Abstract:
Systems and methods control the fluid flow and pressure to provide stable conditions for immersion lithography. A fluid is provided in a space between a lens and a substrate during the immersion lithography process. Fluid is supplied to the space and is recovered from the space through a porous member in fluidic communication with the space. Maintaining the pressure in the porous member under the bubble point of the porous member can eliminate noise created by mixing air with the fluid during fluid recovery. The method can include drawing the fluid from the space via a recovery flow line through a porous member, and maintaining a pressure of the fluid in the porous member below a bubble point of the porous member during drawing of the fluid from the space.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a continuation of International Application No. PCT/US2004/022915 filed Jul. 16, 2004, which claims the benefit of U.S. Provisional Patent Application No. 60/500,312 filed Sep. 3, 2003, and U.S. Provisional Patent Application No. 60/541,329 filed Feb. 2, 2004. The disclosures of these applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The invention relates generally to systems and methods for providing fluid for immersion lithography and, more particularly, for controlling the fluid flow and pressure to provide stable conditions for immersion lithography. 
     An exposure apparatus is one type of precision assembly that is commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, an optical assembly, a wafer stage assembly that retains a semiconductor wafer, a measurement system, and a control system. The resist coated wafer is placed in the path of the radiation emanating from a patterned mask and exposed by the radiation. When the resist is developed, the mask pattern is transferred onto the wafer. In microscopy, extreme ultraviolet (EUV) radiation is transmitted through a thin specimen to a resist covered plate. When the resist is developed, a topographic shape relating to the specimen structure is left. 
     Immersion lithography is a technique that can enhance the resolution of projection lithography by permitting exposures with a numerical aperture (NA) greater than one, which is the theoretical maximum for conventional “dry” systems. By filling the space between the final optical element and the resist-coated target (i.e., wafer), immersion lithography permits exposure with light that would otherwise be totally internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion liquid (or of the resist or lens material, whichever is least) are possible. Liquid immersion also increases the wafer depth of focus, i.e., the tolerable error in the vertical position of the wafer, by the index of the immersion liquid compared to a dry system with the same numerical aperture. Immersion lithography thus has the potential to provide resolution enhancement equivalent to the shift from 248 to 193 nm. Unlike a shift in the exposure wavelength, however, the adoption of immersion would not require the development of new light sources, optical materials, or coatings, and should allow the use of the same or similar resists as conventional lithography at the same wavelength. In an immersion system where only the final optical element and its housing and the wafer (and perhaps the stage as well) are in contact with the immersion fluid, much of the technology and design developed for conventional tools in areas such as contamination control, carry over directly to immersion lithography. 
     One of the challenges of immersion lithography is to design a system for delivery and recovery of a fluid, such as water, between the final optical element and the wafer, so as to provide a stable condition for immersion lithography. 
     SUMMARY 
     Embodiments of the invention are directed to systems and methods of controlling the fluid flow and pressure to provide stable conditions for immersion lithography. A fluid is provided in a space between the lens and the substrate during the immersion lithography process. Fluid is supplied to the space and is recovered from the space through a porous member in fluidic communication with the space. Maintaining the pressure in the porous member under the bubble point of the porous member can eliminate noise created by mixing air with the fluid during fluid recovery. The bubble point is a characteristic of the porous member that depends on the size of the holes in the porous member (the largest hole) and the contact angle that the fluid forms with the porous member (as a parameter based on the property of the porous material and the property of the fluid). Because the bubble point is typically a very low pressure, the control of this low pressure becomes an important issue. 
     An aspect of the invention is directed to a method of recovering a fluid from a space between a lens and a substrate in an immersion lithography system. The method includes drawing the fluid from the space via a recovery flow line through a porous member and maintaining a pressure of the fluid in the porous member below a bubble point of the porous member during drawing of the fluid from the space. 
