Proceedings Template - WORD - Case School of Engineering

unkindnesskindUrban and Civil

Nov 15, 2013 (3 years and 10 months ago)

60 views

Design and Fabrication of Curved Micromirrors

Using the
Mu
l
tiPoly

Process

Bin Mi
, Harold Kahn
*
, Frank Merat, Arthur H. Heuer
*
, Stephen M. Philips

Department of Electrical Engineering and Computer Sc
i
ence,

*
Department of Material Science and Engineering
,

Case Western Reserve University, Cleveland, OH, Email: Bin.Mi@cwru.edu



Abstract

This paper presents the design and fabrication details of a
type of reflective curved micromirror with controllable sta
t-
ic shapes. The fabrication of this device uses con
ve
n
tional
surface micromachining technology and the
Mult
i
Poly

pr
o-
c
ess

[1]
, which is a technique for depositing mult
i
layers of
LPCVD polysilicon in order to control the overall stress
and stress gradient of
the films. The inexpensive fa
b
rication
of these micromirrors allows for a range of d
e
signs that
could address many new applications, including optical
switches.


KEYWORDS

MEMS,
MultiPoly
, Micromirrors, Radius of curvature

INTRODUCTION

Controlling the sta
tic shape of Microelectromechanical Sy
s-
tems (MEMS) components has been challenging but desi
r-
able for many applications, including optical MEMS. In
general, stress gradient induced curvature is unwanted for
MEMS micromirrors, because in many applicaitons op
tical
efficiency is dependent on mirror’s flatness. Ther
e
fore, va
r-
i
ous techniques have been demonstrated to min
i
mize the
static mirror deformation. For example, thick si
n
gle
-
crystal
silicon is used to strengthen mirrors
[2]

and A
r
gon ion m
a-
chining is shown to correct the stress
-
induced out
-
of
-
plane
deformation
[3]
.

While a great deal of effort has been focused on making fla
t
micromirrors, few attempts have been made to research
micromirrors with 3D profiles
[4]
. Less noticed, but pe
r-
haps equally important in the long run, this family of mi
r
ror
structures also has many interesting applications. One e
x-
ample is a

high
-
speed deformable focusing element that
could provide new capabilities, such as collimation corre
c-
tion and vibration compensation of poorly collimated
beams in optical switches, sample height variation compe
n-
sation in scanning confocal microscopy, and

optimiz
a
tion
of signal power in optical detectors. On the other hand, to
achieve a spherical shape micromirror with co
n
trollable
static curv
a
ture is the first step of design and fa
b
ricating an
actuated shaped micromirror
[5]
, which enables curved
-
to
-
flat trans
i
tions on micromirrors to achieve digital extinction
func
tion in optical switches application.

Furthermore, the curved mirror structure by itself is sui
t
able
for many potential applications
including reflectors in ultr
a-
sonic imaging and ultrasonic transducers
[6]
; miniature
m
i
crophones and speakers
[7]
; and biomedical research in
h
u
man eyes, such as corneal surgery and manmade retinas
[8]
.

As one of the most widely used structural material for s
u
r-
face
-
micromachined devices, polysilicon is typically depo
s-
ited by low
-
pressure chemical vapor deposition (LPCVD).
The intrinsic stresses of the polysilicon films vary with the
microstructure. And the microstructure of LPCVD polysil
i-
con films is dependent

on the deposition conditions, such as
deposition temperatures.
Specifically, when the growth
temperature is lower than ~560°C, amorphous films are
formed. When growth temperature is in the range from
~560°C to ~600°C, crystalline films with fine grains ar
e
formed. At higher temperatures (from ~610°C to ~700°C),
columnar films are obtained. Compressive stress is present
in the amorphous and the columnar films, while the fine
-
grained films contain tensile stresses.

