Magnetic sidewalls for write lines in field-induced MRAM and methods of manufacturing them

In one embodiment, there is provided a non-volatile magnetic memory cell. The non-volatile magnetic memory cell comprises a switchable magnetic element; and a word line and a bit line to energize the switchable magnetic element; wherein at least one of the word line and the bit line comprises a magnetic sidewall that is discontinuous.

FIELD

Embodiments of the invention relate to magnetic random access memory (MRAM) devices and methods for their manufacture.

BACKGROUND

Field-induced magnetic random access memory (MRAM) use a current-induced magnetic field generated around metal lines to write data in memory cells. In an MRAM cell one bit of data is stored in a magnetic tunnel junction (MTJ). In field-induced MRAM the MTJ sits in-between two metal lines, the bit line and the word line. Normally, these lines are perpendicular to each other. To write binary data (“0” or “1”) in an MTJ cell, enough current must go simultaneously through the bit line and the word line of that particular cell for a certain amount of time. The sense in which the current flows in both metal lines sets a data value of either a “0” or a “1” in the cell.

It is advantageous to MRAM technology to be able to write data in the memory cells with as low a current as possible. Lower current means lower energy and voltage requirements for the memory device, smaller transistors (which may impact positively the memory density), and higher reliability of the metal lines employed in writing the cells is.

SUMMARY

According to one aspect of the invention, there is provided a non-volatile magnetic memory cell, comprising a switchable magnetic element; and a word line and a bit line to energize the switchable magnetic element; wherein at least one of the word line and the bit line comprises a magnetic sidewall that is discontinuous.

According to second aspect of the invention, there is provided memory device, comprising:

an array of magnetic memory cells, each cell comprising a switchable magnetic element; and

a word line and a bit line to energize the switchable magnetic element; wherein at least one of the word line and the bit line comprises a magnetic sidewall that is discontinuous.

Other aspects of the invention will be apparent from the written description that follows.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details.

Although the following description contains many specifics for the purposes of illustration, one skilled in the art will appreciate that many variations and/or alterations to said details are within the scope of the present invention. Similarly, although many of the features of the present invention are described in terms of each other, or in conjunction with each other, one skilled in the art will appreciate that many of these features can be provided independently of other features. Accordingly, this description of the invention is set forth without any loss of generality to, and without imposing limitations upon, the invention.

Broadly, embodiments of the present invention disclose MRAM structures with metal lines having magnetic sidewalls in different configurations. In a first configuration, the magnetic sidewalls are continuous and extend along the full length of a metal line. In a second configuration, the magnetic sidewalls are discontinuous and are located at portions of metal lines that are close to the MTJ cells. Advantageously, the magnetic sidewalls reduce the current in the word and bit lines needed to switch the MTJ cells. Embodiments of the present invention also disclose techniques for manufacturing the metal lines.

Referring now toFIG. 1(a), in a first configuration metal line2is shown having continuous magnetic sidewalls4extending along its entire length.FIG. 1(b) shows the metal line2clad with discontinuous metal line, portions of which are indicated with reference numeral6.

Referring now toFIG. 2, reference numeral8generally indicates a 3×3 MRAM array, in accordance with one embodiment of the invention. In the array8, each MRAM cell includes a word line10and a bit line12with a MTJ stack/element14disposed at the intersections of the word and bit lines10,12. As will be seen, the word lines10have continuous sidewalls10.1, whereas the bits lines12have continuous sidewalls12.1. Each MTJ14is connected to access circuitry (not shown) through a bit line10, a bottom electrode16, and a stack18.

Referring now toFIG. 3, reference numeral20generally indicates a 3×3 MRAM array, in accordance with another embodiment of the invention. InFIG. 2, the same or similar reference numerals used inFIG. 1are used to indicate the same or similar components. As will be seen, the case of the embodiment ofFIG. 3, the bit line10includes discontinuous magnetic sidewalls, portions of which are indicated with reference numeral10.2. Likewise, the word line12includes discontinuous magnetic sidewalls, portions of which are indicated with reference numeral12.2

In one embodiment, in the case of continuous magnetic sidewalls said sidewalls are magnetically very soft. For this purpose, the magnetic walls may be made of NiFe, NiFeMo alloys or ultrasoft magnetic materials. In one embodiment, the thicknesses of the magnetic layer in the sidewalls are selected to keep the soft properties of the magnetic sidewalls. For example, thin magnetic layers (much less than 10 nm) in the sidewalls are avoided.

