Chemical Transport Deposition of Purified Poly-Si Films from Metallurgical-grade Si Using Subatmospheric-pressure H₂ Plasma

Abstract

Purified Si film is prepared directly from metallurgical-grade Si (MG-Si) by chemical transport using subatmospheric-pressure H 2 plasma. The purification mechanism is based on the selective etching of Si by atomic H. Since most metals are not etched by H, this process is efficient to reduce metal impurities in Si films. It is demonstrated that the concentrations of most metal impurities (Fe, Mn, Ti, Co, Cr, Ni, etc.) in the prepared Si film are in the acceptable range for applying it to solar-grade Si (SOG-Si) material, or below the determination limit of the present measurements. On the other hand B and P atoms, which make volatile hydrogen compounds such as B 2 H 6 and PH 3 , are difficult to eliminate by the present principle. From the infrared absorption measurements of the etching product produced by the reaction between H 2 plasma and MG-Si, it is found that the main etching product is SiH 4 . Therefore, a remote-type chemical transport process is possible to produce SiH 4 gas directly from MG-Si. Combining other purifying principle (such as a pyrolysis filter), this process may have an advantage to eliminate B 2 H 6 and PH 3 from the produced SiH 4 gas.

Purified Silicon Film Formation from Metallurgical-Grade Silicon by Hydrogen-Plasma-Induced Chemical Transport

“…To increase Si etching rate, we performed etching experiments under various plasma conditions. From the result, which has been reported in the previous reports, 17,26) the following guiding principles have been obtained to develop the high-rate SiH 4 generation process. Namely, i) 2.45 GHz microwave plasma is more efficient for H atom generation than 150 MHz VHF plasma, probably because a very high power density can be applied to a small volume in the former case, ii) a higher plasma power is better for obtaining higher Si etching rates, but the temperature of MG-Si should be kept low, iii) a higher H 2 pressure is better to generate high-density H atoms as long as an available input power has no limit, and iv) an appropriate transit time in the plasma, in other words, an appropriate gas velocity is needed to obtain a high Si recovery as SiH 4 .…”

“…To eliminate B-related molecules from SiH 4 gas generated by the H etching of MG-Si, we have developed a pyrolysis filter. 17,33) Since B 2 H 6 decomposes at lower temperatures than SiH 4 , 34,35) we can use a heated filter at an appropriate temperature to eliminate B 2 H 6 from SiH 4 gas. The pyrolysis filter is made of porous carbon material that is heated by direct current flow.…”

“…[12][13][14][15] The other is a new process of SiH 4 gas formation directly from MG-Si using 2.45 GHz microwave H 2 plasma (remote-type APECT). 16,17) Although the experiments were performed in the H 2 pressure range from 100 to 760 Torr, the acronym ''APECT'' is used throughout this article.…”

Effects of Surface Temperature on High-Rate Etching of Silicon by Narrow-Gap Microwave Hydrogen Plasma

“…Based on the Si etching reaction with hydrogen (H), we have proposed a method to produce the high-purity SiH 4 gas directly from metallurgical-grade Si (MG-Si) materials. [1][2][3] This method is based on two important technologies. One is the utilization of narrow-gap microwave plasma system.…”