High Performance RF Research Articles

(Invited) 3D Integration Processes for Advanced Sensor Systems and High-Perfomance RF Components

Sensor systems are the key elements of today’s automotive, health care, environmental and Internet-of-Things (IoT) applications. By using MEMS sensors real-world data like physical or electrical parameters in production equipment, gas concentration in the environment or chemical parameters like th ph-value in fluids can be gathered, digitized and transfered wireless to cloud storage for further processing e.g. by means of big data algorithms. Advanced sensors are in many cases based on nano-technologies for optimizing the data acquisition. These analogue data are processed by using state-of-the-art electronics like ultra-low power microcomputers and transceiver circuits. 3D integration processes are most suitable for high performant and reliable integration of sensor functions and electronic processing and minimize the footprint, weight and formfactor of the sensor/IC product. Key application demonstrators of such heterogeneous 3D integrated systems were evaluated already in 2009 by Infineon, SINTEF and Fraunhofer for automotive sensors [1] and later in the European Integrating Project e-BRAINS by a consortium of Infineon, Siemens, IMEC, CEA, SINTEF, Tyndall, Fraunhofer, EPFL and others for bio-sensors (Magna, Infineon), infrared imagers (Sensonor, SINTEF), active medical implants (Sorin, 3D Plus) and smart gas sensors (Siemens, EPFL). A particular focus of the e-BRAINS project was the development of novel low-temperature processes for highly reliable 3D integrated sensor systems [2]. As another key application, we demonstrated high-performance communication devices with Infineon Technologies Austria, IMEC, Fraunhofer and EPFL. For high frequency components, Fraunhofer EMFT developed a specific fine-pitch 3D-TSV technology for RF-MEMS and RF-IC applications. High-performance RF test structures, antennas and 3D embedded high-Q RF inductors were designed and evaluated by EPFL and IMEC [3-4]. The application IoT is predicted as a third wave of MEMS proliferations, after automotive and mobile phones. A subset of IoT are “smart objects” showing local intelligence and awareness. We are in the phase of rapidly growing markets and increasing competition and we must reach for cost reduction, standardization and increasing functionality and performance. A corresponding key is simplification of processes and explicitly regarding the fabrication of device stacks by wafer bonding. Due to the heterogeneity of the components of the 3D integrated system – typically at least one sensor, one IC for data processing and one IC for data transmission, the bonding processes have to be optimised. Chemical and gas sensors are typically comprised of very sensitive layers as e.g. the coating of capacitive CO2 sensors [5] and sophisticated devices as vertically stacked SiNWs, fin or ISFET bio-sensors [6-7] and capacitive gas sensors based on CNT [8] and other technologies). Therefore new low temperature processes are the key to realize robust heterogeneous sensor systems with high performance and reliability [9]. Various bonding schemes, including oxide-oxide bond and variants of metal bond technologies - as thermocompression bonding, solid-liquid-interdiffusion (EMFT) and DBI® (XPERI) - will be discussed. Additionally a 3D-TSV integration process for realizing RF structures like ultra compact antennas and inductors, which can be applied as a passive RF front-end for MEMS will be demonstrated.

Ka band High RF performance AlGaN/GaN HEMTs

To realize high RF performance AlGaN/GaN high electron mobility transistor (HEMT), two SiNx passivation layers were respectively grown on the surface of AlGaN/GaN epi wafer by inductively coupled-plasma chemical vapor deposition (ICP-CVD) and plasma enhanced chemical vapor deposition (PECVD). The two-stage passivation can effectively suppress the current collapse and decrease the parasitic parameters. To verify the performance improvement, a 6 × 50 μm GaN HEMT device was fabricated and measured. The device shows a maximum drain current of about 1067 mA/mm and a maximum transconductance of about 433 mS/mm, the cut-off frequency (fT) of 45.4 GHz at a gate bias of -1.5 V and a drain bias of 20 V. The device shows a power density of 3.7 W/mm, a power-added efficiency (PAE) of 46.5% and power gain of 8.5 dB by load-pull measurement. In addition, a 34-36 GHz GaN monolithic microwave integrated circuit (MMIC) was fabricated, which exhibits an output power of about 41.5 dBm, a PAE of 20% and a gain of 15.5 dB.

