Electrostatic Force Microscopy - Nano & Pico Characterization Lab
Contents
1. reducing the cantilever resonant frequency Conversely repulsive forces make the cantilever effectively stiffer increasing the resonant frequency A comparison of these force additives is shown in Figure 230 3 230 4 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series f w O Amplitude Frequency Attractive gradient equivalent to additional spring in tension attached to tip reducing the cantilever resonance frequency Amplitude 4 A Frequency Repulsive gradient equivalent to additional spring in compression attached to tip increasing the cantilever resonance frequency Figure 230 3 Comparison of Attractive and Repulsive Forces to Action of a Taught Spring Attached to the Tip Changes in cantilever resonant frequency can be detected in one of the following ways e Phase detection with Extender Electronics Module only e Frequency modulation FM with Extender Electronics Module only e Amplitude detection not recommended due to artifacts All of the above methods rely on the change in resonant frequency of the cantilever due to vertical force gradients from the sample Figure 230 4 shows a diagram of how the Extender Electronics module provides the signal enhancement and feed back allowing gradient detection The best candidates for electric field gradient imaging are samples that have large contrasts in the electric force gradient due to material differe2. Dimension Series 230 4 Electric Field Gradient Detection Procedures NOTE Amplitude detection is the only procedure described here that can be done without the Extender Electronics Module however this method is no longer recom mended see Section 230 4 2 1 Locate the two toggle switches on the backside of the Extender Electronics box Figure 230 16 then verify that they are toggled as shown in Table 230 1 Tip or Sample Mode Voltage FM Phase Gnd Surface Potential Surface Potential Analog 2 cope Figure 230 16 Toggle Switches on back of Extender Electronics Module FM Phase Surface GND Surface Analog 2 Potential Potential TappingMode Contact AFM MFM Surface Potential Apply voltage to tip or sample Use for electric field gradient imaging tunneling AFM Table 230 1 Extender Electronics Module Toggle Switch Settings Support Notes 230 19 Electric Force Microscopy Dimension Series Support Note No 230 NOTE The toggle switch combination of Surface Potential ON and Analog2 ON is not recommended and can produce erratic and undefined results 2 Electrically connect the sample by mounting it to a standard sample disk or stage using conducting epoxy or silver paint Ensure the connection is good a poor con nection will introduce noise 3 Mount a metal coated NanoProbe cantilever into the electric field cantilever holder MFM style cantileve
3. 230 6 2 Applying Voltage to the Sample Directly 29 230 7 Surface Potential Imaging Procedure 30 230 7 1 Troubleshooting the Surface Potential Feedback Loop 33 Document Revision History Support Note 229 Rev Date Section s Ref DCR Approval _ gt Rev A 24MAY96 Initial Release 0062 0110 lA Digital Instruments 1996 230 1 520 E Montecito St Santa Barbara CA 93103 805 899 3380 Electric Force Microscopy Dimension Series Support Note No 230 230 1 Electric Force Microscopy Overview This support note describes how to perform electric force microscopy EFM imag ing on a Dimension series system Similar to magnetic force microscopy MFM and sharing many of it s procedural techniques this mode utilizes the Interleave and LiftMode procedures discussed in the product instruction manual The MFM techniques can be found in Support Note 229 Contact Digital Instruments for a copy of this document Please read those sections prior to attempting electric force measurements All standard Dimension series SPMs are capable of EFM imaging using amplitude detection techniques By adding an Extender electronics module see Figure 14 2 the Dimension system may also be used for frequency modulation or phase detection giving improved results Amplitude detection has largely been superseded by frequency modulation and phase detection This hardware unit is required for surface potential imaging and is strongly
4. Common or negative terminal See below Figure 230 24 Jumper Configuration for Application of Voltage Directly to Sample The sample should be electrically insulated from the chuck Connect the external voltage source directly to the sample by attaching fine gauge wire to appropriate contacts e g on integrated circuits connect electrical leads directly to pads For normal operation the sample chuck is held at ground be certain to carefully insu late any electrical connections from the sample chuck gt 10 MQ External Voltage Source Electrical Insulator Sample Chuck Support Notes 230 29 Electric Force Microscopy Dimension Series Support Note No 230 230 7 Surface Potential Imaging Procedure 1 Locate the two toggle switches on the backside of the Extender Electronics box Figure 230 25 then verify that they are toggled as shown in Table 230 2 Tip or Sample Mode Voltage FM Phase Gnd Surface Potential Surface Potential Analog 2 Tip or Sample Voltage Surface GND Surface Analog 2 Potential Potential TappingMode Contact AFM Apply voltage to tip or sample Use for electric field gradient imaging tunneling AFM Table 230 2 Extender Electronics Module Toggle Switch Settings For Surface Potential Imaging NOTE The toggle switch combination of Surface Potential ON and Analog2 ON is not recommended and can produce erratic and undefined res
5. enter Frequeney 82 23 We ee 0 05 Ekz div Figure 230 17 Phase Detection Cantilever Tune Extender Electronics Module Installed The phase should decrease with increasing frequency and cross the center line 90 point at the peak frequency The phase curve then correctly reflects the phase lag between the drive voltage and the cantilever response Again gradi ents in the electric force will cause a shift AF in the resonance frequency In this case resonance shifts give rise to phase shifts Ag which can then give an image of the electric force gradients see Figure 230 18 180 D D g 90 i w lt oO 0 Drive Frequency Figure 230 18 Shift In Phase at Fixed Drive Frequency Support Notes 230 21 Electric Force Microscopy Dimension Series Support Note No 230 e Under Interleave Controls set the Lift start height to 0 nm and Lift scan height to 100 nm The lift height can later be optimized Set the remaining Interleave parameters Setpoint Drive amplitude Drive frequency and gains to the main Feedback Controls values This can be done by setting the flags left of the Interleave Control column to off grayed bullets Quit Auto Tune and return to Image Mode e Engage the AFM and make the necessary adjustments to obtain a good topog raphy Height image on Channel 2 e Select the Interleave Controls panel Set the Setpoint Drive amplitude Drive frequency to Main off
