Abstract:
A method of forming a protective film to prevent a nitride read only memory (NROM) is disclosed. In the method of the present invention, the protective layers are formed in the inter-level dielectrics (ILD)/inter-metal dielectrics (IMD) layer of the NROM cell, and the protective layers can prevent the NROM cell from being penetrated by the ultra-violet light or plasma, and avoid increasing the ion mobility to cause the charge gain during the process that affects the stability of the electricity of the NROM cell. Additionally, the threshold voltage of the NROM cell can decrease to expand the range of the threshold voltage.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method of forming a protective film to prevent a nitride read only memory (NROM) cell charging, and more particularly, to a method of forming a NROM cell that the protective layers are formed in the inter-level dielectrics (ILD)/inter-metal dielectrics (IMD) layer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Referring to FIG. 1, it shows a cross-sectional view of the structure of the conventional NROM cell. The forming of the NROM cell, firstly the active area is defined on the substrate  100  by using photolithography and such methods as the wet etching. The phosphorous ions (P − ) are doped into the substrate  100  by using the ion implantation to form the channel  124 . The first oxide layer  102 , the nitride layer  104 , and the second oxide layer  106  are deposited on the substrate  100  in turn, and the nitride layer  104  is located between the first oxide layer  102  and the second oxide layer  106 . The first oxide layer  102 , the nitride layer  104 , and the second oxide layer  106  are defined by using the photolithography and the etching process to form an oxide/nitride/oxide (ONO) structure  108  and expose the substrate  100 .  
           [0003]    The polysilicon layer  110  is deposited to cover the second oxide layer  106 , and the silicide layer  112  is deposited to cover the polysilicon layer  110 , then the polysilicon layer  110  and the silicide layer  112  are defined to expose the second oxide layer  106  by using photolithography and the etching process similarly, so that the gate  114  is formed. Next, a material layer, such as the tetra-ethyl-ortho-silicate (TEOS), the silicon dioxide (SiO 2 ), or the silicon nitride (Si 3 N 4 ) etc., is deposited, such as by the chemical vapor deposition (CVD) to cover the substrate  100 , the second oxide layer  106 , and the silicide layer  112 . The material layer is defined by using photolithography and the anisotropic etching process to form the spacer  116 .  
           [0004]    Subsequently, the heavy highly concentrated doping and great depth is executed on the substrate  100  by using the structure consisted of the spacer  116  and the gate  114  as the mask, and the phosphorous (P) or the arsenic (As) that has greater solid solubility to the silicon (Si) as the ion source, so that the drain  122  and the source  126  are formed. An insulated layer  118  is deposited to cover the substrate  100 , the spacer  116 , and the silicide layer  112 , and the ILD/IMD layer  120  is deposited to cover the insulated layer  118 . Developed to this present, the NROM cell structure is completed.  
           [0005]    For the following process, such as ultra-violet light or plasma usually penetrates through the NROM device to excite the atoms, so that the NROM device electrical instability results and damages the NROM device, or increases the ion mobility to cause the charge gain during the process, enhancing the threshold voltage, and affecting the stability of the device.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the conventional method of forming the NROM cell, there is no protective structure formed, so on the following process, the NROM cell may be penetrated through by the ultra-violet light or the plasma, and may increase the ion mobility to cause the charge gain, so that NROM cell damage will result and also result in the NROM cell electrical instability.  
           [0007]    Accordingly, one aspect of the invention is to provide a method of forming the protective film to prevent a NROM cell charging. For NROM cell formation, the method of the present invention forms the protective layers to avoid the ultra-violet light and the plasma penetrating through the NROM device, to prevent the ion mobility increasing, and to keep the electricity&#39;s stability.  
           [0008]    Another aspect of the invention is to provide a method of forming the protective film to prevent a NROM cell charging. For NROM cell formation, the method of the present invention forms one or a plurality of protective layers in the ILD/IMD layer to avoid the charge gain during the process, increasing the ion mobility, and the ultraviolet light or the plasma penetrating, so to enhance the electricity&#39;s stability of the device, decrease the NROM device&#39;s threshold voltage, and expand the threshold voltage range.  
           [0009]    For at least the foregoing aspects discussed above, the present invention provides a method of forming the protective film to prevent a NROM cell charging. The present invention&#39;s method adds one or a plurality of protective layers, such as the silicon nitride (SiN x ) layer or the silicon-oxy-nitride (SiON) layer, and the protective layers can resist the ultra-violet light illumination and the invading plasma, so that the NROM device cannot be excited to discharge in order to the ultra-violet light and the plasma, and decrease the device&#39;s threshold voltage, so the device&#39;s electricity can be controlled and kept constant. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0011]    [0011]FIG. 1 is a cross-sectional view of the structure of the conventional NROM cell;  
         [0012]    [0012]FIG. 2 is a cross-sectional view of the structure of the NROM cell in accordance with a preferred embodiment of the present invention; and  
         [0013]    [0013]FIG. 3 is a cross-sectional view of the structure of the NROM cell in accordance with another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    In contrast with the flash memory and the erasable programmable read only memory (EPROM), the NROM cell can&#39;t be erased to the initial state by the ultraviolet light, but the ultra-violet light&#39;s illumination excites the NROM cell&#39;s atoms to result in the NROM cell&#39;s electrical instability, and damage the device, therefore there is a need to isolate the NROM cell from the ultra-violet light.  
         [0015]    Additionally, the NROM cell can be invaded by plasma ions during process to increase the charges of the NROM cell, so that the NROM cell&#39;s electrical stability is affected similarly and damaged. Therefore, the NROM cell needs a protective structure to prevent the penetrating of the ultra-violet light and the plasma ions, and to maintain the chargers of the NROM cell.  
