Abstract:
A scattering enhanced thin absorber for a EUV reticle and a method of making thereof is disclosed. Embodiments include forming a multilayer on the upper surface of a substrate, forming a capping layer over the multilayer, forming one or more diffuse scattering layers over the capping layer, and etching the diffuse scattering layers to form a stack.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to absorbers in extreme ultra violet (EUV) reflective reticles. The present disclosure is particularly applicable to 20 nanometer (nm), 14 nm, and beyond semiconductor device technology nodes. 
       BACKGROUND 
       [0002]    EUV lithography (EUVL) is a next-generation lithography technology for 1x nm technology nodes. A reflective mask (or EUV reticle) is used in a single-exposure process to produce imaged features on a wafer.  FIG. 1  illustrates a EUV reticle  100 , according to a conventional design. A reflective multilayer stack  101  on a substrate  103  reflects EUV radiation at unmasked portions by Bragg interference. Masked (non-reflective) areas  105  of EUV reticle  100  are formed by etching buffer layer  107  and absorbing layer  109 . Capping layer  111  is formed over the reflective multilayer stack  101  and protects it during the etching. The thickness of absorbing layer  109  ranges from 51 to 77 nm and may be obtained from the commercial market. 
         [0003]      FIG. 2A  illustrates a conventional EUVL single-exposure process and a corresponding mask shadowing effect. EUV reticle  200  is irradiated by incident EUV  201  via non-telecentric optics (not shown for illustrative convenience) and is reflected only at unmasked portions of reflective multilayer  203  to produce imaging radiation  205 . Due to the non-telecentric optics, the incident EUV  201  is at an offset angle  207  (conventionally set to six degrees) to a Z-axis normal. A mask shadowing effect  209  is induced by the interaction of the off-axis illumination with the mask topography. 
         [0004]    Adverting to  FIG. 2B , the mask shadowing effect  209  varies depending on the orientation of the mask features with respect to incident EUV  201 . Specifically, the imaged features on an exposed wafer indicate a printing difference (H-V print difference) between the horizontally oriented (H) features  211  and the vertically oriented (V) features  213  of EUV reticle  200  (orientation is with respect to the plane formed by the incident EUV  201  and plane normal Z; this plane is parallel to the vertical features and perpendicular to the horizontal features). The H-V print difference becomes even greater if either the offset angle  207  or the thickness of absorber layer  215  increases. 
         [0005]    With the absorber thickness commercially available today, it is possible to compensate the H-V print difference for 1x nm technology nodes, but it does not scale well to smaller critical dimensions, especially for half-pitch values below 25 nm. Neither simple rule-based optical proximity correction (OPC) techniques nor using a thinner absorber layer maintains the printability and defectivity at beyond 1x nm technology nodes. In particular, it is difficult to compensate for the larger H-V print difference using simple rule-based OPC, and absorber layer  215  cannot be made arbitrarily thin without engendering reduced image contrast, process window, normalized image log-slope (NILS), and increased defectivity (e.g., pinholes) caused by increased residual light reflected by reflective multilayer  203  at masked portions. 
         [0006]    A need therefore exists for methodology enabling EUV lithography for beyond 1x nm technology nodes while enhancing printability and improving defectivity, and the resulting device. 
       SUMMARY 
       [0007]    An aspect of the present disclosure is a method for fabricating a thin absorber exhibiting enhanced printability and low defectivity. 
         [0008]    Another aspect of the present disclosure is a thin absorber exhibiting enhanced printability and low defectivity. 
         [0009]    Additional aspects and other features of the present disclosure will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present disclosure. The advantages of the present disclosure may be realized and obtained as particularly pointed out in the appended claims. 
         [0010]    According to the present disclosure, some technical effects may be achieved in part by a method including: forming a multilayer on an upper surface of a substrate, forming a capping layer over the multilayer, forming one or more diffuse scattering layers over the capping layer, and etching the diffuse scattering layers to form a stack. 
