High-End Finishing Correction Processes

For highest precision and to minimize form deviations, optics can be subjected to a final correction step, the so-called finishing. This serves for even more precise surface treatment by polishing and enables zonal corrections of even the finest deviations. asphericon offers three different high-end finishing processes for refining the surface structure of optical components:

• Ion beam process (ION-FinishTM),
• Magnetorheological polishing (Magnetorheological Finishing-Technology®) and
• Ångström Polishing.

These three procedures will be explained and compared on the following pages. Initially, the focus will be on ION Finishing.

ION-Finishing - What is behind it?

With the ion beam process (ION-Finish^TM), extreme accuracies in polishing error correction can be achieved. Through localized “treatment” of the optical surfaces by means of a focused ion beam, a precisely controlled ablation with a depth resolution in the sub-nanometer range is realized (see fig. 1). The technology enables shape accuracy up to λ/50.

Figure 1: Spot processing of optical surfaces using a focused ion beam

The Procedure

For ION-Finishing, an element to be machined (maximum area of 200 x 200 mm² edge length 100 or 200 mm) is held in a special fixture. The measurement of the optical element mounted in the special holder allows the elimination of the positioning error and thus forms the basis for highest machining accuracies. A residence time matrix of the ion beam is calculated based on the static removal profile of the ion beam and the polishing error of the surface of the optical element measured over the entire area (cf. Fig. 2 a). The system consisting of an optical element and holder is fed fully automatically via a vacuum lock into a vacuum processing chamber. The form error can be minimized to the desired target geometry by means of dwell time of controlled localized ion irradiation (cf. Fig. 2 b). The use of an aperture revolver, with apertures of different diameters, enables the exact and efficient correction of various long-wave errors (cf. Fig. 2 c). The following applies: The smaller the aperture diameter, the narrower the ion beam, the finer the ablation possibilities. ION-Finishing is characterized by the highest shape accuracy (λ/50). Additionally, our state-of-the-art production facilities guarantee effective and high-precision products that are adapted to individual requirements.

Figure 2: a) Completely measured polishing error; b) Spot ion irradiation of the polishing error; c) Exact correction of long-wave and short-wave polishing errors

Advantages of the ION Beam Process

ION finishing is particularly gentle on the material as it is free of chemical influences. It is therefore also suitable for polishing more fragile materials (e.g. materials that shouldn’t come into contact with H2O). The complete measurement of the substrate holder with the clamped optical element guarantees an elimination of the positioning error. Thus, the ION-Finishing achieves the most precise results with optimally polished surfaces.

The following advantages can be summarized for ION-Finishing:

Application

Due to the particularly high-quality requirements, ION-Finishing has dominated the production of optical high-performance components for years, e.g. for lithography optics, astro- and X-ray optics, laser optics for optical measuring instruments and many other areas. High-performance optics with an ION-Finish can be used in the short-wave visible and UV spectrum as well as for optical components with high radiation densities.