Flexure mount for a MEMS deformable mirror for the GPI Planet Imager

Alexis Hill1, Steven Cornelissen2, Daren Dillon3, Charlie Lam2, Dave Palmer4

, Les Saddlemeyer1

1National Research Council of Canada, Herzberg Institute of Astrophysics 2 Boston Micromachines Corporation, 30 Spinelli Place, Cambridge, MA 02138, ph. 617-868-4178

fax. 617-868-7996 3 University of California, Santa Cruz, Laboratory for Adaptive Optics, 1156 High Street, Santa

Cruz, CA 95064 ph.831-459-1257 Fax. 831-459-3382 4Lawrence Livermore National Laboratory, 7000 East Ave., Livermore CA 94550-9234, phone 925-

422-1100 fax. 925-422-1370

ABSTRACT

Small deformable mirrors (DMs) produced using microelectromechanical systems (MEMS) techniques have been used in thermally stable, bench-top laboratory environments. With advances in MEMS DM technology, a variety of field applications are becoming more common, such as the Gemini Planet Imager’s (GPI) adaptive optics system. Instruments at the Gemini Observatory operate in conditions where fluctuating ambient temperature, varying gravity orientations and humidity and dust can have a significant effect on DM performance. As such, it is crucial that the mechanical design of the MEMS DM mount be tailored to the environment. GPI’s approach has been to mount a 4096 actuator MEMS DM, developed by Boston Micromachines Corporation, using high performance optical mounting techniques rather than a typical laboratory set-up. Flexures are incorporated into the DM mount to reduce deformations on the optical surface due to thermal fluctuations. These flexures have also been sized to maintain alignment under varying gravity vector orientations. This paper is a follow-up to a previous paper which presented the preliminary design. The completed design of the opto-mechanical mounting scheme is discussed and results from finite element analysis are presented, including predicting the stability of the mirror surface in varying gravity vectors and thermal conditions. Keywords: flexures, MEMS, opto-mechanical mount, deformable mirror, adaptive optics

1. INTRODUCTION The Gemini Planet Imager is being built for the Gemini Observatory for the purpose of directly detecting and characterizing extrasolar Jovian planets. GPI incorporates its own AO subsystem, including an 11x11 actuator “woofer” DM to reduce residual wave front error to a level controllable by a finer “tweeter” mirror. A MEMS DM is used as the “tweeter” and is bracketed by two off-axis parabolas as shown in Figure 1.

Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II, edited by Ramón Navarro, Colin R. Cunningham, Eric Prieto, Proc. of SPIE Vol. 8450, 84500H

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Mirror surface

Chip carrier

Connectors

Figure 1. GPI AO bench.

The MEMS DM was developed by Boston Micromachines Corporation (BMC) in cooperation with Lawrence Livermore National Labs[1]. The MEMS DM reflects a corrected, collimated beam from the first parabola (OAP3) to the second parabola (OAP4).

Small (typically <1k actuators) MEMS DMs have been frequently been tested and operated in stable, bench-top, laboratory environments. The MEMS DMs are typically attached to a printed circuit board (PCB) using a zero-insertion force (ZIF) socket, which provides contact between the pins of the deformable mirror and the PCB. Generally the PCBs are made of some type of plastic and are in turn attached to an aluminum mount and aligned to a system on an optical bench. This has been generally adequate for the requirements of these MEMS systems as temperature and position of an optical bench in a lab is stable.

The MEMS DM in GPI has an increased size and number of actuators compared to prior MEMS DMs and will be operating in an environment that includes fluctuating temperature, humidity and changing orientation as the telescope tracks the sky. BMC has developed the MEMS DM with the mirror embedded in an alumina chip carrier, with connectors wire-bonded on the back (Figure 2).

Figure 2. MEMS DM, embedded in chip carrier.

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MEMS DM Chip Carrierand Flex Cables

FlexureAssembly

Micrometer

Base BracketAssembly

M6 MountingBolt

40

The chip carrier/MEMS DM assembly is then supported in a traditional opto-mechanical mount using flexures and a rigid base (Figure 3). Flexures are incorporated into the DM mount to reduce deformations on the optical surface due to thermal fluctuations to maintain alignment under varying gravity vector orientations.

Figure 3. CAD rendering of opto-mechanical mount of MEMS DM.

A previous paper[2] on this topic discussed the preliminary design of the flexure-based mount. In this paper, the completed design of the opto-mechanical mounting scheme is discussed and results from finite element analysis are presented, including predicting the stability of the mirror surface in varying gravity vectors and thermal conditions.

