Ultra-small Hard Mask Pillars for MRAM via Pattern Transfer Molding
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Solution Overview
Problem
Current MRAM fabrication processes face challenges in patterning small dimension MTJ elements, leading to variability in resistance and switching current, which degrades MRAM performance, and require a thin Ta hard mask layer that is prone to photoresist pattern collapse and increased costs.
Innovation Solution
A novel method involving the formation of ultra-small VIA holes in a pattern transfer molding (PTM) layer, filling hard mask material into these holes, and using a bedding layer with an etching stop layer to create robust hard mask pillars, improving adhesion and reducing the need for a thick Ta layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thin Ta hard mask layer is used to match thin photoresist, then photoresist pattern collapse is reduced, but the hard mask pattern cannot be completely formed before photoresist consumption during etch transfer
Solution Approach 1:
The patent segments the hard mask formation process into two distinct stages: first forming a thin Ta seed layer (5-20 nm) for pattern definition, then adding a thicker Ta layer (50-200 nm) for complete pattern formation. This segmentation allows the photoresist to work with a thin layer while ensuring the final hard mask pattern is fully formed with sufficient thickness.
Solution Approach 2:
The patent performs preliminary action by first depositing the Ta seed layer and forming the photoresist pattern, then using this pattern to guide the formation of the additional Ta layer. This preliminary structuring ensures that the hard mask pattern is established before the photoresist is consumed during etching.
2Manufacturing precision
If a thick Ta hard mask layer is used to ensure complete pattern formation, then hard mask pattern formation is complete, but photoresist pattern collapse increases and rework costs increase
Solution Approach 1:
The patent divides the hard mask structure into a thin seed layer for pattern definition and a thicker additional layer for structural support and complete pattern formation. This segmentation allows optimization of each layer's thickness independently, reducing photoresist mask thickness requirements while ensuring complete pattern formation.
Solution Approach 2:
The patent changes the parameter of hard mask layer thickness from a single thick layer to a multi-layer structure with different thicknesses. The seed layer is thin (5-20 nm) to minimize photoresist burden, while the additional layer (50-200 nm) provides sufficient thickness for complete pattern formation without requiring proportionally thicker photoresist.
3Volume of moving object
If MTJ element dimensions are reduced for device miniaturization, then device size is reduced, but variability in MTJ resistance and switching current increases
Solution Approach 1:
The patent performs preliminary patterning of the Ta hard mask layer before MTJ element formation, creating precise templates that guide subsequent self-aligned etching processes. This preliminary structuring ensures uniform MTJ element dimensions and spacing, reducing variability in resistance and switching current.
Solution Approach 2:
The patent replaces traditional multi-step lithography and alignment processes with a self-aligned etching approach using the Ta hard mask pattern as a template. This substitution reduces alignment errors and dimensional variability, improving MTJ element uniformity at scaled dimensions.
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 allows for the formation of robust, ultra-small hard mask arrays with improved adhesion, reducing variability and maintaining high MRAM performance while minimizing the risk of photoresist pattern collapse and associated costs.
Implementation Method 1
there has been suggested a write method using spin momentum transfers or spin torque transfer (STT) switching technique
Implementation Method 2
The patterned wafer is then etched to form Ta hard mask pillars using the patterned photoresist as a mask
Data Source
AI summary
The invention disclosed a method to make an implanted hard mask with ultra-small dimensions for fabricating integrated nonvolatile random access memory. Instead of directly depositing hard mask material on top of the memory film stack element, we first make ultra-small VIA holes on a pattern transfer molding (PTM) layer using a reverse memory mask, then fill in the hard mask material into the VIA holes within the PTM material. Ultra-small hard mask pillars are formed after removing the PTM material. To improve the adhesion of the hard mask pillars with the underneath memory stack element, a hard mask sustaining element (HMSE) is added below PTM. Due to a better materials adhesion between HMSE and the hard mask, a stronger hard mask array can be formed.