Formation of ceramic thin films using electrospray in cone-jet mode

An electrostatic atomization technique has been developed to generate ultrafine spray droplets of ZrO₂ and SiC ceramic suspensions in a range of a few micrometers with a narrow size distribution. The aim of this paper is to deposit uniform thin films (from a few micrometers to a few tens of micrometers) of these ceramic materials on alloy substrates. Compared to some other thin-film deposition techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and plasma spray (PS), etc., the thin-film deposition process using electrostatic atomization is not only cheap but also capable of depositing a very thin multilayer with abrupt interfaces. CVD and PVD are expensive techniques. They require either a high vacuum, even an ultrahigh vacuum environment or complex gas handling system. Their deposition rate is also low. PS is normally used to grow thermal barrier coatings which usually have a thickness of a few tens to a few hundreds micrometers. Its application is limited by the quality of the coatings (high porosity, coarse and nonuniform microstructure). Preliminary results in this work have shown that, for low through-put atomization, the cone-jet is the most suitable method to produce a fine charged aerosol with a narrow size distribution, which is crucial to produce uniform thin films. It was found that the size of ceramic particles in ZrO₂ and SiC thin films is less than 10 μm. Microstructures of these thin films show very homogenous morphologies. These results indicate that ceramic thin films with high homogeneity can be deposited using electrostatic atomization. It was also observed that the morphology of the underlayer has some influence on the morphology of the top layer     

Formation of ferroelectric thin films on ruthenium electrodes

Abstract

Sr_0.8Bi_2.4Ta₂O₉ (SBT) and Bi_3.25+xLa_0.75Ti₃O₁₂ (BLT) films were prepared on Ru electrodes by a sol-gel spin-coating method. It was found in the case of SBT/Ru that the oxidation of Ru was suppressed by annealing at temperatures lower than 650°C in O₂ atmosphere or by annealing lower than 700°C in N₂. However, the remanent polarization values of the films were as low as 2.3 μC/cm2 (2Pr) for the former case and 4.4 μC/cm2 for the latter case. On the other hand, the BLT/Ru samples crystallized at 650°C showed good crystallinity without oxidation of the Ru electrode. It was found that the ferroelectric properties depended on the amount of excess-Bi sensitively, and the film prepared under the optimum Bi composition showed an excellent P-V hysteresis loop with 2 Pr of 25 μC/cm2.     

Theory of size effects in ferroelectric ceramic thin films on metal substrates

The aim of this paper is to show how a Landau thermodynamic theory might be utilized to study size effects in ferroelectric thin films on metal substrates via reflectivity measurements that could be carried out with terahertz radiation, particularly in the far-infrared region. The approach taken is to minimize a Landau free energy functional that includes a gradient term to describe the size effects. Landau-Khalatnikov equations together with Maxwell’s equations for the electromagnetic field are then solved simultaneously to describe how the radiation interacts with the film. From this reflectivity curves can be calculated and related to experimental studies. Attention is paid to how the metal substrate can influence the reflectivity curves compared to free standing films without substrates. The significance of the work lies in the fact that ferroelectric ceramic thin films are becoming of increasing technological importance, and films on metal substrates such as electrodes are of obvious relevance to applications such as memory devices which rely on applied electric fields to change the polarization direction. The main conclusion is that terahertz wave measurements in the far-infrared provide an informative and sensitive probe of the size effects and substrate influence.     

Adaptive-learning neuron circuits using ferroelectric thin films

Abstract

Adaptive-learning neuron circuits are reviewed, in which a pulse frequency modulation (PFM) system is used and the interval of output pulses is changed through the learning process. Key devices of the circuits are MFSFETs (Metal Ferroelectric Semiconductor Field Effect Transistors). They are used for representing the synaptic weights of neurons and the polarity of the films is gradually changed by applying input pulses to the gates. In order to produce PFM signals, circuit using a UJT (unijunction transistor) is discussed. As a preliminary experiment for realization of MFSFETs, the electrical properties of metal ferroelectric metal (MFM) capacitor using sol-gel derived PZT films are discussed.     

