Abstract:
A photoresist layer is exposed two or more times. At least one exposure is conducted through a regular mask, and at least one exposure through a modified mask with a clear region overlapping the position of a non-clear region of the first mask. The radiation dose used with the modified mask is insufficient by itself to create a resist pattern on the substrate. The exposure through the modified mask alleviates the resist underexposure in concave corners of the opaque pattern of the regular mask. Instead of the modified mask, an exposure without a mask can be performed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to photolithography. 
     Photolithography is widely used to form patterns on semiconductor wafers in fabrication of integrated circuits. A wafer  110  (FIG. 1) is coated with a photoresist layer  120 . Photoresist  120  is irradiated from a light source  130 . A mask or reticle  140  is placed between source  130  and resist  120 . Mask  140  carries a pattern consisting of opaque and clear features. This pattern defines which areas of resist  120  are exposed to the light from source  130 . After the exposure, the resist  120  is developed so that some of the resist is removed to uncover the underlying substrate  110 . If the resist is “positive”, then the resist is removed where it was exposed to the light. If the resist is “negative”, the resist is removed where it was not exposed. In either case, the remaining resist and the exposed (uncovered) areas of substrate  110  reproduce the pattern on mask  140 . The wafer is then processed as desired (e.g. the exposed areas of substrate  110  can be etched, implanted with dopant, etc.). 
     The resist pattern on wafer  110  is not always a faithful reproduction of the mask. In FIG. 2, an opaque feature  210 M on mask  140  has a concave corner  220 M. Feature  210 M should ideally be printed (reproduced) in resist  120  as feature  210 R, with a corner  220 R. In fact, the resist region  230  in the corner&#39;s cavity gets underexposed. As a result, the corner is smoothened in the resist pattern, as shown by line  240 . See U.S. Pat. No. 6,280,887 issued Aug. 28, 2001 to Lu. 
     The resist pattern can be corrected with a serif  310  (FIG.  3 ). The serif is a region cut out in opaque feature  210 M to increase the exposure of region  230 . However, if the feature  210 M is narrow, i.e. the dimensions D 1 , D 2  are small, the serif can be difficult to form on the mask. 
     SUMMARY 
     The invention is defined by the appended claims which are incorporated into this section in their entirety. The rest of this section summarizes some features of the invention. 
     Some embodiments of the present invention provide alternative techniques to reduce underexposure of the resist. In some embodiments, the resist is exposed twice. One exposure is through a mask like in the prior art, for example, as in FIG. 2 or  3 . The other exposure is conducted through a different (“modified”) mask which exposes resist regions which correspond to opaque regions of the first mask. This “modified” exposure is not conducted with a sufficient light energy dose to create a resist pattern on the wafer. For example, in the case of the positive resist, the modified exposure dose is insufficient to cause the resist to be removed during the developing process. However, the modified exposure increases the total energy dose delivered to regions such as  230 . A more faithful pattern reproduction results in some cases. 
     The modified and non-modified exposures can be performed in any order. 
     In some embodiments, the modified exposure does not use a mask. The entire resist surface is exposed. 
     Other features and embodiments are described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view illustrating a prior art photoresist exposure system suitable for some embodiments of the present invention. 
     FIG. 2 is plan view of a prior art mask and a corresponding pattern formed in photoresist. 
     FIG. 3 is a plan view of a prior art mask. 
     FIG. 4 is a plan view of a mask used in some embodiments of the present invention. 
     FIG. 5 is a plan view of a resist pattern corresponding to the mask of FIG.  4 . 
     FIGS. 6-10 are plan views of masks used in some embodiments of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 4 illustrates an exemplary mask or reticle  140  which will now be used to illustrate some aspects of the invention. (The terms “mask” and “reticle” are used interchangeably herein.) The mask has a number of clear regions  410 M surrounded by an opaque region  420 M. In one embodiment, the mask was used with a positive resist to define isolation trenches formed in a monocrystalline silicon substrate for a memory array. Exemplary dimensions are as follows. Each clear region  410 M is a rectangle of a height H=1.38 μm and a width W=0.18 μm. The vertical gaps  430 M between the adjacent rectangles  410 M have each a height of V=0.22 μm. Each horizontal gap A is 0.3 μm. Light source  130  (FIG. 1) is a deep ultraviolet light source (DUV) having a wave length of 248 nm. (The dimensions and other details are given for illustration and are not limiting. Also, in one embodiment, the dimensions were 4 times larger than given above because the mask was used with a projection lens reducing the image on the wafer by a factor of 4.) 
     Of note, each rectangle  410 M has two convex ends  440 . Each end  440  defines a cavity in opaque region  420 M. Each angle  450  of rectangle  410 M also defines a cavity in region  420 M. 
     The resist pattern formed on the wafer is shown in FIG.  5 . Regions  410 R,  420 R,  430 R correspond to respective mask regions  410 M,  420 M,  430 M. The rectangles  410 R are rounded, their height H is reduced, and the width W is increased. In one embodiment using a prior art resist exposure, the vertical gap V was 0.456 μm in the resist pattern, i.e. more than twice as large as the 0.22 μm gap on mask  140 . 
     A more faithful image can be obtained with an additional exposure through a modified mask  602  (FIG.  6 ). In mask  602 , each clear region  610  extends vertically through an area corresponding to an entire column of rectangles  410 M (FIG.  4 ). The positions of the short sides of rectangles  410 M are shown by dashed lines. Each region  610  covers the areas corresponding to the rectangles  410 M and the vertical gaps  430 M in one column. The clear regions  610  do not have cavities at the position of rectangle ends  440  (FIG.  4 ). 
     In one embodiment, the resist pattern is formed as follows: 
     
