報告者：廖信宏 指導教授：楊燿州 博士 2010.10.5

1. 微機電式數位類比轉換反射鏡模組之研製及在白光干涉系統之應用A Micromirror Module using a MEMS Digital-to-Analog Converter and its Application for White-Light Interferometry System 報告者：廖信宏 指導教授：楊燿州 博士 2010.10.5

2. 個人簡歷 • 姓名：廖信宏 • 籍貫：台灣省雲林縣 • 學歷： • 國立中正大學 機械工程系學士班 (1999/9 ~ 2003/6) • 國立臺灣大學 機械工程所碩士班 (2003/9 ~ 2004/6) • 國立臺灣大學 機械工程所逕讀博士班 (2004/9 ~ )

3. Outline Introduction of Micromirror Review of Micromirror Devices Motivation MEMS-DAC Micromirror Module White-Light Interferometry System Conclusions

4. Introduction • Micromirrors, which are realized by MEMS technologies, have been widely used in many applications: • Projection displays • Maskless lithography • Optical communications • Biomedical imaging systems Optical Coherence Tomography (Santec Corp.) DLP Cinema®(Texas Instruments Inc.) Optical Switch/OXC (Lucent Technologies) Zone-Plate-Array Lithography (LumArray Inc.)

5. Outline • Introduction of Micromirror • Review of Micromirror Devices • Electrostatic Micromirrors • Electrothermal Micromirrors • Electromagnetic Micromirrors • Other Micromirrors • Motivation • MEMS-DAC Micromirror Module • White-Light Interferometry System • Conclusions

6. Electrostatic Micromirrors

7. Electrostatic Micromirrors

8. ElectrothermalMicromirrors

9. Electromagnetic Micromirrors

10. Other Micromirrors

11. Other Micromirrors

12. Outline Introduction of Micromirror Review of Micromirror Devices Motivation MEMS-DAC Micromirror Module White-Light Interferometry System Conclusions

13. Motivation (1/2) • Based on actuation methods, micromirror devices can be classified into two types: • Digital type • The devices need binary or bi-stable mechanisms for switching mirror positions. • Analog type • The positions of mirrors are active through the whole operation range. • The mirror motions are usually feed-back controlled if accuracy is an important issue for applications.

14. Motivation (2/2) MEMS digital-to-analog converters (M-DAC), which are analogous to the electrical DAC counter-part, provide highly repeatable and accurate displacements without the need of feed-back control circuits. In this work, a mirror module integrated with an M-DAC device which precisely actuates a large-area mirror in out-of-plane motion is developed.

15. Outline • Introduction of Micromirror • Review of Micromirror Devices • Motivation • MEMS-DAC Micromirror Module • Principle and Design • Fabrication Process • Experiment Results • White-Light Interferometry System • Conclusions

16. R/2R Ladder D/A Converters In this circuit, only two resistor values are required, which lends itself nicely to the fabrication of ICs with a resolution of 8, 10, or 12 bits, and higher.

17. Binary-Weighted D/A Converters Coming up with accurate resistances over a large range of values is very difficult. This limits the practical use of this type of D/A converter for any more than 4-bit conversions.

18. Principle of M-DACMM (1/2) • MEMS Digital-to-Analog Converter Mirror Module (M-DACMM) • An analog output can be generated by the combination of the bits actuated by the parallel-plate actuators.

19. Principle of M-DACMM (2/2) The displacement of the platform is G. Zhou et al., “Micromechanical digital-to-analog converter for out-of-plane motion,” Journal of Microelectromechanical Systems, Vol. 13, No. 5, 2004, pp. 770-778

20. Electrostatic Parallel-Plate Actuator • The parallel-plate actuator is operated in a two-state mode. • Contact state: V > Vpull-in • Suspended state: V = 0

21. Connection Springs The spring constant can be estimated as: D. Perouliset al., “Electromechanical considerations in developing low-voltage RF MEMS switch”, IEEE Transactions on Microwave Theory and Techniques, Vol. 15, No. 1, 2003, pp. 259-270

22. Design of the 4-bit M-DACMM

23. Simulation Results The displacement of the mirror platform is simulated using CoventorWare®.

24. Structure of the M-DACMM Device • The device is composed of • Platform Layer • Electrode Layer

25. Fabrication Process – Platform Layer LPCVD nitride deposition on a SOI wafer Backside RIE etching Backside KOH anisotropic wet etching Front-side RIE etching

26. Fabrication Process – Platform Layer Front-side ICP-RIE etching Remove nitride and Release oxide Cr/Au deposition

27. Fabrication Process – Electrode Layer Pyrex 7740 glass wafer HF/HCl etching HF/HCl etching Cr/Au deposition and pattern

28. Fabrication Process – Assembly The platform layer is flipped and assembled to the electrode layer by eutectic bonding. Platform Layer Electrode Layer

29. Fabrication Results – Platform Layer

30. Fabrication Results – Electrode Layer

31. Chip Assembly The fabricated device was assembled and wire-bonded on a PCB for testing. Electrode Layer Platform Layer PCB

32. Architecture for Transient Behavior Measurements Laser Doppler Vibrometer

33. Transient Response (1/2) The switching time between two input binary states is less than 80 ms.

34. Transient Response (2/2) There is no oscillation during the dynamic switching between the suspended state and contact state, which indicates that the system is overdamped due to large mirror area without perforations.

35. Digital-to-Analog Convert The displacement increases linearly with the input binary code. The measured full-scale displacement is 1050 nm and the motion step (the least significant bit, LSB) is 72 nm.

36. Summary • 4-bit MEMS Digital-to-Analog Converter Mirror Module • Advantages • High position accuracy • High repeatability • Small size • Actuators • Electrostatic parallel-plate actuator • Operating voltage: 60 V • Measured results • Full-scale displacement: 1050 nm • Motion step: 72 nm • Switching time: < 80 ms

37. Outline • Introduction of Micromirror • Review of Micromirror Devices • Motivation • MEMS-DAC Micromirror Module • White-Light Interferometry System • Principle • Experiment Results • Conclusions

38. White-Light LinnikInterferometry

39. White-Light Interferometry (WLI) System with M-DACMM The interferograms of different phases are obtained by moving the reference mirror on the M-DACMM.

40. Interference For single-wavelength fringe pattern, each fringe pattern has a unique spatial frequency. At zero optical path difference, constructive interference always occurs.

41. White-Light Interference The intensity can be expressed as:

42. Seven-Point Algorithm (1/2) The corrected intensity can be expressed as Seven measurements are made of the intensity as follows: where is the average wavelength of the light source.

43. Seven-Point Algorithm (2/2) Assume the coherence envelope can be considered as locally linear, therefore The phase can be expressed by The surface height can be calculated as

44. Phase Unwrapping The calculated always stay within the range. From the phase equation, it can be expressed as where , The phase can be represented as

45. Architecture of WLI System KT1008D, KYOTTO

46. Preliminary Setup of WLI System

47. Setup of WLI System with M-DACMM The M-DACMM device is installed in this system to replace the reference mirror and the piezoelectric stage.

48. Procedures for Surface Profile Measurement

49. Graphical User Interface The procedures of mirror actuation, image acquisition and surface profile calculation are developed in LabVIEW software.

50. Sample Preparation • Two samples are fabricated for the surface profile measurements. • The grating structure is fabricated by etching a silicon substrate with RIE. • The optical lens is made by glass and coated with Au of 3000Å. • Grating Structure • Optical Lens