We have investigated by scanning force microscopy the growth of surface relief structures on thin diblock copolymer films for a infinity x infinity geometry (homogeneous substrate) and a L x infinity geometry (heterogeneous substrate). We characterize the growth by a length scale obtained from the correlation function describing the fluctuations. Two-dimensional viscous flow within a continuous layer dominates the process. On the homogeneous substrate we find a power law growth with an exponent of alpha = 2/3, while the growth on the stripe pattern in a direction parallel to the stripes is characterized by an exponent that is much lower.