Piranha solution

Piranha solution, also known as piranha etch, is a mixture of sulphuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂), used to clean organic residues off substrates. Because the mixture is a strong oxidizer, it will remove most organic matter, and it will also hydroxylate most surfaces (add OH groups), making them extremely hydrophilic (water compatible).

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Applications

Piranha solution is used frequently in the microelectronics industry, to clean, for example, photoresist from silicon wafers. It is used in scientific research to make highly hydrophilic surfaces. It is sometimes used to passivate glassware prior to doing sensitive chemical reactions.

Piranha solution is sometimes used to clean laboratory glassware, though it should not be done routinely due to its dangers. Unlike chromic acid solutions, piranha will not contaminate glassware with heavy metal ions.

Preparation and use

Many different mixture ratios are commonly used, and all are called piranha. A typical mixture is 3:1 concentrated sulphuric acid to hydrogen peroxide solution (such as a 30% hydrogen peroxide stock solution). Other protocols may use a 4:1 or even 7:1 mixture. A closely related mixture, sometimes called "base piranha" is a 3:1 mixture of ammonium hydroxide (NH₄OH) with hydrogen peroxide.

To create the piranha bath, one typically starts with a bath of sulphuric acid, to which the peroxide is carefully to be added. One must always add the peroxide to the acid, not the other way around. The mixture reaction is exothermic, hence the solution will become hot. Once the mixture has stabilized, it can be further heated to sustain its reactivity.

Piranha should be used promptly after it is prepared (after waiting for the initial mixing reaction to stabilize). The hot (often bubbling) solution will clean organics off of substrates, and oxidize/hydroxylate most metal surfaces. Cleaning usually requires about 10 to 40 minutes, after which time the substrates can be removed from the solution. Anything removed from the solution should be rinsed with a large amount of deionized water. The substrates should now be hydrophilic, which is easily verified by ensuring that the rinse water is wetting (spreading out over) the substrate.

Immersing a substrate (such as a wafer) into the solution should be done slowly to prevent thermal shock that may crack the substrate material.

Generally, it is bad practice to store piranha for long periods of time, in part because decomposition of the hydrogen peroxide converts it to a mislabeled concentrated sulphuric acid. Old solution should be disposed of promptly, with a fresh batch being made when required.

Safety

Piranha solution can be explosive. Mixing the solution is exothermic. The resultant heat can bring solution temperatures up to 120°C. One must allow the solution to cool reasonably before applying any heat. The sudden increase in temperature can also lead to violent boiling, or even splashing of the extremely acidic solution. Also, explosions may occur if the peroxide solution concentration is more than 50%. A 30% peroxide in water solution is more reasonable.

Piranha solution that is no longer being used should never be left unattended if hot. It should not be stored in a closed container. Mixing piranha with organic solvents (acetone, isopropyl alcohol, etc.) will cause an explosion.^[1] Adding anything to the piranha solution (such as a substrate that may have organic residue), must be done slowly and carefully, giving the solution time to stabilize.

Protection equipment

When handling piranha solution, one must be adequately protected. At a minimum:

• The solution should be prepared and kept at all times inside a fumehood.
• The user should wear safe lab attire:
  • a full face shield
  • rubber gloves (regular latex gloves are not sufficient)
  • labcoat
  • acid apron
  • long pants (no shorts)
  • closed-toe shoes
• Always use glass containers to handle the solution. Piranha will melt and react with plastics.

Disposal

The gases from the piranha solution must be allowed to dissipate, and the solution allowed to cool. Hydrogen peroxide is easily converted to water and oxygen, and after use is largely concentrated sulphuric acid. Nevertheless, some quantity of hydrogen peroxide will still be present, and used solution should be treated as active piranha solution.

Used piranha solution should be diluted with extreme care into distilled water; the usual rules regarding acid and water mixing will apply. A catalytic amount of manganese dioxide will help the remaining hydrogen peroxide self-decompose. After dilution the solution should be neutralised and disposed of as acid waste.

Care must be taken not to allow the solution to be mixed with organic solvents, as this will cause a violent reaction and quite possibly a substantial explosion.

Chemistry

The effectiveness of piranha solution in removing organic residues is due to two distinct processes that operate at noticeably different rates. The first and faster process is removal of hydrogen and oxygen as units of water by the concentrated sulphuric acid. This occurs because hydration of concentrated sulphuric acid is thermodynamically strongly favorable, with an ΔH of -880 kJ/mol. It is this rapid dehydrating property, rather than acidity per se, that makes both concentrated sulphuric acid, and so piranha solution, very dangerous to handle.

The dehydration process exhibits itself as the rapid carbonisation of common organic materials, especially carbohydrates, when immersed in piranha solution. Piranha solution was named in part for the vigour of this first process, since large quantities of organic residues immersed piranha solution are dehydrated so violently that the process resembles a piranha feeding frenzy. The second and more definitive rationale for the name, however, is the ability of piranha solution to "eat anything," including in particular elemental carbon in the form of soot or char.

This second and far more interesting process can be understood as the sulphuric-acid boosted conversion of hydrogen peroxide from a relatively mild oxidizing agent into one sufficiently aggressive to dissolve elemental carbon, a material that is notoriously resistant to room temperature aqueous reactions. This transformation can be viewed as the energetically favourable dehydration of hydrogen peroxide to form hydronium ions, bisulfate ions, and, transiently, atomic oxygen:

H₂SO₄ + H₂O₂ → H₃O⁺ + HSO₄^- + O

It is this extremely reactive atomic oxygen species that allows piranha solution to dissolve elemental carbon. Carbon allotropes are difficult to attack chemically because of the highly stable and typically graphite-like hybridized bonds that surface carbon atoms tend to form with each other. The most likely route by which piranha solution disrupts these stable carbon-to-carbon surface bonds is for an atomic oxygen first to attach directly to a surface carbon to form a carbonyl group:

   C                 C
    \     /           \       / 
     C = C  +  O   →   C=O  =C
    /     \           /       \ 
   C                 C

In the above process, the oxygen atom in effect "steals" an electron bonding pair from the central carbon, forming the carbonyl group and simultaneously disrupting the bonds of the target carbon atom with one or more of its neighbours. The result is a cascading effect in which a single atomic oxygen reaction initiates significant "unraveling" of the local bonding structure, which in turn allows a wide range of aqueous reactions to affect previously impervious carbon atoms. Further oxidation, for example, can convert the initial carbonyl group into carbon dioxide and create a new carbonyl group on the neighbouring carbon whose bonds were disrupted:

   C                   C-
    \       /                      /
     C=O  =C  +  2O  →   O=C=O  O=C
    /       \                      \
   C                   C-

The carbon removed by piranha solution may be either original residues or char from the dehydration step. The oxidation process is slower than the dehydration process, taking place over a period of minutes. The oxidation of carbon exhibits itself as a gradual clearing of suspended soot and carbon char left by the initial dehydration process. In time, piranha solutions in which organic materials have been immersed typically will return to complete clarity, with no visible traces of the original organic materials remaining.

A final minor contribution to the piranha solution cleaning is its high acidity, which dissolves deposits such as metal oxides and carbonates. However, since it is safer and easier to remove such deposits using milder acids, piranha solution is more typically used in situations where high acidity complicates cleaning instead of assisting it. For substrates with low tolerance for acidity, the alkaline oxidising solution known as base piranha is preferred.

References

1. ^ Discussion of piranha solution usage, safety and an accident description.