Exploring new metrology for complex photomask patterns

Photomask pattern sizes are usually defined by a one-dimensional Critical Dimension (CD). As mask pattern shapes become more complex, a single CD no longer provides sufficient information to characterize the mask feature. For simple square contacts, an area measurement is generally accepted as a better choice for determining contact uniformity. However, the area metric may not adequately characterize complex shapes; it does not lend itself to CD metrology and it ignores pattern placement. This paper investigates new ways of measuring complex mask shapes with aggressive Optical Proximity Correction (OPC). An example of more informative metric is center of gravity. This new metric will be compared to more traditional mask characterization variables like CD mean to target, CD uniformity, and Image Placement (IP). Wafer simulations of the mask shapes will be used to understand which mask pattern metrics are most representative of the image transferred to wafer images. The results will be discussed in terms of their potential to improve mask quality for 32nm technology and beyond.