Abstract:
Methods of fabricating a photomask, methods of treating a chemically amplified resist-coated photomask blank, a photomask blank resulting from the methods, and systems for fabricating a photomask are provided. The method is useful for recovering the exposure sensitivity of a chemically amplified resist disposed on a photomask blank from a post-coat delay effect.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 10/391,708, filed Mar. 19, 2003. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to semiconductor processing methods, and more particularly to processes for fabricating a photomask that can be used in a photolithography process. 
     BACKGROUND OF THE INVENTION 
     Photolithography using a patterned masking layer is commonly used for formation of electronic components such as semiconductor integrated circuits. A pattern is transferred to a reactive photoresist layer on the semiconductor substrate by exposure of the patterned masking layer overlying the substrate to light or other energy source (such as ultraviolet light or electron beam radiation) which passes through open areas of the patterned masking layer. 
     The masking layer can be, for example a photomask or reticle, prepared from a photomask blank that typically comprises a thin layer of an opaque or nontransparent material, typically a metal-containing material such as a chromium-, molybdenum- or tungsten-containing material, deposited on a transparent plate of glass or quartz by sputtering or vacuum evaporation, for example. A pattern can be formed on the photomask blank structure by standard photolithography processes to selectively remove portions of the nontransparent layer (e.g., opaque chromium film) and create open areas. The pattern can be etched, for example, by forming a photoresist material over the nontransparent layer, forming a pattern in the photoresist material with an electron beam or laser beam, and transferring the pattern to the underlying nontransparent material with an etch that removes exposed portions of the nontransparent material. 
     With the fast pace of reduction of the micro-lithography design rule and the significant increase of data volume, the requirement on critical dimension (“CD”) control and throughput on a photomask manufacture is becoming more and more challenging. Currently, photomasks are patterned with either 50 kV e-beam or laser pattern generation tools, and chemically amplified photoresists (“CA resists”) that are sensitive to shorter wavelengths of light are being used to reduce the beam exposure (dose) and settling times, thereby improving the throughput of photomask pattern exposure. CA resists are more sensitive to radiation than non-chemically amplified resists such as ZEP-7000, thus needing a smaller dose of radiation (e.g., 8-9 μcoulomb versus about 25 μcoulomb) and a shorter development time. 
     Typical CA resists are composed of an acid generator that produces acid upon exposure to radiation (e.g., ultraviolet (UV) light, laser light, X-rays, electron beam), and acid-labile compounds or polymers that have changed solubility in a developer solvent through acid-catalyzed reactions. The irradiated area forms an image in the CA resist, and an annealing step (post exposure bake, or PEB) is performed after exposure. The photoresist is then exposed to a developing solvent to remove portions of the resist, the underlying opaque chrome or other nontransparent layer is etched, and the CA resist is removed. The patterned photomask is used to transfer a pattern to a photoresist layer disposed on a wafer during a fabrication step. The patterned photomask layer is exposed to blanket radiation that passes through open areas of the photomask layer onto the surface of the photoresist. The photoresist is then developed providing open areas exposing a material layer on the wafer. A fabrication step, for example, an ion implantation or an etch of the material layer, can be performed to form an underlying semiconductor device structure. 
     Ideally, it is advantageous to coat the photomask blank with a CA resist and immediately expose the resist on the blank. However, in practice, resist-coated blanks are often stored for several hours up to weeks before exposure. A difficulty with the use of CA resists is the change in stability and radiation dose sensitivity that occurs when a CA resist is coated on a substrate that is left standing inside or outside an irradiation apparatus. This effect is commonly known as “post coat delay” (PCD) effect. 
     Development of CA resists depends on the catalytic action of the acid generator, and if acid loss (neutralization) occurs, which decreases the radiation dose sensitivity of the resist, abnormalities will occur during pattern formation resulting in a significant difference between critical dimensions (CD) (i.e., minimum feature dimensions) of the patterned features compared to the required pattern dimensions. The acid loss is believed to be due to the presence of nitrogen-containing compounds such as atmospheric ammonia or ammonium ions, which neutralize the acid generator resulting in incomplete solubility of the resist, thus limiting shelf life of a photomask blank with an applied CA resist layer. 
