Patent Publication Number: US-6699758-B2

Title: Semiconductor device and method for manufacturing the same

Description:
This is a Divisional of application Ser. No. 09/173,616 filed Oct. 16, 1998, now U.S. Pat. No. 6,340,829. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method for manufacturing the same, and more particularly to a technique for improvement in characteristics of an input protection circuit in an LSI. 
     2. Description of the Background Art 
     In a MOSFET, a well-known method for effectively enhancing a proof stress against variation in voltage amplitude of an input signal which is applied to a source/drain region is formation of a silicide protection. This method is, for example, to form an SiO 2  film over a gate electrode and side walls and then forming silicide on a surface portion of an n + -type layer uncovered with the SiO 2  film as shown in FIG. 33, thereby raising a resistance of the source/drain region near the gate electrode without forming silicide on the region. 
     In a MOS structure using SOI (Silicon On Insulator), however, since the SOI layer is very thin (e.g., its thickness is about 1000 Å), the SOI layer is etched in an etching to form the SiO 2  film for silicide protection. If the SOI layer is etched, too, part of the SOI layer becomes thin because of level difference and the silicide layer which should originally extend from a surface of the SOI layer to the inside thereof reaches an interface between the SOI layer and a buried oxide film, to disadvantageously cause a leakage current and remove the silicide film. 
     This will be discussed below with reference to cross-sectional step views of FIGS. 29 to  35 . 
     In order to avoid silicification of the source/drain region near the gate electrode by covering the region with the SiO 2  film, usually, a series of steps shown in FIGS. 29 to  33  are carried out. Specifically, a gate electrode and source/drain regions are formed as shown in FIG. 29, and an SiO 2  film is deposited as shown in FIG.  30 . Next, a resist is formed on a portion of the SiO 2  film which is to serve as the silicide protection portion as shown in FIG. 31, and a dry etching is performed to form an SiO 2  film to serve as the silicide protection portion. After that, the unnecessary resist is removed. Then, a silicide layer is formed as shown in FIG.  34 . 
     Since the Si layer as the SOI layer is very thin, about 1000 Å, the Si layer is also etched in the dry etching and as a result a level difference as shown in FIG. 34 is created locally in a surface of the Si layer. In this condition, when silicide is formed on a prescribed uncovered portion of the source/drain region, the buried oxide film and a silicide layer come into contact as shown in FIG.  35 . Since the silicide layer weakly adheres to the buried oxide film in this condition, there is a possibility that the silicide layer may be removed depending on the strength of thermal stress applied in later steps. Further, even if the silicide layer is not removed, a leakage current may be produced between two silicide layers through the buried oxide film, and therefore there may arise an appreciable influence on characteristics of a transistor such as malfunction in a transistor operation. 
     Regarding this point, Japanese Patent Application Laid Open Gazette 64-20663 discloses that in a dry etching for forming a side wall of a gate electrode of a MOS transistor, an Si 3 N 4  film is formed as an etching stopper film in advance on a surface of a semiconductor layer to cover both sides of the gate electrode and a gate insulating film and then a side wall is so formed as to cover the Si 3 N 4  film. This prior art, however, which essentially suggests a side wall of double-layered structure consisting of the Si 3 N 4  film and the SiO 2  film, can not be an effective solution of the above problem. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a semiconductor device. According to a first aspect of the present invention, the semiconductor device comprises: an underlying layer; a semiconductor layer provided on a surface of the underlying layer; a gate insulating film provided on a first region in a flat surface of the semiconductor layer; a gate electrode provided on a surface of the gate insulating film; a side wall provided on second and third regions adjacent to the first region in the flat surface of the semiconductor layer, covering side surfaces of the gate insulating film and side surfaces of the gate electrode; a first insulating film provided on fourth and fifth regions adjacent to the second and third regions, respectively, in the flat surface of the semiconductor layer, on surfaces of the side walls and on a surface of the gate electrode; a second insulating film provided on a surface of the first insulating film, being different in material from the first insulating film; a first impurity layer of the first conductivity type extending from a center portion of the first region to the inside of the semiconductor layer; a second impurity layer of the second conductivity type adjacent to the first impurity layer, extending from one of peripheral portions of the first region, the second region, the fourth region, a sixth region externally adjacent to the fourth region to the inside of the semiconductor layer; a third impurity layer of the second conductivity type adjacent to the first impurity layer, extending from the other of the peripheral portions of the first region, the third region, the fifth region, a seventh region externally adjacent to the fifth region to the inside of the semiconductor layer; a first silicide layer provided on the sixth region and inside the second impurity layer located immediately below the sixth region, of which a bottom surface is located inside the second impurity layer; and a second silicide layer provided on the seventh region and inside the third impurity layer located immediately below the seventh region, of which a bottom surface is located inside the third impurity layer. 
