Abstract:
A miniature contact is incorporated in a semiconductor device for transferring an electric signal between a conductive wiring and an impurity region, and a titanium silicide and a single crystal silicon region doped with an impurity forms an ohmic contact; in order to form the ohmic contact, a surface portion of the single crystal silicon region is made amorphous by using an ion-bombardment, thereafter, titanium is deposited on the amorphous silicon to have the thickness ranging between 3 nanometers and 10 nanometers, and the titanium layer is converted to a titanium silicide layer through an annealing at 400 degrees to 500 degrees in centigrade, thereby forming the low-resistive ohmic contact without changing the impurity profile of the single crystal silicon region.

Description:
FIELD OF THE INVENTION 
     This invention relates to a process of fabricating a semiconductor device and, more particularly, to a process of fabricating a semiconductor device with a low-resistive contact without a high temperature heat treatment. 
     DESCRIPTION OF THE RELATED ART 
     In order to form a low-resistivity contact with a semiconductor substrate in a contact hole, refractory metal film is deposited so as to cover the contact area of the semiconductor substrate, and the refractory metal reacts with the semiconductor through a heat-treatment so as to produce a refractory metal silicide layer. Titanium is attractive, because the titanium forms a low Schottky barrier together with any one of the p-type semiconductors and the n-type semiconductors. Moreover, the titanium easily reduces natural oxide unavoidably covering the contact area, and the silicidation smoothly proceeds. 
     An experiment is reported by Akihiro Sakamoto et al in “Lower Thickness Limit of Ti Film in BLK-W Contact”, Proceedings of 38th. Spring Conference of Applied Physics Society, 30p-W-7. According to the paper, a titanium film was deposited form 5 nanometers to 40 nanometers thick on a single crystal silicon substrate, and was annealed at 800 degrees centigrade for 30 seconds. Sakamoto et. al. taught that the contact resistance was drastically increased when the titanium film was equal to or less than 10 nanometers thick. 
     A process of forming a metal-semiconductor ohmic contact is disclosed in Japanese Patent Publication of Unexamined Application No. 4-215424. According to the Japanese Patent Publication of Unexamined Application, arsenic was ion implanted into a semiconductor layer so as to make the semiconductor layer amorphous, and titanium was deposited to 100 nanometers thick on the amorphous semiconductor layer. The titanium layer was annealed at a low temperature equal to or less than 500 degrees centigrade. 
     The first prior art ohmic contact disclosed in the paper encounters a problem in high contact resistance, because the ohmic contact is formed with the single crystal silicon. The high temperature annealing is another problem. The high temperature annealing causes the dopant impurity to be diffused into the silicon substrate, and destroys the impurity profile. 
     The impurity profile is less affected in the second prior art process, because the titanium layer is annealed at the relatively low temperature. However, the amorphous silicon layer requires the thick titanium layer of 100 nanometers thick, and the deposition consumes time and a large amount of titanium. Thus, the second prior art process encounters another problem in high production cost. 
     A problem inherent in the second prior art contact is a high contact resistance. The high contact resistance is derived from shortage of dopant impurity in the contact area. The dopant impurity tends to be diffused into the thick refractory metal silicide layer, and makes the dopant concentration in the contact area light. This tendency is conspicuous in a miniature contact hole of the order of 0.5 micron by 0.5 micron square or 0.5 micron in diameter. 
     The aspect ratio of the contact hole is getting larger and larger together with the integration density. It is impossible to properly deposit refractory metal on the bottom surface of a miniature contact hole with a large aspect ratio through sputtering. Device manufacturers try to use a chemical vapor deposition so as to perfectly grow a refractory metal layer or a refractory metal silicide layer over the surface defining the miniature contact hole with the large aspect ratio. However, the refractory metal grows differently on the contact area depending upon the conductivity type of the contact area. When the refractory metal is concurrently deposited on a heavily doped p-type contact area and a heavily doped n-type contact area, the refractory metal layer on the heavily doped p-type contact area is different in thickness from the heavily doped n-type contact area. If one of the refractory metal layers is optimized, the other refractory metal layer is so thin that the electric resistance is increased. On the other hand, if the other refractory metal layer is optimized, the refractory metal layer is too thick, and leakage current is increased. 
     SUMMARY OF THE INVENTION 
     It is therefore an important object of the present invention to provide a process of fabricating a semiconductor device through which the contact resistance of a miniature contact hole is decreased by using a low temperature heat treatment. 
     It is also an important object of the present invention to provide a process of fabricating a semiconductor device through which contact areas different in conductivity type are properly covered with refractory metal layers or refractory metal silicide layers. 
     The present inventors contemplated the problems, and investigated influences of crystal structure on the contact resistance. The present inventors formed p-type single crystal impurity regions exposed by miniature contact holes of 0.5 micron square and p-type amorphous impurity regions also exposed by miniature contact holes of 0.5 micron square. The present inventors deposited titanium on the p-type single crystal impurity regions and the p-type amorphous impurity regions, and varied the thickness of the titanium layers. The titanium layers were treated with heat at 500 degrees centigrade for 30 minutes in nitrogen ambience, and titanium silicide layers formed contacts with the p-type single crystal impurity regions and the p-type amorphous impurity regions. The present inventors measured the contact resistance, and plotted the contact resistances in FIG.  1 . Plot PL 1  represents the contact resistance of the p-type single crystal impurity regions, and plot PL 2  was indicative of the contact resistance of the p-type amorphous impurity regions. 
