Abstract:
A multi-gate field effect transistor includes: a plurality of semiconductor layers arranged in parallel on a substrate; source and drain regions formed in each of the semiconductor layers; channel regions each provided between the source region and the drain region in each of the semiconductor layers; protection films each provided on an upper face of each of the channel regions; gate insulating films each provided on both side faces of each of the channel regions; a plurality of gate electrodes provided on both side faces of each of the channel regions so as to interpose the gate insulating film, provided above the upper face of each of the channel region so as to interpose the protection film, and containing a metal element; a connecting portion connecting upper faces of the gate electrodes; and a gate wire connected to the connecting portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-83680 filed on Mar. 27, 2008 in Japan, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a multi-gate field effect transistor and a method for manufacturing the multi-gate field effect transistor. 
   2. Related Art 
   Highly-integrated LSIs with high performances have been achieved by reducing the sizes of field effect transistors (FETs) as the fundamental elements of LSIs and improving the performances of those field effect transistors. The performance of each FET is determined by how high the driving current is during an ON operation and how low the leakage current from the channel is in an OFF state. According to the International Semiconductor Roadmap, novel techniques for realizing higher driving current and lower leakage current are necessary after the 45-nm generation. 
   To reduce the leakage current, fully-depleted (FD) devices each having its channel region fully depleted are expected as the next-generation fundamental devices, as the FD devices have high resistance to short channel effects. Particularly, attention is drawn to multi-gate field effect transistors among those devices. For example, a multi-gate field effect transistor has such a structure that a very small channel region is surrounded by gate electrodes, which is different from a single-gate field effect transistor that controls the potential in the channel from one direction only by a gate electrode formed on the surface of the channel as disclosed in JP-A 2005-86024 (KOKAI). The advantages of this structure are that the potential controllability in the channel region is made higher, a reduction of the potential barrier due to the shorter channel of each device is prevented, and the leakage current caused in an OFF state is lowered. 
   Here, controlling the threshold value of each device is essential. A FD device characteristically has inversion charges generated at a lower voltage than in a case of a bulk-type device. Therefore, if a conventional gate electrode material is used in a FD device, the leakage current in an OFF state (at 0 volt) becomes too high, and a new gate electrode material is required. At present, such gate electrodes include a type that uses a metal as a gate electrode (a metal gate), and a type that uses a compound of a metal and a semiconductor. The former is referred to a metal gate, the latter is referred to a silicide gate when the semiconductor is silicon and the compound is a silicide. 
   In a case where a metal gate and a silicide gate are used in a three-dimensional multi-gate field effect transistor, problems are caused in the manufacturing process. Particularly, metal contamination is always the biggest problem. To achieve a very small gate length, a flat surface is necessary when lithographic patterning is performed before the gate electrode is processed. However, the metal used in the metal gate or the silicide gate might enter the base semiconductor (such as silicon) or the gate insulating film, and the metal might become the center of carrier recoupling, charge traps, or a leakage current path. As a result, the device performance is greatly degraded. Therefore, flattening cannot be performed directly on the metal gate electrode or the silicide electrode by CMP (Chemical Mechanical Polishing). Also, the existence of a natural oxide film becomes a problem when a gate electrode is silicided. However, by any conventional manufacturing method, the natural oxide film cannot be removed. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of these circumstances, and an object thereof is to provide a multi-gate field effect transistor that can prevent metal contamination even if a flattening process is carried out, and a method for manufacturing the multi-gate field effect transistor. 
