We demonstrate a glass microwave microfluidic device for determining the permittivity of a wide range of liquid chemicals from 100 MHz to 30 GHz with associated uncertainties. Conventional microwave microfluidic devices use polymer-based microfluidic layers for fluid delivery, but these polymers swell in organic solvents and are not suitable for many applications. Our device incorporates glass microfluidic channels with platinum coplanar waveguides (CPWs) to provide a solvent-resistant architecture for broadband dielectric spectroscopy. We utilize broadband scattering parameter measurements with a vector network analyzer (VNA) on a wafer probing station and multiline thru-reflect-line (mTRL) calibrations to extract the distributed circuit parameters of transmission lines and solve for fluid permittivity. In this work, we demonstrate the utility of the device by measuring the permittivity of four organic solvents difficult to measure otherwise: hexane, heptane, decane, and toluene.

Polyimide thin films are commonly used as encapsulation and insulation material in implantable medical devices. Fluid transport can result in unwanted electrical or ionic pathways, adversely affecting device function. However, it is unknown how fluid transport in or under polyimide films contributes to the composite broadband dielectric properties. Notably at microwave frequencies, dielectric loss contributed by the fluid is large due to the characteristic relaxation frequency of water (similar to 20 GHz). We fabricated polyimide coplanar waveguides and immersed them in deionized water or phosphate-buffered saline and measured the frequency-dependent, broadband dielectric properties of the films subject to fluid exposure across 5 months. We observed changes in broadband permittivity (similar to 3.1 to 3.25) and peak loss tangent (0.006 to 0.011), tracking the dispersive dielectric properties of water. We observed no film delamination in the span of the study. Ultimately, these results suggest that fluid transport has a considerable effect on film dielectric properties and integrity. Quantification of such effects are crucial for evaluating the long-term use of polyimide in wireless, implantable, biomedical micromechanical systems (MEMS) that may operate at microwave frequency ranges (e.g., 2.45 GHz).

We evaluate the accuracy of small-signal on-wafer device characterization in the sub-THz frequency range when employing conventional and advanced calibration methods with state-of-the-art design of on-chip standards. We report that major discrepancies resulting from an interlaboratory comparison experiment are mainly attributed to the influence of microwave probes. When performing conventional multiline thru-reflect-line (mTRL) calibrations with six different models of probe from 140 to 325 GHz, we observe significant variations in the measured scattering-parameters (S-parameters) and show that probes made by various manufacturers induce disparate crosstalk responses that vary between -70 and -15 dB on open-open standards. After applying a crosstalk correction approach as a second-tier calibration, we obtain significantly better agreement between the RF performance of the same heterojunction bipolar transistor (HBT) measured with two different probe models. While the difference between the maximum stable gain (MSG) reaches 1.8 dB at 210 GHz after applying a conventional mTRL calibration, we reduce the error to similar to 0.5 dB after crosstalk correction. This study shows that crosstalk correction methods must be implemented in the sub-THz frequency range to accurately estimate the RF performance of active devices and circuits.

We report on broadband measurements of the effective permittivity of conductor-dielectric flexible composite materials over the frequency range 0.05 GHz - 67 GHz. The materials under test are comprised of silver nanoparticles dispersed in an elastomer host material (polydimethylsiloxane) with variable volume fraction from 0.75 % - 5.8 %. Broadband calibration and de-embedding techniques are applied, along with finite-element simulations, to determine the complex effective permittivity for four different silver nanoparticle volume fractions from the measured scattering parameters. The resulting effective permittivity is well described by Maxwell-Garnett mixing formula, and the material coefficients extracted from fits include the conductivity, permittivity, and shape factor of the silver nanoparticle inclusions.

The dielectric response of an ionic solution includes ionic diffusion, solvent molecule reorientation, and the intermolecular interactions between solvent, cations, and anions. While ionic diffusion and solvent reorientation produce large responses in the dielectric spectra, the contributions of the ion-ion and ion-water interactions can be difficult to measure and compute. Here, we combine computational and experimental techniques to investigate the impact of ion-ion and ion-water interactions in aqueous sodium fluoride solutions and determine whether an ion pairing peak exists in the dielectric spectra. We use our recently developed direct electric field approach to perform frequency-resolved configurational analysis of ion pairing. From this analysis, we identify a Debye peak associated with the reorientation of ion pairs. Our broadband microwave microfluidic spectra of sodium fluoride solutions further support the existence of a low-frequency Debye relaxation due to ion pairs in the solution.

