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A G R E A T E R M E A S U R E O F C O N F I D E N C E

Introduction 2 | Nanotech Testing Challenges 2 | Electrical Measurement Considerations 5 | Electrical Noise 6 | Source-Measure Inst

Pulsing Technologies 8 | Avoiding Self-Heating Problems 9 | Application Example: Graphene 10 | Summary 12 | Glossary 13 | Selector Guide 16 | For More Inf

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ENSURING THE ACCURACY OF NANOSCALE ELECTRICAL MEASUREMENTS A G R E A T E R M E A S U R E O F C O N F I D E N C E

Introduction ...........................

Nanotech Testing Challeng

Electrical Measurement Co

Electrical Noise .....................

Source-Measure Instrumen

Pulsing Technologies ..........

Avoiding Self-Heating Prob

Application Example: Grap

Summary ................................

Glossary ..................................

Selector Guide ......................

For More Information ..........

2 Ask Us Your Application Or

Nanotech Testing Challenges

Nanotechnology has the potential to improveour quality of life in diverse ways, such as fasterelectronics, huge memory/storage capacitiesfor PCs, cheaper energy through more efficientenergy conversion, and improved securitythrough the development of nanoscale bio-and chemical-detection systems.

With nanoelectronic materials, sensitive electricalmeasurement tools are essential. They providethe data needed to understand the electricalproperties of new materials fully and the electricalperformance of new nanoelectronic devices andcomponents. Instrument sensitivity must be muchhigher because electrical currents are much lowerand many nanoscale materials exhibit significantlyimproved properties, such as conductivity. Themagnitude of measured currents may be in thefemtoamp range and resistances as low as micro-

ohms. Therefore, measurement techniques andinstruments must minimize noise and other sourcesof error that might interfere with the signal.

The nature of nanotech materials requires somenovel testing techniques. Because these materialsare built at the atomic or molecular level, quantummechanics come into play. As a result of smallparticle sizes, the atoms and molecules of thesenew materials may bond differently than theymight otherwise in bulk substances. There may benew electronic structures, crystalline shapes, andmaterial behavior. Nanoparticles with these newproperties can be used individually or as buildingblocks for bulk material. Although the di scovery ofbulk properties remains important, measurementsalso need to uncover the characteristics unique tonanoscale structures.

Particle size and structure have a major influenceon the measurement techniques used to investigatea material. The material’s chemical and electricalcharacteristics change as particle sizes are reducedto nanometer dimensions. This even applies tobiological materials. Therefore, most of thesematerials require chemical and electrical testing tocharacterize them for practical product applications.For many of them, the actual quantity beingmeasured is a low level current or voltage that wastranslated from another physical quantity.1 Direct

electrical measurements are possible on manysubstances with the probing instruments and nano-manipulators now available.

As a substance is reduced to nanoscopic dimensions,both the bandgap and the distance betweenadjacent energy levels within the material’s electronenergy bands are altered. These changes, along with a particle’s nanoscopic size with respect to thematerial’s mean free path (the average distance anelectron travels between scattering events), directlyaffect the electrical resistance of a nanoparticle.More generally, a material’s bandgap directlyinfluences whether a particle is a conductor, aninsulator, or a semiconductor. These influentialelectronic properties allow, for example, a carbonnanotube (CNT) to be used to create a transistorswitch.2 One way to do this is by connecting a

semiconducting CNT between two electrodes thatfunction as a drain and source. A third electrode (thegate) is placed directly under the entire length of thecarbon nanotube channel. For a semiconductingCNT, the introduction of an electric field through thechannel (via the insulated gate placed in proximityto the CNT channel) can be used to change the CNTfrom its semiconducting state to its insulating stateby increasing the gate voltage. Decreasing the gate voltage will transition the device into a conductingstate. This conduction mechanism is analogous tothe operation of a silicon MOSFET transistor switch,

which i s created by doping silicon with either a nelectron acceptor or donor to alter the material’selectronic conductivity in specific localities.

Introduction

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Nanotech Testing Challenges (continued)

For macroscopic particles, electrons take ondiscrete quanta of energy that lie within energybands, with each band consisting of many energylevels that electrons can share through theirthermal energies. For a conducting material,

electrons can be thermally excited into theconduction band (i.e., electrons are present inthe valence as well as in the conduction band).For an insulator (bandgap > thermal energy ofthe electron), enormous energy is required foran electron to transition from the valence tothe conduction band separated by the materialbandgap. If a suitable amount of energy is absorbed(> bandgap), then electrons can jump bands.

