Top Electrode Thickness Research Articles

Peculiar effect of mechanical stress on polarization stability in micrometer-scale ferroelectric capacitors

Piezoresponse force microscopy (PFM) has been used to study the polarization stability in micrometer size Pb(Zr,Ti)O3 capacitors. It is shown that the top electrode thickness has a profound effect on the equilibrium polarization state of poled capacitors triggering spontaneous polarization backswitching in the absence of an applied electric field and leading to the formation of an abnormal domain pattern. PFM examination of poled capacitors with thick (250 nm) top electrodes reveals domain patterns with the central regions always oriented in the direction opposite to the applied field. It is suggested that the driving force behind the observed effect is a transient response to the residual shear stress created by the top electrode in the poled capacitors during field-induced polarization switching. The proposed mechanism is quantified using finite element ferroelectric phase field modeling. The observed effect provides valuable insight into the polarization retention behavior in micrometer size ferroelectric capacitors.

Imaging mechanism of piezoresponse force microscopy in capacitor structures

The image formation mechanism in piezoresponse force microscopy (PFM) of capacitor structures is analyzed. We demonstrate that the spatial resolution is a bilinear function of film and top electrode thicknesses and derive the corresponding analytical expressions. For many perovskites, the opposite contributions of d31 and d33 components can result in anomalous domain wall profiles. This analysis establishes the applicability limits of PFM for polarization dynamics studies in capacitors and applies them to other structural probes, including focused x-ray studies of capacitor structures.

Fabrication and analysis of ZnO thin film bulk acoustic resonators

This study employs RF magnetron sputter technique to deposit high C-axis preferred orientation ZnO thin film on silicon substrate, which is then used as the piezoelectric thin film for a thin film bulk acoustic resonator (FBAR). Electrical properties of the FBAR component were investigated by sputtering a ZnO thin film on various bottom electrode materials, as well as varying sputter power, sputter pressure, substrate temperature, argon and oxygen flow rate ratio, so that structural parameters of each layer were changed. The experimental results show that when sputter power is 200 W, sputter pressure is 10 mTorr, substrate temperature is 300 °C, and argon to oxygen ratio is 4:6, the ZnO thin film has high C-axis preferred orientation. The FBAR component made in this experiment show that different bottom electrode materials have great impact on components. In the experiment, the Pt bottom electrode resonant frequency was clearly lower than the Mo bottom electrode resonant frequency, because Pt has higher mass density and lower acoustic wave rate. The component resonant frequency will decrease as ZnO thin film thickness increases; when top electrode thickness is higher, its resonant frequency also drops, due to top electrode mass loading effect and increased acoustic wave path. Therefore, ZnO thin film and top/bottom electrode thickness can be fine-tuned according to the required resonant frequency.

Piezoelectric micromachined ultrasonic transducers: modeling the influence of structural parameters on device performance

Piezoelectric micromachined ultrasonic transducers (pMUTs), a potential alternative for conventional one-dimensional phased array ultrasonic transducers, were investigated. We used a modeling approach to study the performance of lead zirconate titanate (PZT)-driven pMUTs for the frequency range of 2-10 MHz, optimized for maximum coupling coefficient, as a function of device design. Using original tools designed for the purpose, a comprehensive build-test finite element model was developed to predict and measure the device performance. In particular, the model estimates the device coupling coefficient and the acoustic impedance, besides the readily extractable resonance frequency and bandwidth. To validate the model, a prototype device was built and tested, showing good agreement between the model predictions and experimental results. Modeling results indicate that the coupling coefficient is significantly affected by silicon membrane, PZT, and top electrode thickness as well as the top electrode design. Results also indicate considerable flexibility in maximizing the coupling coefficient while maintaining the device acoustic impedance at a level matching that of the human body. The bandwidth proved to be superior to that of conventional transducers, reaching 102% in some cases.

Effect of AlN Film Thickness and Top Electrode Materials on Characteristics of Thin-Film Bulk Acoustic-Wave Resonator Devices

Highly c-axis-oriented aluminum nitride (AlN) thin films were deposited on Mo electrodes by reactive RF magnetron sputtering and their thickness effects on thin-film bulk acoustic-wave resonator (FBAR) characteristics are presented. The results obtained in this study show that thick AlN films have a strong c-axis orientation and tend to promote a small rocking curve full-width at half-maximum (FWHM), but a high grain size and a high surface roughness, hence degrading the characteristics of FBARs. The characteristics of FBARs depend not only the thickness and quality of AlN films, but also the thickness of the top electrode and the type of material used. Specifically, an increase in either AlN film or top electrode thickness increases parallel resonant quality factor (QP), but reduces effective electromechanical coupling coefficient (keff2). However, the resonance provides a maximum keff2 when AlN film thickness approaches the fundamental one-half-wavelength. The effects of the type of top-electrode material and AlN film thickness on the keff2 and quality factor of the fabricated FBARs are also discussed.

The use of MIM tunnel junctions to investigate kinetic electron excitation in atomic collision cascades

A novel technique is introduced to investigate the kinetic excitation of electrons in a solid by bombardment with energetic ions. The sample is prepared as a metal-insulator-metal (MIM) tunnel junction which opens the possibility to detect hot electrons with excitation energies well below the vacuum barrier. The excitations produced by the projectile impact onto the top electrode are detected as a tunnel current into the underlying base electrode. By varying the top electrode thickness, the elastic transport of hot electrons towards the tunnel junction can be studied.

