 36 at the junction between the upper chamber 11 and lower chamber 30, provides a continuous flow of a fine stream of liquid silicon into the chamber 30.

Typicaily the insert is made of a high temperature resistant material such as silicon carbide.

The silicon produced in the chamber 1 1 may be drawn directly from the chamber for further use, such as in the making of vitreous silica as disclosed in the above-mentioned U.S. Patent Application Serial No. 684 108, for making castings, and for other uses as desired. When making castings, it is preferred to use the chamber 30 for accumulating a quantity of the liquid silicon sufficient for the desired casting, with the liquid silicon added to the chamber 30 from the chamber 11 in a substantially continuous stream, while being drawn from the chamber 30 periodically for filling a mould.

One mould handling construction is shown in the drawing. It is desirable to perform the casting operation in an inert atmosphere. This may be accomplished by inserting a mould 40 into a chamber 41 through a door 42. With the door closed, the chamber 41 is evacuated through a line 43 and is then filled with an inert gas through a line 44. A sliding door 45 is then opened permitting movement of the mould from the chamber 41 to another chamber 46, positioning the mould below an outlet of the chamber 30.

An orifice, preferably in an insert 50 at the lower end of the chamber 30, provides flow of liquid silicon from the chamber 30 into the mould 40. Typically the insert is made of a high temperature resistant material such as silicon carbide. An induction heating coil 51 is positioned around the insert and may be used for controlling flow through the orifice. With the power to the heating coil 51 off, the silicon solidifies at the orifice and blocks flow from the chamber 30.

When it is desired to make a casting, the heating coil 51 is energized, liquifying the silicon at the orifice and permitting flow of liquid silicon from the chamber 30 into the mould 40. When the mould is filled to the desired level, the power to the coil 51 is turned off, permitting the silicon to solidify in the orifice. The mould 40 is now ready for removal from the chamber 46. The mould may be removed through the chamber 41, or another exit chamber may be provided so that an empty mould can be introduced via the chamber 41 thereby reducing the time interval between pourings.

The mould typically is made of titanium because of its high melting point, with the inner surface thereof preferably coated with a layer of silica which may be sprayed thereon. Alternatively, the mould may be of silicon with a silica lining. As another alternative, the mould may be entirely of silica similar to the standard crucibles used for Czochralski crystal growing. The portion of the structure adjacent the orifice outlet, including a sleeve 55 and a plate 56, are preferably made of a refractory metal such as tungsten, and a water cooling coil 57 may be mounted on the plate 56.

Other portions of the chambers 41, 46 may be made of steel or other metals as desired. A plate 58 forming the bottom of the chambers 41 and 46 may be cooled by a water cooling coil 59. Cooling for the chambers 11 and 30 may be provided by another water cooling coil 60 carried on a sleeve 61 positioned about the chambers.

The chamber 11 is heated to maintain the silicon in a molten state, typically in the range of 1700 to 19000K. It is desirable that the reactant gases be pre-heated prior to entering the chamber 11 for enhancing the reaction and this may be accomplished by the induction heating coil 21.

Other methods of gas pre-heating may be utilized, but it is preferred to have the separate gas streams enter the chamber directly over the liquid silicon. The incoming gas streams provide a pressure in the chamber 11 which aids in ejecting the stream of liquid silicon into the chamber 30.

While the present invention is directed to method and apparatus for making pure silicon, various modifiers and dopants may be incorporated if desired. When the end product is to be an oxide, such as silica, modifiers are often used. Certain modifiers are described below by way of example only.

The sag point of fused silica may be raised about 1000K by adding about 0.20 to 0.25% alumina to the silica. This may be accomplished by including an aluminium halogen, such as aluminium chloride, in the trichlorosilane gas input.

The inclusion of about 10% titania reduces the coefficient of thermal expansion of silica from 55 x 10-8 to approximately zero. Titania also increases the index of refraction of silica. Titania may be added in the form of titanium tetrachloride.

The incorporation of about 4 to 21% of neodymium oxide produces a silica suitable for use as a laser glass. The neodymium may be introduced as neodymium chloride. All of these metallic halogens become gaseous when heated and are easily handled in the apparatus disclosed herein.

When the end product is to be a semiconductor, dopants can be used. Typical dopants are boron, aluminium, gallium, phosphorus, arsenic and antimony, which are used in conventional quantities for semi-conductors. The dopant may be introduced in gaseous compound form in the silane gas stream, typical compounds being diborane, phosphene and arsene.

