Applying reactive ion beam etching (RIBE) processes at the Leibniz Institute of Surface Modification (IOM), several reference samples to be used in industry for calibrating of roughness testing equipment have been generated with the smoothest sample featuring 0.1 nm rms Sq. Subsequently these reference samples have been measured cross-site applying atomic force microscopy (AFM), white light interferometry (WLI), Nomarski1 microscopy (NM) and scatterometry (iTIRM2) determining the appropriate range of measurable rms surface roughness for each industrial measuring device.
Standard UV materials, such as ArF-grade fused silica, have impurities that lead to low transmittance, high absorption,
and fluorescence when exposed to high irradiance. Calcium fluoride (CaF2), on the other hand, is a promising material
for use as an optical diffuser for applications at 157nm, 193nm, and 248nm due to its low defect density and high
transmission in the deep UV regime. In this paper, we discuss our method for fabricating Gaussian homogenizers in
calcium fluoride using a grayscale photolithography process. Refractive microlens array homogenizers and Gaussian
homogenizers have been fabricated in CaF2 and tested at 193nm for efficiency and uniformity. Using an excimer laser,
uniformity results were obtained for cylindrical lens arrays in tandem and crossed to observe the homogeneity in an
imaging configuration and for producing a square output. Efficiency, uniformity, and zero order measurements are
provided for the Gaussian homogenizers.
KEYWORDS: Beam shaping, Polarization, Optical components, Near field optics, Polarizers, Optical testing, Near field, Lithographic illumination, Silica, Binary data
Micro and nano optics enable the control of light for producing intensity distributions with given profiles, propagation properties and polarization states. The higher the requirements on the optical function, the more complicated will be its realizing with a single element surface or a single element class. Combinations of refractive and diffractive, both diffractive or sub-wavelength structures with each other give the ability to link the advantages of different element classes or different element functions for realizing the optical functionality. In the paper we discuss two different examples of combinations for DUV applications. In detail we present a diffractive - diffractive beam homogenizer with NA of 0.3 that show no zero order. A binary phase grating for polarization control combined with a beam shaping element will be presented. The polarized order of this grating shows an efficiency of about 90% with a degree of polarization better than 90%. Wave optical and rigorous design strategies and simulations as well as the optical measurements will be discussed for the given examples.
In the optimization process of DUV-illumination systems for inspection tools and lithographic devices, more and more an exact control of angular distribution and homogeneity of the illumination will be required. On the one hand, diffractive homogenizers enable homogeneous illumination of areas with almost arbitrary shape with a high numerical aperture. On the other hand, diffractive optical elements produce a zero order or so called “hot spot”. If the optical axis is within the illuminated area, this hot spot will decrease the homogeneity of illumination. The zero order is caused by profile aberrations and its intensity can be decreased by increasing the fabrication accuracy. But the higher the numerical aperture, the larger the ratio between zero order brightness and brightness of the surrounding homogenized area. I.e., in cases of high NA the zero order of a homogenizer cannot be reduced to the brightness of the surrounding area. We present a novel approach of beam homogenization using a combination of two serially arranged diffractive optical elements that produces an intensity distribution without hot spot. Such compact two-element homogenizers have been realized for wavelengths down to 193nm. A homogenizer for 193nm producing a homogeneously illuminated rectangle with 0.3 NA will be presented.
We report on a new approach in analog photolithography for the manufacturing of optical elements with a continuous profile. It is based on a phase-only mask used in a mask aligner.The advantage of this new approach is that it is contact free, i.e.there is a gap between the mask and the substrate during the exposure. That allows
a non-destructive mask lithography.
We report on a novel technique for hardening micro-optical resist elements, like beam-shaping elements or micro lenses, with large profile depths before proportional transfer into the fused silica substrate. This technique allows to harden the resist with only small distorsions of the height profile. For demonstration a refractive beam shaper was designed and fabricated in photo-resist using gray-tone lithography. This element was transfered into fused silica with high etch rates using an optimized set of parameters in an ICP-etcher.
In many laser diode applications, it is necessary to make a beam shaping or beam transformation. One example is the collimation, but often we wish to achieve additional properties like special shapes of the beam. Such beams can be designed with high efficiency and signal quality by means of refractive beam shaping elements. Frequently, we have to vary the beam propagation parameters significantly to fulfil the beam shaping task. If we want to use refractive beam shaping elements, the design results in an element with a large profile depth. A well suited fabrication method for refractive beam shaping elements is the gray tone lithography, however, it is limited by the achievable depth of profile. This means that design and fabrication methods should be taken into account to achieve the advantages of refractive elements. On the one hand, we have to improve fabrication technique for enlarging the producible profile depth. On the other hand, we have to use all of the design freedoms to reduce the profile depth. We will present results of the design and fabrication of a refractive beam shaping element with a profile depth up to 60micrometers to transform a laser diode beam into a line intensity distribution.
The transformation of a given incoming wave front into a certain intensity distribution is an interesting task for micro optics with applications, for instance, in material processing and display illumination. For the design of a beam shaping element, the amplitude and the phase of the illumination wave have to be known at least in one plane. If the light source for the beam shaping problem is a high power laser or a multi mode fiber, the illumination wave is not the fundamental mode but the superposition of many modes with fluctuating fractions of intensity. This means that the amplitude and the phase of the beam can not be expressed analytically. However, the average amplitude can be calculated using the measured intensity distribution while the phase is unknown. Therefore we applied a method where the phase of the real illumination wave is approximated by a representative phase distribution which can be assumed from the propagation of the multi mode beam. With this approximation, we calculated refractive beam shaping elements using well known numerical methods based on wave optics. The beam shaping elements were fabricated using gray tone lithography which is a well suited technology for the fabrication of continuous surface profiles. We will present results of the application of such an element to shape the beam from a multi mode fiber.
