Download A high force low area MEMS thermal actuator

A high force low area MEMS
thermal actuator
2004.10.9
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Abstract

This paper presents a new type of mems(micro-electro-mechanical
systems)actuator consisting of an array of in-plane micro-fabricated thermal
buckle-beam actuators

The technology used in MEMS actuators is typically magnetic,electrostatic or
thermal.


Magnetic actuators may require special materials in the fabrication process
Electrostatic actuators typically require high voltages, large chip areas and
produce very low forces

Thermal actuators typically generate on the order of a few micro-Newtons each
but can be combined for larger forces by linking with small tendons.

A disadvantage of this type of actuator is that it moves in an arc where most
desired movements are linear. Also, when combined in an array, the linking
tendons consume much of the energy in bending them. Also, arrays of these can
still occupy a fairly large chip area.
Abstract

The electro-thermal actuator described here resembles a chevron
where an array of buckle-beams are packed close together and link
two common anchored arms with a movable third arm.

Arrays can be made within a single released micromachined layer and
generate many mN of force. Additional actuators can be arrayed with
no coupling penalty and occupy much less area that an equivalent
pseudo-bimorph actuator.

Preliminary tests indicate that a 450 x 120 μm array consumes 240 mW
of power, deflection up to 14 μm and can produce many milli-newtons.
A chip of actuator geometry variations and different applications has
been fabricated and tested.
Introduction

The desired attributes of an internal actuator are small chip real estate,
large deflection (>lO μm) and an electrical requirement compatible
with today’s CMOS circuitry.

MEMS actuators are typically used for either one-time deployment of
structures for automatic assembly, an in-use adjustment such as
focusing or tweaking an optical parameter or constant periodic
actuation as in the case of micro-optic scanners.

Electrostatic actuators rely on the attractive forces between oppositely
charged conductors in close proximity. Magnetic actuation uses the
force of attraction or repulsion between a magnetic field produced by
an electric current and a magnetic material or other electromagnet.

Electro-thermal actuators rely on the joule heating and resulting small
mechanical expansion of a conductor when a current is passed
through it.
Introduction

Conversely, employing the thermal actuator array proposed by Reid
[2], one can achieve about 450 μN per square mm of MEMS chip area.

The electrical power required is 3.87 mW per μN. These actuators
depend on the differential thermal expansion of two polysilicon arms
to produce a pseudobimorph that deflects in an arc. For an array,
these devices may be coupled to a beam through bending yokes.

The actuator array presented here consists of only one thermal
expansion beam per actuator and can produce about 3700 μN per
square mm and 1.53 mW per μN. As structures become more
complicated, especially in the case of free-space optical devices [3]

the one-time deployment required for assembly becomes more
important and reliant on high-force, low-area actuators. Many of the
deployment actuators today are of the comb drive type and typically
occupy many times the area of the device they are deploying.
Device fabrication
The tested actuators were fabricated
using the Multi-User MEMS Processes
(MUMPs) [4].
MUMPs is a surface micromachining
process employing a substrate, an
insulating nitride layer and three structural
polysilicon layers separated by two
sacrificial oxide layers as shown in Fig. 1
Figure 1. Cross-section view of the MUMPs
fabrication process showing three
polysilicon layers with a single anchor point.
•
•
The second and third polysilicon layers (Poly1 and Poly2) are both
releasable to act as movable structures. A 0.5 μm gold layer can be
pattem-deposited on the Poly2 layer for optical reflection or increased
conductivity.
The final step performed is an HF etch of the intervening sacrificial
oxide layers and subsequent drying.
Actuator design
a voltage is applied between the
mechanical anchors, ohmic heating of
the two half-beams causes them to
expand and ultimately buckle.
 The resistivity of polysilicon allows
the actuator to operate at voltages and
currents compatible with standard
integrated circuitry (CMOS).
Figure 2. Single buckle-beam actuator. The
applied voltage causes ohmic heating and
expansion between the two fixed anchors,
buckling the beam at the midpoint.
The beam is normally designed with
a pre-bend angle a so buckling will
have an affinity to move in-plane
The actuator displacement d is given by
d = [ L2 + 2 ( L ) L' - Lcos(a)2]1/2 - Lsin(a)
Actuator design
•Arrays of buckle-beam devices can be easily
designed by arranging them in a pattern
resembling a chevron as shown in Fig. 3.
•A center beam is added to stiffen the midpoint
and allow mechanical coupling of the individual
beams as well as providing a method of
transmitting the linear force to another device.
Figure 3. An array of four
buckle-beam actuators
with the addition of a
coupling beam. The output
force is linear – four times
that of a single actuator
•There is no theoretical limit to the number of
beams added as long as the device and
conductors can handle the current and heat,
•the beams can lose heat rapidly and there is no
cross coupling of heat from one beam to
another.
•Most of the actuator arrays explored in this
paper consist of pairs of 218μm half-beams in
varying number and thickness.
Actuator design
•If more than one actuator array is connected
(mechanically and electrically) to a single
micro-structure, care must be taken to
eliminate any common mode currents that
arise when the actuators are excited
differently
•Fig. 4 shows an alternative method of
lectrical connection to the buckle-beam array
that can help eliminate this problem.
Figure 4. Alternative connection
methods to minimize cross currents.
If more than one actuator array is
connected to a microstructure, the
common-mode current must be
minimized or damage could result.
• The current in Fig.3 passes from one anchor
to the other, placing the centercoupling beam
at the half-resistanceholtagk point.
•The current shown in Fig. 4 is fed from both
anchors toward the coupling beam, which is
at ground or a common mode potential
Actuator design
 This alternate connection could also be used to cause unequal
currents to flow in both halves of the array, moving the center
coupling beam a small distance either way and orthogonal to
the primary displacement direction.
 This action is presently being explored in an application where
the smaller displacement can be used to accurately position an
external structure clamped by the larger displacement.
Force and deflection measurement
Figure 5. CAD image of a typical
chevron actuator array. Note the
capture bearings used to prevent
the buckle-beams from deflecting
out-of-plane.
Figure 6. Cross-section view of the
actuator array of Fig. 5.Note the
dimple bearing used to reduce
stiction.
Force and deflection measurement


