Elimination of Resist Poisoning in Via-First Dual Damascene Processes

Resist poisoning in via-first dual damascene was observed after trench lithography. The resist poisoning was more likely to occur when a low-k material was used. To examine the relationship between resist poisoning and the amount of basic contaminants, we analyzed basic molecules by capillary electrophoresis (CE), ion chromatography, and thermal desorption-atmospheric pressure ionization-mass spectrometry (TD-API-MS). It was found that three amines were adsorbed on a low-k wafer. The total amount of amines was related to the resist poisoning. We also conducted TD-API-MS experiments on wafers with a variety of via densities. These experiments well explained why isolated vias are more likely to suffer from resist poisoning. As a process approach toward the elimination of resist poisoning, we confirmed that quick heat treatment at a high temperature was effective in removing the amines from the low-k wafer. As a material approach, we tried to make resists with high resistance to resist poisoning. The addition of a quencher (organic base) in the resists effectively reduced the resist poisoning. The mechanism was verified by nonaqueous titrations of a model resist system. It was found from the model experiments that a buffer system composed of the photoacid and its conjugate base was working to keep the pH changes from external basic contaminants small.