Abstract:
A base-loaded polymer is applied to a semiconductor feature formed after exposing and developing a photoresist layer in order to reduce line edge roughness caused by a residual acid collecting on the edges of the feature during the post-exposure bake of the photoresist. Alternatively, a polymer is applied containing grains that are of suitable for smoothing the line edge roughness.

Description:
FIELD  
         [0001]    Embodiments of the invention relate to the field of semiconductor manufacturing. More particularly, embodiments of the invention relate to the reduction of line edge roughness in features formed in a semiconductor device from exposure and development of photoresist within the features.  
         BACKGROUND  
         [0002]    As feature sizes continue to decrease in modern photolithographic semiconductor manufacturing processes effects, such as undesired line edge roughness, insufficient lithographical resolution, and limited depth of focus problems, may increase. Specifically, photoresist image footprints typically become increasingly difficult to control as semiconductor device features become smaller and closer together, because these undesired effects seldom scale with the reduced feature size. More specifically, the molecular size in the polymer backbone used in the photoresist must typically meet certain physical/mechanical requirements and, therefore, should not be scaled with the reduction in feature size.  
           [0003]    Line edge roughness is typically at least partially caused by acid diffusion that results when a photoresist is exposed to an incident radiation, such as ultra-violet light. The acid typically serves as a catalyst for chemical reactions (deprotection) that occur within the photoresist at high temperatures, such as a post-exposure bake. The acid may be produced by photoacid generator molecules that are blended into the photoresist and diffuse the acid upon exposure to incident radiation.  
           [0004]    [0004]FIG. 1 is a photograph illustrating the effects of line edge roughness in a semiconductor device feature. Note that the line edge roughness results in a non-uniform feature line edge, thus limiting the feature size of the device.  
           [0005]    Prior art methods for controlling line edge roughness include adding a base quencher to the photoresist that has a relatively low base concentration with respect to the acid concentration. Acid concentrations less than or equal to the base concentration would be quenched (neutralized) by the base.  
           [0006]    During a post-exposure bake, at least some of the acid diffusion at a feature line edge formed by the exposed and developed photoresist would be neutralized by the base. This results in a better-defined feature edge and reduced line edge roughness. Using a base quencher prior to exposing and developing the photoresist, however, can result in a decrease in photoresist sensitivity, thereby increasing the incident radiation energy necessary to produce the same result.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    Embodiments and the invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements in which:  
         [0008]    [0008]FIG. 1 is a photograph illustrating the effects of line edge roughness in a semiconductor device feature.  
         [0009]    [0009]FIG. 2 illustrates a cross-section of line edge roughness according to one embodiment of the invention.  
         [0010]    [0010]FIG. 3 illustrates a cross-section of line edge roughness according to one embodiment of the invention.  
         [0011]    [0011]FIG. 4 is a flow diagram illustrating a semiconductor process according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]    Embodiments of the invention described herein are intended to help reduce semiconductor process effects, such as undesired line edge roughness, insufficient lithographical resolution, and limited depth of focus problems associated with the removal of a photoresist layer from a semiconductor device feature. More particularly, embodiments of the invention relate to reducing line edge roughness in a semiconductor device by applying a base-loaded polymer to the feature after the photoresist layer has been exposed and developed.  
         [0013]    For one embodiment of the invention, the feature line edge roughness is reduced by applying a base-loaded polymer to a semiconductor wafer using a spin process after a feature-defining photoresist layer has been exposed and removed (developed). The base-loaded polymer may be applied before a post-exposure bake (“PEB”) process step or afterward. Furthermore, the polymer may be applied after a short PEB or one that is performed at a lower temperature than normal such that the photoresist is partially unprotected (“deprotection”) before the polymer is applied.  
         [0014]    Deprotection is a process wherein regions of the photoresist where acid formed by exposure to light serves as a catalyst for a chemical reaction typically during PEB, thereby removing protection groups attached to the polymer and renders them soluble in a developer.  
         [0015]    [0015]FIG. 2 illustrates an embodiment of the invention. After the PEB and/or develop process step, a conformal coating of base-loaded polymer  201  may be applied to the semiconductor wafer and baked. During this second bake, the residual acid  205 , diffuses out of the feature and reacts with the base. At least some of the low acid concentration at the edge of the feature is neutralized  210  by the base and reduces the line edge roughness of the feature.  
         [0016]    By applying the base-loaded polymer after developing the photoresist, as in the above embodiment, the photoresist sensitivity is not significantly affected, and therefore no significant amount of incident radiation energy need be applied during exposure.  
         [0017]    Alternatively, a sealant may be implied instead of a base-loaded polymer in a similar way as described above. The sealant, however, serves to fill in imperfections of the feature line edge rather than neutralize the acid within the line edge in order to reduce line edge roughness. FIG. 3 illustrates one embodiment of the invention in which a sealant is applied. As illustrated in FIG. 3, the original resist line edge roughness  301  is smoothed by application of the sealant  305 .  
         [0018]    [0018]FIG. 4 is a flow chart illustrating one embodiment of the invention. First, at operation  401 , a post-exposure bake is performed on a semiconductor wafer to partially deprotect the photoresist layer. Next, the photoresist layer is developed at operation  405 , and a conformal coating of base-loaded polymer is applied to the wafer at operation  410 . At operation  415 , a second bake is then performed on the wafer.  
         [0019]    Various materials may be used in various embodiments of the invention. In some embodiments of the invention, the polymer may be a sealant, such as aliphatic and/or aromatic oligomer, monomer, and/or macromonomer with epoxide, such as acrylic, styrenic, or vinyl. Oligomer, monomer, and macromonomer can also induce solubility in alkane, alcohol, aqueous, or a fluorocarbon solvent system. Furthermore, polymer blends and/or blends of polymer and composite filler materials may also be used.  
         [0020]    Filler materials may be colloidal inorganic or crosslinked organic material, such as silica, alumina, CaF 2 , colloidal cross-linked polystyrene, fullerines, metal clusters, etc., with a grain size sufficient to “fill in” imperfections of the feature line edge in order to create a smoother line edge. For other embodiments of the invention, the polymer is loaded with a base such as pyridine, piperidineethanol, or amine based quencher such as triethyl, tripropyl, tripentyl, diethanol, or triethanol amine.  
         [0021]    While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments , which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention.