Charging Phenomena During Medium Current Ion Implantation of Carbonized Photo-Resist Surface Layers

Charging phenomenon caused by ion implantation into the photo-resist has been evaluated with use of a surface potential measurement tool to clarify the mechanism of burst-like discharge of the accumulated charge in medium current implantation machines. The molecular bond of the photo-resist is cleaved by the kinetic energy of the implanted impurity ions selectively at the portion of the smallest bond energy, and cross-linking develops from this point. Bond breaking and cross-linking is accompanied by out-gassing of hydrogen released by collisions with the dopant atom and recoil cascade atoms. Finally, a carbon-rich layer with low resistance is formed on the photo-resist surface. When arsenic ions are implanted, the degraded layer of low resistance including the carbonization is being formed even under low dose (1 × 10¹⁴/cm² or less) conditions. This is called the medium current charging phenomenon with a current of a few hundred μA. The charges are accumulated on the photo-resist, until the low resistance layer is formed on the surface of the photo-resist. When the induced voltage reaches a critical value, they rush into the open area through the layer. As a result, an explosive local melting of silicon occurs, with the critical value depending on the mask-pattern arrangement.