Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/307,230, filed Jan. 27, 2006, now U.S. Pat. 7,446,859, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The present invention relates generally to semiconductor device manufacturing, and, more particularly, to an apparatus and method for reducing contamination in immersion lithography. 
     Lithography is one of the most important techniques utilized in semiconductor manufacturing, and is particularly used to define patterns, such as those employed in a wiring layer patterning process or a doped-region defining process for example. A lithography process generally includes an exposure step and a development step, wherein the exposure step utilizes a light source to irradiate a photoresist layer directly or through a photomask to induce chemical reactions in exposed portions. The development step is conducted to remove the exposed portion in positive resist (or the unexposed portion in negative resist) and form photoresist patterns, thus completing the transfer of photomask patterns or virtual patterns to the resist material. 
     Immersion lithography (IL) is rapidly emerging as the technique of choice for printing sub-100 nm photoresist structures while still using 193 nm exposure sources. By increasing the index of refraction of the medium between the last lens element of the exposure tool and the resist-coated substrate, the numerical aperture of the lithography system is increased and thus the printable minimum feature size for a given exposure wavelength can be reduced in accordance with the well-known Rayleigh equation. Accordingly, existing immersion lithography processes are conducted in a liquid phase environment, and thus a higher resolution is achieved since the refractive index of the immersion liquid (e.g., ultra pure water) is higher than that of air (about 1.47 versus 1.0). Therefore, the dimensions of the formed IC devices can be further scaled using an immersion lithography technique. 
     However, one drawback associated with immersion lithography stems from the physical contact between the immersion fluid and the resist material, which can potentially lead to partial image integrity failure and contamination embedded in or below the resist. More specifically, evaporation of the immersion fluid off the resist surface on the trailing edge of the shower head during exposure can lead to the concentration of trace contaminants, which can be transferred during the subsequent processing steps and finally affect device yield and performance in a severe manner. For example, traces of colloidal silica present in the immersion fluid can be concentrated in areas where immersion fluid evaporation is verified. 
     In addition, the trailing edge of the water pool contained by the showerhead can easily leave behind a residual immersion fluid layer, or eventually break down into droplets of variable size, under specific scanning conditions. For example, with typical wafer stage speeds in the order of 500-1000 mm/s, any discontinuity present on the scanned surface will affect the mechanical stability of the fluid pool and lead to the formation of fluid droplets. Similarly, a low contact angle between the immersion fluid and the scanned surface will increase the shape and size of the trailing fluid edge, thus increasing the chances of forming a residual fluid layer. Either the presence of a residual fluid layer or droplets can easily lead to the formation of defects. Extractable components from the topcoat or resist layers (e.g., oligomeric material, photoacid generator, photogenerated acid, base quencher) can be extracted by the immersion fluid and result in micromasking or watermark-like defects upon fluid drying. 
     Accordingly, it would be desirable to be able to reduce or eliminate altogether the contamination left behind by immersion lithography. 
     SUMMARY 
     In one embodiment, a method for reducing contamination in immersion lithography includes retaining a semiconductor wafer on a support surface of a wafer chuck, the wafer chuck having a gap therein, the gap located adjacent an outer edge of the wafer, and the gap containing a volume of immersion lithography fluid therein; providing a fluid circulation path within the wafer chuck so as to facilitate the radial outward movement of the immersion lithography fluid in the gap, thereby maintaining a meniscus of the immersion lithography fluid at a selected height with respect to a top surface of the semiconductor wafer; introducing pressurized fluid through an inlet path included in the fluid circulation path, the inlet path in communication with the immersion lithography fluid in the gap; and removing excess fluid immersion lithography fluid from the gap through a return path included in the fluid circulation path; wherein the chuck further comprises a lip adjacent the gap and an outer channel adjacent the lip, the gap in fluid communication with the inlet path and the outer channel in communication with the return path, wherein excess immersion fluid in the gap travels radially outward from the gap, over the lip, and into the outer channel; and wherein one or more grooves are formed within the lip, the one or more grooves configured to permit the immersion fluid to pass between the gap and the outer channel. 
