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The evolution of the ALTA (R) series of laser mask pattern generators has increased the relative contribution of intensity errors on critical-dimension (CD) control to those from placement errors. This paradigm shift has driven a change in rasterization strategy wherein aerial image sharpness is improved at the cost of a slight decrease in the averaging of column-to-column placement errors. Print performance evaluation using small-area CD test patterns show improvements in stripe-axis local CD uniformity (CDU) 3σ values of 15-25% using the new strategy, and systematic brush-error contributions were reduced by 50%. The increased importance of intensity errors, coupled with the improvement of ALTA system performance, has also made the mask-blank and process-induced errors a more significant part of the overall error budget. A simple model based on two components, a pattern-invariant footprint and one related to the exposure density ρ (x, y), is shown to describe adequately the errors induced by these sources. The first component is modeled by a fourth-order, two-dimensional polynomial, whereas the second is modeled as a convolution of ρ (x, y) with one or more Gaussian kernels. Implementation of this model on the ALTA 4700 system shows improvements in global CDU of 50%. |
