ATOMIC FORCE MICROSCOPY

Transcription

1 B47 Physikalisches Praktikum für Fortgeschrittene Supervision: Prof. Dr. Sabine Maier ATOMIC FORCE MICROSCOPY Version: E1.4 first edit: 15/09/2015 last edit: 05/10/2018

2 2 CONTENT 1. Introduction Preparation The easyscan2 AFM Experiment Changing the Cantilever Measurements in Dynamic Mode Scan-parameter settings and tip-approach Measurement of the calibration grid: optimization of the scan parameter XY-Scanner calibration Calibration of the free vibration amplitude A mplitude-distance and resonance curves Contact mode measurements Exchanging the cantilever Measurement of an optical storage disc Measurement of Collagen Data Analysis Dynamic Mode Contact Mode References... 21

3 Experiment B47: Atomic Force Microscopy 3 1. INTRODUCTION The atomic force microscope (AFM) is a crucial analysis instrument in modern nanotechnology. Surface properties like topography, elasticity or adhesion can be determined by AFM measurements on microscopic scales. The AFM was invented in 1986 by Binning and Rohrer (IBM research lab Zürich and Stanford University). Like the scanning tunnelling microscope (STM) the AFM belongs to the family of the scanning probe methods. All members of this family have the same basic working principle: A microscopic probe scans over a surface and interacts with it. By recording a measurement parameter, which is related to the interaction between tip and surface, a local map of the surface properties is created. In the case of the AFM, the probe is a tip with a length of some micrometers and a tip radius of about 10 nm. The tip is mounted at a µm long cantilever that is used as force gauge (Fig. 1). Figure 1: Scanning electron microscope image of a rectangular AFM cantilever with the dimensions l = 445µm, w = 43µm, t = 4.5µm, h =14.75 µm (figure adapted from ref. [1]) Usually the whole system (cantilever plus tip) is produced as one part by etching it from a silicon piece. In Contact-Mode AFM the tip is approached to the surface until the (repulsive) interaction forces lead to a deflection of the cantilever, which can be detected and used as signal. To image the surface, the deflection of the cantilever is kept constant by a feedback control. In Dynamic-Mode AFM, the cantilever is oscillating and the change in the amplitude or the frequency of the oscillation is measured. The dynamic mode can be subdivided into different classes, especially into tapping-mode and non-contact AFM. In this experiment you will use contact mode and tapping mode to characterize different samples.

4 4 Preparation 2. PREPARATION To carry out this experiment successfully, an understanding of the following topics is necessary: The working principle and setup of an AFM Measuring modes of an AFM, especially the dynamic vs. static mode Properties of a damped harmonic oscillator under influence of an external force Forces between tip and sample You should be able to answer the following questions: 1. How does an AFM work? 2. What are the important measuring modes and their advantages and disadvantages? 3. How does the feedback control system (PI controller) work while imaging with an AFM? 4. Which forces exist between tip and sample? Which ones are short-ranged and which long-ranged? 5. How can the corresponding potential be approximated? How does the force look like? In which ranges of the potential are the different measurement modes settled? 6. What is the typical appearance of a force-distance-curve? What are the differences when measuring it under ambient conditions, in water, and in vacuum? 7. When do instabilities ("jump in", "jump out") take place in a force distance curve and how can they be explained? 8. In which order of magnitude are the forces measured by an AFM? 9. How does the resonance peak of a one-dimensional harmonic oscillator change under the influence of a constant external force or a constant external force gradient? The associated equation of motion is:.

