Abstract:
An etching signal layer which is formed by a sequential gas phase deposition with a layer thickness of less than 20 nanometers, and which is composed of a metal oxide or of an oxide of rare earths is provided between a substrate, which is located underneath it, and a process layer. The etching signal layer produces an etching signal, which is independent of the stack layer systems that are to be removed, and contains two or more materials that contain silicon, and can be removed quickly and with narrow process tolerances. One substrate surface of the substrate is protected irrespective of the topography. Etching methods based on the etching signal layer can be carried out precisely, and can be used in a variable manner.

Description:
BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0001]    The invention relates to an etching method for removing material in the course of processing semiconductor wafers. The method includes providing a semiconductor wafer as a substrate, providing an etching signal layer at least on sections of one substrate surface of the substrate, providing a process layer at least on sections of the etching signal layer, and removing at least sections of the process layer in the course of an etching process. An etching signal is produced upon an exposure of sections of the etching signal layer that are located underneath the removed sections of the process layer. The etching process is ended as a function of the etching signal.  
           [0002]    The invention also relates to an etching signal layer.  
           [0003]    The processing of semiconductor wafers generally includes two or more process steps with etching processes, by which either layers are removed over the entire surface, or mask patterns that are produced lithographically are transferred to layers or structures located underneath them.  
           [0004]    In wet-chemical etching processes, solid materials are dissolved in a chemical solution. In chemical-mechanical polishing processes (CMP), wet-chemical etching is assisted by the mechanical influence of a polishing agent on the layer or structure to be removed. In dry chemical etching processes, the material to be removed reacts with components of an etching gas, forming volatile reaction products. In the course of physical dry etching processes, the material to be removed is subjected to a particle beam, under the influence of which the material to be removed is vaporized, and individual atoms are detached from the layer or structure to be removed. For reactive ion etching (reactive ion beam etching, RIE), ions are accelerated in the direction of the material to be removed, and initiate a chemical etching process when they strike it.  
           [0005]    Isotropic etching processes, for example wet-chemical etching, remove material independently of the direction while anisotropic etching processes such as RIE and CMP in contrast remove the material in a preferred direction.  
           [0006]    The etching processes generally stop at etching stop layers. The etching stop layers are in each case provided from a material which does not recede as a result of the respectively used etching process, or which recedes at an etching rate which is very slow in comparison with that of the material to be removed.  
           [0007]    It is also normal to use measurement techniques to detect an etching end point by use of an etching stop signal layer. In the case of dry etching processes by way of example, the process surface which has been etched back by the etching process is for this purpose evaluated spectroscopically, or, in the case of wet-chemical etching processes, the etching rate is recorded continuously, and any significant change is evaluated.  
           [0008]    The material of the etching stop layer and/or of the etching stop signal layer is chosen such that the etching process for the material to be removed is as selective as possible with respect to the material of the etching stop layer and of the etching stop signal layer. Admittedly, silicon, silicon oxide (SiO 2 ) and silicon nitride (Si 3 N 4 ) and silicides can in principle be etched selectively. If materials containing silicon are provided as etching stop layers with respect to one another, then the etching stop layers are also attacked by the etching process and must be provided with a sufficient layer thickness in order to reliably prevent the etching process from penetrating into the material which is located under the etching stop layer. If silicon nitride is used as the etching stop layer or etching stop signal layer when removing silicon oxide or silicon, then the silicon nitride layer must be provided with a thickness of several tens of nanometers. In order to assist in the removal of the etching stop layer itself, a further barrier layer is then required between the etching stop layer and the substrate, for example silicon oxide, which is located underneath it, in order to protect the substrate which is located underneath during removal of the etching stop layer.  
           [0009]    U.S. Pat. No. 6,593,633 discloses a method for producing a semiconductor device, in the course of which a silicon carbide layer (SiC) is deposited in a plasma-enhanced gas phase deposition method (plasma-enhanced chemical vapor deposition, PECVD) and an etching stop layer that remains in places is in consequence provided. After being used as an etching stop layer, the silicon carbide layer remains in places as an isolator layer with low permittivity between two semiconductor device structures that need to be decoupled.  
