Photosensitive glass, also known as photostructurable glass (PSG) or photomachinable glass, is a glass that belongs to the lithium-silicate family of glasses, in which images can be captured by exposure to short wave radiations such as ultraviolet light.[1] Photosensitive glass was first discovered by S. Donald Stookey in 1937.[2][3][4]
Exposure process
When the glass is exposed to UV light in the wavelength range 280–320 nm, a latent image is formed. The glass remains transparent at this stage, but its absorption in the UV range of the spectrum increases. This increased absorption is only detectable using uv transmission spectroscopy.[5]
The reason behind this is suggested to be an oxidation reduction reaction that occurs inside the glass during exposure in which cerium ions are oxidized to a more stable state and silver ions are reduced to silver.[5]
Post-exposure heat treatment
The latent image captured in the glass is made visible by heating.[6][2][4] This heat treatment is done in two stages: the temperature is first raised to about 500 °C to allow for the completion of the oxidation-reduction reaction, and formation of silver nanoclusters. In the following stage, when the temperature is raised to 550–560 °C, lithium metasilicate (Li2SiO3) forms on the silver nanoclusters. This material forms in the crystalline phase.[6]
HF chemical etching
The lithium metasilicate that forms in the exposed regions of the glass has the unique property of being strongly etched in hydrofluoric acid (HF). Hence allowing a three-dimensional image of the mask to be produced, the resulting glass microstructures have a surface roughness in the range 5 μm[1] to 0.7 μm.[6]
