A High Stroke Actuator Micro-mirror Array Designed for Adaptive

Optics

Quan Sun

1,2,3

, Baozhu Yan

1,2,3

and Yi Yang

1,2,3

College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha, China

State Key Laboratory of Pulsed Power Laser Technology, Changsha, China

Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha, China

Keywords: Adaptive Optics, MEMS, High Stroke, Micro-mirror Array.

Abstract: A micro-mirror with large out-of-plane displacement actuator with three polysilicon layers fabrication process

is designed for adaptive optics application. The optimized micro-mirror actuating structure consists of three

individual levers, each of which is actuated by electrostatic attractive force with the plane-parallel structure

to produce a large upward displacement at the end of the long arm of the lever with the lever principle. Finite

element analysis (FEA) models are built to calculate the maximum upward displacement of the long arm. The

actuator with 320 um long arm and 80 um short arm were designed by 2um thick Poly1 layer, while the

electrodes were designed by Poly0 and mirror plate was designed by Poly2 with a polish process afterwards.

The micro-mirrors can be tightly arranged in a hexagonal array to be applied in adaptive optics (AO) system

as a deformable mirror. An AO simulation system is built to test the aberration correction effect of the micro-

mirror array. The results showed that the 61 micro-mirror array is better than 37 micro-mirror array in

aberration correction, which showed good application prospect of this high stoke micro-mirror array in AO

systems.

1 INTRODUCTION

Recently, microelectromechanical systems (MEMS)-

based micromirrors have received much attention.

They have been applied in a wide range of areas, such

as in optical switches (

Chen W. C., et al., 2003) (Tsai

., et al., 2015) and displays

(Yan J., et al., 2001)

(Freeman M. O., 2003), high performance imaging

including biomedical imaging (Zhang Y. H., et al.,

2006) (

Manzanera S. , et al., 2011) and astronomy

imaging (Blain Celia, 2013) (

Morzinski K. M. et al.,

2006), and laser-based communication (L. MC A., et

al., 2002) (

Li J., et al., 2005) in adaptive optics.

MEMS-based micromirrors have higher operating

speed and lower mass than traditional technology

fabricated deformable mirrors, and a potential for

lower cost and integration with electronics through

batch micro-fabrication processes. The latest

developments in adaptive optics for compensating

large amplitude, high order wavefront aberrations

have pushed for high stroke, high spatial resolution

deformable mirrors. Many recent papers have

addressed design, modeling and fabrication of various

types of micromirrors with large stroke (

Dagel D. J., et

al., 2006) (Sun Q., et al., 2010) (Lin P. Y., et al.,

2011). The various micromirror prototypes are

fabricated with different processes: commercial

standard processes or custom-designed processes.

Compared with custom-designed processes,

commercial standard processes provide mature and

stable runs, with a low cost and short fabrication cycle

but with strict design rules which limit the

performance of the fabricated device. One of the

industry's longest-running standard processes is

Multi-User MEMS Processes (MUMPs)

(Carter J., et

al., 2005). It provided a three layers fabrication

process and has been widely chosen for micromirror

prototypes fabrication

(Zhang X. M., et al., 2001) (Sun

Q., et al., 2009). The defect of this process is that it is

difficult to make a smooth and flat mirror surface

without polish after Ploy3 deposition. Most of the

micromirrors reported to date employ electrostatic

actuators because of their low power consumptions

and fast response time (Zhang J. L., et al., 2003)

(Chiou J. C., et al., 2007). In most applications,

electrostatic actuators are preferred because of

relatively simple in terms of design and fabrication;

however, it suffers from the pull-in phenomenon,

which limits its useful scan range. In this paper, we

168

Sun, Q., Yan, B. and Yang, Y.

A High Stroke Actuator Micro-mirror Array Designed for Adaptive Optics.

DOI: 10.5220/0009171501680173

In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 168-173

ISBN: 978-989-758-401-5; ISSN: 2184-4364

proposed a micromirror with large-stroke (about 8

um) electrostatic actuators designed by upgraded

three layers polysilicon surface micromachining

technology. The micromirror structure has a

hexagonal mirror plate actuated by three levers and

controlled by three electrodes, which exhibits a large

stroke and tip/tilt/piston motion.

