Abstract:
A memory device is disclosed. A pillar structure comprises a first electrode layer, a dielectric layer overlying the first electrode layer, and a second electrode layer overlying the dielectric layer. A phase change layer covers a surrounding of the pillar structure. A bottom electrode electrically connects the first electrode layer of the pillar structure. A top electrode electrically connects the second electrode layer of the pillar structure.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a memory device and fabrication thereof, and in more particularly to a phase change memory device and a fabrication thereof. 
   2. Description of the Related Art 
   Phase change memory devices have many advantages, such as high speed, lower power consumption, high capacity, greater endurance, better process integrity and lower cost. Thus, phase change memory devices can serve as independent or embedded memory devices with high integrity. Due to the described advantages, phase change memory devices can substitute for volatile memory devices, such as SRAM or DRAM, and non-volatile memory devices, such as Flash memory devices. 
   Phase change memory devices write, read or erase according to different resistance of a phase change material between crystal state and non-crystal state. For example, a phase change layer is applied with a relative high current and short pulse, such as 1 mA with 50 ns, to change from a crystal state to a non-crystal state. Because the non-crystal state phase change layer has higher resistance, such as 105 ohm, the phase change memory device presents a smaller current when applied with a voltage to read. When erasing, the phase change layer is applied with a low current, such as 0.2 mA, for a longer duration, such as 100 ns, to change from a non-crystal state to a crystal state. Since the crystal state phase change layer has lower resistance, such as 103-104 ohm, the phase change memory device presents a higher current when applied with a voltage to read. The phase change memory device operates according the mechanism described. 
     FIG. 1  shows a conventional T shaped phase change memory device. Referring to  FIG. 1 , a conventional T-shaped phase change memory device sequentially comprises a bottom electrode  102 , a heating electrode  104 , a phase change layer  106  and a top electrode  108 , wherein the columnar heating electrode  104  connects the phase change layer  106 . In a standard phase change memory device, current is determined according to a contact area between an electrode and a phase change layer thereof. In the conventional T shaped phase change memory device, the contact area between the heating electrode  104  and the phase change layer  106  is determined by limits of photolithography, rendering reduction of dimension difficult. 
     FIG. 2  shows another conventional phase change memory device, in which a heating electrode  202  is disposed horizontally. As shown in  FIG. 2 , a planar heating electrode  202  is formed on a bottom electrode  204  and an inter-layer dielectric layer  206 . The planar heating electrode  202  is patterned in a first direction by lithography, and a phase change layer  208  is then formed to contact the patterned planar heating electrode  202 . Thereafter, the phase change layer  208  is patterned in a second direction by lithography to define memory cells of the memory device. Next, a top electrode  210  is formed, electrically connecting the phase change layer  208 . In the memory device, the heating electrode  202  is disposed horizontally, and the size of the contact area between the heating electrode  202  and the phase change layer  208  is determined by thickness of the heating electrode  202 , which is not limited by lithography. Phase change layer  208  of the phase change memory device, however, is formed by gap filling, negatively affecting endurance and uniformity of contact between the phase change layer  208  and the heating electrode  202  of the phase change memory device. 
   BRIEF SUMMARY OF THE INVENTION 
   A detailed description is given in the following embodiments with reference to the accompanying drawings. These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by the invention. 
   The invention provides a method for forming a memory device. A first inter-layer dielectric layer is formed on a substrate. A bottom electrode is formed in the inter-layer dielectric layer. A first electrode layer is formed on the first inter-layer dielectric layer and the bottom electrode. A dielectric layer is formed on the first electrode layer. A second electrode layer is formed on the dielectric layer. The first electrode layer, the dielectric layer and the second electrode layer are patterned to form a pillar structure, corresponding to a memory cell of the memory device. A phase change layer is formed on the pillar structure and the substrate. The phase change layer is patterned to separate the patterned phase change layer of the memory cell from another patterned phase change layer of an adjacent memory cell. A top electrode is formed to at least electrically connect the second electrode layer of the pillar structure. 
