Abstract:
A semiconductor device manufacturing method includes forming a silicon layer by epitaxial growth over a semiconductor substrate having a first area and a second area; forming a first gate oxide film by oxidizing the silicon layer; removing the first gate oxide film from the second area, while maintaining the first gate oxide film in the first area; thereafter, increasing a thickness of the first gate oxide film in the first area and simultaneously forming a second gate oxide film by oxidizing the silicon layer in the second area; and forming a first gate electrode and a second gate electrode over the first gate oxide film and the second gate oxide film, respectively, wherein after the formation of the first and second gate electrodes, the silicon layer in the first area is thicker than the silicon layer in the second area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-121678 filed on Jun. 12, 2014, which is incorporated herein by references in its entirety. 
     FIELD 
     The disclosures herein relate to a method of manufacturing a semiconductor device. 
     BACKGROUND 
     In order to reduce variation in threshold voltage due to statistical fluctuation in channel impurity, a non-doped epitaxial silicon layer is formed over a highly doped channel layer that has a steep distribution of impurity density. It is proposed to control the threshold voltage of a transistor by adjusting the thickness of the non-doped epitaxial silicon layer. See, for example, Japanese Laid-open Patent Publication No. 2012-79746. An epitaxial silicon layer formed in an area assigned to high-threshold-voltage transistors is made thinner than an epitaxial silicon layer formed in an area assigned to low-threshold-voltage transistors. With this arrangement, different types of transistors with different performances can be arranged on the same substrate. 
       FIG. 1  illustrates a conventional process for forming epitaxial silicon layers with different thicknesses. An epitaxial silicon layer  112  is formed over a highly-doped layer  111  on a silicon substrate  131  (S 101 ). Then, a photoresist mask  113  is formed by a photolithographic technique (S 102 ) and a part of the epitaxial silicon layer  112  is etched to produce an epitaxial silicon layer  114  with a different thickness (S 103 ). Then, a first gate oxide film (Gox 1 )  116  is formed over the entire surface including a trench  115  to form a device-isolating layer  117  (S 104 ). A photoresist layer  123  with an opening pattern in an area not illustrated is formed and an unnecessary portion of the first gate oxide film  116  is removed by etching (S 105 ). This step is referred to as “oxide film etching  1 ”. Then, the photoresist layer  123  is removed and second gate oxidation is performed to form a second gate oxide (Gox 2 ) film (S 106 ). In an area having been covered with the photoresist layer  123  during the process of oxide film etching  1 , a gate oxide layer (Gox 1 +Gox 2 )  119  is formed. In the area (not illustrated) to which the oxide film etching  1  was applied, a gate oxide film that is thinner than the gate oxide film  119  is formed. Unnecessary portions of the gate oxide film  119  are removed (S 107 ). Then, a third gate oxide film (Gox 3 )  24  is formed by gate oxidation  3  (S 108 ). 
     The conventional method partially changes the thickness of the epitaxial silicon layer using a photolithography technique and an etching technique. Accordingly, at least two additional steps, i.e., a photography step and an etching step are added. It is desirable to create non-doped epitaxial layers with different thicknesses in an ordinary process without adding extra steps. 
     SUMMARY 
     According to an aspect of the embodiments, a method of manufacturing a semiconductor device include steps of
         forming a silicon layer by epitaxial growth over a semiconductor substrate having a first area and a second area;   forming a first gate oxide film by oxidizing the silicon layer;   removing the first gate oxide film from the second area, while maintaining the first gate oxide film in the first area;   after the removal of the first gate oxide film from the second area, increasing a thickness of the first gate oxide film in the first area, and simultaneously forming a second gate oxide film by oxidizing the silicon layer in the second area; and   forming a first gate electrode and a second gate electrode over the first gate oxide film and the second gate oxide film, respectively,   wherein after the formation of the first gate electrode and the second gate electrode, the silicon layer in the first area has a first thickness, and the silicon layer in the second area has a second thickness less than the first thickness.       

