Abstract:
A memory for an integrated circuit and method of fabricating same are provided, comprising providing an array of magnetic memory devices, preferably TMR junctions, that are configured as individual studs and protrude from a substrate. A layer of insulating spacer material is deposited over the array of magnetic memory devices, and a spacer etch is performed to remove the spacer material preferentially from the top surfaces of the magnetic memory devices and from substrate surface areas between the magnetic memory devices. Preferably, the insulating spacer material is low k and/or a barrier to outdiffusion of species from the TMR junctions. Examples include silicon carbide (BLOk™), low temperature silicon nitride or diamond-like carbon. In another embodiment, the insulating spacer material is also a magnetic material and may comprise magnesium-zinc ferrites or nickel-zinc ferrites.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed generally to integrated circuit magnetic memory devices for storing information and, more particularly, to methods and structures for insulating the devices. 
     2. Description of the Related Art 
     The memory integrated circuit most commonly used in computers and computer system components is the dynamic random access memory (DRAM), wherein voltage stored in capacitors represents digital bits of information. Electric power must be supplied to these memories to maintain the information because, without frequent refresh cycles, the stored charge in the capacitors dissipates, and the information is lost. Memories that require constant power are known as volatile memories. 
     Non-volatile memories do not need frequent refresh cycles to preserve their stored information, so they consume less power than volatile memories. There are many applications where non-volatile memories are preferred or required, such as in cell phones or in control systems of automobiles. 
     Magnetic random access memories (MRAMs) are non-volatile memories. Digital bits of information are stored as alternative directions of magnetization in a magnetic storage element or cell. The storage elements may be simple, thin ferromagnetic films or more complex layered magnetic thin-film structures, such as tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) elements. 
     Memory array structures are formed generally of a first set of parallel conductive lines covered by an insulating layer, over which lies a second set of parallel conductive lines, perpendicular to the first lines. Either of these sets of conductive lines can be the bit lines and the other the word lines. In the simplest configuration, the magnetic storage cells are sandwiched between the bit lines and the word lines at their intersections. More complicated structures with transistor or diode configurations can also be used. When current flows through a bit line or a word line, it generates a magnetic field around the line. The arrays are designed so that each conductive line supplies only part of the field needed to reverse the magnetization of the storage cells. Switching occurs only at those intersections where both word and bit lines are carrying current. Neither line by itself can switch a bit; only those cells addressed by both bit and word lines can be switched. In some arrangements, one or both of a sense line and a bit line cooperates with a word line to flip the bit. 
     FIG. 1 illustrates, by way of example, the three functional layers of a simple TMR device. TMR devices  10  work by electron tunneling from one magnetic layer to another through a thin barrier layer  12 . The tunneling probability is greatest when the magnetic layers  14 ,  16 , on either side of the barrier layer  12 , have parallel magnetizations and least when the magnetizations are anti-parallel. In order for the devices to function properly, these layers must be electrically isolated from one another. Any short-circuiting of the tunnel dielectric layer prevents proper reading of the layers&#39; relative magneto-resistance, which represents the data storage of the device. 
     Materials used in layers of the TMR devices, especially metals such as nickel, iron and cobalt can diffuse out from the devices and into other functional areas of the integrated circuit, especially during subsequent processing steps at elevated temperatures. 
     In the prior art, the dielectric layer  20  shown in FIG. 1 served both as electrical insulation around the magnetic memory devices and as a barrier to outdiffusion of TMR device species. Silicon nitride is the material most often used for this dual purpose. Silicon nitride is a very hard material and tends to make conformal layers. When deposited over the magnetic memory array  10 , the top surface of the silicon nitride  20  is not flat, and a chemical mechanical planarization step is performed before additional processing to form conducting lines that contact the top surfaces of the devices. 
     The high current density carried by the bit and word lines makes copper conductors desirable for MRAM arrays to reduce the likelihood of electromigration. Copper conducting lines are made usually using a damascene process. A copper conducting line  18 , in contact with the bottom of the TMR devices  10  is shown in the plane of the paper in FIG.  1 . To make conducting lines at the top of the devices, first the silicon nitride layer  20  is deposited over the MRAM array. Trenches (FIG. 2) are etched into the silicon nitride layer to make contact with the top surfaces  22  of the TMR devices  10 . 
