Reactive Ion Etching and Ion Beam Etching for Ferroelectric Memories

The fabrication of the FeRAM requires the development of etching processes for ferroelectric thin films and electrodes as well as deposition processes. But different etching methods have different impacts such as resolution and process-induced damages. In this paper, reactive ion etching (RIE) and ion beam etching (IBE) for ferroelectric thin films and Pt electrodes have been studied in details. Different silicon-based ferroelectric thin films and their corresponding electrodes have been etched successfully. To compensate the possible oxygen loss caused by the etching process, the samples should be annealed in O₂ at 550°C for 30 min after RIE and IBE etching.     

Reactive ion etching of sol-gel deposited lead zirconate titanate (pzt) thin films in sf6 plasmas

Abstract

Sol-gel deposited lead zirconate titanate piezoelectric thin films were reactive ion etched in SF₆ plasmas. Etch rates increased approximately linearly with cathode power, reaching 65 nm/min at 300 W, Etch rate was independent of SF₆ flow rate but increased as pressure was reduced, indicating an etch process that is dependent on ion-induced mechanisms. Photoresist masks could be removed subsequent to etching, but mask erosion led to gradually sloped sidewalls. Use of Pt masks provided for anisotropic etching with vertical sidewalls and smooth etched surfaces.     

EFFECT OF ANNEALING ON THE ELECTRICAL PROPERTIES OF SrTiO3 THIN FILMS PRODUCED BY ION BEAM SPUTTERING

ABSTRACT

Thin film capacitors with SrTiO₃ (STO) as dielectric and Pt as electrode material have been prepared by ion beam sputtering. The as-deposited film is amorphous and exhibits a crystallization temperature around 321°C as proved by X-ray diffraction. The effect of post annealing on the crystalline quality of the films was systematically studied by x-ray diffraction and Atomic Force microscopy (AFM). The temperature and frequency dependent dielectric properties were measured from 30°C to 200°C and 0.01 Hz to 10⁵ Hz, respectively. The influence of the microstructure of SrTiO₃ thin films on their electrical properties was investigated through an extensive characterization. The electrical properties of SrTiO₃ films appear to be strongly depending on the annealing temperatures. The capacitance voltage (C-V) characteristics reveal an improvement of capacitance density with increasing the annealing temperature.     

PIEZOELECTRIC RESPONSE OF PZT NANOSTRUCTURES OBTAINED BY FOCUSED ION BEAM

ABSTRACT

Pb(Zr_x,Ti_{1 - x})O₃ (PZT) microscale island (1μ m～ 100 nm) was fabricated by Focused Ion Beam (FIB) before and after its crystallization. In the first case the FIB etching is realized on amorphous films and a post annealing treatment, at the crystallization temperature of the PZT films, is necessary to cristallize the film in the perovskite phase. In the second case the etching are made on crystallized films. Local electrical properties were evaluated by piezoresponse force microscopy (PFM) technique and the degradations induce in the films are studies by Raman spectroscopy. Compared to the PZT island fabricated after crystallization, the result shows that there is noticeable enhancement in nanoscale electrical properties of PZT island fabricated before crystallization, especially when the island size decreases.     

Etching effects on ferroelectric capacitors with multilayered electrodes

Abstract

Dry etching of PZT thin film capacitors with RuOx/Pt multilayered electrodes was studied to examine the etching effects. PZT films were deposited on RuOx/Pt/Ti/SiO₂/ Si substrates by sol-gel process and Pt films were prepared by DC magnetron sputtering. PZT and Pt thin films were etched with Cl₂/C₂F₆/Ar gas combination in an Inductively Coupled Plasma (ICP) by varying the etching parameters such as coil RF power, DC bias to wafer susceptor, and gas pressure.

Etching effects were investigated in terms of etch rate, etch selectivity, etch profiles, and electrical properties of etched capacitors. Quantitative analysis of the etching damage was obtained by calculating the shift of the coercive field and the switchable polarization in hysteresis loops. Finally, the etching damage mechanism was discussed and the optimization of etching processes for the fabrication of PZT capacitors was attempted to minimize the etching damage to ferroelectric capacitors.     

