1.

Introduction

The performance of multilayer dielectric (MLD) coatings of high-reflector mirrors in high-peak-power laser systems including the National Ignition Facility, Laser Megajoule, and OMEGA^1–3 can be limited by surface contamination. While handling and processing may contribute to unwanted particulate generation, the major contamination sources are derived from components that surround the mirrors within the laser system,^4–6 such as beam dumps and fasteners. Typical contaminants that have been found include both organic and inorganic materials, such as oil, polyester fibers, ceramics, metals, and metal oxides. In addition to having a large range of chemical composition, contaminants also have a variety of morphologies including spherical, disc-like, or irregular shape.

For highly reflective MLD coatings, adding an absentee overcoat or protective capping layer above the outermost layer has been shown to increase the resistance to laser-induced damage.^7–9 A similar strategy of adding a protective capping layer has also been adopted to extend the lifetime of multilayer mirrors in extreme ultraviolet lithography applications.¹⁰ However, the underlying mechanism by which the coupling between the laser and contaminant causes damage to the protective layer is still elusive. In order to facilitate effective selection and design of capping materials and to develop a practical solution for contaminant removal and post mitigation,^11,12 an improved understanding of the physical principles is desirable.

An early study showed that the laser-induced damage to the reflective surface is strongly dependent on the contaminant composition and size, as well as the laser fluence.¹³ In the present work, we extend that study to explore how the laser-induced damage depends on the contaminant shape. We use Ti particles, a common surface contaminant found on MLD mirrors residing in close proximity to Ti-based beam dumps in high energy laser systems. One selected shape is spherical, and another shape is irregular (sickle-shaped). To account for the role of the highly reflective surface, we utilize an oblique angle of incidence of the laser beam to reveal the impact of field intensification from multiple reflections between the MLD coating surfaces and the surface of the particle. Specifically, in the given geometry, we investigate the response of the capping layer under 45 deg oblique irradiation of a single pulse of laser light (1053 nm, p-polarized, fluence $\sim 10\mathrm{J}/{\mathrm{cm}}^2$, pulse length 14 ns) in the presence of both spherically and irregularly shaped Ti particles. The high reflector ($>99.5\%$ HR) samples were silica-hafnia multilayer coatings fabricated by e-beam physical vapor deposition (e-PVD). Silica-hafnia multilayers are commonly used for high reflectors in high peak-power laser systems that require high laser damage resistance.^14,15 The capping layer materials used in this study are amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ and the previously used ${\mathrm{SiO}}_2$⁸. ${\mathrm{Al}}_2{\mathrm{O}}_3$ is a large bandgap material with a large thermal expansion coefficient ($8\mathrm{ppm}/\mathrm{K}$) and high mechanical strength and has been shown to have high laser-damage resistance.^16–18 Recent studies have demonstrated that this material can be successfully integrated with silica-hafnia multilayer coatings by e-PVD for large-aperture high-fluence laser systems.¹⁵

We find that under similar exposure conditions, the response of the capping layer to laser–particle interaction demonstrates great contrast and is strongly dependent on the Ti particle shape. As shown in our previous work,¹⁹ for a spherically shaped Ti particle, the laser-induced damage is more pronounced on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping surface than on the ${\mathrm{SiO}}_2$ capping surface. The observed difference is attributed to heating of the substrate by the ablated plasma and the large disparity in the thermal expansion coefficients of the two capping materials, with that of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer being about 15 times greater than that of ${\mathrm{SiO}}_2$.¹⁹ On the other hand, for irregularly shaped Ti particles as investigated in the current work, plasma mainly ablates outward, producing recoil momentum, and the laser-induced damage is more severe on the ${\mathrm{SiO}}_2$ surface than on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer. This difference is related to the large difference in mechanical toughness between the two materials, with that of ${\mathrm{Al}}_2{\mathrm{O}}_3$ much greater than that of the ${\mathrm{SiO}}_2$. Our findings provide fundamental criteria for the selection of high laser damage resistance materials for different applications.

2.

Experimental Method

3.

