Abstract:
A method for moving resist stripper across the surface of a semiconductor substrate. The method includes applying a wet chemical resist stripper, such as an organic or oxidizing wet chemical resist stripper, to at least a portion of a photomask positioned over the semiconductor substrate. A carrier fluid, such as a gas, is then directed toward the semiconductor substrate so as to move the resist stripper across the substrate. The carrier fluid may be directed toward the substrate as the resist stripper is being applied thereto or following application of the resist stripper. A system for effecting the method is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/639,550, filed Aug. 16, 2000, now U.S. Pat. No. 6,440,871, issued Aug. 27, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods for stripping photoresist from a semiconductor device structure and, in particular, to methods for continuously moving resist stripper across the surface of a semiconductor device structure. More particularly, the present invention relates to resist stripper application methods that include exposing the resist stripper to a gas, to move the resist stripper across the semiconductor device structure, to thin the resist stripper, or to otherwise increase the rate at which the resist stripper removes photoresist from the semiconductor device structure. 
     2. Background of Related Art 
     In fabricating semiconductor devices, several material layers, including electrically conductive and insulative layers, are formed and patterned to build various structures upon an active surface of a semiconductor substrate, such as a wafer or other large-scale substrate formed from semiconductive material (e.g., silicon, gallium arsenide, or indium phosphide), thereby forming a semiconductor device structure. 
     The material layers formed over a semiconductor substrate are typically patterned by forming masks thereover. Photomasks are often employed. The formation of photomasks involves the use of a photoresist material that takes on a specific pattern as the photoresist material is exposed to radiation, such as one or more visible wavelengths of light, through a reticle. In this manner, the reticle and the radiation transmitted therethrough together define the specific pattern of the photoresist. The photoresist is then developed, or cured, so as to maintain the pattern and to form a photomask, which is commonly referred to in the art as a “photoresist” or simply as a “resist.” Once the photomask has been formed, one or more underlying material layers may be patterned through the photomask, such as by way of wet or dry etching processes. 
     After one or more layers underlying a photomask have been patterned through the photomask to form a semiconductor device structure, the photomask is typically removed. Various processes are known for removing photomasks. Typically, a thin layer of a resist stripper is applied to the semiconductor device structure, such as by spraying the resist stripper onto the semiconductor device structure. Alternatively, a semiconductor device structure bearing a photomask is immersed, or dipped, into a bath of wet chemical resist stripper. 
     One type of resist stripper that may be used to remove a photomask from a semiconductor device structure is a wet chemical resist stripper, such as an organic resist stripper (e.g., phenol-based and phenol-free organic strippers) or an oxidizing resist stripper (e.g., solutions of sulfuric acid (H 2 SO 4 ) and an oxidant, such as hydrogen peroxide (H 2 O 2 ) or ammonium persulfate). Wet chemical resist strippers typically remove, or dissolve, the photomask with selectivity over (i.e., at a faster rate than) the material of the structures and material layers that underlie and that may be exposed through the photomask or upon removal of the photomask material from the semiconductor device structure. Some such wet chemical resist strippers include one or more types of active chemicals that remove photomasks by reacting with the material or materials of the photomasks. Thus, the concentrations of active chemicals in these wet chemical resist strippers decrease over time, thereby reducing the effectiveness of these resist strippers. Moreover, as the concentrations of reaction products increase in locations where further stripping is desired, the rate at which further reactions between the resist stripper and the photoresist may occur and, thus, the rate at which the photoresist is removed from the semiconductor device structure, are reduced. 
     As another example, ozonated water may be used as a resist stripper to remove a photomask from a semiconductor substrate. Typically, the water is heated to enhance the ability of the ozone dissolved therein to remove a resist layer from a semiconductor substrate. The heated, ozonated water may be applied to the resist-covered substrate by spraying. As those of skill in the art are aware, ozone effervesces from water relatively quickly. Thus, ozonated water resist strippers lose their effectiveness over time. In addition, as with other types of resist strippers, the rates at which ozonated water resist strippers remove photoresists may be reduced as the concentrations of reaction products increase in the resist stripper. 
