Thomas Mitra

Beam shaping of line generators based on high power diode lasers to achieve high intensity and uniformity levels

Beam shaping improvements of line generators based on high power diode lasers in combination with newly designed and produced high precision micro-optics lead to new applications such as hardening, metallization and annealing of different materials. Two aspects are mainly needed to be focused on for getting the best results and throughput in these applications. The first one is the overall power content along the narrow axis of the line, namely the peak intensity in combination with the beam shape. The second one is the intensity homogeneity along the long axis of the line. Herewith, a beam shaping concept that fulfils the desired requirements in a variable modality is presented. The concept consists of macro-lenses and newly designed micro-optics and results in a passively cooled high power diode laser emitting at 808nm. The laser has an output power of 1000W. The generated line has a length of 13mm and a width of <100μm at a remarkably large working distance of about 80mm. We attained an intensity distribution along the line length with a peak power density >80kW/cm2 and uniformity >97%. To achieve such an extraordinary homogeneity level, several approaches based on cylindrical lens arrays were designed and tested. Methods to reduce inhomogeneities caused by diffraction effects and effects based on geometric optics are presented as well as their results. Additionally, the potential of this concept with regard to modularity, expandability and variability is reviewed. Finally, an application example - crystallisation of a thin film of a-Si on a glass substrate - is presented.

Beam shaping of high power diode lasers benefits from asymmetrical refractive micro-lens arrays

Micro-lens arrays are widely used for beam shaping, especially beam homogenization of various laser sources. Monolithic arrays of cylindrical lenslets made of glass, semiconductors or crystals provide great advantages in laser applications, e.g. high efficiency, intensity stability and very low absorption. However, up to now, mainly symmetrical micro-lens surfaces are utilized in most applications due to design and manufacturing restrictions. The manufacture and application benefits of asymmetrical cylindrical-like micro-lens surfaces are enabled by LIMOs unique production technology. The asymmetrical shape is defined by uneven-polynomial terms and/or an asymmetrical cut-off from an even polynomial surface. Advantages of asymmetrical micro-lenses are off-axis light propagation, the correction of aberration effects or intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis. Additionally, the opportunities in simultaneous illumination from numerous light sources to one target are extended by just geometrical arrangement without the need for collinear beam alignment. First application results of such micro-lens arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100 W laser with tilted illumination at an angle of 35° is shown. A multi-kW line generator based on the superposition of over 50 diode laser bars under different illumination angles is demonstrated as well. Thus, laser material processing like plastics welding, soldering or annealing becomes much more convenient and less demanding regarding beam steering.

Free form micro-optics enables uniform off-axis illumination and superposition of high power laser devices

High power laser sources are used in a large variety of applications for material processing, such as ablation, welding, soldering, cutting, drilling, laser annealing, micro-machining and deep-UV lithography. Using high performance optics in the laser systems to generate the appropriate beam profile becomes a key factor for getting the best results and throughput in an application field. Refractive micro-lens arrays made of glass, semiconductors or crystals provide great advantages in laser applications, by improving efficiency, precision, intensity stability and performance. With LIMOs unique production technology, free form surfaces on monolithic arrays exceeding 200 mm edge length can be manufactured with high precision and reproducibility. Each lens of the array can be designed individually and can also be shaped asymmetrically. The asymmetric shape is defined by odd- and even-polynomial terms and/or an asymmetric cut-off from a polynomial surface. Advantages of asymmetric micro-lenses are off-axis light propagation, the correction of aberration effects, or the correction of the intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis. The applications results of such micro-lens arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100 W laser with tilted illumination under an angle of 30°-50° is shown. A multi-kW line generator based on the superposition of over 50 diode laser bars under different illumination angles is demonstrated as well. Novel microoptical beam shapers in lithographic applications reduce the complexity of macrooptics in hyper-NA illumination systems. Extremely uniform intensity distribution can be created without using field lenses or by using simple spherical field lenses instead of complex aspheres.

