Process for preparing polysilicon using exothermic reaction

Details Process for preparing polysilicon using exothermic reaction Process for preparing polysilicon using exothermic reaction

1999-09-10

2002-05-28

Griffin, Steven P. (Department: 1754)

Chemistry of inorganic compounds

Silicon or compound thereof

Elemental silicon

C423S350000, C422S146000, C427S213000, C427S215000

Reexamination Certificate

active

06395248

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a process for preparing polysilicon to be used as a raw material for large-diameter single crystals to be processed to silicon wafers or for solar-cell application. More specifically, it relates to a process for preparing polysilicon in large scale, which comprises depositing silicon onto the surface of seed silicon through thermal decomposition or hydrogen reduction of reactant gas containing silicon element, wherein an exothermic reaction can be additionally introduced so that the heat of reaction generated by the exothermic reaction is utilized in the deposition reaction of silicon.
BACKGROUND ART
In general, the high-purity polysilicon (or polycrystalline silicon) used as a raw material for solar cells as well as for large-diameter single crystals to be processed to silicon wafers. The polysilicon is prepared in large scale by continuously depositing silicon onto the surface of seed silicon, through thermal decomposition or hydrogen reduction of raw material gas containing silicon element. In the large-scale production of polysilicon the deposition rate of silicon is normally greater than about 0.01 &mgr;m/min.
For commercial manufacture of polysilicon, the Siemens process is widely used. This processes is carried out by depositing silicon onto the surface of electrically heated high-temperature silicon core rods from silicon element-containing gas such as trichlorosilane (SiHCl
3
: referred as “TCS” hereinafter), dichlorosilane (SiH
2
Cl
2
) or monosilane (SiH
4
) in a bell-jar type reactor. It is conceivable to heat a silicon core rod with a high-temperature radiation as well as with an electromagnetic wave including high-frequency wave on behalf of the electrical resistance heating via electrode. Therefore, polysilicon can be prepared regardless of the shape of the reactor if the silicon core rod is heated.
As halogen-containing silane gases for preparing polysilicon, TCS has widely been used commercially. By suing TCS as a raw material, polysilicon is prepared in the Siemens reactor through following procedure. First, a lot of thin core rods (or slim rods) made of silicon are normally installed in the reactor, with respective top-side ends of two core rods being connected with each other by placing an additional core rod as illustrated in FIG.
1
and their bottom ends being connected to two electrodes, respectively. Here, the core rods are required to be preheated to about 400 to 700° C. by a separate heating means in advance to electrical heating via electrode. Thereby the specific resistance of silicon core rods becomes so low that a large amount of electric current can be supplied through them, which renders electric heating via electrode possible. Maintaining their temperature sufficiently high, namely, about 1,000° C. or more, the silane gas is then introduced into the reactor as a reactant gas and the silicon deposition initiates. Although the high-temperature silicon deposition can be obtained only by thermal decomposition of silane gas, in many cases the reaction gas may in the deposition procedure in view of the reaction mechanism and the physical property of the product. Many elementary reactions can occur in a high temperature reactor, but are generally represented by the deposition reaction, which deposits the silicon element of silane gas on the surface of core rods and enlarges the silicon rods as time passes.
When the diameter of a silicon rod is increased, the temperature at its core section should be higher than that at its outer surface in order to maintain the surface temperature necessary for the deposition reaction; accordingly, the electric current via electrode should be increased with time. The thermal energy due to electricity should provide at least the heat required for: i) heating of the reaction gas provided into the reactor; ii) making up the heat loss emitted outward the reactor; and iii) the heat of reaction for the deposition reaction on the surface of silicon rod. In the meanwhile, it is very difficult to preheat the reaction gas sufficiently, i.e., to the required reaction-temperature level, before feeding into a reactor. Most silane gases thermally decompose by themselves at an incipient decomposition temperature, namely, at around 400° C. This causes undesirable silicon deposition on surrounding high-temperature surface, leading to a blocking inside a preheater or connection tubes. Moreover, the reaction gas is vulnerable to contamination during the preheating step. The insufficiently preheated reaction gas should then be heated further inside the bell-jar type reactor. This removes much from the surface of silicon rods. Accordingly, there is a temperature gradient in radial direction of each rod; the temperature is lowest at its surface for silicon deposition, while the temperature is highest at its core axis. Although the total surface area of the silicon rods in contact with the reaction gas increases with time, the conversion of silane gas into silicon is low because the larger the rod becomes, maintaining its surface temperature becomes more difficult. Therefore, the deposition yield of silicon is normally much lower than a thermodynamic equilibrium value of about 20 to 25 mol %. Although the total surface area of the silicon rods changes with time, the overall deposition rate of silicon is normally greater than about 0.1 to 1.0 &mgr;m/min in case of commercial-scale bell-jar type reactor.
If a silicon rod becomes larger than a certain size, its surface temperature cannot be maintained by its electrical heating alone, because its core axis cannot be heated above the melting point of silicon, 1,410° C. Although the surface area for silicon deposition increases with rod diameter, and at the same time the overall deposition rate in the reactor can be increased further by increasing the feed rate of the reaction gas, the deposition reaction should be terminated by the limitation on the heating of the enlarged silicon rod. When the diameter of the silicon rod reaches a maximum of about 10 to 15 cm, the reaction should be terminated, the reactor is dismantled, and the rod-type polysilicon products are separated from the electrodes. Thus, continuous preparation of polysilicon is impossible by using a bell-jar type reactor. Therefore, for reducing the specific electric power consumption and preparation cost, it is essential to maintain the surface temperature of the silicon rod in the limited reactor space as high as possible and to enhance thereby the silicon deposition as much as possible although the yield may be less than that achievable at a thermodynamic equilibrium.
Recently, a process for preparing polysilicon in the form of granule by using a fluidized-bed type reactor has been developed. Chemical reactions occurring in this process are basically the same as those in the bell-jar reactor. But the fluidized bed process is characterized by the fact that silicon particles are fluidized by the reaction gas provided from the lower-side of the reactor, and silicon is deposited on the surface of heated particles; thus the average size of heated particles increases with the deposition reaction. If small-size seed crystals (or seed particles) become larger in the course of the continuous deposition procedure, the degree of fluidization is reduced; thereby the larger silicon particles tend to gradually precipitate to the bottom-side of the reactor. In such a fluidized bed reactor, granule-type polysilicon can be continuously prepared, by providing seed crystals continuously or periodically into the reactor, and then by withdrawing the enlarged particles from the bottom of the reactor. Some of the particles obtained (i.e., product granules) split into smaller particles by a milling procedure, and the seed crystals thus prepared are introduced again into the reactor. As mentioned for the bell-jar type reactor, the silane gas or hydrogen gas contained in the reaction gas cannot be sufficiently preheated before being introduced into the fluidized bed reactor. In addition, the portions of th

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