Patent Publication Number: US-8969184-B2

Title: Method for fabricating a porous semiconductor body region

Description:
RELATED APPLICATIONS 
     This application is a Continuation application of co-pending application Ser. No. 13/407,728, which was filed on Feb. 28, 2012. The co-pending application claimed priority benefit to German Patent Application 102011012721, which was filed on Mar. 1, 2011. The entire contents of the co-pending application and the German Patent Application are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Exemplary embodiments of the present invention relate to a method for fabricating a deep porous semiconductor body region in a semiconductor body. Further exemplary embodiments relate to the fabrication of a deep foreign substance region of a semiconductor body using a porous semiconductor body region. 
     BACKGROUND 
     The fabrication of porous semiconductor body regions is known. By way of example, the production of a porous semiconductor body region by means of anodic oxidation has been shown. In this case, the porous semiconductor body region is fabricated by means of anodic oxidation of the semiconductor body from the rear of the semiconductor body into the semiconductor body. 
     It is desirable to provide a method for fabricating a locally bounded porous semiconductor body region deep in the semiconductor body. Furthermore, it is desirable to provide a method for fabricating a deep foreign substance region using a porous semiconductor body region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  and  FIG. 1   b  illustrate a method for fabricating a porous semiconductor body region in a trench. 
         FIG. 2  shows the fabrication of a porous semiconductor body region in a plurality of trenches. 
         FIG. 3  shows the fabrication of a porous semiconductor body region along a portion of the side wall in a trench. 
         FIG. 4  shows the fabrication of a continuous porous semiconductor body region. 
         FIG. 5   a  to  FIG. 5   c  illustrate the production of a foreign substance region in a semiconductor body along porous semiconductor body regions. 
         FIG. 6  shows a semiconductor element having continuous diffusion structures. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are explained in more detail below with reference to the appended figures. However, the invention is not limited to the specifically described embodiments but rather can be modified and adapted in suitable fashion. It is within the scope of the invention to combine individual features and combinations of features from an embodiment with features and combinations of features from another embodiment in a suitable fashion in order to arrive at further embodiments according to the invention. 
     Before the exemplary embodiments of the present invention are explained in more detail below with reference to the figures, it is pointed out that elements which are the same in the figures have been provided with the same or similar reference symbols and that a repeated description of these elements is omitted. In addition, the figures are not necessarily true to scale. The focus is instead on explaining the fundamental principles. 
       FIG. 1  shows an exemplary embodiment of a method for fabricating a porous semiconductor body region in a semiconductor body. In this case,  FIG. 1   a  shows an interim result in which a porous semiconductor body region  14  has been produced along a trench in a monocrystalline semiconductor body  10 . The monocrystalline semiconductor body  10  may in this case comprise any known semiconductor material, such as silicon. By way of example, the monocrystalline semiconductor body may be a conventional semiconductor wafer, also called a wafer. Alternatively, it may be just one portion of such a wafer. 
     First of all, a trench  13  is produced in the semiconductor body  10  starting from a first surface  11  of the semiconductor body  10 . In this case, the trench extends in a direction towards a surface  12  of the semiconductor body  10  which is opposite the surface  11 . The trench  13  can extend very deep into the semiconductor body  10  in comparison with an opening width of the trench  13  at the surface of the semiconductor body. 
     When the trench  13  has been produced, a porous semiconductor body region  14  is produced, starting from this trench, in the semiconductor body  10  along the trench  13 . By way of example, the porous semiconductor body region  14  is produced by means of electrochemical etching of the semiconductor body in the trench  13 . By way of example, this etching may be anodic oxidation of the semiconductor body  10 . Hence, the porous semiconductor body region  14  is produced in the semiconductor body  10  from the surface of the trench  13  in a direction perpendicular thereto. The extent of the porous semiconductor body region  14  into the semiconductor body from a side wall of the trench  13  in a direction perpendicular thereto is called the width B of the porous semiconductor body region  14 . Furthermore, the porous semiconductor body region  14  produced has a length L along the side walls of the trench  13  which is perpendicular to the width B. The length L thus extends in a direction from the surface  11  to the opposite surface  12 . The ratio L/B of the porous semiconductor body region  14  may be greater than 3 in this case. 
