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Birefringent magnesium oxide thin films are formed by glancing-angle deposition to perform as quarter-wave plates at a wavelength of 351 nm. These films are being developed to fabricate a large-aperture distributed-polarization rotator for use in vacuum, with an ultimate laser-damage–threshold goal of up to 12 J/cm2 for a 5-ns flat-in-time pulse. The laser-damage threshold, ease of deposition, and optical film properties are evaluated. While the measured large-area laser-damage threshold is limited to ~4 J/cm2 in vacuum, initial results based on small-spot testing in air (>20 J/cm2) suggest MgO may be suitable with further process development.
© 2016 Optical Society of America
1. Introduction
A development effort is underway to fabricate a large-aperture distributed-polarization rotator (DPR) for beam smoothing at the National Ignition Facility (NIF) in preparation for polar-direct-drive implosion experiments [1–4]. This component would operate in a vacuum environment with a 351-nm, 5-ns incident laser pulse operating at a fluence of up to 12 J/cm2. As part of this effort, columnar thin films formed by glancing-angle deposition (GLAD) are being studied as a means of spatially patterning a fused-silica substrate with ± π/2-phase retardance regions at a wavelength of 351 nm, as described previously [3,4]. This results in regions of right-and left-hand-circular polarization states being transmitted, allowing for polarization smoothing of a large-aperture beam with a single substrate using coated regions resistant to high fluences.
A GLAD film of sufficient thickness to provide the desired phase retardance may be multiple microns in thickness, depending on the difference in refractive index (Δn) for the orthogonal polarization components of the linearly polarized light incident on the given layer material. The linear polarization state of the normally incident light is defined relative to the film structure, with the orthogonal components oriented at ± 45° to the fast axis of the birefringent coating. Such a thick coating has rapid oscillations in the ultraviolet spectral transmittance and is poorly suited to be accurately incorporated as part of a multilayer interference coating. Consequently, the baseline design concept selected for the GLAD DPR incorporates a birefringent film with an average refractive index equivalent to that of the fused-silica substrate [3,4]. This requires the ideal materials for this application to have a bulk refractive index in the 1.6 to 1.8 range in order to create a porous, structured film with an average refractive index of 1.48 to match fused silica at a 351-nm wavelength [5]. Higher refractive index materials could potentially be used, but laser-damage thresholds of dielectric oxide materials are typically greater for low-refractive-index materials, requiring the lowest possible refractive index material to be used for application in the ultraviolet beam of the NIF [6,7].
This work describes the development of magnesium oxide coatings produced with a GLAD process to achieve ± π/2 phase retardance for a 351-nm wavelength, large-aperture, high-fluence laser. This material study is undertaken in support of the nominally 0.5-m DPR process development effort detailed elsewhere [3,4]. Magnesium oxide (MgO) is pursued for its high-laser-damage threshold in the ultraviolet region of the spectrum, near-neutral film stress, and ease of deposition. Other characteristics that must be evaluated as possible concerns include the overall component spectral reflectance, optical scatter, and modulation of the transmitted or reflected beam, which may lead to damage of other laser system components. Substrates have been limited to a maximum of 50.8-mm-diam so far in this material development effort, but the process utilized is scalable for meter-class components for the NIF.
2. Background
GLAD is a process in which an incident vapor flux condenses with low surface mobility on a substrate oriented at a high incidence angle, forming a film microstructure that is highly anisotropic because of self-shadowing effects [7–13]. Related processes to deposit structured films include oblique-angle deposition (OAD), reactive-ballistic deposition (RBD), sculptured thin films (STF’s), as well as other designations, with these processes being means of producing oriented-column structures exhibiting birefringence as well as other optical properties [13–16]. Through precise control of the substrate orientation and rotation during deposition, complex structures may be realized with reflective, antireflective, filtration, and polarization-control properties [8–11,13,17]. GLAD coatings have typically been limited to small areas since the film structure and corresponding performance are highly dependent on the angle at which the evaporant condenses on the surface of the substrate. However, homogeneous phase retardation can be achieved over larger areas by linearly translating a substrate behind an aperture, forming oriented columns over the entire scanned region [3,4,18]. This has been previously demonstrated as a part of this development effort [3,4].
