Abstract:
A megasonic immersion lithography exposure apparatus includes an optical transfer chamber for containing an exposure liquid, at least one megasonic plate operably engaging said optical transfer chamber for propagating sonic waves through the exposure liquid, and an optical system provided adjacent to said optical transfer chamber for projecting light through a mask and said exposure liquid and onto a wafer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Division of U.S. patent application Ser. No. 11/237,651 filed Sep. 29, 2005, which is a Continuation of International Application No. PCT/US2004/010309 filed Apr. 2, 2004, which claims the benefit of U.S. Provisional Patent Application No. 60/462,556 filed Apr. 11, 2003 and U.S. Provisional Patent Application No. 60/482,913 filed Jun. 27, 2003. The disclosures of these applications are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     This invention relates to an immersion lithography system and more particularly to methods, as well as systems, for cleaning up the optical element that contacts and absorbs water in the process of immersion lithography. 
     Immersion lithography systems, such as disclosed in W099/49504, which is herein incorporated by reference for describing the general background of the technology as well as some general considerations related thereto, are adapted to supply a liquid into the space between a workpiece such as a wafer and the last-stage optical element of an optical system for projecting the image of a reticle onto the workpiece. The liquid thus supplied improves the performance of the optical system and the quality of the exposure. 
     The liquid to be supplied may be water for light with wavelength of 193 nm although different liquids may be necessary for light with other wavelengths. Because the last-stage optical element of the optical system is exposed to the liquid, there is a possibility that some of the liquid may be absorbed. This possibility is particularly high if the last-stage optical element of the optical system is a lens because calcium fluoride is a common lens material for lithography systems while it is a hygroscopic material that is capable of absorbing water from the surrounding environment. 
     The absorbed water may cause several problems. First, it may degrade the image projected by the lens by changing the refractive properties of the lens or by causing the lens to swell to thereby change the geometry of the lens. Second, it may cause long-term degradation of the lens due to chemical effects. 
     Conventional air-immersion exposure lithography systems require the optical elements to be made detachable for maintenance work such as when they are cleaned. It is a cumbersome and time-consuming operation, however, to remove an optical element and to reset it after it is cleaned or to exchange an optical element for a new one. 
     It is therefore an object of this invention to provide systems and methods for periodically removing the water from the lens such that the amount of absorbed water will not reach a critical level and the degradation of the image and the long-term damage to the lens can be prevented. 
     It is another object of the invention to provide systems and methods for making the maintenance of the optical element of an immersion lithography apparatus easier and thereby improve the useful lifetime of the optical element. 
     SUMMARY 
     Immersion lithography apparatus of this invention may include a reticle stage arranged to retain a reticle, a working stage arranged to retain a workpiece, an optical system including an illumination source and an optical element opposite the workpiece for projecting an image pattern of the reticle onto the workpiece by radiation from the illumination source while defining a gap between the optical element and the workpiece, and a fluid-supplying device for providing an immersion liquid between and contacting both the optical element and the workpiece during an immersion lithography process. The apparatus also includes a cleaning device to clean the optical element. The term “cleaning” will be used throughout this disclosure to mean both removing immersion liquid that has been absorbed into the optical element and removing dirt, debris, salts and the like from the optical element. 
     Many different kinds of cleaning devices may be used within the scope of this invention. For example, the cleaning device may use a cleaning liquid having affinity to the immersion liquid to be contacted with the optical element. If the immersion liquid is water, ethanol may serve as the cleaning liquid. As another example, the cleaning device may include a heat-generating device for heating the optical element and/or a vacuum device for generating a vacuum condition on the optical element. 
     Ultrasonic vibrations may be used for removing the absorbed liquid. An ultrasonic vibrator such as a piezoelectric transducer may be attached to the housing for the optical element or placed opposite the optical element such that the vibrations may be transmitted to the optical element through a liquid maintained in the gap. 
