Wavefront improvement by IBF-processed correction surfaces

0.5 keV Xe+ ion beam nano smoothing of ULE® substrate after processing with 3.0–10.0 keV Xe+ ion beam

The ULE® substrates used in projection optics of extreme ultraviolet lithography (EUVL) tools are mechanically prefinished with shape accuracy of several nanometer rms (specification: under 0.15 nm rms) with high-spatial frequency roughness (HSFR) (spatial wavelength: under 1 μm) of 0.06 nm rms. The ion beam figuring is used for the final shape error correction of the substrates at low spatial wavelength of greater than 1 mm using high energy (5–10 keV) Ar+ ion beam with 1 mm beam diameter. However, the surface roughness values of the ULE® substrates when machined by 5 and 10 keV Ar+ ion beams result in 0.15 and 0.17 nm rms, respectively; also it is to be noted that these values are greater than the required 0.15 nm rms specification of HFSR. Therefore, the authors developed a method where low energy (0.3 and 0.5 keV) ion beams are used for smoothening the surface of ULE® substrates, previously treated with high energy (5 and 10 keV) ion beams for figuring. By this two-stage operation of ion beam figuring and followed by smoothening, ULE® substrates with HSFR of 0.1 nm rms were obtained. Therefore, the authors conclude that the method presented here is one of the most effective methods for the figure error correction of ULE® substrate and will be useful in EUVL projection optics.

Ion Beam Figuring (IBF) for surface form error correction can be used for optics of different size. Thereby by a focused ion beam scans over the mirror surface in a meander-like path and removes the exact amount needed to correct form errors. We will give an overview about the technical aspects regarding system design for doing IBF on small and big optics. Next generation x-ray telescopes consist of ten thousands of single silicon mirrors. Each of those mirrors need individual ion beam figuring. Since the mirrors are about 100 mm in size and light-weight, they can be easily moved in front of the ion beam source. In contrast the single mirror sizes for larger telescopes, like the Extremely Large Telescope (ELT) build by the ESO, is much bigger. The primary mirror of the ELT consists of 798 single elements, each 1.5 m in diameter. Such large mirrors cannot be moved around due to its weight, so the ion beam source will be moved in front of the mirror. A system for mass production of such big mirrors will be introduced.

The convex hyperbolic secondary mirror is a Φ520-mm Zerodur lightweight hyperbolic convex mirror. Typically conventional methods like CCOS, stressed-lap polishing are used to manufacture this secondary mirror. Nevertheless, the required surface accuracy cannot be achieved through the use of conventional polishing methods because of the unpredictable behavior of the polishing tools, which leads to an unstable removal rate. Ion beam figuring is an optical fabrication method that provides highly controlled error of previously polished surfaces using a directed, inert and neutralized ion beam to physically sputter material from the optic surface. Several iterations with different ion beam size are selected and optimized to fit different stages of surface figure error and spatial frequency components. Before ion beam figuring, surface figure error of the secondary mirror is 2.5λ p-v, 0.23λ rms, and is improved to 0.12λ p-v, 0.014λ rms in several process iterations. The demonstration clearly shows that ion beam figuring can not only be used to the final correction of aspheric, but also be suitable for polishing the coarse surface of large, complex mirror.

Ion beam figuring (IBF) is a deterministic and powerful optical figuring process to high-end optics. To perform an IBF process, some computations are inevitably involved, such as to extract beam removal functions, to calculate dwell times, to determine scan velocities and to recognize the removal rate. These computations influence the process time and the process result of the IBF. In this paper, the computations involved in an IBF process are introduced and the software IBFCAM, which is developed by us to implement all the computations, is also introduced. IBFCAM is well designed and it consists of 6 modules, including IBFCAM. Read, IBFCAM. Removal, IBFCAM.BRF, IBFCAM. Time, IBFCAM. Code and IBFCAM. PostAnalysis. Each module deal with a relatively independent function. Particularly, The IBFCAM. Post Analysis is newly added to IBFCAM in version 5.0. It is used to recognize the actual the removal rate. especially useful to a new material or a new optics. This is very useful to a new optics, especially to a new material.

The ion beam figuring is a kind of advanced technology of mirror processing. It has the advantages of high processing precision, high speed and no damage to the mirror surface. The ion beam figuring machine is established by using a one meter diameter vacuum coating plant in this research project. Mechanical scanning device, ion source and workpiece make up of the machine. Water, electricity and gas will be imported to the vacuum chamber. The computer software, extracting the error function between the ion beam processing function and mirror surface function by using the data of interferometer measuring, will calculate the dwell-time function in the course of processing. The computer will control the whole process based on the dwell-time function. The experiment processing indicates that PV will reach 1/14λ and RMS will reach 1/70λ by once ion beam figuring.

