Making Ultra-Low Current Measurements with the Low-Noise Model 4200-SCS

Making Ultra-Low Current Measurements with the Low-Noise Model 4200-SCS
Number 2241
Application Note
Series
Making Ultra-Low Current Measurements
with the Low-Noise Model 4200-SCS
Semiconductor Characterization System
Introduction
Parametric characterization of semiconductor devices typically
requires making extremely low current measurements. For
MOSFET devices, the gate voltage controls the on/off of the
MOSFET, in other words, the drain current flow. The off state
current and the transition from the on state to the off state (the
subthreshold current) are both of critical importance in the performance of a ULSI CMOS circuit. In modern, highly integrated
circuits, the off state current is typically on the order of just
femtoamps. Furthermore, there are a number of device phenomena that are of critical importance in device characterization, such
as Gate Induced Drain Leakage (GIDL). Therefore, it is very
important to have a tightly integrated parametric characterization
system that is capable of making these ultra-low current
measurements.
A typical semiconductor characterization system will
include a DC characterization system such as the Keithley
4200-SCS, a switch matrix, a probe station, and cabling. Too
often, however, when a semiconductor characterization system is
configured, the system specifier will tend to concentrate on the
DC parametric instrumentation while neglecting the rest of the
system. In fact, the switch matrix and probe station chosen can
have a significant impact on the system’s overall measurement
performance. In addition, even with a properly configured system, the system implementation itself can significantly affect
measurement integrity.
This application note will discuss several important aspects
of making low current measurements with the Model 4200-SCS,
including grounding and shielding, noise in the measurement,
and system settling time.
Grounding and Shielding Issues
Instrument Common vs. Chassis Ground
Before discussing grounding in detail, it is important to distinguish between the instrument common and the chassis ground.
The Model 4200-SCS and similar instruments have two grounds
available. One is called the common; the other is the power
ground. (See Figure 1.) When shipped from the factory, these
two grounds are connected, but they are different. The common
is the ground for the complete measurement circuit; it will affect
the system’s low-level measurement performance. In contrast,
the chassis ground is connected to the power line ground and is
mainly used for safety reasons. Usually, there are no problems
associated with connecting these grounds together. Sometimes,
however, the power line ground can be noisy. In other cases, a
test fixture and probe station connected to the instrument may
create a ground loop that generates additional noise. Due to these
concerns, ensuring low-level measurement accuracy demands
that the system grounding be thought out carefully.
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Figure 1. Common vs. Earth Ground (Chassis)
System Grounding and Shielding
Technically, although grounding and shielding are closely
related, they are actually two different issues. In a test fixture or
probe station, the device and probing are typically enclosed in
soft metal shielding. This metal enclosure is used to eliminate
interference from power lines and high frequency radiation (RF
or microwave) and to reduce magnetic interference. Most semiconductor devices are also relatively light sensitive, so the
enclosure also prevents light from striking the device under test,
which could produce a low level current flow that would interfere with making accurate low current measurements.
This metal enclosure is normally grounded for safety
reasons. However, when an instrument is connected to the probe
station through triaxial cables, the point where ground connections are made is very important. The configuration in Figure 2a
illustrates a common grounding design error.
Note that the instrument common and the chassis ground
are connected. The probe station is also grounded to the power
line locally. Even more significantly, the measurement
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Figure 2a. Ground loops
Figure 2b. Eliminating ground loops
instrument and the probe station are connected to different power
outlets. The power line grounds of these two outlets may not be
of the same level all the time. Therefore, a fluctuating current
may flow between the instrument and the probe station. This
creates what is known as a ground loop. To avoid ground loops,
a single point ground must be used. Figure 2b illustrates a better
grounding scheme when a probe station is used.
it is amplified. When measuring extremely low currents, remote
mounting of the PreAmp offers significant measurement accuracy advantages.
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The most likely sources of noise are the other components
of the system, such as long cables or switching hardware that is
inappropriate for the application. Therefore, it is advisable to use
the best switch matrix available, such as Keithley’s 7174A ultralow current switch, and to keep all connecting cables as short
as possible.
