01-08-2012, 12:55 PM
CMOS Fabrication
CMOS Fabrication.ppt (Size: 631 KB / Downloads: 29)
CMOS transistors are fabricated on silicon wafer
Lithography process similar to printing press
On each step, different materials are deposited or etched
Easiest to understand by viewing both top and cross-section of wafer in a simplified manufacturing process
Inverter Cross-section
Typically use p-type substrate for nMOS transistors
Requires n-well for body of pMOS transistors
Well and Substrate Taps
Substrate must be tied to GND and n-well to VDD
Metal to lightly-doped semiconductor forms poor connection (used for Schottky Diode)
Use heavily doped well and substrate contacts
Fabrication Steps
Start with blank wafer
Build inverter from the bottom up
First step will be to form the n-well
Cover wafer with protective layer of SiO2 (oxide)
Remove layer where n-well should be built
Implant or diffuse n dopants into exposed wafer
Strip off SiO2
n-well
n-well is formed with diffusion or ion implantation
Diffusion
Place wafer in furnace with arsenic gas
Heat until As atoms diffuse into exposed Si
Ion Implanatation
Blast wafer with beam of As ions
Ions blocked by SiO2, only enter exposed Si