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Hafnium dioxide films deposited using electron-beam evaporation tend to exhibit high tensile stresses, particularly when deposited on low-thermal-expansion substrates for use in a low-relative-humidity environment. Hafnia has been shown to be a critical material, however, for use in high-peak-power laser coatings, providing exceptional deposition control and laser-damage resistance. To correct for tensile thin-film stresses in hafnia/silica multilayer coatings, alumina compensation layers were incorporated in the multilayer design. Determination of the stresses resulting from alumina layers in different coating designs has led to the realization of the influence of water diffusion and the diffusion-barrier properties of alumina that must be considered. The inclusion of alumina layers in a hafnia/silica multilayer provides the ability to produce low-compressive-stress, high-laser-damage-threshold coatings.
©2012 Optical Society of America
1. Introduction
Optical coatings are a critical technology for successful construction and operation of high-peak-power laser systems. Optical coatings must provide appropriate spectral and photometric performance while maintaining high laser-damage resistance at the wavelength(s) and pulse duration(s) of interest. Additionally, the surface figure of the optics to be coated must be maintained quite accurately to preserve the optical performance of the system. Thin-film stresses from the optical coating process, both compressive and tensile, pose a risk to the performance and longevity of the coated components.
Electron-beam deposition of optical coatings has been the standard process for fabricating multilayer coatings for high-peak-power–laser applications [1–3]. The ability to scale the process to large substrates, flexibility in source materials and coating designs, and relatively low cost encourage the selection of this deposition process. Ultimately, however, the determination to date that such coatings produce the highest laser-damage thresholds has led to the use of electron-beam evaporation as the deposition process for large, high-peak-power–laser components for systems such as OMEGA, OMEGA EP, the National Ignition Facility (NIF), Laser Megajoule, and others [1–8].
Multilayer coatings consisting of hafnium dioxide and silicon dioxide have been the standard choice for applications at both 1053 nm and 351 nm for these laser systems [1–6]. These materials provide good spectral and uniformity control while maintaining high laser-damage thresholds. Hafnia/silica multilayers exhibit high tensile stresses, however, particularly on low-thermal-expansion substrates in low-relative-humidity environments, sufficient to provide significant substrate deformation and cracking of the coated surface [2,9]. These stresses result from many sources, including differences in coefficients of thermal expansion of the substrate and film materials, intrinsic stresses related to the deposited film structure, and other contributions related to growth and interactions of thin-film materials [10]. Stresses may be irreversible, based on the film structure, materials, and method of deposition, or reversible, such as stress resulting from moisture content in the porous structure of an evaporated thin film [10]. By matching the temperature and humidity of the measurement and use environment of a coating, the resulting film stress from all sources may be evaluated and efforts may be undertaken to adjust it as needed. The use of evaporated coatings in pulse-compressed laser systems, which operate in a vacuum environment, necessitates coating stresses that are neutral or slightly compressive, in order to avoid tensile failure in the form of cracking, or crazing, of the film. Modification of the electron-beam–deposition process for hafnia/silica coatings has been explored elsewhere, through both evaporation parameters and energetic assistance, in order to modify the overall film stress [9,11,12]. In this work, the use of aluminum oxide is investigated as a means of adjusting the stress in multilayer reflective coatings to achieve a more neutral or slightly compressive film.
Aluminum oxide has a high bandgap with a corresponding high laser-damage resistance [13,14]. Its moderate refractive index makes it unattractive, however, as a selection for either the high- or low-index material in interference coating designs since such a refractive index leads to significantly thicker coatings with far greater numbers of layers. Alumina films deposited by electron-beam evaporation have been shown to exhibit tensile film stresses while significantly restricting the diffusion of water through the film, suggesting a relatively dense film structure without large, columnar pores in the coating [15]. The limited number of available coating materials with sufficiently high laser-damage resistance requires the exploration of all available choices. The diffusion behavior of alumina, coupled with its high bandgap and laser-damage resistance, suggests further investigation of alumina performance could be beneficial to modify the performance of hafnia/silica coatings.