An interesting scope in micro optics is the transformation of an arbitrary incoming wave front into another arbitrary intensity distribution. This task includes simple focusing as well as the sophisticated control of the propagation properties of the illumination wave. For the reasons of wave length independence and high efficiency, it is necessary to use refractive elements. The design of these beam shaping elements is made by well-known numerical methods based on wave optics. One technology for the fabrication of refractive micro optical elements is gray tone lithography which is capable of the realization of continuous surface profiles with a total height of up to 65 μm. With the use of gray tone lithography on a preform, even higher profiles are capable of being produced. We fabricated high quality lens arrays and different beam shaping elements. The generation of top-hat intensity distributions with different (non separable) shapes or a line of constant intensity are examples of application. We used a single mode fiber as well as a laser diode and a multi mode fiber as light sources. The illumination waves are Gaussian or Gaussian-like beams.
To realize a well defined binary grating (for grating originals as well as grating masks), it is helpful to use a rectangular resist profile because of the importance of the fill factor. This can be achieved easier by e-beam writing than by holography. On the opposite, well-known handicaps of e-beam writing are large writing times and grating ghosts (caused e.g. by stitching errors). We fabricated chromium grating masks (period 500 nm, size 100 mm x 100 mm) by an extremely fast and specialized e-beam direct writing process. The typical and critical parameters, like fill factor and wave front, were locally measured in the whole grating area. The paper will show the method of e-beam writing and the results of lateral grating quality. The remaining errors are separated in problems caused by the writing process and in problems related to resist technology and etching process.
Recording holographic optical elements usually requires a good illumination uniformity as well as a spherical or plane phase. To fulfill the uniformity demand, an expansion of the Gaussian beam is necessary. This leads to a loss of intensity connected with an essential extension of the recording time. Alternatively, the recording efficiency can be increased by using a beam transformation the beam from a Gaussian into a top hat intensity distribution. We designed, realized and tested a setup for a more efficient hologram recording which can be used for full color application. The heart of the setup is a refractive beam shaping element fabricated by a gray tone lithography and proportional transfer into quartz glass. The beam shaping element shows a conversion efficiency of $GTR99,5% (like a refractive lens) in the whole visible spectral range and an intensity uniformity <5%RMS.
The potential of 3D integrated optics based on different technological schemes is investigated. Theoretical and experimental results for waveguide geometries with stacked waveguide layers and with waveguide circuits prepared on topological structures are reported as well. Within waveguide geometries including individual guides in a sequence of stacked layers directional coupler arrays allow for short length passive signal distribution, and various schemes of single and multipath switching can be identified. Cost effective preparation technologies as spin coating of polymer and PECVD of SiON layers and their patterning by UV- exposure or RIE, respectively, have been prove to fulfill the critical tolerance requirements of a simultaneous directional coupling in two transversal directions. To realize waveguides with smooth height variation gray scale lithography was used to produce topological surfaces. Upon those surfaces waveguide paths and devices can be defined subsequently, which are useful e.g. for non-planar to planar fan out structures or interferometer configurations for sensing applications. The topological surfaces can be replicated very efficiently by reaction molding, a technology widely used for micro-optical structures, too.
Several algorithms can be used in order to calculate a phase function that shapes a light beam into a certain intensity distribution. In cases of smooth phase functions, e.g. for the conversion of a Gaussian beam into a flat top distribution, the optical element can be realized by a smooth surface profile without 2 pi jumps. Even though the disadvantages of diffractive elements are well known, often the limitations of the fabrication technologies permit the realization of only diffractive elements. For the beam shaping elements in micro optics, methods exist which allow the fabrication of smooth surface profiles. Such refractive elements have the advantages of a wavelength independent behavior and a conversion efficiency of nearly 100% as known from refractive lenses. In some cases, the design of refractive result in thick surface profiles and the fabrication technique do not fit to this. Therefore, the objective of the paper is to show some possibilities to make the design meet the fabrication capabilities. We used lithographic and etch techniques as well as replication technologies for the fabrication. The basis of the presented technology is gray tone technique, the generation of a proper pre-form, and its correction in order to achieve maximum coincidence with the desired surface profile. Several refractive beam shaping elements have been realized.
Analogue contact lithography is a suitable technology for the fabrication of continuous surface profiles. In this field HEBS-glass gray scale masks have a great potential, for instance for producing microoptical elements. This paper summarizes detailed investigations on the electron beam exposure of HEBS-glass masks. At first we give an idea of the effects we obtained by exposing HEBS-glass masks with different kinds of e-beam writers (Gaussian beam and variable shape e-beam writer). We found thermal effects and a bottleneck effect which have different consequences for the gray level of the exposed mask. To understand its physical causes the bottleneck effect was investigated in detail. Based on this knowledge we introduce two different strategies to overcome the problems caused by the different concepts of the e-beam writer. Selected examples of fabricated profiles demonstrate the facilities of HEBS-glass using these strategies.
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