     The present inventors noticed that the contact resistance was varied between the single crystal and the amorphous silicon regions. The single crystal region required a thick titanium layer for a low resistivity contact, while the amorphous region formed a low-resistivity contact between 3 nanometers thick and 10 nanometers thick. The present inventors first concluded that the amorphous refractory metal layer between 3 nanometers and 10 nanometers thick was desirable for a low-resistivity contact. 
     The present inventors further investigated growing technologies, and found that a chemical vapor deposition was available for refractory metal deposited on an amorphous dopant impurity region exposed by a miniature contact hole with a large aspect ratio. A contact region was changed to amorphous by using an ion-implantation. Boron/boron difluoride (BF 2 ) or phosphorous/arsenic was desirable for the ion-implantation. If TiCl 4  was reduced in the chemical vapor deposition, titanium was deposited at a certain temperature not higher than 600 degrees centigrade. 
     In accordance with one aspect of the present invention, there is provided a process of fabricating a semiconductor device on a semiconductor substrate, comprising: a) preparing a semiconductor layer; b) forming an insulating layer over the semiconductor layer; c) forming an opening in the insulating layer so that the semiconductor layer is exposed to the opening; d) making the semiconductor layer exposed to the opening amorphous; e) depositing a refractory metal layer from 3 nanometers to 10 nanometers thick on the semiconductor layer; and f) treating the refractory metal layer with heat so as to convert the refractory metal layer to a refractory metal silicide layer. 
     In accordance with another aspect of the present invention, there is provided a process of fabricating a semiconductor device, comprising the steps of: a) preparing a semiconductor layer; b) forming an insulating layer having an opening where the semiconductor layer is exposed; c) making the semiconductor layer exposed to the opening amorphous; and d) depositing a refractory metal layer by using a chemical vapor deposition so as to grow a refractory metal silicide on the semiconductor layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the process according to the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a graph showing relation between contact resistance and the thickness of a titanium layer on the bottom surface of a contact hole; 
     FIGS. 2A to  2 F are cross sectional views showing a process of fabricating a semiconductor device according to the present invention; 
     FIGS. 3A to  3 F are cross sectional views showing another process of fabricating a semiconductor device according to the present invention; 
     FIGS. 4A to  4 H are cross sectional views showing yet another process of fabricating a semiconductor device according to the present invention; 
     FIGS. 5A and 5B are cross sectional views showing essential steps of still another process of fabricating a semiconductor device according to the present invention; 
     FIGS. 6A to  6 E are cross sectional views showing still another process of fabricating a semiconductor device according to the present invention; and 
     FIG. 7 is a graph showing relation between the thickness of a titanium layer and the conductivity type of a silicon layer thereunder. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIGS. 2A to  2 F illustrate a process of fabricating a semiconductor device embodying the present invention. The process starts with preparation of a p-type single crystal silicon substrate  1   a , and a field oxide layer  1   b  is selectively grown on the major surface of the p-type silicon substrate  1   a . The field oxide layer  1   b  defines a plurality of active areas in the major surface, and two active areas are shown in FIGS. 2A to  2 F. A lightly doped n-type well  3   a  is formed in one of the active areas, and a heavily doped p-type impurity region  3   b  and a heavily doped n-type impurity region  4   a  are formed in the lightly doped n-type well  3   a  and the other of the active areas, respectively. The heavily doped p-type impurity region  3   b  is electrically isolated from the heavily doped n-type impurity region  4   a  by means of the field oxide layer  1   b.    
     Boro phospho-silicate glass is deposited to 1.5 microns thick over the entire surface of the resultant semiconductor structure by using a chemical vapor deposition, and forms an inter-level insulating layer  5   a.    
     Photo-resist solution is spun onto the inter-level insulating layer  5   a , and is baked so as to form a photo-resist layer on the inter-level insulating layer  5   a . A pattern image representative of contact holes is optically transferred to the photo-resist layer so as to form a latent image therein. The latent image is developed, and the photo-resist layer is patterned into a photo-resist etching mask (not shown) for the contact holes. Thus, the photo-resist etching mask is provided on the inter-level insulating layer  5   a  through a lithography. 
     Using the photo-resist etching mask, the inter-level insulating layer  5   a  is selectively etched so as to form contact holes  5   b  and  5   c  therein. The heavily doped n-type impurity region  4   a  and the heavily doped p-type impurity region  3   b  are respectively exposed to the contact holes  5   b / 5   c  of 0.5 micron square as shown in FIG.  2 A. In this instance, openings of the photo-resist etching mask are slightly deviated from target positions over the heavily doped n-type impurity region  4   a  and the heavily doped p-type impurity region  3   b , and, for this reason, the contact holes  5   b / 5   c  are not exactly nested in the heavily doped n-type impurity region  4   a  and the heavily doped p-type impurity region  3   b , respectively. 
     Subsequently, a photo-resist ion-implantation mask  6   a  is formed on the inter-level insulating layer  5   a  by using lithography, and the heavily doped n-type impurity region  4   a  is covered with the photo-resist ion-implantation mask  6   a . Boron difluoride (BF 2 ) is ion-implanted through the contact hole  5   c  into the heavily doped p-type single crystal impurity region  3   b  at dose of 5×10 14  cm −2  to 5×10 15  cm −2  under acceleration energy of 10 KeV to 30 KeV. A surface portion of the heavily doped p-type impurity region  3   a  becomes amorphous due to the ion bombardment expressed by mark “x”, and a p-type amorphous silicon layer  3   c  is formed in the heavily doped p-type single crystal impurity region  3   b  as shown in FIG.  2 B. After the ion-implantation, the photo-resist ion-implantation mask  6   a  is stripped off, and the heavily doped n-type single crystal impurity region  4   a  is exposed again. 