     According to a second aspect of the present invention, the semiconductor device of the first aspect further comprises: a third insulating film provided on a surface of the second insulating film. 
     According to a third aspect of the present invention, in the semiconductor device of the second aspect, the first insulating film and the third insulating film are of the same material. 
     According to a fourth aspect of the present invention, in the semiconductor device of the first aspect, the first insulating film is an SiO 2  film. 
     According to a fifth aspect of the present invention, in the semiconductor device of the first aspect, the first insulating film is an Si 3 N 4  film. 
     According to a sixth aspect of the present invention, the semiconductor device comprises: a semiconductor layer; a gate insulating film formed on a surface of the semiconductor layer; a gate electrode formed on a surface of the gate insulating film; a side wall formed on the surface of the semiconductor layer to cover side surfaces of the gate insulating film and side surfaces of the gate electrode; and first and second insulating layers formed on the surface of the semiconductor layer in this order by dry etching to cover surfaces of the side walls and a surface of the gate electrode, and in the semiconductor device, an etching rate of the second insulating layer is set larger than that of the first insulating layer in the dry etching. 
     According to a seventh aspect of the present invention, in the semiconductor device of the sixth aspect, a portion of the first insulating layer uncovered with the second insulating layer after the dry etching is removed by wet etching. 
     According to an eighth aspect of the present invention, in the semiconductor device of the seventh aspect, the first insulating layer comprises first and second insulating films of different materials, and the etching rate of the second insulating layer is set larger than that of the second insulating film adjacent to the second insulating layer. 
     The present invention is also directed to a method for manufacturing a semiconductor device. According to a ninth aspect of the present invention, the method comprises the steps of: (a) providing a semiconductor layer, a gate insulating film formed on a surface of the semiconductor layer, a gate electrode formed on a surface of the gate insulating film, a side wall formed on the surface of the semiconductor layer to cover side surfaces of the gate insulating film and side surfaces of the gate electrode; (b) forming a first insulating layer on surfaces of the side wall, a surface of the gate electrode and an exposed portion of the surface of the semiconductor layer; (c) forming a second insulating layer on a surface of the first insulating layer; (d) forming a resist layer on a surface of the second insulating layer and patterning the resist layer so as to be located above a surface region within a predetermined range surrounding a region in which the side wall is provided in the surface of the semiconductor layer; (e) etching the second and first insulating layers by dry etching with an etching rate of the second insulating layer set larger than that of the first insulating layer; (f) removing an exposed portion of the first insulating layer uncovered with the second insulating layer after the dry etching by wet etching; and (g) removing the resist layer. 
     According to a tenth aspect of the present invention, in the method of the ninth aspect, the first insulating layer comprises a first insulating film and a second insulating film, the etching rate of the second insulating layer is set larger than that of the second insulating film, the step (b) comprises the steps of (b-1) forming the first insulating film equivalent in material to the second insulating layer on the surface of the semiconductor layer; and (b-2) forming the second insulating film different in material from the second insulating layer on a surface of the first insulating film, and the step (f) comprises the steps of (f-1) removing an exposed portion of the second insulating film after the dry etching by a first wet etching; and (f-2) removing an exposed portion of the first insulating film after the first wet etching by a second wet etching. 
     According to an eleventh aspect of the present invention, in the method of the tenth aspect, the second insulating film is an SiO 2  film. 
     According to a twelfth aspect of the present invention, in the method of the tenth aspect, the second insulating film is an Si 3 N 4  film. 
     According to the present invention, the semiconductor device, which has a silicide protection portion covering a gate insulating film, a gate electrode and a side wall covering the side surfaces of the gate insulating film and the gate electrode, has a characteristic feature that the silicide protection portion is of layered structure, consisting essentially of a plurality of insulating films. 
     In the semiconductor device of the first to fifth aspects, since a plurality of insulating films are sequentially provided on the semiconductor layer, the surface of the semiconductor layer has no level difference and is flat, and the film thickness of the semiconductor layer is uniform from the first region to the sixth region. Therefore, the bottom surface of the silicide layer provided on the sixth region and in the second impurity layer never reaches the underlying layer and the problems such as the generation of leakage current and the removal of silicide film never arise. 
