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Surface figuring using chemically reactive plasma jet machining (PJM) is a promising non-conventional technology for deterministic ultra-precision machining of optical components. Based on chemical reactions between plasma generated radicals and the surface atoms this technology is capable to fabricate complex shaped free form surfaces. Since the material removal rate during PJM depends strongly on the surface temperature which itself is influenced by the jet heat flux to the surface, the arising nonlinear effects on the etch result have to be regarded. Conventionally applied dwell time calculation algorithms do not consider those effects leading to significant machining errors in some cases. In order to improve the machining procedure with respect to deterministic material removal yielding predictable results a process simulation model has been developed. This model considers spatio-temporal variations of surface temperature and temperature dependent material removal and is able to predict the final workpiece topography after machining.

Plasma Jet Machining (PJM) is a surface figuring technology based on atmospheric plasma assisted chemical etching or deposition, respectively. In both cases a sub‐aperture plasma jet source is used combined with a CNC multi‐axes system for the processing of curved surfaces. It is under development for the surface figuring of a variety of optical materials by IOM for about 15 years. PJM is capable to figure deep aspheric or free‐form substrates with high material removal rate and high spatial resolution. Based on chemical reactions between plasma generated radicals and the surface PJM does not introduce any damage to the processed surface and sub‐surface region in contrast to abrasive techniques. Deterministic deposition of SiOx layers and subsequent proportional transfer using ion beams or polishing is another plasma jet based technique for surface figuring that extends the range of machinable materials. The article gives an overview on the current state of PJM development in IOM and shows examples of its application.

The material removal of SiC utilizing a 2.45 GHz microwave‐driven plasma jet source in comparison with a 13.56 MHz RF excited plasma jet source at atmospheric pressure has been investigated. A coaxial nozzle with a central tube for helium, CF4 and O2 feeding the plasma and the outer ring‐shaped nozzle for N2 to shield the plasma jet from the surrounding air is applied. Additionally an O2 gas flow is provided and its effect on the etching rate is discussed for varied [CF4]/[O2] ratios. By optimizing the ratio of CF4 and O2 gas flow an improvement in etching rates and a change in surface roughness have been found. An increase of the etching rate with a decrease of the CF4 ratio has been detected for both jets. The etching rate of the microwave excited jet has been improved additionally by heating the SiC sample up to 350 °C.

Surface figuring using reactive plasma jet machining is a promising technology for the manufacture of optical elements. Due to the pure chemical etching mechanism the material removal rate during plasma jet treatment strongly depends on the workpiece surface temperature, which is influenced by the jet heat flux. This paper presents the basis for an enhanced process simulation by introducing a transient heat transfer model including a moving heat source. A comprehensive method of determination for corresponding model parameters has been developed. It is based on temperature measurements during a well defined etching experiment. The model is able to predict the workpiece surface temperature distribution at any given time.