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Achieving higher optical power in UV laser systems is a challenging task due to the limited performance of their built-in optical elements. As a rule of thumb, interference coatings of such elements are found to be the weakest links by the means of laser-induced damage threshold (LIDT). The optical resistance is directly attributed to the fundamental absorption properties of deposited layers. Unfortunately, there are only a limited set of available materials with discrete refractive indices that are also compatible with UV applications. In this study, an attempt is made to employ sculptured layers in order to produce durable anti-reflective (AR) coatings by using the so-called glancing angle deposition (GLAD) method. Spectral, structural, mechanical and stress properties of GLAD coatings were investigated in detail. AR coatings produced by GLAD were found to be three times more laser damage resistant at 355 nm wavelength as compared to those prepared by ion beam sputtering (IBS).
© 2017 Optical Society of America
1. Introduction
Anti-reflection coatings have a long history of technological development [1]. Conventional designs of anti-reflection coatings contain at least two materials with different refractive indices. The suppressed reflection of light is obtained by using destructive interference. Such coatings are manufactured by combining at least two layers, which construct the electromagnetic wave having the same intensity but the opposite phase as the one reflecting from the substrate-air interface. An alternative approach is to use single layer coating produced out of the material with well-matched refractive index equal to square root of substrates refractive index [2,3]. The choice of such discrete refractive indices is limited in nature by the availability of transparent pure materials. To match the desired refractive index, an effective medium [4] concept can be employed. Porous anti-reflection coatings were prepared chemically and deposited via Sol-gel technology, that could be optimized to reach refractive index of anti-reflection film [5]. Recently, another similar approach has been reported that is based on the direct chemical etching of the glass substrate and thus also producing porous effective medium layer [6–8]. In this case, the gradient of refractive index is achieved. Such microstructures also suppress the reflection in a wide wavelength range [9]. Surprisingly, both approaches sharing porosity feature also share good performance of laser irradiation. Porous coatings are known to feature low effective refractive index. This is in agreement with recent findings of [10] where high LIDT is directly correlated with low refractive index and vice versa. Unfortunately, latter two methods have technological limitation. When more complicated spectral performance is needed, it is hard to produce more than one porous layer with distinct refractive indices. Physical vapor deposition or highly energetic sputtering methods do not have such limitations. Furthermore, standard coatings have intrinsic tensions as the by-product of the growth process [11], which could lead to cracking. High power laser systems often use crystal, small dimension and very thin optics. Ability to manufacture multilayer anti-reflection coatings with low stress levels would be beneficial to them. This motivated us to create a low-stress multilayer AR coating that could also benefit from the low refractive index in porosity layers leading to higher optical resistance.
For our study, we employed widely studied artificial materials, namely sculptured thin films, produced by Glancing Angle Deposition (GLAD) method. Well-controlled porosity allows achieving of the desired refractive index of any material in the range between the air and material indices. In this manner, high refractive index materials (Al2O3, HfO2, ZrO2 and etc.) can be avoided in designing anti-reflection coatings. Several attempts to produce anti-reflection coatings were already made by using GLAD method. Optical coatings, produced with GLAD, exhibited high optical performance as broadband [12, 13] and broad-angle [14] anti-reflection coatings. The reader can find the other extensive measurements of optical parameters being demonstrated in the literature during last two decades [15,16], whereas to our best knowledge the mechanical and optical resistance properties of such coatings were not investigated in detail. In this work, we address this gap and study GLAD anti-reflection coatings by the means of their optical, mechanical, structural and optical resistance properties.
The paper is organized as follows, first of all, experimental methods are described. Next, single layer GLAD coatings were characterized in order to find optimal conditions for deposition of more complex designs. Finally, several multilayer GLAD AR coatings were produced with tailored optical properties. The properties of such experimental coatings were directly compared with those prepared by state of the art Ion Beam Sputtering (IBS) technology.
