Abstract:
An amorphous semiconductor film comprising silicon is provided with a metal element which is capable of promoting a crystallization of silicon. Then, the semiconductor film is crystallized by hating at a relatively low temperature. After introducing impurity ions into source and drain regions of the semiconductor film, the source and drain regions are recrystallized by heating. During the recrystallization, the channel region having crystallinity functions as crystalline nuclei. Accordingly, it is possible to avoid defects occurring in the boundary regions between the channel region and source/drain regions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a thin film transistor (in general, called “TFT”) and a method of manufacturing the same, and particularly to a method of forming source and drain regions in a thin film transistor.  
           [0003]    In recent years, there have been known active matrix liquid-crystal display unit using a thin film transistor. FIGS. 2A to  2 D show a process of manufacturing a general thin film transistor. First, a silicon oxide film or silicon nitride is formed on a glass substrate  201  as a first coating film  202 . A Corning 7059 glass or the like is used as a glass substrate. After the formation of the first coating film  202 , a silicon semiconductor film which forms an active layer is formed on the first coating film  202 . An amorphous silicon film is usually formed through the plasma CVD technique or low pressure thermal CVD technique, and thereafter the amorphous silicon film is crystallized by heating or the application of laser beam. Then, a silicon film subjected to a crystal property (hereinafter referred to as “crystalline silicon film”) is patterned to thereby form an active layer  203 . (FIG. 2A).  
           [0004]    After the formation of the active layer  203 , a silicon oxide film is formed as a gate insulating film  204  through the plasma CVD technique or the sputtering technique. Then, a gate electrode  205  is formed of material mainly containing metal or semiconductor. After the formation of the gate electrode  205 , impurity ions are injected thereinto so as to form a source region  207  as well as a drain region  209 . This process is executed using the gate electrode  205  as a mask. As the ions injected, P (phosphorus) is used in the manufacture of an n-channel thin film transistor, whereas B (boron) is used in the manufacture of a p-channel thin film transistor. Also, a channel formation region  208  is formed simultaneously during this process. (FIG. 2B).  
           [0005]    After the formation of the source region  207  and the drain region  209  as well as the channel formation region  208 , the source region  207  and the train region  209  are recrystallized by application of a laser beam or an infrared ray, and the impurity ions injected into those region are activated. The recrystallization of the source region  207  and the drain region  209  are made because the source region  207  and the drain region  209  have been made amorphous by the bombardment of injected ions at the time of the preceding ion injection.  
           [0006]    The above-mentioned recrystallization and activation of the source and drain regions may be performed by heating. However, in the case of heating, its effect could not be obtained without heating at temperature of 700° C. or higher (preferably 800° C. or higher). Taking the heat-resistivity of a glass substrate (a substrate made of Corning 7059 glass must be dealt with at 600° C. or lower) into account, such a heat treatment is improper.  
           [0007]    Subsequently, an interlayer insulating film  211  is formed of silicon oxide or other insulating materials. Further, after forming contact holes, a source electrode  212  and a drain electrode  213  are formed of a proper metallic material.  
           [0008]    The thin film transistor manufactured through the foregoing processes suffers from such a problem that its characteristics are deteriorated or largely dispersed. This problem results from the fact that defects concentrate in the vicinity of interfaces between the source region  207  and the channel formation region  208  and between the drain region  209  and the channel formation region  208 .  
           [0009]    In other words, the source region  207  and the drain region  209 , which have been made amorphous by the injection of ions in the process of FIG. 2B, are recrystallized by the application of a laser beam in the process of FIG. 2C, during which the channel formation region  208  remains crystalline. Therefore, the crystallization of the source and drain regions, which progresses by the application of a laser beam, stops at the interfaces between the source and drain regions and the channel formation region having the crystal property from the first. As a result, a large number of defects resulting from mismatching of lattices are produced in the vicinity of the interfaces between the source and drain regions and the channel formation region. The existence of those defects makes not only the characteristics dispersed and unstable but also an off-state current increase.  
           [0010]    As a manner of solving the foregoing problem, it has been found that the recrystallization of the source and drain regions and the activation of the impurity ions are performed at a temperature of 700° C. or higher, preferably 800° C. or higher. If the recrystallization of the source and drain regions and the activation of the impurity ions are performed at a temperature of 700° C. or higher, preferably 800° C. or higher, energy is also applied to the channel formation region  208 . Hence, mismatching of lattices produced in the vicinity of the interfaces between the source and drain regions and the channel formation region can be released, as a result of which the defects can be prevented from concentrating in the vicinity of the interfaces between the source and drain regions and the channel formation region.  
           [0011]    However, in order that processes for the recrystallization of the source and drain regions and the activation of impurity ions injected are performed by a process of heating at 700° C. or higher, a substrate capable of resisting a temperature of 700° C. or higher must be used. However, such a substrate is expensive, resulting in a large obstacle to the use of the thin film transistor in a liquid-crystal display apparatus. In other words, in the use of an inexpensive glass substrate having a heat-resistant temperature of 600° C. or lower, the processes for the recrystallization of the source and drain regions and the activation of impurity ions cannot be realized by heating for all practical purposes.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention has been made in view of the above problems with the prior art, and an object of the invention is to execute the recrystallization and activation of source and drain regions at a temperature lower than that in the prior art.  
