Low-loss Substrate Research Articles

Design of Broadband W-Band Waveguide Package and Application to Low Noise Amplifier Module

In this paper, the broadband millimeter-wave waveguide package, which can cover the entire W-band (75–110 GHz) is presented and applied to build a low noise amplifier module. For this purpose, a broadband waveguide-to-microstrip transition was designed using an extended E-plane probe in a low-loss and thin dielectric substrate. The end of the probe substrate was firmly fixed on to the waveguide wall in order to minimize the performance degradation caused by the probable bending of the substrate. In addition, we predicted and analyzed in-band resonances by the simulations that are caused by the empty spaces in the waveguide package to accommodate integrated circuits (ICs) and external bias circuits. These resonances are removed by designing an asymmetrical bias space structure with a radiation boundary at an external bias connection plane. The bond-wires, which are used to connect the ICs with the transition, can generate impedance mismatches and limit the bandwidth performance of the waveguide package. Their effect is carefully compensated for by designing the broadband two-section matching circuits in the transition substrate. Finally, the broadband waveguide package is designed using a commercial three-dimensional electromagnetic structure simulator and applied to build a W-band low noise amplifier module. The measurement of the back-to-back connected waveguide-to-microstrip transition including the empty spaces for the ICs and bias circuits showed the insertion loss less than 3.5 dB and return loss higher than 13.3 dB across the entire W-band without any in-band resonances. The measured insertion loss includes the losses of 8.7 mm-long microstrip line and 41.8 mm-long waveguide section. The designed waveguide package was utilized to build the low noise amplifier module that had a measured gain greater than 14.9 dB from 75 GHz to 105 GHz (>12.9 dB at the entire W-band) and noise figure less than 4.4 dB from 93.5 GHz to 94.5 GHz.

A wideband, single layer reflectarray antenna with cross loop and square ring slot loaded patch elements

AbstractThis paper presents the design and analysis of a wideband X/Ku and Ku band reflectarray antenna. The proposed unit cell of the reflectarray antenna comprises a patch loaded with two distinct slots, viz. a square ring and a cross loop, printed on a low loss substrate, which is backed by a foam-loaded ground plane. The unit cell element offers a linear and large dynamic reflection phase range, which is achieved by optimizing the shape, location, and geometrical parameters of the two slots loaded on the patch. A 324 element microstrip reflectarray antenna of size 200 × 200 mm2 is constructed and analyzed for its radiation characteristics by simulation and measurement. The reflectarray offers a 3 dB gain bandwidth of 50.75% with the operating frequency range of 10–16.8 GHz. It offers a peak gain and aperture efficiency of 25.4 dB and 40% at 12.6 GHz, respectively. The cross-polarization level is below −40 dB over the entire operating frequency range.

Low-Loss and Light Substrate Integrated Waveguide Using 3D Printed Honeycomb Structure.

This article proposes a low-loss and light 3D-printed substrate-integrated waveguide (SIW). Despite the use of lossy polylactic acid (PLA) material, insertion loss is reduced, and bandwidth is increased due to a honeycomb substrate similar to air. To demonstrate the proposed concept, we fabricated microstrip-fed SIWs with solid PLA and honeycomb substrates, and compared their performance numerically and experimentally. Average measured insertion loss from 3.4 to 5.5 GHz for the honeycomb SIW is 1.38 dB, whereas SIW with solid PLA is 3.15 dB. Light weight is an additional advantage of the proposed structure.

FSS Based Circular Polarizer for High-Speed Wireless Communication at 75 GHz

An FSS based circular polarizer for high-speed wireless communication at 75 GHz is presented. It has been designed on a low loss substrate with cross-dipole elements. Both simulation and measured results showed more than 98% circular polarization at 75 GHz. Moreover, 3 dB axial-ratio bandwidth of 6.8 GHz (Simulation) and 7.8 GHz (Measured) has been achieved. The proposed design has many advantages over the recently published research such as simplicity, low-profile, percentage bandwidth, frequency of operation and relative insertion loss.