2. Methods
2.1. Sample preparation
Three series of experimental samples were fabricated in order to analyse the SiO2 thin films produced by GLAD method. Information about all samples is presented in Table 1. An electron beam evaporation apparatus (“Sidrabe”, Latvia) was used to deposit all layers by GLAD method. For the first series the thin films of 300 nm thickness were produced to model their refractive indices by evaporating silica at angles of 0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80° and 85°. The value of the angle was considered between vapour flux and substrates normal. Fused silica substrates were continuously rotated around substrate holder axis at 0.5 Hz to avoid birefringence effect when light is incident parallel to substrates normal. Every evaporation of SiO2 material in GLAD method was done in room temperature. The chamber was pumped with mechanical and turbomolecular pumps and resulted in base pressure of 1.7×10−3 Pa. The distance between evaporation source and the substrates was 30 cm. For the second series additional evaporations were made at angles of 0°, 30°, 50°, 70° and 85° for SEM, LIDT and stress analysis. The thickness of experimental samples in this series were maintained around 1 μm.
Table 1. Information about the samples.
Using the modelled refractive indices from single layer thin films of 300 nm thickness, the anti-reflection coatings were designed and fabricated in the third series of experimental samples. The multilayer coatings were devised to gain low (<0.5%) reflection for the wavelengths of 1064 nm, 532 nm and 355 nm. Such anti-reflection coatings are usually used to coat various crystals for the generation of third harmonic combining the first and second. In this experiment fused silica (FS) substrates with typical surface roughness of 0.5 nm were used for all anti-reflection coatings. The great care was intended for substrate surface preparation before evaporation process. First step was the precise mechanical cleaning with ethanol. Then ultrasonic cleaning in an aqueous surfactant with commercial alkaline cleaning solution was used. Afterwards, the substrate was rinsed twice in distilled warm water and dried with IR heater. Two designs were fabricated using GLAD method and evaporating solely SiO2 material. Samples were named GLAD1 and GLAD2 and differ in thickness of the consisting layers. Additional process was proceeded and named IBS1 using ion-beam sputtering (IBS) technology by sputtering Al and Si targets with additional oxygen flow. Al2O3 was picked because of its high refractive index and low absorption in UV region. GLAD1 sample contained three pairs of layers with high and low refractive index thin films with thickness of 18 nm and 27 nm, 47 nm and 55 nm, 23 nm and 148 nm deposited at the angles of 0° and 85°, respectively. GLAD2 sample contained also six layers with thickness 17 nm and 24 nm, 56 nm and 48 nm, 27 nm and 145 nm deposited at the angles of 30° and 85°, respectively. During the deposition of each film in GLAD1 and GLAD2 coatings, the substrate was rotated at 0.5 Hz around its axis. For each layer the films growth rate was maintained at 3 Å/s controlling it with quartz crystal monitor. IBS1 sample contained four pairs of Al2O3 and SiO2 as high and low refractive index materials with thickness of 102 nm and 63 nm, 26 nm and 158 nm, 170 nm and 40 nm, 38 nm and 86 nm, respectively. Designs were chosen as typical coatings for suppression of reflected light to values lower than 0.5 % at the wavelengths of 355 nm, 532 nm and 1064 nm. All anti-reflection coatings were deposited on one side of the substrates.
2.2. LIDT measurements
Two setups were used for the measurement of laser induced damage threshold (LIDT) in ns regime for the wavelength of 355 nm. Both were operated according to 1-on-1 method described in ISO standard (ISO 21254:2011 [17]) with micro-focusing approach.
