Scanning Tunneling Microscopy (STM)
Etching Tungsten Tips
Rev. B
MultiMode SPM Instruction Manual
165/(166 Blank)
9.6.1 Procedure
1. Mix a 5% (by weight) solution of Sodium Nitrite in water.
2. Pour
≈
40ml of the Sodium Nitrite solution into a beaker.
3. Pour
≈
40ml of WD 40 into a beaker.
4. Construct an electrode out of the platinum wire and insert it into the beaker.
5. Adjust the variac for 30V, and with it Off, connect one output to the platinum electrode.
6. Cut 10 to 12 pieces of tungsten wire
≈
1.25cm long. Before etching, check to make sure that
at least one end of the wire has not split by inserting the ends into a tip holder. If an end has
split, you will not be able to insert it into the tipholder. You can etch the end that has split,
however, preserve the unspoiled end.
7. Place the tungsten tips into a holder. We like to use an IC socket (the low cost, edge-grip
square contact type, not the machine-grip round contact type), with all the pins soldered
together. The tips will then be held in place while inverting them over the solution. We also
find it helpful to solder the IC socket to the back of a proto-board (perf-board). You can then
invert the tips over a beaker with the proto-board sitting on the rim of the beaker.
8. Invert the tips over the Sodium Nitrite solution with
≈
2mm of the tips’ surface submerged.
More than 2mm will cause excessive foaming of the solution during etching, and less than
2mm will result in tips that are too blunt.
9. Connect the other output of the variac to the common of all the tips.
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 it stops.
11. Re-submerge the tips
≈
1mm into the solution for
≈
15 seconds at 30V. Turn on the variac and
re-etch the tips. There should be only slight bubbling from the tip and it should not glow.
Note:
It possible to vary the tip shape at this point by lowering the voltage on the
variac and increasing or decreasing the amount of time the tips are submerged.
Longer time gives blunter tips.
12. Dip the tips into ethyl alcohol to clean them. If you plan to keep the tips around for more than
a day, dip them into the WD 40 after cleaning.
13. Examine the tips under the optical microscope. Ones 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. Of course, this is a
subjective process. As your experience in etching grows, you will get better at throwing out
the bad ones.
14. Repeat the etching procedure. Replace the etching solution when a fairly large amount of
residue is present. Typically, you can etch 60 to 80 tips in a 40ml solution.
