2.1

Visible/Near-IR antireflection coating for normal incidence

The first antireflection coating that has been studied is an antireflection optimized for the spectral band between 400 and 1100 nm operating at normal incidence. For this study, we considered Nb₂O₅ as the high refractive index material and SiO₂ as the low refractive index material and fused silica as the substrate. The minimum number of layers required to achieve such performances is equal to ~10. With such a stack the average reflection coefficient is equal to ~1%. Increasing the number of layers of the stack allows slightly decreasing the residual reflection to a level of ~0.8%. Such a residual reflection coefficient is the lowest value that can be achieved with a multilayer structure (see first and second rules of thumb). In this paper, we considered a formula with 20 alternated high and low refractive index layers (Table 1).

Table 1.

Layers thicknesses of visible/Near-IR antireflection coating for zero incidence.

It can be seen that the formula has no periodicity as well as large variations in thickness (30× change). There are also very thin layers: the first layer has a thickness of 5 nm and 8 layers have thicknesses lower than 20 nm. Finally, such a structure is very sensitive to an error on the thickness of each layer. This means that fabricating such structures requires a very good control of the thickness of each of the layers.

The technology chosen to fabricate this structure was PARMS (Plasma Assisted Reactive Magnetron Sputtering) technology. The depositions were therefore carried out using the Bühler HELIOS machine within the Espace Photonique platform of Institut Fresnel. The control of the thickness of each of the layers was carried out using a Bühler OMS 5000 optical monitoring system interfaced with the HELIOS machine. Two different optical monitoring strategies were considered: one with a single test glass and one with multiple test glasses. Indeed, the HELIOS machine is equipped with a load-lock system that allows changing the test glass during the fabrication in between the deposition of two layers. To begin with, we considered the use of a single test glass to monitor the entire stack. Using a dedicated software for the determination of optimal optical monitoring strategy combined with a Virtual Deposition Process software, the following optical monitoring strategy was determined:

Given the complexity of stack, the determination of an optimal strategy remains complex and no all-optical strategy (i.e. with no time monitored layers) could be found. Such a prototype of antireflection coating was fabricated on the HELIOS machine and the transmission spectrum was measured using a Perkin Elmer Lambda 1050 spectrophotometer (Figure 1).

Fig. 1.

Spectral transmission of visible/near-IR antireflection coating for normal incidence monitored with a single test glass.

We can first see that the proposed monitoring strategy allowed to precisely control the thickness of each layer. The overall performances are close to the theoretical one. However, it can be seen that there are some small disagreements on the short wavelength range, which are most certainly due to thickness errors in the last layers of the time-controlled stack. We therefore wondered whether it would be possible to further minimize production errors. In order to be able to generate an all-optical monitoring strategy, we opted for the use of two test glasses. A new optical monitoring strategy was therefore created:

We then fabricated a new prototype on the HELIOS machine using this new strategy. The transmission spectrum was then measured on this new component (Figure 2). It can be seen that using this method, we have been able to fabricate a visible/near-IR antireflection coating for normal incidence with very low manufacturing errors, thus making it possible to minimize the Fresnel reflection of a fused silica substrate with performances close to those predicted by the theory.

Fig. 2.

Spectral transmission of visible/near-IR antireflection coating for normal incidence monitored with two test glasses.

To further characterize the performances of the antireflection coatings and also test the repeatability of the manufacturing process, a new deposition run was performed (Run 2). Two types of samples were generated: a new single side antireflection coated sample and dual side antireflection coated samples (Run 1 + 2). The performances of the new single side antireflection coating were characterized and compared with the one of the first coating run (Figure 3).

Fig. 3.

Spectral transmission of visible/near-IR antireflection coating for normal incidence monitored with two test glasses. (Run 1 and 2).

It can be seen that the stability of the deposition rates of the HELIOS machine combined with the efficient developed optical monitoring strategy make it possible to guarantee optimal repeatability from one deposition to another. Finally, we measured the spectral response of the dual side component (Figure 4). It can be seen that the developed multilayer structures can guarantee an average reflectivity of the order of 98% with fluctuation not exceeding 1% over the spectral range 400-1100 nm.

Fig. 4.

Spectral transmission of dual side visible/near-IR antireflection coating for normal incidence monitored with two test glasses (depositions 1 and 2).

2.2

Visible/Near-IR antireflection coating for oblique incidence

The second antireflection coating structure that has been studied was optimized for the spectral band between 400 and 900 nm and allows minimizing the residual reflection at oblique incidence between 0 and 45°. In this particular case, the structure was optimized in order to minimize the residual reflection in S polarization. For this study, we again considered Nb₂O₅ as the high refractive index material and SiO₂ as the low refractive index material and fused silica as the substrate. The designed structure is given in Table 2. It is composed with 34 layers with a wide range of thicknesses. In this case, it is not possible to design more simple structures in order to achieve high performances. This can be easily explained by the fact that when the angle of incidence is increased, there is a blue shift of the whole spectrum that needs to be compensated by increasing the spectral range of the designed antireflection structure.

