Optimizing absorber thickness of attenuating phase-shifting masks for hyper-NA lithography

Full-field ArF immersion scanners with NA above 1.20 are expected to be available in 2006. 45-nm-halfpitch lines and spaces (LS) are feasible by using these scanners with polarized dipole illumination. However, from the standpoint of design flexibility, using c-quadrupole or annular illumination is the better alternative to the dipole illumination. An attenuating phase-shifting mask (attPSM) has been widely used for device manufacturing to improve resolution because of the simple layer structure without rigid specifications for mask topography such as undercut biasing in case of alternating PSM. In this study the impact of the absorber thickness of single-layer and bi-layer attPSMs, i.e. MoSi, Cr/SiON, and Ta/SiO2, on the imaging performance of ArF immersion lithography is investigated by using a rigorous electro-magnetic field simulator, Prolith version 9.0.1 with advanced mask package (KLA-Tencor), using a lumped parameter resist model. Exposure latitude (EL) of critical dimension (CD), depth of focus, mask-error-enhancement factor (MEEF), and sensitivity to polarization are compared to find an optimum mask structure for the c-quadrupole illumination in hyper- NA lithography. In the hyper-NA lithography, the standard absorber thickness giving 6% transmittance is not necessarily the optimum from the viewpoint of the imaging performance. For example, to delineate 45 nm 1:1 LS with NA of 1.20 by using the Cr/SiON attPSM, the aerial-image contrast in resist of the standard 25-nm-thick Chromium is 20% smaller than that of 50-nm-thick Chromium. Optimizing the Chromium thickness increases EL by 29%, though it has few impacts, less than 1%, on 65 nm 1:1 LS with NA of 0.85. The EL of a thickness-optimized Ta/SiO2 attPSM is 33% larger than that of the standard 68-nm-thick MoSi. Supposing the dose error, the focus error, and wafer CD error, we can relax the specification of mask CD error by using the 30-nm-thick Tantalum. Ta/SiO2 is 63% more sensitive to the polarization state of the incident light than MoSi, but the impact on CD is only 0.8 nm with a 10% change of the polarization. Calculating diffraction efficiencies for the 45 nm LS shows probable causes of the better imaging performance of Ta/SiO2; intensity of the 0th- and 1st-order diffracted light becomes larger than for MoSi and Cr/SiON as the illumination angle increases, phase difference between the 0th and 1st order is smaller in the large illumination angle, and amount of the flare-like exposure by light sources of the c-quad illumination parallel to the LS is smaller. As a summary, the three attPSMs with the nominal 6% transmittance and 180o phase difference give different imaging performances. Optimizing mask materials and thicknesses is important to have better image quality at NA above 1.0.