We fabricated stochastic antireflective structures (ARS) and analyzed their stability against high power laser irradiation and high temperature annealing. For 8 ps pulse duration and 1030 nm wavelength we experimentally determined their laser induced damage threshold to 4.9 (±0.3) J/cm2, which is nearly as high as bulk fused silica with 5.6 (±0.3) J/cm2. A commercial layer stack reached 2.0 (±0.2) J/cm2. An annealing process removed adsorbed organics, as shown by XPS measurements, and significantly increased the transmission of the ARS. Because of their monolithic build the ARS endure such high temperature treatments. For more sensitive samples an UV irradiation proved to be capable. It decreased the absorbed light and reinforced the transmission.
© 2012 OSA
Since Bernhard and Miller  found very small periodic structures on the compound eye of moths and thus discovered the antireflective behavior of these so-called motheye structures, many have theoretically analyzed [2–7] and attempted to synthesize those subwavelength nanostructures. These structures show some appreciable properties. Other than interference coatings they have a broad spectral and angular action band that depends on the lateral structure size and the depth of the structures. Due to their monolithic build they have a high damage threshold and temperature stability, as we will show in this paper. First, deterministically ordered structures were fabricated by interference lithography , and later electron beam lithography  was used. In addition to lithographic techniques methods were found that could create hexagonally or stochastically ordered antireflective structures (ARS) by using self-organization processes [10–15]. Many different optical materials have been treated by such processes, such as fused silica, glasses, silicon, and plastics. Often nanoparticles were used which were deposited on the surface and acted as an etching mask. In our approach we developed a bottom-up technique that renders the use of nanoparticles or any other masking step unnecessary [15, 16]. Using the so-called self-masking effect the mask is created out of the etching chemistry itself. Thus, in one single process step completely stochastic nanostructures arise that show considerable antireflective properties. By varying our etching parameters we are able to fabricate stochastic ARS in fused silica for a wide spectral range of light to increase the transmission in the IR, VIS, and UV range of light significantly (see Fig. 1). We recently showed that these stochastic structures can be produced on strongly curved fused silica surfaces such as lenses  and micro lense arrays . Another important point of research now is the stability and reliability of such stochastic ARS. We will show that due to their monolithic build ARS show some remarkable advantages in high power stability. In addition, we will present annealing and UV irradiation as two simple methods to remove atmospheric adsorptions and renew the optical performance of the ARS.
2. Removing adsorptions by annealing and UV irradiation
Especially for the DUV range it is known that atmospheric adsorptions can decrease the transmission of optical glasses which is due to the absorption of electromagnetic radiation by hydrocarbons. This problem exists for nearly every optical surface used in regular atmosphere. In a first measurement we used a spectrometer (Perkin Elmer® Lambda 900) to investigate the transmission behavior of a fused silica surface with one day-old ARS. We determined a significantly increased transmission in the UV range caused by the ARS (Fig. 2). Now we stored the sample for 8 weeks at a regular atmosphere and repeated the measurement. We found a broad spectral decrease in transmission (of about 1% at 200 nm wavelength) while the reflection at the surface had not changed. Since we do not expect the scattering proportion to change, this loss must have occured due to increased absorptions on the surface. To remove the absorbing hydrocarbons we annealed the sample at 500°C for one hour. Since the ARS are of monolithic build and the glass transition temperature of fused silica is about 1200°C one can easily use high temperature annealing without deforming or destroying the nanostructures. The annealed sample was measured again and showed a significantly increased transmission of about 2% (at 200 nm wavelength), which is even higher than the initial value.
Since we used a plasma etching process to fabricate the ARS we assumed that some polymers generated by the etching chemistry remained on the surface. Next to atmospheric adsorptions, those polymers would decrease the transmission value of a very new sample as well. To detect such polymers on the surface we performed XPS measurements (X-ray photoelectron spectroscopy). The information depth of the measurement is about 2 nm to 6 nm. As Fig. 3 shows we found fluorine contaminations which derived from the fluorine gas compound during the etching. In addition, some CF-CO bindings were found on a high binding energy of C1s. But regarding the Auger C signal at the kinetic energy of 270 eV the C-F contamination was not very high, about one or two monolayers. After the annealing the CF-CO binding signal vanished as well as the fluorine peak. Some weak carbon signal remained due to residual atmospheric contaminations. This information combined with the transmission measurements showed that contaminations from the etching process and atmospheric hydrocarbon adsorptions weaken the transmission behavior of ARS. An annealing process can easily remove those adsorbates and renew the transmission enhancement of the ARS.
As a second way to remove adsorptions we used ultraviolet radiation. First, we fabricated ARS on one side of the fused silica sample and stored it in the clean room at regular atmosphere. After 24 hours and 8 days, respectively, we measured the transmission and reflection at 193 nm wavelength and then performed a third measurement after the so-called UV burning. Therefore, the samples were placed in a UV cure chamber (Loctite® UVALOC, using a mercury vapor lamp) and irradiated with UV-A light (315 nm – 400 nm) for 12 min. These measurements were done for a plain fused silica sample as well. As shown in Fig. 4(a) the ARS increased the transmission by about 3.2% for a single surface. The older the samples, the more the transmission decreased for both the plain substrate and the sample with ARS. To show that the adsorbates were successfully removed, we assume thatFigure 4(b) illustrates that the ARS show a higher absorption than the plain surface. We presume that this is due to the increased surface of the ARS which allows more hydrocarbons to be adsorbed. Through showing that the sum of R + T increases we proved that UV burning can easily remove adsorbed organics, comparable to the tempering process. It is a fast and simple technique for conditioning ARS samples and thus, should be the preferred cleaning approach for temperature sensitive setups.
