Abstract:
A method and a structure. The structure includes: a solid core comprising a first photoresist material, the core having a bottom surface on a substrate, a top surface and opposite first and second side surfaces between the top surface and the bottom surface; and a shell comprising a second photoresist material, the shell on the top surface of the substrate, the shell containing a cavity open to the top surface of the substrate, the shell formed over the top surface and the first and second side surfaces walls of the core, the core completely filling the cavity. The core is stiffer than the shell. The method includes: forming the core from a first photoresist layer and forming the shell from a second photoresist layer applied over the core. The core may be cross-linked to increase its stiffness.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of micro-photolithography; more specifically, it relates to a method for generating composite photoresist structures. 
     BACKGROUND OF THE INVENTION 
     As the dimension of the structures of integrated circuits become ever smaller the photoresist images used to define those structures during fabrication are also becoming smaller. Photoresist images have become so small that the photoresist pattern can collapse during the development step of photolithographic fabrication processes resulting in permanent defects in the integrated circuit being fabricated. Accordingly, there exists a need in the art to eliminate or reduce the phenomenon of photoresist pattern collapse. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a structure, comprising: a solid core comprising a first photoresist material, the core having a bottom surface on a substrate, a top surface and opposite first and second side surfaces between the top surface and the bottom surface; and a shell comprising a second photoresist material, the shell on the top surface of the substrate, the shell containing a cavity open to the top surface of the substrate, the shell formed on and completely covering the top surface and the first and second side surfaces walls of the core, the core completely filling the cavity. 
     A second aspect of the present invention is a method, comprising, in the order recited: forming a first photoresist layer on a top surface of a substrate, the first photoresist layer comprising a first photoresist material; exposing the first photoresist layer through a first photomask and developing the exposed first photoresist layer to form a solid core comprising the first photoresist material, the core having a bottom surface on the substrate, a top surface and opposite first and second side surfaces between the top surface and the bottom surface; the second photoresist layer completely covering the top surface and opposite first and second side surfaces of the core, the second photoresist layer comprising a second photoresist material; and exposing the second photoresist layer through a second photomask and developing the exposed second photoresist layer to form a shell comprising the second photoresist material, the shell on the top surface of the substrate, the shell containing a cavity open to the top surface of the substrate, the shell formed over the top surface and the first and second side surfaces walls of the core, the core completely filling the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a cross-section diagram illustrating the cause of pattern collapse; 
         FIG. 2  is a cross-section diagram illustrating one form of pattern collapse; 
         FIGS. 3A through 3E  are cross-section diagrams illustrating fabrication of a first composite photoresist structure according to the present invention; 
         FIGS. 3F and 3G  are cross-section diagrams illustrating an alternate fabrication of the first composite photoresist structure according to the present invention 
         FIG. 4  is a cross-section diagram illustrating the resistance of composite structures according to the present invention to pattern collapse; 
         FIG. 5  is a cross-section diagram illustrating no pattern collapse of a composite structure photoresist structure according to the present invention; 
         FIG. 6  is cross-section of a first exemplary process that may be performed using a composite photoresist structure according to the present invention; 
         FIG. 7  is cross-section of a second exemplary process that may be performed using a composite photoresist structure according to the present invention. 
         FIG. 8  is a cross-section of a second composite photoresist according to the present invention; 
         FIG. 9  is a cross-section of a third composite photoresist according to the present invention; and 
         FIG. 10  is a cross-section of a fourth composite photoresist according to the present invention; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A positive photoresist is a type of photoresist in which regions of photoresist exposed to actinic radiation (e.g. ultra-violet light, electron beam or X-ray) become soluble to a photoresist developer and the regions of the photoresist that are unexposed remain insoluble in the photoresist developer. A negative photoresist is a type of photoresist in which regions of photoresist exposed to actinic radiation become insoluble in the photoresist developer and the regions of the photoresist that are unexposed are soluble in the photoresist developer. A dual-tone photoresist is a photoresist that can act as either a positive or negative photoresist depending upon processing conditions. The term positive photoresists is intended to include dual-tone photoresists processed to behave as a positive photoresist and the term negative photoresists is intended to include dual-tone photoresists processed to behave as a negative photoresist. Photoresists also include photoactive polymers such as photoactive polyimides. 