     The semiconductor device of the fourth or fifth aspect has an advantage of using a flexible and practicable insulating film, such as an SiO 2  film or an Si 3 N 4  film, as a base. 
     In the semiconductor device of the sixth aspect, since such a selection ratio as (the etching rate of the second insulating layer)&gt;(the etching rate of the first insulating layer) is used in the dry etching, the portion of the second insulating layer to be etched is removed and then the etching is stopped at the surface of the first insulating layer. Specifically, the first insulating layer can be used as an etching stopper layer and it is possible to prevent the surface of the semiconductor layer from being etched by dry etching in forming the first and second insulating layers. 
     In the semiconductor device of the seventh aspect, since only the portion of the first insulating layer uncovered with the second insulating layer is removed by wet etching, a structure where the first insulating layer is formed on the surface of the semiconductor layer and the second insulating layer is formed on the surface of first insulating layer is obtained. Therefore, the exposed surface of the semiconductor layer after the wet etching is not etched and a flat surface of the semiconductor layer can be obtained. That makes it possible, for example, to form the silicide layer only on the surface of the semiconductor layer and inside the semiconductor layer when the silicide layer is formed in the semiconductor device. 
     In the semiconductor device of the eighth aspect, an etching stopper layer of triple-layered structure is achieved. 
     By the method of the ninth to eleventh aspects, (i) since the first insulating layer can work as the stopper layer for the dry etching, it is possible to prevent the surface of the semiconductor layer from being etched in the dry etching step, and (ii) since the exposed portion of the first insulating layer after the step (e) is removed by wet etching, a flat surface of the semiconductor layer that has never been etched through the process can be eventually obtained. Since the silicide layer can be thereby formed only inside the semiconductor layer without coming into contact with the interface when the silicide layer is further formed in the semiconductor device, the problems such as the generation of leakage current and the removal of silicide film never arise. 
     The method of the tenth aspect has an advantage that the second insulating film far away from the surface of the semiconductor layer can work as the stopper layer for dry etching. 
     Since the method of eleventh and twelfth aspects use the SiO 2  film and the Si 3 N 4  film as a base, it is possible to provide a flexible and practicable manufacturing technique. 
     In present invention, since the silicide protection causes no difference in level to be created in the surface of the semiconductor, a flat surface of the semiconductor layer can be obtained and a god silicide layer can be obtained. 
     An object of the present invention is to provide a semiconductor device in which a surface region of a semiconductor layer for forming a silicide layer and that for forming a silicide protection portion are even as one surface, and to provide a method for manufacturing the semiconductor device. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section showing a semiconductor in accordance with a first preferred embodiment of the present invention; 
     FIG. 2A is a plan view showing a semiconductor in accordance with the preferred embodiment of the present invention; 
     FIGS. 2B-2D are cross-sectional views of the device shown in FIG. 2A; 
     FIGS. 3 and 4 are cross sections showing a first example; 
     FIGS. 5 and 6 are cross sections showing a second example; 
     FIGS. 7 and 8 are cross sections showing a third example; 
     FIGS. 9 and 10 are cross sections showing a fourth example; 
     FIGS. 11 to  28  are cross sections showing steps for manufacturing a semiconductor device in accordance with a second preferred embodiment of the present invention; and 
     FIGS. 29 to  35  illustrate problems of the background art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The First Preferred Embodiment 
     FIG. 1 is a cross section showing a structure of a MOSFET on an SOI layer which is an exemplary semiconductor device in accordance with the present invention, and FIG. 2A is a plan view of the MOSFET of FIG.  1 . As understood from FIGS. 1 and 2A, FIG. 1 is a cross section taken along the line I-II of FIG.  2 A. FIGS. 2B-2D are cross sectional views of the device shown in FIG. 2A, drawn through respective lines IIB—IIB, IIC—IIC, and IID—IID. 
     FIGS. 1 and 2A show an Si single crystal wafer  1 , an underlying layer  2  which is a buried oxide film (BOX) and an SOI (Silicon On Insulator) layer or semiconductor layer  3  provided on a surface  2 S of the underlying layer  2 . Thus, in the first preferred embodiment, the SOI layer  3  is formed by SIMOX. Specifically, atoms of oxygen are injected into the Si single crystal wafer from its surface and the wafer is thereafter annealed at a constant temperature to diffuse the injected atoms of oxygen inside the wafer from its surface and the vicinity, thereby forming the SiO 2  film having a thickness of several thousand Å inside the wafer. As a result, the SOI layer  3  as the semiconductor layer having a thickness of about 1000 Å is formed between the surface  2 S of the buried oxide film  2  and a surface  3 S of the Si single crystal wafer  1 . 