As a par tic le’s siz e is reduced to nanosco picdimensions, the allowable energies within thecontinuous bands separate into discrete levels

(because there are far fewer atoms in the mix). Thisoccurs when the separation between energy levelsapproaches the thermal energy of the electrons( Figure 1 ). W ith fewer energy levels with in thespecific energy band, the density of states of thematerial changes.

The density of states is a measure of the numberof energy options available to an electron as it fallsinto a lower energy level by giving up energy or asit ascends to a higher energy level after absorbingenergy. A corollary is that if the density of states isknown, the size of the particle can be deduced.

Fi gu re 1. As ma te ri al is redu ced fr om macroscopic dimension s to nanoscopic size,its continuous energy bands (a) separate into discrete energy levels within the band (b) andthe bandgap increases.

Characterizing the density of states is a fundamentalactivity in nanoscopic material research. Density ofstates (3D dimensionality) as a function of energy

can be expressed as:

This represents the number of electron states perunit volume per unit energy at energy E, where:

m = the effective mass of the particle,h = Planck’s constant, andE = the energy (electron orbital location) in

electron volts.

Although the resu lt is indep endent of volume(can be applied to any size particle), this equationis of limited value if the particle size/structure isunknown. However, other ways are available todetermine the density of states experimentally, from which the particle size can be found.

Because the density of states can be used to predictthe electrical behavior of materials, it is also possibleto use electrical impedance measurements to derivedensity of states information. The density of statesis found by plotting differential conductance vs.

applied voltage. Differential conductance is simply(di/dv). When this conductance is plotted against voltage, the graph indicates the material’s densityof states. Highly conductive materials possess anabundance of free energy levels in the conductionband, i.e., greater density of states (more individualallowed energy levels per unit energy). Insulatingmaterials have an electronic structure with a dearthof occupied energy levels in the conduction band.Because density of states corresponds to the densityof these energy levels, a plot of conduction vs. voltage provides a direct measure of the electronicdensity of states at each energy level (voltage across

the device).

One approach to this technique is to use a nano-manipulator that makes low resistance contactsto the nanoparticle. Such an arrangementallows charge transport and density of statesmeasurements. This works well into the conductionregion thanks to the low resistance directconnections of the nano-probes on the material(particle) being tested.

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4

The nano-manipulator and its probes, along with a source-measure unit (SMU), are used toapply a current or voltage stimulus directly to thenanoparticle and measure its corresponding voltageor current response ( Figure 2 ). T he advantage

of electrical source-measure testing is rooted inthe fact that a specific SMU measurement mode(source current/measure voltage or vice versa) canbe chosen based on the relative impedance of thematerial or device under test (DUT). Furthermore,the measurement mode can change dynamicallyas the impedance changes, such as occurs in CNTsacting as semiconductor switches. This allows amuch wider dynamic range of voltage and currentstimuli and measurements, thereby optimizingparametric test precision and accuracy. SMU voltageand current sensitivity can be as good as 1 microvoltand 100 atto-amps.

Electrical measurements on nanoscopic materialsplace stringent requirements on the instrumentation.In order to measure conductivity, impedance,or other electrical properties, and relate thosemeasurements to the density of states, a galvanicconnection must be made to the nanoscopic DUT. 3 This represents one of the major hurdles to beovercome in the field of nanotechnology testing.There are only a few tools available and few deviceconstructs that facilitate connections of this type.

Particle self-assembly can be accomplishedfrom silicon to silicon, where conventionalphotolithographic techniques are used to makeelectrical connection pads for probing. Particles thatare long enough to straddle such pads (for example,carbon nanowires) can be connected to the padsthrough externally generated electrostatic fields.

Although the properties of quantum wells, wires,and dots differ, it’s possible that information abouta particular material in the form of a quantum dotcan be inferred by examining the same material

fashioned as a quantum wire or well (nano-film).Nano-films are particularly easy to measure becauseonly one dimension is small. Such a film mightbe deposited on a conductive substrate, allowing

measurements through the volume as well as overthe surface, using appropriately placed macroscopictest pads formed on the material surface. Forconductive materials, separate pads for source a ndmeasure can be deposited to create a Kelvin (4-wire)connection.4 This t ype of circuit eliminates test leadresistance from the measurement and improvesaccuracy. In any case, a quantum well (nano-film)can be tested like any other bulk material.

