Overview

1
Overview

• Quick look at some common MEMS actuators
• Piezoelectric
• Thermal
• Magnetic
• Next
• Electrostatic actuators
• Actuators and mechanism
• Beams

2
MEMS Actuation Options

• Piezoelectric
• Thermal
• Magnetic
• Electrostatic
• Dynamics
• Beam bending
• Damping

3
Ferroelectrics (piezoelectrics)

• Huge energy densities
• Good efficiency
• Huge force, small displacement
• Major fabrications challenges
• Continuously promising technology

4
Piezoelectric effect

• Polyvinylidene flouride (PVDF)
• Zinc oxide - ZnO
• Lead zirconate titanate PZT
• PMNPT

5
Piezoelectric products
V
A
L

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

6
Bimorph for STM and AFM
Aluminum electrodes
ZnO
After Akamine, Stanford, 90
7
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

8
Thermal Expansion
L
A

• g DT is the thermal expansion strain (dL/L)
• E e is the thermal expansion stress
• F A s is the thermal expansion force
• gsilicon 2.3x10-6/K

.
9
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
• g DT
• E e
• F A s
• dL e L

10
Thermal expansion The heatuator
Plot by R. Conant, UCB.
11
Thermal Actuators
Uses thermal expansion for actuation
Very effective and high force output per unit area
Actuator translates in this direction
Cold arm
Current output pad
Hot arm
Cascaded thermal actuators for high force
Current input pad
12
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
13
Bubble actuators (thermal and other)

• Lin, Pisano, UCB, 92?
• HP switch
• Papavasiliu, Pisano, UCB - electrolysis

14
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

15
Magnetic actuators

• Lorentz force
• Internal current in an external (fixed) magnetic
  field
• Dipole actuators
• Internal magnetic material in an external
  (varying) field

16
Magnetic Actuation (external field)
External magnetic field
NiFe electroplated on polysilicon
Silicon substrate

• Fabrication NiFe electroplating
• Switching external field
• Packaging

17
Magnetic Parallel Assembly
Parallel assembly of Hinged Microstructures Using
Magnetic Actuation Yong Yi and Chang
Liu Microelectronics Laboratory University of
Illinois at Urbana-Champaign Urbana, IL 61801
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.
Solid-State Sensor and Actuator Workshop Hilton
Head 1998

18
Parallel assembly
Parallel assembly of Hinged Microstructures Using
Magnetic Actuation Yong Yi and Chang
Liu Microelectronics Laboratory University of
Illinois at Urbana-Champaign Urbana, 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 gt 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.
Solid-State Sensor and Actuator Workshop Hilton
Head 1998

19
Magnetic actuators Onix switch?

• Magnetic actuation, electrostatic hold

20
Magnetic actuators in CMOS
Resonant Magnetometer B. Eyre, Pister, Judy
Lorentz force excitation Piezoresistive detection
21
LIGA synchrotron lithography, electroplated metal
Closed Loop Controlled, Large Throw, Magnetic
Linear Microactuator with 1000 mm Structural
Height H. Guckel, K. Fischer, and E. Stiers U.
Wisconsin
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany

22
Magnetic Actuation in LIGA
U. Wisconsin
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany

23
Magnetic Actuation in LIGA
U. Wisconsin
Micro Electro Mechanical Systems Jan., 1998
Heidelberg, Germany

24
Maxell (Hitachi) RF ID Chip
25
Magnetic actuator summary

• High current, low voltage (contrast w/
  electrostatics)
• Typically low efficiency
• Potentially large forces and large displacements
• Some process integration issues