Abstract:
A memory device using a multi-layer with a graded resistance change is provided. The memory device includes: a lower electrode; a data storage layer being located on the lower electrode and having the graded resistance change; and an upper electrode being located on the data storage layer.

Description:
[0001]     Priority is claimed to Korean Patent Application No. 10-2004-0029675, filed on Apr. 28, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a memory device using a multi-layer material with a graded resistance change, and more particularly, to a nonvolatile memory device using a multi-layered resistance change material or a multi-layered resistance change film with a graded resistance change, and a method thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     Semiconductor memory device has been developed for the purpose of a large number of memory cells per unit area (high integration), a high operation speed and lower power driving. A various memory devices are being developed.  
         [0006]     The semiconductor memory device generally includes many memory cells having a circuit connection. In the case of Dynamic Random Access Memory (DRAM), a unit memory cell generally includes one transistor and one capacitor. The DRAM has an advantage of the high integration and the high operation speed, but is a volatile memory in which all data are erased after power is turned-off.  
         [0007]     A typical example of the nonvolatile memory device is a flash memory in which the stored data can be preserved even after the power is turned-off. The flash memory has a nonvolatile characteristic unlike the DRAM, but has a low integration and a low operation speed in comparison with the DRAM.  
         [0008]     As a present nonvolatile memory device being much studied, there are Magnetic Random Access Memory (MRAM), Ferroelectric Random Access Memory (FRAM), Phase-change Random Access Memory (PRAM) and the like. The MRAM stores data by using a change of a magnetization direction in a tunnel junction. The FRAM stores data by using a polarization characteristic of ferroelectric substance. The PRAM stores data by using a phase change of a specific material. All of them have respective advantages and disadvantages, but are being developed for the high integration, the high operation speed, the low power driving and a good data retention.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a newly structured nonvolatile semiconductor memory device using a resistance change in which a multi-layer material with a graded resistance that changes over a thickness of the multi-layered material is used to perform low power driving and high-speed operation.  
         [0010]     According to an aspect of the present invention, there is provided a memory device using a multi-layer material with a graded resistance change, the device including: a lower electrode; a data storage layer being located on the lower electrode and having the graded resistance that changes over a thickness of the data storage layer; and an upper electrode being located on the data storage layer.  
         [0011]     The data storage layer may have a resistance layer containing a transition metal material.  
         [0012]     The data storage layer may include: a first resistance layer being located on the lower electrode; a second resistance layer being located on the first resistance layer, and having the transition metal material; and a third resistance layer being located on the second resistance layer.  
         [0013]     The second resistance layer may have a larger resistance value than the first resistance layer and the third resistance layer.  
         [0014]     The data storage layer may include: the first resistance layer being located on the lower electrode and having the transition metal material; and the second resistance layer being located on the first resistance layer, and having a lower resistance value than the first resistance layer.  
         [0015]     The data storage layer may include: the first resistance layer being located on the lower electrode; and the second resistance layer being located on the first resistance layer and having the transition metal material, and having a larger resistance value than the first resistance layer.  
         [0016]     The transition metal material may be selected from the group consisting of nickel, titanium, hafnium, niobium, zinc, tungsten and cobalt.  
         [0017]     According to another aspect of the present invention, there is provided a memory device using a multi-layer with a graded resistance change, the device including: a semiconductor substrate; a first impurity region and a second impurity region being located in the semiconductor substrate; a gate oxide film and a gate electrode layer being sequentially located on the semiconductor substrate to be in contact with the first impurity region and the second impurity region; a lower electrode being located on the second impurity region to be electrically connected with the second impurity region; a data storage layer being located on the lower electrode, and having the graded resistance that changes over a thickness of the data storage layer; and an upper electrode located on the data storage layer.  
         [0018]     The data storage layer may be located to have a resistance layer containing the transition metal material.  
         [0019]     The transition metal material may be selected from the group consisting of nickel, titanium, hafnium, niobium, zinc, tungsten and cobalt.  
