Abstract:
A method for fabricating a split gate flash memory includes depositing a second conductive layer for forming a control gate on a semiconductor substrate having a first conductive layer, an insulating layer, and an oxide layer on both sides of the first conductive layer formed thereon, filling an anti-implant protective layer in a depression of the second conductive layer, performing ion implant on the second conductive layer, removing the anti-implant protective layer filled in the depression of the second conductive layer, forming a photoresist pattern by depositing a photoresist layer on the second conductive layer for forming a control gate, and treating the photoresist layer with a light exposure and a development process, and forming the control gate by etching the second conductive layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a memory device, and more particularly, to a method for fabricating a split gate flash memory. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing polymer residue from being formed in a space between the split gate area on both sides of the flash memory device and for enhancing the electrical characteristics of the flash memory device. 
   2. Discussion of the Related Art 
   Flash memory technology has been continuously developed by enhancing the cell structure in various ways. Such variety of forms includes the stacked gate cell structure and the split gate cell structure. The stacked gate cell structure is formed of a sequential deposition of a floating gate and a control gate. The stacked gate cell has the problem of over-erasing, which occurs when the floating gate is excessively discharged. A threshold voltage of the excessively discharged cell has a negative (−) value. Accordingly, an electric current flows through even when a cell is not selected. In other words, the current flows when a read voltage is not applied to the control gate. The split gate cell structure has been proposed in order to prevent the problem of over-erasing. 
     FIG. 1  illustrates a cross-sectional view of a general split gate cell structure. Referring to  FIG. 1 , the split gate (type) cell structure includes an oxide-nitride-oxide (ONO) layer  20 , a first conductive layer  30 , being the floating gate, and an insulating layer (i.e., a nitride layer)  40  sequentially deposited on a semiconductor layer  10 . An oxide layer  50  is formed on each side wall of the first conductive layer  30 . A second conductive layer  60 , being the control gate, is formed to cover only one side of the first conductive layer  30 . 
     FIG. 2  illustrates a cross-sectional view of a control gate etching process in the related art method for fabricating a split gate flash memory. Referring to  FIG. 2 , a first conductive layer  20 , a nitride layer  40 , and an oxide layer  50  are formed. Then, a second conductive layer  60  is formed on the entire surface thereof. Subsequently, a photoresist layer  70  is deposited on the entire surface of the second conductive layer  60 . After treating the photoresist layer  70  with a photolithography process and an etching process, a portion of the second conductive layer  60  remains, so as to form the control gate. The etching process of the second conductive layer is completed by removing the photoresist layer  70 . 
   As described above, a split gate having a horizontally symmetrical structure is formed in a memory cell area. Herein, when depositing the second conductive layer  60  for forming the control gate pattern, the floating gate pattern, i.e., a deposition of the first conductive layer  30 , the insulating layer  40 , and the oxide layer  50 , is already formed on the substrate. Therefore, step differences between the first conductive layer  30 , the insulating layer  40 , and the oxide layer  50  are formed, thereby causing a hollow groove (or depression) to be formed in the space between the two split gate areas, as shown in the dotted line of  FIG. 2 . 
   At this point, a photoresist layer  70  is deposited on the second conductive layer  60 , and the photoresist layer  70  is treated with a light exposure process and a development process. Then, due to the above-described step difference, photoresist scum (Ps) may remain in the hollow groove portion (or depression area) of the second conductive layer  60 . In addition, after forming the photoresist  70  pattern, a native oxide layer may be formed on the surface of the second conductive layer  60  in the portion having the photoresist layer  70  removed. Therefore, during a following control gate etching process, a reaction between the photoresist scum and the native oxide layer disturbs the etching process, thereby producing a polymer residue (Pr), which is a non-etched residue, in the space between the two split gate areas. However, the above-described space between the two split gate areas is a common drain area, which may cause deficiency in forming a silicide layer when the residue remains in this specific area, thereby increasing contact resistance, which in turn deteriorates the electrical characteristic of the device. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a method for fabricating a split gate flash memory that substantially obviates the above-identified and other problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide a method for fabricating a split gate flash memory that can prevent polymer residue from being produced in a space between two split gate areas, thereby enhancing the electrical characteristics of the flash memory device. 