   A method for manufacturing a multi-gate field effect transistor according to a first aspect of the present invention includes: forming a plurality of semiconductor layers in parallel on a substrate; forming a protection film on an upper face of each semiconductor layer; forming a first insulating layer on the semiconductor layers, the first insulating layer covering side faces of each semiconductor layer and covering side faces and an upper face of the protection film; forming a first interlayer insulating film on an entire surface and flattening the first interlayer insulating film to expose an upper face of each first insulating layer; removing the first insulating layers to form a plurality of holes at locations from which the first insulating layers are removed; forming a gate insulating film on both side faces of each semiconductor layer inside the holes; depositing a first polysilicon film on an entire surface to fill the holes; flattening the first polysilicon film to divide the first polysilicon film into a plurality of second polysilicon films and expose an upper face of the first interlayer insulating film; forming masks having shapes of gate electrodes on the second polysilicon films; patterning the second polysilicon films with the use of the masks; forming sidewalls made of an insulator so that the sidewalls sandwich the patterned second polysilicon films in a gate length direction; patterning the protection film with the use of the masks and the sidewalls to selectively expose an upper face of each of the semiconductor layers; implanting an impurity into the selectively exposed semiconductor layers to form source and drain regions; removing the masks to expose an upper face of each second polysilicon film; turning each of the second polysilicon films into a silicide, the second polysilicon films each having the upper face exposed; forming a second interlayer insulating film on an entire surface, forming an opening continuing to each of the silicides, and filling the opening to form a metal connecting portion connecting the silicides; and forming a wire connected to the connecting portion. 
   A method for manufacturing a multi-gate field effect transistor according to a second aspect of the present invention includes: forming a plurality of semiconductor layers in parallel on a substrate; forming a protection film on an upper face of each semiconductor layer; forming a first insulating layer on the semiconductor layers, the first insulating layer covering side faces of each semiconductor layer and side faces and an upper face of the protection film; forming a first interlayer insulating film on an entire surface and flattening the first interlayer insulating film to expose an upper face of each first insulating layer; removing the first insulating layers to form a plurality of holes at locations from which the first insulating layers are removed; forming a gate insulating film on both side faces of each semiconductor layer inside the holes; forming a gate metal film to cover side faces and a bottom face of each hole, the gate insulating film, and the protection film inside the holes; forming a polysilicon film to cover the gate metal film; etching the polysilicon film and the gate metal film existing in each hole, so as not to expose the gate metal film on the protection film; depositing again a polysilicon film to fill each hole; flattening the polysilicon film to divide the polysilicon film into a plurality of polysilicon layers and expose an upper face of the first interlayer insulating film; forming masks having shapes of gate electrodes on the polysilicon layers; patterning the polysilicon layers with the use of the masks; forming sidewalls made of an insulator so that the sidewalls sandwich the patterned polysilicon layers in a gate length direction; patterning the protection film with the use of the sidewalls to selectively expose an upper face of each semiconductor layer; implanting an impurity into the selectively exposed semiconductor layers to form source and drain regions; removing the masks to expose an upper face of each polysilicon layer; forming a second interlayer insulating film on an entire surface, forming an opening continuing to each of the polysilicon layers on the second interlayer insulating film, and filling the opening to form a metal connecting portion connecting the polysilicon layers; and forming a wire connected to the connecting portion. 
   A multi-gate field effect transistor according to a third aspect of the present invention includes: a plurality of semiconductor layers of a first conductivity type arranged in parallel on a substrate; source and drain regions of a second conductivity type formed at a distance from each other in each of the semiconductor layers; channel regions each provided between the source region and the drain region in each of the semiconductor layers; protection films each provided on an upper face of each of the channel regions; gate insulating films each provided on both side faces of each of the channel regions; a plurality of gate electrodes provided on both side faces of each of the channel regions so as to interpose the gate insulating film therebetween, provided above the upper face of each of the channel region so as to interpose the protection film therebetween, and containing a metal element; an interlayer insulating film provided on the substrate so as to cover side faces of each of the gate electrodes; a connecting portion connecting upper faces of the gate electrodes; and a gate wire connected to the connecting portion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A to 3C  illustrate the procedures for manufacturing a conventional multi-gate FET; 
       FIGS. 4A to 5B  illustrate the problems with the procedures for manufacturing the conventional multi-gate FET; 
       FIG. 6  is a plan view of a multi-gate FET in accordance with a first embodiment of the present invention; 
       FIGS. 7A to 17  are cross-sectional views illustrating the procedures for manufacturing the multi-gate FET in accordance with the first embodiment; 
       FIGS. 18A and 18B  illustrate the effects of the multi-gate FET in accordance with the first embodiment; 
       FIGS. 19A to 21D  are cross-sectional views illustrating the procedures for manufacturing a multi-gate FET in accordance with a second embodiment of the present invention; and 
       FIG. 22  is a cross-sectional view of the multi-gate FET in accordance with the second embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1A to 5B , the problems with conventional multi-gate field effect transistors are described in detail to clarify the characteristics of a multi-gate field effect transistor in accordance with an embodiment of the present invention, before the multi-gate field effect transistor is described. Although the problems with n-type channel transistors are described in the following, the same problems as below are found in p-type channel transistors. 