         [0020]     According to another aspect of the present invention, there is provided a memory device using a multi-layer with a graded resistance change, the device including: a semiconductor substrate; a diode being located on the semiconductor substrate and having a switching function; and a data storage layer being connected to the diode, and having the graded resistance that changes over a thickness of the data storage layer.  
         [0021]     The data storage layer may be formed to have a resistance layer containing a transition metal material with content being varied.  
         [0022]     The transition metal material may be selected from the group consisting of nickel, titanium, hafnium, niobium, zinc, tungsten and cobalt. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0024]      FIGS. 1A through 1C  are views illustrating multi-layer materials with graded resistance changes according to exemplary embodiments of the present invention;  
         [0025]      FIG. 2  is a view illustrating an example of a transistor structure to which a memory device using a multi-layer with a graded resistance change is applied;  
         [0026]      FIG. 3A  is a graph illustrating a voltage-current characteristic of a multi-layer material with a graded resistance change shown in  FIGS. 1A through 1C ;  
         [0027]      FIG. 3B  is a graph illustrating a resistance change of a nickel oxide depending on a weight percent of oxygen (O 2 ), in a nickel oxide used for a multi-layer according to a preferred embodiment of the present invention;  
         [0028]      FIG. 4A  is a graph illustrating an operation characteristic of a memory device having a single-layered film with a single regular resistance distribution; and  
         [0029]      FIG. 4B  is a graph illustrating an operation characteristic of a memory device having a multi-layer with a graded resistance change according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0031]      FIGS. 1A through 1C  are views illustrating multi-layers with graded resistance changes according to embodiments of the present invention.  
         [0032]     Referring to  FIG. 1A , a multi-layered structure is formed to have a first resistance layer  12   a  having a resistance value R 1 , a second resistance layer  12   b  having a resistance value R 2 , and a third resistance layer  12   c  having a resistance value R 3 , which are sequentially layered on a lower electrode  11 . Additionally, an upper electrode  13  is formed on the third resistance layer  12   c.    
         [0033]     Here, the lower electrode  11  and the upper electrode  13  generally use a conductive material, which can be used as an electrode material of a semiconductor device, for example, as a metal material. Particularly, the lower electrode  11  can be selectively determined depending on a kind of material formed thereon. A multi-layer resistant body with the graded resistance change (variable resistance) is formed on the lower electrode  11 . The first resistance layer  12   a,  the second resistance layer  12   b  and the third resistance layer  12   c  basically use an insulating material having a low electrical conductivity. The present invention is characterized in that the multi-layer with the graded resistance change is formed between the lower electrode  11  and the upper electrode  13 . Here, the resistance values R 1  and R 2  of the first resistance layer  12   a  and the second resistance layer  12   b  are different from each other, and the resistance values R 1  and R 3  of the second resistance layer  12   b  and the third resistance layer  12   c  are different from each other.  
         [0034]     In an embodiment of  FIG. 1A , the resistance value R 2  of the second resistance layer  12   b  may be larger than the resistance values R 1  and R 3  of the first resistance layer  12   a  and the third resistance layer  12   c  (R 2 &gt;R 1 , R 3 ). Additionally, the second resistance layer  12   b  having a relatively high resistance value R 2  may be formed of material containing a transition metal (for example, a transition metal oxide). The transition metal oxide is a nickel oxide (NiO), a titanium oxide (TiO 2 ), a hafnium oxide (HfO), a niobium oxide (NbO 2 ), a zinc oxide (ZnO), a tungsten oxide (WO 3 ), and a cobalt oxide (CoO), for example.  
         [0035]     The first resistance layer  12   a  and the third resistance layer  12   c  can use the same transition metal oxide as the second resistance layer  12   b,  but can be formed of other insulating materials. In case where the first resistance layer  12   a  and the third resistance layer  12   c  use the transition metal oxide, they have a resistance value lower than the resistance value R 2  of the second resistance layer  12   b.  The first resistance layer  12   a,  the second resistance layer  12   b  and the third resistance layer  12   c  are arranged as follows.  
         [0036]     First, the second resistance layer  12   b  is formed including the transition metal material, and may be formed of the transition metal oxide.  
