Piranha solution

Piranha solution, also known as piranha etch, is a mixture of sulfuric 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).

Applications

Piranha solution is used frequently in the microelectronics industry, e.g. to clean photoresist residue from silicon wafers. It is sometimes used to clean laboratory glassware, though it is discouraged in many institutions and it should not be done routinely due to its dangers. [cite web | work = Sci.chem FAQ | title = 16. Laboratory Procedures | url = http://www.faqs.org/faqs/sci/chem-faq/part4/section-1.html | accessdate = 2008-01-11] Unlike chromic acid solutions, piranha will not contaminate glassware with heavy metal ions.

Piranha solution is used to make glass hydrophilic by hydroxylating the surface, thus increasing the number of silanol groups on the surface. [cite journal | author = K. J. Seu, A. P. Pandey, F. Haque, E. A. Proctor, A. E. Ribbe and J. S. Hovis | title = Effect of Surface Treatment on Diffusion and Domain Formation in Supported Lipid Bilayers | year = 2007 | journal = Biophysical Journal | volume = 92 | issue = 7 | pages = 2445–2450 | doi = 10.1529/biophysj.106.099721 | pmid = 17218468]

It is sometimes used to passivate glassware prior to doing sensitive chemical reactions.Fact|date=January 2008

Piranha solution may also be used to etch titanium.

Preparation and use

Many different mixture ratios are commonly used, and all are called piranha. A typical mixture is 3:1 concentrated sulfuric acid to 30% hydrogen peroxide 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.

Piranha solution may be prepared by adding the peroxide to the acid (as opposed to the usual rule of adding acid to water).cite web | publisher = Princeton University | title = Section 10: Chemical Specific Information — Piranha Solutions | work = Laboratory Safety Manual | accessdate = 2008-01-11 | url = http://web.princeton.edu/sites/ehs/labsafetymanual/cheminfo/piranha.htm] Once the mixture has stabilized, it can be further heated to sustain its reactivity. The hot (often bubbling) solution will clean organic compounds off 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.

Due to the self-decomposition of hydrogen peroxide, piranha solution should be used freshly-prepared. Piranha solution should not be stored. Waste piranha solution should be neutralized and disposed "in situ", instead of being accumulated in carboys. Oxygen given off during the self-decomposition, as well as the oxidation products of organic compounds can cause the container to rupture.

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

afety

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. 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.

Disposal

The gases from the piranha solution must be allowed to dissipate, and the solution allowed to cool.

Local regulations may vary; used piranha solution may be disposed off by allowing to cool, and flushing down the drain with copious amounts of water.

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 sulfuric acid. This occurs because hydration of concentrated sulfuric 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 sulfuric 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 sulfuric-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.

ee also

* Peroxymonosulfuric acid or Caro's acid
* Peroxydisulfuric acid

References

External links

* [http://www.enma.umd.edu/LAMP/Sop/Piranha_SOP.htm University of Maryland piranha guidelines]
* [http://web.princeton.edu/sites/ehs/labsafetymanual/cheminfo/piranha.htm Princeton guidelines]
* [http://yarchive.net/chem/piranha_solution.html Piranha solution] (including accounts of an accident)