Abstract:
Fabrication methods for MRAM are described wherein any re-deposited metal on the sidewalls of the memory element pillars is cleaned before the interconnection process is begun. In embodiments the pillars are first fabricated, then a dielectric material is deposited on the pillars over the re-deposited metal on the sidewalls. The dielectric material substantially covers any exposed metal and therefore reduces sources of re-deposition during subsequent etching. Etching is then performed to remove the dielectric material from the top electrode and the sidewalls of the pillars down to at least the bottom edge of the barrier. The result is that the previously re-deposited metal that could result in an electrical short on the sidewalls of the barrier is removed. Various embodiments of the invention include ways of enhancing or optimizing the process. The bitline interconnection process proceeds after the sidewalls have been etched clean as described.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to fabrication methods for semiconductor Back-End-Of-Line (BEOL) memories including Magnetic Random Access Memory (MRAM) and more particularly to fabrication methods for MRAM that include ways of preventing interlayer shorting from metallic re-deposition material on sidewalls of the memory elements. 
       BACKGROUND 
       [0002]    The BEOL memories such as RRAM (Resistive Random Access Memory), PRAM (Phase Change Random Access Memory), and MRAM have a resistive device as a memory element. These memories are expected to have wide application because of their high access speed and non-volatility at power-off. 
         [0003]    The memory device includes a multilayer magnetoresistive memory element in between a top electrode and a bottom electrode. The resistive memory element of MRAM is a Magnetic Tunnel Junction (MTJ) including a free layer and a fixed layer separated by a barrier layer. A magnetic moment of the free layer is manipulated by an electric current between the electrodes to be parallel or antiparallel to the fixed layer by an electric current between the electrodes. Whether the magnetic vector of the free layer is parallel or antiparallel to the fixed layer determines the low or high resistance state of the MTJ, which can be read using an electrical current that does not alter magnetic orientation. The two resistance states can be used as memory states “0” or “1”. 
         [0004]      FIG. 1  illustrates a cross sectional view of the thin film layers at a selected stage of the fabrication process of MTJ memory cells according to the prior art. At the stage shown in  FIG. 1 , the unpatterned layers of MTJ film stack  20  have been deposited over the wafer onto the previously fabricated landing pads  21  or contact studs that connect to a control structures  19  like an FET and/or diode. The MTJ layer stack  20  includes unpatterned layers for the bottom electrode layer  22 , lower magnetic layer  23 , barrier layer  24 , upper magnetic layer  25  and top electrode  26 . Either one of the magnetic layers  23 ,  25  can be designed to work as a pinned layer with a fixed magnetic field and the other one acts as a free layer with a magnetization orientation that is manipulated by a vertical current that flow through the memory element between the electrodes. The barrier layer  24  is a dielectric film such as MgO or Al 2 O 3  designed as efficiently manipulating TMR. The bottom electrode  22  and the top electrode  26  are primarily Ta or Ta alloys. A hard mask, which is not shown, can also be deposited as part of the standard fabrication process. 
         [0005]    During the fabrication process, after the stage shown in  FIG. 1 , arrays of MTJ pillars  30  containing the patterned layers for the memory elements are formed on the wafer using conventional lithography and dry etching as shown in  FIG. 2 . The MTJ pillar  30  includes a bottom electrode  22 ′, a lower magnetic layer  23 ′, a barrier layer  24 ′, an upper magnetic layer  25 ′ and a top electrode  26 ′. The dry etching process often sputters metal and re-deposits it on MTJ pillar sidewall which is especially deleterious on the barrier layer. The metallic re-deposition material  27  can electrically short the pinned layer and the free layer across the sidewall area  28  of barrier layer  24 ′ rendering the device inoperable. 
         [0006]    Sources of the re-deposition metal can be the MTJ stack itself or the landing pad metal that was deposited and patterned before the MTJ layer stack. After the etching process has removed the layers down through the barrier layer, the exposed sidewalls of the layers for the memory element become susceptible to being shorted with the re-deposited metal. The MTJ pillar is exposed to the process environment at this point in the process. After the unprotected material in bottom electrode layer is etched away, additional metal structures that were deposited and patterned below the MTJ layer are now exposed to the etching ambient. These previous structures include not only the MTJ landing pad but also peripheral circuitry. Thus there are several sources of metal that can be sputtered out and then re-deposited on the sidewall of the pillar. 
