Low-loss RF Research Articles

Demonstration of superconducting micromachined cavities

Superconducting enclosures will be key components of scalable quantum computing devices based on circuit quantum electrodynamics. Within a densely integrated device, they can protect qubits from noise and serve as quantum memory units. Whether constructed by machining bulk pieces of metal or microfabricating wafers, 3D enclosures are typically assembled from two or more parts. The resulting seams potentially dissipate crossing currents and limit performance. In this letter, we present measured quality factors of superconducting cavity resonators of several materials, dimensions, and seam locations. We observe that superconducting indium can be a low-loss RF conductor and form low-loss seams. Leveraging this, we create a superconducting micromachined resonator with indium that has a quality factor of two million, despite a greatly reduced mode volume. Inter-layer coupling to this type of resonator is achieved by an aperture located under a planar transmission line. The described techniques demonstrate a proof-of-principle for multilayer microwave integrated quantum circuits for scalable quantum computing.

A wide-band magnetic tunable bandstop filter prototype with FeGaB/Al2O3 multilayer films

In this paper, we demonstrate a prototype of magnetic tunable bandstop filter by using magnetic multilayers of [FeGaB 25 nm/Al2O3 5 nm]4 prepared by magnetron sputtering. The multilayer structure exhibits a relatively low coercive field and RF loss compared to the single layer film with the same thickness. By placing a single transmission line on the top of magnetic multilayers, a prototype of tunable bandstop filter was fabricated and operates from 4.5 GHz to 7.08 GHz under very low magnetic bias fields from 100 Oe to 400 Oe. These low loss multilayer films are good candidates for RF/microwave applications.

Development of cap-free sputtered GeTe films for inline phase change switch based RF circuits

Germanium telluride (GeTe) films have been recently demonstrated as the active element in low-loss RF switches where a 7.3 THz cut-off frequency (Fco) was achieved. In order to simultaneously realize the low ON-state transmission loss and large OFF-state isolation required for this application, significant optimization of the GeTe films was required. In particular, minimizing contact resistance (Rc) and sheet resistivity (Rsheet) without the use of a capping layer is a necessity. Varying the GeTe deposition conditions led to a wide range of structural, chemical, and electrical properties, which ultimately enabled the demonstration of a capless GeTe inline phase change switch (IPCS) structure. Conversely, improper deposition conditions led to extensive oxidation which would push Rc and Rsheet to unacceptable levels. In addition to its relevance for IPCS devices, this work has implications for the environmental stability of GeTe as a function of its physical morphology.

Highly Linear and Highly Efficient Dual-Carrier Power Amplifier Based on Low-Loss RF Carrier Combiner

This paper presents a novel power amplifier (PA) architecture for amplifying dual-carrier signals with simultaneous high efficiency and high linearity. This dual-carrier PA is based on two high-efficiency PAs and an innovative RF carrier combiner, which is a diplexer implemented with bandstop filters (BSFs). This special BSF diplexer is capable of combining two carriers with ultra-narrow frequency spacings, e.g., down to 0.25% fractional bandwidth, while maintaining a low combining loss of 1 dB. Compared to conventional diplexers implemented with bandpass filters, this new technology leads to minimized resource usage for achieving the same combining performance. The integrated PA exhibits excellent performance when amplifying dual-carrier signals in terms of linearity and efficiency. Specifically, third-order intermodulations of 40 dBc were measured with 70.2% (60.8%) power-added efficiency for carrier spacing of 10 MHz (5 MHz).

Design of novel compact anti-stiction and low insertion loss RF MEMS switch

A novel torsional RF MEMS capacitive switch design on silicon substrate is presented. The optimized switch topology such as reduction in up-state capacitance results in insertion loss better than ź0.1 dB till 20 GHz. Off to on state capacitance ratio is also improved by 18 fold and isolation is better than ź43 dB at 9.5 GHz. The achieved on state return loss is ź38 dB as compared to ź21 dB at 9.5 GHz. An optimized reduction in contact area and use of floating metal layer increases the switching speed from 56 to 46 μsec. It also increases the switch reliability by alleviating the stiction.