     Conventional processes for applying resist strippers to resist, such as spraying or immersion, do not facilitate continuous movement of the resist strippers across the semiconductor device structure following application and may, therefore, permit the resist strippers to sit, or stagnate, on the resist. Stagnation of resist strippers is somewhat undesirable, however, as the concentrations of reaction products may increase during stagnation and stagnation may, therefore, reduce the rate at which the resist strippers remove photomasks from semiconductor device structures. In addition, when a wet etchant is employed as the resist stripper, the active chemical reactant or reactants of the resist stripper may react with the photomask and, therefore, decrease in concentration. As a result, in a stagnant area, the rate at which such a wet etchant resist stripper removes the photomask and, thus, the ability of such a wet etchant resist stripper to remove the photomask, decreases over time. 
     In the ozonated water example of a resist stripper, when the resist stripper is at rest, ozone escapes from the water into the atmosphere over time. As the concentration of ozone in the ozonated water resist stripper decreases, the effectiveness of the resist stripper, as well as the rate at which a photomask is removed from a semiconductor substrate therewith, are reduced. 
     When conventional stripping methods are employed, ozonated water resist strippers typically remove hard-baked photoresist at a rate of about 4,000 Å per second to less than about 8,000 Å per second. 
     The art lacks teaching of methods for introducing one or more gases into or onto a resist stripper to maintain a desired rate for stripping resist from a semiconductor device structure, as well as stripping systems for effecting such methods. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention includes a method and system for stripping resists from semiconductor substrates while maintaining a desired rate of resist stripping. The method of the present invention includes applying a quantity of resist stripper onto a semiconductor device structure and directing at least one carrier fluid, such as a gas other than ozone, toward the resist stripper or forming at least one gas other than ozone in the resist stripper. 
     The resist stripper is applied to the semiconductor device structure in a manner that the resist stripper contacts at least a portion of a photomask, or resist, to be removed. For example, the resist stripper may be hot ozonated water that includes a concentration sufficient to remove the resist at a desired rate. Other types of resist strippers, such as wet chemical resist strippers, may also be used in accordance with teachings of the present invention. The resist stripper may be applied to at least a portion of the semiconductor device structure by spraying, in a stream, by dipping the semiconductor device structure in the resist stripper, or otherwise, as known in the art. 
     The resist stripper may be exposed to one or more gases or other carrier fluids prior to, during, or after application thereof onto a photoresist on the semiconductor device structure. The one or more other, non-ozone gases may thin the layer of resist stripper over the semiconductor device structure or move the resist stripper across a surface of the semiconductor device structure, both of which prevent the formation of or eliminate macroscopic drops of resist stripper on the semiconductor device structure. Alternatively, the one or more gases may be the product of one or more chemical reactions effected by or in the resist stripper, in which case the one or more gases are formed in the resist stripper. In any event, by exposing the resist stripper to one or more gases or other carrier fluids, the rate at which the resist stripper removes photoresist from the semiconductor device structure is increased. 
     As an example of the manner in which resist stripper may be moved across the surface of the semiconductor device structure, one or more gases under pressure, such as in a jet or stream of liquid or gas, may be directed at least partially across the surface of the semiconductor device structure so as to force resist stripper across the semiconductor device structure. This movement of resist stripper across the surface of the substrate prevents stagnation of the resist stripper and, consequently, prevents a reduction in the rate at which the resist stripper removes the photomask from the semiconductor substrate. 
     The one or more gases may be directed across the surface of the semiconductor device structure substantially simultaneously with the resist stripper, either by combining the gas or gases with the resist stripper or separately from the resist stripper. Alternatively, the gas or gases may be directed across the semiconductor device structure after the resist stripper has been applied to the semiconductor device structure. When the one or more gases are directed across the surface of the semiconductor device structure, the one or more gases force the resist stripper to move across the device structure. 