     Another photo-resist ion-implantation mask  6   b  is provided on the inter-level insulating layer  5   a  by using the lithography, and the p-type amorphous silicon layer  3   c  is covered with the photo-resist ion-implantation mask  6   b . Phosphorous is ion implanted through the contact hole  5   b  into the heavily doped n-type single crystal impurity region  4   a  at dose of 3×10 14  cm −2  to 1×10 15  cm −2  under acceleration energy of 10 KeV to 70 KeV. A surface portion of the heavily doped n-type single crystal impurity region  4   a  becomes amorphous due to the ion bombardment expressed by mark “x”, and an n-type amorphous silicon layer  4   b  is formed in the surface portion as shown in FIG.  2 C. After the ion-implantation, the photo-resist ion-implantation mask  6   b  is stripped off. 
     Subsequently, titanium is sputtered over the entire surface of the resultant semiconductor structure, and a titanium layer  7   a  topographically extends over the entire surface. A collimator plate (not shown) is provided between the titanium target and the semiconductor structure shown in FIG. 2C for preventing the semiconductor structure from titanium particles obliquely radiated from the target, and has an aspect ratio of 3 in this instance. When the titanium is deposited through the collimator plate over the entire surface containing the bottom surfaces of the contact holes  5   b / 5   c  and the upper surface of the interlevel insulating layer  5   a , the titanium layer  7   a  is not uniformly deposited, and the ratio of the titanium layer on the bottom surface to the titanium layer on the upper surface is of the order of 13 percent. In order to regulate the titanium layer  7   a  from 3 nanometers to 10 nanometers thick, the titanium layer  7   a  is deposited from 23 nanometers to 77 nanometers thick on the upper surface of the inter-level insulating layer  5   a.    
     Titanium nitride is sputtered over the titanium layer  7   a , and the titanium layer  7   a  is overlain by a titanium nitride layer  7   b  as shown in FIG.  2 D. The titanium nitride layer  7   b  is 100 nanometers thick over the upper surface of the inter-level insulating layer  5   a . The titanium nitride layer  7   b  defines secondary holes  5   d  and  5   e  in the contact holes  5   b  and  5   c , respectively. The titanium nitride layer  7   b  does not allow the titanium layer  7   a  to be nitrided in an annealing step, and the titanium nitride layer  7   b  allows the manufacturer to decrease the thickness of the titanium layer  7   a . The titanium nitride layer  7   b  further serves as a barrier layer against tungsten in a later stage. 
     The resultant semiconductor structure is placed in nitrogen ambience, and is heated to between 400 degrees and 500 degrees centigrade. The semiconductor structure is maintained in the high temperature nitrogen ambience for 10 minutes to 60 minutes, and the titanium reacts with the p-type amorphous silicon  3   c  and the n-type amorphous silicon  4   b  so as to form titanium silicide layers  3   d / 4   c  as shown in FIG.  2 E. Thus, the titanium reacts with the silicon at 400 degrees to 500 degrees centigrade, because the silicon is amorphous. 
     The p-type amorphous silicon and the n-type amorphous silicon are recrystallized during the annealing. The annealing temperature is low enough that the heavily doped n-type single crystal impurity region  4   a  and the heavily doped p-type single crystal impurity region  3   b  maintain the impurity profiles. Moreover, the heat activates the boron difluoride and the phosphorous both implanted beneath the periphery of the field oxide layer  1   b , and the boron difluoride and the phosphorous form an additional p-type impurity region  3   e  and an additional n-type impurity region  4   d  merged into the heavily doped p-type single crystal impurity region  3   b  and the heavily doped n-type single crystal impurity region  4   a . Thus, the contact holes  5   b / 5   c  are finally nested into the heavily doped n-type single crystal impurity region  4   a / 4   d  and the heavily doped p-type single crystal impurity region  3   b / 3   e , respectively, and the mis-alignment of the photo-resist etching mask for the contact holes does not deteriorate electric characteristics of contacts between the heavily doped p-type/n-type single crystal impurity regions  3   b / 3   e / 4   a / 4   d  and conductive plugs formed in the secondary holes  5   d / 5   e  at a later stage. The ion-implanted impurities are activated at 10 to 50 percent through the annealing at 400 degrees to 500 degrees centigrade with respect to the annealing at 850 degrees centigrade, and reverse leakage current across the p-n junction is ten times larger than that annealed at 850 degrees centigrade. However, the reverse leakage current is admittable in a standard semiconductor integrated circuit. 
     Subsequently, tungsten is deposited over the titanium nitride layer  7   b . The tungsten fills the secondary holes  5   d / 5   e , and swells into a tungsten layer of 0.5 micron thick over the inter-level insulating layer  5   a . A photo-resist etching mask (not shown) is formed on the tungsten layer by using the lithography, and the tungsten layer is selectively etched away. As a result, tungsten plugs  7   c / 7   d  are left in the secondary holes  5   d / 5   e . The photo-resist etching mask is stripped off. 
     Aluminum alloy target (not shown) is prepared, and contains copper at 0.5 percent. The aluminum alloy is deposited to 0.5 micron thick over the entire surface of the resultant semiconductor structure by using a sputtering, and a photo-resist etching mask (not shown) is provided on the aluminum alloy layer by using the lithography. The aluminum alloy layer, the titanium nitride layer and the titanium layer are selectively etched away, and form multi-layer metal wirings  8   a  and  8   b  as shown in FIG.  2 F. 
