Patent Publication Number: US-7211484-B2

Title: Method of manufacturing flash memory device

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to a method of manufacturing a flash memory device and, more specifically, to a method of manufacturing a flash memory device that can improve effective field oxide height (hereinafter, referred to as “EFH”) variation between a cell region, a high-voltage transistor region and a low-voltage transistor region in a flash memory device using a self-aligned shallow trench isolation (hereinafter, referred to as “SA-STI”) scheme. 
   2. Discussion of Related Art 
   A flash memory is provided with a high-voltage transistor and a low-voltage transistor for driving cells in view of a device&#39;s characteristic. A gate oxide film of the high-voltage transistor has a thick thickness, a gate oxide film of the low-voltage transistor has a thin thickness, and a gate oxide film of the cell has the same or similar thickness as those of the low-voltage transistor. For example, in a 120 nm level NAND flash memory device, the gate oxide film in the cell may be about 80 Å in thickness, the gate oxide film in the high-voltage transistor may be 350 Å in thickness, and the gate oxide film in the low-voltage transistor may be about 80 Å in thickness. A difference in a topology depending on the thickness of the oxide film in each region results in EFH variation between the high-voltage transistor region and the cell region or the low-voltage transistor region after a chemical mechanical polishing (hereinafter, referred to as “CMP”) process for performing a field oxide film, a subsequent process, is performed. In the above, EFH refers to an effective height of a field oxide film that is protruded upwardly from the interface between a first polysilicon layer for a floating gate and a second polysilicon layer for a floating gate. 
     FIG. 1  is a cross-sectional view illustrating a method of manufacturing a flash memory device using the SA-STI scheme according to a related art. 
   Referring to  FIG. 1 , a semiconductor substrate  11  in which a cell region CELL, a high-voltage transistor region HV and a low-voltage transistor region LV are defined is provided. A high-voltage gate oxide film  12 H is thickly formed on the semiconductor substrate  11  of the high-voltage transistor region HV, and a low-voltage gate oxide film  12 L and a cell gate oxide film  12 C are thinly formed on the semiconductor substrate  11  of each of the low-voltage transistor region LV and the cell region CELL. A first polysilicon layer  13  for a floating gate is formed on the oxide films  12 C,  12 H and  12 L. A SA-STI process is then performed to form a number of trenches  15  for isolation in the semiconductor substrate  11 . The trenches  15  are buried with oxide for isolation to form field oxide films  160 . A second polysilicon layer  17  for a floating gate is then formed on the entire structure including the field oxide films  160 . Though not shown in the drawing, an etch process using a mask for a floating gate, a dielectric film formation process, a process of forming a conductive layer for a control gate, and an etch process using a mask for a control gate are performed to form gates in the respective regions CELL, HV and LV. 
   If the flash memory device is fabricated by the above-mentioned method, however, EFH variation takes place among the field oxide films  160  each formed in the regions CELL, HV and LV due to a difference in a topology of the oxide films  12 C,  12 H and  12 L, which are formed in the cell region CELL, the high-voltage transistor region HV and the low-voltage transistor region LV, respectively. It results in EFH variation of about 300 Å or more, even if a nitride film strip process that is used in a SA-STI process after a CMP process and a cleaning process that is performed before the second polysilicon layer  17  is deposited are performed. The EFH of the field oxide film  160  in the high-voltage transistor region HV is about 50 to 200 Å, while the EFH of the field oxide film  160  in the cell region CELL or the low-voltage transistor region LV is 300 to 800 Å. The EFH of the cell region CELL and the low-voltage transistor region LV are high and wide in value. Such values vary depending on conditions of the CMP process. Variation in the EFH between the high-voltage transistor region HV and other regions CELL and LV not only causes many problems such as making it difficult to set a gate etch target of each of the regions CELL, HV and LV, making it impossible to obtain a good gate pattern profile, causing a fail in a device due to polysilicon remnant, and the like. These problems become critical, as the devices is higher integrated. An attempt to solve these problems has been made. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a method of manufacturing a flash memory device, which can secure stability of a process and reliability of a device, by improving EFH variation that is caused among a cell region, a high-voltage transistor region and a low-voltage transistor region due to a protrusion of a field oxide film of each of the regions. 