Electronic waste is the fastest growing solid waste stream in the world, accounting for 54 million tons of waste globally. Such waste contains valuable chemical elements, such as gold, palladium, copper, silver, tin, and nickel that could be recycled back into the economy. Here, we present a proof-of-concept device to test a new electric-acoustic force balance to separate particles based on their electrical and mechanical properties. The device applies an acoustic force and electric force simultaneously causing microparticles to move to a position where the forces are equal. We used interdigitated transducers to generate a standing acoustic wave in the channel, collimating the particles at the pressure nodes. We then applied an electric field by applying a voltage between two electrodes patterned on either side of the microfluidic channel. This electric force changed the position where the microparticles collimated. The vision is to test microparticles from new waste recovery processes and show how they separate in our force balance to inform large scale separation technology.

On-wafer calibrations are critical for measurements of embedded devices at the correct reference planes. A major challenge in on-wafer calibrations is the development of accurate calibrations that cover a frequency range from MHz to THz. Another challenge facing on-wafer measurements is the lack of calibrations that are directly traceable to the SI. The multiline Thru-Reflect-Line (TRL) calibration is a promising approach for high-frequency traceable on-wafer measurements. However, space considerations limit this approach to higher frequencies. Here, we compare the performance of several calibrations with different footprint requirements. Specifically, we compare three calibration kits: TRL fabricated with Au conductors, TRL fabricated with Pt conductors, and a series resistor (SR) calibration kit with Au conductors. We find that both the Au and Pt TRL calibration kits perform well above approximately 100 MHz. Then, we compare the performance of the different TRL calibration kits with reduced numbers of lines to assess the impact of device footprint on calibration quality. We find that changing the conductor material to more resistive Pt does not substantially improve the quality of the TRL calibration at low frequencies, while increasing the length of the longest line does.

We present a method for accurately determining the permittivity of dielectric materials in 3D integrated structures at broadband RF frequencies. With applications of microwave and millimeter-wave electronics on the rise, reliable methods for measuring the electrical properties of dielectrics used in integrated circuits are critical. We outline an on-wafer method for extracting the permittivity of a 3D multilayer glass structure from 100 MHz to 30 GHz using S-parameter measurements of different calibration chips. Our method can be used to inform better design of metrology for dielectric materials for 3D integrated circuit technologies.

Electrosynthesis plays an important role in chemical manufacturing with over 2 billion dollars in annual sales worldwide of products such as chlor-alkali and aluminum. Synthesis with an electrochemical driving force can afford unique chemical products that are difficult to achieve with conventional synthesis techniques. Electrosynthesis can also provide a more sustainable alternative to energy-intensive processes. For example, the most profitable electro-organic synthesis in industry is adiponitrile production, a precursor to Nylon 6,6, and the electrochemical route consumes less energy than conventional thermochemical methods. However, electrosynthesis reaction mechanisms are often complex and involve multiple biproducts that are difficult to distinguish, limiting product yield and cost efficiency. Prior work involves characterization with techniques such as NMR spectroscopy, mass spectrometry, and optical spectroscopy to understand product evolution during the reaction. These methods tend not to capture intermolecular interactions between substances, which is important for understanding the influence of the dielectric environment on reaction outcomes. Here, we propose to analyze the dielectric environment of electrosynthesis reaction mixtures with on-chip broadband microwave microfluidic spectroscopy (MMS) from 40 kHz – 110 GHz (see attached figure). MMS has been implemented previously for capturing broadband fluid effects such as electric double layer formation, ionic conduction, ion pairing, and dipole relaxations in a single measurement. We want to use MMS to diagnose the interactions between species during an electrolysis reaction and correlate these interactions with specific electrochemical mechanisms. We plan to test our idea by analyzing a model reaction, namely the Shono oxidation of N-Boc pyrrolidine, which is a well-studied electrosynthesis reaction. The goal is to validate our method with a known electrosynthesis reaction mechanism, then extend the technique to systems with an unknown mechanism. We performed broadband MMS of the reaction mixture at different time points to capture product evolution during the electrolysis. Then, we calibrated the resulting S-parameters and fit the data to equivalent circuit models. The equivalent circuit parameters were correlated to signatures from specific entities in the product mixture and used to develop a picture of the evolving dielectric environment. After validating our models with known literature about the Shono oxidation mechanism, we plan to extend our technique to other electrolysis reactions with unknown mechanisms. The goal is to provide a high-throughput technique for identifying key species and interactions that influence the outcome of electrolysis reactions. Figure 1

