SrTiO3 thin films by mocvd for 1 gbit DRAM application

Abstract

Three important aspects of the preparation of SrTiO₃ thin films by MOCVD are discussed in detail in view of the application of these films as the capacitor dielectric of Gbit-scale DRAMs: CVD reactions in the Sr(DPM)₂-Ti(i-OC₃H₇)₄-O₂ system, step coverage and relations between microstructure and electrical properties. The effect of the substrate temperature on the Sr and Ti deposition rates was first investigated for thermal and ECR CVD SrTiO₃ films. SrO and TiO₂ deposition by thermal CVD above 550°C were found to be controlled by the surface reaction and gas transport, respectively, whereas both SrO and TiO₂ deposition are controlled by gas transport for ECR CVD at 450 to 600°C. The influence of the Sr and Ti deposition regimes on the step coverage of SrO, TiO₂ and SrTiO₃ were then assessed. SrO films prepared by thermal CVD at 600°C exhibited the best step coverage, indicating that a relation exists between reaction controlled deposition and good step coverage. The effect of the film composition and film thickness on the microstructure of SrTiO₃ thin films were finally investigated and correlations were made to other analyzed physical and electrical properties. Polycrystalline perovskite phase SrTiO₃ films were obtained for a composition 0.7 ≤ Sr/Ti ≤ 1.2. The best crystallinity, maximum permittivity and maximum refractive index were obtained for Sr/Ti = 0.95. Titanium rich films are thought to be composed of a mixture of a titanium rich amorphous phase and crystalline SrTiO₃, and strontium rich films are believed top correspond to a (SrTiO₃)_m (SrO)_n structure. The dielectric constant slowly decreased as the film thickness was reduced. The sharp decrease observed near 400–500 Å could be due to the existence of some perturbed layer at the interface with one or both of the electrodes     

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.     

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.     

Scaling Effect on the Dielectric Constant in Ba(Ti x Zr1−x )O3 Thin Films

Recent work on PZT and BST thin films reveal a thickness dependence of the dielectric constant for a film thickness below 100 nm. This effect is commonly attributed to an interfacial layer between the electrode and the dielectric film (dead layer). In this contribution we report on the influence of the film thickness on the dielectric constant of Ba(Ti_xZr_{1 – x})O₃ thin films with different Zr-contents (x = 0–30 at.%). The films were prepared by chemical solution deposition (CSD) with thickness between 30 and 350 nm.The electrical characterization was performed in a temperature range between 25 and 200C. Results were interpreted with respect to the formation of a serial dead layer capacitance.     

Frequency and Voltage Dependent Dielectric Properties of Ni-doped Ba0.6Sr0.4TiO3 Thin Films

Highly (100) preferred undoped and 1–5% Ni-doped Ba_1–xSr_xTiO₃ (BST) thin films were deposited onto MgO (100) single crystal substrate at 750°C using pulsed laser deposition. BST thin film-based interdigital capacitors (IDC) were prepared by standard photolithography process. The microwave properties of BST films were measured at 10 GHz. Ni-doped BST films showed better dielectric properties by exhibiting improved dielectric Q while retaining an appropriate capacitance tuning compared to undoped BST films. 1% Ni-doped BST film showed the maximum figure of merit of 2896.1. It is suggested that 1 mol% Ni doped BST film is an effective candidate for high performance tunable device applications.     

Growth and Dielectric Properties of Ferroelectric BaTiO3 Thin Films for Cantilever-Type Microsensors

ABSTRACT

Ferroelectric BaTiO₃ (BTO) thin films were deposited on Si, silicon-on-insulator (SOI) and MgO substrates by pulsed laser deposition. The orientations of the films, polycrystalline and epitaxial phase, were controlled by the lattice mismatch between the BTO film and substrates. The structural properties and surface morphologies were examined using X-ray diffractometer and atomic force microscope. The dielectric properties of BTO films were investigated using metal-ferroelectric-metal (MFM) and interdigital co-planar capacitors. Conductive oxide layers, SrRuO₃(SRO) and La_0.5Sr_0.5CoO₃ (LSCO), were grown on Si and SOI substrates as bottom electrodes. For MFM capacitors based on Au/BTO/SRO/Si and Au/BTO/LSCO/SOI layer structures, a little asymmetric capacitance-voltage curves were obtained with about 36% capacitance tunability. The remanent polarizations were about 21 μC/cm² and the coercive fields were about 71 kV/cm. For an interdigital capacitor based on Au/BTO/MgO layer structure, a little lossy capacitance-voltage curve was obtained with about 64% capacitance tunability.     

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.     

