Reinhard Steiner

New solutions to realize complex optical systems by a combination of diffractive and refractive optical components

Diffractive optical elements (DOEs) have a great potential in the complete or partial substitution of refractive or reflective optical elements in imaging systems. The greater design flexibility compared to an all-refractive/reflective solution allows a more convenient realization of the optical systems and additionally opens up new possibilities for optimizing the performance or compactness. To demonstrate the opportunities of the hybrid optical concept we discuss different imaging systems for various applications. We present the lens design of a hybrid microscope objective which is especially applicable for wafer inspection technologies. Meeting the requirements for such a system used in the deep-UV regime (248 nm) is very challenging. The short wavelength limits the material selection and demands cement free optical groups. The additional requirement of an autofocus system, working at a wavelength in the near infrared region, is fulfilled by the special combination of two selected and adjusted DOEs. Furthermore, we discuss the opportunities of the hybrid concept c of a slit lamp used for ophthalmologic examinations. The DOEs are the basic elements of this hybrid concept. We demonstrate that holographic lithography is an appropriate technology to realize a wide variety of elements with different profile geometries. We address in particular the additional possibilities of an UV-laser system as an exposure tool. Additionally to the high spatial frequencies, the 266 nm exposure wavelength allows the use of novel photo resists with advantageous development behavior.

Immersion mask inspection with hybrid-microscopic systems at 193 nm

The capability of a high NA, large working distance, microscope objective was demonstrated by investigating different mask features. The microscope objective is based on a hybrid concept combining diffractive and refractive optical elements. Resolution down to 125 nm lines and spaces (L/S) is demonstrated by investigating periodic chrome on glass structures. A significant additional improvement of the resolution is achieved by inducing a solid immersion lens (SIL).

Fabrication of low straylight holographic gratings for space applications

The main challenges of fabricating diffraction gratings for use in earth monitoring spectrometers are given by the requirements for low stray light, high diffraction efficiency and a low polarization sensitivity. Furthermore the use in space also requires a high environmental stability of these gratings. We found that holography in combination with ion beam plasma etching provides a way to obtain monolithic, robust fused silica gratings which are able to meet the above mentioned requirements for space applications. Holography accompanied by plasma etching allows the fabrication of a wide range of different grating profiles to optimize the efficiency including the polarization behavior according to a wealth of applications. Typical profile shapes feasible are blazed gratings, sinusoidal profiles and binary profiles and this allows to tailor the efficiency and polarization requirements exactly to the spectral range of the special application. Holographic gratings can be fabricated on plane and also on curved substrates as core components of imaging spectrometers. In this paper we present our grating fabrication flow for the example of plane blazed gratings and we relate the efficiency and stray light measurement results to certain steps of the process. The holographic setup was optimized to minimize stray light and ghosting recorded by the photoresist during the exposure. Low wave front deviations require the use of highly accurate grating substrates and high precision optics in the holographic exposure.

Microstructured optics for excimer-based systems: applications for imaging, beam shaping, and coherence management

Imaging diffractive optical elements (DOEs), randomly distributed microlenses and sub-λ-structures allow the improvement of the performance of Excimer-based imaging and illumination systems. Here we present the concept study of a hybrid imaging system for Excimer laser high power application at a working wavelength of 308 nm. In this hybrid approach a focus has to be put onto the impact of the non-desired diffraction orders to the optical performance and also to the amount of light remaining in the system and causing lens heating. Hereby the diffraction efficiency of the DOE is of enormous importance. Especially the influence of the passive facet of blazed transmission gratings has to be considered. For illumination systems we discuss the transfer of the uneven raw profile of an Excimer-laser into a Gaussian far-field distribution by introducing a microlens array with a 2-dimensional Voronoi-pattern. The holographic record of the microlens array allows additionally to influence the coherence parameters.