High Quality Silicon-Based AlN Thin Films for MEMS Application

ABSTRACT

Aluminum nitride is an attractive piezoelectric material for MEMS devices such as bulk acoustic wave (BAW) devices. (002)-oriented AlN films were deposited on Si, Al and Pt by reactive sputtering. Optimized AlN (002) peak reaches a full width at half maximum (FWHM) of 5.6°. Auger electron spectroscopy is used to analyze the oxygen contamination of films. To find the suitable electrode material for device application, the growth mechanism of AlN crystallites on different substrates is also discussed. Based on sputtered AlN films, the prototype of AlN thin film bulk acoustic resonator (TFBAR) was fabricated successfully.     

Effect of Zr/Ti Ratio on Microstructure and Electrical Properties of Lead Zirconate Titanate Thin Films Derived by Pulsed Laser Deposition

PZT films were fabricated using various targets of Pb(Zr_xTi_{1 – x})O₃ with Zr/Ti ratios of 70/30, 58/42, 52/48, 45/55 and 30/70, and with excess PbO of 20 wt% on Pt/Ti/SiO₂/Si(100) substrates. The rosette structure was observed in the films derived from the target with a Zr/Ti ratio of 70/30 and disappeared with increasing titanium composition. The observations on surface and cross-sectional microstructure were consistent with a higher perovskite nucleation for the higher Ti content films. The PZT films derived from the target with a Zr/Ti ratio of 45/55 had a polycrystalline columnar microstructure extending throughout the thickness of the film and no pyrochlore phase on the surface was observed. The PZT films derived from the target with a Zr/Ti ratio of 45/55 exhibited better electric properties than those derived from the target with a Zr/Ti ratio of 52/48.     

Effect of Ultra-Thin SrTiO3 Seed Layer on the Microwave Surface Resistance of YBa2Cu3O7 Films Deposited on (1 0 0) MgO

RHEED controlled ultra-thin buffer layers of SrTiO₃ have been deposited on (1 0 0) MgO by pulsed laser deposition to grow YBa₂Cu₃O₇ films for microwave applications. A buffer layer with a thickness from 5 to 40 unit cells of SrTiO₃ is sufficient to expand to more than 60°C the range of deposition temperatures for which a low microwave surface resistance [Rs(10 GHz, 77 K)<0.5 m] is obtained. The Rs values are as low as those obtained on LaAlO₃ substrates, furthermore they present a slightly lower magnetic field dependency. The XRD -scans show that the SrTiO₃ seed layer induces an oriented epitaxial growth with the [1 0 0] axis of YBCO parallel to the one of MgO over this broadened range of deposition temperatures. This seed layer promoting effect is not observed with other oxides such as Ba₀.15Sr_0.85TiO₃, CeO₂, Ce₁–x _{La x} O₂.     

Epitaxial thin film heterostructures of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3

Abstract

We have grown stoichiometric pure perovskite phase Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) thin films and PMN-PT/SrRuO₃ heterostructures on miscut (100) SrTiO₃ substrates by pulsed laser deposition. X-ray diffraction θ–2θ scans show that the PMN-PT films are purely c-axis oriented. The off-axis Φ scans show that the heterostructures grow “cube-on-cube” with an in-plane epitaxial arrangement of PMN-PT[100], [010] // SrRuO₃[001] // SrTiO₃[100] and PMN-PT[010], [100] // SrRuO₃[110] // SrTiO₃[010]. The crystalline quality of the films found to be comparable to that of bulk single crystals. The AFM images show that the SrRuO₃ and PMN-PT layers have smooth surfaces with root mean square roughness of 9Å. These epitaxial heterostructures can be used for the fabrication of piezoelectric devices.     

Thin films of novel ferroelectric composites

Abstract

Composites of BSTO combined with other non electrically active oxides have demonstrated adjustable electronic properteis which can be tailored for use in various electronic devices.^[1,2] These novel composites of barium strontium titanate (BSTO) and oxide III compounds have already exhibited promising results in their ceramic form.^[3] The additive oxides modify the dielectric constant, tunability (change in the dielectric constant with applied voltage), and dielectric loss of the material. One application has been for use in phased array antennas and insertion has been accomplished into several working antenna systems.^[4] To further accommodate the frequencies required by these phased array antennas, thin films of the composites have been fabricated. Preliminary studies have indicated that thin films of such composites exhibit similar behavior as their bulk ceramic counterparts.^[5] The purpose of this study is to investigate the properties of the BSTO/oxide III based compounds in thin film form.     

