Glass, ceramics, and sapphire putting laser beams through their paces

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1 Glass, ceramics, and sapphire putting laser beams through their paces Ditzingen, 31. January Glass, sapphire, and ceramics are popular materials for microtechnology and precision engineering applications. However, they increasingly pose challenges for conventional manufacturing processes. This opens up interesting opportunities for powerful ultrashort pulse lasers. These materials outstanding properties make them indispensable for many products: glass is used as the substrate for smartphone displays and in its toughened guise as a cover for the displays; ceramics are hard, chemically stable, and for instance used as substrates for electronic components and circuits; as well as being an electrical insulator, sapphire is extremely hard and scratch-resistant, making it suitable for use in semiconductor and LED technology. But there s one thing that glass, ceramics, and sapphire have in common: they are all difficult to process. Given that they are brittle and often very hard materials, they push conventional manufacturing processes such as milling, drilling, and grinding to their limits. The tools wear quickly, and several process steps are often required to obtain adequate quality. Using laser beams on hard, brittle materials Laser beams can display their advantages to particularly good effect on hard, brittle materials. They are wear-free and can be focused down to the smallest diameters. Galvanometer scanners flexibly position the beam, enabling virtually any contour geometry one could wish for. Ultrashort pulse lasers (Figure 1) are especially suited to processing brittle materials. With laser pulses that are less than 10 picoseconds long, material is vaporized before heat can transfer to the surrounding material. As long as the laser parameters are precisely adjusted to match the application, there is no need at all for rework. Many wide-band-gap dielectrics such as glass are transparent to light in the visible and near-infrared spectral range. However, the high intensity of picosecond pulses can produce free electrons Figure 1 In order to transfer the excellent properties of ultrashort pulse lasers to the requirements of highproductivity industrial manufacturing, TRUMPF scaled up its ultrashort pulse lasers from the TruMicro 5000 series to deliver higher outputs 1

2 by means of multi-photon ionization. Then avalanche ionization releases further charge carriers, which are in turn capable of breaking up chemical bonds in the material, with the result that material is ablated. Avoiding cracks by means of low heat input When processing brittle materials, it is always a particular challenge to avoid the occurrence of micro-cracks, as these weaken the component. One cause of cracks is when there is too much heat input into the component. The heat generates tensions in the material, which facilitate the formation of cracks. Suitable processing strategies can counteract the formation of micro-cracks. These involve carefully defining parameters such as pulse energy, pulse overlap, repetition rate, focus diameter, and the number of times the laser passes over the workpiece. The optimum operating point depends on the material, the geometry, and time and quality requirements, and is identified in application trials. A joint project between TRUMPF in Ditzingen, southern Germany, and the Fraunhofer Institute for Laser Technology ILT in Aachen, western Germany, is working out the theoretical foundations for optimizing processes on brittle materials. Various criteria can be used to evaluate quality: the flexural strength of the component as a measure for the fracture strength diminished by crack formation; the roughness of the cut edge; and the visual impression of the edge. The condition of the cut edge can be assessed under an optical microscope and with the aid of a scanning electron microscope (Figure 2). Figure 2 SEM image of a laserprocessed sapphire microstructure A common method for measuring the flexural strength of a component is the four-point bending test (cf. DIN EN 843-5). This involves positioning processed specimen sheets of material of defined geometries on two cylindrical supports. Two additional cylinders are used to apply a continuously rising force at a defined speed to the specimen sheet until it snaps. The force at the point of destruction is measured, and the breaking stress can be calculated in MPa via the geometry of the component. Although the occurrence of micro-cracks and their effect on breaking stress vary from component to component, they can be described using Weibull statistics. Typically, 10 to 15 specimens are processed and subjected to the four-point bending test. Figure 3 shows the Weibull distribution of a cell phone display glass ( Corning Eagle XG ) which was cut using a TRUMPF ultrashort pulse laser. The point at which the breaking probability is 10 percent can be taken as the characteristic value for the 2

3 flexural strength of the component. Different processing methods can be compared in this way. Processing toughened glass The smartphone boom has increased the importance of displays. Touch-sensitive displays (touchscreens) have superseded cell phone keypads as the main user interface. A typical smartphone contains up to four glass panels: two panels form the display itself and contain the thin-film transistors and liquid crystals; another panel provides the touch functionality; and a chemically toughened cover panel protects the layers beneath against scratches, impact damage, and dirt. As users want their smartphones to be light and thin, ever thinner glass panels are being used. A typical glass display panel is 0.3 mm thick; a chemically toughened cover panel, 0.7 mm. This pushes conventional cutting processes to their limits. Cutting wheels are not up to the task of processing glass that has undergone special chemical toughening, whereas milling it requires extensive rework in the form of grinding and polishing. Instead, ultrashort pulse lasers in the infrared and green ranges can be used. Their picosecond pulses reduce the formation of micro-cracks and create a cut edge that is far superior to the quality achieved by milling. The laser beam passes over the material multiple times to produce the cut (Figure 4). Speed, edge quality, and edge angle can be determined via the choice of process strategy. The ablation process is very robust compared to alternative laser processes: for example, a slight deflection of the glass does not affect the results. In tests using a green picosecond laser, flexural strengths of up to 280 MPa were achieved on Corning Eagle XG. Tests using an infrared picosecond laser revealed only marginally lower flexural strength at three times the speed. Figure 3 Weibull distribution of a cell phone glass display panel ( Corning Eagle XG ), which was cut using a TRUMPF ultrashort pulse laser. The characteristic value for the flexural strength of the component corresponds to a breaking probability of 10 percent Figure 4 Cutting glass using a picosecond laser 3

