Abstract:
A semiconductor device comprises: a channel region of a transistor formed in a predetermined region of silicon layer formed on insulation film; a gate electrode formed on the channel region via gate insulation film; and source/drain regions formed in the silicon layer thicker than said channel region located out of the channel region, wherein the transistor is a memory element constituting the channel region as a floating body cell.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2005-259494, filed on Sep. 7, 2005; the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device and more particularly to a semiconductor memory device using a silicon on insulator (SOI) substrate. The present invention also relates to a manufacturing method of the semiconductor device.  
         [0003]     The SOI substrate in which an insulation film (buried insulation film) is provided on a silicon substrate and a silicon layer is provided on the insulation film has been known in, for example, Japanese Patent Application Laid-Open Publication No. 2001-298171 and Japanese Patent Application Laid-Open Publication No. 5-257356(1993).  
         [0004]     A semiconductor memory device having a floating body cell (FBC) memory cell with a portion corresponding to a channel region of MOS transistor formed in the silicon layer on the buried insulation film of such an SOI substrate used as a floating body so as to accumulate electric charges has been developed.  
         [0005]     In such a semiconductor memory device, the area of its memory cell can be extremely reduced because a capacitor may be eliminated, so that high integration can be achieved and accordingly the memory capacity per unit area can be increased.  
         [0006]     In the FBC, as far as SOI film thickness concerned, as the degree of depleted condition is intensified to full depleted (FD) condition, namely, the thinner the SOI film be, the higher ΔVth which can serve as a criterion when detecting information accumulated in the memory cell may be increased. This ΔVth indicates a difference between Vth when “0” data is stored and Vth when “1” data is stored.  
         [0007]     Because this FD condition is easier to be obtained as the thickness of the SOI film is decreased, the thickness of the SOI film is required to be decreased as much as possible from viewpoints of improvement (increase of ΔVth) of the performance of the memory cell of the FBC cell.  
         [0008]     Furthermore, the BOX film which is buried oxide film is preferred to be as thin as possible in order to improve coupling between the body and a substrate electrode (hereinafter referred to a plate electrode) of the memory cell. Therefore, both of the buried oxide film and the SOI film thereon are preferred to be as thin as possible.  
         [0009]     However, manufacturing of a SOI wafer with the BOX film set to less than 60 Angstrom in thickness and the SOI film set to less than 300 Angstrom in the SOI substrate requires highly accurate control of wafer manufacturing process, thereby leading to increase in wafer manufacturing cost, which is a large obstacle to achievement of an excellent FBC.  
         [0010]     If the thickness of the SOI film is too small, when a contact is formed, not only electric short-circuit is likely to occur between the contact and plate potential but also contact resistance is increased, which is a problem to be solved. Although the SOI film can be thinned uniformly, if the SOI film is thinned to about 20 nm, the ΔVth is decreased, so that an intention of improving performance using a device under a fully depleted condition by thinning the SOI film is not achieved.  
         [0011]     Additionally, if the BOX film is thinned extremely, diffusion capacitance increases so that sharing of process with a logic section, which is a large advantage of the FBC, becomes impossible.  
       SUMMARY OF THE INVENTION  
       [0012]     According to a first aspect of the present invention, there is provided a semiconductor device comprising:  
         [0013]     a channel region of a transistor formed in a predetermined region of silicon layer formed on insulation film;  
         [0014]     a gate electrode formed on the channel region via gate insulation film; and  
         [0015]     source/drain regions formed in the silicon layer thicker than said channel region located out of the channel region, wherein the transistor is a memory element constituting the channel region as a floating body cell.  
         [0016]     According to a second aspect of the present invention, there is provided a semiconductor device comprising:  
         [0017]     a channel region of transistor formed in a predetermined region of silicon layer formed on insulation film;  
         [0018]     a gate electrode formed on the channel region via gate insulation film; and  
         [0019]     a source region and a drain region formed in the silicon layer out of the channel region, said silicon layer having the same thickness of the channel region in the drain side while having thicker thickness than the channel region in the source side, wherein the transistor is a memory element constituting the channel region as a floating body cell.  
