Abstract:
In the case of the cost-effective method according to the invention for fabricating a power transistor arrangement, a trench power transistor arrangement ( 1 ) is fabricated with four patterning planes each containing a lithography step. The power transistor arrangement according to the invention has a cell array ( 3 ) with cell array trenches ( 5 ) each containing a field electrode structure ( 11 ) and a gate electrode structure ( 10 ). The field electrode structure ( 11 ) is electrically conductively connected to the source metallization ( 15 ) by a connection trench ( 6 ) in the cell array ( 3 ).

Description:
BACKGROUND 
   The invention relates to a method for fabricating a power transistor arrangement and a mask for carrying out the method. Moreover, the invention relates to a power transistor arrangement. 
   Transistor arrangements fashioned as MOS (Metal Oxide semiconductor) power transistors are provided for controlling switching currents having high current intensities (up to several tens of amperes) by means of low control voltages. The dielectric strength of such power transistors may be as much as several 100 V. The switching times are usually in the region of a few microseconds. 
   MOS power transistors take the form of trench MOS power transistors, for example. A trench MOS power transistor is formed in a semiconductor substrate having, in at least one active cell array, in each case a plurality of trench transistor cells arranged next to one another. 
   Depending on the fashioning of the trench transistor cells, it is possible to realize for example normally on and normally off p-channel and n-channel trench MOS power transistors. 
     FIG. 1  shows a conventional power transistor arrangement  1  embodied as a trench MOS power transistor with a schematic illustration of the source, drain and gate connections, which is embodied as an n-channel MOSFET with a vertical double-diffused trench structure (VDMOSFET, vertical double-diffused metal oxide semiconductor field effect transistor). In this case, a drain metallization  231  connected to the drain connection is arranged on a rear side of a semiconductor substrate  16 . An n ++ -doped drain layer  23  adjoins the drain metallization  231  in the semiconductor substrate  16 . A drift zone  232  adjoins the drain layer  23  opposite to the drain metallization  231 . The drift zone  232  is generally formed from a weakly n-doped semiconductor substrate  16  that generally comprises silicon applied epitaxially. A space charge zone forms in the drift zone  232  during off-state operation of the trench MOS power transistor, the extent of said space charge zone essentially determining the maximum reverse voltage. 
   In a cell array  3 , cell array trenches  5  are arranged in the semiconductor substrate  16 . The cell array trenches  5 , which are illustrated in cross section, in this example extend parallel in a direction perpendicular to the cross-sectional area. Gate electrode structures  10  and field electrode structures  11  are arranged in the cell array trenches  5 . The field electrode structure  11  is insulated from the semiconductor substrate  16  by an insulation layer  18 , which may comprise a field oxide, for example. The gate electrode structure  10  is insulated from the field electrode structure  11  and the semiconductor substrate  16  by a gate insulation layer  20 , which may be a silicon oxide, for example. The drift zone  232  of the semiconductor substrate  16  is adjoined by p-doped body zones in regions between the cell array trenches  5 , said body zones approximately being situated opposite the gate electrode structures  10 . n ++ -doped source regions  8   a  are provided between the body zones and a substrate surface  17 . The field electrode structures  11  reduce a parasitic capacitance between the gate electrode structures  10  and the drift zone  232 . A source metallization  15  is electrically conductively connected to the source regions  8   a  by means of source contact trenches  8 . The source metallization  15  is electrically insulated with respect to the gate electrode structures  10  by an intermediate oxide layer  22 . The material both of the gate electrode structures  10  and of the field electrode structures  11  is heavily doped polysilicon, for example. The conductivity of the gate electrode structure  10  may be improved for example by an additional layer in the gate electrode structure  10 , for instance a silicide layer. The cell array trench  5  with the gate electrode structure  10  and the field electrode structure  11  forms, together with the adjoining doped regions of the semiconductor substrate  16 , a trench transistor cell  2  extending as far as the drain layer  23 . 
   If a positive potential is applied to the gate electrode structure  10  in such an active trench transistor cell  2 , then an n-conducting inversion channel forms in the p-doped body zone from the minority carriers (electrons) of the p-doped body zone that have accumulated there. 
   In an edge region  4  of the power transistor arrangement  1  formed as a trench MOS power transistor, on the one hand the field electrode structures  11  arranged in the cell array trenches  5  are contact-connected to the source metallization  15 , and on the other hand the gate electrode structures  10  arranged in the cell array trenches  5  are contact-connected to a gate metallization  14 . Furthermore, an example of a shielding electrode  12  is illustrated in the edge region  4 . 
