Test and Measurement
to an LCR/C-V meter. Note that the guard is
allowed to float and the outer shields are all
interconnected at the prober. (Interconnecting
the outer shields does not adversely affect I-V
measurements. In some cases, it may even
improve I-V measurement performance.) The
triaxial cables replace the coaxial cables shown
in Figure 2. It’s possible to switch between I-V
and C-V measurements by re-connecting only
the instrument end of the cable. There’s also no
need to disconnect the wafer from the prober to
make the switch, which saves time and ensures
consistent probe needle to wafer contact. Figure 7: Connections between a DUT and an
Pulse testing (as well as other test types) LCR/C-V meter
requires 50Ω transmission lines to the DUT. If the
triaxial cables are designed to have a
characteristic impedance of 100Ω from their
center connectors to their outer shields and two
of these cables are connected in parallel, the
combination has a characteristic impedance of
50Ω. In addition, pulsing usually requires one or
more DUT pins to be connected to ground. The
28
center connector can be connected to the outer
shield by adding a jumper, as shown in Figure 8,
www
to make the ground connection. Figure 8: Pulse testing usually requires
.eur Attaching this jumper at the end of the connecting one or more DUT pins to ground. A
oasiasemiconductor
probe arm would require disconnecting the jumper can be used to tie the center connector
probe needle from the wafer to protect the to the outer shield to make this ground
wafer; unfortunately, the probe needle area can connection
be difficult to access. Although applying the
ground connection at the end of a short cable
might seem like a simpler alternative, this would
.com
reduce the effective bandwidth of the ground.
To obtain a clean 10ns rise time, the ground
cable must have an electrical length that is less
square4
Issue VIII 2009
than 1.5ns (approximately 30cm), which would
allow the ground to be applied at the mounting
base of the probe manipulator. Adding
connectors in the cables near the probe
manipulator mounts allows inserting a shorting
cap as shown in Figure 9.
Figure 5: When two transmission lines are The shorting cap can be added without
connected in series, the characteristic disturbing the probe needle, making it possible
impedance of the combined transmission line is to switch from I-V and/or C-V measurements to
the sum of the two individual transmission lines pulsed measurements without the need to re-
probe a wafer site. By allowing an operator to
make quick, easy set-up changes while the
probe needles are in contact with a wafer, the
cap reduces pad damage and maintains the
same contact impedance for all three types of
measurements.
In order to simplify switching from Kelvin to
Figure 6: A triaxial cable non-Kelvin measurements, it was necessary to
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