Movable Drop-In Mirror for EUV Mask Inspection
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Solution Overview
Problem
The existing mask inspection apparatuses for EUV lithography face challenges with reduced reflectance due to carbon contaminants on reflective surfaces, leading to frequent cleaning needs and interruptions during inspections, especially with high-intensity EUV light sources.
Innovation Solution
A mask inspection apparatus with a movable drop-in mirror having a multi-layer film and a silicon wafer substrate, where the illuminated spot is moved to prevent local temperature increases and carbon contaminant accumulation, and uses cleaning light with a different wavelength to decompose contaminants, allowing continuous operation without mirror exchange.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high-intensity EUV light is used for inspection, then inspection speed and productivity are improved, but carbon contaminants accumulate on the reflective surface causing reflectance to decrease
Solution Approach 1:
The patent applies the dynamics principle by making the drop-in mirror movable instead of stationary. The mirror can be shifted in the optical path to move the illuminated spot to different areas of the reflective surface. This dynamic positioning prevents carbon contaminants from accumulating in a single location, thereby maintaining reflectance while allowing continuous high-intensity EUV inspection without interruptions for cleaning or mirror replacement.
2Ease of operation
If the illuminated spot remains stationary on the reflective surface, then alignment is simplified, but local temperature increases and carbon contaminant accumulation occurs
Solution Approach 1:
The drop-in mirror is configured to be movable, allowing the illuminated spot to be dynamically repositioned across different areas of the reflective surface. This prevents stationary heating and contaminant accumulation while maintaining relatively simple alignment procedures. The mirror can be shifted to fresh areas of the reflective surface that have not yet been exposed to high-intensity EUV light.
Solution Approach 2:
The patent introduces spatial movement in another dimension by allowing the drop-in mirror to shift position in the optical path. This transforms the problem from a two-dimensional stationary illumination scenario to a three-dimensional dynamic scenario where the illuminated spot can be repositioned across the reflective surface area, distributing thermal and contamination effects across multiple locations.
3Reliability
If the drop-in mirror is frequently cleaned or replaced, then reflectance is maintained, but inspection interruptions increase and productivity decreases
Solution Approach 1:
By making the drop-in mirror movable rather than stationary, the system can dynamically reposition the illuminated spot to avoid areas with accumulated carbon contaminants. This eliminates the need for frequent cleaning or replacement operations, enabling continuous high-productivity inspection while maintaining high reflectance through spatial redistribution of the illumination.
Solution Approach 2:
The movable drop-in mirror enables continuous inspection operations without interruptions for cleaning or mirror replacement. The mirror can be continuously repositioned to fresh areas of the reflective surface, allowing the useful action of inspection to proceed uninterrupted while maintaining high reflectance performance.
4Reliability
If a larger reflective surface area is used, then contaminant distribution is improved, but the device complexity and cost increase
Solution Approach 1:
Rather than simply increasing the physical size of the drop-in mirror, the patent employs a dynamic approach where a standard-sized mirror is made movable. This allows the illuminated spot to be repositioned across different areas of the reflective surface, effectively utilizing the available surface area without the complexity and cost associated with manufacturing and handling larger mirrors. The mobility mechanism enables contaminant distribution across the existing mirror surface.
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
This configuration maintains high reflectance and prevents interruptions by effectively moving the illuminated area and using cleaning light to decompose contaminants, ensuring continuous operation and improved image quality.
Implementation Method 1
a multi-layer film for reflecting extreme ultraviolet (extreme ultraviolet used in EUVL will be hereinafter referred to as EUV light) is formed on a substrate made of a low thermal expansion glass
Implementation Method 2
a part that was in the illuminated spot on the reflective surface, which has been moved outside the illuminated spot when the drop-in mirror is moved, is configured to be irradiated with cleaning light including VUV light with a wavelength different from a wavelength of the illumination light
Data Source
AI summary
Provided are a mask inspection apparatus and a mask inspection method that can prevent a reduction in a reflectance of a drop-in mirror, which is caused by carbon contaminants. A mask inspection apparatus according to the present invention includes a drop-in mirror including multi-layer film and a reflective surface. The drop-in mirror is configured to reflect illumination light incident on the reflective surface and illuminate the mask. An area of the reflective surface is configured to be greater than an area of an illuminated spot irradiated with the illumination light on the reflective surface. The drop-in mirror is configured to be movable. A position of the illuminated spot on the reflective surface is configured to be moved when the drop-in mirror is moved.