Section 5-Applications; Introduction; Applications Of The Currentfunction; Highresistancemeasurements - Keithley 595 Instruction Manual

SECTION 5
APPLICATIONS
5.1 INTRODUCTION
The Model 595 Quasi&tic CV Meter facilitates the measti-
ment of semiconductor device parameters, specifically,
quasistatic capacitance and current. Using the built-in
voltage source and an analog or digital plotter, plots of C
or I versus V can be easily generated.
This section describes in detail some typical applications
of the Model 595. Current and capacitance applications will
be discussed separately. The current section will give in-
formation about measurement of high resistances and IV
characteristics of vaiious 'devices. The capacitance section
will present guidelines for proper use of the function and
valid interpretation of data for MOS device characteriza-
tion. It will also compare the feedback charge method used
by the Model 595 with two other techniques: the ramp
method and the static or QV method. Lastly, a bibliography
is provided on CV and related topics.
5.2 APPLICATIONS OF THE CURRENT
FUNCTION
5.2.1 High Resistance Measurements
High impedance levels are frequently encountered in
semiconductor devices and in ftiures used for device
characterization. Some ewmples are MOS gate oxide,
devices to~~device isolation, probe-to-pedestal insulation
material, and co&al cable insulation. High resistances such
as these can be measured using the Model 595 voltage
source and the current function.
The xixeaswement circuit used by the Model 595~ is shown
in Figure 5-1. The resistor is connected from the input of
the Model 595 in the current t?mction to the voltage source.
Because the input of the Model 595 is a virtual ground, the
voltage, V, applied to the resistor is equal to the voltage
source output to within one millivolt. The current, I, flow
Since very high resistance measurements are subject to
leakage Nzent errors, circuit connections must be carefully
made. To avoid unsuitably low leakage resistance, the
device under test should be placed in a properly designed
test fixture, such as the Keith@ Model 6104 Test Box. The
test box, when connected to the GUARD terminal of the
Model 595, acts as an electrostatic shield, and to some ex-
tent an RFl shield, intercepting signals which would other-
wise interfere with the input. It has the additional benefit
of shielding the device from light, as many devices are
photosensitive. The meter input of the Model 595 should
be connected to the test fixture with low noise coaxial cable
such as the supplied Model 4801. The voltage source may
be connected to the test box with ordinary banana plug test
leads, using a BNC to banana plug adapter such as a
Pomona Model 1296.
USING MODEL 'B'S VOLTAGE SOURCE
------~---w
--
+OEL
REAR
PANEL
"ETA= BOX
Figure 5-1. Cunent Measurement Corrections Using
the Model 595 Voltage Source
5.2.2 Voltage Coefficients of High Resistances
High value resistances (above lO%) often exhibit substan-
tial variation in resistance with applied voltage. This
resistance variation is known as the voltage coefficient. The
Model 595 is an ideal instrument for obtaining data to deter-
mine voltage coefficients because of its built-in variable
ing through the resistor is measured by the Model 595. The
unknown resistance, R, is calculated from the sourced
voltage and measured current using Ohm's law: R = V 1 I.
For example, the highest resistance which can be measured
voltage source and its highly sensitive -nt preamplifier.
in this manner is W5n. This would be done by setting the
voltage source to 2OV and measuring 0.02OpA.
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