Abstract:
Immersion lithography system and method using direction-controlling fluid inlets are described. According to one embodiment of the present disclosure, an immersion lithography apparatus includes a lens assembly having an imaging lens disposed therein and a wafer stage configured to retain a wafer beneath the lens assembly. The apparatus also includes a plurality of direction-controlling fluid inlets disposed adjacent to the lens assembly, each direction-controlling fluid inlet in the plurality of direction-controlling fluid inlets being configured to direct a flow of fluid beneath the lens assembly and being independently controllable with respect to the other fluid inlets in the plurality of direction-controlling fluid inlets.

Description:
CROSS-REFERENCE 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/670,860, filed on Feb. 2, 2007, which claims priority from U.S. Provisional Patent Application Ser. No. 60/864,204, filed on Nov. 3, 2006, both of which are-hereby incorporated by reference in their-entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates generally to immersion photolithography and, more particularly, to an immersion photolithography system using a sealed wafer bottom. 
         [0003]    Immersion lithography is a relatively new advancement in photolithography, in which the exposure procedure is performed with a liquid filling the space between the surface of the wafer and the lens. Using immersion photolithography, higher numerical apertures can be built than when using lenses in air, resulting in improved resolution. Further, immersion provides enhanced depth-of-focus (DOF) for printing ever smaller features. It is understood that the present disclosure is not limited to immersion lithography, but immersion lithography provides an example of a semiconductor process that can benefit from the invention described in greater detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
           [0005]      FIG. 1A  illustrates an LBC immersion system. 
           [0006]      FIG. 1B  illustrates an alternative design of an LBC immersion system. 
           [0007]      FIG. 2  illustrates a WBC immersion system. 
           [0008]      FIG. 3  illustrates a top view of a full immersion lithography system wherein a seal ring is disposed in contact with a bottom edge of a wafer in accordance with one embodiment. 
           [0009]      FIG. 4  illustrates a side view of the full immersion lithography system of  FIG. 3 . 
           [0010]      FIG. 5  is an enlarged side view of the full immersion lithography system of  FIG. 3 . 
           [0011]      FIG. 6  illustrates the full immersion lithography system of  FIG. 3  after the retaining wall thereof has been lowered to drain the immersion fluid therefrom. 
           [0012]      FIG. 7  illustrates a drying head for use in removing residual fluid from a wafer. 
           [0013]      FIGS. 8A and 8B  illustrate one implementation of a proximity cover including direction-controlling fluid inlets. 
           [0014]      FIGS. 9-11  illustrate fluid direction control implemented using the direction-controlling fluid inlets of  FIGS. 8A and 8B . 
           [0015]      FIGS. 12A and 12B  illustrate an alternative implementation of a proximity cover including direction-controlling fluid inlets. 
           [0016]      FIG. 13  illustrates an alternative arrangement of the full immersion lithography system of  FIG. 3 . 
           [0017]      FIG. 14  illustrates a full immersion lithography system in accordance with another alternative embodiment. 
           [0018]      FIG. 15  illustrates a full immersion lithography system in accordance with yet another alternative embodiment. 
           [0019]      FIG. 16  illustrates a double-nozzle direction-controlling fluid inlet arrangement. 
           [0020]      FIG. 17  illustrates the double-nozzle direction-controlling fluid inlet arrangement of  FIG. 16  disposed on the full immersion lithography system of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The present disclosure relates generally to the liquid immersion photolithography systems, and, more particularly, to an immersion photolithography system using a sealed wafer bottom. It is understood, however, that specific embodiments are provided as examples to teach the broader inventive concept, and one of ordinary skill in the art can easily apply the teachings of the present disclosure to other methods and systems. Also, it is understood that the methods and systems discussed in the present disclosure include some conventional structures and/or steps. Since these structures and steps are well known in the art, they will only be discussed in a general level of detail. Furthermore, reference numbers are repeated throughout the drawings for the sake of convenience and example, and such repetition does not indicate any required combination of features or steps throughout the drawings. 
         [0022]    Generally, there are two system configurations in immersion lithography, including lens-based (“LBC”) systems and wafer-based (“WBC”) systems. With LBC systems, immersion fluid is selectively applied to and extracted from a small region between the lens and the wafer and the immersion assembly is stationary with respect to the lens as the wafer is stepped or scanned. 
