How substrate temperature and oxygen flow rate affect surface quality and ring structure of HfO₂ film on SiO₂ substrate

Designing low-absorption HfO₂ optical thin-film components (OTCs) on fused silica substrates (FSS) for high-power laser systems remains challenging. Electron beam evaporation (EBE) can introduce micro-defects in the HfO₂ films and simultaneously alter the amorphous network of FSS, compromising optical performance. This work investigates how substrate temperature (T) and oxygen flow rate (Q) control the relationship among process, structure and properties of OTCs, and how these relationships can be utilized to guide OTC design. Reactive EBE is conducted under controlled T (23 ∼ 100 °C) and Q (140 ∼ 190 sccm). Film surface quality is quantified by AFM roughness (Sa), while the nanoscale FSS network is assessed by Raman spectroscopy. The resulting structure changes are validated at the property level using optical constants and weak absorption (β) at the wavelength of 1064 nm. The novelty lies in a systematic cross-scale correlation that explicitly links film-induced stress and morphology with the evolution of the substrate ring structure during EBE. This aspect has not been widely explored for HfO₂/fused-silica OTCs. Increasing T reduces the average compressive stress from 112 to 95 MPa, lowers Sa by 24.2%, decreases the Raman intensity by 21.6%, and reduces β by 41.2% from 6.92 to 4.07 ppm. Increasing Q further stabilizes hydroxyl-related structures and suppresses unfavorable ring conversion, leading to additional reductions of 11.8% in Sa , 11.1% in Raman intensity, and 29.2% in β . These results provide a critical process-parameter design guideline for co-optimizing film morphology, substrate network structure, and low- β performance in high-reliability OTCs.