This application is a continuation of prior U.S. application Ser. No. 08/559,813, now U.S. Pat. No. 5,843,829, which was a divisional of U.S. application Ser. No. 08/265,040, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to fabrication of semiconductor devices, and more particularly to the method for fabricating a semiconductor device including the step for forming an amorphous silicon layer followed by a crystallization step thereof. 
     In the fabrication of semiconductor devices, it is known to deposit a semiconductor layer in the amorphous state and to form a polycrystalline semiconductor layer from such an amorphous semiconductor layer. For example, a dynamic random access memory (DRAM) is formed by depositing a thin amorphous silicon layer first, followed by the formation of thin, fin-shaped electrodes of polysilicon. It should be noted that a stacked fin capacitor includes a number of electrode fins stacked upon each other. In the fabrication of such a stacked fin capacitor electrode, one or more amorphous silicon layers are deposited one after another, with intervening silicon oxide layers. As such a fabrication step includes the step of heating the amorphous silicon layer to a temperature of 800-1000° C., the amorphous silicon layer generally experiences crystallization as a result of the high temperature. In other words, the amorphous silicon layer is converted to a polysilicon layer as a result of the heating. By forming the electrode of the stacked fin capacitor by way of deposition of an amorphous silicon layer, it becomes possible to obtain an electrode having an extremely smooth surface. Thus, by using the electrode having such a smooth surface, it becomes possible to deposit a very thin dielectric layer on the surface of the electrode without introducing therein defects such as a pinhole. 
     FIG.1 shows the structure of a conventional DRAM including a stacked fin capacitor. 
     Referring to FIG.1, the illustrated device is constructed on a p-type substrate  1  covered by a field oxide film  2 , wherein the field oxide film  2  includes an aperture defining a device region  2   a . Thus, the surface of the substrate  1  is exposed in correspondence to the device region  2   a , and a polysilicon pattern forming a word line WL extends over the surface of the substrate thus exposed. 
     In the device region  2   a , n + -type diffusion regions  4  and  7  are formed at both sides of the word line WL acting as the gate of a MOS transistor Tr, as source and drain regions. Further, a p-type channel region CH is formed immediately below the word line. Although not illustrated, it should be noted that there is formed a thin gate insulation film underneath the word line WL acting as the gate, similarly to other MOS transistors. Further, another word line WL extends on the field oxide film  2  in parallel to the word line WL described previously. 
     On the field oxide film  2 , an insulator layer  3  of silicon oxide is deposited such that the insulator layer  3  covers the word line WL as well as the device region  2   a . Further, the insulator layer  3  is formed with contact holes  5  and  8  such that the contact holes expose the diffusion regions  4  and  7 . On the insulator layer  3 , a polysilicon pattern is provided as a bit line BL such that the bit line BL passes over the contact hole  8 , wherein the bit line BL establishes a contact to the diffusion region  7  at the contact hole  8 . On the other hand, a stacked fin capacitor Q is formed on the insulator layer  3  in correspondence to the contact hole  5  such that the capacitor Q is connected to the diffusion region  4  via the contact hole  5 . 
     It should be noted that the stacked fin capacitor Q includes a hollow polysilicon trunk  6   a  contacted to the diffusion region  4  at the contact hole  5  and a fin region  6  formed of a number of thin polysilicon fins  6   b - 6   d  connected commonly to the trunk  6   a , wherein the polysilicon fins  6   b - 6   d  extend laterally. Further, a thin dielectric film  6   e  of silicon oxide or silicon nitride (Si 3 N 4 ) is formed on the surface of the foregoing polysilicon fins  6   b - 6   d  as well as on the inner surface of the hollow trunk  6   a . Further, there is provided a polysilicon layer forming an opposing electrode  6   f  at the outside of the dielectric film  6   e  such that the dielectric film  6   e  is sandwiched between the fin electrode  6  and the opposing electrode  6   f . Generally, the electric charges accumulated in a capacitor increase with decreasing thickness of the dielectric film of the capacitor. Thus, it is desirable to reduce the thickness of the dielectric film  6   e  as much as possible in the stacked fin capacitor Q. In fact, a silicon nitride film having a thickness of about 7 nm is used for the dielectric film  6   e.    