         [0020]     According to a third aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising:  
         [0021]     forming stacked layers of a buried oxide film, a silicon film and silicon nitride film on a silicon substrate;  
         [0022]     etching the nitride film to remain it for element region;  
         [0023]     etching the silicon film and the buried oxide film to remain them for the element region;  
         [0024]     filling an insulating film in a portion removed by the etching;  
         [0025]     forming a trench in the silicon layer and the silicon nitride layer in the element region, the bottom of the trench being positioned in the thickness of the silicon film;  
         [0026]     filling a gate material in the trench;  
         [0027]     selectively removing the silicon nitride layer;  
         [0028]     ion implanting to form source/drain regions; and  
         [0029]     forming interconnections for leading out the source/drain and gate regions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIGS. 1A-1L  are sectional views of a semiconductor device on respective steps showing a manufacturing method of the semiconductor device according to a first embodiment of the present invention;  
         [0031]      FIGS. 2A-2L  are sectional views of a semiconductor device of respective steps showing a manufacturing method of the semiconductor device according to a second embodiment of the present invention;  
         [0032]      FIG. 3  is a sectional view of the semiconductor device showing the structure of the semiconductor device according to a third embodiment of the present invention; and  
         [0033]      FIG. 4  is a sectional view of a semiconductor device showing the structure of the semiconductor device according to a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     Hereinafter, the embodiments of the present invention will be described in detail by taking a semiconductor memory device having a FBC memory cell as an example. Drawings shown below are not accurate representation of film thickness and the like but schematic ones.  
       (1) First Embodiment  
       [0035]     An example of first manufacturing process for manufacturing a semiconductor device according to a first embodiment of the present invention will be described with reference to  FIGS. 1A-1L .  
         [0036]     First, a SOI substrate  100  in which buried oxide film (BOX film)  102  in the thickness of 250 Angstrom is provided on a silicon semiconductor base material  101  and silicon film (SOI film)  103  in the thickness of 1500 Angstrom is formed thereon is prepared ( FIG. 1A ).  
         [0037]     Next, thermally-oxidized film  104  about 20 Angstrom thick is formed by heating the SOI substrate  100  in oxidation atmosphere and subsequently, SiN film  105  in the thickness of 1700 Angstrom is deposited and further, boro silicate glass (BSG) film  106  in the thickness of 3000 Angstrom is deposited ( FIG. 1B ).  
         [0038]     Next, resist pattern is formed for forming a device region of a desired pattern. That is, photo resist  107  is applied on its entire surface and the resist pattern is formed by patterning using photo lithography method. This pattern is a shape for keeping a portion corresponding to a device isolation expected region of a transistor to be formed later remained ( FIG. 1C ).  
         [0039]     The BSG film and SiN film are etched and removed using anisotropic etching method like RIE with employing the patterned resist  107  as masking material. As a consequence, the SOI film  103  and thermally-oxidized film  104  in the device isolation expected region are exposed outside ( FIG. 1D ).  
         [0040]     Subsequently, after the resist film  107  remained on the SOI substrate is removed, the SOI film  103  and the thermally-oxidized film  104  are etched using reactive ion etching (RIE) method with employing the BSG/BiN films  105 - 106  as masking materials. This etching depth is set to, for example, 1500 Angstrom. Therefore, the SOI film out of the device formation area is removed.  
         [0041]     After this etching, reaction product at the time of etching is removed by washing and subsequently, the BSG film existing on the SiN film  105  is removed according to, for example, wet etching method ( FIG. 1E ).  
         [0042]     Then, thermally-oxidized film  108  is formed in the thickness of 40 Angstrom on the side face of the exposed SOI film  103  by thermal oxidation. The side portion of this thermally-oxide film is filled with TEOS film  109  because there is a trench with respect to a device formation area of an adjacent cell. Subsequently, according to CMP method (chemical mechanical polishing method) with the SiN film on the SOI used as stopper film, TEOS of a unnecessary portion is removed and flattened so as to form shallow trench isolation (STI). At this time, the remained film on the wafer of the SiN film is about 1500 Angstrom ( FIG. 1F ).  