CWRU researchers have developed a
MultiPol
y

process
[1]

which
uses multilayer deposition to
precisely

control the
residual stress and stress gradient of large silicon structures.
Polysilicon deposited at 570°C or 615°C, which individ
u
a
l-
ly possess i
ntrinsic stresses but of opposite sign, are co
m-
bined into multilayer films. By proper engineering of the
individual layer thicknesses, films with controlled curv
a-
tures can be produced.

This, in turn, leads to the capabi
l
ity
of controlling the static shape
of MEMS comp
o
nents
.

The approach of this work is to study the general controll
a-
bility of the shape of a static circular mirror by researching
different designs for the mirror structure, incorporating the
MultiPoly

process and multilayer plate theory, and
stud
y
ing
the static shape by experiments.

DESIGN OF STATIC
CURVED

MICROMIRRORS

To make static curved micromirrors with different static
shapes using
MultiPol
y
, the multilayer film design is a key
step. The idea is similar to a bimetallic mirror where the
shape is controlled by computing stresses in different films,
but the result is a static, thermally stable mirror

an inhe
r-
ent advantage of
Mu
l
tiPo
ly.



Figure 1. Free body diagrams of individual layers of
laminated films (after
[9]
)


The multilayer micromirrors were designed using a m
e-
cha
n
ical model (Figure

1), where a linear elastic laminate
analysis was applied
[9]
. Individual layers’ thicknesses are
chosen by the calculation in the model so that the released
micromirror can achieve prescribed curvature. In this e
x-
pe
r
iment, 8
-
layer micromirrors with predicted radius of
cu
r
vature equal to 8.3mm are designed.
The total thickness
is 5 microns with individual layer thicknesses (starting with
the bottom
615°C layer) of 0.5, 1.0,0.3,1.0,0.35, 0.75, 0.5
and 0.6 microns.



Figure 2. Micromirror structure before release. The
basic structure is an 8
-
layer 5 µm thick M
ultiPoly film
with a small anchor at the bottom



Figure 3. Structure of the micromirror of Fig.1 after
release (side view, not to scale, and the curvature is
exagge
r
ated)



FABRICATION

Table 1 describes the detailed fabrication process. Only
standard sur
face micromachining processing for polysilicon
was used throug
h
out this study.

Figure 2 and 3 are side view sketches of a 300um
-
diameter
mirror before and after release. The micromirrors are a
n-
chored to the substrate at their centers. After released, the
m
icromirrors curved up into spherical shapes.


RESULTS AND DISCUSSI
ON

Axisymmetric curved micromirrors of different sizes (from
300um to 1mm in diameter) were produced by
MultiPoly
film fabrication. Figure 4 is a scanning electron micrograph
of two large c
ircular micromirrors (diameter=700um and
1mm, respectively) fabricated from an 8
-
layer 5 µm thick
MultiPoly
film. Figure 5 is a top view interferometer i
m
age
of a mirror with of a 9mm radius of curvature resulting
from the designed stress gradient in the f
ilm.


Table
1
. Processing steps used to fabricate the shaped
MEMS mirror


Name

Description

1

Si
3
N
4
deposition
-

100nm

LPCVD

2

LTO


200nm

Releasing oxide

LPCVD


3

Photolithography

mask (anchor holes)

4

Etch SiO
2
,

Strip PR


Reactive ion etch
(RIE)

6

Polysilicon De
p-
osition

LPCVD,
MultiPoly

recipe

7

Masking Oxide
Dep
o
sition

LPCVD


400nm

8

Photolithography

mask (stru
c
tures)

9

Masking Oxide
Etch


RIE using CHF
3

and
C
2
F
6

10

Polysilicon
Structure Etch

RIE using Cl
2

11

R
elease

Dissolve release o
x-
ide in aqueous HF


n
n
m
n
v
E
,
,

P
n
P
n
M
n
M
n
P
i
M
i
M
i
P
1
M
1
M
1
P
i
P
1
1
1
1
,
,
v
E
m

i
i
m
i
v
E
,
,

z

Figure 4.
Scanning electron micrograph of two circular m
i-
cromirrors fabricated from an 8
-
layer 5 µm thick Mult
i
Poly
film, diam
e
ter=700um and 1mm, respectively



