2.1.

Over-Coating: Bury the Unsuitable Mirror Coating Under Another Mirror Coating

The electric field magnitude in a high reflection coating quenches rapidly within the outermost coating layers, as shown in Fig. 1, which is an electric field model generated by OptiLayer software of the 42-layer quarter wave design. The reasoning behind repair method (i) is that the deposition of a correct mirror coating on top of an incorrect mirror coating would even further diminish the amount of light penetrating into the incorrect coating. Therefore, the incorrect coating may be prevented from greatly impacting the LIDT. This repair method is therefore most suitable for mirror coatings rather than high transmission coatings. The primary advantage of over-coating is that it is straightforward and faster than the ion milling approaches that we also tested. A disadvantage of this method concerns the possibility of delamination and crazing as a result of stress mismatch between the incorrect coating and the correct over-coating.

Fig. 1

Electric field magnitude model of the 42-layer ${\mathrm{HfO}}_2/{\mathrm{SiO}}_2$ quarter-wave type coating design, shown at 1054 nm, 45 deg in P-pol and S-pol. The vertical dashed lines denote layer boundaries. When this coating was deposited, a lack of oxygen resulted in the deposition of Hf metal instead of ${\mathrm{HfO}}_2$ for layer 35. At the boundaries of layer 35, the electric field intensity, as a fraction of the incident intensity, is 30% at the start of the layer and 15% at the end of the layer.

2.2.

Ion Milling: Etch the Unsuitable Coating Completely from the Optic Surface with Ion Milling, and then Recoat the Optic

Ion milling is a physical etching process that involves the ionization of a gas (or combination of gases) that is accelerated by an ion source.³ The ion source is typically directed at the surface to be etched, and the bombardment of the ions against the surface removes particles from the surface.³ It is possible to remove an optical coating from a substrate using ion milling, but mention of this practice is virtually absent from the literature. More common applications of ion milling for optical coatings include ion-assisted deposition,⁴ substrate cleaning,^4,5 optical fabrication,⁶ in situ coating layer thickness control,⁷ and distributed phase plate manufacture.⁸

Although ion milling is slower compared with reactive ion etching and various chemical etching processes, it is promising for the removal of optical coatings because of its relative simplicity: it does not rely on maintaining complex chemical conditions with hazardous materials, and the mechanical force on the substrate due to the ion bombardment is negligible compared with polishing.⁶ However, ion milling can also increase the surface roughness of the substrate, and create an altered substrate layer as a result of preferential sputtering and decomposition.⁴ While these factors can degrade the performance of an optic, we tested ion milling anyway, considering that surface defects on the substrate may be less damaging to high reflection coatings compared with transmissive coatings. Coating systems that include an ion source for ion-assisted deposition are already equipped to perform ion milling. Following ion milling, the bare optical substrate can then be recoated with the correct optical coating.

2.3.

Ion Milling: Etch Through a Number of Unsuitable Layers with Ion Milling, Leaving the Rest of the Coating Intact, and then Recoat the Layers that Were Etched

This process is the same as repair method (ii) except that it involves ion milling to only etch through undesired layers, then recoating those etched layers. This has the advantage of being faster than repair method (ii), which requires more time to accommodate the entire removal of the optical coating. In our case, we used this method to etch through all the layers leading up to and past the incorrect Hf metal layer (a total of 10 layers were etched). Then, we recoated those 10 layers.

3.1.

Over-Coating: Bury the Unsuitable Mirror Coating Under Another Mirror Coating

A test optic was prepared with the 34-layer mirror coating having the intentional Hf metal layer at layer 27. Following the deposition of this 34-layer coating, the test optic was removed from the coating chamber. While it would be prudent to allow the optic to remain in the coating chamber for the immediate deposition of the correct mirror coating over the incorrect one, we opted to remove the test optic in order to mimic the conditions that our 65-cm diameter BK7 mirror had gone through. More specifically, when we learned that the coating on the 65-cm diameter optic was incorrect, we removed it from the coating chamber because we lacked experience in the repair of optical coatings and, therefore, needed to conduct tests to determine the best approach for repairs. After the 34-layer test optic was removed from the coating chamber, it was washed according to our protocol¹⁰ and returned to the coating chamber for the over-coating process. The over-coating was a 35-layer mirror coating that was equivalent to the 34-layer coating except (1) the coating did not contain any Hf metal layers, and (2) the first layer was a quarter wave of ${\mathrm{SiO}}_2$ to maintain the quarter-wave stack characteristics of the coating, since the outermost layer of the incorrect 34-layer coating was a half-wave of ${\mathrm{SiO}}_2$. What this over-coating method amounted to was essentially a 41-layer quarter-wave stack coating on top of an Hf metal layer. The 41-layer quarter wave stack begins with a ${\mathrm{SiO}}_2$ quarter-wave (layer 28 of the incorrect 34-layer coating) and ends with a ${\mathrm{SiO}}_2$ half-wave (layer 35 of the correct over-coating).

