Gate oxide reliability in an integrated metal-oxide-semiconductor field-effect transistor-microelectromechanical system technology

Abstract

We have demonstrated a simple technique for building n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) and complex micromechanical systems simultaneously instead of serially, allowing a more straightforward integration of complete systems. The fabrication sequence uses few additional process steps and only one additional masking layer compared with a microelectromechanical system (MEMS)-only technology, but exposes the MOSFET gate oxide to the extreme temperatures of MEMS processing. We find that the high-temperature MEMS anneals can greatly change the current-voltage characteristics of the SiO2 gate oxide film. Defects are introduced by the high-temperature processing which result in considerable positive-charge trapping at short times that leads to increased currents using high bias fields and short bias times. At longer times, the current across the oxide decays as 1/time suggesting electron traps that are uniformly distributed in the bulk of the oxide. A traditional reliability evaluation of the thick oxides using time-dependent dielectric breakdown is greatly hampered by excessive impact ionization that occurs at high gate voltages. This makes a rigorous evaluation of gate oxide lifetime at use fields difficult. We do note that while the current below breakdown is altered by the positive-charge trapping, the voltages at breakdown seen in ramps are not severely impacted by the high-temperature annealing. Breakdown voltages of films annealed at the highest of the temperatures studied here are about the same as those of unannealed films. The breakdown fields of these stressed oxides are 10–11MV∕cm, suggesting that reliable SUMMiT™ field-effect transistor process gate oxides can be fabricated as long as the use voltage is well below the onset for impact ionization.