     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive to the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a conventional technique for controlling the thickness of an epitaxial silicon layer; 
         FIG. 2  illustrates a technique of controlling the thickness of an epitaxial silicon layer according to an embodiment; 
         FIG. 3  is a schematic diagram of a semiconductor device fabricated using a film-thickness control technique for epitaxial silicon layers according to the first embodiment; 
         FIG. 4A  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 4B  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 4C  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 4D  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 4E  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 4F  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 4G  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 4H  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 5  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 6A  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 6B  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 6C  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 6D  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 6E  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 6F  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 7  is a schematic diagram of a semiconductor device fabricated using a film-thickness control technique for epitaxial silicon layers according to the second embodiment; 
         FIG. 8  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9A  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9B  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9C  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9D  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9E  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9F  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9G  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 9H  illustrates a manufacturing process of the semiconductor device of the second embodiment; 
         FIG. 10  illustrates a manufacturing process of the semiconductor device of the first embodiment; 
         FIG. 11  is a schematic diagram of a semiconductor device to which the third embodiment is applied; 
         FIG. 12  illustrates variations in thickness and property of epitaxial silicon layer depending on positions in a furnace; 
         FIG. 13  is a diagram to explain setting of a thickness of an initial oxide-film for each wafer; 
         FIG. 14  is a diagram to explain setting of a thickness of an initial oxide-film for each wafer; 
         FIG. 15  illustrates a profile of impurity concentration in the depth direction of a semiconductor wafer; and 
         FIG. 16  illustrates a relationship between thickness of an epitaxial silicon layer and thickness of an initial oxide film. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A novel technique of forming non-doped epitaxial layers with different thicknesses is provided. Non-doped epitaxial layers with different thicknesses are fabricated in an ordinary semiconductor process without introducing additional photolithography and etching steps. 
     In addition, the inventors found that the thickness of an epitaxial silicon layer varies during epitaxial growth depending on a position in a furnace. The variations in thickness of epitaxial silicon layers cause variations in transistor characteristics (such as threshold voltage) between wafers in a lot and between lots. Accordingly, it is also desirable to reduce variations in the thickness of epitaxial silicon layers. 
     The embodiments make use of an oxide film forming process to control the thickness of epitaxial silicon layers to fabricate a semiconductor device with epitaxial silicon layers of different thicknesses. In addition, variation in the thickness of epitaxial silicon layers is reduced by controlling the thickness of initial oxide films. 
       FIG. 2  illustrates a basic idea of the embodiments to control the thickness of epitaxial silicon layers. A highly-doped layer  11  is formed in a predetermined area on a silicon substrate  31 , and a non-doped silicon layer is formed over the silicon substrate  31  by epitaxial growth (S 1 ). The non-doped silicon layer formed by epitaxial growth is referred to as a “epitaxial silicon layer”. 
     Then, a gate oxide film (Gox 1 )  16  is formed over the entire surface including a trench  15 , and a device-isolating layer  17  is formed (S 2 ). Unlike the conventional method, additional photolithography and etching steps are not introduced to change the thickness of the epitaxial silicon layer  12  prior to the formation of the gate oxide film  16 . 
     Then, a photoresist layer  23  is formed and an opening  23 A is formed during the ordinary photoresist patterning process. The opening  23 A is arranged in an area that is not exposed by the conventional method (see  FIG. 1 ). The gate oxide film  16  exposed in the opening  23 A is etched (S 3 ). The opening  23 A is formed in a region in which the thickness of the epitaxial silicon layer  12  is to be altered. 
     Then, the photoresist layer  23  is removed and the second gate oxidation (gate oxidation  2 ) is performed to form a gate oxide film (Gox 2 +Gox 1 )  19  in the area having been covered with the photoresist layer  23 . During the process of gate oxidation  2 , a gate oxide film (Gox 1 )  18  is formed simultaneously with the gate oxide film  19 , in the area having been exposed in the opening  23 A (S 4 ). During the formation of the gate oxide film (Gox 1 )  18 , a surface region of the epitaxial silicon layer  12  is oxidized in the area of the opening  23 A and the oxidized surface region becomes a part of the gate oxide film  18 . Consequently. The thickness of the epitaxial silicon layer  12  is reduced. By making use of the process of gate oxidation  2  (S 4 ), epitaxial silicon layers  12  and  22  with different thicknesses are formed without carrying out etching on the epitaxial silicon layer  12 . 
     Then, unnecessary portions of the gate oxide film  18  and the gate oxide film  19  are removed by etching (S 5 ). Then, a third gate oxide film (Gox 3 )  24  is formed in a desired area (S 6 ). 
     In the embodiments described below, the thickness of the epitaxial silicon layer is controlled by making use of an oxidation process, and epitaxial silicon layers with different thicknesses can be formed in accordance with threshold voltages of transistors, without introducing additional photolithography and etching processes. 