3.2.

Ion Milling: Etch the Unsuitable Coating Completely from the Optic Surface with Ion Milling, and then Recoat the Optic

We performed ion milling using our 16-cm diameter RF ion source (manufactured by Veeco), which is normally used for ion-assisted deposition. As shown in Fig. 2, the ion beam is oriented diagonally to aim at the center of the rotating planet can when it is located on the opposite side of the chamber. We performed a preliminary ion milling test on a slab of float glass that was 94-cm diameter truncated to 44-cm wide in order to test different ion milling parameters and establish the settings to use for etching the large 65-cm diameter optic. The slab of float glass was prepared with the 34-layer test coating having the intentional Hf metal layer located at layer 27. We etched the float glass until layer 25 was removed. Several lessons were learned as a result of this initial ion milling test with float glass, which are summarized in the points later.

Fig. 2

The RF ion source.

The nonuniform etch rate of our ion milling process posed the most significant challenges, and we considered what effect this would have on the feasibility of repair method (iii) as we observed the removal of the Hf metal layer (layer 27) on the float glass. The float glass ion milling test was stopped after etching through layer 25, to help ensure that no remnants of Hf metal remained at the center of the optic. However, additional layers were exposed simultaneously in a bull’s eye pattern due the etch rate being fastest at the center of the optic compared to the edges. Starting from the center of the float glass we observed layer 24, and in the bull’s eye rings surrounding layer 24, we observed layer 25, 26, and 27. If we were to pursue repair method (iii) and recoat the layers that were removed, we would begin with recoating layer 25, but that means the final coating would only be correct over the center of the bull’s eye area ($\sim 15\text{-}\mathrm{cm}$ diameter) where layer 24 was exposed after ion milling. This partial correction of the coating would not adequately serve the function of the large optic that we were trying to repair. Therefore, repair method (iii) was no longer an option for large optics, but we tested it anyway at a later time because it may be suitable for small optics, which we describe in Sec. 3.3.

At this point, our investigation of repair method (i) was also complete and we learned that the LIDT of the repaired coating was too low to be appropriate for operation in the beam train.

By process of elimination, we could already appreciate that repair method (ii) was the best option for large optics, and this informed our decision to promptly utilize this method to repair the 65 cm diameter optic without further testing, especially because this optic was required urgently. A few unknowns that were not addressed were (1) what the etch rate of the BK7 substrate was, (2) how much curvature would be etched into the substrate as a result of the nonuniform etch rate, and (3) how would the surface roughness of the substrate be affected by the long ion milling process.

In preparation to determine the etch rate of BK7, we measured the thickness of two uncoated BK7 test substrates in five locations each using a micrometer with $1\text{-}\mu \mathrm{m}$ resolution. Then, one test substrate was placed at the center of a planet can in the coating chamber, and the other was placed 25 cm from the center of the planet can, allowing us to measure differences in etch uniformity at locations analogous to the center of a large optic and 25 cm from the center of the large optic (the 25-cm distance is based on an existing opening in the tooling plate that secures the test substrates in the chamber). These uncoated test substrates were then etched during the ion milling process that completely removed the 42-layer mirror coating from the 65-cm diameter BK7 optic.

We paused the etch process for the 65-cm diameter BK7 optic when layer 1 appeared (${\mathrm{HfO}}_2$). During the pause, we measured the diameter of each ring in the bull’s eye pattern, which included layers 1 through 9. A model of the bull’s eye pattern, based on the diameter measurements, is shown in Fig. 3, and a plot of the curvature of the etched layers is shown in Fig. 4. A radius of curvature of 32 km, also plotted in Fig. 4, was loosely fitted to the layer thickness data. However, because the substrate etches at a slower rate compared with the coating materials, it was practical to assume that the radius of curvature etched into the substrate would be even greater than 32 km. However, even if the substrate had a radius of curvature of 32 km, this would not have a significant effect on the performance of the mirror.

Fig. 3

Model of the etched coating layers in a bull’s eye pattern on the 65-cm diameter substrate just after layer 1 (${\mathrm{HfO}}_2$) was exposed. Layer 1 is the central light circle, surrounded by layers 2 through 9. Light colored rings represent ${\mathrm{HfO}}_2$ and dark colored rings represent ${\mathrm{SiO}}_2$. The outermost dark ring is the portion of the BK7 substrate that was masked by the tooling that holds the optic in the coating chamber.

Fig. 4

The thicknesses of the layers present on the substrate with respect to substrate radius just after layer 1 was exposed during the ion milling process of the 65-cm diameter substrate. A radius of curvature of 32 km was loosely fitted to the data to give a conservative approximation of the curvature that would be etched into the substrate. The labeled brackets show the approximate span of each layer, starting with layer 1 and ending with layer 9.

