Sub-100 nm gate technologies for Si/SiGe-buried-channel RF devices

A novel fabrication process of sub-100 nm self-aligned T-gates for heterostructure field-effect transistors (HFETs) using optical contact lithography is presented. A 500-nm-wide polyimide fin is used as an implantation mask, shrunk by dry etching and subsequently replaced by a gate metal. A low-resistive gate head to form a T-shape is independently defined by wet chemical etching. Using this method, Si/SiGe modulation-doped field-effect transistors (MODFETs) have been prepared, having a gate length of 90 nm. The self-alignment enables the realization of very small source/gate and gate/drain spacings of 200 nm. This yields, together with an optimized salicide (self-aligned silicide) ohmic contact, a much lower access resistance compared to conventional gates defined by e-beam lithography. A record transit frequency f_T of 90 GHz and a very high transconductance of 570 mS/mm have been achieved for MODFETs.