MSE PhD Final Defense of Rohan Soni

TITLE: Surface nitridation of aluminum alloys for protective nitride coating

ADVISOR: Soumendra Basu ME, MSE

COMMITTEE: Vinod Sarin ME, MSE; Srikanth Gopalan ME, MSE; Daniel Cole ME; Dr. Pratima Rao

CHAIR: Joerg Werner ME, MSE

ABSTRACT: Fluorine plasma etching is a common process in semiconductor wafer processing. The plasma operations degrade the chamber components that are primarily made of aluminum alloys such as Al-6061. A technology to prolong life of chamber components was sought with the goal of resisting fluorine diffusion. AlN is a well-known diffusion barrier to fluorine in the semiconductor industry. However, there is no available process that can impart protective AlN layer over large aluminum parts with complex geometries. Moreover, existing processes to deposit AlN on samples with simple geometries require stringent conditions such as high vacuum, toxic reactants, high temperatures or mechanical processing that disturb the component geometry.

A process of carrying out surface nitridation of Al-6061 alloy substrates was developed, that allowed for the growth of a dense aluminum nitride coating on samples having complex geometries. The method uses nitrogen gas alone as the nitrogen carrier, and not ammonia derivatives used in conventional processes. Mg powders in a powder bed placed before the sample along the gas flow path supplied a rapid burst of magnesium vapor to the sample during exothermal nitridation (combustion) of Mg powders. This burst of supersaturated magnesium vapor converts 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. The process led to uniform coating coverage over the flat sample surfaces as well as over the surfaces of drilled holes in the sample. The coatings were stable when subjected to several 400°C thermal cycles. The advantages of this process are that the process reactants are inexpensive, non-toxic, and is carried out at atmospheric pressure.

The AlN coatings were examined using XRD, SEM, EDS, and TEM. The AlN coating were found to be dense, well adhered, and a two-phase mixture of dendritic AlN interspaced with Al grains, making the coating electrically conductive. Uncoated and AlN coated Al-6061 alloy coupons were exposed to a fluorine plasma to assess the effectiveness of the AlN coatings. The results showed that the AlN coated Al-6061 samples resisted fluorine attack significantly better than the uncoated samples. Although fluorine did react with a top layer of the coating forming AlF3, the remainder of the coating and the substrate were free of fluorine attack, validating the effectiveness of the AlN coatings.