     The titanium layer  7   a  ranges from 3 nanometers to 10 nanometers thick according to the present invention. If the titanium layer  7   a  is thinner than 3 nanometers, the titanium silicide layer  4   c / 3   d  are too thin to decrease the contact resistance. On the other hand, if the titanium layer  7   a  is thicker than 10 nanometers, the p-type dopant impurity and the n-type dopant impurity are diffused from the heavily doped p-type impurity region  3   b  and the heavily doped n-type impurity region  4   a  into the titanium silicide layers  3   d / 4   c , and the resistance is increased in the n-type impurity region  4   a  due to shortage of the dopant impurities. Especially, the contact resistance over the n-type impurity region  4   a  tends to be increased. Moreover, it is hard to deposit titanium more than 10 nanometers thick on the bottom surfaces of the contact holes  5   b / 5   c  by using the sputtering. 
     In this instance, boron difluoride and phosphorous are ion implanted. However, boron and arsenic are available for the ion-implantation. 
     The present inventors evaluated the contact formed through the process described hereinbefore. The contact resistance over the heavily doped p-type single crystal impurity region  3   b / 3   e  was of the order of 150 ohms, and the contact resistance over the heavily doped n-type single crystal impurity region  4   a / 4   d  was of the order of 100 ohms. 
     Second Embodiment 
     FIGS. 3A to  3 F illustrate another process embodying the present invention. The process starts with preparation of a p-type single crystal silicon substrate  11   a , and a field oxide layer  11   b  is selectively grown on the major surface of the p-type silicon substrate  11   a . The field oxide layer  11   b  defines a plurality of active areas in the major surface, and two active areas are shown in FIGS. 3A to  3 F. A lightly doped n-type well  13   a  is formed in one of the active areas, and a heavily doped p-type impurity region  13   b  and a heavily doped n-type impurity region  14   a  are formed in the lightly doped n-type well  13   a  and the other of the active areas, respectively. The heavily doped p-type impurity region  13   b  is electrically isolated from the heavily doped n-type impurity region  14   a  by means of the field oxide layer  11   b.    
     Boro phospho-silicate glass is deposited to 1.5 microns thick over the entire surface of the resultant semiconductor structure by using a chemical vapor deposition, and forms an inter-level insulating layer  15   a.    
     A photo-resist etching mask (not shown) is provided on the inter-level insulating layer  15   a  through a lithography. Using the photo-resist etching mask, the inter-level insulating layer  15   a  is selectively etched so as to form contact holes  15   b  and  15   c  therein. The heavily doped n-type impurity region  14   a  and the heavily doped p-type impurity region  13   b  are respectively exposed by the contact holes  15   b / 15   c  of 0.5 micron square as shown in FIG.  3 A. In this instance, openings of the photo-resist etching mask are slightly deviated from target positions over the heavily doped n-type impurity region  14   a  and the heavily doped p-type impurity region  13   b , and, for this reason, the contact holes  15   b / 15   c  are not exactly nested in the heavily doped n-type impurity region  14   a  and the heavily doped p-type impurity region  13   b , respectively. 
     Phosphorous is ion implanted through contact holes  15   b / 15   c  into the heavily doped n-type single crystal impurity region  14   a  and the heavily doped p-type single crystal impurity region  13   b  at dose equal to or greater than 3×10 14  cm −2  under acceleration energy of 70 KeV. Surface portions of these single crystal impurity regions  14   a / 13   b  become amorphous due to the ion bombardment marked with “x”, and the heavily doped n-type single crystal impurity region  14   a  and the heavily doped p-type single crystal impurity region  13   b  are overlain by an n-type amorphous silicon layer  14   b  and a p-type amorphous silicon layer  13   c , respectively, as shown in FIG.  3 B. If the dose is less than 3×10 14 cm   −2 , the ion bombardment hardly makes the single crystal impurity regions  14   b / 13   c  amorphous. 
     Subsequently, a photo-resist ion-implantation mask  16   a  is formed on the inter-level insulating layer  15   a  by using lithography, and the heavily doped n-type impurity region  14   a  is covered with the photo-resist ion-implantation mask  16   a . Boron is ion-implanted through the contact hole  15   c  into the heavily doped p-type single crystal impurity region  13   b  at dose of 1×10 15  cm −2  to 5×10 15  cm −2  under acceleration energy of 20 KeV to 50 KeV as shown in FIG.  3 C. 
     The boron is so light that the minimum dose for the amorphous silicon is 1×10 16  cm −2 . For this reason, the surface portion of the heavily doped p-type impurity region  13   b  is converted to an amorphous through the bombardment of phosphorous. However, the boron is expected to change the n-type well  13   a  beneath the field oxide layer  11   b  to a p-type region so as to cure the mis-alignment of the photoresist etching mask for the contacts  15   b / 15   c . After the ion-implantation, the photo-resist ion-implantation mask  16   a  is stripped off. 
     Subsequently, titanium is sputtered over the entire surface of the resultant semiconductor structure, and a titanium layer  17   a  topographically extends over the entire surface. The titanium layer  17   a  is not uniform in thickness as described hereinbefore, and the titanium layer  17   a  on the n-type amorphous layer  14   b  and the p-type amorphous layer  13   c  ranges from 3 nanometers to 10 nanometers thick. 
     Titanium nitride is sputtered over the titanium layer  17   a , and the titanium layer  17   a  is overlain by a titanium nitride layer  17   b  as shown in FIG.  3 D. The titanium nitride layer  17   b  is 100 nanometers thick over the upper surface of the inter-level insulating layer  15   a . The titanium nitride layer  17   b  defines secondary holes  15   d  and  15   e  in the contact holes  15   b  and  15   c , respectively. The titanium nitride layer  17   b  behaves similarly to the titanium nitride layer  7   b.    
