how fabrication steps will be in vlsi

Fabrication Steps:

First of all 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). then Remove layer where n-well should be built. Implant or diffuse n dopants into exposed wafer–Strip off SiO2.

Oxidation: Grow SiO2 on top of Si wafer about 900 to 1200 C with H2O or O2 in oxidation furnace

Photoresist: Spin on photoresist,Photoresist is a light-sensitive organic polymer it Softens where exposed to light

Lithography: in lithography procss firstly Expose photoresist through n-well mask and then Strip off exposed photoresist

Etch: In this etching process firstly Etch oxide with hydrofluoric acid (HF). It Seeps through skin and eats bone; nasty stuff!!! Only attacks oxide where resist has been exposed

Strip Photoresist: Strip off remaining photoresistand Use mixture of acids called piranah etch.

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.

Strip Oxide: Strip off the remaining oxide using HFBack to bare wafer with n-well Subsequent steps involve similar series of steps

Polysilicon: Deposit very thin layer of gate oxide<>

Polysilicon Patterning: Use same lithography process to pattern polysilicon

Self-Aligned Process: Use oxide and masking to expose where n+ dopants should be diffused or implanted. N-diffusion forms nMOS source, drain, and n-well contact

N-diffusion: Pattern oxide and form n+ regions. Polysilicon is better than metal for self-aligned gates because it doesn’t melt during later processing

P-Diffusion: Similar set of steps form p+ diffusion regions for pMOS source and drain and substrate contact

Contacts:Now we need to wire together the devicesCover chip with thick field oxideEtch oxide where contact cuts are needed