           [0010]    U.S. Pat. No. 6,518,671 (Yang et al.) names tantalum oxide (Ta 2 O 5 ) and aluminum oxide (Al 2 O 3 ) as materials for the etching stop layers. The layer thickness of the etching stop layer is 20 to 70 nanometers. The etching stop layer is applied to a planar surface and is structured photolithographically. The etching stop layer limits material removal in an anisotropic chemical-physical dry etching process.  
         SUMMARY OF THE INVENTION  
         [0011]    It is accordingly an object of the invention to provide an etching method and an etching signal layer for processing semiconductor wafers that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which two or more materials which contain silicon can be removed in a single etching process, irrespective of the etching process that is used and irrespective of the surface structure of a layer or structure which is located underneath and needs to be protected against removal.  
           [0012]    With the foregoing and other objects in view there is provided, in accordance with the invention, an etching method for removing material in a course of processing semiconductor wafers. The method includes providing a semiconductor wafer as a substrate and performing a sequential gas phase deposition process or a molecular beam epitaxy process for producing an etching signal layer at least on sections of one substrate surface of the substrate. The etching signal layer is a dielectric layer composed of a metal oxide or an oxide of rare earths. A process layer is provided at least on sections of the etching signal layer. At least sections of the process layer are removed in a course of an etching process. An etching signal is produced upon exposure of sections of the etching signal layer disposed underneath removed sections of the process layer and results in exposed sections. The etching process is ended in dependence on the etching signal.  
           [0013]    In the course of the etching method according to the invention for removal of material in the course of processing semiconductor wafers, a semiconductor wafer is first provided as the substrate. At this time, the semiconductor wafer is typically already structured with different materials. An etching signal layer is provided at least in places on one substrate surface of the substrate. A process layer is provided at least in places on the etching signal layer. The process layer is either integral and homogeneous, or is an unstructured or intrinsically structured layer system. In the course of an etching process, the process layer is removed completely or in places. If, by way of example, the process layer is produced by a deposition process, in the course of which trench structures that extend into the substrate are filled, then the process layer is removed completely above the etching signal layer. If a primary structure of an etching mask is mapped into the process layer, by a photolithographic process, then the process layer is removed only in places above the etching signal layer. An etching signal is produced during or after the exposure of sections of the etching signal layer that are located under those sections of the process layer which are to be removed. The etching process is ended as a function of the etching signal. The etching signal can be evaluated by an etching or polishing apparatus.  
           [0014]    According to the invention, the etching signal layer is provided by a sequential gas phase deposition (atomic layer deposition (ALD)) or molecular beam epitaxy as a dielectric layer composed of a metal oxide, including oxides of rare earths.  
           [0015]    Metal oxides and oxides of rare earths can be etched in one step with high etching selectivity with respect to all normal materials that contain silicon in semiconductor process technology, such as crystalline silicon, amorphous silicon, silicon dioxide and silicon nitride. Sequential gas phase deposition can be used to produce the etching signal layer even on structured substrates with a uniform layer thickness and with good edge coverage. Layers deposited by sequential gas phase deposition and composed of metal oxide or oxides of rare earths are on the one hand robust against normal chemical etching methods, but on the other hand can effectively be pulverized and removed in a simple manner by mechanical processes, for example by particle bombardment, particularly when the layers are very thin.  
           [0016]    The etching method according to the invention allows not only standardization of the etching processes which are required in the course of processing the semiconductor wafers, but also makes it very simple to remove etching signal layers which are used only temporarily.  
           [0017]    The etching signal layer is preferably provided with a layer thickness for which charge carrier transport based on the quantum-mechanical tunnel effect through the etching signal layer is still possible to a significant extent. Charge carrier transport to a significant extent may be assumed to be current densities of more than 10 −5  A/cm 2  for isolating applications and more than 10 −2  A/cm 2  for conductive applications, depending on the purpose and the material that is used.  
