Abstract:
According to the present invention, there is provided a semiconductor memory having a memory cell array region and peripheral circuit region, comprising, a gate electrode formed on a semiconductor substrate via a first insulating film in each of said memory cell array region and peripheral circuit region, and including a conductive layer which at least partially includes a silicon layer, and a second insulating film, a first oxide film formed on side surfaces of said conductive layer included in said gate electrode and on said semiconductor substrate in said memory cell array region, a second oxide film formed on side surfaces of said conductive layer included in said gate electrode and on said semiconductor substrate in said peripheral circuit region, and having a film thickness smaller than that of said first oxide film, a first nitride film formed on side surfaces of said gate electrode in said memory cell array region, and a second nitride film formed on side surfaces of said gate electrode in said peripheral circuit region, and having a film thickness larger than that of said first nitride film.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims benefit of priority under 35 USC 119 from the Japanese Patent Application No. 2002-303859, filed on Oct. 18, 2002, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor memory and a method of manufacturing the same and, more particularly, to a DRAM (Dynamic Random Access Memory: a random access memory requiring data storage) or a DRAM embedded memory having a DRAM function, and a method of manufacturing the same.  
         [0003]     It is increasingly demanded to maintain the cell characteristics (pause/retention) of a DRAM and increase the operating speed of a peripheral circuit at the same time. However, it is very difficult to satisfy both of these two requirements.  
         [0004]     To increase the operating speed of a transistor in a peripheral circuit, it is possible to improve the short channel effect by decreasing the depth of source and drain layers, and increase the driving power by reducing the channel resistance. To this end, however, an impurity must be ion-implanted at low acceleration when source and drain diffusion layers are formed, thereby requiring a thin oxide film on the substrate surface.  
         [0005]     To decrease the thickness of the oxide film on the substrate surface, the thickness of an oxide film on the side surfaces of a gate electrode must also be decreased. Unfortunately, this intensifies the gate edge electric field. Consequently, in the state in which data is stored by applying a voltage of about 0 or −0.4 [V] to the gate electrode, a GIDL (Gate Induced Drain Leakage) current by which electric charge passes through the substrate increases. This worsens the data holding characteristic.  
         [0006]     In the conventional manufacuring method, a gate electrode side wall oxidation process is performed at once after gate electrodes are formed. Therefore, the thickness of the sidewall oxide film, the shape of an end portion of the gate electrode in contact with the substrate surface, and the thickness of the oxide film on the substrate surface in a memory cell array region are the same as those in a peripheral circuit region.  
         [0007]     To improve the characteristics, particularly, the data holding time characteristic of a DRAM cell, it is preferable to increase the thickness of the sidewall oxide film of a transistor and increase the oxide film thickness near the end portion of the gate electrode in the memory cell array region. This is so because the GIDL current can be reduced by alleviating field concentration between the gate electrode end portion and the substrate.  
         [0008]     In the conventional device, however, the oxide film thickness in the memory cell array region is the same as that in the peripheral circuit region. Therefore, when an impurity is ion-implanted to form source and drain layers, this ion-implantation must be performed at relatively high acceleration so that the impurity is well implanted even if a thick oxide film is present. This makes it difficult to increase the operating speed of a transistor in the peripheral circuit region by decreasing the thickness of diffusion layers of the transistor.  
         [0009]     In contrast, if the sidewall oxide film thickness of the gate electrode is reduced to decrease the thickness of the source and drain diffusion layers of the transistor in the peripheral circuit region, the end-portion film thickness of the gate electrode of the transistor in the memory cell array region also decreases. This increases the GIDL current and deteriorates the data holding characteristic.  
         [0010]     Examples of references disclosing the conventional semiconductor memories are as follows.  
         [0000]     [Patent Reference 1] 
         [0011]     Japanese Patent Laid-Open No. 2002-43549  
         [0000]     [Patent Reference 2] 
         [0012]     U.S. Pat. No. 6,235,574B1  
         [0013]     In the conventional semiconductor memory as described above, the optimum conditions of the sidewall oxide film thickness of the gate electrode in the memory cell array region have a tradeoff relationship with those in the peripheral circuit region. Accordingly, it is necessary to sacrifice one of these gate electrodes or to form them under intermediate conditions. This makes it impossible to improve the holding characteristic of the memory cell and increase the operating speed of the peripheral circuit at the same time.  
       SUMMARY OF THE INVENTION  
       [0014]     According to an aspect of the present invention, there is provided a semiconductor memory device having a memory cell array region and peripheral circuit region, comprising:  
         [0015]     a gate electrode formed on a semiconductor substrate via an insulating film in each of said memory cell array region and peripheral circuit region, and including a conductive layer;  
         [0016]     a first oxide film formed on side surfaces of said conductive layer of said gate electrode and on said semiconductor substrate in said memory cell array region;  
         [0017]     a second oxide film formed on side surfaces of said conductive layer of said gate electrode and on said semiconductor substrate in said peripheral circuit region, and having a film thickness smaller than that of said first oxide film;  
         [0018]     a first nitride film formed on side surfaces of said gate electrode in said memory cell array region; and  
         [0019]     a second nitride film formed on side surfaces of said gate electrode in said peripheral circuit region, and having a film thickness larger than that of said first nitride film.  
