Abstract:
Disclosed are a phase-change memory device and its manufacturing method, which can reduce a contact area between a bottom electrode and a phase-change layer, thereby reducing the quantity of current necessary for phase change. The phase-change memory device comprises: a bottom electrode formed on a contact plug; a phase-change layer formed on the bottom electrode and having a shape of a character ‘π’; and a top electrode formed on the phase-change layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the invention 
   The present invention relates to a phase-change memory device and method of manufacturing the same, and more particularly to a phase-change memory device and method of manufacturing the same, which can reduce a contact area between a bottom electrode and a phase-change layer, thereby reducing quantity of current. 
   2. Description of the Prior Art 
   As generally known in the art, memory devices can be classified into Random Access Memories (RAMs) which are volatile and lose stored information when power supplied thereto is interrupted and Read Only Memories (ROMs) which are non-volatile and keep information stored therein even when power supplied thereto is interrupted. The volatile RAMs include Dynamic RAMs (DRAMs) and Static RAMs (SRAMs) and the non-volatile ROMs include flash memory devices such as Electrically Erasable and Programmable ROMs (EEPROMs). 
   However, the DRAM, which is an excellent memory device as is well known in the art, requires a high charge-storage capacity for its periodic refresh operation, which consequently requires an increase of a surface area of an electrode, thereby making it difficult to achieve a high circuit integration. Further, the flash memory device requires higher operation voltage than a power supply voltage in relation to the lamination structure of two gates. Therefore, in the flash memory device, a separate voltage-booster circuit is necessary in order to obtain a voltage necessary for write and erase operations. In this regard also, the flash memory device disturbs high integration of a circuit. 
   Therefore, researches have been made into the development of a new non-volatile memory device capable of achieving high-integration and having a simple construction, one proposal for which is a phase-change memory device (phase-change RAM). 
   In the phase-change memory device, a phase-change layer interposed between a top electrode and a bottom electrode experiences a phase change from a crystalline state to an amorphous state by current flow between the top electrode and the bottom electrode, so that the phase-change memory device discriminates the information stored in the cell by means of resistance difference between the crystalline state and the amorphous state. 
   In other words, the phase-change memory device employs a chalcogenide film as the phase-change layer. The chalcogenide film is a compound material layer (hereinafter, referred to as “GST layer”) consisting of germanium (Ge), stibium (Sb), and tellurium (Te) and causes a phase change between the crystalline state and the amorphous state by an applied current or Joule Heat. Here, the phase-change layer has a higher specific resistance in the amorphous state than in the crystalline state. Therefore, whether the information stored in the phase-change memory cell refers to a logic ‘1’ or ‘0’ can be determined by detecting the current flowing through the phase-change layer in the ‘read’ mode. 
     FIG. 1  is a sectional view of a conventional phase-change memory cell. 
   In the conventional phase-change memory cell as shown in  FIG. 1 , a interlayer dielectric  5  is formed on a bottom electrode  3  formed on a semiconductor substrate  1 . Then, the interlayer dielectric  5  is etched to form a contact plug  7  electrically connected with source regions and a phase-change layer  9  is then formed on the resultant substrate having the contact plug  7  formed thereon. Thereafter, a top electrode  11  is formed on the phase-change layer  9 . 
   When voltage is applied in order to program the phase change memory cell, heat is generated at an interface between the phase-change layer  9  and the contact plug  7 , so that a portion  9   a  of the phase-change layer  9  is transformed into an amorphous state. The heat generated at the periphery C of the contact plug  7  and the phase-change layer  9  may be spread to the neighboring contact plug  7  and be unable to reach the temperature necessary for the phase change. Therefore, even after the phase-change layer is phase-changed into an amorphous state, there may remain an abnormal region (which is not changed into the amorphous state) at the periphery of the phase-change layer  9 . 
   Further, in the ‘read’ and ‘write’ operations of the phase-change memory device, the large contact area between the bottom electrode and the phase-change layer requires an increased quantity of current for the phase change, thereby having a bad influence on the speed of the phase-change memory device. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a phase-change memory device and its manufacturing method, which can reduce a contact area between a bottom electrode and a phase-change layer, thereby reducing the quantity of current necessary for phase change. 