     First Embodiment 
       FIG. 3  is a schematic diagram of a semiconductor device  30  fabricated under the control of film thickness making use of a gate oxidation process according to the first embodiment. Different-type transistors Tr 1 , Tr 2 , Tr 3  and Tr 4  are mounted on the semiconductor device  30 . For the transistors Tr 1  to Tr 4 , the combinations of the effective film thicknesses t 1 , t 2 , t 3 , and t 4  of epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d  and the thicknesses of the gate oxide films  61 - 63  are different from each other. The transistor Tr 1  is a low-threshold and low-voltage transistor, and the transistor Tr 3  is a high-threshold and low-voltage transistor. The transistors Tr 2  and Tr 4  are high voltage and low-threshold voltage with different threshold voltages and different operating voltages. The transistor Tr 1  is arranged in a circuit area that performs high-speed operations. The transistor Tr 3  is arranged in a circuit area in which leakage currents are reduced. The transistors Tr 2  and Tr 4  are arranged in an area to which a high voltage is applied. 
     In the respective areas, a highly doped impurity layer  38  is formed under the epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d.  The highly doped impurity layer  38  is provided to control the threshold voltages and prevent the punch-through effect. For convenience sake, the highly doped impurity layer  38  is referred to as “punch-through stop layer  38 ”. 
     Comparing transistor Tr 1  and transistor Tr 3 , the depth of the punch-through stop layer  38  located under the channel with the effective thickness t 1  is greater than the depth of the punch-through stop player  38  located under the channel with the effective thickness t 3  (t 1 &gt;t 3 ). In the transistors Tr 2  and Tr 4 , the punch-through stop layer  38  are located at appropriate depths to control the associated threshold voltages. 
     In this example, the effective thicknesses of the epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d  are varied making use of a process of forming gate oxide films of the high-voltage transistors Tr 2  and Tr 4 . The combination of the effective thickness of the epitaxial silicon layers  45   a  through  45   d  and the thickness of the gate oxide films  61 - 63  differs among the transistors Tr 1 , Tr 2 , Tr 3  and Tr 4 . 
       FIG. 4A  through  FIG. 4H ,  FIG. 5 , and  FIG. 6A  through  FIG. 6F  illustrate a manufacturing process of the semiconductor device  30  according to the first embodiment. In  FIG. 4A , a silicon oxide film  32  is formed by, for example, thermal oxidation, over the entire surface of the silicon substrate  31 . The silicon oxide film  32  protects the surface of the silicon substrate  31 . 
     In  FIG. 4B , a photoresist film  33  with an opening  34  is formed. An area for low-voltage NMOS (n-channel metal-oxide-silicon) transistors is exposed in the opening  34 , while the other area is covered with the photoresist film  33 . Ion implantation is carried out using the photoresist film  33  as a mask to form an embedded N-well  35  in the low-voltage NMOS transistor forming area. The embedded N-well  35  may be fabricated by injecting phosphor ions at an acceleration energy of 700 keV with dose of 1.5×10 13  cm −2 . Then, the photoresist film  33  is removed by, for example, a wet process. 
     In  FIG. 4C , a photoresist film  36  with an opening  37  is formed by photolithography. The low-voltage NMOS transistor forming area is again exposed in the opening  37 , while the other area is covered with the photoresist film  36 . Ion implantation is performed using the photoresist film  36  as a mask to form a P-well  38 W and a highly doped p-type impurity layer  38 . The P-well  38 W may be formed by injecting, for example, boron ions (B+) at an acceleration energy of 150 keV with dose of 7.5×10 12  cm −2  in four oblique incident directions with respect to the normal of the silicon substrate  31 . The highly doped p-type impurity layer  38  may be formed by injection of germanium ions (Ge+), carbon ions (C+), boron ions (B+), and boron fluoride (BF). For example, germanium ions are injected at an acceleration energy of 30 keV with dose of 5×10 14  cm −2 . Carbon ions may be injected at an acceleration energy of 5 keV with dose of 5×10 14  cm −2 . Boron ions may be injected at an acceleration energy of 20 keV with dose of 1.8×10 13  cm −2 . Boron fluoride ions (BF 2 +) may be injected at an acceleration energy of 10 to 25 keV with dose of 2×10 12  cm −2  to 6×10 12  cm −2 . 
     Germanium turns the silicon substrate  31  amorphous and prevents channeling of boron ions, while increasing the probability of carbon atoms positioned at lattice points. The carbon atoms placed at the lattice points prevent diffusion of boron ions. From this viewpoint, germanium ions may be injected before carbons ions and boron ions are injected. The P-well  38 W and the highly doped p-type impurity layer  38  may be formed simultaneously, or alternatively, the P-well  38 W may be formed before the formation of the highly doped p-type impurity layer  38 . 