         [0020]     According to an aspect of the present invention, there is provided a semiconductor memory device having a memory cell array region and peripheral circuit region, comprising:  
         [0021]     a gate electrode formed on a semiconductor substrate via an insulating film in each of said memory cell array region and peripheral circuit region, a first oxide film formed on side surfaces of said gate electrode in said memory cell array region;  
         [0022]     a second oxide film formed on side surfaces of said gate electrode in said peripheral circuit region, and having a film thickness smaller than that of said first oxide film;  
         [0023]     a first nitride film formed on said first oxide film on the side surfaces of said gate electrode in said memory cell array region; and  
         [0024]     a second nitride film formed on said second oxide film on the side surfaces of said gate electrode in said peripheral circuit region, and having a film thickness larger than that of said first nitride film.  
         [0025]     According to an aspect of the present invention, there is provided a method for manufacturing semiconductor memory device having a memory cell array region and peripheral circuit region, comprising:  
         [0026]     forming a first insulating film on a semiconductor substrate in the memory cell array region and peripheral circuit region;  
         [0027]     forming, on the first insulating film, a conductive layer which at least partially includes a silicon layer, and a second insulating film;  
         [0028]     forming a gate electrode by patterning the conductive layer and second insulating film into a gate electrode shape;  
         [0029]     forming a first oxide film by annealing side surfaces of the conductive layer included in the gate electrode and a surface of the semiconductor substrate in an oxidizing ambient;  
         [0030]     forming a first nitride film on an entire surface;  
         [0031]     patterning the first nitride film such that the first nitride film is removed from the memory cell array region and left behind in the peripheral circuit region;  
         [0032]     forming a second oxide film by annealing side surfaces of the conductive layer included in the gate electrode and a surface of the semiconductor substrate in the memory cell array region in an oxidizing ambient, while the peripheral circuit region is covered with the first nitride film;  
         [0033]     etching the first nitride film remaining in the peripheral circuit region to leave the first nitride film behind on side surfaces of the gate electrode in the peripheral circuit region;  
         [0034]     forming a second nitride film on an entire surface; and  
         [0035]     etching the second nitride film to leave the second nitride film behind on the side surfaces of the gate electrodes in the memory cell array region and peripheral circuit region.  
         [0036]     According to an aspect of the present invention, there is provided a method for manufacturing semiconductor memory device having a memory cell array region and peripheral circuit region, comprising:  
         [0037]     forming a first insulating film on a semiconductor substrate in the memory cell array region and peripheral circuit region;  
         [0038]     forming a conductive layer which at least partially includes a silicon layer on the first insulating film;  
         [0039]     forming a gate electrode by patterning the conductive layer into a gate electrode shape;  
         [0040]     forming a first oxide film by annealing side surfaces of the gate electrode and a surface of the semiconductor substrate in an oxidizing ambient;  
         [0041]     forming a first nitride film on an entire surface;  
         [0042]     patterning the first nitride film such that the first nitride film is removed from the memory cell array region and left behind in the peripheral circuit region;  
         [0043]     forming a second oxide film by annealing the side surfaces of the gate electrode and the surface of the semiconductor substrate in the memory cell array region in an oxidizing ambient, while the peripheral circuit region is covered with the first nitride film;  
         [0044]     etching the first nitride film remaining in the peripheral circuit region to leave the first nitride film behind on the side surfaces of the gate electrode in the peripheral circuit region;  
         [0045]     forming a second nitride film on an entire surface; and  
         [0046]     etching the second nitride film to leave the second nitride film behind on the side surfaces of the gate electrodes in the memory cell array region and peripheral circuit region. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]      FIGS. 1-12  are the longitudinal sectional views showing the sections of a semiconductor memory in the respective manufacturing steps according to the first embodiment of the present invention.  
         [0048]      FIGS. 13-17  are the longitudinal sectional views showing the sections of a semiconductor memory in the respective manufacturing steps according to the second embodiment of the present invention.  
         [0049]      FIGS. 18-32  are the longitudinal sectional views showing the sections of a semiconductor memory in the respective manufacturing steps according to the third embodiment of the present invention.  
         [0050]      FIGS. 33-34  are the longitudinal sectional views showing the sections of a semiconductor memory in the respective manufacturing steps according to the fourth embodiment of the present invention.  
         [0051]      FIGS. 35-36  are the longitudinal sectional views showing the sections of a semiconductor memory in the respective manufacturing steps according to the fifth embodiment of the present invention.  