Low frequency and microwave performances of Ba0.6Sr0.4TiO3 films on atomic layer deposited TiO2/high resistivity Si substrates

We report on high tunablity of Ba_0.6Sr_0.4TiO₃ (BST) thin films realized through the use of atomic layer deposited TiO₂ films as a microwave buffer layer between BST and a high resistivity (HR) Si substrate. Coplanar waveguide (CPW) meander-line phase shifters using BST/TiO₂/HR-Si and BST/MgO structures exhibited a differential phase shift of 95^∘ and 24.4^∘, respectively, at 15 GHz under an electric field of 10 kV/cm. The figure of merit of the phase shifters at 15 GHz was 30.6^∘/dB for BST film grown on a TiO₂/HR-Si substrate and 12.2^∘/dB for BST film grown on a MgO single crystal substrate. These results constitute significant progress in integrating BST films with conventional silicon technology.     

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.     

INTEGRATED INFRARED DETECTORS BASED ON (Ba0.65Sr0.35)TiO3 THIN FILMS

ABSTRACT

The integrated single element infrared detectors were developed for high-loading smart munition application using (Ba_0.65Sr_0.35)TiO₃ (BST) pyroelectric films and SiO₂ thermal insulating layer. (Ba,Sr)RuO₃ (BSR) seeding-layer was deposited on the Pt/Ti/SiO₂/Si to induce BST films c-axis preferred orientation growth. The capacitance dependant with temperature of BST films were measured at the temperature ranging from 230 K to 340 K, releasing that the temperature coefficient of capacitance was 1.6%/K. The pyroelectric coefficient of BST films was 7.45 × 10^{? 7} C/K cm² measured by dynamic method. Infrared response evaluation of the fabricated sensor has been carried out with D* of 6.34 × 10⁷ cm Hz^1/2/W indicating that the integrated infrared detectors have potential application as gun-launched smart munition seeker.     

Thickness dependent ferroelectric properties of BSTO thin films deposited by RF magnetron sputtering

The thickness dependence of ferroelectric permittivity of (Ba, Sr)TiO₃ has been investigated. The BST films could be obtained to have a simple cubic perovskite structure, space group Pm3m, and practically c-axis epitaxial structure deposited at 800^∘C. Through post-annealing process, we have improved the dielectric properties; dielectric permittivity, dielectric loss, and tunability. The change in dielectric properties before and after annealing is attributed to the change in film strain and the contraction in film lattice. As the thickness of BST films increases from 55 nm to 350 nm, the dielectric constant of BST films increases from about 100 to above 670 due to the reduction of interfacial dead layers with low dielectric constant between films and top electrodes. The dielectric loss of BST thin films decreased as the thickness increases. The existence of interfacial dead layers in a thinner film had a larger effect on the effective dielectric constant than tensile strain between the BST films and MgO substrate.     

Variations of interface potential barrier height and leakage current of (Ba,Sr)TiO3 thin films deposited by sputtering process

Abstract

Variations of the leakage current behaviors and interface potential barrier height (Φ_B) of rf-sputter deposited (Ba, Sr)TiO₃ (BST) thin films with thicknesses ranging from 20 nm to 150 nm are investigated as a function of the thickness and bias voltages. The top and bottom electrodes are dc-sputter-deposited Pt films. Φ_B critically depends on the BST film deposition temperature, postannealing atmosphere and time after the annealing. The postannealing under N₂ atmosphere results in a high interface potential barrier height and low leakage current. Maintaining the BST capacitor in air for a long time reduces the Φ_B from about 2.4 eV to 1.6 eV due to the oxidation. Φ_B is not so dependent on the film thickness in this experimental range. The leakage conduction mechanism is very dependent on the BST film thickness; the 20 nm thick film shows tunneling current, 30 and 40 nm thick films show Schottky emission current.     

The resistance degradation of (Ba0.5Sr0.5)TiO3 thin films

Abstract

The leakage current and dielectric properties of (Ba_0.5Sr_0.5)TiO₃(BST) thin films prepared by pulsed laser deposition (PLD) were investigated. It was found that leakage currents for positive bias voltage were higher than that for negative bias voltage, which was attributed to the lattice mismatch between bottom Pt electrode and BST thin film. The time-dependent breakdown process under positive voltage was observed, which was interpreted as the increase of the internal electric field in the film near the bottom electrode. However, the internal electric field can be decreased and eventually recovered by applying negative bias voltage. It was found that internal electric field near the interface can influence the capacitance of the BST thin film capacitor. An explanation for the thickness effect of BST thin films was given.     

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.     