     In some embodiments, maintaining the pressure is accomplished by providing an overflow container kept at a preset pressure and directing the fluid drawn from the space through the porous member via the recovery flow line to the overflow container. Maintaining the pressure can further include siphoning the fluid from the overflow container to a collection tank. The fluid is siphoned down by gravity to the collection tank disposed below the overflow container. In other embodiments, maintaining the pressure includes providing a fluid level buffer, drawing the fluid from the space via a buffer flow line through the porous member to the fluid level buffer, sensing a pressure or a fluid level at the fluid level buffer, and controlling the fluid flow drawn from the space via the recovery flow line through the porous member based on the sensed pressure or fluid level at the fluid level buffer. Controlling the fluid flow can include controlling a variable valve disposed in the recovery flow line downstream of the porous member. In still other embodiments, maintaining the pressure includes providing a fluid level buffer, drawing the fluid from the space via a buffer flow line through the porous member to the fluid level buffer, sensing a pressure or a fluid level at the fluid level buffer, and controlling a vacuum pressure at an outlet of the recovery flow line through the porous member based on the sensed pressure or fluid level at the fluid level buffer. Controlling the vacuum pressure can include controlling a vacuum regulator in a collection tank at the outlet of the recovery flow line. 
     In accordance with another aspect of the invention, an apparatus for recovering a fluid from a space between a lens and a substrate in an immersion lithography system includes an inner part that includes a lens opening to accommodate a portion of the lens and to position the lens apart from the substrate separated by the space to receive a fluid in the space between the lens and the substrate. An outer part is disposed around the inner part, and includes a porous member fluidicly coupled with the space and with a fluid recovery outlet to draw fluid from the space via the porous member to the fluid recovery outlet. A pressure control system is fluidicly coupled with the porous member to maintain a pressure at the surface of the porous member below a bubble point of the porous member during drawing of the fluid from the space via the porous member. 
     In some embodiments, the pressure control system includes an overflow container fluidicly coupled with the porous member and a vacuum regulator that regulates a pressure in the overflow container. A collection tank is fluidicly coupled to and disposed below the overflow container. In other embodiments, the pressure control system includes a fluid level buffer fluidicly coupled with the porous member, a sensor that senses a pressure or a fluid level at the fluid level buffer and a controller that adjusts a flow rate of the fluid drawn from the space through the fluid recovery outlet, based on a sensor signal output from the sensor, to maintain a pressure at the surface of the porous member below a bubble point of the porous member during drawing of the fluid from the space via the porous member. The pressure control system can include a valve disposed downstream of the fluid recovery outlet, and the controller controls the valve to adjust the flow rate of the fluid drawn from the space through the fluid recovery outlet. In still other embodiments, the pressure control system includes a collection tank fluidicly coupled to the fluid recovery outlet and a controllable vacuum regulator that regulates a pressure in the collection tank. The controller controls the controllable vacuum regulator to adjust the flow rate of the fluid drawn from the space through the fluid recovery outlet to the collection tank by controlling the pressure in the collection tank. 
     In specific embodiments, the inner part is spaced from the outer part by an intermediate spacing. The inner part includes an inner cavity forming a part of the spacing between the lens and the substrate, and the inner part includes apertures disposed above the inner cavity for at least one of introducing fluid into and drawing fluid from the inner cavity. The inner part includes apertures disposed on opposite sides of the lens opening for introducing fluid into the inner cavity. The inner part includes a pair of buffer slots disposed on opposite sides of the lens opening in a direction of scan of the immersion lithography system. The inner part includes purge holes and each of the pair of buffer slots is fluidicly coupled to at least one of the purge holes. The porous member is selected from the group consisting of a mesh, a porous material, and a member having etched holes therein. 
     In accordance with another aspect of the invention, an apparatus includes an optical projection system having a last optical element and that projects an image onto a workpiece, and a stage that supports the workpiece adjacent to the optical projection system when the image is being projected onto the workpiece. A gap is provided between the last optical element and the workpiece and is filled with an immersion fluid. A porous material is positioned adjacent to the gap and recovers fluid exiting the gap. A control system maintains a pressure on the porous material. The pressure is at or below the bubble point of the porous material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in conjunction with the accompanying drawings of exemplary embodiments in which like reference numerals designate like elements and in which: 
         FIG. 1  is a simplified elevational view schematically illustrating an immersion lithography system according to an embodiment of the invention; 
         FIG. 2  is a perspective view of a nozzle for fluid delivery and recovery for immersion lithography according to one embodiment of the invention; 
         FIG. 3  is a simplified cross-sectional view of the nozzle of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of the inner part of the nozzle of  FIG. 2 ; 
         FIG. 5  is a simplified cross-sectional view of the nozzle according to another embodiment; 
         FIG. 6  is a simplified view schematically illustrating a pressure control system for fluid recovery in an immersion lithography system according to one embodiment of the invention; 
         FIG. 7  is a simplified view schematically illustrating a pressure control system for fluid recovery in an immersion lithography system according to another embodiment of the invention; 
         FIG. 8  is a simplified view schematically illustrating a pressure control system for fluid recovery in an immersion lithography system according to another embodiment of the invention; and 
         FIG. 9  is a simplified view schematically illustrating a pressure control system for fluid recovery in an immersion lithography system with water stagnation prevention according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows an immersion lithography system  10  including a reticle stage  12  on which a reticle is supported, a projection lens  14 , and a wafer  16  supported on a wafer stage  18 . An immersion apparatus  20 , which is sometimes referred to herein as a showerhead or a nozzle, is disposed around the final optical element  22  of the projection lens  14  to provide and recover a fluid, which may be a liquid such as water or a gas, between the final optical element  22  and the wafer  16 . In the present embodiment, the immersion lithography system  10  is a scanning lithography system in which the reticle and the wafer  16  are moved synchronously in respective scanning directions during a scanning exposure. 
