  We fabricated superconducting MgB2 thin films on (001) MgO substrates. The
samples were prepared by magnetron rf and dc co-sputtering on heated
substrates. They were annealed ex-situ for one hour at temperatures between
450{\deg}C and 750{\deg}C. We will show that the substrate temperature during
the sputtering process and the post annealing temperatures play a crucial role
in forming MgB2 superconducting thin films. We achieved a critical onset
temperature of 27.1K for a film thickness of 30nm. The crystal structures were
measured by x-ray diffraction.
