Preparation and application of the 3C SiC substrate to piezoelectric micro cantilever transducers

Transcription

1 Appl Phys A (2012) 108: DOI /s x Preparation and application of the 3C SiC substrate to piezoelectric micro cantilever transducers Kiyong Choi Duck Kyun Choi Dong-Yeon Lee Jaesool Shim Sungho Ko Jae Hong Park Received: 10 January 2012 / Accepted: 21 February 2012 / Published online: 27 March 2012 Springer-Verlag 2012 Abstract This study examines the fabrication process and mechanical properties of piezoelectric films with the substrate, which is made from silicon carbide. After depositing the PZT thick film on silicon carbide substrate and silicon substrate respectively, it was shown that silicon carbide substrate formed a stable interface with PZT thick film up to 950 C, compared with silicon substrate. In addition, the dielectric constant of the PZT thick film sintered at 950 C on a silicon carbide substrate was 843, and this value was about over 25 % improved value compared with that on a silicon substrate. A thick film piezoelectric micro transducer of a micro cantilever type was fabricated by using a multifunctional 3C SiC substrate. The fabricated micro cantilever was a micro cantilever with multiple thin films K. Choi Technology Analysis Team, Defense Agency for Technology and Quality, Seoul , Republic of Korea D.K. Choi Division of Advanced Materials Science, Hanyang University, Seoul , Republic of Korea D.-Y. Lee J. Shim School of Mechanical Engineering, Yeungnam University, 214-1, Dae-dong, Gyeongsan-si, Gyeongsangbukdo, , Republic of Korea S. Ko ( ) Department of Applied Bioscience, CHA University, Seongnam, Gyeonggi-Do , Republic of Korea shko7@cha.ac.kr J.H. Park ( ) Division of Nano-Convergence Technology, Korea National NanoFab Center, Deajeon , Republic of Korea jhpark@nnfc.re.kr on either silicon or silicon carbide substrate. The piezoelectric thick-film micro cantilever that was fabricated by using a SiC substrate showed excellent mechanical and thermal properties. The piezoelectric micro cantilever on the SiC substrate shows an excellent sensitivity towards the change of mass compared with the piezoelectric micro cantilever on the Si substrate. 1 Introduction As a functional material, SiC has high thermal conductivity, stability at high temperature, high breakdown electric field, high electron saturation velocity and low coefficient of thermal expansion, so that it is adaptable to extreme conditions [1 5]. However, it is difficult to engineer mechanically and chemically because SiC has high mechanical strength and is chemically stable. If SiC could be engineered easily, it could be considered as the optimal material for MEMS devices due to its stability. However, silicon carbide is not easily etched due to its chemical inertness. Therefore, it needs to establish dry etching process to apply SiC to the MEMS process. Also, during heat treatment at high temperature, it is necessary to study the stability between silicon carbide substrate and piezoelectric material. In this research, successful formation, engineering and integration of silicon carbide material as the substrate with a piezoelectric film were demonstrated. We fabricated the piezoelectric micro cantilever with silicon carbide as a supporting substrate that has high thermal stability and high elastic coefficient. The resonant property of a piezoelectric micro cantilever transducer on silicon carbide substrate was evaluated. It has characteristics different than the piezoelectric micro cantilever fabricated on a silicon substrate. Also, the piezoelectric micro cantilever on the SiC substrate shows

