Abstract:
A liquid immersion photolithography system includes an exposure system that exposes a substrate with electromagnetic radiation and includes a projection optical system that focuses the electromagnetic radiation on the substrate. A liquid supply system provides liquid flow between the projection optical system and the substrate. An optional plurality of micronozzles are arranged around the periphery of one side of the projection optical system so as to provide a substantially uniform velocity distribution of the liquid flow in an area where the substrate is being exposed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/053,328, filed on Feb. 9, 2005, entitled IMMERSION PHOTOLITHOGRAPHY SYSTEM AND METHOD USING MICROCHANNEL NOZZLES which is also a continuation of U.S. patent application Ser. No. 10/464,542, filed on Jun. 19, 2003, entitled IMMERSION PHOTOLITHOGRAPHY SYSTEM AND METHOD USING MICROCHANNEL NOZZLES, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to liquid immersion photolithography, and more particularly, to a method and a system for controlling velocity profile of liquid flow in an immersion photolithographic system. 
     2. Description of the Related Art 
     The practical limits of optical lithography assume that the medium through which imaging is occurring is air. This practical limit is defined by the effective wavelength equation 
                 Λ   eff     =     λ     2   ·   n   ·   NA         ,         
where 8 is the wavelength of incident light, NA is the numerical aperture of the projection optical system, and n is the index of refraction of the medium. Now, by introducing a liquid (instead of the air) between a last lens element of the projection optical system and a wafer being imaged, the refractive index changes (increases), thereby enabling enhanced resolution by lowering the effective wavelength of the light source. Lowering a light source&#39;s wavelength automatically enables finer resolution of smaller details. In this way, immersion lithography becomes attractive by, for instance, effectively lowering a 157 nm light source to a 115 nm wavelength, thereby gaining resolution while enabling the printing of critical layers with the same photolithographic tools that the industry is accustomed to using today.
 
     Similarly, immersion lithography can push 193 nm lithography down to 145 nm. In theory, older technology such as the 193 nm tools can now still be used. Also, in theory, many difficulties of 157 nm lithography—large amounts of CaF 2 , hard pellicles, a nitrogen purge, etc.—can be avoided. 
     However, despite the promise of immersion photolithography, a number of problems remain, which have so far precluded commercialization of immersion photolithographic systems. These problems include optical distortions. For example, during immersion lithography scanning, sufficient g-loads are created that can interfere with system performance. These accelerative loads can cause a vibrational, fluidic shearing interaction with the lens resulting in optical degradation. The up and down scanning motions within the lens-fluid environment of Immersion Lithography can generate varying fluidic shear forces on the optics. This can cause lens vibrational instability, which may lead to optical “fading”. Other velocity profile non-uniformities can also cause optical distortions. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an immersion photolithography system with a near-uniform velocity profile of the liquid in the exposure area that substantially obviates one or more of the problems and disadvantages of the related art. 
     There is provided a liquid immersion photolithography system including an exposure system that exposes a substrate with electromagnetic radiation, and includes a projection optical system that focuses the electromagnetic radiation on the substrate. A liquid supply system provides liquid flow between the projection optical system and the substrate. A plurality of micronozzles are optionally arranged around the periphery of one side of the projection optical system so as to provide a substantially uniform velocity distribution of the liquid flow in an area where the substrate is being exposed. 
     In another aspect there is provided a liquid immersion photolithography system including an exposure system that exposes an exposure area on a substrate with electromagnetic radiation and includes a projection optical system. A liquid flow is generated between the projection optical system and the exposure area. A microshower is at one side of the projection optical system, and provides the liquid flow in the exposure area having a desired velocity profile. 
     Additional features and advantages of the invention will be set forth in the description that follows. Yet further features and advantages will be apparent to a person skilled in the art based on the description set forth herein or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGS. 
       The accompanying drawings, which are included to provide a further understanding of the exemplary embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  shows a side view of a basic liquid immersion photolithography setup. 
         FIG. 2  shows a plan view of the setup of  FIG. 1 . 
         FIG. 3  shows the basic liquid immersion photolithography setup with liquid flow direction reversed, compared to  FIG. 1 . 
         FIG. 4  shows additional detail of the liquid immersion photolithography system. 
         FIG. 5  shows a partial isometric view of the structure of  FIG. 4 . 