     On the flat surface  3 S of the SOI layer  3  and inside the SOI layer  3 , the MOSFET is provided. Specifically, a gate insulating film  4  is formed on a first region R 1  (a center portion R 1 C, other than peripheral portions, which occupies most of the region R 1 , corresponds to a surface region of a p −  layer  9  discussed later) in the uniformly flat surface  3 S, and further, a gate electrode or a control electrode  6  of polysilicon and the like is formed on a surface  4 S of the film  4 . A side wall  5  is so formed as to cover side surfaces  6 W of the gate electrode  6  and side surfaces  4 W of the gate insulating film  4  on a second region R 2  and a third region R 3  adjacent to the first region R 1  inside the flat surface  3 S and on regions R 8  and R 9  where no p −  layer  9  is formed shown in FIG.  2 A. Further, the first impurity layer  9  (p −  layer) having first impurities of the first conductivity type (p type) at relatively low concentration is formed inside the SOI layer  3 , extending from the center portion R 1 C of the first region R 1  in the flat surface  3 S to the second surface  2 S of the buried oxide film  2  immediately therebelow. A second impurity layer  10  (e.g., source region) having second impurities of the second conductive type (n type) is so formed as to be adjacent to the first impurity layer  9 , extending from the regions in the flat surface  3 S, specifically one of peripheral portions RS 1  of the first region R 1  adjacent to the center portion R 1 C, the second region R 2 , a fourth region R 4  externally adjacent to the second region R 2  and a sixth region R 6  externally adjacent to the fourth region R 4 , to the surface  2 S of the buried oxide film  2 . 
     The second impurity layer  10  consists of {circle around (1)} an n −  layer  14  having the second impurities at relatively low concentration, extending from the above one peripheral portion RS 1  of the first region R 1  and a portion R 21  of the second region R 2  having an interface with the side wall  5  to the inside of the layer  10  and {circle around (2)} an n +  layer  12  having the second impurities at relatively high concentration, extending from the remainder of the second region R 2  (externally adjacent to the portion R 21 ) and the fourth region R 4  in the flat surface  3 S adjacent to the remainder of the region R 2  to the inside of the layer  10 . Further, a third impurity layer (e.g., drain region) having the second impurities of the second conductivity type is so formed as to sandwich the first impurity layer  9  with the second impurity layer  10 , extending from the regions in the flat surface  3 S, specifically the other peripheral portion RS 2  of the first region R 1  adjacent to the center portion R 1 C, the third region R 3 , a fifth region R 5  externally adjacent to the region R 3  and a seventh region R 7  externally adjacent to the fifth region R 5 , to the surface  2 S of the buried oxide film  2 . 
     A silicide protection portion  8  consisting of a plurality of layers is so layered on the flat surface  3 S as to cover the gate electrode  6  and the side wall  5 . Specifically, a first insulating film  81  is formed on the fourth region R 4  and the fifth region R 5  in the flat surface  3 S having no difference in level, surfaces  5 W of the side wall  4  and a surface  6 S of the gate electrode  6 , and a second insulating film  82  different in material from the film  81  is so formed on a surface  81 S of the film  81  as to cover the surface  81 S. The films  81  and  82  are made of different materials. The silicide protection portion  8  has the layered structure consisting of a plurality of insulating films and covers the regions R 1  to R 5  in the flat surface  3 S which are not covered with a silicide layer discussed later. The portion  8  does not reach high-resistance portions  16  and  17  immediately below the regions R 4  and R 5  of the n +  layers  12  and  13  in the second and third impurity layers  10  and  11 . Therefore, the high-resistance portions  16  and  17  of the n +  layers  12  and  13  each have the same thickness as the layers  9 ,  14  and  15  in the SOI layer  3 . The thickness is represented as d 1  in FIG.  1 . 
