Bichop R.H. (Ed.) Mechatronic Systems, Sensors, and Actuators: Fundamentals and Modeling
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20-134 Mechatronic Systems, Sensors, and Actuators
to an output amplifier by a set of switches connected to the horizontal shitch register. Using two separate
vertical shift registers and separating row select and row reset functions enables a rolling curtain type of
electronic shutter to be implemented with an exposure ratio equal to the number of rows.
Photodiode arrays generally have less extensive electrode structures over each sensing element compared
with CCD arrays and consequently the spectral response is smoother and extends further at the blue end
of the spectrum, which is an advantage for color sensors. The peak quantum efficiency is also higher
ranging from 60% to 80% compared with 10% to 60% for photogates, leading to almost twice the electrical
output power for a given light input power. On the other hand, photodiodes have higher noise levels than
CCDs because of the reverse-bias leakage current. The photodiode can be operated in integrating mode
or in continuous mode. In the former, the photodiode capacitance is reset to a reference reverse bias and
then allowed to float. The charge on the photodiode capacitance is then discharged by photon-generated
current and leakage currents. After a specified integration time, the remaining charge can be read and
the difference from the reference value is proportional to the diode irradiance if the leakage sources are
negligible. Both passive and active pixel devices have been developed. In the latter, the charge on the photo-
diode is read out through a MOS field effect transistor (MOSFET), which converts charge to voltage and
provides gain. Figure 20.110 illustrates a typical three-transistor active pixel. Transistor T
1
resets the
photodiode and after an integration period the pixel signal is read out through the source-follower
transistor T
2
and the selection transistor T
3
. Two disadvantages of this approach are a low fill-factor,
because of the pixel area used by the amplifying transistor, and the increased pixel nonuniformity because
of variations in transistor characteristics. In the ibis range of CMOS sensors designed by FillFactory, a
small area photodiode is used which reduces dark current and kTC noise while also increasing the charge-
to-voltage conversion factor (9
µ
V per electron at output in ibis1). The pixel architecture introduces a
small potential barrier, which prevents photon-induced electrons from being collected by structures in
the pixel other than the photodiode. This allows the photodiode to collect most of the electrons generated
in the substrate beneath the pixel effectively increasing the fill-factor [13]. In integrating devices, the fixed
pattern noise resulting from variations in MOSFET thresholds can be reduced by correlation double
sampling. The pixel is sampled at the end of the integration period and the pixel is reset and sampled
again. The difference in the two samples is the measure of the light intensity and is free of pixel offsets.
Photoresponse nonuniformity is harder to control. It is caused by variations in the photodiode collection
volume, junction capacitance, and gate capacitance of the MOSFET amplifier. In the ibis4 1280 × 1024
pixel sensor, PRNU is quoted as less than 10% peak-to-peak with half saturation in the neighborhood.
A light sensor with a continuous pixel response has the advantage that pixels can be accessed in any order
without an integration time so an image processing algorithm could decide, on-the-fly, which pixel or group
of pixels to read next. In the Fuga range of sensors designed by FillFactory, continuous mode operation is
achieved by passing the photon-generated current through a series resistance [14]. Because the photon-
induced current is very small, the series resistance must be very large and it is realized by a MOSFET operated
in weak inversion. The resulting current-to-voltage conversion is logarithmic and the devices have a very
wide dynamic range (six orders of magnitude or 120 dB) with a quoted dark limit of 10
−4
W m
−2
. The
output signal of an individual pixel cannot respond instantaneously to a change in irradiance because of
the RC time constant of the photodiode capacitance and the series resistance. A typical value for this time
constant is a fraction of a millisecond. The fixed pattern noise of these devices due to pixel nonuniformity
is very large, around 50–100%, and cannot be reduced by correlated double sampling because of the
continuous response. However, it is static and can be greatly reduced using correction values stored in a
look-up table implemented in PROM, so that fully corrected monochrome or color cameras with 1024 ×
1024 pixels are available with Fuga sensors.
