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Highly sensitive picoampere meter
Conference Paper · July 1996
DOI: 10.1109/CPEM.1996.547100 · Source: IEEE Xplore
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WEl A-2.
HIGHLY SENSITWE PICOAMPERE METER
Gert Rietveld and Hans Heimeriks
NMi Van Swinden Laboratorium
P.O. Box 654, 2600 AR Delft, The Netherlands
Abstract
We have developed a sensitive picoampere meter
for accurate room temperature measurement of
currents below 10 PA. The main characteristics of the
instrument are an extremely low input current, far
below 1 fA, and fully automated operation. Preliminary calibration of the instrument with a separately
developed facility has been done at the accuracy level
of a few parts in io4.
The ability to control the motion of single
electrons through a nanostmctured device [ 11 has
started worldwide activities on the development of a
current standard based on this effect. One of the main
problems encountered in the realisation of such a
standard is the fact that the current generated by the
devices is limited to a few picoampere. An accurate
measurement of such a low current requires an
extremely low leakage current of the input stage of the
measurement setup. This can only be realised using
cryogenic equipment [2].
reed capacitor. The slope in Vout is measured by a
digital voltmeter DVM1, triggered by an accurate
timebase, and is related to the input current by
where E represents the integrator error, e.g., caused by
leakage.
Note that this method of single slope integration
is very similar to that used for decades in accurate
teraohmmeters [4]. There the current is generated by
applying a voltage across a highohmic resistor.
Circuit considerations
In order to limit leakage currents and noise we
have paid extensive attention to correct shielding and
guarding, especially of the input terminal. The cable
between the current source and the picoampere meter
was kept as short as possible.
As a first step towards such a cryogenic current
measurement, we have built a room temperature
picoampere meter. Essential part of the instrument is
the input head that was taken from an old vibrating
reed electrometer. Since the input stage of such an
electrometer only contains two capacitors, leakage
currents of less than 100 electrons per second can be
realised. A completely new electronic measurement
system has been built around the input head in order to
achieve a higher accuracy than the few parts in lo3 of
the old electrometer [ 3 ] .
Picoamperemeter
LIA
LPF
Vout
VibratinP Reed Picoamperemeter
The block dlagrm of the new instrument is shown
schematically in the top part of Fig. 1. The DC input
current I,nput
charges the vibrating reed capacitor q,,
which consequently transforms the DC charge into an
AC voltage. This voltage is amplified and then
rectified by a lock-in amplifier (LIA), that is using the
function generator driving C, (not shown in the fibwe)
as reference. The output voltage of the LIA is
integrated and the resulting voltage V,, is fed back to
the feedback capacitor C, via a low pass filter (LPF).
If Iinput
is constant, Vout will increase linearly in time,
thus continuously nulling the charge on the vibrating
Figure 1 Schematic diagram of the picoampere meter
with calibration facility. Essential elements of the
meter are the vibrating reed capacitor C, and the
feedback capacitor C,. Cr, transforms the DC input
current into an AC voltage, and a linearly increasing
voltage Vout is applied across C;, such that the effect
of the incoming charge on C, is compensated. Further
explanation is given in the text.
332
each applied current the value of AVOUt/At is
determined by a linear fit through the measured values
of VOUt.
With respect to the original electrometer the
sensitivity was considerably increased by changing the
driving frequency to the resonance frequency of the
vibrating reed capacitor, by improving the
preamplifier, and by using a modem LIA. The use of
a sinusoidal driving signal instead of the original
square wave helped reducing the noise.
The measured leakage current of the picoampere
meter when no input is applied amounts to (15 f
10) aA. If the calibration setup is connected to the
meter, without an intermelate cable, the leakage
increases significantly to approximately 0,3 fA. The
reproducability of current measurements with the
calibration setup is 0,11 fA.
Essential part of the new meter is the integrator in
the feedback loop, which makes the vibrating reed
capacitor a null-indicating amplifier and makes
continuous measurements of current possible. Another
important aspect is the automation of instrument
(including sources) so that many measurement cycles
can be repeated automatically.
Using the calibrator, the value of the integrator
error E was found to be (Sk 2).10-4.Contributions to
h s non-zero value include a possible AC/DC error in
the value of C,, a non-optimised measurement
procedure, and leakage in the capacitors C,, C,, and
Cst
Calibration of the instrument
The basic requirements for accurate
measurements with the picoampere meter are directly
clear fi-omEq. 1. The two limiting factors for obtaining
a high accuracy are determination of the value of C,
and the influence of integration errors E .
We have developed a room temperature
picoampere meter, based on a vibrating reed capacitor,
for measurement of currents below 10 PA. Features of
the instrument are: a low leakage current, automated
control, and an accuracy in the range of a few parts in
lo4. Calibration can be carried out with a separately
developed setup for generating currents in the pArange.
The feedback capacitor is an air capacitor, where
the distance between the plates determined by a
sapphire spacer. The construction is such that edge
effects are important, so that an in situ calibration is
necessary. The value of C,, as measured with an AC
capacitance bridge operating at 1000 Hz, is (20,6023
f 0,001) pF.
Note that the instrument probably can not be used
for measurement of the current generated by single
electron devices since part of the AC signal generated
by the vibrating reed capacitor returns into the current
source (even though this effect is limited by an input
resistor of 200 MQ, see Fig. 1).
Since it is difficult to accurately estimate the
effect of integrator errors, we have built a separate
facility for generating currents in the range of a few
PA. The idea is to reverse the principle of the
picoampere meter, namely to apply a linearly ramping
voltage across a capacitor. The setup is schematically
shown in the bottom part of Fig. 1. The ramp voltage
is generated by integrating the DC input voltage Vin .
The output of the integrator is applied to a standard
capacitor C,, (General Radio, type 1404-C; value 10
pF) and simultaneously measured with a digital
voltmeter, DVM2.
References
-111 L.J. Geerligs, V.F. Anderegg. P.A.M. Holweg,
J.E. Mooij, H. Potlhier, D. Esteve, C. Urbina, and
M. H. Devoret, “Frequency-Locked Turnstile
Device for Single Electrons”, Phys. Rev. Lett.,
Vol. 64, pp. 269 1-2694, 1990.
121 S.M. Verbrugh, “Development of a Single
Electron Turnstile as a Current Standard’, Thesis,
Delft University of Technology, Ch. 6, pp. 109122, 1995.
[3] “Model 40 1 vibrating reed electrometer”, Cary
Instruments, instrument manual.
[4] G.C.C. Chen, W.Y.C. Lin, J.C.M. Hsu, S.H.
Tsao, “Accurate Stelf-Checking Digital Teraohmmeter”, IEEE Trans. Instrum. and Meas., Vol.
IM-44, No. 2, pp. 192-195, April 1995.
Measurement results
The procedure for measurement of a certain
current I is an at least 10 times repetition of the cycle
of values +I, 0, -I, 0. The measurements with zero
applied current give the possibility to correct for the
effect of the leakage current. The voltage ramp during
the current measurement always is within symmetrical
limits, that vary from f 2 V to f 10 V, depending on
the value of the current. The measurement time ranges
tiom 40 to 200 seconds. The driving frequency of the
vibrating reed capacitor is 575 Hz, and the time
constant of the feedback loop is a few seconds. For
333
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