ASML AH53 and AH74 with higher odd-order diffraction light are the widely used alignment marks in industry to achieve better alignment accuracy by reducing mark damage noise. During lithography alignment process, decent diffraction light power is the basic demand. However, with the use of some high absorption (k is not equal to 0 for detective wavelength) material, it is difficult to detect the light power reflecting from the thick and opaque film stacks with these standard alignment marks. Here we optimized four alignment marks with higher odd-order diffraction power with comparing with AH53 and AH74. One software based on Fourier optical theory is built to quickly calculate the wafer quality (WQ) of different film stacks and different alignment marks. ASML SMASH alignment system can accept customized alignment mark, with new mark type configuration file. In order to demonstrate the effectiveness of new alignment marks, we put the marks on a mask and do the experiments to compare with simulation results. All the experiments results show that new designed alignment marks have larger WQs of odd-order diffraction.
Source-mask optimization (SMO) is used in advanced computational lithography to further enlarge the process margin. SMO provides the source for subsequent optical proximity correction (OPC) to generate the mask with reasonable manufacturability and functionality. Little attention is paid to the mask optimization procedure of SMO. The procedure may potentially cause significant mismatch between a source-mask optimized mask (SMOed mask) and an optical proximity-corrected mask (OPCed mask), which affects the efficiency of the optimization. We investigate and report a specific example of an efficient method to align the SMOed mask to the OPCed mask so as to reduce the cycles of computational lithography and improve the predictability of SMO. This method incorporates techniques of retargeting and manipulating the cost function (CF) into SMO to modify the CF and eventually change the mask shapes. Various defects can also be corrected to minimize the needed number of hotspots, which also improves the effectiveness of SMO and decreases the cycles of computational lithography. Our sample simulations performed on a metal layer with both diffractive optical element (DOE) and freeform illumination demonstrate that the proposed SMO further enhances the process window (PW) by more than 30% compared with conventional SMO. The optimized mask shape is also more similar to OPCed mask. Experimental verification is also performed to validate the proposed method.