6. grayed bullets Verify that Interleave scan is set to Lift e Choose a Lift scan height of 100 nm This will be optimized later Set the Channel 1 image Data type to Phase and choose Retrace for the scan Line direction on both Channel and 2 images e Switch Interleave mode to Enable to start LiftMode Set the Channel 1 Scan line to Interleave to display interleave data This screen should now display the cantilever phase change due to electrical force gradients from the sample in the left image and topography in the right image e Optimize the Lift height For high resolution make the Lift scan height as small as possible without crashing the tip into the surface If the tip crashes into the surface it creates bright or dark streaks across the image Also if the Lift scan height is set extremely low the tip may continuously tap on the surface during the LiftMode scan Check this by toggling between the Interleave and main scan lines for the phase image The two images will look very similar if the tip is continuously tapping on the surface during the LiftMode scan In this case increase the Lift scan height until the Interleave scan image abruptly changes indicating that it is now oscillating above the surface and not continu ously tapping e Adjust the sample or tip voltage to confirm that contrast is due to electrical force gradients On very rough samples contrast in LiftMode images may be from air damping between the tip and sur
7. holder is clean and the cantilever is tightly secured 6 Engage the AFM and make the necessary adjustments for a good TappingMode image while displaying height data 7 Select the Interleave Controls command This brings up a new set of scan parameters that are used for the interleaved scan where surface potential is mea sured Different values from those on the main scan may be entered for any of the interleaved scan parameter To fix any of the parameters so they are the same on the main and interleave scans click on the green bullets to the left of particular param eter The green bullet changes to off gray and the parameter value changes to the main Feedback Controls value Set the interleave Drive frequency to the main feedback value Enter an interleaved Setpoint of 0 V Set Interleave scan to Lift 8 Enter an Interleave Controls Drive amplitude This is the ac voltage that is applied to the AFM tip Higher Drive amplitude produces a larger electrostatic force on the cantilever and this makes for more sensitive potential measurements Conversely the maximum total voltage ac dc that may be applied to the tip is 10 V So a large Drive amplitude reduces the range of the DC voltage that can be applied to the cantilever If the sample surface potentials to be measured are very large it is necessary to choose a small Drive amplitude while small surface poten tials can be imaged more successfully with large Drive amplitudes To st
8. jumper configuration instructions for basic microscope models operating with the Extender Electronics Module See Jumper Configuration Sections E F G H The backplane board used with Dimension series SPMs is shown below in Figure 230 5 At the center of the board is located a header supplied with jumpers For non EFM applications and surface potential operation jumpers are usually left in their original positions or returned to their original positions Figure 230 5 Dimension series Microscope Backplane Jumpers are circled Support Notes 230 7 Electric Force Microscopy Dimension Series Support Note No 230 1 Carefully examine the following figures and identify which jumper configura tion if any is appropriate for your application 2 Power down the NanoScope III controller and turn off all peripherals Unplug the NanoScope III power cable from the microscope s electronic box 3 Remove the back panel on the microscope s electronic box Locate header and jumpers per Figure 230 5 on the main electronics backplane As shipped from the factory jumpers on systems without the Extender Electronics Module should appear as shown in Figure 230 6 whereas jumper systems with the Extender Elec tronics option should appear as in Figure 230 11 If the microscope is to be used for EFM imaging in cases where voltage is applied to the tip or sample it is necessary to change the jumpers 4 Depending upon whether
9. recommended for electric field gradient imaging Two types of electric force microscopy are available using the Dimension Series system 1 electric field gradient imaging and 2 surface potential imaging Each of the electric field measurement techniques are based on a two pass Lift Mode measurement LiftMode allows the imaging of relatively weak but long range magnetic and electrostatic interactions while minimizing the influence of topography see Figure 230 1 Measurements are taken in two passes each con sisting of one trace and one retrace across each scanline First topographical data is taken in TappingMode on one trace and retrace The tip is then raised to the final scan height and a second trace and retrace performed while maintaining a constant separation between the tip and local surface topography Both methods of electric force measurement are explained in this support note 1 This support note describes how to perform electric force microscopy EFM imaging on a Dimension series system and replaces Support Note 206 Mag netic amp Electric Force MFM amp EFM Imaging with SPMs Dimension series Mi croscopes Support Note 229 provides information on magnetic force microscopes MFM for Dimension series systems and Support Note 231 provides information on EFM for MultiMode systems 230 2 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series e E MS Force Gradi
