Abstract:
Charged-particle-beam (CPB) optical systems, and CPB microlithography apparatus including CPB optical systems, are disclosed that include a “shaping aperture” that absorbs a very low percentage of incident charged particles and hence does not experience excessive temperature increases due to bombardment by and absorption of incident charged particles. Nevertheless, the shaping apertures are effective for trimming and shaping a charged particle beam to produce a downstream-propagating beam having a desired transverse profile. The aperture opening in the shaping aperture is defined in a conductive thin-film membrane. The membrane thickness is configured to cause charged particles incident on the membrane to experience scattering (e.g., forward-scattering). CPB optical systems including the shaping aperture also include a “screening aperture” downstream of the shaping aperture to block (absorb) scattered charged particles. The screening aperture is made from a relatively thick conductive sheet and is situated where the shaped beam forms a crossover downstream of the shaping aperture.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention pertains to charged-particle-beam (CPB) “optical” systems as used, for example, in CPB microlithography apparatus. Microlithography is a key technique used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to CPB optical systems comprising at least one aperture serving to “trim” or shape the charged particle beam as the beam passes through an opening defined by the aperture by absorption of outlying particles of the beam.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional charged-particle-beam (CPB) optical systems typically include at least one “shaping aperture” constructed of an aperture plate defining an opening through which the charged particle beam passes. The opening is sized such that, as the beam passes through the opening, peripheral regions of the transverse profile of the beam are clipped by respective edges of the opening. Hence, shaping apertures generally are used, for example, for trimming the beam, shaping the transverse profile of the beam, or aligning the beam.  
           [0003]    Conventionally, the aperture plate of a shaping aperture is fabricated from a sheet of metal (e.g., molybdenum) having a thickness sufficient to absorb the charged particles of the clipped portions of the beam. This absorption causes heating of the aperture plate. Excessive heating results in distortion and/or damage to the aperture plate, which causes undesired changes in the size and/or geometry of the opening. The heating also can extend to neighboring structural components that can be deformed or damaged by the heat. For example, elastomeric O-rings located near the aperture can be deformed or damaged from heat.  
           [0004]    The conventional approach to the problem of heating of the shaping aperture is to cool the aperture plate actively, such as by circulating a heat-exchange fluid through passages in the aperture plate and surrounding structures. Unfortunately, this approach results in substantial apparatus complexity and cost.  
         SUMMARY OF THE INVENTION  
         [0005]    In view of the shortcomings of conventional apparatus as summarized above, an object of the invention is to provide charged-particle-beam (CPB) optical systems including a beam-trimming and/or profile-shaping aperture (termed generally herein a “shaping aperture”) exhibiting substantially reduced absorption of incident charged particles compared to conventional systems. Another object is to provide CPB optical systems including at least one shaping aperture that exhibits substantially less heating during normal operation than conventional systems. Yet another object is to provide CPB optical systems (including at least one shaping aperture) having less complexity and lower cost, compared to conventional systems, without compromising performance. Yet another object is to provide CPB optical systems in which temperature control of the shaping aperture(s) and neighboring components is significantly easier to achieve, compared to conventional systems.  
           [0006]    To such ends, and according to a first object of the invention, CPB optical systems are provided. An embodiment of such a system comprises a shaping aperture and a screening aperture. The shaping aperture is situated and configured to receive a beam of charged particles propagating along an optical axis from a CPB source. The shaping aperture comprises a conductive thin-film membrane defining an aperture opening that transmits at least a portion of the beam incident on the shaping aperture. The thin-film membrane is configured to scatter the charged particles of the beam incident on the membrane without absorbing the incident charged particles, so as to form a shaped beam propagating downstream of the shaping aperture. The screening aperture is situated downstream of the shaping aperture at a location at which the shaped beam forms a crossover. The screening aperture comprises a conductive sheet defining an aperture opening having a width dimension corresponding to a width dimension of the crossover. The conductive sheet is sufficiently thick in an optical-axis direction so as to absorb charged particles incident on the sheet.  
