Abstract:
Disclosed is a non-volatile memory cell including a first conductive electrode region, a second conductive electrode region and a memory region disposed therebetween. The memory region includes one or a plurality of metal oxide nanoparticles, which contact and electrically connect the first and the second electrode region via contact locations and which exhibit a bistable resistance properties when applying an external voltage.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to non-volatile, resistive memory cells based on the bistable resistance properties of metal oxide nanoparticles, a process for manufacturing such a non-volatile memory cell and an arrangement of a plurality of such memory cells to a memory cell arrangement.  
       BACKGROUND OF THE INVENTION  
       [0002]     Conventional storage technologies (floating gate memories such as flash and DRAM) are based on the storage of charges in inorganic, silicon-based materials. These technologies for storing electric charges will reach their scaling limits within the near future. Thus, there is increased research for alternative methods for storing information. Herein, the principle of resistive memories based on the bistable resistance change in metal oxide nanoparticles has proven to be promising.  
         [0003]     However, with conventional methods for manufacturing resistively switching metal oxide storage materials, active memory cells of few nanometers in dimension are unattainable. The currently used technologies for the deposition of the micro-crystalline storage substance are conventional thin-film techniques, such as vapor deposition or sputtering followed by the oxidation of the metal films (J. F. Gibbons and W. E. Beadle, Solid-State Electron., 7, 785 (1964); W. R. Hiatt and T. W. Hickmott, Appl. Phys. Lett., 6, 106 (1965) and F. Argall, Solid-State Electron., 11, 535 (1968)). During the deposition, induced inner mechanical stress, which may partly be considerably high, may occur in the layers. In addition, this stress may be increased by inhomogeneous heating of a larger layer volume due to the thermistor effect, so that the adhesiveness of the layers often limits the switching cycles, and the layers may peel off (S. Seo, M. J. Lee, D. H. Seo, E. J. Jeoung, D.-S. Suh, Y. S. Joung, I. K. Yoo, I. R. Hwang, S. H. Kim, I. S. Gyun, J.-S. Kim, J. S. Choi and B. H. Park, Appl. Phys. Lett., 85, 5655 (2004) and S. Seo, M. J. Lee, S. K. Choi, D. S. Suh, Y. S. Joung, I. K. Yoo, I. S. Buyn, I. R. Hwang, S. H. Kim, B. H. Park, Appl. Phys. Lett., Vol. 86 (2005), S. 093509). Up to now, bistable switching in nickel oxide has been observed only in connection with chemical conversion or chemical reactions of the oxide material in a narrow filament, wherein the layer was destroyed after a limited number of cycles.  
         [0004]     Furthermore, a high storage density cannot be realized with this technology, since a reduction in size is difficult.  
         [0005]     The mechanisms for monostable or temporally limited bistable switching are based on isolator breakdown and thermistor effects coupled with the diffusion of contact material into the memory cell and thus allow only a small number of switching cycles. This severely limits the lifetime.  
         [0006]     Thus, it is an object of the present invention to provide a novel type of memory cell that can be easily manufactured. In accordance with the present invention, this object is achieved by the subject matter of claims  1 ,  5  and  14 .  
       DISCLOSURE OF THE INVENTION  
       [0007]     In accordance with a first aspect of the present invention, a non-volatile memory cell comprises metal oxide nanoparticles arranged between two electrode regions, wherein the metal oxide nanoparticles conductively connect the electrode regions and exhibit bistable resistance properties.  
         [0008]     Due to the reduction of the switching and storage effect to only a few or even a single nanoparticle, the memory cell according to the present invention attains a significantly higher storage density at lower cost. Thus, it is possible to realize a lower switching power and faster switching times than in the storage technology with conventionally deposited NiO 1-x  layers.  
         [0009]     The inventive use of metal oxide nanoparticles enables switching between two bistable resistance states in the nanoparticles. Applying a corresponding voltage within a narrow range allows switching between these resistance states. Due to thermal-electronic interaction with trap centers and metal ion diffusion along the nano-crystalline grain boundaries, the state of high conductivity is stabilized. The formation and disruption of the filament region of high conductivity is furthermore coupled to a thermistor effect. This thermistor effect enhances the diffusion process and the charge and discharge processes in the defective states, due to an inhomogeneous temperature distribution in the metal oxide storage material when applying a voltage.  
