Abstract:
A method for manufacturing a semiconductor device including forming a gate electrode over a substrate; forming a pate insulating film over the pate electrode and over the substrate; forming a semiconductor film on the gate insulating film; providing the semiconductor film with a metal element; crystallizing the semiconductor film provided with the metal element; doping an element which is used for gettering into a portion of the crystallized semiconductor film; and heating the crystallized semiconductor film whereby the metal element contained in a channel region of the semiconductor film is gettered by the portion.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a bottom-gate-type, thin film transistor, and also to a method for producing it. 
   BACKGROUND OF THE INVENTION 
   Known are thin film transistors (hereinafter referred to as TFT) having an active layer of a silicon-film formed on a glass or quartz substrate. 
   Various types of thin film transistors are known, of which, at present, bottom-gate-type ones are being most industrialized for practical use. 
   In view of their productivity, it is desirable to develop much in future bottom-gate-type TFT to which the production process in part, the designing rule and the production equipment are common. 
   A bottom-gate-type, thin film transistor comprises a gate electrode, a gate-insulating film and an active layer of a silicon film as formed on a substrate in that order. 
   As the silicon film constituting the active layer, generally used is an amorphous silicon film. However, in order to obtain TFT of higher quality, it is desirable to use a crystalline silicon film. 
   To obtain a crystalline silicon film, much used is a technique of crystallizing an amorphous silicon film through exposure to laser radiations. 
   For the crystallization, also employable is heating, which, however, does not apply to the production of bottom-gate-type TFT. 
   This is because, in the process for producing bottom-gate-type TFT, the heating shall follow the formation of the gate electrode, by which the gate electrode material will diffuse unfavorably. 
   However, from the viewpoint of the quality of the crystalline silicon film produced and of the process of producing the film, heating is preferred to laser irradiation. 
   As the material of the gate electrode, most preferred is aluminum with low resistance. 
   However, the electrode material of aluminum is problematic in that it will diffuse, as being influenced by the heating of the active layer for crystallization and activation, to often form projections of so-called hillocks and whiskers. 
   In particular, in the production of bottom-gate-type TFT, the gate electrode is first formed and thereafter the active layer is formed thereon. In this, therefore, the influence of heat on the layers to be formed in different steps is often problematic. 
   SUMMARY OF THE INVENTION 
   The subject matter of the present invention disclosed herein is to provide a constitution of a bottom-gate-type TFT having an active layer of a crystalline silicon film. 
   It is also to provide a novel constitution of a bottom-gate-type TFT having a gate electrode comprising aluminum. 
   Specifically, the subject matter of the invention is to provide a constitution of a bottom-gate-type TFT having a gate electrode of an aluminum material and having an active layer of a crystalline silicon film. 
   One aspect of the invention disclosed herein is a semiconductor device, which comprises; 
   a gate electrode, 
   a gate-insulating film as formed over said gate electrode, and 
   an active layer of a crystalline silicon film as formed on said gate-insulating film, 
   and is characterized in that; 
   said active layer is so constituted that the crystal growth therein extends from the source and drain regions to the channel-forming region, and 
   said source and drain regions contain a higher concentration of a metal element capable of promoting the crystallization of silicon than said channel-forming region. 
   In that constitution, the metal element capable of promoting the crystallization of silicon is most preferably nickel. 
   Apart from nickel, the metal element may be selected from Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. 
   In place of the crystalline silicon film, employable is a compound film of silicon and germanium. 
   Another aspect of the invention is a method for producing a semiconductor device, comprising; 
   a step of forming a gate electrode on a substrate, 
   a step of forming a gate-insulating film on said gate electrode, 
   a step of forming an amorphous silicon film on said gate-insulating film, 
   a step of forming a mask on said amorphous silicon film above said gate electrode, 
   a step of-introducing a metal element capable of promoting the crystallization of silicon, into said amorphous silicon film via said mask, 
   a step of heating the thus-layered substrate to induce crystal growth in said amorphous silicon film in the direction from the region into which said metal element has been introduced to the region below said mask, 
   a step of doping phosphorus into the region into which said metal element has been introduced, via said mask, and 
   a step of again heating the substrate to thereby concentrate said metal element in said phosphorus-doped region. 
   The invention disclosed herein is characterized in that the gate electrode has a laminate structure composed of a thin titanium film and a thick aluminum film, and that the aluminum film constituting the gate electrode is subjected to anodic oxidation. 
   In the process of producing the device having the constitution noted above, the layered substrate is heated prior to forming the oxide film through anodic oxidation to thereby intentionally make the aluminum film have projections of so-called hillocks and whiskers on its surface. This is to prevent the formation any other projections on the surface of the aluminum film in the subsequent steps. 
   In the process, in addition, the heating induces the crystallization of the aluminum film due to the action of the titanium film, thereby retarding the formation of hillocks and whiskers on the surface of the aluminum film in the subsequent steps. 
   Therefore, the constitution noted above is advantageous in that, even when the layered substrate is heated in the subsequent steps, aluminum elements constituting the aluminum film do not diffuse into any other films, or the aluminum film is not melted to flow out anywhere, or hillocks and whiskers are not formed on the surface of the aluminum film. 
   It is important to side-etch the titanium film and to fill the side-etched region with the aluminum oxide as formed through anodic oxidation. This is because the aluminum diffusion and melt flow to be caused by the heating occurs essentially in the interfacial boundary between the aluminum pattern and the substrate below the pattern. 
   The constitution of the invention disclosed herein is extremely useful in bottom-gate-type TFT in which the active layer is made of a crystalline silicon film. This is because the process of producing such bottom-gate-type TFT requires a step of forming the crystalline silicon film after the formation of the gate electrode. 
   In the invention disclosed herein, aluminum can be used to form the gate electrode, and the substrate having an aluminum pattern to be the gate electrode formed thereon can be heated. 
   Another aspect of the invention disclosed herein is a semiconductor device, which comprises; 
   a gate electrode of a laminate film composed of a titanium film and an aluminum film, 
   and is characterized in that; 
   said titanium film is side-etched via the mask of said aluminum film, and 
   said aluminum film is subjected to anodic oxidation to form an oxide film on its surface. 
   Still another aspect of the invention is a semiconductor device provided with a thin film transistor, which comprises; 
   a gate electrode of a laminate film composed of a titanium film and an aluminum film, 
   and is characterized in that; 
   said titanium film is side-etched via the mask of said aluminum film, and 
   said aluminum film is subjected to anodic oxidation to form an oxide film on its surface. 
   In those constitutions noted above, the aluminum film may contain impurities. 
   In those, the side-etched region of the titanium film is filled with the anodic oxide film of aluminum. 
   Still another aspect of the invention is a method for producing a semiconductor device, comprising; 
   a step of forming a pattern of a laminate film composed of a titanium film and an aluminum film, 
   a step of side-etching said titanium film, 
   a step of forming an oxide film on the surface of said aluminum film through anodic oxidation, 
   a step of forming an amorphous silicon film above said pattern, and 
   a step of crystallizing said amorphous silicon film under heat. 
   Still another aspect of the invention is a method for producing a semiconductor device, comprising; 
   a step of forming a pattern of a laminate film composed of a titanium film and an aluminum film, 
   a step of side-etching said titanium film, 
   a step of intentionally forming projections on the surface of said aluminum film under heat, 
   a step of forming an oxide film on the surface of said aluminum film through anodic oxidation, 
   a step of forming an amorphous silicon film above said pattern, and 
   a step of crystallizing said amorphous silicon film under heat. 
   In those constitutions, the heating may be effected in a heating furnace. It may also be attained through exposure to strong radiations. 
   The heating temperature is preferably not lower than 500° C. to produce better results in the invention. 
   In the invention, it is desirable to use aluminum having an extremely high purity. Using aluminum having a higher purity retards the formation of hillocks and whiskers on the surface of the aluminum film. 
   In order to retard the formation of hillocks and whiskers, impurities may be added to the aluminum film in an amount not larger than a few % by weight. The impurities include, for example, Si, Sc, Ti and Y, and even other elements selected from lanthanides and actinides. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1(A)  to  FIG. 1(D) ,  FIG. 2(A)  to  FIG. 2(C) , and  FIG. 3(A)  to  FIG. 3(D)  show a process for producing an N-channel TFT. 
       FIG. 4(A)  to  FIG. 4(C) ,  FIG. 5(A)  to  FIG. 5(C) ,  FIG. 6(A)  and  FIG. 6(B) ,  FIG. 7 ,  FIG. 8 , and  FIG. 9  show a process for producing a combination of a P-channel TFT and an N-channel TFT. 
       FIG. 10(A) ,  FIG. 10(B) , and  FIG. 11  show a process for producing a combination of a P-channel TFT and an N-channel TFT. 
       FIG. 12(A)  to  FIG. 12(C)  show a process for producing an N-channel TFT. 
       FIG. 13(A)  to  FIG. 13(F)  are graphical views of semiconductor devices comprising TFT. 
       FIG. 14(A)  to  FIG. 14(C) , and  FIG. 15(A)  to  FIG. 15(D)  show a process for producing a TFT. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Preferred embodiments of the invention are described below, with reference to the drawings attached hereto. 
   As illustrated, a pattern to be a gate electrode, which is a laminate of a titanium film  102  and an aluminum film  103 , is formed on a glass substrate  101 . ( FIG. 1(A) ) 
   Next, the titanium film  102  is side-etched, as so indicated by  104  ( FIG. 1(B) ) 
   This is to fill the side-etched region with an oxide film to be formed through anodic oxidation in the subsequent step. As being so side-etched, the anodic oxidation of the aluminum pattern can extend to the bottom edge of the pattern. 
   Next, the thus-layered substrate is heated to intentionally form hillocks and whiskers on the surface of the aluminum pattern  103 . The heating induces abnormal growth of aluminum to give those projections. This is effective for retarding the formation of any other hillocks and whiskers in the subsequent steps. 
