Abstract:
A method for forming a contact of a semiconductor deices is disclosed. More specifically, in the method for forming a contact of a semiconductor device, an interlayer dielectric (hereinafter, referred to as “ILD”) layer is polished using a CMP slurry having high selectivity to an oxide film in a STI (shallow trench isolation) etching process for forming a line-type storage node contact (hereinafter, referred to as “SNC”, and an ILD layer having a predetermined thickness is re-formed on the semiconductor substrate to secure a sufficient etching margin to a subsequent etching process, thereby preventing loss of a hard mask nitride film of a bit line and reducing fail of a self-aligned contact (hereinafter, referred to as “SAC”) between a storage node and a bit line.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a method for forming a contact of a semiconductor device, and more specifically, to a method for forming a contact of a semiconductor device wherein an interlayer dielectric (hereinafter, referred to as “ILD”) layer is polished using a CMP slurry having high selectivity to an oxide film in a STI (shallow trench isolation) etching process for forming a line-type storage node contact (hereinafter, referred to as “SNC”), and an ILD layer having a predetermined thickness is re-formed on the semiconductor substrate to secure a sufficient etching margin to a subsequent etching process, thereby preventing loss of a hard mask nitride film of a bit line and reducing fail of a self-aligned contact (hereinafter, referred to as “SAC”) between a storage node and a bit line. 
   2. Description of the Prior Art 
   The development of fine pattern formation technology has affected the high-integration trend of semiconductor devices, and the size of a unit cell of a semiconductor memory device has been reduced depending on high integration and high capacity of the semiconductor memory device. 
   Specifically, in case of DRAM (Dynamic Random Access Memory) which leads increase of integration degree, vertical structures become complicated according to reduction of memory cell size. As a result, in order to increase the effective area of a capacitor, a capacitor is formed after a bit line is formed. 
   In addition, when the bit line is formed, it is important to form a SNC for performing an electrical operation between a transistor and a capacitor. 
   The SNC is formed by a line-type SAC process. In the line-type SAC process, a bit line is patterned, an ILD layer is formed, and the semiconductor substrate is etched except a part of the ILD layer for separating a contact. 
   Here, in order to secure an etching margin for the etching process, the ILD layer having a predetermined thickness is required to remain on the bit line in the planarization process on the ILD layer. However, as a semiconductor device becomes microscopic, it is difficult to perform the planarization process on the ILD layer on the bit line, thereby forming the ILD layer of non-uniformity. 
     FIGS. 1   a  to  1   f  are diagrams illustrating a conventional method for forming a contact of a semiconductor device. 
   Referring to  FIG. 1   a , a first ILD layer  5  using an oxide film is formed on a semiconductor substrate  1  having a cell transistor (not shown) and a lower poly silicone plug  3 . 
   As shown in  FIG. 1   b , the stacked structure of a barrier layer (not shown) material for bit line, a conductive layer (not shown) for bit line and a hard mask nitride film (not shown) are formed on the first ILD layer  5  of  FIG. 1   a , and a selectively etching is performed the stacked structure to form a bit line  13  comprising a barrier layer pattern  7  for bit line, a conductive layer pattern  9  for bit line and a hard mask nitride film pattern  11 . 
   As shown in  FIG. 1   c , a bit line spacer  15  is formed at a sidewall of the bit line  13  of  FIG. 1   b.    
   As shown in  FIG. 1   d , a second ILD layer  17  is formed on the semiconductor substrate including bit line of  FIG. 1   c.    
   The second ILD layer  17  of  FIG. 1   d  is polished using common CMP slurry. Due to the polishing process, an ILD layer having a predetermined thickness remains on the bit line  13 . 
   The common CMP slurry including a colloidal or fumed SiO 2  abrasive and additive such as KOH/NH 4 OH has a pH ranging from 10 to 11 and a polishing selectivity in the range of 1:4 for a nitride film: oxide film. 
   As shown in  FIG. 1   f , a SNC etching process is performed on the planarized second ILD layer  17  shown in  FIG. 1   e  until the poly silicone plug  3  is exposed to form a SNC opening  19 . 
   When the ILD layer is polished using the common slurry so that the ILD layer having a predetermined thickness remains on the bit line, the ILD layer on the bit line has large difference in the thickness, thereby causing loss of the hard mask nitride film in a subsequent SNC etching process. 
   For example, if an etching process is performed after an etching target for forming a SNC opening is determined in the ILD layer having a thick thickness formed on the bit line, the hard mask nitride  11  is severely lost on the ILD layer  17  having a thick thickness as shown in  FIG. 2   a . In this way, since the thickness of the hard mask nitride film becomes thinner by a SNC CMP process, errors of the bit line and SAC are generated in the etching process for forming a subsequent SN. 
   On the other hand, if an etching process is performed after an etching target is determined in the ILD layer having a thin thickness formed on the bit line, the upper portion of the ILD layer  17  having a thin thickness is not polished but remains on the bit line as shown in  FIG. 2   b . As a result, the SNC is not open. 
   This shortcoming generates between area difference of the SNC region and size difference of the bottom region in the etching process. As a result, it is difficult to embody uniform device characteristics on the whole surface of the wafer. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a method for forming a contact of a semiconductor device wherein an ILD layer is polished using a CMP slurry having more excellent polishing selectivity to an oxide film than to an nitride film and an ILD layer having a predetermined thickness is re-deposited to form the uniform ILD layer on a bit line, thereby securing a sufficient etching margin for performing a subsequent SNC etching process. 
