An apparatus for processing a flexible substrate, comprising: a vacuum chamber having a first chamber portion, a second chamber portion and a third chamber portion; an unwinding shaft for supporting the flexible substrate to be processed and a winding shaft supporting the flexible substrate having the thin film deposited thereon, wherein the unwinding shaft and the winding shaft are arranged in the first chamber portion; at least one gap sluice for separating the first chamber portion from the second chamber portion, wherein the gap sluice is configured such that the flexible substrate can move there through and the gap sluice can be opened and closed for providing a vacuum seal; a coating drum having a rotation axis and a curved outer surface for guiding the substrate along the curved outer surface through a first vacuum processing region and at least one second vacuum processing region, wherein a first portion of the coating drum is provided in the second chamber portion and the remaining portion of the coating drum is provided in the third chamber portion; a first processing station corresponding to the first processing region and at least one second processing station corresponding to the at least one second vacuum processing region, wherein the first processing station and the second processing station each comprises: a flange portion for providing a vacuum connection; wherein the third chamber portion has a convex shaped chamber wall portion, wherein the third chamber portion has at least two openings provided therein, particularly wherein the at least two openings are essentially parallel to the convex shaped chamber wall portion; and wherein the first processing station and the at least one second processing station are configured to be received in the at least two openings, wherein the flange portions of the first processing station and the second processing station provide a vacuum tight connection with the third chamber portion. a vacuum chamber having a first chamber portion, a second chamber portion and a third chamber portion; an unwinding shaft for supporting the flexible substrate to be processed and a winding shaft supporting the flexible substrate having the thin film deposited thereon, wherein the unwinding shaft and the winding shaft are arranged in the first chamber portion; at least one gap sluice for separating the first chamber portion from the second chamber portion, wherein the gap sluice is configured such that the flexible substrate can move there through and the gap sluice can be opened and closed for providing a vacuum seal; a coating drum having a rotation axis and a curved outer surface for guiding the substrate along the curved outer surface through a first vacuum processing region and at least one second vacuum processing region, wherein a first portion of the coating drum is provided in the second chamber portion and the remaining portion of the coating drum is provided in the third chamber portion; a first processing station corresponding to the first processing region and at least one second processing station corresponding to the at least one second vacuum processing region, wherein the first processing station and the second processing station each comprises: a flange portion for providing a vacuum connection; wherein the third chamber portion has a convex shaped chamber wall portion, wherein the third chamber portion has at least two openings provided therein, particularly wherein the at least two openings are essentially parallel to the convex shaped chamber wall portion; and wherein the first processing station and the at least one second processing station are configured to be received in the at least two openings, wherein the flange portions of the first processing station and the second processing station provide a vacuum tight connection with the third chamber portion.