Advanced optical coatings of a segmented ... - SPIE Digital Library

Advanced optical coatings of a segmented MEMS DM Michael A. Helmbrecht*, Min He Iris AO, Inc., 2680 Bancroft Way, Berkeley, CA, 94706, USA ABSTRACT The performance of microelectromechanical systems deformable mirrors (MEMS DMs) continues to improve in areas of stroke, open-loop positioning, actuator count, and drive electronics. However, a key area lacking in the development of these devices is good optical coatings suitable for a broad spectrum of applications and wavelengths. This paper discusses the progress Iris AO has made towards coating its MEMS DMs with gold, protected aluminum, protected silver, and dielectric coatings. Keywords: Optical Coating, Deformable Mirror, Adaptive Optics, Spatial Light Modulator, MEMS

1. INTRODUCTION Although MEMS DM technology has matured, MEMS DMs are typically only sold with aluminum or gold coatings as these are easy coatings to deposit with tooling common to all MEMS fabrication facilities. The coating techniques used for these mirrors, typically thermal evaporation or e-beam evaporation, have high defect densities compared to state-ofthe-art coatings deposited onto conventional optics. In contrast, high-quality coating vendors use either ion-beamassisted e-beam evaporation or ion-beam sputtering to deposit very dense films with low defect densities onto conventional optics.

Mirror Surface Height (nm)

Another striking difference between coatings for conventional optics and MEMS DMs is the thickness. To counter deformation in the mirrors caused by of Mirror-Surface Deformation with 100 nm Au Coating (-nm/°C) residual stresses and thermal coefficient 10 of expansion (TCE) differences in the 1.0 μm Segment coatings, MEMS designers desire 9 coatings much thinner than what would 2.5 μm Segment be deposited onto a conventional optic. 8 This is necessary because the aspect 7 ratios for MEMS mirrors, the segment or actuator area divided by substrate 6 thickness, can be 10 to 100 times larger than those of conventional mirrors. 5 5 μm Segment Furthermore, the bow caused by residual stresses in the coatings is proportional to 4 the coating thickness and the square of the aspect ratio as described in Stoney’s 3 equation.1,2 Figure 1 plots Stoney’s equation using the stress from TCE mismatches for a 1°C change in temperature between a 10 nm/100 nm titanium/gold coating and a silicon substrate of various thicknesses. The vertical axis is the surface deformation in nanometers for the 1°C change and the horizontal axis is the *

2 10 μm Segment 1

15 μm Segment Iris AO DM

0 -400

-300

-200

-100

0 Position (μm)

100

200

300

Figure 1: Theoretical plot of height deformation along the length of the mirror segment caused by CTE mismatches of the optical coating and silicon for a 1°C change in temperature. Experiments show that the Iris AO DM deforms only 0.56 nm/°C as shown by the plotted data point.

[email protected], phone (510) 849-2375, www.irisao.com MEMS Adaptive Optics III, edited by Scot S. Olivier, Thomas G. Bifano, Joel A. Kubby, Proc. of SPIE Vol. 7209, 72090M · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.813821

Proc. of SPIE Vol. 7209 72090M-1

400

position along the mirror. The bow of membrane and continuous-facesheet deformable mirrors is reduced because the end constraints are different. However, the mirror stiffness for these types of devices is a function of residual stresses, stresses from TCE mismatches in the films, as well as the thickness cubed. The optical coatings can be as much as 510% of the total mirror thickness. Thus, these coatings can have a large effect on the mechanical response of membrane and continuous mirrors. Clearly, thinner coatings are better to reduce unwanted bow and changes in stiffness. However, the coatings cannot be made arbitrarily thin. Thinner coatings lead to increased transmission through the coating, thus increasing interactions with the underlying MEMS substrate. These interactions can change the phase of the reflected light at the interface and also change the polarization of the light. Typical coating thicknesses for gold and aluminum films on MEMS DMs are 50 nm and 100 nm respectively. In contrast, on a conventional optic these coatings are often 150 nm and 300 nm respectively.