         [0037]     Second, the first resistance layer  12   a  and the third resistance layer  12   c  are formed of an insulating material having a lower resistance value than the resistance value R 2  of the second resistance layer  12   b.    
         [0038]     Third, like the second resistance layer  12   b,  the first resistance layer  12   a  and the third resistance layer  12   c  can be also formed of the transition metal oxide, but is formed to have a lower resistance value than the resistance value of the second resistance layer  12   b.    
         [0039]     For reference, the first resistance layer  12   a,  the second resistance layer  12   b  and the third resistance layer  12   c  can be also formed of the same transition metal oxide. However, the second resistance layer  12   b  is also formed to have the resistance value R 2  higher than the resistance values R 1  and R 3  of the first resistance layer  12   a  and the third resistance layer  12   c.  For example, in case where the nickel oxide is used as one of the transition metal oxide, if a concentration of oxygen is controlled, even the same material has a different resistance value depending on a concentration distribution of oxygen. Accordingly, a multi-layer material with the graded resistance change is formed using the above characteristic.  
         [0040]     Unlike  FIG. 1A ,  FIGS. 1B and 1C  are views illustrating the multi-layers with the graded resistance changes having the two resistance layers. However, it should be noted that the resistance layer  12   b,  which is formed of the transition metal oxide, represents a resistance layer region containing the transition metal having a higher resistance value than those of other resistance layers in the multi-layer with the graded resistance change.  
         [0041]     The multi-layer material with the graded resistance change according to embodiments of the present invention can be easily manufactured using a general semiconductor technique. In case where the first resistance layer  12   a,  the second resistance layer  12   b  and the third resistance layer  12   c  can be formed of the same transition metal oxide (for example, the nickel oxide), an amount of oxygen can be suitably controlled such that the multi-layer material is consecutively formed to have different resistance values in a direction of thickness.  
         [0042]     In order to embody the multi-layer material with the graded resistance change as the memory device according to the embodiment of the present invention, the multi-layer material connected with a transistor structure having a switching function is illustrated in an embodiment of  FIG. 2 . Here, a diode can be used as a switching device excepting the transistor.  
         [0043]     Referring to  FIG. 2 , a first impurity region  25   a  and a second impurity region  25   b  having predetermined doped impurities are formed at both sides of a portion of a semiconductor substrate  24 . A channel region (without a reference number) is formed in the semiconductor substrate  24  between the first impurity region  25   a  and the second impurity region  25   b.  A gate insulating layer  26  and a gate electrode layer  27  are formed on the channel region. The multi-layer material with the graded resistance change can be directly formed on the first impurity region  25   a  (e.g., source) and/or the second impurity region  25   b  (e.g., drain).  FIG. 2  illustrates the multi-layer material connected on the second impurity region  25   b.    
         [0044]     A lower electrode  21  is formed on the second impurity region  25   b.  The first resistance layer  22   a,  the second resistance layer  22   b  and the third resistance layer  22   c  are sequentially formed, correspondingly to  FIG. 1A , on the lower electrode  21  to form a variable resistance structure. An upper electrode  23  is formed on the third resistance layer  22   c.    FIG. 2  illustrates the multi-layer material with the graded resistance change directly formed on the second impurity region  25   b  to include the lower electrode  21 . However, it should be noted that a connection structure through the substrate  24  to the second impurity region  25   b  and the conductive plug (not shown) can be formed in  FIG. 2 .  
         [0045]     The memory device of  FIG. 2  has a 1T-1R structure including one resistance in one transistor, and the multi-layer structure  12   a,    12   b  and  12   c  is a data storage layer. The above-structured memory device can be easily manufactured on the basis of a general manufacture process of the semiconductor memory device.  
         [0046]     The description of an exemplary fabrication method is as follows.  