     As  FIG. 1   b  shows, the production of the porous semiconductor body region  14  is followed by the trench  13  being filled again with a material  15  of the semiconductor body  10 . This filling can be accomplished by epitaxially depositing the semiconductor material  15  in the trench, for example. A porous semiconductor body region  14  has multiple adjacent pores with pore sizes in the range from 2 nm to 200 nm, for example. In this context, both what are known as mesopores, with pore diameters in the range from 2 nm to 50 nm, and what are known as macropores, with pore diameters in the range from 50 nm to 200 nm, may arise. The pore walls situated between the pores comprise the material of the semiconductor body  10 . The proportion of the remaining semiconductor body material in the porous semiconductor body region can be set to a proportion in the range from 10% to 70% of the volume of the porous semiconductor body region, for example. This allows epitaxial deposition of the semiconductor material  15 . 
       FIG. 2  shows an embodiment of the method, in which first of a plurality of trenches  13 ′,  13 ″,  13 ′″ are produced in the semiconductor body  10 . Along all the exposed surfaces of these trenches, porous semiconductor body regions  14 ′,  14 ″,  14 ′″ are then produced in each trench. These adjacent porous semiconductor body regions  14 ′,  14 ″,  14 ′″ can be produced between at least two of such trenches  13 ′,  13 ″,  13 ′″ with a width B such that adjacent porous semiconductor body regions  14 ′,  14 ″,  14 ′″ touch and can merge to form one cohesive semiconductor body region. 
     A further exemplary embodiment is shown in  FIG. 3 , in which the porous semiconductor body region  14  is produced only along a portion of the side walls of a trench  13 . This can be achieved by producing a mask  16  in the trench, for example. A portion  16 ′ of the mask  16  may cover the base and a lower portion of the side wall of the trench  13 , for example. A further portion  16 ″ of the mask  16  may alternatively or additionally also cover an upper portion of the side wall of the trench  13 . The porous semiconductor body region  14  is then produced during the anodic oxidation, for example, only at the still exposed surfaces of the semiconductor body  10  in the trench  13 . 
       FIG. 4  shows a variant embodiment of the method, in which the porous semiconductor body region is formed up to the rear surface  12  of the semiconductor body  10 . The continuous structure—ranging from the surface  11  to the surface  12 —of the porous semiconductor body region  14  can be achieved by already forming the trench  13  from the surface to the surface  12  and hence subsequently producing a continuous structure for the porous semiconductor body region in the already continuous trench  13 . Alternatively, the trench can first of all be produced to a depth in the semiconductor body  10 , as already described above, and the porous semiconductor body region  14  can be formed along the side walls of this trench. Subsequently, the rear surface  12  of the semiconductor body  10  can then be removed mechanically and/or chemically at least as far as the porous semiconductor body region  14 . 
     An exemplary embodiment of a method for introducing a foreign substance into a semiconductor body  10  using a porous solid-state region  14  will now be described with reference to  FIGS. 5   a  to  5   c.    
       FIG. 5   a  shows a semiconductor body  10 , the surface  11  of which has a mask  20  formed on it. The mask  20  covers the surface  11  of the semiconductor body  10 . The mask  20  contains a mask opening  21  in which the surface  11  of the semiconductor body  10  remains uncovered. As  FIG. 5   b  shows, elongate porous semiconductor body regions  14 —extending into the semiconductor body  10  in the direction of the surface  12 —are produced beneath the mask opening  21  in the semiconductor body  10 , as described above. These porous semiconductor body regions  14  have a higher diffusion length for the foreign substance that is to be introduced, in comparison with the undisturbed monocrystalline semiconductor body  10 . On account of the faster diffusion of foreign substances along the porous semiconductor body regions  14  in comparison to the diffusion of the foreign substances in the undisturbed monocrystalline semiconductor body  10 , foreign substance regions  22  are formed in the semiconductor body  10  along the porous semiconductor body regions  14 , said foreign substance regions having a substantially greater extent in the vertical direction from the surface  11  to the surface  12  than in a lateral direction parallel to the surface  11 . 
