by Marthe Linnerud ([email protected])

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Etching Si with 30 wt% KOH

KOH is a simple way to etch silicon. This is the protocol that I use for etching (001) Si in KOH with LPCVD Si3N4 as the etch mask. This protocol uses 30 wt% KOH at 80C, but it can be easily adapted to other concentrations (<45wt%) or temperatures (<80C). The etch rate of (001) Si is commonly reported to be 1.1 um/min. However, it is strongly recommended to check the etch rate for your wafer and your KOH solution.

Patterning and preparing the Si3N4 etch mask

I make the Si3N4 etch mask by patterning SPR with photolithography and etching the Si3N4 iwith ICP-RIE.

Lithography

• Solvent clean: 5 min acetone clean in USB followed by 2 min IPA clean

• Dehydration bake: 150C for 5 min

• Plasma cleaning: 50/50/100, O2/Ar/Power for 2 min

  Ar is used to avoid oxidation of top surface of the Si3N4 film.

• HMDS, 15 min.

  My experience is that there is no adhesion between Si3N4 and SPR unless HMDS is applied.

• Spin coat SPR-700. 3500 rpm, 500 rpm/sec, 47 sec

• Soft bake: 95C for 1 min

• Exposure in MLA 150: 90 mJ/cm2

• Post-exposure bake: 115C for 1 min

• Development: MaD-332/S for 40 sec.

ICP-RIE Si3N4 etch

The Si3N4 is patterned by ICP-RIE to create an etch mask. With the old ICP-RIE chiller, I used this recipe. I typically overetch a little to make sure all of the Si3N4 is gone at the unmasked areas before the KOH etch.

The protocol will be updated with a new ICP-RIE recipe later

After dry-etching the Si3N4, remove the SPR by solvent and plasma cleaning.

Optional: Front-side protection using AR-PC 5040

If you are making a device and etch the silicon wafer from the back side, it is necessary to protect the front side. See protocol #46 for applying AR-PC 5040 as a protective coating for KOH etching.

KOH etch (long etches)

For long etches, it is important to keep the evaporation of water minimal to ensure a constant etch rate. Therefore, we have come up with a set-up utilizing a closed reaction flask and a condenser. An alternative set-up for shorter etches is described below.

[!CAUTION]

• ONLY choose beakers/tweezers/other equipment from the KOH shelf. The K+ ions are very mobile and may contaminate other processes. Therefore, we use designated equipment.
• The etch should be carried out in the corrosive fume hood.
• A buddy is needed for this process.

You need:

• The corrosive hot plate with a temperature probe
• KOH reaction flask (top and bottom part with seal and clamp)
• Stand
• Stage/jack
• Condenser
• Teflon wafer/chip holder
• Magnetic stirrer
• 2x Designated beakers for water bath
• Grease (Flammable solvents cabinet in chemical area)
• Plastic tweezer
• Timer
• Wipes for cleaning
• PPE: Apron, yellow gloves, face shield

The set-up in the corrosive fumehood

The KOH wafer holder

Etching

• Prepare the setup as depicted in the photo.
  • Apply grease (Found in chemical area) on the outlet of the condenser and trap with a blunt swab
  • Make sure that the condenser and top lid of the reaction flask are held tightly by separate clamps and that these are aligned.
  • Make sure the corrosive hotplate is connected to N2 gas flow.
  • Test that lid and cooler are tight by lowering the stand
• Put the wafer holder in the reaction flask without any solution in it. With a permanent marker, draw a line just above the wafer position.
• Remove the wafer holder and insert the magnetic stirrer
• Fill the reaction flask with KOH to the marking (USE PPE!)
• Raise the stage (it helps holding a hand under the stage while you raise it), and secure the clamp. Make sure you don’t squeeze your fingers!
• Heat the KOH to 80C, stirrer at 350 rpm (you need 500 mL)
• While you wait for the KOH solution to warm up, mount your wafer on the wafer holder.
• When hot (use PPE!)
  • Open the clamp
  • With one hand under the jack, lower the stage Make sure you don’t squeeze your fingers!
  • Insert the wafer holder –note the time
  • With one hand below the stage, raise the stand
  • Fasten the clamp
• Write an ongoing experiment sign
• When the etch is finished:
  • Turn off the heat
  • Open the clamp
  • Lower the stage
  • Move the wafer holder to a water bath, making sure not to spill on the display of the hot plate– mentally note the time
  • Rinse the gloves
  • With one hand below the stage, raise the stand
  • Fasten the clamp
  • Move the stage to a new water bath (5 min)
  • Use a tweezer to open the screws and remove the wafer.
  • Rinse with fresh water
  • Dry with N2 gun
• Clean up when the KOH has cooled down to 30C after approximately 40 min. The waste goes in the diluted inorganic bases container. Rinse everything in DI water, followed by IPA. Lightly dry the equipment with a wipe.
• Place everything back in the KOH cabinet. DO NOT PUT IN THE DISHWASHER. We do not want to contaminate other people’s processes.

