Abstract:
A lithography system may utilize a biased sidewall chrome alternating aperture mask (SCAAM). Glass steps in the mask may be positioned at the center of the chrome sidewalls in chrome lines rather than the center of the chrome lines themselves.

Description:
BACKGROUND 
     A binary photomask may include glass and chrome features which form a pattern. Light may pass through the clear glass areas and be blocked by the opaque chrome areas. Light that passes through the mask may continue through a lens, which projects an image of the mask pattern onto a wafer. The wafer is coated with a photosensitive film (photoresist), which undergoes a chemical reaction when exposed to light. After exposure, the areas on the photoresist exposed to the light may be removed in a developing process, leaving the unexposed areas as features on the wafer. 
     The quality of an image produced with a binary mask may be degraded by light from clear areas on the mask diffracting into regions that ideally would be completely dark. A nominally dark region may have light diffracted into it from the adjacent nominally bright regions. Consequently, contrast between bright and dark regions on the imaging plane may be degraded, thereby degrading image quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an optical lithography system. 
         FIG. 2  is a block diagram showing the effect of an alternating phase shift mask (alt-PSM) on the amplitude and intensity of light passing through the mask. 
         FIG. 3A  is a cross sectional view through a line and space pattern on a sidewall chrome alternating aperture mask (SCAAM). 
         FIG. 3B  is a top view of the SCAAM shown in  FIG. 3A . 
         FIG. 4  is a cross sectional view through a line and space pattern on a biased SCAAM. 
         FIGS. 5A and 5B  are graphs showing intensity of light at the wafer between shifted and unshifted space regions produced by adjacent 0° and 180° apertures in the SCAAM and biased SCAAM. 
         FIGS. 6A and 6B  are graphs showing differences in size between shifted and unshifted space regions over a range of pitches in the SCAAM and biased SCAAM. 
         FIGS. 7A and 7B  are graphs showing the sensitivity of the difference in the shifted and unshifted spaces to misalignment of the chrome line with respect to the phase region etched in the glass in the SCAAM and biased SCAAM. 
         FIG. 8  is a flowchart describing a process flow for fabricating a biased SCAAM. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an optical lithography system  100 . Light from an illumination source  105  is transferred to a patterned mask  110 , or reticle, by an illumination system  115 . Light passes through the mask and into the entrance pupil of an imaging system  120 . The resulting pattern is imaged onto a photoresist covered wafer  125  by a lens  130  in the imaging system. 
     The mask may be a biased sidewall chrome alternating aperture mask (SCAAM). A SCAAM is a type of alternating phase shift mask (alt-PSM). The quality of an imaged mask pattern produced with a typical binary mask may be degraded by light from clear areas on the mask diffracting into regions that ideally would be completely dark. The nominally dark region may have light diffracted into it from the adjacent nominally bright regions.  FIG. 2  shows an exemplary alt-PSM  200 . In the alt-PSM, alternating clear regions  205  and  210  may have different step heights which cause the light to be phase-shifted 180°. As a consequence, the light diffracted into the nominally dark area  215  from the clear area  205  to the left will interfere destructively with the light diffracted from the right clear area  210 . This may improve image contrast on the wafer. 
     A SCAAM may be manufactured by taking a chromeless mask with an appropriate phase pattern, re-coating the mask with chrome and resist, and generating the desired exposure pattern in the chrome layer. The result is a PSM structure  300  where all glass walls  305  are covered by opaque chrome  310  and all chrome is supported by glass, as shown in  FIGS. 3A and 3B . With all glass sidewalls covered and all chrome supported, the SCAAM may minimize phase and amplitude anomalies which may be associated with other PSM structures. 
     In the SCAAM structure  300  shown in  FIGS. 3A and 3B , the glass step  305  may be positioned in the center  315  of the chrome line  310  of the mask. It has been found that, in this arrangement, the mask structure may result in sizeable focus-dependent changes in the difference between spaces  225 ,  220  on the wafer printed with 0° and 180° phase apertures at tight pitches (see  FIG. 2 ). 
     In an embodiment shown in  FIG. 4 , the position of a glass step  405  under a chrome line  410  may be adjusted so that the center  425  of a chrome sidewall  420  is positioned at the center  415  of the chrome line  410  (including chrome oxide anti-reflective layer  417 ). It has been found that, in this arrangement, the focus-dependent changes in the wafer image may be much reduced compared to the structure  300  of  FIGS. 3A and 3B , in which the glass step  305  is positioned in the center  315  of the chrome line  310 . Improvements in performance are demonstrated by the graphs shown in  FIGS. 5-8 . 
       FIGS. 5 and 6  show graphs describing the performance of simulated SCAAM and biased SCAAM masks. Both masks were modeled as 4×reduction masks designed for 193 nm exposure wavelength and imaged with a numerical aperture (NA) of 0.85 and a partial coherence factor (σ) of 0.4. The width of a chrome line on the masks was 280 nm (4×) and the pitch of the printed images is 160 nm on the wafer (1×). 
       FIGS. 5A and 5B  show the intensity of light at the wafer between shifted (S 180 ) and unshifted (S 0 ) space regions produced by adjacent 0° and 180° apertures in the SCAAM and biased SCAAM, respectively. The biased SCAAM produces a higher intensity in the space regions at best focus  500  and lower space size imbalance S 0 -S 180  at +85 nm  505  and −85 nm  510  defocus. 
       FIGS. 6A and 6B  show the difference in size between shifted and unshifted space regions over a range of pitches. The biased SCAAM shows lower space size imbalance S 0 -S 180  between best focus  600  and +85 nm  605  and −85 nm  610  defocus at 160 nm pitch. 
     The biased SCAAM structure  400  ( FIG. 4 ) may be less sensitive than the SCAAM structure  300  ( FIG. 3 ) to chrome-to-glass etch pattern overlay errors encountered during mask production.  FIGS. 7A and 7B  show the sensitivity of the difference in the shifted and unshifted spaces to misalignment of the chrome line with respect to the phase region etched in the glass on the SCAAM and the biased SCAAM, respectively. Both masks were designed for 193 nm exposure wavelength and imaged with a numerical aperture (NA) of 0.85 and a partial coherence factor (σ) of 0.3. The lines and spaces were printed at a pitch of 160 (1×) and at best focus. The exposure dose was chosen to produce approximately equal lines and spaces on the wafer at 160-nm pitch. The space difference S 0 -S 180  is given for several choices of the chrome line width on the mask. For most cases, the difference in spaces for the biased SCAAM varies less over different degrees of misalignment than for the SCAAM. 
       FIG. 8  shows a process flow  800  for making a biased SCAAM. The phase topography for a mask pattern may be etched in a glass substrate or layer (block  805 ). The design data for the biased SCAAM may be adjusted, or biased, from that of a typical SCAAM in that the size of the etched glass regions may be made larger in each linear direction such that the center of the chrome sidewalls are positioned at the center of corresponding chrome lines. The exact value of the suitable bias may depend on the actual process and materials used to fabricate the mask. The glass may then be coated with a layer of chrome (block  810 ). The chrome may be covered with a resist (block  815 ), which is exposed with the desired mask pattern (block  820 ). The exposed resist may be developed (block  825 ) and etched to expose transparent openings in the chrome layer to define the desired mask pattern (block  830 ). 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, blocks in the flowchart may be skipped or performed out of order and still produce desirable results. Accordingly, other embodiments are within the scope of the following claims.