10. the com mon or negative terminal of an external power supply to the Dimension system ground Access to ground and other Dimension signals is also made by removing the back panel of the electronics box as shown in the jumper configuration draw ings below 1 Power down the NanoScope controller and turn off all peripherals Unplug the NanoScope power cable from the microscope s electronic box 2 Remove the back panel on the microscope s electronic box Locate jumpers on the main electronics backplane panel per Figure 230 5 As shipped from the factory the backplane jumpers should appear as shown in Figure 230 11 and reproduced here Figure 230 22 for convenience 4 ChuckGround Bias L Chuck V Ni Analog 2 or m cane Gnd OSC DC signal Qa E Unused a 6 lt Unused STM signal Auxiliary 2 Output Ground Ground oa Indicates jumpers Figure 230 22 Normal jumper configuration as shipped from factory for Dimension systems with Extender Electronics Module installed 1 0 ee 00 3 Depending upon whether voltage is to be applied to the sample directly or indi rectly reconfigure jumpers on the backplane header according to either Figure 230 23 or Figure 230 24 below Support Notes 230 27 Electric Force Microscopy Dimension Series Support Note No 230 230 6 1 Applying Voltage to the Sample Indirectly When voltage is to be applied to the sample via the sample chuck for indirect sur face potential im
11. voltage is to be applied to the tip or sample and the amount of voltage to be used reconfigure jumpers on the backplane header using the appropriate jumper configuration as shown in the appropriate sections below 5 After the backplane jumpers are correctly configured replace the cover on the electronics box Apply power to the microscope and all peripherals Boot the com puter and start the NanoScope software 6 If the backplane jumpers are configured to use an external voltage source click on the Microscope Calibrate Detector option to display the Detectors Param eters window Switch the Allow in attenuation field to Allow 7 Imaging can now be accomplished using the procedures in Section 230 4 230 8 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series 230 3 1 Jumper Configurations for Systems Without the Extender Electronics Module As shipped from the factory the jumper configuration on a Dimension series sys tem without the Extender Electronics Module should appear as shown in Figure 230 6 below y Sample Chuck Ground Bias Sample Chuck a N Gain Select a a la N 2 j Le lt x Ground Unused NI vA STM signal from Dimension head Auxiliary D to NanoScope III controller Ground Ground F Indicates jumpers Figure 230 6 Normal Jumper Configuration For Systems Without the Extender Electronics Module As Shipped From Factory
12. 11 Electric Force Microscopy Dimension Series Support Note No 230 C Field Gradient Imaging External Voltage Source Applied to the Tip In some cases it may be advantageous to use voltages greater than 12 VDC or to use a pulsed power supply If an external source of voltage is to be applied to the tip configure jumpers as shown in Figure 230 9 gt 10 MQ Sample Chuck Ground Bias lh O External Voltage Source Sample Chuck Analog 2 Gain Select Q BOF gt 0 lt Figure 230 9 Jumper Configuration for Applying External Voltage to Tip For Systems Without the Extender Electronics Module Unused STM signal from Dimension head Auxiliary D to NanoScope III controller Ground Ground C Indicates jumpers A current limiting resistor e g 10 100 MQ should be placed in series with the external voltage supply as shown to protect the tip and sample from damage Cur rent limited power supplies may also be used Voltage leads should be connected to pins on the header using soldered push on connectors Do not solder leads directly to the header pins as the heat may cause damage and or make jumpering the pins difficult 230 12 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series D Field Gradient Imaging External Voltage Source Applied to the Sample In some cases it may be advantageous to use voltages greater than 12 VDC o
13. Notes Support Note No 230 Electric Force Microscopy Dimension Series B Field Gradient Imaging Voltage Applied to the Sample The jumper configuration in Figure 230 8 connects the Analog 2 signal from the NanoScope II controller 12 VDC range to the sample chuck Enabling the Analog 2 Voltage Line The Analog 2 voltage line is normally used by the NanoScope to control the attenuation 1x or 8x of the main feedback signal This application of EFM imag ing uses the Analog 2 signal for EFM data Therefore input attenuation must be disabled for the duration of the EFM experiments To do this click on the Micro scope Calibrate Detector option to display the Detectors Parameters window Switch the Allow in attenuation field to Disallow Return to the main Feedback Controls panel the Analog 2 field should now be enabled This signifies that volt age is now being supplied via the Analog 2 pin located on the backplane header Remember to restore Allow in attenuation upon completion of EFM imaging Sample Chuck Ground Bias Tip Sample Chuck g b Analog 2 i ws x ees ed es Cc z A Gain Select V Analog 2 Ground AFM Tip STM signal from Dimension head Auxiliary D to NanoScope III controller Ground Ground Cc Indicates jumpers Figure 230 8 Jumper Configuration for Application of Voltage to Sample For Systems Without the Extender Electronics Module Support Notes 230
14. aging configure the jumpers as shown in Figure 230 23 External Voltage Source Oph Chuck Ground Bias Chuck 10 MQ Cd Analog 2 or 3 Nai Gnd OSC DC signal Q g m Unused gt 9 lt m Unused STM signal C Auxiliary 2 Output m Ground m Ground E Indicates jumpers Figure 230 23 Jumper Configuration for Application of Voltage to Sample via Sample Chuck A current limiting resistor e g 10 100 MQ should be placed in series with the external voltage supply to protect the tip and sample from damage Current limited power supplies may also be used Voltage leads should be connected to pins on the header using soldered push on connectors Do not solder leads directly to the header pins Heat may cause damage and or make jumpering the pins difficult The sample should be electrically connected directly to the chuck or a standard sample puck using conductive epoxy or silver paint as shown below Conductive Epoxy or Paint l Sample Chuck 230 268 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series 230 6 2 Applying Voltage to the Sample Directly If voltage is to be applied directly to the sample configure jumpers as shown in Figure 230 24 Y Chuck Ground Bias C Indicates jumpers Sample Chuck V g Analog 2 or J Gnd OSC DC signal N Q g E m Unused 5 5 z m Unused Xf STM signal Auxiliary 2 Output LA y p Connection to Power Supply
