Abstract:
The present invention has as its object to provide a technique of forming a surface of a thin-film semiconductor having corrugations and smoothing the same. This is achieved by a fabrication method for thin-film semiconductors which smooths a surface of a silicon film having corrugations, comprising the steps of forming an oxidized silicon film on the surface of the silicon film, removing the oxidized silicon film which has been formed in protruding portions among the corrugations and exposing at least part of protruding portions in the silicon film, and removing the protrusions in the silicon film exposed in the previous step. In the above structure, the silicon film having corrugations has an average thickness of about 100 Å to 1000 Å (e.g. an amorphous silicon film or a crystalline silicon film crystallized by thermal processing) which is irradiated by a laser beam and crystallized or a silicon layer promoting crystallization.

Description:
BACKGROUND OF THE INVENTION 
   The invention disclosed in this specification relates to a fabrication method for a thin-film semiconductor utilized in devices using thin-film semiconductors (for example, thin-film transistors, photo-electric conversion devices, etc.). 
   In recent years liquid crystal display devices utilizing thin-film transistors have become well-known. These are known as active matrix type devices, and have thin-film transistors respectively arranged in each pixel disposed in a matrix shape, these thin-film transistors controlling input and output of charges maintained in the pixel electrode of each pixel. These types of active matrix liquid crystal display devices are compact and light-weight, and in addition since they can display a high speed picture in minute detail, are expected to become the main force in future display devices. 
   Thin-film transistors which are utilized in active-matrix liquid crystal display devices require to be formed on the surface of a substrate having translucence. This is because light is required to pass through the substrate forming the liquid crystal display. 
   As a substrate having translucence, a glass or quartz substrate, or even a plastic substrate may be cited. In forming a thin-film semiconductor, since heating must be performed to a certain extent, utilizing a plastic substrate is inappropriate. Also, since a quartz substrate can withstand high temperatures in the order of 1000° C., it is appropriate as a substrate for forming a thin-film semiconductor, although it is generally unsuitable due to its high cost (in particular, over a large area it can be ten times the cost of a glass substrate or more). 
   Consequently, a glass substrate is generally used, the thin-film semiconductor being formed on the surface of this glass substrate. Currently, as a thin-film semiconductor, an amorphous silicon film is generally used. The amorphous silicon film can be formed by a plasma CVD method and heated to about 200 to 400° C., therefore a low-cost glass substrate can be utilized. 
   Also, where fabricating a thin-film transistor using an amorphous silicon film, there is the problem that the characteristics thereof are low. Accordingly, in order to achieve an active matrix liquid crystal display device having a display characteristic which is more effective than that obtainable under current circumstances, a thin-film transistor having an even higher characteristic is necessary. 
   In attaining a thin-film transistor having an even higher characteristic than a thin-film transistor using an amorphous silicon film a crystalline silicon film may be used as the thin-film semiconductor. A crystalline silicon film can be achieved by thermal processing of an amorphous silicon film. However, in such a case the following problems occur. Namely, although generally the withstand temperature of a glass substrate is 600° C. or less, crystallization of an amorphous silicon film requires temperatures of 600° C. and more. Thus techniques of performing thermal processing at a temperature of around 600° C. to crystallize an amorphous silicon film formed on a glass substrate are currently being researched. However, where crystallizing an amorphous silicon film at a temperature of about 600° C., it is necessary to perform thermal processing for some tens of hours or more (generally 24 hours or more), therefore there is the problem that practicality and productivity are extremely low. 
   As a technique for solving this problem, there is a technique of deforming the amorphous silicon film into a crystalline silicon film by irradiating it with a laser beam. Since irradiation by laser beam does not incur thermal damage to the lower level (base) glass substrate, the problem of thermal resistance of the glass substrate accompanying a method using thermal processing does not occur. 
   However, where an amorphous silicon film of about 1000 Å or less is irradiated by a laser beam, it is clear that corrugations form in the surface of the crystalline silicon film thus obtained. This tendency is particularly strong where the amorphous silicon film, which is the starting film, is thin at 1000 Å or less. Alternatively, from the problem of laser beam absorption the result that the thinner the film thickness (particularly 500 Å or less) of the amorphous silicon film which is the starting film the more favorable for crystallization. 
   Namely, where the thickness of the amorphous silicon film which is the starting film is made thin in order to facilitate crystallization, there exists the dilemma that the surface of the thus-obtained crystalline silicon film will have large corrugations. 
