Method of manufacturing a vertical MOSFET having a gate electrode of polycrystalline silicon
This application is a division of application Ser. No. 08/012,419, filed Feb. 2, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a vertical metal oxide semiconductor field effect transistor (MOSFET) and a manufacturing method thereof and more particularly, to a vertical MOSFET which has a gate electrode made of polycrystalline silicon doped with p-type and n-type impurities and a manufacturing method thereof. 
     2. Description of the Related Art 
     Generally, a vertical MOSFET has a source electrode and a gate electrode made of polycrystalline silicon which are laminated on the surface of an epitaxial layer formed on a silicon substrate. The MOSFET has a drain electrode on the reverse surface of the substrate. A source region is formed in the surface area of the epitaxial layer and a channel is formed in a surface region, which is in contact with the gate oxide film, of the epitaxial layer. The substrate and epitaxial layer serve as a drain region. 
     For a conventional vertical MOSFET, phosphorus (P) is ion-implanted into a polycrystalline silicon film forming a gate electrode to make the conductivity type of the MOSFET n, so that the sheet resistance of the film becomes about 10 Ω/□ to lessen the gate resistance, namely, gate electrode resistance. If the gate resistance is small, a high switching speed is provided as understood From the following expression: 
     
         t=f(1/CR.sub.G)                                            (1) 
    
     where t is the switching speed, C is the gate capacitance, and R G  is the gate resistance. 
     On the other hand, the threshold voltage of an FET, V T , is generally represented by the following expression: 
     
         V.sub.T =φ.sub.MS -(Q.sub.SS /C.sub.0)+2φ.sub.f -(Q.sub.B /C.sub.0)(2) 
    
     where φ MS  is the work function difference between the gate electrode material and the substrate semiconductor, Q SS  is the charge on the surfaces of the gate oxide film side of the substrate semiconductor, Q B  is the charge of the depletion layer of the substrate semiconductor, C O  is the gate oxide film capacitance, and φ f  is the Fermi level of the substrate semiconductor. 
     The work function difference φ MS  is substantially 0 V between the n-type polycrystalline silicon of the gate electrode and the n-type single crystal silicon of the substrate. As understood from the expression (2), when the gate electrode is made of n-type polycrystalline silicon, the absolute value of the threshold voltage V T  is greater by about 0.9 V than the case where it is made of p-type polycrystalline silicon. Thus, it is difficult to drive the FET at low voltage although the switching speed of the FET becomes high. 
     If boron (B) rather than phosphorus is ion-implanted into a polycrystalline silicon film as a gate electrode to make conductivity type p, some boron ions pass through the gate oxide film even if thermal processing for the p-type polycrystalline silicon film is performed at a temperature of about 900° C. as a result, the FETs vary in threshold voltage V T . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide vertical MOSFET which is low in threshold voltage. 
     Another object of the invention is to provide a manufacturing method of the MOSFET which is less susceptible to threshold voltage fluctuation. 
     According to a first aspect of the invention, there is provided a vertical MOSFET, which includes a p-type gate electrode made of polycrystalline silicon formed through a gate insulating film on a first or second surface of a conductive semiconductor substrate. The gate electrode contains an n-type impurity in addition to a boron (B) impurity as a p-type impurity. 
     Preferably, the n-type impurity is at least one of phosphorus (P) and arsenic (As) impurities. Preferably, the concentration of the n-type impurity ranges from 5×10 18  to 1×10 20  cm -3 . If only one of the phosphorus and arsenic impurities is only contained, the concentration of the impurity needs to range from 5×10 18  to 1×10 20  cm -3 . If both the phosphorus and arsenic impurities are contained, the total concentration of the impurities may range from 5×10 18  to 1×10 20  cm -3 . 
     A channel, which is formed around a surface in the gate insulating film side of the semiconductor substrate, preferably has a positive polarity. 
     Manufacturing conditions and desired FET characteristics should be considered to determine the boron impurity concentration. 
