TEM observations of magnetic domains and grain boundaries on spin‐sprayed ferrite films exhibiting high permeability usable for gigahertz noise suppressors

Lorentz TEM observations of magnetic domain wall motion, as well as TEM observations of grain boundaries, were performed on spin‐sprayed ferrite films #1 (Ni_0.17Zn_0.22Fe_2.61O₄) and #2 (Ni_0.19Zn_0.20Co_0.03Fe_2.58O₄), both 0.5 µm in thickness. They exhibit much higher natural resonance frequencies than the bulk ferrite and thus have been applied to gigahertz noise suppressors. Films #1 and #2 exhibit prominent and weak in‐plane uniaxial magnetic anisotropy, respectively, which is induced along the liquid flow direction during spin‐spraying. Both films have columnar crystallites with 100‐200 nm widths aligned perpendicular to the film plane, and the boundaries of the crystallites have no pores or impurity phases. Therefore, the crystallites are magnetically exchange‐coupled, which is responsible for the unusually high permeability and high natural resonance frequencies of the films. Under zero bias magnetic field, film #1 exhibits mosaic‐shaped magnetic domains, whereas film #2 exhibits magnetic domains elongated along the easy magnetization axis, both several hundred nanometers in width. For both films the domain structure remains unchanged when an in‐plane bias DC magnetic field,H_dc, of up to 10 Oe is applied along the hard axis. Under a stronger H_dc, the domain structure prominently changes, and the domain walls disappear when H_dc exceeds ∼100 Oe. This confirms our previous finding that the initial permeability is ascribed only to magnetization rotation, with no contribution from domain wall motion [J. Magn. Magn. Mater., 278 , 256 (2004)]. © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Complex permittivity and permeability of Co-substituted NiCuZn ferrite at rf and microwave frequencies

NiCuZn ferrite has recently attracted a lot of attention for its application in high frequency (up to a few GHz) multilayer chip inductors (MLCIs) and for other microwave devices owing to their favorable electromagnetic properties and low densification temperature. In order to study the effect of substitution of cations by cobalt in small concentration on the dielectric and magnetic properties at low and high frequencies, bulk polycrystalline ferrite samples of starting composition (Ni_0.2Cu_0.2Zn_0.6)_{1 − x} Co _x Fe₂O₄, having x = 0, 0.01, 0.03 and 0.05, were prepared by citrate precursor method. Pure spinel (cubic) ferrite formation was confirmed by powder X-ray diffraction technique. Complex permittivity and permeability were measured at microwave frequencies (X-band) using the cavity perturbation method, which is a non-contact method. The values of real part of permittivity (ε ′) vary in the range 7–9.6 and of the imaginary part (ε ″) vary from 0.020–0.120, whereas real part of the permeability (μ′) lies in the range 2.6–14.0 and the imaginary part of permeability (μ″) varies from 0.5–6.0. It is observed that there is an increase in μ′ and decrease in the magnetic loss (tan δ _μ) on increasing the cobalt concentration from x = 0 to x = 0.05. The variation of these parameters, both with frequency in X-band and with the cobalt concentration, is discussed in this paper.     

Asymmetrical single-mode silica waveguide deposited by PECVD

Silicon oxynitride (SiON) layer and SiO₂ buffer layer were deposited on silicon wafers by PECVD technique using SiH₄, N₂O and N₂. The refractive index of SiON films measured at a wavelength of 1552 nm using a prism coupler, could be continuously varied from 1.4480 to 1.4508. Optical planar waveguides with a thickness of 6 μm and a refractive index contrast (Δn) of 0.36% have been obtained. In addition, etching experiments were performed using ICP dry etching equipment on thick SiON films grown on Si substrates covered with a thick SiO₂ buffer layer. In order to measure optical properties, a polarization maintaining single-mode fiber was used for the input and a microscope objective for the output at 1.55 μ m. A low-loss and low propagation SiON-based waveguide was fabricated with easily adjustable refractive index of core layer.     

