Professor Richard S. MullerMichael A. Helmbrecht MEMS for Adaptive Optics Michael A. Helmbrecht...

Professor Richard S. MullerMichael A. Helmbrecht

MEMS for Adaptive OpticsMEMS for Adaptive Optics

Michael A. Helmbrecht

Professor R. S. Muller


MEMS for Adaptive OpticsMEMS for Adaptive OpticsAdaptable-Mirror Section

Mirror Substrate

Solder-BumpElectrical Connections

Movable Mirror ElementsScale ~10s to 100s m

Scale ~ mm to cm

Current-DriverElectronics

Array of Elevating Mirrors


How Can We Elevate the Mirror Elements?

How Can We Elevate the Mirror Elements?

• Magnetic Actuation– Solenoidal structures

• Electrostatic Actuation– Vertical comb-drives

• Thermal Actuation– Thermal-bimorph structures– Our initial design approach


Thermal-Bimorph ActuationThermal-Bimorph Actuation

Large force Low voltage Long life

• 107 cycles without failure measured at BSAC

Actuators can be hidden under mirror

- Heat dissipation• Possible mirror

distortions• Possible electronics

problems

- Complicated linkage• Must be compliant when

bending• Must result in pure

translation of mirrors


Actuator DesignActuator Design

Static anchor anddrive electrodes

Mirror-bondingsiteTorsional flexure

Bimorphactuator

Side View

Front View



Bimorphactuator

Side View

Front View



BimorphactuatorStatic anchor - and -drive electrodes

Mirror-bondingsiteTorsional flexureBimorphactuator Side ViewFront View

Static anchor - and -drive electrodes

Mirror-bondingsite

Torsional flexure

Bimorphactuator

Side View

Front View


Mirror-Element LayoutMirror-Element Layout

Trench Flexure

Bimorph Layer 2

Bimorph Layer 1

Mirror Bond

Mirror

Contact


Thermal-Bimorph ActuatorsThermal-Bimorph Actuators• Tip deflects when 1

• is the coefficient of expansion

• hT-T0), (2-1), 1/w• Theoretical plot shown for:

• T-T0 =70°C• (2-1)=23.4 ppm/°C• w=1m

Theoretical Bimorph Deflection

0

10

20

30

40

50

0 100 200 300 400

Beam Length (m)

(Mirror Height

)m

h=1.5m

1

1

2

2

1

2

012

21

5))((2

3

−

√√↵

√√↵

+

+−−=

EE

EE

TTwl

h 2

ww

h

l


Bimorph Mechanical ModelBimorph Mechanical Model• Mirror element size l• Bimorph length l• Au- or Al-film on 10 m-

thick silicon (SOI)– Neglect bimorph mass

• Each bimorph layer thickness w

• Bimorph width b=20 m– Power scales with b

303)( TTl

h

bfR −∝

l

w

Mirror Mm

kb

wm

Torsional flexure


Mechanical-Resonance Frequency (h=10 m)

Mechanical-Resonance Frequency (h=10 m)

432

865

1.3E3

1.73E32.16E3

2.6E3

3.03E3 3.46E3

3.9E3 4.33E3

4.76E3 5.2E3 5.63E3

100 120 140 160 180 200 220 240 260 280 30010

20

30

40

50

60

Mechanical Resonance Frequency (Hz)

Bimorph Length (m)

T-T

0 (K

)

T-T

(K

) 0.15

0.29

0.44

0.59

0.73

0.88

1.0

1.2

1.3

1.5

1.6

1.8

100 120 140 160 180 200 220 240 260 280 30010

20

30

40

50

60Bimorph-Layer Thickness (m)

0

Bimorph Length (m)


Bimorph Thermal ModelBimorph Thermal Model

• Losses through the substrate dominate– Convection losses are negligible

• Mirror heats up to a fraction of the bimorph temperature (T - T0)mirror = (0.33 down to 0.125) (T - T0), for 100 m < l < 300 m

l

w

Mirror

Rbsgap

Rbmgap

Rb

Cb

IR2

Rmsgap

wm

Torsional flexure


1.2

2.4

3.6

4.8 6.07.2

8.4

9.6 11 12 13 14

100 120 140 160 180 200 220 240 260 280 30010

20

30

40

50

60Power (mW)

T-T0

K

Bimorph Length (m)

Thermal-Time-Constant -3dB Frequency and Power DensityThermal-Time-Constant -3dB Frequency and Power Density

2.33E3

4.66E3

7E3

9.33E3

11.7E3

14E3

100 120 140 160 180 200 220 240 260 280 30010

20

30

40

50

60Thermal Actuation -3dB Frequency (Hz)

Bimorph Length (m)

T-T

0 (K

)


• Mirror

Berkeley Bimorph ProcessBerkeley Bimorph Process

• Single-mask process

• Low-stress nitride (SixNy) and aluminum bimorph

• Other processes under consideration– Hidden actuators – Electrostatic actuators– Bonded mirrors

Current path

Mirror

Die photo - actuated, released mirrors


ConclusionsConclusions

• Bimorph actuator is small and can be fabricated under the mirror– Fill factor > 98%

• Bimorph actuators can be tiled in an arrays to actuate large numbers of mirrors

• Bimorph actuator provides large tip deflection (h = 10 m)• Models show frequency requirements are met (f>1 kHz)• Bimorph actuators have been fabricated

– Testing is underway

Professor Richard S. MullerMichael A. Helmbrecht MEMS for Adaptive Optics Michael A. Helmbrecht...

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