     If the mask that defines the trenches is even slightly misaligned, the etching step can cause an overetch along the side of the MRAM device. As shown in FIG. 2, when the copper  24  is deposited, it can fill the overetched area as well, making copper regions  26  along the sides of the TMR devices  10  and short-circuiting the devices  10 . 
     Accordingly, structures for and methods of making MRAM arrays with damascene copper conducting lines with greater design tolerance and which will result in increased yield and reliability for MRAM arrays using TMR devices are needed. 
     SUMMARY OF THE INVENTION 
     In accordance in one aspect of the present invention, a memory for an integrated circuit and method of fabricating same are provided. An array of magnetic memory devices, preferably TMR junctions, are configured as individual studs and protrude from a substrate. A layer of insulating spacer material is deposited over the array of magnetic memory devices and a spacer etch is performed to remove the spacer material preferentially from top surfaces of the magnetic memory devices and from substrate surface areas between the magnetic memory devices. Next, a filler dielectric layer is deposited to fill at least regions over the substrate and between the magnetic memory devices. 
     Preferably, the insulating spacer material is also a barrier to outdiffusion of species from the TMR junctions and may consist of silicon carbide (e.g., BLOk™), low temperature silicon nitride or diamond-like carbon. In another embodiment, the insulating spacer material is also a magnetic material and may comprise magnesium-zinc ferrites or nickel-zinc ferrites. 
     The material for the filler dielectric layer may comprise spin-on-glass, borophosphosilicate glass, fluorinated silicate glass or hydrogen silsesquioxane glass. Preferably, the filler dielectric layer can be etched selectively relative to the spacer material. Trenches are etched into the filler dielectric layer to make contact to the TMR devices as for a damascene process. 
     In accordance with another aspect of the invention, an array of magnetic memory cells are provided on a substrate wherein each magnetic memory cell comprises a tunneling magnetoresistance (TMR) structure in a stud configuration, a spacer surrounding the TMR structure and at least two electrodes that make contact to the TMR structure. Regions between and over individual cells are filled with a dielectric material that reflows easily. 
     In one arrangement, the spacer comprises a low k insulator and is a barrier to diffusion of TMR structure species. In another arrangement, the spacer comprises a material with high magnetic permeability. In some arrangements, regions between individual cells have spacer material between the dielectric layer and the substrate, in addition to filler dielectric thereover. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross section of TMR cells in a portion of an MRAM array and having a stud configuration on a substrate, over which a dielectric layer has been deposited according to the prior art. 
     FIG. 2 is a schematic cross section of the MRAM array of FIG. 1 after trench etch and copper deposition, wherein misalignment of the etch mask has caused overetching along one side of the TMR cells. 
     FIG. 3 is a schematic cross section of TMR cells in a stud configuration on a substrate, which form a portion of an MRAM array, constructed in accordance with the preferred embodiments. 
     FIG. 4 is a schematic cross section of the TMR cells of FIG. 3 after a layer of spacer material has been deposited, in accordance with a preferred embodiment of the present invention. 
     FIG. 5 is a schematic cross section of the TMR cells of FIG. 4 showing spacers along the sides of the TMR cells after a preferred spacer etch has been performed. 
     FIG. 6 is a schematic cross section of the TMR cells of FIG. 5 after deposition of a filler dielectric layer to fill the areas between the cells. 
     FIG. 7 is a schematic cross section of the TMR cells of FIG. 6 after etching trenches that expose the top surfaces of the TMR cells. 
     FIG. 8 is a schematic cross section of the TMR cells of FIG. 4 showing spacers along the sides of the TMR cells and residual spacer material on horizontal surfaces after a partial spacer etch has been performed in accordance with an alternative embodiment. 
     FIG. 9 is a schematic cross section of the TMR cells of FIG. 8 after deposition of a dielectric layer to fill the areas between the cells and after trenches have been etched and residual spacer material has been removed, thus exposing the top surfaces of the TMR cells. 
     FIG. 10 is a schematic cross section of the TMR cells of FIG. 9 after copper has been deposited to fill the trenches and the top surface has been planarized. 
