Chemical vapor deposition of Pb1−xLaxTiO3

Abstract

We report chemical vapor deposition (CVD) of PbLaTiO₃ films for integrated pyroelectric devices. Pb(thd)₂, La(thd)₃ and Ti(O-Pr)₂(thd)₂ were introduced to the reactor via a single liquid precursor solution that is vaporized. Substrate temperatures were approximately 535°C and post deposition annealing was not used. Films were deposited on fused silica and Pt metallized Si substrates. The liquid delivery technique permitted excellent composition control and films on fused silica were predominantly [100] oriented with trace amounts of [110] and [111] present. Pyroelectricity for 0.7μm thick films deposited on Pt metallized Si was measured using a modified Byer-Roundi technique and pyroelectric coefficients as high as 90 nC/cm²·K were observed. The high crystalline quality and pyroelectric properties are attributed to the excellent composition control afforded by the liquid delivery CVD technique.     

MOCVD (BaxSr1−x)Ti1+yO3+z (BST) thin films for high frequency tunable devices

Abstract

We have investigated the structural and electrical characteristics of (Ba_xSr_1?x)Ti_1+yO_3+z (BST) thin films synthesized at 650°C on Pt/SiO₂/Si substrates using a large area, vertical metalorganic chemical vapor deposition (MOCVD) reactor equipped with a liquid delivery system. Films with a Ba/Sr ratio of 70/30 were studied, as determined using X-ray fluorescence spectroscopy (XRF) and Rutherford backscattering spectrometry (RBS). A substantial reduction of the dielectric loss was achieved when annealing the entire capacitor structure in air at 700°C. Dielectric tunability as high as 2.3:1 was measured for BST capacitors with the currently optimized processing conditions.     

Characterization of BaTiO3—BaZrO3 Solid Solution Thin Films Prepared by MOCVD

Thin films of the solid solution system Ba(Ti_{1 ? y} Zr _y )O₃ (y = 0, 0.37, 1) have been grown on ?111? Pt-coated silicon substrates by means of metal-organic chemical vapor deposition (MOCVD). Commercially available liquid precursors typical for the fabrication of perovskite thin films were used: Ba(thd)₂(diethylene-triamine), Ti(OⁱPr)₂(thd)₂, and Zr(OⁱPr)₂(thd)₂ (thd = C₁₁H₁₉O₂) dissolved in diglyme (C₆H₁₄O₃). Growth took place in a horizontal type reactor (AIX-200). The vaporizer technology based on a TriJet? liquid delivery system provided by AIXTRON. It allows to feed in four metal-organic species in a pulsed injection mode. We combined process-related studies (growth rate, rate-limiting factors) with material-related properties (crystallinity and morphology) using various analytical techniques (X-ray analysis, scanning electron microscopy). Furthermore we characterized the samples electrically with respect to the voltage dependence of permittivity and dielectric losses.     

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.     

High quality BaxSr1-xTiO3 films grown by mocld and novel ferroelectric/ferrite structures for dual-tuning microwave devices

Abstract

Excellent single crystal Ba_xSr_1-xTiO₃ (BST) films were grown on LaAlO₃ substrates using the metal-organic chemical liquid deposition (MOCLD) method. Very low losses (tanδ ∽0.002-0.008) were measured from these films at 400 KHz. Biaxially oriented BST films were successfully grown on polycrystalline YIG substrates using both MOCLD and pulsed laser deposition methods with biaxially oriented MgO and YSZ buffer layers. The dielectric losses of the films range from 0.005 to 0.015 while 25% of dielectric constant change was observed with 40V bias voltage up to 10 MHz. Both the dissipation and dielectric constant of the films remained nearly constants over a wide temperature range (77 K to 380 K). A dual-tuning microwave coplanar phase shifter using a BST film grown on a MgO buffered polycrystalline YIG substrate was fabricated. A significant phase shift was observed in GHz frequency range when an electric bias or a magnetic field was applied to the device.     

