Feature Article

MANUFACTURING

Driving improvements

Microstructures and nanostructures are pivotal in today’s medical technology. They are found in the electrodes of cardiac pacemakers, precision mechanical components for implants and products for analytics and diagnostics. They provide new diagnostic methods and therapies and support development and improvement of medical treatments. To maintain this trend, known technologies must be advanced and new processes developed. As a result, new technologies such as excimer and ultra-short-pulsed lasers come more to the fore and displace traditional manufacturing methods.

Excimer lasers

Excimer lasers are already reliable components of industrial micro-machining processes. These lasers have a gas mixture as the gain medium, which typically contains a noble gas and a halogen as the buffer gas. Established gases are argon fluoride, krypton fluoride, xenon chloride and xenon fluoride.

These different types of gas mixture typically emit wavelengths of between 157 and 351 nm. Thus, excimer lasers almost always operate in the ultraviolet spectral region. The pulse durations are in the range of 4 to 200 ns, with pulse energies up to 300 mJ. The combination of these short pulses and high pulse energies allows the excimer laser to reach and surpass the ablation threshold of many materials, thus these lasers are able to process a wide range of different materials.

Another advantage of these lasers is the ease with which the beam can be shaped. Based on a Gaussian input beam, it is possible to create a homogenised laser beam using refractive optical elements or micro lenses. This homogenised beam, referred to as a flat-top or top-hat beam, has the same optical intensity in the whole profile. Thus, at whatever position it is in, the beam will ablate the same amount of material and produce a smooth surface on the ablated area and on the walls perpendicular to it.

The optical imaging of the laser beam on the surface of the work piece takes place with the help of an aperture mask and an objective. According to the form of the aperture mask, the laser beam can be shaped into different geometrical shapes such as rectangles, triangles, circles and ellipses as well as complex pictures. These characteristics make the excimer laser ideal for processing thermally sensitive materials. Some applications of these systems are described below.

Stripping polymer coated tubes

Minimally invasive surgery requires the production of smaller and more accurate tools, including, electrical supply lines, electrical ports and different catheters. These catheters, which consist of metal tubes with polymer layers, require different holders for tools such as cameras and lights. These holders are manufactured with an excimer laser (Figure 1). The high absorption coefficient of ultraviolet radiation in organic materials such as polymers means the ablation threshold can be easily reached even with low pulse energies. By the instant transition from the solid to the gas phase, the ablated material is expelled from the processed area and the formation of melted material can be prevented. The short pulse means that the impact of heat on the surrounding material can be avoided almost completely. There is no processing of the metal, because its ablation threshold is much higher than the threshold of the polymer. The depth of penetration amounts to only a few nanometres per pulse, which makes it possible to create high precision structures inside the polymer. Depending on the required tool and application, individual holders can be manufactured in an easy, fast and reliable way.

High precision assemblies

Figure 2: Cavities to handle small amounts of fluids machined with excimer laser.

Excimer lasers can also be used in the production of assemblies for handling small amounts of liquids employed in analytics and diagnostics. The quick analysis of different substances on known or unknown viruses, proteins or bacteria can only be achieved by precisely controlled handling of many small liquid samples; the evaluation typically involves the measurement of different electrical characteristics.

The first step in the production of micro cavity sensors is the creation of cavities inside the basic material (Figure 2). The shapes of these cavities range from rectangles and cylinders to turned truncated pyramids. The dimensions vary in the range of 100 to 1000 µm with depths of 50 to 500 µm. The ablation itself occurs layer by layer. With this technique, it is possible to create different shapes with different angles. Then the whole structure is coated with a thin electroconductive metal layer a few nano-metres thick.

Figure 3: Micro drill hole inside a polymer tube, diameter 40 μm.

The next step is manufacturing the electrodes. Unnecessary metal is ablated by the excimer laser so that only thin conductor tracks are left on the surface. This means that if a drop of liquid is brought inside the cavity, it touches the ends of the conductors. By measuring different electrical characteristics, it is possible to make a statement about the chemical composition or the microbial activity of the liquid sample. To analyse small amounts of liquid, it is necessary to use accurate metering devices. With the excimer laser, it is possible to create drill holes with a minimum diameter of 40 µm in polymers with no heat affected zones and no leftover melted material (Figure 3). The size and the shape of drill holes can be changed by the aperture mask.