     The resultant semiconductor structure is placed in nitrogen ambience, and is heated to between 400 degrees and 500 degrees centigrade. The semiconductor structure is maintained in the high temperature nitrogen ambience for 10 minutes to 60 minutes, and the titanium reacts with the p-type amorphous silicon  13   c  and the n-type amorphous silicon  14   b  so as to form titanium silicide layers  13   d / 14   c  as shown in FIG.  3 E. Thus, the titanium reacts with the silicon at 400 degrees to 500 degrees centigrade, because the silicon is amorphous. 
     The p-type amorphous silicon and the n-type amorphous silicon are recrystallized during the annealing. The annealing temperature is low enough that the heavily doped n-type single crystal impurity region  14   a  and the heavily doped p-type single crystal impurity region  13   b  maintain the impurity profiles. Moreover, the heat activates the boron and phosphorous both implanted beneath the periphery of the field oxide layer  11   b , and the boron and the phosphorous form an additional p-type impurity region  13   e  and an additional n-type impurity region  14   d  merged into the heavily doped p-type single crystal impurity region  13   b  and the heavily doped n-type single crystal impurity region  14   a . Thus, the contact holes  15   b / 15   c  are finally nested into the heavily doped n-type single crystal impurity region  14   a / 14   d  and the heavily doped p-type single crystal impurity region  13   b / 13   e , respectively, and the mis-alignment of the photo-resist etching mask for the contact holes does not deteriorate electric characteristics of the contacts between the heavily doped p-type/n-type single crystal impurity regions  13   b / 13   e / 14   a / 14   d  and conductive plugs formed in the secondary holes  15   d / 15   e  at a later stage. 
     In this instance, the p-type amorphous layer  13   c  is formed by using the boron instead of the boron difluoride. The boron allows the p-type amorphous layer  13   c  and the n-type amorphous layer  14   b  to be recrystallized at lower temperature within a shorter time period than the boron difluoride, and the ion-implanted dopant impurities are rapidly activated. The fluorine impedes the recrystallization and the activation. However, the fluorine is never introduced into the amorphous layers  13   c / 14   b  in this instance. 
     Subsequently, the secondary holes  15   d / 15   e  are plugged with pieces of tungsten  17   c / 17   d , and multi-layer wirings  18   a / 18   b  are patterned on the inter-level insulating layer  15   a  as similar to the first embodiment. The resultant semiconductor structure is illustrated in FIG.  3 F. 
     The process implementing the second embodiment achieves all the advantages of the first embodiment. Moreover, the ion-implantation of phosphorous is carried out without a photo-resist etching mask, and the process of the second embodiment is simpler than that of the first embodiment. For this reason, the production cost is decreased. 
     Third Embodiment 
     FIGS. 4A to  4 H illustrate yet another process of fabricating a semiconductor device embodying the present invention. The process starts with preparation of a p-type single crystal silicon substrate  21   a , and a field oxide layer  21   b  is selectively grown on the major surface of the p-type silicon substrate  21   a . The field oxide layer  21   b  defines a plurality of active areas in the major surface, and two active areas are shown in FIGS. 4A to  4 H. A lightly doped n-type well  23   a  is formed in one of the active areas, and a heavily doped p-type impurity region  23   b  and a heavily doped n-type impurity region  24   a  are formed in the lightly doped n-type well  23   a  and the other of the active areas, respectively. The heavily doped p-type impurity region  23   b  is electrically isolated from the heavily doped n-type impurity region  24   a  by means of the field oxide layer  21   b.    
     Boro phospho-silicate glass is deposited to 1.5 microns thick over the entire surface of the resultant semiconductor structure by using a chemical vapor deposition, and forms an inter-level insulating layer  25   a.    
     A photo-resist etching mask (not shown) is provided on the inter-level insulating layer  25   a  through lithography. Using the photo-resist etching mask, the inter-level insulating layer  25   a  is selectively etched so as to form contact holes  25   b  and  25   c  of 0.5 micron square therein. The heavily doped n-type impurity region  24   a  and the heavily doped p-type impurity region  23   b  are respectively exposed by the contact holes  25   b / 25   c  as shown in FIG.  4 A. In this instance, openings of the photo-resist etching mask are slightly deviated from target positions over the heavily doped n-type impurity region  24   a  and the heavily doped p-type impurity region  23   b , and, for this reason, the contact holes  25   b / 25   c  are not exactly nested in the heavily doped n-type impurity region  24   a  and the heavily doped p-type impurity region  23   b , respectively. 
     Subsequently, a photo-resist ion-implantation mask  26   a  is formed on the inter-level insulating layer  25   a  by using lithography, and the heavily doped n-type impurity region  24   a  is covered with the photo-resist ion-implantation mask  26   a . Boron difluoride is ion implanted through the contact hole  25   c  into the heavily doped p-type single crystal impurity region  23   b  under the conditions similar to the first embodiment. A surface portion of the heavily doped p-type impurity region  23   a  becomes amorphous due to the ion bombardment expressed by mark “x”, and a p-type amorphous silicon layer  23   c  is formed in the heavily doped p-type single crystal impurity region  23   b  as shown in FIG.  4 B. After the ion-implantation, the photo-resist ion-implantation mask  26   a  is stripped off, and the heavily doped n-type single crystal impurity region  24   a  is exposed again. 
     Another photo-resist ion-implantation mask  26   b  is provided on the inter-level insulating layer  25   a  by using lithography, and the p-type amorphous silicon layer  23   c  is covered with the photo-resist ion-implantation mask  26   b . Phosphorous is ion implanted through the contact hole  25   b  into the heavily doped n-type single crystal impurity region  24   a  under the conditions similar to the first embodiment. A surface portion of the heavily doped n-type single crystal impurity region  24   a  becomes amorphous due to the ion bombardment expressed by mark “x”, and an n-type amorphous silicon layer  24   b  is formed in the surface portion as shown in FIG.  4 C. After the ion-implantation, the photo-resist ion-implantation mask  26   b  is stripped off. 
