Piranha solution
Encyclopedia
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 hydroxyl
ate most surfaces (add OH groups), making them extremely hydrophilic (water compatible).
(NH4OH) with hydrogen peroxide.
Piranha solution must be prepared with great care. It is highly corrosive and oxidating. Make sure surfaces are reasonably clean, and completely free of organic solvents from previous wash steps, before coming into contact with piranha solution. Piranha solution cleans by dissolving organic contaminants, and a large amount of contaminants will cause violent bubbling and a release of gas that can cause an explosion.
Piranha solution may be prepared by adding the peroxide to the acid — although very few authorities disagree. 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%. Once the mixture has stabilized, it can be further heated to sustain its reactivity. The hot (often bubbling) solution will clean organic compound
s 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.
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 be used freshly prepared. Piranha solution should not be stored.
Immersing the substrate (such as a wafer) into the solution should be done slowly to prevent thermal shock that may crack the substrate material.
industry, e.g. to clean photoresist
residue from silicon
wafers.
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 will not contaminate glassware with heavy metal ions.
Piranha solution is particularly useful when cleaning sintered (or 'fritted') glassware, however this can be incredibly dangerous due to the risk of explosion from organic contaminants. The size of the pores of sintered glassware is critical for its function, so it should not be cleaned with strong bases, which gradually dissolve the sinter. Sintered glass also tends to capture material deep within the structure, making it difficult to remove. 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 accomplished by allowing piranha solution to percolate backward through the sintered glass.
Piranha solution is used to make glass hydrophilic by hydroxylating the surface, thus increasing the number of silanol
groups on the surface.
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 in 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:
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:
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 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 oxide
s and carbonate
s. 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.
Piranha solution that is no longer being used should never be left unattended if hot. It should not be stored in a closed container. Piranha solution should not be disposed with organic solvents (e.g. in waste solvent carboys), as this will cause a violent reaction and a substantial explosion.
Used piranha solution remains very acidic and oxidizing. It should be allowed to cool, and oxygen gas should be allowed to dissipate prior to disposal, dissolution of the hydrogen peroxide can be catalysed with a few grains of manganese dioxide. When cleaning glassware, it is both prudent and practical to allow the piranha solution to react overnight. This allows the spent solution to degrade prior to disposal. Spent piranha solution may be disposed of by slowly pouring down the drain with copious amounts of water, where it is greatly diluted. Some organizations insist that spent piranha solutions should be collected and disposed separately.
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
(H2SO4) and hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
(H2O2), used to clean organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
residues off substrates. Because the mixture is a strong oxidizer, it will remove most organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
, and it will also hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
ate most surfaces (add OH groups), making them extremely hydrophilic (water compatible).
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 hydroxideAmmonium hydroxide
Ammonia solution, also known as ammonium hydroxide, ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or simply ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3...
(NH4OH) with hydrogen peroxide.
Piranha solution must be prepared with great care. It is highly corrosive and oxidating. Make sure surfaces are reasonably clean, and completely free of organic solvents from previous wash steps, before coming into contact with piranha solution. Piranha solution cleans by dissolving organic contaminants, and a large amount of contaminants will cause violent bubbling and a release of gas that can cause an explosion.
Piranha solution may be prepared by adding the peroxide to the acid — although very few authorities disagree. Mixing the solution is exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
. 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%. Once the mixture has stabilized, it can be further heated to sustain its reactivity. The hot (often bubbling) solution will clean organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s off substrates, and oxidize/hydroxylate most metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
surfaces. Cleaning usually requires about 10 to 40 minutes, after which time the substrates can be removed from the solution.
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 be used freshly prepared. Piranha solution should not be stored.
Immersing the substrate (such as a wafer) into the solution should be done slowly to prevent thermal shock that may crack the substrate material.
Applications
Piranha solution is used frequently in the microelectronicsMicroelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
industry, e.g. to clean photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....
residue from silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
wafers.
In the laboratory, this solution is sometimes used to clean glassware
Laboratory glassware
Laboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...