         [0043]     Next, resist pattern in which only a cell portion is exposed is formed and after that, the resist is removed by ion-implantation of phosphor for plate electrode formation. SiN film  110  is deposited on the SOI wafer in the thickness of 200 Angstrom and next, BSG film  111  is deposited in the thickness of 1000 Angstrom ( FIG. 1 (G)). Resist pattern  112  is formed such that the BSG film corresponding to a gate electrode is exposed and with this resist  112  used as masking material, the BSG/SiN  111 ,  110 , the SiN film  105 , and the thermally-oxidized film  104  are etched up to the face of the SOI substrate  103  so as to form a trench  113  with the top face of the SOI  103  exposed outside ( FIG. 1G ).  
         [0044]     After the remained resist  112  is removed, a pattern in which the SOI portion expected to be formed into thin film is exposed is formed and by etching the exposed SOI film  103  according to the CDE method, a deeper trench  114  is formed so that the thickness of the SOI film  103  is adjusted to a desired thickness, for example, 400 Angstrom.  
         [0045]     Next, the BSG film  111  remained on the SiN film  110  on the SOI substrate is removed by etching according to vacuum plasma method (VPC) and if necessary, ion implantation for Vt adjustment of Tr is carried out ( FIG. 1H ).  
         [0046]     Therefore, the thickness at a gate electrode formation expected location is reduced from the initially 1500 Angstrom to 400 Angstrom.  
         [0047]     Next, thermally-oxidized film  115  is formed in the thickness of 60 Angstrom on the bottom and side wall of the trench  114  by thermal oxidation with this SOI  103  exposed and amorphous silicon doped with no impurity is deposited on the entire surface in the thickness of 2000 Angstrom so as to fill the trench  114 . Then, gate electrode  116  is formed according to damascene process for flattening using the CMP method.  
         [0048]     Next, the SiN film  105  remained on the SOI substrate is removed using a chemical such as H 3 PO 4  and oxide film  117  is formed on the surface of the SOI substrate  103  and the surface of the gate electrode  116  by post oxidation and ion implantation is carried out for doping for formation of desired diffusion layer. This ion implantation is carried out under a condition that dose of phosphor or boron is 3×10 13  (hereinafter expressed as 3E13)/cm 2  and acceleration voltage is 10 keV ( FIG. 11 ).  
         [0049]     Next, layered film in the thickness of TEOS/SiN/TEOS 200/200/400 Angstrom are deposited on the entire surface of the SOI substrate and etching is carried out for leaving spacer  118  adjacent to the gate electrode  116  so as to expose the top face of the SOI substrate and polysilicon electrode. Then, phosphor or boron is ion-implanted into a desired region at a dose of 3E15/cm 2  or more at an acceleration voltage of 10 keV so that impurity is activated at high temperature for a short time according to RTA method ( FIG. 1J ).  
         [0050]     After activation is completed, the post oxidized film  117  is removed, cobalt (Co) is deposited on the entire surface of the SOI substrate and salicide film  119  is formed at a portion making direct contact with silicon by heat treatment. Excess cobalt at a portion in which no silicon exists is removed by etching.  
         [0051]     After the salicide film is formed, SiN is deposited in the thickness of 200 Angstrom on the entire surface of wafer and next non-doped silicate glass (NSG) film  120 , which is silicate glass doped with no impurity, is deposited in the thickness of 6000 Angstrom and then it is flattened according to the CMP method ( FIG. 1K ).  
         [0052]     Next, resist pattern for forming a contact between source/drain region and gate electrode is formed and the NSG film  120  is etched according to the RIE method until the top face of the SiN is exposed so as to form a contact hole  121  and remained resist is removed. Next, the SiN film exposed on the bottom of the contact hole  121  is removed by etching so that the top face of SOI/gate polysilicon is exposed within the contact hole  121  and then, three-layered film of Ti/TiN/W are deposited in the thickness of 100/200/3000 Angstrom respectively. Then, the Ti/TiN/W of other portion than the contact portion is removed according to the CMP method so that the Ti/TiN/W layered film  122  is left only inside the contact. Next, NSG film  123  about 3000 Angstrom is deposited and a channel  124  is formed at a portion corresponding to a wiring layer pattern and Cu is deposited therein by plating. Then, SL electrode  125  is formed by etching the Cu at an unnecessary portion according to the CMP method ( FIG. 1L ).  