Serial bideposition is a process modification for oriented column structures developed by Hodgkinson and Wu that allows film birefringence to be maintained more consistently throughout the thickness of the deposited layer by alternating the deposition direction between ± θ rather than remaining at a constant angle θ [12,17]. This process modification leads to vertical column formation while maintaining the small-scale film orientation that provides optical birefringence. Another method of controlling column broadening and maintaining uniform retardance throughout the film thickness is phi-sweep, a process wherein the incident vapor flux is maintained at a constant angle, but the substrate is continuously rotated in an oscillating manner about its surface normal over an angular range of ± α [19]. This may be considered in the future, but was not evaluated as part of this effort.
Material selection is critical to the performance of GLAD structures for high-fluence applications. Since magnesium oxide sublimes rather than evaporating, particulate defect ejection is minimized during deposition which is expected to improve laser-damage resistance [20]. The bulk refractive index of MgO is approximately 1.77 near 351 nm, making it a potential candidate to index-match to a fused-silica substrate by the controlled porosity of a GLAD process [21]. Magnesium oxide has found limited application as a traditional optical thin-film material, forming mid-refractive-index layers with a haze developing from interaction with atmospheric carbon dioxide, a potential source of optical scatter [20,22]. However, the benefits associated with low-energy deposition and a precise index match to fused silica make the use of MgO particularly suitable for this application.
Thin-film stresses have been studied extensively, but the performance of MgO GLAD-type structures in a dry or vacuum environment has not been previously reported. Typically, film stresses in dry or vacuum environments become more tensile, potentially leading to cracking of the coating for sufficiently thick films with high tensile stress [23]. Highly porous GLAD films generally possess a near-neutral film stress since the film is not continuous; as the film is deposited in a more-dense manner, by using a lower incidence angle for the vapor flux, a tensile film stress results with significant concern for tensile failure even in ambient environments [24]. An example of a tensile failure is shown in Fig. 1, with a GLAD film of Al2O3 being imaged in the vacuum environment of a scanning electron microscope (SEM) [4]. The film cracking is readily apparent, consistently occurring orthogonal to the fast axis of the birefringent film (i.e., parallel to the tilted-column deposition direction).
Fig. 1 Cracking in an aluminum oxide GLAD film is apparent because of tensile stress failure. The image has been colorized to enhance the visibility of the film texture Materials and deposition processes must be selected to mitigate changes to the GLAD film structure in a dry environment.
There are three primary ways that the stress of a structured film can be addressed: cap it with a solid layer as discussed by Hawkeye et al., modify the structure and resulting mechanical properties of the film via ion assist, or maintain isolated columns in a high-porosity film, leaving no continuous structure to crack [24–26]. As shown by Dohnálek et al., GLAD MgO tends to form in isolated columns, removing the potential for film cracking by tensile stress [15]. This provides a significant benefit for films intended for use in low-relative-humidity environments since tensile-stress mitigation can be extremely challenging. Given that MgO naturally forms a structure resistant to tensile failure, it is highly desirable for such applications.
A final consideration for these films is that use in a vacuum environment necessitates evaluation of the film performance in a 0% relative humidity environment. This results in a different refractive index for the film since voids in the film may be filled with water of index 1.3 instead of air or vacuum of index 1.0 [27]. Potential mixtures of bulk MgO can be evaluated with either void or water using a Maxwell–Garnett effective medium approximation, indicating the necessary change in pore volume for the film based on the relation
where neff is the effective refractive index of the composite medium, nc and nd are the indices of the continuous (MgO) and dispersed (void or water) phases, respectively, and ϕ is the porosity of the structure [28,29].As shown in Fig. 2, achieving a refractive index equivalent to that of fused silica with n = 1.48 requires the MgO film to have a significantly different pore fraction (35%) for a dry environment relative to that possible in the case where all pores are filled with water (60%) based on calculated values using Eq. (1). This confirms the importance of evaluating the film in the final use environment, not only for stresses, but also for refractive index, retardance, and laser-damage thresholds.
