Removal of Boron from Silicon by Moist Hydrogen Gas
- PDF / 596,345 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 92 Downloads / 226 Views
TRODUCTION
TRADITIONALLY, off-spec polycrystalline silicon from the semiconductor industry, produced through the Siemens process, has been supplying the solar industry.[1] This silicon is both costly ( < $35 per kg, 2009) and far purer (9 nines) than that what is needed for PV applications (6 nines).[2] To make solar cells less expensive, it has been necessary to develop production methods for solar silicon that improve cost and efficiency.[3] Many methods have been patented the last decades by companies like Elkem, Fesil/SINTEF, and Kawasaki/NEDO,[4–6] where the target has been to produce 6-nines pure solar grade silicon (SoG-Si) at approximately $15 per kg. These methods have often been referred to as the metallurgical route compared with chemical route by the Siemens process. In the metallurgical route, boron is particularly difficult to remove because of its high segregation coefficient and low vapor pressure.[7] From the metallurgical production of silicon (MG-Si), approximately 95 pct of the boron put into the process remains in the product, and boron is for the most part linked to the impurity levels in coke (61 pct) and quarts (37 pct). Carefully choosing the charge materials is, therefore, the first step in lowering the boron content.[8] To meet the limit of the accepted boron content in SoG-Si ( < 0.5 ppmw B), a purification step is needed. A practical way to do this is through plasma refining.[9–11] In plasma refining, the heat of the plasma torch activates gases such as oxygen and hydrogen on the silicon surface to produce volatile H-B-O species.[12] This method requires less handling of materials compared with slag refining.[13,14] Still, the process is relatively expensive ERLEND F. NORDSTRAND, Scientist, and MERETE TANGSTAD, Professor and Scientist, are with the Norwegian University of Science and Technology, 7491 Trondheim, Norway. Contact e-mail: [email protected] Manuscript submitted November 9, 2011. Article published online May 1, 2012. 814—VOLUME 43B, AUGUST 2012
because of the high energy consumption of the plasma torch. It has, therefore, been desirable to develop new and cheaper ways for boron removal using reactive gasses. In this work, an evaporation unit will be used to add water vapor to H2 gas, which subsequently will be sparged on top of silicon melts. From these results, the effect of temperature and gas composition during gaseous boron removal will be studied.
II.
THEORETICAL BACKGROUND
It was already demonstrated in 1956 by Theuerer[15] in a zone refining reactor that boron could be removed by treating the surface with moist hydrogen. However, it was not until the late 1980s and the entry of metallurgical production of SoG-Si that reactive gas blowing was shown considerable interest. Suzuki et al.[16] used plasma to purify 5 g MG-Si on a water-cooled block and achieved 99 pct removal of boron using an Ar-H2O mixture. The removal rate was relatively slow because of the small reaction area. Later, Ikeda et al.[17] demonstrated, using a rotational torch that enlarged the contact area, that the
Data Loading...
