Patent Publication Number: US-2011051113-A1

Title: Lithography system and optical module thereof

Description:
BACKGROUND OF THE INVENTION  
     1. Field of the Invention 
     The present invention relates generally to the field of integrated circuit manufacturing and, more particularly, to a lithography system with lenses immersed in a liquid medium. 
     2. Description of the Prior Art 
     The manufacture of integrated circuits requires multiple photolithographic steps to define and create specific circuit features and components layer-by-layer onto a semiconductor wafer. 
     For instance, patterns can be formed from a photo resist layer disposed on the wafer by passing light energy through a mask having an arrangement in order to image the desired pattern onto the photo resist layer. As a result, a latent pattern is transferred to the photo resist layer. In areas where the photo resist layer is sufficiently exposed, after a development cycle, the photo resist layer can become soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photo resist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer. Afterwards, the remaining portions of the photo resist layer will be removed. 
     There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. As a result, there is a corresponding need to increase the resolution of lithography systems. 
     A conventional method for improving resolution includes the methods of: off-axis illumination, immersion lithography and increasing the numerical aperture of the lens. In addition, some methods involve adjusting equipment parameters, such as adapting exposure energy and exposure time in order to achieve a better resolution and achieve a compromise between resolution and depth of focus. However, satisfactory results have not yet been obtained. 
     Therefore, it is important to develop a lithography system with improved resolution that has compatibility with current equipment. 
     SUMMARY OF THE INVENTION  
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its purpose is merely to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     According to a preferred embodiment of the invention, an optical module of a lithography system comprises: a container; a liquid medium positioned within the container; and a first set of lenses immersed in the liquid medium. 
     According to another preferred embodiment of the invention, a lithography system comprises: a light source; a photo mask positioned downstream of the light source; and an optical module having a front surface positioned downstream of the photo mask, wherein the optical module comprises: a container; a liquid medium situated in the container; and a first set of lenses immersed in the liquid medium. The lithography system further comprises: a wafer stage positioned downstream of the optical module for supporting a wafer, wherein the wafer comprises a dry film; and a first medium positioned between the front surface of the optical module and a surface of the dry film. 
     A feature of the present invention is that the lenses in the optical module of the lithography system are immersed in the liquid medium to thereby improve the numerical aperture. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of a lithography system according to a preferred embodiment of the present invention. 
         FIG. 2  is a magnified localized view of the second lens module shown in  FIG. 1  according to the first embodiment of the present invention. 
         FIG. 3  is a magnified localized view of the second lens module shown in  FIG. 1  according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION  
       FIG. 1  depicts a schematic diagram of a lithography system  10  according to a preferred embodiment of the present invention. As shown in  FIG. 1 , a lithography system  10  includes: a light source  12 , a first lens module  14  positioned downstream of the light source  12 , a photo mask  16  positioned downstream of the first lens module  14 , a second lens module  18  having a front surface  20  facing away from the photo mask  16 , and a wafer stage  22  positioned downstream of the second lens module  18  for supporting a wafer  24 , wherein the wafer  24  includes a dry film  26 . In addition, there is a medium  28  disposed between the front surface  20  and the top surface of the dry film  26 . 
     The light source  12  can have, for example, a deep ultraviolet wavelength (e.g., about 248 nm or about 193 nm), or a vacuum ultraviolet (VUV) wavelength (e.g., about 157 nm), although other wavelengths (e.g., an extreme ultraviolet wavelength) are possible and are also considered to fall within the scope of the invention described and claimed herein. The photo mask  16  selectively blocks light source  12  such that a pattern defined by the photo mask  16  is transferred towards the dry film  26 . 
     The medium  28  may be air. According to a preferred embodiment of the present invention, the medium  28  may be a liquid, a supercritical fluid, or other medium having a refractive index that is greater than 1.4 at a wavelength of 193 nm. 
     It is noteworthy that the second lens module  18  of the present invention has a special design to increase the resolution of the lithography system  10 . 
       FIG. 2  is a magnified localized view of the second lens module  18  shown in  FIG. 1  according to the first embodiment of the present invention. As shown in  FIG. 2 , the second lens module  18  includes a container  30 , a liquid medium  32  within the container  30  and a set of lenses  34  immersed in the liquid medium  32 . The shape of the container  30  may be cylindrical or other shapes. The shape of the container  30  given in  FIG. 2  is for illustrative purposes only. 
