IMPLEMENTING VERTICALLY
STANDING MIRRORS USING SOI
MUMPS TECHNOLOGY
Simon Fraser University (Engineering Science)
Group Members:
Desmond Cheung
Sunjaya Djaja
Nima Jahedi
Daniel Kim
Abstract
The purpose of this particular project is to implement vertically standing mirrors using
SOI MUMPS technology, offered by Cronos. These mirrors are relatively small,
approximately 100 µm × 100 µm built on a SOI substrate. An important aspect of this
design is the trench (holes) in the SOI substrate. Such technology has been classified in
general terms as MEMS (Micro-Electro-Mechanical Systems). More specifically, this
implementation is categorized as optical MEMS. It is envisioned that such systems would
be used in telecommunication/photonic switching systems. The designs that are presented
in this paper are novel and the existence of anything similar if they do exist is unknown
to the authors.
Table of Contents
Table of Contents................................................................................................................ 2
Table of Figures .................................................................................................................. 2
1
Introduction................................................................................................................. 3
2
SOI MUMPS Technology........................................................................................... 3
3
Previous Attempt ........................................................................................................ 4
4
Design Approach ........................................................................................................ 5
5
Design Philosophy ...................................................................................................... 5
6
New Designs ............................................................................................................... 5
6.1
Desmond’s Design .............................................................................................. 5
6.1.1
Explanation ................................................................................................. 5
6.1.2
Layout of Desmond’s Design ..................................................................... 8
6.2
Sun’s Design ....................................................................................................... 9
6.2.1
Explanation ................................................................................................. 9
6.2.2
Layout of Sun’s Design .............................................................................. 9
6.3
Daniel’s Design................................................................................................... 9
6.3.1
Explanation ................................................................................................. 9
6.3.2
Layout of Daniel’s Design ........................................................................ 10
6.4
Saewon’s Design............................................................................................... 10
6.4.1
Explanation ............................................................................................... 10
6.4.2
Layout of Saewon’s Design ...................................................................... 11
6.5
Nima’s Design .................................................................................................. 11
6.5.1
Layout of Nima’s Design.......................................................................... 12
7
Future Work and Conclusion .................................................................................... 13
Appendix........................................................................................................................... 14
Calculations................................................................................................................... 14
Length of torsion bar................................................................................................. 14
Cantilever beam ........................................................................................................ 15
Guided-end beam ...................................................................................................... 16
Spring........................................................................................................................ 17
Table of Figures
Figure 1: Typical cross section of SOI MUMPS structure ................................................. 3
Figure 2: Failed structure designed in previous attempt where (a) and (b) are parts of the
same structure ............................................................................................................. 4
Figure 3: Propping up the micro-mirror in Desmond’s design........................................... 6
Figure 4: Cross-section of propping up the micro-mirror in Desmond’s design................ 6
Figure 5: The locking mechanism in Desmond’s design.................................................... 7
Figure 6: Layout of Desmond's design ............................................................................... 8
Figure 7: Layout of Sun's design ........................................................................................ 9
Figure 8: Layout of Daniel's design.................................................................................. 10
Figure 9: Layout of Saewon's design................................................................................ 11
1 Introduction
The purpose of this project is to design vertically standing mirrors using SOI MUMPS
technology offered by Cronos. These mirrors are in the order of 100 µm × 100 µm each.
To erect these mirrors would require the application of a mechanical force on “handles”
on these structures.
These types of technology have been categorized more generally as MEMS (MicroElectro-Mechanical Systems). Such systems are characterized by the construction of
mechanical structures which are in the micron range and in some ways controlled in an
electrical manner. Because the project is to be used for its optical properties, these types
of MEMS are referred to as optical MEMS.
MEMS is forecasted to gain a revenue of up to $10 billion by 2006 (Nanoelectronics
Planet). As a result, this is a subject of much current research.
2 SOI MUMPS Technology
SOI is an abbreviation for Silicon-On-Insulator. The origin of this word is clearly shown
in Figure 1. There is silicon on top of an oxide layer which is the insulator layer. Below
the insulator layer is the substrate layer.
Figure 1: Typical cross section of SOI MUMPS structure
The dimension of each layer has been defined together with its tolerances.
The most crucial reason why the SOI MUMPS technology was chosen instead of the
other MUMPS technologies is because this technology provides highly planarized
surfaces. This is essential for the creation of mirrors because mirrors need to be flat. The
mirrors are then formed by depositing metal on the flat surface.
Several ensuing advantages come from the choice of using SOI MUMPS, namely
increased reliability in fabrication as well as lower cost. These advantages can be
attributed to the fact that because less number of layers are needed, less fabrication steps
are needed. SOI MUMPS has only one structural layer whereas other technologies like
PolyMUMPS has three structural polysilicon layers.
