Abstract:
Phase-changeable memory devices include non-volatile memory cells. Each of these non-volatile memory cells may include a phase-changeable diode on a semiconductor substrate and a phase-changeable memory element having a first terminal electrically coupled to a terminal of the phase-changeable diode. This phase-changeable diode may include a lower electrode pattern on the semiconductor substrate, a first phase-changeable pattern on the lower electrode pattern and a gate switching layer pattern on the first phase-changeable pattern. The phase-changeable memory element includes a second phase-changeable pattern electrically coupled to the terminal of the phase-changeable diode and a memory switching layer pattern on the second phase-changeable pattern. The memory switching layer pattern may include a composite of a titanium layer pattern contacting the phase-changeable memory element and a titanium nitride layer pattern contacting the titanium layer pattern.

Description:
REFERENCE TO PRIORITY APPLICATION 
   This application is a divisional of U.S. application Ser. No. 11/145,154, filed Jun. 3, 2005 now U.S. Pat. No. 7,105,870, which claims priority to Korean Application No. 2004-49820, filed Jun. 29, 2004. The disclosure of U.S. application Ser. No. 11/145,154 is hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to non-volatile memory devices and methods of forming non-volatile memory devices and, more particularly, to memory devices having phase-changeable materials therein and methods of forming same. 
   BACKGROUND OF THE INVENTION 
   Conventional phase-changeable random access memories (PRAMs) may utilize a metal-oxide semiconductor (MOS) field effect transistor to control switching within a PRAM cell having a phase-changeable memory element therein. A phase-changeable memory element may utilize a phase-changeable material such as germanium-antimony-tellurium (GST), which is susceptible to phase changes in response to Joule heating. These phase changes enable the material to operate as a non-volatile storage medium for binary data. However, the use of a MOS field effect transistor within each PRAM cell may result in an unnecessarily large layout footprint for each cell and thereby reduce integration density of large PRAM arrays. The use of a MOS field effect transistor may also increase fabrication costs. 
   One example of a non-volatile phase changeable storage device is illustrated in U.S. Pat. No. 6,750,469 to Ichihara et al. Another non-volatile storage device is illustrated in U.S. Patent Publication No. 2003/0193053 to Gilton. This storage device may include a diode and a memory cell having chalcogenide glass therein. This chalcogenide glass may be formed as a germanium selenide layer. 
   SUMMARY OF THE INVENTION 
   Phase-changeable memory devices according to some embodiments of the invention include non-volatile memory cells. Each of these non-volatile memory cells may include a phase-changeable diode on a semiconductor substrate and a phase-changeable memory element having a first terminal electrically coupled to a terminal of the phase-changeable diode. This phase-changeable diode may include a lower electrode pattern on the semiconductor substrate, a first phase-changeable pattern on the lower electrode pattern and a gate switching layer pattern on the first phase-changeable pattern. The phase-changeable memory element includes a second phase-changeable pattern electrically coupled to the terminal of the phase-changeable diode and a memory switching layer pattern on the second phase-changeable pattern. The memory switching layer pattern may include a composite of a titanium layer pattern contacting the phase-changeable memory element and a titanium nitride layer pattern contacting the titanium layer pattern. The first phase-changeable pattern may be a material selected from the group consisting of Ge x As y Te z  and Al x As y Te z  and the second phase-changeable pattern may be a material selected from the group consisting of Ge x Sb y Te z . 
   Still further embodiments of the invention include methods of forming non-volatile memory devices and cells having phase-changeable diode and phase-changeable memory elements therein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a layout view of a phase-changeable random access memory (PRAM) cell according to embodiments of the invention. 
       FIG. 2  is a cross-sectional view of the PRAM cell of  FIG. 1 , taken along line  2 - 2 ′. 
       FIGS. 3-14  are cross-sectional views of intermediate structures that illustrate methods forming the PRAM cell of  FIGS. 1-2 , according to embodiments of the present invention. 
       FIG. 15  is a current versus voltage graph illustrating characteristics of phase-changeable materials according to embodiments of the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   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. This 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. 