   As shown in  FIGS. 1A ,  1 B, and  1 C, an insulating film  4  is formed on a supporting substrate  2 , and a SOI layer  6  is formed on the insulating film  4  to form a SOI substrate. A 100-nm thick silicon nitride film as a protection film  8  for the channel is deposited on the SOI substrate by LPCVD (Low Pressure Chemical Vapor Deposition) or the like, and device isolation is performed by a known device isolating technique. Further, patterning is performed on the SOI layer  6  by a known patterning technique, so as to form the channel. The channel width is 20 nm, for example.  FIG. 1A  is a plan view of this structure.  FIG. 1B  is a cross-sectional view of the structure, taken along the line A-A′ of  FIG. 1A .  FIG. 1C  is a cross-sectional view of the structure, taken along the line B-B′ of  FIG. 1A . 
   As shown in  FIGS. 2A ,  2 B, and  2 C, a 1-nm thick silicon dioxide film is formed as a gate insulating film  9  by RTO (Rapid Thermal Oxidation) or the like, and plasma nitridation is then performed to increase the dielectric constant. A 250-nm thick polysilicon film  10 A to be a gate electrode is deposited by LPCVD or the like. To flatten the surface, CMP is performed with the silicon nitride film  8  serving as an etching stopper, so as to deposit a 60-nm thick polysilicon film  10 B. At this point, a natural oxide film  100  is formed on the surface of the polysilicon film  10 A.  FIG. 2A  is a plan view of this structure.  FIG. 2B  is a cross-sectional view of the structure, taken along the line A-A′ of  FIG. 2A .  FIG. 2C  is a cross-sectional view of the structure, taken along the line B-B′ of  FIG. 2A . 
   As shown in  FIGS. 3A ,  3 B, and  3 C, a hard mask layer (not shown) formed with a silicon nitride film is deposited on the structure. Patterning is then performed on the hard mask layer by a photolithography technique or the like. With the patterned hard mask layer serving as a mask, patterning is performed on the polysilicon film  10 B by RIE or the like, so as to form a gate electrode portion. Here, an offset spacer may be further formed, but is not shown in the drawings. 
   To perform patterning to form a gate electrode having a gate length of 50 nm or less by a photolithography technique, a flat surface is necessary. Therefore, the CMP procedure illustrated in  FIGS. 2A ,  2 B, and  2 C is carried out. However, if a fully-silicided gate or a metal gate is to be formed by CMP, a problem is caused during the procedure. For example, in the case of a fully-silicided gate, a metal (such as Ni)  11  is deposited on the polysilicon film  10 B by sputtering, and a heat treatment is carried out at a temperature between 400° C. and 500° C. to silicide the polysilicon film  10 B, as shown in  FIG. 4A . However, since the natural oxide film  100  is not uniformly formed, as shown in  FIG. 4B , the natural oxide film  100  becomes the barrier against diffusion of the metal  11  in the regions having the natural oxide film  100  formed therein, and the siliciding reaction in the region of the polysilicon film  10 A located lower than the natural oxide film  100  becomes uneven. As a result, the polysilicon film  10 A is divided into a portion  13  to be the silicide and a portion to remain the polysilicon film  10 A, as shown in  FIG. 4B . In this case, in the transistor having the channel  6  on the left-hand side in  FIG. 4B , the silicide  13  exists on either side of the channel  6 , and accordingly, the threshold values on both sides of the channel  6  are the same. However, in the transistor having the channel  6  on the right-hand side, the silicide  13  exists on the left side of the channel  6 , but the polysilicon film  10 A exists on the right side of the channel  6 . Therefore, the threshold values are different from each other by the difference between the work functions of polysilicon and silicide. 
