A patterning technique of lead zirconate titanate thin film by ultraviolet-light

The patterning technique of Pb(Zr, Ti)O₃ (PZT) thin film is an essential process in device fabrication processes for application in microsensors and microactuators. In this paper, a novel pattern technique is proposed for PZT thin film by UV photolysis processes. PZT thin films were first spin coated on the substrate and exposed by UV light for photolysis step. The UV photolysis step defined exposed and unexposed area by mask, and the pattern will be transferred to PZT thin film. After photolysis, PZT films were placed in non-ionic surfactant to remove unexposed area. Finally, PZT films were sintered at 650 °C in the furnace for crystallization. Experimental results showed that remnant polarization of patterned PZT film by UV photolysis was 21.4 μc cm^?2, which was compared to 17.24 μc cm^?2 by hot plate prolysis. Coercive fields were 45 and 104 kV cm^?1 by UV photolysis and hot plate prolysis, respectively. Dielectric loss was 0.027 by UV photolysis which was much smaller than 0.043 by hot plate prolysis. PZT thin films patterned by UV photolysis showed satisfactory geometries.     

Development of novel PZT thin films for active sliders based on head load/unload on demand systems

 Micromachined active sliders based on head load/unload on demand systems is an interesting concept technology for ultra-high magnetic recording density of more than 100 Gb/in². The active sliders that we proposed use PZT thin films as a microactuator and control the slider flying height of less than 10 nm. It is necessary to develop high performance microactuators in order to achieve active sliders operating at very low applied voltage. This paper describes the development of novel PZT thin films for active sliders. The sol–gel fabrication process for PZT thin films is developed and the fundamental characteristics for the PZT thin films are investigated. It is confirmed that the PZT thin films have good ferroelectric properties. Furthermore, novel thin film microactuators are proposed. The feature is that the sol–gel PZT thin films (thickness 540 nm) are deposited on the sputtered PZT thin films (thickness 300 nm) fabricated on bottom Pt/Ti electrodes. Therefore, the novel thin films consist of a thermal SiO₂ layer and the sputtered and sol–gel PZT thin films layers sandwiched with upper Pt and bottom Pt/Ti electrodes on a Si slider material. Fabricating the diaphragm microactuator, the piezoelectric properties for the novel composite PZT thin films are studied. As a result, the piezoelectric strain constant d ₃₁ for the novel PZT thin films is identified to be 130 × 10⁻¹² m/V. This value is higher than conventional monolithic PZT thin films and it is found that the novel composite PZT thin films have the good piezoelectric properties. This suggests the feasibility of realizing active sliders operating at lower voltage under about 10 V. Received: 22 June 2001/Accepted: 17 October 2001     

Two-axis micro fluxgate sensor on single chip

We present a two-axis micro fluxgate sensor on single chip for electronic compassing function. To measure X- and Y-axis magnetic fields, functional two fluxgate sensors were perpendicularly aligned and connected each other. The fluxgate sensor was composed of square-ring shaped magnetic core and solenoid excitation and pick-up coils. The solenoid coils and magnetic core were separated by benzocyclobutane which had high insulation and good planarization characters. Copper coil patterns of 10 μm width and 6 μm thickness were electroplated on Ti (300 Å)/Cu (1,500 Å) seed layers. 3 μm thick Ni_0.8Fe_0.2 (permalloy) film for the magnetic core was also electroplated under 2,000 gauss. Excellent linear response over the range of ?100 μT to +100 μT was obtained with the sensitivity of ～280 V/T. Actual chip size was 3.1×3.1 mm². The sine and cosine signals of two-axis fluxgate sensor had a good function of azimuth compass.     

Flexible piezoelectric micromachined ultrasonic transducer (pMUT) for application in brain stimulation

We propose a new flexible piezoelectric micromachined ultrasonic transducer (pMUT) array integrated on flexible polydimethylsiloxane (PDMS) that can be used in studying brain stimulation by ultrasound. To achieve the technical demands of a high sound pressure level and flexibility, a diaphragm-type piezoelectric ultrasound transducer array was manufactured with 55 μm-thick bulk lead zirconate titanate (PZT) that was thinned after bonding with a silicon wafer. The ultrasound transducer array was then strongly bonded onto a PDMS substrate using an oxygen-plasma treatment followed by precise dicing with a fixed pitch to achieve flexibility. The radius of curvature was smaller than 5 mm, which is sufficient for attachment to the surface of a mouse brain. After a thinning process for the PZT layer, we observed that the PZT layer still maintained a high ferroelectric property. The measured remnant polarization (P_r) and coercive field (E_c) were 28.26 μC/cm² and 79 kV/cm, respectively. The resonant frequencies of fabricated pMUT elements with different membrane sizes of 700, 800, 900, 1200 μm in diameter were measured to be 694.4, 565.4, 430.8, and 289.3 kHz, respectively. By measuring the ultrasound output pressure, a pMUT showed a sound intensity (I_sppa) of 44 mW/cm² at 80 V, which is high enough for low-intensity ultrasound brain stimulation.

