Abstract:
An exposure apparatus includes a projection optical system for projecting a pattern on a mask onto an object to be exposed, a final surface of the projection optical system and a surface of the object being immersed in a fluid, and a plane-parallel plate arranged between the projection optical system and the object, wherein the plane-parallel plate includes a first surface and a second surface that serves as a back surface of the first surface and opposes to the object, both the first and second surfaces being immersed in the fluid.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to an exposure apparatus and method used to fabricate various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pickup devices such as CCDs, as well as fine patterns used for micromechanics, and more particularly to an immersion type exposure method and apparatus for immersing the final surface of the projection optical system and the surface of the object in the fluid and exposing the object through the fluid.  
         [0002]     The projection exposure apparatus employed to manufacture a semiconductor device, a liquid crystal display device, etc. is required to use an increased numerical aperture (“NA”) of a projection optical system and a shortened wavelength of an exposure light. A KrF stepper (with λ=about 248 nm) uses a NA of 0.65 in 2000. The projection exposure apparatus is being shifted from the stepper to a scanner to which a high NA projection lens and KrF and ArF (λ=about 193 nm) lasers are applicable. The current scanners use a projection lens having a NA up to 0.8 with a light source, such as KrF and ArF lasers. A prospective scanner that will appear in the market within several years will use a projection optical system having a NA of 0.85 and a light source of KrF, ArF and even F 2  (λ=about 157 nm) lasers. Discussions have started about whether it is possible to develop a projection optical system having a NA up to 0.95 of a dry system that uses the gas for a medium between lenses as an ultimate projection optical system for the photolithography.  
         [0003]     An antireflection coating for an optical element progresses as the projection optical system progresses when the projection exposure apparatus appeared with a light source of KrF and ArF excimer lasers in 1990, the coating material for the antireflection coating became restricted, such as SiO 2  and MgF 2  as a low refractive index material having a refractive index between 1.45 and 1.55, and Al 2 O 3  and LaF 3  as a middle refractive index material having a refractive index between 1.65 and 1.75. This limit increases the design difficulty and requires a reduction of the transmission loss due to absorptions in the coating, contaminated substrate, and scattering in the coating layer, which has conventionally been negligible.  
         [0004]     In this background, the immersion exposure has attracted attention as one measure to improve the resolution while using the ArF and F 2  lasers (see, for example, Japanese Patent Application, Publication No. 10-303114). The immersion exposure promotes the higher numerical aperture (“NA”) by replacing a medium (typically air) at the wafer side of the projection exposure with the fluid The projection exposure apparatus has an NA of n·sin θ where n is a refractive index of the medium, and the NA increases when the medium that has a refractive index higher than the air&#39;s refractive index, i.e., n&gt;1. For example, when the fluid is the pure water (with a refractive index n=1.33), and the maximum incident angle of the light that images on the wafer is the same between the dry system and the wet system, the wet system&#39;s resolving power improves by 1.33 times. In other words, the NA in the wet system corresponds to 1.33 times as large as the NA of the dry system. If the apparatus&#39;s limit of the incident angle of the light that images on the wafer is 70°, the NA of the dry system has a limit of 0.94 whereas the NA of the wet system has a limit of 1.25 (when converted into the dry system), providing a projection optical system with a high NA. When the projection lens&#39;s final surface is contaminated in the immersion projection optical system, the immersed optical element should be easily replaced. Therefore, Japanese Patent Application, Publication No. 10-303114 proposes a structure that provides the projection lens&#39;s final surface with a plane-parallel plate.  
         [0005]     As described by this inventor in Japanese Patent Application No. 2003-135578, the P-polarized light reflectance of the antireflection coating in the high NA projection optical system remarkably increases, as shown in  FIG. 7 , when the antireflection coating receives the light from the air layer and exceeds the Brewster angle that is determined by the refractive index of the antireflection coating&#39;s final layer (at the air side). In general, it is usual to use a low refractive index material for the antireflection coating&#39;s final layer that contacts the air so as to design the reflectance to be reduced in the wide incident-angle range. Even when the basic coating design changes, as shown in  FIG. 8 , the reflectance becomes similar values as the incident angle increases if the material of the final layer that contacts the air is the same. Although the phase change between the P-polarized light and the S-polarized light is negligible in a range below the Brewster angle, their transmission phase changes become large once the angle exceeds the Brewster angle and the influence to the aberration of the projection optical system cannot be negligible.  
