Abstract:
A method for fabrication a memory having a memory area and a peripheral area includes forming a first gate insulating layer with a first thickness over a substrate of a first region in the peripheral area and a second insulating layer with a second thickness over the substrate of the memory region. Thereafter, a buried diffusion region is formed in the substrate of the memory area. A charge trapping layer and a third insulating layer are formed over the substrate. A gate insulating layer is formed in the second region in the peripheral area, wherein the first thickness is greater than a second thickness after removing the charge trapping layer and third insulating layer on the first and second region in the peripheral area. A conductive layer is formed over the substrate of the memory area and the peripheral area substantially after the gate insulating layer is formed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a method for fabricating a semiconductor device. More particularly, the present invention relates to a method for fabricating a non-volatile memory. 
         [0003]    2. Description of Related Art 
         [0004]    As the integration of semiconductor devices increases, the line width of the semiconductor devices must be reduced. However, as the size of the semiconductor devices, for example, memory devices decreases, a variety of problems is resulted. For example, as the line width of a memory device is reduced, the channel length between the source and the drain is correspondingly reduced, leading to the short channel effect. 
         [0005]    Normally, a memory device integrates the memory cell arrays and the peripheral circuit elements onto a single chip. The peripheral circuit elements include high voltage (HV) devices and (LV) low voltage devices. The prior art method in forming such a memory device involves first forming a silicon oxide-silicon nitride-silicon oxide (ONO) composite structure in the memory array area, followed by forming the buried diffusion regions in the substrate adjacent to the ONO structure. The buried diffusion regions are formed by implanting dopants in the substrate. Thereafter, thermal oxidation processes are respectively performed to form the gate insulating layers for the high voltage devices and the low voltage devices in the peripheral circuit area. 
         [0006]    According to the prior art fabrication method, subsequent to the formation of the buried diffusion regions in the memory array area, the high thermal budget processes, such as the fabrication of the thick gate insulating layers for the high voltage devices, can seriously affect the size of the buried diffusion region. In essence, the high temperature budget leads to an expansion of the buried diffusion regions. Consequently, the short channel effect is intensified. Further, the device may become defective. 
         [0007]    To remedy the problems of diffusion junction expansion, a fabrication method has been proposed by forming thick gate insulating layer in the peripheral circuit area first, followed by the tunnel oxide in the memory array area and thin gate insulating layer in the peripheral circuit area. Buried diffusion regions are then formed by implanting ions in the substrate of the memory array area. Since the gate insulting layers are already formed in the peripheral circuit area, the size of the buried diffusion regions will not be affected by the high thermal budget processes. Thereafter, a silicon nitride layer and a top oxide layer are formed in both the memory array area and the peripheral circuit area to complete the formation of the ONO structure of the memory device. However, the silicon nitride layer and the top oxide layer in the peripheral circuit area have to be removed eventually. The removal of the silicon nitride layer and the top oxide layer, which requires the application of etchants such as hydrofluoric acid and hot phosphoric acid, may induce defects on the gate insulating layers underneath. Consequently, the quality of the devices in the peripheral circuit area is compromised. Defect control is especially critical for the LV gate insulating layer due to the delicate nature of the LV devices. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the foregoing, the present invention provides a method for fabricating a non-volatile memory in which the expansion of the buried diffusion region is prevented. Ultimately, the short channel effect is mitigated and the device characteristics of the memory can be better controlled. 
         [0009]    The present invention also provides a method for fabricating a non-volatile memory, wherein the gate insulating layer of the low voltage devices in the peripheral circuit area is obviated of defects. Consequently, the yield is improved. 
         [0010]    As embodied and broadly described herein, the present invention provides a method for fabricating a non-volatile memory that includes forming a first insulating layer with a first thickness over a first region of the peripheral area, forming a tunnel oxide over the memory area and thereafter a buried diffusion region in the memory area. A charge trapping medium including a charge storage layer is subsequently formed over the memory area. Ultimately, a second insulating layer with a second thickness is formed over a second region of the periphery circuit area, wherein the first thickness of the first insulating layer is greater than the second thickness of the second insulating layer. 