   In order to accomplish this object, there is provided a phase-change memory device comprising: a semiconductor substrate having a bottom structure; a interlayer dielectric formed on the semiconductor substrate to cover the bottom structure; a contact plug formed in the interlayer dielectric; a bottom electrode formed on the contact plug and a portion of the interlayer dielectric adjacent to the contact plug; a first oxide layer formed on the interlayer dielectric and the bottom electrode and having a contact hole for exposing the bottom electrode formed in the first oxide layer; a second oxide layer formed on the bottom electrode within the contact hole such that the bottom electrode is partially exposed; a phase-change layer formed between the second oxide layer and the side of the contact hole and on the second oxide layer and the partially exposed bottom electrode; and a top electrode formed on the phase-change layer. 
   It is preferred that each of the bottom electrode and the top electrode is formed of a polysilicon film or a metal film. 
   It is also preferred that the phase-change layer is formed on a portion of the first oxide layer adjacent to the contact hole as well as between the second oxide layer and the surface of the contact hole and on the second oxide layer. 
   In accordance with another aspect of the present invention, there is provided a phase-change memory device comprising: a bottom electrode formed on a contact plug; a phase-change layer formed on the bottom electrode and having a shape of a character ‘π’; and a top electrode formed on the phase-change layer. 
   In accordance with still another aspect of the present invention, there is provided a method of manufacturing phase-change memory device, comprising the steps of: providing a semiconductor substrate having a bottom structure; forming a interlayer dielectric on the semiconductor substrate to cover the bottom structure; forming a contact plug within the interlayer dielectric; forming a bottom electrode on the contact plug and a portion of the interlayer dielectric adjacent to the contact plug; forming a first oxide layer on the interlayer dielectric layer and the bottom electrode; etching the first oxide layer to form a contact hole for exposing the bottom electrode; forming a spacer on a side surface of the contact hole; forming a second oxide layer inside of the spacer within the contact hole; removing the spacer; forming a phase-change layer between the second oxide layer and the side surface of the contact hole and on the second oxide layer and the partially exposed bottom electrode; and forming a top electrode on the phase-change layer. 
   Preferably, the surface of the first oxide layer is flattened by a CMP process after being formed. 
   Also, the each of the bottom electrode and the top electrode may be formed of a polysilicon film or a metal film. 
   More preferably, the spacer may be formed of a nitride film. 
   In accordance with still another aspect of the present invention, there is provided a method of manufacturing phase-change memory device, comprising the steps of: providing a semiconductor substrate having a bottom structure; forming a interlayer dielectric layer on the semiconductor substrate to cover the bottom structure; forming a contact plug within the interlayer dielectric; forming a bottom electrode on the contact plug and a portion of the interlayer dielectric adjacent to the contact plug; forming a first oxide layer on the interlayer dielectric and the bottom electrode; etching the first oxide layer to form a contact hole for exposing the bottom electrode; forming a spacer on a side surface of the contact hole; forming a second oxide layer inside of the spacer within the contact hole; removing the spacer; forming a phase-change layer between the second oxide layer and the side surface of the contact hole and on the second oxide layer and the partially exposed bottom electrode; forming a conductive layer on the phase-change layer; and etching the conductive layer to form a top electrode simultaneously while etching the phase-change layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a sectional view of a conventional phase-change memory cell; 
       FIG. 2  is a sectional view of a phase-change memory device according to an embodiment of the present invention; 
       FIGS. 3A through 3F  are sectional views showing a process of manufacturing a phase-change memory device according to an embodiment of the present invention; and 
       FIG. 4  is a sectional view illustrating a process of manufacturing a phase-change memory device according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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, and so repetition of the description on the same or similar components will be omitted. 
     FIG. 2  is a sectional view of a phase-change memory device according to an embodiment of the present invention. 
   Referring to  FIG. 2 , in the phase-change memory device according to the present invention, a interlayer dielectric  22  is formed to cover a bottom structure (not shown) of a semiconductor substrate  21  and a contact plug  23  is then formed within the interlayer dielectric  22 . Then, a bottom electrode  24  is formed on the contact plug  23  and a portion of the interlayer dielectric  22  adjacent to the contact plug  23 . Then, a first oxide layer  25  is formed on the interlayer dielectric  22  and the bottom electrode  24 , and a contact hole  26  for exposing the bottom electrode  24  is formed in the first oxide layer  25 . A second oxide layer  28  is formed on the bottom electrode  24  within the contact hole  26  such that the bottom electrode  24  is partially exposed. A phase-change layer  29  is formed between the second oxide layer  28  and the surface of the contact hole  26  and on the second oxide layer  28 . Also, the phase-change layer  29  may be formed on a portion of the first oxide layer  25  adjacent to the contact hole  26 . A top electrode  30  is formed on the phase-change layer  29 . 
