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Optimized Impedance Standard Substrate Designs for Dual and Differential Applications Tim Lesher, Leonard Hayden, Eric Strid Cascade Microtech, Inc. 2430 NW 206th Avenue, Beaverton, OR, 97006 Email: [email protected] Fax:503-601-1601 This paper was presented by Tim Lesher of Cascade Microtech at the 2003 ARFTG Conference. Abstract -- Optimized dual signal Impedance Standard Substrate (ISS) designs are demonstrated. The optimal designs had loop- under grounds, were selected for minimum II. LOOP-UNDER GROUNDS deviation from lumped element behavior and used mode dampening structures. A comparison of existing design approaches is given and the quality A coplanar waveguide (CPW) structure will have of the designs is illustrated to 50GHz. an imbalance of energy if the length of one slot is longer than another as in [3] and the grounds are not tied together. This imbalance will lead to I. INTRODUCTION mode conversion, and if the line is long enough or if the frequency is high enough resonant Historically, most wafer probe measurements use behavior may be observed. A 50 micron signal two single transmission line probes opposing and 25 micron gap CPW on alumina has a 130 each other. The most popular probe micron per picosecond propagation velocity [4], configuration is ground-signal-ground (GSG), or approximately 7.2 microns per degree at due to it's well behaved launch from the probe 50GHz. A number of effects, including probe tips to the substrate [1]. Undesired modes that asymmetry, slot lengths and widths and probe can occur at the launch are microstrip mode placement can add up to result in a significant (sometimes referred to as parallel plate mode, to phase difference in the slots and undesired mode the DUT ground plane [2]) or slotline mode conversion. between the two grounds [3]. Both probe calibration structure and DUT transitions need to The mode conversion problem is compounded be well designed to minimize mode conversion with dual signal probes and structures since the by virtue of their symmetry and small size outside grounds are that much farther away for a relative to a wavelength. given pitch. As well, the probe grounds are longer on the outside than in the center as shown To characterize planar differential/mixed-mode in the photograph in figure 1 below. Regardless or multiport devices, more transmission lines are of how the probe is manufactured, the length of required in each probe head. O