The disadvantage of using this technology is that there is only one structural layer,
namely the silicon layer. This excludes the use of conventional hinges and joints, making
the design more difficult and more restricted.
3 Previous Attempt
An important part in coming up with new designs is studying previous attempts to do the
same thing. In this case, the previous attempts have failed. Even so, studying where
things have gone wrong can provide valuable insight.
(a)
(b)
Figure 2: Failed structure designed in previous attempt where (a) and (b) are parts of the same
structure
Referring to Figure 2, the principle of operation is as such: when the pushing pad is
pushed down with a micro-probe, this would lift the mirror up. The torsion hinge would
allow the mirror to move up while at the same time being attached to the silicon layer.
Then when the mirror is in the upright position, the two pulling bars would be pulled by
using the pulling rings. This ensures that the locking mechanism latch thus making sure
that the mirrors would stand vertically. There are a total of three operations needed, one
push and two pulls. Suspensions are present to make sure that the whole pulling bar
remains in the surface plane, so that the locking mechanism will be in the same plane as
the pulling bar.
The mechanism for latching the mirror in place is faulty. Looking at Figure 2, one could
see that the dimensions for the locks are incorrect. The suspension springs broke resulting
in important structures floating away during fabrication. Several critical features that are
supposed to be in the same plane are misaligned as was shown in Figure 2.
4 Design Approach
Intuitive ideas of how micro-mirrors can be propped were made. Then cardboard models
were made to verify intuition. The models filter off what are obviously unrealizable
designs. Calculations were then made for the selected models that were to be
implemented.
Taken into account were the scaling effects. Scaling effects refer to the effects of taking
models in human scale and scaling them down to the micron range. An example of this
would be objects of human scale where gravity is the predominant force to micro scale
where forces of adhesion and friction are more predominant, while that of gravity is
negligible in the presence of these other forces.
5 Design Philosophy
In the process of designing, a number of design points were emphasized, although, in
some cases, they might have been compromised for some other reasons. These points
constitute the group’s design philosophy.
The designs must be reliable, simple and requires minimal number of operations to prop
up the micro-mirrors. The designs were made in keeping with this spirit.
6 New Designs
The group’s designs will be categorized based on the designer’s name. There is Desmond
Cheung, Sunjaya Djaja, Daniel Kim, Saewon Lee and Nima Jahedi.
6.1 Desmond’s Design
6.1.1 Explanation
To get an idea of Desmond’s design, please take a look at Figure 3, Figure 4 and Figure 5
first. Details of the necessary calculations are shown in Appendix 1.
(a)
(b)
(c)
Figure 3: Propping up the micro-mirror in Desmond’s design
d2
d1
(a)
(b)
(c)
Figure 4: Cross-section of propping up the micro-mirror in Desmond’s design
Figure 5: The locking mechanism in Desmond’s design
The mirror is formed on top of a trench, which is a hole in the substrate. A microprobe
will be used to lift the mirror from the backside of the mirror as shown in Figure 3(a) and
3(b). Once in the upright position, there will be a counter-torque that seeks to push the
mirror back into the original position. To ensure that the mirror stands upright, any one of
the two forces applied at different positions with respect to the rotational axis (torsion
hinge) need to be applied. The first force as shown by the orange arrow in Figure 3(c) is
applied below the rotational axis and is in front of the mirror. The second force as shown
by the yellow arrow in Figure 3(c) is applied above the rotational axis and is behind the
mirror. In theory any one of these two forces could be applied exclusively, however, in
Desmond’s design, the two forces are applied, one above the rotational axis and the other
one below the rotational axis.
These two forces are applied by the locking mechanism shown in Figure 5. When the
mirror is in the upright position and then the pulling bar is pulled by means of exertion of
force on the pulling ring, Feature (2) as shown in Figure 5 moves into Feature (1) which
is under the rotational axis, while at the same time Feature (4) moves into Feature (3),
which is above the rotational axis. The combination of Feature (1) and (2) is to exert a
force under the rotational axis, whereas Feature (3) and (4) applies a force above the
rotational axis. Once in the U-shaped enclave of Feature (1) and Feature (3), the pulling
bar is released, and the spring will exert a force, pulling the bar in the direction in which
it has been pulled, locking the mirror into an upright position.
6.1.2 Layout of Desmond’s Design
(a)
(b)
(c)
Figure 6: Layout of Desmond's design
6.2 Sun’s Design
6.2.1 Explanation
These set of designs are essentially variants of Desmond’s designs. The emphasis is on
making Feature (2) and (4) as shown in Figure 5 shorter. The rationale in making these
shorter is so that it will reduce the chance of them breaking. As the designs are almost the
same as Desmond’s not much more details would be given and hence only the layouts
will be shown.
