By WDQ OPTICS | 22 April 2021 | 0 Comments
Development Of Thin Film Filters
Development Of Thin Film Filters
TiO2 and SiO2 thin film systems evaporated by electron beam (EB) have important applications. However, with conventional evaporation technology, even if the substrate temperature is more than 300 ℃, the film still shows obvious columnar structure characteristics. Because the film layer of this columnar structure contains a large number of voids, with the moisture absorption of the film filter, the refractive index of the film increases and the central wavelength of the filter will drift obviously. In order to characterize this structural characteristic, the aggregation density p is proposed, which is defined as the ratio of the volume of the solid part of the film to the total volume. Therefore, it is a physical quantity to describe the degree of film porosity.
With the development of ion plating technology, such as ion-assisted deposition (IAD), reactive ion plating (RIP), and ion beam sputtering (IBS), the aggregation density of films has been significantly improved. Even experiments have reported that the aggregation density of some films is greater than 1. This means that the density of films is higher than that of bulk materials in nature. The reason is that films with high aggregation density often show large compressive stress, resulting in higher aggregation density. However, even if the aggregation density of the film is greater than 1, the central wavelength of the filter will still drift. It has been recognized that not only the aggregation density but also the temperature refractive index and thermal expansion coefficient of the film and substrate affect the central wavelength drift of the thin film filter. Therefore, the central wavelength shift of the filter can be simply expressed as Δλ= Drift caused by moisture absorption in film gap + drift caused by temperature refractive index change + drift caused by thermal expansion.
Obviously, when the aggregation density is increased to 1 by ion technology, the central wavelength shift caused by moisture absorption can be ignored, while the other two factors rise to be the main factors. Starting from the general process, this paper focuses on the relationship between the optical stability of the three cavity filters composed of TiO2 / SiO2 and the above three factors. The experimental results show that in the visible region, for the film system with an aggregation density of about 0.92, the central wave growth caused by moisture absorption is about 10 nm. For the glued film system, the refractive index of water vapor in the gap of the film system decreases with the increase of temperature, resulting in a short shift of the central wavelength of about 1 × 10-2nm / ℃. The drift caused by thermal expansion is about 1 × 10-3nm / ℃.
TiO2 and SiO2 thin film systems evaporated by electron beam (EB) have important applications. However, with conventional evaporation technology, even if the substrate temperature is more than 300 ℃, the film still shows obvious columnar structure characteristics. Because the film layer of this columnar structure contains a large number of voids, with the moisture absorption of the film filter, the refractive index of the film increases and the central wavelength of the filter will drift obviously. In order to characterize this structural characteristic, the aggregation density p is proposed, which is defined as the ratio of the volume of the solid part of the film to the total volume. Therefore, it is a physical quantity to describe the degree of film porosity.
With the development of ion plating technology, such as ion-assisted deposition (IAD), reactive ion plating (RIP), and ion beam sputtering (IBS), the aggregation density of films has been significantly improved. Even experiments have reported that the aggregation density of some films is greater than 1. This means that the density of films is higher than that of bulk materials in nature. The reason is that films with high aggregation density often show large compressive stress, resulting in higher aggregation density. However, even if the aggregation density of the film is greater than 1, the central wavelength of the filter will still drift. It has been recognized that not only the aggregation density but also the temperature refractive index and thermal expansion coefficient of the film and substrate affect the central wavelength drift of the thin film filter. Therefore, the central wavelength shift of the filter can be simply expressed as Δλ= Drift caused by moisture absorption in film gap + drift caused by temperature refractive index change + drift caused by thermal expansion.
Obviously, when the aggregation density is increased to 1 by ion technology, the central wavelength shift caused by moisture absorption can be ignored, while the other two factors rise to be the main factors. Starting from the general process, this paper focuses on the relationship between the optical stability of the three cavity filters composed of TiO2 / SiO2 and the above three factors. The experimental results show that in the visible region, for the film system with an aggregation density of about 0.92, the central wave growth caused by moisture absorption is about 10 nm. For the glued film system, the refractive index of water vapor in the gap of the film system decreases with the increase of temperature, resulting in a short shift of the central wavelength of about 1 × 10-2nm / ℃. The drift caused by thermal expansion is about 1 × 10-3nm / ℃.
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