   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 and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for fabricating a split gate type flash memory device includes depositing a second conductive layer for forming a control gate on a semiconductor substrate having a first conductive layer, an insulating layer, and an oxide layer on both sides of the first conductive layer formed thereon, filling an anti-implant protective layer in a depression of the second conductive layer, performing ion implantation on the second conductive layer, removing the anti-implant protective layer filled in the depression of the second conductive layer, forming a photoresist pattern by depositing a photoresist layer on the second conductive layer for forming a control gate, and treating the photoresist layer with a light exposure process and a development process, and forming the control gate by etching the second conductive layer. 
   The filling of an anti-implant protective layer in a depression of the second conductive layer may include forming a photoresist layer as the anti-implant protective layer on the second conductive layer including the depression, and performing an etching process on the photoresist layer, so as to remove the photoresist layer excepting for the depression. Alternatively, the filling of an anti-implant protective layer in a depression of the second conductive layer may also include forming a photoresist layer as the anti-implant protective layer on an area of the second conductive layer corresponding to the depression. 
   On the other hand, the filling an anti-implant protective layer in a depression of the second conductive layer may include depositing a nitride layer as the anti-implant protective layer on the second conductive layer, and performing an etch back process on the nitride layer, so as to remove the nitride layer excepting for the depression. And, the performing an etching process on the photoresist layer, so as to remove the photoresist layer excepting for the depression may include performing a wet-etch process to remove the nitride layer filled in the depression. And, the ion implant process may include nitrogen (N+) implant. 
   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: 
       FIG. 1  illustrates a cross-sectional view of a conventional split gate cell structure; 
       FIG. 2  illustrates a cross-sectional view of a conventional control gate etching process in the related art method for fabricating a split gate flash memory; 
       FIGS. 3A to 3G  illustrate cross-sectional views showing the process steps of the method for fabricating a split gate flash memory according to a first embodiment of the present invention; 
       FIGS. 4A to 4F  illustrate cross-sectional views showing the process steps of the method for fabricating a split gate flash memory according to a second embodiment of the present invention; and 
       FIGS. 5A to 5G  illustrate cross-sectional views showing the process steps of the method for fabricating a split gate flash memory according to a third embodiment of 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, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIGS. 3A to 3G  illustrate cross-sectional views showing the process steps of the method for fabricating a split gate flash memory according to a first embodiment of the present invention. Referring to  FIG. 3A , a first conductive layer  130  is formed on a semiconductor substrate, an insulating layer  140  is formed on the first conductive layer  130 , and an oxide layer  150  is formed on each side of the first conductive layer  130 . Then, a second conductive layer  160  is formed on the entire surface thereof. When depositing the second conductive layer  160  as described above, due to the step difference between the first conductive layer  130 , the insulating layer  140 , and the oxide layers  150  on both sides of the conductive layer  130 , a depression  160   a  may be formed between the two floating gates. 
   Then, referring to  FIG. 3B , in order to neutralize (or eliminate) the topology caused by the depression  160   a  in the second conductive layer  160 , a photoresist layer  170  is formed on the second conductive layer  160 . Subsequently, as shown in  FIG. 3C , the photoresist layer  170  is etched-back, so as to produce a photoresist residue  170   a  within the depression  160   a  of the second conductive layer  160 . Thereafter, referring to  FIG. 3D , an ion implant process is performed on the surface of the second conductive layer  160  and the photoresist residue  170   a . Herein, it is preferable that the ion implant is performed with nitrogen (N+) ions. 
   As described above, when performing the ion implant process, the photoresist residue  170   a  filled in the depression  160   a  acts as a protective layer against the ion implant (i.e., an anti-implant protective layer). Therefore, nitrogen ion can be injected into the second conductive layer  160  with the exception of the depression  160   a . Accordingly, the depression  160   a  of the second conductive layer  160  (i.e., the area perpendicular to the photoresist residue  170   a ) becomes a non-doping area, and the rest of the second conductive layer  160  excluding the depression  160   a  becomes a doping area. 
   Subsequently, referring to  FIG. 3E , the photoresist residue  170   a  is removed from the depression  160   a . Thereafter, as shown in  FIG. 3F , a photoresist pattern  180  for forming a control gate is formed on the second conductive layer  160 . Then, referring to  FIG. 3G , an etching process for forming a control gate is performed, thereby forming the control gate. Generally, when performing the etching process, the etch rate in the doping area is fast, whereas the etch rate in the non-doping area is slow. By using such characteristic, the present invention decreases the etch rate in the area having the step difference (i.e., the depression  160   a ), so as to repress non-etched material from being formed during the etching process. 
