Abstract:
A method of manufacturing a semiconductor device includes steps (a) to (d). The step (a) is a step of forming a first insulating film and a nitride film on a semiconductor substrate in this order. The step (b) is a step of removing said first insulating film and said nitride film in a first region while leaving said first insulating film and said nitride film in a second region. The step (c) is a step of forming a second insulating film on said semiconductor substrate in said first region. Here, a thickness of said second insulating film is different from that of said first insulating film. A third insulating film is formed on said nitride film in said second region along with the formation of said second insulating film. The step (d) is a step of removing said third insulating film and said nitride film in said second region.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of manufacturing a semiconductor device, and more particularly relates to a method of manufacturing a semiconductor device with regard to a film thickness of a gate oxide film. 
   2. Description of the Related Art 
   There are techniques known for forming a plurality of transistors in which film thicknesses of gate oxide films are different from each other on the same substrate. For example, Japanese Laid-Open Patent Application (JP-P2005-5668A) discloses a method of manufacturing a dual gate oxide film. This conventional technique will be described below. 
     FIGS. 1A to 1C  and  FIGS. 2A to 2C  are sectional views showing the method of manufacturing the dual gate oxide film, in this conventional technique. In each of the drawings, the left side indicates the region where the transistor having the thick gate oxide film is formed (hereafter, referred to as the thick film Tr region), and the right side indicates the region where the transistor having the thin gate oxide film is formed (hereafter, referred to as the thin film Tr region). 
   With reference to  FIG. 1A , an isolation region (STI: Shallow Trench Isolation)  110  is firstly formed in a silicon substrate  101 . Next, with reference to  FIG. 1B , the surface of the silicon substrate  101  is oxidized to form a first gate oxide film  102  such that the first gate oxide film  102  covers the surface of the silicon substrate  101 . After that, with reference to  FIG. 1C , a lithography operation is used to mask the thick film Tr region with a resist  103 , and the resist  103  is patterned so as to open only the thin film Tr region. Next, with reference to  FIG. 2A , the first gate oxide film  102  in the thin film Tr region is wet-etched by using an acidic chemical solution. Consequently, the first gate oxide film  102  in the thin film Tr region is removed. 
   At this time, in the boundary between the thin film Tr region and the thick film Tr region, the chemical solution invades the interface between the resist  103  and the first gate oxide film  102 . For this reason, an end  120  of the first gate oxide film  102  in the thick film Tr region is partially etched. After that, with reference to  FIG. 2B , the resist  103  is removed. The end  120  of the first gate oxide film  102  in the thick film Tr region becomes thinner towards the thin film Tr region, in the boundary between the thick film Tr region and the thin film Tr region. Then, with reference to  FIG. 2C , a second gate oxide film  104  is formed. Consequently, the gate oxide film having the thin film thickness, which is constituted by only the second gate oxide film  104 , can be formed in the thin film Tr region. On the other hand, the gate oxide film having the thick film thickness, in which the first gate oxide film  102  and the second gate oxide film  104  are laminated, can be formed in the thick film Tr region. However, in a region P of the boundary of the thick film Tr region, the film thickness becomes thin. For this reason, since the transistor cannot be formed in this region P, this is defined as a forbidden region where the placement of the transistors is forbidden when a circuit is designed. In future, as the miniaturization of a semiconductor circuit is advanced, this forbidden region exhibits the severe influence as the new subject of the miniaturization. The technique is desired which can reduce the forbidden region in the boundary between the thin film Tr region and the thick film Tr region. The technique is desired which can attain the miniaturization of the semiconductor circuit efficiently without any waste of regions in a semiconductor chip. 