15. art choose a Drive amplitude of 5 V Support Notes 230 31 Electric Force Microscopy Dimension Series Support Note No 230 9 Set the Channel 2 image Data type to Potential Set the scan Line direction for the main and interleave scans to Retrace Remember to choose the Retrace direc tion because the lift step occurs on the trace scan and data collection occurs on the retrace 10 Choose a Lift start height of 0 nm and a Lift scan height of 100 nm The Lift scan height can be readjusted later 11 Switch Interleave mode to Enable to start LiftWode Now when the micro scope completes a topographic scan line trace and retrace the system turns off the TappingMode piezo and switches the oscillator signal to the cantilever The cantile ver is driven electrostatically according to the interleave Drive amplitude that has been selected Also when Potential is selected as the Data type for the Channel 2 image a feedback circuit is enabled in the Extender box which adjusts the dc volt age on the tip to maintain the cantilever oscillation amplitude at zero To do this the feedback circuit uses the lock in signal of the cantilever oscillation and tries to keep this value at zero volts As detailed in the Section 230 5 above when the cantilever oscillation amplitude has returned to zero the dc voltage on the tip and sample are the same The NanoScope records the dc voltage applied to the tip and this signal is displayed in the Potential
16. d Digital 8 instruments Support Note No 230 Rev A Electric Force Microscopy EFM Applicable to Dimension Series Systems Support Note Table of Contents 230 1 Electric Force Microscopy Overview 2 230 1 1 Electric Field Gradient Imaging Overview 4 230 1 2 Surface Potential Imaging Overview 4 230 2 Electric Field Gradient Detection Theory 4 230 3 Electric Field Gradient Detection Preparation 7 230 3 1 Jumper Configurations for Systems Without the Extender Electronics Module 9 A Field Gradient Imaging Voltage Applied to the Tip 10 B Field Gradient Imaging Voltage Applied to the Sample 11 C Field Gradient Imaging External Voltage Source Applied to the Tip 12 D Field Gradient Imaging External Voltage Source Applied to the Sample 13 230 3 2 Jumper Configurations For Microscopes With the Extender Electronics Module 14 E Field Gradient Imaging Voltage Applied to the Tip 15 F Field Gradient Imaging Voltage Applied to the Sample 16 G Field Gradient Imaging External Voltage Source Applied to the Tip 17 H Field Gradient Imaging External Voltage Source Applied to the Sample 18 230 4 Electric Field Gradient Detection Procedures 19 230 4 1 Phase Detection 20 230 4 2 Amplitude Detection 23 With Extender Electronics Module 23 Without Extender Electronics Module 23 230 5 Surface Potential Detection Theory 25 230 6 Surface Potential Detection Preparation 26 230 6 1 Applying Voltage to the Sample Indirectly 28
17. data type 12 Adjust the FM gains The feedback loop that is used by the Extender Electronics for surface potential measurements is the same as the one used in Frequency Modu lation FM for magnetic and electric force gradient detection as described previ ously The feedback loop should be tuned in a similar manner to the FM setup Select Other Controls and adjust the FM gains Setting both FM Integral gain and FM Proportional gain to 15 0 is a good starting point As with the topography gains the scan can be optimized by increasing the gains to maximize feedback response but not so high that oscillation sets in The gains often need to be much lower for potential measurements than for standard FM measurements More infor mation on tuning the feedback loop is given in the Troubleshooting section below 13 Optimize the Lift heights Set the Lift scan height at the smallest value possible that does not make the Potential feedback loop unstable or cause the tip to crash into the sample surface When the tip crashes into the surface during the Potential measurement dark or light streaks appear in the Potential image In this case increase the Lift scan height until these streaks are minimized 14 For large sample voltages or qualitative work select Data type Phase instead of Potential When the Extender box has been configured for surface potential measurements the phase signal is actually the cantilever amplitude signal as measur
18. ed by a lock in amplifier If the feedback loop is not enabled by selecting the Data type Potential the lock in cantilever amplitude depends on the voltage dif ference between the tip and sample in a roughly linear fashion The lock in ampli fier produces a voltage that is proportional to the cantilever amplitude Qualitative 230 32 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series surface potential images can be collected using this lock in signal Also if the sam ple has a surface potential that exceeds 10 V greater than the range of the Poten tial signal it is possible to use the lock in signal to provide qualitative images that reflect the sample surface potential To view the lock in signal with the reconfig ured Extender box select the Data type Phase 230 7 1 Troubleshooting the Surface Potential Feedback Loop The surface potential signal feedback loop can be unstable This instability can cause the potential signal to oscillate or become stuck at either 10 V or 10 V Here are some tips to see if the feedback loop is working properly with no oscillation e Go into Scope Mode and look at the Potential signal If oscillation noise is evi dent in the signal reduce the FM gains If oscillations persist even at very low FM gains try increasing the Lift scan height and or reducing the Drive ampli tude until oscillation stops If the tip crashes into the surface the Lock in signa