   By way of example, the field electrode structures  11  arranged in the cell array trenches  5  are contact-connected in a cross-sectional plane VII parallel to the cross-sectional plane VI. In the cell array trenches  5 , running perpendicular to the cross-sectional plane VI, the gate electrode structures  10  do not extend over the entire length of the cell array trenches  5 , so that the respective field electrode structure  11  is contact-connected in a connection region of the cell array trenches  5 , as shown in the plane VII. Each field electrode structure  11  pulled over the substrate surface  17  is electrically conductively connected to the source metallization  15 . Moreover, a shielding electrode  12  extending above the substrate surface  17  is formed. 
   In a further cross-sectional plane VIII extending between the first cross-sectional plane VI and the second cross-sectional plane VII parallel thereto, the gate electrode structures  10  are electrically connected to an edge gate structure  13 . The edge gate structure  13  is electrically conductively connected to the gate metallization  14 . The edge gate structures  13  and the shielding electrodes  12  are generally formed from doped polysilicon. The source metallization  15 , the gate metallization  14 , the edge gate structure  13 , the shielding electrode  12 , and also the semiconductor substrate  16  are mutually insulated from one another in each case by an insulation layer  18 , an intermediate oxide layer  22  and also a further insulation layer  18 . 
   In order to fabricate a complex structure, such as the power transistor arrangement described in  FIG. 1 , in which both the gate electrode structure and the field electrode structure are led out into the edge region and connected there in each case to a gate metallization, and a source metallization, respectively, at least five to seven patterning planes are employed in the present-day fabrication methods. A patterning plane generally comprises a lithographic imaging of structures that are predefined on an exposure mask onto the semiconductor substrate to be patterned and subsequent etching, deposition or growth and planarization steps. 
   The at least seven patterning planes for fabricating a power transistor arrangement such as has been described in  FIG. 1 , for example, contain a trench patterning, during which cell array and edge trenches are introduced into the semiconductor substrate, a patterning of deposited polysilicon for formation of the field electrode structure, a patterning of a gate insulation layer (gate oxide), a patterning of a second deposited polysilicon layer for formation of the gate electrode structure, a patterning of body and source regions, a patterning of contact holes, and a patterning of a metal plane. 
   A major cost factor in each patterning plane is the lithographic imaging, since the requisite devices are technically very complicated and cost-intensive. Moreover, the entire imaging process requires a high precision and is thus highly susceptible to error. For the reasons mentioned, it is endeavored to reduce the number of lithographic imagings and thus also the number of patterning planes. 
   A fabrication method with only five patterning planes has already been proposed. In the method, the body and source patterning and also gate electrode patterning planes are eliminated. Lithographic imagings are then not used any longer either for body patterning, source patterning or for gate electrode patterning. The remaining five patterning planes comprise the trench patterning, the field electrode patterning, the patterning of the gate insulation layer, the contact hole patterning and the patterning of the metal plane. 
   The present invention is based on the object of providing a cost-effective method with a further reduced number of patterning planes for fabrication of a power transistor arrangement. Moreover, it is an object of the invention to provide a mask for carrying out the method. The object further encompasses a power transistor arrangement fabricated by the method. 
   This object is achieved by means of a method having the features of patent claim  1  and by means of a mask for carrying out the method in accordance with patent claim  14 . Furthermore, the object is achieved by means of a power transistor arrangement in accordance with patent claim  23 . Advantageous developments of the invention emerge from the respective subclaims. 
   SUMMARY 
   A method for fabricating a power transistor arrangement is provided, in which a cell array and an edge region adjoining the cell array are provided in a semiconductor substrate. There are introduced, within the cell array, cell array trenches, and also at least one connection trench crossing the cell array trenches, and, in the edge region, at least one edge trench adjoining the cell array trenches. In this case, the edge trench is provided such that it is wider than the cell array trenches and the connection trench. An insulation layer is applied, and a first conductive layer is applied to the insulation layer, the cell array trenches and the connection trench at least being filled and the wider edge trench not being completely filled. The first conductive layer is completely removed from the edge trenches and is caused to recede in the cell array essentially as far as the substrate surface. A mask covering the edge region and the connection trench is applied. In sections not covered by the mask, the first conductive layer is caused to recede in the cell array trenches. A gate insulation layer is provided in sections not covered by the mask in the cell array trenches above the first conductive layer that has been caused to recede and forms a field electrode structure. A contact connection of the field electrode structure is implemented in the region of the connection trench. 