Parameters and Factors Affecting Reliability and Accuracy in Measuring Electro-Textile Conductivity Using Transmission Line Method

Designing and analyzing a textile-based antenna requires precise representation of the material in electromagnetic solver. Due to its complex and inhomogeneous thread structure, the exact cross-sectional structure of the conductive textile segment was very difficult to be modeled, and thus the representation was simplified to bulk conductivity characteristic. Incorrect conductivity value will cause significant deterioration in antenna impedance and radiation performance. Therefore, the conductivity for an electro-textile material used for a wearable antenna design shall be accurately measured. Previously, a transmission line technique was proposed, however, the reliability of the method in correlation to operational frequency and other test conditions were not clarified. Furthermore, the practicality of that technique and the proposed equation for an actual electro-textile sample was not demonstrated. In this paper, the validity of the method over wide range of frequencies from 500 MHz to 5GHz was validated through a more practical and simplified equation derived from the two-port transmission line concept. The accuracy of the proposed technique and equation was demonstrated through comprehensive electromagnetic simulation, measurement and analysis. For validation, strip line samples that consist of an off-the shelf conductive fabric called SHIELDIT were fabricated on two low-loss substrates with slightly different dielectric characteristics; Rogers RO5880 and Chukoh CGP-500A at 500 MHz, 2.45 GHz, 3.5 GHz and 5 GHz. For comparison, the applicability of the technique for other samples such as pure laminated copper and self-manufactured electro-textile was also tested as reference. Through analysis, the reliability of the technique for different material's loss characteristics, material's conductivity range and operational frequencies was compared through simulation and measurement. The threshold level for conductivity of the test sample was found to be less than 10 7 S/m, which has been validated from 0.5 GHz until up to 5 GHz as shown by the measurement results in this paper.

Thermal Characterization of Quasi-Vertical GaAs Schottky Diodes Integrated on Silicon Using Thermoreflectance and Electrical Transient Measurements

Thermal management and design have been understood, for many years, as critical factors in the implementation of submillimeter-wave Schottky-diode-based circuits and instruments. Removal of heat is particularly important for frequency multipliers, as these circuits generally exhibit low-to-modest conversion efficiencies, and are usually driven with high-power sources to achieve usable output power in the submillimeter-wave region of the spectrum. Elevated diode junction temperature due to inadequate heat sinking is known to degrade performance, accelerate aging effects (for example, due to electromigration, ohmic contact deterioration, or thermally-induced stress), and can ultimately lead to device failure. The relatively-low thermal conductivity of GaAs (the predominant material technology for submillimeter-wave diodes), coupled with restrictions on diode anode size and geometry needed to minimize parasitics and achieve the device impedances required for high-frequency operation, present significant challenges and trade-offs between electrical and thermal designs of these devices. Recognition that heating is a major factor limiting efficiency and output power has prompted a number of approaches to mitigate excessive temperature rise in the junction of planar Schottky diodes, including the use of AlN or diamond as low-loss substrates that act as heat spreaders. A new diode topology, based on a quasi-vertical geometry that is realized through heterogeneous integration of GaAs with high-resistivity silicon, was recently developed at the University of Virginia for submillimeter-wave applications. Unlike planar diodes, the device structure of the quasi-vertical diode consists of a metal contact that underlies the diode's anode and epitaxial mesa, thus providing a large-area ohmic cathode contact that also serves as an integrated heat sink. Measurement of high-efficiency multipliers based on this technology suggest the quasi-vertical architecture provides an effective approach for heat removal and thermal management in Schottky diodes. This paper presents the first results reporting thermal performances of terahertz quasi-vertical GaAs Schottky diodes integrated on silicon. The devices are characterized using a thermoreflectance measurement technique, a method based on the change in refractive index, and therefore surface reflectivity, with changes in temperature. Heating and cooling temperature profiles and 2-D temperature maps are obtained for 3.5 micron and 5.5 micron diameter diodes. From these measurements, the device thermal resistances, junction temperatures, and thermal time-constants are extracted. Equivalent thermal circuit and finite element models are developed to study the device geometry, and extract material thermal parameters. The devices are also characterized using an electrical transient method, and the temperature and cooling transients found from this technique are found to be comparable to those obtained from thermoreflectance measurements. The quasi-vertical diodes studied in this work are shown to demonstrate a faster transient thermal response compared to flip-chip bonded terahertz diodes reported in the literature.

