Patent Publication Number: US-7589372-B2

Title: Nonvolatile memory device and method for fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 11/206.257, filed Aug. 15, 2005 now U.S. Pat. No.7,371,639, which is incorporated herein by reference in its entirety. This application also claims the benefit of Korean Application No. P2004-63869, filed on Aug. 13, 2004, which is hereby incorporated by reference as if fully set forth herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a nonvolatile memory device, and more particularly, to a method for fabricating a nonvolatile memory device, which decreases power consumption and prevents contamination of an insulating layer. 
   2. Discussion of the Related Art 
   Nonvolatile memory devices are advantageous in that there is no loss of data even with power supply interruption, which has led to their widespread use for data storage of computer BIOS chips, set-top boxes, printers, network servers, digital cameras, and mobile phones. Among nonvolatile memory devices, an electrically erasable programmable read-only memory (EEPROM) device enables data erase operations from individual memory cells or by sectors. A cell transistor of an EEPROM device is programmed by increasing the transistor&#39;s threshold voltage by generating channel hot electrons on the drain side, which are then stored in a floating gate, and is erased by lowering the threshold voltage by generating a high voltage potential between the floating gate and the source/substrate to thereby discharge the stored channel hot electrons. 
   Meanwhile, flash memory is a specific form of nonvolatile memory by which data bits are stored in units of memory or memory cells. A grouping of memory cells can be termed a word, a grouping of words can be termed a page, and a grouping of pages can be termed a sector. Data can be accessed for reading and programming by word or page, while an entire sector is commonly accessed for erasing, as in a “flash.” 
     FIGS. 1A and 1B  illustrate a process for fabricating a flash memory device according to the related art, which has a stacked gate structure comprising a floating gate and a control gate. The floating gate stores electrical charges for the gate, and the control gate receives a drive voltage. 
   Referring to  FIG. 1A , a tunneling oxide layer  11   a  is formed on a predetermined portion of a semiconductor substrate  11 , and a stacked gate  12  is formed on the tunneling oxide layer by a series of deposition steps to form layers of a stacked gate structure. The stacked gate  12  comprises a polysilicon layer for the formation of a floating gate  12   a , a gate insulating layer  12   b  having an oxide-nitride-oxide structure, and a polysilicon layer for the formation of a control gate  12   c , which are sequentially formed on the tunneling oxide layer  11   a . The polysilicon layer for floating gate formation has a thickness of 800˜1200 Å formed by low-pressure chemical mechanical deposition, and the polysilicon layer for control gate formation has a thickness of 2000˜2200 Å, also formed by low-pressure chemical mechanical deposition. 
   Subsequently, a photolithography process is used to form the stacked gate  12  structure, comprised of the control gate  12   c , the gate insulating layer  12   b , and the floating gate  12   a , by etching the structure obtained through previous deposition steps. That is, the photolithography process is used to remove selectively (i.e., etch) portions of the polysilicon layer for control gate formation, the gate insulating layer  12   b , and the polysilicon layer for floating gate formation, thereby forming the stacked gate  12 . Liner layers (oxide sidewalls)  13  are then formed on lateral sides of the stacked structure, that is, on each of the control gate  12   c , the gate insulating layer  12   b , and the floating gate  12   a.    
   Referring to  FIG. 1B , insulating sidewalls  14  are formed outwardly beside the liner layers  13 . Then, impurity ions are implanted using the stacked gate  12  and the insulating sidewalls  14  as a mask, thereby forming source/drain regions  15  and  16 . Subsequently, a silicide layer  17  is formed on exposed surfaces of the source/drain regions  15  and  16  and the control gate  12   c . An insulating interlayer  19  is formed on the entire surface of the resulting structure, and a plurality of contact holes is formed in the insulating layer to expose the silicide layer  17  above the source/drain regions  15  and  16  and the control gate  12   c . A plurality of plugs  18  is formed by filling the contact holes. 
   In the above process for fabricating a flash memory device according to the related art, the etching of the photolithography process to form the stacked gate structure includes two steps, namely, wet-etching and dry-etching, which is a complex procedure. This two-step process is performed to minimize plasma damage during photolithography. The gate insulating layer may nevertheless become contaminated as a result of plasma damage or due to the multi-step etching conditions themselves. 
   To program the above flash memory device according to the related art, a programming voltage is applied to the control gate  12   c  through a word line and to the drain  16  through a bit line. Thus, electrons of the drain  16  are injected toward the floating gate  12   a  through the tunneling oxide layer  11   a  by way of a hot-carrier method. In erasing data, an erasing voltage is applied to the source  15  through a source line. Thus, electrons injected to the floating gate  12   a  are discharged to the channel through the tunneling oxide layer  11   a.    