1 Bioimpedance Bioelectricity Basics, Wiley 2003.

2 Applied Physics Letters, Single and Mul tiwalle d Car bon Nanot ube Fie ld Ef fectTransistors, volume 17, number 73, October 26, 1998, IBM Research Division.

3 I-V Measurements of Nanoscale Wires and Tubes with the Model 4200-SCS and ZyvexS100 Nanomanipulator, Application Note #2481, Keithley Instruments, 200 4.

4 Four-Probe Resistivity and Hall Voltage Measurements with the Model 4200-SCS, Application Note #2475, Keithley Instruments, 2004.

Figure 2. Nano-manipulator probing of nanoscale structures: Microscopic view of low impedance probecontact to a CNT for direct electrical measurements. Photo of a nano-manipulator head assembly.

Photos courtesy of Zyvex Corporation

Nanotech Testing Challenges (continued)

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Electrical Measurement Considerations

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Electrical measurements on passive devices (anydevice that is not a source of energy) are made by

following a simple procedure: stimulate the samplein some way and measure its response to thestimulus. This method also works for devices thathave both passive and active properties with linearor non-linear transfer functions. With appropriatetechniques, a source-measure algorithm can beuseful for characterizing sources of energy.

For nanoscopic particles, this general methodtakes the form of source-measure testing toquantify impedance, conductance, and resistance, which reveal cr itical material properties. Th is testmethodology is useful even if the end application is

not an electronic circuit.

Several considerations are important in thecharacterization of nanoscopic particles:

n Nanoscopic particles will not support themagnitude of currents that macroscopic devicecan carry (unless they are superconducting).This means that when a device is interrogated,the magnitude of a current stimulus must becarefully controlled.

n Nanoscopic particles will not hold off as much voltage from adjacent devices as a conventional

electronic component or material (such as atransistor). This is because smaller devicescan be and are placed closer together. Smallerdevices also have less mass and may be affectedby the forces associated with large fields. Inaddition, internal electric fields associated withnanoscopic particles can be very high, requiringcareful attention to applied voltages.

n Given that nanoscopic devices are so small,they typically have lower parasitic (stray)inductance and capacitance. This is especiallyuseful when they are used in an electronic

circuit, enabling faster switching speeds andlower power consumption than comparablemacroscopic devices. However, this also meansthat instrumentation for characterizing their I-Vcurves must measure low currents while trackingthe short reaction time.

Because nanoscopic test applications oftenrequire low current sourcing and measurement,appropriate instrument selection and use i s criticalfor accurate electrical characterization. In additionto being highly sensitive, the instrumentation musthave a short response time (sometimes referred to

as high bandwidth), which is related to a DUT’s lowcapacitance and ability to change state rapidly atlow currents.

The switching speed of a source-measure testcircuit may be limited by the instrumentation used

to follow the state of the device. This is especiallytrue if a non-optimal measurement topology isused to observe the device. The two possibletopologies are source current/measure voltage orsource voltage/measure current.

When considering the measurement of low imped-ance (<1000 ohms) devices, the source current/ measure voltage technique will generally yieldthe best results. Current sources are stable whenapplied to lower impedances, and a good signal-to-noise ratio can be achieved without great di fficulty.This allows for accurate low voltage response

measurements.

When measuring high impedance (>10,000 ohms)devices, the source voltage/measure currenttechnique is best. Stable voltage sources to drivehigh impedances are easily constructed. Whena well-designed voltage source is placed across ahigh impedance, it will quickly charge the straycapacitance of the DUT and test cables and rapidlysettle to its final output value. The small currentresponse of the DUT can be accurately measured with an appropriate ammeter.

Dr. Kang WangDirector of the Engineered NanUniversity of Ca

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Electrical Noise

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Measurement topology also has an impact onelectrical noise, which is the ultimate limitationon measurement sensitivity and accuracy. For lowimpedance voltage measurements with a currentsource, the measurement circuits will be sensitive toDUT voltage noise and impedance. For macroscopicdevices, such as a resistor, the Johnson noise voltageat room temperature (270K) is expressed as:

where k = Boltzmann’s constantT = Absolute temperature of the source

in degrees KelvinB = Noise bandwidth in HertzR = Resistance of the source in ohms

which can be further simplified to:

This equation shows that as DUT resistance (R)decreases, the Johnson voltage noise generated bythe DUT also decreases. Conversely, high impedancedevices stimulated with a voltage source are limitedby current measurement noise.