     Alternatively, cavitating bubbles may be used for the removal of the absorbed liquid. A pad with fins may be used to generate cavitating bubbles in a liquid maintained in the gap between the pad and the optical element. 
     According to another embodiment of the invention, the nozzles through which the immersion liquid is supplied into the gap between the workpiece and the optical element may be used to alternately supply a cleaning liquid by providing a flow route-switching device such as a switch valve. 
     With a system and method of this invention, the cleaning procedure becomes significantly easier and faster because there is no need to detach the optical element to be cleaned and the cleaning process improves the useful lifetime of the optical element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in conjunction with the following drawings of exemplary embodiments in which like reference numerals designate like elements, and in which: 
         FIG. 1  is a schematic cross-sectional view of an immersion lithography apparatus to which methods and systems of this invention may be applied; 
         FIG. 2  is a process flow diagram illustrating an exemplary process by which semiconductor devices are fabricated using the apparatus shown in  FIG. 1  according to the invention; 
         FIG. 3  is a flowchart of the wafer processing step shown in  FIG. 2  in the case of fabricating semiconductor devices according to the invention; 
         FIG. 4  is a schematic drawing showing a side view of a portion of the immersion lithography apparatus of  FIG. 1 ; 
         FIG. 5  is a schematic side view of a portion of another immersion lithography apparatus having an ultrasonic transducer attached so as to serve as its cleaning device; 
         FIG. 6  is a schematic side view of a portion of another immersion lithography apparatus having a piezoelectric cleaning device below its optical system; 
         FIG. 7  is a schematic diagonal view of an example of a piezoelectric device; 
         FIG. 8  is a schematic side view of a portion of another immersion lithography apparatus having two mutually attached piezoelectric planar members as the cleaning device; 
         FIG. 9  is a schematic side view of a portion of another immersion lithography apparatus having a bubble-generating pad as the cleaning device; and 
         FIG. 10  is a schematic side view of a portion of another immersion lithography apparatus having a switching device incorporated in the fluid-supplying device. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows an immersion lithography apparatus  100  to which cleaning methods and systems of this invention may be applied. 
     As shown in  FIG. 1 , the immersion lithography apparatus  100  comprises an illuminator optical unit  1  including a light source such as an excimer laser unit, an optical integrator (or homogenizer) and a lens and serving to emit pulsed ultraviolet light IL with wavelength 248 nm to be made incident to a pattern on a reticle R. The pattern on the reticle R is projected onto a wafer W coated with a photoresist at a specified magnification (such as ¼ or ⅕) through a telecentric light projection unit PL. The pulsed light IL may alternatively be ArF excimer laser light with wavelength 193 nm, F 2  laser light with wavelength 157 nm or the i-line of a mercury lamp with wavelength 365 nm. In what follows, the coordinate system with X-, Y- and Z-axes as shown in  FIG. 1  is referenced to explain the directions in describing the structure and functions of the lithography apparatus  100 . For the convenience of disclosure and description, the light projection unit PL is illustrated in  FIG. 1  only by way of its last-stage optical element (such as a lens)  4  disposed opposite to the wafer W and a cylindrical housing  3  containing the rest of its components. 
     The reticle R is supported on a reticle stage RST incorporating a mechanism for moving the reticle R in the X-direction, the Y-direction and the rotary direction around the Z-axis. The two-dimensional position and orientation of the reticle R on the reticle stage RST are detected by a laser interferometer (not shown) in real time and the positioning of the reticle R is affected by a main control unit  14  on the basis of the detection thus made. 
     The wafer W is held by a wafer holder (not shown) on a Z-stage  9  for controlling the focusing position (along the Z-axis) and the tilting angle of the wafer W. The Z-stage  9  is affixed to an XY-stage  10  adapted to move in the XY-plane substantially parallel to the image-forming surface of the light projection unit PL. The XY-stage  10  is set on a base  11 . Thus, the Z-stage  9  serves to match the wafer surface with the image surface of the light projection unit PL by adjusting the focusing position (along the Z-axis) and the tilting angle of the wafer W by the auto-focusing and auto-leveling method, and the XY-stage  10  serves to adjust the position of the wafer W in the X-direction and the Y-direction. 
