- © Fraunhofer
Light is a universal tool which can be used for cutting, drilling, melting, joining and metal removal. What’s more, its potential is far from exhausted. Tailor-made light can open up an ongoing stream of new technical applications. Photonics will be one of the key technologies in the 21st century.
“For the rest of my life I will reflect on what light is,” said Albert Einstein. The Nobel Prize winner was not alone in being fascinated by light. As a phenomenon, it has held the attention of all cultures throughout the ages. Scientists like Joseph von Fraunhofer, Max Planck and Einstein attempted to unlock light’s secrets. Its unique properties were first put to better use in the second half of the 20th century, when the development of the laser made it possible to control photons and turn light into a universal tool of unmatched precision and performance. It can cut, drill and weld a host of different materials, store data on DVDs and transport information. Laser light is strong enough to cut through the toughest steel and can be dosed so carefully that it can be used to gently treat the eyes and skin.
“Given the unique physical properties of the photon, industrial production in the 21st century will be shaped by the laser. With the shortest energy pulses, highest energy densities and smallest interaction zones, no other tool can be controlled in time and space more quickly and precisely than light,” states Prof. Reinhart Poprawe, Head of the Fraunhofer Institute for Laser Technology ILT in Aachen. The enormous potential of this universal tool has not yet been fully exploited. Experts estimate that only about 50 percent of all potential laser applications have been realized. Completely new possibilities are opened up, for example, by a special type of solid-state laser: the fiber laser consists of an optical fiber which is doped with small quantities of an active material – mostly ions from the group of rare earths. When activated they release energy in the form of laser radiation. Optical elements – such as laser mirrors – are integrated directly in the light-conducting fiber.
This makes the system highly compact and robust, and the new fiber lasers achieve excellent beam quality. They develop hardly any heat, since the large surface area of the fibers, which are often several meters long, ensures even and efficient cooling. A further advantage: fiber lasers do not require elaborate adjustment. From beam generation in the diode through to the processing optics the laser beam does not leave the fiber. “Compared with conventional technologies, fiber lasers offer huge cost-cutting potential and are generating a steady stream of new applications.
In addition to high efficiency, their thermal and mechanical stability, small size and scalability make fiber lasers very attractive for production operations,” explains Prof. Andreas Tünnermann, Head of the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena.
The fiber laser – a new production tool
Fiber lasers are regarded as a breakthrough development in laser technology with enormous innovation potential. This new laser generation is suitable for marking, inscribing, cutting and welding. But it can also remove or structure material. Fiber lasers have reached an output power of several kilowatts, with close to diffraction-limited beam quality. This makes the new laser generation increasingly interesting for use in production.
For a fiber laser to become an efficient tool, however, the energy contained in the beam must be optimally guided, bundled and shaped. This task is performed by the optical system. Lenses, prisms, mirrors and other elements give the laser beam its direction, shape and power-density distribution. Scientists at the IOF have developed a new type of beam shaping optical system based on non-regular microlens arrays. This system considerably improves the distribution homogeneity of the laser beam.
High precision is essential to micromaterial processing. In future, particularly powerful, ultra-short-pulsed fiber lasers should make it possible to achieve drill-hole geometries that no longer require secondary processing. The pulse duration of some 100 femtoseconds to a few picoseconds prevents the materials from melting or cracks from forming. IOF researchers have already succeeded in drilling high-quality microholes in metals with a thickness of up to 1 mm (by comparison: in one femtosecond light travels a distance of 0.3 µm). Working in cooperation with their colleagues at the Fraunhofer Institute for Material and Beam Technology IWS in Dresden, the IOF researchers have also developed a new microstructuring system. This fiber laser operates with a high mean output of about 30 W, a pulse repeat rate of up to 100 kHz and extremely short pulses, enabling a very wide range of materials to be structured without being damaged. The researchers achieved the outstanding parameters of the system by means of a new fiber design. The microstructured crystal fiber – with an outer diameter of a few millimeters and a length of a few 10 centimeters – contains the waveguide structures for the pump and laser radiation.
The new laser systems are also interesting for the manufacture of components for use in medicine, biotechnology and bioanalysis. These products have to be made of biocompatible materials which do not react with other media. This represents a particular challenge for the joining technology, which cannot use any fillers and must not influence the material or cause contamination. Researchers at the ILT have developed the “TWIST®” technique, based on fiber lasers, which enables weld seams with a width of 100 µm to be produced at a speed of up to 18 m/min. The technique is even suitable for welding transparent plastics. Thanks to the new beam sources, the properties of the laser radiation can be adapted to the absorption behavior of polymers. Additional absorbers are not required. With this new technique, microfluidic chips can be produced which feature very small channel widths and channel distances.
Fiber lasers also open up new potential in material cutting. “Compared with conventional lasers of the same power, the focal radii of fiber lasers are ten times smaller. This means that higher intensities can be generated on the tool, and the cutting speed rises,” explains Dr. Thomas Himmer from the IWS. The researchers at the IWS are even using fiber lasers in a remote process to quickly and reliably cut complex structures – for example electrical sheets for drive systems or generators. In this process the beam of light is moved by swivel-mounted deflection mirrors in a scanner optical system. The laser spot moves at the rate of several meters per second, enabling parts of complex shape to be cut in just a few seconds.
The performance capability of the process has been demonstrated by the IWS researchers. They cut a hole matrix of 100 circles with a diameter of 6.5 mm in various thicknesses of stainless steel sheet. The 50 µm thin sheets were cut in just 1.2 seconds. The time taken to cut the 0.2 mm thick sheets was 2.6 seconds. The researchers were able to make complex cuts at a speed of 100 meters per minute. By comparison: conventional methods only reach an average cutting speed for multi-layered contours of about 20 meters per minute. In the BRIOLAS project funded by the German Research Ministry, IWS scientists are now working on further fundamentals for remote laser cutting with brilliant high-power lasers. Initial successful experiments have shown that the remote laser cutting technique represents a highly productive approach with great future potential for the flexible processing of metal sheets and strips.
Remote laser cutting also increases productivity in the manufacture of airbags. New cars are fitted with a number of different airbag types for the front, sides and windows. This variety of types requires flexible and highly productive manufacturing plant. Up to now, gas lasers have cut the airbag parts from lengths of polyamide fabric up to three meters wide. The thermal cut melts the edge of the fabric so that the material does not fray. Up to 20 layers of material can be cut at the same time, but the cut quality of the individual layers differs considerably. Researchers at the IWS have combined remote processing with continuous feeding of the fabric lengths. In cooperation with the company Held Systems Deutschland GmbH, the new generation of airbag laser cutting systems has been built. They enable fabric with a width of 2.5 meters to be cut at a material throughput speed of up to 20 meters per minute with an accuracy of 0.5 mm. The first systems are already in use in the industry, and companies have been able to increase productivity by 50 to 90 percent compared with conventional multi-layer cutting.
Shortwave light for tiny structures
Fraunhofer researchers regularly present the results of their work in seminars, in order to accelerate transfer into wide application. At the end of September the institutes will be holding the 5th International Fiber Laser Workshop in Dresden. Since the earliest days of the semiconductor industry, microchips have been produced by means of light exposure. So that smaller structures with more transistors can be produced in future, however, a leap in technology needs to be made to a new generation of lithography. Extreme ultraviolet EUV lithography is a promising candidate. The principle behind this technology is that light with a wavelength of 13 nanometers is guided by a reflection mask onto the silicon wafers to produce nanometer structures. If successful EUV technology will revolutionize the entire semiconductor industry and extend the validity of Moore´s Law that semiconductor performance doubles every 18 months. Research scientists at the ILT, the Department of Laser Technology at RWTH Aachen University, and the companies AIXUV and Philips Extreme UV are developing just such EUV light sources.
The work on EUV requires new tools and production techniques. The short-wave EUV radiation is absorbed by all materials, and even by air, which means that the entire light exposure process has to take place in a vacuum. Transparent masks and lenses cannot be used, calling for the development of new systems. For example, special chucks are needed to pick up the silicon wafer and exposure mask and hold them so that they are stable in the vacuum. Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF have developed electrostatic chucks for the EUV lithography process that are extremely flat. The IOF uses special glass materials and has developed new technologies to increase the flatness of the chucks. The height deviations of more than 100 nanometers previously measured have been reduced to 74 nanometers with the new material.
The use of highly reflecting mirrors is a particularly efficient means of guiding EUV light. Researchers at the IOF are working on new collector mirrors for EUV lithography, with the aim of improving their long-time stability and reflection properties. For this purpose, the researchers coated the collector substrates with high-temperature gradient systems. Molybdenum-silicon coatings are optimized for a maximum reflection of 13.5 nm and a working temperature of 400 degrees Celsius. The production of high-precision projection optics is an ambitious task when this involves reflectors for EUV light. IWS researchers achieved a reflection level of over 70 percent, which is the highest obtained anywhere in the world.
New production technologies are made possible not only by new laser generations and new light sources. The skilful use of conventional laser systems also opens up new processing techniques. The laser can be used, for example, to apply tiny structures quickly and cheaply. Researchers at the Fraunhofer Institute for Production Technology IPT in Aachen, working in cooperation with the company Freudenberg Anlagen- und Werkzeugtechnik GmbH, have developed a laser-structuring system that makes it possible, for example, to create ventilation structures in molding tools, or tribological structures on three-dimensional surfaces, without any distortion – and 80 percent faster than with conventional etching techniques. A further advantage is that the manufacturing costs are about 70 percent lower.
Plastic and metal are not very compatible, but thanks to a new laser-based technique they can form a good partnership. Plastic-metal-hybrid components can be produced using the LIFTEC® method. The metallic component is heated through the plastic using laser radiation, until the metal starts to gently melt the plastic and can be pressed into the plastic component. The metal is heated again and pressed deeper into the plastic by further mechanical pressure. This enables the two otherwise incompatible partners to be combined. “The key element of the process is a component with a higher melting temperature than the plastic joining partner. Apart from metals and ceramics, high-temperature plastics can also be used for the more temperature-resistant material,” explains Dr. Arnold Gillner, Head of the Microtechnology Department at the ILT.
The generative production process Selective Laser Melting (SLM) has already proved successful in some sectors of industry for the direct manufacture of metallic functional components. A laser beam is used to selectively melt consecutive layers of metal powder, allowing a component to be produced directly from 3-D CAD data. Extensive knowledge of SLM process control and the resulting mechanical properties has already been obtained for processing titanium and steel materials. The dental industry is using the technique to make dentures and in the tool and die industry it is being used to produce inserts with near-net-shape cooling channels for injection-molding and die-casting tools. Now researchers at the ILT are working on the qualification of the SLM technique for aluminum alloys such as AlSi10Mg.
The German Research Ministry is supporting the “Alugenerativ” alliance project. Aluminum alloys are used in particular in the auto industry, mechanical engineering and the aircraft industry. Series-identical functional prototypes, individual parts and small series are made from aluminum, mainly by die casting or conventional prototyping. On the example of a valve made of AlSi10Mg, researchers at the ILT working in close cooperation with the industrial partner Festo have proved that components can be produced much more quickly with SML. It only takes seven working days to produce six series-identical functional prototypes with the generative technique. By comparison: this takes 120 working days with die casting and 30 with conventional prototyping by milling, erosion and turning. “The mechanical properties are equal to those attained by conventionally produced components, and some properties are even improved. This is a decisive criterion for use in industry,” explains Dr. Wilhelm Meiners from the ILT.
Scientists at the Fraunhofer Institute for Applied Solid State Physics IAF in Freiburg are working on innovative, infrared-emitting semiconductor disk lasers. The system combines the advantages of a semiconductor-based diode laser with those of a classic solid-state laser. As the active area consists of a semiconductor heterostructure, the emission wavelength can be varied over a very wide range. The external resonator produces very good beam quality. These new lasers have been realized with emission wavelengths of between 1.9 and 2.8 µm, longer-wave lasers are planned. The high output power (up to 5 W ) opens up a wide range of applications, from laser surgery and medical diagnosis to remote detection of gases and air turbulence. For example, semiconductor lasers which radiate light with a wavelength of around 2.2 µm make it possible to see clearly through human blood and create new potential in blood vessel endoscopy, a fact jointly discovered by research scientists at the Fraunhofer Institutes IAF and IMS.
The examples show that research scientists succeed again and again in extending the repertoire of light as a tool. The optical technologies are among the most innovative in Germany. On average companies in this sector invest around nine per cent of their sales revenue – and in some cases as much as 25 per cent – in R&D. In other sectors of industry only three per cent of sales revenue is expended on R&D.
Many other branches of industry also benefit from the wide-ranging innovations in the optical technologies sector. “Optical science and engineering represents a key, cross-sectional and enabling technology for the future markets of the 21st century,” says Prof. Eckhard Beyer, Chairman of the Fraunhofer Surface Technology and Photonics Alliance and Director of the Fraunhofer IWS. Optical technologies can be found in virtually all sectors – from nanotechnology and medical engineering to communication and information systems. The optical technologies are a major driving force of economic activity in Europe.
Articles and Research News
Fraunhofer Group Light & Surfaces
Surface technology and photonics are two of the core competences of the Fraunhofer-Gesellschaft. Their complex interrelationship is derived from the fact that surface technology is of essential importance in the manufacture of optical and optoelectronic components and products, while laser technology is gaining ever-greater significance in production processes and metrology in connection with surface technology.