8.2 Wet etches: acids and bases
The simplest etches use a liquid solvent that converts the material into a soluble compound or a gas. Unfortunately, most materials used in micro-devices have few soluble compounds, so some very aggressive chemicals are needed to attack them. Here is a list of some of the most commonly used ones:
Hydrofluoric acid (chemical formula HF) is used to convert silicon dioxide into water-soluble H2SiF6 (plus some hydrogen and water). It attacks silicon so slowly that for practical purposes it cannot be said to etch it at all. Dilute HF is used as a dip to remove native oxide from bare silicon surfaces before a silicon etch.
HNA is a mixture of hydrofluoric and nitric acids, diluted with acetic acid. It will eat into silicon very rapidly (up to 100 μm min−1) at around room temperature – the nitric acid oxidises the silicon, allowing the HF to attack. This etch is isotropic (i.e. not at all directional), so the etched hole bulges out sideways below the mask opening, as shown in Figure 34.
Hot potassium hydroxide (KOH) solution does not etch silicon as quickly (around 1 μm min−1), but has the advantage that it is anisotropic (etching faster along some crystal planes than others). This allows angled facets to be cut into the silicon surface (as shown in Figure 35) giving better control over where the etch goes.
Tetramethyl ammonium hydroxide (TMAH) solution, although less anisotropic than KOH, is a useful alternative when integrated circuits are to be made on the same chip (potassium is fatal to integrated circuits).
Phosphoric acid (H3PO4) solution will dissolve aluminium oxide, and together with some oxidising nitric acid can also etch aluminium. When heated to 160 °C, this acid will also etch silicon nitride. Silicon and silicon dioxide are unaffected by H3PO4, hot or cold.
Table 4 summarises the characteristics of these etchants.
Table 4 Etch rates and target materials for a selection of wet etchants
|Etchant||Target material||Approximate etch rate/μm min−1||Preferred mask material|
|HF (20 °C)||SiO2||0.1||photoresist, Si3N4|
|HNA (20 °C)||Si||4–100||Si3N4, SiO2|
|KOH (80 °C)||Si||1||Si3N4, SiO2|
|TMAH (80 °C)||Si||0.5–1||SiO2, Si3N4|
|H3PO4 (160 °C)||Si3N4||0.004–0.01||SiO2|
A quick glance at the safety sheets shows that all of these are dangerous chemicals (except for acetic acid, which you can sprinkle on your food), but there is another problem with using liquids in device production. Once the process is over, how do you dry the device off? It is essential that all of the reactants are removed – we cannot have the etch continuing merrily onwards throughout the life of the device – but they may be difficult to remove without leaving residues that can interfere with later processing. Microsystems manufacture also has an extra difficulty: surface tension forces at the liquid surface can rip apart or glue together a lovingly created and delicate suspended structure. Much better, therefore, if both the reactants and the products of the etch process stay as gases throughout.