           [0018]    In conjunction with the application of the etching signal layer by sequential gas phase deposition, as a consequence of which the etching signal layer has a virtually uniform layer thickness over its entire extent, remanent sections of the etching signal layer may also be disposed between semiconductor wafer structures that have to be conductively connected to one another. Therefore, for certain applications, for example when forming vertical connecting structures (plugs), there is no need to remove the etching signal layer.  
           [0019]    The etching signal layer is provided in a particularly advantageous manner with a layer thickness of less than 20 nanometers. This results in the electrical contact resistance between conductive structures being increased to a comparatively minor extent. Furthermore, an etching signal layer is removed quickly and easily in comparison to conventional etching stop layers, for example silicon nitride layers. There is no need to provide a further barrier layer underneath the etching stop layer, to protect the substrate that is located underneath against removal of the etching stop layer. Suitable metal oxides for forming the etching signal layer are, for example, HfO 2 , ZrO 2 , Nb 2 O 5 , Ta 2 O 5  and Al 2 O 3 . Suitable oxides of rare earths are, for example, Pr 2 O 5  and La 2 O 3 .  
           [0020]    The etching signal layer is preferably formed from a material that has a signal element that is not contained in the process layer, for example a metal. A process surface which is in each case actually subjected to the etching process (wet-chemical, RIE, CMP) and which is formed as a function of the progress of the etching process of the process layer and/or of the etching signal layer is observed and evaluated spectroscopically. The etching signal is produced when the signal element is detected spectroscopically. The etching signal is thus produced in a simple manner, and precisely synchronized to the progress of the etching process.  
           [0021]    According to another preferred embodiment of the etching method according to the invention, an etching rate of the etching process is recorded continuously, for example during CMP processes, and an etching signal is produced when a significant change, normally a significant reduction, occurs in the etching rate.  
           [0022]    The etching process, irrespective of whether this is a wet-chemical process, RIE or CMP, can be ended at the same time as the etching signal or with a slight run-on, so that the etching process stops on the surface of the etching signal layer. However, it is particularly advantageous for the etching process to be ended after a first run-on time, which is of such duration that the etching signal layer in the exposed sections is actually completely removed during the first run-on time. This is in fact possible because the etching signal layer has a virtually uniform and very thin layer thickness since it is produced by sequential gas phase deposition. The timing for the removal of the etching signal layer with respect to the previous etching signal can be controlled with very narrow tolerances, so that the substrate which is located underneath it is not damaged in the process.  
           [0023]    As an alternative to this, the etching process is ended after a second run-on time has elapsed. The second run-on time is then of such a duration that the substrate which is located underneath the etching signal layer is removed during the second run-on time as far as the depth which is required for the purpose, in sections which are located underneath the exposed sections of the etching signal layer. This type of processing, for example in conjunction with RIE and CMP processes, results in relatively narrow manufacturing tolerances, since the etching signal layer is itself removed within narrow tolerances, owing to its thin layer thickness and high conformity.  
           [0024]    It is also advantageous to apply a first process layer element at least to sections of the etching signal layer before the provision of the etching signal layer on the substrate, and to apply a second process layer element at least to sections of the etching signal layer after the provision of the etching signal layer. The second run-on time may then be of such a duration that, during the second run-on time, the second process layer element is actually completely removed in the sections that are located underneath the exposed sections of the etching signal layer.  
           [0025]    In this case, the etching signal that is produced by the etching signal layer is used for fine adjustment of an etching process that acts on the entire process layer formed by the two process layer elements.  
           [0026]    In certain cases, adaptation of parameters of the etching process as a function of detection of the etching signal is advantageous for the various variants mentioned.  
           [0027]    The etching method according to the invention is particularly advantageous when the process layer is formed from more than one stack layer, in which case at least one of the stack layers may be provided with a secondary structure having two or more stack sections composed of different materials. Layer systems such as these, which are structured jointly at least in places, and are removed above an etching signal layer, can occur in semiconductor process technology in conjunction, for example, with connecting lines which cross in different levels, but not only in this case.  
           [0028]    The advantages of the etching method according to the invention are particularly applicable when the substrate is provided with a relief, which is structured on the basis of a basic structure, with trenches and webs on the substrate surface. The etching method according to the invention ensures that the etching signal layer is provided with a high degree of uniformity, with a thin layer thickness, homogeneously and with good edge coverage, even on the structured substrate surface, so that the advantages mentioned above are also still obtained for structured surfaces, in contrast to known etching methods.  
