Substrate bias dependent leakage in LDD MOSFETs

A new leakage mechanism that is a strong function of substrate bias has been observed on lightly doped drain (LDD) MOSFETs. The substrate bias dependence on this drain-to-substrate leakage current is shown to be related to the formation of sidewall spacers. Devices with a conventional oxide spacer exhibit substrate-bias-dependent leakage with a wide variation in leakage characteristics. Devices with a disposable spacer, however, do not exhibit such leakage and have a small variation in leakage characteristics. The deleterious leakage characteristics observed with conventional oxide spacers are caused by defects produced in the silicon along the spacer edge during spacer formation. These defects can be avoided with a spacer formation process that does not etch into silicon in the source/drain regions.<>