32
CHAPTER 2:
System Overview
Model CRX-6.5K Probe Station
2.4.8 High Impedance/
Low Leakage
Measurements
The Model CRX-6.5K can accommodate resistance measurements greater than
100 G
)
, but not without special consideration given to probe station configuration
and external electronics. High impedance measurements are difficult for several
reasons. The current used to excite the sample must be very small, so even tiny
amounts of leakage current can create a large percentage reading error. High
resistance lead arrangements are more susceptible to environmental fields, which
easily induce current noise. Probe to sample contacts are difficult to establish and
verify.
2.4.8.1 Grounding and Shielding
A general discussion of grounding and shielding is in section 2.4.1. These concepts
become more important for high resistance measurements. High resistance samples
do not short circuit induced noise the way low resistance samples do. When
measuring high resistance, measurement electronics tend to convert common mode
noise, which is easily canceled, to normal mode noise, which is difficult to separate
from the signal.
2.4.8.2 Driven Guards
Driven guards are used to minimize the leakage current that typically flows between
conductors in the leads used to connect the sample to measurement electronics. The
most common leakage path is between the signal leads and their respective shield or
ground. Properly configured, a guarded measurement system can reduce leakage
current by three orders of magnitude or more, which would allow a system capable of
accurately measuring 10 M
)
to 100 M
)
to measure 10 G
)
to 100 G
)
.
Guarding works by surrounding signal leads with coaxial conductors and driving
them with a guard voltage close to the signal voltage. Very little current crosses the
insulation resistance leaks, because the voltage difference is low. Guarding does not
provide adequate shielding so the signal and guard are often surrounded by a shield,
requiring triaxial cable and connectors (FIGURE 2-25).
The entire experiment must be set up with guarding in mind. A key element of most
guarded systems is the excitation source. Keithley Instruments Model 6220 DC
precision current source is an excellent example of a guarded source that can
combine with a precision voltmeter or electrometer to make high resistance
measurements. The critical elements needed to carry the guarding inside the probe
station are described in section 2.4.8.3.
FIGURE 2-25
Recommended circuit for measuring high-resistance devices
R
100 nA
V
Feedthrough (triax)
Cable (triax)
Driven guard
Measurement electronics
Cryostat (vacuum system)
DVM
