Patent Publication Number: US-11658037-B2

Title: Method of atomic layer etching of oxide

Description:
This application is a continuation of U.S. patent application Ser. No. 16/401,441, entitled “Method of Atomic Layer Etching of Oxide,” filed May 2, 2019, which claims priority to U.S. Provisional Patent Application No. 62/670,459, entitled, “Method of Atomic Layer Etching of Oxide,” filed May 11, 2018 and U.S. Provisional Patent Application No. 62/684,878, entitled, “Method of Atomic Layer Etching of Oxide,” filed Jun. 14, 2018 the disclosure of which is expressly incorporated herein, in its entirety, by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to the processing of substrates in plasma process equipment. In particular, it provides a method to control plasma etching of layers comprising oxides. 
     The use of plasma systems for the processing of substrates has long been known. For example, plasma processing of semiconductor wafers is well known. One well known use of plasma processing is for etching of substrates. Plasma etching presents numerous technical challenges. Further as geometries for structures and layers on substrates continue to shrink, tradeoffs between etch selectivity, profile, aspect ratio dependent etching, uniformity, etc. become more difficult to manage. In order to achieve desired process performance, variable settings of the plasma processing equipment can be adjusted to change the plasma properties. These settings include, but are not limited to gas flow rates, gas pressure, electrical power for the plasma excitation, bias voltages, etc., all as is known in the art. However, as geometries continue to shrink it has been found that sufficient control of ion energy, ion flux, radical flux, etc. that results from the settings of the plasma processing equipment is not satisfactory to achieve the desired etch results. 
     One technique to improve plasma etching has been to utilize atomic layer etch (ALE) plasma processes. ALE processes are general known to involve processes which remove thin layers sequentially through one or more self-limiting reactions. Thus, ALE processes offer improved performance by decoupling the etch process into sequential steps of surface modification and removal of the modified surface, thereby allowing the segregation of the roles of radical flux and ion flux and energy. Such processes often include multiple cyclic series of layer modification and etch steps. The modification step may modify the exposed surfaces and the etch step may selectively remove the modified layer. Thus, a series of self-limiting reactions may occur. As used herein, an ALE process may also include quasi-ALE processes. In such processes, a series of modification and etch step cycles may still be used, however, the removal step may not be purely self-limiting as after removal of the modified layer, the etch substantially slows down, though it may not completely stop. In either case, the ALE based processes include a cyclic series of modification and etch steps. 
     It would be desirable to provide an improved ALE process. More specifically, it would be desirable to provide an improved ALE process for etching of oxides. 
     SUMMARY 
     In one exemplary embodiment, described herein is an ALE process for etching oxides. In one embodiment, an ALE process for etching silicon oxide is provided. However, it will be recognized that the concepts described herein may be applicable to the etching of other oxides, for example, metal oxides, germanium dioxide, silicon oxynitride, etc. In an embodiment, an ALE modification step includes the use of a fluorinated hydrocarbon such as a carbon tetrafluoride (CF4) based plasma, wherein the fluorinated hydrocarbon can be perfluorinated hydrocarbon and is gaseous at the working temperatures of the methods described herein. This modification step preferentially removes oxygen from the surface of the silicon oxide, providing a modified surface layer, which can be a silicon rich surface and can be a monolayer. The ALE removal step includes the use of a hydrogen (H2) based plasma. This removal step removes the silicon enriched layer formed in the modification step. The silicon oxide etch ALE process utilizing CF4 and H2 steps may be utilized in a wide range of substrate process steps. For example, the ALE process may be utilized for, but is not limited to, self-aligned contact etch steps, silicon fin reveal steps, oxide mandrel pull steps, oxide spacer trim, and oxide liner etch. 
     In one embodiment, a method for etching a substrate is provided. The method may comprise providing a first layer comprising silicon oxide, the first layer to be etched selective to a second layer. The method further comprises exposing the first layer to a first plasma comprising CF4 to modify at least a surface of the first layer to form a modified surface layer, the modified surface layer being silicon rich compared to the remainder of the first layer. The method further comprises exposing the modified surface layer to a second plasma comprising H2, the plasma comprising H2 removing at least a portion of the modified surface layer. A combination of use of the first plasma and the second plasma reduces at least a portion of a thickness of the first layer. 
