Piranha solution, also known as
piranha etch, is a mixture of
sulfuric acidSulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
(H
2SO
4) and
hydrogen peroxideHydrogen peroxide is a very pale blue liquid, slightly more viscous than water, that appears colorless in dilute solution. It is a weak acid, has strong oxidizing properties, and is a powerful bleaching agent. It is used as a disinfectant, antiseptic, oxidizer, and in rocketry as a propellant...
(H
2O
2), used to clean
organicAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
residues off substrates. Because the mixture is a strong oxidizer, it will remove most organic matter, and it will also
hydroxylIn chemistry, hydroxyl is composed of molecules consisting of an oxygen atom and a hydrogen atom connected by a covalent bond . The neutral form is a hydroxyl radical. The hydroxyl anion is called hydroxide; it is a diatomic ion with a charge of negative one...
ate most surfaces (add OH groups), making them extremely hydrophilic (water compatible).
Applications
Piranha solution is used frequently in the
microelectronicsMicroelectronics is a subfield of electronics. Microelectronics, as the name suggests,is related to the study and manufacture, or microfabrication, of electronic components which are very small . These devices are made from semiconductors...
industry, e.g. to clean
photoresistA 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
siliconSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
wafers. It is sometimes used to clean
laboratory glasswareLaboratory 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 done routinely due to its dangers. Unlike
chromic acidChromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...
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
silanolSilanols are compounds containing silicon atoms to which hydroxy substituents bond directly. They are similar to alcohols just as silanes are similar to alkanes....
groups on the surface.
It is sometimes used to passivate glassware prior to doing sensitive chemical reactions.
Piranha solution may also be used to etch
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
.
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 , also known as ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, or aqueous ammonia, is a solution of ammonia in water...
(NH
4OH) 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). Once the mixture has stabilized, it can be further heated to sustain its reactivity. The hot (often bubbling) solution will clean
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
s off substrates, and oxidize/hydroxylate most
metalA metal is a chemical element that is a good conductor of both electricity and heat, forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions...
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. 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.
Safety
PiranhaA piranha or piraña is a member of a family of omnivorous freshwater fish which live in South American rivers. In Venezuelan rivers, they are called caribes. They are known for their sharp teeth and a voracious appetite for meat....
solution, as suggested by the name, is very dangerous, with the majority of its components acidic and highly corrosive. It must be handled extremely carefully.
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 (
acetoneAcetone is the organic compound with the formula OC2. This colorless, mobile, flammable liquid is the simplest example of the ketones. Owing to the fact that acetone is miscible with water it serves as an important solvent in its own right, typically as the solvent of choice for...
,
isopropyl alcoholIsopropyl alcohol is a common name for a colorless, flammable chemical compound with a strong odor...
, 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.
Used piranha solution is diluted sulfuric acid and is considered hazardous waste.
Under no circumstances should used piranha solution ever be disposed of by flushing it down the drain. However, some facilities have specialized waste handling equipment that neutralizes the waste solution, allowing it to be flushed down certain drains for convenience.
Care must be taken not to allow the solution to be mixed with organic solvents, as this will cause a violent reaction and might even cause a substantial explosion.
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 is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
by the concentrated sulfuric acid. This occurs because hydration of concentrated sulfuric acid is thermodynamically strongly favorable, with an ΔH of -880 k
JThe joule , named for James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy exerted by a force of one newton acting to move an object through a distance of one metre...
/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
piranhaA piranha or piraña is a member of a family of omnivorous freshwater fish which live in South American rivers. In Venezuelan rivers, they are called caribes. They are known for their sharp teeth and a voracious appetite for meat....
feeding frenzyIn 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
carbonCarbon 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
sootSoot 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
charCharring is a chemical process of incomplete combustion of a solid when subjected to high heat. The resulting residue matter is called Char. Charring can be either a deliberate and controlled reaction used in manufacturing or it may be the result of naturally-occurring processes...
.
This second and far more interesting process can be understood as the sulfuric-acid boosted conversion of hydrogen peroxide from a relatively mild
oxidizing agentAn oxidizing agent can be defined as either:#a chemical compound that readily transfers oxygen atoms, or#a substance that gains electrons 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
hydroniumIn chemistry, hydronium is the common name for the aqueous cation , the simplest type of oxonium ion, produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton to the surrounding water...
ions,
bisulfateAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
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 bondsIn chemistry, hybridisation is the concept of mixing atomic orbitals within an atom to form new hybrid orbitals. It is an integral part of the valence bond theory.Hybridisation of orbitals will change the shape of the atom...
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
carbonylIn organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex In organic chemistry, a carbonyl group is a functional group...
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
oxideAn oxide is a chemical compound containing at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides. Oxides result when elements are oxidized by oxygen in air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon...
s and
carbonateIn chemistry, a carbonate is a salt or ester of carbonic acid, characterized by the presence of the carbonate ion, or a carbonate functional group O=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.
See also
- Peroxymonosulfuric acid
Peroxymonosulfuric acid, also known as persulfuric acid, peroxysulfuric acid, or as Caro's acid, is H2SO5, a liquid at room temperature...
or Caro's acid
- Peroxydisulfuric acid
Peroxydisulfuric is a sulfur oxoacid with the chemical formula H2S2O8 . In structural terms it can be written HO3SOOSO3H. It is one of a group of sulfur oxoacids, its salts, commonly known as persulfates, are industrially important but the...
- RCA clean
The 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...
External links