Mosfet having a variable gate oxide thickness and a variable gate work function, and a method for making the same

摘要

A transistor has a gate with a variable work function and a gate oxide layer with variable thickness. The gate oxide layer has an area of reduced thickness at its center, and the gate is made from central and peripheral portions. The central portion is formed over the central (thinner) portion of the gate oxide layer, and the peripheral portions are formed over the thicker areas of the gate oxide layer. The gate, gate oxide layer, and two source/drain regions may be formed in a damascene trench for improved performance, and lightly doped drain (LDD) regions preferably extend from the source/drain regions in overlapping relationship with the peripheral portions of the gate. Additionally, a method for making an asymmetrical transistor is presented, which involves applying a gate oxide layer on a semiconductor layer in contact with a sidewall structure. A first spacer made of a gate material is formed on the structure and gate oxide layer. An LDD region is then formed in the semiconductor layer, using the first spacer as a mask for alignment purposes. This is followed by formation of a second spacer on the gate oxide layer in overlapping relationship with the LDD region. The second spacer contacts the first spacer and is made of a gate material, and thus the first and second spacers collectively form the gate of the transistor. Final processing steps are performed to finish the device.