Abstract:
In fabricating a buried p-channel MOS transistor using an n-type substrate, a shallow n-type diffused layer is formed by ion implantation in each of intended source and drain regions so as to become oppositely adjacent to the shallow p-type diffused layer under the gate electrode. After that p-type diffused layers to serve as source and drain are formed by ion implantation through the n-type diffused layers, and the implanted impurities are activated. In consequence, impurity concentration at the substrate surface becomes lower in the section right under each end of the gate electrode than in the gate middle sections. This measure brings about suppression of the short channel effect inherent to conventional buried-channel MOS transistors and makes it possible to shorten the physical gate length.

Description:
This application is a division of application Ser. No. 08/747,745, filed Nov. 12, 1996, now U.S. Pat. No. 5,719,430 which is a continuation of application Ser. No. 08/681,516, filed Jul. 23, 1996, abandoned, which is a continuation of application Ser. No. 08/230,778, filed Apr. 21, 1994, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an improved buried-channel MOS transistor and a process of producing the improved transistor. 
     Conventional buried p-channel MOS transistors have a shallow p-type diffused layer in an n-type substrate surface under the gate electrode and gate oxide film in order to adequately lower the threshold voltage. The shallow p-type diffused layer provides a channel between source and drain regions which are more heavily doped p-type diffused layers. 
     An advantage of buried-channel MOS transistors over surface channel MOS transistors is increased mobility of electric charge because of little scattering at the interface between the substrate and the gate oxide film. On the other hand, a disadvantage of buried-channel MOS transistors is susceptibility to the so-called short channel effect. That is, since the source and drain regions and the channel region are of the same type conductivity, the application of a voltage to the drain causes wider expansion of the depletion layer than in surface channel MOS transistors and hence is liable to result in a phenomenon called punch-through, i.e. flow of current between source and drain as a result of the drain voltage directly forcing down the potential energy at the boundary between source and channel. In this case the punch-through occurs in the vicinity of the substrate surface and hence is called surface punch-through. The fundamental reason for the liability to punch-through is that effective channel length becomes considerably shorter than the physical length of the gate electrode. Therefore, it is difficult to further miniaturize buried-channel MOS transistors by shortening the physical length of the gate electrode. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved buried-type MOS transistor in which effective channel length relative to the physical gate length is extended so that susceptibility to the short channel effect is diminished. 
     It is another object of the invention to provide a process of fabricating a buried-channel MOS transistor improved in the above stated point. 
     A buried-channel MOS transistor according to the invention has a semiconductor substrate of first-type conductivity, a gate dielectric film formed on a surface of the substratea in a limited area, a first diffused layer of opposite second-type conductivity which is formed in a surface region of the substrate under the gate dielectric film so as to provide a channel region, a gate electrode which is formed on the gate dielectric layer, a pair of second diffused layers of the second-type conductivity which are formed in the substrate on two opposite sides of the first diffused layer so as to provide source and drain regions and are higher in impurity concentration and greater in depth from the substrate surface than the first diffused layer. As the principal feature of the invention, the channel region includes two border regions each of which is a diffused layer in a surface region of the substrate, is lower in impurity concentration than the first diffused layer and intervenes between the first diffused layer and one of the second diffused layers such that impurity concentration at the surface of the substrate is lower in a section right under each end face of the gate electrode than in sections under a central part of the gate electrode. 
     A fabrication process according to the invention essentially comprises the steps of (a) forming a gate dielectric film on a surface of a semiconductor substrate of first-type conductivity, (b) forming a first diffused layer of opposite second-type conductivity in a surface region of the substrate under the gate dielectric film by ion implantation of a second-type impurity, (c) forming a gate electrode having two opposite end faces on the gate dielectric film, (d) forming a pair of auxiliary diffused layers of the first-type conductivity in surface regions of the substrate so as to become oppositely adjacent to the first diffused layer existing under the gate electrode by ion implantation of a first-type impurity by using the gate electrode as mask, (e) forming a pair of second diffused layers of the second-type conductivity in intended source and drain regions of the substrate by ion implantation of a second-type impurity through major areas of the auxiliary diffused layers by using the gate electrode as mask, the second diffused layers being higher in ion concentration and greater in depth from the substrate surface than the first diffused layer, and (f) activating the impurities already implanted into the substrate by a heat treatment. 
     In this fabrication process the novel point is the step (d). That is, a shallow diffused layer of the same type conductivity as the substrate is formed in each of intended source and drain regions before forming the diffused layers of opposite type conductivity to provide source and drain regions. In consequence, the obtained MOS transistor has channel border regions wherein impurity concentration at the substrate surface is lower than in the main part of the channel, and the effective channel length extends. Therefore, a buried-channel MOS transistor according to the invention is hardly susceptible to the short channel effect, and hence the invention makes it possible to further miniaturize buried-channel MOS transitors by shortening the physical length of gate electrode. For example, in a buried p-channel MOS transistor it is possible to shorten the gate length to about 0.25 μm without suffering from punch-through. 
     Besides, the invention has the effect of suppressing deterioration of MOS transistors by the influence of hot electrons, because lowering of impurity concentration at the boundaries between the channel and the source and drain regions brings about relaxation of electric field. In this regard, the invention makes a contribution to enhancement of reliability of buried-channel MOS transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(A) to 1(E) illustrate, in schematic cross-sectinal views, a first process of fabricating a buried-channel MOS transistor according to the invention; 
     FIG. 2 is a chart showing simulated net impurity profiles in a MOS transistor produced by the process of FIGS. 1(A) to 1(E); 
     FIG. 3 is a graph showing the result of an experiment as to shortening of gate length that can be accomplished by the invention; 
     FIGS. 4(A) to 4(D) illustrate, in schematic cross-sectional views, a second process of fabricating a buried-channel MOS transistor according to the invention; 
     FIGS. 5(A) to 5(C) illustrate, in schematic cross-sectional views, a third process of fabricating a buried-channel MOS transistor according to the invention; and 
     FIGS. 6(A) to 6(D) illustrate a conventional process of fabricating a buried-channel MOS transistor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1(A) to 1(E) illustrate a process of fabricating a buried p-channel MOS transistor as a first embodiment of the invention. The completed structure of the transistor is seen in FIG. 1(E). 
     Referring to FIG. 1(A), first a relatively thick oxide film 12 is formed on an n-type silicon substrate 10 so as to provide field regions by which active regions are defined, and in each active region a gate oxide film 14 is formed on the silicon substrate surface by a suitable method such as thermal oxidation. Next, a shallow p-type diffused layer 16 is formed under the gate oxide film 14 by low-energy ion implantation of a p-type impurity into the n-type silicon substrate 10 so as to realize a desired threshold voltage. The p-type diffused layer 16 is relatively low in impurity concentration. For example, boron is implanted at 10-30 keV at a dose of 10 12  to 10 13  /cm 2 . 
     Referring to FIG. 1(B), a gate electrode 18 of polycrystallin silicon is formed on the gate oxide film 14 by using a patterning technique. Next, using the gate electrode 18 as mask, an n-type impurity is ion-implanted into the n-type silicon substrate 12 at low energy to thereby form a shallow n-type diffused layer 20 on each side of the gate electrode 18. For example, phosphorus is implanted at 10-30 keV at a dose of 10 12  to 10 14  /cm 2 . The p-type diffused layer 16 remains in the region right under the gate electrode 18. Next, an oxide film is grown on the substrate surface over the entire area inclusive of the gate electrode 18, for example, to a thickness of 150-200 nm, and then that oxide film is mostly removed together with the initially formed oxide film 14 by anisotropic etching so as to leave an oxide sidewall 22 on each of two opposite end faces of the gate electrode 18. Of course the oxide film 14 remains under the gate electrode 18. 