Parylene C is a widely used dielectric barrier in implantable medical devices because it conforms well to surfaces and insulates against biological environments. However, multiple studies have shown that moisture can intrude into Parylene C films through defects and intrinsic diffusion, leading to delamination and device failure. While many studies have tested device integrity in vitro, few have isolated the influence of specific degradation mechanisms on device failure. Here, we use a broadband impedance technique called Microwave Microfluidic Spectroscopy (MMS) to measure fluid permeation in targeted regions of Parylene C films that are free of defects and have optimal adhesion to the substrate. We found no changes in the broadband S-parameters from 100 MHz–110 GHz for Parylene C coated coplanar waveguides soaked in water or phosphate buffered saline at 20 °C or 37 °C for two months. Furthermore, there was no delamination induced by fluid soaking. Our study helps to clear debate about the influence of water and ion diffusion on Parylene C device lifetime and inform better fabrication of Parylene C coatings for implantable devices.

Parylene C thin films are commonly used as a passivation layer, protective coating, or substrate material in implantable medical devices. However, fluid or vapor may permeate through Parylene C films over time through defects, film edges, or bulk diffusion and lead to device failure in vivo. Such failure can be difficult to detect using electrochemical impedance spectroscopy (0.1 Hz – 1 MHz) which does not capture the dielectric contributions of fluid at higher frequencies.We present a method for tracking broadband (1 MHz – 110 GHz) microwave dielectric properties of polymer-fluid interfaces. Using 2D electromagnetic modeling, we simulated measurement sensitivity to potential changes in Parylene C film dielectric properties and estimated sensitivity to changes in film relative permittivity as small as ±0.1 and fluid layers under films as small as 5 nm. Results were used to aid in the interpretation of measurements of soaked Parylene C films. These results are informative development of robust, hermetic polymer films in implantable medical devices and other RF applications in the analysis of complex fluids.

Flexible and stretchable materials have attracted significant interest in wearable electronics and bioengineering fields. Recent developments also incorporate embedded microwave circuits and systems with engineered flexible materials that operate over a broad frequency range (≈1–100 GHz). Herein, a simple flip‐chip technique is used to evaluate frequency‐dependent electromagnetic properties of flexible materials and applied to evaluate engineered microwave absorbers based on self‐biased barium hexaferrite composites. On‐wafer error correction and de‐embedding techniques are applied to determine broadband electromagnetic properties of the material‐loaded transmission lines by placing the materials on top of coplanar waveguide transmission lines. Finite‐element simulations along with broadband measurements were employed to estimate the electromagnetic material properties. To demonstrate, flexible polydimethylsiloxane (PDMS) composites are fabricated with barium hexaferrite nanoparticles and complex permittivity and permeability of the composites are quantified under zero magnetic field bias up to 110 GHz. Frequency‐dependent composite permeability is fitted to models describing the ferromagnetic resonance of the self‐biased barium hexaferrite nanoparticles in polydimethylsiloxane, and constituent nanoparticle properties are estimated using the Maxwell–Garnett mixing model. This study paves the way to exploit a wide range of engineered materials in flexible, wearable, and biomedical electronics applications and presents a convenient methodology to extract important broadband electromagnetic properties of nanoparticles for customized electromagnetic applications.

The environment around a host-guest complex is defined by intermolecular interactions between the complex, solvent molecules, and counterions. These interactions govern both the solubility of these complexes and the rates of reactions occurring within the host molecules and can be critical to catalytic and separation applications of host-guest systems. However, these interactions are challenging to detect using standard analytical chemistry techniques. Here, we quantify the hydration and ion pairing of a FeII4L4 coordination cage with a set of guest molecules having widely varying physicochemical properties. The impact of guest properties on host ion pairing and hydration was determined through microwave microfluidic measurements paired with principal component analysis (PCA). This analysis showed that introducing guest molecules into solution displaced counterions that were bound to the cage, and that the solvent solubility of the guest has the greatest impact on the solvent and ion-pairing dynamics surrounding the host. Specifically, we found that when we performed PCA of the measured equivalent circuit parameters and the solubility and dipole moment, we observed a high (>90%) explained variance for the first two principal components for each circuit parameter. We also observed that cage-counterion pairing is well-described by a single ion-pairing type, with a one-step reaction model independent of the type of cargo, and that the ion-pairing association constant is reduced for cargo with higher water solubility. Quantifying hydration and cage-counterion interactions is a critical step to building the next generation of design criteria for host-guest chemistries.

