Relaxation of Thermal Stress in Direct Bonding by Partitioning the Bonding Area for Fabrication of Optical Isolator with Semiconductor Guiding Layer

The improvement of a direct bonding technique is important in realizing a waveguide optical isolator with a III–V compound semiconductor guiding layer. By applying a surface-activated bonding technique, a magneto-optic garnet, CeY 2 Fe 5 O 12 , was successfully bonded on GaInAsP at an elevated temperature of 450 °C with a large bonding area. However, a marked increase in propagation loss was observed in the waveguide after the bonding process owing to cracks formed by the difference in thermal expansion between garnets and compound semiconductors. To solve this problem, we demonstrate the effectiveness of partitioning the bonding area for reducing the thermal stress.