SPM Training Manual

SPM Training ManualVersion 2.0July 2003Marc Taub, Brian Menzel, Gaurav Khanna, and Erica Lilleodden Laboratory for Advanced Materials, Stanford UniversityI. Introduction:Scanning Probe Microscopy (SPM) is a general term referring to surface characterization techniques that utilize probes in close proximity to the sample surface. Surface topographical characterization is perhaps the most common use of SPM techniques. These devices are capable of characterizing the topography of a sample with vertical features ranging in size from 10-10to 10-6 meters. The lateral dimensions of the scanned region may be on the order of 10-10to 10-4 meters.Scanning Tunneling Microscopy (STM), the first of the SPM techniques was invented in 1981 by G. Binnig and H. Rohrer. Sub-atomic resolution was achieved and the Nobel Prize was awarded for their work in 1986. G. Binnig, C. Quate, and C. Gerber introduced atomic Force Microscopy (AFM) in 1986. This technique allowed for the surface characterization of non-conductive samples. Various SPM techniques are being developed; among these are Lateral Force Microscopy, Magnetic Force Microscopy, andElectric Force Microscopy. Future developments may include the use of probes that can simultaneously perform chemical spectroscopic analysis.While Stanford’s Center for Materials Research possesses a MultiMode SPM, capable of accepting heads allowing a wide range of characterization techniques, this manual will focus on the AFM capabilities of this instrument.II. Basics of AFM Operation:The Atomic Force Microscope is able to measure small variations in surface topography by measuring the deflection of a tip on the end of a cantilever beam, as the sample surface is rastered below the tip. In these microscopes, a laser is focused on to the tip of a highly reflective cantilever beam such that the laser is reflected onto the surface of a position-sensitive photodetector. The sample is rastered underneath the tip via a piezoelectric scanner. A feedback loop is used to maintain either a constant deflection (contact mode) or oscillatory amplitude (tapping mode) of the cantilever. The mirror is used to increase the path length between the cantilever and the detector in order to amplify deflections of the laser beam.A. Cantilevers and Tips:Tips for use on the AFM are generally short and thin. They are short so that there is a high resonant frequency in order to reduce noise vibrations. They are thin so that a small force constant is provided, such that large deflections are obtained with small forces.Tips are pyramidal in shape, with a tip half angle of 35 degrees. The radius of curvature of the tip is between 20 and 60 nm. The force constant for these cantilever beams is approximately 0.5 N/m and the cantilever length is between 100 and 200 µm.Scanning electron microscope images of an AFM cantilever and tipB. AFM MultiMode HeadThe optical head of the MultiMode SPM contains the laser and tip holder assemblies.KnobsA – X-axis head position knobB – Y-axis head position knobC – X-axis laser position knobD – Y-axis laser position knobE – Photodetector position knobF – Tip holder set screwG –Mirror tilt leverBCA DE F GC. Basics of Contact Mode Operation:Overview:While in contact mode, the tip of the cantilever beam is maintained in contact with the surface of the sample. Essentially, constant force is maintained between the sample and the tip. These interactive forces follow Hooke’s law and can range from nN to µN. As the sample is moved below the cantilever tip, changes in surface topography cause the location of the reflected laser spot on the photoelectric scanner to deviate from the set position. The piezoelectric scanner then responds by vertically deflecting the sample in order to reflect the laser spot back on the set position. This vertical deflection of thepiezoelectric scanner is associated with a change in voltage. As such, the surface topography of the sample can be obtained by mapping out the corresponding deviations in this voltage.Instructions for Operation:Pretest Procedure• All SPM Lab users must record their name and use of equipment time in the logbookfor billing and contact records.• To power up the SPM, turn the switch on the rear of the NanoScopeIIIa controller(lower controller) to the ON position.• The power to the optical microscope monitor and light source should be turned onand off as needed and are located directly on those instruments.voltage field (∆V piezo )displacement field (topography) piezoelectricscannerStep 1: Contact Surface, V photodetector = 0 Step 2: Scanner moves laterally, tip deflection changes Step 3:Scanner moves vertically to maintain deflectionTesting Procedure•Select an appropriate piezoelectric scanner based upon the relevant feature size of the sample.Scanner Maximum Lateral Range J~ 150 µm x 150 µm E~ 20 µm x 20 µm D~ 20 µm x 20 µm A ~ 1 µm x 1 µmAll scanners have a maximum vertical range of ~ 5 µm.