Results

The typical morphologies of Ti particles used for the current study are shown in Fig. 1. While the commercially available Ti particles are all smooth surface spheres [Fig. 1(a)], the homemade filings exhibit irregular, mostly sickle-shaped morphologies with relatively rough surfaces [Fig. 1(b)]. The sprinkled Ti spherical particles on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer surface are shown in Fig. 2(a). While variations in size are inevitable, the majority of the particles on the surface were $\sim 30\mu \mathrm{m}$ in diameter. The apparently larger particles were usually aggregates of smaller ones. Exposure to a single laser shot at 45 deg off normal with an average fluence of $10\mathrm{J}/{\mathrm{cm}}^2$ leads to damage on the capping surface which is characterized by the distorted oval features (light areas) in Fig. 2(b). It is noted that part of the results from the spherical Ti particle-induced damage on capping layers have been discussed in detail in Ref. 19. For the purpose of clear comparison of results obtained from different Ti particle shapes, relevant results from the previous work with Ti spherical particles are adopted in the current presentation [Figs. 2(b) Fig. 3 and 4(b)].

Fig. 1

Morphology of representative surrogate Ti particles from (a) commercially available Ti spheres and (b) homemade Ti filings. The Ti sphere shown has a diameter of $22\mu \mathrm{m}$. The length of the Ti fillings shown is $\sim 150\mu \mathrm{m}$; its width ranges between 45 and $55\mu \mathrm{m}$.

Fig. 2

Optical images of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer surface: (a) with spherical Ti particle sprinkled on before laser shot and (b) after a single laser shot.

Fig. 3

Optical images of the ${\mathrm{SiO}}_2$ capping layer surface: (a) with spherical Ti particle sprinkled on before laser shot and (b) after a single laser shot.

Fig. 4

Optical images of the capping layer surface after one laser shot at the presence of irregularly shaped Ti filings: (a) ${\mathrm{Al}}_2{\mathrm{O}}_3$ surface and (b) ${\mathrm{SiO}}_2$ surface.

A comparison of Figs. 2(a) and 2(b) shows that nearly all of the damage sites are at the locations of a Ti particle. However, some damage sites appear at locations where no Ti particles are found, suggesting that some particles were relocated during transportation of the test sample between microscope imaging and laser damage testing. However, no original particles were observed after the laser shot. The size of these damage sites ranged from a few $\mu \mathrm{m}$ to $\sim 400\mu \mathrm{m}$ along the longer dimension. The damage sites had an approximate depth relative to the pristine top layer ranging between 250 and 350 nm consistent with previous observations.¹⁹ Since the capping layer was $\sim 360\text{-}\mathrm{nm}$ thick, the result suggests that damage was caused by a partial to full removal of the capping materials at the damage sites.

The distribution of Ti spheres on the ${\mathrm{SiO}}_2$ capping layer surface [Fig. 3(a)] was similar to that on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer surface. The surface was also modified at the locations of a Ti particle after one laser shot of identical conditions [Fig. 3(b)]. However, the change in the capping layer is drastically different from that observed on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ surface shown in Fig. 2(b). Significantly, there is no surface layer delamination. Instead the modified sites all show an apparent depression or erosion from the original surface with the depth of $\sim 150\mathrm{nm}$.¹⁹ Similar plasma-etched pits have been reported for both input and exit surface-fused silica surfaces with metal contaminants under pulsed laser exposures.^24,25 On both capping layers, associated with the surface modification, there are trails of Ti droplets appearing along the beam direction in the vicinity of the damage.

The response of the two capping layers to the laser–Ti irregular particle interaction is opposite to that of the laser–Ti spherical particle interaction. With Ti filing particles, the laser interaction-induced damage is more pronounced on ${\mathrm{SiO}}_2$ than on ${\mathrm{Al}}_2{\mathrm{O}}_3$. Figure 4(a) shows the surface of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer after the laser exposure. At the Ti filing location, there is essentially no surface modification except one as indicated by the arrow in Fig. 4(a). Instead, the surface contains fragmented Ti particles near the original Ti particle locations. The shape of the resulting fragments (the dark spots in the image) resembles the original sickle shape of the Ti filings. Figure 4(b) shows the surface of the ${\mathrm{SiO}}_2$ capping layer after the laser exposure. Near the original particle location, fragmented Ti particles (from submicrons to a few microns) are observed. Interestingly, along the beam propagation direction, the surface at the location where the fragmented particle resides is clearly fractured. The side toward the beam direction, on the other hand, shows optical features similar to those observed in Fig. 3(b), which may result from a similar surface depression. These dramatic differences argue for further analysis of the response of the capping materials to the interaction between laser and particles of different shapes.

