Abstract:
Flash memory and methods of fabricating flash memory are disclosed. A disclosed method comprises: forming a first floating gate; and extending the first floating gate by forming a second floating gate adjacent a first sidewall of the floating gate. The second floating gate extends upward above the first floating gate. The method also includes depositing a dielectric layer on the first floating gate and the second floating gate; and forming a control gate on the dielectric layer.

Description:
FIELD OF THE DISCLOSURE 
   The present disclosure relates to flash memory and, more particularly, to flash memory and methods for fabricating flash memory. 
   BACKGROUND 
   A flash memory generally comprises a floating gate and a control gate. A dielectric layer such as an oxide-nitride-oxide (hereinafter referred to as “ONO”) layer is positioned between the floating gate and the control gate. A tunneling oxide layer is positioned between the floating gate and a silicon substrate. Flash memory usually stores or removes data by placing or removing electrons or holes in or from the floating gate. Because the floating gate is completely isolated by the tunneling oxide layer and the dielectric layer, once electrons or holes enter the floating gate, they cannot escape from the floating gate even though no power is supplied. Therefore, data can be stored in the flash memory for long time periods without using power. 
   In order to write or erase data into or from the flash memory, a bias voltage is applied to terminals accessible from outside the memory. That is, a sufficiently strong voltage must be applied to the control gate and to a junction or substrate to generate a strong electric field at both ends of the tunneling oxide. The coupling ratio (hereinafter referred to as “CR”) is a ratio of the voltage applied to the control gate and the junction or substrate, and the voltage induced in the floating gate. As the coupling ratio CR increases, the efficiency of programming and erase operations of a memory cell improves such that the voltage which must be applied from outside to effect such operations decreases. 
   The CR is defined by the ratio between the capacitance of the tunneling oxide layer and the capacitance of the ONO layer. When the tunneling oxide capacitance is represented as C TUN  and the ONO capacitance is represented as C ONO , the coupling ratio CR may be expressed as follows: 
   
     
       
         
           CR 
           = 
           
             
               C 
               ONO 
             
             
               ( 
               
                 
                   C 
                   TUN 
                 
                 + 
                 
                   C 
                   ONO 
                 
               
               ) 
             
           
         
       
     