         [0007]    The MTJ and bottom electrode layers are etched conventionally with ion milling or high-biased reactive ion etching where elements are mainly removed mechanically. Freed electrically conductive material generated by mechanical etching is easily re-deposited on the exposed sidewalls and can cause a short defect. However, during the etching process material is also being removed from the sidewalls, so it is the net result of these opposing processes that determines the final amount of re-deposited material that remains on the sidewalls. One variable is the slope of the sidewall, because the removal rate is faster on a shallow sloped sidewall. Also the re-deposition rate lowers with a decrease in remaining re-deposition source material. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the invention include fabrication methods for MRAM wherein any re-deposited metal on the sidewalls of the memory element pillars is cleaned before the interconnection process is begun. In embodiments the memory element pillars are first fabricated, then a dielectric material is deposited on the memory element pillars over the re-deposited metal on the sidewalls. The dielectric material substantially covers any exposed metal and therefore reduces sources of re-deposition during subsequent etching. Etching is then performed to remove the dielectric material from the top electrode and the sidewalls of the pillars down to at least the bottom edge of the barrier. The result is that the previously re-deposited metal that could result in an electrical short on the sidewalls of the barrier is removed. Various embodiments of the invention include ways of enhancing or optimizing the process. 
         [0009]    A first embodiment of the invention includes directionally depositing the dielectric film to create a thinner film on MTJ pillar sidewalls than on the planar areas. Because directional deposition allows a thinner film to be formed on the sidewalls it is preferable to non-directional deposition. The embodiment then uses physical etching using an inert or chemically inactive gas is used to remove re-deposited metal together with the dielectric material from the sidewalls. A relatively high electrical bias is preferably applied to the substrate during the etching. The etching process also removes the dielectric material from the top electrode to allow subsequent interconnection to bitlines. 
         [0010]    A second embodiment deposits an oxygen-free dielectric film on the sidewalls using high density plasma CVD. Vertical etching with appropriate chemistry systems, such as CH 3 OH or CO/NH 3 , is then used to remove re-deposited metal together with the dielectric material from the sidewalls and the top electrode. 
         [0011]    A third embodiment first deposits a relatively thin oxygen-free dielectric film and then a thicker film of, for example, silicon oxide. are deposited on the sidewall and planarized with CMP. The first dielectric layer is preferably thin and roughly conforms to the topography of the pillars. The second dielectric film is relatively thick to allow the wafer to be planarized with CMP. Vertical etching with appropriate chemistry systems, such as CH 3 OH or CO/NH 3 , is then used to remove re-deposited metal together with the dielectric material from the sidewalls of the barrier layer and the top electrode. 
         [0012]    In each embodiment, the bitline interconnection process phase of the fabrication proceeds after the sidewalls have been etched as described. This process embodiment deposits an oxygen-free dielectric material and an inter-layer dielectric (ILD) material over the pillars on the wafer. The ILD material is selected to have a higher etching rate in the selected dry etching process than the oxygen-free dielectric material. The wafer is planarized using CMP. The trench lines are patterned across the top electrodes using the selected dry etching process to remove inter-layer dielectric (ILD) material down to the oxygen-free dielectric material which acts as an etch stop. The top electrodes in the pillars are exposed using a second selected dry etching process to remove the oxygen-free dielectric material. The fabrication process continues with the bitline interconnection process is which metal is deposited metal over the exposed top electrodes in trench lines to form bitlines for interconnection. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0013]      FIG. 1  illustrates a cross sectional view of the thin film layers at a selected stage of the fabrication process of MTJ memory cells according to the prior art. 
           [0014]      FIG. 2  illustrates a cross sectional view of the thin film layers at a selected stage of the fabrication process of MTJ memory cells that is subsequent to the stage of  FIG. 1 .  FIG. 2  shows arrays of MTJ pillars  30  containing the layers for the memory elements formed on the wafer according to the prior art that have undesirable re-deposition material on the sidewalls. 
           [0015]      FIG. 3-FIG .  8  illustrate cross sectional views of process stages for the first embodiment of the invention. In  FIG. 3  a dielectric layer is formed over the MTJ pillars with directional deposition. 
           [0016]      FIG. 4  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 3  after sidewall cleaning has been performed. 
           [0017]      FIG. 5  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 4  after dielectric films  31 ,  32  have been deposited on the cleaned MTJ pillars. 
           [0018]      FIG. 6  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 5  after trench lines  42  have been formed in the dielectric  32 . 
           [0019]      FIG. 7  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 6  after etching is performed to remove enough of the dielectric layer  31  to expose the upper surface of top electrode  26 ′. 
           [0020]      FIG. 8  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 7  after the interconnection process is resumed with a damascene process filling the trench lines with Cu to form bitlines. 