     Typically, to compensate for the loss in sensitivity, the amount of decrease in dose sensitivity over time is estimated and the exposure process is adjusted accordingly. The relative dose sensitivity of the CA resist layer can be determined through experimentation and/or theoretical calculation. The difference in dose sensitivity may be a linear function of time difference between exposure of regions, a logarithmic difference, or related by some other mathematical correlation, depending on the type of radiation utilized, the type of resist utilized, and the tolerance for differences in minimum feature size between a first blank and a second blank. 
     Several methods have been used in an attempt to extend the shelf time or length of time a photomask substrate with a CA resist applied can be stored prior to exposure to the imaging/patterning radiation. Such methods have included the use of protective coatings and treatment with oxygen plasma to reduce the exposure of a CA resist to nitrogen-containing compounds. While use of these known methods produces some improvement over an untreated substrate, significant profile defects are still evident. 
     It would be desirable to develop a process for reducing the PCD effect on a chemically amplified (CA) resist coating disposed on a substrate such as a photomask blank. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods of fabricating a photomask, methods of treating a chemically amplified resist-coated photomask blank, a photomask blank resulting from the methods, and systems for fabricating a photomask. 
     In one aspect, the invention provides, in a method of fabricating a photomask, a method of recovering exposure sensitivity of a chemically amplified resist disposed on a photomask blank from a post-coat delay effect. In one embodiment, the method comprises heating the photomask blank to dehydrate the resist, and cooling the photomask blank in a nitrogen-purged environment to about room temperature, such that the resist has a level of exposure sensitivity approximate to the resist as when initially applied onto the photomask blank; and patterning an image into the photoresist. For the heating step, for example, the resist-coated photomask blank can be positioned in a chamber that is heated to up to about 80° C. above room temperature or, in a preferred embodiment, the photomask blank can be supported on a temperature-controllable substrate (e.g., hotplate) and heated to about 90-180° C. for up to about 30 minutes. In the cooling step, for example, the heated photomask blank can be positioned in a nitrogen-purged chamber on a chill plate and cooled to about room temperature for about 10-30 minutes or more. In a preferred cooling step, the heated photomask blank is positioned in a nitrogen-purged chamber at about room temperature and cooled over an extended time period of about 6-12 hours. 
     In another aspect, the invention provides a method of treating a chemically amplified resist-coated photomask blank. In one embodiment, the method comprises heating the photomask blank to dehydrate the chemically-amplified layer, prior to patterning the resist. The treated resist layer possesses a level of exposure sensitivity approximate to the level of the resist when initially coated onto the photomask blank. 
     In another aspect, the invention provides a system for fabricating a photomask. In one embodiment, the system comprises a unit for treating a chemically amplified resist layer disposed on a photomask blank prior to patterning the resist layer, the treating unit comprising: means for supporting the photomask blank within a chamber of the treating unit, and a heating element (means). In one embodiment of the system, the heating means comprises an apparatus operable to heat the chamber to an elevated temperature. In another embodiment, the system comprises a heating element incorporated into the supporting means, which preferably comprises a temperature controllable plate having concentric heaters disposed on a plate with the temperature of each of the heaters being controlled independently. 
     The system can further comprise a unit for cooling the resist layer on the photomask blank, the cooling unit comprising: means for supporting the photomask blank within a chamber of the cooling unit; means for purging nitrogen from the chamber; and a cooling element (means). In one embodiment, the cooling means comprises an apparatus operable to cool the chamber to at least about room temperature. In another embodiment, the system comprises a cooling element incorporated into the supporting means, for example, a chill plate. 
     In an embodiment of a system according to the invention, a treating unit is provided that comprises a support platform for the photomask blank that incorporates a temperature adjustable element that can provide both heating and cooling; and a mechanism for purging nitrogen from the chamber. In another embodiment, the treating unit comprises a support for the photomask blank that incorporates an adjustable heating element; a device for maintaining the temperature of the chamber at about room temperature; and a mechanism for purging nitrogen from the chamber. The temperature controlled chamber can be used to maintain the temperature of the chamber and a N 2 -purged environment. The system can also be structured with a unit for heating the photomask blank and a separate unit for chilling the photomask blank before patterning. 
