VLSI Design(Fabrication)

Very-large-scale integration (VLSI) is the process of creating an integrated circuit by combining
thousands of transistors into a single chip.
HISTORY
During the mid-1920s, inventors attempted devices that were
intended to control current in solid-state diodes and convert them
into triodes.
With the invention of transistors at Bell Labs in 1947, the field of
electronics shifted from vacuum tubes to solid-state devices.
Electrical engineers of the 1950s saw the possibilities of
constructing far more advanced circuits.
Jack Kilby at Texas Instruments (in September 1958)discovered
the first integrated circuit, where he combined all the components
and the chip on the same block of semiconductor material.
Although the first integrated circuit was crude and had some
problems, the idea was groundbreaking.
All these led to development in > SSI (early 1960s) > MSI (late
1960s) > LSI > VLSI (970s and 1980s).

Cleaning (Acid process, Dry
cleaning)
Oxidation
Photolithography
Diffusion
Metallization

• It protects the junction from moisture, and also serves
as an insulator on the wafer surface.
• It is extremely necessary for the designing and
fabrication during diffusion and metallization.
In oxidation:
• Wafer is exposed to oxygen & Oxygen molecules
diffuse into the wafer.
• A chemical reaction occurs between oxygen and silicon
& a layer of oxide grows on the wafer surface.
• Si(solid) + 2H2O SiO2 (Solid) +2 H2

Photolithography, also termed optical
lithography or UV lithography, is a process
used in microfabrication to pattern parts of
a thin film or the bulk of a substrate. It uses
light to transfer a geometric pattern from a
photomask to a light-sensitive chemical
"photoresist", or simply "resist," on the
substrate.
Materials used:
Mask, Photo resist, Developer, 10% HF,
Acetone

DIFFUSION
Requirements for
diffusion:
• Temperature: 10000C
• Gas : N2= 1 L/minute,
O2= 1L/minute.
• Boron nitride
• Time :
Pre-dip- 15 minutes
Driving – 3 hours.
• 10%HF.

SiB
N2
O2
Nitrogen is blown over Si,
forming a layer of B.
Now B layer is removed,
and Oxygen is blown over
the sample. Since Si
reacts well with O2, so B
penetrates.
1000 C
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.
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.
..
.
.
.
.
.
.
..
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.. ..
.
.
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.

n-substrate
BSiO3(BORON GLASS)
[O2+Si+B] BSiO3
P
n-substrate
P
Boron glass eached
out with 10% HF

METALIZATION
• Metallization is the final step in the wafer
processing sequence. Metallization is the
process by which the components of IC’s
are interconnected by aluminum
conductor.
• Metalization is used to create contacts
with the silicon and to make
interconnections on the chip.
• Desired properties are
– low resistivity
• in ohms/square
– good adhesion to silicon and
insulators
– good coverage of steps in chip surface
– immunity to corrosion
– ductility (so temperature cycles don’t
cause failures)

• For metallization in case of p-type we choose Al and for n-
type we choose Ag.
• The process by which metallization is done is known as
vacuum evaporation system. We choose it for the following
reasons:
• To avoid the oxide of the metal.
• Mean path should be free.
Types of EVAPORATION
•Vacuum thermal evaporation system
•Vacuum electron beam evaporation system
•Vacuum radio frequency generator
•Vacuum plasma system

CMOS FABRICATION PROCESS
P-type Substrate
Silicon Wafer
Si-O2 Layer
A Si-O2 Layer is created by oxidation on top of the wafer
P-type Substrate
Start with clean p-type substrate (p-type wafer)

CMOS FABRICATION PROCESS
P-substrate
Si02
photoresist
A Photoresist is coated
over the total thing
Opaque
area
Transparent
area
mask
P-substrate
UV Ray
Masking and exposure
under UV light(E)
Resist dissolved after
developed (D)
Pre-shape the well pattern at resist layer

PHOTOLITHOGRAPHY
P-substrate
Removing the unwanted
pattern by wet etching
P-substrate
Resist clean
Desired pattern formed

DIFFUSION
P-substrate
Phosphorous ion
P-substrate
N-Well
Ion bombardment by ion implantation
SiO2 as mask, uncovered area will exposed to dopant ion

P-substrate
N-Well
P-substrate
N-Well
P-substrate
N-Well
P-substrate
N-Well
Deposit polisilicon
layer
Grow very thin
gate oxide
Photolithography
and etching to
form gate pattern