A compact and high gain substrate integrated horn array antenna system

AbstractIn this paper, a novel compact, lightweight and high gain array antennae system is demonstrated which is based on substrate integrated dielectric filled waveguide concepts. It is a planar design which can be easily fabricated with a common two‐layer substrate. The overall goal was to have a high gain so that this array antenna could be potentially used in autonomous unmanned aerial vehicle systems for collision avoidance. An initial model was designed, simulated and fabricated and, upon evaluation, was further modified to achieve a higher performance specification. Additionally, the design can be easily and inexpensively fabricated using standard substrates with a rapid prototyping platform. A two element array resulted in a simulated gain of 9.6 dB with a radiation pattern which had a maximum lobe between a broadside and an end‐fire. This was with a low loss substrate such as Rogers RT duroid 5880. For the design, a low dissipation factor material is preferable as it minimizes losses and also increases compactness of the entire antenna array.

Post-CMOS Fabrication Technology Enabling Simultaneous Fabrication of 3-D Solenoidal Micro-Inductors and Flexible I/Os

In this paper, co-fabrication and characterization of micro-inductors with mechanically flexible interconnects (MFIs) as I/Os is presented. A double-lithography and double-reflow process is used to obtain different heights of reflowed photoresist domes for the micro-inductors and the MFIs. The developed process allows fabrication of MFIs with different heights, pitches, and materials. The fine-pitch copper MFIs allow for dense connections between chips using interconnect bridging or stitching technologies (embedded multi-die interconnect bridge, heterogeneous interconnect stitching technology, etc.), while the large nickel tungsten (NiW) MFIs enable connections with the base substrate. The interconnect stitch chip can also host integrated passive devices, allowing close integration with active chips. Furthermore, the stitch chip can be fabricated out of low-loss substrate, enabling high-quality passives along with low-loss electrical interconnections. Copper micro-inductors with two different numbers of turns were fabricated on glass to minimize substrate losses; inductance, resistance, and Q-factor values of 0.45 nH, $4~\Omega $ , and 25 were obtained at 33 GHz for the five-turn inductor with a self-resonant frequency of >40 GHz. The compliance of the fabricated 9- $\mu \text{m}$ -thick Cu MFI was measured to be 1.1 mm/N, while that of the 7- and 10- $\mu \text{m}$ -thick NiW MFIs were measured to be 10 and 2.6 mm/N, respectively.

Numerical and Experimental Time-Domain Characterization of Terahertz Conducting Polymers

A comprehensive framework for the theoretical and experimental investigation of thin conducting films for terahertz applications is presented. The electromagnetic properties of conducting polymers spin-coated on low-loss dielectric substrates are characterized by means of terahertz time-domain spectroscopy and interpreted through the Drude-Smith model. The analysis is complemented by an advanced finite-difference time-domain algorithm, which rigorously deals with both the dispersive nature of the involved materials and the extremely subwavelength thickness of the conducting films. Significant agreement is observed among experimental measurements, numerical simulations, and theoretical results. The proposed approach provides a complete toolbox for the engineering of terahertz optoelectronic devices.

Integration of Wireless Coil and Bluetooth Antenna for High Charging and Radiation Efficiencies

In this paper, a wireless charging coil integrated with a Bluetooth antenna is proposed. The equivalent circuit model is developed to analyze this multifunctional coil. The outermost turn of the coil is appropriately designed in order to improve the radiation efficiency of the antenna. In this way, the radiation frequency of the coil is determined by the length of the outermost coil. Meanwhile, the inductance of the coil is adjusted to achieve high charging efficiency. When the coil is taken as a Bluetooth antenna, the measured reflection coefficient has achieved a fractional bandwidth of 8.3%, covering a frequency range of 2.3–2.5 GHz, with $\vert {S} _{11}\vert dB. The measured radiation efficiency of the antenna is about 62% at 2.4 GHz. When the coil is applied, the system charging efficiency is about 61%. In particular, the entire circuit module is fabricated on the low-cost FR4 printed circuit board. It can be expected that both the radiation and charging efficiencies can further be improved by using low-loss substrate.

