Abstract:
A phase-changeable memory device comprises a substrate and an access transistor formed in and/or on the substrate. Laterally spaced apart first and second conductive patterns are disposed on the substrate and have opposing sidewalls. A conductor electrically connects the first conductive region to a source/drain region of the access transistor. A phase-changeable material region is disposed between the first and second conductive patterns and contacts the opposing sidewalls of the first and second conductive patterns. Contact areas between the conductive patterns and the phase-changeable material region are preferably substantially smaller than contact areas at which the conductive patterns contact conductors (e.g., vias) connected thereto, such that high current densities may be developed in the phase-changeable material. Methods of fabricating such devices are also discussed.

Description:
RELATED APPLICATION  
         [0001]    This application claims priority from Korean Patent Application No. 2002-52728 filed on Sep. 3, 2002 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to semiconductor memory devices and methods of fabrication therefor, and more particularly, to phase-changeable memory devices and methods of fabrication therefor.  
         BACKGROUND OF THE INVENTION  
         [0003]    Semiconductor memory devices can be categorized as either volatile memory devices or nonvolatile memory devices according to whether or not data is retained when power supplies are interrupted. The volatile memory devices may be classified into a dynamic random access memory (DRAM) and a static random access memory (SRAM). The nonvolatile memory devices include flash memory devices. These memory devices represent a logic value, such as “0” or “1,” based on stored charge. Because a periodic refresh operation is typically needed for such devices, a DRAM may require a high charge storage capacitance. Consequently, there have been attempts to increase a surface area of a capacitor electrode to increase storage capacitance. However, an increase in the surface area of the capacitor electrode can interfere with an increase in integration of the DRAM.  
           [0004]    A typical flash memory device has gate patterns including a gate insulating layer, a floating gate, a dielectric layer, and a control gate, which are sequentially stacked on a semiconductor substrate. To write and erase data, a method of tunneling charges through the gate insulating layer is used at a voltage that is higher than the normal power supply voltage. Accordingly, flash memory devices typically require a booster circuit in order to produce the voltage for erase and write operations.  
           [0005]    As memory devices become highly integrated, many efforts have been underway to develop a new memory device having nonvolatile and random access characteristics, and a simple structure. Phase-changeable memory devices are one type of such memory devices. A typical phase-changeable memory device has a cell made of a phase-changeable material. Depending on a provided current density (i.e., Joule heating), the phase-changeable material can be electrically switched between amorphous and crystalline states and/or between variously resistive crystalline states.  
           [0006]    [0006]FIG. 1 is a graph showing a method of programming and erasing a phase-changeable memory cell. Here, a horizontal axis represents time (T) and a vertical axis represents temperature (TMP: ° C.) applied to the phase-changeable material cell. The phase-changeable material is heated at a temperature that is higher than a melting temperature Tm during a relatively short first time T 1 . Next, the phase-changeable material is rapidly quenched. In this case, the phase-changeable material may be changed into the amorphous state (curve  1 ). During a second time T 2  that is longer than the first time T 1 , the phase-changeable material is heated at a temperature that is lower than the melting temperature Tm and is higher than a crystallization temperature Tc. Next, the phase-changeable material is quenched. In this case, the phase-changeable material is changed into the crystalline state (curve  2 ). The resistivity of the phase-changeable material with the amorphous state is higher than that of the phase-changeable material with the crystalline state. Accordingly, currents flowing through the phase-changeable material may be detected in a read mode, and it is possible to discriminate whether information stored in the phase-changeable memory cell has a logic value of “1” or “0”. Generally, a chalcogenide material is used as the phase-changeable material, in particular, a compound (hereinafter a “GST layer”) including germanium (Ge), stibium (Sb), and tellurium (Te).  
           [0007]    As described above, heat is typically needed in order to switch the state of the phase-changeable material. In the typical phase-changeable memory device, if currents of a high density flow through an area contacting the phase-changeable material, a crystallization state of the phase-changeable material at a contact area is varied. The smaller the contact area is, the lower the current density for changing the state of the phase-changeable material.  
           [0008]    [0008]FIG. 2 is a cross-sectional view for explaining a typical phase-changeable memory device structure, which schematically shows a phase-changeable memory cell. Referring to FIG. 2, the typical phase-changeable memory device includes a lower conductive pattern  10 , a phase-changeable material pattern  16  and an upper conductive pattern  18 . The phase-changeable material pattern  16  is electrically connected to the lower conductive pattern  10  through a contact plug  14  formed in an insulating layer  12 . The insulating layer  12  is disposed on the lower conductive pattern  10 . The upper conductive pattern  18  is formed on the phase-changeable material pattern  16 . In the typical phase-changeable memory device, if currents flow between the lower conductive pattern  10  and the upper conductive pattern  18 , the crystallization state of the phase-changeable material varies in accordance with a pulse (i.e., heat) of the currents flowing through an area (hereinafter referred to as “active contact area”)  20  where the contact plug  14  is in contact with the phase-changeable material pattern  16 . The heat (i.e., energy) required to change the state of the phase-changeable material is directly affected by the active contact area  20  where the phase-changeable material pattern  16  is in contact with the contact plug  14 . Preferably, the active contact area  20  should be made as small as possible.  