2. DESIGN FEATURES 2.1 Semi-kinematic mount

The base bracket assembly (Figure 3 and Figure 4) is attached to the optical bench (not shown) using three M6 screws. The MEMS DM is aligned with respect to the pupil according to the requirements in Table 1. Small micrometers (can be seen in Figure 3) are used to position the base bracket assembly. Each micrometer provides 1µ of resolution and 10mm of range for alignment purposes. Shims (not shown) provide height alignment between the bench and the base of the mount.

Figure 4. Mount for MEMS DM.

The interface between the base bracket and flexure assemblies is via three flat, machined pads, providing a repeatable semi-kinematic joint. Flatness in the joint was achieved by lapping the two parts together by hand with a small amount of diamond grit.

Clocking of the flexure assembly with respect to the optical path is provided using a micrometer . A spring-loaded set screw maintains contact between the shaft of the flexure assembly and a V-groove machined into the bore of the base

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bracket assembly. This locates the flexure assembly in the base bracket for repeatable positioning, especially during clocking adjustments. Once aligned, the assembly is secured by tightening all screws.

Table 1 – the MEMS DM Alignment tolerances

DOF Resolution Range of motion

Linear X 4 um (1% of actuator pitch) +/-4mm

Linear Y 4 um (1% of actuator pitch) +/-4mm

Linear Z (Piston) 50 um +/-2mm

Tip/tilt (rotX/rotY) 1 mrad +/-1deg

Clocking (RotZ) 10 mrad +/-5deg

The MEMS is a 64x64 array of actuators on 400micron pitch. The “best” 44x44 patch of actuators, that is, the patch with the highest number of working actuators, is positioned in the light path to provide AO correction. Sufficient clearance in Linear X and Y motions is provided through large clearance holes in the base bracket. Additionally, the mount is designed to allow the entire chip carrier to be rotated 180° about the optical axis to provide more flexibility for choosing the best patch of actuators with which to illuminate the pupil (Figure 5).

Figure 5. Pupil on MEMS DM.

2.2 Bond pads/flexures

The flexure assembly (Figure 6) consists of a flexure support and three blade flexures to compensate for temperature changes in the Gemini environment. The flexure support is made of 6061-T6 aluminum and the flexures are 303 stainless steel (shim stock). Flexures are attached to Kovar mounting pads, which are bonded to the surface of the chip carrier.

Figure 6. Flexure assembly.

The material for these pads is Kovar because it has a coefficient of thermal expansion similar to the alumina ceramic (the material used for the chip carrier). The Kovar pads are bonded to the alumina with Armstrong A12 adhesive using four

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small bond patches (to avoid the stress caused by larger bond areas), see Figure 7. A flexure assembly (shown in Figure 5), consisting of a flexure support ring and three flexures, are secured to the Kovar pads using stainless steel screws.

Figure 7. Four bond areas on pads.

Because working with adhesives successfully can be a “black art”, the process was refined and practiced on a blank substrate as shown in Figure 7. This bond was tested using a weight to ensure a nominal bond formed a sufficiently strong bond with the process used. Mass upward of 10kg was supported by the flexure without breaking the adhesive bond. The bond thickness of 0.074 um was maintained by using small glass beads mixed in with the adhesive.

Figure 8. Test bond

The bond pads were attached to the surface of the chip carrier by first assembling the flexure assembly, independent of the base bracket. The bond pads were set on a flat surface to ensure the surfaces were all coplanar as shown in Figure 5. The flexure assembly was then bonded to the surface of the chip carrier in a jig which ensures fairly good centering with the DM. Centering did not have to be extremely accurate as there is sufficient adjustment ability during alignment to compensate. This process was practiced on a “dummy” MEMS (Figure 9), then an engineering grade DM and finally the science grade DM (Figure 9).

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SPIGPI-0115SSCH0231-Hire HMditWPre.re

HI 2OO

Figure 9. Bonding jig set-up.

Figure 10. Science graded MEMS in bonding jig

2.3 Humidity seal

The relative humidity within GPI’s optical enclosure is nominally controlled to approximately 30%RH but can vary depending on environmental factors and is not closely controlled. The MEMS DM performance can be severely degraded if operated in a humid environment. Rather than controlling the humidity in GPI’s entire opto-mechanical enclosure, BMC supplied the MEMS DM with an enclosure sealed to the face of he chip carrier using RTV. The enclosure has an inlet and outlet port in which pressurized dry air is supplied, giving a slightly positive pressure to the enclosure. Small, permanently attached fittings are the interface to the GPI air system services (can be seen in Figures 8 and 9).