Magnetron sputter-deposited multilayer (BaxSr1x)Ti1+yO3+z thin films for passive and active devices

Abstract

High permittivity (Ba_xSr_1?x)Ti_1+yO_3+z(BST) thin films are being investigated for integration into charge storage dielectrics and electric-field tunable elements for high frequency devices. For the latter application, it is desirable to have BST capacitors with high tunability and low losses. Therefore, we investigated the use of multilayer BST thin films consisting of very low dielectric loss BST/electrode interfacial layers ((Ba+Sr)/Ti = 0.73) sandwiching a high tunability, high permittivity primary BST layer ((Ba+Sr)/Ti = 0.9). BST capacitors with multiple layers of controlled composition can be effectively produced insitu by magnetron sputter deposition, using a single stoichiometric target and controlling the layer composition by changing the total process gas (Ar+O<₂) pressure. The layered BST film capacitors exhibit simultaneous low loss (tan Δ = 0.005), high tunability (76%), high charge storage energy density (34 J/cm³), low leakage, and high dielectric breakdown (>2.8 MV/cm).     

Epitaxial Ba0.5Sr0.5TiO3 thin films for microwave phase shifters

Abstract

A tunable phase shifter was fabricated with epitaxial Ba_0.5Sr_0.5TiO₃ (BST) thin film and gold coplanar waveguide. BST thin film of the thickness ～0.5 μm was deposited by laser ablation on the MgO(OOl) single crystalline substrate. Gold electrode of the thickness ～2 μm was prepared by the sequence of thermal evaporation, electroplating, and wet etching. Epitaxial quality of the BST thin film was confirmed by X-ray diffraction. The microwave performance of phase shifter was measured at room temperature in the frequency range of 8–12 GHz, and with applied bias voltage of up to 30 V. Effect of Mn dopant in the epitaxial films was also considered.     

Interface potential barrier height and leakage current behavior of Pt/(Ba,Sr)TiO3/Pt capacitors fabricated by sputtering process

Abstract

Variations of the leakage current behaviors and interface potential barrier height (φ _B ) of rf-sputter deposited (Ba, Sr)TiO₃ (BST) thin films, with thickness ranging from 20nm to 150 nm are investigated as a function of the thickness and bias voltages. The top and bottom electrodes are dc-sputter-deposited Pt films. φ _B critically depends on the BST film deposition temperature, postannealing atmosphere and time after the annealing. The postannealing under N₂ atmosphere results in a high interface potential barrier height and low leakage current. Maintaining the BST capacitor in air for a long time reduces the φ _B from about 2.4 eV to 1.6eV due to the oxidation. φ _B is not so dependent on the film thickness in this experimental range. The leakage conduction mechanism is very dependent on the BST film thickness; the 20nm thick film shows tunneling current, 30 and 40 nm thick films show Schottky emission current and the thicker films show a mixed characteristics and bulk and interface limited currents although the mechanism is not clearly understood at this moment.     

Analysis of saturation currents and barrier height of Ta2O5 doped based on Ba0,5S0,5TiO3 photodiode

Ba_0,5Sr_0,5TiO₃ (BST) thin films have been prepared on Si (100) p-type substrates using a chemical solution deposition (CSD) method and doped with 0%, 2.5%, 5%, 7.5%, 10% tantalum pentaoxide (Ta₂O₅). Chemical Solution Deposition Method (CSD) used the spin coating techniques with a rotational speed of 3000?rpm for 30?seconds. BST thin films annealed at a temperature 850?°C. Various electrical parameters such as saturation current, series resistance and barrier height have been calculated from the analysis of experimental I–V results and discussed in detail. The series resistance was found from the experiment of 42.8 MΩ, 7.9 MΩ, 7.2 MΩ, 2.03 MΩ, 1.2 MΩ for variation doping content. The saturation current of 22.3 μA, 2.7 μA, 9.7 μA, 4.82 μA, 4.50 μA was obtained in BST thin film with variation doping content. Then, the barrier height of 0.56?eV, 0.62?eV, 0.58?eV, 0.54?eV, 0.60?eV was obtained in BST thin film with variation doping content. The optical characterization and analysis microstructure such as XRD, EDX were discussed in detail.     

Microwave Characterization of BST Thin Films on LAO Interdigital Capacitor

The dielectric properties of (Ba_0.5Sr_0.5)TiO₃ (BST) thin films on LaAlO₃ (LAO) single crystals were studied over a wide frequency range. The samples with interdigital electrodes were prepared by microelectronic processing. The dielectric characterizations were carried out in the following steps: 1) the standard calibration of the instrument, 2) the removal of parasitic capacitance and the extraction of the capacitance of the interdigital capacitor (IDC) and 3) the extraction of the dielectric permittivity (?) of BST. It was found that ? of BST has a constant value (about 400) from 50 MHz to 2 GHz; at higher frequencies, ? gradually decreases. The BST interdigital capacitors exhibited good dielectric tunability.     

Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract

Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002–0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 °C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO₂/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1–0.2 urn. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO₂ interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 μm. Devices with dielectric Q factors over 150 at 100 MHz (tan δ ～ 0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.