19. ent Scope Data Interleave scan ONAN Nean Topographic Scope Data Main scan Electric Fields Cantilever measures surface topography on first main scan Cantilever ascends to lift scan height Cantilever follows stored surface topography at the lift height ak sample while responding to electric influences on second interleav scan Figure 230 1 EFM LiftMode Principles Figure 230 2 Extender Electronics Module required for frequency phase detection MFM and EFM Support Notes 230 3 Electric Force Microscopy Dimension Series Support Note No 230 230 1 1 Electric Field Gradient Imaging Overview Electric field gradient imaging is a technique which measures variations in the electric field gradient above a sample The sample may be conducting nonconduct ing or mixed Since the electric field gradient is also shaped by the surface topogra phy e g sharp points on the surface concentrate the field gradient large differences in topography can make it difficult to distinguish electric field varia tions In general the best samples for electric field gradient imaging are samples that have applied voltages of roughly 1 volt or more samples with fairly smooth topography or samples with trapped charge Samples with insulating layers passi vation on top of conducting regions may also be good candidates for electric field gradient imaging 230 1 2 Surface Potential Imaging Overview Surface potential i
20. es Optical interference appears as evenly spaced sometimes wavy lines with about 1 2um spacing superimposed on the lift image This occurs when ambi ent laser light i e light passing around or through the cantilever then reflect ing off the sample interferes with laser light reflecting from the cantilever Interference can be alleviated by moving the beam spot up the cantilever away from the tip about one third of the cantilever length from the tip usually works well On the Dimension head the adjustment can be refined by carefully mov ing the beam spot laterally on the cantilever while scanning until interference fringes are minimized NOTE Optical interference is essentially eliminated by using Phase Detection or Frequency Modulation available only with the Extender Electronics Module 230 24 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series 230 5 Surface Potential Detection Theory NOTE Surface potential detection EFM is only possible using the Extender Elec tronics Module This section does not apply to microscopes which are not equipped with the Extender Electronics Module The Extender Electronics Module allows measurement of local sample surface potential This is similar to techniques called Scanning Maxwell Stress Microscopy and Kelvin Probe Microscopy Surface potential detection is a two pass system where the surface topography is obtained in the first pass and the surface
21. face It is often useful to look at the phase data in Scope Mode while adjusting the tip or sample voltage up and down Contrast due to electrical force gradients should increase or decrease as the tip sample voltage is changed e For more quantitative results switch the to the frequency Data Type for Chan nel 1 This technique provides a direct measure of the change in resonant fre quency felt by the cantilever It may be necessary to optimize the FM frequency modulation gain to properly track the shifts in resonant frequency This is described in detail in Chapter 13 of the appropriate product instruction manual 230 22 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series 230 4 2 Amplitude Detection Amplitude Detection With Extender Electronics Module MFM EFM To set up for Amplitude Detection field gradient imaging on systems with the Extender module installed follow the instructions in Section 230 4 1 Phase Detec tion with the exception that the Channel 1 Data Type should be set to Amplitude Without Extender Electronics Module Note This imaging method although described here is not recommended without the Extender Electronics module due to the presence of artifacts Amplitude Detection unlike Phase Detection is available with or without the optional Extender Electronics Module This section describes the differences in software set up and imaging for EFM systems without t
22. he Extender module When EFM imaging without the Extender module changes in the cantilever amplitude provide an indirect measure of shifts in the cantilever resonance frequency as shown in Figure 230 19 Amplitude Drive Frequency Figure 230 19 Shift In Amplitude at Fixed Drive Frequency Extender Electronics Module not installed e Set the Drive frequency to the left side of the cantilever resonance curve as shown in Figure 230 20 Support Notes 230 23 Electric Force Microscopy Dimension Series Support Note No 230 Orfeat Doce In Zoon Gut Setpoint Executa Clear Find Peak Jero Phoce Frequency Sweep Cantilever espornse 0 25 um div Setpoint Contor Frequency 2 35 KHI 0 05 Hae daly Figure 230 20 Amplitude Detection Cantilever Tune Extender Electronics Module not Installed e For maximum sensitivity set the Drive frequency to the steepest part of the resonance curve As the tip oscillates above the sample a gradient in the mag netic force will shift the resonance frequency F see Figure 230 19 Tracking the variations in oscillation amplitude while in LiftMode yields an image of the electric force gradients Either side of the resonance may be used though we have obtained slightly better results on the low side as shown in Figure 230 19 e When using Amplitude Detection optical interference may sometimes appear in the lift magnetic force image when imaging highly reflective sampl
