Etching Tungsten Tips

You can purchase tungsten tips for Scanning Tunneling Microscopy (STM) imaging from Bruker or make them yourself. This section describes the process of etching tungsten tips for use with the Dimension Icon.

Materials Required

• Variac auto transformer
• Optical microscope (20–100X)
• Sodium nitrite (NaNO₂)
• Distilled water
• Ethyl alcohol
• WD-40 (anti-oxidant)
• Two 50 ml beakers
• Tip holder
• Platinum wire
• Tungsten Wire, 0.010" diameter
• Miscellaneous wire/clips

Procedure

1. Mix a 5 percent (weight/volume) solution of sodium nitrite in water.
2. Pour ~40 ml of the sodium nitrite solution into a beaker.
3. Pour ~40 ml of WD-40 into a second beaker.
4. Construct an electrode out of the platinum wire and insert it into the beaker containing sodium nitrite solution.
5. Adjust the variac for 30 V, and with it Off, connect one output to the platinum electrode.
6. Cut 10 to 12 pieces of tungsten wire ~1.25 cm long.

7. Place the tungsten tips into a holder for inversion over the solution.

8. Invert the probes over the sodium nitrite solution with ~ 2 mm of the tips’ surface submerged. More than 2 mm will cause excessive foaming of the solution during etching, and less than 2 mm will result in tips that are too blunt.
9. If you are using the IC socket to etch multiple tips, connect the other output of the variac to the common wire connecting the tips. Otherwise, connect the output of the variac to the back end of the probe you are etching.
10. Turn on the variac and etch the tips. While the tips are etching, the solution will foam, and the tips will start to glow. As the tips etch towards the surface, the foaming will be reduced. Continue to etch the tip until foaming stops.
11. Re-submerge the tips ~1 mm into the solution for ~15 seconds at 30 V. Turn on the variac and re-etch the tips. There should be only slight bubbling from the tip and it should not glow.

12. Dip the probes into ethyl alcohol to clean them. If you plan to keep the probes around for more than a day, then dip them into the WD-40 after cleaning.
13. Dispose of unwanted probes in an appropriately labeled solvent-contaminated waste container.
14. Examine the tips under the optical microscope. Tips that are too long, too blunt, or split at the end will hardly ever be good tips and can be thrown out at this time. This is a subjective process; as your experience in etching grows, you will get better at throwing out bad tips.
15. For more probes, repeat the etching procedure. Replace the etching solution when a large amount of residue is present. Typically, you can etch 60 to 80 tips in a 40 ml solution.

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