Phase Grating for EUV Mask Inspection Spectral Purity
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Solution Overview
Problem
Existing optical systems for mask inspection using ultra-violet light above 193 nm are insufficient to resolve features and defects below 22 nm node, and existing Spectral Purity Filter methods are not applicable for EUV mask inspection with Xenon LPP sources, leading to thermal damage and image flare issues.
Innovation Solution
The development of phase grating designs for both near normal incidence and grazing incidence mirrors, specifically configured for EUV mask inspection, which include multiple bilayers and optimized grating structures to suppress out-of-band radiation and reduce the intensity of the 1030 nm drive laser, while maintaining high reflectivity for the 13.5 nm EUV light.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Laser Produced Plasma (LPP) light source is used for EUV mask inspection, then the resolution of features and defects below 22 nm node is improved, but thermal damage and image flare are caused by the 1030 nm drive laser entering the inspection system
Solution Approach 1:
A phase grating is introduced as an intermediary optical element between the LPP light source and the inspection system. The grating is specifically designed to reflect EUV light (13.5 nm) while suppressing the 1030 nm drive laser radiation, thereby mediating between the harmful laser radiation and the inspection system to enable high-resolution imaging without thermal damage and image flare
Solution Approach 2:
The phase grating employs local quality by creating spatially varying optical properties through its grating structure. The grating lines and spaces are designed with specific dimensions and materials that selectively interact with different wavelengths, allowing EUV light to pass through while blocking the 1030 nm laser radiation in specific spatial regions
2Reliability
If Spectral Purity Filter (SPF) methods developed for EUV lithography are used, then the spectral purity is improved, but the method is not applicable for mask inspection due to significant difference in laser wavelength and use case
Solution Approach 1:
The phase grating design incorporates parameter changes by optimizing the grating dimensions, material composition, and layer structure specifically for the 1030 nm laser wavelength and EUV inspection application. The grating parameters (such as line width, spacing, and depth) are calculated to achieve the desired spectral suppression ratio for the specific wavelength difference between the drive laser and EUV light
Solution Approach 2:
The phase grating is segmented into multiple functional layers and grating elements that work together to achieve spectral purity. The grating structure is divided into fine lines and spaces that create multiple reflection paths, effectively separating the EUV signal from the laser radiation through spatial and spectral discrimination
3Object-affected harmful factors
If the intensity of the 1030 nm drive laser is significantly reduced, then thermal damage and image flare are minimized, but the overall light output and inspection quality are compromised
Solution Approach 1:
The phase grating acts as an intermediary that selectively transmits EUV light while blocking the 1030 nm laser radiation. This allows the system to maintain high EUV light output for quality inspection while the harmful laser intensity is suppressed by the grating's wavelength-selective properties
Solution Approach 2:
The phase grating exhibits wavelength-dependent optical properties, effectively changing its transmission characteristics differentially across the spectrum. The grating is designed to be transparent to EUV wavelengths (13.5 nm) while being reflective or absorptive to the 1030 nm laser wavelength, enabling selective passage of useful light while blocking harmful radiation
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 phase grating designs effectively reduce the intensity of the 1030 nm light by a factor of 50 and the 1030 nm to 13.5 nm light ratio by a factor of 2000, minimizing thermal damage and image flare, while ensuring high reflectivity for EUV light, thus enhancing the accuracy of EUV mask inspection.
Implementation Method 1
phase grating designs... to suppress out-of-band radiation and reduce the intensity of the 1030 nm drive laser
Implementation Method 2
maintaining high reflectivity for the 13.5 nm EUV light
Implementation Method 3
ensuring high reflectivity for EUV light
Implementation Method 4
a plurality of gratings positioned on the plurality of continuous base bilayers, wherein each of the plurality of gratings is formed using between 10 and 200 bilayers
Data Source
AI summary
Spectral Purity Filters, or SPFs, are disclosed. Such SPFs are designed to block out the 1030 nm drive laser and other undesired out of band light in a EUV mask inspection system. Different phase grating configurations for near normal incidence and grazing incidence are provided in the present disclosure and are configured specifically for EUV mask inspection.