Table 2.

Layers thicknesses of visible/Near-IR antireflection coating for oblique incidence.

Based on the results shown in Section 2.1 and in order to further minimize the errors of realization, we opted for an all optical monitoring strategy based on the use of three consecutive test glasses. A first test glass is used to monitor the first 12 layers, a second glass for the next 12 and a third glass for the last 10. We then determined the following monitoring strategy:

This strategy was then validated by performing simulations using Virtual Deposition Process software. We then made the deposition of the antireflection structure using the Bühler HELIOS machine. The transmission spectrum was finally measured at normal incidence (Figure 5).

Fig. 5.

Spectral response in transmission at normal incidence of a visible/near-IR antireflection coating optimized for S polarization in oblique incidence, deposited on one side, and optically monitored using 3 test glasses.

The transmission spectra were also measured at a 45° angle of incidence for the S and P polarizations (Figure 6). It should be noted that at a 45° angle of incidence, the reflection on the uncoated rear face induces a sharp decrease in the transmission of the order of 8% in S polarization while this contribution is negligible in polarization P (0.6%).

Fig. 6.

Spectral response in transmission at a 45° angle of incidence of a visible/near-IR antireflection coating optimized for S polarization in oblique incidence, deposited on one side, and optically monitored using 3 test glasses.

We see that with this optical monitoring method, we have been able to minimize the errors on the layer thicknesses during the deposition of the coating, which guarantees a very good agreement between theory and experimental data.

We also repeated the manufacturing and monitoring process in order to fabricate dual-side coated samples. Figure 7 compares the transmission responses measured at normal incidence of the two coating runs. It can again be seen that the stability of the deposition rates of the HELIOS machine, combined with the optical monitoring process repeatability, ensures optimum repeatability from one deposition to another.

Fig. 7.

Spectral response in transmission at normal incidence of visible/near-IR antireflection coating for oblique incidence monitored with three test glasses (Run 1 and 2).

Finally, we measured the spectral response of the dual-side coated substrate (Figure 8). It can be seen that with these structures, it is possible to guarantee an average reflectivity of the order of 97% with fluctuations lower than 1.5% over the spectral range 400-900 nm. Transmission fluctuations a little larger than those predicted by the theory can be observed. This effect is probably due to a higher sensitivity of the stack, at oblique incidence, on deposition errors than at normal incidence.

Fig. 8.

Spectral response in transmission at a 45° angle of incidence of visible/near-IR antireflection coating for oblique incidence monitored with three test glasses (Run 1 and 2).

2.3

Near-IR/Mid-IR antireflection coating for normal incidence

The last antireflection coating structure that has been studied was optimized for the spectral band between 1.5 and 15μm and allows minimizing the residual reflection at oblique normal incidence. In this case, the chosen materials were no longer oxide materials but ZnS as the high refractive index material and YF3 as the low refractive index material. The structure was optimized to minimize Fresnel reflection of a ZnSe substrate. The designed structure is given in Table 3. It is composed with 14 layers with thicknesses from 30 to 593 nm and a total thickness of 2.9 μm.

Table 3.

Layers thicknesses of Near-IR/Mid-IR antireflection coating for normal incidence.

The technology chosen to fabricate this structure is PIAD (Plasma Ion Assisted Deposition) technology. The depositions were therefore carried out using the Bühler SYRUSpro 710 machine within the Espace Photonique platform of Institut Fresnel. The control of the thickness of each of the layers was carried out using a Bühler OMS 5000 optical monitoring system interfaced with the SYRUSpro machine. Using a dedicated software for the determination of optimal optical monitoring strategy combined with a Virtual Deposition Process software, the following optical monitoring strategy was determined:

We then made the deposition of the antireflection structure on a ZnSe substrate using the Bühler SYRUSpro 710 machine. The transmission spectrum was finally measured at normal incidence using a Fourier Transform Infrared Spectrometer (Figure 9).

Fig. 9.

Spectral response in transmission at normal incidence of a near-IR/mid-IR antireflection coating optimized normal incidence, deposited on one side of a ZnSe substrate.

One can see that using this strategy, we have been able to fabricate a broadband antireflection coating covering the whole 1.5-15 μm spectral range. There is a very good agreement between theory and experiment. It is also worth noting that the dispersion models that we used account for water absorption in near-IR ranges and also for long wavelength absorption, explaining the decrease of transmission in this wavelength range, not because of the design but because of the material properties. However, with such a structure, the average residual reflection on the coated surface is equal to about 3% and up to 5% at 15 μm. Using only a multilmayer structure, it is not possible to achieve lower residual reflection coefficient using these materials. In order to achieve further decrease of the residual reflection coeffient, it becomes mandatory to generate a gradient of refractive index at the boundary between the last layer of the stack and air. This can be achieved by structuring the last layer of the stack and adapting the design of the stack. Such activities are under development.