Tetramethylammonium hydroxide
Tetramethylammonium hydroxide (TMAH or TMAOH) is a quaternary ammonium salt with the molecular formula (CH3)4NOH. It is used as an anisotropic etchant of silicon. It is also used as a basic solvent in the development
Photographic processing
Photographic processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image...

 of acidic 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....

 in the photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...

 process. Since it is a phase transfer catalyst
Phase transfer catalyst
In chemistry, a phase transfer catalyst or PTC is a catalyst that facilitates the migration of a reactant from one phase into another phase where reaction occurs. Phase transfer catalysis is a special form of heterogeneous catalysis. Ionic reactants are often soluble in an aqueous phase but...

, it is highly effective in stripping photoresist. It is also used as a surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...

 in the synthesis of ferrofluid
A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field.Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid . Each tiny particle is thoroughly coated with a surfactant to inhibit clumping...

, to prevent agglomeration
In the study of human settlements, an urban agglomeration is an extended city or town area comprising the built-up area of a central place and any suburbs linked by continuous urban area. In France, INSEE the French Statistical Institute, translate it as "Unité urbaine" which means continuous...



TMAH solution is a strong base. The tetramethylammonium ion can damage nerves and muscles, causing difficulties in breathing and possibly death in a short period of time after exposure by contact, even with only a small amount. It is structurally similar to acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

, an important peripheral neurotransmitter at both the neuromuscular junction
Neuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...

 and autonomic ganglia. This structural similarity is reflected in its mechanism of toxicity - it binds to and activates the nicotinic acetylcholine receptors, although the response may subsequently densentize in the continued presence of this agonist. The action of tetramethylammonium is most pronounced in autonomic ganglia, and so tetramethylammonium is traditionally classed as a ganglion-stimulant drug. The ganglionic effects may contribute the deaths that the have followed accidental industrial exposure, although the "chemical burns" induced by this strong base are also severe.

It also smells like dead fish if it is not pure, from trimethylamine
Trimethylamine is an organic compound with the formula N3. This colorless, hygroscopic, and flammable tertiary amine has a strong "fishy" odor in low concentrations and an ammonia-like odor at higher concentrations...

 impurity. TMAH has virtually no odor when pure.

Wet anisotropic Etching

TMAH belongs to the family of quaternary ammonium hydroxide (QAH) solutions and is commonly used to anisotropically etch 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...

. Typical etching temperatures are between 70°-90°C and typical concentrations are 5-25 wt% TMAH in water. (100) silicon etch rates generally increase with temperature and decrease with increasing TMAH concentration. Etched silicon (100) surface roughness decreases with increasing TMAH concentration, and smooth surfaces can be obtained with 20% TMAH solutions. Etch rates are typically in the 0.1-1 micron per minute range.

Common masking materials for long etches in TMAH include silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

 (LPCVD and thermal) and silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...

. Silicon nitride has a negligible etch rate in TMAH; the etch rate for silicon dioxide in TMAH varies with the quality of the film, but is generally on the order of 0.1 nm/minute.
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