Center for Adaptive Optics 15 Nov 1999 Meeting Major William D. Cowan, Ph.D. Air Force Research Laboratory Materials and Manufacturing Directorate, AFRL/ML Wright-Patterson AFB, Ohio Microfabricated Segmented Micromirror Arrays 1

2 Introduction Foundry Processes MUMPs 19 MEM-DM Continuous Facesheet Designs Micromirror Surface Figure Proposed CfAO SUMMiT Design Overview

3 Problem: Make practical deformable mirrors (DMs) for adaptive optics (AO) in foundry microfabrication processes DMs among the most expensive components in AO systems: $1000/channel Microelectromechanical systems (MEMS) ideally suited for optical applications - deflections consistent with optical wavelengths - photolithographic (parallel) fabrication of parts with identical characteristics Deflection uniformity critical for low cost AO (eliminate 100% testing) Use foundry fabrication processes to reduce cost for low volume applications Lessons learned applicable to specialized microfabrication processes (Reduce cost, size, weight, power dissipation) Introduction

4 Foundry Process Descriptions SUMMiT MMPOLY3(2  m) SACOX3(1.5-2  m, CMP) MMPOLY1+2(2.5  m) SACOX1(2  m) MMPOLY0(0.3  m) SiN(0.8  m) Oxide(0.6  m) Substrate MUMPs Metal(0.5  m) Poly2(1.5  m) Oxide2(0.75  m) Poly1 (2.0  m) Oxide1(2  m) Poly0(0.5  m) SiN(0.6  m) Substrate Trade fill factor, mirror size, array size(wiring depth) Self-planarization may help fill factor Planarization decouples mirror and actuator design etch access holes $3k 2 mos. $10k ? mos.

5 partial Poly2 self-planarization 1.5  m wide Poly1 anchors Incomplete etch of 1.5  m wide Poly1 gap wiring (MMPOLY0) anchor actuator upper electrode (MMPOLY1+2) flexure mirror (MMPOLY3) mirror to actuator vias etch access holes 3  m  3  m MUMPs self-planarizationSUMMiT with CMP Planarization MUMPs vs. SUMMiT Planarizaton

6 Deflection of electrostatic piston micromirror Electrostatic Piston Micromirror d g t Movable top electrode Fixed bottom electrode k, spring constant V A top electrode, mirror plate flexure anchor to bottom electrode k is a function of flexure number, geometry, and material stiffness (note how unidirectional layout mitigates the effect of residual stress) t is fixed by sacrificial layer thickness of process d is defined by optical modulation requirements Trade k and A for desired V, uniformity, yield, etc., for d=0 to ~t/3

7 Testing Piston Micromirrors Good deflection uniformity on die (wafer) but not necessarily die to die Dynamic laser interferometer testing is expensive in time/complexity Deflection (nm) Control voltage (V) Static fringe measurement V 316 =18 V dynamic laser interferometer measured modeled Static fringe technique developed for interferometric microscope is very fast Simple procedure: Toggle electrode voltage between 0 and V, fringe lines appear static for deflection= /2,,…, where  is test wavelength Interferometric microscope video also provides rapid characterization of yield and deflection uniformity With good fit to model, only need one data point for characterization Only need one data point from one device in an array But why not model this simple structure and avoid characterization testing?? Material Properties??

8 Segmented MEM-DM (M19) 12  12 Array 203  m center-to-center mirror spacing Stroke ~0.6  m Trapped oxide plate Poly0 wires under flexures Post foundry metallization required Fill Factor: ~77% M19 Piston Micromirror Element M19 MEM-DM Image

9 HeNe M1M1 Iris BS1 LlLl LsLs MEM-DM MEMS Control PC Beam Expander Aberrating Lens L a L t1 LFLF LMLM BS2 PSF Image Camera PC L t2 PSF Camera PC Optical Attenuator Optical Power Meter L w1 L w2 Adaptive Optics Test Bed M19 Optical Measurements Optical input power normalized using attenuator and power meter Increase magnification of far field pattern on PSF camera PSF camera frame rate used to scale measured intensities

10 M19 MEM-DM Aberration Correction Incident Optical Signal Plane ROC=0.70 m ROC=0.35 m ROC=1.60 m PlaneROC=0.80 m MEM-DM Figure 1.0 Hz) 0.07 Hz)0.04 Hz) 0.09 Hz)0.76 Hz)0.05 Hz) 0.18 Hz) 0.04 Hz) 0.27 Hz)

11 M19 MEM-DM Demo

12 Status of MUMPs 19 Design Still have the same device operating in the AFIT AO testbed Approaching 2 years of intermittent operation exposed to laboratory air Stan Rogers using to demonstrate phase retrieval Delivered 2 packaged devices to Dr Wild and Dr Kibblewhite at University of Chicago, Yerkes Observatory Don’t know status of their work, but recently had inquiry from MEMS Optical who had seen MUMPs 19 devices while visiting U of C May have a couple left - have been requested by USAF Academy For quick (~4 months), moderate performance, low-cost devices this design can be shoehorned into a 0.5 cm square die with 4 copies per MUMPs die site Will yield >50 devices for $3k + packaging costs Still need post foundry metallization

 m406  m  m406  m Height (nm) Heights (nm) 18 V21 V MUMPs Continuous Facesheet DM Influence Function Interferometric Microscope Image Observed actuator coupling ~40% in good agreement with predicted

14 Single element of MUMPs 21 CF DM 144 actuators Wired as a defocus corrector - elements equidistant from center are connected Only 16 voltages required Actuators can flatten residual stress induced deformation 0 V applied deformation due to residual stress 21 V applied to center 4 elements 21 V applied to 8 elements Interferometric microscope images of MUMPs 21 DM center etch holes print-through of actuator structure MUMPs 21 CF MEM-DM

15 Potential applications - optical aberration correction - laser communication - direct write photolithography - laser machining - consumer electro-optics Optical Efficiency/ Imaging Performance - fill factor (% reflective surface area) - mirror surface figure -- curvature -- print through -- reflectivity - array surface figure (uniformity) Ideal Curvature FF<100% Print-through Micromirror array surface Far field Micromirror Surface Figure

16 12  12 Trapped oxide plate Poly0 wires under flexures Post foundry metal Fill Factor: 77% M19M19_A M19_BM19_C 8  8 Trapped oxide plate MUMPs metal Fill Factor: 67.4% 8  8 Trapped oxide plate Post foundry metal Fill Factor: 67% 8  8 Poly2 mirror plate attached to actuator by vias Post foundry metal Fill Factor: 71.9% MUMPs Mirror Designs All arrays employ 203  m center-to-center mirror spacing MUMPs flexures 4  m wide for better yield and deflection uniformity

17 SUMMiT Mirror Design wiring (MMPOLY0) anchor actuator upper electrode (MMPOLY1+2) flexure metallization stop & actuator interconnect mirror (MMPOLY3) mirror to actuator vias 10  m  10  m etch access holes 3  m  3  m As-drawn fill-factor: 95% Post foundry metallization required 203  m center-to-center mirror spacing gap 3  m

18 Micromirror Surface Characterization Instrument: Zygo Maxim 3-D Laser interferometric microscope Accuracy: 3 nm RMS Manual scan of mirror middle to get Peak-to-Valley (PV) MUMPs devices - only Poly0 electrode under mirror - curvature due to residual material stresses in plate structure - metal ~50 MPa tensile - polys ~10 MPa compressive - trapped oxide ? M19_A False color image of surface height Mesh of surface figure Scan line PV=303 nm

19 Micromirror Surface Characterization False color image of surface height Mesh of surface figure Scan line PV=291 nm SUMMiT - design employs actuator and wiring under mirror plate - planarization incomplete - print-through of underlying structures - some residual stress curvature Zygo results confirmed by checking an unreleased die on stylus surface profilometer Note!: Devices fabricated on early SUMMiT runs Planarization targeted at mechanical vice optical flatness Sandia has now fixed problem (new SUMMiT Optical process)

20 Image PSF Optical Perf vs. Micromirror Figure M19_A MUMPs Metal nm PV concave M19_B AFIT Metal 55.6 nm (convex) SUMMiT AFIT Metal nm PV (print-through + concave) < 

