Thin diamond films have been grown by the microwave chemical vapor deposition method on polished silicon substrates using a methane concentration of 1.5% in hydrogen and deposition times between 7 and 60 min. The films were studied in air by scanning tunneling microscopy (STM). For short deposition times we have found small isolated diamond crystals (100 nm wide and 20 nm high), growing on the scratches produced by the initial polishing of the silicon surface, along with a smooth granular structure. As the deposition time increases to 15 min, the granular structure shows a slight faceting whereas the crystal size increases appreciably. This deposition period is characterized by a lower growth rate than that observed for longer times, which is explained as due to the presumably high nondiamond component of the granular structure. At 30 min a continuous film is formed with predominant {111} faces. The same trend is found for samples grown after 60 min. STM images show that {111} surfaces are rougher than {100} ones supporting the 2 X 1 reconstruction of the {100} surface during diamond growth.