Photomask Blank Conductive Layer Grounding for EB Lithography
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Solution Overview
Problem
Current photomask blanks face challenges in achieving high accuracy and optical properties for pattern transfer using exposure light of sub-200 nm wavelength, particularly due to issues with resist film thickness, etch resistance, and charge buildup during electron beam lithography, leading to inaccuracies and potential substrate contamination.
Innovation Solution
A photomask blank structure with a transparent substrate, a resistance layer having a resistivity of at least 0.1 Ω·cm, and a conductive layer with a resistivity of less than 0.1 Ω·cm, where the resistivities and thicknesses of these layers satisfy a specific equation, allowing for effective grounding and reducing electrical charging during electron beam writing, ensuring low resistance values and accurate pattern transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the photoresist film thickness is reduced to form finer photomask patterns, then the pattern resolution is improved, but the etch resistance of the resist pattern deteriorates
Solution Approach 1:
The patent divides the single photoresist film into two separate layers: a lower photoresist film for etching the first light-shielding film, and an upper photoresist film for etching the second light-shielding film. This segmentation allows each layer to be optimized independently - the lower layer can be thicker for better etch resistance, while the upper layer can be thinner for finer pattern resolution, thereby resolving the contradiction between etch resistance and pattern resolution
Solution Approach 2:
The patent uses different types of photoresist materials for the upper and lower layers. The lower photoresist film uses a positive-type chemically amplified resist with specific etch resistance properties, while the upper photoresist film uses a different composition optimized for fine pattern formation. This composite structure allows each layer to exhibit its optimal properties, simultaneously achieving both high etch resistance and fine pattern resolution
2Reliability
If a single thick photoresist film is used to ensure etch resistance, then the etch resistance is improved, but the pattern resolution and accuracy deteriorate
Solution Approach 1:
The patent segments the photoresist system into two layers with different thicknesses and functions. The lower photoresist film has greater thickness (50-200 nm) providing sufficient etch resistance for the first light-shielding film, while the upper photoresist film has smaller thickness (20-100 nm) enabling fine pattern resolution. This segmentation eliminates the need for a single thick film, thereby maintaining etch resistance while improving pattern resolution
Solution Approach 2:
The patent applies different photoresist properties to different locations in the vertical structure. The lower layer is designed with properties optimized for etch resistance and structural support, while the upper layer is designed with properties optimized for fine pattern fidelity. This local optimization of properties at different positions in the resist structure simultaneously achieves both high etch resistance and fine pattern resolution
3Reliability
If the photomask blank is grounded during electron beam writing, then charge buildup is prevented, but mechanical contact may cause scratches on the multilayer film
Solution Approach 1:
The patent introduces a conductive layer as an intermediary between the photoresist film and the substrate. This conductive layer provides a pathway for charge dissipation during electron beam writing, preventing charge buildup that would otherwise require mechanical grounding. The conductive layer eliminates the need for direct mechanical contact with the multilayer film, thereby preventing scratches while maintaining effective charge control
Solution Approach 2:
The patent replaces the mechanical grounding system with an electrical conduction system. Instead of using a grounding pin to make mechanical contact with the photomask blank, the conductive layer provides an electrical pathway for charge dissipation. This substitution eliminates mechanical contact and potential scratches while achieving the same charge control function through electrical conduction
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed solution enables the production of photomasks with necessary optical properties and low resistance grounding, preventing electrical charging and ensuring high accuracy in pattern transfer, even with reduced multilayer film thickness, thus addressing the limitations of existing technologies.
Implementation Method 1
a conductive layer disposed contiguous to the surface of the resistance layer adjacent to the substrate and having a resistivity of less than 0.1 Ω·cm
Data Source
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AI summary
In a photomask blank comprising a transparent substrate, a resistance layer, and a conductive layer, the resistivity and thickness of the conductive layer and the resistivity and thickness of the resistance layer are selected so as to meet a specific equation (1). In EB lithography, a ground can be established at a necessary fully low resistance value, and EB writing be carried out at a high accuracy.