Abstract:
A Fabry-Perot thin absorber for an extreme ultraviolet (EUV) reticle and a method of making is disclosed. Embodiments include forming a molybdenum/silicon (Mo/Si) multilayer on an upper surface of a substrate; forming a ruthenium (Ru) capping layer over the Mo/Si multilayer; forming an absorber cavity layer over the Ru layer; forming two or more pairs of a silicon (Si) layer and an absorbing layer over the absorber cavity layer; and etching the Si layers, absorbing layers, and the absorber cavity layer 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 1×nm technology nodes. A reflective mask (or reticle) is used in a single-exposure process to produce imaged features on a wafer.  FIG. 1  illustrates a EUVL 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 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, which 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, incident EUV  201  is at offset angle  207  (conventionally set to six degrees) to a Z-axis normal. Mask shadowing effect  209  is induced by the interaction of the off-axis illumination with the mask topography. 
         [0004]    Adverting to  FIG. 2B , 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 between the horizontally oriented (H) features  211  and the vertically oriented (V) features  213  (H−V print difference) of EUV reticle  200  (orientation is with respect to the plane formed by 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 offset angle  207  or the thickness of absorber layers  215  increases. 
         [0005]    With the absorber thickness commercially available today, it is possible to compensate the H−V print difference for 1×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 1×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  109  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 multilayer  101  at masked portions. 
         [0006]    A need therefore exists for methodology enabling EUV lithography for beyond 1×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 Fabry-Perot thin absorber exhibiting enhanced printability and low defectivity. 
         [0008]    Another aspect of the present disclosure is a Fabry-Perot 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 molybdenum/silicon (Mo/Si) multilayer on an upper surface of a substrate, forming a ruthenium (Ru) layer over the Mo/Si multilayer, forming an absorber cavity layer over the Ru layer, forming two or more pairs of a silicon (Si) layer and an absorbing layer over the absorber cavity layer, and etching the Si layers, absorbing layers, and the absorber cavity layer to form a stack. 
         [0011]    Aspects of the present disclosure include forming the stack to a height above the capping layer of 10 nanometer (nm) to 60 nm. Further aspects include forming the absorber cavity layer and each absorbing layer of Ru. Other aspects include forming each absorbing layer to a thickness of 3 nm to 5 nm, and forming each Si layer to a thickness of 2 nm to 3 nm. Another aspect includes forming the absorber cavity layer to a thickness of 2 nm to 50 nm. Other aspects include forming the absorber cavity layer and each absorbing layer of tantalum nitride (TaN). Additional aspects include forming each absorbing layer to a thickness of 3 nm to 5 nm, and forming each Si layer to a thickness of 2 nm to 3 nm. Another aspect includes forming the absorber cavity layer to a thickness of 2 nm to 50 nm. A further aspect includes forming the Ru layer to a thickness of 2 nm to 5 nm. 
         [0012]    Another aspect of the present disclosure is a device including a Mo/Si multilayer on an upper surface of a substrate, a Ru layer over the Mo/Si multilayer, and a stack of an absorber cavity layer and two or more pairs of a Si layer and an absorbing layer formed on the Ru layer. 
         [0013]    Aspects include the stack having a height of 10 nm to 60 nm above the Ru layer. Another aspect includes the absorber cavity layer and each absorbing layer including Ru. Further aspects include each absorbing layer being formed to a thickness of 3 nm to 5 nm, and each Si layer being formed to a thickness of 2 nm to 3 nm. An additional aspect includes and the absorber cavity layer being formed to a thickness of 2 nm to 50 nm. Further aspects include the absorber cavity layer and each absorbing layer including TaN. Other aspects include each absorbing layer being formed to a thickness of 3 nm to 5 nm, and each Si layer being formed to a thickness of 2 nm to 3 nm. Another aspect includes the absorber cavity layer being formed to a thickness of 2 nm to 50 nm. An additional aspect includes the Ru layer being formed to a thickness of 2 nm to 5 nm. 
         [0014]    Another aspect of the present disclosure is a method including forming a Mo/Si multilayer on an upper surface of a substrate, forming a Ru layer to a thickness of 2 nm to 5 nm over the Mo/Si multilayer, and forming a stack having a thickness of 10 nm to 60 nm on the Ru layer by forming an absorber cavity layer of Ru or TaN to a thickness of 2 nm to 50 nm over the Ru layer, forming a Si layer over the absorber cavity layer forming an absorbing layer of Ru if the absorber cavity layer is formed of Ru or of TaN if the absorber cavity layer is formed of TaN, forming one to nine additional pairs of a Si layer and an absorbing layer over the absorbing layer, and etching the Si layers, the absorbing layers, and the absorber cavity layer. Other aspects include forming each absorbing layer to a thickness of 3 nm to 5 nm, and forming each Si layer to a thickness of 2 nm to 3 nm. 
         [0015]    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  
         [0016]    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: 
           [0017]      FIG. 1  schematically illustrates a background art EUV reticle employing a conventional absorber; 
           [0018]      FIGS. 2A and 2B  schematically illustrate a shadow masking effect observed with a background art EUV reticle employing a conventional absorber; 
           [0019]      FIGS. 3A through 3D  schematically illustrate a process flow for forming a EUV reticle employing a Fabry-Perot thin absorber, in accordance with an exemplary embodiment of the present disclosure; and 
           [0020]      FIG. 4  schematically illustrates a configuration of a EUV reticle employing a Fabry-Perot thin absorber, in accordance with an exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION  
       [0021]    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.” 