     When producing insulating layers, for example, the porous semiconductor body regions  14  can be converted into an insulating material, e.g. by means of oxidation. In this case, the conversion process in the semiconductor body regions  14  that have been rendered porous takes place considerably faster than in the undisturbed monocrystalline semiconductor body  10 . Such porous semiconductor body regions  14  converted into insulating material can be regarded in the manner of homogeneous insulating regions. In the case of power semiconductor elements, for example, there is no or only little reduction in the breakdown field strength in the semiconductor body regions  14  converted into insulating regions in comparison with an insulating region which is converted from undisturbed monocrystalline semiconductor body  10 . This applies particularly so long as the pore size of the porous semiconductor body regions  14  is in the same order of magnitude or smaller than the free path length of charge carriers in air. Insulating regions in the semiconductor body  10  which still have pores have the advantage of a stress-reducing action by the mechanical stress brought about on account of the different material properties of the insulating material and of the semiconductor material. 
     Exemplary applications of the presented method for introducing a foreign substance into a semiconductor body are as follows: 
     Formation of an insulating layer between two semiconductor body regions, particularly lateral insulation of integrated circuits in which it is possible to cut down on trenches for dielectric insulation and to replace them with junction isolation, for example. 
     Formation of a connection for a buried dopant region, for example in an integrated circuit. The connection, e.g. a sinker, may have very low lateral out-diffusion in this case. 
     Production of a plated-through hole for a semiconductor body, as required for drain-up and source-down semiconductor elements or for two-way inhibiting semiconductor elements, for example. 
     Formation of a dopant column, for example for superjunction semiconductor elements. 
     Formation of deep insulating layers, as are needed in novel semiconductor elements such as a TEDFET, for example. 
     An exemplary application of the method is shown in  FIG. 6 . The example shows a silicon substrate which is n-doped, for example, in the semiconductor body  10  and which has a high-voltage diode  30  formed in it. The high-voltage diode  30  has a cathode electrode  31  on a cathode connection region  32 . The cathode connection region  32  and the cathode electrode  31  are formed at a first surface  11  of the semiconductor body  10 . An anode region  33  with an anode electrode  34  fitted to it is formed at a second surface  12  of the semiconductor body  10 , which is opposite the first surface  11 . In this case, the anode region  33  has p-type doping and forms a pn-junction with the n-doped silicon substrate. Extending through the semiconductor body  10  is an isolating diffusion region  35  which has been produced on the basis of the previously described method for introducing a foreign substance into the semiconductor body  10  and which bounds the high-voltage diode  30 . The isolating diffusion region  35 , having a porous semiconductor body region  14  and a diffused foreign substance region  22 , divides the semiconductor body  10  into a subregion  10   a , in which the high-voltage diode is formed, and an adjacent subregion  10   b  of the semiconductor body  10 . 
     The inhibiting pn-junction of the high-voltage diode is situated between the anode and the n-doped substrate in  FIG. 6 . In the case of conventionally produced diodes, the anode is provided by driving in a pn-junction from the front. This results in an excessive field increase at the edges of the p-type anode on account of the geometric effect of the doping. In the case of isolation-diffused edges, the excessive field increase at the edges of the p-type anode is not present. The geometric effect lowers the field strength in the case of inhibiting. The highest field strength occurs at the cathode-side surface. This field strength is not higher than in the homogeneous region of the anode, however. Edge terminations with isolation diffusion can be dimensioned much more easily than conventional ones. Isolation diffusion are also used in thyristors, GTOs, bipolar transistors, IGBTs or other high-voltage elements, for example. 
