AIMS and resist simulation

Abstract

The AIMS™45-193i is the established tool for mask performance qualification and defect printing analysis in the mask shop under scanner conditions. Vector effects are taken into account by the proprietary Zeiss vector effect emulator. In several studies an excellent correlation to wafer prints has been reported. However, a systematic offset to wafer prints in terms of mask error enhancement factor (MEEF) and exposure latitude has been observed which is attributed to well known resist effects. The AIMS™ measures the aerial image in resist whereas in a real lithography process further image blur of the latent image is caused by photo acid diffusion during wafer processing and resist development. To explain the gap between the AIMS™ and wafer prints we have investigated aerial images in combination with an easy to use resist model. It does take resist effects into account with sufficient accuracy to explain printing behavior of photo masks but without the need to calibrate lots of parameters of the actually used resist which usually are not known to a mask shop. The resist effects predominantly reduce the image contrast and thus increase the MEEF and the sensitivity to mask defects. This somewhat counterintuitive behavior is labeled “contrast enhancement by contrast reduction”. Additionally application of the resist model improves the agreement of e.g. the exposure latitude or MEEF measured by the AIMS™ compared to wafer prints.