20.8.7 Vision Systems
Machine vision is used in a wide variety of applications including manufacturing operations, measure-
ments in science and engineering, remote surveillance, and robotic guidance. In machine vision systems
the principal imaging component is not just a sensor chip, but a complete camera, like those shown in
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Sensors 20-135
Figure 20.111, including sensing array, associated electronics, output signal format, and lens. Depending
on the application the camera could be RS-170/CCIR monochrome, NTSC/PAL color, progressive scan,
variable scan, or line scan. Five major system parameters which govern the choice of camera are field of
view, resolution, working distance, depth of field, and image data acquisition rate. Color may also be
important to the application, but otherwise monochrome images are preferred because they require less
memory and process faster. As a rule of thumb, for size measurement applications, the sensor should
have a number of pixels at least equal to twice the ratio of the largest to smallest object sizes of interest.
Lighting should be arranged to illuminate the objects of interest so that the best possible images can be
acquired. Lighting might be ambient, high-frequency fluorescent, LED, incandescent, or quartz halogen.
A frame grabber or video capture card, usually in the form of a plug-in board which is installed in
the computer, is often required to interface the camera to a host computer. Camera suppliers can
recommend compatible frame grabbers. The frame grabber will store the image data from the camera
in on-board, or system memory, sampling and digitizing analog data as necessary. In some cases the
camera may output digital data, which is compatible with a standard computer interface like USB 2.0 or
IEE-1394 Fire Wire, so a separate frame grabber may not be needed. The computer is often a PC or
Macintosh and should be as fast as possible to keep the time needed to process each image as short as
possible, or to allow more processing to be done in the time available. Machine vision software is needed
to create the program which processes the image data. This may come in many forms, including C libraries
of device drivers and functions, ActiveX controls, and point and click programming environments which
allow easy assembly of image processing operations. When an image has been analyzed the system must
be able to communicate the result to control the process or to pass information to a database. This
requires a digital I/O interface or network connection. The human eye and brain can identify objects
and interpret scenes under a wide variety of conditions. Machine vision systems are far less versatile so
the creation of a successful system requires careful consideration of all elements of the system and precise
identification of the goals to be accomplished, which should be kept as simple as possible.
References
1. Wilson, J. and Hawkes, J. F. B., Optoelectronics: An Introduction, Prentice-Hall International,
London, 1983.
2. Jenkins, F. A. and White, H. E., Fundamentals of Physical Optics, 4th ed., McGraw-Hill, New York,
1981.
FIGURE 20.111 Three solid state cameras are shown. The nearest is an inexpensive single board camera with a
CMOS sensor and 4-mm lens. The middle one is a miniature CCIR interline frame-transfer CCD camera, dwarfed
by the 9.5- to 75-mm C-mount zoom lens. The rear camera is a high quality Cohu 4712 monochrome frame-transfer
CCD camera fitted with a 16-mm C-mount lens and 20-mm extension tube.
9258_C020_Sect_7-9.fm Page 135 Tuesday, October 9, 2007 9:09 PM
20-136 Mechatronic Systems, Sensors, and Actuators
3. Hewitt, H. and Vause, A. S., Lamps and Lighting, Edward Arnold, London, 1966.
4. Fraden, J., Pyroelectric thermometers, in The Measurement, Instrumentation and Sensors Handbook,
Webster, J. G., Ed., CRC Press, 1999, chap. 32.
5. Schuermeyer, F. and Pickenpaugh, T., Photoconductive sensors, in The Measurement, Instrumen-
tation and Sensors Handbook, Webster, J. G., Ed., CRC Press, 1999, chap. 56.
6. Sze, S. M., Semiconductor Devices, John Wiley and Sons, New York, 1985.
7. Ray, S. F., Applied Photographic Optics, 2nd. ed., Focal Press, Oxford, 1994
8. CCIR, Characteristics of Monochrome and Colour Television Systems, Recommendations and Reports
of the CCIR, Vol. XI, Part 1: Broadcasting Service (Television), Section IIA, 1982.
9. Amelio, G. F., Charge coupled devices, Scientific American, 176, 1974.
10. Sheu, L. and Kadekodi, N., Linear CCDs, Advances in linear solid-state sensors, Electronic Imaging,
August, 72, 1984.
