Abstract:
Photoresists may be formed over a structure that has been modified so as to poison a lower layer of the photoresist. Then, when the photoresist is patterned, it is only patterned down to the poisoned layer. The poisoned layer may be removed subsequently. However, because of the use of the modification process, the critical dimensions of the photoresist may be improved in some embodiments.

Description:
BACKGROUND 
     This invention relates generally to fabricating integrated circuits and, particularly, to techniques for using photoresist to pattern features on semiconductor structures. 
     A photoresist may be formed over a semiconductor structure. The photoresist may be selectively exposed to radiation. Regions of the photoresist that are exposed to radiation may be more or less susceptible to attack by subsequent etching processes. Thus, by exposing a pattern on the photoresist, the photoresist may be patterned to have a particular profile. This profile may then act as a mask for subsequent processes to remove underlying material, for example, in the desired pattern. 
     In photoresist patterning techniques for forming relatively small features, it is desirable to control the photoresist profile to prevent undercutting of a developer sensitive substrate. A substrate is any material that is positioned under the photoresist and can include antireflective coatings and resist release layers. A resist release layer is a layer that is utilized to facilitate the release of the resist after the patterning is completed. In particular, it may be desirable to control resist profiles to enhance pattern fidelity at the bottom of printed features, such as trenches and holes. 
     One technique for using photoresist is called the Shipley lift-off layer (LOL). The LOL facilitates removal of the photoresist after patterning and etching and is one type of resist release layer. The LOL is easily attacked by the photoresist developer. The photoresist developer is utilized to develop the material after exposure. Since the LOL is easily attacked by that developer, lifting or peeling of the photoresist may occur prior to etching. 
     However, intentionally underdeveloping the photoresist leaves a thin layer of photoresist over the LOL, preventing the developer from attacking the LOL. This may be followed by a descum step to break through the remaining resist and LOL. This process may be followed by standard etching of the underlying substrate. While this procedure has promise, it is difficult to control and has relatively little process margin. 
     Thus, there is a need for better ways to control photoresist profiles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an enlarged cross-sectional view in an early stage of fabrication; 
         FIG. 2  is an enlarged cross-sectional view at a subsequent stage of fabrication in accordance with one embodiment of the present invention; 
         FIG. 3  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
         FIG. 4  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention; 
         FIG. 5  is an enlarged cross-sectional view of another embodiment of the present invention; and 
         FIG. 6  is an enlarged cross-sectional view at a subsequent stage in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a substrate  12  may be covered with an anti-reflective coating or a resist release layer  14 , such as a lift-off layer (LOL) in one embodiment of the present invention. A photoresist  16  may then be formed over the layer  14 , for example, by spin coating. The acid/base properties of the substrate  12  and/or layer  14  may be modified to control the profile of the photoresist  16  in the vicinity of the resist/substrate interface. 
     In some embodiments the layer  14  may be used due to processing constraints such as chemical attack, normal resist strip temperatures, or because dry etch may cause damage to polymeric, metallic, organic, or inorganic coatings on the wafer. Therefore, a resist release layer  14  may be utilized to facilitate removal of the photoresist  16  after patterning. In one embodiment, a Shipley lift-off layer, such as LOL-2000, available from Shipley Company, Marlborough, Mass., may be utilized. In other embodiments the resist release layer  14  may not be used. 
     An acid, a base, an acid analog, or a base analog may be added to a structure, such as the resist release layer  14  or the substrate  12 , to modify the resist  16  in the vicinity of the resist  16  structure interface. A base analog is a substance which, when exposed to certain ambient characteristics, forms a base. Examples of such ambient characteristics include photo exposure or thermal exposure. Similarly, an acid analog is a substance that forms an acid when exposed to certain ambient characteristics such as a thermal condition or a photo exposure. 
     If a base or base analog is added to the resist release layer  14 , resist  16  poisoning may result in a lack of resist development in the poisoned area of the resist  16 . Therefore, in some embodiments, standard processing conditions can be used to pattern the photoresist  16 , providing good critical dimension control and controllable critical dimension bias while maintaining a protective layer of photoresist  16  to prevent premature dissolution of the resist release layer  14 . 
     As shown in  FIG. 2 , the photoresist  16  may be exposed and developed using standard processes to form a trench  18 . The trench  18  stops at the poisoned region  20  of the photoresist  16 . However, the rest of the trench  18  profile may be relatively vertical, indicating that good critical dimension control may be maintained throughout the remainder of the process. 
     Next, referring to  FIG. 3 , a descum process may clear the photoresist  16  and resist release layer  14  from the bottom of the trench  22 . Thereafter, the substrate  12  may be etched, as indicated in  FIG. 4 . 
     Exposure dependent poisoning of the photoresist  16  may be accomplished through the application of a photo-acid generator (PAG) or a photo-base generator (PBG). 
     Photo-acid generators are commonly added to photoresist to catalyze decomposition upon exposure to radiation. Thus, the addition of a photo-base generator to a portion of the resist  16  that relies on a photo-acid generator for decomposition may reduce the decomposition rate of that portion effecting a modulation of the resist&#39;s contrast curve. Examples of suitable photo-acid generators include onium salts, sulfides, nitroaryl derivatives or aryl sulfates (for example,or tosylates). Photo-acid generators may include sulfide and onium type compounds such as diphenyl iodide hexafluorophosphate, diphenyl iodide hexafluoroarsenate, diphenyl iodide hexafluoroantimonate, diphenyl p-methoxyphenyl triflate, diphenyl p-tert-butylphenyl triflate, diphenyl p-isobutylphenyl triflate, diphenyl p-tert-butylphenyl triflate, triphenylsulfonium hexafluorophosphate, triphenylsulfonium hexafluoroarsenate, triphenylsulfonium hexafluoroantimonate, triphenylsulfonium triflate, triphenylsulfonium nonafluorobutylsulfonate, diphenyliodonium heptadecafluorooctylsulphonate, and dibutylnaphthysulfonium triflate. 