Fig. 2 Maxwell–Garnett effective medium approximation for MgO with pores filled with either void or water. To achieve a refractive index of 1.48 to match that of fused silica, a pore fraction of 35% (void) or 60% (water) is required. EMA: Effective medium approximation.
It is expected that the water content of the film will be between the two lines shown in Fig. 2 for a typical ambient environment, but will shift to the dashed MgO/void line in a 0% relative humidity environment. For a given film with a defined pore fraction, use in a dry environment has the effect of reducing the refractive index of the GLAD layer.
3. Experimental procedure
Magnesium oxide films were deposited using a Telemark electron-beam gun with a slowly rotating pan at ~3 hours per rotation in a cryopumped 45-in. coating chamber as shown in Fig. 3. Since MgO sublimes, the emission current of the electron-beam gun was operated at only about 50 mA for deposition [20]. Deposition rate and overall layer thickness were controlled by quartz crystal monitoring, using an Inficon IC5 with a Crystal 12 rotary crystal head. The chamber was evacuated to approximately 2 × 10−6 Torr and heated with quartz lamps to 150°C. The substrate was mounted to a single-axis variable-angle stage, making it possible to control the arrival angle of the incident vapor flux over the range ± 90° relative to the substrate normal. MgO films were deposited in an oxygen backfill pressure of 1 × 10−4 Torr using a serial bideposition process with the substrate oriented at θ for 60 s, then rotated to –θ (the opposite position) in 4 s to dwell for 60 s, with continued oscillation in this manner throughout the entire deposition [12,17].
Fig. 3 Electron-beam deposition system to fabricate structured MgO films. The substrate mount oscillates throughout the deposition between ± θ, dwelling at each position for 60 s.
Reflectance and transmittance were measured using a 351-nm laser and detector, also enabling the calculation of loss caused by scatter. Refractive indices were characterized with a Woolam variable-angle spectroscopic ellipsometer (VASE) in an ambient environment and a nitrogen-purged environment. Laser-damage testing was performed on cleaved float glass, polished fused silica, and polished fused silica processed with an “Advanced Mitigation Process” (AMP) using 1- to 5-ns pulses at a wavelength of 351 nm [30–32]. Vacuum laser-damage testing was performed at LLNL on the Optical Sciences Laser (OSL), using a 2.5-cm2 beam in air and vacuum [33].
A wide range of deposition angles were evaluated, since the refractive index of the resulting film is based on the porosity of the coating, and the porosity is a function of the incident angle of the vapor flux on the substrate. The oxygen backfill is required to maintain film stoichiometry, with a balance between deposition rate and available oxygen required to provide sufficiently high-laser-damage thresholds while simultaneously minimizing scattering of the evaporant flux, which can disrupt the film structure [34].
4. Results and discussion
Characterization of the MgO films was performed on different substrates for different testing purposes; silicon wafers were used for ellipsometry and SEM imaging, polished fused silica was used for retardance measurements and optical performance, and cleaved float glass or AMP substrates were used for laser-damage testing. While the film properties are anticipated to vary somewhat for the different substrate types, the use of different substrate materials at this time is necessary to better understand the film. Cleaved float glass is not of sufficient optical quality to allow retardance or optical measurements, and fused silica has too low of a refractive index difference from the MgO film to allow accurate ellipsometry. The porous nature of the film makes electron microscopy very difficult on fused silica, and cross-sectional sample preparation is significantly improved on crystalline silicon. As the process continues to be refined, measurements of optical performance on AMP substrates will guide the process optimization, although understanding of the changes in film structure with respect to process conditions will be severely reduced. Ellipsometric analysis of the MgO films indicates the presence of a gradient in the refractive index, as shown in Fig. 4. Ellipsometric measurements were performed in both ambient (40%) relative humidity, as well as within a nitrogen-purged enclosure monitored with a hygrometer to simulate the low-relative-humidity conditions of vacuum since a vacuum-environment spectral ellipsometer was not available for this work.
