Multi-Level Memory Cell Using Varying MTJ Thickness
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Solution Overview
Problem
Multi-Level Memory Cells (MLCs) with multiple Magnetic Tunnel Junction (MTJ) structures face high manufacturing costs due to the need for multiple masks and processes to fabricate MTJs with different areas, limiting their scalability and memory density.
Innovation Solution
The use of MTJ structures with varying thicknesses for the tunnel barriers and free layers, allowing for a single mask to pattern all MTJ structures and enabling serial connection in a vertical stack, which reduces area occupancy and fabrication costs while maintaining four possible states for increased memory density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If multiple masks and processes are used to fabricate MTJs with different areas, then MTJ structures with different resistance values can be achieved, but manufacturing cost increases
Solution Approach 1:
The patent applies local quality by varying the thickness of specific layers (tunnel barrier layer and/or free layer) in different MTJ structures while keeping the planar area identical. This allows each MTJ to have different resistance characteristics through localized thickness modification rather than requiring different areas, thereby achieving resistance differentiation without needing multiple masks for area variation.
Solution Approach 2:
The patent changes the physical parameter of layer thickness to differentiate MTJ resistance values. By controlling the thickness of the tunnel barrier layer and/or free layer during fabrication, the patent achieves different resistance characteristics for different MTJs using a single mask pattern, thus avoiding the need for multiple masking processes while maintaining precise resistance control.
2Adaptability or versatility
If multiple masks are used to pattern MTJs with different areas, then diverse MTJ characteristics are achieved, but mask cost increases
Solution Approach 1:
The patent uses local quality by maintaining identical planar areas for all MTJ structures while introducing local variations in layer thickness. This allows diverse MTJ characteristics to be achieved through localized thickness control in the vertical dimension rather than through area variation in the planar dimension, enabling single-mask fabrication with multiple MTJ types.
Solution Approach 2:
The patent transitions from varying MTJ characteristics in the planar dimension (different areas) to varying them in the vertical dimension (different thicknesses). This dimensional shift allows all MTJs to be patterned with the same mask while achieving different resistance values through controlled thickness variations of specific layers.
3Adaptability or versatility
If MTJ structures with different areas are fabricated, then four states can be achieved, but area occupancy increases
Solution Approach 1:
The patent applies local quality by using identical planar areas for all MTJ structures while creating local variations in layer thickness to achieve different resistance states. This allows four memory states to be encoded through thickness variations rather than area variations, minimizing the required die area while maintaining full functionality.
Solution Approach 2:
The patent moves the differentiation of MTJ states from the planar dimension to the vertical dimension. Instead of using different areas to encode different states, the patent uses different layer thicknesses, thereby achieving the same four-state functionality in a compact area footprint.
4Ease of manufacture
If MTJ structures with varying thicknesses are used, then single mask fabrication is enabled, but manufacturing precision requirements increase
Solution Approach 1:
The patent changes the controlled parameter from planar area to layer thickness. By using standard thin-film deposition techniques with controlled thickness parameters, the patent achieves precise resistance differentiation. The thickness parameter can be accurately controlled during fabrication using conventional semiconductor manufacturing equipment, making the process both simple and precise.
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 reduces die costs and increases memory density by allowing low-voltage write operations without additional circuits, while minimizing mask costs and enabling efficient switching between states, thus enhancing the cost-effectiveness and performance of MLCs.
Implementation Method 1
The tunneling current depends on the relative orientation of the free layer 12 and the pinned layer 14. When using a Spin-Transfer Torque (STT) MTJ, the difference in the tunneling current as the spin alignment of the free layer 12 and the pinned layer 14 is switched between parallel (P) and anti-parallel (AP) states is known as the Tunnel Magnetoresistance Ratio (TMR).
Implementation Method 2
The free layer 12 and the pinned layer 14 can store information even when the magnetic H-field is '0' due to a hysteresis loop 18 of the MTJ 10.
Implementation Method 3
When using a Spin-Transfer Torque (STT) MTJ, the difference in the tunneling current as the spin alignment of the free layer 12 and the pinned layer 14 is switched between parallel (P) and anti-parallel (AP) states
Data Source
AI summary
A Multi-Level Memory Cell (MLC) using multiple Magnetic Tunnel Junction (MTJ) structures having one or more layers with varying thickness is disclosed. The multiple MTJ structures, which are vertically stacked and arranged in series, may have substantially identical area dimensions to minimize fabrication costs because one mask can be used to pattern the multiple MTJ structures. Further, varying the thicknesses associated with the one or more layers may provide the multiple MTJ structures with different switching current densities and thereby increase memory density and improve read and write operations. In one embodiment, the layers with the varying thicknesses may include tunnel barriers or magnesium oxide layers associated with the multiple MTJ structures and/or free layers associated with the multiple MTJ structures.