Low-Current Measurement Challenges - Agilent Technologies B1500A Training Manual

Low Current Measurement
Challenges
NOISE FROM PROBER
Faraday enclosure required
Eliminate all electrical interference
STRAY CAPACITANCE
Full guarding, including the probe
Guarded chuck to measure substrate current
MEASUREMENT SETUP
Hold time, integration, auto ranging
CALIBRATION & ZERO CANCEL
Measurement below 1pA is not possible without special attention to the prober and cabling. The
wafer must be in total darkness and in an electrically isolated metal box. Motors must be turned off
or completely shielded.
Stray capacitance kills any hope of making good low current measurements. High cable capacitance
which is unguarded can produce pA level currents during voltage steps. The probes themselves must
be guarded.
The B1500A defaults at bootup to setup conditions which are not optimized for ultra low current.
This is desirable because there is a large trade-off between speed and ultra-low current accuracy.
The typical use is moderate current levels where settings can be optimize for speed, not accuracy.
There are some features such as zero cancel, which can eliminate the last 10 or 20 fA of error from
your measurement. Before zeroing, you should check that SMU CALIBRATION has been
performed. SMU Calibration removes most error and makes the final zero cancel easier.
6-4
Module 6
Low Current Measurement