     

A new hybrid fabrication process for a high sensitivity MEMS microphone

A novel multi-layer stacking capacitive type microphone is designed in this study based on theoretical analysis and numerical simulations, while fabricated via two standard stable silicon-based MEMS processes—PolyMUMPs and SOIMUMPs. The adoption of two standardized processes helps greatly to increase yield rate. The sensitivity of the microphone is first determined by an analytical model based on an equivalent circuit, which is followed by finite element (FEM) analyses on the capacitance value, static pull-in voltage and dynamic characteristics. Based on the developed analytical model, varied dimensions of the microphone are optimized and then the performance is validated by analytical simulations. In the next step, micro-fabrication of the microphone is accomplished using two standard processes, PolyMUMPs and SOIMUMPs provided by MEMSCAP. Experiments are conducted to acquire the information of pull-in voltage for safe operation and frequency response in sensitivity for performance evaluation. In the static case, experimental results show a good agreement with the analytical results with 90 Mpa residual stress assumed. As for dynamic validation, the frequency response is measured in an anechoic room adopting the exciting frequency as the audible range from 100–10 kHz. The measured sensitivity is as around 0.78 and 1.7 mV/Pa from 100 to 10 kHz, under the biases of 2 and 4.5 V, respectively. Within the audible frequency range, the proposed device maintains the loss as less as 2.7 dB (ref. to V/Pa), under 3 dB—the commonly acceptable drop within audible frequency range.     

Monolithic integration of Pb(Zr,Ti)O3 thin film based resonators using a complete dry microfabrication process

Monolithic fabrication of lead zirconate titanate [Pb(Zr,Ti)O₃ or PZT] based thin film resonant devices such as microcantilevers, Lamb wave and bulk acoustic wave resonators are demonstrated. High-performance PZT thin films with a thickness of 2.6 μm are prepared on a silicon on insulator wafer by a sputtering deposition process. A highly selective reactive ion etching process is employed for micro-patterning of PZT, platinum electrodes, and SiO₂ insulation layer. Self-actuation of the PZT microcantilevers is demonstrated and the frequency response is characterized using a laser Doppler vibrometer. The frequency response of the Lamb wave resonator is evaluated by measuring its transmission characteristic using a network analyzer. For a Lamb wave resonator with a length of 240 μm and an interdigital period of 80 μm, the 1st order and 2nd resonance frequencies are 15.3 and 41.8 MHz, respectively.     

Piezoelectric unimorph miroactuators with X-shaped structure composed of PZT thin films

In this study, we report piezoelectric microactuators composed of Pb(Zr,Ti)O₃ (PZT) films for low-voltage RF-MEMS switches. In order to realize a flat beam shape as well as a large displacement, we have adopted an X-shaped connector at the center of the beam. Finite element method (FEM) simulation indicates that the bending motion of the beam is almost same as two connected cantilevers, and the maximum displacement reaches 3.2 μm/5 V. To fabricate the microactuators, piezoelectric PZT films were deposited on Si substrates using rf-sputtering and microfabricated into the PZT/Cr unimorph actuators of 800 μm in length and 200 μm in width, respectively. Although the X-shaped connector effectively releases the stress of the multilayered beam so that the beam shape is almost flat, small residual stress caused slight concave curvature along the width. The displacement at the center of the beam was measured using a laser Doppler vibrometer. The measurement revealed that the displacement was 0.5 μm/5 V which was lower than the FEM result. The reduction of the displacement is attributed to the increase of the stiffness of the beam due to the concave curvature of the beam width.     