   The formation of hillocks and whiskers results from the non-uniform distribution of aluminum atoms in the aluminum film and from the residual stress of the film. Accordingly, the formation of hillocks and whiskers around the aluminum film by previously heating the film retards the formation any further hillocks and whiskers therearound in the subsequent steps. 
   Next, the aluminum pattern  103 , which acts as an anode, is subjected to anodic oxidation to form an oxide film  105  thereon, as in  FIG. 1(C) . In this step, the anodic oxidation extends to the bottom edge of the aluminum pattern  103  (at which the titanium film  102  was side-etched in the previous step of  FIG. 1(B) ). 
   Next, as in  FIG. 1(D) , a gate-insulating film  106  and an amorphous silicon film  107  are formed. 
   Next, as in  FIG. 2(B) , formed is a mask  110  of a silicon oxide film. Then, nickel is applied to the layered substrate to thereby make nickel kept in contact with the surface, as so indicated by  111 . 
   Next, the amorphous silicon film  107  is crystallized under heat. In this step, the crystal growth starts from the left-side and right-side regions that are contacted with nickel, and meets to form a grain boundary at the site indicated by  112 , as in  FIG. 3(A) . 
   This crystal growth is referred to as lateral crystal growth, which gives a crystal structure continuing in the direction of the crystal growth. In the crystal structure of that type, the movement of carriers in the crystal growth direction is hardly influenced by crystal defects or crystal level spacing. 
   Concretely, by aligning the axis of the crystal growth direction with that of the carrier movement direction in TFT operation, the degree of carrier movement can be increased. For example, according to this technique, it is easy to obtain N-channel TFT having a high degree of carrier movement of 100 cm 2 /Vs or higher. 
   Where a large number of TFT are formed on one and the same substrate, the channel-forming region in each TFT shall always have a grain boundary at the site indicated by  112 , by which the plural TFT are prevented from having ununified characteristics. 
   In this process, an oxide film is formed as in  FIG. 1(C) , prior to the crystallization of silicon under heat. In the heating step in this process, therefore, the aluminum film is prevented from being melted to flow out anywhere and aluminum atoms in the film are also prevented from diffusing to any other layers. Accordingly, the heat resistance of the semiconductor device comprising the aluminum electrode is improved. 
   Now, the invention is described in more detail with reference to the following Examples, which, however, are not intended to restrict the scope of the invention. 
   EXAMPLE 1 
     FIG. 1(A)  to  FIG. 1(D) ,  FIG. 2(A)  to  FIG. 2(C) , and FIG.  3 (A) to  FIG. 3(D)  are referred to, which show the process of this example. First formed is a gate electrode on the glass substrate  101 . 
   As the substrate, also usable is a quartz substrate, or a semiconductor or metal substrate having an insulating film formed thereon. The substrate of that group will be generically referred to as a substrate having an insulating surface. 
   In this, first, a titanium film is formed on a glass substrate through sputtering to have a thickness of 20 nm, and then an aluminum film containing 0.2% by weight of titanium is formed on the titanium film also through sputtering to have a thickness of 400 nm. 
   Next, the laminate film of the titanium film and the aluminum film is patterned to obtain the laminate pattern as in  FIG. 1(A) . The laminate pattern is composed of the titanium film pattern  102  and the aluminum film pattern  103 . 
   To obtain this pattern, used is dry etching as combined with taper etching. The combined etching gives the laminate pattern, of which the cross section is tapered, as illustrated. 
   Next, this is heated in an inert atmosphere at 400° C. for 1 hour. This heating produces the following results. 
   (1) Crystallization of aluminum due to the action of the titanium film. 
   (2) Intentional formation of hillocks and whiskers on the surface of the aluminum film. 
   (1) enriches the crystal structure of aluminum, and is effective for preventing the formation of hillocks and whiskers in the subsequent steps. In addition, it is also effective for increasing the heat resistance of the gate electrode formed herein. 
   As in (2), hillocks and whiskers are formed in this stage, whereby any further hillocks and whiskers are prevented from being formed in the subsequent steps. 
   One factor of forming hillocks and whiskers is based on the intrinsic stress of aluminum and the non-uniform composition of the aluminum film. As so mentioned hereinabove, the hillocks and whiskers once formed around the aluminum film act to reduce the residual stress and the non-uniform composition of the aluminum film. 
   Next, the titanium film pattern  102  is selectively side-etched according to a wet etching method. After having been thus side-etched, the area of the cross section of the titanium film is reduced to give a reduced titanium film pattern  104 . ( FIG. 1(B) ) 
   Next, the aluminum film pattern  103  acting as an anode is subjected to anodic oxidation to form an oxide film  105  around the exposed surface of the aluminum pattern. ( FIG. 1(C) ) 
   The anodic oxidation in this step extends outside and inside the aluminum pattern. The oxide film is also formed on the side-etched site of the titanium film, which is thus filled with the aluminum oxide film. 