   The method according to the invention for fabricating a power transistor arrangement advantageously requires just four pattering planes with a respective lithographic imaging. In the first patterning plane, the cell array trenches, the connection trenches and the edge trenches are introduced into the semiconductor substrate by means of a lithographic imaging and subsequent etching processes. In this case, the width of the trenches is to be provided such that the edge trenches are wider than the cell array trenches and the connection trench. By way of example, the edge trenches may be provided with 1.5 to 2 times the width of the cell array and connection trenches. An insulation layer, for example a field oxide, is applied to the then patterned substrate surface. Furthermore, a first conductive layer is applied to the insulation layer in the first patterning plane. This may be done by conformal deposition of doped polysilicon, the cell array trenches and the connection trench at least being filled and the wider edge trench being lined but not completely filled with the doped polysilicon. The width ratio of the edge trenches to the cell array trenches and the connection trenches is designed such that the first conductive layer is completely removed from the edge trenches by means of an etching process and is caused to recede in the cell array essentially as far as the substrate surface. In the subsequent second patterning plane, a mask covering the edge region and the connection trench is patterned by means of a lithographic imaging. In the sections not covered by the mask, the first conductive layer is caused to recede in the cell array trenches and a field electrode structure is formed. Furthermore, in this patterning plane, wet-chemical etching of the field oxide defines a region on which a gate insulation layer is later formed, for example by growth of a gate oxide. A contact connection of the field electrode structure in the region of the connection trench, and also the contact connection of source regions in the cell array and the contact connection of a gate electrode in the edge region of the power transistor arrangement are effected in a third patterning plane. The provision of a metal plane is effected in a fourth patterning plane. 
   In the method according to the invention, two patterning planes, namely the patterning of the field electrode structure and the patterning of the gate insulation layer, are advantageously combined into one patterning plane. An item of information that was conventionally transmitted by a lithographic imaging is communicated in the trench width. The edge trenches and the cell array trenches are patterned with different widths, which has the effect that the edge trenches remain open when the cell array trenches and the connection trench, which is provided for example with the same width as the cell array trenches, are completely filled with the first conductive layer. The edge trenches are completely emptied after the first conductive layer has been etched back. By means of the lithographically patterned mask according to the invention that is employed in the method, both the field electrode structure and the gate insulation layer are patterned in one patterning plane. 
   The method additionally provides a connection trench according to the invention which crosses the cell array trenches and connects the field electrode structure directly in the cell array to the source metallization arranged above the cell array. 
   This avoids leading out the field electrode structure into the edge trench, whereby it is possible to reduce a complexity in the patterning and thus also the requirements made of the lithography processes of the subsequent patterning planes. 
   An essential advantage of the method according to the invention is that the number of patterning planes is reduced from five to four, thereby saving a lithographic imaging. As a result, the number of error sources is reduced and both costs and time are saved. 
   In an advantageous manner, for the patterning of the gate insulation layer, sections of the insulation layer that are covered neither by the mask nor by the first conductive layer are removed. The mask is then removed and the gate insulation layer is provided by means of an oxidation of semiconductor material. Both the mask and the first conductive layer mask the insulation layer. The non-masked sections of the insulation layer are removed. After the removal of the mask, the gate insulation layer is applied by means of a virtually self-aligning process since a gate oxide layer formed by the oxidation is formed only in connection with the semiconductor material. The gate oxide layer is formed on the semiconductor substrate and on, for example, a polysilicon of the first conductive layer forming the field electrode structure. 
   Preferably, after the application of the gate insulation layer, a second conductive layer is applied for forming a gate electrode structure. The second conductive layer is etched back as far as the substrate surface, so that the cell array trenches are completely filled. An intermediate oxide layer is applied for insulation purposes. There are provided, in the intermediate oxide layer, in the cell array, source contact trenches for the contact connection of source regions and of the connection trench, and also, in the edge region, gate contact holes for the contact connection of the gate electrode structure. A gate metallization is then provided above the edge region and a source metallization is provided above the cell array. Both source contact trenches and gate contact holes may be filled with a metal or with a doped polysilicon. 
   In an advantageous manner, provision is made of gate connection trenches adjoining the edge trenches, the gate connection trenches being provided with the width of the edge trenches. The gate connection trenches and the edge trenches can thereby be treated in the same way. The gate connection trenches are also completely freed of the first conductive layer and completely filled with the second conductive layer forming the gate electrode structure. 
   Preferably, a field oxide is deposited or grown for the purpose of applying the insulation layer. 
