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Electrostatic Microactuator for Dual Stage Positioning System

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Electrostatic Microactuator for Dual Stage Positioning System Powered By Docstoc
					MEMS Actuators


   Dr. Chen Bangtao
    A/P Francis Tay

btchen@ibn.a-star.edu.sg
   ehtay@nus.edu.sg
                     Outline
 Introduction of microactuators
  – Piezoelectric microactuators
  – Magnetic microactuators
  – Electrostatic microactuator
  – Thermal microactuators
  – others

 Electrostatic microactuators
  – Principle
  – Design of Microactuator for HDD
  – Fabrication
  – Testing
  – Applications
                                      2
                      Introduction
 Core components in MEMS:
   Sensors, actuators, transduction unit /control logic

 Sensor: convert one physical quantity /energy to another
 Actuator: convert electric energy into mechanical energy
     Input          Power           Power
     signal         supply          supply



  Microsensing   Transduction     Transduction   Microactuation
    element          unit             unit          element



                   Output                           Output
                                                    Output
                   signal                            signal
                                                    signal
    MEMS as microsensor            MEMS as microactuator          3
                   Microactuators
 Categorized by principle
  •Thermal forces;
  •Piezoelectric forces;
  •Electromagnetic forces;
  •Electrostatic forces;
  •Other forces, such as shape
  memory alloy, magnetrostrictive
   force, ultrasonic force;

 Applications
  Micropump, micromirror,
  micromotor, microvalve,
  microrelay, microgripper,
  optical switch, etc.              4
                Piezoelectric effect
Principle: piezoelectric crystals produce electrical charge
with applied mechanical stress; the converse applies too:
an applied voltage generates deformation of crystals.
Polarization:



                                                         Induced mechanical
           Mechanical forces
                                                         deformation


                                Applied
   V                           voltage V



  Mechanical-force-induced                 Electric-voltage-induced
  electric voltage                         mechanical deformation         5
                         Piezoelectric actuators
 Piezoelectric effect with E-field and stress is
   D d 
        i   i       
                         D and E is the electric displacement and field,  and  is
    d E
           i   i
                         mechanical stress and strain, d is piezoelectric constant

Piezoelectric crystals: Quartz (crystal SiO2), Barium titanate ceramics
(BaTiO3), Lead zirconate titanate, PZT(PbTi1-xZrxO3), PbZrTiO6, PbNb2O6,
PZT-polymer composite, PZT-metal composite, etc.

Applications: ultrasonic emitter, buzzer, resonator/ actuators




                                                                                 6
         Electromagnetic microactuators
The electromagnetic force can be generated with one or two
magnets or coils.
High power consumption and heat dissipation, not suitable
for miniaturization.
Widely used in focusing system of optical storage disk and
magnetic head positioner of hard disk.




  Electromagnetic mobile microactuator   Electromagnetic microactuator for hard disk
                                                                                   7
             Thermal microactuators
Thermal actuation is widely used for actuation cantilever or
membrane structures.
The thermal deformation is generated by the distinct thermal
expansion coefficient in different materials.
The strain caused by the temperature change is
                    dT     T
                  Th


                  Tl   1        2   1   2



                           1               1>2


                           2
The actuation speed is slow, which is related to the heating
speed and reversion speed of the materials.
Applications: inkjet head, microvalve, microgripper, etc.      8
              Shape memory alloy
Shape-Memory Alloys are metals that, after being strained, at
a certain temperature revert back to their original shape.
SMA materials: NiTi, CuZnAl , CuAlNi, etc.
Pros: bio-compatibility, diverse filed of application
Cons: hysteresis, low transition temp, slow relaxation speed


                               Two phases transformation
                               occur in SMA: Martensite and
                               Austenite at certain temps.


                              Applications: medical industry,
                              aerospace and marine

                                                          9
         Electrostatic microactuators
Principle: electrostatic force (attraction) generated between
moving and fixed plate as voltage applied.
Two major configurations: (a). parallel-plate capacitors (out-
of-plane), (b) interdigitated fingers /comb drive (in-plane)
Pros: simplicity, low power, fast response, low cost
Cons: non-linear dependence of force on gap, contamination,
sticking instability
Applications: micromotors, accelerometers, optical switches,
micromirrors, comb-drive microactuators
                  Parallel-plate
                  capacitor

                                   Comb drive
                                                             10
           Electrostatic microactuators




