Abstract:
A device structure and method for forming an interconnect structure in a magnetic random access memory (MRAM) device. In an exemplary embodiment, the method includes defining a magnetic stack layer on a lower metallization level, the magnetic stack layer including a non-ferromagnetic layer disposed between a pair of ferromagnetic layers. A conductive hardmask is defined over the magnetic stack layer, and selected portions of the hardmask and the magnetic stack layer, are then removed, thereby creating an array of magnetic tunnel junction (MTJ) stacks. The MTJ stacks include remaining portions of the magnetic stack layer and the hardmask, wherein the hardmask forms a self aligning contact between the magnetic stack layer and an upper metallization level subsequently formed above the MTJ stacks.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to magnetic memory devices and, more particularly, to a process sequence of fabricating magnetic random access memory (MRAM) devices. 
     Magnetic (or magneto-resistive) random access memory (MRAM) is a promising technology in the development of non-volatile random access memory that could begin to replace the existing dynamic random access memory (DRAM) as the standard memory for computing devices. The use of MRAM as a non-volatile RAM will eventually allow for “instant on” systems that come to life as soon as the system is turned on, thus saving the amount of time needed for a conventional PC, for example, to transfer boot data from a hard disk drive to volatile DRAM during system power up. 
     A magnetic memory element (also referred to as a tunneling magneto-resistive, or TMR device) includes a structure having ferromagnetic layers separated by a non-magnetic layer, and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the memory element as directions of magnetization vectors in the magnetic layers. More specifically, magnetic vectors in one magnetic layer (also referred to as a reference layer) are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer (also referred to as a “free” layer) may be switched between the same direction and the opposite direction with respect the fixed magnetization direction of the reference layer. The magnetization directions of the free layer are also known “parallel” and “antiparallel” states, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an antiparallel state refers to opposing magnetic alignments therebetween. 
     Depending upon the magnetic state of the free layer (parallel or antiparallel), the magnetic memory element exhibits two different resistances in response to a vertically applied current with respect to the TMR device. The particular resistance of the TMR device thus reflects the magnetization state of the free layer, wherein resistance is “low” when the magnetization is parallel, and “high” when the magnetization is antiparallel. Accordingly, a detection of changes in resistance allows an MRAM device to provide information stored in the magnetic memory element (i.e., a read operation). In addition, an MRAM cell is written to through the application a bi-directional current in a particular direction, in order to magnetically align the free layer in a parallel or antiparallel state. 
     A practical MRAM device integrates a plurality of magnetic memory elements with other circuits such as, for example, control circuits for the magnetic memory elements, comparators for detecting the states in the magnetic memory elements, input/output circuits, etc. As such, there are certain microfabrication processing difficulties to be overcome before high capacity/density MRAM products become commercially available. For example, in order to reduce the power consumption of the device, CMOS switching technology is desirable. As is known in the art, various CMOS processing steps (such as depositing dielectric and metal layers and annealing implants) are carried out at relatively requires high temperatures (e.g., in excess of 300E C). On the other hand, magnetic layers employ ferromagnetic material, such as CoFe and NiFeCo, that requires processing temperatures below 300E C in order to prevent intermixing of magnetic materials. Thus, the magnetic memory elements need to be fabricated at a different stage after CMOS processing. 
     Moreover, magnetic memory elements contain components that are easily oxidized and also sensitive to corrosion. To protect magnetic memory elements from degradation and keep the performance and reliability of the MRAM device, a passivation layer is typically formed thereupon. In addition, a magnetic memory element includes very thin layers, some of them on the order tens of angstroms thick. Because the performance of the magnetic memory element is particularly sensitive to the surface conditions on which magnetic layers are deposited, it is desirable to maintain a relatively flat surface to prevent the characteristics of an MRAM device from degrading. 
     Notwithstanding the above described processing variations between ferromagnetic materials and conventional DRAM elements, it is desirable to be able to simplify the MRAM fabrication process and increase the compatibility thereof with conventional back-end-of-line (BEOL), e.g. copper, metallization techniques. 
     SUMMARY OF INVENTION 
     The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a method for forming an interconnect structure in a magnetic random access memory (MRAM) device. In an exemplary embodiment, the method includes defining a magnetic stack layer on a lower metallization level, the magnetic stack layer including a non-ferromagnetic layer disposed between a pair of ferromagnetic layers. A conductive hardmask is defined over the magnetic stack layer, and selected portions of the hardmask and the magnetic stack layer, are then removed, thereby creating an array of magnetic tunnel junction (MTJ) stacks. The MTJ stacks include remaining portions of the magnetic stack layer and the hardmask, wherein the hardmask forms a self aligning contact between the magnetic stack layer and an upper metallization level subsequently formed above the MTJ stacks. 
