Atomic Force Microscopy (AFM)

Introduction

Background Information:
Brief AFM Principles of Operation
Adorable AFM Flash Animation

Training

• Basic training with another LASE member first

• Certification with Marylene

• Additional certification with Marylene for tip change

AFM 1 Measurement Procedure

1. Log into AFM1

2. Turn on Camera

   1. Turn on old screen on the left side of the desk

   2. Turn on illumination source by twisting knob on desktop tower

3. Put sample on the stage and turn on vacuum

   1. Note: the stage only moves halfway under, so samples might need to be placed farther forward. Also note that the stage can be rotated

4. Open the software on the desktop called V614r1

   1. Turn on the laser

      1. Click on the real-time button that looks like a gold laser at the top left side of the window

   2. Open the following windows by double clicking on their icons (left side of screen):

      1. "Meter"

      2. "Navigate"

      3. "Scan single"

5. Align the system

   1. Click "Locate Tip"

      1. This focuses on the tip

      2. Adjust z and the focus/zoom if needed

      3. Press "OK" if the tip is sufficiently focused

   2. Move sample under the laser using the XY stage control buttons

   3. Decide what you want to focus on when bringing the tip near the sample. Pick between "Tip reflection" or "Surface". For samples that have a reflective surface, you want to focus on "Tip reflection"

   4. Adjust the z height

      1. Start with a quick speed, like 100, when lowering until some of the features come into view

      2. Adjust speed to a lower value, like 50, and lower until the tip or the surface come into better focus

      3. Lower the speed again, like 25, until closer

      4. Lower speed, like 10, for a fine adjustment

   5. Align laser using the laser alignment knobs until it is centered on the tip (might want to dim the sight source box power to make it easier to see)

   6. Align the photodetector using the the photodetector knob until the vertical deflection is below 0.1 V

6. Tune the system

   1. Click the tuning fork button

   2. Change the peak offset to 5%

   3. Click 'Auto tune'

   4. Look for a Gaussian amplitude graph and sigmonic (i.e. backward S) for the phase. If it looks bad, the tip is probably bad

7. Perform the scan

   1. Click on the "Real Time 1 - single scan" window

   2. Click the "Engage" button

   3. Set up the scan

      1. Change the scan size to the size you care about, e.g. 30 um

      2. Make sure the scan is in 'tapping' mode

      3. The scan is working when the 'trace' and 'retrace' graphs are aligned

         1. Adjust the amplitude setpoint to align the curves.

         2. Note: the force of the cantilever on the samples is inversely proportional to the amplitude setpoint. This means the curves will be better aligned for a lower amplitude (i.e. higher force) because the surface is being traced better

8. Capturing images

   1. Under the "Real Time tab", choose "capture filename"

   2. Choose saved location in your directory, give the file a useful name, and hit "ok"

   3. In Real Time 1 single scan window 'Capture' will show and there will be a 'capture: next' in the bottom left side of the software window

   4. Image will start capturing once arrow reaches top or bottom of frame. To speed this up, hit "frame up" or "frame down" button

   5. Bottom right of screen reads "Capture Next" when waiting for arrow to reach end of frame, "Capture on" during capture, and "Capture done" once complete

   6. Do not adjust the amplitude error or other scan parameters during capture! This will end capturing of your frame. Adjusting the scale bar in any scan window is ok.

9. Ending scan

   1. Change scan size to zero

   2. When stabilized (trace and retrace are flat and constant), hit "withdraw" or "disengage" button. I like to hit this twice to be safe.

   3. Raise AFM tip with z motor control up and down arrows

      1. Make sure the speed is set to slow at first, then you can change to fast mode once you are confident you are well clear of the surface

      2. Make sure you press the up arrow!!!

   4. Once tip is raised, move stage out with xy.

      1. Do not hit the hard stop at the edge of the marble

      2. Replace sample if necessary.

10. Image processing

    1. Flatten (steamroller button): smooths out image (gets rid of contrast artifacts from tip crossing large features). Usually 1st order flatten is appropriate.

    2. Roughness (ruler button): measures surface roughness of image or of area. Rq is the typically reported value.

    3. Step height (step button): measure step height of feature

    4. Copy images from "capture" folder to z drive- can be accessed from computer outside of clean room with a thumb drive.

11. When finished at the AFM

    1. Close all software windows (automatically happens if you close the main window)

    2. Turn off the light control box

    3. Turn off the tip computer monitor

    4. Sign out of the lab access system

AFM 2 Measurement Procedure

• Setup

  • Open software

  • Open workspace (if you don't have an existing workspace, copy Emily's "Tapping_2015.wks" file to your own directory). When copying someone's workspace, always make sure it's set up for tapping mode.

  • Click on navigate to activate workspace

  • Center your sample on the stage, turn on vacuum

  • Use xy arrows in software to move your sample under the laser beam, roughly in the area of interest

• Focusing on Tip

  • Click "locate tip"

  • Use front knobs on AFM to move crosshair if not centered (see picture)

  • Use up and down arrows to focus on tip (use slowest speed). Confirm focus.

  • In the "Focus on:" drop down menu choose "tip reflection". This avoids crashing the tip into the surface when you bring the tip down. If your surface is not adequately reflective, you may have to choose to focus on the surface, but this should be avoided.

