Ion Beam Figuring System Research Articles

Sub-nanometre quality X-ray mirrors created using ion beam figuring.

Ion beam figuring (IBF) is a powerful technique for figure correction of X-ray mirrors to a high accuracy. Here, recent technical advancements in the IBF instrument developed at Diamond Light Source are presented and experimental results for figuring of X-ray mirrors are given. The IBF system is equipped with a stable DC gridded ion source (120 mm diameter), a four-axis motion stage to manipulate the optic, a Faraday cup to monitor the ion-beam current, and a camera for alignment. A novel laser speckle angular measurement instrument also provides on-board metrology. To demonstrate the IBF system's capabilities, two silicon X-ray mirrors were processed. For 1D correction, a height error of 0.08 nm r.m.s. and a slope error of 44 nrad r.m.s. were achieved. For 2D correction over a 67 mm × 17 mm clear aperture, a height error of 0.8 nm r.m.s.and a slope error of 230 nrad r.m.s. were obtained. For the 1D case, this optical quality is comparable with the highest-grade, commercially available, X-ray optics.

Development of a position–velocity–time-modulated two-dimensional ion beam figuring system for synchrotron x-ray mirror fabrication

With the rapid evolution of synchrotron x-ray sources, the demand for high-quality precision x-ray mirrors has greatly increased. Single nanometer shape accuracy is required to keep imaging capabilities at the diffraction limit. Ion beam figuring (IBF) has been used frequently for ultra-precision finishing of mirrors, but achieving the ultimate accuracy depends on three important points: careful alignment, accurate dwell time calculation and implementation, and accurate optical metrology. The Optical Metrology Group at National Synchrotron Light Source II has designed and built a position-velocity-time-modulated two-dimensional IBF system (PVT-IBF) with three novel characteristics: (1) a beam footprint on the mirror was used as a reference to align the coordinate systems between the metrology and the IBF hardware; (2) the robust iterative Fourier transform-based dwell time algorithm proposed by our group was applied to obtain an accurate dwell time map; and (3) the dwell time was then transformed to velocities and implemented with the PVT motion scheme. In this study, the technical aspects of the PVT-IBF systems are described in detail, followed by an experimental demonstration of the figuring results. In our first experiment, the 2D RMS in a $ 50\;{\rm mm} \times 5\;{\rm mm} $50mm×5mm clear aperture was reduced from 3.4 to 1.1 nm after one IBF run. In our second experiment, due to a 5 mm pinhole installed in front of the source, the 2D RMS in a $ 50\;{\rm mm} \times 5\;{\rm mm} $50mm×5mm clear aperture was reduced from 39.1 to 1.9 nm after three IBF runs, demonstrating that our PVT-IBF solution is an effective and deterministic figuring process.

Machining position-attitude optimisation in ion beam figuring of high-precision off-axis aspheric surfaces

High-precision off-axis aspheric components are widely applied in various high-performance optical systems. However, the complicated manufacturing features of off-axis aspheric make it difficult to achieve high-precision surface for traditional optical manufacturing methods. Ion beam figuring (IBF) is a non-contact and highly deterministic method for the final precision optical figuring. In this paper, two different machining position-attitude models are discussed, which have great influence on the machining difficulty and figuring accuracy of IBF. The child mirror coordinate position-attitude model (CCPAM), which is based on the robustness of IBF removal function to small disturbance of target distance and incidence angle, can effectively decrease the sag height and incidence angle, thereby reduce the beam errors and machining difficulty. A linear three-axis IBF system is successfully applied for the figuring of a large-aperture parabolic off-axis aspheric mirror based on the CCPAM and its best-fit sphere. With two iterations of 113 min, surface error is down to 7.658 nm RMS (effective aperture 310 mm × 310 mm). The figuring results indicate IBF can realise the rapid fabrication of high-precision off-axis aspheric based on the machining position-attitude optimisation.

A one-dimensional ion beam figuring system for x-ray mirror fabrication.

We report on the development of a one-dimensional Ion Beam Figuring (IBF) system for x-ray mirror polishing. Ion beam figuring provides a highly deterministic method for the final precision figuring of optical components with advantages over conventional methods. The system is based on a state of the art sputtering deposition system outfitted with a gridded radio frequency inductive coupled plasma ion beam source equipped with ion optics and dedicated slit developed specifically for this application. The production of an IBF system able to produce an elongated removal function rather than circular is presented in this paper, where we describe in detail the technical aspect and present the first obtained results.

Development of ion beam figuring system with electrostatic deflection for ultraprecise X-ray reflective optics.

We developed an ion beam figuring system that utilizes electrostatic deflection. The system can produce an arbitrary shape by deterministically scanning the ion beam. The scan of the ion beam, which can be precisely controlled using only an electrical signal, enables us to avoid degradation of the mirror shape caused by imperfect acceleration or deceleration of a mechanically scanning stage. Additionally, this surface figuring method can easily be combined with X-ray metrology because the workpiece remains fixed during the figuring. We evaluated the figuring accuracy of the system by fabricating a plano-elliptical mirror for X-ray focusing. A mirror with a shape error of 1.4 nm root mean square (RMS) with a maximum removal depth of 992 nm, which corresponds to figuring accuracy of 0.14% RMS, was achieved. After the second shape corrections, an elliptical shape with a shape error of approximately 1 nm peak-to-valley, 0.48 nm RMS could be fabricated. Then, the mirror surface was smoothed by a low-energy ion beam. Consequently, a micro-roughness of 0.117 nm RMS, measured by atomic force microscopy, was achieved over an area of 1 × 1 μm(2).

Ion Beam Figuring System for Ultra-Precise Optics

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.

Corrective capability analysis and machining error control in ion beam figuring of high-precision optical mirrors

In deterministic ion beam figuring (IBF) technology, the application of small ion beam enhances the corrective capability for mid-to-high spatial frequency errors on the optical surface, which directly determines the surface accuracy of the figuring process. But when the diameter of the ion beam becomes smaller, the machining errors will have a stronger influence on the final figuring result, so these errors must be controlled through corresponding methods. We investigate the corrective principle in IBF for surface errors of different spatial frequencies and establish the selection criterion for removal function in different figuring stages to realize the rapid convergence of surface accuracy. Then, through analyzing and controlling the machining errors in the figuring process, high-precision mirrors can be rapidly obtained. Finally, experiments on fused silica planar and spherical samples are conducted on our self-developed IBF system, and their final surface accuracy are both smaller than 1.1 nm RMS (root-mean-square) and 12.0 nm PV (Peak-to-Valley) after several iterations within 20 min.

Realization of Velocity Mode in Flat Optics Machining Using Ion Beam

In order to overcome the disadvantages such as more extra process time and lower machining precision in current ion beam figuring process mode, which is referred as position mode, a new process mode is proposed, which is referred as velocity mode. The method to determine the feed velocity in velocity mode is discussed in details. To demonstrate the proposed velocity mode process, a φ 98 flat neoceramic glass sample is figured on the ion beam figuring system KDIFS-500, which is developed by us. Its figure error is improved from initial 33.2 nm root mean square to 3.1 nm root mean square through one figuring process of 17.4 min with convergence ratio reaching 10.7. The figuring result shows that the velocity mode would save process time, improve machining precision and achieve higher convergence ratio than position mode.

IBFによる光学素子の加工 第1報:高精度な形状修正加工

IBFによる光学素子の加工 第1報:高精度な形状修正加工