tmah etch

Tetramethylammonium hydroxide (TMAH, (CH3)4NOH) is an alkaline solution used in semiconductor manufacturing for two primary applications: as a photoresist developer and as an anisotropic silicon etchant. As a developer, 2.38% by weight TMAH in deionized water (also known as MF-319 or equivalent) is the industry-standard aqueous base developer for positive and negative chemically amplified photoresists. The hydroxide ions dissolve exposed positive resist where deprotection has created hydrophilic carboxylic acid groups. TMAH replaced earlier KOH-based developers because it is a metal-ion-free organic base, eliminating the risk of alkali metal contamination that would degrade gate oxide reliability. As a silicon etchant, TMAH solutions at higher concentrations (5-25 wt%) and elevated temperatures (70-90°C) provide anisotropic wet etching of crystalline silicon with properties similar to KOH but with the advantage of being CMOS-compatible (no mobile ion contamination). TMAH etching exploits the different etch rates of silicon crystal planes: the (100) plane etches approximately 10-30× faster than the (111) plane, enabling the fabrication of precisely shaped V-grooves, pyramidal cavities, and membrane structures used in MEMS devices, AFM probe tips, and microfluidic channels. Typical TMAH silicon etch rates range from 0.5 to 1.5 μm/min for (100) silicon at 80°C with 25% concentration. Adding surfactants such as Triton X-100 or isopropyl alcohol to TMAH solutions can improve etch surface quality by reducing hydrogen bubble adhesion and hillock formation. TMAH selectivity of silicon over SiO2 is approximately 5,000:1, enabling oxide masks for silicon etching. While TMAH offers lower etch rates and higher cost compared to KOH, its metal-free composition makes it essential for post-CMOS MEMS processing where device contamination must be avoided. Safety considerations include its high toxicity (it is a potent neurotoxin through skin absorption) and strong basicity.