           [0007]    By configuring the shaping aperture using a conductive thin-film membrane, the current of absorbed charged particles of the incident beam is limited to at most several percent of the current of charged particles absorbed by a conventional shaping aperture configured using a metal sheet. Hence, a shaping aperture according to this embodiment experiences much less heating than a conventional shaping aperture, thereby eliminating any need for an active cooling system for the shaping aperture. Also, temperature control of components near the shaping aperture is much simpler than conventionally.  
           [0008]    Charged particles that have been forward-scattered by the thin-film membrane of the shaping aperture are blocked by the screening aperture. Effective blocking is achieved by absorption of the forward-scattered charged particles by the relatively thick conductive sheet of the screening aperture (the conductive sheet can be, for example, 500 to 1000 μm thick), and by positioning the screening aperture at a crossover. Thus, the screening aperture prevents scattered charged particles from reaching the sensitive substrate.  
           [0009]    By way of example, the charged particle beam can be an electron beam. In such an instance, the thin-film membrane desirably has a thickness that is 10 to 100 times a mean-free-path length of electrons in the thin-film membrane. With a thickness in this range, most of the electrons in the beam incident on the membrane pass through (with scattering) the membrane without being absorbed by the membrane.  
           [0010]    The CPB optical system can include a first condenser lens and a second condenser lens situated at respective positions along the optical axis. The first condenser lens desirably is situated and configured to converge the charged particle beam, propagating from the CPB source, to form a “crossover” on the optical axis at a principal plane of the second condenser lens. The shaping aperture desirably is situated along the optical axis at the same position as the second condenser lens. The shaping aperture can be a beam-trimming aperture configured to determine an aperture angle of the charged particle beam emitted from the CPB source.  
           [0011]    The system also can include a profile-shaping aperture situated downstream of the second condenser lens but upstream of the screening aperture. The profile-shaping aperture desirably comprises a conductive thin-film membrane defining an aperture opening that transmits at least a portion of the beam incident on the profile-shaping aperture. The thin-film membrane scatters the charged particles of the beam incident on the membrane without absorbing the incident charged particles, so as to form a shaped beam propagating downstream of the profile-shaping aperture. The profile-shaping aperture can be situated at an axial position at which an image of a CPB-emitting surface of the CPB source is formed. This system also can include a third condenser lens situated downstream of the profile-shaping aperture and upstream of the screening aperture.  
           [0012]    The shaping aperture can be a beam-trimming aperture configured to determine an aperture angle of the charged particle beam emitted from the CPB source. In such a configuration, if the charged particle beam is an electron beam, the thin-film membrane desirably has a thickness that is 10 to 100 times a mean-free-path length of electrons in the thin-film membrane. CPB optical systems in which the shaping aperture is a beam-trimming aperture also can include a first condenser lens and a second condenser lens situated at respective positions along the optical axis. The first condenser lens is situated and configured to converge the charged particle beam, propagating from the CPB source, to form a “crossover” on the optical axis at a principal plane of the second condenser lens. The system also can include a profile-shaping aperture, as described above, situated downstream of the second condenser lens but upstream of the screening aperture.  
           [0013]    According to another aspect of the invention, CPB microlithography apparatus are provided. An exemplary embodiment of such an apparatus comprises an illumination-optical system comprising a CPB optical system as summarized above. The apparatus also includes a projection-optical system situated downstream of the illumination-optical system. The projection-optical system desirably comprises first and second projection lenses, and a contrast aperture situated axially at a beam crossover between the first and second projection lenses. The contrast aperture desirably includes a conductive sheet that defines an aperture opening corresponding to the beam crossover, the conductive sheet being sufficiently thick in an optical-axis direction so as to absorb charged particles incident on the sheet. The illumination-optical system desirably comprises a first condenser lens and a second condenser lens situated at respective positions along the optical axis. The first condenser lens desirably is situated and configured to converge the charged particle beam, propagating from the CPB source, to form a “crossover” on the optical axis at a principal plane of the second condenser lens. The shaping aperture desirably is situated along the optical axis at the same position as the second condenser lens. The shaping aperture can be a beam-trimming aperture configured to determine an aperture angle of the charged particle beam emitted from the CPB source. A profile-shaping aperture can be included, situated downstream of the second condenser lens but upstream of the screening aperture. If present, the profile-shaping aperture desirably is configured to include a conductive thin-film membrane defining an aperture opening that transmits at least a portion of the beam incident on the profile-shaping aperture. The thin-film membrane scatters the charged particles of the beam incident on the membrane without absorbing the incident charged particles, to form a shaped beam propagating downstream of the profile-shaping aperture.  
