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CMOS Fabrication Techniques

The document discusses CMOS fabrication processes. It describes the key steps in CMOS fabrication which include crystal growth, photolithography, oxidation, diffusion, ion implantation, etching, and metallization. It then explains the n-well and p-well CMOS fabrication processes which involve doping wells or tubs through diffusion or implantation and creating contacts. Finally, it briefly introduces the twin-tub process which allows independent optimization of n-device and p-device parameters.

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Prajeeth Babu Kodru
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303 views38 pages

CMOS Fabrication Techniques

The document discusses CMOS fabrication processes. It describes the key steps in CMOS fabrication which include crystal growth, photolithography, oxidation, diffusion, ion implantation, etching, and metallization. It then explains the n-well and p-well CMOS fabrication processes which involve doping wells or tubs through diffusion or implantation and creating contacts. Finally, it briefly introduces the twin-tub process which allows independent optimization of n-device and p-device parameters.

Uploaded by

Prajeeth Babu Kodru
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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CMOS Fabrication

Dr. K. R. Pasupathy
Overview
• CMOS Fabrication technologies
• N-well
• P-well
• Twin-tub processes
• CMOS process steps (fabrication steps)
• Crystal growth
• Photolithography
• Oxidation
• Diffusion
• Ion implantation
• Etching
• Metallization
CMOS inverter cross section
• Input is A (two gates are connected)
• Output is Y (two drains are connected)
• Other than contact regions – SiO2 (field oxide)
• Substrate – low potential; n-well – high potential
• Substrate and well contacts in semiconductor are heavily doped
• Ohmic contact (low resistance and bidirectional)
Masks
• Masks specify where the
components will be
manufactured.
• To define the regions where
it should be formed
n-well process
CMOS inverter fabrication using n-well
process
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
Working of photoresist

• Light sensitive material


• When UV light falls, it
undergoes chemical changes
• Becomes more soluble or less
soluble according to whether it
is positive or negative
• Photoresist is dipped into a
solvent called “developer
solution”
• Soluble portions are dissolved
• Use – to etch selected
portions
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
Procedure for doping
• Whenever you want dope,
• Grow SiO2
• Add photo resist
• Expose light through mask
• Wash it with developer solution
• Etch SiO2 selectively
• Dope the region through diffusion
or ion implantation
• Finally etch the oxide completely
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process (cont’d)
CMOS inverter fabrication using n-well
process - Final structure
P-well process
P-well process steps

• eBook :- Neil Weste and Kamran Eshraghian “Principles of CMOS VLSI


Design – A systems perspective”
P-well process steps (cont’d)

• Dopants can
penetrate thin oxide
but not thick oxide
P-well process steps (cont’d)
P-well process steps (cont’d)
Cross section of p-well cmos
(a) Schematic

(b) layout

(c) Cross section of p-well process

(d) More realistic cross-section


P-well substrate contacts
Twin-tub process
Twin tub process
• Can be viewed as independent
devices
• Device parameters of n-device and p-
devices can be independently
optimized
• Major steps
• Tub formation
• Thin oxide etching
• Source and drain implantation
• Contact cut definition
• metallization
Twin tub process
Twin tub process
Twin tub process
Twin tub process

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