Abstract:
For particularly simple and targeted formations of a diffusion region, an interfacial region of a semiconductor substrate is subjected to a thermal transformation process and thereby carry out the thermally activated diffusion of a dopant in a substantially directed form, in particular in substantially a preferential direction, by interaction of a provided dopant with a transforming interfacial region.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a method for forming a diffusion region and in particular to a method for forming a diffusion contact or the like in a semiconductor substrate and in particular for connecting a DRAM memory cell to a vertical MOSFET or the like.  
           [0003]    In the case of integrated semiconductor circuits, a large number of components bearing different functions are formed in a very confined space in a semiconductor substrate or the like. In this case, various components must either be connected to one another or adequately insulated from one another.  
           [0004]    Certain contacts are made for example as diffusion contacts. In this case, a region of the semiconductor substrate that is initially non-conducting is prestructured and then locally enriched in a targeted manner by a dopant in a doping operation, to thereby increase the conductivity locally in the initially non-conducting subregion of the semiconductor substrate by introducing corresponding charge carriers.  
           [0005]    The disadvantage of forming contacts of this type by a diffusion process is that the diffusion process as such generally proceeds more or less isotropically. Therefore, a locally introduced high dopant concentration spreads more or less uniformly in all spatial directions during the thermal outdiffusion or annealing. To be able nevertheless to use the formation of contact regions by diffusion, it is necessary to introduce in the semiconductor substrate a minimum distance between components which absolutely have to be insulated or to form an additional electrical insulation, for example in the form of an oxide region, in order that unwanted instances of contacting or even short circuits are avoided.  
           [0006]    The provision of an additional insulating region, for example in the form of an oxide or the like, hinders the individual process steps and consequently increases the costs of production. The maintenance of minimum distances is at odds with the aim and desire of making integrated semiconductor circuits as highly integrated and effective as possible.  
         SUMMARY OF THE INVENTION  
         [0007]    It is accordingly an object of the invention to provide a method for forming a diffusion region that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which the risk of unwanted instances of contacting or short circuits is particularly low.  
           [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for forming a diffusion region. The method includes providing a semiconductor substrate, and forming a first conductivity region and a second conductivity region in the semiconductor substrate. The first conductivity region and the second conductivity region are spatially separated from each other by an intermediate region of the semiconductor substrate. The diffusion region is formed between the first conductivity region and the second conductivity region in the intermediate region by thermally activated diffusion of at least one dopant into the intermediate region. The thermally activated diffusion of the dopant is conducted substantially in a directed manner by an interaction of the dopant with a subregion of the intermediate region. A transformation process is thermally initiated at least in the subregion of the intermediate region.  
           [0009]    In the case of the method according to the invention for forming the diffusion region, in particular a diffusion contact or the like, in a semiconductor substrate or the like, in particular for connecting a DRAM memory cell on a vertical MOSFET, a first conductivity region and a second conductivity region are spatially separated from each other by an intermediate region of the semiconductor substrate. The diffusion region or the diffusion contact is formed between the first conductivity region and the second conductivity region, in particular in the intermediate region. The forming of the diffusion region or the diffusion contact takes place by thermally activated diffusion of at least one dopant into the intermediate region of the semiconductor substrate. Furthermore, a transformation process is thermally initiated and/or conducted at least in a subregion of the intermediate region, in particular substantially at the same time as the diffusion, and the thermally activated diffusion of the dopant is thereby carried out substantially in a directed manner, in particular substantially in or along a preferential direction, by interaction of the dopant with the transforming subregion of the intermediate region.  
           [0010]    A central idea of the present invention is consequently to control the direction of the thermally activated diffusion of the dopant particles by bringing the dopant particles into specific interaction with the subregion of the intermediate region. The interaction is carried out by initiating and conducting a transformation process of the subregion of the intermediate region. By initiating the transformation processes by thermal activation, the corresponding interaction of the dopant particles with the subregion of the intermediate region and the material present there is also achieved at the same time.  
           [0011]    More preferably, a chemical transformation process, a crystallization process and/or the like is/are carried out as the transformation process in the subregion of the intermediate region of the semiconductor substrate.  
           [0012]    In this case, an oxidation operation is particularly preferred, in particular an oxidation operation using oxygen or with oxygen.  
