Process variations of thin films and antireflective coatings

Abstract

The common baseline for lithography process control is the assumption of the presence of a uniform substrate. This assumption may lead one to disregard variations introduced by the substrate thickness and optical properties. With the reduction of lithographic feature size, the impact of the substrate optical parameters, neglected until now, increases and reaches a level that has a tangible contribution to the CD budget. Non-uniformity of substrate and/or ARC create CD variations. A first step should be the measurement of the actual reflectivity variations, then compare the process reflectivity variations relative to the exposure latitude. The reason is the impact of the reflectivity of the substrate/photoresist interface on the amount of absorbed energy inside the photoresist itself. In this paper we present measurement data from the NovaScan 420 Integrated Thickness Monitoring (ITM) system that has been gathered during CMP process steps of several 0.18 and 0.25 technologies. Data was extracted from a large database collected worldwide. We analyzed the actual thickness scatter of the top polished oxide layer as well as measurements of the thickness variations of layers below the top oxide (e.g. TiN, Oxynitrides). These data were used for the analysis of the substrate/photoresist interface reflectivity variations at lithography steps. The analysis enabled simulation of 'across the wafer' reflectivity profiles at the DUV wavelengths. It appeared that some applications (e.g. photoresists on dielectrics) presented 20% 'across the wafer' variations in substrate reflectivity while others (e.g. photoresist on metal) did not show any variations. 'Wafer-to-wafer' trends reveal that the average values of the wafer indicate small changes, while site-to-site differences are consistent and non-negligible. Usually the tests that investigate the CD sensitivity to ARC or other substrate layers reflectivity do not reveal the expected correlation since the substrate optical reflectivity is not the dominant factor. PEB temperature, develop uniformity, dose control, focus control and metrology noise may have similar contributions that hide the CD correlation to reflectivity. Estimations for the actually achieved improvements for each application will be presented, together with the data collection requirements necessary to show the reduction of CD variations (such as: large sampling, integrated tool, angstrom level repeatability). Using the measured ITM system data, we suggest to use 'wafer-to-wafer' trends and wafer reflectivity maps to create compensation for the reflectivity variations by adjusting the coating thickness or the exposure dose respectively. In some cases the 'across the wafer' and 'wafer to wafer' data will illustrate the need for Integrated Metrology. This will enable the monitoring of the ARC thickness, refractive index, optical extinction coefficient, and also other layers that have an important contribution to the reflectivity.