         [0052]      FIGS. 37-39  are the longitudinal. sectional views showing the sections of a semiconductor memory in the respective manufacturing steps according to the sixth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0053]     Embodiments of the present invention will be described below with reference to the accompanying drawings. The first to third embodiments are trench capacitor type DRAMs, and the fourth to sixth embodiments are stacked capacitor type DRAMs.  
         [0000]     (1) First Embodiment  
         [0054]     As shown in  FIG. 1 , in a surface portion of a semiconductor substrate  1 , an element isolation groove  2  about 3,000 Å deep is formed by photolithography and RIE. After a thermal oxide film is formed by thermally oxidizing silicon side surfaces in the element isolation groove  2 , a silicon oxide film such as TEOS is buried as an element isolation buried oxide film  3 . An element region and element isolation region are formed through a planarizing process such as chemical mechanical polishing (to be referred to as CMP hereinafter).  
         [0055]     This embodiment is a trench capacitor type DRAM. In a memory cell array region, therefore, a trench  19  is formed in the surface portion of the semiconductor substrate  1 , and a buried plate electrode  20 , insulating oxide film  21 , and storage node contact  22  are formed to obtain a trench capacitor  18 .  
         [0056]     On the surface of the semiconductor substrate  1 , a sacrificial oxide film  17  (about 70 Å thick) is formed by thermal oxidation. After that, a well/channel impurity is implanted in the memory cell array region and in a peripheral circuit region by photolithography and ion implantation. Finally, activation annealing is performed.  
         [0057]     After the sacrificial oxide film  17  on the semiconductor substrate  1  is peeled, as shown in  FIG. 2 , a gate oxide film  4  (about 70 Å thick) is formed by thermal oxidation. A phosphorus-doped, n-type amorphous silicon film  5  (about 1,000 Å thick) and tungsten silicon (WSi) film  6  (about 550 Å thick), for example, as gate electrode materials, and a cap silicon nitride film  7  (about 2,000 Å thick) are sequentially deposited by low-pressure chemical vapor deposition (to be referred to as LP-CVD), sputtering, or the like. The amorphous silicon film  5  can also be a p-type film.  
         [0058]     A photoresist film (not shown) having a gate electrode pattern is formed by photolithography. Reactive ion etching (to be referred to as RIE hereinafter) is then performed to etch the cap silicon nitride film  7 , and the amorphous silicon film  5  and tungsten silicon film  6  as the gate electrode materials, thereby forming the gate electrode pattern.  
         [0059]     To suppress abnormal oxidation of tungsten, an annealing step is performed by RTA (Rapid Thermal Anneal). After that, as shown in  FIG. 3 , RTO (Rapid Thermal Oxidation) is performed to form an oxide film  8  on the gate electrode side surfaces.  
         [0060]     Since the surface portion of the semiconductor substrate  1  also oxidizes, a silicon oxide film forms. The film thickness of the gate sidewall oxide film  8  is about 50 Å or less, preferably, about 20 Å, so as not to increase the oxide film thickness on the surface of the semiconductor substrate  1  such that low acceleration conditions are applicable when a source/drain impurity for a transistor in the peripheral circuit region is ion-implanted.  
         [0061]     Note that annealing for the gate sidewall oxide film  8  is sometimes unnecessary, depending on the conditions such as the gate electrode materials.  
         [0062]     As shown in  FIG. 4 , in the peripheral circuit region, a photoresist film (not shown) is formed and an impurity is ion-implanted in order to form an extended portion in an LDD structure of the source/drain diffusion layers of the transistor. After that, the resist film is peeled.  
         [0063]     After the impurity is ion-implanted, activation annealing for activating the implanted impurity is performed in, e.g., a nitrogen ambient at about 950° C. for about 10 sec. Note that this annealing step need not always be performed.  
         [0064]     As shown in  FIG. 5 , a silicon nitride film  10  for protecting the peripheral circuit region is deposited to have a thickness of, e.g., 100 Å by LP-CVD. The film formation temperature can be, e.g., about 750° C. However, if film formation at a lower temperature is possible, the film formation temperature is not limited to the above value. Then, a photoresist film  11  for protecting the peripheral circuit region is formed. As shown in  FIG. 6 , the silicon nitride film  10  deposited in the memory cell array region is peeled by isotropic etching (e.g., wet etching or CDE), and the photoresist film  11  is peeled after that.  
         [0065]     As shown in  FIG. 7 , while the silicon nitride film  10  is left behind only in the peripheral circuit region, a thermal oxide film  12  is formed on the side surfaces of a gate electrode  5  and on the surface of the semiconductor substrate  1  in the memory cell array region by thermal oxidation. The final thickness of the thermal oxide film  12  must be controlled to, e.g., about 100 to 120 Å. Note that this film thickness can be freely changed by combining the device characteristics and is not limited to the range of 100 to 120 Å. Since the peripheral circuit region is covered with the silicon nitride film  10 , the surface of the semiconductor substrate  1  and the side walls of the gate electrode  5  do not oxidize in the peripheral circuit region.  