         FIG. 6  shows an exemplary liquid velocity profile. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     One major problem in immersion photolithography is the non-uniformity of the liquid flow, particularly its gradient in the vertical direction. The non-uniformity is due primarily to the fact that near a moving surface, the liquid is in contact with that surface (e.g., a surface of a wafer). For example, during scanning, the wafer moves relative to the exposure system, creating a “dragging effect” near its surface. Thus, the laws of fluid dynamics dictate that the fluid velocity relative to the wafer surface is zero in those areas (or at least close to zero), while fluid velocity is maximum further away from the wafer surface. Similarly, the fluid velocity relative to the bottom surface of the lens is zero. These fluid velocity variations are known as “boundary layer” velocity profiles. The combination of these effects produces a shearing force in the liquid that creates a twofold optical distortion problem: 1) the generation of inertial vibrational forces upon the aperture hardware (resulting in optical distortion), and 2) the formation of velocity striations within the fluid, which cause additional optical distortions. 
     Additionally, injection of liquid into the exposure area also provides a liquid flow with potential additional non-uniformities in the velocity distribution. For example, a number of striations can exist within the fluid, further degrading exposure quality. Similarly, air bubbles, opto-fluidic vibrations, or turbulence in the liquid flow also can degrade the overall performance of the photolithographic system because of the introduction of optical distortions into the exposure process. Thus, dealing with velocity profile non-uniformities is important from the perspective of the quality of imaging in a photolithographic system. In the ideal case, the velocity profile of the liquid is substantially uniform everywhere. 
       FIG. 1  illustrates a liquid immersion photolithographic system of the present invention in a block diagram form. As shown in  FIG. 1 , a projection optical system  100  of a photolithographic tool includes a lens  102  (which is typically comprised of multiple lens elements). In this figure, the lens  102  has a flat bottom surface  108 , although that need not be the case. Lens height  409  (see  FIG. 4 ) may be adjustable to maintain a specific distance to the wafer  101 . 
     The projection optical system  100  also includes a housing  103  (only the lower portion is shown). The housing  103  includes an annular liquid channel  105 A, and optionally a plurality of other such channels  105 B, etc. Liquid flows through the channels  105  (flowing in through the channel  105 A in this figure, and flowing out through the channel  105 B). The arrows  107 A,  107 B designate the direction of liquid flow over a wafer  101 , as the wafer  101  is being scanned across a field of view of the projection optical system  100 . 
       FIG. 2  illustrates a bottom-up view of the structure shown in  FIG. 1 . As shown in  FIG. 2 , a clear aperture area  216  defines an exposure area of the projection optical system  100  and the lens  102 . The various arrows  107 A- 107 D,  211 A- 211 D illustrate possible liquid flow directions at any given time. As may be further seen in  FIG. 2 , the housing  103  also includes a number of pressurized chambers  215 A- 215 D. Each pressurized chamber  215  may also be referred to as a “plenum.” The plenum  215  therefore acts as a pressure source, as discussed below. It will also be appreciated that the liquid flow can be turned off completely when no exposure is taking place, or when the wafer  101  is being swapped. 
     Further, as shown in  FIG. 2 , the lower portion of the housing  103  may be divided into a number of sections. In this figure, there are four such sections (quadrants), separated by gaps  217 A- 217 D. It will be appreciated that the number of such sections may be more or fewer than four, although, in most applications, it is expected that four quadrants is an optimal number. For example, for motion only along one axis, dividing the housing  103  into two sections may be sufficient. For X-Y motion, four sections (quadrants) are preferred. For even greater control, eight sections may be needed. This sectioning permits control over liquid flow direction, as also discussed further below. Controlling the direction of liquid flow makes it possible to counteract mechanical strains on the lens  102 , therefore the flow profile in the X direction (especially during a step) may be different from the flow profile in the Y direction (especially during a scan). 
       FIG. 3  illustrates the same structure as in  FIG. 1 , except that the direction of the liquid flow is reversed. As will be appreciated by one of ordinary skill in the art, the ability to reverse the direction of liquid flow is important in a practical photolithographic system, since the direction of wafer motion is normally not limited to just one direction. Similarly, it will be appreciated by one of ordinary skill in the art that, as in  FIG. 2 , the wafer  101  can move both in the X direction and the Y direction. Thus, dividing the housing  103  into quadrants permits the direction of liquid flow to be adjusted for any direction of wafer movement. 
       FIG. 4  illustrates an embodiment of the present invention in additional detail. As shown in  FIG. 4 , the lens  102  is mounted in the housing  103 . The housing  103  has the annular channels  105 A,  105 B, through which liquid flows in and out from a liquid supply system (not shown in these figures). From the channel  105 A, the liquid then enters a first large plenum  215 A. It then flows through a diffuser screen  412 A, into a first small plenum  414 A (which is typically smaller than the first plenum  215 A). The diffuser screen  412 A helps remove the turbulence and air bubbles that may be present in the first large plenum  215 A. The diffuser screen  412  also acts as a pressure drop screen. 
