Ion beam processing for ultra-smooth glass-ceramic optics

In modern optical systems (e.g., space x-ray optics and extreme UV lithography), the specifications for surface accuracy and smoothness of mirrors are becoming ever more stringent. In general, nanometer—or even sub-nanometer—magnitudes are required for these parameters. Advanced fabrication technologies are therefore needed for the manufacture of high-accuracy and high-quality optical surfaces. Ion sputtering processes—such as ion beam figuring (IBF)—are promising techniques for the fabrication of ultra-smooth surfaces. The use of ion sputtering for ultra-smooth fabrication, however, is strongly dependent on processing conditions and material characteristics. In this process, ion-induced self-organization may also create nanopatterns that can be observed on some material surfaces. For instance, ion sputtering of glass-ceramic materials often results in the formation of surface nanoscale microstructures¹ that can seriously affect the quality of the optical surface.

IBF is a highly deterministic (i.e., the process is predictable, quantitative, and controlled), highly stable, non-contact method, and can provide an ultra-precise fabrication method for optical surfaces. All these characteristics present advantages over conventional figuring technologies.² In high-performance optics that require nanometer or sub-nanometer surface accuracy, materials with a low coefficient of thermal expansion are typically used as a substrate. Zerodur (produced by Schott AG) is a lithium aluminosilicate glass-ceramic that contains both non-crystalline (amorphous silica) and crystalline (quartz) composites. It can match zero thermal expansion very closely within a large temperature range and is therefore a prime candidate material for telescope mirrors and lithography optics. The difference in sputtering rates of dual-phase materials such as Zerodur, however, is an important factor in the evolution of their nanoscale morphology. During IBF of Zerodur, the amorphous phase is sputtered faster than the crystalline phase, which causes the granular quartz to protrude from the Zerodur surface (see Figure 1). At even greater removal depths, the distribution of granular patterns becomes dense and the height of the nanopatterns is amplified. This causes serious roughening of the optical surface. The preferential sputtering effect, however, disappears for some amorphous materials (or surfaces that can be amorphized). In these cases, ion-induced smoothing mechanisms dominate the surface morphology evolution instead.

Figure 1. Illustration of the ion beam sputtering process. Atomic force microscopy (AFM) images of a Zerodur surface before (left) and after (right) ion beam figuring (IBF). The IBF process increases the surface roughness of the material, as illustrated by the associated surface profiles. Ar⁺: Argon ion. D: Diameter. SiO₂: Silica. RMS: Root mean square.

We previously showed that ultra-smooth surfaces can be obtained on fused silica and on silicon surfaces. We were able to achieve surface roughness values down to 0.1nm root mean square (RMS)—see Figure 2—for these materials.^2,3 From the different ion sputtering results for these two materials, compared with Zerodur, we conclude that morphology evolution at microscopic scales has an inseparable relationship with the behavior of surface materials. This is especially obvious during the fabrication of ultra-smooth surfaces. Fortunately, glass-ceramic optics are mostly used in reflective systems, where a multilayer film is deposited on the substrate to improve reflectivity. We have therefore developed a new IBF method for the fabrication of ultra-smooth Zerodur optics.

Figure 2. AFM images of fused silica and silicon (Si) surfaces before and after IBF, showing surface morphology. (a) Original fused silica surface. (b) Fused silica surface after IBF. (c) Original Si surface. (d) Si surface after IBF.

In our technique, we use a deterministic method to deposit a layer of a suitable material on the Zerodur substrate. This method (see Figure 3) is known as deterministic ion beam adding (IBA). We then use an IBF step to smooth this surface layer. The addition of the thin layer of material on the Zerodur substrate has no influence on later processing of the optical component.

Figure 3. Schematic illustration of the combined ion beam processing technique for glass-ceramic optics surfaces, using a deterministic ion beam adding (IBA) and removal method.

We do not use IBA for the deterministic removal of material from the optical surface, or to control surface error. Rather, we aim to add material deterministically to the local low areas of the optical surface, and to correct the surface error. During the material adding process, we can maintain, and even improve, the original surface figure accuracy.⁴ IBA therefore presents an advantage over conventional coating techniques. In addition, it is a feasible method for the addition of suitable material layers on extremely precise Zerodur surfaces.

To test our new combined methodology, we used ion beam sputtering to prepare two patterned Zerodur samples. We observed granular nanostructures—as shown in Figure 4(a) and (d)—that had formed on the surfaces of these samples after direct ion sputtering. We used our IBA technique to add a layer of silicon or silica to the samples. Through this process the granular nanostructures were covered with the added surface layers and the surface roughness of both samples was slightly reduced, as shown in Figure 4(b) and (e). In the final step, we used IBF runs to smooth the added surface layers further. Our experimental results show that the surface quality of the Zerodur optics was substantially improved—see Figure 4(c) and (f)—and we have therefore demonstrated the feasibility of our method.

Figure 4. AFM images showing the surface morphology of Zerodur optic surfaces used in experimental testing of the new sputtering technique. Si and SiO₂ target materials have been applied to the Zerodur samples in the top and bottom rows, respectively.The samples are covered with embryonic granular patterns in (a) and (d). In (b) and (e) the granular nanostructures have been covered by the added surface layer to reduce the surface roughness. IBF was used to smooth the samples further, as shown in (c) and (f).

We have developed a new IBF method for Zerodur optics that involves deterministic IBA technology. With this methodology, we can achieve ultra-smooth Zerodur optics, with surface roughness values down to 0.15nm RMS. Our next step is to investigate wider applications for our combined processing technology. This will include fabrications of large silicon carbide and glass-ceramic mirrors for space-based optical systems.

This work was supported by the National Natural Science Foundation of China (under contracts 51175504 and 91023042).