Abstract:
Methods of reversing the tone of a pattern having non-uniformly sized features. The methods include depositing a highly conformal hard mask layer over the patterned layer with a non-planar protective coating and etch schemes for minimizing critical dimension variations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e)(1) of U.S. Provisional No. 62/254,891 filed on Nov. 13, 2015, which is incorporated by reference herein. 
     
    
     BACKGROUND INFORMATION 
       [0002]    Nano-fabrication includes the fabrication of very small structures that have features on the order of 100 nanometers or smaller. One application in which nano-fabrication has had a sizeable impact is in the processing of integrated circuits. The semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area formed on a substrate; therefore nano-fabrication becomes increasingly important. Nano-fabrication provides greater process control while allowing continued reduction of the minimum feature dimensions of the structures formed. 
         [0003]    An exemplary nano-fabrication technique in use today is commonly referred to as nanoimprint lithography. Nanoimprint lithography is useful in a variety of applications including, for example, fabricating layers of integrated devices such as CMOS logic, microprocessors, NAND Flash memory, NOR Flash memory, DRAM memory, or other memory devices such as MRAM, 3D cross-point memory, Re-RAM, Fe-RAM, STT-RAM, and the like. Exemplary nanoimprint lithography processes are described in detail in numerous publications, such as U.S. Pat. No. 8,349,241, U.S. Pat. No. 8,066,930, and U.S. Pat. No. 6,936,194, all of which are hereby incorporated by reference herein. 
         [0004]    A nanoimprint lithography technique disclosed in each of the aforementioned U.S. patents includes formation of a relief pattern in a formable (polymerizable) layer and transferring a pattern corresponding to the relief pattern into an underlying substrate. The substrate may be coupled to a motion stage to obtain a desired positioning to facilitate the patterning process. The patterning process uses a template spaced apart from the substrate and a formable liquid applied between the template and the substrate. The formable liquid is solidified to form a rigid layer that has a pattern conforming to a shape of the surface of the template that contacts the formable liquid. After solidification, the template is separated from the rigid layer such that the template and the substrate are spaced apart. The substrate and the solidified layer are then subjected to additional processes to transfer a relief image into the substrate that corresponds to the pattern in the solidified layer. 
         [0005]    An additional nanoimprint lithography technique involves forming a planarized layer over the previously solidified patterned layer and then subjecting the substrate, the solidified patterned layer, and the planarized layer to additional process to transfer a relief image into the substrate the corresponds to the inverse or reverse of the solidified layer pattern. Such processes have become increasingly important in nanoimprint lithography, as well as in other lithography processes, that are used in integrated device fabrication. However, difficulties in achieving adequate planarization of planarized layer while retaining adequate etch selectivity have limited the effectiveness of such processes, especially when pattern features with critical dimensions of 20 nm and below are required. 
       SUMMARY OF INVENTION 
       [0006]    The present invention provides for methods for creating a relief pattern that is the inverse or reverse of an original relief pattern, including original relief patterns having non-uniformly-sized features. In one aspect of the invention, the method includes depositing a conformal hard mask layer by low temperature deposition (e.g., by atomic layer deposition (ALD)) over an originally-patterned layer followed by applying a non-planar protective layer over the conformal layer. In various aspects of the invention, the degree of non-planarity can be less than 95% or 90% or 80% or 70% or 60%, or in some cases even less than 50% or 40% or 30% planar. In further aspects of the invention, etch rates of the protective layer, conformal layer, and patterned layer can all be selected to enhance critical dimension (CD) uniformity (i.e., minimize CD variability) of the reversed features. In certain aspects, the protective layer has an etch rate selectivity ξ1&gt;5 as to the conformal layer, the conformal layer has an etch selectivity ξ2&gt;1 as to the protective layer, and the patterned layer has an etch selectivity ξ3&gt;5 as to the conformal layer, under respective etch process conditions. In a particular aspect, the conformal layer is silicon oxide, SiO 2 , or aluminum oxide, Al 2 O 3  and the non-planar protective layer is spin-on-glass (SOG). 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]    So that features and advantages of the present invention can be understood in detail, a more particular description of embodiments of the invention may be had by reference to the embodiments illustrated in the appended drawings. It is to be noted, however, that the appended drawings only illustrate typical embodiments of the invention, and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0008]      FIG. 1  illustrates a simplified side view of a nanoimprint lithography system having a template and a mold spaced apart from a substrate. 
           [0009]      FIG. 2  illustrates a simplified view of the substrate illustrated in  FIG. 1 , having a solidified patterned layer formed thereon. 
           [0010]      FIGS. 3-6  illustrate a simplified cross-sectional view of a reverse tone process. 
           [0011]      FIGS. 7A-7D  illustrate a simplified cross-sectional view of critical dimension variations resulting from the process of  FIGS. 3-6 . 
           [0012]      FIGS. 8A-8D  illustrate a simplified cross-sectional view of a reverse tone process according to an embodiment of the invention. 
           [0013]      FIGS. 9A-9C  illustrate a simplified cross-sectional view of a different type of lithographic process. 
           [0014]      FIG. 10A-10C  illustrate a schematic cross-sectional view of a process according to an embodiment of the invention. 
           [0015]      FIG. 11  illustrates a reverse tone process flow according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to the figures, and particularly to  FIG. 1 , illustrated therein is nanoimprint lithography system  10  used to form a relief pattern on substrate  12 . Substrate  12  may be coupled to substrate chuck  14 . As illustrated, substrate chuck  14  is a vacuum chuck. Substrate chuck  14 , however, may be any chuck including, but not limited to, vacuum, pin-type, groove-type, electrostatic, electromagnetic, and/or the like. Exemplary chucks are described in U.S. Pat. No. 6,873,087, which is hereby incorporated by reference herein. 
         [0017]    Substrate  12  and substrate chuck  14  may be further supported by stage  16 . Stage  16  may provide translational and/or rotational motion along the x, y, and z-axes. Stage  16 , substrate  12 , and substrate chuck  14  may also be positioned on a base (not shown). 
         [0018]    Spaced-apart from substrate  12  is template  18 . Template  18  may include a body having a first side and a second side with one side having a mesa  20  extending therefrom towards substrate  12 . Mesa  20  may have a patterning surface  22  thereon. Further, mesa  20  may be referred to as mold  20 . Alternatively, template  18  may be formed without mesa  20 . 
