Page 15 - Chip Scale Review_May June_2021-digital

MEMS impedance matching
As IC frequency increases, impedance
can also become a significant factor and
it can be a huge challenge coupling the
probe head to the next interconnect level.
However, by using impedance matching
when developing the probe head, it
is possible to achieve a transparent
transition between the tester, the PCB and
probe head, and the wafer.
This process once again begins with
the customized MEMSFlex™ probe head
stack-up and fine tuning the inductance and
capacitance. The primary goal is to achieve
a reflection loss of -20dB to -15dB with
a transmission loss of -1dB at the highest
frequency requested. Additional parameters
Figure 6: MEMS probe head guide plate stack-up examples for optimal heat dissipation. that must be considered aside from the

Figure 7: Current measurements at various test temperatures.
MEMS coaxial probe head for RF
Designed specifically for high-speed
RF applications, we also are developing
a coaxial probe head with a better
grounding strategy. Just like any of the
other MEMSFlex™ probe designs, the
coaxial solution begins with the selection
of the materials and the overall grounding
scheme of the design (Figure 8). After
performing mechanical, electrical,
and thermal simulations, the coaxial
probe head demonstrates much cleaner
transmission loss optimized by moving
the ground reference closer to the RF
signal probe (Figure 9). The probe heads
developed with this coaxial technology
not only provide a cleaner transmission
loss, but a target reflection loss of -10dB
at high speeds [1].
Figure 8: MEMS coaxial probe layout with shielding.

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