KOH vs TMAH Anisotropic Etching of Silicon: Etch Rate, Masks and CMOS Compatibility
KOH and TMAH are both alkaline solutions that dissolve silicon at very different rates depending on the crystal plane. The (111) planes etch far more slowly than the others, so on a (100) wafer they appear as sidewalls sloped at about 54.7°, which lets you make V-grooves, pyramid-shaped pits and thin diaphragms. KOH is cheap, has a large rate difference between planes and has long been used for bulk micromachining of MEMS. However, potassium ions disturb transistor characteristics, so KOH cannot be brought into CMOS processing. TMAH contains no metal ions, so it is easier to combine with CMOS and can use oxide masks, but its rate difference between planes is smaller. It is also absorbed through the skin and can cause severe poisoning, so it needs especially careful handling.
| Aspect | KOH | TMAH |
|---|---|---|
| Composition and conditions | Aqueous potassium hydroxide, typically about 20 to 40 wt% at about 60 to 90 °C | Aqueous tetramethylammonium hydroxide (a metal-free organic alkali), typically about 5 to 25 wt% at about 70 to 90 °C |
| Typical (100) etch rate | Typically around 1 µm per minute (varies strongly with concentration and temperature) | Typically about 0.3 to 1 µm per minute, often slower than KOH |
| (100) to (111) rate ratio | Large: typically over 100, and several hundred under some conditions | Smaller: typically in the tens (from about ten upward) |
| Usable masks | Silicon nitride (LPCVD) is best; SiO2 etches at a rate that is not negligible, so long etches need a thick film | SiO2 etches very slowly, so thermal oxide works well as a mask; silicon nitride can also be used |
| CMOS compatibility | Poor: potassium ions enter the gate oxide as mobile ions and shift the threshold voltage, so KOH is kept out of CMOS processing; it also attacks aluminum quickly | Good: it contains no metal ions, so it can be used for MEMS processing after the CMOS circuits are built (post-CMOS); it attacks aluminum, but conditions that limit the attack, such as dissolving silicon into the solution, are known |
| Surface roughness | Relatively smooth surfaces are easier to get at higher concentrations | At low concentration, small pyramid-shaped hillocks tend to form on (100) surfaces; higher concentration gives smoother surfaces but slower etching |
| Safety and handling | A strong alkali that causes serious damage to skin and eyes; cheap and easy to handle | Besides being a strong alkali, it can be absorbed through the skin and block signal transmission from nerves to muscles, which can cause systemic poisoning such as respiratory arrest; concentrated solutions are especially dangerous |
| Typical uses | Bulk machining of MEMS without circuits, such as pressure-sensor diaphragms, V-grooves for aligning optical fibers, inkjet nozzles and through-wafer holes | Releasing MEMS structures built on the same chip as circuits (such as thermal sensors), and anisotropic silicon etching where metal-ion contamination must be avoided |
Figures are typical orders of magnitude and vary by process generation and product.