         [0053]     After that, wiring metal layer and contact are formed using dual damascene process of executing these at the same time as filling a contact with metal (Cu) and finally, a semiconductor device is completed.  
         [0054]     Because in the semiconductor device of this embodiment, the thickness of the SOI film at the channel portion is sufficiently small, FD state can be attained so as to increase ΔVth. This is particularly advantageous when this semiconductor device is used as a semiconductor device containing FBC cell.  
         [0055]     Further, because according to the manufacturing process described above of this semiconductor device, when FBC memory cell is manufactured using a thick SOI substrate, the gate electrode of a memory cell is formed by using damascene process in which a channel is formed by reducing the film thickness of the SOI portion in a channel region until a desired thickness is reached before formation of gate insulation film and this channel  
         [0000]     is filled with gate electrode material, stabilized thin SOI film having a high reliability can be obtained.  
         [0056]     Further, because the thickness of the SOI film in other region than the channel region of the gate electrode bottom portion is a thickness which may be formed easily using current SOI wafer manufacturing technology, increase in manufacturing cost of the SOI wafer can be prevented.  
         [0057]     Furthermore, because the SOI film thickness in other region than the channel region is sufficiently large, electric short-circuit between the contact and plate potential upon formation of a contact can be prevented easily.  
         [0058]      FIGS. 2A-2L  are sectional views of a semiconductor device for explaining a second manufacturing process for manufacturing the semiconductor device according to the second embodiment of the present invention.  
         [0059]     Here, SOI substrate in  200  which BOX film  202  in the thickness of 250 Angstrom and SOI film  203  in the thickness of 1500 Angstrom are provided on silicon substrate  201  like the first embodiment is used ( FIG. 2A ).  
         [0060]     Next, thermally-oxidized film  204  about 20 Angstrom is formed by heating the SOI substrate  200  in oxidation atmosphere and subsequently, SiN film  205  in the thickness of 1700 Angstrom is deposited and further, boron silicate glass (BSG) film  206  in the thickness of 3000 Angstrom is deposited ( FIG. 2B ).  
         [0061]     Next, resist pattern is formed for forming a device region of a desired pattern. That is, photo resist  207  is applied on its entire surface and the resist pattern is formed by patterning using photo lithography method. This pattern has a shape for keeping a portion corresponding to a device isolation expected region of a transistor to be formed later remained ( FIG. 2C ).  
         [0062]     The BSG film  206  and SiN film  205  are etched and removed according to anisotropic etching method like RIE by using this patterned resist  207  as masking material. As a consequence, the SOI film  203  and thermally-oxidized film  204  in the device isolation expected region are exposed outside ( FIG. 2D ).  
         [0063]     Subsequently, after the resist film  207  remained on the SOI substrate is removed, the SOI film  203  and the thermally-oxidized film  204  are etched according to reactive ion etching (RIE) method by using the BSG/BiN films  205 - 206  as masking materials. This etching depth is set to, for example, 1500 Angstrom. Therefore, the SOI film  203  out of the device formation area is removed.  
         [0064]     After this etching, reaction product at the time of etching is removed by washing and subsequently, the BSG film  206  existing on the SiN film  205  is removed according to, for example, wet etching method ( FIG. 2E ).  
         [0065]     Then, thermally-oxidized film  208  is formed in the thickness of about 40 Angstrom on the side face of the exposed SOI film  203  by thermal oxidation. The side portion of this thermally-oxide film is filled with TEOS film  209  because there is a trench with respect to a device formation area of an adjacent cell. Subsequently, according to CMP method (chemical mechanical polishing method) with the SiN film on the SOI used as stopper film, TEOS of a unnecessary portion is removed and flattened so as to form shallow trench isolation (STI). At this time, the remained film on the wafer of the SiN film is about 1300 Angstrom ( FIG. 2F ).  
         [0066]     Next, resist pattern in which only a cell portion is exposed is formed and after that, the resist is removed by ion-implantation of phosphor for plate electrode formation. SiN film  210  is deposited on the SOI wafer in the thickness of 200 Angstrom.  