3D Integrated High-Precision Passives on Thin Glass Substrates for Miniaturized and High-Performance RF Components

Abstract Double-side or 3-D integration of high-precision and high-performance bandpass and lowpass filters that are interconnected with through-vias were designed and demonstrated on 100-micron thin glass substrates for ultra-miniaturized diplexer components. A novel process for achieving high precision with large-area fabrication was developed to achieve much improved tolerance in electrical performance. High-precision, high quality factor, and high component densities with thin-film layers on glass were used to realize innovative topologies on glass for high out-of-band rejection and low insertion loss. Low-loss 100-μm thick glass cores and multiple layers of 15-μm thin polymer films were used to build the filters on substrates. The demonstrated diplexers have dimensions of 2.3 ×2.8 ×.2 mm. Aided by the dimensional stability of glass and process control with semiadditive patterning, the performance of the fabricated filters showed excellent correlation with the simulation. The impact of process-sensitivity analysis on diplexer performance was also analyzed. Finally, a unique and innovative process solution was demonstrated to control the process deviation and achieve good diplexer tolerance. The performance deviation was controlled by ~3.5X with the new process.

High RF Performance AlGaN/GaN HEMT Fabricated by Recess-Arrayed Ohmic Contact Technology

In this letter, a high-performance gate-recessed AlGaN/GaN high electron mobility transistor (HEMT) fabricated by the recess-arrayed ohmic contact technology is presented. An array of square columns was etched in the ohmic region, using Ti/Al/Ni/Au metal deposition and rapid annealing to fabricate a low-resistance ohmic contact. By using the transmission line method measurement, an ohmic contact resistance ( ${R}_{c}$ ) of 0.12 $\Omega$ • mm is measured, which is 70% lower than that of the reference device. The patterned ohmic contact HEMT (POC HEMT) exhibits the saturation current of 1130 mA/mm at ${V}_{ds}=10$ V and ${V}_{gs}=1$ V, with a shift of the knee voltage from 6 V to 3 V. The RF measurement shows the power-added efficiency (PAE) of 71.6% at 5 GHz. After the third harmonic tuning, the PAE of 85.2% and the corresponding power density of 11.2 W/mm and power gain of 16.9 dB are obtained. According to the achieved results, the recess-arrayed ohmic contact processed GaN HEMT has a great potential for high RF efficiency and power applications.

Miniaturized and high-performance RF packages with ultra-thin glass substrates

Advanced RF packages are demonstrate with active (low-noise amplifier, RF switch) and passive integration in ultra-thin 3D glass packages with miniaturization and enhanced performance. The novelty of this RF packages is three-fold: 1) Ultra-thin 100 μm glass, 2) Double-side thinfilm RF circuits interconnected with Through-Package Vias (TPVs), and 3) Direct assembly of the glass-core package to the board with Land Grid Array (LGA) connections. An innovative double-via process, starting from prefabricated vias in bare glass, polymer filling and via drilling, is utilized for a robust and high-yield substrate fabrication process. Scalable and low-cost panel laminate processes are utilized to form the RF circuits on the build-up layers. The performance benefits are demonstrated through interconnect loss, impedance match, electrical gain and noise figure measurements. Compared to existing RF substrates, the glass substrates show 2.5X miniaturization in substrate thickness with extensibility to thinner substrates.

A high-Q floating active inductor using 130 nm BiCMOS technology and its application in IF band pass filter

In this work, the performance of floating active inductor is studied using 130 nm BiCMOS technology. A new design of floating type active inductor based on gyrator-C topology using MOSFETs and npn SiGe HBTs has been proposed. The quality factor, power consumption, noise figure, S11 and inductance value of the proposed design have been obtained and results are compared. The tradeoff between design parameters are discussed and optimal values of design components are found. The maximum value of quality factor obtained is 300 with inductive bandwidth of 0.32–2.65 GHz and power consumption of 269 µW. The design was also used in IF band pass filter for receiver applications and performance is studied in terms of gain, 1 dB compression point and IIP3. From the study, it has been observed that proposed floating type active inductor can be used in high performance RF applications compared to the applications that uses passive inductor.

Efficient parasitic-aware hybrid sizing methodology for analog and RF integrated circuits

In this paper, a highly efficient parasitic-aware hybrid sizing methodology is proposed. It involves geometric programming (GP) as the first phase, both single-objective and many-objective evolutionary algorithms (EA) as the second sizing phase. The circuit performance constraints and layout-induced parasitics are considered simultaneously right from the GP sizing phase, while the optimization accuracy is significantly improved in the EA sizing phase. The proposed methodology features an effective integration of layout information into both sizing phases. It has been used to optimize several high-performance analog and RF circuits in different CMOS technologies. The experimental results demonstrate high efficacy of our proposed parasitic-aware hybrid sizing methodology.