Dielectric constant measurement of thin films using goniometricterahertz time-domain spectroscopy

Goniometric time-domain spectroscopy (GTDS), employing an ultrashort electromagnetic (EM) pulse technique, has been developed for measuring the dielectric constant of thin films in a broad band of gigahertz to terahertz. An ultrafast optoelectronic system, including an emitter and a detector unit, is constructed with a &thetas;-2&thetas; goniometer. A silicon wafer was analyzed as the reference substrate material. A sharp π phase-shift of the reflected EM wave was observed at the Brewster angle of 73.5° for a bare silicon wafer. The phase shift for a film on the Si substrate is relatively smooth due to its two surfaces' providing a complex reflectance. The dielectric constant of the film on Si, related with angular dependency of the phase shift, can be extracted by means of fitting the curve or measuring slope of the curve near the Brewster angle. The measured dielectric constants of FLARE, TiO_x, and PZT film are reported     

High temperature processing of ferroelectric thin films using interconnect wafer technology

Abstract

There is interest in ferroelectric thin films for uncooled IR detector applications. Currently the processing of these devices takes a fully integrated approach where the thin films are deposited directly onto underlying CMOS readout circuitry, thereby imposing severe limits on the thermal budget available for the crystallisation of the ferroelectric material. This is incommensurate with obtaining the best ferroelectric properties from materials such as lead scandium tantalate (PST) which requires elevated temperature processing to attain the highest merit figures for IR detection. In this paper thin film PST processed within the CMOS survivability envelope will be compared to that processed at temperatures up to 850°C. A novel interconnect wafer technology will be outlined which enables processing to be extended to such temperatures. It will be shown that elevated temperature processing of the PST film can result in dramatic improvement of the materials merit figure for IR detection     

The effect of ceramic target processing on pulsed laser deposited thin films of barium strontium titanium oxide

Abstract

The effect of ball-milling on the powder, electronic, and optical properties of the barium strontium titanium oxide (Ba_1-xSr_xTiO₃, BSTO; 0≤×≤1) ceramics have been studied in detail.^[1] Thin films of these BSTO compositions were synthesized by the Pulsed Laser Deposition (PLD) method. The ablation targets were prepared from unmilled and milled powder of BSTO. The structural, optical, and electronic properties of the bulk ceramics and the thin films were investigated. A detailed report of these studies will be presented.     

Poling of pyroelectric thin films

Abstract

Poling of calcium and zirconium modified lead titanate pyroelelectric thin films has been studied. Poling temperature and electrical field were optimized to achieve high figure of merits. It was found that thermal activation is important to obtain highly poled films. Defect-dipole complexes are supposed to play a major role to retain the polarization and also to enhance it. Porous calcium substituted films achieved an up to three times higer voltage response than zirconate substituted thin films, because porosity decreased the dielectric constant significantly.     

Improvement in microstructure of SBT thin films

Abstract

Various attempts to improve the microstructure of SBT thin films were carried out. One was to employ ultra-thin BT film as a top layer on the conventional SBT thin film. After optimization of the BT top layer thickness, a very smooth SBT surface was successfully achieved. Particularly, the insulation break down field was improved to more than 1.0 MV/cm. Next we used UV exposure during the baking process. By optimizing the UV assisted process, surface morphology was successfully improved with fine grain microstructure. The break down field was improved to more than 1.0 MV/cm. Also, these two process combinations successfully lead to more reliable SBT thin films. The break down field was drastically improved to more than 1.2 MV/cm.     

Microstructures and dielectric properties of Zn2SnO4 thin films by sputtering from a ZnO doped ceramic target

The dielectric properties of Zn₂SnO₄ thin films with various degrees of ZnO dopant concentration were investigated. Zn₂SnO₄ thin films were prepared using the radio frequency magnetron sputtering. The X-ray diffraction patterns of the 0 and 75 mole % ZnO doped Zn₂SnO₄ thin films revealed that Zn₂SnO₄ is the main crystalline phase, which is accompanied by a little SnO₂ as the second phase. The second phase SnO₂ in specimens vanished when the extent of ZnO additive was increased to 100 mole%. A dielectric constant of 15–40 and a loss factor of 0.10–0.14 of Zn₂SnO₄ thin films were measured at 1 MHz with ZnO dopant concentration in the range of 0–100 mole%.     

