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 of acidic photoresist in the photolithography process. Since it is a phase transfer catalyst, it is highly effective in stripping photoresist. It is used to prevent the nanoparticles in a ferrofluid from sticking together.

Toxicity

TMAH solution is a strong base. Its 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 a small amount. It also smells like dead fish, if it is not pure, from Trimethyl amine impurity. TMAH has virtually no odor when pure.

Wet anisotropic Etching

TMAH solution is used to anisotropically etch silicon along its crystallographic axis. TMAH doesn't etch silicon nitride or silicon oxide. They are often used as mask for long Silicon etches.

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