     Referring to FIG. 1(C), using the gate electrode 18 and sidewalls 22 as mask, a p-type impurity is obliquely ion-implanted into the substrate 10 to thereby form a p-type diffused layer 24, which is deeper than the precedingly formed p-type and n-type diffused layers 16 and 20, on each side of the gate electrode 18. For example, boron is implanted at an angle of about 45° with the substrate surface at 30-60 keV at a dose of 10 13  to 10 14  /cm 2 . On each side of the gate electrode 18 a major portion of the n-type diffused layer 20 turns into a surface portion of the p-type diffused layer 24, but under the sidewall 22 an edge portion 20a of the n-type diffused layer 20 remains between the p-type diffused layer 24 and the p-type diffused layer 16 under the gate electrode 10. In the region under each sidewall 22 the concentration of the implanted p-type impurity becomes highest at a small depth from the substrate surface. 
     Referring to FIG. 1(D), using the gate electrode 18 and sidewalls 22 as mask, a p-type impurity is ion implanted into the substrate 10 at a relatively high dose to thereby form a pair of p-type diffused layers 26. In this case the ion implantation is performed perpendicularly to the substrate surface or at a small angle with the perpendicular, and the p-type diffused layers 26 are made deeper than the precedingly formed p-type diffused layers 24. For example, boron difluoride BF 2  is implanted at 40-70 keV at a dose of 10 15  to 10 16  /cm 2 . 
     In the above described process the n-type diffused layers 20 and the p-type diffused layers 24, 26 are formed in a self-aligned manner. 
     After that a heat treatment is made to activate the impurities implanted by the steps illustrated in FIGS. 1(A) to 1(D). 
     Referring to FIG. 1(E), a dielectric layer 32 for interlayer insulation is formed on the intermediate in the state of FIG. 1(D). Then contact holes 33 are formed in the dielectric layer 32 to provide access to the p-type diffused layers 26, and by using the contact holes 33 metal wirings 34 are made for the respective p-type diffused layers 26. 
     In the MOS transistor shown in FIG. 1(E) a complementary diffused layer 30, which originates from an edge portion 20a of the n-type diffused layer 20, intervenes between the p-type diffused layer 16 under the gate electrode 18 and the p-type diffused layer 24 on each side. The complementary diffused layers 30 are p-type or n-type diffused layers lower in impurity concentration than the p-type diffused layer 16, because p-type impurities diffuse into the edge portions 20a of the precedently formed n-type diffused layer 20 during the ion implantation operations to form the p-type diffused layers 24 and 26 and/or the heat treatment for activation of the implanted impurities. The diffused layer 30 on each side exists right under an end of the gate electrode 18 and a part of the sidewall 22. In this MOS transistor the p-type diffused layers 24 and 26 on each side constitute a source or drain region. For each source/drain region the p-type diffused layer 24, which is relatively low in impurity concentration, provides a junction region adjacent to the gate. That is, this MOS transistor employs a so-called lightly-doped drain (LDD) structure. The p-type diffused layer 16 and the complementary diffused layers 30 jointly provide a channel region. That is, the complementary diffused layers 30 serve as channel border regions. Therefore, in this MOS transistor the boundary between either of the source and drain regions and the channel region is not right beneath an end of the gate electrode 18: the boundary is shifted to some extent toward the source or drain region. In consequence, effective channel length is extended without extending the gate length. 
     The primary merit of the presence of the above described diffused layers or channel border regions 30 is that the possibility of surface punch-through is lessened. Besides, the provision of the lightly doped diffused layers 30 brings about relaxation of electric field at the boundaries between the channel region and the source and drain regions and hence is effective for suppression of the generation of hot electrons which are liable to deteriorate the MOS transistor. The effects of suppressing the occurrence of surface punch-through and the generation of hot electrons become highest when the depth of the n-type diffused layer 20 in FIG. 1(B) from the substrate surface is nearly the same as the depth of the p-type diffused layer 16. 
