Abstract:
A mask-pellicle assembly is disclosed. The mask-pellicle assembly includes a mask substrate having an absorber pattern and a hard pellicle attached to the mask substrate by exterior gas pressure.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to pellicles which reduce the propagation of defects in integrated circuits by shielding a mask from particles during photolithography. More particularly, the present invention relates to a new and improved double-decker mask-pellicle assembly which is characterized by enhanced durability and less susceptibility to distortion after mounting to a mask.  
       BACKGROUND OF THE INVENTION  
       [0002]     Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.  
         [0003]     During the photolithography step of semiconductor production, light energy is applied through a mask onto the photoresist material previously deposited on the wafer to define circuit patterns which will be etched in a subsequent processing step to define the circuits on the wafer. Because these circuit patterns on the photoresist represent a two-dimensional configuration of the circuit to be fabricated on the wafer, minimization of particle generation and uniform application of the photoresist material to the wafer are very important. By minimizing or eliminating particle generation during photoresist application, the resolution of the circuit patterns, as well as circuit pattern density, is increased.  
         [0004]     Masks must remain meticulously clean for the creation of perfect images during its many exposures to pattern a circuit pattern on a substrate. The mask may be easily damaged such as by dropping of the mask, the formation of scratches on the mask surface, electrostatic discharge (ESD), and particles. ESD can cause discharge of a small current through the chromium lines on the surface of the mask, melting a circuit line and destroying the circuit pattern. Therefore, a pellicle is typically attached to a mask to prevent particles from accumulating on the mask.  
         [0005]     Pellicles are necessary to prevent the propagation of particle-related defects in semiconductor device components during the use of steppers and scanners. The pellicle includes a membrane which covers the mask to keep unwanted particles safely out of focus from the patterned side of the mask. Particles which land on the pellicle or on the other side of the mask only contribute slightly to the patterning process since they are far away from the object plane of the imaging system.  
         [0006]     Generally, two different types of pellicles are used in semiconductor fabrication: soft pellicles and hard pellicles. Soft pellicles, which are easy to manufacture and handle, are fabricated by dropping an organic solution onto a high-speed spinning device to form a membrane. This membrane will be attached to a rigid frame, which in turn is attached to a mask. Soft pellicles are used for  193  nm or longer wavelength exposures. For wavelengths shorter than  193  nm, the existing materials used for soft pellicles are not suitable. These materials decay within hundreds of laser illumination exposures.  
         [0007]      FIGS. 1 and 2  illustrate a mask  8  on which is mounted a conventional soft pellicle  10 . The mask  8  includes an absorber pattern  16  which is provided on a transparent substrate  14  such as quartz and defines the circuit pattern image to be transferred to a photolithography layer (not shown) on a wafer. The pellicle  10  includes a pellicle frame  12  which is attached to the substrate  14  and surrounds the absorber pattern  16 . A transparent pellicle film  13  spans the pellicle frame  12  and extends over the absorber pattern  16 . An air cavity  17  is defined between the pellicle film  13  and the substrate  14 .  
         [0008]      FIGS. 3 and 4  illustrate a mask  18  on which is mounted a conventional hard pellicle  20 . The mask  18  includes an absorber pattern  26  provided on a transparent substrate  14  such as quartz. The pellicle  20  includes a pellicle frame  22  which is attached to the substrate  24  and surrounds the absorber pattern  16 . The pellicle  20  is mounted on the pellicle frame  22  and extends over the absorber pattern  26 . An air cavity  27  is defined between the pellicle  20  and the substrate  24 .  
         [0009]     Hard pellicles are difficult to manufacture and to mount on a flat planar surface of a mask. For an ordinary 150-nm mask, a hard pellicle includes a transparent plate having a length of 140 mm, a width of 120 mm and a thickness on the order of 1 mm. Because of its non-negligible thickness, the hard pellicle is considered an optical element. Therefore, its smoothness and flatness must be kept within a fraction of the exposure wavelength. Moreover, the pellicle tilt must be within optical limits. Because of these strict requirements, hard pellicles are very expensive. In some extreme cases, a high-quality hard pellicle is more expensive than the mask to which the pellicle is attached.  