       FIGS. 2 and 3  show the apparatus or nozzle  20  for delivery and recovery of the fluid between the final optical element  22  and the wafer  16  for immersion lithography.  FIG. 2  shows the bottom perspective view of the nozzle  20 , which includes an outer part  30  and an inner part  32 . The inner part  32  defines an inner cavity  34  to receive the fluid between the final optical element  22  and the wafer  16 . The inner part  32  includes apertures  38  for fluid flow into and out of the inner cavity  34 . As seen in  FIG. 2 , there are apertures  38  disposed on both sides of the final optical element  22 . The inner part  32  has a flat portion  33  surrounding the inner cavity  34 . The flat portion  33  is substantially parallel to the wafer  16 . The distance D 1  between the end surface of the final optical element  22  and the wafer  16  is greater than the distance D 2  between the flat portion  33  and the wafer  16 . The distance D 1  could be 1.0-5.0 mm, and the distance D 2  could be 0.5-2.0 mm. In another embodiment, the distance D 1  is substantially equal to the distance D 2 . The inner part  32  further includes a pair of buffers or buffer slots  40  with purge holes  42  (see  FIG. 4 ). The buffers  40  are arranged at or near the flat portion  33 . The buffers  40  are disposed on opposite sides of the final optical element  22 . A cross-sectional view of the inner part  32  in the direction of scan  44  is illustrated in  FIG. 4 . 
     The outer part  30  is spaced from the inner part  32  by an intermediate spacing or groove  48 , which may be referred to as an atmospheric groove. The outer part  30  includes one or more fluid recovery openings  50  disposed on opposite sides of the final optical element  22 . A porous member  51  is disposed in a slot or outer cavity  53  that extends around the inner part  32  and fluidicly communicates with the pair of fluid recovery openings  50 . The porous member  51  may be a mesh or may be formed of a porous material having holes typically in the size range of about 50-200 microns. For example, the porous member  51  may be a wire mesh including woven pieces or layers of material made of metal, plastic, or the like, a porous metal, a porous glass, a porous plastic, a porous ceramic, or a sheet of material having chemically etched holes (e.g., by photo-etching). The outer part  30  further includes a fluid buffer outlet  56  and a fluid recovery outlet  58 . In another embodiment of the nozzle  20 ′ as seen in  FIG. 5 , the inner part  32  does not contact or form a seal with the final optical element  22 , but is spaced from the final optical element  22 . The gap prevents nozzle vibrations from being transmitted to the final optical element  22 . However, the gap may allow the fluid to be exposed to air. 
     One feature of the nozzle  20  is that it is made in two pieces, namely, the outer part  30  and the inner part  32 . The inner part  32  keeps the fluid between the lens and the wafer surface, and the outer part  30  is mainly provided for fluid recovery. Vibration might be introduced during fluid recovery from the outer part  30  through the porous member  51  to the other components of the lithography system, including the inner part  32  which may be used to direct an autofocus beam to the wafer  16 . A damping material can be mounted between the outer part  30  and the mounting piece to which the outer part  30  is mounted to minimize the transmission of vibration from the outer part  30 . In addition, the outer part  30  that includes the porous member may be prone to contamination and thus needs to be replaced for maintenance. Making the outer part  30  as a separate part facilitates easier maintenance. It can also minimize readjustment and recalibration time after replacement of the outer part as opposed to replacing the entire nozzle  20 . Manufacturability of the nozzle  20  can also be improved if the nozzle  20  is made in two separate parts. It is understood that the nozzle  20  may be made of a single piece in alternative embodiments. 