Metal-based piezoelectric microelectromechanical systems scanner composed of Pb(Zr, Ti)O3 thin film on titanium substrate

We have developed a titanium (Ti)-based piezoelectric microelectromechanical systems scanner driven by a Pb(Zr, Ti)O₃ (PZT) thin film for the development of laser scanning displays. The 2-μm-thick PZT thin film was directly deposited on a 50-μm-thick Ti substrate by radio frequency magnetron sputtering. Prior to PZT deposition, the Ti substrate was microfabricated into the shape of a horizontal scanner by wet etching; therefore, we could fabricate a piezoelectric microactuator without using the photolithography process. We confirmed the growth of the polycrystalline PZT film with perovskite structures on the Ti substrate. We achieved an optical scanning angle of 22° at a resonant frequency of 25.4?kHz using a driving voltage of 20?V _pp. These horizontal scanning properties can be applicable for laser displays.     

Ferroelectric properties of direct-patterned half-micron thick PZT film

Ferroelectric properties of direct-patterned PZT(PbZr_0.52Ti_0.48O₃) films with 460 μm × 460 μm size and 510 nm thick were analyzed for applying to micro-detecting devices. A photosensitive solution containing ortho-nitrobenzaldehyde was used for the preparation of direct-patterned PZT film. PZT solution was coated on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrate for three times to obtain half-micron thick film and three times of direct-patterning process were repeated to define a pattern on multi-layer PZT film. Through intermediate and final anneal procedure of direct-patterned PZT film, any shrinkage along horizontal direction was not observed within this experimental condition, i.e., the size of the pattern was preserved after annealing, only a thickness reduction was observed after each annealing treatment. Ferroelectric properties of direct-patterned PZT film with 460 μm × 460 μm size and 510 nm thick were compared with those of un-patterned conventional PZT film and shown to be almost the same. Through this work, the high potentiality of direct-patternable PZT film for applying to micro-devices without the introduction of physical damages from dry-etching could be confirmed.     

Piezoelectric thin films formed by MOD on cantilever beams for micro sensors and actuators

Novel piezoelectric cantilever beams for micro sensors and actuators based on PZT thin films have been batch fabricated by surface micromachining. Lead zirconate titanate (PZT) thin film is formed by metalorganic deposition (MOD) on Pt/Ti/SiO₂/Si (1 0 0) substrates and Pt/Ti/LTO/Si₃N₄ cantilever beams and then annealed at 700 °C in air. The PZT thin film is 0.5 m thick and has dielectric permittivity of 1698, remanent polarization of 13.66 C/cm², and coercive field of 44.5 kV/cm. The influence of deposition temperatures on PZT thin film stress has been investigated. When continuously controlling the deposition temperatures, the stress of the thin film is reduced from 0.313 × 10⁸ to 0.269 × 10⁸ N/m, that is 16.4% decrease. With the total 120 designed devices on 4-inch wafers, the number of functional devices is increased from 82 to 97, that is 12.47%.Tianhong Cui joined the faculty of Institute for Micromanufacturing at Louisisana Tech University in 1999. He received his B.S. from Nanjing University of Aeronautics & Astronautics in 1991, and his Ph.D. from Chinese Academy of Sciences in 1995. He has conducted research and development work for the realization of microsensors and microactuators since 1992. Prior to joining the IfM in 1999, he was at the National Laboratory of Metrology in Japan as a Research Fellow under STA fellowship, and previous to that, served as a Postdoctoral Research Associate at the University of Minnesota. His current research interests include MEMS, polymer micro/nanoelectronics, and nanotechnology.     

Fabrication of piezoelectric multilayer thin-film actuators

In this study, we fabricated multilayer ceramics (MLCs) composed of multilayered Pb(Zr,Ti)O₃ (PZT) piezoelectric thin films with internal electrodes and evaluated their dielectric and piezoelectric properties. The stack of PZT ferroelectric layers (550 nm) and SrRuO₃ (SRO, 80 nm) electrodes were alternatively deposited on Pt/Ti-coated silicon-on-insulator substrates by radio-frequency magnetron sputtering. The MLCs composed of one, three, and five PZT layers were fabricated by the alternate sputtering deposition of PZT ferroelectric layers and SRO electrodes through the movable shadow mask. The capacitances of MLCs were proportionally increased with the number of PZT layers, while their relative dielectric constants were almost same among the each MLC. The MLCs exhibited symmetric and saturated P–E hysteresis loops similar to the conventional PZT thin films. We estimated that the piezoelectric properties of MLCs by FEM simulation, and confirmed that the effective transverse piezoelectric coefficients (d _31,eff ) increased with the number of PZT layers. The piezoelectric coefficients calculated to be d _31,eff  = ?2964 pC/N at 25 PZT layers, which is much higher than those of conventional single-layer piezoelectric thin films.     