The entire ion milling process was spread over 10 days with few breaks. The substrate itself was etched for 52.5 h. In other words, after the substrate first appeared, it was exposed to the ion beam for 52.5 h while the coating layers between the center and edge of the substrate were finally etched away, due to the slower etch rate at the edges.

After the ion milling process finally concluded, we visually inspected the surfaces of the 65-cm diameter substrate and test substrates. Every substrate was covered with small pits, and the 50-cm diameter test substrates were subsequently measured with a microscope to determine the pit sizes (microscope: Zeiss Axioskop 2 with Basler A631 fm camera). Also, the thicknesses of the 50-cm diameter test substrates were measured again to calculate the etch rate of BK7, but the amount of material removed was so small that no change in thickness was measured due to the limited resolution of the micrometer.

Following the surface inspection, the etched optics (two test substrates and the 65-cm diameter BK7 substrate) were cleaned using our standard protocol.¹⁰ The pitting did not hamper our ability to clean the substrates in any discernable way. Moreover, the surface tension of the substrates as we washed them indicated they were quite clean, which is a testament to the effectiveness of ion milling as an in situ cleaning process. The most characteristic evidence of this was how easily the deionized water sheeted off the substrates without beading up. In our experience, contamination on substrates causes water to bead up, and these beads can leave additional residue on the substrate. We did not experience this issue with any of the etched substrates.

After cleaning, the substrates were returned to the coating chamber. As before, one of the test substrates was located in the center of the planet can and the other was located 25 cm away from the center of the planet can. The correct 42-layer coating was then deposited.

3.3.

Ion Milling: Etch through a Number of Unsuitable Layers with Ion Milling, Leaving the Rest of the Coating Intact, and then Recoat the Layers that were Etched

As explained above, repair method (iii) is not an appropriate method for large optics because the etch rate is uniform only within a central 15-cm diameter region. However, for small optics, repair method (iii) is applicable and faster than repair method (ii). For this ion milling test, we prepared a single test substrate with the 34-layer test coating having the intentional Hf metal layer located at layer 27. Ion milling was then used to remove all layers through layer 25, in the same ion milling test that was used to etch layers from the float glass substrate reported above. Layers 25 through 34 (10 layers total) were then recoated at a later date. Test coatings of the recoated layers were conducted to obtain a spectral match between the recoated layers and the underlying layers. When an appropriate test coating was realized, it was then recoated onto the test substrate.

5.1.

Over-Coating: Bury the Unsuitable Mirror Coating under Another Mirror Coating

As shown in Fig. 6, the LIDT of the repair method (i) coating is just $7.0\mathrm{J}/{\mathrm{cm}}^2$. This repaired coating consists of the incorrect 34-layer coating and a correct 35-layer overcoat. The LIDT of just the 35-layer overcoat is $70.0\mathrm{J}/{\mathrm{cm}}^2$, which indicates that the low LIDT of the repaired coating may still be influenced by the Hf metal layer in the incorrect coating even though that layer is 42 layers deep in the coating. An electric field model of this coating could help us understand whether the deeply buried yet absorbing Hf metal layer may play a large role in reducing the LIDT of the coating. Unfortunately, there was not an adequate amount of refractive index or absorption data available for Hf metal to create an electric field model of this coating in our wavelengths of interest.

An alternative explanation of the poor LIDT of this coating is the possibility of high residual stress. Depositing 35 additional layers on top of an existing 34-layer coating more than doubles the thickness of the original coating and therefore increases its susceptibility to residual stress problems. Furthermore, the original coating was deposited at an earlier time compared with the additional 35 layers, which could lead to a stress mismatch between the original and recoated layers. Fortunately, we did not observe crazing, delamination, or any other obvious physical defects, but these problems could be more apparent on larger optics.

The optical transmission scans shown in Fig. 5 of both the repaired and unrepaired coatings are similar, owing to the broad reflectivity of the Hf metal layer in the incorrect coating. The coatings have low transmission, below 0.04% at both 1054 and 1064 nm. Even so, the low LIDT of the repaired coating indicates that enough light may still be reaching the incorrect Hf metal layer to induce damage. As a consequence of the low LIDT improvement from $1.0\mathrm{J}/{\mathrm{cm}}^2$ to $7.0\mathrm{J}/{\mathrm{cm}}^2$, repair method (i) is not adequate for our laser system.

5.2.