   According to a preferred embodiment of the present invention, there is provided a method of manufacturing a flash memory device, including the steps of providing a semiconductor substrate in which a cell region, a high-voltage transistor region and a low-voltage transistor region are defined; forming a field oxide film having a high EFH in the semiconductor substrate of each of the cell region and the low-voltage transistor region and forming a field oxide film having a low EFH in the semiconductor substrate of the high-voltage transistor region, due to variation in a topology of the gate oxide films formed in the respective regions; and etching the field oxide films having the high EFH by a given thickness by means of a field oxide film recess process, whereby the EFHs of the field oxide films formed in the regions become same or similar. 
   According to another preferred embodiment of the present invention, there is provided a method of manufacturing a flash memory device, including the steps of forming a cell gate oxide film, a high-voltage gate oxide film and a low-voltage gate oxide film on a semiconductor substrate in which a cell region, a high-voltage transistor region and a low-voltage transistor region are defined; forming a first polysilicon layer and a nitride film on the gate oxide films; sequentially etching the nitride film, the first polysilicon layer, the gate oxide films and the semiconductor substrate to form a number of trenches for isolation in the respective regions; depositing an oxide film on the entire structure including the trenches, and then polishing the oxide film and the nitride film by a given thickness by means of a polishing process, whereby a field oxide film having a high EFH is formed in the semiconductor substrate of each of the cell region and the low-voltage transistor region and a field oxide film having a low EFH is formed in the semiconductor substrate of the high-voltage transistor region; stripping the nitride film left after the polishing process; etching the field oxide films having the high EFH by a given thickness by means of a field oxide film recess process, whereby the EHFs of the field oxide films formed in the regions become same or similar; and forming a second polysilicon layer on the first polysilicon layer including the field oxide films having the same or similar EFH. 
   According to still another preferred embodiment of the present invention, there is provided a method of manufacturing a flash memory device, including the steps of forming a cell gate oxide film, a high-voltage gate oxide film and a low-voltage gate oxide film on a semiconductor substrate in which a cell region, a high-voltage transistor region and a low-voltage transistor region are defined; forming a first polysilicon layer and a nitride film on the gate oxide films; sequentially etching the nitride film, the first polysilicon layer, the gate oxide films and the semiconductor substrate to form a number of trenches for isolation in the respective regions; depositing an oxide film on the entire structure including the trenches and then polishing the oxide film and the nitride film by a given thickness by means of a polishing process, whereby a field oxide film having a high EFH is formed in the semiconductor substrate of each of the cell region and the low-voltage transistor region and a field oxide film having a low EFH is formed in the semiconductor substrate of the high-voltage transistor region; etching the field oxide films having the high EFH by a given thickness by means of a field oxide film recess process, whereby the EHFs of the field oxide films formed in the regions become same or similar; stripping the nitride film left after the polishing process and the field oxide film recess process; and forming a second polysilicon layer on the first polysilicon layer including the field oxide films having the same or similar EFH. 
   In the above embodiments, the field oxide film recess process includes the steps of forming a photoresist pattern that closes the high-voltage transistor region in which the field oxide films having the low EFH are formed; etching the field oxide film having the high EFH by a given thickness using a BOE solution, by using the photoresist pattern as an etch mask; stripping the photoresist pattern and organic contaminant using a PIRANHA cleaning solution; and stripping particles and organic contaminant using a SC-1 cleaning solution. In this case, the photoresist pattern is hardened by means of a descum process at a temperature of 80 to 140° C. The BOE solution is a solution in which NH 4 F and HF are mixed in the ratio of 9:1, 100:1 or 300:1. Further, a field oxide film etch target using the BOE solution is set according to variation in the EFH between the field oxide film having the low EFH and the field oxide film having the high EFH. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a flash memory device for explaining a method of manufacturing the device according to a related art; 
       FIGS. 2 to 6  are cross-sectional views of flash memory devices for explaining a method of manufacturing the device according to one embodiment of the present invention; and 
       FIGS. 7 to 12  are cross-sectional views of flash memory devices for explaining a method of manufacturing the device according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Now, the preferred embodiments according to the present invention will be described with reference to the accompanying drawings. Since preferred embodiments are provided for the purpose that the ordinary skilled in the art are able to understand the present invention, they may be modified in various manners and the scope of the present invention is not limited by the preferred embodiments described later. Further, in the drawing, the thickness and size of each layer are exaggerated for convenience of explanation and clarity. Like reference numerals are used to identify the same or similar parts. Meanwhile, in case where it is described that one film is “on” the other film or a semiconductor substrate, the one film may directly contact the other film or the semiconductor substrate. Or, a third film may be intervened between the one film and the other film or the semiconductor substrate. 