23. ins on the header using soldered push on connectors Do not solder leads directly to the header pins as the heat may cause damage and or make jumpering the pins difficult Support Notes 230 17 Electric Force Microscopy Dimension Series Support Note No 230 H Field Gradient Imaging External Voltage Source Applied to the Sample In some cases it may be advantageous to utilize voltages greater than 12 VDC or to utilize a pulsed power supply If an external source of voltage is to be applied to the sample configure jumpers as shown in Figure 230 15 External Voltage Source T Chuck Ground Bias gt 10 MQ Chuck g Analog 2 or m ei Gnd OSC DC signal Ip Unused m Unused a STM signal J Auxiliary 2 Output m Ground Ground eS Indicates jumpers Figure 230 15 Jumper Configuration for Applying External Voltage to Sample For Systems With the Extender Electronics Module Ground AFM Ti a A current limiting resistor e g 10 100 MQ should be placed in series with the external voltage supply as shown to protect the tip and sample from damage Cur rent limited power supplies may also be used Voltage leads should be connected to pins on the header using soldered push on connectors Do not solder leads directly to the header pins as the heat may cause damage and or make jumpering the pins difficult 230 18 Support Notes Support Note No 230 Electric Force Microscopy
24. l becomes unstable and can cause the feedback loop to malfunction Increasing the Lift height and reducing the Drive amplitude can prevent this problem Once oscillation stops the FM gains may be increased for improved perfor mance e In Scope Mode if the Potential signal is perfectly flat and shows no noise even with a small Z range the feedback loop is probably stuck at 10 V You can verify this by changing the value of Realtime planefit to None in the Channel 1 panel Reduce the Scan rate and watch the display monitor which indicates the cantilever amplitude On the topographic trace the voltage displayed should be the setpoint selected for the Main scan On the Potential trace this voltage drops close to zero if the cantilever oscillation is being successfully reduced If the value on the display monitor instead goes to a large nonzero value the feedback loop is probably not working properly In this case try reducing the Drive amplitude and increasing the Lift scan height It may also be helpful to momentarily turn the Interleave mode to Disabled then back to Enabled Also try reducing any external voltage that is being applied to the sample to stabilize the feedback loop then turn the voltage back up Support Notes 230 33
25. maging measures the effective surface voltage of the sample by adjusting the voltage on the tip so that it feels a minimum electric force from the sample In this state the voltage on the tip and sample is the same Samples for surface potential measurements should have an equivalent surface voltage of less than 10 volts and operation is easiest for voltage ranges of 5 volts The noise level of this technique can be as low as a few mV Samples may consist of conduct ing and nonconducting regions but the conducting regions should not be passi vated Samples with regions of different materials will also show contrast due to contact potential differences Semi quantitative voltage measurements can be made on samples if the system is carefully calibrated on a sample at a known voltage This method requires the Extender Electronics Module and version 3 1 or later of the NanoScope III software For more information regarding installation of the Extender Electronics Module see sections at the end of this support note 230 2 Electric Field Gradient Detection Theory Electric field gradient imaging is analogous to standard MFM except that gradients being sensed are due to electrostatic forces In this method the cantilever is vibrated by a small piezoelectric element near its resonant frequency The cantile ver s resonant frequency changes in response to any additional force gradient Attractive forces make the cantilever effectively softer
26. mek Support Notes 230 9 Electric Force Microscopy Dimension Series Support Note No 230 A Field Gradient Imaging Voltage Applied to the Tip The jumper configuration in Figure 230 7 connects the Analog 2 signal from the NanoScope II controller 12 VDC range to the tip Enabling the Analog 2 Voltage Line The Analog 2 voltage line is normally used by the NanoScope to control the attenuation 1x or 8x of the main feedback signal This application of EFM imag ing uses the Analog 2 signal for EFM data Therefore input attenuation must be disabled for the duration of the EFM experiments To do this click on the Micro scope Calibrate Detector option to display the Detectors Parameters window Switch the Allow in attenuation field to Disallow Return to the main Feedback Controls panel the Analog 2 field should now be enabled This signifies that volt age is now being supplied via the Analog 2 pin located on the backplane header Remember to restore Allow in attenuation upon completion of EFM imaging Sample Chuck Ground Bias Sample Chuck I m J Analog 2 Gain Select N ca Ground AFM Tip o0 0 0 Unused STM signal from Dimension head Auxiliary D to NanoScope III controller Ground Ground N Indicates jumpers Figure 230 7 Jumper Configuration for Application of Voltage to Tip For Systems Without the Extender Electronics Module 230 10 Support
27. ment method described below NOTE In most cases it is necessary to apply a voltage across the tip or sample to achieve a high quality image Various methods for applying voltages to the tip and sample are included in the sections that follow Samples with permanent electric fields may not require the application of voltage 230 6 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series 230 3 Electric Field Gradient Detection Preparation This section explains how to conduct electric field gradient imaging by applying a voltage to the tip or sample to generate electric fields If the sample being imaged has a permanent electric field which does not require the external application of voltage the steps below are not required and you can proceed to Section 230 4 Note Before attempting to reconfigure the jumpers carefully read the follow ing Jumper Configuration sections When it is necessary to apply voltage to the tip or sample minor changes must be made to the jumpers on the microscope s backplane and the toggle switches on the Extender Electronics Module if equipped Original jumper configurations and jumper changes are dependent on the microscope being used and the measurements desired Section 230 3 1 provides jumper configuration instructions for basic microscope models operating without the Extender Electronics Module See Jumper Configuration Sections A B C D Section 230 3 2 provides
