Focused Ion Beam Fabrication of SU-8 Waveguide Structures on Oxidized Silicon
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Focused Ion Beam Fabrication of SU-8 Waveguide Structures on Oxidized Silicon Swagata Samanta1, Pallab Banerji2 and Pranabendu Ganguly1 1 Advanced Technology Development Centre, Indian Institute of Technology Kharagpur, West Bengal, Kharagpur 721302 , India. 2 Materials Science Centre, Indian Institute of Technology Kharagpur, West Bengal, Kharagpur 721302, India. ABSTRACT This work deals with SU-8 waveguides and waveguide structures fabricated on an oxidized silicon substrate using ‘Focused ion beam (FIB) lithography’. From our experimentation it seems that FIB method is practically not suitable for fabricating long SU-8 waveguide structures, rather it is more suitable for nanoscale modification of already fabricated waveguides, such as, to fabricate photonic crystal structures. INTRODUCTION Conventional UV lithography has been the mainstay for fabrication of micro-devices for decades. High resolution, which is required for more compact nano-devices, can be somewhat obtained with the expense of depth of focus, hence the device designs involve a tradeoff between short wavelength and large numerical apertures [1-3].The limitation of resolution due to diffraction results in searching alternative lithography techniques, which are being developed in research laboratories throughout the world. Moreover, dependency on masks complicates the fabrication as a new mask must be generated for each new design. Generating a new mask is neither cost nor time efficient. Different mask-less lithography techniques are developed whose key advantage is the ability to change lithography patterns from one run to the next, without incurring the cost of generating a new photomask. Some of the maskless techniques include electron beam lithography (EBL), focused ion beam (FIB) lithography, scanning probe lithography (SPL), direct laser writing (DLW) technique, and dip pen lithography (DPL) [4-8]. Optical waveguides and devices using SU-8 material are of today's interest due to their potential applications in optical communications, optical interconnect, and integrated optics. SU-8 exhibits a huge potential for micro-technologies as it can be spin coated at a thickness ranging from a millimeter to a few microns and it gives very high aspect ratios when patterned [9, 10]. SU-8 being a low cost material and optically transparent in the visible region as well as at telecommunication wavelengths of 1.3–1.6µm makes it a suitable material for optical waveguides. Both single- and multi-mode waveguides using SU-8 were fabricated before by many research groups with variations in cladding material (air, polymers) using different fabrication techniques (both mask and maskless approaches) [8, 11, 12]. In this work, we have used maskless focused ion beam lithography, where focused beam of ions (Ga+) hit the sample surface and mills the required portion of material to create air-cladded SU-8 waveguide structures. To the best of our knowledge, this is the first attempt to fabricate SU-8 polymer waveguide structures by using FIB technique. The limitations of
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