Abstract:
A method for manufacturing a semiconductor device includes forming an oxide film uniformly in a trench in the device isolation by, for example, a radical oxidation process. The method also includes increasing the thickness of the oxide film positioned at recess sidewalls by forming a gate oxide film. Manufacturing the device according to this method will prevent junction leakage and maintain a gate oxidation intensity characteristic that will improve the refresh characteristic of the device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The priority benefit of Korean patent application No. 10-2008-0045812, filed on May 16, 2008, is hereby claimed and the disclosure thereof is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to a method for manufacturing a semiconductor device, and more specifically, to a method for manufacturing a semiconductor device that improves the refresh characteristic of a memory device. 
     A threshold voltage (Vt) is the most important parameter in the manufacture of a transistor. The threshold voltage depends on the thickness of the gate oxide film, the channel doping concentration, the oxide charge, and the gate material. The threshold voltage shows phenomena that are not identical with theoretical values as a high integration of the semiconductor device. 
     Due to the high integration of the semiconductor device, a device that can be operated at a lower operating voltage ranging from 1V to 2V with an improved speed is required. The threshold voltage also requires a lower voltage. However, it is difficult to control a device due to the short channel effect when the threshold voltage is decreased. The short channel effect causes a Drain Induced Built-in Leakage (DIBL) by hot carriers. Although various attempts have been made to reduce the short channel effect, a satisfactory solution has not been found to prevent increasing the short channel effect due to the high integration of a semiconductor device. 
     For example, the short channel effect has not been addressed by regulation of the doping concentration. The current solutions include a method for forming an ion-implant channel and a Super Steep Retrograde Channel (SSR) through vertically abrupt channel doping, a laterally abrupt channel doping method, and a method for forming a channel having a halo structure through a large angle tilt implant. 
       FIGS. 1   a  to  1   d  are plane views illustrating a conventional method for manufacturing a semiconductor device. Referring to  FIG. 1   a , a pad nitride film  110  is formed over the semiconductor substrate  100 . The pad nitride film  110  and the semiconductor substrate  100  are etched to form a trench  120 . 
     Referring to  FIGS. 1   b  and  1   c , an oxide film  130  is formed at sidewalls of the trench  120  by a dry oxidation process in the trench  120 . A nitride film  140  is formed over the resulting structure including the trench  120 . 
     Referring to  FIG. 1   d , an insulating film  150  is filled in the trench  120  to form a device isolation film that defines an active region. The active region is etched to form a recess, and a radical oxidation process is performed on the resulting structure including the recess to form a gate oxide film. 
     In the conventional method for manufacturing a semiconductor device, after forming a trench, a void  155  is generated in a subsequent process because the oxide film formed by the dry oxidation process is not deposited with a uniform thickness in the trench. After the active region is etched to form a recess, the junction leakage is generated at sidewalls of the recess because of the gate oxide film formed uniformly in the recess by the radical oxidation process. As a result, it is difficult to make the refresh time of a memory device to be less than 50 nm. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a method for manufacturing a semiconductor device. The method includes etching a semiconductor substrate to form a trench, and forming an oxide film uniformly over the etched substrate (including the trench). The method also includes forming a nitride film over the oxide film, and filling the trench with an insulating film and planarizing the insulating film and the nitride film to form a device isolating film that defines an active region. The method also includes etching the active region to form a recess, and forming a gate oxide film over the etched active region so that the thickness of the gate oxide film formed at sidewalls of the recess is greater than that of the gate oxide film formed over the semiconductor substrate. 
     The oxide film can be formed by a radical oxidation process. The radical oxidation process preferably is performed in a chamber including N 2  ranging from 10 slm to 20 slm, and O 2  ranging from 0.2 slm to 0.1 slm. The radical oxidation process preferably is performed with a ratio of H 2 :O 2 =2:1. Furthermore, the radical oxidation process preferably is performed with a chamber pressure ranging from 0.1 Torr to 1 Torr. The formed oxide film preferably has a thickness ranging from 60 Å to 120 Å. 
     The nitride film can include a liner nitride film. The formed nitride film preferably has a thickness ranging from 50 Å to 100 Å. 
     The insulating film is a film selected from the group consisting of a HDP film, a SOD/HDP film, and combinations thereof. 
     The gate oxide film can be formed by a dry oxidation process. The dry oxidation process preferably is performed in a chamber including N 2  ranging from 10 slm to 20 slm, and O 2  ranging from 0.2 slm to 0.1 slm. Preferably the thickness of the gate oxide film formed at sidewalls of the recess is greater than that of the gate oxide film formed over the semiconductor substrate. 
     Additional features of the disclosed invention may become apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
         FIGS. 1   a  to  1   d  are cross-sectional views illustrating a conventional method for manufacturing a semiconductor device. 