   In a LSI, variations among the threshold values of transistors become a serious problem. Therefore, the different threshold values of the channels of the respective transistors are a critical problem. 
   Furthermore, in such a situation, the resistances of the gate electrodes also vary, causing trouble during a high-frequency operation. 
   In a case where the gate electrode is made of a metal, on the other hand, a metal electrode  17  and a polysilicon film  10  are deposited to surround a semiconductor layer  6  to be the channel region. An interlayer insulating film  18  is then deposited. As a result, the upper face of the interlayer insulating film  18  becomes uneven. Therefore, to perform fine lithographic patterning, it is necessary to flatten the surface, as shown in  FIG. 5B . In the flattening, only the silicon nitride film  8  serves as the stopper when the CMP procedure is carried out. As a result, the metal electrode  17  is also etched. As the metal electrode  17  is etched by CMP, the particles of the metal are scattered about on the entire wafer surface, and the wafer is contaminated with the metal. This is not allowable in the manufacture of LSIs. Therefore, in a case where a metal gate is used, lithographic patterning needs to be performed to form the gate electrode, without a flattening process. As a result, a small-sized gate cannot be formed. 
   The inventors made an intensive study to solve those problems, and developed multi-gate field effect transistors. The multi-gate field effect transistors are described below as embodiments of the present invention. 
   First Embodiment 
     FIG. 6  is a plan view of a multi-gate field effect transistor in accordance with a first embodiment of the present invention. As shown in  FIG. 6 , the multi-gate field effect transistor of this embodiment includes two fins  50   1  and  50   2 , an n-type source region  60   a  formed at one end of each of those fins, an n-type drain region  60   b  formed at the other end of each of those fins, and a gate  70  formed to bridge the two fins  50   1  and  50   2 . Each of the fins  50   1  and  50   2  includes a p-type semiconductor layer to be the channel region, a gate insulating film formed on the side faces of the channel region, and a protection film formed on the upper face of the semiconductor layer. An n-type semiconductor layer is formed on the portion of the semiconductor layer located on either side of the channel region of each fin. The n-type semiconductor layer is connected to the n-type source region  60   a  or the n-type drain region  60   b,  and is also called the n-type source region  60   a  or the n-type drain region  60   b.  The gate  70  is formed to cover the gate insulating film on the channel region. 
   Referring now to  FIGS. 7A to 19D , a method for manufacturing the multi-gate field effect transistor of this embodiment is described. In this embodiment, an n-type channel field effect transistor is described. However, a p-type channel field effect transistor can be produced in the same manner as below, if ion species to be used for ion implantation are changed.  FIGS. 7A ,  8 A,  9 A,  10 A,  11 A,  12 A,  13 A,  14 A,  15 ,  16 ,  17 ,  18 , and  19 A are cross-sectional views of the multi-gate field effect transistor, taken along the line A-A′ of  FIG. 6 .  FIGS. 7B ,  8 B,  9 B,  10 B,  11 B,  12 B,  13 B,  14 B, and  19 B are cross-sectional views of the multi-gate field effect transistor, taken along the line B-B′ of  FIG. 6 .  FIGS. 7C ,  8 C,  9 C,  10 C,  11 C,  12 C,  13 C,  14 C, and  19 C are cross-sectional views of the multi-gate field effect transistor, taken along the line C-C′ of  FIG. 6 .  FIGS. 7D ,  8 D,  9 D,  10 D,  11 D,  12 D,  13 D,  14 D, and  19 D are cross-sectional views of the multi-gate field effect transistor, taken along the line D-D′ of  FIG. 6 . 