Generally, system noise will have the greatest impact on
measurement integrity when the signal to be measured is very
small. That’s because when the signal is amplified, the noise is
also amplified. The key to low level measurement accuracy is
increasing the signal-to-noise ratio. The Model 4200-SCS’s
Remote PreAmp increases the instrumentation’s low current
measurement capability. The PreAmp can be mounted remotely
on a probe station platen so that the signal need only travel a
very short distance (just the length of the probe needle) before
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Even assuming that a characterization system is properly shielded and grounded, it’s still possible for noise to get incorporated
unintentionally into the measurement results. Typically, measurement instruments like the Model 4200-SCS contribute very little
noise to the overall error total. Its noise specification is only
about 0.2% of the range, which means the p-p noise on the lowest range is just a few femtoamps. Noise can be further reduced
with the use of proper signal averaging (through filtering and/or
increasing the number of power line cycle integrations).
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PREAMP
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Techniques for Minimizing
System Noise
PreAmp
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PreAmp
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Figure 3. PreAmp mounted remotely on a probe station
Chuck Noise
Ambient temperature and thermal chucks are both commonly
used in device characterization applications. Each type offers a
choice of regular (coaxial) or guarded (triaxial) chuck designs,
with coaxial chucks generally being less costly. However, coaxial chucks tend to have higher leakage and noise specifications
than triaxial designs. Low current measurements require the use
of guarded chucks. In this design, the guard of the chuck is connected to the inner shield (guard) of the triaxial cable. The inner
shield is held at the center pin voltage by a unit gain buffer in the
measurement instrument.
The heating element found in thermal chucks tends to add
considerable noise to the measurement, especially if the heating
element uses AC current. Whenever possible, a DC heater should
be used to minimize the noise introduced into the measurement.
To reduce noise still further, once the desired temperature has
been reached, the heater can be switched off during the measurement. The drawback of this technique is that the temperature
may drop slightly after the heater is turned off. However, if the
measurement can be performed quickly, measurement accuracy
should not suffer significantly.
Test System Settling Time
As mentioned previously, overall characterization system performance depends not only on the specifications of the measurement instrument itself, but in large part on the switch hardware,
prober, and cabling. This is particularly true for high speed or
low current measurements. Often, erroneous signals are measured that are unrelated to the real device parameters. It is important to note that measurement errors introduced by the settling
current can not be eliminated by signal averaging.
Settling of the System
A step voltage test is typically used to characterize the problems
associated with system settling. A 10V step is applied to the
whole system under test, then current is monitored continuously.
The resulting I-t (current vs. time) plot (Figure 4) can illustrate
several important system characteristics.
Once the transient current has settled to its steady value, it
corresponds to the system leakage current. Typically, leakage
current is characterized as amp per volt (A/V). To determine the
leakage current of the system, simply measure the steady-state
current and divide by the voltage step. In a semiconductor test
system, leakage current typically comes from the switch relays
or the probe card. Most low-level characterization systems now
use triaxial cables, so leakage current is rarely the result of
the cabling.
There are two types of leakage associated with the switch
or probe card:
• The Path-to-Ground leakage is the leakage path from the
relay to the GND of the instrument or from the probe
card pin to the ground.
• The Path-to-Path leakage is mostly the leakage between
adjacent switch relays or probe card pins.
1e–10
Determining the Proper Measurement Speed
1e–11
For an SMU (Source Measure Unit) based instrument, the proper
amount of delay must be added between the sourcing and measuring functions. In addition to the instrument-only delay, even
more delay time may be needed to accommodate system settling.
As discussed previously, the step voltage test can be used to
determine the proper delay time.
1e–12
Current
(Amps) 1e–13
1e–14
1e–15
1e–16
0.0
is high, then a significant portion of the current measured is actually transient current, not the leakage current. This may lead the
engineer to conclude erroneously that the material is leaky.
