Patent Publication Number: US-6707602-B2

Title: Reflective spectral filtering of high power extreme ultra-violet radiation

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a Divisional Application of U.S. patent application Ser. No. 09/965,170, filed Sep. 27, 2001 now U.S. Pat. No. 6,577,442. This Divisional Application claims the benefit of the U.S. patent application Ser. No. 09/965,170. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates to lithography. In particular, the invention relates to extreme ultra-violet (EUV) lithography. 
     2. Description of Related Art 
     Extreme ultraviolet (EUV) lithography is an imaging technology with higher resolution capabilities than are available from longer wavelength exposure tools. The very short exposure wavelength requires an all reflective lens system because refractive materials are not sufficiently transparent. The EUV technique bounces EUV photons off a system of mirrors, including a mask made of reflective materials, that is ultimately focused on a resist-coated silicon wafer. By doing so, EUV systems can pattern features smaller than 0.05 micrometer. 
     Present EUV radiation sources have a very broad-band emission spectrum. For example, Xe laser pulsed plasma (LPP-Xe) sources have in excess of 40% of their radiation at wavelengths longer than 125 nanometers (nm) and approximately 60% of the energy is at wavelengths that are longer than 17 nm. EUV lithography tools for high volume manufacturing may require about 13.4 nm +/−1% wavelength radiation. Power levels are expected to be in the range between 50 and 100 Watts. Thus, kilowatts of energy potentially need to be filtered from the source spectra. Otherwise, it would unreasonably distort the mask and mirrors used in EUV imaging tools. 
     Existing techniques include use of ultra-thin transmission filters based on coating of a membrane on support structures. This technique has low efficiency, typically passing only about 50% of the desired wavelength. In addition, the membrane filters are easy to rupture at high power levels because of absorption. Another technique uses mono-chrometer and diffraction of the actinic 13.4 nm light. This technique is substantially less efficient and is generally used with only synchroton sources. Yet, another technique uses cooled Mo/Si multi-layer coated mirrors. This technique operates only through absorption and selective reflection, leading to problems in heating and a lack of long wavelength filtering. 
     Therefore, there is a need to have an efficient technique for spectral filtering of high power EUV radiation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following detailed description of the present invention in which: 
     FIG. 1 is a diagram illustrating a system using a reflective spectral filter in which one embodiment of the invention can be practiced. 
     FIG. 2 is a diagram illustrating a process to fabricate the reflective spectral filter shown in FIG. 1 according to one embodiment of the invention. 
     FIG. 3 is a diagram illustrating a process to fabricate a reflective spectral filter shown in FIG. 1 according to another embodiment of the invention. 
     FIG. 4 is a diagram illustrating reflectance and absorption spectra for the multi-layer coatings according to another embodiment of the invention. 
     FIG. 5 is a diagram illustrating a system using a transmitting spectral filter in which one embodiment of the invention can be practiced. 
     FIG. 6 is a diagram illustrating a process to fabricate the transmitting spectral filter shown in FIG. 5 according to one embodiment of the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention is a technique for efficient spectral filtering of EUV radiation. The spectral filter includes a grating structure and a multi-layer coating. The multi-layer coating is designed to reflect a narrow band around a short wavelength (e.g., 13.4 nm) and also reflects long wavelengths (e.g., greater than approximately 60 nm). The grating structure has a grating period responsive to the long wavelength band. The multi-layer coating is formed by layers of materials having high and low atomic numbers (e.g., Mo and Si) interspersed by layers of a compound (e.g., SiC). Regions of the longer wavelength rays are removed from the optical path by diffraction. The actinic rays have a wavelength much shorter than the grating period and undergoes simple reflection. The combined properties of the multi-layer coating and the grating structure generate the desired spectral characteristics. 
     In the following description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the present invention. In other instances, well-known structures are shown in block diagram form in order not to obscure the present invention. 
