With the rapid development of ultraviolet optics and ultraviolet technology, ultraviolet monochromator as an important tool in the development of ultraviolet technology provides strong support for related technological innovation. As an important optical component in the ultraviolet monochromator, the ultraviolet filter performance seriously affects the test accuracy of the monochromator, and its optical performance mainly includes transmittance, cut-off depth, and steepness of the transition zone, among which the influence of steepness is particularly important. In recent years, in-depth research has been carried out on the preparation of high-performance ultraviolet filter films at home and abroad, most of which focus on center transmittance, cut-off depth, and cut-off band width. Meanwhile, although the optical performance has been improved to a certain extent, in the transition zone there are still some stray rays that have not been effectively filtered out. To this end, based on the utilization requirements of the ultraviolet monochromator, a deep ultraviolet high-steepness filter film is developed to filter the interference of incoherent light and improve the measurement accuracy of the monochromator.
By analyzing material properties and studying the thin film design theory, Al2O3 and AlF3 are selected as high and low refractive index materials respectively, and a vacuum ultraviolet wide cut-off, deep ultraviolet to visible high transmission filter film is designed on the fused silica substrate by double-sided split design method. During thin film preparation, the control variable method is adopted to optimize the preparation and thin film stress analysis, and the optimal deposition process parameters are selected, which solves the problem of thin film cracking caused by excessive stress of the prepared high-steep filter film. Additionally, the monitoring error of film thickness is inverted and analyzed via repeated experiments, and the proportion coefficient of film thickness is corrected to realize the accurate monitoring of film thickness and improve the steepness of the transition zone.
Due to the large number of layers of high-steepness filter films, the accumulated stress is too large, which seriously affects the mechanical properties of the film and causes film cracking. To solve this problem, we discuss the influence of different deposition process parameters on the film quality (Table 2) and demonstrate the surface shape changes of the substrate before and after coating in Table 3. Meanwhile, the power value of the coating surface becomes larger after coating, and the residual stress of the film is expressed as tensile stress on the fused silica substrate, which is calculated by the simplified Stoney formula (Fig. 12). The process adjustment before and after increasing ion-assisted deposition energy can effectively reduce the thin film stress. Additionally, observations on the cracking degree of the film under different ion source parameters (Fig. 13) show that when the ion source power is increased to 200 V/2 A, the crack fringes of the film disappear completely, with significantly improved film properties. After the thin film deposition, the test spectral curve (Fig. 14) deviates greatly from the theoretical design spectrum. The inversion analysis reveals that this difference is mainly due to the quenching effect during the deposition of the thin film, which leads to the thin thickness of 33-54 layers, and the error inversion spectral curve is shown in Fig. 15. After repeated experiments to correct the crystallized thickness of the layer, the test spectral curve is in good agreement with the design spectral curve (Fig. 16).
Based on the theory of thin film structure design, the reasonable selection of thin film structure parameters is realized, and the film layer thickness is appropriately adjusted and optimized, which effectively reduces film preparation difficulty. By employing the control variable method, the influence of ion-assisted deposition energy on the thin film quality is emphatically discussed. When the ion source power increases from 0 V/0 A to 200 V/2 A, the film stress decreases from 219 MPa to 178 MPa, and the problem of film cracking is effectively solved. After the thin film deposition is completed, the transmission spectrum curve is inverted and analyzed, and the proportion coefficient of the film thickness is adjusted to achieve accurate film thickness control. Finally, the deep ultraviolet high-steepness filter film has a transmittance of 3.05% at 227.7 nm, a transmittance of 89.91% at 231.3 nm, a steepness of 3.6 nm, and an average transmittance of 97.67% and 0.61% at 232-400 nm and 115-228 nm respectively, which meets the needs of the ultraviolet monochromator.
