|Section2={{Chembox Properties
| C=4 | H =13 | N = 1 | O = 1
| Appearance=
| Density=
| MeltingPt=
| BoilingPt=
| Solubility=
}}
|Section3={{Chembox Hazards
| MainHazards=
| FlashPt=
| Autoignition=
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| Section8 = {{Chembox Related
| OtherAnions =
tetramethylammonium chloride
| OtherCations =
tetraethylammonium 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 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 only 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. 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)
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.
See also
References