193nm CD shrinkage under SEM: modeling the mechanism

As photolithography platforms move from 248nm to 193nm resist systems, the industry's established dimension measurement technique (CD-SEM) causes significant shrink-age of the resist structures during measurement. Many studies have been done to characterize this effect and look for the factors that influence / reduce this shrink-age. While numerous anecdotal mechanisms have been proposed to explain the shrink-age, few theoretical / empirical equations have been proposed to connect the observed effects to fundamental mechanisms. Models are proposed relating physical properties (accelerating voltage, photoresist density, resist e-beam film shrink-age) to the commonly observed CD 'hammer test' shrinkage profiles. The validity of the model assumptions is tested via Monte Carlo simulations, FITR, e-beam curing, SPM and ellipsometry. These models explain the shape of the CD response to repeated measurements (exponential decay curve) and the magnitude of the shrinkage. These models also offer insight into why lower accelerating voltages cause reduced CD shrinkage, although the models predict that accelerating voltage should be a much more dominant parameter for CD shrink-age than literature has shown to date. Mass loss and density changes were also characterized during e-beam cure to check the validity of the model assumptions.