Observation of artificial nano-domains in ferroelectric thin films using nonlinear dielectric imaging and piezo imaging

Abstract

We have developed scanning nonlinear dielectric microscopy (SNDM) that is the first successful purely electrical method for observing polarization distributions of ferroelectric materials. Now the resolution of SNDM has been improved to sub-nanometer.

On the other hand, the piezoelectric response imaging (piezo-imaging) using scanning force microscopy (SFM) is well known as the method for observing the polarization distributions. In this study, we compare the resolution of SNDM with that of the piezo-imaging and confirm that the resolution of SNDM is much higher than that of piezo-imaging. Then, as the fundamental study to apply the SNDM system to a ferroelectric reading-recording system, we switched and observed the ferroelectric domains using the SNDM system.     

Low-temperature synthesis in vacuum of c-axis oriented ferroelectric YMnO3 thin films using alkoxy-derived precursors

Abstract

Hexagonal YMnO₃ thin films were prepared on Pt(111)/TiO_x/SiO₂/Si and Pt(111)/Al₂O₃(0001) substrates using alkoxy-derived precursor solutions. The films were prepared by spin coating the YMnO₃ precursor solutions and then, the films were calcined and crystallized via rapid thermal annealing in oxygen or vacuum ambient. The annealing conditions and substrates were critical for crystallization of ferroelectric YMnO₃ films. When annealed in vacuum, the films both on Pt(111)/TiO_x/SiO₂/Si and Pt(111)/Al₂O₃(0001) substrates crystallized to hexagonal YMnO₃ and the orientation depended on the substrates. The film on Pt(111)/Al₂O₃(0001) had c-axis orientation and the film on Pt(111)/TiO_x/SiO₂/Si had no preferred orientation. In addition, it was found that crystallization behavior, orientation and morphology of YMnO₃ films on Pt(111)/TiO_x/SiO₂/Si substrates depended on the annealing condition. The heat-treatment in vacuum at initial stage for crystallization affected the restraint of perovskite phase and formation of hexagonal phase. The following heat-treatment in oxygen promoted the c-axis orientation and grain growth. The optimum annealing procedure for crystallization of the c-axis oriented YMnO₃ films on Pt(111)/TiO_x/SiO₂/Si was addressed.     

Landau theory of thin ferroelectric films

Abstract

Landau-Devonshire theory¹ is a useful phenomenological model to describe the properties of ferroelectric phase transitions. Below the transition temperature, the Landau model can be generalized to describe the thermodynamic stability of a ferroelectric crystallite in a bistable polarized configuration, and to predict the response of the crystallite to external fields and charges. The three primary elements to be considered in modeling thin-film ferroelectric devices are the polar response of the ferroelectric itself, the contribution of electrode interfaces, and the interaction of mobile and immobile charged defects and carriers with the ferroelectric and the electrodes. First, the hysteresis properties of a single domain or crystallite are derived. This result is generalized to find the polar response in a polycrystalline film where there may be variations in the size and orientation of the crystallites and in the coercivity, remanence and offset of the domains. After postulating that metal electrodes form Schottky barriers with respect to the ferroelectric, we can then calculate the electric fields and potentials throughout the ferroelectric film. These calculations show that space charges form near the electrodes and the magnitude of the electric field in these regions is large. A further examination of the space charges results in a model for the C-V response of the ferroelectric capacitor, as the C-V response is dominated by space charge effects. The charge concentrations, contact potentials, high-field permittivity, and space charge widths can be extracted from the C-V data. Finally, the interactions between defects and domains leading to domain pinning and fatigue are investigated.     

GHZ polarization dynamics in ferroelectric thin films

Abstract

We present results from experiments which measure the local dielectric response of ferroelectric thin films driven by microwave-frequency electric fields. The repetition rate of a mode-locked Ti:Sapphire laser is used to generate a microwave drive signal that is phase-locked to an optical probe pulse and applied to the ferroelectric thin film. The induced polarization change in the ferroelectric film is measured stroboscopically via the electro-optic effect. Polarization images are acquired by scanning the laser beam across the sample in a confocal geometry. Time resolution is achieved by changing the delay between the electrical pump and the optical probe. Initial results show large local phase shifts in the ferroelectric response of closely separated regions of a Ba_0.5Sr_0.5TiO₃ thin film. This new experimental technique may help to understand the physical mechanisms of dielectric loss in these materials.     