2. Background
Tensile stresses pose significant challenges for integrating and utilizing coated optical components. First, any film stress leads to a deformation of the optic surface in accordance with the mechanical properties of the film and substrate, as described by Stoney’s equation [16]:
where σ is the film stress, R is the radius of curvature of the surface, Es is Young’s modulus of the substrate, νs is Poisson’s ratio for the substrate, and tf and ts are the thickness of the film and substrate, respectively. This describes the impact of the stress on the radius of curvature of the optic surface, leading to changes in the flatness and corresponding optical performance of the component. While mechanically stiff substrate materials of sufficient thickness ts will exhibit minimal bending from film stress, tensile stresses remain a problem if they lead to cracking, or crazing, of the coating [17].Fracture of the surface of a coating results from sufficiently high tensile stresses such that the fracture toughness of the film is exceeded. Fracture will initiate at a defect in the coating, whether initiated by a scratch at the edge from coating tooling or optic mounting, or at a defect within the film such as shown in Fig. 1 . Fracture also requires a sufficiently thick film, such that the stress in the film can be relieved through fracture at the surface, given the strain at that point. This relationship is given by Hutchinson and Suo [18]:
where hc is the critical coating thickness, Γ is the fracture resistance of the film, Ef is Young’s modulus of the film, Z is a geometrical constant dependent on the fracture type (1.976 for film crazing), and σ is the tensile stress in the film. Compressive stresses will not lead to fracture of the coating surface; instead, excessive compressive stresses may lead to a buckling of the coating, potentially with delamination from the surface, and is not a concern in this work. The dependence on the thickness of the film relative to the film stress in Eq. (2) provides a means for understanding failure mechanisms in the coating.
Fig. 1 Scanning electron microscope (SEM) imaging of the initiation site and crack that forms as a result of the high tensile stress in the film. A defect site in the coating provides an initiation site for tensile stress failure, while tearing of the film is evident within the crack that forms.
Given the relationships in Eqs. (1) and (2), it is important to reduce the film stress such that optical performance of the component is preserved while fracture of the coating is avoided. Thicker substrates may aid in maintaining flatter optical surfaces, and thinner coatings help to prevent cracking, but both of these approaches result in a cost in substrate size and/or achievable coating performance [19]. To provide optimal performance of the optical coating, the magnitude of the tensile film stress must be kept low or ideally moved to a compressive state. Compressive stress also must be kept low in order to maintain surface flatness of the optical component in accordance with Eq. (1).
Stresses in a multilayer coating are a function of the film thickness of each constituent material in the coating, as well as the stress from all sources in each material. Equation (1) may be modified to account for the individual stress in each layer by [20]
where σi and ti are now the stress and thickness of each layer of the coating, respectively. Likewise, the total stress in the multilayer will be given bysince the individual stress contributions are simply weighted by the relative layer thicknesses of each. Modification of selected layers provides a means of adjusting the overall stress, with the use of three materials providing the ability to calculate the stress according towhere Ti is now the total thickness of a given material in the entire multilayer, since the stress is assumed to be constant for all layers of the same material deposited in the same manner. Subscripts H, L, and A denote hafnia (high refractive index), silica (low refractive index), and alumina, respectively. Given individual material stresses, coating designs may then be modified to yield the desired overall stress.Equation (5) assumes that the stresses in individual layers are identical for a given material, and that the stress is homogeneous throughout the thickness of each layer. Given that electron-beam–deposited films exhibit inhomogeneity, roughness, and columnar-growth structure, the influence of anisotropic properties in the composite film stress is expected to be present but of unknown magnitude. The influence of other layers, through roughness or capping effects, is also unknown, as are interfacial strains. The complexity in film structure for porous, evaporated coatings is anticipated to influence the film stress, suggesting additional complexity in quantitative understanding beyond that in Eq. (5).
