UNIT 1 IC FABRICATION

UNIT-I

IC FABRICATION

INTEGRATED CIRCUITS

An integrated circuit (IC) is a miniature ,low cost electronic circuit consisting of active and
passive components fabricated together on a single crystal of silicon. The active components are
transistors and diodes and passive components are resistors and capacitors.

Advantages of integrated circuits
1. Miniaturization and hence increased equipment density.
2. Cost reduction due to batch processing.
3. Increased system reliability due to the elimination of soldered joints.
4. Improved functional performance.
5. Matched devices.
6. Increased operating speeds.
7. Reduction in power consumption

Basic processes involved in fabricating Monolithic ICs
1. Silicon wafer (substrate) preparation
2. Epitaxial growth
3. Oxidation
4. Photolithography
5. Diffusion
6. Ion implantation
7. Isolation technique
8. Metallization
9. Assembly processing & packaging

Silicon wafer (substrate) preparation

1.Crystal growth &doping
2.Ingot trimming & grinding
3.Ingot slicing
4.Wafer policing & etching
5.Wafer cleaning

Epitaxial growth

Epitaxy means growing a single crystal silicon structure upon a original silicon substrate, so that
the resulting layer is an extension of the substrate crystal structure.
The basic chemical reaction in the epitaxial growth process of pure silicon is the hydrogen
reduction of silicon tetrachloride.

Oxidation

SiO2 is an extremely hard protective coating & is unaffected by almost all reagents except by
hydrochloric acid. Thus it stands against any contamination.
By selective etching of SiO2, diffusion of impurities through carefully defined through windows in
the SiO2 can be accomplished to fabricate various components.

Photolithography
The process of photolithography makes it possible to produce microscopically small circuit
and device pattern on si wafer
Two processes involved in photolithography
a) Making a photographic mask
b) Photo etching

Photographic mask


The development of photographic mask involves the preparation of initial artwork and its diffusion.
reduction, decomposition of initial artwork or layout into several mask layers.

Photo etching
Photo etching is used for the removal of SiO2 from desired regions so that the desired2impurities can
be diffused

Diffusion
The process of introducing impurities into selected regions of a silicon wafer is called diffusion. The
rate at which various impurities diffuse into the silicon will be of the order of 1μm/hr at the
temperature range of 9000 C to 11000C .The impurity atoms have the tendency to move from regions
of higher concentrations to lower concentrations

Ion implantation technique
It is performed at low temperature. Therefore, previously diffused regions have a lesser tendency for
lateral spreading.
In diffusion process, temperature has to be controlled over a large area inside the oven, where as in
ion implantation process, accelerating potential & beam content are dielectrically controlled from
outside

Dielectric isolation
In dielectric isolation, a layer of solid dielectric such as SiO2 or ruby completely surrounds each
components thereby producing isolation, both electrical & physical. This isolating dielectric layer is
thick enough so that its associated capacitance is negligible. Also, it is possible to fabricate both pnp
& npn transistors within the same silicon substrate

Metallization
The process of producing a thin metal film layer that will serve to make interconnection of the various
components on the chip is called metallization.

Aluminium is preferred for metallization
1. It is a good conductor
2. it is easy to deposit aluminium films using vacuum deposition.
3. It makes good mechanical bonds with silicon
4. It forms a low resistance contact

IC packages available
1. Metal can package.
2. Dual-in-line package.
3. Ceramic flat packages.