In the basic measurement system (B-LIDT) the third harmonics (pulse duration 3.1 ns) of pulsed (frequency of 15 Hz) nanosecond Nd:YAG laser (model Ekspla) was used for the comparative measurements. The energy of Gaussian beam shaped laser pulse was controlled by attenuator. In order to avoid damaging backward surface of samples due to laser beam self-focusing, short distance of f = 20 cm focusing lense was chosen for sample LIDT testing. Focused beam diameter at the target plane was evaluated using knife-edge scanning method and was around 60 μm. Scattered light was projected through ∼0.5 mm aperture to photodetector using ∼40 mm diameter of f = 150 mm lens. Additionally, 10 mm diameter metal circle was placed in the path of direct laser beam in front of the lens to avoid direct laser light detection. In order to obtain better precision of measured data, after laser exposition the small laser pulse energy was used to additionally scan the same area in a few hundred places. This method allowed us to achieve automated LIDT registration (damaged or not damaged). The results of automated evaluation of damaged areas and those tested with microscope fully coincided. This method considerably increased measurements speed and enabled us to avoid possible mistakes. Using B-LIDT setup, we were interested in comparable LIDT results only, therefore real energy fluencies were not calculated.
For absolute numbers of LIDT, multilayer anti-reflection coatings were measured using calibrated setup (C-LIDT). This system used Nd:YAG InnoLas Laser (λ=355 nm, pulse duration 6.8 ns, spatial beam profile - Gaussian, beam diameter in target plane (1/e2): 160.4 ± 6.3 μm, single longitudinal mode). The main elements, operational principle of the system and statistical 1-on-1 testing were described in recent bibliographic reference [18].
3. Results and discussion
Refractive indexes were modeled using spectrophotometric transmission data measurements of 300 nm thickness single layers and fitting them with commercial software “OptiLayer”. Deposition at 0° angle results in layer properties close to the fused silica substrate, since no shadowing effect occurs during the film growth process. The decrease of index is observed when deposition angle is changed from 0° to 85° as shown in Fig. 1(a). The decrease is because of low energy process and adatom mobility. Self-shadowing effect dominates when atoms are impinging the substrate at increased angle of deposition [15]. The refractive index diminishes from 1.45 to 1.13 for the wavelength of 532 nm, the latter being deposited at the angle of 85°. Similar behaviour was also observed by W. J. Kong et al. [12].
Figure 1 (a) Modelled dispersions of refractive indices and (b) calculated porosity of GLAD SiO2 thin films deposited at various angles. (c) SiO2 thin films porosity for different refractive indexes at 355 wavelength according to Bruggeman model [19].
In order to determine the porosity of experimental thin films, calculations were performed using modelled refractive indices. The increased self-shadowing effect in GLAD technique resulted in increased porosity of deposited thin films as seen from Fig. 1(b). In order to determine the volumetric fraction of films material within the layer, the dielectric constants of air, experimental sample and silica film from IBS process were used in Bruggeman model [19]. The thin film, deposited at normal angle of incidence, has 11% of voids within. Referring to previously reported [15] findings, porosity is most likely due to combination of shadowing effect and low energy adatoms during the evaporation (no high energy ion- or plasma-assistance is included in the process). Increasing deposition angle by ten degrees, the porosity remains the same, but by changing the angle further to 50° it grows to 24%. The increase is more effective when changing the angle from 60° to 85°. The porosity reaches the value of 73% at such oblique angle. The change in inclination angle of the curve is due to the same domination of the self-shadowing effect at oblique angles of deposition. A linear dependence of effective index of refraction as a function of porosity at the wavelength of 355 nm is presented in Fig. 1(c). Bruggeman approach typically corresponds to the “homogeneous mixture” situation [20]. The features of columnar thin films are much smaller than visible wavelength. Therefore, an assumption of isotropic mixture was used. For anisotropic thin films with larger features, anisotropic Bruggeman approach should be considered [21].
Fabricated anti-reflection coatings of three designs were measured by spectrophotometer in reflection and transmission mode. The measured spectra of transmission and reflection are presented in Fig. 2. As seen from the images, the reflection of the multilayer coating, deposited by IBS technology, does not exceed 3.8% value for the wavelengths of 355 nm, 532 nm and 1064 nm. When neglecting the substrates backside reflection, the value of remaining anti-reflection coating does not exceed 0.3%. The reflection losses of both GLAD1 and GLAD2 samples are similar and do not exceed 0.2% for all three wavelengths when the substrates backside reflection is also neglected. Broadband anti-reflection coatings were also produced by Kennedy group in University of Alberta [13]. Transmission values higher than 99.5% were obtained in spectral range from 500 nm to 1000 nm. Unfortunately, UV spectral region had light transmission lower than 99%. The authors highlight that better optical performance is viable using such sculptured thin films with better optical monitoring during the process, but it was not the goal of this work.