6.2.2 Layout of Sun’s Design
(a)
(b)
Figure 7: Layout of Sun's design
6.3 Daniel’s Design
6.3.1 Explanation
The complete layout of Daniel’s design is shown in Figure 8. The design consists of two
separate structures: a 300umX300um mirror and a supporting structure with two arms
surrounding the mirror. Two push pads attached at end of each structures form work
spaces to manipulate these structures. In order to bring up the mirror out of the plane, the
following steps are taken: Using a microprobe, push up and hold the mirror well above
60 degrees. Next using another microprobe, push up and hold the supporting structure
around 40 to 50 degrees. If the supporting structure hits the mirror on the way up, push
up the mirror some more until the mirror does not stand in the way. Finally, release the
mirror and hope for the tooth on the supporting structure finds its way into one of the
holes on the mirror. If the tooth cannot get into one of the holes, adjust the position of
the supporting structure by pushing it up using the microprobe. After locking the tooth
inside the hole, the current position is retained even after fully releasing the microprobes.
The angle of the vertical standing mirror depends on which holes the teeth lock into.
Lower the teeth out of the plane, higher the angle of the mirror.
The major advantage of this design is simplicity. Both the concept and actual layout are
easy and simple. Also, the operations required with microprobes are very simple and
thus eases testing. Because of this design simplicity, this design has high probability of
working compared to other designs. The design, however, suffers from two main
disadvantages: only has maximum angle ≈ 60 deg, and requires two microprobes.
There are several variations with Daniel’s design. Tooth sizes, size of holes on the mirror,
and length of the torsion bars are varied in different designs. The calculations for the
length of the torsion bars are shown in Appendix.
6.3.2 Layout of Daniel’s Design
Mirror
Tooth
Support
(a)
(b)
Figure 8: Layout of Daniel's design
6.4 Saewon’s Design
6.4.1 Explanation
Saewon’s design is a variant of Daniel’s design. However, this design resolves one of the
disadvantages of Daniel’s design that is, a limited angle of the standing mirror. Saewon’s
design can acquire maximum 85 degrees out of plane standing mirror. By extending the
location of teeth beyond the axis of rotation of the mirror, greater angle is achieved when
teeth locks into the holes. One major drawback of this approach is that, the mirror has to
be pushed up more than 90 degrees to give space for the teeth to come out of the plane.
If not, the teeth will hit the bottom of the mirror when brought up, preventing them from
getting into the holes.
As the designs are almost the same as Daniel’s not much more details would be given and
hence only the layouts will be shown.
6.4.2 Layout of Saewon’s Design
Figure 9: Layout of Saewon's design
6.5 Nima’s Design
Nima’s design implements a vertical standing mirror with only one push operation. The
mirror and the supporting structure are connected with connection bars (shown in Figure
10) to make them one structure. In this design, two push-pads are placed just in case of
failure of one push design so we can cut a connection bars with an optical laser and
experiment with two forces as Daniel’s design.
We have overall of six different implementations of this design, one of which is
shown in layout Figure10(a). The underlying concept here is, by connecting the mirror to
the supports, supports will follow the mirror. On the way up, the teeth on the supporting
structure will hit the bottom of the mirror in the way up and manage to pass it, then by
moving down the mirror again the teeth can get lock into the holes. In this particular
design the failure probability is higher because the teeth might miss the bottom of the
mirror completely or get stuck.
For implementation variations, we changed the connecting bar to a HSpring
shown in Figure10 (b) and varied the shape of the holes. In one of the implementations,
we used HSpring, which give us a higher degree of “out of plane” flexibility. A new
locking mechanism (shown in Figure 10(c)) is used in some different variations. This
mechanism guides the teeth to get into the provided area, but does not let them out. This
mechanism does not have previous problem and as long as teeth hit the mirror the lock
will work. Unfortunately, we did not have any simulation program, which can support
our theories. We put four different variations of the design with different position of this
lock mechanism, so for future attempts designers have a good intuition on what are the
appropriate distances and bars for this lock mechanism.
6.5.1 Layout of Nima’s Design
Lock (new)
Connecting
Bar
HSpring
(a)
(b)
This distance is
8u
The tooth is 10u x 10u and
the gap is 8u, which can
increase one way.
Can move in one
direction only
(downwards)
(c)
Figure 10: Layout of Nima's design
7 Future Work and Conclusion
The design was submitted to Cronos for fabrication on 23 June 2003. It is anticipated that
the fabricated devices would be returned to SFU sometime in October 2003. Testing will
then commence. Depending on the success of the design, publications might be made in
the process.
