Piranha solution, which contains but is not to be confused with Caro's acid, also known as piranha etch, is a mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2). The resulting mixture is used to clean organic residues off substrates, for example silicon wafers. Because the mixture is a strong oxidizing agent, it will decompose most organic matter, and it will also hydroxylate most surfaces (by adding –OH groups), making them highly hydrophilic (water-compatible). This means the solution can also easily dissolve fabric and skin, potentially causing severe damage and chemical burns in case of inadvertent contact. It is named after the piranha fish due to its tendency to rapidly dissolve and 'consume' organic materials through vigorous chemical reactions.
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; other protocols may use a 4:1 or even 7:1 mixture. A closely related mixture, sometimes called "base piranha", is a 5:1:1 mixture of water, ammonia solution (NH4OH, or NH3(aq)), and 30% hydrogen peroxide. As hydrogen peroxide is less stable at high pH than under acidic conditions, NH4OH (pH c. 11.6) also accelerates its decomposition. At higher pH, H2O2 will decompose violently. Piranha solution must be prepared with great care. It is highly corrosive and an extremely powerful oxidizer. Surfaces must be reasonably clean and completely free of organic solvents from previous washing steps before coming into contact with the solution. Piranha solution cleans by decomposing organic contaminants, and a large amount of contaminant will cause violent bubbling and a release of gas that can cause an explosion. Piranha solution should always be prepared by adding hydrogen peroxide to sulfuric acid slowly, never in reverse order. This minimises the concentration of hydrogen peroxide during the mixing process, helping to reduce instantaneous heat generation and explosion risk. Mixing the solution is an extremely exothermic process. If the solution is made rapidly, it will instantly boil, releasing large amounts of corrosive fumes. Even when made with care, the resulting heat can easily bring the solution temperature above 100 °C. It must be allowed to cool reasonably before it is used. A sudden increase in temperature can also lead to a violent boiling of the extremely acidic solution. Solutions made using hydrogen peroxide at concentrations greater than 50 wt % may cause an explosion. The 1:1 acid–peroxide mixtures will also create an explosion risk even when using common 30 wt. % hydrogen peroxide. Once the mixture has stabilized, it can be further heated to sustain its reactivity. 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. 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. 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. 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 backwards 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 recommended due to the risk of explosion from reacting with traces of organic compounds, such as acetone. Piranha solution is also used to make glass more hydrophilic by hydroxylating its surface, thus increasing the number of silanol groups present on its surface.
Mechanism
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. The dehydration process exhibits itself as the rapid carbonization of common organic materials, especially carbohydrates, when they enter in contact with sulfuric acid.
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