Foturan

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.

Foturan demonstrator in different processing steps (UV-exposure, tempering, etching, ceramization)

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.[1]

Composition and Properties
Composition
Ingredient SiO2 LiO2 Al2O3 K2O Na2O ZnO B2O3 Sb2O3 Ag2O CeO2
Share [%] 75-85 7-11 3-6 3-6 1-2 0-2 0-1 0,2-1 0,1-0,3 0,01-0,2
Mechanical Properties
Knoop-Hardness in N/mm² (0.1/20)480
Vickers-Härte in N/mm² (0.2/25)520
Density in g/cm³2,37
Thermal Properties
Coefficient of mean linear thermal expansion a20-300 in 10−6·K−18,49
Thermal Conductivity at 90 °C in W/mK1,28
Transformation Temperature Tg in °C455
Electrical Properties
Relative Permittivity
Frequency [GHz] 1.1 1.9 5
Glass-state (annealed at 40 °C/h) 6.4 6.4 6.4
Ceramic-state (ceramized at 560 °C) 5.8 5.9 5.8
Ceramic-state (ceramized at 810 °C) 5.4 5.5 5.4
Dissipation factor tanα(·10−4)
Frequency [GHz] 1.1 1.9 5
Glass-state (annealed at 40 °C/h) 84 90 109
Ceramic-state (ceramized at 560 °C) 58 65 79
Ceramic-state (ceramized at 810 °C) 39 44 55
Chemical Properties
Hydrolytic resistance acc. to DIN ISO 719 in µgNa2O/g (class)578 (HGB 4)
Acid resistance acc. to DIN 12116 in mg/dm² (class)0,48 (S1)
Alkali resistance acc. to DIN ISO 695 in mg/dm² (class)100 (A2)
Optical properties
Refractive Index
wavelength [nm], λ= 300 486.1 (nF) 546.1 (ne) 567.6 (nd) 656.3 (nC)
Glass-state (annealed at 40 °C/h) 1.549 1.518 1.515 1.512 1.510
Ceramic-state (ceramized at 560 °C) n/a 1.519 1.515 1.513 1.511
Ceramic-state (ceramized at 810 °C) n/a 1.532 1.528 1.526 1.523
Spectral Transmittance
τ(λ) t250 t270 t280 t295 t350
in [%, 1mm] 0.1 3 11 29 89

Foturan is a lithium aluminosilicate glass system doped with small amounts of silver oxides and cerium oxides.[2]

Processing

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.[3]

Exposure
Foturan - Processing steps (schematic view)

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 Ce3+ transforms into Ce4+ and frees an electron.[4]

Tempering

During the nucleation tempering (~ 500 °C), the Silver-ion Ag+ will be transferred into Ag0 by scavenging the electron released from Ce3+.

This activates the agglomeration of atomic silver to form nanometer-scale silver clusters

During the subsequent crystallization tempering (~560-600 °C), lithium metasilicates (Li2SiO3 glass-ceramic) forms on the silver cluster nucleation in the exposed areas. The un-exposed glass, otherwise amorphous, remains unchanged.[4]

Etching

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.[4]

Ceramization (Optional)

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 dicilicate Li2Si2O5.[4]

Product characteristics
Foturan (ceramized)
  • Small structure size: Structure sizes of ~ 25 μm are possible
  • High aspect ratio: Etchingratios of > 20:1 make aspect ratio of > 10:1 and a wall angle of ~ 1-2° possible
  • High optical transmission in visible and non-visible spectrum: More than 90% transmission (substrate thickness 1 mm) between 350 nm and 2.700 nm
  • High temperature resistance: Tg > 450°Celsius
  • Pore-free: Suitable for biotech / microfluidics application
  • Low self fluorescence
  • Hydrolytic resistance (acc. to DIN ISO 719): HGB 4
  • Acid resistance (acc. to DIN 12116): S 1
  • Alkali resistance (acc. to DIN ISO 695): A 2

Foturan in the scientific community

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.[5]

Many of those deal with topics such as

Applications

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.

References
  1. "Schott Press Release 02-16-2016". 2016-02-16. Retrieved 2016-02-16.
  2. "Foturan Schott Website". Retrieved 2016-02-12.
  3. Höland, Wolfram (1999). Glass Ceramic Technology (1 ed.). Wiley. p. 236. ISBN 0470487879.
  4. Livingston, F.E.; Adams, P.M.; Helvajian, Henry (2005). "Influence of cerium on the pulsed UV nanosecond laser processing of photostructurable glass ceramic materials". Applied Surface Science. 247 (1–4): 527. Bibcode:2005ApSS..247..526L. doi:10.1016/j.apsusc.2005.01.158.
  5. "Foturan on Google Scholar". Google Scholar. Retrieved 30 October 2015.
  6. Rajta, I. (September 2003). "Proton beam micromachining on PMMA, Foturan and CR-39 materials". Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 210: 260–265. Bibcode:2003NIMPB.210..260R. doi:10.1016/s0168-583x(03)01025-5.
  7. Wang, Zhongke (October 2008). "Fabrication of integrated microchip for optical sensing by femtosecond laser direct writing of Foturan glass". Applied Physics A. 93 (1): 225–229. Bibcode:2008ApPhA..93..225W. doi:10.1007/s00339-008-4664-2. S2CID 19174537.
  8. An, R. (March 2007). "Optical waveguide writing inside Foturan glass with femtosecond laser pulses". Applied Physics A. 86 (3): 343–346. Bibcode:2007ApPhA..86..343A. doi:10.1007/s00339-006-3773-z. S2CID 98597105.
  9. He, Fei (December 2009). "Rapid fabrication of optical volume gratings in Foturan glass by femtosecond laser micromachining". Applied Physics A. 97 (4): 853–857. Bibcode:2009ApPhA..97..853H. doi:10.1007/s00339-009-5338-4. S2CID 98824071.
  10. Kim, Joohan (January 25, 2003). Pique, Alberto; Sugioka, Koji; Herman, Peter R; Fieret, Jim; Bachmann, Friedrich G; Dubowski, Jan J; Hoving, Willem; Washio, Kunihiko; Geohegan, David B; Traeger, Frank; Murakami, Kouichi (eds.). "Fabrication of microstructures in FOTURAN using excimer and femtosecond lasers". SPIE Conference Volume 4977. Photon Processing in Microelectronics and Photonics II. 4977: 324. Bibcode:2003SPIE.4977..324K. doi:10.1117/12.479239. S2CID 137683384.
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