Abstract:
A method of fabricating MIS transistors starts with formation of gate electrode portions. Then, high-speed ions are irradiated through an insulating film to implant impurity ions into a semiconductor region by a self-aligning process, followed by total removal of the insulating film. The laminate is irradiated with laser light or other similar intense light to activate the doped semiconductor region. Another method of fabricating MIS transistors begins with formation of a gate-insulating film and gate electrode portions. Then, the gate-insulating film is removed, using the gate electrode portions as a mask. The semiconductor surface is exposed, or a thin insulating film is formed on this surface. High-speed ions are irradiated to perform a self-aligning ion implantation process. A further method of fabricating MIS transistors starts with formation of a gate-insulating film and gate electrode portions. Then, the gate-insulating film is etched by a self-aligning process, using the gate electrode portions as a mask, to thin the film. Subsequently, high-speed ions are irradiated to form doped regions in a semiconductor region by a self-aligning process.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method of fabricating MIS transistors and, more particularly, to a method of fabricating MIS transistors having improved crystallinity by illuminating a high-speed ions to implant impurities into a semiconductor region and then improving the crystallinity by laser annealing, lamp annealing, or illumination of other equivalent intense light. 
   BACKGROUND OF THE INVENTION 
   A structure comprising a semiconductor layer (S) on which a thin insulating film (I) and metal control electrodes (M) are formed is known as a MIS structure. A transistor having such a structure to control the electrical current flowing through the semiconductor layer is referred to as a MIS transistor. Where the insulating film is made of silicon oxide, the transistor is called a MOS transistor. 
   In the past, the activation step (for removing crystal defects produced on impurity implantation) conducted after the implantation of impurities into such MIS transistors has been carried out by thermal annealing. For this step, a high temperature as high as more than 1000° C. is needed. In recent years, there is a demand for lower-temperature processes. Accordingly, alternatives to such high-temperature thermal annealing have been discussed. One promising method is to illuminate laser light or other intense light, for effecting activation. Depending on the used light source, this method is called laser annealing or lamp annealing. 
   A conventional method of fabricating MIS transistors, using laser annealing, is now described by referring to  FIGS. 4(A)-4(E) . An insulating film  402  is deposited as a base layer on a substrate  401 . Then, a substantially intrinsic crystalline semiconductor film is deposited. This is photolithographically patterned into island-shaped semiconductor regions  403 . Thereafter, an insulating film  404  acting as a gate-insulating film is deposited. Subsequently, gate electrodes  405  are deposited ( FIG. 4(A) ). 
   If necessary, the gate electrodes are anodized to form an anodic oxide  406  on the top and side surfaces of the gate electrodes and conductive interconnects. This method for forming such an anodic oxide and its merits are described in detail in Japanese Patent application Ser. Nos. 30220/1992, 34194/1992, 38637/1992, etc. Of course, this anodization step may be omitted if not necessary ( FIG. 4(B) ). 
   Then, an impurity is implanted by ion implantation or ion (plasma) doping. In particular, the substrate is placed in a fast stream of ions. Using the gate electrode portions, i.e., the gate electrodes and the surrounding anodic oxide, as a mask, an impurity is implanted into the island-shaped semiconductor regions  403  by a self-aligning process. In this way, doped regions  407  which will act as source and drain are formed ( FIG. 4(C) ). 
   Thereafter, intense light such as laser light is illuminated to recover the crystallinity which was deteriorated by the previous impurity implantation step ( FIG. 4(D) ). 
   An interlayer insulator  408  is then deposited, and contact holes are formed in it. Source and drain electrodes  409  are formed, thus completing MIS transistors ( FIG. 4(E) ). 
