Chapter Eight - Ferrites for RF Passive Devices

Retirement of Associate Editor Lawrence J. Varnerin

Background: Radiofrequency (RF) devices have widespread use in skin rejuvenation. Although they are used noninvasively, recently minimally invasive RF devices are being added to the inventory to increase their efficiency. Because RF devices do not operate on a light basis, their effects are independent of skin color and type. Therefore, they have a broader spectrum of patients compared to other noninvasive and minimally invasive devices. Skin rejuvenation with RF devices will continue to be important for plastic surgeons to pursue the nonsurgical operations. With RF application, heat is generated at different levels and different degrees under the skin. Methods: Shrinkage and denaturation of the collagen with temperature increase the likelihood of desired rejuvenation effects. The degree of temperature increase in RF applications depends on the frequency of the devices, the power of the devices used, and the characteristics of the headers. Today, different types of RF devices are offered by manufacturers. Heating with an RF device in a therapeutic dose of the skin is possible if appropriate frequency and adequate power are provided. When the therapeutic temperature is close to the complication limit, the user needs to know the device well, be aware of the skin structure at the application site and skin thickness, as well as can adjust the application doses well to get better therapeutic results. Conclusion: The wide variety of RF devices has led to the development of different application methods for users. In this article, RF devices, mechanisms of action, methods of use, clinical practice techniques, and results are reviewed. Even though the results are good, RF applications are not an alternative to a surgery.

Monolithic 3D-IC is one of the solutions to relieve Moore’s law with vertically integrating circuits for sub-1nm technology nodes. Therefore, thin-film transistors (TFTs) play an important role in this trend because of their low fabrication temperature to realize back-end circuits. On the other hand, 3D integrating filter, duplexer, switch, and so on is necessary as antennas array requirements increase in 5G or beyond. Consequently, it is foreseeable to adopt TFTs to implement radio frequency (RF) devices. Fig. 1 shows the schematic ideal 3D SoC for sub-1nm technology. Our previous research tried to demonstrate high-frequency back-end devices based on the gate-all-around stacked nanosheet low-temperature polycrystalline silicon channel (GAA NS LTPS). Fig.2 shows the current-voltage transfer characteristics of different width designs of LTPS and α-IGZO devices. The only way to enhance GAA NS LTPS RF devices with the same process is by increasing channels. However, it leads to a larger footprint, and the frequency doesn’t boost with increasing channels in proportion because of the capacitance between the multiple channels. In the meantime, the LTPS gate controllability becomes poor, and threshold voltage shifts significantly when the drive current is improved by widening channel width. Consequently, a-IGZO is adopted as the channel material of RF devices to solve the problem mentioned above. The a-IGZO film is back-end compatible and has transparency and high uniformity. The most important is that the gate controllability decay phenomenon is negligible no matter what the channel width is, which is helpful for different width designs. In contrast, IGZO devices can keep their threshold voltage and have ultra-low leakage current due to a large bandgap. According to the system on panel (SoP) trend, we attempt to integrate RFIC with the peripheral circuit on a substrate. Therefore, a-IGZO is also introduced as a pull-down transistor in a CMOS for power reduction and process simplicity. To further minimize the footprint, the a-IGZO devices are nanoscale and stacked on the p-type LTPS as the defined heterogeneous CFET (HCFET), which is demonstrated in the previous study [1]. In this talk, we will discuss HCFET architecture in detail. We compared junctionless mode (JL) and inversion mode (IM) for bottom p-type LTPS in HCFET. In our results, IM is better than JL as the junction structure for bottom PMOS because the requirement of the bottom channel in a HCFET is thin and width flexible for design. In addition, a trench gate of the bottom device plays an important role in HCFET. The trench tri-gate structure can avoid gate dielectric damage by plasma in the etching process, keep the top IGZO layer continuously and enhance performance compared to the general tri-gate structure. On the other hand, the gate of top n-type IGZO can be bottom gate only or dual gate for different requirements. Finally, HCFET can significantly reduce the distance between IGZO and p-type LTPS channels to save power and lower latency in the circuit.[1] S. -W. Chang et al., "First Demonstration of Heterogeneous IGZO/Si CFET Monolithic 3-D Integration With Dual Work Function Gate for Ultralow-Power SRAM and RF Applications," in IEEE Transactions on Electron Devices, vol. 69, no. 4, pp. 2101-2107, April 2022, doi: 10.1109/TED.2021.3138947. Figure 1