Texas Instruments, optical switch        Mehregany, MIT, electrostatic motor




Sandia lab, linear comb drive actuator      Sandia lab, rotary actuation
                                                                               11
                     Outline
 Introduction of microactuators
  – Piezoelectric microactuators
  – Magnetic microactuators
  – Electrostatic microactuator
  – Thermal microactuators
  – others

 Electrostatic microactuators
  – Principle
  – Design of Microactuator for HDD
  – Fabrication
  – Testing
  – Applications
                                      12
               Electrostatic principle
Parallel plate capacitor
                                  The capacitance between two plates:
                                                        Q
                                                     C
                                                        V
                                                                          Q
                                  The electric field:         E
                                                                          A
The capacitance between the two           Q    Q       A
                                       C           
plates:                                   Ed Q A  d d

                                                1
The electrostatic force between                 CV                   2

                                           U         1 C     1 C
the plates is defined by:               F     2         V       V          2        2


                                           x    x   2 x     2 d

                                             AV      2
                                                            CV    2

Solved the electrostatic force:         F        2
                                                          
                                             2d              2d
                                                                                   13
      Lateral comb drive microactuator
Lateral actuation: direction of finger movement is parallel to
the direction of fingers.




                                              2 t  x  x     
Total capacitance in the comb drive:    C  N            0
                                                              C 
                                                                
                                                               p
                                                      d
                                           1 C               t
The Electrostatic force generated:      F      V  2
                                                        N V       2


                                           2 x
                                                       x 0
                                                              d
The electrostatic force is proportional to the finger thickness and finger
number, square proportional to the applied voltage, while is inversely
proportional to the separation gap.                                       14
 Transverse comb drive microactuator
Transverse actuation: direction of finger movement is
orthogonal to the direction of fingers.
                             The capacitance in the comb drive:
                                    lt               lt                        
                             C  N
                                  x x   C  C  N
                                                      x  x C                    
                                                                                    
                                                                                 
                              l                     f       r                   f

                                      0                                 0




                              The electrostatic force generated:
                                   1                    1        1    
                              F     N x tV   2
                                                   
                                                    ( x  x) ( x  x) 
                                                                      
                                                                      
                                                                2           2
                                   2                    0           0




 • Pros: Frequently used for sensing for the sensitivity and
 ease of fabrication
 • Cons: not used as actuator because of the physical limit
 of distance.
                                                                                15
    Equilibrium analysis of comb drive
What happens to a parallel plate capacitor when the applied
voltage is gradually increased?
                            The mechanical restoring spring with
                            spring constant Km is associated with
                            the suspension of the top plate.
                            According to Hooke’s law, Fm= - Kmx
                            At equilibrium, Fm + Fe = 0, i.e.,
                                         t
                                      N V K x
                                            2
                                                  m
                                         d
As V↑,  Fe↑, top plate moving downwards,  x ↑, gap↓.
Pull-in instability: As the voltage increases across parallel
plates, the separation gap would decrease until they reach
2/3 of the original spacing, at which point the two plates
would be suddenly snapped into contact.
                                                                  16
Comb drive microactuator




                           17
             Comb drive fabrication
Bulk-Micromachining:
                              Surface micromachining:
Comb drive actuator can be
made on SOI (or other 2-layer
structure, by wafer bonding)




   Very expensive, thin structure   Very thin structure, low yield   18
          A new design of microactuator
Principle: laterally driven comb drive microactuator
Characteristics:
Bulk 3D silicon-on-glass device
high-aspect-ratio microstructure
high electrostatic driving force
Low driving voltage
Good toughness and reliability

Fabrication method:
Bulk-micromachining
e.g.., deep reactive ion etching and wafer bonding
                                                       19
Application in dual-stage actuator servo
      system for hard disk drives




    First actuation stage: Voice coil motor
     large movement and coarse head-positioning.
    Second actuation stage: Microactuator
     High-bandwidth and fine positioning.          20
     Actuating animation and objective
               V
   Fixed
 electrode



 Support
 flexure

 Stopper


                            Movable electrode
Measures to enhance the electrostatic force
Design objective
  A stoke of ±0.5 μm, Servo bandwidth of 2 kHz.
Two affecting factors
  Electrostatic force, stiffness of the microflexure.   21
       Dynamics of the microactuator
Scheme model of spring-mass-damper system

                                   where m is the mass of the moving
                                   part, C is the damping coefficient, K
                                   is the stiffness of the suspension
                                   flexure, and Fe is the electrostatic
                                   force of actuators.