   In the method described above, when impurities are implanted, a large amount of impurities is introduced also into the gate-insulating film  404 . These impurities themselves act as cores absorbing the laser light. In addition, defects produced by the impurity implantation absorb the laser light strongly. Especially, UV light is absorbed much, and light strong enough to activate the doped semiconductor regions  407  does not reach these regions. Usually, the insulating film is made of silicon oxide. The laser light is emitted from an excimer laser which has an excellent mass-producibility. If the silicon oxide is pure, it is sufficiently transparent to UV light emitted from an excimer laser. However, if impurities such as phosphorus and boron are present, the transparency deteriorates greatly. Hence, the activation is not sufficiently done. 
   If the doped regions are not sufficiently activated in this way, their resistivities are increased. It substantially follows that a resistor is inserted in series between source and drain. That is, the apparent mobility of the transistor drops. Also, the rising characteristics, or steepness, obtained when the transistor is turned on deteriorate. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing problems, the present invention has been made. It is an object of the present invention to provide a method of efficiently carrying out an activation step using laser illumination. 
   If the thickness of the gate-insulating film described above is increased, then the breakdown voltage of the transistor is improved. However, this also requires that the accelerating voltage for the impurity ions be increased and that the implantation time be increased. Especially, where shallow doped regions are formed, a quite highly monochromatic ion beam is needed. In consequence, the dosage per unit time deteriorates severely. 
   On the other hand, where the gate-insulating film is removed and the semiconductor surface is exposed to efficiently conduct the implantation step, the surface is roughened when laser light or other intense radiation is illuminated and the doped regions are activated. As a result, the contact holes are deteriorated to an intolerable level. 
   It is another object of the invention to provide a method of efficiently carrying out an implantation step and a laser activation step. 
   In one embodiment of the present invention, an insulating film is formed as a gate-insulating film. Impurity ions are implanted into a semiconductor region through all or parts of the insulating film by irradiation of high-speed ions. Then, the insulating film is removed excluding the portions located under the gate electrode portions, thus exposing the semiconductor region. Laser light or other equivalent intense light is illuminated to perform an anneal. The above-described absorption of light by the insulating film does not take place. Activation can be done quite efficiently. 
   In another embodiment of the invention, a first insulating film is formed as a gate-insulating film. Using the gate electrode portions as a mask, the first insulating film is removed by a self-aligning process to expose the semiconductor surface. Then, impurity ions are implanted into the exposed semiconductor layer by irradiation of high-speed ions. Thereafter, a second insulating film of an appropriate thickness is formed on the exposed semiconductor layer. Thereafter, the semiconductor layer is irradiated with laser light or equivalent intense light through the second insulating film to perform an anneal. In this method, the above-described deterioration in the implantation efficiency is prevented. Rather, an ion implantation step and a subsequent activation step can be accomplished quite efficiently. 
   In a further embodiment of the invention, a first insulating film is formed as a gate-insulating film. Using the gate electrode portions as a mask, the first insulating film is removed by a self-aligning process to expose the semiconductor layer surface. Then, a second insulating film of an appropriate thickness is formed on the semiconductor layer. Impurity ions are implanted into the semiconductor region through the second insulating film by irradiation of high-speed ions. Thereafter, the semiconductor layer is irradiated with laser light or other equivalent intense light to perform an anneal. In this method, the above-described decrease in the implantation efficiency is prevented. Rather, an ion implantation step and a subsequent activation step can be accomplished quite efficiently. 