     When using such a thin dielectric film for the stacked fin capacitor, it is necessary to form the surface of the polysilicon fin as smooth as possible to avoid formation of defects such as a pinhole. Thus, it has been practiced to form the polysilicon fin by first depositing an amorphous silicon film by a vapor phase deposition process such as CVD (chemical vapor deposition), followed by a step of providing a conductivity thereto by way of ion implantation or other suitable doping process. As the amorphous silicon layer has an extremely smooth surface, the polysilicon layer formed as a result of crystallization of such an amorphous silicon layer also has a smooth surface. 
     On the other hand, the fabrication of semiconductor devices having a complex structure as in the case of the device of FIG. 1, includes a large number of fabrication steps, and there is a tendency that the throughput of production is reduced. In order to improve the throughput as much as possible, recent semiconductor plants use so-called cluster type production systems wherein unnecessary transport or storage process is minimized or eliminated. In the cluster type production system, various processing stations such as vapor phase deposition apparatuses and etching apparatuses are connected with each other by a transportation chamber. The product in fabrication is transported between various processing stations one by one via the transportation chamber, without contacting the air outside. 
     FIG. 2A shows an example of such a cluster type vapor phase deposition apparatus. 
     Referring to FIG. 2A, the apparatus includes a first reaction chamber  11   a  and a second reaction chamber  11   b , as well as a transportation chamber  12  connecting the foregoing first and second reaction chambers  11   a  and  11   b . Further, each of the reaction chambers has a construction shown in FIG.  2 B. 
     Referring to FIG. 2B, the reaction chamber includes a shower nozzle  11   1  supplied with a source gas via a piping  11   2 , and a substrate  11   4  is held on a holder  11   3  that includes therein a heater mechanism (not shown) such that the substrate  11   4  faces the shower nozzle  11   1 . The source gas is introduced from the shower nozzle  11   1  to the surface of the substrate  11   4  heated by the heater mechanism, wherein the constituent atoms of the semiconductor layer to be deposited are released as a result of the pyrolytic decomposition of the source gas in the vicinity of the substrate  11   4 . In the vapor phase deposition apparatus of FIG. 2A, one may conduct the deposition of the amorphous silicon layer forming the fins  6   b - 6   d  in the first reaction chamber  11   a  and conduct the deposition of a dielectric film  6   e  on the foregoing amorphous silicon layer. 
     The substrate processed in the reaction chamber  11   a  is transported one by one to the reaction chamber  11   b  via the transport chamber  12 , while the loading and unloading of the substrate to and from the transportation chamber  12  is carried out via a load gate  13 . The transportation chamber  12  is filled with an inert gas such that the substrate does not contact with the air in the external environment even when it is transported between processing stations. By using such a cluster type vapor phase deposition apparatus, it becomes possible to eliminate the conventional processes, employed in the conventional stand alone type apparatuses, for storing half products obtained after the processing in the reaction chamber  11   a  and for transporting such half products for a long distance as a batch. As a result, the throughput of the production of semiconductor devices increases substantially. 
     FIG. 3 shows another example of the cluster type vapor phase deposition apparatus. 
     Referring to FIG. 3, the apparatus includes a first reaction chamber  11   a ′ and a second reaction chamber  11   b ′ connected with each other by a transportation chamber  12 ′ and processes a plurality of substrates simultaneously in each of the reaction chambers. In correspondence to each of the reaction chambers  11   a ′ and  11   b ′, there is provided an external heating device such as an infrared lamp for heating the substrates in the reaction chamber. Thus, the apparatus of FIG. 3, capable of processing a plurality of substrates simultaneously, can realize a high throughput of production. 
     Meanwhile, the inventors of the present invention have discovered that the surface of the electrode fins  6   b - 6   d  becomes rough when a DRAM shown in FIG. 1 is fabricated by using such a cluster type vapor phase deposition apparatus, particularly in the step of crystallizing the amorphous silicon layer deposited in the reaction chamber  11   a  as the electrode fins  6   b - 6   d . When the surface of the fins  6   b - 6   d  becomes rough, the chance that defects such as pinholes are formed in the thin dielectric film  6   e  on the amorphous silicon layer, increases substantially. When such a pinhole is formed, the capacitance of the capacitor decreases substantially. It is believed that such a rough surface develops as a result of the grain growth of silicon crystals at the time of crystallization of the amorphous silicon layer. 
