Abstract:
A method for forming interconnect structures in a magnetic random access memory (MRAM) device includes defining an array of magnetic tunnel junction (MTJ) stacks over a lower metallization level. A encapsulating dielectric layer is formed over the array of MTJ stacks and the lower metallization level. Then, a via opening is defined in the encapsulating dielectric layer, and a planar interlevel dielectric (ILD) layer is deposited over the encapsulating dielectric layer and within the via opening. Openings are then formed within ILD layer, over the array of MTJ stacks and the via opening.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to magnetic memory devices and, more particularly, to a maskless array protection (AP) process flow that enables formation of interconnect vias and self-aligned contact to magnetic random access memory (MRAM) devices. 
     Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile random access memory technology that could potentially replace the 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 (barrier), 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, the magnetic moment of one magnetic layer (also referred to as a reference layer) is fixed or pinned, while the magnetic moment 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 orientation of the magnetic moment 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 resistance values in response to a voltage applied across the tunnel junction barrier. 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 a MRAM device to provide information stored in the magnetic memory element (i.e., a read operation). In addition, a 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 and miscellaneous support circuitry. 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 and provide the variety of support functions CMOS technology is required. As is known in the art, various CMOS processing steps (such as annealing implants) are carried out at relatively high temperatures (e.g., in excess of 300Â° C.). On the other hand, ferromagnetic materials employed in the fabrication of MRAM devices, such as CoFe and NiFeCo for example, require substantially lower process temperatures in order to prevent intermixing of magnetic materials. Thus, the magnetic memory elements are designed to be integrated into the back end wiring structure following front end CMOS processing. 
     Magnetic memory elements contain components that are easily oxidized and also sensitive to corrosion. To protect magnetic memory elements from degradation and ensure the performance and reliability of the MRAM device, it is desirable to form a passivation layer thereupon. In addition, a magnetic memory element includes very thin layers, some 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 an atomically flat surface to prevent degradation of the MRAM device characteristics. 
     Notwithstanding the above described processing variations between ferromagnetic materials and conventional DRAM elements, it is nonetheless desirable to simplify the MRAM fabrication process and increase the compatibility thereof with conventional back-end-of-line (BEOL) metallization process sequences. The BEOL metallization process sequence commonly utilizes copper as the metallic conductor, but is not limited to this conductor material. Wiring features are formed by filling etched recesses in the interlevel dielectric (ILD) with metal and removing the extraneous metal by polishing the wafer to a flat surface leaving the filled features separated by ILD. The mesa structure of the fabricated MTJ device results in a step height differential between the MTJ device array and surrounding support area. For a variety of reasons, it is important to maintain the planarity of each level of back end wiring, and the array step height necessitates an additional planarization step before the next metallization level is formed. It is highly desirable to reduce the cost and seamlessly integrate the MRAM array and support structure into a back end wiring process. Moreover, this also requires the MRAM process to be compatible with low dielectric constant (low-k) interlevel dielectric materials. 
     In general, the material chosen to encapsulate the MTJ device may not be the ILD material used to support the BEOL wiring structure. Prior art teaches a fabrication method that separates the array region comprising the MTJ storage devices and the surrounding support area. While the prior art is a workable method, there are cost and performance issues associated therewith, as addressed hereinafter. 
     SUMMARY OF INVENTION 
     The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a method for forming interconnect structures in a magnetic random access memory (MRAM) device. In an exemplary embodiment, the method includes defining an array of magnetic tunnel junction (MTJ) stacks over a lower metallization level. An encapsulating dielectric layer is formed over the array of MTJ stacks and the lower metallization level. Then, a via opening is defined in the encapsulating dielectric layer, and a planar interlevel dielectric (ILD) layer is deposited over the encapsulating dielectric layer and within the via opening. Openings are then formed within ILD layer, over the array of MTJ stacks and the via opening. 
     In another aspect, a method for forming back end of line (BEOL) interconnect structures in a magnetic random access memory (MRAM) device includes defining an array of magnetic tunnel junction (MTJ) stacks over a lower metallization level. A first dielectric layer is formed over the array of MTJ stacks and the lower metallization level. Thereafter, the first dielectric layer is planarized down to the top of the MTJ stacks. A via opening is then defined in the first dielectric layer, thereby exposing a portion of said lower metallization layer. A second dielectric layer is formed over the first dielectric layer and over the exposed portion of said lower metallization layer. Then, a planar interlevel dielectric (ILD) layer is deposited over the second dielectric layer, and openings are formed within the ILD layer, over the array of MTJ stacks and the via opening. 
     In still another aspect, a method for forming back end of line (BEOL) interconnect structures in a magnetic random access memory (MRAM) device includes defining an array of magnetic tunnel junction (MTJ) stacks over a lower metallization level. A passivation layer is formed over the array of MTJ stacks and the lower metallization level. An encapsulating dielectric layer is then formed over the passivation layer, the encapsulating dielectric layer and the passivation layer being planarized to the top of the MTJ stacks. A dielectric mask layer is formed over the passivation layer, the encapsulating dielectric layer and the MTJ stacks, and a via opening is defined in the dielectric mask layer, thereby exposing a portion of the encapsulating dielectric layer. Then, a planar interlevel dielectric (ILD) layer is deposited over the dielectric mask layer, and openings are formed within the ILD layer and over the array of MTJ stacks and the via mask openings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
     FIGS.  1 ( a ) through  1 ( i ) illustrate an existing process for implementing via integration following the formation of the MTJ stacks of an MRAM device, in which a masked array protection step is used; 
     FIGS.  2 ( a ) through  2 ( d ) illustrate a maskless array protection method for implementing via integration following the formation of the MTJ stacks of an MRAM device, in accordance with an embodiment of the invention; and 
     FIGS.  3 ( a ) through  3 ( e ) illustrate another maskless array protection method for implementing via integration following the formation of the MTJ stacks of an MRAM device, in accordance with an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIGS.  1 ( a ) through  1 ( i ), there is illustrated an existing process for implementing via integration following the formation of the MTJ stacks of an MRAM device. FIG.  1 ( a ) illustrates the formation of an MRAM structure  100  up to the second level of metallization, 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 doped silicon dioxide, 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 via feature  110 . 
     The first metallization layer M 1  is formed within a liner or barrier layer  112  such as, for example, tantalum/tantalum nitride. The first metallization level may or may not be formed upon a nitride layer  114  covering the first interlevel dielectric (ILD) layer  108  and via contact feature  110 . 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. 
     FIG.  1 ( b ) illustrates the formation of a plurality of MTJ stacks  122 , comprising a buffer layer, pinning layer, bottom pinned ferromagnetic layer, a tunnel barrier layer and top free ferromagnetic layer deposited over the second metallization level M 2 . A additional hardmask layer is deposited to provide an etch mask during the metal etching process for the MTJ stack. The example shown here utilizes silicon dioxide, which is subsequently removed during the wiring trench etch process, selective to the silicon nitride MTJ stack encapsulation. However, other hardmask materials, such as titanium nitride or tantalum nitride may also be used. If other dielectric materials are used as the hardmask material, then such material should have etch selectivity with respect to a subsequent encapsulating dielectric formed thereupon. It should also be noted at this point that the MTJ stacks need not necessarily be formed upon M 2 , and could also be formed upon M 1  or on a metallization level above M 2 . Once the MTJ stacks  122  are formed, a blanket nitride layer  124  (encapsulating dielectric) is deposited thereupon, as shown in FIG.  1 ( c ), and thereafter planarized down to the top of the MTJ stacks  122 , as shown in FIG.  1 ( d ). 
     At this point during the conventional processing, an array protection sequence is employed to protect the MRAM array elements (i.e., the MTJ stacks  122 ) during the formation of the next metallization level, as well as the vias connecting thereto from M 2 . Thus, referring now to FIG.  1 ( e ), there is shown an array protection (AP) lithography step in which a photoresist layer  126  is applied and exposed over the array region and the unmasked region of nitride layer  124  thereunder are removed from the non-array areas. However, in so doing, there is a non-planar step height that results from the removal of nitride layer  124 , and upon which a subsequent nitride layer  128  and a fourth ILD (oxide) layer  130  are formed in preparation for the next metallization level. 
