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Lecture 2. Working Principles of Microsystems
2.1 Microsensors
 Definition of Sensor:
A sensor is a device that converts one form of energy into another and provides
the user with a usable output in response to a specific measurable input.
2.1.1 Acoustic Wave Sensors
 Principal application:
- To detect chemical components in a gas.

Basic idea:
- To fabricate a sensor in which acoustic waves are propagated, and where
some aspect of that propagation (e.g., velocity, amplitude, etc.) is changed by
-
the adsorption/reaction or viscosity of the sensed species. (Kovacs, 2000)
Generation of acoustic waves: piezoelectric (the most popular),
magnetostrictive, etc.
2.1.2 Biomedical Sensors and Biosensors
 BioMEMS encompasses:
1.
2.
3.

Biosensors;
Bioinstruments and surgery tools;
Biotesting and analytical systems
Biomedical Sensors:
 To detect biological substance.
 Example: one for glucose (血糖) concentration
- Glucose + O2 → gluconolactone + H2O2
- Then, H2O2 → O2 + 2H+ + 2e- By measuring the current or the pH, the glucose concentration can be
determined.
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表一 二十位糖尿病專家對糖尿病診斷之分歧標準
(http://www.ilshb.gov.tw/dm/)

血糖 mg/dl
正常人最高限
糖尿病人最低限
空腹血糖
105-120
120-160
1 小時血糖
160-209
185-250
2 小時血糖
120-250
105-180
Biosensors
 Definition: Any measuring devices that contain a biological element.
 The biomolecules (such as enzymes, antibodies, etc.), when attached to the
sensing elements, can alter the output signals of the sensors.
 Schematic of biosensors:
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
Biotesting and Analytical Systems
 These systems separate various species in biological samples.
 Electrohydrodynamics involves the driving of an ionized fluid by the
application of electric fields.
2.1.3 Chemical Sensors
 Used to sense particular chemical components, such as various gas species.

4 types:
I. Chemiresistor sensors
- Chemiresistros are simply structures in which the resistance (or impedance)
between two electrical contacts is modified depending on the quantity of an
unknown in the environment. (Kovacs, 2000)
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-
Example: A polymer called phthalocyanine is used with copper to sense NH3
and NO2.
II. Chemicapacitor sensors:
- Some polymers can be used as dielectric materials in a capacitor.
- Exposed to certain gases → its dielectric constant changes → capacitance
changes
III. Chemimechanical sensors
- Certain materials (e.g., polymers) would change shape when exposed to
-
chemicals including moisture.
One may detect such chemicals by measuring its dimension change.
Example: A moisture sensor using pyraline PI-2722.
IV. Metal xodie gas sensors
- The working principle is similar to that of chemiresistor sensors.
- Several semiconducting metals, such as SnO2, change their electric resistance
after absorbing certain gases.
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
Catalyst deposition can speed up the reaction and hence increase the sensitivity
of the sensor.
2.1.4 Optical Sensors
 Figure 2.6(a):
The photovoltaic junction can produce an electric potential when the more
transparent substrate of semiconductor A is subjected to incident photon energy.

Figure 2.6(b):
A special material changes its electric resistance when exposed to light.

Incident photon energy can be converted into electric current output from these
devices.
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2.1.5 Pressure Sensors
 Principle:
- Mechanically induced diaphragm deformation and stresses are then
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converted into electrical signal output through several means of transduction.

Two types: absolute and gage pressure sensors
- Absolute: has an evacuated cavity on one side of the diaphragm so that the
measured pressure is the absolute value with vacuum as the reference.
- Gage: no evacuation is necessary.

