Abstract:
A method and apparatus for forming a tapered photoresist edge. The method includes: forming a photoresist layer on a substrate; exposing a first annular region of the photoresist layer adjacent to a perimeter of the substrate to actinic radiation, the first annular region having a first outer perimeter proximate to a perimeter of the substrate and a first inner perimeter away from the perimeter of the substrate, the actinic radiation gradually decreasing in intensity from the first outer perimeter to the first inner perimeter; and developing the exposed first annular region of the photoresist layer to form a tapered profile in a second annular region of the photoresist layer, the second annular region having a second perimeter proximate to the perimeter of the substrate and a second inner perimeter away from the substrate perimeter, the profile gradually increasing in thickness from the second outer perimeter to the second inner perimeter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of integrated circuit fabrication; more specifically, it relates to a method and apparatus for forming a photoresist layer with a tapered edge for use in immersion photolithography. 
       BACKGROUND OF THE INVENTION 
       [0002]    Photolithography is a technique by which a pattern on a photomask is projected through a lens onto a photoresist layer on a substrate such as a semiconductor substrate. After a subsequent development cycle the pattern on the photomask is transferred to the photoresist layer. Conventional photolithography is unable to produce the smallest of the photoresist images sizes that current integrated circuit designs call for. Immersion lithography utilizes an immersion fluid placed between the lens and the photoresist layer. This increases the numerical aperture of the exposure system and improves depth of focus and resolution. However, immersion photolithography has been found to have defect generation mechanisms not found in conventional lithography. Accordingly, there exists a need in the art for improved immersion lithography processes that overcome immersion lithography related defect mechanisms. 
       SUMMARY OF THE INVENTION 
       [0003]    A first aspect of the present invention is a method, comprising: forming a photoresist layer on a substrate; exposing a first annular region of the photoresist layer immediately adjacent to a perimeter of the substrate to actinic radiation, the first annular region having a first outer perimeter proximate to a perimeter of the substrate and a first inner perimeter away from the perimeter of the substrate, the actinic radiation gradually decreasing in intensity from the first outer perimeter to the first inner perimeter; developing the exposed first annular region of the photoresist layer to form a tapered profile in a second annular region of said photoresist layer, the second annular region having a second outer perimeter proximate to the perimeter of the substrate and a second inner perimeter away from the perimeter of the substrate, the profile gradually increasing in thickness from the second outer perimeter to the second inner perimeter; and after the developing, no portion of the photoresist layer remaining on the substrate between the second outer perimeter and the perimeter of the substrate. 
         [0004]    A second aspect of the present invention is a method, comprising: forming a photoresist layer on a substrate by spin application of a photoresist material; rotating the substrate while exposing the photoresist layer to a spot of actinic radiation placed proximate to a perimeter of the substrate to form an exposed annular region of the photoresist layer immediately adjacent to the perimeter of the substrate, the exposed annular region having a first outer perimeter proximate to the perimeter of the substrate and a first inner perimeter away from the perimeter of the substrate, the spot of actinic radiation gradually decreasing in intensity from a region of the spot of actinic radiation nearest to the perimeter of the substrate to a region of the spot of actinic radiation nearest a center of the substrate; after the exposing, rotating the substrate while spraying a stream of developer onto the perimeter of the substrate to form a tapered profile in a tapered annular region in the photoresist layer, the tapered annular region having a second outer perimeter proximate to the perimeter of the substrate and a second inner perimeter away from the perimeter of the substrate, the profile gradually increasing in thickness from the second outer perimeter to the second inner perimeter; and after the developing, no portion of the photoresist layer remaining on the substrate between the second outer perimeter and the perimeter of the substrate. 
