MTJ Etching Uniformity via Ta Hard Mask Passivation
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Solution Overview
Problem
Existing etching methods for magnetic tunneling junction (MTJ) structures, particularly those using tantalum (Ta) hard masks, face challenges in achieving smooth sidewalls and uniformity due to fencing issues and limitations in Ta thickness caused by the use of dielectric hard masks like SiO2, which result in rough sidewalls and poor uniformity.
Innovation Solution
A method involving a Ta/PR etching mask stack with a passivation step using O2, H2O vapor, or air to form a smooth metal hard mask, allowing the Ta layer to remain as a top electrode without increasing the photoresist thickness, thereby improving etching profile and uniformity by reducing pattern transfer steps and avoiding SiO2 striation effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If SiO2 hard mask is used with high SiO2/PR selectivity chemical etching (e.g., C4F8), then etching selectivity is improved, but severe sidewall striations are generated which transfer to Ta and MTJ sidewalls leading to rough sidewalls and poor uniformity
Solution Approach 1:
The patent changes the etching chemistry from fluorine-based (C4F8) to oxygen-based (CF4), which fundamentally alters the etching mechanism. This parameter change reduces sidewall striation while maintaining adequate selectivity, directly resolving the contradiction between selectivity and sidewall smoothness
Solution Approach 2:
The patent introduces a TaN layer as an intermediary between the SiO2 hard mask and the Ta layer. This intermediate layer acts as a buffer that prevents the transfer of sidewall striations from the SiO2 etching process to the Ta layer, thereby maintaining smooth Ta sidewalls while still benefiting from the high selectivity of SiO2 etching
2Shape
If CF4 etching is used for SiO2, then sidewall striation is reduced, but poor SiO2/PR selectivity limits the thickness of SiO2 and Ta that can be etched
Solution Approach 1:
The patent modifies the etching parameters by using CF4 with adjusted power settings and gas flow ratios. By optimizing these parameters, the patent achieves a balance where sidewall striation is minimized while maintaining sufficient etching selectivity to etch thicker SiO2 and Ta layers
Solution Approach 2:
The TaN intermediate layer enables the use of CF4 etching by providing a protective barrier. This intermediary allows the SiO2 to be etched with CF4 (achieving smooth sidewalls) while the TaN layer protects the underlying Ta from excessive etching, effectively decoupling the selectivity requirement from the sidewall smoothness requirement
3Device complexity
If photoresist is used directly to pattern Ta, then process steps are reduced, but fencing issues occur
Solution Approach 1:
The patent introduces TaN as an intermediary hard mask layer between the photoresist and the Ta layer. This intermediate layer serves as a protective barrier during photoresist removal, preventing the fencing effect from occurring while still allowing the photoresist to effectively define the pattern
Solution Approach 2:
The patent performs preliminary etching of the Ta layer through the photoresist mask before complete photoresist removal. This preliminary action creates a protective Ta hard mask that prevents fencing during subsequent photoresist strip, thereby maintaining pattern fidelity while using a simplified direct-photoresist-patterning approach
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 smooth sidewalls and uniform MTJ devices with the ability to etch a thicker Ta layer using the same PR thickness, enhancing etching uniformity and profile within wafers and chips, and allowing for a larger CMP process window.
Implementation Method 1
The metal hard mask is passivated during or after etching to form a smooth hard mask profile
Implementation Method 2
The electrode layer is etched away where it is not covered by the photoresist mask to form a metal hard mask
Data Source
AI summary
A hard mask stack for etching a magnetic tunneling junction (MTJ) structure is described. An electrode layer is deposited on a stack of MTJ layers on a bottom electrode. A photoresist mask is formed on the electrode layer. The electrode layer is etched away where it is not covered by the photoresist mask to form a metal hard mask. The metal hard mask is passivated during or after etching to form a smooth hard mask profile. Thereafter, the photoresist mask is removed and the MTJ structure is etched using the metal hard mask wherein the metal hard mask remaining acts as a top electrode. The resulting MTJ device has smooth sidewalls and uniform device shape.