     Then, the photoresist film  36  is removed by ashing. Thermal treatment is performed in the inactive atmosphere for damage recovery of the silicon substrate  31  due to ion implantation. The thermal treatment may be performed at 600° C., 150 seconds in the nitrogen atmosphere. 
     In  FIG. 4D , a natural oxide film is removed by hydrofluoric acid. Then, wet oxidation is performed on the surface of the silicon substrate  31  using, for example, in-situ stream generation (ISSG) under low pressure, at 810° C., 20 seconds to form a silicon oxide film  39  with a thickness of 3 nm. Then, a photoresist film  40  with an aperture  41  is formed. An area for low-voltage PMOS transistors is exposed in the aperture  41 , while the other area is covered with the photoresist film  40 . Ion implantation is performed using the photoresist film  40  as a mask to form an N-well  43 W and a highly doped n-type impurity layer  43  in the low-voltage PMOS transistor area. The N-well  43 W may be formed by, for example, phosphorous ion implantation (P+) at an acceleration energy of 360 keV with dose of 7.5×10 12  cm −2  in four oblique incident directions with respect to the normal of the silicon substrate  31 . The highly doped n-type impurity layer  43  may be formed by injecting antimony ions (Sb+) at an acceleration energy of 80 keV with dose of 3×10 12  cm −2  in four oblique incident directions with respect to the normal of the silicon substrate  31 , then injecting antimony ions (Sb+) at an acceleration energy of 130 keV with dose of 1.5×10 12  cm −2  in four oblique incident directions with respect to the normal of the silicon substrate  31 , and injecting antimony ions (Sb+) at an acceleration energy of 20 keV with 7×10 12  cm −2  dose in four oblique incident directions. 
     In  FIG. 4E , the photoresist film  40  is removed by, for example, ashing. Then, silicon oxide film  39  is removed by wet etching using a hydrofluoric acid. Then, wet oxidation is performed on the surface of the silicon substrate  31  using, for example, ISSG, to form a silicon oxide film (not illustrated). Then, the silicon oxide film is removed by wet etching using hydrofluoric acid. Then, a non-doped silicon layer  45  with a thickness of 25 nm is epitaxially grown over the silicon substrate  31  by chemical vapor deposition (CVD) or other suitable methods. This silicon layer  45  is referred to as the “epitaxial silicon layer  45 ”. 
     Then, wet oxidation is performed on the surface of the epitaxial silicon layer  45  using, for example, ISSG, under low pressure, 810° C., 20 seconds to form a silicon oxide film  46  with a thickness of, for example, 3 nm. Then, a silicon nitride film  47  with a thickness of 70 nm is formed over the silicon oxide film  46  by, for example, low pressure chemical vapor deposition (LPCVD) under the conditions of 700° C., 150 minutes. 
     In  FIG. 4F , the silicon nitride film  47 , the silicon oxide film  46 , the epitaxial silicon layer  45 , and the silicon substrate  31  are processed by anisotropic dry etching using the photoresist film  48  as a mask formed by photolithography. By this process, a device isolating trench  49  is formed in a predetermined region. 
     In  FIG. 4G , using, for example, a thermal oxidation technique, the surfaces of the epitaxial silicon layer  45  and the silicon substrate  31  are oxidized at 650° C. via a free radical. By this process, a silicon oxide film is formed as a liner film on the inner wall of the device isolating trench  49 . Then, a silicon oxide film with a thickness of 500 nm is deposited by high-density plasma CVD to fill the device isolating trench  49  with the silicon oxide film. Then, chemical mechanical planarization or polishing (CMP) is performed to remove the surplus CVD oxide from the top of the silicon nitride film  47 , whereby an isolation region  51  is formed. 
     In  FIG. 4H , the silicon nitride film  47  is removed by wet etching using hot phosphoric acid. 
     The steps of  FIG. 5  and subsequent are explained focusing on the area  50  in which NMOS transistors are to be formed; however, the similar steps are carried out in the PMOS transistor area. In  FIG. 5 , the silicon oxide film  46  is removed by wet etching using a hydrofluoric acid. Then, wet thermal oxidation is performed on the surface of the epitaxial silicon layer  45  to form a silicon oxide film (Gox 1 )  53  with a thickness of 6 nm. This step corresponds to gate oxidation  1  (S 2 ) of  FIG. 2 . The conditions of the wet thermal oxidation are, for example, 750° C. and 30 minutes. After  FIG. 5 , illustration of the P-well  38 W is omitted. 
