DYNAMIC ANALYSIS OF CMUTs IN DIFFERENT REGIMES OF OPERATION Baris Bayram, Edward Hæggström, A. Sanli Ergun, Goksen G. Yaralioglu, and Butrus T. Khuri-Yakub Ginzton Laboratory, Stanford University, CA 2003 IEEE Ultrasonics Symposium, Honolulu, Hawaii This work is funded by Office of Naval Research
Outline Motivation FEM model of a single CMUT cell Dynamic FEM analysis Results Conclusion
Motivation Goal: Low voltage, high frequency CMUTs with more output power & less nonlinearity Method: Investigate different operation regimes for CMUTs: Conventional (no contact) (always in contact) Collapse-snapback (intermittent contact)
Different Operation Regimes
FEM model of a single CMUT cell Axisymmetric 2-D model Electrical Properties Structural Properties Fluid-structure Interface Acoustic Wave Equation Exact Absorbing Boundary (Grote et al) Basic Properties Collapse: 80 V Snapback: 50 V Resonance: 5 MHz
Dynamic FEM Analysis Commercially available software (ANSYS 7.1) Transient Analysis Coupled electrical & structural analysis Fluid medium Contact capability Exact absorbing boundary (Grote et al) Large Signal Characterization Pulse and sinusoidal (AC) excitation Displacement and pressure output Nonlinear distortion: 2 nd harmonic
Average Displacement ( m) Pressure (MPa) Results: Pulse Excitation -0.074-0.076 0.4 0.2 0-0.078-0.2-0.08 0 0.2 0.4 0.6 0.8 1-0.4 0 0.2 0.4 0.6 0.8 1-0.02 Conventional 0.05 Conventional -0.022 0-0.024 0 1 2 3 4 5 6 7 Time ( s) -0.05 0 1 2 3 4 5 6 Time ( s) V BIAS =70V, V PULSE =+5V, t PULSE =20ns Conventional Displacement (p-p) (Å) 39 70 Resonance freq. (MHz) 3.84 8.75
Results: Pulse Excitation operation V BIAS =65 V Pulse excitation: V PULSE =15V t PULSE =20ns Displacement (p-p): 200 Å, 13 Å/V Pressure (p-p): 2000 kpa, 133 kpa/v
Displacement (p-p) (A) Resonance Frequency (MHz) Results: Bias Voltage 70 60 50 40 10 8 6 4 30 Conventional 20 55 60 65 70 75 80 Bias Voltage(V) 2 Conventional 0 55 60 65 70 75 80 Bias Voltage(V) V PULSE =+5V, t PULSE =20ns operation: - Higher resonance frequency : contact radius effect - Larger displacement & output pressure
Results: AC Excitation operation V BIAS =70 V AC excitation: f EXC =1 MHz V P-P =30V 2 nd harmonic: -23 db Displacement (p-p): 200 Å, 7 Å/V Pressure (p-p): 36 kpa, 1.2 kpa/v
2nd order harmonic level (db) Average Displacement (p-p) (A) Results: AC Amplitude -14-16 -18-20 -22-24 -26 Conventional -28 10 15 20 25 30 AC Amplitude (p-p) (V) 240 220 200 180 160 140 120 100 Conventional 80 10 15 20 25 30 AC Amplitude (p-p) (V) V BIAS =65 V, f EXC =1 MHz operation: MORE LINEAR RESPONSE
Results: Collapse-snapback Collapse-snapback operation V BIAS =70 V AC excitation: f EXC =1 MHz V P-P =40 V 2 nd harmonic: -18 db 3 rd harmonic: -10 db Displacement (p-p): 1200 Å, 30 Å/V Pressure (p-p): 3600 kpa, 90 kpa/v
Results: Collapse-snapback Collapse-snapback operation V BIAS =65 V AC excitation: f EXC =5 MHz V P-P =60 V Displacement (p-p): 1600 Å, 26 Å/V Pressure (p-p): 9500 kpa, 158 kpa/v
Comparison of Operation Regimes Performance Conventional Collapse- Snapback V BIAS High Low Low V TOTAL <V COLLAPSE >V SNAPBACK >V COLLAPSE <V SNAPBACK Output Power Low Medium High Linearity Good Better Worse
Conclusion Investigated different regimes of operation using transient analysis in FEM operation: More linear Low voltage bias Higher frequency Collapse-snapback operation: Higher output pressure Future work: 3D analysis of CMUT arrays