Gregory T. Stauf

Voltage-controlled RF filters employing thin-film barium-strontium-titanate tunable capacitors

Tunable lowpass and bandpass lumped-element filters employing barium-strontium-titanate (BST)-based capacitors are presented. A new metallization technique is used, which improves the quality factor of the tunable BST capacitors by a factor of two. The lowpass filter has an insertion loss of 2 dB and a tunability of 40% (120-170 MHz) with the application of 0-9 V DC bias. The bandpass filter (BPF) has an insertion loss of 3 dB and a tunability of 57% (176-276 MHz) with the application of 0-6 V DC. The third-order intercept point of the BPF was measured to be 19 dBm with the application of two tones around 170 MHz.

Tunable barium strontium titanate thin film capacitors for RF and microwave applications

The measurement results for thin film barium strontium titanate (BST) based voltage tunable capacitors intended for RF applications are reported. At 9 V DC, BST capacitors fabricated using MOCVD (metalorganic chemical vapor deposition) method achieved 71% (3.4:1) tunability. The measured device quality factor (Q) for BST varactors is comparable with the device Q for commercially available varactor diodes of similar capacitance. The typical dielectric loss tangent was in the range 0.003-0.009 at VHF. Large signal measurement and modeling results for BST thin film capacitors are also presented.

High‐mobility InSb grown by organometallic vapor phase epitaxy

The highest mobility InSb epilayer grown to date by organometallic vapor phase epitaxy has been achieved by utilizing high‐purity organometallic sources, choosing a reactor geometry to reproducibly control the source concentrations above the substrate, and using a high‐ integrity reactor system. On a p‐type InSb substrate, an unintentionally doped layer had a 77 K n‐type carrier concentration and mobility of 1.4×1015 cm−3 and 2.53×105 cm2/V s. Growths on GaAs substrates were greatly affected by the lattice mismatch and had 77 K carrier concentrations similar to the InSb case but with mobilities of (5.0–9.0)×104 cm2/V s. The crystal quality, morphology, and cyclotron resonance characteristics are reported and found to be comparable to state‐of‐the‐art molecular beam epitaxy layers.

Low‐temperature organometallic vapor phase epitaxy of InSb using the novel Sb precursor triisopropylantimony

InSb has been grown by organometallic vapor phase epitaxy using triisopropylantimony (TIPSb) and trimethylindium (TMIn) at temperatures as low as 300 °C, the lowest yet reported for a stable Sb source. Unintentionally doped films were n type, and the best electronic transport result was obtained for a growth at 350 °C yielding a 77 K carrier concentration of 3.7×1016 cm−3 and mobility of 6.3×104 cm2/V s; these impurities are thought to be from the reagent grade chemicals used to synthesize the TIPSb. The morphology, growth rate, and transport properties of these films are reported as a function of growth temperature, molar ratios, and substrate (GaAs or InSb). By comparing these transport properties with Hall measurements of ∼1 μm InSb epilayers on GaAs grown from trimethylantimony and TMIn, it was found that these properties were greatly improved because of the low‐temperature epitaxy enabled by this source.

Tunable RF filters using thin film barium strontium titanate based capacitors

Tunable lowpass and bandpass filters based on barium strontium titanate (BST) capacitors are reported. For the lowpass filter, 40% tunability was achieved at 9 V. Intermodulation distortion generated by the lowpass filters was also measured and predicted using a nonlinear model for the BST capacitors. The bandpass filter showed 45% tunability with an applied DC bias of 10 V.

Liquid-delivery MOCVD: chemical and process perspectives on ferro-electric thin film growth

MOCVD processes for (Ba,Sr)TiO3 (BST) and Pb(Zr,Ti)O3 (PZT) are described based on different metal – organic precursors. In the case of BST, processes using polyamine and tetraglyme adducts of group II metals were compared. Similar incorporation efficiencies were obtained at 640 °C for both adducts, and electrical properties were comparable. Delivery methodologies were compared using the tetraglyme adducts for BST. Composition control was equivalent using liquid mass flow controllers versus a positive displacement pump. For PZT, Zr(thd)4 and Zr(OiPr)2(thd)2 were compared as source materials in combination with Ti(OiPr)2(thd)2 and Pb(thd)2(pmdeta). The mixed ligand Zr source increased Zr incorporation efficiency by a factor of nearly eight for otherwise equivalent processes centred at 590 °C, and run to run repeatability was 0.1 at% for Pb and 0.6 at% for Zr (1 σ) over a series of more than 300 films. The electrical properties of thin film PZT deposited with Zr(OiPr)2(thd)2 compared favourably with those obtained previously with Zr(thd)4. Copyright

Low voltage tunable barium strontium titanate thin film capacitors for RF and microwave applications

The measurement results for thin film barium strontium titanate (BST) based voltage tunable capacitors intended for RF applications are reported. At 9 V DC, BST capacitors fabricated using MOCVD (metalorganic chemical vapor deposition) method achieved 71% (3.4:1) tunability. The measured device quality factor (Q) for BST varactors is comparable with the device Q for commercially available varactor diodes of similar capacitance. The typical dielectric loss tangent was in the range 0.003-0.009 at VHF. Large signal measurement and modeling results for BST thin film capacitors are also presented.In this paper, we report on the fabrication and testing of thin film barium strontium titanate (BST) based capacitors for RF and microwave components. At 7 V DC applied, MOCVD BST parallel plate capacitors yielded 60% (2.4:1) tunability between 45 MHz and 200 MHz. In this frequency range, the dielectric loss tangent was typically 0.004. Measured results for the frequency dependent dielectric permittivity and tunability at large RF voltage amplitudes are presented.

Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002–0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 °C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO2/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1–0.2 urn. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO2 interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 μm. Devices with dielectric Q factors over 150 at 100 MHz (tan δ ∼ 0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.

Microwave characterization of thin film BST material using a simple measurement technique

Thin film barium strontium titanate (BST) in the parallel plate capacitors is characterized at microwave frequencies using a simple measurement technique. Short standards are fabricated on the same wafer as the BST capacitors to remove the parasitics of pads, lines, discontinuities and electrodes. The dielectric constant of the patterned BST thin film in the parallel plate capacitor is found to be frequency independent up to 10 GHz. The average loss tangent of BST thin film for the sample under test is approximately 0.006 and also found to be frequency independent up to 10 GHz.

Single molecular precursor metal-organic chemical vapor deposition of MgAl2O4 thin films

MgAl 2 O 4 films have been grown epitaxially on both Si(100) and MgO(100) by a novel single source metal-organic chemical vapor deposition (MOCVD) process. A single molecular source reagent [magnesium dialuminum isopropoxide, MgAl 2 (OC 3 H 7 ) 8 ] having the desired Mg: Al ratio was dissolved in a liquid solution and flash-vaporized into the reactor. Both thermal and plasma-enhanced MOCVD were used to grow epitaxial MgAl 2 O 4 thin films. The Mg: Al ratio in the deposited films was the same as that of the starting compound (Mg: Al = 1:2) over a wide range of deposition conditions. The deposition temperature required for the formation of crystalline spinel was found to be significantly reduced and crystallinity was much improved on Si by using a remote plasma-enhanced MOCVD process. The epitaxial nature of the MgAl 2 O 4 films was established by x-ray pole figure analysis.