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We present a fabrication method for silicon nitride solid immersion lenses (SILs) integrated with atomic force microscope (AFM) cantilevers. We demonstrate a scanning optical microscope based on the microfabricated SIL that operates in reflection and transmission modes at a wavelength of λ = 400 nm. In this microscope, light is focused to a spot in a high refractive index SIL held close to the sample. The minimum spot size of a SIL-based microscope, which determines the transverse optical resolution, is λ/(2n) where n is the refractive index of the SIL. This is smaller than the minimum spot size of λ/2 in air. The SIL, therefore, makes possible optical resolution better than the diffraction limit in air. The full-width at half-maximum (FWHM) spot size of the SIL-based microscope is measured to be ∼133 nm in transmission mode, which is ∼1.98 times better than the spot size measured without the SIL (264 nm). This improvement factor is close to the refractive index of the silicon nitride SIL (n = 1.96).