Comparative Characteristics of Na0.5K0.5NbO3 Films on Pt by Pulsed Laser Deposition and Magnetron Sputtering

Ferroelectric Na_0.5K_0.5NbO₃ (NKN) thin films were grown on the Pt₈₀Ir₂₀ polycrystalline substrates by pulsed laser deposition (PLD) and radio frequency-magnetron sputtering (RF) technique using the same stoichiometric Na_0.5K_0.5NbO₃ ceramic target. X-ray diffraction proved both PLD- and RF-made Na_0.5K_0.5NbO₃/Pt₈₀Ir₂₀ films are single phase and have preferential c-axis orientation. Temperature dependence of dielectric permittivity reveals the presence of two phase transitions around 210 and 410°C. Capacitance vs. applied voltage C-V @ 100 kHz, I-V, and P-E hysteresis characteristics recorded for the vertical capacitive structures yielded loss tanδ = 0.026 and 0.016, tunability about 44.5 and 30% @ 100 kV/cm, Ohmic resistivity 6.7 × 10¹² Ω·cm and 0.2 × 10¹² Ω·cm, remnant polarization 11.7 and 9.7 μC/cm², coercive field 28.0 and 94.6 kV/cm for PLD- and RF-films, respectively. Piezoelectric test carried out in hydrostatic conditions showed piezoelectric coefficient d _H = 21 for PLD-NKN and 15 pC/N for RF-NKN film.     

光学薄膜制备用的多离子束电子束系统

本文介绍一种新型的光学薄膜制备用多离子束电子束系统。     

Ferroelectric Lead Zirconate Titanate and Barium Titanate Nanotubes

Wetting of the pore walls of porous templates is a simple and convenient method to prepare nanotubes. Ferroelectric lead zirconate titanate and barium titanate nanotubes were fabricated by wetting of porous silicon templates of polymeric precursors. The ferro- and piezoelectric properties of an individual ferroelectric either of a PZT or a BaTiO₃ nanotube were electrically characterized by measuring the local piezoelectric hysteresis. A sharp switching at the coercive voltage of about 2 V was shown from the hysterisis loop. The corresponding effective remnant piezoelectric coefficient is about 90 pm/V. We also expect that free-standing ferroelectric nanotubes obtained by partial etching of the silicon template will be used as building blocks of miniaturized devices and can have a significant impact in the field of nano-electromechanical systems.     

Formation of Silicate-Added (Bi,La)4Ti3O12 Films on LaAlO3/Si(100) Structures

We fabricated MFIS (metal-ferroelectric-insulator-semiconductor) diodes with ((Bi,La)₄Ti₃O₁₂: BLT) films and lanthanum silicate (La₂SiO₅: LSO)-added BLT films formed on LaAlO₃/Si(100) structures. LaAlO₃ films were prepared by an MBD (molecular beam deposition) method. After the film deposition, they were subjected to ex site N₂ annealing in a rapid thermal annealing (RTA) furnace at 800°C for 1 min. BLT films and LSO-added BLT films were deposited on these LaAlO₃/Si structures using a sol-gel technique. The memory windows of BLT and LSO-added BLT films were 3.0 V and 2.1 V, respectively. It was found from the current density-voltage (I-V) characteristics that the insulation property of the LSO-added BLT film was superior to that of the BLT film. We conclude from these results that LaAlO₃ is an excellent candidate of a buffer layer for forming ferroelectric-gate FETs and that the LSO-added BLT film is suitable for low voltage operation of the FETs.     

Microwave Dielectric Properties of (111) Oriented Ferroelectric (Ba1 − xSrx)TiO3 Thin Films

Epitaxial (111) oriented ferroelectric (Ba_{1 ? x}Sr_x)TiO₃ (BST) films were deposited on MgO (111) single crystals using pulsed laser deposition. Structural properties of BST films were investigated using X-ray diffractometer. The dielectric properties of BST films were investigated under the dc bias field of 0–40 V using interdigital capacitors (IDT) fabricated by photolithography and etching process. The small signal dielectric properties of BST films were calculated by modified conformal mapping both the measured data using an impedance gain/phase analyzer and the reflection coefficient data measured using a HP 8510C vector network analyzer in 0.05–10 GHz at room temperature. The IDT capacitor based on (111) oriented BST film exhibits about 40% of capacitance change with an dc bias of 40 V which value is somewhat smaller than that of the IDT device based on (001) oriented BST film. But the dielectric quality factor value is about twice that of the device based on (001) oriented BST film.     

Miniature Microphone with Silicon-Based Ferroelectric Thin Films

A miniature microphone with silicon-based lead zirconate titanate (PZT) thin films has been fabricated and tested. The main structure of the device is composed of Al/Pt/PZT/Pt/Ti/SiO₂/Si₃N₄/SiO₂/Si multi-layer diaphragm. The PZT thin films have been prepared using an improved sol-gel method. Optimized fabrication process of the device has been developed, especially, RIE (reactive ion etching) and IBE (ion beam etching) processes have been used to etch the PZT thin film and electrode metal successfully. The sensitivity of microphone is 16 mV/Pa at 1 KHz and 158 mV/Pa at the resonant frequency of 17.3 KHz. The electrical and thermal reliability of the microphone is satisfactory. This miniature microphone can be widely used in hearing aids, mobile phones, and many other applications.     