     After a photoresist layer is exposed through a patterned mask (or the pattern directly written into the photoresist layer without the use of a photomask) a latent image of the pattern is formed in the photoresist layer. A chemical developer is used to remove the soluble portions of the photoresist layer as described supra. The developer is then rinsed away. It is during the rinsing process, particularly when water is used as the rinsing agent that photoresist pattern collapse occurs. Pattern collapse becomes more prevalent the greater the aspect ratio (height to width) of the photoresist features formed after development. In one example, photoresist features having aspect ratios of about 3:1 or greater are prone to pattern collapse. 
       FIG. 1  is a cross-section diagram illustrating the cause of pattern collapse. In  FIG. 1 , a substrate  100  has a top surface  105  on which an exemplary process layer  110  has been formed. Process layer  110  may be a dielectric layer, an electrically conductive layer, or a semi-conducting layer. Formed on a top surface  115  of process layer  115  are a first photoresist feature  120 A and a second photoresist feature  120 B. A drop of rinse fluid  125 , in this example, water, is formed between the first and second photoresist features  120 A and  120 B. First and second photoresist features  120 A and  120 B have a height H 1  and a width W 1  and are spaced apart a distance S 1 . A meniscus  130  is formed between first and second photoresist features  120 A and  120 B at a tangential angle θ to sidewalls  122  of the first and second photoresist features. First photoresist feature includes a virtual core  123 A and a virtual shell  124 A and second photoresist feature includes a virtual core  123 B and a virtual shell  124 B for the purposes of deriving an equation (2) that may be compared to an equation (3) derived infra. Virtual shells are spaced apart distance S 1  and virtual cores are spaced apart a distance S 2 . A liquid between two upright structures exerts a capillary force ΔP between the two structures according to the LaPlace equation:
 Δ P =(2γ cos θ)/ S   (1) 
     where ΔP has been defined supra;
         S is the distance between the structures; and   γ is the surface tension of the liquid,
 
The following equation can be derived for calculating a value of ΔPh from known or measurable parameters when photoresist features  120 A and  120 B are homogenous:
 
Δ Ph=S 1 E 1/4 a (( a/b )+1)  (2)
       

     where ΔPh capillary force ΔP between photoresist features  120 A and  120 B;
         S 1  is the distance between photoresist features  120 A and  120 B;   E 1 =Young&#39;s modulus (of elasticity) of the photoresist material of first and second photoresist features  120 A and  120 B;   a=the cross-sectional area of virtual shell  124 A or  124 B (assuming they are the same); and   b=the cross-sectional area of a virtual core  123 A or  123 B (assuming they are the same).       

       FIG. 2  is a cross-section diagram illustrating one form of pattern collapse. In  FIG. 2 , as the rinse fluid  125  (see  FIG. 1 , evaporates, first and second photoresist features  120 A and  120 B are pulled toward each other and distorted. Note, if second photoresist feature  120 B were very much wider than first photoresist feature  120 A, only the first photoresist feature would distort because the aspect ratio of the second photoresist feature would be small. In extreme cases, the photoresist features can become detached from surface  115 . 
       FIGS. 3A through 3E  are cross-section diagrams illustrating fabrication of a first composite photoresist structure according to the present invention. In  FIG. 3A , an exemplary photomask  140  includes a substrate  145  having a front surface  150  and opaque islands  155  formed on the front surface. A positive photoresist layer  160  having a thickness T 2  is formed on top surface  115  of process layer  110 . Photomask  140  is exposed to actinic radiation (hυ), which passes through substrate  145 , is blocked by opaque islands  155  and impinges on photoresist layer  160 . 