         [0016]    Referring to FIG. 2, it shows a cross-sectional view of the structure of the NROM cell in accordance with a preferred embodiment of the present invention. On the formation of the NROM cell, the active area is defined first on the substrate  200  by using photolithography and the etching method, such as the wet etching method, and the phosphorous ions (P − ) are doped into the substrate  200  by using the ion implantation to form the channel  226 . Subsequently, the first oxide layer  202 , the nitride layer  204 , and the second oxide layer  206  are deposited on the substrate  200  in turn, and the nitride layer  204  is located between the first oxide layer  202  and the second oxide layer  206 . The first oxide layer  202 , the nitride layer  204 , and the second oxide layer  206  are defined by using photolithography and the etching process to form an oxide/nitride/oxide (ONO) structure  208  and expose the substrate  200 .  
         [0017]    Then, the polysilicon layer  210  is deposited to cover the second oxide layer  206  such as by the low pressure chemical vapor deposition (LPCVD), and the silicide layer  212 , such as tungsten silicide (WSi 2 ) or titanium silicide (TiSi 2 ), is deposited to cover the polysilicon layer  210  such as by the LPCVD. The polysilicon layer  210  and the silicide layer  212  are defined to expose the second oxide layer  206  by using photolithography and the etching process similarly, so that the gate  214  is formed. Next, a material layer, such as the tetra-ethyl-ortho-silicate (TEOS), the silicon dioxide (SiO 2 ), or the silicon nitride (Si 3 N 4 ) etc., is deposited by such as the CVD to cover the substrate  200 , the second oxide layer  206 , and the gate  214 . The material layer is defined by using the photolithography and the anisotropic etching process to form the spacer  216  on the sidewall of the gate  214 .  
         [0018]    Subsequently, the heavy doping of higher concentration and greater depth is executed on the substrate  200  by using the structure consisted of the spacer  216  and the gate  214  as the mask, and the phosphorous (P) or the arsenic (As) that has greater solid solubility to the silicon (Si) as the ion source, to dope the nitrogen ions (N + ) into the substrate  200 , so that the drain  224  and the source  228  are formed. An insulated layer  218  is deposited to cover the substrate  200 , the spacer  216 , and the gate  214 , and the protective layer  220 , such as the silicon nitride (SiN x ) or the silicon-oxy-nitride (SiON) etc., is deposited to cover the insulated layer  218 , wherein the protective layer  220  can prevent the penetrating of the ultra-violet light and the plasma. Next, the ILD/IMD layer  222  is deposited.  
         [0019]    Because the aforementioned method of forming the protective film to prevent the NROM cell charging forms a protective layer  220  that can protect the NROM cell to resist the ultra-violet light&#39;s illumination and the invading plasma, and avoid affecting the NROM cell&#39;s electricity. Therefore, the stability of the NROM cell can be maintained, and the threshold voltage of the device can decrease to about less than 0.2 volts, so that the range of the threshold voltage can expand.  
         [0020]    Referring to FIG. 3, it shows a cross-sectional view of the structure of the NROM cell in accordance with another present invention preferred embodiment. The structure of the NROM cell is to deposit the first protective layer  270  on the insulated layer  218  of the NROM cell structure in FIG. 2, and deposit the first ILD/IMD layer  272  to cover the first protective layer  270 . Next, the second protective layer  274  is deposited to cover the first ILD/IMD layer  272 , the second ILD/IMD layer  276  is deposited to cover the second protective layer  274 , and the third protective layer  278  is deposited to cover the second ILD/IMD layer  276 , wherein the material of the first protective layer  270 , the second protective layer  274 , and the third protective layer  278  can be the silicon nitride (SiN x ) or the silicon-oxy-nitride (SiON), etc.. Then, the insulated layer  218 , the first protective layer  270 , the first ILD/IMD layer  272 , the second protective layer  274 , the second ILD/IMD layer  276 , and the third protective layer  278  are defined to form the contact/via  284  and expose the gate  214 by using the photolithography and the etching process, and the plug  280  is formed on the contact/via  284 , such as tungsten (W). Afterward, a metal layer  282  is deposited to cover the third protective layer  278  and the plug  280 .  
         [0021]    Because the aforementioned NROM structure has formed three protective layers between the ILD/IMD layers, i.e. the first protective layer  270 , the second protective layer  274 , and the third protective layer  278 , so the NROM structure has more protection to resist the ultra-violet light penetration, plasma, and keep the device&#39;s stability.  
         [0022]    According to the preferred embodiments of the present invention, while the process permits, there can be a plurality of protective layers formed in the NROM of the present invention, and the number of the protective layers is not limited. Furthermore, the locations of the protective layers are not limited while the protective layers are between the ILD/IMD layers and the insulated layer of the NROM.  
         [0023]    The advantage of the present invention is to provide a method of forming the NROM having the protective structure. The method of the present invention forms at least one protective layer, such as the silicon nitride (SiN x ) or the silicon-oxy-nitride (SiON), to prevent the ultra-violet light illumination and the plasma from invading during the process, and to prevent the device&#39;s excitement, and inducing the charges that increases the ion mobility. Therefore, the application of the present invention can resist the ultra-violet light and the plasma penetration through the NROM cell, so it is not only can avoid increasing the ion mobility to maintain the stability of the electricity of the NROM device, but also can decrease the threshold voltage to expand the threshold voltage range.  
         [0024]    As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrations of the present invention rather than limitations of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.