Abstract:
A phase-change memory device more precisely controls electrical current required to accomplish a phase change by using contact holes that extend between phase change layers that are sized differently from each other.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a phase change memory device and a method of manufacturing the same. More particularly, the present invention relates to a phase-change memory device and a method of making a phase change memory device, in which a contact area between a bottom electrode and a phase-change layer is reduced.  
       BACKGROUND  
       [0002]     As is generally known in the art, semiconductor memory devices are mainly classified into a volatile random access memory (RAM), which loses information stored therein when power is cut off, and non-volatile read only memory (ROM), which continuously retains information stored therein even if power is cut off. Volatile semiconductor RAM includes dynamic RAM (DRAM) and static RAM (SRAM). The non-volatile semiconductor ROM includes flash memory such as an electrically erasable and programmable ROM.  
         [0003]     However, as generally known to those skilled in the art, even though the DRAM is a memory device having superior performance, the DRAM stores data using a capacitance effect and therefore it requires relatively high storage capacitance. Since the storage capacity of an electrical capacitor is directly proportional to the spacing between electrodes the material between electrodes and the surface area of the electrodes, DRAMs need relatively large areas just to accommodate the capacitors needed to store charges that represent data. Put another way, as a semiconductor data storage device, the DRAM presents a problem in view of high integration.  
         [0004]     Prior art flash memory devices present a different set of problems. Flash memory requires operating voltage higher than supply voltage because the flash memory has a stacked structure of two gates. For this reason, the flash memory device must be equipped with a separate voltage-booster circuit in order to obtain voltages required for writing or erasing operations. In this regard, the flash memory device disturbs high integration of a circuit.  
         [0005]     Relatively recent research has developed new highly integrated memory devices having a simple structure that provides a non-volatile memory. One example of a new highly integrated non-volatile memory device is a a phase change RAM device.  
         [0006]     A phase change RAM device determines information stored in a cell according to a resistance difference between a crystalline state and an amorphous state of a phase change layer. This phase-change layer is interposed between electrodes and undergoes a phase change from the crystalline state to the amorphous state as current flows between the electrodes.  
         [0007]     Phase-change memory devices employ a chalcogenide film as the phase-change layer. The chalcogenide film is a compound material layer consisting of germanium (Ge), stibium (Sb), and tellurium (Te). Electrical current through the material causes it to undergo a phase change between the crystalline state and the amorphous state by reason of Joule heat caused by the electrical current. The phase-change layer has a higher electrical resistance in the amorphous state than in the crystalline state. Information stored in the phase-change memory cell can be either a logic “1” or “0” by detecting the current flowing through the phase-change layer in writing or reading modes.  
         [0008]      FIG. 1  is a sectional view of a conventional phase-change memory device.  
         [0009]     As shown in  FIG. 1 , gates  4  are formed on a semiconductor substrate  1 , and a junction area (not shown) is formed on the surface of the semiconductor substrate at both sides of the gate  4 . An insulating interlayer  5  is formed on the entire surface of the semiconductor substrate  1  in such a manner that the insulating interlayer  5  covers the gates  4 . A first tungsten plug  6   a  and a second tungsten plug  6   b  are formed at predetermined portions of the insulating interlayer  5  where a phase change cell is formed and a ground voltage (Vss) is applied, respectively.  
         [0010]     A first oxide layer  7  is formed on the insulating interlayer  5  including the first and second tungsten plugs  6 A and  6 B. Although it is not shown in detail, in the first oxide layer  7 , a dot-type metal pad  8  is formed in a predetermined area where the phase-change cell is formed, such that the dot-type metal pad  8  is in contact with the first tungsten plug  6 A, and a bar-type ground line (Vss line)  9  is formed in a predetermined area to which a ground voltage is applied, such that the bar-type ground line is in contact with the second tungsten plug  6 B.  
         [0011]     A second oxide layer  10  is formed on the first oxide layer  7  including the metal pad  8  and the ground line  9 . A plug-type bottom electrode  11  is formed in a predetermined area of the second oxide layer  10 , onto which the phase-change cell will be formed, such that plug-type bottom electrode  11  is in contact with the metal pad  8 .  
         [0012]     A phase-change layer  12  and a top electrode  13  are sequentially stacked in a predetermined pattern on the second oxide layer  10 , so as to be in contact with the bottom electrode  11 , thereby forming the phase-change cell including the plug-type bottom electrode  11 , phase-change layer  12  and top electrode  13  which have been sequentially stacked.  
         [0013]     Then, a third oxide layer  14  is formed on the second oxide layer  10  so as to cover the phase-change cell, and a metal line  15  is formed on the third oxide layer  14  so as to be in contact with the top electrode  13 .  