   First, as shown in  FIGS. 7A ,  7 B,  7 C, and  7 D, an insulating film  4  is formed on a supporting substrate  2 , and a SOI layer  6  is formed on the insulating film, to complete a SOI substrate. A 100-nm thick silicon nitride film is deposited as a protection film  8  on the SOI substrate by LPCVD (Low Pressure Chemical Vapor Deposition) or the like, and an insulating film  19  made of TEOS (Tetra Ethyl Ortho Silicate) or the like is deposited on the protection film  8 . After that, device isolation is performed by a known device isolating technique. Further, patterning is performed on the SOI layer  6  by a known patterning technique, so as to form the channels. The width of each channel is 20 nm, for example. Here, the protection film  8  and the insulating film  19  exist on the upper face of each channel  6 . 
   As shown in  FIGS. 8A ,  8 B,  8 C, and  8 D, an insulating film  19  is further deposited, and etching is performed on the insulating film  19  by RIE (Reactive Ion Etching), so as to form sidewalls  19   a  formed from the insulating film  19  on the side faces of each channel  6  and each protection film  8 . The insulating film  19  to be the sidewalls  19   a  also covers the upper face of each protection film  8 . Those procedures are repeated several time, so that the width of the sidewalls  19   a  (the thickness of each sidewall from the corresponding side face of the channel  6  in a direction perpendicular to the side face of the channel  6 ) is maximized (50 nm to 100 nm, for example), and the height (the height from each protection film  8 ) is also maximized (300 nm, for example). Here, the insulating film  19   a  to be the sidewalls has round upper faces. 
   As shown in  FIGS. 9A ,  9 B,  9 C, and  9 D, an insulating film  20  is deposited to cover the insulating film  19   a  to be the sidewalls. The insulating film  20  is then flattened by CMP, so as to expose the upper faces of the insulating film  19   a.  Here, the etching of the insulating film  19   a  is not stopped at the upper faces, but over-etching is performed to shave off the round top portions of the insulating film  19   a  and widen the opening portions. 
   As shown in  FIGS. 10A ,  10 B,  10 C, and  10 D, the insulating film  19   a  covering the side faces and the upper faces of the channels  6  and the protection films  8  is completely removed by etching with the use of a liquid solution or the like, and holes are formed. If a diluted HF solution or the like is used, the insulating film  4  is also slightly etched. However, in a case where the insulating film  4  has a thickness of 1 μm, for example, the difference in etching rate between the TEOS (the material of the insulating film  19   a ) and the silicon thermally-oxidized film (the insulating film  4 ) is utilized to adjust the etching time. In this manner, the remaining insulating film  4  can have a sufficient thickness. After that, with the holes being formed, a 1-nm thick silicon dioxide film to be a gate insulating film  9  is formed on the side faces of each channel region  6  by RTO, for example. Plasma nitridation is then performed to increase the dielectric constant of the gate insulating film  9 . Further, a polysilicon film  10  to be a gate electrode is deposited by LPCVD or the like. Here, the furnace temperature of LPCVD is set at a low temperature, and polysilicon is deposited through a rate-limiting reaction, so as to completely fill the holes. This is because, if deposition is performed at a high temperature, a polysilicon film is formed through a rate-limiting supply of the raw material, and the polysilicon film is formed preferentially on the insulating film  20 , resulting in voids remaining in the holes. After that, the polysilicon film  10  is flattened by CMP, so as to obtain a flat surface suitable for the gate lithographic patterning. 
   As shown in  FIGS. 11A ,  11 B,  11 C, and  11 D, a gate mask material is deposited, and patterning is performed on the gate mask material by a lithography technique, so as to form a mask  21 . 
   After that, with the use of the mask  21 , patterning and slimming is performed on the polysilicon film  10 , so as to form the polysilicon film  10  into the shape of a gate electrode. Here, the polysilicon film  10  and the gate insulating film  9  existing on the semiconductor layers  6  to be the source and drain regions are removed by RIE ( FIGS. 11B and 11C ). 