3. Damage threshold of stochastic antireflective structures
The damage threshold of optical elements used in high power laser systems is of special interest. Today, even ultra short pulsed lasers with pulse durations down to a few femtoseconds are state of the art. Here, already relatively low pulse energies result in very high pulse peak powers and intensities. Since coated antireflective dielectric layer stacks are damage sensitive to such high power densities and because their performance is often limiting high power laser applications [18, 19], another suitable antireflection technology for those surfaces has to be found. As we will show ARS can provide such high power stable antireflective surfaces and ensure durable antireflective properties. We fabricated stochastic ARS and measured their laser induced damage threshold (LIDT). According to the ISO standard 11254-2  the LIDT is the power density right before modification or damage of the sample by the laser radiation or the pulse, respectively. Here the probability of damage is 0% (LIDT0%, see also Fig. 5(a). Our measurement setup was based on the ISO standard 11254-2 and the measurements followed the S on 1 method, at which S was 10,000 pulses, i.e. 10,000 pulses incident on one spot. We used a commercial ultra short pulse laser (Trumpf TruMicro 5050) with 1030 nm wavelength and 8 ps pulse duration as well as a spatio-temporal gaussian beam profile. The laser was operated at a repetition rate of 2 kHz. Typically, 600 to 760 sample spots were tested for every measurement with at least 15 different energy densities. The sample’s damage was detected by measuring the increase of the scattered light and was cross-checked by Nomarski microscopy.
We determined the damage threshold of plain fused silica wafers (Schott Lithosil®) and our stochastic ARS, fabricated on the same type of wafers. The incidence angle was 0° and the 1/exp(2) beam diameter measured 28 μm. For the bulk fused silica we found the LIDT at 6.8 (±0.5) J/cm2 (600 sample spots), which is consistent with previously published results [21, 22]. For the ARS we measured a LIDT of 6.2 (±0.5) J/cm2, where 700 sample spots were analyzed. The survival curve for that measurement for 10,000 pulses is shown in Fig. 5(a). Here, the ARS were on the front side of the wafer, facing the laser system. For a setup where the ARS were on the back side of the wafer we determined the LIDT at 6.3 (±0.6) J/cm2 with 700 evaluated sample spots as well. During these experiments we noticed in some places damages inside the bulk material of the plain wafers and wafers with ARS. To prevent a self-focusing effect  we repeated the measurements and used thinner silica wafers (0.5 mm instead of 1.0 mm) and increased the beam diameter to 55 μm. We measured 5.6 (±0.3) J/cm2 as the LIDT for plain fused silica and 4.9 (±0.3) J/cm2 for front sided ARS each with 640 sample spots. For ARS on the back side of the wafers we found the LIDT to be 5.5 (±0.3) J/cm2 (760 sample spots). Table 1 shows the measured LIDT. The slight decrease of the LIDT compared to the previous results are caused by the increased beam diameter. Larger beam cross-sections involve larger irradiated areas on the surface. Thus, the probability of damage inducing defects rises and the sample is already being destroyed at lower fluences [24, 25].
Next to the beam diameter the number of pulses imposed on every sample spot affects the LIDT. Due to the accumulating damage behavior of the material the LIDT is higher for small number of pulses and converges towards a minimal LIDT for high number of pulses [26, 20]. We determined the LIDT of the ARS for several pulse numbers, as shown in Fig. 5(b). It is noticeable that the LIDT for pulse numbers above 1,000 pulses is nearly constant. Thus, the LIDT we measured at 10,000 pulses will be nearly the same for infinite number of pulses.
To compare our ARS not just to plain fused silica we performed further measurements with antireflective dielectric layers. We used a conventional antireflective layer stack designed for 1030 nm wavelength at 41° incidence angle and built of Ta2O5 and SiO2 layers deposited on fused silica (P18, Tafelmeier GmbH). For 0° incidence angle we found the LIDT at 2.0 (±0.2) J/cm2 (640 sample spots), which is less than half of the ARS value. Since the layer stack was designed for 41° incidence we repeated our measurement for 41° incidence angle and determined the LIDT at 1.7 (±0.2) J/cm2 (TE polarization, 640 sample spots).
These results show that our stochastic ARS in fused silica are nearly as stable as the unstructured plain surface and are much more resistant to high power densities than dielectric layer stacks.
In summary, we analyzed the damage threshold of stochastic antireflective nanostructures (ARS) against high power laser pulses and the possibility to eliminate surface adsorbates on those nanostructures by temperature treatment and UV irradiation. Regarding the former, we found the laser induced damage threshold LIDT0% of ARS to be nearly as high as for plain bulk fused silica with 4.9 (±0.3) J/cm2 and 5.6 (±0.3) J/cm2, respectively. A standard antireflective layer stack reached only 2.0 (±0.2) J/cm2. Additionally, we could confirm the assumption that higher number of pulses decrease the LIDT. Regarding the elimination of adsorbates we could significantly increase the transmission in the UV range by annealing the ARS and removing the light absorbing organics without damaging the nanostructures. For more temperature sensitive surfaces we found UV burning to be an equally suitable procedure to remove those adsorbates and to increase the transmission of the ARS. At 193 nm wavelength our ARS showed a transmission increase of 3.2% per interface compared to plain fused silica. The theoretical maximum value for a perfect single antireflective surface would be 4.7% at that wavelength. By improving our manufacturing process we expect to be able to advance our ARS and get even closer to that theoretical value.
The authors thank Dr. Bernd Schröter for the XPS measurements and acknowledge financial support from the Federal Ministry of Education and Research (BMBF), projects ”OncoOptics” (grant number 03ZIK455) and ”OpMiSen” (grant number 16SV5577), as well as the German Research Foundation (DFG, Leibniz program).
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