     With the MOSFET of the invention, an n-type impurity in addition to boron as a p-type impurity is contained in the gate electrode made of polycrystalline silicon for preventing boron ions from passing through the gate oxide film into the semiconductor substrate; thus the MOSFET can be less susceptible to threshold voltage fluctuation. 
     Besides, since the conductivity type of the polycrystalline silicon film is p, the absolute value of the threshold voltage is smaller than that when the conductivity type is n. Therefore, the MOSFET can be driven at a low voltage. 
     According to a second aspect of the invention, there is provided a method of manufacturing a vertical MOSFET, which comprises the steps of forming an insulating film on a surface of a conductive semiconductor substrate; forming a polycrystalline silicon film on the insulating film; doping the polycrystalline silicon film with an n-type impurity; patterning the polycrystalline silicon film doped with the n-type impurity and the gate insulating film; producing an n-type polycrystalline silicon gate electrode and a gate insulating film and doping the n-type polycrystalline silicon gate electrode with a p-type impurity such as boron to thereby produce a p-type polycrystalline silicon electrode. 
     Preferably, the polycrystalline silicon film doped with the n-type impurity is doped with the p-type impurity at the same time as a step of forming a p-type region is formed in the semiconductor substrate. Apart from this step, another step of doping the polycrystalline silicon film with the p-type impurity may be added. In this case, the amount of the p-type impurity with which the polycrystalline silicon film is doped can be set independently of the impurity concentration required for forming the p-type region, so that the resistance value of the gate electrode can be easily controlled. Making the gate resistance small provides high switching speed. 
     Further, doping the polycrystalline silicon film with the p-type impurity may be suppressed at the step of forming the p-type region in the semiconductor substrate. In this case, polycrystalline silicon film is doped with the p-type impurity at a separate step from the step of forming the p-type region. 
     Preferably, the type and concentration of the n-type impurity are made the same as those for the above-mentioned FET. 
     The FET according to the first aspect of the invention can be easily provided by the manufacturing method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIGS. 1A-1C are fragmentary sectional views showing process steps of a method of manufacturing vertical MOSFET according to a first embodiment of the invention respectively; and 
     FIGS. 2A-2D are fragmentary sectional views showing process steps of a method of manufacturing vertical MOSFET according to a second embodiment of the invention respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the accompanying drawings, preferred embodiments of the invention will be described below. 
     [First embodiment] 
     FIG. 1C shows a p-channel vertical MOSFET according to a first embodiment of the invention. Here a plurality of the FETs integrated on a p +  -type silicon substrate is partially shown. 
     The FET of the first embodiment has an epitaxial layer 2 of p -  -type silicon (about 20-100 nm thick) on a surface of a p +  -type silicon substrate 1 and n-type base regions 6 in the epitaxial layer 2. An n +  -type region 7 which acts as a back gate is formed in the approximate center near the surface of the inside of each base region 6, and p +  -type source regions 8 are formed on both sides of the n +  -type region 7. 
     On the surface of the epitaxial layer 2, gate oxide films 3 made of silicon oxide are formed, each of which is straddling its adjacent base regions 6 and overlapping the peripheries of the base regions 6 and parts of the source regions 8. A channel is formed in the surface of the n-type base region 6 just below each gate oxide film 3 and has a positive polarity. Formed just above each gate oxide film 3 is a gate electrode 4 made of polycrystalline silicon (about 600 nm thick). 
     The gate electrodes 4 are doped with phosphorus ions as an n-type impurity and boron ions as a p-type impurity. The polycrystalline silicon concentration of the phosphorus ions as dopant ranges from 5×10 15  to 1×10 20  cm -3 , which is slightly lower than that of the boron ions as dopant; therefore, the conductivity type of each gate electrode 4 is p. 
     The concentration of the boron ions needs only to be set to that at which the conductivity type of each gate electrode 4 becomes p; it can be selected as desired within the range satisfying the condition according to the desired characteristics of the FET. 