Back-side and front-side pressurization (Fig. 2.7):
- Back side: no interference with signal transducer.
- Front side: used under special circumstances because of interference of
pressurizing medium with the signal transducer

In Fig. 2.8, four piezoresistors implanted beneath the silicon die, and a
Wheatstone bridge circuit is used.
- By an applied pressure, the values of R1 and R3 increase, whereas those of R2
and R4 decrease.
- The output voltage from the Wheatstone bridge is:
 R1
R2 

Vo  Vin 

 R1  R4 R2  R3 
-
[eq. 2.1]
Advantages: high gains and good linearity.
Disadvantage: temperature sensitive.
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R2 (-v)
R3 (+v)

Capacitive type (Fig. 2.9):
- Principle:
Any deformation of the diaphragm due to the applied pressure will
narrow the gap between two electrodes, leading to a change of capacitance
across the electrodes.
-
Capacitance:
C   r 0
A
d
[Eq. 2.2]
where εr: relative permittivity, ε0: permittivity of vacuum, d: gap, and A:
area.
-
Output Voltage:
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Vo 
-
C
Vin
2(2C  C )
Advantages: independent of the operating temperature.
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[eq. 2.3]
-
Disadvantages:



Not as sensitive as those with piezoresistors.
The relation between the output and the capacitance change is
nonlinear.
Vibrating-beam type (Fig. 2.12):
- has a n-type silicon beam and p-type electrode.
- The beam is made to vibrate at its resonant frequency by applying an ac
signal to the electrode before the application of pressure.
- The stress induced in the diaphragm (and the die) will be transmitted to the
-
vibrating beam, causing a shift of natural frequency.
The shift of natural frequency can be correlated to the applied pressure.
Advantage: (1) insensitive to temperature; and (2) good linearity.
-
Disadvantage: costly on fabrication.
2.1.6 Thermal Sensors

Thermocouples:
- A voltage is produced at the open ends of two dissimilar metallic wires when
the junction of the wires is heated (Fig. 2.13a).
- Seebeck effect:
 A thermocouple is arranged with both hot and cold junctions (Fig.
2.13b), and the following voltage is generated:
V=βΔT
where β is the Seebeck coefficient, and ΔT is the temperature
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difference between hot and cold junctions.

Microthermopile:
- Thermopiles operate with both hot and cold junctions, but they are arranged
with thermocouples in parallel and voltage output in series (Fig. 2.14).
V=NβΔT
where N is the number of thermocouple pairs.
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
In Fig. 2.15, a total of 32 polysilicon-gold thermocouples were used in the
thermopile.
- The chip dimension: 3.6mm X 3.6mm X 20μm
-
A typical output signal of 100 mV was obtained from a 500 K blackbody
radiation source of Q=0.29 mW/cm2.
2.2 Microactuation
 Four commonly used means for microdevice actuation are:
(1) thermal forces;
(2) shape memory alloys;
(3) piezoelectric crystals;
(4) electrostatic forces.
2.2.1 Actuation Using Thermal Forces
 Principles:



Bimetallic strips are made by binding two materials with distinct
thermal expansion coefficients (also called thermal bimorph actuator).
The strip will bend when heated or cooled from the initial reference
temperature (Fig. 2.16).
Examples: microclamps or valves.
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2.2.2 Actuation Using Shape Memory Alloys
 Principles:
- The shape memory alloys (SMA) tend to return to their original shape at
a preset temperature.
-
Example in Fig. 2.17,
 A SMA strip is in a bent shape at a designed preset temperature T.
 The silicon beam is set straight at the room temperature.
 Heating the SMA to temperature T would prompt the SMA strip to
return to its original bent shape, causing the beam to deform with
the strip.
2.2.3 Actuation Using Piezoelectric Crystals
 Principles:
- Certain crystals deform with the application of an electric voltage.
- The reverse is also valid. Deforming the crystal would generate a
voltage.
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-
Example:
2.2.4 Actuation Using Electrostatic Forces
 Coulomb’s Law:
Two charge particles with charges q and q’ would exert an electrostatic
force to each other:
1 qq'
4 r 2
where ε: permittivity of the material.
F
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[Eq. 2.5]

Electrostatic Force in Parallel Plates
-
In Fig. 2.21, the induced capacitance between the plates is
C   r 0
A
WL
  r 0
d
d
[Eq. 2.6]
-
The energy associated with the electric potential can be expressed as:
-
  WLV 2
1
U   CV 2   r 0
2
2d
The associated force that is normal to the plates is
-
U
1  r  0WLV 2
Fd  

d
2
d2
The associated forces in the W and L directions are
-
[Eq. 2.7]
[Eq. 2.8]
FW  
U 1  r  0 LV 2

W 2
d
[Eq. 2.10]
FL  
U 1  r  0WV 2

L 2
d
[Eq. 2.11]
Drawback:
The force magnitude is low.
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2.3 MEMS with Microactuators
2.3.1 Microgrippers
 Principle:
The required gripping forces in a gripper can be provided either by normal
force (Fig. 2.24a) or by the in-plane forces from pairs of misaligned plates (Fig.
2.24b).
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
Disadvantage of the design in Fig. 2.24a:
The electrodes occupy excessive space.

