Abstract:
A method for forming a bit line of a semiconductor device wherein a first opening in an interlayer insulation film is formed in a P+ S/D (source/drain) region, a post etch treatment (PET) for stabilizing the resistance in the P+ S/D opening is performed, followed by the subsequent formation of a second opening in the N+ S/D region, such that any increase of the resistance of the N+ S/D opening by the PET is thereby prevented.

Description:
BACKGROUND 
   1. Technical Field 
   The present disclosure relates to a method for forming a bit line of a semiconductor device, and more particularly to a method for forming a bit line of a semiconductor device wherein a first opening in a P+ S/D region, and then forming a second opening in an N+ S/D region to prevent increase of the resistance in the N+ source/drain (S/D) region opening during a post etch treatment (PET) for stabilizing the resistance in a P+ S/D region opening. 
   2. Description of the Related Art 
   Recently, the size of a unit cell of a semiconductor memory device has decreased continuously due to high integration and large capacity of the semiconductor memory device. As the size of a memory cell of a dynamic random access memory (DRAM) which leads improvements in integration density is decreased, the vertical structure because extremely complex, which calls for a method for increasing an effective area of a capacitor. 
   A conventional semiconductor device employs a bit line consisting of polycide which is composed of doped polysilicon and tungsten silicide as a data input/output path to increase integration of the device and improve information processing capability. 
   However, the resistance of the conventional bit line structure in increased as the size of an opening in decreased. In order to solve the foregoing problem, a tungsten bit line having a low resistance value has been introduced. 
   However, even in the case of the tungsten bit line, the resistance of the opening for forming the bit line shows large changes during a subsequent thermal process. Specifically, since the resistance of a P+ S/D region remarkably increases, a subsequent PET is performed only for the P+ S/D region opening. 
   The PET is performed after simultaneously forming in the P+ S/D region and the N+ S/D region openings. Thus, while the resistance of the P+ S/D region opening is decreased, the resistance of the N+ S/D region opening is increased. 
   The resistance of the opening in the N+ S/D region is contrary to the resistance of the opening in the P+ S/D region. Generally, when the resistance of the P+ S/D region opening is reduced and stabilized, the resistance of the N+ S/D region opening is increased, and when the resistance of the N+ S/D region opening is reduced, the resistance of the P+ S/D region opening is increased. 
   Specifically, when the PET is performed to reduce the resistance of the P+ S/D region opening having an average resistance value of 1200Ω, the resistance of the P+ S/D region opening is stabilized at an average value of 900Ω. However, the resistance of the N+ S/D region opening having a average resistance value of 280 to 300Ω is increased to 450Ω(see FIGS.  1  and  2 ). 
   A conventional method for forming a bit line will be described with reference to  FIGS. 3   a  to  3   g.    
   Referring to  FIG. 3   a , a conductive layer for a word line (not shown) and a nitride film (not shown) are sequentially formed in a cell region of a semiconductor substrate  1 . 
   Thereafter, the conductive layer for the word line and the hard mask nitride film are pattered via a photolithography process using a mask for a gate electrode to form a word line pattern  4  including a conductive layer pattern  2  for the word line and a nitride film pattern  3  which is a hard mask layer. A spacer  5  is then formed at a sidewall of the word line pattern  4 . 
   A polysilicon layer (not shown) is formed on the entire surface of the resulting structure including the word line pattern  4  and the spacer  5  and then etched to form a plug  6 . 
   Next, P+ S/D region  7  and an N+ S/D region  8  are formed in a peripheral region of the resulting structure, and an interlayer insulating film  9  is then formed on the entire surface of the resulting structure using an oxide film. 
   Referring to  FIG. 3   b , openings  11   a ,  11   b  and  11   c  exposing the plug region  6 , the P+ S/D region  7  and the N+ S/D region  8 , respectively, are formed by etching the interlayer insulating film  9 . Thereafter, the opening  11   b  in the P+ S/D region  7  is subjected to a PET. 
   Referring to  FIG. 3   c , a photoresist layer  15  is formed on the entire surface of the resulting structure. 
   Referring to  FIG. 3   d , a portion of the photoresist layer  15  on the P+ S/D region  7  is removed to form an opening  11   b - 1  exposing the P+ S/D region  7 . 
