Abstract:
A method which is particularly advantageous for improving a Self-Aligned Pattern (SAP) etching process. In such a process, facets formed on a spacer layer can cause undesirable lateral etching in an underlying layer beneath the spacer layer when the underlying layer is to be etched. This detracts from the desired vertical form of the etch. The etching of the underlying layer is performed in at least two steps, with a passivation layer or protective layer formed between the etch steps, so that sidewalls of the underlying layer that was partially etched during the initial etching are protected. After the protective layer is formed, the etching of the remaining portions of the underlying layer can resume.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Provisional Application No. 61/831,011 filed Jun. 4, 2013, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to improved etching methods. The invention can be particularly advantageous in etching with Self-Aligned Patterning (SAP) techniques. 
       BACKGROUND OF THE INVENTION 
       [0003]    Techniques herein relate to semiconductor device fabrication. Self-Aligned Patterning (SAP) is a known technique for decreasing feature sizes on semiconductor substrates. Due to the semi conformal nature of spacer materials, however, SAP techniques can lead to undesirable bowing of underlying layers. Techniques herein address such bowing by providing a technique to protect underlying layers while maintaining critical dimensions. 
         [0004]    In a typical SAP method, a spacer layer is formed upon mandrels, the top portion of the spacer layer is removed, and then the mandrel is removed, thereby leaving spaced portions of the spacer layer upon an underlying layer. The underlying layer is then etched between the portions of the spacer layer. However, the spacer layer can often have facets, for example, curved or inclined portions. When the underlying layer is subsequently etched, ions deflect from such facets and can impinge upon sidewalls of the underlying layer. As a result, the sidewalls are etched, and the desired vertical profile for etching of the underlying layer is not achieved. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention provides an improved etching method which is particularly advantageous when used with SAP techniques. Typically, when etching an underlying layer underneath a spacer layer, the underlying layer is etched to the complete depth of the etch in one etching step. In accordance with an example of the invention, a first portion of the underlying layer is etched followed by the deposition or growth of a protective layer on the sidewalls of the partially etched underlying layer. After the protective layer is formed, the remainder of the depth of the underlying layer is then etched. The protective layer thereby prevents or reduces lateral etching in the sidewalls of the underlying layer, so that bowing is avoided or minimized, and a more vertical etch is achieved. 
         [0006]    In accordance with a preferred example, after the first portion of the underlying layer is etched, etching is discontinued, followed by formation of the protective layer. The protective layer can be formed not only on the sidewalls of the partially etched underlying layer, but also on bottom surfaces. Therefore, in accordance with an example, when etching begins again, a breakthrough etch step can be first performed followed by continuing of etching through to the remainder of the depth of the underlying layer. In the breakthrough etch, the etching conditions can be modified to be more advantageous in etching through the protective layer on the bottom surface of the trench in the partially etched underlying layer. For example, the pressure can be decreased and/or the etch chemistry can be modified. 
         [0007]    The initial etching, protective layer forming, and then the continued etching or completion of etching of the remainder of the underlying layer, can be formed in the same process chamber, with a plasma continuously maintained throughout the process. For example, the process chemistry can be varied so that after the initial etching of the underlying layer, the etching is discontinued and the process chemistry or plasma chemistry is changed so that the protective layer is deposited or grown, and thereafter, the process chemistry can be again changed (e.g., back to that used for the initial etching) so that etching resumes to etch the remainder of the underlying layer, with a plasma maintained throughout the process. Alternately, the plasma can be extinguished after each of the steps, and then resumed for the next step. By way of example, and not to be construed as limiting, maintaining a plasma continuously throughout the process will typically be desirable for production of commercial products. However, discontinuing or extinguishing of the plasma might be desirable, for example, during process development. 
         [0008]    By way of example, the etching of the first portion of the underlying layer and the etching of the remaining portion of the underlying layer can use the same process gas chemistry, because the material of the underlying layer is the same. As noted above, when the etching initially resumes after deposition or growth of the protective layer, the process gas chemistry could also be modified during breakthrough of the protective layer on the bottom wall of the partially etched underlying layer. Because the etching is highly directional with vertical ions, this modification during breakthrough etching is optional. 
