Abstract:
A key hole structure and method for forming a key hole structure to form a pore in a memory cell. The method includes forming a first dielectric layer on a semiconductor substrate having an electrode formed therein, forming an isolation layer on the first dielectric layer, forming a second dielectric layer on the isolation layer, and forming a planarization stop layer on the second dielectric layer. The method further includes forming a via to extend to the first dielectric layer and recessing the isolation layer and the stop layer with respect to the second dielectric layer, depositing a conformal film within via and over the stop layer, forming a key hole within the conformal film at a center region of the via such that a tip of the key hole is disposed at an upper surface of the second dielectric layer, and planarizing the conformal film to the stop layer.

Description:
BACKGROUND 
     The present invention relates generally to phase change memory cells, and more specifically, to a planarization stop layer for forming a phase change memory device. 
     Phase change material (PCM) has a variety of applications in microelectronic devices such as optical storage media and solid state phase change memory devices. Phase change random access memory (PRAM) devices, for example, store data using a phase change material, such as, for example, a chalcogenide alloy, that transforms into a crystalline state or an amorphous state during cooling after a heat treatment. Each state of the phase change material has different resistance characteristics. Specifically, the phase change material in the crystalline state has low resistance and the phase change material in the amorphous state has high resistance. The crystalline state is typically referred to as a “set state” having a logic level “0”, and the amorphous state is typically referred to as a “reset state” having a logic level “1”. A current passed through the phase change material creates ohmic heating and causes the phase change material to melt. Melting and gradually cooling down the phase change material allows time for the phase change material to form the crystalline state. Melting and abruptly cooling the phase change material quenches the phase change material into the amorphous state. 
     Pore integration provides a uniform and sub-lithographic patterning for PCM application. Conventionally, a key hole method is performed to form a pore within phase change memory cell.  FIG. 1  illustrates a conventional key hole method. In  FIG. 1 , a substrate  10  including a bottom electrode  12  is provided. A first dielectric layer  14  is formed on the substrate  10 , an isolation layer  16  is formed on the first dielectric layer  14 , and a second dielectric layer  18  is formed on the isolation layer  16 . A photo resist layer (not shown) is formed over the second dielectric layer  18 . A via  20  is formed to extend to the first dielectric layer  14 . Next, the photo resist layer is removed and the isolation layer  16  is recessed, creating overhang portions of the second dielectric layer  18 . A conformal film  22  e.g., an amorphous silicon layer is deposited within the via  20  and pinched to form a void (i.e., a keyhole  24 ) in the via  20 . Typically, the conformal film  22  is recessed and the keyhole  24  is transferred down into the first dielectric layer  16  by a reactive ion etching (RIE) operation, to form a pore. There are variations in center to edge thickness of the conformal film  22  due to the nature of the process. The difference in the conformal film  22  thickness can be coupled into the pore diameter and depth formed during the RIE process, causing the CD variation. A planarization operation e.g., a silicon chemical mechanical polishing (CMP) operation is typically used to attempt to improve the uniformity across the wafer prior to the performance of the RIE process. However, when the CMP operation is performed it may cause a tip  24   a  of the key hole  24  to open, thereby causing CMP slurry to get into the key hole  24  and block the RIE process for transferring the key hole  24  to the first dielectric layer  14  to form the pore. 
     SUMMARY 
     The present invention obviates the above-mentioned problems, by providing a method which includes forming a planarization stop layer e.g., a chemical mechanical polishing (CMP) stop layer for forming a phase change memory device. The CMP stop layer stops the CMP before it approaches a tip of the key hole formed within the conformal film. 
     According to an embodiment of the present invention, a method for forming a key hole structure to form a pore in a memory cell is provided. The method includes forming a first dielectric layer on a semiconductor substrate having an electrode formed therein, forming an isolation layer on the first dielectric layer, forming a second dielectric layer on the isolation layer, and forming a planarization stop layer on the second dielectric layer. The method further includes forming a via to extend to the first dielectric layer and recessing the isolation layer and the planarization stop layer with respect to the second dielectric layer, depositing a conformal film within via and over the planarization stop layer, forming a key hole within the conformal film at a center region of the via such that a tip of the key hole is disposed at an upper surface of the second dielectric layer, and planarizing the conformal film to the planarization stop layer. 
