Interference lithography for 3D photonic band gap crystal layer by layer fabrication
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Interference lithography for 3D photonic band gap crystal layer by layer fabrication. A Feigel, Z. Kotler and B. Sfez Electro-Optics Division, NRC Soreq, 81800 Yavne, Israel, A. Arsh, M. Klebanov and V. Lyubin Physics Dept, Ben-Gurion University, 84105 Beer-Sheva, Israel, ABSTRACT We present fabrication of 3D photonic band gap woodpile crystals from photosensitive chalcogenide glass with the help of interference lithography and layer by layer construction. The alignment method is described, which is scalable to extremely small feature sizes required for photonic crystals in the visible region. INTRODUCTION Optical spectrum photonic band gap crystals require high refraction index materials and technology for 3D sub-micron structure fabrication. The most flexible approaches are based on semiconductor lithography processes1,2. Most of them require sophisticated equipment and are confined by the current state of the art feature size limit. Thus there is demand for scalable lithography technologies and new types of materials, offering easy treatment and high refractive index. Chalcogenide glasses are highly promising materials for photonic crystals3. Their refractive index varies from 2.5 to 3, together with transparency from 800nm to 12 micron. They are photosensitive, and can be used as positive or negative photoresists. In addition they are amorphous materials that can be deposited on almost any other material by vacuum vapor deposition at low temperature. EXPERIMENTAL DETAILS Maskless interference lithography can replace classical mask lithography in the case of photonic crystals due to their periodical structure. The required periodic pattern of illuminated and dark regions can be produced by interference of two or more coherent laser beams. The advantages of interference over mask lithography are higher resolution and deep focus. The latter allows the usage of interference lithography on curved surfaces. The woodpile photonic band gap crystal can be considered as a multi layer construction, where each different layer is a diffraction grating (see Figure 1). Hence each layer can be produced by the interference pattern of only two beams. For formation of each layer, the first
Figure 1: Woodpile Photonic Crystal
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step is vacuum vapor deposition of thin chalcogenide film. The grating is obtained by using layer as a negative photoresist, after illumination and selective etching. The next step is the planarization of this grating by spin coating of Shipley photoresist, with subsequent thinning to the width of the grating. Afterwards the subsequent layers are formed in the same way. The final step is washing out of photoresist using a stripper. All layers must be accurately aligned with each other, meaning their periods, directions and relative positions should be precisely fitted. Otherwise the desired optical properties will suffer. A technique for alignment between the two-beam interference pattern and the existing grating was developed4. During interference lithography the already produced layers of woodpile
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