     FIG. 2  shows the state of the surface of an amorphous silicon film obtained by irradiating an amorphous silicon film of 500 Å thickness formed on a glass substrate with a laser beam.  FIG. 2  is a photograph taken when observing the surface of the amorphous silicon film with an atomic microscope. 
   Where a thin-film transistor is fabricated using a thin-film semiconductor, the state of the surface of the thin-film semiconductor is extremely important. This is because carriers are conducted in the surface of the thin-film semiconductor. If corrugations exist in the surface of the thin-film semiconductor, potential barriers, traps, etc. exist which give rise to disconnection or warping of the lattice, the moving carrier being dispersed, trapped, etc. 
   Also, where a thin-film transistor is fabricated using a thin-film semiconductor, although it is necessary to form a gate insulation film or other insulation film in contact with the thin-film semiconductor, if corrugations exist in the surface of the thin-film semiconductor step coverage of the insulation film is unsatisfactory, causing unfavorable insulation and instability. In addition, the corrugations in the surface of the thin-film semiconductor as described above become hindrances to fabrication of thin-film diodes, photo-electric conversion devices, etc. Consequently, it is preferable that the surface of the thin-film semiconductor be as smooth as possible. 
   SUMMARY OF THE INVENTION 
   The invention disclosed in this specification has as its object to provide a technique of forming a surface of a thin-film semiconductor having depressions and protrusions and smoothing the same. 
   One of the main inventions disclosed in this specification is a fabrication method for thin-film semiconductors which smooths a surface of a silicon film having depressions and protrusions, comprising the steps of forming an oxidized silicon film on the surface of the silicon film, removing the oxidized silicon film which has been formed in protruding portions among the depressions and protrusions and exposing at least part of protruding portions in the silicon film, and removing the protrusions in the silicon film exposed in the previous step. In the above structure, as the silicon film having depressions and protrusions, an example can be given of a silicon film having an average thickness of about 1000 Å or less (e.g. an amorphous silicon film or a crystalline silicon film crystallized by thermal processing) which is irradiated by a laser beam and crystallized or a silicon layer promoting crystallization. Note that in practice a silicon film with an average thickness of 100 Å or more is preferable. 
   Where the silicon film with an average thickness of 1000 Å or less is irradiated with a laser beam (e.g. an excimer laser having a wavelength of infra-red light or less) a crystalline silicon film with extremely favorable crystallization can be obtained. However, on the other hand as shown in  FIG. 2  the surface thereof has large depressions and protrusions. 
   Upon irradiating the silicon film with a laser beam, the surface of the silicon film reaches an instantaneous molten state. Then when it is cooled and hardened instantaneous crystallization advances and a crystalline silicon film is formed. 
   Comparing a silicon film in a crystallized state and a silicon film in a solution state, the solution state silicon film has the greater density. Consequently, where advancing from an instantaneous molten state to crystalline state, local expansion occurs and as a result depressions and protrusions are formed in the surface thereof. However, looking at this from a different point of view, as a result of the formation of these depressions and protrusions, since internal stress caused by the crystallization is alleviated, it can be said that this results in obtaining a silicon film having favorable low internal stress crystallinity. 
   In this manner, by irradiating the silicon film with a laser beam, corrugations forming in the surface of the silicon film are an unavoidable phenomenon in order to obtain a favorable low internal stress crystalline silicon film. It is clear that this phenomenon also is particularly remarkable where a silicon film having a thickness of 1000 Å or less is irradiated with a laser beam. 
   Also, where a metal element is added to promote crystallization of the silicon in the laser irradiated silicon film, since crystallization proceeds very effectively from the molten state due to laser irradiation, the corrugations form even more remarkably. 
   As the metal for promoting crystallization, one type or a of types from among Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au can be used. The element among these which can in particular achieve the most significant affect is nickel (Ni). 
   As the silicon film irradiated by laser beam, an amorphous silicon film, a crystalline silicon film crystallized by thermal processing, or a silicon film decrystallized by injection of ion impurities or the like may be cited. Also, as the silicon film irradiated by laser beam, an amorphous silicon film, using a material having a thickness of 1000 Å or less is one condition for obtaining favorable crystallinity. However, film thickness of 100 Å or less is generally not practical because it cannot maintain the state of a thin film. 
   In the invention disclosed in this specification, in order to remove the corrugations in the surface of the crystalline silicon film formed by laser irradiation as described above, the following steps are employed. 
