Question

Draw a complete step-by-step CMOS process flow with a p-type substrate for n well formation including a layout mask for each lithography step, showing an NMOSFET, a PMOSFET, and an undoped polysilicon resistor. The size of both MOS transistors is W=25 micro-meters, L=0.25micro- m, and the length of the source/drain regions are Ls=0.5 micrometers. The resistor value is R=35k.and the strip width for resistors is set at 0.5micro meters.

Answer 1

Step 1) Start with a P-substrate.

Step 2 ) Oxidation -- Grow SiO2 on top of Silicon wafer at -900 to -1200C with H2O or O2 in oxidation furnace, So that layer of SiO2 will be formed on top of P-substrate

Step 3) Photoresist -- Photoresist is light sensitive organic polymer that will soften when exposed to light

Step 4) Lithography -- For lithography, expose the photoresist so that it will soften and then strip off the part to form n-well

Step 5) Etch -- The wafer is exposed to hydrofluoric acid (HF), to etch the SiO2 part

Step 6) Strip Photoresist -- Strip the remaining photoresist

Step 7) Diffusion and Ion implantation to form n-well

n-well is formed with diffusion and ion implantation. Diffusion is a process in which the wafer is placed in furnace and exposed to Arsenic gas. When heat is applied to the furnace, the Arsanic (As) gas atoms will diffuse into the exposed Si wafer to form n-well. These As atoms are blocked by SiO2 layer.

Step 8) Strip Oxide -- Strip off the remaining SiO2 using HF

Step 9 ) Polysilicon -- Deposit a thin layer of gate oxide then on top of gate oxide, polysilicon is deposited by process of CVD (Chemical Vapor Deposition).

Step 10) Polysilicon Patterning -- Same lithography process is followed to form required pattern