Periodic Leaky-Wave Antenna With Circular Polarization and Low-SLL Properties

A periodic leaky-wave array antenna with circular polarization and low sidelobe level (SLL) for millimeter-wave application is proposed. The unit cell of the array consists of four circular split rings located on both sides of a low-loss substrate, and a planar Goubau line is used to excite the split rings to generate radiation. The antenna can realize an impedance bandwidth larger than 67%, and the 3 dB axial-ratio bandwidth is about 8.9%. The maximum gain is about 18 dBi. To reduce the SLL, the leakage loss factor α along the radiation aperture is tapered, using beamforming technique, by controlling the aperture field distribution into a cosine function. The results show that the SLL of the proposed antenna is about 6 dB lower than that of the antenna with uniform aperture field distribution. The antenna was fabricated and measured, and the experimental results are in good agreement with those of simulation.

Small- and Large-Signal Performance Up To 175 °C of Low-Cost Porous Silicon Substrate for RF Applications

This paper focuses on the comparison of the RF performances of various advanced trap-rich (TR) silicon-on-insulator (SOI) and porous silicon (PSi) substrates. The PSi substrates are produced by electroporisification starting either from cheap standard resistivity ( $10~\Omega \cdot $ cm) silicon (PSi-S) or from the most widespread highly doped milliohm-centimeter Si wafers (PSi-M). Through small-signal RF measurements, it is shown that TR-enhanced high-resistivity silicon and both types of PSi are acceptable for RF applications, showing high effective resistivities and low RF substrate losses. What is more the substrate effective permittivity is decreased in the PSi substrates providing higher coupling isolation at high frequencies. Furthermore, large-signal harmonic distortion measurements reveal very strong linearity for both PSi substrates, with PSi-S achieving 50-dB improvement over the already highly linear TR substrate. Finally, RF characterization was carried out over temperature from 25 °C to 175 °C. While the performances of TR and PSi-M substrate rapidly deteriorate above 120 °C, the cost-effective PSi-S substrate maintains high performance all the way up to 175 °C, allowing it a wider range of potential applications.

Novel Broadband Antennas for Future Mobile Communications

Compact planar antipodal endfire broadband antennas based on novel windmilllike shape are proposed in this paper for future mobile applications. Asymmetric structure of the proposed antennas makes these antennas to radiate in boresight direction, while antipodal structure provides wideband behavior. In the first phase, an ultrawideband antenna is designed using an FR4 substrate with a size of 34 mm $\times$ 30 mm, for the frequency range from 4 to 10 GHz and a peak gain of 5 dBi. This antenna is then upgraded to Antenna-II, to shift its frequency band to more than 10 GHz and up to >150 GHz. To improve the performance of Antenna-II, a low-loss substrate Rogers RO4232 is used. The final optimized antenna achieves at least 175% fractional bandwidth in the frequency range from 10 to >150 GHz and exhibits a peak gain of 8.7 dBi while maintaining an overall average gain of 6.4 dBi in the entire frequency range. Design as well as the performance analysis of this antenna is also investigated for low earth orbit-to-low earth orbit intersatellite link applications with its power analysis.

Non‐linear characteristics of an optically reconfigurable microwave switch

A novel, high-power optically controlled microwave switch is presented. The switch uses integrated illumination of a silicon superstate through a low-loss glass substrate which reduces losses related to the plasma conductivity tail in the silicon. Numerical electromagnetic modelling is used to design the switch and good agreement between measured and simulated results has been achieved. The switch is then characterised using a two-tone non-linearity test at 2 GHz and a third-order intercept point of +72 dBm is obtained with 10 W per tone.

Tunable terahertz filter composed of an array of subwavelength metallic ring apertures

We demonstrate a reconfigurable structure on a low-loss polystyrene foam substrate functioning as a tunable terahertz filter. The device consists of periodic metallic rings with a tunable lateral spacing between the outer holes on the upper layer and the inner pillars that are attached to the substrate. The device works in the transmission mode and its polarization-dependent resonance frequency can be adjusted by moving the perforated metallic membrane with respect to the pillars. The transmission characteristic of this periodic structure is studied in detail and used to optimize its configuration. It is shown that the tuning range, resonance frequency, and bandwidth of the filter can be controlled by properly scaling the geometrical parameters of the filter, i.e., the inner and outer radii of the holes, the height of the upper metallic membrane and the periodicity of the structure. This device is polarization-sensitive and can be tuned over a wide frequency range from 1.03 THz to 1.4 THz for terahertz waves polarized perpendicular to the displacement direction of the membrane.