The actuator’s displacement x produced by an electrostatic
force Fe is            m  Cx  Kx  F
                        x               e




                                    1   K
The resonant frequency is    f 
                                   2
                              0
                                        m
                                                F
The frequency response is x f                 e


                                      2f  m  j 2fC  K
                                             2




                                                                    22
     Flexures in the microactuators
Straight flexure (fixed-fixed)                            3
                                      12 EI 2 Etb
                                 k 2 x
                                             3       3
                                        l     l
                                    2 EA        b
                                 k 
                                  y
                                          2E t
                                      l         l
                                 K   l    2


                                     y

                                          2
                                 K b  x




Folded flexure
                                                  3
                                      2 Etb
                                 k 
                                  x           3
                                        l
                                              b
                                 k  2E t
                                  y
                                              l
                                 K   l    2


                                     y

                                          2
                                 K b  x
                                                              23
 Flexure analysis in the microactuator
 Requirements:
  Large stiffness effects Kx, Ky and large stiffness ratio Ky/Kx.
 Types of flexures compared in the microactuator




  Straight-plate flexure   Folded-plate flexure   Interlapped-L flexure
                                                                          24
              Finite element method
•FEM is a computer-based numerical technique for calculating
the strength and behavior of engineering structures.
•To calculate and analyze the deflection, stress, vibration,
buckling behavior and many other phenomena.




Schematic
flow of FEA




                                                         25
Modeling of the flexures’ deflection




                   Influence factors to stiffness:
                   • Flexure plate width b
                   • Flexure joint height hjoint
                   • Overlapped length loverlap
                   • Plate gap of the flexure lgap


                                                26
Example: Influence of the flexure plate width
                                                                                                                          550k
                                      140
                                                                                                                          500k                 Straight-plate flexure
                                                Straight-plate flexure
                                      120                                                                                 450k                 Folded-plate flexure
                                                Folded-plate flexure
                                                                                                                                               Interlapped-L flexure
                                                Interlapped-L flexure                                                     400k
       Stiffness of Kx


                                      100




                                                                                              Stiffness of Ky
                                                                                                                          350k
                                       80                                                                                 300k
                                                                                                                          250k
                                       60
                                                                                                                          200k
                                       40                                                                                 150k
                                                                                                                          100k
                                       20
                                                                                                                           50k
                                        0                                                                                       0
                                            3        4             5          6       7                                                 3            4               5      6        7
                                                Plate width of the flexures b, m                                                                Plate width of the flexures b, m



                                                                                                                          2.8
                                      4.8
                                      4.6                                                                                                            Frequency response
        Stiffness ratio, Log(Ky/Kx)




                                                                                                                          2.4
                                      4.4




                                                                                              Natural frequency fn, kHz
                                      4.2                                                                                                   Straight-plate flexure
                                      4.0
                                                                                                                          2.0               Folded-plate flexure
                                      3.8                                                                                                   Interlapped-L flexure
                                                         Straight-plate flexure
                                      3.6                                                                                 1.6
                                                         Folded-plate flexure
                                      3.4
                                                         Interlapped-L flexure
                                      3.2                                                                                 1.2
                                      3.0
                                      2.8                                                                                 0.8
                                      2.6
                                      2.4                                                                                 0.4
                                      2.2
                                            3        4              5             6       7                                         3               4            5          6        7
                                                Flexure width of the flexure b, m                                                          Plate width of the flexures b, m




      The plate width of the flexure should be 5~6 μm.
                                                                                                                                                                                         27
Example: Influence of flexure plate gap

                                                                                                                                 70k
                             50
                                                  Interlapped-L flexure                                                          60k

                             40                                                                                                  50k
      Stiffness of Kx




                                                                                                               Stiffness of Ky
                                                                                                                                 40k
                             30                                                                                                                             Interlapped-L flexure
                                                                                                                                 30k

                             20                                                                                                  20k

                                                  Folded-plate flexure                                                           10k
                             10                                                                                                                   Folded-plate flexure
                                                                                                                                  0
                                      4       6         8         10      12    14     16       18   20   22                           4   6     8     10      12     14     16     18   20   22
                                                  Plate gap of the flexure lgap, m                                                            Plate gap of the flexure lgap, m


                             1400

                             1200
     Stiffness ratio Ky/Kx




                             1000                                                                              The flexure gap should be
                              800

                              600
                                                                        Interlapped-L flexure                  no less than 10 μm.
                              400

                              200                               Folded-plate flexure

                                  0
                                          4       6         8      10      12    14     16      18   20   22
                                                      Plate gap of the flexure lgap, m




                                                                                                                                                                                                   28
    Bulk-micromachining and SOG
Glass substrate instead of SOI wafer:
• Facilitate to fabricate 3-D bulk silicon structures
• Good isolation between the different electrodes
• Strong structure and good reliability
• Much lower cost than SOI

Eutectic bonding VS anodic bonding:
• Intermediate metal of Cr/Au acts as not only the adhesive
layer, but also the electrode pad.
• low temperature and no voltage bonding process.