   Preferably, doped polysilicon is deposited conformally for the purpose of applying the first conductive layer. In the case of a conformal deposition process, the cell array trenches and the connection trench are advantageously filled more rapidly than the wider edge trenches. Given a suitable width ratio between edge trenches and cell array trenches, the process can be set such that after a specific time during which the polysilicon is deposited conformally, the cell array trenches and the connection trench are completely filled, while the wider edge trenches are lined with the polysilicon with a defined layer thickness and an opening remains. 
   Preferably, an isotropic etching process is employed for the first instance of causing the first conductive layer to recede before the application of the mask. 
   Preferably, an isotropic dry etching process is used for the purpose of forming the field electrode structure during the second instance of causing the first conductive layer to recede using the mask. An undercut of the mask can advantageously be achieved by means of an isotropic etching process. This is favorable in partial regions of the connection trench since the polysilicon of the first conductive layer is to be caused to recede at the crossover locations between the cell array trenches and the connection trenches in order that the polysilicon of the second conductive layer forming the gate electrode structure is located in the cell array trench at the crossover locations above the polysilicon of the first conductive layer and the gate electrode structures are through-connected in the region of the crossover locations on the left and right of the connection trench. 
   Preferably, the uncovered insulation layer is removed by means of a wet-chemical etching process. 
   Preferably, doped polysilicon is provided as material for the second conductive layer. 
   In an advantageous manner, the second conductive layer is applied by means of a conformal deposition process. A thickness of the second conductive layer is provided in such a way that the edge trenches and the gate connection trenches are essentially filled. This is necessary since the gate electrode structures are electrically conductively connected by the edge trenches and the gate connection trenches to the gate metallization by means of gate contact holes filled with conductive material. 
   Preferably, body and source regions are introduced into the semiconductor substrate by an implantation of atoms and a subsequent thermal step. 
   Preferably, for application of the mask, a photosensitive resist layer is applied to a surface to be processed. A structure provided for the mask is imaged onto the resist layer by means of an exposure mask having the structure and a lithography method, and the resist layer is subsequently patterned by means of an etching process. 
   The mask according to the invention for carrying out the method according to the invention essentially covers the edge region and coves the connection trench at least in sections. In the case of etching processes that are customary at the present time, it is expedient to cover the connection trench at least in the region of crossover locations between the connection trench and source contact trenches that are processed later. Since, at said crossover locations, the connection trench is contact-connected to the source metallization by the source contract trench, the polysilicon of the first conductive layer in the connection trench, at least in the region of said crossover location, is led to the substrate surface and should not be removed during the etching process for forming the field electrode structure. 
   Preferably, the mask is provided with a ridge covering the connection trench and having a width of essentially the width of the connection trench. When carrying out the method according to the invention using this mask, an undercut under the ridge covering the connection trench may occur if the ridge is provided such that it is too narrow. As a result, both the insulation layer and the material of the first conductive layer in the connection trench may be removed to such an extent that there is no longer a connection to the source contact trench that contact-connects the source regions. On the other hand, however, the ridge cannot be made arbitrarily wide either since otherwise the insulation layer is not removed on the semiconductor substrate beside the connection trench. This would have the consequence that the full body charge would not be implanted in the region of the residual sections of the insulation layer, which would in turn result in a reduced breakdown voltage. Furthermore, when using this mask there is a process window between the isotropic undercutting of the polysilicon of the first conductive layer under the ridge and the spacing of the cell array trenches, since an etching attack is also effected from the cell array trench. This is desirable, on the one hand, since the polysilicon of the first conductive layer is to be removed in the region of crossover locations between the cell array trenches and the connection trenches, so that the. polysilicon of the second conductive layer, from which the gate electrode structure is formed, is located at the crossover location between cell array trench and connection trench above the polysilicon of the first conductive layer. This is expedient in order that as many connection trenches as desired can be integrated in the cell array. Otherwise the gate electrode structure would have no connection to the gate metallization in the cell array trenches between the connection trenches. On the other hand, however, the undercut must also not be so large that, in the region of the later source contact trench, the material of the first conductive layer in the connection trench no longer reaches to the substrate surface. 
   Preferably, the ridge covering the connection trench is widened in the region of a crossover location between connection trench and cell array trench, so that the cell array trench is covered in the region of the crossover location. With this mask it is possible to delimit the etching attack from a direction of the cell array trenches. By varying a widening of the ridge in the region of the crossover location, an isotropic undercutting under the ridge may advantageously be set in a controlled fashion. 
   Preferably, the mask is provided with a ridge covering the connection trench and having a width in a range between the width and triple the width of the connection trench. 
