Self-Aligned Via-First Interconnects for Sub-40 nm Spacing
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Solution Overview
Problem
The formation of interconnect structures with metal lines and vias becomes challenging due to critical dimension uniformity and overlay control issues, especially at pitches smaller than 40 nm, leading to leakage and bridging problems in conventional trench-first and via-first processes.
Innovation Solution
A via-first process is employed where via patterns are formed in a first hard mask, followed by trench patterns in a second hard mask, allowing via holes to be self-aligned to the trenches, thereby maintaining consistent spacing and preventing leakage and bridging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If trench-first process is used, then trench patterns are formed before via patterns, but via-formation process window is limited due to surface topography and via-to-trench space window is adversely affected by via overlay shift
Solution Approach 1:
The patent inverts the conventional trench-first sequence by implementing a via-first process. Via patterns are formed in the first hard mask before trench patterns are formed in the second hard mask. This inversion eliminates the surface topography issues that limit the via-formation process window in trench-first approaches, as vias are formed on a planar surface rather than on top of pre-formed trenches.
Solution Approach 2:
The via patterns are formed as a preliminary action before trench formation. By completing via patterning first while the surface is still planar and accessible, the process avoids subsequent overlay shifts between via and trench formation. The via holes are self-aligned to trenches through the dual hard mask approach, eliminating the need for additional overlay control steps.
2Manufacturing precision
If via-first process is used, then via patterns are formed before trench patterns, but overlay shift of trench patterns from via patterns may cause leakage between metal lines
Solution Approach 1:
The patent introduces a second hard mask as an intermediary layer formed over the first hard mask. The via patterns are defined in the first hard mask, while trench patterns are defined in the second hard mask. This intermediary structure allows independent patterning of vias and trenches without direct overlay dependency, as the second hard mask serves as the active patterning layer for trenches while the first hard mask defines via locations. The etch selectivity between the two hard masks enables precise control of via-to-trench spacing.
Solution Approach 2:
The patterning process is segmented into two independent stages: via patterning in the first hard mask and trench patterning in the second hard mask. This segmentation separates the via formation step from the trench formation step, allowing each to be optimized independently. The via holes are formed through etching the first hard mask and underlying dielectric, while trenches are formed by etching the second hard mask and dielectric, with the spacing controlled by the thickness and pattern of the first hard mask remaining as an etch stop.
3Productivity
If pitch is reduced below 40 nm, then device density increases, but critical dimension uniformity control and overlay control become more difficult
Solution Approach 1:
The via-first process with dual hard masks creates a self-aligned structure where the via holes are automatically positioned relative to trenches. The first hard mask serves dual purposes: defining via patterns and acting as an etch stop layer that controls the spacing between vias and trenches. This self-service mechanism eliminates the need for additional overlay alignment steps, which become increasingly difficult at pitches below 40 nm, while maintaining consistent via-to-trench spacing.
4Length of moving object
If via size is reduced, then via pitch can be reduced, but via size reduction is limited due to optical effects and stochastic effects impact CD uniformity
Solution Approach 1:
The patent replaces the conventional approach of directly patterning vias on the dielectric surface with a dual hard mask system. Instead of relying solely on optical lithography resolution to define small via dimensions, the via size and position are defined by the first hard mask pattern, while the second hard mask defines trench positions. The etch selectivity between hard masks and the physical presence of the first hard mask as an etch stop provide mechanical control over via dimensions and spacing, reducing dependence on optical effects and stochastic variations.
Data Source
AI summary
A structure includes a dielectric layer, and a metal line in the dielectric layer. The metal line has a first straight edge and a second straight edge extending in a lengthwise direction of the metal line. The first straight edge and the second straight edge are parallel to each other. A via is underlying and joined to the metal line. The via has a third straight edge underlying and vertically aligned to the first straight edge, and a first curved edge and a second curved edge connecting to opposite ends of the third straight edge.