     FIG. 1 is a diagram illustrating a system using a reflective spectral filter in which one embodiment of the invention can be practiced. The system  100  includes an EUV radiation source  110 , a condenser mirror  120 , a reflective spectral filter  130 , and a baffle  140 . The system  100  is typically used in EUV lithography. 
     The EUV radiation source  110  is a laser pulsed plasma source. A Xenon (Xe) gas jet is hit with a high power laser pulse. Ionization and recombination generates spectral lines ranging from X-ray to Infra-red. As is known by one skilled in the art, any other methods to provide the EUV radiation source can be employed. 
     The condenser mirror  120  is an optical subsystem to collect the EUV radiation from the EUV radiation source  110 . 
     The reflective spectral filter  130  receives the source radiation as provided by the condenser mirror  120  and to reflect actinic rays  160  and diffracted rays  150 . In one embodiment, the actinic rays are radiation at 13.4 nm and the diffracted rays  150  are at wavelengths longer than the actinic wavelength (e.g., 60 nm). The reflective spectral filter  130  will be described in FIGS. 2 and 3. 
     The baffle  140  is a metal plate to stop the diffracted rays  150 . The baffle  140  has a hole  145  aligned with the principal optical path of the spectral filter  130 . The actinic rays  160  are on the optical path and go through the hole  145 . 
     FIG. 2 is a diagram illustrating a process  200  to fabricate the reflective spectral filter  130  shown in FIG. 1 according to one embodiment of the invention. 
     The process  200  starts with polishing a blank mirror substrate  210 . The mirror substrate  210  may have a flat or curved surface. The mirror substrate  210  may be made of Zerodur, ULE, or composites with low coefficients of thermal expansion that can be polished well. Then, a relief material or a photo-resist layer  220  is deposited on the mirror substrate  210 . 
     Next, the relief layer  220  is lithographically patterned and etched to form a grating structure  220 . The grating structure  220  has a plurality of ridges spaced at a grating period T. It is noted that although the preferred embodiment has a periodic pattern of ridges, it is contemplated that non-periodic pattern may also be used. The ridge shape is not restricted to the rectangular cross-section shown and could be triangular or other possibilities. The grating structure  220  may have a one-dimensional layout or a two-dimensional layout. The layout pattern  250  shows a representative two-dimensional layout having ridges  252  and grooves  254 . The grating period T is selected to be responsive to a band around a design filter wavelength and selected harmonics. The grating period T is selected to cause diffracting, out of an optical path, an incident radiation within the band of the design filter wavelength. The design filter wavelength may be 60 nm corresponding to when coating reflectivity begins to increase, 140 nm corresponding to significant emission spectra of the EUV source, or other possibilities. The ridges have a ridge width W and height H. The ridge width W is typically less than the grating period T. In one embodiment, the ridge width W is approximately proportionally to the grating period T with a proportionality constant α. In one embodiment, the proportionality constant α may be approximately equal to one-half. The ridge height H may be any suitable number. In one embodiment, the ridge height H is approximately proportionally to the grating period T with a proportionality constant β. The proportionality constant β may be an odd integer number times a quarter of the design filter wavelength. Typical values of the ridge height H may be 35 nm, 105 nm, and 175 nm. 
     Then, a multi-layer coating  230  is deposited conformally on the grating structure  220  and the mirror substrate  210 . The multi-layer coating  230  has a number of layers, or stack, of first and second materials having either high and low atomic numbers, respectively, or high and low densities of charge carriers, respectively. 
     In one embodiment, the first material is molybdenum (Mo) and the second material is silicon (Si) or beryllium (Be). The multi-layer coating  230  may also have a number of layers of a compound interspersed within the layers of the first and second materials. In one embodiment, the compound is silicon carbide (SiC). The incorporation of the compound SiC in the stack improves heating durability with minimal reduction (e.g., 3% to 5%) in reflectivity. 