Low temperature preparation of ferroelectric SrBi2Ta2O9 thin films by a modified RF magnetron sputtering technique

Abstract

Bi–layered ferroelectric SrBi₂Ta₂O₉ (SBT) films were successfully prepared on Pt/Ti/SiO₂/Si substrates at 650°C by a modified rf magnetron sputtering technique. The SBT films annealed for 1 h in O₂ (760 torr) and again for 30 min in O₂ (5 torr) at 650°C show a average grain size of about 49 nm. The SBT films annealed at 65 0°C have a remanent polarization (P_r) of 6.0 μC/cm² and coercive field (E_c) of 36 kV/cm at an excitation voltage of 5 V. The films showed fatigue–free characteristics up to 4.0 × 10¹⁰ switching cycles under 5 V bipolar pulse. The retention characteristics of SBT films looked very promosing up to 1.0 × 10⁵ s.     

Influence of substrate orientation on structural,ferroelectric and piezoelectric properties of hexagonal YFeO<Subscript>3</Subscript> films

In this study, we fabricated hexagonal YFeO₃ films on Si(100) and Si(111) substrates by a sol-gel method, and investigated the effect of substrate orientation on the structural, ferroelectric and piezoelectric properties of the YFeO₃ films. Grazing incidence X-ray diffraction reveals that the YFeO₃ films on Si(100) and Si(111) have slight preferred-orientation in (110) and (004) direction, respectively. Both films possess granular microstructure with good crystallization, as well as low surface roughness. The (004)-oriented film has higher density than the (110)-oriented film, resulting in lower leakage current. The two films show weak ferroelectric properties and different domain structures. Local amplitude and phase response loops manifest that the film on Si(111) possesses larger phase change and coercive field. While influenced by polarization rotation towards the applied field direction, larger effective piezoelectric coefficient d₃₃ of 67 pm/V has been found in the film on Si(100). These findings suggest that preferred orientation can account for the physical properties of multiferroic YFeO₃ films.     

Low-Temperature Process of Ferroelectric (Y0.95,Bi0.05)MnO3 Thin Films and Their Structural and Electrical Properties

[(Y_0.95,Bi_0.05)MnO₃] (YBM) films have been grown on Y₂O₃ buffered Si (001) by pulsed-laser deposition (PLD). We have compared the structural and dielectric properties of YBM films with those of typical YMnO₃ films from the viewpoint of lowering the process temperature. The highly c-axis oriented YBM film have been obtained on Y₂O₃/Si (001) at 700°C, which is a significantly reduced growth temperature from that of typical YMnO₃ films (850°C). The Bi modification was effective for the low temperature processing of YBM films. These highly c-axis oriented YBM films was obtained only at high ambient oxygen pressures, for example above 100 mTorr, contrary to YMnO₃ films which requires low ambient oxygen pressure for the growth of c-axis preferred orientation. The dielectric constant and dissipation factor was 29 and 0.017 at 1 MHz, respectively. The memory window due to ferroelectric polarization switching was found in a capacitance-voltage (C-V) characteristic. The YBM/Y₂O₃/Si structure with above characteristics of YBM films exhibited the C-V memory window of 1.2 V at a sweep voltage of 5 V. The flat-band voltage shifted symmetrically with increasing the sweep voltage up to 8 V due to little charge injection from Si. As a result, the memory window increased progressively with increasing the sweep voltage and amounted to 2 V at a sweep voltage of 8 V. The leakage current density was below 5 × 10^?7 A/cm² at a bias voltage of 8 V.     

The role of buffer layer in strontium bismuth tantalate based ferroelectric gate mos structures for non-volatile non destructive read out memory applications

Abstract

Metal Ferroelectric Insulator Semiconductor (MFIS) structure has been fabricated with strontium bismuth tantalate (SBT) as the ferroelectric thin film and zirconium oxide (ZrO₂) as the insulating buffer layer. SBT film was deposited by spin-on metal organic deposition (MOD) technique. ZrO₂ film was deposited by electron beam evaporation. The capacitance versus voltage characteristics(C-V) of the MFIS structure shows hysteresis and the direction of hysteresis corresponds to ferroelectric polarization. The C-V characteristics of MFIS structure shows memory window of 1.8 volts for a write/erase voltage of 9V at a sweep rate of 1 sec/1.8V. In order to understand the role of coercive voltage on the memory window in MFIS structures, C-V characteristics metal-ferroelectric-metal (MFM) structures with various SBT film thickness’ were also studied.     

