We report on the advanced optical characterizations of microfabricated solid immersion lenses with 2-mu m diameter, operating at lambda = 642 nm. The main feature, the spot size reduction, has been investigated by applying a focused Gaussian beam of NA = 0.9. Particular illuminating beams, e. g., Bessel-Gauss beams of the zeroth and the first order, a doughnut-shape beam and its decompositions, i.e. two-half-lobes beams, have also been used to influence the shape of the immersed focal spot. Detailed optical characterizations have been conducted by measuring the amplitude and phase distributions with a high-resolution interference microscope (HRIM) in volume around the focal spot. The immersion effect of the SiO2 solid immersion lens leads to a spot-size reduction of approximately 1.5 which agrees well with theory. Particularly shaped incident beams exhibit a comparable size reduction of the immersed spots. Such structured focal spots are of significant interest in optical trapping, lithography, and optical data storage systems.