Improving mask uniformity (including CDU) is a direct way to improve wafer uniformity because mask errors are repeatedly transferred through every exposure made with that mask. Unlike many process-induced variations, mask-induced variation is reproduced on every exposure on every wafer, making tighter mask control especially valuable.
Conventional mask process correction (MPC) solutions have always focused on linearity. The D2S solution, while providing leading-edge linearity correction, focuses on uniformity. Uniformity is key because it is generally more difficult to compensate for local and global random variations than for predictable, systematic linearity errors.
Mask uniformity is typically measured for local uniformity and global uniformity separately. This is because the causes of manufacturing variation and the various mitigation mechanisms in the mask writers that correct for these effects – proximity effect correction (PEC), loading effect correction (LEC), fogging effect correction (FEC), etc. – mostly affect global uniformity. Traditionally, machine-resident corrections have not addressed local uniformity. Rather local uniformity has been about the mask process and writer performance.
The D2S MPC solution, pixel-level dose correction (PLDC), extends correction capability to smaller scales and can therefore improve both local and global uniformity.
Dose margin (DM), exposure latitude in wafer terminology, has empirically proven to be a useful indicator of mask uniformity because it reflects sensitivity to multiple sources of variation, including resist thickness and temperature, residual backscatter and fogging effects, and model errors in the applied corrections. Increasing dose improves dose margin in isolated areas. But with dense leading-edge chips, increasing dose everywhere also increases backscatter, which has a negative impact on dose margin because of interactions with PEC. So, simply increasing dose everywhere with the intent of improving dose margin everywhere is counterproductive. In addition, increasing injected dose also has a negative effect on mask write times.
By strategically improving dose margin while reducing backscatter without increasing mask write times, the D2S PLDC solution improves mask uniformity while simultaneously improving linearity. Superior mask uniformity leads to better wafer uniformity and process windows.
