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IntroductionA MEMS vertical levitation comb drive actuator has been created for the measurement of piezoelectric coefficients in thin/thick films or piezoelectrically active micro-scale components of other MEMS devices. The device exerts a dynamic force of 33 μN at an applied voltage of 100 V. The charge developed on the piezoelectric test device is measured using a charge sensitive pre-amplifier and lock-in technique, enabling measurements down to 1×10-5 pC. The system was tested with ten different piezoelectric samples with coefficients in the range 70–1375 pC N-1 and showed a good correlation (r = 0.9997) to measurements performed with macroscopic applied stresses, and piezoelectric impedance resonance techniques.

MEMS DesignThe device measures the longitudinal piezoelectric coefficient (d33) through the direct piezoelectric effect. The MEMSCAP MetalMUMPS process [1] was chosen to manufacture the devices, because of the ability to mechanically connect isolated Ni components, allowing electrical isolation between the sample contact and charge measurement circuit, and the comb drive actuation circuit (figure 1). The free displacement of the device was modelled using Coventor [2] (figure 2), and validated with laser Doppler vibrometry.

CalibrationThe static and dynamic force output of the system was measured with a femtoTools capacitive sensor, which was calibrated against a CSM Instruments Ultra Nanoindentation Tester (figure 3). The blocking force of the MEMS was estimated through measurements of the relative spring stiffnesses of the sensor and MEMS, as:

Where VM is the voltage applied to the stators of the actuator (all other parts are grounded).The piezoelectric charge on the test sample was measured off-chip with an Amptek A250F charge sensitive preamplifier and lock-in amplifier. The behaviour of the amplifier was characterised as a function of input charge and frequency using a 2.0 ± 0.1 pF test capacitor (figure 4).

MEMS sensors for microscale piezoelectric metrology

Jenny Wooldridge, Andres Muniz-Piniella, Mark Stewart, Tamaryn A. V. Shean, Paul M. Weaver, Markys G. Cain National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK

ResultsThe MEMS system was verified using ten macro-scale samples (table 1), for which the piezoelectric coefficients were also measured with a Berlincourt tester and impedance resonance methods. Mechanical contact was made through a 2 mm diameter sapphire sphere affixed to the bottom of the sample, and the charge measurement circuit connected to the top and bottom electrodes of each sample.

Sample Type Shape Size (mm)

1 PZT – PC4D (Hard) Cylinder 15.054(1) : 3.018(8)

2 PZT – SP8 (Hard) Cylinder 15.036(1) : 6.30(3)

3 PZT – SP4 (Hard) Cylinder 15.039(2) : 6.31(3)

4 PZT – SP88 (Hard) Cylinder 15.397(1) : 6.31(8)

5 PZT – PZT4D (Hard) Cylinder 15.875(1) : 6.33(1)

6 PZT – PC4D (Hard) Cylinder 15.070(9) : 3.008(8)

7 PZT – PZT5A (Soft) Cylinder 15.038(2) : 6.354(2)

8 PZT – PC5H (Soft) Cylinder 15.021(2) : 3.03(1)

9 PZT – PC5H (Soft) Cylinder 15.074(2) : 3.02(1)

10 PMN-0.32PT Cube 3.4(1) : 2.0(1) : 4.1(1)

The results of the MEMS measurements showed good correlation (r=0.9997) to the macro-Berlincourt and impedance resonance results, as shown in figure 5. The hard PZT samples show a d33 value of ~104 ± 6% of the values measured via the other two techniques, and the soft PZT results were slightly higher at 110 ± 5%. Similarly the PMN-PT sample piezoelectric coefficient was calculated as 1480 ± 30 pC N−1, 107 ± 2% of the value measured in the macro-Berlincourt meter. The small differences are readily explained by the frequency dependence of the extrinsic piezoelectric response, when considering the frequency of the applied stress fields between the MEMS methods, and electrical impedance methods (on which the macro-Berlincourt system is calibrated) [3].

Finally, a scanning measurement was performed on a micro-scale PZT sample that had been partially thermally depoled (figure 6). Contact in this case was made by mounting a 95 μm diameter Al-coated glass microsphere to the central plate of the MEMS. A drop of 60% in the measured d33 value across the sample correlates with overall polarisation drop on the same sample as measured with Laser Intensity Modulation Method [4].

ConclusionsA MEMS piezoelectric measurement system has been designed and tested, for the evaluation of d33 coefficients in small-scale systems. The results are in good agreement with measurements made with a Berlincourt meter, impedance resonance methods and LIMM. This new test system enables the rapid evaluation of micro-scale piezoelectric materials, structures and devices which are currently being developed for applications within ICT, energy harvesting and sensors technologies.

AcknowledgementsThis work was supported by the UK’s National Measurement System.

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Sample NumberFigure 5. Top panel: Summary of measured PZT d33 values from the MEMS micro-Berlincourt, piezoelectric resonance, macro-Berlincourt measurements and the manufacturer’s datasheet values. Bottom panel: Comparison of the MEMS measurements to the other two experiments.

Figure 6. Image of the thermally damaged region of the sample (top panel) and the measured d33 at 8 points across the surface (bottom panel).

References[1] http://www.memscap.com/products/mumps/metalmumps[2] http://www.coventor.com/[3] Damjanovic D, Phys. Rev. B, 55, R649–52 (1997)[4] M. Stewart and M. G. Cain, J. Am. Ceram. Soc., 91(7), 2176 (2008)

Table 1. Details of the samples used in this study. For the cylindrical samples, the first dimension refers to the length, and the second is the diameter.

Figure 4. Panel A: the gain of the charge amplifier decreases rapidly below the RC frequency of the charge amplifier circuit. Panel B: sample dataset at 5 kHz.

Figure 3. Calibration of the force output of the actuator.

Figure 2. Simulation of the free displacement of the actuator.

Figure 1. Schematic of the MetalMUMPS process; cross-section taken though the central plate of the device (not to scale).