2. CMOS Fabrication
@kuntala das Slide 2
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
2/1/2024
3. Inverter Cross-section
@kuntala das Slide 3
Typically use p-type substrate for nMOS
transistors
Requires n-well for body of pMOS transistors
n+
p substrate
p+
n well
A
Y
GND VDD
n+ p+
SiO2
n+ diffusion
p+ diffusion
polysilicon
metal1
nMOS transistor pMOS transistor
2/1/2024
4. Well and Substrate Taps
@kuntala das Slide 4
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 / taps
n+
p substrate
p+
n well
A
Y
GND VDD
n+
p+
substrate tap well tap
n+ p+
2/1/2024
5. Inverter Mask Set
@kuntala das Slide 5
Transistors and wires are defined by masks
Cross-section taken along dashed line
GND VDD
Y
A
substrate tap well tap
nMOS transistor pMOS transistor
2/1/2024
6. Detailed Mask Views
@kuntala das Slide 6
Six masks
n-well
Polysilicon
n+ diffusion
p+ diffusion
Contact
Metal
Metal
Polysilicon
Contact
n+ Diffusion
p+ Diffusion
n well
2/1/2024
7. Fabrication Steps
@kuntala das Slide 7
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
p substrate
2/1/2024
8. Oxidation
@kuntala das Slide 8
Grow SiO2 on top of Si wafer
900 – 1200 C with H2O or O2 in oxidation furnace
p substrate
SiO2
2/1/2024
9. Photoresist
@kuntala das Slide 9
Spin on photoresist
Photoresist is a light-sensitive organic polymer
Softens where exposed to light
p substrate
SiO2
Photoresist
2/1/2024
10. Lithography
@kuntala das Slide 10
Expose photoresist through n-well mask
Strip off unexposed photoresist
p substrate
SiO2
Photoresist
2/1/2024
11. Etch
@kuntala das Slide 11
Etch oxide with hydrofluoric acid (HF)
Seeps through skin and eats bone; nasty stuff!!!
Only attacks oxide where resist has been
exposed
p substrate
SiO2
Photoresist
2/1/2024
12. Strip Photoresist
@kuntala das Slide 12
Strip off remaining photoresist
Use mixture of acids called piranah etch
Necessary so resist doesn’t melt in next step
p substrate
SiO2
2/1/2024
13. n-well
@kuntala das Slide 13
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
n well
SiO2
2/1/2024
14. Strip Oxide
@kuntala das Slide 14
Strip off the remaining oxide using HF
Back to bare wafer with n-well
Subsequent steps involve similar series of steps
p substrate
n well
2/1/2024
15. Poly-silicon
@kuntala das Slide 15
Deposit very thin layer of gate oxide
< 20 Å (6-7 atomic layers)
Chemical Vapor Deposition (CVD) of silicon layer
Place wafer in furnace with Silane gas (SiH4)
Forms many small crystals called polysilicon
Heavily doped to be good conductor
Thin gate oxide
Polysilicon
p substrate
n well
2/1/2024
16. Polysilicon Patterning
@kuntala das Slide 16
Use same lithography process to pattern
polysilicon
Polysilicon
p substrate
Thin gate oxide
Polysilicon
n well
2/1/2024
17. N-diffusion
@kuntala das Slide 17
Use oxide and masking to expose where n+
dopants should be diffused or implanted
N-diffusion forms nMOS source, drain, and n-well
contact
p substrate
n well
2/1/2024
19. N-diffusion (cont.)
@kuntala das Slide 19
Historically dopants were diffused
Usually ion implantation today
But regions are still called diffusion
n well
p substrate
n+
n+ n+
2/1/2024
21. P-Diffusion
@kuntala das Slide 21
Similar set of steps form p+ diffusion regions for
pMOS source and drain and substrate contact
p+ Diffusion
p substrate
n well
n+
n+ n+
p+
p+
p+
2/1/2024
22. Contacts
@kuntala das Slide 22
Now we need to wire together the devices
Cover chip with thick field oxide
Etch oxide where contact cuts are needed
p substrate
Thick field oxide
n well
n+
n+ n+
p+
p+
p+
Contact
2/1/2024
23. Metalization
@kuntala das Slide 23
Sputter on aluminum over whole wafer
Pattern to remove excess metal, leaving wires
p substrate
Metal
Thick field oxide
n well
n+
n+ n+
p+
p+
p+
Metal
2/1/2024