   The mask is advantageously provided with pads that cover crossover locations between the connection trench and the source contract trenches. In the case of this mask, the polysilicon of the first conductive layer in the connection trench is no longer provided in a manner reaching continuously to the substrate surface. The problem of causing the first conductive layer to recede at crossover locations between the cell array trenches and the connection trench is solved in a simple manner. 
   In an advantageous manner, in the case of the method for fabricating a power transistor arrangement, provision is made of a mask having a ridge that covers the connection trench and is widened in the region of crossover locations between the connection trench and the cell array trenches. The undercut of the ridge can be set in a controlled fashion with the aid of this mask. 
   In an advantageous manner, the mask is provided with a ridge covering the connection trench and having a width in a range between the width and triple the width of the connection trench. The body and source regions are implanted before the application of the mask and after the first instance of causing the first conductive layer to recede. The width of the ridge is chosen such that the polysilicon of the first conductive layer in the connection trench reaches to the source contact trench and the undercut in the region of the crossover location between the connection trench and the cell array trench is favorable for the connection of the gate electrode structure. Since the insulation layer on the semiconductor substrate now cannot be removed in a region of the ridge and this is harmful to the implantation of body and source regions, in this method variant the body and source regions are implanted before the application of the mask, after the first instance of causing the first conductive layer to recede. 
   In an advantageous manner, the mask is provided with pads covering crossover locations between the connection trench and the source contract trenches. An anisotropic dry etching process is carried out for formation of the field electrode structure during the second instance of causing the first conductive layer to recede and, in this case, the first conductive layer is etched back until the cell array trenches are filled as far as a predetermined height. In this method variant, the material of the first conductive layer is etched back anisotropically. In this case, the etching is masked with a resist pad, for example, only in the regions in which the subsequently processed source contact trenches cross the connection trench. In the case of this mask, too, the edge region is covered with a resist layer. The advantage of this variant is that the connection trench does not have to be formed in continuous fashion. It is even possible to integrate as many short connection trenches as desired between the cell array trenches with any desired density in the cell array. 
   Preferably, one of the masks described is provided and an etching process having anisotropic and isotropic components is employed for causing the first conductive layer to recede for formation of the field electrode structure. With a combination of isotropic and anisotropic components during an etching process, an undercut under the masking of the connection trench and of the edge region can be set in a targeted manner. 
   The power transistor arrangement according to the invention has at least one cell array formed in a semiconductor substrate, and an edge region adjoining the cell array. Cell array trenches are formed within the cell array and trench transistor cells are formed along the cell array trenches. Two electrode structures that are insulated from one another and from the semiconductor substrate are arranged within a respective cell array trench, one electrode structure being formed as a field electrode structure and the other electrode structure being formed as a gate electrode structure. A gate metallization is arranged above the edge region at least in sections. In this case, the cell array trenches are led out into the edge region and the gate electrode structure is electrically conductively connected to the gate metallization. According to the invention, at least one connection trench crossing the cell array trenches is provided in the cell array. In the region of the connection trench, the field electrode structure is electrically conductively connected to a source metallization arranged above the active cell array. 
   The advantage of this power transistor arrangement is that, through the connection trench provided in the cell array, the field electrode structure is no longer led out from the trench and connected to a shielding electrode, for example, at the edge, rather the field electrode structure is short-circuited directly through the connection trench with the source metallization arranged above the cell array. A complexity of a structure of the power transistor arrangement is thus reduced. Consequently, the requirements made of lithography processes are also simplified. This leads to a reduction of the susceptibility of the entire process to error and thus to a saving of costs and time in the fabrication of the power transistor arrangement. Since the field electrode structure is short-circuited directly in the cell array with a source metallization, the edge region can be made narrower than in conventional power transistor arrangements. This leads to a further saving, since the power transistor arrangements can be provided such that they are smaller by this area. 
   Preferably, the edge region encloses the cell array. 
   In an advantageous manner, the cell array is surrounded at least in sections by edge trenches provided in the edge region, the cell array trenches opening into the edge trenches or being lengthened by the edge trenches. Gate connection trenches adjoining the edge trenches are provided in the edge region. The edge trenches and the gate connection trenches are provided such that they are wider than the cell array trenches and the connection trenches. By virtue of the cell array trenches opening directly into the edge trenches, the gate electrode structure is connected in a simple manner directly to the edge trenches and the gate connection trenches adjoining the latter. 
   In the text above, the invention has been explained in each case using the example of a trench power transistor arrangement. Over and above this the invention can be extended in an obvious manner to IGBTs, transistor arrangements with a planar structure and those with a drain-up structure. 
