Patent Publication Number: US-2005130066-A1

Title: Method of forming single sided conductor and semiconductor device having the same

Description:
FIELD OF INVENTION  
      The present invention generally relates to a method of forming a semiconductor device, and more particularly, to a method of forming a single sided conductor of a semiconductor device by using a tilted mask layer.  
     BACKGROUND OF THE INVENTION  
      The formation of semiconductor devices often requires processing on one side of a trench. For example, this may involve an isolation structure of dielectric layer or a conductive structure on one side of the trench, whereas the other side of the trench remains unchanged. However, as the feature size of the semiconductor device shrinks, the use of lithography technique to define the single sided conductor becomes difficult to control, or even fails to comply with the need of practical applications. Therefore, to form a sub-lithographic single sided conductor without using any extra lithography process is an advance development.  
      A conventional single sided conductor is generally formed by filling the trench with a polysilicon layer. Then, a nitride liner and an amorphous silicon layer are deposited thereon. By controlling the angle of implantation, a portion of the amorphous silicon layer along one side of the trench remains unimplanted. Then, the implanted amorphous silicon layer is removed and the nitride liner thereunder is exposed. The exposed nitride liner and the polysilicon layer thereunder are etched by using the unimplanted amorphous silicon layer as a mask to expose one side of the trench for subsequent processes, and the other side of the trench is protected by the unimplanted amorphous silicon layer. However, the etching process is difficult to control so that the residual amorphous silicon layer and the nitride liner might be thinner or even eliminated making the polysilicon etched profile become poor and reducing the reliability of devices and the production yield.  
      Therefore, it is desire to provide a method of forming a single sided conductor with excellent profile and without using extra lithography processes.  
     SUMMARY OF THE INVENTION  
      One aspect of the present invention is to provide a method of forming a single sided conductor, which can be a sub-lithographic feature without implementing extra lithography processes.  
      Another aspect of the present invention is to provide a method of forming a single sided conductor by using a tilted mask layer as a mask to form a selective deposited oxide layer on the sidewall of an opening where the single sided conductor is to be formed.  
      A further aspect of the present invention is to provide a method of forming a semiconductor device having a single sided conductor, such as a trench capacitor, which controls the feature size of the single sided conductor by adjusting the tilt angle of the substrate and the thickness of a photoresist layer.  
      In one embodiment, a method of forming a single sided conductor includes providing a substrate having an opening. The opening exposes a sidewall and an opening base surface. A tilted mask layer is formed in the opening exposing the sidewall and a portion of the opening base surface. A dielectric layer, such as liquid phase deposition formed oxide layer, is formed on the exposed sidewall and the exposed opening base surface. To accomplish the formation of the single sided conductor, the tilted mask layer is then stripped, and a conductive layer is formed.  
      The step of forming the tilted mask layer includes coating a layer of photoresist over the substrate. It is noted that a portion of the photoresist is removed so that the opening is partially filled with the photoresist layer. Then, the substrate is tilted and the photoresist layer is reflowed to form the tilted mask layer in the opening. Furthermore, the substrate is preferably heated to a temperature about 100 to 150° C. for about 100 to 150 seconds during the reflow of the photoresist layer. After the photoresist layer is reflowed, the tilted mask layer is hardened by ultraviolet. Moreover, before the dielectric layer is formed, a native oxide is formed on the exposed sidewall and the exposed opening base surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
       FIGS. 1A  to  1 D illustrate cross-sectional views of forming a single sided conductor in a first embodiment of the present invention; and  
       FIGS. 2A  to  2 F illustrate cross-sectional views of forming a semiconductor device having a single sided conductor in a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention discloses a method of forming a single sided conductor, which utilizes a tilted mask layer and a selective deposition technique to form the single sided conductor having sub-lithographic feature size and excellent profile.  FIGS. 1 and 2  illustrate preferred embodiments of the present invention.  
      Referring to  FIGS. 1A  to  1 D, in one embodiment, the present invention provides a method of forming a single sided conductor. The method includes providing a substrate  100 , which has an opening  110 . The opening  110  exposes a sidewall  102  and an opening base surface  104 . As shown in  FIG. 1B , a tilted mask layer  120  is formed in the opening  110 . The tilted mask layer  120  exposes the sidewall  102  and a portion  104   a  of the opening base surface  104  and covers the other portion  104   b  of the opening base  104 . The step of forming the tilted mask layer  120  includes forming a layer of photoresist, which partially fills the opening  110 . Then, the substrate  100  is titled to reflow the photoresist layer, so that the tilted mask layer  120  is formed in the opening  110 , as shown in  FIG. 1B .  
      As shown in  FIG. 1C , a dielectric layer  130  is formed on the exposed sidewall  102  and the exposed portion  104   a  of the opening base surface  104 . The dielectric layer can be formed by selective deposition technique, such as liquid phase deposition, so that the dielectric layer  130  is selectively deposited on predetermined surfaces, such as the exposed sidewall  102  and the exposed portion  104   a  of the opening base surface  104 , and not deposited on the tilted mask layer  120 . Furthermore, by selecting appropriate surface material of the substrate  100 , the dielectric layer  130  is selectively not deposited on the surface of the substrate  100 . Then, the tilted mask layer  120  is stripped, and a conductive layer  140  is formed, as shown in  FIG. 1D . Therefore, the single sided conductor of sub-lithographic feature size is formed without implementing extra lithography processes.  
