Fabrication of PZT Thick Films on Silicon Substrates for Piezoelectric Actuator

In recent years, there has been an increased interest in ferroelectric lead zirconate titanate (PZT) films for applications in piezoelectric devices. Many potential applications require a film thickness of about 10 μm for higher force, better sensitivity and stability. In this study we fabricate lead zirconate titanate (PZT) thick films by screen printing on silicon substrates with a platinum bottom electrode. Various substrates were studied. The longitudinal piezoelectric coefficient, d₃₃, was measured by the normal load testing method. Breakdown voltage, tan δ, P-E hysteresis loop and permittivity were measured on the PZT thick films. The results are promising for the use of PZT thick films in various applications, for example, in silicon micromachined micro-pump.     

Basic investigations on a piezoelectric bending actuator for micro-electro-mechanical applications

Abstract

This article deals with basic investigations into the development and manufacturing process of a piezoelectric animated micro actuator. The concept of the fabrication of a piezoelectric microactuator will be introduced and first fundamental investigations on materials characterization and process technology for the Pt bottom electrode and piezoelectric PZT layer will be presented. PZT thin film have been deposited with the chemical solution deposition (CSD) technique and have been characterized with dielectric and ferroelectric measurements. For optimization of actuator properties an analytical approach and a simulation with finite element method was carried out. This shows that the cantilever must have a length above 300 μm to reach a tip deflection higher than 10 μm at voltages comparable to those used in integrated circuits (IC). Additionally, stress measurements of each layer have been used to characterize the films mechanically.     

Fabrication of PZT Thick Films on Silicon Substrates for Piezoelectric Actuator

In recent years, there has been an increased interest in ferroelectric lead zirconate titanate (PZT) films for applications in piezoelectric devices. Many potential applications require a film thickness of about 10 m for higher force, better sensitivity and stability. In this study we fabricate lead zirconate titanate (PZT) thick films by screen printing on silicon substrates with a platinum bottom electrode. Various substrates were studied. The longitudinal piezoelectric coefficient, d₃₃, was measured by the normal load testing method. Breakdown voltage, tan , P-E hysteresis loop and permittivity were measured on the PZT thick films. The results are promising for the use of PZT thick films in various applications, for example, in silicon micromachined micro-pump.     

Hysteresis nonlinearity compensator for piezoelectric actuator

The purpose of this paper is to improve the hysteresis characteristics of a stack type piezoelectric actuator using the hysteresis nonlinearity compensator. Recently, several printing methods that cost less and are faster than previous semiconductor processes have been developed for the production of electric paper and RFID. The system proposed in this study prints by spraying the molten metal, and consists of a nozzle, heating furnace, operating actuator, and an XYZ 3-axis stage. As an operating system, the piezoelectric (PZT) method has very valuable uses. However, the PZT actuator has a hysteresis nonlinearity due to the ferroelectric characteristics of the PZT element. This causes problems in the system position control characteristics and deteriorates the performance of the system. This study proposed an inverse hysteresis model, a mathematic modeling method that can express the geometric relationship between voltage and displacement, in order to reduce the hysteresis of the PZT actuator. In addition, system identification and PID control methods were examined. Also, it was confirmed that the proposed control strategy gives good tracking performance.     

Piezoelectric and Pyroelectric Properties of Pb(Zr,Ti)O3 Films for Micro-Sensors and Actuators

The piezoelectric and the pyroelectric properties of PZT films are systematically investigated for tetragonal (Zr/Ti = 30/70), morphotropic (52/48), and rhombohedral (70/30) compositions. The magnitude of the effective longitudinal piezoelectric coefficient (d₃₃) and pyroelectric coefficient (p) of these films is measured by atomic force microscopy and Byer-Roundy method, respectively. All films are consistently highly textured (111) orientation and have dense microstructures. Slightly less degree of texture in higher Zr-rich composition is observed due to the lattice mismatch between PZT and Pt bottom electrode and higher activation energy for nucleation. Squareness of polarization hysteresis loops is optimized in tetragonal composition, which indicates the tetragonal PZT is closer to the ideal hysteresis behavior than other compositions. It is shown that the piezoelectric coefficient and the pyroelectric figure of merit are dependent on the dielectric properties of the films. The morphotropic PZT films with high dielectric constant and low pyroelectric figure of merit show the largest piezoelectric coefficient values, while the tetragonal PZT films with low dielectric constant and high remanent polarization values show the largest pyroelectric figure of merit compared to other compositions, which indicate the suitability for PIR sensor devices.     

