Hendrik Paetzelt

Individual GaAs nanorods imaged by coherent X‐ray diffraction

Using scanning X-ray diffraction microscopy with a spot size of 220 x 600 nm, it was possible to inspect individual GaAs nanorods grown seed-free through circular openings in a SiN(x) mask in a periodic array with 3 microm spacing on GaAs[111]B. The focused X-ray beam allows the determination of the strain state of individual rods and, in combination with coherent diffraction imaging, it was also possible to characterize morphological details. Rods grown either in the centre or at the edge of the array show significant differences in shape, size and strain state.

Precision asphere and freeform optics manufacturing using plasma jet machining technology

Atmospheric Plasma Jet Machining is a non-conventional deterministic sub-aperture surface processing technique that employs a local chemical etching process for material removal. It was shown to have great technological potential for the manufacturing and correction of precision optical surfaces made preferably from fused silica, ULE(R), silicon, or silicon carbide. A plasma based processing chain suitable for freeform generation or surface correction comprises high-rate plasma etching for freeform generation (removal rate 1-30 mm3/min), bonnet polishing for smoothing, plasma fine correction (removal rate 0.01-1 mm3/min) to achieve minimal surface figure error and a post-polishing step utilizing a soft tool. Since plasma jets are not only a source of reactive species but also of heat, the chemical reaction during processing depends on the resulting local surface temperature distribution. A coupled model involving finite elements for temperature field analysis and sophisticated dwell time calculation algorithms has been developed to simulate the complex interplay of surface heating and material removal. With it, the convergence of the etching process is significantly increased. Sub-aperture polishing on freeforms suffers from inhomogeneities in material removal depending on local surface curvatures. An analytical approach has been chosen to estimate the material removal during polishing and appropriate measures have been undertaken to preserve the plasma-generated surface shape. By combination of the processing steps and applying the theoretical modeling fast and efficient precision freeform manufacturing aiming at residual errors of less than 30 nm PV becomes possible.

Nonconventional ultra-precision manufacturing of ULE mirror surfaces using atmospheric reactive plasma jets

In this paper we present a non-abrasive surface manufacturing technology suited for fast and efficient figuring of optical surfaces made of ULE (Corning Ultra Low Expansion) glass. Plasma Jet Machining (PJM) technology is based on an atmospheric chemical reactive plasma jet tool that locally interacts with the surface in order to remove material by chemical reactions forming volatile species. ULE has been proven to be suited for the PJM process. It has been found that the volume removal rate is approximately 25% higher than for fused silica and values up to 50mm3/min can be reached with our setup. Thus, figuring and figure error correction of large optics like mirror segments for earth based telescopes can be realized within a reasonable time. In the paper principles of the PJM process as well as ULE specific issues are discussed and machining results are presented.