           [0013]    A particularly strong influence of the transformation process in the subregion of the intermediate region is obtained if, according to a preferred embodiment of the method according to the invention for forming a diffusion region, lattice imperfections, reactive centers and/or the like are produced, provided with increased mobility and/or brought into interaction with the dopant particles by the transformation process.  
           [0014]    This measure consequently achieves the particular effect that both the mobility of the dopant particles and the mobility of possible lattice imperfections are increased by thermal activation. As a result, the probabilities of certain mass transfers, for example exchange processes or the like, can be correspondingly increased.  
           [0015]    In the case of a particularly preferred embodiment of the method according to the invention, a silicon substrate, in particular a bulk silicon substrate, a p-doped silicon or the like, is provided as the semiconductor substrate, as the intermediate region and/or as the subregion thereof.  
           [0016]    In the case of a further embodiment of the method according to the invention, the dopant is chosen to match the material of the intermediate region or of the semiconductor substrate and in particular to match the material of the subregion of the intermediate region, in particular with regard to a particularly high mobility of the dopant particles or of the dopant with respect to a preferential direction.  
           [0017]    Particularly advantageous properties are obtained if phosphorus or the like is chosen as the dopant. Phosphorus has, for example in comparison with arsenic, a much stronger tendency to interact with lattice imperfections. It is therefore much easier with phosphorus, in comparison with arsenic or the like, to accomplish a directed diffusion along a preferential direction by local forming of mobile lattice imperfections.  
           [0018]    Although the dopant can be introduced subsequently into already existing structures, for example locally by corresponding implantation or the like, it is of particular advantage if the dopant is supplied by a depot region, in particular in the semiconductor substrate. The depot region is advantageously created as a material subregion during the production of the basic structures and is then distributed in a desired way during the thermal outdiffusion from the depot region in a directed manner while interacting with the thermal transformation region.  
           [0019]    In this case, the depot region may be provided as a separate region in the region of the semiconductor substrate.  
           [0020]    It is also conceivable for the depot region to be provided in substantially direct spatial proximity to the first conductivity region, the subregion and/or intermediate region of the semiconductor substrate and/or the second conductivity region.  
           [0021]    In the case of a particularly advantageous embodiment of the method according to the invention, the depot region is formed and provided as the first conductivity region or as part thereof, in particular as what is known as a buried-strap region or the like.  
           [0022]    In the case of a particularly preferred embodiment of the method according to the invention, a polysilicon region or the like is respectively provided as the first conductivity region and/or as the second conductivity region.  
           [0023]    According to another preferred embodiment of the method according to the invention, it is provided that an interfacial region or edge region of the semiconductor substrate or of the intermediate region thereof, in particular with respect to a neighboring medium, is provided as a subregion for the transformation process, in particular in substantially direct connection, in particular along a preferential direction, between the first conductivity region and the second conductivity region.  
           [0024]    In this case, a medium having an oxidizing agent and/or a medium supplying an oxidizing agent is provided in an advantageous way as the neighboring medium, in particular an oxygen-containing atmosphere or the like.  
           [0025]    With regard to the distribution of the dopant in a directed form, it is of particular advantage that, during the transformation process, a substantially electrically insulating interfacial region and/or edge region, which contains in particular an oxide, a silicon oxide and/or the like, is formed in the subregion.  
           [0026]    It is further of advantage that, after the transformation process, a third conductivity region is provided in the direct proximity of the subregion of the intermediate region or of the semiconductor substrate in such a way that a gate region which can be driven via the third conductivity region is formed by the subregion and/or the interfacial region between the first conductivity region and the second conductivity region with the diffusion region or diffusion contact as a connection contact, in particular in the form of a vertical MOSFET or the like.  
           [0027]    In this case, the first conductivity region is provided in an advantageous way as a line device, a top electrode device, in particular of a storage capacitor or the like, as a buried-strap region (BS region) and/or as part thereof or a combination thereof.  
           [0028]    In the case of another embodiment, the second conductivity region is advantageously formed as part of a bit line or word line.  
           [0029]    In a corresponding way, the depot region for the dopant may be provided as part of a top electrode region, of a buried-strap region (BS region) or the like, in particular as a BS-divot-fill region or the like.  
           [0030]    The connection of trench memory cells to MOSFETs disposed above them and vertically aligned requires that a diffusion region or diffusion contact is produced. The diffusion region must on the one hand have an adequate material overlap with the gate region or gate oxide of the vertical MOSFET and on the other hand be horizontally localized and restricted on account of the proximity of further elements of the circuit configuration which must not be contacted.  