5 Experiment B47: Atomic Force Microscopy 5 Literature for preparation (all literature chapters are also available in printed form from the supervisors on request): [1] B. Bhushan, Springer Handbook of Nanotechnology, Springer, Berlin 2007 As E-Book: Chapter 22: Principle of Operation, Instrumentation, and Probes Chapter 27: Dynamic modes of AFM [2] Nanosurf easyscan2 AFM Operating Instructions (see below, StudOn) [3] E. Meyer, H. Hug und R. Bennewitz, Scanning Probe Microscopy: The Lab on a tip, Springer Verlag [4] Further material for the experiment including software and manuals: On StudOn ( Online-Angebote» 4. Nat» 4.5 Physik» Physik der Kondensierten Materie» Professur für Experimentalphysik (Rastersondenmikroskopie)» Rasterkraftmikroskopie Password: B47AFM

6 6 The easyscan2 AFM 3. THE EASYSCAN2 AFM The easyscan2 AFM from Nanosurf is used in this experiment (Figure 2). It is easy to handle and manages many different measurement modes. In contrast to most other AFMs the easyscan2 uses an electromagnetic scanner instead of a piezoelectric one. The advantages of this system are a high linearity and the fact, that a high voltage supply is not necessary. (a) (b) Figure 2: (a) Overview of the easyscan2 (b) Top and bottom view of the scan head (Figures adapted from [2]). The force detection of the easyscan2 is based on a deflection sensor (see Figure 3). A laser spot is focused on the front part of the cantilever and its reflection is centered on a two-segment photodiode. The monitored signal is the difference of both photo currents. Before scanning, the system is normally adjusted to zero. Due to the bending of the cantilever during the scan, the reflection of the laser spot moves vertically on the diode and changes the difference between the photo currents. The sign of the difference is related to the direction of the bending and the value to the magnitude. (a) (b) Figure 3: (a) Schematic of an AFM with deflection sensor. (b) Assembly of the easyscan2 scan head (Figure adapted from [2]).

7 Experiment B47: Atomic Force Microscopy 7 Attention: The laser of the easyscan2 is a class 2M laser: DO NOT STARE INTO THE BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS. The easyscan2 scan head has to be placed during the entire experiment either on the scan table or the storage plate.

8 8 Experiment 4. EXPERIMENT 4.1 CHANGING THE CANTILEVER The first measurement is performed in the dynamic mode with a Budget Sensor Tap190Al-G cantilever (f R = 190 khz; k = 48 N/m). Exchange the cantilever ONLY under the supervision of the supervisor. Never touch the cantilever with bare hands. It can be easily damaged or destroyed. Always use tweezers when handling the cantilever. Always mount the Dropstop (see Figure 4a) before exchanging the cantilever. Otherwise the cantilever could drop into the scan head and cause damage. In addition the Dropstop shields the laser. The price of one cantilever is 40 Euro. How to exchange the cantilever: 1. Cantilever exchange: Remove the old cantilever (Figure 4) Switch off the scan electronic Turn the scan head upside down and mount the Dropstop (Figure 4a). Use the Push Rod to push down the cantilever fixing spring (Figure 4b). Remove the cantilever with tweezers and put it into the storage box (Figure 4c). Figure 4:: (a) Mounting the Dropstop. (b) Mounting the Push Rod. (c) Replacing the cantilever (Figure adapted from [2]).

9 Experiment B47: Atomic Force Microscopy 9 2. Inserting a new cantilever Use tweezers to take a new cantilever from the storage box. Place the cantilever carefully on the alignment chip (Figure 5). Figure 5:: Left: Alignment chip on the microscope. Right: Bottom side of the cantilever chip (Figure adapted from [2]). Carefully move the cantilever-chip into the correct position by slightly tapping the cantilever chip on its top side. The position is correct, when the cantilever chip symmetrically fits to the alignment chip (Figure 6, left). Figure 6:: Left: Correct cantilever position. The cantilever chip and alignment-chip form small triangles in two corners which should be symmetric. The light reflection continuous. Middle/right: wrong cantilever positions. Remove carefully the Push Rod. If the cantilever chip still moves, it is not inserted correctly. Remove the Dropstop. Place the scan-head on the scan table. Connect all cables with the scan-head and activate the electronics. IMPORTANT: Always activate the electronic before starting the software since otherwise a proper communication between computer and electronics cannot be assured. In the case, there is no connection between computer and electronics, the software says "Simulation" in the bottom status bar, and both electronics and computer have to be restarted.