     Another feature of the nozzle  20  is the atmospheric groove  48  between the inner part  32  and the outer part  30 . This atmospheric groove  48  functions as a breaking edge to prevent fluid in the inner part  32  from being drawn out by the porous member  51  on the outer part  30  if the fluid recovery rate is faster than the fluid supply rate. In the situation when there is no breaking edge, a balance between the fluid recovery rate and the fluid supply rate has to be maintained so that fluid can be kept within the inner part  32  at all times during scanning. Having the atmospheric groove  48  allows the recovery rate to be set at a maximum to minimize fluid leakage out of the outer part  30  during scanning. The atmospheric groove  48  also acts as a buffer for fluid to go in and out during scanning, minimizing water supply and recovery requirements. 
     In the process of immersion lithography, a fluid is to be filled between the projection lens  14  and the wafer  16  from a dry state and, at other times, the fluid is to be recovered. For example, in the beginning of exposure of a new wafer, the fluid is to completely fill the inner cavity  34  of the inner part  32  before starting exposure. During this process, ideally no air bubbles can exist between the projection lens  14  and wafer  16  or other optical paths such as the auto focus beam. The fluid supply in the inner cavity of the inner part  32  is designed to be at the highest point in the cavity (via apertures  38 ) so that the fluid is filled from top down, allowing air bubbles to be pushed out of the inner cavity during the filling process. The fluid desirably is initially supplied from one side in this embodiment (the set of apertures  38  on one side), so that the fluid is filled from one side to the other, again allowing air bubbles to be pushed out to avoid trapping air therein. Other arrangements are also possible, as long as the fluid is being filled from the inside out. 
     On occasion, the fluid has to be fully recovered from the inner cavity of the inner part  32 . In  FIG. 4 , there are small holes  42  in each of the buffers  40  in the inner cavity. These holes  42  are provided for fast fluid recovery or fluid purge when the fluid has to be fully recovered. Sucking the fluid out from these holes  42  using high vacuum with the combination of some movement in the wafer stage  18  allows all the fluid to be recovered within a reasonable time. 
     The inner part  32  has two groups or rows of holes  38  for supplying or recovering the fluid. Each row can be independently controlled to either supply or recover the fluid. In the case where both rows are chosen for fluid supply, all the fluid is recovered through the porous member  51  in the outer part  30 . Because both rows are supplying fluid, a pressure can build up in the inner cavity causing deformation of the final optical element  22  of the projection lens  14  or the wafer  16  or both. The fluid flow across the final optical element  22  may also be limited, and thus the temperature of the fluid between the final optical element  22  and the wafer  16  may eventually rise, causing adverse effects. On the other hand, if one row is chosen for supply and the other for recovery, a fluid flow will be driven across the final optical element  22 , minimizing temperature rise. It can also reduce the pressure otherwise created by supplying fluid from both rows. In this case, less fluid needs to be recovered through the porous member  51 , lowering the fluid recovery requirement in the porous member. In other nozzle configurations, multiple fluid supplies and recoveries may be provided so as to optimize the performance. 
     During scanning motion of the wafer stage  18  (in the direction of scan  44  in  FIG. 2 ), the fluid may be dragged in and out of the inner cavity of the inner part  32 . When the fluid is dragged out, it is recovered through the porous member  51  in the outer part  30 . When the wafer stage  18  is moved in the opposite direction, air may be dragged into the inner cavity of the inner part  32 . During this time, the fluid in the buffers  40 , as well as the fluid supplied from within the inner cavity, helps to refill the fluid that is dragged along the scanning direction, preventing air from getting into the inner cavity. The buffers  40  and the porous member  51  work together to minimize fluid leaking out from the outer part  30 , and air dragging into the inner cavity of the inner part  32  during scanning motion of the wafer stage  18 . 
     Recovering fluid through the porous member  51  by maintaining the pressure in the porous member  51  under the bubble point can eliminate noise created by mixing air with the fluid during fluid recovery. The bubble point is a characteristic of the porous member  51  that depends on the size of the holes in the porous member  51  (the largest hole) and the contact angle that the fluid forms with the porous member  51  (as a parameter based on the property of the porous material and the property of the fluid). Due to the fact that the bubble point is typically a very low pressure (e.g., about 1000 pascal), the control of this low pressure becomes an important issue.  FIGS. 6-7  illustrate three specific ways of maintaining the pressure below the bubble point during fluid recovery. 