Figure 7. Photomicrograph of a chevron
actuator array. The probe at the bottom is
used to shorten the force-measuring
beam to 60 pm
The actuator is deflected about 7
μm. a probe is shown shortening
the 130 μm force beam to 60 μm.
By measuring the deflection of
the bending-beam, the applied
force can be computed. Note
that the applied force is not the
same as the total force capability
of the actuator.
For small deflections of the force beam, the applied force is calculated
F = Etdw3/4 l3
where F = applied force ,E = Young’s modulus - 160 Gpa [7]
t = beam thickness in μm , w = beam width in μm
l =beam length in μm
Test results
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
Figure 8. Deflection distance vs. pre-bend
angle results for 2.0 μm thick polysilicon
actuators. Two different forcemeasuring
beam lengths were used.
A graph of actuator deflection for
various pre-bend angles is shown in
Fig. 8
The data was taken from a number of
2 μm thick structures at two different
applied forces by selecting force
beam lengths of 130 and 60 μm.
It can be seen (and predicted) that
actuators with a small pre-bend
angle (<0.5 degrees) exhibited little
or no deflection and hence little
output force.
For very small pre-bend angles, the
actuator sometimes refused to move
in-plane and, instead, buckled outof-plane.
Predictably, with larger pre-bend
angles, the deflection was reduced
but the available force increased. Fig.
8 also indicates that, for a 60 μm
force beam, the optimal pre-bend is
around 1degree for maximum
deflection.
Test results
Figure 9. Actuator deflection vs.excitation
voltage. All actuators appear to be linear
for excitations greater than 2 volts. The
slope is pre-bend angle dependent.

Fig. 9 is a graph of deflection for a
series of 2 μm actuators with various
pre-bend and excitation voltages.

It indicates that the actuators exhibit
a linear response when the excitation
is above 2 volts. The slope of the
curves indicates the higher the prebend angle, the lower the deflection.

Deflection response was measured in
all actuators to be around 2KHz.
Conclusion





These actuators can be used almost anywhere MEMS linear motion is
required.
They produce high force though consume considerable power. The
small deflection (~10 μm) can be extended through leverage, gearing
or clutcWfriction drive.
Using the technique described here, a buckle-beam actuator array
consisting of 48 two pm thick beams would have an output force of
around 240 pN and occupy an area of 65,OO sq. μm. This same force
would require an array of pseudo bi-morph devices occupying
533,000 sq. μm of chip real estate or 12,000,000 sq. μm for an
electrostatic actuator array.
Future work will consist of exploring other geometry variations of these
actuators such as longer beam lengths, beam-to-beam spacing and
methods to avoid out-of-plane buckling.
Applications will include deployment systems for optical devices,
rotary motion, mirror scanners and other display devices.
Application

Figure 10. Photomicrograph of a
thermally actuated gear motor.
The actuators are connected in a
common mode configuration and
excited with two sources in phase
quadrature that causes the drive
gear to move in a circular pattern.