In an attempt to understand the fundamental chemistry of the refractory metal/Si interface and the influence of other chemical constituents, we have investigated the microscopic chemical bonding and reactions which occur at the Ti/Si and Ti/oxygen/Si interface under atomically clean interface conditions and with controlled, well-characterized oxygen interlayers produced on Si prior to the Ti deposition. Surface spectroscopy techniques (UPS, XPS, AES) were used to identify the interface electronic structure, composition, and chemical reactions. Although bulk silicide contacts (TiSi and TiS2) have not previously been observed to form below ~500–600 °C, we find that significant atomic mixing across the interface occurs over significant distances (~100 A) at temperatures as low as 300 °C. The UPS spectra differentiate clearly between Ti-O and Si-O chemical bonds and reveal the microscopic chemistry in the Ti/oxygen/Si interfacial system. With oxygen already present on the initial Si surface, deposition of Ti at 25 °C reduces the Si, replacing Si-O bonds by Ti-O bonds. However, upon sufficient annealing to promote the Ti-Si reaction, the Ti atom bonding is changed to Ti-Si. As a result, the oxygen is then rebonded to Si, primarily that Si which has segregated to the overlayer surface during Ti-Si reaction. Thus this refractory transition-metal/Si interface displays strong chemical reactions well below normal silicide formation temperatures (~550 °C)—first, in Ti-Si intermixing at ~300 °C, and second, in reduction of the Si-O bonds at ~25 °C. © 1983, American Vacuum Society. All rights reserved.