INTEGRATED CIRCUITS

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INTEGRATED CIRCUITS
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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.
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ADVANTAGES OF ICS

• 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

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DISADVANTAGES OF ICS

• 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.
• AT PRESENT COILS, INDUCTORS AND TRANSFORMERS CAN
  NOT BE PRODUCED IN IC FORM.

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CLASSIFICATION OF ICS

• On the basis of fabrication techniques used
• On the basis of the chip size
• On the basis of applications

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ON BASIS OF FABRICATION

• Monolithic ICs
• Thin and Thick Film ICs.
• Hybrid or Multi-chip ICs.

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MONOLITHIC ICS
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.
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THIN AND THICK FILM ICS
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 thick-film 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.
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HYBRID ICS
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
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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)

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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
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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
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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.
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ON BASIS OF APPLICATIONS

• LINEAR INTEGRATED CIRCUITS
• DIGITAL INTEGRATED CIRCUITS

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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.

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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.
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FABRICATION STEPS

• The base on which ic is formed is a p-type
  substrate.
• The 2nd layer is a thin layer grown as single
  n-type 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.

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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 --gt
Si 4 HCl For n-type doping of epitaxial layer
PH4 N2 is fed to the reactor .
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OXIDE GROWTH

• SiO2 layer is grown and removed from surface
  of silicon slice many times during manufacture of
  ICs
• 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

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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
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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.
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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 .
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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.
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FABRICATNG A MONOLITHIC CIRCUIT
Fabrication steps involved are

1. Epitaxial Growth
2. Isolation Diffusion
3. Base Diffusion
4. Emitter diffusion
5. Aluminum Metallization

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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.
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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.
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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.
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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.
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VACUUM DEPOSITION
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MONOLITHIC DIODE
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MONOLITHIC RESISTOR
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MONOLITHIC CAPACITOR
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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.

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Advantages of thin film

• Good high frequency response
• High component package density
• Resistors can be trimmed to precision
• Simple processing techniques

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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

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Advantages of thick film

• Low fabrication cost
• Good high frequency response
• Very low tolerance cause of trimming
• Highly stable and reliable
• Simple fabrication steps

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DIFFUSION
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Sputtering apparatus
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