Troy R. Taylor

Realization of high tunability barium strontium titanate thin films by rf magnetron sputtering

Ferroelectric thin films are currently being used to develop tunable microwave circuits based on the electric-field dependence of the dielectric constant. (Ba0.5Sr0.5)TiO3 (BST) films prepared by sputtering on Pt/TiO2/SiO2/Si substrates are found to exhibit a capacitance change (tunability) of nearly 4:1. Higher tunability has been attributed to the (100) texturing of the BST films and is a result of the biaxial tensile stress imposed by Si on BST making the polar axis oriented in plane. Electrical characterization shows that the dielectric permittivity increases with increase in film thickness (up to ∼200 nm).

A new high performance phase shifter using Ba/sub x/Sr/sub 1-x/TiO 3 thin films

In this paper, a new device topology has been proposed to implement parallel plate capacitors using Ba/sub x/Sr/sub 1-x/TiO/sub 3/ (BST) thin films. The device layout utilizes a single parallel capacitor and minimizes conductor losses in the base electrode. The new design simplifies the monolithic process and overcomes the problems associated with electrode patterning. An X-band 180/spl deg/ phase shifter has been implemented using the new device design. The circuit provided 240/spl deg/ phase shift with an insertion loss of only 3 dB at 10 GHz at room temperature. We have shown a figure of merit 93/spl deg//dB at 6.3 GHz and 87/spl deg//dB at 8.5 GHz. To our knowledge, these are the best figure of merit results reported in the literature for distributed phase shifters implemented using BST films at room temperature.

Monolithic Ka-band phase shifter using voltage tunable BaSrTiO/sub 3/ parallel plate capacitors

Monolithic Ka-band phase shifter circuit that employs voltage tunable BaSrTiO/sub 3/ (BST) parallel plate capacitors is presented here. The circuit is capable of continuous 0/spl deg/-157/spl deg/ phase shift at 30 GHz with an insertion loss of only 5.8 dB and return loss better than 12 dB. In addition to promising loss performance (27.1/spl deg//dB) at 30 GHz, the circuit reported here has several advantages over previously reported BST phase shifters such as moderate control voltages (20 V), room temperature operation, and compatibility with monolithic fabrication techniques.

Impact of thermal strain on the dielectric constant of sputtered barium strontium titanate thin films

Barium strontium titanate thin films were deposited by sputtering on Pt/SiO2 structures using five different host substrates: magnesium oxide, strontium titanate, sapphire, silicon, and vycor glass. These substrates were chosen to provide a systematic change in thermal strain while maintaining the same film microstructure. All films have a weakly textured microstructure. Temperature dependent dielectric measurements from 100–500 K determined that decreasing thermal expansion coefficient of the host substrate (i.e., larger tensile thermal strain) reduced the film dielectric permittivity. The experimentally determined Curie–Weiss temperature decreased with increasing tensile thermal strain and the Curie–Weiss constant increased with tensile strain as predicted by Pertsev et al. [J. Appl. Phys. 85, 1698 (1999)].

BaSrTiO/sub 3/ interdigitated capacitors for distributed phase shifter applications

In this letter, BaSrTiO/sub 3/ (BST) interdigitated capacitors on sapphire substrates have been investigated. The tunability and quality factor of interdigital capacitors have been optimized for microwave and millimeter wave applications. A monolithic K-band phase shifter circuit that employs voltage tunable BaSrTiO/sub 3/ (BST) interdigitated capacitors is presented here.

Microwave integrated circuits using thin-film BST

We discuss the development and potential applications of a microwave varactor technology using thin-film BST on inexpensive substrates. BST thin-films have been developed and optimized specifically for microwave integrated circuits, using both MOCVD-grown and RF magnetron-sputtered films. The material optimization efforts have concentrated on achieving high tunability and simultaneous low loss, and also developing robust electrode systems for circuit fabrication on silicon substrates. We present advances in monolithic microwave integrated capacitors and application to high performance and low-cost phase-shifters circuits. Efforts in frequency multiplication devices will also be presented.

