Abstract:
Provided is a method of manufacturing a semiconductor device including at least two processes. Under an atmosphere comprising hydrogen and oxygen, a sacrificial oxide film is formed on a silicon substrate that is provided with at least one nitride region. Then, the sacrificial oxide film and the nitride region are removed from the silicon substrate.

Description:
This Application claims priority from Japanese Application 2006-319728, filed on Nov. 26, 2006. 
   BACKGROUND OF THE INVENTION 
   This invention relates to a method of manufacturing a semiconductor device and, in particular, to a manufacturing method for removing undesirable nitride regions by using a sacrificial oxide film. 
   In the process of manufacturing semiconductor devices such as field effect transistors, the semiconductor devices on the silicon substrate are separated by a field insulation film or an STI (Shallow Trench Isolation) film. 
   JP H8-107205A discloses a technique for separating the semiconductor devices by the field oxide film which is formed by the LOCOS (Local Oxidation of Silicon) method. During the field oxidation process, nitride films are also oxidized so that nitrogen atoms (N) or ammonium molecules (NH 3 ) are produced and form undesirable nitride regions in field regions. In order to remove the undesirable nitride regions, a sacrificial oxide film is formed on the silicon substrate. Then, the silicon substrate is soaked in hydrofluoric acid solution to remove the undesirable nitride regions together with the sacrificial oxide film. 
   According to JP H8-107205A, the sacrificial oxide film is formed in the atmosphere comprising ozone. However, the temperature of the atmosphere should be lower than 800° C. because the thermal-resistivity of ozone is very poor. 
   The semiconductor devices on the silicon substrate may be separated by the STI film. During the process of forming the STI film, undesirable nitride regions are also formed on the silicon substrate. One of existing methods of removing the nitride region is to carry out dry or wet oxidation. However, during the dry or wet oxidation, the oxidizing species may diffuse into the STI film and oxidize the silicon substrate in the trench. 
   The oxidized STI film will expand and apply stresses to the device formation regions adjacent thereto. Such stresses may result in the increase of the junction leakage current. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a method of manufacturing a semiconductor device comprises at least two processes. Under an atmosphere comprising hydrogen and oxygen, a sacrificial oxide film is formed on a silicon substrate that is provided with at least one nitride region. Then, the sacrificial oxide film and the nitride region are removed from the silicon substrate. 
   An appreciation of the objectives of the present invention and a more complete understanding of its structure may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view showing a process of a method of fabricating a semiconductor device in accordance with an embodiment of the present invention; 
       FIG. 2  is a cross-sectional view showing a subsequent process to that of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view showing a subsequent process to that of  FIG. 2 ; 
       FIG. 4  is a cross-sectional view showing a subsequent process to that of  FIG. 3 ; 
       FIG. 5  is a cross-sectional view showing a subsequent process to that of  FIG. 4 ; 
       FIG. 6  is a cross-sectional view showing a subsequent process to that of FIG.  5 ; 
       FIG. 7  is a graph showing a relationship between oxidation time of a sacrificial oxide film and a thickness of the oxide film; 
       FIG. 8  is a cross-sectional view showing a subsequent process to that of  FIG. 6 ; 
       FIG. 9  is a cross-sectional view showing a subsequent process to that of  FIG. 8 ; 
       FIG. 10  is a graph showing a relationship between a thickness of a sacrificial oxide film and a junction leakage current. 
       FIG. 11  is a cross-sectional view for used in describing a problem of the conventional semiconductor device. 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a pad oxide film  11  made of SiO 2  is formed on the main surface of a silicon substrate  10 , and a nitride film  12  made of Si 3 N 4  is formed on the pad oxide film  11 . In this embodiment, the thickness of the pad oxide film  11  and the nitride film  12  are 9 nm and 150 nm, respectively. Next, using a resist film (not shown) as a mask, the pad oxide film  11  and the nitride film  12  are patterned so as to expose a predetermined region of the silicon substrate  10 . The resist film is removed after the patterning. 
   Referring to  FIG. 2 , using the patterned nitride film  12  as a mask, the predetermined region is etched to form a trench  13 . In this embodiment, a depth of the trench is 250 nm. 
   Referring to  FIG. 3 , the trench  13  is subjected to the wet oxidation process to form a liner oxide film  14  on the surface of the trench  13 . In this embodiment, the liner oxide film  14  has a thickness of 15 nm. During the wet oxidation process, the patterned nitride film  12  is also oxidized so that nitrogen atoms are moved out of the nitride film. The nitrogen atoms react with H 2 O molecules, which serve as oxidizing species, to form NH 3  molecules. The NH 3  molecules are diffused into the pad oxide film  11  and azotize the silicon substrate  10  so that undesirable nitride regions  15  are formed in the silicon substrate  10 , wherein the undesirable nitride regions (Si 3 N 4 )  15  are positioned around the trench  13  and directly under the patterned pad oxide film  11 . 
   Referring to  FIG. 4 , a high density plasma (HDP) oxide film  16  is formed to fill the trench  13 . 
   Referring to  FIG. 5 , the HDP oxide film  16  is polished and removed by the CMP (Chemical Mechanical Polishing) method using the patterned nitride film  12  as the stopper. By removing the patterned nitride film  12  and the patterned pad oxide film  11 , an STI film  17  is obtained, wherein the STI film  17  is comprised of the liner oxide film  14  and the DHP oxide film  16 . 
   The STI film  17  defines a plurality of device formation regions  18  into the silicon substrate  10 . The undesirable nitride regions  15  are exposed on the edges of the device formation regions  18 . If gate oxide films are formed without removing the undesirable nitride regions  15 , the undesirable nitride regions  15  inhibit the formation of the gate oxide films. As the result, the gate oxide films have thinner regions on the undesirable nitride regions  15 , respectively; the thinner regions of the gate oxide films cause withstand voltage failures of the gate oxide films. 
   Referring to  FIG. 6 , sacrificial oxide layers  19  are formed on the device formation region  18  in order to remove the undesirable nitride regions  15 . In this embodiment, the thickness of each of the sacrificial oxide layers  19  is 9 nm. The sacrificial oxide layers  19  are formed by placing the silicon substrate  10  comprising the STI film  17  within a reaction chamber, followed by supplying a hydrogen gas (H 2 ) and an oxygen gas (O 2 ) into the reaction chamber simultaneously. In this embodiment, the ratio of H 2  and O 2  is 5:95. The atmosphere within the reaction chamber is kept at 1000° C. and 7 Torr during the formation process of the sacrificial oxide layers  19 . Preferably, the temperature of the atmosphere within the reaction chamber is 800° C. or higher and, more preferably, 1000° C. 
   H 2  and O 2  react with each other in the reaction chamber to form water. During this process, H 2  and O 2  generate radical species which contain oxygen (O) atomic radicals and hydroxyl (OH—) molecular radicals. In comparison with the conventional wet or dry oxidation, the radical species according to this embodiment need a shortened time to oxidize exposed regions of the silicon substrate  10 , i.e., the device formation regions  18  with no oxide film. In addition, the radical species will lose their energy at the thick oxide region such as the STI film  17  and turn into the oxide species which have the oxidation characteristic same as that of the oxide species generated by wet or dry oxidization. In other words, according to the present embodiment, only the surfaces of the device formation regions  18  can be oxidized without further oxidizing the inner surface of the trench  13  in the silicon substrate  10 . 
   The thickness of the sacrificial oxide film  19  may be adjusted so that it is proportional to the thickness of the liner oxide film  14 , in order to ensure complete removal of the undesirable nitride regions  15 . Specifically, it is preferable that the thickness of the sacrificial oxide film  19  is 6 nm, 9 nm, and 12 nm when the thickness of the liner oxide film  14  is 10 nm, 15 nm, and 20 nm, respectively. It is more preferable that the thickness of the sacrificial oxide film  19  is three-fifth of the thickness of the liner oxide film  14 . 
   As understood from  FIG. 7 , the required time for the formation of the sacrificial oxide film  19  having a thickness of 6 nm, 9 nm, or 12 nm is about 19 seconds, 47 seconds, or 86 seconds. In the earlier technique of wet or dry oxidation, the required time for the formation of the sacrificial oxide film  19  having a thickness of 6 nm, 9 nm, or 12 nm is about 800 seconds, 1500 seconds, or 2100 seconds. In other words, the time required for the formation of the sacrificial oxide film  19  in accordance with the present embodiment becomes one-twentieth of the conventional oxidation time. Since the required time for the sacrificial oxide film formation is very short, the inner surface of the trench  13  in the silicon substrate  10  can be prevented from being further oxidized. In accordance with the earlier technique, the sacrificial oxide film formation expands the STI film to cause the stresses on the PN junctions, as shown in  FIG. 11 . On the other hand, the sacrificial oxide film formation according to the present embodiment can prevent the expansion of the STI film so that the junction leakage current due to the stress of the expansion can be avoided, too. 
   Referring to  FIG. 8 , impurities are injected or implanted into the device formation regions  18  through the sacrificial oxide films  19  to form impurity diffusion regions  20  within the device formation regions  18 . Thereafter, the sacrificial oxide films  19  are removed therefrom by using the hydrofluoric acid solution. According to the removal of the sacrificial oxide films  19 , the undesirable nitride regions  15  are also removed from the silicon substrate  10 . 
   Referring to  FIG. 9 , gate oxide films  21  are formed on the device formation regions  18 . Then, devices or elements are formed in a manner well known. 
   For evaluation of the effect of the present embodiment, two samples were fabricated and measured, wherein one of the samples was formed in accordance with the earlier technique, while the other was formed in accordance with the present embodiment. Each of the samples had a structure similar to that of  FIG. 11 . In detail, each sample had a p-type silicon substrate, in which an STI film was formed to define a device formation region. In the device formation region on the surface of the silicon substrate, an n-type diffusion layer is formed to constitute a PN junction. Each of the samples had a shape of 100 nm square, as seen from above. In addition, an electrode was formed on the device formation region. The junction leakage current was measured with a predetermined voltage applied between the electrode and the substrate. The measurement result is shown in  FIG. 10 . 
   As apparent from  FIG. 10 , if the thickness of the sacrificial oxide film is 10 nm, the junction leakage current of the conventional device was 1.2×10 −8  A/cm 2  higher than the standard of 1.0×10 −8  A/cm 2 , while the junction leakage current of the present invention was 9.0×10 −9  A/cm 2  lower than the standard because the STI film is prevented from being expanded during the formation process of the sacrificial oxide layer. 
   The present application is based on Japanese patent applications of JP2006-319728 filed before the Japan Patent Office on Nov. 28, 2006, the contents of which are incorporated herein by reference. 
   While there has been described what is believed to be the preferred embodiment of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such embodiments that fall within the true scope of the invention.