Abstract:
An integrated circuit having at least two vertical MOS transistors, and method for manufacturing same, wherein first source/drain regions of the two vertical MOS transistors are located in an upper region of sidewalls of a trench. A second source/drain region is shared by both MOS transistors and is adjacent at a floor of the trench. Gate electrodes of the MOS transistors that are arranged at the sidewalls of the trench can be individually contacted via parts of a conductive layer that are arranged above the first source/drain regions. In a manufacturing method, such arrangement is made possible by the deposition of a conductive layer of doped polysilicon before the generation of the trench. The area of an MOS transistor can amount to 4F 2 .

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of The Invention  
           [0002]    The present invention is generally directed to integrated circuits that require individually controllable MOS transistors.  
           [0003]    2. Description of the Prior Art  
           [0004]    An increased packing density as well as a shortening of the connections between components are desirable in the development of new integrated circuits. This is typically accomplished using planar silicon technology.  
           [0005]    A diminution of the areas of MOS transistors can be achieved, for example, with a vertical, instead of a horizontal, channel course. An earlier German Letters Patent proposed a vertical MOS transistor whose source and drain regions are arranged laterally and in differing depth. The channel thereof proceeds substantially perpendicularly relative to the surface of the circuit along the sidewalls of a depression. The MOS transistor is surrounded by an insulation structure. The saving in area per transistor amounts to approximately 4F 2  compared to the area of planar transistors. At approximately 16F 2 , however, the area of this vertical MOS transistor continues to be large.  
           [0006]    U.S. Pat. No. 5,376,575 disclosed the employment of vertical MOS transistors for DRAM cell arrangements; i.e., memory cell arrangements having dynamic random access. Each vertical MOS transistor in the disclosed manufacturing method includes two oppositely residing sidewalls of a trench situated in a substrate. First doped regions that act as first source/drain regions of the MOS transistor are provided in the upper region of the sidewalls. Second doped regions that act as second source/drain regions are arranged under the first source/drain regions. Second source/drain regions of MOS transistors neighboring along the trench are connected to one another via a bit line. Surfaces of the sidewalls are provided with a gate oxide. A gate electrode that covers surfaces of the gate oxides lying opposite one another is provided for the MOS transistor. Shallow trenches are provided in the substrate which proceed transversely relative to bit lines, and word lines proceed transversely to the bit lines arranged therein. The word lines are laterally adjacent to gate electrodes and are thus connected thereto. Gate electrodes of MOS transistors neighboring along a word line are connected to one another via the word line. The smallest area of a memory cell obtainable with this method amounts to 6F 2 . What is disadvantageous about vertical MOS transistors of this type is that they can only be employed for circuit arrangements wherein the gate electrodes of the MOS transistors are connected to one another. The MOS transistors cannot be individually driven.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed toward an integrated circuit having MOS transistors that can be individually driven and which can occupy a relatively small area as well as a manufacturing method for such a circuit.  
           [0008]    In an embodiment of the present invention, a first MOS transistor is adjacent to a first sidewall of a trench and a second MOS transistor is adjacent to a second sidewall of the trench lying opposite the first sidewall. The first MOS transistor and the second MOS transistor lie opposite one another. First source/drain regions of the two MOS transistors are located in the upper region of the sidewalls. A second source/drain region is divided by the two MOS transistors and is adjacent to a floor of the trench. The sidewalls of the trench are provided with a gate dielectric. The gate electrodes of the MOS transistor are arranged in the trench at the sidewalls of the trench. The gate electrodes can be individually driven via parts of a conductive layer that are arranged above the first source/drain regions.  
           [0009]    An inventive MOS transistor can be manufactured with an area of 4F 2 . It is important for the manufacturing method that a conductive layer is produced before generation of the trench. The gate electrodes can be individually contacted via parts of the conductive layer.  
           [0010]    The present invention offers the advantage that the MOS transistors can be individually driven. Such MOS transistors can thus be employed, for example, for radio-frequency circuit arrangements or for inverters. It is within the scope of the present invention to have more than two MOS transistors at a trench for manufacturing large circuit arrangements with enhanced packing density. It is also within the scope of the present invention to generate a plurality of trenches arranged parallel to each other at which MOS transistors are formed for manufacturing large circuit arrangements with enhanced packing density.  
           [0011]    It is further within the scope of the present invention that vertical transistors of a first trench are complimentary to vertical transistors of a second trench. Also, the present invention contemplates generating a doped, well-shaped first region by implantation in which the first trench is generated and a doped, well-shaped second region in which the second trench is generated. In this case, it is advantageous to employ a first mask for the implantation of the first region that subsequently serves as mask for producing a second mask which is employed in the implantation of the second region.  
