EUV Mask Inspection Focus Adjustment via Visible Light Autofocus
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Solution Overview
Problem
Current inspection methods for EUV masks face challenges in accurately adjusting the focus point due to the stringent requirements for the shape accuracy of concave mirrors and the need for forming known-sized focusing patterns, making it difficult to detect microscopic defects effectively.
Innovation Solution
An inspection apparatus utilizing an EUV light source, an illumination optical system with a concave mirror and a convex mirror, and an AF light source with a wavelength of 450 nm to 650 nm to adjust the focus point on the EUV mask, allowing for a simpler configuration and relaxation of the concave mirror's shape accuracy, while using a pellicle made of silicon to prevent light reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a focusing pattern with known size is formed on the EUV mask to adjust focus, then the focus point can be adjusted, but the configuration becomes complex and the concave mirror requires high shape accuracy
Solution Approach 1:
The invention extracts the focus adjustment function from the EUV optical path by using a separate visible light autofocus optical system. The visible light autofocus optical system includes its own light source, condensing lens, and photodetector, which are independent from the EUV illumination and detection systems. This allows focus adjustment without requiring modifications to the EUV mask or optical components.
Solution Approach 2:
The visible light autofocus optical system serves multiple functions: it provides focus adjustment for the EUV inspection system, enables determination of the inspection area on the mask, and works independently of the EUV optical path. The same visible light system can be used for both autofocus and inspection area identification purposes.
2Measurement precision
If a focusing pattern with known size is formed on the EUV mask, then focus adjustment is possible, but the manufacturing process becomes more difficult
Solution Approach 1:
The invention removes the requirement for pre-formed focusing patterns on the EUV mask by extracting the focus measurement function to a separate visible light system. The autofocus optical system uses visible light to detect focus status through the mask substrate without requiring any special pattern structures on the mask itself.
Solution Approach 2:
The visible light autofocus optical system acts as an intermediary between the EUV inspection system and the mask. Instead of directly measuring EUV focus requirements on the mask surface, the system uses visible light that can penetrate the mask substrate to detect focus status indirectly through the mask structure itself.
3Measurement precision
If the concave mirror has high shape accuracy requirements, then focus adjustment precision is improved, but the device complexity and cost increase
Solution Approach 1:
The invention segments the optical measurement functions into two independent systems: an EUV inspection optical system and a visible light autofocus optical system. The visible light system handles all focus measurement and adjustment functions, while the EUV system focuses solely on inspection. This segmentation allows the concave mirror in the EUV system to have relaxed accuracy requirements.
Solution Approach 2:
The visible light autofocus optical system serves as an intermediary that performs the sensitive focus measurements. By using visible light with a separate photodetector system, the invention transfers the precision measurement requirements from the EUV concave mirror to the visible light optical components, which can be manufactured with standard tolerances.
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
Enables accurate focus adjustment and defect detection with a simpler configuration, reducing the complexity and cost of the inspection apparatus while maintaining high precision in detecting microscopic irregularities on EUV masks.
Implementation Method 1
an EUV light source configured to generate EUV light
Implementation Method 2
a concave mirror with a hole formed therein, the concave mirror with the hole being configured to reflect the EUV light reflected on the EUV mask
Implementation Method 3
a convex mirror configured to reflect the EUV light reflected on the concave mirror with the hole toward the hole of the concave mirror with the hole
Implementation Method 4
an AF light source configured to generate AF light having a wavelength of 450 nm to 650 nm
Implementation Method 5
an AF photodetector configured to detect the AF light reflected on the EUV mask through the concave mirror with the hole and the convex mirror
Data Source
AI summary
An inspection apparatus according to an aspect of the present invention includes an EUV light source 11, an illumination optical system 10 provided to apply the EUV light to an EUV mask 60, a concave mirror and a convex mirror 22 configured to reflect the EUV light reflected on the EUV mask 60, a camera 32 configured to detect EUV light reflected on the convex mirror 22 and thereby take an image of the EUV mask 60, an AF light source 16 configured to generate AF light having a wavelength of 450 nm to 650 nm, first and second detectors 27 and 30 configured to detect the AF light reflected on the EUV mask 60 through the concave mirror with the hole 21 and the convex mirror 22, and an processing device 31 configured to adjust a focus point of the EUV light on the EUV mask 60.