Abstract:
A trench structure is produced in a substrate wafer in a two-step trench process. A trench mask is produced in a first etching step and the trench structure is realized in the substrate wafer in a second etching step. An auxiliary lithography structure is produced in the substrate wafer in the trench process. A protective structure that protects the substrate wafer in the region of the auxiliary lithography structure against an etching attack in the second etching step is formed in the region of the auxiliary lithography structure in the manufacture of the trench mask.

Description:
BACKGROUND OF THE INVENTION 
     Complex process sequences having a plurality of lithography steps are required for manufacturing integrated circuits. Auxiliary lithography structures in every mask level are produced in the process steps in the substrate in which the integrated circuit is also manufactured. These auxiliary lithography structures are employed for the alignment of the next mask level as well as for the evaluation of the process steps. The auxiliary lithography structures are thus subject to the same process influences as the structures of the integrated circuit to be manufactured (see, for example, S. Wolf et al., Silicon Processing for the VLSI-Eras, Lattice Press, 1987, pp. 473-476). 
     Trench structures are being increasingly employed in circuit arrangements having an increased packing density. Among other things, such trench structures are filled with insulating material and are utilized as insulating trenches for insulation between neighboring circuit elements, for example between bipolar transistors or between power components and logic components. The trench structure is etched into the substrate. The auxiliary lithography structure is likewise subjected to the trench-etching process in the manufacture of the trench structure. 
     When the auxiliary lithography structures are broader than the trench structures--this being frequently the case in current technologies--a complete filling of the auxiliary lithography structures does not occur when filling deep trenches. The surface of the structure then comprises steps in the region of the auxiliary lithography structures, these steps leading to the fact that a reliable process management suitable for fabrication is no longer possible. This problem is especially serious in smart power technology on SOI material. Logic components and power components that are each respectively surrounded by an insulating trench that extends onto the surface of the insulating layer are thereby realized in a single-crystal silicon layer that is arranged on an insulating layer (see, for example, A. Nagakawa et al., ISPS 1990, pp. 97-101). In this employment, trench structures having widths of 1-3 μm and depths from 20-30 μm arise. Auxiliary lithography structures, whose geometry is usually prescribed by the apparatus manufacture, by contrast, comprise widths in the range from 3-30 μm. 
     SUMMARY OF THE INVENTION 
     The invention is based on the problem of specifying a method for manufacturing an integrated circuit arrangement wherein trench structures and auxiliary lithography structures are manufactured :such that a reliable process management suitable for fabrication is possible. In particular, high steps in the surface should be avoided for that purpose. 
     In the method of the invention, a trench structure is produced in a two-stage trench process that comprises at least two etching steps. A trench mask is thereby first produced at the surface of a substrate wafer by deposition of a layer and structuring of the layer. The structuring of the layer represents the first etching step of the trench process. Subsequently, the trench structure is realized in the substrate wafer in a second etching step. The material of the substrate wafer is attacked in the second etching step. In the trench process, an auxiliary lithography structure is simultaneously produced in the substrate wafer outside the integrated circuit arrangement. For example, auxiliary lithography structures are alignment marks, resolution patterns for focal and dimensional definition as well as verniers and other superimposition structures for identifying positional errors. In the manufacture of the trench mask, a protective structure that protects the substrate wafer in the region of the auxiliary lithography structure against an etching attack in the second etching step is formed in the region of the auxiliary lithography structure. For that purpose, for example, a layer that resists the second etching step is locally applied at the surface of the substrate wafer in the region of the auxiliary lithography structure. 
     Given a substrate wafer that comprises silicon at least in the region of a principal surface, the protective structure in the region of the auxiliary lithography structure can occur by surface-wide application of a layer sequence on the principal surface. The layer sequence comprises at least one SiO 2  layer arranged on the principal surface and a silicon layer arranged on the SiO 2  layer, and preferably composed of polysilicon. The silicon layer is now locally oxidized at least in the region of the trench structure, whereas the silicon layer in the region of the auxiliary lithography structure is not oxidized and remains in place here as a silicon layer. 
     The first etching step for manufacturing the trench mask is now implemented selectively relative to silicon, so that the etching in the region of the trench structure stops on the principal surface of the substrate wafer and stops on the silicon layer in the region of the auxiliary lithography structure. Both the oxide that arose due to local oxidation as well as the SiO 2  layer are etched through in the region of the trench structure in the first etching step. The SiO 2  layer is protected by the silicon layer in the region of the auxiliary lithography structure. 
     In the second etching step, the trench structure is produced by etching into the silicon of the substrate wafer. For that purpose, an etching selective relative to SiO 2  is employed that attacks only the silicon layer in the region of the auxiliary lithography structure, whereas it stops on the SiO 2  layer arranged therebelow. The SiO 2  layer acts as a protective structure here. 