     The two-dimensional position and orientation of the Z-stage  9  (and hence also of the wafer W) are monitored in real time by another laser interferometer  13  with reference to a mobile mirror  12  affixed to the Z-stage  9 . Control data based on the results of this monitoring are transmitted from the main control unit  14  to a stage-driving unit  15  adapted to control the motions of the Z-stage  9  and the XY-stage  10  according to the received control data. At the time of an exposure, the projection light is made to sequentially move from one to another of different exposure positions on the wafer W according to the pattern on the reticle R in a step-and-repeat routine or in a step-and-scan routine. 
     The lithography apparatus  100  described with reference to  FIG. 1  is an immersion lithography apparatus and is hence adapted to have a liquid (or the “immersion liquid”)  7  of a specified kind such as water filling the space (the “gap”) between the surface of the wafer W and the lower surface of the last-stage optical element  4  of the light projection unit PL at least while the pattern image of the reticle R is being projected onto the wafer W. 
     The last-stage optical element  4  of the light projection unit PL may be detachably affixed to the cylindrical housing  3  and is designed such that the liquid  7  will contact only the last-stage optical element  4  and not the cylindrical housing  3  because the housing  3  typically comprises a metallic material and is likely to become corroded. 
     The liquid  7  is supplied from a liquid supply unit  5  that may comprise a tank, a pressure pump and a temperature regulator (not individually shown) to the space above the wafer W under a temperature-regulated condition and is collected by a liquid recovery unit  6 . The temperature of the liquid  7  is regulated to be approximately the same as the temperature inside the chamber in which the lithography apparatus  100  itself is disposed. Numeral  21  indicates supply nozzles through which the liquid  7  is supplied from the supply unit  5 . Numeral  23  indicates recovery nozzles through which the liquid  7  is collected into the recovery unit  6 . The structure described above with reference to  FIG. 1  is not intended to limit the scope of the immersion lithography apparatus to which the cleaning methods and devices of the invention are applicable. In other words, the cleaning methods and devices of the invention are applicable to immersion lithography apparatus of many different kinds. In particular, the numbers and arrangements of the supply and recovery nozzles  21  and  23  around the light projection unit PL may be designed in a variety of ways for establishing a smooth flow and quick recovery of the immersion liquid  7 . 
     A method embodying this invention of removing the portion of the liquid  7  such as water absorbed by the last-stage optical element  4  made of a hygroscopic material, as well as dirt, debris, etc., is explained next with reference to  FIGS. 1 and 4 . After the wafer W is exposed with light from the illuminator optical unit  1  through the light projection unit PL in the presence of the liquid  7  as shown in  FIG. 1 , the liquid  7  is removed from underneath the light projection unit PL and a cleaning device  30  is brought into contact with the last-stage optical element  4  as shown in  FIG. 4 . In the case of a portable kind, as shown in  FIG. 4 , the cleaning device  30  may be placed on the Z-stage  9  or the aforementioned wafer holder thereon, as shown in  FIG. 4 , in place of the wafer W. 
     Different types and kinds of cleaning devices  30  can be used for the purpose of this invention. As a first example, the cleaning device  30  may be a container containing a liquid (“cleaning liquid”) having a strong affinity to the immersion liquid  7  that is absorbed by the optical element  4 . If the immersion liquid  7  is water, the cleaning device  30  may contain ethanol because ethanol has a strong affinity to water. Any cleaning liquid may be used provided it has a sufficiently strong affinity to the liquid to be removed and does not damage the optical element  4  or its coating. The bottom surface of the optical element  4  is soaked in the cleaning liquid for a period of time sufficiently long to reduce the level of the absorbed immersion liquid. The cleaning device  30  is removed thereafter and the optical element  4  is ready to be exposed to the liquid  7  again. 
