The vapour etching of silicon sacrificial layers is often a critical process in the fabrication of micro/nanosystems. This method has a number of attractive features, in particular, high etch rates of sacrificial silicon layers and good selectivities associated with photoresist, SiO 2 , stoichiometric Si 3 N 4 and a number of regularly used metal films. However, materials that are commonly inert to XeF 2 are etched when located in the proximity of a silicon sacrificial layer. This proximity is a common situation in the fabrication of such systems and can become a critical issue affecting process control and device reliability. This work uses test structures that have been designed to be very sensitive, thereby delivering much lower selectivities then are typically reported in the literature. This sensitive quantification of the proximity effect is used to evaluate methods designed to improve the selectivity. This work suggests that a reduction in the processing temperature from 25°C to 10°C increases the Si: PECVD SiN selectivity by 68%. However, a more easily implemented modification is to flow hydrogen into the reaction chamber. This method improves the Si: PECVD SiN selectivity by an order of magnitude and the Si: LPCVD SiN selectivity between 200% and 600%.