Composition control and dielectric properties of bismuth zinc niobate thin films synthesized by radio-frequency magnetron sputtering

Radio-frequency sputtering was used to deposit near-stoichiometric Bi1.5Zn1.0Nb1.5O7 (BZN) films. Composition, crystallinity, and phase purity of the films were investigated by Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy (TEM). X-ray diffraction detected cubic pyrochlore in films that were annealed above 400 °C. Films were fully crystallized at 750 °C. TEM and electron diffraction confirmed the cubic pyrochlore structure of the grains. Dielectric constant and loss were measured using planar Si/SiO2/Pt/BZN/Pt and Al2O3/Pt/BZN/Pt capacitor structures, respectively. After annealing at 750 °C, films on Pt coated silicon wafers showed a permittivity of 170, low dielectric losses, and a large electric field tunability of the dielectric constant at a measurement frequency of 1 MHz. Dielectric loss tangents improved when substrates were moderately heated during deposition.Radio-frequency sputtering was used to deposit near-stoichiometric Bi1.5Zn1.0Nb1.5O7 (BZN) films. Composition, crystallinity, and phase purity of the films were investigated by Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy (TEM). X-ray diffraction detected cubic pyrochlore in films that were annealed above 400 °C. Films were fully crystallized at 750 °C. TEM and electron diffraction confirmed the cubic pyrochlore structure of the grains. Dielectric constant and loss were measured using planar Si/SiO2/Pt/BZN/Pt and Al2O3/Pt/BZN/Pt capacitor structures, respectively. After annealing at 750 °C, films on Pt coated silicon wafers showed a permittivity of 170, low dielectric losses, and a large electric field tunability of the dielectric constant at a measurement frequency of 1 MHz. Dielectric loss tangents improved when substrates were moderately heated during deposition.

Effect of thermal strain on the ferroelectric phase transition in polycrystalline Ba0.5Sr0.5TiO3 thin films studied by Raman spectroscopy

We have applied Raman spectroscopy to study the influence of thermal strain on the vibrational properties of polycrystalline Ba0.5Sr0.5TiO3 films. The films were grown by rf magnetron sputtering on Pt∕SiO2 surface using different host substrates: strontium titanate, sapphire, silicon, and vycor glass. These substrates provide a systematic change in the thermal strain while maintaining the same film microstructure. From the temperature dependence of the ferroelectric A1 soft phonon intensity, the ferroelectric phase transition temperature, TC, was determined. We found that TC decreases with increasing tensile stress in the films. This dependence is different from the theoretical predictions for epitaxial ferroelectric films. The reduction of the ferroelectric transition temperature with increasing biaxial tensile strain is attributed to the suppression of in-plane polarization due to the small lateral grain size in the films.

Influence of stoichiometry on the dielectric properties of sputtered strontium titanate thin films

The dielectric permittivity, dielectric quality factor (inverse dielectric loss), and lattice parameter of 140 nm sputtered SrTiO3 films were dependent on the oxygen partial pressure and total chamber pressure (O2+Ar) during film growth. Films were grown at 25 and 75 mTorr (mT) in an oxygen rich and oxygen deficient sputtering gas environment concurrently on (100) SrTiO3 and (111) Pt/(0001) Al2O3 substrates. Films were deposited on platinized sapphire for electrical characterization and the homoepitaxial films were used as a structural and chemical standard. High resolution triple axis x-ray diffraction results showed an increase in mismatch between the film and substrate (200) peak in homoepitaxial SrTiO3 films with higher total growth and lower oxygen pressures. Dielectric quality factors of the SrTiO3 films on platinized sapphire at 1 MHz for the 25 mT (50 sccm Ar/50 sccm O2), 25 mT (90 sccm Ar/10 sccm O2), 75 mT (50 sccm Ar/50 sccm O2), and 75 mT (90 sccm Ar/10 sccm O2) film growths were 320, 251, 209...

High-isolation BST-MEMS switches

In this paper, emerging (Ba,Sr)TiO/sub 3/ thin film technology was investigated for enhancing RF-MEMS capacitive switches. Materials properties of high-permittivity BST thin films and fabrication issues are discussed. Prototype BST-MEMS switches for K-/Ka-band applications were fabricated and measured. This measured data is compared with measurements from conventional SiN-based MEMS switches, showing improved isolation at lower frequencies due to the higher down-state capacitance density.