, though it is discouraged in many institutions and it should not be used routinely due to its dangers. Unlike chromic acid
Chromic acid
The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...
solutions, piranha will not contaminate glassware with heavy metal ions.
Piranha solution is particularly useful when cleaning sintered (or 'fritted') glassware, however this can be incredibly dangerous due to the risk of explosion from organic contaminants. The size of the pores of sintered glassware is critical for its function, so it should not be cleaned with strong bases, which gradually dissolve the sinter. Sintered glass also tends to capture material deep within the structure, making it difficult to remove. 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 accomplished by allowing piranha solution to percolate backward through the sintered glass.
Piranha solution is used to make glass hydrophilic by hydroxylating the surface, thus increasing the number of silanol
Silanol
Silanol, also known as silyl alcohol, is a chemical with formula SiH3OH. It is the simplest silicon alcohol, and is a heavy, volatile, colorless, flammable liquid. At room temperature it is a polar liquid...
groups on the surface.
Mechanism of action
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 waterWater
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
by the concentrated sulfuric acid. This occurs because hydration of concentrated sulfuric acid is thermodynamically strongly favorable, with an ΔH of -880 kJ
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
/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 in piranha solution are dehydrated so violently that the process resembles a piranha
Piranha
A piranha or piraña is a member of family Characidae in order Characiformes, an omnivorous freshwater fish that inhabits South American rivers. In Venezuela, they are called caribes...
feeding frenzy
Feeding frenzy
In ecology, a feeding frenzy is a situation where oversaturation of a supply of food leads to rapid feeding by predatory animals. For example, a large school of fish can cause nearby sharks to enter a feeding frenzy. This can cause the sharks to go wild, biting anything that moves, including each...
. The second and more definitive rationale for the name, however, is the ability of piranha solution to “eat anything,” including in particular elemental carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in the form of soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
or char
Charring
Charring is a chemical process of incomplete combustion of certain solids when subjected to high heat. The resulting residue matter is called Char. By the action of heat, charring removes hydrogen and oxygen from the solid, so that the remaining char is composed primarily of carbon...
.
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
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
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
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
ions, bisulfate
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
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
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
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
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
group:
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 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 oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s and carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
s. 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.
Safety and disposal
Piranha solution is very dangerous, being both strongly acidic and a strong oxidizer.Piranha solution that is no longer being used should never be left unattended if hot. It should not be stored in a closed container. Piranha solution should not be disposed with organic solvents (e.g. in waste solvent carboys), as this will cause a violent reaction and a substantial explosion.
Used piranha solution remains very acidic and oxidizing. It should be allowed to cool, and oxygen gas should be allowed to dissipate prior to disposal, dissolution of the hydrogen peroxide can be catalysed with a few grains of manganese dioxide. When cleaning glassware, it is both prudent and practical to allow the piranha solution to react overnight. This allows the spent solution to degrade prior to disposal. Spent piranha solution may be disposed of by slowly pouring down the drain with copious amounts of water, where it is greatly diluted. Some organizations insist that spent piranha solutions should be collected and disposed separately.
See also
- Peroxymonosulfuric acidPeroxymonosulfuric acidPeroxymonosulfuric acid, also known as persulfuric acid, peroxysulfuric acid, or as Caro's acid, is H2SO5, a liquid at room temperature. In this acid, the S center adopts its characteristic tetrahedral geometry; the connectivity is indicated by the formula HO-O-S2-OH...
or Caro's acid - Peroxydisulfuric acidPeroxydisulfuric acidPeroxydisulfuric acid is a sulfur oxoacid with the chemical formula H2S2O8. It is also called Marshall's acid. In structural terms it can be written HO3SOOSO3H. It contains sulfur in its +6 oxidation state, but it also contains peroxide ions, which is why it appears to be in a higher oxidation...
- RCA cleanRCA cleanThe RCA clean is a standard set of wafer cleaning steps which needs to be performed before high temp processing steps of silicon wafers in semiconductor manufacturing. RCA cleaning includes RCA-1 and RCA-2 cleaning procedures...