PZT thick films by diol chemical solution deposition

Process optimization and properties of lead zirconate titanate (PZT) films for piezoelectric micromachined ultrasonic transducers (pMUTs) for scanning probe devices will be presented. The goal of the work was a replacement of the tetragenic and mutagenic solvent and a decrease of time-consuming PZT 2–methoxy ethanol (2MOE) route. An alternative diol-based solution synthesis process was developed and “Design Of Experiment” (DOE) was used to achieve processing optimization for thick and crack free films. Tight parameter control allowed to develop a highly reproducible PZT diol process. The crystallization behaviour of crack-free PbZr_0.53Ti_0.47O₃ films (1–5 μm) with oriented perovskite structure was examined by X-ray diffraction and surface analysis using scanning electron microscopy. Piezoelectric and dielectric properties were examined. The effective transverse piezoelectric coefficient e _31,f of sol–gel processed films was investigated for 4 μm thick layers. Best properties were achieved with {1 0 0}-textured films, where a remanent e _31,f value of −7.3 C/m² was measured for 4.1 μm thick films.     

Piezoelectric PZT films for MEMS and their characterization by interferometry

Piezoelectric films can be used in micro-electro-mechanical system (MEMS) devices because the piezoelectric effect can provide high forces with relatively low energy losses. The energy output by a piezoelectric film per unit area is proportional to the film thickness, so it is desirable to have relatively thick films. Chemical solution deposition (CSD) techniques were used to prepare lead zirconate titanate (PZT) thin films with Zr/Ti ratios of 30/70 and 52/48. Usually CSD processing is restricted to making crack-free single layer films of ca 70 nm thick, but modifications to the sol-gel process have permitted the fabrication of dense, crack-free single layers up to 200–300 nm thick, which can be built-up into layers up to 3 μm thick. Thicker PZT films (> 2 μm single layer) can be produced by using a composite sol-gel/ceramic process. Knowledge of the electro-active properties of these materials is essential for modeling and design of novel MEMS devices and accurate measurement of these properties is by no means straightforward. A novel double beam common path laser interferometer has been developed to measure the piezoelectric coefficient in films and the results were compared with the values obtained by Berlincourt method. A laser scanning vibrometer was also used to measuring the longitudinal (d ₃₃) and transverse (d ₃₁) piezoelectric coefficients for PZT films and ceramics and the results were compared to those obtained by the other methods. It was found that for thin film samples, the d _33,f values obtained from the Belincourt method is usually larger than those obtained from the interferometer method but smaller than those from the vibrometer method and the reasons for this are discussed.     

Ferroelectric properties of (Bi3.15 Nd 0.85 )Ti 3 O 12 with decreasing film thickness

Film texture and ferroelectric behaviors of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNdT) of layered-perovskite structure deposited on Pt/TiO₂/Si substrate are dependent on the film thickness. When the film thickness is reduced from ∼240 to ∼120 nm, BNdT grains evolve from a rod-like morphology to a spherical morphology, accompanied by a decrease in average grain size. At the same time, P-E hysteresis transforms from a square-shaped hysteresis loop (2Pr ∼24.1 μC/cm² at 240 nm) to a relative slimmer hysteresis loop (with a lower 2Pr = 19.8 μC/cm² at 120 nm). The nonvolatile polarization (Δ P) shows a maximum at the film thickness of 160 nm, where Δ P was measured to be 14.7 μC/cm² and 6.8 μC/cm² at 5 V and 3 V, respectively. A small amount of excess bismuth in the film thickness of 130 nm, introduced by co-sputtering, can lead to a much enhanced remanent polarization (2Pr of 21.3 μC/cm² at 5 V and 15.2 μC/cm²at 3 V), which is promising for low voltage FRAM applications.     

Resonance properties and mass sensitivity of monolithic microcantilever sensors actuated by piezoelectric PZT thick film