         [0019]    Template  18  and/or mold  20  may be formed from such materials including, but not limited to, fused-silica, quartz, silicon, organic polymers, siloxane polymers, borosilicate glass, fluorocarbon polymers, metal, hardened sapphire, and/or the like. As illustrated, patterning surface  22  comprises features defined by a plurality of spaced-apart recesses  24  and/or protrusions  26 , though embodiments of the present invention are not limited to such configurations (e.g., planar surface). Patterning surface  22  may define any original pattern that forms the basis of a pattern to be formed on substrate  12 . 
         [0020]    Template  18  may be coupled to chuck  28 . Chuck  28  may be configured as, but not limited to, vacuum, pin-type, groove-type, electrostatic, electromagnetic, and/or other similar chuck types. Further, chuck  28  may be coupled to imprint head  30  which in turn may be moveably coupled to bridge  36  such that chuck  28 , imprint head  30  and template  18  are moveable in at least the z-axis direction. 
         [0021]    Nanoimprint lithography system  10  may further comprise a fluid dispense system  32 . Fluid dispense system  32  may be used to deposit formable material  34  (e.g., polymerizable material) on substrate  12 . Formable material  34  may be positioned upon substrate  12  using techniques, such as, drop dispense, spin-coating, dip coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), thin film deposition, thick film deposition, and/or the like. Formable material  34  may be disposed upon substrate  12  before and/or after a desired volume is defined between mold  22  and substrate  12  depending on design considerations. For example, formable material  34  may comprise a monomer mixture as described in U.S. Pat. No. 7,157,036 and U.S. Pat. No. 8,076,386, both of which are herein incorporated by reference. 
         [0022]    Referring to  FIGS. 1 and 2 , nanoimprint lithography system  10  may further comprise energy source  38  that directs energy  40  along path  42 . Imprint head  30  and stage  16  may be configured to position template  18  and substrate  12  in superimposition with path  42 . Camera  58  may likewise be positioned in superimposition with path  42 . Nanoimprint lithography system  10  may be regulated by processor  54  in communication with stage  16 , imprint head  30 , fluid dispense system  32 , source  38 , and/or camera  58  and may operate on a computer readable program stored in memory  56 . 
         [0023]    Either imprint head  30 , stage  16 , or both vary a distance between mold  20  and substrate  12  to define a desired volume therebetween that is filled by formable material  34 . For example, imprint head  30  may apply a force to template  18  such that mold  20  contacts formable material  34 . After the desired volume is filled with formable material  34 , source  38  produces energy  40 , e.g., ultraviolet radiation, causing formable material  34  to solidify and/or cross-link conforming to a shape of surface  44  of substrate  12  and patterning surface  22 , defining patterned layer  46  on substrate  12 . Patterned layer  46  may comprise a residual layer  48  and a plurality of features shown as protrusions  50  and recessions  52 , with protrusions  50  having a thickness t 1  and residual layer having a thickness t 2 . 
         [0024]    The above-mentioned system and process may be further employed in imprint lithography processes and systems referred to in U.S. Pat. No. 6,932,934, U.S. Pat. No. 7,077,992, U.S. Pat. No. 7,179,396, and U.S. Pat. No. 7,396,475, all of which are hereby incorporated by reference in their entirety. 
         [0025]    Additional nanoimprint lithography techniques involve, in particular, formation of a planarized layer over the formed patterned layer, in order to transfer a relief image into the substrate the corresponds to an inverted or reversed pattern of the original relief pattern of the formed patterned layer. A process that results in creating such an inverted or reversed-pattern in the substrate is sometimes known as or referred to as a “reverse tone” process. Reverse tone processes have become increasingly important in nanoimprint lithography, as well as in other lithography processes. However, difficulties in achieving adequate planarization while also retaining adequate etch selectivity relative to the underlying patterned layer have limited the effectiveness of reverse tone processes that require planarized layers, especially when underlying pattern features have critical dimensions (or CDs) of 20 nm and below. 
         [0026]    The difficulties in selecting an effective planarizing material and achieving a planar layer using such material are particularly acute when the original relief pattern contains non-uniformly sized features, i.e., patterns including both small and large features. Depending on the properties of the planarizing material, its thickness, and the technique used to deposit the layer there is a characteristic spatial parameter, sp, which is used for characterization of the planarization effectiveness depending on the feature size. For example, in case of 300 nm spin-on-carbon (SOC) material deposited by spin-on technique the characteristic spatial parameter sp˜1 micron. As used herein, “small” features refer to features having a lateral size, s, in at least in one in-plane direction that less than 1 um (i.e., s&lt;1 um), whereas “large” features refer to features having lateral sizes in both in-plane dimensions, s1 and s2, that are at least 1 um (i.e., s1, s2≧1 um). As further used herein, the term features includes features that protrude or extend from the pattern as well as recesses or open areas within the pattern. For example, a 20 nm/20 nm line/space periodic pattern represents a set of equally spaced small features, whereas an open (i.e., free of any other features inside) square area of 15 um×15 um, or a rectangular open area 10 um×60 um are considered large features. An example of a pattern of non-uniformly sized features, includes a pattern having two groups or areas of 50 nm/50 nm lines/spaces (small features) which are spaced apart by a 5 um gap or open area (a large feature).  FIG. 3  illustrates a typical difficulty associated with such scenario. Patterned layer  202  (not to scale) is formed over substrate  204  and contains small features  101 ,  102 ,  103 ,  104 ,  105 ,  106 , and  107 , with features  101 ,  102 ,  103  and  104  separated from features  105 ,  106 , and  107  by open area  108 , and with open area  109  similarly extending beyond feature. Here open areas  108  and  109  can each be considered as a large feature relative to features  101 ,  102 ,  103 ,  104 ,  105 ,  106 , and  107  such that features  104 ,  105  and  107  in particular can be considered to be located at the transition zone between small and large features. Layer  200  is formed overtop patterned layer  202  as a planarizing layer. However, due to practical limitations as further described herein, layer  200  is not fully planar, and instead includes formed depressions over open areas  108  and  109 , such that the planarization efficiency for layer  200  is less than 100%. As further used herein, the term “planarization efficiency” is defined as 
         [0000]    
       