     In forming the n-type diffused layer 20 in FIG. 1(B) by ion implantation it is necessary that the projection range of the ion implantation is smaller than that of the ion implantation for forming the p-type diffused layer 24. 
     With respect to a MOS transistor of the structure shown in FIG. 1(E), FIG. 2 shows the result of calculation of distributions of net impurity concentrations (numbers/cm 3 ) in the silicon substrate in the thickness direction. The curve A represents impurity concentrations in the middle section of the channel region and the curve B impurity concentrations in the section right beneath each end of the gate electrode. It is seen that at the substrate surface the impurity concentration in the channel border region right under each end of the gate is lower than the impurity concentration in the channel middle region beneath the center of the gate electrode. Under the conditions employed for this calculation the channel border regions right beneath the gate ends are p-type diffused layers very low in impurity concentration, but under some other conditions the channel border regions become n-type diffused layers very low in impurity concentration. 
     An experiment was conducted to confirm the possibility of shortening the physical length of the gate electrode 18 in a buried p-channel MOS transistor according to the invention. In the experiment the gate electrode 18 was formed in various lengths, and in forming the n-type diffused layers 20 by ion implantation of phosphorus at constant energy of 20 keV the phosphorus implantation dose was varied. On each of the obtained MOS transistor samples a constant voltage was applied to the drain while the source, gate and the substrate were grounded to test whether surface punch-through occurs or not. As a result of the experiment, FIG. 3 shows the relationship between the phosphorus implantation dose and a minimal length of the gate electrode to satisfy the condition that surface punch-through should be avoided. It is evident that the physical gate length can be shortened by forming the n-type diffused layers 20 before forming the p-type diffused layers which become the source and drain regions. 
     For comparison, FIGS. 6(A) to 6(d) illustrate a conventional process of fabricating a buried p-channel MOS transistor. The completed structure of the transistor is seen in FIG. 6(D). 
     As shown in FIG. 6(A) which is identical with FIG. 1(A), a field oxide film 12 and a gate oxide film 14 are formed on an n-type silicon substrate 10, and then a shallow and lightly doped p-type diffused layer 16 is formed under the gate oxide film 14 by ion implantation. 
     Referring to FIG. 6(B), a gate electrode 18 of n-type polycrystalline silicon is formed on the gate oxide film 14. Next, using the gate electrode 18 as mask, ion implantation is performed to form a pair of p-type diffused layers 24 which are deeper than the p-type diffused layer 16 and higher in impurity concentration. 
     Referring to FIG. 6(C), oxide sidewalls 22 are formed on the end faces of the gate electrode 18 by first growing an oxide film on the substrate surface over the entire area inclusive of the gate electrode 18 and then removing most of the oxide film by anisotropic etching. Next, using the gate electrode 18 and sidewalls 22 as mask, a p-type impurity is ion-implanted into the substrate to form a pair of p-type diffused layers 26 which are higher in impurity concentration than the precedingly formed p-type diffused layers 24. After that a heat treatment is made to activate the impurities already implanted into the substrate 10. Thus, in this process no n-type impurity is implanted into the substate. 
     Finally, as shown in FIG. 6(D), a dielectric layer 32 for interlayer insulation is formed over the entire area, and metal wirings 34 for the p-type diffused layers 26 are made by using contact holes 33 in the dielectric layer 32. 
     In the thus produced MOS transistor the p-type diffused layers 24 and 26 on each side of the gate electrode 18 constitute a source or drain region, and the shallow p-type diffused layer 16 under the gate electrode 18 becomes a channel region. Therefore, in this MOS transistor the channel length does not exceed the length of the gate electrode. 
     As a second embodiment of the invention, FIGS. 4(A) to 4(D) illustrate another process of fabricating a buried p-channel MOS transistor. 
     Referring to FIG. 4(A), in the same way as in the first embodiment, a field oxide film 12, a gate oxide film 14 and a gate electrode 18 are formed on an n-type silicon substrate 10, and a shallow and lightly doped p-type diffused layer 16 is formed under the gate oxide film 14, and then an n-type diffused layer 20 is formed on each side of the gate electrode 18. 
