Coating apparatus – Gas or vapor deposition – Means to coat or impregnate particulate matter
Reexamination Certificate
2000-12-26
2004-12-07
Lund, Jeffrie R. (Department: 1763)
Coating apparatus
Gas or vapor deposition
Means to coat or impregnate particulate matter
C422S110000, C422S129000, C422S139000, C422S145000, C422S146000, C422S147000, C422S198000, C422S198000
Reexamination Certificate
active
06827786
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of deposition of silicon by chemical vapor deposition, and more particularly to a Machine for Production of Granular Silicon which is of lower cost, more convenient, more reliable, more efficient, provides better quality granules and is better integrated into the overall silicon purification process than existing methods. The use of a pulsing gas flow to circulate granules between a heater and reactor section solves both the granule heating problem and the granule sintering problems that have prevented prior methods from operating for extended time and producing good quality granules. This approach enables use of cheap and reliable resistance heating in contrast to more expensive and less reliable techniques such as microwave and laser heating. Use of an inline non contaminating sieve technique and pulsing gas flow removes silicon product with a more uniform size and returns undersize material which reduces seed generation problems. Online adjustment of the gas pulse shape and flow distribution provides additional control of attrition and seed generation. An optional feedstock recovery system for hydrohalosilane feedstock allows a more efficient method of recycling the silicon tetrahalide by product, allows use of cheaper methods for the production of the hydrohalosilane and provides flexibility in balancing the overall product slate of a silicon purification facility.
This invention relates generally to the field of silicon purification, and more particularly to a machine for production of high purity silicon granules by the decomposition of a high purity silicon containing gas, such as silane, trichlorosilane or tribronosilane, which can be designed to optimize the overall efficiency of such a silicon purification process.
The production of high purity electronic grade silicon is the critical first step of the entire multi-billion dollar semi-conductor industry. The basic process, used by most manufacturers consists of three steps; conversion of metallurgical grade silicon into a hydrohalosilane such as trichlorosilane, purification of this material by distillation and other means, and decomposition of the material back to silicon. The Ethyl process, directly reduces silicon tetrafluoride to silane with a byproduct of aluminum trifluoride.
The decomposition reactors are all rod reactors except for fluid bed reactors operated on silane as part of the Ethyl Process. Fluid bed reactor have significant capital, operating and energy advantages but have proved difficult to implement. The only operating fluid bed units produce a dusty product contaminated with hydrogen that is not widely accepted.
There are two decomposition reactions for hydrohalosilanes; thermal decomposition and hydrogen reduction. (Trichlorosilane is used in the examples but bromine or iodine can be substituted for chlorine, fluorine cannot)
4SiHCl
3
→Si+3SiCl4+2H2 (thermal)
SiHCl
3
+2H
2
→Si+3HCl (hydrogen reduction)
All halosilane reactors incorporate both and consequently produce an effluent, which has a range of silicon hydrohalides and tetrahalides and hydrogen halides and hydrogen.
The essence of the process is impure silicon in, pure silicon out plus small impurity streams. To accomplish this there are large recycle streams of hydrogen, silicon and halide containing streams and is important not to produce low value by-products or waste streams. Union Carbide developed an approach of producing silane by disproportionation then decomposing the silane
4SiHCl
3
→SiH4+3SiCl4
SiH4→Si+2H2
it can be seen that the overall reaction is the same as the thermal decomposition reaction.
The major use for the polycrystalline silicon is in production of single crystal silicon via melting and growth of single crystal silicon boules in Czochralski crystal pullers. Such pullers have specific requirements with regard to feeding the granules (also known as beads), contamination, and ease of melting etc. which must be met in order to use silicon beds. Kajimolo et al documents some of these issues in U.S. Pat. No. 5,037,503.
The purity requirements for electronic grade silicon are severe with specifications for hydrogen at about 30-50 ppma, parts per million atomic, oxygen at 0.5-1.5 ppma and carbon at 0.1-0.25 ppma with specifications for donors such as boron, phosphorus and aluminum in the ppba, parts per billion atomic, and metals in the ppta. parts per trillion atomic. Thus all materials which come in contact with the silicon must be virtually free of metals and donors and have very small amounts of oxygen, hydrogen and carbon which are transferable to the silicon. Historically such specifications have progressively tightened and this trend can be expected to continue. Other trends in the industry are to larger and larger wafer diameters with the current transition from 200 mm to 300 mm wafers being underway. This trend has led to the need to pull larger and larger diameter crystals which in turn leads to the desire to add silicon to the crystal growing furnace white the crystal is being pulled. This can be done conveniently with silicon granules which melt easily and are very pure and hence there is a need for such high purity granules. A further historical trend is the decreasing availability of cheap hydro-electric power which has been the prime source of energy for the very inefficient rod reacts which leads to the increasing need to improve energy efficiency in the deposition process.
Because of the lower energy, capital cost and operating cost of fluid bed reactors much work has been done to develop this technology but the problem of meeting the above ever tighter purity specifications is more acute with the use of fluid bed reactors because they are more susceptible to materials problems as the silicon product is in physical contact with the wall, which thus must be at or close to the deposition temperature. This requires hot walls in contrast to the rod reactors, which typically have cooled walls. Furthermore fluid bed reactors do not have the internal heat generation provided by the electrical healing of the rod in rod reactors and so must add heat in some other way. If this heat is added through the walls, the walls must be hotter than the silicon product. A further problem is that the materials coming into the reactor can only be preheated to a temperature below their thermal decomposition temperature which is 350-450° C. for most feedstock materials. For high tout fluid bed reactors putting in the additional heat to bring the temperature up to the desired decomposition temperature of greater than 800° C. is very difficult. The major operational problem is sintering of the beads in the reactor and the resultant plugging of the reactor, the major purity problems are metals, carbon, oxygen and hydrogen in the bulk and surface of the product and the major problem in feeding beads to the crystal puller is difficulty in controlling the bead flow due to variation in shape and size.
The sintering appears to be more prevalent as the temperature, deposition rate, silicon containing gas concentration and bead size increases and less prevalent as the fluidizing gas flow rate increases. Hence a violently fluidized bed will have a lower tendency to sinter but may tend to blow over more dust and will require more heat.
It has been accepted that it is important to have a reactor that does not contaminate the product and that the use of metal reactors is not feasible, see Ling U.S. Pat. No. 3,012,661 and Ingle U.S. Pat. No. 4,416,913 and hence metal contamination can be resolved by not contacting the beads with any metal parts. Similarly contact with carbon or carbon containing materials leads to carbon contamination so graphite or silicon carbide parts are usually coated with silicon, carbon can also come in through contaminants in the inlet gases such as carbon monoxide, carbon dioxide and methane. Oxygen normally comes in through oxygen containing compounds such as water, carbon monoxide and carbon diox
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