   Furthermore, the invention can be applied to power transistor arrangements with in each case normally on and normally off p-channel and n-channel transistor cells. 
   The invention is explained in more detail below with reference to  FIGS. 1 to 7 , in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a simplified schematic cross section through a conventional transistor arrangement in the transition region between cell array and edge region, 
       FIG. 2  shows a plan view of a power transistor arrangement in accordance with an exemplary embodiment of the invention, 
       FIG. 3  shows schematic cross sections through trenches of a power transistor arrangement according to the invention in different stages of an exemplary embodiment of the method according to the invention, 
       FIGS. 4 to 7  show exemplary embodiments of masks according to the invention. 
   

   DESCRIPTION 
     FIG. 1  has already been explained in greater detail in the introduction to the description. 
   Reference symbols not represented in the respective figures will be found in  FIG. 1 . 
   The power transistor arrangement  1  illustrated as an exemplary embodiment in  FIG. 2  has a cell array  3  formed in a semiconductor substrate  16 , said cell array being surrounded by an edge region  4 . Cell array trenches  5  are formed within the cell array  3  in the semiconductor substrate  16  and trench transistor cells  2  are formed along the cell array trenches  5 . The cell array trenches  5  are provided with a width of 0.75 micrometer. Two electrode structures that are insulated from one another and from the semiconductor substrate  16  are arranged in the cell array trenches  5 . In this case, one electrode structure is formed as a field electrode structure  11  and the other electrode structure is formed as a gate electrode structure  10 . Edge trenches  7  surrounding the cell array  3  and gate connection trenches  7   a  adjoining the edge trenches  7  are formed in the edge region  4 . The edge trenches  7  and the gate connection trenches  7   a  are provided with a width in the range between 1.5 and 2 times the width of the cell array trenches  5 . The gate electrode structure  10  is electrically conductively connected to a gate metallization  14  by the gate connection trenches  7   a  filled with conductive material and gate contact holes  9  introduced in the region of the gate connection trenches  7   a.  The cell array trenches  5  open into the edge trenches  7  and are crossed by a connection trench  6 , having the same width as the cell array trenches  5 , in the cell array  3 . The field electrode structure  11  is electrically conductively connected to the connection trench  6  in the cell array  3 . Above the connection trench  6 , a source contact trench  8  that contact-connects source regions  8   a  and the connection trench  6  runs parallel to the cell array trenches  5 . The connection trench  6  is filled with doped polysilicon, for example, so that it can be contact-connected by the overlying source contact trench  8  and be short-circuited with the source metallization  15 . 
   In the layout illustrated in  FIG. 2 , the cell array  3  and the edge region  4  are in each case demarcated by a broken line. The edge trenches  7 , the gate connection trenches  7   a,  the cell array trenches  5  and the connection trench  6  are introduced into the semiconductor substrate  16 . The source contact trenches  8  run parallel to the cell array trenches  5 . Situated above the gate connection trenches  7   a  are gate contact holes  9  filled with conductive material, which connect the gate electrode structure  10  to the gate metallization  14 . The source contact trench  8  filled with conductive material produces the connection of the source regions  8   a  and of the connection trench  6  to the source metallization  15 . 
   In order to fabricate a power transistor arrangement  1  in accordance with the layout in  FIG. 2 , edge trenches  7 , gate connection trenches  7   a,  and at least one connection trench  6  are introduced into the semiconductor substrate  16  in a first patterning plane by means of a lithographic imaging and etching process. An insulation layer  18 , which may comprise a field oxide, for example, is deposited or grown onto a now patterned substrate surface  17 . A first conductive layer  19  made of a highly doped polysilicon is subsequently applied by means of a conformal deposition process. Since the edge trenches  7  and the gate connection trenches  7   a  are provided such that they are wider than the cell array trenches  5  and the connection trench  6 , the wider edge trenches  7  are not completely filled when the cell array trenches  5  and the connection trench  6  are completely filled. In general, the cell array trenches  5  and the connection trenches  6  have the same width. 
     FIGS. 3   a  to  3   g  show the four trenches in each case in cross section: I edge trench  7 , II gate connection trench  7   a,  III connection trench  6 , V cell array trench  5 . The drawing IV illustrates a longitudinal section through the connection trench  6 .  FIG. 3   a  illustrates the semiconductor substrate  16  after patterning of the edge trenches  7 , the gate connection trenches  7   a,  the connection trench  6  and the cell array trenches  5 . The insulation layer  18  is applied on the patterned substrate surface  17 . The first conductive layer  19  is provided on the insulation layer  18 . In the processing state depicted, the connection trench  6  and the cell array trench  5  are completely filled with the conductive layer  19 , while the edge trenches  7  and the gate connection trench  7   a  have an opening. 