In this paper, we proposed an upgraded process

with polish operation after the third polysilicon layer

to design a large stroke micro-mirror. The paper is

organized as follow. Section 2 describes the design

details of micromirror structure with simulation

results by finite element method (FEM) in COMSOL

Multiphysics software which is provided software

solutions for multiphysics modeling by the COMSOL

Group. Section 3 reports two kind of micromirror

arrays applied in adaptive optics simulation system to

compare their compensation effect. And conclusions

are presented in Section 4.

2 DESIGN OF MICROMIRROR

In the fabrication process, polysilicon is used as the

structural material and phosphosilicate glass (PSG) is

used as the sacrificial material. Twelve lithographic

masks are used to pattern seven physical layers. The

physical layers, from the substrate up, are as follows:

0.6μm of nitride (Nitride), 0.5μm of polysilicon

(Poly0), 2.0μm of PSG (Oxide1), 2.0μm of

polysilicon (Poly1), 0.75μm of PSG (Oxide2), 2.5μm

of polysilicon (Poly2) and finally 0.5μm of gold

(Metal), deposited on top of a thin adhesion layer of

chromium. Poly1 and Poly2 are usually used to form

the releasable structural layers, while Poly0 is fixed

and generally used to form addressed electrodes and

electrical interconnects. Nitride is used as electrical

insulation between the polysilicon layer and the

substrate. Metal layer is coated on top of Poly2 to

serve as reflective surface. In order to reach a high fill

rate of micro-mirror array reflect surface, we used

Poly2 to design the mirror plate to a hexagon shape.

As we know, there is residual stress after deposited

polysilicon released, which will make the plate to

bending deformation. One way to reducing the

deformation is increasing the thickness of plate.

While Poly1 is left to be made actuator, we design a

3um wide frame with Poly1 to connect the plate at its

edge. In order to simulate the characters of our

designed micromirror structure, a 3D solid model is

built in COMSOL Multiphysics. The typical material

parameters are acquired from COMSOL material

library. Figure 1 shows the FEM simulation result of

stress-induced deformation of mirror plate. The

hexagonal side length of mirror plate model is 450um.

The bowing value from the edge of plate to the centre

is 0.517um.

Figure 1: Simulation result of mirror plate deformation with

FEM in COMSOL.

Figure 2 shows the bowing value versus mirror plate

size. As shown, the bowing value increase with plate

size. The AO applications need the mirror surface

flatness no more than wavelength/5. So that we

decide to make the mirror plate size to be 250um.

0 100 200 300 400 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

The bowing value (um)

Size of plate (um)

Figure 2: Simulation result of the bowing value versus

mirror plate size.

We chose Poly1 to make electrostatic actuators. In

order to enlarge the upwards displacement of mirror

plate, lever structure is decided to be used as actuator.

The enlarge factor is equal to leverage ratio. And the

lever actuator shape is also influenced by the mirror

plate shape. As the mirror plate is hexagonal, and side

length is 250um, three actuators need to be arranged

symmetrically to achieve the mirror plate have three

degrees of freedom motion. In order to obtain the

longest lever length and the largest control electrode

area, three lever actuators are designed to cross each

other and have a special shape layout as shown in

Figure 3(a) which could improve the effective

utilization of Poly1 and reduce the print effect to

above layer. Each lever has two anchor which connect

the lever structure to the substrate and support

cantilever to serve as rotation axis of the lever. The

A High Stroke Actuator Micro-mirror Array Designed for Adaptive Optics

169

joint connects the Poly1 lever to Poly2 mirror plate.

Figure 3(b) shows the section view of lever actuator.

The lengths of two lever arms is represented by L1

and L2. While L represent the whole length of the

lever, which equals 400um. H represents the lifting

height, and g represents the gap between Poly2 and

Poly1 after the sacrificial layer is etched. L3 represent

the width of electrode fabricated by Poly0. Figure

3(c) shows the 3D model that we used to make

simulation analysis by FEM.

(a) top view of three lever actuator layout

(b) section view of lever actuator

Figure 3: Lever actuators structure design detail.

