Ultraprecise surface processing comprises all manufacturing processes allowing manufacturers to produce macroscopic bodies, whose surfaces are produced with extremely high precision and smoothness. Ultra precise surfaces exhibit improved functionalities for a multiplicity of applications.
Manufacturing of Ultraprecise Surfaces
Mechanical/chemical and optical manufacturing processes as well as ion beam and plasma procedures belong to the most important procedures for ultraprecise surface figuring and form correction. Ion beam and plasma processes allow form correction and/or shaping of large surfaces (cm² to m²) with accuracies within the nanometer range and a roughness reduction in a sub nanometer range. Due to the low working speed and the high costs for the manufacturing equipment, the application range is limited, so far, to the production of high performance optics. Also optical procedures using UV lasers are applied for ultraprecise surface treatment in particular for polymer surfaces.
Industry Applications for Ultraprecise Surface Figuring
The main application field of ultraprecise surface figuring is optics. Apart from ever smoother and more precise lenses in the visible range, there is an increasing demand for optics of other spectral ranges, i.e. infrared, UV and x-ray. Also in the range of joining techniques, especially in microelectronics, ultraprecise surface processing plays an important role. For the cost-advantageous manufacturing of microsensors and actuators, the direct bonding of silicon wafers and other components gains importance. This concerns both the joining of silicon and other semiconductor elements (optical elements on III/V semiconductor basis) on a chip, and the assembly of different optical and mechanical microcomponents (e.g. quartz micro lenses, piezoelectric actuators etc.). During direct bonding two ultraprecise surfaces are brought into contact, so that they can be irreversibly connected through a pressure and a temperature treatment without additional adhesives.
Defined Microstructures in the Surfaces of Components
A high innovation and market potential lies further in the production of defined microstructures in the surface of components, and in figuring more complex component geometries. By manufacturing microscopic bars, pyramids or cube segments, special optical, chemical, mechanical, tribological or thermal surface properties can be achieved. These applications are particularly relevant in lighting, communication and measuring technologies. As examples, micro radiators, sealing surfaces or microstructured air bearings should be mentioned.
Ultraprecise Surface Processing in Optical Satellite Communication, Optics and X-Ray Mirrors
In space technology ultraprecise surface processing is important for the production of components for optical satellite communication or of optics (IR to x-ray range) for the earth observation and astronomy. Telescope optics must be manufactured more precisely, the smaller the wavelength of the observed light is, in order to avoid light scatterings. Particularly high demands are made on X-ray optics. X-ray mirrors, which are important components of observation instruments for the detection of X-ray sources in space, require ultra smooth surfaces with a roughness less than 1 nm to avoid light scattering. By means of pulse laser deposition, these mirrors can be manufactured in high vacuum with a precision of a few hundredths nanometers. The optics of the 1990 commissioned German X-ray satellite ROSAT (ROentgen SATellite) counted with a surface roughness of 0,35 nm at that time as the smoothest mirror in the world.
Benefits of Using Modern X-Ray Mirrors
Conventional x-ray mirrors, according to the Wolter principle, are manufactured from glass ceramics with a thin metal coating. The disadvantages of these conventional glass-ceramic, monolithic optics are a relatively high weight and a limited collecting surface. Modern x-ray mirrors are based on very thin single mirrors and mirror foils with a nested design. Thus the collecting surface of the optics can be substantially increased. Examples for this design are the European XMM (X-ray Multi Mirror) x-ray telescope, which consists of three telescopes each of which have 27 single nickel mirrors, and the Astro-E with 5 telescopes and 180 mirror foils each. Such mirror foils, with a diameter of typically about 200 µm, permit a very close nesting and thus an increased collecting area in particular for high-energetic radiation, as well as a drastic weight reduction compared with monolithic optics.
X-Ray Telescopes Currently Used in Space Projects
At present, three efficient x-ray telescopes are deployed in space with the American Chandra, the European XMM and the Japanese-American Astro-E. For future missions in x-ray astronomy, such as Constellation X of NASA or XEUS of the ESA, still higher demands are made with regard to the performance of the telescopes. For their manufacturing, the ultraprecise surface figuring of foil substrates will play a key role.
Characterisation of Nano-Surfaces
A further important field in the range of ultraprecise surfaces is the characterisation of the mechanical-physicochemical properties of surfaces, including local defects. For space applications, the behaviour of nanosurfaces under space conditions is of particular interest. During exposition in space, different effects on nanosurfaces occur, which can lead to a functional deterioration, for example, through crystal growth, increasing roughness or droplet formation. In the frame of the DLR (Deutsche Zentrum für Luft und Raumfahrt - the German Space Agency) project SESAM (Surface Effects Sample Monitor), a measuring device was developed, making it possible to analyse changes of surfaces under space conditions on a nanometer scale, and to correlate them with the ambient conditions, e.g. influence of atomic oxygen (Toebben 1999). Here, different nanoanalytic procedures are applied such as scanning probe and scanning force microscopy, quantitative Nomarski microscopy and field emission scanning electron microscopy.