Aligning the Cantilever, Laser and Photodetector

Cantilever too far forward Cantilever too far backCantilever just right! Positioning Cantilever Correctly in Nose Cone

When a cantilever is first mounted in the scanner, the laser will be close to it, but in some random location. By observing the position and motion of the laser spot projected below the microscope stand (in the 5500), the user can quickly and easily position the laser on the back of the cantilever. In the schematic diagram to the right, a top- down view of the laser adjustment knobs is at the top and the frosted screen found on the scanner is in the middle. At the bottom is the table beneath the microscope, with the red laser spot shown. The cantilever-shaped object shown there will of course not be visible but is drawn in as a reference to aid in understanding what is happening as the laser spot is moved with the adjustment knobs. Placing a white piece of paper beneath the microscope aids in seeing the laser spot. Adjustment knobs Frosted screen Table below microscope stand

Turn back adjustment knob CW until cantilever chip blocks laser and spot disappears from below table. The first step is to find the main part of the cantilever chip. Cantilever chip now blocks laser from reaching table. Click anywhere to see animation.

Do this by turning the back knob CCW until the laser spot just reappears on the table below the microscope. Next, the location of the edge of the cantilever chip needs to be found. As soon as the entire laser spot is visible, stop turning the knob! We want the laser to be right at the edge of the chip, especially when trying to align on short cantilevers. Click anywhere to see animation.

Turn front knob in either direction and watch for laser to appear and disappear quickly on the frosted screen at the same time as it disappears and re- appears on the table. When the laser spot appears on the frosted screen and disappears from the table, it is on the cantilever. The laser spot is now near the base of the cantilever. We need to find the cantilever itself. If the laser totally disappears and does not reappear, it is most likely hitting the side of the window in the nose cone. Turn the knob in the other direction until the spot reappears. Click anywhere to see animation.

Now turn the knob in the opposite direction to put the laser spot on the cantilever. When the laser is on the cantilever, the spot will disappear from the table and show up on the frosted screen The laser is now on the cantilever, but near the base, not at the tip where we want it. Click anywhere to see animation.

To find the end of the cantilever, turn the back knob CCW until the laser spot disappears from the frosted screen and reappears on the table. This means the laser is just off the end of the cantilever. Do not turn the knob any further than required to make the spot appear on the table! Click anywhere to see animation.

Finally, bring laser back on the end of cantilever by turning the back knob CW until laser just disappears from the table and reappears on the frosted screen. Again, do not turn the knob any further than necessary to make the spot appear. At this point slight tweaking may be necessary to make the spot on the frosted screen as small, symmetrical and as bright as possible. Make only small adjustments at this point! Photodetector may now be inserted and adjusted. Click anywhere to see animation.

The Laser Alignment window shows the position of the laser spot on the 4- quadrant photodiode in the detector housing and is used as an aid in aligning the photodiode with the laser position. Remember, the laser spot stays stationary while the user moves the photodiode. The Deflection signal is the output of the top two photodiode quadrants minus the output of the bottom two quadrants. The large positive value shown above means the laser is mostly on the top two quadrants and thus the photodiode must be moved up to bring the bottom quadrants closer to the laser spot. The Friction signal is the difference between left two quadrants and the right two quadrants. The negative value means the photodiode must be moved to the right. Adjusting Photodetector

Deflection signal adjustment knob. Turn to left to make signal more negative The knob on the end of the detector housing moves the photodiode up and down, changing the Deflection signal. When Deflection signal is close to zero, photodiode is centered top-to-bottom on the laser spot. Click anywhere to see animation. Note how as photodiode moves up, laser spot in Laser Alignment window moves down and the Deflection signal approaches zero. Turn knob to left to move photodiode up and make Deflection signal more negative. Turn knob to right to move photodiode down and make Deflection signal more positive. Back of photodetector not shown for clarity

Friction signal adjustment knob. Push up to make signal more positive. The knob on the side of the detector housing moves the photodiode left and right, changing the Friction signal. When Friction signal is close to zero, photodiode is centered left-to-right on the laser spot. Click anywhere to see animation. Note how as photodiode moves to the right, laser spot in Laser Alignment window moves to the right and the Friction signal approaches zero. Push knob up to move photodiode to the right and make Friction signal more positive. Pull knob down to move photodiode to the left and make Friction signal more negative. Back of photodetector not shown for clarity Laser and photodetector are now aligned and correctly adjusted!