Spatial coherence properties of an LED-based illumination system for mask-aligner lithography

A high-power LED-based illumination system has been developed as a replacement for the mercury arc lamps used in mask-aligner lithography. LEDs are arranged in a grid array and placed in the entrance aperture of individual reflectors. Those reflectors decrease the angular extent of the light. With this multisource approach, different groups of LEDs can be switched on independently. The illumination patterns created determine the illumination angles and the spatial coherence in the mask plane. The spatial coherence is measured in the mask plane by using a circular double slits approach. The interference pattern for different illumination patterns are measured, showing the effect of the asymmetry and size of the angular extent of the illumination light. The effect of the different illumination patterns on the quality of the prints are also illustrated with print tests.

High power modular LED-based illumination system for lithography applications

Johana Bernasconi, Marcel Groccia, Hans Peter Herzig, Raoul Kirner, Wilfried Noell, Toralf Scharf

Mask-aligner lithography is a technology used to transfer patterns with critical dimensions in the micrometer range from below 1 micron for contact printing to a dozen of microns in proximity printing. This technology is widely used in the fabrication of MEMS, micro-optical components, and similar ﬁelds. Traditionally, the light sources used for mask-aligners are high-pressure mercury arc lamps, which emit in the UV rang of the spectrum with peaks at 365 nm, 405 nm and 435 nm, respectively the g-, h- and i- lines. These lamps suﬀer from several disadvantages (ineﬃcient, bulky, dangerous), which makes alternatives interesting. In recent years, high power UV LEDs at the same wavelengths appeared on the market, opening the door to new illumination systems for mask-aligners. We have developed a modular 250 W LED-based illumination system, which can advantageously replace a 1 kW mercury arc lamp illumination. LEDs, arranged in a 7x7 grid array, are placed in the entrance apertures of individual reﬂectors, which collimate the individual irradiation to an output angle of 10◦. A subsequent ﬂy’s eye integrator homogenizes the illumination in the mask plane. It is followed by a Fourier lens, superimposing the individual channels in the mask plane, and a ﬁeld lens to ensure telecentric illumination. This multisource approach allows the shaping of the source by switching individual illumination channels, determining the illumination angles and the spatial coherence in the mask plane. This concept can be used, for example, to do source-mask optimization. Compared to mercury arc lamp illumination, our system is simultaneously more eﬃcient, compact, versatile, economic and sustainable. In our contribution, we present the design of the system as well as lithographic test prints done with diﬀerent illumination patterns.

2018

High power ultraviolet LED illumination systems: coherence properties and applications in photolithography

Johana Bernasconi

Mask-aligner photolithography is a technology used to replicate patterns from a mask to a photosensitive substrate. It is widely used in the fabrication of MEMS and micro-optical components, and for other applications with dimensions in the micrometer range. Traditionally, the light sources used for mask-aligners are high-pressure mercury arc lamps, which emit in the ultraviolet range with peaks at 365nm, 405nm and 435nm, the so-called g-, h- and i- lines. These lamps suffer from several disadvantages, such as a low efficiency, bulkiness, a short lifetime, and the toxicity of mercury. Finding an alternative to mercury arc lamps would be highly beneficial. In addition, specific techniques in mask-aligner technologies like Talbot lithography, multiple exposures, mask-source optimization or optical proximity correction lithography require high power sources with an increased control over the angular distribution and the spatial coherence. This is not easily done with a mercury arc lamp illumination. A method to easily measure the angular distribution and the spatial coherence in the mask-plane would be of great interest. In recent years, high power ultraviolet LEDs at the same wavelengths have appeared on the market. LEDs possess a smaller Etendue, they can be electronically driven at high frequencies and have a superior lifetime. This makes them ideal candidates to overcome the limitations of the mercury arc lamp illumination systems. The work focuses on the development and study of a novel LED-based illumination system for mask aligner lithography. This illumination system consists of an array of 7x7 LEDs, with individual reflectors. They form a modular 250W source which can replace a 1kW mercury arc lamp. The light is collected by the reflectors and brought onto a fly¿s eye integrator with two subsequent lenses which shape and homogenize the light field. Different patterns can be created by the source, determining the angles and the spatial coherence in the mask plane. The first part of the work presents the design and a complete set of characterizations of the final prototype. The achieved irradiance uniformity in the mask-plane of a MA/BA8 Gen3 SUSS mask-aligner is within ±1.2-2%. Prints tests in proximity printing, with a gap of 30µm, demonstrate a resolution of 3.5µm. In the second part of the thesis, the development of a method to measure the spatial coherence, which is based on a double slit approach and backed up by simulation, is described. The link between the spatial coherence length and the angular extent is made and the measurements show a good agreement with the analytical expression. A compact system is described, which enables the measurement of the spatial coherence in the mask plane, where only a little space is available. An original method is put into practice in order to measure the directional spatial coherence in two dimensions in the mask plane of a mask-aligner. This is achieved by using a circular double slits system. Prints are also made which illustrate the importance of the angular distribution and the spatial coherence. This novel LED-based illumination system is implementable in current, commercial, mask-aligners. It enables a greater efficiency, more functionalities and a longer lifetime compared to standard mercury arc lamp illumination systems. In particular, the spatial coherence properties of the source can be precisely managed and monitored thanks to the directional spatial coherence measurement.