           [0013]    Another object of the invention is to provide a thin film transistor with a structure in which defects on interfaces between source and drain regions and a channel formation region are reduced.  
           [0014]    A still another object of the invention is to provide a thin film transistor which is sufficiently high in a crystal property of the source and drain regions.  
           [0015]    In order to achieve the above objects, according to one aspect of the invention, there is provided a method of manufacturing a thin film transistor, comprising the steps of:  
           [0016]    introducing a metal element for promoting crystallization into an amorphous silicon film;  
           [0017]    subjecting said amorphous silicon film to a heat treatment to form a crystalline silicon film;  
           [0018]    forming an active layer using said crystalline silicon film;  
           [0019]    selectively injecting impurity ions into a part of said active layer; and  
           [0020]    subjecting said active layer to a heat treatment to grow crystal from a region into which said impurity ions have not been injected toward a region into which said impurity ions have been injected.  
           [0021]    In the foregoing structure, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au are applicable as the metal element which promotes crystallization. In particular, the use of Ni (nickel) makes it possible to obtain a remarkable effect.  
           [0022]    An amorphous silicon film is formed on a glass substrate, a quartz substrate, or a semiconductor substrate or metal substrate having an insulating surface. The amorphous silicon film is formed through a vapor phase technique such as a plasma CVD technique or low pressure thermal CVD technique, or the sputtering technique.  
           [0023]    As the impurity ions, ions of phosphorus or boron are used.  
           [0024]    In the foregoing structure, it is effective to apply a laser beam or an intense light beam to the formed film before or after the heat treatment. In particular, the application of a laser beam after the heat treatment makes the crystal property of the film effectively enhanced.  
           [0025]    According to another aspect of the invention, there is provided a method of manufacturing a thin film transistor, comprising the steps of:  
           [0026]    introducing a metal element for promoting crystallization into an amorphous silicon film;  
           [0027]    subjecting said amorphous silicon film to a heat treatment to form a crystalline silicon film;  
           [0028]    forming an active layer using said crystalline silicon film;  
           [0029]    selectively injecting impurity ions into regions of said active layer; and  
           [0030]    subjecting said active layer to a heat treatment to recrystallize the impurity doped regions with the region which is not introduced with the ions used as crystal nuclei.  
           [0031]    According to still another aspect of the invention, the active layer has such a structure that a crystal grows from said channel formation region toward the source and drain regions adjacent to said channel formation region with the channel formation region functioning as a crystalline nucleus.  
           [0032]    For example, in a thin film transistor as shown in FIG. 3D, a crystal growth develops as indicated by arrows  301  with a channel formation region  208  being a crystalline nucleus as shown in FIG. 3C, as a result of which a source region  207  and a drain region  209  are crystallized.  
           [0033]    According to yet still another aspect of the invention, the active layer has such a structure that a crystal grows from a peripheral region of the channel formation region toward the source and drain regions adjacent to said peripheral region with said peripheral region being a crystalline nucleus.  
           [0034]    For example, in a thin film transistor shown in FIG. 1D, a crystal growth develops from an offset gate region  108  which is a periphery region of the channel formation region, as a result of which a source region  107  and a drain region  110  are crystallized in a process of FIG. 1C.  
           [0035]    As said peripheral region of the channel formation region, an offset gate region, a light doped region and a non-doped region each having a crystal property without being subjected to ion injection are applicable.  
           [0036]    As a metal element which promotes crystallization useable to the present invention disclosed in this specification, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au, which are elements interstitial with respect to silicon, are usable. Atoms interstitial with respect to silicon are dispersed into a silicon film during a heat treatment. While the interstitial elements are dispersed thereinto, the crystallization of silicon progresses simultaneously. In other words, the interstitial metal makes the crystallization of an amorphous silicon film promoted with a catalytic action of the dispersed metal.  
           [0037]    The introduced amount (added amount) of the interstitial elements becomes important since the interstitial elements are allowed to be rapidly dispersed in the silicon film. In other words, the small introduced amount of the interstitial elements makes the effect of promoting crystallization low so that an excellent crystal property cannot be obtained. On the other hand, the excess amount of the interstitial atoms makes the semiconductor characteristic of silicon spoiled.  
           [0038]    Therefore, there exists an optimum amount of introducing the interstitial metal elements into the amorphous silicon film. For example, it has been found that, in the case of using Ni as a metal element which promotes the crystallization, if the density of Ni in a crystallized silicon film is 1×10 15  cm −3  or more, the effect of promoting crystallization can be obtained, whereas, if the density of Ni in the crystallized silicon film is 1×10 19  cm −3  or less, the semiconductor characteristic is not hindered. The “density” used here is defined by the minimum value obtained through SIMS (Secondary Ion Measurement System). Also, the above recited metal elements other than Ni can obtain their effects in the same density range as that of Ni likewise.  
