Patent Publication Number: US-7709844-B2

Title: Semiconductor device equipped with semiconductor circuits composed of semiconductor elements and processes for production thereof

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a structure of a semiconductor device equipped with semiconductor circuits composed of semiconductor elements such as insulated gate transistors, and also to a process for producing the same. 
   More particularly, the present invention relates to a semiconductor device equipped with semiconductor circuits composed of semiconductor elements having wiring of tantalum material and also to a process for producing the same. The semiconductor device of the present invention includes not only such elements as thin film transistors (TFT) and MOS transistors but also display units having semiconductor circuits composed of said insulated gate transistors and electro-optical units such as image sensors. Moreover, the semiconductor device of the present invention also includes electronic machines and equipment equipped with such display units and electro-optical units. 
   2. Description of the Related Art 
   Much attention is being devoted to active matrix liquid crystal displays in which the pixel matrix circuits and drive circuits are constructed of thin film transistors (TFT) formed on an insulating substrate. Liquid crystal displays in use have a size ranging from 0.5 to 20 inches. 
   One of the developmental works for liquid crystal displays is directed to increasing their display area. Unfortunately, according as the display area increases, the pixel matrix circuits for pixel displays also increase in area. As the result, the source wiring and gate wiring arranged in matrix become longer, resulting in an increased wiring resistance. Moreover, in order to meet the requirement for finer pitches, it is necessary to make wiring smaller. This causes the wiring resistance to increase remarkably. Since TFTs are connected to the source wiring and gate wiring for individual pixels, an increased number of pixels leads to an increased parasitic capacity. Liquid crystal displays are usually have the gate wiring and gate electrode formed integrally, and hence the gate signal delay becomes significant according as the panel area increases. 
   Therefore, if the gate electrode wiring is made of a material having a lower resistivity, then it would be possible to make the gate wiring thinner and longer accordingly. This leads to panels of large area. Conventional materials for gate electrode wiring are Al, Ta, and Ti. Of these, aluminum is most common because of its low resistivity and capability of anodic oxidation. Aluminum forms anodic oxidized film which contributes to heat resistance but suffers whiskers and hillocks, wiring deformation, and diffusion into the insulating film and active layer even at low process temperatures of 300-400° C. This is the major cause to deteriorate TFT&#39;s action and characteristic properties. 
   What is necessary for larger panels and finer pixels is an electrode structure which has a lower resistivity and better heat resistance. 
   Properties currently required of TFT are high mobility. It is expected that this requirement would be met if crystalline semiconductor film, which has higher mobility than amorphous semiconductor film, is used as the active layer. In the past, it was necessary to use a quartz substrate having a high strain point in order to obtain a crystalline semiconductor film by heat treatment. Attempts have been made to reduce the crystallization temperature so that expensive quartz substrates are replaced by cheap glass substrates. 
   Accordingly, the present inventors developed a technology to produce a crystallized semiconductor film from an amorphous semiconductor film (typically that of amorphous silicon film or Ge-containing amorphous silicon film) by introduction of a small amount of metal element and subsequent heat treatment. (Japanese Patent Laid-open No.6-232059 and 7-321339) Examples of the metal element to promote crystallization include Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. They may be used alone or in combination with another. This technology enables the production of crystalline semiconductor film at a process temperature low enough for the glass substrate to withstand. Other metals that can be used include Ge and Pb, which undergo substitutional diffusion into amorphous semiconductor film. 
   The disadvantage of this technology is that the metal used for crystallization remains in the crystalline semiconductor film, producing an adverse effect on TFT&#39;s characteristic properties (particularly, reliability and uniformity). So, the present inventors further developed a technology to form wiring from aluminum and subsequently remove the metal elements from the crystalline semiconductor film by gettering. (Japanese Patent Laid-open No.8-330602) According to this technology, gettering is accomplished by performing heat treatment while using the phosphorus-doped source region and drain region as the gettering sink so that the catalyst elements in the channel forming region are captured in the source region and drain region. 
   However, the above-mentioned technology has the disadvantage of being limited in the temperature range for heat treatment (about 300-450° C.) because wiring is made of aluminum with low heat resistance. For satisfactory gettering, heat treatment at 400° C. and above, preferably 550° C. and above, is necessary. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a semiconductor device with a new electrode structure which has a low resistivity and withstands gettering satisfactorily. It is another object of the present invention to provide a process for producing said semiconductor device. 
   The first aspect of the present invention is a semiconductor device equipped with semiconductor circuits composed of semiconductor elements, wherein said semiconductor element comprises:
     a substrate with an insulating surface,   a gate electrode of multi-layer structure over said substrate,   a protective film covering said substrate and the top and sides of said gate electrode,   a gate insulating film covering said protective film, and   a source region, a drain region, and a channel forming region (between said source region and said drain region) which are formed on said gate insulating film.   

   In the above-mentioned construction, the gate electrode of multi-layer structure has at least one layer whose principal component is at least one kind of element selected from tantalum, molybdenum, titanium, chromium, and silicon. 
   In the above-mentioned construction, the gate electrode of multi-layer structure is composed of three layers arranged on top of another, with a first layer being composed mainly of tantalum and containing nitrogen, a second layer being composed mainly of tantalum, and a third layer being composed mainly of tantalum and containing nitrogen, the first layer being adjacent to the substrate. 
   The second aspect of the present invention is a semiconductor device equipped with semiconductor circuits composed of semiconductor elements, wherein said semiconductor element comprises:
     a substrate with an insulating surface,   a gate electrode over said substrate,   a protective film covering said substrate and the top and sides of said gate electrode,   a gate insulating film covering said protective film,   a source region, a drain region, and a channel forming region (between said source region and said drain region) which are formed on said gate insulating film,   an inorganic insulator in contact with said channel forming region, and   an organic resin film in contact with said source region and drain region.   