The PZT thick film cantilever devices fabricated via MEMS process have much attraction because they are appropriate for biological transducer or sensor, resulting from their large actuating force and relatively high sensitivity especially in liquid. By means of resonance behavior, theoretical calculation and experimental verification of the PZT thick film cantilever devices have not been studied before. Accordingly, we focused on the sensitivity analysis and interpretation of the PZT thick film cantilevers in this study. Especially, the investigation for mass sensitivity of the PZT thick film cantilever is of importance for physical, chemical and biological sensing application. The PZT thick film cantilever devices were constructed on Pt/TiO₂/SiN _X /Si substrates using screen printing method and MEMS process. The harmonic oscillation response (resonance frequency) was measured using an optical laser interferometric vibrometer. The effect of cantilever geometry on the resonance frequency change was investigated. Compared with the theoretical resonant frequency change by mass loading, the experimental resonant frequency change of the PZT micromechanical thick film cantilever shows a variation of less than 2%. Mass sensitivities are estimated to be 30.7, 57.1 and 152.0 pg/Hz for the 400 × 380 μm, 400 × 480 μm and 400 × 580 μm cantilever, respectively.     

Spray pyrolysis of La0.6Sr0.4Co0.2Fe0.8O3-δ thin film cathodes

Spray pyrolysis has been used to prepare La_0.6 Sr_0.4Co_0.2Fe_0.8O_3-δ thin film cathodes for solid oxide fuel cell (SOFC) applications. The films are polycrystalline with nano-meter sized grains and less than 1 μm in thickness. Deposition parameters for film deposition have been established. The ratio of deposition temperature to solvent boiling point is found to be the most important processing parameter that determines whether a crack free homogeneous and coherent film is obtained. The morphology can be tailored by the deposition parameters. Annealing at 650^∘C for four hours in air results in coherent films of the desired perovskite phase. The films are potential cathodes for thin film micro-solid oxide fuel cells.     

Photoresist patterned thick-film piezoelectric elements on silicon

A fundamental limitation of screen printing is the achievable alignment accuracy and resolution. This paper presents details of a thick-resist process that improves both of these factors. The technique involves exposing/developing a thick resist to form the desired pattern and then filling the features with thick film material using a doctor blading process. Registration accuracy comparable with standard photolithographic processes has been achieved resulting in minimum feature sizes of <50 μm and a film thickness of 100 μm. Piezoelectric elements have been successfully poled on a platinised silicon wafer with a measured d ₃₃ value of 60 pCN⁻¹.     

Novel patterning of composite thick film PZT

There is a clear need for thick PZT films (10–100 μm) in micro-electromechanical systems (MEMS). Some applications, like high frequency transducers operating in the thickness mode, require frequencies in the MHz region and thus thicker films, which in addition will provide more power. Thicker films are also important in actuator systems and sensors as it will generate more force and voltage, respectively. Integration of complex structures of thick films in thick films in MEMS is challenging. The use of normal thin film patterning techniques is difficult for thick films due to the amount of material that has to be removed and the isotropic nature of wet etching. A new patterning technique suitable for composite thick films using an epoxy mould is presented. By filling a micro mould of SU-8 photoresist with PZT paste details down to 20 μm with vertical feature walls could be patterned in a 15 μm thick film.     

Characteristics of PMW-PNN-PT-PZ thick films on various bottom electrodes

Characteristics of piezoelectric thick films on various bottom electrodes prepared by screen printing method were investigated. The composition of the ceramics used in this study was 0.01Pb(Mg_1/2W_1/2)O₃–0.41Pb(Ni_1/3Nb_2/3)O₃–0.35PbTiO₃–0.23PbZrO₃ + 0.1wt% Y₂O₃ +2.0 wt.%ZnO(PMW-PNN-PT-PZ). The Ag and the Ag-Pd electrodes were coated on SiO₂/Si substrate by screen printing method and Pt electrode was deposited on Ti/ SiO₂/Si substrate by DC sputtering system. The piezoelectric PMW-PNN-PT-PZ thick films were fabricated on each electrode and annealed by rapid thermal annealing (RTA). The PMW-PNN-PT-PZ piezoelectric thick films on Ag/SiO₂/Si has higher remanent polarization (P _r) of 22.4 μC/cm².     