2 162 K. Choi et al. an excellent sensitivity towards the change of mass compared with the piezoelectric micro cantilever on the Si substrate. 2 Experimental details 2.1 Fabrication of piezoelectric thick films on SiC substrates 6000-Å-thick SiO 2 film was deposited on a silicon carbide substrate; then a 300-Å-thick TiO 2 adhesion layer was deposited, using RF magnetron sputter Å-thick Pt was coated as a bottom electrode using a sputtering system. After that, using screen printing, 30-μm-thick PZT paste was printed. Organic additives were burned out for 10 minutes at 400 C, and then were removed. Lastly, the screen printed PZT paste was sintered at C for 10 minutes. The sintered PZT was coated with 1500-Å-thick Pt for a top electrode. Dielectric constant of piezoelectric film was measured at 100 khz using HP 4924A LF impedance analyzer. Interfacial structure of PZT thick film and silicon carbide was observed using the FE-SEM (Field Emission Scanning Electron Microscope, HITACHI S-4100). 2.2 Conditions for etching SiC substrates for MEMS engineering In order to determine the optimal etching process of silicon carbide, ICP (Inductive Coupled Plasma) power was determined to be the first variable on the condition of the amount of 30-sccm SF 6 gas, and 30-mTorr process pressure during etching process. Then it was evaluated for its etching properties at ICP power of W for 10 minutes. From the experiments for investigating the mixing ratio of SF 6 and O 2, proper ratio of mixing was determined. Lastly, the condition for the optimal process was determined by changing the process pressure. The depth of etching for silicon carbide was measured through α-step. The cross section and the surface were observed through field emission scanning electron microscope. 2.3 Fabrication of the PZT thick-film micro cantilever on Si substrate Figure 1 shows the fabrication process of the PZT thickfilm micro cantilever on a silicon substrate. Before screenprinting PZT thick film, a diffusion barrier, an adhesion layer and a bottom electrode were deposited. First, as a diffusion barrier, 1.4 μm ofsin X was deposited on a silicon substrate by using LPCVD (low pressure chemical vapor deposition). A 5000-Å-thick Pt bottom electrode on the 300-Å-thick TiO 2 (an adhesion layer) was deposited by using an RF sputtering method. The bottom electrode was etched by using AOE (advanced oxide etcher) after patterning of Pt/TiO 2. About 25-μm-thick PZT thick film paste was coated on the substrate of Pt/TiO 2 /SiNx/Si structure, using screen printing method. Organic materials were removed from screen-printed PZT thick film through 600 C thermal treatment. After that, the PZT thick film was sintered at 850 C for 10 minutes to minimize the reaction between silicon substrate and PZT thick film. Using an RF sputtering system, Ti and Pt were deposited with 300 and 4000 Å, respectively, in thickness as the top electrode on Fig. 1 The fabrication process of the PZT thick-film micro cantilever on the Si wafer

3 Preparation and application of the 3C SiC substrate to piezoelectric micro cantilever transducers 163 Fig. 2 The fabrication process of the PZT thick-film micro cantilever on the SiC wafer the sintered PZT thick film. Then it was patterned through a lift-off. In order to make windows on the back side of the silicon substrate, following the patterning of the back side window, we removed the open SiN X film using an RIE (Reactive Ion Etcher). Then opened back side of silicon substrate was etched in 30 % KOH solution to be a silicon membrane. Lastly, the released micro cantilever was completed by etching the front side window following a pattering process for the front side window. 2.4 Fabrication of the PZT thick-film micro cantilever on SiC substrate Figure 2 shows the process flow for fabricating the PZT thick-film micro cantilever on a silicon carbide substrate. For silicon carbide, it is difficult to perform a wet etching because of chemical inertness of SiC, so it has a process flow different than the process on the silicon substrate. It is necessary to use ionic plasma to perform an etching process of silicon carbide. Therefore, it is desirable that the etching process for front side windows is carried out first. Also, etch mask that can sustain at high temperature is introduced by depositing 1-μm-thick aluminum using an E-beam evaporator. Then it was patterned and etched via an AOE process. The etching process for front side windows was carried out by etching 20-μm-thick silicon carbide by using an ICP (Inductively Coupled Plasma) etcher. Since the back side of silicon carbide also needs to be etched by using ICP process, the