     As another example, the cleaning device  30  may contain a heat-generating device and/or a vacuum device (not separately shown). The combination of heat and vacuum on the surface of the optical element  4  causes the absorbed liquid to undergo a phase change into vapor, or to evaporate from the surface. The reduction in liquid density on the surface of the optical element  4  draws the liquid  7  that is absorbed more deeply in the element  4  to the surface of the optical element  4 . 
       FIG. 5  shows a third example in which use is made of an ultrasonic transducer (or ultrasonic vibrator)  32  attached to the housing  3  of the light projection unit PL. As the ultrasonic transducer  32  (such as a piezoelectric transducer) is activated, pressure waves are generated and propagated, serving to clean the surface of the optical element  4 . 
     During the cleaning operation in  FIG. 5 , the gap adjacent to the optical element  4  is filled with the immersion liquid  7 . In this case, the supply and recovery nozzles can continue to supply and collect the immersion liquid  7 , or the supply and recovery nozzles can stop supplying and collecting the immersion liquid  7 . Also during the cleaning operation, the optical element  4  can face a surface of wafer W, a surface of the Z-stage  9 , or a surface of another assembly. 
       FIG. 6  is a fourth example using a vibratory tool  34  placed below the optical element  4  to be cleaned. The tool  34  may be shaped like the wafer W with thickness more or less equal to that of the wafer W, or about 0.5-1 mm, and may be made entirely of a piezoelectric material such that its thickness will fluctuate when activated. As the tool  34  is placed below the optical element  4 , like the wafer W as shown in  FIG. 1 , and the gap between the optical element  4  and the tool  34  is filled with the liquid  7 , pressure waves are generated in the immersion liquid  7  to clean the optical element. 
     During the cleaning operation of  FIG. 6 , the gap adjacent to the optical element  4  is filled with the immersion liquid  7 . In this case, the supply and recovery nozzles can continue to supply and collect the immersion liquid, or the supply and recovery nozzles can stop supplying and collecting the immersion liquid  7 . In another example, the vibrator tool  34  may be a ultrasonic transducer attached to the wafer holder on a Z-stage  9 , or another assembly. 
       FIG. 7  shows another tool  36 , structured alternatively, having a plurality of piezoelectric transducers  38  supported by a planar supporting member  39 . 
       FIG. 8  shows still another example of a cleaning device having two planar members  40  of a piezoelectric material attached in a face-to-face relationship and adapted to oscillate parallel to each other and out of phase by 180° with respect to each other. As a result, these members  40 , attached to each other, will vibrate in the transverse directions, as shown in  FIG. 8  in a very exaggerated manner. The vibration has node points at constant intervals where the members  40  are not displaced. The members  40  are supported at these node points on a supporting member  41 . As voltages are applied to these members  40  so as to cause the vibrations in the mode described above, ultrasonic pressure waves are thereby generated and propagated through the liquid  7 , and the optical element  4  is cleaned, as desired. 
       FIG. 9  shows still another example of a cleaning device that cleans the optical element  4  by creating cavitating bubbles. Cavitating bubbles trapped and energized by ultrasound are high-temperature, high-pressure microreactors and intense energy released by the implosive compression of the bubbles is believed to rip molecules apart. The example shown in  FIG. 9  is characterized as comprising a pad  43  with fins protruding upward and rapidly moved horizontally as shown by an arrow below the optical element  4  with a bubble-generating liquid  17  filling the gap in between (structure for moving the pad  43  not being shown). As the pad  43  is thus moved, the fins serve to stir the liquid  17  and to generate cavitating bubbles that in turn serve to clean the optical element. 