     FIGS. 4A to 4F  illustrate cross-sectional views showing the process steps of the method for fabricating a split gate flash memory according to a second embodiment of the present invention. The second embodiment of the present invention is similar to the process described in the first embodiment of the present invention, apart from the structure of the anti-implant protective layer and the treatment process. 
   More specifically, referring to  FIG. 4A , a first conductive layer  230  is formed on a semiconductor substrate, an insulating layer  240  is formed on the first conductive layer  230 , and an oxide layer  250  is formed on each side of the first conductive layer  230 . Then, a second conductive layer  260  is formed on the entire surface thereof. Then, as shown in  FIG. 4B , a photoresist pattern  270  is formed, as an anti-implant protective layer, on an area corresponding to a depression  260   a  of the second conductive layer  260 . After forming the photoresist pattern  270  on the depression  260   a , the photoresist pattern  270  prevents doping from occurring in the depression  260   a  area of the second conductive layer  260 . 
   Subsequently, referring to  FIG. 4C , an ion implant process is performed on the surface of the second conductive layer  260  and the photoresist residue  270   a . When performing the ion implant process, as described above, the photoresist pattern  270  formed on the depression  260   a  acts as an anti-implant protective layer, thereby allowing doping to occur in the second conductive layer  260  with the exception of the depression  260   a . After performing the ion implant process, as shown in  FIG. 4D , a photoresist strip (PR strip) process is performed to remove the photoresist pattern  270 . Thereafter, referring to  FIG. 4E , a photoresist pattern  280  for forming the control gate is formed on the second conductive layer. Finally, as shown in  FIG. 4F , an etching process for forming the control gate is performed, thereby forming the control gate. 
     FIGS. 5A to 5G  illustrate cross-sectional views showing the process steps of the method for fabricating a split gate flash memory according to a third embodiment of the present invention. The third embodiment of the present invention is similar to the process described in the first and second embodiments of the present invention, apart from the structure of the anti-implant protective layer and the treatment process. 
   Referring to  FIG. 5A , a first conductive layer  330  is formed on a semiconductor substrate, an insulating layer  340  is formed on the first conductive layer  330 , and an oxide layer  350  is formed on each side of the first conductive layer  330 . Then, a second conductive layer  360  is formed on the entire surface thereof. Then, as shown in  FIG. 5B , a nitride layer  370  is formed, as an anti-implant protective layer on the second conductive layer  360 . Subsequently, referring to  FIG. 5C , the nitride layer  370  is treated with a nitride etch back process, so as to remove the nitride layer  370  with the exception of the area of the depression  360   a , thereby forming a nitride residue  370   a  in the depression  360   a.    
   Thereafter, as shown in  FIG. 5D , an ion implant process is performed on the surface of the second conductive layer  360  and the nitride residue  370   a . When performing the ion implant process as described above, the nitride residue  370   a  prevents doping from occurring in the depression  360   a  of the second conductive layer  360 . In other words, doping is performed on the entire surface with the exception of the depression  360   a  area. After performing the ion implant, as described above, referring to  FIG. 5E , a nitride wet-etch process is performed, so as to remove the nitride residue  370   a  from the depression  360   a . Subsequently, as shown in  FIG. 5F , a photoresist pattern  380  for forming the control gate is formed on the second conductive layer. Finally, as shown in  FIG. 5G , an etching process for forming the control gate is performed, thereby forming the control gate. 
   As described above, the method for fabricating the split gate flash memory according to the present invention has the following advantages. By using the difference in etch rate between the doping area and the non-doping area, a different etch rate is applied to a depression part, which is caused by step differences between floating gates occurring during the forming of the control gate, and the area excluding the depression part, thereby preventing non-etched polymer residue from being produced during the etching process, and, accordingly, enhancing the electrical characteristics of the flash memory device. 
   This application claims the benefit of Korean patent application No. 10-2003-0101753, filed on Dec. 31, 2003, the entire contents of which is hereby incorporated by reference as if fully set forth herein. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.