           [0014]    According to yet another aspect of the invention, methods are provided for microlithographically exposing a pattern, defined by a reticle, onto a sensitive substrate. The methods are performed using a charged particle beam propagating from a source through an illumination-optical system to the reticle, and from the reticle through a projection-optical system to a sensitive substrate. In this context, the methods are directed especially to shaping the charged particle beam. In an embodiment of such a method, a shaping aperture is provided and situated so as to receive the charged particle beam. The shaping aperture comprises a thin-film membrane defining an aperture opening that transmits at least a portion of the charged particle beam incident on the shaping aperture. The thin-film membrane is configured to scatter the charged particles of the beam incident on the membrane without absorbing the incident charged particles. The charged particle beam is passed through the aperture opening of the shaping aperture to form a shaped beam propagating downstream of the shaping aperture. A screening aperture is provided and situated downstream of the shaping aperture at a location at which the shaped beam forms a crossover. The screening aperture comprises a conductive sheet defining an aperture opening having a width dimension corresponding to a width dimension of the crossover. The conductive sheet is sufficiently thick in an optical-axis direction so as to absorb charged particles incident on the sheet. The charged particle beam is passed through the aperture opening of the screening aperture.  
           [0015]    The invention also encompasses CPB microlithography methods that comprise beam-shaping methods as summarized above, as well as device-manufacturing methods that comprising CPB microlithography methods as summarized above.  
           [0016]    The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic elevational diagram of an exemplary embodiment, according to the invention, of a charged-particle-beam (CPB) optical system as used in a CPB microlithography apparatus, wherein the CPB optical system includes both a beam-trimming aperture and a profile-shaping aperture (with screening aperture) according to the invention.  
         [0018]    [0018]FIG. 2 is a schematic elevational diagram depicting absorption of charged particles, scattered by impingement on an upstream profile-shaping aperture, by a downstream screening aperture.  
         [0019]    FIGS.  3 ( a )- 3 ( f ) are schematic elevational diagrams showing the results of respective steps in an exemplary method for manufacturing a beam-shaping aperture according to the invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]    This invention is described below in the context of a representative embodiment. It will be understood, however, that the described embodiment is not intended to be limiting in any way.  
         [0021]    [0021]FIG. 1 is a schematic elevational diagram of an exemplary embodiment, according to the invention, of a charged-particle-beam (CPB) optical system configured for use in a CPB microlithography apparatus. The FIG. -  1  apparatus is described in the context of employing an electron beam as a representative charged particle beam. It will be understood that the general principles of the FIG. 1 embodiment can be applied with equal facility to use of an alternative charged particle beam such as an ion beam.  
         [0022]    The FIG. 1 apparatus includes an electron gun  1  situated at an extreme upstream end of the CPB optical system. The electron gun  1  emits an electron beam downward in the figure (i.e., the beam emitted from the electron gun propagates in a downstream direction). The electron beam as emitted from the electron gun  1  propagates along an optical axis A.  
         [0023]    The FIG. 1 apparatus is configured for performing CPB microlithography, and hence comprises an “illumination-optical system” IOS and a “projection-optical system” POS. The illumination-optical system IOS is situated between the electron gun  1  and a “reticle”  19  that defines a pattern to be projected microlithographically onto a “sensitive substrate”  27  (e.g., semiconductor wafer coated with a suitable “resist”). The projection-optical system POS is situated between the reticle  19  and the substrate  27 .  
         [0024]    The illumination-optical system IOS comprises a first condenser lens  3 , a second condenser lens  5 , a beam-trimming aperture  7 , a profile-shaping aperture  9 , a third condenser lens  13 , a “screening” aperture  15 , and an illumination lens  17 . The projection-optical system POS comprises a first projection lens  21 , a contrast aperture  25 , and a second projection lens  23 .  
         [0025]    Although this embodiment has both a beam-trimming aperture  7  and a profile-shaping aperture  9 , in an alternative embodiment, the beam-trimming aperture  7  could be omitted.  
