Mathies:Metal Etching

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Metal Etching

We can etch Ti and Pt by Aqua regia or Ion mill.

Aqua regia etch of Ti/Pt

The aqua regia etch solution is 3 parts HCl to 1 part HNO3.

For Ti/Pt, etch the wafer for 2-3minutes in aqua regia heated to 90 - 95°C.

1. Before etching, pattern the wafer according to standard photolithography techniques. After developing, hard-bake the wafer for 20 minutes in a 120°C oven. This will minimize undercutting and etching of the photoresist layer.

2. For this etch, use the pyrex dish in the Microlab labeled "For Aqua Regia Only"; the proportions are marked on the side of the dish.

3. CAUTION: this is a hazardous solution, so wear full safety gear (face shield, apron, and chem-resistant gloves) and BE CAREFUL.

4. Set up a hotplate in a fume hood. Prepare the etch solution in the pyrex dish, adding 3 parts HCl first, then 1 part HNO3.

5. Set the hotplate temperature to 250°C and allow the etch solution to heat for ~30min. Cover with aluminum foil so that the aqua regia does not evaporate away too quickly. Measure the temperature periodically by poking a thermometer through a small hole in the foil.

6. Once the aqua regia has reached 90 - 95°C, remove the foil and gently agitate the wafer in the solution for 4.5 minutes. You will be able to see the Ti/Pt coating dissolve away. (Etching for 2.5-3 minutes removes most of the Ti/Pt, but sometimes leaves small specks of metal on the wafer.)

7. After 4.5 minutes, remove the wafer and rinse well.

8. If there are still specks of Ti/Pt remaining, you can etch the wafer a second time for 1-2 minutes. Note that the aqua regia evaporates and cools fairly quickly during the etching process, so the solution may need to be reheated or may become less effective. You can also etch for longer periods of time, but the etch undercuts the photoresist pattern and may degrade your feature quality.

Ionmill metal etching

Please refer to the lab manual for ionmill protocol.

General Information

The ionmill is excellent for anisotropically etching metals.

This instrument has been used specifically for Ti/Pt etching, and the protocol here is tailored for etching Ti/Pt. Minor adjustments may be necessary for other metals.

  • 20 nm/200 nm of Ti/Pt will etch in ~10 min.
  • Shipley 1818 photoresist (soft baked for 20 min at 90°C and hard baked for 20 min at 120°C) will mill at 2.0 µm/h, so samples coated with the normal procedure (30 s at 2500 rpm which yields ≈ 1.8 µm thick photoresist) should not be ion milled any longer than 45 min since overetching may begin to round corners (of course over-etching may be desired in special circumstances).
  • Glass will etch at ≈ 1.5 µm/h using the following settings. The thick resist is typically used to ensure that that only the features are etched.

If nothing else, make sure that the vent is off before opening any other valves. There are no interlocks. A vent open with cryo pump is an excellent way to dump the cryo and lead to Ionmill downtime (>2 days). Also, make sure the roughing valve is closed before opening the cryo. Don’t leave the water on after use. Occasionally a leak is sprung and this also leads to cryo dumping. All of these are bad for the machine.

Ion Mill Equipment Operation

General Preparation

Check cryo pump temperature (it must be 0 develops, compensate with neutralizer. Also, keep an eye on the suppressor, sometimes it extends past 250 V (i.e., off the gauge range), so you will need to adjust it. Fluctuations in the controls tend to occur in the early stages of milling. You should keep an eye on the controls for the first 20-30 min of milling, but if you need to mill for an extended time (say a few hours) check it every 20-30 min after the initial time. Close shutter after desired time period.

System Shutdown

1. Turn down the acceleration all the way, push HV off button, and then turn off main.

2. Turn down the arc all the way.

3. Turn cathode down all the way.

4. Turn the magnet, suppressor, and neutralizer off in any order. Always turn one knob down, then turn that control off before proceeding to the next control.

5. Turn off the isolation transformer.

6. Turn the argon pressure setting to 2 × 10-5 torr.

7. Switch to valve closed mode (press control/closed).

8. Turn off the filament and then the power to the argon controller.

9. Let system cool for 5-10 min, meanwhile, turn off table rotation.

10. Turn off master power (breaker), coolant, and filament.

Removing Sample and Leaving Chamber under High Vacuum Conditions

1. Close high vacuum valve (again wait for pop to insure it is closed).

2. Vent chamber.

3. Raise hoist and turn off vent, make sure gasket (giant O-ring) is in place after hoist has risen.

4. Remove sample/plate.

5. Clean stage with razor blade and wipe with acetone or isopropanol. Also, clean off the copper plate if you used it.

6. Lower hoist.

7. TURN VENT OFF. Open roughing valve.

8. When pressure is between 100 and 200 mtorr, close roughing valve, then open high vacuum valve.

(notes prepared by Nick Toriello, 2/6/06)


or instead, discuss this protocol.