[!WARNING] Make sure to rinse gloves after you insert and remove the holder from the reaction flask.

[!TIP] It can also be useful to measure the exact etch rate to calculate when the etch is ready. If you wish to do that, retrieve the wafer after 30 minutes and measure the etch rate with a profiler, before continuing the etch. Do not turn off the hotplate and remember to close the reaction flask while you are measuring the etch depth to avoid any water evaporation!

Example images of the sample with protective coating before (left) and after (right) etching. Here you see the protective coating on top of a chip design after approximately 3h in the KOH bath. The edges of the protective coating might delaminate slightly. Any scratches in the Si3N4 etch mask on the back side will cause etching of the Si in those regions and may introduce regions where the protective coating might start to delaminate. But overall, there will be good protection of the front side.

Some reference SEM images of what to expect

The KOH etch (shorter etches)

If you are interested in etching for a shorter periods of time (my guess is <30 min), it might be better to use a simpler set-up.

You need:

• The corrosive hot plate with a temperature probe
• Beaker for the KOH bath
• Teflon lid with a hole for the temperature probe and the dipper
• Designated beaker for water bath
• A beaker with water that is placed on top of the lid for keeping it in place
• Plastic tweezer
• Teflon dipper
• Timer
• Wipes for cleaning
• PPE: Apron, yellow gloves, face shield

Set-up:

• Mount the hotplate with N2 gas flow.
• Place the beaker with 30 wt% KOH on the hotplate with the lid, and insert the probe
• Set the heater to 80C. Wait until the temperature reaches 80C.

While you wait for the solution to heat up, you can prepare the sample for the etch.

• If both sides of the wafer are coated in Si3N4, you should clean it quickly with a simple solvent clean, and plasma clean both sides using Ar/O2 50/50/100 for 2 min to improve hydrophilicity. This is to avoid that the sample starts floating in the KOH bath.
• If the back side of your sample is coated with a protective coating (like the AR-PC 5040), you should only plasma clean the front side.

Etching:

• When the heater has reached 80C

  • Turn off the heat
  • Remove the temperature probe
  • Start the timer and insert the sample
  • Place the lid back on
  • Insert the temperature probe
  • Turn the heater on again
  • Place the beaker with water on the lid to keep it in place.

    I turn off the heat to avoid overheating the hot plate when I remove the temperature probe to insert the sample. With this approach, there will be some batch-to-batch inconsistencies, since it is more or less impossible to do it exactly the same every time. Another possibility is to program the hot plate to a constant temperature increase and insert the sample before starting the program.

• Etch the Si for the desired amount of time.

• Remove the sample and place it in the DI water bath for 3 min. Actively shower the sample in DI water for 30 sec, and place it in a new water bath for an additional 3 min.

• Dry with N2 gun

Tidying up

• Clean up when the KOH has cooled down after approximately 40 min. The waste goes in the diluted inorganic bases container. Rinse everything in DI water, followed by IPA. Lightly dry the equipment with a wipe.
• Place everything back in the KOH cabinet.

[!CAUTION] DO NOT PUT THE BEAKERS IN THE DISHWASHER. We do not want to contaminate other people’s processes.

[!WARNING] While taking the lid on and off, there will be some KOH condensation on it. Therefore, make sure to rinse the lid and gloves after you take it off.

[!NOTE] This set-up is simple and straightforward to use. However, it does not provide perfect control during longer etches due to water evaporation (increasing the concentration of the KOH in the duration of the etch).