           [0009]    However, because the lower conductive pattern  10  is connected to the phase-changeable material pattern  16  through the contact plug  14 , a size of the active contact area  20  may be limited by a photolithography resolution for the contact hole, i.e., the size of the active contact area  20  generally cannot be reduced beyond the photolithography resolution. Furthermore, it may be difficult to form such contact holes uniformly in the memory device, which can result in variance in current flow for changing the state of the phase-changeable material for each contact area. Thus, mis-operation in read mode can easily occur. Also, because only one active contact area  20  is formed at the area that is in contact with the contact plug  14 , a resistance variation resulting from the variation of the crystallization state is small.  
         SUMMARY OF THE INVENTION  
         [0010]    According to some embodiments of the present invention, a phase-changeable memory device comprises a substrate and an access transistor formed in and/or on the substrate. Laterally spaced apart first and second conductive patterns are disposed on the substrate and have opposing sidewalls. A conductor electrically connects the first conductive region to a source/drain region of the access transistor. A phase-changeable material region is disposed between the first and second conductive patterns and contacts the opposing sidewalls of the first and second conductive patterns.  
           [0011]    In some embodiments, the access transistor comprises first and second source/drain regions in the substrate and a gate electrode disposed on the substrate between the first and second source drain regions. The memory device further comprises an insulating layer disposed on the substrate and overlying the gate electrode and the first and second source/drain regions. The first and second conductive patterns are disposed on the insulating layer, and the conductor passes through the insulating layer and connects the first conductive pattern to the first source/drain region. The phase-changeable material layer is disposed on the insulating layer between the first and second conductive regions. The first and second conductive patterns may overlie respective ones of the first and second source/drain regions, and the phase-changeable material region may overlie the gate electrode.  
           [0012]    According to further aspects, the conductor extends from the first source/drain region to contact the overlying first conductive pattern at a first contact area, and the first conductive pattern contacts the phase-changeable material region at a second contact area that is substantially smaller than the first contact area. The insulating layer may comprise a first insulating layer, the conductor may comprise a first conductor, and the memory device may further comprise a second insulating layer disposed on the substrate and overlying the first and second conductive patterns, and a second conductor passing through the second insulating layer and contacting the second conductive pattern at a third contact area. The second conductive pattern may contact the phase-changeable material region at a fourth contact area that is substantially smaller than the third contact area. For example, each of the first and second conductive patterns may comprise a body portion that contacts the corresponding one of the first and second conductors and a smaller tab portion that extends from the body portion to contact the phase-changeable material region.  
           [0013]    A bit line conductor may be disposed on the second insulating layer, wherein the second conductor electrically connects the second conductive pattern to the bit line conductor. A common drain line conductor may be disposed in the first insulating layer between the second conductive pattern and the second source/drain region, and a third conductor may extend from the second source/drain region to contact the common drain line conductor.  
           [0014]    According to further embodiments of the present invention, a phase-changeable memory device has active contact areas at opposite sidewalls of a phase-changeable material pattern. On the same plane as the phase-changeable material pattern, a pair of conductive patterns is disposed so as to be in contact with the active contact areas. Accordingly, a size of the active contact area depends on a thickness and a width of the conductive pattern. The thickness of the conductive patterns at the contact areas can be made smaller than the resolution of a photolithography resolution, so size of the active contact areas can be reduced in relation to conventional processes.  
           [0015]    In accordance with further embodiments of the present invention, a phase-changeable memory device includes a pair of conductive patterns that are disposed on the same plane. The conductive patterns are spaced apart from each other. A variable resistivity material pattern is disposed between the conductive patterns so as to be in direct contact with the sidewalls of the conductive patterns at active contact areas. An upper metal interconnection for detecting resistivity of the variable resistivity material pattern is disposed over the conductive patterns and the variable resistivity material pattern so as to be electrically connected to one pattern of the conductive patterns. A semiconductor substrate having an impurity diffusion region is disposed under the conductive patterns and the variable resistivity material pattern so as to be electrically connected to another pattern of the conductive patterns. The variable resistivity material pattern is a phase-changeable material (e.g., a chalcogenide material) whose crystallization state is changed depending on applied heat (or energy).  
           [0016]    Preferably, the upper metal interconnection is electrically connected to one conductive pattern through an upper contact plug. The upper contact plug penetrates an upper insulating layer disposed on the conductive patterns and the variable resistivity material pattern. Preferably, the impurity diffusion region is electrically connected to the other conductive pattern through a lower contact plug. The lower contact plug penetrates a lower insulating layer disposed under the conductive patterns and the variable resistivity material pattern.  