Humidity inside the MEMS DM enclosure is monitored with a humidity sensor Honeywell HIH 4000 mounted in a hermetic plug in the enclosure air supply. The sensor connects with 3 electrical leads to a socket in the 1-wire Humidity/Pressure Sensor Board (U3 in Figure 10). Positive pressure inside the sealed enclosure is monitored with a differential pressure sensor Honeywell ASDX001 mounted on the same HSB. The ‘+’ port of the sensor connects to a pressure sensing port in the enclosure with flexible tubing and the ‘–‘ port is open to atmospheric pressure. The HSB is mechanically mounted to the outside of MEMS DM enclosure and connects electrically with a 1-Wire cable to the OMSS OME bulkhead.

Figure 11. Humidity/Pressure Sensor Board (HSB)

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ir

8 8

0 z

Mouse hole forrouting cables

Grid of M6x1holes N

Cable supportsfor Flex Cables

LJu

2.4 Cable management/strain relief

Flex cables are supplied by BMC to provide communication with the DM Controller in a dedicated enclosure at a distance from the MEMS DM. The flex cables (Figure 11) have eight 528-pin Meg-array connectors at one end (to mate with the chip carrier connectors) and sixteen 300-pin plugs at the other end to interface to the DM controller connectors (see Figure 3). It should also be noted that the cable connectors at the chip carrier and at the DM controller are rated for only 50 uses before failure and should not be connected and disconnected frequently. The flex cables are supported at the optical table close to the MEMS DM so they are not damaged and so they do not cause any motion of the MEMS DM during telescope slewing.

Figure 12. Flex cables and Meg-array connectors

Cables are routed through a “mouse hole” in the optical table. Due to a supporting rib directly behind the MEMS mount, the mouse-hole is offset from the MEMS position (Figure 13), causing the cables to be pulled away from the MEMS DM. For this reason and because of space limitations once the WFS was installed (see Figure 14), the cable clamp assembly has gone through a couple of design iterations. Finally, a simple cable clamp that straddles the mouse hole was incorporated (Figure 13).

Figure 13. Flex cables on optical bench

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Figure 14. Flex cables on AO bench.

GPI is currently in the integration and test phase at University of California, Santa Cruz and will be operational early next year.

3. ANALYSIS Instruments at the Gemini Observatory operate in conditions where fluctuating ambient temperature, varying gravity orientations and humidity are significant considerations for instrument designers. GPI is subjected to temperature variations during operations of -5° to +20° C and a gravity vector that is continuously variable in all orientations. Despite the stiffness of the chip carrier, it is expected that the differential contraction of the aluminum base bracket compared to the alumina chip carrier flexures will cause deformation of the mirror surface, pulling it into a “bowl” shape.

The physical position (at the optical origin) of the MEMS DM must be stable during observations to within the tolerances shown in Table 1, in the presence of all operational temperature variations, vibrations and gravity orientations.

Table 1. MEMS DM Mount positional stability (microns).

DOF X Y Z

MEMS DM 4 8 12

NOTE: the X-Y plane is nominally located on the pupil created by OAP3 (on the surface of the DM) and centered about the optical axis

Material properties were applied as shown in Table 2, on a simplified model, imported from CAD (Figure 15). The base assembly is not included as it provides only a stable mount for the flexure ring support. Imported simplified CAD model to ANSYS Workbench

Table 2. Material properties.

Material Density (g/cc)

Modulus of elasticity (GPa)

Yield/flexural strength (MPa)

Coefficient of thermal expansion (ppm/K @

20°C)

Alumina ceramic 3.7 – 3.97 360 300 6.6

Kovar 8.36 138 345 5.86

Aluminum 6061 2.71 70 275 23.9

303 Stainless steel 8.0 193 240 17.3

Macor 2.52 66.9 94 12.6

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Bondedinterface

Macorenclosure(RTV seal)

Fastener interfaces

I

Chip carrier

Flat (x3)Fixed in Z

BarrelFixed in X, V

Fastener interfaces were modeled as stiff (bonded) contact regions rather than including screws in the model to make the analysis more efficient. The adhesive is also modeled as bonded contact at the interface between the chip carrier and the Kovar bond pads.

Figure 15. Simplified model used for FEA analysis.

The model in ANSYS was meshed (Figure 16) with a fairly dense mesh on chip carrier (part of most interest) and at the adhesive bonds. The rest of the model was meshed with relatively larger element sizes as only their gross position was important and the analysis could be more efficient.

Figure 16. Meshed model.

The assembly was constrained at three mounting “flat” locations in Z (focus) direction but allowed to move in X, Y for thermal contraction. Then the center of “barrel” was constrained in X, Y to allow the entire flexure ring to contract with thermal changes. One point was constrained in Y only to prevent whole assembly from rotating about the Z-axis.

Figure 17. Constrained model.

