High volume manufacturing with extreme ultraviolet (EUV) lithography requires mask induced overlay errors of less than 1.5nm for the N7 node. The use of electrostatic chucking and reflective optics causes the reticle backside flatness and reticle thickness to directly affect the placement of the pattern at wafer through both in-plane (IPD) and out of plane distortions (OPD). The minimization of reticle flatness alleviates some of the image placement distortion caused by the reticle’s shape, however to be within the image placement error budget, N7 EUV blanks must have flatness <16nm p-v. With the manufacturing challenges associated with generating such flat blanks, compensation may be an option for imaging improvements; such methodologies will likely be essential for EUV to meet the stringent image placement and overlay specifications needed for high volume manufacturing (HVM).
Numerous compensation approaches can be utilized to minimize flatness related image placement errors including write compensation of the reticle, feed forward of reticle flatness data to the scanner corrections, and high-order empirical scanner corrections. This study investigates the benefits and limitations of each of these approaches, and seeks to better define which types of errors can be compensated and which will need further reticle flatness development in order to meet N7 and N5 specifications. Additionally, attention is given to the reticle’s shape as it relates to the limitations to the depth of focus required within the scanner systems. Utilizing an array of substrates and blanks from different vendors, we provide an assessment on which type of compensation method is most effective for addressing the various topographies for each specific reticle, and further explore for which node such schemes may be necessary.
This investigation seeks to provide a guide for the industry to work towards the implementation of functional tolerances related to both the compensation scheme used in manufacturing, and the reticle’s resulting non-correctable flatness (residual).