THE PT SI(111) INTERFACE AND THE PROPERTIES OF THIN PT LAYERS ON SI/

Thin Pt- and Pt-silicide films on Si are currently used in microelectr onics as Schottky diodes with high values of the electrical barrier (s imilar to 0.8eV). Such films also have metallic or near metallic sheet conductivities and are suited for interconnects. The choice of Pt is indicated by the reaction between Pt and Si which seems easy to contro l, at low temperatures, enabling so-called self-alignment of the silic ide pattern. This is somewhat unexpected, however, when considering th e bulk thermodynamic phase diagram of these elements. From this, proce ssing temperatures of over 830 degrees C should be needed for reaching the eutectic temperature and an even higher temperature (1210 degrees C) needed to reach the most stable silicide composition, PtSi. Many t imes, studies of the Pt/Si system under UHV conditions of processing a nd purity have been interpreted as forming silicide at the Si surface, even at room temperature, However, it was noted that no bulk silicide is grown from thin Pt deposits by heating in UHV. Normally, an inhomo geneous composition profile is found with the top surface being enrich ed in silicon. In the present studies Pt was deposited slowly on the S i(lll) 7 x 7 surface. This was done to follow its reaction and in-dept h distribution profile during deposition, and the formation of the Sch ottky barrier in this step. Subsequent annealing was then studied. Fur ther experiments studied the sensitivity of annealing to impurities (C and O), and to oxidation. It was found that during deposition of Pt s ome Si atoms are retained in the top surface, bonding with or imbeddin g themselves in the Pt with a silicide-like appearance of the valence band, Si(2p) core levels, and the Si (L(2,3) VV) Auger spectrum (XAES) . However the distribution of Pt and Si below the surface during depos ition is indicative of an almost homogeneous Pt film, with some signs of a combined island and layer growth, past the first monolayer of Pt. The dislodging of Si atoms occurs during completion of the first mono layer, and the reaction starts only after this is completed. Impuritie s in a Pt him, present before annealing and were found to create a hom ogeneous, stable metallic phase. Oxidation of various Pt/Si structures differs from that of clean Si surfaces at intermediate temperatures, where evidence for the formation of an oxide with (weak) features of S iO2 is obtained.