Characterization of critical dimension uniformity through in-situ detection of aerial images in a scanner

Abstract

We introduce a novel in-scanner aerial image sampling technique using a sensor wafer that can be loaded into a production scanner to acquire data at the wafer plane, and under exact production conditions in terms of all optical settings and parameters of the actual scanner and an actual reticle. We demonstrate the applicability of this system to CD uniformity characterization of a production scanner in combination with a test reticle. CD estimates can be directly obtained from the image data by applying a fixed threshold, by employing a calibrated resist model to the sensor data, or by using the sensor data to accurately calibrate a complete lithography model. The linear response of the sensor provides complete information on the imaging process, and CD data can be immediately correlated to other image parameters, such as contrast, ILS, peak signal values, etc. We demonstrate the ability of the system to characterize CD variations and through-pitch curves, and to generate CD uniformity maps across the exposure field. We have extensively studied the repeatability and reproducibility of the system, and show its ability to detect changes in imaging performance over time in a production environment, differences between exposure tools, or different mask manufacturing processes.

Characterization and applications of an in-scanner aerial image detection system

“…A more detailed investigation of the sensor wafer system for CD uniformity detection, including across field CD uniformity and prediction of actual resist CDs by addition of a calibrated resist model will be presented elsewhere. 6 Here, a fixed threshold is simply chosen to match approximately the smallest line width to the target, i.e. the drawn CD.…”