Electromagnetic Properties of Materials

Project Goals

High-frequency evanescent microwave probe.

This project develops, improves, and analyzes measurement methods, uncertainties, and theory for the characterization of the complex permittivity and permeability of naturally occurring and artificial materials in the radio-frequency, microwave, and millimeter-wave spectrum as a function of temperature and bias fields. Emphasis is on metrology, thin films, liquids, biological materials, high frequencies, small scales, and artificial materials.

Customer Needs

Current research is in the areas of high frequencies, variable temperatures, and nanoscale dimensions. Currently, thin films, frequencies, variable temperatures, and nanoscale dimensions. Currently, thin films, artificial and biological materials, substrates, and liquids are perceived as the most important measurement areas. Substrate-based components employing thin films form the basis for microelectronic circuitry. Electronic substrate materials are used in printed wiring boards (PWB), low-temperature cofired ceramics (LTCC), CPU chips, and microwave components. Industry requires new measurement methods with well-characterized uncertainties, at microwave and millimeter frequencies and over variable temperatures. Data on temperature-dependent dielectric and loss properties of ceramics, substrates, and crystals at microwave and millimeter frequencies is crucial in the wireless and time-standards arena. For example, computer-based design methods require very accurate data on the dielectric and magnetic properties of these materials over wide ranges of frequency and temperature. An understanding of loss mechanisms in low-loss crystals is important when interpreting measurement results. Artificial dielectrics are becoming commonplace and can be designed to obtain properties not exhibited in nature. New dielectric metrology is needed for these areas.

Various applications require composite dielectrics that emulate the human body's electrical properties for testing metal detectors and analyzing electromagnetic interference (EMI) to implanted medical devices. Measurements of liquid permittivity are needed to support biotechnology research. To support the evolving microelectronics industry, methods for characterizing nanoscale and metamaterial properties will be necessary for the development of novel new technologies. On-chip microscale-to-nanoscale measurements of permittivity are important for the microelectronics industry. Both solid and liquid dielectric reference materials are needed to provide measurement traceability to NIST. Measurement intercomparisons provide assessments of the quality of material characterization.

Newly developed thin-film materials such as hightemperature superconductors, ferroelectrics, and magnetoelectrics hold great potential for improved functionality in microwave devices, but are still in the critical stage of material development. Accurate characterization of the microwave properties of these emerging materials at this stage can have a large impact on the development of future electronic systems. Such materials can provide novel device concepts useful to the commercial, military, and metrological communities.

Technical Strategy

The project's main thrusts in 2005 are to develop measurement methods to support the health care industries, measure materials at higher frequencies, broaden our measurement temperature range, and measure advanced materials over smaller dimensions. The current specific areas of research are thin films and printed wiring boards, applications to biotechnology, low-loss dielectric and magnetic crystals, probing methods for micro-nanoscale permittivity, dielectric metrology of advanced materials such as metamaterials, superconductors, and ferroelectrics, and theoretical modeling of dielectric relaxation.

In response to needs in the microelectronics industry, we are developing accurate methods for measuring the dielectric properties of thin films using both transmission-line and resonator methods. Using a previously developed on-wafer transmissionline model, we will extend measurements of thin films to frequencies above 40 gigahertz, and will also develop a new resonator method. We will also aid the PWB and LTCC industries in measuring the permittivity of substrates at high frequencies. To this end, we will further enhance our wideband, variable-temperature metrology. We will extend the capability of our Fabry-Perot measurement system to include variable temperatures, and will complete the model for the split-post resonator. We will measure a wide spectrum of ceramic materials commonly used in the electronics industry as a function of temperature. We will continue to work with and support IPC Tasks Groups and the LTCC Working Group through measurement assistance.

To satisfy documented needs in the health care, biotechnology, and metal-detector industries, we will characterize materials that emulate the electrical properties of the human body. We will also develop a coaxial probe for in-vitro measurements in support of research on detection of breast cancer. In addition, in support of the biotechnology industry, we will improve our liquid measurement metrology and will compare our measurements with NPL's using the liquid measurement methods we have developed.