         [0009]    The invention will be better appreciated with reference to the detailed description of examples of embodiments herein. It is to be understood that the examples can be modified, and that certain features or combinations of features can be utilized without utilizing other features. Thus, the invention could be practiced using subsets of features or advantages of the disclosed examples, or with modifications to suit particular materials or processes to which the invention can be applied. In addition, it is to be understood that, unless specified, the sequence of steps of the invention could also be modified or could overlap with one another, or could include additional steps. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]      FIGS. 1A and 1B  illustrate forming and etching of a spacer layer in a conventional SAP process; 
           [0011]      FIGS. 2A and 2B  illustrate mandrel removal and etching of an underlying layer in a conventional SAP process; 
           [0012]      FIGS. 3A-3D  illustrate an example of etching in an SAP process of the invention; and  FIG. 4  is a flowchart representing an example of a process of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0013]      FIGS. 1A-B  and  2 A-B show a conventional self-aligned patterning (SAP) process and accompanying drawbacks. For such patterning, a mandrel  10  can be used as a core, and then a spacer layer  12  is deposited over the mandrel core. The mandrel and spacer layer are deposited on an underlying layer  14 . During deposition and/or spacer etch—prior to removal of the mandrel—the spacers can result in a faceted surface  12   a  on one side (the side that is not adjacent to the mandrel). This can be curved, angled, or otherwise not parallel to an opposing side. In  FIG. 2A , with the mandrel removed, the faceted portions  12   a  of the spacer can be easily distinguished. Such faceting can be undesirable during a subsequent underlayer etching process. During an underlayer etch, the faceted spacer/mask can cause ions, radicals, or other species to be deflected and strike sidewalls of the underlying layer as represented by arrows A in  FIG. 2B . This is undesirable because such deflected ions laterally etch the underlying layer, resulting in bowing. The undercut or bowing is primarily on the non-mandrel side, while the mandrel side (i.e., the sidewall surfaces adjacent to where the mandrel was located) has relatively little or no bowing. 
         [0014]    In accordance with the invention, a method of self-aligned pattern etching is provided that reduces bowing of an underlying layer in the presence of a faceted spacer. Such techniques include executing a partial etch, followed by a sidewall protection step to protect the partially etched underlying layer, and then an etch of the remaining underlying layer. Note that various different types of materials and etch/protection chemistries can be used. For convenience, techniques herein will identify certain specific examples of materials. For example, the spacer layer  12  can comprise silicon oxide (S 102 ), the underlying layer  14  can be silicon, and the substrate (or a further layer beneath the underlying layer)  16  can be silicon oxide or silicon nitride or other material. 
         [0015]    The mandrel  10  is formed by conventional techniques, for example, by depositing a layer of the mandrel material and then etching the layer so that a plurality of mandrels remain upon the substrate. Thereafter, the spacer material is formed upon the mandrel, portions of the spacer material are etched, and the mandrel is then removed, for example, by etching as discussed previously with respect to  FIGS. 1A-B  and  2 A. 
         [0016]    Referring to  FIGS. 3A-D , a substrate is provided or received having a spacer layer  112  (mandrel removed), an underlying layer  114 , and a substrate or further layer  116 . This substrate stack includes faceted/angled sidewalls  112   a  of spacers  112 . Using the spacers in the spacer layer as a mask, a top portion of the underlying layer is etched as show in  FIG. 3B . This partial etch can be executed by using an etch chemistry that is selective to the underlying layer relative to the spacers. After the partial etch, a protection layer  115  can be deposited and/or grown on the now created sidewalls ( 114   a ) of the underlying layer  114 . By way of a non-limiting example, when the underlying layer is silicon, then the deposited/grown protection layer can be silicon dioxide formed, for example, with an oxygen plasma. Thus, an oxidation step can be used for creating a protection layer. After silicon dioxide is protecting upper sidewalls of the underlying layer, then an etch step can be continued that etches the underlying layer  114  down to the substrate or target layer  116  below. 
         [0017]    As mentioned, various chemistries and protection layers can be selected based on particular material compositions of the various layers. For example, if the underlying layer  114  is a nitride, then a fluorocarbon chemistry can be used to create a protection or passivation layer  115 . When the underlying layer  114  is an organic layer, then corresponding chemistries can be used that resist etching the mask ( 112 ,  115 ) while etching the underlying layer. Also note that the invention is not limited to a partial etch followed by a single deposition, which is then followed by a full/remaining etch. For example, there can be several etch/deposition/etch cycles for a particular application. 