High-Performance MMIC Inductors for GaN-on-Low-Resistivity Silicon for Microwave Applications

Novel MMIC spiral inductors on GaN-on-low-resistivity silicon (LR-Si) substrates (σ <; 40 Ω · cm) are demonstrated with enhanced self-resonance frequency ( fSRF) and Q-factor. The developed technology improves inductor performance by suppressing substrate coupling effects using air-bridge technology above benzocyclobutene dielectric as an interface layer on the lossy substrate. A 0.83-nH spiral inductor with peak Q-factor enhancement of 57% (Q = 22 at 24 GHz) and maximum fSRF of 59 GHz was achieved because of the extra 5-μm elevation in air. An accurate broad-band model for the fabricated inductors has been developed and verified for further performance analysis up to 40 GHz. The proposed inductors utilize cost-effective, reliable, and MMIC-compatible technology for the realization of high-performance RF GaN-on-LR Si MMIC circuits for millimeter-wave applications.

Compact Silicon-Micromachined Wideband 220–330-GHz Turnstile Orthomode Transducer

This paper reports on a turnstile-junction orthomode transducer (OMT) implemented by silicon micromachining in the 220-330-GHz band. Turnstile OMTs are very wideband and allow for co-planar ports but require accurate and complex geometries, which makes their fabrication challenging at higher frequencies. The compact 10 mm × 10 mm × 0.9 mm OMT-chip presented in this paper is the first micromachined full-band OMT in any frequency range and only the second turnstile OMT implemented above 110 GHz. The measured insertion loss (0.3 dB average, 0.6 dB worst case) and the cross polarization (60 dB average, 30 dB worst case) over the whole waveguide band represent the best performance of any wideband OMT, regardless of design or fabrication technology, in the 220-330-GHz band. The return loss with 22 dB average (16 dB worst case) is comparable with or better than previous works. This paper discusses design considerations and compromises of this complex 9-layer silicon micromachined device, including the influence of side-wall slopes, underetching, and postbonding misalignment between the chips. It is shown that for a device that is very sensitive to geometrical variations, such as a turnstile OMT, it is necessary to anticipate and compensate for any fabrication imperfections in the design to achieve high RF performance.

DC and RF Performance of AlGaN/GaN/SiC MOSHEMTs With Deep Sub-Micron T-Gates and Atomic Layer Epitaxy MgCaO as Gate Dielectric

In this letter, we report on the dc and RF performance of AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors (MOSHEMTs) with various gate lengths ( ${L}_{\mathsf {G}}$ ) from 90 to 500 nm using atomic-layer-epitaxy single crystalline Mg0.25Ca0.75O as gate dielectric. The 90-nm T-gate MOSHEMT simultaneously demonstrates a ft/fmax of 113/160 GHz with high on/off ratio of $5\times 10^{8}$ . The on/off ratio increases to $2\times 10^{11}$ at ${L}_{\mathsf {G}}=350$ nm by reducing short channel effects. The gate leakage current is around 10−11 A/mm at off-state and 10−5 A/mm at on-state. A 160 nm ${L}_{\mathsf {G}}$ MOSHEMT also exhibits an output power density of 4.18 W/mm at ${f} = 35$ GHz and ${V}_{\mathsf {DS}}=20$ V. MgCaO demonstrates to be a promising dielectric for GaN MOS technology in serving as the surface passivation layer and reducing the gate leakage current while maintaining high RF performances for high-power applications.

Characterization of 0.18-μm gate length AlGaN/GaN HEMTs on SiC fabricated using two-step gate recessing

We fabricated a 0.18-μm gate-length AlGaN/GaN high electron mobility transistor (HEMT) on SiC substrate fabricated by using two-step gate recessing which was composed of inductively coupled plasma (ICP) dry etching with a gas mixture of BCl3/Cl2 and wet chemical etching using the oxygen plasma treatment and HCl-based cleaning. The two-step gate recessing process exhibited an etch depth of 4.5 nm for the AlGaN layer and the clean surface of AlGaN layer at the AlGaN/gate metal contact region for the AlGaN/GaN HEMT structure. The recessed 0.18 μm × 200 μm AlGaN/GaN HEMT devices showed good DC characteristics, having a good Schottky diode ideality factor of 1.25, an extrinsic transconductance (g m ) of 345 mS/mm, and a threshold voltage (V th ) of −2.03 V. The recessed HEMT devices exhibited high RF performance, having a cut-off frequency (f T ) of 48 GHz and a maximum oscillation frequency (f max ) of 130 GHz. These devices also showed minimum noise figure of 0.83 dB and associated gain of 12.2 dB at 10 GHz.