   In a still other embodiment of the invention, an insulating film is formed as a gate-insulating film. Using the gate electrode portions as a mask, the insulating film is etched to reduce its thickness to such an extent that ions of appropriate energy penetrate the film. Impurity ions are implanted into a semiconductor region by irradiation of high-speed ions through the thinned insulating film. Then, the semiconductor layer is irradiated with laser light or other equivalent intense light to perform an anneal. Prior to the laser irradiation, a transparent insulating film may be formed on the semiconductor layer surface. In this method, the above-described decrease in the implantation efficiency is prevented. Rather, an ion implantation step and a subsequent activation step can be accomplished quite efficiently. Other objects and features of the invention will appear in the course of the description thereof, which follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1(A)-1(E)  are cross-sectional views of MIS transistors, illustrating steps performed to fabricate the transistors according to the invention; 
       FIGS. 2(A)-2(E)  are cross-sectional views of other MIS transistors, illustrating steps performed to fabricate the transistors according to the invention; 
       FIGS. 3(A)-3(D)  are cross-sectional views of further MIS transistors, illustrating steps performed to fabricate the transistors according to the invention; 
       FIGS. 4(A)-4(E)  are cross-sectional views of MIS transistors, illustrating steps performed to fabricate the transistors by the prior art method; 
       FIGS. 5(A)-5(E)  are cross-sectional views of yet other MIS transistors, illustrating steps performed to fabricate the transistors according to the invention; 
       FIGS. 6(A)-6(E)  are cross-sectional views of still other MIS transistors, illustrating steps performed to fabricate the transistors according to the invention; 
       FIGS. 7(A)-7(D)  are cross-sectional views of still further MIS transistors, illustrating steps performed to fabricate the transistors according to the invention; 
       FIGS. 8(A)-8(E)  are cross-sectional views of yet other MIS transistors, illustrating steps performed to fabricate the transistors according to the invention; and 
       FIGS. 9(A)-9(D)  are cross-sectional views of additional MIS transistors, illustrating steps performed to fabricate the transistors according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   EXAMPLE 1 
   The present example is illustrated in  FIGS. 1(A)-1(E) . Silicon oxide was deposited as an insulating base film  102  having a thickness of 1000 Å on a non-alkali glass substrate  101  made of Corning 7059 or the like. A substantially intrinsic amorphous silicon film having a thickness of 1500 Å was formed on the insulating film  102 . This amorphous film was annealed at 600° C. for 12 hours to crystallize this film. This crystallized film was photolithographically patterned into island-shaped semiconductor regions  103 . A silicon oxide film  104  having a thickness of 1200 Å was deposited as a gate-insulating film. Gate electrodes  105  having a thickness of 6000 Å were fabricated from aluminum ( FIG. 1(A) ). 
   Then, the gate electrodes were anodized to form an anodic oxide  106  on the top and side surfaces of the gate electrodes and conductive interconnects. The method of fabricating such an anodic oxide and its merits are described in detail in above-cited Japanese Patent application Ser. Nos. 30220/1992, 34194/1992, 38637/1992, etc. Of course, this anodization step may be omitted if not necessary ( FIG. 1(B) ). 
   Thereafter, a stream of plasma of phosphorus/hydrogen accelerated at 65-100 keV (e.g., 80 keV) was irradiated to implant phosphorus ions into the island-shaped semiconductor regions  103  by a self-aligning process. Thus, doped regions  107  which would become source and drain were formed ( FIG. 1(C) ). 
   Using the anodic oxide as a mask, the gate-insulating film  104  was etched, thus exposing the surfaces of the doped semiconductor regions  107 . Laser light (having a wavelength of 248 nm) emitted from a KrF excimer laser was illuminated to recover the crystallinity of the semiconductor regions  107  which was deteriorated by the previous impurity implantation step. At this time, the energy density was 150 to 300 mJ/cm 2 , e.g. , 200 mJ/cm 2  ( FIG. 1(D) ). 
   Subsequently, an interlayer insulator  108  was deposited, and contact holes were formed in it. Source and drain electrodes  109  were formed. In this way, N-channel transistors were completed ( FIG. 1(E) ). 
   P-channel transistors can be similarly built. If well-known CMOS fabrication techniques are used, N- and P-channel transistors can be both formed on the same substrate. The typical mobility of N-channel MOS transistors fabricated in the present example was 120 cm 2 /V·s. The typical mobility of P-channel MOS transistors fabricated in the present example was 80 cm 2 /V·s. With respect to CMOS shift registers consisting of 5 stages and comprising N- and P-channel transistors formed on the same substrate, synchronization of 15 MHz was confirmed at a drain voltage of 20 V. 