         [0005]    A third aspect of the present invention is an apparatus, comprising: a rotatable wafer chuck; a spot exposure source positioned over a first peripheral region of the wafer chuck, the spot exposure source capable of directing a beam of radiation toward a less than whole portion of the wafer chuck proximate to a perimeter of the wafer chuck, the beam of radiation having a graded intensity that gradually decreases from a region of the spot nearest to the perimeter of the wafer chuck to a region of the spot nearest a center of the wafer chuck; and a spot develop spray head positioned over a second peripheral region of the wafer chuck, the spot develop spray head capable of directing a localized stream of developer toward a less than whole portion of the wafer chuck proximate to the perimeter of the wafer chuck. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
           [0007]      FIGS. 1A through 1D  are cross-sectional drawings illustrating formation of a photoresist layer with a tapered edge according to the present invention; 
           [0008]      FIGS. 2A through 2D  are cross-sectional drawings illustrating formation of a photoresist layer between antireflective coatings and having a tapered edge according to the present invention; 
           [0009]      FIG. 3A through 3C  are cross-sectional drawings illustrating alternative edge bead profiles for the photoresist layer of  FIG. 1D ; 
           [0010]      FIGS. 4A and 4B  are cross-sectional drawings illustrating alternative edge bead profiles for the photoresist layer of  FIG. 2D ; 
           [0011]      FIG. 5A  is a schematic diagram of an first apparatus and  FIG. 5B  is a schematic diagram of a second apparatus for generating a graded exposure for use with positive tone photoresists according to an embodiment of present invention; 
           [0012]      FIG. 6A  is a top view and  FIG. 6B  is a cross-sectional view through line  6 B- 6 B of  FIG. 6A  illustrating an exemplary apparatus for practicing the present invention; 
           [0013]      FIGS. 7A and 7B  are cross-sectional drawing illustrating formation of photoresist images by immersion lithography according to the present invention; and 
           [0014]      FIG. 8A  is a schematic diagram of an first apparatus and  FIG. 8  is a schematic diagram of a second apparatus for generating a graded exposure for use with negative tone photoresists according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIGS. 1A through 1D  are cross-sectional drawings illustrating formation of a photoresist layer with a tapered edge according to the present invention. In  FIG. 1A , a circular substrate has a top surface  105 , an edge  110  and a central axis  115 . Central axis  115  is perpendicular to top surface  105  of substrate  100 . In one example, substrate  100  is a semiconductor substrate, examples of which include, but are not limited to single-crystal bulk silicon substrates and silicon-on-insulator (SOI) substrates. Circular semiconductor substrates are also called wafers. Substrate  100  is advantageously a wafer having a diameter of between about 100 mm and about 300 mm. However, the present invention may be practiced on other shaped substrates, such a rectangular substrates. In one example, substrate  100  is a circular or rectangular glass or quartz substrate. Formed on top surface  105  is a photoresist layer  120 . Photoresist layer  120  is formed by spin application. Photoresist is generally comprised of a polymer, a sensitizer and a casting solvent. In one example photoresist layer  120  is a chemically-amplified positive tone resist. A positive tone resist is made soluble in a developer by exposure to actinic radiation (e.g., 193 nm ultraviolet light). By contrast, a negative tone resist is made insoluble in a developer by exposure to actinic radiation. In one example, photoresist layer is between about 150 nm and about 250 nm thick. After spin application, photoresist layer  120  may be heated to a temperature above room temperature to drive out any remaining casting solvent. This is called a “pre-bake” or “soft-bake” and is optional. 
         [0016]    Because of the dynamics of spin application (depositing a puddle of photoresist in the center of a substrate and spinning the substrate to spread out the photoresist into a thin layer) a bead of photoresist is formed proximate to the edge of the substrate. This bead of photoresist is thicker than the rest of the photoresist layer and can cause process control problems as well as induce defects. Therefore, various edge bead removal processes have been developed. All these processes remove the photoresist from a ring proximate to the edge of the substrate. However, the newly formed edge of the photoresist is vertical (i.e., perpendicular relative to the top surface of the substrate). The Inventor has found that during immersion lithography, as the immersion head translates across the vertical edge of the photoresist, chips of the photoresist are removed from the photoresist edge and deposited on the top surface of the photoresist layer where they subsequently cause printing defects. The Inventor believes one possible defect mechanism is that impact of the immersion fluid on the vertical sidewall of the photoresist layer generates shock waves in the immersion fluid as the substrate bearing the photoresist layer is moved while in contact with the immersion fluid, which cause the photoresist edge chipping. The present invention reduces or eliminates this problem by forming a tapered photoresist edge which is less susceptible to immersion fluid impact damage. 