         [0011]    Aspects of the present disclosure include forming a single diffuse scattering layer with a roughened surface. Further aspects include forming an absorber layer over each diffuse scattering layer of the one or more diffuse scattering layers. Additional aspects include roughening a surface of each of the one or more diffuse scattering layers to a high frequency roughness adjacent to the overlying absorber layer. Another aspect includes forming nano-particles between each absorber layer and the underlying diffuse scattering layer or embedding nano-particles in each diffuse scattering layer or absorbing layers. Other aspects include the nano-particles including nickel (Ni), chromium (Cr), carbon (C), tantalum (Ta), or titanium (Ti). 
         [0012]    A further aspect includes forming each diffuse scattering layer of silicon (Si) and each absorbing layer of a high EUV-absorbing material such as tantalum nitride (TaN), tantalum nitride oxide (TaNO), tantalum borate (TaBO), Ni, gold (Au), silver (Ag), C, tellurium (Te), platinum (Pt), palladium (Pd), or Cr. Additional aspects include forming each diffuse scattering layer to a thickness of 5 nm to 50 nm and forming each absorber layer to a thickness of 2 nm to 20 nm. A further aspect includes forming the total absorber stacks to a height of 60 nm or less above the capping layer. An additional aspect includes forming the capping layer of ruthenium (Ru) to a thickness of 1 nm to 5 nm. Other aspects include forming the diffuse scattering layer by depositing nanoparticles on an absorber and etching the absorber to form a porous topology. 
         [0013]    Another aspect of the present disclosure is a reflective mask (or EUV reticle) including: a multilayer on an upper surface of a substrate, a capping layer over the multilayer, and a stack of one or more etched diffuse scattering layers over the capping layer. 
         [0014]    Aspects of the present disclosure include the stack including a single diffuse scattering layer with a roughened surface. Another aspect includes each etched diffuse scattering layer of the stack being formed to a thickness of 5 nm to 50 nm. A further aspect includes an etched absorber layer formed over each of the one or more etched diffuse scattering layers of the stack. Another aspect includes roughened surfaces between each etched absorber layer and the underlying etched diffuse scattering layer. Other aspects include nano-particles between each etched absorber and the underlying diffuse scattering layers of first stack or embedded in each etched diffuse scattering layer of the stack. Additional aspects include the nano-particles being Ni, Cr, C, Ta, or Ti nano-particles. Another aspect includes each diffuse scattering layer being Si, and each absorbing layer such as TaN, TaNO, TaBO, Ni, Au, Ag, C, Pt, Pd, Te, or Cr. 
         [0015]    Another aspect of the present disclosure is a method including: forming a multilayer on the upper surface of a substrate, forming a Ru capping layer over the multilayer, and forming a stack of two to twenty pairs of a diffuse scattering layer and an absorbing layer over the Ru capping layer, wherein each diffuse scattering layer includes nanoparticles embedded in or formed on a Si layer, each absorbing layer includes high absorbing materials such as TaN, TaNO, TaBO, Ni, Au, Ag, C, Pt, Pd, Te, or Cr, and the thickness of the stack is 60 nm or less. 
         [0016]    Additional aspects and technical effects of the present disclosure will become readily apparent to those skilled in the art from the following detailed description wherein embodiments of the present disclosure are described simply by way of illustration of the best mode contemplated to carry out the present disclosure. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
           [0018]      FIG. 1  schematically illustrates a background art EUV reticle employing a conventional absorber; 
           [0019]      FIGS. 2A and 2B  schematically illustrate a shadow mask effect observed with a background art EUVL reticle employing a conventional absorber; 
           [0020]      FIGS. 3A through 3C  schematically illustrate a process flow for forming a EUV reticle employing a scattering enhanced thin absorber, in accordance with an exemplary embodiment of the present disclosure; 
           [0021]      FIGS. 4A through 4C  schematically illustrate a process flow for forming a EUV reticle employing a scattering enhanced thin absorber, in accordance with another exemplary embodiment of the present disclosure; 
           [0022]      FIG. 5  schematically illustrates a EUV reticle employing a scattering enhanced thin absorber, formed in accordance with an exemplary embodiment of the present disclosure, as EUV radiation is incident upon it; 
           [0023]      FIGS. 6A through 6C  schematically illustrate modifications to a scattering enhanced thin absorber, in accordance with various additional exemplary embodiments of the present disclosure; 
           [0024]      FIG. 7  schematically illustrates a scattering enhanced thin absorber material, in accordance with an exemplary embodiment of the present disclosure; and 
           [0025]      FIGS. 8A and 8B  schematically illustrate a process flow forming another scattering enhanced thin absorber material, in accordance with an exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments. It should be apparent, however, that exemplary embodiments may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring exemplary embodiments. In addition, unless otherwise indicated, all numbers expressing quantities, ratios, and numerical properties of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” 
         [0027]    A perfectly diffusing surface eliminates most of the specular reflected light. The intensity of light reflected from a diffuse surface is approximated by I=k d ·I i ·R ML , where k d  is the diffuse reflection coefficient between 0 and 1, I i  is the intrinsic intensity of a point source, and R ML  is the reflection amplitude of a reflective multilayer utilized in EUVL reticles. For a perfect diffuse reflection, k d →0. In order to increase the total integrated scatter (the ratio between scattered power and reflected power) at the absorptive portions of the reticle, materials or structures with high internal inhomogeneity (large number of sub-surfaces) may be used. 
