MEMS 3
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Transcript of MEMS 3
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Introduction to Sensing And Actuation Methods
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Sensing & Actuation Methods
Sensing
• Electrostatic• Thermal• Magnetic• Piezoelectric• Piezoresistive
Actuation
• Electrostatic• Thermal• Magnetic• Piezoelectric• Shape Memory Alloys
Tunneling ,Optical, FET, RF Resonance Sensing
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Design considerations
Sensor
• Sensitivity• Linearity• Responsivity• SNR• Dynamic Range• Bandwidth• Drift• Reliability• Cross talk• Cost
Actuator
• Torque or force output capacity
• Range of motion• Dynamic response• Ease of fabrication• Power consumption &
energy efficiency• Linearity of displacement
as a function of driving bias
• Cross sensitivity & environmental stability
• Foot Print
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Electrostatic Sensing and Actuation
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Principle of operation
• A capacitor is broadly defined as two conductors that can hold opposite charges
• If the distance/relative position or dielectric medium between two conductors change as a result of applied stimulus,the capacitance value will change.This forms the basis of capacitive (Electrostatic) sensing.
• If a voltage or electric field is applied across two conductors,an electrostatic force would develop between these two objects resulting in actuation. This is defined as electrostatic actuation.
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Principle of operation ……• Two Types of capacitive electrode geometries
* Parallel Plate Capacitors
* Interdigitated Finger (Comb Drive) Capacitors
• Two Parallel plates can move with respect to each other
* Normal Displacement
* Parallel sliding displacement
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Equilibrium position of Electrostatic Actuator under Bias
Electromechanical model of a // plate capacitor
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Electrostatic Actuation
Electrostatic energy stored by a capacitor
Maximum Energy stored is
Where Eb is the breakdown electric field
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When a voltage V is applied, a force Felectric develops between the plates.The magnitude of force equals the gradient of the stored energy W
• The spatial gradient of Electric Force is defined as electrical spring constant, Ke
Ke changes with position (d) and the biasing voltage (V)
Effective spring constant of the structure: Km-Ke
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Calculation of equilibrium displacement : x
Mechanical restoring force is
At equilibrium,
Equilibrium distance x can be calculated by solving this quadratic equation with respect to x.
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Electrical and Mechanical Force as a Function of Spacing
Graphical solution
Amplitude of electrostatic & mechanical force
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Balance of Electrical and Mechanical Force
Effect of different bias voltages on equilibrium distance, x
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Balance of Forces at the Pull-in voltage
•At tangent,Magnitude of Fe equals Fm
• Pull-in Voltage
•The // plate electrostatic actuator becomes unstable for V greater than Vp
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At Pull in Voltage, magnitudes of electrical and mechanical balance forces are same.By equating these two forces
Analytical Solution
We know
Only solution for x when Ke=Km is satisfied: Independent of Vp &
Spring constant
Putting V2 from above
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We get
And consequently we get
Putting x = xo/3 in
at V = V pull in or Vp
For V>Vpull in, Snap in condition,•There is no equilibrium position and the two plates ‘snap in’ or come in contact
• Idealized case: Two sources of deviation - Fringe caps. & Restoring force considered linear
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Two Types -Transverse - Longitudinal
• Many Parallel plates can increaseActuation force.
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Perspective view of comb-drive sensors and actuators
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Transverse Comb drive
Capacitance at Rest Csl=Csr=є0 l0 t/x0
Capacitance after movement x Csl= є0 l0 t/x0-x Csr= є0 l0 t/x0+x Total value of capacitance= Csl+Csr+Cf
The displacement sensitivity Sx=∂Ct ot/ ∂ xMagnitude of force(Actuator)Fx= | ∂ U/ ∂ x | = | ∂ / ∂ x (1/2CtotV2)I
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Longitudianal Comb Drive
With lateral movement y,the capacitance of single finger Csl=Csr=є0 (l0 - y) t/x0
The displacement sensitivity: Sy= ∂Ct ot/ ∂ y Force (Actuator) Fy= ∂E/ ∂ y= ∂/∂y (1/2CtotV2)
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Applications
• Electrostatic Motor• Inertia Sensor - Parallel plate- capacitive accelerometer - Torsional plate- capacitive accelerometer
• Pressure Sensor - Membrane parallel plate pressure sensor - Membrane capacitive condenser microphone
• Flow sensor• Tactile sensor
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MEMS Electrostatic Actuators
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MEMS Electrostatic Actuators
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An out of plane accelerometer based on comb drive actuation
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Typical Calculations
• The force constant associated with the mass is twice that of each individual fixed-guided cantilever. The overall force constant is
K= 24EI/L3
• The total capacitance at rest is contributed by eight fixed electrodes and therefore 16 vertical wall capacitors .The value of total capacitance is
C(t)= 16 (єo loto/d)
• The displacement in Z axis which is a function of the applied acceleration causes the effective thickness (t) to change. Upon displacement z,the capacitance becomes
C(t)= 16 {єo lo(to-z)/d} and
z= ma/K= maL3/24EI
The relative change of capacitance with respect to acceleration a is
∂C/ ∂a= 2 єo lomL3/3dEI
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Fabrication Process of Torsional Acceleration Sensor
)2( fmfr ll
d
lnC
Change in capacitance under angular displacement
Where,lm :length of inertia masslf: length of sensing fingerd: gap distancen: number of sense fingers
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Example: Force Balanced ADXL-50
ADXL-50
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Accelerometer with Capacitive Sensing
Bulk micromachined capacitive accelerometer. Inertial mass in the middle wafer forms the moveable electrode of a variable differential capacitive Circuit.
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Accelerometer with Capacitive Sensing
Fabrication Process Steps:
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Parallel-Plate Capacitive Accelerometer
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Surface Micromachined Parallel Plate Capacitor as an Accelerometer
RT Process
Ni Plate Size1x0.6mm2 in area5um Thick
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Fabrication Process of Pressure Sensor with Sealed Cavity
Oxidation + Patterning
Anisotropic Etch ( 9um)
Oxide Etch
Oxidation
Patterning
B Diffusion 15um
Reoxidise+Pattern
Thin B-Doping
Dielectric + Patterning
Poly + Doping
CMP + Cr/Au deposition+Oxide Dep+Pattern
Flip chip Bonding
Silicon Etch
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Surface Micromachined Pressure Sensor
Capacitance changes with deflecting membrane which can be measured using AC circuitry.
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Comb-Drive Actuator for Optical Switching
Linearly graded comb teeth
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MEMS Electrostatic Actuators
Electrostatic Optical Switch
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Bulk Micromachined Parallel-Plate Capacitor as Differential Mode Tactile Sensor
dd
LC r
2
20
Capacitance change under normal force
Ld
LC r
5.0
20
Total Capacitance under shear force
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Fabrication Process of Tactile Sensor
Buried n type layer (3um) +6um thick n epi layer
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Scratch Drive Actuator
Square Pulse
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SDA Supported by Elastic Beams
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Fabrication Process: SDA
Oxidation+Poly Si+P Implant+Photolithography+ Nitride deposition
Sacrificial oxide+ Two step Lithography
Poly Si ( Buckling beam)+ P Implant +Photolith. + Dry Etch
Stress Anneal with thin oxide+Sacrificial layer removal
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SDA Actuator and Linked Buckling Beam
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Maximum Deflection Vs Horizontal Displacement
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Generated Force By SDA and Applied Voltage
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3-D Self-Assembled Polysilicon Structure
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MEMS Electrostatic Actuators
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MEMS Electrostatic Actuators
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MEMS Electrostatic Actuators
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