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MEMS Design & Fab

Overview

• Quick look at some common MEMS actuators• Piezoelectric• Thermal• Magnetic

• Next:• Electrostatic actuators• Actuators and mechanism• Beams

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MEMS Design & Fab

MEMS Actuation Options• Piezoelectric

• Thermal

• Magnetic

• Electrostatic

• Dynamics• Beam bending• Damping

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Ferroelectrics (piezoelectrics)• Huge energy densities

• Good efficiency

• Huge force, small displacement

• Major fabrications challenges• Continuously promising technology

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MEMS Design & Fab

Piezoelectric effect

• Polyvinylidene flouride (PVDF)

• Zinc oxide - ZnO

• Lead zirconate titanate – PZT

• PMNPT

V d L0A

FL

A

d - piezoelectric coefficient rank 2 tensor: e.g. d11, d31

V

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MEMS Design & Fab

Piezoelectric products

• Quartz resonators (single crystal)• E.g. crystal oscillators• ~10Million/day, $0.10 each, vacuum packaged

L

AV

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MEMS Design & Fab

Bimorph for STM and AFM

ZnO

Aluminum electrodes

After Akamine, Stanford, ~90

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MEMS Design & Fab

Piezoelectric Actuator Summary• High voltage, low current

• ~100V/um• No static current (excellent insulator)

• Highest energy density of any MEMS actuator but• Large force, small displacement• Typically very difficult to integrate with other

materials/devices

• “Continuously promising”

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MEMS Design & Fab

Thermal Expansion

.

= T is the thermal expansion strain (L/L)= is the thermal expansion stressF = A is the thermal expansion force

silicon ~ 2.3x10-6/K

AL

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MEMS Design & Fab

Thermal actuator worksheet• Assume that you have a silicon beam that is 100 microns long,

and 1um square. You heat it by 100K. How much force do you get if you constrain it? How much elongation if you allow it to expand? TCE for silicon is 2.3x10^-6/K .

Area== T == =F = A =L= L=

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MEMS Design & Fab

Plot by: R. Conant, UCB.

Thermal expansion: The heatuator

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MEMS Design & Fab

Thermal Actuators

Current input pad

Actuator translatesin this direction

Cold arm

Hot arm

Current output pad

Uses thermal expansion for actuationVery effective and high force output per unit area

Cascaded thermal actuators for high force

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MEMS Design & Fab

Thermal actuators in CMOS

Shen, Allegretto, Hu, Robinson, U. Alberta

Joule heating of beams leads to differential thermal expansion, changing the angle of the mirror

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MEMS Design & Fab

Bubble actuators (thermal and other)• Lin, Pisano, UCB, ~92?

• HP switch

• Papavasiliu, Pisano, UCB - electrolysis

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MEMS Design & Fab

Thermal actuator summary• Easy process integration!

• Large forces, small displacements

• Need lever mechanisms to trade off force for displacement

• Typically very inefficient

• Time constants ~1ms

• Substantial conduction through air

• Minimal convection in sub-millimeter designs

• Radiation losses important above ~300C

• Instant heating, slow cooling• Except when radiative losses dominate

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MEMS Design & Fab

Magnetic actuators• Lorentz force

• Internal current in an external (fixed) magnetic field

• Dipole actuators• Internal magnetic material in an external (varying)

field

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Magnetic Actuation (external field)

Silicon substrate

NiFe electroplatedon polysilicon

External magneticfield

• Fabrication: NiFe electroplating

• Switching external field

• Packaging

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MEMS Design & Fab

Magnetic Parallel Assembly

Solid-State Sensor and Actuator WorkshopHilton Head 1998

Figure 1. (a) An SEM micrograph of a Type I structure. The flap is allowed to rotate about the Y- axis. (b) Schematic cross-sectional view of the structure at rest; (c) schematic cross-sectional view of the flap as Hext is increased.

Figure 2. (a) SEM micrograph of a Type II structure. (b) Schematic cross-sectional view of the structure at rest; (c) schematic cross-sectional view of the structure when Hext is increased.

Parallel assembly of Hinged Microstructures Using Magnetic Actuation

Yong Yi and Chang LiuMicroelectronics Laboratory

University of Illinois at Urbana-ChampaignUrbana, IL 61801

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MEMS Design & Fab

Parallel assembly

Solid-State Sensor and Actuator WorkshopHilton Head 1998

Parallel assembly of Hinged Microstructures Using Magnetic Actuation

Yong Yi and Chang LiuMicroelectronics Laboratory

University of Illinois at Urbana-ChampaignUrbana, IL 61801

Figure 8. Schematic of the assembly process for the flap 3-D devices. (a) Both flaps in the resting position; (b) primary flap raised to 90º at Hext = H1; (c) full 3-D assembly is achieved at Hext = H2 (H2 > H1 ).

Figure 9. An SEM micrograph of a 3-D device using three Type I flaps. The sequence of actuation is not critical to the assembly of this device.

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Magnetic actuators – Onix switch?

• Magnetic actuation, electrostatic hold

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MEMS Design & Fab

Magnetic actuators in CMOS

Resonant Magnetometer

B. Eyre, Pister, Judy

Lorentz force excitation

Piezoresistive detection

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MEMS Design & Fab

LIGA: synchrotron lithography, electroplated metal

Micro Electro Mechanical SystemsJan., 1998 Heidelberg, Germany

Closed Loop Controlled, Large Throw, Magnetic Linear

Microactuator with 1000 m Structural Height

H. Guckel, K. Fischer, and E. Stiers

U. Wisconsin

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Magnetic Actuation in LIGA

Micro Electro Mechanical SystemsJan., 1998 Heidelberg, Germany

U. Wisconsin

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MEMS Design & Fab

Magnetic Actuation in LIGA

Micro Electro Mechanical SystemsJan., 1998 Heidelberg, Germany

U. Wisconsin

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MEMS Design & Fab

Maxell (Hitachi) RF ID Chip

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MEMS Design & Fab

Magnetic actuator summary• High current, low voltage (contrast w/

electrostatics)

• Typically low efficiency

• Potentially large forces and large displacements

• Some process integration issues