           [0021]      FIGS. 9 and 10  illustrate a second embodiment of the invention using high density plasma CVD for sidewall cleaning.  FIG. 9  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 2  after the deposition of dielectric material  51 . 
           [0022]      FIG. 10  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 9  after etching the dielectric material and removing the undesirable re-deposition material from the sidewall of the barrier layer. 
           [0023]      FIGS. 11 to 13  illustrate a third embodiment of the invention that uses CMP for sidewall cleaning after the pillars have been patterned as illustrated in  FIG. 2 .  FIG. 11  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 2  after the deposition of dielectric material  53 . 
           [0024]      FIG. 12  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 11  after dielectric film  61  has been deposited and then planarized with CMP. 
           [0025]      FIG. 13  illustrates a cross sectional view of a process stage subsequent to that of  FIG. 12  after the dielectric films have been etched down to below the barrier level. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    In the following description of the embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration of specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized without departing from the scope of the present invention. It should be noted that the figures discussed herein are not drawn to scale and thicknesses of lines are not indicative of actual sizes. The cross section view in the figures is generally taken through the approximate center the memory cell in a plane perpendicular to the substrate except where otherwise noted. Although only one or two cells are shown in the figures, the method may used for the simultaneous fabrication of many cells on a wafer according to standard techniques. 
         [0027]    First Embodiment: 
         [0028]      FIGS. 3-8  will be used to illustrate the first embodiment of the invention. The process stage in  FIG. 3  follows that of  FIG. 2 .  FIG. 3  illustrates the result of the deposition of dielectric film  29  over the pillars. The dielectric layer  29  has been formed over the MTJ pillars  30  by vertical directional deposition such as down-flow CVD or biased CVD. Accordingly the dielectric layer  29  is thicker in flat area such as the space between the pillars and is thinner on the sidewalls. The dielectric layer  29  completely covers the undesirable re-deposition material  27  and generally conforms to the existing topography of the pillars. 
         [0029]      FIG. 4  illustrates a cross sectional view of a process stage, subsequent to that of  FIG. 3 , after sidewall cleaning has been performed using an etching technique that results in the top surface of the pillar being etched faster than the other surfaces which are etched isotropically. Although the dielectric material  29  on top of the pillars is thicker than on the sidewall, the top is more exposed to the etching ambient. This etching requirement is met by dry etching using an inert gas such as argon (Ar) along with a relatively high electrical bias being applied to the wafer. In some embodiments, ion milling etching may also be applied by itself or in combination with dry etching. The removal rate for this type of etching is not sensitive to the type of material, so the rate does not vary greatly across different materials. Thus, this etching technique first removes substantially all of the thinner dielectric material  29  and re-deposition material  27  on the sidewalls as shown in  FIG. 4 . Then as the etching proceeds, all of the dielectric material  29  is removed from the top surface of the MTJ pillar and some rounding of the outer edges of the top electrode  26 ′ occurs. In this way the sidewall of each pillar is cleaned of unwanted material that could short the layers of the MTJ without damaging the essential structures which include the top electrode. When this phase of the process ends, residual dielectric material  29 ′ remains on the planar surfaces between the pillars, and small areas of re-deposition material  27 ′ also remain on the sidewalls of the bottom electrode layer  22 ′ and possibly a small portion of the lower magnetic layer  23 ′. As long as the barrier layer sidewall is completely cleaned of material, the re-deposition material  27 ′ causes no problems. 
         [0030]      FIG. 5  to  FIG. 8  will be used to describe the interconnect process to connect the top electrode  26 ′ to a standard bitline. A relatively thin oxygen free dielectric film  31  such as a silicon nitride or a silicon carbide is deposited on the cleaned MTJ pillar as shown in  FIG. 5 . This dielectric film  31  substantially conforms to the topography of the pillars. Next a relatively thick inter-layer dielectric (ILD)  32  such as silicon oxide is deposited and planarized with CMP. The materials for layers  31 ,  32  are selected so that the etching rate of dielectric material  31  in the ILD etching ambient is much smaller than the etching rate of the ILD material  32 . This allows the dielectric material  31  to work as an etch stopper when conventional trench lines  42  (as shown in  FIG. 6 ) are formed in the ILD  32  using conventional lithography and dry etching. Layer  31  can also be composed of more than one layer to make selection of etching ambient more flexible. The additional layers can be selected according to the prior art. 
