Equivalent modeling techniques for predicting pattern transfer in EPL masks

Abstract

One of the primary technologies proposed for the manufacturing of microcircuit devices in the sub-100 nm regime is electron-beam projection lithography (EPL). To meet the stringent error budgets, it is necessary to eliminate, minimize or correct for the sources of mask-related distortions. Among them is pattern transfer in-plane distortion (IPD). It is driven by the residual stresses of the multilayers deposited on the mask substrate during the fabrication process. This paper focuses on EPL technologies (such as SCALPEL and PREVAIL) that employ a freestanding membrane as a key component of the mask. To facilitate pattern transfer simulations, equivalent modeling techniques have been developed using finite element methods, and applied to specific test patterns. Because of the relative size of IC pattern features, it is impractical to model all details of the design. Consequently, equivalent modeling techniques are needed to simulate the pattern-specific distortions induced during pattern transfer. For repeated patterns, such as memory chips, it is sufficient to simulate basic stress/strain testing of a unit cell representative of the design to determine the equivalent stiffness of the membrane. Equivalent orthotropic properties (i.e. the elastic modulus in mutually orthogonal directions, Poisson’s ratio and the shear modulus) were determined for the IBM Nighteagle/Falcon format (an SRAM device at 175 nm). Analytical calculations for a theoretical test case were also conducted to benchmark the FE simulations.