     A first silicide layer  71  which is a silicified n +  layer is formed on the sixth region R 6  in the flat surface  3 S and in the n +  layer  12  of the second impurity layer  10  to a depth of d 2  (&lt;d 1 ) from the region R 6 . A second silicide layer  72  which is a silicified n +  layer is formed on the seventh region R 7  in the flat surface  3 S and in the n +  layer  13  to the depth of d 2  from the region R 7 . Thus, bottom surfaces  7 B of the first and second silicide layers  71  and  72  are located in the n +  layers  12  and  13  without coming into contact with the surface  2 S, and the first and second silicide layers  71  and  72  serve as respective low-resistance portions (the sheet resistance is e.g., 10 Ω/□) of the second and third impurity layers. The sheet resistance of high-resistance portion in the n +  layer  12  and  13  where no silicide layer  71  or  72  is formed is e.g., 100 Ω/□. In the semiconductor device of this preferred embodiment, no portion in the SOI layer  3  is ever etched and the SOI layer  3  has a uniform thickness d 1 . And the silicide layers  71  and  72  do not reach the interface (surface  2 S) between the SOI layer  3  and the underlaying layer  2 . Therefore, the problems in the prior art, such as the generation of leakage current and the removal of silicide film are not caused at all. 
     Further, as indicated by a broken line in FIG. 1, an insulating film equivalent in quality to the first insulating film  81  may be formed as a third insulating film  83  on a surface  82 S of the second insulating film  82 . Furthermore, more insulating films, e.g., a fourth insulating film, a fifth insulating film and so on, may be used to constitute the silicide protection portion  8  of a plurality of films. These structures can produce the same advantages as that of FIG.  1 . 
     The above-discussed structure of the semiconductor device of this preferred embodiment, especially the structure of the silicide protection portion  8 , can be specified as below from the viewpoint of manufacturing process. Specifically, the first and second insulating films  81  and  82  of FIG. 1 are formed in this order as first and second insulating layers to cover the surfaces  5 S of the side wall  5 , the surface  6 S of the gate electrode  6  and the uncovered flat surface  3 S. Further, the second insulating layer is first anisotropically etched by dry etching and the etched second insulating layer covers the surfaces  5 S and  6 S and the regions R 4  and R 5 . After that, a wet etching is performed on a range from an exposed surface of the first insulting film through the first insulating layer immediately therebelow to remove the portion. The films  81  and  82  finally have the above-formed structure. In the above dry etching, a selection ratio of the etching rates of the first and second insulating films is so set as to hold such a relation as (the etching rate of the second insulating film)&gt;(the etching rate of the first insulating film). With this setting, the dry etching is stopped at the surface of the first insulating layer and the flat surface  3 S is not dry-etched at all, keeping the flatness as it was before the dry etching also after the dry etching and the following wet etching. 
     Further, if the third insulating film  83  of FIG. 1 is formed, it is necessary to set the selection ratio, by appropriately choosing the etchant, so as to hold such a relation as (the etching rate of the third insulating film  83 )&gt;(the etching rate of the second insulating film  82 ). In this sense, it may be considered that the third insulating film  83  acts as “the second insulating layer” and a combination unit of the second and first insulating films  82  and  81  act as “the first insulating layer”. 
     As the first and third insulating films  81  and  83  of FIG. 1, an SiO 2  film, for example, having flexibility may be used. In this case, as the second insulating film  82 , an SiN film, a polysilicon film undoped with impurity or other insulating film may be used. 
     Conversely, when an Si 3 N 4  film which also has flexibility is used as the first and third insulating films  81  and  83 , an SiO 2  film, a polysilicon film undoped with impurity or other insulating film may be used. 
     Specific examples will be discussed below. 
     In a structure of the silicide protection portion  8  of FIG. 3, an Si 3 N 4  film  8 B is inserted into the SiO 2  film which corresponds to the prior-art silicide protection portion, being sandwiched by SiO 2  films  8 A 1  and  8 A 2 . 
     Such a structure can be achieved by setting the selection ratio of the SiO 2  film  8 A 2  and the Si 3 N 4  film  8 B as follows in the dry etching for the silicide protection portion  8  of layered structure of FIG.  4 . Specifically, when the etching rate of the Si 3 N 4  film  8 B is set smaller than that of the SiO 2  film  8 A 2  in the dry etching, the anisotropic etching is performed on the SiO 2  film  8 A 2  and then stopped at a surface of the Si 3 N 4  film  8 B. After that, an exposed portion of the Si 3 N 4  film  8 B other than a portion to form the silicide protection portion  8  is removed by wet etching with thermal phosphoric acid and a portion of the SiO 2  film  8 A 1  immediately therebelow is removed by wet etching, to form the silicide protection portion  8 . 