Another potential problem is that because the current is transient,
it may exhibit wide fluctuations. Sometimes, this phenomenon
can be misinterpreted as the result of a noisy system.
0.5
Step voltage
applied
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Time (seconds)
Figure 4. System settling time
There are two regions of interest on this I-t curve.
Immediately after the voltage step, there is a transient current.
This current will gradually decay to a steady value. The time this
current requires to settle down to the steady value is the settling
time. Typically, the time it takes to settle down to 1/e of the
initial value is used as the time constant. This time constant can
vary widely for different configurations or systems. Settling is
mainly the result of the capacitance inherent in the switch relays,
cables, etc. It may also be a function of the dielectric absorption
of the insulating materials used in the system. If insulators with
high dielectric absorption are used, the system settling time may
be very long.
Extended settling times can have serious consequences in
the measurement, such as when making a leakage current measurement on a high resistivity material. If the measurement speed
From a measurement perspective, we are only concerned
about the overall effect. Therefore, a step voltage test is the most
convenient and rather simple test. The first step is to determine
the tolerable measurement error. If a picoamp-level leakage
current is tolerable, then the delay time can be set to the point
that the transient current is at a sub-picoamp level. If the
expected device current is in the tens of femtoamps, then the
delay must be set so that the transient current is lower than the
expected value.
It is important to note that there is a trade-off between the
measurement speed and measurement level. If the current measurement level is low, then the delay between measurement steps
must be longer, which means the measurement speed will be
lower. On the other hand, if the user can tolerate a somewhat
higher transient current, then a higher speed can be used.
Choosing the Proper System
Configuration
Proper system configuration is the most critical aspect of avoiding measurement problems. A system of this type can only be as
good as its worst-performing component. For instance, if a good
instrument is configured with a poor performing switch or
prober, then the switch or prober will determine the overall
measurement performance, no matter how good the instrument
is. The following examples illustrate the importance of proper
component selection in ensuring system accuracy.
Switch Selection
The Model 4200-SCS has 100aA current resolution and 10fA
current measurement accuracy. However, if the system is configured with a switch card with 100fA offset current (such as the
Model 7072A Semiconductor Matrix Card), its measurement
accuracy will be limited to approximately 100fA and extended
settling times will be required. However, cards based on new
technologies, such as the “air matrix” design used in the Model
7174A, offer significantly better leakage performance and faster
settling times. In fact, the Model 7174A offers a 10fA offset
current specification (typical) and a settling time of <2.5s to
400fA after application of 10V (typical).
Prober Card or Manipulator Selection
Not all prober cards and manipulators are created equal. While
epoxy-based cards are suitable for many applications and are
relatively economical, they have higher leakage current and
longer settling time specifications than newer card designs can
provide. For example, for ultra-low current measurement applications, high-performance, two-layer cards ensure low leakage
and minimal dielectric absorption with Teflon®-insulated coaxial
feed-throughs and ceramic blade needle mounts. Keithley offers
both epoxy-based and high-performance card designs to simplify
matching the card used to the requirements of the application.
If a manipulator is used, consider using a specially
designed triaxial guarded manipulator. While a coaxial manipulator may be more economical or settle slightly faster, the low
current performance of these manipulators typically can’t
compare with triaxial designs.
Conclusion
Cable Selection
To reduce settling times, short cables should generally be used in
the system. Reducing excessive cabling in the system will speed
up the measurement. In addition, not all cables are the same,
even though they may all be called triaxial cables. It is a good
practice to order cables directly from the instrument vendor to
ensure they all have been tested thoroughly.
Making accurate low current measurements demands a thorough
understanding of the factors that contribute to low level measurement errors. This understanding allows the system specifier to
choose appropriate grounding and shielding techniques, chucks,
switches, cabling, probe cards, etc., which will make the most of
the capabilities inherent in modern device characterization
systems.
Specifications are subject to change without notice.
All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
All other trademarks and trade names are the property of their respective companies.
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© Copyright 2006 Keithley Instruments, Inc.
Printed in the U.S.A.
No. 2241
11063KGW
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