         [0047]     First, a gate insulating layer  26  and a gate electrode layer  27  are sequentially deposited on the semiconductor substrate  24 . Both sides of the gate insulating layer  26  and the gate electrode layer  27  are removed to expose both side upper portions of the semiconductor substrate  24 . Next, predetermined impurities are doped, using implantation, at both sides of the exposed semiconductor substrate  24 . By doing so, the first impurity region  25   a  and the second impurity region  25   b  are formed, thereby completing a transistor structure. Additionally, an interlayer insulating layer (not shown) can be deposited on the transistor structure. The second impurity region  25   b  is exposed. The lower electrode  21 , the first resistance layer  22   a,  the second resistance layer  22   b,  the third resistance layer  22   c,  and the upper electrode  23  are sequentially formed on the exposed second impurity region  25   b  such that the memory device of the  1 T- 1 R structure can be manufactured as shown in  FIG. 2 .  
         [0048]     Hereinafter, an operation characteristic of the memory device using the multi-layer material with the graded resistance change according to an exemplary embodiment of the present invention is in detail described with reference to the attached drawings.  
         [0049]      FIG. 3A  is a graph illustrating an electrical characteristic of the multi-layer with the graded resistance change in which the resistance layers  22   a,    22   b  and  22   c  are formed using the nickel oxide. A horizontal axis represents a voltage applied to the resistance layers  22   a,    22   b  and  22   c,  and a vertical axis represents a current flowing in the resistance layer with respect to the applied voltage.  
         [0050]     Referring to  FIG. 3A , in the case of the memory device using the multi-layer material with the graded resistance change according to an exemplary embodiment of the present invention, two operation characteristics are shown.  
         [0051]     First, if a zero voltage applied to the resistance layers  22   a,    22   b  and  22   c  is gradually increased, the current is increased in proportion to the applied voltage. However, if the voltage of more than V 1  is applied, the resistance is rapidly increased to decrease the current. Additionally, if the voltage of more than V 2 (V 2 &gt;V 1 ) is applied, the resistance is rapidly decreased to increase the current, thereby again following a graph G 1 .  
         [0052]     Alternatively, the electrical characteristic of the resistance layers  22   a,    22   b  and  22   c  depending on the voltage larger than V 1  applied thereto influences an electrical characteristic of when a voltage of less than V 1  is subsequently applied. This is in detail described as follows.  
         [0053]     First, in case where a voltage of V 1  to V 2  is applied to the resistance layers  22   a,    22   b  and  22   c  and then, the voltage of less than V 1  is again applied to the resistance layers  22   a,    22   b  and  22   c,  the current is measured following a graph G 2  of  FIG. 3A . Additionally, in case where a voltage (V 3 ) larger than V 2  is applied to and then, a voltage of less than V 1  is again applied to the resistance layers  22   a,    22   b  and  22   c,  the current is measured following the graph G 1  of  FIG. 3A . By this, it can be appreciated that the electrical characteristic remains without disappearance to influence the resistance layers  22   a,    22   b  and  22   c  in dependence upon a magnitude (a voltage of V 1  to V 2  or a voltage of more than V 2 ) of the voltage of more than V 1 .  
         [0054]     Accordingly, the transition metal oxide can be employed in the resistance layers  22   a,    22   b  and  22   c  to apply the multi-layer with the graded resistance change to the nonvolatile memory device. That is, in case where the voltage of V 1  to V 2  of  FIG. 3A  is applied, the resistance layers  22   a,    22   b  and  22   c  are designated to be in a state of ‘0’, and in case where the voltage larger than V 2  is applied, the resistance layers  22   a,    22   b  and  22  are designated to be in a state of ‘1’, thereby recording data. In case where data is reproduced, the voltage of less than V 1  is applied to measure a drain current (Id), thereby identifying whether or not data stored in the resistance layers  22   a,    22   b  and  22   c  are in a state of ‘0’ or ‘1’. Of course, the state of ‘1’ or ‘0’ is selectively designated.  
         [0055]      FIG. 3B  is a graph illustrating the graded resistance change of the nickel oxide depending on a content (wt %) of oxygen (O 2 ) of the nickel oxide (NiO) used for the multi-layer material as shown in  FIG. 2 . As described above, the first resistance layer  22   a,  the second resistance layer  22   b  and the third resistance layer  22   c,  which constitute the multi-layer material with the graded resistance change of  FIG. 2 , can be formed of the same transition metal oxide having a varied content of oxygen. Since the resistance characteristic of the transition metal oxide is different depending on the content of oxygen, the oxygen content of the resistance layers  22   a,    22   b  and  22   c  should be controlled to represent the electrical characteristic as in  FIG. 3A .  