Growth of (Ba,Sr)TiO 3 Thin Films by MOCVD: Stoichiometry Effects

(Ba x Sr 1 m x )TiO 3 thin films were deposited in a planetary multi-wafer MOCVD reactor combined with a liquid delivery system using 0.35 molar solutions of Ba(thd) 2 and Sr(thd) 2 and a 0.4 molar solution of Ti(O-i-Pr) 2 (thd) 2 . The film growth on Pt-(111) was investigated within a wide parameter field, e.g., the deposition temperature was varied between 560C and 650C, which yields films with microstructures ranging from randomly oriented polycrystalline to perfectly (100)-textured columnar structures. Special emphasis is given to film stoichiometry: starting with (Ba 0.7 Sr 0.3 )TiO 3 the Group-II/Ti content was varied from 0.9 to 1.1 and the Ba content was reduced to the limit of pure SrTiO 3 films. The electrical properties of MIM structures were investigated after deposition of Pt top electrodes. The nominal thickness of the films was varied between 5 and 100 nm and permittivity and leakage current both are shown to depend strongly on the film thickness. These dependencies on the film thickness are analyzed within the phenomenological dead layer model. The dependence of the electrical properties on stoichiometry are discussed in detail.     

Microstructural Properties of Ba0.6Sr0.4TiO3/RuO2 Multi-Layers Grown on MgO and YSZ by Pulsed-Laser Deposition

The microstructure of Ba_0.6Sr_0.4TiO₃ (BST)/RuO₂ multi-layers grown on (100) MgO and (100) YSZ substrates, respectively, by pulsed-laser deposition (PLD) has been studied by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The RuO₂ films deposited at 700°C adopt epitaxial relationships with both substrates. The epitaxial films on (100) MgO consist of two variants with an orientation relationship given by (110) RuO₂//(100) MgO and [001] RuO₂//[011] MgO. The epitaxial films on (100) YSZ contain four variants with an orientation relationship given by (200) RuO₂//(100) YSZ and [011] RuO₂//[001] YSZ. The BST films deposited on the RuO₂ electrode are epitaxial on the (200) RuO₂ films deposited on YSZ, and non-epitaxial on the (110) RuO₂ films deposited on MgO. The epitaxial relationship between the BST and (200) RuO₂ films can be described as (111) BST//(200) RuO₂ and [1&1macr;0] BST//[011] RuO₂. The BST films contain at least four variants. The growth and microstructural properties of the multi-layer structures can be understood based on geometrical consideration of the crystal structures.     

Characterization of Ni/Ce Co-Doped BST Thin Film Grown on LSCO/Pt Electrodes Prepared by Pulsed Laser Deposition

Perovskite (Ba_0.6Sr_0.4)TiO₃ (BST) thin films doped with Ni, Ce and Ni/Ce codopants were prepared on LSCO/Pt/SiO₂/Si substrates by pulsed laser deposition method. In this study, La_0.5Sr_0.5CoO₃ (LSCO) bottom electrodes were used to improve the crystallinity and dielectric properties of BST films. Single ion doped(1 mol%Ni doped,1 mol%Ce doped) BST films showed more improved crystallinity, smoother surface, and smaller grain size than that with 1 mol%Ni/1 mol%Ce. The dielectric constant and loss of Ni/Ce co-doped BST films were about 298 and 1.8%, respectively. In addition, tunability and figure of merit of co-doped BST films showed minimum values of approximately 9.3% and 5, respectively. With 1% Ni-doped BST thin films, results gave a tunability of 54.2% and a loss tangent of 1.8% while a figure of merit was 30. Correlation of the material properties with dielectric and tunable properties suggests the 1 mol%Ni-doped BST films are effective potential candidate for tunable device applications.     

A comparison of mocld with PLD BaxSr1−xTiO3 thin films on LaAIO3 for tunable microwave applications

Abstract

Historically, tunable dielectric devices using thin crystalline Ba_xSr_1-x TiO₃ (BST) films deposited on lattice-matched substrates, such as LaAlO₃, have generally been grown using pulsed laser deposition (PLD). Highly oriented BST films can be grown by PLD but large projects are hampered by constraints of deposition area, deposition time and expense. The Metal-Organic Chemical Liquid Deposition (MOCLD) process allows for larger areas, faster turnover and lower cost. Several BST films deposited on LaAlO₃ by MOCLD have been tested in 16 GHz coupled microstrip phase shifters. They can be compared with many PLD BST films tested in the same circuit design. The MOCLD phase shifter performance of 293° phase shift with 53 V/μm dc bias and a figure of merit of 47°/dB is comparable to the most highly oriented PLD BST films. The PLD BST films used here have measured XRD full-width-at-half-maxima (FWHM) as low as 0.047°. The best FWHM of these MOCLD BST films has been measured to be 0.058°.     