         [0018]    The amount of etching of the underlying layer  114  before forming a protective or passivation layer can vary depending upon the materials and the depth to be etched. However, preferably the passivation or protective layer  115  is deposited when there has been an initial etch of 50% or less of the total depth to be etched. 
         [0019]    After the first etching or initial etching, a trench is formed in the layer  114  having both sidewalls  114   a  and a bottom wall  114   b  in the trench. When the protective layer is formed, it typically will be formed on both the sidewalls  114   a  and bottom walls  114   b  of the trench of the partially etched underlying layer, with respective protective layer portions  115   a  and  115   b.  The portion  115   b  on the bottom wall  114   b  of the underlying layer  114  must therefore be etched through before continuing to etch the remaining portions of the underlying layer  114 . Preferably, the etching of the underlying layer  114  is conducted using a low pressure, for example, 1-100 mTorr which produces ions that are highly directional, so that the protective layer  115   b  at the bottom of the trench can be etched through while the protective layer portion  115   a  is sufficient to protect the sidewalls  114   a  with respect to deflected ions. 
         [0020]    To further assist in etching through or breaking through the portion  115   b , the process can optionally be modified for a breakthrough etch of the portion  115   b,  and thereafter, the etching of the remaining portions of the underlying layer  114  can resume (e.g., with the same plasma chemistry and conditions as used in the initial underlying layer partial etch). For example, the pressure and/or etch chemistry can be modified during the initial etching after the protective layer  115  has been formed to further assist in etching through the bottom portion  115   b  of the protective layer. As noted earlier, the plasma processing is preferably conducted at low pressure. During the deposition or growth of the protective layer, the pressure can be slightly higher than that used during etching of the underlying layer, as a slightly higher pressure is beneficial for formation or growth of the protective layers (during the deposition or growth of the protective layer  115 , the pressure remains low, for example, in the 1-100 mTorr range, however the pressure is preferably slightly higher than the pressure used during etching). During the etching through the bottom portion  115   b,  the pressure is again reduced, and is preferably lower than the pressure used for the deposition/growth of the protective layer. This reduced pressure for etching through the bottom portion  115   b  can be the same as or could be slightly lower than the pressure used for etching the remainder of underlying layer  114 . In addition, the etch chemistry for etching portion  115   b  can be modified as compared with the etch chemistry used for the remaining portion of the underlying layer  114 . For example, during the etching of the bottom portion  115   b,  fluorine or a fluorine containing gas, such as CF x  or C x F y  could be utilized, and thereafter, the gas chemistry can be changed back to the gas chemistry utilized for etching the underlying layer  114 . The gas chemistry used for etching the underlying layer  114  can be the same in the initial etching and for etching of the final or remaining portions of the underlying layer  114 . 
         [0021]    The plasma chemistry will depend upon the materials of the underlying layer being etched, however, by way of example, and to not be construed as limiting, an HBr or Cl plasma can be used for etching the underlying layer. By way of example, and not to be construed as limiting, the initial etching of the underlying layer can proceed for about 20-40 seconds, with the deposition or passivation forming/growth then proceeding for 2-30 seconds, followed by a breakthrough etch (to etch the portion  115   b ) of approximately 2-10 seconds, with the final or remaining etch proceeding for approximately 30-60 seconds. As noted earlier, rather than providing a two-step etch of the underlying layer with one deposition/growth in between, multiple cycles of alternating etching and protective layer forming at steps could be utilized. 
         [0022]    Beneath the underlying layer  114 , the substrate  116  or substrate layers  116  are illustrated. Various layers  116  could be present underneath the underlying layer  114 . The target layer  116  can serve as an etch stop layer so that once the underlying layer  114  is etched fully through, the etching discontinues. By way of example, the layer  116  can be SiO or SiN. Of course, various layers can also be present underneath the layer immediately below the underlying layer  114 . The layer or substrate  116  could be the silicon wafer substrate or substrate base itself. The layer  116  could also be a layer or substrate that is subsequently etched using the underlying layer  114  as a mask. For example, the underlying layer  114  could be formed of a mask material so that, after the etching of the underlying layer  114 , the underlying layer  114  (e.g., as shown in  FIG. 3B ) provides a mask for subsequent etching of the substrate  116 , with the etching of the substrate  116  (or other layer present beneath the underlying layer  114 ) proceeding in the spaces or apertures formed between the portions of the underlying layer  114  remaining upon the substrate  116 . 
