Metal etching, a technique used to, as its name suggests, etch designs into metal, can be either dry or wet. This dry or wet state is determined by the etchant used to dissolve the metal. Etchants used are frequently chemicals, but etching via chemicals is not to be confused with chemical milling, a similar but less effective process.
In addition to being dry or wet, etching can be anisotropic, when an enchant removes material vertically, or isotropic, when an enchant removes material uniformly in all directions. Also, anisotropic etching may be completely or only partially so. Anisotropic dry etches tend to have a finer aspect ratio and finer resolution than their isotropic counterparts. To learn more about dry etching, read the section below.
The basic method by which dry metal etchings are made consists of a combination by gas and kinetic energy. First, gas or plasma removes the excess material, then kinetic energy created particle beams and/or the chemical reaction that follows, etches into what remains.
Following this basic category, dry etching may be further broken down into the subcategories: physical, chemical and reactive ion based etching.
In the process of physical dry etching, no chemical reaction takes place. Rather, the atoms are kicked out as the material evaporates, and kinetic energy only, of ions, electrons or photons creates the etching.
Chemical dry etching, and its close relative vapor phase etching, both attack silicon surfaces with chemical reactions. Usually, these chemical reactions are brought about using fluorine, tetrafluoro methane, nitrogen trifluoride, sulfur hexafluoride and chlorine gas.
The last type of dry metal etching to talk about is reactive ion etching, which is actually a combination of the aforementioned physical and chemical dry etchings. This type of etching works by engaging positively charged ions, produced from reactive gases, which are then exposed to the metal surfaces at high speeds and then, finally, react chemically with the silicon. Reactive ion etching works more rapidly and is used more frequently than standalone methods in both industrial and lab settings.
Wet etching, of course, is making a metal etching using an etchant in liquid form. In wet etching, the planned patterns and configuration are covered using lithography, and liquid etchant etches away at the exposed material. The material is then etched away by liquid etchant. During this process, multiple chemical reactions occur, as the original reactants are consumed and new reactants created. Common etchants used in wet anisotropic etching of silicon include: ethylenediamine pyrocatechol (EDP), potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH). In contrast, isotropic wet etching of silicon employs acetic acid, nitric acid and hydrofluoric acid in combination with one another. The concentration of these etchants determines the etching’s rate.
To understand the process of wet etching better, note these three, simplified steps: