Water-Immersible Micromachined Pb(Zr, Ti)O3 Thin Film Actuators

We demonstrate a water-immersible thin film lead zirconate titanate, Pb(Zr, Ti)O₃, [PZT] actuator, without special passivation layer, towards in-vivo or in-vitro scanning probe microscope (SPM) measurements of living cells in water or biological fluids. In order to be water-immersible, the electrodes need to be electrically insulated and the piezoelectric layer needs to be protected against direct water contact. This paper describes our design solution with a simple fabrication process for a water-immersible piezoelectric device, which separates the bottom electrode from the top electrode by having a narrow ditch covered with PZT film. The PZT film is then encapsulated with the top metal electrode without insulation layer. In this structure, the PZT is sandwiched between the top and bottom metal electrodes to prevent water permeation. The device is fabricated using lift-off processing for the bottom and top electrodes, sol-gel spinning for the PZT thin film and wet etching for the PZT patterning. The piezoelectric constant, d₃₁, is about –100 pC/N. The dielectric polarization and fatigue properties of the devices were measured in air and water. The spontaneous polarization, remnant polarization, coercive field and dielectric constant are 54 C/cm², 15 C/cm², 60 kV/cm and 1200, respectively. The polarization property of the device was unchanged in either air or water up to 1 × 10⁹ continuous cycles.     

Preparation of Pb(Zr,Ti)O3 Thin Films on Silica Substrate by Metallo-Organic Decomposition

In this study, we report the deposition of crack-free transparent PZT films (up to ～859 nm) by metallo-organic decomposition (MOD) process on amorphous silica substrate. Effect of SrTiO₃ (STO) buffer layer on the growth behavior of PZT thin films deposited on SiO₂-coated silicon substrates was systematically studied. Perovskite phase, which cannot be formed directly on SiO₂/Si substrates, has been obtained when a thin STO film (～150 nm) was used as buffer layer. A SIMS examination indicates that the upward diffusion of Si-species into PZT layer is minimal, although downward diffusion of Pb-species into the SiO₂ layer is still observable.     

Characteristics of ferroelectric gate mos and mosfets

Abstract

Electrical characteristics of metal-insulator-semiconductor (MIS) capacitors of a variety of ferroelectric materials like lead zirconate titanate (PZT), lead titanate (PT) and barium magnesium fluoride (BMF) on p-silicon have been studied. PZT was deposited by r.f. magnetron sputtering from a composite target and PT from co-evaporation. The films were annealed in oxygen atmosphere in the temperature range 550–700°C for various times. PZT and PT films which are directly deposited on silicon showed low effective dielectric constant.¹⁰ For normal applied bias voltages (±5 V), the C-V curves did not show significant hysteresis. The effective dielectric constant was improved significantly by the incorporation of a buffer layer. BMF film was deposited in ultra high vacuum on a heated substrate and the film was encapsulated by a zirconium oxide layer. The C-V curves for these MIS capacitors shows hysteresis and the direction of hysteresis corresponds to ferroelectric polarization.     

Analytical model and optimal design of a d33-mode active layer for the lightweight unimorph piezo-composite actuator

This paper presents an analytical model, simulation and optimal design of a d₃₃-mode lightweight piezo-composite unimorph actuator (d₃₃-mode LIPCA). The actuator consists of a PZT layer bonded to several other composite prepreg layers. In which, the interdigitated electrodes (IDE) are deposited on the surfaces of the PZT material. The PZT layer is poled in-plane and operated in the semi d₃₃-mode. Since the d₃₃ coefficient is normally twice of d₃₁ value, the actuation performance of d₃₃-mode LIPCA is expected to be improved significantly. Performance predictions of the d₃₃-mode LIPCA are done by using the finite element method with a coupled-field model. Design of the IDE system patterned on the PZT surfaces is optimized in a poling process analysis to achieve the highest portion of PZT material poled in the longitudinal direction. Effect of the design variables such as electrode width, gap between electrodes and applied voltage for poling process is also performed numerically.     