   EXAMPLE 2 
   The present example is illustrated in  FIGS. 2(A)-2(E) . Silicon oxide was deposited as an insulating base film  202  having a thickness of 1000 Å on a sheet of non-alkali glass  201 . A substantially intrinsic amorphous silicon semiconductor film having a thickness of 500 Å was formed on the insulating film  202 . The amorphous semiconductor film was illuminated with KrF laser light to crystallize the film. Preferably, the energy density of the laser light was 250 to 400 mJ/cm 2 . Where the substrate was maintained at 300-550° C. during the laser irradiation, good characteristics were obtained. 
   The silicon film crystallized in this way was photolitho-graphically patterned into island-shaped semiconductor regions  203 . Silicon oxide was deposited as a gate-insulating film  204  having a thickness of 1200 Å on the island-shaped regions  203 . Then, gate electrodes  205  of aluminum having a thickness of 6000 Å were formed ( FIG. 2(A) ). 
   Then, the gate electrodes were anodized to deposit an anodic oxide  206  on the top and side surfaces of the gate electrodes and conductive interconnects. Of course, this anodic oxidation step may be omitted if not necessary ( FIG. 2(B) ). 
   Thereafter, using the gate electrode portions as a mask, the gate-insulating film  204  was etched to a depth of about 200 to 700 Å by vapor phase etching, thus decreasing the thickness of the gate-insulating film. In this way, a thin insulating film  207  was formed. A stream of phosphorus/hydrogen plasma accelerated at 25 to 70 keV, e.g., 50 keV, was irradiated to implant phosphorus ions into the island-shaped regions  203  by a self-aligning process. As a result, doped regions  208  which would become source and drain were formed ( FIG. 2(C) ). 
   Generally, the breakdown voltage of a gate-insulating film can be increased as the thickness is increased. However, if ions are implanted through a thick insulating film, the accelerating energy for the ions is increased. Especially, where these ions contain ions having different masses or different scattering cross sections, unwanted ions may be implanted deep into the substrate. For example, in order to implant monovalent phosphorus ions to a depth of 1500 Å, an energy of 100 keV is necessary. Hydrogen ions accelerated at the same time are implanted to a depth of 5000 Å. That is, there is the possibility that the ions are transmitted through the gate electrodes and reach the gate-insulating film and even the underlying semiconductor region. This problem can be solved by reducing the thickness of the insulating film on the doped regions down to a required value. 
   After the completion of the ion implantation, the gate-insulating film  204  was etched, using the anodic oxide as a mask. In this way, the surfaces of the doped semiconductor regions  208  were exposed. KrF excimer laser light was illuminated to recover the crystallinity of the semiconductor regions  208  which deteriorated in the previous impurity implantation step ( FIG. 2(D) ). 
   Thereafter, an interlayer insulator  209  was deposited, and contact holes were formed in this insulator. Source and drain electrodes  210  were formed. Thus, N-channel transistors were completed ( FIG. 2(E) ). 
   EXAMPLE 3 
   The present example is illustrated in  FIGS. 3(A)-3(D) . Silicon oxide was deposited as an insulating base film  302  having a thickness of 1000 Å on a sheet of non-alkali glass  301 . A substantially intrinsic amorphous silicon semiconductor film having a thickness of 500 Å was formed on the insulating film  302 . The amorphous semiconductor film was illuminated with KrF laser light to crystallize the film. Preferably, the energy density of the laser light was 250 to 400 mJ/cm 2 . Where the substrate was maintained at 300-550° C. during the laser irradiation, good characteristics were obtained. 
   The silicon film crystallized in this way was photolitho-graphically patterned into island-shaped semiconductor regions  303 . Silicon oxide was deposited as a gate-insulating film  304  having a thickness of 1200 Å on the island-shaped regions  203 . Then, gate electrodes  305  of aluminum having a thickness of 6000 Å were formed. 