28. nces or regions at substantially different potentials voltage differ ences of 1 0 or more For other samples having rough surface topography or small voltage variations this technique may be undesirable because topographic features will appear in the LiftMode image Support Notes 230 S5 Electric Force Microscopy Dimension Series Support Note No 230 Cantilever Deflection Signal Photodiode signal Conditionin F g RMS Detector Amplitude Signal Phase Signal Phase Detector Servo Controller Reference Signal Feedback loop adjusts oscillation frequency until phase lag is zero Frequency Control lines High Resolution Oscillator Oscillator Signal Extender Electronics Module Sample Figure 230 4 Diagram of Extender Electronics Module in Phase and Frequency Measurement Mode Topographic features appear in the LiftMode image because local force gradients are heavily influenced by surface structure That is sharp features on sample sur faces concentrate the local force gradient This happens on all roughness scales so the local force gradient will also vary in much the same way as does the surface topography Thus electric field LiftMode images measured by amplitude phase or frequency detection often show contrast that is very similar to the surface topogra phy Samples with rough surface topography or with smaller potential variations are more successfully imaged by the surface potential measure
29. ocated on the backplane header Remember to restore Allow in attenuation upon completion of EFM imaging Chuck Ground Bias Da Ico Sample Chuck Nf Analog 2 or A Gnd OSC DC signal Qa g EE Unused 3 6 lt Unused STM signal Auxiliary 2 Output Ground Ground a Indicates jumpers Figure 230 12 Jumper Configuration for Application of Voltage to Tip Standard Configuration of Systems With the Extender Electronics Module as Shipped from Factory mec Ae Support Notes 230 15 Electric Force Microscopy Dimension Series Support Note No 230 F Field Gradient Imaging Voltage Applied to the Sample The jumper configuration in Figure 230 13 connects the Analog 2 signal from the NanoScope III controller 12 VDC range to the sample Enabling the Analog 2 Voltage Line The Analog 2 voltage line is normally used by the NanoScope to control the attenuation 1x or 8x of the main feedback signal This application of EFM imag ing uses the Analog 2 signal for EFM data Therefore input attenuation must be disabled for the duration of the EFM experiments To do this click on the Micro scope Calibrate Detector option to display the Detectors Parameters window Switch the Allow in attenuation field to Disallow Return to the main Feedback Controls panel the Analog 2 field should now be enabled This signifies that volt age is now being supplied via the Analog 2 pin located on the back
30. onfiguration as shown in Figure 230 11 4 ChuckGround Bias Y N ca Chuck Nf Analog 2 or J Gnd OSC DC signal Qa E Unused 3 6 z Unused STM signal Auxiliary 2 Output Ground Ground lt Indicates jumpers Figure 230 11 Normal jumper configuration as shipped from factory for Dimension systems with Extender Electronics Module installed SECO 230 14 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series E Field Gradient Imaging Voltage Applied to the Tip Notice that the jumper configuration in Figure 230 12 connects the Analog 2 signal from the NanoScope III controller 12 VDC range to the tip and is exactly the same as the jumper configuration shown in Figure 230 11 the standard configura tion as shipped from the factory Enabling the Analog 2 Voltage Line The Analog 2 voltage line is normally used by the NanoScope to control the attenuation 1x or 8x of the main feedback signal This application of EFM imag ing uses the Analog 2 signal for EFM data Therefore input attenuation must be disabled for the duration of the EFM experiments To do this click on the Micro scope Calibrate Detector option to display the Detectors Parameters window Switch the Allow in attenuation field to Disallow Return to the main Feedback Controls panel the Analog 2 field should now be enabled This signifies that volt age is now being supplied via the Analog 2 pin l
31. or z in signal Potential Signal Sample High Resolution Oscillator Figure 230 21 Simplified Block Diagram of Extender Electronics Module in Surface Potential Mode The key here is that the force on the cantilever depends on the product of the ac drive voltage and the dc voltage difference between the tip and the sample And when the tip and sample are at the same dc voltage Vqc 0 the cantilever will feel no oscillating force The Extender Electronics Module uses this fact to determine the effective surface potential on the sample Vsample The Extender determines the local surface potential by adjusting the dc voltage on the tip Vtip until the oscillation amplitude becomes zero At this point the tip voltage will be the same as the unknown surface potential The voltage applied to the cantilever tip Vtip is recorded by the NanoScope III to construct a voltage map of the surface 230 6 Surface Potential Detection Preparation It is often desirable to apply a voltage to one or more areas of a sample This may be done in two ways by connecting a voltage to the sample mounting chuck or by 230 26 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series making direct contact to the sample In both cases jumper configurations on the backplane of the microscope must be changed to match the environment desired NOTE In addition to any reconfigured jumpers remember to connect