Substrate effect on in-plane ferroelectric and dielectric properties of Ba0.7Sr0.3TiO3 thin films

Heteroepitaxial Ba_0.7Sr_0.3TiO₃ thin films were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (001) (LSAT) and SrTiO₃ (001) (STO) single crystal substrates using pulsed laser deposition (PLD). X-ray diffraction characterization revealed a good crystallinity and a pure perovskite structure for films grown on both LSAT and STO substrates. The in-plane ferroelectric and dielectric properties of the films were studied using interdigital electrodes (IDE). The film grown on LSAT substrate exhibited an enhanced in-plane ferroelectricity, including a well-defined P-E hysteresis loop with the remnant polarization P _r = 10.5 μC/cm² and a butterfly-shaped C-V curve. Nevertheless, only a slim hysteresis loop was observed in the film grown on STO substrate. Curie temperature T _c of the film grown on LSAT substrate was found to be ∼105^∘C, which is nearly 70^∘C higher than that of the bulk Ba_0.7Sr_0.3TiO₃ ceramics. T _c of the film grown on STO substrate has almost no change compared to the bulk Ba_0.7Sr_0.3TiO₃ ceramics. The dielectric tunabilities were found to be 64% and 52% at 1 MHz for the films grown on LSAT and STO substrates, respectively.     

PATTERNED HIGH FREQUENCY THICK FILM MEMS TRANSDUCER

ABSTRACT

Ferroelectric properties of BiFeO₃ (BFO) thin films epitaxially grown on SrRuO₃ (SRO)/(001)SrTiO₃ (STO) structure were investigated. First, bottom SRO electrodes were deposited on STO substrates by metalorganic chemical vapor deposition (MOCVD) or by sputtering. Then, BFO thin films were deposited on SRO/STO structures by chemical solution deposition. X-ray diffraction analysis showed that the SRO films deposited by both methods grew epitaxially on STO substrates as a single phase perovskite structure, but the out-of-plane lattice parameters of SRO were different, that is, they were 0.396 nm in MOCVD and 0.399 nm in sputtering. The leakage current densities were higher than 1 A/cm² at room temperature in BFO films on both MOCVD-and sputtered-SRO, but the current density in the film on sputtered SRO decreased to 2 × 10^?4 A/cm² at 80 K. The remanent polarization of approximately 50 μC/cm² was observed at 80 K in the BFO thin film on sputtered-SRO/STO.     

Characterization of hetero-epitaxial PZT capacitor

Abstract

A hetero-epitaxial Au/PbZr_0.48Ti_0.52O₃(PZT)/SrRuO₃(SRO) capacitor was fabricated on a single crystal SrTiO₃ (STO) substrate by pulsed laser deposition. An SRO buffer layer (a nucleation layer) was formed at the SRO/STO interface to ensure the highly epitaxial growth of the PZT and SRO films. An X-ray diffraction measurement revealed that the (00l) planes of the PZT and SRO grew parallel to the substrate surface. A transition layer of ～ 5 nm thickness was observed at the SRO/STO interface by high-resolution transmission electron microscopy (HR-TEM). This transition layer corresponds to the nucleation layer intentionally grown at the interface. Remanent polarization of the capacitor was 32.1 μC/cm² due to the good epitaxial growth of the films.     

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.     

Large area deposition of Pb(Zr,Ti)O<Subscript>3</Subscript> thin films for piezoelectric MEMS devices

Pb(Zr,Ti)O₃ (PZT) thin films deposited on insulating ZrO₂ buffered silicon wafer are intended to be employed for in-plane polarized piezoelectric MEMS devices. Multi-target reactive sputtering system for large area deposition of in-situ crystallized PZT thin films and the ZrO₂ buffer layer has been employed. The interface analysis of multilayer structures by high resolution transmission microscope, X-ray diffraction, optical refraction, and absorption spectra studies has been presented. At a substrate temperature of 520°C and excess lead deposition condition, the formation of a PZT superstructure has been revealed. The substrate temperature of 580°C leads to the crystallization of PZT directly into a single phase perovskite crystal structure. A pronounced Urbach behavior in our PZT thin films has been observed by optical absorption studies. The surface roughness of PZT films deposited on a ZrO₂ buffer layer is much higher than that on conducting platinized silicon wafer.     

Bilayered Pb(Zr,Ti)O3/(Bi,Nd)4Ti3O12 thin films

Bilayered thin films consisting of Pb(Zr_0.52Ti_0.48) O₃ (PZT) and (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) layers are successfully deposited on Si(100)/SiO₂/Ti/Pt substrate by a combined process involving sol-gel and RF-sputtering. Their dielectric properties cannot be described by the simple rule of mixture on the basis of the series connection model. There occurs a dielectric layer of lower dielectric permittivity in the bilayered thin film, which degrades the polarization behaviors. The bilayered film gives rise to an improvement in fatigue resistance up to 10¹⁰ switching cycles. Moreover, the domain pinning effect after polarization switching is reduced greatly as compared to that of single layered PZT and BNT thin films.     