           [0039]    Apart from the above recited metal atoms, the use of Al or Sn can make the crystallization of an amorphous silicon film promoted likewise. However, Al or Sn causes an alloy to be formed in combination with silicon so as not be dispersed and interstitial into a silicon film. Then, the crystallization progresses in such a manner that a crystal growth develops from a portion where Al or Sn is alloyed with silicon with that portion being a crystalline nucleus. In this case, as a result that Al or Sn is not dispersed into the silicon film, the crystallization progresses from the portion of the crystalline nucleus. In this way, in the use of Al or Sn, a crystal growth is conducted only from a portion where Al or Sn is introduced (that is, an alloy layer consisting of those elements with silicon). This causes a problem such that its crystal property is generally lowered in comparison with a case of using the foregoing interstitial elements such as Ni. For example, there arises such a problem that this makes it difficult to obtain a crystalline silicon film which is uniformly crystallized.  
           [0040]    As a result that a metal element promoting the crystallization is introduced and a semiconductor layer with a region having a crystal property as well as a region having an amorphous property is subjected to a heat treatment, the amorphous region can be recrystallized with the crystalline region being a crystalline nucleus. In this situation, since the crystallization progresses from the crystalline region toward the amorphous region, mismatching of lattices is not produced in the vicinity of the interfaces between the crystalline region and the amorphous region, thereby being capable of preventing the defects from concentrating.  
           [0041]    For example, a silicon semiconductor layer is crystallized by a metal element which promotes crystallization and includes a specified region which has been made amorphous with the injection of impurity ions giving one conductivity type thereinto. Such a silicon semiconductor layer is subjected to a heat treatment so that crystal growth progresses from the region having the crystal property toward the region which has been made amorphous by the injection of impurity ions giving one conductivity type. Thus, the region which has been made amorphous can be crystallized.  
           [0042]    In this situation, since the crystallization progresses from the region having the crystal property toward the region having the amorphous property, defects resulting from mismatching of lattices can be prevented from being produced in the vicinity of the interfaces between the respective regions. Thus, the semiconductor layer having the crystal property where the defects do not concentrate in a specified region can be obtained.  
           [0043]    Also, in a process of manufacturing a thin film transistor including a manufacturing process shown in FIGS. 3A to  3 D, the active layer  203  is constituted by a silicon film which has been crystallized by the heat treatment due to the action of the metal element which promotes crystallization. During the process of FIG. 3B, impurity ions giving one conductivity type are injected into such an active layer  203  with a mask of the gate electrode  205  so that the source region  207  and the drain region  209  are formed in a self-aligning manner. When the source and drain regions have been made amorphous, a heat treatment is conducted at 550° C. for about 2 hours in the process of FIG. 3C. As a result, the crystal growth is made, as indicated by the arrows  301 , toward the source and drain regions with the channel formation region  208  having the crystal property functioning as a crystalline nucleus.  
           [0044]    The crystal growth indicated by the arrows  301  is conducted at a relatively low temperature for a short period of time with the channel formation region  208  functioning as the crystalline nucleus with the following causes.  
           [0045]    (1) A catalyst element which promotes the crystallization of silicon has been introduced into the active layer  203 .  
           [0046]    (2) The source region  207  and the drain region  209  have been doped with an element having one conductivity type which is of a catalyst element for promoting crystallization.  
           [0047]    Since the foregoing crystal growth is conducted from the channel formation region toward the source and drain regions as indicated by the arrows  301 , the defects resulting from the mismatching of lattices do not concentrate in the vicinity of the interface between the channel formation region  208  and the source region  207 , as well as in the vicinity of the interface between the channel formation region  208  and the source region  209 .  
           [0048]    The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0049]    [0049]FIGS. 1A to  1 D show a process of manufacturing a thin film transistor in accordance with one embodiment of the present invention;  
         [0050]    [0050]FIGS. 2A to  2 D show a process of manufacturing a thin film transistor in accordance with a prior art;  
         [0051]    [0051]FIGS. 3A to  3 D show a process of manufacturing a thin film transistor in accordance with another embodiment of the present invention;  
         [0052]    [0052]FIGS. 4A to  4 D show a process of manufacturing a thin film transistor in accordance with still another embodiment of the present invention; and  
         [0053]    [0053]FIGS. 5A to  5 D show a process of manufacturing a thin film transistor in accordance with yet still another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]    Now, a description will be given to embodiments of the present invention with reference to the accompanying drawings.  
         [0055]    (First Embodiment)  
         [0056]    [0056]FIGS. 1A to  1 D show a process of manufacturing a thin film transistor in accordance with a first embodiment of the present invention. The thin film transistor described in this embodiment can be used for an active matrix type liquid-crystal display unit, an image sensor or other thin-film integrating circuits.  