10 10 Experiment 4.2 MEASUREMENTS IN DYNAMIC MODE Scan-parameter settings and tip-approach Use the video-option in side view. Figure 1: Video image of the cantilever. Place the calibration grid on the scan table by using tweezers (NEVER touch the surface of the sample). Align the edges parallel to the scan table. Use the following parameter settings: Figure 2: Parameter settings for dynamic mode measurements. Check the "SPM Parameters" by clicking "More " and choosing the "Imaging" tab:

11 Experiment B47: Atomic Force Microscopy 11 Approach the cantilever to the surface by using the following procedure: 1. Use the adjustment screws to approach the cantilever to a cantilever-surface distance of 1-2 mm. Make sure the scan head stays parallel to the scan table. 2. Use "Advance" to further reduce the distance between cantilever and sample. Use the shadow of the cantilever on the surface as orientation: A gap between shadow and cantilever should still be visible. 3. Now activate the auto approach function of the microscope by clicking "Approach". 4. After short time a pop up window indicates that the approach was successful: 5. Check the LEDs at the control unit: The "probe-status-led" should be green. In case of a red LED the approach failed. Then retract the cantilever, check the placing of the cantilever on the alignment-chip and try it again. Important: Touching the scan-head or scan-table while the cantilever is engaged to the sample can distort the measurement, and most likely damages cantilever and sample.

12 12 Experiment Measurement of the calibration grid: optimization of the scan parameter Plane correction: The surface of the sample (i.e. measurement plane) and the plane of the xyscanner are ideally parallel to each other. In practical application this is rarely the case. A mismatch between both planes reduces the efficiency of the z-controller, and thereby also reduces the ability to resolve small details of the sample. A slope correction is able to correct this mismatch: Slope correction procedure: Start a scan and take a measurement without slope correction. Measure the angle between the horizontal and a scanned line in x-direction during scanning. Use the angle measurement tool from the Analysis-panel and apply it to a line scan in the cross-section window. Correct the slope in x-direction by inserting the measured angle in the "Slope X" field in the "Imaging" tab of the SPM parameters (click "More...", choose the "Imaging" tab). Rotate the scan angle by 90 to repeat the same procedure for the y-direction. Tasks: Measure the calibration grid before the slope correction. Do a slope correction and measure the calibration grid again. What is the difference between the "line-fit-filter" and the slope correction? Why are there stripes when the "line-fit-filter" is activated? Change the scan parameter (time per line, free vibration amplitude, P-/Igain) in a controlled way and find optimal settings. Describe your observations for too high and too low settings of the controller gain. Save one image with optimal parameters.

13 Experiment B47: Atomic Force Microscopy XY-Scanner calibration The scanner moves in x- and y-direction by applying a voltage to the scanner. In order to connect a distance to the applied voltage, the scanner has to be calibrated by using a calibration grid with known dimensions (Hint: A parallel alignment between scan direction and grid edges simplifies the calibration of the xy-scanner. If this is not the case, Retract, change the orientation of the grid and approach again). The xy-scanner is calibrated by using the following procedure: Use the "Measure Length" tool from the "Analysis" panel to determine the size of the calibration grid in your last measurement. Calculate the calibration factors for x- and y-direction by dividing the reference size of the grid by the measured size of the grid. Open the "Scan head calibration editor" ("Settings" -> "Calibration" -> "Edit") Insert the factors for x- and y-direction in the corresponding fields by clicking "Set". Confirm by clicking "Set". Close the "Scan head calibration editor" again by clicking "OK". The scanner should be calibrated now. Measure the calibration grid again. Do not forget to save the measurement. Check the size of the measured grid by using the "Measure Length" tool. Task: Calibrate the xy-scanner using the procedure described above. Write your calibration factors down?