   (1) Laminating an oxidized silicon film on the surface of the silicon film having corrugations. 
   (2) Removing the oxidized silicon film which has been formed in protruding portions among the corrugations and exposing at least a part of the protruding portions in the silicon film. 
   (3) Removing the exposed protruding portions in the silicon film. 
   Why oxidized silicon film is used in this series of steps is because its etching rate with silicon is extremely high with respect to a predetermined etchant. For example, where hydrazine is used as the etchant, although the silicon is easily removed by etching, the oxidized silicon is mostly not etched away. 
   Also, as a method of selectively etching the silicon, using halogen fluoride gas as indicated by ClF 3 , ClF, BrF 3 , IF 3 , BrF, BrFs and IF 5  is effective. Using ClF 3  is particularly effective. 
   In step (2), removing the oxidized silicon film which has been formed in protruding portions among the corrugations is to expose the protruding portions and remove only these exposed protruding portions in a subsequent step. A representative diagram showing a state where these protruding portions are exposed is given in FIG.  3 (B). What is shown in  FIG. 3  is a state where the oxidized silicon film  303  in the depressions indicated by  305  is left and the protruding portions indicated by  304  are exposed. 
   Then, in this state, by performing etching using an etchant which can selectively etch away only the silicon (e.g. hydrazine, ClF 3 , etc.), the state shown in FIG.  4 (A) can be attained. Then by performing an etching process using an etchant which can further selectively etch the oxidized silicon film  303  (e.g. a buffer hydrofluoric acid), a smoothed surface as shown in FIG.  4 (B) can be attained. Here, if the thickness of the oxidized silicon film  303  is approximately 100 Å, a surface having corrugations with a height difference of 100 Å or less can be achieved as the smoothness of the surface of the attained silicon layer. 
   Here, although a case where an oxidized silicon film is used as the film indicated by  303  has been explained, a material which can selectively leave silicon in the predetermined etching step of another film such as silicon nitride or the like can be used. 
   Namely, a material having a masking property when etching the silicon can be used in place of the oxidized silicon film explained here. As this type of masking material, a material having a lower etching rate than the silicon film in the predetermined etching method can be used. 
   Another structure of the invention is a method which smooths a surface of a silicon film having depressions and protrusions, comprising the steps of filling depressed portions with a filler and exposing protruding portions among the depressions and protrusions, and removing the protruding portions exposed in the previous step. 
   In the above structure, as the step of filling the depressed portions among the corrugations with a filler and exposing the protruding portions, the step illustrated in FIG.  3 (B) may be cited. In this step, by forming an oxidized silicon film over the corrugations and thereafter removing the oxidized silicon formed on the protruding portions, a state where the protruding portions indicated by  304  are exposed and the oxidized silicon film  303  is left in the depressed portions indicated by  305 , i.e. a state where the depressed portions indicated by  305  are filled with the oxidized silicon film  303 , is exhibited. 
   Then after reaching this state, by selectively removing the protruding portions indicated by  304 , protrusions in the surface are eliminated (these cannot be completely eliminated as shown in  FIG. 4 ) and a smoothed silicon film can be attained. 
   Another structure of the invention is a method which smooths a surface of a silicon film having depressions and protrusions, comprising the steps of forming an oxidized silicon film on the surface of the silicon film, and simultaneously removing the oxidized silicon film and the protruding portions of the silicon film. 
   The above structure achieves a state wherein the oxidized silicon film is formed on the surface of the silicon film having corrugations and these corrugations are sufficiently smoothed, and further, a silicon film having a smooth surface is ultimately achieved by performing etching with an etching method in which the etching rates of the silicon film and oxidized silicon film are sufficiently low. Namely, by forming an oxidized silicon film, after sufficiently smoothing the corrugations in the surface of the silicon film, etching progresses while maintaining the smoothness of the surface and ultimately a silicon film having a smooth surface is obtained by performing etching with a method or under conditions in which the etching rates of the silicon film and oxidized silicon film do not differ. 
   Concrete examples of the above steps are shown in FIG.  5  and FIG.  6 . Firstly, as shown in FIG.  5 (A) a silicon film (crystalline silicon film)  107  having corrugations is obtained. Thereafter, as shown in FIG.  5 (B) the oxidized silicon film  303  is formed and a state in which the surface is sufficiently smooth is reached. This state can be reached by forming the oxidized silicon film under a formation method or forming conditions in which step coverage is insufficient. Note that where the oxidized silicon film  303  is formed under a formation method or forming conditions in which step coverage is favorable, because film formation progresses with lower corrugations remaining as it is, caution is necessary. 
