Abstract:
A method for manufacturing a stitched space in a semiconductor circuit implements a photolithographic process for printing one or more image fields on a wafer surface, each image field corresponding to a portion of a circuit or device and including a space that is to be stitched in adjacent image fields. The space to be stitched that is produced from an image field is overlapped onto the space to be stitched produced from the adjacent image field, however, the overlapped space from the adjacent image fields is intentionally misaligned. The stitched space is then subject to the double light exposure dose to print the stitched space, with the result that an overlay tolerance of the stitched space is improved.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor photolithographic processes and techniques for forming integrated circuit chips, and particularly, to a novel, non-intuitive lithographic-based technique for stitching two spaces together spaces that have a tighter tolerance than a standard space stitching technique. 
     DESCRIPTION OF THE PRIOR ART 
     In semiconductor lithographic processes, circuits, which may comprise electronic devices including active and passive elements interconnected by one or more conductors (lines), are typically formed by successively imprinting patterns onto a semiconductor wafer utilizing one or more photolithographic masks, and performing series of etches and depositions of materials that form on and/or react with the wafer. Typically, circuits or devices are defined on a photoresist layer that is subject to one or image fields by exposing the photoresist to illumination through a reticle. The reticle or image fields may be formed on a plurality of lithographic masks and image fields may be “stitched” together during a stepper process to produce a complete image field when the device or circuit boundary exceeds an image field size of the stepper apparatus. In conventional techniques, the stepper apparatus aligns itself to registration marks, e.g. that may have been previously placed on the wafer to accurately indicated the orientation of the image fields produced with the stepper. 
     Subsequent to the placement of the registration marks by photolithography, the wafer is subject to placement or orientation by the stepper apparatus to enable the photolithographic process to utilize the registration marks to orient the wafer relative to the print image. For each layer to be printed, any adjacent images must be aligned with extremely high precision such that there is optical continuity along a line or a space or a patterned device region. The method utilized is called image field stitching. 
     In these semiconductor lithographic processes, it is well known that isolated lines print with tighter tolerances than isolated spaces. This is illustrated in the description and analysis of  FIGS. 1A ,  1 B and  2 A,  2 B to be described.  FIGS. 1A ,  1 B particularly show how an example metal line feature pattern (reticle design)  10  ( FIG. 1A ), at each of successively smaller feature sizes, is stitched and printed at those feature sizes ( FIG. 1B ). In  FIG. 1A , each metal line feature pattern  10  comprises lines  11  and spaces  12  therebetween. A line from the pattern  10  is to be stitched when at a butted (no or zero overlap) position at a stitch boundary indicated at the arrow  15  depicted in the figure.  FIG. 1B  shows optical images of the resulting printed stitched design pattern  10  including the resulting stitched metal line  19  on the wafer surface  25  at each of the successively smaller feature sizes  25   a ,  25   b ,  25   c  and  25   d . As shown in  FIG. 1B , there is noted the smooth seamless appearance at the stitched boundary of stitched line  19  at each of various feature sizes, i.e., the metal lines stitch with no error, or at least error that is imperceptible at the optical resolutions of the imaging device for the feature sizes  25   a ,  25   b ,  25   c  and  25   d  shown in  FIG. 1B . However, for spaces, as shown in  FIGS. 2A ,  2 B, this is not the case. That is, given the metal space feature pattern  10 ′ ( FIG. 2A ), at each of successively smaller feature sizes in this illustrative example, a space  12  from the pattern is to be stitched (with no or zero overlap) at a stitch boundary indicated at the arrow  30  depicted in the figure.  FIG. 2B  shows the resulting optical image of the stitched metal space  29  on the wafer surface  25 ′. It is clear from  FIG. 2B  the appearance of errors at the stitched boundary as they print at each of various feature sizes  35   a ,  35   b ,  35   c  and  35   d  and particularly the smaller feature sizes  35   c ,  35   d  as shown in  FIG. 2B . That is, the spaces between the metal lines stitch with error, e.g., jogs and other type errors that are perceptible at the optical resolutions of the imaging device, for feature sizes at 0.32 um (when two 160 nm spaces are butted) and smaller because of space resolution issues. 
     This is driven by interference from the light from each edge of the isolated line that is not possible with an isolated space that is essentially single slit diffraction. Isolated spaces are desirable as they define the emitter in a bipolar device (e.g., SiGe or otherwise) for example, and while the emitter might not be a minimum space, it needs to be controlled very tightly from an image size and overlay standpoint to reduce the parasitics associated with the extrinsic base. 
     Prior art references U.S. Pat. No. 6,225,013 and WO/2003/042759 are representative of stepped photolithographic processes using a plurality of mask region image fields that are to be “stitched” to define a device, e.g., by aligning the stitched image fields in manner such that they overlap during the stepper process. However, each of these patents address stitching of line features, and not spaces, and are directed to profiling of the image areas so as to minimize optical losses. 
     It would thus be highly desirable to provide a semiconductor lithographic “stitching” process that exhibits tighter tolerance for stitched spaces as opposed to a standard stitching process for spaces. 