     The resultant semiconductor structure is placed in inert ambience containing nitrogen. The p-type amorphous silicon layer  23   c  and the n-type amorphous silicon layer  24   b  are heated to between 800 degrees and 900 degrees centigrade, and are maintained in the high-temperature inert ambience for 10 minutes to 30 minutes. The boron of the boron difluoride and the phosphorous are perfectly activated, and the p-type amorphous silicon layer  23   c  and the n-type amorphous silicon layer  24   b  are recrystallized again. Moreover, an additional p-type region  23   d  and an additional n-type region  24   c  are merged with the heavily doped p-type single crystal impurity region  23   b  and the heavily doped n-type single crystal impurity region  24   a . Thus, the mis-alignment between the heavily doped single crystal impurity regions  23   b / 24   a  and the contact holes  25   c / 25   b  is cured through the heat treatment. The resultant semiconductor structure is illustrated in FIG.  4 D. 
     Subsequently, arsenic is ion implanted through the contact holes  25   b / 25   c  into the heavily doped p-type single crystal impurity region  23   b / 23   d  and the heavily doped n-type single crystal impurity region  24   a / 24   c . The dose is more than 2×10 14  cm −2  and much less than the dose of the boron difluoride. The acceleration energy is regulated to 30 KeV. If the dose of the arsenic is less than 2×10 14  cm −2 , an amorphous silicon layer is not produced. On the other hand, if the dose of the arsenic is greater than the dose of the boron difluoride, an n-type amorphous silicon layer is formed on the heavily doped p-type single crystal impurity region  23   b / 23   d . The bombardment of arsenic makes a surface portion of the heavily doped p-type single crystal impurity region  23   b / 23   d  and a surface portion of the heavily doped n-type single crystal impurity region  24   a / 24   c  amorphous. Thus, the heavily doped p-type single crystal impurity region  23   b / 23   d  and the heavily doped n-type single crystal impurity region  24   a / 24   c  are respectively overlain by a p-type amorphous silicon layer  23   e  and an n-type amorphous silicon layer  24   d  as shown in FIG.  4 E. 
     Subsequently, titanium is sputtered over the entire surface of the resultant semiconductor structure, and a titanium layer  27   a  topographically extends over the entire surface. The titanium layer  27   a  is not uniform in thickness, and the p-type amorphous silicon layer  23   e  and the n-type amorphous silicon layer  24   d  are covered with the titanium layer  27   a  ranging from 3 nanometers to 10 nanometers thick. 
     Titanium nitride is sputtered over the titanium layer  27   a , and the titanium layer  27   a  is overlain by a titanium nitride layer  27   b  as shown in FIG.  4 F. The titanium nitride layer  7   b  is 100 nanometers thick over the upper surface of the inter-level insulating layer  25   a . The titanium nitride layer  27   b  defines secondary holes  25   d  and  25   e  in the contact holes  25   b  and  25   c , respectively. The titanium nitride layer  27   b  serves as a protective layer as similar to that of the first embodiment. 
     The resultant semiconductor structure is placed in inert ambience containing nitrogen, and is heated to between 400 degrees and 500 degrees centigrade. The semiconductor structure is maintained in the high temperature inert ambience for 10 minutes to 60 minutes, and the titanium reacts with the p-type amorphous silicon  23   e  and the n-type amorphous silicon  24   d  so as to form titanium silicide layers  23   e / 24   d  as shown in FIG.  4 G. Thus, the titanium reacts with the silicon at 400 degrees to 500 degrees centigrade, because the silicon is amorphous. The p-type amorphous silicon and the n-type amorphous silicon are recrystallized during the annealing, and the ion-implanted arsenic is activated. The annealing temperature is low enough that the heavily doped n-type single crystal impurity region  24   a  and the heavily doped p-type single crystal impurity region  23   b  maintain the impurity profiles. Even though the arsenic is activated, the single crystal impurity region  23   b / 23   d  are maintained in the p-type. 
     Subsequently, the secondary holes  25   d / 25   e  are plugged with pieces of tungsten  27   c / 27   d , and multi-layer wirings  28   a / 28   b  are patterned on the inter-level insulating layer  25   a  as similar to the first embodiment. The resultant semiconductor structure is illustrated in FIG.  4 H. 
     The process implementing the third embodiment achieves all the advantages of the first embodiment. Moreover, the boron and the phosphorous are activated at approximately 100 percent during the heat treatment described in conjunction with FIG. 4D, and the reverse leakage current is decreased from those of the first and second embodiments. 
     If the arsenic increases the contact resistance between the heavily doped p-type single crystal impurity region  23   b / 23   d  and the titanium silicide layer  23   e  to non-acceptable level, boron difluoride may be ion implanted into the heavily doped p-type single crystal impurity region  23   b / 23   d  instead of the arsenic. The p-type amorphous silicon layer  23   e  and the n-type amorphous silicon layer  24   d  may be formed through an ion-implantation of boron difluoride and an ion-implantation of phosphorous, respectively. 
     Fourth Embodiment 
     FIGS. 5A and 5B illustrate still another process of fabricating a semiconductor device embodying the present invention. The process implementing the fourth embodiment is available for impurity regions wide enough to nest contact holes therein. 