     FIG. 1  is a layout view of a PRAM according to an embodiment of the invention, and  FIG. 2  is a sectional view of a PRAM taken along line  2 - 2 ′ of  FIG. 1 . Referring now to  FIGS. 1 and 2 , a device isolation layer  20  is disposed in a semiconductor substrate  10 , while isolating at least one semiconductor active region  25 . A lower electrode layer pattern  32  is disposed on the active region  25  of the semiconductor substrate  10 . The lower electrode layer pattern  32  is disposed to traverse the active region  25 . A pad layer pattern  45  is disposed on the semiconductor substrate  10 . The pad layer pattern  45  may surround the lower electrode layer pattern  32 . The pad layer pattern  45  can be disposed to contact sidewalls of the lower electrode layer pattern  32 . The pad layer pattern  45  may be an electrically insulating layer having an etching ratio different from that of the device isolation layer  20 . Alternatively, the pad layer pattern  45  may be an insulating layer having the same etching ratio as that of the device isolation layer  20 . The pad layer pattern  45  may be a TEOS (tetra-ethyl-orthosilicate), or a HDP (high density plasma) oxide layer. The lower electrode layer pattern  32  may be formed as a titanium nitride (TiN) pattern. Alternatively, the lower electrode layer pattern  32  may be a tungsten (W) pattern. 
   A gate switching pattern  63  is disposed on the lower electrode layer pattern  32 . The gate switching pattern  63  may include a gate phase-change layer pattern  54  and a gate switching layer pattern  62 , which are sequentially stacked. The gate switching layer pattern  62  is preferably a titanium nitride (TiN) pattern. The gate phase-change layer pattern  54  can be a composite of germanium, arsenic and tellurium (Ge x As y Te z ). Or, the gate phase-change layer pattern  54  can be a composite of aluminum, arsenic and tellurium (Al x As y Te z ). The lower electrode layer pattern  32  has a greater width than that of the gate switching pattern  63 , and is in contact with the main surface of the semiconductor substrate  10 . Alternatively, a gate interlayer insulating layer (not shown) may be interposed between the lower electrode layer pattern  32  and the semiconductor substrate  10 . In this case, the lower electrode layer pattern  32  has a greater width than that of the gate switching pattern  63 , and is in contact with the gate interlayer insulating layer. 
   A buried interlayer insulating layer  70  is formed on the pad layer pattern  45  and the lower electrode layer pattern  32 , while covering the gate switching pattern  63 . A memory switching pattern  93  is disposed on the buried interlayer insulating layer  70 . The memory switching pattern  93  preferably includes a memory phase-change layer pattern  84  and a memory switching layer pattern  92 , which are sequentially stacked. A gate landing pad  78  is disposed in the buried interlayer insulating layer  70 , to electrically connect the memory switching pattern  93  and the gate switching pattern  63 . The gate landing pad  78  may be preferably a titanium nitride (TiN) layer. The memory switching layer pattern  92  preferably includes a titanium (Ti) pattern and a titanium nitride (TiN) pattern, which are sequentially stacked. The memory phase-change layer pattern  84  is preferably a composite of germanium, antimony and tellurium (Ge x Sb y Te z ). The buried interlayer insulating layer  70  is preferably an electrically insulating layer having an etching ratio different from that of the pad layer pattern  45 . Alternatively, the buried interlayer insulating layer  70  may be an insulating layer having the same etching ratio as that of the pad layer pattern  45 . The buried interlayer insulating layer  70  may be a PEOX (plasma-enhanced oxide) layer. 
   A planarized interlayer insulating layer  100  is disposed on the buried interlayer insulating layer  70 , while covering the memory switching pattern  93 . An upper electrode layer pattern  110  is disposed on the planarized interlayer insulating layer  100 . The upper electrode layer pattern  110  is disposed perpendicular to the lower electrode layer pattern  32 . A memory landing pad  108  is disposed in the planarized interlayer insulating layer  100 , while being in contact with the upper electrode layer pattern  110  and the memory switching pattern  93  concurrently. The memory landing pad  108  preferably includes a titanium nitride (TiN) layer and a tungsten (W) layer, which are sequentially stacked. The upper electrode layer pattern  110  is preferably an aluminum (Al) layer or a copper (Cu) layer, for example. The planarized interlayer insulating layer  100  is preferably an insulating layer having the same etching ratio as that of the buried interlayer insulating layer  70 . Or, the planarized interlayer insulating layer  100  may be an insulating layer having an etching ratio different from that of the buried interlayer insulating layer  70 . The planarized interlayer insulating layer  100  may be a TEOS layer or a USG (undoped silicate glass) layer. 