The Johnson current noise of a resistor at 270K is:

indicating that the noise goes down as DUT resis-tance increases.

For all particle sizes, in addition to Johnsonnoise, there could be a noise gain associated with

the measurement topology chosen. Noise gainis a parasitic amplification of the noise of themeasurement system that is not present when

the correct measurement topology is chosen.For example, consider a source voltage/measurecurrent topology. An operational amplifier is usedin many current measurement (ammeter) circuits,as shown in Figure 3.

To minimize noise gain, the ammeter circuit mustoperate at a low gain with respect to its non-invertinginput terminal.

(a) (b)

Figure 3. (a) Circuit model for the source voltage/measure current technique; ( b) Modified model illustrating the noise gain (op-amp noise “gained up”) when the DUT impedance is lowcompared to the measurement impedance.

Dr. Virginia AyeHead, The ElectNanostructures

Michigan State U

V n= √ (4kTBR)

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Source-Measure Instruments

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A commercial DC source-measure un it (SMU) i s aconvenient test tool for many nanoscopic material

and device measurements. SMUs change measure-ment topology automatically (that is, they can rapidlyswitch between sourcing voltage/measuring currentand sourcing current/measuring voltage). Thismakes it easier to minimize measurement noise while maximizing measurement speed and accuracy.

Some nanoparticles can change state with theapplication of an external field. When investigatingsuch materials, an SMU can be configured to source voltage and measure current for a nanoparticle in itshigh impedance state. When the material is in its lowimpedance state, more accurate results are achieved

by sourcing current and measuring voltage.Furthermore, the SMU has a current compliancefunction that can automatically limit the DC currentlevel to prevent damage to the material or deviceunder test (DUT). Similarly, there is a voltagecompliance function when voltage is being sourced.

When using the compliance function, an SMU wil lsatisfy the source value unless the user’s compliance

value is exceeded. For example, when an SMU isconfigured to source voltage with a preset currentcompliance, if that compliance value is exceeded, theSMU automatically starts acting as a constant currentsource. Its output level then will be the compliancecurrent value. Alternately, if the SMU is set to sourcecurrent with a compliance voltage, it will automati-cally switch to sourcing voltage (the compliance voltage) if the DUT impedance and the current itdraws begin to drive the voltage higher than thecompliance value.

Although a nanoscopic device, such as a CNT switch,

can change states rapidly, the change in instrumentstate is not instantaneous. Depending on the SMUmodel, the switching time can range from 100nanoseconds to 100 microseconds. Although suchswitching speeds are not fast enough to track ananoparticle as it changes state, the time is shortenough to allow accurate measurements of bothstates while limiting DUT power dissipation toacceptable levels.

Ryan MajorR&D Project ManagerHysitron, Inc.

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Pulsing Techniques

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Choosing the correct measurement topology toimprove measurement speed and minimize noise

may still be insufficient to the test needs for somenanoscopic materials. For example, it appearsthat some CNTs can switch 1000 times faster thanconventional CMOS transistor switches. This istoo fast for the nano-amp ranges of commercialpicoammeters. Demanding devices like these mayrequire other techniques to improve the speed ofimpedance measurements.

Low power pulsing techniques may offer a partialsolution to this problem and are available in some

SMU designs. The idea is to use a much higher testcurrent or test voltage and apply this large stimulusfor a short sourcing cycle. The larger stimulus willlower the sourcing noise (by improving the signal-to-noise ratio) and improve the rise or settle timefor a voltage pulse or current pulse, respectively.Quieter sources require less filtering and permit ashorter sourcing cycle time (narrower pulse width). A larger source stimulus also increases the responsecurrent or voltage so that higher instrument rangescan be used, further minimizing the effects ofnoise. Because there is less noise, the measurement

acquisition time (integration period) can beshortened, allowing for faster measurements.

DC offsets due to thermal voltages and meteroffsets can give significant errors in the measured voltage.

Performing a 2-point delta measurement cancelsoffset error. The measured delta voltage givescorrect voltage response to the current pulse.

An optional third measurement point can helpcancel moving offsets.