           [0017]    In some embodiments, the phase-changeable memory device includes the other impurity diffusion region, a gate line, and a common drain electrode. The other impurity diffusion region is apart from the impurity diffusion region to which the lower contact plug is connected. The gate line is disposed on a semiconductor substrate between the impurity diffusion region and the other impurity diffusion region, and in the lower insulating layer. The common drain electrode is disposed in the lower insulating layer and is electrically connected to the other impurity diffusion region through a predetermined region of the lower insulating layer. When currents flow between the upper metal interconnection and the common drain electrode, a crystallization structure of a variable resistivity material at the active contact areas is changed in response to a density of the currents flowing across the active contact areas. For example, if predetermined currents are supplied in a state of grounding the upper metal interconnection and applying a turn-on voltage to the gate line, the heat (or energy) is applied to the active contact areas and, thus, the crystallization structure at the active contact areas is changed. By doing so, a program operation is performed. A read operation is performed by grounding the common drain electrode and applying a read voltage to the upper metal interconnection to detect resistance of the variable resistivity material pattern.  
           [0018]    In some embodiments, the conductive patterns comprise titanium nitride. Each of the lower contact plug and the upper contact plug may respectively comprise titanium, titanium nitride and tungsten, which are sequentially stacked.  
           [0019]    In accordance with further embodiments of the present invention, a phase-changeable memory device includes a phase-changeable material pattern, a pair of conductive patterns, an upper insulating layer, a bit line, a lower insulating layer, and a semiconductor substrate. The conductive patterns are disposed on the same plane as the phase-changeable material pattern and are in direct contact with the phase-changeable material pattern at active contact areas. The upper insulating layer is disposed on the phase-changeable material pattern and the conductive patterns. The bit line is disposed on the upper insulating layer and is electrically connected to one conductive pattern through an upper contact plug penetrating the upper insulating layer. The lower insulating layer is disposed under the phase-changeable material pattern and the conductive patterns. The semiconductor substrate includes a source region that is electrically connected to the other conductive pattern through a lower contact plug penetrating the lower insulating layer.  
           [0020]    In some embodiments, the phase-changeable memory device includes a drain region, a gate line, and a common drain electrode. The drain region is apart from the source region to be formed in the semiconductor substrate. The gate line is disposed on a semiconductor substrate between the source and drain regions. The common drain electrode is disposed in the lower insulating layer and is electrically connected to the drain region through a predetermined region of the lower insulating layer.  
           [0021]    In accordance with still other aspects of the present invention, a phase-changeable memory device includes a phase-changeable material pattern, a first electrode pattern, a second electrode pattern, resistance detection interconnection and a semiconductor substrate. The first electrode pattern is disposed on the same plane as the phase-changeable material pattern and is in contact with the phase-changeable material pattern at one active contact area. The second electrode pattern is disposed on the same plane as the phase-changeable material pattern and is in contact with the phase-changeable material pattern at another active contact area. Also, the second electrode pattern is formed symmetrically in mirror-image relations with respect to the first electrode pattern. The resistance detection interconnection is electrically connected to the first electrode pattern. The semiconductor substrate has an impurity diffusion region that is electrically connected to the second electrode pattern.  
           [0022]    In some embodiments, an upper insulating layer is interposed between the phase-changeable material pattern and the resistance detection interconnection and between electrode patterns and the resistance detection interconnection. The resistance detection interconnection is electrically connected to the second electrode pattern through an upper contact plug penetrating the upper insulating layer. A lower insulating layer is interposed between the phase-changeable material pattern and the semiconductor substrate and between electrode patterns and the semiconductor substrate. The impurity diffusion region is electrically connected to the first electrode pattern through a lower contact plug penetrating the lower insulating layer. The resistance detection interconnection detects resistivity of the phase-changeable material pattern to provide information about a logic value.  
           [0023]    In phase-changeable memory devices according to further embodiments of the present invention, each electrode pattern includes a plug contact region contacting a contact plug, and a material pattern contact region protruding from a middle portion of sidewalls of plug contact region toward the phase-changeable material pattern to contact the phase-changeable material pattern at an active contact area. A width of the material pattern contact region is preferably less than a thickness thereof. Accordingly, it is possible to reduce dimensions of the active contact areas.  
           [0024]    According to further embodiments of the present invention, a memory device can be formed with an active contact area that is not defined by a contact plug process. The active contact area is formed at a side surface of a variable resistivity material pattern. Accordingly, a size of the active contact area is dependant upon a thickness of a conductive pattern contacting with the active contact area. The thickness of the conductive pattern can be thinner to decrease the size of the active contact area. According to further embodiments of the present invention, a memory device having two active contact areas can be provided. The active contact areas may be symmetrically formed at both sides of a variable resistivity material layer pattern.  
