Self-Aligned MTJ Pillar and SOT-Line Fabrication
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Solution Overview
Problem
The challenge in fabricating magnetic tunneling junction (MTJ) devices is the difficulty in aligning the spin-orbit torque (SOT) layer with the MTJ pillar during scaling, leading to current circumvention and reduced SOT-effect due to the SOT-layer being larger than the MTJ pillar, which affects the switching efficiency.
Innovation Solution
A method is developed to form self-aligned MTJ and SOT layers by using a multi-step etching process with mask lines and sidewall spacers, allowing the MTJ pillar and SOT-line to be coextensive, thereby controlling their dimensions and reducing current circumvention, while enabling flexible angle selection for different shapes and orientations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the SOT-layer is made larger than the cross-section of the MTJ pillar to ensure contact, then alignment between SOT-layer and MTJ pillar is improved, but current circumvention occurs and SOT-effect is reduced
Solution Approach 1:
The patent transitions from conventional planar patterning to three-dimensional self-aligned patterning using sidewall spacers. The first etch mask defines initial patterns, then sidewall spacers are formed on the sides of etched structures to create vertically extended alignment references. This dimensional transition enables the SOT-layer and MTJ pillar to be precisely aligned through vertical spacer structures rather than relying solely on lateral mask alignment, eliminating current circumvention while maintaining contact.
Solution Approach 2:
The sidewall spacers serve as self-aligned structures that automatically position the SOT-layer relative to the MTJ pillar. Instead of requiring separate alignment processes, the spacers form conformally on the etched structures and inherently define the precise lateral position of subsequent layers. This self-service mechanism ensures perfect alignment without additional alignment steps or oversized layers, preventing current circumvention while guaranteeing contact.
2Ease of manufacture
If conventional independent patterning of SOT-layer and MTJ pillar is used, then manufacturing process is simpler, but alignment precision deteriorates
Solution Approach 1:
The patent merges the patterning of the SOT-layer and MTJ pillar into a single self-aligned process sequence. Instead of independently patterning each layer with separate lithography and etch steps, the method combines them by using sidewall spacers formed from the first etch mask patterns as the alignment reference for the second etch mask. This merging of patterning operations maintains process efficiency while achieving superior alignment precision through the physical connection established by the spacers.
Solution Approach 2:
The first etch mask and subsequent sidewall spacer formation are performed as preliminary actions that pre-establish the alignment geometry before the final SOT-layer patterning. The sidewall spacers are formed in advance on the etched structures, creating permanent alignment references that guide the positioning of the second etch mask and subsequent SOT-layer deposition. This preliminary structuring ensures precise alignment is built into the process flow rather than relying on final-step alignment adjustments.
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 approach ensures effective alignment and reduced current circumvention, enhancing the SOT-effect and switching efficiency of the MTJ devices, allowing for precise control over the dimensions and shapes of the MTJ pillars and SOT-lines, which can improve the performance of MTJ devices in memory technologies like MRAM.
Implementation Method 1
The free layer magnetization direction may alternatively be controlled by a spin-orbit torque (SOT) effect wherein the magnetization direction is changed by passing a current through an additional SOT-generating layer arranged below the MTJ layer structure.
Implementation Method 2
The free layer magnetization direction may be controlled by a spin-torque transfer (STT) effect, wherein the magnetization direction is changed by passing a relatively high current through the MTJ layer structure, parallel to the stacking direction of the layers.
Data Source
Figure 1a~1f
Figure 2a~2b
Figure 3
AI summary
According to an aspect there is provided a method for forming an MTJ device, comprising: forming a layer stack comprising an MTJ layer structure and a spin-orbit torque layer, SOT-layer, below the MTJ layer structure; forming a first etch mask over the layer stack, the first etch mask comprising a first mask line extending in a first horizontal direction; patterning the layer stack to form an MTJ line extending in the first horizontal direction, the patterning comprising etching while the first etch mask masks the layer stack, and stopping etching on or above the SOT-layer; forming sidewall spacers on either side of the MTJ line; while the sidewall spacers mask the SOT-layer, etching the SOT-layer to form a patterned layer stack comprising the MTJ line and a first patterned SOT-layer; forming a second etch mask over the patterned layer stack, the second etch mask comprising a second mask line extending in a second horizontal direction across the MTJ line; and patterning the patterned layer stack to form a twice patterned SOT-layer, the twice patterned SOT-layer comprising an SOT-line extending in the second horizontal direction, and to form an MTJ pillar on the SOT-line, the patterning comprising etching while the second etch mask masks the patterned layer stack.