     In another embodiment, a method for reducing contamination in immersion lithography includes retaining a semiconductor wafer on a support surface of a wafer chuck, the wafer chuck having a gap therein, the gap located adjacent an outer edge of the wafer, and the gap containing a volume of immersion lithography fluid therein; providing a fluid circulation path within the wafer chuck so as to facilitate the radial outward movement of the immersion lithography fluid in the gap, thereby maintaining a meniscus of the immersion lithography fluid at a selected height with respect to a top surface of the semiconductor wafer; introducing pressurized fluid through an inlet path included in the fluid circulation path, the inlet path in communication with the immersion lithography fluid in the gap; and removing excess fluid immersion lithography fluid from the gap through a return path included in the fluid circulation path; wherein the chuck further comprises a lip disposed beneath the gap, the lip separating the inlet path and the return path, and a pressure sensor in communication with the fluid circulation path, wherein excess immersion fluid in the gap travels radially outward from the gap, over the lip, and into the return path. 
     In still another embodiment, a method for reducing contamination in immersion lithography includes retaining a semiconductor wafer on a support surface of a wafer chuck, the wafer chuck having a gap therein, the gap located adjacent an outer edge of the wafer, and the gap containing a volume of immersion lithography fluid therein; providing a fluid circulation path within the wafer chuck so as to facilitate the radial outward movement of the immersion lithography fluid in the gap, thereby maintaining a meniscus of the immersion lithography fluid at a selected height with respect to a top surface of the semiconductor wafer; introducing pressurized fluid through an inlet path included in the fluid circulation path, the inlet path in communication with the immersion lithography fluid in the gap; and removing excess fluid immersion lithography fluid from the gap through a return path included in the fluid circulation path, and wherein the chuck further includes a first fluid column, defined by the gap, a second fluid column, disposed radially outward with respect to the first fluid column, the second fluid column including the inlet path and the return path passing therethrough, and a passage configured for fluidly connecting the first and the second fluid columns, wherein the meniscus of the immersion lithography fluid is passively controlled through fluid level control within the second fluid column. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  is a partial cross-sectional view of a portion of a conventional chuck assembly that may be used in conjunction with immersion lithography; 
         FIG. 2  is a partial cross-sectional view of a chuck assembly configured for immersion lithography, in accordance with an exemplary embodiment of the invention; 
         FIG. 3  illustrates an optional groove feature that may be introduced within the chuck assembly of  FIG. 2 ; 
         FIG. 4  is a partial side cross-sectional view of the optional groove feature, taken along the lines  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a partial cross-sectional view of a chuck assembly configured for immersion lithography, in accordance with an alternative embodiment of the invention; 
         FIG. 6  is partial cross-sectional view of a chuck assembly configured for immersion lithography, in accordance with still another embodiment of the invention; 
         FIG. 7  is a cross-sectional view of a chuck assembly of the passive control type illustrated in  FIG. 6 , depicting an exemplary two-piece construction embodiment of the same; 
         FIG. 8  is a top view of the chuck assembly of  FIG. 7 ; 
         FIGS. 9(   a ) through  9 ( d ) illustrate various possible cross-sectional shapes for sealing rings used in the chuck assembly embodiments; and 
         FIG. 10  depicts an alternate location of a sealing ring with respect to the wafer edge support portion of a chuck assembly. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is an apparatus and methodology for reducing contamination associated with immersion lithography. Generally speaking, wafer contamination is left behind near the wafer edge, and in a manner that such contamination is produced as a result of interaction of the immersion fluid with the topography between the wafer edge and the chuck. Recent simulations have shown that reducing topography on the surface covered by the tool showerhead helps to maintain the meniscus, and to avoid bubble formation. Thus, it is advantageous to artificially extend the wafer surface, so as to make the transition between the wafer and the chuck as flat as possible without sharp transitions. 