   Then, the gate electrodes were anodized to deposit an anodic oxide  306  on the top and side surfaces of the gate electrodes and conductive interconnects. Using the gate electrode portions as a mask, the whole gate-insulating film  304  was etched by a wet etching process ( FIG. 3(A) ). 
   Silicon oxide was deposited as a film  307  having a thickness of 200 to 300 Å. A stream of phosphorus/hydrogen plasma accelerated at 10 to 40 keV, e.g., 20 keV, was irradiated to implant phosphorus ions into the island-shaped, semiconductor regions  303  by a self-aligning process. As a result, doped regions  308  which would become source and drain were formed ( FIG. 3(B) ). 
   The present example of method is adapted for the case in which the insulating film on the source and drain is made extremely thin. Specifically, in the method of Example 2, the thickness of the remaining insulating film cannot be accurately controlled where the gate-insulating film is thick. For example, where a silicon oxide film of 1200 Å was thinned to 200 Å by the method of Example 2, variations exceeding 200 Å occurred. In the method where the insulating film was deposited as in the present example, variations were less than 50 Å. 
   After the completion of the ion implantation, the silicon oxide film  307  was totally etched away, using the anodic oxide as a mask. In this way, the surfaces of the doped semiconductor regions  308  were exposed. KrF excimer laser light was illuminated to recover the crystallinity of the semiconductor regions  308  which deteriorated in the previous impurity implantation step ( FIG. 3(C) ). 
   Subsequently, an interlayer insulator  309  was deposited, and contact holes were formed in this insulator. Source and drain electrodes  310  were formed. Thus, N-channel transistors were completed ( FIG. 3(D) ). 
   EXAMPLE 4 
   The present example is illustrated in  FIGS. 5(A)-5(E) . Silicon oxide was deposited as an insulating base film  502  having a thickness of 1000 Å on a sheet of non-alkali glass  501  made of Corning 7059 or the like. A substantially intrinsic amorphous silicon semiconductor film having a thickness of 1500 Å was formed on the insulating film  502 . The amorphous semiconductor film was annealed at 600° C. for 12 hours to crystallize it. This crystallized silicon film was photolithographically patterned into island-shaped semiconductor regions  503 . Silicon oxide was deposited as a gate-insulating film  504  having a thickness of 1200 Å on the island-shaped regions  503 . Then, gate electrodes  505  of aluminum having a thickness of 6000 Å were formed ( FIG. 5(A) ). 
   Then, the gate electrodes were anodized to deposit an anodic oxide  506  on the top and side surfaces of the gate electrodes and conductive interconnects. The method of fabricating such an anodic oxide and its merits are described in detail in the above-cited Japanese Patent application Ser. Nos. 30220/1992, 34194/1992, 38637/1992, etc. Of course, this anodization step may be omitted if not necessary ( FIG. 5(B) ). 
   The gate-insulating film was then etched by dry etching. Carbon tetrafluoride was used as an etching gas. At this time, the anodic oxide, or alumina, was not etched. As a result, the gate-insulating film was etched excluding the portions located under the gate electrodes  505  and the anodic oxide  506 . A stream of phosphorus/hydrogen plasma accelerated at 5 to 20 keV, e.g., 10 keV, was irradiated to implant phosphorus ions into the island-shaped semiconductor regions  503  by a self-aligning process. As a result, doped regions  507  which would become source and drain were formed ( FIG. 5(C) ). 
   Subsequently, silicon oxide was deposited as an interlayer insulator  508  having a thickness of 5000 Å. KrF excimer laser light having a wavelength of 248 nm was illuminated to recover the crystallinity of the semiconductor regions  507  which deteriorated in the previous impurity implantation step. At this time, the energy density was 150 to 300 mJ/cm 2 , e.g., 200 mJ/cm 2  ( FIG. 5(D) ). 
