Foturan (notation of the manufacturer: FOTURAN) is a photosensitive glass by SCHOTT Corporation developed in 1984. It is a technical glass-ceramic which can be structured without photoresist when it is exposed to shortwave radiation such as ultraviolet light and subsequently etched.
In February 2016, Schott announced the introduction of Foturan II at Photonics West. Foturan II is characterized by higher homogeneity of the photosensitivity which allows finer microstructures.
Foturan is a lithium aluminosilicate glass system doped with small amounts of silver oxides and cerium oxides.
Foturan can be structured via UV-exposure, tempering and etching: Crystal nucleation grow in Foturan when exposed to UV and heat treated afterwards. The crystalized areas react much faster to hydrofluoric acid than the surrounding vitreous material, resulting in very fine microstructures, tight tolerance and high aspect ratio.
If Foturan is exposed to light in the ultra-violet-range with a wavelength of 320 nm (eventually via photomask, contact lithography or proximity lithography to expose certain patterns), a chemical reaction is started in the exposed areas: The containing Ce<sup>3+</sup> transforms into Ce<sup>4+</sup> and frees an electron.
During the nucleation tempering (~ 500 ðC), the Silver-ion Ag<sup>+</sup> will be transferred into Ag<sup>0</sup> by scavenging the electron released from Ce<sup>3+</sup>.
This activates the agglomeration of atomic silver to form nanometer-scale silver clusters
During the subsequent crystallization tempering (~560-600 ðC), lithium metasilicates (Li<sub>2</sub>SiO<sub>3</sub> glass-ceramic) forms on the silver cluster nucleation in the exposed areas. The unexposed glass, otherwise amorphous, remains unchanged.
After tempering, the crystallized areas can be etched with hydrofluoric acid 20 times faster than the unexposed, still amorphous glass. Thus, structures with an aspect ratio of ca. 10:1 can be created.
After etching, a ceramization of the entire substrate after a 2nd UV-exposure and thermal treatment is possible. The crystalline phase in this stage is lithium disilicate Li<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>.
Foturan is a widely known material in the material science community. As of October 30, 2015, Google Scholar showed more than 1.000 results of Foturan in scholarly literatures across an array of publishing formats and disciplines.
Many of those deal with topics such as
Foturan is mainly used for microstructure applications, where small and complex structures have to be created out of a solid and robust base material. Overall there are five main areas for which Foturan is used:
By thermal diffusion bonding it is possible to bond multiple Foturan layers on top of each other to create complex 3-dimensional microstructures.