   In the above-mentioned second construction, the gate electrode is of three-layer structure, with a first layer being composed mainly of tantalum and containing nitrogen, a second layer being composed mainly of tantalum, and a third layer being composed mainly of tantalum and containing nitrogen. 
   In each of the above-mentioned constructions, the protective film is a silicon nitride film and has a film thickness of 10-100 nm. 
   In each of the above-mentioned constructions, the source region and drain region are at least partly silicide. 
   In each of the above-mentioned constructions, the source region and drain region are incorporated with an impurity to impart the n-type conductivity. 
   In each of the above-mentioned constructions, the source region and drain region are incorporated with an impurity to impart the n-type conductivity and an impurity to impart the p-type conductivity. 
   In each of the above-mentioned constructions, the channel forming region contains a catalyst element to promote crystallization of silicon, with the concentration of said catalyst element being higher in the source region and drain region than in the channel forming region. 
   In each of the above-mentioned constructions, the catalyst element is at least one member selected from Ni, Fe, Co, Pt, Cu, Au, and Ge. 
   The third aspect of the present invention is a process for producing a semiconductor device equipped with semiconductor circuits composed of semiconductor elements, wherein said process comprises:.
     a step of forming wiring over a substrate with an insulating surface,   a step of forming a protective film that covers said wiring,   a step of forming a gate insulating film on said protective film,   a step of forming on said gate insulating film a crystalline semiconductor film containing a catalyst element to promote crystallization of silicon,   a step of irradiating said crystalline semiconductor film with a laser light,   a step of forming a mask of insulating film on part of said crystalline semiconductor film,   a step of doping with phosphorus the region which is to become the source region or drain region,   a step of performing heat treatment for gettering of said catalyst element, and   a step of patterning said crystalline semiconductor film, thereby forming an active layer.   

   The fourth aspect of the present invention is a process for producing a semiconductor device equipped with semiconductor circuits composed of semiconductor elements, wherein said process comprises:
     a step of forming wiring over a substrate with an insulating surface,   a step of forming a protective film that covers said wiring,   a step of forming a gate insulating film on said protective film,   a step of forming on said gate insulating film a crystalline semiconductor film containing a catalyst element to promote crystallization of silicon,   a step of patterning said crystalline semiconductor film, thereby forming an active layer,   a step of irradiating said crystalline semiconductor film with a laser light,   a step of forming a mask of insulating film on part of said crystalline semiconductor film,   a step of doping with phosphorus the region which is to become the source region or drain region,   a step of performing heat treatment for gettering of said catalyst element.   

   A process for producing a semiconductor device equipped with semiconductor circuits composed of semiconductor elements as defined in the above-mentioned third or fourth construction, wherein the step of forming wiring on a substrate with an insulating surface includes the substeps of forming continuously a first tantalum layer containing nitrogen, a second tantalum layer, and a third tantalum layer containing nitrogen (on top of another, with the first layer being adjacent to the substrate), and performing patterning. 
   In the above-mentioned third or fourth construction, the step of forming a crystalline semiconductor film on a gate insulating film consists of the substeps of forming an amorphous semiconductor film in contact with the surface of said gate insulating film, causing the amorphous semiconductor film to hold a catalyst element to promote crystallization of silicon, and performing heat treatment, thereby crystallizing the amorphous semiconductor film and forming a crystalline semiconductor film. 
   In the above-mentioned third or fourth construction, the step of forming a crystalline semiconductor film on a gate insulating film consists of the substeps of forming an amorphous semiconductor film in contact with the surface of said gate insulating film, causing the amorphous semiconductor film to hold a catalyst element to promote crystallization of silicon, and irradiating a laser light, thereby crystallizing the amorphous semiconductor film and forming a crystalline semiconductor film. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view showing one example of the structure according to the present invention (Example 1). 
       FIGS. 2A-2B  are top views showing one example of the structure according to the present invention (Example 1). 
       FIGS. 3A-3D  are sectional views showing one example of the process for fabrication according to the present invention (Example 1). 
       FIGS. 4A-4C  are sectional views showing one example of the process for fabrication according to the present invention (Example 1). 
       FIGS. 5A-5C  are sectional views showing one example of the process for fabrication according to the present invention (Example 1). 
       FIGS. 6A-6B  are sectional views showing one example of the process for fabrication according to the present invention (Example 1). 
       FIGS. 7A-7C  are sectional views showing one example of the process for fabrication according to the present invention (Example 2). 
       FIGS. 8A-8C  are sectional views showing one example of the process for fabrication according to the present invention (Example 2). 
       FIGS. 9A-9B  are sectional views showing one example of the process for fabrication according to the present invention (Example 2). 
       FIG. 10  is a sectional view showing one example of the structure according to the present invention (Example 3). 
       FIG. 11  is a sectional view showing one example of the structure according to the present invention (Example 6). 
       FIGS. 12A-12B  are external appearances of AMLCD. 
       FIGS. 13A-13F  are diagrams showing electronic machines and equipment. 
       FIGS. 14A-14D  illustrates front and rear-type projections in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   According to the present invention, the gate wiring and the gate electrode are made of tantalum or a material composed mainly of tantalum. Tantalum is one of the desirable materials because it has a work function close to that of silicon and hence it shifts the threshold value of TFT only slightly. 
   It is known that tantalum has two kinds of crystal structure (or body-centered cubic structure [α-Ta] and tetragonal lattice structure [β-Ta]). Thin film of tetragonal lattice structure [β-Ta] has a resistivity of about 170-200 μΩ·cm, and thin film of body-centered cubic structure [α-Ta] has a resistivity of about 13-15 μΩ·cm. It is known that tantalum thin film usually takes on the β-Ta structure but it also takes on the α-Ta structure (also called bcc-Ta) if it is incorporated with a trace amount of impurity such as nitrogen. 
   According to the present invention, TaN film and Ta film are formed consecutively (or laminated on top of the other) so as to obtain the α-Ta structure. In this case, the TaN film should be thicker than 30 nm, preferably thicker than 40 nm, depending on the constituent of the TaN film. 
   Unfortunately, tantalum or a material composed mainly of tantalum is liable to occlude hydrogen and become oxidized. Hence, when formed into film, it changes in quality (or increases in resistance) due to oxidation and hydrogen occlusion. 
   In order to address this problem, the present invention employs the gate wiring and the gate electrode of three-layer structure, in which a TaN film (thickness of 30 nm or more, preferably 40 nm or more), a Ta film, and a TaN film are formed (laminated) consecutively on top of another. The three-layer structure is covered with a protective film after patterning. 
   The consecutively formed three-layer structure in combination with the protective film prevents hydrogen occlusion and oxidation. 
   The tantalum multi-layer structure (TaN/Ta/TaN=50/250/50 nm) changes in resistance after heat treatment at 450° C., 500° C., 550° C., and 600° C. for 2 hours, as shown in Table 1. The heat treatment was carried out as follows. The temperature was raised from 400° C. to the treating temperature minus 10° C. at a rate of 9.9° C./min and then further raised to the treating temperature at a rate of 5° C./min, and the treating temperature was kept for 2 hours. After gradual cooling, the resistance was measured. 
   
