Photoresist induced contrast loss and its impact on EUV imaging extendibility

In order to meet the CDU specifications for the 22, 16 and 11nm technology nodes, EUV systems can be designed that provide sufficiently high aerial image contrast. This can be done by higher NA designs and/or by applying off-axis illumination. The contrast loss from the exposure system can be minimized to less than 10-20% by controlling the lens aberrations, flare and vibrations. However, EUV resist model calibration studies revealed that resist induced contrast loss exceeds 50% thus limiting resolution capability. Experiments were performed to assess state-of-the-art photoresist that showed significant improvements in EUV photoresist contrast while improving sensitivity. Finally, a method to experimentally quantify resist contrast loss was proposed.