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Significant discrepancies between target and coating stoichiometry during sputtering are widely observed in literature, especially for compounds with a large difference in atomic mass and radius. None of the numerous explanations proposed so far, reaching from sole gas scatter processes to resputtering, is consistent with the reported experimental data. Thus, a detailed study will be presented here, where the variation in stoichiometry with gas pressure, target-substrate distance and geometry is determined via elastic recoil detection for depositions from TixB targets of a diameter of 51 mm sputtered in Ar. For deposition along the target normal a pronounced Ti deficiency of up to 30% is detected. Increasing the pressure or distance from 0.5 to 2 Pa and 5 to 10 cm, respectively, leads to an almost linear congruent increase in Ti/B ratio surpassing even the target stoichiometry, which is initially surprising given the smaller mean free path of the larger Ti. Depositions at lower angles (30 and 60°) on the other hand revealed a higher Ti/B ratio supporting the notion that the lighter B is preferentially emitted along the target normal. Therefore, the arrival rates of B and Ti on the substrate surface are not just determined by their respective mean free path and scattering angles but also to a large extent by their respective kinetic energy distribution as well as emission angles during sputtering. This experimentally identified sensitive interplay is consistent with results obtained from Monte-Carlo simulations combining the respective emission characteristics from the sputter process as well as the transport through the gas phase. The latter exhibits a transition from a directional flux to thermal diffusion of the film forming species within for sputtering typical pressures and distances. Therefore, the energy distribution functions obtained from mass-energy analysis were used for a further validation of the here presented model. |