     This embodiment of the method of the invention can be advantageously utilized in process sequences wherein field oxide regions are produced before the trench etching. In particular, these field oxide regions are formed in a modified LOCOS method that is known, for example, from R. Burroester et al., ESSCERC 1988. The local oxidation is implemented upon employment of a layer sequence of SiO 2 , polysilicon and Si 3  N 4 . The described embodiment of the invention can be introduced into such a process without requiring an additional photo technique. The thickness of the SiO 2  layer that is arranged at the surface of the substrate wafer must be set to the selectivity of the etching utilized in the second etching step, and must also be set to the depth of the trench structure. 
     Both a SOI substrate as well as a single-crystal silicon substrate are suitable as a substrate wafer in the method of the invention. Given a single-crystal silicon substrate, the depth of the trench structure is set via the etching time. 
     The invention shall be set forth in greater detail below with reference to an exemplary embodiment and to the drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a substrate wafer having a SiO 2  layer, a polysilicon layer and a Si 3  N 4  layer; 
     FIG. 2 shows the substrate; wafer after the structuring of the Si 3  N 4  layer for a local oxidation; 
     FIG. 3 shows the substrate wafer before the structuring of layers for producing a trench mask; 
     FIG. 4 shows the substrate; wafer after the first etching step; 
     FIG. 5 shows the substrate; wafer after the second etching step; 
     FIG. 6 shows the substrate wafer after the formation of spaces along the walls of the trench structure; and 
     FIG. 7 shows the substrate wafer after the filling of the trench structure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of the invention is set forth with reference to the example of a 500 volt smart power process. The initial material is a substrate wafer 1 of SOI material. The substrate wafer 1 comprises a single-crystal silicon substrate 11, an insulating layer 12 of SiO 2  arranged thereon that has a thickness of, for example 2 μm, and a lightly n-doped monocrystalline silicon layer 13 arranged on the insulating layer 12 that, for example, is 20 μm thick. The substrate wafer 1 is preferably manufactured according to the direct wafer bonding (DWB) method that is known, for example, from A. Nagakawa et al., ISPS 1990, pp. 97.101. 
     A SiO 2  layer 2 is produced on the surface of the monocrystalline silicon layer 13, for example by thermal oxidation of the surface (see FIG. 1). The SiO 2  layer 2 is produced in a thickness in the range between 50 and 300 nm. 
     A polycrystalline silicon layer 3 having a thickness of, for example, 50-250 nm, is deposited onto the SiO 2  layer 2, and a Si 3  N 4  layer 4 having a thickness of, for example 50-250 nm, is deposited on this polycrystalline silicon layer 3. 
     In FIG. 1, a region for an auxiliary lithography structure is referenced as double arrow L and a region for a trench structure is referenced as double arrow G. The auxiliary lithography structure is implemented as an inverse superimposition structure by contrast reversal. 
     A photo resist layer 5 is applied surface-wide and is exposed and developed such that the surface of the Si 3  N 4  layer 4 is uncovered in the region G for the trench structure, whereas the photo resist remains on the Si 3  N 4  layer 4 in the region L for the auxiliary lithography structure. The Si 3  N 4  layer 4 is structured in an anisotropic dry etching process (see FIG. 2). The anisotropic dry etching process is preferably selective relative to polysilicon, so that the etching process stops at the surface of the polysilicon layer 3. For example, a plasma etching process with CHF 3  /O 2  chemistry is suitable as an etching process. 
     Field oxide regions 6 are produced in the regions not covered by the structured Si 3  N 4  layer 4. They are produced by local oxidation by use of the Si 3  N 4  layer 4 as an oxidation mask (see FIG. 3). In the local oxidation, the uncovered parts of the polysilicon layer 3 are completely oxidized. Since the region L for the auxiliary lithography structure is covered by the Si 3  N 4  layer 4, the polysilicon layer 3 is preserved in this region. The surface of the polysilicon layer 3, by contrast, is uncovered in the region G for the trench structure, so that the polysilicon here is completely oxidized. 
     After the removal of the photo resist layer 5, a further Si 3  N 4  layer 7 is deposited in surface-wide fashion with a thickness of, for example 20-200 nm, and a further SiO 2  layer 8 is deposited thereon iin a thickness of, for example 200-1,000 nm. The further SiO 2  layer 8 is deposited in a CVD method. The further SiO 2  layer 8 must be suitable for a trench mask. In particular, it is required for setting the sidewalls of the trench structure. The further Si 3  N 4  layer 7 serves as an etch stop for the later removal of the SiO 2  layer 8 in a wet-chemical etching process. 
     A further photo resist layer 9 is applied surface-wide, and is exposed and developed such that both the region G for the trench structure as well as the region L for the auxiliary lithography structure are uncovered. 