• Mount the scanner onto the MultiMode base. Align the hexagonal back leg of thescanner with the matching foot on the base. Plug the scanner cable into the base.• Select an appropriate tip. For contact mode, use a silicon nitride probe tip. Thisprobe appears gold in color and has two cantilever tips on either side.Remove the two tips on the side opposite of those to be used with a pair of tweezers. This is done because the tips on the opposite side are easily damaged and this ensures that future users will not be using potentially damaged tips.• Mount the tip in the tip holder by placing the tip under the spring-loaded gold ring.Note that the flat side of tip should be facing upwards, such that the beam and tip are pointing up towards you.• Insert the tip holder into the AFM head. The tip holder assembly must be flippedbefore being inserted, such that the beam and tip are now pointing downwards. Secure the tip holder assembly by finger-tightening the large setscrew located on the back of the head.• Place the head on top of the piezoelectric scanner to verify that there is adequateclearance between the tip and where the sample surface will be. Check to see if the head is relatively level. If either of these conditions is not met, adjust the height/level of the head using the three adjustable legs on the piezoelectric scanner.• Verify that the two springs that will be used to secure the head are of the appropriatesize and are securely fastened.• Remove the head. K=0.12 N/m K=0.58 N/mK=0.06 N/m K=0.32 N/m• Attach the sample to be examined onto a puck with double-sided tape. The sample should fit entirely on the 0.6-inch diameter puck.• Place the puck onto the piezoelectric scanner. The puck is held securely in place by a magnet, so that the sample can be rastered underneath the tip.• Mount the head onto the piezoelectric scanner. Hold the head firmly and secure the head with the two springs.• Bring the head down by adjusting all three legs of the piezoelectric scanner.Continue lowering the head only so long as you are very comfortable that the tip is not in contact with the surface.• Turn on the optical microscope and light source. Use the coarse adjustment of the microscope to lower the microscope towards the head. Focus on the cantilever. Over focus on the reflection of the cantilever. Then focus in between the actual tip and the reflection of the tip.• Continue lowering the head using all three legs until the actual cantilever and the reflected image come close to being in focus together. The tip is almost at the surface.• Focus on the real tip image.• Plug in the laser cable.• Turn down the illumination of the optical microscope so that the laser spot can be seen.• Switch the toggle switch on the MultiMode base to read AFM/LM.• Use the x and y laser positioning knobs on the head to focus the laser onto the tip of the cantilever.• Optimize parameters.Goal: Maximize sum (histogram band in lower LCD) >6 VReduce top LCD readout (which represents the difference in signalbetween the upper and lower portions of the photodetector) to between –2and –4 VIterate between x and y laser positions (while maintaining a reasonable position of the laser on the tip).Rotate the lever on the back of the head to move the mirror. A reasonable position for the mirror is an angle between 30 and 45 degrees from the horizontal.Adjust the photodetector position using the knob on the head.• Access the SPM software. Log into WindowsNT (press CTRL-ALT-DEL) as Username: spmuserPassword: spmlabDouble-click on the NanoScope icon.A log file will appear. Please enter your full name and click ok, this willcomplete the log in process.Click on the di icon.Click on microscope select-- select Quadrexed MultiMode and then click on Edit.♦ Click on Scanner to select appropriate scanner (A, D, E, J) then click ok.• Select initial scan parametersOther Controls panel: set AFM Mode to ContactScan Controls panel: set the initial Scan Size to 1um, X and Y Offsets to 0, and Scan Angle to 0, Scan Frequency to 2 HzFeedback Controls panel: set Integral Gain to 5.0, Proportional Gain to 5.0, and Setpoint to 0Channel 1 panel: set the Data Type to Height, Line Direction to Trace, Real Time Plane Fit to Line, Offline Plane Fit to Offset, High Pass Filter Off, Low PassFilter Off.