   It is preferred that each of the bottom electrode  24  and the top electrode  30  is formed of a polysilicon film or a metal film. 
   In a ‘read’ or ‘write’ operation of the phase-change memory device, when heat is generated at a contact surface of the phase-change layer  29 , the state of the phase-change layer  29  changes into an amorphous state or a crystalline state. In the phase-change memory device according to the present invention, since the phase-change layer  29  is formed between the second oxide layer  28  and the surface of the contact hole  26 , on the second oxide layer  28 , and on a portion of the first oxide layer  25  adjacent to the contact hole  26 , the contact area A between the bottom electrode  24  and the phase-change layer  29  can be reduced. As a result, electric current necessary for phase change can be reduced and the speed of the phase-change memory device can be improved. 
     FIGS. 3A through 3F  are sectional views showing a process of manufacturing a phase-change memory device according to an embodiment of the present invention. 
   As shown in  FIG. 3A , the interlayer dielectric  22  is formed on the semiconductor substrate  21  having a bottom structure (not shown) to cover the bottom structure. Then, the interlayer dielectric  22  is etched to form the contact plug  23 . Thereafter, the bottom electrode  24  is formed on the contact plug  23  and a portion of the interlayer dielectric  22  adjacent to the contact plug  23 . Here, the bottom electrode  24  is formed of a polysilicon film or a metal film. 
   Referring to  FIG. 3B , the first oxide layer  25  is formed on the interlayer dielectric  22  and the bottom electrode  24 . Then, the surface of the first oxide layer  25  is flattened by a CMP process after being formed. Thereafter, the contact hole  26  for exposing the bottom electrode  24  is formed in the first oxide layer  25  by etching the first oxide layer  25 . 
   Referring to  FIG. 3C , a spacer  27  is then formed on a side surface of the contact hole  26 . Here, the spacer  27  is formed of a nitride film. 
   Referring to  FIG. 3D , the second oxide layer  28  is then formed inside of the spacer  27  within the contact hole  26 . 
   Next, as shown in  FIG. 3E , the spacer  27  formed on the side surface of the contact hole  26  is removed by wet etching. 
   Referring to  FIG. 3F , the phase-change layer  29  is then formed between the second oxide layer  28  and the surface of the contact hole  26  and on the second oxide layer  28 . Thereafter, the top electrode  30  is formed on the phase-change layer  29 . Herein, the phase-change layer  29  is formed of a polysilicon film or a metal film. 
     FIG. 4  is a sectional view illustrating a process of manufacturing a phase-change memory device according to another embodiment of the present invention. 
   As shown in  FIG. 4 , the interlayer dielectric  22  is formed to cover a bottom structure of the semiconductor substrate  21 . Then, the contact plug  23  is formed within the interlayer dielectric  22  and the bottom electrode  24  is then formed on the contact plug  23  and a portion of the interlayer dielectric  22  adjacent to the contact plug  23 . Thereafter, the first oxide layer  25  is formed on the interlayer dielectric  22  and the bottom electrode  24 , and the first oxide layer  25  is etched to form the contact hole  26  for exposing the bottom electrode  24 . 
   Thereafter, a spacer (not shown) is formed on a side surface of the contact hole  26  and the second oxide layer  28  is then formed inside of the spacer within the contact hole  26 . Then, the spacer is removed, and the phase-change layer  29  is formed between the second oxide layer  28  and the surface of the contact hole  26 , on the second oxide layer  28 , and on a portion of the first oxide layer  25  adjacent to the contact hole  26 . Thereafter, a conductive layer for a top electrode is formed on the phase-change layer  29  and is then etched to form the top electrode  30  simultaneously while the phase-change layer  29  is etched. 
   According to the present invention as described above, a spacer is formed on a side surface of a contact hole, an oxide layer is formed to fill the contact hole, the spacer is then removed, and a phase-change layer is then formed in the spacer-removed portion and the oxide layer. As a result, the contact area between the bottom electrode and the phase-change layer is reduced, so that electric current necessary for the phase change can be reduced. 
   Also, such reduction of the electric current necessary for the phase change can consequently improve speed of the phase-change memory device. 
   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.