EPFL2018

Polymer multilayer photonic micro- and nanostructures with tailored spectral properties

Amazing and beautiful optical effects are present in Nature. Some examples are the iridescent colors produced by peacocks, butterflies and beetles. While the simulations of some of these structures have already been realized, only a few elements have been fabricated. We are interested to understand and to reproduce some of these amazing optical properties. Applications such as security or decorative elements in different fields like jewelery or horology can be imagined. SEM pictures of the structure of these animals show a frequent combination of periodic or pseudo-periodic elements such as interference filters, multilayer reflectors, microlenses and gratings. The different elements combine angular and spectral effects. This combination enables strong optical effects for a large range of viewing angles and spectra. Different technologies such as standard photolithography, recording of interference patterns, transfer of photoresist structures in glass substrates, replications, hot embossing and spincoating were used to fabricate a variety of optical elements. The fabricated elements were produced with polymer materials because of their ease of use, and the possibility to build multilevel structures with replication methods. A drawback is the small refractive index difference possible between polymers and compared to air. In the first part, different combinations of polymer micro- and nano-optical elements such as corrugated gratings, multilayer Bragg reflectors, microlens arrays, micro-prisms arrays and diffusers were successfully fabricated. Poly (vinyl alcohol) and poly (N-vinycarbazole) with refractive indices of 1.56 and 1.72, respectively, at a wavelength of 500nm were used to fabricate the polymer multilayer reflectors. The micro-optical elements used were: microlenses with diameter between 32 μm and 250 μm and around 20 μm height; prisms with 50 μm period and 25 μm height. The angularly dependent reflectivity of the different fabricated elements was studied. Thanks to the close combination of diffractive, refractive and reflective micro- and nano-optical elements, non-standard artificial visual color effects were produced. The fabricated elements were modeled with different tools according to the dimensions of the different optical elements. Ray tracing analysis was used in the case with micro-optical elements while the Fourier modal method permitted simulation of the interaction of the light with the periodic nano-structures. The specific effects of the variation of the different parameters were highlighted, the basic principles and the limitation of the polymer technology were identified. In the second part, the optical modeling tools and the fabrication technologies developed were used to model and fabricate polymer light emitting diodes in a distributed feedback regime. The optical properties of the different layers were modeled and the physical dimensions of elements with active conductive polymers me-LPPP and F8 were calculated, fabricated and tested. The sensitive dimensions and parameters were underlined. Polymer materials permitted rapid fabrication of complex and innovative optical elements with very simple technologies like spin-coating and replication methods. These technologies allow the combination of nano-optical elements with dimensions of the order of a hundred nanometers with micro-optical elements with dimension of the order of about ten to one hundred microns.

Institut de Microtechnique, Université de Neuchâtel2008