         
           
             
               
                 
                   P 
                   = 
                   
                     
                       ( 
                       
                         
                           H 
                           2 
                         
                         - 
                         
                           H 
                           1 
                         
                       
                       ) 
                     
                     
                       H 
                       FH 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where H FH  is the original feature (or step) height, H 1  is the thickness of the planarizing material overtop the feature (or step), and H 2  is the thickness of the planarizing material at the bottom of the feature (e.g. in the open bottom area near the step). Thus planarization efficiency of a layer is a measure of its variability away from the ideal condition of completely 100% planar layer. 
         [0027]    Planarization variability is, to a large part, a naturally occurring phenomenon. That is, upon application of the planarizing material (typically e.g. by spin coating), the material will resolve to the same coating thickness on all large open areas or surfaces where the effect of boundaries is minimal. To a similar extent the material will likewise resolve to the same relative thickness across large features having full feature height where boundary effects are also likewise minimal, albeit at a slight overall elevation as compared to the material coating the open areas or surfaces. Tightly-spaced small features having full feature height produce a similar effect as large full height features with an effective thickness depending on the specific duty cycle of the pattern in that place. The phenomena of thickness variations is illustrated in a simplified form in  FIG. 3 , with thickness variations of layer  200  seen at open areas  108  and  109 , and above tightly spaced small features  101 ,  102 ,  103 ,  104 , and  105 ,  106 ,  107 . 
         [0028]    Planarization variability, in particular, produces adverse effects in transition zones where a pattern shifts from small to larger, or large to smaller features. In  FIG. 3 , such transition zones are located between features  104 ,  108 , between features  108 ,  105 , and between features  107 , and  109 . As another example, such transition zones can occur where there are clusters of nanometer scaled features separated by micron-sized open areas. Such transition zones tend to have different planarization profiles relative to remainder of the pattern. On further processing, this can lead to features in the transition zone having variable critical dimensions (CDs) as compared to the features inside more uniform size area. Referring to  FIGS. 4-5 , as layer  200  is etched back to expose the tops of features  101 ,  102 ,  103  and  106 , features  104 ,  105  and  107  become more exposed relative to features  101 ,  102 ,  103  and  106  as a result of the variation in planarity profile at the transition zones (see  FIGS. 4, 5 ). Then upon subsequent etching away of the exposed features (see  FIG. 6 ), the resultant trenches  114 ,  115  and  117  (corresponding to features  104 ,  105  and  107 ) will have slightly larger CDs than resultant trenches  111 ,  112 ,  113 , and  116  (corresponding to features  101 ,  102 ,  103  and  106 ) and this same undesirable CD variation will persist upon further etching of the pattern into the substrate (not shown). This phenomenon is attributable to the fact that the patterned feature sidewalls are not completely vertical (i.e., the patterned features sidewalls have slopes less than 90 degrees). This can be more clearly seen with reference to  FIGS. 7A-7D .  FIG. 7A  depicts substrate  204  with patterned layer  202  including feature  210  extending therefrom having height h 0  and sidewall slope θ (less than 90°) such that the base of feature  210  is wider than its top  210 . Hard mask  200  has been etched back to expose top  212  of feature  210 .  FIG. 7B  depicts the result of a through etch of the feature and underlying residual layer (rlt) to expose substrate  204 . The CD of the resultant feature when etched into the substrate will be CD 0 , which corresponds to the width of feature  201  at its top  212 . However, if hard mask  200  is initially etched back further, as shown in  FIG. 7C , such that the feature  210  is reduced to height h 1  (with h 1 &lt;h 0 ), then the through etch of feature  210  results in a wider CD 1  (CD 1 &gt;CD 0 ), as shown in  7 D. With reference to  FIGS. 3-6 , the same phenomena occurs in the transition zones where the planarity profile variations of layer  200  lead to a greater etch back of transition features  104 ,  105  and  107  which then leads to subsequent wider CDs of resultant features  114 ,  115 ,  117  as compared to resultant features  111 ,  112 ,  113 , and  116  in the non-transition areas. 
         [0029]    In addition to planarization variability, an additional difficulty in imprint reverse tone processes is achieving adequate etch selectivity of the planarization layer relative to the cured imprint resist, as the planarization layer acts as hard mask. Imprint resists are typically formed of an organic material. High etch selectivity typically requires the composition of the hard mask to be different than that of the resist. For example, a silicon-containing material such as spin-on glass (SOG) is a commonly used planarizing material and can theoretically have the etch selectivity necessary to be an effective hard mask for an organic imprint resist that is further subjected to, for example, an oxygen plasma etch. Unfortunately, the spin-on glass needs to be baked at a high temperature (˜300-400° C.) for full conversion to a silicon oxide-like material that has the required high etch selectivity characteristics. But this conversion temperature is well above imprint resist transition temperatures (˜80° C.), i.e., the temperature at which the imprint resist starts to melt and flow. Thus SOG is not useful as an etch mask without damaging the resist features. Conversely, if the SOG is not baked at a high enough temperature, the conversion to a silicon oxide-like material will not occur. Thus while the resist features will survive, the material will not have good enough etch selectivity to be useful for imprint reverse tone processes. 
