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Activated He:H2 Strip of Photoresist over Porous Low-k Materials

Introduction As design rules go beyond 90nm the introduction of porous low-k materials in Cu/low-k integration has forced dramatic changes in photoresist stripping and residue removal in BEOL applications. The major challenge is to remove photoresist and composite residues without corroding the copper or changing the dielectric constant of the porous ILD materials. This requires that the dry strip process not only can effectively remove photoresist and organic BARC materials but also can preserve the physical, chemical, and electrical properties of the ILD materials. Further, the dry strip process needs to be compatible with subsequent wet cleans. Currently there are two approaches of dry strip in Cu\low-k integration applications. One approach is the popular low pressure, low temperature anisotropic strip, which use O2 or N2H2 plasma chemistry with physical and ion enhancement to achieve lateral selectivity. The problems associated with the directional O2 strip are sidewall carbon depletion due to backscattered ions/electrons, cap rounding and etch stop loss due to sputtering, and CD shift due to etch stop redeposition. The other approach is to use downstream plasma isotropic strip with reducing chemistries. The key to success for the isotropic strip is to correctly choose the gas chemistry and process conditions. At Axcelis we have developed an activated He:H2 strip process excited by a downstream microwave plasma that has shown excellent performance on multiple Cu\low-k applications [1-4]. The He:H2 strip process substantially minimizes the damages to the ILD materials and enables post-strip wet cleans.