     In another embodiment, a method for etching a substrate is provided. The method comprises providing a first layer comprising silicon oxide. The method further comprises performing an atomic layer etch process to etch the first layer, the atomic layer etch process comprising multiple cycles of (1) a surface modification step comprising a first plasma, the first plasma comprising CF4 and (2) a removal step following the surface modification step, the removal step comprising a second plasma, the second plasma comprising H2. 
     In another embodiment, a method for reducing a thickness of a silicon oxide layer on a substrate is provided. The method comprises at least one cycle of (a) exposing the silicon oxide layer to a perfluorinated hydrocarbon plasma to modify the surface of the silicon oxide layer followed by (b) exposing a product of step (a) to an elemental hydrogen plasma to remove at least a portion of the surface modified in step (a). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present inventions and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features. It is to be noted, however, that the accompanying drawings illustrate only exemplary embodiments of the disclosed concepts and are therefore not to be considered limiting of the scope, for the disclosed concepts may admit to other equally effective embodiments. 
         FIG.  1    illustrates one exemplary process flow utilizing the etch methods described herein. 
         FIGS.  2 A- 2 C  illustrate the surface mechanisms which may occur in the steps of the methods of one embodiment described herein. 
         FIG.  3    illustrates one exemplary table contrasting the amount of silicon oxide etched by carbon tetrafluoride plasma alone, hydrogen plasma alone, and carbon tetrafluoride plasma followed by hydrogen plasma. 
         FIG.  4    illustrates a graph of silicon oxide removed by the hydrogen plasma after the carbon tetrafluoride plasma. 
         FIG.  5    illustrates a graph comparing the total silicon oxide etched and the amount of silicon oxide etched per cycle as a function of the number of first and second plasma cycles described herein. 
         FIG.  6    illustrates a graph showing exemplary amounts of silicon oxide, silicon nitride, and polysilicon using the methods described herein. 
         FIGS.  7 A- 7 C  illustrate a representative application of the method described herein in a self-aligned contact application. 
         FIGS.  8 A- 8 B  illustrate a representative application of the method herein in a fin reveal application. 
         FIGS.  9 A- 9 B  illustrate a representative application of the method described herein in an oxide mandrel pull application. 
         FIGS.  10 A- 10 B  illustrate a representative application of the method described herein in a silicon oxide spacer trim application. 
         FIGS.  11 A- 11 B  illustrates a representative application of the method described herein in a silicon oxide liner etch application. 
         FIGS.  12 - 14    illustrate a representative flow diagrams of the methods disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     In one exemplary embodiment, described herein is an ALE process for etching oxide. In one embodiment, an ALE process for etching silicon oxide is provided. However, it will be recognized that the concepts described herein may be applicable to the etching of other oxides. For example, metal oxides where formation of volatile metal hydride and silicon oxynitride may be applicable. The ALE modification step includes the use of a fluorinated hydrocarbon plasma such as a carbon tetrafluoride (CF4) based plasma. However, it will be recognized with the benefit of this disclosure that other fluorocarbon gases may be utilized singularly or in combination with CF4 to achieve the modification step. For example, other fluorocarbons may include, but are not limited to, hexafluorobutadiene (C4F6) and octafluorocyclobutane (C4F8). This modification step preferentially removes oxygen from the surface of the silicon oxide, providing a silicon rich layer on the surface of the substrate. The ALE removal step includes the use of a hydrogen (H2) based plasma. This removal step removes the silicon enriched layer formed in the modification step. The silicon enriched layer can be a monolayer. The silicon oxide etch ALE process utilizing CF4 and H2 steps may be utilized in a wide range of substrate process steps. For example, the ALE process may be utilized for, but is not limited to, self-aligned contact etch steps, silicon fin reveal steps, oxide mandrel pull steps, oxide spacer trim steps, and oxide liner etch steps. 