         [0030]    The present invention addresses these and other concerns by replacing the single planarization layer with a first conformal hard mask layer followed by a second protective layer on top of the first conformal hard mask layer. The first conformal hard mask layer preferably achieves: (1) highly conformal coating of the underlying patterned resist layer at (2) high thickness uniformity, while also having (3) high etch selectivity as to the resist in a first etch chemistry such that the first conformal hard mask etches slower than the resist, as well as (4) high etch selectivity relative to the second protection layer in a second etch recipe such that the first conformal hard mask etches slower than the second protective layer. In one embodiment, the first conformal hard mask layer is silicon oxide, SiO 2 , which is deposited by a low temperature (˜50° C.) atomic layer deposition (ALD) technique. Such a technique can deposit a highly conformal, uniform thickness SiO 2  layer over the patterned resist at a low temperature t˜50° C., i.e., below the resist transition temperature. Other oxides, such as aluminum oxide, Al 2 O 3 , can also be deposited using a room temperature ALD process. Such ALD techniques achieve a highly conformal coating, with a uniformity thickness within one to two monolayers. Also, oxides such as SiO 2  and Al 2 O 3  have very high etch selectivity to both imprint and optical resists, especially in an oxygen-based plasma etch. The terms “etch rate selectivity” and “etch selectivity” are used herein interchangeably. 
         [0031]    The second protective layer preferably achieves (1) high etch selectivity as to the first highly conformal layer, with etch selectivity ξ1 of at least 5 (ξ1≧5) using a selected etch chemistry such that the protective layer etches slower than the first conformal layer, and (2) achieves some level of planarization efficiency, (e.g., at least 15% or 20% or 25%, or 35% or 50%), but full 100% planarization efficiency is not required (e.g., planarization efficiency can be less than 95% or 90% or 80% or 70% or 60%, or in some cases even less than 50% or 40% or 30%). In other words, the planarization efficiency is relaxed as compared to processes requiring a fully planar layer. In one embodiment, the protective layer can be a spin-on carbon layer (SOC). SOC has high etch selectivity with respect to the above oxides where the SOC etches slower than the above oxides. For example, SOC has an etch selectivity ξ1≧5 with respect to silicon oxide, SiO 2 , in an oxygen-based plasma etch. In another example, a protective layer can be an adhesion layer as described in U.S. Pat. No. 8,557,351 incorporated herein by reference. In another example a protective layer can be made of Level® M10 material from Brewer Science, Inc. (Rolla, Mo.). 
         [0032]    Referring to  FIGS. 8A-8D , a reverse tone process according to the invention is further depicted. In  FIG. 8A , patterned layer  402  is formed over substrate  404  and contains features  301 ,  302 ,  303 ,  304 ,  305 ,  306 , and  307  extending from residual layer  310 . Features  301 ,  302 ,  303  and  304  are separated from features  305 ,  306 , and  307  by open area  308 , and with open area  309  similarly extending beyond feature  307 , similar to the pattern in  FIG. 3 . Conformal hard mask layer  406  is layered over patterned layer  402  at a uniform thickness t HM . Protective layer  408  is layered over hard mask layer  406 . Notably, protective layer  408  includes planarity variations at transition zones associated with features  304 ,  305  and  307 , similar to the situation in  FIG. 3 . Turning to  FIG. 8B , protective layer is etched back ( FIG. 8B ) to at least expose the top  410  of conformal hard mask layer that extends across the tops of features  301 ,  302 ,  303 ,  304 ,  305 ,  306 , and  307 , while portions  412  and  414  of protective layer  408  remain over open areas  308  and  309  respectively. Because of the thickness uniformity of the hard mask  406  and its high etch selectivity to protective layer  408 , the top surface of that portion of hard mask  406  that extends over pattern features  301 ,  302 ,  303 ,  304 ,  305 ,  306 , and  307  is uniformly opened and exposed across the entirety of the pattern, including across those pattern features in the transition zones, that is, pattern features  304 ,  306 , and  307 . The high uniformity of the underlying hard mask layer effectively overcomes the defectivity concerns otherwise associated with planarizing layers that are not fully planar. In a sense, the thickness variations of the protective layer are “rectified” by the top surface portions of the hard mask. Because of the high etch selectivity, the protective layer can be slightly over-etched to open all the tops of the hard mask features uniformly over the whole pattern. The remaining protective layer portions  412  and  414  can remain non-planar as their primary purpose is to protect the underlying hard mask during the next etch step. Turning to  FIG. 8C , hard mask layer  406  is then etched back to expose the tops of pattern resist features  301 ,  302 ,  303 ,  304 ,  305 ,  306 , and  307 . The highly uniform thickness of the hard mask leads to highly uniform CDs of the resultant reverse tone pattern features because the etching of the hard mask starts and stops across the pattern at the same height. The sidewall slope of the features does not impact CDs because there is no variability between feature etching in the transition zones vs. non-transition zones; instead, all features have consistent and uniform CDs, as further shown in  FIG. 8D . In  FIG. 8D , the pattern features and underlying residual layer have been etched away (as has remaining protective layer portions  412  and  414 ) resulting in corresponding trenches ( 311 ,  312 ,  313 ,  314 ,  315 ,  316 , and  317 ) that have highly uniform CDs. Further, the etch selectivity of resist used to form the resist features to the conformal hard mask in a specific etch recipe can be denoted ξ3. For example, a typical imprint resist has an etch selectivity ξ3≧5 with respect to silicon oxide, SiO 2 , in an oxygen-based plasma etch. The CD uniformity of the obtained similar size features, CDU1, has contribution from CD uniformity of the original resist features, CDU0; protection layer thickness uniformity, PLU; and, in the case of an imprinted pattern, the residual layer thickness uniformity of the resist under the features, RLTU, and is described by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     CDU 
                      