   Then, by uniformly promoting etching in the perpendicular direction as shown in FIG.  6 (A), the protruding portions and the oxidized silicon film can be etched away simultaneously and ultimately a silicon film  601  having a smooth surface can be obtained (FIG.  6 (B)). 
   This etching step can be performed by using an RIE method utilizing a gas mixture of CF 4  and oxygen for example. It is important that in this etching step a method or conditions be selected under which the etching rates of the silicon film and the oxidized silicon film are substantially equal. 
   Another structure of the invention is a method which smooths a surface of a first silicon film having depressions and protrusions, comprising the steps of forming a second silicon film on the surface of the first silicon film, and removing the first silicon film and second silicon film by performing etching so that the surface obtains a smoothed first silicon film. 
   In the above structure, by forming a further silicon film (generally an amorphous silicon film) on the surface of the silicon film having corrugations and thereafter by executing etching which is uniform in the direction of the thickness of the film, a smooth silicon film is obtained. 
   Concrete examples of the above structure will be explained using FIG.  5  and FIG.  6 . Firstly, an amorphous silicon film  303  (here  303  indicates an amorphous silicon film) is formed in the surface of the silicon film  107  having corrugations as shown in FIG.  5 (A) with a formation method in which step coverage is insufficient. As a result the surface reaches a flat state as shown in FIG.  5 (B) Then, as shown in FIG.  6 (A), by performing etching using an etching method having anisotropy in the perpendicular direction (e.g. RIE method), etching can be performed uniformly in the perpendicular direction and a silicon film having a smooth surface (smoothed) as shown in FIG.  6 (B) can be obtained. In this structure, the silicon film  107  having corrugations and the amorphous silicon film  303  can be made to have a substantially equal etching rate. Consequently, if the smoothness of the amorphous silicon film  303  can be ensured, smoothing of the surface of the silicon film  107  can be reliably realized. 
   Note that it is necessary to perform etching so that all of the amorphous silicon film  303  (refer to FIG.  5  and  FIG. 6 ) serving as the second silicon film is removed. This is because where the silicon film  107  having corrugations is crystalline, it is preferable for there to be no residual amorphous silicon film  303  on the surface thereof. 
   By forming an oxidized silicon film whose step coverage is unsatisfactory on the surface of a silicon film having corrugations, removing the thin oxidized silicon film formed on the protruding portions and selectively etching the exposed protruding portions immediately thereafter, a crystalline silicon film whose surface is smoothed can be obtained. 
   Also, by forming an oxidized silicon film whose step coverage is unsatisfactory on the surface of a silicon film having corrugations in a state where the surface thereof has the necessary flatness, and performing etching with the silicon and the oxidized silicon having the same etching rate, a smoothed silicon film having flatness can be obtained. 
   Further, by forming an upper silicon film whose step coverage is unsatisfactory on the surface of a silicon film having corrugations in a state where the surface thereof has the necessary flatness and performing further etching, a smoothed silicon film having flatness can be obtained. 
   By using the smoothed silicon film obtained by utilizing the invention disclosed in the present specification, an thin-film device having excellent electrical characteristics can be obtained without manufacturing difficulties. Also, the invention disclosed in the present specification can be utilized not only for silicon films formed with corrugations due to being irradiated by a laser beam, but for any case generally where a silicon film having corrugations is to be smoothed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description, appended claims and accompaning drawings, wherein: 
     FIGS.  1 (A) to  1 (C) show fabrication steps for a crystalline silicon film, 
       FIG. 2  shows a thin-film of silicon irradiated by a laser beam, 
     FIGS.  3 (A) and  3 (B) show steps for smoothing the surface of a silicon film having corrugations, 
     FIGS.  4 (A) and  4 (B) show steps for smoothing the surface of a silicon film having corrugations, 
     FIGS.  5 (A) and  5 (B) show steps for smoothing the surface of a silicon film having corrugations, 
     FIGS.  6 (A) and  6 (B) show steps for smoothing the surface of a silicon film having corrugations, 
     FIGS.  7 (A) to  7 (D) show fabrication steps for a crystalline silicon film, and 
     FIGS.  8 (A) to  8 (D) show fabrication steps for a thin-film transistor. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
   The formation step of a crystalline silicon film formed in a glass substrate having a flat surface in the present embodiment will be explained using FIG.  1 . Firstly, an oxidized silicon film  102  serving as a lower level film (base film) is deposited on the glass substrate  101  to a thickness of 3000 Å by a sputtering method. Then an amorphous silicon film  103  is deposited to a thickness of 500 Å by a plasma CVD method or a low pressure thermal CVD method. 
