Hybrid Piezoelectric MEMS Resonators for Application in Bio-Chemical Sensing

Transcription

1 Journal of Applied Science and Engineering, Vol. 17, No. 1, pp (2014) DOI: /jase Hybrid Piezoelectric MEMS Resonators for Application in Bio-Chemical Sensing J. Lu*, L. Zhang, H. Takagi, T. Itoh and R. Maeda Research Center for Ubiquitous MEMS and Micro Engineering (UMEMSME), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, , Japan Abstract MEMS resonator exhibits extraordinary high sensitivity when used as bio-chemical sensor for detecting weight of specimen, adsorption of molecules or cells, etc. by resonant frequency shift, in which piezoelectric transduction is effective to reduce driving voltage as well as power consumption for portable applications. However, sensitivity of pizeoelectric MEMS resonator is deteriorated by piezoelectric film and complicated resonant structure due to its large energy dissipation and residual stress. This paper reviews and summerizes our recent developed several piezoelectric MEMS resonators, including cantilever actuated by PZT thin film and detected by piezoresistive gauge, beam resonator actuated by PZT thin film and detected by electrostatic sensor, disk/ring resonator actuated and detected by PZT thin film, for the pursuit of high quality-factor (Q-factor), high resonant frequency, and thus better device sensitivity. The detailed device structures were presented in this paper, and the performance of each device was evaluated, investigated, and listed for comparisons. Our experimental results clearly demonstrated that by seperating the PZT thin film from the resonant strcuture or by modulating the resonant mode through novel structure design, preferred high Q-factor were achieved with reasonably high resonent frequency. According to above results, the advantageous and weaknesses of those devices were discussed for application as bio-chemical sensors. Key Words: MEMS Resonator, Freqency Shift, Q-Factor, Energy Dissipation, Bio-Chemical Sensor 1. Introduction The quartz crystal microbalance (QCM) is a simple, cost effective, high-resolution resonant-based mass sensing technique, which has been well studied and used for several decades as a methodology to detect mass, to online measure surface coverage by monolayer or thin film, to analyze energy dissipating properties of the bonded surface mass, etc. [1,2]. In recent years, the advances of microelectromechanical systems (MEMS) technology enable miniaturizing the conversional QCM sensors from several mm to micrometer range. Those reported MEMS resonators utilize not only piezoelectric but also piezoresistive or electrostatic for actuation and detection *Corresponding author. jian-lu@aist.go.jp [3,4]. Since sensitivity of MEMS resonator was proved extraordinary high due to its reduced volume and mass by several orders when compared to QCM sensor, MEMS resonator has been well studied and applied for gas identification [5], biological detection [6], and intact microorganisms recognition [7], etc. Besides mass of the resonator itself (m 0 ), other essential parameters that decide sensitivity of the MEMS resonators are the resonant frequency (f 0 ) and the quality factor (Q-factor) of the resonator, because minimum detectable loading mass ( m) is reversely proportional to f 0 and Q-factor as explained by equation (1): (1) where f is the frequency shift after loading mass ( m)

