CMOS Fabrication
• CMOS transistors are fabricated on a silicon wafers of diameter
around 200-300 mm
• Lithography process is used which is similar to a printing press.
• Where on each step different materials are deposited, patterened and
etched.
Step 1 - Substrate
1. on a blank wafer a p type substrate is created through diffusion.
Step 2 - Oxidation
• Grow SiO2 in a oxidation furnace at high temperature.
Step 3 - Lithography
• Apply a photoresist layer
Step 4 - n well mask
• Apply an n-well mask and expose it to uv rays.
Step 6 - removal of photoresist by acids and basic solutions.
Step 7 - Removal of SiO2 using acid like hydroflouric
Step 8 - Complete layer of photoresist is removed
Step 9 - Formation of n well using diffusion or ion implantation
• Step 10- Removal of SiO2 completely
• Step 11- Deposit very thin layer of gate oxide using CVD process and
then a layer of polysilicon
• Step 12- Remove the extra polysilicon barring the gate terminals
• Step 13- Oxidation process is done including the gate terminals
• Step 14 - Masking and n-diffusion
• By using masking small gaps are made for n regions
• n-type dopants are diffused or ion implanted in the n regions
• Step 15 - the oxide layer is again removed.
• Step 16- similar to n rions p regions are diffused using oxide and
masks.
• Step 17- a thick field oxide is formed except on terminals of pmos and
nmos where it is etched off.
• Step 18 - Metallisation - Al is sputtered on the whole wafer and then
mask is made to remove the unwanted bits for terminals.
• Step 19- Removal of excess metal
• Step 20- Terminals are inserted
END