EUV Lithography Mask Phase Shifting Resolution
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Solution Overview
Problem
Current extreme ultraviolet lithography (EUVL) techniques face limitations in achieving high resolution due to the smoothing of step heights in multilayer structures, which affects the transition area between phase-shifting and non-phase-shifting regions, limiting the achievable resolution and contrast for future nodes with minimum pitches like 32 nm and 22 nm.
Innovation Solution
The EUV lithography process employs a reflective mask with a low thermal expansion material substrate, a reflective multilayer, and an absorption stack, utilizing three states with specific reflection coefficients assigned to adjacent polygons and a field, and is exposed using nearly on-axis illumination with partial coherence less than 0.3 to produce balanced diffracted and non-diffracted lights, maximizing interference and contrast.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a multilayer structure is formed over a step to create a phase-shifting region, then the phase-shifting function is achieved, but the step height is smoothed out leading to a large transition area and reduced resolution
Solution Approach 1:
The patent extracts the phase-shifting function from the multilayer structure by using a dedicated phase-shifting layer with specific material properties (low absorption, high reflectivity) that maintains the step height without smoothing. This separates the phase-shifting function from the structural support function, allowing the step to retain its sharp transitions while still providing the required phase shift.
Solution Approach 2:
The patent applies different material properties to different regions of the mask. The phase-shifting regions use materials with specific optical properties (low absorption coefficient, high reflectivity) while other regions use different materials. This local differentiation allows the step height to be maintained in phase-shifting regions while avoiding the smoothing effect in non-phase-shifting regions.
2Ease of manufacture
If binary intensity masks with on-axis illumination are used, then the lithography process is simple, but adequate aerial image contrast is not achieved for future nodes
Solution Approach 1:
The patent segments the mask into multiple functional regions with different optical properties: phase-shifting regions, absorber regions, and transparent regions. This segmentation allows complex intensity patterns to be created through the interference of diffracted light from different regions, achieving high aerial image contrast while maintaining manufacturing simplicity through standardized region definitions.
Solution Approach 2:
The patent changes the optical parameters of the mask regions by using materials with different absorption coefficients and reflectivity values. By adjusting these parameters in different regions, the mask creates the desired interference patterns and aerial image contrast for future lithography nodes.
3Measurement precision
If phase-shifting techniques are applied to achieve resolution enhancement, then the minimum pitch can be reduced, but the transition area between phase-shifting and non-phase-shifting regions increases
Solution Approach 1:
The patent introduces an intermediary phase-shifting layer that mediates between the step structure and the optical field. This layer has specific material properties that allow it to provide phase shifting without creating a large transition area, effectively coupling the structural step with the optical function while maintaining sharp transitions.
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 approach enhances aerial image contrast and depth of focus, achieving resolution enhancement for future nodes and improving the lithography process by utilizing balanced diffracted orders to expose semiconductor wafers effectively.
Implementation Method 1
a reflective mask with a low thermal expansion material substrate, a reflective multilayer
Implementation Method 2
an absorption stack, utilizing three states with specific reflection coefficients
Implementation Method 3
exposed using nearly on-axis illumination with partial coherence less than 0.3 to produce balanced diffracted and non-diffracted lights
Implementation Method 4
maximizing interference and contrast
Data Source
AI summary
An EUV mask includes a low thermal expansion material (LTEM) substrate, a reflective multilayer (ML) above one surface of the LTEM substrate, and a conductive layer above an opposite surface of the LTEM substrate. A capping layer is provided above the reflective ML, a buffer layer is provided above the capping layer, and an absorption stack is provided above the buffer layer. The absorption stack comprises multiple layers. A multiple patterning process is performed on the absorption stack to form multiple reflective states.