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2.4 Considerations
for DC/RF Electrical
Measurements
Nearly every DC or RF measurement done in a Model CRX-6.5K has some unique
configuration or requirement. Although it is impossible to predict every application,
this section provides information on how to optimize the probe station for some of
the most common measurement challenges.
2.4.1 Grounding
Shielding, and Isolation
for DC/RF
Measurements
The quality and repeatability of DC and RF measurements is greatly influenced by the
integrity of the ground system. Components of the probe station are integral to the
overall ground system, but so are signal sources and meters making the actual
sample measurements. Careful consideration should be given to how these
components work together when setting up any experiment. The following sections
describe features of the probe station that relate to grounding, shielding and
isolation, with some suggestions on how to use them effectively.
2.4.1.1 Ground Reference
The ground reference of a measurement system should be determined first. Signal
paths, signal return paths, and shielding build off of that foundation. In most cases
earth ground is the ground reference for the experiment. The vacuum chamber is
typically tied to earth ground to form a shield around the sample and probes. The
Model CRX-6.5K is configured this way if it is assembled according to the instructions
in Chapter 3. The shield conductor of the temperature controller cable is used to give
the chamber a low impedance path to earth ground (FIGURE 2-20).
Grounding the vacuum chamber through the Model 336 is not appropriate for all
experiments, so the connection is designed so that it can be changed easily. The outer
shell of the BNC and triaxial signal connectors and FT-BNC feedthrough connector are
electrically connected to the vacuum chamber. Any one of these connectors can be
used to establish a ground reference through the measurement electronics if the
ground connection to the Model 336 is removed.
If the vacuum chamber ground reference connection is removed from the Model 336, it is
important to reestablish the ground reference through the measurement
instrumentation. Leaving the chamber ungrounded often causes unpredictable
measurement results.
The probe station sample cooling assembly and included grounded sample holder are
also electrically connected to the vacuum chamber. This will ground the back side of
the sample substrate during normal operation. The quality of this ground is very
dependent on sample mounting technique. Optional sample holders are available to
completely isolate the sample if necessary.