     The process starts with preparation of a p-type single crystal silicon substrate  31   a , and a field oxide layer  31   b  is selectively grown on the major surface of the p-type silicon substrate  31   a . The field oxide layer  31   b  defines a plurality of active areas in the major surface, and two active areas are shown in FIGS. 5A and 5B. A lightly doped n-type well  33   a  is formed in one of the active areas, and a heavily doped p-type impurity region  33   b  and a heavily doped n-type impurity region  34   a  are formed in the lightly doped n-type well  33   a  and the other of the active areas, respectively. The heavily doped p-type impurity region  33   b  is electrically isolated from the heavily doped n-type impurity region  34   a  by means of the field oxide layer  31   b.    
     Boro phospho-silicate glass is deposited to 1.5 microns thick over the entire surface of the resultant semiconductor structure by using a chemical vapor deposition, and forms an inter-level insulating layer  35   a.    
     A photo-resist etching mask (not shown) is provided on the inter-level insulating layer  35   a  through lithography. Using the photo-resist etching mask, the inter-level insulating layer  35   a  is selectively etched so as to form contact holes  35   b  and  35   c  therein. The heavily doped n-type impurity region  34   a  and the heavily doped p-type impurity region  33   b  are respectively exposed by the contact holes  35   b / 35   c  as shown in FIG.  5 A. The photo-resist etching mask is then stripped off. 
     There is an margin between the heavily doped single crystal impurity regions  34   a / 33   b  and the contact holes  35   b / 35   c  as described hereinbefore. For this reason, even if the photo-resist etching mask is mis-aligned with the target areas over the heavily doped single crystal impurity regions  34   a / 33   b , the contact holes  35   b / 35   c  are still nested in the heavily doped single crystal impurity regions  34   a / 33   b , respectively. 
     Arsenic or boron difluoride is ion implanted through the contact holes  35   b / 35   c  into the heavily doped n-type single crystal impurity region  34   a  and the heavily doped p-type single crystal impurity region  33   b  as similar to the third embodiment. The ion bombardment makes a surface portion  34   b  of the heavily doped n-type single crystal impurity region  34   a  and a surface portion  33   c  of the heavily doped p-type single crystal impurity region  33   b  amorphous. If the arsenic is ion implanted, the dose should be selected in such a manner that the implanted arsenic does not change the conductivity type of the heavily doped p-type single crystal impurity region  33   b . Similarly, if the boron difluoride is ion implanted, the dose should be selected in such a manner that the implanted boron does not change the conductivity type of the heavily doped n-type single crystal impurity region  34   a.    
     Thereafter, the process proceeds to the steps shown in FIGS. 4F to  4 H. 
     Fifth Embodiment 
     FIGS. 6A to  6 E illustrate still another process of fabricating a semiconductor device embodying the present invention. The process starts with preparation of a p-type single crystal silicon substrate  41   a , and a field oxide layer  41   b  is selectively grown on the major surface of the p-type silicon substrate  41   a . The field oxide layer  41   b  defines a plurality of active areas in the major surface, and two active areas are shown in FIGS. 6A to  6 E. A lightly doped n-type well  43   a  is formed in one of the active areas, and a heavily doped p-type impurity region  43   b  and a heavily doped n-type impurity region  44   a  are formed in the lightly doped n-type well  43   a  and the other of the active areas, respectively. The heavily doped p-type impurity region  43   b  is electrically isolated from the heavily doped n-type impurity region  44   a  by means of the field oxide layer  41   b.    
     Boro phospho-silicate glass is deposited over the entire surface of the resultant semiconductor structure, and forms an inter-level insulating layer  45   a.    
     A photo-resist etching mask (not shown) is provided on the inter-level insulating layer  5   a  through lithography. Using the photo-resist etching mask, the inter-level insulating layer  45   a  is selectively etched so as to form contact holes  45   b  and  45   c  therein. The heavily doped n-type impurity region  44   a  and the heavily doped p-type impurity region  43   b  are respectively exposed by the contact holes  45   b / 45   c  as shown in FIG.  6 A. 
     Subsequently, a photo-resist ion-implantation mask  46   a  is formed on the inter-level insulating layer  45   a  by using lithography, and the heavily doped n-type impurity region  44   a  is covered with the photo-resist ion-implantation mask  46   a . Boron difluoride (BF 2 ) is ion-implanted through the contact hole  45   c  into the heavily doped p-type single crystal impurity region  43   b , and a surface portion of the heavily doped p-type impurity region  43   a  becomes amorphous due to the ion bombardment expressed by mark “x”. Thus, a p-type amorphous silicon layer  43   c  is formed in the heavily doped p-type single crystal impurity region  43   b  as shown in FIG.  6 B. After the ion-implantation, the photo-resist ion-implantation mask  46   a  is stripped off, and the heavily doped n-type single crystal impurity region  44   a  is exposed again. 
