The morphologies of the micro-defects are observed using focused ion beam scanning electron microscopy (FIB-SEM). The feature points are cut with an ion beam to obtain the cross-sectional morphology and structure, and the element composition in the designated area is analyzed using an energy dispersive spectrometer (EDS). Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) images of the micro-defects are obtained using a 200 keV field emission transmission electron microscope (TEM). Three-dimensional reconstruction is conducted to analyze the submicron defects. Samples are prepared for the TEM observations using FIB-SEM. The LIDT is tested by 1-on-1. A 2ω Nd:YAG laser with a pulse width of 8 ns and a 3ω Nd:YAG laser with a pulse width of 8 ns are used for the 532 nm and 355 nm LIDT measurements, respectively. The effective beam sizes on the sample surface for the 532 nm and 355 nm LIDT measurements are approximately 0.32 mm2 and 0.30 mm2, respectively. Ten sites are tested for each energy step.
Results and Discussions This study predominantly analyzes the micro-defects with the micron and sub-micron (hundred nanometers) scale. The morphologies and structures of the micro-defects produced by coating (Figs.2 and 3) and substrate polishing (Figs.6-8) are characterized by means of precise cutting and three-dimensional reconstruction, and the element distributions before and after laser irradiation are analyzed using EDS analysis.The results demonstrate that the seeds of submicron nodule defects arise from the ejection of SiO2 during the deposition process (Figs.4 and 5), whereby the seed diameter is 100 nm. The vacuum pumping and heating during the coating process cause the Na and K ions in the substrate to diffuse and concentrate in layers with a high refractive index (Fig.5). Further, the seeds of micron-scale nodule defects originate from the ejection of SiO2 or HfO2 during the deposition process, whereby damaged pits are formed after the nodule defect is irradiated by laser, the edge of pits becomes molten, and the HfO2 layer near the edge of the damaged pits has a prominent porous columnar structure, of which the atomic fraction ratio of O and Hf is less than 2:1 (Fig.9). Impurity defects in the substrate produce plasma after being irradiated by laser, and the eruption temperature of the plasma is higher than the gasification temperature of HfO2, which causes the gasified HfO2 to enter the substrate crack (Fig.10).
Thin-film components of laser systems require excellent optical performance and a high laser-induced damage threshold (LIDT). The micro-defects in the components (such as coating material ejection defects and substrate defects) are the critical cause for the reduced LIDT. To control micro-defects, we must first detect the defects and trace the formation process of micro-defects. In this study, the morphologies and structures of the micro-defects produced by coating and substrate polishing are characterized by precise cutting and three-dimensional reconstruction. Additionally, the element distributions before and after laser irradiation are analyzed. The results clarify the direction of the increasing LIDT of thin-film components.
High-reflection films at 355 nm and 532 nm are deposited on BK7 substrates using the electron beam evaporation. The BK7 substrates are also used for surface topography measurements and additional measurements. Before deposition, the coating chamber is heated to 473 K and evacuated to a base pressure of 1×10-3 Pa. The deposition rates of HfO2 and SiO2 in the multilayer coatings are 0.1 nm/s and 0.2 nm/s, respectively. H and L represent the HfO2 and SiO2 layers with a quarter-wavelength optical thickness (QWOT) at 355 nm or 532 nm, respectively (Fig.1).
The results provide detailed data and evidence for improving coating and substrate polishing processes. The analysis results show that to improve the LIDT, the SiO2 pre-melting and deposition process should be further optimized to avoid 100-nm SiO2 defects, high-purity quartz substrates should be used to avoid the diffusion of metal ions in substrates, and ion beam polishing technology can be utilized to remove the substrate surface defects caused by traditional polishing processes.