     As is outlined in greater detail hereinafter, the exemplary embodiments herein maintain fluid meniscus integrity in the topography gap of a wafer chuck by introducing an internal fluid circulation path within the chuck. The circulation path includes the gap itself, thus flowing immersion fluid through the gap (between the outer edge of the wafer and the chuck) radially outwards, and maintaining the water level at same height as the wafer surface. Moreover, the fluid level may be maintained at the same height as wafer surface with active or passive control of the fluid circulation path. It should be noted that any suitable fluid may be used for the fluid circulation path within the chuck, so long as it optically matches the immersion fluid of the lithography process and does not damage the surrounding surfaces of the wafer. 
     Referring initially to  FIG. 1 , there is shown a partial cross-sectional view of a portion of a chuck assembly  100  that may be used in conjunction with, for example, immersion lithography. As is shown, a wafer chuck  102  has a semiconductor wafer  104  held thereupon, with the wafer  104  depicted as having a thin layer of photoresist material  106  formed thereon. It will be noted that the relative dimensions of the chuck  102 , wafer  104  and resist layer  106  are not to scale and are depicted as such for illustrative purposes only. However, as is shown in  FIG. 1 , there exists a substantial topography at the outer edge of the wafer  104  as a result of the air gap  108  formed within the chuck  102 . 
     Accordingly,  FIG. 2  is a partial cross-sectional view of a chuck assembly  200  configured for immersion lithography, in accordance with an exemplary embodiment of the invention. As is shown, a wafer chuck  202  is modified to include an immersion fluid supply line  210  and an outer channel  204  that facilitates the outward flow of immersion fluid from the gap  108 , thus forming a liquid surface that extends from the edge of the wafer  104  top surface to the wafer chuck surface, thereby filling the air gap with the added fluid. While the outermost portion  206  of the chuck has a height that roughly correlates to the height of the wafer  104 , it will be noted that the intermediate portion (lip)  208  of the chuck between the gap  108  and the outer channel  204  has a height that is lower than the top of the outermost portion  206  and the wafer  104 . This allows fluid to travel over the top of the lip  208 , leading to a reduction in splashing and thus contamination. 
     In addition to the outer channel  204 , a fluid circulation path is also formed within the chuck  202 . Particularly, an inlet path  210  allows a pressurized source of fluid to flow into the bottom of the gap  108 , while a negative pressure return path  212  originates from a sidewall formed within the outer channel  204 . In an exemplary embodiment, the fluid used and circulated through the fluid circulation path has the same optical characteristics as that used for the immersion lithography, so as to avoid any changes in optical characteristics from any mixing therebetween. Furthermore, in order to prevent optical fluid from entering beneath the surface of the wafer  104  where it contacts the chuck  202 , a seal  214  (e.g., an O-ring) is positioned between the bottom of the wafer and the bottom of the gap  108 . As will be discussed in further detail hereinafter, a variety of seal shapes and materials may be implemented. 
       FIG. 3  illustrates an optional feature that may be introduced within the chuck assembly  200  of  FIG. 2 , for the purpose of eliminating waves as the chuck moves about. More specifically, a plurality of side grooves  216  is formed (at periodic intervals) within the lip  208  of the chuck  202 . Thereby, an additional level of fluid level control is achieved since, in addition to flowing over the top of the lip, the optical fluid can also flow through each of the side grooves  216 .  FIG. 4  is a partial side cross-sectional view, taken along the lines  4 - 4  of  FIG. 3 , illustrating in further detail one of the grooves  216  formed in the lip  208 . 