         [0066]     Consequently, as shown in  FIG. 7 , bird&#39;s beaks  100  formed by the thermal oxide film  12  below the side surfaces of the gate electrode  5  in the memory cell array region are larger than bird&#39;s beaks  101  formed by the oxide film  8  below the side surfaces of the gate electrode  5  in the peripheral circuit region.  
         [0067]     In addition, the thickness of the oxide film formed on the side surfaces of the gate electrode  5  in the memory cell array region is larger than that in the peripheral circuit region. This brings about the same effect for the oxide film thickness on the surface of the semiconductor substrate  1 ; the oxide film on the substrate in the memory cell array region is thicker than that in the peripheral circuit region. This contributes to the formation of a shallow source/drain junction by ion implantation at low acceleration in the peripheral circuit region.  
         [0068]     After that, the silicon nitride film  10  is etched back by RIE under the conditions by which the selectivity is high for the thermal oxide film  12 . Consequently, as shown in  FIG. 8 , the silicon nitride film on the silicon substrate surface and on the element isolation buried oxide film in the peripheral circuit region are etched away, and the silicon nitride film side walls remain on the side surfaces of the transistor gate in the peripheral circuit region.  
         [0069]     In this state, a photoresist film (not shown) is formed by photolithography. As shown in  FIG. 9 , a source/drain impurity is ion-implanted only in the memory cell array region, thereby forming source/drain diffusion layers  13 . After that, annealing for activating the impurity is performed in, e.g., a nitrogen ambient at about 950° C. for about 10 sec.  
         [0070]     A silicon nitride film about 200 Å thick is then deposited by LP-CVD, and the entire surface is etched back by RIE. Consequently, as shown in  FIG. 10 , a spacer silicon nitride film  14  is formed on the side surfaces of the gate electrodes  5  of the transistors in both the memory cell array region and peripheral circuit region. The thickness of the spacer silicon nitride film of the transistor in the peripheral circuit region is about 300 Å, i.e., larger than that (about 200 Å) in the memory cell region, since this spacer silicon nitride film includes both the silicon nitride film  14  and the silicon nitride film  10  formed in the step shown in  FIG. 8 .  
         [0071]     As described above, the film thickness of the gate electrode side walls in the peripheral circuit region is large, so the source/drain diffusion layers can be formed away from the conductive layer  5  of the gate electrode. Accordingly, it is possible to suppress the short channel effect and increase the driving power of this transistor in the peripheral circuit region.  
         [0072]     In both the peripheral circuit region and memory cell array region, a photoresist film (not shown) is formed by photolithography, an impurity for forming a source/drain is ion-implanted, and activation annealing for activating the impurity is performed in, e.g., a nitrogen ambient at about 950° C. for about 10 sec. Consequently, as shown in  FIG. 11 , it is possible to form source/drain diffusion layers  15  in the transistor of the memory cell array region, and source/drain diffusion layers  16  in the transistor of the peripheral circuit region.  
         [0073]     After that, a barrier silicon nitride film  23  which functions as a stopper when contact hole etching is performed is deposited on the entire surface to have a film thickness of, e.g., about 80 Å by LP-CVD. In addition, a dielectric interlayer  28  made of BPSG is deposited and planarized by CMP. Contact holes are then formed by photolithography and RIE. In the peripheral circuit region, a contact  25  is formed by burying a metal material such as titanium/titanium nitride-tungsten. In the memory cell array region, a memory cell bit line contact  24  is formed.  
         [0074]     After that, aluminum or the like is deposited on the entire surface, and photolithography and RIE are used to form a wiring layer  26  such as a bit line in the memory cell array region, and a peripheral circuit wiring layer  27  in the peripheral circuit region, thereby finally completing a DRAM.  
         [0075]     In this embodiment, the gate electrode sidewall film thickness can be increased in the memory cell array region and decreased in the peripheral circuit region. In particular, the bird&#39;s beaks formed below the side walls of the gate electrode in the memory cell array region are larger than those in the peripheral circuit region. Since this alleviates field concentration in this vicinity, the GIDL current reduces, so the data holding characteristic improves. Furthermore, in the peripheral circuit region, the source/drain structure is formed as a shallow junction, and this realizes a high operating speed.  
         [0076]     In this embodiment, when the gate electrodes are formed, patterning is performed while the conductive layer made up of the polysilicon film  5  and tungsten film  6  and the cap silicon nitride film  7  are stacked. This makes the application of a self-alignment contact formation process feasible, and thereby improves the applicability to a device having a high integration degree, such as a general-purpose DRAM memory cell.  
         [0000]     (2) Second Embodiment  
         [0077]     This embodiment differs from the above first embodiment in the formation of source/drain diffusion layers of a transistor in a peripheral circuit region.  