     It would thus be highly desirable to provide a semiconductor lithographic stitching process that exhibits tighter tolerance for stitched spaces that are parallel to the stitched boundary and result in a seamless stitch. 
     It would therefore be desirable to provide a semiconductor lithographic stitching process that exhibits tighter tolerance for stitched spaces at 0.18 μm technology and smaller feature sizes. 
     SUMMARY OF THE INVENTION 
     The invention relates generally to an improved, non-intuitive lithographic-based technique for stitching two spaces together spaces that have a tighter tolerance than a standard space stitching technique. 
     Thus, according to one aspect of the invention, there is provided a method for generating a stitched spaces defined by adjacent image patterns in a photolithographic system. 
     According to one aspect of the invention, there is provided a method for stitching segments designed by adjacent image patterns of a photolithographic system during the manufacture of a semiconductor circuit or device. 
     The method exploits a novel, non-intuitive recognition that a stitched space has a tighter tolerance than a standard space when two spaces are stitched together with a double exposure. Even with the added tolerance associated with stitching, the final space width tolerance is shown to be improved to the level of the unstitched line. Stitching can be done with one mask exposed twice with a small off-set in x- and y-directions. This gives a tighter tolerance in the space than standard lithography according to conventional solutions. 
     Thus, there is provided a method for manufacturing a stitched space in a semiconductor circuit having improved overlay tolerance, where the stitching is carried out with a double light exposure dose of a single mask with intentional offset of the mask from the space to be stitched, wherein a tighter tolerance control for a stitched space is achieved as compared to an unstitched space. 
     The method for manufacturing a stitched space in a semiconductor circuit implements a photolithographic process for printing one or more image fields on a wafer surface, each image field corresponding to a portion of a circuit or device and including a space that is to be stitched in adjacent image fields. According to the invention, the space to be stitched that is produced from an image field is overlapped onto the space to be stitched produced from the adjacent image field, however, the overlapped space from the adjacent image fields is misaligned according to the intentional offset. The stitched space is then subject to the double light exposure dose to print the stitched space, with the result that an overlay tolerance of the stitched space is improved. 
     Advantageously, the methods employed by the present invention can be utilized for printing space width features of at least 180 nm technology with a greater degree of control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features and advantages of the present invention will become apparent to one skilled in the art, in view of the following detailed description taken in combination with the attached drawings, in which: 
         FIGS. 1A ,  1 B particularly show how an example metal line feature pattern (reticle design)  10  ( FIG. 1A ), at each of successively smaller feature sizes, is stitched at a metal line stitch boundary and printed on a wafer at those feature sizes ( FIG. 1B ) according to conventional methods; 
         FIGS. 2A ,  2 B particularly show how an example metal space feature pattern (reticle design)  10  ( FIG. 2A ), at each of successively smaller feature sizes, is stitched at a space stitch boundary and printed on a wafer at those feature sizes ( FIG. 2B ) according to conventional methods; 
         FIG. 3A  depicts an example test pattern image layout on the wafer as found during a precursor step of determining overlay tolerance for the photolithographic tooling used in accordance with the present invention; 
         FIG. 3B  depicts a graph  90  of the overlay error induced by the intentional stepping errors provided as a result of the example stepper process performed as described with respect to  FIG. 3A ; 
         FIG. 4  depicts the well-behaved (linear) response  100  when line images form as overlay is varied; 
         FIG. 5  particularly depicts the quadratic response  200  of how space feature images form as overlay is varied; and, 
         FIG. 6  depicts example summary data  300  of the example linewidth and space width measurement results for both stitched and unstitched lines. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In a methodology for printing semiconductor device and circuit images comprising lines (i.e., conductors) and spaces in between the lines, the present invention exploits a novel and non-intuitive recognition that a stitched space has a tighter tolerance than a standard space by stitching two spaces together with a double exposure. Even with the added overlay tolerance associated with stitching, the final space width tolerance is improved to the level of the unstitched line. Stitching can be performed with one mask exposed twice with a small off-set in x and y axes. This gives a tighter tolerance in the space than standard lithography, which would be the conventional solution. 
       FIG. 3A  illustrates a resulting printed layout  50  of example test pattern images  75  that have been “stitched” together on the surface of a test wafer  99 . To generate the layout  50  of  FIG. 3 , a test photolithographic mask (not shown) has been imaged on the wafer using a photolithographic stepper process to produce image fields  75 . In the process, the mask is stepped with intentional errors to generate respective image fields that are stitched together at a stitch boundary with various x-direction offsets  78  and y-direction offsets  79 . In this manner, the mask overlay tolerance is determinable. That is, by intentionally generating overlay errors, e.g., by a stepping the mask with intentional errors in either X-step direction, Y-step direction, or both, as indicated in the  FIG. 3A , overlay error may be determined.  FIG. 3B  depicts a graph  90  of the overlay error induced by the intentional stepping errors provided as a result of the example stepper process performed as described with respect to  FIG. 3A . The overlay error is plotted as a graph  90  of the measured overlay error in nanometers (y-axis) versus the intended overlay errors (corresponding to the programmed intentional stepping errors) of the photolithographic stepping process. This graph  90  allows for the evaluation of stitching optimization—centering the frame overlap—and process window evaluation for any level. 