           [0031]    Until now, arsenic-doped polysilicon has been used for contacting in the outdiffusion of the buried-strap region, the arsenic provided forming the corresponding dopant. The dopant diffuses to the same degree in vertical and horizontal directions, so that, when spanning a vertical distance to form the overlap with respect to the gate of the vertical MOSFET, a corresponding diffusion distance is substantially also covered in the lateral direction to neighboring circuit elements which, however, are not to be contacted. To actually avoid undesired contacting, it is therefore necessary to provide correspondingly large distances between neighboring elements of the circuit configuration between which contacting must not occur.  
           [0032]    In particular, the use of phosphorus-doped polysilicon in the upper region of the trench cell with subsequent thermally oxidative production of the gate dielectric of the vertical MOSFET allows an anisotropic diffusion to develop. Consequently, the overlap with respect to the gate can be significantly improved, the lateral outdiffusion in the direction of circuit elements which are not to be contacted remaining limited at the same time on account of the reduced necessary diffusion time.  
           [0033]    In an advantageous way, phosphorus is consequently introduced in the upper part of a DRAM trench cell and the contact with the vertical MOSFET lying above it is established by utilizing the effect of what is known as oxidation-enhanced diffusion.  
           [0034]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0035]    Although the invention is illustrated and described herein as embodied in a method for forming a diffusion region, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0036]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    FIGS.  1 - 3  are diagrammatic, lateral cross-sectional views of two structures that are obtained in the case of an exemplary embodiment of a method according to the invention; and  
         [0038]    FIGS.  4 - 6  are diagrammatic, side-sectional views of intermediate stages that are achieved with the forming of conventional diffusion regions. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a starting point for forming a diffusion region  50  or diffusion contact  50  in the way according to the invention in a lateral cross-sectional view.  
         [0040]    In a semiconductor substrate  20 , for example a p-doped bulk silicon or the like, a first conductivity region  30  is formed in a trench  22 , for example in the form of polycrystalline silicon or the like. The first conductivity region  30  may be, for example, a connecting lead, or else, as in the case of a DRAM cell with a vertical cell structure, the lead to a top electrode of a capacitor device to be provided further down in the structure.  
         [0041]    At a vertical distance away from the first conductivity region  30  is a second conductivity region  40 , which may be formed for example as part of a word line or bit line. Provided between the first conductivity region  30  and the second conductivity region  40  is an intermediate region  25  of the semiconductor substrate  20 . In the intermediate region  25 , the diffusion region  50  or diffusion contact  50  is to be produced by introducing a dopant P. The diffusion region  50  or the diffusion contact  50  serves for the direct and/or switchable contacting—for example by channel-forming over a channel path or gate path K—of the first conductivity region  30  with the second conductivity region  40 .  
         [0042]    The first conductivity region  30  is electrically insulated laterally with respect to the semiconductor substrate  20  by a first insulating region  24 . The first insulating region  24  may also serve as an oxide collar or collar region, to suppress the forming and switching of parasitic semiconductor switching elements.  
         [0043]    In the upward direction, the first conductivity region  30  is electrically insulated by a second insulating region  23  and closed off with respect to the trench  22 . The second insulating region  23  is often also referred to as the trench top oxide (TTO) and serves for the electrical insulation of the first conductivity region  30  from a gate terminal  70  to be formed in the trench  22 .  
         [0044]    Next to the upper side region of the first conductivity region  30 , in the first insulating region  24  there is a recess that is filled with a polysilicon region  32  doped with a dopant p (FIG. 2) forming a depot region  32 . The depot region  32  is referred to in the case of vertical DRAM cell configurations as a buried-strap region, the filling itself as a BS-divot-fill region. It serves decisively for the forming of the diffusion region  50  by an outdiffusion of the doping material P into the region of the semiconductor substrate  20 .  
         [0045]    The configuration of FIG. 1 is the starting point of the method according to the invention. A similar configuration is the starting point in conventional methods for forming diffusion regions. There, the BS-divot-fill region  32  is, for example, undoped polysilicon, and the first conductivity region  30  is arsenic-doped polysilicon.  
         [0046]    This provision is intended to explain initially the conventional procedure when forming diffusion regions or diffusion contacts.  