           [0025]    In accordance with yet further embodiments of the present invention, there are provided methods of forming phase-changeable memory devices. A semiconductor substrate is formed with an impurity diffusion region. A conductive layer is formed that is electrically connected to the impurity diffusion region. Next, the conductive layer is patterned to form a symmetrical pair of conductive patterns so that one conductive pattern is electrically connected to the impurity diffusion region. A variable resistor pattern is then formed between the pair of conductive patterns. The variable resistor pattern direct contacts sidewalls of the conductive patterns at active contact areas. A metal interconnection is formed to be electrically connected to the other conductive pattern.  
           [0026]    The pair of conductive patterns can be formed by following steps according to some embodiments. The conductive layer is patterned to form a intermediate conductive pattern so as to be electrically connected to the impurity diffusion region. Next, an insulating layer is formed on the resultant structure having the intermediate conductive pattern. Thereafter, the insulating layer and the intermediate conductive pattern are patterned to form the conductive patterns so that the intermediate conductive pattern is apart from the pair of conductive patterns. The insulating layer and the intermediate conductive pattern are patterned to form the variable resistivity resistor material layer over the insulating layer, at a space between the pair of conductive patterns. Thereafter, the variable resistivity material layer is patterned to form the variable resistivity material pattern.  
           [0027]    The pair of the conductive pattern can be formed by forming an insulating layer on the conductive layer and patterning the insulating layer and the conductive layer. The variable resistor material pattern may be formed by patterning the insulating layer and the intermediate conductive pattern, forming the variable resistor material layer over the insulating layer at a space between the pair of conductive patterns, and patterning the variable resistor material layer.  
           [0028]    In some embodiments, forming the conductive layer includes forming a lower insulating layer on the semiconductor substrate, patterning the lower insulating layer to form a lower contact hole exposing the impurity diffusion region, filling the contact hole to form a lower contact plug, and forming the conductive layer on the contact plug and the lower insulating layer. In further embodiments, forming the metal interconnection includes forming an upper insulating layer on the resultant structure with the variable resistor material pattern, patterning the upper insulating layer to form an upper contact hole which exposes the other conductive pattern that is not electrically connected to the impurity diffusion region, forming an upper contact plug filling the upper contact hole, forming a metal layer on the upper insulating layer and on the upper contact plug, and patterning the metal layer. In other embodiments, providing the semiconductor substrate includes forming another impurity diffusion region and a gate line to be positioned between the impurity diffusion region and another impurity diffusion region, forming a common drain electrode to be electrically connected to another impurity diffusion region.  
           [0029]    In accordance with further embodiments of the present invention, there are provided methods of forming phase-changeable memory devices. A source region, a drain region and a gate electrode are formed on a semiconductor substrate. A common drain electrode is formed to be isolated from the semiconductor substrate by an insulating layer and to be electrically connected to the drain region through a predetermined region of the insulating layer. A lower insulating layer is formed on the resultant structure having the common drain electrode. A lower contact plug is formed in the lower insulating layer to be electrically connected to the source region. A phase-changeable material layer and a protective insulating layer are sequentially formed on the lower insulating layer and the lower contact plug. The protective insulating layer and the phase-changeable material layer are successively patterned to form a protective insulating pattern and a phase-changeable material pattern so as not to be in contact with the lower contact plug. A conductive layer is formed on the lower insulating layer, the lower contact plug and the patterns. Next, the conductive layer is patterned such that an upper portion of the phase-changeable material pattern is partially exposed, to form a pair of conductive patterns. The pair of conductive patterns is spaced apart from each other by the phase-changeable material pattern and in direct contact with both sidewalls of the phase-changeable material pattern. One conductive pattern is electrically connected to the lower contact plug. An upper insulating layer is formed on the lower insulating layer, the conductive patterns and the exposed phase-changeable material pattern. An upper contact plug is then formed to be electrically connected to the other conductive pattern that is not electrically connected to the lower contact plug. A metal interconnection is formed on the upper insulating layer to be electrically connected to the upper contact plug.  
           [0030]    In accordance with further embodiments of the present invention, methods of forming phase-changeable memory devices are provided. A source region, a drain region and a gate electrode are formed at a semiconductor substrate. A common drain electrode is formed to be electrically connected to the drain region. A lower insulating layer is formed on the resultant structure having the common drain electrode. A lower contact plug is then formed in the lower insulating layer to be electrically connected to the source region. A conductive layer is formed on the lower insulating layer and the lower contact plug. The conductive layer is patterned to form a pair of conductive patterns, which are apart from each other, such that one conductive pattern is in contact with the lower contact plug. A chalcogenide material pattern is formed between the pair of conductive patterns so as to be directly contact with sidewalls of the pair of conductive patterns. An upper insulating layer is formed on the resultant structure having the material patterns. An upper contact plug is formed to be electrically connected to the other conductive pattern that is not electrically connected to the lower contact plug. A metal interconnection is formed on the upper insulating layer to be electrically connected to the upper contact plug.  