     More specifically,  FIG.  1    illustrates an exemplary ALE process for etching oxide according to the techniques disclosed herein. In  FIG.  1   , the process  100  is illustrated by the initial delivery  110  of a substrate into a plasma processing region. Next a carbon tetrafluoride plasma is ignited in step  1 , block  120 . The substrate is then subjected to step  2 , block  130  where a hydrogen plasma is ignited and the substrate exposed to the hydrogen plasma. It should be noted that argon or other inert gas can be used as a co-feed with the carbon tetrafluoride and hydrogen. If additional etching is desired, the substrate is returned to step  1  block  120  via line  125  for an additional cycle of steps  1  and  2 . If etching is complete, the substrate is removed from the plasma processing region as shown in removal block  140 . 
     More specifically, as shown in  FIG.  1   , the ALE process starts with a CF4/Argon plasma step  1  block  120  to operate as a layer modification step. Then a H2/Argon plasma step is performed as shown in step  2  block  130  to remove the modified layer generated in the layer modification step. The modification and removal steps may then be repeated a sufficient number of cycles so as to complete the removal of the desired amount of oxide. In one embodiment, the oxide is silicon oxide. 
       FIGS.  2 A- 2 C  illustrate the exemplary mechanisms that occur in each step of the ALE process of  FIG.  1   . It will be recognized that the mechanisms disclosed are merely exemplary, and other mechanisms may occur.  FIGS.  2 A- 2 C  are illustrative and not intended to show precise substrate modifications. As shown in  FIG.  2 A , substrate  210  includes silicon atoms  211  and oxygen atoms  212 . As shown in  FIG.  2 A  an upper oxygen layer  213  is provided. After being exposed to a carbon tetrafluoride plasma, the substrate  210  is modified to form a silicon-enriched layer  225  on the substrate  220 , which includes a oxygen depleted zone  213 A, as shown in  FIG.  2 B . Notably, the upper oxygen layer  213  of  FIG.  2 A  has been at least partially reduced resulting in a silicon-enriched layer  225 . The silicon-enriched layer  225  of substrate  220  is then subjected to a hydrogen plasma, resulting in a final substrate  230  as shown in  FIG.  2 C . As shown in  FIG.  2 C , the removal of a silicon-enriched layer  225  is selective to silicon oxide. 
     Thus, as shown in  FIGS.  2 A- 2 C , regions at the surface of the silicon oxide become silicon rich as oxygen is removed from the silicon oxide surface in the modification step  215 . Then, in the removal step (step  2 ), shown in  FIG.  2 C  exposure to hydrogen plasma results in the silicon being removed due to the etching action of the hydrogen plasma, which can be a in one exemplary embodiment, a H2/Argon plasma. This process may be repeated in multiple cycles to incrementally remove oxygen and then remove the silicon rich layer down through the silicon oxide layer until the preferred amount of silicon oxide removal is achieved. 
     The impact of using a two-step ALE process versus merely using one or the other steps is shown in  FIG.  3   . As shown in the graph of  FIG.  3   , the amount of oxide etched by just step  1  and step  2  alone, as depicted by step  1 , bar  310  and step  2 , bar  330  is contrasted to the use of both steps  1  and  2  in combination as described above, as depicted in steps  1  and  2  combination, bar  320 . The Y axis is denoted as oxide etched (Angstroms). 
     The self-limiting effect of the two-step ALE process is shown in  FIG.  4   .  FIG.  4    illustrates the amount of oxide that is removed by the second step (hydrogen plasma) after performance of the first step (carbon tetrafluoride plasma). As shown in the line  410  of the graph of  FIG.  4   , for increasing etch times of the second step, the amount of silicon oxide removed from the surface relatively saturates over time. In the graph, the Y axis is denoted as oxide EA (Angstroms) with the X axis shown as hydrogen plasma time in seconds. 
       FIG.  5    illustrates the total oxide etched and the amount of oxide etched per cycle as a function of the number of cycles of the ALE steps. The graph in  FIG.  5    depicts oxide etched (Angstroms) in the left Y axis and etch per cycle (Angstroms per cycle) in the right Y axis. The X axis shows the number of cycles of steps  1  and  2 . The oxide etched is denoted as line  520  and the etch per cycle is denoted as line  510 . 