   The invention provides a memory device. A pillar structure comprises a first electrode layer, a dielectric layer overlying the first electrode layer, and a second electrode layer overlying the dielectric layer. A phase change layer covers a surrounding of the pillar structure. A bottom electrode electrically connects the first electrode layer of the pillar structure. A top electrode electrically connects the second electrode layer of the pillar structure. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  shows a conventional T shaped phase change memory device. 
       FIG. 2  shows another conventional phase change memory device. 
       FIG. 3A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 3B  is a cross section of  FIG. 3A . 
       FIG. 4A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 4B  is a cross section of  FIG. 4A . 
       FIG. 5A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 5B  is a cross section of  FIG. 5A . 
       FIG. 6A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 6B  is a cross section of  FIG. 6A . 
       FIG. 7A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 7B  is a cross section of  FIG. 7A . 
       FIG. 8A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 8B  is a cross section of  FIG. 8A . 
       FIG. 9A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 9B  is a cross section of  FIG. 9A . 
       FIG. 9C  show a top view of a memory device comprising a plurality of memory cells. 
       FIG. 10A  shows a top view of an intermediate stage of a method for forming a phase change memory device of an embodiment of the invention. 
       FIG. 10B  is a cross section of  FIG. 10A . 
       FIG. 11  shows a three dimensional view of a memory cell of an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF INVENTION 
   The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. Embodiments of the invention are described with reference to the drawings that accompany the invention. It is noted that in the accompanying drawings, like and/or corresponding elements are referred to by like reference numerals. The invention is not limited to any particular fluid driving device or driving method, which is not particularly mentioned in the specification. 
     FIGS. 3A-10B  illustrate a method for forming a phase change memory device of an embodiment of the invention. Referring to  FIG. 3A  and  FIG. 3B , in which  FIG. 3A  is a top view of  FIG. 3B , a substrate comprising necessary elements is provided. The elements can be gates, dielectric layers and/or conductive vias, but the substrate, elements thereon or fabrications thereof are known in the art, which are not shown in the figures for simplicity. Next, a first inert layer dielectric layer  302  and a bottom electrode  304  are formed on the dielectric layer and/or the conductive via (not shown) over the substrate. The first inert layer dielectric layer  302  can be silicon oxide, silicon nitride, silicon oxynitride or low k dielectric materials. The bottom electrode  304  can comprise low conductivity materials, such as aluminum, cupper or tungsten. The formation of the bottom electrodes  304  can comprise forming openings in the first inert layer dielectric layer  302  by lithography and etching, and filling the openings with conductive materials. Alternatively, the bottom electrodes  304  can be formed by patterning a conductive layer, blanketly depositing a first inert layer dielectric layer  302 , and then etching back the first inert layer dielectric layer  302 . 
   Next, referring to  FIG. 4A  and  FIG. 4B , in which  FIG. 4A  is a top view of  FIG. 4B , a first electrode layer  306  is formed on the bottom electrode  304  and the first inter-layer dielectric layer  302  by physical vapor deposition, PVD or atomic layer deposition, ALD. The first electrode layer  306  can be TiN, TiW or TiAlN. Note that the first electrode layer  306  cannot be too thick, which is preferably about 5 Å-500 Å, and more preferably about 100 Å-300 Å. Next, a dielectric layer  308  is formed on the first electrode layer  306  by low pressure chemical vapor deposition (LPCVD), atmosphere pressure chemical vapor deposition (APCVD), sub-atmospheric chemical vapor deposition (SACVD), plasma enhanced chemical vapor deposition (PECVD) or other depositing methods. The dielectric layer  308  can be silicon oxide, silicon nitride, silicon oxynitride or the like. Thereafter, a second electrode layer  310  is formed on the dielectric layer  308  by physical vapor deposition, PVD or atomic layer deposition, ALD. The second electrode layer  310  can be TiN, TiW, TiAl, TaN or TiAlN. In a preferred embodiment of the invention, the second electrode layer  310  is thicker than the first electrode layer  306 . For example, the second electrode layer is twice or triple thickness that of the first electrode layer, in which the second electrode layer can be about 100 Å-3000 Å thick. 
   Referring to  FIG. 5A  and  FIG. 5B , a resist layer (not shown) is formed on the second electrode layer  310  by a coating method, such as spin coating. Next, the resist layer is defined by lithography to form a patterned resist layer  312  according to predetermined design. 
