How is the needle/tip of an STM actually made? I was wondering how the needle/tip of an STM is made. Any links would be also great, I couldn't find any.
 A: Here is an example of etching STM tip (Cr tip specifically): https://aip.scitation.org/doi/10.1063/1.4976567
In principle, the easiest way to make a tip is that you just need to pull a metallic wire by holding its opposite ends, and then you'll get a change to have a atomic sharp tip at where it's broken. You can also cut while pull it with a scissor, the break point might be atomically sharp, too. I've done that and use such tip to see clear atoms before. People now use such method only on tip wires which cannot be etched.

More professionally, STM guys etch a metallic wire (~1mm diameter for tungsten wire for instance) with usually NaOH (HCl) solution. First of all, you cover part of the wire with rubber tube, and insert the covered part into the solution to form a circuit (see pics above). The corrosion happens after you turn on the power for the etching circuit. The corrosion happens only at where the solution and wire touches but neither in the rubber tube nor above the solution, so when the wire is about to break (now the diameter here is super tiny!), the covered part will pull itself down by its own gravity and finally break the wire into two. This pulling process by gravity will most of the time shape the breakpoint to be atomic sharp, meaning literally only several atoms on the top of the wire. And finally you get a atomically sharp STM tip.