     The reflective spectral filter  130  in this embodiment therefore includes a multi-layer coating  230 , the grating structure  220 , and the mirror substrate  210 . The multi-layer coating  230  is designed to be reflective at around the actinic wavelength (e.g., 13.4 nm) in an optical path and wavelengths longer than the actinic wavelength (e.g., 60 nm, 140 nm). With this construction, wavelengths that are both near the grating period T and that reflect from the multi-layer coating  230  are diffracted away (diffracted rays  150  shown in FIG. 1) from the path of the actinic rays (actinic rays  160  shown in FIG.  1 ). 
     FIG. 3 is a diagram illustrating a process  300  to fabricate the reflective spectral filter  130  shown in FIG. 1 according to another embodiment of the invention. 
     The process  300  starts with polishing a mirror substrate  310 . This step is much similar to the step shown in FIG.  2 . Next, a first multi-layer coating  320  is deposited on the mirror substrate  310 . The first multi-layer coating  320  is made of materials similar to the multi-layer coating  230  in FIG.  2 . It includes a number of layers, or stack, of first and second materials having either high and-low atomic numbers, respectively, or high and low densities of charge carriers, respectively. 
     In one embodiment, the first material is molybdenum (Mo) and the second material is silicon (Si) or beryllium (Be). The multi-layer coating  230  may also have a number of layers of a compound interspersed within the layers of the first and second materials. In one embodiment, the compound is silicon carbide (SiC). 
     Then, an etch stop layer  330  is optionally deposited on the multi-layer coating  320 . The etch stop layer  330  may be made by SiC or any other suitable material. The etch stop  330  is used so that subsequent etching does not cut into the multi-layer coating  320 . 
     Next, a metal spacer layer  340  is deposited on the etch stop layer  330  to provide grating relief layer. Then, a second multi-layer coating  350  is deposited on the metal spacer layer  340 . This second multi-layer coating  350  is essentially the same as the first multi-layer coating  320 . Both the multi-layer coatings  350  and  320  are designed to be reflective at around the actinic wavelength (e.g., 13.4 nm). Note that the metal spacer layer  340  may not be needed. In addition, the second multi-layer coating  350  may not be needed leaving only the metal spacer layer  340 . 
     The process  300  then lithographically patterns and etches a grating structure  360  from the second multi-layer coating  350  and the metal spacer layer  340 . The grating structure  360  may have a one-dimensional layout or a two-dimensional layout. The layout pattern  370  shows a representative two-dimensional layout having ridges  372  and grooves  374 . The grating structure  360  has a grating period T responsive to a band around a longer design filter wavelength (e.g., 60 nm) and selected harmonics. The grating period T is selected to cause diffracting, out of an optical path, an incident radiation within this band around of the design filter wavelength. Finally, the portion of the etch stop layer  330  that is exposed is removed. Alternatively, the etch stop layer  330  may be left on the first multi-layer coating if it is thin and transparent. The grating structure  360  includes a number of ridges spaced at the grating period T. Similar to the grating structure  220  in FIG. 2, the ridges have a ridge width W and height H. The ridge width W and height H may be any suitable values. In one embodiment, the ridge width W is approximately proportionally to the grating period T with a proportionality constant α. The ridge height H is approximately proportionally to the grating period T with a proportionality constant β. 
     Similar to the embodiment shown in FIG. 2, the reflective spectral filter  130  in this embodiment therefore includes a first multi-layer coating  320 , the grating structure  360 , and the mirror substrate  310 . The grating structure  360  may include a metal spacer  340  only, a second multi-layer coating  350  only, or a combination of the metal spacer  340  and the second multi-layer coating  350  as shown. The first and second multi-layer coatings  320  and  340  are designed to be reflective at around the actinic wavelength (e.g., 13.4 nm) in an optical path. With this construction, wavelengths that are both near the grating period T and that reflect from the multi-layer coatings  320  and  340  are diffracted away (diffracted rays  150  shown in FIG. 1) from the path of the actinic rays (actinic rays  160  shown in FIG.  1 ). 
     An electromagnetic (EM) simulation is performed to study the effects of the reflective multi-layer coating. The results of the simulation are shown in Table 1 and FIG.  4 . Table 1 shows components of the multi-layer coating as used in FIGS. 2 and 3. It is noted that the materials and thickness are merely for illustrative purposes. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Multi-layer coating components 
               