         [0014]     Meanwhile, the conventional phase-change memory device requires a very high current (e.g. 1 mA or more) in order to achieve stable phase change. Therefore, in order to decrease the current required for the phase change of the phase-change layer, it is necessary to reduce the contact area between the phase-change layer and the electrodes.  
         [0015]     The conventional exposure and etching techniques present limitations to reduce the contact area between the phase-change layer and the electrodes.  
         [0016]     In addition, according to the conventional phase-change memory device shown in  FIG. 1 , although the phase-change layer  12  is in contact with the top electrode  13  as well as the bottom electrode  11 , only the area being in contact with the bottom electrode  11  is generally used as a phase-change area because it is impossible to use both of the contact areas as phase-change areas. Accordingly, the phase change of the phase-change layer  12  depends on a contact resistance between the phase-change layer  12  and the bottom electrode  11 . However, as described above, because of the limitations of the conventional techniques, it is difficult to stably form the contact area between the phase-change layer  12  and the bottom electrode  11 , so that variation of the contact resistance becomes increased, thereby deteriorating reliability of products.  
       SUMMARY OF THE INVENTION  
       [0017]     In light of the foregoing, At least one object of the present invention is to provide a phase-change memory device, in which the contact area between a bottom electrode and a phase-change layer is reduced. In order to accomplish this and other objects, there is provided a phase-change memory device comprising: a semiconductor substrate formed with a lower pattern and a first insulating layer covering the lower pattern; a first electrode formed on the first insulating layer; a second insulating layer formed on the first insulating layer so as to cover the first electrode, and formed with a first contact hole for exposing a predetermined portion of the first electrode; a first phase-change layer formed at an inner portion of the first contact hole and on a predetermined portion of the second insulating layer adjacent to the first contact hole; a third insulating layer formed on the second insulating layer so as to cover the first phase-change layer, and formed with a second contact hole for exposing a predetermined portion of the first phase-change layer; a second phase-change layer formed at an inner portion of the second contact hole; and a second electrode formed on the second phase-change layer and on a predetermined portion of the third insulating layer adjacent to the second phase-change layer, wherein one hole of the first and second contact holes has a relatively larger size.  
         [0018]     Herein, the second contact hole may have a size larger than that of the first contact hole, so that a phase change of the first phase-change layer is generated at an interfacial portion between a first portion of the first phase-change layer filled in the first contact hole and a second portion of the first phase-change layer formed on the second insulating layer.  
         [0019]     Also, the first contact hole may have a size larger than that of the second contact hole, so that a phase change of the second phase-change layer is generated at an interfacial portion between the first phase-change layer and the second phase-change layer.  
         [0020]     The first electrode is a bottom electrode, and the second electrode is a top electrode.  
         [0021]     The first phase-change layer and the second phase-change layer are made from materials different from each other.  
         [0022]     In accordance with another aspect of the present invention, there is provided a method of manufacturing a phase-change memory device, the method comprising the steps of: forming a first insulating layer on a semiconductor substrate having a lower pattern; forming a first electrode on the first insulating layer; forming a second insulating layer on the first insulating layer so as to cover the first electrode; forming a first contact hole for exposing a predetermined portion of the first electrode by etching the second insulating layer; forming a first phase-change layer at an inner portion of the first contact hole and on a predetermined portion of the second insulating layer adjacent to the first contact hole; forming a third insulating layer on the second insulating layer so as to cover the first phase-change layer; forming a second contact hole for exposing a predetermined portion of the first phase-change layer by etching the third insulating layer; forming a second phase-change layer at an inner portion of the second contact hole; and forming a second electrode on the second phase-change layer and on a predetermined portion of the third insulating layer adjacent to the second phase-change layer, wherein one hole of the first and second contact holes has a relatively larger size.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0024]      FIG. 1  is a sectional view of a conventional phase-change memory device;  
         [0025]      FIGS. 2A  to  2 D are sectional views for showing a non-volatile memory device and a method of manufacturing a non-volatile phase-change memory device according to an embodiment of the present invention; and  
         [0026]      FIG. 3  is a sectional view of a phase-change memory device manufactured according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]     Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the following description and drawings, the same reference numerals are used to designate the same or similar components, so repetition of the description on the same or similar components will be omitted.  
         [0028]      FIGS. 2A  to  2 D are sectional views for showing a method of manufacturing a phase-change memory device according to an embodiment of the present invention.  FIGS. 2A and 2D  also show a non-volatile memory device.  
         [0029]     Referring to  FIG. 2A , after a semiconductor substrate  21  having a lower pattern including a transistor is prepared, a first insulating layer  22  is formed on the semiconductor substrate  21  so as to cover the lower pattern. Then, according to the typical process, a contact plug  23  being in contact with the lower pattern or semiconductor substrate  21  is formed in the first insulating layer  22 . Next, a conducting layer is deposited on the first insulating layer  22  including the contact plug  23 , and is then subjected to a patterning process so as to form a first electrode (i.e. a bottom electrode)  24  on the contact plug  23  and on a predetermined portion of the first insulating layer  22  adjacent to the contact plug  23 .  