     The one or more gases may be directed across the surface of the semiconductor device structure from a central region thereof toward an outer periphery thereof. Alternatively, the one or more gases may be directed onto a surface of a semiconductor device structure near a peripheral edge thereof so as to move resist stripper across the semiconductor device structure. Application of one or more gases may also be effected in any alternative manner that facilitates the substantially continuous movement of resist stripper across the surface of the semiconductor device structure. 
     The one or more gases will preferably not react with (e.g., oxidize or otherwise react with or change the nature of) materials of structures or layers of the semiconductor device structure that are exposed through the photoresist or as the photoresist is removed from the semiconductor device structure. Gases that may be used in the method of the present invention include, without limitation, inert gases or gas mixtures, such as nitrogen or noble gases (e.g., argon), air, and gaseous hydrochloric acid. 
     The present invention also includes systems for applying resist stripper to semiconductor device structures in a manner that effects the inventive method. An example of such a system includes a source of resist stripper, a stripper application component for introducing resist stripper onto a surface of a semiconductor device structure, a gas source, and a gas application component for directing gas at least partially across the surface of the semiconductor device structure so as to move resist stripper across the surface. The stripper application and gas application components may be separate from one another or comprise the same component. 
     Other features and advantages of the present invention will become apparent to those of skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic representation of a semiconductor device structure including a photomask with resist stripper applied thereto; 
     FIG. 1B is a schematic representation illustrating another method of applying photoresist to a photomask on a semiconductor device structure; 
     FIGS. 2A and 2B are schematic representations illustrating movement of resist stripper across the surface of the semiconductor device structure by a gas under pressure; 
     FIG. 3 is a schematic representation of a variation of the use of a gas under pressure to move resist stripper across a semiconductor device structure when the method of the present invention is used; 
     FIG. 4 is a schematic representation of the use of gas bubbles in the resist stripper to effect movement of the resist stripper across a semiconductor wafer in accordance with teachings of the present invention; 
     FIG. 5 is a schematic representation illustrating the alternative, nonhorizontal orientation of a semiconductor device structure as a method according to the present invention is being effected; 
     FIG. 6 schematically illustrates rotation of a semiconductor device structure as a method according to the present invention is being effected; and 
     FIGS. 7A and 7B schematically depict systems for effecting methods that incorporate teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A schematically illustrates a semiconductor device structure  10 , in this case a semiconductor wafer, including a photomask  14  over an active surface  12  thereof. Photomask  14  is formed from polymerized, or consolidated, photoresist and may be either soft-baked or hard-baked, as known in the art. Resist stripper  16  preferably includes ozone (e.g., ozonated water), but may be any other type of resist stripper known and used in the art. Resist stripper  16  may be applied to photomask  14  by known processes, such as by spraying resist stripper  16  onto photomask  14 , as shown in FIG.  1 A. As an alternative, resist stripper  16  may be applied to photomask  14  by at least partially immersing photomask  14  in a quantity of resist stripper  16 , as illustrated in FIG.  1 B. In any event, resist stripper  16  forms a layer  18  over semiconductor device structure  10  and over any photomask  14  on active surface  12  of semiconductor device structure  10 . 
     In order to effect the method of the present invention, resist stripper  16  is exposed to one or more gases  20  or other, nongaseous carrier fluids, which effect movement of resist stripper  16  across semiconductor device structure  10  or thin layer  18  of resist stripper  16 , as shown in FIGS. 2A and 2B. Both of these effects of exposing resist stripper  16  to gas  20  facilitate the transfer of reaction products of the reaction between resist stripper  16  and the photoresist of photomask  14  away from the location where such a reaction is occurring. For example, as resist stripper  16  is moved laterally across semiconductor device structure  10 , fresh resist stripper  16  that includes little or no reaction products is continuously supplied to locations where resist stripper reacts with the photoresist of photomask  14 . As another example, by thinning layer  18  of resist stripper  16 , reaction products of the reaction between resist stripper  16  and the photoresist of photomask  14  may more readily escape through layer  18  than if layer  18  were thicker, thus permitting the action between resist stripper  16  and the photoresist of photomask  14  to occur at a faster rate than would be possible if higher concentrations of these reaction products were present at the locations where this reaction is occurring. Other methods of exposing resist stripper  16  to gas  20  in a manner that may increase the rate at which resist stripper  16  removes photoresist of photomask  14  from semiconductor device structure  10  are also within the scope of the present invention. 