         FIG. 2  is a plane view illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention. 
         FIGS. 3   a  to  3   g  are cross-sectional views illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention. 
     
    
    
     While the disclosed method is susceptible of embodiments in various forms, there are illustrated in the drawings (and will hereafter be described) specific embodiments of the invention, with the understanding that the disclosure is intended to be illustrative, and is not intended to limit the invention to the specific embodiments described and illustrated herein. 
     DETAILED DESCRIPTION 
     The present invention will be described in detail with reference to drawing  FIGS. 2 and 3   a  to  3   g . In the drawings, the thickness of layers and regions is exaggerated for accuracy, and a layer can be directly formed over a different layer or a substrate or a third layer can be formed between the different layer and the substrate. The same reference numbers represent the same components. 
       FIG. 2  is a plane view illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention. Referring to  FIG. 2 , relative locations of an active region  260 , a device isolation film  270 , and a recess  280  are shown. 
       FIGS. 3   a  to  3   g  are cross-sectional views illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.  FIGS. 3   a  to  3   d  show cross-sectional views taken along A-A′ of  FIG. 2 , and  FIGS. 3   e  to  3   g  show cross-sectional views taken along B-B′ of  FIG. 2 . 
     Referring to  FIG. 3   a , a pad nitride film  210  is formed over a semiconductor substrate  200 . After a photoresist film is formed over the pad nitride film  210 , a photoresist pattern (not shown) is formed by an exposing and developing process using a trench mask. The pad nitride film  210  and the semiconductor substrate  200  are etched with the photoresist pattern as a mask to form a trench  220 . 
     Referring to  FIGS. 3   b  and  3   c , a radical oxidation process preferably is performed on the trench  220  to form an oxide film  230  at sidewalls of the trench  220 . 
     The radical oxidation process facilitates the formation of the uniform thickness of the oxide film  230  of the silicon pattern, and enables space security in a gap-fill process of an insulating film during a subsequent process. The radical oxidation process preferably is performed in a chamber including N 2  ranging from 10 slm to 20 slm, and O 2  ranging from 0.2 slm to 0.1 slm. The radical oxidation process preferably is performed with a ratio of H 2 :O 2 =2:1. The chamber pressure preferably ranges from 0.1 Torr to 1 Torr. 
     As shown, the formed oxide film  230  has the same (i.e., uniform) thickness throughout the trench  220 . The formed oxide film  230  preferably has a thickness ranging from 60 Å to 120 Å. 
     A nitride film  240  is formed over (the resulting structure including) the oxide film  230 . The formed nitride film  240  preferably has a thickness ranging from 50 Å to 100 Å. The nitride film  240  can include a liner nitride film. It is possible to secure a sufficient amount of space in the trench  220 , so that the thickness of the nitride film  240  can be deposited thickly. Because the nitride film  240  can prevent a plasma attack in the top edge of the active region (not shown) in the gap-fill process of the insulating film during the subsequent process. 
     Referring to  FIG. 3   d , an insulating film  250  is deposited over the resulting structure including the nitride film  240  and fills the trench  220 . The insulating film  250  preferably is a film selected from the group consisting of a HDP (High Density Plasma) film, a SOD (Spin On Dielectric)/HDP film, and combinations thereof. 
     Referring to  FIG. 3   e , after the insulating film  250  of  FIG. 3   d  is filled, the insulating film  250 , the nitride film  240 , and the pad nitride film  210  are planarized to expose the semiconductor substrate  200 , thereby forming the device isolation film  270  that defines the active region  260 . The active region  260  is etched to form a recess  280 . 
     Referring to  FIG. 3   f , a gate oxide film  290  is formed over the etched active region. The gate oxide film can be formed by a dry oxidation process. The dry oxidation process preferably is performed in a chamber including N 2  ranging from 10 slm to 20 slm, and O 2  ranging from 0.2 slm to 0.1 slm. The thickness of the gate oxide film  290  formed at sidewalls of the recess preferably is greater than that of the gate oxide film  290  formed over the semiconductor substrate  200 . 
       FIG. 3   g , which is an enlarged view of the region ‘A’ of  FIG. 3   f , illustrates the gate oxide film  290  formed by the dry oxidation process over the resulting structure, including the recess  280 . The sidewall thickness of the gate oxide film  290  formed over the recess  280  reduces a junction  300  leakage formed in the semiconductor substrate, thereby improving the refresh characteristic of a semiconductor device. 
     The above embodiments of the disclosure are illustrative and not limitative. Various alternatives and equivalents are possible. The invention is not limited by the type of deposition, etching polishing, and patterning steps described herein. Nor is the invention limited to any specific type of semiconductor device. For example, the disclosure may be implemented in a dynamic random access memory (DRAM) device or nonvolatile memory device. Other additions, subtractions, or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.