       
         
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 1. Coat the wafer 110 with a 0.078 μm layer of antireflective coating (not 
               
               
                 shown) of type AR2 available from Shipley Company, L.L.C., of 
               
               
                 Marlborough, Massachusetts. Bake and cool down the wafer. 
               
               
                 (This is a standard step.) 
               
               
                 2. Deposit a 0.0609 μm layer of a positive photoresist of type UV6 
               
               
                 (Trademark) available from Shipley. 
               
               
                 3. Expose the wafer through mask 602 of FIG. 6 to ultraviolet light with 
               
               
                 one half of the dose normally used in prior art. In some embodiments, 
               
               
                 the exposure is performed with a scanner of type ASML 500 available 
               
               
                 from ASML. The “best dose” recommended by ASML is 27.5 mJ/cm 2 , 
               
               
                 so the exposure at this step occurs with a dose of 13.75 mJ/cm 2 . 
               
               
                 This dose is insufficient for the subsequent development step (step 7 
               
               
                 below) to remove the resist in the exposed regions and uncover the wafer. 
               
               
                 4. Bake and cool down the wafer. 
               
               
                 5. Expose the wafer through the mask 140 of FIG. 4 to ultraviolet light 
               
               
                 with a dose of 13.75 mJ/cm 2 . 
               
               
                 6. Bake and cool down the wafer. 
               
               
                 7. Develop the resist. 
               
               
                   
               
             
          
         
       
     
