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.     

Fabrication and characteristics of PZT thick films on Pt/Ti foil substrates for piezoelectric vibrators

Crack free perovskite PbZr_0.53Ti_0.47O₃ (PZT (53/47)) thick films up to 30 μm were prepared on flexible platinum-coated titanium foil substrates (Pt/Ti) by a metal organic decomposition (MOD) process. The dielectric, ferroelectric and piezoelectric properties of the films were examined and discussed. A well-saturated hysteresis loop of the thick film was present in almost rectangular shape (Pr?=?35 μC/cm²; Ec?=?32 kV/cm). The efficient piezoelectric coefficient d _{33, f} of the thick film is about 448 (1 kHz). PZT piezoelectric vibrators were made in bimorph mode. The displacements of the vibrators were investigated as the functions of the applied electric field and the substrate thickness. Under the same condition, the vibrator made from the thinnest Pt/Ti substrate gives the largest displacement.     

Piezoelectric micromachined ultrasonic transducers with thick PZT sol gel films

The fabricated micro machined ultrasonic transducers (pMUT) was based on piezoelectric laminated plates operating at flexural modes. The fabricated bimorph pMUT transducers were composed of 5-layers. A 4 μm thick lead zirconate titanate (PZT) thin film deposited by a sol–gel method was used. The piezoelectric layer exhibited a capacitance corresponding to a permitivity of ɛ _r = 1,200. The electromechanical coupling coefficient (k ²) and quality factor (Q) were measured as k ² = 4.4% and Q = 145 in air for a low frequency transducer (240 kHz). The effect of DC bias voltage on frequency and k ² has been studied. The 16.9 MHz transducer yielded values of Q = 25 in air and k ² = 3%.     

Dielectric Characteristics of In-Plane Polarized Lead Zirconate Titanate Thin Films on Oxide Layers

Sol-gel derived lead zirconate titanate (PZT) films have been prepared on STO-passivated silicon substrates. The STO buffer layer of thickness about 55 nm is prepared by rf-magnetron sputtering. XRD results reveal that the PZT film has well-crystallized perovskite phase, indicating that the thin STO layer can effectively prevent reaction and inter-diffusion between the PZT film and silicon substrate. Interdigitated electrodes (IDEs) have then been deposited on the PZT film by magnetron sputtering and patterned using the standard photolithography. With the IDEs, the dielectric and ferroelectric properties of the PZT film under transverse or in-plane electric fields have been investigated. By assuming a uniform distribution of electric field (in-plane electric field model), the estimated relative permittivity of the PZT film is about 2100, while the dielectric loss is less than 1%. Good in-plane polarization hysteresis loop is observed, showing an observed remanent polarization value of 21 μC/cm², which is comparable to that of a PZT film with both top and bottom electrodes. The in-plane polarized PZT/STO/SiO₂/Si film can be used to fabricate d₃₃-mode unimorph bending transducers, which will have much better performance than the conventional bending transducers driven electromechanically through the piezoelectric d₃₁ mode.     

An integrated microelectromechanical microwave switch based on piezoelectric actuation

In the field of microwave applications, microelectromechanical systems (MEMS) are attractive devices in order to force miniaturization by on chip integration. Here, we describe the design, fabrication and testing of a silicon based micromachined switch using piezo-electrically actuated elements. The microwave circuit consists of a coplanar waveguide (CPW) design with two piezoelectric activated beams integrated between the middle line and the ground planes. During operation the beams short the CPW by two overhanging bridge contacts and therefore the transmission characteristics of the microwave circuit change. The CPW is realized by 3 µm thick electroplated copper to yield good transmission characteristics, whereas the clamped—clamped beams benefit from a 250 nm thin PZT film between 100 nm thin Pt electrodes on top of a SiO₂ layer. By the use of double side clamped beams awkward stress compensation of the piezoelectric stack is omitted. Instead the system relies on some initial mechanical stress. Measurements prove deflections of more than 13 µm for a 1400 µm long beam with operation voltages below 10 V. This is in good agreement with finite element simulations. The novel RF-MEMS is predicted to reach an isolation (in “on” state) of more than 20 dB up to 15 GHz.     

