A study of CD-SEM suitability for CD metrology of modern photomasks

The requirements of the semiconductor industry, as evidenced by the SLA roadmap, are driving the reticle development cycle at an ever-increasing rate. With the current trends towards employing optical proximity corrections (OPC) to features to improve image transfer to the wafer, as well as development of phase shift masks (PSM), both targeting to extend the range of optical lithography, even more emphasis is being placed on photomask quality. Along with enhanced performance pattern generation and inspection tools, metrology tools capabilities need to be up to the task. This paper chronicles the development and optimization of a CD-SEM (Critical Dimension - Scanning Electron Microscope) as a metrology tool for mask production. Accuracy, linearity and precision were investigated with the emphasis on improving both dynamic and static precision. Algorithm evaluation was focused at improving the confidence in the measurement, and its correlation with the on-wafer CD. Accuracy was compared to pitch values written by an advanced e-beam lithography tool as well as an AFM. New algorithms were developed to address the growing requirement in two-dimensional metrology, as well as pattern fidelity issues that are facing the mask industry.