Fabrication of Sonic Sensors Using PZT Thin Film on Si Diaphragm and Cantilever

1. Lab. Of Intelligent Material and Technology Fabrication of Sonic Sensors Using PZT Thin Film on Si Diaphragm and Cantilever S. Murakami1, K. Inoue1, Y. Suzuki1, S. Takamatst2, T. Kitano2, M. Kinoshita2, K. Yamashita3 and M. Okuyama3 1Technology Research Institute of Osaka Prefecture,Japan 2Technical Research Institute, Hitachi Zosen Corporation, Japan 3Graduate School of Engineering Science, Osaka University, Japan (c) 2002 IEEE

2. Abstract • Micro sonic sensors with a high quality factor (Q-value) using piezoelectric diaphragm and cantilever have been developed for detecting a specific sonic wave which is emitted due to anomalous behavior of rotary machines such as turbines, motors and engines. • A piezoelectric PbZr0.52Ti0.4803 thin film was prepared on a micromachined SOI wafer by sol-gel method. • The dependence of the output voltage of the both diaphragm and cantilever-sensors on the sonic frequency was examined. • The both sensors were found to have resonant peaks in the frequency region over 5kHz with a high Q-value (>100), and the typical sensitivity for the sonic wave at the resonant frequency was estimated to be 10 mV/Pa, which is comparable to that of a commercial bulk ceramics sensor.

3. Instruction 1 • Recently, extensive studies have been carried out on lead zirconate titanate (PZT) thin film because of its excellent piezoelectric and ferroelectric properties. • A variety of thin film preparation techniques such as sputtering, laser ablation, chemical vapor deposition (CVD), metal organic decomposition (MOD) and sol-gel method have been used for fabrication of the PZT thin film. • The PZT thin film has mainly two distinct applications: - one is a ferroelectric memory application where the low circuit voltages necessitate the use of submicron films (<0.2 μm) - another is piezoelectric application using the Si micromachining, for example, integrated on- hip micro actuators and ultrasonic sensors in air or underwater. • We have done so far some research works on the preparation of the PZT film by facing target sputtering or sol-gel method and have developed the high directivity ultrasonic micro array sensor, which consists of the sopgel derived PZT thin film on eighteen diaphragms of micromachined SOI wafer and shows a clear ultrasonic image for three-dimensional objects.

4. Instruction 2 • Inthis study, we fabricated the micro sonic diaphragm and cantilever-sensors using piezoelectric PZT thin film for detecting a specific weak sonic wave under a large background noise. • The sonic frequency dependence of the output voltage of the both sensors was investigated. • The quality factors (Q-values) and sensitivities at the resonant frequencies were estimated from the experimental results. • The relations between the resonant frequencies and the size of the diaphragm or cantilever were also examined.

5. STRUCTURES AND FABRICATION PROCESS • Cross sections of the diaphragm and cantilever sensors are shown in Fig. l(a) and (b), respectively. • The starting material was an SOI wafer by SUMITOMO METAL INDUSTRIES, LTD. with 2.5 μm thick Si, 1.0 μm thick SiO2 (insulator layer), and 625 μm thick Si-bulk. • We successfully fabricated 0.5-2 mm square diaphragms and cantilevers with 0.5-2mm beam length and 0.48 mm beam width. • As shown in Fig. 1(a) and (b), the both diaphragm and cantilever consist of Pt/Ti upper electrode, PZT film, Pt/Ti lower electrode, SiO2 and Si, whose thicknesses are 0.3, 1.0, 0.3, 0.7 and 2.2 μm, respectively. • Both of the structures had no crack or deformation that is originated from stress unbalance in a total stacked structure.

6. STRUCTURES AND FABRICATION PROCESS Fig. 1. Cross sections of (a) the diaphragm- and (b) cantilever-sensors. Fig. 2. Fabrication process of the micro diaphragm and cantilever- structures.

