High Frequency Electrical Characterization of Dielectric Materials

(Faculty mentor: Michael Lanagan)

Relevance to System Level Packaging: The requirement for faster data transmission has led to increased interest in the microwave and mm-wave frequency ranges, and has prompted research on dielectric materials and components at high frequencies (Figure 1). Cell phone communications in the 900 MHz to 2 GHz range are common and there is new interest in data communications at 2.45 GHz. Personal computer processors are projected to reach 5 GHz within the decade. Optoelectronic modulators have reached data high rates and require materials and components that operate well above 10 GHz. At higher frequencies, wireless local area networks operate at 60 GHz and smart cruise control systems at 77 GHz have been installed in European automobiles.

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Within today’s high-speed digital and analog circuitry, fundamental connector components and packages rely on dielectric materials not only to position conductors mechanically, but also to maintain specific electrical properties. The electrical properties of these materials must be known to high accuracy during the analysis, design, and modification cycles of new high-speed components. Further, each of the material properties should be known at many discrete frequencies across a wide frequency band. The faculty advisors at Penn State will work closely with an undergraduate to develop the material characterization tools necessary for their future electronic components and systems.

To date, no comprehensive study has been completed that compiles important electrical values for commonly used high-speed dielectric materials for connectors. Engineers are left to design components based upon extrapolations from dielectric material properties that are often listed at lower frequencies. In addition, lot-to-lot variation in material dielectric properties will affect design rules for printed wire boards. A clear need exists for a high-frequency dielectric material characterization to be completed in a stringent manner. An established measurement facility at Penn State has been established to characterize specific materials of interest to industry (Figure 2). The information from such a study would be extremely valuable across numerous sub-segments of the electronics industry. This project will build a database for packaging materials for information and communications systems.

Summary of Proposed Work: Dielectrics play enabling roles in advanced wireless, computer, automobile, aerospace, and medical systems. The challenge for industry is to understand how the dielectric material properties above 1 GHz will influence device functionality and design. The short design-to-market cycle has forced electronic component manufacturers to rely more on accurate simulation, which is dependent on material property data. Material properties that must be understood include relative permittivity, relative permeability, conductivity, electrical loss tangent, and magnetic loss tangent. Anisotropic materials, such as most PWB laminates and many plastics, need to be characterized in several directions.

The Center for Dielectric Studies (CDS) at Penn State has developed a suite of characterization tools for dielectric measurement. For low frequency measurements (≤1MHz), standard single-layer capacitor structures are fabricated by depositing electrodes on both sides of a dielectric substrate. An LCR meter (Hewlett-Packard 4284) in conjunction with a computer interfaced temperature chamber is used for dielectric measurement at temperatures ranging from +150^oC to -170^oC. A number of microwave dielectric measurement techniques have been developed to accommodate a large range of dielectric constant, dielectric loss, frequency, and dimensional specifications. High-frequency measurement techniques are separated into two categories, transmission and resonant methods. Transmission techniques have swept frequency capability, which allow for measurement over the range of the network analyzer (1 MHz to 26 GHz). A wide range of dielectric constant values can be measured; however, dielectric loss values are limited to tan d > 0.01. Resonant cavity techniques are generally limited to a single frequency, which is defined by the cavity dimensions. Low loss samples of (tan d < 0.01) range can be measured by resonant techniques. Resonant techniques to be used in this study are resonant post and split cavity.

In summary, resonant techniques are generally employed for low-loss materials and transmission/reflection methods are useful for high-loss specimens. The most accepted high-frequency measurement techniques consist of resonant post, spit cavity, ring resonator, and waveguide transmission techniques. All of these techniques are available at the CDS microwave dielectric characterization laboratory.