         [0026]    The electron beam emitted from the electron gun  1  is converged by the first condenser lens  3  to form a “crossover” on the optical axis at the principal plane of the second condenser lens  5 . The beam-trimming aperture  7  is situated at the same axial position as the second condenser lens  5 . The beam-trimming aperture  7  typically defines a circular opening that transmits the beam, thereby determining the downstream aperture angle of the beam. The beam-trimming aperture  7  can be made from a thin, electrically conductive membrane of, e.g., silicon or the like.  
         [0027]    The thickness of the membrane of the beam-trimming aperture  7  (i.e., the Z-dimension) generally is sufficient to cause charged particles, incident on the membrane, to be scattered rather than absorbed by the membrane. The membrane is also sufficiently thick to have adequate mechanical strength to provide adequate service as a beam-trimming aperture. For an incident beam of electrons, the membrane thickness typically is within the range of 10 to 100 times the length of the mean free path of electrons of the beam in the membrane material. By way of example, the mean free path of electrons in a silicon membrane is 150 nm for a 100 keV electron beam. Under such conditions, the thickness of the silicon beam-trimming aperture  7  can be about 2 μm.  
         [0028]    At 2 μm thickness, the beam-trimming aperture  7  is configured as a “membrane.” Because the membrane transmits (with scattering) incident charged particles, rather than absorbing the particles, the membrane experiences very little heating from impingement of incident charged particles. Hence, a beam-trimming aperture  7  configured as a membrane is not subject to thermal deformation.  
         [0029]    The profile-shaping aperture  9  is disposed downstream of the second condenser lens  5  at an axial position at which an image of the electron-emission surface (cathode) of the electron gun  1  is formed. The profile-shaping aperture  9  defines the transverse profile of the electron-beam flux and determines the transverse sectional area of the beam illuminating a region on the reticle  19 . The profile-shaping aperture  9  desirably is a membrane made of silicon or the like, similar to the beam-trimming aperture  7 . With such a configuration, the profile-shaping aperture  9  does not exhibit significant temperature increases from impingement of incident charged particles. Hence, the profile-shaping aperture  9  is not subject to thermal deformation.  
         [0030]    The third condenser lens  13  is disposed downstream of the profile-shaping aperture  9 , and the screening aperture  15  is disposed at a crossover position downstream of the third condenser lens  13 . The screening aperture  15  is fabricated from a sheet of metal, such as molybdenum or tantalum, desirably approximately 500 to 1000 μm thick. The screening aperture  15  functions in conjunction with the beam-trimming aperture  7  and/or profile-shaping aperture  9 , and serves to block (by absorption) charged particles scattered by the beam-trimming aperture  7  and/or the beam-shaping aperture  9 . By relegating the task of charged-particle absorption to the screening aperture  15  (which does not have to define an aperture opening accurately that otherwise would be deformed by heating), the beam-trimming and profile-shaping apertures are relieved of having to be subject to heating.  
         [0031]    The illumination lens  17  is disposed downstream of the screening aperture  15 . The electron beam passing through the illumination lens  17  forms an image of the profile-shaping aperture  9  on the reticle  19  situated downstream of the illumination lens  17 .  
         [0032]    The first and second projection lenses  21 ,  23 , respectively, are disposed downstream of the reticle  19 . The contrast aperture  25  is disposed at a crossover location between the projection lenses  21 ,  23 . The contrast aperture  25 , similar to the screening aperture  15 , is fabricated from a sheet of metal, such as molybdenum or tantalum, desirably approximately 500 to 1000 μm thick. The contrast aperture  25  serves to block (by absorption) charged particles scattered by the membrane portion of the reticle  19 . An image of the illuminated portion of the reticle  19  is formed, with demagnification, on a corresponding region of the substrate  27  by the first and second projection lenses  21 ,  23 , respectively.  
         [0033]    Although not described or shown herein, it will be understood that each of the illumination-optical system IOS and projection-optical system POS includes one or more deflectors and corrective coils as used for beam scanning, beam-position adjustment, and aberration control, for example.  
         [0034]    [0034]FIG. 2 is a schematic elevational diagram depicting absorption of charged particles, scattered by an upstream profile-shaping aperture, by the downstream screening aperture. The figure shows an enlargement of an area around the profile-shaping aperture  9 , third condenser lens  13 , and screening aperture  15 .  
