Two-dimensional plastic microlens arrays by deep lithography with protons: fabrication and characterization

We present a quantitative study of the fabrication process of two-dimensional plastic microlens arrays fabricated using deep lithography with protons. Our process involves the proton irradiation of a PMMA (poly(methyl methacrylate)) sample in regions with a circular footprint followed by a diffusion of MMA vapour into the bombarded zones to cause a lens-shaped volume expansion. In the first part of this paper we give a detailed description of our fabrication technique and of the calibration procedure that goes with it. We demonstrate the flexibility of our approach with the fabrication of different types of array: highly uniform microlens arrays and arrays of microlenses with varying sags (maximum height of the spherical lenscap) and pitches. All lenses under test feature diameters of 200 +/- 2 mum, root-mean-square (RMS) roughnesses on the top of the lenses of lambda/30@632 nm and lens sags ranging from 10 to 70 mum. We also present the optical performances and the aberrations of the microlenses, measured using a dedicated transmission Mach-Zehnder interferometer. The focal lengths of the lenses under study range from 166 to 1444 mum, corresponding to a range of sags between 9.77 and 69.73 mum and to focal numbers between 0.83 and 7.22. Typical values for the RMS and peak-to-valley aberrations of 0.209lambda and 1.057lambda respectively were observed. To conclude, we analyse and discuss the strengths and weaknesses of this fabrication method.