2 18 J. Lu et al. adsorption, and Q is the Q-factor, which can be calculated by the ratio of resonant frequency to bandwidth of the resonant peak at -3 db (Q = f 0 / f -3dB ). Although the interests to piezoelectric MEMS devices are increasing, especially for portable or wireless sensor nods because of its low or no power consumption and low driving voltage [8], the practical application of piezoelectric MEMS resonators as bio-chemical sensors is still underway. Energy dissipation from piezoelectric thin film was found comparable to air-damping under atmospheric pressure, which greatly degraded Q-factor of the device. Moreover, the integration of piezoelectric thin film, e.g., lead zirconate titanate (PZT), into MEMS is difficult at present due to its high residual stress in multi-layered structure and process compatibilities, which may also deteriorate sensitivity of the device. For the pursuit of high Q-factor, high resonant frequency, and thus better device sensitivity, we have engaged in developing hybrid MEMS resonators for several years. This paper reviews and compares our recent developed piezoelectric MEMS resonators, including (1) micro cantilever which was actuated and detected by PZT thin film with reduced PZT size; (2) micro cantilever with separated PZT thin film actuators and detected by piezoresistive gauge; (3) beam resonator which was actuated by PZT thin film and detected by electrostatic sensor; and (4) disk/ring resonator which was actuated and detected by PZT thin film. The performance of each device was evaluated, investigated, and summarized for comparisons, which fit well with our theoretical estimations. Based on theoretical analysis and experimental results, the advantageous and weaknesses of above devices were analyzed and discussed herein for application as bio-chemical sensors. Besides, the future research subjects to future improve sensitivity of piezoelectric MEMS resonators were also proposed from both device performances and fabrication processes point-of-view. loss (e.g., air damping [9]), direct coupling to the supporting structure [10], internal mechanical-thermal coupling [11], and other losses. Since Q-factor of a resonator is inversely proportional to the rate of energy dissipation, it can be calculated by equation (2): (2) where Q air is the air damping Q-factor, Q sup is the support loss Q-factor, and Q TED is the TED Q-factor. The Q-factor of other energy losses is denoted by Q others. The energy dissipation of a piezoelectric MEMS resonator can be therefore calculated by equation (3): (3) where Q Piezoelectric is the PZT thin film induced energy loss Q-factor, including energy dissipation from piezoelectric PZT thin films and energy dissipation from multi-layered PZT-electrode stack structures. Figure 1 shows a traditional piezoelectric micro cantilever and its application as a bio-chemical sensor. In our previous studies, by using PZT cantilevers with various cantilever lengths, PZT thicknesses as well as numbers of structure layers, we have concluded that energy dissipation from piezoelectric thin film is dominant in various mechanisms [12]. Then, the relation between 2. Energy Dissipation in Piezoelectric MEMS Resonators Q-factor of a resonator is defined as the ratio of stored energy to energy dissipation in one cycle of vibration. Energy in a MEMS resonator can be dissipated through a variety of mechanisms, including viscous drag Figure 1. Structure of a traditional piezoelectric micro cantilever and its typical application as bio-chemical sensors. The cantilever is actuated and detected by on-cantilever-integrated PZT thin film.

3 Hybrid Piezoelectric MEMS Resonators for Application in Bio-Chemical Sensing 19 each energy dissipation mechanism under atmospheric pressure can be estimated by (4): (4) When under reduced pressures (for example: < 30 Pa) where air damping effects become ignorable, the relation between each energy dissipation mechanism can be estimated by (5): (5) Atmospheric pressure or liquid environmental is usually required for application in bio-chemical sensing. Above theoretical analysis clearly suggested that piezoelectric PZT thin film should be separated from the resonant structure and size of the PZT pattern should be reduced to the lowest possibility to reduce energy dissipation (improve Q Piezoelectric ) for high Q-factor as well as to avoid degradation of the sensitivity by residual stress. Besides, modulating the resonant mode through novel structure design to reduce air damping effects (improve Q air ) may also quite effective to improve sensitivity of piezoelectric MEMS resonator. 3. Design of Hybird MEMS Resonators According to above theoretical analysis, we have developed different types of piezoelectric hybrid MEMS resonator for the pursuit of high device sensitivity. Figure 2 shows structure of these devices. In Figure 2(a), the PZT-electrode stack was limited only at the fixed end of the cantilever to suppress energy loss from the PZT film and multi-layered structure [13]. The device in Figure 2(a) is to investigate the effects of PZT-electrode stack size on device sensitivity, and compared to traditional PZT cantilever as shown in Figure 1. The device in Figure 2(b) is to study the effects of PZT thin film when PZT-electrode stack was separated from the resonant structure. In Figure 2(b), two PZT actuators were designed near the fixed-end of the cantilever and then connected to cantilever by narrow beams for actuation. Vibration amplitude of the PZT actuator was limited within a few nm by 2 support beams on each Figure 2. Structure of the proposed hybrid MEMS resonators: (a) micro cantilever actuated and detected by PZT with reduced PZT pattern size; (b) micro cantilever with separated PZT actuator and detected by piezoresistive gauge; (c) beam resonator actuated by PZT and detected by electrostatic sensor; and (d) disk/ring resonator actuated and detected by PZT.

