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A GREAT ER M EA SU R E O F C O N F I D E N C E voltage of the transistor increases due to the
built-in voltage in the gate capacitor; there-
fore, the drain current decreases.
Pulse testing can be one of two different
types: voltage or current pulsing.
Voltage pulsing produces much narrow-
er pulse widths than current pulsing. This
makes it more suitable for experiments in
thermal transport where the time frame of
interest is shorter than a few hundred nano-
seconds. High amplitude accuracy and pro-
grammable rise and fall times are necessary
to control the amount of energy delivered to
a nanodevice. Voltage pulsing is useful for
transient analysis, charge trapping, and AC
stress tests during reliability testing, as well
Pulse Testing for
as generating clock signals and simulating
repeating control lines such as in memory
read and write cycles.
Current pulsing is very similar to voltage
pulsing. In this method, a specified current
pulse is applied to the DUT and the resulting
voltage across the device is measured. Cur-
rent pulsing is often used to measure very
Jonathan Tucker low resistances or to obtain an I-V curve
Keithley Instruments, Inc. without putting significant power levels into
the DUT that would otherwise damage or
destroy a nanoscale device.
Introduction Pulse Testing Techniques Both voltage and current pulse testing
Nanotechnology research works with Pulsed electrical testing is a measure- have many benefits but are not without some
matter at the molecular level, atom by atom, ment technique that reduces the total energy drawbacks. For example, the speed charac-
to create structures with fundamentally new dissipated in a device. It reduces the joule teristics of an ultra-short voltage pulse are
properties. In particular, the field of nano- heating effects (such as I2R and V2/R) that in the radio frequency (RF) domain, so it is
electronics is developing especially rapidly could potentially damage small nanoscale very easy to introduce errors in the measure-
with potential impact across a wide range of devices. The device under test (DUT) is ex- ment if the test system is not optimized for
industries. Nanoelectronics research today cited for a very short interval with a source high bandwidth. There are three main sourc-
includes devices that utilize carbon nano- high enough to produce a quality measurable es of errors: signal losses due to cables and
tubes, semiconductor nanowires, molecular signal, and then the source is removed. connectors, losses due to device parasitics,
organic-based electronics, and single-elec- Pulsing also gives engineers more data and contact resistance.
tron devices. about the device, allowing for better char- Current pulsing is usually subject to
Unfortunately, these smaller devices acterization and understanding of device slower rise times, perhaps as low as a few
can't be tested using standard test techniques behavior. For example, pulse testing can be hundred nanoseconds. This is usually lim-
for a number of reasons. One key reason is used for transient testing of a nanodevice to ited by the inductance and capacitance in the
the physical size of the devices. The nano- determine its transfer function and thereby experimental setup.
scale dimensions of some of the new `be- characterize the material under test. Pulse
yond CMOS' devices can be susceptible to test measurements are essential for devices Pulse I-V Testing
damage from even small amounts of current with isothermal limitations, such as SOI de- Performing current versus voltage (I-V)
used in the measurement process. In addi- vices, FinFETs, and nano devices, in order pulse characterization on nanoscale de-
tion, traditional DC test techniques are not to avoid self-heating effects that could mask vices often requires measuring very small
always adequate to reveal how devices really the response that the researcher is seeking. voltages or currents due to the necessity of
operate. Consequently, designers need new Pulsing also helps the device engineer to un- applying a very small current or voltage, re-
testing techniques and test tools. One such derstand charge trapping effects. The effect spectively, to control power or to reduce the
technique is pulse testing, and it is essential of charge trapping is decreased drain current joule heating effects. Here, lowlevel meas-
for the new generation of nanoelectronic after a transistor is turned on. As charges are urement techniques become important, not
devices. trapped in the gate dielectric, the threshold only for I-V characterization of devices but
Pulse Testing for Nanoscale Devices May 2007 1
also for resistance measurements of highly conductive materials. For measurements effectively at low levels, pulse testing techniques
researchers and electronic industry test engineers, this power limita- should be used in combination with line frequency synchronization.
tion makes characterizing modern devices and materials, as well as By synchronizing the pulse measurement with the line frequency,
future devices, challenging. any 50/60Hz line frequency noise is eliminated.
Unlike I-V curve generation on micro-scale components and ma- With applications requiring greater voltage sensitivity, even small
terials, measurement on nanoscale materials and devices requires errors can become important. One common way of avoiding these er-
special care and techniques. I-V DC characterizations are typically rors is to use a delta method. This is the difference between "before"
performed using a two-point electrical measurement technique. The and "during" readings, which corrects for DC offsets. Unfortunately,
problem with this method is that if sourcing a current and measuring DC offsets tend to drift. This problem can be corrected with a simi-
voltage, the voltage is measured not only across the device but in- lar technique known as the three-point delta method. Here, taking a
cludes the voltage drop across the test lead and contact, as well. If third measurement after the pulse can correct for the drift.
the goal is to measure the resistance of a device using a typical ohm
meter to measure resistance greater than a few ohms, this added re- Tools for Nano Testing
sistance is usually not a problem. However, when measuring low re- With nanoelectronic and semiconducting materials and films,
sistances on conductive nanoscale material or components, obtaining sensitive electrical measurement tools are essential. They provide
accurate results with a two-point measurement can be a problem even the data needed to fully understand the electrical properties of new
when using pulse testing. materials and the electrical performance of new device and compo-
If the Pulse I-V characterization or resistance measurement in- nents. Instrument sensitivity must be much higher because electrical
volves low voltage or low resistance, such as with molecular wires currents and voltages are much lower and many nanoscale materi-
and semiconducting nanowires, a four-wire or Kelvin measurement als exhibit significantly improved properties such as conductivity.
technique with a probe station will yield more accurate results. With The magnitude of measured current may be in the femtoamp range,
a Kelvin measurement, a second set of probes is used for sensing. voltage in the nanovolt range, and resistance as low as micro ohms.
Negligible current flows in these probes due to the high impedances Therefore, measurement techniques and instruments must minimize
associated with the sensing inputs; therefore, only the voltage drop noise and other sources of error that might interfere with the signal.
across the DUT is measured. As a result, the resistance measurement One such solution is the Keithley Model 4200-SCS Semiconduc-
or I-V curve generation is more accurate. Source and measurement tor Characterization System with 0.1fA (or 100attoamps) and 1