Fig. 4 Ellipsometric analysis in a nitrogen-purged environment of an MgO film deposited at 58° indicates the presence of a gradient in the refractive index profile of the layer structure next to the substrate. Measurement in a wet versus dry environment shows a lower nominal refractive index in the nitrogen environment with a slightly greater birefringence.
Such a change in the film refractive index is consistent with the findings of Aboelfotoh, where a densified layer of approximately 50 nm was found next to the substrate, although the ellipsometry models for this work indicate the dense layer is somewhat thicker [35]. Consistent film density as a function of the depth in the layer is very important to achieve controlled reflectance from the coated surface. Ideally, the nominal MgO layer would be index matched to the substrate to minimize the Fresnel reflectance from the film/substrate interface. An antireflection coating can then be used on the film/air interface to minimize overall reflectance loss. The ellipsometry models for ambient and dry environments at different angles do not indicate the presence of a clear trend in the refractive indices, the film birefringence, or even the overall thickness given the complexity of the film structure. To simplify the model, film thicknesses were based on SEM cross-sectional imaging and applied to models for both ambient and dry conditions, and the refractive index was held constant from the initial dense gradient to the air interface of the film. The models for different flux angles and conditions exhibit too much variation to provide confidence in the modeled film properties; the overall trend indicates lower refractive indices at higher deposition angles, but precise modeling of the films has not been achieved at this time. In the future, depositions will be performed on several substrate types simultaneously to provide multiple ellipsometric measurements to be fit concurrently with a single film model in an effort to improve the model quality. The expected range of refractive indices for the MgO film theoretically results in up to 0.5% Fresnel reflectance from the substrate/coating interface once the overall component is antireflection coated.
As shown in Fig. 5, the MgO column tops exhibit a characteristic “roof-tile” shape as observed by Ma et al., with the columns isolated from one another [36]. The densified region near the substrate is also apparent. This profile leads to difficulties in matching the refractive index of the film to the substrate, since a higher refractive index next to the substrate will increase the Fresnel reflectance at that interface; matching the refractive index at the substrate results in too low of a refractive index at the air interface to successfully antireflect using a single-layer coating.
Fig. 5 (a) Cross-sectional view of an MgO film on silicon, showing column growth with broadening as the film is deposited, with a more-dense region near the substrate. Deposition at a 70° incidence angle, substrate temperature of 150°C, with a rate of 0.06 nm/s and an O2 backfill of 1 × 10−4 Torr. (b) View of the MgO columnar structure normal to the substrate surface clearly indicates the porosity of the film.
Transmission and reflection measurements were performed at 351 nm to estimate optical losses caused by scatter of the structured film, as shown in Table 1. Significant variations were found based on the incident linear polarization, backfill pressure, deposition rate, substrate temperature, and run-to-run variations. Further investigation is being pursued to better characterize the scatter from MgO GLAD coatings, but losses vary from 2% to 4% for the depositions being studied.
Table 1. Deposition parameters surveyed for MgO GLAD films. Deposition rate, backfill pressure, and angle were optimized to yield high-laser-damage thresholds with minimal scatter, while matching the refractive index of fused silica at 351 nm. Measurements of each condition were limited based on the substrate choice, with selected angles deposited on silicon only for refractive index characterization.
Retardance mapping measurements of the MgO GLAD films were carried out on a Hinds Instruments Exicor 450XT Mueller Matrix Polarimeter operating at a wavelength of 355 nm. Since retardance is simply the product of the film thickness and the difference in refractive index Δn for the birefringent film, it was found that achieving the required retardance was rather trivial; the film thickness was simply scaled to achieve the desired value. Retardance of 87.8 ± 5 nm was routinely achieved, leaving the focus of this material-development effort on the trade-offs between scatter losses, laser-damage thresholds, film stress, and low reflectance.
