Rand Biggers

A Si MMIC compatible ferroelectric varactor shunt switch for microwave applications

This letter describes a ferroelectric thin-film based varactor shunt switch, for microwave and millimeterwave switching applications. Our implementation is based on a coplanar waveguide transmission line shunted by a ferroelectric varactor. The concept of switching ON and OFF is based on the dielectric tunability of the ferroelectric barium strontium titanium oxide (BST) thin-films. From experimental verification, the isolation of a switch with a varactor area of 75/spl mu/m/sup 2/ was approximately 20dB at 35GHz and the insertion loss was below 4.5dB up to 35GHz. This letter addresses the design and experimental verification of the first ferroelectric varactor shunt switch with an Si monolithic microwave integrated circuit compatible process.

Synthesis and Characterization of Nanostructured BSTO Thin-Films for Microwave Applications

Nanophase synthesis of ferroelectric thin-films of Ba0.6Sr0.4TiO3 (BSTO) was studied systematically for applications in tunable microwave components. Synthesis of nanostructured BSTO was performed using a pulsed-laser deposition system with real-time in-situ process control. The main research goal was to utilize the pulsed laser deposition parameters to control the grain growth for low microwave loss nanostructured BSTO thin-films on crystalline substrates such as LaAlO3. These parameters include the energy density of the laser pulses, wavelength, oxygen partial pressure, distance between the target and the substrate, and the substrate temperature. The nanostructural characterization was performed using XRD, SEM and AFM. Microwave characterization was done using coplanar waveguide lines to characterize the frequency dependent dielectric properties (ϵr and tan δ). BSTO films were grown at the same measured temperature and energy density but in different oxygen ambient pressures from 19 mTorr through 300 mTorr. Using contact mode AFM, the grain size was found to decrease as the oxygen ambient pressure was reduced from 150 mTorr to 38 mTorr. The growth process changed when the pressure was increased above 150 mTorr. Nanocluster structures rather than nanoparticles were found at 225 mTorr. Average grain sizes less than 100 nm were obtained to oxygen pressures below 75 mTorr. The XRD spectra indicate the highly crystalline nature of the film. Microwave measurements, performed between 9–18 GHz, suggest the nano-structured BSTO thin-films on LaAlO3 (LAO) substrates are highly tunable (up to 25%).

New Research Directions in Tunable Microwave Dielectrics

Nano-structured Ba0.6Sr0.4TiO3 (BST) thin-films are currently being investigated by our group for applications in tunable microwave devices and circuits. This work is performed in collaboration with Air Force Research Laboratory Materials Directorate. We have synthesized low microwave-loss nano-structured columnar BST thin-films using a self-directed process-controlled pulsed laser deposition system. The average grain size of the films was controllable from ∼ 20 nm to ∼ 150 nm without any clustering by increasing the oxygen ambient pressure from 38 mT to 150 mT. The films with lower average grain size have been found to be tunable and low loss up to 50 GHz. Precise grain size control has resulted in low loss tunable high-K dielectric for variety of applications. Two new research applications are pursued by our group. One is in the design and development of electromagnetic band gap structures (EBGs) and the other in the design and development of capacitive shunt switches for microwave applications.

Development of nickel alloy substrates for Y-Ba-Cu-O coated conductor applications

Fabrication of long-length, textured substrates constitute a critical step in the successful application of coated High Temperature Superconductors (HTS). Substrate materials stronger than nickel are needed for robust applications, while substrates with non-magnetic characteristics are preferred for AC applications. The present work is thus focused on development of texture in high strength, non-magnetic substrate materials. As the development of cube texture is easier in medium to high stacking fault energy materials, binary alloys based on nickel were evaluated for the present application. High purity alloys were melted and hot/cold worked to obtain thin tapes. The development of texture in these alloys as a function of processing parameters was studied by X-ray diffraction and metallographic techniques. Orientation Imaging Microscopy (OIM) was used to quantify the extent of texture development in these substrates. Results to date on the development of texture by thermo-mechanical processing of these alloys are presented.

Characterization of local dielectric properties of superconductor YBa2Cu3O7-δ using evanescent microwave microscopy

A near-field evanescent microwave microscope based on a coaxial transmission line resonator with a tungsten tip protruding through an end-wall aperture is used to measure local dielectric properties of thin film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// in superconducting state below critical temperature T/sub c/=91 K at T=79.4 K and in normal state at room temperature (T=298 K). The dielectric property of the superconductor within the near field of the tip frustrates the electric field and measurably changes the transmission lines resonant frequency. The shift of the resonators frequency is measured as a function of tip-sample separation and associated change in quality factor (Q) image scans of the thin film is obtained. A quantitative relationship between the real and imaginary parts of the local dielectric constant and the frequency shift using the method of images is established. The comparison between experimental data and theory based on this method is given and discussed for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film deposited on LaAlO/sub 3/ substrate.