4 20 J. Lu et al. side of the PZT actuators to eliminate its energy dissipation. The resonant structure was therefore made by silicon only to reduced energy dissipation from multi-layered structure. Piezoresistive Wheatstone bridge was introduced to detect its resonant frequency shift when bio-chemical specimens were adsorbed onto the sensing area [14]. Beam resonator which was actuated by PZT at its node point and detected by electrostatic sensor at the free-end was also developed as shown in Figure 2(c). It is an extension of the device in Figure 2(b) by introducing electrostatic sensor to detect the resonant frequency, since piezoresistive gauge in Figure 2(b) may be degraded during high temperature PZT annealing process. Besides, electrostatic detection was believed more sensitive with lower power consumption than piezoresistive detection. Bio-chemical applications usually require air or liquid working conditions, where the energy dissipation from environmental viscosity damping becomes more essential than PZT. In Figure 2(d), PZT-electrode stacks were used for both device actuation and detection, same as the device in Figures 1 and 2(a). However, in Figure 2(d), PZT-electrode stacks were integrated onto the surface of disk/ring resonator at certain locations to modulate its resonant mode to in-plane vibration, which is believed can reduce air damping significantly. The location and size of the PZT pattern were optimized by FEAusing ANSYS to achieve preferred resonant mode and thus compress the viscosity damping [15]. 4. Resonator Farication & Evaluation In our works, insulator on silicon (SOI) wafer with 5 m-thick n-type active silicon layer was used to fabricate above devices. Boron was diffused at 950 C for 160 min for making p-type piezoresistive gauge onto SOI wafer for the device as shown in Figure 2(b). Sol-gel PZT (PZT-20, Kojundo Chemical Co.) with the thickness of 1 m was prepared on the SOI wafer to fabricate PZT actuator and detector. Sputtered Pt/Ti film and Au/ Ti film with the thickness of 200/50 nm were used as bottom electrode and top electrode of the PZT, respectively. Dry-etching by Ar plasma was then used to pattern PZTelectrode stack, followed by 300 nm-thick SiO 2 deposition by sputter for device passivation. After making Au/Ti electrodes for PZT-electrode stack and Al electrodes for piezoresistive gauge, the device shape was defined by deep reactive ion etching (DRIE) and finally the device was released from backside of the wafer by DRIE dry etching to remove Si and buffered hydro-fluoride (BHF) wet etching to remove SiO 2 layer. The detailed PZT and device fabrication process have been published elsewhere [14,16]. Figure 3 shows SEM images of the fabricated hybrid MEMS resonators. Figure 4 shows the packaged devices for evaluation. To compare the results with each other, resonant frequency and Q-factor of above resonators were measured under atmospheric pressure by a laser Doppler vibrometer (MLD-221D, Neoark Crop.) and a network analyzer (HP4395A, Agilent Technologies). To evaluated electrical performances of those devices, I-V curve of the piezoresistive gauge and equivalent capacitance of PZT thin film were measured by using a semiconductor parameter analyzer (HP4155C, Agilent Technologies). 5. Results and Discussion Table 1 listed the measured resonant frequency and Q-factor of those fabricated hybrid MEMS resonators. Clearly, Q-factor was improved to 808 at the resonant frequency of khz by Cantilever B (cantilever length: 100 m: width: 30 m) as shown in Figure 3(a) [13], which is several times higher than traditional PZT cantilever (~300 by Cantilever A) [17] as shown in Figure 1. However, it is still much lower than Si cantilever by which the Q-factor was measured as ~1100 with the same dimensions. The compression of energy dissipation by reducing the PZT-electrode pattern size was identical, but limited. The measured Q-factor of Cantilever C clearly reveals that by separating PZT actuators from the resonant structure, excellent Q-factor of 1113, which is same as Si cantilever, can be obtained at the resonant frequency of 719 khz (cantilever length: 100 m: width: 30 m) [14]. After normalizing Q-factor, m 0, and dimensions of above cantilevers with traditional PZT cantilever, we could obtain ~17% and ~60% improvement in sensitivity of the device as shown in Figures 3(a) and (b), respectively. Figure 5 shows measured I-V curve of the piezoresistive gauge, indicating that the diffused area can survive from

5 Hybrid Piezoelectric MEMS Resonators for Application in Bio-Chemical Sensing 21 Figure 3. SEM images of the fabricated hybrid MEMS resonators: (a) micro cantilever actuated and detected by PZT (marked as Cantilever B); (b) micro cantilever with separated PZT actuator and detected by piezoresistive gauge (Marked as Cantilever C); (c) beam resonator actuated by PZT and detected by electrostatic sensor (Marked as Beam); and (d) disk/ring resonator actuated and detected by PZT (Marked as Disk/Ring). Figure 4. Photos of the packaged devices for mechanical and electrical evaluation. The insert shows a package of disk/ring resonators. PZT annealing process and can be used for cantilever detection. The beam as shown in Figure 3(c) is expected to reduce the fabrication difficulties in Cantilever C. However, it was found that Q-factor was measured as ~400 at the resonant frequency of 500 khz. It is believed due to the variation of dimension by residual stress and mis- micromachining, because energy dissipation from support loss strongly depends on how close of the actuation points to node points. The structure optimization is under considering in our research to insure an enough allowance for beam dimension. It worth noticing that both Q-factor and resonant frequency was dramatically improved to 1319 and 1.34 MHz by disk/ring resonator as shown in Figure 3(d) [15]. The in-plane vibration may greatly reduce viscosity damping. Figure 6 shows measured equivalent capacitance and resistance of the PZT films of a disk/ring resonator at resonant frequency. The results in Table 1 indicate that by optimizing the PZT pattern to reduce piezoelectric loss and by optimizing the support beams to compress clamped loss, the device performance may be further improved. By summarizing above results, it suggested that that Cantilever C exhibits excellent Q-factor and reasonably high resonant frequency, but further improvement needs numerical estimation on energy dissipation from air damping and support loss. The disk/ring resonator shows very attractive potential capability in high sensitivity,