The goal of this work is to alter the tensile stress state in the hafnia/silica multilayer coatings, ideally shifting it to a low-magnitude compressive stress to eliminate the risk of cracking of the coating while minimizing substrate deformation. Observations of current hafnia/silica coatings deposited by traditional electron-beam evaporation for use in a vacuum environment on the OMEGA EP Laser System have indicated that multilayers of greater than 5-μm total film thickness consistently exhibit tensile stress failures in accordance with Eq. (2), providing the motivation for this effort. In addition, such coatings have tensile stresses of 80 MPa or greater, significantly deforming the substrate surface [4,12]. Since polarizer coatings in particular cannot be realized with coatings of less than 5-μm thickness, the stress in the film must be shifted to a more-compressive state [4]. This is pursued through process changes that will influence one or more sources of stress in any of the film materials, leading to an overall multilayer stress of the desired sign and magnitude.
3. Experimental procedure
Coating depositions were performed in a 54-in. coating chamber equipped with quartz heater lamps, dual electron-beam guns, and planetary substrate rotation. Granular silicon dioxide was evaporated from a continuously rotating pan, while hafnium metal or aluminum oxide was deposited from a stationary six-pocket electron-beam gun. The baseline coating is a 32-layer hafnia/silica quarter-wave mirror centered at λ0 = 1053 nm with a half-wave silica overcoat on fused-silica substrates. Alumina layers were substituted for selected hafnia layers, uniformly distributing the alumina layers throughout the coating. In addition, the first high-index layer on the incident side of the coating was always replaced by alumina to take advantage of the higher bandgap of alumina in the region of highest electric-field intensity. In this manner, the coating has alumina/silica interfaces, but no hafnia/alumina interfaces. The refractive index profile of such a coating is shown in Fig. 2 , with the outermost layers on the air side being a half-wave optical thickness of silica and a quarter-wave optical thickness of alumina immediately beneath it.
Fig. 2 Refractive-index profile of a hafnia/silica/alumina high-reflector coating. Selected hafnia layers are replaced with equivalent-optical-thickness alumina layers, with the alumina layers being equally distributed throughout the overall thickness of the coating.
The primary means of altering the coating in this study are the amount of alumina introduced in the coating design, the location of the alumina in the coating structure, and the number of interfaces of each coating material. Depositions were performed with different overall coating thicknesses, relative numbers of layers and associated interfaces, and individual layer thicknesses as described in Table 1 . The convention for describing the multilayer coating design is that A, L, and H represent quarter-wave optical thicknesses at 1053 nm of alumina, silica, and hafnia, respectively; coefficients indicate a multiple of quarter-wave thickness; and superscripts signify the repetition of that portion of the coating design. Deposition parameters such as oxygen back-fill pressures, deposition rate, and substrate temperature remained constant throughout to maintain consistent film properties and corresponding film stress for each material deposition. The alumina was inserted in quarter-wave optical thickness layers to contribute to the optical performance of the coating while potentially modifying the mechanical properties.
Table 1. Quarter-wave high-reflector coatings for 1053 nm with different numbers of alumina layers replacing hafnia layers. Film stress is determined from surface flatness measurements on a Zygo New View white-light interferometer, with a negative stress being compressive and a positive stress being tensile.
Spectral measurements were performed on a Perkin-Elmer Lambda 900 spectrophotometer operating in transmission mode at normal incidence. The spectrophotometer environment was maintained at approximately 0% relative humidity by purging the instrument with nitrogen gas in order to eliminate optical-thickness variations as a result of the film’s water content. Photometric measurements were executed on a laser-based reflectometer system, again in a nitrogen-purged environment to achieve 0% relative humidity; measurements were performed at a constant wavelength while scanning the incident angle on the substrate. The measurement procedure incorporates a dual-beam configuration, using lock-in amplifiers and a chopped signal to minimize signal noise. Extended integration times at each point in the measurement scan further improve the quality of the measured result.