Preparation and characterization of PZT ferroelectric thin films by plasma enhanced metalorganic chemical vapor deposition

Abstract

PZT thin films with a uniform distribution of components were prepared by plasma enhanced chemical vapor deposition (PECVD) using Pb(C₂H₅)₄, Z (O-i-C₄H₉)₄, Ti(O-i-C₃H₇)₄, and oxygen. The crystallization of films was occure after annealing in the temperature range between 450 and 550°C under O₂ ambient for 1 hr. The significant change of Pb concentration in PECVD PZT thin films was not observed in the relation to annealing temperature and time. The dielectric constant PECVD PZT thin films increased with the Ti content, showed the maximum value in the vicinity of morphotropic phase boundary (MPB) composition of PZT material, and decreased with the Ti content. The leakage current density of PZT (65/35) thin film of 180 nm in thickness was 3·37 × 10^?7 A/cm² at the applied voltage of 3 V. Remanent polarization increased with increasing of Zr content in the film and coercive field was nearly independent of the composition. The typical values of electrical properties were ε_r = 570, E_c = 90 kV/cm, and P_r = 19 μC/cm² in the PECVD PZT (54/46) thin film of 220 nm in thickness.     

Fabrication and evaluation of PZT/Ag composites and functionally graded piezoelectric actuators

Metallic Ag as the second phase was added into PZT ceramic matrix to fabricate piezoelectric composites and functionally graded actuators by gradually altering Ag concentration, aimed to improve mechanical properties and to solve possible interfacial debonding usually observed in conventional bimorph-type piezoelectric actuator. The PZT/Ag composites were obtained by directly co-firing PZT and Ag powders at 1200^∘C for 1 h. The fracture strength σ _f and fracture toughness K _IC , as well as the corresponding piezoelectric properties, were firstly evaluated upon the PZT/Ag composites for Ag concentrations of 0–30 vol%. The mechanical properties for the PZT/Ag composites were found to be greatly enhanced compared with pure PZT ceramics: from 69 to 129 MPa for σ _f and from 1.0 to 3.7 MPa.m^1/2 for K _IC . With increasing Ag concentration, the piezoelectric constant d ₃₃ of PZT/Ag composites was found to decrease from 419 to 86 pC/N. Then, a functionally graded actuator was fabricated and evaluated in terms of electric-induced curvature k. The PZT/Ag FGM actuator with size of 12 mm × 3 mm × 1 mm has a curvature k of 0.03–0.17 m⁻¹ that corresponds to applied voltages of 100–500 V. A comprehensive comparison was made on the mechanical property enhancements by the metal particles dispersion, and the bending displacements produced by the FGM actuators between the PZT/Ag and previously fabricated PZT/Pt systems.     

Fine patterning of ceramic thick layer on aerosol deposition by lift-off process using photoresist

Ceramic fine patterning using aerosol deposition (AD) method and lift-off process was reported. Pre-baking conditions and thickness of photoresist layer were selected carefully to minimize the dimensions of the patterned structure. As a result, a pattern width less than 10 µm for 2 µm thick PZT and α-Al₂O₃ AD-deposited layers was obtained on Si substrate at room temperature.     

Fabrication of dome-shaped PZT-epoxy actuator using modified solvent and spin coating technique

Thick film two phase, 0–3 composite PZT-epoxy dome-shaped structures have been fabricated for the first time using a modified solvent and spin coating technique, where a PZT and ethanol solution was dispersed in an epoxy matrix, combined with a hardener, spin coated onto stainless steel sheets, and poled at ~2.2 kV/mm. The electro-mechanical performances of the films were investigated as a function of volume fraction of PZT. The volume fraction of PZT was varied from 0.1 to 0.7 and the piezoelectric coefficients d ₃₁ and d ₃₃ , and the capacitance, C, were measured, and used to calculate the effective dielectric constants. The values for d₃₃, d₃₁, C and dielectric constant were 1.06 pC/N, 0.74 pC/N, 6.0 pF and 76.1 respectively, at 70 % volume fraction of PZT. The surface topography and morphology were examined via AFM and SEM. The piezoelectric strain coefficients, capacitance and effective dielectric constant increased with increasing PZT content, in addition to the surface roughness. Agglomeration of PZT particles and surface crevices were observed on sample surfaces, which are most likely due to surface tension and air bubbles formed during the mixing process.     

