Isotropic Wet Chemical Etching of Silicon Glass

Whenever there is chemical erosion of glass, tvater or its dissociation products, or OH ions, will be involved. Because of this, a distinction is made between the resistance of glass to water (its hydrolytic resistance) and to alkali or acid. Under attack from water or acids, small numbers of cations, particularly monovalent and divalent, are released. On resistant types of glass, a very thin layer of silica gel forms in this way on the glass surface and usually inhibits further erosion. In contrast, hydrofluoric acid, alkaline solutions, and, under certain circumstances, phosphoric acid will slowly remove the inhibitor layer and thus the entire surface. Nonaqueous solutions (organic solutions), however, are practically nonreactive with glass. 

The pinhole problem, adhesion difficulty, and low aspect ratio all restrict the depth of the etch for the structuring. Depths down to 120 pm are achieved simply in Ref. [12], there is a report of 300 pm achieved by using a 1 pm Cr/Au masking system. 

Through-holes are only possible in very thin substrates. This process can be combined with other means of structuring glass for example, microsandblasting or ultrasonic lapping. 

The process followed with glass is more complicated than described below, adequate enough to understand the sequence. The glass is melted under reducing conditions so that cerium is present in the form of Ce3+ and silver in the form of Ag+. The Ce3+ will emit another electron if illuminated with UV light (X = 300-320 nm), and this electron can be taken up by the Ag+ to form Ag°. The amount of energy 

Light exposure (initiation), tempering/heat treatment and etching (development, structuring, fixation). 

---

Figure 2.14 Isotropic wet chemical etching of silicon glass (with friendly permission of the Institute of Physical High Technology).

---