Recipe for STM Tip Electrochemcial Etch of Tungsten Wire in KOH

Before we begin, we apparently have to remind the users of the obvious: Clean up after you have used the setup – after use, empty the reservoir of the used KOH and flush it well with deionized (DI) water. KOH is etching, so wipe off droplets from the table/equipment after you.

• Use a freshly cut wire -do not touch the wire roll with your fingers, since the grease will contaminate it for future users (some use the w-wire in UHV equipment where a greasy finger print can ruin quite a lot, so take care of it).
• Prepare a solution of 16.8g KOH pellets in 100ml deionized (DI) water or use the prepared solution in the plastic bottle if available.
• Fill the teflon cup well up with the solution.
• Clip the needle onto the holder and set the needle on positive and the solution on negative bias.
• Dip the needle about 0.5mm into the solution – the deeper you dip, the longer time it will take…
• Etch with about 10V and 2mA stop current. You can increase voltage if you think nothing happens to break through eventual oxide layers.
• Examine the tip in the microscope and etch again if necessary - sometimes just a quick dip can do wonders.

One reservoir of about 50mL KOH solution can efficiently etch about 3 tips, then it begins to take longer time. Stir the liquid when the etch rate drops to get fresh etchants into the reaction zone.

Tips get coated in oxide in some 10h time. They can be cleaned of oxide by flushing a bit of KOH solution over them, or in case of thick oxide by doing a quick electrochemcial etch again. A thin film of liquid remains on the tip after etching – clean in DI water and drying in hot air usually helps on this.