   Now, hereinafter, a method of forming a PRAM having a gate phase-change layer pattern according to the invention will be described as follows.  FIGS. 3 through 14  are sectional views illustrating a method of forming a PRAM taken along line I-I′ of  FIG. 1 , respectively. Referring to  FIG. 1  and  FIGS. 3 through 5 , a device isolation layer  20  is formed in a semiconductor substrate  10 . The device isolation layer  20  is formed to isolate at least one active region  25 . A lower electrode layer  30  is formed on the semiconductor substrate having the device isolation layer  20  therein. The lower electrode layer  30  is preferably formed by using a titanium nitride (TiN) layer. The lower electrode layer  30  may also be formed as a tungsten (W) layer. 
   Then, a photoresist pattern  34  is formed on the lower electrode layer  30 . The photoresist pattern  34  is formed on the active region  25  of the semiconductor substrate  10 . By using the photoresist pattern  34  as an etching mask, an etching process  38  can be performed on the lower electrode layer  30 . The etching process  38  forms a lower electrode layer pattern  32  on the active region  25  of the semiconductor substrate  10 . The lower electrode layer pattern  32  is formed to traverse the active region  25 . 
   A pad layer  40  is formed to cover the lower electrode layer pattern  32 . The pad layer  40  is preferably formed by using an electrically insulating layer having the same etching ratio as that of the device isolation layer  20 . Or, the pad layer  40  may be formed by using an insulating layer having an etching ratio different from that of the device isolation layer  20 . The pad layer  40  may be formed by using a TEOS (tetra-ethyl-orghosilicate) or a HDP (high density plasma) process. 
   Referring to  FIG. 1  and  FIGS. 6 through 8 , By using the lower electrode layer pattern  32  as an etching buffer layer, a planarization process (not shown) is performed on the pad layer  40 . The planarization process is performed until the upper surface of the lower electrode layer pattern  32  is exposed, thereby forming a pad layer pattern  45 . The planarization process can be performed by using CMP (chemical mechanical polishing) or an etching-back technique. 
   A gate phase-change layer  50  and a gate switching layer  60  are sequentially formed on the semiconductor substrate having the pad layer pattern  45 . The gate switching layer  60  is formed as a titanium nitride (TiN) layer. The gate phase-change layer  50  is preferably formed using a composite of germanium, arsenic and tellurium (Ge x As y Te z ). Alternatively, the gate phase-change layer  50  may be formed using a composite of aluminum, arsenic and tellurium (Al x As y Te z ). 
   Then, a photoresist pattern  64  is formed on the gate switching layer  60 . The photoresist pattern  64  is formed to be disposed above the lower electrode layer pattern  32 . By using the photoresist pattern  64  as an etching mask, an etching process  68  is sequentially performed on the gate switching layer  60  and the gate phase-change layer  50 . The etching process  68  forms a gate switching pattern  63  on a predetermined region of the lower electrode layer pattern  32 . The gate switching pattern  63  is preferably formed using a gate phase-change layer pattern  54  and a gate switching layer pattern  62 , which are sequentially stacked. The gate switching pattern  63  can secure a switching characteristic of a diode by using a phase-change of the gate phase-change layer pattern  54 . Therefore, the gate switching pattern  63  can replace a CMOS transistor. Further, the gate switching pattern  63  can simplify semiconductor fabrication processes of a PRAM. 
   A buried interlayer insulating layer  70  is formed to cover the gate switching pattern  63 . The buried interlayer insulating layer  70  is preferably formed using an insulating layer having an etching ratio different from that of the pad layer  40 . The buried interlayer insulating layer  70  may be formed by using an insulating layer having the same etching ratio as that of the pad layer  40 . The buried interlayer insulating layer  70  may be formed using a PEOX (plasma-enhanced oxide) process. 
   Referring to  FIG. 1  and  FIGS. 9 and 10 , a gate switching contact hole  74  is formed in the buried interlayer insulating layer  70 . The gate switching contact hole  74  is formed to expose the gate switching pattern  63 . A gate landing pad  78  is formed to fill the gate switching contact hole  74 . The gate landing pad  78  is preferably formed as a titanium nitride (TiN) pad. 
