UNIT III- DEVICE CORROSION
3.1 Introduction
Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with
its environment, chemical attack accompanies physical deterioration. Corrosion causes the
malfunctioning of controls and switching gear that results in more than 50% of electrical system
downtime, causing a huge loss. Harsh environments can cause electrochemical deterioration and
chemical attack on metallic wire terminals. Overall, the corrosion of current-carrying parts and
switching, as well as controlling equipment, affects the reliability and uninterrupted operation of
all kinds of electrical components and production facilities. This type of corrosion caused to
electrical and electronic devices are called as Device Corrosion.
The careful selection of coatings and substrate materials based on the service exposure of
electrical equipment and controls can improve the reliability and cost effectiveness of the entire
system. Electronic devices are presently used under service conditions that were never thought off
few years back. Consumer electronics are used under highly variable environmental parameters
(e.g. mobile phones brought to the seaside or sound system installations in kitchens or bathrooms).
Industrial electronics also experience a variety of environments due to their wide spread
use. In many cases precautions for protecting electronics against aggressive conditions are not met.
The demand for electronics is not lifetime performance, but reliability, since replacing electronics
is a costly task. The increased use of electronics has also increased the demand for corrosion
reliability. Corrosion failures can be difficult to diagnose and cause unexpected production loss
due to plant shut down. Overall size of electronic equipment and therefore the component size
have been decreasing at a faster rate.
3.2 Chemistry of IC (Integrated Circuit)
The chemistry involved in fabrication of IC (fig 3.1) chips:
Substrate preparation
Epitaxial growth
Silicon dioxide growth
Photo-etching
3.1
, Diffusion
Fig.3.1 Integrated circuit and related components
3.2.1 Substrate Preparation
It involves the following steps:
Selection of Raw Material: Silicon of about 98% purity is obtained on reduction of silicon dioxide
on heating it along with carbon at a suitably high temperature in a furnace.
Zone refining: The order of purity needed in manufacture of IC chips is 1 impurity atom in
109 silicon atoms. This high order of purity is obtained on repeated zone refining process.
Crystal Growth: Silicon of desire purity is heated in a furnace and p-type impurity in proper
proportion is added to it. The p-type silicon crystal is then grown from the melt using a seed crystal.
Slicing and Polishing: The p-type silicon crystal is sliced into wafer each of thickness about 6
mils. Each wafer is then lapped to remove saw marks and then polished mechanically or chemically
to produce mirror like smoothness.
3.2.2 Epitaxial Growth
3.2
, Epitaxial growth consists in formation of a single crystal thin film from the gas phase on a
wafer (p-type in this case) of the same material (silicon). This epitaxial layer forms a
continuation of the single crystal structure of the substrate.
Epitaxial growth uses a chemical reaction whereby silicon is precipitated from a gaseous
solution to form a thin film of single crystal (mono crystalline) silicon on the surface of
silicon wafer placed in the gaseous solution.
The basic chemical reaction involved in epitaxial growth of pure silicon is hydrogen
reduction of silicon tetrachloride .The epitaxial layer may be made either n-type or p-type.
Salient Features of Epitaxial Process
The atom is arranged on crystal substrate in the single crystal fashion forming an extension
of the substrate layer.
Epitaxial layer is grown from gas phase whereas large size crystals are grown from liquid
phase. Further, in epitaxial process, no part pf the system has temperature approaching the
melting point.
Epitaxial layer has highly uniform resistivity (better than disused layer).
Epitaxial layer may also be grown on diffused surface.
Doping may be either p-type or n-type.
Doping concentration may be varied over large range and complex impurity profile may
be grown.
3.2.3 Silicon Dioxide Growth
Before formation of a fresh SiO2 layer on the wafer surface, the wafer is washed in a suitable
solution to remove the previous SiO2 layer. Then the wafer is heated to a suitably high temperature
in an atmosphere of oxygen. Silicon oxidises to form a thin layer of SiO2.
