Abstract:
An extreme ultraviolet lithography mask may be formed of a multilayered stack covered by a spacer layer, such as silicon or boron carbide, in turn covered by a thin layer to prevent inter-diffusion, and finally covered by a capping layer of ruthenium. By optimizing the spacer layer thickness based on the capping layer, the optical properties may be improved.

Description:
BACKGROUND  
       [0001]     This invention relates generally to extreme ultraviolet lithography mask blanks.  
         [0002]     In extreme ultraviolet lithography, a mask is formed from a blank. The blank provides a reflective surface which defines features. Extreme ultraviolet radiation is shined on the blank and is reflected therefrom to transfer features from the blank to a semiconductor wafer in a repeatable fashion.  
         [0003]     Generally, extreme ultraviolet lithography masks are reflective masks fabricated by depositing interference multilayers such as molybdenum and silicon in alternating layers. The very top ending layer is referred to as a capping layer. Typically a silicon layer is used as a capping layer.  
         [0004]     The thicker silicon capping layer is needed because of the mask patterning process control requirements. In the mask patterning process, the silicon capping layer serves as the etch stop layer for the buffer layer etch. During the buffer layer etch, when the etch selectivity to the multilayer capping layer is low, the capping layer is partially and non-uniformly removed. For example, one promising buffer layer for extreme ultraviolet lithography mask patterning is silicon dioxide. However, the etch selectivity to the silicon capping layer in a square mask etcher is rather low, for example, about 3 to 1.  
         [0005]     Thus, there is a need for better ways to make blanks for extreme ultraviolet lithography. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a partial, enlarged cross-sectional view of one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0007]     An upper masking layer  18  and a lower masking layer  16  have an aperture  22 . Radiation, indicated by the lines L, is reflected from the bottom of the aperture  22 . The radiation may be extreme ultraviolet irradiation in one embodiment of the present invention. A pattern of a large number of apertures  22  may be transferred to a semiconductor wafer (not shown) by reflecting radiation from those apertures  22 .  
         [0008]     The radiation is actually reflected from the capping layer  14  that, in one embodiment of the present invention, may be formed of ruthenium. In one embodiment, the layer  14  may be of a thickness of from about 1 to 4.5 nanometers and, particularly, greater than 2 nanometers.  
         [0009]     A ruthenium capping layer  14  is resistant to the oxidation. In addition, the etch selectivity of the buffer silicon dioxide layer to ruthenium is much larger than that of a silicon capping layer. The ruthenium layer also has better chemical cleaning resistance than a silicon capping layer.  
         [0010]     While ruthenium has a higher extreme ultraviolet absorption coefficient than silicon, a sufficiently thin ruthenium capping layer may be utilized without dramatically reducing the multilayer reflectivity.  
         [0011]     A ruthenium capping layer  14  of 2 nanometers may be susceptible to damage during mask patterning processes. However, a thicker ruthenium capping layer may reduce the multilayer reflectivity, and may produce a larger multilayer reflectivity variation, if the ruthenium capping layer happens to be non-uniform.  
         [0012]     The ruthenium capping layer  14  may be deposited over an interface layer  20 . The interface layer  20  may be molybdenum or boron carbide, to mention two examples. The layer  20  may reduce or prevent inter-diffusion between the layers  14  and  22 . In one embodiment, the layer  20  may be 5 Angstroms in thickness.  
         [0013]     A spacer layer  22 , below the layer  20 , may have a thickness between about 2.4 and about 3.8 nanometers in one embodiment of the present invention. In this range, the multilayer reflectivity variation, as a result of any ruthenium capping layer  14  thickness variation, may be controlled. Advantageously, the spacer layer  22  has lower extreme ultraviolet absorption. The spacer layer  22  may be silicon, in one embodiment.  
         [0014]     Below the layer  22  is the multilayer stack  12 . In one embodiment, the multilayer stack includes a first layer of silicon of approximately 4.2 nanometers covered by a layer of molybdenum of 2.8 nanometers. This may be followed by another layer of silicon and thereafter another layer of molybdenum in one embodiment of the present invention.  
         [0015]     In some embodiments, optimizing the spacer layer  22  may enable the use of a thicker capping layer  14  that may be used to protect the multilayer stack  12  from damage from the patterning process steps. The optimized spacer layer  22  not only can optimize the peak multilayer reflectivity, but can also reduce or even minimize, for a given capping layer material and thickness, the multilayer reflectivity variation when the capping layer  14  is partially and non-uniformly removed. This may result in larger mask patterning process margins in some embodiments.  
         [0016]     For example, simulation indicates that with a standard multilayer stack (the spacer layer  22  having a standard silicon layer of 4.14 nanometers as used in the Mo/Si multilayer stack)and a very thin interface layer  20  (&lt;5 A), the optimized capping layer  14  thickness is around 2 nanometers. This thickness provides maximum multilayer blank peak reflectivity of about 75 percent and minimum reflectivity variation of 0.5 percent when layer  14  thickness varies from 2-0.4 nanometers. However, this relatively thin capping layer  14  thickness gives rise to the problems with capping layers described above. To increase the capping layer  14  thickness beyond 2 nanometers without optimizing the spacer layer  22 , the blank peak reflectivity will be reduced drastically as layer  14  thickness increases. The average reflectivity reduction per nanometer of capping layer  14  thickness increase for capping layer  14  thicknesses in the range of 2-4 nanometer is about 3.5 percent. As a result, larger reflectivity variation will result when the capping layer  14  thickness variation exists.  
         [0017]     However, with a 3.8 nanometer silicon spacer layer  22 , the optimized capping layer  14  thickness can go up to 2.3 nanometers with only a slight increase in the multilayer blank peak reflectivity. The multilayer reflectivity variation is still within 0.5 percent when the capping layer  14  thickness variation is between 2.3-0.7 nanometers. Similarly, with a 2.9 nanometer silicon spacer  22 , the optimized capping layer  14  thickness can go up to 3.3 nanometers with slight treadoff for the peak reflectivity (about 1.0 percent reflectivity loss). For maintaining the multilayer reflectivity variation within 0.5 percent, the capping layer  14  thickness variation can be between 3.3-1.7 nanometers. Finally, with a 2.4 nanometer silicon spacer  22 , the optimized capping layer  14  thickness can go up to 3.8 nanometers, again with a small tradeoff for the peak reflectivity (about 2.5 percent reflectivity loss). The capping layer  14  thickness can vary from 3.8-2.4 nanometers with a reflectivity variation of less than 0.5 percent. Thus, it is clear that the optimization of the spacer layer  22  enables the use of a significantly thicker capping layer  14 .  
         [0018]     In the actual multilayer fabrication, due to interlayer diffusion effect, the calculated optimized spacer thickness may deviate from that of experimentally obtained value. However, the optimization theory/principle remains the same.  
         [0019]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.