7. STRUCTURES AND FABRICATION PROCESS • The schematic fabrication process of the sensors is shown in Fig. 2. The sensor structures were fabricated as follows; • (1) An SO1 wafer was oxidized on both sides with thickness of 0.7 μm. • (2) Thin diaphragm structures were formed by using anisotropic etching technique with TMAH (22 wt%, 104°C) etchant. An important point here is to quit the etching, saving a final diaphragm with about 50 μm thick Si-bulk so as to take care of process- handling in the post-process flow. • (3) A Pt/Ti thin film was deposited by sputtering, and patterned using a lift-off technique in order to form the lower electrode. • (4) A piezoelectric PZT thin film was prepared on the front surface by sol-gel method as described in the next section. The PZT film was patterned by etching with HF and HNO3 (HF:HNO3:H2O=1:1:100) etchant. • (5) The upper electrode was formed in the same way as the lower electrode was.

8. STRUCTURES AND FABRICATION PROCESS • (6) The Si-bulk and insulator layer below the PZT capacitor were all etched out by RIE with SF6 reactive ion gas and by BHF etchant, respectively. • Here, the diaphragm-sensor was completed. • In order to form the cantilevers, the SiO2 layer and Si layer on the front surface were removed completely by BHF etchant and by RIE with SF6 reactive ion gas, respectively, as can be seen in Fig. 2. • In this process, it is possible to fabricate the both diaphragm- and cantilever-sensors on a monolithic chip.

9. PREPARATION OF PZT THIN FILM BY SOL-GEL METHOD • In the present study, we have followed a procedure previously reported for the preparation of the PZT thin film by sol-gel method. • Sol-gel precursor of Pb:Zr:Ti=1.15:0.52:0.48 in 15 wt% solution by Mitsubishi Material Corporation has been used to prepare the PZT thin film. • The PZT thin film with this composition close to the morphotropic phase boundary between rhombohedral and tetragonal phase fields has been extensively studied because of its excellent piezoelectric and ferroelectric properties. • The procedure to prepare the PZT film is to spin-coat the solution on the micromachined SOI wafer at 4000 rpm for 20 seconds; then the film is baked on hot plate at 300°C for 10 min in air to remove the solvent. Each three layers are annealed at 600°Cfor 10 minutes in pure oxygen to make the film become crystallized. • Finally, the PZT thin film is formed with twelve layers up to 1.0pm in thickness, and shows smooth and crack-free surface.

10. PREPARATION OF PZT THIN FILM BY SOL-GEL METHOD Fig. 4 P-E hysteresis loop for the PZT film onPt/Ti/SiO2/SOI at 20°C. Fig. 3 XRD patterns of the PZT thin film onPt/Ti/SiO2/SOI. • Figure 3 shows the XRD pattern of the PZT film onPt/Ti/SiO2/SOI. • It is found in this figure that the PZT filmshows perovskite PZT (111) and (110) peaksdominatingpattern, and does not show any peaks of pyrochlore orexcess PbO2 phase.

11. PREPARATION OF PZT THIN FILM BY SOL-GEL METHOD • These facts imply that highly (111)and (1 10) oriented perovskite PZT film successfullybecomes crystallized. It is well known that the highlyoriented PZT film permits the excellent piezoelectricproperty; therefore we could expect to obtain the microsonic sensors with a high sensitivity. • The ferroelectric property of the PZT film wasevaluated at 20°Cby RT600system (Radient Co.).Figure 4shows the Polarization (P)-Electric field (E)hysteresis loop of the PZT film on Pt/Ti/SiO2/SOI with anupper electrode consisting of an array of sputtered Pt dotsof 0.1 mm in diameter. • It can be obsered in this figurethat the PZT film shows a good ferroelectric property. Theremanent polarization +Pr-(-Pr)and coercive electric fieldEc are determined t be 33 μC/cm2 .and 36 kV/cm,respectively. • These values parallel those of the PZT filmprepared by multi-step process using the facing targetsputtering system or sol-gel method.