         [0030]     Referring to  FIG. 2B , a second insulating layer  25  is formed on the first insulating layer  22  so as to cover the bottom electrode  24 . Next, a first contact hole  26  for exposing a predetermined portion of the bottom electrode  24  by etching the second insulating layer  25 . Then, a phase-change material is deposited on the second insulating layer  25  such that the first contact hole  26  is fully filled with the phase-change material, and is then subjected to a patterning process so as to form a first phase-change layer  27  at an inner portion of the first contact hole  26 , and on a predetermined portion of the second insulating layer  25  adjacent to the first contact hole  26 .  
         [0031]     Herein, it can be understood that the first phase-change layer  27  includes a plug-type portion formed at an inner portion of the first contact hole  26 , and a layer-type portion formed on the second insulating layer  25 . Herein, it is preferred that the layer-type portion formed on the second insulating layer  25  is formed to be as thin as possible due to a need to take into consideration a third insulating layer to be formed in the following process.  
         [0032]     Referring to  FIG. 2C , a third insulating layer  28  is formed on the second insulating layer  25  so as to cover the first phase-change layer  27 . Next, a second contact hole  29  is formed by etching the third insulating layer  28  to exposed a predetermined portion of the first phase-change layer  27 . In this case and as shown in  FIG. 2C , the second contact hole  29  has a cross-section or size that is larger than that of the first contact hole  26  such that a phase change occurs at a specific portion of the first phase-change layer  27 .  
         [0033]     Then, a phase-change material is deposited on the third insulating layer  28  such that the second contact hole  29  is fully filled with the phase-change material, and is then subjected to an etch back or CMP (Chemical Mechanical Polishing) process so as to form a second phase-change layer  30  of a plug shape at an inner portion of the second contact hole  29 . In this case, it is preferred that the second phase-change layer  30  is made from a material different from that of the first phase-change layer  27 .  
         [0034]     Referring to  FIG. 2D , a conducting layer is deposited on the third insulating layer  28  including the second phase-change layer  30 , and is then subjected to a patterning process so as to form a second electrode (i.e. top electrode  31 ) on the second phase-change layer  30  and a predetermined portion of the third insulating layer  28  adjacent to the second phase-change layer  30 .  
         [0035]     Thereafter, although it is not shown, a series of typical processes is subsequently performed to complete the phase-change memory device according to an embodiment of the present invention.  
         [0036]     According to the phase-change memory device disclosed herein and as described above, since the second contact hole has a size larger than that of the first contact hole, a phase change is generated at a predetermined portion of the first phase-change layer that is located at the interface (an interfacial portion) between a first portion of the first phase-change layer filled in the first contact hole and a second portion of the first phase-change layer formed on the second insulating layer, when the device is driven. This is because the first and second phase-change layers serve as a self-heater (i.e. a resistance component) between the bottom electrode and top electrode, and current profiles applied to the first and second phase-change layers differ from each other.  
         [0037]     Therefore, the phase-change memory device disclosed and claimed herein controls a current density (i.e. current profile) of a phase-change layer itself, instead of controlling the contact area between an electrode and a phase-change layer, thereby efficiently reducing a current required for the phase change of the phase-change layer, while not being influenced by the limitations of the exposure and etching techniques.  
         [0038]     The present invention has been disclosed in that the second contact hole is formed to have a size larger than that of the first contact hole such that a phase change may occur at a predetermined portion of the first phase-change layer. However, according to another embodiment of the present invention, as shown in  FIG. 3 , it is also possible to form first and second contact holes  26   a  and  29   a , in which the first contact hole  26   a  has a size larger than that of the second contact hole  29   a . In this case, a phase change of a second phase-change layer  30  is generated at an interfacial portion between a first phase-change layer  27  of a layer shape formed on the second insulating layer  25  and the second phase-change layer  30  formed in a plug shape.  
         [0039]     In this case also, it is possible to efficiently reduce a current required for the phase change of the phase-change layer by controlling a current profile, while not being influenced by the limitations of the exposure and etching techniques.  
         [0040]     As described above, according to the present invention, a phase-change layer interposed between a bottom electrode and a top electrode is formed so as to have a stacked structure while two stacked layers of the phase-change layer serve as a self-heater (i.e. resistance component). Accordingly, it is possible to efficiently reduce a current required for the phase change of the phase-change layer, while not being influenced by the limitations of the exposure and etching techniques. Accordingly, it is possible to uniformly control the operation of the phase-change memory device, thereby providing the phase-change memory device having high reliability.  
         [0041]     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.