   In an embodiment, a method for forming a contact of a semiconductor device comprises the steps of: 
   forming a first interlayer dielectric (ILD) layer on a semiconductor substrate having a cell transistor and a lower poly silicon plug thereon; 
   forming a first interlayer dielectric (ILD) layer on a semiconductor substrate having a cell transistor and a lower poly silicon plug thereon forming a stacked structure of a barrier layer, a conductive layer and a hard mask nitride film on the first ILD layer, and selectively etching the stacked structure to form a bit line forming a second ILD layer on the semiconductor substrate including the bit line polishing the second ILD layer using CMP slurry having high selectivity for oxide film to expose the hard mask nitride film forming a third ILD layer on the exposed hard mask nitride film and the second ILD layer, and performing an etching process to form an opening for storage node contact exposing the lower poly silicon plug. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a  to  1   f  are diagrams illustrating a conventional method for forming a contact of a semiconductor device. 
     FIGS  2   a  and  2   b  are cross-sectional diagrams illustrating a thickness of an ILD layer on a bit line formed according to the conventional method. 
       FIGS. 3   a  to  3   g  are diagrams illustrating a method for forming a contact of a semiconductor device according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be described in detail with reference to the accompanying drawings. 
   Referring to  FIG. 3   a , a first ILD layer  25  is formed by depositing an oxide film on the semiconductor substrate  21  having a cell transistor (not shown) and a lower poly silicone plug  23 . 
   As shown in  FIG. 3   b , the stacked structure of a barrier layer (not shown) material for bit line, a conductive layer (not shown) for bit line and a hard mask nitride film (not shown) are formed on the first ILD layer  25  of  FIG. 3   a , and a selective etching is subjected to the stacked structure to form a bit line  33  comprising a barrier layer pattern  27  for bit line, a conductive layer pattern  29  for bit line and a hard mask nitride film pattern  31 . 
   The barrier layer material for bit line is formed of Ti/TiN, and the conductive layer is tungsten. 
   As shown in  FIG. 3   c , a bit line spacer  35  is formed at a sidewall of the bit line  33  of  FIG. 3   b.    
   As shown in  FIG. 3   d , a second ILD layer  37  is formed on the entire surface of the semiconductor substrate of  FIG. 3   c . Here, the first ILD layer and the second ILD layer are preferably formed using a common oxide film. 
   As shown in  FIG. 3   e , a CMP process is performed on the second ILD layer  37  of  FIG. 3   d  by using CMP slurry having selectivity for oxide film to expose the hard mask nitride film  31  as a polishing barrier film. 
   The CMP slurry for oxide film, CMP slurry composition for a STI process, comprises Ceria (CeO2) as an abrasive and has a pH ranging from 4 to 10, preferably from 6 to 8. Distilled water or ultra-pure water is used as the solvent. 
   Also, the CMP slurry for oxide film comprises an organic polymer as an additive which is preferably polyacrylic acid salt. 
   An amount of the abrasive ranges from 0.5 to 10 wt %, preferably from 1 to 5 wt %, and an amount of the additive ranges from 0.5 to 10 wt %, preferably from 1 to 5 wt %. 
   The selectivity ratio of slurry composition for a nitride film to oxide film according to an embodiment of the present invention ranges 1:10˜200, preferably 1:30˜200. 
   The second ILD layer  37  is polished using CMP slurry for oxide film having high selectivity ratio of 1:10˜200 for a nitride film to oxide film to exposed the hard mask nitride film. 
   As shown in  FIG. 3   f , a third ILD layer  38  is uniformly formed on the exposed hard mask nitride film and the second ILD layer  37  planarized by the polishing process of  FIG. 3   e.    
   Here, the third ILD layer  38  is formed from a source selected from a group consisting of HDP PSG (high density plasma phosphosilicate glass), BPSG (borophosphosilicate glass), PSG (phosphosilicate glass), HDP USG (high density plasma undoped silicate glass), FSG (fluorosilicate glass), PE-SiH 4  (plasma enhanced-silane), LP-TEOS (low pressure-tetraethoxysilicate glass) and PE-TEOS (plasma enhanced-tetraethoxysilicate glass). 
   Here, a thickness of the third ILD layer ranges from 500 to 5000 Å, preferably from 500 to 2000 Å. 
   The SNC etching process is subjected to the third ILD layer  38  and the second ILD layer  37  of  FIG. 3   f  to form an opening  39  for the SNC exposing the lower poly silicone plug shown in  FIG. 3   g.    
   In this way, after the hard mask nitride film is exposed and the third ILD layer having a predetermined thickness is re-deposited on the hard mask nitride film, the ILD layer having the uniform thickness can be formed on the bit line. As a result, loss of the hard mask nitride film can be prevented in a subsequent etching process for forming the SNC, thereby securing a sufficient etching margin and reducing errors between the SN and the bit line. 
   As discussed earlier, in an embodiment of the present invention, after a second ILD layer is polished, a third ILD layer having a predetermined thickness is formed on the second ILD layer. As a result, loss of a bit line hard mask nitride film can be prevented in a subsequent etching process for forming a SNC, thereby a sufficient etching margin and reducing errors between the SN and the bit line. Accordingly, stable devices can be manufactured.