LASER MICROPROCESSING POWERED BY UV PHOTONS Paper #P109 Ralph Delmdahl, Rainer Paetzel Coherent GmbH, Hans-Boeckler-Str.12, Goettingen, 37079, Germany Abstract Lasers with ultraviolet (UV) output offer unique benefits for processing a wide range of materials. In particular, the short wavelength, high energy UV photons experience less diffraction than longer visible or infrared wavelengths, which enables materials processing at higher spatial resolution. In addition, UV lasers micromachine primarily by non-thermal means, thus minimizing the heat affected zone during laser processing. Excimer lasers emitting in the UV region deliver pulse energies scalable up to 1000 mj and output powers up to many hundred watts. Thus, they are the key to fast and effective large area ablation and to machining structures as small as one micron. As a consequence, excimer lasers are the preferred laser technology when it comes to high-performance laser microprocessing with unsurpassed quality and repeatability. As a result, excimer lasers are increasingly employed for many demanding, high precision tasks in semiconductor fabrication, display manufacture, medical device production and scientific research. This article explores some of these applications as well as recent advances in the excimer laser technology that has been developed to service them. Introduction UV wavelengths are particularly advantageous in laser microprocessing because the high energy photons can remove material by direct bond breaking in most materials, including plastics and glasses. This photoablation process generates virtually no heat and hence only marginal peripheral thermal damage as compared to longer wavelength lasers, particularly in the infrared [1]. affected zone (HAZ). In addition, most plastics have a UV absorption spectrum characterized by sharp peaks. The availability of several output wavelengths means that it is often possible to match one of these peaks very closely, leading to highly efficient material removal. Due to diffraction effects, short wavelengths can also be imaged to a much smaller feature size than longer wavelengths, which enables the production of features at much higher spatial resolution. This has proven to be a critical benefit in many applications requiring very small features down to a few microns. Pulsed UV Laser Landscape With industrial processes, throughput rate is as important as process quality. The excimer laser offers excels in this area because it delivers both very high pulse energy and very high average power. Simply stated, the higher the pulse energy, the more material is removed with each pulse or the larger the area covered with each single excimer laser pulse [2]. The high output power in combination with the unmatched resolving power enables thus the fast production of micron-sized features over large areas as required e.g. for flat panel display manufacturing. The latest generation of excimer lasers is now available with output powers of over five hundred watts. As shown in figure 1, other pulsed ultraviolet (UV) laser technologies relying on frequency conversion do not even come close to excimer technology as to the achievable average output power level. In contrast, longer wavelength lasers remove material by intense local heating, which inevitably causes a heat
Achievable Output Power [W] 600 500 400 300 200 308 nm High Power UV Technology Landscape Excimer Lasers 248 nm 308 nm 308 nm 248 nm necessary for most industrial high-speed, high-quality annealing purposes.in order to realize sufficient UV energy densities of the order of 0.5J/cm2 at the substrate which is to be annealed, excimer lasers with high pulse energies of several hundred mj up to 1J are employed. 100 308 nm 248 nm UV-DPSSL 355 nm Lamppumped Nd:YAG laser 355 nm 0 UV Laser Technologies Figure 1. Comparison of pulsed UV power levels (355nm and below) provided by pulsed energy lasers. Large Area UV Beam Concepts Treating large areas in minimum times is often an essential prerequisite in industrial processing [3]. While excimer lasers offer intrinsic advantages for precise large-area treatment, exploiting their full potential in industrial UV processing relies on the appropriate beam delivery architecture. Depending on the application highest through-puts are achieved with enabling beam delivery concepts such as line-beam scanning and step and repeat mask imaging [4]. The most effective large-area excimer laser beam architectures and respective applications will be described in the following section. Line-Beam Scanning Figure 2. Layout of an optical train for shaping large line-beams from high power UV excimer lasers. Step and Repeat Mask Projection High-resolution UV mask illumination of large substrate panels or wafers aiming at producing precise surface micro-patterns such as matrices of holes, grooves or complex circuit structures is effectively done with a high power step and repeat mask imaging concept as schematically shown in figure 3. Line beam optics convert the rectangular excimer laser beam profile which has dimensions of ca. 30mm x 12mm into a line-shaped beam profile which covers entire flat panels or wafers in a scanning process. Depending on the application requirements the illumination field size can vary from 50mm to 500mm in length and from 0.3mm to 0.6mm in width. The building blocks of a high performance line beam optics setup are visible in figure 2 where a typical beam path used for low temperature silicon anneal-ing is depicted. The UV excimer laser beam enters from the middle-left in the picture where it passes an attenuator and is shaped by telescope optics. Using separate elements for long and short axis homogenization leads to the extremely homogeneous and long-term stable line-shape field which is Figure 3. UV mask projection concept for coverage of large-areas with dense and complex patterns.