An Electronically-Scanned 1.8–2.1 GHz Base-Station Antenna Using Packaged High-Reliability RF MEMS Phase Shifters

This paper presents the first 4-element base-station antenna that is passively scanned using low-loss RF MEMS phase shifters. The design is based on packaged RF MEMS SPDT switches, which are capable of handling Watts of RF power with high reliability. The results agree very well with simulations and show pattern scanning to 9° with a measured gain of 8.6-8.3 dB at 2 GHz. The array is capable of handling 5-10 W of power with no distortion.

RF devices: from WLSCP to smart interposers

RF wireless devices such as 3G/4G, Wifi, Bluethooth etc... represent today 32% of the whole WLCSP products which know a steady grow above 10% per year [1]. However, a clear trend of combining multi RF chips in one WLCSP module is emerging. This allows shorter RF connections, reduction of BOM and diminution of the overall cost. Complex and heterogeneous RF systems will become possible allowing soon tunability. A silicon package directly reported on board is a good candidate to meet these next challenges of RF WLCSP. Indeed, low cost and multi-chips interposers will soon be produced by TSV fabs or OSATs and tunables devices could be reported on them. But the direct report on board as well as the high frequency signal integrity must be first evaluated carefully. In this work, we describe promising results of silicon package based on high resistive silicon interposer encompassing 2 levels of Cu Damascene, Tx/Rx antenna, one RF chip and TSV last for 60GHz applications. A new wafer-level molding [2] is performed on front side by lamination at wafer level. On the backside of the interposer, thick Cu RDL and soft layer passivation is deposited before the balling. Just like in WLSCP, different kinds of solder balls are mounted and analyzed with respect to their size (350 or 400 μm) or type (solder or polymer/solder). The passivated interposers are diced and then reported on a PCB, thus forming the silicon package. Low Rf loss (<1dB) attenuation is demonstrated for the routing layers while the antennas exhibit a suitable gain of 5dBi over the 55–65 GHz band. The assembly characteristics between the silicon package and the board will be discussed with respect to the different chosen design rules. These first results pave the way to a new kind of smart package based on Si interposers, moving forward to more complex WLCSP RF devices.

Whole Body Screening Using High-Temperature Superconducting MR Volume Resonators: Mice Studies

High temperature superconducting (HTS) surface resonators have been used as a low loss RF receiver resonator for improving magnetic resonance imaging image quality. However, the application of HTS surface resonators is significantly limited by their filling factor. To maximize the filling factor, it is desirable to have the RF resonator wrapped around the sample so that more nuclear magnetic dipoles can contribute to the signal. In this study, a whole new Bi2Sr2Ca2Cu2O3 (Bi-2223) superconducting saddle resonator (width of 5 cm and length of 8 cm) was designed for the magnetic resonance image of a mouse's whole body in Bruker 3 T MRI system. The experiment was conducted with a professionally-made copper saddle resonator and a Bi-2223 saddle resonator to show the difference. Signal-to-noise ratio (SNR) with the HTS saddle resonator at 77 K was 2.1 and 2 folds higher than that of the copper saddle resonator at 300 K for a phantom and an in-vivo mice whole body imaging. Testing results were in accordance with predicted ones, and the difference between the predicted SNR gains and measured SNR gains were 2.4%∼2.7%. In summary, with this HTS saddle system, a mouse's whole body can be imaged in one scan and could reach a high SNR due to a 2 folds SNR gain over the professionally-made prototype of copper saddle resonator at 300 K. The use of HTS saddle resonator not only improves SNR but also enables a mouse's whole body screen in one scan.

Design of a Low Loss RF Mixer in Ku-Band (12 - 18 GHz)

The goal of this paper is to design and develop low conversion loss mixers at Ku Band (12 - 18 GHz) in microstrip line configuration. The Ku-band (Kurtz-under band) is primarily used for satellite Communications, particularly for editing and broadcasting satellite television. CAD tools (Agilent’s Advance Design System software) have been used for simulation and optimization of the circuits. 180 degree balanced mixer configuration has been adopted due to better RF to LO isolation requirement. The circuit is designed on microstrip line using RT Duroid (Dielectric Constant = 2.22). Initially a rat race coupler (180 degree hybrid) was designed and simulated and optimized for required performance successfully.

