INTEGRATED CIRCUITS
INTEGRATED CIRCUITS
In electronics, an integrated circuit (also known as IC,
microcircuit, microchip, silicon chip, or chip) is a miniaturized
electronic circuit (consisting mainly of semiconductor devices,
as well as passive components) that has been manufactured in
the surface of a thin substrate of semiconductor material.
Integrated circuits are used in almost all electronic equipment
in use today and have revolutionized the world of electronics.
The first integrated circuit was
developed in the 1950s by Jack Kilby of
Texas Instruments and Robert Noyce
of Fairchild Semiconductor.
ADVANTAGES OF IC’S
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SMALL SIZE
LOW COST
IMPROVED PERFORMANCE
HIGH RELIABILITY AND RUGGEDNESS
LOW POWER CONSUMPTION
LESS VULNERABILITY TO PARAMETER VARIATION
EASY TROUBLESHOOTING
INCREASED OPERATING SPEED
LESS WEIGHT,VOLUME
EASY REPLACEMENT
DISADVANTAGES OF IC’S
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AS IC IS SMALL IN SIZE ITS UNABLE TO DISSIPATE
LARGE AMOUNT OF POWER. INCREASE IN CURRENT
MAY PRODUCE ENOUGH HEAT WHICH MAY
DESTROY THE DEVICE.
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AT PRESENT COILS, INDUCTORS AND
TRANSFORMERS CAN NOT BE PRODUCED IN IC
FORM.
CLASSIFICATION OF IC’S
On the basis of fabrication techniques used
On the basis of the chip size
On the basis of applications
ON BASIS OF FABRICATION
Monolithic IC’s
Thin and Thick Film IC’s.
Hybrid or Multi-chip ICs.
MONOLITHIC IC’S
Monolithic circuit is built into a
single stone or single crystal i.e. in
monolithic ICs, all circuit
components, and their
interconnections are formed into or
on the top of a single chip of silicon.
Monolithic ICs are by far the most
common type of ICs used in
practice, because of mass
production , lower cost and higher
reliability.
THIN AND THICK FILM IC’S
These devices are larger than monolithic
ICs but smaller than discrete circuits. These
ICs can be used when power requirement is
comparatively higher. With a thin-or thickfilm IC, the passive components like
resistors and capacitors are integrated, but
the transistors and diodes are connected as
discrete components to form a complete
circuit. The essential difference between the
thin- and thick-film ICs is not their relative
thickness but the method of deposition of
film. Both have similar appearance,
properties and general characteristics.
HYBRID IC’S
The circuit is fabricated by
interconnecting a number of
individual chips.
Hybrids ICs are widely used for high
power audio amplifier applications .
Have better performance than
monolithic ICs
Process is too expensive for mass
production
ON BASIS OF CHIP SIZE
SSI (small-scale integration)
MSI (medium-scale integration)
LSI (large-scale integration)
VLSI (very large-scale integration)
ULSI (ultra large-scale integration)
SSI AND MSI
Small scale integration (SSI)
has 3 to 30 gates/chip orUp
to 100 electronic components
per chip
Medium scale integration
(MSI) has 30 to 300
gates/chip or 100 to 3,000
electronic components per
chip
LSI AND VLSI
Large scale integration (LSI)-300
to 3,000 gates/chip or 3,000 to
100,000 electronic components per
chip
Very large scale integration
(VLSI)-more than 3,000
gates/chip or 100,000 to 1,000,000
electronic components per chip
ULSI
Ultra Large-Scale Integration
(ULSI)- More than 1 million
electronic components per
chip The Intel 486 and
Pentium microprocessors, for
example, use ULSI technology.
The line between VLSI and
ULSI is vague.
ON BASIS OF APPLICATIONS
LINEAR INTEGRATED CIRCUITS
DIGITAL INTEGRATED CIRCUITS
LINEAR INTEGRATED
CIRCUITS
When the input and output relationship
of a circuit is linear, linear ICs are used.
Input and output can take place on a
continuous range of values.
Example operational amplifiers, power
amplifiers, microwave amplifiers
multipliers etc.
DIGITAL INTEGRATED
CIRCUITS
When the circuit is either in on-state
or off-state and not in between the
two, the circuit is called the digital
circuit. ICs used in such circuits are
called the digital ICs. They find
wide applications in computers and
logic circuits.
Example logic gates, flip flops,
counters, microprocessors, memory
chips etc.
FABRICATION STEPS
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The base on which ic is formed is a p-type
substrate.
The 2nd layer is a thin layer grown as single ntype crystal and also known as epitaxial growth.
Active and passive devices of the circuits are
formed in this layer by diffusion.
Selective diffusion is needed at each diffusion
stage.
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SiO2 layer is formed on the top of epitaxial
layer.
n+ emitter is diffused into p-type base by
photolithographic masking and etching.
Another SiO2 layering , masking and etching
exposes n+ and p areas for forming metallic
contacts.
A thin Al layer is then deposited over entire chip
surface leaving inter & external connections, the
rest of Al is etched away.