• Engage the tip.Go to Motor/Engage (or click on the Engage icon) to engage the tip (the motor value located at the bottom of the monitor displays the number of microns the motor has moved down from its original position).Wait until the tip engages the surface.Once contact with the sample surface occurs, go to View/Scope Mode.• Adjust scan parametersGo to View/Scope Mode (or click on the Scope Mode icon).Check to see if Trace and Retrace are tracking each other well (i.e. look similar).If they are tracking, the lines should look the same, but they will not necessarily overlap each other, either horizontally or vertically.Once engaged, click on Setpoint and use the right arrow key to gradually increase the Setpoint value. Meanwhile, monitor the Z-Center position located on the right hand side of the monitor. If the line in the Z-Center moves close to the limit of the extended end, the tip has made a false engage. In such a case, withdraw the tip once, reduce the top LCD signal readout back to between –2 and –4 V, and re-Engage.Adjust the Scan Rate, Gains, and/or Setpoint to improve the tracking.Set desired scan size, scan rate, scan angle, and offsets.Once the scan parameters are optimized, scan size and other features can be adjusted for capturing images for analysis.• Image CaptureSelect Capture/Capture File Name to name the file.Select Capture/Capture (this will save your to the capture directory).Once images are captured, they may then be analyzed and prepared for exporting offline.To view your captured image, disengage the tip, click the di icon and select offline analysis. Open your image by double clicking on its file name or by using the open command located under the File menu.Remember to move your images from the capture directory to your user folder.Posttest Procedure• Select Motor/Withdraw 4-5 times to sufficiently withdraw the tip from the sample surface.• Unplug the laser cable.• Carefully remove the head restraint springs and lift the head assembly off of the piezoelectric scanner.• Remove the tip holder assembly from the head.• Remove the tip from the tip holder.• Unplug the scanner cable from the base.• Remove the piezoelectric scanner.• Return all SPM components to their appropriate locations.• Exit the NanoScope software.• Turn off power to the NanoscopeIII controller (bottom controller), optical microscope monitor, and light source.• Log-out of Windows NT• Please be sure to record in the logbook the length of time spent on the SPM.D. Basics of Tapping Mode Operation:Overview:In tapping mode, the cantilever beam is oscillated at its resonant frequency. A prescribed amount of damping is used to represent sample contact. As the sample is moved below the cantilever tip, changes in surface topography cause the amplitude of the reflected laser spot on the photoelectric scanner to deviate from its target amplitude. The piezoelectric scanner then responds by vertically deflecting the sample such that the amplitude of the reflected laser spot is maintained at the set amplitude. This vertical deflection of the piezoelectric scanner is associated with a change in voltage. As such, the surface topography of the sample can be obtained by mapping out the corresponding deviations in this voltage.Step 1: Contact surface, V photodetector = 0Step 2:Scanner moves laterally,AMPLITUDE of tip changesStep 3:Piezo moves verticallyto maintain amplitudeInstructions for Operation:Pretest Procedure• All SPM Lab users must record their name and use of equipment time in the logbook for billing and contact records.• To power up the SPM, turn the switch on the rear of the NanoScopeIII controller (lower controller) to the ON position.• The power to the optical microscope monitor and light source should be turned on and off as needed and are located directly on those instruments.Testing Procedure• Select an appropriate piezoelectric scanner based upon the relevant feature size of the sample.Scanner Maximum Lateral RangeJ ~ 150 µm x 150 µmE ~ 20 µm x 20 µmD ~ 20 µm x 20 µmA ~ 1 µm x 1 µmAll scanners have a maximum vertical range of ~ 5 µm.• Mount the scanner onto the MultiMode base. Align the hexagonal back leg of the scanner with the matching foot on the base. Plug the scanner cable into the base.• Select an appropriate tip. For tapping mode, use an etched single crystal silicon probe tip. This probe appears silver in color.• Mount the tip in the tip holder by placing the tip under the spring-loaded gold ring.Note that the flat side of tip should be facing upwards, such that the beam and tip are pointing up towards you.• Insert the tip holder into the AFM head. The tip holder assembly must be flipped before being inserted, such that the beam and tip are now pointing downwards.Secure the tip holder assembly by finger-tightening the large setscrew located on the back of the head.