Dual-channel additive compensation of mask-plane illumination uniformity for i-line lithography

To satisfy the requirement of two-dimensional (2D) uniformity for rectangular flat-top illumination at the mask plane in i-line projection lithography, this paper proposes a dual-channel additive ultraviolet illumination compensation method. The system comprises a main homogenized illumination channel and an independent compensation channel, and it achieves fine correction of the mask-plane illumination distribution through the linear superposition of intensities from the two channels. To avoid the global inversion problem caused by spatial convolution in conventional defocused transmissive compensation schemes, a reflective compensation element is introduced into the compensation channel. A conjugate mapping relationship is established between the compensation plane and the mask plane, which allows the compensation effect to be approximated as a local multiplicative modulation. On this basis, a pointwise inverse solution for the reflectance distribution of the compensation element is derived. Ray-tracing simulations based on LightTools show that the proposed method reduces the 2D illumination nonuniformity at the mask plane from 3.122% to 1.011%, which demonstrates its effectiveness for 2D illumination uniformity correction in i-line lithography.