     
       
         
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
                 
                 
                 
               Film thickness 
             
             
               Conditions of 
               Resistance before 
               Resistance after 
               after heat 
             
             
               heat treatment 
               heat treatment 
               heat treatment 
               treatment 
             
             
                 
             
           
          
             
               450° C. for 2 hours 
               17.5 kΩ 
               25 kΩ 
               400 nm 
             
             
               500° C. for 2 hours 
               10.5 kΩ 
               50 kΩ 
               530 nm 
             
             
               550° C. for 2 hours 
                 19 kΩ 
               ∞ 
               700 nm 
             
             
               600° C. for 2 hours 
               10.5 kΩ 
               ∞ 
               1000 nm  
             
             
                 
             
          
         
       
     
   
   It is noted from Table 1 that the tantalum multi-layer film increases in resistance and film thickness with increasing temperature due to deterioration (oxidation). 
   The tantalum multi-layer structure (TaN/Ta/TaN) coated with a protective film (SiN, 25 nm thick) changes in resistance after heat treatment at 450° C., 500° C., 550° C., and 600° C. for 2 hours, as shown in Table 2. The heat treatment was accomplished in the same way as in the case of Table 1. 
   
     
       
         
             
             
             
             
           
             
               TABLE 2 
             
             
                 
             
             
                 
                 
                 
               Film thickness 
             
             
               Conditions of 
               Resistance before 
               Resistance after 
               after heat 
             
             
               heat treatment 
               heat treatment 
               heat treatment 
               treatment 
             
             
                 
             
           
          
             
               450° C. for 2 hours 
               21.3 kΩ 
               19.1 kΩ 
               350 nm 
             
             
               500° C. for 2 hours 
               19.8 kΩ 
               19.0 kΩ 
               350 nm 
             
             
               550° C. for 2 hours 
               10.3 kΩ 
               11.7 kΩ 
               360 nm 
             
             
               600° C. for 2 hours 
                 50 kΩ 
                 40 kΩ 
               340 nm 
             
             
                 
             
          
         
       
     
   
