One important component for STM is the quality of the probe tip. In order to effectively obtain atomic resolution, the probe tip must itself be on the atomic scale. Ideally, that implies that the tip should end in exactly one atom. Using electrochemical etching, we've been able to approach that ideal. Here are several Scanning Electron Microscope (SEM) images of UHV tips made by our group.
The
procedure:
For use in ultra-high vacuum (UHV), the tips are created by electro-chemically
etching tungsten wire. We start with a short piece of tungsten wire that has a
diameter of 0.020 inches. This wire is then partially submerged into a strong
base (either potassium hydroxide, KOH, or sodium hydroxide, NaOH) for
electro-chemical etching. A potential difference is then applied to the solution
(with the tungsten being an anode) for a specified amount of time. The
particular voltage is empirically determined while the duration is chosen by
observing the drop event when the submerged portion of the wire drops off.
The results:
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This tip was made by electro-chemically etching tungsten wire in KOH at 3 Volts and 18 milliAmps for 67 minutes. |
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This tip was made by electro-chemically etching tungsten wire in KOH at 4 Volts and 19 milliAmps for 45 minutes. |
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Here is a demonstration of what we refer to as our SUPER Micro-Tip. It was made using an electronic circuit to detect the drop event and turn off the bias voltage to the tip after a delay on the order of a few microseconds. |
