Proc. SPIE 4770, Lithography and photoablation systems for microelectronics and optoelectronics: importance of laser beam shaping in system design, 0000 (24 October 2002); https://doi.org/10.1117/12.451644
As microelectronic and optoelectronic devices continue to make advances in speed, functionality, and levels of integration, demands on the fabrication equipment for higher resolution, large-format processing, and lower cost continue to accelerate. Excimer lasers, due to their high power, short wavelength, and low spatial coherence, are ideally suited for these systems. In the optical design of the illumination and imaging systems in lithography tools, laser beam shaping is a critical factor in optimizing three key characteristics: beam uniformity, beam geometry, and numerical aperture. In state-of-the-art systems, various beam homogenization techniques are employed to produce high uniform intensity distributions; a variety of beam geometries are implemented depending on the imaging system configurations; and the partial coherence factor is carefully controlled by optimum design of the illumination and imaging numerical apertures. In this paper we review several advanced lithography and photoablation system designs used in microelectronics and optoelectronics fabrication, and show how advanced beam shaping criteria have played an integral role in the overall tool design, making it possible to achieve large-area patterning with higher resolutions as well as higher throughputs.
In the multi-photon dissociation process of Carbon isotope enrichment, IR photons are used to selectively excite a molecule with the given isotopic base element. This enrichment process is very sensitive to the beam's intensity and wavelength. Because the intensity is determined by the propagation of the field, the enrichment factors are also very dependent on the field propagation. In this paper, the influence of the wavelength and intensity of the beam, on the isotope selective dissociation of a CFC compound is investigated both experimentally and theoretically. Consideration is also given to some of the factors that influence the delivery of various beams to the reactor chamber, and their subsequent propagation through the reactor. The results show that suitable beam forming can lead to an improved isotope separation process.
An overview of the history and current practices of laser beam shaping is presented. When diffraction effects are not important, geometrical methods for laser beam shaping (ray tracing, conservation of energy within a bundle of rays, and the constant optical path length condition) can be used to determine system configurations, including aspheric elements and spherical-surface GRIN lenses, which are required to transform an input laser beam profile into a more useful form of illumination. This paper also summarizes applications of these techniques to the optical design of a two-plano-aspheric lens system for shaping a rotationally symmetric Gaussian beam, a two-mirror system with no central obscuration for shaping an elliptical Gaussian input beam, and a three-element spherical surface GRIN system for shaping a rotationally symmetric Gaussian beam.
When a pair of aspheric lenses is used to transform the transverse intensity profile of a laser beam, a deviation of the actual lens surfaces from their ideal shape causes the output beam to deviate in intensity and phase from the intended profile. In this paper, geometrical optics and the paraxial approximation are applied to derive simple expressions for the effect of small errors in the slope and curvature of the lens surfaces on the output intensity profile produced by a refractive beam reshaper. The results are applied to a Gaussian-to-flattop reshaper, which has been described in detail in previous publications,and provide insight into the tolerances required of the optics. For example, if the output intensity is required to be within 5% of the design value, then the tolerance on the lens curvature at the optic axis is less than 2%
This contribution focuses on the study and comparison of different design approaches for designing phase-only diffractive optical elements (PDOEs) for different possible applications in laser beam shaping. Especially, new results and approaches, concerning the iterative Fourier transform algorithm, are analyzed, implemented, and compared. Namely, various approaches within the iterative Fourier transform algorithm (IFTA) are analyzed for the case of phase-only diffractive optical elements with quantizied phase levels (either binary or multilevel structures). First, the general scheme of the IFTA iterative approach with partial quantization is briefly presented and discussed. Then, the special assortment of the general IFTA scheme is given with respect to quantization constraint strategies. Based on such a special classification, the three practically interesting approaches are chosen, further-analyzed, and compared to eachother. The performance of these algorithms is compared in detail in terms of the signal-to-noise ratio characteristic developments with respect to the numberof iterations, for various input diffusive-type objects chose. Also, the performance is documented on the complex spectra developments for typical computer reconstruction results. The advantages and drawbacks of all approaches are discussed, and a brief guide on the choice of a particular approach for typical design tasks is given. Finally, the two ways of amplitude elimination within the design procedure are considered, namely the direct elimination and partial elimination of the amplitude of the complex hologram function.
