Threshold voltage control of Pt-Ti-0 gate Si-metal-insulator semiconductor field-effect transistors hydrogen gas sensors by using oxygen invasion into Ti layers

摘要

Although the threshold voltages (Vth) of the as-processed Pt(15nm)/Ti(5nm)-gate Si-MOSFETs under same channel ion dose conditions show a large variation such as 0.846 V among several wafers, the air-annealing and succeeding hydrogen post-annealing procedure for the FETs hydrogen gas sensors leads to excellent uniform Vth distributions and large sensing amplitude AVg. The oxygen invasion process through Pt grain boundaries to amorphous Ti layers at 400 ℃ air-annealing for two hours is not a simple dopant diffusion process but super-heavily oxygen-doped process partly to grow nano-crystalline TiOx. The oxygen-invaded Ti layers change to a kind of new materials; novel mixing layers of nano-crystalline TiOx and super-heavily oxygen-doped amorphous Ti formed on SiO_2/Si substrates. The Ti mixing layers change from metals to semiconductors or insulators. As the Ti layers are so thin like 5 nm, the total amount of oxygen invaded into Ti layers will be saturated and stabilized. From the device operation point of view, it is crucial to control the Vth precisely that the Ti novel mixing layers are thin and fully depleted. This is supported by the fact that the Vth change before and after air-annealing procedures can be well explained by the difference of vacuum work function between Pt and Ti.