Ångstrom-Level Piezoelectric Measurement

1. Ångstrom-Level Piezoelectric Measurement Joe T. Evans, Jr., Scott Chapman, Bob Howard, Spencer Smith, Michelle Bell Radiant Technologies, Inc. July 8, 2013

2. Summary • Motivation • Equipment Architecture • Piston Displacement • Calibration • MEMs Measurement • Future Objectives All of the test samples shown below were fabricated by Radiant.

3. Why Measure Ångstroms? • Thin ferroelectric and piezoelectric films have great potential to impact the future economy. • More sensitive sensors • Tiny machines • Autonomous memory • Radiant already offers advanced piezoelectric tasks to interface existing AFMs, laser vibrometers, and other displacement meters with its testers. • After years of planning, we have now an affordable instrument to measure absoluteÅngstroms.

4. Butterfly vs PFM • The goal is to accurately measure the butterfly displacement loop of a piezoelectric element under test. • The butterfly loop is a Large Signal Measurement • PFM is a Small Signal Measurement PFM is to the Butterfly Loop as Small Signal Capacitance is to the Polarization Hysteresis Loop.

5. Butterfly Measurement Techniques • Atomic Force Microscope • These These are plots of the piston motion of capacitor surface. • Laser Vibrometer Asylum SA Polytec OFV534/OFV5000

6. Noise is Ever Present • Averaged and Smoothed • Original Data Even on an exceptionally quiet commercial AFM, noise is significant.

7. Goal • Find a method to • Inexpensively measure ferroelectric and piezoelectric displacement loops of thin films. • Create a system that the average university lab can afford. • Guarantee a level of absolute displacement accuracy.

8. System Architecture Digital microscope camera Light lever Sample holder Manual sample positioning Z-piezo actuator

9. Quad Cell Laser - + Reference GPID  HVA Light Lever Control System Chuck PZT Actuator • Negative feedback to the piezoelectric actuator under the chuck maintains the position of the reflected laser spot at the reference position on the Quad Cell.

10. Quad Cell Laser - + Reference GPID  HVA Piezoelectric Test Feedback Signal (pMEMs) Precision Tester Chuck PZT Actuator Error Signal (Piston) • For Ångstrom-level displacements, the PNDS looks directly at the Error Signal. The test is run faster than the GPID can respond. • For MEMs-level displacements, the PNDS looks at the Feedback Signal. The test is run slowly so the GPID can respond.

11. Piston Displacement The photosensor error signal measured at high speed (~1kHz) is used to capture Ångstrom-level displacements.

12. Compare 1m-thick 4/20/80 PNZT with platinum electrodes – 1kHz. The Polytec laser vibrometer has an absolute distance reference in the wavelength of its laser light. Amplitudes of the PNDS measurements when properly calibrated compare well with the laser vibrometer.

13. piezoMEMs Displacement Large displacements such as the 2µ-thick PNZT membrane in the photo below are measured at 1Hz or slower while monitoring the feedback signal controlling the chuck vertical position. The feedback signal is the position of the chuck and must be multiplied by “-1” to properly orient the loop.

14. Finding Small Electrodes • All butterfly loops are made in non-tapping contact mode. • The PNDS has a manual X:Y stage but some capacitors may be smaller than the cantilever. • Surface scans in contact mode are sufficient to find small top electrode features for test using a conductive or non-conductive cantilever. An image of the error function can be efficient at finding features and determining their coordinates.

15. In-Contact Surface Scan • High fidelity is not required in the scan image to successfully identify the target coordinates for the test.

16. Noise Reduction • The measurements are noisy at such small displacements. Noise reduction procedure: • Make multiple measurements • Move each loop to the origin • Remove “tilt” in the loop due to Z-drift • Average all of the loops. • Smooth the averaged loop.