Structural and dielectric properties of BaTiO3SrTiO3-multilayers deposited by PLD

Abstract

Dielectric superlattices are characterized by the structural variability of the film periodicity additional to the stochiometry. They have interesting dielectric properties different from the solid solutions of the same overall composition.

In this paper the structural and dielectric properties of BaTiO₃/SrTiO₃-superlattices prepared by pulsed laser deposition on single crystalline substrates are characterized in comparison to a (Ba_0.5Sr_0.5)TiO₃ solid solution. The films were characterized using X-ray diffraction, SEM and dielectric measurements.

The BaTiO₃/SrTiO₃-superlattices have a different frequency and a weaker bias and temperature dependence of the dielectric properties than the solid solution. The mechanical stresses caused by the lattice mismatch between the different layers could be one reason for this behaviour.     

Aging and Annealing Effects of ZnO Thin Films on GaAs Substrates Deposited by Pulsed Laser Deposition

Zinc oxide (ZnO) thin films were deposited on GaAs (100) substrates at different temperatures in the pulsed laser deposition (PLD) system. From the measurements of X-ray diffraction (XRD) at room temperature, 300–500°C were found to be good condition for the crystallization of the thin films. From the photoluminescence (PL) measurements, 500°C was found to be the optimized temperature for its optical property. Samples grown at 100, 200, 300, and 400°C showed near band-edge (NBE) emissions and deep-level emissions. The intensity of deep-level emissions decreased as time goes on, which is believed to originate from oxygen vacancies or zinc interstitials in thin films. While for the sample grown at 500°C, bright NBE emissions were observed at room temperature, and no deep-level emissions observed. This means that the high-optical-quality thin film was grown at 500°C. At the same time, annealing process of ZnO thin films grown at room temperature was carried out in PLD chamber. It was found that the annealing temperature of 600°C has strong effects on its PL. Aging and annealing effects in ZnO thin films grown on GaAs substrates by PLD were observed for the first time.     

脉冲激光沉积(PLD)法制备NiZn铁氧体多晶薄膜研究

采用脉冲激光沉积（PLD）法分别在硅和玻璃基片上沉积了NiZn铁氧体多晶薄膜。实验表明：基片温度、氧气压以及热处理对薄膜的沉积速率、磁性能有很大影响。     

Studies of ZnO Thin Films On Sapphire (0001) Substrates Deposited by Pulsed Laser Deposition

ZnO thin films are deposited on sapphire (0001) substrates at different temperatures in the pulsed laser deposition (PLD) system. By measurements of X-ray diffraction (XRD), atomic force microscopy (AFM), and Photoluminescence (PL) at room temperature, fabrication temperatures higher than 400C is found to be the optimum condition for the structural and optical properties of ZnO thin films. With the increase of the fabrication temperature, the grain size becomes bigger and the thin film becomes more homogeneous. In order to get the high-quality ZnO thin film at low temperature, ZnO thin films are deposited at room temperature and annealed in a rapid thermal annealing (RTA) system. It is found that the optical property of the thin film can be greatly improved by annealing in RTA system.     

Pulsed Laser Deposition of Ba0.6Sr0.4TiO3 Thin Films and Their Optical Properties

ABSTRACT

Barium strontium titanate [Ba_0.6Sr_0.4TiO₃ or BST (60/40)] thin films were deposited on MgO (100) substrates using pulsed laser deposition. X-ray diffraction (XRD) measurements revealed that the BST thin films had epitaxially grown on the MgO (100) substrates. The surface morphology of the thin films was observed using an atomic force microscope and the grain size was found to be about 100–150 nm. The surface roughness was around 4.9 nm for a 250 nm thick film. The optical transmittance of the BST thin film was measured using a transmission mode ellipsometer. The BST/MgO configuration was highly transparent in the visible region. The optical band gap energy of the BST film, calculated by applying the Tauc relation, was 3.56 eV. Optical waveguide characteristics of the BST (60/40) thin film were determined using a prism coupler. The electro-optic (E-O) properties were measured at 632.8 nm wavelength using a phase modulation detection method. The BST film exhibited a predominately quadratic E-O behavior and the quadratic E-O coefficient was found to be 0.58 × 10^{? 17} m²/V².     