   It is noted from Table 2 that the protective film (SiN) prevents the tantalum multi-layer film from increasing in resistance and film thickness after heat treatment. 
   The foregoing indicates that heat treatment at high temperatures (400-700° C.) is possible if a tantalum film or a tantalum-based film (which has good heat resistance) is used for the wiring and the wiring is coated with a protective film. Such heat treatment permits the gettering of metallic elements in a crystalline semiconductor film. The gate wiring (with a wiring width of 0.1-5 μm) withstands such heat treatment at specific temperatures without being oxidized and hence retains its low resistance because it is coated with a protective film. 
   The content of nitrogen in the TaN film ranges from 5 to 60%, depending on the sputtering apparatus and conditions. Incidentally, it is desirable to obtain the α-Ta film by using argon or xenon plasma. 
   Moreover, tantalum may be replaced by Mo, Ti, Nb, W, Mo—Ta alloy, Nb—Ta alloy, W—Ta alloy, or the like. These materials may be used in the form of nitrogen-containing metal or silicide (which is a metallic compound with silicon). 
   According to the present invention, the protective film may be an inorganic insulating film (such as silicon nitride film and silicon nitride oxide film) in a single layer or multiple layers. The thickness of the protective film ranges from 10 to 100 nm. In addition, the protective film may be amorphous silicon film or crystalline silicon film. 
   Since TaN film is less liable to hydrogen occlusion and oxidation than Ta film, it is used as the uppermost layer to cover tantalum layer so as to ensure good ohmic contact when contact holes are formed. 
   Another desirable structure to give good ohmic contact for wiring connection is a multi-layer wiring composed of a tantalum-based layer  1101  and a titanium-based layer  1102  laminated thereon, as shown in  FIG. 11 . This titanium-based layer  1102  protects the tantalum-based layer  1101  from oxidation and hydrogen occlusion when contact holes are made. The titanium-based layer also provides good ohmic contact because it does not become an insulator when exposed and oxidized and it is easily removed. In other words, the titanium-based layer protects the tantalum-based layer and also facilitates the formation of contact holes (openings) because it provides sufficient margin for etching. 
   High-heat-resistant tantalum film or tantalum-based film used for wiring permits heat treatment at high temperatures (400-700° C.) for the gettering of metallic elements in crystalline semiconductor film. During high-temperature heat treatment, the protective film prevents the diffusion of impurities from the substrate due to heating, thereby permitting the gate insulating film to retain good insulating properties. The result is that TFTs having good characteristic properties can be produced without being affected by the concentrations of impurities contained in the substrate. 
   Thus, the semiconductor device according to the present invention has a lower resistivity than conventional ones (with tantalum film of β-Ta). The process of the present invention enables the production of good TFTs regardless of the concentration of impurities in the substrate even when heat treatment is carried out at high temperatures (400-700° C.). 
   The invention will be explained with reference to the following examples, which are not intended to restrict the scope thereof. 
   EXAMPLE 1 
   A semiconductor device according to the present invention is constructed as shown in  FIG. 1 . It has peripheral driving circuits and pixel matrix circuits on the same substrate. For easy illustration, there are shown in  FIG. 1  a CMOS circuit  202  constituting part of the peripheral driving circuits and a pixel TFT  203  (n-channel type TFT) constituting part of the pixel matrix circuits. 
     FIG. 2  is a top view corresponding to  FIG. 1 . The section taken along the thick line A-A′ represents the structure of the pixel matrix circuit  201  in  FIG. 1  and the section taken along the thick line B-B′ represents the structure of the CMOS circuit  202  in  FIG. 1 . 
   In each of thin-film transistors (TFTs)  203 - 205 , gate electrodes  101 - 104  are formed on a substrate  100  according to a prescribed pattern. To be more specific, gate electrodes  101 - 104  are formed on an underlying film (not shown) and are of multi-layer structure (TaN film [50 nm thick]/Ta film [250 nm thick]/TaN film [50 nm thick]) to prevent the increase of resistance. The substrate and the gate electrodes are covered with an inorganic protective film  105 . On this protective film is formed a gate insulating film  106   a  or  106   b . On this gate insulating film are formed active layers  107 - 114  of crystalline semiconductor film. On the active layer are formed thin oxide films  115 - 117  by irradiation with a laser light in an oxidizing atmosphere. 
   In the case of p-channel type TFT  205  for CMOS circuit, the active layer consists of a p + -type region  113  (source region or drain region) with a high impurity concentration, a channel forming region  110 , and a p + -type region  114  with a low impurity concentration which is formed between the above-mentioned two regions. On the channel forming region is formed an etching stopper  118 . The above-mentioned layers are covered with a first interlayer insulating film  119  (which is flat). In this interlayer insulating film  119  are formed contact holes for wiring  124  connected to the region  113  with a high concentration of impurity. On the first interlayer insulating film is formed a second interlayer insulating film  125 . Wiring  124  is connected to wiring  128 . On the second interlayer insulating film is formed a third interlayer insulating film  129 . 
   On the other hand, the active layer in the n-channel type TFT  204  consists of an n + -type region  111  with a high impurity concentration, a channel forming region  109 , and an n + -type region  112  with a low impurity concentration which is formed between the above-mentioned two regions. In any active layer, the region with a high impurity concentration functions as the source region or drain region. To the source region and drain region are connected wiring  122  and  123 . Other part than the active layer has the same structure as in the p-channel type TFT. 
   The n-channel type TFT  203  in the pixel matrix circuit  201  is formed in the same manner as the n-channel type TFT in the CMOS circuit until the first interlayer insulting flat film  119  is formed. Finally, the wiring  121  is connected to the source region and the wiring  120  is connected to the drain region. On them are formed a second interlayer insulating film  125  and the black mask  126 . The black mask  126  covers the pixel TFT to form the auxiliary capacity in conjunction with the wiring  120 . On it is formed a third interlayer insulating film  129 , and a pixel electrode  130  (which is a transparent electrically conductive film of ITO or the like) is connected. 
   The semiconductor device shown in  FIG. 1  is produced by the process which is detailed below with reference to  FIG. 3 . 
   First, a substrate  100  having an insulating surface is made ready. The substrate may be of glass, quartz, ceramics, or semiconductor. In this example, a quartz substrate was used as the substrate  100 . In order to ensure flatness, it is desirable to coat the substrate with an underlying film (of silicon oxide, silicon nitride, or silicon nitride oxide). The underlying film prevents peeling due to strain by the substrate and the gate wiring material under stress. 
   Then, the gate wiring and the gate electrode are formed in laminate structure. In this example, a tantalum nitride (TaN) film, a tantalum (Ta) film, and a tantalum nitride (TaN) film are consecutively formed by sputtering on the insulating film. After patterning, the gate electrode of three-layer structure is formed. ( FIG. 3(A) ) 
   In this example, the TaN film (which should preferably be thicker than 40 nm) is consecutively laminated with the Ta film so as to form low-resistance α-Ta. 
   Since the Ta film is more subject to oxidation and hydrogen occlusion than the TaN film, the following layer structure (as shown in  FIG. 3(A) ) is adopted to prevent it from increasing in resistance. 
   TaN [ 101   a ,  102   a ,  103   a ,  014   a;  50 nm thick]/Ta[ 101   b ,  102   b ,  103   b ,  104   b;  250 nm thick]/TaN[ 101   c ,  102   c ,  103   c ,  104   c;  50 nm thick] 
   The TaN layer as the uppermost layer is intended to protect the Ta film from being exposed for oxidation and hydrogen occlusion, thereby providing good ohmic contact, when contacts with other wiring are made. Also, forming a TiN film as the uppermost layer is desirable because it forms no insulator even though it becomes oxidized. 