     According to another embodiment of the present invention, a method for forming a pore in a memory cell is provided. The method includes forming a first dielectric layer on a semiconductor substrate having an electrode formed therein, forming an isolation layer on the first dielectric layer, forming a second dielectric layer on the isolation layer, and forming a planarization stop layer on the second dielectric layer. The method further includes forming a via to extend to the first dielectric layer and recessing the isolation layer and the planarization stop layer with respect to the second dielectric layer, depositing a conformal film within via and over the planarization stop layer, forming a key hole within the conformal film at a center region of the via such that a tip of the key hole is disposed at an upper surface of the second dielectric layer, planarizing the conformal film to the planarization stop layer, and etching the conformal film and transferring the key hole to the first dielectric layer to form a pore within the first dielectric layer. 
     According to yet another embodiment of the present invention, a key hole structure for forming a pore cell within a memory cell is provided. The key hole structure includes a first dielectric layer on a semiconductor substrate having an electrode formed therein, an isolation layer formed on the first dielectric layer, a second dielectric layer formed on the isolation layer, a planarization stop layer formed on the second dielectric layer, the isolation layer and the planarization stop layer recessed with respect to the second dielectric layer, and a via extending to the first dielectric layer. The via having a conformal film deposited therein and including a key hole within the conformal film at a center region of the via such that a tip of the key hole is disposed at an upper surface of the second dielectric layer. The conformal film is planarized to an upper surface of the planarization stop layer. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagram illustrating a conventional key hole method. 
         FIG. 2  is a diagram illustrating a fabrication operation of a key hole structure that can be implemented within embodiments of the present invention. 
         FIG. 3  is a diagram illustrating a recessing operation of a via that can be implemented within embodiments of the present invention. 
         FIG. 4  is a diagram illustrating deposition operation of a conformal film that can be implemented within embodiments of the present invention. 
         FIG. 5  is a diagram illustrating a planarization operation of the conformal film shown in  FIG. 4  that can be implemented within embodiments of the present invention. 
         FIG. 6  is a diagram illustrating the formation of a pore that can be implemented within embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a key hole structure and method for forming a key hole structure used to form a pore of a phase change memory cell. With reference now to  FIG. 2 , a substrate  100  including a bottom electrode  102  is provided. The bottom electrode  102  may be formed of titanium nitride (TiN) or tungsten (W), for example. The bottom electrode  102  is connected to access circuitry (not shown) such as an access transistor or diode. A first dielectric layer  104  is formed on the substrate  100  and an isolation layer  106  is formed on the first dielectric layer  104 . According to an embodiment of the present invention, the first dielectric layer  104  may be formed of silicon nitride (SiN), for example. The isolation layer  106  may be formed of silicon dioxide (SiO 2 ), for example. Further, shown in  FIG. 2 , a second dielectric layer  108  is formed on the isolation layer  106 . The second dielectric layer  108  may be formed of silicon nitride (SiN), for example. An alternative material for the second dielectric layer may be titanium nitride (TiN). The choice of the materials is based on the material system. For instance, if silicon dioxide (SiO2) is used for the first dielectric layer  104 , and silicon (Si) for the isolation layer  106 , then silicon dioxide (SiO2) may be used for the second dielectric layer  108  as well. As long as the wet etch can selectively recess the isolation layer  106  and the planarization stop layer  110 , and not the second dielectric layer  108  and the first dielectric layer  104 . 
     Also shown in  FIG. 2 , a third dielectric layer  110  e.g., a planarization stop layer  110  is formed on the second dielectric layer  108 . According to an embodiment of the present invention, the planarization stop layer  110  may be formed of a low temperature silicon nitride (SiN), silicon dioxide (SiO 2 ), oxy-nitride, oxide or other suitable material that has a faster buffered oxide etch (BOE) etch rate compared to approximately 400 degree Celsius silicon nitride (SiN). According to an embodiment of the present invention, the planarization stop layer  110  is formed of a material having a faster etch rate than that of the second dielectric layer  108  and a similar etch rate as that of the isolation layer  106 . According to one embodiment of the present invention, the second dielectric layer  108  and the planarization stop layer  110  may both be formed of silicon nitride (SiN) based on different deposition conditions (e.g., temperature) thereby having different etch rates. For example, the second dielectric layer  108  may be formed of silicon nitride (SiN) formed at a 400 degree Celsius processing temperature and the planarization stop layer  110  may be formed of silicon nitride (SiN) formed at a 200 degree Celsius processing temperature. According to an embodiment of the present invention, the planarization stop layer  110  may be of a predetermined thickness equal to or greater than that of the second dielectric layer  108 . For example, the second dielectric layer  108  may be of a predetermined thickness of approximately 30 nanometers (nm) while the planarization stop layer  110  may be of a predetermined thickness of approximately 30 nanometers (nm) or greater. According to another embodiment of the present invention, the second dielectric layer  108  may be of a predetermined thickness ranging from approximately 10 nanometers (nm) to 50 nanometers (nm) and the planarization stop layer  110  may be of a predetermined thickness ranging from approximately 30 nanometers (nm) to approximately 60 nanometers (nm). Further, in  FIG. 2 , a via  112  is formed extending to the first dielectric layer  104  using lithographic techniques. According to an embodiment of the present invention, a minimum thickness of the planarization stop layer  110  (“third dielectric layer”) is determined by the material recess amount after wet etch and CMP stop range. The thickness of the third dielectric layer should not be too thick that it changes the aspect ratio of the via  112  and changes the subsequent conformal film deposition profile. 
