MPC strategies for multi-beam mask writers

Abstract

The benefit of complex and curvilinear mask shapes as well as the demand for fast mask production cycles has been driving the development of Multi-Beam Mask Writers (MBMW) for several years. Meanwhile, those writers have reached a quality level where they can be integrated into mask production flows at various nodes and even be used for writing imprint lithography templates at wafer scale. 50 keV e-beam writers whether Multi-Beam or Variable Shaped Beam (VSB), are affected by scattering effects at various length scales and require significant corrections in order to print mask features on target. Correction methods for long-range effects such as PEC (Proximity Effect Correction) and FEC (Fogging Effect Correction) have been developed for VSB machines and can be applied to MBMWs in the same way. Similarly, long-range mask process effects like loading (LEC) can be corrected using the same methods as developed for VSB machines. Besides long-range scattering and etch effects, critical masks for the 14 nm technology node and below are affected by short-range scattering and etch effects like e-beam forward scattering and etch micro-loading. Those effects increase at length scales below several 100 nm and change printed CDs significantly at minimum feature sizes on DUV and EUV masks where SRAFs are targeted around 60 nm and 30 nm, respectively. Figure 1 (left) shows an example of a typical mask CD error signature where the range of CD errors from small, isolated features to large nested features can easily cover 15 nm or more. Those short range distortions are generally corrected using so called Mask-Process-Correction (MPC) tools which compensate mask errors by moving edges of the input design and optionally adjust the dose of printed features locally. Simulated mask contours before and after MPC are shown in Figure 1 (center, right) demonstrating the large effect of MPC on SRAFs but also the non-negligible effect on main feature CDs, especially on line-ends and narrow lines.