     Turning again to FIGS. 2A and 2B, as an example of a way in which resist stripper  16  may be exposed to one or more gases  20 , a quantity of gas  20 , such as nitrogen, air, or gaseous hydrochloric acid, or another, nongaseous carrier fluid, is directed under pressure (either positive (FIG. 2A) or negative (FIG.  2 B)) at least partially across layer  18 . Gas  20  does not include ozone. By directing gas  20  at least partially across layer  18 , resist stripper  16  of layer  18  is moved across semiconductor device structure  10  in a plane substantially parallel to the plane of semiconductor device structure  10 , such as in the directions of arrows  22 . In this manner, products of the chemical reaction between the ozone of resist stripper  16  and the material or materials of photomask  14  are continuously moved away from photomask  14 , thereby facilitating subsequent reactions between the ozone of resist stripper  16  and the photoresist of photomask  14 , as well as the removal of photoresist from semiconductor device structure  10 , to proceed at a faster rate. In addition, by directing one or more gases  20  at least partially across layer  18 , the thickness of layer  18  may be reduced, which permits reactants of the reaction between the ozone of resist stripper  16  and the photoresist of photomask  14  to more readily pass through layer  18  and disperse. 
     As shown in FIGS. 2A and 3, gas  20  may be directed toward layer  18  in a pressurized jet  30 . Pressurized jet  30  may be directed toward layer  18  from substantially the same location as that from which resist stripper  16  is introduced over semiconductor device structure  10 , as shown in FIG. 3, or from a different location than that from which resist stripper  16  is directed over semiconductor device structure  10 , as depicted in FIG.  2 A. 
     With reference to FIG. 4, gas  20  may alternatively be directed across layer  18  of resist stripper  16  in the form of bubbles  32  on the surface of or residing within layer  18 . The movement of bubbles  32  across layer  18  may effect the substantially continuous movement of resist stripper  16  over semiconductor device structure  10 . As bubbles  32  move through layer  18 , products of the reaction between resist stripper  16  and the photoresist of photomask  14  may be carried by bubbles  32  away from the locations in which said photoresist removal reactions are occurring. In addition, bubbles  32  may move layer  18  across semiconductor device structure  10  as bubbles  32  flow therethrough. Both of these effects of bubbles  32  facilitate the passage of products of the reaction between resist stripper  16  and the photoresist of photomask  14  away from the locations in which this reaction is occurring, thereby increasing the overall reaction rate. 
     FIG. 5 depicts the nonhorizontal orientation of a semiconductor device structure  10 . When semiconductor device structure  10  is nonhorizontally oriented, gravity further facilitates thinning of layer  18  and movement of resist stripper  16  in layer  18  over semiconductor device structure  10 . 
     Preferably, semiconductor device structure  10  remains substantially stationary as the method of the present invention is being effected. Thus, existing automated wet bench equipment may be used to conduct the method of the present invention. 
     As another alternative, which is illustrated in FIG. 6, a semiconductor device structure  10  may be rotated in a plane thereof to further effect thinning of layer  18  and movement of resist stripper  16  in layer  18  over semiconductor device structure  10  by centrifugal force. Again, the additional thinning and movement of layer  18  provided by such rotation further accelerate the rate at which resist stripper  16  removes the material or materials of photomask  14  from semiconductor device structure  10 . 
