<title>Multilevel diffractive optical element manufacture by excimer laser ablation and halftone masks</title>

Abstract

A novel method is presented to manufacture multilevel diffractive optical elements (DOEs) in polymer by single-step KrF excimer laser ablation using a halftone mask. The DOEs have a typical pixel dimension of 5 µm and are up to 512 by 512 pixels in size. The DOEs presented are Fresnel lenses and Fourier computer generated holograms, calculated by means of a conventional iterative Fourier transform algorithm. The halftone mask is built up as an array of 5 µm-square pixels, each containing a rectangular or L-shaped window on an opaque background. The mask is imaged onto the polymer with a 5x, 0.13 NA reduction lens. The pixels are not resolved by the lens, so they behave simply as attenuators, allowing spatial variation of the ablation rate via the window size. The advantages of halftone mask technology over other methods, such as pixel-by-pixel ablation and multi-mask overlay, are that it is very fast regardless of DOE size, and that no high-precision motion stages and alignment are required. The challenges are that the halftone mask is specific to the etch curve of the polymer used, that precise calibration of each grey-level is required, and that the halftone mask must be calculated specifically for the imaging lens used. This paper describes the design procedures for multilevel DOEs and halftone masks, the calibration of the various levels, and some preliminary DOE test results.

<title>Laser micromachining for manufacturing MEMS devices</title>

“…This grey scale can be used to control the excimer laser etch depth. As shown in Figure 8, such masks can be used to produce structures like diffractive optical elements (DOE';s) that have 2 1 / 2 D depth topography (16) .…”

Excimer laser micromachining with half-tone masks for the fabrication of 3-D microstructures

“…The above approach has been used successfully in the fabrication of laser micromachined multi-level diffractive optical elements in polycarbonate [8]. In this kind of application, where the maximum depth is of the order of 1 mm, the accuracy to which the desired surface can be produced depends mainly on the quality of the etch function data, and on control of the maximum fluence.…”

“…Half-tone masks have been used previously with conventional projection lithography to produce microlens arrays and other microstructures [6,7]. They have also been used with excimer lasers to fabricate multi-level diffractive optical elements [8], and in the formation of apodised gratings in optical fibres [9]. In this paper we investigate the use of half-tone ablation to define deep polymer microstructures with complex surface profiles.…”

Fabrication of micro-lenses using excimer laser ablation by means of laser-generated grey-tone-masks

“…Instead, excimer laser ablation, which has been well proven for prototyping of polymer micro-structures with good surface quality, was going to be applied [3,4,5]. Excimer laser ablation by means of grey-tone mask has already been demonstrated in the literature [6]. However, commercially fabricated grey-tone masks, e.g.…”