In the case of the configuration with discontinuous magnetic sidewalls, said sidewalls may be thin (<10 nm) so as not to overpower the MTJ.

In one embodiment, the aspect ratio of these patterned sidewalls is set carefully and consistently across the memory device. Setting the aspect ratio of the patterned walls involves considering the magnetic switching field of the cell, the cell stability against thermal fluctuations, stray magnetic fields, and half-select. In one embodiment, the patterned magnetic sidewalls have an aspect ratio 1 or close to 1 with the longer side oriented along a top-bottom direction inFIG. 3. The magnetic field for switching the cells as well as the cell stability tends to increase with the aspect ratio of the magnetic sidewalls. For the magnetic sidewalls it is preferable to use materials with very low magneto-crystalline anisotropy, like NiFe, NiFeMo or CoFeB alloys.

In one embodiment, the magnetic sidewalls may be made of several layers.FIG. 4shows a cross-section through a metal line (word or bit) having three layer magnetic sidewalls. The layers include an outer layer20, an inner layer22, and a middle layer21sandwiched between the outer layer20and the inner layer22. The layers20and22are non-magnetic, whereas the middle layer21is the magnetic one. The purpose of the outer22and the inner20layer is to protect the integrity of the magnetic layer21, so that its thickness is not affected by processing. The innermost layer20also helps protect the metal line23during processing and helps reduce electro-migration in the metal line23. The outer and the inner layers may be composed of Ta, or other materials that fit the purpose.

Manufacturing of the magnetic sidewalls can be accomplished by different methods. In case of metal lines defined by etching a metallic layer; like AlCu and W lines, the process flow for manufacturing is shown inFIG. 5. The first step is to define the metal line30(FIG. 5a), which is shown in cross-section. In one embodiment the metal line30is defined with the assistance of a hard mask, the remaining of which is denoted as31. The next step is the deposition of the layers32composing the magnetic sidewalls, as shown inFIG. 5b). In one embodiment the deposition can be made by Physical Vapor Deposition (PVD). The next step (c) is the definition of the walls through anisotropic etching of the deposited layers. In one embodiment this can be accomplished with Reactive Ion Etching (RIE) using for example: chlorine gas mixed with argon. At this point the continuous magnetic sidewalls are already defined along the metal lines. For discontinuous magnetic sidewalls, after step c) a photo-lithography process follows, as shown in plan view inFIG. 6d). This step is to protect with photo-resist40the parts of the magnetic sidewalls that are going to remain. The next step (e) is the etching away of the exposed magnetic sidewalls. After stripping the photo-resist (f), the discontinuous magnetic sidewalls41are defined.

In the case of metallic lines defined by the Damascene method, like Cu lines, the process flow for manufacturing the magnetic sidewalls is shown inFIG. 7. Starting from the groove50etched in the dielectric layer51, the next step is the deposition of the layers52composing the magnetic sidewalls. In one embodiment the deposition can be made by PVD. The following step (c) is the definition of the walls through anisotropic etching of the deposited layers: In one embodiment this can be accomplished with RIE using chlorine gas mixed with argon gas. The following step (d) is the filling of the groove with metal and the chemical-mechanical polishing (CMP) down to the dielectric layer. For metal filling a thin metallic seed layer53is deposited first. In a different embodiment the innermost metallic layer54deposited in b) serves as seed layer for metal filling. For that purpose, step c) is replaced with step e), where the innermost deposited layer is left after RIE. After that, step f) follows which implies metal filling and CMP. Either after step d) or after step f) the result is the definition of continuous magnetic sidewalls along the metal lines. For discontinuous magnetic sidewalls, after step c) or e) a photo-lithography process follows, as shown in plain view inFIG. 8g). This step is to protect with photo-resist60the parts of the magnetic sidewalls that are going to remain. The following step (h) is etching away the exposed magnetic sidewalls. After stripping the photo-resist (i), the discontinuous magnetic sidewalls61are defined.

One skilled in the art would be aware of the requirements and specificities of the techniques mentioned above for the purpose of manufacturing the magnetic sidewalls. The manufacturing techniques mentioned herein are not intended to limit the scope of the invention.