YBa2Cu3O7−x–Ag thick films deposited by pulsed laser ablation

Abstract Ag-doped YBa 2 Cu 3 O 7− x films, with thickness ranging from 0.06 to 2.5 μ m, were deposited by pulsed laser ablation onto (100) LaAlO 3 single-crystal substrates. The target was YBa 2 Cu 3 O 7− x with 5 wt.% Ag addition. The presence of Ag in the films in concentrations of ∼1 at.% was detected by X-ray fluorescence and secondary ion mass spectrometry (SIMS) analysis. Biaxial alignment of the films was indicated by φ scans with full-width-half-maximum (FWHM) spread of 1–2° for various thicknesses. Utilizing a standard deposition process, most films showed a critical transition temperature ( T c ) >90 K as measured by the ac susceptibility technique. Film critical current densities ( J c ) on the order of 10 6 A/cm 2 were measured at 77 K with a four-probe technique on a 100- μ m-wide patterned microbridge.

Spectral-component monitoring and control of pulsed laser deposition of YBCO films

We examine the time-resolved spectral components emitted at approximately 327 nm and approximately 550 nm in YBCO plumes during pulsed laser deposition of thin films using a KrF excimer laser at (lambda) equals 248 nm. The studied emission signals last for approximately 20 microsecond(s) ec, and show variations when process parameters such as laser power, laser excitation voltage, beam focus, chamber pressure, substrate temperature, pulse repetition rate, and target rotation rate are changed. These signals are also dependent on other factors such as target wear and age of the laser gas mixture. Spectral-component monitoring is a supplementary method of real-time plume evaluation, and allows observation of changes both prior to deposition and during the actual deposition. Adjustments can be made to the process parameters to make the plume conform to criteria necessary for the growth of films with specific qualities. The use of these spectral components as real-time process- control state variables for more reproducible fabrication of high quality thin films will be assessed.

Effect of grain orientation and local strain on the quality of polycrystalline YBa 2 Cu 3 O 7 superconductive films

The critical current densities of superconducting thin films and their dependence on the film structural characteristics has been a major research interest for more than a decade. Controlling this relationship is crucial if large-scale high-quality YBa 2 Cu 3 O 7 (YBCO) tapes are to be produced. Two major keystones of information have been established in this field. Firstly, there is a direct relationship between the critical current density and the grain-boundary angle in polycrystalline YBCO films. Grain boundaries with a mismatch angle higher than 5° usually result in reduced critical current densities. This detrimental effect of large-angle grain boundaries to the quality of YBCO films has been attributed to strain fields resulting from such grain boundaries. Secondly, the quality of the YBCO film can be enhanced by straining its lattice in specific direction. Here, we report, for the first time, direct experimental results coupling local grain orientation and local strain maps of thin YBCO films deposited on a (001) biaxially textured nickel substrate. These results were correlated to the quality of the film and showed how grain structure in the nickel substrate affects the grain structure in the YBCO films even in the presence of several buffer layers. More importantly, the data show that highquality films with high critical current densities can be produced, in spite of large-angle grain boundaries, if the film is compressed in the range of 0.5% strain normal to the a axis.

A New Ferroelectric Varactor Shunt Switch for Microwave and Millimeterwave Reconfigurable Circuits

Abstract : This paper presents a ferroelectric varactor shunt switch which can be useful for microwave/millimeter wave switching as well as for the design of reconfigurable circuits. The device operation is based on nonlinear dielectric tunability of a ferroelectric thin-film sandwiched between two metal layers in the parallel plate configuration. A CPW based design allows for MMIC compatible shunt switches with low insertion loss and high isolation. Experimental performance of the varactor shunt switch indicates good switching performance with ^24 dB isolation 41 GHz, and insertion loss below 7 dB up to 45 GHz.

XPS depth profiling studies of YBCO layer on buffered substrates

XPS studies of a typical YBCO coated conductor architecture were conducted to investigate the chemical and microstructural profiles. The detailed XPS depth profiling study was performed on one sample (YBCO/CeO2/YSZ/CeO2/Ni) of coated conductor. The chemical depth profiling involved bombarding a small area of the specimen surface with 3 keV Ar+ ions and analyzing the freshly exposed surface after each bombardment. Results of the process show that the Y(3d) photo-electronic peak shape in these films is very different from bulk (sintered and oxygen annealed) YBCO superconductors and surface analysis of YBCO thin films. This may indicate a possible difference in the atomic co-ordination between some laser-ablated films and bulk sintered ones. The correlation between chemical binding states of the ions and superconducting properties need to be investigated in details. The lower portion of the YBCO film showed distinct signs of contamination including Ce, and traces of Ni. An initial report of Zr diffusion into...