Fluidized bed reactor modeling for production of silicon by silane pyrolysis

VAPOR
DEPOSITION
AND
HONUGENEOUS
NUCLEATION
IN
PYROLYSISWITEIN INTERPARTICLESPACES -APPLICATICN OF FINES FORIIATI~ ANALYSISTDFLDIDI2%DBF.DCVD-
ቤተ መጻሕፍቲ ባይዱ
Takehiko Department *Department
ABSTRACT
The rate of silane pyrolysis was measured over the temperature ranging from 823K to 973K and over in quartz tube reactors with a relatively wide range of monosilane and hydrogen concentrations The homogeneous nucleation rate and heterogeneous CVD rate and without seed silicon particles. of temperature, and silane and hydrogen were separately evaluated and formulated as functions concentrations. The previously reported experimental observation of fines formation within bed CVD reactor was quantitatively explained using the present kinetics. fluidized
Chemical
vapor
deosition:
fluidized
bed:
monosilane
pyrolysis:
polycrystalline
silicon;
fines
INTRoDucMoN The commercial production of polycrystalline silicon is conducted by hydrogen reduction of trichlorosilane within Siemens bell-jar reactor. This conventional filament process has several disadvantages including (1) significantly reduced thermal efficiency which is caused by heat loss through the reactor wall maintained at relatively low temperature. (2) low production capacity due to relatively small reaction interface per unit volume of reactor, and (3) inevitable batch An increasing demand for silicon of higher purity requires also pyrolysis of operation. whose purity exceeds several orders of magnitude purity of trichlorosilane. A monosilane. commercial process of monosilane pyrolysis by use of a bell-jar reactor similar to Siemens process has been developed to a point of commercial availability. The employment of a reaction path starting from monosilane has various merits over the conventional reaction path starting from trichlorosilane. The significant advantages include (1) high one pass yield of polycrystalline silicon from metallic silicon, (2) high one path conversion of monosilane without by-product such as hydrogen chloride as by a product, by-product recycle, (3) no corrosive (4) lowered deposition temperature which results in reduced heat loss, and (5)the thermodynamic The development of low cost silane production processes made this alternative yield of 100%. However the introduction of high process more advantageous (Kojima and Furusawa, 1987). concentration monosilane into the bell-jar reactor results in the significantly high loss of due to homogeneous nucleation. which may be enhanced in monosilane pyrolysis more than in fines trichlorosilane reduction by hydrogen (Herrick and Woodruff, 1984). Therefore a bell-jar reactor with water-cooled wall is employed to reduce the temperature of the gas phase and to maintain the This leads to a significant reduction in formation of fines below a certain acceptable level. energy efficiency. Therefore a fluidized bed CVD (chemical vapor deposition) process with increased surface of solids, on which heterogeneous CVD reaction takes place. is considered to be Extensive development of fluidized bed potentially advantageous over the conventional process. CVD process has been carried out (Hsu et al.. 1984, Hsu. 1986. Iya, 1986). however. this process is faced with several problems. The most important break-through for the development of this process is to reduce the formation of fines while maintaining the smooth fluidization. Hsu et al.(1984)conducted fluidised bed experiments and measured the elutriation of fines from the The observed formation of fines was far below the level (Hsu et al., 1984) estimated by reactor. both the heterogeneous reaction rate (Iya et al. 1982) and homogeneous reaction rate (Hogness et the observed fines formation which was enhanced by the increased silane Furthermore, al. 1936). concentration has not been well elucidated (Hsu et al., 1984). Several mathematical models taking account of coalescence, coagulation, scavenging and heterogeneous growth on fines (Hsu et Laiet al., 1986)as wall as heterogeneous and homogeneous decomposition have been al., 1987. presented for the analysis of the fluidized bed CVD process. However, the fines formation enhanced with increased silane concentration has not been so far sufficiently explained. Since the formation of fines is highly and homogeneous nucleation rate within the two reaction rates under conditions authors conducted separate evaluation dependent on the relative importance of CVD reaction rate interparticle spaces of the bed, a separate evaluation of pertinent to practical operation is needed. The present of both homogeneous and hetergeneous reaction rates over