Abstract:
An attenuated phase-shifting photomask (APSM) provides adjustment of attenuation from mask to mask. The APSM includes a multilayer substrate, a buffer thin film coupled to the substrate, and a top layer thin film on top of the buffer thin film. The thin films are etched with a circuit pattern to form a photomask, and are chosen to have certain thicknesses which would provide adjustment of attenuation within a specified attenuation operating range and appropriate phase shift without changing said buffer thin film and said top layer thin film.

Description:
BACKGROUND 
     This disclosure relates to photomasking. 
     Optical lithography is a technology used to print patterns that define integrated circuits onto semiconductor wafers. Typically, a pattern on an attenuation photomask is imaged by a highly accurate camera. The image is transferred onto a silicon wafer coated with photoresist. Continued improvements in optical lithography have enabled the printing of ever-finer features. This has allowed the integrated circuit industry to produce more powerful and cost-effective semiconductor devices. 
     A conventional binary mask that controls the amplitude of light incident upon a wafer is often inadequate when the integrated circuit (IC) feature size is small. 
     Phase-shifting masks can be used for optical lithography for the generation of IC feature sizes below one micron such as 0.25 micron. Under these subwavelength conditions, optical distortions as well as diffusion and loading effects of photosensitive resist and etch processes cause printed line edges to vary. Phase shifting improves the resolution that optical lithography can attain, producing smaller, higher-performance IC features by modulating the projected light at the mask level. 
     Successors to optical lithography are being developed to further improve the resolution. Extreme-ultraviolet (EUV) lithography is one of the leading successors to optical lithography. It may be viewed as a natural extension, since it uses short wavelength optical radiation to carry out projection imaging. However, EUV lithography (EUVL) technology is different from the technology of optical lithography in that the properties of materials with respect to EUV radiation are different from their properties with respect to visible and UV ranges. For example, the EUV radiation is strongly absorbed in virtually all materials, including gas. Thus, EUVL imaging systems often utilize entirely reflective optical elements rather than refractive elements, such as lenses. 
     SUMMARY 
     A photomasking technique provides adjustment of attenuation from mask to mask within a specified attenuation operating range and appropriate phase shift without changing the material of thin film layers disposed on a substrate. 
     The photomask includes a substrate, a first film coupled to the substrate, and an upper layer film on top of the first film. The first film has a first thickness of a particular material. The upper layer film has a second thickness, of another material. The films are etched with a circuit pattern to form a photomask, and are chosen to have certain thicknesses which would provide adjustment of attenuation within a specified attenuation operating range and appropriate phase shift without changing the material of the first film and the upper layer thin film. In one embodiment, the specified attenuation operating range is about 6% to 20% attenuation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will be described in reference to the accompanying drawings wherein: 
     FIGS. 1A through 1E show a fabrication process flow for a photomask; 
     FIGS. 2A through 2E show a fabrication process flow for an extreme-ultraviolet (EUV) attenuated phase-shifting mask (APSM) according to an embodiment of the invention; 
     FIGS. 3A through 3F show a process flow for the inspection and repair of the EUV APSM according to an embodiment of the invention; and 
     FIG. 4 shows a process flow diagram for fabricating an integrated-circuit wafer according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     A Conventional binary photomask can be made with a layer of chrome  100  deposited on a quartz or glass substrate  102  as shown in FIG.  1 A. The chrome is covered with a photosensitive resist  104  (FIG.  1 B). A beam writer then writes the circuit pattern onto the photomask by exposing the photosensitive resist with an electron beam or laser, which changes the molecular composition of the resist  104 . During the developing process, any positive resist that has been exposed  106  will be removed as shown in FIG.  1 C. In an alternative embodiment, the process can be performed with a negative resist, which removes the non-exposed area. Next, the mask is etched in a process that removes the chrome  100  from the area where the resist has been removed  106  (FIG.  1 D). Finally, FIG. 1E shows all the resist removed from the photomask  108 . 