   Then, contact holes were formed in the interlayer insulator  508 . Source and drain electrodes  509  were formed. Thus, N-channel transistors were completed ( FIG. 5(E) ). 
   P-channel transistors can be similarly built. If well-known CMOS fabrication techniques are used, N- and P-channel transistors can be both formed on the same substrate. The typical mobility of N-channel MOS transistors fabricated in the present example was 120 cm 2 /V·s. The typical mobility of P-channel MOS transistors fabricated in the present example was 80 cm 2 /V·s. With respect to CMOS shift registers consisting of 5 stages and comprising N- and P-channel transistors formed on the same substrate, synchronization of 15 MHz was confirmed at a drain voltage of 20 V. 
   EXAMPLE 5 
   The present example is illustrated in  FIGS. 6(A)-6(E) . Silicon oxide was deposited as an insulating base film  602  having a thickness of 1000 Å on a sheet of non-alkali glass  601 . A substantially intrinsic amorphous silicon semiconductor film having a thickness of 500 Å was formed on the insulating film  602 . The amorphous semiconductor film was illuminated with KrF laser light to crystallize the film. Preferably, the energy density of the laser light was 250 to 400 mJ/cm 2 . Where the substrate was maintained at 300-550° C. during the laser irradiation, good characteristics were obtained. 
   This crystallized silicon film was photolithographically patterned into island-shaped semiconductor regions  603 . Silicon oxide was deposited as a gate-insulating film  604  having a thickness of 1200 Å on the island-shaped regions  603 . Then, gate electrodes  605  of aluminum having a thickness of 6000 Å were formed ( FIG. 6(A) ). 
   Then, the gate electrodes were anodized to deposit an anodic oxide  606  on the top and side surfaces of the gate electrodes and conductive interconnects. Of course, this anodic oxidation step may be omitted if not necessary. Subsequently, using the gate electrode portions as a mask, the gate-insulating film  604  was etched by vapor phase etching to expose the surfaces of the semiconductor regions  603  ( FIG. 6(B) ). 
   As a protective film working in an ion implantation process, silicon oxide or silicon nitride  607  was deposited to a thickness of 500 Å over the whole surface A stream of phosphorus/hydrogen plasma accelerated at 25 to 70 keV, e.g., 50 keV, was irradiated to implant phosphorus ions into the island-shaped regions  603  by a self-aligning process. As a result, doped regions  608  which would become source and drain were formed ( FIG. 6(C) ). 
   Subsequently, silicon oxide was deposited as an interlayer insulator  609  having a thickness of 5000 Å. KrF excimer laser light was illuminated to recover the crystallinity of the semiconductor regions  608  which deteriorated in the previous impurity implantation step ( FIG. 6(D) ). 
   Then, contact holes were formed in the interlayer insulator  609 . Source and drain electrodes  610  were formed. Thus, N-channel transistors were completed ( FIG. 6(E) ). 
   EXAMPLE 6 
   The present example was illustrated in  FIGS. 7(A)-7(D) . Silicon oxide was deposited as an insulating base film  702  having a thickness of 1000 Å on a sheet of non-alkali glass  701 . A substantially intrinsic amorphous silicon semiconductor film having a thickness of 500 Å was formed on the insulating film  702 . The amorphous semiconductor film was illuminated with KrF laser light to crystallize the film. This crystallized silicon film was photolithographically patterned into island-shaped semiconductor regions  703 . Silicon oxide was deposited as a gate-insulating film  704  having a thickness of 1200 Å on the island-shaped regions  703 . Then, gate electrodes  705  of aluminum having a thickness of 6000 Å were formed. 
   Then, the gate electrodes were anodized to deposit an anodic oxide  706  on the top and side surfaces of the gate electrodes and conductive interconnects. Subsequently, using the gate electrode portions as a mask, the gate-insulating film  704  was totally etched by wet etching ( FIG. 7(A) ). 