Ultra-short-pulse lasers

Ultra-short-pulse laser systems are mainly used for processing metals and ceramics. These laser systems are solid state lasers based on a solid-state gain media such as crystals or glasses doped with rare earth or transition metal ions. Important gain media are yttrium–aluminium garnet and yttrium-vanadate doped with neodymium. Present industrial ultra-short-pulse lasers generate pulses with a duration lower than 12 ps, power between 2.5 and 50 W and frequency up to 1000 kHz. Depending on the model, wavelengths of 355, 532 and 1064 nm are available. It is therefore possible to process between the ultraviolet and the close infrared spectral region with the same gain medium.

Figure 4: Micro drill hole in stainless steel (cross section), diameter 80 μm, machined with picosecond laser.

The biggest advantage of these ultra-short-pulse laser systems, in contrast to the nanosecond laser, is their ability to reach high pulse energy of several megawatts. In just a few picoseconds the whole energy is inserted into the material. Because of this short reaction time, almost all of the energy is converted into ablation, which prevents heat affected zones and melted material. The results are sharp and precisely cut edges. Applications are fine blanking (precise cutting that produces cut widths of 10 µm to 15 µm), micro structuring, micro engraving and micro drilling.

Micro hole drilling

Helical drilling is used to produce micro drill holes in cannulae, nozzles and metal or ceramic implants. Compared with other drilling techniques, the laser beam accomplishes an additional rotation around its own axis. By changing the beam diameter and the angle of impact, it is possible to drill cylindrical and conical holes with positive and negative tapers. This technology achieves diameters of 50 µm in 1 mm thick steel (Figure 4). Helical drilling in combination with an ultra-short-pulse laser system can produce high precision drill-holes in almost all materials.

For the production of thin filters and membranes another technique is used. To reach the high processing time that is necessary for this method, the laser is combined with high speed scanning systems. This achieves ultra fast positioning of the laser beam without moving the sample. Using standard CAD software, the arrangement of the drill-holes can be changed quickly and individually. Depending on the chosen laser system, drill-holes with a diameter of 30 µm can be achieved in 50 to 100 µm thick metal foils. The drilling rate can reach values of more than 1000 holes per second.

Subsurface engraving

Traceability of products using inextinguishable identification codes is gaining importance because of drug-tracking regulations pursued by the European Medicines Agency and the United States Food and Drug Administration. In addition, the manufacturers of medical devices are increasingly dependent on the traceability of their products for product optimisation and cost reduction or protection against counterfeiting and product piracy. A subsurface engraving technology for transparent materials can guarantee this traceability. Ultra-short-pulse lasers allow the creation of 2 µm imperfections inside the material. Depending on the material, these imperfections appear as white or black dots. With a special arrangement of these dots, different levels of contrasts can be created. If necessary, the engraving can be downscaled so that it can only be detected with a special camera system.

The engraving is protected against environmental influences because it is inside the material and has no contact with any surface. In contrast with surface engraving, the marking cannot be changed or damaged and will be readable for its entire economic lifetime. The technology also guarantees a high resistance to thermal shock, because it does not create micro cracks or tensions inside the material. Studies have shown that a temperature change from –20 °C to +100 °C does not influence the material or the marking, thus it is possible to fill devices with hot fluids after engraving and to employ steam or heat sterilisation. There is no limit with regard to the shape and design of the markings and this technology allows the creation of two-dimensional and three-dimensional symbols and codes. Organic glasses such as polycarbonate and mineral glasses such as quartz glass can be engraved.

Ever increasing possibilities

Thanks to the further development of high-frequency laser systems, current laser systems surpass traditional technologies in speed, accuracy and cost-efficiency. Choosing the most suitable laser allows users to realise almost every application, whether in polymers, ceramics or metals.

Yves Rausch, Dipl.-Ing., is Applications Engineer at 3D-Micromac AG, Annaberger Strasse 240, D-09125 Chemnitz, Germany, tel. +49 371 400 430, e-mail: info@3d-micromac.com, www.3d-micromac.com