         [0023]    Referring to  FIG. 1A , one embodiment of an LBC system  100  includes an immersion head  102  comprising an imaging lens  104 , a fluid inlet  106 , and a fluid outlet  108 . As shown in  FIG. 1A , immersion fluid is disposed in an area  110  beneath the imaging lens  104  and above a wafer  112 , which is secured to a wafer stage  114  via a vacuum system  116 . The fluid is injected into the area  110  via the fluid inlet  106  and expelled via the fluid outlet  108 , which process may result in fluid temperature control issues and fluid evaporation problems. 
         [0024]    Advantages to LBC systems include the fact that the wafer stage thereof is essentially identical to that of a dry system, thereby saving development time and expense. Additionally, with LBC systems it is possible to maintain the same alignment, focus, and leveling setup as used in dry systems. Finally, with LBC systems, the volume of immersion fluid used is small, so that filling up the fluid-retaining cavity can be performed very quickly, thereby maintaining high wafer throughput volume. 
         [0025]    Problems associated with LBC systems include the fact that, near the edge of the wafer, the immersion region includes the wafer and areas outside the chuck, such that maintaining the hydrodynamics in the fluid cavity and managing fluid extracting can be more difficult. Another problem is that particles at the backside of the wafer tend to be washed to the surface. Additionally, the LBC immersion head tends to leave trace amounts of fluid behind on the wafer surface as the wafer moves during the step-and-scan operation. This is a root cause of fluid stains on the wafer. Yet another problem associated with LBC systems is that the photoresist will have inconsistent fluid-contact history at different locations. Specifically, as the wafer is stepped from field to field, the neighboring fields, or parts thereof, are covered by fluid. This may occur to the same field multiple times and not necessarily in the same sequence or the same number of times for each field. Finally, in some LBC system designs, such as illustrated in  FIG. 1B , immersion fluid flows over the wafer edge into a fluid drain  120  located along the edge of the wafer  112 . While this reduces particle trapping, it results in wafer cooling at the edge, distorting the wafer and affecting overlay accuracy. 
         [0026]    Referring to  FIG. 2 , in contrast to LBC systems, in WBC systems, the wafer is completely immersed in immersion fluid in a circulating tank in the wafer stage. In a WBC system  200 , immersion fluid is selectively introduced into and expelled from a small region  204  between a lens  206  and a wafer  208  via a fluid inlet  210  and a fluid outlet  212 , respectively. The immersion fluid circulates in the region  204  under and over the wafer stage continuously and is filtered and temperature-regulated as it moves across the surface area of the wafer  208 . The fluid can be completely drained from the region  204  to allow for loading and unloading of the wafer  208 . A cover  214  prevents immersion fluid  202  from spilling over and foreign particles from falling into the fluid. 
         [0027]    Advantages of WBC systems include the fact that exposure at the edge of the wafer is the same as that at the center thereof. Moreover, each field contacts the wafer for the same amount of time. Additionally, there is no possibility of fluid stains caused by an immersion head and there is no issue of bubble generation from poor hydrodynamics near the edge of the wafer. WBC systems do, however, suffer certain deficiencies, including the fact that pre- and post-exposure soaking times of each exposure field are different. Moreover, it takes more effort or more time to fill and drain the immersion fluid and focusing, tilting, and alignment have to be performed in the immersion mode if twin stage is not used. Finally, substantial redesign of the wafer stage, as compared to a dry system, is necessary. 
         [0028]    Two additional problems affect both LBC and WBC systems. These include the fact that the resist at the wafer edge within several millimeters (the “edge bead”), is usually removed because it is thicker than the rest of the resist coating. This leaves the possibility of broken resist fragments under the flushing of the fluid, thus contributing to particulate defects. Moreover, the fluid can seep into the underside of the wafer, making it a contamination source and susceptible for contamination as well. The evaporation of this fluid can contribute to uneven cooling and overlay errors. 
         [0029]    Referring now to  FIGS. 3 and 4 , illustrated therein are top and side views of a full immersion lithography system  300  in which a seal ring is disposed such that it is in contact with a bottom edge of a wafer in accordance with one embodiment. Such a full immersion lithography system may alternatively be referred to herein as a “WISBOT” system. As best shown in  FIG. 4 , the system  300  comprises a wafer stage  302  to which a wafer  304  may be secured via a vacuum system  306 . A lens assembly  308  is disposed over the wafer  304 . In accordance with one embodiment, immersion fluid  309  is disposed in an area, or tank,  310  over and around the wafer  304  between the wafer and the lens assembly  308 . The immersion fluid is retained within the tank  310  by a fluid retaining wall  311 . In one embodiment, the refractive index of the immersion fluid is substantially 1.34. A seal ring  312  constructed of rubber or similar material is disposed on the wafer stage  302  such that it contacts a bottom edge of the wafer  304  disposed on the stage. In one embodiment, the thickness of the seal ring  312  is between 1 and 10 millimeters. The top edge of the seal ring  312  extends slightly above the bottom of the wafer  304  so that when the wafer is secured to the wafer stage  302  by the vacuum system  306 , the edge of the wafer is sealed against fluid seepage by the seal ring. In other words, the seal ring  312  seals what might otherwise be a gap between the wafer  304  and the wafer stage  302 . 