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Avoiding Self-Heating Problems

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A possi ble sourc e of error in nano research isself-heating due to excessive electrical current

through the DUT. Such currents may even leadto catastrophic failure of the sample. Therefore,instrumentation must automatically limit sourcecurrent during device testing. Programmablecurrent and voltage compliance circuits arestandard features of most SMU-based testsystems with pulsed current capabilities and maybe required to avoid self-heating of some lowresistance structures.

When an elevated test current is required, it mustbe short enough so that it does not introduceenough energy to heat the DUT to destructive

temperatures. (Nanoscopic devices tolerate verylittle heat, so the total energy dissipated in themmust be maintained at low levels.) In addition,care must be taken that the magnitude of the testcurrent is low enough that the DUT’s nanoscopicchannel does not become saturated. (For instance,a current channel that’s 1.5 nanometers in diameterseverely limits the number of electrons that canpass through it per unit of time.) Some nanoscopicdevices can support only a few hundred nano-ampsof current in their conductive state. Thus, a device’ssaturation current may define the maximum test

current, even in pulsed applications.

The following equation illustrates how duty cycleand measurement time in pulse mode affect DUT

power dissipation. To calculate power dissipationin pulse mode, multiply the apparent powerdissipation (V·I) by the test stimulus time anddivide by the test repetition rate:

where: Pp = Pulse power dissipationPa = Appa rent power (i.e., V·I)Tt = Test timeT

r

= Test repetition rate

Pulse mode is also useful for density of statemeasurements using a low impedance connection,such as through a nano-manipulator. Pulsing allowsmeasurements at I/V locations that were previouslyuncharacterizable due to pa rticle self-heating.

P p = Pa × T t / T r

Jonathan TuckerSenior Marketer, Nanotechnology Keithley Instruments, Inc.

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Graphene, the single-atom-thick crystal of carbon,has outstanding electrical conductivity. It also has

extremely strong, yet flexible bonds. Its hardness isgreater than the hardness of diamond. Until relativelyrecently, physicists did not believe that a solid crystaljust a single atom thick could exist. ProfessorsNovoselov and Geim proved otherwise with thediscovery of graphene in 2004; for their achievement,they won the 2010 Nobel Prize in Physics.

For the semiconductor industry, the excitingthing about graphene is that electrons travelthrough it unimpeded, and these electrons behaveaccording to quantum electrodynamic principles.Carrier mobilities through graphene are on the

order of 10,000cm2 /V -s at room temperat ure,and mobility values as high as 200,000 cm2 /V- son suspended samples of graphene have beenreported. Graphene’s high mobility hasalready led to the development of veryhigh frequency (100GHz and higher) RFtransistors. Unfortunately, graphene doesnot have a natural bandgap, so manyresearchers are investigating methodsto create one so graphene’s high speedproperties and nano scale size couldreplace silicon in next-generation FETsfor digital circuitry, thereby extending thelife of Moore’s Law.

Researchers characterizing graphene andgraphene-based materials use Hall effect

measurements and study longitudinalresistance to assess carrier mobility and lookfor evidence of the quantum Hall effect, whereby longitudinal resistivity decrea ses tonear 0Ω-cm. These measurements require ver y low cur rent , prec isio n sour cing , onthe order of nano-amps. However, themost important aspect of tight control oversourcing is ensuring that excessive powerdoes not develop across the graphene samplein order to avoid destroying it. Furthermore,at nano-amp source current levels, theresulting voltages developed across thesample are extremely small, on the orderof ten to hundreds of nanovolts. These type of nanovolt-level measurements require special

instrumentation with sufficient resolution andextremely high sensitivity.

In nanovolt-level measurements, thermoelectric voltages and noise sources can significantly impactmeasurement accuracy, so it’s important to employtechniques designed to minimize these effects.For example, using a current source that allowsreversing the polarity of its signal can eliminatemeasurement errors due to thermal voltageoffsets. Furthermore, a current source that canoutput low duty cycle, narrow pulses will minimizemeasurement errors due to resistivity changesresulting from self-heating of the graphene sample.

Graphene: The Semiconductor Industry’sReplacement for Silicon?

Graphene

DC Current Source

Nanovoltmeter

Nanovoltmeter Vxx = LongitudinalVoltage

I

Vxx= Rxx

Vxy = Transversal Voltage,

Hall Voltage withapplied B

Vxy

Vxx

Configuration for simultaneous measurement of Hall effect voltage and longitudinal resistance of a graphene sample in a Hall bar configuration.

A graphene single electron transistor (SET).

