Effect of process parameters on the growth of AlN coatings on Al-based alloy

Semiconductor fabrication equipment extensively uses Al alloys which form an AlF layer when exposed to fluorine gas used in semiconductor processing. The AlF layer can flake off, rendering the chamber components unfit for semiconductor manufacturing. With the goal of resisting fluorine attack, the growth of protective AlN coatings on Al-6061 substrates was investigated, and this paper reports on the effects of process parameters on coating quality. It was found that Mg powders in a powder bed placed before the sample along the gas flow path can supply a rapid burst of magnesium vapor to the sample during exothermal nitridation (combustion) of Mg powders. This burst of supersaturated magnesium vapor can convert the native protective Al₂O₃ to non-protective MgO on the sample surface if the extent of magnesium supersaturation, the temperature of the sample, and the residence time of the vapor around the sample are high enough. At the same time, the magnesium supersaturation should not be so high as to get significant gas phase nucleation of Mg₃N₂ particulates that can stick to the front edge of the sample causing a ‘front edge anomaly’. This balance is achieved by using a bimodal distribution of magnesium powders. Conversion of Al₂O₃ to MgO is accompanied by the formation of a Mg₃N₂ layer above the MgO layer, with incomplete surface coverage. Microstructural analysis suggests that AlN nucleation is preferred on this Mg₃N₂ layer, with uncovered areas being regions of outward Al diffusion from the alloy. The coating grows outward with the AlN dendrites growing outwards and laterally leading to a dense coating with a dendritic network of AlN in an Al matrix. Even a small concentration of oxygen or water vapor in the reaction chamber leads to excessive MgO formation on the AlN coating surface, particularly during the sample cooldown. Excessive MgO formation on the coating surface, termed as ‘MgO poisoning’, inhibits further coating growth. The residual Mg and Mg₃N₂ in the powder bed getter the oxygen and moisture, respectively, thereby keeping the oxygen content sufficiently low to avoid MgO poisoning provided the chamber has good hermetic integrity.