A deposition apparatus (100, 101) for coating a substrate (10, 10b) is described. The deposition apparatus includes a first spool chamber (110) housing a storage spool (112) for providing the flexible substrate (10), a deposition chamber (120) arranged downstream from the first spool chamber (110), and a second spool chamber (150) arranged downstream from the deposition chamber (120) and housing a wind-up spool (152) for winding the flexible substrate (10) thereon after deposition. The deposition chamber (120) includes a coating drum (122) for guiding the flexible substrate past a plurality of deposition units (121) including at least one deposition unit (124) having a graphite target (125). Further, the deposition apparatus (100) includes a treatment device (160) configured to treat the flexible substrate, upstream or downstream from the at least one deposition unit. The treatment device (160a, 160b, 160c) comprises a linear ion source (161), which comprises a power source (176) electrically connected to an extraction electrode (168) and to ground potential, wherein the power source (176) is adapted for operation at a frequency in the range from about 1 kHz to about 500 kHz.