Spectral and Laser-Induced Damage Characteristics of Atomic Layer Deposited SiO2 Films on Fused Silica Glass
CHENG Hai-peng1,2, GENG Feng2, LIU Min-cai2, ZHANG Qing-hua2, LI Ya-guo1*
1. Fine Optical Engineering Research Center, Chengdu 610041, China
2. Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
Abstract：In this paper, single-layer SiO2 films of homogeneous material were deposited on the surface of fused silica glasses by atomic layer deposition (ALD) technology. The physical and chemical properties of the optical films and the laser induced damage performance under laser irradiation were deeply researched. Bis-tert-butylaminosilane (BTBAS) and ozone (O3) were chosen as reaction precursors in the experiment, and ALD prepared a series of film samples under different temperature conditions. Firstly, a study on the characteristics of ALD and the uniformity of the films was carried out. It was found that the film growth thickness and the number of deposition cycles conformed to the linear growth, which verified the atomic layer-by-layer growth characteristics of the ALD. The uniformity of the deposited film on the surface is fine, while the error does not exceed 2%. Then, for the SiO2 films deposited at different temperatures, the roughness and various spectral characteristics have been tested. The comparison results show that the surface roughness of the sample is slightly decreased after coating. The ALD film samples have excellent transmittance in the range of 200 to 1 000 nm, both exceeding 90% and gradually approaching 93.3%, and their transmission spectrum is not significantly different from the spectrum measured on a bare fused silica substrate. The difference between fluorescence spectrum and Fourier transform infrared (FTIR) spectrum before and after coating confirms the existence of point defects (non-bridging oxygen, oxygen vacancies, hydroxyl, etc.) in the SiO2 films deposited by ALD, which will affect the film damage resistance performance. Finally, ultraviolet laser-induced damage tests were performed on the substrate and film samples. Results of damage performance show that the laser induced damage threshold of the thin film deposited on the surface is reduced, and the zero-probability damage threshold is decreased from 31.8 J·cm-2 to about 20 J·cm-2, which is consistent with the characterization of spectral defects. The point defect in the film will absorb ultraviolet laser energy, causing the local temperature to rise, and then the threshold of laser damage resistance is reduced while the phenomenon of laser-induced damage occurs. Within the selected deposition temperature range, SiO2 films deposited at higher temperature seem to have better damage performance. The deposition temperature conditions can be controlled to make the samples’ damage performance closer to the substrate itself. It is expected that the optimization of other reaction parameters will further improve the film damage performance.
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