The basic building blocks of a mask projection UV beam delivery system are of similar order as in the line-beam optics shown before. The main difference is, however, that a mask containing the pattern information is homogeneously illuminated and imaged down to the substrate with a projection lens by a factor of typically 5 to 25. Since the energy density achieved on the substrate increases with the square of the demagnification factor energy densities measured on the substrate range from some hundred mj/cm 2 to some ten J/cm 2. On account of the high UV power and short wavelength of today s industrial excimer lasers (see figure 1), such mask imaging optical systems are capable of covering large substrate areas of up to 400mm 2 within a single excimer laser pulse. Thus providing lateral (horizontal) feature resolutions to the micron level and depth (vertical) resolution to the sub-hundred nanometer level. In low-temperature excimer laser annealing of amorphous silicon layers the line-beam profile at a wavelength of 308nm is directed at a silicon coated substrate which is then scanned relative to the pulsing line-beam. With large line-beam dimensions for high process throughput, a high excimer laser pulse energy of ca. 1J is necessary to reach the threshold for near-complete melt of a ca. 50 nm silicon layer depth. Recrystallization direction is upwards initiated by amorphous silicon remainders left at the bottom of the layer as depicted in figure 4. With large line-beam dimensions for high process throughput, a high excimer laser pulse energy of ca. 1J is necessary to reach the threshold for near-complete melt of a ca. 50nm silicon layer depth. Recrystallization direction is upwards initiated by amorphous silicon remainders left at the bottom of the layer as depicted in figure 3. Step and repeat mask projection type large-area treatment is used for both surface annealing and surface abla-tion processing tasks and provide a high degree of pattern reproducibility. The following section is aimed at illustrating the capabilities of large-area UV micro-processing as to achievable production rate and potential for process upscaling giving real-world industrial application examples. Large-Area UV Applications Precision and productivity are indispensable factors in industrial microprocessing. Sophisticated UV beam delivery systems fuelled by the enabling UV output power of today`s mature excimer laser technology meet these requirements as will be demonstrated with the upcoming production floor applications. Figure 4. Schematic view of the excimer laser silicon annealing process transforming amorphous into polycrystalline silicon. Silicon Annealing In low-temperature excimer laser annealing of amorphous silicon layers the line-beam profile at a wavelength of 308nm is directed at a silicon coated substrate which is then scanned relative to the pulsing line-beam. State-of-the-art line-beam optics in combination with high power excimer lasers deliver line-beam dimensions of 465mm x 0.4mm. In the flat-panel display industry, the excimer laser line-beam architecture enables fast silicon recrystallization speeds of over 30cm²/s at an excimer laser pulse repetition rate of 300Hz. The result is a homogeneous polycrystalline silicon backplane with about 0.3µm x 0.3μm grain size shown
in figure 5 enabling electron mobilities greater than 100cm²/Vs. This value is two orders of magnitude higher compared to employing amorphous silicon and represents the basis for next generation high performance flat panel display devices. Dopant ion activation by low-thermal budget excimer laser annealing is pivotal for the development of highperformance miniaturized electronic switching devices [5]. Full surface melt annealing activation of boron ions implanted at a dose of 1.6*10 14 cm - ² and with an energy of 15keV yield wafer surface resistances as low as 110mV/mA [6]. Due to the single crystalline nature of the wafers melt annealing is done at relatively high energy density of 3J/cm² in a single pulse process. Large field sizes of several square milimeters and high power excimer lasers support throughput rates of 200 wafers per hour. Laser Direct Patterning A particularly effective concept is combining mask projection with reel-to-reel processing as shown in figure 7. Figure 5. Microscopic view of grain boundaries in an excimer laser recrystallized silicon surface. Low-temperature melt annealing of silicon wafer surfaces is effectively conducted by means of step and repeat large-field mask projection as visible in figure 6 where homogeneous 2.5mm x 2.5mm square fields at 308nm were stepped over a 5 inch silicon wafer surface. Figure 7. Layout of an excimer laser based mask projection system with reel-to-reel sample handling and flexible substrate. Figure 6. Microscope image of one-shot excimer laser annealed Si (100) wafer surface using step and repeat processing. Laser direct patterning enables repetitive production of complex patterns such as sensor circuits on flexible substrates. In this technique, the homogenized excimer laser beam passes a mask containing the pattern for one or even several complex circuits and is imaged onto a flexible polymer substrate on which a thin 50-150nm metal layer has been deposited.