         [0011]    According to one aspect of the invention, the first insulating layer and the second insulating layer are gate dielectric films formed by methods including thermal oxidation, for example. 
         [0012]    According to one aspect of the invention, a conductive layer is formed over the substrate substantially after the fabrication of the second insulating over the second region. 
         [0013]    According to one aspect of the invention, the first region of the peripheral area is formed with high voltage devices, while the second region of the peripheral area is formed with low voltage devices. 
         [0014]    According to one aspect of the invention, the first insulating layer is also formed over the memory area, wherein the first insulating layer in the memory area is subsequently removed, and a tunnel oxide is formed on the memory area before the buried diffusion region in the memory area is formed. 
         [0015]    According to the one aspect of the invention, a first notch is formed in an isolation structure configured between the peripheral area and the memory area when the first insulating layer in the memory area is removed. 
         [0016]    According to other aspect of the invention, the first insulating layer is also formed over the second region of the peripheral area, wherein the first insulating layer in the second region is removed before the second insulating layer in the second region of the peripheral area is formed. 
         [0017]    According to the other aspect of the invention, a second notch is formed in an isolation structure configured between the first region and the second region when the first insulating layer in the peripheral area is removed. 
         [0018]    According to yet another aspect of the invention, the charge trapping medium in the peripheral area is subsequently removed and an undercut of the charge storage layer is formed at the periphery of the memory area. 
         [0019]    In the present invention, the fabrication of the gate dielectric film for the high voltage devices in the peripheral area is conducted before the formation of the buried diffusion region in the memory area. Therefore, the size of the buried diffusion region remains unaffected by the high thermal budget process used in forming the gate dielectric film of the transistors in the peripheral area. The expansion of the buried diffusion region is thus confined. Accordingly, the short channel effect can be minimized. Further, the device characteristics of the memory can be better controlled. 
         [0020]    Moreover, the fabrication of the gate dielectric film for the low voltage devices is conducted after the formation of the charge trapping medium in the memory area and the formation of the gate dielectric film for the high voltage devices. Further, a conductive layer is formed substantially after the fabrication of the gate dielectric film for the low voltage devices, covering the gate dielectric film of the low voltage devices. Accordingly, the gate dielectric film of the low voltage devices remains uncompromised by other processes of the fabrication of the memory device. Ultimately, the quality and the property of the low voltage devices can be better maintained. 
         [0021]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0023]      FIGS. 1A-1H  are cross-sectional views showing selected process steps for fabricating a non-volatile memory comprising a memory array area and a corresponding peripheral circuit area in accordance to one embodiment of the invention. 
           [0024]      FIG. 2  is a flow chart of steps in exemplary processes that may be used in the fabrication of a non-volatile memory comprising a memory array area and a peripheral circuit array according to one embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]      FIGS. 1A-1H  are cross-sectional views showing selected process steps for fabricating a memory comprising a memory array area and a corresponding peripheral circuit area in accordance to one embodiment of the invention. 
         [0026]    Referring to  FIG. 1A , a semiconductor substrate  100  is provided. The semiconductor substrate  100  may be a silicon wafer, for example. The surface of the semiconductor substrate  100  is divided into at least a memory array area  100   a  and a peripheral circuit area  100   b  formed adjacent to each other. The peripheral circuit area  100   b  may further divide into at least a high voltage device region (HV)  102   a  and a low voltage device region (LV)  102   b . Additionally, wells  103   a ,  103   b  may also form in the memory array area  100   a  and the high voltage device region (HV)  102   a , respectively. The wells  103   a ,  103   b  are formed by implanting dopants in the substrate  100 , for example. The wells may be either an N-well for forming PMOS devices or P-well for forming NMOS devices. Further, isolation structures  104 , for example, shallow trench isolation structures, are used to separate the different regions and areas. 