   Then, silicon oxide was deposited as a new silicon oxide film  707  having a thickness of 200 to 300 Å. A stream of phosphorus/hydrogen plasma accelerated at 10 to 40 keV, e.g., 20 keV, was irradiated to implant phosphorus ions into the island-shaped semiconductor regions  703  by a self-aligning process. As a result, doped regions  708  which would become source and drain were formed ( FIG. 7(B) ). 
   After the completion of the ion implantation step, KrF excimer laser light was illuminated to recover the crystallinity of the semiconductor regions  708  which deteriorated in the previous impurity implantation step ( FIG. 7(C) ). 
   Then, an interlayer insulator  709  was deposited, and contact holes were formed in it. Source and drain electrodes  710  were formed. Thus, N-channel transistors were completed ( FIG. 7(D) ). 
   EXAMPLE 7 
   The present example is illustrated in  FIGS. 8(A)-8(E) . Silicon oxide was deposited as an insulating base film  802  having a thickness of 1000 Å on a substrate of non-alkali glass  801  made of Corning 7059 or the like. A substantially intrinsic amorphous silicon semiconductor film having a thickness of 1500 Å was formed on the insulating film  802 . The amorphous semiconductor film was annealed at 600° C. for 12 hours to crystallize it. This crystallized silicon film was photolithographically patterned into island-shaped semiconductor regions  803 . Silicon oxide was deposited as a gate-insulating film  804  having a thickness of 1200 Å on the island-shaped regions  803 . Then, gate electrodes  805  of aluminum having a thickness of 6000 Å were formed ( FIG. 8(A) ). 
   Then, the gate electrodes were anodized to deposit an anodic oxide  806  on the top and side surfaces of the gate electrodes and conductive interconnects. The method of fabricating such an anodic oxide and its merits are described in detail in the above-cited Japanese Patent application Ser. Nos. 30220/1992, 34194/1992, 38637/1992, etc. Of course, this anodization step may be omitted if not necessary ( FIG. 8(B) ). 
   The gate-insulating film was then etched by dry etching. Carbon tetrafluoride was used as an etching gas. At this time, the anodic oxide, or alumina, was not etched. As a result, the gate-insulating film was etched excluding the portions located under the gate electrodes  805  and the anodic oxide  806 . The etching step was interrupted when the thickness of the gate-insulating film  804  reached 500 Å, and a thin insulating film  807  was formed. A stream of phosphorus/hydrogen plasma accelerated at 15 to 50 keV, e.g., 30 keV, was irradiated to implant phosphorus ions into the island-shaped semiconductor regions  803  by a self-aligning process. As a result, doped regions  808  which would become source and drain were formed ( FIG. 8(C) ). 
   Subsequently, KrF excimer laser light having a wavelength of 248 nm was illuminated to recover the crystallinity of the semiconductor regions  808  which deteriorated in the previous impurity implantation step. At this time, the energy density was 150 to 300 mJ/cm 2 , e.g., 200 mJ/cm 2  ( FIG. 8(D) ). 
   Then, an interlayer insulator  809  was deposited and contact holes were formed in the interlayer insulator  809 . Source and drain electrodes  810  were formed. Thus, N-channel transistors were completed ( FIG. 8(E) ). 
   P-channel transistors can be similarly built. If well-known CMOS fabrication techniques are used, N- and P-channel transistors can be both formed on the same substrate. The typical mobility of N-channel MOS transistors fabricated in the present example was 120 cm 2 /V·s. The typical mobility of P-channel MOS transistors fabricated in the present example was 80 cm 2 /V·s. With respect to CMOS shift registers consisting of 5 stages and comprising N- and P-channel transistors formed on the same substrate, synchronization of 15 MHz was confirmed at a drain voltage of 20 V. 
