Noncontact Modal Excitation of Small Structures Using Ultrasound Radiation Force Society for Experimental Mechanics Annual Meeting Springfield, MA June 4, 2007 Thomas M. Huber, Scott D. Hagemeyer, Eric T. Ofstad Physics Department, Gustavus Adolphus College Mostafa Fatemi, Randy Kinnick, James Greenleaf Ultrasound Research Laboratory, Mayo Clinic and Foundation

2 Introduction  Overview of Ultrasound Stimulated Excitation  Uses ultrasound radiation force for non-contact modal excitation  Selective Excitation by Phase Shifted Pair of Transducers  Results for simple cantilever  Results for MEMS Gyroscope  Results for MEMS mirror  Conclusions

3 Ultrasound Stimulated Radiation Force Excitation  Vibro-Acoustography Developed in 1998 at Mayo Clinic Ultrasound Research Lab by Fatemi & Greenleaf  Difference frequency between two ultrasound sources causes excitation of object. Detection by acoustic re-emission  Technique has been used for imaging in water and tissue  We have also used the ultrasound radiation force for modal testing of organ reeds and hard drive suspensions (IMAC 2006)

4 Ultrasound Stimulated Amplitude Modulated Excitation  Dual sideband, carrier suppressed amplitude modulated signal centered, for example, at 550 kHz  Difference frequency of Δf of 100 Hz to over 50 kHz  Difference frequency Δf between ultrasound beams produces radiation force that causes vibration of object  Vibrations were detected using a Polytec laser Doppler vibrometer  In some experiments, comparison of ultrasound excitation and mechanical shaker  Transducer used in this experiment had 1.5 mm diameter focus spot size

5 Photos of Setup

6 Selective Excitation using Phase-Shifted Pair of Transducers  To illustrate this technique, consider first a simple cantilever in air  Instead of using a single transducer, use a pair of ultrasound transducers to allow selective excitation of transverse or torsional modes  If radiation force from both transducers are in phase, selectively excites transverse modes while suppressing torsional modes  If radiation force is out of phase, selectively excites torsional modes while suppressing transverse modes  Demonstrated for cantilevers, MEMS mirror and hard drive suspensions

7 Phase-shifted selective excitation: Detailed Description  Two 40 kHz transducers, each with dual sideband suppressed carrier AM waveform  Modulation frequency swept from 50 – 5000 Hz  Difference frequency Δ f leads to excitation from 100 Hz – 10 kHz  Modulation phase difference of 90 degrees leads to 180 degree phase difference in radiation force

8 Phase Shifted Selective Excitation  Use scanning vibrometer to measure deflection shape  Adjust amplitudes of two 40 kHz transducers to give roughly equal response

9 Phase Shifted Selective Excitation  Adjust amplitudes of two 40 kHz transducers to give roughly equal response  When both transducers turned on simultaneously with same modulation phase  Enhanced Transverse Mode  Suppressed Torsional Mode

10 Phase Shifted Selective Excitation  Driving in-phase excites transverse but suppresses torsional modes (dashed blue curve)  Driving out-of-phase (phase difference near 90 degrees) excites torsional while suppressing transverse modes (red curve)  This technique allows information about mode shape to determined even from a single point vibrometer  Can differentiate two overlapping modes (if, for example, 2 nd transverse and 1 st torsional mode were at nearly identical frequencies)

11 Case Study: MEMS Gyroscope  Analog Devices ADXRS MEMS Gyroscope  Pair of Test Masses ¾ mm square separated by ½ mm  Test masses have in-plane resonance frequency of 14 kHz.  Question: What about out-of-plane motion

12 Ultrasound Excitation of MEMS Gyroscope  Scanning vibrometer detects motion of test masses & nearby regions  Ultrasound transducer focused on gyroscope.  Central frequency of 600 kHz, with Δf = 13.5 kHz  Maximum velocity of 250 μm/s  Measured out-of-plane displacement amplitude of 2.5 nm!

13 Ultrasound Excitation of MEMS Gyroscope Ultrasound transducer centered Ultrasound transducer Moved ½ mm right Ultrasound transducer Moved ½ mm left Base Excitation with Mechanical Shaker Vibrates entire structure Demonstrates capability of this technique for non- contact selective excitation without exciting the base

14 Another Device Tested: 2-d MEMS Mirror  Manufactured by Applied MEMS  Mirror is 3mm on Side - Gold plated Silicon  Three vibrational modes  X Axis torsion mode: 60 Hz  Y Axis torsion mode: 829 Hz  Transverse mode (forward/back): 329 Hz (incidental – not used for operation of mirror)

15 Selective Ultrasound Excitation of MEMS Mirror  Ultrasound focus ellipse about 1x1.5 mm  Focus position can be moved horizontally or vertically  Changing transducer position allows selective excitation  Upper figure: All modes present when focus near center of mirror.  Red line shows excitation using mechanical shaker.  Middle: X-torsional mode increases when ultrasound focus near top of mirror.  Bottom: Z-Torsional mode increases when focus near right edge

16 Selective Ultrasound Excitation of MEMS Mirror  X-Torsional mode peaks when focus near top/bottom of mirror  Transverse mode decreases as transducer moved vertically (smaller fraction of beam on mirror)  Ratio of amplitudes of X-Torsional to Transverse modes changes by over factor of 10x as vertical position is varied

17 Phase-Shifted Selective Excitation of MEMS Mirror  Driving in-phase excites transverse and Y-Torsion modes but suppresses X- torsional mode (blue curve)  Driving with 90 degree phase shift excites X-torsional mode while suppressing other modes (red curve)  By varying phase, the relative amplitude of the modes can be adjusted

18 Conclusions  Ultrasound excitation allows non-contact modal testing of MEMS mirror, MEMS gyroscope and other devices  Selective excitation  Insensitive to vibration of base or other parts of system  Selectively excite modes by moving ultrasound focus point  Phase-shifted pair of transducers allows transverse/torsional selectivity  May be especially useful for devices with nearly overlapping modes  Future possibilities:  Other MEMS devices???  In-plane excitation

19 Acknowledgements This material is based upon work supported by the National Science Foundation under Grant No Thank You