         [0027]    Referring to  FIG. 1B , an insulating film  106  is to be provided on substrate  100  of the memory array area  100   a  and the peripheral circuit area  100   b , including both the high voltage device region  102   a  and the low voltage device region  102   b . The insulating film  106  is, for example, an oxide layer of about 150 to 170 angstroms thick. The insulating film  106  is formed by performing a thermal process in an oxidation furnace, for example. In one embodiment, the insulating film  106  serves as a gate dielectric film of the high voltage devices. In one embodiment, a cleaning process may perform to clean the substrate  100  before the formation of the insulating film  106 . 
         [0028]    Referring to  FIG. 1C , a patterned mask (not shown) is formed, wherein this patterned mask conceals the peripheral circuit area  100   b  while exposes the memory array area  100   a . Using the patterned mask as an etching mask, the insulating film  106  in the memory array area  100   a  is removed. The insulating film  106  is removed by wet etching, for example, with etchant including but not limited to BOE (buffer oxide etchant) or hydrofluoric acid. The removal of the insulating film  106  may result with a notch  107  being formed at the top edge surface of the isolation structure  104  configured between the memory array area  100   a  and the peripheral circuit area  100   b . Thereafter, an insulating film  108  is formed on the substrate  100  in the memory array area  100   a . The insulating film  108 , which serves as a tunneling oxide layer, is formed with silicon oxide to a thickness of about 50 to 100 angstroms, for example. One method of forming the insulating film  108  is by thermal oxidation, for example. In one embodiment, a cleaning process, for example, a pre-gate oxide cleaning process may perform to clean the substrate  100  before the formation of the insulating film  108 . In one embodiment, the insulating film  106  in the high voltage device regions  102   a  and  102   b  may concurrently provide with an additional thickness. 
         [0029]    Continuing to  FIG. 1D , a patterned mask layer  110  is formed over the substrate  100  of the memory array area  100   a  and the peripheral circuit area  10   b . The patterned mask layer  110  conceals the peripheral circuit area  100   b  and a portion of the memory array area  100   a . An ion implantation process is then performed to implant dopants into the substrate  100  exposed by the patterned mask layer  110  to form a plurality of buried bit lines  112  in the substrate  100 . 
         [0030]    Now referring to  FIG. 1E , the patterned mask layer  110  is removed. Then, a dielectric layer  114  and an insulating film  116  are sequentially formed over the insulating film  108  in the memory array area  100   a  and the insulating film  106  in the peripheral circuit area  100   b . A material that constitutes the dielectric layer  114  includes nitrides, but also may include, for example but not limited to oxynitride, hafnium oxide, aluminate oxide, the silicate thereof or other materials having similar properties. The insulating film  116  is, for example, an oxide layer. The dielectric layer  114  and the insulating film  116  are respectively formed by deposition methods including but not limited to, for example, low pressure chemical vapor deposition (LPCVD) at a temperature ranging from 600 to 800 degrees Celsius. In one embodiment, the dielectric layer  114  and the insulating film  116  serve as a charge storage layer and an inter-gate dielectric film, respectively, and the insulating film  108 , the dielectric layer  114  and the insulating film  116  together form a charge-trapping medium of the memory. 
         [0031]    Continuing to  FIG. 1F , a patterned mask layer  118  is formed concealing the memory array area  100   a  while exposing the peripheral circuit area  100   b . Using the patterned mask layer  118  as an etching mask, the insulating film  116  in peripheral circuit area  100   b  is removed via wet etching, for example. The etchant used in the process includes but not limited to diluted hydrofluoric acid or BOE (buffer oxide etchant). 
         [0032]    Subsequent to the removal of the insulating film  116  in the peripheral circuit area  100   b , the patterned mask layer  118  is removed as shown in  FIG. 1G . Thereafter, the dielectric layer  114  in the peripheral circuit area  100   b  is selectively removed. The removal of the dielectric layer  114  is accomplished by wet etching using hot phosphoric acid as an etchant, for example. Since the dielectric layer  114  in the memory array area is protected by the insulating film  116 , the dielectric layer  114  in the memory array area is prevented from being etched. Further, an undercut  117  of the dielectric layer  114  is formed under the insulating film  116  at the boundary between the memory array area  100   a  and the peripheral circuit area  100   b.    