     It should be noted that such a roughening of the surface of the silicon layer associated with the crystallization of the amorphous silicon layer appears characteristically when a cluster type production facility is used for increasing the throughput of production. As long as a conventional stand alone type apparatus is used, the problem does not emerge. Thus, there has been a problem in the conventional fabrication process of semiconductor devices, an example being a DRAM, in that the cluster type apparatus cannot be used effectively for improving the throughput of production. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful fabrication method of a semiconductor device wherein the foregoing problems are eliminated. 
     Another and more specific object of the present invention is to provide a method for fabricating a semiconductor device that uses a cluster type apparatus such that a plurality of semiconductor substrates, deposited with an amorphous silicon layer as a result of a processing in a first processing station, are subjected to a heat treatment process in a second station, while transporting the substrates one by one from said first station to said second station each time the processing for a substrate is completed in said first station without exposing the substrates to external atmosphere, such that a crystallization occurs in the amorphous silicon layer in said second station, wherein the problem of roughening of the surface of said amorphous silicon layer is substantially eliminated. 
     Another object of the present invention is to provide a method for fabricating a semiconductor device, comprising the steps of: 
     setting the temperature of a substrate held in a vapor deposition apparatus to a first temperature; 
     depositing an amorphous silicon layer on a surface of said substrate by introducing a source gas containing silicon to said vapor deposition apparatus such that said source gas is decomposed in the vicinity of a surface of said substrate; 
     elevating the temperature of said substrate, after said step of deposition for depositing said amorphous silicon layer, from said first temperature to a second, higher temperature, said second temperature being chosen such that a crystallization occurs in said amorphous silicon layer; and 
     crystallizing said amorphous silicon layer by holding the temperature of said substrate at said second temperature to convert said amorphous silicon layer to a polysilicon layer; 
     wherein said step of elevating the temperature comprises an oxidation step for introducing a gas that contains oxygen in gas molecules to said vapor deposition apparatus before a crystallization occurs in said amorphous silicon layer, such that an oxide film is formed on the surface of said amorphous silicon layer. 
     Another object of the present invention is to provide a method for fabricating a semiconductor device, comprising the steps of: 
     setting the temperature of a substrate held in a vapor deposition apparatus to a first temperature; 
     depositing an amorphous silicon layer on a surface of said substrate by introducing a source gas containing silicon to said vapor deposition apparatus such that said source gas decomposed in the vicinity of a surface of said substrate; 
     elevating the temperature of said substrate, after said step of depositing said amorphous silicon layer, from said first temperature to a second, higher temperature, said second temperature being chosen such that a crystallization occurs in said amorphous silicon layer; and 
     crystallizing said amorphous layer by holding the temperature of said substrate at said second temperature; 
     wherein there is provided an oxidation step, after said step of depositing the amorphous silicon layer and before a moment in which crystallization occurs in said amorphous layer, for introducing a gas containing oxygen in gas molecules, such that an oxidation film is formed on a surface of said amorphous silicon layer. 
     According to the present invention, it is possible to suppress the diffusion of silicon atoms along the surface of the amorphous silicon layer by providing an oxide film on the surface of the amorphous silicon layer. As a result, the growth of silicon crystals in the direction perpendicular to the plane of the substrate is effectively suppressed at the time of crystallization of the amorphous silicon layer. It is believed that such an artificial oxide film performs the role of the native oxide film that prevents the grain growth of silicon crystals and hence the development of rough surface in the crystallization of amorphous silicon layer in the conventional production system that uses a stand alone type deposition apparatuses. 
     It should be noted that the substrate has been left in the air for a long time period in the conventional batch type production system that uses a stand alone type vapor deposition apparatuses, after the step for depositing an amorphous silicon layer. Thus, the conventional fabrication process of semiconductor devices implicitly and inevitably includes the step of forming a native oxide film on the surface of the amorphous silicon layer that suppresses the diffusion of the silicon atoms along the surface of the amorphous silicon layer. Thus, such a native oxide film has suppressed the development of the rough surface in the crystallization of the amorphous silicon layer. When a cluster type apparatus is used for increasing the throughput, on the other hand, the substrate deposited with the amorphous silicon layer does not have a chance to contact with the air outside the deposition apparatus. Thus, no formation of native oxide film occurs on the surface of the amorphous silicon layer. In the present invention, an oxide film acting the role of the native oxide film is formed artificially on the surface of the amorphous silicon layer by introducing an oxidation gas to the vapor phase deposition apparatus for preventing the development of the rough surface. 