         [0078]     The manufacturing process is similar to that of the first embodiment up to the step shown in  FIG. 7  (i.e., a trench capacitor  18  is formed, a gate electrode sidewall oxide film  8  is formed through the element isolation step and gate electrode formation step, transistor source/drain diffusion layers  9  are formed in the peripheral circuit region, a silicon nitride film  10  is formed only in the peripheral circuit region, and a thermal oxide film  12  is formed on the side surfaces of a gate electrode  5  in a memory cell array region by thermal oxidation).  FIG. 13  shows the section of the device in this stage.  
         [0079]     After that, the silicon nitride film  10  is etched back by RIE under the conditions by which the selectivity is high for the thermal oxide film  12 . Consequently, the silicon nitride film  10  on the surface of a semiconductor substrate  1  and on an element isolation buried oxide film  3  in the peripheral circuit region are etched away, and the silicon nitride film  10  remains on the side surfaces of a transistor gate electrode  5  in the peripheral circuit region.  
         [0080]     In this state, a photoresist film (not shown) is formed by photolithography, and a source/drain impurity is ion-implanted only in the memory cell array region, thereby forming source/drain diffusion layers  13  as shown in  FIG. 14 .  
         [0081]     Unlike in the first embodiment, a photoresist film (not shown) is then formed by photolithography, and a source/drain impurity is ion-implanted in a specific transistor region in the peripheral circuit region. Annealing for activating the impurity is performed at, e.g., about 950° C. for about 210 sec to form source/drain diffusion layers  30 .  
         [0082]     Subsequently, a silicon nitride film about, e.g., 200 Å thick is deposited by LP-CVD, and the entire surface is etched back by RIE. Consequently, as shown in  FIG. 15 , a spacer silicon nitride film  14  is formed on the side surfaces of the gate electrodes  5  in the transistor regions in both the memory cell array region and peripheral circuit region. The thickness of the spacer silicon nitride film of the transistor in the peripheral circuit region is about 300 Å, i.e., larger than that (about 200 Å) of the silicon nitride film  14  in the memory cell array region, since a film thickness of about 100 Å of the silicon nitride film  10  is added. This structure facilitates the formation of a source/drain structure for suppressing the short channel effect.  
         [0083]     In both the peripheral circuit region and memory cell array region, a photoresist film (not shown) is formed by photolithography, an impurity for forming a source/drain is ion-implanted, and activation annealing for activating the impurity is performed at, e.g., about 950° C. for about 210 sec. Consequently, it is possible to form source/drain diffusion layers  15  in the transistor region of the memory cell array region, and source/drain diffusion layers  16  in the transistor region of the peripheral circuit region.  
         [0084]     After that, as in the first embodiment, a barrier silicon nitride film  23  which functions as a stopper when contacts hole etching is performed is deposited on the entire surface to have a film thickness of, e.g., about 80 Å by LP-CVD. In addition, a dielectric interlayer  28  made of BPSG is deposited and planarized by CMP. Contact holes are then formed by photolithography and RIE. In the peripheral circuit region, a contact  25  is formed by burying a metal material such as titanium/titanium nitride-tungsten. In the memory cell array region, a memory cell bit line contact  24  is formed.  
         [0085]     After that, aluminum or the like is deposited on the entire surface, and photolithography and RIE are used to form a wiring layer  26  such as a bit line in the memory cell array region, and a peripheral circuit wiring layer  27  in the peripheral circuit region, thereby finally completing a DRAM.  
         [0086]     As in the above first embodiment, the gate electrode sidewall film thickness in the memory cell array region and that in the peripheral circuit region are different. This improves the data holding characteristic in the memory cell array region, and increases the operating speed in the peripheral circuit region by forming the source/drain structure as a shallow junction. Also, a self-alignment contact formation process can be applied when the gate electrodes are formed. This achieves high applicability to a device having a high integration degree, such as a general-purpose DRAM memory cell.  
         [0000]     (3) Third Embodiment  
         [0087]     In the first and second embodiments described above, the gate electrode has a stacked structure including a conductive layer and insulating layer. In this embodiment, the gate electrode has only a conductive layer.  
         [0088]     As shown in  FIG. 18 , to form an element region in a surface portion of a semiconductor substrate  1  through steps similar to those of the first embodiment, an element isolation groove  2  about 3,000 Å deep is formed by photolithography and RIE. After a thermal oxide film is formed by thermal oxidation on the side surfaces of the element isolation groove  2 , a silicon oxide film such as TEOS is buried as an element isolation buried oxide film  3  in the element isolation groove  2 . An element region and element isolation region are then formed through a planarizing process such as CMP. In addition, a trench capacitor  18  is formed in a memory cell array region.  
         [0089]     On the surface of the semiconductor substrate  1 , a sacrificial oxide film  17  (about 70 Å thick) is formed by thermal oxidation. After that, a well/channel impurity is implanted in the memory cell array region and in a peripheral circuit region by photolithography and ion implantation. Finally, activation annealing is performed.  
         [0090]     After the sacrificial oxide film  17  on the semiconductor substrate  1  is peeled, as shown in  FIG. 19 , a gate oxide film  4  about 70 Å thick is formed by thermal oxidation. As a gate electrode material, a phosphorus-doped amorphous silicon film  5 , for example, is deposited by LP-CVD. A photoresist film (not shown) having a gate electrode pattern is formed by photolithography. RIE is then performed to etch the gate electrode material to form gate electrodes  31 .  
         [0091]     As shown in  FIG. 20 , RTO is performed to form an oxide film  8  on the side surfaces of the gate electrodes  31 . Since the surface of the semiconductor substrate  1  and the surfaces of the gate electrode  31  are also oxidized, the silicon oxide film  8  is formed on the entire surface. The film thickness of the sidewall oxide film of the gate electrodes  31  is preferably about 20 Å, so as not to increase the oxide film thickness on the surface of the semiconductor substrate  1  such that low acceleration conditions can be applied when a source/drain impurity for a transistor in the peripheral circuit region is ion-implanted.  
         [0092]     To form an LDD-structure extended region in source/drain diffusion layers of the transistor in the peripheral circuit region, a desired impurity is ion-implanted by using a photoresist film (not shown), thereby forming source/drain diffusion layers  9 . After that, the resist film is peeled.  
         [0093]     After this diffusion layer formation impurity is ion-implanted, activation annealing for activating the implanted impurity is performed in, e.g., a nitrogen ambient at about 950° C. for about 10 sec. Note that this annealing step need not always be performed.  
         [0094]     As shown in  FIG. 22 , a silicon nitride film  10  for protecting the peripheral circuit region is deposited to have a thickness of, e.g., 100 Å by LP-CVD. The film formation temperature can be, e.g., about 750° C. However, if film formation at a low temperature is possible, a lower temperature is more preferable.  
         [0095]     Then, a photoresist film  11  is so formed as to protect the peripheral circuit region. After that, the silicon nitride film  10  deposited in the memory cell array region is peeled by isotropic etching (e.g., wet etching or CDE), and the photoresist film  11  is peeled after that.  
         [0096]     As shown in  FIG. 23 , while the silicon nitride film  10  is deposited only in the peripheral circuit region, a thermal oxidation step is performed. By this step, as shown in  FIG. 24 , a thermal oxide film  12  is formed on the side surfaces of the gate electrode  31  and on the surface of the semiconductor substrate  1  in the memory cell array region. The final thickness of the thermal oxide film  12  must be controlled to, e.g., about 100 to 120 Å. Note that this film thickness can be freely changed by combining the device characteristics and is not limited to the range of 100 to 120 Å. Since the peripheral circuit region is covered with the silicon nitride film  10 , the surface of the semiconductor substrate  1  and the side surfaces of the gate electrode  31  in the peripheral circuit region do not oxidize, so the thermal oxide film  12  does not form.  
         [0097]     After that, the silicon nitride film  10  is etched back by RIE under the conditions by which the selectivity is high for the thermal oxide film  12 . Consequently, as shown in  FIG. 25 , the silicon nitride film  10  on the semiconductor substrate  1  in the peripheral circuit region and on the buried oxide film  3  in the element isolation groove  2  is etched away, and the silicon nitride film  10  remains only on the side surfaces of the gate electrode  31  in the peripheral circuit region.  
         [0098]     In this state, as shown in  FIG. 26 , a photoresist film (not shown) is formed by photolithography. A source/drain impurity is then ion-implanted only in the memory cell array region to form source/drain diffusion layers  13 . After that, annealing for activating the impurity is performed in, e.g., a nitrogen ambient at about 950° C. for about 10 sec.  
         [0099]     As shown in  FIG. 27 , a silicon nitride film about 200 Å thick is deposited by LP-CVD, and the entire surface is etched back by RIE, thereby forming a spacer silicon nitride film  14  on the side surfaces of the gate electrodes  31  in both the memory cell array region and peripheral circuit region. As in the first and second embodiments, the thickness of the spacer silicon nitride films  10  and  14  in the peripheral circuit region is about 300 Å, i.e., larger than that (about 200 Å) of the silicon nitride film  14  in the memory cell region. Accordingly, it is possible to obtain a structure which facilitates the formation of a source/drain structure for suppressing the short channel effect.  
         [0100]     A photoresist film (not shown) is formed by photolithography in both the peripheral circuit region and memory cell array region. As shown in  FIG. 28 , an impurity is ion-implanted, and activation annealing for activating the impurity is performed in a nitrogen ambient at about 950 ° C. for about 10 sec. As a consequence, it is possible to form source/drain diffusion layers  15  in the memory cell array region, and source/drain diffusion layers  16  in the peripheral circuit region.  
         [0101]     As shown in  FIG. 29 , a liquid chemical such as hydrofluoric acid is used to etch away the thermal oxide films  4  and  12  on the surface of the semiconductor substrate  1  and on the gate electrodes  31 . In this manner, the silicon on the surface of the semiconductor substrate  1  and the polysilicon on the upper surfaces of the gate electrodes  31  are exposed.  