Surface flatness measurements of the 1-in.-diam substrates were performed on a Zygo New View white-light interferometer in a nitrogen-purged enclosure to achieve approximately 0% relative humidity [21]. Samples were purged for 15 h prior to measurement to stabilize the coating stress; measurement routines were automated to ensure consistent purge times. Samples measured after only 6 h of purging exhibited irregular measurement results, with a significant decrease in correlation with deposition parameters. Measurements were corrected for cavity irregularity by referencing a λ/50 calibration flat, and all measurements subtracted the pre-coating flatness measurement of the individual substrate. Samples were supported on a three-point ball-bearing mount, with each point positioned 120° apart at 65% of the radius of the substrate to minimize distortion caused by gravity while mounted. The uncoated surface of the samples was measured to avoid interferometric phase errors from the coating. Film stresses based on these surface measurements were compared to those carried out on 310-mm-diam × 14-mm-thick fused-silica substrates measured on an 18-in. Zygo interferometer, with calculated stresses agreeing to within ± 8MPa. Film stresses were also compared to measurements taken in a custom vacuum cell on the Zygo New View; vacuum film stresses were measured to be approximately 8 MPa more tensile than those determined in a nitrogen environment. Given the relative difficulty of making in-vacuum measurements, this shift in determined film stress was considered negligible at this time.
Laser-damage testing was performed using 1-ns pulses at a wavelength of 1053 nm. The irradiation spot size, illuminated by a 2-m-focal-length lens, was 600 μm, allowing for the use of fluences up to 100 J/cm2. The sample was inspected under 110 × magnification using dark-field microscopy, with an observable change in the surface being defined as damage. Testing may be targeted on defects present in the coating, as a means of identifying the weakest points in the film structure, or on sites that appear pristine, as a means of evaluating the maximum-possible damage threshold for a clean substrate and zero-particulate process. Modes of testing included 1-on-1, where each site on the substrate is illuminated only once, and N-on-1, where the fluence is gradually ramped through a series of shots until damage is observed [22].
4. Results and discussion
Evaporated alumina films have been reported in the literature as both tensile and compressive [14,23]. The films being studied were found to have a tensile stress when deposited as a monolayer, with a stress of the order of 70 ± 15 MPa. This would suggest that alumina is not a viable material to compensate for tensile stresses in high-damage-threshold coatings since it would not offset the high tensile stress generated in hafnia layers. However, while composite film stresses in hafnia/silica multilayers were tensile, alumina/silica multilayers remain quite compressive. This could in part be a result of the magnitude of the tensile stress in alumina being less than that in hafnia, allowing it to be compensated by the compressive silica stress. It is also possible that the use of alumina is altering the properties of the silica in the multilayer structure. This could be a physical change, such as the alteration of the film structure, or a chemical change related to the water content in the film. Regardless, the stress state is altered from tensile to compressive, making the use of evaporated alumina films of significant interest.
The film stresses as measured in the hafnia/alumina/silica multilayers indicate an unexpected result for the different layer material combinations. The three-material coating design is much more compressive than can be explained based on the constituent materials; given the relative proportion of materials in each design, this should not be possible in order for Eq. (5) to be valid for all coatings using comparable stress values for the materials in each [12]. This failure of Eq. (5) to appropriately describe the multilayer stress suggests two potential modifications: inhomogeneous layer stresses resulting from interfacial effects and growth irregularities, or reduced penetration of water into the film [24]. The water’s dependence on the overall film stress may be due to an impact on the irreversible portion of the film stress, resulting from a chemical change in one or more of the coating materials or an expansion of the film structure. Water content may also alter the reversible stress by occupying pores within the film and straining the film structure. The influence of interfacial or water-dependent stresses will be evaluated separately to determine how well each fits the experimental data, which, in turn, will determine which terms must be included in the model.
