Lutz Aschke

Novel continuously shaped diffractive optical elements enable high efficiency beam shaping

LIMOs unique production technology is capable to manufacture free form surfaces on monolithic arrays larger than 250 mm with high precision and reproducibility. Different kinds of intensity distributions with best uniformities or customized profiles have been achieved by using LIMOs refractive optical elements. Recently LIMO pushed the limits of this lens production technology and was able to manufacture first diffractive optical elements (DOEs) based on continuous reliefs profile. Beside for the illumination devices in lithography, DOEs find wide use in optical devices for other technological applications, such as optical communications, laser technologies and data processing. Classic lithographic technologies lead to quantized (step-like) profiles of diffractive micro-reliefs, which cause a decrease of DOEs diffractive efficiency. The newest development of LIMOs microlens fabrication technology allows us to make a step from free programmable microlens profiles to diffractive optical elements with high efficiency. Our first results of this approach are demonstrated in this paper. Diffractive beam splitters with continuous profile are fabricated and investigated. The results of profile measurements and intensity distribution of the diffractive beam splitters are given. The comparison between theoretical simulations and experimental results shows very good correlation.

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.

Beam shaping: top hat and customized intensity distributions for semiconductor manufacturing and inspection

Enabling the next technology nodes with optical technologies means further reduced error budgets for optical systems and new optical approaches for higher precision and increased throughput. This contribution discusses important aspects and features of laser beam shaping in optical systems for semiconductor manufacturing and inspection. Beam shaping principles for different types of lasers and illumination requirements are explained.

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 approach for manufacturing of continuously shaped diffractive optical elements

Optical lithography with its 193nm technology is pushed to reach and shift its limits even further. There is strong demand on innovations in illumination part of exposure tools. Current illumination systems consisting of diffractive and refractive optical elements offer numerous benefits such as optimized laser beam shape with high homogeneity and high numerical aperture enabling high efficiency. LIMOs unique production technology is capable to manufacture free form surfaces on monolithic arrays larger than 250mm with high precision and reproducibility. Different kinds of intensity distributions with best uniformities or customized profiles have been achieved by using LIMOs refractive optical elements. Recently LIMO pushed the limits of this lens production technology and was able to manufacture first diffractive optical elements (DOE) based on continuous reliefs profile. Beside for the illumination devices in lithography, DOEs find wide use in optical devices for other technological applications, such as optical communications and data processing. Up to now DOE designs follow the principle of phase diffraction gratings. Its diffraction structure with a periodic phase profile performs a superposition of beams with predefined energy ratios. Due to the application for high precise laser-beam shaping and beam splitting in optical technologies and optical fiber networks, number of grating orders is increased up to some tens or even hundreds. Classic lithographic technologies lead to quantized (step-like) profiles of diffractive micro-reliefs, which causes a decrease of beam splitters diffractive efficiency. The newest development of LIMOs microlens fabrication technology allows us to make a step from free programmable microlens profiles to diffractive optical elements with high efficiency. Our first results of this approach are demonstrated in this paper. Diffractive beam splitters are presented. A special mathematical method is used to design diffractive optical elements with continuous surface profiles. Comparison between theoretical simulations and experimental results shows very good correlation.

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.

Front Matter: Volume 9343

This PDF file contains the front matter associated with SPIE Proceedings Volume 9343, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.