         [0010]     Another drawback of hard pellicles is their fragility. Hard pellicles suffer distortion on the order of 4 μm from center to edges when attached to a mask. Furthermore, hard pellicles are easy to damage during the mounting and dismounting processes.  
         [0011]     Therefore, a mask-hard pellicle assembly is needed which is characterized by enhanced durability and less susceptibility to distortion after mounting to a mask.  
         [0012]     An object of the present invention is to provide a novel mask-pellicle assembly which is durable.  
         [0013]     Another object of the present invention is to provide a novel mask-pellicle assembly which is low-cost.  
         [0014]     Still another object of the present invention is to provide a novel mask-pellicle assembly which are resistant to distortion.  
         [0015]     Yet another object of the present invention is to provide a novel mask-pellicle assembly which does not require glue or other adhesives for mounting.  
         [0016]     A still further object of the present invention is to provide a mask-pellicle assembly which is recyclable.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention is generally directed to a novel double-decker mask-pellicle assembly which includes a hard pellicle attached to a mask by vacuum pressure. Various sealing mechanisms are provided between the pellicle and the mask to prevent the leakage of atmospheric air between the pellicle and the mask. The pellicle-mask assembly is characterized by low cost, enhanced strength and distortion resistance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0019]      FIGS. 1 and 2  are top and sectional views, respectively, of a typical conventional mask-pellicle assembly;  
         [0020]      FIGS. 3 and 4  are top and sectional views, respectively, of another conventional mask-pellicle assembly;  
         [0021]      FIGS. 5 and 6  are top and sectional views, respectively, of a mask-pellicle assembly according to a first embodiment of the present invention;  
         [0022]      FIGS. 7 and 8  are top and sectional views, respectively, of a mask-pellicle assembly according to a second embodiment of the present invention;  
         [0023]      FIGS. 9 and 10  are top and sectional views, respectively, of a mask-pellicle assembly according to an alternative second embodiment of the mask-pellicle assembly shown in  FIGS. 7 and 8 ;  
         [0024]      FIG. 11  is a cross-sectional view of a mask-pellicle assembly according to a third embodiment of the present invention;  
         [0025]      FIG. 12  is a cross-sectional view of a mask-pellicle assembly according to a fourth embodiment of the present invention;  
         [0026]      FIGS. 13 and 14  are top and sectional views, respectively, of a mask-pellicle assembly according to a fifth embodiment of the present invention;  
         [0027]      FIG. 15  is a top view of a mask-pellicle assembly according to a sixth embodiment of the present invention;  
         [0028]      FIGS. 16 and 17  are top and sectional views, respectively, of a mask-pellicle assembly according to a seventh embodiment of the present invention;  
         [0029]      FIG. 18  is a sectional view of a mask-pellicle assembly according to an eighth embodiment of the present invention; and  
         [0030]      FIG. 19  is a top view of a mask-pellicle assembly according to a ninth embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     Referring initially to  FIGS. 5 and 6 , a first embodiment of a pellicle-mask assembly of the present invention is generally indicated by reference numeral  29 . The pellicle-mask assembly  29  includes a mask  30  having a transparent substrate  31  which may be quartz, for example. An absorber pattern and/or phase-shift pattern  32  is formed on the surface of the substrate  31  using techniques known by those skilled in the art. In fabrication of the pellicle-mask assembly  29 , a hard pellicle  34 , having a transparent pellicle body  35  which is typically quartz, is secured against the absorber pattern  32  using vacuum pressure. Preferably, the hard pellicle  34  has a thickness of at least about 1 mm. Accordingly, attachment of the hard pellicle  34  to the mask  30  may be carried out in a conventional vacuum chamber (not shown). In the fabricated pellicle-mask assembly  29 , vacuum spaces  33  exist in the interstices defined by the absorber pattern  32 , whereas air spaces  36  are defined between the substrate  31  and the pellicle body  35  at the edges of the absorber pattern  32 . Therefore, the absorber pattern  32  abuts against the pellicle body  35  to form a seal which contains the vacuum pressure in the vacuum spaces  33  that secures the pellicle  34  to the mask  30 . Atmospheric air pressure presses against the pellicle body  35  and mask substrate  31  to maintain the structural integrity of the pellicle-mask assembly  29 .  