Membrane microcantilever arrays fabrication with PZT thin films for nanorange movement

Lead zirconate titanate (PZT) piezoelectric thin films have been prepared by sol-gel method to fabricate microcantilever arrays for nano-actuation with potential applications in the hard disk drives. In order to solve the silicon over-etching problem, which leads to a low production yield in the microcantilever fabrication process, a new fabrication process using DRIE etching of silicon from the front side of the silicon wafer has been developed. Silicon free membrane microcantilevers with PZT thin films of 1 μm in thickness have been successfully fabricated with almost 100% yield by this new process. Annealing temperature and time are critical to the preparation of the sol-gel PZT thin film. The fabrication process of microcantilever arrays in planar structure will be presented. Key issues on the fabrication of the cantilever are the compatible etching process of PZT thin film and the compensation of thin film stress in all layers to obtain a flat multi-layer structure.     

Fabrication and characterization of a piezoelectric micromirror using for optical data tracking of high-density storage

A micromirror actuated by three piezoelectric microcantilevers is presented for optical data tracking of high-density storage application. The microcantilevers are actuated by 2.5-μm-thick lead zirconate titanate (PZT) films which are deposited on the silicon-based substrate by a compatible sol–gel route. The X-ray diffraction result shows that the PZT film is perovskite structure and has a typical good ferroelectric loop. The quasi-static displacement of the mirror plate increases linearly with increasing the driving voltage and the tracking resolution on disk is as high as 8 nm/V. The micromirror also provides a high bandwidth of about 21 kHz, which is high enough to support the optical data tracking of future high-density storage.     

An analytical study of the effect of a support geometry on frequency shift and support loss of piezoelectric ring-shaped resonators for healthcare and environmental applications

Ring-shaped resonators with one support have been designed in this work. The ring-shaped resonator reacts with a mass perturbation to provide eigenstate or frequency shifts which could transfer to electrical signals by piezoelectric effect. The aforementioned ring-shaped resonator is mainly comprised with a multilayer of Pt/Ti/PZT/Pt/Ti/SiO₂ deposited on the silicon-on-insulator wafer and expected to be a contour mode. In order to estimate the sensitivity of the ring-shaped resonator against the mass perturbation, the theoretical analysis was conducted by ANSYS from two aspects including: (a) the effect of support geometry on frequency shift and support loss (from view point of vibration mode shape); and (b) the mass application methodology. It is found that low-amplitude vibration area is smaller at narrow support, and frequency shift of trapezoid support is higher than that of rectangle support.     

Submicron polymer structures with X-ray lithography and hot embossing

This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiO _x membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λ _c ≈ 0.4 nm) at a dose of 4 kJ/cm³ using a Si₃N₄ membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.     

A low cost,high performance insulin delivery system based on PZT actuation

A low driving voltage, low cost, high performance insulin delivery system based on PZT actuation is presented in this paper, which consists of two functional units, namely, micropump unit and electronic control unit. The PZT micropump is the core of micropump unit and is the key base to ultimately realize insulin precision delivery of the whole system. The electronic control unit is the important auxiliary unit for the realization of the whole system function. To obtain a higher working performance under low voltage, a serial structure with two chambers and three check valves is adopted in the design of PZT micropump. In place of silicon and glass, main parts of micro-pump unit are manufactured using the polymers which have good biocompatibility, stability and low cost. Through the systematic experimental test for the prototype of PZT insulin delivery system in lab, the maximum backpressure of 14.64 kPa is recorded at applied voltage of 36 V and working frequency of 160 Hz, the maximum flow rate of 5.74 ml/min is obtained in the condition of 36 V and 300 Hz. Under the voltage of 36 V and working frequency of 200 Hz, the micro-dosage pumped by PZT micro-pump displays a good linear characteristic with the number of driving impulses. The minimum resolution of insulin delivery can obtain 3 × 10^?4 ml (0.03 U insulin at the concentration of 100 U).     