            
           
           
               
               
               
            
               
                   
                   
                 Thickness 
               
               
                 No. 
                 Layer 
                 (nm) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 Si 
                 3.48 
               
               
                 2 
                 Mo 
                 3.69 
               
               
                 3 
                 SiC 
                 3.40 
               
               
                 4 
                 Mo 
                 3.60 
               
               
                 5 
                 Si 
                 3.41 
               
               
                 6 
                 Mo 
                 3.58 
               
               
                 7 
                 Si 
                 3.41 
               
               
                 8 
                 Mo 
                 3.63 
               
               
                 9 
                 SiC 
                 3.46 
               
               
                 10 
                 Mo 
                 3.53 
               
               
                 11 
                 Si 
                 3.45 
               
               
                 12 
                 Mo 
                 3.51 
               
               
                 13 
                 Si 
                 3.47 
               
               
                 14 
                 Mo 
                 3.59 
               
               
                 15 
                 SiC 
                 3.49 
               
               
                 16 
                 Mo 
                 3.46 
               
               
                 17 
                 Si 
                 3.52 
               
               
                 18 
                 Mo 
                 3.44 
               
               
                 19 
                 Si 
                 3.54 
               
               
                 20 
                 Mo 
                 3.54 
               
               
                 21 
                 SiC 
                 3.52 
               
               
                 22 
                 Mo 
                 3.38 
               
               
                 23 
                 Si 
                 3.59 
               
               
                 24 
                 Mo 
                 3.35 
               
               
                 25 
                 Si 
                 3.62 
               
               
                 26 
                 Mo 
                 3.49 
               
               
                 27 
                 SiC 
                 3.56 
               
               
                 28 
                 Mo 
                 3.29 
               
               
                 29 
                 Si 
                 3.66 
               
               
                 30 
                 Mo 
                 3.26 
               
               
                 31 
                 Si 
                 3.69 
               
               
                 32 
                 Mo 
                 3.45 
               
               
                 33 
                 SiC 
                 3.60 
               
               
                 34 
                 Mo 
                 3.21 
               
               
                 35 
                 Si 
                 3.73 
               
               
                 36 
                 Mo 
                 3.17 
               
               
                 37 
                 Si 
                 3.77 
               
               
                 38 
                 Mo 
                 3.40 
               
               
                 39 
                 SiC 
                 3.64 
               
               
                 40 
                 Mo 
                 3.13 
               
               
                 41 
                 Si 
                 3.80 
               
               
                 42 
                 Mo 
                 3.10 
               
               
                 43 
                 Si 
                 3.83 
               
               
                 44 
                 Mo 
                 3.36 
               
               
                 45 
                 SiC 
                 3.67 
               
               
                 46 
                 Mo 
                 3.07 
               
               
                 47 
                 Si 
                 3.86 
               
               
                 48 
                 Mo 
                 3.04 
               
               
                 49 
                 Si 
                 3.89 
               
               
                 50 
                 Mo 
                 3.33 
               
               
                 51 
                 SiC 
                 3.70 
               
               
                 52 
                 Mo 
                 3.02 
               
               
                 53 
                 Si 
                 3.90 
               
               
                 54 
                 Mo 
                 2.99 
               
               
                 55 
                 Si 
                 3.93 
               
               
                 56 
                 Mo 
                 3.31 
               
               
                 57 
                 SiC 
                 3.72 
               
               
                 58 
                 Mo 
                 2.98 
               
               
                 59 
                 Si 
                 3.93 
               
               
                 60 
                 Mo 
                 2.95 
               
               
                 61 
                 Si 
                 3.96 
               
               
                 62 
                 Mo 
                 3.29 
               
               
                 63 
                 SiC 
                 3.74 
               
               
                 64 
                 Mo 
                 2.95 
               
               
                 65 
                 Si 
                 3.95 
               
               
                 66 
                 Mo 
                 2.93 
               
               
                 67 
                 Si 
                 3.98 
               
               
                 68 
                 Mo 
                 3.28 
               
               
                 69 
                 SiC 
                 3.75 
               
               
                 70 
                 Mo 
                 2.94 
               
               
                 71 
                 Si 
                 3.97 
               
               
                 72 
                 Mo 
                 2.91 
               
               
                 73 
                 Si 
                 3.99 
               
               
                 74 
                 Mo 
                 3.27 
               
               
                 75 
                 SiC 
                 3.75 
               
               
                 76 
                 Mo 
                 2.92 
               
               
                 77 
                 Si 
                 3.98 
               
               
                 78 
                 Mo 
                 2.90 
               
               
                 79 
                 Si 
                 4.00 
               
               
                 80 
                 Mo 
                 3.26 
               
               
                 81 
                 SiC 
                 3.76 
               
               
                 82 
                 Mo 
                 2.91 
               
               
                 83 
                 Si 
                 3.99 
               
               
                 84 
                 Mo 
                 2.89 
               
               
                 85 
                 Si 
                 4.01 
               
               
                 86 
                 Mo 
                 3.26 
               
               
                 87 
                 SiC 
                 3.76 
               
               
                 88 
                 Mo 
                 2.91 
               
               
                 89 
                 Si 
                 3.99 
               
               
                 90 
                 Mo 
                 2.89 
               
               
                 91 
                 Si 
                 4.01 
               
               
                 92 
                 Mo 
                 2.6 
               
               
                   
