MEMS 3

Introduction to Sensing And Actuation Methods

Sensing & Actuation Methods

Sensing

• Electrostatic• Thermal• Magnetic• Piezoelectric• Piezoresistive

Actuation

• Electrostatic• Thermal• Magnetic• Piezoelectric• Shape Memory Alloys

Tunneling ,Optical, FET, RF Resonance Sensing

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

Electrostatic Sensing and Actuation

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.

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

Equilibrium position of Electrostatic Actuator under Bias

Electromechanical model of a // plate capacitor

Electrostatic Actuation

Electrostatic energy stored by a capacitor

Maximum Energy stored is

Where Eb is the breakdown electric field

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

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.

Electrical and Mechanical Force as a Function of Spacing

Graphical solution

Amplitude of electrostatic & mechanical force

Balance of Electrical and Mechanical Force

Effect of different bias voltages on equilibrium distance, x

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

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

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

Two Types -Transverse - Longitudinal

• Many Parallel plates can increaseActuation force.

Perspective view of comb-drive sensors and actuators

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

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)

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

MEMS Electrostatic Actuators


An out of plane accelerometer based on comb drive actuation

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

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

Example: Force Balanced ADXL-50

ADXL-50

Accelerometer with Capacitive Sensing

Bulk micromachined capacitive accelerometer. Inertial mass in the middle wafer forms the moveable electrode of a variable differential capacitive Circuit.

Accelerometer with Capacitive Sensing

Fabrication Process Steps:

Parallel-Plate Capacitive Accelerometer

Surface Micromachined Parallel Plate Capacitor as an Accelerometer

RT Process

Ni Plate Size1x0.6mm2 in area5um Thick

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

Surface Micromachined Pressure Sensor

Capacitance changes with deflecting membrane which can be measured using AC circuitry.

Comb-Drive Actuator for Optical Switching

Linearly graded comb teeth


Electrostatic Optical Switch

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

Fabrication Process of Tactile Sensor

Buried n type layer (3um) +6um thick n epi layer

Scratch Drive Actuator

Square Pulse

SDA Supported by Elastic Beams

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

SDA Actuator and Linked Buckling Beam

Maximum Deflection Vs Horizontal Displacement

Generated Force By SDA and Applied Voltage

3-D Self-Assembled Polysilicon Structure




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MEMS 3