Comb drives as Fig. 2.24b or:
2.3.2 Micromotors
 Two types: linear and rotary motors
- Linear motors:
 Energizing the pair of electrodes A and A’ causes the top plate to
move to the left until they are fully aligned.

Then, energize the pairs of electrodes B and B’, C and C’, and D
and D’ in sequence.
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-
Rotary motors:
2.3.3 Microvalves
 Applications:
-

Precision control of gas flow for manufacturing processes;
Biomedical systems such as in controlling the blood flow in an artery
(動脈).
The microvalve in Fig. 2.29 (Jerman, 1991) uses the thermal bimorph actuation
(like the one in Fig. 2.16).
- Heating the two electrical resistor rings can cause a downward
movement of the diaphragm to close the flow passage.
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
Figure 2.30,
- The downward bending of the silicon diaphragm (to close the valve) is
activated by heat supplied to a special liquid in the sealed compartment
(隔間) above the diaphragm.
2.3.4 Micropumps
 Figure 2.31,
- Use the electrostatic actuation to deform the diaphragm.
- Apply a voltage across the electrodes
 Diaphragm: deform upward
 Chamber volume: increase
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 Chamber pressure: reduce
 Inlet check valve: open (fluid flows in)
Then, cut off the applied voltage
 Diaphragm: return to its initial position
 Chamber volume: reduce
 Chamber pressure: increase
 outlet check valve: open (fluid flows out)
- Zengerle (1992) reported
 Diaphragm size: 4mm×4mm×25μm thick
 Gap between the diaphragm and electrode: 4μm


Actuation frequency: 1 to 100 Hz
Pumping rate: 70μL/min at 25 Hz
2.4 Microaccelerometers
 Functions:
- measure accelerometer;
- measure the applied force by F=ma.

Applications:
- ±2g range for the car’s suspension system and antilock braking system.
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-
±50g range for the actuation of air bags.

Principal components of an accelerometer (Fig. 2.32):

Figure 2.33 shows a microaccelerometer:
- Silicon beam: serve as a spring.
- Mass M: also called a seismic mass or proof mass.
- Air in the surrounding: produce damping.
-
Piezoresistor: measure the deformation
 correlated to the acceleration
(Details: discussed in Chapter 4.)

Figure 2.34: use differential capacitance
- Components: a proof mass (thin beam), two springs (tethers [拴繩,拴鏈]
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made of elastic material), and a differential capacitive sensing.
-

Principles:
 In the event of acceleration, the proof mass will displace in the
direction opposite to the acceleration. (??)
 The differential capacitive sensor cell senses the movement.
Figure 2.36: dual-axial-motion sensor.
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2.5 Microfluidics
 Applications:
- Chemical analysis
- Biological and chemical sensing
- Drug delivery
- Molecular separation such as DNA analysis
- Amplification, sequencing or synthesis of nucleic acids
- Environmental monitoring


Principal advantages:
-
Small samples
Better performance with reduced power consumption
Can be combined with traditional electronic systems for a lab-on-a-chip
-
(LOC)
Produced in batches s.t. disposable (可丟棄的)
Major components of a microfluidic system:
1. Microsensors: used to measure fluid properties (pressure, temperature, and
flow rate, etc.).
2.
3.
4.
Actuators: used to alter the state of fluids (microvalves, micropumps, and
compressors, etc.).
Distribution channels: regulate flows in various branches in the system
(capillary networks in Fig. 2.3, microchannels in Fig. 2.37, etc).
System integration: integrate 1, 2, 3, and electrical systems that provides
electrohydrodynamic (電之流體動力學) forces, the circuits for transducing
and processing the electronic signals, and control of the microfluid flow in
the system.
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