         [0067]     Next, Resist pattern  212  is formed such that the BSG film corresponding to a gate electrode and a spacer adjacent to the gate is exposed and with this resist  212  used as masking material, the SiN film  210 ,  205 , and the thermally-oxidized film  204  are etched up to the face of the SOI substrate so as to form a trench  213  with the top face of the SOI  203  exposed externally ( FIG. 2G ).  
         [0068]     After the remained resist  211  is removed, a pattern for exposing the SOI portion whose film is expected to be thinned is formed again and by etching the exposed SOI film  203  according to CDE method, a deeper trench  214  is formed so as to turn the thickness of the SOI film  203  to a desired thickness, for example, 400 Angstrom ( FIG. 2H ). Therefore, the thickness at a gate electrode formation expected location is reduced from the initially 1500 Angstrom to 400 Angstrom ( FIG. 2H ).  
         [0069]     Next, by thermal oxidation with this SOI film exposed, thermally-oxidized film is formed in the thickness of 60 Angstrom on the bottom and side wall of the trench  214  and the trench  214  is filled with amorphous silicon doped with no impurity such that it is deposited in the thickness of 2000 Angstrom and then, damascene process is carried out for flattening using the CMP method. Further, by removing the SiN film remained on a portion on the source/drain of a portion formed by burying and the SOI substrate with chemical like H 3 PO 4 , gate electrode  215  is obtained. After thermally-oxidized film  216  is formed on an exposed surface by thermal oxidation, ion implantation is carried out for doping for formation of desired diffusion layer. This ion implantation is carried out under a condition that dose of phosphor or boron is 3E13/cm 2  and acceleration voltage is 10 keV ( FIG. 2I ).  
         [0070]     Next, TEOS/SiN/TEOS 200/200/400 Angstrom thick are deposited as layered film on the entire surface of the SOI substrate and etching is carried out for leaving spacer  218  adjacent to the gate electrode  215  within the trench  214  so as to expose oxide film  216  on the top face of the SOI substrate  203  and polysilicon electrode  215 . Then, phosphor or boron is ion-implanted into a desired region at a dose of 3E15/cm 2  or more at an acceleration voltage of 10 keV, so that impurity is activated at high temperature for a short time using RTA method ( FIG. 2J ).  
         [0071]     After activation is ended, the oxidized film  216  is removed, cobalt (Co) is deposited on the entire surface of the SOI substrate and salicide film  219  is formed at a portion making direct contact with silicon by heat treatment. Excess cobalt at a portion in which no silicon exists is removed by etching.  
         [0072]     After the salicide film is formed, SiN is deposited in the thickness of 200 Angstrom on the entire surface of wafer and next non-doped silicate glass (NSG) film  120 , which is silicate glass doped with no impurity, is deposited in the thickness of 6000 Angstrom and then it is flattened using the CMP method ( FIG. 2K ).  
         [0073]     Next, resist pattern for forming a contact between source/drain region and gate electrode is formed and the NSG film  220  is etched using the RIE method until the top face of the SiN is exposed so as to form a contact hole  221  and remained resist is removed. Next, the SiN film exposed on the bottom of the contact hole  121  is removed by etching so that the top face of SOI/gate polysilicon is exposed within the contact hole  221  and then, three-layered film of Ti/TiN/W are deposited in the thickness of 100/200/3000 Angstrom respectively. Then, the Ti/TiN/W of other portion than the contact portion is removed using the CMP method so that the Ti/TiN/W stacked film  222  is left only inside the contact. Next, NSG film  223  about 3000 Angstrom is deposited and a channel  224  is formed at a portion corresponding to a wiring layer pattern and Cu is deposited therein by plating. Then, SL electrode  225  is formed by etching the Cu at an unnecessary portion using the CMP method.  
         [0074]     After that, wiring metal layer and contact are formed using dual damascene process of executing these at the same time as filling a contact with metal (Cu) and finally, a semiconductor device is completed.  
         [0075]     Because in the semiconductor device of this embodiment, the thickness of the SOI film at the channel portion is sufficiently small, FD state can be attained so as to increase ΔVth. This is particularly advantageous when this semiconductor device is used as a semiconductor device containing FBC cell.  