     According to the present invention, it becomes possible to produce semiconductor devices such as a DRAM that include a polysilicon layer formed as a result of crystallization of an amorphous silicon layer, with a high throughput, by using the cluster type production apparatus, without causing the development of rough surface of the polysilicon layer. When fabricating a DRAM, in particular, it becomes possible to maintain the surface of the fin electrodes forming a capacitor smoothly even when there occurred a crystallization of the amorphous silicon layer. Thus, it becomes possible to form an extremely thin dielectric film on the surface of the polysilicon layer thus crystallized, with a uniform thickness and without introducing defects. As the capacitor having such a thin dielectric film has a large capacitance, the DRAM fabricated as such also provides a large capacitance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the cross section of a conventional stacked fin capacitor; 
     FIGS. 2A and 2B are diagrams showing the schematical construction of the cluster type vapor deposition apparatus used in the present invention; and 
     FIG. 3 is a diagram showing the construction of another cluster type vapor deposition apparatus. 
     FIGS. 4A-4D are diagrams showing the fabrication of an MIM capacitor according to a first embodiment of the present invention; 
     FIG. 5 is a diagram showing the surface morphology of a polysilicon layer obtained as a result of the present invention; 
     FIG. 6 is a diagram showing the surface morphology of a polysilicon layer crystallized immediately after the deposition of an amorphous silicon layer; 
     FIG. 7 is a diagram showing the surface morphology of a polysilicon layer crystallized after a short exposure to the air after the step of depositing an amorphous silicon layer; and 
     FIGS. 8A-8H are diagrams showing the fabrication process of a DRAM according to a second embodiment of the present invention; 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, a first embodiment of the present invention for the fabrication process of an MIM capacitor will be described with reference to FIGS. 4A-4D. 
     In the step of FIG. 4A, an n + -type diffusion region  21   a  is formed on a substrate  21  of a p − -type silicon single crystal. Further, an amorphous silicon layer  22  is deposited on the substrate  21  by a vapor deposition apparatus shown in FIG. 2A or  2 B so as to cover the foregoing diffusion region  21   a . Typically, the amorphous silicon layer  22  is formed by holding the substrate  21  in a reaction chamber  11   a  of FIG. 2A and a source gas such as disilane (Si 2 H 6 ) is introduced into the reaction chamber  11   a . The amorphous silicon layer  22  is formed with a thickness of about 100 nm by causing a pyrolytic decomposition in the source gas thus introduced at a first temperature of about 450° C. Typically, the internal pressure of the reaction chamber  11   a  is set to about 10 Torr. In the illustrated example, the source gas of disilane and a carrier gas of N 2  were supplied with respective flowrates of 10 cc/min and 1000 cc/min. The amorphous silicon layer  22  thus deposited has no grain boundary and shows a very smooth surface with a surface roughness of less than several nanometers, in correspondence to the surface of the single crystal substrate  21 . 
     Next, in the step of FIG. 4B, the temperature of the substrate  21  is elevated from 450° C. to a second temperature of 600° C. or more, preferably 800° C. or more, while holding the substrate  21  in the reaction chamber  11   a . Further, an oxidation gas containing oxygen in the molecules is introduced into the reaction chamber  11   a  concurrently to or after the onset of the temperature elevation process. In the illustrated example, a nitrogen monoxide (N 2 O) gas is introduced to the reaction chamber  11   a  for 30 minutes at the moment when the substrate temperature has reached 800° C., with a flowrate of 1000 cc/min. The N 2 O gas thus introduced into the reaction chamber  11   a  experiences a pyrolytic decomposition at the foregoing second temperature, and oxygen atoms are released as a result. The oxygen atoms thus released cause an oxidation of the surface of the amorphous silicon layer  22 , and an oxide film  23  having a thickness less than about 5 nm is formed on the surface of the amorphous silicon layer  22  as indicated in FIG. 4B as a result of the oxidation. In order to facilitate the pyrolytic decomposition of the oxidation gas, the foregoing second temperature is set above the temperature of pyrolytic decomposition of the oxidation gas. It should be noted that the temperature of pyrolytic decomposition is about 600° C. in the case N 2 O is used for the oxidation gas. Thus, the foregoing second temperature is set to 600° C. or more in the temperature elevation process of FIG.  4 B. 
