Piranha solution
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Piranha solution, also known as piranha etch, is a mixture of
Preparation and use
Many different mixture ratios are commonly used, and all are called piranha. A typical mixture is 3 parts of concentrated sulfuric acid and 1 part of 30 wt. % hydrogen peroxide solution;
Piranha solution must be prepared with great care. It is highly
Piranha solution should always be prepared by adding hydrogen peroxide to sulfuric acid slowly, never in reverse order.
Once the mixture has stabilized, it can be further heated to sustain its reactivity.[6] The hot (often bubbling) solution cleans organic compounds off substrates and oxidizes or hydroxylates most metal surfaces. Cleaning usually requires about 10 to 40 minutes, after which the substrates can be removed from the solution and rinsed with deionized water.
The solution may be mixed before application or directly applied to the material, applying the sulfuric acid first, followed by the peroxide. Due to the self-decomposition of hydrogen peroxide, piranha solution should always be used freshly prepared (extemporaneous preparation). The solution should not be stored, as it generates gas and therefore cannot be kept in a closed container because of the risk of overpressure and explosion.[3][7] As the solution violently reacts with many oxidizable substances commonly disposed of as chemical waste, if the solution has not yet been completely self-decomposed, or safely neutralized, it must be left in an open container under a fume hood, and clearly marked.
Applications
Piranha solution is used frequently in the microelectronics industry, e.g. to clean photoresist or organic material residue from silicon wafers. It is also widely employed in wet etching of wafers in the semiconductor fabrication process.[1]
In the laboratory, this solution is sometimes used to clean glassware, though it is discouraged in many institutions and it should not be used routinely due to its dangers.[8] Unlike chromic acid solutions, piranha does not contaminate glassware with Cr3+ ions.
Piranha solution is particularly useful when cleaning sintered (or "fritted") glass filters. A good porosity and sufficient permeability of the sintered glass filter is critical for its proper function, so it should never be cleaned with strong bases (NaOH, Na3PO4, Na2CO3, ...) which dissolve the silica of the glass sinter and clog the filter. Sintered glass also tends to trap small solid particles deep inside its porous structure, making it difficult to remove them. Where less aggressive cleaning methods fail, piranha solution can be used to return the sinter to a pristine white, free-flowing form without excessive damage to the pore dimensions. This is usually achieved by allowing the solution to percolate backward through the sintered glass. Although cleaning sintered glass with piranha solution will leave it as clean as possible without damaging the glass it is not recommend due to the risk of explosion.[citation needed][clarification needed]
Piranha solution is also used to make glass more
Mechanism
This section relies largely or entirely on a single source. (May 2021) |
The effectiveness of piranha solution in decomposing organic residues is due to two distinct processes operating at noticeably different rates. The first and faster process is the removal of hydrogen and oxygen as units of water by the concentrated sulfuric acid. This occurs because hydration of concentrated sulfuric acid is strongly thermodynamically favorable, with a standard enthalpy of reaction (ΔH) of −880 kJ/mol. It is this rapid dehydrating reaction, rather than its acidity itself, that makes concentrated sulfuric acid, and so piranha solution, dangerous to handle.
This dehydration process exhibits itself as the rapid carbonisation of common organic materials, especially carbohydrates, when they enter in contact with a piranha solution.
A second and far more interesting process is dissolution. It can be understood as the sulfuric-acid boosted conversion of hydrogen peroxide from a relatively mild
It is this extremely reactive atomic oxygen species that allows piranha solution to dissolve elemental carbon. Carbon
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:
The carbon removed by the 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. With time, piranha solutions in which organic materials have been immersed typically return to complete clarity, with no visible traces of the original organic materials remaining.
A last secondary contribution to the piranha solution cleaning is its high acidity, which dissolves deposits such as metal oxides, hydroxides, and carbonates. However, since it is safer and easier to remove such deposits using milder acids, the solution is more typically used in situations where high acidity facilitates cleaning instead of complicating it. For substrates with low tolerance for acidity, an alkaline solution consisting of ammonium hydroxide and hydrogen peroxide, known as base piranha, is preferred.