         [0028]    The present disclosure addresses and solves the current problem of H-V print difference attendant upon EUV lithography beyond 1x nm technology nodes. In accordance with embodiments of the present disclosure, a scattering enhanced thin absorber is utilized to reduce H-V print difference and improve printability for beyond 1x nm technology nodes. 
         [0029]    Methodology in accordance with embodiments of the present disclosure includes utilization of one or more diffuse scattering layers interleaved with an absorber layer in the masked portions of a EUV reticle. Additional aspects include utilization of roughened surfaces, nano-particles, and an absorber material with an increased number of diffuse scattering sub-surfaces. 
         [0030]    Still other aspects, features, and technical effects will be readily apparent to those skilled in this art from the following detailed description, wherein preferred embodiments are shown and described, simply by way of illustration of the best mode contemplated. The disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
         [0031]      FIGS. 3A through 3C  schematically illustrate various process steps for forming EUV reticle  300  employing a scattering enhanced thin absorber, in accordance with an exemplary embodiment of the present disclosure. 
         [0032]      FIG. 3A  illustrates capping layer  301  and reflective multilayer  303  deposited over substrate  305 . Capping layer  301  may be formed of Ru and may have a thickness from 1 nm to 5 nm. 
         [0033]    Adverting to  FIG. 3B , diffuse scattering layer  307  and absorber layer  309  are formed over capping layer  301 . Diffuse scattering layer  307  may include, for example, a material with a high degree of surface roughness. The thickness of diffuse scattering layer  307  may be from 5 nm to 50 nm and the thickness of the absorber layer may be from 2 nm to 20 nm. Alternatively, absorber layer  309  may be omitted, if diffuse scattering layer  307  is a perfect scattering material. 
         [0034]    Finally, as shown in  FIG. 3C , diffuse scattering layer  307  and absorber layer  309  are etched to leave behind a masked non-reflective portion  311  and an unmasked reflective portion  313  of EUV reticle  300 . 
         [0035]      FIGS. 4A through 4C  schematically illustrate a process flow for forming a EUV reticle employing a scattering enhanced thin absorber, in accordance with another exemplary embodiment of the present disclosure. 
         [0036]      FIG. 4A  illustrates capping layer  401  and reflective multilayer  403  formed over substrate  405 . Capping layer  401  may be formed of Ru and may have a thickness from 1 nm to 5 nm. 
         [0037]    Adverting to  FIG. 4B , one or more diffuse scattering layers  407  and a corresponding number of absorber layers  409  are formed interleaved over capping layer  401 . The number of repeats for the diffuse scattering layer  407  and absorber layer  409  pairs may, for example, be between 2 and 20. Diffuse scattering layer  407  may have a thickness of 5 nm to 50 nm and may be formed of, for example, Si. Alternatively, diffuse scattering layer  407  may be formed of a strong absorber material filled with nano-particles. Strong absorber materials may include, for example, Si, TaN, TaNO, TaBO, Cr, Ni, etc. The nano-particles may be formed of, for example, Ni, Cr, C, Ta, or Ti. Absorber layer  409  may have a thickness of 2 nm to 20 nm and may be formed of, for example, TaN, TaNO, TaBN, TaBO, Ni, Au, Ag, Te, C, Pt, Pd or Cr. 