           [0029]    The method according to the invention is suitable for photolithographic methods, in the course of which a primary structure of a photolithographic mask is mapped by an etching process into the process layer. A mask which is structured on the basis of the primary structure is for this purpose provided on the process layer, and the primary structure is transferred by the etching process at least into the process layer. In this case, the secondary structure can be provided independently of the primary structure, and in a different manner to it.  
           [0030]    Alternatively, the etching process can also be carried out as a chemical-mechanical polishing method. In this case, sections of the process layer above a substrate edge that is defined by an upper edge of a web that is the highest with respect to the substrate surface are removed in the course of the etching process.  
           [0031]    In certain cases, the etching signal layer may also be vertically structured or may be deposited such that it is already structured in the course of the etching method according to the invention.  
           [0032]    The etching signal layer is for this purpose provided in a particularly preferred manner by a sequential gas phase deposition, in which two or more molecular individual layers are deposited successively, and in which the deposition process is controlled such that each individual layer is deposited in the direction from the substrate edge into the depth of the relief. The process of depositing each individual layer is ended prematurely before the respective individual layer completely covers the substrate that is located underneath it. Therefore, the etching signal layer is deposited in such a way that it is already vertically structured, so that there is no need for subsequent vertical structuring, for example by modification of the etching resistance by oblique implantation of dopants which influence the etching resistance of the etching signal layer.  
           [0033]    The etching signal layer is in certain cases compressed and hardened by a tempering or heat step. In conjunction with the etching of a process layer composed of material containing silicon, an aluminum oxide layer with a thickness of about 5 nanometers is provided which is hardened in a tempering step at 700 to 1,100 degrees Celsius for at least 5 seconds to a maximum duration of about 10 hours.  
           [0034]    The aluminum produces a unique spectroscopic etching signal. The aluminum oxide layer which is formed from crystallites of different crystal structure has chemical/physical characteristics which are so largely different to silicon and silicon compounds that silicon, silicon oxide and silicon nitride can be etched back jointly with high etching selectivity with respect to the etching signal layer. Aluminum oxide is stable in response to conventional chemical/physical etching processes, so that layer thicknesses of less than 20 nanometers and less than 10 nanometers are sufficient for it to act as an etching signal layer. Aluminum oxide layers of up to 5 nanometers or 10 nanometers can be effectively pulverized and can be removed comparatively easily, and in a manner which protects the substrate, by mechanical processes, just by particle bombardment. A similar situation applies to hafnium oxide layers used as etching signal layers.  
           [0035]    The process of carrying out the etching method according to the invention is associated with the etching signal layer according to the invention. The etching signal layer according to the invention is composed of a metal oxide or an oxide of rare earths. The etching signal layer is produced by sequential gas phase deposition, or by a molecular beam epitaxial method. The etching signal layer is provided as a layer system containing two or more monomolecular individual layers. The layer thickness of the etching signal layer is characterized in that a significant charge carrier transport, based on the quantum-mechanical tunnel effect, is still possible through the etching signal layer. The layer thickness is preferably less than 20 nm.  
           [0036]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0037]    Although the invention is illustrated and described herein as embodied in an etching method and an etching signal layer for processing semiconductor wafers, 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.  