11. Rutherford, D. A., A new generation of cameras tackles tomorrow’s challenges, Photonics Spectra,
September, 119, 1989.
12. Asano, A., MOS sensors continue to improve their image, Advanced Imaging, 42-44f, 1989.
13. Dierickx, B., Meynants, G., and Scheffer, D., Near 100% fill factor in CMOS active pixels, in Proc.
IEEE Workshop on Charge-Coupled & Advanced Image Sensors, Brugge, Belgium, P.1, 1997.
14. Ricquier, N. and Dierickx, B., Pixel structure with logarithmic response for intelligent and flexible
imager architectures, Microelectronics Engineering, 19, 631, 1992.
20.9 Integrated Microsensors
Chang Liu
20.9.1 Introduction
The purpose of this section is to provide the general audience in the mechatronics field with information
about micro-integrated sensors. It is my wish that an avid reader interested in the sensors area would be
able to understand common fabrication techniques and sensing principles, and develop rudimentary
background to guide the selection of commercialized sensors and development of custom sensors in the
future.
Contents for this section are organized as follows. First, the general history of microsensors is discussed.
This is followed by a brief discussion about major fabrication methods for microsensors. Commonly
used principles for sensors are reviewed next. Sensing of a physical parameter of interest can be achieved
using various structures and under different sensing principles. Examples of sensors, along with their
structures and fabrication techniques, are provided to familiarize the readers with the configurations and
fabrication methods for each.
The microsensors research area covers diverse disciplines such as materials, microfabrication, elec-
tronics, and mechanics. A comprehensive coverage of all aspects is beyond the scope of this book. We
will focus on a few primary sensing principles and frequently used sensors examples. A reader would be
able to grasp a glimpse of the sensors field from the perspectives of sensing principles and of application
areas. References for further in-depth studies are provided when appropriate.
20.9.1.1 Definition of Integrated Microsensors
Integrated microsensors refer to sensors or arrays of sensors that are developed using microfabrication
technology. The characteristic length scale of individual microsensors ranges between 1
µ
m and 1 mm.
Nanosensors refer to sensors with characteristic length scale on the order of 1 nm to 1 mm. In this text,
we are mainly concerned about sensors for detecting physical variables such as force, pressure, tactile
contact, acceleration, rotation, temperature, and acoustic waves. Chemical sensors, used for sensing the
concentration of chemicals or pH values, are beyond the scope of this book.
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A brief historical overview of microsensors development is presented here. The microsensors are made
possible by using integrated microfabrication technology, first developed for making integrated circuits.
Since the invention of the first transistor in 1947 and the successful demonstration of the integrated
circuits in 1971, technologies and equipment for building integrated and miniature circuit components
on semiconductor substrates (e.g., silicon and GaAs) have improved rapidly. The integration density
doubles every 12–18 months following the empirical Moore’s law. The integrated circuit technology
revolutionized the modern society by enabling low cost analog signal processors, digital logical units,
computer memories, and CCD cameras. These achievements should serve as evidence of the power of
integration and miniaturization technology.
Advanced signal processors such as analog ASIC (application specific integrated circuits) and CPU
(central processing units) are merely one aspect of a highly intelligent mechtronics system. Sensors are
of critical importance for mechantronics systems to interact with the physical world. In the 1970s, a few
researchers experimented with using IC fabrication technology to realize mechanical transduction ele-
ments on a silicon chip. H. Nathansan [1] developed floating gate transistors where the gate is made of
a suspended cantilever beam and its distance to the conducting channel can be adjusted using electrostatic
forces. Work by several pioneering researchers resulted in the first commercial pressure sensors [2],
accelerometers [3], integrated gas chromatometers, as well as the ink jet printer nozzle array [4,5].
There are several important advantages associated with integrated microsensors compared with con-
ventional macroscopic sensors. Miniaturization of sensors means that such sensors offer better spatial
resolution. In many cases, reduced inertia and thermal mass translate into higher mechanical resonant
frequency and lower thermal time constants. Since such sensors are fabricated using photolithography
methods, their costs can be low, as many identical units are made in parallel (if the demand is sufficiently
large). Further, since the geometric features of sensor components are defined by precision photolithog-
raphy, the uniformity and repeatability of the performance of such sensors are significantly improved
over conventional sensors. The capability to monolithically integrate sensors and integrated circuits
reduces the path length between sensors and circuits and increases the signal-to-noise ratio.