• Place the head on top of the piezoelectric scanner to verify that there is adequate clearance between the tip and where the sample surface will be. Check to see if the head is relatively level. If either of these conditions is not met, adjust the height/level of the head using the three adjustable legs on the piezoelectric scanner. Verify that the two springs that will be used to secure the head are of the appropriate size and are securely fastened.• Remove the head.• Attach the sample to be examined onto a puck with double-sided tape. The sample should fit entirely on the 0.6-inch diameter puck.• Place the puck onto the piezoelectric scanner. The puck is held securely in place by a magnet, so that the sample can be rastered underneath the tip.• Mount the head onto the piezoelectric scanner. Hold the head firmly and secure the head with the two springs.• Bring the head down by adjusting all three legs of the piezoelectric scanner.Continue lowering the head only so long as you are very comfortable that the tip is not in contact with the surface.• Turn on the optical microscope and light source. Use the coarse adjustment of the microscope to lower the microscope towards the head. Focus on the cantilever. Over focus on the reflection of the cantilever. Then focus in between the actual tip and the reflection of the tip.• Continue lowering the head using all three legs until the actual cantilever and the reflected image come close to being in focus together. The tip is almost at the surface.• Focus on the real tip image.• Plug in the laser cable.• Turn down the illumination of the optical microscope so that the laser spot can be seen.• Switch the toggle switch on the MultiMode base to read TMAFM.• Use the x, y laser positioning knobs on the head to focus the laser onto the tip of the cantilever.• Optimize parameters.Goal: Maximize sum (histogram band in lower LCD) >4 VReduce difference signal (lower LCD) to 0.0 VIterate between x and y laser positions (while maintaining a reasonable position of the laser).Rotate the lever on the back of the head to move the mirror. A reasonable position for the mirror is an angle between 30 and 45 degrees from the horizontal.Adjust the photodetector position using the knob on the head.• Access the SPM software. Log into WindowsNT (press CTRL-ALT-DEL) as Username: spmuserPassword: spmlabDouble-click on the NanoScope icon.A log file will appear. Please enter your full name and click ok, this willcomplete the log in process.Click on the di icon.Click on microscope select-- select Quadrexed MultiMode and then click on Edit.♦ Click on Scanner to select appropriate scanner (A, D, E, J).• Select initial scan parametersOther Controls panel: set AFM Mode to TappingScan Controls panel: set the initial Scan Size to 1um, X and Y Offsets to 0, Scan Angle to 0, and Scan Frequency to 2 Hz (Note: Setpoint will be automatically setin later procedure.)Feedback Controls panel: set Integral Gain to 0.5, Proportional Gain to 0.5Channel 1 panel: set the Data Type to Height, Line Direction to Trace, Real Time Plane Fit to Line, Offline Plane Fit to Offset, High Pass Filter Off, Low PassFilter Off.• Tune the cantileverGo to View/Cantilever Tune (or click on the Cantilever Tune icon).For Auto Tune Controls, make sure the Start Frequency is at 100kHz and the End Frequency is at 500kHz.Target Amplitude for smooth surfaces should be 2-3 V.Target Amplitude for rough surfaces should be 0.5-1 V.Click on Auto Tune (a "Tuning" sign should appear then disappear once Auto Tune is done).A single peak will appear with a resonance frequency between 200-450 kHz (Ifthis does not occur, change the tip).Verify that the Drive Amplitude is between 50 mV and 200 mV.Verify that the Setpoint is between 0.5 and 2.0 VWhen done, quit the Cantilever Tune menu.• Engage the tip.Go to Motor/Engage (or click on the Engage icon) to engage the tip (the motor value located at the bottom of the monitor displays the number of microns the motor has moved down from its original position).Wait until the tip engages the surface.Once contact with the sample surface occurs, go to View/Scope Mode.• Adjust scan parametersGo to View/Scope Mode (or click on the Scope Mode icon).Check to see if Trace and Retrace are tracking each other well (i.e. look similar).If they are tracking, the lines should look the same, but they will not necessarily overlap each other, either horizontally or verticallyOnce engaged, click on Setpoint and use the right arrow key to gradually increase the Setpoint value. Meanwhile, monitor the Z-Center position located on the right hand side of the monitor. If the line in the Z-Center moves close to the limit of the extended end, the tip has made a false engage. In such a case, withdraw the tip once, reduce the lower LCD signal readout back to 0 V, and re-Engage.Adjust the Scan Rate, Gains, and/or Setpoint to improve the tracking.Set desired scan size, scan rate, scan angle, and offsets.Once the scan parameters are optimized, scan size and other features can be adjusted for capturing images for analysis.