Tetramethylammonium hydroxide

Tetramethylammonium hydroxide (TMAH) 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.

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.