         [0010]     In accordance with a second aspect of the present invention, a process for manufacturing a non-volatile, resistive memory element comprises the following steps: providing a first conductive electrode region on a substrate; arranging metal oxide nanoparticles on the first electrode region; filling the gaps between the metal oxide nanoparticles with a dielectric so that the metal oxide nanoparticles and the dielectric form a common surface; and arranging a second conductive electrode region on said surface.  
         [0011]     The invention provides a process for generating metal oxide nanoparticles, e.g. made of non-stoichiometric NiO 1-x , by chemical deposition of diblock copolymers on a CMOS-compatible contact, using a simple inexpensive low-temperature process.  
         [0012]     Due to the smaller dimensions of the metal oxide nanoparticles and the density and homogeneity of the deposition, memory cells with higher density and improved switching and storage properties can be realized. Therein, it is possible to perform bistable switching with only a single particle of about 3 nm×3-5 nm. Thus, the conductive filament can be reduced in the on-state to dimensions below 3 nm. This size corresponds to the limit for thermo-electronic switching and storage effects.  
         [0013]     Even when contacting several nanoparticles with a contact pair, the bistable switching is carried out by only one nanoparticle. The nanoparticle that first forms a conductive filament undertakes the storage of information. Thus, when a nanoparticle fails, another nanoparticle can immediately assume the function of the failed nanoparticle. The minimal dimensions for a memory cell are thus limited merely by current CMOS-technology.  
         [0014]     The current invention thus provides a non-volatile memory cell based on the bistable resistance property of metal oxide nanoparticles. The process for manufacturing this memory element is based on the formation of nanocrystals which are applied via diblock copolymer monolayers.  
         [0015]     According to a further aspect of the present invention, a memory cell arrangement is provided which comprises a plurality of non-volatile memory cells.  
         [0016]     According to a preferred embodiment, the metal oxide nanoparticles comprise NiO 1-x  nanoparticles. Here, x is in the range of 0.5 to 0.95, and preferably between 0.7 and 0.9.  
         [0017]     Among metal chalcogenides, nickel oxide has an exceptional position because of its large band gap of about 4.5 eV. At room temperature, nickel oxide in stoichiometric composition is an isolating semiconductor. Furthermore, because of the steepness of its resistance-temperature characteristics when applying an adequate voltage, nickel oxide exhibits a region with negative differential resistance and monostable switching in its I-U-characteristics, due to the thermistor effect.  
         [0018]     On the one hand, electronic defects are formed in layers of NiO 1-x  nanoparticles due to the oxygen deficiency, and on the other hand, there is an increased nickel ion diffusion along the defects and grain boundaries due to the excess nickel. The diffusion of nickel is also a thermally activated process. The result is a very complex process of thermally enhanced interactions of nickel ion diffusion and electric charging and discharging of deep defects which are generated through the oxygen deficiency. Due to the non-stoichiometric composition of NiO 1-x , this composition is oxygen-deficient.  
         [0019]     According to a further preferred embodiment, the first electrode region is a tungsten electrode.  
         [0020]     In accordance with a further improved embodiment, the second electrode region is made of aluminum, titanium or platinum.  
         [0021]     According to a further preferred embodiment, the substrate is a silicon substrate.  
         [0022]     In accordance with a further preferred embodiment, the applied metal oxide nanoparticles are NiO 1-x  nanoparticles. Herein, x is in a range of 0.5 to 0.95, and preferably 0.7 to 0.9.  
         [0023]     According to a further preferred embodiment, the first conductive electrode region is a tungsten bottom contact.  
         [0024]     According to a further preferred embodiment, the second conductive electrode region is made of aluminum, titanium or platinum.  
         [0025]     According to a further preferred embodiment, the dielectric between the metal oxide nanoparticles includes SiO 2 , Si 3 N 4  or Al 2 O 3 .  
         [0026]     In accordance with a further preferred embodiment, the dielectric SiO 2  is deposited through dissociation of hexamethyldisiloxane.  
         [0027]     In accordance with a further preferred embodiment, the metal oxide nanoparticles have a size of 2.5 to 15 nm, and preferably 3 to 8 nm.  
         [0028]     In accordance with a further preferred embodiment, the memory cells are arranged in a crosspoint array.  
