A MTJ memory element is also referred to as a MTJ cell and is a key component in magnetic recording devices, and in memory devices such as MRAM and spin torque transfer (STT)-MRAM. An important step in fabricating an array of MTJ cells is the etch transfer of a pattern in an overlying hard mask through a MTJ stack of layers to form an array of MTJ cells 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 involves a plurality of etch steps involving one or both of RIE and IBE.
A MTJ stack of layers includes two ferromagnetic layers called the free layer (FL) and pinned layer (PL), and a tunnel barrier layer comprised of one or more dielectric layers between the FL and PL. Conductive layers (electrodes) above and below the PL/tunnel barrier/FL stack serve as electrical connections to a bit line and source line that are above and below the MTJ, respectively. The PL 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 (P) or anti-parallel (AP) to the PL magnetization direction thereby establishing a “0” or “1” memory state for the MTJ. The magnetoresistive ratio (DRR) is expressed by dR/R where dR is the difference in resistance between the P and AP magnetic states when a current is passed through the MTJ, and R is the minimum resistance value. The bottommost MTJ layer is usually a non-magnetic seed layer that promotes uniform growth in overlying layers, and enhances PMA in the overlying PL or FL. A capping layer (also referred to as a top electrode) such as Ta is generally formed as the uppermost MTJ layer and serves as a protective layer during subsequent physical and chemical etches.
Precise patterning techniques including photolithography and RIE are typically involved to define millions of MTJ cells in a MRAM array. The etching process to transfer the pattern in a photoresist mask through the underlying MTJ stack of layers is challenging since there are a variety of materials (magnetic alloys, non-magnetic metals, and dielectric films) in a MTJ stack of layers that each have a different etch rate when subjected to IBE or RIE. Also, due to a chemical reaction during RIE, portions of the MTJ layers adjoining the sidewall are easily damaged because of exposure to moisture, oxygen, and other oxidants such as methanol thereby lowering DRR and coercivity (Hc). This damage is cell size dependent meaning that the problem becomes more severe as cell size decreases.
To avoid chemical damage to MTJ sidewalls, pure physical etching techniques such as Ar based RIE or IBE have been applied. However, due to their non-volatile nature, metals such as Ta from the top and bottom electrodes, or ferromagnetic material from the PL or FL are easily redeposited on MTJ sidewalls, and cause electrical shorting that renders the device unusable. Physical damage to the sidewalls may also occur because of the highly energetic ions in physical etching. To remove physical damage to the sidewall or redeposited materials, additional steps such as horizontal RIE or IBE trimming have been employed, but these additional steps add to the fabrication cost and cycle time. The feasibility of surface trimming is also limited by MTJ cell density.
Another issue with conventional MTJ etch processing is that the volumes (width×thickness) of the FL and PL are equal or substantially the same. Thus, as cell size shrinks below 60 nm, the PL magnetization becomes too weak to stabilize the FL internal magnetic state. Furthermore, data retention is affected if the PL size and energy barrier (EB) continue to decrease. Note that EB in a magnetic layer is related to thermal stability (A) shown in equation (1) belowΔ=kV/kBT  Eq. (1)where k is a constant, V is the volume of the magnetic layer (PL), kB is the Boltzmann constant, and T is temperature.
In order to overcome the aforementioned issues that are associated with conventional MTJ patterning technology, new fabrication process flows are required so that MTJ cells having a CD substantially less than 60 nm may be formed while maintaining magnetic properties such as DRR, the integrity of MTJ sidewalls, and the pinning strength of the PL on the FL. Also, the new fabrication sequence must have high throughput and low cost in order to be competitive with other memory devices.
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