20.9.1.2 Fabrication Process of Integrated Microsensors
As mentioned previously, the microfabrication technology for microsensors was developed based on
integrated circuits-compatible platforms. As a result, silicon has historically been a predominant material
for microintegrated sensors. In other words, a majority of sensors are now made with silicon wafer as a
substrate. However, in recent years, new materials such as polymers are being applied to microfabrication.
Polymer materials offer lower costs than single crystal silicon wafers and, in some cases, simpler process-
ability, compared with semiconducting silicon. There are few examples of sensors that are entirely based
on polymer these days, because certain key sensing elements are not yet available in polymer format.
However, with the advancement of polymer microfabrication technology and organic semiconductors,
all-polymer sensors can be predicted for future use.
Microsensors and mechanical elements (notably cantilevers and diaphragms) can be made from silicon
substrates in a variety of ways. The two primary categories of fabrication methods are called bulk
micromachining and surface micromachining. In bulk machining, a portion of the silicon substrate is
removed using chemical wet etch or plasma-assisted dry etch to render freestanding mechanical members.
For silicon substrates, the following wet chemical etchants are frequently used: potassium hydroxide
(KOH), ethylene-diamine pyrocatecol (EDP), or tetramethyl ammonium hydroxide (TMAH). Dry etch-
ing methods use AC-excited plasma as an energetic source to selectively and anisotropically remove the
substrate materials. A review of etching solutions commonly used in the silicon microfabrication industry
and their respective etch rates on various materials can be found in reference 6.
In surface micromachining methods, a freestanding structure typically resides within a thin region
near the substrate surface. The fabrication process involves only layers on the surface of a substrate, hence
the name surface micromachining. The fabrication process is typically referred to as a sacrificial etching
method as well. First, a thin-film solid layer called the sacrificial layer is placed on the wafer surface. This
is followed by the deposition of a structural layer, which constitutes the mechanical structure
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(e.g., cantilever or membrane). An etchant that selectively etches the sacrificial layer with a much greater
rate than the structural layer is used to remove the sacrificial layer without damaging the structure layer,
leaving the structural material freestanding.
In the ensuing section, the fabrication process related to specific examples of sensors will be discussed
to illustrate specific uses of bulk and surface micromachining methods. Both surface and bulk micro-
machining techniques offer advantages and disadvantages. For example, the surface micromachining
process is generally compatible with established integrated circuit foundries because no substrate etching
is involved. The bulk micromachining also involves somewhat lengthy substrate removal. However, a
bulk silicon micromachining process is capable of realizing single crystal silicon structures with extremely
low intrinsic mechanical stress and bending. For development of custom sensors, the selection of a
fabrication process must be done carefully to achieve desired device characteristics and fabrication yield
and to reduce overall sensor costs.
20.9.1.3 Resources Regarding Sensors
The community that develops microfabrication methods and microintegrated sensors frequently pub-
lishes archival results in the Journal of Microelectromechanical Systems (MEMS), Journal of Sensors and
Actuators, and the Journal of Micromechanics and Microengineering. Major conferences in this area include:
(1) the IEEE workshop on solid-state sensors and actuators (held biannually at the Hilton Head island,
South Carolina); (2) the International Conference on Solid-State Sensors and Actuators (held biannually
at international venues); and (3) International Conference on Micro-Electro-Mechanical Systems (held
annually at international venues).
Interested readers can find more in-depth discussions on microsensors in a number of reference books
[7,8] and websites [9].
20.9.2 Examples of Micro- and Nanosensors
20.9.2.1 Basic Sensing Principles
Microsensors are based on a number of transduction principles, including electrostatic, piezoresistive,
piezoelectric, and electromagnetic (including optical sensing). The fundamental principles of these sens-
ing methods are discussed in the following.
