Disclosed are semiconductor packages and their fabrication methods. The semiconductor package includes a redistribution substrate that includes an organic dielectric layer and a metal pattern in the organic dielectric layer, and a semiconductor chip on the redistribution substrate. The organic dielectric layer has a maximum absorbance equal to or greater than about 0.04 at a first wavelength range, and a fluorescence intensity equal to or greater than about 4×103 at the first wavelength range. The first wavelength range is about 450 nm to about 650 nm.