   Next an extremely thin oxidized silicon film (not shown) is formed on the surface of the amorphous silicon film  103  by irradiating it with UV light in an oxidized atmosphere. This oxidized silicon film is for improving wettability of a solution coating the device in a later step. 
   Then, the film is coated with a solution containing the element nickel which is a metal element for facilitating crystallization of the silicon. Here, a nickel acetate solution containing nickel to a predetermined density is used as the solution containing the element nickel. Then after forming a water film  104  of the nickel acetate solution, spin coating is performed using a spinner  105 , the nickel element reaches a state where it is maintained in contact with the surface of the amorphous silicon film  103  (FIG.  1 (A)). 
   Introducing the nickel element makes the average nickel density within the silicon film in its final state 1×10 16  cm −3  to 5×10 19  cm −3 . Specifically, the nickel density within the nickel acetate solution is adjusted so that the average nickel density within the silicon film in its final state (state where a crystalline silicon film is attained) becomes the above-described density. Note that the value of this density may be a value measured by a SIMS (secondary ion mass spectrometer). 
   If the nickel density in the silicon film is 1×10 16  cm  −3  or less, the effect of facilitating crystallization cannot be achieved and if the nickel density in the silicon film is 5×10 19  cm −3  or more, the characteristic of the obtained silicon film as a semiconductor will be lost (a characteristic as a metal will appear), therefore caution is necessary. 
   When a state where the nickel element is maintained in the surface of the amorphous silicon film is obtained, thermal processing is performed as shown in FIG.  1 (B). A crystalline silicon film  106  can be attained by this thermal processing. Here, this thermal processing is performed in a nitrogen atmosphere. The conditions for thermal processing are 550° C. for 4 hours. 
   This thermal processing can be performed at a temperature of 450° C. or more. However, at temperatures in the order of 450° C. to 500° C., since even though the amorphous silicon film is crystallized it takes some tens of hours to perform, it is preferable to perform it at a temperature of about 550° C. Also, although crystallization can be achieved in less time if thermal processing is performed at a temperature of more than 550° C., when considering deformation, shrinkage, etc. of the glass substrate due to heating, thermal processing at a temperature of approximately 550° C. or less is preferable. 
   The crystalline silicon film  106  obtained by thermal processing as shown in FIG.  1 (B) has not attained sufficiently crystallization and is unsuitable to be used in an active layer of a thin-film transistor as it is. Thus, the crystallinity of the silicon film is further improved by irradiating it with a laser beam as shown in FIG.  1 (C). Here, since the thickness of the crystalline silicon film  106  is about 500 Å (slightly shrunk by thermal processing), the energy of radiated laser beam is effectively absorbed in the silicon film (especially in the vicinity of the surface). Then a crystalline silicon film  107  can be attained in a state where crystallization has been further facilitated. 
   The crystalline silicon film  107  obtained in the step shown in FIG.  1 (C) has extremely favorable crystallinity. However, as shown in  FIG. 2 , the surface thereof has corrugations in the order of several hundreds of angstroms. 
   An enlarged portion of the device in this state is shown in FIG.  3 (A). What is shown in FIG.  3 (A) is a crystalline silicon film  107  whose average film thickness indicated by  302  is 500 Å, and having corrugations whose height is indicated by  301 . The corrugations are formed by irradiation by the laser beam shown in FIG.  1 (C). These corrugations are about 100 Å to 600 Å where the film thickness of the starting film (silicon film at the stage prior to laser beam irradiation) is 500 Å. 
   Next, the oxidized silicon film  303  is deposited to a thickness of 100 Å by a vapor deposition or plasma CVD method. It is necessary for the depositing method and depositing conditions of this oxidized silicon film  303  to be insufficient in step coverage. 
   In this manner, the oxidized silicon film cannot but be deposited extremely thin on the sides of the protruding portions indicated by  304  due to the step coverage problem. Also, the oxidized silicon film as indicated by  303  is deposited relatively thickly in the lower portions (depressions) of the indented portions indicated by  305  (FIG.  3 (B)). 
   Then etching is performed on the oxidized silicon film using an etchant. Here etching is performed using ammonia fluoride to remove the extremely thin oxidized silicon film deposited on the protruding portions indicated by  304 . With this step, although the oxidized silicon film  303  deposited in the indented portions (depressions) indicated by  305  is also somewhat etched, because the thickness of the oxidized silicon film deposited on the protruding portions  304  is extremely thin there is no great problem regarding etching of the oxidized silicon film  303  deposited in the indented portions (depressions) indicated by  305 . 
   Thus a state wherein the protruding portions  304  of the crystalline silicon film  107  are exposed is reached. In this state the  303  in the indented portions (depressions) is left behind. Then, by performing etching using an etchant on the silicon, the protruding portions can be selectively removed as shown in FIG.  4 (A). At this time, if etching is overdone holes as indicated by  401  will be formed, therefore caution is required. 
   Hydrazine may be used as the etchant with respect to the silicon as described above. Also one or a number of types of gas selected from among ClF 3 , ClF, BrF 3 , IF 3 , BrF, BrF 5  and IF 5  can be used. 
   Next, by performing etching using an etchant on the oxidized silicon, the oxidized silicon film  303  remaining in the indented portions is etched. Here etching is performed using a buffer hydrofluoric acid as the etchant. Thus a crystalline silicon film  107  whose surface is smoothed to a certain extent as shown in FIG.  4 (B) is obtained. 
   The crystalline silicon film  107  obtained by the steps shown in the present embodiment and shown in FIG.  4 (B) still has some protruding portions as indicated by  402 . These protruding portions can be reduced in size by thinning as much as possible the thickness of the oxidized silicon film  303  in the step shown in FIG.  3 (B). However, the thickness of the oxidized silicon film  303  must be sufficiently thick compared to the thickness of the oxidized silicon film formed in the protruding portions indicated by  304 . 
   In the present embodiment, since the thickness of the oxidized silicon film  303  in the step of FIG.  3 (B) is made 100 Å, the difference in height of the corrugations in the surface of the crystalline silicon film  107  shown in FIG.  4 (B) can be made about 100 Å. Note that, as shown in the present embodiment, where the protruding portions of the silicon film having corrugations are selectively removed, caution is required as the average film thickness becomes thinner. 
   Second Embodiment 
   The present embodiment relates to a structure which achieves a flat crystalline silicon film using dry etching having perpendicular anisotropy. Firstly a crystalline silicon film is obtained on a substrate (glass substrate) having an insulative surface through a step such as that shown in FIG.  1 . This crystalline silicon film is formed by being irradiated by a laser beam as shown in FIG.  1 (C), therefore the surface thereof has corrugations. 
   FIG.  5 (A) shows an enlarged view of this state. In FIG.  5 (A) the average thickness indicated by  302  and the crystalline silicon film having corrugations having a height difference indicated by  301  are shown. The average thickness indicated by  302  is for example 500 Å, and the height difference of the corrugations indicated by  301  is about 600 to 700 Å for example. As shown in FIG.  5 (A), where the thin-film silicon of about 500 Å is irradiated by a laser beam, the height difference indicated by  301  is more than the average film thickness at its largest. 
   When the crystalline silicon film having the surface condition shown in FIG.  5 (A) is obtained, an oxidized silicon film  303  is deposited by a plasma CVD method. It is necessary to deposit the oxidized silicon film  303  thickly enough that the surface thereof is flat. For example, where the height difference indicated by  301  is maximum at 600 to 700 Å or thereabouts, the oxidized silicon film indicated by  303  must be deposited to a thickness of about 3000 Å or more. Also, the deposition method and conditions must incur extremely bad step coverage. Thus the state shown in FIG.  5 (B) is reached. 
   When the state shown in FIG.  5 (B) is reached, dry etching is performed by an RIE method using a gas which is a mixture of CF 4  and oxygen. The dry etching using a gas which is a mixture of CF 4  and oxygen has an etching rate which is roughly the same for both the silicon and the oxidized silicon. Thus etching can be performed while maintaining the flatness of the exposed surface as shown in FIG.  6 (A). Then, etching is performed until all of the oxidized silicon film  303  is etched. Thereby, a crystalline silicon film  601  having flatness as shown in FIG.  6 (B) can be achieved. 
   Where the method shown in the present embodiment is utilized, a crystalline silicon film having a substantially flat surface can be attained. However, on the other hand there is a disadvantage in that the conditions for dry etching are delicate. 