     FIG. 2  to  FIG. 6  are cross-sectional views of flash memory devices for explaining a method of manufacturing the device using a self align shallow trench isolation (SA-STI) scheme according to one embodiment of the present invention. 
   Referring to  FIG. 2 , a semiconductor substrate  21  in which a cell region CELL, a high-voltage transistor region HV and a low-voltage transistor region LV are defined is provided. A high-voltage gate oxide film  22 H is thickly formed on the semiconductor substrate  21  of the high-voltage transistor region HV. A low-voltage gate oxide film  22 L and a cell gate oxide film  22 C are thinly formed on the semiconductor substrates  21  of the low-voltage transistor region LV and the cell region CELL, respectively. A first polysilicon layer  23  for a floating gate is formed on the oxide films  22 C,  22 H and  22 L. A nitride film  24  is formed on the first polysilicon layer  23 . The nitride film  24 , the first polysilicon layer  23 , the oxide films  22 C,  22 H and  22 L, and the semiconductor substrate  21  are then etched by means of a SA-STI etch process, thereby forming a number of trenches  25  for isolation in the semiconductor substrate  21  of the cell region CELL, the high-voltage transistor region HV and the low-voltage transistor region LV. Next, an oxide film  26  for isolation is formed on the entire structure including the trenches  25  for isolation, whereby the trenches  25  are sufficiently buried. 
   In the above, the high-voltage gate oxide film  22 H is formed in a thickness of 300 to 500 Å, and the low-voltage gate oxide film  22 L and the cell gate oxide film  22 C are each formed in A thickness of below 100 Å. The first polysilicon layer  23  is formed in a thickness 300 to 700 Å. The nitride film  24  is formed in a thickness of 800 to 1200 Å. The trenches  25  are formed in depth of 2500 to 5000 Å. An oxide film  26  for isolation may be formed using a material having a good gap filing capability and a good insulating property, for example, HDP oxide, but may be formed in a single layer or a multi layer structure using various insulating substance. 
   By reference to  FIG. 3 , a CMP process is performed to form field oxide films  260  within the trenches  25 . In the above, it is preferred that the CMP process is performed right before the surface of the first polysilicon layer  23  in the high-voltage transistor region HV, which has a high topology due to the thick high-voltage gate oxide film  22 H, is exposed. 
   From  FIG. 3 , it can be seen that the thickness of the nitride film  24  left in the cell region CELL or the low-voltage transistor region LV after the CMP process is much thicker than that of the nitride film  24  left in the high-voltage transistor region HV. The thickness of the nitride film  24  left in these regions CELL, HV and LV becomes a factor to decide an EFH value of each of the cell region CELL, the high-voltage transistor region HV and the low-voltage transistor region LV. In other words, the EFH value of the high-voltage transistor region HV is low, and the EFH value of each of the cell region CELL and the low-voltage transistor region LV is high. For this reason, there is variation in the EFH among these regions CELL, HV and LV. This causes the above-mentioned conventional problems to occur. 
   By reference to  FIG. 4 , the remaining nitride film  24  is pre-treated using an oxide etch solution containing HF and the remaining nitride film  24  is completely stripped in a solution containing H 3 PO 4 . During the HF pre-treatment process and the nitride film strip process, the top of the field oxide films  260  formed in respective regions CELL, HV and LV is lost a little. Due to this, although the EFH value of each of the regions CELL, HV and LV is a little lowered, variation in the EFH is not improved. A photoresist pattern  29  through which the high-voltage transistor region HV is closed and the cell region CELL and the low-voltage transistor region LV are opened, is formed on the field oxide film  260  and the first polysilicon layer  23  in the high-voltage region HV. In order to prevent the occurrence of attach on the substrate and defects due to a subsequent field oxide film recess process, the photoresist pattern  29  is hardened by means of a descum process. In this case, the descum process is performed at a temperature of 80 to 140° C. for 10 or less minutes. 
   By reference to  FIG. 5 , the field oxide film  260  in each of the cell region CELL and the low-voltage transistor region LV is etched by a given thickness by means of a field oxide film recess process using the photoresist pattern  29  as an etch mask. This makes same or similar the protrusion and height of the field oxide film  260  in the high-voltage transistor HV that is protected by the photoresist pattern  29 . While the field oxide film recess process is performed, the photoresist pattern  29  is stripped. 