         [0035]    The charged particle beam transmitted through the opening defined by the profile-shaping aperture  9  (representative trajectories of transmitted charged particles are denoted by the solid lines in the figure) are converged by the third condenser lens  13  for passage through the opening defined by the screening aperture  15 . Meanwhile, charged particles impinging on the profile-shaping aperture  9  are scattered as they pass through the membrane of the profile-shaping aperture  9  (representative trajectories of scattered charged particles are denoted by the dashed lines in the figure). Most of the scattered charged particles are not converged sufficiently by the third condenser lens  13  for passage through the opening in the screening aperture  15 . Rather, these scattered charged particles impinge on and are absorbed by the aperture plate of the screening aperture  15 . By having the screening aperture  15 , rather than the profile-shaping aperture  9 , perform the task of particle absorption, the profile-shaping aperture  9  does not experience significant absorption-based heating.  
         [0036]    FIGS.  3 ( a )- 3 ( f ) are schematic elevational diagrams showing the results of respective steps in an exemplary method for manufacturing a beam-trimming or profile-shaping aperture according to the invention. In a first step, a boron-doped oxide film  33 D′ and a silicon membrane layer  32 A are laminated on a major surface (top surface in the figure) of a base substrate  31  (FIG. 3( a )). The base substrate  31  desirably is made of silicon. A film of silicon nitride (SiN)  35  is formed on the opposing major surface (bottom surface in the figure) of the base substrate  31 . The SiN film  35  serves as a mask during later etching of the base substrate  31  from the bottom.  
         [0037]    A resist film  36  is applied to the SiN film  35  (FIG. 3( b )). A pattern for forming support struts  31 A in the base substrate  31  is exposed into the resist film  36  and the resist is developed. Using the developed resist film  36  as a mask, the SiN film  35  is dry etched as shown in the figure to form a corresponding pattern in the SiN film  35 . The pattern defines the locations at which the support struts  31 A will be formed.  
         [0038]    Next, the base substrate  31  is wet-etched from its bottom surface (“backetched”) according to the pattern in the SiN film  35  to form the girder-like support struts  31 A and a bilayer membrane consisting of the silicon membrane layer  32 A and the boron-doped oxide layer  33 D′. The support struts  31 A support the bilayer membrane and form a membrane region  31 B. The boron-doped oxide film  33 D′ is exposed in the membrane region  31 B. The SiN film  35  and resist film  36  remaining at the “lower” ends of the support struts  31  A (opposite from the boron-doped oxide film  33 D′) are stripped away (FIG. 3( c )).  
         [0039]    A film of resist  37  is applied to the membrane layer  32 A. A pattern defining the desired aperture opening (e.g., square) for the membrane layer  32 A is exposed into the resist film  37 , and the resist is developed (FIG. 3( d )).  
         [0040]    The membrane layer  32 A is dry-etched using the developed resist film  37  as a mask, thereby forming the respective aperture opening in the membrane layer  32 A of the membrane region  31 B. Remaining resist film  37  is stripped away (FIG. 3( e )).  
         [0041]    The boron-doped oxide film  33 D′ is removed in the membrane region  31 B using hydrofluoric acid, thereby completing fabrication of the beam-shaping aperture (FIG. 3( f )).  
         [0042]    By making the beam-trimming and profile-shaping apertures from respective thin films according to the invention, absorption of CPB beam current by these apertures is reduced to at most several percent of the absorption that otherwise would be exhibited by a beam-trimming or profile-shaping aperture made of a relatively thick sheet of metal. As a result, the need to perform active cooling of the beam-trimming or profile-shaping aperture(s) in a CPB optical system according to the invention is eliminated, resulting in simplification and cost reduction of the overall system. Also, control of the temperature of components surrounding the beam-trimming and profile-shaping apertures is simplified. In addition, because charged particles forward-scattered from the beam-trimming or profile-shaping aperture are blocked by a downstream metal screening aperture, the scattered charged particles are prevented from reaching the substrate, thereby achieving maximal image contrast on the substrate.  
         [0043]    Whereas the invention has been described in connection with a representative embodiment, it will be understood that the invention is not limited to that embodiment. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.