     Subsequently, a first etching step is implemented for producing a trench mask 10 (see FIG. 4). For that purpose, an anisotropic dry etching process is employed that etches SiO 2  and Si 3  N 4  with nearly the same etching rates, but that etches silicon with a substantially lower etching rate. The further SiO 2  layer 8 and the further Si 3  N 4  layer 7 are therefore structured in the first etching step. The etching stops on the surface of the polysilicon layer 3 in the region L of the auxiliary lithography structure. By contrast thereto, the field oxide region 6 is likewise etched through in the region G for the trench structure, and the etching stops on the surface of the monocrystalline silicon layer 13 (see FIG. 4). The trench mask 10 is formed in the region G for the trench structure by the field oxide region 6, by the further Si 3  N 4  layer 7, as well as by the further SiO 2  layer 8. For example, a CHF 3  /O 2  plasma is suitable as an etching process for the first etching step. 
     The further photo resist layer 9 is removed after the first etching step. A second etching step is implemented. For that purpose, an anisotropic dry etching process is employed that etches silicon highly selectively relative to SiO 2 . In particular, a plasma etching process with HBr/NF 3  +He/O 2  chemistry is suitable for this purpose. This process has a selectivity of approximately 100:1 for etching silicon relative to SiO 2 . 
     The etching in the second etching step therefore stops on the surface of the insulating layer 12 of SiO 2  in the region G for the trench structure, as a result whereof a trench structure 14 is produced in the region of the monocrystalline silicon layer 13. The trench structure 14 comprises a depth corresponding to the thickness of the monocrystalline silicon layer 12, i.e. a depth of 20 μm and a width of 2 μm. The uncovered polysilicon layer 3 is removed in the region L for the auxiliary lithography structure in the second etching step. The etching in the second etching step then stops on the surface of the SiO 2  layer 2 that acts as a protective structure for the auxiliary lithography structure in the region L. The thickness of the SiO 2  layer 2 must therefore be set to the depth of the trench structure 14 and to the selectivity of the etching employed in the second etching step. Particularly when manufacturing trench structures having depths greater than 20 μm, the required thickness of the SiO 2  layer 2 can thereby be greater than required for the local oxidation (LOCOS) for the formation of the field oxide regions in a standard CMOS process in which the inventive method for producing the trench structure is embedded. 
     If it does not seem justifiable for purposes of the overall process to select the thickness of the SiO 2  layer correspondingly greater, it lies within the scope of the invention to locally produce an additional SiO 2  structure (not shown) in the region L for the auxiliary lithography structure before the deposition of the SiO 2  layer 2. This can occur by local oxidation or by deposition of SiO 2 . A mask is required for this purpose. It is especially advantageous to implement this SiO 2  structure in a preceding process step of the overall process, for example an implantation, with a mask employed for that purpose, so that a further mask is in fact required for the manufacture of the SiO 2  structure. This further mask, however, is not an additional mask in the overall process. 
     The trench structure 14 is subsequently filled in conformity with the planned use. For that purpose, for example, an amorphous silicon layer having an essentially conformal edge coverage is first deposited in surface-wide fashion and anisotropically etched, as a result whereof silicon spaces 15 are formed along vertical sidewalls (see FIG. 6). 
     Subsequently, the further SiO 2  layer 8 is wet-chemically removed, whereby the further Si 3  N 4  layer 7 acts as an etching stop (see FIG. 7). SiO 2  spacers 16 are formed in the trench structure 14 as well as in the region of the auxiliary lithography structure by oxidation of the silicon spacers 15. By repeated deposition of amorphous silicon having essentially conformal edge coverage, the trench structure 14 is filled with a silicon fill 17. The trench structure 14 is closed with a SiO 2  termination 18 by oxidation of the silicon fill 17. Simultaneously, further SiO 2  spacers 19 are formed in the region L of the auxiliary lithography structure (see FIG. 7). 
     The conventional process steps of an overall process can be continued after the removal of the further Si 3  N 4  layer 7, of the Si 3  N 4  layer 4, of the polysilicon layer 3, as well as of the SiO 2  layer 2. 
     Both the SiO 2  termination 18 as well as the field oxide regions 6 are thinned in the removal of the SiO 2  layer 2. Since the thickness of the SiO 2  layer 2 is prescribed by the oxide erosion in the second etching step, this must be intercepted by a longer local oxidation for producing the field oxide regions 6, or for producing the SiO 2  termination 18. In applications where this is not possible, for example because of an enlargement of the lateral expanse of the field oxide regions 6 due to an enlarged bird&#39;s bill length, a SiO 2  structure can also be locally produced here in the region L of the auxiliary lithography structure before the, deposition of the SiO 2  layer 2. 
     Due to the SiO 2  spacers 16, 19, elevations remain in the region L of the auxiliary lithography structure after the removal of the layer sequence. These elevations can be employed in common with the neighboring field oxide regions for the alignment of the next mask level as well as for the evaluation of the process steps. These elevations cause irregularities in the surface in the range of a few 100 nm, much like the irregularities also caused by gate electrodes or other elevations in the circuit structures. Irregularities on this order of magnitude are acceptable for the purpose of a reliable and fabrication suited process management. 
     Although various minor changes and modifications might be proposed by those skilled in the art, it will be understood that we wish to include within the claims of the patent warranted hereon all such changes and modifications as reasonably come within our contribution to the art.