INTERACTION OF COPPER-CATALYSTS AND SI(100) FOR THE DIRECT SYNTHESIS OF METHYLCHLOROSILANES

Single crystal Si(100) surfaces with a native oxide layer were reacted with methyl chloride to investigate the direct synthesis of dimethyld ichlorosilane. These high purity silicon surfaces are excellent models of the reacting powders us ed industrially for direct synthesis. The oxide layer did not appear to inhibit reaction significantly. The copp er catalyst was added to the surface by various methods, and the form of the catalyst necessary for selective reaction was determined. React ion was carried out at atmospheric pressure in a recirculating batch r eactor and the copper-silicon surfaces were characterized before and a fter reaction by XRD, SEM, EDS, AES, and optical microscopy. Catalysts that contained only metallic Cu or only Cu2O did not catalyze dimethy ldichlorosilane formation; both Cu and Cu2O were needed. A mixture con taining 82 wt% Cu and 18 Wt% Cu2O yielded the best selectivity (65 mol % (CH3)2SiCl2, 33 mol% CH3SiCl3, and 2 mol% (CH3)3SiCl). This selectiv ity is comparable to those obtained in fluidized bed reactors for copp er-silicon powders without promoters. Both CuCl and Cu(HCOO)2 . 2H2O c atalysts were also selective for dimethyldichlorosilane formation init ially, but methyldichlorosilane formed at longer reaction times. Coppe r formate dihydrate solution decomposed to form Cu/Cu2O mixtures on Si (100). Because the Cu percentage was lower than 82%, however, and beca use formate decomposition also formed a Cu film on the surface, the se lectivity was lower. Most of the catalysts reacted with silicon to for m the stoichiometric alloy Cu3Si, but some of the resulting surfaces d id not react to form methylchlorosilanes. The competition between Cu3S i formation and consumption to form methylchlorosilanes was different for the different catalysts. A correlation was seen between epitaxial growth of Cu3Si on Si(100) and poor selectivity for dimethyldichlorosi lane formation. The most selective surfaces had a randomly oriented Cu 3Si phase. The Si(100) surface reacted by forming square pyramidal pit s with Si(111) sides; the pits contained Cu3Si. (C) 1994 Academic Pres s, Inc.