Preparation and characterization of PZT thin films deposited on PZT buffer layer with different lead content

Lead zirconate titanate (PZT) thin films deposited on Pt electrode and Pb_1+x(Zr_0.52,Ti_0.48)O₃ (x = 0.10, 0.15, 0.25, 0.30) buffer layer have been prepared by sol-gel methods to investigate the effects of lead content in the buffer layer on crystalline orientation, electric and fatigue properties of PZT films. XRD and SEM showed that all films exhibited dense perovskite structure with (100) preferential orientation. The maximum dielectric constant (1571 at 100 Hz) was obtained in the PZT film with buffer layer containing 25% excess lead, which increased by 42.5% compared with the film without buffer layer. Fatigue resistance was improved by introducing buffer layer.     

Mechanical and electric fatigue of PZT(53/47) films on metallic substrates

Abstract

We have investigated the fatigue of electromechanical and dielectric properties of sol-gel derived PZT(53/47) thin films deposited on metallic substrates by means of electric and mechanical cycling. For the mechanical cycling a two point bending method was used to apply transversal stress to the samples. During mechanical cycling the piezoelectric coefficient d₃₁ remained constant up to about 10⁵ cycles, for a higher number of cycles a strong decrease was observed. During electric cycling no significant changes in the ferroelectric and electromechanical hysteresis loops could be found up to about 3×10⁵ cycles. Above this number the coercive field increases, the maximum strain and the remanent polarization decrease.

Obviously each electric cycling of the investigated films is accompanied by a mechanical cycling. It is assumed, that microcracks induced by mechanical stress are the main reason for the deterioration of the physical properties films during electric and mechanical cycling both.     

EFFECTS OF PZT BUFFER LAYERS ON PZFNT FERROELECTRIC THIN FILMS

ABSTRACT

PZFNT thin films were fabricated on 5-inch Pt/Ti/SiO₂/Si and PZT/Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering method and rapid thermal annealing process. By investigating the two thin films at various annealing temperatures, the results show that the annealing temperature of PZFNT thin films without PZT buffer layers is about 730°C, which is higher than that of PZFNT films with PZT buffer layers. By use of PZT buffer layers, the annealing temperature of PZFNT films is decreased greatly, and the dielectric and ferroelectric properties are improved. In the optimum process, the thin films with PZT buffer layers have a dielectric constant of 1199 and dielectric loss of 3.0% at 1 KHz. The remanent polarization and coercive field of the thin films are 21.1 μC/cm² and 53.5 KV/cm respectively. The films have the significant potential for FRAM and pyroelectric infrared detectors.     

Preparation of Textured Growth Pb(Zr,Ti)O3 Thin Films on Si Substrate Using SrTiO3 as Buffer Layers

PZT thin films are deposited on SiO₂/Si substrate by metallo-organic decomposition (MOD) process, using SrTiO₃ (STO) as buffer layer for textured growth. The STO layers deposited on SiO₂/Si substrate by pulsed laser deposition process show (100)/(200) preferred orientation, whereas the STO buffer layer deposited on silica substrate using spin-coating technique show random orientation behavior. The use of STO as buffer layers enhanced the crystallization and the preferred orientations of the PZT films. The PZT on STO buffered SiO₂/Si substrates thus obtained possess high refractive index, (n)_PZT/STO = 2.1159, and are of good enough quality for optical waveguide applications.     

Determination of Cross Sectional Variation of Ferroelectric Properties for Thin Film (Ca. 500 nm) PZT (30/70) via PFM