   Opening  11   b - 1  in the P+ S/D region  7  is subject to a P+ ion implantation process  17  to stabilize the resistance of the opening  11   b - 1 . Thereafter, the photoresist layer  15  is stripped. 
   Referring to  FIG. 3   e , a rapid thermal annealing “RTA”process is performed to compensate for damaged oxide film in the P+ S/D region  7  due to the ion implantation process. 
   When subsequent processes are performed without the RTA process, the damaged portions are etched faster than the undamaged portions during the subsequent cleaning process, and thus a step difference  23  is generated between the P+ ion implant region and the non-implant region, resulting in a bridge  25  (see FIG.  4 ). 
   Referring to  FIG. 3   f , a barrier metal layer  19  is formed on the entire surface of the interlayer insulating film  9  including the opening using Ti/TiN. The Ti layer reacts with the Si substrate by an RTA process to form TiSi 2 , thereby stabilizing the resistance of the opening. 
   Thereafter, a tungsten layer  21  is formed on the entire surface of the resulting structure. 
   Referring to  FIG. 3   g , the tungsten layer  21  is etched to form a tungsten bit line  21   a.    
   In accordance with the conventional method for forming the bit line of the semiconductor device, since the openings are simultaneously formed in the P+ S/D region and the N+ S/D region, the resistance of the N+ S/D region opening is increased due to the subsequent PET. Moreover, the RTA process must be introduced in order to compensate for damages of the interlayer insulating film during the additional P+ ion implantation process. These complicate the manufacturing process and decrease the integration and operation speed of the device. 
   SUMMARY OF THE DISCLOSURE 
   It is an object of the present invention to provide a method for forming the bit line of the semiconductor device, and to form openings for the bit lines having stable resistances, by forming a first opening in a P+ S/D region, performing a PET, and forming a second opening in an N+ S/D region. 
   To achieve the above-described object of the invention, a method for forming bit line of semiconductor device, comprising the steps of:
         (a) forming a P+ S/D region and an N+ S/D region on a semiconductor substrate;   (b) forming a planarized interlayer insulating film on the entire surface of the resulting structure;   (c) etching the interlayer insulating film to form a first opening exposing the P+ S/D region;   (d) subjecting the entire surface of the resulting structure, including the first opening, to a P+ ion implantation process;   (e) etching the interlayer insulating film to form a second opening exposing the N+ S/D region;   (f) forming a barrier metal layer on the entire surface of the resulting structure including the first and second openings;   (g) forming a tungsten layer filling the first and second openings on the entire surface of the resulting structure; and   (h) selectively etching the tungsten layer and the barrier metal layer to form a bit line.       

   Preferably, the etching process in the step (c) uses CF 4 , CHF 3 , O 2 , Ar, CO, or mixed gas thereof, and further comprises over-etching the exposed P+ S/D region. 
   The P+ ion implantation process in the step (d) uses BF 2  gas with energy of 10 to 30 KeV, preferably 10 to 25 KeV and a dose of 1.0×10 −15  to 5.0×10 −15  atom/cm 2 , preferably 2.0×10 −15  to 4.0×10 −15  atom/cm 2 . 
   Preferably, the second opening is formed in the N+ S/D region in the same manner as the first opening in the P+ S/D region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a graph showing resistances of a P+ S/D region opening manufactured in accordance with the conventional art with and without a PET process. 
       FIG. 2  is a graph showing resistances of a N+ S/D region opening manufactured in accordance with the conventional art with and without a PET process. 
       FIGS. 3   a  through  3   g  are cross-sectional diagrams illustrating a conventional method for forming a bit line of a semiconductor device. 
       FIG. 4  shows a step difference generated due to an ion implantation process in accordance with the conventional method. 
       FIGS. 5   a  through  5   f  are cross-sectional diagrams illustrating a method for forming a bit line of a semiconductor device in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present disclosure will be described in detail with reference to the accompanying drawings. 
   Many changes and modifications to the embodiments described herein could be made. The scope of some changes is discussed above. The scope of others will become apparent from the appended claims. 
   A method for forming a bit line of a semiconductor device will be described. 