Table 1 shows the FEM simulation results of pull-in

voltage and maximum lifting height of levers with

different structural parameters by Comsol. The

conclusion showed that increasing resisting arm

length would improve the max lifting height while

would bring in high pull-in voltage. Higher voltage

would cause more complicated control circuit and

higher safety risk. On balance, we chose the

parameters with L1=320um, L2=80um. And Figure 4

shows the simulation results of lifting height with the

voltage applied on the electrode at the model

parameters identified above. We can draw the

conclusion that with this lever actuators, the micro-

mirror could get a large out-of-plane piston

displacement up to 8um.

Table 1: Simulation results of pull-in voltage and maximum

lifting height of levers with different structural parameters.

Length of

resisting arm

L1 (um)

Length of

power arm

L2 (um)

Pull-in

Voltage

(V)

Lifting

height

(um)

335 65 420 10.30

330 70 357 9.42

325 75 299 8.66

320 80 246 8.00

315 85 204 7.41

310 90 169 6.88

305 95 141 6.42

300 100 119 6.00

0 50 100 150 200 250

Lifting Height (um)

Voltage (V)

Figure 4: Simulation results of lifting height of lever with

the voltage.

3 MICROMIRROR ARRAY

APPLIED IN AO

Two size of micro-mirror arrays were built with our

designed high stroke micro-mirror. As shown in

Figure 5, one array consists of 37 micro-mirrors to

form an about 1.75mm side length hexagon, while the

other consists of 61micro-mirrors to form an about

2.2mm side length hexagon. These arrays would be

use in adaptive optic (AO) system after fabrication.

We tested these arrays in an AO simulation

system. The simulation system is built in SeeLight, a

software tool for high fidelity modelling of advanced

PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology

170

(a) 37 micro-mirror array

(b) 61 micro-mirror array

Figure 5: Layouts of 37 micro-mirror array and 61 micro-

mirror array.

optical systems such as laser active illumination and

object detection systems which is developed by

National University of Defense Technology and

Institute of software, Chinese academy of Sciences.

The principle of the software is based on wave optics

theory with performing propagation by the angular

spectrum theory and fast Fourier transform. As each

micro-mirror in the array could make tip/tilt/piston

motion. The max piston height is 8um. We used its

characteristics to build a micro-mirror array model in

the simulation system. Figure 6 shows the models

schematic of AO simulation system. In the simulation

system, the plane wave model outputs a plane wave

beam, which transmits the aberration model and

distorts its wavefront with the aberration in medium.

The Hartmann Sensor model will measure the

distorted wavefront, and calculates the control orders

with the Centroid Algorithm model and the Control

Operation model. The Micromirror Array Model

receives the control voltage data from the Closed-

loop Feedback model and operates every micromirror

to the required height to correct the aberration

wavefront of the input beam.

Figure 6: The simulation model schematic of AO system

built in SeeLight software.

Figure 7 shows the aberration wavefront map

generated in simulation system which PV aberration

is about 6um. The size of map is transformed to be

equal to the two sizes of micromirror array, separately.

And Figure 8(a) shows the piston motion heights of

37 micromirror array, while Figure 8(b) shows that of

61 micromirror array. The RMS of corrected

residuals were 293nm and 123nm respectively.

Apparently, the micro-mirror number of the array is

larger, the AO aberration correction is better. But

larger number of micro-mirror would bring out more

complex control circuit and closed-loop algorithm.

Figure 7: The aberration wavefront map for AO to measure

and correct.

A High Stroke Actuator Micro-mirror Array Designed for Adaptive Optics

171

(a) 37 micro-mirror piston motion heights

(b) 61 micro-mirror piston motion heights

Figure 8: The aberration wavefront map for AO to measure

and correct.

4 CONCLUSIONS

In this paper, a micro-mirror with large out-of-plane

displacement actuator is designed for adaptive optics

application. The micro-mirror with three lever

actuators is designed by optimized three layers

fabrication process. Micro-mirror structure models

were built in Finite element analysis (FEA) software

Consol to get the optimum structural parameters. The

micro-mirrors were tightly arranged in a hexagonal

array to serve as a deformable mirror. An AO

simulation system was built to test the aberration

correction effect of the micro-mirror array. The

results showed that the 61 micro-mirror array was

better than 37 micro-mirror array in aberration

correction, which showed good application prospect

of this high stoke micro-mirror array in AO systems.

ACKNOWLEDGEMENTS

The main author would like to acknowledge Professor

Edmond Cretu at University of British Columbia in

Canada who provided the opportunity for me to visit

and start the research on this area in his lab.

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