Electromagnetic Properties of Materials
Project Goals
The primary objectives of this program are to develop, improve, and analyze relevant measurement methods, uncertainties, and the underlying physics of the complex permittivity and permeability of bulk to nanoscale, naturally occurring and artificial materials in the radio-frequency through terahertz-wave spectrum as functions of temperature, frequency, and bias fields. The emphasis is on the metrology of substrates, thin films, liquids, biological materials, artificial materials, all at high frequencies and small scales.
Customer Needs
As microprocessor speeds steadily increase and new high-frequency technologies come on-line there is a continual and essential need for high-frequency materials characterization. Industries and researchers require new measurement methods on a broad array of materials with well characterized uncertainties, at microwave frequencies through terahertz, and over a range of temperatures. In addition to applications involving traditional bulk materials, trends in microwave materials are moving toward thinner layered substrates, biological materials, artificial dielectrics, and nanoscale materials.
At present, the frequencies of primary interest to the microelectronic industry are peaked in the 5 to 10 gigahertz region, but satellite communications, radars, and homeland security applications now use much higher frequencies, up into the terahertz bands. The primary driver for dielectric measurements in the microelectronics arena originates in the fact that as the operational speeds of devices increase, the dielectric losses in substrate materials severely influence microcircuit operation through signal degradation and heating.
Microelectronic circuitry is packaged on multilayered substrates and thin films. Electronic substrate/ thin film materials are used in printed wiring boards (PWB), low-temperature cofired ceramics (LTCC), central processing unit (CPU) chips, and microwave components. The dielectric parameters influence the propagation speed, impedance, heating, and phasing characteristics of the substrates. In order to achieve circuit miniaturization, new substrates are being developed that incorporate artificial, tunable, and layered structures. Emerging substrates that exhibit electrical properties that do not occur in naturally occurring materials are being designed with metamaterials and nanoscale composites. Such materials can provide novel device concepts useful to the commercial, military, and metrological communities. This field is in serious need of metrology, and each of these materials requires new measurement methods and well characterized uncertainties. Newly developed thin film materials such as high-temperature superconductors, ferroelectrics, and magnetoelectrics hold great potential for improved functionality in microwave devices, but are still in the critical stage of materials development. Accurate characterization at this stage of the microwave properties of these emerging materials can have a large impact on the development of future electronic systems.
Security needs and biological research require high-frequency characterization of liquids. Hence, the need for reference liquids and basic metrology has increased. Both solid and liquid dielectric reference materials are needed to provide measurement traceability to NIST. Measurement comparison provides assessments of the quality of material characterization.
Data on temperature-dependent dielectric and loss properties of ceramics, substrates, and crystals from cryogenic temperatures to 300 degrees Celsius, at microwave and millimeter frequencies, are crucial in the wireless and the 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. When interpreting measurement results, an understanding of loss mechanisms in low-loss crystals is important. Meshing of an understanding of the underlying physics with dielectric spectroscopy is becoming increasingly important in novel material research. In addition, nondestructive methods for permittivity measurement are needed throughout industry.
Various applications require composite dielectrics that simulate the human body’s electrical properties for security applications such as in metal detectors and also for analyzing the effect of electromagnetic interference (EMI) on implanted medical devices and cell phones.
To support future needs in the microelectronics industry for the development of novel new technologies, methods for characterizing nanoscale composites will be necessary. Needs for on-chip microscale-to-nanoscale measurements of permittivity and permeability have been highlighted by the microelectronics industry.
Technical Strategy
The program’s main thrusts are in the areas that support homeland security, biotechnology, microelectronics industry, and nanoscale metrology. To support the microelectronics industries in their quest for lower-loss materials, we will develop higher frequency measurement methods and broaden our measurement temperature range with added humidity control. Current research materials under study are liquids, thin films and printed wiring boards, low-loss dielectrics, magnetic crystals, and synthetically polarized films.
To support the nanotechnology and artificial material efforts, we will develop metrology for nanoscale composites and advanced materials such as metamaterials, multiferroics, thin films, superconductors, and ferroelectrics. We have an on-going effort focused on determining the permittivity and permeability of thin-film samples that are difficult to measure accurately with conventional cavity-based microwave measurement techniques. Material systems of interest are ferroelectric thin films and multiferroic materials, which display different properties depending on the application of an electric- or magnetic-field bias. Also of interest are thin-film materials that display relaxation behaviors within the broad frequency range of our measurement systems.
In order to understand the underlying physics of our novel material measurements, we plan to have an ongoing theoretical modeling effort that supports the interpretation of our broadband spectroscopy measurements.
 Evanescent microwave probe for on-chip dielectric measurements. |
In response to homeland security needs at airports, we will perform research on the identification of liquids from relaxation spectrum measurements. This research is presently funded through the NIST Office of Law Enforcement Standards.
In response to a need from the microelectronics industry for on-chip characterization, we will extend our evanescent probe system to incorporate a dielectric resonator, enhance our theoretical model, and perform measurements on relevant materials. 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 and extend the capability of our Fabry-Perot measurement system to include variable temperatures.
To aid the semiconductor industry for materials characterization we will measure a wide spectrum of semiconductor materials commonly used in the electronics industry as a function of temperature and frequency.
To satisfy documented needs in the health care, biotechnology, and metal-detector industries, we will characterize materials that simulate the electrical properties of the human body. In addition, in support of the biotechnology industry, we will improve our liquid measurement metrology and will compare our measurements with those of the U.K.’s National Physical Laboratory, using the liquid measurement methods we have developed.