   In conjunction with the above technique, Japanese Laid Open Patent Application (JP-P2005-129711A) discloses a semiconductor device and a method of manufacturing the same. This method of manufacturing the semiconductor device includes: a step of forming a bottom oxide film on a semiconductor substrate of a memory transistor formation region and a peripheral circuit transistor formation region; a step of forming a nitride film on the bottom oxide film; a step of forming a top oxide film on the nitride film; a step of removing the top oxide film, the nitride film and the bottom oxide film in the peripheral circuit transistor formation region to expose the surface of the semiconductor substrate in the peripheral circuit transistor formation region; a step f executing a heat treatment in the atmosphere having nitrogen and oxygen in each of the semiconductor substrate of the peripheral circuit transistor formation region and the top oxide film of the memory transistor formation region; and a step of forming a gate insulating film on the semiconductor substrate in the peripheral circuit transistor formation region. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a method of manufacturing a semiconductor device that can reduce the forbidden region in the boundary between the thin film Tr region and the thick film Tr region. 
   Also, another object of the present invention is to provide a method of manufacturing a semiconductor device that can attain the miniaturization of the semiconductor device efficiently without any waste of regions in a semiconductor chip. 
   This and other objects, features and advantages of the present invention will be readily ascertained by referring to the following description and drawings. 
   In order to achieve an aspect of the present invention, the present invention provides a method of manufacturing a semiconductor device comprising: (a) forming a first insulating film and a nitride film on a semiconductor substrate in this order; (b) removing said first insulating film and said nitride film in a first region while leaving said first insulating film and said nitride film in a second region; (c) forming a second insulating film on said semiconductor substrate in said first region, wherein a thickness of said second insulating film is different from that of said first insulating film, and a third insulating film is formed on said nitride film in said second region along with the formation of said second insulating film; and (d) removing said third insulating film and said nitride film in said second region. 
   In the method of manufacturing a semiconductor device, said step (d) may include: removing said third insulating film by using a chemical solution which dissolves said third insulating film rather than said nitride film; and removing said nitride film by using a chemical solution which dissolves said nitride film rather than said second insulating film. 
   In the method of manufacturing a semiconductor device, said step (d) may include: removing said third insulating film and said nitride film in said second region by a dry-etching method. 
   The method of manufacturing a semiconductor device, said step (d) may further include: executing a heat treatment of said semiconductor substrate in an atmosphere having nitrogen and oxygen after said removing by the dry-etching method. 
   In the method of manufacturing a semiconductor device, said step (b) may include: removing said first insulating film and said nitride film in said first region by a dry-etching method. 
   In the method of manufacturing a semiconductor device, said step (b) may further include: executing a heat treatment of said semiconductor substrate in an atmosphere having nitrogen and oxygen after said removing by the dry-etching method. 
   In the method of manufacturing a semiconductor device, said step (d) may include: removing said third insulating film by using a chemical solution which dissolves said third insulating film rather than said nitride film; and removing said nitride film by using a chemical solution which dissolves said nitride film rather than said second insulating film In the method of manufacturing a semiconductor device, said step (d) may include: removing said third insulating film and said nitride film in said second region by a dry-etching method. 
   In the method of manufacturing a semiconductor device, said step (d) may further include: executing a heat treatment of said semiconductor substrate in an atmosphere having nitrogen and oxygen after said removing by the dry-etching method. 
   In the method of manufacturing a semiconductor device, said first insulating film may be one of a silicon oxide film and a silicon oxynitride film. Said second insulating film may be one of a silicon oxide film and a silicon oxynitride film. Said nitride film may be a silicon nitride film. 
   In the method of manufacturing a semiconductor device, said step (b) may include: removing said first insulating film and said nitride film in said first region by a dry-etching method. 
   In the method of manufacturing a semiconductor device, said step (b) may further include: executing a heat treatment of said semiconductor substrate in an atmosphere having nitrogen and oxygen after said removing by the dry-etching method. 
   In the method of manufacturing a semiconductor device, said step (c) may include: executing an oxidation of said semiconductor substrate. Said second insulating film may be composed of an oxide film formed by said heat treatment and an oxide film formed by said oxidation on said semiconductor substrate. Said third insulating film may be composed of an oxide film formed by said heat treatment and an oxide film formed by said oxidation film on said nitride film. 
   In the method of manufacturing a semiconductor device, said step (d) may include: removing said third insulating film by using a chemical solution which dissolves said third insulating film rather than said nitride film; and removing said nitride film by using a chemical solution which dissolves said nitride film rather than said second insulating film. 