           [0031]    The pair of conductive patterns may be formed by the following steps: The conductive layer is patterned to form an intermediate conductive pattern that is electrically connected to the lower contact plug. Next, an insulating layer is formed on the resultant structure where the intermediate conductive pattern will be formed. Thereafter, the insulating layer and the intermediate conductive pattern are patterned to the pair of conductive patterns. The chalcogenide material pattern may be formed by forming a chacogenide layer on the insulating layer and exposed lower insulating layer between the pair of conductive patterns, and patterning the chacogenide layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a graph illustrating operations for programming and erasing a phase-changeable memory cell.  
         [0033]    [0033]FIG. 2 is a cross-sectional view showing a portion of a typical conventional phase-changeable memory device.  
         [0034]    [0034]FIG. 3A is a cross-sectional view showing a portion of a phase-changeable memory device according to first embodiments of the present invention.  
         [0035]    [0035]FIG. 3B is a top plan view of the phase-changeable memory device of FIG. 3A, taken along a line I-I′.  
         [0036]    [0036]FIG. 3C is a perspective view of the phase-changeable memory device of FIGS. 3A and 3B.  
         [0037]    [0037]FIG. 4 is an equivalent circuit view of the phase-changeable memory device of FIGS.  3 A- 3 C.  
         [0038]    FIGS.  5 A- 5 G are cross-sectional views showing exemplary operations for forming a phase-changeable memory device according to some embodiments of the present invention.  
         [0039]    FIGS.  6 A- 6 E are cross-sectional views showing exemplary operations for forming a phase-changeable memory device according to further embodiments of the present invention.  
         [0040]    FIGS.  7 A- 7 E are cross-sectional views showing exemplary operations for forming a phase-changeable memory device according to still further embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0041]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Like numbers refer to like elements throughout.  
         [0042]    A phase-changeable memory device according to some embodiments of the present invention includes a phase-changeable material region with two active contact areas that are disposed on the same plane. That is, the active contact areas exist at opposing sidewalls of a phase-changeable material region. On the same plane as the phase-changeable material layer, conductive patterns are in contact with the phase-changeable material region at the active contact areas. The conductive patterns serve as a current path that supplies predetermined currents to the sidewalls of the phase-changeable material region. The phase-changeable material region is interposed between the symmetrical pair of conductive patterns on the same plane. Accordingly, currents in the phase-changeable material region flow in a lateral (horizontal) direction.  
         [0043]    [0043]FIG. 3A is a cross-sectional view of a phase-changeable memory device according to some embodiments of the present invention. FIG. 3B is a top plan view of the phase-changeable memory device of FIG. 3A, taken along a line I-I′. FIG. 3C is a perspective view of the phase-changeable memory device of FIGS. 3A and 3B.  
         [0044]    Referring to FIGS. 3A through 3C, the phase-changeable memory device includes a pair of conductive patterns  260   a  and  260   b  and a phase-changeable material pattern  300   a . The pair of conductive patterns  260   a  and  260   b  are disposed on opposite sides of the phase-changeable material pattern  300   a  on the same plane, i.e., on a lower insulating layer  250 . The pair of conductive patterns  260   a  and  260   b  are spaced apart from each other by a predetermined length L. The phase-changeable material pattern  300   a  is interposed between the conductive patterns  260   a  and  260   b , and is in direct contact with sidewalls of the conductive patterns  260   a  and  260   b . One conductive pattern (here, the conductive pattern  260   a ) is electrically connected to a first source/drain region, e.g., an impurity diffusion region  140 , of a semiconductor substrate  100 . The other conductive pattern  260   b  is electrically connected to a metal interconnection  360  (e.g., a bit line). Preferably, the conductive pattern  260   a  is electrically connected to the impurity diffusion region  140  through a contact plug  240  passing through the lower insulating layer  250 . The other conductive pattern  260   b  is electrically connected to the metal interconnection  360  through a contact plug  340  passing through an upper insulating layer  330  that is interposed between the conductive pattern  260   b  and the metal interconnection  360 . The upper insulating layer  330  is interposed between the metal interconnection  360  and the conductive pattern  260   a  and the phase-changeable material pattern  300   a . In the semiconductor substrate  100 , another source/drain region, e.g., another impurity diffusion region  160 , is spaced apart from the impurity diffusion region  140  that is electrically connected to one conductive pattern  260   a . A gate electrode  120  is disposed on the semiconductor substrate  100  between the impurity diffusion regions  140  and  160 . The impurity diffusion regions  140  and  160  and the gate electrode  120  form an access transistor Ta. The gate electrode  120  acts as a word line. A common drain electrode  200  is disposed in the lower insulating layer  250  so as to be electrically connected to the other impurity diffusion region  160  through a contact plug formed in the lower insulating layer  250 . The conductive patterns  260   a  and  260   b  are electrically isolated from the gate electrode  120  and the common drain electrode  200  by the upper insulating layer  250 .  
