Study of work function of CVD WSi x thin film on high K dielectric.
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E6.32.1
Study of work function of CVD WSix thin film on high K dielectric. S.Maîtrejean(1), S.Allégret(2), F.Fillot(1), T.Farjot(1), B.Guillaumot(2), G.Passemard(2) (1) (2)
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ABSTRACT To meet requirements of CMOS circuits at sub 45nm scale, gate oxide thickness shall decrease. Thus high K materials are needed as dielectric gate. In this setting, due to gate depletion effect, metallic material should be used as an alternative to poly silicon gate. Moreover, specifications on threshold voltage require modulation of gate material work function with respect to nMOS or pMOS transistor. WSix work function is known to be sensitive to material stoichiometry. In this work, WSix thin films with x between 2.2 and 2.5 are evaluated as metal gate on HfO2 and SiO2 dielectric. Film chemical characteristics are correlated with work function measurements. Thin films are deposited using WF6 and dichlorosilane on a 200mm wafer industrial chamber. Thermal treatments are applied to sample in order to recrystallise the film and confirm it stability. MOS Capacitors are processed. Electrical characterizations (capacitance vs voltage) are used to extract work function with respect to film composition. Films are chemically and morphologically stable up to 800°C. A W/Si ratio gradient is observed between surface and dielectric/film interface. Whatever the nominal stoichiometry, Si/W ratio is constant at this interface. This result is correlated with identical work function measurement for different nominal stoichiometry. Moreover no differences are observed between Vfb vs equivalent oxide thickness curves for HfO2 and SiO2 dielectric. High EOT variations have been identified for SiO2 capacitors in contrast with SiO2/HfO2 capacitors. These results characterize WSix as a suitable metal gate for nMOS transistors. INTRODUCTION To meet requirements of CMOS circuits at sub 45nm scale, capacitor gate shall decrease to obtain equivalent SiO2 oxide thickness (EOT) below 1nm. The capacitance associated with the depleted layer in poly silicon gate becomes significant and cannot be solved by simple increase of the gate doping level [1]. Moreover, poly silicon instability on High K dielectric have been observed. It is thus necessary to consider alternative gate electrodes. The use of metal as gate electrode is now a widely studied option that could solve the poly silicon issues. However, to process high performance transistors, gate work function (φm) shall be adjusted: a value around 4.4eV is needed for nMOS transistor whereas a value around 4.8eV is needed for pMOS one [2]. Thus two materials or a work function adjustable one is required. The second option shall make integration easier. Tungsten silicide (WSix) is the most commonly used silicide for polycide processes because of high thermal stability, low resistivity and ease of patterning with dry etch processes [3]. Previous works have shown that work function can be modulated with Si/
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