Influence of Stress on the Performance of PZT Thin Films for Microphone Application

ABSTRACT

A series of microphone cells based on integrated PZT thin films were fabricated by typical MEMS process. The thickness of back silicon as a load of the vibrating diaphragm was controlled by changing ICP (Inductive Coupling Plasma) etching time and power. Concaved diaphragms with different radius were formed due to the balanced tensile stress among the multi-layer films structure. The remnent and saturated polarization strength of microphone cells decreased with the increased deformation of diaphragm. The phenomenon was explained as the weakening of mechanical-electrical coupling in piezoelectric thin films in stressed state.     

High-rate sputtering of thick PZT thin films for MEMS

Crack and void free polycrystalline Lead Zirconate Titanate (PZT) thin films in the range of 5 µm to 10 µm have been successfully deposited on silicon substrates using a novel high rate sputtering process. The sputtered PZT layers show a high dielectric constant ε_r between 1,000 and 1,800 with a moderate dissipation factor tan (δ) = 0,002???0,01 measured at f = 1 kHz, a distinct ferroelectric hysteresis loop with a remanent polarisation of 17 µC/cm² and coercive field strength of 5.4 kV/mm. The piezoelectric coefficients d_33,f = 80 pm/V are measured by using a Double Beam Laser Interferometer (DBLI). Based on this deposition process a membrane actuator mainly consisting of a SOI layer and a sputtered PZT thin film was prepared. The deflection of this membrane actuator depending on the driving voltage was measured with a white light interferometer and compared to the results of finite element analysis (FEA). With this approach a transverse piezoelectric coefficient of about e₃₁?=??11.2 C/m² was calculated, whereas all the other material parameters in the model were lent from PZT-5A.     

Investigation of top electrode for PZT thick films based MEMS sensors

In this work processing of screen printed piezoelectric PZT thick films on silicon substrates is investigated for use in future MEMS devices. E-beam evaporated Al and Pt are patterned on PZT as a top electrode using a lift-off process with a line width down to 3 μm. Three test structures are used to investigate the optimal thickness of the top electrode, the degradation of the piezoelectric properties of the PZT film in absence of a diffusion barrier layer and finally how to fabricate electrical interconnects down the edge of the PZT thick film. The roughness of the PZT is found to have a strong influence on the conductance of the top electrode influencing the optimal top electrode thickness. A 100 nm thick top electrode on the PZT thick film with a surface roughness of 273 nm has a 4.5 times higher resistance compared to a similar wire on a planar SiO₂ surface which has a surface roughness of less than 10 nm. It is found that the piezoelectric properties of the PZT thick film are degraded up to 1,000 μm away from a region of the PZT thick film that is exposed directly to the silicon substrate without a diffusion barrier layer. Finally, ferroelectric hysteresis loops are used to verify that the piezoelectric properties of the PZT thick film are unchanged after the processing of the top electrode.     

Electrical properties of 3-component piezoelectric thick films by screen printing method

Abstract

PZT(52/48) thick films with Pb-based complex oxide (PCW) additive were prepared on Pt/TiO₂/YSZ/SiO₂/Si substrate by screen printing method. PCW addition and PZT sol application are performed to fabricate high density PZT thick film and to lower sintering temperature. With the increase of sintering temperature, electrical properties of screen-printed films were improved. Further, for the sol-gel treated thick films, the electrical properties were improved as compared to only screen-printed films. For the PZT-0.12PCW thick films with sol-treated and sintered at 900°C, the remanent polarization (P_r) was about 23.8 μC/cm² at the applied filed of 150 kV/cm², the dielectric permittivity (Ω_r) was 1024 at the frequency of 100 kHz, and the piezoelectric coefficient (d₃₃) was 339 pC/N at the applied pressure of 1 atm. Finally, the application of these PZT thick films to piezoelectric actuator is described.     

Self-polarization effect in Pb(Zr,Ti)O3 thin films

Abstract

The self-polarization effect is investigated in Pb(Zr,Ti)O₃ (PZT) thin films deposited by sol-gel and magnetron sputtering techniques. The effective piezoelectric coefficient of as-grown films, which is proportional to their initial polarization (self-polarization), is measured by a sensitive interferometric technique as a function of the annealing temperature, PZT composition, film thickness and bottom electrode material. The results indicate that the films are self-polarized by an internal bias field upon cooling through the phase transition temperature. It is suggested that a built-in field of a Schottky barrier between the PZT film and the bottom electrode is responsible for the observed effect. Self-polarization of the films is found to be very stable and in some cases to be as high as 90% of that produced by the subsequent room temperature poling. This property is very useful for piezoelectric and pyroelectric applications of PZT films since the poling procedure can be avoided. The properties of self-polarization are found to be similar for the films produced by sol-gel and sputtering techniques, suggesting that the same mechanism is operative in both cases.     