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Graphene: The Semiconductor Industry’s Replacement for Silicon? (continued)

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Silicon

V

Vgate

Graphene

SiO2

Configuration of a measurement system for asses sin g the ban dgap in gra phe ne and graphene-based structures.

10

4K14T

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0-2

ρxx (kΩ) σ

xy (4e2ih)

0 2 4

+7/2

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n (1012 cm-2)

Hall VoltageV XY

LongitudinalVoltageV XX

Plot of Hall voltage and longitudinal voltage across a magnetic field of varying intensity. Note how the Hall Voltage is constant at specific points of magnetic field intensity;

at those points, the longitudinal voltage drops to near 0, indicating extremely highconductivity. This demonstrates that graphene exhibits the quantum Hall effect.

Plot courtesy of Neto, Novoselov, Geim, et, al. The Electronic Properties of Graphene. Jan. 2009

Thus, using a current source and nanovoltmetercombination that can synchronize sourcing and

measurement simplifies the elimination of the ther-mal offsets and the of averaging out noise signals.

For graphene or a graphene-based material toreplace silicon, it must have a bandgap so that a

FET channel can be turned on and off. A precisionSourceMeter ® instrument is needed to modulatethe substrate or “gate” voltage to characterizethe sample’s performance across a range of gate voltages. Aga in, a low level current source a nd ananovoltmeter are required to provide low power,low level measurements.

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Summary

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The electronic structure of nanoscopic particles isa reflection of the atomic electron energies and the

distribution of orbitals for both molecularly sharedand free electrons. This kind of information can beused to describe how such materials will interactin the presence of energy and other materials. Thedensity of states in a material is directly related toits electronic structure and is useful in predictingor manipulating its properties.

It can be found through direct electrical measure-ments of differential conductance. Thus, the

density of states can predict a material’s electricalimpedance and vice versa.

However, there is a right way and a wrong wayto interrogate a nanoscopic material electrically,depending on its impedance. For a low impedancematerial, the source current/measure voltagemethod will result in the least electrical noise andallow the most accurate response measurement with the w idest bandwidth. For a high impedancematerial, the source voltage/measure currentmethod is more appropriate for similar reasons. Attimes, the appropriate measurement mode must be

used in unison with yet another voltage or currentsource to activate or stimulate the device.

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Absolute Accuracy. A measure of the closeness of

agreement of an instrument reading compared to thatof a primary standard having absolute traceabilityto a standard sanctioned by a recognized standardsorganization. Accuracy is often separated into gain and

offset terms. See also Relative Accuracy .

A/D (Analog-to-Digital) Converter. A circuit used to

convert an analog input signal into digital information. Alldigital meters use an A/D converter to convert the inputsignal into digital information.

Analog Output. An output that is directly proportional tothe input signal.

Assembler. A molecular manufacturing device that can

be used to guide chemical reactions by positioningmolecules. An assembler can be programmed to build

virtually any molecular structure or device from simpler

chemical building blocks.

Auto-Ranging. The ability of an instrument toautomatically switch among ranges to determine the

range offering the highest resolution. The ranges a reusually in decade steps.

Auto-Ranging Time. For instruments with auto-rangingcapability, the time interval between application of astep input signal and its display, including the time fordetermining and changing to the correct range.

Bandwidth. The range of frequencies that can beconducted or amplified within certain limits. Bandwidth

is usually specified by the –3dB (half-power) points.

Bias Voltage. A voltage applied to a circuit or device toestablish a reference level or operating point of the device

during testing.

Capacitance. In a capacitor or system of conductors and

dielectrics, that property which permits the storage ofelectrically separated charges when potential differences

exist between the conductors. Capacitance is related tothe charge and voltage as follows: C = Q/V, where C is the

capacitance in farads, Q is the charge in coulombs, and Vis the voltage in volts.

Carbon Nanotube. A tube-shaped nanodevice formed

from a sheet of single-layer carbon atoms that has novelelectrical and tensile properties. These fibers mayexhibit electrical conductivity as high as copper, thermalconductivity as high as diamond, strength 100 times

greater than steel at one-sixth of steel’s weight, and highstrain to failure. They can be superconducting, insulating,semiconducting, or conducting (metallic). Non-carbon

nanotubes, often called nanowires, are often created fromboron nitride or silicon.