     After the step of FIG. 4B, the atmosphere of the reaction chamber  11   a  is switched to nitrogen (N 2 ), and the substrate is applied with a heat treatment process at a temperature of 800-1000° C., while flowing N 2  with a flowrate of 1000 cc/min. As a result of the heat treatment process, the amorphous silicon layer  22  experiences crystallization as indicated in FIG. 4C, and a polysilicon layer  22 ′ replaces the amorphous silicon layer  22 . It should be noted that, because of the fact that the oxide film  23  covers the surface of the amorphous silicon layer  22 , the diffusion of the silicon atoms along the surface of the layer  22  is effectively suppressed during the process of crystallization of the amorphous silicon layer  22 . Thus, the grain growth of the silicon grains in the polysilicon layer  22 ′ in the direction vertical to the surface of the layer  22 ′ is effectively suppressed. As a result, the polysilicon layer  22 ′ shows a smooth surface similar to the smooth surface of the original amorphous silicon layer  22 . 
     After the step of FIG. 4C, the temperature of the reaction chamber  11   a  is lowered to the room temperature. Further, the substrate  21  is transported to the second reaction chamber  11   b  via the transportation chamber  12  filled with an inert gas such as nitrogen, without exposing to the air. In the reaction chamber  11   b , a silicon oxide film  24  acting as the capacitor dielectric film is deposited on the polysilicon layer  22 ′ as indicated in FIG.  4 D. Typically, the pressure of the reaction chamber  11   b  is set to about 10 Torr, and the SiO 2  layer  24  is deposited with a thickness of about 20 nm as a result, of reaction between silane (SiH 4 ) and nitrogen monoxide (N 2 O). Further, a polysilicon layer  25  forming the opposing electrode of the capacitor is deposited with a thickness of about 1.00 nm, as a result of pyrolytic decomposition of disilane (Si 2 H 6 ) at a temperature of 400-650° C. Further, the layered body thus obtained is subjected to a patterning process in the step of FIG. 4D to form the desired capacitor, such that the n + -type region  21   a  acting as a capacitor electrode is exposed. 
     In the foregoing steps of FIGS. 4A-4D, one may conduct the step of FIG. 4C in the second reaction chamber  11   b . In this case, the substrate  21  is immediately transported to the reaction chamber  11   b  via the transportation chamber  12  without exposing to the air, after the process in the reaction chamber  11   a  for forming the oxide film  23  is completed. In the reaction chamber  11   b , the SiO 2  layer  24  and the polysilicon layer  25  are deposited directly on the oxide film  23  covering the amorphous silicon layer  22 . As the substrate  21  is held at the substrate temperature of 800° C. or more, a crystallization occurs in the amorphous silicon layer  22  similarly to the step of FIG. 4C, during the deposition of the layers  24  and  25 . 
     FIG. 5 shows the surface morphology of the polysilicon layer  22 ′, more strictly the surface of the oxide film  23  covering the polysilicon layer  22 ′, observed by an atomic force microscope. As will be seen clearly from FIG. 5, the polysilicon layer  22 ′ shows an extremely flat and smooth surface, with projections and depressions less than several nanometers in magnitude. 
     FIG. 6 shows the surface morphology of the polysilicon layer  22 ′ in which the step for forming the oxide film  23  on the surface of the amorphous silicon layer  22  is omitted. As will be seen from FIG. 3, there occurs a conspicuous grain growth in the amorphous silicon layer  22  as a result of crystallization of amorphous silicon, when the formation of the oxide film  23  is omitted. It will be noted that the projections and depressions thus formed in the layer  22 ′ have a magnitude of more than several tens of nanometers, some reaching even 100 nanometers. 
     As the polysilicon layer  22 ′ formed by the steps of FIGS. 4C and 4D has an extremely smooth surface, the dielectric film  24  formed thereon is substantially free from defects such as pinholes, even when the thickness of the film  24  is reduced. As a result, it becomes possible to increase the capacitance of the capacitor substantially. 
     FIG. 7 shows the surface morphology of a polysilicon layer  23  crystallized from the amorphous silicon layer without the step of FIG. 4B for forming the oxide film  23 . In this case, it should be noted that the substrate  21  is taken out from the vapor phase deposition apparatus to the air, after the step of FIG.  4 A and before the step of the crystallization, for about 30 minutes. It is expected that such an exposure to the air causes a formation of native oxide film on the surface of the amorphous silicon layer  22 . The result of FIG. 4 indicates, however, that the exposure to the air for such a short time is insufficient for the formation of the native oxide film that effectively prevents the grain growth of the silicon crystals by suppressing the atomic diffusion along the surface of the amorphous silicon layer. 