2. IRIS AO MIRROR SEGMENTS: A ROBUST SUBSTRATE To enable the use of thick coatings without suffering the deleterious effects of coating residual stresses and TCE mismatches on optical quality, the Iris AO S37-X DM uses a relatively thick mirror segment compared to other MEMS DMs. Figure 2 below shows the schematic of an individual piston/tip/tilt (PTT) mirror segment. Tens, hundreds and even thousands of these can be tiled into an array to form a segmented DM. Figure 2b shows a die photograph of a 37-PTTsegment DM. Operation of the device is discussed in depth in another paper in these proceedings as well as in prior papers.3,4 The critical aspect of the design for this paper is the thick, 20-50 µm, single crystal silicon mirror segment that is assembled onto the underlying actuator platform. The thick segment resists deformation from the optical coatings extremely well. Thus, these mirrors are coated with the better-performing thicker coatings. Coatings with protection layers can be deposited onto the mirrors as well. Further, because we have access to the backside of the mirror prior to assembly, we can add stress compensation layers as is necessary when using dielectric coatings.

%

Rigid High-Qua lity Mirror Segment Bondsites Actua tor Pla tform Electrodes Tempera ture Insensitive Bimorph Flexure

Figure 2: a) Schematic diagram of a 700 µm diameter (vertex-to-vertex) mirror segment and b) die photograph of a 111-actuator 37-piston/tip/tilt-segment DM with 3.5 mm inscribed aperture.

3. OPTICAL COATING RESULTS The following describes results of coating Iris AO S37-X DMs with standard coatings provided by an optical coating vendor. All of the coatings were deposited using the ion-beam-assisted e-beam coating technique. The goal of this work was to demonstrate high-quality coatings commonly used with conventional optics rather than ones optimized for the MEMS process at the expense of optical quality. The two ubiquitous coatings for MEMS mirrors are gold and aluminum. We therefore start with these and then extend the work to protected-silver and dielectric coatings. One compounding factor with the S37-X process is that after coating the DMs, they are exposed to a 10 minute, 120°C die


attach curing step. All of the flatness results described here are of mirror arrays after the die attach step. Thus, the measurements incorporate changes in the residual stresses as a result of the relatively high temperature step. The gold coatings consisted of a 10 nm titanium adhesion layer and a 125 nm gold layer. After packaging, the mirror array had residual surface figure errors of 14.9 nm rms. In prior coating runs, a 10 nm chromium adhesion layer was used with similar results. Normally chromium is not used with MEMS devices because the residual stress in these films can be many hundreds of MPa. For the S37-X, the thick segments provide a rigid substrate that resists excessive bowing from the chromium layer. Figure 3a shows the measured surface figure of a gold-coated S37-5 DM.

eec

Mag: 1.4X Mode: VSI

Surface Data

Dat:

r

Tim

e ec1:iii Mode: PSI

Surface Statistics: Ra: 11.O2nm

Surface Statistics:

Rq: 14.00nn: Rz: 104.66:tt:: Rt: 170.67nm

Rq: 9.21 11111

Set-up P:sc:stttetecs:

Sct-sp P:sc:sssetecs:

ORe: 06X48::

ONe: 56X480

See:pticg: 6.06 cc:

So::phoo: 6.06:o::

Pecee::ed OpSuts'. Tee::: Rcc::ecd:

Pcooe::ed Opsoss'. To:::: Ro:::oed:

Tilt

Till

FilteR:g:

Pillooiog:

N

N 0110

Mag: 1.4X

II)at

Surface Data

Tim

Ra: 7.28:11:1 Rz: 07.0511111 RI: 90.05 11111

Figure 3: a) Measured surface figure after a 125 nm gold coating with a 10 nm titanium adhesion layer. Surface figure errors are 14.9 nm rms. b) Measured surface figure after a 250 nm aluminum coating with a 10 nm titanium adhesion layer and a 12 nm protection layer. Surface figure errors are 9.2 nm rms.