PZT Thin Film Bi-Layer Devices for Phase Controlled Actuation in MEMS

A potential application for ferroelectric thin films is micro positioning and actuation. For using PZT films as micro-actuators it is desirable to have film thicknesses of comparable size to the underlying structure. The amount of actuation possible is determined by a number of factors: the piezoelectric coefficient d ₃₁, geometric factors and the compliance of both the actuator and cantilever and the electric field across the film. Using a bi-layer should therefore increase the amount of actuation for a given drive voltage. Bi-layer devices can also be driven at constant voltage, and their actuation varied by the phase difference of the drive voltage between the two layers. PZT films of thickness 0.5 m have been deposited as a bi-layer. Micro-actuators have been fabricated using these structures, their electric properties measured and their electro-mechanical properties characterised and evaluated using optical beam deflection.     

Comparing macroscopic and microscopic properties of seeded ferroelectric thin films

The comparison of macroscopic and microscopic properties of ferroelectric thin films in the systems of lead zirconate titanate (PZT) and strontium bismuth tantalate (SBT) with and without seeds is carried out. Microscopic properties were studied by piezo-response force microscopy (PFM). The local piezoelectric properties with and without seeds are compared with their macroscopic electric properties measured by conventional techniques. Previous microstructure analysis of PZT thin films showed that an intermetallic Pt _x Pb layer between PZT and Pt, formed during the annealing process, was reduced and even eliminated in seeded PZT films. In SBT films, the addition of SBT seeds suppressed the interdiffusion of Pt and film components. Hence, the interfaces of PZT/substrate and SBT/substrate are modified by the presence of seeds, and their electrical properties are improved. In both PZT and SBT films, the remanent polarization values are higher in seeded films than in unseeded ones. Similarly, local piezo-response signal of single grain showed higher longitudinal piezoelectric coefficient d ₃₃ in seeded films than in unseeded ones. The critical voltage in which the ferroelectric domain starts to switch is lower in seeded films than in unseeded ones. The analysis of nanoscale switching in PZT and SBT films by PFM is presented and related to the corresponding macroscopic electric properties.     

Integration of Piezoelectric PZT Thin Films with Internal Electrodes into an Actuator Structure for MEMS Applications

This paper presents the Finite-Element-Method (FEM) analyses, the processing, and the characterization of micro cantilever beams, which are driven by the transversal piezoelectric effect of two PbZr 0.45 Ti 0.55 O 3 (PZT) thin films with an internal platinum electrode. SiO 2 /Si 3 N 4 is used as elastic substrate for this stack. For the development of this thin film bimorph silicon bulk micro machining is used in combination with chemical solution deposition (CSD), sputter technology and reactive ion etching (RIE). For electrical and piezoelectric characterization of the PZT films CV-, hysteretic, and double beam laser interferometer measurements are carried out. The findings are compared to the FEM analyses and the results of a single piezoelectric layer design [1].     

Ultrasonic Micromotors Based on PZT Thin Films

The integration of piezoelectric Pb(Zr,Ti)O₃ thin films on silicon substrates for ultrasonic motor applications is reviewed. With suitable buffer and bottom electrode layers the problems due to high processing temperatures in oxygen ambient can be handled. Reproducibility can be increased by stabilized electrode systems and seeding layers for PZT nucleation. Elastic fin micromotors of millimeter size have been fabricated. They achieve the necessary torques, low speeds, and battery voltage operation for applications in wristwatches. However, their efficiency needs to be improved. Better figure of merits of the material still can be achieved. Better designs with larger coupling factors are desirable.     

Mechanism for slow switching effect in advanced low-voltage, high-speed Pb(Zr/sub 1-x/Ti/sub x/)O/sub 3/ ferroelectric memory

Slow-switching effect in PZT ferroelectric memory under low-voltage and high-speed operation is observed. The slow-switching effect becomes worse at lower operation voltage and elevated temperature. This effect significantly reduces the sensing margin and causes severe reliability issue for advanced ferroelectric memory, particularly for low-voltage and high-speed applications. This slow-switching effect is believed to be attributed to slowing down of polarization switching caused by band bending from Schottky built-in potential at the electrode/ferroelectric interface. The proposed mechanism is supported by the polarity dependence in an asymmetric LNO/PZT/Pt sample.     

Piezoelectric Ring-Morph Actuators for Valve Application

This paper presents piezoelectric ring-morph actuators designed to produce a large flexural displacement for valve actuation application. The ring-shaped piezoelectric plate in the actuator produces a radial contraction or expansion and causes the center part of the metal disc to generate a large flexural displacement. The PZT ring creates a space under the metal disc which permits it to bend with a curvature larger than that of the conventional bimorph in the axis direction of the ring-morph when the PZT ring under the metal disc contracts. A prototype with diameter of 25.4 mm and thickness of 0.55 mm could produce a total stroke (static displacement) of over 130 m under a driving voltage of ±300 V and a load of 2 N. The maximum generative force of ring-morph actuator was about 30 N.     

