Ernest A. Fardin

Enhanced tunability of magnetron sputtered Ba0.5Sr0.5TiO3 thin films on c-plane sapphire substrates

Thin films of Ba0.5Sr0.5TiO3 (BST) were deposited on c-plane (0001) sapphire by rf magnetron sputtering and investigated by complementary materials analysis methods. Microwave properties of the films, including tunability and Q factor were measured from 1to20GHz by patterning interdigital capacitors (IDCs) on the film surface. The tunability is correlated with texture, strain, and grain size in the deposited films. An enhanced capacitance tunability of 56% at a bias field of 200kV∕cm and total device Q of more than 15 (up to 20GHz) were achieved following postdeposition annealing at 900°C.

A varactor tuned branch-line hybrid coupler

This paper introduces a novel branch-line 90/spl deg/ hybrid coupler incorporating varactor diodes which allow tuning of the frequency response. A design covering the DCS, PCS and IMT2000 cellular frequency bands (1710-2170 MHz) is presented. Given a varactor tunability of 2.5:1, simulations suggest 20 dB return loss and 3/spl plusmn/1 dB coupling is achievable across each transmit and receive sub-band by tuning the varactors. These results offer an improvement over a conventional single-section branch-line hybrid centred at 1950 MHz, and some miniaturisation is also achieved due to the capacitive loading. A prototype is constructed using commercially available varactor diodes, and reasonable agreement between the measured and simulated results is achieved.

Polycrystalline Ba0.6Sr0.4TiO3 thin films on r-plane sapphire : Effect of film thickness on strain and dielectric properties

Polycrystalline Ba0.6Sr0.4TiO3 (BST) films grown on r-plane sapphire exhibit strong variation of in-plane strain over the thickness range of 25–400nm. At a critical thickness of ∼200nm, the films are strain relieved; in thinner films, the strain is tensile, while compressive strain was observed in the 400nm film. Microwave properties of the films were measured from 1to20GHz by the interdigital capacitor method. A capacitance tunability of 64% was observed in the 200nm film, while thinner films showed improved Q factor. These results demonstrate the possibility of incorporating frequency agile BST-based devices into the silicon on sapphire process.

Frequency Agile 90° Hybrid Coupler Using Barium Strontium Titanate Varactors

This paper presents a tunable quadrature hybrid coupler realized using integrated lumped components. The centre frequency of the hybrid is tuned by controlling the DC bias across barium strontium titanate (BST) varactors. The hybrid was designed to tune from 1710-2170 MHz. Varactors were fabricated by sputtering a 700 nm BST film on a c-plane sapphire substrate and patterning interdigital electrodes on top of the BST film. Promising results were achieved from the first prototype, with a maximum insertion loss of 3.5 dB and minimum return loss of 12.2 dB across the tuning range. Phase quadrature between the coupled ports is within 90plusmn5deg from 1710-2170 MHz.

Nanolithography by elastomeric scattering mask: An application of photolithographic standing waves

An application is demonstrated for the much maligned standing wave in photolithography that is responsible for the sidewall corrugations in photoresist patterns. We demonstrate the realization of a polydimethylsiloxane (PDMS) scattering mask through the casting of these sidewall corrugations and their application as the masking components in an otherwise transparent bulk of PDMS. Photoresist structures with widths in the order of 80nm are realized by the application of this mask, demonstrating excellent correlation with the lateral depths of the sidewall corrugations. The continuity of the sidewall corrugations around corners makes both straight and curved lines realizable with equal simplicity.

Barium strontium titanate thin film varactors on r-plane sapphire

This paper presents the microwave properties of barium strontium titanate (BST) thin films on r-plane sapphire substrates. A series of films with thickness 25-400 nm was prepared by pulsed laser deposition (PLD). Microwave properties of the films, including capacitance tunability and loss tangent, were extracted by patterning interdigitated capacitors (IDCs) on the film surface. The highest tunability of 64% was observed in the 200 nm film. These results demonstrate the possibility of integrating BST into the silicon on sapphire process.

Structural and Microwave Characterization of Magnetron Sputtered Ba0.5Sr0.5TiO3 Films on c-plane Sapphire Substrates

Many studies of Barium Strontium Titanate (BST) thin films for RF / microwave applications have employed MgO, LaAlO,3 or Pt/Si as the substrate material for BST deposition. However, there have been relatively few reports of BST films grown on sapphire, despite the excellent microwave properties of this material. In this investigation, BST thin films were deposited by RF magnetron sputtering on (001) single crystal c-plane sapphire substrates. Interdigitated capacitors (IDCs) patterned on the film surface were used to measure the dielectric tunability and loss tangent at microwave frequencies. Thick Au conductors were electroplated to minimize conductor losses. Post deposition annealing in air was found to significantly improve the tunability of the sputtered films.

Low Cost Interdigital BST Varactors for Tunable Microwave Applications

In this paper, a simple and inexpensive technique for the fabrication of barium strontium titanate (BST) varactors is presented. BST films were deposited using RF magnetron sputtering on platinum coated silicon substrates. The advantages of silicon include low cost and ease of integration with existing fabrication processes. The microstructural properties of the BST layer were examined with scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. Gold top layer electrodes were deposited using DC sputtering and patterned with a conventional photolithographic process. The S-parameters of the varactor were measured from 40 MHz to 20 GHz on a vector network analyser. From this data, the capacitance, tunability and Q-factor of the device were extracted. The BST-based varactors show potential for integration into electronically tunable microwave circuits.

RF magnetron sputter deposition and analysis of strontium-doped lead zirconate titanate thin films

The paper investigates conditions for depositing perovskite-oriented strontium-doped lead zirconate titanate (PSZT) thin films using RF magnetron sputtering. PSZT is a material that can exhibit high piezoelectric and ferroelectric properties. The deposition was conducted using an 8/65/35 PSZT sputtering target. The effects of sputtering conditions and the deposition rates for films sputtered onto several surfaces (including gold and platinum coated substrates) were studied. Combinations of in-situ heating during sputtering and post-deposition Rapid Thermal Annealing (RTA) were performed and resulting phases determined. RTA was carried out in argon to observe their effects. The sputtered films were analyzed by Scanning Electron Microscopy (SEM), X-ray Diffractometry (XRD), and X-Ray Photoelectron Spectroscopy (XPS). Results show dramatic differences in the grain structure of the deposited films on the different surfaces. The stoichiometry of the sputtered films is demonstrated using XPS. In the case of gold and platinum coated substrates, sputtering was also carried out for different durations, to establish the growth rate of the film, and to observe the variation in grain size with sputtering duration. The deposited thin films were resistant to most chemical wet etchants and were Ion Beam Etched (IBE) at 19 nm/min.