Figure 5. Top view interferometer image of a

micromirror. It curves upward with a 9 mm radius of
curvature, calc
u
lated by curve fitting


The radius of curvature can be measured using an interfe
r-
ometer and calculated from the number of the interferom
e-
ter fringes and the diameter D of the m
i
cromirror
by

z
D
R
8
2



(1)

where z=n×λ, n is the number of fringes and λ is the wav
e-
length used by the interferometer. In this case, n =13,
λ=540nm and D=500um. This static mirror has a radius of
curvature of 9mm, which matches the prediction of 8.3mm

well.

For some different designs of cirlular micromirrors, both
axisymmetric dome shape (or spherical shape) and saddle
shape have been observed. For example, Figure 6 shows
two micromirrors fabricated with the same multipoly film
(3.1 microns in thicknes
s), but (a) is an axisymmetric mu
l-
tipoly micromirror with diameter=500 microns while (b) is
in a saddle shape with micromirror diameter=700 microns.


100

m

(a)

100

m

(b)

Figure 6. Two micromirrors with the same multilayer
design and different shapes (a) D=500 mic
rons, a
x-
i
symmeric shape and (b) D=700 microns, saddled
shape


Applying thin plate theory with an ener
g
y
-
based Rayleigh
-
Ritz approach gives the explanation for the different shapes
-

whether spherical domed shape or saddled shape
[10]
. In
fact, which shape occurs depends on the diameter to thic
k-
ness ratio and individual layer thickness to t
otal thickness
ratio. Generally, reducing the mirror’s diameter and the
i
n
dividual layer thickness and increasing the total thickness
will contribute to the stability of an axisymmetric stru
c
ture.
Another work
[4]

confirmed this approach by
optimi
z
ing
the stress layer thickness to the structure thickness ratio and
making the micromirror in a stable spherical shape without
buckling.

With the increased total thickness comparing to previous
saddle shape designs (total thickness= 3.1 microns), o
ur 5
-
micron micromirrors fabricated by the same 8
-
layer
Mu
l
t
i-
Poly

film are axisymmetric for all four different sizes.

Another interesting result is the distribution of the radii of
curvature vs. the micromirrors’ sizes.
Figure 7 shows the
measured radii o
f curvature (best fit) vs. diameters of the
micromi
r
rors.

Although the linear laminate model provides only one r
a
d
i-
us of curvature for each multilayer design, it is only an e
s-
tim
a
tion when geometrically nonlinear effects are small
[11]
. Specifically, a linear analysis over
-
predicts the out
-
of
plane
displacements. For smaller micromirror (such as
D=300 micron and D=500um), closer agreement between
the linear model prediction and the experimental results can
be reached. But for larger micromirrors, the increases of the
radii of curvature with the mirro
r size become more obv
i-
ous.

Radius of Curvature Results
4
5
6
7
8
9
10
300
500
700
1000
Diameter of the micromirrors(um)
Radiuis of Curvature (mm)

Figure 7. Measured radii of curvature (best fit) vs. d
i-
ameters of a total of 18 fabricated micromirrors on the
same wafer. Variation within the same size group is
shown using a bar and the mean value is
marked with a
square dot.



SUMMARY


The design and fabrication of a static curved micromirror is
presented. The measured radii of curvature agree to the
pr
e
diction that was calculated with a linear laminate model
using the design parameters. Conventional

surface
-
micromachining fabrication processes are used. Results of
different static shapes and the distribution of radius of cu
r-
vature vs. mirror sizes are discussed.

A future study will use underlying electrodes to apply ele
c-
trostatic force on the mirror

structure and study effect on
the shape of the actuated mirror.