         [0017]    In  FIG. 1B , the photoresist proximate to edge  110  of substrate  100  is exposed to actinic radiation (e.g., ultraviolet light)  125  while the substrate is rotated about central axis  115 . This is called an edge bead exposure. Alternatively, the ultraviolet light could be rotated and the wafer not rotated. The ultraviolet light gradually decreases in intensity from edge  110  toward center axis  115  of substrate  100 . The exposure extends a distance D 0  from edge  110  toward center axis  115  of wafer  100 . In one example, D 0  is between about 2 millimeters (mm) to about 6 mm. In one example, substrate  100  is rotated at between about 100 and about 1000 RPM during the edge bead exposure. 
         [0018]    In  FIG. 1C , a spray of developer solution  130  is directed onto the exposed region of photoresist layer  120  proximate to edge  110 . This is called an edge bead development. In one example, the developer is diluted to about 10% of a concentration that will be used to develop a device pattern in the remainder of photoresist layer  120  after an immersion lithography step. See  FIG. 7B  and description infra. One exemplary developer is aqueous tetramethylammonium hydroxide (TMAH). A dilute edge bead developer (compared to the device pattern developer) has the advantage of being less likely to leach photoresist components from the regions of photoresist layer adjacent to tapered edge region  135 , though the same developer may be used for both. In one example, substrate  100  is rotated at between about 100 and about 1000 RPM during the edge bead development. 
         [0019]    In  FIG. 1D , after the edge bead development step of  FIG. 1C , a tapered edge region  135  of photoresist layer  120  is formed. A peripheral region  140  of substrate  100  is also exposed. Peripheral region  140  has a width D 1  measured from edge  110  of substrate  100  and tapered edge region  135  has a width D 2  measured from peripheral region  140  toward central axis  115 . In one example, D 1  is between about 1 mm and about 3 mm. In one example, D 2  is between about 1 mm and about 3 mm. In  FIG. 1D , by way of example, tapered edge region  135  is uniformly (linearly) tapered. Alternatively, the soft-bake described supra, or another soft-bake may be performed after the edge bead development step, to further “smooth” tapered edge region  135 . 
         [0020]      FIGS. 2A through 2D  are cross-sectional drawings illustrating formation of a photoresist layer between antireflective coatings and having a tapered edge according to the present invention.  FIGS. 2A through 2D  are similar, respectively, to  FIGS. 1A through 1D  except that a bottom antireflective coating (BARC)  141  is formed on top surface  105  of substrate  100 , photoresist layer  120  is formed on a top surface  142  of BARC  141  and a top antireflective coating (TARC)  143  is formed on a top surface  150  of photoresist layer  120 . BARC  141  and TARC  143  may be formed by spin application. In one example, a soft-bake as described supra is performed after formation of TARC  143 . In one example both BARC  141  and TARC  143  are organic materials between about 100 nm to about 200 nm thick. In one example, BARC  141  comprises an under-layer (on substrate  100 ) and a silicon containing layer on top of the under-layer. In one example, either or both of BARC  141  and TARC  143  are photosensitive. Alternatively only BARC  141  and photoresist layer  120  is formed in  FIG. 2A . Alternatively, only photoresist layer  120  and TARC  143  is formed in  FIG. 2A . In  FIG. 2D , BARC  141  extends only to peripheral region  140  and TARC  143  extends only to tapered edge region  135 . 
         [0021]      FIG. 3A through 3C  are cross-sectional drawings illustrating alternative edge bead profiles for the photoresist layer of  FIG. 1D . In  FIG. 3A , tapered edge region  135  is concave. In  FIG. 3B , tapered edge region  135  is linearly tapered and extends to edge  110  of substrate  100  (i.e., D 1  of  FIG. 1D  is about zero). In  FIG. 3C , tapered edge region  135  is concave and extends to edge  110  of substrate  100  (i.e., D 1  of  FIG. 1D  is about zero). 