The complete chip layout is shown below:
Appendix
Calculations
Length of torsion bar
Torsion bar allows the mirror to be flipped up 90˚ (vertical) up from the horizontal plane.
A minimum length of the torsion bar is required for it to be able to twist 90˚. On the
other hand, a torsion bar too long would allow too much of undesired movement along
the x or z plane.
The thickness of the bar is determined by the technology, in this case, 10µm. A square
cross-section creates the optimum geometry for twisting motion. The following
calculation shows how to obtain the length1.
a = 5µm, K=2.25a4
τ max = max . shear stress
1.35aGφ
=
l
1.35(5µm)(73GPa)(π / 2)
=
< yeild strength < 0.7 × 7GPa
l
∴ l>
l
2a
2a
1.35(5µm)(73GPa)(π / 2)
= 165µm
4.7GPa
We have used a minimum of 200µm in our designs to give some slack for bars to
minimize the chance of breaking them. This calculation applies to all the designs above.
1
J. Judy, Batch-fabricated ferromagnetic microactuators with silicon flexures. PhD thesis, University of
California, Berkeley, 1996.
Cantilever beam
A cantilever beam is used in the locks for both Desmond’s and Sun’s designs. Since the
locks go out of plane (above or below) to hold the mirror, it will bend to a certain degree.
The amount of bending depends on the length and thickness of the beam.
The thickness of the beam is fixed (10µm). We calculate the beam length as shown
below.
wh 3
Iy =
12
Fl 3
Max. deflection = δ max =
3EI
h
Since stress = strain × E = 0.036 E
w
F
F
and Stress σ = =
A hw
2
Thus, Fmax = 0.036 EA
δ max =
l
Fl 3
0.036 EAl 3
0.036hwl 3
=
=
3EI 3E ( wh 3 / 12)
wh 3 / 4
4(0.036)l 3
=
h2
l=
3
δ max h 2
0.144
For a deflection of 10µm, the length required is 19.1µm. However, the amount of
deflection with this length is not realistic as the calculation assumes small deflection. We
made the length a lot longer and verified it with the efunda.com3 to make sure that it is
realistic.
If we use 1% of the yield strain for the above calculation, which would give better
reliability, we would need a length of 88.55µm.
2
J. Judy, Batch-fabricated ferromagnetic microactuators with silicon flexures. PhD thesis, University of
California, Berkeley, 1996.
3
www.efunda.com
Guided-end beam
Guided-end beam is used for the springs maily for holding the lock to prevent it from
floating and partly for providing a force pulling back the locks after they have locked the
mirrors.
The thickness of the beam is fixed (10µm). We calculate4 the beam length as shown
below. To increase the reliability of the beams, we have used 1% of the yield strain.
Iz =
hw 3
12
Fl 3
12 EI
Since stress = 1% × strain × E = 0.00036 E
Max. deflection = δ max =
F
F
=
A hw
= 0.00036 EA
and Stress σ =
Thus, Fmax
δ max
l
h
w
Fl 3
0.00036 EAl 3 0.00036hwl 3
=
=
=
12 EI 12 E (hw 3 / 12)
hw 3
=
0.00036l 3
w2
l=3
δ max w 2
0.00036
100 µm(4 µm) 2
0.00036
= 164.41µm
=3
For a deflection of 100µm, the length is 164.41µm. We used serpentine pattern to replace
one guided-end beam to decrease the required length of the support to reduce the total
area of the design.
4
J. Judy, Batch-fabricated ferromagnetic microactuators with silicon flexures. PhD thesis, University of
California, Berkeley, 1996.
Spring
A spring is used to provide the main force to keep the lock in place after the mirror has
been locked. This is implemented with a serpentine spring. The following calculation
shows the number of bars needed for springs of width 4µm and length of 72µm.
To increase the reliability of the springs, we give use 1% of the theoretical yield strain.
Fmax = -kx = -x/C = (1%)(0.036)EA
and
C = (L3/EI) (N/12 + 1/48)
5
N is the number of bars, C is the compliance, L is the length, and E is the elastic modulus.
x
= 0.00036 EA
C
 L3 N 1 
− x = 0.00036 EA ( + ) 
 EI 12 48 
 L3
N 1 
( + ) 
= 0.00036hw 3
 hw / 12 12 48 
−
 12 L3 N 1 
= 0.00036 2 ( + ) 
 w 12 48 
For L = 72 µm, h = 4 µm, x = 100 µm

xw 2
1 
− 
N = (12)
3
 (0.00432)( L ) 48 
 100 µm(4 µm) 2
1 
= (12)
− 
3
48 
 (0.00432)(72µm)
N = 12
We have used almost triple the number of bars for the springs to minimize the chance of
breaking them.
5
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