Etymology
Piranha solution is named after the piranha fish. It is so first because the vigor of the dehydration process, since large quantities of organic residues immersed in the solution are dehydrated so violently that the process resembles the fish's reputed feeding frenzy. The second and more definitive rationale for the name, however, is the dissolution ability of piranha solution, capable of "eating anything", in particular, elemental carbon in the form of soot or char.
Safety and disposal
Piranha solution is dangerous to handle, being both strongly acidic and a strong oxidizer. Solution that is no longer being used should never be left unattended if hot. It should never be stored in a closed receptacle because of the risk of gas overpressure and explosive burst with spills (especially with fragile thin wall volumetric flask). Piranha solution should never be disposed of with organic solvents (e.g. in waste solvent carboys), as this will cause a violent reaction and a substantial explosion, and any aqueous waste container containing even a weak or depleted piranha solution should be labelled appropriately to prevent this.[3]
The solution should be allowed to cool, and oxygen gas should be allowed to dissipate prior to disposal. When cleaning glassware, it is both prudent and practical to allow the piranha solution to react overnight taking care to leave the receptacles open under a ventilated fume cupboard. This allows the spent solution to degrade prior to disposal and is especially important if a large portion of peroxide was used in the preparation. While some institutions believe that used piranha solution should be collected as hazardous waste, others consider that it can be neutralized and poured down the drain with copious amounts of water.[3][11][12] Improper neutralization can cause a fast decomposition, which releases pure oxygen (increased risk of fire of flammable substances in a close space).
One procedure for
See also
- Aqua regia (HNO3 + 3 HCl)
- Fenton's reagent (H2O2 + Fe2+)
- Green death (xH2SO4 + yHCl + zFeCl3 + wCuCl2)
- Peroxydisulfuric acid, or Marshall's acid (H2S2O8)
- Peroxymonosulfuric acid, or Caro's acid (H2SO5)
- RCA clean (silicon wafer cleaning procedure)
- Chromic acid (H2CrO4)
- Superhydrophilicity
- Ultrahydrophobicity
References
- ^ a b c admin (2016-12-28). "How Piranha Etch is Used in Silicon Wafer Cleaning". Modutek. Retrieved 2022-01-02.
- ^ "Piranha". University of Pennsylvania. Archived from the original on 18 July 2010. Retrieved 4 May 2011.
- ^ a b c d "Section 10: Chemical Specific Information — Piranha Solutions". Laboratory Safety Manual. Princeton University.
- ^ "Standard Operating Procedure for Piranha Solutions" (Microsoft Word). MIT. Retrieved 12 May 2016.
- ISBN 9781616650414.
- ^ "Procedure on handling and using Acid Piranha solution" (PDF). University of Cambridge. Archived from the original (PDF) on 15 June 2015. Retrieved 12 June 2015.
- ^ Kemsley, Jyllian (16 January 2015). "Pirahana Solution Explosions". Chemical & Engineering News. The Safety Zone. American Chemical Society. Archived from the original on 4 December 2020. Retrieved 30 September 2021.
- ^ "16. Laboratory Procedures". Sci.chem FAQ. Retrieved 2008-01-11.
- PMID 17218468.
- .
- ^ "Piranha Waste Fact Sheet, University of Illinois at Urbana-Champaign" (PDF).
- ^ "Pirana Solution Use Policy, University of Illinois at Urbana-Champaign" (PDF). Archived from the original (PDF) on 2017-08-13. Retrieved 2017-08-13.
- ^ "Division of Research Safety | Illinois". drs.illinois.edu. Retrieved 2020-11-08.
External links
- How to make Piranha Solution on YouTube
- How to real etching in plated copper: H2SO4 + H2O2 + Cu on YouTube
- Piranha solution dissolves a hot dog on YouTube