     In other words, the morphology control of the amorphous silicon layer during the crystallization process by forming an artificial oxide film is effective not only in the case wherein the crystallization occurs in the amorphous silicon layer immediately after the formation of the amorphous silicon layer without contacting to the air as in the case of the cluster type apparatus, but also in the case wherein the amorphous silicon layer is contacted to the air for a short time. 
     Next, a second embodiment of the present invention will be described with reference to FIGS. 8A-8H. 
     Referring to FIG. 8A, a thin oxide film  33  is formed on the surface of a p-type silicon substrate  31  in correspondence to a memory cell region. Further, a thick field oxide film  32  is formed so as to define the memory cell region. Typically, the field oxide film has a thickness of about 400 nm, while the oxide film  33  has a thickness of about 10 nm. After the field oxide film  32  and the oxide film  33  thus formed, a plurality of polysilicon word lines WL is formed on the surface of the substrate  31  so as to extend in a parallel relationship. Thus, the part of the word line WL extending over the oxide film  33  acts as a gate electrode  35  of the transfer gate transistor Tr, and the oxide film  33  underneath the word line WL acts as a gate oxide film. Typically, the polysilicon word lines WL are formed by depositing a polysilicon layer with a thickness of about 200 nm and conducting an ion implantation process for introducing phosphorus ions with a concentration. of about 1×10 20 /cm 3 , followed by a patterning step. Further, n + -type diffusion regions  36  and  37  are formed. at both sides of the gate electrode  35  as the source region and the drain region of the transfer gate transistor Tr, by conducting an ion implantation process while using the gate electrode  35  as a self-alignment mask. Thus, the transfer gate transistor Tr includes a source region formed of the diffusion region  36 , a drain region formed of the diffusion region  37 , and a gate formed of the polysilicon gate electrode  35  connected to the word line, as usual. 
     On the surface of the layered body thus formed, a silicon oxide layer  38  is formed by a CVD process with a thickness of about 100 nm as usual, and a contact hole  39  is formed for exposing the surface of the diffusion region  36 . Further, a polysilicon layer  40  and a tungsten silicide (WSi) layer  41  are deposited on the silicon oxide layer  38  consecutively with respective thicknesses of 50 nm and 100 nm, such that the polysilicon layer  40  establishes an electrical contact to the surface of the diffusion region  36  at the contact hole  39 . After the layers  40  and  41  are deposited, the polysilicon layer  40  is provided with conductivity by introducing phosphorous ions to the polysilicon layer  40  with a dose of 4×10 15 /cm 2  under an acceleration voltage of 70 keV. Further, the layers  40  and  41  are patterned to form bit lines BL in connection to the diffusion region  36 . It should be noted that the layers  40  and  41  thus patterned act also as a source electrode  42  of the transfer gate transistor Tr. See the structure of FIG.  8 B. 
     The structure of FIG. 8B is then introduced to a vapor deposition apparatus, and an etching stopper layer  43  of Si 3 N 4  is deposited thereon with a thickness of about 50 nm. Further, a silicon oxide layer  44  and an amorphous silicon layer  45  are deposited on the etching stopper layer  43  by a vapor deposition process with respective thicknesses of 30 nm and 20 nm. The amorphous silicon layer  45  is deposited as a result of pyrolytic decomposition of disilane at a temperature of 400-550° C., preferably at 450° C., similarly to the previous embodiment. Of course, the source gas for forming the amorphous silicon layer  45  is not limited to disilane but other gases such as silane (SiH 4 ) or trisilane (Si 3 H 8 ) may be employed. 
     In the present invention, the nitrogen monoxide (N 2 O) is introduced to the vapor deposition apparatus after the amorphous silicon layer  45  is formed, and the substrate temperature is raised subsequently above the pyrolytic decomposition temperature of N 2 O. As a result, a thin oxide film  45   a  is formed on the surface of the amorphous silicon layer  45  in correspondence to the oxide film  23  of FIG.  4 B. 
     After the oxide film  45   a  is thus formed on the surface of the amorphous silicon layer  45 , the substrate  31  is transported from the vapor phase deposition apparatus to the ion implantation apparatus via the transportation chamber of the cluster apparatus, without being exposed to the air. Further, an ion implantation of phosphorus is conducted in the ion implantation apparatus with a dose of 4×10 15 /cm 2  under an acceleration voltage of 5 keV. 
