Refractory metal gate processes for VLSI applications

The conventional n-channel silicon-gate MOS technology faces limitations due to poly-sheet resistance, patterning, and overall process compatibility as feature dimensions shrink and circuit sizes increase. This work describes an investigation of refractory metals as alternate gate material due to their potential advantages in terms of patternability, film sheet resistance that is 50 to 100 times lower comparable to polysilicon, and circuit layout options with improved performance and cost effectiveness. A process was developed for fabrication of NMOS devices using electron-beam direct writing, refractory metal gates and interconnects. Evaporated as well as sputtered molybdenum and tungsten show columnar crystalline structure allowing reproducible patterning of 1-µm lines with the conventional plasma etch techniques. Sheet resistivities of 0.25 Ω/□ and 0.4 Ω/□ were achieved for 3500-Å Mo and W films, respectively. Basic parameters such as electronic work function, interface, and surface state charge densities and MOS transistor threshold voltages were determined from metal gate devices with 200-800-Å oxides. Flat-band voltages of -0.2 V with mid 10
¹⁰
cm
^-2
interface charges were achieved. Mo-Al-Si double-level metal structure with contacts as small as 2 µm
²
were fabricated. Considerable improvement in the speed of the LSI circuits fabricated with these high-conductivity gates was demonstrated through computer simulation. Improved circuit layouts of static and dynamic memory cells for speed, power, and density optimization can be achieved. A four-mask process with the simplicity of aluminum gate technology and the performance of current n-channel silicon gate MOS process is proposed.