   After the conformal deposition of the doped polysilicon for the first conductive layer  19 , the polysilicon is completely removed again from the edge trenches  7  and the gate connection trenches  7   a  by means of an isotropic etching process and caused to recede in the cell array trenches  5  and in the connection trench  6  as far as the substrate surface  17 . 
   The second patterning plane then ensues. For patterning of a mask  24 , a photosensitive resist layer is applied and patterned by means of a lithographic imaging and subsequent etching steps. The resist layer is patterned in such a way as to form a mask  24  covering the edge region  4  and the connection trench  6 . 
     FIG. 3   b  illustrates the edge trench  7  and the gate connection trench  6  after the complete removal of the first conductive layer  19  and after the application of the mask  24 . The polysilicon of the first conductive layer  19  is situated in the connection trench  6 , said polysilicon just about reaching the substrate surface  17 . The polysilicon is isolated from the semiconductor substrate  16  by the insulation layer  18 . The resist ridge  241  can be seen on the polysilicon of the first conductive layer  19  in the connection trench  6 . The cell array trench  5  differs from the illustration in  FIG. 3   a  by virtue of the etched-back polysilicon of the first conductive layer  19 . 
   After the application of the mask  24 , for formation of the field electrode structure  11 , the polysilicon of the first conductive layer  19  is caused to recede as far as a predetermined height in the cell array trenches  5  by means of an isotropic etching process. The field oxide of the insulation layer  18  is then removed wet-chemically at the locations at which the insulation layer  18  is not covered by the mask  24  or by the polysilicon. 
     FIG. 3   c  represents the trenches after the removal of the non-masked insulation layer  18  and after causing the polysilicon of the first conductive layer  19  to recede in the cell array trench. 
   After the removal of the mask  24 , a gate insulation layer  20  is applied, which is formed by an oxidation of the semiconductor substrate  16 , for example silicon. This is a virtually self-aligning process since the oxide forms only on silicon. 
     FIG. 3   d  illustrates the trenches after this process step. The gate insulation layer  20  is formed on the semiconductor substrate  16  in the cell array  3 , as can be seen in  FIG. 3   d  I, III, IV and V. The gate insulation layer  20  has been formed on the polysilicon of the first conductive layer  19 , as can be gathered from  FIG. 3   d  III, IV and V. 
   The formation of the gate insulation layer  20  is followed by conformal deposition of a doped polysilicon for the second conductive layer  21  for formation of a gate electrode structure  10 . 
     FIG. 3   e  shows the trenches after the conformal deposition of the polysilicon of the second conductive layer  21 . The edge trench  7  and the gate connection trench  7   a  are completely filled with polysilicon. In the cell array  3 , the cell array trench  5  is filled with the polysilicon of the second conductive layer  21 . The polysilicon of the second conductive layer  21  has been deposited above the connection trench  6 . 
   In order to form the gate electrode structure  10 , the polysilicon of the second conductive layer  19  is caused to recede as far as the substrate surface  17  by means of an isotropic etching process in the edge trenches  7  and gate connection trenches  7   a.  The gate insulation layer  20  on the polysilicon of the first conductive layer  19  in the connection trenches  6  acts as an etching stop layer in this case. In the cell array trenches  5 , the doped polysilicon of the second conductive layer  21  is caused to recede to just below the substrate surface  17 . The illustration of the trenches in  FIG. 3   f  differs from the illustration in  FIG. 3   e  by the fact that the second conductive layer  21  has been caused to recede. 
   An intermediate oxide layer  22  is deposited, which insulates regions that are not intended to be conductively connected to metal planes that are still to be processed. A third patterning plane with a lithographic imaging is necessary in order to introduce gate contact holes  9  and source contact trench  8  into the intermediate oxide layer  22 . After a patterning of the gate contact holes  9  and of the source contact trench  8 , they are filled with a doped polysilicon or with a sputtered metal. A fourth patterning plane is subsequently effected in order to provide a gate metallization  14  and a source metallization  15 . 
     FIG. 3   g  illustrates the trenches after the process steps mentioned have been carried out. The illustration reveals the intermediate oxide layer  22 , gate and source metallization  14 ,  15 , the gate contact holes  9  and the source contact trench  8 . The first conductive layer  19  formed as field electrode  11  is illustrated in the cell array trench  5 , which layer is isolated from the second conductive layer  21 , formed as gate electrode  10 , by the gate insulation layer  20 . Furthermore, the source regions  8   a  and the p-doped body region are introduced into the semiconductor substrate  16 . 