etch mask was electroplated with 10-μm-thick Ni, Then it was patterned and etched via an AOE process. In order to insulate the silicon carbide electrically, 6000-Å-thick SiO 2 was deposited by using a PECVD (Plasma-Enhanced Chemical Vapor Deposition) system. After that, thermal treatment at 650 C for 10 minutes was performed for improving the crystal structure. Before screen-printing PZT thick film, an adhesion layer and a bottom electrode were deposited on the silicon substrate. Firstly, a 300-Å-thick TiO 2 film was deposited as an adhesion layer. Subsequently, 5000-Å-thick Pt was deposited by using a RF sputtering system. The bottom electrode was etched by using AOE after patterning. The 25-μm-thick PZT film was formed through screen-printing on the substrate of Pt/TiO 2 /SiO 2 /SiC structure. Organic materials were removed from screen-printed PZT thick film through 600 C thermal treatment. After carrying out the thermal treatment, the PZT thick film was sintered at 850 C for 10 minutes. In order to deposit the top electrode on the fabricated PZT thick film, Ti and Pt were deposited in 300 and 4000 Å thickness, respectively, by using RF sputtering. The top electrode was patterned by a lifting-off process. In order to make window on the back side of silicon carbide substrate, SiN X and SiC were etched in order to use ICP after patterning of the back side window; then the micro cantilever was completed. 2.5 Design of the PZT micro cantilever transducers The PZT thick-film micro cantilever fabricated for this study was designed to have a width of 500 μm and a length of

4 164 K. Choi et al. either 500 or 600 μm. A photo mask was composed of 5 layers and designed to be 4.8mm 9.6 mm. A total number ofdevicesona4inchwaferwas72.also,theedge margin between each layer is 20 μm. As shown in the design, the geometry of the PZT thick-film micro cantilever was two types: of a width 500 μm and of a length 500 or 600 μm. 3 Results and discussion 3.1 Optimal process conditions for engineering SiC substrates In order to apply silicon carbide to micro cantilever device, it is important to evaluate its crystallographic properties. The used silicon carbide substrate was fabricated by CVD (Chemical Vapor Deposition) method. Figure 3 shows the X-ray diffraction (XRD) pattern (CuK α, Ni filter) of a fabricated silicon carbide. It is shown in Fig. 1 that the fabricated silicon carbide substrate has the polycrystalline structure of 3C SiC. In order to obtain flatness of the surface of SiC substrate, the substrate was treated with chemical-mechanical polishing (CMP). In Fig. 4, (a) (b) and (c) (d) are the images of SEM (Scanning Electron Microscope, HITACHI- 4700) before and after the CMP process, respectively. From Fig. 4(a) (b) it is clear that the surfaces of silicon carbide are quite rough before the CMP process. However, it is shown that the roughness of surfaces has much decreased after the CMP treatment (Fig. 4(c) (d)). Decreasing roughness on the surface is clearly shown in the images of atomic force microscope (AFM) before (Fig. 5(a)) and after (Fig. 5(b)) the CMP. After the CMP process, rms (root-mean-square) roughness of silicon carbide surface clearly decreased from μm to 0.95 nm. This value means that roughness is sufficiently small to deposit the next film on it. Also, in applying SiC to a device as a functional substrate, the value Fig. 3 XRD pattern of the 3C S1C wafer of modulus of elasticity is important. In general, it is known that Young s modulus of the silicon carbide is higher than that of the Si substrate. In order to investigate this property using nano-indenter test, Young s modulus and harness of both silicon and silicon carbide were measured. Additionally, coefficients of thermal expansion and thermal conductivity were examined (Table 1). Young s modulus of silicon carbide has a value of 290 GPa, which is about 1.5 times as high as Young s modulus of silicon, 190 GPa. In terms of strength, it was confirmed that silicon carbide is twice as strong as silicon. Also, for the coefficient of thermal expansion, SiC has the value twice as low as Si does. For thermal conductivity, SiC wafer