     The photomask  108 , shown in FIG. 1E, is used in much the same way that a photographic negative is used to make a photograph. A beam of light projects the image patterned on the photomask  108 , several times, side by side, onto small areas called “dice” on a silicon wafer. Areas where the chrome has been removed pass the beam of light  110  through without any attenuation. The remaining areas attenuate the light  112  and create dark spots on the wafer. 
     An attenuated phase-shifting mask (APSM), which combines attenuation with a phase-shifting technique, is fabricated by replacing the chrome absorber layer  100  with a synthesized material. The material is synthesized at a given thickness to meet both phase shift and attenuation requirements. The patterning of such an APSM mask is similar to that of a conventional optical mask, but uses a different absorber material. This difference requires a different etch process. The achievable attenuation range is very narrow because of the limitation of available material. For example, it is difficult to achieve attenuation on the order of about 10 to 15%, which offers better performance for certain mask layers than the case of traditional 6% attenuation. Therefore, a change of attenuation for different mask layers means a change of absorber material, and hence, an imposition of new process development which involves new film selection, deposition, etch, and cleaning parameters for each layer of absorber material. 
     An APSM process for extreme-ultraviolet (EUV) lithography involves choosing a single material, for use at an EUV wavelength (13.4 nm), which matches both the phase-shift and the attenuation requirements. This task is difficult to achieve because a different range of attenuation is often required for different applications. The difficulty is compounded by the high absorption characteristic of condensed material at 13.4 nm, and very little difference in the refractive index between the absorber and vacuum, which determines the phase difference between the attenuated region and the reflective region. 
     FIGS. 2A through 2E show an improved fabrication process flow for an EUV APSM. The process starts with a multi-layer substrate  200  (FIG.  2 A). In some embodiments, the substrate includes silicon, ultra-low expansion material, etc. This is followed by the deposition of first  202  and second  204  thin film layers (FIG.  2 B). In some embodiments, the second thin film layer  204  comprises at least one layer of thin film disposed on top of the first thin film layer  202 . FIGS. 2C through 2E illustrate the rest of the process, which includes resist coating and patterning (FIG.  2 C), consecutive etching of the top and bottom thin-film layers (FIG.  2 D), and stripping of the resist (FIG.  2 E). The resultant APSM provides appropriate attenuation and 180-degree phase shift where the thin-film layers remain  206  and no attenuation and zero phase-shift where the layers have been stripped  208 . 
     In one embodiment, an inspection and repair process follows the top layer etching. The process flow for the inspection and repair of the EUV APSM is shown in FIGS. 3A through 3F. The process starts with a multi-layer substrate  200 , as before. The first thin-film layer  202 , which acts as a buffer layer, is then deposited as shown in FIG.  3 A. This is followed by the deposition of the second layer film  204  (FIG. 3B) and the resist coating and patterning (FIG.  3 C). 
     The repair process starts after the top layer  204  thin film etching and resist removal has been completed, as shown in FIG.  3 D. The top layer  204  has been etched using the bottom layer  202  as the etch stop buffer layer. Once the top layer etching is done, the thin film is inspected for defects. FIG. 3D shows two defects  300 ,  302  typically found during inspection. Filling the gap  304  with same material as the top layer film repairs the defect  300 . Chipping away the leftover blob repairs the defect  302  (FIG.  3 E). Finally the bottom layer is etched to produce an EUV APSM as shown in FIG.  3 F. 
     The selection of the type and thickness for each thin-film layer  202 ,  204  should satisfy certain conditions as follows: 
     