     In  FIG. 6A , a portion of the silicon oxide film  53  is removed by photolithography and hydrofluoric acid wet etching, using a photoresist film  54  as a mask. This step corresponds to oxide film etching  1  (S 3 ) of  FIG. 2 . 
     In  FIG. 6B , the photoresist film  54  is removed, and wet thermal oxidation is performed on the surface of the epitaxial silicon layer  45  to form a silicon oxide film (Gox 2 )  56  with a thickness of 6 nm. The wet thermal oxidation may be performed under the conditions of 750° C. and 20 minutes. This step corresponds to gate oxidation  2  (S 4 ) of  FIG. 2 . 
     During this process, the surface area of the epitaxial silicon layer  54  that has not been covered with the photoresist film  54  is oxidized to a certain depth and becomes a part of the silicon oxide film  56 . The epitaxial silicon layer  45  of this area becomes an epitaxial silicon layer  45   I  with a reduced thickness. On the other hand, in the area that has been covered with the photoresist film  54 , the second silicon oxide film (Gox 2 ) is formed over the silicon oxide film (Gox 1 )  53 . As a result, a silicon oxide film (Gox 1 +Gox 2 )  57  is produced. The thickness of the silicon oxide film  57  is about 8.5 nm to 9 nm, which thickness is less than the total thickness of the silicon oxide film  53  and the silicon oxide film  56 . 
     In  FIG. 6C , a part of the silicon oxide film  56  and a part of the silicon oxide film  57  are removed by photolithography and hydrofluoric acid wet etching, using a photoresist film  58  as a mask. This step corresponds to oxide film etching  2  (S 5 ) in  FIG. 2 . 
     In  FIG. 6D , a silicon oxide film (Gox 3 )  61  with a thickness of 2 nm is formed over the entire surface by, for example, thermal oxidation under the conditions of 810° C. and 8 seconds. At this point of time, silicon oxide films with different thicknesses, namely, the silicon oxide film (Gox 3 )  61 , the silicon oxide film (Gox 1 +Gox 2 +Gox 3 )  62 , and the silicon oxide film (Gox 2 +Gox 3 )  63  are formed in the associated areas. During this thermal oxidation, the surface area of the epitaxial silicon layer  45  that has been exposed from the silicon oxide films  56  and  57  is oxidized to a certain depth, and epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d  with different thicknesses t 1 , t 2 , t 3  and t 4  are produced. The epitaxial silicon layer  45   d  is thinner than the epitaxial silicon layer  45   b,  and the epitaxial silicon layer  45   c  is thinner than the epitaxial silicon layer  45   d  (t 2 &gt;t 4 &gt;t 3 ). The epitaxial silicon layer  45   a  is thinner than the epitaxial silicon layer  45   b  (t 2 &gt;t 1 ). 
     In  FIG. 6E , a non-doped polycrystalline silicon film with a thickness of 100 nm is formed over the entire surface by LPCVD at 605° C. The polycrystalline silicon film is patterned by photolithography and dry etching to form gate electrodes  64  in corresponding transistor fabrication areas. In  FIG. 6F , sidewall spaces  65  are formed on the side walls of the gate electrodes  64 , and source and drain impurity diffusion regions  21  are formed. Thus, a semiconductor device  30  with different types of transistors Tr 1 , Tr 2 , Tr 3 , and Tr 4  is fabricated. 
     Second Embodiment 
       FIG. 7  is a schematic diagram of a semiconductor device  70  fabricated under the control of film thickness making use of a gate oxidation process according to the second embodiment. In the semiconductor device  70 , different types of transistors Tr 1 , Tr 2 , Tr 3  and Tr 4  are mounted together with a flash memory cell transistor FL on the same chip. The effective thicknesses A, B, C and D of the epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d,  and the thicknesses of the gate oxide films  81 ,  82 ,  83  and  84  of the transistors Tr 1 , Tr 2 , Tr 3  and Tr 4  are varied making use of the gate oxide film forming process of the high-voltage transistors Tr 2  and Tr 4 . Accordingly, the transistors Tr 1  through Tr 4  with different threshold voltages and different operating voltages are fabricated simultaneously with the flash memory cell transistor FL. 
