Abstract:
A phase shift mask may include boundaries between phase shift regions with continuous sloped phase edges. The continuous sloped phase edges may be produced by introducing a predetermined degree of defocus into a beam used during production of the mask to image the pattern on the mask. Such a phase shift mask may be “trimless”, i.e., not require a corresponding binary “trim” mask for a second exposure to remove phase conflicts after exposure with the phase shift mask.

Description:
BACKGROUND 
     A phase shift mask (PSM) may include alternating clear regions (which may be designated as zero (0) and pi (π) regions) with different step heights. The different step heights cause the light to be phase-shifted 180° between the two regions. As a consequence, the light from the clear zero region interferes destructively with the light diffracted from the adjacent pi region, causing an unexposed (dark) area on the photoresist layer. 
     “Phase conflict” is a potential problem in PSM technology. Phase conflict arises from unintentionally joining two regions that transmit light with opposite phases. Destructive interference of the light from these two regions may create artificial features on the wafer (e.g., unwanted lines). 
     Phase conflict may be avoided by using a double exposure/two mask method. In a first exposure, a PSM is used to image a pattern, which may include undesired features caused by phase conflict. In a second exposure, a binary (e.g., chrome-on-glass) “trim” mask is used to clear the undesired features. However, the addition of the second (trim mask) exposure may introduce overlay errors, reduce flexibility of physical design, and increase cycle time, labor, and production costs. These problems may worsen as the process margins of photolithography decrease and the pattern densities of integrated circuits increase. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a photolithography system. 
         FIG. 2  is a sectional view of a phase shift mask (PSM). 
         FIG. 3A  is a sectional view of a PSM having an abrupt boundary between two phase shift regions. 
         FIG. 3B  is a sectional view of a PSM having a sloped boundary between two phase shift regions. 
         FIGS. 4A-4D  show a process flow for producing a PSM including a sloped phase edge architecture. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an optical lithography system  100 . Light from an illumination source  105  is transferred to a patterned mask  110  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  110  may be a phase shift mask (PSM). The quality of an imaged 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. A PSM may be used to reduce such diffraction and improve image contrast on the wafer. 
       FIG. 2  shows an exemplary PSM  200 . In the PSM, alternating clear regions (which may be designated as zero (0) regions  210  and pi (π) regions  205 ) may have different step heights which cause the light to be phase-shifted 180° (90°/270° phase steps may also be used). As a consequence, the light from the clear zero region interferes destructively with the light diffracted from the adjacent pi region, causing an unexposed (dark) area on the photoresist layer. This may improve image contrast on the wafer. 
     PSMs may be fabricated using photolithography techniques. A glass substrate is coated with a layer of photoresist material. The photoresist may be exposed with a pattern on a mask to define phase shifted regions in the glass substrate. Depending on the tone of the photoresist (i.e., positive or negative), the exposed areas of the photoresist will be made either be more soluble (positive) or less soluble (negative). The more soluble areas of the photoresist are then removed in a development process, leaving portions of the substrate exposed. The exposed surfaces (and the remaining resist) may then be etched to form features with step heights corresponding to a phase shift at the wavelength used for the lithography tool in which the mask will be used (e.g., 193 nm). 
     “Phase conflict” is a potential problem in PSM technology. Phase conflict arises from unintentionally joining two regions that transmit light with opposite phases. Destructive interference of the light from these two regions may create artificial features on wafer (e.g., unwanted lines). 
     Phase conflict may be avoided by using a double exposure/two mask technique. In a first exposure, a PSM is used to image a pattern, which may include undesired features caused by phase conflict. In a second exposure, a binary (e.g., chrome-on-glass) “trim” mask is used to clear the undesired features. However, the addition of the second (trim mask) exposure may introduce overlay errors, reduce flexibility of physical design, and increase cycle time, labor, and production costs. 
     Phase conflict may be caused by the abruptness in the boundary (step)  302  between two adjacent phase shift regions  304  (0),  306  (Π), as shown in  FIG. 3A . In an embodiment, phase conflict may be avoided by lengthening a lateral distance  308  over which the phase transition occurs to form a continuous sloped phase edge between adjacent phase shift regions, as shown in  FIG. 3B . The gradual transition may prevent unwanted features from printing at the boundaries between phase shift regions. The lateral distance of the boundary may be on the order of the wavelength of light used in the lithography tool in which the mask will be used (e.g., order of 10 2  nm). 
     A continuous sloped phase edge between phase shift regions may be produced during the fabrication of the PSM using a “controlled blur” imaging technique. The imaging system used for mask lithography (e.g., optical or electron beam) may include a defocus control to control the amount of defocus at the imaging plane. The imaging system may be operated in a deliberate “out-of-focus” condition, creating a sloped dose gradient in the exposed resist at the boundary between exposed and unexposed regions. 
       FIGS. 4A-4D  show a process flow for producing a PSM including a sloped phase edge architecture. An exposure dose  400  maybe applied to a resistance material  402  on a glass substrate  404 , as shown in  FIG. 4A . The “slope” of this dose gradient can be adjusted by the degree of defocus applied to the electron beam. The “slope” of the dose gradient may be selected based on the value required to manufacture the slope in the etched glass. The dose gradient creates a slope  406  in the resist  402  under suitable resist development conditions, as shown in  FIGS. 4B and 4C . When the resist  402  is etched, the slope  406  is imposed on the final glass profile  408 , as shown in  FIG. 4D . In the embodiment shown in  FIGS. 4A-4D , the glass/resist etch rate is approximately 1:1 such that the glass and resist edge at approximately the same rate. However, other glass/resist etch rates may produce satisfactory results. 
     In an embodiment, the degree of defocus may be adjusted for individual shot exposures allowing the dose gradient to vary over a large range. For example, the degree of defocus may be adjusted for different figures in the mask. In addition, the defocus may be asymmetric, enabling the dose gradient to be, e.g., steep for vertical edges while gradual for horizontal edges. 
     The use of sloped phase-edge architecture in the PSM may eliminate the need for a second exposure with a trim mask. Accordingly, alignment problems associated with the second exposure may be avoided. Also, the single exposure process may be more compatible with high speed manufacturing. 
     Another technique for generating a gradual phase transition is to provide a stepped boundary (e.g., 0°/90°/270° or 0°/60°/120°/180°). However, such stepped boundaries may result in printing out-of-focus and may require a more complex manufacturing process. The embodiments described above may produce a relatively smooth and gradual phase transition which may avoid these problems. 
     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. Accordingly, other embodiments are within the scope of the following claims.