         [0033]    Referring to  FIG. 1H , dopants are implanted in the substrate  100  of the low voltage device region  102   b  in the peripheral circuit area  100   b  to form a well  103   c  in the low voltage device region  102   b . Thereafter, the insulating film  106  in low voltage device region  102   b  is removed, followed by forming an insulating film  120  on the substrate  100  in the low voltage device region  102   b . The insulating film  106  is removed by wet etching methods, for example. The removal of the insulating film  106  in the low voltage device region  102   b  results with a notch  121  formed at the top edge surface of the isolation structure  104  configured between the low voltage device region  102   b  and the high voltage device region  102   a . The insulating film  120  is an oxide layer formed to a thickness of about 15 to 75 angstroms, for example. The insulating film  120  is formed by growing in an oxidation furnace, for example. In one embodiment, the insulating film  120  serves as a gate dielectric film of the low voltage devices. In one embodiment, the insulating film  106  in the high voltage device region  102   a  may concurrently provide with an additional thickness. Subsequent to the formation of the insulating film  120 , a polysilicon layer  122  is formed over both the memory array area  100   a  and the peripheral circuit area  100   b.    
         [0034]    It should be understood that the fabrication of the non-volatile memory is subsequently completed by resorting to known techniques and thus will not be reiterated herein. 
         [0035]      FIG. 2  is a flow chart of steps in exemplary processes that may be used in the fabrication of a memory comprising a memory array area and a peripheral circuit area according to one embodiment of the present invention. The same reference numbers are used in the flow chart shown in  FIG. 2  and in the description illustrated by  FIGS. 1A-1H  to refer to the same or like parts. 
         [0036]    First considering step S 200  in  FIG. 2 , isolation structures  104  are formed to divide a semiconductor substrate  100  into a memory array area  100   a  and a peripheral circuit area  100   b  and to further divide the peripheral circuit area  100   b  into at least a high voltage (HV) device region  102   a  and a low voltage (LV) device region  102   b . Wells  103   b ,  103   a  are respectively formed in the high voltage device region  102   a  and the memory array area  100   a  in step S 202 . Thereafter, an insulating film  106  is formed in the high voltage device region (HV)  102   a  in step S 204 . In step S 206 , an insulating layer  108  is formed in the memory array area  100   a . Then, a plurality of buried bit lines  112  is formed in the substrate  100  in the memory array area  100   a  in step S 208 . After this, a dielectric layer  114  and an insulating film  116  are sequentially formed in the memory array area  100   a  in step S 210 . Continuing to step S 212 , well  103   c  is formed in the low voltage device region  102   b . Then, an insulating film  120  is formed on the substrate  100  in the low voltage device region  102   b  in step  214 . Ultimately, a polysilicon layer  122  is formed over the memory array area  100   a  and the peripheral circuit area  100   b  in step S 216 . 
         [0037]    In the invention, the buried diffusion region is formed subsequent to the formation of the gate dielectric film of the high voltage device devices. The buried diffusion region is thus prevented from being affected by the thermal process used in forming the gate insulating film. In other words, the buried diffusion region is prevented from unwanted expansion due to the high thermal budget process used in forming the thick gate dielectric film of the high voltage devices in the peripheral circuit area. Accordingly, even the size of the memory cell is further reduced, the short channel effect is minimized. Moreover, the device characteristics of the memory can be better controlled. 
         [0038]    Additionally, in the present invention, the gate dielectric film of the low voltage devices is formed after the formations of the charge trapping medium and the buried drain lines in the memory array area. Accordingly, the quality of gate dielectric film of the low voltage devices is not compromised by being exposed to the other process steps in the fabrication of the memory. For example, the gate dielectric film of the low voltage devices can be prevented from being damaged by etchants used in the removal of dielectric layer  114  and the insulating film  116  in the periphery circuit area. As a result, defects are less likely to be induced to the gate dielectric film of the low voltage devices. Moreover, since the well region in the low voltage device region is formed after the charge trapping medium and the buried drain lines in the memory array area are completed, the well junction in the low voltage device region is well defined. In essence, the quality and the property of the low voltage devices can be better preserved. 
         [0039]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.