Thin-film stresses are an important performance specification since excessive tensile stresses will lead to fracture of the coating, a common difficulty for GLAD films deposited at lower angles with greater density [37]. Stresses in the GLAD layer are calculated based on the change in curvature of a substrate measured with a Zygo New View white-light interferometer, using the back surface to avoid phase effects of the coating [23,38]. Measurements of MgO films on thin substrates indicate tensile stresses remain relatively stable at 24 MPa ± 10 MPa in a 0% relative humidity environment—a magnitude that has not been found to exhibit cracking; the low tensile stresses are likely a result of the isolated-column structure of the coating, as shown in Fig. 6(a) (imaged in the vacuum environment of an SEM). The film stress (σ) in a 40% relative humidity environment remains 0 < σ < 10 MPa (compressive). There is limited texturing of the coating with a corresponding increase in scatter apparent after vacuum exposure, as shown in Fig. 6(b). This is being investigated further in an attempt to mitigate observed transmission losses. A “nanocarpet” effect has been described by Hawkeye et al. as a clustering of the isolated columns resulting from capillary forces as the liquid is removed from the structure, with mitigation strategies including mechanical strengthening of the columns and removal of the moisture by critical-point drying [39]. Such an effect is a potential source of concern and is apparent in the vacuum environment of an SEM, as shown in Fig. 6(b); separation of the column texture is apparent when viewing the tips of the columns from above.
Fig. 6 (a) MgO film on silicon exhibiting isolated columnar structure, reducing the potential for film fracture in vacuum (tensile stress failure). Deposition at 62° angle of incidence, heat at 150°C, with a rate of 0.06 nm/s with O2 backfill at 1 × 10−4 Torr. (b) After vacuum exposure, any texturing or separation of the GLAD columnar structure correlates to an increase in scatter loss, which is different for different porosity MgO films.
Laser-damage thresholds are highly dependent on intrinsic absorption, the electronic bandgap of the layer material(s), and the presence of any defects in the film, which can locally increase the optical absorptance or focus the incident laser intensity [40]. Since MgO deposition is carried out with a very low electron-beam–gun current, it is unlikely there will be sufficient energy to eject particles to be embedded in the coating; as shown in Fig. 7, an SEM image of the coated surface shows no particles visible in the deposited film. It is expected that any localized defects will be sub-stoichiometric defects, which likely can be mitigated by reducing the deposition rate, increasing the oxygen backfill pressure, and/or by using post-deposition annealing.
Fig. 7 MgO film on silicon, exhibiting a consistent structure without the incorporation of deposition defects since the electron-beam emission current is maintained at a very low level. The image has been colorized to enhance the visibility of the film texture. Deposition at 62°, heat at 150°C, with a rate of 0.06 nm/s with O2 backfill at 1 × 10−4 Torr.
Small-spot laser-damage tests at 351 nm using a 300-μm-diam, (1/e2), 4.8-ns flat-in-time pulse indicate that damage thresholds up to 30 J/cm2 may be achieved with an N:1 testing protocol on cleaved float glass or an AMP substrate, as shown in Fig. 8 [31]. Note that film-deposition rates are held constant on the quartz crystal monitor, leading to changes in the effective deposition rate for different incidence angles of the evaporant flux as shown in Fig. 8; this leads to a difference of more than a factor of 2 in the oxygen:MgO ratio for the condensing film. Damage sites are highly localized, without significant catastrophic damage in the surrounding area.
Fig. 8 Laser-damage thresholds of MgO GLAD films tested at 351 nm with a 4.8-ns flat-in-time pulse in 1:1 and N:1 configurations. The points A–C correspond to the deposition conditions noted in Table 1. Deposition rates are compensated by the cosine of the incident evaporant flux.
Large-area testing was performed on the OSL facility at LLNL at both atmospheric pressure and 2.5 Torr; the method employed exposes a relatively large area of the sample (~2.5 cm2) with a single laser pulse. The 351-nm laser spot on the sample is spatially flat with approximately 20% contrast [33]. Temporally, the pulse is of near-constant intensity for 5 ns with 140-ps rise and fall times. The location of each damage test is first exposed to a number of pulses of increasing fluence to condition the surface. The conditioning ramps began at 3 J/cm2 and increased in 1 J/cm2 increments, terminating at 8 J/cm2 and 9 J/cm2 for the 2.5 Torr and air test locations, respectively. The vacuum and air damage tests were conducted with an average fluence of 9 J/cm2 and 10 J/cm2, respectively.