Figure 5 shows the microscope images of selected sites on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer after the single laser exposure at $10\mathrm{J}/{\mathrm{cm}}^2$. Figure 5(a) is small region on the capping layer surface selected from the optical microscope image shown in Fig. 4(a) which displays the marked features on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping surface at the Ti particle location after single laser shot. At this resolution, it is difficult to discern the nature of the marked spots. To reveal the exact nature of these spots on the surface, high-resolution SEM images were collected and their images marked by the circles are shown in Figs. 5(b)–5(d). These images clearly show that the dark regions seen in Fig. 5(a) are made of small Ti particles, fragmented from the parent Ti particle upon the laser shot. It is speculated that the recoil momentum from the ejected plasma causes the remnants of the heated particle to strike the capping layer surface and lead to fragmentation and that no damage occurs on the capping surface, even at the particle locations. The others locations show similar characteristics.

Fig. 5

Microscope images of particles on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping surface after one laser shot at the presence of irregularly shaped Ti filings: (a) optical image showing the surface features and (b)–(d) SEM images showing a closer view of the specific sites marked in (a). The sites are clearly from fragmented Ti particles and are not surface damage.

4.

Phenomenology and Modeling

Laser light impinging on a metal can elevate its surface temperature through energy deposition within the optical skin depth. If the fluence is sufficiently high, the temperature can rise beyond the vaporization temperature. Subsequent interaction between the laser and vapor can cause further temperature elevation and result in plasma formation. In our earlier estimate,¹⁹ the fluence threshold for plasma formation was about $4.4\mathrm{J}/{\mathrm{cm}}^2$ for a 14-ns pulse. Thus under the present irradiation of $10\mathrm{J}/{\mathrm{cm}}^2$, plasma formation is expected. The plasma ejected from the Ti surface carries both thermal energy and momentum. Depending on the beam direction and the shape of the metal particle, the thermally active plasma plume may intercept the surface on which the metallic particle resides which can lead to surface modification including plasma scalding.²⁶ In addition to transporting thermal energy, the plasma can also lead to recoil of the fragmented droplets and the remaining particle itself. The recoil momentum depends on the ejection direction of the plasma, which we will show to be strongly correlated to the shape of the particle. The shape dependence, in turn, leads to a marked difference in the response of the capping layer to the laser–particle interaction.

The combination of oblique incidence and reflective surfaces between the sphere and the substrate leads to an increased absorption on the Ti surface, as we have shown for a spherical particle.¹⁹ The analysis utilized optical simulations with FRED. In short, for a spherical particle, the maximum absorbed intensity is shifted from 45 deg above the equator to 10 deg above the equator, as shown in Fig. 8 of Ref. 19. In the presence of the mirror, the total power absorbed by the sphere is increased by 81%, and the maximum local absorbed intensity is increased by about 26% (shifted in location). Recall that the overall enhancement of absorption is increased by the mirror. Thus, in the presence of the reflective or mirror surface, it is sufficient to have only $3.5\mathrm{J}/{\mathrm{cm}}^2$ instead of $4.4\mathrm{J}/{\mathrm{cm}}^2$ in the incident beam in order to generate plasma from the Ti particles.

The physical mechanisms of the plasma effects are shown in Fig. 6. The incident laser radiation absorbs into the metal, rapidly heating the surface and producing the initial plasma which is enhanced by the radiation reflected from the mirror. This increases the plasma generation which shifts the peak of intensity and plasma production downward toward the equator, as we noted. The hot plasma ejected as a result of rapid evaporation is partially deposited on the capping layer, resulting in heating and thermal expansion. While the thermal diffusivity of thin film is strongly dependent on the deposition process, the trends between the two capping materials are thought to remain the same. The thermal diffusion time of the capping layer is estimated to be $\sim 10\mu \mathrm{s}$ which is much longer than that of the laser pulse width. The short duration of the interaction leaves the next layer cold, and the thermal expansion of the capping layer produces a delamination on the area affected by the plasma, which can be larger than the particle size (see Figs. 2 and 3). Since the thermal expansion coefficient of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ is much higher (by a factor of about 15) than that of fused silica, the delamination of the alumina layer is much more pronounced. However, if the capping layer is thicker than the thermal diffusion length during the plasma lifetime, delamination is suppressed and the mechanical strength of the material determines the extent of the damage. Further investigation is underway to explore the critical thickness of the capping layer at which the delamination is suppressed.

