Patent Publication Number: US-2006001851-A1

Title: Immersion photolithography system

Description:
FIELD OF THE INVENTION  
      This invention relates to an immersion photolithography system, and to a method of performing immersion photolithography.  
      Photolithography is an important process step in semiconductor device fabrication. In photolithography, a circuit design is transferred to a wafer through a pattern imaged onto a photoresist layer deposited on the wafer surface. The wafer then undergoes various etch and deposition processes before a new design is transferred to the wafer surface. This cyclical process continues, building up the multiple layers of the semiconductor device.  
      The minimum feature that may be printed using photolithography is determined by the resolution limit W, which is defined by the Rayleigh equation as:  
             W   =         k   1     ⁢   λ     NA             (   1   )             
 
 where k 1  is the resolution factor, λ is the wavelength of the exposing radiation and NA is the numerical aperture. In lithographic processes used in the manufacture of semiconductor devices, it is therefore advantageous to use radiation of very short wavelength in order to improve optical resolution so that very small features in the device may be accurately reproduced. Monochromatic visible light of various wavelengths have been used, and more recently radiation in the deep ultra violet (DUV) range has been used, including radiation at 193 nm as generated using an ArF excimer laser. 
 
      The value of NA is determined by the acceptance angle (α) of the lens and the index of refraction (n) of the medium surrounding the lens, and is given by 
 
 NA=n  sin α  (2) 
 
      For clean dry air (CDA), the value of n is 1, and so the physical limit to NA for a lithographic technique using CDA as a medium between the lens and the wafer is 1, with the practical limit being currently around 0.9.  
      Immersion photolithography is a known technique for improving optical resolution by increasing the value of NA. With reference to  FIG. 1 , in this technique a liquid  10  having a refractive index n&gt;1 is placed between the lower surface of the objective lens  12  of a projection device  14  and the upper surface of a wafer  16  located on a moveable wafer stage  18 . The liquid placed between lens  12  and wafer  16  should, ideally, have a low optical absorption at 193 nm, be compatible with the lens material and the photoresist deposited on the wafer surface, and have good uniformity. These criteria are met by ultra-pure, degassed water, which has a refractive index n≈1.44. The increased value of n, in comparison to a technique where the medium between lens and wafer is CDA, increases the value of NA, which in turn decreases the resolution limit W, enabling smaller features to be reproduced.  
      Whilst ultra-pure water is ideal for the current generation of lens geometries, even higher refractive index liquids will be required for hyper-NA lens geometries. For example, an organic liquid having the required refractive index can replace the ultra-pure water. A more attractive alternative is to add one or more compounds to the water to increase its refractive index. A problem associated with the use of a saturated solution is that, during immersion lithography, there will be evaporation of ultra-pure water at the interface between the lens and the liquid solution and at the interface between the wafer and the liquid solution, leading to the deposition at these interfaces of solute from the solution.  
      It is an object of the present invention to provide a system which inhibits evaporation from immersion liquid located between the lens and wafer in an immersion photolithography system.  
     SUMMARY OF THE INVENTION  
      In a first aspect, the present invention provides an immersion lithography system comprising a wafer stage, a lens for projecting an image onto a wafer located on the wafer stage, immersion fluid supply means for supplying immersion fluid between the lens and the wafer, and purge fluid conveying means for conveying about the supplied immersion fluid a purge fluid saturated with a component of the immersion fluid.  
      By conveying about the immersion fluid a purge fluid saturated with a component of the immersion fluid, evaporation from the immersion fluid can be inhibited. This can prevent the deposition during photolithography of particulates at the interfaces between the immersion fluid and the lens, wafer and/or purge fluid. Where the immersion fluid is a pure liquid, such as ultra-pure water, saturating the purge fluid with the liquid can prevent the deposition at these interfaces of particulates formed within the liquid, for example, from the photoresist layer, during photolithography. Where the immersion fluid is a solution, saturating the purge fluid with the solvent can also inhibit the deposition of solute at these interfaces.  
      The purge fluid may comprise one of clean, dry air (CDA), nitrogen, or any other liquid or gas which does not react adversely with the immersion fluid, an example of which is a water-based solution containing an inorganic or organic solute.  
      In another aspect of the present invention, the immersion lithography system comprises an enclosure housing the wafer stage and the lens, the purge fluid supply system being configured to supply to the enclosure a stream of purge fluid. This enclosure can assist in maintaining a saturated environment about the immersion fluid, and so in a second aspect the present invention provides an immersion lithography system comprising an enclosure housing a wafer stage and a lens for projecting an image onto a wafer located on the wafer stage, immersion fluid supply means for supplying into the enclosure immersion fluid through which, during use, the lens projects an image onto the wafer, and purge fluid conveying means for conveying through the enclosure a purge fluid saturated with a component of the immersion fluid.  
      In another aspect of the present invention, a method is provided for performing immersion photolithography comprising the steps of locating an immersion fluid between a wafer and a lens, projecting an image onto the wafer through the immersion fluid, and conveying about the immersion fluid a purge fluid saturated with a component of the immersion fluid.  
      In yet a further aspect of the present invention, a method is provided for performing immersion photolithography comprising the steps of providing an enclosure housing a lens, positioning within the enclosure a wafer such that the lens projects an image onto the wafer, maintaining within the enclosure an immersion fluid between the lens and the wafer, and conveying through the enclosure a purge fluid saturated with a component of the immersion fluid.  
      Features described above in relation to system aspects of the invention are equally applicable to method aspects, and vice versa. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates schematically a known immersion photolithography system; and  
       FIG. 2  illustrates schematically the immersion photolithography system in accordance with the present invention. 
    