     Each thickness of the SiO 2  films  8 A 1  and  8 A 2  and the Si 3 N 4  film  8 B is in a range of several hundred Å to several thousand Å. 
     In the silicide protection portion  8  of FIG. 5, the Si 3 N 4  film  8 B is provided as the first insulating film and the SiO 2  film  8 A is provided thereon as the second insulating film. 
     To achieve such a double-layered structure, a dry etching is performed on the SiO 2  film  8 A with the etching rate of the Si 3 N 4  film  8 B of FIG. 6 set smaller than that of the SiO 2  film  8 A, line the example 1. After removing a portion of the SiO 2  film not to form the silicide protection portion  8  by anisotropic etching, the dry etching is stopped at the surface of an exposed surface of the Si 3 N 4  film  8 B and a portion of the Si 3 N 4  film  8 B other than the silicide protection portion  8  is removed by wet etching with thermal phosphoric acid. Thus, the silicide protection portion  8  of FIG. 5 is formed. 
     Each thickness of the SiO 2  film  8 A and the Si 3 N 4  film  8 B is in a range of several hundred Å to several thousand Å. 
     In a structure of the silicide protection portion  8  of FIG. 7, the SiO 2  film  8 A is provided as the first insulating film under the Si 3 N 4  film  8 B as the second insulating film. 
     In the example 3, contrary to the examples 1 and 2, the etching rate of the SiO 2  film  8 A is set smaller than that of the Si 3 N 4  film  8 B, and the SiO 2  film  8 A and the Si 3 N 4  film  8 B which are layered in the order of FIG. 8 are dry-etched. The dry etching is thereby stopped at the surface of the SiO 2  film  8 A, and a portion of the SiO 2  film  8 A other than the silicide protection portion  8  is removed by wet etching, to form the silicide protection portion  8  of FIG.  7 . 
     Each thickness of a lower layer, the SiO 2  film  8 A, and an upper layer, the SiN film  8 B, is in a range of several hundred Å to several thousand Å. 
     In a structure of the silicide protection portion  8  of FIG. 9, contrary to the example 1, the Si 3 N 4  film  8 B 1  as the first insulating film and the Si 3 N 4  film  8 B 2  as the third insulating film sandwich the SiO 2  film  8 A as the second insulating film. It may be considered in this structure that the SiN film  8 B 2  acting as the second insulating layer is formed on a combination unit of the Si 3 N 4  film  8 B 1  and the SiO 2  film  8 A acting as the first insulating layer. 
     In this example, the selection ratio of the Si 3 N 4  film  8 B 2  and the SiO 2  film  8 A is so set as to hold such a relation as (the etching rate of the film  8 B 2 )&gt;(the etching rate of the film  8 A), to control an for a source/drain region (see FIG.  10 ). 
     Each thickness of the SiO 2  film  8 A and the Si 3 N 4  films  8 B 1  and  8 B 2  is in a range of several hundred Å to several thousand Å. 
     The Second Preferred Embodiment 
     An exemplary method for manufacturing a semiconductor device (MOSFET) having the silicide protection portion as discussed in the first preferred embodiment will be discussed with reference to the cross sections of FIGS. 11 to  24 . 
     (Preparation Step a) 
     FIG. 11 shows a step of injecting a channel doping ion. 
     First, the silicon single crystal wafer  1  is prepared as a support substrate, an SiO 2  film  2  having a thickness of about 4000 Å is formed by the above-mentioned SIMOX in the silicon wafer  1  as the buried oxide film (underlying layer) and then the SOI layer  3  having a thickness of about 1000 Å is formed. 
     After that, through the injection step of FIG. 11, the first impurity layer having the first impurities of the first conductivity type is formed in the SOI layer  3 . What is used as the first impurity and its injection concentration depend on whether an nMOSFET or a pMOSFET is to be formed. Specifically, in a case of formation of an nMOSFET, a p-type impurity is used as the first impurity of the first conductivity type to be injected, and the SOI layer  3  is implanted with, for example, arsenic having a concentration of about 4E13 cm −2  as the first impurity at an acceleration energy of 60 keV. On the other hand, in a case of information of a pMOSFET, an n-type impurity is used as the second impurity of the second conductivity type, and the SOI layer  3  is implanted with, for example, boron having a concentration of about 4E13 cm −2  at an acceleration energy of 10 keV. 
