What Are Thin Film Filters?

Filters are optical devices used to select the required radiation band. A common feature of filters is that no filter can make the imaging of celestial bodies brighter, because all filters will absorb certain wavelengths, making objects darker.

Its main feature is that the size can be made quite large. Thin film filters generally have longer wavelengths and are more commonly used as infrared filters. The latter is on a certain substrate, using

A variety of interference filters with arbitrary wavelengths from ultraviolet to infrared with of 1 to 500 angstroms. Peak of metal-dielectric film filter

Filter products are mainly classified according to their spectral bands, spectral characteristics, film materials, and application characteristics.

Spectral band : UV filter, visible filter, infrared filter;

Spectral characteristics : bandpass filter,

TiO ₂ and SiO ₂ thin film systems evaporated by electron beam (EB) have important applications. However, with conventional evaporation technology, even if the temperature of the substrate is as high as 300 ° C or more, the film still exhibits obvious columnar structural characteristics. Since the film of this columnar structure contains a large number of voids, as the film filter absorbs moisture, the refractive index of the film layer increases, and the center wavelength of the filter will have a significant drift. In order to characterize this structural characteristic, the aggregation density P has been proposed, which is defined as the ratio of the volume of the solid portion in the film to the total volume. So it is a physical quantity describing the degree of film looseness.

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, and even experimental reports have been reported. Some The aggregate density of the film is greater than 1. This means that the density of the film is higher than the density of bulk materials in nature, because high-density films often show larger compressive stress, which results in films with higher aggregate density. However, even if the film's aggregate density is greater than 1, the center wavelength of the filter will still drift. It has been recognized that it is not only the aggregation density that affects the center wavelength shift of the thin film filter, but also the temperature refractive index and thermal expansion coefficient of the thin film and the substrate. Therefore, the center wavelength drift of the filter can be simply expressed as = drift caused by moisture absorption in the film gap + drift caused by temperature refractive index change + drift caused by thermal expansion.

Obviously, when the ion density is used to increase the aggregation density to 1, the center wavelength drift caused by moisture absorption is negligible, and the rise of the other two factors is the main factor. This article starts from the general process, and focuses on the relationship between the optical stability of the three-cavity filter composed of TiO ₂ / SiO ₂ and the above three factors. The experimental results show that in the visible light region, for the film system with an aggregation density of about 0.92, the central wavelength caused by moisture absorption is the largest among these three factors, and the order of magnitude is about 10 nm. For the glued film system, the short-wavelength shift of the central wavelength caused by the decrease of the refractive index of water vapor in the film space with the increase of temperature is in the order of 1 × 10 ^-2 nm / . The thermal expansion is about 1 × 10 ^-3 nm / .

Moisture-induced drift

Because the film is a columnar structure, there are gaps between the columnar structures.

Table 1 Calculated values of the center wavelength shift caused by the moisture absorption effect of materials with different aggregation densities

The refractive index of the internal air is 1. After the moisture absorption, the void is filled with water vapor, and the refractive index becomes 1.333. Therefore, the refractive index of the film layer, and thus the optical thickness and spectral characteristics, are changed.

Substitute the film structure (HLH2LHLHL) ³ and the corresponding refractive index we prepared, and according to our process conditions, the aggregation density of TiO ₂ and SiO ₂ is about 0.92, so for red, green, and blue with different center wavelengths Filter, can calculate the center wavelength drift caused by the corresponding moisture absorption. In the case of f = 1 (that is, full moisture absorption), a series of center wavelength drifts calculated for different aggregation densities of TiO ₂ and SiO ₂ are shown in Table 1.

It can be seen from the table that the aggregation density of the low-refractive-index material SiO ₂ plays a major role in shifting the central wavelength in the case of moisture absorption. The difference in center wavelength drift caused by the high-refractive-index material's aggregation density is only about 1 nm, while the low-refractive-index material has a change of about 3 nm. The reason is that after the low refractive index material absorbs moisture, the ratio of the refractive index rise to the original refractive index is high, which is equivalent to a large increase in the optical thickness, resulting in large drift. More importantly, SiO ₂ is used as the spacer layer of the film system, and the influence of the spacer layer on the center wavelength shift is the largest.

In summary, the theory that the central wavelength is short-shifted can be explained by the theory that the water vapor that originally occupied the voids in the film was evaporated due to the temperature increase, which can better explain our experimental data, and from this we can deduce the aggregation density of our prepared SiO ₂ It is between 0.92 and 0.95. The theoretical analysis is consistent with the analysis of process conditions.

Temperature-induced drift

In addition to the center wavelength shift caused by moisture absorption,

Table 2 Temperature coefficient of refractive index of quartz crystal

Changes, as well as changes in thickness due to thermal expansion of the film system, also cause changes in the optical thickness of the film layer, which causes the center wavelength to drift. Not only that, because the thermal expansion coefficient of the substrate is different from the thermal expansion coefficient of the film system, under the condition of heat, the film system will be elastically deformed by the substrate stress, which will cause the aggregation density to change and the center wavelength to shift. The theory can be used to quantitatively analyze the center wavelength drift caused by temperature rise. The main factors are the refractive index temperature coefficient of the material, the linear thermal expansion coefficient of the substrate, the Poisson's ratio of the material, the linear thermal expansion coefficient of the film system, the film

Table 3 Variation of refractive index of water at different temperatures with wavelength