RF power limiter using capacitively-coupled contacts III-nitride varactor

Reported is a low-loss RF power limiter built using a voltage-controlled capacitor formed by two planar Schottky contacts deposited over an AlGaN/GaN heterostructure. The symmetrical structure of the varactor enables dual-polarity voltage clamping. Small electrode size and electrode spacing of 2 μm ensures a low impedance and low loss. Varying the varactor width in the range 0.25 - 1 mm allows tuning of the limiting powers in the range 17 - 40 dBm, also dependent on the operating frequency. The small-signal loss at 10 GHz is 0.2 - 0.67 dB. The varactor structure fabrication does not require gate alignment or annealing; the device is robust and fully compatible with MMICs; it also provides the DC block with around 95 V breakdown. The PL operation in the temperature range 25 - 200°C without significant parameter degradation was demonstrated. Power limiter CW stress during 100 hat 24 dBm revealed no performance degradation.

High-Performance Low-Leakage-Current AlN/GaN HEMTs Grown on Silicon Substrate

In this letter, ultrathin-barrier AlN/GaN high-electron mobility transistors (HEMTs) capped with in situ metal-organic-chemical-vapor-deposition-grown SiN have been successfully fabricated on 100-mm Si substrates. Output current density exceeding 2 A/mm has been reached, which represents, to the best of our knowledge, the highest value ever achieved for GaN-on-Si HEMTs. This results from the high 2DEG density of the optimized AlN/GaN heterostructure. Despite the ultrathin barrier of 6 nm, low gate and drain leakage currents of about 10 μA/mm are obtained without the use of a gate dielectric that generally induces reliability issues. Furthermore, the high aspect ratio (gate length <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> /gate-to-channel distance) and low RF losses (at the buffer/Si substrate interface) are reflected in excellent RF performances. The current gain extrinsic cutoff frequency <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> and the maximum oscillation frequency <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> were 52 and 102 GHz with a 0.2- μm gate length, respectively, resulting in an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> · <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> product as high as the reported state-of-the-art GaN-on-Si HEMTs.

Demonstration of a Linearized Traveling Wave Y-Fed Directional Coupler Modulator Based on Electro-Optic Polymer

In this paper, we demonstrate a traveling wave linear modulator for high speed analog optical link applications. A two-section Y-fed directional coupler modulator based on electro-optic polymer is linearized with Δβ-reversal technique to suppress the third-order intermodulation distortions. A traveling wave electrode with low RF loss and excellent velocity matching is designed and demonstrated with the bandwidth-length product of 125 GHz ·cm. The 3-dB electrical bandwidth of 10 GHz and the spurious free dynamic range of 110 ±3 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> over the modulation frequency of 2-8 GHz are measured form the fabricated device.

RF MEMS Capacitive Switches for Wide Temperature Range Applications Using a Standard Thin-Film Process

In this paper, the design, fabrication, and measurements of a capacitive RF microelectromechanical systems (RF MEMS) switch with very low sensitivity to thermal stress is presented. The switch is built by thin-film technology (0.8-μm Au) and shows <;50-mV/°C variation in the pull-down voltage at 25 °C-125 °C. The effects of the residual biaxial stress and stress gradients are studied in detail and the final switch design is very tolerant to a wide range of stress. The switch exhibits excellent RF and mechanical performances, and a capacitance ratio of about 51-57 (Cu = 54-66 fF, Cd = 3.1 - 3.4 pF) is reported. The mechanical resonant frequency and quality factor are fο = 105 - 115 kHz and Qm ≈ 8, respectively, with a measured switching time of about 3-3.8 μs. The applications areas are in low-loss RF MEMS, phase shifters, and reconfigurable networks.

PHEMT 공정을 이용한 저손실, 고전력 4중 대역용 SP6T 스위치 칩의 설계 및 제작

In this paper, low-loss and high-power RF SP6T switch chips are designed, fabricated and measured for GSM/EGSM/DCS/PCS applications using WIN Semiconductors 0.5 pHEMT process. We utilized a combined configuration of series and series-shunt structures for optimized switch performance, and a common transistor structure on a receiver path for reducing chip area. The gate width and the number of stacked transistors are determined using ON/OFF input power level of the transceiver system. To improve the switch performance, feed-forward capacitors, shunt capacitors and parasitic FET inductance elimination due to resonance are actively used. The fabricated chip size is . S-parameter measurement shows an insertion loss of 0.5~1.2 dB and isolation of 28~36 dB. The fabricated SP6T switch chips can handle 4 W input power and suppress second and third harmonics by more than 75 dBc.