Aluminum coatings oxidize instantaneously when they are exposed to oxygen. The oxide growth on the surface of the aluminum is quenched after a thin oxide is formed. However, to insure long-term stability, it is best to protect the aluminum coating with a thin dielectric coating. Figure 3b shows the measured surface figure errors of an S37-5 DM coated with a 10 nm titanium adhesion layer followed by a 250 nm aluminum layer and a 12 nm protection layer. This mirror had surface figure errors of only 9.2 nm rms. Protected-silver coatings are compelling given that they are more reflective than aluminum and extend into much shorter wavelengths than gold coatings do. However, protected silver coatings can be difficult to deposit. Again, following the approach of using coatings that are routinely done for conventional optics, we had protected silver coatings deposited onto blank silicon wafers to act as test coupons. The coating vendor’s standard recipe produced excellent coatings on a BK7 test coupon, however, the coatings did not adhere to the silicon well. The coatings were ridden with hillocs. It is likely that the standard adhesion layer did not form a good barrier between the silicon and the silver, allowing the silver to diffuse into the silicon. This is likely the case as the coatings degraded over time. Further development is under way to eliminate these problems. For laser line applications, dielectric coatings are compelling because they are far more resilient to damage than metallic coatings and they can achieve reflectivity in excess of 99%. To demonstrate dielectric coatings, we coated silicon wafers with a 99.5% reflective dielectric coating for 355 nm and 589 nm. The 355 nm coating was approximately 1.5 µm thick and the 589 nm coating was nearly 3 µm thick. Stress levels in these films were as high as 70 MPa. One wafer with the 355 nm coating passed a damage test performed by Spica Techologies to 160 kW/cm2. For this test, the beam straddled a gap machined into the test wafer to test if the segmentation may lead to premature damage. The test was run on a wafer only and not on an actual DM. We expect the suspended segments will become too hot when exposed to more than many tens of Watts of incident laser power. Iris AO in conjunction with the Center for Adaptive Optics will be testing power handling of a DM sometime in 2009 for laser-guide-star uplink correction. The 1.5 µm thick 355 nm coating was applied to an S37-5 DM. Even the thick mirror segments of the S37-5 DMs bowed excessively. For 25 µm-thick segments, the 355 nm coating resulted in surface figure errors of nearly 80 nm rms as seen


eec

Mag: 1.4X Mode: VSI

Dat:

Surface Data

Tim

Surface Statistics:

Ra: 66.65nm Rq: 78.52:1111 Rz: 582.2211111 Rt: 635.75 11111 Sct-sp P:sc:sssctccs: Oae:

06X48::

01111ph1111 6.66:111:

Pcccc::cd Oiltlss: Te:::: Re:::::cd: Till

Filleilcg: N :1111

Figure 4: a) Measured surface figure after a ~1.5 µm dielectric coating. Surface figure errors are 78.5 nm rms. Mirrors with stress compensation to reduce figure errors to less than λ/20 rms are currently being fabricated.

in Figure 4. Iris AO is nearly complete with the fabrication of 50 µm thick stress-compensated mirror wafers to reduce the surface figure errors to less than λ/20 rms.

4. CONCLUSIONS Attempts to coat the Iris AO 111-actuator S37-X DMs with high-quality coatings used on conventional optics proved successful for both gold and aluminum coatings. Despite the thicker coating thicknesses and added protection layer of the aluminum, the mirrors stayed flat to better than 15 nm rms for both gold aluminum coatings. Tests with protectedsilver coatings normally deposited onto conventional optics showed that these coatings do not adhere well to silicon substrates. Furthermore, the coatings worsen with time. Further development will be required to coat silicon substrates with protected silver. Finally, high-quality dielectric coatings can be deposited onto the S37-X DMs. However, stress compensation techniques that are under development will need to be employed to keep surface figure errors to less than λ/20 rms for these multiple-micron-thick coatings.

5. ACKNOWLEDGEMENTS This work has been supported by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST-9876783 and by a NIH Phase I SBIR, 1R43EY016264-01A2.

6. REFERENCES 1. 2. 3. 4.

Stoney, G. G.,. In Proceedings of the Royal Society of London Series A, volume 82, page 172, (1909). Helmbrecht, M. A. , He, M. , Juneau, T. , Hart, M. , Doble, N. P. , “Segmented MEMS Deformable-Mirror for Wavefront Correction,” Invited Presentation, Proc. of SPIE, Vol. 6376, 2006). Helmbrecht, M. A. , Kempf, C.J. , He, M. , “Scaling of the Iris AO segmented MEMS DM to larger arrays,” Invited Presentation, Proc. of SPIE, Vol. 7209, (2009). Helmbrecht, M. A., Juneau, T., Hart, M. and Doble, N., “Performance of a High-Stroke, Segmented MEMS Deformable-Mirror Technology,” Invited Presentation, Proc. of SPIE, Vol. 6113, (2006).
