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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. One common the gaps or the probe fingers will never be
configuration is a ground-signal-ground-signal- perfectly matched.
ground (GSGSG) but the additional conductors
and the increased size of the standards increases
the likelihood of mode conversion. This paper
details the optimization of short, open, load and
thru calibration structures for GSGSG probe
configurations in the pitch range of 100 to 250
microns. The calibration structures make use of
a number of features to dampen the mode
conversions that are inherent in the larger
structures necessary for dual signal probes.
These include loop-under grounds, opens on the
Fig. 1