Precision optical asphere fabrication by plasma jet chemical etching (PJCE) and ion beam figuring

We develop a Plasma Jet Chemical Etching (PJCE) technique for high rate precision machining of optical materials aiming in a technology mature for precision asphere and free-form surface topology fabrication. The present contribution summarizes the achievements after about twelve months experience with a prototype production tool facility. PJCE is performed with the help of a microwave driven reactive plasma-jet working in a broad pressure range (10-600mbar). We developed a moveable lightweight microwave plasma jet source for dwell time techniques performed in a roughly pumped process chamber equipped with a six axis system for precision workpiece and plasma source movement. Volume etch rates of some 10 mm(3)/nm have been achieved for fused silica and silicon, respectively, using reactive (CF4, SF6, O-2) and inert (Ar, He) gas mixtures and applying a microwave (2.45GHz) power in the 100-200 W range. Large quartz plates (circle divide80-160mm) have been figured using dwell time methods to achieve aspheric deformations of some 10mum. The figured surfaces show shape errors of 1-2 mum and a microroughness of 50-100nm RMS but no sub-surface damage enabling a small tool shape conserving post polishing up to the sub-nanometer roughness level. Thus, ion beam finishing can do surface shaping to the nanometer error range.