In preparation for high NA immersion lithography, researchers found that electromagnetic field (EMF) effects due to mask topography were causing a significant reduction in process window performance. To reduce the EMF effects, mask film thickness had to be reduced. For a phase shift mask, the film thickness cannot be thinned without altering the degree of phase shifting. And the absorber thickness on conventional COG films has already been reduced to the optical density limit. A new solution was required.
Together with ShinEtsu and IBM, Toppan has developed a new binary photomask material called OMOG (or Opaque MoSi on Glass). Using MoSi as the absorber layer, OMOG provides sufficient optical density to appear opaque at 193nm wavelengths while simultaneously being thin enough to reduce the EMF effects that previously plagued high NA immersion lithography. Since the entire industry has decades of experience using MoSi films, OMOG is a perfect solution for advanced lithography.
The OMOG material stack utilizes a MoSi absorber with a higher extinction coefficient, which allows us to further reduce the film thickness while maintaining sufficient optical density. Improved pattern fidelity and better resolution are achieved through the combination of a super thin chrome hard-mask and thinner resist. The MoSi absorber, with its high anisotropic etch behavior, helps deliver superior CD uniformity in the final product. Furthermore, the low film stress inherent to the OMOG film stack helps to improve flatness and reduce process-induced pattern placement errors. In summary, OMOG photomasks provide finer resolution, increased fidelity, tighter CD uniformity and better registration.
The thin OMOG material inherently helps improve your wafer printing performance. OMOG's thin topography reduces the electromagnetic field bias, which has been shown to degrade process window overlap across features and magnify the impact of mask errors (MEEF).
The combination of reduced EMF bias, improved pattern fidelity and finer resolution help reduce mask pattern constraints. These relaxed mask pattern constraints, in turn, allow you more OPC flexibility on critical features such as sub-resolution assists and corner-to-corner gaps. In other words, OMOG allows you to extend the mask further in order to perfect the printed wafer image.
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