At 300Hz repetition rate single-pulse direct patterning can generate 18,000 circuits per minute at substrate feed rates of tens of meters per second [7]. Laser direct patterning can be used with several different flexible plastic substrates (PET, polyimide, PEN, and PMMA) and a full range of conductors including copper, gold, silver, platinum, aluminum, and even titanium. due to their short 193nm output wavelength. The high photon energy of 6.4eV associated with this wavelength effectively breaks the molecular bonds present in glass, diamond or polymer substrates and thus selectively removes the material by evaporating it without melting or heat transfer into the bulk material as indicated in figure 9. Also for high reel-to-reel feed rates structure sizes of better than 10µm are easily obtained using excimer laser wavelengths of 248 or 308nm as is shown in figure 8. Figure 9. Laser material interaction regimes of far UV laser wavelengths far infrared laser wavelengths in transparent substrates. Figure 8. Single-shot ablation of 30nm thick aluminum on polyethylene terephthalate (PET) foil at 248nm and 250mJ/cm 2. There is growing demand for low unit cost, miniaturized electrical circuits and large-area, reel-toreel UV processing lends the required precision and productivity for antennae circuits, disposable medical sensors and the like. UV Laser Marking and Engraving Imprinting tiny brand logos as well as providing miniaturized functional inscriptions on hard and transparent high value parts such as eye glass lenses and diamonds requires a material-friendly and highresolution marking and engraving tool. Recently, excimer lasers have emerged as the enabling technology in highest precision marking and engraving The resulting accuracy and resolution obtained in the marks and engravings is unachieved by the thermal interaction of longer wavelength lasers such as CO 2 and Nd:YAG lasers. Excimer lasers are easy and flexible to use and deliver their nanosecond width, ultrashort 193nm wavelength pulses with up to 500Hz pulse frequency. The inherently cold characteristics of the lasermaterial-interaction enable excellent repeatability and process control in industrial precision marking and engraving tasks. Additives for adjusting the absorptivity of polymers or glasses to the laser wavelength as often is the case width visible or IR wavelength lasers are not required. By appropriate selection of the laser beam fluence and spot size, the contrast and appearance of the marks and engraving remains freely selectable. Commercial eye glass marking systems based on 193nm excimer lasers typically run five days a week in a three shift operation marking about 1,500 lenses a day [8]. Diamond marking proceeds at a rate of about 1s/character.