           [0038]    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  
       [0039]    [0039]FIGS. 1A and 1B are diagrammatic, sectional views of a first exemplary embodiment of an etching method according to the invention, with an anisotropic etching process that stops on an etching signal layer;  
         [0040]    [0040]FIG. 2 is a sectional view of the exemplary embodiment shown in FIGS. 1A and 1B, after a first etching run-on duration;  
         [0041]    [0041]FIG. 3 is a sectional view of the exemplary embodiment shown in FIG. 2, after a second etching run-on duration;  
         [0042]    [0042]FIGS. 4A and 4B are sectional views of an exemplary embodiment of the etching method according to the invention, with fine adjustment of the etching process by use of an etching signal layer;  
         [0043]    [0043]FIGS. 5A and 5B are sectional view of an exemplary embodiment of the etching method according to the invention, for removal of multilayer structures;  
         [0044]    [0044]FIGS. 6A and 6B are sectional views of an exemplary embodiment of the etching method according to the invention, with a chemical-mechanical etching process;  
         [0045]    [0045]FIGS. 7A and 7B are sectional views showing an exemplary embodiment of the etching method according to the invention as shown in FIGS. 6A and 6B, with a layer system to be removed composed of different materials;  
         [0046]    [0046]FIGS. 8A and 8B are sectional views showing an exemplary embodiment of the etching method according to the invention with an isotropic etching process; and  
         [0047]    [0047]FIGS. 9A and 9B are sectional views showing an exemplary embodiment of the method according to the invention with an etching signal layer which is deposited in a vertically structured manner. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1A thereof, there is shown a substrate  1 . The substrate  1  is composed of monocrystalline silicon, silicon oxide, silicon nitride, and/or some other material that is normally used in semiconductor process technology. The substrate  1  is homogeneous or has structures that are not shown in FIG. 1A or in the following figures.  
         [0049]    An etching signal layer  2  with a thickness of less than 20 nm is applied to one substrate surface  10  of the substrate  1 . The etching signal layer  2  was produced by sequential gas phase deposition, and is composed of two or more molecular individual layers. A process layer  3  is provided on the etching signal layer  2 . An etching mask  4 , composed of a photoresist material, which has been structured by a photolithographic process is located on the process layer  3 .  
         [0050]    The structure of the etching mask  4  is transferred by an etching process to the process layer  3  which is located underneath it.  
         [0051]    In the example in FIG. 1B, the removal of the material of the process layer  3  ends at an upper edge of the etching signal layer  2 . Since it is thin, the etching signal layer  2  can be removed easily and quickly. However, by virtue of its dielectric character, the etching signal layer  2  may remain underneath a dielectric structure, which is adjacent in exposed sections, in the process layer  3  or above a dielectric structure in the substrate  1 . As a result of its conductive characteristics owing to its thin layer thickness, it may then also remain on the substrate surface  10  if it is provided between two conductive structures that need to be connected to one another.  
         [0052]    [0052]FIG. 2 shows the result of the etching method when the etching process, possibly in the event of a change to the process parameters, continues for a predefined time synchronized to the first occurrence of the etching signal, before it is interrupted. As a result of the thin, uniform layer thickness, resulting from the method, of the etching signal layer  2 , the etching signal layer  2  is removed uniformly in a comparatively short time during the run-on time, with narrow tolerances and without any damage to the substrate surface  10  of the substrate  1 . This results in the structured etching signal layer  2 ′ as shown in FIG. 2.  
         [0053]    The narrow etching tolerances during removal of the etching signal layer  2  also make it possible, as is shown in FIG. 3, to etch back the substrate  1  in a second run-on time in a defined manner in those sections which are located underneath the exposed sections of the etching signal layer  2  and with narrow tolerances, by a predetermined run-on dx.  
         [0054]    In contrast to the previous examples, the process layer element  3  in the exemplary embodiment of the etching method according to the invention illustrated in FIGS. 4A, 4B is applied in the form of two process layer elements  31 ,  32 , as is shown in FIG. 4A. In this case, the second process layer element  32  is located underneath the etching signal layer  2 . The first process layer element  31  is located on the etching layer signal  2 . Particularly when, for example, the layer thickness of the first process layer element  31  is not known precisely, or the first process layer element  31  is intended to be removed comparatively quickly, the second process layer element  32 , or parts of it, can be removed synchronized to an etching signal that is generated by the etching signal layer  2 . This method is particularly advantageous in conjunction with chemical-mechanical polishing methods used as the etching process.  
         [0055]    [0055]FIG. 4B shows the process layer elements  31 ′,  32 ′, structured by the etching process as well as the structured etching signal layer  2 ′.  