      The present invention can be applied to the manufacture of any semiconductor device in need of single sided conductor, for example, a single sided buried strap of a capacitor or a vertical transistor, but not limited thereto. Therefore, another embodiment is described hereinafter in detail.  
      Referring to  FIG. 2A , in a second embodiment, the present invention provides a method of forming a semiconductor device having a single sided conductor, such as a trench capacitor. The method includes providing a semiconductor substrate  200 , such as a silicon wafer or a silicon-containing substrate. The semiconductor substrate  200  has a pad dielectric layer  210  formed thereon, a storage node  220  formed therein, and an opening etched therein  230 . The opening  230  exposes a sidewall  202 , and a surface  204  of the storage node  220 . The pad dielectric layer  210  includes a pad oxide layer  212  and a pad nitride layer  214 , which can be formed by conventional deposition processes and act as a hard mask in subsequent processes. The storage node  220  of the capacitor is formed in the semiconductor substrate  200  by conventional processes, such as lithography, etch, deposition, oxidation, etc. The storage node  220  of the capacitor includes a capacitor dielectric layer  222 , such as oxide/nitride layer, a capacitor conductor  224 , such as a polysilicon layer, and other layers formed thereon, such as a conductive plug  226  and a collar dielectric layer  228 . The layers of storage node  220  of the capacitor can be formed by conventional lithography, etch, deposition, oxidation, etc, and not elaborated hereinafter.  
      A layer of photoresist  240  is coated on the pad dielectric layer  210 . A portion of the photoresist is removed, so that the opening  230  is partially filled with the photoresist layer  240 , as shown in  FIG. 2B . In other words, the photoresist layer is firstly coated over the semiconductor substrate  200  and fills the opening  230 . The, the photoresist on the pad dielectric layer  210  and a portion of the photoresist in the opening  230  are removed, so that the photoresist layer  240  having a predetermined thickness is remained in the opening  230 . Then, the semiconductor substrate  200  is tilted to reflow the photoresist layer  240  to form a tilted photoresist layer  242  in the opening  230 . The tilted photoresist layer  242  exposes the sidewall  202  and a portion  204   a  of the surface  204  of the storage node  220 , as shown in  FIG. 2C . It is noted that by adjusting the tilt angle of the semiconductor substrate  200  and the thickness of the photoresist layer  240  remained in the opening  230  can control the size (lateral width) of the single sided conductor. For example, if the thickness of the photoresist layer  240  is about half-filled the opening  230 , and the semiconductor substrate  200  is tilted about 45 degree, the photoresist is flowed toward the lower sidewall of the opening  230  and exposes about half the surface  204  of the storage node  220 . That is, the exposed portion  204   a  is about equal to the covered portion  204   b . Furthermore, if the tilted angle is less than 45 degree, the exposed portion  204   a  is smaller then the covered portion  204   b . It is noted that when adjusting the tilt angle of the semiconductor substrate and the thickness of the photoresist layer, the thickness of photoresist layer  240  is preferably a thickness that the photoresist can cover the predetermined surface or expose the predetermined surface in a predetermined tilt angle, but not reflow out of the opening  230 . Therefore, problems induced due to the overflow of photoresist can be eliminated.  
      Moreover, during the reflow of the photoresist layer  240 , the semiconductor substrate  200  is preferably heated to improve the reflow process of the photoresist layer  240 . The semiconductor substrate  200  is preferably heated to a temperature about 100 to 150° C. for about 100-150 seconds. It is noted that the temperature and the time of heating the semiconductor substrate  200  can vary with the selection of the photoresist so as to form the tilted photoresist layer in a predetermined profile. Moreover, after the photoresist layer  240  is reflowed, the photoresist is hardened by ultraviolet. In this step, the solvent in the photoresist is removed and the profile of the titled photoresist layer  242  is enhanced.  
      As shown in  FIG. 2D , a dielectric layer  250  is formed on the exposed sidewall  202  and the exposed surface  204   a . The dielectric layer  250  can be a selectively deposited dielectric layer, such as a liquid phase deposition formed oxide layer, which is selectively deposited on surfaces of a predetermined material. Therefore, the dielectric layer  250  can be controlled to deposit on the exposed sidewall  202  and the exposed surface  204   a  of the storage node  220 , but not on the tilted photoresist layer  242 . For example, before the dielectric layer  250  is formed, a native oxide layer  255  is grown on the exposed sidewall  202  and the exposed surface  204   a  of the storage node  220 . Therefore, the oxide layer formed by liquid phase deposition is selectively deposited on the native oxide layer  255 . It is noted that before the dielectric layer  250  is formed, an ozone ashing step is performed to remove the residual photoresist on the exposed sidewall  202  and the exposed surface  204   a  of the storage node  220 . A portion of the tilted photoresist layer  242  may be removed in the ozone ashing step to further adjust the profile of the exposed portion  204   a  of the storage node  220 .  
      Referring to  FIG. 2E , the tilted photoresist layer  242  is removed to expose the covered portion  204   b  of the storage node  220 . As shown in  FIG. 2F , a conductive layer  260 , such as a polysilicon layer, is formed over the semiconductor substrate  200  upon the exposed surface  204   b  of the storage node  220  and electrically coupled to the conductive plug  226 .  
      According to different design needs, by adjusting the tilt angle of the substrate and the thickness of the photoresist in the opening, the feature size of the single sided conductor of the present invention can be controlled. Moreover, the single sided conductor can be a sub-lithographic feature formed without implementing extra lithography processes.  
      Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from what is intended to be limited solely by the appended claims.