The influence of an inhomogeneous layer stress will be a function of the number of interfaces of the given material and the overall thickness of each. The potential changes resulting from inhomogeneous stresses can be modeled by interfacial effects. The influence of the alumina thickness and the number of layer interfaces can be included by altering Eq. (5) to a form
where σH/L and σA/L are the stresses resulting from interfacial and film-growth effects at each of the hafnia/silica (IH/L) and alumina/silica (IA/L) interfaces, respectively. The number of interfaces in the coating design for each combination of materials is counted, and since both hafnia and alumina have silica layers above and below each layer, the directional dependence is not required.It is possible to determine values for each of the five stresses included in Eq. (6) by establishing a linear series of five equations for simultaneous solution. The first six depositions were selected to provide a means of calculating the stresses six times by eliminating one of the depositions from consideration for each calculation. The calculation is most readily constructed in matrix form, for five given depositions:
where σtotali are the measured stress values of the multilayer coatings, after having stabilized with age. Rearranging this for solution yieldsThe six possible solutions, based on elimination of each of depositions 1–6, are summarized in Table 2 .Table 2. Solutions to the system of equations incorporating interfacial stresses describing the individual stress contributions. All stresses are expressed in MPa.
Analysis of the results shows none of the mathematical solutions fit the physical parameters of the problem. Based on single-layer stress measurements, σH is expected to be of the order of 200 MPa and σL is of the order of –80 MPa. In evaluating the potential solutions, σH is consistently compressive while σL is consistently tensile. The solutions are also unstable, which together with the incorrect signs on the stresses indicates that interfacial effects alone do not adequately describe the influence on the film stress. While additional solutions could be found by selecting different combinations of depositions, the failure of the model is apparent.
The changes in coating stress attributable to the inclusion of alumina layers may also be a result of the reduced diffusion of water through the coating structure. As the density of the film is increased, the amount of water that may penetrate the film structure is reduced, altering the water-dependent irreversible and reversible film stresses. If alumina is sufficiently dense to impede the diffusion of water into the multilayer film structure, the presence of alumina would alter the overall coating stress by modifying the amount of water present in the film at any given time. Such an effect would be dependent on where the alumina is located in the multilayer, since reduced water penetration of the coating would only result beneath a given alumina layer. As shown in Fig. 3 , the stress in an alumina/hafnia/silica coating exhibits a very slow drift as the coating is dried in a nitrogen-purged environment. This leads to a strong time dependence for all measured values of stress, which stabilizes the coating performance as much as possible. This uncertainty makes precise stress determination of coatings containing alumina very difficult since even after a week of drying time the stress may not be fully stabilized.
Fig. 3 Change in stress in an alumina/silica coating as a function of time in dry nitrogen. Note that the stress changes quite slowly, corresponding to a change in the reversible film stress as water is removed from the pores of the coating, leading to instability in the optical performance over an extended period of time as the surface flatness continues to change.
Given the limitations of the interfacial model as expressed in Eq. (5), a diffusion-based stress model was developed to describe the influence of alumina layers on the stresses of hafnia and silica. The relatively dense structure of alumina inhibits the movement of water through the coating structure, so it is likely that the presence of alumina layers influences the hydrolysis of porous silica layers contained beneath the alumina, with the aging of the film to a more tensile state over time being largely avoided [4,6]. This model assumes that the presence of a given alumina layer alters the stresses of any hafnia and silica present between that layer and the substrate by inhibiting the diffusion of water from moist air through the coating structure, as shown in Fig. 4 , reducing the available water in the film. This may alter both the reversible and irreversible stresses in the film; the reversible stress relies on water content in the film pores and is reduced by the presence of dense, diffusion-barrier layers of alumina. The irreversible stress results not only from thermal and intrinsic film-structure stresses, but also the stress resulting from the chemical change of silica from hydrolysis, a process which is a function of the water vapor content in the film. A reduction of water penetrating into the coating by inserting diffusion barrier layers would influence the hydrolysis of silica in particular, leading to a change in the silica stress state by altering the stress aging of the coating [6]. It is also assumed that the coated area is large enough that moisture penetration from the edges of the coated region has minimal effect.