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.     

Effects of Al2O3 on the piezoelectric properties of Pb(Mn1/3Nb2/3)O3-PbZrO3-PbTiO3 ceramics

Piezoelectric properties of Al₂O₃-doped Pb(Mn_1/3Nb_2/3)O₃-PbZrO₃-PbTiO₃ ceramics were investigated. The constituent phases, microstructure, electromechanical coupling factor, dielectric constant, piezoelectric charge and voltage constants were analyzed. Diffraction peaks for (002) and (200) planes were identified by X-ray diffractometer for all the specimens doped with Al₂O₃. The highest sintered density of 7.8 g/cm³ was obtained for 0.2 wt% Al₂O₃-doped specimen. Grain size increased by doping Al₂O₃ up to 0.3 wt%, and it decreased by more doping. Electromechanical coupling factor, dielectric constant, piezoelectric charge and voltage constants increased by doping Al₂O₃ up to 0.2 wt%, and it decreased by more doping. This might result from the formation of oxygen vacancies due to defects in O^{2 −} ion sites and the substitution of Al³⁺ ions.     

Fabrication of MFIS diodes using sol-gel derived SBT films and LaAlO3 buffer layers

Abstract

We demonstrate the ferroelectric behavior of Sr_0.8Bi_2.2Ta₂O₉ (SBT) films grown on Si(100) substrates by using lanthanum aluminate (LaAlO₃) buffer layers. LaAlO₃ films were prepared by vacuum evaporation method. Then, they were subjected to ex situ dry N₂ annealing in a rapid thermal annealing (RTA) furnace. From the capacitance-voltage (C-V) measurement, the dielectric constant of LaAlO₃ was estimated to be 20～25. On these substrates, SBT films (210nm) were deposited by sol-gel method and they were characterized by XRD analysis after annealing under various conditions. It was found from C-V characteristics that the memory window of an SBT film annealed at 750°C for 30min in O₂ atmosphere was about 3.0V for the voltage sweep of ±10V. It was also found from the retention measurement that the capacitance values of the SBT film annealed at 750°C did not change over 12hours. It is concluded from these results that the SBT/LaAlO₃/Si(100) structure is one of the most promising structures for realizing MFISFETs (metal-ferroelectric-insulator-semiconductor field-effect-transistors).     

Epitaxial ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films on La0.7Sr0.3MnO3 bottom electrode

Epitaxial (001)-oriented 0.7Pb(Mg_0.33Nb_0.67)O₃-0.3PbTiO₃ (PMN-PT) thin films were deposited by pulsed laser deposition on vicinal SrTiO₃ (001) substrates using La_0.7Sr_0.3MnO₃ as bottom electrode. Detailed microstructural investigations of these films were carried out using X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). Polarization-field hysteresis curves were measured at room temperature. Spontaneous polarization P _s , remnant polarization P _r and coercive voltage V _c were found to be 25 μC/cm², 15 μC/cm² and 0.81 V, respectively. Field dependent dielectric constant measurements exhibited butterfly shaped curves, indicating the true ferroelectric nature of these films at room temperature. The dielectric constant and the dielectric loss at 100 kHz were found to be 238 and 0.14, respectively. The local piezoelectric properties of PMN-PT films were investigated by piezoelectric force microscopy and were found to exhibit a local piezoelectric coefficient of 7.8 pm/V.     

Preparation and properties of Pb(Zr, Ti)O3 ferroelectric thin films and compositionally graded thin films on LaNiO3/Si substrates

LaNiO₃ (LNO) thin films were prepared on Si (100) wafer by MOD method. Pb(Zr, Ti)O₃ ferroelectric thin films and their compositionally graded thin films were prepared on LNO/Si (100) substrates by a modified sol–gel method. The composition depth profile of a graded film was determined by using a combination of Auger electron spectroscopy and Ar ion etching. The results confirmed that the processing method produces graded composition change. XRD analysis showed that the graded thin film possessed a composite structure of tetragonal and rhombohedral. The dielectric constant of the graded thin films was higher than that of each thin film unit, but the loss tangent was near to each other at 10 kHz. The temperature characteristics of the dielectric constant of the graded thin films at different frequencies showed three peaks and ferroelectric relaxor feature to some extent. Hysteresis loops showed that graded thin film had higher remanent polarization, smaller coercive field than each thin film unit. The pyroelectric coefficient of the graded thin films increased gradually with temperature, and was higher than that of each thin film unit.