Instruments and Instrumentation 25
Sputtering is similar to vacuum deposition. In this method, an inert gas
such as argon or helium is introduced into a chamber that contains anode
and cathode electrodes supplied by an external high-voltage source. The
anode contains the sample to be deposited on and the cathode contains the
deposited material. The principle is that the high voltage ignites a plasma
effect in the inert gas and the gas ions bombard the target containing the
material to be deposited. When the kinetic energy of the bombarding ions
is sufficiently high, some of the atoms from the target surface are freed and
carried by the gas to the surface of the sample. The sputtering technique
yields better uniformity, particularly in the presence of a magnetic field. This
method does not require high temperatures, so virtually any type of material,
including organic materials, or mixtures of different materials can be sput-
tered.
Chemical vapor deposition (CVD) is one of the most common methods
used for the fabrication of semiconductor-based sensors. It is a widely
applied technique, particularly in the production of optical and optoelec-
tronic devices. The CVD process takes place in a reaction chamber where
substrates or wafers are positioned on stationary or rotating tables. The
dopants are allowed to enter the chamber mixed together with a carrier gas
such as hydrogen. The substrate is kept at an elevated temperature that helps
the additives to be deposited on the surface of the sample. The thickness of
the deposition is controlled by the amount of dopant in the gas, the pressure
at the inlet, and the temperature of the substrate.
1.4.3 Trends in Sensor Technology and IC Sensors
The present trend in sensor technology has shifted toward IC sensors in the
form of microsystems, intelligent sensors, nanosensors, and others. Micro-
systems refer to the dimensions of devices in the micrometer (10
–6
m) range,
whereas nanotechnology refers to the dimensions of devices in the nanom-

causing a current to flow in the circuit. The output voltage of photodiodes
is highly nonlinear, thus requiring suitable linearization and amplification
circuits, which can be included on the same chip.
Integrated circuit sensors can be grouped according to their signal domains:
• Radiant domain: sensors contain a wide spectrum of electromagnetic
radiation, visible spectrum, and nuclear radiation. Some examples
are photovoltaic, photoelectric, photoconductive, and photomag-
neto effect sensors.
• Mechanical domain: sensors include a wide range of devices from
MEMS to tactile sensors. Some examples are piezoresistive, photo-
electric and photovoltaic sensors, and micromachined devices.
• Thermal domain: sensors are largely semiconductor-based devices
that exhibit sensitivity to temperature effects. Although sensitivity
to temperature is undesirable in many applications, the temperature
dependence of semiconductors can be useful for temperature mea-
surements and control. Some of these devices are based on the See-
back and Nernst effects.
• Magnetic domain: sensors are made from magnetically sensitive
semiconductors that are obtained by using doping techniques and
FIGURE 1.9
Typical structure of a photodiode.
SiO
2
V
out
Photon
Metal contact
n-type
Intrinsic material
p-type

devices that are already equipped with embedded internal sensors. They are
produced by combining bipolar and metal oxide semiconductor (MOS) cir-
FIGURE 1.10
An IC temperature sensor.
A0
A1
A2
V
+
(2.7V to 5.5V)
O.S.
SDA
SCL
Temperature
Sensor
Delta-Sigma
A/D
Limit
Comparison
Control
Logic
Hysteresis
Register
Over temp.
Shutdown
IIC Interface
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28 Wireless Sensors and Instruments
cuitry with metal oxide semiconductor field effect transistor (MOSFET) tech-

sensing arrays for the integration of all necessary signal conditioning com-
ponents and computational capabilities on the same chip.
Complementary metal oxide semiconductor technology allows the inte-
gration of many sensors on a single chip, thus it is a common method applied
in sensing arrays. Some examples of CMOS sensing arrays include photo-
diode arrays, ion detectors, moisture sensors, electrostatic discharge sensors,
strain gauges, edge damage detectors, and corrosion detectors.
Photodiode arrays are a typical example of a sensor array. A photodiode
consists of a thin surface region of p-type silicon formed on an n-type silicon
substrate. A negative voltage applied to a surface electrode reverses the bias
of the pn junction. This creates a depletion region in the n-type silicon, which
contains only an immobile positive charge. Light penetrating into the deple-
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© 2006 by Taylor & Francis Group, LLC
Instruments and Instrumentation 29
tion region creates electron-hole pairs, which discharge the capacitor linearly
in time. There are two basic types of photodiodes: serially switched photo-
diode arrays, shown in Figure 1.11, and charge-coupled photodiode arrays.
In these arrays, the basic principle is to use light intensity to charge a capac-
itor and then read the capacitor voltage by shifting it through the registers.
Such solid-state image sensors can be considerably complex when they are
manufactured in IC forms.
Integrated multisensor chips are attracting considerable R&D attention.
Multipurpose integrated sensor chips have been manufactured for the simul-
taneous measurement of physical and chemical variables. IC technology
allows the design of complex systems on a single chip that incorporate high-
performance analog subsystems such as op amps and data converters on the
same die with digital circuits. These devices, generally manufactured by
MOS technology, include signal conditioning, array accessing, and output
buffering along with infrared sensing arrays, chemical sensors, accelerome-