   EXAMPLE 8 
   The present example is illustrated in  FIGS. 9(A)-9(D) . Silicon oxide was deposited as an insulating base film  902  having a thickness of 1000 Å on a substrate of non-alkali glass  901 . A substantially intrinsic amorphous silicon semiconductor film having a thickness of 500 Å was formed on the insulating film  902 . This amorphous film was crystallized by well-known laser annealing and photolithographically patterned into island-shaped semiconductor regions  903 . Silicon oxide was deposited as a gate-insulating film  904  having a thickness of 1200 Å on the island-shaped semiconductor regions  903 . Then, gate electrodes  905  of aluminum having a thickness of 6000 Å were formed. Then, the gate electrodes were anodized to deposit an anodic oxide  906  on the top and side surfaces of the gate electrodes and conductive interconnects ( FIG. 9(A) ). 
   The gate-insulating film was then etched by dry etching. Carbon tetrafluoride was used as an etching gas. At this time, the anodic oxide, or alumina, was not etched. As a result, the gate-insulating film was etched excluding the portions located under the gate electrodes  905  and the anodic oxide  906 . The etching step was interrupted when the thickness of the gate-insulating film  904  reached 500 Å. As a result, a thin insulating film  907  was formed. A stream of phosphorus/hydrogen plasma accelerated at 15 to 50 keV, e.g., 30 keV, was irradiated to implant phosphorus ions into the island-shaped semiconductor regions  903  by a self-aligning process. As a result, doped regions  908  which would become source and drain were formed ( FIG. 9(B) ). 
   Subsequently, silicon oxide was deposited as an interlayer insulator  909  having a thickness of 5000 Å. KrF excimer laser light having a wavelength of 248 nm was illuminated to recover the crystallinity of the semiconductor regions  908  which deteriorated in the previous impurity implantation step. At this time, the energy density was 150 to 300 mJ/cm 2 , e.g., 200 mJ/cm 2 . Where only a thin insulating film coats a semiconductor surface on laser irradiation as in Example 7, the bombardment which is caused by crystallization of the semiconductor roughens the surface. This presents problems in forming contacts. These problems did not occur in the present example, because a thick insulating film was formed ( FIG. 9(C) ). 
   Then, contact holes were formed in the interlayer insulator  909 . Source and drain electrodes  910  were formed. Thus, N-channel transistors were completed ( FIG. 9(D) ). 
   In the present example, the thick insulating film acting also as an interlayer insulating film was formed on the thin insulating film  907 . The thick insulating film may also be formed after removing the thin insulating film completely. When impurity ions are irradiated, a large amount of impurities is introduced into the insulating film, whereby laser light is absorbed. Accordingly, the insulating film containing such impurities is removed completely. Thus, the efficiency of the later laser annealing step can be enhanced. 
   The present invention provides a method of efficiently conducting ion implantation or ion doping and then laser annealing or lamp annealing. Obviously, the present invention can contribute to a decrease in the temperature of the process and thus yield great industrial advantages. In the illustrated examples, the present invention has been described in connection with MIS transistors having an active layer in the form of a thin film. These transistors are known as thin-film transistors. Restrictions tend to be imposed on the substrates of these thin-film transistors and so a low-temperature process is essential for these transistors. However, it is obvious that where the present invention is applied to MIS transistors formed on a semiconductor substrate consisting of a single crystal, similar advantages can be derived. 
   In the present invention, semiconductor species constituting the semiconductor regions include silicon, germanium, silicon carbide, silicon-germanium alloys, and gallium arsenide. Examples of the material of the gate electrodes include doped silicon, molybdenum, tungsten, titanium, aluminum, their alloys, silicides, and nitrides. Lasers preferably used in the present invention include excimer lasers such as ArF laser having a wavelength of 193 nm, KrF laser having a wavelength of 248 nm, XeCl laser having a wavelength of 308 nm, and XeF laser having a wavelength of 350 nm, Nd:YAG laser having a wavelength of 1064 nm, its second harmonic (532 nm), its third harmonic (354 nm), and its fourth harmonic (266 nm). Of course, use of other laser or light source is embraced in the scope of the present invention.