         [0022]    The present disclosure addresses and solves the current problem of H−V print difference attendant upon EUV lithography beyond 1×nm technology nodes. In accordance with embodiments of the present disclosure, a Fabry-Perot thin absorber is utilized to reduce H−V print difference and improve printability for beyond 1×nm technology nodes. 
         [0023]    A Fabry-Perot thin absorber is proposed to minimize residual reflection from masked portions of a EUV reticle. The thin absorber employs resonance to increase absorption of EUV radiation in the masked portions. A Fabry-Perot cavity is utilized to induce resonance. Transmission (T e ) in a Fabry-Perot cavity is a function of the mirror reflection coefficients r 1  and r 2 : T e =(1−R 2 )/(1+R 2 −2·R·cos(δ/2)), where R=(r 1 ·r 2 ), and δ is the phase shift which is function of cavity material and length. Energy conservation requires that the sum of transmission, reflection, and loss coefficients is equal to one. Therefore, reflection within the cavity may be minimized by increasing the relative amount of transmission and loss. 
         [0024]    Methodology in accordance with embodiments of the present disclosure includes utilization of an absorber cavity layer to form a Fabry-Perot cavity in the masked portions of a EUV reticle. Additional aspects include a stack of one or more absorber layers interleaved with a resonating layer formed on the absorber cavity layer. 
         [0025]    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. 
         [0026]      FIGS. 3A through 3D  schematically illustrate a process flow for forming a EUV reticle employing a Fabry-Perot thin absorber, in accordance with an exemplary embodiment of the present disclosure. 
         [0027]      FIG. 3A  illustrates capping layer  301  and reflective multilayer  303  formed over substrate  305 . Capping layer  301  may be formed of Ru and may have a thickness from 2 nm to 5 nm. Reflective multilayer  303  may be made of 6.9-7 nm-pitch bilayers of Mo and Si (reflective Mo/Si multilayer). The Mo and Si sub-layers may have a thickness of approximately 2.8 nm and 4.1 nm, respectively. In one embodiment, the first and second sub-layers of reflective multilayer  303  may be formed of lanthanum nitride and boron (LaN/B), respectively, or lanthanum and boron carbide (La/B 4 C), respectively. Each of the sub-layers for the LaN/B or La/B 4 C stack may have a thickness from 1 nm to 3.4 nm. 
         [0028]    Adverting to  FIG. 3B , absorber cavity layer  307  is next formed over capping layer  301 . Absorber cavity layer  307  may have a thickness from 2 nm to 50 nm and may be formed of, for example, TaN, tantalum boro nitride (TaBN), gold (Au), silver (Ag), platinum (Pt), palladium (Pd), tellurium (Te), carbon (C), nickel (Ni), or Ru. 
         [0029]    Adverting to  FIG. 3C , one or more resonating layers  309  and a corresponding number of absorber layers  311  are formed interleaved over absorber cavity layer  301 . The number of repeats for resonating layer  309  and absorber layer  311  pairs may, for example, be between 2 and 10. Resonating layers  309  may each have a thickness of 2 nm to 3 nm and may be formed of, for example, Si. Absorber layers  311  may each have a thickness of 3 nm to 5 nm and may be formed of, for example, TaN or Ru. The resonating and the absorber layers are formed by low and high absorbing materials, respectively. For example, the resonating layer may be formed by Si, C, Mo, etc. The absorbing layer may be formed by TaN, TaBN, Au, Ag, Pt, Pd, Te, C, Ni, or Ru. 
         [0030]    Resonating layers  309  and absorber layers  311  together work as a mirror with respect to the Fabry-Perot cavity formed by absorber cavity layer  307  and capping layer  301 . Similarly, reflective multilayer  303  works as another mirror. Absorber layers  311  increase loss associated with radiation from within the cavity, thus decreasing overall reflection of EUV radiation reflected within the cavity. 
         [0031]    Finally, as shown in  FIG. 3D , resonating layers  309 , absorber layers  311 , and absorber cavity layer  307  are etched to leave behind a masked non-reflective portion  313  and an unmasked reflective portion  315  of EUV reticle  300 . 
         [0032]      FIG. 4  schematically illustrates a configuration of EUV reticle  400  employing a Fabry-Perot thin absorber, in accordance with an exemplary embodiment of the present disclosure. In this configuration, capping layer  401  is formed of Ru and has a thickness of 2.5 nm, and reflective multilayer  403  is formed of 40.5 pairs of Mo/Si sub-layers. A stack formed in a non-reflective portion  405  of EUV reticle  400  includes absorber cavity layer  407  formed of TaN and has a thickness of 19 nm. Four pairs of resonating layers  409  and absorber layers  411  are formed over absorber cavity layer  407 . Each resonating layer  409  is formed of Si and has a thickness of 2.78 nm. Each absorber layer  411  is formed of TaN and has a thickness of 4.16 nm. The combined height of absorber cavity layer  407 , resonating layers  409 , and absorber layers  411  is 46.75 nm. 
         [0033]    In accordance with another exemplary embodiment, absorber cavity layer  407  may be replaced with a Ru capping layer having a thickness of 21 nm, each Si resonating layer  409  may be formed to a thickness of 3.33 nm, and each absorber layer  411  may be replaced with a Ru absorber layer having a thickness of 3.61 nm. A EUV reticle employing a Fabry-Perot absorber using a Ru configuration is easier to fabricate than the EUV reticle of  FIG. 4 . However, a EUV reticle using a Fabry-Perot absorber with a TaN configuration, as in  FIG. 4 , demonstrates better performance. 
         [0034]    The embodiments of the present disclosure can achieve several technical effects, including improved printability and low defectivity at sub 1×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. 
         [0035]    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.