         [0038]    Finally, as shown in  FIG. 4C , diffuse scattering layers  407  and absorber layers  409  are etched to leave behind a masked non-reflective portion  411  and an unmasked reflective portion  413  of EUV reticle  400 . 
         [0039]      FIG. 5  illustrates incident EUV radiation  501  on EUV reticle  500  as it is diffuse scattered, according to an exemplary embodiment. As shown, some portion of the radiation is scattered at surface  505  of absorber  503 . Most of the radiation goes through first absorber layer  507  and first diffuse scattering layer  509 . The scattering reflection at first diffuse scattering layer  509  is absorbed by first absorber layer  507 . The residual radiation is bounced back and forth in the remaining absorber layers and diffuse scattering layers until completely absorbed. Even though the height  511  of absorber  503  above capping layer  513  may be equal to or less than 60 nm, the overall optical path is made much greater than 60 nm due to the multiple reflections at the absorber and diffuse scattering layers. 
         [0040]      FIGS. 6A through 6C  schematically illustrate modifications to a scattering enhanced thin absorber, in accordance with various additional exemplary embodiments of the present disclosure. 
         [0041]    Adverting to  FIG. 6A , absorber layer  601  and diffuse scattering layer  603  are shown formed with their respective adjacent surfaces  605  and  607  having a high frequency surface roughness to induce further diffuse scattering of incident EUV radiation. 
         [0042]    Adverting to  FIG. 6B , absorber layer  601  and diffuse scattering layer  603  are shown with nano-particles  609  filled between their respective adjacent surfaces. Nano-particles  609  may be formed of Ni, Cr, C, Ta, Ti, hafnium (Hf), or zirconium (Zr), all of which have large refractive index difference than the surrounding material. 
         [0043]    Adverting to  FIG. 6C , diffuse scattering layer  603  may be filled with nano-particles  611  to create additional diffuse scattering sub-surfaces. Nano-particles  611  may be formed of Ni, Cr, C, Ta, Ti, Hf, or Zr, which have large refractive index difference than the surrounding material. 
         [0044]    In addition to increasing diffuse scattering at surfaces of the diffuse scattering and absorber layers, diffuse scattering may also be enhanced by increasing the number of sub-surfaces within a single absorber layer.  FIG. 7  schematically illustrates an enhanced thin absorber material, in accordance with an exemplary embodiment of the present disclosure 
         [0045]    As shown in  FIG. 7 , an absorber material  701  may be filled with nano-particles  703 , which produces a diffuse scattering material  700  with a large number of sub-surfaces. Diffuse scattering material  700  may be formed by adding nano-particles to absorber material  701  in powdered form or solution and then depositing the mixture. Nano-particles  611  may be formed of Ni, Cr, C, Ta, Ti, Hf, or Zr, which have large refractive index difference than the surrounding material. Absorber material  701  may include Si, TaN, TaNO, TaBN, TaBO, Cr, Au, Ag, C, Pt, Pd, Te or Ni. 
         [0046]      FIGS. 8A and 8B  schematically illustrate various process steps for forming another enhanced diffuse scattering material, in accordance with an exemplary embodiment of the present disclosure. As shown in  FIG. 8A , nano-particles  801  are sparsely deposited on an absorber material  803 , which is then etched to create a highly porous absorber material  805 . Diffuse scattering is enhanced at the large number of sub-surfaces  807 . 
         [0047]    The embodiments of the present disclosure can achieve several technical effects, including improved printability and low defectivity at sub 1x nm technology nodes produced by EUVL. The present disclosure enjoys industrial applicability in any of various EUVL systems used to produce devices for various industrial applications as, for example, microprocessors, smart phones, mobile phones, cellular handsets, set-top boxes, DVD recorders and players, automotive navigation, printers and peripherals, networking and telecom equipment, gaming systems, and digital cameras. The present disclosure therefore enjoys industrial applicability in any of various highly integrated semiconductor devices. 
         [0048]    In the preceding description, the present disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not as restrictive. It is understood that the present disclosure is capable of using various other combinations and embodiments and is capable of any changes or modifications within the scope of the inventive concept as expressed herein.