Tetramethylammonium hydroxide

Tetramethylammonium hydroxide
IUPAC name	tetramethylammonium hydroxide
Identifiers
CAS number	75-59-2 Y
PubChem	60966
SMILES	C[N+](C)(C)C.[OH-]
Properties
Molecular formula	C4H13NO
Molar mass	91.15 g mol−1
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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 also used as a surfactant in the synthesis of ferrofluid, to prevent agglomeration.

Toxicity

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 a small amount. It also smells like dead fish, if it is not pure, from trimethylamine 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. 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. ^[1]

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.

References

1. ^ J. T. L. Thong, W. K. Choi, C. W. Chong, TMAH etching of silicon and the interaction of etching parameters, Sensors and Actuators A: Physical, Volume 63, Issue 3, December 1997, Pages 243-249, ISSN 0924-4247, DOI: 10.1016/S0924-4247(97)80511-0. (http://www.sciencedirect.com/science/article/B6THG-3S9D11W-3V/2/46a49827456cb9320e8b0173ba32b1bf)

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