         [0102]     As shown in  FIG. 30 , a stacked film  32  made up of, e.g., cobalt/titanium/titanium nitride (Co/Ti/TiN) (120/200/200 Å thick) is deposited by sputtering or the like. Annealing is then performed in a nitrogen ambient at about 480° C. for about 60 sec, and the stacked film  32  is removed by decomposition with sulfuric acid. As shown in  FIG. 31 , annealing is performed again in a nitrogen ambient at about 800° C. for about 30 sec to selectively form a salicide film  33  on the surface of the semiconductor substrate  1  and on the upper surfaces of the gate electrodes  31 .  
         [0103]     To prevent deterioration of the data holding characteristic, it is also possible to add a process by which, e.g., no salicide film  33  forms on a storage node contact  22 . That is, a salicide process meeting the device characteristics can be applied.  
         [0104]     After that, as shown. in  FIG. 32  as in the first and second embodiments, a barrier silicon nitride film  23  which functions as a stopper when contact hole etching is performed is deposited on the entire surface to have a film thickness of, e.g., about 80 Å by LP-CVD. In addition, a dielectric interlayer  28  made of BPSG is deposited and planarized by CMP. Contact holes are then formed by photolithography and RIE. In the peripheral circuit region, a contact  25  is formed by burying a metal material such as titanium/titanium nitride-tungsten. In the memory cell array region, a memory cell bit line contact  24  is formed.  
         [0105]     After that, aluminum or the like is deposited on the entire surface, and photolithography and RIE are used to form a wiring layer  26  such as a bit line in the memory cell array region, and a peripheral circuit wiring layer  27  in the peripheral circuit region, thereby finally completing a DRAM.  
         [0106]     In this embodiment, as in the first and second embodiments, the gate electrode sidewall film thickness is small in the memory cell array region and large in the peripheral circuit region. This improves the data holding characteristic in the memory cell array region, and at the same time increases the operating speed in the peripheral circuit region by forming the source/drain structure as a shallow junction.  
         [0107]     Unlike in the first and second embodiments described previously, only the conductive layer such as a polysilicon film is deposited and patterned into the shape of an electrode. Since this improves the compatibility with a silicide process, this embodiment is particularly useful for a device which contains both a logic circuit requiring a high operating speed and a DRAM.  
         [0000]     (4) Fourth Embodiment  
         [0108]     In this embodiment, a trench capacitor in the first embodiment described previously is replaced with a stacked capacitor. The other constituent elements and their manufacturing steps are the same as in the first embodiment.  
         [0109]     As shown in  FIG. 33 , in a surface portion of a semiconductor substrate  1 , an element isolation groove  2  about 3,000 Å deep is formed by photolithography and RIE. After a thermal oxide film is formed by thermally oxidizing silicon side walls in the element isolation groove  2 , a silicon oxide film such as TEOS is buried as an element isolation buried oxide film  3 . An element region and element isolation region are formed through a planarizing process such as CMP.  
         [0110]     After that, transistors are formed in a memory cell array region and peripheral circuit region through the same steps as in the first embodiment shown in FIGS.  2  to  11 .  
         [0111]     In addition, as in the first embodiment, a barrier silicon nitride film  23  which functions as a stopper when contact hole etching is performed is deposited on the entire surface to have a film thickness of, e.g., about 80 Å by LP-CVD. Furthermore, a dielectric interlayer  28  made of BPSG is deposited and planarized by CMP. Contact holes are then formed by photolithography and RIE. In the peripheral circuit region, a contact  25  is formed by burying a metal material such as titanium/titanium nitride-tungsten. In the memory cell array region, a memory cell bit line contact  24  is formed.  
         [0112]     After that, aluminum or the like is deposited on the entire surface, and photolithography and RIE are used to form a wiring layer  26  such as a bit line in the memory cell array region, and a peripheral circuit wiring layer  27  in the peripheral circuit region.  
         [0113]     A DRAM is completed by forming a stacked capacitor  34  including a plate electrode  35 , capacitor dielectric film  36 , and storage node electrode  37 .  
         [0114]     In this embodiment, as in the first, second, and third embodiments described above, the gate electrode sidewall film thickness is small in the memory cell array region and large in the peripheral circuit region. This improves the data holding characteristic in the memory cell array region, and at the same time increases the operating speed in the peripheral circuit region by forming the source/drain structure as a shallow junction.  
         [0115]     Also, as in the first and second embodiments, a self-alignment contact formation process can be applied when the gate electrodes are formed. This achieves high applicability to a device having a high integration degree, such as a general-purpose DRAM memory cell.  
         [0000]     (5) Fifth Embodiment  
         [0116]     In this embodiment, a trench capacitor in the second embodiment described previously is replaced with a stacked capacitor. The other constituent elements and their manufacturing steps are the same as in the second embodiment.  
