Constraints on removal of Si₃N₄ film with conformation-controlled poly(acrylic acid) in shallow-trench isolation chemical–mechanical planarization (STI CMP)

The effect of changes in poly(acrylic acid) (PAA) conformation on removal of Si₃N₄ film was investigated. PAA was used as a passivation agent by adsorption on an Si₃N₄ film in shallow-trench isolation chemical–mechanical planarization (STI CMP). Adsorption behavior of PAA on the Si₃N₄ film and the conformation transition were determined by adsorption isotherms and force measurements using atomic force microscopy (AFM) as a function of ionic strength. AFM results revealed that, as ionic strength increases, the repulsive force between the negatively charged carboxylate groups along the backbone of PAA is reduced due to counterion screening and to the changes of PAA conformation from a stretched to a coiled configuration. At high ionic strength, the coiled conformation of PAA formed a dense passivation layer on the Si₃N₄ film, which led to suppression of the removal rate of Si₃N₄ film from 72 to 61 Å/min in the STI CMP process.