   Third Embodiment 
   The present embodiment is an example wherein the corrugations in the surface of a silicon film where crystal growth is performed in the surface direction of the film (a direction parallel to the substrate) from a region in which a metal element is introduced by selectively introducing a metal element for promoting crystallization of silicon into an amorphous silicon film are removed. 
   Firstly, as shown in  FIG. 7 , an oxidized silicon film  102  is deposited to a thickness of 3000 Å by a sputtering method or plasma CVD method as a lower level film (base film) on a glass substrate  101 . Next by means of a plasma CVD method or low pressure thermal CVD method, an amorphous silicon film  103  is deposited to a thickness of 500 Å. Then this is irradiated by UV light in an oxidized atmosphere to form an extremely thin oxidized film (not shown) on the surface of the amorphous silicon film  103 . 
   Next a resist mask  702  is formed. This resist mask  702  has a structure which exposes the regions indicated by  701 . The regions indicated by  701  have a slit shape which has its long side extending along the front side in the drawing and the opposite side to the front side. Then the substrate  101  is arranged on a spinner  105  and coated by a nickel acetate solution containing nickel in a predetermined density, thus forming a water film  703 . Then spin coating is performed using the spinner  105 . 
   Subsequently, the resist mask  702  is removed and the state shown in FIG.  7 (B) is reached. In this state, nickel is introduced into the regions indicated by  701  (the surface of the amorphous silicon film  103  exposed in slit shapes). In this state the nickel reaches an extremely thin film state as indicated by  704  or a state where it is maintained in contact with the amorphous silicon film  103  in a diffused state. 
   Then thermal processing is performed for 4 hours at 550° C. in a nitrogen atmosphere to crystallize the amorphous silicon film  103 . In this thermal processing, as shown by the arrow  705  in FIG.  7 (C), crystal growth is carried out in a direction parallel to the substrate  101  from the regions where the nickel indicated by  701  is introduced. This crystal growth can be performed from several tens of μm to 100 μm or more. 
   This crystal growth in the direction parallel to the substrate and indicated by the arrow  705  advances in a pin or column shape. Also, that amorphous components remain in gaps where crystal growth occurs in pin or column shapes is clear from observation by a TEM (transparent electron microscope). 
   After the crystal growth shown in FIG.  7 (C) is performed, this is irradiated by a laser beam (KrF excimer laser) as shown in FIG.  7 (D) to further promote crystallization of the silicon film. Thus a region  706  which particularly promotes crystallinity by laser beam irradiation is obtained as shown in FIG.  7 (D). Note that the region indicated by  707  is a region in which crystal growth indicated by  705  has not occurred (region beyond the region  705  of the crystal growth) in the thermal processing step shown in FIG.  7 (C). Note that this region is crystallized by laser beam irradiation in the step shown in FIG.  7 (D) (crystal growth also being advanced solely by laser beam irradiation). 
   The surface of the thus-obtained crystalline silicon film has similar corrugations to those shown in FIG.  2 . These corrugations are formed in all regions irradiated by a laser beam. Namely, the surface of the obtained crystalline silicon film has a shape such as is shown in FIG.  3 (A) or FIG.  5 (A). Also, by passing through the steps shown in FIG.  3  and  FIG. 4  or the steps shown in FIG.  5  and  FIG. 6 , a crystalline silicon film having a smooth surface can be attained. 
   Fourth Embodiment 
   The present embodiment relates to a structure for obtaining a thin-film transistor using the crystalline silicon film whose surface is smoothed which is obtained in the first embodiment or the second embodiment. Firstly a crystalline silicon film  107  with a substantially flat surface is formed on the glass substrate  101  by the method disclosed in the first embodiment. According to the method shown in the first embodiment a crystalline silicon film in which the height difference of corrugations in the surface is 100 Å or less can be obtained (FIG.  8 (A)). 
   Next the crystalline silicon film  107  is patterned, to form an active layer  801  of the thin-film transistor. Then an oxidized silicon film  802  which serves as a gate insulation film is deposited by a plasma CVD method to a thickness of 1000 Å. Further, a film having aluminum as the main component and containing a small amount of scandium is deposited to a thickness of 6000 Å by an electron beam vapor deposition method. Then the film having aluminum as the main component is patterned to form a gate electrode  803 . Thereafter, by performing anode oxidation with the gate electrode  803  as an anode in an electrolyte solution, an oxide layer  804  is formed to a thickness of 2000 Å (FIG.  8 (B)). 