17. Noise Filtering Raw data vs filtered loop.

18. Repeatability

19. Conductive Cantilever • The PNDS works in DC contact mode with a top electrode. • Only a few organizations in the world right now are capable of making micron-scale capacitors and connecting them to bond pads. Therefore, a conductive cantilever must be used to make electrical contact with the top electrode of very small capacitors. • The bottom electrode must be contacted by a wire in order to allow the surface scan motion of the chuck.

20. Conductive Cantilever • The traditional 40nm-diameter probe tip coated with metal cannot last long when switching ferroelectric capacitors. • The current density is too high, causing the metal coating to evaporate. • A better choice is to use a tip with a wider diameter and coat with thicker metal. • Radiant uses the 3m “Plateau” tip from App Nano coated with 1000Å of platinum. • This tip will execute hundreds of hysteresis loops on large area capacitors without burning out.

21. Absolute Calibration Step #1: Do a surface scan of a calibrated reference sample to calibrate the piezo Z-element. Step #2: Execute a FORCE-DISTANCE curve by pushing on the cantilever with the now calibrated piezoelectric Z-element to calibrate the photo-sensor App Nano SHS-0.1-3 1000Å Reference Grid Force-Distance Measurement [Y-axis = Photosensor voltage. X-axis = Z-element displacement.]

22. Calibrating the Chuck Actuator • 30µ x 30µImage • Measure a calibrated step reference to calibrate the piezoelectric actuator of the chuck. • App Nano SHS-01.3 1000Å reference grid. • ±5% vertical accuracy on step height • 10µ grid of 5µ holes

23. Calibrating the Chuck Actuator 5µ x 5µ Image • Execute a 5µ scan over an edge to find the step. • Use a fast scan and the error image to identify edge.

24. Calibrating the Chuck Actuator 2µ x 2µ Image • Use a small scan box over an edge to provide an accurate step reference. • Slow scan to prevent overshoot. • Small area to eliminate Z-distortion by X:Y actuators

25. Calibrating the Chuck Actuator Extract Vertical Profile

26. Calibrating the Chuck Actuator Extract Vertical Profile • De-rotate the data • Use tools to measure vertical height. • Enter any adjustment (in this case zero) into the PNDS GUI to adjust Z-sensitivity.

27. Calibrating the Photosensor Force-Distance Curve • Chuck piezoelectric actuator is now calibrated to ±5%. • Measure Force-Distance curve to determine the sensitivity of the photosensor to vertical displacement. • Use piezoelectric actuator to push up on the cantilever a specific distance. • The slope of the photosensor response is the scale factor to enter into Vision to convert PNDS photosensor output to sample displacement.

28. Calibrating the Photosensor Force-Distance Curve ~1100Å/volt ~0.9mV/Å PII/PMF = 0.3mV single-pass resolution Vz z • Y-axis Voltage output of photosensor • X-axis Vertical displacement of chuck actuator

29. Estimated Accuracy • Piezoelectric Actuator ±5% • Photosensor ±5% • Best case accuracy ±10% • Radiant is comfortable with a claim of ±15%.

30. MEMs Cantilever Measurement 1.2mm

31. Cantilever Motion Average of ten 1-second loops. PNDS cantilever capturing butterfly motion of resonator edge.

32. Complex Cantilever Motion By measuring at different parts of the resonator as it is flexed in pseudo-static piezoelectric motion, more complex behavior is revealed.

33. Complex Cantilever Motion Construct of cantilever motion. Single-sided voltage application should make the cantilever bend upwards in a smooth curve. This cantilever does not.

34. True AFMs and Vibrometers • The PNDS is intended to be an inexpensive system for small materials programs. • All of the measurement techniques described here work with all of the more advanced commercial AFMs and with sensitive laser vibrometers.

35. Future Efforts • Gain more experience with the technique on various samples. • Investigate the variation of the butterfly loop across the face of a capacitor. • Follow evolution of local butterfly loop with fatigue, imprint, retention, and temperature.