The placement of ele
c-
trodes on the substrate will enable electrostatic actuation to
flatten this circular structure or change its radius of curv
a-
ture. This device can then act as a reflector

with variable
focal length, which has applications in optical switching.


ACKNOWLEDGMENTS

This work was supported in part by NASA under grants
NAG3
-
2578 and NAG3
-
2799.

REFERENCES

[1]

J. Yang, H. Kahn, A.
-
Q. He, S. M. Phillips, and A.
H
. Heuer, "A new technique for producing large
-
area as
-
deposited zero
-
stress LPCVD polysilicon
films: the MultiPoly process,"
Jou
r
nal of Micro
e-
lectromechanical Systems
, vol. 9, pp. 485
-
494,
2000.

[2]

R. A. Conant, J. T. Nee, K. Y. Lau, and R. S. Mu
l-
ler, "A
Flat High
-
frequency Scanning Micr
o-
mi
r
ror,"
Proceedings of the Solid
-
State Sensor and
Actuator Worshop
, pp. 6
-
9, 2000.

[3]

T. G. Bifano, "Elimination of stress
-
induced cu
r-
vature in thin
-
film structures,"
Journal of Micro
e-
lectromechanical Systems
, vol. 11, p
p. 592
-
597,
2002.

[4]

M. T.
-
K. Hou, K.
-
M. Liao, H.
-
Z. Yeh, P.
-
Y.
Hong, and R. Chen, "Design and fabrication of
surface
-
micromachined spherical mirrors," pr
e-
sented at IEEE/LEOS International Conference on
Optical MEMS, 2002.

[5]

B. Mi, D. A. Smith, F. Merat
, H. Kahn, A. H.
Heuer, and S. M. Philips, "Static and Electrically
Actuated Shaped MEMS Mirrors," In preparation.

[6]

C.
-
H. Han and E. S. Kim, "Micromachined Pi
e-
zo
e
lectric Ultrasonic Transdu
c
ers on Dome
-
Shaped
-
Diaphragm in Silicon Substrate," pr
e
sen
t-
ed at

IEEE Ultrasonics Symposium, Orlando,
1999.

[7]

E. S. Kim, R. S. Muller, and P. R. Gray, "Int
e
gra
t-
ed Microphone with CMOS Circuits on a Si
n
gle
Chip," presented at IEEE International Ele
c
tron
Devices Meeting, 1989.

[8]

D. Scribner, M. Humayun, B. Justus, C.

Merritt, R.
Klein, J.G. Howard, M. Peckerar, F. Perkins, L.
Johnson, W. Bassett, P. Skeath, E. Margalit, Kah
-
Guan Au Eong, J. Weiland, E. de Juan, J. F. Jr., R.
Graham, C. Trautfield, and S. Taylor,
"INTRAOCULAR RETINAL PROSTHESIS
TEST DEVICE," presented
at 23rd Annual Inte
r-
national Conference of the IEEE Engineering in
Medicine and Biology Society, Istanbul, Turkey,
2001.

[9]

A. Ni, D. Sherman, R. Ballarini, H. Kahn, B. Mi,
S. M. Phillips, and A. H. Heuer, "Optimal Design
of Multilayered Polysilicon Films

for Pr
e
scribed
Curvature," in
Mechanical Properties of MEMS
Structures,
, R. B. a. S. M. A. W.O. Soboyejo, Ed.:
Kluwer Academic Publis
h
ers, In print.

[10]

M. W. Hyer and A. B. Jilani, "Deformation cha
r-
acteristics of circular RAINBOW actuators,"
Smart Mater
ials and Structures
, vol. 11, pp. 175
-
195, 2002.

[11]

M. W. Hyer and A. B. Jilani, "Predicting the A
x-
i
symmetric Manufacturing Deformatons of Disk
-
Style Benders,"
Journal of Intelligent Material
Systems and Structures
, vol. 12, 2000.