Evaluation of surface roughness of ULE® substrates machined by Ar+ ion beam

To improve the effective deformation of the unimorph deformable mirror (DM), ion beam figuring (IBF) is applied for manufacturing the initial surface figure. However, heat produced during IBF can cause comparable internal stress and thermal deformation on the adhesive layer, which causes the mirror’s figure to be unsatisfactory. Consequently, the whole manufacturing process, especially thermal distribution on the adhesive, should be monitored and controlled. A 19-unit thin-layer unimorph DM is adopted as the object to simulate and measure the distribution law of temperature field on the adhesive during the IBF process. IBF parameters are optimized according to the characteristics of the adhesive and the manufacturing simulation results. With a ϕ3-mm diaphragm integrated on IBF, the processing temperature is reduced below the adhesive’s glass transition temperature Tg. Actual figuring results report over 50% reduction on both peak-to-valley (PV) and root-mean-square (RMS) of DM with the value from 310 to 141 nm and from 38 to 17 nm, respectively. This precision has satisfied the self-corrected requirement of general unimorph DMs, which is usually <20 nm RMS. The voltage fitting coefficient of self-correcting for most electrodes is reduced to <0.05, which means the significant improvement in the effective deformation and correction ability.

Ion beam figuring (IBF) is a deterministic method for final figuring process of optical surface. The removal functions of IBF should be measured and calibrated strictly to guarantee the accuracy of the figuring results. The incident parameters of ion, surface profile and ion density distribution on the surface are the main factors which affect the material removal rate within the removal function. For the curved surfaces these factors vary from point to point so that the removal functions keeps changing when figuring curved surface especially for steep surface. Unlike the first two factors, the ion density distribution can’t be described by mathematic expression directly. Moreover, the ion is not in Gaussian distribution on the curved surface any more, which will result in variation of removal functions. Faraday cup is used to get the ion space distribution. So that the ion density on an arbitrary point can be obtained exactly. Based on the material removal features, revision method is proposed to fix the difference of the removal functions between the curved surface and the flat surface. Through experiments and theoretical analysis, the removal functions changes distinctly and not in Gaussian distribution. The computation method can fit the variety of the removal function effectively.

Ion beam figuring (IBF) is a novel technology for Ultra-precise optics. Material is removed from optic surface in atomic or molecular form by physical sputtering. Due to non-contact between the tool and the work piece, the problems involved in the conventional process are avoided, such as edge-effect and tool-wear. The ion beam figuring process is of high determinacy and high efficiency. All these properties make ion beam figuring one of the promising methods for producing mirrors of high precision with nm-rms accuracy. In this article, a new ion beam figuring system which contains doubled vacuum chambers is set up. Optics can be exchanged by a transport vehicle shuttling between the two vacuum chambers without opening the primary vacuum chamber and waiting for the ion source to cool completely, which means the efficiency can be increased greatly. A high performance processing robot contains three linear axes and two angular axes of motion, providing 5-axis ion source positioning capability with high accuracy. The angle can be up to 50° to figure very steep spherical and aspherical surfaces. Then, the beam removal function of Gaussian shape is obtained by an experimental method and it is extremely stable for a long time. Finally, two sample mirrors are figured by the ion beam figuring system: one is a fused silica flat mirror with a 100 mm diameter (90% effective aperture) and an ultra-precise flat mirror with a surface error of 0.89 nm rms, 14.7 nm PV is obtained; the other fused silica concave spherical mirror with a 100 mm aperture (90% effective aperture) and 420 mm radius of curvature is figured and a concave spherical mirror with 1 nm rms, 16.9 nm PV is obtained, which prove that the ion beam figuring system is favourable for the figuring process.

The laser damage threshold of fused silica optics is affected by the surface/subsurface defects in optical fabrication and is related to surface contamination. In this paper, the evolution of surface roughness and photothermal weak absorption of fused silica treated by plasma cleaning and ion beam figuring (IBF) is studied. The results show that plasma cleaning has a certain change on the surface roughness of fused silica, while the change of photothermal weak absorption depends on the initial surface quality. The surface roughness of fused silica has a certain regularity after ion beam figuring, and the photothermal weak absorption is basically consistent with the initial.