         [0029]     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  diagrammatically shows an arrangement of metal oxide nanoparticles embedded in a dielectric between two electrodes in an embodiment of the present invention;  
         [0031]      FIG. 2   a  shows a nanostructure with 2.9 nm particle size manufactured in accordance with an embodiment of the invention;  
         [0032]      FIG. 2   b  shows a nanostructure with 7.9 nm particle size manufactured in accordance with an embodiment of the invention;  
         [0033]      FIG. 3  illustrates the formation of a conductive filament in a nanoparticle;  
         [0034]      FIG. 4  shows a memory cell arrangement in the form of a crosspoint array serving as an embodiment of the present invention;  
         [0035]      FIG. 5   a  shows a cross-sectional view through  FIG. 4 , taken along the line A-A′;  
         [0036]      FIG. 5   b  shows a cross-sectional view through  FIG. 4 , taken along the line B-B′;  
         [0037]      FIG. 6  illustrates a manufacturing process for a bottom contact. 
     
    
       [0038]     Identical reference symbols in the Figures denote identical or functionally identical components.  
       DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0039]      FIG. 1  illustrates a memory element S. Two nanoparticles  10  of, for example, NiO 1-x  are embedded in an isolating matrix  11  of a dielectric. The nanoparticles  10  contact a bottom contact  9  via contact locations K 1  and a top contact  12  via contact locations K 2 .  
         [0040]      FIGS. 2   a  and  2   b  illustrate a nanoparticle structure with a particle size of 2.9 nm and 7.9 nm, respectively, manufactured in accordance with the embodiment.  
         [0041]      FIG. 3  shows a NiO 1-x  nanoparticle  10  in an isolating matrix  11  without (left) and with (right) conductive filament. The nanoparticle is contacted by the bottom electrode  9  and the top electrode  12 .  
         [0042]      FIGS. 4 and 5  illustrate the configuration of a memory element array.  FIG. 4  shows a top view of a memory element array. Word lines  9 - 1 ,  9 - 2  and  9 - 3  as well as bit lines  12 - 1 ,  12 - 2  and  12 - 3  are arranged on a substrate  1  made of e.g. silicon. An isolating matrix  11  in which nanoparticles are embedded is arranged between the word lines and the bit lines. The individual word lines and bit lines are in contact with the memory elements S 1 -S 9 .  
         [0043]      FIG. 5   a  shows a cross-sectional view taken along the line A-A′, and  FIG. 5   b  shows a cross-sectional view taken along the line B-B′ in  FIG. 4 . In  FIG. 5   a , a wordline  9 - 1  is arranged on a substrate  1 . An isolating matrix  11  in which nanoparticles are embedded is arranged on this wordline. The individual memory elements S 7 , S 8  and S 9  are contacted via bit lines  12 - 1 ,  12 - 2  and  12 - 3  which run, for example, perpendicularly to the wordline  9 - 1 .  
         [0044]      FIG. 5   b  shows a cross-sectional view along the line B-B′ in  FIG. 4 . A layer of silicon oxide  2 , an etch stop layer of silicon nitride  2   a  and another layer of silicon oxide  3  are arranged on the substrate  1 . Wordline contacts  9 - 1 ,  9 - 2  and  9 - 3  are arranged in the layer made of silicon oxide  3 . An isolating matrix layer  11  is arranged on the layer of silicon oxide  3 . Nanoparticles which define the memory elements S 7 , S 4  and S 1  via the word lines  9 - 1 ,  9 - 2  and  9 - 3  are embedded in this layer  11 . A bitline  12 - 1  which contacts the memory elements S 7 , S 4  and S 1  is arranged on the layer of the isolating matrix  11 .  
         [0045]      FIG. 6   a  to  6   d  illustrate the manufacturing process of a bottom electrode. A layer of silicon oxide  2 , an etch stop layer made of silicon nitride  2   a  and another layer of silicon oxide  3   a  are arranged on a silicon substrate  1 .  FIG. 6   b  illustrates how tungsten  9 - 1   a  is deposited into the layer of silicon oxide  3   a .  FIG. 6   c  illustrates a layer of silicon oxide  3   b  arranged on the layer of silicon oxide  3   a . This layer of silicon oxide  3   b  completely covers the deposition of tungsten  9 - 1   a . The layers  3   a  and  3   b  can be combined to layer  3 .  FIG. 6   d  shows how a sublithographic plug  9 - 1   b  has been arranged in the layer of silicon oxide  3   b , contacting the deposition of tungsten  9 - 1   a . Both elements  9 - 1   a  and  9 - 1   b  can be combined to a bottom contact  9 - 1 .  