   The above operation is achieved in accordance with a coupling among elements of the stacked gate, specifically, between interfaces of the control gate and the floating gate and between interfaces of the floating gate and the drain, and the coupling ratio should be maximized to decrease power consumption. Therefore, an improvement of the coupling ratio is needed to decrease power consumption of the device, so that low-power flash memory devices can be provided for mobile products requiring low power consumption. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a nonvolatile memory device and a method for fabricating the same that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide a nonvolatile memory device to decrease power consumption, and a method for fabricating the same. 
   Another object of the present invention is to provide a nonvolatile memory device to prevent an insulating layer from becoming contaminated by plasma damage, and a method for fabricating the same. 
   Another object of the present invention is to provide a method for fabricating a nonvolatile memory device, which enables a simplified photolithography process for the formation of a stacked gate, and a nonvolatile memory device suitable for the same. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these objects and other advantages in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a nonvolatile memory device, comprising a semiconductor substrate; a tunneling oxide layer formed on a predetermined portion of the semiconductor substrate; a floating gate formed on the tunneling oxide layer, the floating gate having a trench structure; a control gate formed inside the trench structure of the floating gate; and a gate insulating layer disposed between the floating gate and the control gate. 
   In another aspect, there is provided a method for fabricating a nonvolatile memory device, comprising forming a tunneling oxide layer on a predetermined portion of a semiconductor substrate; forming a first polysilicon layer for a formation of a floating gate on the tunneling oxide layer; forming a trench in the first polysilicon layer for floating gate formation, the trench having a predetermined depth; forming a gate insulating layer in the trench formed in the first polysilicon layer for floating gate formation; forming, on the gate insulating layer, a second polysilicon layer for a formation of a control gate; performing chemical mechanical polishing with respect to the second polysilicon layer for control gate formation; forming, over the floating gate, the gate insulating layer, and the control gate, a photoresist pattern for forming the floating gate; and etching the first polysilicon layer for floating gate formation a using the photoresist pattern. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIGS. 1A and 1B  are cross-sectional views illustrating a process for fabricating a nonvolatile memory device according to the related art; and 
       FIGS. 2A-2E  are cross-sectional views illustrating a process for fabricating a nonvolatile memory device according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, like reference numbers will be used throughout the drawings to refer to the same or similar parts. 
     FIGS. 2A-2E  illustrate a process for fabricating a nonvolatile memory device according to the present invention. As an initial step in the process, a sacrificial oxide film (not shown) is formed on the semiconductor substrate and is removed, by wet-etching, after the formation of a well and a channel layer by respective ion-implantation processes. 
   Referring to  FIG. 2A , a tunneling oxide layer  22  is formed on a semiconductor substrate  21  to a thickness of 90˜100 Å, using a furnace thermal process performed at a temperature of 700˜800° C. Low-pressure chemical vapor deposition is then carried out to form a first polysilicon layer  23  having a thickness of 4500˜5500 Å on the tunneling oxide layer  22 . The first polysilicon layer  23  thus formed will later serve, after etching, as the floating gate of the nonvolatile memory device. A trench  24  is formed in the first polysilicon layer  23  for floating gate formation by removing, using a chlorine gas (Cl 2 ) etching process, a predetermined interior portion of the first polysilicon layer, which will later serve as an encasement for the control gate of the nonvolatile memory device. After etching, the trench  24  preferably has a depth of 2500˜3500 Å, such that, assuming an initial formation thickness of 4500˜5500 Å for the first polysilicon layer  23 , about 1000˜3000 Å of its thickness remains to form the lower side of the trench. 
   Referring to  FIG. 2B , a gate insulating layer  25  is formed on the surface of the first polysilicon layer  23  so as to cover the inner walls of the trench  24  with an oxide-nitride-oxide structure including a sequential formation of an oxide layer, a nitride layer, and another (i.e., upper) oxide layer by, for example, a known method. The lower oxide layer of the gate insulating layer  25  is formed to a thickness of 50˜70 Å by low-pressure chemical vapor deposition at a temperature of about 700˜800° C.; the nitride layer of the gate insulating layer is formed to a thickness of 60˜80 Å, also by low-pressure chemical vapor deposition but at a temperature of about 650˜750° C.; and the other oxide layer is formed by a furnace thermal process at a temperature of about 800˜900° C. Low-pressure chemical vapor deposition is performed to deposit a second polysilicon layer  26 , for control gate formation, on the entire surface of the resulting structure. The second polysilicon layer  26  may have a thickness of 3500˜4500 Å, which is substantially thicker than the depth of the trench  24 . 
