Selective area growth of thin films reduces the number of steps in microfabrication processing and enables novel device structures. Here, we report, for the first time, selective area epitaxy of an oxide material on a GaN surface. Chlorination of the GaN surface via wet chemical processing is found effective to disrupt Mg adsorption and selectively prevent molecular beam epitaxy growth of MgO. MgO films grown on neighboring, nonchlorinated surfaces are epitaxial with a (111) MgO∥(0001) GaN crystallographic relationship. Better than 3 μm lateral resolution for the selective area growth of MgO on GaN is demonstrated.

Ba0.7Sr0.3TiO3 thin polycrystalline films with an ultra-high capacitance tunability approaching 5: 1 at 175 kV/cm were made possible by a flux-assisted synthesis approach. In this process, a small volume fraction of a low melting temperature glass is added during low-temperature sputter deposition. Subsequent annealing activates the liquid phase, which in turn provides the mass transport needed to approach full density, to increase grain size, and to improve crystallinity, and, in so doing, achieves a stronger non-linear dielectric response. Ba0.7Sr0.3TiO3 films with 0%, 1%, 4%, and 7% BaO-3B(2)O(3) flux exhibited grain sizes of 25 nm, 28 nm, 48 nm, and 56 nm, and dielectric tunabilities of 25%, 33%, 64%, and 80% respectively. These values represent substantial improvements when compared to conventionally processed tunable dielectric films. (C) 2014 AIP Publishing LLC.

The addition of a liquid-forming flux to barium titanate thin films promotes densification and grain growth, improves nonlinear dielectric properties, and allows residual strain to be sustained in polycrystalline films without cracking at thicknesses relevant to device fabrication. Relative tuning, an excellent indicator of crystalline quality and an important material property for tunable microwave devices, increases from 20% to 70%. Films exhibit 0.15% residual differential thermal expansion mismatch strain, resulting in a shift to the paraelectric-ferroelectric phase transition of 50 °C. This result is in excellent agreement with theory, demonstrating the ability to tune ferroic transitions without epitaxial approaches.

A long‐standing challenge to the widespread application of complex oxide thin films is the stable and robust integration of noble metal electrodes, such as platinum, which remains the optimal choice for numerous applications. By considering both work of adhesion and stability against chemical diffusion, it is demonstrated that the use of an improved adhesion layer (namely, ZnO) between the silicon substrate and platinum bottom electrode enables dramatic improvements in the properties of the overlying functional oxide films. Using BaTiO 3 and Pb(Zr,Ti)O 3 films as test cases, it is shown that the use of ZnO as the adhesion layer leads directly to increased process temperature capabilities and dramatic improvements in chemical homogeneity of the films. These result in significant property enhancements (e.g., 300% improvement to bulk‐like permittivity for the BaTiO 3 films) of oxide films prepared on Pt/ZnO as compared to the conventional Pt/Ti and Pt/TiO x stacks. A comparison of electrical, structural, and chemical properties that demonstrate the impact of adhesion layer chemistry on the chemical homogeneity of the overlying complex oxide is presented. Collectively, this analysis shows that in addition to the simple need for adhesion, metal‐oxide layers between noble metals and silicon can have tremendous chemical impact on the terminal complex oxide layers.

On page 2295, Jon F. Ihlefeld and co-workers reveal how proper metallization adhesion layer selection results in chemically homogeneous complex oxide films with enhanced ferroelectric and dielectric responses. The image shows a cross-sectional scanning electron microscopy image of a lead zircon ate titanate thin film on a platinized silicon wafer.

We report on the geometric limits associated with tunability of interdigitated capacitors, specifically regarding the impact of a parasitic non-tunable component that necessarily accompanies a ferroelectric surface capacitor, and can dominate the voltage-dependent response as capacitor dimensions are reduced to achieve the small capacitance values required for impedance matching in the X band. We present a case study of simple gap capacitors prepared and characterized as a function of gap width (i.e., the distance between electrodes) and gap length (i.e., the edge-to-edge gap distance). Our series of measurements reveals that for gap widths in the micrometer range, as gap lengths are reduced to meet sub-picofarad capacitance values, the non-tunable parasitic elements limit the effective tunability. These experimental measurements are supported by a companion set of microwave models that clarify the existence of parallel parasitic elements.

Polycrystalline BaTiO_3 thin films have been prepared by hydrothermal reaction with sputter-deposited nanostructured reactive Ti templates designed to control net diffusion direction and distance. Templates were prepared in two morphologies, i.e., planar and nanopillar. The samples produced from flat templates showed sluggish transformation kinetics and an eventual termination of reaction without fully consuming the Ti metal. Templates with pillar morphology, on the other hand, could be transformed to phase-pure BaTiO_3 independent of the template thickness. In the as-precipitated state, those films revealed a permittivity of ~1000 and loss tangent values around 0.1 with mild dispersion in the kilohertz frequency range. Annealing these films under forming gas containing 1 vol% H_2 balance N_2 for 3 h at 200 °C decreased high-field losses to 0.06 and reduced dispersion. Mn incorporation as an in situ acceptor dopant was also explored. Addition of Mn during hydrothermal treatment further improved the electrical properties. Annealing under the same postgrowth conditions virtually eliminated the frequency dispersion in the range of 1 kHz to 1 MHz, while maintaining permittivity values in the range of 350.