   Tantalum as the wiring material may be replaced by Mo, Nb, W, Mo—Ta alloy, Nb—Ta alloy, W—Ta alloy, or the like. Moreover, these materials may be used in the form of nitrogen-containing metal or silicide (which is a metallic compound with silicon). 
   A protective film  105  of silicon nitride is formed such that it covers the gate electrode. This inorganic protective film protects the tantalum film in the gate electrode from oxidation and hydrogen occlusion. In addition, the protective film prevents the diffusion of impurity from the substrate by heating, thereby keeping the gate insulating film with good insulating properties, when high-temperature treatment (such as gettering) is performed. Further, the protective film  105  protects the gate electrode and wiring from laser light and heat. The thickness of the protective film ranges from 10 to 100 nm; in this example, it is 20 nm. ( FIG. 3(B) ) 
   Gate insulating films  106   a  and  106   b  are formed such that they cover the protective film. In this example, the insulating film  106   a  is 125 nm thick and the insulating film  106   b  is 75 nm thick, and they are made of silicon oxide nitride (SiOxNy). The gate insulating film is formed such that the region to become the gate insulating film for high voltage circuits is selectively made thicker than the gate insulating film for high-speed driving circuits. The resulting structure withstands high voltages. The insulating films with different thickness may be formed by any known method. This is accomplished by, for example, forming an insulating film (75 nm thick) over the entire surface and then selectively laminating another insulating film (50 nm thick) thereon. The insulating films  106   a  and  106   b  may be of silicon oxide, silicon nitride, or silicon oxide nitride, or in the form of their laminate, with the film thickness being 50-300 nm. 
   On the gate insulating film is laminated an amorphous semiconductor film so as to form an active layer on the insulating films  106   a  and  106   b . It is desirable to form the protective film  105 , the insulating film  106 , and the amorphous semiconductor film consecutively so as to reduce impurities and to increase throughput. The active layer should be a crystalline semiconductor film (typically, a crystalline silicon film), 20-100 nm thick, preferably 25-70 nm thick. The crystalline semiconductor film may be formed by any known method, such as laser crystallization or thermal crystallization. In this example, a catalyst element (nickel) is added to promote crystallization. This technology is disclosed in Japanese Patent Laid-open No.7-130652 and 9-312260. 
   In this example, an amorphous silicon film (55 nm thick) is formed by reduced pressure CVD method. Then, a solution of Ni acetic acid is applied by using a spinner. Upon drying, there is obtained a Ni layer  302 . ( FIG. 3(C) ) The Ni layer is not in the form of complete layer. The concentration of Ni in the Ni acetate should be 1-1000 ppm. In this example, this concentration is 100 ppm. In this state, Ni is held on the surface of the amorphous silicon film. Upon heating at 550° C. for 8 hours in an inert or oxidizing atmosphere, there is obtained a crystalline silicon film. ( FIG. 3(D) ) 
   The crystalline silicon film is irradiated with a laser light in an oxidizing atmosphere for laser annealing and oxidation to form a thin oxide film  401 . ( FIG. 4(A) ) This thin oxide film contributes to adhesion between the crystalline silicon film and a resist or etching stopper to be formed later. However, this oxide film is not obtained if laser irradiation is carried out in an inert atmosphere. 
   A silicon oxide film (120 nm thick) is formed, and it is patterned to form an etching stopper  118 . A doping mask  402  of resin is formed. Incidentally, the etching stopper  118  may also be formed from amorphous silicon film, crystalline silicon film, silicon nitride film, or silicon oxide nitride film. 
   The first doping with phosphorus is carried out by non-self-alignment process employing the resist  402  as a mask. ( FIG. 4(B) ) In this example, phosphorus is added into the N +  region indicated by  403  such that the concentration is 1×10 20 -8×10 21  atoms/cm 3 . 
   The resist mask  402  is removed. The second doping with phosphorus is carried out by using the etching stopper  118  as a mask. ( FIG. 4(C) ) In this example, this doping is carried out such that the concentration is 1×10 15 -1×10 17  atoms/cm 3  in the N-region  406 . In an n-channel type TFT, the n + -type region  407  becomes the source region or drain region and the N −  region becomes the region  406  with low impurity concentration. 
   Then, the n-channel type TFTs  203  and  204  are covered with a resist  501 . Boron is added to the active layer of the p-channel type TFT so as to form the p-type region  502  (in which phosphorus exists in high concentrations) and the p-type region  503  (in which phosphorus exists in low concentration). ( FIG. 5(A) ) The dose of boron is such that the concentration of boron in p-type region is about 1.3-2 times the concentration of phosphorus ions added to the n + -type region. Incidentally, in this example, any known method may be used to add phosphorus ions or boron ions. It includes ion implantation, plasma doping, application of a solution containing phosphorus ions or boron ions, followed by heating, and forming a film containing phosphorus ions or boron ions, followed by heating. 
   The p-type regions  502  and  503  become the source region or drain region of the p-channel type TFT. The region in which phosphorus ions or boron ions have not been injected becomes the intrinsic (or substantially intrinsic) channel forming region which subsequently serves as the carrier moving path. 
   Incidentally, in this specification, the “intrinsic region” means a region which does not contain any impurity at all which changes the Fermi level of silicon, and the “substantially intrinsic region” means a region in which electrons and holes are completely balanced to cancel the conductivity type, that is, a region which contains an impurity to impart the n-type or p-type in concentrations (1×10 15 -1×10 17  atoms/cm 3 ) to permit control of the threshold value or a region in which the conductivity type is cancelled by intentionally adding a reverse conducting type impurity. 
   Then, heat treatment is carried out at 450° C. or above for 0.5-12 hours (at 550° C. for 2 hours in this example) in an inert atmosphere or dry oxidizing atmosphere. ( FIG. 5(B) ) 
   This heat treatment causes Ni (which has been intentionally added to crystallize the amorphous silicon film) to diffuse from the channel forming region to the source region and drain region as schematically indicated by arrows in  FIG. 5(B) . Upon arrival at the source region and drain region, Ni is captured there (for gettering). Heat treatment at 400-600° C. for 0.5-4 hours is enough for Ni gettering. 
   As the result, it is possible to reduce the Ni concentration in the channel forming region  110 . The Ni concentration in the channel forming regions  107 - 110  may be reduced below 5×10 17  atoms/cm 3 , which is the detection limit of SIMS. On the other hand, the Ni concentration in the source region and drain region which have been used as the gettering sink becomes higher than that in the channel forming region. ( FIG. 5(C) ) 
   An impurity to impart the n-type conductivity includes phosphorus as well as antimony and bismuth. Phosphorous is most capable of gettering, and antimony comes next. 
   It has been experimentally confirmed that the region  505  in which the boron concentration is about 1.3-2 times higher than the phosphorus concentration because of the addition of both phosphorus and boron is more capable of gettering than the source region and drain region  504  of n-channel type TFT to which only phosphorus has been added. 
   Further, this heat treatment not only performs gettering but also activates phosphorus and boron added to the source region and drain region and to the region of low impurity concentration. In the past, it was only possible to heat up to about 450° C. because the wiring material (aluminum) is poor in heat resistance. In this example, however, it is possible to sufficiently activate the dopant only by heat treatment at 500° C. or above, thereby reducing resistance in the source region and drain region. 
   In addition, this heat treatment recovers crystallinity in the region in which crystallinity has been destroyed by ion doping. 
   In other words, the heat treatment in an oxidizing atmosphere in  FIG. 5(B)  permits simultaneously
     1) gettering to reduce the concentration of catalyst element in the channel forming regions  107 - 110 ;   2) activation of impurity in the source regions and drain regions  504  and  505 ; and   3) annealing to recover the damage to crystal structure that has occurred during ion implantation.   