There is much speculation about the origin of fatigue in ferroelectric thin films that have been grown by sol-gel on various substrates. One of the most important substrates for growing thin film ferroelectric materials is native SiO₂. After the deposition of suitable electrode, most common are Ti-Pt, a thin film of a ferroelectric material can be grown on the substrate. Procedures to grow thin film lead zirconate titanate (PZT 30-70) have been well publicized and it is now routine that almost 100% [111] orientated PZT with a columnar structure of ca. 100 nm is grown on SiO₂ substrates. Recent studies using PFM have shown that it is possible to determine a variation in hysteresis loops with a spatial accuracy of 50 nm on the surface of a ferroelectric material. This study was to determine the variation in d₃₃ for a thin film sample of ferroelectric in terms of a depth profile. It was not suitable to just turn the sample on it's side and analyze the d₃₃ in terms of depth as the continuous back electrode would mean that the resulting vibration of the ferroelectric surface would be a combination of d₃₃ and d₃₁. The de-convolution of this signal was outside the scope of the current investigation. In order to determine the d₃₃ of the material in terms of depth the sample was machined (with an accuracy of 10 nm) to release the ferroelectric from the underlying electrode and electroded in the correct orientation for d₃₃ to be investigated in cross section. Samples of perovskite PZT(30/70) have been produced via sol-gel using a spin coating technique that are ca. 500 nm in cross sectional thickness on Pt/Ti/SiO₂ back electrodes. The PZT is shown to be highly [111] orientated and has in the past been well characterized both at the macro and nanoscale using a variety of techniques. Using focused ion beam (FIB) milling the PZT layer has been released from the SiO₂/Ti/Pt substrate and electroded in such a way as to determine the δ₃₃ properties of the film in terms of thickness of the as-deposited film. Figure 1 indicates the machining process used to generate the free standing PZT block. In Fig. 2, the results of the FIB machining process are shown. The ‘bridge’ structure has the dimensions of 500 nm in cross section and is long enough to ensure that electrode cross talk between the d₃₃ and d₃₁ modes is reduced. Hysteresis loops have been generated using the technique of piezo force microscopy (PFM) at discrete locations across the surface of the film corresponding to a depth profile of the as-deposited PZT. Variations in the shape of the hysteresis loops, calculated δ₃₃ and coercive field for the film have been related to the variation in oxygen defect density in the film. This study shows that the PZT thin film has a variety of fundamental piezoelectric constants that are associated with the variation in local crystal chemistry.     

Application of fluorinated SiO2 interlayer dielectrics for ferroelectric memory

Abstract

Fluorine-doped silicon oxide (SiOF) as interlayer dielectric (ILD) was deposited over PZT capacitors by electron cyclotron resonance (ECR) chemical vapor deposition using SiF₄ and N₂O gases. In the conventional deposition of SiO₂ ILD layer using hydrogen-contained source gases, the properties of ferroelectric capacitors are known to be degraded during the formation of SiO₂ layer. In this study, we examined the degradation of electrical properties of SiOF-deposited PZT capacitors. The remnant polarization and leakage currents were not degraded after the deposition of SiOF. We observed that the fluorine atoms were not diffused into the metal electrode in both cases of the SiOF deposited PZT capacitors and post-deposition annealed capacitors. The SiOF films deposited in the high CF₄ flow rate exhibited rough columnar structure on the metal electrodes. We can successfully deposit SiOF in a smooth morphology by introducing TiO₂buffer layer or using the novel deposition method of changing the SiF₄ flow rate, namely two-layer-deposition method.     

Microfabrication of Piezoelectric MEMS

In this paper we present an overview of processes for fabrication of piezoelectric thin film devices using PZT (Pb(Zr _x Ti_{1 ? x} )O₃) in planar structures. These structures are used in cantilever-like and membrane configurations for sensing and actuation. Elaboration of a compatible wet and dry etching sequence for patterning of PZT, electrodes, SiO₂ and silicon substrate is the key issues. The method for compensation of mechanical stresses to obtain flat, multilayer structures is demonstrated. Definition of membrane thickness and release of the structures are obtained by Deep Reactive Ion Etching of silicon (SOI—silicon on insulator substrates) or by surface micromachining. The complete process has been used for fabrication of cantilever arrays, ultrasonic transducers and pressure sensors. Excellent permittivity and transverse piezoelectric coefficient of PZT have been obtained with the final devices. Other examples of applications like: ferroelectric memories, nanopatterning and local growth of PZT are presented as well. The microfabrication of piezoelectric MEMS was found to be a complex task where all aspects from device design, material properties and microfabrication to assessment of performance are closely interconnected.     

High performance thin film PZT ultrasonic transducer by CSD for distance measurements in water

Piezoelectric thin film ultrasonic transducers were realised and tested for short range distance measurements. Displacements in air and water as a function of frequency were modelled by Comsol Multiphysics finite element modelling (FEM) and transducer configurations with a two electrode layout were manufactured to enable larger displacements than with the conventional design. The transducer was fabricated on a silicon wafer by chemical solution deposition (CSD) with total PZT (Pb(Zr_0.53Ti_0.47)O₃) thickness of 2 µm. Subsequently, a cavity underneath the PZT was wet etched creating a bending membrane with a total thickness of ??13 µm. The displacements of the transducers as a function of frequency were modelled and measured by fiber-optic laser vibrometer. The effective piezoelectric d₃₃ coefficient of 300 nm/V and 144 nm/V in air and 48 nm/V and 18 nm/V in water was obtained for 260?×?260 µm² and 390?×?390 µm² membranes, respectively. The accuracy of the modelled resonance frequencies both in air and water was relatively good, of ??4-13% and ??5-20%, respectively.     

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.     

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.