     In  FIG. 3B , after developing and rinsing, photoresist features  165  are formed on top surface  115  of process layer  110 . Each photoresist feature  165  has a height H 2  and a width W 2  and are spaced apart a distance S 2 . Referring back to  FIG. 1 , H 2  is less than H 1 , W 2  is less than W 1  and S 2  is greater than S 1 . 
     In  FIG. 3C , photoresist images  165  (see  FIG. 13B ) are hardened (e.g. cross-linked) by exposure to ultra-violet (UV) light to form photoresist cores  170 . The cross-linking increases the Young&#39;s modulus of cores  170  relative photoresist images  165 . 
     Alternatively, hardening may be accomplished by heating or electron or ion bombardment of photoresist images  165 . The hardening/cross-linking process is optional, as described infra. 
     In  FIG. 3D , a positive photoresist layer  175  having a thickness T 1  (where T 1  is greater than T 2 , see  FIG. 3A ) is formed on top surface  115  of process layer  110  and covering cores  170 . Again, photomask  140  is exposed to actinic radiation (hυ), which passes through substrate  145 , is blocked by opaque islands  155  and impinges on photoresist layer  175 . However, the exposure dose of the actinic radiation is adjusted so as to produce a wider (W 1 &gt;W 2 ) photoresist image, (see  FIG. 3E ) than was produced by the exposure of  FIG. 3A . 
     In  FIG. 3E , after developing and rinsing, photoresist shells  180  are formed on top surface  115  of process layer  110 , on a top surfaces  182  and sidewalls  183  and  184  of cores  170  to form composite photoresist features  185 . Each composite photoresist feature  185  has a height H 1  and width W 1  and are spaced apart distance S 1  the same as in  FIG. 1  and the Young&#39;s modulus of cores  170  is greater than Young&#39;s modulus of shells  180 . 
       FIGS. 3F and 3G  are cross-section diagrams illustrating an alternate fabrication of the first composite photoresist structure according to the present invention.  FIG. 3F  us similar to  FIG. 3D  except a second exemplary photomask  190  is used to expose photoresist layer  175 . Photomask  190  includes a substrate  195  having a front surface  200  and opaque islands  200  formed on the front surface. Photomask  190  is exposed to actinic radiation (hυ), which passes through substrate  195 , is blocked by opaque islands  200  and impinges on photoresist layer  175 . Opaque islands  200  are wider than opaque islands  155  of photomask  140  (see  FIG. 3D ).  FIG. 3G  is the same as  FIG. 3E . 
       FIG. 4  is a cross-section diagram illustrating the resistance of composite structures according to the present invention to pattern collapse. In  FIG. 4 , substrate  100  has top surface  105  on which exemplary process layer  110  has been formed. Formed on top surface  115  of process layer  115  are a first composite photoresist feature  185 A including a core  170 A and a shell  180 A and a second composite photoresist feature  185 B including a core  170 B and a shell  180 B. First and second composite photoresist features were formed as illustrated in  FIGS. 3A through 3G  and describe supra. A drop of rinse fluid  125 , in this example, water, is formed between the first and second composite photoresist features  120 A and  120 B. First and second composite photoresist features  185 A and  185 B have the same height H 1  and width W 1  and are spaced apart the same distance S 1  as first and second photoresist images  120 A and  120 B of  FIG. 1 . Meniscus  130  is formed between first and second photoresist features  185 A and  185 B at tangential angle θ to sidewalls  207  of the first and second photoresist features. Rinse fluid  125  exerts a capillary force ΔPc between first and second photoresist features  185 A and  185 B. Photoresist shells  180  have a Young&#39;s modulus of E 1  (the same photoresist features  120 A and  120 B of  FIG. 1 ). Photoresist cores  170  have a Young&#39;s modulus of E 2 , with E 2  greater than E 1  (photoresist cores  170  are stiffer than photoresist shells  180  or first and second photoresist features  120 A and  120 B of  FIG. 1 ). Again, starting with equation (1) for ΔP described supra, the following equation can be derived for calculating a value of ΔPc when photoresist features are composite comprising a core and shell of material each having a different Young&#39;s Modulus:
 Δ Pc=S 1 E 1/4 a (( a/b )+ y )  (3) 
     where ΔPh capillary force ΔP between photoresist features  185 A and  185 B;
         S 1  is the distance between shells  180 A and  180 B;   E 1 =Young&#39;s modulus of the photoresist material shells  180 A and  180 B (assuming E 2 =Young&#39;s modulus of the photoresist material of cores  170 A and  170 B and E 2 &gt;E 1 );   a=the cross-sectional area of shell  180 A or  180 B(assuming they are the same); and   b=the cross-sectional area of core  170 A or  170 B (assuming they are the same).       

