Abstract
Traditionally, aberration correction in extreme ultraviolet (EUV) projection optics requires the use of multiple lossy mirrors, which results in prohibitively high source power requirements. We analyze a single spherical mirror projection optical system where aberration correction is built into the mask itself, through Inverse Lithography Technology (ILT). By having fewer mirrors, this would reduce the power requirements for EUV lithography. We model a single spherical mirror system with orders of magnitude more spherical aberration than would ever be tolerated in a traditional multiple mirror system. By using ILT, (implemented by an adjoint-based gradient descent optimization algorithm), we design photomasks that successfully print test patterns, in spite of these enormous aberrations. This mathematical method was tested with a 6 plane wave illumination source. Nonetheless, it would have poor power throughput from a totally incoherent source.
Highlights
Extreme ultraviolet (EUV) lithography is the leading contender to become the industrial scale lithography technology in the semiconductor industry
To design masks with built-in aberration correction, we employ the optimization approach called Inverse Lithography Technology (ILT), which was developed by Luminescent Inc [2]. and Intel [3,4,5,6,7], independently
We have found that even ~0.1λ phase shift at the edge of the mirror produces ~10% errors in the test pattern features, unless the mask is redesigned to account for the newly shifted mirror surface
Summary
Extreme ultraviolet (EUV) lithography is the leading contender to become the industrial scale lithography technology in the semiconductor industry. We consider a single mirror system in which the aberration correction is built in to the mask design This could result in (1-0.75) = 83% reduction in EUV source power required, but the mathematical procedure will constrain the source incoherence. To design masks with built-in aberration correction, we employ the optimization approach called Inverse Lithography Technology (ILT), which was developed by Luminescent Inc [2]. We present a specific way to apply the adjoint method to Inverse Lithography Technology We apply this form of ILT to a single spherical mirror system with orders of magnitude greater aberrations than would ever be tolerated in a traditional multiple mirror system. The adjoint method allows us to design photomasks with non-intuitive shapes that successfully print test patterns, in spite of these enormous aberrations
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