         [0006]     Therefore, the antireflection coating having a limited usable coating material for the F 2 , ArF and KrF lasers disadvantageously deteriorates both the transmittance and the imaging performance of the projection optical system once the NA of 0.85 or 58° of the light exceeds the Brewster angle. It is also necessary to consider the limits of the antireflection coating in the projection optical system of the wet system. While the immersion exposure apparatus disclosed in Japanese Patent Application, Publication No. 10-303114 uses the plane-parallel plate for the final optical element in the projection optical system and facilitates the replacement, an immersion structure of the plane-parallel plate at only the wafer side poses a problem of the total reflection at the interface between the plane-parallel plate and the air layer at the side of the projection optical system when the NA of the projection optical system exceeds 1.0.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     Accordingly, it is an exemplary object of the present invention to provide an exposure method and apparatus, which maintains the high transmittance and high imaging performance using a NA of 0.85 or greater.  
         [0008]     An exposure method according to one aspect of the present invention includes a projection optical system for projecting a pattern on a mask onto an object to be exposed, a final surface of the projection optical system and a surface of the object being immersed in a fluid, and a plane-parallel plate arranged between the projection optical system and the object, wherein the plane-parallel plate includes a first surface and a second Surface that serves as a back surface of the first surface and opposes to the object, both the first and second surfaces being immersed in the fluid.  
         [0009]     An exposure method according to another aspect of the present invention for immerses, in a fluid, a surface of an object to be exposed, and a final surface of a projection optical system, and for projecting a pattern on a mask onto the object via the projection optical system, includes the steps of immersing, in the fluid, a first surface of a plane-parallel plate arranged between the projection optical system and the object, immersing, in the fluid, a second surface of the plane-parallel plate which serves as a back surface of the first surface and opposes to the object, and projecting the pattern on the object via the plane-parallel plate.  
         [0010]     A device manufacturing method according to another aspect of the present invention includes the steps of exposing an object using the above exposure apparatus, and developing an object that has been exposed. Claims for a device fabricating method for performing operations similar to that of the above exposure apparatus cover devices as intermediate and final products. Such devices include semiconductor chips like an LSI and VLSI, CCDs, LCDs, magnetic sensors, thin film magnetic heads, and the like.  
         [0011]     Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic block diagram of an exposure apparatus according to one embodiment of the present invention.  
         [0013]      FIG. 2  is a schematic enlarged section of an immersed portion of a projection optical system shown in  FIG. 1 .  
         [0014]      FIG. 3  is the reflectance of an antireflection coating for the immersion system at the maximum incident angle.  
         [0015]      FIG. 4  is a partial explanatory view of an inventive immersion, high-NA projection optical system.  
         [0016]      FIG. 5  is a flowchart for explaining a device manufacturing method using the inventive exposure apparatus.  
         [0017]      FIG. 6  is a detailed flowchart of a step  4  in  FIG. 5 .  
         [0018]      FIG. 7  shows a relationship between reflectance changes and the Brewster angle to the incident angle upon the antireflection coating.  
         [0019]      FIG. 8  shows the reflectance changes relative to the incident angle. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     Referring now to  FIGS. 1 and 2 , a description will be given of an exposure apparatus  1  according to one embodiment according to the present invention. Here,  FIG. 1  is a schematic block diagram of a projection exposure apparatus (scanner)  1 . In  FIG. 1, 10  denotes a light source, such as an excimer laser, another continuously oscillating laser, and a Hg lamp.  20  denotes an illumination optical system.  30  denotes a reticle (or a mask) placed on a reticle stage  32 .  50  denotes a projection optical system that transfers a circuit pattern on the reticle  30  onto a resist applied wafer  70  placed on a wafer stage  72 . The projection optical system  50  has an aperture stop  52  inside it, and projects the diffracted light within a predetermined aperture, from the circuit pattern on the reticle  30  onto the resist applied wafer  70  at a reduced size. A plane-parallel plate  60  is arranged between the projection optical system  50  and the wafer  70  so that the plane-parallel plate  60  can be replaced when contaminated. This plane-parallel plate  60  is made of a quartz or calcium fluoride substrate. When the quartz substrate is used, the material deteriorates due to the laser irradiation and thus a structure preferably facilitates the replacement of the substrate. When the calcium fluoride substrate is used, the coating is preferably applied on the substrate surface to prevent the deliquescence. Two sheet glasses  40  and  42  are located below the reticle, and are used for aberrational adjustments. A total thickness of the two corrects the spherical aberration. An inclination of the two together corrects the on-axis astigmatism. The two sheet glasses angled in a wedge shape correct the on-axis coma.  