    
      With reference to  FIG. 2 , an immersion photolithography system  20  comprises an enclosure  22  housing an imaging lens  24  and a wafer stage  26  in a controlled environment. The imaging lens  24  is the final optical component of an optical system for projecting an image onto a photoresist layer formed on the surface of wafer  28  located on the wafer stage  26 . The wafer stage  26  may comprises any suitable mechanism for holding the wafer  28  to the wafer stage, for example a vacuum system, and is moveable to position accurately the wafer  28  beneath the imaging lens  24 .  
      Immersion fluid  30  is maintained between the lens  24  and the wafer  28  by an immersion fluid supply system. This system comprises an immersion fluid dispenser  32  surrounding the lens  24  to dispense the immersion fluid  30  locally between the lens  24  and the wafer  28 . One or more differential air seals (not shown) may be used to prevent the ingress of immersion fluid into other parts of the system, for example, the mechanism used to move the wafer stage  26 .  
      Due to outgassing from the photoresist layer and the generation of particulates during photolithography, it is desirable to maintain a steady flow of immersion fluid between the lens  24  and the wafer  28 . As depicted in  FIG. 2 , the immersion fluid supply system comprises an evacuation system, shown generally at  34 , for drawing the immersion fluid  30  from between the lens  24  and the wafer  28 , the dispenser  32  serving to replenish the immersion fluid  30  so that a substantially constant amount of immersion fluid  30  is maintained between the lens  24  and the wafer  28 . An immersion fluid supply, shown generally at  36 , serves to supply the immersion fluid to the dispenser  32  from a source  38  thereof. Optionally, the immersion fluid drawn from the enclosure  22  may be recycled and recirculated back to the dispenser  32 .  
      An example of a suitable immersion fluid is ultra-pure, degassed water, due to its relatively high refractive index of 1.44 compared to air (having a refractive index of 1) and its compatibility with the lens material and photoresist. In order to increase the refractive index further, inorganic or organic compounds may be added to the water to form a saturated solution. Such a compound may be an organic, polar compound or an inorganic ionic compound. Inorganic salts having relatively large ions can be used such as caesium sulphate. In order to achieve as high a refractive index as possible, the solution of ultra-pure water and inorganic salt should be blended so as to have a high saturation level. In either case, evaporation of water during the photolithographic process can cause deposits to be formed at the interface between the lens  24  and the immersion fluid  30 , and at the interface between the wafer  28  and the immersion fluid  30 . Where the immersion fluid is a pure liquid, such as ultra-pure water, the sources of these deposits are particulates formed during photolithograpy, whereas where the immersion fluid is a solution, these particulates can additionally comprise micro crystals of the solute.  
      In order to inhibit the evaporation of the liquid, or solute, from the immersion fluid  30  during photolithography, a purge fluid supply system is provided for supplying to the enclosure  22 , and in particular about the immersion fluid  30  within the enclosure  22 , a purge fluid saturated with the liquid, or solute as the case may be, of the immersion fluid  30 . The purge fluid is conveyed from a source  40  into the enclosure  22  via conduit  42  communicating with an inlet  44  of the enclosure  22 . In order to maintain a steady flow of purge fluid within the enclosure  22 , a purge fluid evacuation system is provided from drawing the purge fluid from the enclosure  22  via conduit  46  communicating with an outlet  48  of the enclosure  22 .  
      Where the liquid, or solute, is water, for example, the purge fluid may conveniently comprise water-saturated CDA. This can be produced in the source  40  by passing a stream of CDA over one side of a membrane contactor in fluid communication with ultra-pure water on its other side. The water-saturated CDA is then conveyed into the enclosure  22  to purge the interface between the lens  24  and the immersion fluid  30  and the interface between the wafer  28  and the immersion fluid  30  to inhibit the evaporation of water from the immersion fluid  30 .  
      While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.