We extended the study of a diffractive laser beam shaping system which is originally designed for continuous waves (SPIE vol. 2863, pp 237-45, 1996). The beam profile of an ultra-short laser pulse passing through this optical system is calculated. Two different numerical methods, Monte Carlo and Gaussian method, are applied to this problem. We find that the Gaussian method yields better results because the step size algorithms used in this method are well suited for this specific problem. The Gaussian numerical simulation shows that the fluence still yields a top-hat radial distribution. Beam shaping of ultra-short laser pulses using this optical scheme is feasible.
Conventionally laser beam shaping problems are defined by the required intensity and/or phase distribution in a single 2-D output plane, although recently there have also been examples for beam shaping solutions where the system output had to satisfy constraints in a small number of axially separated planes. For a number of application areas it is beneficial to be able to work with beams that have a particular intensity distribution that is specified in a 3-D volume. Laser material processing, optical microscopy and laser trapping (optical tweezers) are a few examples for these. We will discuss how diffractive optical elements can be used to generate beams with prescribed 3-D intensity profiles, with particular emphasis on techniques for the design of such diffractive optics. Practical examples will be given for the implementation of the diffractive optical elements using programmable spatial light modulators and for the application of the 3-D beams.
We present a novel technical approach to build a multiwavelength/superbroadband laser source based on a combination of spectral and spatial domains. This approach provides a direct transformation of spatial domain of the pumping beam into spectral domain of the output oscillation. Several spatially dispersive laser cavities were designed and studied both theoretically and experimentally for optimal spatial to spectral transformation. This spatial to spectral transformation was demonstrated for LiF:F2+** color center laser, single broad-stripe diode laser operating at 660nm and 1560 nm multi-stripe diode laser.
Analyzing the transversal modal content of a laser beam is a promising tool to evaluate the quality of a beam and to understand its propagation behavior. Whereas a complete description of the beam by superposition of its transversal modal components usually requires knowledge about strength and relative phase delay of the individual modes, for certain important applications information about relative strength of individual modes is sufficient to evaluate beam quality. The present work discusses in detail an approach to designate the strength of few pre-selected modes contained in a given laser beam by applying computer generated amplitude holograms as correlation filters. Quality of different coding methods for the complex transmission function of the filters is compared, and experimental results obtained from various optical set-ups are presented.
High power LDA's in general have an emitter line dimension of 1cmx1μm and a beam divergence of 40°x10°, which could not be delivered or focused effectively with common optics. With the ever-increasing needs of high power diode laser sources, it is crucial to obtain high power high brightness diode laser beam with improved optical quality. Although it has been known that beam shaping techniques can be used to achieve this aim, it has been challenging to shape the laser beam effectively and efficiently to deliver it through an optical fiber with high brightness. By using a new beam shaping technology developed recently, however, brightness over 2.4 MW/cm2-str has been achieved by delivering 28W from a 0.1mm multimode fiber. Diode lasers are the most efficient laser comparing with other lasers, and its application in various industrial sectors will become more and more extensive.
Propagation-invariant fields of the third kind are defined on the base of a modal theory proposed recently by authors. The physical nature of these fields is discussed and their examples are illustrated by numerical simulation.
The technique of optical generating the light string and light capillary beams predicted recently by authors in a framework of the modal theory of propagation-invariant fields is proposed and its capacity is demonstrated in experiment.
This paper discusses the novel adaptive optical closed loop system with bimorph mirror as a wavefront corrector and Shack-Hartmann wavefront sensor to compensate for the aberrations of the laser beam occurred during the distribution of the beam from laser to processed material. Adaptive system can correct for the low-order aberrations in the real-time - the frequency of corrected aberrations is less then 25 (30) Hz. The amplitude of such aberrations - about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations - up to 15th Zernike polynomial (excluding tip-tilt).
The intracavity way of intensity distribution formation is discussed. The examples of a such formation by means of bimorph flexible corrector are shown. The whole structure of adaptive system for the intracavity given intensity formation is considered.
This report discusses the design and installation of a phase optic inserted in the near field of the HELEN high power glass laser. The element is designed to shape the intensity distribution at the focal spot of the laser to produce an increase in the peak intensity through correction of static and thermally induced wavefront errors on the beam. A phase element has been fabricated commercially using a magneto-rheological finishing tool. Test data is presented.