Chemical Etching
Chemical etching is the removal of material from a wafer by dissolving it in a liquid reagent, used throughout semiconductor fabrication to clean surfaces, thin crystals, open windows in films and carve shapes into silicon. Wet etching depends on a chemical reaction that converts the solid into soluble products, so the choice of reagent determines what is removed and what is left alone. It is the oldest patterning method in the field, older than the integrated circuit, and it remains in use today even though dry plasma etching has taken over most fine-line work.
The early transistor era leaned on etching to shape and clean germanium and silicon, and a few recipes became famous. In the 1950s, transistor makers used acid mixtures to thin crystal slices, to remove lapping damage and to polish and clean junction surfaces; a widely used germanium etch known as CP-4 combined hydrofluoric, nitric and acetic acids, with a little bromine in some versions. The nitric acid oxidizes the germanium, the hydrofluoric acid dissolves the oxide as it forms, and the acetic acid moderates the reaction, so the surface is smoothed rather than pitted. Because device performance depended on surface condition, etching was a form of process control as much as a shaping step, and etch recipes were closely guarded, tweaked and named. The general principle of oxidize-then-dissolve underlies most semiconductor etches to this day.
Different materials call for different reagents, and the pairing of reagent with film is what makes selective patterning possible. Hydrofluoric acid, usually buffered with ammonium fluoride, dissolves silicon dioxide readily while barely touching silicon, which is why it is the standard way to open windows in oxide after photolithography, or to strip a thin native oxide just before a metal contact is made. Hot phosphoric acid dissolves silicon nitride, and a mixture of phosphoric, nitric and acetic acids etches aluminium. Selectivity, the ratio of the etch rate of the target film to that of the material beneath or beside it, is the central figure of merit: an oxide etch with high selectivity to silicon will stop on the wafer surface rather than eating into it. Etch rates are typically quoted in nanometres or micrometres per minute and are very sensitive to temperature, concentration and agitation, so baths are controlled and monitored closely.
The difference between isotropic and anisotropic etching is the key to understanding both its power and its limits. An isotropic etch, such as an HF-nitric acid mixture on silicon, attacks equally in every direction, so it cuts sideways under the mask by roughly as much as it cuts downward; this is acceptable for thin films and wide features but ruins resolution when line widths approach the film thickness. An anisotropic etch exploits the crystal itself: potassium hydroxide solutions etch silicon far faster along the (100) planes than along the (111) planes, so a rectangular opening in a mask on a (100) wafer produces a pit with flat, sloped sidewalls meeting at about 54.7 degrees, bounded by those slow planes. This crystallographic etching is the foundation of silicon micromachining, producing pressure-sensor diaphragms, inkjet nozzles and microfluidic channels, with silicon dioxide or silicon nitride serving as the mask because KOH attacks them only slowly.
Wet etching has been displaced for the finest patterning, but not eliminated, and its pitfalls are well known. As features shrank below a few micrometres in the 1970s, isotropic undercutting made wet etching unsuitable for defining narrow lines, and plasma and reactive-ion etching, which use ionized gases to remove material directionally, took over. Still, wet chemistry is used wherever its selectivity and low damage are valuable: wafer cleaning with sequences such as the RCA clean developed by Werner Kern at RCA in the 1960s, oxide removal, nitride stripping, and releasing micromechanical structures. Misconceptions are common: a stronger or hotter bath is not necessarily a better one, because etch rate rises steeply with temperature and uniformity suffers; and KOH, though effective, contains mobile potassium ions that are damaging to transistors and must be kept away from active device areas. Safety is a practical concern too, since hydrofluoric acid penetrates skin and is dangerous in ways that its mild appearance does not suggest.
Read chemical etching as controlled dissolution, where the reagent, the mask and the crystal orientation together decide which atoms leave and which stay, trading the speed and selectivity of liquids against the directional precision of plasma.