     In another aspect, the invention provides a photomask blank. In one embodiment, the photomask blank comprises an unpatterned, chemically-amplified resist layer coated on the photomask blank for an extended time and thus subjected to post coat delay effect, the resist layer being heat-treated and substantially dehydrated such that the resist has a level of exposure sensitivity approximate to the resist as when initially applied onto the photomask blank. 
     The present invention solves the problem of maintaining a uniform and reproducible pattern critical dimension in a CA resist from the time of application of the resist onto a substrate to and during a writing process for imaging a photomask. The process of the invention eliminates, or substantially reduces, the sensitivity of a CA photoresist to the environment and maintains the pattern critical dimension of the resist to extend the shelf life of the CA resist during storage in handling which may be for a time period up to weeks, and during direct write imaging of the photoresist by either optical or e-beam radiation. The process of the invention significantly improves the lithographic performance in terms of critical dimension and integrity of the photoresist. The invention advantageously eliminates unnecessary work to calibrate the post coat delay (PCD) effect, and increases the accuracy and reproducibility of CD performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described below with reference to the following accompanying drawings, which are for illustrative purposes only. Throughout the following views, the reference numerals will be used in the drawings, and the same reference numerals will be used throughout the several views and in the description to indicate same or like parts. 
         FIGS. 1-3  are diagrammatic cross-sectional views of a fragment of a photomask blank at sequential processing steps showing treatment of a chemically-amplified resist layer according to an embodiment of the method of the invention.  FIG. 1  illustrates a resist-coated photomask blank at a preliminary processing step.  FIG. 2  depicts a heat treatment to dehydrate the resist layer on the photomask blank.  FIG. 3  depicts a processing step to pattern the resist layer. 
         FIG. 4  is a top plan view of a prior art heating device. 
         FIG. 5  is a block diagram of an embodiment of a system according to the invention incorporating a unit for treating a chemically amplified resist layer disposed on a photomask blank. 
         FIG. 6A-6B  are depictions of embodiments of a treating unit that can be utilized in the system of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described generally with reference to the drawings for the purpose of illustrating the present preferred embodiments only and not for purposes of limiting the same. The figures illustrate processing steps for fabricating photomask in accordance with the present invention. It should be readily apparent that the processing steps are only a portion of the entire fabrication process. 
     In the current application, the terms “semiconductive substrate”, “semiconductor substrate”, “wafer fragment” or “wafer” will be understood to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive materials such as a semiconductor wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. 
     The terms “photomask blank” or “patterned masking layer,” as used herein, refers to either a photomask or reticle as those terms are known and used in the art. 
     The present invention addresses and solves problems of deterioration of radiation dose sensitivity of a chemically amplified (CA) photoresist layer coated on a photomask blank. 
       FIGS. 1-3  illustrate steps in an embodiment of a method of the invention. Referring to  FIG. 1 , a portion of an embodiment of a resist-coated binary photomask blank  10  is shown in cross-sectional view. The photomask blank  10  comprises a substrate  12  and a nontransparent film  14  formed on the surface  16  of the substrate  12 , and a CA resist layer  18  formed on the nontransparent film layer  14 . The substrate  12  typically comprises a quartz plate as the base of the mask blank, but can comprise a fluorinated quartz, borosilicate glass, soda lime glass, for example, or other substantially transparent material. The nontransparent film  14  typically comprises an opaque chrome-containing layer. The layer of opaque chromium is generally about 50-120 nm thick, with commercially available chromium thicknesses of about 50, 79, and 100 nm. The opaque chromium layer can be formed on the substrate by known techniques such as by reactive sputtering or vacuum evaporation. In the illustrated embodiment, an opaque chrome layer is disposed on a quartz plate, but it is understood that the nontransparent layer can be separated from the substrate by various intervening materials. 
     The CA photoresist layer  18  can be formed over the nontransparent layer  14  by suitable means known and used in the art, for example, by spinning the CA resist material onto the surface of the nontransparent layer. In the illustrated embodiment, the photoresist layer  18  is disposed on the nontransparent layer  14  but can be separated from the nontransparent layer by various materials, for example, an anti-reflective coating (ARC) (not shown), among others. Exemplary ARC materials include chrome oxynitride, titanium nitride, silicon nitride, or molybdenum silicide, or other material suitable for use to reduce light reflection from the substrate surface into the resist during photolithography. Inorganic or organic ARCs can be used. A suitable means of forming the ARC is to sputter it onto the surface of the layer. 
     The relative time difference between application of the CA resist layer  18  on the photomask blank and exposure of the CA resist to radiation development can be up to 24 hours or more to up to several weeks, for example, with a resulting deterioration in dose sensitivity of the CA resist. 
     According to the invention, prior to loading the resist-coated photomask blank  10  onto a pattern generation tool, a process step is conducted to recover or restore the dose sensitivity of the CA resist to about the dose sensitivity level of the CA resist as initially coated on the blank. It has been found that heating of the CA resist layer on the photomask blank prior to a radiation exposure step dehydrates the resist layer and eliminates the need to compensate for the PCD effect and the deterioration of the dose sensitivity of the resist material, which would otherwise require increasing the radiation dosage level during pattern generation. The heating step, depicted in  FIG. 2 , thus improves the reliability of the resist. 
     In one embodiment, the heating step comprises placing the CA resist-coated photomask blank  10  into a nitrogen-purged environment (e.g., container) at an elevated temperature effective to heat (arrows  20 ) the CA resist layer  18  and dehydrate the resist. Generally, the temperature within the nitrogen-purged environment is maintained at about 1° C. to about 80° C. above room temperature (i.e., 20° C.), or about 21° C. to about 100° C., with a preferred temperature of about 50° C. The resist-coated blank  10  can be maintained in the heated nitrogen-purged environment to dehydrate the CA layer, which can range from about 180 seconds up to about 180 minutes, for example, depending on the time period from when the resist was initially applied to the photomask blank, the thickness of the resist layer, the resist used, among other factors. The CA resist-coated blank can be stored in the heated, nitrogen-purged container for an extended period of time until a subsequent patterning step. 
     In a preferred embodiment, the heating step of the invention comprises a bake treatment of the CA resist-coated photomask blank  10  to dehydrate the CA resist layer  18  prior to exposing the CA resist layer to patterning radiation. The baking step ( FIG. 2 ) comprises exposing the resist-coated photomask blank  10  to a temperature of about 90° C. to about 180° C., typically about 120° C. to about 140° C., for a time period of about 30 seconds to about 30 minutes, preferably about 5 to about 10 minutes. The bake step can be conducted in either a N 2 -purged or atmospheric environment. 
     The baking step can be conducted using the same equipment as utilized in a post-exposure bake (PEB) technique, which commonly employs a temperature-controlled plate (e.g., hotplate). A preferred bake unit is described, for example, in U.S. Pat. No. 6,441,351 (Hayasaki), the disclosure of which is incorporated by reference herein, and illustrated in  FIG. 4 . As shown, an exemplary bake unit  22  comprises three concentric heaters  24  provided in a heat equalizing plate  26 . The temperature of each of the heaters  24  is controlled independently by an embedded thermocouple to provide a homogeneous bake treatment of the CA resist layer. A heat treatment device having a single ring heater as the heat source (not shown) can also be used. 
     The subsequent writing or patterning step typically takes about 3-20 hours and precise pattern placement on the CA resist layer  18  is a critical consideration in preparing the photomask. Normally, the actual alignment of the pattern on the CA resist layer is offset to some extent from the set data points for pattern placement, commonly known as a “registration offset.” The thermal effect from a heated photomask blank can interfere with pattern placement accuracy and increase the registration offset. 
     To reduce such thermal effects, the photomask blank can be cooled, preferably to about room temperature (i.e., about 20-25° C.). The photomask blank can be cooled, for example, by transferring the heated photomask blank to a chill plate that is set at about room temperature to cool the photomask blank for about 10-30 minutes or more. In a preferred cooling step, the photomask blank is maintained in a nitrogen-purged environment (e.g., chamber, container) at about room temperature for an extended time period to acclimate the photomask blank to about room temperature, which is generally about 6 hours to up to about 12 hours. Cooling of the resist layer helps prevent distortion of the etch pattern and loss of image quality, and helps ensure that the pattern is properly transferred to the resist. 
     Referring to  FIG. 3 , the photomask blank  10  is then patterned by exposure of the CA resist to radiation in a direct write process that produces an image in the CA resist, for example, using laser-produced ultraviolet (UV) radiation, including deep ultraviolet (DUV) radiation, or electron beam (e-beam) radiation. Exposure of the CA photoresist to radiation will cause release of a catalyst (typically acid) within a region of the resist, which will subsequently catalyze chemical reactions to change the solubility of the region in a developer solvent relative to the solubility of an unexposed region. 
     After exposure of the CA resist to e-beam radiation, for example, the photomask blank is transported to a bake unit and “baked” at a suitable temperature to enhance (i.e. speed up) the chemical reaction within the CA resist so that the image will be transferred throughout the entire thickness of the CA resist and the pattern can be subsequently properly developed. The bake treatment is typically referred to as a “Post Exposure Bake” (PEB). 
     Subsequent to the bake step, the CA resist layer can be patterned and developed using standard lithography techniques to form openings  28  in the resist layer  18 . Typically the CA resist layer is exposed to a developing solvent applied over the surface of the resist, for example, by a spinning technique. In the use of a positive CA resist, the radiation exposed areas of the resist are removed to produce open spaces  28  leaving non-irradiated areas in place over the nontransparent layer  14  (e.g., opaque chrome layer). In the use of a negative CA resist, the unexposed areas of the resist are removed, leaving the radiation exposed areas in place. A pattern transfer from the CA  18  resist into the underlying nontransparent layer  14  is then performed, typically using an anisotropic plasma dry etching technique, to create openings in the nontransparent layer that expose the substrate. The CA resist is then stripped away using known techniques in the art. 
     The resulting photomask is used to transfer a pattern to a photoresist layer disposed on a wafer during a fabrication step. The photomask is exposed to blanket radiation that passes through the open areas onto the surface of the photoresist. The photoresist is then developed providing open areas exposing the underlying material layer on the wafer, and an ion implantation or an etch of the material layer can be performed, for example, to form an underlying semiconductor device structure. The photomask can be utilized in the photolithography of a semiconductor device such as a logic device, microprocessor, DRAM, SRAM, and the like. 
     Subsequent processing may then be conducted on the semiconductor device structure, as known in the art. 
       FIG. 5  is a diagram of an embodiment of a system  30  that can be utilized in implementing the method of the invention. The system  30  can include a processing unit  32  for fabricating the photomask blank; a treating unit  34  (chamber, etc.) comprising a support  36  for the photomask blank, a heating element  38 , a chill or cooling element  40 , and a nitrogen-purging apparatus  42 ; a processing unit  44  for patterning and developing the photoresist layer; and a conveyor mechanism  46  for conducting the photomask blank through the system. The various units can be electrically coupled to a microprocessor  48 , which may be programmed to carry out particular functions as is known in the art. For example, a bake unit such as a hotplate within a heating chamber (or chill plate within a cooling chamber) can be connected to a controller such as a computer or other processor to control the temperature of the plate, etc. 
     In one embodiment, depicted in  FIG. 6A , the treating unit  34  is structured with a temperature controlled plate support  36  which temperature can be elevated or lowered to function as a heating element  38  and a cooling element  40 , and a device  42  to purge nitrogen from the chamber  50 . In a preferred embodiment, as depicted in  FIG. 6B , the treating unit  34 ′ can be structured with a bake unit in the form of a hotplate  38 ′ as a support  36 ′ for the photomask blank in the chamber  50 ′, an interconnected chilling unit  40 ′ to cool the chamber  50 ′ to room temperature, and an interconnected device  42 ′ operable to provide a nitrogen-purged atmosphere in the chamber  50 ′. In a simple application, the room temperature controlled chamber can provide both cooling and a N 2 -purged environment. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.