                     
                         
                     
                      
                     
                       
                         1 
                         2 
                       
                       ~ 
                       CDU 
                     
                      
                     
                         
                     
                      
                     
                       0 
                       2 
                     
                   
                   + 
                   
                     A 
                      
                     
                       
                         PLU 
                         2 
                       
                       
                         
                           ( 
                           
                             ξ 
                             1 
                           
                           ) 
                         
                         2 
                       
                     
                   
                   + 
                   
                     A 
                      
                     
                       
                         RTLU 
                         2 
                       
                       
                         
                           ( 
                           
                             ξ 
                             3 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where coefficient A depends on side wall slope of the original resist features. If both etch selectivities ξ1 and ξ3 are maximized, the contributions of protective layer thickness uniformity, PLU, and residual layer thickness uniformity, RLTU, (in the case of an imprinted pattern) to the resulted feature CD uniformity, CDU1, are minimized. For instance, if ξ1=10, and ξ3=10, the contribution of PLU and RLTU to (CDU1) 2  will be 100 times smaller compared to the etch processes with no etch rate selectivity between the materials, i.e. ξ1=1, ξ3=1. Thus the reverse tone features of similar sizes will have highly uniform CDs. Note that in the case of patterned resist with no residual layer the term with RLTU in equation (2) is omitted. Thus in  FIG. 8D , when the structure is subjected to further etching into substrate  404 , the result is an inverse (reverse tone) of the original pattern with highly uniform CDs, regardless of whether or not the respective original pattern features were located in transition zones. 
         [0033]      FIGS. 9A-9C  further illustrate the importance of the protective layer to the reverse tone processes according to the present invention.  FIG. 9A  depicts a conformal hard mask layer  606  deposited over patterned layer  602 , similar to hard mask layer  406  as shown in  FIG. 8A  but without added protective layer  408 .  FIG. 9B  depicts an etch back of hard mask layer  606  to expose the tops of features  501 ,  502 ,  503 ,  504 ,  505 ,  506  and  507 , but such an etch back of the hard mask layer along tops of these features also removes the hard mask layer at open features  508  and  509 . These features  508  and  509  thus are lost upon further reverse tone processing steps, as shown in  FIG. 9C . That is, with mask layer  606  etched back at open features  508  and  509  such that features  508  and  509  are no longer protected, the subsequent etching of patterned layer  602  will not generate an inverse (or reverse tone) pattern of open features  508  and  509  into substrate  604 . In other words, the pattern tone is not fully inverted or reversed. 
         [0034]    With reference to  FIGS. 10A-10C , the minimum planarization efficiency of the protective layer under given parameters, including given etch selectivity of hard mask relative to the protective layer material, can be determined.  FIG. 10A  depicts substrate  704  with patterned layer  702  having feature  710  formed thereon, with hard mask layer  706  formed over patterned layer  702 , e.g., by SiO 2  atomic layer deposition (ALD) to conformally pattern layer  702  with a uniform layer thickness, h HM . Feature  710  provides for a feature that determines a height step in the hard mask layer denoted as FH. Protective layer  708  is applied over hard mask layer  706 , e.g., by spin-on process. The thickness of protective layer  708  on top of feature  710  area is denoted as h top , with the thickness of protective layer at the bottom the step, i.e., adjacent to feature  710 , denoted as h bottom . The corresponding height step established in protective layer  708  is denoted as Δ. The planarization efficiency, PE, according to equation (3) is thus expressed as: 
         [0000]    
       
         
           
             
               
                 
                   PE 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             h 
                             bottom 
                           
                           - 
                           
                             h 
                             top 
                           
                         
                         ) 
                       
                       FH 
                     
                     = 
                     
                       1 
                       - 
                       
                         Δ 
                         FH 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0035]    Turning to  FIG. 10B , protection layer  708  has been etched back to expose the portion of hard mask layer  706  covering the top of feature  710 , with the thickness of protective layer  708  at the bottom of the step reduced to thickness h protect . With reference to  FIG. 10C , hard mask  706  is then etched away to reveal feature  710  of patterned layer  702 , with protective layer  708  further reduced to h final . For a given etch selectivity, ξ2, of the hard mask material to the protective layer material, then for a given thickness, h HME , of hard mask layer  706  that is etched away, the protective layer  708  will correspondingly be etched away a thickness h HME /ξ2. In general, the hard mask can be subjected to etch condition that will remove a thickness, h HME , that is more or less than the actual hard mask thickness, h HM . The change in protection layer  708  thickness during the hard mask etch is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       h 
                       protect 
                     
                     - 
                     
                       h 
                       final 
                     
                   
                   = 
                   
                     
                       h 
                       HME 
                     
                     ξ2 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    If the hard mask etch is stopped when the top of feature  710  is first exposed, then: 
         [0000]        h   HME   =h   HM   (5)
 