         [0047]    In the transition to the structure just described to the structure of FIG. 4, a third insulating region  26  is formed at side wall regions  22   b  of the trench  22 , for example in the form of an oxide or the like, which then serves later as the gate oxide, as it is known.  
         [0048]    In the transition to the configuration of FIG. 5, the still exposed recess of the trench  22  is then filled with what is known as the gate terminal  70 .  
         [0049]    Then, in the transition to the structure of FIG. 6, an annealing process, which possibly involves a number of steps, is carried out, in which process the entire configuration of FIG. 5 is heated and thereby brings about the outdiffusion of the doping material P of the depot region  32  into the region of the semiconductor substrate  20 .  
         [0050]    In FIG. 6, the diffusion fronts I, II and III at three different successive points in time of the diffusion operation are represented. It can be clearly seen that, although a vertical diffusion of the dopant P, starting from the depot region  32 , into the intermediate region  25  and towards the second conductivity region  40  takes place for the forming of the local diffusion region  50 , it can nevertheless also be seen that a lateral diffusion in the direction of the arrows also takes place, in the direction of a third conductivity region  60  that is provided. The third conductivity region  60  serves for illustrating by way of example the presence of further switching elements which are to be electrically insulated with respect to the first conductivity region  30  or the second conductivity region  40  to ensure a desired functionality of the configuration.  
         [0051]    It can be seen more clearly that, as a result of the lateral outdiffusion of the dopant p from the depot region  32  toward the third conductivity region  60 , the insulating region between the diffusion region  50  and the further conductivity region  60 , which indeed is not to be contacted, has the relatively low value d, so that, in the case of the configuration of FIG. 6, there is a considerable risk of electrical contacts forming between the first conductivity region  30  and the third conductivity region  60 .  
         [0052]    In the case of the procedure according to the invention, on the other hand, starting from the configuration of FIG. 1, the gate oxide region  26  is not explicitly deposited. Instead, with the trench  22  open and in the presence of an oxygen-containing atmosphere as a neighboring medium  100  (FIG. 2) of the intermediate region  25  or edge region  25   b  thereof, a thermal transformation operation is initiated by heating up the entire structure of FIG. 1. The transformation operation preferably takes place in an edge region  25   b  of the semiconductor substrate  20  or of the intermediate region  25  of p-doped polysilicon.  
         [0053]    At the same time, in the transition to the structure of FIG. 2, the outdiffusion of the dopant p then takes place from the depot region  32  into the region of the semiconductor substrate  20 .  
         [0054]    In a way analogous to the intermediate state shown in FIG. 6, it is also the case with the configuration of FIG. 2 that various diffusion fronts I, II which are assigned successively at different points in time of the diffusion process are represented. As in the case of the configuration of FIG. 6, a marked lateral outdiffusion of the dopant p from the depot region  32  towards the third conductivity region  60  also takes place in the case of the procedure according to FIG. 2.  
         [0055]    On account of the oxidation operation in the edge region  25   b  and the accompanying production and/or mobilization of lattice imperfections, an intensive interaction of the dopant particles P with the lattice imperfections of a transformation region  22   b  takes place in particular, so that the dopant P has a greater mobility in the vertical direction along the edge region  25   b  than in the lateral direction toward the third conductivity region  60 . As a consequence, in comparison with the configuration of FIG. 6, the dopant P spreads out more easily in the vertical direction than in the lateral direction. As a consequence, the annealing process, and consequently the diffusion operation, may already be interrupted when the diffusion front II is reached, that is at a very much earlier point in time, because there is then already a marked overlap of the diffusion region  50  with the oxide region  22   b  formed up until then as the transformed subregion  25   b.    
         [0056]    This aspect is illustrated by a depicted distance D between the diffusion region or diffusion contact  50  and the third conductivity region  60 . The distance D is very much greater than the corresponding distance d of the configuration of FIG. 6.  
         [0057]    The limit of the diffusion contact or diffusion region  50  is defined by the front II, and the limit corresponds accordingly to the limit of a pn junction. In the upper region of the diffusion contact or diffusion region  50 , a corresponding channel path K must then be formed in the direct proximity of the second conductivity region  40  by corresponding application of an electric potential.  
         [0058]    [0058]FIG. 3 shows in greater detail the configuration of FIG. 2 in the region of the diffusion region  50 , with the filling of the trench  22  with a corresponding gate contact region in the form of the gate terminal  70  also already being represented.