• Image CaptureSelect Capture/Capture File Name to name the file.Select Capture/Capture (this will save your to the capture directory).Once images are captured, they may then be analyzed and prepared for exporting offline.To view your captured image, disengage the tip, click the di icon and select offline analysis. Open your image by double clicking on its file name or by using the open command located under the File menu.Remember to move your images from the capture directory to your user folder. Posttest Procedure• Select Motor/Withdraw 4-5 times to sufficiently withdraw the tip from the sample surface.• Unplug the laser cable.• Carefully remove the head restraint springs and lift the head assembly off of the piezoelectric scanner.• Remove the tip holder assembly from the head.• Remove the tip from the tip holder.• Unplug the scanner cable from the base.• Remove the piezoelectric scanner.• Return all SPM components to their appropriate locations.• Exit the NanoScope software.• Turn off power to the NanoscopeIII controller (bottom controller), optical microscope monitor, and light source.• Log-out of Windows NT.• Please be sure to record in the logbook the length of time spent on the SPM.E. Comparison of Contact and Tapping Modes:Contact Mode AFM Tapping Mode AFMPrinciple:Deflection of cantilever is kept constant Principle:Oscillation amplitude of cantilever is kept constantAdvantages:Cheaper tips, simpler set-up, and faster scanning rates Advantages:Able to image soft samples, such as biological and polymeric materialsIII. Tip-Sample Interaction Effects:The probe that is used in SPM techniques may introduce tip-sample interactions that can lead to artifacts in the obtained images. Typically, sample contamination or tip rounding may serve to limit feature resolution. The geometry of the probe tip itself may also influence the obtained images. As such, it may be necessary to deconvolve tip geometry from obtained images. Software is available on the NanoScope to accomplish this task.IV. Offline Data Analysis/Modification:Please be referred to the Digital Instruments manual for a complete discussion of image analysis and modifying filters.A wide variety of analysis functions are available in the Analyze menu. These include Section , which allows for depth, height, and width measurements to be taken across a section of the sample and Roughness , which allows for roughness measurements to be performed over the entire image or a selected region of interest.Typical image modifications listed in the Modify menu include Planefit Auto (x = 2, y =2) and Flatten Auto (order = 0). Planefit is commonly used to remove tilt or bow from images. Planefit calculates a single polynomial fit for the entire image and then subtracts the polynomial fit from the image. Flatten is used to remove image artifacts due to vertical scanner drift, image bow, skips and anything else that may have resulted in vertical offsets between scan lines.V. Offline access to your files:Your data files, the instrument manuals and the data analysis software can be downloaded off the laboratory computer. To access the laboratory computer make a short cut directly to the AFM computer (if you are running Microsoft 2000 or better) by creating a new network place (\\) under the network neighborhood. (IP address ofthe computer is 171.64.107.41) This shortcut will also give you access to your user folderEffect of tip rounding on sharpfeature resolution Effect of sample contamination on feature resolutionand hence your AFM images. Alternatively you can simply type the above link in the run box under the start menu or in the address box in an explorer window.VI. Six Rules of Safety:Here is a summary of precautions to follow during your learning phase. If you follow the six rules below, the MM-SPM can come to little harm.• 1. Read the manualsIt is recommended that you read chapters 1-5 in the DI MultiMode-V.431 manual before doing any imaging work. Each of the remaining chapters is dedicated to specific types of imaging. Chapter 6 provides an introduction to contact AFM, which is a good place for beginners to start. Other chapters may be read as required. Also consult the Command Reference Manual for a complete explanation of software controls.• 2. Follow good “rules of engagement.”