Particulate dispersion and freeform fabrication of BaTiO3 thick films via direct inkjet printing

A piezoelectric drop-on-demand inkjet printer was used to print BaTiO₃-Ni functionally graded thick films onto porous substrates of computer-defined patterns without the need of prior mold tooling and the use of masks. The inkjet deposits were built up from a sequential delivery of ink droplets expelled from the print-head orifice with a precise location-registration capability upon multiple printings. A uniform dispersion of the particle-filled liquid inks was a prerequisite for a successful printing, and an addition of propylene glycol facilitated the nanoparticles dispersion in evaporative ethanol-isopropanol solvents. Typical line width of the inkjet-printed patterns was as narrow as 200 μm, and a maximum film thickness attainable exceeded 20 μm. A uniform concentration gradient of the dissimilar phases was evident along the thickness direction of the printed functional films.     

Fabrication of piezoelectric MEMS devices-from thin film to bulk PZT wafer

Integrating patterned functional piezoelectric layer onto silicon substrate is a key technique challenge in fabrication of piezoelectric Micro Electro Mechanical System (pMEMS) devices. Different device applications have different requirements on the thickness and in-plane geometry of the piezoelectric layers and thus have their own processing difficulties. In this paper, the techniques of integrating piezoelectric function into pMEMS has been discussed together with some diaphragm-based pMEMS devices which have relatively lenient requirement on patterning of the piezoelectric layers. Sol-gel thin film can meet the requirement of most of the sensor applications. The composite thick film is one of the promising solutions for thick film devices due to its good processing compatibility. Si/Pb(Zr _x Ti_1 − x )O₃ wafer bonding technique makes it possible to thin down the ceramic wafer to less than 10 μm by using chemical mechanical polishing, which, therefore, provide us another approach to integrate thick piezoelectric layer on silicon to cover the need of most of the thick film devices. Directly make double side aligned electrode patterns on bulk piezoelectric wafer/plate by using photolithography opens up a new area of pMEMS. The advantage of using bulk piezoelectric wafer/plate in pMEMS is that we can select commercial available ceramics or single crystals with excellent piezoelectric properties and thus ensure the overall performance of the devices.     

Preparation and electrical properties of 0.4Pb(Zn1/3Nb2/3)O3-0.6Pb(Zr0.4Ti0.6)O3 thin films by 2-step annealing method

Films of (1−x)Pb(Zn_1/3Nb_2/3)O₃-xPb(Zr_0.4Ti_0.6) O₃ (x = 0.6, 40PZN-60PZT) were deposited on Pt/TiO₂/ SiO₂/Si substrate through spin coating. Using a combination of homogeneous precursor solution preparation and two-step pyrolysis process, we were able to obtain the 40PZN-60PZT thin films of perovskite phase virtually without pyrochlore phase precipitation after annealing above 650^∘C. But since annealing done at the high temperatures for extended time can cause diffusion of Pt, TiO₂ and Si, and precipitation of nonstoichiometric PbO, we adopted 2-step annealing method to circumvent these problems. The 2-step annealed films show dense microstructure than the 1-step films annealed at higher temperature. Furthermore, the root-mean-square surface roughness of 220 nm thick films which are annealed at 720^∘C for 1 min and then annealed at 650^∘C for 5 min was found to be 3.9 nm by atomic force microscopy as compared to the 12 nm surface roughness of the film annealed only at 720^∘C for 5 min. The electrical properties of 2-step annealed films are virtually same and those of the 1-step annealed films annealed at high temperature. The film 2-step annealed at 720^∘C for brief 1 min and with subsequent annealing at 650^∘C for 5 min showed a saturated hysteresis loop at an applied voltage of 5 V with remanent polarization (P _r) and coercive voltage (V _c) of 25.3 μC/cm² and 0.66 V respectively. The leakage current density was lower than 10⁻⁵A/cm² at an applied voltage of 5 V.     

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.     

Preparation and characteristics of LiCoO2 paste electrodes for lithium ion micro-batteries

The thick film electrodes (cathode) have been developed via screen printing using LiCoO₂ paste to improve the discharge capacity of lithium ion micro-batteries. The LiCoO₂ pastes were formulated using the mixtures of LiCoO₂ powder as a functional material, carbon black as a conducting agent, ethyl cellulose and terpineol as a vehicle, and Emphos PS-21A as a dispersant. Depending on the amount of carbon black, the average and maximum surface roughnesses varied from 0.54 to 1.00 μm and 6.2 to 18.7 μm respectively. The internal resistance in the paste electrodes could also be controlled to 120 KΩcm⁻² by the addition of carbon black in the pastes. The thickness of the printed film is independent of the paste composition, but it decreased from 15 to 6 μm with the increase in screen mesh number from 250 to 500. The initial specific discharge capacity of the printed cathode which was prepared using the mixture Ag-coated LiCoO₂ powder improved to 180 μAhcm⁻².     

