A novel mask structure for measuring the defocus of scanner

A new focus monitor mask having novel grating structure is proposed to measure the focus variation of the scanner. The grating pattern composes of transparent line, opaque line, π-phase shift groove and π/2 -phase shift groove with their width ratio equivalent to 1:4:1:2. By using this structure, one of the first order and one of the second order of the diffraction spectrum are eliminated. Therefore, the lithography image is formed by the interference of the zeroth order and the left positive (or negative) 1st and 2nd orders, which is more sensitive to the subtle change of focus. The basic principle and characteristic of the proposed mask is described in this paper. Simulations with the lithography simulator PROLITH shows that the monitoring accuracy is improved more than 25%, compared with the conventional phase grating focus monitor (PGFM). The novel mask proposed in our job has potential to be an efficient candidate for measuring the defocus of scanner in the immersion lithography with hyper NA.