               
            
           
         
       
     
     FIG. 4 is a diagram illustrating reflectance and absorption spectra for the multi-layer coatings according to one embodiment of the invention. The diagram shows high reflectivity at the design wavelength of 13.4 nm (92.5 eV) and also unwanted wavelengths longer than 60 nm (less then 20.7 eV). 
     The diagram shows the reflectance and absorption spectra for a white (flat) source from a {Si (Mo SiC Mo Si Mo Si){circumflex over ( )}15 Mo Si} multi-layer with thickness optimized for a 5-degree incidence. The total number of layers is 92. The total Mo thickness is 147.97 nm. The total Si thickness is 117.31 nm. The total number of SiC thickness is 54.54 nm. 
     It is clear from the diagram that the reflectance is maximum at two places: a narrow band around 13.4 nm and a narrow band around 60 nm. The absorption spectrum is correspondingly minimum at these two bands. 
     FIG. 5 is a diagram illustrating a system  500  using a transmitting spectral filter in which one embodiment of the invention can be practiced. The system  500  includes an EUV radiation source  510 , a condenser mirror  520 , a fold mirror  530 , and a transmitting spectral filter  540 . The system  500  is typically used in EUV lithography. 
     The EUV radiation source  510  and the condenser mirror  520  are the same as the EUV radiation source  210  and the condenser mirror  220  shown in FIG.  2 . The fold mirror  530  is standard mirror that reflects the incident radiation from the condenser mirror  520 . The fold mirror  530  may have multi-layer coating that reflects a narrow band around a design wavelength (e.g., 13.4 nm) and wavelengths longer than a second wavelength (e.g., 60 nm). The reflected radiation forms an optical path  550 . 
     The transmitting spectral filter  540  is inserted into the optical path  550 . The transmitting spectral filter  540  has a grating structure or a metal mesh with a low aspect ration supported on an ultra-thin layer of transmitting material (e.g., Nitride, Oxide). The desired actinic rays (e.g., the 13.4 nm rays) are much shorter than the period of the mesh or the grating structure and pass through the transmitting spectral filter  540  with little blocking. The rays having longer wavelengths that are near the period of the mesh or the grating structure are not transmitted and undergo reflection and diffraction, forming the reflected and diffracted rays  560 . 
     FIG. 6 is a diagram illustrating a process  600  to fabricate the transmitting spectral filter  540  shown in FIG. 5 according to one embodiment of the invention. 
     The process  600  starts with polishing a wafer blank  610 . Then, an ultra-thin film layer  620  is deposited on the wafer blank  610 . The ultra-thin film layer  620  is made of a transmitting material such as Nitride or Oxide or any other material that has high transmittance. 
     Next, a metal layer  630  is deposited on the ultra-thin film layer  620 . Then, the metal layer  630  is lithographically patterned and etched to become a metal mesh or a grating structure  640 . The grating structure  640  may have a one-dimensional layout or a two dimensional layout. The grating structure  640  has a grating period T responsive to rays that have wavelengths longer than a design wavelength (e.g., 13.4 nm). Specifically, the grating structure  640  has a grating period selected to reflect or diffract incident radiation having wavelengths longer than the second wavelength (e.g., 60 nm). The grating structure  640  has a number of ridges spaced at the grating period T. The ridges have a ridge width W and height H. The ridge width W and height H may be of any suitable values to provide the desired characteristics. In one embodiment, the ridge width W is approximately proportionally to the grating period T with a proportionality constant of γ. A typical value of γ is much less than 0.5. 
     Then, the process  600  lithographically patterns and etches the back side of the wafer  610  to form a support structure  615 . The support structure  615  provides support or frame for the thin-film layer  620  and the grating structure  640 . The layout pattern  660  shows a representative two-dimensional layout having ridges  662 , grooves  664 , and support structure  615 . 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention.