Micro processing with laser radiation

Transcription

1 Micro processing with laser radiation Trends and perspectives Miniaturization and highly integrated functionalization are the driving factors in the production of innovative products in almost every industrial area today: Electronic components require conducting traces smaller than 100 µm und 3-dimensional interconnections, highly fuel efficient cars are only possible by micro structured functional surfaces and injection nozzles with geometries < 100 µm, medical products require selective functionalization for improved fluid control and micro optical components increase the functionality in a large variety of consumer products like mobile phones and miniaturized cameras. For all this applications manufacturing methods are needed, which are able to perform specific processing steps without influencing the overall properties of the materials the product is made from and on the other hand allow the easy integration into mass manufacturing lines at high production rates. Moreover the different processes should be used in a flexible way to be able to customize the functions and properties of the part with respect to increasing mass customizing requirements. Laser processes always have been used for highly selective processing technologies with minimized heat input to provide low distortion or material specific property changes. Recent developments on new laser sources allow even further decrease of unintentional production and extensive processing areas. New lasers like Fiber lasers with process adapted wavelengths and ultimate beam quality Ultrashort pulsed lasers in the ps and fs range and with very high repetition rates Lasers with highly desirable energy distribution in time and space allow new manufacturing methods which meet the demands of flexible micro produc- DER AUTOR ARNOLD GILLNER Arnold Gillner is heading the department of micro technology at the Fraunhofer-Institute for Laser Technology in Aachen, Germany. He is working on laser based micro structuring with UVand short pulse lasers, micro joining technologies with laser radiation and laser applications in biotechnology and medicine. Dr.-Ing. Arnold Gillner Fraunhofer Institut f. Lasertechnik Steinbachstr Aachen Germany Phone: Fax: arnold.gillner@ilt.fraunhofer.de Website: tion processes at high processing speeds. Moreover those lasers have become more reliable and efficient, so that they can be used in processing lines with high demands on yield and cost effective production. This holds especially for the manufacturing of micro parts, where packaging of single components requires the ability to join different materials with joining geometries < 50 µm or structuring is necessary in the micron and submicron range in almost all kinds of materials. Under these conditions laser processes have been established in a wide range of new applications beside the already existing use in classical fields like turbine drilling, fine mechanic parts welding, selective soldering of electronic parts and marking. Recent process developments based on the new laser sources have resulted in product optimizations with new features and higher efficiency especially in: Solar cell production Mechanical engineering with improved surface functionalities and wear resistance Biotechnical and medical products Miniaturized mechanical parts like mini motors and gears Electronic part manufacturing Moulding and forming tools and processes Most of these processes require mass manufacturing capabilities which are fulfilled by the new laser sources and high speed beam guiding and scanning devices. Micro ablation, drilling and surface functionalization with short pulse lasers and nanoscale approaches In the production of micro devices, the surface properties become more and more important for highly loaded mechanical devices, biotechnology components and medical products with respect to wear resistance, wetting properties and chemical composition of the surface. New laser processes provide appropriate solutions for the change and setting specific surface properties. Typical applications are gliders, gears, bearings for micro mechanical devices as well as micro fluidic systems or miniaturized devices for DNA- and proteome analysis (bio-chips) and implants. For the processing of metallic or ceramic parts high repetition rate short pulse lasers in the ns- and psrange with minimized heat input are used for micro structuring the surface providing lubricant containers and improved gliding characteristics. With newly designed laser technologies based on nanoscale sized UV-laser treatment of polymers for surface processing, wetting properties, cell growth behaviour and im- LTJ 21

2 mobilization of functional molecules with high spatial resolution can be set and modified. Depending on the processing parameters and used polymers either hydrophobic or hydrophilic properties can be enhanced (i.e. laser induced lotus / anti-lotus effect). Enhanced roughness and changes of the chemical composition have also influence on cell growth on polymer surfaces. Thus guiding aids for cells e.g. on medical implants can be generated by laser irradiation. Due to the outstanding properties of high quality and high energy laser light, micro and nano structured surfaces can be processed and combined with chemical modifications, which are produced by photochemical processes. By a topographic and morphologic modification of polymer surfaces, their characteristics can be changed and certain functionalities can be set, which the actual materials do not provide. Compared to conventional surface functionalization methods laser treatment allows a selective and laterally structured processing. By scanning and mask imaging, only those regions of a part can be changed in its properties, which are of interest for the later use. Depending on the type of structuring and functionalization as well as the number of parts to be produced different laser technologies can be used to process either the part direct or the tool, with which the part is manufactured. For direct processing the following laser processes can be used: Direct nano structuring of polymer surfaces by interference ablation methods THE INSTITUTE Fraunhofer Institut für Lasertechnik Aachen, Germany Fraunhofer ILT is developing for more than twenty years lasers and industrial applications for various sectors like automobile industry, mechanical engineering, chemical and electrical engineering, medical technology and optics. The four business areas cover innovative diode and solid state lasers, materials applications like cutting, ablation, drilling, welding, surface treatment and micro processing, process control and laser measurement and testing technology processes for inspection of surfaces and for materials analysis FIGURE 1: Micro structured steel replication tool (left), Wetting test at replicated polymer (right). Photochemistry based nano functionalization of polymer surfaces Combination of microscopic topographic change with functional surface photochemistry Laser based surface chemistry and molecule binding For indirect processing the following laser processes can be used: Micro scale structuring of injection molding tools Nano structuring of tools by laser ablation of surface layers and subsequent etching Especially the indirect methods allow low cost solutions for mass production and the equipment of consumer parts with functional nano structures and enhanced functionalities. Nano-structures with an extreme surface to volume ratio show a particularly high potential, since over the manufacturing processes both chemical and structural characteristics can be combined. Thus super hydrophobic surface properties can be adjusted in the sense of a synthetic lotus effect by a locally selective structuring of this surface in the submicrometer or nanometer range e.g. on certain polymers. On medical dosing equipment the adhesion of fluids can be avoided and thus an accurate dosage can be guaranteed with medical dosing assistance. In biological chips for medical analytics the flow characteristics and capillary effects can be functionally influenced. For the production of micro and nanostructures on three-dimensional replication tools a new laser-based process for the nano and micro structuring has been developed based on high speed scanning ablation using UV-lasers with interference imaging and direct focusing. With this technologies micro and nano scale structures in the size from 300 nm to several µm can be produced with which aimed functional structures can be produced on the polymer parts. In Fig. 1 the surface of a replication tool is shown, where by laser ablation in tungsten carbide micro pits with sizes between 1-5 µm have been produced. Due to the process characteristics of laser ablation sub micrometer scaled substructures are produced, which further increase the surface area. Tools like this have been successfully tested for the replication of polymers to achieve functional surfaces. In Fig. 1 the result of a wetting test shows, that by micro structuring a super hydrophobic effect can be produced. Further investigations on steel structuring are related to combined processes, where interference methods are used for nano structuring of coated surfaces. In a second process step this structured layers are used as a conformal mask within etching procedures to transfer the nano structures into the tool steel. For the use on medical implants surface structuring has been performed for improved cell growing with cell guiding structures in PDMS (Fig. 2). The structure sizes vary from 2 to 30 µm in height and distance. The analysis of cells on micro patterned polymer substrates revealed that there is an influence of laser treated zones on cell behaviour. Enhanced roughness due to ripples and recondensed debris material in the nanometer scale seem to improve the conditions for cells adherence which can be seen from the much higher cell density around the laser structured areas. The influence of laser generated patterns on polymer surfaces is clearly indicated. Using the above mentioned new short pulse lasers for micro ablation allow also the production of micro meshes for filter purposes and micro nozzles in fuel injection systems, micro dosing and micro chemistry. Spinnerets for example used in the textile industry and injection nozzles for common rail diesel injection systems are just a few examples of 22 LTJ January 2007 No. 1

3 FIGURE 2: Improved cell spreading along PDMS pillars (left), Cell Growth on nanostructured surface (in Cooperation with Brown-University). FIGURE 3: Laser drilled micro filter in stainless steel (Hole diameter 15 µm). extremely fine holes in metal parts with typical dimensions of µm at hole depths up to 2 mm. Compared to conventional hole drilling technologies laser hole drilling allow new processing capabilities and geometries because the process is non-contact and flexible. In addition, there are fewer process limitations, no need for expensive waste disposal, and tooling costs are reasonable. Compared to EDM machining laser drilling provides high aspect ratios and the capability to drill all kinds of material, including ceramics, silicon, diamond and polymers. With a flexible scanning laser beam even non circular holes with complex shapes are possible. With a newly developed drilling technology highly transparent metals with minimized thermal distortion can be produced providing hole dimensions smaller than 10 µm at drilling speeds of more than 1000 holes/ second. In Fig. 3 a sample for micro filter is shown in stainless steel with holes dimensions of 15 µm and a pitch of 50 µm. To produce high aspect ratio holes with diameters of µm at material thicknesses of 1 mm and more by means of helical drilling, a special drilling optics is necessary which rotates the laser beam with variable rotation diameters and high rotational speed. For this applications, a new type of laser drilling head has been developed by Fraunhofer-ILT using a rotating Dove-Prism, which is spinning the laser beam on a circle with frequencies up to 600 times per second. By tilting and moving the laser beam with respect to the rotational axis of the prism conical shaped holes with positive and negative taper can be produced. With this optics and a Q-switch Nd:YAG-Laser with 15 ns pulse duration and pulse energy of µj Fraunhofer-ILT researchers have processed tool-quality and highgrade steels up to two millimeter thick. The pulsed laser pierces a hole 50 micrometers in diameter through the metal sheets in less than a quarter of a minute. The residual melt FIGURE 4: Helical drilling optics and sample of laser drilled micro hole in steel, hole diameter 60 µm, material thickness 1 mm. thickness in the hole is as low as 1-2 µm and the melt on top of the material can be easily removed by ultrasound cleaning. In Fig. 4 the compact drilling optics and an example for a micro hole in steel is shown. Micro joining of metals, semiconductors and polymers with laser processes Joining in electronic device manufacturing today is still dominated by conventional joining techniques like soldering, press fitting, crimping and resistance welding. Due to the ongoing miniaturization of parts and higher required strength with respect to thermal and mechanical stability new joining processes are needed, which meet the demands of mass production and stabilities at elevated temperatures and at even smaller joining geometries. Laser beam joining techniques have been under intensive investigations leading to new processes for mass manufacturing and high accuracy assembling. Improved micro welding with modified pulsed lasers and innovative fiber lasers provide new solutions for selective joining of metals, semiconductors, polymers and even dissimilar materials. With the newly developed SHADOW welding technology technical aspects such as tensile strength, geometry and precision of the weld have been improved. This technology provides highest flexibility in weld geometry with a minimum of welding time as well as new possibilities in using application adapted materials. Different parts and even different metals can be joined in a non-contact process. The application of a relative movement between the laser beam and the part to be joined at feed rates of up to 60 m/ min produces weld seams with a length up to 30 mm using a pulsed Nd:YAG laser with a pulse duration of up to 100 ms. Due to the LTJ 23

4 low energy input, typically 1 J to 6 J, a weld width as small as 50 µm and a weld depth as small as 20 µm have been attained. This results in low distortion of the joined watch components. Fig. 5 shows an example of an electric connector from copper, which has been welded to a sintered copper alloy with ring shaped joining geometry within one pulse at pulse length of 17 ms. FIGURE 6: Fiber laser micro welding of different materials. FIGURE 5: SHADOW welding of electrical connector, welding time 17 ms. New high accuracy fiber lasers with outstanding beam quality allow even further miniaturization of the joining geometry. Due to very small fibers with 10 µm diameter spot sizes of µm can be achieved at working distances of more than 100 mm. Combined with high speed x-y-scanners the set up of flexible joining tools are possible, which easily can be integrated into manufacturing lines. Fiber lasers with powers up to 200 W can be used for micro welding steel, titanium and even copper materials at high speeds with a minimum of energy input and resulting distortion. Due to the small melt volume and deep penetration of the laser beam by using the well known key hole welding technology from macro applications joining of dissimilar materials become possible even for material combinations, which show typically the formation of brittle intermetallic phases. With this technology welding of steel-copper, copper-aluminium, steel- brass and other combinations is possible. Part combinations, where a very small component has to be joined to a large part, which hardly could be made with conventional pulsed lasers, now can be realized by fiber laser welding because of the very high intensity, leading to simultaneous melting of both parts even with totally different heat capacities. In Fig. 6 two different butt welds in steel and copper are shown, which has been performed at laser powers up to 200 W and with significant welding speed. With this technology miniaturized parts can be welded with almost no distortion proving clean and smooth surfaces of the joint even at critical dimensions as shown in Fig. 7, where a thin tube has been welded into a larger component. FIGURE 7: Micro joining of miniaturized tubes with fiber lasers. Laser welding of polymers is an already well established technology for a large variety of thermoplastic polymer parts. In principle there are currently 4 different processing technologies used like contour welding, quasi simultaneous welding, simultaneous welding and mask welding. Diode lasers and cw-nd:yag-lasers are used for contour and quasi simultaneous welding whereas diode lasers are used for the remaining two technologies. In all technologies substantial energy is deposited into the material leading to melting depth of typically several 100 µm which can be used for larger parts and wide weld seams. For thermo sensitive parts and micro joining geometries a new laser process for welding polymer parts has been developed, using an ultra fast scanned fiber laser beam. The very fine focused laser beam is scanned in the form of circles or other geometries to form the weld width while moving simultaneously the scanned laser beam across the joining geometry. With this technology the interface of the parts to be joined are heated very selective just below the degradation temperature but avoiding overheating. In this way very high welding speeds of more than 20 m/min at laser powers less than 10 W can be achieved. Due to the short interaction time, the energy deposition is concentrated to a very small volume, so that in the cross section the welding geometry almost cannot be seen (Fig 8). Also very small welding beads in the range 100 µm can be produced without degradation the FIGURE 8: Micro polymer welding of micro fluidic devices, cross section of welded area. 24 LTJ January 2007 No. 1

5 centre of the bead, which occurs when using just a fine focused Gaussian beam. Therefore this technology is well suited for packaging of micro fluidic devices (Fig. 8). For micro system applications the laser joining technology has been modified to join even silicon and glass parts without any melting, based on the formation of a thermally induced oxygen bond. For joining silicon and glass Nd:YAG radiation is transmitted through the glass part and absorbed on the silicon surface at the interface between two parts. At this interface the joining process is performed by production of oxygen bridges and adhesion. The application of this technology can be found in the assem- FIGURE 9: Laser bonded micro optical part on silicon substrate. bly of sensors and micro system components as well as the assembly of micro fluidic parts with biological components for biological essays (Fig. 9). Conclusions Laser processes has been shown as a versatile tool for high precision manufacturing of small parts and geometries in the micrometer and sub micrometer range. The main advantages of all laser processes minimized energy input with a maximum of selectivity allows new design principles and product features. Due to new laser developments with ultra precision continuous fiber lasers, high repetition rate short pulse lasers with ps- and fs-pulse duration and compact high power laser diode sources production rate and quality for manufacturing micro scale parts can be increased continuously under economic conditions. 2-AXIS LASER BEAM DEFLECTION UNIT SUPERSCAN -9 NEW & COMPACT RAYLASE AG Fon: +49-(0)8153/ info@raylase.com DON T MISS OUT. ORDER A TEST UNIT TODAY! Extremely compact design perfect for small laser systems Low drift and high accuracy High throughput matched with stability and reliability Robust and dust proof (CE) for industrial conditions PLEASE VISIT US AT PHOTONICS WEST! JANUARY 23 25, 2007 BOOTH # LTJ 25