6 22 J. Lu et al. Table 1. The summarized device structure and performances for comparison. The advantageous and weaknesses of those devices were also analyzed for application as bio-chemical sensors Device description Cantilever A (Ref. [17]) Cantilever B (Figure 3(a)) Cantilever C (Figure 3(b)) Beam (Figure 3(c)) Disk/ring (Figure 3(d)) Actuation/detection Q-factor Resonant frequency Performances Advantageous To be improved PZT/PZT ~300 Few hundreds of KHz Low/no power consumption Sensitivity PZT/PZT ~800 Few hundreds Low/no power consumption Sensitivity of KHz PZT/Piezoresistive ~1,100 Up to 1 MHz Low actuation voltage, high Power consumption sensitivity (current flow in resistors) PZT/Electrostatic ~400 Up to 1 MHz Low actuation voltage Residual stress in device, sensitivity PZT/PZT ~1,300 Few to tens of Low/no power consumption, Impedance matching at MHz high sensitivity high frequency Figure 5. Measured I-V curve of the piezoresistive gauge in cantilever C. since Q-factor can be further improved by optimizing the PZT pattern (size and location) and support beam (shape and size) to compress energy dissipation by preferred in-plane vibration mode. This work is undergoing. The latest results will be presented in our future publications. For summary, above experimental results along with theoretical analyze clearly demonstrated that energy dissipation of piezoelectric MEMS resonator can be dramatically compressed, if locations and patterns of the electrode-piezoelectric-electrode stack is well designed for the pursuit of lowest air-damping and support loss. Although resonant frequency of the devices presented in this paper is not as high as QCM or other silicon resonators, the reasonably high-q achieved in this paper, Figure 6. Measured equivalent capacitance (upper image) and resistance (lower image) of PZT film on a disk/ring resonator. especially under atmospheric conditions, may greatly improves sensitivity of those devices as for bio-sensing applications. Other advantageous of above piezoelectric MEMS resonators are low driving voltage, low power consumption, and easy to be integrated as sensor array. The practical application of above devices for biosensing needs further works on adsorption/dis-adsorption coatings which has high selectivity to target specimen and less effect to resonator itself. The results will be presented in our future publications. 6. Conclusions In this paper, we discussed and compared different

