Optical bonding with fast sol-gel

Abstract

We investigate here the properties of fast sol-gel for optical bonding. The precursors of the fast sol-gel material are organically modified alkoxides generating a transparent hybrid (organic-inorganic) substance with silica glass-like properties whose index of refraction can be modified by the addition of various metal-oxides. The fast sol-gel method consists of rapid fabrication of a viscous resin and its subsequent dilution for long shelf life use. This material, when used as an adhesive offers the option of either a thermal or UV curing procedure. We demonstrate a bonding strength of ∼ 10 MPa when a 15 µm layer is applied between two glass elements. The bonding remained stable after an extensive-40 • C-120 • C temperature cycling with minimal residual solvent evaporation at 150 • C. The fast sol-gel material was tested for optical bonding between silica bulks, between silica bulk and silicon wafers and as an adhesive in silica fibre couplers.

UV-curable glassy material for the manufacture of bulk and nano-structured elements

“…Typical layer thickness was 5 µm. Such bonding was stable under thermal cycling in the range -40˚C up to 120˚C (no reduction in transparency or adhesive strength) [26].…”

“…The resulting material possesses the following properties: free of cracks and shrinkage, ability to control its refractive index, high thermal stability, high laser damage threshold: 5 KW/cm 2 (CW), 2.5 GW/cm 2 (pulse), strong optical bonding (above 10 MPa), and low optical loss (less than 0.01 dB/cm in the visible range). A summary of these properties is presented in Table 1 below [23]- [26].…”

“…For patterned relief, coins were used for demonstrating macro structures, while for the smaller scales micron-scale photo-resist molds were prepared by photo-lithography in a mask-aligner using metal masks. As example, we fabricated a stripe pattern structure on a thin FSG film on silicon substrate [26]. For sub-micron molds we used patterned gold mold prepared by focused ion beam (FIB) or electron beam lithography (EBL).…”

“…The UV-curing process was also tested for bonding applications (replacing the thermal curing in the bonding procedure reported in reference [26]). Laser damage threshold was measured on FSG disc sample using a 532 nm Nd:YAG laser (Surelite, Continuum) with 7 ns pulses focused to 15 µm diameter as irradiation source.…”

“…These materials are prepared by the fast solgel (FSG) process which leads to solidification without shrinkage or formation of cracks [24]. Such materials with organic residues of less than 30% [25] present high thermal stability, high optical quality and high adhesive strength and may be also used as optical adhesives [26]. The FSG material was demonstrated to have only 4% weight loss at 400˚C, due to continued inorganic cross-linking [26].…”