   In addition, the heat treatment may be accompanied simultaneously by or followed by or preceded by laser annealing, infrared light annealing, or UV light annealing. 
   Then, the active layer is patterned into a desired shape, as shown in  FIG. 6(A) . 
   After that, the region  111  of high impurity concentration is reduced in resistance. For this purpose, a metal film is formed on the active layer to make it selectively into silicide and the metal film is heated so that the region indicated by  111  is made into silicide. This step reduces the resistance to such an extent the resulting semiconductor device operates at high frequencies of the order of GHz. The metal film to make silicide may be the one which is composed mainly of cobalt, titanium, tantalum, tungsten, and molybdenum. Incidentally, in order to make silicide effectively, it is desirable to remove the thin oxide films  115 - 117  on the region of high impurity concentration before the metal film is formed. Alternatively, it is desirable to remove the etching stopper  118 . 
   Then, a first interlayer insulating film  119  is formed from a transparent organic resin (acrylic resin) over the entire surface of the substrate. In this example, a first interlayer insulating film  119  (1 μm thick) is formed by spin coating. It has a flat surface as shown if it is made of a transparent resin such as acrylic resin, polyimide resin, and BCB (benzocyclobutene). It may also be made of silicon oxide or silicon oxide nitride. 
   Contact holes are formed, and metal film (not shown) is formed which constitutes electrodes for contacts. This metal film is of three-layer structure, composed titanium film, aluminum film, and titanium film, formed by sputtering. This metal film (or laminated film) is patterned to form the electrodes and wirings indicated by  120 - 124 . 
   A second interlayer insulating film  125  (1 μm thick) of organic resin is formed by spin coating. A desired part is made thin by etching to form the auxiliary capacity. A metal film of Ti (300 nm thick) is formed. This metal film is patterned to form the black mask  126  and the lead wirings  127  and  128 . 
   A third interlayer insulating film  129  (1 μm thick) is formed from an acrylic resin by spin coating. The resulting resin film has a flat surface as shown. 
   A contact hole is formed, and a pixel electrode  130  is formed. In this example, an ITO film (100 nm thick) is formed, and it is patterned to form the pixel electrode  130 . 
   Finally, heat treatment is carried out at 350° C. for 1 hour in a hydrogen atmosphere so as to reduce defects in the semiconductor layer. The results are shown in  FIG. 6(B) . 
   In this example, the pixel TFT  203  for the pixel matrix circuit has the gate electrode of double gate structure. However, the gate electrode may be of multi-gate structure (or triple-gate structure) in order to reduce the fluctuation of off current. Moreover, the gate electrode may be of single-gate structure for large opening. 
   The TFT structure shown in this example is an example of bottom gate type (etching topper type). The TFT structure is not limited to the one shown in this example. For example, it may be of channel etch type structure. In this example, the production of transmission LCD is demonstrated; however, this is merely an example of semiconductor devices. The ITO pixel electrode may be formed from a highly reflective metal film, and the pixel electrode may be patterned differently so as to produce a reflection LCD. The reflection LCD may have an underlying film of laminate structure composed of heat resistant metal film and insulating film or composed of aluminum nitride film and insulating film. In this case the metal film under the insulating film effectively functions as a heat radiating layer. The sequence of the above-mentioned steps may be changed adequately by those who practice the invention. 
   EXAMPLE 2 
   In Example 1, patterning is carried out after the step of laser irradiation ( FIG. 6(A) ). In this example, however, patterning is carried out before the step of laser irradiation. This is shown in  FIG. 7-9 . These two examples are the same in base structure; only differences are explained. 
   The steps up to the formation of the crystalline semiconductor film are the same as shown in  FIG. 3(D)  in Example 1; therefore, their explanation is omitted. The intermediate product shown in  FIG. 3(D)  is patterned according to the desired shape and irradiated with a laser beam in an oxidizing atmosphere to give an intermediate product shown in  FIG. 7(A) . As shown in  FIG. 7(A) , the surface of the active layers  701 - 703  is covered with thin oxide films  704 - 706 . 
   Subsequent steps are the same as those in Example 1. They are doping with phosphorus in high concentration (FIG.  7 (B)), doping with phosphorus in low concentration (FIG.  7 (C)), doping with boron (FIG.  8 (A)), and gettering ( FIG. 8(B) ). The intermediate product up to this step is shown in  FIG. 8(C) . 
   The etching stopper  707  placed on the top of the channel forming region is removed, as shown n  FIG. 9(A) . The step of removing the etching stopper  707  may be omitted. 
   This step may be preceded or followed by or accompanied simultaneously by the step of removing the thin oxide films  704 - 706 . After the removal of the thin oxide film, it is desirable to form selectively a metal film on the region of high impurity concentration, and this metal film is heated to be made into silicide. In this way it is possible to reduce resistance in the source region and drain region so as to permit operation at high frequencies of the order of GHz. The metal film to be made into silicide may be the one which is composed mainly of cobalt, titanium, tantalum, tungsten, or molybdenum. 
   The subsequent steps are identical with those in Example 1 and hence their explanation is omitted. Thus there is obtained an intermediate product shown in  FIG. 9(B) . The advantage of this structure is that the thin oxide films  704 - 706  protect the active layers  701 - 703  from impurity which has diffused from the interlayer insulating film. 
   EXAMPLE 3 
   This example demonstrates a structure which is almost identical with that in Example 1 except for slight differences explained below. The structure in Example 1 is characterized by a difference in thickness between the gate insulating film  106   b  of CMOS circuit  202  and the gate insulating film  106   a  of the pixel matrix circuit  201 , both circuits constituting part of the peripheral drive circuits. The gate insulating film according to this example is shown in  FIG. 10 . It has the same thickness as in Example 1. 