     In the respective areas, the depth of the highly doped impurity layer  38 , which may be referred to as the punch-through stop layer  38 , provided under the epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d  varies. Comparing Tr 1  with Tr 3 , the depth of the punch-through stop layer  38  located under the channel with the effective thickness A is greater than that of the punch-through stop layer  38  located under the channel with the effective thickness B (A&gt;B). The threshold voltage of Tr 3  is greater that of Tr 1 . 
       FIG. 8 ,  FIG. 9A  through  FIG. 9H  and  FIG. 10  illustrate a manufacturing process of the semiconductor device  70  according to the second embodiment. The process up to  FIG. 4H  is common between the first embodiment and the second embodiment, and explanation is made focusing on the NMOS transistor area after formation of the isolating region  51 . 
     In  FIG. 8 , the silicon oxide film  46  is removed by wet etching using a hydrofluoric acid. Then, wet thermal oxidation is performed on the surface of the epitaxial silicon layer  45  to form a tunneling oxide film (TN—OX)  71  with a thickness of 10 nm. The wet thermal oxidation may be performed under the conditions of 750° C., 65 minutes under ordinary pressure, while supplying nitrogen (N 2 ), oxygen (O 2 ) and hydrogen (H 2 ). 
     In  FIG. 9A , a phosphorus-doped amorphous silicon layer (D-aSi)  72  is formed over the entire surface by, for example, LPCVD, and patterned into a prescribed shape. The pattered amorphous silicon layer  72  becomes a floating gate. Then, a silicon oxide film  73  with a thickness of 5 nm is formed by LPCVD at 750° C. A silicon nitride film  74  with a thickness of 10 nm is formed over the silicon oxide film  73  at 700° C. Then silicon oxide film  75  is formed over the silicon nitride film  74  by low-pressure radical oxidation at 750° C. The layered structure of the oxide film  75 , nitride film  74  and the oxide film  73  is referred to as an “ONO (oxide-nitride-oxide) film.” 
     In  FIG. 9B , unnecessary portions of the ONO film are removed from the area other than the flash memory cell area. 
     In  FIG. 9C , wet thermal oxidation is performed on the surface of the epitaxial silicon layer  45  at 800° C. for 30 minutes to form a silicon oxide film (Gox1) 76 with a thickness of 11 nm. During the oxidation, the thickness of the epitaxial silicon layer  45  is reduced and epitaxial silicon layer  45   I  remains. Because the thickness of the silicon oxide film (Gox 1 )  76  is greater than that of Gox 1  of the first embodiment, the change in the thickness of the epitaxial silicon layer  45  to epitaxial silicon layer  45   I  is also different from the first embodiment. 
     In  FIG. 9D , a portion of the silicon oxide film  76  is removed by photolithography and hydrofluoric acid wet etching using a photoresist mask  78 . This step corresponds to oxide film etching  1  (S 3 ) of  FIG. 2 . 
     In  FIG. 9E , the photoresist mask  78  is removed, and wet thermal oxidation is performed on the surface of the epitaxial silicon layer  45   I  to form a silicon oxide film (Gox 2 )  79  with a thickness of 7 nm. The wet thermal oxidation may be performed under the conditions of 800° C. and 20 minutes. This step corresponds to gate oxidation  2  (S 4 ) of  FIG. 2 . The surface area of the epitaxial silicon layer  45   I  is oxidized to a certain depth and becomes a part of the silicon oxide film  79 . The epitaxial silicon layer  45   I  of this area becomes an epitaxial silicon layer  45   II  with a reduced thickness. On the other hand, in the area that has been covered with the photoresist mask  78 , the second silicon oxide film (Gox 2 ) is formed over the silicon oxide film (Gox 1 )  76 . As a result, a silicon oxide film (Gox 1 +Gox 2 )  80  is produced. The increase in the thickness of the silicon oxide film  76  to the silicon oxide film  80  is equal to or less than the increase in the thickness of the silicon oxide film  53  in the first embodiment. 
     In  FIG. 9F , a part of the silicon oxide film  79  and a part of the silicon oxide film  80  are removed by photolithography and hydrofluoric acid wet etching, using a photoresist film  82  as a mask. Then the photoresist film  82  is removed by ashing. This step corresponds to oxide film etching  2  (S 5 ) in  FIG. 2 . 
