1. 2005 년 1 학기 논문세미나 A Novel Micromachined Pump Based On Thick-Film Piezoelectric Actuation Michael Koch, Nick Harris, Alan G. R. Evans, Neil M. White and Arthur Brunnschweiler University of Southampton, Department of Electronics and Computer Science, Southampton SO17 lBJ, UK, M.Koch@soton.ac.uk ©1997IEEEM.Koch@soton.ac.uk Lab. Of Intelligent Material and Technology

2. 2005 년 1 학기 논문세미나 Summary A new silicon-based micropump is described in this paper. →The key element of the device is a thick-film/ silicon micromachined hybrid actuator. The actuation principle relies on the flexure of a screen printed piezoelectric lead zirconate titanate (PZT) layer on a silicon membrane (8 mm x 4 mm x 70μm). Inlet and outlet valves are of the cantilever type and use deep boron diffusion together with KOH etching. Pump rates of up to 120 μl min -1 have been achieved. A maximum backpressure of 2kPa was measured when using a 600 V pp sinusoidal drive voltage at 200 Hz across a 100 μm thick PZT layer.

3. 2005 년 1 학기 논문세미나 Instruction Two major classes of these pumps are now available → membrane actuated pumps and electric field induced flow pumps Membrane actuated pumps can be further subdivided into electrostatically, thermopneumatically and piezoelectrically actuated pumps. As small geometries require accurate positioning of the piezoelectric element on the membrane, the advantage of cheap mass fabrication In this paper case the combination of screen printing and silicon micromachining allows a return to inexpensive fabrication technologies. Other methods like sol-gel deposition and physical vapour deposition of PZT have been described, but lack the possibility of producing thick layers of 100 μm and more. The basic principles and some simulations of the hybrid actuator have already been reported but this is the first presentation of a working micropump using hybrid actuation.

4. 2005 년 1 학기 논문세미나 Fabrication The pump consists of a stack of three silicon chips. The two lower layers form the inlet and outlet valves. The top layer is the membrane actuator. The operation of the pump depends on the oscillating membrane varying the size of the chamber volume. When the chamber volume is increased, liquid is sucked through the inlet valve while the outlet remains shut. If the chamber volume decreases, the inlet valve closes and liquid is pumped through the outlet. Fig. 1. hybrid actuated micropump stack

5. 2005 년 1 학기 논문세미나 Fabrication The fabrication of the passive cantilever valves has already been presented elsewhere. It is based on a B+ anisotropic etch stop layer for the definition of the cantilever and the creation of a conical duct on the same wafer. For ease of testing, two identical chips were placed on top of each other to complete both inlet and outlet valve. ▲ Passive Cantilever Valves ▲ Membrane Actuator Standard silicon micromachining processes were used for the membrane. → A 70 μm thick membrane was formed with a time-controlled etch in KOH at 70°C and 30% wt using LPCVD silicon nitride as an etch mask. After removal of the etch mask the wafers were oxidized in a wet atmosphere. Processing was then continued with thick-film technology, allowing mass fabrication of up to 4” wafers. In our case the printing was done on quartered 4” silicon wafers to allow for several tests per wafer. Reasonable alignment between thick-film and silicon micromachined structures is achieved with alignment marks in each corner of the quartered wafer and the screens

6. 2005 년 1 학기 논문세미나 Fabrication ▲ Membrane Actuator The generation of a PZT capacitor between two electrodes : → cermet gold bottom electrode of 10 μm thickness → dried at 12O o C for 10 min fired in a conveyor belt furnace at 95O o C for 1 hour bottom electrode has been made larger than the diaphragm → reason : to reduce stress in the diaphragm for temperature coefficient mismatch Designs with 4 mm x 8 mm bottom electrodes led to maximum deflections in the centre of the membrane of 30μm on 70 μm thick Si membranes For 5 mm x 9 mm bottom electrodes, part of the stress is absorbed by the bulk-Si. → The bending was around 7μm and thus significantly reduced. Both measurements were taken after the whole printing cycle of bottom electrode, PZT and top electrode Experiments have been carried out with different insulation layers and inks for the bottom electrode, Three different surface layers were investigated: → Native oxidized surface → 500 nm thick layer of thermally grown Si0 2 → 160 nm thick layer of Si 3 N 4 on 80 nm of Si0 2

7. 2005 년 1 학기 논문세미나 Fabrication ▲ Membrane Actuator In parallel two different types of thick-film ink for electrodes were investigated: → Resinate inks give fired thicknesses of the order of 1μm → cermet inks give a fired thickness of the order of 10μm. Three inks were tested: → a silver palladium cermet ink, a gold cermet ink and a gold resinate ink. Measurements of the PZT capacitance on an evaporated Au electrode gave only 0.4% of the bulkvalue. → no further tests for both evaporated gold and resinate inks were carried out Printing the electrodes with gold cermet ink gave good results on SiO 2 → small areas of the fired electrode became discoloured: non-conducting → Bubbles : after firing the cermet gold ink on Si 3 N 4 covered wafers and gold printed on silicon with a native oxide formed an eutectic making it unsuitable for further printing. → This suggests that the discolouring and therefore loss in electrical conduction is due to defects in the insulation layer