     FIG. 12 shows a step of depositing a gate oxide film (gate insulating film) and a polysilicon film for gate electrode. In this step, a gate oxide film  4 A having a thickness of about 70 Å and a polysilicon film  6 A having a thickness of about 2000 Å are layered in this order on the flat surface  3 S of the SOI layer  3 . 
     FIG. 13 shows a step of injecting doping ions to form a gate electrode. Through this step, the polysilicon film  6 A of FIG. 12 becomes a conductive polysilicon film  6 B. In a case of nMOSFET, the film  6 A is implanted with phosphorus having a concentration of about 5E15 cm −2  at an acceleration energy of 15 keV. On the other hand, in a case of pMOSFET, the film  6 A is implanted with boron having a concentration of about 5E15 cm −2  at an acceleration energy of 10 keV. 
     FIG. 14 shows a step of dry etching for gate patterning. Specifically, a resist pattern  20  is formed in accordance with the gate pattern and a dry etching is performed with this pattern  20  as a mask, to form the gate oxide film  4  as the gate insulating film and the polysilicon film  6  as the gate electrode as shown in FIG.  14 . 
     FIG. 15 shows a step of removing the resist pattern  20  of FIG.  14  and injecting impurity ions for the source/drain. 
     As the second impurity of the second conductivity type, an n-type impurity (e.g., arsenic: 60 keV, about 4E13 cm −2 ) is used in an nMOSFET, and a p-type impurity (e.g., boron: 10 keV, about 4E13 cm −2 ) is used in a pMOSFET. 
     FIG. 16 shows a step of depositing a SiO 2  film  5 A for the side wall. 
     In this step, the SiO 2  film  5 A having a thickness of 800 Å is formed on the flat surface  3 S of the SOI layer  3 , side surfaces of the gate oxide film  4  and surfaces of the polysilicon film  6  for the gate electrode. 
     FIG. 17 shows the second injection step of impurity ions for the source/drain after the dry etching for the SiO 2  film  5 A for the side wall. Specifically, the SiO 2  film  5 A of FIG. 16 is dry-etched to form the side wall  5  for covering both side surfaces of the gate insulating film  4  and the gate electrode  6  and after that, the second impurities (ions) of the second conductivity type are injected into the source/drain regions. For example, in the case of the nMOSFET shown in FIG. 17, arsenic ions having a concentration of about 4E15 cm −2  are injected at an acceleration energy of 60 keV. On the other hand, in the case of a pMOSFET (not shown), boron ions having a concentration of about 4E15 cm −2  are injected at an acceleration energy of 10 keV. By this injection, portions uncovered with the side wall  5  in the second and third impurity layers serving as the source/drain regions become highly-concentrated impurity layers (n + ). 
     (The First Insulating Layer Formation Step b) 
     FIG. 18 shows a state where the first insulating layer  81 A is formed. 
     Specifically, the first insulating layer  81 A is formed on the surfaces of the side wall  5 , the surface of the gate electrode  6 , and an exposed portion of the flat surface  3 S of the SOI layer  3  uncovered with the side wall  5 . The layer  81 A is an SiO 2  film or an Si 3 N 4  film. If the Si 3 N 4  film is used, the Si 3 N 4  film must have a thickness of 3000 Å or less, lest an stress between the Si 3 N 4  film and the SiO 2  film becomes large. 
     (The Second Insulating Layer Formation Step c) 
     Next, as shown in FIG. 19, the second insulating layer  82 A is layered on a surface of the first insulating film  81 A. When an SiO 2  film, for example, is used as the first insulating layer  81 A, an Si 3 N 4  film, an undoped polysilicon film or other insulating film is used as the second insulating layer  82 A. Conversely, when the Si 3 N 4  film is used as the first insulating layer  81 A, an SiO 2  film, an undoped polysilicon film or other insulating film is used as the second insulating layer  82 A. 
     (Resist Patterning Step d) 
     A resist layer (not shown) is formed on an entire surface of the second insulating layer  82 A, and as shown in FIG. 20, the resist layer is patterned to form a resist layer  21  which is located above a portion SR 1  covered with the side wall  5  and the gate insulating film  4  in the flat surface  3 S of the SOI layer  3  and above a surface region SR 2  within a predetermined range surrounding the portion SR 1 . 