         [0000]    Thus for a given etch selectivity, ξ2, the minimum required thickness of protection layer  708  to protect the underlying hard mask at a specific location is a thickness such that when feature  710  is first exposed, i.e. h HME =h HM , the final thickness, h final , of protection layer in the specific location is approaching zero. This can be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     h 
                     
                       
                         protect 
                          
                         _ 
                          
                         min 
                       
                        
                       
                         _ 
                          
                         local 
                       
                     
                   
                   = 
                   
                     
                       h 
                       HM 
                     
                     ξ2 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where the minimal protective layer thickness is denoted h protect   _   min   _   local . 
         [0036]    The effective minimum required thickness of protection layer  708  is further dependent upon two additional variables, the total or global thickness variation of the protective layer across the entirety of a substrate (e.g., wafer), denoted as Δh protect   _   global , and the global feature height variation across the entirety of the patterned layer, denoted as ΔFH global . Taking these variable into consideration, the required maximum global thickness of the protection layer across the entirety of the substrate or wafer, denoted h protect   _   max   _   global , can be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     h 
                     
                       
                         protect 
                          
                         _ 
                          
                         ma 
                          
                         x 
                       
                        
                       
                         _ 
                          
                         global 
                       
                     
                   
                   = 
                   
                     
                       
                         h 
                         HM 
                       
                       ξ2 
                     
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         h 
                         
                           protect 
                            
                           _ 
                            
                           global 
                         
                       
                     
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         FH 
                         global 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0000]    From here, the minimum planarization efficiency, or PE min , for the protective layer can be expressed as: 
         [0000]    
       
         
           
             
               
                 
                   
                     PE 
                     min 
                   
                   = 
                   
                     
                       h 
                       
                         
                           protect 
                            
                           _ 
                            
                           ma 
                            
                           x 
                         
                          
                         
                           _ 
                            
                           global 
                         
                       
                     
                     FH 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     PE 
                     min 
                   
                   = 
                   
                     
                       1 
                       FH 
                     
                      
                     
                       ( 
                       
                         
                           
                             h 
                             HM 
                           
                           ξ2 
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             h 
                             
                               protect 
                                
                               _ 
                                
                               global 
                             
                           
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             FH 
                             global 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In other words, the minimum planarization efficiency, PE, for a given protective layer is dependent upon the etch selectivity, ξ2, feature height, FH, hard mask thickness, h HM , and the global protective layer thickness variation and global feature height variation, Δh protect   _   global  and ΔFH global . For a given hard mask thickness to be etched away, h HME , the minimum planarization efficiency for the protective layer can be expressed by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     PE 
                     min 
                   
                   = 
                   
                     
                       1 
                       FH 
                     
                      
                     
                       ( 
                       
                         
                           
                             h 
                             HME 
                           
                           ξ2 
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             h 
                             
                               protect 
                                
                               _ 
                                
                               global 
                             
                           
                         
                         + 
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             FH 
                             global 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where the minimum etch thickness of the hard mask, h HM , is replaced by the required thickness etch of the hard mask, h HME . 
         [0037]    Referring now to  FIG. 11 , process  800  depicts a work flow process that encompasses aspects of the invention. In step  810 , a patterned resist layer is formed on a substrate (or other underlying layer). This can be accomplished through known lithography techniques, including, but not limited to, optical and imprint lithography techniques. In step  820 , a conformal hard mask layer is deposited on the patterned resist layer at room or low temperature, i.e., at a temperature below the resist transition temperature. The conformal hard mask layer should be highly conformal and have a highly uniform thickness, preferably with less than 5 nm thickness variability. The conformal hard mask layer should also have a high etch selectivity to the resist. For example, silicon oxide, SiO 2 , or aluminum oxide, Al 2 O 3 , deposited by low temperature atomic layer deposition (ALD) techniques can achieve such requirements. To achieve the highest CD uniformity for the smallest features, it is further preferable to set the conformal layer thickness to a thickness that is larger than one-half of the smallest pitch size of the patterned layer features (i.e., thickness t&gt;0.5 smallest feature pitch p). In step  830  a protective layer is formed over the conformal hard mask layer by known techniques, including but not limited to spin-on processes. The protection layer should have a high etch selectivity to the conformal hard mask layer, e.g., etch selectivity ξ1≧5, in a first selected etch chemistry where the conformal hard mask layer etches slower than the protective layer. For example, a protective layer of spin-on-carbon (SOC) over a hard mask layer of SiO 2  has the requisite etch selectivity in an oxygen-based etch process. The protective layer and conformal hard mask layer pair should also be selected such that the protective layer likewise has an etch selectivity to the conformal hard mask layer, e.g., etch selectivity ξ2&gt;1, in a second selected etch chemistry where the protective layer etches slower than the conformal hard mask. In the same SOC protective layer and SiO 2  hard mask layer example, the SiO 2  has the requisite etch selectivity as to SOC in a plasma etch using a CF 4 /CHF 3  mixture. The protective layer must also be applied so as to achieve a requisite minimum planarization efficiency, PE min  (see equation (9) herein), although complete planarization (100%) of the protective layer is not required, as previously indicated. For example, an SOC protective layer can be formed by a spin-on process with a PE=50%, which corresponds to variations in the protection layer thickness of one-half the feature height, FH, of the patterned layer. 
         [0038]    Once the protective layer has been applied, etch steps  840 ,  850 ,  860  and  870  are performed to generate an inverse pattern (reverse tone) of the patterned layer into the substrate. In step  840 , the protective layer is etched back until the tops of the hard mask features are opened uniformly over the entirety of the substrate (whole wafer). A slight over-etching of the protective layer is acceptable if necessary to open all the hard mask features. As previously noted, for a SOC protective layer on top of silicon oxide SiO2 hard mask layer, the requisite high etch selectivity, ξ1≧5, exists for an oxygen-based etch process. In step  850 , the tops of the hard mask features are etched away until the tops of the resist features are opened. Here also a slight over-etch may be required until all the patterned resist layer features over the entirety of the substrate (whole wafer) are opened to the required level (height). In some cases the features can be opened to ˜0.7 level (height) of the full resist feature height. High etch selectivity, ξ2, of protective layer to hard mask layer is also required here, such that the protection layer shields the hard mask from being etched away at open spaces extending between features or feature clusters. The protection layer here etches slower than the hard mask. As previously noted, for a SiO 2  hard mask layer and a SOC protective layer, the requisite etch selectivity exists for a plasma etch using a fluorine-based chemistry, e.g., C 4 F 8 , CF 4 , CHF 3  or a mixture thereof. In step  860 , the patterned resist is then etched through using a highly anisotropic etch process to vertically etch away the resist and maintain the feature critical dimensions (CDs), as defined by hard mask opening cross-sections, under high control. High etch selectivity, ξ3, of hard mask to resist, e.g., ξ3≧5, is required. In this case the hard mask etches slower than the resist. For example, when using an organic imprint resist and silicon oxide, SiO 2 , as the hard mask, the requisite etch selectivity is achieved in a plasma etch using e.g. oxygen, oxygen/argon, and/or an oxygen/helium gas mixture. Finally, in step  870  the substrate (or other underlying layer) is etched to transfer a reverse tone (inverse pattern) of the original pattern into the substrate, with highly uniform feature CDs. The etch requirement here depends on the hard mask material, e.g., SiO 2 , and on the substrate e.g., Si, or other underlying material, e.g., another SOC layer. 
       Examples 
       [0039]    In the following examples, silicon wafer substrates are patterned by imprint lithography techniques, with the resultant patterned layers having differing global feature height variations, ΔFH global . Patterned layers are then coated with SiO 2  conformal hard mask layer, with the SiO 2  deposited on the patterned layers by ALD technique to varying thicknesses, h HM . A spin-on carbon (SOC) protection layer is then deposited over the hard mask layer by spin-on process to an average thickness of 300 nm, with the protection layer having varying global thickness variations, Δh protect   _   global . Etch selectivities, ξ2, are likewise varied. 
       Example 1 
       [0040]    In a first example, the hard mask layer thickness that is etched away h HME =20 nm; the etch selectivity ξ2=5; the global variations of the protection layer thickness above the same type of feature, Δh protect   _   global =3 nm, and global variations of the feature height, ΔFH global =2 nm, and the feature height, FH=40 nm. From here, equation (10) above becomes: 
         [0000]      PE min =1/40(20/5+3+2)=0.225=22.5%  (11)
 
         [0000]    Thus with the above parameters, the minimum planarization efficiency of 22.5% for the protective layer is adequate to safely and completely reverse the pattern tone. Table 1 below shows PE min  values for the above given parameters but with different etch rates. 
         [0000]                                                TABLE 1                   Relation between given etch selectivity, ξ2, and the required minimum       planarization efficiency according to equation (10) with remaining       parameters constant.                ξ2   PE min , %                            20   15           10   17.5           6   20.8           5   22.5           4   25           3   29.2           2   37.5           1   62.5                        
As shown, if the etch rate selectivity is very high, e.g., ξ2=10, the minimum planarization efficiency can be fairly low, e.g., PE min =17.5%. And vice versa, if the planarization efficiency is fairly high (but not still not fully planar), e.g., PE min =62.5%, the etch rate selectivity can fairly low, e.g., ξ2=1.
 
       Example 2 
       [0041]    In this example, the variables remain the same as in Example 1 with the exception that the global variation of the protection layer thickness is increased to Δh protect   _   global =10 nm. From here equation (10) above becomes: 
         [0000]      PE min =1/40(20/5+10+2)=0.4=40%  (12)
 
         [0000]    Here, given the above parameters and chosen etch selectivity ξ2=5, the minimum planarization efficiency of 40% is enough to for the protective layer to safely and completely reverse the pattern tone. However, it can also be seen that non-ideal spin-on protection layer thickness variations of 10 nm are causing more stringent requirements for planarization efficiency. For ξ2=5, the minimum planarization efficiency is increased from 22.5% for Δh protect   _   global =3 nm (in Example 1) to 40% for Δh protect   _   global =10 nm in this example. Table 2 below shows PE min  values for the above given parameters but with different associated etch rate selectivity. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Example 2 relation between given etch selectivity, ξ2, and the required 
               
               
                 minimum planarization efficiency remaining parameters constant 
               
             
          
           
               
                   
                 ξ2 
                 PE min , % 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 32.5 
               
               
                   
                 10 
                 35 
               
               
                   
                 6 
                 38.3 
               
               
                   
                 5 
                 40 
               
               
                   
                 4 
                 42.5 
               
               
                   
                 3 
                 46.7 
               
               
                   
                 2 
                 55 
               
               
                   
                 1 
                 80 
               
               
                   
                   
               
             
          
         
       
     
       Example 3 
       [0042]    In this example, the variables remain the same as in Example 1 with the exception that hard mask thickness is increased to h HME =30 nm. From here equation (10) above becomes: 
         [0000]      PE min =1/40(30/5+3+2)=0.275=27.5%  (13)
 
         [0000]    Here, given the above parameters and chosen etch selectivity ξ2=5, the minimum planarization efficiency of 27.5% is enough to for the protective layer to safely and completely reverse the pattern tone. It is apparent that increased thickness of the hard mask (i.e., increased etch depth of the hard mask) causes more stringent requirements for minimum planarization efficiency. For ξ2=5 the minimum planarization efficiency is increased from 22.5% for h HME =20 nm (Example 1) to 27.5% for h HME =30 nm in this example. Table 3 shows PE min  values for the above given parameters but with different associated etch rate selectivity. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Example 3 relation between given etch selectivity, ξ2, and the required 
               
               
                 minimum planarization efficiency with remaining parameters constant. 
               
