Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-02-22
2003-03-25
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492200, C250S398000, C250S397000
Reexamination Certificate
active
06538255
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to electron guns as used in apparatus and methods utilizing an electron beam, especially apparatus and methods in which an electron beam is used to perform projection of an image of a pattern (such as an integrated circuit pattern), as defined on a reticle, to a sensitive substrate (such as a semiconductor wafer). The invention also pertains to apparatus including such guns and to methods for manufacturing devices (e.g., semiconductor integrated circuits), wherein the methods utilize such projection apparatus.
BACKGROUND OF THE INVENTION
A key technology in manufacturing integrated circuits and displays is microlithography (image-transfer and imprinting technology). Feature sizes and line widths of integrated circuits progressively are becoming more miniaturized and have now reached the resolution limit of light (visible and ultraviolet light as used in “optical” microlithography). Electron-beam microlithography currently is under intensive investigation as a possible successor to optical microlithography, especially in view of the potentially greater resolving power of electron-beam microlithography compared to optical microlithography.
In electron-beam microlithography, an electron beam is produced by an electron gun. The beam is directed to a reticle (sometimes termed a “mask”) that defines the pattern to be transferred. The beam illuminates the pattern, or a selected portion thereof on the reticle, and the portion of the beam passing through the illuminated portion of the reticle is directed to a selected region of the substrate. More specifically, the electron beam propagating from the electron gun to the reticle is termed the “illumination beam,” which passes through an “illumination-optical system” to the reticle. The illumination-optical system typically includes multiple electromagnetic lenses that converge the illumination beam appropriately for illuminating the desired region of the reticle. Upon passing through the reticle, the illumination beam acquires an ability to form an image of the illuminated portion of the reticle; thus, the beam propagating downstream of the reticle is termed the “patterned beam.” The patterned beam passes through a “projection-optical system” to the substrate. The projection-optical system typically includes a pair of electromagnetic projection lenses that form a focused image, of the illuminated portion of the reticle, of a desired size on a corresponding region of the substrate. Hence, the image defined by the reticle is projected onto the substrate, usually portion-by-portion. This general process is also termed “pattern transfer” because the pattern defined by the reticle effectively is “transferred” to the substrate.
Conventional microlithography apparatus as summarized above normally produce a “demagnified” (or “reduced”) image on the substrate. This means that the image as formed on the substrate is smaller, usually by an integer factor, than the corresponding illuminated region on the reticle. The reciprocal of the integer factor is termed the “demagnification ratio,” of which a representative value is 1/4 or 1/5.
Electron guns used in conventional electron-beam microlithography apparatus of the type summarized above generally include three electrodes. The first electrode is a cathode used within a temperature-limitation region of its intensity-temperature (I-T) profile (FIG.
3
). The second electrode is an anode that is charged appropriately to pull electrons away from the cathode to propagate through an axial aperture defined by the anode. The third electrode is a Wehnelt electrode (also termed a “Wehnelt cylinder”) that serves, inter alia, to guide electrons from the cathode through the anode aperture and thus, by preventing impingement of the electrons on the anode, reduce heating of the anode. In this conventional electron-gun configuration, the cathode and Wehnelt electrode are insulated electrically from each other, and have different electrical potentials (voltages) applied to them.
Many types of conventional electron-beam microlithography systems (e.g., variable-shaped pattern systems, character-projection systems, and divided-pattern projection systems) utilize a “solid” electron beam having a transverse profile (e.g., gaussian or rectangular) in which the beam intensity at the contrast aperture is greatest at the center of the beam. However, it has been found that, in such systems, a solid beam is subject to “space-charge effects” that are manifest as, e.g., focal-point shift, increase in beam blur, and distortion of the pattern as projected onto a wafer or other suitable substrate. In effort to solve problems associated with space-charge effects, electron guns have been investigated that produce a “hollow beam” in which, at the contrast aperture, the most intense portion of the beam is not located at the center of the beam, but rather at peripheral regions of the beam.
Unfortunately, no effective methods or apparatus exist to date for evaluating or adjusting a hollow beam.
As shown in
FIG. 3
, within the temperature-limitation region of the cathodic I-T profile, even a slight change in cathode temperature causes a substantial change in the intensity of beam current produced by the cathode. Consequently, in an electron gun in which the cathode is operated under temperature-limitation conditions, any irregularity in cathode-surface temperature or irregularity in the work function of the cathode surface causes the intensity distribution of the electron beam to be not uniform. An electron beam produced under such conditions does not provide a desired uniform illumination of the reticle. As a result, the dimensional accuracy of the pattern as transferred onto the substrate is degraded. This problem is difficult especially whenever a hollow beam is used.
For example, in order for an electron-beam microlithography apparatus to be viable commercially for high-volume production, it must have a per-shot exposure area of at least 1 mm×1 mm at the reticle, which requires a cathodic surface having an area of 3 to 10 mm
2
. Variations arising from the cathode being operated under temperature-limitation conditions can be substantial, especially with a gun having a cathode configured to produce a hollow beam.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide electron guns, for use in electron-beam microlithography apparatus that produce a demagnified image of the reticle pattern on the substrate, in which the transverse distribution of electron-beam intensity can be made uniform or otherwise adjusted as required.
To such end, and according to a first aspect of the invention, electron guns are provided that comprise a cathode, an anode, and a filament array. The cathode comprises an electron-emitting surface that emits a beam of electrons whenever the cathode is energized electrically. The anode is situated downstream of the cathode and can be energized at a voltage appropriate for drawing electrons from the cathode. The filament array is situated adjacent the cathode (e.g., adjacent an upstream-facing surface of the cathode) and is configured to energize respective regions of the cathode in a selective manner. The filament array comprises multiple filaments that are controllable independently to allow independent adjustment of electrical energy from the filaments to respective regions of the cathode.
Typically, the cathode and anode are arranged on an axis (“optical axis”), and the multiple filaments are arranged equidistantly from one another radially around the axis. For example, the filament array can comprise eight independently controllable filaments. In a particularly advantageous configuration, the electron-emitting surface is ring-shaped about the axis so as to emit a hollow beam of electrons, where each filament is adjacent a respective region of the ring-shaped electron-emitting surface.
The electron gun can include a control anode situated between the cathode and the anode. The el
Berman Jack
Klarquist & Sparkman, LLP
Nikon Corporation
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