Applications Of The Capacitance Function; Characterizing The Mos Capacitor In Depletion And Accumulation - Keithley 595 Instruction Manual

APPLICKTIONS
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Figure 5-8. Capacitor Leakage Tests
5.3 APPLICATIONS OF THE CAPACITANCE
FUNCTION
The Model 595 has been optimized for the measurements
of quasistatic capacitance. This measurement is most~com=
manly made on silicon MOS capacitors. The following
discussion will thus focus primarily on MOS devices,
however, much of the information presented can also be
applied to measurements of other types of capacitors.
53.1 Considerations for Capacitance
Measurements
The method used by the Model 595 to measure quasistatic
capacitance is described in Section 7, Principles of Opera-
tion. Basically, the method consists of applying a voltage
step across the capacitor under test and measuring the
charge step induced on the capacitor. From the voltage steps
applied and the charge step measured, the capacitance
value is determined according to the relation: AQ = C (AV).
To make a valid measurement of differential capacitance on
a MOS device, the voltage steps applied should be in the
small
signal range of the device under test. The small signal
range is defined as the increment of voltage which is small
enough such that the capacitance of the device does not
vary significantly across the voltage increment.
Step~voltages selectable~on the Model 595 are 0.01,9.02,0.05,
and O.lOV. Since full range capacitances from 2OOpF to 201-Z
may be measured, the full range charge steps induced are
between 2pC and 2nC. From these charge measurements,
4%
digit readings are calculated. Therefore, very small
charges, even as low as OAK, can affect the capacitance
reading. Furthermore, since measurement delay time
can
be
varied from 0.07 to 19999sec, leakage currents of
femtoamps or less can cause appreciable errors in the
capacitance measurement. To keep interference from
stray
currents low, insulation resistance in the device under test
and the fixture must beg kept-very high. Paragraph 5.2.1
describes the procedure for verifying insulation resistance.
When current flows in addition to the displacement charge
through the capacitor into the meter input of the Model
595, this current will cause an error in the capacitance
reading. If the current is constant during the measurement,
and the current is measured using Q/t, then the error in
capacitance is given by:
C(error) = (Q/t)(Step time) / (Step V).
The range of capacitance which may be measured is con-
sequently reduced by C(error).
Note that the capacitance reading can be corrected for the
effect of constant current error sources by the use of the
corrected capacitance feature. See paragraph 5.3.4 for
guidelines on the proper use of this feature.
5.3.2 Characterizing the MOS Capacitor in
Depletion and Accumulation
MQS capacitors come in many varieties, according to the
application for which they are intended. For simplicity, the
MOS construction assumed for this discussion will be a
silicon substrate, on which is grown
a thin insulating film
such as SiO, , which in torir has a conducting gate material
deposited upon it. The capacitance of the device is
measured from an ohmic contact on the silicon substrate
to the conducting gate material. It is good measurement
practice to connect the gate to the meter input, since there
are typically fewer stray low impedance paths to other
signal sources at this terminal.
The capacitance of the MOS device is voltage-dependent
as ilhr$mted in Figure 5-9. Three distinct regions of the CV
curve..arQ discemable:~ accumulation, depletion, and inver-
sion. In addition to the material presented here regarding
CV curves, many of the sources presented in the
bibliography, paragraph 5.4 desaibe regions of the CV curve
in detail.
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