PFM Experiments With Highvoltage DCAC Bias

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PFM Experiments with

High Voltage DC/AC Bias

Support Note

IntroductionPiezoelectric force microscopy (PFM)has found major applications in thestudy of ferroelectric materials,particularly for high resolution imaging,domain switching, local hysteresismeasurements, and switching offerroelectric capacitors. For example,ferroelectric materials are used in manyfield effect transistor applications due totheir high permittivity. With the demandof miniaturization of electronic devices,it becomes important to study the sizeeffect of ferroelectric materials, i.e.,to study the critical size range wheresignificant deviation from bulk propertiesoccurs. The high spatial resolution ofPFM also provides a unique opportunityto study the fundamental processof domain switching, including the

thermodynamics and kinetics of domainnucleation, growth, and relaxation.By analyzing local hysteresis loopmeasured by PFM, information aboutthe piezoelectric properties of smallferroelectric domains can be obtained,including coercive voltages, nucleationvoltages, forward and reverse saturationand remanent responses, as well asthe effective work of switching definedby the area included in the hysteresisloop. Simultaneously recorded VPFMand LPFM nanoscale hysteresis loopswere used to differentiate 900 domain

switching from 1800 domain switching,and to study the dependence of PFM oncrystallographic orientations.

Even though a wide range of materialscan be studied by PFM with a relativelylow AC/DC bias to the tip or sample,

within +/-10 V typically, there is ademand for being able to apply higherbias, up to several hundred volts, in manycases. One reason for such a requestis the need of high voltages for domainswitching in some materials. In order tostudy the piezoelectric behavior of thosematerials, high enough voltages haveto be applied to switch the polarizationand to observe the hysteresis of therelaxation process. A second reasonfor the need of high voltage bias is toincrease the signal-to-noise ratio ofPFM experiments on materials of lowerpiezoelectric response. Since the PFMsignal measured is proportional to thebias voltage, it will increase with theAC voltage of the applied modulationbias. Therefore, one can apply higherbias to study the piezoelectric properties

of materials with lower piezoelectriccoefficient, provided the materialunder test can sustain the voltageapplied without damage and pertinentproperty change.

In this technical note, we will brieflydescribe a number of differentconfigurations for doing PFMmeasurement using the Agilent5420/5500/5600 AFM systems,particularly about the configurationthat allows the user to perform PFMexperiments with high AC/DC bias with

the help of an external high voltageamplifier. The setup discussed insection A involves only the standardhardware including a MAC III controller.This configuration is the most directapproach, every system equippedwith a MAC III controller can do PFM

Shijie Wu and John AlexanderAgilent Technologies


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function generator for AC modulationbias and the lock-in amplifier for PFMsignal analysis. The MAC III has threedual phase lock-in amplifiers (LIA)converting the AC inputs to amplitudeand phase. These digitally-controlledanalog LIA have a broad bandwidth (upto 6 MHz) that covers the operationbandwidth of the photodetectoremployed in the microscope. The electricconnection for a simple PFM experimentis similar to that of a current sensingAFM experiment, except that a contact/AAC mode nosecone is used insteadof a CSAFM nosecone. The conductivecantilever is connected to tip bias

experiments with this setup. Thediscussion in section A also helps user tounderstand the various sources of input/output signals in the system, thus betterunderstand the operational principle ofPFM, However, this configuration is notthe best configuration for doing PFMdue to the possible coupling betweensignals. The approach in section Bseparates the modulation bias fromthe PFM signal by the use of a MACIII Signal Access Box, thus minimizesthe coupling between the AC bias andAFM signals, and should be used as therecommended approach for low voltagePFM experiments. The configuration insection C shows a simple approach forPFM experiments with high voltage bias,using a commercially available stand-alone high voltage amplifier.

A. PFM setup for5420/5500/5600 AFM system

with default hardwareThe default configurations for PFMexperiments using the 5420/5500/5600system are essentially all the same.The typical setup for PFM experiment isillustrated in a block diagram (excludingthe shaded part) in Figure 1, involvingthe use of MAC III controller as both the

Figure 1. Block diagram showing the PFM setupwith default hardware and with a M AC III SignalAccess Box (shaded part of the diagram) for Agilent5420/5500/5600 AFM system.

Figure 2. Software control window for setting DC biasto sample or to Cantilever.

