Glass direct bonding

Gerhard Kalkowski, Carolin Rothhardt, Ramona Eberhardt, Paul-Johannes Jobst and Mark Schürmann

Fraunhofer Institut für Angewandte Optik und Feinmechanik, Albert-Einstein-Str. 7,

07745 Jena (Germany)

ABSTRACT We report on applications of the direct wafer bonding technology to join glass substrates for use as optical devices in high power laser applications. Coated and uncoated planar glass substrates were bonded to each other by plasma-activated hydrophilic “direct” bonding and –for comparison– sodium-silicate solution bonding, also known as “hydroxide-catalysis” bonding. Both methods are expected to generate covalent Si-O-Si bonds through water condensation and extraction from the bonding area. Important benefits are the high stiffness and temperature stability of these bonds. Fused silica substrates of 25 mm diameter and up to 10 mm thickness were used, with flatness better than λ/5 peak-to-valley (λ=633 nm) and roughness better than 0.6 nm root-mean-square. Oxide films coatings up to 150 mm thickness were generated by reactive dual-magnetron sputtering from various targets (Si, Ti, Nb, Al etc.). Direct bonding was performed at temperatures of about 250 °C in vacuum under compressive pressures in the MPa range (see Fig. 1). Sodium silicate solution bonding was performed by joining under very low compressive pressure (10-2 MPa), extended room temperature drying and finally curing at temperatures >100 °C. Optical reflection and transmission data in the spectral range of 200…1200 nm were obtained and absorption was determined from the difference to 100 %. The optical data (see Fig. 2) show, that absorption of uncoated samples is negligible for direct bonding, while sodium-silicate solution bonding shows a marked increase in absorption at UV wavelengths below about 400 nm. For the coated samples, direct bonding generally resulted in visibly clear “defect free” bonds, while sodium-silicate solution bonding of the same coatings showed visible defects (spots and stain) in several cases. Bonding strength was determined from 3-point bending tests on rectangular beams of about 5 mm*5 mm cross-section, cut out of the bonded sample pairs by diamond sawing. For both bonding methods, the mean breaking stresses for uncoated bonded samples amounts to >50 % of bulk fused silica prepared the same way. For coated samples, preliminary (uncompleted) results reveal a markedly reduced bonding strength (see Fig. 3). Currently, laser damage threshold tests on uncoated samples at a wavelength of about 1064 nm are prepared. Keywords: glass direct bonding, hydrophilic bonding, silicate-solution bonding, optical transmission

Fig.1: Directly bonded glass substrates (fused silica) with various thin film coatings (from left to right): none, TiO2, Nb2O5, Al2O3, ITO.

Fig.2: Spectral reflection and transmission (measured) and absorption data (calculated, 10-fold enhanced) for uncoated fused silica samples. Note difference between Direct-Bond (pink) versus Silicate-Bond (red) values.

Fig.3: Breaking stresses as determined from 3-pt-bending tests for uncoated and coated samples after bonding. Note low values for directly bonded coated samples.

Abstract #2982, Honolulu PRiME 2012, © 2012 The Electrochemical Society

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