             
          
           
               
                   
                 ξ2 
                 PE min , % 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 16.3 
               
               
                   
                 10 
                 20 
               
               
                   
                 6 
                 25 
               
               
                   
                 5 
                 27.5 
               
               
                   
                 4 
                 31.3 
               
               
                   
                 3 
                 37.5 
               
               
                   
                 2 
                 50 
               
               
                   
                 1 
                 87.5 
               
               
                   
                   
               
             
          
         
       
     
       Example 4 
       [0043]    In this example, the variables remain the same as in Example 1 with the exception that the global variation of the protection layer thickness is increased to Δh protect   _   global =10 nm as in Example 2 and the hard mask thickness is increased to h HME =30 nm as in Example 3. From here equation (10) above becomes: 
         [0000]      PE min =1/40(30/5+10+2)=0.45=45%  (14)
 
         [0044]    For the four chosen here parameters used in equation 10, and chosen etch selectivity ξ2=5, the minimum planarization efficiency of 45% is enough safely and completely reverse the pattern tone. Here it is apparent that the increased thickness of the hard mask (or increased etch depth of the hard mask), and simultaneous degradation in spin-on film uniformity cause more stringent requirements for minimum planarization efficiency. For ξ2=5, the planarization efficiency is increased to 45%. Table 4 shows PE min  values for the above given parameters but with different associated etch rate selectivity. Here, for very low etch selectivity, e.g. ξ2=1, there is no acceptable solution. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Example 4 relation between given etch selectivity, ξ2, and the required 
               
               
                 minimum planarization efficiency with remaining parameters constant. 
               
             
          
           
               
                   
                 ξ2 
                 PE min , % 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 33.8 
               
               
                   
                 10 
                 37.5 
               
               
                   
                 6 
                 42.5 
               
               
                   
                 5 
                 45 
               
               
                   
                 4 
                 48.8 
               
               
                   
                 3 
                 55 
               
               
                   
                 2 
                 67.5 
               
               
                   
                 1 
                 105 (not achievable) 
               
               
                   
                   
               
             
          
         
       
     
       Example 5 
       [0045]    In this example, the variables remain the same as in Example 2 with the exception that the feature height is increased to FH=60 nm. From here equation (10) above becomes: 
         [0000]      PE min =1/60(20/5+10+2)=0.267=26.7%  (15)
 
         [0000]    Example 5 uses parameters from Example 2 plus increased feature height from 40 nm to 60 nm. For etch selectivity ξ2=5, the minimum planarization efficiency of 26.7% is enough to safely and completely reverse the pattern tone. Table 5 shows PE min  values for a range of different etch rate selectivity. It is apparent that increased feature height relaxes the requirements for the minimum planarization efficiency (compare to Table 2). For instance, for ξ2=5 the planarization efficiency requirement is down to 26.5% from 40% in Example 2. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Example 5 relation between given etch selectivity, ξ2, and the required 
               
               
                 minimum planarization efficiency with remaining parameters constant. 
               
             
          
           
               
                   
                 ξ2 
                 PE min , % 
               
               
                   
                   
               
             
          
           
               
                   
                 20 
                 21.7 
               
               
                   
                 10 
                 23.3 
               
               
                   
                 6 
                 25.6 
               
               
                   
                 5 
                 26.7 
               
               
                   
                 4 
                 28.3 
               
               
                   
                 3 
                 31.1 
               
               
                   
                 2 
                 36.7 
               
               
                   
                 1 
                 53.3 
               
               
                   
                   
               
             
          
         
       
     
       Example 6 
       [0046]    In this example, the formed patterned layer consisted of the smallest features being 30 nm line/space with 1:1 duty cycle pattern, with large open areas between the features as large as 30 microns. The feature height for all the features, FH, was 57 nm. Feature height variations on the template (and thus the resulting patterned layer) were ΔFH global =2 nm. The hard mask layer was SiO 2  layer deposited by ALD technique. The hard mask thickness was h HM =20 nm. A spin-on carbon (SOC) protection layer with an average thickness of 300 nm was deposited by spin-on process. The global thickness variations along the whole wafer were Δh protect   _   global =5 nm. The planarization layer had a maximum measured height step Δ=28 nm. Thus, the measured planarization efficiency PE according to the equation (3) was: 
         [0000]      PE measured =1−28/57=0.49=49%  (16)
 
         [0000]    The etch selectivity of SOC to SiO 2  was ξ1=20, using an oxygen plasma recipe to etch back the SOC (i.e., etch step 1). CF4/CHF3 mixture plasma recipe was used to etch back the silicon oxide hard mask, with the etch selectivity ξ2=4 (i.e., etch step 2). From formula (10) above, the minimum PE required for successful tone reversal with the above parameters is expressed as: 
         [0000]      PE min =1/57(20/4+5+2)=0.21=21%  (17)
 
         [0000]    The measured planarization efficiency, PE measured =49%, which is significantly larger than the minimum planarization efficiency, PE min =21%, required for reverse tone processing: 
         [0000]      PE measured &gt;PE min   (18)
 
         [0000]    The observed planarization efficiency obtained by spin-on coating (49%) was enough to successfully reverse the whole pattern, including the small features (30 nm line/space) and large features (30 micron open areas). 
         [0047]    Further modifications and alternative embodiments of various aspects will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only. It is to be understood that the forms shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description.