Regarding the wavelength of the laser, the optical penetration depth of the material has to be taken into account. Figure 10 shows the penetration depth (reciprocal of absorption coefficient) of hard material versus the wavelength by orders of magnitude. While the long wavelengths penetrate into the material as much as many microns, the short-wavelength 193nm radiation is effectively absorbed already in the first nanometers. The upper axis in Figure 10 converts the optical penetration depth into wavelengths, the right axis shows the pulse durations that goes with the thermal diffusion lengths on the left axis. Commercially available marking lasers, their optical penetration depth and thermal diffusion length As a consequence, the ideal laser source should have a very short wavelength in the far-uv in conjunction with short pulse duration. Such a favorable combination of parameters is provided by 193nm excimer lasers and minimizes both optical penetration depth and thermal load as shown in Figure 10. Today s excimer lasers for marking and engraving markets are extremely compact with the footprint of a notebook and a weight of about 50 kg. An integrated premix gas bottle with less than 0.2% halogen content eliminates the need for gas installation and complies with general gas and pressure safety regulations. Pulse energies and pulse frequency can be varied over a large dynamic range without affecting other parameters such as pulse width and beam profile dimensions. Under typical marking and engraving operation conditions, solid-state compact 193 nm excimer lasers provide hands-free running periods of more than 10 million pulses on each single gas fill which corresponds to periods of 2 to 4 weeks between automated gas fills. The laser tube and optics can be used over years of operation before a replacement is necessary. Modern state-of the-art excimer lasers for marking and engraving are driven by solid state switches and employ advanced corona preionization technology in combination with cleanroom assembled, metalceramic tube design. Figure 10. Thermal diffusion lengths and optical penetration depths of commercially available laser sources used in marking and engraving. As a result, not only superior operational gas lifetime is achieved, as shown in figure 11, but also exceptional long-term pulse-to-pulse energy stability of less than 1%, rms. Investigations regarding the wavelength dependence of the laser induced damage showed that a damage zone of 20μm in depth is produced using an infrared laser of 1064nm and still a damage of many microns in depth is observable applying a frequency converted Nd:YAG laser of 532nm. Accordingly, a wavelength in the far-uv range is most appropriate for achieving high marking resolution, since the pulse energy is absorbed essentially at the surface over a small volume. Of equal importance for marking and engraving applications is the pulse duration of the laser source. The longer the pulse duration the deeper the thermal energy is conducted into the bulk and thus a larger material volume is thermally damaged. Figure 11. Gas life run of a marking excimer laser at 500Hz and at a wavelength of 193nm.
Conclusions Increasing product miniaturization has created challenges for manufacturers in many branches. Often, traditional production processes are too slow, unable to achieve the necessary precision, or simply not costeffective. Miniaturization and use of thin film technology in particular is an ongoing trend seen in industrial manufacturing. In selectively patterning or annealing thin and functional layers which often exhibit a thickness between 50nm and 1µm over large areas the excimer laser with its unparalleled UV power and appropriate beam delivery concepts is a key enabler in industrial large-area micro-processing. The maximum penetration depth up to which the material is influenced by the laser radiation correlates with the optical and thermal properties of the materials. When considering transparent substrates such as diamonds, eye glass blanks or dielectrics, highest marking and engraving precision is obtained using 193nm excimer lasers. [4] Masters, A., Geuking, T. (2005) Beam-shaping optics expand excimer-laser applications, Laser Focus World 41, 99-101. [5] Rajendran, B., Shenoy, R. S., Witte, D. J., Chokshi, N. S., DeLeon, R. L., Tompa, G. S., Pease, R. W. (2007) Silicon Devices - Low Thermal Budget Processing for Sequential 3-D IC Fabrication, IEEE Transactions on Electron Devices 54, 707-714. [6] Paetzel, R., Turk, B., Brune, J., Govorkov, S., Simon, F. (2008) Laser solutions for wafer and thin film annealing, physica status solidi 10, 3215-3220. [7] Delmdahl, R. (2010) The excimer laser: Precision engineering, Nature Photonics 4, 286. [8] Paetzel, R., Haidl, M. (2005) Excimer lasers make the reliability mark, Industrial Laser Solutions 11, 5-8. Due to their extremely short wavelength and correspondingly high photon energy in conjunction with the nanosecond-scale pulse width, heat conduction plays a minor role and the absorption coefficient rises strongly toward the far-uv spectral range. Therefore, efficient large-area laser processing in a depth ranging from sub-micron (engraving of eye glasses and diamonds) to about 10µm to 20µm (photochemical marking of polymers) is achieved in a controlled and crack-free manner leading to highest laser machining precision and process reproducibility. References [1] Ihlemann, J., Schulz-Ruhtenberg, M., Fricke- Begemann, T. (2007) Micropatterning of fused silica by ArF- and F 2 -laser ablation, Journal of Physics: Conference Series 59, 206-209. [2] Delmdahl, R., Paetzel, R. (2008) Pulsed laser deposition-uv laser sources and applications, Journal of Applied Physics A 93, 611-615. [3] Usoskin, A., Kirchhoff, L., Knoke, J., Prause, B., Rutt, A., Selskij, V., Farrell, D. (2007) Processing of long-length YBCO coated conductors based on stainless steel tapes, IEEE Transactions on Applied Superconductivity 17, 3235-3238.