Figure 2 (a) Transmittance and (b) reflectance of experimental samples: IBS1 (red curve), GLAD1 (blue curve), GLAD2 (cyan curve) and fused silica substrate (black curve). Multilayer anti-reflection coatings were deposited only on one side of the substrate.
Fig. 3(a) presents stress measurements, obtained by Fizeau interferometer, revealed the evolution of tension in nano-sculptured coatings deposited by various angles. The compressive stress was found for the thin film deposited at 0° angle and was measured to be −30 MPa after the coating process. By increasing the angle of deposition, the intrinsic tension undergoes from compressive to tensile and reaches the maximum value of 50 MPa when deposited at 50° angle. Increasing the deposition angle results in decreased stress values, which diminishes at the angle of 85°. The time related tensions were measured only at one more point two weeks after the deposition. The values of stress changed to more tensile for the denser films. The change was negligible for porous coatings, deposited at 70° and 85° angles. Non-linear stress behaviour of columnar thin films of SiO2 can be related to hydroxyl covering when exposed to environment [22]. Water-induced stresses change in time [23] and can have larger influence to more densely packed columns in the film. At oblique deposition angles competition between separate columns is negligible due to bigger distances between them. It allows for thin film to grow with low intrinsic stress. The overall stress of anti-reflection coatings was measured to be five times lower for GLAD process than produced by IBS technology. Sputtering process results in densely packed thin films with high tension values, which can be eliminated only by various post-deposition treatments [24]. The GLAD1 and GLAD2 coatings mainly consist of porous thin films with low stress values, which result in overall low stress anti-reflection coatings.
Figure 3 Measured stress values of (a) single layer thin films deposited at various angles and (b) multilayer anti-reflection coatings.
The results of laser damage threshold measurements are presented in Fig. 4. From (a) part of the figure the increasing LIDT values by changing the deposition angle from 0° to 85° can be observed. The increase in fluency threshold is most likely related to increased porosity of thin films deposited by GLAD process. In pulsed nanosecond regime thermal effects occur in material. Usually thermally heated materials have a tendency to expand, which is the cause of a thermoelastic breakdown in dielectric layers [25]. Porous structures can withstand more heating because of voids in the thin films, which allows the coating to expand without a breakdown. In other case, non-linear effects such as multiphoton absorption may also be the cause of laser induced damage. Materials with larger bandgap value are more resistant to non-linear effects [26]. Nano-sculptured thin films of SiO2 material are mixed with voids and have smaller effective refractive index (and thus possibly higher effective bandgap value). Such outcome is also possible explanation of increased LIDT values. The LIDT values of normally and at 85° angle deposited thin films differ four times. The laser damage values of coated anti-reflection coatings are presented in Fig. 4(b). Both GLAD1 and GLAD2 coatings have similar LIDT values equal to 16.8 J/cm2 and 15.3 J/cm2 respectively and the multilayer, produced by IBS technology, has three times lower laser resistance equal to 5.1 J/cm2. Higher LIDT values were obtained by Hobbs and MacLeod [9] when they etched FS substrates to suppress the reflection from substrate surface. The values around 20 J/cm2 were measured for the wavelength of 355 nm in nanosecond regime. Higher values could be measured because of different laser parameters. Damage probability curves of anti-reflection coatings are presented in Fig. 5. A sudden transition is observed for both coatings, indicating distinct threshold.
Figure 4 LIDT measurement results for (a) single layer thin films deposited at various angles and (b) multilayer anti-reflection coatings.