4 The availability of ever more powerful picosecond lasers presents big opportunities for greater efficiency in glass processing. This becomes clear if we compare the ablation rates and process efficiencies of steel and glass (Figure 5). Whereas the process efficiency of steel ablation already begins to fall at fluences (pulse energy densities) over 5 J/cm2 due to plasma shielding, glass permits significantly higher fluences, until process efficiency reaches its maximum level. When processing glass, therefore, higher pulse energy can be translated into more efficient ablation. When people want to further reduce the weight and thickness of glass, they may consider using ultra-thin glass. With a thickness of up to 50 µm, this glass is even more delicate to handle and extremely sensitive to mechanical processing. In fact, it is impossible to process it without using lasers. Sapphire for very tough cases Figure 5 Top: the ablation rate for steel already begins to drop from fluences of 5 J/cm2 on; Bottom: the ablation rate for glass does not fall at higher fluences. Thus, higher pulse energy increases the efficiency of the process Sapphire is the second-hardest material on Earth after diamond, which makes it very difficult to process by mechanical means. Cutting sapphire with lasers is a standard process today in LED manufacturing, where sapphire is used as a substrate. On account of its scratch resistance and optical transparency, sapphire is used, for example, to produce wristwatch crystals and protective windows for optical instruments. Ultrashort pulse lasers come into their own precisely when processing small contours i.e. when cutting out small disks and drilling small holes. Flexible contours can be negotiated with the galvanometer scanner. The ultrashort pulses take heat influence out of the equation, resulting in excellent edge quality. Figure 6 shows 0.4 mm thick circular, square, and triangular contours cut out of sapphire using the TruMicro 5070 infrared picosecond laser. The smallest contour size is 0.2 mm. Chipping and cracking can be avoided by virtue of a clever processing strategy combined with the ultrashort pulses. Experience has shown pulse energies Figure 6 Circular, square, and triangular contours, 0.4 mm thick, cut out of sapphire using the TruMicro 5070 picosecond laser with infrared wavelength 4

5 of around 100 µj to be optimal. Higher throughput can be obtained, for example, by splitting a laser beam with a pulse energy of 250 µj into two separate beams with 125 µj each and then processing two components in parallel. Achieving excellent quality depends on finding the optimum interplay of processing strategy, suction, and workpiece fixture. Ceramics resistant to wear and temperature Technical ceramics are always used whenever components are exposed to high temperatures or low wear is an important factor. For many applications, the material s electrical insulation properties are prized. For instance, technical ceramics are used as PCB material for sensor chips in the automobile industry, as low-wear storage material for pumps, or as material for nozzles in the food industry. The best-known technical ceramics include aluminum oxide, aluminum nitride, and zirconium oxide. Lasers are the right tool for drilling, cutting, or structuring thin ceramics. For example, 0.3 mm thick sheets of zirconium oxide can be rapidly cut at rates of up to 10 mm/s with infrared ultrashort pulse lasers. The ultrashort pulses produce a smooth, crack-free cut edge. With their sharp focusing properties, ultrashort pulse lasers are especially useful for drilling functions: they are capable of drilling holes with a diameter of 60 µm in 0.5 mm thick aluminum oxide and aluminum nitride (Figures 7, 8, and 9). They achieve drilling rates of up to 20 holes a second. In the case of scribing and breaking processes whereby a laser scribes the material to up to a third of its thickness and then the ceramics are broken mechanically speeds as high as 50 mm/s are possible. Figure 7 Laser-drilled aluminum oxide sheet Figure 8 Aluminum nitride, thickness: 500 µm, bore diameter: 60 µm Figure 9 Laser-cut geometry in aluminum oxide Efficient, high-power laser beam sources Throughput is a decisive variable in industrial manufacturing, albeit with the proviso that the required processing quality is obtained. In order to transfer the excellent properties of ultrashort pulse lasers to the requirements of high-productivity industrial manufacturing, 5

6 TRUMPF scaled up its ultrashort pulse lasers from the TruMicro 5000 series to deliver higher outputs. The new TruMicro 5070 and TruMicro 5270 models provide 100 W infrared and 60 W green laser power with pulse lengths of under 10 picoseconds. The regenerative amplifiers are based on disk laser technology, where thermal effects are reduced in the laser medium, enabling good beam quality and scaling to high average outputs. As all TRUMPF ultrashort pulse lasers feature two-stage power regulation, processing results are reproducible under all conditions. The pulse energy and frequency are varied by an external modulator outside the amplifier this ensures that the beam properties are not affected by variations in the laser parameters. Right of usage: Attached to this press release are digital photos at resolution suitable for printing. They may only be used for editorial purposes. Use is free of charge when credit is given as Photo: TRUMPF Group and a complimentary copy is sent to the above address. Graphic editing except for the purpose of extracting the main motive are prohibited. Additional photos can be accessed on the company website: About TRUMPF TRUMPF is a leading global technology company with Machine Tools, Laser Technology, Electronics and Medical Technology as its business fields. Products manufactured with the company's technology can be found in almost every sector of industry. TRUMPF is the world technological and market leader for machine tools used in flexible sheet metal processing, and also for industrial lasers. In 2011/12 the company which has approximately 9,600 employees achieved sales of 2.33 billion Euros. With around 60 subsidiaries and branches, the TRUMPF Group is represented in almost all the countries of Europe, North and South America, and Asia. It has production facilities in Germany, China, France, Great Britain, Japan, Mexico, Austria, Poland, Switzerland, Singapore, the Czech Republic and the USA. For more information about TRUMPF go to 6