Fig. 4 Diffusion-based model assumes a given alumina layer acts as a water-diffusion barrier, influencing the water content and the corresponding stress of all layers below it. As depicted, alumina layer #1 would influence the stress of layers 2 to 8. Buried within the multilayer coating structure, layer #5 would affect the stresses in only layers 6 to 8.
Mathematically, the diffusion-based model can be expressed by the thickness-weighted sum of the stresses of the constituent materials adjusted by a change in the stress as a function of the alumina thickness acting as a diffusion barrier. This is established with a diffusion-based constant ki to account for the relative impact of different thicknesses of alumina on the diffusion of water, as well as an additional constant Di to adjust the magnitude of the stress based on a given change in the water content of the film.
The difficulty with this model lies in its added complexity; by including seven variables, it is now necessary to have a minimum of seven high-quality data sets for unique depositions to solve the system of equations. Given the slow drifts in stress measurements resulting in measurement inaccuracies, the refinement of such a model will be essential to making it useful. Furthermore, numerous data sets may be necessary to overcome the impact of measurement error. This may be pursued further in future work to refine the material properties of the thin-film materials to better predict surface deformation from hafnia/silica/alumina coatings.
The additional degrees of freedom present in Eqs. (9) and (10) have a physical basis related to the movement of water through the structure. Di represents the shift in the irreversible stress for a given material resulting from water-induced changes; these changes can be expected to be different depending on the relative porosity and any chemical changes that may take place in hafnia versus silica. Since all of the measurements of coating stress are performed in a nitrogen-purged atmosphere, with the coating dried for 15 h, the reversible stress resulting from the presence of water in the pores of the film is negligible. The alumina layers are acting as diffusion barriers, but the exponential reduction in diffusion because of such a barrier must be appropriately scaled to account for the alumina film structure and the ability of water to penetrate it. Ideally, this should result in a value of kH equal to that of kL, and this should be explored within the model.
The added complexity of the model and the relatively low accuracy of the stress measurements suggest the use of an optimization routine to best fit all available data to the model, based on varying the individual material stresses σH, σL, and σA, as well as the diffusion-related constants DH, DL, kH, and kL. Further improvements to this model will be possible as additional data sets become available, ideally with unique coating designs containing alumina layers of different thicknesses and placement. Potential simplifications can be sought by requiring kH = kL, or even an assumed constant value for kH and kL such as 1 μm–1. Assuming kH = kL = 1, the diffusion-based model yields the material properties shown in Table 3 for hafnia/silica/alumina multilayers.
Table 3. Calculated values for individual material stresses incorporating the influence of alumina layers as water-diffusion barriers. Solution assumes kH = kL = 1 μm–1.
Evaluating the individual material stresses in a 0% relative-humidity environment, this model appears to represent the physical performance of the coating much more closely. When considering single layers of hafnia or silica (alumina thickness equal to 0), it is important to note that Eq. (10) will yield σH + DH or σL + DL, respectively. The stress for a hafnia monolayer is now 212 MPa (tensile), while that for a monolayer of silica would be –228 MPa (compressive), both consistent with experimental results. The stress in a silica monolayer is somewhat more compressive than that measured, but this may be a result of the aging influence of silica hydrolysis, shifting from a more-compressive film to a less-compressive film. The time between film deposition and measurement can have a significant impact on silica stress, with delays in measurement shifting the stress in the tensile direction. Evaluating samples 1–8, noting that this data were also used to determine the constants within the model, the modeled multilayer stresses are shown in Table 4 .
Table 4. Measured versus modeled multilayer stresses using the water-diffusion stress model. All samples coated in the primary deposition system model the stress within 6 MPa of the measured value, within the margin of error of the stress measurement.