Novel high-throughput micro-optical beam shapers reduce the complexity of macro-optics in hyper-NA illumination systems

Uniform illumination of the mask is a key factor for the lithography process. The requirements of Immersion Lithography make illumination systems even more complex e.g. by adding additional parameters like polarization and improved throughput. Arrays of refractive microoptics are the ideal solution for high transmission homogenizing elements since several tool generations. These arrays can provide very steep intensity profiles (top hat and other profiles), enable lossless polarization control and do not suffer from zero order losses like diffractive elements. Usually refractive microlens arrays are used with macrooptical field lenses in order to illuminate a field very uniformly or with a customized intensity distribution. High numerical apertures create the necessity for aspherical surfaces which leads to significantly higher lens cost especially for the macrooptics. In this paper we present novel microoptical homogenizers which create extremely uniform intensity distributions for high numerical apertures without any field lens or at least only with spherical field lenses. Especially multi-pole off-axis illumination can be improved with less optical components. An important prerequisite for these special types of homogenizers is that LIMO can produce free form surfaces on monolithic arrays larger than 200 mm with high precision and reproducibility. Every lens can be designed individually and can also be shaped asymmetrically. We will present surface test methods and the final UV tests, guaranteeing the performance for the applications. Example data gained with these tests will be shown with regard to: meeting the design parameters, reproducibility over one wafer and reproducibility in large lots. Monolithic elements based on crossed cylindrical lenses provide a fill factor close to 100%. Simulations and measurements prove that microoptic arrays can be produced which provide a uniformity of the homogenized laser light of significantly better than 1% P-V at numerical apertures above 0.35. Refractive microoptic arrays do not change the polarization state of the transmitted light which is an important prerequisite in immersion exposure tools. LIMO homogenizer sets are manufactured from fused silica and Calcium Fluoride thus they are suitable for all DUV wavelengths at highest laser fluxes.

Overview: Process-optimized beam transformers and their impact on high-power laser applications

Micro-lenses and micro-lens arrays are widely used for various applications. Monolithic arrays of cylindrical lenslets made of glass, semiconductors or crystals provide great advantages to laser applications, e.g. high efficiency, intensity stability and very low absorption. However, up to now, mainly symmetrical micro-lens surfaces are utilized in most applications due to design and manufacturing restrictions. The manufacture and application benefits of asymmetrical cylindrical-like micro-lens surfaces are enabled by LIMOs unique production technology. The asymmetrical shape is defined by uneven-polynomial terms and/or an asymmetrical cut-off from an even polynomial surface. Advantages of asymmetrical micro-lenses are off-axis light propagation, the correction of aberration effects or intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis. First application results of such microlens arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100 W laser with tilted illumination under an angle of 30-50° is shown. A homogeneity of better than 90% was achieved for a field size of 270 mm x 270 mm. In laser direct write processes a top hat profile has several advantages compared to a Gaussian beam profile, especially the throughput of the system and quality of the structures can be improved. Novel patterning results with TopHat-converted single mode lasers and a special Gaussian-to-TopHat galvo scan system are demonstrated for solar cell technology.

Inspection and metrology tools benefit from free-form refractive micro-lens and micro-lens arrays

LIMOs unique production technology based on computer-aided design enables the manufacture of high precision asphere single lenses and arrays, where every single lens can be individually shaped. These free form micro-optical cylindrical lens and lens arrays find their application in various types of metrology systems. Due to the high precise manufacturing of specially designed surface, single lenses can be bond directly onto sensor or sensor arrays, performing efficient projection of signal onto detector. Optical modules based on micro-lenses arrays enable special intensity distribution, as well as highly homogeneous illumination with inhomogeneity less then 1% (peak to valley) used in illumination parts of inspection tools. Due to the special free form profile, a special case of asymmetric lens arrays can offer extreme uniformity illumination at the target non orthogonal to the illumination path. The feature under inspection can be uniformly illuminated even if it lies at a specific angle to the illumination. This allows better conditions for measurement devices arranged orthogonal to the mask or wafer. Furthermore the use of micro-optics enables more sufficient inspection of laser beam parameters for excimer or CO2 lasers. Additionally very accurate metal patterns can be applied on the optics and used as alignment marks, apertures or bonding features.