One of the methods for obtaining higher LIDT values is the optimization of electric field within the multilayer coating [27]. It allows to shift electric field peaks, denoted by |E|2, to layers with higher laser resistance. During the optimization of layers thickness in experimental samples, E-field was not managed to reach any goals. This can be observed in Fig. 6, where distribution of |E|2 is presented across coating thickness. Peaks appear in both coatings at the layers with relatively high refractive index. Such result shows that higher damage thresholds can be obtained only by manipulating the properties of materials and not its distribution of electric field in multilayer. It simplifies the optimization process and at the same time encourages to extend future work in optimising E-field in nano-sculptured coatings for even better results. Such optimisation would be beneficial in optics for ultrashort pulses where nonlinear effects are more dominant.
Figure 6 Modelled electric field for anti-reflection coating produced by (a) IBS technology and (b) GLAD process (GLAD1 sample). Blue line represents dense Al2O3 material, red line — dense SiO2, pale red line — porous SiO2.
SEM imaging was used for analysis of inner structure of nano-sculptured thin films. Figure 7 shows the columnar growth which is visible in all experimental samples when observing layer surface. The cross-section image of GLAD1 sample indicates the porosity of coating. Unfortunately, distinct layers are hard to confirm, because thickness of dense layers are very thin (from 18 nm to 47 nm). Evidence of competitive growth is apparent in structures corresponding to low-energy evaporation. This effect together with self-shadowing dominates during thin film growth in GLAD process. It is also observed when material is deposited at larger angles. The radius of the cone tops in thin film deposited at 0° angle is 8 nm. It increases to 9.5 nm at 30° deposition angle, 14 nm at 50°, 27 nm at 70° and 109 nm at 85°. The diameters of columns increase more than ten times when deposition angle is changed from 30° to 85°. Larger gaps between columns are forming due to their expansion. Such observation explains a sudden increase in porosity and decrease in refractive index at 85° angle. Analysis of cross-sections of thin film deposited at 70° indicates that characteristic feature sizes of the columns remain constant during growth. Different results were observed in thin film, deposited at 85° angle. At the beginning of growth, the diameter is small and equal to 43 nm. During the evolution, the diameter extends to 83 nm in the middle of the 1 μm thin film. Near the top of the layer, the columns width is 114 nm, which is three times larger than at the beginning. Columnar growth evolution of SiO2 material at the deposition angle of 87° was observed also by Vick and co-authors from University of Alberta [28]. A close look at SEM images in the paper allows to confirm that diameter of individual columns expand to more than 120 nm when film thickness is larger than 420 nm. The increasing column diameter when film thickness is increasing is also observed in this paper. Experimental anti-reflection coatings contain layers with thickness lower than 150 nm. Columnar growth can be considered continuous within such layer. Since features of individual layers are smaller than 100 nm, it is worth mentioning, that laser-induced breakdown threshold should not be lowered due to the structure of thin films [29].
Figure 7 SEM measurements of GLAD1 coating (cross-section) and thin films deposited at (from left to right) 0°, 30°, 50°, 70°, 70° (cross-section), 85° and 85° (cross-section).
4. Conclusions
Single layers of SiO2 material produced by GLAD technology exhibit different stress values at different deposition angles. Measured values undergo the transition from tensile to compressive tension when changing the angle from 0° to 50°. Residual stress values changes over time: after fourteen days all tensions of thin films are compressive. The minimal stress values and changes during time are observed for thin films deposited at oblique angles. Porous SiO2 single layers, produced by GLAD method, also experience the porosity change from 11%, when deposited at normal angle, to 73%, when deposited at 85°. An increase of porosity results in enhancement of LIDT values in nano-sculptured thin films.
Anti-reflection coatings fabricated by GLAD process showed superior stress and LIDT performance when compared with coatings made by IBS technology. Because of high porosity both GLAD coatings have 50 times lower stress values than IBS analogous coating. The induced porosity also enabled nano-sculptured anti-reflection coatings to withstand three times higher laser radiation fluence.
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