C MOS: Omplementary Fabrication

8-1
Complementary MOS fabrication

• CMOS Technology depends on using both N-Type and P-Type devices on the
same chip.
• The two main technologies to do this task are:
– P-Well (Will discuss the process steps involved with this technology)
• The substrate is N-Type. The N-Channel device is built into a P-Type well within the parent
N-Type substrate. The P-channel device is built directly on the substrate.
– N-Well
• The substrate is P-Type. The N-channel device is built directly on the substrate, while the P-
channel device is built into a N-type well within the parent P-Type substrate.
• Two more advanced technologies to do this task are:

Becoming more popular for sub-micron geometries where device performance and density must be
pushed beyond the limits of the conventional p & n-well CMOS processes.
– Twin Tub
• Both an N-Well and a P-Well are manufactured on a lightly doped N-type substrate.
– Silicon-on-Insulator (SOI) CMOS Process
• SOI allows the creation of independent, completely isolated nMOS and pMOS transistors
virtually side-by-side on an insulating substrate.
CMOS fabrication process overview

8-2
P-well on N-substrate
Steps :
• N-type substrate
• Oxidation, and mask (MASK 1) to create P-well (4-5m deep)
• P-well doping
P-well acts as substrate for nMOS devices.

The two areas are electrically isolated using thick field oxide (and often
isolation implants [not shown here])
SiO2
P-well
N-type substrate

8-3
Polysilicon Gate Formation
Steps :
• Remove p-well definition oxide
• Grow thick field oxide
• Pattern (MASK 2) to expose nMOS and pMOS active regions
• Grow thin layer of SiO2 (~0.1m) gate oxide, over the entire chip surface
• Deposit polysilicon on top of gate oxide to form gate structure
• Pattern poly on gate oxide (MASK 3)
Thin gate oxide Gate (patterned

(SiO2) polysilicon on thin oxide)
Thick field
oxide nMOS active region
P
pMOS active region N-type substrate

8-4
nMOS P+ Source/Drain difusion – self-aligned to Poly gate
Implant P+ nMOS S/D regions (MASK 4)
P+ implant/diffusion
P+ mask
Thick field
oxide
P
N-type substrate

8-5
pMOS N+ Source/Drain difusion – self-aligned to Poly gate
Implant N+ pMOS S/D regions (MASK 5 – often the inverse of MASK 4)
N+ implant/diffusion
N+ mask
P+ N+ P
N-type substrate

8-6
pMOS N+ Source/Drain difusion, contact holes & metallisation
Oxide and pattern for contact holes (MASK 6)

Deposit metal and pattern (MASK 7)
Passivation oxide and pattern bonding pads (MASK 8)
P-well acts as substrate for nMOS devices.

Two separate substrates : requires two separate substrate connections
Definition of substrate connection areas can be included in MASK 4/MASK5
Vin
Vdd Vout Vss

N+ for N-substrate P+ (for P-substrate
contact) contact)
P
P+ P channel N+
Device N channel
Device
N-type substrate

8-7
CMOS N-well process
An N-well process is also widely used
Vin
P+ for P-substrate
contact) Vdd Vout Vss N+ (for N-
substrate contact)
N-well
N + N channel P+
Device P channel
Device
P-type substrate

8-8
Twin-Tub (Twin-Well) CMOS Process
This technology provides the basis for separate optimization of the nMOS and pMOS transistors,
thus making it possible for threshold voltage, body effect and the channel transconductance of
both types of transistors to be tuned independently. Generally, the starting material is a n+ or p+
substrate, with a lightly doped epitaxial layer on top. This epitaxial layer provides the actual
substrate on which the n-well and the p-well are formed. Since two independent doping steps are
performed for the creation of the well regions, the dopant concentrations can be carefully
optimized to produce the desired device characteristics. The Twin-Tub process is shown below.
In the conventional p & n-well CMOS process, the doping density of the well region is typically
about one order of magnitude higher than the substrate, which, among other effects, results in
unbalanced drain parasitics. The twin-tub process avoids this problem.

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8-1

Complementary MOS fabrication

• Two more advanced technologies to do this task are:

CMOS fabrication process overview

P-well acts as substrate for nMOS devices.

CMOS fabrication process overview

Thin gate oxide Gate (patterned

CMOS fabrication process overview

Implant P+ nMOS S/D regions (MASK 4)

CMOS fabrication process overview

Implant N+ pMOS S/D regions (MASK 5 – often the inverse of MASK 4)

CMOS fabrication process overview

Oxide and pattern for contact holes (MASK 6)

P-well acts as substrate for nMOS devices.

Vdd Vout Vss

CMOS fabrication process overview

An N-well process is also widely used

CMOS fabrication process overview

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