Materials properties are an essential component for the accurate modeling of integrated devices and circuits. The accuracy of such models depends explicitly on the accuracy of the input material parameters and interfaces between them. With the trend toward increasing heterogeneous integration, the relationships between electromagnetic, thermal, and mechanical material properties of heterogeneously integrated devices are even more important. Recent trends toward co-design emphasize the optimization of all aspects of circuit performance from the beginning, rather than sequentially optimizing the electromagnetic, thermal, mechanical characteristics. It can be critical for modeling success to understand, for example, where losses due to an electromagnetic signal are significant, as those losses can lead to energy dissipation with the subsequent temperature rise being a function of local thermal properties such as the thermal conductivity and heat capacity. Beyond losses, nonuniform temperature distributions generate mechanical stress that can impact interfaces between materials with dissimilar coefficients of thermal expansion. Furthermore, change in temperature and stress can lead to changes in the linear electromagnetic properties, resulting in changes in signal propagation and the generation of nonlinear effects. Material properties are also important as they connect device response to underlying materials physics. This connection allows one to exploit different physical phenomena to add functionality at materials level, and to understand and mitigate non-idealities such as nonlinear response. As such, it is critically important to quantify nonlinear electromagnetic and electro-thermo-mechanical properties of heterogeneous integrated devices. In Fig. 1, the Heckmann diagram shows the electro-thermo-mechanical relations in a crystal, where T, S, E, D, θ, and σ are stress, strain, electric field, electric displacement, temperature, and entropy, respectively. This diagram illustrates the various nonlinear interactions that can be important for determining the overall response of microelectronic devices composed of a wide range of material systems. Here, we present an overview of experimental efforts designed to accurately characterize the linear electromagnetic properties of materials relevant for microelectronics, including dielectrics and conductors as a function of frequency from 100 kHz through 220 GHz. Dispersion and absorption imply frequency dependence of complex quantities such as the dielectric permittivity and magnetic permeability, and this in turn necessitates broadband characterization techniques. We describe efforts to characterize broadband frequency-dependent linear electromagnetic properties over a wide range of temperatures, including cryogenic temperatures relevant for quantum computing, and augment these techniques with approaches to characterize the relevant thermal material parameters. We then describe measurements of nonlinear response of different material systems to quantify the nonlinear relationships between different thermodynamic fields in integrated structures. We conclude with a discussion of the needs for additional metrology to characterize these complex interactions inside complex 3D and packaged microelectronic devices and at buried interfaces within these heterogeneous integrated structures. Figure 1

Next-generation millimeter-wave (> 30 GHz) telecommunications electronics must be compact, energy efficient, and have good thermal management. Tunable materials may play a role in meeting these requirements for millimeter-wave front-end devices, but there are few models or even measurements of tunable dielectrics at these frequencies. Here, we report on the adaptation and development of high-frequency dielectric spectroscopy techniques for composition-spread thin films from 100 MHz to 110 GHz. Our comprehensive technique sequentially probes the composition, frequency, and electric field dependence of the complex permittivity in a combinatorial thin film library, which provides a platform to rapidly explore functional materials for emerging telecommunications electronics. This is achieved by modifying existing on-wafer transmission line permittivity measurement techniques to obtain a compact set of test devices that can be patterned to extract the complex permittivity in multiple regions of a thin film. We demonstrate this technique by applying it to composition-spread Ba1-xSrxTiO3 thin films spanning compositions from x = 0 to x = 1. The systematic approach to materials growth inherent in combinatorial synthesis allows for a comprehensive picture of the Ba1-xSrxTiO3 system. Our continuous, quantitative measurements provide an encompassing view of the composition- and voltage-dependent trends in the room temperature dielectric properties at millimeter-wave frequencies-from strong, few-picosecond relaxations to no relaxation, and from large relative tunability (nr > 50% at 75 kVcm-1) to zero tunability. Our work underscores both the utility of our technique, and the need to discover lower-loss, highly tunable electronic materials for next-generation telecommunications.