   A gate sidewall material  12  made of SiN or the like is then deposited on the entire surface. After that, RIE is performed on the gate sidewall material  12 , so as to form sidewalls  12  at the side portions of the polysilicon film  10  ( FIGS. 12C and 12D ). Here, over-etching is performed, so that the sidewalls of the polysilicon film  10  remain ( FIG. 12C ), but the sidewalls of the semiconductor layers  6  to be the source and drain regions are removed ( FIG. 12B ), due to the difference in height between the gate electrode  10  and each of the semiconductor layers  6  to be the source and drain regions. Also, the channel protection film  8  on each of the semiconductor layers  6  to be the source and drain regions is removed by the over-etching, so that the semiconductor layers  6  to be the source and drain regions are exposed ( FIGS. 12B and 12C ). By the over-etching, the insulating film  20  is recess-etched in the regions not covered with the mask  12 , and the height of the insulating film  20  is reduced, as shown in  FIG. 12D . 
   As shown in  FIGS. 13B and 13C , selective etching is performed on the exposed semiconductor layers  6 , with the sidewalls  12  serving as masks. In this manner, the height of each of the semiconductor layers  6  measured from the insulating film  4  is reduced. Phosphorus (P) ions as the dopant for forming the n-type source and drain regions are implanted from a diagonal direction at an angle of 7 degrees or more with respect to the direction perpendicular to the film plane ( FIG. 13C ). By doing so, the n-type source region  60   a  and the n-type drain region  60   b  are formed, and each p-type semiconductor region  6  between the source region  60   a  and the drain region  60   b  becomes the channel region. Here, the mask  21  is not removed, as shown in  FIGS. 13A ,  13 C, and  13 D. If p-type source and drain regions are to be formed, boron (B) is used as the dopant. 
   As shown in  FIGS. 14A ,  14 B,  14 C, and  14 D, after the gate mask  21  is removed, metal sputtering using Ni and a heat treatment are performed, so as to fully silicide the polysilicon film  10 . In this manner, gate electrodes  13  are formed. Since a natural oxide film does not exist on the polysilicon film  10 , uniform Ni diffusion is caused, and the entire gate electrodes  13  are fully silicided. At the same time, the source region  60   a  and the drain region  60   b  are also silicided ( FIGS. 14B and 14C ). 
   In this situation, as shown in  FIG. 14A , the gate electrode  13  is divided into two, and a voltage cannot be applied to the gate electrodes  13 . Therefore, it is necessary to connect the gate electrodes  13  with an upper-layer wire. After the siliciding procedure illustrated in  FIGS. 14A to 14D  is completed, a SiN film  22   a  of several tens of nanometers is deposited as the contact etching stopper. An interlayer insulating film  22   b  is then deposited on the SiN film  22   a,  and flattening is performed by CMP ( FIG. 15 ). Even if the concavities and convexities of the upper face of the interlayer insulating film  22   b  are large before CMP is performed, the SiN film  22   a  serves as the etching stopper during the CMP. 
   A contact hole (not shown) continuing to the gate electrodes  13  is then formed through the insulating film  22  formed with the interlayer insulating film  22   b  and the SiN film  22   a  by a lithography technique and RIE. The contact hole is filled with TiN—W or the like, so as to form a plug  23  ( FIG. 16 ). Further, an interlayer insulating film (not shown) is deposited, and a wire processing groove is formed in the interlayer insulating film by a lithography technique and RIE. This groove is filled with a metal wire  24  made of Al—Cu or the like, so that the gate electrodes  13  are connected to a voltage applying terminal (not shown) via the plug  23  and the metal wire  24  ( FIG. 17 ). Thus, the multi-gate FET of this embodiment is completed. 
   When gate wire resistances are compared between the multi-gate FET of this embodiment formed in this manner and a conventional FUSI-gate Fin FET, signals are transmitted through very thin gate electrodes in the conventional FUSI-gate Fin FET, while signals are transmitted through the thick metal wires  24  in this embodiment. Accordingly, in this embodiment, the adverse influence of the wire resistance on the gate electrodes having high-frequency signals applied thereto during the driving of the transistors can be greatly reduced. For example, where the width (the gate length) is 20 nm, the height of the portion of each FUSI electrode covering the upper portion of each Fin is 60 nm, the width of the Al—Cu wire is 120 nm, and the height of the Al—Cu wire is 1 μm, as shown in  FIGS. 18A and 18B , the area S through which signals are transmitted can be increased 100 times or more, and the gate resistance can be further lowered, with the difference in resistivity being taken into consideration. 
