Mask Blank Glass Substrate PSD Control for EUV Reflectance
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Solution Overview
Problem
Existing EUV lithography mask blank substrates fail to achieve stable high reflectance and flatness due to inadequate control of surface roughness and titanium concentration distribution, leading to reduced throughput and accuracy in exposure steps.
Innovation Solution
A mask blank glass substrate with controlled circularly averaged power spectral density and root mean square roughness, utilizing titania-doped synthetic quartz glass, and precise polishing and cleaning processes to ensure high reflectance and flatness, measured using atomic force microscopes and white light interferometers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If surface roughness is controlled only by RMS value, then surface smoothness is improved, but reflectance of EUV light is not sufficiently improved
Solution Approach 1:
The patent changes the parameter for evaluating surface roughness from simple RMS value to power spectral density (PSD) distribution across multiple spatial frequencies. This parameter transformation reveals that controlling specific spatial frequency components (particularly around 0.1 μm⁻¹) is more effective for improving EUV light reflectance than merely reducing overall RMS roughness.
Solution Approach 2:
The patent introduces a new dimension for surface characterization by analyzing PSD across the spatial frequency domain rather than only in the spatial domain. This dimensional transformation from simple height measurement to frequency-domain analysis enables identification of specific roughness components that affect optical performance.
2Strength
If titanium concentration is increased to improve material properties, then strength is improved, but periodic structures occur on surface causing deteriorated flatness
Solution Approach 1:
The patent applies local quality control by specifying that titanium concentration distribution must be controlled within ±5 wt% of the average concentration. This local uniformity constraint prevents the formation of periodic surface structures while maintaining the overall high titanium content needed for material strength.
Solution Approach 2:
The patent implements feedback control by measuring actual titanium concentration distribution and using this information to adjust processing parameters. The concentration distribution is monitored and controlled to prevent periodic surface structures from forming during manufacturing.
3Manufacturing precision
If PSD is controlled only in specific spatial frequency range, then certain surface characteristics are improved, but comprehensive requirements for reflectance and flatness are not satisfied
Solution Approach 1:
The patent creates a universal surface control standard by establishing PSD limits across multiple spatial frequency ranges (0.01-10 μm⁻¹). This multi-frequency control approach simultaneously satisfies multiple requirements including EUV light reflectance, surface flatness, and defect reduction, making the substrate suitable for various leading-edge lithography applications.
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 substrate achieves high EUV light reflectance and flatness, suitable for leading edge applications, enhancing throughput and exposure accuracy in EUV lithography.
Implementation Method 1
An atomic force microscope is generally used to measure the surface roughness of a substrate
Implementation Method 2
measured using atomic force microscopes and white light interferometers
Implementation Method 3
a reflective mask is expected as a transfer mask used in the EUVL
Data Source
AI summary
A mask blank glass substrate having a maximum value of a circularly averaged power spectral density of 1,000 nm4 or less at a spatial frequency of 0.1 µm-1 or more and 20 µm-1 or less, the maximum value being obtained by measuring a surface morphology of a region of 10 µm × 10 µm with an atomic force microscope.