           [0012]    It is advantageous to employ structured layers that contain silicon nitride as masks when insulating structures are produced by thermal oxidation since such masks are heat-resistant and impermeable for oxidants.  
           [0013]    Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and from the Drawings.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 shows a first substrate after a first mask, a first photoresist mask as well as a doped, first region have been produced as the surface of the substrate.  
         [0015]    [0015]FIG. 2 shows the first substrate after a second mask was formed with the assistance of the first mask and after a doped, second region was produced with the assistance of the second mask.  
         [0016]    [0016]FIG. 3 shows the first substrate after a first insulating structure, a doped, third region and a doped, fourth region were produced. The first insulating structure separates the third region from the fourth region.  
         [0017]    [0017]FIG. 4 shows the first substrate after a first insulating layer, a layer of doped polysilicon, a second insulating layer as well as a first trench and a second trench have been produced. First source/drain regions were thereby defined.  
         [0018]    [0018]FIG. 5 shows the first substrate after the generation of both a second source/drain region and a third source/drain region with the assistance of a second insulating structure SiO 2 .  
         [0019]    [0019]FIG. 6 shows the first substrate after the generation of a third insulating layer and after the generation of spacers of doped polysilicon.  
         [0020]    [0020]FIG. 7 shows the first substrate after a gate dielectric was formed from the third insulating layer and gate electrodes were generated and insulated from one another.  
         [0021]    [0021]FIG. 8 shows a second substrate after a first trench, a second trench, a first region, a second region, first source/drain regions, a second source/drain region, a third source/drain region, a first insulating structure, a second insulating structure, a layer of doped polysilicon and a second insulating structure were generated analogous to FIG. 5 and a structure of silicon nitride was generated.  
         [0022]    [0022]FIG. 9 shows the second substrate after a gate dielectric was generated.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    According to a first embodiment, a first substrate  1  is a semiconductor wafer that contains silicon at a surface O. An n-doped first region Ge 1  and a p-doped second region Ge 2  are generated at the surface O by implantation such that they laterally adjoin one another (see FIG. 2). The dopant concentration of the first region Ge 1  amounts to approximately 10 17  cm −3 . The dopant concentration of the second region Ge 2  amounts to approximately 10 17  cm −3 . The first region Ge 1  and the second region Ge 2  will include channel regions of four MOS transistors yet to be produced.  
         [0024]    So that the first region Ge 1  and the second region Ge 2  laterally adjoin one another to increase the packing density, silicon nitride having a thickness of approximately 150 nm is deposited on the surface O before they are generated. Silicon nitride is etched with the assistance of a first photo-resist mask P 1  that is produced by a photolithographic method, as a result whereof a first mask M 1  having the same shape as the first photoresist mask P 1  is formed and which is more heat-resistant than the first photoresist mask P 1  (see FIG. 1). A mixture of O 2  and SF 6  is suitable as etchant. The first photoresist mask P 1  also serves as a mask in the subsequent generation of the first region Ge 1  by implantation (see FIG. 1).  
         [0025]    The first photoresist mask P 1  is subsequently removed. A second mask M 2  (see FIG. 2) is formed at the surface O above the first region Ge 1  by thermal oxidation with the assistance of the first mask M 1  that is heat-resistant and impermeable to oxidants. As a result, the first mask M 1  adjoins the second mask M 2 . The thickness of the second mask M 2  amounts to approximately 400 nm.  
         [0026]    Subsequently, silicon nitride is selectively etched to SiO 2 , for example with H 3 PO 4 , as a result whereof the first mask M 1  is removed. The second region Ge 2  is generated by implantation, whereby the second mask M 2  serves as a mask (see FIG. 2). Since the first mask M 1  adjoined the second mask M 2 , the first region Ge 1  adjoins the second region Ge 2 . Subsequently, SiO 2  is selectively etched relative to silicon such that the second mask M 2  is removed. HF, for example, is suitable as etchant.  
         [0027]    A first insulating structure I 1  that encompasses boundary regions between the first region Ge 1  and the second region Ge 2  is formed by thermal oxidation with the assistance of a third mask (not shown) of silicon nitride that does not cover boundary regions between the first region Ge 1  and the second region Ge 2  and that is produced—analogous to the first mask M 1 —with the assistance of a second photoresist mask (not shown) having the same shape as the third mask (see FIG. 3). Subsequently, the third mask is removed by etching. The first insulating structure I 1  will separate a first source/drain region S/D 1   b  (yet to be produced) of a second MOS transistor formed in the first region Ge 1  from a first source/drain region S/D 1   c  (yet to be produced) of a third MOS transistor in the second region Ge 2 .  