     The liquid medium  32  may be de-ionized water, a mixture of phosphoric acid (H 3 PO 4 ) and water, a phosphoric acid solution, “Delphi”, which is available from Mitsui Chemical, oil (e.g., perfluorinated polyethers (PFPE)) or other liquids having a refractive index that is greater than 1.4 at a wavelength of 193 nm. The refractive index of the liquid medium  32  corresponds to the refractive index of the set of lenses  34 , such that the refractive index of the liquid medium  32  matches or approaches the refractive index of the set of lenses  34 . The refractive index of the liquid medium  32  depends on the refractive index of the whole lithography system including lenses, the photo resist, and other mediums. The refractive index of the liquid medium  32  can be any value which matches the refractive indices of the whole lithography system. Moreover, the liquid medium  32  may be identical to the medium  28 . 
     According to anther preferred embodiment of the present invention, the second lens module includes two sets of lenses immersed in different mediums respectively.  FIG. 3  is a magnified localized view of the second lens module  18  in  FIG. 1  according to a second embodiment of the present invention, wherein like reference numerals are used to refer to like elements throughout. 
     As shown in  FIG. 3 , the second lens module  18  includes a container  18 , a liquid medium  40  in the container  18 , a medium  44  in the container  18  which is adjacent to the liquid medium  40 , a first set of lenses  36  immersed in the liquid medium  40  and a second set of the lenses  38  immersed in the medium  44 . A glass  42  may be optionally disposed between the liquid medium  40  and the medium  44  to separate the liquid medium  40  and the medium  44 . 
     The liquid medium  40  may be de-ionized water, a mixture of phosphoric acid (H 3 PO 4 ) and water, a phosphoric acid solution, “Delphi”, which is available from Mitsui Chemical, oil (e.g., perfluorinated polyethers (PFPE)) or other liquids having a refractive index that is greater than 1.4 at a wavelength of 193 nm. The refractive index of the liquid medium  40  depends on the refractive indices of the whole lithography system including lenses, the photo resist, and other mediums. The refractive index of the liquid medium  40  can be any value which matches the refractive indices of the whole lithography system. Generally, the refractive index of the medium  40  is greater than 1.4 at a wavelength of 193 nm. 
     The medium  44  may be air. According to a preferred embodiment of the present invention the medium  44  may be preferably de-ionized water, a mixture of phosphoric acid (H 3 PO 4 ) and water, a phosphoric acid solution, “Delphi”, which is available from Mitsui Chemical, oil (e.g., perfluorinated polyethers (PFPE)), supercritical fluid or other mediums having a refractive index that is greater than 1.4 at a wavelength of 193 nm. For instance, the first set of lenses  36  may be immersed in de-ionized water and the second set of lenses  38  may be immersed in mixture of phosphoric acid and water. 
     Although the above embodiments merely describe one and two medium to immerse one and two sets of lenses, respectively, other combinations of lenses and mediums can be used to implement the invention. For example, there may be three, four or more than four sets of lenses to be immersed in different kinds of mediums respectively. Taking three sets of lenses as an example, the three sets of lenses immersed in de-ionized water, a mixture of phosphoric acid, and Delphi, respectively, or the three sets of lenses immersed in de-ionized water, air, and a mixture of phosphoric acid and water, respectively, are considered to fall within the scope of the invention described and claimed herein. In addition, although only the second lens module  18  is described in the above embodiment, the lenses in the first lens module  14  can also utilize the same design used by the second lens module  18 . 
     In particular, the resolution can be defined as: resolution=κλ/NA where κ is a lithographic constant, λ is an exposure radiation wavelength, and NA is a numerical aperture. Furthermore, the numerical aperture can be derived as follows: NA=n sin θ where n is a refractive index of the medium in which the lenses are working and 2θ is an angle of acceptance of a lens. Thus, the resolution can be increased by increasing the refractive index and/or decreasing the lithographic constant. 
     Therefore, the liquid medium  32 ,  40  of the first and second embodiment possess a refractive index greater than 1.4 at a wavelength of 193 nm, which increases the numerical aperture. If the medium  44  of the second embodiment also possesses a refractive index greater than 1.4 at a wavelength of 193 nm, the numerical aperture can be further increased. As the numerical aperture increases, the resolution is improved. Furthermore, for a traditional lens module, one skilled in the art should know that the complexity of the lenses is increased as the numerical aperture increases. However, compared to the traditional lens module, the lenses in the present invention require a less complex design to reach the same numerical aperture. 
     Moreover, the mediums  32 ,  40 ,  44  that immerse the lenses can be chosen depending on different designs. Usually, mediums that immerse the lenses may have refractive indices of greater than 1.4 at a wavelength of 193 nm. In fact, the lithography system includes many elements with different refractive indices; if the refractive indices of two adjacent elements are enormously different, total reflection may occur. The mediums in the present invention can also be a bridge to compromise the different refractive indices of the elements in the lithography system. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.