Finite element analysis and experiments on a silicon membrane actuated by an epitaxial PZT thin film for localized-mass sensing applications

In this paper, we investigate the performance of a piezoelectric membrane actuated by an epitaxial piezoelectric Pb(Zr_0.2Ti_0.8)O₃ (PZT) thin film for localized-mass sensing applications. The fabrication and characterization of piezoelectric circular membranes based on epitaxial thin films prepared on a silicon wafer are presented. The dynamic behavior and mass sensing performance of the proposed structure are experimentally investigated and compared to numerical analyses. A 1500 μm diameter silicon membrane actuated by a 150 nm thick epitaxial PZT film exhibits a strong harmonic oscillation response with a high quality factor of 110-144 depending on the resonant mode at atmospheric pressure. Different aspects related to the effect of the mass position and of the resonant mode on the mass sensitivity as well as the minimum detectable mass are evaluated. The operation of the epitaxial PZT membrane as a mass sensor is determined by loading polystyrene microspheres. The mass sensitivity is a function of the mass position, which is the highest at the antinodal points. The epitaxial PZT membrane exhibits a mass sensitivity in the order of 10⁻¹² g/Hz with a minimum detectable mass of 5 ng. The results reveal that the mass sensor realized with the epitaxial PZT thin film, which is capable of generating a high actuating force, is a promising candidate for the development of high performance mass sensors. Such devices can be applied for various biological and chemical sensing applications.     

Design and fabrication of microtunnel and Si-diaphragm for ZnO based MEMS acoustic sensor for high SPL and low frequency application

The purpose of the paper is to design and fabricate a ZnO-based MEMS acoustic sensor for higher sound pressure level (SPL) measurement in the range of 120–200 dB and low frequency infrasonic wave detection. The thickness of silicon diaphragm was optimized for higher SPL using MEMS-CAD-Tool COVENTORWARE. The microtunnel which relates the cavity to the atmosphere was designed and simulated analytically for low cut-off frequency of the sensor in infrasonic band. The resonance frequency of the sensor was obtained using modal analysis. The sensitivity of the sensor was also estimated using COVENTORWARE. The optimized Si-diaphragm thickness for the intended SPL range was determined and found to be 50 μm. The lower cut-off frequency of the sensor for a 10 μm-deep microtunnel was found to be 0.094 Hz. The resonance frequency of the sensor was obtained using modal analysis and found to be 78.9 kHz. Based on simulation results, the MEMS acoustic sensor with 10 μm-deep microtunnel was fabricated. The optimum sensitivity of sensor was calculated using simulated results and found to be 116.4 μVolt/Pa. The lower cut-off frequency of the sensor can be utilized to detect low frequency sounds. The high SPL sensing capability of the device up to 200 dB facilitates detection of high sound pressure level in launch vehicles, rocket motors and weapons’ discharge applications.     

A MEMS sandwich differential capacitive silicon-on-insulator accelerometer

A differential capacitive accelerometer with simple process is designed, simulated, and fabricated. To achieve a precision structure dimension with fewer processing steps, the silicon device layer transfer technology is being used to built a sandwich accelerometer based on a silicon-on-insulator (SOI) wafer, which was assembled by glass-si-glass multilayer anodic bonding. Deep reactive ion etching is being used to define symmetric beams and large mass block of equal thickness together in SOI device layer (up to 100 μm) in a single step to avoid alignment error in double side process. An actual accelerometer which is designed for 50 g measure range is fabricated with six lithography steps. Measurement results show 0.1166 V/g sensitivity and 0.022 % nonlinearity error in ±1 g gravity static response test. The accelerometer also provides a power spectrum less than 10.49 μVrms/Hz^1/2 (89.97 μg/Hz^1/2) in a non-isolated laboratory environment with a capacitive interface circuit.     

Manufacture of microfluidic glass chips by deep plasma etching, femtosecond laser ablation, and anodic bonding

Two dry subtractive techniques for the fabrication of microchannels in borosilicate glass were investigated, plasma etching and laser ablation. Inductively coupled plasma reactive ion etching was carried out in a fluorine plasma (C₄F₈/O₂) using an electroplated Ni mask. Depth up to 100 μm with a profile angle of 83°–88° and a smooth bottom of the etched structure (Ra below 3 nm) were achieved at an etch rate of 0.9 μm/min. An ultrashort pulse Ti:sapphire laser operating at the wavelength of 800 nm and 5 kHz repetition rate was used for micromachining. Channels of 100 μm width and 140 μm height with a profile angle of 80–85° were obtained in 3 min using an average power of 160 mW and a pulse duration of 120 fs. A novel process for glass–glass anodic bonding using a conductive interlayer of Si/Al/Si has been developed to seal microfluidic components with good optical transparency using a relatively low temperature (350°C).