     Referring now to  FIG. 5 , there is shown a partial cross-sectional view of a chuck assembly  500  configured for immersion lithography, in accordance with an alternative embodiment of the invention. As is shown, a wafer chuck  502  is modified to include an interior fluid circulation path. However, in contrast to the embodiment of  FIGS. 2 through 5 , the interior fluid circulation path of the chuck  502  in  FIG. 5  is configured directly beneath the existing gap  108 . As such, when immersion fluid within the gap  108  is accelerated, it is caused to flow from the direction of the pressurized inlet path  510 , over a lip  508  beneath the gap  108 , to the return path  512 . Moreover, the fluid level of the embodiment of  FIG. 5  is maintained through an active control approach in that a pressure sensor  504  is configured within the chuck  502  in order to sense the fluid pressure within the path. In this manner, the positive pressure of the inlet path  510  and the negative pressure of the return path  512  may be independently controlled to adjust for changes in pressure in the immersion fluid in the gap, thereby maintaining water level  110  and meniscus integrity of the passing immersion fluid. 
     One particular advantage associated with the embodiment of  FIG. 5  is a simpler flow of fluid within the chuck  502 . As is the case with  FIG. 2 , the chuck  502  also includes a sealing ring  514  to prevent immersion fluid from coming between the bottom surface of the wafer  104  and the chuck  502 . 
     In addition to active control, the fluid level within an immersion lithography chuck assembly can also be maintained through passive control means. For example,  FIG. 6  is partial cross-sectional view of a chuck assembly  600  configured for immersion lithography, in accordance with still another embodiment of the invention. As is shown, the passive control embodiment provides a modified wafer chuck  602  that incorporates a first water column (i.e., the existing gap  108 ) and a second column  604  formed at an outer location with respect to the radius of the chuck  602 . Similar to the earlier embodiments, a fluid circulation path is once again provided within the chuck  602  for maintaining the integrity of the fluid meniscus  110 . 
     The fluid circulation path, including inlet path  610  and return path  612 , is directed through the second column  604 , which further includes an overflow lip  608 . Thus, fluid traveling in an outward direction will flow over the lip  608  and into the return path  612  of the second column  604 . The passive control of the fluid level in the gap  108  is achieved through the control of the second column  604 , since the gap  108  is fluidly connected to the second column  604  through passage  606  formed within the chuck  602 . In addition, a Venturi tube  607  is formed at the bottom of the gap  108 , connecting the gap  108  to the inlet path  610  and thus allowing for the circulation of fluid through the gap  108  as well. As is the case with the embodiments of  FIGS. 2 and 5 , the chuck  602  also includes a sealing ring  614  to prevent immersion fluid from coming between the bottom surface of the wafer  104  and the chuck  602 . 
     Accordingly, in operation of the passively controlled chuck assembly  600 , fluid passes by the first column (i.e., gap  108 ) and through the Venturi tube  607 , which sucks fluid from the gap  108 . Thereby, the excess fluid left over from the passing of the meniscus  110  over the gap  108  is removed from the gap  108 . Thereafter, the excess fluid joins the inlet path  610  where it then flows over the lip  608  and into a drain (i.e., return path  612 ), thus maintaining the level of the fluid at the top surface of the chuck  602 . Because the two columns (gap  108 , second column  604 ) are connected (e.g., through passage  606 ) in zones of equal pressure at equal height, the fluid in the gap  108  will be maintained at the same level as that present in the second column  604 . It will be noted that the flow of fluid within the chuck  602  need not be continuous, and may instead be made to occur at selected locations along the circumference of the chuck  602 . 
     Thus configured, the passively controlled chuck assembly  600  allows for very fast control of fluid levels adjacent to the wafer  104 , by minimizing the topography that the meniscus  110  crosses in a stable manner. Since the Venturi tube  607  provides for circulation of immersion fluid through the chuck gap  108 , contamination of the fluid is less likely to accumulate, which in turn results in a smaller probability of contaminant particles being deposited on the surface of the wafer  104 . A second advantage of having a series of Venturi tubes  607  and connecting passages  606  at a specified intervals is that they provide for a method to prevent undue increase in fluid pressure in the gap  108  during chuck acceleration. 