     FIG. 11 is a collapsed top view of an array of TMR cells showing metal lines making contact to the cells both above and below the cells. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It is well-known that cells employing the tunneling magnetoresistance (TMR) effect can be used for making magnetic random access memory arrays (MRAMs). The embodiments of this invention can be used to advantage on TMR MRAMs having any of a number of TMR stack configurations and fabricated by any of a number of methods. The figures show the simplest configuration for illustrating the preferred embodiment. Generally, a TMR stack comprises a stack of layers that includes a layer of pinned hard magnetic material, a stack of layers that includes a sense layer of soft magnetic material and a tunneling dielectric layer separating the two stacks. An exemplary stack below the dielectric includes a tantalum seed layer, a nickel-iron seed layer, a magnesium oxide pinning layer and a nickel-iron or nickel-iron-cobalt pinned layer. An exemplary tunneling dielectric layer over this stack comprises aluminum oxide. An exemplary stack over the tunnel dielectric includes a nickel-iron or nickel-iron-cobalt sense layer, a tantalum barrier layer and a tungsten nitride layer. 
     As shown in FIG. 3, the description of the first embodiment will use, as a starting point, an array of TMR cells  10  having stud configurations and protruding from a substrate that may contain other devices consistent with integrated circuit manufacture. The cells  10  have a similar construction to that described above with respect to FIG.  1 . It is preferred that the top layer  30  of the TMR cells comprises a conductive barrier layer comprising tantalum in the illustrated embodiment. Just below the TMR cells and making electrical contact with them are a series of parallel conductive lines, preferably comprising copper, one of which  18  is shown contacting the devices and in the plane of the page in FIG.  3 . 
     In the illustrated embodiment of the current invention, a blanket layer of spacer material  40  is deposited over the MRAM array, as shown in FIG.  4 . In one arrangement, the spacer material comprises Si 3 N 4 . Preferably, the spacer material is a low k electrical insulator to prevent shorting out the memory devices. Preferably the value of k is less than 3.5, more preferably less than 3.0. The thickness of the deposited spacer layer is preferably between about 5 nm and 100 nm, more preferably between about 20 nm and 40 nm. Preferably, the spacer material comprises a material that acts as a barrier to outdiffusion of metals from the TMR stack. Materials that have these characteristics include low temperature silicon nitride (apart from the low k characteristic), diamond-like carbon and silicon carbide, preferably as produced by the BLOk™ (Barrier LOw k) chemical vapor deposition process of Applied Materials in Santa Clara, Calif. 
     Alternatively, the spacer layer  40  comprises a material that, in addition to having a low k and being a diffusion barrier, has high magnetic permeability and low coercivity. In one arrangement, the magnetic permeability is between about 10 and 1 gauss/oersted, and the coercivity is less than about 0.1 oersted. In another arrangement, the magnetic permeability is between about 1 and 2 gauss/oersted, and the coercivity is less than about 1.0 oersted. Such a material additionally provides protection from stray magnetic fields for data stored in the device. Preferred materials for magnetic protection include dielectric magnetic materials, such as magnesium-zinc ferrites ((MnZn) O. FezO 3 ) and nickel-zinc ferrites ((NiZn) O. Fe 2 O 3 ). The spacer layer is deposited by any suitable process, including physical vapor deposition or chemical vapor deposition. 
     The spacer layer is etched, preferably using an anisotropic etch process that preferentially etches the horizontal portions  42  of the spacer layer  40 . More preferably, the spacer etch is selective against etching tantalum, thus allowing the tantalum layer  30  to act as an etch stop and protecting the TMR memory structures. A physical process, such as ion milling, can be used for all spacer materials. Preferably, the spacer etch has a chemical component and is a reactive ion etch (RIE). Preferred etchants for silicon carbide include CF 4 , CH 2 F 2  and C 2 F 6 . Preferred etchants for silicon nitride include CF 4  and CHF 3 . Preferred etchants for diamond-like carbon include oxygen-based plasma. Preferred etchants for magnesium-zinc ferrites and nickel-zinc ferrites include Cl 2 - and CF 4 -based RIE. 
     The spacer etch leaves a plurality of spacers  50  over sidewalls of the MRAM devices  10 , as shown in FIG. 5, wherein all spacer material has been removed from horizontal substrate surfaces  52  and from top surfaces  54  of the devices  10 . 