         [0045]    The conductive pattern  260   a  includes a plug contact region, e.g., a relatively large body portion  260   ac , and a material pattern contact region, e.g., a relatively small tab portion  260   ai  extending from the body portion. The plug contact region  260   ac  is in contact with the contact plug  240 . The material pattern contact region  260   ai  is in contact with the phase-changeable material pattern  300   a . The conductive pattern  260   b  also includes a similar plug contact region  260   bc  and a similar material pattern contact region  260   bi . The plug contact region  260   bc  is in contact with the contact plug  340 . The material pattern contact region  260   bi  is in contact with the phase-changeable material pattern  300   a . The material pattern contact regions  260   ai  and  260   bi  are extended from a middle portion of sidewalls of the plug contact regions  260   ac  and  260   bc  toward the phase-changeable material pattern  300   a . It is preferable that the width of the material pattern contact regions  260   ai  and  260   bi  are made as narrow as possible to the resolution of the photolithography. Because the plug contact regions  260   ac  and  260   bc  are connected to the contact plugs  240  and  340 , the plug contact regions  260   ac  and  260   bc  are relatively larger than the material pattern contact regions  260   ai  and  260   bi.    
         [0046]    The length L between the pair of conductive patterns (i.e., a size of Y axis dimension of the phase-changeable material pattern  300   a ) depends on the photolithography resolution. The conductive patterns  260   a  and  260   b  are in contact with the phase-changeable material pattern  300   a  at active contact areas  270   a  and  270   b  at opposite sidewalls of the phase-changeable material pattern  300   a . It is preferable that the active contact areas  270   a  and  270   b  be made as small as possible in order to reduce currents required to vary a crystallization state of phase-changeable material pattern  300   a.    
         [0047]    According to some embodiments of the present invention, the dimensions of the active contact areas  270   a  and  270   b  is dependent upon a thickness T (i.e., a Z axis dimension) of the conductive patterns  260   a  and  260   b  and a width W of the conductive patterns  260   a  and  260   b  (i.e., the material pattern contact regions  260   ai  and  260   bi ). The width W of the conductive patterns  260   a  and  260   b  may be limited to the photolithography resolution. Because the thickness T of the conductive patterns generally is not limited by the photolithography resolution, the thickness T can be lower than the photolithography resolution. Furthermore, the phase-changeable memory device includes two active contact areas  270   a  and  270   b . Therefore, although the same currents (i.e., joule heat) are used, a resistivity variation can be doubled.  
         [0048]    Program and read operations of the phase-changeable memory device will now be described. FIG. 4 is an equivalent circuit of the phase-changeable memory device of FIGS.  3 A- 3 C. Referring to FIGS.  3 A- 3 C and  4 , the bit line BL (i.e., the metal interconnection  360 ) is grounded and a turn on voltage is applied to the word line WL (i.e., the gate electrode  120 ). Predetermined current flows are applied to the common drain electrode CDL ( 200  in FIGS.  3 A- 3 C). Accordingly, a current path is formed between the common drain electrode CDL and the bit line BL. The crystallization state of the active contact areas  270 A and  270 B of the phase-changeable material pattern Rc is changed by Joule heat (Joule energy) of the current flow. The amount of current applied to the common drain electrode CDL is adjusted, so that heat (energy) applied to the active contact area is determined. Thus, phase-changeable materials have a crystalline state or an amorphous state depending on current flow. For example, if a temperature of 650° C. is applied during very short time to the active contact areas, the phase-changeable materials of the active contact areas are transformed into the amorphous state. If a temperature of 400° C. is applied for a relatively long time to the active contact areas, the phase-changeable materials of the active contact areas are transformed into the crystalline state. Alternatively, the program operation can be performed by grounding the common drain electrode CDL, and applying predetermined current flows to the bit line BL.  
         [0049]    Next, the read operation will be explained. The common drain electrode CDL is grounded, and a read voltage is applied to the bit line BL. The bit line BL detects resistivity (i.e., currents flowing through the phase-changeable material pattern) of the phase-changeable material pattern Rc. Therefore, a logic value can be discriminated.  
         [0050]    [0050]FIGS. 5A through 5G illustrate methods for forming a phase-changeable memory device as shown in FIG. 5G according to first embodiments of the present invention. Referring to FIG. 5A, after a device isolation process is performed by a conventional method, an access transistor is formed on a semiconductor substrate  100 . The access transistor includes a source region  140 , a drain region  160  and a gate electrode  120 . A lower first oxide layer  190  is formed on the semiconductor substrate  100  having the access transistor. A common drain electrode  200  is formed on the lower first oxide layer  190  and is electrically connected to the drain region  160  of the access transistor. A lower second oxide layer  230  is formed on the common drain electrode  200  and the lower first oxide layer  190 . The lower first oxide layer  190  and the lower second oxide layer  230  form a lower insulating layer  250 . An intermediate conductive pattern  260 ′ is formed on the lower second oxide layer  230  and is electrically connected to the source region  140  of the access transistor.  
