Abstract

The minimum spatial resolution of optical systems in the diffraction limit is approximately the free space wavelength divided by twice the numerical aperture (NA) of the system. NA is defined as the product of the index of refraction at the focal point and the sine of the maximum convergence angle of the light. Resolution below the diffraction limit in air can be achieved with a solid immersion lens (SIL) by scanning a sample within the near field of a spot formed in a high refractive-index lens material in the manner of Mansfield and Kino (1990). This paper presents a technique for microfabricating high-NA SILs in silicon with diameters on the order of 10 μm. Silicon has a higher index than previously demonstrated SILs, and it transmits well in the mid-infrared and near-infrared wavelength ranges, making it an ideal choice for high-resolution thermometry and spectroscopy. However, traditional methods for manufacturing SILs are time consuming, labor intensive, and expensive and cannot typically be used to make lenses smaller than 1 mm in diameter. We review current microlens fabrication techniques and describe the fabrication process developed for this work. We include a method for lens formation using acetone vapor to reflow photoresist pillars that can be used to make aspherical as well as spherical lenses. Microlenses etched in single-crystal silicon with diameters on the order of 10 μm and NAs as high as 3.0 are shown. Wafer-scale fabrication offers the opportunity to integrate microlenses onto MEMs structures such as scanning probes for optical imaging, lithography, spectroscopy, and thermometry with high optical efficiency and spatial resolution.

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