Arsenic ion
P-substrate
N-Well
P-substrate N-Well
P-substrate N-Well
Boron ion
P-substrate N-Well
Ion implantation with Arsenic
ion for n+ dopant.
Nmos’s Source and drain
with VDD contact formation
Ion implantation with
boron for p+ dopant
Pmos’s source and drain
formation with GND contact
n+ type silicon P+ type silicon

P-substrate N-Well
P-substrate N-Well
P-substrate N-Well
Deposit CVD Oxide layer
through out wafer surface
Photo and etching process
to make contact
Metal deposition
throughout wafer surface
n+ type
silicon
P+ type
silicon

COMPLETE CMOS
Vin
P-substrate N-Well
Ground
Vou
t Vdd
Photo and etching processes to
pattern interconnection
n+ type
silicon
P+ type
silicon
Metal
contact
SiO2
Layer
Polysilicon

BASIC CMOS INVERTER
• For NMOS: VGSN= Vin
VDSN=Vout
• For PMOS: VGSP= Vin-VDD
VDSP= Vout-VDD

BASIC CMOS INVERTER
VDSN=Vout
VDSP= Vout-VDD
How is it a digital inverter?

BASIC CMOS INVERTER
VDSN=Vout
VDSP= Vout-VDD
CASE 1: Vin=0V
VGSN=0V<VT (NMOS IS OFF)
VSGP=VDD>VT (PMOS IS ON)

BASIC CMOS INVERTER
VDSN=Vout
VDSP= Vout-VDD
CASE 1: Vin=0V
CASE 2: Vin= VDD

BASIC CMOS INVERTER
VDSN=Vout
VDSP= Vout-VDD
CASE 1: Vin=0V
CASE 2: Vin= VDD
VSGP= 0V<VT (PMOS IS OFF)

BASIC CMOS INVERTER
VDSN=Vout
VDSP= Vout-VDD
CASE 1: Vin=0V
CASE 2: Vin= VDD
VSGP= 0V<VT (PMOS IS OFF)
VGSN= VDD>VT (NMOS IS ON)

INPUT (Vin) PMOS NMOS
0 linear cut off
VTN linear saturation
VM saturation saturation
VDD-VTP saturation linear
VDD cut off linear

BY DUALITY & COMPLEMENTATION PROPERTIES
•By Duality: F’=(A’.B’)’ = A+B
(PMOS)
•By Complementation:
F’=AB (NMOS)

GATE REALISATION
BY SUTTON’S METHOD
NMOS: (Bubbled
output)
PMOS: (Bubbled
input)

India’s Contribution to VLSI Designing:
 Indian Institute of Technology and Intel together are
working for bringing advancement on VLSI in India.
 Lots of conferences on VLSI are going in India in
every month and the organization named VLSI
Society of India working with industry and upcoming
engineer providing finance for their project on VLSI.
 The Indian govt. has also took an initiation by
launching a VLSI education program across 32
institutes to increase the availability of chip design
talent.

Future of VLSI
 Technology is evolving everyday
and VLSI is the most progressing
one it is moving to ULSI.
 It has been predicted that VLSI will
develop more in the coming
decade.

Advantage of VLSI Designing:
 Compactness: Reduces the Size of
Circuits.
.
 Reliability: higher reliability.
 Mobility: Increases the Operating speed
of circuits
 Requires less power than discrete
components.
 Occupies a relatively smaller area.
 Easily available productivity

Disadvantage of VLSI Designing
• Previously the cost was high.
• Still the basic things like mobile phone and
other related products are cheaper but high
end products are pocket eater.
• Advancement in Indian market is required.
• Lack of training institute, so affects on
production in India.

Application of VLSI Designing
 From a simple mobile phone to the
server used in large companies.
 Recent example is Intel’s new upcoming 45
nm integration processor.
 Communications ,Microwave and RF
Cryptography.
Consumer Electronics
Automobiles
Space Applications
Robotics
Health domain.....and list continues to grow.

CONCLUSION
 We have learnt the steps of Fabrication
on Si Wayfer.
 We have learnt the CMOS Fabrication
process.
 Learned about the CMOS inverter and
GATE Realisation.
 Also learned about the applications of
VLSI design, its advantages and
disadvantages.
 There is a tremendous scope and growth
for those who choose VLSI design as a
career.

VLSI Design(Fabrication)

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