         [0031]    The result of the next phase of the process is illustrated in  FIG. 7 . The result of the prior process etching phase exposed the dielectric layer  31  at the bottom of the trench lines  42 . Another etching step is performed to remove enough of the dielectric layer  31  to expose the upper surface of top electrode  26 ′. The material for layer  31  was selected to be resistant to the etching process used for the ILD  32  material. Therefore, a different etching chemistry, one appropriate for the material is now used to vertically etch exposed dielectric film  31 . The etching continues until a sufficient area of the top electrode  26 ′ has been exposed to allow the electrical interconnect to be made. Like a conventional spacer etching process, the vertical etching leaves dielectric material  31  on the sidewall of the barrier layer  24 ′ for protection. After the top electrode  26 ′ is exposed, the interconnection process is resumed with a conventional damascene process filling the trench lines with copper (Cu) to form bitlines  44  as shown in  FIG. 8 . It should be noted that this process formed the bitlines without the need for a via mask. Thus, in this embodiment of the invention the MTJ is directly connected to the bit line without the use of a via or the process steps for forming the via. 
         [0032]    Second embodiment: 
         [0033]      FIGS. 9 and 10  illustrate a second embodiment of the invention which uses high density plasma CVD for sidewall cleaning. As for the first embodiment, the pillars are patterned as shown in  FIG. 2 .  FIG. 9  shows that a relatively thick dielectric material  51  has been deposited over the pillars  30  which include metallic re-deposition material  27  on the sidewalls as described above. The dielectric material  51  is selected to be an oxygen-free material such as silicon nitride or silicon carbide. The dielectric material  51  is thinner on top of the pillars than in the valley areas between the pillars. Next a dry etching is used to etch the dielectric material  51  and re-deposition material  27 . The chemistry system is selected for more rapid removal of the dielectric material  51  and slower removal of the material in the top electrode  26 ′. Appropriate systems include CH 3 OH and CO/NH 3  and the like. The dielectric material  51  is vertically etched to the stage as shown in  FIG. 10  and the re-deposition material  27  is removed where it is exposed as well. The etching process should continue until the sidewalls of the barrier layers  24 ′ have been cleaned of undesirable re-deposition material. Residual re-deposition material  27 ′ can remain on the sidewall of the lower magnetic layer  23 ′ and bottom electrode  22 ′ without deleterious effect. From the stage shown in  FIG. 10 , the process proceeds according to the first embodiment to connect a metal bitline to the top electrodes as illustrated in  FIG. 5  to  FIG. 8  and described above. 
         [0034]    Third embodiment: 
         [0035]      FIG. 11  to  FIG. 13  illustrate a third embodiment of the invention that uses CMP for sidewall cleaning. The pillars are first patterned as illustrated in  FIG. 2 .  FIG. 11  shows that a dielectric material  53  has been deposited over the pillars  30  in a relatively thin layer that roughly conforms to the topography of the pillars. The dielectric material  53  is selected to be an oxygen-free material such as silicon nitride or silicon carbide and like materials. The next part of the process is illustrated in  FIG. 12  which shows that a relatively thick dielectric film  61 , such as one of the silicon oxides, has been deposited and then the wafer has been planarized with CMP. Next the dielectric layers  53 ,  61  are vertically etched. The chemistry system is selected to have high selectivity for more rapid removal of the dielectric material  53 ,  61  and slower removal of the material in the top electrode  26 ′. Appropriate systems include CH 3 OH and CO/NH 3  and the like. 
         [0036]    The dielectric material  53 ,  61  are vertically etched to the stage as shown in  FIG. 13  leaving residual dielectric material  53 ′,  61 ′. As in the second embodiment the etching continues until the dielectric material and the re-deposition material  27  has been removed from the top electrode  26 ′, as well as the sidewalls of upper magnetic layer  25 ′ and barrier layer  24 ′. At this point the sidewalls of the barrier layers  24 ′ have been cleaned of undesirable re-deposition material. The surface levels of residual dielectric material  53 ′,  61 ′ are below the level of the barrier layers  24 ′. Because the first dielectric layer  53  was thin, the residual dielectric material  61 ′ filled in the central area between the pillars and the residual dielectric material  53 ′ forms a cup-like surround for the residual dielectric material  61 ′. 
         [0037]    As noted for the second embodiment, residual re-deposition material  27 ′ can remain on the sidewall of the lower magnetic layer  23 ′ and the bottom electrode  22 ′ without deleterious effect. From the stage shown in  FIG. 13 , the process proceeds according to the first embodiment to connect a metal bitline to the top electrodes as illustrated in  FIG. 5  to  FIG. 8  and described above.