1. Electrochemical Etching of W and Ag Tips for STM and STM-Light Emission Studies
Conor Omand
Supervisor: Sarah Burke

2. Introduction Defined by Radius of Curvature (RoC)
Ideally 1 atom at the apex
RoC < 200nm usable (with manipulation)
Last atom wins
Not Good Enough
Good Enough

3. Mechanical Manipulation
Cutting, grinding, fracturing
Irreproducible and asymmetric with RoC not well defined
Generally look bad under characterization, but can be extremely sharp
Mechanical Manipulation

4. Electrochemical Etching
More reproducible way to make sharp, symmetric tips
Look good under characterization
RoC well defined
Electrochemical Etching

5. Tip Materials Commonly made of W or Pt-Ir
W is mechanically stable and easy to etch, but oxidizes readily in air.
Pt-Ir is inert in air, but difficult to etch
Can be made from almost any metal (Au, Ag, Ni, Cr, Fe, etc.) or from carbon nanotubes
I focus on W and Ag

6. Method Applied power triggers an electrolytic redox reaction
The metal is oxidized
into an ion and leaves into
solution
Method

7. Reactions For W: Cathode: 6H2O + 6e- -> 3H2(g) + 6OH-
Anode: W(s) + 8OH- -> WO4-2 +4H2O + 6e-
Overall: W(s) + 2OH- + 2H2O -> WO H2(g)
For Ag:
Cathode: 2H+ + 2e-  ->  H2(g)
Anode: Ag(s)  ->  Ag+ + e-
Overall: 3Ag(s) + C3H4OH(COOH)3  ->  Ag3C3H4OH(COO)3 + 3/2H2(g)

8. Step 3: Scanning Electron Microscope (~500 000X)
Tip Characterization
Step 1: Stereo-Optical Microscope (8X)
Step 2: Compound Optical Microscope (500X)
Step 3: Scanning Electron Microscope (~ X)
Step 4: STM use

9. In-Situ Manipulation Pre-Scan: Annealing Sputtering Field Evaporation
During Scan:
Voltage Pulses
Controlled Crash
Field Emission

10. Goal To find an etching recipe for each material that:
Gives reproducible results
Gives tips with RoC < 200nm
Is quick and simple
Parameters to vary:
Applied voltage
Electrolyte Concentration
Gap Size
Voltage type (AC/DC)

11. Preliminary Results Recipe for W Etching: 3M KOH
0.5mm or 0.7mm gap size
4-10V DC or 2.5-4V AC
Recipe for Ag Etching:
40% or 50% w/w Citric Acid
40-60V AC

12. Some things that can go wrong
Preliminary Results
Some things that can go wrong
Bending
Curling
Fracture
Contamination

13. More things that can go wrong
Preliminary Results
More things that can go wrong
Crystal Deposits
Hair
->
Bluntness
SEM-Induced Carbon Growth

14. Preliminary Results Success? Nanometer Scale Micron Scale Not too bad
Not too good

15. Preliminary Results Success! W, RoC ~ 150nm W, RoC < 50nm
Ag, RoC < 50nm

16. Preliminary Results 13 tips with RoC < 200nm (12 W, 1 Ag)
Reproducible etching times with W
2 tips left SEM uncharacterized for STM use

17. What's to Come? Cleaner, more controlled etching
Test recipes that look promising
Etch enough tips to find a success rate for each recipe
Get a reproducible time for Ag etching
Characterize tips by using them in STM
Experimentation documentation for those who wish to repeat the experiment

18. Thank You for Listening
Any Questions?