Selective Dry Etch for Defining Ohmic Contacts for High Performance ZnO TFTs

摘要

Recently, gigahertz RF performance has been demonstrated in zinc oxide (ZnO) TFT. However, the need arises for sub-micron channel length (Lc) dimensions to extend these results into X-band frequency range of operation. This thesis is a pioneering effort identifying device access materials to be selectively etched to ZnO via plasma-assisted etch (PAE) to avoid processing limitations from traditional optical lithography channel definition methods. A subtractive etch process using CF4/O2 gas mixture was completed with various Ohmic contact materials to ZnO providing foundational research upon which nano- scale, high-frequency ZnO thin-film transistors (TFTs) could be fabricated. Molybdenum, tantalum, titanium tungsten 10-90, and tungsten metallic contact schemes to ZnO are investigated for their etch selectivities to ZnO and etch profiles. Tungsten displayed promising device scalability results with excellent aspect ratio and 200nm Lc. A new semiconductor-semiconductor contact interface to ZnO using nc-Si is initially reported with 15mA/mm current density and 18mS/mm transconductance. Nc-Si also displays promising scaling results through the subtractive etch process defined with e-beam lithography. Results included 157nm channel length, high aspect ratio, and high extrapolated current density of nearly 1A/mm at 100nm Lc and gate and drain voltages of 10V.