An ultra high vacuum chamber was constructed to study thin film growth using scanning tunneling microscopy (STM). The sample stage is specially designed so that samples can be transferred in situ among various film preparation techniques. For first studies, Ru thin films were deposited at sub-monolayer coverages on C(0001) substrates. From our STM studies, it is found that Ru diffuses easily along the graphite surface, that Ru atoms tend to nucleate at defect sites, and that the first monolayer of Ru grows homogeneously on graphite. These results are in agreement with our recent Auger electron spectroscopy data.