         [0032]     In use of the pellicle-mask assembly  29 , the assembly  29  is placed on a mask stage in a scanner (not shown) or stepper (not shown). UV light  37  is directed through the pellicle  34 , absorber pattern  32  and mask substrate  31 , respectively, and onto the surface of a photoresist layer (not shown) provided on a wafer. The UV light  37  transfers the circuit pattern image defined by the absorber pattern  32  onto the photoresist layer, which is developed to define the circuit pattern image to be etched in an underlying layer, as is known by those skilled in the art.  
         [0033]     Referring next to  FIGS. 7-10 , a second embodiment of a pellicle-mask assembly of the present invention is generally indicated by reference numeral  39  and includes a mask  40  having a transparent substrate  41  and an absorber pattern and/or phase-shift pattern  42  on the surface of the substrate  41 . In fabrication of the pellicle mask assembly  39 , a hard pellicle  44 , having a transparent pellicle body  45 , is secured against the absorber pattern  42  using vacuum pressure, and this step may be carried out in a conventional vacuum chamber (not shown). In the fabricated pellicle-mask assembly  39 , vacuum spaces  43  exist in the interstices defined by the absorber pattern  42 . A soft sealing frame  46 , which may be plastic, for example, is interposed between the mask substrate  41  and the pellicle body  45  along the edges or perimeter of the absorber pattern  42 . In the embodiment of the pellicle-mask assembly  39   a  shown in  FIGS. 9 and 10 , the sealing frame  46   a  is rubber. An alternative material for the sealing frame  46  includes an oxide. The vacuum pressure in the vacuum spaces  43  secures the pellicle  44  to the mask  40 .  
         [0034]     Referring next to  FIG. 11 , a third embodiment of a pellicle-mask assembly of the present invention is generally indicated by reference numeral  59  and includes a mask  60  having a transparent substrate  61  and an absorber pattern and/or phase shift pattern  62  on the surface of the substrate  61 . A hard pellicle  64 , having a transparent pellicle body  65 , is secured against the absorber pattern  62  using vacuum pressure. Vacuum spaces  63  exist in the interstices defined by the absorber pattern  62  and at the edges or perimeter of the absorber pattern  62 . A flat O-ring  66 , which may be rubber or plastic, for example, is provided along the edges of the pellicle-mask assembly  59 , and tightly engages the edges of the mask substrate  61  and pellicle body  65 . The vacuum pressure in the vacuum spaces  63  secures the pellicle  64  to the mask  60 . The O-ring  66  prevents air from entering between the mask substrate  61  and pellicle body  65 , thus maintaining the integrity of the vacuum pressure in the vacuum spaces  63 .  
         [0035]     Referring next to  FIG. 12 , a fourth embodiment of a pellicle-mask assembly  69  of the present invention includes a mask  70  having a transparent substrate  71  and an absorber pattern  72 . A hard pellicle  74 , having a transparent pellicle body  75 , is secured against the absorber pattern  72  by vacuum pressure. Vacuum spaces  73  are defined by the interstices in the absorber pattern  72  and at the edges of the absorber pattern  72 . A round O-ring  76 , which may be rubber or plastic, for example, is provided along the edges of the pellicle-mask assembly  69 . The round O-ring  76  is interposed between the mask substrate  71  and pellicle body  75 . The O-ring  76  prevents air from entering between the mask substrate  71  and pellicle body  75  and maintains the integrity of the vacuum pressure in the vacuum spaces  73 .  