   Referring to  FIG. 6A  and  FIG. 6B , the second electrode layer  310 , the dielectric layer  308  and the first electrode layer  306  are sequentially and anisotropically etched to form a pillar structure  314  with closed surroundings using the patterned resist layer  312  as a mask. Thereafter, the patterned resist layer  312  is removed. The pillar structure  314  is preferably column-shaped, but the invention is not limited thereto. The pillar structure  314  can be any closed-shaped structure, such an oval-shaped pillar or a square, etc. Note that the pillar structure  314  with a closed surrounding corresponds to a single memory cell of the memory device of an embodiment of the invention. 
   Referring to  FIG. 7A  and  FIG. 7B , a phase change layer  316  is formed on the first inter-layer dielectric layer  302  and top and sidewalls of the pillar structure  314  by physical vapor deposition (PVD) or atomic layer deposition (ALD). The phase change layer  316  can be Ag, In, Te, Sb or combinations thereof, or Ge, Te, Sb or combinations thereof. In a preferred embodiment of the invention, the phase change layer  316  is Ag x In y Te z Sb w  or Ge x Te y Sb w , and about 500 Å thick. Note that the phase change layer  316  directly contacts the surrounding of the pillar structure  314 . Specifically, the phase change layer  316  directly contacts the surrounding of the first electrode layer  306  of the pillar structure  314 . 
   Referring to  FIG. 8A  and  FIG. 8B , a resist layer (not shown) is formed on the phase change layer  316 , and then defined by lithography to form a patterned resist layer  318 . 
   Referring to  FIG. 9A ,  FIG. 9B  and  FIG. 9C , the phase change layer  316  is etched using the patterned resist layer  318  as a mask to form a patterned phase change layer  320  of the memory cell  300 , which is separated from other patterned phase change layers  307 ,  309 ,  311  of adjacent memory cells  301 ,  303 ,  305 . 
   Referring to  FIG. 10A  and  FIG. 10B , a second inter-layer dielectric layer  330  is formed to cover the patterned phase change layer  320  and the first inter-layer dielectric layer  302  by a depositing method, such as chemical vapor deposition. The second inter-layer dielectric layer  330  can be silicon oxide, silicon nitride or silicon oxynitride. Next, the second inter-layer dielectric layer  330  is polished. Thereafter, the second inter-layer dielectric layer  330  and the patterned phase change layer  316  are patterned to form an opening, exposing the second electrode layer  310 . Next, a conductive layer, such as Al, Cu or W is deposited on the second inter-layer dielectric layer  330  and fills the opening to form a top electrode  332 , electrically connecting the second electrode layer  310  of the pillar structure  314 . 
     FIG. 11  shows a three dimensional view of a memory cell of an embodiment of the invention, explaining the structure more detail. In this embodiment, a major portion of the memory cell is the pillar structure  314 , comprising a first electrode layer  306 , a dielectric layer  308  and a second electrode layer  310 . The pillar structure  314  is covered by the patterned phase change layer  320 . In addition, the first electrode layer  306  and the second electrode layer  310  of pillar structure  314  electrically connect the top electrode  332  and the bottom electrode  304  respectively. 
   According to the embodiments described, because the first electrode layer  306  of the pillar structure  314  is much thicker than the second electrode layer  310 , the first electrode layer has higher resistance. Therefore, heat generated from passage of current mainly neighbors the first electrode layer  306 . When the pillar structure  314  is column-shaped, the interface between the first electrode layer  306  (heating electrode) and the phase change layer  320  forms a ring. For example, the columnar structure  314  has a diameter cd and a thickness t. The area A of the interface between the heating electrode  306  and the phase change layer  320  is equal to cd×π×t. Note that the area A is not limited to lithography process. In addition, only one lithography step is required to determining the contact area between the heating electrode  306  and the phase change layer  320  of a phase change memory device of the embodiment of the invention. Accordingly, variations and/or affection generated from lithography steps can be reduced. Additionally, the phase change layer  320  is not further processed or modified, for example by heating, thus composition change could be reduced. Furthermore, in an embodiment of the invention, because the heating electrode (first electrode layer  306 ) is formed on a plane, it is more easily fabricated than conventional technology. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.