Electrical CD characterisation of binary and alternating aperture phase shifting masks

Many of the recent advances in optical lithography have been driven by the utilisation of complex photomasks using Optical Proximity Correction (OPC) or phase shifting technologies. These masks are difficult and expensive to manufacture so the ability to test and characterise the mask making process is very important. This paper examines the issues involved in the use of relatively low cost Electrical Critical Dimension (ECD) measurement of mask features. Modified cross-bridge test structures have been designed to allow the on-mask measurement of dense and isolated, binary and phase shifted layouts. The results of electrical and Critical Dimension Scanning Electron Microscope (CD-SEM) testing of these structures are presented and indicate the lower variability associated with ECD measurements. In particular the adverse effect of phase shifting elements on the accuracy of SEM measurements is highlighted.