Enhanced UV-assisted quartz wafer bonding: unveiling the critical role of humidity gradient in surface hydroxylation and interfacial strength

Relative humidity (RH) is a critical but frequently overlooked variable that significantly impacts the reliability of ultraviolet (UV)-activated quartz-crystal wafer bonding. This study examined how relative humidity during UV activation influences hydroxyl- and water-related surface chemistry and measured interfacial strength. Exposing quartz-crystal wafers to combined 185/254 nm UV irradiation under controlled relative humidity (15–90%) revealed a non-monotonic humidity dependence of hydroxyl- and water-related spectroscopic responses, with the largest response at 45% RH. The mean measured tensile strength increased from 2.64 MPa at 15% RH to a maximum of 9.88 MPa at 45% RH. It then decreased with further increasing humidity, reaching 2.01 MPa at 90% RH. This decline is consistent with excess adsorbed water hindering intimate interfacial contact and subsequent silanol condensation. Cross-sectional TEM and EDS compared local interfaces formed at 30%, 45%, 60% and 75% RH, revealing differences in interfacial contrast and Si/O signal profiles. These findings identify a practical humidity-control window for improving the reproducibility and strength of atmospheric-pressure UV-assisted quartz-crystal wafer bonding, supporting its development for wafer-level packaging of quartz crystal resonators.