Tetramethylammonium hydroxide

Chemistry

Anhydrous TMAH has never been isolated. The only relatively stable solid form in which this substance exists is as the pentahydrate, N(CH₃)₄OH·5H₂O, and this has been assigned the CAS# 10424-65-4. A trihydrate, C₄H₁₃NO·3H₂O, has also been reported, and this has been assigned the CAS# 10424-66-5. TMAH is most commonly encountered as an aqueous solution, in concentrations from ~2–25%, and less frequently as solutions in methanol. These solutions are identified by the CAS# 75-59-2.

Preparation

One of the earliest preparations of TMAH reported in the literature is that of Walker and Johnston, who made it by the salt metathesis reaction of tetramethylammonium chloride and potassium hydroxide in dry methanol, in which TMAH is soluble, but potassium chloride is not:

NMe₄⁺Cl⁻ + KOH → NMe₄⁺ OH⁻ + KCl

Where Me stands for the methyl group, CH₃-.

This report also provides details for isolation of TMAH as its pentahydrate, noting the existence of a trihydrate, and emphasizes the avidity which even the former exhibits for atmospheric moisture and carbon dioxide. These authors reported a m.p. of 62–63 °C for the pentahydrate, and a solubility in water of 220 g/100 mL at 15 °C.

Reactions

TMAH undergoes simple acid-base reactions with strong or weak acids to produce tetramethylammonium salts whose anion is derived from the acid, e.g.

NMe₄⁺ OH⁻ + HCl → NMe₄⁺Cl⁻ + H₂ONMe₄⁺ OH⁻ + CO₂ → NMe₄⁺ HCO₃⁻

Solutions of TMAH may be used to make other tetramethylammonium salts in a metathesis reaction with ammonium salts, whereby the anion is derived from the ammonium salt. The reaction is driven in the desired direction by evaporative removal of ammonia and water. For example, tetramethylammonium thiocyanate may be made from ammonium thiocyanate, thus:

NMe₄⁺ OH⁻ + NH₄⁺SCN⁻ → NMe₄⁺SCN⁻ + NH₃ + H₂O

TMAH, in common with many other TMA salts containing simple anions, decomposes on heating into trimethylamine. Dimethyl ether is a major decomposition product rather than methanol. The idealized equation is:

2 NMe₄⁺ OH⁻ → 2 NMe₃ + MeOMe + H₂O

Properties

TMAH is a very strong base.

Uses

One of the industrial uses of TMAH is for the anisotropic etching of silicon. It is used as a basic solvent in the development of acidic photoresist in the photolithography process, and is highly effective in stripping photoresist. TMAH has some phase transfer catalyst properties, and is used as a surfactant in the synthesis of ferrofluid, to inhibit nanoparticle aggregation.

TMAH is the most common reagent currently used in thermochemolysis, an analytical technique involving both pyrolysis and chemical derivatization of the analyte.

Wet anisotropic etching

TMAH belongs to the family of quaternary ammonium hydroxide (QAH) solutions and is commonly used to anisotropically etch silicon. Typical etching temperatures are between 70 and 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 micrometer per minute range.

Common masking materials for long etches in TMAH include silicon dioxide (LPCVD and thermal) and silicon nitride. 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.

Toxicity

The tetramethylammonium ion affects nerves and muscles, causing difficulties in breathing, muscular paralysis and possibly death. It is structurally related to acetylcholine, an important neurotransmitter at both the neuromuscular junction and autonomic ganglia. This structural similarity is reflected in its mechanism of toxicity - it binds to and activates the nicotinic acetylcholine receptors, although they may become densensitized in the continued presence of the 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 to the deaths that have followed accidental industrial exposure, although the "chemical burns" induced by this strong base are also severe. There is evidence that poisoning can occur through skin-contact with concentrated solutions of TMAH.