An Integrated Aerial Image Sensor for Lithography Diagnostics

The subject of this paper is a novel aerial image sensor (IAIS) suitable for integration within the surface of an autonomous test wafer. The IAIS could be used as a lithography processing monitor, affording a "wafer\´s eye view" of the process, and therefore facilitating advanced process control and diagnostics without integrating (and dedicating) the sensor to the processing equipment. The main IAIS challenge is to retrieve nm-scale aerial image detail, while utilizing micrometer scale photo detector pixels. To address this problem, we propose a design of an on-wafer aperture mask that, when combined with an appropriate periodic aerial image, will produce a low spatial frequency interference pattern. While we demonstrate a design example aimed at the 65nm technology node through TEMPEST simulation, this concept is easily scalable to several future technology. We also detail the IAIS modeling techniques based on the Abbe\´s formulation. The performance of the IAIS under different lithography settings can be predicted accordingly. Five metrology metrics are proposed to facilitate resonant analysis on the aerial image variation and the detector image correspondence. Best focus plane calibration is discussed in some detail finally using partial coherent image modeling techniques.