14 14 Experiment Calibration of the free vibration amplitude The free vibration amplitude (deflection of the free oscillating cantilever) is by default given as a voltage. In order to determine its value in nanometer, a conversion factor needs to be derived from an amplitudedistance curve. The amplitude calibration: After each successful approach the easyscan2 software uses the current z-position as reference and sets it to zero. A negative value means a z-position above the sample surface and a positive value means a z-position below the surface. Always begin with "Start values" well above the surface (i.e. negative ones) and then decrease the "Start value" stepwise. Avoid pressing too strongly on the surface. Change to the spectroscopy mode by clicking "Spectroscopy" in the lower left panel. Use the following parameters: Start value -200 nm and End value -50 nm. Check that the "Amplitude - Spec forward"-option is activated. Measure amplitude-distance-curves by increasing the "Start value" and "End value" step by step (for example in 10 nm steps, and keep the range in constant ~150nm) until you observe a change in the amplitude (i.e. a change of the slope of the amplitude-distance-curve). Always confirm each change with the enter key, otherwise the change is not applied to the measurement. If the mean value of the amplitude is smaller than 50 mv, do not further increase the "End value, but ask your supervisor for help. Save one measurement and retract the cantilever from the sample by clicking "Withdraw".

15 Experiment B47: Atomic Force Microscopy 15 To calibrate the free vibration amplitude, measure the slope (= sensitivity) of the curve with the "Measure Length" tool from the "Analysis" panel. Open the "Scan head calibration editor" (Settings -> Calibration -> Edit) and go to the "I/O Signals" tab. Calculate from the measured slope the value for the deflection, which would be expected for an amplitude of 10 V and insert it in the corresponding field. Tasks: Determine the calibration factor for the vibration amplitude in Volt per Nanometer. How large is free amplitude of 200 mv (as used in the previous measurements) in nm?

16 16 Experiment Amplitude-distance and resonance curves Acquisition of resonance curves: Approach the cantilever again to the surface. When pressing the approach button, a frequency sweep is done automatically by the software. Use the "Freq. Sweep" button in the "Acquisition" panel to display the resonance curve. Click "Capture" in order to save the resonance curve. How large is the excitation amplitude, which is needed to have a free vibration amplitude of 200 nm? Measure two amplitude-distance curves, one at a frequency above the resonance frequency and one with the frequency below the resonance frequency. You can change the excitation frequency in the "Vibration frequency search dialog" by shifting the marker in the "Amplitude-Frequency sweep" window (see Figure 9). Select the new frequency such, that the resulting amplitude is approximately the same as before. Figure 9: Resonance curve with chosen excitation frequency above (left) and below (right) the resonance frequency. The amplitude is in both cases approximately the same (blue horizontal line). Tasks: Examine the dependence of the amplitude-distance curves on the chosen excitation frequency. Measure therefore two amplitude-distance-curves, one with a frequency above the resonance frequency and one with the frequency below the resonance frequency (use the procedure explained above). Save at least one resonance curve to estimate the Q-factor in your report. On which variables does the Q- factor depend on? Which excitation amplitude of the cantilever is necessary to realize a free vibration amplitude of 200 mv?

