MTJ Etch Selectivity via Noble Gas Oxidant Ratios
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Solution Overview
Problem
Current etching processes for MTJ nanopillars face challenges in achieving high etch selectivity between the hard mask and underlying MTJ layers, leading to excessive hard mask thickness, chemical damage, and redeposition of materials on sidewalls, which complicates the formation of high-density arrays with critical dimensions below 100 nm.
Innovation Solution
A reactive ion etch process using a combination of noble gases and oxidants, such as Ar and methanol, is employed, with a main etch step followed by an over etch step and an optional ion beam etch cleaning step, to minimize chemical damage and redeposition while enhancing etch selectivity, thereby allowing for a thinner hard mask and improved device yields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If RIE with pure Ar plasma or IBE is used, then chemical damage along MTJ sidewalls is avoided, but etch selectivity between hard mask and MTJ layers is poor requiring a hard mask up to 50% thicker
Solution Approach 1:
The patent changes the chemical composition parameters of the etching plasma by introducing oxidants (O2, N2O, NF3, CF4, or mixtures) to the noble gas plasma. This modifies the plasma chemistry to achieve both minimal chemical damage and improved etch selectivity. The oxidant concentration is controlled at specific ranges (e.g., 1-10% O2 in Ar, or 5-20% NF3 in Ar) to optimize the balance between sidewall protection and selective etching.
Solution Approach 2:
The patent uses composite plasma compositions combining noble gases (Ar, Kr, Xe) with oxidant gases (O2, N2O, NF3, CF4). This composite approach leverages the physical sputtering capability of noble gases to minimize chemical damage while the oxidant components enhance etch selectivity through selective oxidation of MTJ layer materials, achieving both benefits simultaneously.
2Object-affected harmful factors
If IBE or RIE based only on noble gas is used, then physical etching occurs without chemical damage, but substantial redeposition of hard mask and bottom electrode metals on MTJ sidewalls occurs causing shorts and low yields
Solution Approach 1:
The patent modifies the plasma chemical environment by adding oxidants to the noble gas, which changes the surface chemistry during etching. The oxidized surface conditions reduce metal redeposition by forming volatile metal oxides that are removed during the etch process, while maintaining the physical sputtering mechanism that minimizes chemical damage to the sidewalls.
3Object-generated harmful factors
If RIE with oxidants such as methanol is used, then MTJ sidewalls are substantially free of residue, but chemical and plasma damage occurs on MTJ sidewalls
Solution Approach 1:
The patent optimizes the oxidant type and concentration parameters to achieve clean sidewalls with minimal damage. Instead of using pure organic oxidants like methanol that cause significant chemical damage, the patent uses controlled amounts of inorganic oxidants (O2, N2O, NF3) or fluorocarbon oxidants (CF4) at lower concentrations (1-10% or 5-20% respectively in Ar), which provide residue-free sidewalls while minimizing chemical etching damage through milder oxidation chemistry.
Solution Approach 2:
The patent employs short-lived reactive species generated in situ during plasma generation rather than introducing long-lived chemical etchants. The oxidant gases are continuously supplied and consumed during the plasma process, providing temporary protective oxidation on sidewalls that prevents residue formation without causing cumulative chemical damage, as the reactive species are continuously regenerated and removed.
4Manufacturing precision
If a thicker hard mask is used to compensate for poor etch selectivity, then etch selectivity is effectively maintained, but the aspect ratio increases making formation of high density MTJ arrays with CD < 100 nm difficult
Solution Approach 1:
The patent changes the etching process parameters by introducing oxidants to the noble gas plasma, which dramatically improves etch selectivity between the hard mask and MTJ layers. This enhanced selectivity allows the use of thinner hard masks (reducing the required thickness by up to 50% compared to pure noble gas processes), thereby reducing the aspect ratio and enabling the fabrication of high-density MTJ arrays with critical dimensions below 100 nm.
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 process achieves reduced chemical damage, minimized redeposition, and increased etch selectivity, enabling the formation of MTJ nanopillars with critical dimensions below 100 nm and higher device yields by maintaining a noble gas to oxidant flow ratio that optimizes etch rates and selectivity between the hard mask and MTJ layers.
Implementation Method 1
RIE with pure Ar plasma, or IBE cause no chemical damage along MTJ sidewalls
Implementation Method 2
RIE plasma that is generated from oxidants such as methanol is known to provide MTJ sidewalls substantially free of residue
Implementation Method 3
RIE plasma that is generated from oxidants such as methanol
Data Source
AI summary
A process flow for forming magnetic tunnel junction (MTJ) nanopillars with minimal sidewall residue and damage is disclosed wherein a pattern is first formed in a hard mask or uppermost MTJ layer. Thereafter, the hard mask sidewall is etch transferred through the remaining MTJ layers with a RIE process comprising main etch and over etch portions, and a cleaning step. The RIE process features noble gas and an oxidant that is one or more of CH3OH, C2H5OH, NH3, N2O, H2O2, H2O, O2, and CO. Noble gas/oxidant flow rate ratio during over etch may be greater than during main etch to avoid chemical damage to MTJ sidewalls. The cleaning step may comprise plasma or ion beam etch with the noble gas and oxidant mixture. Highest values for magnetoresistive ratio and coercivity (Hc) are observed for noble gas/oxidant ratios from 75:25 to 90:10, especially for MTJ nanopillar sizes ≤100 nm.