Channel (switching). One of several signal paths on a

switching card. For scanner or multiplexer cards, thechannel is used as a switched input in measuring circuits,or as a switched output in sourcing circuits. For switchcards, each channel’s signals paths are independent of

other channels. For matrix cards, a channel is establishedby the actuation of a relay at a row and column crosspoint.

Coaxial Cable. A cable formed from two or more coaxialcylindrical conductors insulated from each other. Theoutermost conductor is often earth grounded.

Common-Mode Rejection Ratio (CMRR). The abilityof an instrument to reject interference from a common

voltage at its input terminals with respect to ground.

Usually expressed in decibels at a given frequency.

Common-Mode Current. The current that flowsbetween the input low terminal and chassis ground of an

instrument.

Common-Mode Voltage. A voltage between input low

and earth ground of an instrument.

Contact Resistance. The resistance in ohms between thecontacts of a relay or connector when the contacts are

closed or in contact.

Contamination. Generally used to describe the unwanted

material that adversely affects the physical, chemical, orelectrical properties of a semiconductor or insulator.

D/A (Digital-to-Analog) Converter. A circuit used to

convert digital information into an analog signal. D/Aconverters are used in many instruments to provide anisolated analog output.

Dielectric Absorption. The effect of residual charge

storage after a previously charged capacitor has beendischarged momentarily.

Digital Multimeter (DMM). An electronic instrument

that measures voltage, current, resistance, or otherelectrical parameters by converting the analog signal todigital information and display. The typical five-function

DMM measures DC volts, DC a mps, AC volts, AC amps,and resistance.

Drift. A gradual change of a reading with no change in input

signal or operating conditions.

Dry Circuit Testing. The process of measuring a device while keeping the voltage across the device below a

certain level (e.g., <20mV) in order to prevent disturbanceof oxidation or other degradation of the device beingmeasured.

Electrochemical Effect. A phenomenon whereby currentsare generated by galvanic battery action caused bycontamination and humidity.

Electrometer. A highly refined DC multimeter. Incomparison with a digital multimeter, an electrometer

is characterized by higher input resistance and greatercurrent sensitivity. It can also have functions not generallyavailable on DMMs (e.g., measuring electric charge,sourcing voltage).

EMF. Electromotive force or voltage. EMF is generallyused in context of a voltage difference caused by

electromagnetic, electrochemical, or thermal effects.

Electrostatic Coupling. A phenomenon whereby a currentis generated by a varying or moving voltage source near a

conductor.

Error. The deviation (difference or ratio) of a measurement

from its true value. True values are by their natureindeterminate. See also Random Error and

Systematic Error.

Fall Time. The time required for a signal to change froma large percentage (usually 90%) to a small percentage(usually 10%) of its peak-to-peak value. See also Rise Time.

Faraday Cup. A Faraday cup (sometimes called a Faraday

cage or icepail) is an enclosure made of sheet metal ormesh. It consists of two electro des, one inside the other,separated by an insulator. While the inner electrode isconnected to the electrometer, the outer electrode is

connected to ground. When a charged object is placedinside the inner electrode, all the charge will flow intothe measurement instrument. The electric field inside a

closed, empty conductor is zero, so the cup shields theobject placed inside it from any atmospheric or strayelectric fields. This allows measuring the charge on theobject accurately.

Feedback Picoammeter. A sensitive ammeter that uses anoperational amplifier feedback configuration to convertan input current into voltage for measurement.

Floating. The condition where a common-mode voltage existsbetween an earth ground and the instrument or circuit of

interest. (Circuit low is not tied to earth potential.)

Four-Point Probe. The four-point collinear proberesistivity measurement technique involves bringing four

equally spaced probes in contact with the material ofunknown resistance. The array is placed in the center ofthe material. A known current is passed through the two

outside probes and the voltage is sensed at the two insideprobes. The resistivity is calculated as follows:

π V r =

____ ×

__ × t × k

ln2 I

where: V = the measured voltage in volts, I = the sourcecurrent in amps, t = the wafer thickness in centimeters,

and k = a correction factor based on the ratio of theprobe to wafer diameter and on the ratio of waferthickness to probe separation.

Four-Terminal Resistance Measurement. Ameasurement where two leads are used to supply acurrent to the unknown, and two different leads are used

to sense the voltage drop across the resistance. The four-terminal configuration provides maximum benefits when

measuring low resistances.

Glossary

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16

Troubled by overheating problems?

The Model 4225-PMU option for the Model 4200-SCS performspulsed I-V testing on a variety of devices for many different purposes,including preventing device self-heating by using narrow pulses and/orlow duty cycle pulses rather than DC signals.