   A gate electrode  100  is formed in that manner. In this step, the aluminum oxide film formed through anodic oxidation shall have an overall grown length of 100 nm. 
   The gate electrode may be made of titanium nitride, tantalum nitride, a laminate of tantalum and tantalum nitride, or a laminate of a tungsten silicide layer and an N-type silicon layer. 
   After having obtained the condition of  FIG. 1(C) , a silicon oxide film  106  that shall be a gate-insulating film is formed, as in  FIG. 1(D) . The film  106  may be formed through plasma CVD to have a thickness of 500 nm. In this step, it is to be noted that the laminate film of the silicon oxide film  106  and the oxide film  105  shall be the gate-insulating film. 
   Next, an amorphous silicon film  107  is formed to have a thickness of 50 nm, through thermal reduced-pressure CVD. ( FIG. 1(D) ) 
   Next, a silicon oxide film  108  is formed to have a thickness of 150 nm through plasma CVD, and then a resist mask  109  is formed. ( FIG. 2(A) ) 
   The resist mask  109  is formed by exposing the back surface of the substrate to light, in which the gate electrode pattern acts as a mask for the light exposure. The formation of the resist mask  109  is effected in a self-aligning manner, and therefore does not require any additional mask. 
   Next, as in  FIG. 2(B) , the silicon oxide film  108  is patterned via the resist mask  109 . Thus is obtained a pattern  110  of the silicon oxide film. 
   After having obtained the condition of  FIG. 2(B) , a nickel acetate solution having a controlled nickel concentration of 10 ppm by weight is applied to the layered substrate in such a manner that nickel is kept in contact with the surface of the layered structure. ( FIG. 2(C) ) 
   In that condition, nickel is not contacted with the surface of the amorphous silicon film  108  in the region coated with the mask  110  of the silicon oxide film, but is contacted with the other region. ( FIG. 2(C) ) 
   To introduce nickel, employed is any of CVD, sputtering, ion implantation, gas adsorption, plasma processing or the like. 
   As the metal element capable of promoting the crystallization of silicon, nickel is the best. Apart from nickel, however, also employable is an element selected from Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. 
   In that condition, the thus-layered substrate is heated in a nitrogen atmosphere at 550° C. for 4 hours. The heating may be effected in a heating furnace equipped with a resistance heater. 
   As a result of this heating treatment, the amorphous silicon film is crystallized by the action of the nickel element. In this step, nickel diffuses in the direction as indicated by the arrows in  FIG. 3(A) , while promoting the crystallization of silicon. 
   At the site indicated by  112 , the crystal growth extending from the both sides meets to form a grain boundary. 
   In that manner, the region of the amorphous silicon film with which nickel is kept in contact is crystallized, and the crystal growth extends from that region to the other region not contacted with nickel. 
   In crystallization of that manner, the grain boundary is always formed in the middle of the region indicated by  112 , which is therefore effective for preventing a plurality of elements, if formed on the substrate, from having ununified characteristics. In other words, all the plural elements formed on one and the same substrate could have the same and unified characteristics owing to that grain boundary formed in each element. 
   Next, as in  FIG. 3(B) , phosphorus is doped under the condition that the doped regions shall be source and drain regions. 
   For the phosphorus doping, employed is a means of plasma doping or ion implantation. 
   In this step, phosphorus is doped into the exposed silicon film. That is, phosphorus is doped into the regions  113  and  114 , as in  FIG. 3(B) . 
   Next, the thus-layered substrate is heated in a nitrogen atmosphere at 550° C. for 2 hours. In this step, nickel moves from the region  115  to the regions  113  and  114  along the arrows illustrated. Thus, nickel having existed in the region  115  is gettered away into the regions  113  and  114 . ( FIG. 3(C) ) 
   In this step, it is desirable that the heating is effected at a temperature falling between 500° C. and 650° C. This is because, if the heating temperature is lower than the defined range, the nickel diffusion will be poor, but if higher than the same, aluminum could not be resistant to the heating. 
   Phosphorus and nickel having bonded to each other give various forms of NiP, NiP 2  and Ni 2 P, and the bonding in those compounds is extremely stable. The compounds have a melting point of 900° C. or higher. 
   The phosphorus diffusion requires a temperature not lower than 800° C. or so. 
   Accordingly, in this heating step, nickel moves actively and bonds to phosphorus to give passivated nickel-phosphorus compounds. ( FIG. 3(C) ) 
   The resulting nickel-phosphorus compounds do not decompose and phosphorus moves nowhere, resulting in that nickel is stably gettered by phosphorus. 
   In other words, as a result of the heat treatment, the nickel concentration in the region  115  is reduced, while that in the regions  113  and  114  is increased. 
   The region  115  from which nickel has been gettered, or that is, from which nickel has been removed shall be the channel region of the TFT to be produced herein. 