“Engagement” refers to the process of bringing the tip and surface together. This is harder than it sounds, and the software routine for controlling the process is complex. Some probes (especially crystal silicon TappingMode™ probes) are prone to breakage if engaged too quickly or too hard. Ensure that engagement settings never exceed the limits of safety and never attempt to engage manually using coarse adjustment screws.• 3. Never move the head while imaging.The head contains the tipholder, laser and photodiode array. An X-Y translation stage is provided for moving the head and tip several millimeters across the sample for coarse adjustment. Even for relatively smooth samples, the head should NEVER be moved with the tip engaged. This almost always results in tip breakage. Always disengage first before using the X-Y stage to move the tip.• 4. Never leave your controller ON while the computer is turned OFF.Operators are advised to turn OFF their controller and computer when finished with imaging. If the controller is left ON for an extended period without an energized computer, damage to the scanner may result. (This is especially true if the scan has been heavily offset in X and Y.)• 5. Do not unplug cables to/from energized hardware. Turn OFF first. Unplugging energized hardware is not recommended and may result in damage to the MM-SPM. Always turn OFF hardware before making connections.• 6. Ask Permission.Basically do not use the instrument in a way different than covered in this manual (i.e. basic contact or tapping mode AFM) without first asking a lab manager for permission VII. Scanning techniques with the MultiMode SPM.The MultiMode is so called because it offers multiple SPM modes, including AFM, ECAFM, ECSTM, STM and TappingMode. While many early SPM s offered only one dedicated operating mode (e.g., STM), the MultiMode was the world’s first multiple-use SPM. It remains one of Digital Instrument’s most versatile instruments. A complete range of Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) techniques is available with the MultiMode SPM. Some of these techniques are available only through Digital Instruments.• Contact AFM— Measures topography by sliding the probe’s tip across the sample surface. Operates in both air and fluids. See Chapter 6.• TappingMode™ AFM— Measures topography by tapping the surface with an oscillating tip. This eliminates shear forces that can damage soft samples and reduce image resolution. TappingMode is available in air and fluids. See Chapter 8.• Phase Imaging— Provides image contrast caused by differences in surface adhesion and viscoelasticity. See Chapters 8 and 13.• Non-contact AFM— Measures topography by sensing Van der Waals attractive forces between the surface and the probe tip held above the surface. Provides lower resolution than either contact AFM or TappingMode.• Magnetic Force Microscope (MFM)— Measures magnetic force gradient distribution above the sample surface. Performed using LiftMode to track topography. See Chapter 13.• Electric Force Microscope (EFM)— Measures electric field gradient distribution above sample surfaces. Performed using LiftMode to track topography (Extender Electronics Module recommended). See Chapter 13.• Surface Potential Microscopy— Measures differences in local surface potential across the sample surface. Performed using LiftMode to track topography. See Chapter 13.• LiftMode™— A combined, two-pass technique that separately measures topography (using TappingMode) and another selected property (e.g., magnetic or electric force), using the topographical information to track the probe tip at a constant height above the surface. See Chapter 12.• Force Modulation— Measures relative elasticity/stiffness of surface features. Force modulation is only one of several types of force imaging which are possible. See Chapter 11 of the DI manual.• Lateral Force Microscopy (LFM)— Measures frictional forces between the probe tip and sample surface. See Chapter 10 of the DI manual.• Scanning Tunneling Microscopy (STM)— Measures topography of surface electronic states using a tunneling current that is dependent on the separation between the probe tip and a conductive sample surface. Tunneling spectroscopy may also be performed. See Chapter 9.• Electrochemical Microscopy (ECSTM and ECAFM)— Measures the surface structure and properties of conducting materials immersed in electrolyte solutions with or without potential control. See ECSTM/ECAFM manuals.• Lithography— Use of a probe tip to mechanically scribe or indent a sample surface. May be used to generate patterns, test surfaces for microhardness, etc. Performed using AFM and STM. See the Command Reference Manual and Support Note 225.。