Nanoscale characterization of polycrystalline ferroelectric materials for piezoelectric applications

In this work, the nanoscale electromechanical properties of several important piezoelectric materials [as exemplified by PbZr _x Ti_1−x O₃ (PZT)] suitable for both bulk actuator and microelectromechanical system (MEMS) applications are reported. The investigations are performed by the piezoresponse force microscopy (PFM) that is currently the most suitable tool for both ferroelectric domain imaging and local piezoelectric studies. The local piezoresponse of individual grains is measured in PZT films and compared with average piezoelectric behavior. Frequency dependencies of local piezoelectric coefficients are presented and analyzed. The results on local piezoelectric nonlinearity, as well as on nanoscale fatigue and aging are briefly discussed. These measurements demonstrate that PFM is promising for studying local piezoelectric phenomena in polycrystalline ferroelectrics where defects and other inhomogeneities are essential for the interpretation of macroscopic piezoelectric properties. Finally, local electromechanical properties of polycrystalline relaxors (PMN-PT, PLZT, doped BaTiO₃) are briefly outlined.     

PVDF actuator for high‐frequency fatigue test of thin‐film metals

This paper suggests a poly(vinylidene fluoride) (PVDF) piezoelectric diaphragm actuator used in a novel fatigue test method for thin metal films. A thin‐film metal specimen is stamped on top of the actuator using a stamping epoxy. As the actuator vibrates, the stress in the specimen increases until it fails under fatigue. A finite element model of the actuator is built, and its vibration amplitude is confirmed to be in a good agreement with experiment. Then, a model of the specimen is added to this model to simulate the vibration of the specimen for fatigue test. Stress analysis of the specimen at a driving voltage of 200 V_0−p confirms that this actuator can increase the stress in the specimen to near 1 GPa, which is high enough for the fatigue test of metals such as titanium. In the experiment, a thin‐film titanium specimen is stamped on top of the actuator which is then vibrated. The stress in the fatigue gauge on thin‐film specimen increased until the specimen failed under fatigue. This shows that the proposed PVDF actuator is suitable for the fatigue test of thin‐film metals such as titanium. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

An investigation of thick PZT films for sensor applications: A case study with different electrode materials

Lead zirconate titanate (PZT) is a piezoelectric material that can sense or respond to mechanical deformations and can be used in ceramic micro-electro-mechanical systems (C-MEMS). A thick-film paste was prepared from a pre-reacted PZT powder (PbZr_0.53Ti_0.47O₃) and thick-film technology (screen-printing and firing) was used to deposit the PZT layers on LTCC tapes and on alumina substrates. The microstructural, electrical and piezoelectric characteristics of the thick PZT films on relatively inert alumina substrates and on LTCC tapes were studied. Preliminary experiments indicated that due to the interaction between the printed PZT layers and the LTCC substrates during firing the electrical characteristics deteriorate significantly. To minimise the influence of substrate-film interactions different electrode materials and the use of additional intermediate layers as a barrier were evaluated. The dielectric permittivities, dielectric losses, and piezoelectric coefficients (d ₃₃) were measured. The dielectric permittivities of the thick films fired on LTCC substrates were lower (210 with gold electrodes and 430 with silver electrodes) than those measured on alumina substrates (500). The piezoelectric coefficients d₃₃ were measured with a Berlincourt piezometer. The d ₃₃ values measured on the LTCC substrates were relatively low (60–80 pC/N) compared with the values obtained for the alumina substrates (around 140 pC/N). The lower dielectric constants and piezoelectric coefficients d ₃₃ of the films on LTCC substrates are attributed to the formation of phases with a lower permittivity. This was a result of the diffusion of SiO₂ from the LTCC into the active PZT layer. The diffusion of silica was confirmed by the SEM and EDS analyses.     

Cryogenic Cofired Multilayer Actuator Development for a Deformable Mirror in the Next Generation Space Telescope

Xinetics is working with NASA to develop a cryogenic deformable mirror technology to meet the specific needs of the Next Generation Space Telescope. One of the critical technical issues is the development of a cryogenic actuator with sufficient displacement and temperature stability. This paper discusses the two year effort to achieve a cofired electroceramic multilayered cryogenic actuator. The development began by testing materials from 300 to 35 K via a cut and bond actuator technology that led to a cryogenic electroceramic material down selection. After selecting a doped SrTiO₃, a cofired actuator process specific to the cryogenic ceramic was developed. The assembled cryogenic actuators achieved the 3 m displacement (stroke) between 35 and 65 K required by the deformable mirror design. The discrete cryogenic actuators were assembled into an engineering model cryogenic 349-channel deformable mirror that was delivered to NASA in October 2001.