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

Professor Richard S. MullerMichael A. Helmbrecht MEMS for Adaptive Optics Adaptable-Mirror Section Mirror Substrate Solder-Bump Electrical Connections Movable Mirror Elements Scale ~10s to 100s  m Scale ~ mm to cm Current-Driver Electronics Array of Elevating Mirrors

Professor Richard S. MullerMichael A. Helmbrecht 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

Professor Richard S. MullerMichael A. Helmbrecht Thermal-Bimorph Actuation  Large force  Low voltage  Long life 10 7 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

Professor Richard S. MullerMichael A. Helmbrecht Actuator Design

Professor Richard S. MullerMichael A. Helmbrecht Mirror-Element Layout

Professor Richard S. MullerMichael A. Helmbrecht Thermal-Bimorph Actuators Tip deflects when     1  is the coefficient of expansion h  T-T 0 ), (  2 -  1 ), 1/w Theoretical plot shown for: T-T 0 =70°C (  2 -  1 )=23.4 ppm/°C w=1  m

Professor Richard S. MullerMichael A. Helmbrecht Bimorph 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

Professor Richard S. MullerMichael A. Helmbrecht Mechanical-Resonance Frequency (h=10  m) T-T (K)

Professor Richard S. MullerMichael A. Helmbrecht Bimorph Thermal Model Losses through the substrate dominate –Convection losses are negligible Mirror heats up to a fraction of the bimorph temperature (T - T 0 ) mirror = (0.33 down to 0.125) (T - T 0 ), for 100  m < l < 300  m

Professor Richard S. MullerMichael A. Helmbrecht Thermal-Time-Constant -3dB Frequency and Power Density

Professor Richard S. MullerMichael A. Helmbrecht Mirror Berkeley Bimorph Process Single-mask process Low-stress nitride (Si x N y ) and aluminum bimorph Other processes under consideration –Hidden actuators –Electrostatic actuators –Bonded mirrors Current path Mirror Die photo - actuated, released mirrors

Professor Richard S. MullerMichael A. Helmbrecht Conclusions 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