     In  FIG. 9G , a silicon oxide film (Gox 3 ) with a thickness of 2 nm is formed over the entire surface by, for example, thermal oxidation under the conditions of 810° C. and 8 seconds. At this point of time, silicon oxide films with different thicknesses, namely, the silicon oxide film (Gox 3 )  81 , the silicon oxide film (Gox 1 +Gox 2 +Gox 3 )  82 , the silicon oxide film (Gox 3 )  83 , and the silicon oxide film (Gox 2 +Gox 3 )  84  are formed in the associated areas. During this thermal oxidation, the surface areas of the epitaxial silicon layers  45   I  and  45   II  that have been exposed from the silicon oxide films  79  and  80  are oxidized to a certain depth, and epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d  with different thicknesses are produced. The epitaxial silicon layer  45   d  is thinner than the epitaxial silicon layer  45   b,  and the epitaxial silicon layer  45   c  is thinner than the epitaxial silicon layer  45   d.  The epitaxial silicon layer  45   a  is thinner than the epitaxial silicon layer  45   b.    
     Then, thermal treatment is performed in the nitric monoxide (NO) atmosphere at 870° C. for 13 seconds to introduce nitrogen into the silicon oxide films  81 ,  82 ,  83  and  84 , and another thermal treatment is performed at 1050° C. for 3 seconds. 
     In  FIG. 9H , a non-doped polycrystalline silicon film with a thickness of 100 nm is formed over the entire surface by LPCVD at 605° C. A stacked gate  87  including the polycrystalline silicon film  85  is fabricated for the flash memory cell. The stacked gate  87  is oxidized by low-pressure radical oxidation at 750° C., and silicon nitride sidewall spacers  88  are formed. Then the polycrystalline silicon film is patterned into a prescribed shape in the area other than the flash memory cell area to form gate electrodes  86  in the corresponding transistor fabrication areas. 
     In  FIG. 10 , a silicon oxide film with a thickness of 80 nm is formed over the entire surface by CVD at 520° C. Anisotropic etching is performed on the silicon oxide film to selectively form sidewall spacers  89  on the side walls of the gate electrodes  86  and the stacked gate  87 . Then, ion implantation is performed selectively using the gate electrode  86  and  87  and the sidewall spacers  88  and  89  as a mask to form impurity regions  91 , which regions becomes source and drain regions in the subsequent thermal process. The impurities are also injected in the polycrystalline silicon gates electrode  85  and  86 . Then thermal treatment is performed for a short time in the inactive gas atmosphere for activation and diffusion of the impurities. The impurities diffuse into the gate electrodes  85  and  86 . Thus, the semiconductor device  70  is fabricated. 
     In the second embodiment, the thickness of the epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d,  and the thickness of the gate oxide films  81 ,  82 ,  83  and  84  of the transistors Tr 1 , Tr 2 , Tr 3  and Tr 4  are varied making use of oxidation processes. Different types of transistors with different threshold voltages and operating voltages can be mounted on the same chip. 
     The silicon oxide film (Gox 1 ) formed by the process of gate oxidation  1  is thicker than that of the first embodiment. Accordingly, the difference in the film thickness can be increased between the epitaxial silicon layers  45   a,    45   b,    45   c  and  45   d,  as compared with the first embodiment. 
     Third Embodiment 
       FIG. 1  through  FIG. 16  illustrate control on the film thickness of an epitaxial silicon layer according to the third embodiment.  FIG. 11  is a schematic diagram of a transistor to which the third embodiment is applied. A non-doped epitaxial silicon layer  103  is positioned under the gate electrode  106  formed over the impurity doped region  102 . A gate oxide film  105  is provided between the gate electrode  106  and the epitaxial silicon layer  103 . With this transistor structure, the thickness of the epitaxial silicon layer  103  varies depending on the wafer position in a furnace. The variations in the transistor characteristics among wafers or among lots become an issue. 
       FIG. 12  is a diagram illustrating variations in transistor characteristic due to fluctuation in the thickness of the epitaxial silicon layer  105 . The horizontal axis represents wafer position in a furnace during growth of epitaxial silicon layer  105 . The vertical axis of the left-hand side represents thickness of the epitaxial silicon layer  105 , and the vertical axis of the right-hand side represents threshold voltage of transistor. The thickness of the epitaxial silicon layers  105  is measured by an optical measuring system (ASET-F5x manufactured by KLA Tencor Corporation). The thickness of the epitaxial silicon layer  105  varies up to 0.7 nm among wafers depending on the wafer position in the furnace, and the threshold voltage characteristic varies up to 15 mV. In the third embodiment, the variations in the thickness of the epitaxial silicon layer  105  is reduced by controlling the thickness of the initial oxide film for each wafer. 