   Referring to  FIG. 5   a , a conductive layer for a word line (not shown) and a nitride film (not shown) are sequentially formed in a cell region of a semiconductor substrate  50 . 
   Thereafter, the conductive layer for the word line and the hard mask nitride film are patterned via a photolithography process using a mask for a gate electrode to form a word line pattern  54  including a conductive layer pattern  52  for the word line and a nitride film pattern  53  which is a hard mask layer. 
   Preferably, the conductive layer for the word line consists of doped silicon, polysilicon, W, WN, WSi X  or TiSi X . 
   A spacer  55  is formed at a sidewall of the word line pattern  54 . Thereafter, a polysilicon layer (not shown) is formed on the entire surface of the resulting structure including the word line pattern  54  and the spacer  55  and then etched to form a plug  56 . 
   Next, P+ S/D region  57  and an N+ S/D region  58  are formed in a peripheral region of the resulting structure, and an interlayer insulating film  59  is then formed on the entire surface of the resulting structure using an oxide film. 
   Referring to  FIG. 5   b , the interlayer insulating film  59  is etched to form a first opening  60  exposing the P+ S/D region  57 . Preferably, the etching process is performed using a gas selected from the group consisting of CF 4 , CHF 3 , O 2 , Ar, CO, and combinations thereof, and more preferably CF 4  gas. 
   The etching process for forming the first opening  60  is preferably an over-etching process so that the substrate is etched as well as the interlayer insulating film  59 . The thickness of the substrate etched in the etching process is 20 to 50%, preferably, 30 to 50% of the thickness of the interlayer insulating film  59 . 
   Thereafter, the first opening  60  is subjected to a PET using at least one gas selected from the group consisting of CF 4 , Ar and O 2  and the resulting structure then subjected to a P+ ion implantation process  61  to stabilize the first opening  60 . 
   The PET removes portions of the substrate damaged during the formation of the first opening. In the PET process, 20 to 150 Å, preferably 50 to 100 Å of semiconductor substrate in depth is etched. During the PET, the O 2  gas used in the PET oxidize the silicon substrate to form an oxide. The oxide is removed in a subsequent cleaning process before forming a barrier metal layer to improve bonding capability of the silicon substrate and the barrier metal layer, thereby lowering the resistance of the P+ S/D region opening. 
   Unlike the conventional arts, since a partial damage of the interlayer insulating film does not occur during the PET, a subsequence RTA process is not required. 
   The P+ ion implantation process  61  is performed using BF 2  gas with an energy of 10 to 30 KeV, preferably 10 to 25 KeV and a dose of 1.0×10 −15  to 5.0×10 −15  atom/cm 2 , preferably 2.0×10 −15  to 4.0×10 −15  atom/cm 2 . 
   Referring to  FIG. 5   c , the interlayer insulating film  59  is etched to form a second opening  63  exposing the N+ S/D region  58 . The etching process is performed under the same process conditions as the etching process for forming the first opening  60 . However it is preferable that the PET and the additional ion implantation process are not performed. 
   Referring to  FIG. 5   d , the interlayer insulating film  59  is etched to form a third opening  64  exposing the plug  56  in the cell region. The etching process is performed in the same manner as the etching processes for forming the first opening  60  and the second opening  63 . 
   Referring to  FIG. 5   e , a barrier metal layer  70  is formed on the entire surface of the resulting structure including the first opening  60 , the second opening  63  and the third opening  64  using Ti/TiN. The resulting structure is then subjected to an RTA process to form a TiSi 2  film (not shown) between a Ti layer and the substrate  50  thereby stabilizing the resistances of the openings. 
   Thereafter, a tungsten layer  73  filling the first to third openings  60 ,  63  and  64  is formed on the barrier metal layer  70 . 
   Referring to  FIG. 5   f , the tungsten layer  73  and the barrier metal layer  70  are etched to expose interlayer insulating film  59 , thereby forming a bit line  73   a.    
   As described above, in accordance with the present invention, only the first opening in the P+ S/D region is subjected to the PET, and the subsequent RTA process is not required. Moreover, the additional P+ ion implantation process is performed on the entire surface of the wafer, which prevents local step differences and bridging phenomenon. Accordingly, the openings in the P+ S/D region and the N+ S/D region have stable resistances.