STRESS CONTROLLED FERROELECTRIC (BA0.5Sr0.5)TiO3 THIN FILMS PREPARED ON GD2O3/SRTiO3(001) BY A PULSED LASER DEPOSITION

ABSTRACT

Stress controlled epitaxial ferroelectric Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited on Gd₂O₃/SrTiO₃ by pulsed laser deposition with oxygen background pressure of 200 mTorr at the deposition temperature of 750°C. In order to control the stress in BST films, oxygen pressures for Gd₂O₃ buffer layers have been varied from 0.1 to 100 mTorr, while that of BST films have been fixed at 200 mTorr. It has been found that the lattice parameters of the BST films deposited on Gd₂O₃ were changed. Furthermore, microwave properties of co-planar waveguide (CPW) fabricated on BST films were investigated by a HP 8510C vector network analyzer from 1–20 GHz. Large dielectric tunabilities were observed from the CPW's fabricated on BST films deposited on Gd₂O₃ layers deposited at low and high oxygen pressures, 0.1 and 100 mTorr, respectively.     

Synthesis and characterization of (BaxSr1−x)Ti1+yO3+z thin films and integration into microwave varactors and phase shifters

Abstract

Precise control of composition and microstructure is critical for the production of (Ba_xSr_1?x)Ti_1+yO_3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high frequency devices. Here we review recent results on composition-microstructure-electrical property relationships of polycrystalline BST films produced by magnetron sputter deposition, that are appropriate for microwave devices such as phase shifters. Films with controlled compositions were grown from a stoichiometric Ba_0.5Sr_0.5TiO₃ target by control of the background processing gas pressure. It was determined that the (Ba+Sr)/Ti ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar+O₂) process pressure, while the O₂/Ar ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties as a function of the (Ba+Sr)/Ti ratio are discussed. Optimized BST layers yielded capacitors with low dielectric losses (0.0047), among the best reported for sputtered BST, while still maintaining tunabili-ties suitable for device applications. These BST films were used to produce distributed-cir-cuit phase-shifters, using a discrete periodic loading of a coplanar waveguide with integrated BST varactors on high-resistivity silicon. Phase shifters yielding 30 degrees of phase shift per dB of insertion loss were demonstrated at 20GHz.     

Structure and dielectric properties of Ca-doped (Ba0.7Sr0.3) TiO3 thin films fabricated by sol–gel method

A wide range of Ca-doped (Ba_0.7Sr_0.3)TiO₃ (BST) thin films (from 0 to 20 mol%) have been prepared on Pt/Ti/SiO₂/Si (100) substrates by sol–gel technique. The structural and dielectric properties of BST thin films were investigated as a function of Ca dopant concentration. The results showed that the microstructure and dielectric properties of the BST films were strongly dependent on the Ca contents. With the Ca dopant concentration increasing, the grain size, dielectric constant and dielectric loss of the BST thin films decreased. As the content of Ca dopant reaches 10 mol%, the dielectric constant, dielectric loss, tunability, the value of FOM and the leakage current density are 281, 0.0136, 16.7%, 12.3 and 5.5?×?10^?6 A/cm², respectively.     

Liquid source chemical vapor deposition of high-dielectric-constant (Ba,Sr)TiO3 films

