JY/11/15/99

MTC

Optically flat arrays of micromirrors

June YuJames 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/cm2 (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 - 2.3 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 bondingis comparable to that for bulk Au

Wafer Bond #2

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Time (sec)

Force (N)

Force (N)

Bond failed at 15.3 Newtons

104 Mpa