has the value twice as high as Si wafer does. Table 2 shows the etching properties of silicon carbide in this research. Due to the chemical inactiveness of SiC for wet etching, etching rate is less than 20 Å per hour. For the dry etching, on the contrarily, the etching rate is 500 nm per minute. Therefore, we can conclude that in order to etch silicon carbide, dry etching is more effective. Thus, an experiment to fabricate an integrated device was performed using dry etching of silicon carbide. In order to evaluate the etching properties of 3C SiC, 3C SiC-substrate was used. Using an E-beam evaporator, the silicon carbide substrate was covered with 1-μm-thickaluminumas a blockingmask. The silicon carbide specimen with the aluminum mask was etched by dry etching process. For SF 6 gas, it is seen that the etching rate increased as RF power increased, whereas there was no significant improvement in etch rate in case of CF 4 gas during the RF power. For the maximum allowable power, 450 W, the etching rate of mixed gas of CF 4 and O 2 was 170 Å per minute, whereas that of SF 6 and O 2 was 840 Å per minute. From this result we could know that there is a difference about 5 times of etching rate between the types of gases. The reason of the gas mixture of CF 4 /O 2 having lower etch rate than that of SF 6 /O 2 is believed to be in that the reaction between CF 4 /SF 6 and SiC produces reactive byproducts including CF X and CO x (CO, CO 2 ). For SF 6, however, the reaction does not include any byproduct so the reaction rate is high [6, 7].Basedontheaboveresult, SF 6 gas mixture was chosen as the reactive gas. However, with RIE, it was shown that the etching rate was less than 900 Å/min, which is relatively low. Actually, the higher etching rate is required to fabricate the silicon carbide device efficiently. Therefore, an etching experiment was conducted with ICP etching method which can obtain the higher etching rate. ICP etching has advantages such as the abilities to process at high plasma density at relatively low pressure and to adjust impact energy freely. Therefore, when the ICP etching process is introduced to 3C SiC etching, the ICP etching process can exhibit high etching rate, perpendicular etching, relatively smooth surface without any residue. If the ICP power increases, the etching rate of silicon carbide increases. If the O 2 content increases, the etching rate

5 Preparation and application of the 3C SiC substrate to piezoelectric micro cantilever transducers 165 Fig. 4 SEM micrographs of (a) the cross section of the 3C SiC substrate, (b)the surface of the 3C SiC wafer before the CMP process, and (c) the cross section of the 3C SiC wafer, (d) the surface of the 3C SiC wafer before the CMP process after the CMP process Fig. 5 AFM images of the SiC wafer surfaces; (a) before the CMP process and (b)afterthe CMP process Table 1 Comparison of mechanical and thermal properties of the Si and the SiC wafers Material Young s modulus (GPa) Hardness (GPa) Thermal expansion coefficient ( C) Thermal conductivity (W/cm K) Si wafer SiC wafer

6 166 K. Choi et al. Table 2 Etching properties of the SiC material Etch rate in KOH C Etc rate in TMAH 80 C Etch rate in 40 % HF Etch rate in HF, HNO 3 Etch rate in CF 4,CF 6 and O 2 plasma < 20 Å/h (Si: 72 μm/h) <20 Å/h <10 Å/h Å/h nm/min decreases. It is considered that the condition where the etching rate decreases was caused from residue carbon composites formed on the etched surface. It is considered that carbon was not quickly and sufficiently removed from the etching surface due to the reaction of C F or C O when silicon carbide is etched in this condition. It is clear that as the ICP source power was increased, silicon carbide was etched faster. It showed the high SiC etching rate of 9500 Å/min at 1300 W, which is the maximum ICP power. The etch rate of silicon carbide decreases from 9500 to 5100 Å/min, as the oxygen content increases. The smoothest etched surface was shown when the oxygen content was 3 sccm. When there was not oxygen, it was shown that the etching rate was high but the etched surface was rough. Also, when the oxygen content was 15 