   In this embodiment, the channel protection insulating film  8  may be made of TEOS, the insulating film  19  may be made of SiN, the insulating film  20  may be made of TEOS, and the gate mask material  21  may be made of SiN. 
   Although the number of fins is two in this embodiment, three or more fins may be employed. 
   As described above, in accordance with this embodiment, metal contamination can be prevented even after a flattening process is carried out. Also, the gate resistance can be lowered. 
   Second Embodiment 
   Referring now to  FIGS. 19A to 21D , a method for manufacturing a multi-gate FET in accordance with a second embodiment of the present invention is described. The multi-gate FET manufactured by the method in accordance with this embodiment is of a metal-gate type.  FIGS. 19A ,  20 A, and  21 A are cross-sectional views of the multi-gate FET, taken along the line A-A′ of  FIG. 6 .  FIGS. 19B ,  20 B, and  21 B are cross-sectional views of the multi-gate FET, taken along the line B-B′ of  FIG. 6 .  FIGS. 19C ,  20 C, and  21 C are cross-sectional views of the multi-gate FET, taken along the line C-C′ of  FIG. 6 .  FIGS. 19D ,  20 D, and  21 D are cross-sectional views of the multi-gate FET, taken along the line D-D′ of  FIG. 6 . 
   First, the same procedures as the procedures for manufacturing the fully-silicided gate FET of the first embodiment illustrated in  FIGS. 7A to 9D  are carried out. As shown in  FIGS. 19A ,  19 B,  19 C, and  19 D, a metal film  17  to be a metal gate is formed by MOCVD (Metal-Organic-Chemical-Vapor-Deposition) on the surfaces of the holes formed by removing the insulating film  19  shown in  FIGS. 9A to 9D . The holes are filled with a polysilicon film  10  by LPCVD. At this point, the surfaces of the gate insulating film  9  and the channel protection film  8  inside the holes are covered with the metal film  17  to be a metal gate, and the polysilicon film  10  is formed outside the metal film  17 . The metal film  17  to be a metal gate and the polysilicon film  10  also adhere onto the insulating film  20 . If CMP is performed in this situation, metal contamination is spread in the entire wafer. 
   To avoid such a problem, etching is performed on the polysilicon film  10  and the metal film  17 , so as to remove the polysilicon film  10  and the metal film  17  from the upper face of the insulating film  20 , as shown in  FIGS. 20A ,  20 B,  20 C, and  20 D. At this point, the semiconductor layers  6  to be the fins and the protection film  8  are covered with the polysilicon film  10 , and are not etched by the RIE. Accordingly, the metal film  17  on the upper face of the insulating film  20  can be controlled with efficiency. By taking advantage of the difference in height between the protection film  8  and the buried insulating film  20  measured from the insulating film  4 , the metal film  17  and the polysilicon film  10  remain on the side faces of the protection film  8  and the gate insulating film  9 . 
   As shown in  FIGS. 21A ,  21 B,  21 C, and  21 D, a polysilicon film  10  is again deposited, and CMP is performed to obtain a flat surface suitable for gate lithographic patterning, without causing metal contamination. After that, the same procedures as the procedures in the first embodiment illustrated in  FIGS. 11A to 17  are carried out to complete a multi-gate FET. However, the procedure of forming the polysilicon film  10  illustrated in  FIGS. 11A to 11D  is unnecessary. The procedure of siliciding the polysilicon film  10  is also unnecessary.  FIG. 22  is a cross-sectional view of the multi-gate field effect transistor of this embodiment manufactured in this manner. 
   In this embodiment, the channel protection insulating film  8  may be made of TEOS, the insulating film  19  may be made of SiN, the insulating film  20  may be made of TEOS, and the gate mask material  21  may be made of SiN, as in the first embodiment. 
   As described above, in accordance with this embodiment, metal contamination can be prevented even after a flattening process is carried out. 
   Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concepts as defined by the appended claims and their equivalents.