5. Implementation
A key advantage of this type of change in the coating process is that it can be readily performed in a standard evaporation system with minimal equipment modifications required. This process was implemented in a 1.8-m coating chamber to stress-compensate a 0.9-m–aperture polarizer coating for use in vacuum on the OMEGA EP Laser System, as previously produced with evaporated hafnia/silica [4]. To integrate this process, the standard six-pocket electron-beam gun used for hafnium metal evaporation was replaced with an EB Sources large-capacity, 12-pocket electron-beam gun, providing additional source capacity for alumina evaporation. All deposition monitoring was performed with weighted averaging using an Inficon IC5 and three SensorsTech cartridge-type quartz-crystal monitors mounted beneath stationary uniformity-correction masks, with the substrate mounted in a counter-rotating planetary rotation system [25,26]. Silica deposition was performed using granular silica in a continuously rotating EB Sources large-capacity, pan-type electron-beam gun.
The original 48-layer polarizer coating design developed for this application was replaced with a 50-layer design containing four alumina layers, with all alumina layers adjoining only silica layers [4]. The alumina layers are nominally one quarter-wave optical thickness, except the layer on the substrate, which is approximately three quarter-waves in optical thickness. The coating design was fully optimized with Optilayer refinement, to maximize the photometric coating performance [27]. The alumina layers were inserted every 16th layer, so that the layer on the substrate was alumina and the final high-index layer was alumina. The outermost layer of the coating remained a thick silica layer of greater than one half-wave optical thickness. The overall coating thickness was 9.1 μm, requiring approximately 10 h of deposition time. Cross-sectional scanning-electron micrographs of the completed polarizer coating are shown in Fig. 5 , with the alumina layers appearing very similar to the surrounding silica layers; only the film microstructure differentiates it from the adjoining layers.
Fig. 5 Cross-sectional scanning electron micrographs of the polarizer coating modified with four alumina layers. The alumina layer appears to have a more-columnar structure than the surrounding silica layers, which appear amorphous. The hafnia layers appear columnar and much brighter in the image.
Coating performance was measured using a laser-based photometer, providing a highly collimated source, precise angle of incidence, and high polarization contrast. The performance of this polarizer is shown in Fig. 6 , indicating p-polarized transmission of greater than 98% through the component over an angular range of nearly 9° incidence; polarizer contrast, defined as Tp:Ts, exceeds 200:1 over 8° of this range. In wavelength space, this component has a useful bandwidth of 30 nm after accounting for slight uniformity errors over the 0.9-m aperture and installation alignment tolerances.
Fig. 6 Photometric measurement of a Brewster-angle polarizer installed on OMEGA EP, utilizing alumina for stress control in a dry environment. This polarizer coating provides high transmission and contrast over a wavelength range of 30 nm with incident 1053-nm light.
Laser-damage testing of this coating using a 1053-nm laser with s-polarized light in a 1:1 mode indicated damage thresholds of greater than 74 J/cm2, when tested at 1 ns (clean sites, with no testing performed above this fluence), and 44 J/cm2, when targeting defects visible using dark-field inspection in a microscope as described previously [22]. Transmitted laser-damage tests in p-polarization remained above 20 J/cm2, resulting in an overall laser-damage performance consistent with previous polarizer coatings produced with evaporated hafnia and silica [4]. Stress measurements indicate that the coating on a fused-silica substrate maintains a compressive stress of approximately −70 MPa, when measured in an N2-purged environment at 0% relative humidity. This compares very favorably with the diffusion-based stress model, which predicts a compressive stress of −74 MPa, listed as sample #9 in Table 1. This controlled compressive stress provides a coating that does not fail in tension, even when used in a vacuum environment. As noted previously, the slow drift in film stress as a function of drying time makes it very difficult to accurately determine the stress, with a measurement uncertainty of the order of ± 10MPa.