         [0030]    A proximity cover  314  having a plurality of fluid inlets  316  therethrough is provided for confining the immersion fluid to the area  310  and for maintaining the temperature of the immersion fluid. The fluid inlets  316  are provided for regulating the fluid flow, as will be described in greater detail herein below. The proximity cover  314  is of a size appropriate for keeping the fluid homogeneous between the lens and the wafer. In the present embodiment, it is not too large to unnecessarily enlarge the size of the enclosing cover, because it should not move too close to the fluid retaining wall  311 . An enclosing cover  318  is attached to a lens column of the lens assembly  308  to enclose the tank  310  and create and maintain a fluid-vapor-rich environment therein. 
         [0031]      FIG. 3  best illustrates the relationship between the seal ring  312 , the wafer  304 , and the enclosing cover  318 . As shown in  FIG. 3 , the wafer  304  comprises a plurality of scanned fields  320 . A region  322  represents a lens field of the lens assembly  308 . As also best illustrated in  FIG. 3 , the lens contains a cover  322  comprising a slot  324  that dictates the scanning exposure field. 
         [0032]    As best shown in  FIG. 5 , which is an enlarged and enhanced view of the system  300 , vapor of the immersion fluid  309  is confined within the area  310 , which is bounded by the enclosing cover  318 , the fluid retaining wall  311  and the wafer stage  302  with the wafer  304  pressed against the seal ring  312  by the vacuum system  306 . After a high concentration of fluid vapor has been achieved in a gap above the fluid  309  within the area  310 , sufficient immersion fluid is introduced to cover the entire surface of the wafer  304 . Overflow holes  330  allow excess fluid to flow into a fluid collection trench  332 . The fluid vapor inevitably escapes through a gap between the fluid retaining wall  311  and the enclosing cover  318  and must be replenished periodically. This gap is necessary to ensure free movement between the fluid retaining wall  311  and the enclosing cover  318  and is kept small and uniform to keep fluid vapor loss to a minimum. 
         [0033]      FIG. 6  illustrates the system  300  after the fluid retaining wall  311  has been lowered to empty the area  310  of fluid. After the wafer  304  and wafer stage  302  are removed from beneath the lens assembly  308 , residual fluid and wetness on the wafer  304  may be removed using a drying head comprising an air knife such as that illustrated in  FIG. 7  and designated by a reference numeral  340 . The drying head  340  comprises at least one vacuum outlet  342  for draining immersion fluid and at least one air purge inlet  344  to purge gas for drying. Additional details regarding the drying head  304  and alternative embodiments thereof are provided in related U.S. Patent Application Ser. No. 60/864,241 (Atty. Docket No. 2006-0682/2461.847) entitled “IMMERSION LITHOGRAPHY SYSTEM USING A SEALED WAFER BATH”, which is hereby incorporated by reference in its entirety. 
         [0034]      FIGS. 8A-12B  illustrate regulation of fluid flow via fluid inlets, such as the fluid inlets  316 .  FIGS. 8A and 8B  illustrate one implementation of the use of direction-controlling fluid inlets  350   a - 350   d  disposed in a proximity cover  352 . As shown in  FIGS. 8A and 8B , the four inlets  350   a - 350   d  surround a lens assembly  354  at angles in 90 degree increments. Each of the inlets  350   a - 350   d  directs fluid toward the lens assembly  354  and the inlet opposite it. In particular, the inlet opposite the edge of the proximity cover  354  that is closest to the edge of the wafer (not shown in  FIGS. 8A and 8B ) at a given time is opened to allow for flow of fluid. All of the other inlets are closed via a fluid control valve disposed therein. In this manner, fresh and uniformly flowing fluid always flows under the lens assembly  354  to ensure freedom from particles and a homogenous immersion medium for aberration-free imaging. Any particle near the edge of the wafer is always carried by the fluid to be drained out. Referring again to  FIG. 5 , it is important that the temperature of the fluid  309  is strictly controlled to constitute an isothermal environment in the imaging area that comprises the lens assembly  308 , the enclosing cover  318 , the proximity cover  314 , the immersion fluid, the fluid vapor, the fluid retaining wall  311  the wafer  304 , and the wafer stage  302 . Needless to say, the temperature of the incoming fluid vapor must also be controlled to the same degree of accuracy. 