  • Use the z meter to lower the tip until the tip and the tip reflection overlap. While bringing the tip down, you will pass through a position where the surface is roughly in focus. This is ok. When the tip and tip reflection shadows start to approach each other, switch to the slowest speed. Keep your eyes on both the image and the physical tip in order to prevent crashing.

  • Open the "meter" window in your workspace

  • Decrease the illumination in your image and use the top knobs on the AFM to align the laser. The laser should be centered on the crosshair.

  • Use the side knobs on the AFM to center the detector. The dot on the screen should be red and near the center of the screen. The magnitude of the vertical deflection should be < 0.1. The intensity of the signal depends on the tip, but should be optimized. If the dot is grey and moving randomly, the laser is too far out of alignment, and cannot be detected by the detector.

• Preparing for scan

  • Under "Focus on:" Choose surface

  • Move in xy to find features, if applicable. Use slowest speed.

  • Click on Tune (tuning fork icon)

  • Autotune- top graph should be symmetric bell curve, bottom graph should be backwards S shape (tuning graphs for good quality tip shown below). If tip needs to be changed, contact Emily, Hema, Stefano or Li Tao.

•  

  • Close hood (note- always do this before engaging tip with surface)

  • Check scan parameters under "scan control"

    • Samples/line: 128 lowest, 512 for good quality. More samples = longer scan

    • Microscope mode: tapping

    • XY closed loop: off unless features > 200 nm expected

    • Scan size: 0

    • Aspect ratio: 1

    • Integral gain: 0.6

    • Proportional gain: 1.2

  • Choose scan windows- I typically choose height, amplitude error, and phase

• Scan

  • Click on scan - height window

  • Click engage button. Software will check distance from surface and bring tip into contact with surface

  • Once red and blue (trace and retrace) lines are consistent, increase scan size to desired range for your scan. 5-10 um gives you a good idea of surface roughness for thin films. It's a good idea to scan several areas of the same sample.

  • Adjust amplitude as necessary under scan control to improve match between trace and retrace lines

  • Check "real times status" green status bar to make sure tip is not over or under extended. Should be < 30 in magnitude.

• Capturing images

  • Choose "capture filename"

  • Choose saved location in your directory

  • Image will start capturing once arrow reaches top or bottom of frame. To speed this up, hit "frame up" or "frame down" button

  • Bottom right of screen reads "Capture Next" when waiting for arrow to reach end of frame, "Capture on" during capture, and "Capture done" once complete

  • Do not adjust the amplitude error or other scan parameters during capture! This will end capturing of your frame. Adjusting the scale bar in any scan window is ok.

• Ending scan

  • Change scan size to zero

  • When stabilized, hit "disengage" button. I like to hit this twice to be safe.

  • Raise AFM hood so you can keep an eye on the tip while raising it

  • Raise AFM tip with up and down arrow. Make sure you press the up arrow.

  • Once tip is raised, move stage out with xy. Replace sample if necessary.

• Image processing

  • Flatten (steamroller button): smooths out image (gets rid of contrast artifacts from tip crossing large features). Usually 1st order flatten is appropriate.

  • Roughness (ruler button): measures surface roughness of image or of area. Rq is the typically reported value.

  • Step height (step button): measure step height of feature

  • Copy images from "capture" folder to z drive- can be accessed from computer outside of clean room with a thumb drive.

Operating Procedure from Tutuc's group: File:Digital Instruments AFM.pdf

Tip Change Procedure

• Use z positioner to move AFM tip a few cm above sample

• Unscrew AFM head using side screw. Screw turns opposite way from anticipated.

• Remove tip holder from AFM head, place on holding stage

• Unclip tip and replace with new tip. Make sure new tip is not touching sides of metal channel

• Replace tip holder on AFM head. Make sure dots line up- different spacings on each side

• Holding AFM head, move laser so that it's partially covered by tip. Strategy: turn horizontal displacement knob the full way clockwise, then one turn back- then turn vertical displacement knob until you can see the shadow of the tip covering the laser dot. If the dot totally disappears, it is probably hitting part of the tip holder, not the actual tip.

• Replace AFM head, tighten screw. Make sure it is firmly seated, so that it doesn't drop into place unexpectedly later

• Locate and lower tip

• Fine alignment of laser and detector

• Make sure to autotune the tip

  • Should get vertical deflection below 0.1 and the RMS amplitude between 400 and 500 mV

• Record tip change in log book.

Trouble shooting

• Issue: curve shapes don't look right during autotune

  • Possible causes: Tip probably needs to be changed. To check quality of tip, choose "Compute Q". Q value should match range specified on tip box.

  • In order to change the AFM tip, you need an additional certification from Marylene. The extra tips are in Emily's green toolbox in the PL lab. Users certified to change the tip: Emily, Hema, Tanuj, Stefano

• Issue: triangular artifacts during scan

  • Possible causes: bad tip, sample is wobbling on stage (can happen with larger samples), tip was not initially focused well

  • Disengage from surface

  • Check autotune

  • If autotune is ok, check sample position. Do not move the sample when the tip is lowered, even if not engaged with surface

  • If ok, try re-aligning laser and detector and re-engaging with surface

• Issue: Error message during startup - software can't connect to stage or z positioner

  • Try restarting software

  • Try a different workspace

  • Tools --> Select Microscope --> Should be only one option, select that one.

• Issue: Poor resolution

  • Change the tip! Order sharper tip if necessary.