High-dielectric-constant (Ba, Sr)TiO₃ [BST] films were deposited by the liquid source chemical vapor deposition (CVD) method. The system consisted of a single-wafer, low-pressure thermal CVD reactor, a vaporizer for liquid source materials, and a shower-type gas nozzle head, giving stable BST film deposition on a 6-in. diam. substrate with uniform thickness and uniform chemical composition ratio. The source materials employed were Ba(DPM)₂, Sr(DPM)₂, and TiO(DPM)₂ dissolved in tetrahydrofuran (THF), resulting in conformal step coverage of BST films at lowered substrate temperatures, where DPM denotes dipivaloylmethanate. Moreover, the two-step deposition technique was developed to restart protrusions formed on BST film surfaces at low temperatures, where the BST films consisted of a buffer layer and a main layer; the buffer layer was a layer about 60 Å thick of CVD-BST film annealed in N₂. Thus, the two-step CVD deposition of BST films on Pt and Ru electrodes achieved an equivalent SiO₂ thickness of t_eq ∼ 0.5 nm, a leakage current of J_L ∼ 1.0 × 10⁻⁸ A/cm² (at +1.1 V), and a dielectric loss of tan δ ∼ 0.01 at a total film thickness of 250 Å, along with conformal coverage of 80% for a trench with an aspect ratio of 0.65. Then, for BST films deposited on patterned electrodes 0.24 μm wide, 0.60 μm deep, and 0.15 μm high (each spaced by 0.14 μm), the capacitance was demonstrated to be increased without significant deterioration of the leakage current: the capacitance was increased in comparison with that for films on flat electrodes, by a factor corresponding to the increase in surface area due to sidewalls of storage-node-like pattern features. This capacitance increase reflects the most characteristic advantage of CVD, an excellent step coverage on microscopic pattern features. These electrical properties satisfy the specifications for capacitors for Gb-scale dynamic random access memories (DRAMs), giving a storage capacitance of more than 25 fF/cell for a stacked capacitor having a storage node 0.2 to 0.3 μm high. © 1998 Scripta Technica, Electr Eng Jpn, 125(1): 47–54, 1998     

Preparation of BST thin films on Pt electrode on Si wafer with down-flow LSMCVD reactor

Abstract

A novel type of down-flow LSMCVD (Liquid Source Mist CVD) reactor was developed to prepare a high dielectric BST thin film on Pt electrode on Si wafer. Barium acetate [Ba (OOCCH₃)₂], strontium acetate [Sr (OOCCH₃)₂], and titanium isoproxide [Ti (OC₃H₇ ⁱ )₄] were used as metal sources. Metal sources were dissolved in acetic acid, 1-butanol, or 2-methoxyethanol. BST [Ba/(Ba + Sr) = 0.7] film annealed on Pt/Ti/SiO₂/Si above 650°C was polycrystalline. BST film has a (110) preferred orientation with increasing temperature. Surface roughness of BST film and grain size increased with increasing temperature. The metal-oxygen bond was formed at 650°C as shown in the spectra of FTIR. The depth profiles of elements of BST thin films indicated a uniform composition throughout the film. BST films annealed at 750°C showed a dielectric constant and a tanδ of 390 (thickness: 150 nm) and 0.06 at a frequency of 100 kHz, respectively. The behavior of capacitance of the BST film with bias voltage showed paraelectric property. BST film annealed at 750°C had the leakage current density of 3.2 (μA/cm²) at a bias voltage of 2V.     

The electrical properties of Ba1-x SrxTiO3 thin films grown by sputter deposition

Abstract

Ba_1-x Sr_xTiO₃ (BST) thin films were deposited by reactive rf-magnetron sputtering onto Si substrates. The influence of the deposition parameters such as temperature and oxygen ambient on the dielectric constant of the films is presented. BST films deposited at 450°C and optimum conditions exhibited a dielectric constant of approximately 200 at frequencies as high as 1GHz. In addition, the films were found to have leakage current densities of <0.1μAmp/cm² at fields of 5×10⁵V/cm. An extrapolated lifetime greater than 10 years was obtained from stress tests at elevated temperatures and fields. These films compared favorably with published data.     

Iridium thin film as a bottom electrode for high dielectric (Ba,Sr)TiO3 capacitors

Abstract

We propose Ir thin films as new electrode materials for high dielectric BST capacitors. Ir was found to be superior to Pt in a number of aspects such as resistivity, adhesion and surface roughness. The Pt/BST/Ir/SiO₂/Si capacitors showed leakage currents as low as Pt/BST/Pt/SiO₂/Si ones, but higher capacitance resulted. For endurance properties with +5V unipolar pulse trains, the dielectric constant of BST films on Ir decreased by only 10% below its initial value after switching of 10⁹ cycles while that on Pt degraded by 30% after 10⁸ cycles. Ir bottom electrode effects on BST film properties were well explained by the formation of IrO₂ phases on the surface of Ir electrodes.     