     Examples of photo-base generators may include nitrocarbamate, quaternary ammonium dithiocarbamate, other generators listed, for example, in Prog. Polym.-Sci., volume 21, 1–45 (1996 Elsevier Science, Ltd.) and a polymeric photo-base generator containing oxime-urethane groups described in Chae, Kyo Ho et al. “Image recording material based on the polymeric photobase generator containing oxime-urethane groups,” Macromol. Rapid Commun. 200, 21, 1007–1012. Photo-acid generators are commands added to photoresist to catalyze decomposition upon radiation exposure. Thus, the addition of a photo-base generator to a portion of a resist that relies on a photoacid generator to enable decomposition may disable decomposition in the affected portion of the resist. 
     Examples of thermal base generators include o-{(.beta.-(dimethylamino)ethyl)aminocarbonyl}benzoic acid, o-{(.gamma.-(dimethylamino)propyl)aminocarbonyl}benzoic acid, 2,5-bis{(.beta.-(dimethylamino)ethyl)aminocarbonyl} terephthalic acid, 2,5-bis{(.gamma.-(dimethylamino)propyl) aminocarbonyl}terephthalic acid, 2,4-bis{(.beta.-(dimethylamino)ethyl)aminocarbonyl}isophthalic acid, and 2,4-bis{(.gamma.-(dimethylamino)propyl) aminocarbonyl} isophthalic acid. The synthesis of these thermal base generators is described, for example, in U.S. Pat. No. 6,258,506. 
     General classes of bases commonly used in photolithography, also referred to as quenchers, may be used in lieu of using thermal or photo-base generators including pyrrolidinones, piperidines, trialkylamines, morpholines, pyridines, and anilines. Specific examples include N-methyl pyrrolidinone, 1-piperidineethanol, 1,8-diazabicyclo[5.4.0]undec-7-ene, trioctylamine, N-isobutylmorpholine, and N,N-dimethylaminopyridine. Additionally, polymeric derivatives of these classes of materials may also be used as quenchers, such as poly(pyrrole), polyaniline, poly(2-vinylpyridine), or poly(N-vinyl-2-pyrrolidone). 
     The amount of photo-acid generator, photo-base generator, thermal base generator, or thermal acid generator may be determined using empirical techniques or simulation to give the desired profile when used in conjunction with a specific photoresist in a specific processing method having specified processing times and temperatures. Concentrations for the additives may range from 0.01 to ten times the concentration of the PAG in the photoresist, with a more preferable concentration of 0.02 to two times the concentration of the PAG in some embodiments. In general, a proton donor may be used to neutralize the proximate proton acceptor based moieties in the adjacent layer of photoresist  16 , and vice versa. 
     In accordance with one embodiment of the present invention, the release layer  14  and the photoresist  16  may be subjected to a bake at from 50 to 150° C. with the most advantageous processing temperatures between 80 and 130° C. The bake time for the release layer  14  and the photoresist  16  may be from 15 to 300 seconds, with the most advantageous times between 45 and 120 seconds. 
     The bake time and temperature may be sufficient to cause diffusions of acid or base, acid or base analog or generator into the photoresist  16  from the appropriate substrate, such as the release layer  14 . These baking times and temperatures, in effect, allow the acid, base, analog, or generator to diffuse into the photoresist  16  where it can be effective in disabling the photoresist. 
     In the embodiments described above, generally a portion of the photoresist is poisoned from an underlying layer. This generally means that where the photoresist uses a photo-acid generator, for example, a photo-base generator is caused to diffuse into the photoresist from the underlying layer and vice versa. In general then, when acids are used to break down the photoresist, bases can be used that are also activated by the same mechanism to prevent the breakdown of the photoresist. 
     In other embodiments of the present invention, the underlying layer may supply, for example by diffusion, a similar active component to that used in the photoresist to initiate photoresist breakdown. As a result, the component which diffuses into the photoresist from the underlying layer may be utilized to reduce footing, scumming, and the like. Conversely, undercutting may be reduced by poisoning the photoresist layer from the underlying layer. Thus, in some embodiments it may be desirable to deactivate the component of the photoresist that is responsible for breaking down the photoresist and in other instances it may be desirable to enhance that activity. 
     As shown in  FIG. 5 , a photoresist  16  that uses a photo-acid generator to initiate breakdown of the photoresist may be supplied with additional photo-acid generator from an underlying layer  14   a . Upon heating the photoresist, the photo-acid generator may diffuse from the layer  14   a  into the photoresist to enhance interfacial breakdown of the photoresist upon radiation exposure, for example, to reduce footing in the trench  18   a  at  26 , as shown in  FIG. 6 . 
     Conversely, in a photoresist that uses a photo-base generator, a photo-base generator may diffuse in from an underlying layer to further enhance the breakdown of the photoresist, particularly at the photoresist/layer interface. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.