Electrostatic Sensing
In electrostatic sensing, a physical variable of interest, such as force or vibration, is transduced into
mechanical displacement of a cantilever beam or a membrane. A schematic diagram of a typical trans-
duction structure is shown below (Figure 20.112). The moving cantilever or membrane forms a capacitor
with a reference, typically immobile, electrode. For the structure shown in the diagram below, the
displacement of the top plate induces changes of the capacitance. The electrostatic sensing is also com-
monly referred to as capacitive sensing. The changes in capacitance are used to provide information
about the parameter of interests. The electrostatic sensing principle can be used for accelerometers,
acoustic sensors, rotation gyros, pressure sensors, tactile sensors, and infrared sensors. Respectively in
these examples, the external excitations responsible for member movements are inertia force, air mass
vibration, Coriolis force, pressure, contact force, and thermal bimetallic bending due to absorbed energy
and increased temperature.
The electromechanical model of a simple capacitive sensor shown in Figure 20.112a is illustrated in
Figure 20.112b. The top electrode is supported by two suspension beams with a combined equivalent
force constant (spring constant) of k. The capacitance value of a parallel plate capacitor is expressed as
where A is the area of the electrode, d is the distance between two electrodes,
ε
r
and
ε
0
are the relative
permitivity of the media and the permitivity of vacuum, respectively. When the distance between the
C
ε
r
ε
0
A
d
-------------
=
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Sensors 20-139
two electrodes changes by ∆d, the first-order estimate of the change in capacitance is
When a force F is applied to the top plate, the change of capacitance value is
Piezoresistive Sensing
In a piezoresistive sensor, the magnitude of a mechanical displacement is measured by the amount of
stress it induces in a mechanical member. A stress-sensitive resistor (called a piezoresistor) located strate-
gically on the mechanical member experiences a change of resistance as a result of the applied stress.
Many materials, including metals, alloys, and doped silicon, exhibit piezoresistive characteristics. The
applied stress causes the lattice of a material to deform, thereby inducing changes in the resistivity as
well as the dimensions of a resistor. The change of resistance (∆R) as a function of applied strain
ε
is
where R
0
is the value of the resistor in the unstressed state, and G is the piezoresistive gauge factor.
Using doped silicon as a piezoresistive sensor, the overall footprint of the sensor can be made quite
small and yet have a respectable value, that is, 1 kΩ. Unlike the capacitive sensor method, which requires
significant plate area to achieve significant capacitance value, the piezoresistive sensor is more area
efficient. As a result, the piezoresistive sensing is more likely to be used for sensors with characteristic
length below 10
µ
m. However, the capacitive measurement method is more generally applicable, whereas
the optimal piezoresistive sensors involve silicon with proper doping concentration.
Piezoelectric Sensing
A piezoelectric material is one that produces electrical polarization (internal electric field) when an
external mechanical strain is applied. A piezoelectric material also exhibits reverse piezoresistivity.
Namely, a mechanical strain (or displacement) will result when a voltage (or electric field) is applied on
the material itself. The reverse piezoelectricity is commonly used as an actuation principle for producing
mechanical movement or force.
One advantage of piezoelectric sensing over piezoresistive sensing lies in the fact that piezoelectric
sensors are self-generating, that is, a potential difference will be created without external power supply.
However, high quality piezoelectric films with consistent and uniform performance characteristics require
dedicated machinery and calibrated processes. Such a technical barrier prevents thin film piezoelectric
materials to be as widely used as piezoresistive elements. However, high quality piezoelectric films are
FIGURE 20.112 Schematic diagram of a parallel plate capacitor: (a) perspective view, (b) electromechanical model.
(b)
(a)
Bottom electrode
k
d
d
C
Bottom electrode
Top electrode
Top electrode
Suspension
beam
Suspension
beam
Anchor Anchor
∆C
C
d
---
∆d=
∆C
C
d
---
F
k
---
=
∆R
R
0
-------
G
ε
=
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20-140 Mechatronic Systems, Sensors, and Actuators
increasingly becoming available through commercial services. Commonly used piezoelectric materials in
microfabricated sensors include sputtered zinc oxide and lead zirconate titanate (PZT).