         [0057]    First, a silicon oxide film having a thickness of 2000 Å is formed on a glass substrate (Corning 7059 glass)  101  as a first coating film  102  through the sputtering technique. The first coating film  102  is formed in order that an impurity is not dispersed from the glass substrate into an active layer. Then, an amorphous silicon film having a thickness of 1000 Å is formed on the first coating film  102  through the plasma CVD technique. It goes without saying that low pressure thermal CVD technique may be used instead of the plasma CVD technique.  
         [0058]    After the formation of the amorphous silicon film, nickel is introduced into the amorphous silicon film as a catalyst metal which promotes crystallization. In this example, nickel elements are introduced into the amorphous silicon film using nickel acetate salt solution. In detail, nickel acetate salt is dropped on the surface of the amorphous silicon film and then treated through the spin coating technique using a spinner so that nickel is arranged in contact with the surface of the amorphous silicon film. Thereafter, the amorphous silicon film is subjected to a heat treatment at 550° C. for 4 hours so as to be crystallized. The heat treatment can be conducted at a temperature of 450° C. or more. However, the heating period of time is too long in the case of 500° C. or less whereas the problem on the heat-resistivity of the glass substrate is actually revealed in the case of 550° C. or more. As a result, it is proper to conduct the heat treatment at a temperature ranging from 500 to 550° C.  
         [0059]    The crystalline silicon film obtained by crystallizing the amorphous silicon film is subjected to a patterning process, thereby obtaining an active layer  103  of a thin film transistor (FIG. 1A).  
         [0060]    After the formation of the active layer  103 , a silicon oxide film  104  having a thickness of 1000 Å, which functions as a gate insulating film, is formed thereon through the plasma CVD technique. Then, a thin film having a thickness of 5000 Å which mainly consists of aluminum is formed thereon through a vapor deposition technique and subjected to a patterning process, thereby forming a gate electrode  105 . Subsequently, anodic oxidation is conducted in an electrolyte with the gate electrode functioning as an anode, thereby forming an oxide layer  106 .  
         [0061]    Thereafter, impurity ions are injected into the source region  107  and the drain region  110 . In this example, P (phosphorus) ions are injected thereinto in order to form an n-channel thin film transistor. In this process, the source region  107  and the drain region  110  as well as the channel formation region  109  and the offset gate region  108  are simultaneously formed in a self-aligning manner (FIG. 2B).  
         [0062]    In the foregoing process of injecting the impurity ions, the source region  107  and the drain region  110  are made amorphous. Thereafter, in the process of FIG. 2C, a heat treatment is conducted at 500° C. for four hours so that the source region  107  and the drain region  110  are recrystallized and P (phosphorus) which has been injected is activated. In this situation, the source region  107  and the drain region  110  are crystallized with the crystalline nucleus being formed by the channel formation region  109  and the offset gate region  108  (in particular, offset gate region  108 ) both having the crystal property. In other words, crystallization progresses from the interfaces between the offset gate region  108  and the source and drain regions toward the source and drain regions.  
         [0063]    It is proper to conduct the foregoing heat treatment at a temperature ranging from 350 to 550° C. If the temperature is 350° C. or less, the heating effect is low whereas if it is 550° C. or more, the heat damage to the glass substrate is actually revealed. In the use of a material mainly containing aluminum as a gate electrode, it is preferable to set the upper limit of that temperature to 500° C. This is because a temperature of 500° C. or more causes such a problem that aluminum is dispersed from the gate electrode mainly containing aluminum into the gate insulating film.  
         [0064]    In the foregoing process, nickel elements existing in the channel region  109  and the offset gate region  108  are dispersed into the source region  107  and the drain region  110 , thereby promoting the crystallization. Also, since the source and drain regions have been doped with impurities giving one conductivity type (in this example, P (phosphorus), the crystallization is more facilitated (the crystallization progresses at 500° C. or less with being doped with P or B).  
         [0065]    Hence, the recrystallization of the source and drain regions readily progresses during the heat treatment at 500° C. for four hours.  
         [0066]    After the completion of the heat treatment, a silicon oxide film  111  having a thickness of 5000 Å is formed as an interlayer insulating film, and a piercing process is then conducted, whereby a source electrode  112  and a drain electrode  113  are formed of aluminum. Then, the hydrogenation process is conducted at 350° C. for one hour in hydrogen atmosphere, thereby completing a thin film transistor.  
         [0067]    In the case where the structure described in this embodiment is applied, since the crystal growth progresses from the offset gate region  108  toward the source region  107  as well as the drain region  110  during the process of FIG. 2C, there arises no problem such that defects resulting from the lattice mismatching concentrate in the vicinity of the interface between the offset gate region  108  and the source region  110 , and in the vicinity of the interface between the offset gate region  108  and the drain region  110 . This can provide such effects that the dispersion of the thin film transistor characteristics is lowered, the deterioration or instability of the characteristics is restrained, and the off-state current is lowered.  
