Avoiding Cu hillocks during the plasma process

When Cu wafers are exposed to H₂/N₂ plasma, hillocks are formed on the Cu wafer surface by a plasma cleaner with a surface wave plasma source. Plasma cleaning is divided into the initial stage and the rising temperature stage. Under a supply of H₂/N₂ gas and with the plasma power turned on, the H radicals first restore native copper oxide to pure copper. The N radicals then compete with the H radicals, and diffuse to the Cu grain boundary, which is the initial stage. During the rising temperature stage, plasma energy is absorbed by the Cu surface, and the wafer temperature increases rapidly. Consequently, plasma-enhanced compressive stress leads to the formation of Cu hillocks. For a plasma with a higher N/H radical ratio, more N radicals diffusing to the Cu grain boundary results in more Cu-N compounds, which make up the main source of stress during H₂/N₂ plasma cleaning. Experimental results indicate that using a plasma cleaner with an inductively coupled plasma source can achieve a lower N/H radical ratio, thus avoiding the formation of Cu hillocks.