3.2.4 Photo-Etching
3.3
3.1 Introduction
Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with
its environment, chemical attack accompanies physical deterioration. Corrosion causes the
malfunctioning of controls and switching gear that results in more than 50% of electrical system
downtime, causing a huge loss. Harsh environments can cause electrochemical deterioration and
chemical attack on metallic wire terminals. Overall, the corrosion of current-carrying parts and
switching, as well as controlling equipment, affects the reliability and uninterrupted operation of
all kinds of electrical components and production facilities. This type of corrosion caused to
electrical and electronic devices are called as Device Corrosion.
The careful selection of coatings and substrate materials based on the service exposure of
electrical equipment and controls can improve the reliability and cost effectiveness of the entire
system. Electronic devices are presently used under service conditions that were never thought off
few years back. Consumer electronics are used under highly variable environmental parameters
(e.g. mobile phones brought to the seaside or sound system installations in kitchens or bathrooms).
Industrial electronics also experience a variety of environments due to their wide spread
use. In many cases precautions for protecting electronics against aggressive conditions are not met.
The demand for electronics is not lifetime performance, but reliability, since replacing electronics
is a costly task. The increased use of electronics has also increased the demand for corrosion
reliability. Corrosion failures can be difficult to diagnose and cause unexpected production loss
due to plant shut down. Overall size of electronic equipment and therefore the component size
have been decreasing at a faster rate.
3.2 Chemistry of IC (Integrated Circuit)
The chemistry involved in fabrication of IC (fig 3.1) chips:
Substrate preparation
Epitaxial growth
Silicon dioxide growth
Photo-etching
3.1
, Diffusion
Fig.3.1 Integrated circuit and related components
3.2.1 Substrate Preparation
It involves the following steps:
Selection of Raw Material: Silicon of about 98% purity is obtained on reduction of silicon dioxide
on heating it along with carbon at a suitably high temperature in a furnace.
Zone refining: The order of purity needed in manufacture of IC chips is 1 impurity atom in
109 silicon atoms. This high order of purity is obtained on repeated zone refining process.
Crystal Growth: Silicon of desire purity is heated in a furnace and p-type impurity in proper
proportion is added to it. The p-type silicon crystal is then grown from the melt using a seed crystal.
Slicing and Polishing: The p-type silicon crystal is sliced into wafer each of thickness about 6
mils. Each wafer is then lapped to remove saw marks and then polished mechanically or chemically
to produce mirror like smoothness.
3.2.2 Epitaxial Growth
3.2
, Epitaxial growth consists in formation of a single crystal thin film from the gas phase on a
wafer (p-type in this case) of the same material (silicon). This epitaxial layer forms a
continuation of the single crystal structure of the substrate.
Epitaxial growth uses a chemical reaction whereby silicon is precipitated from a gaseous
solution to form a thin film of single crystal (mono crystalline) silicon on the surface of
silicon wafer placed in the gaseous solution.
The basic chemical reaction involved in epitaxial growth of pure silicon is hydrogen
reduction of silicon tetrachloride .The epitaxial layer may be made either n-type or p-type.
Salient Features of Epitaxial Process
The atom is arranged on crystal substrate in the single crystal fashion forming an extension
of the substrate layer.
Epitaxial layer is grown from gas phase whereas large size crystals are grown from liquid
phase. Further, in epitaxial process, no part pf the system has temperature approaching the
melting point.
Epitaxial layer has highly uniform resistivity (better than disused layer).
Epitaxial layer may also be grown on diffused surface.
Doping may be either p-type or n-type.
Doping concentration may be varied over large range and complex impurity profile may
be grown.
3.2.3 Silicon Dioxide Growth
Before formation of a fresh SiO2 layer on the wafer surface, the wafer is washed in a suitable
solution to remove the previous SiO2 layer. Then the wafer is heated to a suitably high temperature
in an atmosphere of oxygen. Silicon oxidises to form a thin layer of SiO2.
3.2.4 Photo-Etching
3.3