Temperature Sensing
Temperature sensing is used not only for measuring the temperature of the ambient but also for inferring
heat transfer. Temperature sensors can be made of thermal resistors or thermal couples. A thermal resistor
is a thin film resistive element whose resistance changes as a function of the temperature. This is explained
by changes in resistivity as well as dimensions. Doped semiconductor materials (e.g., single crystal silicon
or polycrystalline silicon) exhibit temperature coefficients of resistors (TCR, denoted
α
) on the order of
−0.1%/°C to 5%/°C. For a thermal resistor, the normalized changes in resistance (R) are related to the
change in temperature (T) by
Thermal couples are made of two different materials with different Seebeck coefficients. The voltage
induced by a single thermal couple junction is proportional to the difference in temperatures at the
junction and of the ambient. For more information about thermal couples, readers can refer to references.
Temperature sensing is also commonly achieved using a thermal bimetallic beam. A composite beam
made of two materials with different thermal expansion coefficients will bend as the two parts expand
with different speed. The amount of mechanical bending, sensed electrostatically or using piezoresistors,
corresponds to the applied temperature. Such sensing principle has been used for making uncooled
infrared sensors.
20.9.2.2 Pressure Sensors
Pressure sensors are important for industrial and automotive control and monitoring. Existing micro-
pressure sensors consist of diaphragms that deform in response to pressure differences. Using microma-
chining technology, the diaphragms can be made very thin, hence greatly increasing the sensitivity of
sensors over conventional pressure sensors with thick diaphragms. Integrated microfabrication technol-
ogy also enables sensors to be made in conjunction with signal-processing circuits. Since the distance
between the sensor diaphragm and the signal processors are close, the noise is generally much lower
compared with conventional sensors.
The diaphragm is a critical element in a pressure sensor. It can be made by either surface microma-
chining or bulk micromachining techniques. Hence, we classify micropressure sensors according to the
methods of forming the diaphragms. In each category of pressure sensors, the displacement of the
diaphragm can be determined by using piezoresistive sensing or capacitive sensing.
Over the past two decades, many micromachined pressure sensors have been developed, some com-
mercialized successfully for automotive and machinery applications. The intent of this section is not to
present an exhaustive summary of all the work that has been accomplished, but rather discuss several
representative devices with the purpose of (1) providing the readers with a general overview of the
available technologies; and (2) providing leads to the existing body of literature in this area.
Bulk Micromachined Pressure Sensors
The schematic diagram of a bulk micromachined pressure sensor is illustrated in the diagram below. The
diaphragm will bend when a differential pressure is applied across it. A common technique for sensing
the diaphragm displacement is by using piezoresistors embedded in the diaphragms. However, it should
be noted that other sensing principles are also feasible.
In the diagram below (Figure 20.113), four piezoresistors are embedded near the edge of the dia-
phragm. The locations of sensors are selected carefully such that under a given displacement at the center
of the diaphragm, the magnitude of the stress is the greatest at the sensor locations. There are a number
of choices for the diaphragms and the piezoresistors. A number of possible materials and their relative
merits are summarized in the Table 20.7. Three distinct pressure sensor architectures and their respective
fabrication processes are discussed in the following paragraphs.
∆R
R
-------
α
T=
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Sensors 20-141
The fabrication process for a pressure sensor using plain silicon wafer as the substrate is shown in
Figure 20.114. In the first step, the wafer is selectively doped with boron or phosphorous atoms to create
piezoresistors on the front side (a). The wafer is then passivated with a thermally grown silicon dioxide
thin film (b). In the ensuing step, the silicon dioxide film on the backside is patterned and selectively
etched to expose the silicon (c). The exposed silicon material will be etched when the wafer is immersed
in an anisotropic silicon etchant (d). In order to form the silicon diaphragm with desired thickness,
TABLE 2 0. 7 A List of Possible Materials for Diaphragm and the Piezoresistive Sensors
Diaphragm Material Piezoresistor Material Relative Merits
Single crystal silicon Doped single crystal silicon Relatively difficult to control the
thickness of the diaphragm
Silicon nitride thin film Polycrystalline silicon Easy to form thin diaphragms;
involved LPCVD polysilicon
FIGURE 20.113 Schematic diagram of a bulk micromachined pressure sensor.