RF-MEMS for high-performance and widely reconfigurable passive components – A review with focus on future telecommunications, Internet of Things (IoT) and 5G applications

Since its first discussions in literature during late ‘90s, RF-MEMS technology (i.e. Radio Frequency MicroElectroMechanical-Systems) has been showing uncommon potential in the realisation of high-performance and widely reconfigurable RF passives for radio and telecommunication systems. Nevertheless, against the most confident forecasts sparkling around the successful exploitation of RF-MEMS technology in mass-market applications, with the mobile phone segment first in line, already commencing from the earliest years of the 2000s, the first design wins for MEMS-based RF passives have started to be announced just in late 2014. Beyond the disappointment of all the most flattering market forecasts and, on the other hand, the effective employment of RF-MEMS in niche applications (like in very specific space and defence scenarios), there were crucial aspects, not fully considered since the beginning, that impaired the success of such a technology in large-market and consumer applications. Quite unexpectedly, the context has changed rather significantly in recent years. The smartphones market segment started to generate a factual need for highly reconfigurable and high-performance RF passive networks, and this circumstance is increasing the momentum of RF-MEMS technology that was expected to take place more than one decade ago. On a broader landscape, the Internet of Things (IoT) and the even wider paradigm of the Internet of Everything (IoE) seem to be potential fields of exploitation for high-performance and highly reconfigurable passive components in RF-MEMS technology.This work frames the current state of RF-MEMS market exploitation, analysing the main reasons impairing in past years the proper employment of Microsystem technology based RF passive components. Moreover, highlights on further expansion of RF-MEMS solutions in mobile and telecommunication systems will be briefly provided and discussed.

1.5-V-threshold-voltage Schottky barrier normally-off AlGaN/GaN high-electron-mobility transistors with fT/fmax of 41/125 GHz

In this paper, a normally-off AlGaN/GaN high-electron-mobility transistors (HEMT) fabricated using inductively coupled plasma (ICP) CF4 plasma recessing and an implantation technique is reported. A gate-to-channel distance of ∼10 nm and an equivalent negative fluorine sheet charge density of −1.21 × 1013 cm−2 extracted using a simple threshold voltage (Vth) analytical model result in a high Vth of 1.5 V, a peak transconductance of 356 mS/mm, and a subthreshold slope of 133 mV/decade. A small degradation of channel mobility leads to a high RF performance with fT/fmax of 41/125 GHz, resulting in a record high fT × Lg product of 10.66 GHz·µm among Schottky barrier AlGaN/GaN normally-off HEMTs with Vth exceeding 1 V, to the best of our knowledge.

Radio-Frequency Characteristics of Partial Dielectric Removal HR-SOI and TR-SOI Substrates

High-resistivity silicon-on-insulator (HR-SOI) and trap-rich high-resistivity silicon-on-insulator (TR-SOI) substrates have been widely adopted for high-performance rf integrated circuits. Radio-frequency loss and non-linearity characteristics are measured from coplanar waveguide (CPW) transmission lines fabricated on HR-SOI and TR-SOI substrates. The patterned insulator structure is introduced to reduce loss and non-linearity characteristics. A metal-oxide-semiconductor (MOS) CPW circuit model is established to expound the mechanism of reducing the parasitic surface conductance (PSC) effect by combining the semiconductor characteristic analysis (pseudo-MOS and C–V test). The rf performance of the CPW transmission lines under dc bias supply is also compared. The TR-SOI substrate with the patterned oxide structure sample has the minimum rf loss (<0.2 dB/mm up to 10 GHz), the best non-linearity performance, and reductions of 4 dB and 10 dB are compared with the state-of-the-art TR-SOI sample's, HD2 and HD3, respectively. It shows the potential application for integrating the two schemes to further suppress the PSC effect.

Highly-Efficient Self-Compact Monopulse Antenna System With Integrated Comparator Network for RF Industrial Applications

A highly-efficient monopulse antenna system is proposed for radar tracking system application. In this study, a novel integrated front-end and back-end complicated three-dimensional (3-D) system is realized practically to achieve high-level of self-compactness. A wideband and compact monopulse comparator network is developed and integrated as the back-end circuit in the system. Performance of the complete monopulse system is verified together with the front-end antenna array. To ensure the system's electrical efficiency and mechanical strength, a 3-D metal-direct-printing technique is utilized to fabricate the complicated structure, avoiding drawbacks from conventional machining methods and assembly processes. Experimental results show the monopulse system can achieve a bandwidth of 12.9% with VSWR less than 1.5 in the Ku-band, and isolation is better than 30 dB. More than 31.5 dBi gain can be maintained in the sum-patterns of wide bandwidth. The amplitude imbalance is less than 0.2 dB and null-depths are lower than −30 dB in the difference-patterns. In particular, with the help of the metal-printing technique, more than 90% efficiency can be retained in the monopulse system. It is a great improvement compared with that obtained from traditional machining approaches, indicating that this technique is promising for realizing high-performance RF intricate systems electrically and mechanically.