Figure 3. Software control windows for sett ing the AC bias to sample and the PFM signal output

through the scanner and spring clip onthe nosecone, and the bias switch atthe back panel of the Head-ElectronicsBox (HE) should be set to Tip. Thesample is connected to the sample biasthrough the middle pin of the 3-wire ECcable that is connected to the base ofthe microscope. During an experiment,the DC bias and the AC modulation canbe applied to either the sample or theconductive tip from the software controlin PicoView. As shown in Figure 2, theDC bias is applied to the conductivecantilever when the Apply Bias Tocontrol is set to Tip from the Advancedtab of the Servo control window, andthe sample is set to ground at the sametime. Similarly, the DC bias can be appliedto the sample by set ting the Apply BiasTo control to Sample, and the tip willbe set to ground in this case.

The AC bias is generated from the ACdrive signal from the lock-in amplifier inMAC III; usually the AC drive from LIA2 is used. The AC modulation can beapplied to the sample or the conductivecantilever by choosing the propersettings in the Advance AAC ModeControls window. Figure 3 shows anexample setting for applying AC biasfrom LIA 2 to the sample. In the leftside window of Figure 3, the AC bias is


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defined using the drive output of LIA 2,which in this case was set to a frequencyof 10.123 kHz and a peak amplitude of1V. Other adjustable parameters in thewindow include: the Gain to scale upthe input signal if the piezoelectric effectis weak and the Bandwidth to reducethe signal noise by applying a band-passfilter. The Input signal can be seteither to Deflection for VPFM or toFriction for LPFM. The Phase Shiftshifts the relative phase angle of the ACdrive signal against the internal referencesignal of the lock-in amplifier. The PhaseOffset adds a constant offset to themeasured phase angle for proper display.

In the right side window (Outputs)of Figure 3, the Sample Bias is setto Drive 2, which directs AC biassignal from the drive output of LIA 2 tothe sample. The Sum box beside theSample Bias is checked in this case,thus the bias signal from the MAC III willbe added to the bias from the sample biasDAC (if any) and delivered to the sampletogether. If this Sum box is unchecked,then the bias from the sample bias DACis cut open, and only the bias from theMAC III will be sent to the sample.

If the Tip Bias is set to Drive 2 andthe Sample Bias is set to ground(GND) in this Outputs window, thenthe same bias signal from MAC III willbe applied to the conductive cantilever.

Again, this AC bias signal can be addedtogether with the bias from the t ip biasDAC if the corresponding Sum boxis checked.

It is important to notice that the biasfrom the controller DACs, which is setin the Servo control window, and thebias from the MAC III controller, which isset in the Advanced AC Mode Controlwindow, are from different sources andwork independently. They can be applied

to sample or tip together as one summedsignal; or they can be applied separately,with one to the sample and another tothe tip, and vice versa.

The outputs from the lock-in amplifierused to analyze the PFM signals aregiven in the form of Amplitude, Phase,X Component, and Y Component. Thesesignals can be channeled internally toone of the four auxiliary channels Aux1, Aux 2, Aux 3, and Aux 4 in MAC III,and then accessed by PicoView fordisplay and processing. As shown in theOutputs window in Figure 3, the Aux1, 2, 3, 4 channels are set to Amplitude,Phase, X, and Y Components from LIA2,respectively. Because the input signalto LIA 2 is deflection, thus all those foursignals correspond to the piezoelectricresponse normal to the sample surface,i.e., VPFM. If LIA 3 is set to the samefrequency as Drive 2 with its input setto friction, then the PFM signal givenby LIA 3 will correspond to the piezoeffect parallel to the sample surface,i.e., LPFM. If Aux1 and 2 are assigned toVPFM signals (signals from LIA 2 in thiscase) and Aux 3 and 4 are assigned toLPFM signals (signals from LIA 3 here),then VPFM and LPFM can be measuredsimultaneously for the sample under test.

B. PFM setup with MAC III

Signal Access BoxIn PFM experiments because of the hugedisparity in the signal size between thedrive signal and the tiny deviations in thedeflection signal, maintaining separationis one of the serious requirements forsuccessful measurements. With thedefault setup described above, however,there is a possibility that the applied ACmodulation bias will interfere with otherelectric signals in the system, particularlythose in the same frequency range. In

order to avoid coupling between thePFM bias and other AFM signals, andto minimize the noise level in the ACbias and the PFM signal detected, it isnecessary to isolate the pathway of theAC bias from the rest of the signals.

As described in Reference 2, the isolationof the PFM bias from the rest of thesignals can be done with the help of aMAC III Signal Access Box. The driveoutputs from MAC III are accessiblethrough the MAC III Signal Access Box,which is shown in the shaded block ofthe diagram in Figure 1. The clean biasvoltage is supplied to the sample with aseparate shielded cable connecting theDrive Output of LIA 2 from the SignalAccess Box and to the sample stage. Theconnections between the componentsare supported by the software thatcontrols them using Advanced ACControls window and allows monitoringof the signals via the Aux Channels I/Owindow. Since the drive output from theMAC III Signal Access Box bypassesthe lines and control switches insidethe MAC III controller and connects tothe sample stage directly, the settingsfor Sample Bias in the Outputswindow shown in Figure 3 are no longerrelevant. It can be set to GND just forsafe practice. With the AC bias from theMAC III directly applied to the sample,one still has the choice to apply a DC biasfrom the controller DAC to the tip, similar

to what have been discussed in theprevious section.