         [0051]    More particularly, the common drain electrode  200  is electrically connected to the drain region  160  by a contact plug  180 . The contact plug  180  penetrates the lower first oxide layer  190  and contacts the drain region  160 . A contact plug process includes forming a contact hole by a photolithographic process and depositing a conductive material in the contact hole. Likewise, the intermediate conductive pattern  260 ′ is electrically connected to the source region  140  through contact plug  240  penetrating the lower insulating layer  250 . Preferably, the contact plug  240  includes a first contact plug  180 ′, a conductive pad  200 ′, and a second contact plug  220 . The first contact plug  180 ′ is in contact with the source region  140  through the lower first oxide layer  190 . The conductive pad  200 ′ is formed on the lower first oxide layer  190  and is electrically connected to the first contact plug  180 ′. The second contact plug  220  is in contact with the conductive pad  200 ′ through the lower second oxide layer  230 . In this case, when the common drain electrode  200  is formed, the first contact plug  180 ′ and the conductive pad  200 ′ are formed simultaneously. After depositing the lower second oxide layer  230 , a contact hole is formed to expose the contact pad  200 ′, and then conductive material is deposited in the contact hole to form the second contact plug  220 . Thereafter, a conductive layer is formed on the second contact plug  220  and the lower second oxide layer  230 . The conductive layer is patterned to form the intermediate conductive pattern  260 ′ that is electrically connected to the second contact plug  220 .  
         [0052]    The second contact plug  220  is made of sequentially stacked Titanium (Ti), titanium nitride (TiN), and tungsten (W) layers. For example, the Ti layer may have a thickness of 50 angstroms, the TiN layer may have a thickness of 250 angstroms and the W layer may have a thickness of 2500 angstroms.  
         [0053]    The intermediate conductive pattern  260 ′ is made of TiN with a thickness of 200 angstroms. Preferably, the intermediate conductive pattern  260 ′ is formed as thin as possible, because the thickness of the intermediate conductive pattern  260 ′ is an important factor determining dimensions of an active contact area. Accordingly the thickness of the intermediate conductive pattern  260 ′ can be made to size smaller than the photolithographic resolution.  
         [0054]    Referring to FIG. 5B, an upper first oxide layer  280  is formed on the intermediate conductive pattern  260 ′ and the lower insulating layer  250 . For example, the upper first oxide layer  280  may have a thickness of about 5000 angstroms.  
         [0055]    Referring to FIG. 5C, the upper first oxide layer  280  and the intermediate conductive pattern  260 ′ are etched by a photolithographic etching process. An opening  290  is formed to expose a portion of the lower first oxide layer  230  under a middle portion of the intermediate conductive pattern  260 ′. Thus, the intermediate conductive pattern  260 ′ is divided into a pair of conductive patterns  260   a  and  260   b , which are spaced apart from each other by the opening  290 . Here, one conductive pattern  260   a  is electrically connected to the second contact plug  220 , in particular, a bottom surface of the conductive pattern  260   a  is in contact with the second contact plug  220 . The opening  290  exposes sidewalls  265   a  and  265   b  of the conductive patterns  260   a  and  260   b.    
         [0056]    Referring to FIG. 5D, a variable resistivity material layer  300  is formed to cover the patterned upper first oxide layer  280 ′, the exposed lower first oxide layer  230  and the exposed sidewalls  265   a  and  265   b  of the conductive patterns. The variable resistivity material layer  300  is a material whose crystalline state can be varied depending on temperature. The variable resistivity material layer  300  is made of a phase-changeable material layer, for example, a GST layer. The phase-changeable material layer  300  may have a thickness of about 200 angstroms. The variable resistor material layer  300  is contact with the sidewalls  265   a  and  265   b  of the conductive patterns  260   a  and  260   b  to define active contact areas  270   a  and  270   b.    
         [0057]    Referring to FIG. 5E, for electrical isolation between neighboring cells, the phase-changeable material layer  300  is etched by a photolithographic etching process to form the phase-changeable material pattern  300   a  (i.e., a phase-changeable memory cell). The phase-changeable material pattern  300   a  is formed between the conductive patterns  260   a  and  260   b , so that sidewalls thereof are in direct contact with the sidewalls  265   a  and  265   b  of the conductive patterns  260   a  and  260   b  at active contact areas  270   a  and  270   a.    
         [0058]    Referring to FIG. 5F, an upper second oxide layer  320  is formed on the surface of the semiconductor substrate where the phase-changeable material pattern  300   a  is formed. The upper second oxide layer  320  and the upper first oxide layer  280  form an upper insulating layer  330 .  
         [0059]    Referring to FIG. 5G, the upper insulating layer  330  is patterned to form a contact hole exposing the conductive pattern  260   b  that is not in contact with the second contact plug  220 . Next, a conductive material is deposited to form a contact plug  340 . The contact plug  340  may be formed using the same method used in forming the second contact plug  220 . A metal material is deposited on the contact plug  340  and the upper insulating layer  330  to be patterned. Thereafter, a bit line  360  is formed. The bit line  360  is a metal interconnection electrically contacting with the contact plug  340 . For example, the bit line  360  may be formed of aluminum.  