         [0117]     As shown in  FIG. 35 , in a surface portion of a semiconductor substrate  1 , an element isolation groove  2  about 3,000 Å deep is formed by photolithography and RIE. After a thermal oxide film is formed by thermally oxidizing silicon side walls in the element isolation groove  2 , a silicon oxide film such as TEOS is buried as an element isolation buried oxide film  3 . An element region and element isolation region are formed through a planarizing process such as CMP.  
         [0118]     After that, transistors are formed in a memory cell array region and peripheral circuit region through the same steps as in the second embodiment shown in FIGS.  13  to  17 .  
         [0119]     In addition, as in the second embodiment, a barrier silicon nitride film  23  which functions as a stopper when contact hole etching is performed is deposited on the entire surface to have a film thickness of, e.g., about 80 Å by LP-CVD. Furthermore, a dielectric interlayer  28  made of BPSG is deposited and planarized by CMP. Contact holes are then formed by photolithography and RIE. In the peripheral circuit region, a contact  25  is formed by burying a metal material such as titanium/titanium nitride-tungsten. In the memory cell array region, a memory cell bit line contact  24  is formed.  
         [0120]     After that, aluminum or the like is deposited on the entire surface, and photolithography and RIE are used to form a wiring layer  26  such as a bit line in the memory cell array region, and a peripheral circuit wiring layer  27  in the peripheral circuit region.  
         [0121]     A DRAM is completed by forming a stacked capacitor  34  including a plate electrode  35 , capacitor dielectric film  36 , and storage node electrode  37 .  
         [0122]     In this embodiment, as in the first, second, and third embodiments described above, the gate electrode sidewall film thickness is small in the memory cell array region and large in the peripheral circuit region. This improves the data holding characteristic in the memory cell array region, and at the same time increases the operating speed in the peripheral circuit region by forming the source/drain structure as a shallow junction.  
         [0123]     Also, as in the second embodiment, a self-alignment contact formation process can be applied when the gate electrodes are formed. This achieves high applicability to a device having a high integration degree, such as a general-purpose DRAM memory cell.  
         [0000]     (6) Sixth Embodiment  
         [0124]     In this embodiment, a trench capacitor in the third embodiment described previously is replaced with a stacked capacitor. The other constituent elements and their manufacturing steps are the same as in the third embodiment.  
         [0125]     As shown in  FIG. 37 , in a surface portion of a semiconductor substrate  1 , an element isolation groove  2  about 3,000 Å deep is formed by photolithography and RIE. After a thermal oxide film is formed by thermally oxidizing silicon side walls in the element isolation groove  2 , a silicon oxide film such as TEOS is buried as an element isolation buried oxide film  3 . An element region and element isolation region are formed through a planarizing process such as CMP.  
         [0126]     After that, transistors are formed in a memory cell array region and peripheral circuit region through the same steps as in the third embodiment shown in FIGS.  18  to  32 .  
         [0127]     In addition, as in the third embodiment, a barrier silicon nitride film  23  which functions as a stopper when contact hole etching is performed is deposited on the entire surface to have a film thickness of, e.g., about 80 Å by LP-CVD. Furthermore, a dielectric interlayer  28  made of BPSG is deposited and planarized by CMP. Contact holes are then formed by photolithography and RIE. In the peripheral circuit region, a contact  25  is formed by burying a metal material such as titanium/titanium nitride-tungsten. In the memory cell array region, a memory cell bit line contact  24  is formed.  
         [0128]     After that, aluminum or the like is deposited on the entire surface, and photolithography and RIE are used to form a wiring layer  26  such as a bit line in the memory cell array region, and a peripheral circuit wiring layer  27  in the peripheral circuit region.  
         [0129]     A DRAM is completed by forming a stacked capacitor  34  including a plate electrode  35 , capacitor dielectric film  36 , and storage node electrode  37 .  
         [0130]     In this embodiment, as in the third embodiment described above, the gate electrode sidewall film thickness is small in the memory cell array region and large in the peripheral circuit region. This improves the data holding characteristic in the memory cell array region, and at the same time increases the operating speed in the peripheral circuit region by forming the source/drain structure as a shallow junction.  
         [0131]     Each of the above embodiments is merely an example and hence does not limit the present invention, so each embodiment can be variously modified within the technical scope of the present invention. For example, the material, formation method, formation conditions, and thickness of each film are examples and can be freely selected as desired.  
         [0132]     In the semiconductor memory and the method of manufacturing the same as described above, the gate electrode sidewall film thickness is decreased in the memory cell array region and increased in the peripheral circuit region. This increases the size of bird&#39;s beaks formed below the side surfaces of the gate electrode in the memory cell array region. Since this alleviates the electric field between this portion and the source/drain diffusion layers, the GIDL current reduces, so the data holding characteristic improves. In addition, the source/drain impurity can be ion-implanted at low acceleration in the peripheral circuit region. Therefore, the source/drain structure is realized as a shallow junction. This suppresses the short channel effect and increases the operating speed of the transistor.