     Next, referring to FIG. 4(B), using the gate electrode 18 as mask, a p-type impurity is ion-implanted into the substrate 10 perpendicularly to the substrate surface or at a small angle with the perpendicular to thereby form a pair of p-type diffused layers 24. For example, boron is implanted at 15-70 keV at a dose of 10 12  to 10 14  /cm 2 . 
     Referring to FIG. 4(C), oxide sidewalls 22 are formed on the end faces of the gate electrode 18 by the method described hereinbefore. Next, using the gate electrode 18 and sidewalls 22 as mask, a p-type impurity is ion-implanted into the substrate perpendicularly to the substrate surface or at a small angle with the perpendicular to thereby form a pair of p-type diffused layers 26 which are higher in impurity concentration than the precedingly formed p-type diffused layers 24. For example, BF 2  is implanted at 40-70 keV at a dose of 10 15  to 10 16  /cm 2 . After that a heat treatment is made to activate the impurities already implanted into the substrate 10. 
     As shown in FIG. 4(D), the MOS transistor is completed by forming a dielectric layer 32 for interlayer insulation and then making metal wirings 34 for the source and drain regions by using contact holes 33 in the dielectric layer 32. In this MOS transistor the main part of the channel region is the p-type diffused layer 16, but the channel region includes channel border regions 30 each of which exists right under an end of the gate electrode 18 and the sidewall 22. The channel border regions 30 are p-type or n-type diffused layers lower in impurity concen-tration than the p-type diffused layer 16 as a result of diffusion of p-type impurities into edge portions 20a of the n-type diffused layer 20 during the ion implantation operations and/or the activating heat treatment. Therefore, the MOS transistor of FIG. 4(D) has the same advantages as the MOS transistor of FIG. 1(E). 
     As a third embodiment of the invention, FIGS. 5(A) to 5(C) illustrate another process of fabricating a buried p-channel MOS transistor. 
     FIG. 5(A) is identical with FIG. 1(B). That is, this process incorporates an initial part of the first embodiment until the formation of sidewalls 22 on the gate electrode 18 after forming the n-type diffused layer 20. 
     Referring to FIG. 5(B), using the gate electrode 18 and sidewalls 22 as mask, a p-type impurity is ion-implanted into the n-type substrate 10 perpendicularly or obliquely to the substrate surface to thereby form a pair of p-type diffused layers 26A which are higher in impurity concentration than the p-type diffused layer 16 under the gate electrode 18. For example, BF 2  is implanted at an angle of 30-45°, at 40-80 keV and at a dose of 10 15  to 10 16  /cm 2 . After that a heat treatment is made to activate the impurities already implanted into the substrate. For example, the heat treatment is carried out at 850° C. for about 30 min. 
     Referring to FIG. 5(C), the dielectric layer 32 for interlayer insulation and the metal wirings 34 for the source and drain regions 26A are provided in the same manner as in the preceding embodiments. In this MOS transistor the channel region includes channel border regions 30 each of which is right under an end of the gate electrode 18 and a part of the sidewall 22. The channel border regions 30 are p-type or n-type diffused layers lower in impurity concentration than the p-type diffused layer (the main part of the channel) 16 as a result of diffusion of p-type impurities into the edge portions 20a of the n-type diffused layer 20. Therefore, this MOS transistor has the same advantages as the MOS transistor of FIG. 1(E). 
     In any of the above described embodiments of the invention the material of the gate electrode 18 may be changed from polycrystalline silicon to a silicide of a metal having a high melting point, such as tungsten silicide by way of example. 
     Of course it is optional to modify any of the above described embodiments to a process of fabricating a buried n-channel MOS transistor by using a p-type semiconductor substrate and reversing the &#34;p-type&#34; and &#34;n-type&#34; mentioned in each embodiment.