Patent Number: 
Section: description

The invention provides an EUVL multilayer coating that has high reflectivity, high thermal stability and controlled reaction or interdiffusion between the two different materials at their interface. Such a multilayer consists of alternating layers of an absorber layer (e.g., molybdenum) and a spacer layer (e.g., silicon). The invention is a thin layer of a third compound, e.g., boron carbide (B4C), placed on both interfaces (Mo-on-Si and Si-on-Mo interface). This third layer comprises boron carbide and other carbon and boron based compounds characterized as having a low absorption in EUV wavelengths and soft X-ray wavelengths. Thus, a multilayer film comprising alternating layers of Mo and Si includes a thin interlayer of boron carbide (e.g., B4C) and/or boron based compounds between each layer. The interlayer changes the surface (interface) chemistry, resulting in an increase of the reflectance and increased thermal stability. A unique feature of a Mo/Si multilayer system is that the interlayer regions are asymmetric. For example, the Mo-on-Si interface is considerably thicker than Si-on-Mo interface. This seems to be an intrinsic property of Mo/Si multilayers and has been observed in multilayers grown by magnetron sputtering [1], ion beam sputtering [2] and by electron beam evaporation [3,4]. The present invention also contemplates depositing a thicker layer of the interlayer material on the Mo-on-Si interface and a thinner layer of the interlayer material on the Si-on-Mo interface. The present inventors experimentally confirmed that a thicker boron carbide layer on the Mo-on-Si interface and a thinner layer of boron carbide on the Si-on-Mo interface gave the best reflectivity results. FIG. 1 shows multilayer design of a Mo/Si multilayer with a thicker B4C layer on the Mo-on-Si interface (where Mo is referred to as 12 and Si is referred to as 14) and a thinner B4C layer 16 on the Si-on-Mo interface (where Si is referred to as 18 and Mo is referred to as 12). The interface layer can be deposited using the same methods as for depositing Mo and Si. These methods include magnetron sputtering, ion beam sputtering, electron evaporation and any combination thereof. FIG. 2 shows the reflectance as a function of wavelength for this design, which achieved a peak reflectance of 69.9% at about 13.4 nm. The present inventors also performed time stability testing of these multilayers. Interdiffusion in a Mo/Si multilayer was prevented by depositing B4C on the interfaces once the initial reaction from a Mo, Si, B, C amorphous layer occurred. The interfaces between the three component layers in these structures remained sharp after 2.5 years and the multilayer structurexe2x80x94(Mo/Mo, Si, B, C/Si/Mo, Si, B, C/ remained unchanged. The reflectivity on a high reflectance multilayer dropped by only 0.4% (absolute) in 2.5 years (FIG. 3). The observed reflectance drop is due to surface oxidation of the last Si layer that occurs within a couple of months after the deposition. This multilayer structure (Mo/Si with B4C on interfaces) is stable in reflectance as well as in wavelength over long periods of time. FIG. 4 shows a schematic representation of an embodiment of a multilayer structure of the present invention. FIG. 5 shows a high magnification cross-section TEM of a Mo/B4C/Si/B4C multilayer fabricated by magnetron sputtering. The deposited thickness are: Mo=26.2 xc3x85, Si=42.72 xc3x85, B4C=2.55/2.55 xc3x85. The TEM thicknesses are: Mo=21.2 xc3x85, Si=34 xc3x85, B37Si31Mo23C9=7.35/7.35 xc3x85. The non-crystalline reacted zones at the Mo/B4C/Si interfaces are clearly shown in this magnification TEM of surface layers in the multilayer. Typically, the sharpness of the Mo on Si interface 50 would be about 2.5 times worse than that of the Si on Mo interface; however, due to the deposition of the interlayer of B4C in the Mo on Si interface, such interface sharpness is comparable to that of the Si on Mo interface. The multilayer was terminated with a Si layer that subsequently formed SiO2 on the top ambient surface. Highest reflectance multilayers are achieved with very thin B4C layers (0.1-0.35 nm). Improved lifetime stability of these multilayers can be achieved with 0.2-0.25 nm B4C thick interfaces. To make the multilayer thermally stable at higher temperatures ( greater than 300 degrees Celsius) thicker B4C layers are needed (0.3 nm or thicker). Referring again to FIG. 1, optimum performance is obtained where the thickness for the interlayer 10 is between 0.1 and 1.0 nm and interlayer 16 is between 0.1 and 0.5 nm Another important property of the multilayers is residual stress. Residual stress of Mo/Si multilayers with boron carbide interfaces is about 30% higher than the residual stress of Mo/Si multilayers with no boron carbide on the interfaces. The measured residual stress of Mo/Si multilayers with B4C interfaces is about xe2x88x92560 MPa. Annealing at about 150 degrees Celsius for about 3 hours reduces the residual stress substantially. The present invention contemplates that the residual stress of annealed Mo/Si multilayers with B4C interfaces is less than or equal to the residual stress of Mo/Si multilayers with no B4C on the interfaces. Still another aspect of the invention is that the annealing does not reduce the peak reflectance and does not change the peak wavelength. References: [1] A. K. Petford-Long, R. S. Rosen, M. B. Stearns, C. -H. Chang, S. R. Nutt, D. G. Stearns, N. M. Ceglio, and A. M. Hawryluk, J. Appl. Phys. 61, 1422, 1987. [2] A. Ulyanenkov, R. Matsuo, K. Omote, K. Inaba, and J. Harada, J. Appl. Phys. 87,7255 (2000). [3] J. M. Slaughter, D. W. Schulze, C. R. Hills, A. Mirone, R. Stalio, R. N. Watts, C. Tarrio, T. B. Lucatorto, M. Krumrey, P. Mueller, C. M. Falco, J. Appl. Phys. 76, 2144 (1994). [4] M. B. Stearns, C. -H. Chang, D. G. Stearns, J. Appl. Phys. 71, 187 (1992). The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.