     Another photo-resist ion-implantation mask  46   b  is provided on inter-level insulating layer  45   a  by using the lithography, and the p-type amorphous silicon layer  43   c  is covered with the photo-resist ion-implantation mask  46   b . Phosphorous is ion implanted through the contact hole  45   b  into the heavily doped n-type single crystal impurity region  44   a , and a surface portion of the heavily doped n-type single crystal impurity region  44   a  becomes amorphous due to the ion bombardment expressed by mark “x”. Thus, an n-type amorphous silicon layer  44   b  is formed in the surface portion as shown in FIG.  6 C. After the ion-implantation, the photo-resist ion-implantation mask  46   b  is stripped off. Subsequently, titanium is deposited over the entire surface of the resultant semiconductor structure by using a plasma-assisted chemical vapor deposition. In detail, the resultant semiconductor structure is placed in a reaction chamber of a plasma-assisted chemical vapor deposition system (not shown), and the p-type semiconductor substrate  41   a  is heated to between 450 degrees and 650 degrees centigrade. A gaseous mixture is introduced into the reaction chamber, and is regulated to between 1 torr and 100 torr. The gaseous mixture contains TiCl 4 , H 2  and Ar, and flow rates of TiCl 4 , H 2  and Ar are regulated from 5 sccm to 20 sccm, 1000 sccm to 2000 sccm and 200 sccm to 500 sccm, respectively. TiCl 4  is decomposed, and the entire surface of the semiconductor structure is covered with a titanium layer  47   a . The titanium immediately reacts with the amorphous silicon, and titanium silicide layers  43   d / 44   c  are formed on the p-type amorphous silicon layer  43   c  and the n-type amorphous silicon layer  44   b . There is no substantial time delay between the introduction of the gaseous mixture and the deposition of titanium, and the titanium silicide layer  43   d  is approximately equal in thickness to the titanium silicide layer  44   c  regardless of the conductivity type of the amorphous silicon. When the titanium layer  47   a  on the amorphous silicon layers  44   b / 43   c  reaches a certain thickness equivalent to a titanium layer ranging between 3 nanometers to 10 nanometers thick, the plasma-assisted chemical vapor deposition is completed. When the titanium layer  47   a  is assumed to be uniformly grown, the titanium silicide layers  43   d / 44   c  range between 7.5 nanometers to 25 nanometers thick. 
     Subsequently, titanium nitride is deposited from 30 nanometers to 50 nanometers thick over the titanium layer  47   a  and the titanium silicide layers  43   d / 44   c  by using a low-pressure chemical vapor deposition. The p-type silicon substrate  41   a  is heated to between 450 degrees and 650 degrees centigrade, and a gaseous mixture of TiCl 4 , NH 3  and N 2  is regulated to between 1 torr to 100 torr. The flow rates of TiCl 4 , NH 3  and N 2  are regulated from 20 sccm to 50 sccm, 50 sccm to 100 sccm and 2000 sccm to 5000 sccm, respectively. Thus, the titanium layer  47   a  and the titanium silicide layers  43   d / 44   c  are covered with a titanium nitride layer  47   b  as shown in FIG.  6 D. 
     The p-type amorphous layer  43   c  and the n-type amorphous layer  44   b  are recrystallized during the chemical vapor depositions. Even if the boron and the phosphorous are ion implanted into the peripheries beneath the field oxide layer  41   b , the boron and the phosphorous are activated during the chemical vapor depositions, and an additional p-type region  43   e  and an additional n-type region  44   d  are merged with the heavily doped p-type single crystal impurity region  43   b  and the heavily doped n-type single crystal impurity region  44   a , respectively. 
     If the activation and the recrystallization is insufficient, a large amount of reverse current flows through the junction. A post annealing may be carried out at 450 degrees to 500 degrees centigrade for 30 minutes or 700 degrees centigrade for 30 seconds after the formation of the titanium nitride layer  42 . 
     Subsequently, tungsten is deposited over the titanium nitride layer  47   b . The tungsten fills the secondary holes defined by the titanium nitride layer  47   b , and swells into a tungsten layer over the inter-level insulating layer  45   a . A photo-resist etching mask (not shown) is formed on the tungsten layer by using lithography, and the tungsten layer is selectively etched away. As a result, tungsten plugs  47   c / 47   d  are left in the secondary holes. The photo-resist etching mask is stripped off. 
     Aluminum alloy is deposited over the entire surface of the titanium nitride layer  47   b  by using a sputtering, and a photo-resist etching mask (not shown) is provided on the aluminum alloy layer by using lithography. The aluminum alloy layer, the titanium nitride layer  47   b  and the titanium layer  47   a  are selectively etched away, and form multi-layer metal wirings  48   a  and  48   b  as shown in FIG.  6 E. 
     Boron and arsenic are available for making the single crystal impurity regions amorphous. 
     As will be understood from the foregoing description, the chemically active amorphous silicon layers  43   c / 44   b  are exposed by the contact holes  45   b / 45   c , and no substantial time delay is introduced between the supply of TiCl 4  and the deposition of titanium. As a result, the titanium silicide layers  43   d / 44   c  equal in thickness are concurrently grown on the impurity regions  43   b / 44   a , respectively, and do not deteriorate electric characteristics of the semiconductor device. The delay time is clear at a low temperature less than 60 degrees in centigrade. 
     The titanium is deposited by using the chemical vapor deposition as described hereinbefore. The chemical vapor deposition creates a uniform layer rather than the sputtering, and the titanium on the amorphous layers easily exceeds 10 nanometers thick. It is not necessary to regulate the titanium on the amorphous layers to 10 nanometers or less. However, a thin titanium layer decreases the contact resistance. 
     If titanium is directly deposited on a p-type single crystal impurity region and an n-type single crystal impurity region without the ion-bombardment shown in FIGS. 6B and 6C, the titanium decomposed from TiCl 4  is deposited as indicated by plots PL 3  and PL 4  in FIG. 7, and the titanium layer on the n-type single crystal impurity region becomes much thicker than the titanium layer on the p-type single crystal impurity region. This is because of the fact that long time delay is introduced from the supply of TiCl 4  and the deposition of titanium on the p-type single crystal impurity region. As a result, the contacts suffers from either leakage current or contact resistance as described in conjunction with the prior art. 
     On the other hand, the contacts according to the present invention are small in leakage current and low in contact resistance by virtue of the titanium silicide layers equally deposited on the amorphous layers. 
     Although particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. 
     For example, the present invention is applicable to other refractory metals in so far as the refractory metal is chemically active on an amorphous silicon layer.