| dc.description.abstract | Direct glass-to-glass bonding is important for high-technology components in optics, microfluidics, and microelectromechanical systems applications. The focus of this research is on the design of an ultrafast, pulsed, microchip laser, specifically focusing on the microchip component. For this component, the proposed material is optically coated on both sides, allowing the light from a diode laser to be changed within it. This material is bonded to a saturable absorber, and sectioned into smaller units to accommodate size and mass production requirements.
I studied a number of different materials for this application, including 1 mm x 25 mm x 75 mm soda-lime float glass substrates and 8 mm diameter x 5 mm cylindrical fused silica/yittrium aluminum garnet (YAG) substrates, both optically coated and uncoated. The bonding process used is based on the classic RCA-1 cleaning procedure from the semiconductor industry modified with an ammonium hydroxide rinse, followed by a thermal treatment under unidirectional pressure without the need for a dedicated drying step. RCA-1 uses a solution of ammonium hydroxide and hydrogen peroxide to clean contaminants off the surface of silicon and enable subsequent bonding.
Bond quality was evaluated using two primary methods: destructive shear testing and laser induced damage (LID) testing through irradiation of the substrates by a continuous wave (CW) fiber laser and ultrafast, pulsed, femtosecond (FS) laser. Destructive shear testing of soda-lime glass samples revealed strong bonds (≈7.81 MPa on average) were achieved using a unidirectional pressure of approximately 0.88 MPa and low bonding temperatures between 160 °C and 300 °C applied for 30 min. Similar results were observed in uncoated fused silica samples.
The optical robustness of the bonds was tested in soda-lime glass and shown to be capable of surviving high powered CW fiber laser irradiation of at least 375 W focused for 2 s without delamination. Melting of the substrate was observed at higher powers and longer exposure times. Evolution of laser damage seemed to appear only in conjunction with melting of the bulk material, suggesting that optical robustness of the bonds is similar, if not greater than the bulk material.
When soda-lime glass samples were exposed to FS laser irradiation, the behaviour of a directly bonded sample was very similar to the bulk material. On the other hand, comparing a directly bonded sample to a poorly bonded sample showed very different damage evolution. Uncoated fused silica samples demonstrated this property as well, with greater damage appearing in poorly bonded samples versus directly bonded samples.
Surface roughness and chemistry was also characterized before and after cleaning, as these are influential factors affecting the efficacy of direct bonding. Surface roughness was measured using optical interferometry and atomic force microscopy (AFM). The results show that the surface roughness is comparable to measurements made with more complicated cleaning methods, demonstrating the advantage of our simplified method. The advantage of the ammonium bath as opposed to a rinse with deionized (DI) water is proposed in the surface chemistry analysis through X-ray photoelectron spectroscopy (XPS), which showed nitrogen incorporation at the bonding interface. This offers more opportunity for direct bonding. SEM was used to visually observe the quality of the bonding interface. These images revealed a relatively high quality bonding interface. A cutting process was also developed to test the feasibility of mass production.
These results highlight the simplicity, optimization, and overall strength of the bonds created using the proposed bonding process, showing that it is an excellent option for the creation of ultrafast, pulsed, microchip lasers. More information is also given regarding surface roughness ranges needed for direct bonding, as well as the mechanism of action for bonding using the RCA-1 chemical process. This will be useful in future optimization efforts for different materials. | en |