     Each of the gate electrodes 4 and each of the gate oxide films 3 are covered with an interlayer insulating film 9 (about 500-1000 nm thick). The surface of the interlayer insulating film 9 is covered with a source electrode 10 made of aluminum. The source electrode 10 is connected to the n +  -type region 7 and the p +  -type source regions 8 through corresponding contact holes formed in the interlayer insulating film 9. 
     The reverse surface of the substrate 1 is covered with a drain electrode 11 made of material such as Au or Ni. 
     An electric current flows through the source electrode 10, the source region 8, the channel on the surface of the base region 6, the epitaxial layer 2, the substrate 1 and the drain electrode 11 in this order. 
     The MOSFET of the above structure is manufactured as described below: 
     First, as shown in FIG. 1A, the epitaxial layer 2 is grown on the surface of the silicon substrate 1. Laminated on the epitaxial layer 2 are a silicon oxide film 3a which is to be the gate oxide films 3 and a polycrystalline silicon film 4a which is to be the gate electrodes 4 in this order, for example, by a chemical vapor deposition (CVD) method. 
     Next, as shown in FIG. 1A, phosphorus is ion-implanted into the polycrystalline silicon film 4a at a dose in such a range as from 3×10 14  to 3×10 15  cm -2  for making the conductivity type of the film 4a n. The ion implantation condition is set so that the concentration of the phosphorus ions as dopant ranges from 5×10 15  to 1×10 20  cm -3 . 
     Next, patterning is performed so as to form transparent holes in the silicon oxide film 3a and the n-type polycrystalline silicon film 4a, as shown in FIG. 1B, thereby forming the gate oxide films 3 and the n-type polycrystalline silicon gate electrodes 4. 
     Next, phosphorus is ion-implanted into the epitaxial layer 2 to form the n-type base regions 6. Then the surfaces of the places where the p +  regions 8 are to be formed are masked, and phosphorus is ion-implanted into the base regions 6 to form the n +  regions 7. In the ion-implantation process, phosphorus is not ion-implanted into the gate electrodes 4. 
     Next, only the surfaces of the n +  regions 7 are masked, then boron is ion-implanted at a dose in such a range as from 5×10 15  to 1×10 16  cm -2  to form the p +  regions 8 within each of the base regions 6. In the process, boron is ion-implanted into the gate electrodes 4 of n-type polycrystalline silicon as well as into the base regions 6, so that the conductivity type of each of the gate electrodes 4 changes to p. 
     At this time, each of the gate electrodes 4 contains phosphorus ions as an n-type impurity and boron ions as a p-type impurity at almost identical concentrations, exactly speaking, the concentration of the boron is slightly higher than that of the phosphorus. The phosphorus ions in the gate electrodes 4 prevent boron ions from passing through the gate oxide films 3 into the base regions 6 and as a result, the FETs thus formed can be made stable in threshold voltage. 
     Next, the interlayer insulating film 9 is formed to cover the surfaces of the base regions 6, the gate oxide films 3 and the gate electrodes 4. The contact holes are formed in the insulating film 9 and then, the source electrode 10 and the drain electrode 11 are formed in this order. Thus the FETs of the structure shown in FIG. 1C can be obtained. 
     [Second embodiment] 
     FIG. 2D shows a vertical MOSFET according to a second embodiment of the invention. The FET according to the embodiment has the same structure as that according to the first embodiment except that the boron ion concentration in each gate electrode 24 is higher than that in the first embodiment. The corresponding parts to those shown in FIG. 1C are designated by the same reference numerals in FIG. 2D, and will therefore not be discussed again. 
     The gate electrode 24 made of p-type polycrystalline silicon contains boron ions at a higher concentration as compared with the gate electrode 4 in the first embodiment, so that the electrode 24 is low in resistance thereby making the switching speed of the FET higher than that in the first embodiment. 
     The MOSFET according to the second embodiment are manufactured as described below: 
     First, as with the first embodiment, as shown in FIG. 2A laminated on an epitaxial layer 2 are the silicon oxide film 3a which is to be the gate oxide films 3 and a polycrystalline silicon film 24a which is to be the gate electrodes 24 in this order. 
