Reactive Atom Plasma for Rapid Figure Correction of Optical Surfaces

Abstract

Nanometre-scale figuring technique at atmospheric pressure for large optical surfaces is a high profile research topic which attracts numerous competing state-of-the-art technologies. In this context, a dry chemical process, called Reactive Atom Plasma (RAP), was developed as a prospectively ideal alternative to CNC polishing or Ion Beam Figuring. The RAP process combines high material removal rates, nanometre level repeatability and absence of subsurface damage. A RAP figuring facility with metre-scale processing capability, Helios 1200, was then established in the Precision Engineering Centre at Cranfield University. The work presented in this paper is carried out using Helios 1200 and demonstrates the rapid figuring capability of the RAP process. First experimental tests of figure correction are performed on fused silica substrates over 100 mm diameter areas. A 500 nm deep spherical hollow shape is etched onto the central region of 200x200 mm polished surfaces. The test is carried out twice for reproducibility purposes. After two iterative steps, a residual figure error of ~16 nm rms is achieved. Subsequently, the process is scaled up to 140 mm diameter areas and two tests are carried out. First, the developed algorithm for 500 nm deep spherical hollow test is confirmed. Residual deviation over processed area is ~18 nm rms after three iterations. Finally, a surface characterised by random topography (79 nm rms initial figure error) is smoothed down to ~ 16 nm rms within three iteration steps. All results presented in this paper are achieved by means of an in-house developed tool-path algorithm. This can be described as a staggered meander-type tool motion path specifically designed to reduce heat transfer and consequently temperature gradient on the surface. Contiguously, classical de-convolution methods are adapted to non-linear etching rates for the derivation of the surface scanning speed maps. The figuring procedure is carried out iteratively. It is noteworthy that iteration steps never exceed ~7 minutes mean processing time.