Mechanism of porous low-k film damage induced by plasma etching radicals

The etching damage on nano-clustering silica (NCS) film due to etching radicals was investigated using a method of radical treatments in RIE plasma. NCS coated-side of the wafer was turned downward and put at 0.65 mm above the wafer stage to investigate only the influence of radicals. Etching radicals, which comes from CF4, diffuse into NCS film and reduce Si–CH3 bonds and Si–CH3 loss is proportional to the amount of diffused fluorine in NCS film. Several Si–CH3 bonds are converted to Si–F bonds then. As a result, the low-k performance is degraded and especially the leakage current heavily increases. We proposed a method for estimating the degree of the sidewall damage due to etching radicals using blanket wafers. The degree of sidewall damage is proportional to the value of CR−0.5, where C is the damage diffusion coefficient, which is derived from Si–CH3 decrement ratio from a radical treatment result and R is the etching rate, which is derived from a RIE treatment result under the same plasma condition. The value of CR−0.5 depends on the etching condition and must be decreased as much as possible in order to reduce the sidewall damage during RIE. For example, lower gas pressure, higher RF power, and higher CF4/Ar gas flow ratio were desirable for the sidewall damage reduction.