Binary Resonant Wings Joe Evans, Naomi Montross, Gerald Salazar Radiant Technologies, Inc. January 15, 2018 International Workshop on Piezoelectric MEMS

Introduction To be profitable, products based on piezoelectric MEMS must have high yield with excellent uniformity fabricated using inexpensive processes. Radiant Technologies is exploring an I-beam process concept that meets these three objectives. The performance of a pMEMS with opposing resonant wings fabricated using the I-beam process will be described in detail.

Binary Wings Through-wafer hole Cantilever “Wing” The cantilevers are ~5m thick. The central crossbar holding the two wings is full-wafer thickness. Parallel-Plate capacitors surround the cantilever edges while IDE capacitors form the center spine.

Test Fixturing Two dice packaged and wire bonded per 18-pin DIP. LDV laser Two dice packaged and wire bonded per 18-pin DIP. Polytec OFV-534 Laser Doppler Vibrometer in displacement mode.

Binary Wings 1 Hertz switching hysteresis loop of the parallel plate capacitors Approximately ±6m tip displacement.

Butterfly Motion Parallel-Plate Actuator Cantilever tip double butterfly loop driven by perimeter parallel-plate capacitors at 1Hz/20V with 32,000 points.

Butterfly Motion Interdigitated Electrode Actuator Butterfly flipped upside down due to IDE field direction. Cantilever tip double butterfly loop driven by central IDE capacitors at 1Hz/165V with 32,000 points.

Original pMEMS Process Standard Thin PZT Capacitor Topside DRIE Etch Backside DRIE Etch Release Die boundaries Cantilever Radiant proposed this process at the 2010 ISAF-ECAPD meeting. It requires two DRIE steps and an oxide release etch.

A Picowatt Energy Harvester 1.7mm Radiant proposed this device at the 2010 MRS winter conference based on the ENHARVKAAL design by Dr. Jan Smits. It is very difficult to yield using the 2-DRIE process.

Membrane Single DRIE Process Standard Thin PZT Capacitor Topside Shallow Plasma Etch Backside DRIE Etch Piezoelectric membrane capacitors yield. Radiant uses these capacitors as displacement references in AFMs.

Piezoelectric Membrane Radiant described the performance of this device at the 2014 piezoMEMS Workshop in Kobe.

Single DRIE Cantilever Process Start with the successful membrane capacitor but make the backside DRIE etch pattern wider. This process works but the cantilever formed only from the ferroelectric capacitor is too fragile to survive release or perform work.

I-beam Process Standard Thin PZT Capacitor Topside Shallow Plasma Etch Backside DRIE Etch I-beam metal Apply any combination of metals and/or other materials above the capacitor to provide mechanical strength. Material selection, thicknesses, and geometries are unlimited. This structure was inspired by the PZT-Capacitor-on-Nickel-Film energy harvester proven by Dr. McKinstry’s group at PSU!

Advantages of the I-beam pMEMS Process Robust structures. High Quality factor. Flexible but strong cantilever with large displacement. I-beam multi-layer stacks can mix different materials. Copper – Nickel – SiN – SiOxyN – Multiple layers Cheaper process flow.

Impulse Testing Impulse applied to cantilever parallel-plate capacitor. Identify single or multiple resonant frequencies of the Wings cantilever using classic impulse response test: 5 kHz & 46 kHz.

Displacement sat multiple frequencies. Resonant Testing 5 kHz stimulus This test procedure requires more development but from the results this device has a Q of at least 50 based on its amplitude change during multiple cycles. Displacement sat multiple frequencies. Cantilever tip displacement at constant stimulus cycling into the parallel-plate capacitors. Find the Q!

Actuator/Sensor Combination ~0.83pC/m ~0.6m Actuate the cantilever at resonance using one capacitor while sensing the piezoelectric polarization generated in the second capacitor by the tip motion.

pMEMS Testing Based on our work with these cantilevers, Radiant is adding new functions to its Precision family of testers to support pMEMS evaluation: Impedance measurements with built-in LCR. Parallel Digital I/O port to control pMEMS circuitry from Vision Frequency counter up to 60 MHz Asynchronous pulse generator DC Bias voltage source Vision tasks: FFT Task Impulse Response Task Mechanical Resonance Task Simultaneous displacement and velocity capture from LDV

Conclusion The I-beam process generates high-yield piezoelectric cantilevers with robust mechanical and electrical properties. Future development will explore a variety of device geometries as well as the Mahoh process for dicing and direct PC board soldering of each die without packaging. Radiant is porting newly developed pMEMS test procedures to its testers for use by the research community. Radiant’s pMEMS process will be available to university and corporate researchers for experimental device fabrication.