Patent ID: 8666703

Claim:
A method for evaluating a diffracting structure, the method comprising: receiving measured spectral information for the diffracting structure; receiving a scatterometry model for the diffracting structure, the scatterometry model having a plurality of model parameters (N); computing a Jacobian matrix of the measured spectral information, the Jacobian matrix containing a column for each of the plurality of model parameters; computing a test matrix including a plurality of combinations of parameter columns derived from the Jacobian matrix; computing a relative precision metric for each of the parameter columns in the plurality of combinations, wherein the relative precision metric is a function of hardware precision for the parameter column and expected process variation for the parameter column; associating, for each of the plurality of combinations, a maximum of the computed relative precision metrics with the corresponding combination; identifying one of the plurality of combinations having an associated maximum relative precision metric which is less than a threshold value, and which includes the greatest number of parameter columns, wherein parameters in the identified combination are to be floated in a revised scatterometry model; running a regression on the measured spectral information with the revised scatterometry model with the parameters floated to generate simulated spectral information; and determining a physical characteristic of the diffracting structure based on the simulated spectral information.