Beeby, SP

Harris, NR

Hill, M

Torah, RN

White, NM

English

Southampton (e-Prints Soton)

A PZT MULTILAYER THICK-FILM ACTUATOR FOR ULTRASONIC APPLICATIONS  N.R.Harris1, R.N. Torah1, N.M.White1, M.Hill2 ,S.P. Beeby1  1 ECS, University of Southampton, Highfield, Southampton, SO17 1BJ 2 SES, University of Southampton, Highfield, Southampton, SO17 1BJ  Email nrh@ecs.soton.ac.uk    Abstract  In many MEMS applications there is a requirement for actuating a structure. This paper describes a method of generating acoustic power from a PZT based thick-film structure which gives output power and electrical efficiencies at least as good as, and in some cases better than bulk PZT in the same application.   Keywords  thick-film, actuation, ultrasonic  I  INTRODUCTION The traditional method for actuating microengineered structures using PZT has been to use bulk PZT bonded to the structure in question. Work at Southampton has been progressing towards being able to replace this bonded transducer with a screen printed equivalent[1]. In the past, the piezoelectric activity available from screen printed PZT has been much lower than the bulk material, but recent developments in both material composition [2] and device structure have allowed a screen printed structure to deliver powers equivalent to bulk devices. This new printed transducer has been incorporated into a redesign of an ultrasonic microfluidic particle manipulator[3], where the original bulk PZT drive element has been replaced with a screen-printed thick-film device. This paper details the performance measurements achieved with several thick-film structures.  II  DESCRIPTION OF ACTUATOR  The actuating mechanism is manufactured by co-firing several layers of PZT at 890 degrees, each separated by an electrode layer. Several test devices were constructed, these consisting of either 1, 2 or 3 PZT layers, printed onto a ceramic substrate.   Figure 1 Structure of a 3  layer construction  Figure 1 shows a three layer structure.  The PZT paste is formulated within the department [2]. The electrodes are thick-film gold. The structure requires polarisation and this was achieved by applying a 4MV/m field at 150 degrees C for 30 minutes. III  ADVANTAGES OF A MULTI-LAYER STRUCTURE  The main advantage in producing a multilayered structure as shown in figure 1 is that it enables larger displacements to be generated than would be possible with a single layer of the the same total thickness, SubstratePZTElectrodeeffectively increasing the d33 coefficient. In addition, layering the structure allows lower absolute voltages to generate the required polarising field strength. Figure 2 shows how a two layer structure is connected and polarised.   Substrate30um30um PZTElectrode4MV/m4MV/mFigure 2 Structure and polarisation of a 2 layer structure  It can be seen that for a given applied voltage there is twice the movement as for 1 layer. This is an effective increase in the d33 coefficient of 100%. This comes about because effectively twice the actual voltage is being applied as compared with a single double thickness layer.  30um30um60um0+Va+Va0+VaDisplacement = d (m/V) x 30um x Va/30um(V/m)/m x 2 layersDisplacement = d (m/V) x 60um x Va/60um(V/m)= 2.d.Va= d.Va Figure 3 Comparison of displacement for a 1 layer and a 2 layer structure. Va is the applied voltage  A three layer structure is polarised in a similiar way as shown and in principle gives 3 times the movement than for 1 layer. It is possible to keep increasing the number of layers in this way but in practice the sensible limitation is 3 layers.  IV  PIEZOELECTRIC MEASUREMENTS  d33 coefficients were measured using a Take Control PM35 meter. The results, given in figure 4, show the expected increase in d coefficient, although the results appear to show that the improvement is not a straight multiplication. For example, the triple layer results are approximately 4 times the single layer results, whereas simple theory would predict 3 times. The reasons for this are not apparent at present, and more results are required to determine whether this is due to process variations or a fundamental effect. Most likely it is due to less PZT being degraded during the firing process. Substrate   Figure 4 D coefficient results  V  POWER CALCULATION AND MEASUREMENTS Direct measurements were taken of the acoustic output power using an acoustic force balance (Ohmic Instruments UPM-DT-1). Input power calculations were carried out in order to calculate the efficiency of the devices. The system used to drive the transducers was as shown in figure 5.  