   Referring to  FIG. 2C , the thickly formed second polysilicon layer  26  is planarized by chemical mechanical polishing to form a control gate  26   a  flush with the surface of the first polysilicon layer  23  and separated from the first polysilicon layer by the gate insulating layer  25 . Thus, the respective materials of the second polysilicon layer  26  and the gate insulating layer  25  remain in the trench  24 , and the control gate  26   a  is disposed so as to be encased in the first polysilicon layer  23  interposing the gate insulating layer  25 . In doing so, the opposing surface area of the interface between the control gate  26   a  and the material of the first polysilicon layer  23  for floating gate formation is increased, thereby increasing a coupling ratio and decreasing power consumption of the device accordingly. 
   Referring to  FIG. 2D , a photoresist (not shown) is formed on the entire surface of the resulting structure of  FIG. 2C , including the exposed upper surfaces of the control gate  26   a , the gate insulating layer  25 , and the first polysilicon layer  23 . An exposure and development process (i.e., photolithography) is then carried out to form a mask pattern used in etching the first polysilicon layer  23  to form a floating gate  23   a , which is thus imparted with a trench structure whose interior is filled with the gate insulating layer  25  and the control gate  26   a . Thus, a photoresist pattern for forming the floating gate  23   a  is positioned over the floating gate, the gate insulating layer  25 , and the control gate  26   a , and the first polysilicon layer  23  for floating gate formation is isotropically etched using the photoresist pattern. Therefore, in contrast with the process of the related art, the gate of the nonvolatile memory device is formed by an etching of a polysilicon layer for floating gate formation, through application of a single-step etching process, thereby preventing the possibility of any contamination of the gate insulating layer, which may result from plasma damage during the etching process used in the device fabrication according to the related art. 
   Referring to  FIG. 2E , an insulating layer (not shown) is deposited and then etched back to form insulating sidewalls  27  on the lateral sides of the floating gate  23   a , and impurity ions are implanted using the floating gate and insulating sidewalls as a mask, thereby forming source/drain regions  28  and  29  in the semiconductor substrate  21  on either side of the floating gate. Subsequently, a salicide process is performed with respect to the substrate, thereby imparting a silicide layer  30  to upper surfaces of the floating gate  23   a  and the control gate  26   a  and to the semiconductor substrate  21  in correspondence with the source/drain regions  28  and  29 . An insulating interlayer  39  is formed on the entire surface of the resulting structure, and a plurality of contact holes is formed in the insulating layer to expose the silicide layer  30  above the source and drain  28  and  29  and the control gate  26   a . A plurality of conductive plugs  31  is formed by filling the contact holes. 
   Accordingly, a nonvolatile memory device according to the present invention is shown in  FIG. 2E . The nonvolatile memory device according to the present invention comprises the semiconductor substrate  21 ; the tunneling oxide layer  22  formed in a predetermined portion of the upper surface of the semiconductor substrate; the floating gate  23   a  formed as a trench structure having an interior, the trench structure being formed on the tunneling oxide layer; the control gate  26   a  formed inside the interior of the trench structure of the floating gate; and the gate insulating layer  25 , having an oxide-nitride-oxide structure, disposed between the floating gate and the control gate. Thus, the gate insulating layer  25  and the control gate  26   a  are both disposed inside the trench, to be substantially encased by the floating gate  23   a  itself, i.e., the trench structure. The insulating sidewalls  27  are formed on the lateral sides of the floating gate  23   a , i.e., outside the trench, and the source/drain regions  28  and  29  are formed in the semiconductor substrate  21  in correspondence with the outer sides of the floating gate. The silicide layer  30  is formed on the upper (exposed) surfaces of the control gate  26   a  and the floating gate  23   a , to be disposed on the top surfaces of the trench structure of the floating gate, and is also disposed over each of the source/drain regions  28  and  29 , that is, in the surface of the semiconductor substrate  21  corresponding to the source/drain regions. The insulating interlayer  39  is formed on the entire resulting surface of the semiconductor substrate  21 . The plurality of conductive plugs  31  are connected, through the insulating interlayer  39 , to each of the control gate  26   a  and the source/drain regions  28  and  29 , to provide electrical contact with external circuitry, such as a word line, bit line, or source line (not shown). 
   As described above, by adopting the nonvolatile memory device and method for fabricating the same according to the present invention, it is possible to increase the size of the interface, i.e., opposing surface area, between the control gate  26   a  and the floating gate  23   a , thereby decreasing power consumption owing to the increased coupling ratio. Also, the first polysilicon layer  23  for floating gate formation is etched through a single-step etching process to form the gate of the device, thereby enabling a prevention of a contamination of the gate insulating layer  25  due to plasma damage during an etching process for forming a conventional stacked gate for a nonvolatile memory device such as a flash memory device. 
   It will be apparent to those skilled in the art that various modifications can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers such modifications provided they come within the scope of the appended claims and their equivalents.