Glass Direct Bonding for Optical Applications

The direct wafer bonding technology is applied to join glass substrates for optical devices in high power laser applications. Uncoated as well as coated fused silica substrates were bonded to each other by hydrophilic direct bonding and -for comparison- sodium silicate-solution bonding. Both technologies are expected to generate materials-adapted Si-O-Si bonds at uncoated interfaces. Optical transmission and reflection in the spectral range of 200 nm to 1200 nm were measured and reveal superior transmission for the direct bonding technology in the ultra-violet range. Even in the near infrared at 980 nm, better performance with direct bonding as compared to silicate-solution bonding is evidenced by laser induced damage threshold measurements. For all coated samples, a distinct reduction in bonding strength relative to uncoated ones is observed in 3-point bending tests.