     Comparing equation (2) with equation (3) (assuming E 1 , S 1 , a and b are the same in both equations) is can be seen that since y+a/b must be greater than 1+a/b when E 2 &gt;E 1 , then ΔPc is less than ΔPh with resistance to distortion governed by the value of y. Note, the hardening process illustrated in  FIG. 3C  and described supra, may be eliminated if the Young&#39;s modulus E 2  of cores  170 A and  170 B have a high enough value without cross-linking so composite photoresist features  185 A and  185 B are stiff enough to overcome the force ΔPc. 
     In one example, H 1  is equal to between about 3 times H 2  to about 3/2 times H 2 . In one example, W 2  is equal to between about 1/2 times W 1  to about 3/4 times W 1 . In one example. H 1 /W 1  is about 3 or greater. In one example H 2 /W 2  is about 2 or less. The relationship between H 1  and H 2  and W 1  and W 2  must be selected so that the stiffness force of first and second composite photoresist features  185 A and  185 B is greater than ΔPc. 
     Continuing from  FIG. 3G ,  FIG. 5  is a cross-section diagram illustrating no pattern collapse of a composite structure photoresist structure according to the present invention. In  FIG. 5 , as the rinse fluid  125  (see  FIG. 9 , evaporates, first and second composite photoresist features  185 A and  185 B are not pulled toward each other and not distorted. 
       FIG. 6  is cross-section of a first exemplary process that may be performed using a composite photoresist structure according to the present invention. In  FIG. 6 , the structures illustrated in  FIG. 3E  (or  3 G) and described supra are subjected to a wet or dry etch (e.g. a reactive ion etch (RIE)) forming trenches  235  exposing substrate  100  (or another process layer between the substrate and the process layer) between islands  240  of process layer  110  of  FIG. 3E . Composite photoresist features  185  may be subsequently removed. 
       FIG. 7  is cross-section of a second exemplary process that may be performed using a composite photoresist structure according to the present invention. In  FIG. 7 , the structures illustrated in  FIG. 3E  (or  3 G) and described supra are subjected to ion implantation process to form ion-implanted region  245  (e.g. doped regions if the ions contain arsenic, phosphorus or boron) into substrate  100 . Composite photoresist features  185  may be subsequently removed. 
       FIG. 8  is a cross-section of a second composite photoresist according to the present invention. In  FIG. 8 , composite photoresist features  210  include a negative photoresist core  215  and positive photoresist shell  180 . Since positive and negative tone photoresists are used, a different photomask is used to form core  215  than that used to form shell  180 . 
       FIG. 9  is a cross-section of a third composite photoresist according to the present invention. In  FIG. 9 , composite photoresist features  220  include positive photoresist core  170  and negative photoresist shell  225 . Since positive and negative tone photoresists are used, a different photomask is used to form core  170  than that used to form shell  225 . 
       FIG. 10  is a cross-section of a fourth composite photoresist according to the present invention. In  FIG. 10 , composite photoresist features  230  include negative photoresist core  215  and negative photoresist shell  225 . Since only negative tone photoresists is used, a same photomask may be used to form core  215  and shell  225 . 
     Thus, the present invention provides a method to eliminate or reduce the photoresist pattern collapse. 
     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.