Low-impact resist metrology: the use of ultralow voltage for high-accuracy performance

Previous results have demonstrated that the most significant line slimming occurs during the initial measurement and is a strong function of landing energy. Since it is difficult to accurately estimate the initial CD value (M0), many test protocols rely on the measurement change between the first and second measurement pass (M1-M2), to evaluate line slimming. However, since the slimming behavior of ArF resist systems has been shown to be exponential and dependant upon the resist formulation, using M1-M2 as the metric for comparing between CD SEM suppliers can severely underestimate the impact of a particular system setup on line slimming. The experiments reported here represent an attempt to assess the impact of the initial measurement (M0-M1) on line slimming. A series of experiments were designed to assess the impact of landing energy on line slimming for ArF photoresist. To validate the results of the experiments, an etched poly wafer was used as a control sample to ensure that metrology differences noted on the ArF resist between a high voltage 800 eV and an Ultra-Low Voltage (ULV) 100 eV condition arose purely from the interaction of the E-beam with the resist. The most significant line slimming was observed to occur during the first measurement at 800 eV, with greater than 10 nm of slimming observed on a nominal 120nm lines), followed by relatively stable slimming performance thereafter. The 100 eV condition demonstrated a significantly reduced level of slimming as a result of the first measurement; if there was any slimming, it could not be distinguished from the uncertainty in the estimate of the initial CD (M0). Measurements were also performed dynamically and at the ULV 100 eV condition slimming was indistinguishable from contamination induced linewidth growth, leading to an initial value closely matching the unperturbed linewidth (M0). The superior ArF line slimming performance at ULV is consistent with numerous published results, and demonstrates the need to assess slimming by a meaningful metric through a comparison of the initial measurement (M1) at high and low voltages, or by a comparison of the initial measurement M1 with the unperturbed linewidth (M0). The results of the experiments conducted point to a need for Low Impact Resist Measurement performance, which is fulfilled by Ultra-Low Voltage Metrology.