TECHNICAL FIELD

The present disclosure relates to a method of reducing MTJ sidewall damage during an etch process that transfers a mask pattern through a MTJ stack of layers thereby generating an array of MTJ nanopillars with improved magnetoresistive ratio and other magnetic properties.

BACKGROUND

A MTJ memory element is also referred to as a MTJ nanopillar and is a key component in magnetic recording devices, and in memory devices such as magnetoresistive random access memory (MRAM) and spin torque transfer (STT)-MRAM. An important step in fabricating an array of MTJs 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 involves a plurality of etch steps involving reactive ion etch (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 (referred to as 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 two 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 RL or FL. A capping layer such as Ta is generally formed as the uppermost MTJ layer and serves as a protective layer during subsequent physical and chemical etches. Thus, a single etch transfer process 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 when subjected to IBE with Ar or to conventional CH3OH based RIE. In particular, methanol RIE causes chemical and plasma damage on MTJ sidewalls although there is minimal redeposition of etched material on the sidewalls. On the other hand, IBE produces no chemical damage and leaves minimal plasma damage, but results in a high degree of redeposited material on MTJ sidewalls. When metal such as Ta is redeposited on the tunnel barrier, shorting may easily occur and render the device unusable.

Current technology does not provide a single etch solution for transferring a hard mask pattern through an entire MTJ stack of layers without either a substantial redeposition of one or more MTJ materials on the MTJ sidewalls, or significant damage to the sidewalls. In any case, removal of material from the sidewalls requires one or more extra steps that reduce throughput and add cost. Moreover, damaged sidewalls are difficult to repair and often lead to reduced yield and therefore higher cost per unit of acceptable product. Therefore, a new method for etching a MTJ stack of layers in a single etch process is needed for higher throughput and lower cost, and the method must maintain or improve magnetic properties including the magnetoresistive ratio in the MTJ nanopillar. Furthermore, a process flow for etching MTJ sidewalls is desired that substantially reduces sidewall damage for devices with diameter (CD) around 60 nm or less.

SUMMARY

One objective of the present disclosure is to provide a method for etching all layers in a MTJ stack below the hard mask with a single etch step that leaves minimal residue.

A second objective of the present disclosure is to provide a process flow for MTJ etching that satisfies the first objective and substantially reduces sidewall damage and associated edge effects compared with conventional methanol based RIE thereby enabling improved device performance, especially for MTJ nanopillars with critical dimensions ≤60 nm.

According to a preferred embodiment, the first objective is achieved with a MTJ stack of layers having at least a reference layer, free layer, a tunnel barrier between the free layer and reference layer, and an uppermost hard mask. In some embodiments, a seed layer is employed as the bottommost MTJ layer. A pattern comprising a plurality of island features with the desired critical dimension for the eventual MTJ nanopillars is first defined in a photoresist mask layer above the hard mask layer. Preferably, there is a bottom anti-reflective coating (BARC) or a dielectric anti-reflective coating (DARC) between the hard mask and photoresist mask layer that has better resistance to subsequent etch processes than the photoresist mask. The pattern is etch transferred through the BARC or DARC by a first RIE or IBE step, and is then transferred through the hard mask by continuing the first etch step, or by performing a second RIE step comprised of a fluorocarbon or chlorocarbon gas, or by a second IBE step.

According to one embodiment of the present disclosure, the pattern of island features is then transferred through the remaining MTJ layers by a RIE step comprising an inert gas, and a chemical such as methanol, ethanol, ammonia, or a combination of CO and NH3. In particular, Ar ions or the like provide a physical component to the etch while the one or more chemicals provide a plasma component for chemical etching. As a result, inert gas ions or plasma substantially minimize chemical damage to the MTJ sidewalls, and redeposition of etched residue on the MTJ sidewalls is significantly reduced by the chemical component. In fact, depending on the composition of the MTJ layers, etch conditions may be optimized to yield minimal sidewall residue, and substantially less sidewall damage than in conventional methanol based RIE. Thereafter, an encapsulation layer is deposited on the resulting MTJ nanopillars, and then a chemical mechanical polish (CMP) process is performed to remove all layers above the hard mask. The CMP process forms a hard mask top surface that is coplanar with the surrounding encapsulation layer. From a top-down view, the MTJ nanopillars form an array of circular or elliptical shapes, for example.

In a second embodiment, a process sequence is used to transfer the pattern of island shapes in the hard mask through the remaining MTJ layers thereby generating a plurality of MTJ nanopillars that has minimal sidewall damage and residue. A first process flow comprises an IBE step and then a separate chemical treatment to convert non-volatile residue on MTJ sidewalls into a volatile residue. Thereafter, a second IBE step, plasma sputter etch step, or a thermal treatment is employed to remove the volatile residue. A second process flow includes a RIE step and then a separate chemical treatment wherein the chemical may be applied without plasma to transform any sidewall residue into a volatile form. Next, an optional IBE step, plasma sputter etch step, or thermal treatment is used to remove the volatile residue. In other embodiments, RIE is alternated with IBE before the optional chemical treatment, and optional volatilization step.