   Subsequently, injection of ion impurities is performed to form source and drain regions. Here injection of phosphorus ions is performed to form an N-channel thin-film transistor. In this step phosphorus ions are injected into the regions indicated, by  805  and  808 . Then by irradiating them with a laser beam, recrystallization (the surface is decrystallized by injection of the ion impurities) and activation of the injected ion impurities are performed. Thus a self-aligned source region  805  and drain region  808 , and further an offset gate region  806  and channel forming region  807  can be formed (FIG.  8 (C)). 
   Next, an oxidized silicon film  809  serving as an interlayer insulation film is deposited by a plasma CVD method to a thickness of 7000 Å. Then contact holes are formed and a source electrode  810  and drain electrode  811  are formed with a material whose main component is aluminum. Finally, by performing a hydrogenation process in a hydrogen atmosphere at 350° C., the thin-film transistor shown in FIG.  8 (D) is completed. 
   In the thin-film transistor shown in FIG.  8 (D), carriers are conducted in the surface of the channel forming region  807  (surface of the plane contacting the gate insulation film  802 ) between the source  805  and drain  808 . Consequently, improvement of the smoothness of the surface of the channel forming region  807  is effective. Namely, by ensuring the smoothness thereof, the effect of dispersion and trapping of carriers when the conducting carriers move can be reduced. Also, improvement of the characteristic of the thin-film transistor can be devised. 
   Fifth Embodiment 
   The present embodiment relates to a structure for obtaining a thin-film transistor using a region in which crystal growth occurs in a direction parallel to the substrate obtained in the third embodiment. The thin-film transistor shown in the present embodiment, by having the source and drain regions disposed in the direction of crystal growth parallel to the substrate, can be made so that carriers move along the crystal grain boundary and can attain a large degree of movement. 
   Sixth Embodiment 
   The present embodiment is an example in which, in the steps of the first embodiment, a crystalline silicon film is attained not by performing the step shown in FIG.  1 (B), but by performing crystallization of an amorphous silicon film solely by laser beam irradiation as shown in FIG.  1 (C). Where a thin amorphous silicon film of about 500 Å as in the case of the first embodiment is irradiated by a laser beam, a silicon film which is crystallized or promotes crystallinity can be attained, but the surface thereof has corrugations as shown in FIG.  2 . 
   In the present embodiment, by irradiating the amorphous silicon film with a laser beam, a crystalline silicon film having a surface such as that shown in  FIG. 2  is obtained, and by further passing through the steps shown in  FIG. 3 , is characterized by obtaining a crystalline silicon film whose surface has been smoothed. 
   Seventh Embodiment 
   The present embodiment is an example using an amorphous silicon film as the film indicated by  303  in the structure of the embodiment shown in FIG.  5 . In this case, the etching rates of the crystalline silicon film  107  having corrugations and the amorphous silicon film  303  do not differ significantly, therefore the dry etching step shown in FIG.  6 (A) can be performed relatively easily. 
   The amorphous silicon film may be one deposited by a plasma CVD method or low pressure thermal CVD method. Also, since the depressions among the corrugations are filled with the amorphous silicon film and obtaining a smooth surface as shown in FIG.  5 (B) is preferred, deposition of the amorphous silicon film  303  is preferably performed by a deposition method or under deposition conditions in which step coverage is unsatisfactory. Also, in order for the etching rate of the oxidized silicon film  303  and the etching rate of the crystalline silicon film  107  to match, adding impurities to the amorphous silicon film  303  is effective. 
   After depositing the amorphous silicon film  303 , dry etching is performed by an RIE method using a gas which is a mixture of CF 4  and oxygen as the etching gas (FIG.  6 (A)). 
   In this dry etching step, the protruding portions of the amorphous silicon film  303  and the crystalline silicon film  107  are etched and a crystalline silicon film  601  having a smooth surface as shown in FIG.  6 (B) can be obtained. In this dry etching step it is necessary to perform the etching so that the amorphous silicon film  303  does not remain. 
   Eighth Embodiment 
   The present embodiment is an example where the etching as shown in FIG.  6 (A) is performed using a CMP method. CMP is an abbreviation for chemical-mechanical polishing. 
   Where CMP is used etching having a high level of smoothness can be performed. Consequently, this is an effective means for performing etching which supports smoothness as shown in FIG.  6 (A).