       FIG. 3  is a diagram illustrating a recessing operation of the via  112  that can be implemented within embodiments of the present invention. In  FIG. 3 , according to an embodiment of the present invention, since the isolation layer  106  and the planarization stop layer  110  have a similar etch rate, both are recessed a predetermined distance when an etching operation is performed, for example, a 500:1 buffered oxide etch (BOE) operation, thereby creating overhang portions  108   a ,  108   b  of the second dielectric layer  108 . The present invention is not limited to any particular type of etching process and may vary accordingly. As shown in  FIG. 3 , the isolation layer  106  and the third dielectric layer  110  may be etched back at approximately a same predetermined distance. In the case of forming a 30 nanometers (nm) key hole, the isolation layer  106  may be etched back approximately 15 nanometers (nm) on each side while the third dielectric layer  110  is etched back approximately 5 to 10 nanometers (nm) on each side. Next, the deposition of a conformal film is described below with reference to  FIG. 4 . 
       FIG. 4  is a diagram illustrating a deposition operation of a conformal film that can be implemented within embodiments of the present invention. As shown in  FIG. 4 , a conformal film  120  is deposited within the via  112  and pinched to form a void (i.e., a key hole  130 ) in the via  112 . The conformal film  120  may be made of amorphous silicon (a-Si) or polysilicon, for example. The conformal film  120  may be deposited by a chemical vapor deposition (CVD) or atomic layer deposition (ALD) deposition, for example or any other suitable deposition process. As shown in  FIG. 4 , a tip  130   a  of the key hole  130  is formed at an upper surface of the second dielectric layer  108 . 
       FIG. 5  is a diagram illustrating a planarization operation of the conformal film shown in  FIG. 4  that can be implemented within embodiments of the present invention. As shown in  FIG. 5 , the conformal film  120  is polished via a chemical mechanical polishing (CMP) process, for example, to an upper surface of the planarization stop layer  110 . Therefore, during the CMP process, the planarization stop layer  110  acts as a CMP stop layer to stop the CMP before it approaches the tip  130   a  of the key hole  130 . Since the tip  130   a  of the key hole  130  is formed at the upper surface of the second dielectric layer  108 , the tip  130   a  remains closed and CMP slurry does not enter the key hole  130  and block a subsequent reactive ion etching (RIE) process (as depicted in  FIG. 6 ) for transferring the key hole  130  to the first dielectric layer  104  to form a pore of a memory cell. The formation of a pore of a memory cell will now be described below with reference to  FIG. 6 . 
       FIG. 6  is a diagram illustrating the formation of a pore that can be implemented within embodiments of the present invention. As shown in  FIG. 6 , the conformal film  120  is recessed via a RIE process and the keyhole  130  is transferred down into the first dielectric layer  106  to form a pore  135  of a memory cell. 
     Further, phase change material (not shown) is then deposited within the pore  135  and a top electrode is formed on the phase change material, resulting in the formation of a phase change memory cell. According to an embodiment of the present invention, the pore  135  may be filled with TiN or W to form the sub-lithography diameter bottom electrode for mushroom phase change memory cell. 
     Embodiments of the present invention provide a key hole structure and method for forming a key hole structure to form a pore of a memory cell. The present invention uses a planarization stop layer which provides the advantage of forming a key hole structure that prevents CMP slurry from approaching a tip of the key hole thereby preventing interference of a key hole transfer during a RIE process for forming a pore within a memory cell. The present invention further minimizes the time of the RIE process which contributes to the non-uniformity across the wafer, thereby improving the uniformity across the wafer. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
     The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.