Contactless Excitation of MEMS Resonant Sensors by Electromagnetic Driving

Transcription

1 Presented at the COMSOL Conference 2009 Milan University of Brescia Department of Electronics for Automation Contactless Excitation of MEMS Resonant Sensors by Electromagnetic Driving Marco Baù, VF V. Ferrari, idm D. Marioli ili Introduction The magnetic excitation principle Application to miniaturized resonators Application to MEMS resonators Conclusions

2 Introduction Excitation electrostatic piezoelectric thermoelectric optothermic magnetic Measurand Mechanical Resonator A Detection capacitive piezoelectric piezoresistive optical magnetic Output Frequency Applications that do not allow for cabled solutions Applications in environments incompatible with active electronics Mechanical resonator sensors are in principle suitable: The resonant approach is robust The resonant frequency does not depend on the detection technique adopted Comsol Conference 2009, October 14-16, 2009, Milan, Italy 2/11

3 Contactless Magnetic Excitation Time-Varying Magnetic Field B AC Magnetic Films Magnetic Film Force Holder Incompliant with traditional microfabrication process Driven Currents DC or AC Magnetic Field Driven Current I AC Holder Proposed Approach Force F Unsuitable for contactless operation Contactless excitation of mechanical resonances in microstructures Exploitation ti of the interaction ti between external DC or AC magnetic field with AC currents inductively coupled to the resonator No specific magnetic property is required The resonators are required to be only electrical conductive Comsol Conference 2009, October 14-16, 2009, Milan, Italy 3/11

4 The excitation principle I E (t) Without static magnetic field F z 1 ( t) J i ( E ) BEr sin( ) sin(2 Et ) 2 Excitation signal V E (t) E B Ez J i (ω E ): current density onto the resonator surface. B Er : radial component of the excitation magnetic field. x y B Er 0r z J i F z N S J i F z B Er 0r Resonator conductive surface : Phase difference between the force F z and the current I E caused by the impedence of the resonator surface. With static magnetic field F z 1 ( t) Ji ( E ) BEr sin( ) B0r sin( Et ) 2 B 0r : radial component of the excitation magnetic field. Comsol Conference 2009, October 14-16, 2009, Milan, Italy 4/11

5 Simulation of the excitation principle Vertical force Axial symmetry geometry R c Coil L AC Simulation Evaluation of Trend of the magnetic field Magnitude of the induced eddy current Magnitude of F z component of force h R d Resonator Coil radius R c =2.5 mm Distance h=5 mm Maximum of the force for R d =5 mm Comsol Conference 2009, October 14-16, 2009, Milan, Italy 5/11

6 Simulation of the excitation principle Vertical force R c Coil h R d L Resonator Coil radius R c =2.5 mm Distance h=5 mm Maximum of the force for R d =5 mm Axial symmetry geometry orm (N/m 3 ) f no AC Simulation Evaluation of 5.E-02 4.E-02 3.E-02 2.E-02 1.E-02 0.E+00 Trend of the magnetic field Magnitude of the induced eddy current Magnitude of F z component of force f norm 1 V V F dv 0.E+00 5.E-03 1.E-02 2.E-02 2.E-02 Radius R d (m) z Comsol Conference 2009, October 14-16, 2009, Milan, Italy 6/11

7 Experimental results on miniaturized resonators Clamped-clamped p resonator CANTILEVER Ail Axial Tension Hld Holders Beam Axial Tension Clamped-clamped titanium beam POSITION SENSITIVE DETECTOR A CONDITIONING CIRCUIT LASER NI6023 ADC CLOCK SYNCHRONOUS REF UNDERSAMPLING 2f EXCITATION COIL EX f G PLL CONTROL SOFTWARE Titanium parameters:e=105 x 10 9 Pa, ρ=4940 kg/m 3, σ=7.407 x 10 5 S/m Dimensions: 17 mm x 1.4 mm x 100 μm Excitation: 35 V (rms)/ 26 ma (rms) Excitation distance: 2 mm Optical system for the frequency characterization of resonators Comsol Conference 2009, October 14-16, 2009, Milan, Italy 7/11

