Segmented EUV Mirror Alignment via Phase Offset Interferometry
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Solution Overview
Problem
In microlithographic projection exposure apparatuses, especially those in the EUV range, increasing the image-side numerical aperture of mirrors is limited by difficulties in reducing surface errors, manufacturing challenges, and maintaining a common phase angle in segmented mirrors, which affects imaging quality.
Innovation Solution
A method for aligning segmented mirrors by recording partial interferograms between mirror segments and a reference wave, determining phase offsets, and adjusting the segments to maintain a distance of less than λ/10 from a predetermined surface normal, ensuring spatial coherence and correct phase alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the dimensions of mirrors are increased to accommodate higher image-side numerical aperture, then the numerical aperture is improved, but the manufacturing precision deteriorates due to increased difficulty in reducing long-wave surface errors
Solution Approach 1:
The patent divides a large mirror into multiple smaller mirror segments. Each segment can be manufactured with high precision using existing capabilities, while collectively they provide the large aperture needed for high numerical aperture imaging. The segments are arranged to form a complete optical surface that maintains the required wavefront quality.
2Illumination intensity
If the dimensions of mirrors are increased, then the numerical aperture is improved, but the device complexity increases due to requirements for larger processing machines and stricter processing tool requirements
Solution Approach 1:
By segmenting the mirror, the patent enables manufacturing of individual segments using smaller, more manageable processing equipment. Each segment can be fabricated, tested, and adjusted independently, avoiding the need for extremely large processing machines that would be required for a monolithic mirror of equivalent aperture.
Solution Approach 2:
The patent employs active adjustment mechanisms that allow the mirror segments to be dynamically positioned and aligned. This dynamic capability enables the system to achieve and maintain the required optical performance through real-time adjustment of segment positions and orientations, compensating for manufacturing tolerances and environmental variations.
3Ease of manufacture
If segmented mirrors are used to accommodate manufacturing limitations, then the ease of manufacture is improved, but the imaging quality deteriorates due to wavefront jumps between partial beam paths
Solution Approach 1:
The patent implements active adjustment mechanisms for mirror segments that enable dynamic control of segment positions and orientations. This allows real-time compensation for wavefront errors and phase mismatches between segments, maintaining high imaging quality despite the segmented structure. The adjustable support structures and actuators enable precise control of each segment's degree of freedom.
Solution Approach 2:
The patent employs wavefront sensing and measurement systems that provide feedback on the actual optical performance of the segmented mirror. This feedback information is used to adjust segment positions and orientations to minimize wavefront errors and maintain the required phase coherence across the entire aperture, thereby preserving imaging quality.
4Ease of manufacture
If segmented mirrors are used, then the ease of manufacture is improved, but the reliability deteriorates due to the requirement for common phase angle among segments
Solution Approach 1:
The patent employs active adjustment mechanisms that continuously maintain the common phase angle requirement among segments. The adjustable support structures and actuators enable real-time correction of phase mismatches, ensuring reliable optical performance even as environmental conditions change or minor disturbances occur during operation.
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 method ensures a sharp imaging result by maintaining spatial coherence and correct phase alignment of segmented mirrors, overcoming limitations in increasing numerical aperture and manufacturing challenges, and reducing the impact of wavefront jumps.
Implementation Method 1
recording a first partial interferogram between a wave reflected at a first mirror segment and a reference wave reflected at a reference surface; recording a second partial interferogram between a wave reflected at a second mirror segment and a reference wave reflected at the reference surface
Data Source
AI summary
A method for aligning a mirror of a microlithographic projection exposure apparatus, according to one formulation, involves: recording a first partial interferogram between a wave reflected at a first mirror segment (101) and a reference wave reflected at a reference surface (110, 310, 510), recording a second partial interferogram between a wave reflected at a second mirror segment (102) and a reference wave reflected at the reference surface, determining a phase offset between the first partial interferogram and the second partial interferogram, and aligning the first mirror segment and the second mirror segment in relation to one another in accordance with the determined phase offset, so that the distance of the relevant mirror segments (101, 102) from a respective predetermined, hypothetical surface in the direction of the respective surface normal is less than λ/10 at each point on the mirror segments, where λ denotes the operating wavelength of the mirror.