Boron Elimination Filter from SiH4 and B2H6 Gas Mixture for Purification of Metallurgical Si
Authors: Ohmi Hiromasa , Kakiuchi Hiroaki , Yasutake Kiyoshi 
Published: February 2020

We have proposed the solar-grade Si film formation method from metallurgical-grade (MG) Si source by using a chemical transport induced by high-pressure hydrogen plasma1). In this method, selectivity of hydrogen induced chemical reaction was utilized to eliminate the metal impurities from MG-Si source. However, MG-Si also contains B and P which are electrically active element in Si matrix. These are difficult to remove by using above method, because they can form volatile hydrides. So, the simple process to eliminate these elements is necessary. Fortunately, it can be expected that elimination of PH3 would be easily achieved by using a molecular sieve due to the polarity of PH3
2). However, B2H6 molecule has no dipole moment in common with SiH4, hence molecular sieve is not expected to remove B2H6 efficiently from SiH4 and B2H6gas mixture.
Then, in this study, we have attempted to eliminate B2H6 from the gas mixtures by using difference of the pyrolytic behavior of both gases and demonstrated that the developed filter can operate effectively as B2H6filter.

EXPERIMENTAL

The porous carbon plate with its thickness of 5mm was used as filter element to the benefit of its chemical inertness, electrical conductivity (162μΩcm), relatively wide specific surface area, and its porosity (30%). The plate was settled in the filter vessel and connected to the heater power source to control its temperature by electrical heating. To investigate the elimination property of the pyloritic filter precisely, the SiH4 and B2H6 gases supplying from gas cylinder were used in this study. The gas mixture ratio of SiH4 and B2H6 was controlled through mass flow controller. Moreover, H2 and He are used as carrier gas. The transmission property of SiH4 and B2H6for porous carbon filter at various temperatures was evaluated by gas phase FTIR. To ensure the Boron elimination from Si material by using the filter, Si film was formed on Si substrate by thermal CVD at 800°C through the use of gas mixture after passing through the filter and B concentration in the film was evaluated by SIMS.

RESULTS AND DISCUSSION

Figure 1 shows the filter temperature (Tfilter) dependence of transmission properties of SiH4 and B2H6. He carrier gas flow rate was 10 slm. During the experiment, the filter vessel pressure was kept at 200 Torr. As shown in Fig.1, B2H6 transmittance begins to decrease at 200°C and is reduced to almost zero at 350°C. On the other hand, SiH4 begins to decrease at 450°C and shows the transmittance of 10% even at 550°C. From this result, it is found that B2H6 can be eliminated efficiently without sacrifice of SiH4, if the Tfilteris kept at 400°C.
Figure 2 shows the B removal performance of the porous carbon filter at various temperatures. The carrier gas flow rate was 10slm. The removal performance (RPgas/film) was defined by dividing the concentration ratio in the gas phase(([B]/[Si])gas) by that in the Si film(([B]/[Si])film). The supplying concentration ratio in gas phase was 10-3 in this study. RPgas/film exponentially increases from 60 to 3200 with increasing the Tfilter from 300°C to 500°C. This temperature dependence of RPgas/film assures that the filter operates on the basis of difference of thermochemical decomposition between SiH4 and B2H6.

CONCLUSION

We have proposed and developed the B eliminating filter operating on the basis of difference of pyrolitic behavior between B2H6 and SiH4. B concentration in both gas phase and Si film could be decreased by filtering the gas mixture at the Tfilterof more than 300°C.
REFERENCES
1) H. Ohmi et al., Appl. Phys. Lett. 95(2009) 181506-1-3.
2) A. Yusa et al., J. Electrochem. Soc. 122 (1975) 1700.