     In another aspect, a magnetic random access memory (MRAM) device includes a magnetic stack layer formed on a lower metallization level, the magnetic stack layer having a non-ferromagnetic layer disposed between a pair of ferromagnetic layers. A conductive hardmask is formed over the magnetic stack layer, and an array of magnetic tunnel junction (MTJ) stacks is created by the removal of selected portions of the hardmask and the magnetic stack layer. The MTJ stacks include remaining portions of the magnetic stack layer and the hardmask, wherein the hardmask forms a self aligning contact between the magnetic stack layer and an upper metallization level formed above the MTJ stacks. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several figures: 
         FIGS. 1–11  are sectional views of processing steps in conjunction with a method for forming a magnetic random access memory (MRAM) device, in accordance with an embodiment of the invention, in which a plurality of magnetic tunnel junction (MTJ) stacks includes a metal hardmask layer thereupon. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is an improved process Sequence of fabricating magnetic random access memory (MRAM) devices wherein, among oilier aspects, a metal hardmask is formed over a plurality of magnetic tunnel junction (MTJ) stacks thereby providing a self-aligned contact between the stacics and subsequent tipper metallization lines formed thereupon. In other words, the hardmask (being self aligned between the MTJ stasks and the upper metallization lines) serves as an electrical contact therebetween. The metal hardmask also serves as an etch stop layer for subsequent dual damaseene processing steps used in the formation of the upper metallization lines and vias connecting the ripper metallization lines to lower metallization lines (on which the MTJ stacks are formed). 
     Referring initially to  FIG. 1 , there is shown a sectional view of the formation of the MTJ stacks of an MRAM device  100 . Prior to the stack formation, the fabrication of the MRAM structure  100 , up to the second level of metallization, is implemented in accordance with well known fabrication processes. The lower level, or front end of line (FEOL) structures, include transistor device  102  formed upon a silicon or other suitable substrate  104 , along with isolation regions  106 . An interlevel dielectric layer  108 , such as SiO2, is used to insulate the active substrate devices (e.g., transistor  102 ) from a first metallization layer M 1 , except where the transistor  102  is connected to M 1  by contact area  110 . 
     The first metallization layer M 1  is formed within a liner or barrier layer  112  (e.g., tantalum/tantalum nitride) which in turn is formed upon a nitride layer  114  on a first interlevel dielectric (ILD) layer  108 . A second interlevel dielectric layer  115  is also formed upon nitride layer  114 . Further, a second metallization layer M 2  (and liner) is formed upon a third interlevel dielectric layer  116 , wherein electrical contact between the first and second metallization layers M 1 , M 2  is achieved through via V 1 . As with the second interlevel dielectric layer  115 , the third interlevel dielectric layer  116  is also formed upon a nitride layer  118 . 
     Those skilled in the art will appreciate that the first metallization layer M 1 , as well as the combination of via V 1  and second metallization layer M 2 , may be formed by, for example, by conventional damascene processing and dual damascene processing, respectively. It will also be appreciated that the aforementioned FEOL structures (denoted collectively by  120  in subsequent Figures) are presented by way of example only, and are thus not discussed in further detail hereinafter. 
     The MTJ stack formation process begins with the deposition of a magnetic stack layer (collectively denoted by  122 ) deposited over the M 2  lines and the ILD layer  116 , and comprising a non-ferromagnetic layer sandwiched between a pair of ferromagnetic layers, allowing for spin-dependent tunneling. The ferromagnetic material used in the stack layer  122  may include materials such as IrMn, PtMn, CoFe, CoFeB, Ru, Al 2 O 3 , and NiFe for example. Other types of magnetic material, such as Ni, Co, and various ratios of the compounds mentioned above, may also be used. It should also be noted at this point that the magnetic stack layer  122  need not necessarily be formed upon M 2 , but could also be formed upon M 1  or at a higher metallization level than M 2 . 
