MRAM MTJ Etching via Protective Oxide to Prevent Electrical Shorting
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The patterning of magnetic tunneling junction (MTJ) stacks in MRAM devices is hindered by electrical shorting due to imprecise etching, which results in shorting pathways and re-deposition of etching residues, particularly at the edges of the cells, leading to high failure rates and resistance variations.
Innovation Solution
A method involving a multilayer MTJ stack structure with specific layers and an RIE process using CF4 and oxygen etching, followed by a CH3OH etch, to precisely pattern the MTJ cells, eliminating shorting and re-deposition by forming a protective oxide layer and ensuring etching selectivity, thereby preventing footings and short-circuit paths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional RIE etching is used to pattern MTJ stacks, then manufacturing process simplicity is maintained, but electrical shorting occurs due to imprecise etching and re-deposition of residues
Solution Approach 1:
The etching process is divided into multiple sequential steps: CF4-based RIE etching followed by CH3OH-based RIE etching. Each step targets specific layers with different selectivities, allowing precise removal of materials without damaging underlying layers. This segmentation enables controlled patterning while preventing electrical shorting.
Solution Approach 2:
The patent introduces an intermediary etching step using CH3OH gas that selectively removes etching residues and redeposited materials between the main etching steps. This intermediary process acts as a cleaning mediator that prevents shorting pathways from forming, bridging the gap between rough etching and final precision patterning.
2Manufacturing precision
If etching is performed to pattern MTJ cells, then cell formation is achieved, but electrical shorting pathways form at cell edges due to unetched footing and re-deposition
Solution Approach 1:
The patent changes etching parameters by using different gases (CF4 vs. CH3OH) with different chemical selectivities. The CF4 etching provides main material removal while the CH3OH etching provides selective cleaning of residues. This parameter change approach allows the same etching tool to perform both roughing and cleaning functions, preventing footing and re-deposition issues.
Solution Approach 2:
The etching process is designed as a continuous sequence where CF4 etching removes material followed immediately by CH3OH etching that cleans residues. This continuous action ensures that no shorting pathways have time to form, maintaining both patterning precision and electrical isolation throughout the process.
3Manufacturing precision
If precise control of MTJ cell size and shape is implemented, then magnetic and switching properties are improved, but etching process complexity increases
Solution Approach 1:
The patterning process is segmented into material removal phase (CF4 etching) and surface cleaning phase (CH3OH etching). This segmentation allows independent optimization of each phase: the first achieves dimensional control while the second ensures clean edges. The result is precise cell dimensions without requiring overly complex single-step etching processes.
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
The method significantly reduces electrical shorting and resistance variations, achieving lower failure rates and more consistent sensor resistances across MRAM cells, suitable for mass production.
Implementation Method 1
in an RIE chamber apply a CF4 gas hard mask etch to etch unprotected lateral portions of the Ta hard mask layer
Implementation Method 2
apply a CF4 gas hard mask etch to etch unprotected lateral portions
Implementation Method 3
apply the first step of a continuous high power oxygen process at a 200 W-500 W power level to clean off remaining photoresist and BARC
Implementation Method 4
continue the high powered oxygen process, for between 110 s and 150 s, at a higher power level of between 400 W and 600 W to further etch away the exposed NiFeHf capping layer, the NiFe free layer, the AlOx barrier layer, then stopping approximately midway through the contiguous CoFe pinned layer to make certain that the AlOx barrier layer has been completely etched through, leaving an oxide layer surrounding all exposed surfaces
Implementation Method 5
in another chamber, etch the remaining unetched layers using a CH3 OH etchant under high bias power
Data Source
AI summary
An MTJ cell without footings and free from electrical short-circuits across a tunneling barrier layer is formed by using a Ta hard mask layer and a combination of etches. A first etch patterns the Ta hard mask, while a second etch uses O2 applied in a single high power process at two successive different power levels. A first power level of between approximately 200 W and 500 W removes BARC, photoresist and Ta residue from the first etch, the second power level, between approximately 400 W and 600 W continues an etch of the stack layers and forms a protective oxide around the etched sides of the stack. Finally, an etch using a carbon, hydrogen and oxygen gas completes the etch while the oxide layer protects the cell from short-circuits across the lateral edges of the barrier layer.