Research Highlights of Polymers Division

High speed electronics

High dielectric constant polymer composite materials are being developed by the electronics industry in response to the need for power-ground decoupling to secure integrity of high speed signals and reduce EMI radiated noise. Current technologies utilize surface mounted discrete chip capacitors, which can extinguish the power-bus noise at frequencies below 10 MHz. At higher frequencies, between 10 MHz and 100 MHz, only capacitors with the lowest connection inductance are able to source the charge. As the operating frequency increases above several hundred MHz, all the discrete capacitors become ineffective. In response to these inherent problems NCMS, together with NIST and more than dozen partners, organized a collaborative research consortium aimed at developing and advancing the use of embedded-decoupling-capacitance (EDC) technology. The consortium identified a need to assess the feasibility of this design and to measure the materials dielectric characteristics at frequencies above 1 GHz.

We have developed a specialized test technique to measure the dielectric permittivity of high dielectric constant films in the microwave range using a microstrip test vehicle shown in Fig. 1. The test vehicle comprises a two-layer circuitry and contains a number of microstrip resonators. To improve the accuracy of the measurements, we added to the test pattern appropriate non-coaxial terminations for in-situ calibration. The test pattern is appropriate for testing permittivity in the functional planar configuration of films, 10 µm to 50 µm thick having the dielectric constant ranging from 4 to 40 at frequencies from 100 MHz to 12 GHz. This frequency range is not covered by the existing standard test methods.

The microstrip-resonator technique, which we developed and transferred to the industrial partners, allows for evaluation of permittivity of EDC films at discrete resonant frequencies that do not necessarily overlap with the dielectric relaxation process. In order to address the structural/dielectric attributes in adequate detail, broadband dielectric measurements are needed. However, the existing test procedures for thin films are based on lumped element approximations, which fail to produce meaningful results at microwave frequencies, especially in the case of high-dielectric constant films.

Fig. 1.Permittivity measurements of high-k films using microstrip test vehicle

We have recently achieved a breakthrough in broadband permittivity measurements of dielectric films in coaxial configuration and were able to extended the measurements to frequencies of practical importance, above 1 GHz (Fig. 2.). This has been accomplished by employing an appropriate theoretical model for the wave propagation in the specimen section. Neglecting the propagation, which is the current approach, leads to large systematic errors at microwave frequencies.

We have determined that, contrary to the commonly accepted simplification, the propagation direction takes place along the diameter of the specimen rather than across the thickness of the dielectric. Furthermore, we found that the limitations of the lumped element method originate from singular behavior at the half-wavelength resonance frequency which is determined by the diameter of the film specimen and its complex permittivity. The experimental permittivity results obtained in the broad frequency range compared very well with the microstrip resonance data at frequencies from 100 MHz to 10 GHz. The thin film configuration does not compromise the dielectric loss measurements since the propagation length is independent of the specimen thickness.

Fig. 2. Broad-band high frequency permittivity measurements of high dielectric constant films – comparison of the NIST developed technique with the prior approach.

To the best of our knowledge, this data analysis scheme has not been successfully implemented before.

We evaluated the microwave dielectric properties of several polymer-ferroelectric ceramic composites newly developed by the industry for EDC applications: BC2000.... and EmCap.... from PolyClad, CPly.... from 3M and HiK.... Polyimide from DuPont. The EDC materials measured in a functional test vehicle configuration showed an exceptionally low and flat impedance characteristic, indicating a much lower Q factor than could be expected from the dielectric properties of the organic and ceramic constituents.

Our study discovered a high frequency relaxation behavior which is intrinsic to binary mixtures of organic polymer resins loaded with ferroelectric powders. We found that the position of the loss peak is determined by the dielectric relaxation of the polymer backbone while its magnitude depends on the dielectric dispersion, and therefore, is amplified by the content and permittivity of the ferroelectric component.

We have recently started developing a new method for broad-band impedance characterization of high-k films directly in the time domain. Experiments were conducted in a coaxial configuration to eliminate radiation effects, apparent dispersive behavior, coupling errors and inductive delays. This allowed us to monitor the response to a 12.5 ps rise time voltage step in a 10 ns time window with 2.5 ps resolution. Such resolution and fast responsiveness is essential for the proper evaluation of low impedance circuits. In comparison, other testing configuration that utilize coaxial connectors or coupling probes introduce inductive components that increase the system impedance well above that of measured impedance and therefore, limits the measurement range to slow responses only. It was found that at the time referenced to the incident voltage step Vi (time zero or at highest frequencies) all the EDC specimens appeared as a short circuit indicating a pure capacitive behavior. The best performance, indicated by the response close to that of zero impedance (short) termination, was observed for several industrial test materials. To our knowledge these results represent the most accurate experimental characterization of low impedance planes to date.

We showed that polymer composite films filled with ferroelectric ceramics can provide low impedance through a high capacitance. In addition, such materials can also suppress the undesirable resonant behavior because of their intrinsic high loss. This makes polymer composites films attractive as a broad-band embedded de-coupling capacitance to secure the signal integrity and minimize the EMI noise in high-speed circuits operating at microwave frequencies.

For more information on this topic

J. Obrzut and C. K. Chiang, “ Dielectric measurements of embedded capacitance materials “ Embedded Decoupling Capacitance Project, NCMS Technical Report No.: 091RE00 August 20000, NCMS, 3025 Boardwalk, Ann Arbor, MI 48108.

http://www.ncms.org/index.html

J. Obrzut, R. Nozaki