FIGURE 20.114 Schematic diagram illustrating major steps in the microfabrication process of a bulk microma-
chined pressure sensor.
External pressure
Tensile
stress
Doped
piezoresistor
(c) Side view
A’
A
(b) Side view
Diaphragm
Strain sensor
(a) Top view
Thin diaphragm
Thin diaphragm
(a)
(b)
(c)
(d)
(e)
Piezoresistor
Metal leads
Diaphragm
Diaphragm
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workers resort to timed etch with precise knowledge of calibrated wafer thickness and etch rates. However,
this step must be performed with caution as the etch rate may vary with time and locations on a wafer.
Typically, the resultant thickness of the diaphragm is large (30–50 µm) to ensure sufficient yield of devices.
In the final step, the oxide on the front side of the wafer is patterned to provide contact vias for metal
lead wires (e).
To circumvent the problem of process uncertainty of the aforementioned process, wafers with barrier
layers can be used (Figure 20.115). For example, it is possible to use a silicon-on-insulator (SOI) wafer
with a thin film of silicon on top of a silicon dioxide layer. The silicon and the oxide layers lie on top of
the bulk silicon substrate (a). Following steps similar to the ones discussed above, one can form piezore-
sistors (b) and open windows in silicon oxide on the backside of the wafer (d). The anisotropic etchant
of silicon has minimal etch rate on the silicon oxide, hence the through-wafer etch will automatically
stop when the buried oxide layer is exposed. This allows a professional engineer to perform adequate
overetch to ensure that diaphragms on all devices reach the same thickness (e). This self-limiting etching
behavior reduces the complexity of process control and is conducive to reducing the process costs. The
oxide layer is then selectively removed using hydrofluoric acid, which does not etch silicon. Hence a thin
silicon diaphragm, with the thickness defined by the thickness of the epitaxial silicon layer specified
during the SOI wafer manufacturing, can be formed efficiently. Finally, via holes are opened on the
frontside and metal leads are deposited and patterned (g).
Although this process is advantageous over the one introduced earlier, it has a few shortcomings. For
example, although the process discussed above is much more efficient in terms of controlling the
diaphragm thickness, the SOI wafers used in the process are more expensive than ordinary silicon wafers.
Even with SOI wafers, the thickness of the silicon diaphragm is typically 2–10
µ
m. In order to further
FIGURE 20.115 Schematic diagram of an alternative process for realizing bulk micromachined pressure sensors.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Piezoresistor
Epitaxial silicon
Buried oxide layer
Metal leads
Diaphragm
Diaphragm
Diaphragm
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Sensors 20-143
increase the pressure sensitivity, it is advantageous to reduce the thickness of the diaphragm further.
However, this would be difficult to achieve if silicon is the diaphragm material. In the following, a process
of using silicon nitride thin film will be discussed.
An alternative sensor structure uses silicon nitride thin film as the diaphragm and deposited and doped
polycrystalline silicon as the piezoresistive sensor. The process is described in Figure 20.116. Starting with
a bare silicon wafer (a), a layer of silicon nitride film is deposited using LPCVD methods (b). The wafer
is then coated with a layer of polysilicon with suitable doping concentration (c). The polysilicon is
patterned and defined. This is followed by the deposition of yet another thin film silicon nitride to protect
the polysilicon film during the ensuing silicon etching (d). The thickness of the two LPCVD silicon
nitride layers is the thickness of the finished diaphragm. A window is opened on the backside of the
wafer to expose the silicon material. The silicon is etched in an anisotropic etchant, which does not attack
the silicon nitride film. In other words, the selectivity between silicon and silicon nitride is high. Following
the formation of the diaphragm, the silicon nitride on top of the polysilicon resistors is selectively
patterned and metal leads are formed (g).
Surface Micromachined Pressure Sensors
The surface micromachining process does not require the removal of silicon substrate, which is time-
consuming and not fully compatible with integrated circuit processes at the present because of the silicon
etchants used. For producing low-cost, high-performance integrated sensors, surface micromachining
FIGURE 20.116 Schematic diagram of major process steps for realizing a bulk micromachined pressure sensor with
a silicon nitride diaphragm.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Silicon nitride film
Metal leads
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