纳米晶化对Co-FeNbZr薄膜结构及阻抗的影响

用直流磁控溅射法制备非晶Co-FeNbZr软磁膜,再用独特的快速循环纳米晶化技术(RRTA)对其进行纳米晶化.研究了不同的纳米晶化工艺条件下薄膜的微观结构和阻抗性能.结果表明,500℃退火的Co-FeNbZr软磁薄膜晶粒细化到30～40nm,阻抗值从28Ω增加到110Ω(1400MHz),阻抗共振峰向低频移动200～220 MHz左右,薄膜的软磁性能极大改善.     

The Effect of Stress on the Microwave Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films

Single phase, (1 0 0) epitaxial Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited onto (1 0 0) LaAlO₃ and MgO substrates by pulsed laser deposition (PLD). The capacitance and dielectric losses of as-deposited and annealed films have been measured from 1–20 GHz as a function of electric field (0–80 kV/cm) at room temperature. The dielectric properties are strongly affected by the substrate type, post-deposition annealing time (6 h) and temperature (1200°C). For epitaxial BST films deposited onto MgO, it is observed that, after a post-deposition anneal the dielectric constant and the dielectric loss decreases. For epitaxial BST films deposited onto LAO, a post-deposition anneal (1000°C) results in an increase in the dielectric constant and an increase in the dielectric loss. The dc electric field induced change in the dielectric constant tends to increase with the dielectric constant and is largest for as-deposited films on MgO and post-deposited annealed films on LAO. In general, for epitaxial BST films, a large electric field effect is observed in films that have a large dielectric loss and a small electric field effect in films that have a low dielectric loss. High resolution X-ray diffraction measurements indicate that deposited film exhibit a significant tetragonal distortion which is strongly affected by a by a post deposition anneal. The observed differences in dielectric properties of the epitaxial BST films on MgO and LAO are attributed to the differences in film stress which arise as a consequence of the lattice mismatch between the film and the substrate and the differences in the thermal coefficient of expansion between the film and the substrate. A thin amorphous buffer layer of BST has been used to relieve stress induced by the lattice mismatch between the film and the substrate. Unlike epitaxial films, stress relieved films do not show an inverse relationship between dielectric tuning and Q (1/tan) and may be superior materials for tunable microwave devices.     

Effect of Strain Gradation on Luminescence and Electronic Properties of Pulsed Laser Deposited Zinc Oxide Thin Films

ZnO thin films were grown by ablation of a ZnO ceramic target using pulsed excimer laser (KrF) under 1 mTorr oxygen partial pressure over (0001) -Al₂O₃ substrates held at 750_C. Highly c-axis oriented (0002) ZnO films with visible range optical transparency over 80% were obtained. Inhomogeneous distribution of strain in the film growth direction was studied by line shape analysis of X-ray diffraction and broad luminescence features centered on near band edge transition at 3.3 eV. Strain in the film adversely affects optical gain and excitonic threshold of UV emission. Post-growth oxygen annealing of films at 850°C for 1 h reduces strain and associated defects at ZnO film interface with (0001) Al₂O₃ substrate. FWHM of X-ray rocking curves show corresponding lowering from 12.5 arc min to 9.0 arc min signifying improved ZnO crystal quality. -rocking curves show line features with two superimposed peaks belonging to interfacial layer and bulk ZnO film. Graded strain in ZnO film is related to differently oriented interfacial layer formed at inception stage of film growth. Decrease in conductivity of annealed ZnO films show that O₂-vacancies are primary defects. Formation of strain free (0002) oriented optical quality ZnO films based on combined process of growth in low O₂ pressure and post growth anneal at high O₂ pressure is proposed for UV-optoelectronic applications.     

Growth,Microwave Properties and Microstructure of Ba0.05Sr0.95TiO3 Thin Films

Charged defects were found to have a significant influence over the microwave properties of Ba_0.05Sr_0.95TiO₃ thin films.     

Effect of AC&DC Field on the Dielectric Response of (Pb,La)TiO3 Thin Films

Lanthanum doped lead titanate thin films are the potential candidates for the capacitors, actuators and pyroelectric sensor applications due to their excellent dielectric, and ferroelectric properties. Lanthanum doped lead titanate thin films are grown on platinum coated Si substrates by excimer laser ablation technique. A broad diffused phase transition with the maximum dielectric permittivity (?_max) shifting to higher temperatures with the increase of frequency, along with frequency dispersion below T_c, which are the signatures of the relaxor like characteristics were observed. The dielectric properties are investigated from ?60°C to 200°C with an application of different dc fields. With increasing dc field, the dielectric constant is observed to reduce and phase transition temperature shifted to higher temperature. With the increased ac signal amplitude of the applied frequency, the magnitude of the dielectric constant is increasing and the frequency dispersion is observed in ferroelectric phase, whereas in paraelectric phase, there is no dispersion has been observed. The results are correlated with the existing theories.     