Taking piezoelectric microsystems from the laboratory to production

Reliable integration of piezoelectric thin films into silicon-based microsystems on an industrial scale is a key enabling technology for a wide range of future products. However, current knowledge in the field is mostly limited to the conditions and scale of academic laboratories. Thus, knowledge on performance, reliability and reproducibility of the films and methods at industrial level is scarce. The present study intends to contribute to the development of reliable technology for integration of piezoelectric thin films into MEMS on an industrial scale. A test wafer design that contained more than 500 multimorph cantilevers, bridges and membranes in the size range between 50 and 1,500 μm was developed. The active piezoelectric material was a ∼2 μm thin film of lead zirconate titanate (PZT) deposited by a state-of-the-art chemical solution deposition (CSD) procedure. Automated measurements of C(V) and dielectric dissipation factor at 1 kHz were made on more than 200 devices at various locations across the wafer surface. The obtained standard deviations were 4.5 and 11% for the permittivity and dissipation factor, respectively. Values for the transverse piezoelectric charge coefficient, e _31,f, of up to −15.1 C/m² were observed. Fatigue tests with a 5 kHz signal applied to a typical cantilever at ± 25 V led to less than 10% reduction of the remanent polarisation after 2 × 10⁷ bipolar cycles. Cantilever out-of-plane deflection at zero field measured after poling was less than 1.1% for a typical 800 μm cantilever.     

PZT thick films on different ceramic substrates; piezoelectric measurements

The piezoelectric, microstructural and electrical characteristics of thick PZT films on relatively inert alumina substrates and on two LTCC tapes, i.e., Du Pont 951 and Electro Science Labs 41020 were studied. A thick-film paste was prepared from the pre-reacted PZT powder (PbZr_0.53Ti_0.47O₃) and printed and fired on LTCC tapes and on alumina substrates, respectively. Dielectric permittivities, dielectric losses, remnant polarizations and coercive fields were measured. The dielectric constants (100–150) of thick films fired on LTCC substrates are low. The piezoelectric coefficients d ₃₃ were measured by different methods, i.e. Berlincourt piezometer, interferometry and piezoresponse force microscope (PFM). The d ₃₃ values on LTCC substrates are low (30–70 pm/V) as compared with values obtained on alumina substrates (around 120 pm/V). Lower dielectric constants and piezoelectric coefficients d ₃₃ of films on LTCC substrates are attributed to the formation of phases with a low permittivity due to the diffusion of silica from LTCC substrates into PZT films. The d ₃₃ constants of samples with different thicknesses of PZT layers (from 20 to 160 μm) at first increase with the increasing thickness of PZT layers and then decrease for thicker films. As the cracks in the structure were not observed the reason for the decreasing d ₃₃ values for thicker films is still unclear.     

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%.     

Electron emission of silicon field emitter arrays coated with N-doped SrTiO3 film

Nitrogen doped SrTiO₃ (STO) thin films have been fabricated on Si field emitter arrays (FEAs) using reactive RF magnetron sputtering in Ar-N₂ mixture ambient for electron emission applications. The nitrogen incorporation in STO films was revealed both in Fourier transform infrared (FTIR) spectroscopy and in Auger electron spectroscopy (AES). Low dose incorporation of nitrogen in STO films shows enhanced crystallinity, whereas the overdosed films show the degraded perovskite structure. The results demonstrate that the threshold emission field is lowered tremendously from 36.24 V/μm for uncoated FEAs to 17.37 V/μm for 30-nm-thick STO coated FEAs deposited in 50% N₂ ambient. The enhanced electron emission characteristics are highly correlated with the nitrogen incorporation in STO and film thickness. The substitution of nitrogen for oxygen may result in the band-gap narrowing in STO with enhanced electron emission. The thickness dependence might be related to the formation of space-charge-induced band-bending interlayer at STO/Si interface.