Tunable bandpass filters are critical components in emerging radio frequency front-ends. A system-aware design guideline and figure of merit (FOM) are developed for optimum system-level performance. The optimisation metric discussed here deviates from earlier guidelines as the filter bandwidth is allowed to vary in the tunable range, constrained only by the downstream system analogue to digital converter. The system-aware FOM uses worst-case filter design parameters and a tuning sensitivity term that captures the frequency tunability relative to material tunability. A 6.74-8.23 GHz tunable barium strontium titanate-based filter is presented as an example to illustrate the design methodology.

A theoretical treatment of distributed electro-thermally induced intermodulation distortion is developed for microstrip transmission lines. The growth of passive intermodulation distortion (PIM) along the length of a line is derived accounting for both loss and electrical dispersion. PIM dependencies on width, length, thickness, and substrate parameters are analyzed leading to design guidelines for low distortion lines. Single metal silver transmission lines are fabricated on sapphire and fused-quartz substrates to isolate the electro-thermal effect and validate the model. Electro-thermal PIM is measured in a two-tone test with tone separation ranging from 4 Hz to 10 kHz.

A 3rd-order combline tunable filter with very narrow bandwidth at X-band is implemented utilizing ferroelectric barium strontium titanate (BST) interdigitated varactors. The filter and its varactors are integrated on a ceramic substrate together with a simple resistive biasing circuit. Upon the application of a bias voltage ranging from 0 to 90 V the passband of the filter tuned from 8.127 to 9.973 GHz while maintaining a fractional bandwidth of approximately 4.8-5.9 %. Over the tuning band the passband insertion loss varied from 10.7 dB to 7.5 dB while the return loss was better than 15 dB. The tuning ratio is 23 %.

A 3rd-order combline tunable filter is implemented utilizing ferroelectric barium strontium titanate (BST) interdigitated varactors. The filter and its varactors are integrated on a ceramic substrate using a simple resistive biasing circuit. Upon the application of a bias voltage ranging from 0 to 40 V the passband of the filter tuned from 6.2 to 7.5 GHz while maintaining a fractional bandwidth of approximately 13.8 %. Over the tuning band the passband insertion loss varied from 8.5 dB to 6.3 dB while the return loss was better than 10 dB.

The impact of dc resistance on the performance of X-band filters with ferroelectric varactors was investigated. Two series of combline bandpass filters with specific geometries to isolate sources of conductor losses were designed and synthesized. Combining the changes in filter geometry with microwave measurements and planar filter solver (Sonnet software) simulations quantitatively identified the dependency of insertion loss on overall metallization thickness and local regions of thin metallization. The optimized 8-GHz bandpass filters exhibited insertion losses of 6.8 dB. These filters required 2.5 microm of metal thickness (or 3 effective skin depths) to achieve this loss. The trend of loss with thickness indicates diminishing return with additional metal. The integration scheme requires thin regions of metal in the immediate vicinity of the varactors. It is shown through experiment and simulation that short distances (i.e., 15 microm) of thin metallization can be tolerated provided that they are located in regions where the resonant microwave current is low.

A bandpass filter for the frequency range of 6??9 GHz, based on tunable interdigital BST varactor is presented. This paper reports the processing steps required to obtain a functional and integrated bandpass filter, which include: via drilling and filling purpose, dielectric deposition, chromium/gold deposition and copper electroplating. Microwave measurements reveal an insertion loss range between 7??11 dB, a 20 dB rejection band, a bandwidth of less than 500 MHz, and 1.73 GHz of frequency tunability.

Highly asymmetric metal-containing block copolymers are capable of forming nanotubes in solutions containing a nonpolar solvent. In this study, the time-dependent formation of these nanotubes is investigated and reveals how the aggregate nanostructures develop (see figure for an example at an early stage). These nanotubes are semicrystalline (see the diffraction pattern in the inset), suggesting that crystallization is at least partially responsible for the unique morphology formed by these block copolymers.

Discrete barium strontium titanate (BST) thin-film capacitors in industry standard 0603 footprint are introduced and characterized. BST capacitors have a voltage-dependent permittivity, enabling BST thin-film capacitors to be used as tuning elements in frequency agile devices. The capacitance changed by 1.5:1 at 35 V (116 kV/cm) bias. The temperature dependence of the capacitance was measured to be less than plusmn 20 % from -100 degC to +100 degC. A 2nd-order tunable combline bandpass filter on FR4 substrate has been implemented using the discrete BST varactors. The filter showed a center frequency tuning of 22% from 2.14 GHz to 2.61 GHz upon application of 130 V (433 kV/cm) bias. The zero-bias insertion loss was 4.9 dB which decreased to 2.9 dB at the high bias state. The return loss was better than 11 dB over the tuning range. Nonlinear characterization of the filter using two-tone test and a digitally-modulated CDMA 2000 signal showed an IP3 of +32 dBm and an ACPR of better than -50 dBc up to 26 dBm of input power, respectively

Incorporation of a block copolymer into a thin polymer film is observed to alter both the rate and mechanism by which the film dewets from an immiscible polymer substrate. Films with little or no copolymer dewet by classical nucleation and growth of circular holes, and the dewetting rate decreases with increasing copolymer concentration. Increasing the copolymer content at constant film thickness generates copolymer micelles that adsorb/aggregate along the polymer/polymer interface and promote nonclassical dewetting fluctuations similar in appearance to spinodal dewetting. At higher copolymer concentrations, dewetting proceeds after a lengthy induction period by the nucleation and growth of flower-shaped holes suggestive of film pinning or viscous fingering. Atomic force microscopy of the polymer/polymer interface after removal of the top film by selective dissolution reveals substantial structural development due to copolymer self-assembly.