Ferroelectric properties of SrBi2Ta2O9 thin films on Si (100) with a LaZrO x buffer layer

To obtain a metal–ferroelectric–insulator–semiconductor (MFIS) structure, we fabricated ferroelectric SrBi₂Ta₂O₉ (SBT) film on a p-type Si (100) wafer with a LaZrO _x (LZO) buffer layer by means of a sol–gel technique. The sol–gel deposited LZO film according to the different annealing temperatures had a good surface morphology even though the crystalline phase was not an amorphous phase. In particular, the root-mean-squared (RMS) surface roughness of the 750-°C-annealed LZO film was about 0.365 nm and its leakage current density was about 8.2?×?10^?7 A/cm² at 10 V. A Au/SBT/LZO/Si structure with different SBT film was fabricated. The C–V characteristics of the Au/SBT/LZO/Si structure showed a clockwise hysteresis loop. The memory window width increased as the SBT film thickness increased. The 600-nm-thick SBT film was crystallized in a polycrystalline phase with a highly preferred (115) orientation. The memory window width of the 600-nm-thick SBT film was about 1.94 V at the bias sweep voltage ±9 V and the leakage current density was about 6.48?×?10^?8 A/cm² at 10 V.     

Preparation and characteristics of fine-grained ferroelectric glass-ceramic composites via a modified hybrid route at low temperature sintering

Dense, homogeneous and fully developed fine-grained ferroelectric Pb(Zr_0.53Ti_0.47)O₃?Cbased glass-ceramics have been successfully prepared at a low-sintering temperature of 850?C900°C by a modified hybrid process in air. The influence of the PbO-B₂O₃-SiO₂ (abbreviated as PBS) glass-gel content on the microstructure, dielectric, and ferroelectric properties of such glass-ceramics has been investigated. The temperature dependence of the dielectric constant indicated that the fine-grained Pb(Zr_0.53Ti_0.47)O₃ (abbreviated as PZT(53/47)) based glass-ceramic shows the characteristic dispersion at the Curie point. Ferroelectric hysteresis loop analyses have been performed to manifest the ferroelectric nature of the highly crystallized PZT(53/47) phase prepared by this modified novel hybrid process despite containing higher wt% glass-gel contents. The best dielectric and ferroelectric properties in a typical sample with 5% by weight of glass-gel content were found to have dielectric constant and loss tangent of 920 and 0.02 at 1 kHz, respectively. The saturation polarization (P _s ), and remanent polarization (P _r ) as well as the coercive field (E _c ) are 21.9 ??C/cm², 10.8 ??C/cm² and 2.19 kV/mm, respectively.     

Influence of Post-Annealing on the Characteristics of Pb(Zr,Ti)O3 Thin Films Deposited by Liquid Delivery MOCVD Using a Cocktail Solution

Ferroelectric PZT thin films were deposited by liquid delivery MOCVD using a cocktail solution. The cocktail solution consisted of Pb(METHD)₂, Zr(METHD)₄ and Ti(MPD)(METHD)₂ diluted with ethylcyclohexane. The films deposited on Pt/Ti/SiO₂/Si at a substrate temperature of 500°C consisted of PZT, PbO and PbPt_x, and showed poor properties. However, after annealing at 450°C in air for thirty minutes, the PbPt_x phase disappeared while the volume of the PbO phase increased. The hysteresis properties were also improved by annealing at 450°C. After annealing at 600°C in air for thirty minutes, the PbPt_x and the PbO phases disappeared perfectly and the PZT thin films showed good hysteresis properties with the remanent polarization of 30 μC/cm² and the coercive field of 88 kV/cm.     