   Then, a memory phase-change layer  80  and a memory switching layer  90  are sequentially formed on the buried interlayer insulating layer  70 . The memory switching layer  90  is preferably formed as a composite of a titanium (Ti) layer and a titanium nitride (TiN) layer, which are sequentially stacked. The memory phase-change layer  80  is preferably formed using a composite of germanium, antimony and tellurium (Ge x Sb y Te z ). 
   Referring to  FIG. 1  and  FIGS. 11 through 14 , a photoresist pattern  94  is formed on the memory switching layer  90 . The photoresist pattern  94  is preferably formed to overlap the gate switching pattern  63  above the semiconductor substrate  10 . By using the photoresist pattern  94  as an etching mask, an etching process  98  is sequentially performed on the memory switching layer  90  and the memory phase-change layer  80 . The etching process  98  forms a memory switching pattern  93  on the buried interlayer insulating layer  70 , being in contact with the gate landing pad  78 . The memory switching pattern  93  is preferably formed using a memory phase-change layer pattern  84  and a memory switching layer pattern  92 , which are sequentially stacked. 
   A planarized interlayer insulating layer  100  is formed to cover the memory switching pattern  93 . A memory switching contact hole  104  is formed in the planarized interlayer insulating layer  100 . The memory switching contact hole  104  is formed to expose the memory switching pattern  93 . A memory landing pad  108  is formed to fill the memory switching contact hole  104 . The memory landing pad  108  is preferably formed by using a titanium nitride (TiN) layer and a tungsten (W) layer, which are sequentially stacked. The planarized interlayer insulating layer  100  is preferably formed by using an insulating layer having an etching ratio different from that of the buried interlayer insulating layer  70 . Or, the planarized interlayer insulating layer  100  may be formed by using an insulating layer having the same etching ratio as that of the buried interlayer insulating layer  70 . The planarized interlayer insulating layer  100  may be formed by using a TEOS or a USG (undoped silicate glass) process. 
   An upper electrode layer pattern  110  is formed on the planarized interlayer insulating layer  100 . The upper electrode layer pattern  110  is in contact with the memory landing pad  108 . The upper electrode layer pattern  110  is formed to be disposed perpendicular to the lower electrode layer pattern  32 . The upper electrode layer pattern  10  is preferably formed by using an aluminum (Al) or a copper (Cu). 
     FIG. 15  is a graph illustrating an operation of a PRAM of  FIG. 1 . Referring to  FIG. 1  and  FIGS. 14 and 15 , in the case that the gate switching pattern  63  and the memory switching pattern  93  are not connected to each other and are used independently, electrical characteristics of the gate switching pattern  63  and the memory switching pattern  93  are shown as follows. First, there will be examined a current characteristic of the memory switching pattern  93  by using a current-voltage graph. In the current-voltage graph, a voltage is applied to the memory switching pattern  93 . The memory switching pattern  93  shows an amorphous state having a high resistance depicted as a current trajectory line  143  until reaching a specific voltage V 1  in the graph. Then, the memory switching pattern  93  causes the memory phase-chance layer pattern  84  to make a phase change from an amorphous state to a crystalline state by using Joule heat of current at the specific voltage V 1 . The memory switching pattern  93  shows different current trajectory lines  146 ,  149  in the graph because of a decrease of inner resistance through the phase change of the memory phase-change layer pattern  84 . Current trajectory lines  146  shows a change of currents upward to a lower limit value I 1  of a setting region  130  with the start of the phase change of the memory phase-change layer pattern  84 . The current trajectory line  149  vertically traverses the resetting and setting regions  120 ,  130  with nearly little change of current above the lower limit value I 1  of the setting region  130 , which is because the phase change of the memory phase-change layer pattern  84  is completed, thereby showing an electrical characteristic of a conductor. The resetting region  120  has a lower limit value I 2  and an upper limit value I 3  of current enough to write data ‘ 1 ’ in the memory switching pattern  93 . The setting region  130  has a lower limit value I 1  and an upper limit value I 2  of current enough to write data ‘ 0 ’ in the memory switching pattern  93 . Further, the memory switching pattern  93  does not show the electrical characteristics following along the current trajectory lines  143 ,  146 ,  149  after the phase change of the memory phase-change layer pattern  84 . Instead, under the lower limit value I 1  of the setting region  130 , the memory switching pattern  93  shows another different current trajectory line  140 . While the memory phase-change layer pattern  84  maintains its crystalline state, the memory switching pattern  93  has the electrical characteristic following along the two current trajectory lines  140 ,  149 . 
