Key drivers of change in our mobile lives come from the digital world and its imaging and image processing technologies. Be it autonomous driving, package delivery by drones or space exploration, a key requirement for future developments in these fields is high-resolution optical systems – and the necessary precision can only be provided when production takes place in cleanrooms. Manufacturers will be showing how to configure cleanrooms to satisfy the requirements of modern optics at the Cleanzone cleanroom trade fair on 17-18 October 2017, in Frankfurt am Main, Germany.
The aforementioned examples represent the fulfilment of long-standing dreams: cars will be rendered safer and more comfortable in a revolutionary way. Yet, to mimic and exceed the performance of the human eye, it is necessary to have optics with extreme sensitivity and sharpness of detail – and this means that even the tiniest impurities can cause problems for the systems.
Another example is offered by the European Extremely Large Telescope (E-ELT) in Chile. One primary mirror and two corrective mirrors, not to mention the need to compensate for distortions that result when light from space passes through the Earth’s atmosphere by minimally bending the mirror surface – mean adjustments are made in the magnitude of a few ten-thousandths of a millimetre.
Holistic concepts are also growing in importance. Whereas optics were designed and produced separately in the past, today manufacturers must also solve the follow up-problem: each individual piece of optics has specific faults. For example, the individual rays comprising a beam may not converge on a single point after traversing an optical system, resulting in an aberration; other faults including various types of distortions and chromatic aberrations. Strictly speaking, the perfect depiction of an expansive object is only possible with flat mirrors (James Clerk Maxwell, 1858; Constantin Carathéodory, 1926). Taking into account the subsequently utilised hardware, it is now necessary to develop software that is suitable for compensating for these faults — and the overall system cannot be allowed to grow too large, because even a car has limited space for electronics and optics.
Of utmost importance for optics, both during the production of lenses and in the subsequent steps, is the painstaking care that must be taken to avoid impurities. This includes the bonding of multiple lenses or of lenses and sensors, for example. In chip production, these requirements quickly render the latest 7-nanometre (nm) technology a necessity. It is a type of lithography, with the special distinction that wavelengths in the extreme ultraviolet spectral range are used, and the structures are created in a vacuum. Here, production may take place in cleanroom class 1 mini-environments of just eight cubic metres in size, for example. A 7 nm structure is equivalent to five carbon atoms in a row. For cleanroom technology, this means that impurities as small as a single molecule can be a concern. This includes not only airborne molecular contamination (AMC) but also surface molecular contamination (SMC).
Impurities in cleanrooms
In optics and electronics, one example of something that can seriously disrupt the manufacturing process is ammonia, which can be introduced by personnel or result from solvents containing isopropyl alcohol. Bonding and furnace processes are very sensitive to outgassing substances such as chlorine and silicon tetrabromide (SiBr4). Other familiar “cleanroom impurities” include acetone (source: personnel), hydrogen bromide and hydrogen chloride, water and sulphuric acid (source: processes), the solvent PGMEA (propylene glycol methyl ether acetate), the silylation agent HMDS (bis(trimethylsilyl)amine) and TEOS (tetraethyl orthosilicate), a compound used in sol-gel processes.
Whereas these substances can be registered analytically, the monitoring of airborne hydrocarbons, epoxides and seemingly ubiquitous siloxanes are a focus of research in the fields of aerospace, microelectronics and optics.
New approaches for chemical-analytical monitoring systems
“Accurate measurement of these three types of substances – hydrocarbons, epoxides and siloxanes – has proven to be particularly difficult, and we still have not found a satisfactory solution,” says Markus Thamm, responsible for sales and marketing at cleanroom.de, Heidelberg. “Even so, there are a number of promising approaches that I believe might be ready for successful use sometime this year. For me, the key is to go beyond recording values at intervals, and to institute a system of continuous monitoring for these three types of substances.”
In addition to possible chemical impurities during operation, it is also important that quality controls are capable of detecting mechanical faults. Volker Knorz, KLA-Tencor, Weilburg: “To do this, we have to measure the surfaces of complex lens systems or wafers and be able to detect particles and scratches that are well under 100 nnm in size. This is yet another prerequisite for optics that are suitable for use in autonomous vehicles or space exploration.”
A targeted tour of Cleanzone in Frankfurt am Main, Germany, on 17–18 October 2017, will bring visitors up to date on cleanroom technology. Perhaps some will even get a little bit closer to space travel during their trade fair visit.
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