JY/11/15/99 MTC Optically flat arrays of micromirrors June Yu James A. Folta William Cowan (AFRL) to improve the mirror surface quality and optical fill-factor of existing MEM DM prototypes

JY/11/15/99 MTC There are a number of technical issues to be addressed for MEM DMs for adaptive optics applications Wavefront Quality: < 20nm surface error Fill Factor: > 99% # of actuators: > 2000 Stroke: > 0.5 µm for single, >4µm for multi- Speed:>1KHz Packaging Addressing Coating: > 80% broad-band, >95% narrow-band reflectivity Damage threshold: > 2J/cm 2 (pulsed), up to 1 KW (average) Size Interface electronics The two most critical issues limiting the application of current prototype MEM DM’s are surface quality and fill factor

JY/11/15/99 MTC Two factors affect the optical surface figure of MEM DM’s Residual stress in the fabrication material curvature of mirror surface Topography induced by the underlying layers in the surface micromachining process print-through DMs fabricated with the MCNC MUMPs process no metallization: ~ 150 nm PV curvature with reflective Au: ~ 300 nm PV curvature COWAN DMs with AFRL coating: 55.6nm to 98.3 nm PV curvature

JY/11/15/99 MTC Foundry-fabricated MEM DM’s exhibit stress induced curvature and “print-through” Microscope image of AFRL MEM DM array fabricated in the MUMPs process showing print- through of underlying layers Lineout of a white-light interferometer image of a released MEM DM - mirror surface has a PV curvature on the order of 300 nm across a single pixel. Unreleased mirrors nm P-V flatness (ignoring print-through)

JY/11/15/99 MTC We are developing a process to bond flat mirror arrays to foundry actuator arrays mirror Sacrificial layer Silicon substrate Au bond posts Actuator array Released interface Mirror array on handle wafer

JY/11/15/99 MTC Post-foundry addition of mirrors has a number of advantages By separating mirror elements and the actuators, we can fine tune the mirror surface figure independent of underlying actuator and circuit layers Reduction or elimination of etch access holes from mirror surface Can incorporate a variety of application-specific optical coatings Possibly lower cost than CMP

JY/11/15/99 MTC We have selected the Au bump compression bonding technique Low temperature process Does not require atomically clean and flat interfaces Does not require large bond bumps as does solder bump technology. suitable for fabrication of MEMS structures with small features. Au is inert Able to work with single dies greatly reduces the cost and lowers the development risk by maximizing the number of experiments that can be performed at reasonable cost.

JY/11/15/99 MTC BSAC has successfully used the Au-to-Au compression bonding technique to transfer micromirrors onto foundry fabricated devices Photo courtesy of Michel M. Maharbiz, Roger T. Howe, and Kristofer S. J. Pister

JY/11/15/99 MTC We are applying the Au-to-Au bonding technique for bonding mirrors to the foundry fabricated actuator arrays SEM image of one micro-actuator AFRL actuator arrays: 12 x 12 arrays, 203  m center-to-center spacing, up to 0.7  m vertical stroke. 90 µm circular pads are designed to accept the bonding of a continuous or pixilated mirrors. Photo of 12x12 actuator array

JY/11/15/99 MTC Au-to-Au compression bonding technique requires uniform arrays of electroplated Au-bumps Arrays of electroplated Au bumps, height = 7 µm±100nm, Au bumps are compressed by 1.1 µm under 70Kg load during bonding

JY/11/15/99 MTC We have selected a controlled stress film as the mirror materials Experimental data show we can tune the mirror film stress Pixilated mirror array (before bonding) 197 µm square 1.4 µm thick with Au bumps

JY/11/15/99 MTC Tensile Strength of Au-to-Au compression bonding is comparable to that for bulk Au Bond failed at 15.3 Newtons 104 Mpa