   Next, there will be examined a current characteristic of the gate switching pattern  63  by using the current-voltage graph. In the current-voltage graph, a voltage is applied to the gate switching pattern  63 . The gate switching pattern  63  shows an amorphous state having a high resistance depicted as a current trajectory line  150  until reaching a specific voltage V 2  in the graph. Then, the gate switching pattern  63  causes the gate phase-change layer pattern  54  to make a phase change from an amorphous state to a crystalline state by using Joule heat of current at the specific voltage V 2 . Since the gate switching pattern  63  and the memory switching pattern  93  use different phase-change layers, respectively, a voltage value causing the gate phase-change layer pattern  54  to start its phase change is also different from the case of the memory switching pattern  93 . The gate switching pattern  63  shows different current trajectory lines  154 ,  158  in the graph because of a decrease of inner resistances through the phase change of the gate phase-change layer pattern  54 . The current trajectory line  154  shows a change of current upward to a lower limit value I 1  of a setting region  130  with the start of the phase change of the gate phase-change layer pattern  54 . The other current trajectory line  158  vertically traverses the resetting and setting regions  120 ,  130  with nearly little change of currents above the lower limit value I 1  of the setting region  130 , which is because the phase change of the gate phase-change layer pattern  54  is completed, thereby showing an electrical characteristic of a conductor. Further, the gate switching pattern  63  shows the electrical characteristic following along the current trajectory lines  150 ,  154 ,  158  after the phase change of the gate phase-change layer pattern  54 , according to the reduction of the voltage. 
   In the event that the gate switching pattern  63  and the memory switching pattern  93  are electrically connected together to form the PRAM, the PRAM shows two different electrical characteristics depending on the crystalline state of the memory phase-change layer pattern  84 . When the gate and the memory phase-change layer patterns  54 ,  84  are in an amorphous and a crystalline states, respectively, the PRAM shows a current trajectory line  160  reaching the lower limit value I 1  of the setting region  130  as depicted in the graph. As such, the voltage applied through the upper electrode layer pattern  110  is focused to cause the Joule heat for phase change in the gate phase-change layer pattern  54 . At this time, the PRAM can cause the gate phase-change layer pattern  54  to make the phase change from V 3 . On the contrary, above the lower limit value I 1  of the setting region  130 , the PRAM shows another different current trajectory line  165  passing nearly vertically through the resetting and the setting regions  120 ,  130 . This is because the gate and the memory phase-change layer patterns  54 ,  84  are completely phase-changed to a crystalline state. 
   In the event the gate and the memory phase-change layer patterns  54 ,  84  are in an amorphous state, the PRAM shows a current trajectory line  170  in the graph showing that a voltage applied through the upper electrode layer pattern  110  is spread to the gate and the memory phase-change layer patterns  54 ,  84 , and is focused to cause Joule heat. The PRAM starts to change phases of the gate and the memory phase-change layer patterns  54 ,  84  to a crystalline state at a specific voltage V 4 . The phase-change reduces the inner resistance of the gate and the memory phase-change layer patterns  54 ,  84 . Therefore, the PRAM shows a current trajectory line  174  reaching from the specific voltage V 4  to the lower limit value I 1  of the setting region  130  in the graph. When the current trajectory line  174  reaches the lower limit value I 1  of the setting region  130 , the gate and the memory phase-change layer patterns  54 ,  84  are completely changed to a crystalline state As such, the PRAM shows a current trajectory line  178  passing nearly vertically through the resetting and the setting regions  120 ,  130 . 
   As described above, embodiments of the invention enable the replacement of a conventional CMOS transistor with a diode based on a phase-change of a gate phase-change layer pattern. Accordingly, embodiments of the invention enable high integration and high speed of a PRAM through the simplification of semiconductor fabrication processes. 
   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 purposes of limitation, the scope of the invention being set forth in the following claims.