Microscopic aspects of the region-by-region polarization reversal kinetics of polycrystalline ferroelectric Pb(Zr,Ti)O3 films

The region-by-region polarization switching in ferroelectric Pb(Zr,Ti)O3 thin films sandwiched between Pt electrodes has been directly observed using piezoelectric scanning probe microscopy. A resolution improved by one order-of-magnitude compared to the standard piezoelectric response imaging technique for ferroelectric capacitors was achieved by reducing the top electrode thickness to 10–15nm through polishing. It was demonstrated that the individually switched regions correspond to single grains or clusters of grains where the grain boundaries act as frontiers limiting the propagation of the switched state. The study of the propagation of the reversed polarization state as a function of voltage applied shows a rather discontinuous growth of the switched areas, the movement of the domain walls being triggered abruptly by different threshold voltages. This result agrees with the earlier proposed nucleation-limited switching model. The observation of the frozen regions that do not switch even at higher voltages provides significant insight into the “bits-failure” problem in submicron ferroelectric capacitors used for nonvolatile memory applications.

Performance Characterization of Thin AlN Films deposited on Mo Electrode for Thin-Film Bulk Acoustic-Wave Resonators

In this paper, we discuss the implementation and testing of a film bulk acoustic-wave resonator (FBAR) comprising an AlN piezoelectric thin film sandwiched between two metal electrodes and located on a silicon substrate with a low-stress silicon nitride (Si3N4) support membrane. In the present study, we choose molybdenum as the bottom electrode rather than the conventionally used Pt/Ti, Au/Cr or Al because it has a low acoustic attenuation and a good electrical conductivity, and provides a good adhesion between the thin AlN film and the low-stress Si3N4 membrane. We investigate the influence of AlN film thickness and top-electrode thickness on the resonance frequency of the FBAR device. The results indicate that decreasing the thickness of either the AlN film or the top electrode increases the resonance frequency. This suggests the potential of tuning the performance of the FBAR device by the carefully controlling AlN film thickness. Furthermore, it is also determined that the resonance frequency of the device can be increased by specifying a higher RF power for the magnetron sputtering process used to deposit the AlN film. This study employs an X-ray diffraction (XRD) technique to investigate the influence of AlN film thickness and RF power on the c-axis orientation of the thin film. It is shown that increasing either the thickness of the AlN film or the power of the RF sputtering process improves c-axis orientation. However, scanning electron microscopy (SEM) and atomic force microscopy (AFM) results reveal that a thicker AlN film exhibits a rougher surface and a larger grain size, both of which decrease the coupling factor of the FBAR device.

다결정 다공성 실리콘의 전계방출 특성

This paper reports the optimum structure of the vacuum packaged Porous poly-silicon Nano-Structured (PNS) emitter. The PNS layer was obtained by electrochemical etching process into polycrystalline silicon layer in a process controlled to anodizing condition. Current-voltage studies were carried out to optimize process condition of electron emission properties as a function of anodizing condition and top electrode thickness. Also, we measured in advance the electron emission properties as a function of substrate temperature because the vacuum packaged process was performed under the condition of high temperature ambient (430<TEX>$^{\circ}C$</TEX>). Auger Electron Spectrometer (AES) studies shows that Au as a top-electrode was diffused to PNS layer during temperature experiments. Thus, we optimized the thickness of top-electrode in order to make the vacuum package PNS emitter. As a result, the vacuum Packaged PNS emitter was successfully emitted by optimizing process.

CAD model analysis of the air‐gap type TFBAR

AbstractA CAD model for an air‐gap‐type TFBAR is presented. Aluminum nitride is used as the piezoelectric material with platinum electrodes. To verify the model, simulated and measured results are compared for various top electrode thicknesses, and the agreement is within 0.5% for the parallel resonance frequency. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 384–385, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10469

Ferroelectric properties of crystalline oriented Pb(Zr,Ti)O3 thin films prepared by sol-gel technique

Ferroelectric properties and reliability characteristics of (111) and (100) preferred tetragonal Pb(Zr0.2Ti0.8)O3 (PZT) thin film capacitors have been investigated as a function of top electrode thickness. The (111) preferred films exhibit better squareness of the hysteresis loop with larger remanant polarization and coercive field than the (100) preferred films. Top electrode thickness dependence of switching polarization can be explained by the compressive stress induced by the top electrode annealing. The film with thinner top electrode shows less initial switching polarization, however, better endurance characteristics due to enhancing partial switching region.

Operation of electroluminescent porous silicon diodes as surface-emitting cold cathodes

Cold electron emission properties of porous silicon (PS) electroluminescent diodes have been investigated as a function of the top metal electrode thickness, surface work function, and the thickness of PS layer. It is shown that the best thicknesses for Pt and Au electrodes are 50∼80 Å. The emission is enhanced by introducing an Al–Mg alloy electrode with a relatively low work function. Although the emission efficiency and the threshold voltage show definite dependencies on the thickness of PS layer, the Fowler–Nordheim plots show a linear behavior in every case. The maximum emission current density and efficiency obtained in this study are 10 μA cm −2 and 10 −3, respectively.

Effect of Top Electrode Thickness on the Performance of Microwave Acoustic Transducers

The effect of placing evaporated electrodes on the surface of thin-film microwave acoustic transducers is studied. In particular, it is found that the top electrode produces frequencies of infinite conversion loss (poles) within the usual operating passband of the transducer. The theory of thin-film microwave acoustic transducers is reviewed and utilized to find an analytic solution for the pole frequencies versus thickness of the top electrode. Theoretics results for Au, Ag, and Al top electrodes deposited on thin films of CdS and ZnO are presented. Experimental results for (Au/CdS), (Ag/ZnO), and (A1/ZnO) transducers are obtained by means of a novel swept-frequency pulse-echo method of measuring conversion loss. The theoretical and experimental results are found to agree within five percent. The relative advantages of transducers with the (Au/ZnO) and (A1/ZnO) configurations are compared, and designs for octave bandwidth operation are given for center frequencies ranging from 1.5 to 10 GHz.