         [0046]     The manufacture of the memory elements is based on a silicon wafer that has been pre-cleaned in accordance with conventional technology. The electrode structure on this silicon wafer is fabricated in accordance with CMOS technology. This is diagrammatically shown for the formation of the bottom electrode in  FIG. 6 . In accordance with the current state of the art, the size of the sublithographic plugs which define the effective size of the memory cells is about 40 nm. Thus, the dimensions of the memory elements are determined by the dimensions that are currently possible in CMOS technology. As this technology advances, smaller memory elements will be possible.  
         [0047]     The fabrication of the metal oxide nanoparticles is performed in accordance with R. F. Mulligan, A. Iliadis, P. Kofinas; J. Appl. Polymer Science, vol. 89 (2003) und R. T. Clay, R. E. Cohen; Supramol. Scienc., vol. 5 (1998). In the examples of Mulligan et al., zinc oxide nanoparticles are deposited. However, this process can be analogously performed for e.g. NiO.  
         [0048]     The metal oxide nanoparticles are generated to a process of self-organizing deposition (self-assembled monolayers, SAM) of diblock-copolymer-monolayers. These can be made of polynorbornene and polynorbornene carboxylic acid and form the basis for homogeneous deposition of metal oxide nanocrystallites.  
         [0049]     The nanoparticles are embedded in a diblock copolymer. This diblock copolymer can be made of polynorbornene and poly(norbornenedicarboxylic acid), for example. After synthesizing the copolymer, the copolymer is again added to a solution after drying, and a desired amount of NiCl 2 , which is also dissolved in e.g. tetrahydrofurane, is then added to the solution. The Ni 2+  ions present in the solution associate with the carboxylic groups of one component of the copolymer. Applying this solution to the tungsten bottom contact, a hexagonally ordered layer grows in a self-organizing process. The solution can be applied by spin-coating, for example. However, it is also possible to simply immerse the wafer into the solution. The resulting metal salt is subsequently converted into the corresponding metal oxide by an ammonium hydroxide base. The copolymer is removed by plasma-ashing.  
         [0050]     Thereafter, the nickel oxide particles are partially reduced by a hydrogen processing gas in a plasma process and brought into the oxygen-deficient NiO 1-x  composition. Remaining on the wafer is a homogeneous dense structure of NiO 1-x  nanoparticles, as is illustrated in  FIG. 2  for nanoparticles of 2.9 and 7.9 nm size.  
         [0051]     In order to isolate the nickel oxide nanoparticles  10 , a thin SiO 2  layer  11  is deposited subsequently by dissociation of hexamethyldisiloxane. This layer  11  is removed by chemical-mechanical polishing (CMP) from the NiO 1-x  particles  10 , until the isolating SiO 2  remains only in the gaps therebetween. Providing the dielectric prevents a short-circuit between the top and bottom contacts. The arrangement of a top contact  12  made of, for example, aluminum, titanium or platinum by sputtering completes the memory arrangement.  
         [0052]     Although the present invention has been described by way of preferred embodiments, it is by no means limited thereto, and numerous modifications are possible. In particular, the nanoparticles may also be made of transition metal oxides, particularly oxides of niobium, titanium, tungsten, vanadium and iron.  
       LIST OF REFERENCE NUMERALS  
       [0000]    
       
           1  Silicon substrate  
           2  Silicon oxide layer  
           2   a  Etch stop layer of silicon nitride  
           3  Silicon oxide layer  
           3   a  Silicon oxide layer  
           3   b  Silicon oxide layer  
           4  NiO 1-x  nanoparticle  
           5  Silicon oxide isolating layer  
           6  Top contact  
           9 - 1  Bottom contact  
           9 - 1   a  Tungsten deposition  
           9 - 1   b  Sublithographic plug made of tungsten  
           10  Nanoparticles  
           11  Isolating matrix  
           12  Top contact  
          A, A′ Section line  
          B, B′ Section line  
          K 1  Contact location to bottom contact  
          K 2  Contact location to top contact  
          S Memory element  
          S 1 -S 9  Memory elements