High-Performance RF Devices and Components on Flexible Cellulose Substrate by Vertically Integrated Additive Manufacturing Technologies

This paper aims to demonstrate that novel additive manufacturing (AM) technologies like metal adhesive laminate and multilayer inkjet printing can be effectively exploited to fabricate high-performing radio-frequency passive components on flexible substrates. Both processes are substrate independent and therefore suitable for manufacturing circuits on several unconventional materials, such as photo-paper. In addition, their complementary features can be combined to develop a novel hybrid process. Proof-of-concept AM prototypes of passive components, such as capacitors and inductors, exhibiting quality factors over 70, never achieved before on paper, and self-resonant frequencies beyond 4 GHz are described. The maximum inductance and capacitance per unit area are 1.4 nH/mm 2 and 6.5 pF/mm 2 , respectively. Moreover, an AM RF mixer with a conversion loss below 10 dB is demonstrated still on paper substrate. The mixer, fabricated with the copper adhesive laminate method, operates at 1 GHz and exploits a lumped balun transformer connected to two packaged diodes in series.

What in the World Is Q? [[Distinguished Microwave Lecture}

Everyone acknowledges that quality factor (Q) works as a functional starting point for high-performance RF system design. Nonetheless, students and young engineers can be puzzled by how best to exploit Q?s efficacy in topology creation or circuit parameter optimization. This is because we encounter ambiguous expressions of Q appearing in different places and in various publications [1]. We might, then, wonder how many independent-looking Q formulations there are in our field and how they are physically related. With such a basic (but burning) question in mind, this article knocks at the door to the mysterious world of Q.

High power tests of an electroforming cavity operating at 11.424 GHz

The achievement of ultra high accelerating gradients is mandatory in order to fabricate compact accelerators at 11.424 GHz for scientific and industrial applications. An extensive experimental and theoretical program to determine a reliable ultra high gradient operation of the future linear accelerators is under way in many laboratories. In particular, systematic studies on the 11.424 GHz frequency accelerator structures, R&D on new materials and the associated microwave technology are in progress to achieve accelerating gradients well above 120 MeV/m. Among the many, the electroforming procedure is a promising approach to manufacture high performance RF devices in order to avoid the high temperature brazing and to produce precise RF structures. We report here the characterization of a hard high gradient RF accelerating structure at 11.424 GHz fabricated using the electroforming technique. Low-level RF measurements and high power RF tests carried out at the SLAC National Accelerator Laboratory on this prototype are presented and discussed. In addition, we present also a possible layout where the water-cooling of irises based on the electroforming process has been considered for the first time.

Improved analog and RF performances of gate-all-around junctionless MOSFET with drain and source extensions

In this paper, the analytical investigation of a new design including drain and source extensions is presented to assess the electrical behavior of cylindrical gate-all-around junctionless (GAAJ) MOSFET for high performance RF and analog applications. Analytical models for drain current and performance parameters are derived incorporating the effect of two highly doped extension regions. Various analog and RF parameters like transconductance, cut-off frequency, drain current drivability, voltage gain and linearity characteristics have also been investigated. The proposed design shows excellent ability in improving the analog performance and provides a good solution to enhance the RF behavior and linearity of GAAJ MOSFET for low cost and high performance analog/RF applications. The proposed model results have been validated against the data obtained from a commercially available numerical device simulator. Moreover, the developed analytical approaches are easy to be implemented into microelectronic software simulators and therefore allow the study of the GAAJ-based deep submicron circuits.

High-Performance RF Inductors and Capacitors Using the Reverse Trench Structure of Silicon

We propose new passive device structures using deep silicon trenches. High-aspect-ratio reverse trench structures coated with copper can be used to make signal lines of the spiral inductor, and a deep trench around the MIM capacitor can be used as a signal isolation wall after back-side thinning. A laminated organic is used to fill the trench. It is possible to make a 10:1 ratio metal pattern maximally, and all signal lines are fixed and isolated due to the laminated organic. Though the spiral inductor is realized on lossy silicon, its Q-factor is higher than 40 at 2 GHz. The fabricated 0.9 GHz LPF adjusting the reverse trench process has only 0.5 dB insertion loss with a very small size.