The Pt/Si(111) interface and the properties of thin Pt layers on Si

P. Morgen, B. Jørgensen, J. Gordon

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    Thin Pt- and Pt-silicide films on Si are currently used in microelectronics as Schottky diodes with high values of the electrical barrier (similar 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 control, at low temperatures, enabling so-called self-alignment of the silicide pattern. This is somewhat unexpected, however, when considering the bulk thermodynamic phase diagram of these elements. From this, processing temperatures of over 830°C should be needed for reaching the eutectic temperature and an even higher temperature (1210°C) needed to reach the most stable silicide composition, PtSi. Many times, studies of the Pt/Si system under UHV conditions of processing and 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 inhomogeneous composition profile is found with the top surface being enriched in silicon. In the present studies Pt was deposited slowly on the Si(111) 7 × 7 surface. This was done to follow its reaction and in-depth distribution profile during deposition, and the formation of the Schottky barrier in this step. Subsequent annealing was then studied. Further experiments studied the sensitivity of annealing to impurities (C and O), and to oxidation. It was found that during deposition of Pt some Si atoms are retained in the top surface, bonding with or imbedding themselves in the Pt with a silicide-like appearance of the valence band, Si(2p) core levels, and the Si (L2,3 VV) Auger spectrum (XAES). However the distribution of Pt and Si below the surface during deposition 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 monolayer, and the reaction starts only after this is completed. Impurities in a Pt film, present before annealing, were found to create a homogeneous, 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 SiO2 is obtained.

    TidsskriftPhysica Scripta
    Udgave nummerT54
    Sider (fra-til)278-282
    StatusUdgivet - 1994


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