Extensive electromechanical characterization of PZT thin films for MEMS applications by electrical and mechanical excitation signals

In this work we present a unique measurement method to estimate the effective transverse piezocoefficient e _31,f of piezoelectric thin films which is often used in micro electromechanical systems (MEMS). This method utilizes basically a 4-point bending setup specially adapted to be used with thin film samples. It allows the application of very homogeneous well defined mechanical stresses to the device. Stress and corresponding strain distribution are verified by Finite Element simulations. Measurements are shown to demonstrate the capability and repeatability of the setup on sol-gel processed PZT thin film samples. In conjunction with additional measurement results it is possible to fully determine the electromechanical characteristics.     

Nd–Fe–B thick films prepared at the high deposition rate of 90 µm/h

After a postannealing process, the magnetic properties of pulsed laser deposition (PLD)‐made Nd–Fe–B film magnets prepared at the deposition rate of 20–90 µm/h showed constant values, and the average values of H_c, B_r and (BH)_max were 1050 kA/m, 0.6 T, and 60 kJ/m³, respectively. The obtained results were comparable to those previously reported. In conclusion, it was found that a maximum deposition rate of 90 µm/h could be achieved without deterioration of magnetic and mechanical properties. © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Effect of Thickness on the Electrical and Optical Properties of Sb Doped SnO2 (ATO) Thin Films

This work reports the preparation and characterization of (SnO₂) thin films doped with 7 mol% Sb₂O₃. The films were prepared by the polymeric precursor method, and deposited by spin-coating, all of them were deposited on amorphous silica substrate. Then, we have studied the thickness effect on the microstrutural, optical and electric properties of these samples. The microstructural characterization was carried out by X-ray diffraction (XRD) and scanning tunneling microscopy (STM). The electrical resistivity measurements were obtained by the van der Pauw four-probe method. UV-visible spectroscopy and ellipsometry were carried out for the optical characterization. The films present nanometric grains in the order of 13 nm, and low roughness. The electrical resistivity decreased with the increase of the film thickness and the smallest measured value was 6.5 × 10^{– 3} cm for the 988 nm thick film. The samples displayed a high transmittance value of 80% in the visible region. The obtained results show that the polymeric precursor method is effective for the TCOs manufacturing.     

Effect of Solution Processing on PZT Thin Films Prepared by a Hybrid MOD Solution Deposition Route

Lead zirconate titanate, Pb(Zr_0.53Ti_0.47)O₃ (PZT), thin films were prepared by a hybrid metalorganic decomposition (MOD) solution deposition route; the effects of processing conditions on the film structure and properties were investigated. Solutions were synthesized by mixing and reacting Zr acetylacetonate and Pb acetate trihydrate with a solution prepared from Ti isopropoxide, acetic acid and water. Chemical changes in the solution during preparation and solution storage (i.e., aging) were investigated by visual observation and FTIR, and were evidenced by changes in phase content and properties of the final PZT films. Results suggest that Zr acetylacetonate and Pb acetate trihydrate react with a Ti oxoacetate-based precursor, and that this reaction continues during aging at room temperature. The PZT film quality and properties improved with aging time of the solution before deposition. To achieve good properties and design a convenient processing route, an accelerated aging scheme, including a brief aging at 60°C and freezing to prevent further reaction, was developed. PZT films prepared from these solutions had average dielectric constants of 1040, loss tangents of 0.05, remnant polarizations of 26 C/cm², and coercive fields of 39 kV/cm.     

Ni0.25Zn0.15Fe2.6O4种子层对NiZn铁氧体双层膜性能的影响

采用旋转喷涂法在Si(100)基片上制备Ni0.25Zn0.15Fe2.6O4(100 nm)铁氧体薄膜作为种子层,然后在种子层上采用射频磁控溅射法沉积Ni0.25Cu0.09Zn0.66Fe1.998O4(600 nm)铁氧体薄膜。研究了种子层对NiZn铁氧体双层膜微观形貌、饱和磁化强度、矫顽力、磁导率及截止频率的影响。结果表明,Ni0.25Zn0.15Fe2.6O4种子层的引入促进了NiZn铁氧体双层膜尖晶石相的晶化和晶粒生长。NiZn铁氧体双层膜的饱和磁化强度Ms为420 kA/m,矫顽力Hc为5.9kA/m,截止频率fr为1.37 GHz,磁导率μ’(300 MHz)高达202。