     The first small plenum  414 A also acts as a pressure chamber. From the first small plenum  414 A, the liquid then flows through a plurality of microchannel nozzles (micronozzles)  416 A, arranged in a form of a microshower. Thus, by the time the liquid reaches the micronozzles  416 , the pressure at the entry to all the micronozzles  416  is uniform, and turbulence and gas bubbles have been substantially removed from the liquid. After the micronozzles  416 , the liquid flows into the clear aperture area  216  under the lens  102 , such that the space between the lens  102  and the wafer  101  is filled with the liquid. 
     In the clear aperture area  216 , the liquid flow is uniform with height, and free of turbulence, bubbles, striations and other imperfections that affect optical image quality. 
     On the other side of the clear aperture area  216 , the liquid once again flows through a set of microchannel nozzles  416 B, into a second small plenum  414 B, through a diffuser screen  412 B, into a second large plenum  215 B and out through the channel  105 B. 
     Thus, with the relative motion of the wafer  101  from left to right in  FIG. 4 , the wafer  101  creates a “dragging effect” on the liquid. The direction of the liquid flow therefore needs to be from right to left, to counteract the “dragging effect,” and result in substantially uniform velocity profile. 
     In  FIG. 4 ,  420  designates effective fluid velocity profile within the clear aperture area  216  as induced by wafer  101  motion.  421  designates counter-injected fluid velocity profile from the microchannel nozzles  416 , yielding near net-zero resultant fluid velocity at the interface between the lens  102  and the liquid in clear aperture area  216 . 
     The microchannel nozzles  416  also refresh (i.e., replace) the working liquid from time to time (which may be necessary to prevent its disassociation over time, since exposure to intense electromagnetic radiation may break down the molecules of the liquid), so as to preclude thermal gradients from causing refractive distortions and image quality degradation. Avoiding dissociation of liquid (for example water) due to constant flow is another advantage. At the short exposure wavelength, water can dissociate at approximately 2.86 J/cm 2  RT and normal P turns to 4.75*10 −19  J per molecule. At 193 nm with one photon carries 1.03*10 −18  J. Additionally, keeping the liquid refreshed allows to maintain a constant temperature of the liquid. The liquid may be refreshed during exposure, or between exposures. 
     The micronozzles  416  also act as a buffer against inertial shearing forces between the optics and the liquid. Note that the shearing force is defined by the equation 
               F   =     A   ·   μ   ·       ⅆ   v       ⅆ   x           ,         
where A is the area, is a viscosity parameter, x is a distance variable, and v is the velocity. The shearing force is approximately 1 Newton in the case of a typical 100 micron gap between the wafer  101  and the lens  102 . Neutralizing these shearing forces is accomplished by inertially dampening the relative accelerative motion between the lens  102  and fluid. This is accomplished by simply creating fluidic motion in a direction opposite to scanning. The microchannel nozzles  416  also act as a buffer against inertial shearing forces between the optics and fluid.
 
     Additionally, the housing  103  includes a system for supplying gas to remove any excess liquid from the wafer  101 . The housing  103  includes a supply side annulus  406 A for gas inflow from a gas supply system (not shown in  FIG. 4 ), a gas seal  410 A, which bridges the distance to the wafer  101  and makes a “squeegee” so as to contain and remove any excess liquid, and a return side gas outflow annulus  405 A (through which excess liquid is removed). The excess liquid may be removed through the return side gas outflow annulus  405 A, together with the exhausted gas. A similar structure may be found in an opposite quadrant of the housing  103 , as shown on the left side of  FIG. 4 . The gas supply system works in conjunction with the liquid supply system, whenever there is liquid flow present, and, consequently, need only be turned on when there is liquid flow in the clear aperture area  216 . 
     As noted above, in  FIG. 4 , with the wafer movement from left to right, the liquid flow is “in” at channel  105 A, and “out” at channel  105 B. When the scan direction is reversed, the liquid flow reverses as well. 
       FIG. 5  shows a partial isometric view of the micronozzle structure area of  FIG. 4 . The channels  105 A- 105 D (not shown in  FIG. 5 ) are connected to outer tubes  507 A- 507 D, through which liquid is supplied. Similarly, though not shown in this figure, the annuli  405 ,  406  may be connected to tubular gas couplings. 
       FIG. 6  illustrates an example of a liquid exhaust velocity profile that may be used in the present invention. As will be appreciated by one of ordinary skill in the art, a “natural” velocity profile is not uniform with height in  FIG. 4 , but rather may have a vertical gradient, which can cause optical distortion. To compensate for this natural gradient, different lengths of tubes (micronozzles  416 ) may be used, as shown in  FIG. 6 . In  FIG. 6 , the micronozzle length ranges from a maximum of L 1  to a minimum of L 2 , resulting in approximately the velocity profile at the exit of the micronozzles  416  shown on the left of  FIG. 6 . The longer the micronozzle  416 , the lower the output velocity of the liquid from that particular micronozzle. Furthermore, the micronozzles  416  themselves may have different diameters, if needed to further control the velocity profile. Note further that the tubes of the micronozzles  416  need not necessarily be parallel to the wafer  101 , to further control the velocity profile. 