         [0068]    (Second Embodiment)  
         [0069]    A second embodiment is an example in which a silicon semiconductor is used for a gate electrode. FIGS. 3A to  3 D show a process of manufacturing a thin film transistor in accordance with the second embodiment of the present invention.  
         [0070]    First, a silicon oxide film  202  having a thickness of 2000 Å is formed on a glass substrate (Corning 7059 glass)  201  as a first coating film through the sputtering technique. Then, an amorphous silicon film having a thickness of 1000 Å is formed on the first coating film  202  through the plasma CVD technique or the low pressure thermal CVD technique. Subsequently, a metal element which promotes crystallization is introduced into an amorphous silicon film using a solution in which a silicon compound and an additive are solved in an organic solution. In this example, a solution for forming silicon oxide such as OCD (Ohka Diffusion Source) made by Tokyo Ohka Kogyo Corporation is used for maintaining the nickel compound therein. The OCD solution is generally used in the formation of a silicon oxide film or the like for passivation. The silicon oxide film can be formed by coating the OCD solution on a surface where a film is to be formed using a spinner and then conducting a predetermined baking process.  
         [0071]    In this example, there is prepared solution which is obtained by mixing the OCD solution with solution where nickel (II) acetylacetonate is solved in methyl acetate and by adjusting the mixture so that silicon oxide is 2.0 wt % and nickel is 200 to 2000 ppm. Then, such solution is coated on the amorphous silicon film through the spin coating technique, and subjected to a prebaking process at 250° C. for 3 minutes, thereby forming a silicon oxide film containing nickel therein with a thickness of about 1300 Å.  
         [0072]    Thereafter, the silicon oxide film is subjected to a heat treatment in the nitrogen atmosphere at 550° C. for 4 hours, thereby obtaining a silicon film having the crystal property. During this process, nickel is dispersed from the silicon oxide film which is in contact with the surface of the silicon film into the silicon film. After the heat treatment, the silicon oxide film is removed.  
         [0073]    Subsequently, a patterning process is conducted to form an active layer  203  of the thin film transistor as shown in FIG. 3A. A silicon oxide film having a thickness of 1000 Å is formed as the gate insulating film  204  through the plasma CVD technique. Moreover, a silicon film having an n-type microcrystal structure with a thickness of 5000 Å is formed through the vapor heat CVD technique and then subjected to a patterning process, thereby forming a gate electrode  205 . After the formation of the gate electrode  205 , the source region  207  and the drain region  209  are doped with phosphorus by injecting impurity ions (phosphorus ions) thereinto. During this process, the source region  207  and the drain region  209  are made amorphous by the bombardment of ions. Also, a channel formation region  208  is formed without any injection of impurity ions (FIG. 3B).  
         [0074]    After the completion of the process of injecting the impurity ions in the source and drain regions shown in FIG. 3B, a heat treatment is conducted so that the source and drain regions are recrystallized and the injected impurity ions are activated. The heat treatment is conducted at 550° C. for 2 hours. In this embodiment, since silicon is used as material of the gate electrode, the heat treatment can be conducted with the upper limit of the heat-resistant temperature of the glass substrate. It goes without saying that as the temperature of the heat treatment is higher, the effect is better (FIG. 3C).  
         [0075]    During the heat treatment, the crystallization progresses toward the source and drain regions  207  and  209  as indicated by arrows  302  with the channel formation region  208  having the crystal property serving as a nucleus. In this manner, the recrystallization of the source and drain regions and the activation of the impurity ions which have been injected in the process of FIG. 3B are simultaneously conducted.  
         [0076]    Subsequently, a silicon oxide film  211  is formed as an interlayer insulating film through the plasma CVD technique and subjected to a piercing process so that the source electrode  212  and the drain electrode  213  are formed of aluminum. Finally, the hydrogenation process is conducted at 350° C. for one hour in the hydrogen atmosphere, thereby completing a thin film transistor as shown in FIG. 3D.  
         [0077]    (Third Embodiment)  
         [0078]    A third embodiment is an example in which a laser beam is irradiated together with the heat treatment during the process shown in FIG. 3C. It has been found through measurement using a TEM photograph (Transmittance Electron Microscope photograph) or an ESR (Electron Spin Resonance) that the crystalline silicon film which has been crystallized at a temperature of about 500 to 550° C. using a metal element (for example, nickel element) that promotes crystallization has the following properties.  
         [0079]    (1) The crystal growth progresses in the form of pillars or branches, and an amorphous component remains between the pillars or branches.  
         [0080]    (2) The amorphous component remaining between the pillars or branches can be crystallized by the irradiation of a laser beam after the crystallization due to heating, being capable of improving the crystallizing ratio.  
         [0081]    (3) The application of the laser beam in the above item (2) makes the density of defects in the film not decreased but inversely tend to be increased.  
         [0082]    (4) The density of defects in the film can be decreased by further conducting a heat treatment after the irradiation of the laser beam in the above item (2).  
         [0083]    In view of the above, this embodiment is characterized in that the irradiation of a laser beam starts from the gate electrode  205  side after the completion of the heat treating process during the process shown in FIG. 3C with the result that the crystal property of the source region  207  and the drain region  209  is improved, and also a heat treatment is conducted after the completion of the irradiation of the laser beam with the result that the density of defects in the active layer is decreased.  
         [0084]    In other words, in this embodiment, in a process of annealing the source and drain regions after the injection of impurity ions, the following processes are executed.  
         [0085]    During the annealing process due to a heat treatment, the source and drain regions are crystallized with the channel formation region serving as a crystalline nucleus simultaneously while the impurities are activated.  
         [0086]    A laser beam is irradiated to the source and drain regions after the foregoing heat treatment, thereby improving the crystal property of the source and drain regions.  
         [0087]    A heat treatment is further conducted after the foregoing irradiation of a laser beam, thereby decreasing the density of defects in the active layer.  
         [0088]    A detailed process will be described below. First, processes until a process of injecting impurities into the source and drain regions are conducted in accordance with the manufacturing processes shown in FIGS. 3A to  3 B. Then, the recrystallization of the source and drain regions and the activation of the impurity ions injected are conducted by heating at 550° C. for 2 hours. Thereafter, a KrF excimer laser beam (wavelength of 248 nm) is irradiated from the side of the gate electrode  205  to improve the crystal property of the source region  207  and the drain region  209 . This process may be conducted with the irradiation energy density of the laser beam being 100 to 300 mJ/cm 2 . Other types of laser beams may be used as a laser beam. Also, an intense light such as infrared rays may be irradiated instead of a laser beam. Moreover, it is effective to heat a sample to about 300 to 400° C. during the irradiation of a laser beam.  
         [0089]    After the completion of the irradiation of a laser beam, a heat treatment is again conducted. In this example, a heat treatment is conducted at 550° C. for one hour. The heat treatment thus conducted makes the density of defects in the active layer decreased about one figure.  
         [0090]    Sequentially, in the process shown in FIG. 3D, the interlayer insulating film  211 , the source electrode  212  and the drain electrode  213  are formed and further subjected to a heat treatment at 350° C. in the hydrogen atmosphere to conduct hydrogenation, thus completing a thin film transistor.  
         [0091]    In the foregoing description of this embodiment, the heat treatment, the irradiation of a laser beam and the heat treatment are conducted in the stated order. However, the heat treatment and the irradiation of a laser beam may additionally be repeated. In other words, the process may be repeated in the order of the heat treatment, the irradiation of a laser beam, the heat treatment, the irradiation of a laser beam and the heat treatment.  
         [0092]    (Fourth Embodiment)  
         [0093]    A fourth embodiment is characterized in that the irradiation of a laser beam is conducted through a glass substrate. It has been found that at least 90% of a light having a wavelength of 360 nm or more transmits through a Corning 7059 glass substrate. As a result, in the case of using a Corning 7059 glass substrate as a glass substrate and irradiating a laser beam from the glass substrate side; it is desirable to use an HgCl excimer laser beam (wavelength of 558 nm), an XeF excimer laser beam (wavelength of 483 nm), a KrO excimer laser beam (wavelength of 558 rm) or the like.  
         [0094]    In the case of irradiating a laser beam from the substrate side, an energy (light energy) is also applied to the vicinity of the interfaces between the source and drain regions and the channel formation region. This prevents the generation of defects caused by the lattice mismatching in the vicinity of the interfaces between the source and drain regions and the channel formation region.  
         [0095]    An example of a process in the case where the structure of this embodiment is applied will be described below.  
         [0096]    (1) The processes shown in FIGS. 3A and 3B are conducted in the same manner as those of the third embodiment.  
         [0097]    (2) A heat treatment is conducted at 550° C. for two hours, thereby conducting the recrystallization of the source and drain regions as well as the activation of the impurities injected.  
         [0098]    (3) A XeF excimer laser beam having an irradiation energy density of 200 mJ/cm 2  is irradiated from the gate electrode side, thereby improving the crystal property of the active layer.  
         [0099]    (4) A heat treatment at 550° C. for one hour is conducted, thereby decreasing the density of defects in the active layer.  
         [0100]    After the completion of annealing the foregoing source and drain regions, as shown in FIG. 3D, the interlayer insulating film  211  is formed, subsequently the source and drain electrodes  212  and  213  are formed, and finally hydrogenation is conducted, thereby completing a thin film transistor.  
         [0101]    (Fifth Embodiment)  
         [0102]    A fifth embodiment is an example in which a thin film transistor is formed by using a metal element which promotes crystallization and a crystalline silicon film which has been grown in a direction parallel with a substrate (called a lateral growth).  
