The thickness dependence of interface layers in FeCo-Si multilayers on sputtering voltage, argon pressure and substrate bias potential was examined. The multilayers were characterised by in situ kinetic ellipsometry, X-ray reflection, X-ray diffraction, polarized neutron reflection and atomic force microscopy. A minimum thickness for the sum of both interface layers per period between 23 A and 24 A was found for different combinations of production parameter values. For an argon pressure of 1.3 X 10(-3) mbar the minimum interface thickness was found at a sputtering voltage of 708 V, and for an argon pressure of 2.3 X 10(-3) mbar at 880 V. These values were determined for a floating substrate potential of approximately + 60 V relative to ground. Applying a substrate bias potential of -70 V the minimum interface thickness occurred for a sputtering voltage of 880 V at an argon pressure of 1.3 X 10(-3) mbar. This study confirmed the results of the former empirical optimisation. By revealing the existence of several sets of optimum parameters it opens up a further parameter to adjust other properties like stress. The interface layer on top of the FeCo layer is 16% thicker than the one on top of the Si layer due to the larger roughness of the FeCo layers. (C) 2003 Elsevier B.V. All rights reserved.