     In one embodiment, the resist exposure is conducted with a scanner of type ASML 500 available from ASML of Tempe, Ariz., which uses a 248 nm light source. The resist exposure is performed with the zero and first order illumination and a −0.1 focus. The numerical aperture (NA) of the projection lens is 0.6. It is believed that this embodiment is capable of reducing the height V of the vertical gaps  430 R in the resist pattern to about 0.317 μm. 
     If the mask  140  features are narrow, creating the modified mask  602  of FIG. 6 can be easier than forming a serif. In particular, if the width W of rectangles  410 M is the minimum feature size, the serif would require sub-lithographic dimensions (i.e., smaller than the minimum feature size) but mask  602  can be formed without sub-lithographic features. However, some embodiments of the present invention combine the mask layout of FIG. 4 with serifs. Also, the invention is not limited to the embodiments in which no sub-lithographic features are needed in the modified mask  602 . 
     Removal of gaps  430 M in mask  602  eliminates diffraction and light destructive interference at the short sides of rectangles  410 M, so better resolution can be obtained. 
     In some embodiments, the exposure at step 3 of Table 1 is conducted without a mask. In some embodiments, more than two exposures are conducted. For example, the process of Table 1 can be augmented with an additional step of exposing the resist without a mask. 
     Steps 3 and 5 in Table 1 can be interchanged. 
     In another embodiment, the width W of rectangles  410 M in mask  140  is 0.14 μm. The exposure through mask  602  is conducted using off-axis illumination. (Off-axis illumination is described in “Handbook of Microlithography, Micromachining, and Microfabrication” edited by P. Rai-Choudhury, vol. 1 (1997), pages 71-73, incorporated herein by reference.) Also, an annular pupil is used to block some of the zero order light. In some embodiments, the rectangles&#39; height H in mask  140  is 0.956 μm, the gap height V=0.156 μm, and the horizontal gap width A=0.210 μm. The width of each feature  610  can be slightly larger than 0.14 μm to accommodate a possible misalignment between the masks  602  and  140 . A 0.16 μm width dimension can be used. 
     FIGS. 7-10 show alternative designs for mask  602 . In FIG. 7, the mask is identical to that of FIG. 6 except that each clear region  610  has a horizontal extension  730  at each gap  430 M. Each extension  730  extends horizontally and vertically beyond the corresponding rectangles  410 M (i.e. beyond the positions of rectangles  410 M on mask  140 ) in both directions (left and right). Extensions  730  increase the light exposure at corners  450  and alleviate the need for serifs  310  (FIG. 3) in mask  140  at these corners. Serifs  310  are used in some embodiments however. 
     FIG. 8 shows an identical mask except that the extensions  730  are merged into horizontal strips each of which traverses the entire array of rectangles  410 M. The strips  730  are greater in height than gaps  430 M, so the strips  730  overlap the positions of rectangles  410 M. In other embodiments, the strips  730  do not overlap the rectangles. 
     The mask of FIG. 8 may be easier and faster to write than the mask of FIG.  6 . Also, the mask of FIG. 8 advantageously provides higher exposure in gaps  430  (i.e. the areas corresponding to gaps  430 M). In addition, compared to FIG. 7, diffraction and destructive light interference at the vertical edges of extensions  730  are eliminated. 
     In FIG. 9, the clear region consists of isolated regions  930 . Each region  930  covers a gap  430 M (FIG. 4) and may extend beyond the gap vertically and/or horizontally, but regions  930  do not cover the rectangles  410 M. During the exposure through mask  140 , the radiation dose is 100% of the best dose. 
     This design is advantageous because there may be a slight misalignment (about 20 nm for example) between the masks  602  and  140 . This misalignment may reduce the contrast along the long edges of rectangles  410 M if these edges are exposed twice (as with the masks of FIGS.  6 - 8 ). 
     With the mask of FIG. 9, the contrast loss is less of a problem, so a higher dose (e.g. more than 50% of the best dose) can be used with mask  602 . 
     In FIG. 10, regions  930  are merged into strips each of which traverses the array of rectangles  410 M and covers one row of gaps  430 M. 100% of the best dose is used at step  5  (with mask  140 ), and more than 50% can be used at step 3. This mask can be faster to write than the mask of FIG.  9 . Also, diffraction and destructive light interference are eliminated at the vertical edges of regions  930  compared to FIG.  8 . 
     The above techniques can be combined with other photolithographic techniques. For example, some or all of the opaque regions can be replaced with partially transmitting non-clear regions. See the aforementioned “Handbook of Microlithography, Micromachining, and Microfabrication”, volume 1, page 79, incorporated herein by reference. The masks involved can be binary masks, phase shift masks, or other types of masks, known or to be invented. The invention is not limited to any particular mask patterns or dimensions, resist types, exposure doses, wavelengths, or other parameters, or any particular materials or equipment. The invention is not limited to baking and cooling cycles or use of antireflective coatings. The invention is defined by the appended claims.