Improvement of the Performance of a Helix Slow-Wave Structure With a Copper-Embedded Barrel

The effect of a copper-embedded barrel on the performance of a helix slow-wave structure (SWS) has been studied theoretically and experimentally. The heat dissipation capability and cold-test characteristics of the SWSs with a monel barrel, a normal copper barrel, and a copper-embedded monel barrel have been analyzed and compared. With its combination of high mechanical strength and good thermal conductivity, the copper-embedded monel barrel enables better heat dissipation capability than the other barrel types and has low RF losses.

A Liquid–Solid Direct Contact Low-Loss RF Micro Switch

This paper reports the design, fabrication, and testing of a liquid-metal (LM) droplet-based radio-frequency microelectromechanical systems (RF MEMS) shunt switch with dc-40 GHz performance. The switch demonstrates better than 0.3 dB insertion loss and 20 dB isolation up to 40 GHz, achieving significant improvements over previous LM-based RF MEMS switches. The improvement is attributed to use of electrowetting on dielectric (EWOD) as a new actuation mechanism, which allows design optimized for RF switching. A two-droplet design is devised to solve the biasing problem of the actuation electrode that would otherwise limit the performance of a single-droplet design. The switch design uses a microframe structure to accurately position the liquid-solid contact line while also absorbing variations in deposited LM volumes. By sliding the liquid-solid contact line electrostatically through EWOD, the switch demonstrates bounceless switching, low switch-on time (60 mus), and low power consumption (10 nJ per cycle).

Microwave ferrites, part 1: fundamental properties

Ferrimagnets having low RF loss are used in passive microwave components such as isolators, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or by passing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe12O19 and SrFe12O19, which have high effective internal magnetic anisotropy that also contributes to the permanent bias. Such a self-biased material remains magnetized even after removing the external applied magnetic field, and thus, may not even require an external permanent magnet. In garnet and spinel ferrites, such as Y3Fe5O12 (YIG) and MgFe2O4, however, the uniaxial anisotropy is much smaller, and one would need to apply huge magnetic fields to achieve such high frequencies. In Part 1 of this review of microwave ferrites a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing, are presented. Operational principles of microwave passive components and electrical tuning of magnetization using magnetoelectric coupling are discussed in Part 2.

Structural, electrical, and piezoelectric properties of ZnO films on SiC-6H substrates

Epitaxial ZnO films were grown on c-plane SiC-6H substrates using metal-organic chemical vapor deposition. X-ray diffraction coupled θ-2θ and ϕ-scans show that the n-type ZnO films have c-axis orientation and in-plane registry with the n-type 6H-SiC substrates. This isotype ZnO∕SiC heterojunction shows rectifying characteristics. Electrical measurements exhibit that the reverse current is in the picoampere (10−12–10−10A) range under the reverse bias of less than 5V, the on-off current ratio is ∼107, and the ideality factor is ∼1.23. The surface acoustic wave characteristics in the structure consisting of a piezoelectric ZnO and a semi-insulating SiC-6H substrate were also studied. The structure shows promise for high frequency and low loss rf applications.

Cryogenic RF switch using III-nitride MOSHFETs

The cryogenic operation of a low-loss RF switch using a AlGaN/GaN metal-oxide semiconductor heterostructure field effect transistor is reported. At 77K the channel resistance of the MOSHFET is three times lower and the contact resistance is 20% lower compared to room temperature. As a result, the performance of the MOSHFET RF switch at 77K is even superior to that at room temperature.

Low energy loss rf circuits on nanostructured porous silicon layers

AbstractWe present a work dealing with the study of thick porous silicon (PS) as insulating substrate for radio frequency (rf) circuits. Planar LC resonant circuits of 2 mm x 2 mm were designed on 100 µm thick PS layers for uses in 10 MHz to 1 GHz frequency range. The influence of PS electrical conductivity on the energy loss is determined using an original method based on experimental data combined with numerical simulations. We have clearly shown the increase of PS conductivity with the frequency due to hopping transport mechanisms in disordered materials. The contribution of the porous substrate to the energy loss is dominant for conductivities higher than 10–3 S/cm. Finally, the insertion of a SiO2 layer between an oxidized porous substrate and the integrated micro‐coil is suggested to optimize the quality factor of integrated rf resonnant circuits. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)