       
           I≡I   o |exp [−(2π/λ)(2α 1   d   1 +2α 2   d   2 )]| 2 ,  (1) 
       
     
     
       
         |(2π/λ)(2Δ n   1   d   1 +2Δ n   2   d   2 )|=π,  (2) 
       
     
     where 
     I o  is the incident light intensity, 
     I is the light intensity reflected from the absorber region (round-trip reflection), 
     λ is the wavelength of the light, 
     α 1  and α 2  are absorption coefficients of films  1  and  2 , respectively, 
     d 1  and d 2  are thicknesses of films  1  and  2 , respectively, 
     n 1  and n 2  are indices of refraction (real part) of films  1  and  2 , respectively for small incident angle, θ, which is an angle the incident light and the normal to the surface. For larger θ, n 1  and n 2  must take into consideration cos(θ) contributions. 
     The EUV mask attenuation for a given thickness of a given film is governed by the absorption coefficient α and the film thickness d. The attenuation of the beam in the absorber region after a round-trip reflection from the multi-layer substrate  200  is, in the first order approximation, given by equation (1). Both the film thickness and the real part of the refractive index difference between the absorber material and vacuum govern the phase shift of an absorber. The reflected light from thin film to thin film and thin film to vacuum interfaces (on the order of about 0.01% to 0.02%) are ignored in equation (1) because the real parts of the refractive indices are closely matched at EUV wavelength. 
     Since an APSM with a n phase shift satisfies equation (2), equations (1) and (2) can be solved for d 1  and d 2  as follows:                      d   1     =                  [         -     λ        (     1   -     n   1       )                ln        (     I     I   o       )       /     (     8      π     )         +       λα   1     4       ]     /                                [         (     1   -     n   1       )          α   2       -       (     1   -     n   2       )          α   1         ]     ,                   (   3   )                       d   2     =                  [         -     λ        (     1   -     n   2       )                ln        (     I     I   o       )       /     (     8      π     )         +       λα   2     4       ]     /                                [         (     1   -     n   2       )          α   1       -       (     1   -     n   1       )          α   2         ]     .                   (   4   )                                
     Therefore, any combination of two films with index of refraction that satisfies equations (3) and (4) can be used. The value of d 1  or d 2  must be a positive number. In practice, however, the film selection also needs to consider the process compatibility, etch selectivity of the two films, and the proper film thickness ratio. The film thickness ratio should be configured to satisfy the minimum repair buffer layer thickness. 
     Examples of the two thin film layer selection are shown in Table 1 below for an attenuation operating range of 6% to 15%. The desired attenuation from about 10% to 15% can be achieved by adjusting the thickness of the two film layers without changing the material of each film. The ability to adjust attenuation without a change in the thin film type enables the same deposition and etch processes in the manufacturing of the thin-film layers. Only the deposition and etching time need to be adjusted to accommodate the film thickness change. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Buffer Layer 
                   
                 Top Layer 
               
               
                 Attenuation 
                 Buffer 
                 Thickness (d 1 ) 
                 Top Layer 
                 Thickness 
               
               
                 (%) 
                 Layer Type 
                 [in Å] 
                 Type 
                 (d 2 ) [in Å] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 6 
                 SiO 2   
                 854 
                 Ti 
                 373 
               
               
                 7 
                 SiO 2   
                 564 
                 Ti 
                 506 
               
               
                 7 
                 SiO 2   
                 766 
                 TiN 
                 304 
               
               
                 8 
                 SiO 2   
                 312 
                 Ti 
                 621 
               
               
                 8 
                 SiO 2   
                 560 
                 TiN 
                 372 
               
               
                 9 
                 SiO 2   
                 378 
                 TIN 
                 433 
               
               
                 10 
                 SiO 2   
                 218 
                 TiN 
                 488 
               
               
                 10 
                 SiO 2   
                 1082 
                 Mo 
                 167 
               
               
                 11 
                 SiO 2   
                 1022 
                 Mo 
                 184 
               
               
                 12 
                 SiO 2   
                 968 
                 Mo 
                 200 
               
               
                 13 
                 SiO 2   
                 918 
                 Mo 
                 214 
               
               
                 14 
                 SiO 2   
                 872 
                 Mo 
                 227 
               
               
                 15 
                 SiO 2   
                 828 
                 Mo 
                 239 
               
               
                   
               
             
          
         
       
     
     Some of the advantages of the EUV APSM and the fabrication process are the ability to adjust the desired attenuation without changing the manufacturing process of the thin film layers in the EUV APSM and the consistency of the two-layer absorber process with the current state-of-art EUV mask process flow. Further, the process is consistent with the current state-of-art EUV mask process flow, as well as repair strategy, that comprises a buffer layer and a metal layer with a buffer layer being used to absorb the repair damage. 
     FIG. 4 shows a process flow diagram for fabricating an integrated-circuit wafer. The process begins by providing a substrate, at step  400 . First and second thin film layers are then deposited on top of the substrate (step  402 ). The first and second thin film layers have first and second thicknesses determined by equations (3) and (4), respectively. At step  404 , a photosensitive resist is deposited on top of the conducting material. Next, the resist is exposed and developed with a pattern on an EUV APSM, at step  406 . The APSM used in this step is fabricated using the above-described method. Finally, the conducting material is etched (step  408 ) and the resist is stripped away (step  410 ). If additional layer is to be patterned, the process is repeated at step  402 . 
     An apparatus that execute above-specified process is also disclosed. The apparatus comprises a machine-readable storage medium having executable instructions that enable the machine to perform the steps in the specified process. 
     Other embodiments are within the scope of the following claims. For example, the fabrication process for the EUV APSM described above can also be used for an optical APSM. In this case, equations (3) and (4) need to be modified to include interface reflections due to non-matched real part of the reflective index at optical wavelengths.