Configuration for High-speed Transmission between Flip-chip Packages Using Low Loss and Flexible Substrate

Configuration for High-speed Transmission between Flip-chip Packages Using Low Loss and Flexible Substrate

Low Dk/Df Polyimide Adhesives for Low Transmission Loss Substrates

We developed solvent-soluble polyimides with good heat resistance and low dielectric constant (Dk)/dissipation factor (Df) characteristics by optimizing the composition ratio of the aliphatic, cycloaliphatic, and aromatic groups present in the polyimide back bone. We found that the adhesives prepared using our polyimides showed good adhesion to polyimide films and low profile copper foils and low Dk/Df. Furthermore, we developed a three-layer flexible copper clad laminate (3LFCCL) with our polyimde (PI) adhesives, low-pofile copper foils, and normal PI films. This 3LFCCL showed a transmission loss similar to that of liquid crystal polymer (LCP) FCCLs at frequencies less than 20 GHz. This result indicates that by using our PI adhesives, lower cost FCCLs can be prepared, which can then be used as high-frequency substrates.

Reliability of Strip Line Method for Determination of Conductivity for Lossy Conductive Materials

Designing electro-textile antenna requires complex analysis as the material cannot be simply modeled as a conventional antenna with perfect metallic surface. Electro-textile has relatively lower conductivity compared with an ideal conductor. Due to its inhomogeneous and unique thread structure, exact conductivity, $\sigma _{c}$ , is difficult to be determined, which will result in inaccurate modeling during antenna designing. Significant deterioration in antenna efficiency can be observed when low conductivity element is used, and thus, accurate determination of bulk conductivity for customized electro-textile shall be performed. Previously, a strip-line method was proposed, and however, the reliability of the technique in correlation with material’s loss characteristics was not clarified. Furthermore, the practicality of the technique for higher frequency, such as ISM band, was not demonstrated. In this paper, the validity of the technique for lossless, low-loss, and high-loss materials for 2.45-GHz wearable application is clarified. A more practical and straightforward equation to calculate electro-textile’s bulk conductivity is derived, and the accuracy and reliability range of the equation is demonstrated through comprehensive electromagnetic simulation. Through analysis, the correlation between $\sigma _{c}$ , dielectric loss and attenuation loss is shown. For validation, two samples that consist of two conductors, namely copper and SHIELDIT, are fabricated on a low-loss RO5880 substrate. The $\sigma _{c}$ values of the conductors are calculated from transmission loss (S21) measurement. Good agreement of $\sigma _{c}$ is obtained for both cases, particularly when the actual $\sigma _{c}$ is within the threshold range of around 107 S/m.

A Simple Resonance Method to Investigate Dielectric Constant of Low Loss Substrates for Smart Clothing

A Simple Resonance Method to Investigate Dielectric Constant of Low Loss Substrates for Smart Clothing

Performance Metrics for Frequency Selective Surface-Based Sensors

A frequency selective surface (FSS) is a periodic array of conductive elements (located on a dielectric substrate) that has a specific transmission or reflection response when illuminated with electromagnetic energy. Since the FSS response is sensitive to changes in substrate properties (permittivity, loss tangent, and thickness) as well as element geometry, FSS-based sensors have strong potential as a wireless sensing solution. To this end, this paper proposes three performance metrics (resonant frequency, resonant depth, and quality factor) to quantify the effect of substrate properties on sensor performance. These metrics are applied to a patch- and loop-based FSS sensor. The results show that the patch-based sensor has improved performance when designed using a thinner and high loss substrate, while the loop-based sensor has better performance when a substrate with less loss but increased thickness is used. Additionally, measurements are also provided for sensors utilizing the low loss substrate. Finally, the performance of the two sensors is shown for strain sensing. The results indicate that the resonant frequency shift (when under mechanical loading) of both sensors is largely independent of the substrate permittivity.