     In the pressure control system  100  of  FIG. 6 , a pressure under bubble point is maintained at the surface of the porous member  51  using a vacuum regulator  102  with the assistance of an overflow container or tank  104  fluidicly coupled to the porous member  51  by a recovery flow line  106  (which is connected to the fluid buffer outlet  56 ). The pressure at the surface of the porous member  51  is equal to the pressure maintained by the vacuum regulator  102  subtracting the pressure created by the height of the fluid above the porous member  51 . Maintaining a constant height of fluid above the porous member  51  using the overflow tank  104  allows easy control of the pressure at the surface of the porous member  51 . The fluid that is recovered through the porous member  51  will overflow and be siphoned down along a siphon line  108  to a collection tank  110 , which is disposed below the overflow tank  104 . An optional flow path  112  is connected between the overflow tank  104  and the collection tank  110  to assist in equalizing the pressure between the overflow tank  104  and the collection tank  110  and facilitate flow along the siphon line  108 . One feature of this pressure control system  100  is that it is a passive system without the necessity of control. 
     In the pressure control system  120  of  FIG. 7 , the pressure at the surface of the porous member  51  is maintained below the bubble point using a vacuum regulator  122  at a fluid level buffer  124  which is fluidicly coupled with the porous member  51  by a buffer flow line  126  (which is connected to the fluid buffer outlet  56 ). A pressure transducer or a water level sensor  128  is used to measure the pressure or fluid level at the fluid level buffer  124 . The sensor signal is then used for feedback control  130  to a valve  132  that is disposed in a recovery flow line  134  (which is connected to the fluid recovery outlet  58 ) connected between the porous member  51  and a collection tank  136 . The valve  132  may be any suitable valve, such as a proportional or variable valve. The variable valve  132  is adjusted to control the fluid flow through the fluid recovery line  134  to the collection tank  136  to maintain the pressure or fluid level of the fluid level buffer  124  at a preset value. The collection tank  136  is under a relatively higher vacuum controlled by a high vacuum regulator  138  for fluid recovery. In this fluid control system  120 , no overflow tank is needed and the collection tank  136  can be placed anywhere in the system and need not be disposed below an overflow tank. An on/off valve  140  is desirably provided in the fluid recovery line  134  and is switched off when fluid recovery is not required. 
     In  FIG. 8 , the pressure control system  160  is similar to the system  120  of  FIG. 7 , and like reference characters are used for like parts. Instead of using the valve  132  for the feedback control of fluid recovery, this system  160  employs a controllable vacuum regulator  162  for the feedback control of fluid recovery. The vacuum regulator  162  is typically electronically controllable to adjust the vacuum pressure in the collection tank  136  based on the sensor signal from the pressure transducer or a water level sensor  128 . The vacuum regulator  162  is adjusted to control the fluid flow through the fluid recovery line  134  to the collection tank  136  to maintain the pressure or fluid level of the fluid level buffer  124  at a preset value. The on/off valve  140  in the fluid recovery line  134  is switched off when fluid recovery is not required. 
       FIG. 9  shows a pressure control system for fluid recovery in an immersion lithography system with water stagnation prevention according to another embodiment of the invention. The pressure control system  180  is similar to the system  120  of  FIG. 7  having the same components with the same reference characters. In addition, the fluid level buffer  124  is fluidicly coupled with a fluid supply or fluid recovery  182  to supply fluid to or recover fluid from the fluid level buffer  124  to prevent stagnation. An optional pump or a similar moving part may be used to induce flow between the fluid level buffer  124  and the fluid supply or fluid recovery  182 . There is a possibility of bacteria/fungus growth in stagnated water or fluid over time. Under normal operation, the water at the fluid level buffer  124  is stagnated because water recovered from the mesh  51  will go through the small tube at the mesh level to the collection tank  136 . By inducing flow into or out of the fluid level buffer  124  during normal operation, the bacteria/fungus growth problem can be prevented. 
     It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should not be limited to the above description. 
     Also, the present invention could be applied to Twin-Stage-Type Lithography Systems. Twin-Stage-Type Lithography Systems, for example, are disclosed in U.S. Pat. Nos. 6,262,796 and 6,341,007, the disclosures of which are incorporated herein by reference in their entireties.