Microlens array fabricated by a low-cost grayscale lithography maskless system

This work presents the fabrication of a contiguous f/# = f/15 Fresnel microlens array (MLA) by employing a low-cost home-built maskless exposure lithographic system based on a digital light projector technology by using Texas Instruments' digital micromirror device chip. A continuous diffractive phase relief structure was generated on a photoresist-coated silicon wafer, replicated in polydimethylsiloxane (PDMS) and electrostatically bonded to a glass substrate. The whole exposure time takes 10.8 min to expose a 2.4 x 2.4 mm MLA, with a resolution of 2.5 mu m. This exposure time is relatively short, enabling high throughput or fast prototyping. Optical characterization was carried out using a He-Ne laser source (lambda = 633 nm), by evaluating the maximum intensity of each spot generated at the MLA focal plane, Imax, as well as its sharpness by measuring their full width at half maximum (FWHM) intensity values. The resulting FWHM and maximum intensity spot average values were FWHMAVG = 20 +/- 8% mu m and Imax(AVG) = 0.71 +/- 7% a.u:, respectively. The quality of replication was evaluated by profile characterization of the resulting mold and replica based on step height measurement along 180 mu m. The maximum obtained difference was 32 nm, corresponding to 2.5% of the total mold height or lambda/20. AFM measurements were also carried out to quantify the roughness quality between mold and replica. The resulting RMS roughness was 4.73 nm (lambda/130) and 6.66 nm (lambda/95) for mold and replica, respectively. A comparison between theoretical and measured intensity profiles at the MLA focal plane was also carried out. A good correspondence between the results was found. Such an MLA can be applied as a Shack-Hartmann wavefront sensor in optical interconnects and to enhance the efficiency of detector arrays. (C) 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)