Microlens fabrication in PMMA with scanning excimer laser ablation techniques

Laser ablation is a versatile technique for fabricating microstructures on polymer surfaces. Due to the nature of the process, the fabrication of the microstructure can take place in a very late stage of a heterogeneous assembly. This makes laser ablation very attractive for fabricating micro-optical components on opto-electronic assemblies in comparison to other fabrication techniques like injection molding and embossing. In this paper we will report on the first experimental results of microlens fabrication with excimer laser ablation techniques. By scanning the polymer surface along multiple circular paths with a circular beam of a pulsed excimer laser, one is able to obtain a lens shape with arbitrary focal distance and diameter. Important issues as choice of ablation parameters, selection of scanning path and performance of the resulting laser ablated lens will be discussed. Introduction. An increasing number of opto-electronic applications makes use of 1D or 2D arrays of optical beams. Often these beams need to be collimated or focussed by microlens arrays. Fabrication technologies for these micro-optical components are embossing and injection molding (both mass-production oriented), lithography, deep proton lithography, micro-jet printing, LIGA, laser ablation, ... In literature several methods are described to realize microlenses with excimer laser ablation. In general, they are based on the use of complex mask patterns for fabrication of Fresnel or refractive microlenses and CGHs (Computer Generated Holograms) by direct ablation, or involve the use of selected polymer materials in a process based on ablation or irradiation and diffusion of doping agents in order to achieve a curved surface. The approach we propose here is based on direct ablation and requires little post-processing. The whole fabrication process is an essentially non-contact method and can be performed on a multitude of polymer materials or layers. It can be applied in a final step of the fabrication of an opto-electronic heterogeneous assembly. Microlens fabrication approach. The lens fabrication method we propose in this paper, uses a simple circular aperture for the excimer beam and a high precision translation stage carrying the PMMA substrate, which is able to make circular movements. The pulse energy and the pulse frequency (number of pulses per second, f) remain constant during the process. While the excimer laser is firing pulses, the translation stage makes subsequent circular movements with different radii and speeds. The relationship between ablation parameters and eventual lens profile is explained hereafter. • Profile of the ablated trench. In the ablation set-up we use a projection lens to image the circular aperture on the mask onto the PMMA substrate with a certain demagnification. We assume that the intensity of the laser light is constant on the sample within the illuminated area (diameter ρ). A circular movement of the table will result in the ablation of a closed trench. If R is the radius of the circle, v the speed of the table movement and d the depth of ablation per pulse, the resulting ablated profile can be described as v d f R R s R s R s R s R s R s R s R a s depth ⋅ ⋅ ⋅ ⋅ ⋅ − Φ ⋅ − − Φ ⋅ + − Φ + + − Φ ⋅ + − Φ ⋅ − + Φ ⋅ ⋅ ⋅ − − − ⋅ =