     After the ion implantation, the substrate  31  is returned to the reaction chamber of the deposition apparatus via the transportation chamber without contacting the air. Further, a silicon oxide layer  46  is deposited on the amorphous silicon layer  45  at a temperature of about 1000° C. with a thickness of about 30 nm. Associated with the deposition of the silicon oxide layer  46 , the amorphous silicon layer  45  experiences a crystallization and is converted to a polysilicon layer. As the surface of the amorphous silicon layer  45  is covered by the oxide film  45   a , the crystallization of the layer  45  does not cause the problem of rough surface associated with the grain growth. 
     Next, a step of FIG. 8D is conducted wherein the structure of FIG. 8C is taken out from the vapor phase deposition apparatus, and a contact hole  47  is formed by a photolithographic process such that the contact hole  47  exposes the diffusion region  37  in the substrate  31 . 
     Further, in a step of FIG. 8E, the structure of FIG. 8D is returned to the vapor phase deposition apparatus, and an amorphous silicon layer  48  is deposited at 450° C. with a thickness of about 20 nm, such that the amorphous silicon layer  48  covers the side wall of the contact hole  47  as well as the surface of the exposed diffusion region  37 . Further, the substrate temperature is raised to 800° C. and the nitrogen monoxide gas (N 2 O) is introduced concurrently, to form a thin oxide film  48   a  on the surface of the layer  48 . After the amorphous silicon layer  48  is thus formed, the layer  48  is continuously crystallized by the heat treatment at 800° C. while switching the atmosphere to nitrogen (N 2 ). As a result, the amorphous silicon layer  48  is converted to a polysilicon layer. The illustrated structure is then transported from the deposition apparatus to the ion implantation apparatus via the transportation chamber of the cluster apparatus, and the polysilicon layer thus formed is provided with a conductivity as a result of ion implantation of phosphorus. 
     Further, the layers  45 - 48  thus formed are subjected to a patterning process (see reference numerals  49 ) by way of a reactive ion etching (RIE) while using the silicon oxide layer  44  as an etching stopper, and a stacked fin electrode  50  is formed as indicated in FIG.  8 E. In the structure of FIG. 8E, too, it should be noted that the rough surface does not develop in the layer  48  because of the formation of the oxide film  48   a  that suppresses the grain growth of the silicon crystals. 
     Next, in the step of FIG. 8F, the silicon oxide layers  44  and  46  are removed by way of a selective etching. As a result of the etching, the oxide films  45   a  and  48   a  are also removed simultaneously. Further, a silicon nitride film  53  is deposited on the surface of the polysilicon fin capacitor  50  by way of a vapor deposition process conducted at 800° C. with a thickness of about 7 nm. As the polysilicon capacitor  50  has an extremely smooth surface due to the crystallization conducted while suppressing the grain growth of silicon crystals by means of the oxide films  45   a  and  48   a , there occurs no defects such as a pinhole in the silicon nitride film  53  even when the thickness of the film  53  is extremely small. 
     After the step of FIG. 8G, a polysilicon layer  54  is deposited on the structure of FIG. 8G as indicated in FIG. 8H, such that the polysilicon layer  54  forms an opposing electrode of the stacked fin capacitor. After the deposition of the polysilicon layer  54 , the polysilicon layer  54  is provided with a conductivity by way of a thermal diffusion process of phosphorus that uses POCl 3 . Further, a planarization layer  55  of spin-on-glass or boro-phosphorus silicate glass is formed for planarization, and an electrode  56  is provided thereon. As a result, a dynamic random access memory having a large capacitance is obtained as indicated in FIG.  8 H. 
     In any of the foregoing first and second embodiments, it should be noted that the oxidation gas supplied to the vapor deposition apparatus for forming the oxide layer on the surface of the amorphous silicon layer is not limited to nitrogen monoxide (N 2 O), but oxygen (O 2 ) or other various gases that contain oxygen atom in the molecule, such as NO x , CO, CO 2 , and the like, may be used. 
     Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.
A method for fabricating a semiconductor device includes the steps of depositing an amorphous silicon layer on a substrate, and forming an oxidation film on a surface of the amorphous silicon layer by treating the surface of the amorphous silicon layer with an oxidation gas. The forming step occurs before crystallization of the amorphous silicon layer.