     As shown in FIG.  1 ( f ), due to the non-planarity (step height) of the protected array area, the oxide layer  130  must then be planarized with an additional step, such as by chemical mechanical polishing (CMP), before a new layer of resist  132  is deposited, patterned and etched to form the openings  134  for the third metallization layer. Once the openings  134  are formed, the exposed portions of nitride layer  128  are removed, as shown in FIG.  1 ( g ). Then, in FIG.  1 ( h ), the protective hardmask of the MTJ stacks are etched away, wherein it will be noted that a corresponding portion of the ILD  116  in the non-array section to the right of M 2  and V 1  is also etched during this step. Finally, FIG.  1 ( i ) illustrates the formation of the M 3  wiring level, which runs orthogonal to the M 2  wiring in the MRAM device. The M 3  liner, as well as the metal material (e.g., copper) is deposited and planarized in a conventional fashion, after which final oxide and nitride layers  136 ,  138  are formed to provide isolation from the final BEOL wiring out of the device. Access to the third metallization lines M 3  is provided in the non-array region through openings  140  etched within layers  136  and  138 . 
     Because of the presence of the MTJ stacks  122  in conventional, array protected MRAM processing, an extra planarization step results in additional cost and complexity in the fabrication of the device. In addition, the step height from the array protection sequence becomes even more pronounced with a multi-stack MRAM configuration, likely incurring additional fabrication yield loss and possibly making the process incompatible with ILD materials that are difficult to planarize. 
     Therefore, in accordance with an embodiment of the invention, there is disclosed a maskless array protection method for implementing via integration following the formation of the MTJ stacks of an MRAM device. Referring now to FIG.  2 ( a ), there is shown the MRAM device  200  at the same stage of processing as was the conventionally fabricated device  100  in FIG.  1 ( d ). However, whereas the conventional processes utilize a lithography step and RIE process to remove the MTJ stack encapsulating dielectric  124  in the support regions outside the array, this embodiment utilizes a lithography step and RIE process to first form via openings in the encapsulating dielectric  124 . The example herein particularly illustrates a second level via V 2  (in the non-array area of MRAM device  200 ) to connect M 2  to a subsequently formed M 3  level. FIG.  2 ( b ) depicts the lithography and reactive ion etching (RIE) of encapsulating dielectric (e.g., nitride layer)  124  to form a patterned opening  142  that will later define V 2 . 
     Then, in FIG.  2 ( c ), an additional nitride layer  144 , used to protect the bottom M 2  metal surface, is deposited over the existing nitride layer  124  (now indicated in dashed lines), the MTJ stacks  122  and the exposed portion of M 2  in the V 1  opening. This is followed by a blanket deposition of an ILD layer  146  (for example, silicon dioxide) atop the nitride layer  144 . Because there is no step height created as a result of removal of nitride in the vicinity of the array region, the ILD layer  146  is formed with sufficient planarity so as to not require an additional planarization step. This simplifies the MRAM fabrication process, thereby improving fabrication yield and production cost. 
     Lastly, in FIG.  2 ( d ), the formation of the openings for the third metallization layer M 3  is illustrated. Appropriate openings  148  over the array and over V 2  are patterned and etched for a subsequent formation of damascene metal wiring features. Once the openings  148  in the ILD layer  146  are etched, a subsequent etch is used to remove the exposed portions of nitride layer  144 . It will be noted that the anisotropic etch of nitride layer  144  will result in a negative RIE bias of the V 2  feature relative to the M 3  opening thereabove. Following the completion of the nitride etch, the hardmask layers atop the MTJ stacks  122  may be removed (not shown), and the third metallization layer may be deposited in accordance with conventional damascene/dual-damascene processing techniques. 