         [0076]     Because according to this embodiment, the spacer is formed within the trench whose film thickness is reduced, the thin portion is expanded thereby obtaining a secure FD state.  
         [0077]     Although in the examples of the above-described two manufacturing processes, the dimension of the channel in which the gate electrode is to be formed is based on a dimension specified by lithography, it is permissible to set a portion expected to be thinned of the SOI smaller than a portion specified by lithography by using the RIE method for etching to leave the side wall by depositing the BSG film preliminarily before the CDE process.  
         [0078]      FIG. 3  is a sectional view of a semiconductor showing the structure of the semiconductor according to the third embodiment of the present invention.  
         [0079]     The third embodiment will be described by comparing the structure of the first embodiment shown in  FIG. 1L . In  FIG. 3 , reference numerals of the order from  300  are used and numbers of the order from  10  and from  1  correspond to those in  FIG. 1L .  
         [0080]     Although the structure of  FIG. 3  is almost the same as the first embodiment, a different point is the profile of drain region (D) and source region (S) formed in SOI film  303 . That is, although in  FIG. 3 , the drain region (D) has completely the same profile as the first embodiment and is in contact with thin SOI film in a channel portion, the front end of the source region (S) is located on the surface of the SOI film on the side face of a gate electrode and its diffusion depth is shallow in entire source region.  
         [0081]     To obtain such a structure, ion implantation condition for formation of diffusion layer described in  FIG. 1J  is changed between the drain region and source region and for the source region, the dose and acceleration voltage are reduced as compared to the drain region, as 3E13/cm 2  and 10 keV.  
         [0082]     Because with such a structure, coupling between the drain region and body region can be set larger than coupling between the source region and body region, deterioration of Vth is unlikely to occur even if the SOI in the channel region is thinned, thereby improving memory performance.  
         [0083]      FIG. 4  is a sectional view of a semiconductor showing the structure of the semiconductor according to the fourth embodiment of the present invention.  
         [0084]     The fourth embodiment will be described by comparing the structure of the first embodiment shown in  FIG. 1L . In  FIG. 4 , reference numerals of the order from 400 are used and numbers of the order from 10 and from 1 correspond to those in  FIG. 1L .  
         [0085]     Although the structure of  FIG. 4  is almost the same as the first embodiment, a different point is that the thickness of the SOI film constituting the drain region is equal to the thickness of the channel portion.  
         [0086]     Such a structure can be obtained by applying the manufacturing process described in the second embodiment. That is, an opening formed in the process of  FIG. 2G  is expanded over entire channel portion and drain region to reduce thickness of the SOI film over the entire channel portion and drain region.  
         [0087]     Although in this embodiment, as same as the embodiment shown in  FIG. 3 , the drain region (D) is in contact with the channel portion just below the gate electrode, the front end of the source region (S) is located on the surface of the SOI film on the side face of the gate electrode. Such a difference in profile is obtained by adopting the method described about the structure of  FIG. 3 .  
         [0088]     Because in the semiconductor device of the fourth embodiment also, the coupling between the drain region and body region is set larger than the coupling between the source region and body region, deterioration of Vth is unlikely to occur even if the SOI is thinned in the channel region, thereby improving the memory performance.  
         [0089]     Because in the semiconductor of the above-described embodiments, the SOI film of the channel portion is thinned sufficiently while maintaining the thickness of the BOX oxide film, complete depletion action for writing data “0” and partial depletion action for writing data “1” are enabled so as to increase ΔVth. This is advantageous when this semiconductor device is used as a semiconductor memory device containing FBC cell.  
         [0090]     Further, because the coupling capacity of the source side of the FBC can be increased, the ΔVth can be increased.  
         [0091]     Because according to the present invention, the thickness of the SOI film in the channel portion of a transistor is set smaller than the thickness of the SOI film of the source/drain diffusion layer portion, an excellent transistor characteristic is secured and a semiconductor device easy to manufacture can be provided.  
         [0092]     Such a semiconductor device is effective particularly if it is used as a memory device of the FBC memory cell.