         [0036]     Referring next to  FIGS. 13-15 , a fifth embodiment of a pellicle-mask assembly of the present invention is generally indicated by reference numeral  79 . The assembly  79  includes a mask  80  having a transparent substrate  81  and an absorber pattern and/or phase shift pattern  82 . A hard pellicle  84 , having a transparent pellicle body  85 , is spaced from the absorber pattern  82  by a rigid inner support  86  and a soft or resilient outer frame  87  which surrounds the inner support  86 . In the embodiment of the assembly  79   a  shown in  FIG. 15 , the inner support  86  has rounded corners  86   a  and the outer frame  87  has rounded corners  87   a . A vacuum space  83  is defined between the mask substrate  81  and the pellicle body  85 . The vacuum space  83  is defined by assembling the mask substrate  81  and pellicle body  85  on the inner support  86  and outer frame  87  in a vacuum chamber (not shown). The inner support  86  and outer frame  87  prevent air from leaking into the vacuum space  83  from outside the pellicle-mask assembly  79  and disrupting the integrity of the vacuum pressure in the vacuum space  83 .  
         [0037]     A sixth embodiment of a pellicle-mask assembly of the present invention is generally indicated by reference numeral  99  in  FIGS. 16 and 17  and includes a mask  100  having a transparent substrate  101  and an absorber pattern  102  thereon. A hard pellicle  104  having a transparent pellicle body  105  is spaced from the absorber pattern  102  by a rigid support  106  which typically has a generally “H”-shaped cross-sectional configuration, as shown in  FIG. 17  and extends around the perimeter of the absorber pattern  102 . A sealing material  107  is interposed between the rigid support  106  and the mask substrate  101  and between the rigid support  106  and the pellicle body  105 . A vacuum space  103  is defined between the mask substrate  101  and the pellicle body  105 . The vacuum space  103  is formed by assembling the mask substrate  101  and pellicle body  105  on the rigid support  106  in a vacuum chamber (not shown). The rigid support  106  prevents air from leaking into the vacuum space  103  from outside the pellicle-mask assembly  99  and disrupting the integrity of the vacuum seal in the vacuum space  103 .  
         [0038]     Referring next to  FIG. 18 , a seventh embodiment of a pellicle-mask assembly of the present invention is generally indicated by reference numeral  109  and includes a mask  110  having a transparent substrate  111  and an absorber pattern  112  provided thereon. A hard pellicle  114  having a transparent pellicle body  115  is spaced from the absorber pattern  112  by a rigid support  116  typically having a generally “H”-shaped cross-sectional configuration. A sealing material  117  may be interposed between the rigid support  116  and the mask substrate  111  and between the rigid support  116  and the pellicle body  115 . The pellicle-mask assembly  109  is assembled in a vacuum chamber (not shown) to form a vacuum space  113  between the mask substrate  111  and the pellicle body  115 . A mechanical support bracket  119 , which typically has a generally “C”-shaped configuration, as shown, engages the respective sides of the assembly  109 . A resilient pad  118  is interposed between the pellicle body  115  and the upper segment of each mechanical support bracket  119  and between the mask substrate  111  and the lower segment of each mechanical support bracket  119 . The rigid supports  116  and mechanical support brackets  119  prevent air from leaking into the vacuum space  113  from outside the pellicle-mask assembly  109  and disrupting the integrity of the vacuum seal in the vacuum space  113 .  
         [0039]     A top view of an eighth embodiment of the pellicle-mask assembly of the present invention is generally indicated by reference numeral  129  in  FIG. 19 . The assembly  129  includes a mask  120  having a transparent mask substrate  121  and an absorber pattern  122  provided thereon. A hard pellicle  124  having a transparent pellicle body  125  is spaced from the absorber pattern  122  by a rigid support  126  which typically has a generally “H”-shaped cross-sectional configuration, as heretofore described with respect to the rigid support  116  of  FIG. 18 . A sealing material  127  may be interposed between the rigid support  126  and the mask substrate  121  and between the rigid support  126  and the pellicle body  125 . The pellicle-mask assembly  129  is assembled in a vacuum chamber (not shown) to form a vacuum space  123  between the mask substrate  121  and the pellicle body  125 , in the same manner as heretofore described with respect to the vacuum space  113  of  FIG. 18 . The rigid supports  126  prevent air from leaking into the vacuum space from outside the pellicle-mask assembly  129 . A safety stop  128  is provided on each end of the pellicle body  125  to protect the mask  120  on a stepper or scanner stage (not shown) during a photolithography process.  
         [0040]     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.