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A G R E AT E R M E A S U R E O F C O N F I D E N C E tinues to be a challenge. With standard gate
dimensions of less than 90nm and space
budgets shrinking continuously, the small-
est probe pad dimensions required for most
prober systems remain fixed at about 50
microns. This limitation is largely the result
of the inaccuracy of probe movements and
the size of the probe tips. This challenge is
being solved with new probing tools that
offer nanometer movement precision with
probe tip diameters of less than 50nm and
current measuring capability better than 1pA
Tips for Electrical
(see Figure 1).
This article will focus on measurement
techniques that can be applied to character-
Characterization of
izing carbon nanotubes, low power devices,
and what can be done to overcome various
sources of measurement error.
Carbon Nanotubes and Methods and Techniques
Low Power Nanoscale
Consumers are demanding faster, more
feature-rich products in ever-smaller form
factors. Because the electronics must have
Devices
smaller sizes, the components will also have
limited power handling capability. As a re-
sult, when electrically characterizing these
components, the test signals need to be kept
small to prevent component breakdown
Jonathan Tucker or other damage. Current versus Voltage
Keithley Instruments, Inc. (I-V) characterization on nanoscale devices
may require the measurement of very small
T
voltages due to the necessity of applying a
hE potential uses for carbon nano- A second challenge is how to characterize very small current to control power, or to
tubes are seemingly endless, with these next generation devices when power reduce the Joule-heating effects. Therefore,
plenty of potential applications in limitation is critical. The scaling of devices low level voltage measurement techniques
the semiconductor industry alone. and components to the nano scale forces re- become important, not only for I-V charac-
Researchers are already incor- searchers to limit the levels of electrical sig- terization of devices, but can be extended to
porating carbon nanotubes into FETs for nals that can be applied for characterization. resistance measurements of non-conductive
switches, memory for consumer goods, and Lastly, probing nanoscale devices con- materials and components. For researchers
field emission displays for the next genera-
tion of televisions. Researchers are also look-
ing into applying carbon nanotubes in sensor
applications to detect molecular particles for
applications in homeland security. There is
also serious work being done to use carbon
nanotubes transistors for digital logic.
The semiconductor and nanotechnology
community continues to be faced with chal-
lenges when working with carbon nanotubes
or other low power nanoscale devices. One
challenge is the difficulty of electrically
characterizing extremely small circuit ele-
ments, not only in the current generation of
semiconductors, but in next-generation na-
noscale electronics, as well. Figure 1. I-V curve on a carbon nanotube
Tips for Electrical Characterization of Carbon Nanotubes and Low Power Nanoscale Devices June2006
Figure 1a. Keithley's Model 4200-SCS semiconductor characterization Figure 1b. Zyvex S100 Nanomanipulator.
system.
and electronics industry test engineers, this power limitation makes Offset Voltages
characterizing the modern devices and materials, and future devices Ideally, when a voltmeter is connected to a relatively low-imped-
challenging. ance circuit in which no voltages are present, it should read zero.
Unlike I-V curve generation on macro- and micro-scale com- However, a number of error sources in the circuit may show up as a
ponents and materials, measurements on carbon nanotubes and na- non-zero voltage offset. These sources include thermoelectric EMFs,
noscale devices require such special care and techniques. General- offsets generated by rectification of RFI (radio frequency interfer-
purpose I-V curve characterizations are often performed using a ence), and offsets in the voltmeter input circuit. Steady offsets can
two-point electrical measurement technique. The problem with this generally be nulled out by shorting the ends of the test leads together,
method is that the voltage is measured not only across the device in and then enabling the instrument's zero (relative) feature. However,
question, but includes the voltage drop across the test leads and con- canceling the offset drift may require frequent re-zeroing or using
tacts as well. If your goal is to measure resistance of a device using specific measurement techniques, particularly in the case of thermo-
a typical ohmmeter to measure resistances greater than a few ohms, electric EMFs.
