Radiant energy – Radiant energy generation and sources
Reexamination Certificate
2000-06-20
2003-04-01
Lee, John R. (Department: 2881)
Radiant energy
Radiant energy generation and sources
C250S492100, C250S492300, C250S3960ML, C313S34600R
Reexamination Certificate
active
06541785
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to electron-beam microlithography (projection-transfer of a pattern, defined by a reticle or mask, to a substrate) using an electron beam. Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to electron-beam sources that emit an electron beam, and to electron-beam microlithography apparatus comprising such an electron gun.
BACKGROUND OF THE INVENTION
Most conventional electron guns employed in conventional electron-beam microlithography apparatus, and in electron-optical systems utilized in such apparatus, have simple construction and operate under temperature-limiting conditions. The cathode (electron-emitting surface) of a conventional electron gun typically is circular. Consequently, the transverse sectional profile of the beam produced by the electron gun is circular, and the current density of the beam tends to be relatively high in locations in which such high beam-current density causes problems, as summarized below.
Semiconductor-device patterns progressively have become finer in recent years. Currently, considerable demand exists to develop practical technologies for transferring a pattern having linewidths of 0.1 &mgr;m or less onto a semiconductor wafer. Whenever a conventional electron-beam microlithography apparatus is used in an attempt to meet these demands, image blur (defocusing) and distortion due to space-charge effects become problematic. The space-charge effects are caused largely by the particular transverse profile and localized high beam-current density of the electron beam in conventional electron-beam microlithography apparatus being “pushed” to achieve such performance.
Reducing image blur and defocusing due to space-charge effects can be achieved by reducing the beam-current density of the electron beam. However, reducing the beam-current density causes other problems such as increased time to expose each wafer, resulting in reduced throughput.
A conventional electron gun used under temperature-limiting conditions also exhibits large variances in current from one exposure (“shot”) to the next. This “shot noise” is a key cause of poor linewidth accuracy in transferring a pattern having small linewidths using conventional electron-beam microlithography technology.
SUMMARY OF THE INVENTION
In view of the shortcomings of conventional apparatus and methods as summarized above, an object of the invention is to provide, inter alia, electron-beam sources that are well-suited for use in electron-beam microlithography apparatus intended for use in transferring patterns having linewidths of 0.1 &mgr;m or less. Another object is to provide electron-beam microlithography apparatus comprising these electron-beam sources, as well as methods (for manufacturing semiconductor devices) utilizing such apparatus.
According to a first aspect of the invention, electron-beam sources are provided. An embodiment of such a source comprises an electron gun comprising a cathode including an annular electron-emitting surface and at least one anode. The source also includes a controller to which the electron gun is connected. The controller is configured to energize the electron-emitting surface in a controllable manner and to energize the anode in a manner in which the electron gun operates under a space-charge-limiting condition. The cathode comprises a cathode body having a work function, and the annular electron-emitting surface is situated on a surface of the cathode body.
In one configuration, the surface of the cathode body is coated, in regions other than the annular electron-emitting surface, with a material having a work function that is greater than the work function of the cathode body. In this configuration, the work functions of the cathode body and of the material desirably differ from each other by at least 1 eV. For example, the cathode body is made of rhenium and the coating material is tungsten.
In another configuration, the annular electron-emitting surface comprises a coating of a material on the surface of the cathode body, wherein the material has a work function that is less than the work function of the cathode body. In this configuration, the work functions of the cathode body and of the material differ from each other by at least 1 eV. For example, the cathode body is made of tungsten and the coating material is rhenium.
In a desirable configuration of the electron-beam source, the cathode comprises a cathode body that includes a convex projection extending toward the anode. The projection desirably is symmetrical about an optical axis and is situated inside the electron-emitting surface.
The electron-beam source further can comprise a control anode situated between the anode and the cathode. The control anode desirably is energized in a controllable manner so as to control a beam current of an electron beam produced by the electron gun.
According to another aspect of the invention, electron-beam microlithography apparatus are provided. An embodiment of such an apparatus comprises an illumination-optical system, a projection-optical system, and an electron-beam source as summarized above. The electron-beam source is situated and configured to produce an electron beam that passes through the illumination-optical system and the projection-optical system.
In a desirable configuration of the apparatus, the projection-optical system comprises a contrast aperture. The illumination-optical system is configured to produce a first beam crossover and a region of the electron beam having a flat distribution of beam-current density, wherein the region is situated in the vicinity of the first beam crossover. The illumination-optical system is further configured to form an image of the region, having a flat distribution of beam-current density, on a reticle situated between the illumination-optical system and the projection-optical system. The projection-optical system is configured to form an image of the cathode at the contrast aperture.
Other aspects of the invention are directed to methods for manufacturing a semiconductor device on a wafer. The methods include a wafer-processing step that includes electron-beam microlithography of a pattern, defined on a reticle, to a wafer using an electron-beam microlithography apparatus as summarized above.
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.
REFERENCES:
patent: 4528474 (1985-07-01), Kim
patent: 4551606 (1985-11-01), Inoue
patent: 5532496 (1996-07-01), Gaston
patent: 6218676 (2001-04-01), Nakasuji
patent: 6326633 (2001-12-01), Nakasuji
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
Vanore David A.
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