     The height of the liquid above the wafer  101 , in a typical system, is approximately 100 microns. Greater height generally results in a need for more micronozzles in  416 A due to a larger volume in which velocity profile needs to be controlled. 
     Thus, with careful selection of the lengths, diameters and orientations of the micronozzles  416 , the velocity profile in the clear aperture area  216  of the wafer  101  may be controlled, resulting in a substantially uniform velocity profile throughout the clear aperture area  216 , thereby improving exposure quality. In essence, the velocity profile generated by a structure such as shown in  FIG. 6  may be “opposite” of the “natural” profile that would exist otherwise. Thus, the characteristics of the micronozzles  416  are tailored to result in a substantially uniform velocity profile. 
     During scanning, the wafer  101  moves in one direction, while the liquid is recirculated and injected in the opposite direction. The effect of the present invention is therefore to neutralize the liquid velocity profile induced by the scanning motion, causing inertial dampening between the lens  102  and the liquid. In other words, the net effect is a “zero” net inertia and velocity profile steering away from motion. Depending on the direction of the liquid flow, either a reduction or elimination of shear forces, or a reduction in optical distortions may result. Thus, the immersion lithographic process is capable of performing at peak levels due to constant fluid refresh, avoidance of gas bubbles, and the buffering of opto-fluidic vibrations. 
     Note further that while the liquid in the plenum  215  may have turbulence and gas bubbles, by the time it travels through the diffuser screen  412 , the flow is uniform. Therefore, after passing through the diffuser screen  412 , the plenum  414 , and exiting from the micronozzles  416 , the liquid flow has a desired velocity profile, substantially without imperfections caused by striations, opto-fluidic vibrations, turbulence, gas bubbles, and other non-uniformities, resulting in improved image quality. 
     As noted above, the bottom surface  108  of the lens  102  need not be flat. It is possible to use a lens  102  with a curved bottom surface  108 , and compensate for any induced velocity profile non-uniformities with an appropriate arrangement of micronozzle lengths, diameters, and orientations, to result in a near-uniform velocity profile. 
     The micronozzles  416  may be constructed using conventional lithographic techniques on silicon material. On a microscopic scale, the micronozzles  416  resemble a honeycomb material composed of tubes that are stacked in a staggered formation that exhibits key characteristic dimensions of hydraulic diameter and length. The micronozzles  416  may be flared out into the clear aperture area  216 . 
     Typical tubular diameters of the micronozzles  416  may vary, for example, from a few microns to tens of microns (e.g., 5-50 microns), and in some cases, up to 5 mm in diameter, and lengths of between about 10 to 100 diameters. Other lengths and/or diameters may be used. Slits, rather than round nozzles, may also be used. The number of micronozzles per unit area may also be varied. 
     For 193 nanometer imaging, the liquid is preferably water (e.g., de-ionized water), although other liquids, for example, cycle-octane, Krypton® (Fomblin oil) and perfluoropolyether oil, may be used. 
     The present invention results in a number of benefits to a liquid immersion photolithographic system. For example, in a step and scan system, transmission is improved, and there is less distortion. Dust particles in the air cannot enter the clear aperture area  216  between the lens  102  and the wafer  101 , since the liquid itself does not contain any dust, and the presence of the liquid acts as a barrier to the dust being present in the clear aperture area  216  during exposure. Preferably, the liquid is brought in after the wafer  101  has been loaded onto a wafer stage, and removed before the wafer  101  is unloaded. This minimizes dust and particulate contamination. Additionally, other ways of keeping the liquid from spilling during wafer exchange are possible as well, and the present invention is not limited to just the approach described above. 
     The fluid velocity profile induced by the scanning motion is neutralized, causing inertial dampening between lens  102  and the shearing fluid. Aside from acting as inertial dampers, the micronozzles  416  serve to refresh the working fluid volume, thereby eliminating refractive distortions due to thermal gradients created by the light source. A side benefit of the micronozzles  416  is their ability to discourage the formation of gas-bubbles during volume refresh. Also, the size of these micronozzles  416  prevents the formation of gas-bubbles that plague more conventional refresh techniques. All of these benefits allow the use of generally existing photolithographic tools and wavelengths to define much smaller features on a semiconductor surface. 
     CONCLUSION 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. 
     The present invention has been described above with the aid of functional building blocks and method steps illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks and method steps have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Also, the order of method steps may be rearranged. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. One skilled in the art will recognize that these functional building blocks can be implemented by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.