Electro-Optic Properties of Heteroepitaxial Pb(Zr,Ti)O3/La0.5Sr0.5CoO3 Film Structures

Vertical ferroelectric Pb(Zr,Ti)O₃ (PZT) 1 μm thick film capacitor was fabricated by pulsed laser deposition technique (PLD) onto conducting La_0.5Sr_0.5CoO₃(LSCO) 100 nm thick bottom electrode on both side polished YAlO₃ + 1% Nd₂O₃ (Nd:YAlO₃) single crystal substrate to operate as a Pockels cell optical modulator. On top of the PZT film, semitransparent 30 nm thick Au electrode was deposited by thermal evaporation. Intensity of the chopped 670 nm polarized laser radiation transmitted through the Au/PZT/LSCO/Nd:YAlO₃ cell was measured at various temperatures and bias voltage applied. Applying 20 V (200 kV/cm) across the capacitive cell, modulation of the transmitted light as high as 3% was achieved while the voltage tunability measured at 1 kHz from C-V characteristics was about 70%. Thermo-optical measurements performed for PZT/Nd:YAlO₃ sample in the range up to 400°C showed the phenomenon of critical opalescence in the vicinity of Curie temperature at 208°C. Optical transmission through the PZT film biased with electric field was studied in the range 400 to 1000 nm. Film thickness, refraction index and absorption coefficient have been determined from the interference pattern observed in the PZT transmission spectrum. A simple model yields the dispersion relation for the electro-optic coefficient.     

Fundamental characteristics of DRAM capacitor application of PLZT ultrathin films prepared by MOCVD

Lanthanum-modified lead zirconate titanate (PLZT) thin films (50 nm to 200 nm) were deposited on Pt/SiO₂/Si substrate by metal-organic chemical vapor deposition (MOCVD). The electrical properties of the films were investigated as a function of the La content or the substrate temperature. Ferroelectric PZT(0/50/50) films were obtained at substrate temperatures as low as 500 °C; their electrical characteristics improved with increasing substrate temperature. La exhibited adequate solid solution in the PZT above 650 °C. PLZT(15/45/55) films with a thickness of 100 nm were found to have good properties for application to the capacitors of dynamic random access memory (DRAM), namely, an effective charge density of 80 fF/μm², a permittivity of 1000, an SiO₂ equivalent thickness of 0.4 nm, and a leakage current density of 5 × 10⁻⁸ A/cm. Addition of La to PZT was effective in reducing the leakage current with an increase in the registration rate. RuO₂ and/or IrO₂ bottom electrodes for ferroelectric PLZT films were also investigated. The RuO₂ films were found effective as diffusion barriers for PLZT and MgO. Significant interdiffusion at RuO₂/Si and RuO₂/SiO₂ interfaces occurred during the deposition of PLZT films. Annealing of the RuO₂ film considerably depressed interface reactions. © 1998 Scripta Technica. Electr Eng Jpn, 122(1): 25–36, 1998     

DIRECTIVITY OPTIMIZATION OF PZT BASED MICROSPEAKER ARRAY

ABSTRACT

In this paper, the directivity is optimized for a PZT based microspeaker array. The SiO₂/Pt/Pb(Zr,Ti)O₃/Pt/Ti/SiO₂ multimorph structure has been designed and fabricated to be the microspeaker cell using silicon based micro-machining process. The membrane size of the single transducer cell is 1000× 1000× 2.5 μ m³, and the main resonance frequency is located near 68 kHz (λ = 5 mm). Directivity is a very important parameter for microspeaker array. Dissimilar arrangement of the cells in transducer array could cause different directivity performance. After comparison, when achieving a good transmitting directivity, the square combined plane array has the smallest area.     

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.     