         [0056]    In comparison to FIGS. 1A, 1B, FIG. 5A shows the process layer  3  as a layer stack with a first, a second and a third stack layer  331 ,  332 ,  333 . In the exemplary embodiment, the second stack layer  332  is structured and has first and second stack layer sections  332   a ,  332   b  composed of different materials. The material of the first stack layer is, for example, silicon oxide, and that of the third stack layer is silicon nitride. The materials of the first and second stack layer sections  332   a ,  332   b  are silicon and silicon nitride respectively. The second stack layer  332  is structured on the basis of a secondary structure. An etching mask  4  that is placed on the process layer  3  is structured on the basis of a primary structure, which is independent of the secondary structure. The primary structure of the etching mask  4  is mapped by an etching process, which acts on all the materials of the process layer  3  that contains silicon independently of the secondary structure provided in the process layer  3 , into the process layer  3  with the assistance of the etching signal layer  2  according to the invention.  
         [0057]    [0057]FIG. 5B shows the result of the structuring of the process layer, as well as the structured process layer  3 ′ that is produced in this way.  
         [0058]    The exemplary embodiment illustrated in FIGS. 5A, 5B can be combined with the exemplary embodiments in FIGS.  2  to  4 B.  
         [0059]    [0059]FIGS. 6A, 6B relate to a CMP process as the etching process used according to the invention. Trenches  61  are incorporated in the substrate  1  and, together with webs  62 , structure a substrate surface  10  of the substrate  1  in the form of a relief. An etching signal layer  2  is applied to the substrate surface  10  covering the substrate  1  with a maximum thickness of 20 nm, homogeneously, uniformly and with good edge coverage. The trenches  61  are filled during the course of a gas phase deposition process, with the filling material also being deposited above an upper edge of the trenches  61 , and forming the process layer  3 .  
         [0060]    The process layer  3  is removed above the substrate surface  10  in the course of a chemical-mechanical polishing process. On detection of an etching signal, which is produced by exposed sections of the etching signal layer  2 , the polishing process is ended. FIG. 6B shows the process layer  3 ′ that has been structured in this way and is recessed into the trenches  61 .  
         [0061]    In comparison to the exemplary embodiment illustrated in FIGS. 6A, 6B, the exemplary embodiment in FIGS. 7A, 7B shows the process layer  3  as a layer stack with the first, second and third stack layers  331 ,  332 ,  333 .  
         [0062]    [0062]FIGS. 8A, 8B shows the etching method according to the invention in conjunction with an isotropic etching process. FIG. 8A shows the substrate  1  with the structured surface  10 . The etching signal layer  2  is in the form of a conformal coating on the substrate  1  in the region of the substrate surface  10 . The process layer  3  with a planar surface is adjacent to the etching signal layer  2 . A structured etching mask  4  is applied to the process layer  3 . The process layer  3  is structured in accordance with the structure of the etching mask  4 , using an etching process with an isotropic component. The result is illustrated in FIG. 8B. The conformal configuration of the etching signal layer  2  allows even complex structures to be reliably protected against isotropic etching processes.  
         [0063]    A trench  61  is incorporated in the substrate  1  which has two sections  11 ,  12  in FIG. 9A. A conductive structure is formed within the second layer element  12  of the substrate  1  and can be conductively connected to a filling in the trench  61  with a low contact resistance. The filling in the trench  61  is produced by a layer sequence of different materials, which are each deposited conformally. This results in the process layer  3  being in the form of a layer stack with three stack layers  331 ,  332 ,  333 . The layer stack is etched back, for example, by a chemical-mechanical polishing method, above a substrate edge  7  that is formed by an upper edge of the trench  61 . The etching signal layer  2  is for this purpose deposited in a vertically structured manner. The etching signal layer  2  covers horizontal sections of the substrate surface  10  in the region of the trench edge  7 , and upper vertical sections of the trench  61 .  
         [0064]    The process layer  3 ′ is etched back into the trenches  61  by a chemical-mechanical polishing process. No component of a dielectric etching signal layer which would increase the contact resistance between conductive end structures in the second layer element  12  and in the filling  3 ′ is located between the second layer element  12 , in which conductive structures are formed, and the filling of the trench  61  or the structured process layer  3 ′.