         [0023]    As noted earlier, according to an example, the spacer layer  112  can be SiO 2 , and it can serve as a mask for the etching of the underlying layer  114  which can be, for example, Si. An oxygen plasma can then be used for forming the protective layer  115  so that the protective layer is, for example, a silicon oxide. It is to be understood that alternate materials can be utilized, or in other words, the present invention could also be utilized or applied to configurations or architectures in which alternate materials are used. For example, the spacer or mask layer  112  could be a silicon nitride material. The protective layer can be SiO, or a polymer or fluorocarbon such as C x H y , or a silicon coating, for example, formed with an SiCl plasma. Beneath the underlying layer, the layer  116  can be, for example, SiN or SiO. For etching the underlying layer, by way of example, a C x F y  can be used for the initial etching and for completion of etching of the underlying layer. If desired to use a different chemistry or different gas for the breakthrough of the portion  115   b  a leaner gas (less polymer) could be used during the breakthrough, followed by resuming of etching with the same gas chemistry for etching the remaining portions of the underlying layer  114  as was used for etching the initial portions of the underlying layer. 
         [0024]    Further by way of example, the spacer or mask layer  112  can also be or include a carbon material, an oxide, or nitride, and the underlying layer can be a carbon material such as an OPL (organic planarization layer), and for protection after the initial etch, a polymer or silicon polymer can be formed on the sidewall of the underlying layer. For etching of the underlying layer, an oxygen based plasma, CO, CO 2 , or COS plasmas can be utilized. In addition, if desired, to assist in breakthrough of the bottom portion  115   b  of the protective layer, additional C x F y  could be utilized for etching through the bottom portion  115   b,  followed by resumption of etching with the same gas chemistry as used during the etching of the initial portion of the underlying layer  114 . 
         [0025]      FIG. 4  is a flowchart representing the process of the invention. As shown in step S 100 , mandrels are first formed, for example by etching a layer of the mandrel material to leave mandrels on the underlying layer. After the mandrels are formed, in step S 102 , a spacer layer is formed on the mandrels, followed by etching of the top portions of the spacer layer in step S 104 . Thus, after step S 104 , the spacer layer is present along the sidewall surfaces of the mandrels, but the top of the mandrel is exposed, so that the mandrels can be removed in step S 106 , for example, by etching. In step S 108 , the first portion of the underlying layer is etched, followed by formation of the protective layer on the partially etched underlying layer in step S 110 . Thereafter, a breakthrough etch can be performed in step S 112 . As discussed earlier, the modification of the etching process for the breakthrough etch is optional, and the same etch process as used for etching the underlying layer could also be used to etch through the bottom portion  115   b  of the protective layer. To further assist etching through of the bottom portion of the protective layer, a breakthrough etch step can be performed in which, for example, the pressure is reduced and/or the etch chemistry modified (for example, with a leaner or lower amount of polymer, or with an additional fluorine containing gas) to assist in the etching through the protective layer bottom portion  115   b . Thereafter, etching continues to etch through the underlying layer as indicated in step S 114 . As also discussed earlier, the initial etching, formation of the protective layer, the optional breakthrough etching, and the completion of etching through the underlying layer can be performed in the same process chamber with a plasma maintained continuously throughout, and with the etch chemistry or other processing conditions modified for performing the various steps. Alternatively, the plasma can be extinguished after one or more of the various steps, and then a plasma is re-struck for the next step. 
         [0026]    In the preceding description, specific details have been set forth. It should be understood, however, that techniques herein may be practiced in other embodiments that depart from these specific details, and that such details are for purposes of explanation and not limitation. Embodiments disclosed herein have been described with reference to the accompanying drawings. Similarly, for purposes of explanation, specific numbers, materials, and configurations have been set forth in order to provide a thorough understanding. Nevertheless, embodiments may be practiced without such specific details. Operations described may be performed in a different order than the described embodiments unless otherwise specified. Various additional operations may be performed and/or described operations may be omitted in additional embodiments. 
         [0027]    Those skilled in the art will also understand that there can be many variations made to the operations of the techniques explained above while still achieving the same objectives of the invention. Such variations are intended to be covered by the scope of this disclosure. As such, the foregoing descriptions of embodiments of the invention are not intended to be limiting upon the scope of the following claims.