8. 2005 년 1 학기 논문세미나 Fabrication ▲ Membrane Actuator Printing and firing of the bottom electrode with cermet based silver palladium ink on the silicon nitride substrate was unsuccessful due to bubbles on the surface. → Silver palladium ink was successfully printed onto the other substrates, although there was a certain discolouring of the PZT resulting in a significant drop in the capacitance of the actuator. → Assuming a perfect capacitor, the relative dielectric constant decreases to -50. This figure of merit is around 10% of the value measured for cermet gold ink printed on a Si0 2 covered wafer. In summary, the best results were obtained with the cermet gold electrode on 500 nm SiO 2. All further experiments refer to this type of electrode. Having printed the bottom electrode the wafer was put through two cycles of double printing, drying and firing of a special PZT ink. This ink contains 95% PZT-5H powder and 5% leadborosilicate powder as a binder. An organic vehicle (ESL 400) has to be added to give a printable ink. With a total of four printing steps a thickness of approximately 100 pm was achieved.

9. 2005 년 1 학기 논문세미나 Fabrication ▲ Membrane Actuator Finally, a gold top electrode is printed with the same process as before. Due to condensation and subsequent diffusion of lead on the silicon surface during firing the intermediate oxide between Si and PZT becomes conductive. It is therefore necessary to deposit the top electrode entirely on the PZT layer. The layout for the top electrode as depicted in figure 1 guarantees that top and bottom electrodes are isolated. Visual inspection and scratch-tests were done to judge adhesion of the printed layers after firing. As peeling off did not occur during scratch tests, adhesion of both bottom electrode to Si-substrate and PZT to Au-electrode seems to be good. Finally, the PZT is polarized with an electric field of 3 MV m -l at 130°C for 24 h. In order to verify a successful polarization, charge measurements were made to determine the d 33 coefficient of the screen printed PZT. Screen printed samples without membranes were mechanically stressed with a constant force and the generated charge was amplified and measured. From this, the d 33 value could be determined. For comparison, a PZT-5H bulk sample was measured giving 641 pC N - l, which is 108% of the literature value. The thick-film samples gave d 33 values of 173 pC N -1 being 29% of the values for PZT-5H.

10. 2005 년 1 학기 논문세미나 Measurement results Table 1. Comparison between surface mounted and hybrid actuator Fig. 3 Backpressure dependence of the micropump at 200Hz and 600 Vpp actuation Fig. 2 Frequency dependence of the micropump at zero backpressure

11. 2005 년 1 학기 논문세미나 Measurement results Displacement measurements at the centre of the membrane were taken with an optical fibre laser interferometer. The resolution of this type of interferometer is nominally a quarter of a wavelength or 158 nm for a helium neon laser with a wavelength of 633 nm. The displacement of a 7 x 3 x 0.1 mm 3 sized PZT printed on a silicon membrane of 8 x 4 x 0.07 mm 3 was approximately 1 μm for a voltage of 100 V and actuation frequencies of 500 Hz to 5 kHz. No significant variation of peak displacement with frequency was noted. In comparison, the deflection of a surface mounted bulk PZT actuator (6.25 x 3.25 x 0.2 mm 3 ) was determined to approximately 3 μm with identical test conditions The micropump was tested in a jig with ethanol as the test liquid. For ease of mounting the two lower layers containing the cantilever valves were placed on top of each other after cleaning with Isopropylalcohol. 1 mm thick silicone layer between the valves and the actuator was necessary to seal the pump chamber.

12. 2005 년 1 학기 논문세미나 Measurement results The cantilevers were 1 x 1 mm 2 in size sitting on a cone with a minimum opening of 130 x 130 μm 2. The frequency dependence of the pump rate at zero backpressure is for two different actuation voltages (450 Vpp and 600 Vpp). A maximum pump rate of 61 μl min -1 and 122 μl min -1, respectively was achieved at frequencies of 100 Hz and 200 Hz. The backpressure dependence is depicted in figure 3 for 600 V, and 200 Hz. An almost linear drop can be observed and a maximum backpressure of 2 kPa is achievable. Tests with an identical setup were done with the surface mounted PZT layer for comparison. Here, a maximum pump rate of 150 μl min -1 was achieved with 200 V, at 200 Hz. The maximum backpressure at this actuation was measured to 20 kPa. All results are summarized in table 1.

13. 2005 년 1 학기 논문세미나 Discussion The first generation of thick-film on silicon PZT actuators and pumps achieves approximately 30% of the performance of surface mounted devices. The mechanical power of the first hybrid actuators is already large enough to drive a micropump and the technology itself has the advantage of introducing mass fabrication for various powerful actuators. The working micropump driven with the thick-film / silicon micromachining PZT actuator could be exploited in medicine as a cheap disposable pump for drug dispensing. However, further work is needed to show compatibility between medical drugs like insulin and the pump.