Which Keithley nanotechnology solution is best for yoursourcing or measurement application?

Carbon NanotubeField Effect Transistors

Low I, Pulse

Polymer Nanofibers/Nanowires

High R/Low I, 1MΩ to 1014Ω

SemiconductorNanowires

Low Power, R<10MΩ, Pulse

Carbon Nanotubesand Graphene

Low Power, R < 100kΩ

Single ElectronDevices/Transistors

Low I, Low V

Nanobatteries

Low I, Low Power

Nanophotonics

Low I, Pulse

Synthesized MolecularElectronics/Wires

Low I, Low Power

Nanosensors & Arrays

Low I, Low V

Therm

Low I, Lo

Keithley instrumentation is being used in a growing list ofnanotechnology research and production test settings. Theapplications shown here are only a sampling of the nanotech-

nology test and measurement tasks for which our instrumentsand systems are suitable. If your tests require sourcing ormeasuring low level signals, Keithley instr umentation can help you perform them more accurately and cost-effectively.

Want multiple channels of sourcingand measurement?

The fully integrated Model 4200 SemiconductorCharacterization System brings together all threecore measurement types, DC-IV, AC impedence andtransient I-V, in one easy-to-operate package. It'sused in many phases of nano research, development,characterization, and production.

Need to characterize mobility,carrier density, and device speed?

The Model 4210-CVU Option takes theguesswork out of obtaining valid capacitance-voltage (C-V) measurements quickly andeasily, with intuitive point-and-click setup,complete cabling, and built-in element models.

Need tighter control over your pulses?

The Series 3400 Pulse/Pattern Generators can outputvoltage pulses with widths as short as 3ns with indepen-dently adjustable rise and fall times as short as 2ns.

Want seamless control over current pulse sourcing and measurement?

When linked together, the Model 6221 AC+DC Current Source and Model 2182A Nanovoltmeter are designed to operate like

a single instrument to make high

speed pulse mode measurements.

Testing lots of devices?

Series 2600A System SourceMeter ® instruments leprecision DC, pulse, and low frequency AC source-mequickly, easily, and economically. They offer virtualflexibility to scale the system’s channel count up or dowchanging application needs.

Looking for just a single channel?

Each Series 2400 SourceMeter instrument is asingle-channel DC parametric tester. Choose from ranges and functions to suit specific application needs2430 can be programmed to produce individual pulstrains up to 5ms wide.

Studying highly resistive nanowires?

The Model 6430 Sub-Femtoamp Remote SourceMeter ® instrument's low noise and driftperformance make it ideal. It measurescurrents with 400aA (400×10-18 A) sensitivity.

Want low current meawithout the high price

With <200μV burden voltageModel 6485 Picoammeter encurrent measurements, even in

source voltages. The Model 6Voltage Source adds a 500V resistance and resistivity measu

Trying to characterize highresistance nanomaterials?

The Model 6517B Electrometer/High Resistance Meter's built-in 1kV source,200TΩ input resistance, and low current

sensitivity make it an ideal solution.

Ask Us Your Application Or

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APPLICATION CONFIGURATION ADVICE

Click here to discuss your test and measurement challenge

and your application with one of our support engineers andreceive a free-of-charge consultation.

QUOTATION REQUEST

In order to request a quote on any products choose

our Keithley office nearest you from the list belowand give us a call or send an email.

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.

KEITHLEY INSTRUMENTS, INC. n 28775 AURORA RD . n CLEVELAND, OH 44139-1891 n 440-248-0400 n Fax: 440-248-6168 n 1 -888-KE ITHLEY n www.ke ithley.com

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ITALYPeschiera Borromeo (Mi)Ph: 02-5538421Fax: [email protected] twww.keithley.it

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Ph: 82-2-574-7778Fax: 82-2-574-7838keithley@keithley. co.krwww.keithley.co.kr

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Ph: 0118-9297500Fax: [email protected]

© Copyright 2011 Keithley Instruments, Inc. Printed in the U.S.A. No. 3114 02.15.11

Contact us:

A G R E A T E R M E A S U R E O F C O N F I D E N C E

For all countries not l isted contact the KIEX department in Germany: Keithley Instruments GmbH/KIEX. Ph: +49-89-84 93 07-62. Fax: +49-89-84 93 07-85. E-Mail: [email protected]

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