To enhance the understanding of the physics of high-frequency losses in dielectrics we will test ferroelectric and other crystals over wide temperature and frequency ranges using an in-house model for determination of permittivity. We will also compare the measured losses as functions of temperature and frequency to expressions in the solid-state literature.

To support the microelectronics industry in on-chip dielectric measurement metrology, we will develop methods for evanescent microwave probing and atomic-force microscopy.

To support basic research on advanced composite materials technologies, we will develop measurement metrology on metamaterials.

We will support the development of standards by attending and contributing to standards committee meetings.

To accurately determine the microwave properties of developing materials, we will combine existing on-wafer measurement techniques with lithographically defined device structures to characterize advanced materials such as high-temperature superconductors, ferroelectric, and magnetoelectric thin films. We are also applying these techniques to perform on-chip characterization of small samples of biological materials or fluids using integrated microfluidic channels and reservoirs. Device measurements are then accomplished by utilizing a number of specialized microwave probe stations, such as our cryogenic probe station or 110 gigahertz probe station.

Accomplishments

We developed measurement metrology for measuring the broadband complex permittivity of liquids. The software is based on an in-house theoretical model for the open-circuited sample holder.
A Standard Reference Material (SRM) for both relative permittivity and loss tangent has been completed and submitted to EEEL's Measurement Committee (MCOM). Cross-linked polystyrene samples were characterized in a mode-filtered circular-cylindrical cavity and will be available pending the approval of MCOM.
In collaboration with S. Hagness of the University of Wisconsin, we have developed new measurement and calibration algorithms and a new electromagnetic theory for characterizing coaxial probes for use in breast cancer detection and therapy.
We have developed new theory and software to characterize the microscopic fields in evanescent probe metrology. A new theoretical model has been developed for wire evanescent microwave probes suspended over a multiple-dielectric (thinfilm on dielectric substrate) test structure for performing on-chip permittivity measurements.
We developed from basic statistical mechanics the microscopic and macroscopic relationships for the local and macroscopic fields in materials and then related these to exact expressions for the constitutive parameters. This work has been published in Physical Review E.
With assistance from our collaborators J. Krupka and R. Clarke, we modified our procedure for measuring materials in the Fabry-Perot resonator from 40 to 100 gigahertz, such that low-loss materials (tan ä = 0.00001) are measurable at 60 gigahertz.
We developed software to complete the design of a 10 gigahertz metal cavity used in a new frequency discriminator design for improved phase noise performance. In addition, completed the construction of 4 additional cavities based on this design to be used by NIST and the sponsor. With Craig Nelson of the Physics Laboratory, we co-authored a paper describing a new method of reactive-tuning of resonant cavities for improved performance of temperature-controlled frequency discriminators.
In anticipation of future needs, we completed the design and modeling of coupling loops for deposition on substrates, to be used at frequencies above 40 gigahertz in air-filled metallic cavities. This design was then tested and demonstrated with a numerical model using finite element methods (Maxwell), with very favorable results.
Major progress has been achieved in our goal of developing techniques for measurements on ferroelectric materials. We have made high-accuracy permittivity measurements on potassium tantalate using our cryogenic system as functions of frequency and temperature. This material has low loss and tunability at low temperatures and may be a significant material for tunable antennas and phase shifters.
For our OLES contract, we made measurements on many building materials. We are constructing a model concrete wall for propagation studies in collaboration with the Time-Domain Fields project. A comprehensive database has been initiated. In addition, a rebar scattering model has been developed.
In collaboration with J. Krupka of the University of Warsaw, we used Bragg reflectors in our Fabry-Perot resonator and found that the system quality factor Q was increased from 150 000 to 250 000.
A new standard for the IPC High-Frequency Task Group (D-24) has been submitted for editorial review.
We published two technical notes that summarize techniques for measuring the conductivity and permeability of metals. In addition, the reports contain conductivity and permeability data for metals commonly used in the manufacture of weapons.
