03-11-2012, 01:29 PM
LINEAR INTEGRATED CIRCUITS (EC1313) QUESTION & ANSWERS
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IC Fabrication
1.Define an Integrated circuit.
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.
2.What are the basic processes involved in fabricating ICs using planar technology?
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
3.List out the steps used in the preparation of Si – wafers.
1.Crystal growth &doping
2.Ingot trimming & grinding
3.Ingot slicing
4.Wafer policing & etching
5.Wafer cleaning
4.Wwrite the basic chemical reaction in the epitaxial growth process of pure silicon.
The basic chemical reaction in the epitaxial growth process of pure silicon is the
hydrogen reduction of silicon tetrachloride.
1200oC
SiCl4 + 2H2 <>
Si + 4 HCl
5.What are the two important properties of SiO2?
1.SiO2 is an extremely hard protective coatng & is unaffected by almost all
reagents except by hydrochloric acid. Thus it stands against any contamination.
2.By selective etching of SiO2 , diffusion of impurities through carefully defined
windows in the SiO2 can be accomplished to fabricate various components.
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6.Explain the process of oxidation.
The silicon wafers are stacked up in a quartz boat & then inserted into quartz
furnace tube. The Si wafers are raised to a high temperature in the range of 950 to 1150oC
& at the same time, exposed to a gas containing O2 or H2O or both.The chemical action is
Si + 2H2O >
Si O2+ 2H2
7.What is meant by molecular beam epitaxy(MBE)?
In the molecular beam epitaxy, ilicon along with dopants is evaporated.The
evaporated species are transported at a relatively high velocity in a vacuum to the
substrate.The relatively low vapour pressure of silicon & the dopants ensures
condensation on a low temperature substrate.Usually, silicon MBE is performed under
ultra high vacuum (UHV) condition of 10-8 to 10-10 Torr.
8.What are the advantages of Molecular Beam Epitaxy( MBE )?
( i ) It is a low temperature process, useful for VLSI. This minimises outdiffusion
& autodoping.
( ii ) It allows precise control of doping& permits complucated profiles to be
generated.
( iii )Linear doping profile desirable for varactor diode in FM , can be obtained
with MBE.
( iv )Wider choice of dopants can be used.
9. What are oxidation induced defects in semi conductor?
1.Stacking faults
2.Oxide isolation defects
Stacking faults:
Structural defects in the silicon lattice is called oxidation induced stacking
faults.The growth of stacking faults is a strong function of substrate orientation ,
conductivity type & defect nuclei present.The stacking faults formation can be
suppressed by the addition of HCl.
Oxide isolation defects :
The stress along the edges of an oxidised area produce severe damage in the
silicon. Such defects results in increased leakage in nearby devices.High temperatures
(around 950oC ) will prevent stress induced defect formation.
10.What is bird’s beak?
In local oxidation process, the oxidation of silicon proceeds slightly under the
nitride as well. Also, a large mismatch in the thermal expansion co-efficients of Si3N4 &
Silicon results in damage to the semi conductor during local oxidation.This damage can
be graetly reduced by growing a thin layer of SiO2 prior to placement of the Si3N4 mask.
Typically 100 to 200Ao is used for this puspose. Unfortunately, this greatly enhances the
penetration of oxide under the nitride masked regions , resulting in oxide configurations
called bird’s beak.