         [0022]      FIGS. 4A and 4B  are cross-sectional drawings illustrating alternative edge bead profiles for the photoresist layer of  FIG. 2D . In  FIG. 4A , TARC  143  is separated from tapered edge region of photoresist layer  120  by a distance D 3 . In  FIG. 4B , TARC  143  is separated from tapered edge region of photoresist layer  120  by a distance D 3  and BARC  141  extends a distance D 4  into peripheral region  140 . In one example D 3  is zero and D 4  is not zero. In one example D 4  is equal to D 1  (see  FIG. 2D ) and D 3  is zero. In one example D 4  is equal to D 1  (see  FIG. 2D ) and D 3  is not zero. 
         [0023]      FIG. 5A  is a schematic diagram of a first apparatus and  FIG. 5B  is a schematic diagram of a second apparatus for generating a graded exposure for use with positive tone photoresists according to an embodiment of the present invention. In  FIG. 5A , an edge bead exposure head  155 A includes a housing  160  (which also acts as a collimator), a reflector  165 , an ultraviolet light source  170  (which may be a simple mercury arc lamp), an optional lens  175  and a graded density filter  180 . Graded density filter  180  comprises bands of neutral density filters A through H arranged with band A nearest edge  110  and band H furthest from edge  110 . The optical density of band A is less than the optical density of band B which is less than the optical density of band C though to the optical density of band G which is less than the optical density of band H. Thus, progressively less ultraviolet light is transmitted through graded density filter  180  from edge  110  in a direction from edge  110  toward the center of substrate  100 . Edge bead exposure heads  155 A and  155 B are essentially for use with positive tone photoresists. Edge bead heads  225 A and  225 B of  FIGS. 8A and 8B  described infra are used with negative tone photoresists. 
         [0024]      FIG. 5B  is similar to  FIG. 5A  except an edge bead exposure head  155 B includes a filter  185  formed of uniform optical density material but of varying thickness. Filter  185  is thinnest over edge  110  and increases in thickness toward the center of substrate  100  in a direction from edge  110  toward the center of substrate  100 . Thus, progressively less ultraviolet light is transmitted through graded density filter  185  from edge  110  in a direction from edge  110  toward the center of substrate  100 . In one example, graded density filter  185  has the shape of a wedge prism or an obliquely truncated cylinder or cone. 
         [0025]      FIG. 6A  is a top view and  FIG. 6B  is a cross-sectional view through line  6 B- 6 B of  FIG. 6A  illustrating an exemplary apparatus for practicing the present invention. In  FIG. 6A , an edge bead removal tool  190  includes a bowl shaped body  195 , a drain  200  in the bottom of the body, a rinse nozzle  205 , a spot developer spray nozzle  210 , a wafer chuck  215  rotatable about a central axis and an edge bead exposure head  155  that generates a spot of ultraviolet light. Edge bead exposure head  155  is similar to edge bead exposure heads  155 A of  FIG. 5A  or  155 B of  FIG. 5B  or another type of graded ultraviolet exposure source that generates more light away from the center axis of chuck  215  and gradually less light toward the center axis of the chuck. Edge bead exposure head  155  and spot develop spray nozzle  210  are positioned over the periphery of substrate  100  mounted on wafer chuck  215 . The term spot is defined to mean limited to a small area relative to an entire area. For example, edge bead exposure head  155  generates a spot of graded intensity ultraviolet light of between about 1 mm and about 3 mm in diameter, which is a very small area of a wafer (e.g. substrate  100 ) having a diameter between about 100 mm to about 300 mm. 