                                                              30
                    Fabrication process (1)


1.Deposition of Cr/Au on the glass substrate. 5. Double-side wet etching the glass
                                              substrate.



2. Lithography on the Cr/Au layer.
                                             6. Lithography on the backside silicon.


3. Wet etching of Au, Cr successively.


                                             7. Deep RIE on the backside silicon
                                             wafer to get the cavities.
4. Lithography on the glass substrate.


                                                                                       31
                 Fabrication process (2)


                                              11. Deep RIE of the topside silicon to
8. Eutectic bonding the silicon to glass.
                                              form the structure of microactuator.




9. Thinning the silicon wafer from topside.


                                              12. Dicing to get the microactuator.


10. Lithography on the topside silicon.


                                                                                     32
                   Fabrication result




Top-view of the electrode pad on       Backside Si etching (DRIE)
glass substrate (Cr/Au layer 1.2 μm)


                                                              33
     High-aspect-ratio structure by DRIE




SEM picture of the microactuator with   Close-up view of the high-aspect-
folded-plate flexures                   ratio structure (Trench 110 μm:4 μm)



                                                                         35
Testing of the microactuators

                        Dynamic
    Static test           test




                                  36
      Testing of the microactuators




Static test with probe station and image analysis system
                                                           37
                                                          Characterization results
                                                                                                                         Dynamic testing
                                                                                                             Frequency response of str flexure 15v                                                   Frequency response of lapped flexure 15v



                            Static testing                                                -115

                                                                                          -120
                                                                                                 3.5   4.5         5.5       6.5       7.5       8.5   9.5   10.5
                                                                                                                                                                                    -115

                                                                                                                                                                                    -120
                                                                                                                                                                                           12   13           14        15        16        17     18   19


                                                                                          -125                                                                                      -125

                                                                                          -130                                                                                      -130




                                                                          Magnitude, dB




                                                                                                                                                                    Magnitude, dB
                                                                                          -135                                                                                      -135
                  3.0
                                                                                          -140                                                                                      -140

                  2.5             Mearsured result                                        -145                                                                                      -145

                                  Calculated result                                       -150                                                                                      -150

                  2.0                                                                     -155                                                                                      -155
Displacement m




                                                                                          -160                                                                                      -160
                  1.5                                                                                                       Frequency, kHz                                                                            Frequency, kHz


                                                                                                             Frequency response of str flexure, 15v                                                  Frequency response of lapped flexure, 15v
                  1.0
                                                                                          400                                                                                       150

                  0.5
                                                                                          350                                                                                       120

                  0.0                                                                     300                                                                                        90
                                                                             Phase, deg




                                                                                                                                                                    Phase, deg
                        0    10     20    30    40    50   60   70   80                   250
                                                                                                                                                                                     60

                                         DC voltage (v)                                   200
                                                                                                                                                                                     30
                                                                                          150
                                                                                                                                                                                      0

                   Straight flexure actuator                                              100
                                                                                                 3.5   4.5        5.5       6.5       7.5       8.5    9.5   10.5                   -30
                                                                                                                                                                                          12    13          14        15        16        17     18    19


                                                                                                                            Frequency, kHz                                                                           Frequency, kHz




                                                                          Straight flexure actuator, fn=7.25kHz Lapped-L flexure actuator, fn=15.8kHz

                                                                                                                                                                                                                                                 38
                      Quality factor
 Q = total energy stored in system/energy loss per unit cycle
 If the distance between two half-power points is Δf, and the
resonance frequency is fr, then Q is Q  f
                                                  f
                                              r




 Source of mechanical energy loss
– crystal domain friction
       – direct coupling of energy to surroundings
           – disturbance and friction with surrounding air
     example: slide film damping and squeezed film damping between
   the parallel plate capacitors

        Source of electrical energy loss
            resistance ohmic heating, electrical radiation
The higher the quality factor, the sharper the system response
                                                                 39
Thank You !

              40

				
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