         [0035]      FIG. 9  illustrates a situation in which the inlet  350   c , which is the one of the inlets  350   a - 350   d  opposite the edge of the proximity cover  354  that is closest to the edge of a wafer  360 , is open, while the other inlets  350   a ,  350   b , and  350   d , are closed, so that the flow of immersion fluid is directed in a direction indicated by an arrow  362 . The fluid passes underneath the lens assembly  354  and flows out through the edge of the wafer  360  nearest the lens assembly.  FIG. 10  illustrates a case in which the inlet  350   b , which is the one of the inlets  350   a - 350   d  opposite the edge of the proximity cover  354  that is closest to the edge of the wafer  360 , is open, while the other inlets  350   a ,  350   c , and  350   d , are closed, so that the flow of immersion fluid is directed in a direction indicated by an arrow  364 . Once again, the fluid passes beneath the lens assembly  354  and flows out through an edge of the wafer  360  nearest the lens assembly.  FIG. 11  illustrates a situation in which multiple inlets, in this case, inlets  350   b  and  350   c , are opened to create oblique flow in directions indicated by arrows  366  and  368 . Additionally, the flow rate of the inlets  350   b ,  350   c , in the illustrated case, may be differently adjusted to produce an arbitrarily oblique flow direction. 
         [0036]      FIGS. 12A and 12B  illustrate an alternative implementation of direction-controlling fluid inlets that may be employed in a WISBOT system. As best shown in  FIG. 12A , inlets  370   a - 370   h  disposed in a proximity cover  372  comprise arcs, rather than lines, and they are arranged in two circular formations, with inlets  370   a - 370   d  forming an outer circle and inlets  370   e - 370   h  forming an inner circle around a lens assembly  374 . This arrangement enables greater flexibility in controlling the flow of fluid by facilitating fluid flows in 45 degree, as opposed to 90 degree, increments. 
         [0037]      FIG. 13  illustrates the system  300  of  FIG. 5  in which the area  310  has been filled with fluid  309 , thereby eliminating the fluid-vapor-rich space above the fluid. In the example illustrated in  FIG. 13 , the entirety of the area enclosed by the enclosing cover  318 , the fluid retaining wall  311 , and the wafer stage  302  with the wafer  304  secured thereto is filled with immersion fluid  309 . A vapor saturated environment is not necessary to prevent evaporation.  FIG. 14  illustrates a WISBOT system  390  that differs from the system  300  in that it does not include a proximity cover; instead, fluid-direction control functions are performed through the enclosing cover  318 .  FIG. 15  illustrates a WISBOT system  400  that differs from the system  300  in that it does not include an enclosing cover; rather, stringent fluid temperature control is imposed within the proximity cover  314 . 
         [0038]      FIGS. 16A and 16B  illustrate a double-nozzle direction-controlling fluid inlet arrangement  410 . As shown in  FIGS. 16A and 16B , the arrangement  410  includes a double-nozzle inlet  412  includes a main nozzle  414  for directing fluid in a direction indicated by an arrow  415  and a secondary nozzle  416  for directing fluid in a direction opposite that of the main nozzle, as indicated by an arrow  417 . In this manner, fresh fluid always flows from the fluid inlet toward the edges of the wafer (not shown). Fluid flow from the main nozzle  414  passes under a lens  418  to maintain a clean and homogenous medium thereunder. Fluid flow from the secondary nozzle  416  is directed toward the opposite side of the wafer. Two additional nozzles  422 ,  424 , arranged to direct fluid in directions indicated by arrows  426  and  428 , respectively, toward the respective outside edges of the wafer change the direction of the fluid flow at different relative wafer/lens positions.  FIG. 17  illustrates the double-nozzle direction-controlling fluid inlet arrangement  410  of  FIG. 16  implemented on the WISBOT system of  FIG. 13 . The double-nozzle direction-controlling fluid inlet  416  of  FIG. 16  can also be implemented on WISBOT systems such as those illustrated in  FIGS. 5 ,  14 , and  15 . 
         [0039]    Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. 
         [0040]    It is understood that various different combinations of the above-listed embodiments and steps can be used in various sequences or in parallel, and there is no particular step that is critical or required. Furthermore, features illustrated and discussed above with respect to some embodiments can be combined with features illustrated and discussed above with respect to other embodiments. Accordingly, all such modifications are intended to be included within the scope of this invention.