50dBSignalGenerator 50 ohmZVoutVo ViVi = 250mV pk-pk = 88.4mV rmsVo = 88.4mV * 50dB = 27.954V rms  Figure 5 Experimental system  The amplifier gave a fixed 50dB amplification and had an output impedance of  50 ohms. The impedance of the load was measured using a Hewlett Packard 4192A LF impedance analyser across a range of frequencies. An example plot of the impedance is shown in figure 6. 024681012141618202000 3000 4000 5000 6000Frequency kHzMagnitude (Ohms)-100-80-60-40-20020406080100Phase (degrees)Magnitude PhaseFigure 6 An impedance plot for a three layer device  This allowed the power delivered to the load to be calculated: Power dissipated is given by Re{VI*} where V is the voltage across the load and I is the current through the load. Now we can calculate Vout = (Z/Z+50)*Vo as we know that Vo is fixed and we have measured Z. Now I = V/Z and therefore power dissipated can be calculated as         ⋅ =  =   =∗ ZVZVZVV Power1ReReRe2*2* VI  RESULTS The following figures show the measured power out and calculated power in for examples of the 1, 2 and 3 layer devices. Bulk PZT00.20.40.60.812000 3000 4000 5000 6000Frequency (kHz)Power in (Watts)050100150200Power Out (mW)Power In Acoustic Power Figure 7 Power results for bulk PZT As a comparison, a crystal of bulk PZT has been mounted in the same way and tested.  00.10.20.30.40.50.62000 3000 4000 5000 6000 7000 8000Frequency (kHz)Input Power (W)020406080100120140160Output Power (mW)Power In Power Out Figure 8 Power results for a 1 layer structure  00.10.20.30.40.50.60.72000 3000 4000 5000 6000 7000 8000Freq (kHz)Power In (W)050100150200250300Power Out (mW)Power in Power out Figure 9 Power results for a 2 layer structure  00.10.20.30.40.50.60.72000 3000 4000 5000 6000Frequency kHzPower In (W)050100150200250Power Out (mW)Power in Power out Figure 10 Power results for a 3 layer structure  From these graphs it can clearly be seen that the resonant frequency of the structure changes with the number of layers, moving from about 5.7MHz for a one layer device to 3.9MHz for a three layer device.  The output power from a one layer device is less than for both a two and a three layer device, as expected, but the output from a three layer device is approximately the same as for a two layer device. This result should be compared with the d coefficient results shown in figure 4, indicating that the effective d coefficient for a three layer device is greater than that for a two layer device.  The power output measured from the bulk material is comparable with that from a 1 layer device, although the input power for the bulk material is higher than for the one layer thick-film device. This indicates that the thick-film device is more efficient in this application. Table 1 shows the efficiencies of the tested devices.   Type  d33 typical (pC/N) Resonant frequency Efficiency % Bulk 246  3.4MHz  18 1 layer  81  5.7MHz  36 2 layer  178  4.9MHz  43 3 layer  323  3.9MHz  35 Table 1 Key results for measured devices  It is apparent from the table that any of the thick-film structures give good efficiencies in this type of application when compared to a similarly mounted bulk device.  VII CONCLUSION  It is concluded that multilayer thick-film transducers can perform better than bulk PZT transducers structures, in terms of conversion efficiency of electrical to acoustic energy, when used as the drive elements for certain microsystems. Moreover, they are attractive in MEMS type applications, especially microfluidics, as they are easier to produce in wafer scale processes, as they are printed rather than individually bonded, as is the case with traditional bulk PZT elements.    VIII  REFERENCES  [1]  Maas, R., Koch, M., Harris, N.R., White, N.M., Evans, A. G. R. Thick-film Printing of PZT onto Silicon Materials Letters 31 (1997) pp109-112 [2]   R.N.Torah,S.P.Beeby,N.M.White Improving the piezoelectric properties of thick-film PZT: the influence of paste composition, powder milling process and electrode material. Sensors and Actuators A 110 (2004) pp378-384 [3]  Harris NR, Hill M, Beeby S, Shen Y, White NM, Hawkes JJ, Coakley WT. A  Silicon Microfluidic Ultrasonic Separator Sensors and Actuators B 95 (2003) pp425-434  IX   ACKNOWLEDGEMENTS  The authors with to thank the Engineering and Physical Research Council (EPSRC) for their financial support under grant number GR/R13333/01. We also gratefully acknowledge the Financial support given by Dstl and Microfiltrex Ltd. In addition the authors would like to thank Angela Cotton in the Medical Physics department at Southampton General Hospital for the use of the power balance.  

A PZT Multilayer Thick_film Actuator for Ultrasonic Applications

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