This thesis propose a method for minimizing the focused spot size of an elliptically-diverging laser diode beam by means of a circular aperture and single plano-convex aspherical microlens. The proposed microlens is fabricated using an excimer laser dragging method and has two different profiles in the x- and y-axis directions. The focused spot size of the beam is examined both numerically and experimentally. The results suggest that with an appropriate experimental setup, the focused spot size approaches the optical diffraction limit. The feasibility of proposed approach for beam pen lithography application is demonstrated by dotted, straight lines and spiral patterns.
In this thesis, the focused spot size of laser diode beam with a single aspheric microlens fabricated by excimer laser is examined by both simulation and experiment. Firstly, a model has been demonstrated for laser diode characteristics and beam propagation and then optimized a lens profile to achieve diffraction limited spot size with raytracing software Zemax. A polynomial surface function is used for lens profiles which have three main advantages, (1) it enables the design of different lens profiles in the x- and y-axis directions; (2) it makes possible the realization of complex surface patterns and (3) it is compatible with the excimer laser dragging method. Based on simulated result, the coefficients of polynomial profile function for plano-convex microlens are found. The excimer laser dragging method is then used to fabricate the aspheric microlens. By this method desired surface profile of the microlens is achieved with better surface roughness. The microlens is then easily assembled with laser diode and circular aperture by simple adhesive without any holder or collimating tube which will reduce assembly cost. Finally using this assembly laser diode beam is focused near optical diffraction limit.
The proposed method of beam focusing is used in beam pen lithography to write high resolution patterns on the substrate. To investigate the patterning sizes, the substrate is first coated with PR, then deposit a thin gold layer on it and finally PR is removed by lift-off process. The three types of patterns including dotted, straight lines and spiral are demonstrated. The smallest metal-dot dimension is 2.57 μm and 2.16 μm in the x- and y-axis directions, whereas the smallest line-width for straight line and spiral are 3.05 μm and 2.16 μm, respectively. Thus the method can be used for write any kind of patterns like dotted, line, spiral, characters and so on. Further reduction in spot size can be done by precise stage movement and minimizing fabrication error.

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