sccm, the etched surface was smooth but rougher than when the etching rate was 3 sccm. According to the result above, a mixed gas including the 3 sccm of oxygen and 27 sccm of SF 6 with a proper etching rate and good surface condition was fixed as a proper gas for SiC etching, followed by etching silicon carbide according to the change of process pressure. The etched surface is shown in Fig. 4. As the process pressure increases up to 50 mtorr, etching rate of SiC increases from 4500 to 8100 Å/min. As the etching pressure increases, the relatively increasing amount of reactive gas is expected to increase the reaction between reactive gas and silicon carbide. In particular, the process pressure at 40 mtorr (Fig. 6(c)) showed almost perpendicular face, different from other process pressure conditions. However, even if the etching rate was faster at 50 mtorr compared with other process pressures, it showed inhomogeneous and rough surface. Therefore, we conclude that the optimal etching condition of silicon carbide could be obtained at ICP source power of 1300 W, process pressure of 40 mtorr, and mixing gas rate with SF 6 gas of 27 sccm and O 2 gas of 3 sccm. 3.2 Evaluation of the dielectric properties of PZT thick films on Si substrates and SiC substrates A comparison was conducted for evaluating the process stability of the PZT thick film according to the type of substrates, which was made of the silicon and the silicon carbide. The silicon and the silicon carbide substrates were covered with 6000 Å of SiO 2 by using PECVD (Plasma Enhanced Chemical Vapor Deposition), and then followed by heat treatment at 650 C for the densification on SiO 2.Using a sputtering system, the substrates were deposited with TiO 2 and Pt for 300 and 5000 Å, respectively. After that, they were coated with PZT thick film through the screen printing process. Interfaces between the PZT thick film and the silicon substrate or the silicon carbide substrate were observed after sintering the PZT thick film from 800 to 950 C. Then dielectric constant was measured by using impedance analyzer. Figure 7 shows dielectric constant according to the sintering temperatures for the PZT thick film formed on the silicon substrate and the silicon carbide substrate. The dielectric constant of PZT thick films deposited on silicon increased from 440 to 690 as the sintering temperature increased from 800 to 950 C. The dielectric constant of the PZT thick film deposited on silicon carbide was shown to increase from 450 to 843 as the sintering temperature increased from 800 to 950 C. Based on this result, the properties of the PZT thick film deposited on the silicon carbide substrate were shown to improve more than those on silicon substrate. The degree of improvement increased as the process temperature increased. The reason for this, as mentioned above, is believed to be in that silicon carbide is more stable due to the lower coefficient of thermal expansion so that it was possible to produce excellent quality of piezoelectric device. Also, since silicon carbide has a better heat conductivity than silicon, it is expected that it might have helped sintering a PZT thick film on a silicon carbide substrate. 3.3 Resonance properties of piezoelectric micro cantilever sensors In order to interpret the basic resonance characteristics of piezoelectric micro cantilevers, the experimentally fabricated PZT thick-film micro cantilever was used. For the 500 μm 500 μm sized PZT thick-film micro cantilever which was fabricated on the silicon substrate, the resonance frequency was 42, 57.5, and 100 khz when the thickness of the silicon supporting layer was 7, 15, and 35 μm, respectively. For the 500 μm 500 μm sized PZT thick-film micro cantilever which was fabricated on the silicon carbide substrate, the resonance frequency of the micro cantilever was 47, 74, and 155 khz when the thickness of the silicon carbide supporting layer was 7, 15, and 35 μm, respectively. For the cantilever length of respective 500 and 600 μm, and the substrate of 7-μm thickness, both resonance frequencies of the fabricated PZT thick-film micro cantilever on silicon carbide substrate increased approximately by over 20 % compared to the resonant frequencies of the PZT thick-film micro cantilever that was fabricated with the same thickness (7 μm) of the silicon substrate. Also, for the cantilever length of respective 500 and 600 μm, and the substrate of 35-μm thickness, both resonance frequencies of the fabricated PZT thick-film micro cantilever on silicon carbide

