Radiant energy – Irradiation of objects or material
Reexamination Certificate
2001-02-20
2003-08-05
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
C250S492200, C250S492220, C250S492230, C250S398000, C250S3960ML
Reexamination Certificate
active
06603131
ABSTRACT:
FIELD OF THE INVENTION
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
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.
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.
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
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.
To such ends, and according to a first aspect 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.
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 to perform than in conventional apparatus.
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 &mgr;m thick), and by positioning the screening aperture at a crossover. Thus, the screening aperture prevents scattered charged particles from reaching the sensitive substrate.
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.
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.
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.
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 secon
El-Shammaa Mary
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
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