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18.718 Mx16.97

15.223

13.475

11.728

9.9801

9.2326

6,4851

4.7376

2.9901 Mm

2.5e .004

Se .004

7.5e e004

le .005 (.n)

It is assumed that the surface of the DM takes the shape of the chip carrier as it deforms. That is to say, if the chip carrier bends due to the thermal changes, the mirror bends in the same motion.

The window enclosure was modelled as being attached to the chip carrier via a compliant RTV bond. This does not add a significant contribution to the stiffness of the chip carrier. The enclosure adds mass to the assembly that is supported by the flexures and therefore must be modelled in order to ensure the flexures are stiff enough to avoid any gross defections due to weak flexures.

The worst deflections are assumed to occur over the maximum temperature range (ie, from room temperature to -15C). This is the actual operating conditions and requirement for operation for the DM.

The connectors are not included in the assembly as they are lightweight wrt the rest of the assembly.

3.1 Flexure size

The flexure size was selected by doing the analysis on various flexures. Static analysis was performed in three gravity orientations (X, Y, Z) plus change in temperature of 25C. Looking at Table 3, it is apparent that flexures at thick as 1.0 mm dominate the effect as they are so stiff that they bend the chip carrier into the shape of a bowl (illustrated in Figure 18) with a large deflection in the Z direction. It is apparent that 0.5mm flexures and less do not exhibit this behavior.

Table 3. Flexure sizing results.

Gravity

Flexure Thickness

(mm)

Gross deflection - z

translation Gross deflection - x translation

Gross deflection - y translation

X 1.000 -2.000 -0.077 -0.158 Y 1.000 -2.100 -0.240 0.080 Z 1.000 -2.290 -0.030 0.080 X 0.500 -0.330 -0.200 -0.250 Y 0.500 -0.320 0.170 -0.530 Z 0.500 0.500 0.120 -0.180 X 0.254 -0.197 -0.445 -0.008 Y 0.254 -0.070 -0.004 -0.473 Z 0.254 -0.240 -0.004 0.006

Figure 18. Deflection results.

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U

0.78466 Max0 .62223

0.4500

0.20736

0.13403

-0.027497

-0.19993

-0.3523 6

-0.51479

-0.67722

3e+004

Ox +004 (urn)

3.2 Further analysis

A 0.5mm thick flexure was chosen and is the basis for the rest of the analysis. With the bowl-shape of the chip carrier, there is concern that there will be unacceptable tips and tilts of the mirror surface over short periods of time (ie, an observation). Otherwise, this error can be calibrated out in the system. Requirements are in Table 4.

To estimate the tip over a patch of mirror, these were converted into a differential deflection through a simple tangent relationship. That is, by assuming the mirror surface is bending as a line and not in a curve. This is only an approximation, of course, but gives a bound to the problem.

A typical observation is about an hour and GPI is expected to observe under conditions where the temperature doesn’t change by more than 0.8°C/hour. In this case the differential deflection between a point at the center of the mirror and the edge of the mirror will be the worst deflection in the system. This was measured in ANSYS by probing the surface of the mirror and subtracting the Z, X and Y deflection values for the worst-case results. For the example shown in Figure 19, the differential deflection between two points is ~0.1 um.

Table 4. Tip/tilt and clocking requirements and results.

Angular requirement

Tip/tilt 4 urad

Clocking 4 urad

Differential deflection

Z deflection ~0.10 um

X,Y deflections ~0.11 um

Diff. deflection modified to

angular

~4 urad

~3.6 urad

Figure 19. Deflection results.

4. SUMMARY The alumina chip carrier was chosen as it is a rigid substrate for supporting and protecting the DM. Due to space constraints and for reliability reasons, a set of eight 528-pin meg-array receptacles are wire-bonded to the back surface of the alumina chip carrier, rather than using a PCB.

The flexures will support the chip carrier as the telescope changes orientation on the sky. The flexures must be flexible enough to avoid inducing excessive stress into the chip carrier/mirror assembly but also strong enough to maintain alignment with the rest of the AO system to within the stability requirements as discussed previously.

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Extraordinary measures were required to ensure the MEMS is not damaged while operating in uncontrolled, changing gravity conditions. In theory, the flexures will stiffly hold the chip carrier without inducing unwanted and uncontrolled deformation into the mirror. Flexure testing and thermal cycling will reveal if the theory meets the practice.

REFERENCES

[1] Poyneer, L., “The use of a high-order MEMS deformable mirror in the Gemini Planet Imager,” Proc. SPIE 7931, 793104-1 (2011).

[2] Hill, A., “Stable flexure mounting of a MEMS deformable mirror for the GPI Planet Imager,” Proc. SPIE 7018, 7018-52 (2008).

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