To enhance the understanding of the physics of high-frequency losses in dielectrics, we will test ferroelectrics and crystals over wide temperature and frequency ranges using an in-house model for determination of the permittivity. We will also compare the measured losses as functions of temperature and frequency to expressions in the solid-state literature.
We will support the development of high-frequency standards by attending and contributing to IPC and IEEE standards committee meetings.
In order to investigate the possible applications of nonlinear permittivity effects we will study nonlinear material responses. Nonlinear materials are finding increased use in electronic applications. Examples include the use of ferroelectric materials as voltage-controlled components and high-temperature superconductors in high-order, low-loss filter applications. Such nonlinear materials invariably give rise to effects such as harmonic generation and intermodulation distortion, which can have important consequences at the system level. We will address these problems by evaluating the nonlinear response of broadband device structures, in order to determine the intrinsic nonlinearity of materials such as ferroelectrics and high temperature superconductors.
Accomplishments
- Complex Permittivities — In response to homeland security needs and as a result
of an industrial collaboration,
we developed improved metrology and software for measuring the broadband
complex permittivity of foams and liquids. The new software is based on an in-house theoretical model for the open-circuited sample holder. We measured the permittivity of lossy foams over K-band frequencies. We completed a Standard Reference Material (SRM) for both relative permittivity and loss tangent.
In a basic research study we applied broadband permittivity measurement techniques to a range of thin-film material systems, including ferroelectrics as well as voltage-tunable dielectrics and artificial multiferroics. These measurements included broadband voltage-dependent measurements of SrTiO3 thin films at cryogenic temperatures, as well as measurements of BaxSr1–xTiO3 thin films at room temperature. We have applied our broadband measurement technique to artificial multiferroic thin-film samples, which are multilayers of ferroelectric PbTiO3 and ferromagnetic Co-Fe-O. We performed measurements of these multiferroic samples under combined electric-and magnetic-field biases.
In order to study new approaches for permittivity determination we developed our broadband nonlinear measurement system for characterizing the broadband nonlinear microwave response. We have also developed the detailed nonlinear models used to analyze the measurement results obtained with this measurement system. Application of our measurement and analysis techniques to high temperature superconducting transmission lines has demonstrated experimentally the equivalence of different manifestations of nonlinear response (third harmonic generation, intermodulation distortion) in the same device, and has provided evidence of an inductive origin for the nonlinear response in these materials. Measurements of the nonlinear response of other ferroelectric materials at room temperature have displayed a range of behaviors, including a much larger intermodulation signal in ferroelectric BaSrTiO3, a smaller microwave nonlinear capacitance compared to the DC nonlinear capacitance, and the dominance of the nonlinear conductance term in ferroelectric PbTiO3 samples. - Properties of Advanced/Emerging Materials – In response to internal NIST nanowire efforts we developed a new method for measuring the electrical conductivity of bulk samples of carbon nanotubes using a dielectric resonator and in-house developed software. We performed measurements and published a paper on the resonance phase in metamaterials.
- Evanescent Probe Metrology and Theory – The microelectronics industry roadmap highlights the importance of on-chip probing. In response to this need we have completed a theory for characterizing the microscopic fields present while performing on-chip permittivity measurements. In order to better understand our permittivity measurements, we developed from basic statistical mechanics the microscopic and macroscopic relationships for the entropy in electromagnetic driving; this work has been published in Physical Review E. We also developed a polarization evolution model that relates relaxation times to the permittivity.
- Semiconductor Materials – Through conversations with industry representatives and NIST personnel, we have determined that refined characterization of semiconductors is needed. We obtained a suite of commonly used semiconductors used by researchers in the semiconductor industry and the Optoelectronics Division and performed variable frequency and variable temperature measurements on the materials. In addition, we characterized the permittivity of substrates as a function of frequency and temperature for the LTCC industry.
- High-Frequency Measurement System – In order to meet the needs for measurements at higher frequencies we designed and constructed a new Fabry-Perot mirror, and preliminary measurements on two reference materials were performed. To expand on these results, a confocal Fabry-Perot system has been ordered and will also operate up to 100 gigahertz.
- Phase Noise – In collaboration with the Physics Laboratory, a ceramic 10 gigahertz air- filled cavity was designed and constructed for use in the X-band phase noise measurement system. Preliminary measurements have indicated an improvement of 10 decibels for close-in phase noise. Also, design work and construction of a 40 gigahertz metallic cavity was successfully completed and incorporated into a basic discriminator circuit at 40 gigahertz. Ceramic versions of this cavity are now being constructed, with expected improvement in phase noise equivalent to the results obtained with the 10 gigahertz cavity.
- Standards and Comparisons – In response to documented industry needs, a new standard for the IPC High-Frequency Task Group (D-24) has been completed and related software has been transferred to industry. In order to test our calculated uncertainties, we contributed specimens for the EUROMET International Intercomparison.
- Measurements on Fluids – We have further developed a switched measurement system that allows for nearly simultaneous measurements of fluid-loaded devices using a DC meter, an LCR meter, an RF network analyzer, and a microwave vector network analyzer. We designed four-terminal measurement devices to explore the role of polarization impedances in the low-frequency measurements of conducting fluids. We further developed our modeling capabilities in order to extract permittivity values from our measurements. We have applied our measurement system to an increasingly broad range of fluid systems at different temperatures, including de-ionized water (at temperatures of 20, 37, and 50 degrees Celsius), and methanol, polystyrene beads, and NaCl buffer solutions (at 20 degrees Celsius).