   In the method of manufacturing a semiconductor device, said step (d) may include: removing said third insulating film and said nitride film in said second region by a dry-etching method. 
   In the method of manufacturing a semiconductor device, said step (d) may further include: executing a heat treatment of said semiconductor substrate in an atmosphere having nitrogen and oxygen after said removing by the dry-etching method. 
   The method of manufacturing a semiconductor device may further include: (e) forming a first gate of a first transistor at a predetermined position in said first region, and a second gate of a second transistor at a predetermined position in said second region; and (f) forming a source and a drain of said first transistor with respect to said first gate in a self-alignment manner, and a source and a drain of said second transistor with respect to said second gate in a self-alignment manner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A to 1C  are sectional views showing a method of manufacturing a dual gate oxide film of a conventional technique; 
       FIGS. 2A to 2C  are sectional views showing a method of manufacturing a dual gate oxide film of a conventional technique; 
       FIG. 3  is a sectional view showing a semiconductor device that is manufactured in an embodiment of a method of manufacturing a semiconductor device according to the present invention; 
       FIGS. 4A to 4C  are sectional views showing an embodiment of a method of manufacturing a semiconductor device according to the present invention; 
       FIGS. 5A to 5C  are sectional views showing an embodiment of a method of manufacturing a semiconductor device according to the present invention; and 
       FIGS. 6A to 6C  are sectional views showing an embodiment of a method of manufacturing a semiconductor device according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposed. 
   The embodiment of a method of manufacturing a semiconductor device according to the present invention will be described below with reference to attached drawings. 
     FIG. 3  is a sectional view showing one example of the semiconductor device that is manufactured in the embodiment of the method of manufacturing the semiconductor device according to the present invention, A semiconductor device  20  has a thick film Tr region (the left side of the drawing) and a thin film Tr region (the right side of the drawing). Here, the thick film Tr region is a region where thick film gate transistors are formed. The thick film gate transistor has the gate oxide film of a thick film thickness. The thin film Tr region is a region where thin film gate transistors are formed. The thin film gate transistor has the gate oxide film of a thin film thickness. In the thick film Tr region, a thick film gate transistor  19  is formed between isolation regions (STI)  10  installed on a semiconductor substrate  1 . In the thin film Tr region, a thin film gate transistor  18  is formed between the isolation regions  10  installed on the semiconductor substrate  1 . 
   The thick film gate transistor  19  includes a source/drain region  17   a , a source/drain region  17   b , a thick gate oxide film  5  and a gate  16 . The source/drain region  17   a  and the source/drain region  17   b  are formed at a predetermined interval on the upper surface of the semiconductor substrate  1 . The thick gate oxide film  5  is formed on the semiconductor substrate  1  between the source/drain region  17   a  and the source/drain region  17   b  (a channel region), and has a relatively thick film thickness. The thick gate oxide film  5  includes a first oxide film  5   a  and a second oxide film  5   b . The gate  16  is formed on the thick gate oxide film  5 . 
   On the other hand, the thin film gate transistor  18  includes a source/drain region  7   a , a source/drain region  7   b , a thin gate oxide film  2  and a gate  6 . The source/drain region  7   a  and the source/drain region  7   b  are formed at a predetermined interval on the upper surface of the semiconductor substrate  1 . The thin gate oxide film  2  is formed on the semiconductor substrate  1  between the source/drain region  7   a  and the source/drain region  7   b  (a channel region), and has a relatively thin film thickness. The gate  6  is formed on the thin gate oxide film  2 . 
   Incidentally, the thick film gate transistor  19  and the thin film gate transistor  18  shown in  FIG. 3  are indicated as examples. The present invention is not limited to these examples. If the transistor (including a memory cell) having the gate oxide film of the thick film thickness is formed in the thick film Tr region and if the transistor (including a memory cell) having the gate oxide film of the thin film thickness is formed in the thin film Tr region, the structure of each transistor does not matter. 
   The embodiment of the method of manufacturing the semiconductor device according to the present invention will be described below with reference to attached drawings.  FIGS. 4A to 4C ,  FIGS. 5A to 5C  and  FIGS. 6A to 6C  are sectional views showing the embodiment of the method of manufacturing the semiconductor device according to the present invention. Here, the case of manufacturing the semiconductor device  20  shown in  FIG. 3  will be explained below. 
   As shown in  FIG. 4A , the isolation regions (STI: Shallow Trench: Isolation)  10  are firstly formed on the semiconductor substrate  1 . Next, as shown in  FIG. 4B , the semiconductor substrate  1  after the isolation is cleaned by a cleaning process. After that, the thin gate oxide film  2  is formed by an oxidizing process so as to cover the surface of the semiconductor substrate  1 . The thin gate oxide film  2  has a film thickness of, for example, 3 nm and is exemplified as a silicon oxide film. The oxidization is executed by an ISSG (In Situ Steam Generation) method, for example, at 1050 degrees Celsius and H 2 :5% atmosphere. After that, as shown in  FIG. 4C , a nitride film  3  having a film thickness of about 15 nm is formed by a CVD method so as to cover the surface of the thin gate oxide film  2 . The nitride film  3  is exemplified as a silicon nitride film. 
   Next, as shown in  FIG. 5A , a resist  4  is patterned by a lithography process such that the thin film Tr region is masked with the resist  4  while the thick film Tr region is not masked. Then, as shown in  FIG. 5B , the nitride film  3  and the thin gate oxide film  2  in the thick film Tr region are etched by a dry-etching method. The dry etching is executed, for example, under etching gas: Ar+CF 4 . Since the dry-etching method is used, as compared with the conventional method that removes the oxide film by using the chemical solution, it is possible to exactly form the boundary between the thin film Tr region and the thick film Tr region. That is, since the nitride film  3  is used as the hard mask and the dry-etching method is further used, it is possible to protect the irregularity in the boundary shape that is caused by the invasion of the chemical solution. Consequently, the improper boundary is not generated whose film thickness is improper such as the end  120  shown in  FIG. 2B , and there is no bad influence on the subsequent processes. 
   After the etching, the resist  4  in the thin film Tr region is removed. Next, as shown in  FIG. 5C , annealing is executed in the mixed atmosphere of nitrogen and oxygen. The annealing condition is, for example, 900 degrees Celsius, N 2 :O 2 =1:1, and 30 sec. Since this substrate recovery process is executed through this annealing, it is possible to reduce the damage which may be caused by plasma at the time of the dry-etching. At this time, the thin first oxide film  5   a  is formed on the surface of the semiconductor substrate  1  in the thick film Tr region. At the same time, the oxide film  5   a ′ is formed on the surface of the nitride film  3  in the thin film Tr region. The film thickness of the first oxide film  5   a  is, for example, 0.3 nm, and the first oxide film  5   a  is exemplified as the silicon oxide film. The film thickness of the oxide film  5   a ′ is thinner than that of the first oxide film  5   a , and the oxide film  5   a ′ is exemplified as the silicon oxide film. 
   After that, as shown in  FIG. 6A , a thick second oxide film  5   b  is formed by the oxidizing process so as to cover the surface of the thermal first oxide film  5   a . The second oxide film  5   b  has a film thickness of, for example, 5.5 nm, and is exemplified as the silicon oxide film. The oxidization is executed by the ISSG method, for example, at 1050 degrees Celsius, H 2 :5% atmosphere. Consequently, the thick gate oxide film  5  is formed in the thick film Tr region. Here, in the thick gate oxide film  5 , the thin thermal first oxide film  5   a  and the thick second oxide film  5   b  are laminated in this order. On the other hand, by this oxidizing process, in the thin film Tr region, since the nitride film exist, a thin oxide film  5   b ′ is formed on the thin oxide film  5   a ′. In succession, as shown in  FIG. 6B , the thin oxide films  5   b ′ and  5   a ′ on the nitride film  3  in the thin film Tr region is firstly wet-etched by using an acid chemical solution. This leads to remove the oxide films  5   b ′ and  5   a ′ on the nitride film  3  in the thin film Tr region. At this time, the first and second oxide films  5   a  and  5   b  in the thick film Tr region are wet-etched just a little, which does not cause any problem. After that, the nitride film  3  in the thin film Tr region is removed by using the chemical solution which reacts with only a nitride film without reacting with an oxide film. Such chemical solution is exemplified as a high temperature phosphoric acid. As a result, in the thick film Tr region, the thick gate oxide film  5  is formed on the surface of the silicon substrate  1 . On the other hand, in the thin film Tr region, the thin gate oxide film  2  is formed on the surface of the silicon substrate  1 . 
   In this way, in the present invention, the oxide films  5   b ′ and  5   a ′ are removed by using the chemical solution which reacts with an oxide film rather than a nitride film, and the nitride film  3  in the thin film Tr region is removed by using the chemical solution which reacts with only a nitride film without reacting with an oxide film. Thus, as compared with the conventional method that removes the oxide film by using the chemical solution, it is possible to form exactly and sharply the boundary between the thin film Tr region and the thick film Tr region. Consequently, the improper boundary, whose film thickness is improper such as the end  120  shown in  FIG. 2B , is not generated. In addition, it is possible to suppress and reduce the forbidden region such as the region P shown in  FIG. 2C . 
   Incidentally, in  FIG. 6B , the resist patterning process, the dry-etching process and the annealing process shown in  FIGS. 5A ,  5 B and  5   c  may be used as the processes for removing the oxide films  5   a ′ and  5   b ′ and the nitride film  3  in the thin film Tr region. In that case, it is possible to obtain the effect similar to the case of using the chemical solution. In addition, since the chemical solution is not used at all, the dimensional control can be executed further accurately. 
   Next, as shown in  FIG. 6C , a metal film is formed by using a sputtering method so as to cover a surface of the thick gate oxide film  5  in the thick film Tr region and a surface of the thin gate oxide film  2  in the thin film Tr region. In succession, the metal film and the thick gate oxide film  5  are patterned by the lithography and dry-etching processes so as to form the gate of the thick film gate transistor  19  at the predetermined position in the thick film Tr region. Simultaneously, the metal film and the thin gate oxide film  2  are patterned by the lithography and dry-etching processes so as to form the gate of the thin film gate transistor  18  at the predetermined position in the thin film Tr region. After that, ion implantation is executed in a self-alignment manner so as to form the source/drain regions (diffusion layers)  17   a  and  17   b  of the thick film gate transistor  19  and the source/drain regions (diffusion layers)  7   a  and  7   b  oft the thin film gate transistor  18 . As mentioned above, the thick film gate transistor  19  is formed in the thick film Tr region, and the thin film gate transistor  18  is formed in the thin film Tr region. 
   In the present invention, without any use of the chemical solution to dissolve the oxide film, the gate oxide films having the film thicknesses different from each other can be formed on the same substrate. That is, although the conventional technique uses the chemical solution and performs the wet-etching and partially removes the oxide film, the present invention uses the nitride film as the hard mask and further uses the dry-etching and partially removes the oxide film. Consequently, it is possible to protect the invasion of the chemical solution in the boundary the between the thick film Tr region and the thin film Tr region and also possible to avoid the improper situation of the film thickness in the boundary. 
   According to the present invention, when the thin film gate transistor  18  and the thick film gate transistor  19  which have the gate oxide film thicknesses different from each other are formed on the same substrate, the side-etching caused by the chemical solution is not substantially generated in the boundary between the thin film Tr region and the thick film Tr region. Thus, it is possible to suppress the generating of the forbidden region in the boundary. Hence, it is possible to effectively use the region in the semiconductor device and also possible to attain the miniaturization of the semiconductor device efficiently without any waste of regions in a semiconductor chip. In addition, the dimension control becomes easy in the processes of manufacturing the semiconductor device. 
   In the present invention, at least one of the thin gate oxide film  2  and the thick gate oxide film  5  may be silicon oxynitride film. 
   According to the present invention, it is possible to reduce the forbidden region in the boundary between the thin film Tr region and the thick film Tr region. And, the miniaturization of the semiconductor device can be attained efficiently without any waste of regions in a semiconductor chip. 
   It is apparent that the present invention is not limited to the above embodiment that may be modified and changed without departing from the scope and spirit of the invention.