A novel thermal reflow method for the fabrication of microlenses with an ultrahigh focal number

In this paper, we demonstrate a novel thermal reflow method with an additional near ultraviolet (UV) flood exposure and upside-down reflow configuration for the fabrication of microlenses with an ultrahigh focal number. By using this method, microlenses with a focal number (F-#) as high as 9.7 have been successfully obtained, which is about four fold higher than that can be fabricated with a conventional reflow method. The final profile of the microlenses can be flexibly and accurately tuned by controlling the flood exposure dosage and adopting the appropriate reflow configuration, which enables fabrication not only of spherical microlenses but also of more complex aspheric lenses. The fabricated microlens is characterized by measuring the point spread function (PSF) and the measurement result indicates that the diffraction limited optical performance of the microlens can be achieved. The method developed in this work can be used for the mass and cost-effective fabrication of high performance microlenses with ultrahigh focal numbers, which can find applications such as in accurate optical testing, integration imaging, and laser beam collimating.