Investigation of ultrathin yttrium silicide for NMOS source/drain contacts

Compared to Ti, yttrium (Y) has lower work function and higher effective mass, and Y silicide appears as one of the best candidates for NMOS source/drain contacts. In this work, ultrathin (<= 5 nm) Y films are employed as interlayer between Ti and n(+)-Si to form ultrathin YSix/n(+)-Si contacts, while ultrathin TiSix/n(+)-Si contact is also fabricated as reference. The YSix/n(+)-Si and TiSix/n(+)-Si contacts are investigated in terms of specific contact resistivity (rho(c)). Also, as-formed YSix/n(+)-Si contacts with various Y thicknesses as well as TiSix/n(+)-Si contact were characterized by means of cross-sectional transmission electron microscopy (XTEM), energy dispersive X-ray spectroscopy (EDX), as well as secondary ion mass spectroscopy (SIMS). Compared to TiSix/n(+)-Si contact, YSix/n(+)-Si contacts show higher rho(c), owing to the incorporation of oxygen into YSix/n(+)-Si contacts, reduction of P concentration at YSix/n(+)-Si interface and grooving of poly-YSix. As for TiN(3 nm)/Ti(5 nm)/Y/n(+)-Si contacts with different Y thicknesses, formation of YSixOy films is observed, which indicates that O contamination deteriorates seriously rho(c) of YSix/n(+)-Si contacts. Furthermore, one effective way to reduce O contamination is provided, i.e., thickening Ti layer, leading to an appreciable reduction of rho(c) for YSix/n(+)-Si contact by about one order of magnitude. Although rho(c) of YSix/n(+)-Si contact is higher than control TiSix/n(+)-Si contact, this work provides experimental evaluation of ultrathin YSix to help establish rho(c) reduction strategies for advanced CMOS technology.