     Gas  20  to which layer  18  is exposed may itself increase the rate with which resist stripper  16  removes the material or materials of photomask  14  from semiconductor device structure  10  or otherwise enhances the removal of photomask  14  by resist stripper  16 . Chemicals or chemical mixtures that form gas bubbles in layer  18  may also increase the rate at which the resist stripper  16  of layer  18  removes the photoresist of photomask  14  by action of the bubbles, as discussed previously herein with reference to FIG. 4, or by causing the formation of gaseous products as resist stripper  16  reacts with the photoresist, which products form and are carried away in bubbles. For example, hydrochloric acid decreases the pH of resist stripper  16  and increases the concentration of ozone in resist stripper  16 , while decreasing the concentration in resist stripper  16  of carbonic acid (H 2 CO 3 ), which is a product of the reaction between an ozonated resist stripper  16  and a photoresist. In addition, by adding hydrochloric acid to resist stripper  16 , the concentration of hydrogen ions in resist stripper  16  increases, which forces the carbonic acid in resist stripper  16  to be broken up into carbon dioxide (CO 2 ) gas and water (H 2 O) rather than into carbonate ions (CO 3 ) and hydrogen ions (H + ). As a result, more carbon dioxide, which is a gas, is formed. Carbon dioxide readily diffuses, effervesces, or bubbles out of resist stripper  16 , away from the reaction between resist stripper  16  and the photoresist of photomask  14 , and will, therefore, not increase in concentration in the presence of the reaction or decrease the rate at which this reaction occurs. Correspondingly, the formation of carbonic acid, which is an ion that tends to remain dissolved in resist stripper  16 , is reduced. Thus, carbonic acid concentrations will not increase as rapidly as if the hydrogen ions from hydrochloric acid were not present and, as a result, the rate at which resist stripper  16  reacts with the photoresist of photomask  14  is not significantly decreased. As directing hydrochloric acid toward layer  18  of resist stripper  16  increases the formation of carbon dioxide gas in resist stripper  16 , the direction of one or more gases  20  toward layer  18  may be effected by introducing either gaseous or liquid hydrochloric acid into layer  18 . In addition, when hydrochloric acid is used, the rate at which resist stripper  16  removes the material or materials of photomask  14  may be increased without actually moving layer  18  or thinning layer  18 . 
     FIG. 7A illustrates an exemplary resist stripping system  40  for effecting the stripping method of the present invention. Resist stripping system  40  includes a wafer support  42 , upon which a semiconductor device structure  10  having photoresist thereon is positioned. Wafer support  42  may be configured to orient a semiconductor device structure  10  positioned thereon nonhorizontally. Wafer support  42  may also be configured to rotate a semiconductor device structure  10  positioned thereon. A stripper applicator  44  of resist stripping system  40  obtains resist stripper  16  from a source  46  and applies resist stripper  16  to a photomask  14  on a semiconductor device structure  10  disposed on wafer support  42 . Resist stripping system  40  also includes a gas source  48 , from which gas  20  is supplied and which communicates with a gas output element  50  that directs gas  20  toward wafer support  42  so as to effect the direction of gas  20  at least partially toward resist stripper  16  or to otherwise expose resist stripper  16  to gas  20 . 
     Alternatively, as shown in FIG. 7B, a resist stripping system  40 ′ incorporating teachings of the present invention may include a chemical output element  54  to which directs a chemical  56 , such as hydrochloric acid, from a chemical source  52  toward wafer support  42  so as to expose resist stripper  16  on a semiconductor device structure  10  positioned on wafer support  42  to chemical  56  and to induce the formation of gas bubbles  32  (FIG. 4) in resist stripper  16 . 
     When methods incorporating teachings of the present invention are employed, the rates with which these ozonated strippers  16  remove hard-baked photoresists are at least about 8,000 angstroms per second up to about 12,000 angstroms per second and greater, as compared with conventional resist stripping methods employing ozonated resist strippers, which remove hard-baked photoresists at much slower rates. While the methods of the present invention are particularly useful for increasing the rates with which ozonated resist strippers  16  remove photoresists, the teachings of the present invention may also be employed to increase the rates with which other types of resist strippers remove photoresists. 
     Although the foregoing description contains many specifics, these should not be construed as limiting the scope of the present invention, but merely as providing illustrations of some of the presently preferred embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the spirit or scope of the present invention. Features from different embodiments may be employed in combination. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions and modifications to the invention as disclosed herein which fall within the meaning and scope of the claims are to be embraced thereby.