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Tapping Mode Laser Alignment

The following demo is used in Alignment step 4 under Operating Procedures in Atomic Force Microscopy. Be sure to start with the Preliminary Operating Procedure.

After the Locate Tip step has been completed, a circle of light with a laser spot can be seen projected down near the chip/cantilever. You can assume that the cantilever lies along the axis through the center of the circle.

Sum Signal

The goal of alignment is to position the laser beam on the end of the cantilever in order to produce the maximum "sum signal". What does "sum signal" mean? In tapping mode, the AFM works by bouncing the tip of the cantilever off of the surface of the sample. A laser beam is positioned at the top of the cantilever. It is reflected onto a mirror and into a photodiode detector. The ΔZ measurement is recorded for up and down configurations of the tip. Those signals are processed into a sum signal. The sum signal is maximized for points nearer the tip of the cantilever (point A) than the base (point B). The maximized signal from point A provides more topographic information. The following schematic shows a SIDE VIEW of the cantilever and tip within the apparatus.

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Maximizing the Sum

The procedure is to position the laser beam onto point A. Essentially, the laser beam is used to trace the edges of the chip and cantilever in order to find the tip. The following schematic shows a TOP VIEW of the cantilever and tip in the monitor window.

  1. Move the laser off the horizontal axis by going up or down.
  2. Move to the right to find the flat (vertical) edge of the chip. This will be apparent when the laser disappears and reappears when you move the beam left again.
  3. Move the beam off the edge of the chip again, then go up (if you moved down in step 1) or down (if you moved up in step 1). The beam should disappear as it crosses the cantilever and reappear on the other side. Move the beam back onto the cantilever and position the beam directly on the cantilever. The positioning is successful if the laser appears to "split".
  4. Watch the sum signal reading on the rightmost monitor. Move the beam to the left until the sum signal reading is maximized.

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