With the application spectrum moving from infrared to visible light, aluminum optics with complex forms are difficult to fabricate by the majority of existing processing methods. Possessing the highest machining precision and low processing contamination, ion beam figuring (IBF) is a better method for fabrication of aluminum optics. However, the surface roughness deteriorates with the removal depth during IBF. In this study, the extra material removal during the IBF process is studied systematically. Extra material removal consists of two parts, determined by the convolution process and the limitation of the dynamic performance of machining tools. Extra material removal can be reduced by filtering out the surface residual error with a spatial frequency higher than the cut-off frequency and reducing the iterations of the machining process. Then, the executability of the dwell time matrix and the figuring ability of the removal function are analyzed. Adjusting the working parameters (volume removal rate) reduces the requirements for dynamic performance of machining tools. Finally, a minimal material removal processing strategy for aluminum optics based on power spectral density analysis and a spatial frequency filtering method is proposed. A simulation is conducted to verify the feasibility of the proposed strategy. With the same final precision (59.8 nm PV and 4.4 nm RMS), the maximum material removal decreases nearly 36 nm by applying the strategy, which reduces roughness nearly 10 nm. This study promotes the application of IBF in the field of aluminum optics fabrication as well as improves the machining precision of aluminum optics.

During the ion beam figuring (IBF) of a space mirror, thermal radiation of the neutral filament and particle collisions will heat the mirror. The adhesive layer used to bond the metal parts and the mirror is very sensitive to temperature rise. When the temperature exceeds the designed value, the mirror surface shape will change markedly because of the thermal deformation and stress release of the adhesive layer, thereby reducing the IBF accuracy. To suppress the thermal effect, we analyzed the heat generation mechanism. By using thermal radiation theory, we established a thermal radiation model of the neutral filament. Additionally, we acquired a surface-type Gaussian heat source model of the ion beam sputtering based on the removal function and Faraday scan result. Using the finite-element-method software ABAQUS, we developed a method that can simulate the thermal effect of the IBF for the full path and all dwell times. Based on the thermal model, which was experimentally confirmed, we simulated the thermal effects for a 675 mm×374 mm rectangular SiC space mirror. By optimizing the dwell time distribution, the peak temperature value of the adhesive layer during the figuring process was reduced under the designed value. After one round of figuring, the RMS value of the surface error changed from 0.094 to 0.015λ (λ=632.8 nm), which proved the effectiveness of the thermal analysis and suppression method.

The continuous phase plate (CPP) provides excellent beam smoothing and shaping impacts in the inertial confinement fusion application. However, due to the features of its dispersion, its surface gradient is frequently too large (>2 μm/cm) to process. When machining a large gradient surface with continuous ion beam figuring (IBF), the acceleration of the machine motion axis cannot fulfill the appropriate requirements, and the machining efficiency is further influenced by the unavoidable extra removal layer. The pulsed ion beam (PIB) discretizes the ion beam by incorporating frequency-domain parameters, resulting in a pulsed beam with a controlled pulse width and frequency and avoiding the extra removal layer. This research evaluates the processing convergence ability of IBF and PIB for the large gradient surface using simulation and experiment. The findings reveal that PIB offers obvious advantages under the same beam diameter. Compared with the convergence ratio (γ = 2.02) and residuals (RMS = 184.36 nm) of IBF, the residuals (RMS = 27.48 nm) of PIB are smaller, and the convergence ratio (γ = 8.47) is higher. This work demonstrates that PIB has better residual convergence in large gradient surface processing. It is expected to realize ion beam machining with a higher convergence ratio.

In this paper, an optical fabrication process is introduced in detail, which is about achieving ultra-precision surface by iteration of ion beam figuring (IBF) and supersmooth polishing technology. Developed as a new technology to achieve supersmooth surface, Numerical controlled micro-jet polishing (MJP) technology is also introduced. 100mm-diameter fused silica is used as the sample. In the process, the surface is superpolished with MJP at first, then IBF is used to correct the figure, and finally MJP is applied again to finish iteration process. The path for IBF is raster scanning, and MJP processes in a spiral way. The figure and roughness of the surface are tested and analyzed with interferometer and atomic force microscope (AFM). The results indicate that ultra-precision optical surface is achieved by the iteration process with figure accuracy of 0.879nmrms, mid-spatial frequency and high-spatial frequency roughness of 0.119nmrms and 0.0726nmrms respectively.