       FIG.  6    illustrates exemplary amounts of silicon oxide, silicon nitride and polysilicon for an ALE process as described herein per 120 seconds of the second step (H2/Argon plasma) as a function of the pressure of the second step. As can be seen in the graph in  FIG.  6   , the two-step ALE process may provide a highly selective process for etching silicon oxide or silicon nitride or polysilicon. It will be recognized that the etch amounts, etch rates, materials, and so on of  FIGS.  2 - 6    are merely exemplary and that the concepts described herein may be used with other ALE processes having other characteristics and qualities. In the graph of  FIG.  6   , line  610  denotes the amount of both oxide and silicon nitride etched (as the graph lines for each material substantially overlap) and line  620  denotes the polysilicon etched. 
     The two-step ALE process described herein may be utilized in a wide variety of applications at various points of differing substrate process flows. For example, the ALE process may be used at self-aligned contact etch steps, silicon fin reveal steps, oxide mandrel pull steps, oxide spacer trim steps, and oxide liner etch steps.  FIGS.  7 A- 11 B  provide exemplary uses of the ALE process described herein in a variety of substrate process flows. It will be recognized that the ALE process described herein may be utilized in many other substrate processing applications. For example, a variety of process steps in which selective removal of silicon oxide may be desired may suitably utilize the techniques disclosed herein. In one embodiment, the techniques may be utilized in semiconductor substrate processing, and more particular, semiconductor wafer processing. 
       FIGS.  7 A- 7 C  illustrate an application of the ALE process techniques disclosed herein in a self-aligned contact application. As shown in  FIG.  7 A , a plurality of layers are formed on a substrate  705 . Substrate  705  may be any substrate for which the use of patterned features is desirable. For example, in one embodiment, substrate  705  may be a semiconductor substrate having one or more semiconductor processing layers formed thereon. In one embodiment, the substrate  705  may be a substrate that has been subject to multiple semiconductor processing steps which yield a wide variety of structures and layers, all of which are known in the substrate processing art and may be considered to be part of the substrate  705 . In the exemplary embodiment of  FIG.  7 A , an oxide layer  710 , may be provided under an amorphous silicon layer  715 . A silicon nitride hard mask  720  may be provided along with a silicon nitride spacer  725  as shown. An oxide layer  730  may be formed over and between the structures formed by the amorphous silicon layer  715  as shown. An organic dielectric layer  735  may be provided over which a silicon anti-reflective coating  735  is provided. A patterned photoresist layer  745  is also provided. 
     As shown in  FIG.  7 A , the oxide layer  730  is formed in and over a region where contacts may ultimately be desired to be formed.  FIG.  7 B  illustrates removal of the various layers (which may be performed via conventional process steps) to a point where the oxide layer  730  is partially etched. In one example, a traditional oxide fluorocarbon etch may be utilized to partially etch oxide layer  730  to achieve the structure shown in  FIG.  7 B  having a remaining portion  730 A of the oxide layer  730 . Then the remaining portion  730 A may be removed such as shown in  FIG.  7 C . As shown in  FIG.  7 C , the remaining portion  730 A has been removed by utilizing the highly selective two step ALE process described herein, for example a fluorocarbon plasma etch chemistry such as carbon tetrafluoride followed by a second step such as a hydrogen plasma step. Thus, a process is provided to remove the remainder of the oxide in the contact region in a highly selective manner to the underlying silicon nitride spacer layer to achieve. Thus, post etch a structure having contact regions  750  may be obtained. 
       FIGS.  8 A- 8 B  illustrate an application of the ALE process techniques disclosed herein in a fin reveal application. As shown in  FIG.  8 A , a fin  807  on a substrate is protected by a silicon nitride layer  805 . Silicon oxide  803  is provided around the fin  807  regions as shown in  FIG.  8 A . The silicon oxide  803  may be removed (in this example partially removed) in this application via use of the ALE process disclosed herein. In this manner, the silicon oxide  803  may be removed selectively to the silicon nitride layer  805  to achieve a structure such as shown in  FIG.  8 B . 
