MEMS SENSOR FOR MEMS METROLOGY

Transcription

1 MEMS SENSOR FOR MEMS METROLOGY IAB Presentation Byungki Kim, H Ali Razavi, F. Levent Degertekin, Thomas R. Kurfess 9/24/24

2 OUTLINE INTRODUCTION Motivation Contact/Noncontact measurement Optical interferometer Research goal MICROINTERFEROMETER Modeling and system analysis Fabricated micro lens sample Microphone scanning by fabricated sensor System integration CONCLUSION 9/24/24 2

3 MOTIVATION MEMS Products Small devices Many devices on wafer. Usually periodic in array Dynamic, moving structures Sensor: microphone,accelerometer,pressure sensor Mechanical actuator MEMS Metrology 25% of cost of products sold spend on quality inspection (% in the semiconductor industry) 4-7% product yield (9% in the semiconductor industry) (from The Commercialization of Microsystems 2) 9/24/24 3

4 CONTACT/NONCONTACT GEOMETRY MEASUREMENT Contact Measurement CMM, Mechanical profilometer, AFM Slow. Deforms devices. No dynamic measurement Noncontact Measurement Microscope Needs large lens for high accuracy in depth. No dynamic measurement Interferometer Dynamic measurement. Slow (no parallel operation). Each unit is expensive 9/24/24 4

5 OPTICAL INTERFEROMETER Michelson s Interferometer -4 Å/ Hz Dynamic measurement: ~3MHz Combines with lens for lateral resolution Mirror λ/2 Beam Splitter Coherent source I(x) x Detector x 9/24/24 5

6 RESEARCH GOAL Metrology tool for MEMS Needs small, fast, sensitive, parallel and inexpensive metrology tool Noncontact measurement for dynamic structures Interferometer Enabling technique for making small sensor MEMS processing Integration with electronics Metrology tool MEMS devices 9/24/24 6

7 PHASE SENSITIVE DIFFRACTION BASED DISPLACEMENT DETECTION Reflecting Reflected mode intensity is periodic surface Interferometric precision can be d =λ/2 achieved by monitoring the intensity of any of the reflected orders d g substrate Example, HeNe laser (λ=632.8nm), reflection diffraction reference fingers with d g =2µm Diffraction pattern depends on d Intensity d =λ/2 d =λ/4 d =λ/ Observation plane (mm) Normalized intensity Reference diffraction grating fingers Zeroth order First order Gap thickness (µm) d =λ/4 2 sin ( ) Distribution of the reflected light Variation of the intensity with of the on an observation plane in distance d on an observation 9/24/24 simulation plane in simulation 7 I I 2π d λ 2 cos ( ) 2π d λ

8 MICROINTERFEROMETER Target surface.8 Zeroth order First order Quartz wafer Micromachined lens Diffraction gratings Detector Schematic of a micro-interferometer Normalized intensity Gap thickness (µm) Variation of the intensity with distance d on an observation plane Diffraction gratings Target surface Transparent substrate Micromachined lenses 9/24/24 8 Limitations Not feasible to measure the step more than λ/8 change Inclined angle should be less than 2tan - (2F-number)

9 SYSTEM ANALYSIS (D) Designed lens and 3 µm period diffraction grating having.5 µm blank 2 nd lobe is seen around 26 µm {=tan[sin-(.6328/3)]} Photo detector having width 2µm at z=- µm, x=26µm Normalized intensity Detector output with width 2 µ m, at x=26 µ m, z=- µ m Relative intensity z Intensity variation on the photo detector plane Target location: µm from focal point, PD: µm behind Relative intensity Relative intensity x Target location: µm from focal point, PD: µm behind X coordinate at detector position [µ m] Target location:. µm from focal point, PD: µm behind Distance from focal point [µ m] X coordinate at detector position [µ m] X coordinate at detector position [µ m] 9/24/24 9

10 FEASIBILITY TEST OF SYSTEM Travel distance of λ/2 per one cycle. For HeNe laser (λ=632.8nm), it is about.3 µm Mirror Lens Laser beam Diffracted beams Normalized intensity Diffraction grating Detector Normalized distance((x-f )/λ) Measured diffraction intensity at the detector in normalized distance in experiment 9/24/24

11 FABRICATED LENS SAMPLE Radius of curvature=62. µm, focal length=99 µm, maximum deviation from sphere=.3%, power loss=3% 9/24/24

12 FREQUENCY MEASUREMENT OF MICROPHONE MEMS Microphone 6 µm diameter Electrostatic actuated at 72kHz by V(DC)±6V(AC) 5,,5 burst Detector signal 72kHz cycles/.39x -5 sec Shows ringing Microphone Detector signal 5 burst burst 5 burst x -5 Time (sec) 9/24/24 2

13 DISPLACEMENT MEASUREMENT OF MICROPHONE MEMS Microphone Two times of 5 bursts during moving 5 µm/sec on the stage λ/2 (.3 µm) period ±~5 nm of displacement during bursts Microphone Moving Stage Normalized intensity Detector output with width 2 µ m, at x=26 µ m, z=- µ m Distance from focal point [µ m] Detector Output [mv] Movement of vibrating membrane Burst # Time [msec] Burst # /24/24 3 Detector Output [mv] Burst # Burst # Time [msec]

14 SCANNING OF MICROPHONE MEMS Microphone 5 µm scanning step Sensitive to alignment error Microphone 9/24/24 4

15 INTEGRATED SYSTEM Implementation of the proposed micro-inteferometer structure with integrated electronics Si PN-junction photodiode was fabricated to detect st order Trough-hole etch by ICP (Inductively Coupled Plasma) etcher Target surface Target surface Micromachined lens Diffraction grating Glass wafer Diffracted orders Incident light Photo detectors Silicon wafer Fabricated photodiode 9/24/24 5

16 CONCLUSION Feasibility test results of the sensor show a good agreement with analysis results Fabricated micro lenses was characterized by surface measurement Vibrating frequency of the microphone can be detected using proposed sensor Vibration magnitude of the microphone is also detectable if the sensor is properly calibrated 9/24/24 6