Novel refractive optics enable multipole off-axis illumination

Optical lithography in the deep ultraviolet (DUV) region is being pushed to reach the limits of printing resolution. The effort required to achieve the 32 nm structure with this technology puts very hard conditions and requests on the illumination optics. Different kinds of illumination modes are combined to get into a regime of extreme numerical aperture (hyper NA). Arrays of refractive micro-optics have been and continue to be the ideal solution for high transmission homogenizing elements for several tool generations. Illumination of the masks with high numerical aperture is critical for achieving the smallest structure on the semiconductor material. Exposure tools use different illumination modes to get better imaging of certain mask structures. The beam shaping necessary to create these illumination modes is achieved mostly with diffractive elements. Most of the currently used modes can also be created with arrays of refractive micro-optics, manufactured from fused silica and calcium fluoride. The advantage over the diffractive optical elements lies in efficiency, which comes close to 90%. An important prerequisite for these special types of optical elements is LIMOs unique production technology which can manufacture free form surfaces on monolithic arrays exceeding 200 mm edge length with high precision and reproducibility. These homogenizing elements in the illumination optics can provide a custom designed intensity distribution, and offer the possibility to correct the failure of other optical elements. Each lens can be designed individually and can also be shaped asymmetrically. Thus unusual lens sizes and shapes can be produced, and various far fields such as rectangles, lines, hexagons or multi-poles can be achieved. In this paper we present novel refractive micro-optical elements which create rectangular dipole illumination. They can also be combined in such a way as to create a quadrupole with variable intensity ratio between the vertical and horizontal poles. The huge advantage of such a multipole illumination is polarization control and variable intensity in poles. Working on this combination, the resolution can be enhanced even further.

Laser direct micro-machining with top-hat-converted single mode lasers

Laser direct micro-machining processes are used in a variety of industries like inkjet printing, semiconductor processing, solar technology, flat-panel display production and medicine. Various kinds of materials, e.g. ceramics, metals, isolators, oxides, organics and semiconductors are being structured. In most cases pulsed single mode solid state lasers with an inhomogeneous Gaussian beam profile are employed, like YAG lasers and their harmonics. However, the quality and functionality of the generated structures and micro-systems as well as the speed of the process can be improved by the utilization of homogeneous top hat profiles. The beam shaping principle of refractive Gaussian-to-top-hat converters is shown. Compact beam shaper modules based on this principle have been developed - supporting most direct laser micro-machining applications. The resulting process advantages are demonstrated by selected application results, namely the drilling of holes and patterning of trenches for different kinds of materials.

Ultra-short pulse laser micro patterning with highest throughput by utilization of a novel multi-beam processing head

In the last decade much improvement has been achieved for ultra-short pulse lasers with high repetition rates. This laser technology has vastly matured so that it entered a manifold of industrial applications recently compared to mainly scientific use in the past. Compared to ns-pulse ablation ultra-short pulses in the ps- or even fs regime lead to still colder ablation and further reduced heat-affected zones. This is crucial for micro patterning when structure sizes are getting smaller and requirements are getting stronger at the same time. An additional advantage of ultra-fast processing is its applicability to a large variety of materials, e.g. metals and several high bandgap materials like glass and ceramics. One challenge for ultra-fast micro machining is throughput. The operational capacity of these processes can be maximized by increasing the scan rate or the number of beams – parallel processing. This contribution focuses on process parallelism of ultra-short pulsed lasers with high repetition rate and individually addressable acousto-optical beam modulation. The core of the multi-beam generation is a smooth diffractive beam splitter component with high uniform spots and negligible loss, and a prismatic array compressor to match beam size and pitch. The optical design and the practical realization of an 8 beam processing head in combination with a high average power single mode ultra-short pulsed laser source are presented as well as the currently on-going and promising laboratory research and micro machining results. Finally, an outlook of scaling the processing head to several tens of beams is given.

Optical elements for optimal brightness of single emitter devices

Semiconductor lasers play an important role in many applications. Depending on the wavelength of the emitted laser light in the blue (e.g. 405-445 nm), red (~ 650 nm), near infrared (780 - 1070 nm) and e.g. the eye-safe wavelength region around 1500 nm a manifold of applications exist. Due to their increasing power and brightness single emitter devices are becoming increasingly widely used for the assembly and packaging of high power diode lasers. In the near infrared typical emitter widths are 50, 90 (100) and 200 μm with power levels available > 15 W. Also larger stripes are available - up to 1000 μm - with power levels > 25W. For highest power laser devices not only the power of the emitter is important - but of equal importance is the subsequent optics to collect all the emitted power while maintaining the brightness of the source. High NA acylindrical micro-lenses very well account for the strong asymmetric emitter characteristics of the fast and slow axis and thus, result in best collimation and coupling efficiencies in contrast to spherical lenses. LIMOs cost-effective micro-optics wafer technology is most suited for such acylindrical optics. It allows the manufacture of different materials to cover wavelengths ranges from the UV to the NIR, e.g. 380 - 2000 nm. Since both sides of a wafer can be structured with crossed cylindrical lenses one single monolithic optical element simultaneously shapes the fast and slow axis of the emitted light. Additionally, mechanical reference planes can be integrated in such monolithic optics for precise and simple integration. Application examples for collimation and fiber coupling optics in the near infrared as well as focussing/pump optics in the blue wavelength range are shown.