Piranha etch

Author: black-piranha-fish

Haijun Qiao, 2007.


This SOP is a MUST-KNOW for the users who potentially use the Piranha solution at the wetbench. Piranha solution is a mixture of concentrated sulphuric acid (H2SO4) and hydrogen peroxide (H2O2), and used for removing organic residues on the semiconductor wafer. This solution is VERY VERY energetic and extreme caution must be taken when preparing, handling and disposing it.

This is a dangerous wetbench process and requires qualification for dangerous wetbench processes.  The use of dangerous chemicals requires that the user may not perform the process alone.


First of all, load the buddy system in effect.
[1] Put on the protective apron, latex gloves, and face shield. Tell other users working in the cleanroom that you are going to use Piranha. Since Piranha solution attacks plastic, all containers holding Piranha should not be placed directly on the wetbench. Place them on glass.

[2] Get a clean glass beaker (not plastic) and take desired amount of concentrated H2O2 into it, then pour 5 times that volume of H2SO4 into H2O2 VERY SLOWLY. Since the mixing is an exothermic reaction, a large amount of heat will be released, gently shake the beaker or stir the solution using glass rod if it’s unrestful.

NB: the previous version of the SOP said to pour the H202 into the acid. Upon discussion with chemists, we have come to the conclusion that this is simply wrong. The usual way of pouring the acid in the water (or H2O2 here) is safer since the acid has a higher density and will therefore sink to the bottom of the beaker rather than boil (potentially explode) from the surface.


[3] When the solution becomes calm, put it on the hotplate @ 100 degree Celsius. Caution: the beaker can be very hot.
[4] Put the wafers into the solution when it starts to bubble, and leave it for 15 minutes.
[5] Move the beaker away from the hotplate and take the wafers out using Teflon (or stainless steel) tweezers, rinse them with DI water.
[6] Since the wetbench is plastic and the beaker is hot, DO NOT put your beaker directly on the bench but use a glass spacer (upside down petri dish works well).


[7] Leave the solution to cool down (must be in an OPEN glass container).
[8] Take a large glass beaker and half fill with water, pour the Piranha solution into water VERY SLOWLY.
[9] Rinse the Piranha beaker with water 3 times. SLOWLY dump the diluted Piranha solution into the sink under running water.
[10] Rinse the large beaker 3 times. Rinse your gloves throughout.

NB: if the parts you are cleaning will leave chemicals in the solution that should not be poured down the
drain (eg photoresist) then contact the facility manager to come up with a safe disposal method.


[11] Spills must be immediately advised to other users working in the cleanroom.
[12] Small amount of spill should be wiped off using wipes, rinse the wipes throughout under running water.
Rinse and wipe where the spill took place for 3 times.
[13] If a large amount of spill happens, evacuate the yellow room right away and call the cleanroom managers.

Safety & Emergency:

All ANF safety and procedural regulations must be followed.  Use of Piranha requires at least one other person in the clean room (buddy system). Piranha etch should be performed in a laminar flow bench, using apron, acid gloves and eye protection including full face shield.

[14] Never handle Piranha solution without all the protective equipments.
[15] Never mix Piranha solution with any organics (including acetone, IPA, methanol, PR, etc.). EXPLOSION!!
[16] Never use plastic container and tweezers. They melt in Piranha solution.
[17] Never store Piranha solution in closed container. EXPLOSION!!

In case of exposure to skin, rinse with lots of water for 15 minutes at least, and seek medical attention immediately. In case of contact with eyes, rinse with eyewash for 15 minutes at least, and seek medical attention immediately. The buddy should call help at 2-4444 or 911.  Report to ANF staff or report to UBC Risk Management.