Fig. 6

Schematics showing the laser-induced plasma formation and particle recoil on reflective surface with (a) a spherical Ti particle and (b) an oblate-shaped Ti particle.

The recoil momentum produced by the ejected plasma imparts motion to the particle, and the direction of the force is given by integration of the incident flux over the particle surface.²⁷ The partially melted particles move along the surface, sputtering small droplets and expanding the extent of damage.

In the case of a flattened particle, two modifying effects take place. First, as the oblateness increases, the reflected light couples less efficiently into the particle, and the region of peak intensity becomes smaller and moves toward the horizontal pole of the ellipsoid. As a result, less plasma is ejected, and it is directed increasingly toward the normal of the surface. Thus less plasma hits the substrate, and the thermally induced delamination diminishes. Second, with increasing oblateness, the recoil momentum becomes progressively more vertical. The vertical component of the recoil momentum presses the particles toward the substrate, increasing the mechanical energy coupling and decreasing the force causing motion along the surface.

To understand this behavior, FRED simulations were performed on Ti ellipsoids of successively increasing flatness. Five cases were considered: a spherical particle and five ellipsoids of aspect ratios (ratio of horizontal length to vertical length) 2:1, 4:1, 6:1, and 8:1. A sample absorption pattern on the 2:1 ellipsoid is shown in Fig. 7(a). According to the calculations, the recoil angle with the horizontal increased from 7 deg (for a spherical particle) to 77 deg (for the 8:1 ellipsoid), as shown in Fig. 7(b). Note that the slope decreases as the angle approaches vertical, as expected.

Fig. 7

(a) Absorbed intensity on the surface of an ellipsoidal particle (aspect ratio 2:1) and (b) recoil angle versus aspect ratio.

These simulation results also imply that for the irregular-shaped Ti particle, the vertical component of the recoil momentum can be over four times larger than that with the spherical particle, potentially delivering a large mechanical impact onto the capping surface. The extent of modification or damage by laser–particle coupling on the capping layer thus will be directly related to the mechanical robustness of the capping material. Since the mechanical toughness of ${\mathrm{Al}}_2{\mathrm{O}}_3$ is 10 times larger than that of ${\mathrm{SiO}}_2$, it is reasonable to hypothesize that the large disparity in mechanical toughness is mainly responsible for the observed difference in response to laser-induced damage [see Fig. 4(b)] by the Ti particles of a different shape. Furthermore, the strong mechanical toughness along with the large vertical recoil momentum is responsible for the fragmentation of the Ti filing on the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer, as shown in Fig. 5.

Thus the large damage produced by spherical contaminants is greatly modified for oblate particles. We would note, parenthetically, that spherical particles have the weakest coupling with the surface and so can be most readily removed by cleaning procedures.

5.

Conclusions

In summary, laser damage testing at oblique incidence shows that the laser–contaminant interaction on HR multilayer coatings leads to modification or damage to the surface protective capping layer. The damage behavior is strongly dependent on the shape of the contaminant particles and the composition of the capping material. For laser interaction with a spherical Ti particle, the damage is characterized by delamination of capping layers and can be related to the thermal properties of the capping materials. For laser interaction with irregularly shaped particles, the damage is characterized by substrate fracture and can be related to the mechanical properties of the capping material. Specifically, in the presence of the spherical Ti contaminant, the ${\mathrm{Al}}_2{\mathrm{O}}_3$ capping layer shows much more severe damage than that of ${\mathrm{SiO}}_2$, because ${\mathrm{Al}}_2{\mathrm{O}}_3$ has a thermal expansion coefficient that is 15 times larger than that of the ${\mathrm{SiO}}_2$. On the other hand, in the presence of the Ti filings, the damage on the ${\mathrm{SiO}}_2$ capping layer is more pronounced as the mechanical toughness of ${\mathrm{Al}}_2{\mathrm{O}}_3$ is 10 times larger than that of ${\mathrm{SiO}}_2$. We have presented numerical simulations that demonstrate the dependence of the energy absorption and inferred laser-induced plasma orientation on particle shape. Overall, our results suggest that a material with a strong mechanical toughness, but a small thermal expansion coefficient, may be effective for use as a capping material to protect against debris-induced laser ablation and damage on $1\omega$ high power dielectric coating mirrors.