     Once the magnetic stack layer  122  is deposited, a metal hardmask layer  124  is then deposited thereupon as shown in  FIG. 2 . In a preferred embodiment, the hardmask layer  124  includes a conductive material such as tantalum, tungsten, titanium, and compounds thereof, such as tantalum nitride or titanium nitride. However, other types of conductive materials can also be used. The hardmask layer  124  is deposited by, for example, physical vapor deposition (PVD), chemical vapor deposition (CVD), or other techniques. In addition, the thickness of the hardmask layer  124  is preferably sufficient to serve as a hardmask for etching of the magnetic stack layer  122 . 
     After being deposited, the hardmask layer  124  is then lithographically patterned and the resulting photo resist  125  pattern is transferred to the hardmask  124  layer by RIE, for example, as is also shown in  FIG. 2 . In  FIG. 3 , the pattern is also shown transferred into the magnetic stack layer  122  by reactive ion etching (RIE) or ion milling, for example, to define the individual MTJ stacks  126 . Once the MTJ stacks  126  are defined, a cap layer  128  is deposited to seal the exposed portions of the M 2  surface in subsequent processing steps, as shown in  FIG. 4 . The cap layer  128  may be, for example, a layer of silicon nitride deposited by CVD. 
     Referring now to  FIG. 5 , another interlevel dielectric layer  130  is blanket deposited over the cap layer  128 , in preparation for the formation of an upper metallization layer (M 3 ) and a via level for interconnection between M 2  and M 3 . Thus, the ILD layer  130  is deposited at a sufficient thickness for by M 3  and V 2  formation. As a result of the step heights created by the formation of the MTJ stacks  126  (and in particular due to the thickness of the hardmask layer  124 ), the deposition of ILD layer may result in nonplanarities  132  over the stacks  126 . Accordingly, the ILD layer  130  may be planarized by chemical mechanical polishing (CMP) for example, as shown in  FIG. 6 , so long as a sufficient thickness for M 3  and V 2  is maintained. Alternatively, a thicker cap layer  128  (i.e., having a cap thickness equal to or greater than the total thickness of the MTJ stacks  126 ) may deposited and thereafter planarized. Then, the ILD layer  130  may be deposited at a smaller thickness, since it will already be formed at a sufficient planarity. 
       FIG. 7  illustrates the formation of M 3  trenches  134  for a subsequent damascene metal process by lithography, patterning and RIE to transfer the desired pattern from a photo resist layer to ILD layer  130 , wherein the cap layer  128  may serve as an etch stop for the M 3  trench pattern in certain parts of the pattern. Following the etching of the M 3  trenches  134 , the remaining resist may either be stripped by cleaning or left in place for the next lithography and etch process in which the via openings  136  for V 2  are defined, as shown in  FIG. 8 . As was the case for the definition of the M 3  trenches, the cap layer  128  serves as an etch stop layer for the V 2  definition. Upon completion of the V 2  via opening formation, the remaining resist is stripped by a cleaning step, as shown in  FIG. 9 . 
     In  FIG. 10 , the cap layer  128  is removed (by etching, for example) in order to expose the hardmask layer  124  of the MTJ stacks  126 , as well as those portions of M 2  to be contacted by the V 2  vias. However, as an alternative approach to the steps illustrated in  FIGS. 7 and 8 , the V 2  openings may be lithographically patterned and etched first, with the cap layer  128  being used as an etch stop layer. Then, the M 3  trench lithography and etching may be carried out, followed by the removal of the cap layer  128 . In either case, the resulting structure will be the same as that shown in  FIG. 10 . Finally, as shown in  FIG. 11 , the metal fill of V 2  and M 3 , along with subsequent planarization, is carried out in accordance with existing dual damascene processing techniques. This may include, for example, a copper (Cu) liner and seed layer deposition, followed by Cu plating and CMP. 
     The formation of the hardmask layer  124  as part of the MTJ stack  126  proves beneficial to the overall BEOL processing of the MRAM device in a number of aspects. First, the hardmask serves to define the MTJ stack  126  and is thus self-aligned to the stack. Second, because the hardmask is purposely made of an electrically conducting material, it ultimately serves as a functional part of the working device as an electrical conduit between the magnetic stack layer  122  and the M 3  metallization lines. The conductive nature of the hardmask thereby eliminates the need for a separate processing level to create the connection to M 3  (such as, for example, by a damascene via). The dual function of a hardmask, which itself becomes a self-aligned connective element in the finished device, simplifies the BEOL fabrication. Thus, the BEOL processing of MRAM devices is more enhanced than existing MRAM processing techniques, and is also more simplified and/or compatible as compared with the conventional Cu BEOL processes. 
     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.