7 Hybrid Piezoelectric MEMS Resonators for Application in Bio-Chemical Sensing 23 types of piezoelectric, piezoresistive, and electrostatic hybrid MEMS resonator form both energy dissipation and device sensitivity point-of-view. We have demonstrated that piezoelectric PZT thin film can be applied in MEMS resonator with less negative effects to device performance if PZT size and PZT location was well designed. By optimizing the shape and the vibration mode of the resonator, PZT thin film can be used for both actuation and detection while keeping excellent Q-factor and high resonant frequency. Those results are useful for developing piezoelectric MEMS resonators for application as bio-chemical sensor. Acknowledgment This work is granted by the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), initiated by the Council for Science and Technology Policy (CSTP). References [1] Lu, C. S., Mass Detection with Piezoelectric Quartz Crystal Resonators, J. Vac. Sci. Technol., Vol. 12, No. 1, pp (1975). doi: / [2] Marx, K. A., Quartz Crystal Microbalance: A Useful Tool for Studying Thin Polymer Films and Complex Biomolecular Systems at the Solution-Surface Interface, Biomacromolecules, Vol. 4, No. 5, pp (2003). doi: /bm020116i [3] Konno, M., Ikehara, T., Murakami, S., Maeda, R., Kimura, M., Fukawa, T. and Mihara, T., Novel MEMS Oscillator Using In-Plane Disk Resonator with Sensing Platform and Its Mass Sensing Characteristics, Proc. 16th International Solid-State Sensors, Actuators and Microsystems Conference (Transducers 11), Beijing, China, June 5 9, pp (2011). doi: /TRANSDUCERS [4] Davis, Z. J., Svendsen, W. and Boisen, A., Design, Fabrication and Testing of a Novel MEMS Resonator for Mass Sensing Applications, Microelectric Engineering, Vol. 85, pp (2007). doi: / j.mee [5]Maute,M.,Raible,S.,Prins,F.E.,Kern,D.P., Weimar, U. and Gopel, W., Fabrication and Application of Polymer Coated Cantilevers as Gas Sensor, Microelectronic Engineering, Vol. 46, pp (1999). doi: /S (99) [6] Waggoner, P. S. and Craighead, H. G., Micro- and Nanomechaical Sensors for Enviromental, Chemical, and Biological Detection, Lab Chip, Vol.7,pp (2007). doi: /b707401h [7] Pepper, J., Noring, R., Klempner, M., Cunningham, B., Petrovich, A., Bousquet, R., Clapp, C., Brady, J. and Hugh, B., Detection of Proteins and Intact Microorganisms Using Microfabricated Flexural Plate Silcon Resonator Arrays, Sensor & Actuators B: Chemical, Vol. 96, No. 3, pp (2003). doi: /S (03) [8] Itoh, T., Kobayashi, T., Okada, H., Masuda, T. and Suga, T., A Digital Output Piezoelectric Accelerometer for Ultra-Low Power Wireless Sensor Node, Proc. IEEE Sensors Conference 2008, Christchurch, New Zealand, Oct , pp (2008). doi: /ICSENS [9] Blom, F. R., Bouwstra, S., Elwenspoek, M. and Fluitman, J. H. J., Dependence of the Quality Factor of Micromachined Silicon Beam Resonators on Pressure and Geometry, J. Vac. Sci. Technol. B, Vol. 10, pp (1992). doi: / [10] Hao, Z., Erbil, A. and Ayazi, F., An Analytical Model for Support Loss in Micromachined Beam Resonators with In-Plane Flexural Vibrations, Sens. Actuators A, Vol. 109, pp (2003). doi: /j.sna [11] Zener, C., Internal Friction in Solids I. Theory of Internal Friction in Reeds, Phys. Rev., Vol. 52, pp (1937). doi: /PhysRev [12] Lu, J., Ikehara, T., Zhang, Y., Maeda, R. and Mihara, T., Energy Dissipation Mechanisms in Lead Zirconate Titanate Thin Film Transduced Micro Cantilevers, Jpn. J. Appl. Phys., Vol. 45, pp (2006). doi: /JJAP [13] Lu, J., Ikehara, T., Zhang, Y., Mihara, T., Itoh, T. and Maeda, R., High Quality Factor Silicon Cantilever Transduced by Piezoelectric Lead Zirconate Titanate Film for Mass Sensing Applications, Jpn. J. Appl. Phys., Vol. 46, pp (2007). doi: / JJAP

8 24 J. Lu et al. [14] Lu, J., Ikehara, T., Zhang, Y., Mihara, T., Itoh, T. and Maeda, R., High-Q and CMOS Compatible Single Crystal Silicon Cantilever with Separated On-Chip Piezoelectric Actuator for Ultra-Sensitive Mass Detection, Proc. 21 st IEEE International Conference on Micro Electro Mechanical Systems (MEMS2008), Tucson, USA, Jan (2008). doi: / MEMSYS [15] Lu, J., Zhang, Y., Itoh, T. and Maeda, R., Micro Disk Resonator with On-Disk Piezoelectric Thin Film Transducers for Integrated MEMS Ubiquitous Applications, Proc. 16th International Conference on Solid- State Sensors, Actuators and Microsystems (Transducers 11), Beijing, China, June 5 9, pp (2011). doi: /TRANSDUCERS [16] Lu, J., Kobayashi, T., Zhang, Y., Maeda, R. and Mihara, T., Wafer Scale Lead Zirconate Titanate Film Preparation by Sol-Gel Method Using Stress Balance Layer, Thin Solid Films, Vol. 515, pp (2006). doi: /j.tsf [17] Lee, C., Itoh, T., Maeda, R., Ohashi, T. and Suga, T., Development of a Piezoelectric Self-Excitation and Self-Detection Mechanism of PZT Microcantilevers for Dynamic SFM in Liquid, J. Vac. Sci. Technol. B, Vol. 15, pp (1997). doi: / Manuscript Received: May 1, 2013 Accepted: Nov. 28, 2013