   The field oxide film recess process may be performed by consecutively performing the following steps. 
   A first step includes etching the exposed portion of the field oxide film  260  by a given thickness by using a buffered oxide etchant (BOE) solution in which NH 4 F and HF are mixed in an adequate ratio, for example, 9:1, 100:1 or 300:1. At this time, if an EFH of the field oxide film  260  in the high-voltage transistor region HV has a value of 50 to 200 Å and an EFH of the field oxide film  260  in the cell region CELL or the low-voltage transistor region LV has a value of 300 to 800 Å, a field oxide film etch target is 200 to 600 Å. In other words, the field oxide film etch target is decided by variation in the EFH between the field oxide film  260  having a low EFH and the field oxide film  260  having a high EFH. Due to this, as shown in  FIG. 5 , the EFH of the field oxide film  260  in the cell region CELL and the low-voltage transistor region LV becomes same or similar as that of the field oxide film  260  in the high-voltage transistor HV. 
   A second step includes stripping the photoresist pattern  29  used as the etch mask while stripping organic contaminant generated in the process using the BOE solution in the first step, by using a PIRANHA cleaning solution where H 2 SO 4  and H 2 O 2  are mixed. At this time, a temperature of the PIRANHA cleaning solution is 80 to 130° C. 
   A third step includes maximizing stripping of particles and organic contaminant left after the PIRANHA cleaning process in the second step, by using standard cleaning-1 (SC-1) solution in which NH 4 OH, H 2 O 2  and H 2 O are mixed in an adequate ratio, for example, 1:1:5 or 0.2:1:10. At this time, a temperature of the SC-1 cleaning solution is 40 to 200° C. 
   Referring to  FIG. 6 , a second polysilicon layer  27  for a floating gate is formed on the entire structure including the field oxide films  260  and the first polysilicon layer  23 . Though not shown in  FIG. 6 , an etch process using a mask for a floating gate, a dielectric film formation process, a process of forming a conductive layer for a control gate, and an etch process using a mask for a control gate are performed to form gates in respective regions. 
     FIGS. 7 to 12  are cross-sectional views of flash memory devices for explaining a method of manufacturing the device using the SA-STI scheme according to another embodiment of the present invention. 
   Referring to  FIG. 7 , a semiconductor substrate  31  in which a cell region CELL, a high-voltage transistor region HV and a low-voltage transistor region LV are defined is provided. A high-voltage gate oxide film  32 H is thickly formed on the semiconductor substrate  31  of the high-voltage transistor region HV. A low-voltage gate oxide film  32 L and a cell gate oxide film  32 C are thinly formed on the semiconductor substrates  31  of the low-voltage transistor region LV and the cell region CELL, respectively. A first polysilicon layer  33  for a floating gate is formed on the oxide films  32 C,  32 H and  32 L. A nitride film  34  is formed on the first polysilicon layer  33 . The nitride film  34 , the first polysilicon layer  33 , the oxide films  32 C,  32 H and  32 L, and the semiconductor substrate  31  are etched by means of a SA-STI etch process, thereby forming a number of trenches  35  for isolation in the semiconductor substrate  31  of the cell region CELL, the high-voltage transistor region HV and the low-voltage transistor region LV. An oxide film  36  for isolation is then formed on the entire structure including the trenches  35  for isolation, so that the trenches  35  are sufficiently filled. 
   In the above, the high-voltage gate oxide film  32 H is formed in thickness of 300 to 500 Å, and the low-voltage gate oxide film  32 L and the cell gate oxide film  32 C are each formed in thickness of below 100 Å. The first polysilicon layer  33  is formed 300 to 700 Å in thickness. The nitride film  34  is formed in thickness of 800 to 1200 Å. The trench  35  is formed in depth of 2500 to 5000 Å. The oxide film  36  for isolation may be formed using a material having a good gap filing capability and a good insulating property, for example, HDP oxide, but may be formed in a single layer or a multi layer structure using various insulating substance. 
   By reference to  FIG. 8 , a CMP process is performed to form field oxide films  360  within the trenches  35 . At this time, it is preferred that the CMP process is performed right before the surface of the first polysilicon layer  33  in the high-voltage transistor region HV, which has a high topology due to a thick high-voltage gate oxide film  32 H, is exposed. 