21 Reflected Optical PSF Mirror Description Optical Efficiency Peak Intensity Effective FWHM Power Normalized Normalized Fill Factor Normalized %% % MUMPs Plane Mirror M19 No Metal M19_A MUMPs Metal M19 AFIT Metal M19 AFIT Metal M19_B AFIT Metal M19_C AFIT Metal SUMMiT No Metal SUMMiT AFIT Metal SUMMiT AFIT Metal Optical Measurement Summary

22 Surface Figure Study Results Fill factor and optical efficiency (power) not good metrics - don’t measure imaging performance Surface figure is most important factor for imaging performance “Good” polysilicon piston micromirror arrays require - planarization - residual stress control/characterization Sputtered chromium/gold metallization promising Proposed fabrication approach - design in an initial convex curvature using residual stresses - sample lot (release and measure curvature) - design metallization to yield flat mirror surfaces - metallize lot

23 Latest SUMMiT Optical Design 32  32 Array of segmented micromirrors (1024 total) 100  m pitch (center-to-center), Nominal fill-factor ~95% Employs unproven Row-Column address scheme Only 2N wires for N 2 array Wiring limits maximum array size in foundry processes Row-Column (line) addressing demonstrated for bistable mirror arrays Pulse width & pulse amplitude modulation also demonstrated (Rounsaval AFIT thesis) Status Only a few samples tested - 15 min partial, 30, 45 min release etches (1:1, HF:HCl) Mirror element flatness <30 nm peak to valley Unreleased array(s) shows global convex curvature May be artifact of CMP process, or residual stress in oxide Probably can minimize by design “tricks” Can also correct out or “flatten” array in use Discuss findings with Sandia to determine cause/fix

24 SUMMiT 32 x 32 Row/Col Array One array so far had problems with MMPOLY3 attachment to underlying actuators May suggest non-uniformity of CMP oxide thickness across wafers Have heard CMP “wedge” problem anecdotes Actuator-only global curvature is convex (~120nm peak to valley)

25 Interferometer Images MUMPs 19 MUMPs Plane Mirror (Gold) SUMMiT Optical = 632 nm

26 Testbed Images & PSFs (Preliminary data) MUMPs 19 MUMPs Plane Mirror (Gold) SUMMiT Optical Partial Release(?) SUMMiT Optical Full Release (30/45?)

27 Proposed CfAO SUMMiT Design 128 to 256 element array of segmented micromirrors Single wire per element address scheme (die size/wire bond limited design) Wire-bonded electrical connections Minimum 100  m pitch (center-to-center) Larger element size for increased fill & lower operating voltage Have 128 element 203  m designs on 0.5 cm square die Trade of bond pad space & mirror size required to optimize Minimum fill-factor ~95% Minimum stroke: 0.5  m Mirror element flatness <30 nm peak to valley Optimize global flatness by design and study of process using current arrays Status Have had initial discussions with Sandia about approach Want design that they will agree to release/package/bond Standard module run should yield finished parts (untested) Will explore progress of metallization - use if available Otherwise design for ease of post-foundry (user) metallization

28 “Use of micro-electro-mechanical deformable mirrors to control aberrations in optical systems: theoretical and experimental results,” Opt. Eng., 36(5), , (May 1997). “Experimental demonstration of using micro-electro-mechanical deformable mirrors to control optical aberrations,” in Adaptive Optics and Applications, Proc. SPIE, 3126, , (July 1997). “Modeling of stress-induced curvature in surface-micromachined devices,” in Microlithography and Metrology in Micromachining III, Proc. SPIE 3225, 56-67, (September 1997). “Thermally actuated piston micromirror arrays,” in Optical Scanning Systems: Design and Applications, Proc. SPIE 3131, , (July 1997). “Measuring the frequency response of surface-micromachined resonators,” in Microelectronic Structures and MEMS for Optical Processing III, Proc. SPIE 3225, 32-43, (September 1997). “Vertical thermal actuators for micro-opto-electro-mechanical systems,” in Optical Scanning Systems: Design and Applications, Proc. SPIE 3131, (July 1997). “SPICE modeling of polysilicon thermal actuators,” in Micromachined Devices and Components, Proc. SPIE 3224, , (September 1997). “Design and testing of polysilicon surface-micromachined piston micromirror arrays,” to be published in Spatial Light Modulators, Proc. SPIE 3292 (1998). Reference Publications