this added resistance is usually not a problem. However, when meas-
uring low resistances on conductive nanoscale materials or compo- Thermoelectric Voltages
nents, obtaining accurate results with a two-point measurement may Thermoelectric voltages, or thermoelectric EMFs, are the most
be a problem. common source of errors in low-voltage measurements. These volt-
If your I-V characterization or resistance measurement involves ages are generated when different parts of a circuit are at different
low voltage or low resistance, such as with molecular wires, semicon- temperatures and when conductors made of dissimilar materials are
ducting nanowires, and carbon nanotubes, a four-wire, or "Kelvin," joined together. Constructing circuits using the same material for all
measurement technique with a probe station is preferred and will conductors minimizes thermoelectric EMF generation.
yield more accurate results. With Kelvin measurements, a second set Measurements at cryogenic temperatures pose special problems.
of probes is used for sensing. Negligible current flows in these probes
due to high impedances associated with the sensing inputs; therefore,
only the voltage drop across the DUT is measured (see Figure 2). As
a result, your resistance measurement or I-V curve generation is more
accurate. Source and measurement functions for this measurement
technique are typically provided by Source-Measure Units (SMUs)
(electronic instruments that source and measure DC voltages and
currents).
Typical Sources of Error
Low power electrical characterization on carbon nanotube based
devices and other nanoscale components can be fraught with meas-
urement error. Offset voltage and noise sources that can normally be
ignored when measuring higher signal levels can introduce signifi-
cant error into low-voltage, low current, low power measurements.
We will discuss four factors that can affect measurement perfor-
mance and accuracy. Figure 2. Four-point measurement schematic.
June2006 Tips for Electrical Characterization of Carbon Nanotubes and Low Power Nanoscale Devices
This is because the connections between the Current sources that offer pulse meas- using carbon nanotubes or other nanoscale
sample in the cryostat and the voltmeter are urement capability can also minimize the materials, the need for testing standards be-
often made of metals with lower thermal amount of power dissipated into a DUT. comes more evident. Consistency in meas-
conductivity than copper, such as iron, which Pulse measurement tools allow users to pro- urement technique and reporting of data
introduces dissimilar metals into the circuit. gram the optimal pulse current amplitude, is critical in order for new manufacturing
In addition, because the source may be near pulse interval, pulse width, and other pulse processes to be consistent. Keithley Instru-
zero Kelvin while the meter is at 300 Kel- parameters to reduce potential device heat- ments worked closely with The Institute of
vin, there is a large temperature gradient. By ing and control the energy applied to the de- Electrical and Electronics Engineers (IEEE)
matching the composition of the wires be- vice. Combined with a synchronized nano- to create P1650TM-2005, the world's first
tween the cryostat and the voltmeter and by voltmeter, the combination can synchronize measurement standard for the electrical
keeping all dissimilar metal junction pairs the pulse and measurement--thus reducing characterization of carbon nanotubes. P1650
at the same temperature, nanovolt measure- device heating. and future standards and recommended
ments can be made with good accuracy. guidelines will permit semiconductor manu-
Another approach to controlling ther- Contaminated Probes facturers and materials manufacturers of
moelectric voltages is to use a delta meas- Test signal integrity when probing car- carbon nanotubes and nanoscale materials to
urement technique. A constant thermoelec- bon nanotubes or nanoscale semiconductor precisely manufacture and fabricate the next
tric voltage may be cancelled using voltage devices depends on a high quality probe generation of electronic components.
measurements made at a positive and nega- contact, which is directly related to contact
tive test current. Alternating the test current resistance (Figure 3). Probe contact resis- Conclusion
also increases noise immunity by increasing tance has become increasingly important as This article focused on just a few of the
the signal-to-noise ratio. Over the short- signal voltages drop and contact pressures measurement issues that the semiconductor
term, thermoelectric drift may be approxi- decrease. industry and nanotechnologists must con-
mated by a linear function. The difference front and overcome when designing the next
between consecutive voltage readings is the generation of electronic devices. Traditional
slope