   
   For a high CR, the C ONO  must be relatively high compared to the C TUN . Capacitance is determined by several factors such as the dielectric constant, the thickness of a dielectric layer, and the area of a capacitor. 
   In a known flash memory, the thickness of the tunneling oxide layer is about 80 Å to 120 Å, and the thickness of the dielectric layer is about 150 Å to 300 Å. As a result, for corresponding areas of the tunneling oxide layer and dielectric layer, the C ONO  is smaller than the C TUN  and, therefore, it is difficult to obtain a CR required for acceptable operation of the flash memory. Accordingly, in order to ensure a high CR, methods for increasing the surface area of the floating gate have been suggested. 
   Kim et al., Korean Patent Publication No. 2003-29203, describes a semiconductor device fabricating method to prevent leakage of current through a multiple tunnel junction layer by reducing the width of the multiple tunnel junction layer. 
   Shin et al., Korean Patent No. 375231, describes a method for fabricating a non-volatile memory device to maximize an area in which a floating gate and a control gate electrode overlap by forming a U-type floating gate without increasing a cell area so that a low operating voltage and a fast operating characteristic are realized. 
   Shin et al., U.S. Pat. No. 6,482,728, describes a method for fabricating a floating gate in a non-volatile memory device to reduce a bridge between floating gates and a field loss during processes such as an ONO etching process. In the above-mentioned U.S. Patent, a conductive layer with upper and lower portions is formed over a substrate with field regions formed therein, and the upper portion of the conductive layer is slope-etched, leaving the lower portion of the conductive layer intact. The slope-etched upper portion of the conductive layer is again vertically etched and the lower portion of the conductive layer is concurrently slope-etched. 
     FIGS. 1   a  through  1   g  are cross-sectional views illustrating a conventional flash memory fabricating process to increase the surface area of a floating gate. Referring to  FIG. 1   a , a tunneling oxide layer  2  is deposited on a silicon substrate  1 . A first polysilicon layer  3  to form a first floating gate is deposited on the tunneling oxide  2 . Then, a first mask layer  4  is deposited on the first polysilicon layer  3 . The first mask layer  4  is generally nitride. 
   Referring to  FIG. 1   b , some parts of the first mask layer  4 , the first polysilicon layer  3 , the tunneling oxide  2 , and the silicon substrate  1  are removed by a photolithography process and an etching process to form shallow trench isolation (hereinafter referred to as “STI”) within field oxide areas  5  to be formed in a subsequent process. 
   Referring to  FIG. 1   c , a field oxide is deposited on the first mask layer  4  and the STI. A chemical mechanical polishing (hereinafter referred to as “CMP”) process is then performed so that the field oxide remains only within the STI areas  6 . 
   Referring to  FIG. 1   d , the STI field oxide is etched until its height reaches the height of the floating gate  3  and the first mask layer  4  is also removed. Then, a second polysilicon layer  7  is deposited over the whole surface of the resulting substrate to form a second floating gate so that the first floating gate  3  is in contact with the second floating gate. A second mask layer is deposited on the second polysilicon layer  7  and selectively etched to form a second mask pattern  8 . 
   Referring to  FIG. 1   e , a third mask layer is deposited on the second polysilicon layer  7  and the second mask pattern  8 . The third mask layer is then anisotropically etched to form spacers  9  on the side-walls of the second mask pattern  8 . 
   Referring to  FIG. 1   f , the second polysilicon layer  7  is etched using the second mask pattern  8  and the spacers  9  as a hard mask to form the second floating gate  7 . The second mask pattern  8  and the spacers  9  are then removed. 
   Finally, referring to  FIG. 1   g , a dielectric layer  10  and a third polysilicon layer  11  are sequentially deposited over the whole surface of the resulting structure to form a control gate. 
   In the above-described prior art process, the length of the floating gate  7  is extended over the field oxide area(s)  6  to increase the surface area of the floating gate  7 . In addition, in order to minimize an increase in memory cell size due to the lengthened floating gate  7 , the space between adjacent floating gates is minimized by a hard mask process. Thus, as shown in  FIG. 1   g , the surface area of the floating gate  7  becomes larger because the height and length of the floating gate  7  increase by h and 2 L, respectively, compared to the surface area of the tunneling oxide. 
   Moreover, the conventional flash memory fabricating method described above necessarily includes several processes such as photolithography, etching, and hard mask layer deposition to increase the length of the floating gate  7  to extend over the field oxide area(s)  6 , thereby increasing process costs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a  through  1   g  are cross-sectional views illustrating a conventional flash memory fabricating process to increase the surface area of a floating gate. 
       FIGS. 2   a  through  2   g  are cross-sectional views illustrating a preferred example flash memory fabricating process performed in accordance with the teachings of the present invention. 
