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Contact Mode Imaging

Contact Mode Imaging. Force Diagram. Repulsive Cantilever pushed up. Force. Distance. Attractive Cantilever pulled down. Elements of a Basic Atomic force Microscope. Actual deflection of the probe. The feedback maintains the probe position (the force) at the same location.

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Contact Mode Imaging

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  1. Contact Mode Imaging Contact Mode Imaging

  2. Force Diagram Repulsive Cantilever pushed up Force Distance AttractiveCantilever pulled down Contact Mode Imaging

  3. Elements of a Basic Atomic force Microscope Actual deflection of the probe The feedback maintains the probe position (the force) at the same location Deflection signal Contact Mode Imaging

  4. Constant Height versus constant Force Mode Constant Height Works only with very flat samples in contact mode. Lower noise, higher resolution, faster scans Most common usage. Less resolution but gives topographic quantitative results Constant Force Contact Mode Imaging

  5. Contact Mode Left: Potential diagram showing the region of the probe while scanning in contact mode. Right: In contact mode the probe glides over the surface. Contact Mode Imaging

  6. Continuous - Contact Contact Point Potential Distance Contact Mode Imaging

  7. Set Point Adjustment • Measure the actual deflection in volts • The set point should be slightly more positive (~ 1V) • Deflection signal is the error signal (difference between Actual deflection and Setpoint) Contact Mode Imaging

  8. Optimization P-I Gains Overshoot may be observed in the line profile at the leading and trailing edge of the structure. Contact Mode Imaging

  9. Scan Speed and Resolution • A typical scan speed will be 2-5 lines/second for smooth surfaces. For rougher surfaces a lower scan speed may be needed. • A typical resolution of 256 pixels/line provides good resolution and speed. Increasing the resolution will improve image quality but will require longer imaging times. • One good option is to scan a large region at low resolution and high speed, and then to zoom in on a region of interest for a high resolution scan at lower speed. Contact Mode Imaging

  10. Applications of Contact Mode • For each pixel, the system will record and plot the error signal (the difference in volts between the surface-induced deflection and the Setpoint) as the Deflection Image (in volts). • The correction signal (the voltage that the feedback loop applies to the z-piezo to maintain the deflection at the Setpoint) is scaled by the piezo sensitivity (nm/V) and plotted as Topography (in nanometers). • As the tip passes over regions of varying friction it will twist in the scan direction as well as deflecting in the vertical axis. The detector senses change in the cantilever‘s twisting motion and outputs it as the lateral deflection (Friction) signal, which is plotted as the Friction image (in volts). • Changes in lateral force on the tip can be caused either by changes in frictional properties across the sample or by variations in topography. The Friction signal will therefore be a convolution of these two components. Comparing the friction and topography images helps to differentiate the impact of topography versus friction. • Application of contact regime is for HARDsamples (such as silicon) Contact Mode Imaging

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