   From  FIG. 8 , it can be seen that a thickness of the nitride film  34  left in the cell region CELL or the low-voltage transistor region LV after the CMP process is much thicker than that of the nitride film  34  left in the high-voltage transistor region HV. The thickness of the nitride film  34  left in these regions CELL, HV and LV become a factor to decide the EFH value of each of the cell region CELL, the high-voltage transistor region HV and the low-voltage transistor region LV. In other words, the EFH value of the high-voltage transistor region HV is low and the EFH values of the cell region CELL and the low-voltage transistor region LV are high. For this reason, EFH variation occurs between these regions CELL, HV and LV. This causes the above-mentioned conventional problems to occur. 
   By reference to  FIG. 9 , a photoresist pattern  39  through which the high-voltage transistor region HV is closed and the cell region CELL and the low-voltage transistor region LV are opened is formed on the field oxide film  360  and the first polysilicon layer  33  in the high-voltage region HV. In order to prevent occurrence of attach against the substrate and defects due to a subsequent field oxide film recess process, the photoresist pattern  39  is hardened by means of a descum process. In this case, the descum process is performed at a temperature of 80 to 140° C. for 10 or less minutes. 
   By reference to  FIG. 10 , the field oxide film  360  in each of the cell region CELL and the low-voltage transistor region LV is etched by a given thickness by means of a field oxide film recess process using the photoresist pattern  39  as an etch mask. This makes same or similar the protrusion and height of the field oxide film  360  in the high-voltage transistor HV that is protected by the photoresist pattern  39 . During the field oxide film recess process, the photoresist pattern  39  is stripped. 
   The field oxide film recess process may be performed by continuously performing the following steps. 
   A first step includes etching the exposed portion of the field oxide film  360  by a given thickness by using a buffered oxide etchant (BOE) solution in which NH 4 F and HF are mixed in an adequate ratio, for example, 9:1, 100:1 or 300:1. At this time, if an EFH of the field oxide film  360  in the high-voltage transistor region HV is 50 to 200 Å and an EFH of the field oxide film  360  in the cell region CELL or the low-voltage transistor region LV is 300 to 800 Å, a field oxide film etch target is 200 to 600 Å. In other words, the field oxide film etch target is decided by variation in the EFH between the field oxide film  360  having a low EFH and the field oxide film  360  having a high EFH. For this reason, as shown in  FIG. 10 , the EFH of the field oxide film  360  in the cell region CELL and the low-voltage transistor region LV becomes same or similar as that of the field oxide film  360  in the high-voltage transistor HV. 
   A second step includes stripping the photoresist pattern  39  used as the etch mask while stripping organic contaminant generated in the process using the BOE solution in the first step, by using a PIRANHA cleaning solution in which H 2 SO 4  and H 2 O 2  are mixed. At this time, a temperature of the PIRANHA cleaning solution is 80 to 130° C. 
   A third step includes maximizing stripping of particles and organic contaminant left after the PIRANHA cleaning process in the second step, by using standard cleaning-1 (SC-1) solution in which NH 4 OH, H 2 O 2  and H 2 O are mixed in an adequate ratio, for example, 1:1:5 or 0.2:1:10. At this time, a temperature of the SC-1 cleaning solution is 40 to 200° C. 
   Referring to  FIG. 11 , the surface of the remaining nitride film  34  is pre-treated using an oxide etch solution to which HF is added. The remaining nitride film  34  is completely stripped in a solution to which H 3 PO 4  is added. During the HF pre-treatment process and the nitride film strip process, the top of each of the field oxide films  360  formed in the regions CELL, HV and LV is lost at little. EFHs of the respective regions CELL, HV and LV are made to have the same lowered value. 
   By reference to  FIG. 12 , a second polysilicon layer  37  for a floating gate is formed on the entire structure including the field oxide films  360  and the first polysilicon layer  33 . Though not shown in the drawing, an etch process using a mask for a floating gate, a dielectric film formation process, a process of forming a conductive layer for a control gate, and an etch process using a mask for a control gate are performed to form gates in respective regions. 
   According to the present invention described above, EFH variation caused among a cell region, a high-voltage transistor region and a low-voltage transistor region due to a protrusion of a field oxide film of each of the regions is improved to facilitate setting of a subsequent gate etch target. Therefore, the present invention has effects that it can secure stability of a process and improve reliability of a device. 
   Although the foregoing description has been made with reference to the preferred embodiments, it is to be understood that changes and modifications of the present invention may be made by the ordinary skilled in the art without departing from the spirit and scope of the present invention and appended claims.