         [0056]     Referring to  FIG. 3B , a region (switching region) having the oxygen content of about 2 to 9.5 wt % has the resistance value of the nickel oxide, which is larger than those of other regions (buffer region), and has the electrical characteristic represented as in  FIG. 3A . Accordingly, in the region having the oxygen content of about 2 to 9.5 wt %, the nickel oxide is used as the second resistance layer  22   b.  Additionally, other nickel oxides having the oxygen content may be used as the first resistance layer  22   a  and the third resistance layer  22   c.  Of course, the first resistance layer  22   a  and the third resistance layer  22   c  can be formed of other insulating materials than the transition metal oxide. They can be applied to the multi-layer material with the graded resistance change according to the embodiment of the present invention.  
         [0057]      FIGS. 4A and 4B  are graphs illustrating a comparison of electrical characteristics of a memory device having a single resistance layer and a memory device having a multi-layer with the graded resistance change according to an embodiment of the present invention.  
         [0058]     A horizontal axis represents the magnitude (V) of the voltage applied to the resistance layer, and a vertical axis represents the current measured in the resistance layer. Since the electrical characteristic of  FIG. 3A  is represented even in the memory device having a single-layered transition metal oxide without the graded resistance change, it can function as a memory device. However, in case where the multi-layer with the graded resistance change is used, a stable switching characteristic can be obtained such that the memory device has a more excellent characteristic.  
         [0059]      FIG. 4A  illustrates the electrical characteristic of the memory device using the single resistance layer. In  FIG. 4A , the nickel oxide corresponding to the switching region of  FIG. 3B  is formed between the lower electrode and the upper electrode. Referring to  FIG. 4A , it can be appreciated that it is difficult to specify magnitudes of mutually varied voltages (V 1  and V 2  of  FIG. 3A ) by using the graphs G 1  and G 2 . Additionally, a region (ΔV) corresponding to a difference (V 2 −V 1 ) between V 1  and V 2  of  FIG. 3A  can be set, but is not so large.  
         [0060]      FIG. 4B  illustrates the electrical characteristic of the memory device with the graded resistance change according to the embodiment of the present invention.  
         [0061]     Here, the first resistance layer  22   a  and the third resistance layer  22   c  of an experimental specimen are formed of the nickel oxide with the oxygen content corresponding to the buffer region of  FIG. 3B , and the second resistance layer  22   b  is formed of the nickel oxide with the oxygen content corresponding to the switching region of  FIG. 3B . Additionally, the first resistance layer  22   a,  the second resistance layer  22   b  and the third resistance layer  22   c  are formed to have the same thickness of about 50 mm.  
         [0062]     Referring to  FIG. 4B , it can be appreciated that the magnitudes of the mutually varied voltages (V 1  and V 2  of  FIG. 3A ) can be easily specified using the graphs G 1  and G 2 . Additionally, the region (ΔV) corresponding to the difference (V 2 −V 1 ) between V 1  and V 2  of  FIG. 3A  can be easily set. Additionally, since the inventive memory device can operate at a relatively lower voltage and current region, low power driving can be performed.  
         [0063]     As described above, embodiments of the present invention provide a non-volatile semiconductor device with the new structure of 1T-1R or 1D-1R. Since embodiments of the present invention can use a widely used semiconductor process such as a conventional DRAM manufacture process as it is, manufacture can be easily implemented and productivity can be enhanced. Further, embodiments of the present invention have an advantage in that, since the operational change in resistance characteristic of the multi-layer material with the graded resistance is used to store and reproduce information, integration can be enhanced with a high-speed operation characteristic and a lower power driving.  
         [0064]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.  
         [0065]     For example, the present invention provides the multi-layer material of two or three layers with the graded resistance changes, but can include multiple layers with the graded resistance changes. Further, the inventive memory device provides the multi-layer material with the graded resistance change connected with the transistor, but it is obvious that the multi-layer material can be connected with a diode, which can serve as another switching device, to be used as the memory device of 1D-1R form.