ptics
Brian Patrick Trease
University of Michigan
Advisor: Prof. Sridhar Kota
Department of Mechanical Engineering
Thomas Zipperian, Manager (Org. 01769)
James Allen, Technical Advisor (Org. 01769)
Tuesday, August 7, 2001
Special thanks
to Daryl Dagel and Michael Baker of Sandia
National Laboratories for their technical conversations, review, and advice. B.P. Trease
Thermal Micromechanisms for Out-of-Plane Actuation - 2
ABSTRACT
Many currently hot Micro-Electro-Mechanical System (MEMS) applications, such as micro-
optics, require high-displacement, high-resolution out-of-plane actuation. We are specifically
focusing on the control of adaptive optics via deformable mirror manipulation. Our goal is to
develop electrically-heated thermal microactuators capable of at least 12 microns out-of-plane
motion with 25 nanometer resolution. While many thermal actuators use bimaterial technology
(i.e. unequal expansion due to different coefficients of thermal expansion), we desire to fabricate
solely in polysilicon, simplifying the process steps and creating compatibility with the
SUMMiT V
TM
Process, Sandias 5-layer polysilicon fabrication technology. The new actuator
designs share the principles already seen in in-plane devices: unequal thermal expansion (due to
different cross-sections) of connected parallel beams, resulting in tip displacement. After the
development of an analytical model as a design tool, these actuators will be combined within a
leverage scheme leading to several feasible designs. An array of candidate devices will be
modeled in computer-aided design software, undergo thermo-mechanical analysis in finite
element analysis software, and be submitted for fabrication in the SUMMiT V
TM
process. The
results of this work will aid in the expansion of MEMS technology from simple planar motion to
complex 3-dimensional mechanisms.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of Energy under Contract DE-AC04-94AL85000. B.P. Trease
Thermal Micromechanisms for Out-of-Plane Actuation - 3
TABLE OF CONTENTS
SUMMARY
.................................................................................................................................... 4
1.
INTRODUCTION
................................................................................................................. 4
1.1.
S
COPE OF
R
ESEARCH
........................................................................................................ 4
1.2.
B
ACKGROUND
.................................................................................................................. 4
1.2.1.
Adaptive Optics Background
................................................................................... 4
1.2.2.
MEMS Background
................................................................................................. 5
1.3.
D
ESIGN
G
OALS FROM A
MEMS P
ERSPECTIVE
................................................................. 5
2.
TECHNOLOGIES FOR MIRROR ACTUATION
........................................................... 6
2.1.
A
LTERNATIVES FOR
O
UT
-
OF
-P
LANE
M
OTION
.................................................................. 6
2.2.
T
HERMAL
A
CTUATORS
..................................................................................................... 7
2.2.1.
Bimaterial
................................................................................................................ 7
2.2.2.
Single Material
........................................................................................................ 8
2.2.2.1.
In-Plane Actuators
........................................................................................... 8
2.2.2.2.
Out-of-Plane Actuators
................................................................................... 9
3.
PRELIMINARY PROTOTYPES AND RESULTS
......................................................... 10
3.1.
D
EVICE
D
ESCRIPTION
..................................................................................................... 10
3.2.
M
ETHODS AND
E
QUIPMENT
............................................................................................ 11
3.3.
R
ESULTS
......................................................................................................................... 11
4.
ACTUATOR MODELING
................................................................................................ 13
4.1.
F
OURIER S
L
AW
............................................................................................................. 13
4.2.
T
HERMAL
E
XPANSION
.................................................................................................... 15
4.3.
F
ORCE
M
ETHOD
............................................................................................................. 15
4.4.
V
IRTUAL
W
ORK
............................................................................................................. 16
4.5.
F
INITE
E
LEMENT
A
NALYSIS
........................................................................................... 16
5.
NEW DESIGNS
................................................................................................................... 16
6.
FUTURE WORK
................................................................................................................ 17
7.
CONCLUSIONS
................................................................................................................. 18
BIBLIOGRAPHY
....................................................................................................................... 18 B.P. Trease
Thermal Micromechanisms for Out-of-Plane Actuation - 4
Summary


Thermal micro-electro-mechanical devices are being investigated for their potential as sources of
out-of-plane actuation. Experimental prototypes have given absolute displacements of greater
than 16 microns (13 microns of dynamic displacement) with 12V applied across the devices.
This has further motivated our development of analytical models to predict such displacements.
Several new device designs await to be evaluated with the new analytical tools. One of the new
designs is presented for the reader. The devices are to be fabricated in the SUMMiT V
TM
Process, Sandia National Laboratories 5-layer polysilicon fabrication technology.
1.

Introduction
Where will the field of adaptive optics find new technologies to assist in mirror manipulation
with very high resolution? Several potential solutions exist and it is the purpose of this paper to
explore devices based on Micro-Electro-Mechanical Systems (MEMS). Developing
MEMS-based adaptive optics is very important in realizing inexpensive and lightweight laser
imaging systems and space telescopes. Furthermore, by exploiting the basic theory of thermal
actuation, we will arrive at simplicity and robustness not found in other adaptive optics
strategies. Aside from the applications in optics, out-of-plane actuation is becoming a
requirement in many other MEMS applications. With much of the need for in-plane actuation
being met by current devices, the next logical step is to explore 3-dimensional motion.
1.1.

Scope of Research
To address the topic of mirror control for adaptive optics, out-of-plane thermal actuators are
being investigated. This prompts the question: How can high-displacement out-of-plane
actuation (also referred to as vertical actuation) in MEMS devices be achieved while maintaining
the high resolution required for adaptive optics applications? In pursuit of an answer, prototype
actuators have already been fabricated and tested. Mathematical models have been and are
continuing to be developed. Combined with finite element analysis information, an array of new
devices is being designed for fabrication by Sandia National Laboratories multi-layer
polysilicon fabrication technology, the SUMMiT V
TM
Process (S</b>andia U</b>ltra-planar, M</b>ulti-level
M
EMS T</b>echnology for Five levels).
1.2.

Background
1.2.1.

Adaptive Optics Background
Adaptive Optics (AO) is the field of imaging correction via deformable mirrors, acousto-optic
and electro-optic modulators, or nonlinear crystals. Corrections are needed for a variety of
optical aberrations, both those internal to the optical system and those external, such as
atmospheric conditions. Adaptive optics is currently applied to both ground-based and space-
based telescopes, and to laser imaging and focusing systems. Today, space telescopes use heavy,
fixed, rigid mirrors that are usually impervious to significant deformations. However, the
original, flawed mirrors of the Hubble Space Telescope provide a striking counter-example. Had B.P. Trease
Thermal Micromechanisms for Out-of-Plane Actuation - 5
the Hubble mirror been of an adaptive and deformable nature, no second repair mission would
have been required. The space program would have been saved from spending an enormous
amount of funds and from a great deal of public embarrassment.
Initial adaptive optics schemes included piezoelectric
1
actuators used to deform continuous
membrane mirrors made of glass. Piezoelectric mirrors are usually assembled into macro-scale
devices and are thus too large and heavy for applications such as a space telescope. The number
of mirrors can range from hundreds to thousands. As the number increases, the cost of
fabrication likewise increases, as does the likelihood of flawed parts.
The particular adaptive optics project that motivates this research [1] seeks to reduce the weight
of a primary telescope mirror to less than 10