         [0028]    A third region Ge 3  that is n-doped (see FIG. 3) is generated at the surface O in the second region Ge 2  by implantation with the assistance of a third photoresist mask (not shown) that covers at least the first region Ge 1 . A fourth region Ge 4  that is p-doped (see FIG. 3) is generated by implantation within the first region Ge 1  with the assistance of a fourth photoresist mask (not shown) that covers at least the second region Ge 2 . The first source/drain regions S/D 1   a , S/D 1   b , S/D 1   c , S/D 1   d  of the four MOS transistors arise from the third region Ge 3  and the fourth region Ge 4 .  
         [0029]    An insulating layer S having a thickness of approximately 50 nm (see FIG. 4) is generated at the surface O by, for example, thermal oxidation. The insulating layer S will contribute to the electrical insulation of the first source/drain regions S/D 1   a,  S/D 1   c , S/D 1   d  of the four MOS transistors from gate electrodes Ga 1 , Ga 2 , Ga 3 , Ga 4  of the four MOS transistors to be generated. Subsequently, a conductive layer L of doped polysilicon is deposited. The conductive layer L will serve for the contacting of the gate electrodes Ga 1 , Ga 2 , Ga 3 , Ga 4 , wherein such contacting is generated above the first source/drain regions S/D 1   a , S/D 1   b , S/D 1   c , S/D 1   d.    
         [0030]    A fourth mask M 4  of SiO 2  having a thickness of approximately 200 nm is generated over the conductive layer L in that SiO 2  is deposited and subsequently structured with the assistance of a fifth photoresist mask (not shown). The fourth mask M 4  covers at least the insulating structure I 1 . Subsequently, a first trench G 1  which parts each of the conductive layer L, the insulating layer S and the fourth region Ge 4  and which extends into the first region Ge 1 , and a second trench G 2  which parts each of the conductive layer L, the insulating layer S and the third region Ge 3  and which extends into the second region Ge 2  are generated (see FIG. 2). This occurs with the assistance of the fourth mask M 4 , whereby the doped polysilicon, SiO 2  and silicon are anisotropically etched. The first trench G 1  and the second trench G 2  proceed substantially parallel and exhibit a depth of approximately 600 nm from the surface O, a width of approximately 500 nm and a length of approximately 100 μm. The fourth region Ge 4  is divided by the first trench G 1  into the first source/drain region S/Da of the first MOS transistor and into the first source/drain region S/D 1   b  of the second MOS transistor. The third region Ge 3  is divided by the second trench G 2  into the first source/drain region S/D 1   c  of the third MOS transistor and into the first source/drain region S/D 1   d  of the fourth MOS transistor.  
         [0031]    Subsequently, SiO 2  is conformally deposited in a TEOS method and is anisotropically re-etched, as a result whereof a second insulating structure  12  is formed that covers sidewalls of the first trench G 1  and of the second trench G 2  in the form of spacers. A p-doped, second source/drain region S/D 2   a/b  that belongs to both the first MOS transistor and the second MOS transistor is generated by implantation at the floor of the first trench G 1  with the assistance of a sixth photoresist mask (not shown) that covers at least the second region Ge 2  but not the first trench G 1  (see FIG. 5). An n-doped second source/drain region S/D 2   c/d  that belongs to the third MOS transistor and to the fourth MOS transistor is generated by implantation (see FIG. 5) at the floor of the second trench G 2  with the assistance of a seventh photoresist mask (not shown) that covers at least the first region Ge 1  but not the second trench G 2 . The second insulating structure  12  thereby serves as mask, as a result whereof the lateral expanse of the p-doped second source/drain region S/D 2   a/b  and the lateral expanse of the n-doped second source/drain region S/D 2   c/d  are kept small. Due to the second insulating structure  12 , moreover, the sidewalls of the first trench G 1  and of the second trench G 2  are protected against implantation. Subsequently, SiO 2  is selectively etched relative to silicon, as a result whereof the second insulating structure  12  and the fourth mask M 4  are removed.  