     Next, as shown in FIG. 2A, phosphorus is ion-implanted into the polycrystalline silicon film 24a at a dose in such a range as from 3×10 14  to 3×10 15  cm -2  for making the conductivity type of the polycrystalline silicon film 24a n, as with the first embodiment. The concentration of the phosphorus to be ion-implanted is set in the range of from 5×10 15  to 1×10 20  cm -3 . 
     Next, as shown in FIG. 2B, boron is further ion-implanted into the n-type polycrystalline silicon film 24a into which phosphorus has been ion-implanted. Although boron is ion-implanted into the film 24a as well as into the p +  -type source regions 8 at the step of forming the regions 8 by ion implantation in the first embodiment, a dedicated step of boron ion implantation for the film 24 A is added, apart from the step of forming the p-type regions 8, in the second embodiment. The dose of the boron ions in the dedicated step ranges, for example, from 5×10 15  to 1×10 16  cm -2 . 
     Next, as shown in FIG. 2C, the silicon oxide film 3a and the phosphorus- and boron-doped polycrystalline silicon film 24a are etched to form the gate oxide films 3 and the n-type or p-type gate electrodes 24. At this time, the conductivity type of the gate electrode 24 is made n or p depending on the phosphorus or boron ion concentration, whichever is the higher. 
     Next, as with the first embodiment, phosphorus is ion-implanted into the epitaxial layer 2 to form the n-type base regions 6, then the n +  region 7 are formed in the respective base regions 6. 
     Next, only the surfaces of the n +  regions 7 are masked, then boron is ion-implanted at a dose in such a range as from 5×10 15  to 1×10 16  cm -2  to form the p +  source regions 8 within each of the base regions 6. In the process, boron is ion-implanted into the gate electrodes 24 of polycrystalline silicon as well as into the base regions 6, so that the conductivity type of each of the gate electrodes 24 changes to p if it is n. If it is p, the conductivity type does not change and the boron ion concentration increases only. 
     Next, the interlayer insulating film 9 is formed to cover the surfaces of the base regions 6, the gate oxide films 3 and the gate electrodes 24. Contact holes are formed in the insulating films 9 and thereafter, the source electrode 10 and the drain electrode 11 are formed in this order. The FETs of the structure shown in FIG. 2D is thus obtained. 
     This manufacturing method according to the second embodiment enables resistance of the polycrystalline silicon film 24a to be set independently of the concentration of the boron to be ion-implanted into the source region 8 at the step of ion-implanting boron into the film 24a, so that it can provide advantages of precise control of gate resistance and improvement of the switching speed of the FET. 
     Since boron has already been ion-implanted into the polycrystalline silicon film 24a in the manufacturing method according to the second embodiment, boron is not necessarily ion-implanted at the step of forming the p +  source regions 8. However, if boron is not ion-implanted in this step, the boron ion concentration needs to be set so that the conductivity type of the polycrystalline silicon film 24a changes to p at the step (FIG. 2B) of ion-implanting boron into the film 24a. 
     If no boron is to be ion-implanted into the polycrystalline silicon film 24a at the step of forming the p +  regions 8, the remaining film 24a needs only to be masked. 
     As described above, for the vertical MOSFETs according to the first and second embodiments, phosphorus is ion-implanted into the gate electrode 4 or 24 made of the polycrystalline silicon film 4a or 24a so that the concentration of the phosphorous is in the range of 5×10 18  to 1×10 20  cm -3  and the conductivity type of the gate electrodes 4 and 24 is made p. Thus boron ions can be prevented from passing through the gate oxide films 3. Resultantly, the FETs can be stabilized in threshold voltage while the threshold voltage is reduced approximately by 0.9 V. 
     In both of the above embodiments, the channels are of p type, but may be of n type if the above-mentioned condition on the gate electrodes is satisfied. Although only phosphorus is indicated as a p-type impurity, arsenic having an equivalent nature to phosphorus as a p-type impurity can also be used.