Piranha Etch

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About this Process
Process Details
Equipment Piranha Etch
Technology [[]]
Chemicals Used Sulfuric Acid, Hydrogen Peroxide
Date Created 5/12/15
Date Modified 4/1/18
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.

Hazards

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.

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).

Chemicals

Sulfuric Acid (H2SO4): CAS # 7664-93-9

Sulfuric Acid SDS

Hydrogen Peroxide (H2O2): CAS # 7722-84-1

Hydrogen Peroxide SDS

  • Piranha

EHS SOP

Standard Operating Procedure

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Locations

Characterization

When the two chemicals are mixed a very exothermic reaction will occur. It will heat to 120-150C almost instantly. This is hot enough to soften many plastics, including the deck of the benches.

Etch Rate

Will clean organic residue off of wafer in approximately 10-20 min

Limitations

Will attack any polymers on the surface