         [0060]    FIGS.  6 A- 6 E show methods of forming the phase-changeable memory device shown in FIG. 6E according to second embodiments of the present invention. In the above-described first embodiments, the phase-changeable material pattern  300   a  is formed after the conductive patterns  260   a  and  260   b  are formed. However, in the second embodiments, the phase-changeable material pattern  300   a  is formed before the conductive patterns  260   a  and  260   b  are formed.  
         [0061]    Referring to FIG. 6A, the access transistor, the common drain electrode  200 , and the contact plug  240  are formed by the same method as the first embodiments. Next, the phase-changeable material layer  300  is formed on the lower insulating layer  250  and the second contact plug  220 . The upper first oxide layer  280  is formed on the phase-changeable material layer  300 .  
         [0062]    Referring to FIG. 6B, the upper first oxide layer  280  and the phase-changeable material layer  300  are successively patterned to form the phase-changeable material pattern  300   a  (i.e., a phase-changeable memory cell). The phase-changeable material pattern  300 A is spaced apart from the second contact plug  220 .  
         [0063]    Referring to FIG. 6C, a conductive layer  260  is formed on the surface of the semiconductor substrate having the phase-changeable material pattern  300   a , i.e., on the second contact plug  220 , the lower oxide layer  250 , the patterned upper first oxide layer  280 ′, and both sidewalls of the phase-changeable material pattern  300   a . The conductive layer is in contact with the phase-changeable material pattern  300   a  to define the active contact areas  270   a  and  270   b.    
         [0064]    Referring to FIG. 6D, the conductive layer  260  is patterned to form a symmetrical pair of conductive patterns  260   a  and  260   b . At this time, one conductive pattern  260   a  is electrically connected to the second contact plug  220 . That is, a bottom surface of the conductive pattern  260   a  is in contact with the second contact plug  220 .  
         [0065]    Referring to FIG. 6E, in a manner similar to that for the first embodiments, the upper second oxide layer  320  is formed. Thereafter, the bit line  360  is formed to be electrically connected to the conductive pattern  260   b  through the contact plug  340 .  
         [0066]    [0066]FIGS. 7A through 7E show methods of forming the phase-changeable memory device as shown in FIG. 7E according to third embodiments of the present invention. In the above-described first embodiments, the conductive patterns  260   a  and  260   b  are formed after the intermediate conductive pattern  260 ′ is formed. In the third embodiments, the intermediate conductive pattern is not formed.  
         [0067]    Referring to FIG. 7A, after the access transistor, the common drain electrode  200 , and the second contact plug  220  are formed by the same method as the first embodiments, the conductive layer  260  is formed on the lower second oxide layer  250  and the second contact plug  220 . The upper first oxide layer  280  is formed on the conductive layer  260 .  
         [0068]    Referring to FIG. 7B, the upper first oxide layer  280  and the conductive layer  260  are patterned to form the spaced apart pair of conductive patterns  260   a  and  260   b . The conductive pattern  260   a  is electrically connected to the second contact plug  220 . The conductive patterns  260   a  and  260   b  are spaced apart from each other by the opening  290 , and the sidewalls  265   a  and  265   b  thereof are exposed.  
         [0069]    Referring to FIG. 7C, the phase-changeable material layer  300  is formed on the surface of the semiconductor substrate having the conductive patterns  260   a  and  260   b . At this time, the phase-changeable material layer  300  is in contact with the sidewalls  265   a  and  265   b  of the conductive patterns  260   a  and  260   b , thus defining the active contact areas  270   a  and  270   b.    
         [0070]    Referring to FIG. 7D, the phase-changeable material layer  300  is patterned to be interposed between the pair of conductive patterns  260   a  and  260   b . The phase-changeable material pattern  300   a  is formed to be in direct contact with the sidewalls  265   a  and  265   b  of the conductive patterns  260   a  and  260   b.    
         [0071]    Referring to FIG. 7E, by using the same method as the first embodiments, the upper second oxide layer  320  is formed and the bit line  360  is formed. The bit line  360  is electrically connected to the conductive pattern  260   b  through the contact plug  340 .  
         [0072]    According to some embodiments of the present invention, dimensions of active contact areas for phase-changeable memory materials may be reduced to decrease currents required during a program operation of a phase-changeable memory device. Also, two active contact areas may be provided, so that a resistivity variation with respect to the same currents may be doubled as compared with a typical phase-changeable memory device. Accordingly, a detection margin for read operations may increase and mis-operation may be reduced.  
         [0073]    Furthermore, because the upper and lower electrodes (i.e., conductive patterns) exist on the same plane, a height of a memory cell region can be decreased. Therefore, it is possible to reduce a step difference between the cell region and a peripheral circuit region.  
         [0074]    In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purpose of limitation, the scope of the invention being set forth in the following claims.