Thin film Parylene C has increasingly been employed as a substrate material with metals like platinum (Pt), especially in MEMS implantable devices. To assist in device design, broadband dielectric spectroscopy (up to 110 GHz) can characterize such materials with unique advantages unavailable in more-commonly used electrochemical impedance spectroscopy (up to 1 MHz). In this work, coplanar waveguides (CPWs) fabricated from electron-beam evaporated Pt coated with Parylene C were measured with broadband dielectric spectroscopy to characterize the effect of thermal annealing. We confirmed that annealing caused no significant changes in Parylene C permittivity $(2.85\pm 0.13$ and $2.80\pm 0.18$ before and after annealing; respectively) and extended the upper frequency limit to 110 GHz for the known permittivity value. Pt resistivity was unexpectedly reduced by 20% from annealing. Results and implications herein may inform fabrication-related design considerations of implantable devices using thin film Parylene C and Pt metal with radio frequency (RF) applications such as wireless power and data transfer.

Frequency-dependent linear-permittivity measurements are commonplace in the literature, providing key insights into the structure of dielectric materials. These measurements describe a material's dynamic response to a small applied electric field. However, nonlinear dielectric materials are widely used for their responses to large applied fields, including switching in ferroelectric materials, and field tuning of the permittivity in paraelectric materials. These behaviors are described by nonlinear permittivity. Nonlinear-permittivity measurements are fraught with technical challenges because of the complex electrical coupling between a sample and its environment. Here, we describe a technique for measuring the complex nonlinear permittivity that circumvents many of the difficulties associated with other approaches. We validate this technique by measuring the nonlinear permittivity of a tunable Ba0.5Sr0.5TiO3 thin film up to 40 GHz and comparing our results with a phenomenological model. These measurements provide insight into the dynamics of nonlinear dielectric materials down to picosecond timescales.

Understanding the electrical properties of materials is a necessary part of any microwave circuit design. In this article, we explore the possibility of employing multiple-offset-reflect devices for on-wafer materials characterization at frequencies up to 110 GHz. The objective of this new technique is to provide a one-port characterization technique that does not require a first-tier calibration. To verify our results, we performed companion analyses with multiline thru-reflect-line, extracting the permittivity of fused silica and SU-8, which is a common photocurable polymer. For fused silica, we obtained a relative permittivity of 3.80 ± 0.02 from 5 to 110 GHz. For SU-8, we obtained a relative permittivity of 3.25 ± 0.02 at 28 GHz, which agreed with the literature values.

Early work on carbon nanotube (CNT) antennas indicated that their performance could not match that of metals such as copper. However, recent improvements in fluid phase CNT processing have yielded macroscopic CNT materials with better alignment and conductivity. There is currently a gap in the literature on CNT antennas for direct experimental measurements of radiation efficiency. In this study, we conducted radiation efficiency measurements of microstrip patch antennas made of shear-aligned CNT films. We measured a radiation efficiency of 94% at 10 GHz and 14 GHz, matching equivalent copper antennas. Furthermore, the minimum CNT film thickness required to match the performance of copper drops with increasing frequency due to reduced losses from the skin effect. These findings pave the way for applications of aligned CNT patch antennas in the aerospace industry, where low weight, mechanical durability, and temperature-independent performance are critically important.

Metal-organic supramolecular cages can act as charged molecular containers that mediate reactions, mimic enzymatic catalysis, and selectively sequester chemicals. The hydration of these cages plays a crucial role in their interactions with other species. Here we use microwave microfluidics to measure the hydration and ion pairing of two metal-organic cage assemblies that are isostructural but have different overall anionic charge. We supplement our measurements with density functional theory calculations to compare binding site energies on model metal-organic cage vertices. We find that the cage with dianionic vertices is more strongly hydrated and forms a distinct ion pair species from the cage with trianionic vertices. We evaluate multi-ion species and distinct ion pair solvations as possible sources for differences in ion dynamics and hydration. Broadly, this work highlights the utility of microwave microfluidics to elucidate the consequences of charge states on metal-organic complexes in solution.