This application relates to the general field of magnetic tunneling junctions (MTJ) and, more particularly, to methods for preventing shorts and sidewall damage in the fabrication of MTJ structures.
Spin transfer torque magnetic random access memory (STT-MRAM) is a strong candidate for future memory applications. In magnetic tunnel junctions (MTJ), the free layer, which stores information for the memory bit, has two preferred magnetization orientations that are perpendicular to the physical plane of the layer. The free layer magnetization direction is expected to be maintained during a read operation and idle, but to change to the opposite direction during a write operation if the new information to store differs from its current memory state. The ability to maintain free layer magnetization direction during an idle period is called data retention or thermal stability. It is proportional to the product of the coercivity (Hc) and the free layer's magnetic moment where Hc is the minimum magnetic field needed to reverse the FL magnetization direction. For embedded STT-MRAM, the MTJ must be able to withstand annealing temperatures up to about 400° C. for 30 minutes which are typical of back end of line (BEOL) semiconductor processes. However, this high temperature procedure usually degrades both the Hc and the magnetic moment. A new MTJ device structure which is capable of enhancing the Hc is needed to better integrate this type of magnetic memory to Complementary Metal-Oxide-Semiconductor (CMOS) technologies.
Several patents teach metal surrounding an MTJ structure, including U.S. Pat. No. 10,084,127 (Annunziata et al), U.S. Pat. No. 10,096,768 (Jiang et al), and U.S. Pat. No. 6,929,957, but these methods are different from the present disclosure.
It is a primary object of the present disclosure to provide a method of enhancing coercivity in the fabrication of a MTJ structure without affecting other device performance parameters.
Another object of the present disclosure is to provide a method of forming a magnetic metal shield surrounding a MTJ structure to enhance coercivity of the MTJ without affecting other device performance parameters.
In accordance with the objectives of the present disclosure, a method for fabricating a magnetic tunneling junction (MTJ) structure is achieved. A MTJ stack is deposited on a bottom electrode, the stack comprising at least a pinned layer, a barrier layer on the pinned layer, and a free layer on the barrier layer. A top electrode layer is deposited on the MTJ stack. The top electrode and MTJ stack are etched where not covered by a photoresist pattern to form an MTJ structure. A conformal encapsulation dielectric is deposited over the MTJ structure. Thereafter, a magnetic metal layer is deposited on the encapsulation dielectric. The magnetic metal layer is anisotropically etched leaving a magnetic metal shield on sidewalls of the MTJ structure. A dielectric layer is deposited over the magnetic metal shield and MTJ structure. The dielectric layer and encapsulation dielectric are polished away to expose the top electrode. A top metal contact layer is deposited contacting the top electrode and the magnetic metal shield wherein the magnetic metal shield has no contact with said bottom electrode and MTJ structure but is separated from them by the encapsulation dielectric.
Also in accordance with the objectives of the present disclosure, a magnetic tunneling junction (MTJ) structure comprises a pinned layer on a bottom electrode, and a barrier layer on the pinned layer, a free layer on the barrier layer, and a top electrode on the free layer. Dielectric sidewalls on the pinned layer, barrier layer, free layer, and top electrode separate them from a vertical magnetic metal shield. The dielectric sidewalls also separate the bottom electrode horizontally from the magnetic metal shield. A top magnetic metal shield is formed horizontally on the top electrode and contacting the vertical magnetic metal shield.
In the accompanying drawings forming a material part of this description, there is shown:
In a typical process, patterned MTJ structures are separated from one another simply by the insulating encapsulation and dielectric materials. Therefore, coercivity (Hc) is determined by the materials within the MTJ stack. In the present disclosure, we form a magnetic metal shield surrounding the MTJ structure which can increase the device's “effective” Hc by reducing the external magnetic field. All other device parameters remain the same.
In the process of the present disclosure, using a self-aligned dielectric hard mask, we form a magnetic metal shield surrounding the MTJ. This metal shield is not in contact with the MTJ and the bottom electrode, but is separated from them by the insulating encapsulation material. Since the external magnetic field through the free layer is compensated by the field generated by the shield, the minimum required magnetic field needed to switch the free layer's magnetization direction is increased; i.e., enhancing the device's “effective” Hc. Compared to the traditional way of re-designing different materials in the MTJ stack to increase Hc, the method in the present disclosure is simple, only involving several additional metal/dielectric deposition and plasma etch steps. Moreover, the method to define the magnetic metal is a self-aligned process which does not require the complicated and expensive lithography, overlay, and CD control which becomes particularly challenging when the device size goes down to sub 60 nm.
The present disclosure describes two methods of fabricating the magnetic metal shield. The first method is illustrated in
The first preferred embodiment of the present disclosure will be described in more detail with reference to
The dielectric hard mask and top electrode are then etched by fluorine carbon based plasma such as CF4 or CHF3 alone, or mixed with Ar and N2 or physical RIE or IBE. The MTJ is then completely etched, either by chemical RIE, physical RIE or IBE, or their combination with a pattern size of ˜60 nm or below, as shown in
Now, as shown in
Referring now to
The second preferred embodiment of the present disclosure will be described in more detail with reference to
Now, using the sidewall spacer 52 as a self-aligned hard mask, the portion of the metal shield 30 on the top and bottom of the patterns is etched away, either by RIE, IBE or their combination, only leaving the metal shield 32 on the sidewall as protected by the spacer 52 as illustrated in
Referring now to
In the process of the present disclosure, we increase the STT-MRAM's Hc by forming a magnetic metal shield surrounding the MTJ. All other device parameters are not affected by this method. The process of the present disclosure is especially useful for embedded STT-MRAM chips, the Hc of which is challenging to maintain during the 400° C. back end of line (BEOL) fabrication and post annealing steps. The metal shield fabrication methods of the present disclosure, using direct anisotropic etching or self-aligned dielectric spacer etching, are simpler and more precise than expensive lithography methods. Furthermore, the disclosed methods avoid increasing overlay and CD control challenges of the lithography processes with device scaling.
Although the preferred embodiment of the present disclosure has been illustrated, and that form has been described in detail, it will be readily understood by those skilled in the art that various modifications may be made therein without departing from the spirit of the disclosure or from the scope of the appended claims.