Influence of Y and Mn alternately doped order on the microstructures and dielectric properties of Ba0.6Sr0.4TiO3 films

ABSTRACT

The influences of Y and Mn alternately doped order on the microstructures and dielectric properties of the Ba_0.6Sr_0.4TiO₃ (BST) films were reported in this paper. The Y and Mn alternately doped BST films were designed as YBST/MnBST/… and MnBST/YBST…multilayer films orderly expressed as (Y/Mn)_M and (Mn/Y)_M for short, and prepared on Pt/Ti/SiO₂/Si wafers by an improved sol-gel method, where Y and Mn represent yttrium doped BST layer and manganese doped BST layer, and M is cycle unit, respectively. The microstructures of the alternately doped BST films were observed by SEM and the capacitance-voltage curves at 100 kHz or 1 MHz were measured by a HP4284A LCR meter and the dielectric properties in the range of 1GHz were measured by an E4991A impedance analyzer. Compared to Y or Mn doped multilayer BST film, (Y/Mn)_M and (Mn/Y)_M show higher dielectric tunability and lower dielectric loss with higher dielectric constant. Moreover, (Y/Mn)_M show better dielectric properties than (Mn/Y)_M because (Y/Mn)_M show granular microstructures independent of M, while the (Mn/Y)_M show granular microstructures when M is 2 and add to a surface layer of columnar microstructures on the granular microstructures when M is 4. The related mechanisms were obtained in terms of the XRD phase structures, the cross-sectional SEM microstructures and the AFM morphologies.     

The effects of the LaNiO3 interlayer in the preferential orientation and the dielectrical properties of Ba x Sr1− x TiO3 thin films

Ba _x Sr_1?x TiO₃ (BST) thin films were grown on different substrates with or without LaNiO₃ (LNO) layer by a modified sol–gel process. The BST thin films obtained have full perovskite phase with dense and crack-free surface. BST thin films on different substrates with LNO layer show (100) preferential orientation, the texture coefficient (TC) value is calculated to be about 42.7%, whereas those without LNO layer show a fairly reduced preferential orientation, the TC value is just about 24.8%. It is considered that the preferential orientation is induced by the interface stress between LNO and BST. Electrical property measurements showed that BST thin film with a LNO interlayer has lower capacitance and larger dielectric loss, which is due to smaller grains of the thin film.     

Planar and Parallel Dielectric Properties of Compositionally Graded (Ba,Sr)TiO3 Thin Films for Tunable Microwave Applications

Compositionally graded (Ba_x,Sr_{1 ? x})TiO₃ [BST] ferroelectric thin films have been received much attention in graded ferroelectric devices due to their unique properties, such as large pyroelectric coefficients, large polarization offset and small temperature coefficient of dielectric constant for microwave tunable devices. Compositionally graded BST thin films were deposited epitaxially on LaAlO₃ [LAO] and Nb-doped SrTiO₃ [STO:Nb] substrates by pulsed laser deposition. The planar and parallel dielectric properties of compositionally graded BST epitaxial thin films ware investigated in the frequency ranges of 100 Hz ～ 1 MHz as a function of the direction of the composition gradient with respect to the substrate at room temperature. The dielectric properties of the graded BST films depended strongly on the direction of the composition gradient with respect to the substrate. The graded ST → BT films grown on LAO and STO:Nb substrates exhibited a excellent dielectric properties than the graded BT → ST films.     

Design and implementation of combline bandpass tunable filter using barium-strontium-titanate thin-film capacitors

The design, fabrication and microwave properties of tunable fifth-order combline bandpass filter using etched barium-strontium-titanate (BST) thin films on sapphire (0001) substrates were investigated. At 1 MHz and 1000 kV/cm electric field, the dielectric tunability, the remanent polarization (2P_r) and the coercive electric field (2E_C) of BST films were 45.96%, 2.26 µC/cm² and 81.83 kV/cm, respectively. The loss tangent was 1.36% at zero electric field. After the BST parallel plate capacitors characterization, BST capacitors were loaded at the end of parallel coupled resonators in the design of the tunable filter. With the application of 20 V DC voltage, the center frequency of the filter varied from 1.17 GHz to 1.34 GHz which corresponds to a relative shift of 13.5%.