12. SONIC WAVE RESPONSE OF SENSORS ▲ Frequency dependence of output voltage • Frequency dependence of output voltage of the micro sensors was determined by means of Fourier transformation of the output voltage waveform that was generated by a pulse sound source at a distance of 0.5 m from the sensors. • The pulse sound was generated by electrical discharge. Figure 5 shows the typical transient output voltage waveform of the 1094 μm square diaphragm-sensor, which has delay of the propagation time corresponding to the distance between the sound source and sensors. The cantilever-sensors also show the similar output voltage waveform. • Figures 6(a) and (b) show the frequency dependence of the output voltage of the diaphragm- and cantilever sensors with various sizes, respectively.

13. SONIC WAVE RESPONSE OF SENSORS ▲ Frequency dependence of output voltage • Ascan be seen in Fig. 6(a), the diaphragm-sensors have resonant peaks in the frequency region over 15 kHz. The sensitivity and Q-value of the 2019 μm square diaphragm-sensor at the resonant frequency of 49.7 kHz were determined to be 8.5 mV/Pa and 100, respectively. • From Fig. 6(b), it is observed that the cantilever sensors have resonant peaks in the frequency region over 5 kHz. The sensitivity and Q-value of the cantilever-sensor with 957 μm length and 485 μm width were determined to be 7.1 mV/Pa and 160 at the resonant frequency of 21.4 kHz, respectively. • The both types of the sensors show nearly identical sensitivities on the order of 10 mV/Pa, which are comparable to that of a commercial bulk ceramics sensor. • On the other hand, the Q-values of the diaphragm-sensors tend to be lower than that of the cantilever-sensor; however those of the both types of the sensors are high enough to apply to the sonic sensor for detecting a specific sonic wave under a large background noise.

14. SONIC WAVE RESPONSE OF SENSORS ▲ Frequency dependence of output voltage Fig. 5. Typical transient output voltage of the 1094 μm square diaphragm-sensor. A pulse sound was generated at a distance of 0.5 m. Fig. 6. Frequency dependence of (a) diaphragm sensors with (1) 695, (2) 890, (3) 1094, (4) 2019μm square-diaphragm, and (b) the cantilever sensors with (1) 859, (2) 957, (3) 1159, (4) 1963μm beam length.

15. SONIC WAVE RESPONSE OF SENSORS ▲ Sensor-size dependence of resonant frequency • Figures 7(a) and (b) show the resonant frequency as a function of the diaphragm area and cantilever length, respectively, which are determined from Figs. 6(a) and (b). • From Figs. 7(a) and (b), the resonant frequencies are found to be proportional to the reciprocal of the diaphragm area and to the power of -2 of the cantilever length, respectively. • These results are consistent with the mechanical model given by D. Young. • Accordingly, it is possible to determine the size of the sonic sensor for detecting each specific sound, for example, which is emitted from abnormal rotary machines. • It is also expected to find application in a variable spectral sensitivity microphone with an array of the above sensors such as fishbone structure. • The detailed oscillation mechanisms of the both diaphragm and cantilever is under consideration.

16. SONIC WAVE RESPONSE OF SENSORS ▲ Sensor-size dependence of resonant frequency Fig. 7. Resonant frequency as a function of (a) diaphragm area and (b) cantilever length.

17. CONCLUSION • We have fabricated micro sonic sensors using piezoelectric diaphragm and cantilever, and investigated the sonic wave response. • The PZT film prepared by sol-gel method shows the (111) and (110) peaks of perovskite phase, and have a good ferroelectric property. • The values of +Pr-(-Pr) and Ec at 20°C were estimated to be 33 pC/cm2 and 36 kV/cm, respectively. • The both diaphragm- and cantilever-sensors have been found to show resonant peaks which are proportional to the reciprocal of the diaphragm area and to the power of - 2 of the cantilever length, respectively, in the frequency region over 5 kHz with a high Q-value (>100).

18. CONCLUSION • It is therefore confirmed that we can control the resonant frequency by varying the size of the diaphragm and cantilever. • The sensitivity for the sonic wave at the resonant frequency of the both diaphragm- and cantilever sensors was estimated to be on the order of 10 mV/Pa. • Thus, it is concluded that micro sonic sensors fabricated in the present work is appropriate for detecting a specific sonic wave under a large background noise.