<title>Fabrication of microlens arrays and gratings with a Novolak-type polymer</title>

Abstract

Novolak type polymers are the basic material for most commercial photoresists used in microelectronic processes, but are not often used for micro-optic applications. In this work, three types of optical devices were implemented in AR P322 novolak-based resist, which can be used as a positive photoresist and a positive electron resist. Gratings of parabolic divergent microlenses with f-number of 0.5 were fabricated using traditional optical lithography, employing the diffraction characteristics of de-focused light during the photolithographic exposure. The contrast curve of the AR P322 used an electron sensitive resist, was determined and yielded a gamma factor of 3.02. This relatively low contrast allows to obtain structures with well controlled curved walls. Direct write electron beam lithography was employed to manufacture gratings of parabolic convergent microlenses with different diameter and focal length, what enables one to control the intensity of the different orders of the diffracted light. This technique was also used to obtain convergent parabolic microlenses, with different diameters and different heights, allowing the control of the focal length of these lenses. These structures have several applications in the fields of pattern recognition, robotic vision and optical sensors.