     (Dry Etching Step e) 
     With the selection ratio so set as to satisfy such a condition as (the etching rate of the second insulating layer  82 A)&gt;(the etching rate of the first insulating layer  81 A), a normal dry etching is performed. As a result, as shown in FIG. 21, a portion of the second insulating layer  82 A uncovered with the resist layer  21  is removed by the dry etching and the dry etching is stopped at an exposed surface of the first insulating layer  81 A. Therefore, not only the first insulating layer  81 A but also the SOI layer  3  immediately therebelow is prevented from being dry-etched, and the surface  3 S of the SOI layer  3  keeps flat. 
     (Wet Etching Step f) 
     Next, with thermal phosphoric acid, a wet etching is performed on an exposed portion of the first insulating layer  81 A uncovered with the second insulating layer  82  after the dry etching to remove the portion. By these steps, only the first and second insulating layers  81  and  82  covered with the resist layer  21  are left on the surface  3 S. Since the flat surface  3 S is not wet-etched in this step, however, no difference in level is created on the flat surface  3 S and no change in flatness of the surface  3 S and thickness of the SOI layer  3  is made. 
     (Resist-Layer Removing Step g) 
     Subsequently, the resist layer  21  is removed. By this removal, the silicide protection portion  8  consisting of the first and second insulating layers  81  and  82  is formed on the flat surface  3 S to cover the gate electrode  6  and the side wall  5  as shown in FIG.  22 . 
     (Silicification Step h) 
     As shown in FIG. 23, cobalt is sputtered on the exposed flat surface  3 S and the surface of the silicide protection portion  8 , to form a cobalt layer  70  (having a thickness of 120 Å). After that, this device is annealed (lamp annealing) in N 2  gas at about 800° C. for thirty seconds. After annealing, Co is removed from the SiO 2  film by means of immersion in an acid solution. Through this step, the cobalt silicide layers  71  and  72  (having a thickness of about 500 Å) are formed in the source/drain regions  10  and  11  uncovered with the silicide protection portion  8 . In this case, the source/drain regions  10  and  11  have a uniform thickness and therefore the bottom surfaces of the cobalt silicide layers  71  and  72  never penetrate the interface  2 S between the SOI layer  3  and the underlying layer  2 , and a structure that works well as an input/output protection circuit can be achieved in the present device. 
     Further, the first insulating layer  81  ( 81 A) of FIG. 18 may be formed of at least double-layered structure. Then, discussion will be made below on a method for manufacturing the first insulating layer  81  of double-layered structure, consisting of the first insulating film  81   a  and the second insulating film  82   a.    
     First, in this case, a step of FIG. 25 is further performed between the steps of FIGS. 18 and 19. Specifically, on a surface of the first insulating film  81   a  formed in the step of FIG. 18, the second insulating film  82   a  different in material therefrom is formed. By this formation, the first insulating layer  81  consisting of the films  81   a  and  82   a  is formed as shown in FIG.  19 . 
     In this case, the etching steps of FIGS. 21 and 22 are amended. Specifically, in this case, the selection ratio is set so that the etching rate of the second insulating layer  82 A equivalent in material to the first insulating film  81   a  may be larger than that of the second insulating film  82   a,  and with this selection ratio, the dry etching is performed. By this etching, as shown in FIG. 26, a portion of the second insulating film  82   a  uncovered with the resist layer  21  is exposed and the dry etching is stopped. Next, the exposed portion of the second insulating film  82   a  of FIG. 26 is removed by the first dry etching as shown in FIG. 27, and further the exposed portion of the first insulating film  81   a  of FIG. 27 is removed by the second wet etching as shown in FIG.  28 . After that, by removing the resist layer  21  of FIG. 28, the silicide protection portion  8  is formed to have the first insulating layer  81  consisting of the first and second insulating films  81   a  and  82   a  shown in FIG.  22  and the second insulating layer  82  made of the third insulating film  83   a  shown in FIG.  22 . 
     Also in this case, naturally, the same advantage of manufacture as the above can be produced. 
     (Additional Remark) 
     Though discussion has been made taking the case of a MOSFET using SOI in the first and second preferred embodiments, the “semiconductor layer” and the “underlying layer” of the present invention are not limited to the “SOI layer” and the “BOX layer”. For example, the present invention can be also applied to a case where a p-type MOSFET or an n-type MOSFET is formed in an n well or a p well. In this case, the n well or the p well is the “underlying layer” in place of the “BOX layer”, and in the well, the source and drain regions and an n −  layer or a p −  layer serving as a channel sandwiched by the source and drain regions constitute the “semiconductor layer”. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.