Patent Publication Number: US-2006003541-A1

Title: Method for forming device isolation film of semiconductor device

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to a method for forming device isolation film of semiconductor device and more specifically, to a method for forming device isolation film of semiconductor device wherein a relatively thin sidewall oxide film is formed in a cell region and a relatively thick sidewall oxide film is formed in a peripheral circuit region, to improve retention time characteristics, prevent a hot carrier phenomenon, and reduce stand-by current.  
      2. Description of the Background Art  
       FIGS. 1   a  through id are cross-sectional views illustrating a method for forming a device isolation film according to a prior art.  
      Referring to  FIG. 1   a , a pad oxide layer  20  and a pad nitride layer  30  are sequentially formed on a semiconductor substrate  10  having a cell region and a peripheral circuit region. A predetermined region of the pad nitride layer  30 , the pad oxide layer  20 , and the semiconductor substrate  10  is etched to form a trench  50  using the hard mask layer pattern  40  as an etching mask.  
      Referring to  FIG. 1   b , a sidewall oxide film  60  is formed on a surface of the trench  50 . Generally, the sidewall oxide films  60  in the cell region and a peripheral circuit region have the same thickness.  
      Referring to  FIG. 1   c , a liner nitride film  70  and a liner oxide film  80  are sequentially formed on the entire surface of the semiconductor substrate  10  including the trench  50  and the pad nitride layer  30 .  
      Referring to  FIG. 1   d , a HDP oxide film  90  is deposited to fill up the trench  50 . Next, the HDP oxide film  90  is subjected to a CMP process until the pad nitride layer  30  is exposed. The pad nitride layer  30  is then removed to form a device isolation film of semiconductor device.  
      In accordance with the conventional method for forming device isolation film of semiconductor device, the sidewall oxide film  60  is formed to prevent generation of crystalline defects on the interface of the semiconductor substrate and HDP oxide film. The volume of the HDP oxide film expands in a subsequent heat treatment which imposes compression stress on the semiconductor substrate, thereby generating a junction leakage current. As a result, data retention time characteristics of a semiconductor device are degraded.  
      Moreover, a liner nitride film formed between the HDP oxide film and the semiconductor substrate reduces the compression stress applied to the semiconductor substrate and improves the data retention time characteristics. However, the liner nitride film generates hot electron traps between the sidewall oxide film and the liner nitride film to cause channel shortening and hot carrier phenomena. As a result, the leakage current characteristic of a PMOS transistor is deteriorated and the stand-by current is increased. Heating phenomenon by the leakage current degrades operation speed in a semiconductor device.  
      In order to solve above problems, a gate tab is formed at the channel edges where negative charges are trapped so as to prevent hot carrier effect at the channel edge. However, the gate tap decreases the channel width.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the present invention to provide a method for forming device isolation of semiconductor device wherein a relatively thin sidewall oxide film is formed in a cell region and a relatively thick sidewall oxide film is formed in a peripheral circuit region to reduce the stress applied to a semiconductor substrate by a liner nitride film and improve the retention time characteristic a semiconductor device.  
      Moreover, it is another object of the present invention to prevent accumulation of negative charges at the junction area of the liner nitride film and the sidewall oxide film by forming a thick sidewall oxide film in the peripheral circuit region so as to prevent hot carrier effect generated by a channel-shortening phenomenon in a pMOS transistor and reduce the stand-by current. Accordingly, the characteristic of the semiconductor device is improved.  
      In order to achieve above-described object, there is provided a method for forming device isolation film of semiconductor device, the method comprising the steps of:  
      (a) sequentially forming a pad oxide layer and a pad nitride layer on a semiconductor substrate having a cell region and a peripheral circuit region;  
      (b) etching a predetermined region of the pad nitride layer, the pad oxide layer, and the semiconductor substrate to form a trench;  
      (c) forming an sidewall oxide film on a surface of the trench;  
      (d) etching a predetermined thickness of the sidewall oxide film in the cell region;  
      (e) forming a liner nitride film and a liner oxide film on the semiconductor substrate including the trench and the pad nitride layer;  
      (f) depositing a HDP oxide film to fill up the trench;  
      (g) performing a planarization process to expose the pad nitride layer; and  
      (h) removing the pad nitride layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1   a  through  1   d  are cross-sectional views illustrating a conventional method for forming a device isolation film of semiconductor device.  
       FIGS. 2   a  through  2   f  are cross-sectional views illustrating a method for forming device isolation film of semiconductor device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EXAMPLARY EMBODIMENTS  
      A method for forming a device isolation film of a semiconductor device in accordance with an embodiment of the present invention will now be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
      Referring to  FIG. 2   a , a pad oxide layer  110  and a pad nitride layer  120  are sequentially formed on a semiconductor substrate  100  having a cell region and a peripheral circuit region respectively denoted as ‘A’ and ‘B’. Next, a predetermined region of the pad nitride layer  120 , the pad oxide layer  110 , and the substrate  100  are etched using a photoresist pattern  130  as an etching mask to form a trench  135 . The photoresist pattern  130  is then removed.  
      Referring to  FIG. 2   b , a sidewall oxide film  140  is formed on a surface of the trench  135 . Preferably, the sidewall oxide film  140  has a thickness ranging from 200 Å to 300 Å.  
      Referring to  FIG. 2   c , a photoresist film pattern  150  exposing the cell region A is formed. A predetermined thickness of the sidewall oxide film  140  in the cell region A is then etched using the photoresist film pattern  150  as an etching mask. Preferably, the predetermined thickness of the sidewall oxide film  140  etched ranges from 100 Å to 150 Å. The etching process may comprise a wet etching process.  
      In one embodiment of the present invention, a combination of a dry etching process and a wet etching process is performed to etch the sidewall oxide film  140  in the cell region. Preferably, a thickness of the sidewall oxide film etched by the dry and wet etching processes ranges from 50 Å to 80 Å respectively.  
      Referring to  2   e , a liner nitride film  170  and a liner oxide film  180  are sequentially formed on the entire surface of the semiconductor substrate including the trench  135  and the pad nitride layer  120 . Preferably, the liner nitride film  170  and the liner oxide film  180  are formed at a temperature of 800° C. to 900° C.  
      Referring to  2   f , a HDP oxide film  190  is deposited to fill up the trench  135 , and then planarized to expose the pad nitride layer  120 . The pad nitride layer  120  is then removed to form the device isolation film  190 .  
      As described above, in accordance with the method for forming device isolation film of semiconductor device of the present invention the sidewall oxide film in the cell region is etched so that the thickness of the sidewall oxide film is less than that of the sidewall oxide film in peripheral circuit region to reduce the stress imposed to the semiconductor substrate  100  by the liner nitride film formed in a subsequent process and improve the retention time characteristic of the semiconductor device.  
      Also, the method prevents negative charges at a junction area of the liner nitride film and the sidewall oxide film by forming a thick sidewall oxide film in the peripheral circuit region so as to prevent hot carrier effect generated by a channel-shortening phenomenon in a pMOS transistor and reduce the stand-by current. Accordingly, the characteristics of the device are improved.  
      As the present invention may be embodied in several forms without departing from the spirit or scope thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description. Rather the present invention should be construed broadly as defined in the appended claims. All changes and modifications that fall within the metes and bounds of the claims, or equivalences of such metes and bounds are intended to be embraced by the appended claims.