EPITAXIAL GROWTH
Epitaxial growth is the formation of layer of Si crystal with n-type
doping as an extension of the existing p-type Si substrate. The
process is carried in a reactor at 1000 C where finished wafers the
inserted.
Si is obtained by breaking SiCl4 in presence of H2 :
SiCl4 + 2 H2 -->
Si + 4 HCl
For n-type doping of epitaxial layer PH4 & N2 is fed to the reactor .
OXIDE GROWTH
SiO2 layer is grown and removed from surface of silicon slice many
times during manufacture of IC’s
Characteristics of SiO2 layer :
•
Acts as diffusion mask permitting selective diffusions into Si wafer
• Used for surface passivation i.e. protecting junction from moisture
and other atmospheric contaminants
• Used for insulating the metal interconnections from Si
The Process…
The silicon wafers are kept in quartz boat and inserted into
a quartz tube. the tube is heated to 1000-1200 C such that
temperature is uniform along the length of tube.
Nitrogen,dry oxygen, steam is passed over the slices to
grow the oxide layer .
Si +O2  SiO2
PHOTOLITHOGRAPHY
Photolithography or Optical lithography, is a process used in
micro fabrication to selectively remove parts of a thin film or
the bulk of a substrate. It uses light to transfer a geometric
pattern from a photo mask to a light-sensitive chemical photo
resist, or simply "resist," on the substrate. A series of chemical
treatments then engraves the exposure pattern into the
material underneath the photo resist.
MASKING AND ETCHING
During the photolithographic process the wafer is
coated with a uniform film of a photosensitive
emulsion such as KPR (Kodak photo resist). A large
black-n-white layout of the desired pattern of
opening is made and then reduced photographically.
This negative (or stencil) of the required dimensions
is placed as a mask over the photo resist. By exposing
the KPR to UV light through
the mask , the photo resist becomes polymerized
under the transparent regions of the stencil .
The mask is now removed and wafer is developed
using chemical (such as trichloroethylene) which
dissolves the unexposed portion of the photo resist
film. Now the chip is immersed in an etching
solution of HCl, which removes the oxide from the
areas through which dopants are to be diffused.
After etching and diffusion of impurities , the resist
mask is removed with a hot
H2SO4.
FABRICATNG A MONOLITHIC
CIRCUIT
Fabrication steps involved are:
1)
2)
3)
4)
5)
Epitaxial Growth
Isolation Diffusion
Base Diffusion
Emitter diffusion
Aluminum Metallization
ISOLATION DIFFUSION
Using photolithographic etching process , oxide is removed at
four different places.The wafer is now subjected to so-called
isolation diffusion where p-type impurities penetrate the n-type
epitaxial layer from the above four places and reach the p-type
substrate. N-type region left is known as isolated region because
they are separated by two back-to-back p-n junctions. Their
purpose is to allow electrical isolation between different circuit
components.
BASE DIFFUSION
During this process a new layer of oxide is formed over the
wafer and the photolithographic process is used again to create
the pattern of openings to diffuse p-type impurities (boron). In
this way transistor base regions such as resistors , the anode of
diodes , and junction capacitors are formed. The resistivity of
base layer should be higher than that of the isolated regions.
EMITTER DIFFUSION
A layer of oxide is again formed over the entire surface,
and the masking and etching processes are used again to
the windows in p-type region. Through these openings
are diffused n-type impurities (phosphorus) for the
formation of transistor emitter , the cathode regions for
diode ,and junction capacitors.
ALUMINIUM METALIZATION
Metalisation is used to interconnect the various components
of the IC. to make these connections, a fourth set of
window is opened into a newly formed SiO2 layer at the
points where contact is to be made. The interconnections
are made using vacuum deposition of a thin even coating of
Al over the entire wafer and undesirable Al areas are
removed using photoresist technique.
VACUUM DEPOSITION
MONOLITHIC DIODE
MONOLITHIC RESISTOR
MONOLITHIC CAPACITOR
THIN FILM FABRICATION
Thin films provide greater precision in component values. The film
deposition can be done using any of the following methods:
Vacuum evaporation
Plating technique--Electroplating and Electrolessplating
Sputtering-- material to be deposited on substrate is subjected to
heavy bombardment by the ions of a heavy inert gas.the atoms
are given out from cathode, migrate away from cathode through
low pressure inert gas and finally land on substrate
Screening – uses screen woven from very fine silk threads and
mounted on Al frames. The screen is coated with photo sensitive
emulsions. The screen is placed on substrate and components
are deposited by driving squeegee across the patterned screen.
Advantages of thin film
Good high frequency response
High component package density
Resistors can be trimmed to precision
Simple processing techniques
THICK FILM FABRICATION
Thick film technology is used to fabricate high
density circuits containing resistors, conductors
and capacitors at low cost.
The technique used for depositing thickfilms
over substrate involves:
Screen printing
Substrate firing
Advantages of thick film
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Low fabrication cost
Good high frequency response
Very low tolerance cause of trimming
Highly stable and reliable
Simple fabrication steps
DIFFUSION
Sputtering apparatus
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