CHARACTERIZATION OF OPTICAL PROPERTIES OF SU-8 AND FABRICATION OF OPTICAL COMPONENETS Om Prakash Parida 1 and Navakant Bhat 2 DOI, DRDL, Kanchanbagh, Hyderabad-58 1 , Dept. of ECE, IISc. Bangalore-560012 2 [email protected]1 , [email protected]2 Abstract: SU-8 is a negative tone, near UV resist. Due to its unique properties, it is widely used as a structural material in diverse fields of MEMS. Its excellent optical transparency beyond 400nm makes it a preferred material for the fabrication of large variety of optical components and systems. UV-Visible spectroscopy and Spectroscopic ellipsometry were carried out for characterizing the optical properties of SU-8. Process parameters were optimized for the fabrication of different types of optical components like photonic bandgap structures, optical waveguides, splitters, directional couplers and gratings using Laser and electron beam lithography. 1. INTRODUCTION SU-8 is a relatively new polymer material in the field of MEMS introduced by IBM in 1989. It is an epoxy based chemically amplified, negative tone, near UV photo-resist as well as a low cost X-ray and e-beam resist [1]-[3]. It is a highly functionalized molecule with 8-epoxy groups, polymerized by cationic photo-polymerization which is induced by Lewis acid during UV illumination. Subsequent heating during post-exposure baking (PEB) activates cross-linking and regenerates the acid catalyst which causes significant enhancement of the resist sensitivity and results in high mechanical strength and thermal stability of the structure. Chemical structure of SU-8 monomer and standard SU-8 film processing sequence are presented in figure-1. Figure-1: (a) Structure of SU-8 monomer and (b) basic SU-8 processing sequence Cross-linked SU-8 films are highly resistant to a large number of acids, bases and solvents. Due to its unique physical, chemical, mechanical, electromagnetic and bio-compatible properties, it has been explored as an important material in diverse fields of MEMS manufacturing not only as a resist material but also as the core structural material [4]. Because of its excellent optical transparency and favorable optical parameters, SU-8 has been preferred as a suitable material for the fabrication of optical waveguides, gratings, Bragg filters and reflectors, photonic bandgap structures, on-chip light sources, optical pressure sensors, opto-fluidic systems and MOEMS [5]-[8]. Optical properties and parameters specified in the datasheets are limited and obtained under specific processing conditions, hence cannot be straightway acceptable for a different processing environment. So extensive characteri- zations for optical properties of SU-8 under different processing conditions were carried out. ‘Ultraviolet- Visible spectroscopy’ and ‘Spectroscopic ellipsometry’ [9] - [10] of SU-8 films were carried out to evaluate the absorption characteristics and refractive index of SU-8 respectively. Process flow was optimized for the fabrication of optical components like 2-D photonic bandgap structures, optical waveguides and splitters, 4-port directional couplers and optical gratings. 2. OPTICAL CHARACTERIZATION 2.1 Absorption characteristics of SU-8 Absorption characteristics of SU-8 were studied by carrying out ‘Ultraviolet-Visible (UV-Vis.) Spectroscopy’ of SU-8 films using ‘Cintra 40 UV- Visible Spectrometer’. A new formulation was prepared by thoroughly mixing SU-8 2150 and SU-8 thinner (Microchem, USA) in an optimized ratio and spin coated on a 2.5cm X 2.5cm clean glass substrate to get 10μm SU-8 film. The sample was then soft baked (SB) on a flat leveled hot plate at 65 0 C for 3 minutes followed by 8 minutes at 95 0 C and then exposed to UV radiation for 45 seconds using ‘MJB-3 single-sided mask aligner’. After exposure, the sample was post-exposure-baked (PEB) on the hot plate at 95 0 C for 9 minutes. During UV-Vis. spectroscopy, the PEB SU-8 sample was exposed to radiations varying from λ=190nm to 900nm. The sample was then hard-baked (HB) on hot plate at 200 0 C for 30 minutes and again UV-Vis. spectroscopy was carried out. Absorption spectra of the PEB and HB samples are shown in figure-2. It can be observed from the absorption spectra that
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CHARACTERIZATION OF OPTICAL PROPERTIES OF SU-8 AND
FABRICATION OF OPTICAL COMPONENETS
Om Prakash Parida
1 and Navakant Bhat
2
DOI, DRDL, Kanchanbagh, Hyderabad-581, Dept. of ECE, IISc. Bangalore-560012