     Then, accounting for any calculated overlay errors from the photolithographic stepper apparatus, a photolithographic stitching process was performed whereby a test mask having a metal line pattern resulting in a formed field image pattern on the wafer, is stepped adjacent to the image to form a second field image pattern to be stitched to the first pattern. In this model, the linewidth of the stitched line is measured over a range of frame overlap and underlap conditions. 
     That is, for example, two line feature images at 0.18 μm widths (180 nm nominally butted lines) that are to be stitched are first separated with a gap (underlapped) and then brought together (overlapped) during a stepping process. The resulting linewidth measurements (in nanometers) on the wafer is plotted as a function of the stitched overlap condition (in nanometers) as shown in  FIG. 4 .  FIG. 4  particularly depicts the well-behaved (linear) response  100  of how line images form as overlay is varied. That is, the frame holding the test pattern mask is stepped so that the 180 nm wide lines to be stitched are printed on the wafer and resulting linewidth measured at first underlapped conditions  101 , then at an abutting condition at  102  with no gap separating the lines, and finally at overlapped conditions  103  over a large range. It is understood that there effectively is an extra UV light dose applied to the exposed line feature image when overlapped. 
     Spaces in the pattern image provide a different result. Two spaces that are printed when in a separated (underlapped) condition each get a normal UV light exposure dose. However, when the printed line space features are overlapped in the manner as described with respect to  FIG. 4 , they effectively get a double dose. However, the measured linewidths show a quadratic (i.e., non-linear) response  200  as shown in  FIG. 5 .  FIG. 5  particularly depicts the quadratic response  200  of how space feature images form as overlay is varied. That is, the frame holding the test pattern mask is stepped so that the 180 nm wide space feature to be stitched are printed on the wafer and resulting space width is measured at underlapped conditions  201 , then at an abutting condition at  202  with no gap separating the spaces, and finally at overlapped conditions  203  over a large range. It is understood that there effectively is an extra UV light dose applied to the exposed space feature image when overlapped resulting in a flattening out of the stitched space width response at the overlapped condition. Thus, in the example provided in  FIG. 5 , due to the quadratic response curve for overlapping spaces, in the example described with respect to  FIG. 4 , two 180 nm wide space features resolved to approximately 0.27 μm wide stitched space when subject to a 120 μm overlap as indicated at  210  in  FIG. 5 . 
     In a final experimental test run, a whole wafer was run to stitch two lines and space features of 180 nm width subject to a 144 nm fixed overlap condition, in the example test run. The resulting printed line widths and space widths were measured under stitched and unstitched condition. A summary  300  of the respective linewidth and space width measurement results are found in  FIG. 6 . As shown in  FIG. 6 , the line measurement was made under a no overlap (i.e., abutting) condition at  301 ; and then the line measurement was made of the stitched line under the fixed exemplary 144 nm overlap condition at  305 . As shown in  FIG. 6 , it is clear that the standard deviation (3 Sigma value) increases from 9.0 to 14.2 indicating that the stitched (printed) linewidth response worsens due largely to the overlay error. This result thus indicates that there is reduced linewidth control when stitching lines. However, with the same experiment set up for spaces, i.e., the two space width measurements, e.g., a 180 nm space, the measurement was made under a non-overlap (i.e., abutting) condition at  311 , and then the space measurement was made of the stitched space under the fixed 144 nm overlap condition at  315 . As shown in  FIG. 6 , it is clear that the standard deviation (3 Sigma value) in this case decreases from 10.7 to 7.0 indicating that the stitched (printed) space width response improves despite the presence of overlay errors. Thus, printed stitched spaces have a tighter tolerance than spaces that are printed without the overlapping technique (i.e., unstitched). 
     Aerial images of spaces are generally worse than aerial image patterns for lines, in general. However, due to the shape of the space response curve shown in  FIG. 5 , for spaces, an overlay tolerance will not change the space width when spaces are printed in overlapped condition. It is additionally seen from the results shown in  FIG. 6  that the tolerance for a stitched space is comparable, if not better than, the tolerance of an unstitched line for the current technology (e.g., 0.18 μm). 
     Thus, according to the invention, a line feature may be stitched with good degree of accuracy given the characteristic linear response curve  100  plotted in  FIG. 4 . Likewise, a space feature having a space width may be stitched with good spacewidth results (i.e., tighter tolerance) when there is overlap (and slight misalignment) of the printed space feature images at a given overlay tolerance as the overlap requires a double light exposure dose. That is, using the characteristic quadratic response curve  200  plotted in  FIG. 5 , stitched spaces may be formed with a good degree of accuracy because the stitched space has tighter tolerance control than a non-stitched space. 
     While the embodiments of the invention as described herein assumed the use of positive photoresist, it is the case that using negative resist for this application will render the mask as resembling the positive line feature. Thus, although the aerial image is improved, the negative resists are not as good as the positive resist, so this may be a less effective solution than the applications use with a positive resist. 
     While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.