   On the other hand, the regions  113  and  114  in which nickel has been gettered, or that is, in which nickel has concentrated shall be the source and drain regions of the TFT. The region  115  is a channel-forming region. ( FIG. 3(C) ) 
   Next, a metallic laminate (not shown) composed of titanium/aluminum/titanium films is formed by sputtering. Regarding the thickness of each film to be formed by sputtering, the titanium film shall have a thickness of 100 nm and the aluminum a thickness of 400 nm. 
   Next, this metallic laminate is patterned to give a source electrode  116  and a drain electrode  117 . Then, the exposed semiconductor film is patterned via the masks of those patterned metal electrodes to obtain the condition of  FIG. 3(D) . 
   In that manner, completed is an N-channel TFT. On the other hand, if a P-channel TFT is desired to be produced, the doped phosphorus is canceled after the heat treatment in the step of  FIG. 3(C) , and thereafter boron is doped to give P-type regions  113  and  114 . 
   For this, the regions  113  and  114  are inverted to P-type regions after-the gettering step. 
   If desired, the regions  113  and  114  may be inverted to P-type regions before the gettering step. 
   The basic experiments which we, the present inventors made have verified that the boron doping at a higher dose into the region into which phosphorus was doped does not retard the gettering effect but rather enhances it. 
   EXAMPLE 2 
   This is to demonstrate a modification of Example 1 in which the heat treatment in the step of  FIG. 3(A)  was effected in a heating furnace. In this example, a heating means of so-called RTA, exposure to strong radiations, is employed for the heat treatment. 
   In RTA, infrared rays having been radiated by a lamp are focused on a mirror, and applied to the layered substrate, whereby the region having exposed to the focused rays is heated at a temperature falling between 600° C. and 800° C. within a short period of time. 
   This heat treatment utilizes the phenomenon that the region exposed to light absorbs the light. In this, therefore, the exposed region can be heated within a short period of time, and the heat treatment of the region can be finished within such a short period of time. Concretely, the heat treatment for about 1 to 10 minutes or so produces the crystallization, as in  FIG. 3(A) . 
   If desired, the heat treatment in Example 1 may be combined with RTA, lamp exposure, in this Example 2. 
   EXAMPLE 3 
   This is to demonstrate another modification of Example 1. In this example, a photo-mask is used in the step of forming the resist mask  109  in  FIG. 2(A) . 
   The process of this example has the disadvantage of requiring an increased number of masks, but has the advantage of employing conventional photolithography using a photo-mask, and the process is stable. 
   EXAMPLE 4 
   This is to demonstrate still another modification of Example 1. The means of crystallization employed herein is different from that in Example 1. 
   In this example, nickel is introduced into the entire surface of the amorphous silicon film  107  in the absence of the mask  110  in the step of  FIG. 2(B) . 
   In this case, there occurs no specific crystal growth (lateral crystal growth) such as that in  FIG. 3(A) . Namely, no lateral crystal growth starts from any specific region herein. 
   In this case, the crystal growth starts locally throughout the entire surface of the film. 
   EXAMPLE 5 
   This is to demonstrate still another modification of Example 1. In this example, boron (B) is added to the amorphous silicon film  107  being formed in the step of  FIG. 1(D)  in order to control the threshold value of the TFT to be produced herein. 
   In this case, a minor amount of diborane (B 2 H 6 ) is added to the raw material gas from which the amorphous silicon film  107  is formed. 
   In this, any of plasma doping or ion implantation is employable for the doping. 
   EXAMPLE 6 
   This is to demonstrate still another modification of Example 1. In this example, a germanium-containing, amorphous silicon film (which comprises silicon as the essential ingredient) is formed in place of the amorphous silicon film  107 . 
   The amorphous silicon film comprising silicon as the essential ingredient and additionally containing germanium may be represented by Si x Ge 1−x  (0.5&lt;x&lt;1). 
   In this example, the film shall contain germanium in an amount of 5 atm. %. Depending on its amount, germanium that exists in the amorphous silicon film controls the threshold value of the TFT produced herein. 
   To form the germanium-containing, amorphous silicon film, a raw material gas comprised of silane and germane is used for plasma CVD, thermal reduced-pressure CVD or sputtering. 
   EXAMPLE 7 
   This is to demonstrate a process for producing a combination of a P-channel TFT and an N-channel TFT. This process illustrated herein is applicable, for example, to the production of CMOS circuits. 
   The process of this example is illustrated in  FIG. 4(A)  to  FIG. 4(C) ,  FIG. 5(A)  to  FIG. 5(C) ,  FIG. 6(A)  and  FIG. 6(B) ,  FIG. 7 ,  FIG. 8 , and  FIG. 9 . First, as in  FIG. 4(A) , a gate electrode comprised of a titanium film pattern  402  and an aluminum film pattern  404  is formed on a glass substrate  401 , along with a gate electrode comprised of a titanium film pattern  403  and an aluminum film pattern  405 . 
   In this, the left-handed one shall be the gate electrode of the P-channel TFT, while the right-handed one shall be the gate electrode of the N-channel TFT. In that manner, the P-channel TFT is formed in the left and the N-channel TFT is in the right. 
   Thus is obtained the condition shown in  FIG. 4(A) . Next, the titanium film patterns  402  and  403  are side-etched via the masks of the aluminum film patterns  404  and  405 . 
   In this step, the both titanium film patterns are side-etched below the bottom edge of each aluminum film pattern, as indicated by  406  in  FIG. 4(B) . 
   Next, the aluminum film patterns  404  and  405 , acting as anodes, are subjected to anodic oxidation to obtain the condition of  FIG. 4(C) . In this,  407  and  408  are oxide films formed through the anodic oxidation. 
   As in  FIG. 4(C) , the oxide films both extend to the bottom edge of each aluminum film pattern. In this drawing, the anodic oxidation at the bottom edge of each aluminum film pattern, at which the titanium film was etched, exaggerated in some degree. 
   Next, as in  FIG. 5(A) , a silicon oxide film  409  is formed through plasma CVD. Then, an amorphous silicon film  410  is formed through thermal reduced-pressure CVD. 
   Next, the back surface of the glass substrate is exposed to light to form silicon oxide film patterns  411  and  412 . Then, a nickel acetate solution is applied to the thus-layered substrate to thereby make nickel kept in contact with the surface of the layered structure, as so indicated by  413 . ( FIG. 5(B) ) 
   Next, this is heated in a nitrogen atmosphere at 550° C. for 4 hours, whereby the amorphous silicon film is crystallized. 
   This crystallization brings about the crystal growth as indicated by the arrows in  FIG. 5(C) . For this, preferably, the heating temperature falls between 500° C. and 600° C. This is because aluminum could not be resistant to temperatures higher than the defined range. However, the crystallization could not be attained at temperatures lower than the defined range. 
   Next, phosphorus is doped under the condition that the doped regions shall be the source and drain regions of the N-channel TFT to be produced herein. 
   In this step, phosphorus is doped into the regions  601 ,  602  and  603 , as in  FIG. 6(A) . 
   Next, the thus-layered substrate is heated in a nitrogen atmosphere at 600° C. for 1 hour. In this step, nickel moves from the regions  604  and  605  to the phosphorus-doped regions  601 ,  602  and  603 .  FIG. 6(B) ) 
   Thus, nickel having existed in the regions  604  and  605  is gettered away into the regions  601 ,  602  and  603 . 
   The regions  604  and  605  shall be the channel-forming regions of TFT being produced herein. 
   Next, a resist mask  701  is formed, as in  FIG. 7 . In this condition, boron is doped. In the left-handed TFT site, boron is doped over the previously phosphorus-doped region. Precisely, in the regions  702  and  703 , boron is doped over the previously doped phosphorus. 
   This boron doping is effected under the condition that it cancels the influence of the previously doped phosphorus on those regions  702  and  703  to thereby produce P-type conductivity. As a result of this boron doping, the N-type regions produced by the previous phosphorus doping (in the step of  FIG. 6(A) ) are inverted into P-type regions. 
   After the doping, the resist mask  701  is removed. Then, the layered substrate is exposed to laser rays, thereby annealing the damage in the doped regions and activating the dopant. This step may be effected by exposure to strong radiations. 
   In that manner, formed are the P-type regions  702  and  703 , and N-type regions  704  and  705 . 
   The P-type regions  702  and  703  have been doped with phosphorus and with boron thereover. In those regions, boron acts to determine the conductivity of those regions, while phosphorus functions to getter nickel. 
   On the other hand, in the N-type regions  704  and  705 , phosphorus acts to determine the conductivity of those regions and also to getter nickel. 
   In  FIG. 7 , the P-type region  702  shall be the source region of the P-channel TFT, while the region  703  shall be the drain region thereof. 
   The N-type region  704  shall be the drain region of the N-channel TFT, while the region  705  shall be the source region thereof. 
   Next, as in  FIG. 8 , a metal film  801  is formed through sputtering over the layered substrate. This metal film  801  is of a laminate of titanium/aluminum/titanium films. 
   Next, as in  FIG. 9 , the metal film  801  is patterned to give patterns  901 ,  902 ,  903  and  904 . 
   In this,  901  is the source electrode of the P-channel TFT, and  902  is the drain electrode thereof. 
     903  is the drain electrode of the N-channel TFT, and  904  is the source electrode thereof. 
   After having formed the electrodes  901  to  904  from the metal film  801 , the exposed silicon film (for the source and drain regions of each TFT) is etched. In that manner, formed is a combination of a P-channel TFT (hereinafter referred to as P-channel TFT) and an N-channel TFT (hereinafter referred to as N-channel TFT) on one and the same substrate, as in  FIG. 9 . 
   EXAMPLE 8 
   This is to demonstrate a modification of Example 7. This example is characterized in that the regions constituting the P-channel TFT are doped with phosphorus for gettering and with boron for conductivity determination (that is, for channel determination), and that the regions constituting the N-channel TFT are doped with phosphorus for gettering and for conductivity determination and are heated for gettering. 
   First, obtained is the condition of  FIG. 6(A)  according to the process of Example 7. This is shown in  FIG. 10(A) . In this stage, the regions  601 ,  602  and  603 , except the others to be the channel regions, have been doped with phosphorus. 
   Next, a resist mask  701  is formed, with which the region to be the N-channel TFT is masked. ( FIG. 10(B) ) 
   Then, boron is doped under the condition that the N-type regions  702  and  703  are inverted to P-type ones. In other words, the boron doping is effected under the condition that it cancels the influence of the previously doped phosphorus on those regions  702  and  703  to thereby exhibit the influence of the thus-doped boron on those regions  702  and  703 . 
   As a result of this boron doping, obtained are P-type regions  702  and  703 . Also obtained are N-type regions  704  and  705 . 
   After the doping, the resist mask  701  is removed. Then, the layered substrate is exposed to laser rays, thereby annealing the damage in the doped regions and activating the dopant. 
   This step may be effected by exposure to infrared rays from an infrared lamp (RTA). 
   In this example, nickel having been used for crystallization is not gettered after the step of doping the dopant to determine the conductivity of the source and drain regions in the P-channel and N-channel TFT, but is gettered after the step of doping the dopant (boron) to determine the conductivity of the source and drain regions in the N-channel TFT. 
   After the step of  FIG. 10(B) , the resist mask  701  is removed, and the doped regions are then annealed. Next, the thus-layered substrate is heated at 550° C. for 1 hour to thereby getter nickel. 
   Precisely, as in  FIG. 11 , nickel having existed in the region  604 , which shall be the channel region, is gettered away into the regions  702  and  703 , and nickel having existed in the region  605 , which shall be the channel region, is also gettered away into the regions  704  and  705 . 
   In this, the regions  702  and  703  have been doped with phosphorus and then with boron thereover. In those regions in that condition, nickel is gettered more highly than in the other regions  704  and  705  which have been doped with phosphorus only. 
   In the basic experiments which we, the present inventors made, nickel was not gettered at all in the regions doped with boron only, but was gettered more highly in the regions doped with phosphorus and boron than in the regions doped with phosphorus only. At present, the reason for those phenomena is not clear. 
   In that manner, obtained are the source region  702 , the channel-forming region  604  and the drain region  703  for the P-channel TFT (PTFT). In this, nickel having existed in the channel region  604  is gettered away into the source region  702  and the drain region  703 , resulting in that the nickel concentration in the channel region  604  is lowered. 
   Also obtained are the source region  705 , the channel-forming region  605  and the drain region  704  for the N-channel TFT (NTFT). In this, nickel having existed in the channel region  605  is gettered away into the source region  705  and the drain region  704 , resulting in that the nickel concentration in the channel region  605  is lowered. 
   After having obtained the layered structure as in  FIG. 11 , this is processed as in  FIG. 8  and  FIG. 9  to form the P-channel TFT and the N-channel TFT on one and the same glass substrate. 
   The process of this example is advantageous in that the nickel concentration in the channel regions that are sensitive to the motion of TFT is lowered, and therefore the negative influence of nickel on the motion of TFT is retarded. 
   EXAMPLE 9 
   This is to demonstrate still another modification of Example 1. The mode of example is characterized in that the nickel gettering is effected in two stages. 
   First is obtained the condition of  FIG. 1(D)  in the same manner as in Example 1. That is, the layered structure having the amorphous silicon film  107  is obtained as in Example 1. 
   Next, a mask  1201  of a silicon oxide film is formed, as in  FIG. 12(A) . Then, phosphorus is doped into the regions  1202  and  1203 , as in  FIG. 12(B) . 
   This phosphorus doping does not participate in the formation of source/drain, but is only for nickel gettering. 
   Next, the layered structure is heated in a nitrogen atmosphere at 600° C. for 1 hour. In this step, nickel having existed in the region  1204  is gettered away into the regions  1202  and  1203 , as in  FIG. 12(C) . In this step, the heating is effected via the mask  1201  of the silicon oxide film. 
   Next, the exposed silicon film is etched via the mask  1201  of the silicon oxide film. Thus, the gettering sites  1202  and  1203  are etched off. 
   As a result of this process, the nickel concentration in the region  1204  (this shall be the active layer of TFT) is reduced. 
   After this, the region  1204  is processed in the same manner as in Example 1 to produce TFT. For this, referred to are the steps of  FIG. 2(A)  to  FIG. 2(C)  and  FIG. 3(A)  to  FIG. 3(D) . 
   In the process of this example, nickel having existed in the region to be the active layer is gettered away, as in  FIG. 12(C) , while nickel having existed in the channel-forming region is also gettered away into the source/drain regions as in  FIG. 3(C) . 
   In this process, therefore, the negative influence of nickel on TFT is drastically removed. 
   EXAMPLE 10 
   This is to demonstrate still another modification of Example 1 or Example 10. In this, the structure of the gate electrode is improved. 
