Mask Blank Substrate Flatness Estimation via Simulation
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Solution Overview
Problem
Existing methods for manufacturing mask blanks for exposure masks with wavelengths of 200 nm or less struggle with accurately estimating the flatness of transparent substrates when set in exposure apparatuses, leading to reduced transfer accuracy and increased defects due to complex surface shapes and film stress issues.
Innovation Solution
A method involving precision-polished transparent substrates with surface shape information obtained from measurement points, simulated to calculate flatness, and thin film formation after satisfying specifications, with film stress control to ensure accurate flatness and reduce defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If simulation is performed using flatness and simple surface shape data of mask blank, then calculation is simplified, but the derived flatness does not agree with actual flatness due to complex surface shapes
Solution Approach 1:
The patent performs preliminary measurement and classification of the transparent substrate's main surface shape before simulation. By categorizing the surface shape into one of four types (convex, concave, saddle, or semicylindrical) and selecting corresponding simulation data in advance, the method ensures accurate flatness estimation without requiring complex real-time calculations during mask blank production.
Solution Approach 2:
The patent changes the simulation parameters based on the measured surface shape characteristics. Instead of using fixed simple surface shape assumptions, the method adjusts the simulation input parameters to match the actual surface shape type (convex, concave, saddle, or semicylindrical), thereby improving the accuracy of flatness derivation while maintaining computational feasibility.
2Ease of manufacture
If light-shielding film is formed on transparent substrate before measurement, then mask blank is complete, but many particles adhere to film causing defects
Solution Approach 1:
The patent reverses the conventional sequence by performing surface shape measurement and flatness calculation on the transparent substrate before forming the light-shielding film. This preliminary action allows selection of substrates meeting flatness specifications before the defect-prone film formation step, preventing particle contamination of the film while ensuring mask blank quality.
Solution Approach 2:
The patent inverts the traditional manufacturing sequence where the light-shielding film is formed first and then measured. Instead, it measures and evaluates the transparent substrate's surface shape and flatness before film formation, thereby avoiding particle adhesion to the film while still ensuring the final mask blank meets specifications.
3Manufacturing precision
If exposure wavelength is reduced to 200 nm or less, then resolution is improved, but depth of focus becomes extremely small reducing transfer accuracy
Solution Approach 1:
The patent applies preliminary anti-action by pre-compensating for focus position shifts through accurate flatness measurement and simulation of the transparent substrate before mask blank production. By selecting substrates with appropriate surface shapes and flatness characteristics, the method counteracts potential focus shifts that would occur during exposure, maintaining transfer accuracy despite the extremely small depth of focus at 200 nm or less wavelength.
Solution Approach 2:
The patent changes physical parameters of the transparent substrate (surface shape and flatness) to compensate for the reduced depth of focus. By selecting substrates with specific surface characteristics (convex, concave, saddle, or semicylindrical shapes with controlled flatness), the method optimizes the mask blank's performance to maintain transfer accuracy when using short wavelength exposure light sources.
Data Source
AI summary
A method includes a preparation step of preparing a transparent substrate having a precision-polished main surface, a surface shape information obtaining step of obtaining, as surface shape information, height information at a plurality of measurement points on the main surface of the transparent substrate that contacts a mask stage of an exposure apparatus, a simulation step of obtaining, based on the surface shape information and shape information of the mask stage, height information at the plurality of measurement points by simulating the state where the transparent substrate is set in the exposure apparatus, a flatness calculation step of calculating, based on the height information obtained through the simulation, a flatness of the transparent substrate when it is set in the exposure apparatus, a judging step of judging whether or not the calculated flatness satisfies a specification, and a thin film forming step of forming a thin film as serving as a mask pattern, on the main surface of the transparent substrate whose flatness satisfies the specification.