7 Preparation and application of the 3C SiC substrate to piezoelectric micro cantilever transducers 167 Fig. 6 SEM micrographs of 3C SiC wafer etching profiles with the variation of the gas pressure; (a) 20mTorr, (b)30mtorr,(c)40mtorrand (d) 50 mtorr at the condition of 200 W (bais), 27 sccm : 3 sccm (amount of the gas flow of SFe : Oi), and 1300 W (ICP power) Fig. 7 The dielectric constants of the PZT thick films deposited on the Si wafer and the SiC wafer after sintering process at various temperatures substrate increased approximately by over 30 % compared to the resonant frequencies of the PZT thick-film micro cantilever that was fabricated with the same thickness (35 μm) of the silicon substrate. In general mechanics, the resonance frequency of a micro cantilever is proportional to the spring constant that is under the influence of the elastic coefficient (refer to Eqs. (1) (2) in the Appendix). Therefore, this difference of resonant frequencies between the PZT thick-film micro cantilevers on a Si substrate and on a SiC substrate is considered to be caused from the difference between the spring constants, K, of Si and SiC. Also, as the thickness of supporting layer increases, the higher power of Young s modulus (SiC: 290 GPa and Si: 190 GPa) and the thickness of the supporting layer increase their influence (refer to Eq. (5)intheAppendix). Following this consideration, we could analyze the higher resonant frequencies of the PZT thickfilm micro cantilever on the SiC substrate, comparing those of the Si substrate. Figure 8 shows both theoretical and experimental values of resonance frequency changes in terms of the thickness of supporting layer of the PZT thick-film micro cantilever with the size of 500 μm 500 μm. It was observed that the average of experimental resonance frequency is relatively in accordance with that of theoretical values in terms of the thickness change of the PZT thick-film micro cantilevers, fabricated by using a silicon carbide substrate and a silicon substrate. In order to understand the change of resonance frequency in accordance with the change of the mass on the micro cantilever, the back side of micro cantilever tip was coated with Au. Figure 9 presents the change of resonance frequency of the micro cantilever when the mass of Au was added to the back side of the PZT thick-film micro cantilever tip on the silicon substrate and the silicon carbide substrate. The weight of Au coated on micro cantilever was approximately 50, 100, and 200 ng. For a micro cantilever that was fabricated by using a silicon carbide substrate, when the mass of Au was added, the resonance frequency decreased as the mass continued to increase. For the micro cantilever that was fabricated by using a silicon carbide substrate, when the increase in the mass to 200 from 50 ng on the micro cantilever occurred, 657 Hz of

8 168 K. Choi et al. Fig. 8 Theoretically and experimentally resonant frequencies according to the thickness of the supporting layer in the PZT thick-film micro cantilever (500 jum 500 jum) fabricated on the Si and the SiC substrates the resonant frequency shift was generated. On the contrarily, 282 Hz of the resonant frequency shift was generated in the micro cantilever that was fabricated by using a silicon substrate, by the same mass added. The mass sensitivity of the micro cantilever that was fabricated by using the silicon carbide substrate was 4.38 Hz/ng, which is over 2.3 times higher than that (1.88 Hz/ng) of the micro cantilever that was fabricated by using the silicon substrate. This phenomenon is supposedly caused by the fact that elastic constant per original mass of silicon carbide is higher than that of silicon. For the micro cantilever that was fabricated by using the silicon carbide substrate, we can conclude that the sensitivity of the resonant frequency for the added mass is higher than that of the micro cantilever which was fabricated by using a silicon substrate. Moreover, we can expect that additional stability of thermal properties such as heat conductivity and coefficient of thermal expansion of silicon carbide are more favorable to the fabrication process than those of silicon. 