Mechanics of thin films and microdevices

This paper discusses the latest developments in nanomechanics of thin films with applications in microelectromechanical systems (MEMS) and microelectronics. A precise methodology that combines in situ atomic force microscopy (AFM) surface measurements of uniaxially tension-loaded MEMS specimens and strain analysis via digital image correlation (DIC) achieving 0.1 pixel spatial displacement resolution is presented. By this method, the mechanical deformation of thin films was obtained in areas as small as 4 /spl times/ 4 /spl mu/m and with 1-2 nm spatial displacement resolution supporting the derivation of interrelations between the material microstructure and the local mechanical properties. This methodology provided for the first time the values of Young's modulus and Poisson's ratio from specimens with cross-sections as small as 2 /spl times/ 6 /spl mu/m. The value of properties derived via AFM/DIC demonstrated very limited scatter compared to indirect mechanical property measurement methods. The application of this technique on nonuniform geometries resolved nanoscale displacement and strain fields in the vicinity of ultrasharp elliptical perforations achieving very good agreement with finite element models. Furthermore, the stochastic and deterministic material failure properties described via Weibull statistics and fracture toughness, respectively, are illustrated for brittle thin films. Failure initiated at notches was found to be influenced by the local radius of curvature and the stress concentration factor. Precise fracture toughness values for MEMS materials were obtained from MEMS specimens with atomically sharp cracks. These studies were supported by measurements of displacements/strains conducted for the first time in the vicinity of mathematically sharp cracks via the AFM/DIC method. The method can be applied to a variety of thermomechanical reliability problems in multilayered thin films and inhomogeneous/anisotropic materials.     

Improvement of the electrical properties of PZT thin films using TiO2 buffer layer

Abstract

Thin TiO₂ layers were sputter-deposited on Pt/Ti/SiO₂/Si wafers, as buffer layers for PZT thin film capacitors. It was found that TiO2 buffers of less than 4-nm-thickness could assist in obtaining highly uniform PZT thin films with no second phase. The leakage current behaviors of the PZT based capacitor are improved, while retaining the ferroelectric properties of PZT thin films such as remanent polarization and coercive field. In addition, the uniform distribution of oxygen in PZT on TiO₂/Pt indicates that the TiO₂ buffer layer act as a barrier for lead-platinum reaction, as well as for oxygen diffusion.     

Electrical switching in lithium niobate thin films

The ferroelectric switching properties of thin films of lithium niobate (LiNbO3) on silicon have been investigated. The polarization is shown to be partially reversible with an applied electric field at room temperature. The samples were films of LiNbO3 which were magnetron sputter deposited on silicon substrates. A double pulse method was used to investigate the ferroelectric switching properties of the films. Evidence for partial stable switching of the LiNbO3 film was observed at a field of roughly 500 kV/cm.     

Sputter deposition of ferroelectric thin films

Ferroelectric films are typically deposited by a variety of techniques, the two most common being chemical methods (sol-gel, metalorganic decomposition) and sputtering. In this paper we briefly review the sputtering techniques, and then discuss ion beam sputter deposition in greater detail. In particular, ion beam sputter deposition of epitaxial lead zirconate titanate (PZT) films is described. It is shown that the films with compositions close to the morphotropic boundary typically show well-developed ferroelectric hysteresis loops, Pmax around 45 μC/cm2, and Pr around 20 μC/cm2. In comparison with typical polycrystalline sol-gel PZT films, however, coercive fields of thin epitaxial films are large (120-200 kV/cm for 95 nm films). The pulsed fatigue behavior is remarkably similar to a polycrystalline non-oriented sol-gel PZT film investigated for comparison. The similarities suggest that the aging behavior may be dominated by the electrodes, which were Pt in both systems.