Development of high-power switching and radio frequency (RF) devices based on the ultrawide-bandgap (UWBG) AlGaN alloy system is generating interest in high Al composition alloys. The increase in bandgap as the alloy composition is varied from GaN to AlN is associated with an increase in the critical electric field, and thus the achievable breakdown voltage, of power devices based on these materials. In addition to the higher critical electric field, the relative insensitivity of saturation velocity to alloy composition offers prospects for RF applications. Also, high Al composition AlGaN transistors offer improved performance at high temperatures. Basic challenges, associated with controllable doping, electrical contacts, and passivation, remain for implementation of high Al composition alloys in these applications, but the expected improvement in device performance drives investigation of their properties. To date, both lateral and quasi-vertical power switching device geometries have been demonstrated (1), and the research on UWBG AlGaN transistors in power and RF electronics was recently reviewed (2).High-quality native substrates enable epitaxial growth of device heterostructures with low densities of threading dislocations, due to the low lattice and thermal mismatches to epitaxial active layers. Single crystal AlN substrates possess a high thermal conductivity and a close lattice match to high Al composition alloys, which make them an excellent choice for growth of AlGaN-based power switching and RF devices. Physical vapor transport growth (PVT) of 2-inch diameter AlN substrates free of low angle grain boundaries and with average threading dislocation densities below 103 cm-2 was recently demonstrated (3). However, high below-bandgap optical absorption in the ultraviolet-C (UV-C) region was observed in AlN grown by PVT in carbon-containing atmospheres, due to a deep-level absorption band related to the carbon impurity. This absorption band negatively impacts the efficiency of optoelectronic devices, which have driven adoption of AlN substrates to date, and typically require light extraction through the substrate. In this work, we studied the optical properties of double-side polished, 2-inch, c-plane AlN substrates by UV-Vis spectroscopy. Absorption coefficients were calculated by accurately accounting for reflection losses (4). Spatially uniform absorption coefficients below 30 cm-1 at 265 nm were demonstrated across 2-inch substrates. Furthermore, the calculated absorption and reflection coefficients were used to determine the complex refractive index and relative permittivity for the E⊥ c polarization in the 250-700 nm range, showing good agreement with published data for nominally unstrained, UV-C transparent bulk crystals.Finally, reliable values for the high-frequency and static permittivities, which are needed in the design of high-power electronic devices, were obtained.References R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. R. Dickerson, A. J. Fischer, A. G. Baca, and E. A. Douglas, ECS J. Sol. State Sci. and Technol. 6(2), Q3061 (2017).A. G. Baca, A. M. Armstrong, B. A. Klein, A. A. Allerman, E. A. Douglas, and R. J. Kaplar, J. Vac. Sci. Technol. 38, 20803 (2020).R. Dalmau, J. Britt, H. Fang, B. Raghothamachar, M. Dudley, and R. Schlesser, Mater Sci. Forum 1004, 63 (2020).R. Dalmau, J. Britt, and R. Schlesser, ECS Trans. 98(6), 3 (2020).

Radio Frequency (RF) devices produce some amount of Unintended Electromagnetic Emissions (UEEs). UEEs are generally unique to a device and can be used as a signature for the purpose of detection and identification. The problem with UEEs is that they are very low in power and are often buried deep inside the noise band. The research herein provides the application of Support Vector Machine (SVM) for detection and identification of RF devices using their UEEs. Experimental Results shows that SVM can detect RF devices within the noise band, and can also identify RF devices using their UEEs.

