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The objective of this work is to demonstrate a simple Linear Superposition approach to creating an optimized illumination scheme from commonly available apertures (e.g. conventional, annular, quadrupole, dipole, etc.) that meet a variety of lithographic process metrics. Previous authors have demonstrated a variety of approaches to optimize the illumination for a given lithographic process. One common example is to use an optimized illumination for a specific pitch range and breaking the exposure into multiple reticles designed to print only nested or isolated features. A second example that has been widely demonstrated is to develop a single custom illuminator, which requires long lead times for delivery and a large capital investment. The true on-wafer performance of this custom illuminator can only be determined post-installation, providing limited ability to verify the simulation work a priori. The linear superposition method described here produces an optimal illumination scheme for a given photolithographic process. The success of this approach is due to the acceptably small nature of the electric field interaction terms between individual illumination modes allowing a multiple-exposure system to model a composite source. Images from optimized sub-components can be added to generate a composite image that is superior overall to any one process alone. After each exposure in a multiple exposure system there is a latent image that is only developable after a specific energy or dose level has been surpassed. It is the additive process of these latent images that creates the composite image. The composite image has the additive properties of the sub-components according to their dose fractions. Once the optimal process and dose split have been determined, it is straightforward to create a composite aperture to produce the same process by a single exposure. The composite aperture is the addition of the multiple sub-components with the relative transmission of each related to the illuminated surface area for systems designed to deliver uniform brightness. This approach produces superior pattern fidelity and an optimized common lithography process without the pitfalls of any one of the sub-component illumination modes.
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