       FIG. 10  shows a different approach to the problem of cleaning the last-stage optical element  4  by applying a cleaning liquid on its bottom surface by using the same source nozzles  21  used for supplying the immersion liquid  7 . For this purpose, a switch valve  25  is inserted between the supply nozzle  21  and the liquid unit  5  such that the immersion liquid  7  and the cleaning liquid can be supplied selectively through the supply nozzle  21 . 
     It is again noted that the cleaning methods and systems according to this invention are applicable to immersion lithography apparatus of different kinds and types, for example, having different numbers of source nozzles. A switch valve as described above need not necessarily be provided to each of the source nozzles but may be provided to a group of the source nozzles. 
     The wafer W itself or a pad  18  of a suitable kind may be placed below the optical element  4  to provide a suitable gap in between when the cleaning liquid is thus supplied through the supply nozzles  21 . This embodiment of the invention is advantageous because the same nozzles already present for supplying the immersion liquid can be utilized for the cleaning process. 
     Although various methods have been separately described above, they may be used in combinations, although that is not separately illustrated in the drawings. For example, the pad  43  with fins shown in  FIG. 9  may be used instead of the pad  18  of  FIG. 10 . In other words, the examples described above are not intended to limit the scope of the invention, and many modifications and variations are possible within the scope of this invention. For example, a polishing pad similar to one used in chemical mechanical polishing may be used for this purpose. The cleanup procedure shown in  FIGS. 4-10  may be carried out with ultraviolet light. The light may irradiate the optical element  4 . The light may be normal exposure light from the illuminator optical unit  1  or some other light of an appropriate wavelength for the purpose of the cleanup. In another example, the ultraviolet light for the purpose of the cleanup may be used without the cleanup procedure shown in  FIGS. 4-10 , and may be used under a condition in which the gap adjacent to the optical element  4  is filled with the immersion liquid  7  from the liquid supply unit  5 . All such modifications and variations that may be apparent to a person skilled in the art are intended to be within the scope of this invention. 
     Any of the above described cleaning methods for removing immersion fluid absorbed by the last-stage optical element also may be used to remove salts, deposits, dirt and debris that may have accumulated. The term cleaning therefore refers to both of these phenomena. 
       FIG. 2  is referenced next to describe a process for fabricating a semiconductor device by using an immersion lithography apparatus incorporating a cleaning device embodying this invention. In step  301  the device&#39;s function and performance characteristics are designed. Next, in step  302 , a mask (reticle) having a pattern is designed according to the previous designing step, and in a parallel step  303 , a wafer is made from a silicon material. The mask pattern designed in step  302  is exposed onto the wafer from step  303  in step  304  by a photolithography system such as the systems described above. In step  305  the semiconductor device is assembled (including the dicing process, bonding process and packaging process), then finally the device is inspected in step  306 . 
       FIG. 3  illustrates a detailed flowchart example of the above-mentioned step  304  in the case of fabricating semiconductor devices. In step  311  (oxidation step), the wafer surface is oxidized. In step  312  (CVD step), an insulation film is formed on the wafer surface. In step  313  (electrode formation step), electrodes are formed on the wafer by vapor deposition. In step  314  (ion implantation step), ions are implanted in the wafer. The aforementioned steps  311 - 314  form the preprocessing steps for wafers during wafer processing, and selection is made at each step according to processing requirements. 
     At each stage of wafer processing, when the above-mentioned preprocessing steps have been completed, the following post-processing steps are implemented. During post-processing, initially, in step  315  (photoresist formation step), photoresist is applied to a wafer. Next, in step  316  (exposure step), the above-mentioned exposure device is used to transfer the circuit pattern of a mask (reticle) onto a wafer. Then, in step  317  (developing step), the exposed wafer is developed, and in step  318  (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step  319  (photoresist removal step), unnecessary photoresist remaining after etching is removed. Multiple circuit patterns are formed by repetition of these preprocessing and post-processing steps. 
     While a lithography system of this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and various substitute equivalents which fall within the scope of this invention. There are many alternative ways of implementing the methods and apparatus of the invention.