   In this example, the same steps as in Example 1 are repeated until the protective film shown in  FIG. 3(B)  is formed. Therefore, the explanation of these steps is omitted. After the intermediate product shown in  FIG. 3(B)  has been obtained as in Example 1, the gate insulating film  1001  and the amorphous semiconductor film are formed consecutively. Then, the active layer consisting of crystalline semiconductor film is patterned by the same process as in Example 1. 
   Subsequently, the thin oxide film (adjacent to the active layer) and the etching stopper are removed and a metal film is selectively formed on the region of high impurity concentration. The metal film is made into silicide by heat treatment. The resulting source region and drain region have a low resistance which permits operation at high frequencies of the order of GHz. The metal film to make silicide may be the one which is composed mainly of cobalt, titanium, tantalum, tungsten, or molybdenum. After that, an interlayer insulating film  1002  of silicon oxide is formed. By the same subsequent steps as in Example 1, there is obtained the structure as shown in  FIG. 10 . Incidentally, this example may be combined with Example 2. 
   EXAMPLE 4 
   This example demonstrates a process for producing a crystalline semiconductor film in a way different from that in Example 1. The process includes steps of adding catalyst element by using a mask and performing heat treatment. The process is basically identical with that of Example 1 except for some differences explained in the following. 
   In this example, the same steps as in Example 1 are repeated until the protective film shown in  FIG. 3(B)  is formed. Therefore, the explanation of these steps is omitted. After the amorphous semiconductor film has been formed, a mask of silicon oxide is formed. This mask has an opening. A catalyst element (Ni) in the form of nickel acetate solution is added to the opening. 
   The amorphous silicon film is crystallized by heating at 400-700° C. Crystallization proceeds in the direction from the region with an opening to the substrate. This crystal growth is referred to as lateral growth. After that, the mask is removed. The region which has become crystallized by lateral growth is used as the region in which TFT channel is formed. This provides good characteristic properties. According to the present invention, it is possible to carry out heat treatment at 400° C. or above and hence to obtain a crystalline semiconductor film. By the same subsequent steps ( FIG. 4(A)  and forward) as in Example 1, there is obtained the structure as shown in  FIG. 1 . Incidentally, this example may be combined with Examples 2 and 3. 
   EXAMPLE 5 
   This example demonstrates a process for producing a crystalline semiconductor film in a way different from that in Example 1. In this example, a crystalline semiconductor film is formed by the aid of a catalyst element to promote crystallization of silicon and also by the aid of laser irradiation that employs a square or rectangular laser beam to cover an area of several to hundreds of square centimeters with one shot of irradiation. The process is basically identical with that of Example 1 except for some differences explained in the following. 
   In this example, the same steps as in Example 1 are repeated until a catalyst element is supported on the surface of the amorphous silicon film as shown in  FIG. 3(C) . Therefore, the explanation of these steps is omitted. In the step shown in  FIG. 3(C) , the concentration of nickel in the nickel acetate solution should be 1-1000 ppm. In this example, it is 100 ppm. Nickel is held on the surface of the amorphous silicon film. A crystalline silicon film is formed by irradiation with excimer laser light (wavelength 248-308 nm) in an inert or oxidizing atmosphere. 
   In this example, a laser apparatus (“SAELC” from Sopla) is used to form the crystalline silicon film. This apparatus evolves a square or rectangular laser beam (wavelength 248 nm) which uniformly covers an area of several to hundreds of square centimeters at one time. By the same subsequent steps ( FIG. 4(A)  and forward) as in Example 1, there is obtained the structure as shown in  FIG. 1 . Incidentally, this example may be combined with Examples 2 to 4. 
   EXAMPLE 6 
   This example demonstrates the structure to provide good ohmic contact for connection between wirings, with reference to  FIG. 11 . The pixel matrix circuit is basically the same in structure as that in Example 1, except for some differences explained below. 
   A substrate with an insulating surface is made ready as in Example 1. An underlying film of silicon oxide (not shown) is formed. A layer  1101  of metal material composed mainly of tantalum and a layer  1102  (20-100 nm thick) of metal material composed mainly of titanium are formed consecutively. Patterning is performed to form a multi-layer wiring. After that, the same procedure as in Example 1 is repeated to form the gate insulating film, active layer, interlayer film, and contact hole. 
   The layer composed mainly of titanium protects the layer  1101  composed mainly of tantalum from oxidation and hydrogen occlusion when contact holes (openings) are formed. The layer composed mainly of titanium provides good ohmic contacts because it does not form an insulator despite reaction with oxidation, although it may be partly removed together with the interlayer insulating film when openings are formed. In other words, the layer composed mainly of titanium protects the layer composed mainly of tantalum and also facilitates the formation of openings because it permits sufficient margin. Openings are formed, and then wiring  1103  is formed for connection with the multi-layer wiring indicated by  1101  and  1102 . After that, the same procedure as in Example 1 is repeated to give the structure shown in  FIG. 11 . 
   The layer composed mainly of titanium may be replaced by a layer composed mainly of one element selected from Cr, Mn, Co, Ni, Cu, Mo, and W. 
   Incidentally, in this example, unlike the structure in Example 1, the etching stopper and thin oxide film are removed and the protective film is not formed. This example may be combined with Examples 2 to 5. 
   EXAMPLE 7 
   This example demonstrates an AMLCD which is constructed of a TFT substrate (a substrate on which elements are formed) having the structure shown in Examples 1 to 6. An external appearance of AMLCD is shown in  FIG. 12 . 
   In  FIG. 12(A) , there is shown a TFT substrate  1201 , on which are formed a pixel matrix  1202 , a drive circuit  1203  at the source, and a drive circuit  1204  at the gate. The pixel matrix corresponds to  FIG. 2(A)  and  FIG. 1 , and a part of it is shown. The drive circuit corresponds to  FIG. 2(B)  and  FIG. 1 , and a part of it is shown. It is desirable to constitute CMOS circuit by complimentary combination of n-type TFT and p-type TFT. There is shown an opposite substrate  1205 . 
   The AMLCD shown in  FIG. 12(A)  consists of the active matrix substrate  1201  and the opposite substrate  1205  bonded together, with their edges aligned. A portion of the opposite substrate  1205  is removed, and the FPC (flexible print circuit)  1206  is connected to the exposed active matrix substrate. This FPC  1206  transmits external signals into the circuit. 
   The surface to which the FPC  1206  is attached is utilized for the mounting of IC chips  1207  and  1208 . These IC chips contain various circuits such as video signal processing circuits, timing pulse generating circuits, γ-correcting circuits, memory circuits, and arithmetic circuits formed on a silicon substrate. One or more IC chips may be used, although two chips are shown in  FIG. 12(A) . 
   The structure as shown in  FIG. 12(B)  could also be possible. The same codes are used for the same parts in  FIG. 12(B)  and  FIG. 12(A) . In this example, the signal processing performed by the IC chip shown in  FIG. 12(A)  is performed by the logic circuit  1209  formed from TFT on the same substrate. In this case, the logic circuit  1209  is also constructed basically of the CMOS circuits in the same way as the drive circuits  1203  and  1204 . 
   The AMLCD may have a color filter for color display or may be driven in ECB (electric field control birefringence) mode or GH (guest-host) mode without a color filter. 
   EXAMPLE 8 
   The process of the present invention may be used to form CMOS circuits and pixel matrix circuits. These circuits may be used for various electro-optical apparatuses (such as liquid crystal display of active matrix type, EL display of active matrix type, and EC display of active matrix type). In other words, the present invention can be applied to any electronic machines and apparatus equipped with these electro-optical devices as display media. 
   Examples of these electronic machines and apparatus include video cameras, digital cameras, projectors (of linear type or front type), head-mount display (goggle-type display), car navigation, personal computer, and mobile information terminals (mobile computers, cellular phones, and electronic books). They are illustrated in  FIGS. 13 and 14 . 
     FIG. 13(A)  shows a personal computer which consists of main body  2001 , image input  2002 , display  2003 , and keyboard  2004 . The present invention may be applied to the image input  2002  and the display  2003  and other signal control circuits. 
     FIG. 13(B)  shows a video camera which consists of a main body  2101 , a display  2102 , an audio input  2103 , a switching unit  2104 , a battery  2105 , and an image receiver  2106 . The present invention may be applied to the display  2102  and the video input  2103  and other signal control circuits. 
     FIG. 13(C)  shows a mobile computer which consists of a main body  2201 , a camera unit  2202 , an image receiver  2203 , a switching unit  2204 , and a display  2205 . The present invention may be applied to the display  2205  and other signal control circuits. 
     FIG. 13(D)  shows a goggle type display which consists of a main body  2301 , a display  2302 , and arms  2303 . The present invention may be applied to the display  2302  and other signal control circuits. 
     FIG. 13(E)  shows a player for a recording medium containing programs, which consists of a main body  2401 , a display  2402 , a speaker  2403 , a recording medium  2404 , and a switching unit  2405 . Incidentally, this apparatus may employ a DVD (digital versatile disc) or CD as the recording medium. It is used to enjoy music and movies and internet. The present invention may be applied to the display  2402  and other signal control circuits. 
     FIG. 13(F)  shows a digital camera which consists of a main body  2501 , a display  2502 , an eyepiece  2503 , a switching unit  2504 , and an image receiver (not shown). The present invention may be applied to the display  2502  and other signal control circuits. 
     FIG. 14(A)  shows a front-type projector which consists of a display  2601  and a screen  2602 . The present invention may be applied to the display and other signal control circuits. 
     FIG. 14(B)  is a rear-type projector which consists of a main body  2701 , a display  2702 , a mirror  2703 , and a screen  2704 . The present invention may be applied to the display and other signal control circuits. 
     FIG. 14(C)  shows an example of the structure of the display  2601  and  2702  in  FIGS. 14(A) and 14(B) , respectively. The displays  2601  and  2702  each consist of an optical system for light source  2801 , mirrors  2802 ,  2804 - 2806 , a dichroic mirror  2803 , a prism  2807 , a liquid crystal display  2808 , a phase difference plate  2809 , and a projecting optical system  2810 . The projecting optical system  2810  contains projector lenses. This example shows the one which contains three lenses. The one which has a single lens may also be possible. Incidentally, the example shown in  FIG. 14(C)  may be modified such that the arrowed optical paths may be provided with such optical elements as lenses, polarizing film, phase difference adjusting film, and IR film. 
     FIG. 14(D)  shows an example of the structure of the light source optical system  2801  in  FIG. 14(C) . The light source optical system  2801  consists of a reflector  2811 , a light source  2812 , lens arrays  2813  and  2814 , a polarizing light converting element  2815 , and a condenser lens  2816 . The light source optical system shown in  FIG. 14(D)  is illustrative only but is not limitative. For example, the light source optical system may be provided with such optical elements as lenses, polarizing film, phase difference adjusting film, and IR film. 
   As mentioned above, the present invention may be applied to a broad range of fields, including almost all electronic machines and equipment. And, the electronic machines and equipment in this example may be realized by any combination of Examples 1 to 8. 
   The present invention enables one to produce a semiconductor device having good TFT characteristics even when heat treatment at a high temperature (400° C. and above) is performed after the gate wiring and electrode (wiring width: 0.1-5 μm) have been formed. 
   According to the present invention, the protective film prevents impurity from diffusing from the substrate during heat treatment at a high temperature. This makes it possible to obtain good TFT characteristics without being affected by the concentration of impurity in the substrate. 
   The heat treatment at a high temperature (400° C. and above), which is performed after the addition of impurity to impart p-type or n-type conductivity type, produces the effect of activating impurity, annealing the crystalline semiconductor film which has been damaged by impurity addition, and reducing the catalyst element remaining in the crystalline semiconductor film (gettering effect).