In recent years, the design and implementation of various types of sensors in the radio frequency (RF) and microwave frequency region for material characterization and testing has gained a lot of attention from various researchers around the globe. The characterization and testing of dielectric materials in the RF and microwave frequency band primarily involves estimating the dielectric properties of materials in the specified frequency band for design and development of various types of modern RF devices and circuits. However, the RF dielectric testing procedure has also become quite attractive for several real-word applications apart from estimating the dielectric properties of materials due to the fact that it is basically a non-invasive and non-destructive process. The RF sensor is a generic term quite often used to represent the electronic device or the hardware, which is required to be designed and developed to facilitate the accurate testing of dielectrics in the RF and microwave frequency regime. In the earlier days, the conventional RF sensors were primarily based on the metallic waveguide or the coaxial line structures. The metallic waveguides or the coaxial lines are basically non-planar bulky structures, employed in the past to design various RF circuit components and devices. However, in modern times, these non-planar structures are mostly replaced with the planar configurations in order to realize the compact RF devices and components. It is mainly due to this reason that the planar RF sensor has emerged as the viable alternative to the conventional waveguide and coaxial sensors for the characterization and testing of dielectric materials in the specified frequency range. The planar RF sensors offer small size, lightweight, compact design, and ease of integration with other RF circuits based on the planar technology. Now, as mentioned earlier, apart from the direct applications of material testing, the planar RF sensors are now also being employed for certain indirect applications in the food, agricultural and bio-medical industries. For these industries, the major application emerging in recent years has been to detect the quality of various types of edible and medical products. It appears that in order to achieve accurate dielectric testing and to successfully realize various types of indirect industrial applications mentioned above, the sensitivity of the designed planar sensor should be reasonably high. As a matter of fact, the sensor with higher sensitivity is most suited to detect any type of adulteration in the edible or the medical grade product. In the last few decades, it has emerged that the planar RF sensors employing the concept of electrically small structures have reasonably higher sensitivity, thus being well suited to accurately estimate the complex permittivity of materials. These electrically small configurations are usually realized using the engineered planar structures in order to achieve excellent sensitivity due to the concentrated electric field, which are becoming quite popular in recent years for direct application in the RF industry to perform the dielectric testing of materials and media. The split ring resonator (SRR), the complementary split ring resonator (CSRR), and the interdigital capacitor (IDC) structures may be considered as a few of the basic engineered structures for these types of applications. However, the IDC structure is found to be more appropriate for certain dielectric testing applications than that of the CSRR and SRR due to its relatively high sensitivity and the overall compact size, as it can directly be etched on the main signal line, thus providing quite high sensitivity. This chapter is mainly focused on the IDC based planar RF sensor and its implementation for various dielectric testing applications. A detailed analysis, including the theoretical description involving the quasi static model and the electromagnetic model of a typical IDC based sensor structure, along with the associated dielectric sensing mechanism, is provided. Both the resonant and the non-resonant variants of the IDC structure including their working principle relevant to the dielectric sensing mechanism are discussed in detail. Finally, the use of the planar IDC RF sensors for various industrial, environmental, and biomedical applications based on their design configurations are discussed.

Gallium nitride (GaN) is becoming a mainstream semiconductor for power and radio-frequency (RF) applications. While commercial GaN devices are increasingly being adopted in data centers, electric vehicles, consumer electronics, telecom and defense applications, their performance is still far from the intrinsic GaN limit. In the last few years, the fin field-effect transistor (FinFET) and trigate architectures have been leveraged to develop a new generation of GaN power and RF devices, which have continuously advanced the state-of-the-art in the area of microwave and power electronics. Very different from Si digital FinFET devices, GaN FinFETs have allowed for numerous structural innovations based on engineering the two-dimensional-electron gas or p–n junctions, in both lateral and vertical architectures. The superior gate controllability in these fin-based GaN devices has not only allowed higher current on/off ratio, steeper threshold swing, and suppression of short-channel effects, but also enhancement-mode operation, on-resistance reduction, current collapse alleviation, linearity improvement, higher operating frequency, and enhanced thermal management. Several GaN FinFET and trigate device technologies are close to commercialization. This review paper presents a global overview of the reported GaN FinFET and trigate device technologies for RF and power applications, as well as provides in-depth analyses correlating device design parameters to device performance space. The paper concludes with a summary of current challenges and exciting research opportunities in this very dynamic research field.