Electromechanical performance of structurally graded monolithic piezoelectric actuator

In this paper, the effect of structural gradients in monolithic piezoelectric actuators is investigated. Different cross-section profiles were micro-machined with a laser into commercial PZT 5H bulk discs with thicknesses of 375 µm and 500 µm (? 25 mm). Profiles and curvatures of the actuators were measured which showed both concave and convex structures, thus indicating pre-stress of the actuators. After poling, the distribution of out-of-plane displacement was scanned by a fibre-optic laser vibrometer. Maximum displacements of ～6.3 µm and ～24.8 µm were obtained from a freely moving and clamped ～375 µm thick actuator, respectively, in a ±0.5 V/µm electric field at 10 Hz frequency without load. Furthermore, deflection in the centre of the actuators was measured up to 184 mN load using the same electric field and frequency. Bending of the bulk actuators without any additional layer was a consequence of the gradient in poling and driving electric field via thickness variation of the material. Hence, different regions produced strain distribution and bending in a similar fashion to other benders. Actuators with the highest arch height exhibited the highest displacement and load bearing capabilities derived from the increased area moment of inertia and enhanced piezoelectric response due to pre-stress. The results show that the monolithic bending actuators can be realised by simple structural designing of the actuator. Such structural gradients can be one reason contributing to the higher displacement of RAINBOW actuators compared to other pre-stressed actuators. In a further development, the structural gradients can be utilized in high displacement pre-stressed actuators and in miniaturized monolithic piezoelectric devices.     

Effects of Ti/Ir top electrodes of PZT capacitors on the hydrogen related degradation

Abstract

Hydrogen annealing damages on properties of PZT capacitors and a role of Ti/Ir hybrid structure top electrodes on capacitors are investigated in this study. It is demonstrated that the capacitors with Ti/Ir structure top electrodes improve a resistance against hydrogen related degradation. As the thickness ratio of Ti/Ir increases, the capacitors show enhanced endurance against hydrogen damages. Especially, PZT (350nm) capacitors with Ti(80nm)/Ir(20nm) hybrid top electrodes show only 26% decrease in nonvolatile polarizations (P?_r) under ± 7V, while 67% of P?_r of ferroelectric capacitors with Ir top electrodes is reduced after forming gas annealing at 250°C for 10min. Based on the XPS analysis, ferroelectric characteristics of PZT thin film capacitors are degraded by destruction of Pb-O bond into metallic Pb due to hydrogen anneal on the catalytic top electrodes (Ir, Pt).     

Recent progress in sputtering PZT thin films for ferroelectric memories

Abstract

This paper describes amorphous Pb(Zr, Ti)O₃ (PZT) thin films deposited by cosputtering Pb(Zr_0.5 Ti_0.5)O₃ and PbO targets. By optimizing the amount of the excess Pb and the deposition temperature, PZT thin films with a single perovskite phase were obtained successfully on Ir substrates and Pt substrates at 520°C. 250-nm-thick PZT films crystallized by rapid thermal annealing (RTA) at 600°C for 20 s exhibited excellent ferroelectric properties: a coercive voltage of 1.0 V, a remanent polarization density of about 40 μC/cm², and a polarization switching endurance over 1x109 cycles. Although a heat treatment in a reductive ambient causes degradation of ferroelectric properties of PZT thin films, their degraded ferroelectric properties can be easily recovered from by a 1-min RTA in an oxygen at 400°C.     

Design and implementation of combline bandpass tunable filter using barium-strontium-titanate thin-film capacitors

The design, fabrication and microwave properties of tunable fifth-order combline bandpass filter using etched barium-strontium-titanate (BST) thin films on sapphire (0001) substrates were investigated. At 1 MHz and 1000 kV/cm electric field, the dielectric tunability, the remanent polarization (2P_r) and the coercive electric field (2E_C) of BST films were 45.96%, 2.26 µC/cm² and 81.83 kV/cm, respectively. The loss tangent was 1.36% at zero electric field. After the BST parallel plate capacitors characterization, BST capacitors were loaded at the end of parallel coupled resonators in the design of the tunable filter. With the application of 20 V DC voltage, the center frequency of the filter varied from 1.17 GHz to 1.34 GHz which corresponds to a relative shift of 13.5%.