EFFECTS OF MEASUREMENT FREQUENCY AND TEMPERATURE ANNEAL ON DIFFERENTIAL GATE CAPACITANCE SPECTRA OBSERVED IN HOT-CARRIER STRESSED MOSFETS

Hot carrier generated fixed and interface traps, located at the Si-SiO2 interface near the drain junction, are observed from the gate-to-drain capacitance of the MOS transistor, using an ac measurement signal applied to the drain. When the channel is biased in inversion, the drain junction is forward biased and carriers from the ac signal source are readily injected into the channel, leading to charge exchange between the inversion carriers and the traps located in one half of the band gap. In channel depletion, the drain junction is reverse biased, and charge exchange is between the substrate majority carriers and traps located in the other half of the band gap. The charge interaction manifests itself in a differential gate capacitance, extracted from pre- and post-stress gate capacitance voltage curves. The differential capacitance spectrum shows two distinct peaks, which are attributed to the response of donor and acceptor interface traps, located on either half of the band gap. This model is supported by capacitance measurements at different frequencies. Lower frequencies lead to a proportionally larger increase in the depletion regime response. Prolonged stress results in the convolution of the two peaks. A reverse bias on the drain leads to the deconvolution of the spectrum allowing the two peaks to be clearly resolved. Trap response may be mashed by the fixed charge but this can be overcome by depopulation of trapped electrons or neutralization of trapped holes through elevated temperature anneal. The differential gate-to-drain capacitance allows the electrical identification of both donor and acceptor interface traps in the same device.