         [0026]    In use, a photoresist-coated substrate  100  (may also have BARC and TARC layers) is placed on chuck  215  and the chuck rotation started. Ultraviolet exposure source  155  is turned on for a first preset duration of time and then turned off. Next developer solution is dispensed from develop dispense nozzle  210  on top surface  150  of photoresist layer (or a top surface of a TARC layer) proximate to the edge of substrate  110 . After a second preset duration of time, the developer is turned off and a rinse fluid dispensed from rinse nozzle  205  for a third present duration of time. Examples of rinse fluids include, water, alcohols, nitrogen and air. Two or more rinse fluids may be dispensed in sequence. Then rotation of chuck  215  is stopped and substrate  100  removed. 
         [0027]      FIGS. 7A and 7B  are cross-sectional drawing illustrating formation of photoresist images by immersion lithography according to the present invention.  FIGS. 7A and 7B  continue from  FIG. 1D . Alternatively,  FIGS. 7A and 7B  may continue from  FIG. 2D  with BARC and/or TARC layers. In  FIG. 7A , a substrate  100  having photoresist layer  120  with a tapered edge region  135  is placed on an X-Y stage  225  and placed under an exposure system  230 . Exposure system  230  includes an immersion head  235  filled with an immersion fluid  240 , a lens  245  disposed between immersion fluid  240  and a photomask  255 , and a ultraviolet light source  260  positioned to project ultraviolet light through photomask  255  onto lenses  245  which in turns focuses the ultraviolet light on top surface  150  of photoresist layer  120 . Photomask  255  has a pattern of transparent and opaque regions on a surface of the photomask. Immersion fluid  240  contacts a bottom surface of lens  245  and top surface  150  of photoresist layer  120 . In one example, immersion fluid  240  comprises water. 
         [0028]    In use, X-Y stage positions different regions of substrate  100  under immersion head  235  moving the wafer  100  and the reticle  255  in a synchronized operation while irradiating with ultraviolet light. The process is continued until the entire photoresist layer has been exposed. In some cases, the exposure is performed through a moving slit synchronized with the movement of wafer  100  so as to expose only regions of reticle  255  at any one time. This is known as step and scan immersion lithography. Immersion fluid  240  will pass over tapered edge region  135  of photoresist layer  120  multiple times with many different entry points onto the wafer. 
         [0029]    In  FIG. 7B , photoresist layer  120  (see  FIG. 7A ) is developed to generate a pattern of photoresist islands  120 A in the same pattern as opaque regions on photomask  255  if photoresist layer  120  (see  FIG. 7A ) is a positive photoresist or in the same pattern as clear regions on photomask  255  if photoresist layer  120  (see  FIG. 7A ) is a negative photoresist. After development, photoresist islands  120 A may be optionally heated to a temperature above room temperature to cure (e.g. cross-link or further polymerize) the polymers in the photoresist islands. Note, there may also be a post-exposure and pre-development bake performed at a temperature above room temperature to enhance latent image formation in photoresist layer formed from chemically amplified photoresist formulations. 
         [0030]      FIG. 8A  is a schematic diagram of an first apparatus and  FIG. 8  is a schematic diagram of a second apparatus for generating a graded exposure for use with negative tone photoresists according to an embodiment of the present invention. In  FIG. 8A , an edge bead exposure head  220 A is similar to edge bead exposure head  155 A of  FIG. 5A  except graded density filter  180  is replaced with a graded density filter  225 . Graded density filter  225  comprises bands of neutral density filters A through H arranged with band H nearest edge  110  and band A furthest from edge  110 . The optical density of band A is less than the optical density of band B which is less than the optical density of band C though to the optical density of band G which is less than the optical density of band H. 
         [0031]    In  FIG. 8A , an edge bead exposure head  220 B is similar to edge bead exposure head  155 B of  FIG. 5A  except filter  185  is replaced with a filter  220 . Filter  230  is formed of uniform optical density material but of varying thickness. Filter  230  is thickest over edge  110  and decreases in thickness toward the center of substrate  100  in a direction from edge  110  toward the center of substrate  100 . 
         [0032]    Thus, the embodiments of the present invention provide an improved immersion lithography processes that overcomes immersion lithography unique defect mechanisms. 
         [0033]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.