       FIG. 13  illustrates how the thickness of the epitaxial silicon layer  103  varies among wafers. The epitaxial silicon layer  103   a  grown on the wafer located at position A in the furnace has a thickness “a”. The epitaxial silicon layer  103   b  grown on the wafer located at position B in the furnace has a thickness “b” (a&gt;b). 
     As illustrated in  FIG. 14 , by forming a silicon oxide film  105   a  with a thickness “c” over the epitaxial silicon layer  103   a  with a thickness “a” on wafer W 1 , the epitaxial silicon layer  103   a  turns to epitaxial silicon layer  103   e  with a reduced thickness “e”. By forming a silicon oxide film  105   b  with a thickness “d” (c&gt;d) is formed over the epitaxial silicon layer  103   b  with a thickness “b” (a&gt;b) on wafer W 2 , the epitaxial silicon layer  103   b  turns to epitaxial silicon layer  103   f  with a reduced thickness “f”. The thickness “c” of the silicon oxide film (i.e., the initial oxide film)  105   a  and the thickness “d” of the silicon oxide film (i.e., the initial oxide film)  105   b  are controlled such that the thickness “e” of the epitaxial silicon layer  103   e  becomes equal to the thickness “f” of the epitaxial silicon layer  103   f.    
       FIG. 15  illustrates a concentration profile of dopants in the depth direction of a wafer on which the transistor illustrated in  FIG. 11  is fabricated. Using the above-stated optical measuring system (ASET-F 5   x  manufactured by KLA Tencor Corporation), the thickness of the highly doped impurity layer  102  and the thickness of the epitaxial silicon layer  103  are measured separately. Based upon the difference between the actually measured value and the designed value of the epitaxial silicon layer  103 , the thickness of the initial oxide film  105  is determined. The thickness of the initial oxide film  105  can be adjusted by controlling the process conditions, such as processing time. 
       FIG. 16  is a diagram illustrating a relationship between thickness of non-doped silicon layer epitaxially grown over the highly doped impurity layer  102  and thickness of initial oxide film  105  to be formed by oxidation. For example, a target thickness of the epitaxial silicon layer  103  is acquired under the conditions that a non-doped silicon layer with a thickness of 25.2 nm is grown and then an initial oxide film  105  with a thickness of 3.0 nm is formed by oxidation. The thickness of the initial oxide film  105  is determined, while changing the thickness of the non-doped silicon layer, such that the thickness of the post-oxidation epitaxial silicon layer  103  becomes constant. 
     In the example of  FIG. 16 , assuming that the thickness of the initial oxide film  105  is d [nm] and that the thickness of the non-doped silicon layer grown over the highly doped impurity layer  102  is t [nm], the equation
 
 d= 2 *t− 47.4
 
holds. If the amount of silicon consumption during oxidation changes, then the slope of the graph changes.
 
     Taking the silicon consuming rate during the oxidation into account,
 
( d− 3.0)*(Si consuming rate in oxidation)= t− 25.2
 
holds. Since the oxidation rate is known, the thickness d of the initial oxide film  105  is controlled by adjusting processing time.
 
     In this way, the epitaxial silicon layer  103  with the target thickness can be formed by performing the following steps. 
     (a) Acquiring reference data (e.g., the data of  FIG. 16 ) representing the relationship between the thickness of the non-doped silicon layer to be grown and the thickness of the initial oxide film under prescribed oxidation conditions. 
     (b) Measuring the thickness of the silicon layer of each wafer in a furnace. 
     (c) Determining the thickness of the initial oxide film  105  based upon the reference data. 
     (d) Carrying out oxidation according to the determined thickness of the initial oxide film. 
     Throughout the first through the third embodiments, the thickness of the epitaxial silicon layer can be controlled making use of an oxidation process, without introducing additional photolithography and etching steps. In the first embodiment, various types of transistors with different threshold voltage characteristics are mounted on the same chip by making use of a gate oxidation process during fabrication of high-voltage transistors. The degree of freedom of circuit design is increased. In the second embodiment, the thickness difference of epitaxial silicon layers between transistors can be increased, compared with the first embodiment, by increasing the thickness of the gate oxide film formed by oxidation. Consequently, transistors with a greater characteristic difference can be mounted on the same chip. In the third embodiment, variations in the transistor characteristic due to thickness fluctuation of non-doped silicon layers during epitaxial growth can be cancelled by making use of an oxidation process. Consequently, characteristic variations between wafers and between lots can be reduced. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.