     FIGS.  3 ( a )- 3 ( e ) illustrate an alternative embodiment of a maskless array protection method for implementing via integration following the formation of the MTJ stacks of an MRAM device array. Referring now to FIG.  3 ( a ), there is shown the MRAM device  200  at the same stage of processing as was the conventionally fabricated device  100  in FIG.  1 ( d ). However, instead of utilizing a nitride dielectric  124  to encapsulate the MRAM device stacks  122 , this embodiment initially features a thin passivation layer  150 , followed by an encapsulating dielectric  152 . The passivation layer  150  may include, but is not limited to, material such as alumina, silicon carbide or any other material suitable for providing a stable mechanical and electrical interface to the MTJ barrier sidewall and metal wiring conductor surface. The encapsulating dielectric  152  may include silicon dioxide, doped glass, or a spin-on material. This provides a method for MRAM device stack sidewall passivation and metal wiring passivation using passivation layer  150  separately from the encapsulating dielectric  152 . If a spin-on material is used for the encapsulating dielectric  152 , the application thereof might result in sufficient planarity to the top of the MTJ stacks. Otherwise, an additional planarization step may be implemented to adjust the height of the encapsulating dielectric  152  to the top of the MTJ stacks  122 , as shown in FIG.  3 ( a ). 
     Next, FIG.  3 ( b ) illustrates a thin dielectric mask layer  154  blanket deposited onto the planarized encapsulating dielectric  152  and MTJ stacks  122 . The dielectric mask layer  154  may include silicon carbide or silicon nitride, for example, such that it enables etch selectivity to the encapsulating dielectric material  152 . In contrast to a conventional process, wherein a lithography step and RIE process are used to remove the MTJ stack encapsulating nitride dielectric in the support regions outside the array, this embodiment also utilizes a lithography step and RIE process to form via openings  142  prior to the deposition of the main interlevel dielectric thereupon. It will be noted, however, that via opening  142  is only formed through the dielectric mask layer  154 . 
     As shown in FIG.  3 ( c ), a blanket deposition of an ILD layer  156  (for example, silicon dioxide) atop the dielectric mask layer  154  results in a buried mask layer  154  that will be used to further etch the via openings to the passivation layer  150  covering the MRAM device stacks and bottom metal wiring level M 2 . Because there is essentially no step height created as a result of removal of nitride outside of the array region, as in prior art, the ILD layer  156  is formed with sufficient planarity so as not require an additional planarization step. This simplifies the MRAM fabrication process, thereby improving fabrication yield and production cost. 
     FIG.  3 ( d ) illustrates the formation of the openings for the third metallization layer M 3 . Appropriate openings  158  over the array and over V 2  are patterned and etched for a subsequent formation of damascene metal wiring features. Finally, in FIG.  3 ( e ), once the openings  158  in the ILD layer  156  are etched, a subsequent etch is used to remove the exposed portions of buried mask layer  154  that cover the MTJ stacks  122  and the exposed portions of the passivation layer  150  covering the V 2  contact to the M 2  wiring surface. Following the completion of the mask etch, the hardmask layers atop the MTJ stacks  122  may be removed, and the third metallization layer (not shown) may be deposited in accordance with conventional damascene/dual damascene processing techniques. 
     As will be appreciated by the above described maskless array protection process, a global planarity is achieved by first defining the vias that connect M 2  to M 3  or, more generally, the vias that connect a lower metallization level (where the bottom layer of the MTJ stack is contacted) to an upper metallization level (where the top layer of the MTJ stack is contacted). Thus, when the subsequent nitride and ILD oxide layers are formed, sufficient planarity is maintained for the damascene processing, which eliminates the need for an additional dielectric planarization step. The M 3  wiring, now independent of the M 2  wiring outside the array alleviates a previous drawback of shorting of the M 3  wiring to the M 2  wiring. Furthermore, the elimination of the dielectric removal outside of the array is expected to show substantial yield improvement through lower foreign material and contamination of the exposed copper wiring surface. Although the exemplary embodiments illustrated herein have been described with reference to a cross-point (XPC) MRAM cell, it will be appreciated that the methods are equally applicable to FET MRAM cell configurations as well. 
     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.