4 Conclusion In order to implement the piezoelectric process representing the excellent piezoelectric property, the properties of the piezoelectric films were evaluated by using silicon carbide as its substrate. Also, SiC substrate was confirmed to have polycrystalline structure of 3C SiC. Therefore, in order to prepare a smooth surface, the SiC substrate was polished using the CMP. For implementation of the silicon carbide in MEMS manufacturing process, the smoothly etched surface and high etching rate of silicon carbide are required. ICP process introduced in this study allows ensuring high Fig. 9 Resonant frequency shift according to increasing mass of Au as depositing Au film on the micro cantilever fabricated on (a) thesi wafer and (b) the SiC wafer etching rate. Also, it was possible to obtain the optimal etching condition of silicon carbide at the ICP source power of 1300 W, the process pressure of 40 mtorr, and the mixing gas rate with SF 6 gas of 27 sccm and O 2 gas of 3 sccm. The final etching rate was 7200 Å/min. It was confirmed that the dielectric constant of PZT films on the SiC substrate also improved over 25 % compared to that of PZT thick film formed on silicon substrate. In order to investigate the applicability of the multifunctional 3C SiC material as a substrate for a piezoelectric micro device, a type of a PZT thick micro cantilever was fabricated by using SiC as a substrate. The fabricated micro cantilever is a unimorph micro cantilever with multiple thin films of either silicon or silicon carbide substrate. Both 500 μm 500 μm and 500 μm 600 μm sized micro cantilever devices were fabricated by establishing each unit process. The PZT thick-film micro cantilevers that were fabricated by using both silicon carbide substrate and silicon substrate produced the resonant frequencies that were close to their theoretical resonance frequencies. Also, the piezoelectric micro cantilever that was fabricated by using the silicon

9 Preparation and application of the 3C SiC substrate to piezoelectric micro cantilever transducers 169 carbide substrate responds more sensitively to mass changes than the piezoelectric micro cantilever that was fabricated by using the silicon substrate. Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education Science and Technology (Grant No. NRF ). Also, the authors are grateful for the financial support from the Intelligent Microsystem Center sponsored by the Korea Ministry of Science and Technology as a part of the 21st Century s Frontier R&D Projects (Grant No. MS ). Appendix A.1 Preliminary discussion for resonant frequencies of piezoelectric micro cantilevers Dynamic motion of micro cantilevers using alternating current needs a piezoelectric part (i.e. a capacitor type PZT oscillator) and a non-piezoelectric part (i.e. a cantilever type membrane). The piezoelectric micro cantilever which is combined (i.e. integrated) with both PZT oscillator and cantilever membrane moves and deflects up and down according to the applied AC frequencies. Consequently, the piezoelectric PZT cantilever generates the strongest transverse wave through the maximum deflection and oscillation, if the applied frequency on sweep matches exactly the characteristic frequency of the whole piezoelectric PZT cantilever, whose characteristic frequency is decided by its geometric parameters such as length, width and thickness, and material parameters such as density and Young s modulus. The dependencies between the performance of proper sensors or self-actuators of piezoelectric micro cantilevers and their parameters are presented. The spring constant of the vertical deflection of a rectangular shaped cantilever is represented by the following equation [8 12]: K = F h = Ewd3 4L 3, (1) where K indicates the total spring constant of a cantilever, F is the loaded force on a micro cantilever, h is the deflection caused by the load, E denotes the cantilever s modulus of elasticity, and w, d, and L are the width, thickness, and length of the cantilever, respectively. The primary resonance frequency is: f = 1 K 2π m = d E 2π(0.98)L 2 ρ, (2) where ρ indicates the density of the micro cantilever and m the effective mass of the micro cantilever. The effective mass (m ) is associated with the beam mass and is represented by m = nm b, where n is the geometric factor of the beam and m b is the beam mass. For example, for a rectangular cantilever, the n valueis0.24[9, 10]. The resonant frequency of a piezoelectric unimorph cantilever clamped at one end can be converted as in Eq. (3)[8, 9, 13 17]: K f = 1 2π Me, (3) K m = Dp, and (4) m ν2 n 2πL 2 f n = I n 2 2π D p = {E2 np h4 np + E2 p h4 p + E np h np E p h p (4h 2 np + 6h np h p + 4h 2 p )}, 12(E np h np + E p h p ) where m (kg/m 2 ), K (N/m), and D p (N m) are the effective mass, the spring constant, and the bending modulus per unit width, respectively, and f n and ν n are the nth-mode resonant frequency and the nth-mode dimensionless eigenvalue that depend on the resonant mode of the cantilever, respectively. L (m) is the length of the cantilever, and ρ p (kg/m 3 ), ρ np (kg/m 3 ), h p (m), h np (m), E n (N/m 2 ), and E np (N/m 2 )are the density, thickness, and Young s moduli of the piezoelectric material and non-piezoelectric material, respectively. References 1. K. Choi, Improvement of resonance properties of PZT cantilever using SiC wafer. Ph.D. Thesis, Hanyang University Press, Seoul, Korea, 2007, pp P.M. Sarro, Silicon carbide as a new MEMS technology. Sens. Actuators A 82, (2000) 3. A. Tasaka, K. Takahashi, K. Tanaka, K. Shimizu, K. Mori, S. Tada, W. Shimizu, T. Abe, M. Inaba, Z. Ogumi, T. Tojo, Plasma etching of SiC surface using NF 3. J. Vac. Sci. Technol. A 20, (2002) 4. N. Susumu, A. Sadao, Chemical etching of thermally-grown SiO 2 films on SiC studied by spectroscopic ellipsometry. Jpn. J. Appl. Phys. 33, (1994) 5. J.S. Shor, A.D. Kurtz, I. Grimberg, B.Z. Weiss, R.M. Osgood, Dopant-selective etch stops in 6H and 3C SiC. J. Appl. Phys. 81, (1997) 6. G.Y. Choi, D.K. Choi, J.Y. Park, T.S. Kim, Reactive ion etching characteristics of SiC film deposited by thermal CVD method for MEMS application. J. Korean Inst. Electr. Electron. Mater. Eng. 17, (2004) 7. J. Lu, T. Kobayashi, Y. Zhang, R. Maeda, T. Mihara, Wafer scale lead zirconate titanate film preparation by sol gel method using stress balance layer. Thin Solid Films 515, (2006) 8. X. Li, W.Y. Shih, I.A. Aksay, W.-H. Shih, Electromechanical behavior of PZT-brass bilayers. J. Am. Ceram. Soc. 82, (1999) 9. W.Y. Shih, X. Li, H. Gu, W.-H. Shih, I.A. Aksay, Simultaneous liquid viscosity and density determination with piezoelectric unimorph cantilevers. J. Appl. Phys. 89, (2001) 10. J.H. Lee, Mechanical cantilever studies for protein detection system. Ph.D. Thesis, Korea, Yonsei University, 2004, p F.G. Brath, J.A.C. Humphrey, Sensing in Biological Engineering (Springer, New York, 2002) 12. D. Sarid, Scanning Force Microscopy (Oxford University Press, New York, 1994) (5)

10 170 K. Choi et al. 13. J.H. Park, T.Y. Kwon, D.S. Yoon, H. Kim, T.S. Kim, Fabrication of microcantilever sensors actuated by piezoelectric Pb(Zr 0.52 Ti 0.48 )O 3 thick films and determination of their electromechanical characteristics. Adv. Funct. Mater. 15, (2005) 14. J.H. Park, T.Y. Kwon, H.J. Kim, S.R. Kim, D.S. Yoon, C.-I. Chun, H. Kim, T.S. Kim, Resonance properties and mass sensitivity of monolithic microcantilever sensors actuated by piezoelectric PZT thick film. J. Electroceram. 17, (2006) 15. J.W. Yi, W.Y. Shih, W.-H. Shih, Effect of length, width, and mode on the mass detection sensitivity of piezoelectric unimorph cantilevers. J. Appl. Phys. 91, (2002) 16. J.W. Yi, W.Y. Shih, R. Mutharasan, W.-H. Shih, In situ detection using piezoelectric PZT/stainless steel cantilevers. J. Appl. Phys. 93, (2003) 17. J. Merhaut, Theory of Electroacoustics (McGraw-Hill, New York, 1981), p. 100