Composite Hard Mask for MRAM Etch Residue Prevention
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Solution Overview
Problem
Current MRAM fabrication processes face challenges with metal etch residue buildup during etching, leading to irregular MTJ shapes and electrical shorting issues, particularly when using Ta as a hard mask, which affects device performance and limits the manufacturing of high-density ICs.
Innovation Solution
A composite hard mask design comprising a thin non-magnetic metal spacer, a middle conductive layer with high etch selectivity, and an upper sacrificial dielectric layer is employed to prevent metal etch residue and ensure uniform MTJ shape integrity, while maintaining sufficient thickness to prevent electrical shorting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If Ta is used as a hard mask for etching, then etching can be performed, but metal etch residue builds up during the etch transfer step
Solution Approach 1:
The hard mask is divided into multiple layers: a lower Ta layer for etching capability, a middle sacrificial dielectric layer to prevent metal etch residue, and an upper conductive layer for pattern transfer. This segmentation allows each layer to perform its specific function while preventing the harmful effects of metal etch residue buildup.
Solution Approach 2:
The sacrificial dielectric layer acts as an intermediary between the lower Ta layer and the upper conductive layer. It prevents direct contact between the metal etch residue and the pattern transfer layer, thereby eliminating the harmful effect of metal etch residue while maintaining the etching capability of the Ta layer.
2Manufacturing precision
If the hard mask thickness is reduced, then pattern resolution improves, but electrical shorting occurs
Solution Approach 1:
The hard mask thickness is segmented into multiple functional layers with different thicknesses. The lower Ta layer can be thin for good pattern resolution, while the middle sacrificial dielectric layer provides additional thickness for electrical isolation, and the upper conductive layer maintains pattern fidelity. This segmentation allows achieving both thin overall thickness for resolution and sufficient isolation thickness for preventing shorting.
Solution Approach 2:
Different layers of the hard mask have different local qualities: the lower Ta layer is optimized for etching and can be thin, the middle sacrificial dielectric layer is optimized for electrical isolation and has higher dielectric strength, and the upper conductive layer is optimized for pattern transfer. This local quality differentiation allows each layer to contribute optimally to its specific function while collectively achieving both thinness and electrical isolation.
3Device complexity
If a single layer hard mask is used, then process complexity is reduced, but metal etch residue cannot be prevented
Solution Approach 1:
The hard mask is segmented into three distinct layers, each with a specific function. The lower Ta layer provides etching capability, the middle sacrificial dielectric layer prevents metal etch residue, and the upper conductive layer enables pattern transfer. This segmentation resolves the contradiction by distributing different functions across multiple layers, allowing metal etch residue prevention while maintaining reasonable process complexity.
Solution Approach 2:
The hard mask uses composite material structure combining Ta (for etching), sacrificial dielectric material (for residue prevention), and conductive material (for pattern transfer). This composite approach allows leveraging the advantages of different materials to simultaneously achieve etching capability, prevent metal etch residue, and maintain pattern fidelity.
Data Source
AI summary
A composite hard mask is disclosed that prevents build up of metal etch residue in a MRAM device during etch processes that define an MTJ shape. As a result, MTJ shape integrity is substantially improved. The hard mask has a lower non-magnetic spacer, a middle conductive layer, and an upper sacrificial dielectric layer. The non-magnetic spacer serves as an etch stop during a pattern transfer with fluorocarbon plasma through the conductive layer. A photoresist pattern is transferred through the dielectric layer with a first fluorocarbon etch. Then the photoresist is removed and a second fluorocarbon etch transfers the pattern through the conductive layer. The dielectric layer protects the top surface of the conductive layer during the second fluorocarbon etch and during a substantial portion of a third RIE step with a gas comprised of C, H, and O that transfers the pattern through the underlying MTJ layers.