         [0103]    [0103]FIGS. 4A to  4 D show a process of manufacturing a thin film transistor in accordance with this embodiment of the present invention. First, a silicon oxide film having a thickness of 2000 Å is formed on a glass substrate  401  as a first coating film  402  through the sputtering technique. Then, an amorphous silicon film  403  having a thickness of 1000 Å is formed on the first coating film  402  through the plasma CVD technique or that low pressure thermal CVD technique. Subsequently, a mask  404  is formed of a resist. The mask  404  is constituted so as to expose a region indicated by reference numeral  400 . A portion  400  has a slip shape longer than the width of the active layer. The opening  400  of the mask  404  is arranged so as to be directed longitudinally toward an opposite side of a drawing paper.  
         [0104]    Then, a silicon oxide film  405  containing a nickel element is formed by coating the foregoing OCD solution to which nickel has been added through the spin coating technique and then prebaking it. The silicon oxide film  405  containing nickel is in contact with the amorphous silicon film at the portion  400 . In other words, nickel becomes introduced from the region  400  into the amorphous silicon film.  
         [0105]    In this situation, a heat treatment is conducted at 550° C. for four hours so that the amorphous silicon film  403  is crystallized. During this process, the crystallization progresses from the region  400  into which nickel has been introduced in a direction indicated by an arrow  406 . It has been proved from the observation of a section TEM (Transmittance Electron Microscope) photograph that the crystallization progresses in the form of a pillar, a needle or a branch. In other words, it has been proved that the probability of existence of grain boundaries is low in the direction of growth of a crystal indicated by an arrow  406 .  
         [0106]    It is effective to irradiate a laser beam after the foregoing heat treatment to improve the crystal property. Moreover, it is effective to again conduct a heat treatment after the irradiation of a laser beam to decrease the density of defects in the film.  
         [0107]    After the heat treatment makes the amorphous silicon film  403  formed into a crystalline silicon film, the silicon oxide film  405  and the resist mask  404  are removed therefrom. Then, a patterning process is conducted to thereby obtain an active layer  406  (FIG. 4B).  
         [0108]    After the active layer  406  is obtained, a silicon oxide film constituting the gate insulating film  407  with a thickness of 1000 Å is formed through the plasma CVD technique. Further, a gate electrode  408  mainly containing aluminum is formed. Then, anodic oxidation is conducted in an electrolyte with the gate electrode  408  functioning as an anode, thereby forming an oxide layer  409  having a thickness of 2000 Å. In this example, a material mainly containing aluminum was used, however, a material enabling anodic oxidation such as tantalum may be used.  
         [0109]    Sequentially, ions of phosphorus are injected when the material is of an n-channel type whereas ions of boron are injected when the material is of a p-channel type. During this process, with the gate electrode  408  and the oxide layer  409  in the periphery of the gate electrode  408  both serving as a mask, impurity ions giving one conductivity type are injected into the source region  410  and the drain region  413 . Simultaneously, the offset gate region  411  and the channel formation region  412  are formed in a self-aligning manner. In the injection of ions, the source and drain regions are made amorphous (FIG. 4C).  
         [0110]    Thereafter, a heat treatment is conducted at 45° C. for four hours so that the recrystallization of the source and drain regions and the activation of the impurity ions injected are conducted. Furthermore, a KrF excimer laser beam is irradiated from the gate electrode side, thereby enhancing the crystal property of the source and drain regions (FIG. 4C).  
         [0111]    Subsequently, a silicon oxide film having a thickness of 7000 Å is formed as the interlayer insulating film  414  through the plasma CVD technique, the source electrode  415  and the drain electrode  416  are formed, and a hydrogenation process is conducted in the hydrogen atmosphere at 350° C. for one hour, thereby completing a thin film transistor (FIG. 4D).  
         [0112]    In the case where the structure of this embodiment is applied, the direction in which the crystal growth progresses is made coincide with a direction in which carriers move in the active layer (coincide with a direction coupling the source region with the drain region), thereby being capable of obtaining a thin film transistor having a large mobility and allowing a large on-state current to flow. Also, a structure can be realized in which defects resulting from the lattice mismatching do not concentrate on or in the vicinity of the interface between the offset gate region  411  and the source region  410 , and on or in the vicinity of the interface between the offset gate region  411  and the drain region  413 . As a result, a thin film transistor without any instability and deterioration of the characteristics can be obtained.  
         [0113]    (Sixth Embodiment)  
         [0114]    A sixth embodiment is an example of a thin film transistor where a light doped region is formed in addition to the offset gate region. FIGS. 5A to  5 D show a process of manufacturing a thin film transistor in accordance with the sixth embodiment of the present invention. First, a silicon oxide film  502  having a thickness of 2000 Å is formed on a glass substrate (Corning 7059 glass)  501  as a first coating film through the sputtering technique. Then, an amorphous silicon film having a thickness of 1000 Å is formed on the first coating film  502  through the plasma CVD technique or the low pressure thermal CVD technique. Thereafter, nickel is introduced into the amorphous silicon film by a technique in which the OCD solution described in the second embodiment is used.  
         [0115]    Sequentially, a heat treatment is conducted at 550° C. for four hours so that the amorphous silicon film is crystallized to obtain a crystalline silicon film. Further, an active layer  503  is obtained by subjecting the film to a patterning process. Then, a silicon oxide film having a thickness of 1000 Å is formed as a gate insulating film  504  through the plasma CVD technique. A gate electrode  505  mainly containing aluminum is formed, and anodic oxidation is conducted, thereby forming an aluminum oxide layer  506 . In this example, the thickness of the oxide layer is 2000 Å.  
         [0116]    Subsequently, an amorphous silicon film  507  having a thickness of 1 μm to 2 μm, for example, a thickness of 1.5 μm is formed through the plasma CVD technique or the sputtering technique. In this process, a vertical anisotropy etching is conducted through the RIE technique (Reactive Ion Etching). At this state, the silicon oxide remains in the region indicated by reference numeral  508 . The size of the region where the amorphous silicon remains can be determined in accordance with the film thickness of the amorphous silicon film  507  and the etching conditions. (FIG. 5A).  
         [0117]    The foregoing vertical anisotropy etching is conducted whereby the amorphous silicon can remain at sides of the gate electrode (strictly speaking, in contact with the oxide layer in the periphery of the gate electrode) as shown in FIG. 5B. Then, the gate insulating film  504  is removed with the gate electrode  505 , the oxide layer  506  in the periphery thereof, and a remainder  508  of the amorphous silicon serving as a mask.  
         [0118]    Subsequently, the remainder  508  of the amorphous silicon is removed, and impurities giving one conductivity type is injected. During this process, the impurity ions are not injected into a region  511  as a result of the gate electrode  505  forming a mask. Also, the impurity ions are not injected into a region  500  as a result of the oxide layer  506  forming a mask. Moreover, the impurity ions of a density lower than that of the regions  509  and  513  into which ions are directly injected are injected into the regions  510  and  512  as a result of the remaining gate insulating film  504  forming a mask.  
         [0119]    In this manner, the channel formation region  511 , the offset gate region  500 , the light doped regions  510  and  512 , the source region  509  and the drain region  513  are formed in a self-aligning manner. The light doped region  512  corresponds to what is generally called an LDD (light doped drain) region.  
         [0120]    The injection of ions may be conducted with an acceleration voltage of 10 to 30 keV and a dose of amount of 5×10 14  to 5×10 15  cm −2  using a phosphorus ion as an impurity ion. Thus, the density of phosphorus in the regions  509  and  513  forming the source and drain regions may be set to 1×10 20  to 2×10 21  cm −3 , and the density of phosphorus in the regions  510  and  512  forming the light doped regions may be set to 1×10 17  to 2×10 18  cm −3 . The density of phosphorus is a measured value obtained by SIMS (Secondary Ion Measuring System). A difference in the density of impurities results from the prevention of a part of ions by the remaining gate insulating film  504 .  
         [0121]    In the foregoing process, the source and drain regions  509  and  513  as well as the light doped regions  510  and  512  are made amorphous. In this situation, a heat treatment is conducted at 450° C. for four hours to thereby conduct the recrystallization of the region into which impurity ions have been injected in the preceding process and the activation of the impurities. In the heat treating process, the crystallization progresses from the offset gate region  500  keeping the crystal property toward the light doped regions  510  and  512  and further toward the source and drain regions  509  and  513 .  
         [0122]    In this manner, the crystal growth progresses contiguously from the offset gate region  500  toward the source and drain regions, thereby being capable of obtaining a crystal structure contiguous from the channel formation region  511  to the source region  509  and further from the channel formation region  511  to the drain region  513 . As a result, a structure where the defects resulting from the lattice mismatching do not concentrate in the middle thereof can be realized.  
         [0123]    It is effective that a laser beam or intense light is irradiated from a front surface or rear surface of the substrate after the foregoing treating process to further improve the crystal property of the active layer. It is also effective to further conduct a heat treatment after the irradiation of a laser beam or intense light.  
         [0124]    Thereafter, the interlayer insulating film  514  which is made of silicon oxide is formed and the source electrode  515  and the drain electrode  516  are also formed. Then, a heat treatment is conducted in the hydrogen atmosphere at 350° C. for one hour, to thereby make a hydrogenation. Thus, a thin film transistor is completed (FIG. 5D).  
         [0125]    The source and drain regions which have been made amorphous by the injection of the impurity ions are subjected to crystal growth with the crystallized channel formation region forming a crystalline nucleus, as a result of which defects can be prevented from concentrating between the source and drain regions and the channel formation region. This causes the problems such as the instability of the operation of the thin film transistor, the dispersion of the characteristics and the large off-state current, which result from the defects, to be restrained.  
         [0126]    The invention described in this specification has the following advantages:  
         [0127]    (1) The recrystallization and activation of the source and drain regions can be conducted at a low temperature in comparison with the prior art.  
         [0128]    (2) The structure in which defects in the interfaces between the source and drain regions and the channel formation region are reduced can be provided.  
         [0129]    (3) A thin film transistor sufficiently high in the crystal property of the source and drain regions can be provided.  
         [0130]    The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.