       FIGS.  9 A- 9 B  illustrates an application of the ALE process techniques disclosed herein in an oxide mandrel pull application. As shown in the figures, a silicon oxide mandrel  910  may be surrounded by silicon or silicon nitride layer  912 , such as for example, spacers formed on the sides of the silicon oxide mandrel  910 . The ALE process described herein may be utilized to remove (pull) the silicon oxide mandrel  910  from the substrate, leaving the spaces  931  remaining post-etch in  FIG.  9 B . 
       FIGS.  10 A- 10 B  illustrate an application of the ALE process techniques disclosed herein in a silicon oxide spacer trim application. As shown in the  FIG.  10 A , a silicon oxide spacer  1010 A may be formed around a structure  1012  (for example a silicon or silicon nitride structure). The silicon oxide spacer  1010  may also be provided over an etch stop layer  1015 . The ALE process described herein may be utilized to provide a spacer trimming step to trim a portion of the silicon oxide spacer  1010 A in a controlled manner so as to narrow the silicon oxide spacer  1010 A width to produce a narrower silicon oxide spacer  1010 B as shown in the  FIG.  10 B . 
       FIG.  11    illustrates an application of the ALE process techniques disclosed herein in a silicon oxide liner etch application. As shown in the figure, a silicon oxide liner  1110  may line the sides of a structure  1112  (for example a silicon or silicon nitride structure) as shown in  FIG.  11 A . The silicon oxide liner  1110  may then be removed via oxide liner etch  1122  in a manner selective to the structure  1112  to produce a structure as shown in the  FIG.  11 B  by utilizing the ALE process disclosed herein as an oxide liner etch. 
     It will be recognized that the process flows described above are merely exemplary, and many other processes and applications may advantageously utilize the techniques disclosed herein.  FIGS.  12 - 14    illustrate exemplary methods for use of the processing techniques described herein. It will be recognized that the embodiments of  FIGS.  12 - 14    are merely exemplary and additional methods may utilize the techniques described herein. Further, additional processing steps may be added to the methods shown in the  FIGS.  12 - 14    as the steps described are not intended to be exclusive. Moreover, the order of the steps is not limited to the order shown in the figures as different orders may occur and/or various steps may be performed in combination or at the same time. 
     In  FIG.  12   , a method of etching a substrate is shown. The method includes step  1205  of providing a first layer comprising silicon oxide, the first layer to be etched selective to a second layer. The method further includes step  1210  of exposing the first layer to a first plasma comprising CF4 to modify at least a surface of the first layer to form a modified surface layer, the modified surface layer being silicon rich compared to the remainder of the first layer. The method further includes step  1215  of exposing the modified surface layer to a second plasma comprising H2, the plasma comprising H2 removing at least a portion of the modified surface layer. In the method, a combination of use of the first plasma and the second plasma reduces at least a portion of a thickness of the first layer. 
     In  FIG.  13   , a method for etching a substrate is shown. The method includes step  1305  of providing a first layer comprising silicon oxide. The method further includes step  1310  of performing an atomic layer etch process to etch the first layer. In the method, the atomic layer etch process may include a step of multiple cycles of (1) a surface modification step comprising a first plasma, the first plasma comprising CF4 and (2) a removal step following the surface modification step, the removal step comprising a second plasma, the second plasma comprising H2. 
     In  FIG.  14   , a method for reducing a thickness of a silicon oxide layer on a substrate is shown. The method includes step  1405  of performing at least one cycle of (a) exposing the silicon oxide layer to a perfluorinated hydrocarbon plasma to modify a surface of the silicon oxide layer. Step  1405  is followed by step  1410  of (b) exposing a product of step (a) to an elemental hydrogen plasma to remove at least a portion of the surface modified in step (a). 
     Further modifications and alternative embodiments of the inventions will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the inventions. It is to be understood that the forms and method of the inventions herein shown and described are to be taken as presently preferred embodiments. Equivalent techniques may be substituted for those illustrated and described herein and certain features of the inventions may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the inventions.