TMAH Etching of Silicon - 1
• Tetra Methyl Ammonium Hydroxide
• MOS/CMOS compatible:
– No alkali metals {Li, Na, K, … }.
– Used in positive photoresist developers which do not use choline.
– Does not significantly etch SiO2 or Al! (Bond wire safe!)
• Anisotropy: (111):(100) ~ 1:10 to 1:35
• Typical recipe:
– 250 mL TMAH (25% from Aldrich)
– 375 mL H2O
– 22 g Si dust dissolved into solution
– Use at 90°C
– Gives about 1 mm/min etch rate
• Hydroxide etches are generally safe and predictable, but
they usually involve an alkali metal which makes them
incompatible with MOS or CMOS processing.
• Ammonium hydroxide (NH4OH) is one hydroxide which
is free of alkali metal, but it is really ammonia which is
dissolved into water. Heating to 90°C for etching will
rapidly evaporate the ammonia from solution.
• Ballasting the ammonium hydroxide with a less volatile
organic solves the problem:
– Tetramethyl ammonium hydroxide: (CH3)4NOH
– Tetraethyl ammonium hydroxide: (C2H5)4NOH