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18 18 Experiment 4.3. CONTACT MODE MEASUREMENTS Exchanging the cantilever Exchange the cantilever in presence of the supervisor. Insert a contact mode cantilever (Cont-Al-G, f R = 25 khz, k = 0.2 N/m). Change the following parameters in the software: Figure 10: "Acquisition" und "Z-Controller" parameter for contact mode AFM Measurement of an optical storage disc We provide 3 samples of different optical storage discs (CD, DVD, BluRay). You can choose one of the three to determine which kind of optical storage disc it is. Choose a sample and put it into the microscope. Approach the cantilever carefully to the sample surface. IMPORTANT: The shadow of the cantilever might not be visible as clearly as on the calibration grid before. Tasks: Search a nice area to measure the structure of the optical disc. First perform an overview scan and apply a slope correction. Zoom stepwise into a good spot to determine the minimal bit size and the track pitch in your report (do not forget to save your measurements). Optimize the feedback parameters to get a good image. The optimal parameters vary strongly for each sample, cantilever,... Which kind of optical storage disc do you have? Conduct a force-distance curve with the same procedure as explained in chapter Save at least one successful force-distance curve and discuss it in your report. Are you able to calibrate the amplitude from mv to nanometer from this curve (analogous to chapter 4.2.4)?

19 Experiment B47: Atomic Force Microscopy Measurement of Collagen Insert the collagen sample into the easyscan2 AFM. Carefully approach the cantilever to the sample surface. You can switch the camera view in the videooption to identify a good place to approach on a collagen fibril beforehand. Tasks: Measure the collagen sample analogous to the measurement on the optical disc: Perform an overview scan and zoom stepwise to a reasonable spot until you can see the typical band pattern (see Figure 11 as an example). You may need to try different fibres to find a suitable one. Why is the band pattern of the fibres often better visible in the deflection signal than in the topography signal? Bonus: Conduct one force-distance curve measurement on the fibril and one on the silicon substrate. Figure 11: Topography and deflection signal of a Collagen sample. Check that you have really done all mentioned tasks and that you have saved all your files. For further analyzing your data at home you will need the easyscan2 software. A copy of the software is available on

20 20 Data Analysis StudOn, or ask your supervisor for a copy. Further analysis software can be downloaded without charge from the following homepages: GWYDDION: (looks similar to GIMP, easy to handle) WSxM: (more complex, more functions, usage of the latest version, not the "stable" one, is recommended) 5. DATA ANALYSIS Write a report which includes a description of the tasks from the experimental part. Use your measured AFMtopographies and curves for the documentation. In addition, answer the following questions in your report, and provide a rough determination of the measurement errors DYNAMIC MODE a) Describe what you see on the measurement of the calibration grid. What is the reason for the lines appearing while the "line-fit-filter" is activated? How can these stripes be removed? Describe your observations for too low and too high I- and P-Gain. Figure 12: Calibration grid with (left) and without (right) "line-fit-filter" (Figure adapted from [2]) b) Indicate your calibration factors for the xy-scanner (plus error). How large is the residual xy-error after the calibration? c) Indicate the calibration factor of the amplitude in mv/nm (plus error). What kind of sample do you need to get a meaningful calibration with this method? d) Provide an interpretation of the amplitude-distance curves above and below the resonance frequency: Compare their shape with the theoretical expected shape (as shown in the literature). What is the

21 Experiment B47: Atomic Force Microscopy 21 difference for an excitation below and above the resonance frequency of the cantilever? Estimate the size of the attractive interaction regime between tip and sample from the amplitude-distance curves. e) Determine the Q-factor of the resonance curve CONTACT MODE a) Discuss the shape of the force-distance curve. Explain the hysteresis between approaching and retracting the probe to/from the sample. Determine a suitable conversion factor for the force from the forcedistance curves. Calculate the maximal repulsive force and the maximal adhesion force (error analysis). b) Briefly describe the mean differences between CD, DVD, and BlueRay discs. Determine with the "Measure-Length" tool the minimal bit size and the pitch track. What kind of optical storage disc have you measured? c) Determine the characteristic periodicity of the collagen fibrils from your measurements. Use the crosssection tool to get a cross section across the collagen fibrils. d) Bonus: Force-distance curves: Which is softer, the Si substrate or the collagen fibrils? 6. REFERENCES [1] B. Bhushan, Springer Handbook of Nanotechnology (Springer, 2010). [2] Nanosurf, Nanosurf easyscan 2 AFM Operating Instructions 2011).