8 Experimental results on miniaturized resonators Clamped-clamped p resonator CANTILEVER Ail Axial Tension Hld Holders Beam Axial Tension Clamped-clamped titanium beam POSITION SENSITIVE DETECTOR A CONDITIONING CIRCUIT LASER NI6023 ADC CLOCK SYNCHRONOUS REF UNDERSAMPLING 2f EXCITATION COIL EX f G PLL CONTROL SOFTWARE Titanium parameters:e=105 x 10 9 Pa, ρ=4940 kg/m 3, σ=7.407 x 10 5 S/m Dimensions: 17 mm x 1.4 mm x 100 μm Excitation: 35 V (rms)/ 26 ma (rms) Excitation distance: 2 mm Optical system for the frequency characterization of resonators Comsol Conference 2009, October 14-16, 2009, Milan, Italy 8/11

9 MEMS Design Effects of the downscaling of the dimensions of the resonators Reduction of the total magnetic flux linked to the structures Decrease of the induced eddy-current density Decrease of the Lorentz force Increase of the mechanical stiffness Anchor Conductive path Cantilever Section Area of the coil Collecting flux coil Proposed solution: conductive path on the surface of the cantilever Connection with a collecting flux coil Multiple transversal paths for the distribution of the circulating currents Comsol Conference 2009, October 14-16, 2009, Milan, Italy 9/11

10 Simulation of MEMS devices w i Design of the conductive paths: Choice of width w i and reciprocal distance d ij Constraint of equal distribution of the circulating current in each path d ij h L f ris 2 E Comsol Conference 2009, October 14-16, 2009, Milan, Italy 10/11

11 Simulation of MEMS devices Design of the conductive paths: Choice of width w i and reciprocal distance d ij N I(A) Constraint of equal distribution of p the circulating current in each path AC electrical simulation with unity current impressed Computation of the current in the transversal paths Estimation of the resonant frequencies of the structures with the conductive paths Simulation for a 1500 x 700 x 15 µm cantilever Value from the theoretical predictions: 9200 Hz Value from the simulation: 9404 Hz h L f ris 2 E Comsol Conference 2009, October 14-16, 2009, Milan, Italy 11/11

12 Fabrication of MEMS devices Polysilicon Diffusion Crystal Silicon Buried Oxide Silicon Passivation Metal 2 Via Metal 1 Contact Process of the CNM (Centro Nacional de Microelectronica) of Barcellona Bulk-micromachining process 450 μm-thick N-doped BESOI (Bond and Etch back Silicon on Insulator) substrate with <100> orientation ti 5 photolithographic masks (1 polysilicon, 2 metals) Die with 4 cantilever On-chip conductive paths On-chip collecting flux coil Half-bridge configuration of polysilicon piezoresistor Comsol Conference 2009, October 14-16, 2009, Milan, Italy 12/11

13 Experimental results on MEMS resonators z LATERAL VIEW x I e I Cantilever beam Laser PSD The current I e induces the B current I r Amplitude Vibration Interaction of I with B r Supporting Lorentz force F z substrate Optical system F Z Magnet Excitation signal C1 C2 B r Single turn on-chip coil I F Z y TOP VIEW x Comsol Conference 2009, October 14-16, 2009, Milan, Italy 13/11

14 Experimental results on MEMS resonators z LATERAL VIEW x I e I Cantilever beam Laser PSD The current I e induces the B current I r Amplitude Vibration Interaction of I with B r Supporting Lorentz force F z substrate Optical system F Z Magnet Excitation signal C1 C2 Single turn on-chip coil Contactless Magnetic Excitation Excitation ti coil C1: L=9.8 mh Excitation current : 200 ma Coil C2: L=5.3 mh Magnet: 1.4 T Distance C1-C2: 8 mm 8.00 B r F Z 7.00 I Magnitude (a.u.) y TOP VIEW x Frequency enc (Hz) Comsol Conference 2009, October 14-16, 2009, Milan, Italy 14/ Phase (deg)

15 Conclusions Contactless excitation of miniaturized resonators by means of magnetic fields has been proved The effects of the downscaling of the dimensions of the resonators on the excitation principle have been analyzed Dedicated solutions have been studied and applied to the design of MEMS microresonators Contactless excitation of MEMS microresonators by means of magnetic fields has been proved The principle can be adopted to excite contactless sensors operating on short-range excitation distance of the order of 1 cm The experimental activity is investigating the possibility of extending the principle to vibration readout Contactless excitation and detection of vibrations in conductive microstructures can be in general applied to measure a large variety of physical quantities which can cause a predictable shift in the resonant frequency of the structure Comsol Conference 2009, October 14-16, 2009, Milan, Italy 15/11