         [0032]    For producing a gate dielectric Gd, a third insulating structure  13  is generated by thermal oxidation (see FIG. 6). The third insulating structure  13  is especially thick at the floors of the first trench G 1  and of the second trench G 2  since highly doped silicon oxidizes more highly at temperatures below 900° C. than doped polysilicon or lightly doped silicon. The thickness of the insulating structure  13  amounts to approximately 15 nm at the sidewalls of the first trench G 1  and of the second trench G 2 . Doped polysilicon having a thickness of approximately 50 nm is deposited thereover. The doped polysilicon is anisotropically re-etched so that only spacers Sp (see FIG. 6) remain at the sidewalls of the first trench G 1  and of the second trench G 2 . By isotropic etching of SiO 2  selectively to doped polysilicon, the third insulating structure  13  is structured such that it covers only the floors and the sidewalls of the first trench G 1  and of the second trench G 2  (see FIG. 7). The conductive layer L is thus uncovered. A part of the third insulating structure  13  at the sidewalls of the first trench G 1  and of the second trench G 2  is protected by the spacers Sp during this etching step and serves as gate dielectric Gd. A part of the third insulating structure  13  of the floors of the first trench G 1  and of the second trench G 2  will insulate the p-doped, second source/drain region S/D 2   a/b  and of the n-doped second source/drain region S/D 2   c/d  from the gate electrodes Ga 1 , Ga 2 , Ga 3 , Ga 4  to be produced for the four MOS transistors to be produced.  
         [0033]    For forming the gate electrodes Ga 1 , Ga 2 , Ga 3 , Ga 4 , conductive material is deposited and anisotropically re-etched, so that the gate electrodes Ga 1 , Ga 2 , Ga 3 , Ga 4  arise at the sidewalls of the first trench G 1  and of the second trench G 2  in the form of spacers and are connected to the conductive layer L.  
         [0034]    With the assistance of a seventh photoresist mask (not shown) that covers regions above the first insulation structure  11 , a gate electrode Ga 2  of the second MOS transistor is insulated from a gate electrode Ga 3  of the third MOS transistor (see FIG. 7). The four MOS transistors are formed in this way. Their channels proceed along the sidewalls of the first trench G 1  and of the second trench G 2 . It is thus a matter of vertical transistors. The gate electrodes Ga 1 , Ga 2 , Ga 3 , Ga 4  are individually driveable.  
         [0035]    In a second exemplary embodiment, a second substrate  1 ′ that contains a first trench G 1 ′, a second trench G 2 ′, a structured, first region Ge 1 ′, a structured, second region Ge 2 ′, for first source/drain regions S/D 1   a/′ , S/D 1   b ′, S/D 1   c ′, S/D 1   d ′, two second source/drain regions S/D 2   a/b ′, S/Dc/d′ a first insulating structure I 1 ′, a second insulating structure I 2 ′, a layer of doped polysilicon L′ and a fourth mask (not shown) is provided analogous to the first exemplary embodiment, wherein the second substrate I′ contains silicon at a surface O′. The fourth mask is removed by anisotropic etching of SiO 2  and the second insulating structure I 2 ′ is etched back until sidewalls of the layer of doped polysilicon L′ are uncovered. Subsequently, silicon nitride is grown surface-wide. Silicon nitride thereby grows more thickly on the conductive material than on SiO 2 . As a result of isotropic etching of silicon nitride, a structure N of silicon nitride (see FIG. 8) remains only at the sidewalls and on a surface of the layer of doped polysilicon L′. The second insulating structure I 2 ′ is removed by isotropic etching with, for example, HF. A third insulating structure I 3 ′ is generated by thermal oxidation, whereby the structure N of silicon nitride protects the layer of doped polysilicon L′ (see FIG. 9). The third insulating structure I 3 ′ at sidewalls of the first trench G 1 ′ and of the second trench G 2 ′ serves as gate dielectric Gd′. The structure N of silicon nitride is then removed in an etching step. Subsequently, gate electrodes Ga 1 ′ Ga 2 ′, Ga 3 ′, Ga 4 ′ are generated, analogous to the first exemplary embodiment.  
         [0036]    Many variations of the exemplary embodiments described herein are within the contemplation and scope of the present invention. In particular, the dimensions of the described layers, regions and trenches can be adapted to the respective requirements. The same is also true of the proposed dopant concentrations. Structures and layers of SiO 2  can, in particular, be produced by thermal oxidation or by a deposition method. Instead of employing SiO 2  as material of the gate dielectric, the employment of other dielectrics is also conceivable, such as, for example, silicon nitride, Al 2 O 3  or TaO 5 . The dielectric can also be composed of three sub-layers. In this case, a lower sub-layer and an upper sub-layer contain SiO 2  and a middle sub-layer contains silicon nitride. Polysilicon can be doped both during and after the deposition. Instead of polysilicon, metal silicides and/or metals can be employed. The first insulating structure can also be formed as trenches filled with SiO 2 .  
         [0037]    Although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the invention as set forth in the hereafter appended claims.