Piranha Cleaning

A piranha solution is used to remove organic residues from substrates. The piranha solution is made of a 3:1 mixture of concentrated sulfuric acid (H2SO4) with hydrogen peroxide (H2O2). This solution is dangerous when hot and the reaction in the acid piranha is self-starting.

Introduction

A piranha solution is used to remove organic residues from substrates. The piranha solution is made of a 3:1 mixture of concentrated sulfuric acid (H2SO4) with hydrogen peroxide (H2O2). This solution is dangerous when hot and the reaction in the acid piranha is self-starting.

There are many things which will cause the reaction to accelerate out of control. "Out of control" can mean anything from the piranha foaming out of its bin and on the deck, to an explosion with a huge shock wave including glove and acid-gown shredding glass shards. Piranhas burn organic compounds. If you provide sufficient fuel for them (i.e. photoresist, IPA), they will generate enormous quantities of heat and gas. [1]

Chemicals involved are toxic and dangerous. NEVER do something you are not absolutely sure of. ALWAYS ask in case of doubt.

This document is NOT sufficient training: it is a reference document. Users have to go through regular wet bench training to be allowed to use piranha solution.

 

Associated Documents & References

  • Rules and safety procedures regarding chemical use.
  • MSDS: hydrogen peroxide and sulfuric acid.
  • General Wet Processing

Equipment Used

  • Non-HF wet bench located in the Ech/Dep room.
  • Hotplate (if 90 to 120°C heating is needed)
  • 2 glass beakers or 2 Pyrex trays, 2 graduated cylinders.
  • Chemicals:
    • Hydrogen Peroxide (H2O2). Location: in the blue “Bases” cabinet
    • Sulfuric Acid (H2SO4). Location: in the blue “Acid” cabinet

 If you need to be assisted by technicians check availability with them before reserving the equipment. Users have to go through regular training before using this equipment.

Verifications Prior to Processing 

Wear protective acid yellow apron, full face protection and green acid gloves.

 

Whenever handling Piranha, only use glass containers (preferably Pyrex). Containers used during the experiment must be very clearly labeled (using a microscope slide) and a warning sign, visible by any user working under the flow hood, must be posted at all times to indicate that the glassware contains Piranha mixture.

Mix the solution in the flow hood with the sash between you and the solution. Wear the full protection as described in the General Wet Processing SOP.

When preparing the piranha solution, always add the peroxide to the acid. The H2O2 is added immediately before the etching process because it immediately produces an exothermic reaction with gas (pressure) release. If the H2O2 concentration is at 50% or greater, an explosion could occur, thus necessary quantities must be accurately measured before mixing. 

Piranha solution is very energetic and potentially explosive.  It is very likely to become hot, more than 100°C.  Handle with care.

Wafer(s)/sample(s) should be rinsed and dried before placing them in a piranha bath. Piranhas are used to remove photoresist and acetone residues, not the compounds themselves. DO NOT use this solution to remove photoresist from a wafer/sample. Process your wafer(s)/sample(s) in an O2 plasma (See Photoresist Removal SOP) or acetone/IPA bath before using piranha.

Leave the hot piranha solution in an open container until cool. Approximately 1H is necessary to reach an acceptable temperature.

Never store hot piranha solutions.  Piranha stored in a closed container will likely explode. 

Adding any acids or bases to piranha or spraying it with water will accelerate the reaction.  This includes Photoresist, which is a strong base. 

Mixing hot piranha with organic compounds may cause an explosion.  This includes acetone, photoresist, isopropyl alcohol, and nylon.

Check availability of chemicals.

 

Operations description

Preparation

Follow the standard operating procedure (General Wet Processing) to prepare the 2 chemicals needed.

WARNING: usually acid has to be added at the end. Piranha is an exception: sulfuric acid has be added
first and then slowly pour the peroxide. The solution will heat up to 80°C.

Cleaning

Soak the wafers/samples in the solution for 10min. NEVER leave this solution unattended. After 10min the solution temperature usually reaches 60°C.

For stronger cleanings, a hotplate can be use to heat the solution from 100 to 120°C. It is understandable that at this temperature, the manipulation is more dangerous so more attention must be paid. 

Rinsing

Remove the wafer(s)/sample(s) from the piranha solution and soak it in the DIW bath previously filled with DIW. Start a 3-cycle DIW rinse.

Disposing and cleaning up

Let the piranha solution cool down with proper label identification. Dispose it in the wet bench drain made for this purpose following the General Wet Processing.

Rinse all labware three times in clean water and put them on the drying rack.