   Various embodiments can be specified for the mask  24  employed in the method described, with which mask two patterning planes can be combined into one patterning plane, namely the patterning of the first conductive layer  19  forming the field electrode structure  11  and the patterning of the gate insulation layer  20 . 
   A first exemplary embodiment of the mask  24  is shown in  FIG. 4 . The mask essentially covers the edge region  4  and the connection trench  6  with a ridge  241 . 
   In the case of this embodiment, it is difficult to set the undercut under the ridge  241  in a controlled manner during the second instance of causing the first conductive layer  19  to recede. The undercut must be controlled such that the polysilicon of the first conductive layer  19  is removed in the region of crossover locations  25  between the connection trench  6  and the cell array trenches  5  in order that the polysilicon in the cell array trench  5  of the second conductive layer  21  forming the gate electrode structure  10  can be placed over the first conductive layer  19 . On the other hand, the undercut must not be to such an extent that the polysilicon of the first conductive layer  21  in the connection trench  6  no longer reaches to the source contact trench  8 . 
   A second exemplary embodiment of a mask  24  is illustrated in  FIG. 5 . This mask differs from the mask described in  FIG. 4  through the widening of the ridge  241 . 
   The widening of the ridge  241  ensures that after the polysilicon of the first conductive layer  19  has been etched back a second time by means of an isotropic etching process, the polysilicon in the connection trench  6  reaches to the source contact trench  8 . The width of the ridge  241  varies in a range between the width and triple the width of the connection trench  6  and is chosen such that the undercut in the region of the crossover location  25  between connection trench  6  and cell array trench  5  is favorable for the connection of the gate electrode structure  10 . However, since it is not possible to remove the insulation layer  18  on the semiconductor substrate  16  in the region of the ridge  241  and this is harmful to the implantation of body and source regions, when using this mask  24  the body and source regions are implanted before the application of the mask  24 , after the first instance of causing the first conductive layer  19  to recede. 
   A third exemplary embodiment of a mask  24  is illustrated in  FIG. 6 . The ridge  241  covering the connection trench  6  is provided with approximately the width of the connection trench  6  and is widened only at the crossover locations  25  between the cell array trenches  5  and the connection trench  6 . The etching process can be set in a controlled fashion by varying the width of the ridge  241  at the crossover locations  25 . The edge region  4  in the region of the gate connection trenches  7   a  is covered by the mask  24  in a manner reaching right into the cell array  3 . Variations are possible in that region. 
   A fourth exemplary embodiment of a mask  24  is illustrated in  FIG. 7 . In this exemplary embodiment, the edge region  4  is covered by the mask  24  and the connection trench  6  is covered by pads  242  in the region of crossover locations  25  between connection trench  6  and source contact trench  8 . 
   In the method variant in which this mask  24  is employed, an anisotropic etching process can be employed when the polysilicon of the first conductive layer  19  is etched back a second time. During the anisotropic etching process, an undercut does not occur under the mask  24  and the pads  242  thereof. It suffices for only the region of the crossover location  25 , at which the subsequently processed source contact trench  8  contact-connects the connection trench  6 , to be masked with a pad  242  made, for example, of resist. The resist is also present in the edge region  4 . By virtue of the fact that, in the case of this method variant, the corners at which the cell array trench  5  and the connection trench  6  meet are not masked, the requirements made of the quality of the gate insulation layer  20  increase. 
   LIST OF REFERENCE SYMBOLS 
   
       
         1  Power transistor arrangement 
         2  Trench transistor cell 
         3  Cell array 
         4  Edge region 
         5  Cell array trench 
         6  Connection trench 
         7  Edge trench 
         7   a  Gate connection trench 
         8  Source contact trench 
         8   a  Source region 
         9  Gate contact hole 
         10  Gate electrode structure 
         11  Field electrode structure 
         12  Shielding electrode 
         13  Edge gate structure 
         14  Gate metallization 
         15  Source metallization 
         16  Semiconductor substrate 
         17  Substrate surface 
         18  Insulation layer 
         19  First conductive layer 
         20  Gate insulation layer 
         21  Second conductive layer 
         22  Intermediate oxide layer 
         23  Drain layer 
         231  Drain metallization 
         232  Drift zone 
         24  Mask 
         241  Ridge 
         242  Pad 
         25  Crossover location