Bottom Electrode Surface Roughness Control for Magnetic Tunnel Junctions
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Solution Overview
Problem
The damascene process for forming magnetic tunnel junctions results in surface roughness issues at the interface of the oxide and bottom electrode, leading to performance detriments and requiring thicker touch-up layers, which complicates etching and necessitates larger MTJ sizes to avoid exposure of the bottom electrode material.
Innovation Solution
A process sequence involving bulk layer deposition, patterning, and chemical mechanical polishing (CMP) to achieve an ultra-smooth bottom electrode surface with a bottom electrode buff layer, allowing for precise control and reduced surface roughness, enabling the deposition of magnetic tunnel junctions with sizes that can be less than, equal to, or greater than the bottom electrode pad size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the damascene process is used to form bottom electrodes, then the bottom electrode can be formed over bottom vias, but a step is formed at the interface of the oxide and bottom electrode during CMP polishing, resulting in surface roughness that deteriorates MTJ performance
Solution Approach 1:
A bottom electrode touch-up layer is deposited over the bottom electrode before the final CMP polishing step. This preliminary layer serves as a sacrificial material that fills in surface irregularities and provides a uniform surface for subsequent MTJ deposition, eliminating the harmful step formation at the oxide-bottom electrode interface
Solution Approach 2:
The bottom electrode touch-up layer acts as an intermediary between the bottom electrode and the MTJ structure. This intermediate layer absorbs the surface roughness issues and provides a smooth interface for MTJ deposition, resolving the contradiction between achieving smooth surfaces and managing process complexity
2Manufacturing precision
If a thicker bottom electrode touch-up layer is deposited to accommodate CMP variations, then surface smoothness is improved, but the etching process becomes more difficult and MTJ size must be increased to avoid exposure of bottom electrode material
Solution Approach 1:
The thickness of the bottom electrode touch-up layer is precisely controlled within a narrow range (5-15 nm) rather than using a thicker layer. This parameter optimization allows sufficient surface smoothness for MTJ deposition while maintaining ease of etching and preventing exposure of the bottom electrode material during MTJ fabrication
3Reliability
If the bottom electrode touch-up layer thickness is increased to ensure complete coverage, then surface coverage is improved, but the MTJ size must be enlarged to prevent excessive exposure of bottom electrode material during etching
Solution Approach 1:
The bottom electrode touch-up layer thickness is optimized to 5-15 nm, which provides complete coverage and reliability for MTJ deposition without requiring larger MTJ dimensions. This precise parameter control ensures that the MTJ can be etched completely through the touch-up layer without exposing the bottom electrode material, maintaining both reliability and compact device size
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 achieves a surface roughness of 0.2 nm or less, improving MTJ performance with higher coercivity and tunnel magnetoresistance, and allows for flexible MTJ size configurations by ensuring a smooth bottom electrode surface, enhancing device performance and manufacturing efficiency.
Implementation Method 1
Chemical Mechanical Polishing (CMP) is used to polish the upper surface of the bottom electrodes back to the upper surface of the oxide
Implementation Method 2
bottom electrode pad through bulk layer deposition
Implementation Method 3
Oxide is deposited over the bottom vias
Data Source
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AI summary
A process sequence is provided to provide an ultra-smooth (0.2 nm or less) bottom electrode surface for depositing magnetic tunnel junctions thereon. In one embodiment, the sequence includes forming a bottom electrode pad through bulk layer deposition followed by patterning and etching. Oxide is then deposited over the formed bottom electrode pads and polished back to expose the bottom electrode pads. A bottom electrode buff layer is then deposited thereover following a pre-clean operation. The bottom electrode buff layer is then exposed to a CMP process to improve surface roughness. An MTJ deposition is then performed over the bottom electrode buff layer.