     A filler dielectric layer  70  is deposited over the MRAM array, as shown in FIG. 6 for the illustrated embodiment. The thickness of the filler dielectric layer  70  is preferably between about 50 nm and 150 nm, more preferably, between about 100 and 120 nm. Because the TMR cells  10  are surrounded by the spacers  50 , which are barriers to outdiffusion of TMR species, choice of a dielectric fill material is not limited to those with good diffusion barrier properties. Softer dielectric materials than the silicon nitride can be used. One benefit is that softer materials flow well and are better able to form flat, smooth surfaces  72 , even over varied topography. These kinds of materials are reflowable, so chemical-mechanical planarization is not necessary, thus saving valuable time in processing and avoiding the contamination and other problems associated with a separate planarization step. Another benefit is that dielectric materials can be chosen with k values less than that of silicon nitride, thus lowering the RC delay of the memory array. Furthermore, replacing a thick silicon nitride layer with the softer oxide-based materials, preferably reflowable materials, also reduces stress in the integrated circuit. Reduced stress, in turn, leads to less defects and higher yield for the fabrication process. 
     It is also preferred that the filler dielectric material  70  is selectively etchable in relation to the spacer material. Preferred dielectric materials include SOG (spin-on glass), BPSG (borophosphosilicate glass), FSG (fluorinated silicate glass) and HSG (hydrogen silsesquioxane glass). These materials can be deposited by chemical vapor deposition or any other suitable method. 
     In FIG. 7, a set of parallel damascene trenches  80 , shown perpendicular to the page and to the copper line  18  below the TMR devices  10 , has been etched to expose the top surfaces  54  of the devices. In the illustrated embodiment, the top surface  54  of the TMR structure  10  comprises tantalum with no residual spacer material thereon. The tantalum layer  30  acts as an etch stop, protecting the TMR structure during the dielectric trench etch. The preferred spacers  50  also alleviate problems of mask misalignment. 
     Over the TMR cells  10  are trenches  80  that are also perpendicular to the page. In the final structure, conducting metal lines (not shown) fill the trenches  80 , making electrical contact to the top surfaces  54  of the TMR cells  10  and have top surfaces level with the top surface of the fill dielectric  70 . 
     An alternative embodiment is shown in FIG. 8, wherein a small amount of spacer material  60 ,  62  remains both on the substrate  18  and on the top surface  54  of the device  10 , respectively. As stated above, the spacer material is preferably an electrical insulator, more preferably low k, so it does not short across the tunnel dielectric nor does layer  60  short devices to one another. The remaining spacer material  62  over the memory devices  10  can be removed either during the etch process that creates trenches  80  in the filler dielectric layer  70  for the metal contact or from a second etch process used specifically to remove the spacer material after the trenches  80  have been formed. FIG. 9 shows the structure after a filler dielectric layer  70  has been deposited, trenches  80  have been etched and residual spacer material  62  has been removed. 
     FIG. 10 shows the resulting structure of the alternative embodiment, after damascene processing to make conducting lines  82  in contact with the devices  10 . A layer of copper was deposited over the trenches  80  and the filler dielectric layer  70 . The copper layer was then planarized, leaving copper conductive lines  82  in the trenches and no copper over the dielectric surface. There is residual spacer material  60  between the substrate  18  and the filler dielectric layer  70 . 
     As stated above, with reference to FIG. 4, the filler dielectric material  70  is preferably selectively etchable in relation to the spacer material. Even if the mask that defines the trenches is slightly misaligned, the trench etch step will not cause an overetch along the side of the TMR device as was shown in the prior art of FIG.  2 . For the embodiment of FIG. 10, even if overetch occurs outside the spacers, the barrier  60  prevents shorting. The process used to etch the dielectric material  70  is selective against the spacer  50 . No gaps along the sides of the device will develop, and the copper will not short out the device. Thus greater design tolerance has been achieved for making MRAM arrays with damascene copper conducting lines. 
     FIG. 11 shows a top view of a portion of an MRAM array in accordance with preferred embodiment of the current invention. A few layers can be seen in this drawing. The TMR structures  10  are shown as rectangular, and the spacers  50  make continuous contact along all four sides. Alternatively, the TMR structures can have any anisotropic shape. 
     The top conducting lines  82  are seen as a set of parallel lines making contact to the top surfaces of the TMR cells  10 , which are arranged in an array. In a parallel plane, below the plane of the paper and perpendicular to lines  82  are another set of parallel conducting lines  18  in contact with the bottom surfaces of the TMR cells  10 . Between the conducting lines  18 ,  82 , are regions of filler dielectric material, configured as columns  70  of material perpendicular to the page. Although it cannot be seen in this view, this filler dielectric material also fills the regions between lines  82  at the top and lines  18  at the bottom of the TMR cells  10 . 
     The preferred embodiments have been described herein in considerable detail to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.