|About this Process|
|Chemicals Used||Sulfuric Acid, Hydrogen Peroxide|
|Authored By||Katharine Beach|
The term ‘piranha’ refers to a hot solution of sulfuric acid and hydrogen peroxide (a.k.a SPM). Piranha strips photoresist and other organics by oxidizing them, and removes metals by forming complexes that stay in the solution. The solution can also hydroxylate many surfaces (adding OH groups). This will make the substrates hydrophilic.
Piranha is used at the following benches with material and size restrictions listed next to the bench:
- Acid Bench 02 CMOS Clean, 4" and 6", Silicon
- Acid Bench 12 Metals allowed, 4" and 6", Silicon and Glass
- Acid Bench 73 Pieces to 6" wafers, any material
- Acid Bench 82 Pieces
- Acid Bench 92 Pieces to 4" wafers, any material
Piranha is used frequently in microelectronics or semiconductors to clean photoresist from silicon wafers, to clean glassware, or other cleaning purposes.
The mechanism of piranha works in a few distinct steps.
- The first step is the removal of hydrogen and oxygen as water from the concentrated sulfuric acid. This occurs due to the favorable enthalpy of reaction (-880 kJ/mol). This means that the hydration of concentrated sulfuric acid is thermodynamically favourable.
H2SO4 + H2O2 --> H2SO5 + H2
- The second step in the process is the sulfuric acid conversion of hydrogen peroxide from a mild oxidizer into a more aggressive oxidizer. This can be viewed as the dehydration of hydrogen peroxide into hydronium, bisulfate, and oxygen
H2SO4 + H2O2 --> HSO4- + H3O++O
There are many hazards involved with working with Piranha:
Piranha has the following properties:
- Liquid and vapor forms corrosive
- Colour: Clear and colourless
Both chemicals (Sulfuric Acid and Hydrogen Peroxide) are highly reactive and oxidizing. The mixture is corrosive and extremely exothermic. The solution is nicknamed 'piranha' because it aggressively attacks any organic material. This makes it effective at cleaning organic contamination off wafers; but, it also means that it will eat through other organic materials, such as your flesh.
When transporting piranha from the tank to the water bath, make a series of wet textile wipes to dilute any drips from your samples to the bench. Piranha will oxidize even the bench top.
Piranha is also a serious explosion hazard. Since it evolves oxygen gas as it reacts, it should never be placed within a sealed container. Pressure can quickly build up in a sealed container and cause the container to explode. In addition, if it is given too much organic material to react with (e.g., if piranha is accidentally mixed with a solvent, or if used to strip a significant quantity of photoresist off a wafer rather than used merely to remove trace residues), it can combust or explode.
Piranha will etch most metals, and in doing so, will evolve flammable hydrogen gas; this, combined with the oxygen and heat produced by the reaction, all contribute to the possibility of fire or explosion. Therefore, it should not be used with metal tweezers or to etch metals. In general, piranha should not be used to clean wafers with metal on them (unless the metal is one which is not attacked by piranha). The hydrogen peroxide used should have a concentration in water no greater than 30% (The hydrogen peroxide stocked in the LNF is 30%); if more highly concentrated hydrogen peroxide is used (>50%), explosion can occur. For additional references on the hazards and safe handling of piranha, see the Safety Data Sheets (SDS) or the document provided by the Department of Environment, Health, & Safety (EHS).
Piranha is made up of the following chemicals:
Sulfuric Acid (H2SO4): CAS # 7664-93-9
Hydrogen Peroxide (H2O2): CAS # 7722-84-1
Note that piranha will melt through plastics. Always use glass beakers (pyrex preferred) when working with piranha solution.
Standard Operating Procedure
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When the two chemicals are mixed a very exothermic reaction will occur. It will heat to 120-150°C almost instantly. This is hot enough to soften many plastics, including the deck of the benches.
Place your beaker on a hot pad or hot plate. This is to protect the bench. Piranha can get very hot and can attack plastics, so failure to follow these directions can lead to melting of your container or bench top. Choose the location of your beaker very carefully because once you have mixed the Piranha chemicals, the beaker will become very hot, and it should not be moved.
Will clean organic residue off of wafer in approximately 10-20 min
Will attack any polymers on the surface