     By way of further illustration,  FIG. 7  is a cross-sectional view of a chuck assembly  700  of the passive control type illustrated in  FIG. 6 , and depicts an exemplary two-piece construction embodiment of the same. As is shown, a first chuck section  702   a  includes the interior plumbing for the chuck, as well as the surface to which the wafer  104  is held. In particular, the first chuck section  702   a  includes the fluid inlet path  610  and return path  612  described above, as well as the Venturi tubes  607  at the bottom of the gap  108 . Plugs  704  may be inserted into the first chuck section  702   a  to prevent the fluid from leaking outside the chuck. 
     Further, the first chuck section  702   a  is removably attached (e.g., by means of bolts  706 ) to a second chuck section  702   b . The second chuck section  702   b , once attached, also serves to define a barrier between the first column (gap  108 ) and the second column  604  for passive fluid level control. As can been seen, the second chuck section  702   b  also includes the interior passage  606  so as to bring the first and second columns in fluid communication with one another, and achieve the passive control of the gap fluid. O-rings  708  may also be used to seal the first and second chuck sections together, as also shown in  FIG. 7 . It will be noted that the detailed exemplary embodiment of  FIG. 7  does not illustrate the sealing rings that prevent fluid from coming between the bottom of the wafer  104  and the first chuck section  702   a.    
       FIG. 8  is a top view of the chuck assembly  700  of  FIG. 7 . In addition to illustrating an exemplary fluid distribution path,  FIG. 8  also shows one possible example of the relative number and positioning of inlet and outlet ports in the first column  604  with respect to the inlet and return fluid paths  610 ,  612 . An exemplary distribution of Venturi tubes  607  within the gap  108  is further illustrated, although it will be appreciated that a different number and location of tubes can also be implemented. 
     As stated earlier, and regardless of the particular chuck assembly embodiment utilized, it is desirable to prevent immersion fluid (e.g., water) from getting beneath the wafer, between the bottom of the wafer and the chuck surface. More specifically, since there is vacuum holding down the wafer, the immersion fluid will have a tendency to seep towards the lower pressure. As such, it is advantageous to block this path by (for example) placing a sealing ring at the outer edge of the wafer support. To this end, several types and shapes of such a sealing ring are available, and from various materials. 
     For example,  FIGS. 9(   a ) through  9 ( d ) illustrate various possible cross-sectional shapes for the sealing rings discussed above. In particular, the sealing ring may be of a delta shape as shown in  FIG. 9(   a ), an X-shape as shown in  FIG. 9(   b ); an O-ring as shown in  FIG. 9(   c ); and a square ring as shown in  FIG. 9(   d ). Other cross-sectional shapes, however, are also contemplated. Furthermore, the sealing rings may be made from any suitable material including, but not limited to: nitrile (buna), silicone, fluorosilicone, hydrogenated nitrile, fluorocarbon (e.g., Viton® by DuPont), neoprene, ethylene propylene, butyl, polyurethane, ethylene acrylic (e.g., Vamac® by DuPont), polyacrylate, and tetrafluoroethylene-propylene (e.g., Aflas® by Asahi Glass). 
     Finally,  FIG. 10  depicts an alternate location of the sealing ring  902  with respect to the wafer edge support portion  904  of a chuck assembly. Whereas the previously described embodiments illustrate the sealing ring positioned outside of the wafer edge support  904  (with respect to the center of the wafer), the sealing ring  902  in  FIG. 10  is disposed on the inside of the wafer edge support  904 . If the sealing ring  902  is located outside the wafer edge support, there is a small force present on the wafer (due to the positive external seal pressure) that could possible bend the wafer upwards and cause defocus errors near the edge of the wafer. Although this condition is less likely with small overlap distances between the wafer edge and the wafer edge support, the inside placement of the sealing ring  902  with respect to the wafer edge support  904  would eliminate any such deflection. 
     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Technology Category: 3