While this coating effort was highly effective, far exceeding the performance requirements for this component, the use of alumina poses significant challenges to the successful implementation in the laser system. The diffusion-barrier properties of the coating significantly restrict the movement of water into and out of the film structure, leading to very slow changes in the coating stress and photometric performance as the relative humidity changes. This change in photometric performance was measured for the polarizer coating, initially stored in an ambient-humidity environment, over a period of multiple days in an N2-purged 0% relative humidity environment as shown in Fig. 7 . The coating undergoes a substantial change in photometric performance, requiring days or even weeks of recovery time if the optic is exposed to an ambient-humidity environment.
Fig. 7 Change in photometric performance of a hafnia/silica polarizer coating containing alumina layers. Note that similar to the stress changes in Fig. 4, the optical performance of the coating changes significantly over an extended period of time in a dry nitrogen environment. In this case, measurements were performed over a period of approximately 8 days.
The slow drift in performance may require the storage and transport of such coatings in a dry atmosphere, while minimizing exposure to humid air during installation. Initial evaluation suggests that the movement of water into the coating also takes place over a long time scale, as evidenced by the “mottled” appearance that develops as the coating is exposed to moist, ambient air. Provided the water penetration is slow, short exposure times during installation and alignment can be overcome relatively quickly. It is understood that moisture penetration through defect sites in the coating leads to the localized exchange of water for void in the coating, resulting in an increase in the optical thickness and a change in the color of the coating as shown in Fig. 8 . Over time, diffusion of the water within the coating structure will bring the water content in the coating to equilibrium, with the coating once again appearing to be a consistent color as the individual moisture-penetration sites through the diffusion barrier coalesce, eliminating the mottled appearance. Alumina is a highly effective diffusion barrier to water penetration and, as such, may require many days of exposure to moist air before the coating once again appears uniform. As shown in Fig. 8, a hafnia/silica coating containing alumina barrier layers continues to exhibit a mottled appearance 2 days after deposition. The alumina is quite dense, with isolated defects providing a path for the moisture through the layer, while the surrounding hafnia/silica layers in the multilayer remain relatively porous.
Fig. 8 Multilayer dielectric coating containing alumina layers 2 days after deposition. Note the “mottled” appearance of the coating color in reflection, indicating an irregular absorption of water into the coating structure.
6. Conclusion
The inclusion of alumina layers in standard hafnia/silica high-reflectance coatings leads to a significantly more compressive overall film stress, enabling one to use such coatings in vacuum environments on low-thermal-expansion substrates without the risk of tensile stress failure causing cracking, or crazing, of the film. The use of multiple designs incorporating different numbers of layers, numbers of interfaces, and thicknesses of the constituent materials provides an opportunity to determine the individual contributions of hafnia, silica, and alumina to the overall stress in the multilayer optical coating, a contribution that was found to be very different than that anticipated based on monolayer stresses. A model incorporating interfacial effects was developed to account for the inhomogeneous film stresses as each layer is formed; such a model does not adequately allocate the influence of individual materials to the overall film stress.
The water-diffusion model has been shown to accurately predict multilayer coating stresses for hafnia/alumina/silica films, with good agreement with measured monolayer and multilayer stresses. By limiting the penetration of water into the film structure, the hydrolysis of silica is restricted and the overall film stress remains more compressive. The stress in hafnia/silica coating designs including alumina stress-compensation layers has been demonstrated in agreement with the developed model, without degradation of spectral performance or laser-damage resistance. This process was used to deposit large-aperture polarizer coatings for use in an N2-purged environment on OMEGA EP. Such coatings may be readily implemented using standard electron-beam evaporation systems, are easily scaled to large-aperture substrates, and provide an ideal means of addressing the need for coatings in vacuum for pulse-compressed laser systems. The slow diffusion of water in such coatings poses some difficulties in implementation, but this may be overcome by storage in a low-relative-humidity environment.
Acknowledgment
The authors wish to express their appreciation to Alex Maltsev for his efforts on the fabrication of extremely high quality, high-aspect-ratio substrates for this study and to Jason Keck for his assistance with developing an optimization routine for the diffusion-based model. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.
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