By using the MAC III Signal AccessBox, it is convenient to setup the PFMexperiment with AC bias supplied to thesample. If it is necessary to apply the ACbias from Drive 2 directly to the t ip withan external cable, then one need to usea specially modified nosecone which isdiscussed in the next section.


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C. High voltage PFM with an

external amplifierThe controller DACs and the driveoutputs from the MAC III all have a rangeof +/-10V, thus limiting the experimentsto materials of relatively high responseand low switching potential. Whenthe materials under test become lesssensitive and need high voltage forpolarity switching, then high voltage ACmodulation and DC bias will be required.High voltage bias can only be appliedto the sample through external cablingbecause the increased voltage will alsoincrease the magnitude of interferencewith other AFM signals. Since the defaultvoltage range from the system is +/-10V,a high voltage amplifier is needed toamplify the voltage to the desired value.Because the AC modulation usually runsfrom several kHz to several hundred kHzin frequency, the amplifier used has tohave a relatively high speed as well.

The high speed, high voltage amplifierused in this experiment is the Agilent33502A unit. The Agilent 33502A is atwo-channel, isolated amplifier. Bothchannels can either propagate a signaldirectly or with 5x amplification. It alsoallows the user to select whether thechannel should be AC or DC coupled. Thebandwidth of the unit is > 300 kHz (-3dB)for small signal and 100 kHz @50Vpp forfull power output. The Agilent 33502A

has a very low noise, < 40nV/rt-Hz @1 kHz. The maximum output voltage is50Vpp at maximum power. The use of theAgilent 33502A here is a simple choice ofconvenience. There are other commercialunits available for this application aswell. For example, the Falco SystemsWMA-280 high voltage amplifier and theTrek Model PZD350 M/S piezo driver/amplifier are all suitable candidates.It is up to the user to decide on theone with the right speed and the rightoutput voltage for their experiments.The user should also take proper

safety precautions when using highvoltage amplifiers.

The high voltage amplifier is connectedto the system via the Breakout Boxfor the 5420/5500/5600 system, asillustrated in the block diagram inFigure 4. The Breakout Box is connectedto the MAC III using a short 44-pin cableto minimize signal coupling in the line;and is connected to the HE with a regular44-pin cable. As parameter settingsshown in Figure 5, when the Apply BiasTo control in the Servo window is setto Tip, and the Tip Bias control in the

Outputs window is set to Drive 2with the Sum box checked, the ACdrive signal from LIA2 of the MAC IIIand the DC tip bias from the controllerDAC are mixed by a sum circuit insidethe MAC III controller and sent to theTIP BIAS/OUT BNC connectoron the Breakout Box. The OUT BNCconnector is connected to the input ofthe high voltage amplifier using a regularBNC cable. There is a toggle switchon the Breakout Box beneath the TIPBIAS connectors. This toggle switchshould be pushed towards the IN

connector, in which case the voltagefrom the MAC III will not be sent to the

Figure 4. Block diagram illustrating the setup for PFM with high voltageAC/DC bias using a Breakout Box and an external high voltage amplifier.

Figure 5. Software sett ings for generating the lowvoltage AC/DC bias to be amplified by the highvoltage amplifier.


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tip through the 44-pin cable anymore,rather serves only as the input signal tothe high voltage amplifier. The outputfrom the high voltage amplifier givesthe desired high voltage bias signal forPFM. This high voltage bias signal canbe connected directly to the cantileverthrough an external cable. The sampleneeds to be connected to the microscopeusing the 3-pin EC cable to close thecircuit, i.e., to be grounded or biased bythe sample bias DAC.

The regular nosecone is connected to thetip bias DAC through the steel spring clipand the metal core of the multipurposescanner. In order to supply the bias fromthe amplifier directly to the cantilevervia external cabling, the nosecone has tobe modified. The modification includes1) the cutting and bending of the tail ofthe spring clip so that it will not contactthe metal core of the scanner, i.e.,the electric connection between thecantilever and the internal tip bias DACis broken open; 2) a 3-pin connectoris soldered to the metal spring clip toconnect the cantilever to the outputof the high voltage amplifier. A photopicture of the whole setup includingthe MAC III, the Breakout Box, the highvoltage amplifier, the microscope and thescanner with the modified nosecone isshown in Figure 6. The red wire with the3-pin connector soldered to the noseconeis long enough to avoid any mechanical

stress to the scanner.