     The exemplary embodiments of a method for fabricating a porous semiconductor body region have the following features:
         producing at least one trench in the semiconductor body, starting from a surface of the semiconductor body,   subsequently producing at least one porous semiconductor body region in the semiconductor body starting from the at least one trench at least along a portion of the side walls of the trench,   subsequently filling the trench with a semiconductor material of the semiconductor body.       

     The production of the at least one trench in the semiconductor body as described in the exemplary embodiments allows the production of a porous semiconductor body region which extends deep into the semiconductor body along the trench walls. The access by a tool which is required for producing the porous semiconductor body region, for example in the case of anodic oxidation of the semiconductor body, through the trench allows the production of elongate structures in the porous semiconductor body regions along the (length) of the trench walls with only small extents in a direction perpendicular to the trench walls (width). In this case, the tool required can be used on the entire surface area of the trench walls in order to fabricate the porous semiconductor body region and can penetrate the semiconductor body. As soon as the access by the tool to the semiconductor body through the trench is stopped, the fabrication of the porous semiconductor body region in the trench is also stopped. Hence, it is thus possible to produce a very finely patterned porous semiconductor body region, for example, with a very high ratio L/B of length to width (L/B&gt;&gt;1). The ratio L/B of length to width may be greater than 3 in this case, for example. It is therefore possible to provide porous semiconductor body regions which are locally bounded at the surface of the semiconductor body on account of a small width and which take up only a small amount of surface area, but which extend a long way into the semiconductor body on account of the great length. 
     In one development of the method, it is possible to produce a plurality of adjacent trenches, starting from which it is possible to fabricate a plurality of adjacent porous semiconductor body regions. By way of example, the trenches can be formed to be very narrow. The distances between the trenches may be identical or else may vary. In one embodiment, at least two adjacent porous semiconductor body regions can be produced such that a relatively large cohesive porous semiconductor body region is produced. 
     In one exemplary embodiment, the at least one trench can be filled, for example with the semiconductor material of the semiconductor body, by means of epitaxial deposition. 
     The exemplary embodiments of a method for introducing a foreign substance into a semiconductor body have the following features:
         fabrication of a porous semiconductor body region through at least one trench, as described above,   provision of a foreign substance in the porous semiconductor body region,   heating of the semiconductor body with the porous semiconductor body region produced therein, with the foreign substance diffusing along the porous semiconductor body region.       

     The porous semiconductor body region has a higher diffusion length than the semiconductor body per se. This allows the foreign substance that is to be introduced to diffuse along the porous semiconductor body region more quickly than in the semiconductor body. Since the porous semiconductor body region is formed along a trench, the foreign substance can penetrate deep into the semiconductor body along this porous semiconductor body region. Diffusion of the foreign substance out of the porous semiconductor body region into the pore-free semiconductor body will take place substantially more slowly than the diffusion along the porous semiconductor body region on account of the different diffusion lengths. Therefore, deliberate orientation of the porous semiconductor body region allows diffusion to be advanced more quickly in this direction as a preference. Furthermore, deliberate adjustment of the pore size in the porous semiconductor body region allows the diffusion length to be influenced. Short diffusion times can be achieved if the semiconductor body is heated at least to a temperature in the range from 800° C. to 1200° C., for example at least to a temperature in the range from 900° C. to 1200° C. 
     In one embodiment of the method, the porous semiconductor body region is produced such that the porous semiconductor body region extends from a surface of the semiconductor body into the semiconductor body. This allows the foreign substance to be provided at the surface of the semiconductor body, for example. In one embodiment, this can be accomplished by providing the foreign substance as a solid layer at the surface of the semiconductor body. Alternatively, the foreign substance can be provided in gaseous form at the surface of the semiconductor body. In another embodiment, the foreign substance is implanted in the porous semiconductor body region. 
     The foreign substances provided may be oxygen or a dopant for the semiconductor body, for example. In one embodiment, the foreign substance diffuses out of the solid-state region into the semiconductor body.