       FIG. 3  is a cross-sectional view illustrating an example flash memory device constructed in accordance with the teachings of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 2   a , a tunneling oxide  22  is deposited on a silicon substrate  21 . A first polysilicon layer to form a first floating gate  23  is deposited on the tunneling oxide  22 . A first mask layer  24  required for an STI process is then deposited on the first polysilicon layer. (The first mask layer  24  is preferably a nitride layer.) Some parts of the first mask layer  24 , the first polysilicon layer, the tunneling oxide  22 , and the silicon substrate  21  are removed by means of photolithography and etching processes within field oxide area(s)  25  to be formed in a subsequent process to form STI and the first floating gate  23 . Field oxide is then deposited over the whole surface of the resulting structure including the STI. Next, some parts of the field oxide are removed by CMP so that the field oxide remains only within the STI area(s)  25 . 
   Referring to  FIG. 2   b , the STI field oxide is further etched until about a half of the sidewall of the first floating gate  23  is exposed. Next, a second polysilicon layer  26  is deposited over the whole surface of the resulting structure to form a second floating gate so that the first floating gate can be connected with the second floating gate. 
   Referring to  FIG. 2   c , a second mask layer  27  is deposited on the second polysilicon layer  26 . The second mask layer is preferably an oxide layer. 
   Referring to  FIG. 2   d , the second mask layer  27  is etched to form spacers  28  on the side-wall portions of the second polysilicon layer  26 . More specifically, blanket etching is performed on the second mask layer  27  to form the spacers  28  on the sidewalls of the second polysilicon layer  26 . 
   Referring to  FIG. 2   e , an etching process is performed on the second polysilicon layer  26  using the spacers  28  as a mask to form the second floating gate. The etching process is preferably a reactive ion etch. Through the etching process, the portions of the second polysilicon layer  26  that are both between the spacers  28  and above the first mask layer  24  are completely removed. The remainder of the second polysilicon layer  26  is etched until the first mask layer  24  is exposed. 
   Referring to  FIG. 2   f , the spacers  28  and the first mask layer  24  are removed to complete a floating gate consisting of the first and second floating gates. As shown in  FIG. 2   f , the first floating gate is connected with the second floating gate, and the second floating gate protrudes upward higher than the first floating gate. 
   Referring to  FIG. 2   g , a dielectric layer  29  is deposited over the entire surface of the structure of  FIG. 2   f . A third polysilicon layer  30  is deposited on the dielectric layer  29  to form a control gate. 
   The illustrated floating gate has a part that is protruded upward. As a result, the floating gate achieves a high coupling ratio CR because the capacitance of the dielectric layer  29  (i.e., C ONO ) increases due to the protrusion of the floating gate. 
   In the above-described example methods for fabricating flash memory, the length (L) of the second floating gate on the field oxide  25  may be decreased as the height of the sidewalls (h 1 , h 2 ) are increased (see  FIG. 3 ). Therefore, the length (L) of the floating gate may be reduced without changing the surface area of the floating gate and, therefore, the size of a memory cell including the reduced length floating gate may ultimately be reduced. In other words, the fabrication methods disclosed herein can reduce the size of a flash memory cell and the voltage required for programming and erase operations with respect to such a cell by increasing the surface area of the capacitor formed between the floating gate and the control gate  30 . In addition, the disclosed methods can be used to achieve low cost and high yield semiconductor device production by simplifying the mask processes. 
   In view of the foregoing persons of ordinary skill in the art will appreciate that improved flash memory devices and improved methods of fabricating a flash memory have been disclosed. An illustrated example method of fabricating a flash memory reduces the size of a memory cell by increasing the surface area of the capacitor formed between a floating gate and a control gate. This reduction also reduces the voltage required for programming and erase operations. The illustrated example method also permits low cost and higher yield semiconductor device production by simplifying mask processes. 
   A disclosed example method of fabricating a flash memory comprises: sequentially depositing a tunneling oxide, a first polysilicon layer to form a first floating gate, and a first mask layer over a silicon substrate, etching the silicon substrate, the tunneling oxide, the first polysilicon layer, and the first mask layer using a predetermined pattern to form STI and the first floating gate, depositing a field oxide over the substrate including the STI, removing part of the field oxide, depositing a second polysilicon layer to form a second floating gate over the first mask layer and the field oxide, depositing a second mask layer over the second polysilicon layer, etching the second mask layer to form spacers, removing part of the second polysilicon layer using the spacers as a mask to form the second floating gate, removing the spacers and the first mask layer, and depositing a dielectric layer and a third polysilicon layer to form a control gate over the first floating gate and the second floating gate. 
   It is noted that this patent claims priority from Korean Patent Application Serial Number 10-2003-0066153, which was filed on Sep. 24, 2003, and is hereby incorporated by reference in its entirety. 
   Although certain example methods, apparatus and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.