Photomask Glass Substrate Flatness Control via Local Polishing
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Solution Overview
Problem
The photolithography process faces challenges in minimizing surface topography deformation of photomasks when mounted in exposure tools due to inadequate flatness of silica glass substrates, leading to increased waste and reduced productivity.
Innovation Solution
A glass substrate with specifically defined strip regions and a central square region is prepared by computing least squares planes and adjusting the surface topography through local polishing or etching to ensure minimal deformation when chucked in an exposure tool, maintaining high flatness and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a substrate with satisfactory flatness is selected, then the initial flatness is improved, but the surface topography deforms when mounted in an exposure tool
Solution Approach 1:
The substrate surface is pre-shaped with a predetermined convex curvature before mounting. This preliminary action compensates for the expected deformation that occurs during vacuum chucking, ensuring that the surface returns to a flat state when mounted in the exposure tool. The convex shape is created through polishing or etching processes that raise the central region relative to the peripheral regions.
Solution Approach 2:
The invention applies a counteracting shape (convex curvature) to the substrate before mounting to offset the harmful deformation (concave distortion) that occurs during vacuum chucking. This preliminary anti-action ensures that the net effect when mounted is a flat surface, eliminating the need for post-mounting adjustment or selection of substrates with excessively high initial flatness.
2Measurement precision
If substrates are inspected by contact methods to confirm shape, then measurement accuracy is improved, but the risk of damage and productivity are worsened
Solution Approach 1:
The invention replaces contact-based mechanical inspection methods with non-contact optical measurement methods. Optical flatness testers use light interference patterns to measure substrate surface shape without physical contact, eliminating the risk of damage to fragile substrates while maintaining sufficient measurement accuracy for quality control.
Solution Approach 2:
The invention introduces light as an intermediary medium for measurement instead of direct mechanical contact. Optical measurement systems use light waves to probe the substrate surface, creating interference patterns that reveal surface topology without applying physical force that could cause damage.
3Manufacturing precision
If the substrate surface is polished to high flatness, then manufacturing precision is improved, but the productivity decreases due to increased processing time
Solution Approach 1:
The substrate surface treatment is divided into two stages: (1) bulk flattening through conventional polishing to achieve general flatness, and (2) localized convex shaping of the central region through selective polishing or etching. This segmentation allows most of the substrate to be processed quickly by efficient bulk methods, while only the central region requires the more time-consuming precision work to create the compensating convex curvature.
Solution Approach 2:
The invention applies different surface treatments to different regions of the substrate. The peripheral regions are polished to general flatness using conventional methods, while the central region is selectively processed to create a convex curvature. This local quality approach ensures high precision where needed (central mounting area) while maintaining overall productivity by not over-processing the entire substrate surface.
Data Source
AI summary
A photomask-forming glass substrate having a square major surface is provided wherein two strip regions are defined on the major surface near a pair of opposed sides such that each region spans between 2 mm and 10 mm inward of the side and excludes end portions extending 2 mm inward from the opposed ends of the side, a least squares plane is computed for each of the two strip regions, the angle included between normal lines to the least squares planes of two strip regions is within 10 seconds, and the height difference between two strip regions is up to 0.5 μm.