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Standard Operating Procedure June 6, 2012 Elmer Diaz Atomic Force Microscopy AFM Instrument Model Figure 1. AFM EasyScan 2 AFM model Description: AFM is a characterization tool that can be used by surface morphology including: Surface roughness Changes in surface height by using scratch test Changes in nanoparticles (NPs) which are on surface Operation: Set up Installing the tip Install the tip in the tip holder as well as mount the sample on the sample stage Samples must be flat within approximately 500 nm, this is the limit in which the microscope can measure Once the tip is installed and sample is mounted, approach the sample by rotating 3 screws, this will change tip to sample distance by mm at a time Be careful not to contact tip with surface, when moving few mm distance If approach the tip to the surface, the tip can break and need to re-installed a new tip You can see the tip of the AFM and the surface in the positioning window, you can see the shadow (See Fig. 2) Switch bottom, so you can see the top view of AFM tip surface In side view – approach the tip to the surface by using the advance and retract bottoms, this should move farther away from the tip when you hit the retract bottom, the shadow moves closer (see Fig. 2) The reason that the shadow moves and not the tip is because the camera is fixed relative to the tip and not fixed relative to the substrate. Even though the actual tip is moving up and down, what you see is the shadow moving up and down Click the advance bottom to activate the server model so that on the screen you see approximately a cm between the shadow of the tip & actual tip if get too close the tip will contact surface and the tip will break If too far away, when start scanning there is not enough force between tip and surface, so you need to go back and re-approach the surface When tip and shadow are very close together tip is close to the surface. 1 cm is a good distance (See Fig. 2) Hit the approach bottom, this activates the piezo that is connected to the tip & will move few hundred nms and the tip comes in close contact to the surface Figure 3 shows top view of the tip and surface Figure 2. Side view of screen capture of AFM in operating mode panel Figure 3. Top view of screen capture of AFM in operating mode panel Adjusting settings Once the AFM tip is scanning across the surface you will be able to see the results in this topology window (see Fig. 4) Figure 4. Scanning topography of AFM The topology is showing is the change in the Z height of the piezo compared to the X, Y position of the tip relative to the surface, you see noise To decrease noise, set the amplitude set point, corresponding to what type of tip you have in the AFM For example, we used the Sicona-n type (Si-N-type) The features for this type of tip are as follows: o length of cantilever is 400 µm, with 40 µm thickness 2.5 µm, tip frequency is 11-18 kHz, tip spring constant k (k = 0.1-0.6 N/m), o 0.01-0.025 (Ohm/cm) o The spring constant of AFM cantilevers can vary from 0.001 to 100 N/m, depending on the application, a typical value for contact mode is 0.1 N/m Using F = kx to determine the amount of force needed for the amplitude (A) set point, in this case the A is set up 7 nN. Set the A to 7 nN Change image scan size, if scan in smaller image the tip will move across the surface slower & increase resolution, for example to 5 microns. Change the tip speed number of seconds takes per line, 0.2 /0.5 are good speeds Change the number of points per line to 256, it takes more data points Change the X, Y slope, the Z plane of the substrate compared to the Z plane of the piezo, and thus de tip can be off by few hundreds nms, this results in surface topology that looks like has an incredible high slope, when surface is flat Set up the X and Y offset slope in degrees. It’s difficult to know what offset degrees are used Click on the offset bottoms & will automatically sets up this parameter for you Change the gains (P-Gain and I-Gain), this will cause different changes in the piezo Z height depending on how the amplitude is changing relative to the tip point, or set point If increase the P-Gain, values around 8 to 10 thousand are reasonable, this will increase resolution but also increase the noise Typically the I-Gain is a fifth of the P-Gain, this will further decrease noise Tip Voltage is another parameter that can be changed, depending on scanning mode will not make difference It can be set as high as 1 – 2 Volts, Increase image size to 20 microns, Sample has 10 x 10 features, you should be able to see something Allow the instrument to scan across, re-adjust settings if needed Figure 5 shows a final topography scan after adjusting all these settings Figure 5. Topography scans after adjusting settings