Reactive Atom Plasma Research Articles

Reactive atom plasma (RAP) figuring machine for meter class optical surfaces

A new surface figuring machine called Helios 1200 is presented in this paper. It is designed for the figuring of meter sized optical surfaces with form accuracy correction capability better than 20 nm rms within a reduced number of iterations. Unlike other large figuring facilities using energy beams, Helios 1200 operates a plasma torch at atmospheric pressure, offers a high material removal rate, and a relatively low running cost. This facility is ideal to process large optical components, lightweight optics, silicon based and difficult to machine materials, aspheric, and free form surfaces. Also, the surfaces processed by the reactive atom plasma (RAP) are easy to fine polish through hand conventional sub-aperture polishing techniques. These unique combined features lead to a new capability for the fabrication of optical components opening up novel design possibilities for optical engineers. The key technical features of this large RAP machine are fast figuring capabilities, non-contact material removal tool, the use of a near Gaussian footprint energy beam, and a proven tool path strategy for the management of the heat transfer. Helios 1200 complies with the European machine safety standard and can be used with different types of reactive gases using either fluorine or chlorine compounds. In this paper, first the need for large optical component is discussed. Then, the RAP facility is described: radio frequency R.F generator, plasma torch, and 3 axis computer numerically controlled motion system. Both the machine design and the performance of the RAP tool is assessed under specific production conditions and in the context of meter class mirror and lens fabrication.

Rapid optical surface figuring using reactive atom plasma

In the context of large optics figure correction, the reactive atom plasma (RAP) process constitutes a fast and unique solution for ultra-precise surface figuring. The RAP process combines high material removal rates with the advantages of non-contact machining methods. The RAP technology is based on an inductively coupled plasma torch that produces a sub-aperture near-Gaussian etching footprint, with material removal repeatability at nanometre level. A large-scale RAP figuring facility, Helios 1200, has been developed in the Precision Engineering Centre at Cranfield University to optimize and apply the RAP technology on metre-sized optical surfaces. In this paper, the first examples of figure correction carried out with Helios 1200 are reported. These experimental results were achieved over 100 and 140 mm diameter areas by means of in-house developed timedwell figuring techniques and a dedicated tool motion path. In particular, classical de-convolution techniques were adapted to the non-linear nature of the etching rate in order to derive velocity maps, while the tool-path algorithm was designed to induce a homogeneous temperature distribution on the process surface. Still, surface temperature raises are known to increase the rate of material chemical etching. Therefore, as a form of heat effects compensation, the adapted tool-path algorithm was combined with an iterative figuring procedure in order to assure faster process convergence. This promptly enabled the realization of figure error corrections down to λ/40 rms. High overall rates of convergence between 78 and 89% were attained within a maximum of three iterations. The mean processing times per iterative step were 6.6 minutes over the 140 mm diameter areas, thus confirming the potentiality of RAP as large surface figuring technique. Applied to metre-class optics, the method should then deliver comparable levels of form accuracy within less than ten hours processing time. The scope of the work presented included both investigation and verification of the effects of tool-path parameters, as well as of the modified de-convolution method. The validated figuring procedure can be progressively scaled up to medium and large optical surfaces. The surface texture after plasma machining was characterized. Some deterioration of surface roughness was observed.

Reactive Atom Plasma for Rapid Figure Correction of Optical Surfaces

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.

A New Reactive Atom Plasma Technology (RAPT) for Precision Machining: the Etching of ULE® Surfaces

AbstractResults on reactive atom plasma etching performed on ULE® (Corning Ultra Low Expansion) glass samples at atmospheric pressure are presented for the first time. A reactive atomic plasma technology (RAPT®), has been developed by RAPT Industries and employed for the finishing of optical surfaces. An atmospheric pressure argon inductively coupled plasma (ICP) excites a reactive gas injected through its centre. The plume of hot neutral excited species reacts at the substrate yielding controlled and repeatable trenches. In the case of ULE a material removal (up to 0.55 mm3/s) is obtained without pre‐heating the samples. Among the factors influencing the results, an increase in gas concentration at the same power does not change the sample temperature, indicating that thermo‐chemical effects do not influence the removal rates. Due to the plasma constructive constrains, increasing the gas concentration is more practical and of wider effect than increasing the power. The benefits of the process are illustrated and the extension of the technology to large optical surfaces discussed.