Theoretical modelling and experimental verification of the influence of Cr edge profiles on microscopic-optical edge signals for COG masks

Abstract

Different types of dimensional metrology instrumentation is in use today for production control of photomasks, namely SEM, AFM as well as optical microscopy and optical scatterometry. High resolution optical microscopy is still important as a reference metrology system, especially because it is sensitive to the optical effects induced e. g. by 2D or 3D details of features on photomasks. Particularly with regard to accurate optical CD measurements a thorough modelling of the optical imaging process on the basis of rigorous diffraction calculation is essential, which accounts for both polarisation effects and the 2D or 3D geometry of the structures. At PTB we use two different rigorous diffraction models to calculate the intensity distribution in the image plane, i.e. the rigorous coupled wave analysis method and the finite elements method. The question arises how accurate the influence of edge details on the microscopic-optical edge signals can be modelled. To answer this question we performed systematic experimental studies on COG test structures with varying height, edge angles and edge profiles. These geometric profile parameters of the test structures have been characterised by AFM measurements. Additionally top CD's of the features have been measured using both a CD-SEM and a metrological AFM. We present UV-optical CD measurements of these test structures and analysed them taking into account the measured profile details and, for comparison, using a simple binary structure model. The CD values determined are compared with the corresponding AFM and SEM values. The good agreement obtained for the optical, AFM and SEM top CD values shows that the optical effects of edge profile details can be modelled correctly with the two models applied. The results again demonstrate the necessity of rigorous model based analysis of the optical measurements, taking into account the edge profile details.