ExamplesPeriodically poled lithium niobate (PPLN)is a domain-engineered lithium niobatecrystal with its ferroelectric domainspointing alternatively to the +c and the-c direction. It is obtained by electricalpoling with periodically structuredelectrode. The sample used here hasalternating domains of 15-20m widestrips, as shown in Figure 7. Since the

surface of the PPLN is polished withoutetching to expose the alternating

Figure 6. Photos showing the hardware components for high voltage PFM, particularly the modifiednosecone with electric lead to connect to the output of the high voltage amplifier.

Figure 7. PFM imaging of PPLN with a 20kHz and 40V p-p ac modulation.


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domains, the topography image is fairlysmooth without noticeable structures.The PFM images (amplitude and phase),on the other hand, clearly reveals theexisting alternating c+ and c- domains.Because the two domains differ only inthe direction of polarization, the PFMamplitudes shows only small differencebetween them, and a minimum at thedomain interface due to the cancelationof opposite movements of c+ and c-domains. The 180 phase angle (+/-5V,with 18/V) clearly indicates the totallyopposite polarization direction of theneighboring domains.

As stated earlier, the PFM signalmeasured will increase with the ACvoltage of the applied modulation bias.The effect of the modulation bias isillustrated in Figure 8, where two lineprofiles from the PFM amplitude imagesobtained using a 20V peak-to-peakmodulation and that using a 40V peak-to-peak AC modulation are comparedtogether. The increase in PFM amplitudewith AC bias voltage is quite obviousfrom these data.

Figure 9 shows the results from a polingexperiment on PVDF film depositedon a Si substrate. PVDF thin film istypically 50-60% crystalline. Whenpoled, PVDF is a ferroelectric polymer.The piezoelectric coefficient of thepoled PVDF thin film is about 6-7 pC/N,

which is about 10 times larger than thatobserved for other polymers. Unlikeother popular piezoelectric materials,such as PZT, PVDF has a negative d33value. Physically, this means that PVDFwill compress instead of expand orvice versa when exposed to the sameelectric field. In this experiment, a -25VDC bias was applied to the AFM tip andan area of 8 x 8m was first scannedin contact mode. Then the tip bias wasswitched to +25V and a 4 x 4m areawas scanned in the center of the 8 x 8mregion. Finally, PFM imaging was done

over a 12 x 12m area to visualize theeffect of poling. It is evident from the

Figure 8. Increase of the PFM amplitude with the increase of AC modulation voltage.

Figure 9. Poling of PVDF film and the hysteresis curve at the position indicated by the red marker. The filmwas poled with a -25V t ip bias for an 8m area, then a 4m area in the middle was poled with a +25V tipbias, and finally a 12m area was scanned to reveal the poling ef fect. The hysteresis curve w as obtainedbetween +10 and -10 Volts, the axis shows +/-2V that was amplified 5 x by the external amplifier beforebeing applied to the tip.


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PFM amplitude image that there existsome crystalline phases in the film thatshows stronger piezo effect after poling.There are also some domains that showminimum piezo effect and are affectedlittle by the applied electric field. ThePFM phase image also clearly showsa 180 phase difference between the-25V poled region and the +25V poledregion, indicating that the polarizationdirection can be switched when theapplied electric field change direction.This switching process can be studiedby the hysteresis curve. The hysteresiscurve presented in Figure 9 is collectedat the point indicated by the red arrowtail marker on the image, which is locatedon one of the piezo active phases. Thevoltage applied has been amplified 5xby the external high voltage amplifier(Agilent 33502A ), therefore the readingof 1V on the x axis of the hysteresiscurves corresponds to 5V in reality. Formthe phase curve, it is seen that the filmwill start switching polarization at about+/-3.5V (the coercive field). The shift ofthe center of the hysteresis curve from0 volts suggests the possible existenceof internal bias field in this agedsample. Even though the coercive fieldis much lower than the applied polingvoltage, which means the polarizationswitching starts at a much lower bias,the application of a higher poling voltageincreases the switching rate and thecompleteness of the process.

SummaryA number of PFM configurationsusing the Agilent AFM systems aredescribed and the key parametersinvolved in PFM experiments and theirappropriate settings in the softwareare discussed. The necessity andpossible approach of doing high voltagePFM are demonstrated with sampleslike PPLN and PVDF thin film depositedon Si substrate.


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PFM Experiments With Highvoltage DCAC Bias