In recent years, replacing the external control stimuli with internal control using characteristics of radio frequency (RF) signals (such as waveform or power level) has attracted considerable attention to the design of reconfigurable multifunctional RF devices. However, even with the most exciting techniques, the control process always needs a chip-based system to sense the power level or waveform of the incident RF signal, which is realised by additional supporting electronic components of the sensing and microcontroller circuits. Therefore, to achieve a batteryless structure, a majority of conventional works focus on using energy harvesters to convert energy from external environmental sources to DC energy for the electronic circuits. Herein, we propose a novel alternative approach to replace the traditional energy harvesters in batteryless RF devices with all-passive energy smart-sensing circuits. By exploiting the features of a nonlinear semiconductor device under different incident RF power values, the proposed network can passively self-sense the RF power level and dynamically self-control RF signal flow and power ratio. The operation of the proposed network can be considered as purely self-adaptation with control from the RF power level. Moreover, normal RF devices can be transformed to all-passive and batteryless purely self-reconfigurable devices via integration with the proposed structure. As proof of concept, the proposed network is integrated with a two-port antenna to experimentally demonstrate its purely self-reconfigurable polarisation. The proposed strategy is hereby expected to extend the field of batteryless self-reconfigurable multifunctional RF devices and pave promising new paths for the development of future intelligent, smart RF devices.

Germanium Telluride (GeTe) can be described as a non-volatile (latching state) phase change material (PCM) in memory applications. GeTe also exhibits a volatile (reversible state) region when heated and cooled between 100-180 °C. At temperatures higher than 185 °C the material crystallizes and “latches” until a temperature near to its melting point (725 °C) is reached and cooled rapidly (quenching). Germanium Antimony Telluride (GeSbTe) or also known as GST has similar characteristics as GeTe. GST also exhibits a volatile (reversible state) region when heated and cooled between 100-150 °C. GST crystallizes at 155 °C and its melting point is 600 °C. This paper demonstrates the feasibility of fabricating radio frequency (RF) devices of phase change materials (PCM) and it also presents a comparison between amorphous and crystalline PCMs in the RF spectrum. Previous work focuses on exploiting GeTe and GST as nonvolatile materials in memory applications, and also on characterizing them for their electrical and mechanical properties. The approach here focuses on fabricating RF devices and analyzing their responses. A simulation with resistor-capacitor (RC) and resistor-inductor (RL) circuits is presented to represent the response of the RF devices under testing. The fabrication process includes two-layer and four-layer devices on the Si wafer. PCMs are sputtered and the test pads are deposited using electron beam evaporation. Results show that these RF devices alone can serve as a low pass filter with a cutoff frequency of 10 MHz.

With the increasing demand for compact, lightweight, cost-effective, and high-performance radiofrequency (RF) devices, the development of low-profile antennas becomes crucial. This article presents a study of a novel carbon-cellulose-based paste intended for screen printing RF devices. The investigation specifically explores the application of high-reactivity carbon mixture (HRCM) particles as conductive fillers. The results demonstrate that optimal electrical conductivity values and discrete electromagnetic dipole performances can be achieved at lower concentrations of solid conductive material compared to conventional pastes, for similar applications. This offers benefits in terms of total cost, material consumption, and environmental impact. The paste formulation showcases a complex non-Newtonian behavior, where yielding flow and thixotropicity are found to be independent and dependent on preshear conditions, respectively. This behavior can be attributed to the network orientation and rearrangement of filler structures within the paste system, which in turn are responsible for filler pattern uniformity and overall printing quality. Compared to traditional conductive materials, HRCM pastes are proven to be a viable alternative for RF devices fabrication, including printed Wi-Fi antennas.