32. plane header Remember to restore Allow in attenuation upon completion of EFM imaging Chuck Ground Bias Jis Sample Chuck Analog 2 or N Gnd OSC DC signal Unused Ground D J AFM Tip N o0 Analog 2 Unused STM signal Auxiliary 2 Output Ground Ground Gs Indicates jumpers Figure 230 13 Jumper Configuration for Application of Voltage to Sample For Systems With the Extender Electronics Module 230 16 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series G Field Gradient Imaging External Voltage Source Applied to the Tip In some cases it may be advantageous to use voltages greater than 12 VDC or to use a pulsed power supply If an external source of voltage is to be applied to the tip configure jumpers as shown in Figure 230 14 Chuck Ground Bias oly External Voltage Source Chuck Analog 2 or Gnd OSC DC signal Unused Qa 2 Ss Z i lt m Unused STM signal Auxiliary 2 Output Ground Ground SS Indicates jumpers Figure 230 14 Jumper Configuration for Applying External Voltage to Tip For Systems With the Extender Electronics Module A current limiting resistor e g 10 100 MQ should be placed in series with the external voltage supply as shown to protect the tip and sample from damage Cur rent limited power supplies may also be used Voltage leads should be connected to p
33. potential is measured on the second pass The two measurements are interleaved that is they are each measured one line at a time with both images displayed on the screen simultaneously A block diagram of the surface potential measurement system is shown in Figure 230 21 On the first pass the sample topography is measured by standard Tapping Mode In TappingMode the cantilever is physically vibrated near its resonant fre quency by a small piezoelectric element On the second pass the piezo that normally vibrates the cantilever is turned off Instead to measure the surface poten tial an oscillating voltage V ac COS Ot is applied directly to the cantilever tip This creates an oscillating electrostatic force at the frequency on the cantilever The oscillating force has the following ampli tude dC F Te VdV ac where c is the vertical derivative of the tip sample capacitance z Vac Vtip Vsample the de voltage difference between the tip and the sample and V is the amplitude of the oscillating voltage applied to the cantilever tip Support Notes 230 25 Electric Force Microscopy Dimension Series Support Note No 230 Cantilever Deflection Signal RMS Detector Lock in Amplifier Reference Signal Servo Controller Signals to NanoScope Photodiode Signal Amplitude Signal Feedback loop DC Voltage adjusts DC tip Photo 9 voltage to zero lock detect
34. r to use a pulsed power supply If an external source of voltage is to be applied to the sample configure jumpers as shown in Figure 230 10 External Voltage Source I Sample Chuck Ground Bias gt 10 MQ Sample Chuck o yN FI o o z E J Gain Select 3 O 6 z Unused STM signal from Dimension head JE Auxiliary D to NanoScope III controller Ground Ground E Indicates jumpers Figure 230 10 Jumper Configuration for Applying External Voltage to Sample For Systems Without the Extender Electronics Module oo A current limiting resistor e g 10 100 MQ should be placed in series with the external voltage supply as shown to protect the tip and sample from damage Cur rent limited power supplies may also be used Voltage leads should be connected to pins on the header using soldered push on connectors Do not solder leads directly to the header pins as the heat may cause damage and or make jumpering the pins difficult Support Notes 230 135 Electric Force Microscopy Dimension Series Support Note No 230 230 3 2 Jumper Configurations For Microscopes With the Extender Electronics Module Reminder Power down the microscope and turn off all peripherals Unplug the NanoScope III control and power cables from the system before attempting to adjust jumper configurations As shipped from the factory systems with the Extender Electronics option should have an original backplane jumper c
35. rs 225 micron long with resonant frequencies around 70 kHz usually work well It is also possible to metal coat standard TappingMode cantilevers Make sure that any deposited metal you use adheres strongly to the sili con cantilever 4 Set up the AFM as usual for TappingMode operation In the Channel panels be certain all Highpass and Lowpass filters are Off Set the Rounding parameter in the Microscope Calibrate Scanner window to zero 0 00 5 Select View Cantilever Tune 6 Follow the instructions below for the type of electric force imaging desired Phase Detection or Amplitude Detection 230 4 1 Phase Detection Phase Detection is only available when the Extender Electronics Module has been correctly configured into the system e In the Cantilever Tune window set Start frequency and End frequency to appropriate values for your cantilever e g for 225 um MFM cantilevers set Start frequency to 40 kHz and End frequency to 100 kHz Select Autotune e Two curves appear on the Cantilever Tune graph the amplitude curve in white and the phase curve in yellow In Figure 230 17 the phase curve is the dashed line and the amplitude curve is the solid line Phase Detection 230 20 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series Find Peak Zero Fhica Setpoint Exoonte Clear Frequency Sweep Orfset Zoom n Zoon Gut Camii lever Eozponse 0 35 pw diu Setpoint
36. ults 230 30 Support Notes Support Note No 230 Electric Force Microscopy Dimension Series 2 Mount a sample onto the Dimension s stage Make any external electrical con nections that are necessary for the sample 3 Mount a metal coated NanoProbe cantilever into the electric field cantilever holder MFM style cantilevers 225 micron long with resonant frequencies around 70 kHz usually work well It is also possible to deposit custom coatings on model FESP silicon TappingMode cantilevers Verify that all deposited metal adheres strongly to the silicon cantilever 4 Set up the AFM as usual for TappingMode operation 5 Use Cantilever Tune AutoTune as described in Section 230 4 1 to locate the cantilever s resonant peak Remember however that the Extender box has been reconfigured so that the phase detection circuitry now acts as a lock in amplifier Any procedures that are normally used to view or adjust the phase signal will now affect the lock in signal instead see Figure 230 21 In this case two curves should appear in the Cantilever Tune box the amplitude curve in white and the lock in curve in yellow In the event you find more than one resonance select a resonance that is sharp and clearly defined but not necessarily the largest It is also helpful to select a resonant peak where the lock in signal also changes very sharply across the peak Multiple peaks can often be eliminated by making sure the cantilever
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