Silica has a solubility in molten silicon of about 1-10 parts per million. Hence when a silica lining is used for a chamber containing molten silicon, such as the chamber 11, the molten silicon tends to erode the wall over a period of time. This erosion effect may be reduced or eliminated by maintaining the molten silicon saturated with oxygen.

In the specific embodiment disclosed herein, this may be accomplished by adding a small amount of water vapour to the hydrogen streams.

It is preferred to provide a silica lining inside the tungsten lining 22 of the tube 1 6 to protect the metal from the oxidizing effect of the water vapour.

Alternatively, a small amount of an oxysilane, of the order of 1-10 ppm, may be included with the halogenated silane to provide the oxygen for the silicon.

The optimum amount of the oxygen source material is best determined by experiment with the specific apparatus being used.

Claims (20)

1. A method of producing liquid silicon, comprising maintaining a pool of liquid silicon in a first chamber having an inner wall of silica, continuously mixing hydrogen and at least one halogenated silane in the gaseous state directly over the pool of liquid silicon, heating the hydrogen and halogenated silane to react to produce additional liquid silicon in the pool, maintaining the pool of liquid silicon substantially saturated with oxygen, and continuously withdrawing liquid silicon from the pool.

2. A method as claimed in Claim 1, wherein the reaction also produces exhaust gases, and further comprising maintaining the level of the pool of liquid silicon above the lower edge of an exhaust baffle separating a gas outlet from the reaction section of the first chamber so that exhaust gases bubble from the pool of liquid silicon under the baffle to the gas outlet.

3. A method as claimed in Claim 2, further comprising withdrawing liquid silicon from the pool in the first chamber into a second chamber at a rate so as to maintain the level of the pool in the first chamber above the lower edge of the baffle while accumulating liquid silicon in the second chamber, and heating the silicon in the second chamber to maintain it in the liquid state.

4. A method as claimed in Claim 3, including periodically drawing liquid silicon from the second chamber into a mould for making a silicon casting.

5. A method as claimed in Claims 3 or 4, wherein the said accumulating step is carried out in a chamber lined with silicon dioxide.

6. A method as claimed in any of Claims 1 to 5, wherein the oxygen is provided by incorporating a small amount of an oxysilane in the halogenated silane.

7. A method as claimed in any of Claims 1 to 5, wherein the oxygen is provided by incorporating a small amount of water vapour in the hydrogen.

8. A method as claimed in any of Claims 1 to 7, 'wherein the silane is at least one of trichlorosilane and silicon tetrachloride.

9. A method as claimed in any of Claims 1 to 8, wherein the said mixing step is carried out in a chamber lined with silicon dioxide.

10. A method of producing liquid silicon, substantially as herein described with reference to the accompanying drawing.

11. Apparatus for producing liquid silicon, comprising a first chamber for holding liquid silicon, means for heating the first chamber, means for directing streams of hydrogen and at least one gaseous halogenated silane into the first chamber, and means for removing exhaust gases from the first chamber.

12. Apparatus as claimed in Claim 11, including a baffle depending downward from the upper surface of the first chamber for separating the said incoming gaseous streams and the said exhaust gases.

13. Apparatus as claimed in Claim 11 or 12, including means for directing a source of oxygen into the first chamber with one of the said streams of hydrogen and halogenated silane.

14. Apparatus as claimed in any of Claims 11 to 13, wherein the first chamber has an outer metal shell and an inner lining of silicon dioxide.

1 5. Apparatus as claimed in any of Claims 11 to 14, further comprising a second chamber for accumulating liquid silicon, and means interconnecting the first and second chambers for permitting liquid silicon to flow from the first chamber into the second chamber.

1 6. Apparatus as claimed in Claim 15, wherein the second chamber has an outer metal shell and an inner lining of silicon dioxide.

17. Apparatus as claimed in Claims 1 5 or 16, further comprising a third chamber for supporting a mould positioned below the second chamber, and means interconnecting the second and third chambers for permitting liquid silicon to intermittently flow from the second chamber into the third chamber.

1 8. Apparatus as claimed in any of Claims 11 to 17, wherein the said means for directing streams of hydrogen and halogenated silane into the first chamber include metal tubes lined with silicon dioxide and an induction heating coil positioned about the said tubes.

19. Apparatus as claimed in Claim 18, wherein one of the said tubes is positioned within the other tube and wherein a helical heating coil is positioned about the said tubes.

20. Apparatus for producing liquid silicon, substantially as herein described with reference to, and as shown in, the accompanying drawing.