We published a new theoretical model for the split-cylinder resonator that incorporates higherorder resonant modes, significantly broadening the frequency range of this method. Prior to this new model, complex permittivity measurements could be made only at a single frequency with the splitcylinder resonator.
We performed measurements and collaborated with P. Kabos, C. Holloway and S. Russek on development of a novel metamaterial. The material consists of yttrium garnet spheres in Styrofoam. The permeability of the spheres is tuned using a DC bias magnetic field until left-handed behavior is observed. We plan to further develop and measure left-handed, meta-, magnetoelectric, and photonicband-gap materials.
We established an experimental platform to explore broadband microwave interactions of liquid and solution-based biological samples confined in microfluidic structures. We have fabricated individual components of this platform, and evaluated them using finite-element simulations. This work will provide a new approach to broadband permittivity measurements of fluids and biological samples, and will also allow the development of new techniques for exposing cell populations to well-characterized electromagnetic fields for microwave hyperthermia and specific absorption rate studies.
We measured pair-breaking current density in high transition temperature superconducting thin films. One of the fundamental properties of superconductors is the critical-current density (J_c), which sets the upper bound for the current-carrying capacity of a superconductor. Our measurements of the nonlinear response of YBa₂Cu₃O_7–δ thin films show agreement with theoretical predictions for nonlinear response due to pair-breaking in d-wave superconductors, and our measurement system yields values for the pair-breaking current density that agree remarkably well with theoretical predictions of this quantity for YBCO. This measurement technique could provide an entirely new path to determining critical current densities in superconductors, and may yield intrinsic J_c values closer to theoretical predictions than conventional J_c measurement techniques.
We developed a broadband technique for determining the complex permittivity of dielectric and ferroelectric thin films at variable temperatures. We have succeeded in obtaining consistent values for the complex permittivity of thin dielectric films over the broad frequency range from 100 hertz to 40 gigahertz by combining several different measurement techniques on the same patterned device structures. Measurements have been demonstrated on SrTiO3 thin films at room temperature. Such broadband measurements will be applied to help quantify dispersion in ferroelectric films, and also to evaluate newly developed magnetoelectric thin films, as well as high-permittivity dielectric thin films.
We have continued development of a novel broadband superconducting device that can limit large transient microwave signals encountered in hostile electromagnetic environments. We have measured the response of our superconducting microwave power limiter to high-voltage (50 volts), fast risetime (250 picoseconds) pulses, and demonstrated that the device is effective in limiting signals on timescales of under 1 nanosecond. We are exploring the application of this device to protect sensitive radio-frequency and microwave electronics from high-power transient signals, including ultra-wideband impulse jammers.

Our goals are to extend new measurement software and theory for the evanescent microscope technique (developed in collaboration with Pavel Kabos) and measure substrate materials on-wafer. We will complete a dielectric database for OLES/HS. We will develop software for support of in-vitro breast cancer tissue measurements (in collaboration with Pavel Kabos). We will complete measurements of KTO₃ ferroelectric and publish the results in a journal.

We plan to write a paper and submit for publication on the measurement of high-loss liquids from 50 megahertz to 10 gigahertz to help industry calibrate dielectric measurements on biological materials. We will characterize and measure a metamaterial for Boeing under DARPA funding (with Pavel Kabos). We will characterize reference fluids and biological samples using our broadband microfluidic measurement platform over the frequency range 100 hertz to 100 gigahertz.

To demonstrate application of a superconducting microwave power limiter to mitigate effects of broadband impulse jammers, we will obtain the pair-breaking current density of superconducting MgB₂ films using our nonlinear measurement techniques and explore effects of disorder and nonlocality on pair-breaking current density. We will demonstrate broadband permittivity measurements of ferroelectric thin films at variable temperatures using on-wafer measurement techniques.

Publications

Additional Information