Ion Milling: Etch the Unsuitable Coating Completely from the Optic Surface with Ion Milling, and then Recoat the Optic

The LIDTs of the repair method (ii) coatings are $61.0\mathrm{J}/{\mathrm{cm}}^2$ and $49.0\mathrm{J}/{\mathrm{cm}}^2$, which are adequate for operation in our laser system. The difference between these coatings is that one substrate was located at the center of the planet can (achieving the LIDT of $61.0\mathrm{J}/{\mathrm{cm}}^2$), and the other was located 25 cm from the center of the planet can (achieving the LIDT of $49.0\mathrm{J}/{\mathrm{cm}}^2$), as explained in Sec. 3. Beyond this difference, the exact causes of the LIDT dissimilarity is not known, and could be due to a number of factors, including coating or etch nonuniformity on the coating plane, which warrant further investigation.

The repair method (ii) coating has a transmission of 0.10% at 1054 nm and 0.34% at 1064 nm, which is adequate for operation in the laser system, although these values could be improved with better centering of the high reflection band at 1054 nm at the time of LIDT testing. The repair method (ii) coating is actually centered at 1036 nm where transmission is 0.02%, but this was intentional because aging often allows our high reflective coatings of this type to shift 20 nm to higher wavelength. Also, because of the higher transmission at 1064 nm compared with 1054 nm, the LIDT at 1054 nm is likely greater than the 49.0 to $61.0\mathrm{J}/{\mathrm{cm}}^2$ values reported here.

While the repair method (ii) coating meets both the LIDT and spectral requirements for operation in the laser beam train, the presence of pits on the etched substrate is a concern. As mentioned in Sec. 3, small pits were scattered on all etched substrate surfaces, which included the two test substrates and the 65-cm diameter mirror substrate. The pits appeared to be scattered fairly evenly on these substrates, with slightly higher density in the center, and ranged in size from about 5 to $10\mu \mathrm{m}$, as captured by the microscope images in Fig. 7. A visual inspection revealed the density of pits to be $\sim 40\text{pits}/{\mathrm{cm}}^2$ in the worst case at the center of the substrate.

Fig. 7

Microscope images of the 5 to $10\mu \mathrm{m}$ pits that were scattered on the repair method (ii) substrates.

Optical scattering is a problem associated with pits, but the effect has been negligible since the repaired mirror has been operating in the beam train since April 2014. In other words, the performance of the laser system did not change after the repaired mirror was introduced as a replacement for the original mirror. We suspect that high reflection coatings are more resilient in terms of their ability to perform well on an inferior substrate because only a small fraction of light actually transmits through to the substrate before being reflected back to the incident medium. Also, the relatively large thickness of high reflection coatings may be better able to fill in defects on a substrate surface. On the other hand, the high density of substrate pits could be very harmful to the LIDT of transmissive coatings due to absorption at these defect sites, causing coating damage or removal. However, at a later time we used the same ion milling process to remove an 8-layer coating from a lens (1132-nm thick coating, versus 7758-nm thick for the 42-layer coating in this study) and observed that the density of pits was much lower: $\sim 1$ to $5/{\mathrm{cm}}^2$. The presence of these imperfections on the lens is not ideal, but certainly more suitable for a transmissive coating compared with the mirror substrate analyzed in this study. Ultimately, our experiments demonstrate that etch time has a direct effect on the density of substrate pits. Future research could involve tuning the ion milling process to reduce pitting, perhaps by decreasing the ion energy once the substrate surface becomes visible, though this will increase the time to etch away the remainder of the coating.

5.3.

Ion Milling: Etch Through a Number of Unsuitable Layers with Ion Milling, Leaving the Rest of the Coating Intact, and then Recoat the Layers that were Etched

The repair method (iii) coating has an LIDT of $32.3\mathrm{J}/{\mathrm{cm}}^2$. This is a moderate improvement, but still not as satisfactory as the LIDT of the repair method (ii) coating. The LIDT of the repair method (iii) coating suffers because it was discovered that the 24-layer coating that remained on the substrate after the ion milling process was actually a high reflection coating centered at 1085 nm, rather than 1054 nm. We did not know this from the outset because the reflectance of the original incorrect coating was broad, due to the reflective Hf metal layer, as shown in Fig. 5. To maintain consistency among the coating layers, the 10 layers that were recoated to form the 34-layer repaired coating were also centered at 1085 nm. However, because LIDT testing took place at 1064 nm, the transmission of the repaired coating at this wavelength is 0.47%. For comparison, the repair method (ii) coating has a transmission of 0.34% at 1064 nm and achieved an LIDT that was nearly twice as high as the LIDT of the repair method (iii) coating. Moreover, the transmission of the repair method (iii) coating at 1054 nm is 0.71%, which is not adequate for operation in the laser beam train. Consequently, the spectral shift that is an effect of aging of the repair method (iii) coating will further increase the transmission at 1054 nm. It would therefore be interesting to repeat this experiment, starting with a properly centered “incorrect” coating at 1054 nm, and determine whether the LIDT is still poor after the coating repair has taken place.