Invisible Dummy Features for Overlay Mark Protection
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Solution Overview
Problem
Existing alignment structures in semiconductor fabrication are susceptible to damage during wafer planarization, leading to measurement errors due to mechanical polishing and micro-loading effects, which can cause asymmetrical overlay marks and image blurs, especially as device sizes shrink.
Innovation Solution
The introduction of 'invisible' sub-resolution dummy features around the overlay mark, which are small enough to avoid detection by alignment tools and reduce micro-loading effects, while maintaining pattern density uniformity and minimizing interference with optical measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional overlay marks are used during wafer planarization, then alignment function is provided, but the overlay marks are susceptible to damage from mechanical polishing causing measurement errors
Solution Approach 1:
The patent introduces dummy features as intermediary elements surrounding the overlay mark. These dummy features act as a buffer that absorbs mechanical polishing stress and prevents direct damage to the overlay mark, while being optically invisible to alignment detection tools. This mediator structure protects the critical overlay mark from harmful mechanical effects during CMP processes.
Solution Approach 2:
The patent applies different properties to different regions: the overlay mark region is kept optically visible for alignment detection, while the surrounding dummy feature regions are designed to be optically invisible (sub-resolution) but mechanically present for protection. This local differentiation allows the overlay mark to fulfill its measurement function while being protected by the dummy features that have different optical and mechanical properties.
2Manufacturing precision
If overlay mark pattern is asymmetrical due to film uniformity control and mechanical polishing loading effect, then alignment structure is formed, but relatively large measurement errors are induced
Solution Approach 1:
The dummy features are strategically placed in specific locations around the overlay mark based on anticipated polishing loading patterns. By concentrating dummy features in high-stress regions, the patent locally compensates for expected asymmetrical deformation, maintaining overall pattern symmetry and reducing measurement errors caused by polishing-induced asymmetry.
Solution Approach 2:
The dummy features are pre-positioned around the overlay mark before polishing occurs. These pre-positioned features create a counterbalancing effect that anticipates and prevents asymmetrical deformation during the polishing process, thereby maintaining pattern symmetry and minimizing measurement errors before they can occur.
3Manufacturing precision
If dummy features are added around overlay mark to reduce micro-loading effects, then pattern density uniformity is improved, but alignment detection complexity increases
Solution Approach 1:
The patent changes the dimensional parameters of the dummy features to be below the resolution threshold of alignment detection tools. By making the dummy features sub-resolution (smaller than the tool's minimum detectable size), they become effectively invisible to the alignment system, thus adding pattern density uniformity without increasing the complexity detected by alignment tools.
Solution Approach 2:
The dummy features serve as an intermediary layer between the overlay mark and the polishing process. They provide the necessary mechanical support and pattern density uniformity while remaining optically transparent to alignment detection, thus mediating between the conflicting requirements of mechanical protection and optical detectability without adding detection complexity.
Data Source
AI summary
An overlay mark is formed over a substrate. A plurality of first dummy features is formed outside the overlay mark in a top view. A plurality of second dummy features is formed closer to the overlay mark than the first dummy features in a top view. The first dummy features are sufficiently big to be visible to an optical machine used to scan the overlay mark. The second dummy features are sufficiently small to be invisible to the optical machine.