The layer's aggregation density and so on. Data on the refractive index changes of various materials with temperature is very scarce, especially data on thin film morphological materials. According to literature reports, the refractive index temperature of different materials varies greatly, such as telluride showing negative values, and generally The refractive index of a material increases with increasing temperature. In our film system, since SiO _{2 is} used as the spacer layer, the temperature coefficient of the refractive index of SiO ₂ plays a major role. The literature has the refractive indices of o- and e-light in the visible range of crystalline quartz, see Table 2. There is also a refractive index temperature coefficient of fused silica in the infrared, which is about + 1.1 × 10 ^-5 / ° C at 1550 nm, but it is difficult to find data in the visible light region. Based on the above data, we can infer that the refractive index temperature coefficient of the SiO ₂ film in the visible region is approximately + 0.5 × 10 ^-5 / ° C. The thermal expansion coefficient of the substrate is 74 × 10 ^-7 / ° C for K9 glass in the range of -30 to 70 ° C, and 86 × 10 ^-7 / ° C in the range of 100 to 300 ° C. The thermal expansion coefficient of the membrane system is about 5.5 × 10 ^-7 / , and the Poisson's ratio is 0.1.

According to the above theoretical analysis and parameter setting, the temperature drift of the center wavelength of the green filter is -0.00088 nm / ° C below 70 ° C, and the temperature drift of the center wavelength is -0.001459 nm / ° C above 100 ° C For different color filters, the values are slightly different, but the order of magnitude is -1 × 10 ^-3 nm / , and a temperature change of 10 will only cause a drift of the order of -10 ^-2 nm. The observed drift is on the order of 1 nm for both monolithic and glued samples, so the results of the above calculations are not a major factor.

For the two-piece glued sample, when the aggregate density is not equal to 1, the voids are mostly filled by water vapor. After glueing, these water molecules still exist and cannot be evaporated to leave the film. According to the literature, the temperature change of the refractive index of water is relatively large compared to the film material, the specific data is shown in Table 3. Its order of magnitude is 10 ^-4 / , which is an order of magnitude higher than that of SiO ₂ , and as the temperature rises, the refractive index decreases faster. For the aggregation density of 0.9, the effect of the temperature coefficient of the refractive index of the water molecule is comparable to or even greater than that of the film material.

From the table, we can see that the refractive index of water has decreased from about 20 ° C to 80 ° C by about 0.01. According to the aggregation density of 0.9, the refractive index temperature coefficient of the coating layer is caused by the decrease in the refractive index of water in the coating layer. ^-5 / , it can be seen that it can completely offset the rise of the refractive index of SiO ₂ with temperature, so that the entire film system exhibits a negative refractive index temperature coefficient. At this time, the refractive index of the film system becomes -1.5 × 10 ^-5 nm / The temperature drift from room temperature to 70 ° C is -0.6 nm, which is in the same order of magnitude as the experimental result of 0 to -2 nm. For the cases above 70 ° C, there is no data on the refractive index change of water, but considering that water gradually changes from liquid to gaseous state after 100 ° C, the refractive index will decrease faster, so from this perspective, the center of the cemented filter can be reasonably explained. Short shift of wavelength with temperature.

We believe that for unbonded monolithic filters, the voids in the thin film columnar structure are almost completely filled with water molecules at room temperature. When the temperature rises to 70 ° C, about 80% to 90% of the water molecules in the columnar structure are The film is removed by evaporation, and at 70 ° C to 120 ° C, about 10 to 20% of the remaining water molecules are also evaporated out of the film. This results in a center wavelength drift between 70 ° C and 120 ° C. The value of this drift in the experimental data is between 1 and 2.5 nm, which is indeed about 1/5 of the drift value from room temperature to 70 ° C. The experiment also reflects that the drift from 100 ° C to 120 ° C is less than the drift in the range of 70 ° C to 100 ° C, which is also in line with our analysis.

Analysis conclusion

Through experiments on the center wavelength drift of the three bandpass filters of red, green, and blue under the influence of temperature, we analyze the causes of this drift. Three of these factors play a role. For non-glued filters, the decrease in refractive index caused by the water molecules that were originally filled in the voids of the columnar structure of the film as the temperature rises is the main factor, which causes a short shift in the center wavelength. This short shift varies with the aggregation density of the film. For films with an aggregate density of 0.92, the values of short shifts are on the order of 10 nm. This process of desorption of moisture is most obvious in the range of room temperature to 70 ° C. 80% to 90% of the water is evaporated, and above 70 ° C, the remaining 10% to 20% of the water is also evaporated. For glued filters, the reason for the short shift of the central wavelength is that the refractive index of water vapor filling the film gap decreases with increasing temperature, and the rate of this decline is much greater than the refractive index of the thin film material with increasing temperature and thermal expansion caused by geometric thickness. Increasing speed, thus causing a decrease in optical thickness and a short shift in the center wavelength. The magnitude of this short shift is approximately -1 × 10 ^-2 nm / ° C. Finally, for a film system with a high concentration density, the refractive index temperature coefficient of the material and the thermal expansion coefficient of the substrate are important factors that determine the center wavelength drift. By calculation, for the range of visible light, the magnitude of this drift is around 1 × 10 ^-3 nm / ° C, and the direction is determined by the thermal expansion coefficient of the substrate.

Based on the above analysis, measures to improve the temperature stability of the membrane system can be formulated. First, increasing the aggregation density of the membrane system is one of the most important means. Increasing the aggregation density reduces the effect of moisture absorption, which is the factor that has the greatest effect on stability. It is also a good measure to glue the film between the glass substrates, which can reduce the drift to the order of ^10-2 nm / ° C. In addition to increasing the film's aggregation density, choosing a material with a small refractive index temperature coefficient or materials with opposite refractive index temperature coefficients to prepare the film system, and also selecting a substrate with an appropriate thermal expansion coefficient are also measures. This is especially important when the density is close to one. ^[4]