A MTJ memory element is also referred to as a MTJ nanopillar and is a key component in memory devices such as magnetoresistive random access memory (MRAM) and spin torque transfer (STT)-MRAM. An important step in fabricating an array of MTJ nanopillars is etch transfer of a pattern in an overlying hard mask through a MTJ stack of layers to form an array of MTJ nanopillars with a critical dimension (CD) that in state of the art devices is substantially less than 100 nm from a top-down view. The etch transfer process typically comprises one or more etch steps involving RIE and/or ion beam etch (IBE).
A MTJ stack of layers includes two ferromagnetic layers called the free layer (FL) and reference layer (RL), and a dielectric layer (tunnel barrier) between the FL and RL. The RL has a fixed magnetization preferably in a perpendicular-to-plane direction (perpendicular magnetic anisotropy or PMA) while the FL is free to rotate to a direction that is parallel or anti-parallel to the RL magnetization direction thereby establishing a “0” or “1” memory state for the MTJ. The magnetoresistive ratio is expressed by dR/R where dR is the difference in resistance between the parallel state resistance (Rp) and the anti-parallel state resistance (Rap), and R is the minimum resistance value (Rp).
The bottommost MTJ layer is usually a non-magnetic seed layer that promotes uniform growth in overlying layers, and enhances PMA in the overlying RL or FL. A capping layer (hard mask) such as Ta is generally formed as the uppermost MTJ layer and serves as a protective layer during subsequent physical and chemical etches. An etch process that transfers a pattern in the hard mask through the MTJ stack of layers is challenging since there are a variety of materials (magnetic alloys, non-magnetic metals, and dielectric films) that each have a different etch rate. Although RIE with pure Ar plasma, or IBE cause no chemical damage along MTJ sidewalls, IBE or RIE based only on noble gas has poor etch selectivity between hard mask and MTJ layers such that a hard mask of up to 50% thicker is required than for RIE processes comprising a chemical reactant. Accordingly, formation of high density MTJ arrays with a CD substantially less than 100 nm is difficult because of a high aspect ratio necessary for MTJ nanopillars due to the added hard mask thickness. Moreover, IBE is associated with substantial redeposition of hard mask and bottom electrode metals on MTJ sidewalls. As a result, a shunting path is easily formed on the MTJ sidewalls, which causes shorts and low yields of the memory device.
RIE plasma that is generated from oxidants such as methanol is known to provide MTJ sidewalls substantially free of residue. However, RIE with chemical etchants does cause chemical and plasma damage on MTJ sidewalls. Therefore, improved etch conditions are needed so that redeposition of hard mask and bottom electrode material on MTJ sidewalls is minimized while reducing chemical damage to sidewalls, and exhibiting greater etch selectivity thereby enabling a substantially thinner hard mask thickness and lower aspect ratio, and higher device yields, especially for MTJs with critical dimensions below 100 nm.