MRAM MTJ Cell Etching Hard Mask and Two-Phase Process
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Solution Overview
Problem
The existing etching processes for fabricating Magnetic Tunnel Junction (MTJ) cells in MRAM face issues such as surface passivation of the top electrode, top electrode erosion, critical dimension control, and re-deposition of materials during the etching cycle, which affect the uniformity and functionality of the memory cells.
Innovation Solution
The proposed solution involves using a hard mask buffer layer, a multilayered etching hard mask structure, and a two-phase etching process to prevent surface passivation, maintain electrode thickness, and remove re-deposited materials, respectively. The buffer layer is made of silicon nitride or silicon carbide, and the multilayered hard mask structure includes materials like borazinic film and aluminum oxide for improved selectivity, while the two-phase etching process uses carbonaceous reactive ion etching followed by inert gas and oxygen plasma to achieve a vertical sidewall profile.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional hard mask materials (SiO2, Si3N4) are used for MTJ cell fabrication, then the etching process can proceed, but the hard mask is severely eroded and may be totally removed before completing the etching cycle, causing top electrode erosion
Solution Approach 1:
The patent applies composite materials by combining multiple hard mask layers with different properties: a first hard mask layer (SiO2 or Si3N4) provides initial protection, while a second hard mask layer (borazinic film or aluminum oxide) with higher etch selectivity and erosion resistance is deposited on top. This composite structure maintains adequate hard mask thickness throughout the etching cycle, preventing top electrode erosion while enabling complete etching of the MTJ cell pedestal.
2Manufacturing precision
If the hard mask is thickened to maintain sufficient top electrode thickness, then electrode protection is improved, but thick photoresist is required which results in tapered sidewall shape
Solution Approach 1:
The patent segments the hard mask structure into multiple thin layers instead of using a single thick layer. The first hard mask layer (5-20 nm) and second hard mask layer (5-20 nm) are deposited sequentially, each providing protection while maintaining a thin overall profile. This segmented approach prevents the need for thick photoresist, thereby avoiding tapered sidewall formation while still providing adequate protection to the top electrode.
3Shape
If carbonaceous reactive ion etching is used, then a good profile of MTJ is achieved, but undesirable polymers are produced and re-deposition occurs which degrades functionality
Solution Approach 1:
The patent introduces an intermediary cleaning step using oxygen plasma or vapor HF between the carbonaceous reactive ion etching process and subsequent processing. This intermediary step removes polymer re-deposition and carbonaceous contaminants from the MTJ sidewalls and surfaces, preventing functionality degradation while preserving the good profile achieved by the carbonaceous etching process.
4Object-generated harmful factors
If ion milling is used to produce no carbonaceous polymer, then polymer-free etching is achieved, but MTJ materials are re-sputtered on the sidewall which shorts the MTJ
Solution Approach 1:
The patent introduces an intermediary cleaning step using oxygen plasma or vapor HF after ion milling to remove re-sputtered MTJ materials from the sidewalls. This intermediary treatment prevents shorting of the MTJ while preserving the polymer-free advantage of ion milling, thereby maintaining both cleanliness and functionality.
5Productivity
If the etching process removes the hard mask completely, then the etching cycle can be completed, but the previously masked top electrode area is exposed to etching ambient causing erosion
Solution Approach 1:
The patent uses composite hard mask materials with different erosion resistances: the first hard mask layer (SiO2 or Si3N4) and second hard mask layer (borazinic film or aluminum oxide) work together to maintain adequate thickness throughout the etching cycle. This composite structure allows complete removal of sacrificial layers while preserving the hard mask sufficiently long to prevent top electrode erosion, enabling both cycle completion and dimensional control.
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 solutions enhance the etching process by preventing surface passivation, maintaining electrode thickness, and achieving a straight sidewall profile, thereby improving the fabrication of MTJ cells and ensuring reliable interconnects and memory functionality.
Implementation Method 1
carbonaceous reactive ion etching provides a good profile of MTJ
Implementation Method 2
ion milling produces no carbonaceous polymer
Implementation Method 3
inert gas and oxygen plasma to achieve a vertical sidewall profile
Implementation Method 4
The insulating film includes cross-linked borazine skeletal molecules
Data Source
AI summary
Various embodiments of the invention relate to etching processes used in fabrication of MTJ cells in an MRAM device. The various embodiments can be used in combination with each other. The first embodiment adds a hard mask buffer layer between a hard mask and a top electrode. The second embodiment uses a multilayered etching hard mask. The third embodiment uses a multilayered top electrode structure including a first Cu layer under a second layer such as Ta. The fourth embodiment is a two-phase etching process used for the bottom electrode to remove re-deposited material while maintaining a more vertical sidewall etching profile. In the first phase the bottom electrode layer is removed using carbonaceous reactive ion etching until the endpoint. In the second phase an inert gas and/or oxygen plasma is used to remove the polymer that was deposited during the previous etching processes.