During the laser exposure, the relative locations of engineered fiducials in both the incident beam and on the sample are captured in transmission with a camera imaged on the sample. This allows the registration of the fluence to the sample surface to within 10 μm. An automated microscope is used to image the entire surface exposed to the laser, including the engineered fiducials. After thresholding, a computer algorithm identifies the size and location of each damage site. The fluence profile of the laser pulse used to produce the damage is divided into spatial regions of relatively constant fluence. The number of damage sites in each of these regions is counted and divided by the area of the region to establish a damage density for each fluence bin. Density is then averaged between subregions of similar average fluence. The damage density as a function of fluence, referred to as a ρ(ϕ) curve, is plotted for both 2.5 Torr and air exposure in Fig. 9. Ideally, the damage density would remain ~0 for the intended operating fluence of the component. As apparent in Fig. 9, further improvements in the coating-laser-damage threshold are needed to reach the operational fluence goal. The measured results show a significant reduction in laser-damage resistance relative to the small-spot testing detailed in Fig. 8. Differences between the tests can be attributed to multiple factors, including the statistical distribution of film defects (damage initiation sites) over a much larger area, electric-field enhancement from the second-surface reflection (the second-surface reflection in the small-beam test does not interfere with the first-surface test area), and potential field enhancements from scattered and wave-guided light within the GLAD film. The relative areas of the two test beams indicates ~2500 × as many defects would be contained in the large- area test compared to the small-spot test; given the measured damage-site densities shown in Fig. 9, this would be equivalent to an increase in laser fluence of more than 100%. Modeling of these potential sources will be undertaken in an effort to quantify and test the potential impact of each contribution, while the observed reduction in laser-damage threshold in a vacuum environment will also be explored further in future testing.
Fig. 9 The damage site density as a function of fluence for a MgO GLAD film tested at 351 nm with a 5-ns flat-in-time pulse in both air and 2.5-Torr environments.
Future work will include studies of aging of the film properties, including retardance, scatter, and laser-damage resistance. Improvements in laser-damage thresholds over large areas will be necessary in order to implement such coatings on the NIF. Component testing, including the use of front- and/or rear-surface antireflection coatings will influence the scatter and reflectance loss of the component as well as alter any enhancements in the electric-field intensity within the film. Improvements in refractive-index matching of the GLAD film to air and the substrate are also expected to influence any waveguiding of scattered light within the film, which can potentially alter the standing-wave electric field intensity within the layer. Finally, additional materials will continue to be studied to best meet the system requirements at 351nm.
5. Conclusions
Quarter-wave-retardance GLAD films have been successfully fabricated with magnesium oxide for use at 351 nm in a vacuum environment. Sublimation of the MgO takes place at a very low electron-beam emission current, leading to low-defect-density films with small-spot laser-damage thresholds greater than 20 J/cm2 for a 351-nm, 5-ns pulse; no film defects were apparent in SEM images of the GLAD surface. Large-aperture damage testing indicates a significant reduction in laser-damage resistance relative to small-spot testing, which is likely a function of multiple factors: statistical distribution of film defects, electric-field enhancement from the second-surface reflection, and potential field enhancements from scattered and wave-guided light. The structure of the MgO GLAD film results in scatter losses of 3% to 5%, improvements in which will be pursued to limit losses and beam modulation for large-aperture, high-fluence laser usage. Ellipsometry models of MgO GLAD films indicate the presence of a more-dense, inhomogeneous region near the surface of the substrate that is expected to result in up to 0.5% specular reflectance for the current component design. While the film exhibits suitable birefringence and low mechanical stress, laser-damage thresholds will need to be improved in order for the MgO coatings to be viable for use in fabricating distributed polarization rotators on the NIF.
Acknowledgment
This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944, 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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