29 “Evaluation of microfabricated deformable mirror systems”, published in Adaptive Optical System Technologies, Proc. SPIE 3353 (1998). “Surface Micromachined Segemented Mirrors for Adaptive Optics”, to be published in IEEE Journal of Selected Topics on Quantum Electronics special issue on MOEMS (early 1999). “Aberration correction results using a segmented micro-electro-mechanical deformable mirror and refractive lenslet array”, Optics Letters, 15 April “Optical phase modulation using a refractive lenslet array and micro-electro-mechanical deformable mirror”, Opt. Eng., 37(12), , (December 1998). “Average power control and positioning of polysilicon thermal actuators”, Sensors and Actuators Vol 72, 88-97, January “Automated assembly of micro-electro-mechanical systems”, The International Journal of Advanced Manufacturing Systems (special issue on Micro/Miniature Manufacturing). “MOEMS for adaptive optics”, IEEE/LEOS Summer Topical Meeting on Optical MEMS, July 1998, Monterrey, California. “Simulation and design of electro-thermally actuated MEMS devices, a case study: the AFIT piston micromirror”, Winter Annual Meeting of ASME, November 15-20, 1998, Anaheim Convention Center & Hilton, Anaheim, California. Reference Publications (cont)

30 Measured Micromirror Curvature Die Mirror X scan Y scan Mean Number Type (nm PV) (nm PV) (nm PV) 2Poly Stacked Poly Stacked Poly Stacked redoPoly redoStacked Die to die variation attributed to metal thickness variation (up to 20% reasonable in MUMPs metallization) Change in die #2 after 5 months due to temperature, metal creep?? - 15% decrease in Poly2/metal vs. 4% for Stacked poly/metal Gold/Poly2 Gold/Stacked poly

31 Measurement Procedure Instrument: Zygo Maxim 3-D Laser Interferometer Test die placed on aluminum block on stage Microscope head manually leveled to die Plane best fitting raw height data automatically subtracted by system eliminating residual die tilt and/or actual mirror tilt (i.e. stuck devices) Measured mirror surface profile data manually scanned in X and Y directions through approximate mirror center Discontinuities at dimples used to define end points of scan Measurements repeatable within +/- 5 nm

32 IntelliCAD Modeling SEM Micrograph of Poly2/metal device“Manhattanized” model of Poly2/metal device Finite Element Model Simplifications - Manhattanized design layouts (mesher blew up on hexagonal mirror structure) - Poly0 address electrode, and dimple effects ignored - Materials assumed isotropic

33 Polysilicon Variation Study - Poly layer thickness max. & min., stress max. and min. - Metal layer thickness and stress nominal - Intial results showed qualitative agreement (but off by >10X) Deflection scale magnified 25 times Cross-section line Plot height of gold surface after subtracting min height False color image of deflection IntelliCAD Modeling

34 Measured to modeled agreement improved by - Re-measuring metal stress at foundry (to 50 MPa from 20 MPa) % metal stress variation consistent with MUMPs 5-15 metal stress - Other work tends to confirm MUMPs polysilicon residual stress values - +/- 20% metal thickness variation chosen to bound problem IntelliCAD Modeling

35 Modeling Results Effort undertaken with two goals (1) Assess ability of commercial CAD tools to predict mirror surface curvature (2) Film parameters accurately quantified by wafer bow measurements?? CAD capability is available Characterization of MUMPs residual stresses questionable - Polysilicon stress values appear reasonably accurate (careful duplication of process conditions for monitor wafers) - Simulation and measured results suggest metal stress higher than reported -- wafer to wafer metal thickness variation -- imprecise thickness of chromium and gold -- bare Si vs polysilicon monitor -- stress variation across wafer Aging (i.e. metal creep) and temperature effects must be studied further Best possible materials characterization/process control a must for MEMS CAD!!

Gold Film Thickness (angstroms) Reflectance Reflectance vs. Metal Thickness (Gold on silicon, HeNe) Metallization Thickness Computed using complex indices of refraction for gold and silicon n g =0.162, k g =3.210 [Liao, Microwave Devices and Circuits] n si =3.877, k si = [Klein & Furtak, Optics] n air n g, k g, t n si, k si HeNe