   In this example, silicon is used as the gate electrode for the N-channel TFT. 
   First, as in  FIG. 14(A) , an N-type silicon film is formed on a glass substrate  101  through thermal reduced-pressure CVD, and then patterned into a pattern  1401 . This pattern  1401  is to be the gate electrode. 
   Then, a gate-insulating silicon oxide film  106  is formed over the gate electrode  1401  through thermal reduced-pressure CVD. Further, an amorphous silicon film  107  is formed thereover also through thermal reduced-pressure CVD. 
   Next, another silicon oxide film  108  is formed over the film  107 , and a resist mask  109  is formed through exposure of the back surface of the thus-layered substrate to light. ( FIG. 14(A) ) 
   Next, a mask  110  of the silicon oxide film is formed below the resist mask  109 , as in  FIG. 14(B) . 
   Next, the resist mask  109  is removed, and a nickel acetate solution is applied to the layered structure to thereby make nickel kept in contact with the surface of the structure, as so indicated by  111 . Thus is obtained the condition of  FIG. 14(C) . 
   Next, this is heated to thereby crystallize the amorphous silicon film  107 , as in  FIG. 15(A) . In this step, the layered structure is heated in a nitrogen atmosphere at 630° C. for 4 hours for the intended crystallization. 
   The upper limit of the heating temperature in this step is defined by the heat-resisting temperature of the glass substrate used, since the gate electrode is made of silicon having high heat resistance. 
   For example, where a quartz substrate is used herein, the heating temperature may be higher. 
   After the crystallization, phosphorus is doped through plasma doping, as in  FIG. 15(B) . In this step, phosphorus is doped in the regions  113  and  114 . 
   Next, this is again heated in a nitrogen atmosphere at 600° C. for 2 hours. As a result of this heat treatment, nickel having existed in the region  115  is gettered away into the regions  113  and  114 . ( FIG. 15(C) ) 
   Next, formed are a source electrode  116  and a drain electrode  117 . Via those electrodes, the exposed semiconductor region is etched, as in  FIG. 15(D) . 
   As the gate electrode, also employable is any of tantalum, laminate of tantalum and tantalum nitride, various silicide materials, and various metal materials. 
   EXAMPLE 11 
   Herein shown are examples of various semiconductor devices comprising TFT as produced according to the invention, such as those illustrated in the other examples. 
     FIG. 13(A)  is a portable, information-processing terminal. The body  2001  of this information-processing terminal is provided with an active-matrix type, liquid crystal display or an active matrix-type, EL display, and with a camera member  2002  through which it takes external information. 
   The camera member  2002  is provided with an image-receiving part  2003  and a control switch  2004 . 
   It is expected that information-processing terminals will be thinner and more lightweight for enhancing their portability. 
   In the illustrated constitution, it is desirable that the peripheral drive circuit, arithmetic circuit and memory circuit to be on the substrate on which the active-matrix type display  2005  is formed are integrated with TFT. In  FIG. 13(A) ,  2006  is an integrated circuit. 
     FIG. 13(B)  is a head-mount display. Its body  2101  is provided with an active-matrix type, liquid crystal display or EL display  2102 . The body  2101  can be mounted on the head with the band  2103 . 
     FIG. 13(C)  is a car navigation device. Its body  2201  is provided with a liquid crystal display device  2202  and a control switch  2203 . The car navigation device of that type has a function of displaying geographic information and the like, in accordance with the signals received by the antenna  2204 . 
     FIG. 13(D)  is a portable telephone. Its body  2301  is provided with an active-matrix type, liquid crystal display device  2304 , a control switch  2305 , a voice-inputting member  2303 , a voice-outputting member  2302  and an antenna  2306 . 
   Recently, a combination composed of the portable, information-processing terminal of  FIG. 13(A)  and the portable telephone of  FIG. 13(D)  has been commercialized  FIG. 13(E)  is a portable video camera. Its body  2401  is provided with an image-receiving member  2406 , a voice-inputting member  2403 , a control switch  2404 , an active-matrix type, liquid crystal display  2402 , a battery  2405 , and an integrated circuit  2407 . 
     FIG. 13(F)  is a projection-type, liquid crystal display device. Its body  2501  is provided with a light source  2502 , an active-matrix type, liquid crystal display device  2503 , and an optical system  2504 . This projection device has a function of displaying images on a screen  2505  positioned outside the device. 
   The liquid crystal display device to be used in the examples illustrated herein may be any of transmission-type ones or reflection-type ones. 
   In the devices of  FIG. 13(A)  to  FIG. 13(E) , also usable are active-matrix type, EL displays in place of the liquid crystal display devices. 
   As has been described in detail herein, the present invention disclosed herein provides bottom-gate-type TFT having aluminum electrodes. TFT of the invention are free from the problems with conventional TFT. Concretely, in producing TFT of the invention, the necessary crystallization can be attained under heat, and heating can be effected at any step in the process of producing TFT. 
   While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.