         [0021]     In order to implement the wet type of projection optical system  50  having the NA of 1.0 or greater (when converted into the dry system), this embodiment immerses, in the fluid, the plane-parallel plate  60  at its both sides of the wafer  70  and the projection optical system  50  (or surfaces  62  and  64  in  FIG. 2 , which will be described later).  
         [0022]     Referring now to  FIG. 2 , a more detailed description will be given of the immersed portion of the projection optical system  50 . Here,  FIG. 2  is a schematic enlarged section of the immersed portion of the projection optical system  50 .  54  denotes the final lens in the projection optical system  50 , and the plane-parallel plate  60  is arranged between the final lens  54  and the wafer  70 . A medium M 1  fills a space between the wafer  70  and the plane-parallel plate  60 . A medium M 2  fills a space between the plane-parallel plate  60  and the final lens  54 . A medium M 3  fills a space between the final lens  54  and a next lens (not shown).  
         [0023]     The necessary condition to implement the wet system of the projection optical system having the NA of 1.0 or greater (when converted into the dry system) is that the media M 1  and M 2  that sandwich the plane-parallel plate  60  are made of the fluid. The sufficient condition is that the incident and exit angles of the light upon and from the optical element in the projection optical system maintained in a range below the Brewster angle due to the performance limits of the antireflection coating, as described in Japanese Patent Application No. 2003-135578. When the light proceeds from the medium M 1  to medium M 2 , the Brewster angle θbs (bs means Brewster) generally satisfies the following equation, where n 1  is a refractive index of the medium M 1  and n 2  is a refractive index of the medium M 2 : 
 
θ bs=arc  tan( n 2/ n 1)  (1) 
 
         [0024]     Here, the antireflection coating is applied on the plane-parallel plate  60 . When M 2  is the air (n 1 =1.0) and the final layer of the antireflection coating at the medium M 2  side is SiO 2  (n 2 =1.56), θbs (dry system) becomes 57°. While the refractive index of the antireflection coating&#39;s final layer slightly differs according to wavelengths and materials, the refractive indexes for the KrF, ArF and F 2  lasers are almost the same and the Brewster angle becomes approximately equal to the maximum light angle at the NA of 0.85. In  FIG. 2 , in order for θ 2  to be smaller than θbs (dry system) in  FIG. 2 , the following equation should be met where the medium M 1  is the pure water (having a refractive index of 1.33): 
 
θ1&lt; arc  sin((sin 57°)/1.33)=39°  (2) 
 
         [0025]     When θ 1  in the immersion system is converted into 1.33·sin θ 1  in the dry system, NA becomes almost 0.85. In other words, in the wet system of the projection optical system having the NA between 0.85 and 1.0 where only the medium M 1  is the fluid and the medium M 2  is gas, the transmittance attenuation can occur because the light incident angle exceeds the Brewster angle at the interface between the plane-parallel plate  60  and the medium M 2  (gas) As a solution for this problem, this embodiment uses the fluid (pure water) for the medium M 2 . A single layer coating is sufficient for the antireflection coating for the immersion system and the refractive index of the antireflection coating is preferably between the refractive index of the pure water and that of the glass material of the lens. This is because the glass material of the lens has a refractive index between about 1.5 and 1.6, and a difference of the refractive index between the pure water and the glass material of the lens is small, such as about 0.2 and 0.3. One example of this material is MgF 2 , and the Brewster angle in the immersion system is Gbs (wet system) of 46.5° where the refractive index of the medium M 2  is n 2 =1.4. Therefore, the following equation is met: 
 
θ1=θ2&lt; θbs  (wet system)=46.5°  (3) 
 
         [0026]     When θ1 in the immersion system is converted into 1.33·sin θ1 in the dry system, it corresponds to NA of 0.96. In the wet system of projection optical system having the NA up to 0.96, the light incident angle and exit angle never exceed the Brewster angle only if both sides of the plane-parallel plate  60  or the surfaces  62  and  64  are immersed in the fluid. The surfaces  62  and  64  have a front and back relationship, and the surface  64  opposes to the wafer  70 .  
         [0027]     On the other hand, although the mere configuration that makes the media M 2  and M 1  to be the fluid could cause the light incident angle and exit angle to exceed the Brewster angle at the NA of 0.96 or greater, the instant inventor has discovered that the optimization of the antireflection coating for the immersion system can solve this problem.  
         [0028]      FIG. 3  plots the reflectance of the antireflection coating for the immersion system relative to the abscissa axis NA (that is converted into the dry system). In  FIG. 3 , the plots shown by a broken line sets such a coating thickness that the reflectance becomes minimum in response to the perpendicularly incident light. The plots shown by a solid line sets such a coating thickness that the reflectance minimizes for the light having NA=1.25. It is understood that the plots of the wet system shown by the broken line more moderately increase the reflectance after the angle exceeds the Brewster angle (about NA=0.96) than that of the dry system shown in  FIG. 8  (in which the Brewster angle is about 58°. It is also understood that even the antireflection coating that minimizes the reflectance to the light having the NA of 1.25 can reduce the reflectance below 1% at the NA up to 1.0 in the wet system as shown by the solid line in  FIG. 3 .  
         [0029]     A refractive-index difference between the antireflection coating&#39;s final layer and the immersion medium in the wet system is thus smaller than half that in the dry system. When the limit of the dry system is NA of 0.85, NA of 1.28 or lower is viable in the wet system. Therefore, even in the wet system of projection optical system having the NA up to 1.28 or the apparatus&#39;s limit NA, a problem of the transmittance attenuation does not occur even when the light incident and exist angles increase, only if the both sides of the plane-parallel pate  60  are immersed in the fluid, and the antireflection coating of the plane-parallel plate is optimized.  
         [0030]     Turning back to  FIG. 2 , the final lens  54  in the projection optical system  50  has front and back surfaces r 1  and r 2  and a normal is shown by an alternate long and short dash line. An illustrated arrow indicates a ray having the maximum aperture (or NA) that can pass in the projection optical system  50 . The ray having the maximum aperture has incident angles “a” and “c” on the wafer  70  and the final lens  54 &#39;s r 1  surface, and an exit angle “b” on the final lens  54 &#39;s r 2  surface. The lens  54  includes at least one meniscus lens that satisfies: a&gt;39° (that corresponds to Na of 0.85 when converted into the dry system), c&lt;b≦Brewster angle determined by the antireflection coating&#39;s final layer (at the fluid side) on the surface r 2 , and c&lt;Brewster angle determined by the antireflection coating&#39;s final layer (at the fluid or gas side) on the surface r 1 .  
         [0031]     Although the reflectance of the antireflection coating in the immersion system is lower than that in the dry system, the coating that is designed such that the reflectance is minimum to the perpendicularly incident light shown by the broken line has a much better characteristic than that shown by the solid line, as shown in  FIG. 3 . Therefore, only the plane-parallel plate  60  that is minimum necessary preferably has the surface on which the incident and exit angles exceed the Brewster angle.  
         [0032]     Whether the pure water (or the immersion system) or the air (the dry system) fills the space above the final lens is determined as follows: The fluid is set to the medium (the wet system) in case of Gbs (dry system)&lt;θN, and the air is set to the medium (dry system) in case of θN&lt;θbs (dry system), where θN is the light incident angle upon the N-th lens (when the N-th lens is the final lens, then “c” in  FIG. 5 ).  
         [0033]     Referring now to  FIG. 4 , a description will be given of another embodiment of the present invention. The temperature coefficient of the refractive index of the air is −9×10 −7 /° C. whereas that of the pure water is −8×10 −5 /° C. It is thus known that the pure water has the temperature sensitivity more than 100 times as high as that of the air. It is preferable that the immersed portion is minimum necessary. This embodiment limits the immersed portion only to both sides of the plane-parallel plate  60 . Therefore, the medium M 4  is made of the gas. The surface r 1  is a plane in this embodiment, and the projection optical system  50  has a planoconvex lens as the final lens  54 A. The meniscus final lens  54  shown in  FIG. 2  causes the temperature rise in the medium M 2  as the fluid due to the exposure light, and the fluid that has a higher temperature than the periphery would gather at the vertex part on the surface r 1  and could deteriorate the imaging performance. The planoconvex final lens  54 A does not cause gathering of the fluid that has a higher temperature than the periphery. The planoconvex final lens  54 A satisfies a&gt;39° (corresponding to the NA of 0.85 when converted in the dry system), c&lt;b, and c&lt;Brewster angle determined by the antireflection coating&#39;s final layer (at the gas side) on the surface r 1 . Thus, this embodiment limits the surface on which the incident and exit angles exceed the Brewster angle to both plane surfaces of the plane-parallel plate  60  and the plane of the planoconvex lens  54 A, and the immersed portion to both sides of both sides of the plane-parallel plate  60 . In  FIG. 2 , the medium M 1  that contacts the wafer  70  is configured not to mix the medium M 2  or M 3 . A circulation of each medium is controlled as a closed circulation system so as to prevent erosions of the fluids and gathering of the dissolved impurities. In order to restrain the temperature rise in each medium due to the exposure light, it is preferable that each circulation system is temperature-controlled.  
         [0034]     Conceivably, the immersed portion of the medium M 1  contacting the wafer  70  that is driven at a high speed is subject to the temperature rise due to the absorption of the exposure light and the heat associated with driving. The influence of the refractive-index changes of the fluid to the imaging performance is considered to be proportional to the temperature rise of the fluid and the optical-path length in the fluid. Therefore, a structure preferably maintains the temperature rise of the fluid as low as possible, and to maintain a relationship of d 1 &lt;d 2  where d 1  is a distance of the medium M 1  and d 2  is a distance of the medium M 2 . This is because the wafer side is more subject to the heat from a driving mechanism for the stage, etc. For example, the projection optical system  50  usually has a back focus of about 20 mm. Moreover, the plane-parallel plate needs a thickness of at least 5 mm in order to eliminate the influence of the surface deflection due to its own weight. Then, optimal d 1  and d 2  are d 2 =10 mm or greater d 1 =5 mm or smaller.  
         [0035]     Referring to  FIGS. 5 and 6 , a description will now be given of an embodiment of a device fabricating method using the above exposure apparatus.  FIG. 5  is a flowchart for explaining a fabrication of devices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs, etc.). Here, a description will be given of a fabrication of a semiconductor chip as an example. Step  1  (circuit design) designs a semiconductor device circuit. Step  2  (mask fabrication) forms a mask having a designed circuit pattern. Step  3  (wafer making) manufactures a wafer using materials such as silicon. Step  4  (wafer process), which is referred to as a pretreatment, forms actual circuitry on the wafer through photolithography using the mask and wafer. Step  5  (assembly), which is also referred to as a post-treatment, forms into a semiconductor chip the wafer formed in Step  4  and includes an assembly step (e.g., dicing, bonding), a packaging step (chip sealing), and the like. Step  6  (inspection) performs various tests for the semiconductor device made in Step  5 , such as a validity test and a durability test. Through these steps, a semiconductor device is finished and shipped (Step  7 ).  
         [0036]      FIG. 6  is a detailed flowchart of the wafer process in Step  4  shown in  FIG. 5 . Step  11  (oxidation) oxidizes the wafer&#39;s surface. Step.  12  (CVD) forms an insulating film on the wafer&#39;s surface. Step  13  (electrode formation) forms electrodes on the wafer by vapor disposition and the like. Step  14  (ion implantation) implants ions into the wafer. Step  15  (resist process) applies a photosensitive material onto the wafer. Step  16  (exposure) uses the exposure apparatus to expose a circuit pattern on the mask onto the wafer. Step  17  (development) develops the exposed wafer. Step  18  (etching) etches parts other than a developed resist image. Step  19  (resist stripping) removes disused resist after etching. These steps are repeated, and multilayer circuit patterns are formed on the wafer. The device fabrication method of this embodiment may manufacture higher quality devices than the conventional one. Thus, the device fabrication method using the exposure apparatus, and the devices as finished goods also constitute one aspect of the present invention. The present invention covers devices as intermediate and final products. Such devices include semiconductor chips like an LSI and VLSI, CCDs, LCDs, magnetic sensors, thin film magnetic heads, and the like.  
         [0037]     Further, the present invention is not limited to these preferred embodiments, and various variations and modifications may be made without departing from the scope of the present invention. For example, while the instant embodiment forms the plane-parallel plate  60  as a separate member from the projection optical system  50 , the plane-parallel plate  60  may be part of the projection optical system  50 .  
         [0038]     The immersion exposure apparatus of this embodiment which has an NA of 0.85 or greater provides the projection optical system with the high transmittance from the center to the periphery on the pupil, improves not only the light intensity efficiency of a modified illumination in which the diffracted light enters the periphery on the pupil in the projection optical system, but also the contrast and symmetry of a resolution limit pattern. The projection optical system of a wet type having the NA of 0.85 or greater can manufacture, at a high yield, semiconductor devices that have a resolution limit pattern of the photolithography.  
         [0039]     The present invention can provide an exposure method and apparatus, which maintains the high transmittance and high imaging performance as well as having the NA of 0.85 or greater.  
         [0040]     This application claims foreign priority benefits based on Japanese Patent Application No. 2003-432769, filed on Dec. 26, 2003, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.