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.     

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.     

Spectroscopic ellipsometric determination of optical properties of V-Al co-doped ZnO films by rf magnetron sputtering

Zn_0.9?xV_0.1Al_xO aerogel nanopowders were prepared in thin film form on glass substrates using a rf magnetron sputtering system. The films were characterized by Scanning electron microscopy (SEM) and X-ray diffraction technique (XRD). The XRD results indicate that all the films have c-axis preferred orientation due to self-texturing mechanism. The ellipsometric spectra of the films were recorded in the photon energy range of 1 eV–5 eV. The SE spectra were analyzed with an appropriate model to accurately determine the thickness and optical constants of the ZnO:(V,Al) thin films. The profiles of refractive index and extinction coefficient with photon energy were extracted. The refractive index of the ZnO:(V,Al) film is decreased from 2.14 to 2.07 with increasing Al concentration and then is increased to 2.19 for x?=?0.04. A maximum band gap energy of ~3.57 eV was obtained for x?=?0.02. The optical band gaps of the films were found to vary from 3.57 eV to 3.41 eV, with Al content. It is evaluated that the optical constants of the ZnO:(V,Al) films can be controlled by Al content.     

Structural and dielectric properties of epitaxial Ba0.6Sr0.4TiO3 thin films grown on Si substrates with thin SrO buffer layers

(100) epitaxial Ba_0.6Sr_0.4TiO₃ (BST) thin films were grown on Si substrates using a 9 nm thick SrO buffer layer. The phase shifter fabricated on BST films grown on a SrO buffered Si substrate showed a larger figure of merit (FOM) of 24.7°/dB as a result of improving the phase tuning while retaining an appropriate insertion loss compared to that (15.3°/dB) for the BST/MgO structure. This work demonstrates that a thin SrO buffer layer plays an important role in the successful integration of BST-based microwave tunable devices onto Si wafers.     

Lateral Epitaxial Growth of Ba0.6Sr0.4TiO3 Thin Films

We report a novel growth technique for epitaxial thin films by combination of selective heteroepitaxial growth and lateral homoepitaxial growth. Ba_0.6Sr_0.4TiO₃ (BST) thin films were deposited on the substrates having patterned SiO_x layers at 450°C using pulsed laser deposition. Post annealing was carried out thereafter for lateral epitaxial growth. The difference in the crystallization temperature of BST thin film on the amorphous masking layers and lattice-matched single crystalline substrate enables selective nucleation and heteroepitaxial growth from the regions of single crystalline substrates during the film deposition. Lateral homoepitaxial growth is expected from the crystallized BST thin film toward the amorphous BST on SiO_x during the post annealing process. In this paper, a study on the difference in nucleation and growth behavior of BST thin films on the amorphous masking layers and lattice-matched single crystal substrates is presented.     

Crystalline and optical properties of PLZT films prepared by pulsed laser deposition

Abstract

The PLZT (3/66/34), PLZT (3/46/54), PLZT (9/65/35) and PLZT (28/0/100) thin films were deposited on MgO (100), Sapphire (0001) and fused silica substrates by using pulsed KrF excimer laser deposition technique. The conventional in-situ and 2-step heating processes were utilized to facilitate the synthesis of a large area of uniform PLZT thin film. Pure perovskite phase can be obtained only under very narrow process conditions, the highly textured PLZT films can be easily obtained by in-situ heating process. For PLZT (28/0/100) material, epitaxial films were successfully coated on MgO (100) substrates by 600°C in-situ and RTA 650°C 2-step heating processes. The latter was found to possess higher refractive index and lower extinction coefficient.     

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°.     

Influence of Space Charge Layers on the Dielectric Properties of Ba0.6 Sr0.4TiO3 Nanostructured Thin Films

Barium strontium titanate (Ba_0.6Sr_0.4TiO₃) nanostructured thin films have been deposited on platinized silicon substrates by the sol-gel method. The as-fired films were found to be amorphous, which crystallize to cubic phase after annealing at 550°C in air for one hour. The low-frequency dielectric responses of the BST films were measured as functions of frequency range from 1 kHz to 1MHz. The thickness dependence dielectric constant of the BST thin films were measured in the temperature range from ?150°C to 150°C at 100 kHz and discussed in the light of an interfacial dead layer. All the samples showed a diffuse type phase transitions. Both the dielectric constant and loss tangent showed anomaly peaks at about 10°C, which corresponds to a tetragonal ferroelectric-to-cubic paraelectric phase transition.     

Ferroelectric properties of Ba1−x Sr x TiO3 thin films synthesized by using novel sol–gel technique through carbonates

Ba_1?x Sr _x TiO₃ (BST) thin films were prepared on the substrate of Pt/Ti/SiO₂/Si by using novel sol–gel process through carbonates. The surface morphology and domain contrast of the films were investigated by atomic force microscopy (AFM), and the domain structures of the BST film were observed when AFM were operated in piezoelectric force microscopic (PFM) analysis and in the friction mode (FFM). The ferroelectric properties of the films were also investigated. It is shown that BST films obtained by the new sol–gel process through carbonates exhibit good properties.     

Influence of the growth temperature on the epitaxial quality of PbTiO3 films deposited in-situ by sputtering

Abstract

Thin films of lead titanate were prepared in-situ on SrTiO₃ substrates using radio-frequency magnetron sputter deposition. The epitaxial quality of the films has been studied as a function of the substrate temperature. Stoichiometric films have been obtained in the temperature range [550°C, 600°C]. The films deposited in the equilibrium zone have a high degree of c-axis oriented epitaxial crystalline structure as shown by X-ray diffraction in the 2θ/θ, θ, and ? scans configuration as well as by electron channeling pattern. The optimum conditions for growing epitaxial PbTiO₃ layers were determined. The crystallinity of films deposited at 550°C is suprior to those deposited at 600°C. The PbTiO₃ films grown at 550°C have a rocking curve full width at half maximum (FWHM) of 0.2°; Normaski optical and atomic force microscopy show that the surface is apparently free of grain boundaries and very smooth. The refractive index of these films has been evaluated from transmission spectra; it is very close to the bulk material value.     

Science and Technology of High Dielectric Constant Thin Films and Materials Integration for Application to High Frequency Devices

Thin films of Ba_1?x Sr _x Ti_1+y O_3+z (BST), were fabricated using both by RF-magnetron sputtering and MOCVD to demonstrate application to high frequency devices. Precise control of composition and microstructure is critical for the production of (Ba _x Sr_1?x )Ti_1+y O_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 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. BST films with a multilayer structure were also developed with different Ti-elemental ratio in each layer to minimize losses and leakage current. Interfacial contamination from C and H species was studied and implications on electrical properties are highlighted. Finally, York's group at the University of California-Santa Barbara successfully integrated our BST films onto phase shifter arrays. The results show potential of BST films in such applications. Results from initial work on the integration of Cu-electrodes with BST films are also presented.     

Characterization of Ba0.55Sr0.45TiO3 films as light and temperature sensors and its implementation on automatic drying system model

The growth of Ba_0.55Sr_0.45TiO₃ films on p-type silicon substrate with depletion and enhancement treatments have been conducted in this research. The aims were to examine film sensitivity as light sensor and value range, resolution, acuracy level, and their hysteresis as temperature sensor. The films were annealed at 800, 850, and 900 °C for 15 hours. In this work, enhancement BST of 850 °C was the best quality film and utilized as light and temperature sensors. Its implementation has been successfully conducted on ATMega8535 microcontroller-based automatic drying system model by exploiting the working principle of the BST films as automatic switch.     

Low dielectric dissipation and enhanced tunability of Ba0.6Sr0.4TiO3 thin films by the modified composition and multilayer structure

Ba_0.6Sr_0.4Ti_1+yO₃ (BST, y?=?0.1, 0.15, 0.2, 0.25, 0.3) thin films were fabricated on Pt-coated silicon substrates by modified sol-gel techniques. It was found that the tunability of BST thin films and dissipation factor decreased with the increase of Ti content. The multilayer structure of Ba_0.6Sr_0.4Ti_1+yO₃(200 nm)/Ba_0.6Sr_0.4TiO₃(100 nm)/Ba_0.6Sr_0.4Ti_1+yO₃ (200 nm; y?=?0.1, 0.2, 0.25) was designed to enhance the tunability. Our results indicated that the modified composition and multilayer structure were beneficial to lowering the dielectric dissipation and enhancing the tunability simultaneously. The tunability of 26.7% and dielectric dissipation of 0.013 were achieved for modified BST thin films.     

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.     

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.     

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.     

Microwave Performance of Distributed Analog Phase Shifter Using Ferroelectric (Ba,Sr)TiO3 Thin Films

This work presents the design, fabrication and microwave performance of distributed analog phase shifter (DAPS) fabricated on (Ba,Sr)TiO₃ (BST) thin films for X-band applications. Ferroelectric BST thin films were deposited on MgO substrates by pulsed laser deposition. The DAPS consists of high impedance coplanar waveguide (CPW) and periodically loaded tunable BST interdigitated capacitors (IDC). In order to reduce the insertion loss of DAPS and to remove the alteration of unloaded CPW properties according to an applied dc bias voltage, BST layer under transmission lines were removed by photolithography and RF-ion milling. The measured results are in good agreement with the simulated results at the frequencies of interest. The measured differential phase shift based on BST thin films was 24° and the insertion loss decreased from ?1.1 dB to ?0.7 dB with increasing the bias voltage from 0 to 40 V at 10 GHz.     

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.     

CHARACTERIZATIONS OF W-DOPED (Ba0.5Sr0.5)TiO3 THIN FILMS USING LaNiO3 ELECTRODE

ABSTRACT

The (Ba_0.5Sr_0.5)TiO₃ thin films were deposited onto LaNiO₃ by RF-magnetron sputtering at 550°C. The influence of W content on microstructure and electrical properties of BST films were investigated. The surface and grain size become smoother and smaller as the W content increased. Besides, the dielectric constant, tunability, dissipation factor, and leakage current decreased when the W content increased. The 1% W is the optimal doping concentration. The resultant BST film, with proper dielectric constant and leakage current, has a tunability of 32%, a dissipation factor of 0.006, a FOM value of 53.3 under applied field of 450 kV/cm.