A new calibration method for latent image fidelity

Abstract

With the shrinking of all fundamental features in modern IC manufacturing, the aerial image calculation is becoming insufficient for accurate simulation of the printable wafer. In this presentation, a reliable simulation methodology based on basic principles is being introduced which includes an exact analytical solution of the Maxwell equations inside the photoresist as well as all other relevant layers. This solution contains multi-layers including bottom anti-reflection coating (BARC), silicon dioxide, nitride layers, as well as immersion in the medium between the stepper and the photoresist. The calibration is performed in order of parameter importance. This calibration gives higher weight to the most critical parameters. The latent image in the resist is computed, the resulting acid concentration is derived from the latent image, and the PEB (post exposure baking) is completed by invoking a reaction-diffusion system. The diffusion equation component orthogonal to the resist surface is solved exactly in closed form due to the small dimension of the resist thickness. The development is performed in a similar way. The latent image is compared to SEM images and the simulation parameters are calibrated through a newly developed optimization scheme to produce very accurate simulation fidelity. The methodology given here has been very successfully applied in detection of printing failures (hot-spots) for state of the art compact designs. An accuracy smaller than 1nm has been obtained. The system is very fast, suitable for entire chip analysis and highly parallelized. This calibration can be performed on a dual core laptop. Several practical examples are given.