ABSTRACTBackground: Various radiofrequency (RF) devices are used to treat skin laxity and face contouring, but few studies have examined ultrahigh-frequency (UHF) electric field (40.68 MHz) RF devices. Objective: To evaluate the efficacy and safety of a UHF electric field (40.68 MHz) RF device for skin tightening and face contouring. Methods: Ten patients each underwent four sessions of UHF electric field RF device treatment at 2-week intervals. Clinical improvement was evaluated with the patient satisfaction score using a six-point scale, and clinical photographs taken at every visit and 2 months after the RF treatment were assessed. Skin biopsies were obtained from one patient before the first treatment and immediately after the last treatment. Adverse reactions were recorded at every follow-up visit. Results: All patients were women with a mean age of 51.7 ± 7.2 years. The mean satisfaction score was 4.5 ± 0.9 immediately after the last treatment session. Cheek, jawline, and neck enhancement and tightening were apparent in all patients. Side effects were minimal, and there were no burns or major complications. Conclusions: The UHF electric field RF device was effective for skin tightening and facial contouring, without significant adverse reactions.

Abstract

Background Radiofrequency (RF) devices are widely used in arthroscopic shoulder surgery for tissue ablation and other effects. They have heating effect during tissue application. It is known that chondrocyte death occurs with temperatures above 45 °C. The present study aimed to investigate the effect of various factors on the heating effect of RF probes in a controlled in vitro model. Methods A perspex model was used, in which volume, flow, outflow position, duration of RF probe use and inflow temperature could be closely controlled. Results Temperature rises in the bathing fluid sufficient to cause cell death were seen. The temperature rise was most noticeable when there was no fluid flow through the compartment, at smaller fluid volumes, if the fluid inflow temperature is elevated, and if the outflow is distant from the RF device. Of note, 30 seconds of use in a volume of 20 mL with no flow led to temperatures above 50 °C. Discussion Most of the factors affecting temperature rises using RF devices are directly under the surgeon's control. Further work is needed in this area to define the potential clinical effects of the use of these devices.

Both radiofrequency (RF) and microwave (MW) ablation devices are clinically used for tumor ablation. Several studies report less dependence on vascular mediated cooling of MW compared to RF ablation. We created computer models of a cooled RF needle electrode, and a dipole MW antenna to determine differences in tissue heat transfer.We created Finite Element computer models of a RF electrode (Cooled needle, 17 gauge), and a MW antenna (Dipole, 13 gauge). We simulated RF ablation for 12 min with power controlled to keep maximum tissue temperature at 100 ºC, and MW ablation for 6 min with 75 W of power applied. For both models we considered change in electric and thermal tissue properties as well as perfusion depending on tissue temperature. We determined tissue temperature profile at the end of the ablation procedure and calculated effect of perfusion on both RF and MW ablation.Maximum tissue temperature was 100 ºC for RF ablation, and 177 ºC for MW ablation. Lesion shape was ellipsoid for RF, and tear-drop shaped for MW ablation. MW ablation is less affected by tissue perfusion mainly due to the shorter ablation time and higher tissue temperature, but not due to MW providing deeper heating than RF. Both MW and RF applicators only produce significant direct heating within mm of the applicator, with most of the ablation zone created by thermal conduction.Both RF and MW applicators only directly heat tissue in close proximity of the applicators. MW ablation allows for higher tissue temperatures than RF since MW propagation is not limited by tissue desiccation and charring. Higher temperatures coupled with lower treatment times result in reduced effects of perfusion on MW ablation.

Severe valgus knee deformity caused by chondronecrosis after using a radiofrequency device