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We present results on the impact of iron surface contamination on the firing stability of the bulk lifetime of silicon samples passivated by Al2O3/SiNx stacks. The effect of blistering, hydrogen passivation, and formation and transformation of the interfacial silicon oxide layer, respectively, has been widely discussed. This paper focuses on an up-to-now largely ignored effect that may dominate the observed degradation in frequent cases: bulk degradation by in-diffusion of impurities from the interface between silicon and atomic layer deposition layer during firing. In order to enhance the apparent passivation quality of thin Al2O3 layers, an appropriate predeposition cleaning plays an important role. By experimental data and theoretical calculation, we could prove that the short firing step is sufficient to promote the diffusion of iron from the surface into the bulk. Even low iron surface contamination levels of less than 1.4 × 1011 at/cm2 can have a detrimental impact on bulk recombination. The detected enhancement of the global Shockley-Read-Hall recombination was attributed essentially to interstitial iron contamination of the bulk. This indicates that a close control of iron surface contamination is a key factor to guarantee for firing stable Al2O3/SiNx stack systems.