Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
1999-09-16
2002-06-11
O'Shea, Sandra (Department: 2875)
Radiant energy
Means to align or position an object relative to a source or...
C430S004000, C250S492200, C250S492230
Reexamination Certificate
active
06403971
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to charged-particle-beam (CPB, e.g., electron-beam) microlithography systems. More specifically, the invention pertains to methods and apparatus for adjusting the charged particle beam used in such systems, especially where contrast is obtained in a projected image according to the degree of scattering of the beam from a reticle in which a one-shot transfer field is relatively large, and a large proportion of the beam illuminating the reticle passes with little absorption through the entire reticle.
BACKGROUND OF THE INVENTION
Electron-beam (as a representative charged particle beam) microlithography systems are attracting greater attention for use in the manufacture of semiconductor devices. Currently, practical use of such systems is mainly limited to developing prototypes of semiconductor devices (e.g., integrated circuits) and for making small production runs of specialized and/or custom devices.
In the earliest electron-beam microlithography systems, the beam is narrowed to a fine point and is scanned in a manner by which the pattern is traced line-by-line (i.e., “written”) on the surface of a substrate. These systems are termed “spot-beam scanning” systems. Such systems exhibit extremely low “throughput” (i.e., number of wafers that can be processed per unit time).
More recent electron-beam systems employ a “variable-shaped” beam and tend to exhibit higher throughput. In such systems, the transverse dimensions of the beam are larger than in a spot-beam scanning system. In addition, the transverse profile and area of the beam can be changed to some extent in a variable-shaped-beam system.
Other conventional electron-beam systems, termed “cell-projection” systems, are typically used whenever the pattern to be “transferred” to the substrate comprises a relatively large area in which a particular small portion of the pattern is repeated many times (such as in a pattern for a memory chip comprising a large number of identical memory cells wherein each memory cell represents the repeated small portion). The highly repeated portion of the pattern is represented by a cell (approximately 5 &mgr;m×5 &mgr;m on the substrate) that is exposed multiple times on different respective regions of the substrate.
Yet another conventional approach involves dividing the reticle pattern into multiple “exposure units” or “subfields” each defining a respective portion of the overall pattern. Such a reticle is termed a “divided” or “segmented” reticle. The exposure units are exposed individually in an ordered manner using an illumination-optical system located upstream of the reticle and a projection-optical system located between the reticle and the substrate. Such a system is termed a “divided-pattern” projection-transfer system. As the exposure units are imaged on the substrate, they are “stitched” together in the proper order to form, after all the exposure units have been exposed, the entire pattern on the substrate.
In cell-projection systems, so-called “absorption-stencil reticles” are generally used. In such reticles, pattern features are represented as corresponding cutouts formed in and extending through the thickness dimension of a relatively thick (normally about 20 &mgr;m thick) silicon membrane. When an “illumination beam” impinges on such a reticle, portions of the beam passing through the reticle form a “patterned beam” that propagates downstream away from the reticle. To produce the pattern in the patterned beam, portions of the illumination beam pass through the cutouts (in the same manner as light through a window) and experience little to no scattering or absorption. Other portions of the illumination beam impinge on the non-cutout portions of the reticle (i.e., on the membrane) and are thereby absorbed. Absorption-stencil reticles are also used in divided-pattern projection-transfer systems.
To increase throughput, various schemes for increasing beam current have been investigated. However, with substantial increases in beam current, absorption-stencil reticles are impractical because electrons absorbed by the reticle membrane caused heating of the reticle. Such heating caused major problems with thermal expansion of the reticle. To solve this problem, “scattering-stencil reticles” were proposed.
In a scattering-stencil reticle, most of the electrons impinging on the reticle membrane are transmitted through the reticle rather than absorbed by the membrane. However, such electrons tend to be significantly scattered or diffused as they pass through the membrane. In a scattering-stencil reticle, as in an absorption stencil reticle, electrons passing through the cutouts are not scattered. Image contrast is obtained by placing a contrast aperture (that blocks electrons scattered by the reticle to prevent such electrons from propagating to the substrate) at or near a beam-convergence plane of the projection-optical system. I.e., the contrast aperture is placed at the Fourier plane, in the projection-optical system, of the reticle plane. Thus, scattered electrons that would otherwise impair image contrast are prevented from propagating to the image on the substrate.
With a stencil reticle, an island-shaped membrane feature cannot be disposed at the center of a cutout in the surrounding membrane because the island-shaped membrane feature would have no physical support. This is termed the “donut-feature” problem. To solve this problem, at least the surrounding cutout is split between two “complementary” exposure units of the reticle. Each split portion is separately projected and exposed onto the substrate. During exposure of the two exposure units, each split portion is positioned on the substrate such that the two cutouts are stitched together to form the exposed region surrounding the island-shaped non-exposed region. This method is termed dividing a reticle pattern into complementary pattern portions. Unfortunately, dividing a reticle pattern into complementary pattern portions requires two separate exposures to image the pattern portion including the island-shaped feature. The need to perform two exposures rather than one decreases throughput by a corresponding amount.
To solve the donut-feature problem, a “scattering-membrane reticle” can be used that comprises a relatively thin (e.g., 2 &mgr;m thick) electron-transmissive reticle membrane without cutouts. Pattern features are defined on such a reticle by a corresponding pattern of an electron-scattering material layered on the membrane. As electrons of the illumination beam pass through the membrane, virtually no electron scattering occurs. However, passage of electrons of the illumination beam through the electron-scattering material causes substantial electron scattering. A scattering membrane reticle improves throughput because island features can be projected onto the substrate without the need for complementary exposure units.
In the electron-optical system (comprising the illumination-optical system and the projection-optical system) of a conventional electron-beam microlithography apparatus, it is necessary occasionally to adjust the axial alignment, focal position, and/or the amount of astigmatism correction exhibited by the electron-optical system. For axial alignment, the respective excitation currents or voltages applied to the projection lenses and/or deflectors are set to “standard conditions” that cause the electron beam to pass through the center of the contrast aperture. Such an axial alignment is typically performed whenever, for example, an exposure-pattern lot is changed or whenever periodic adjustments are made to the microlithography apparatus.
In spot-beam scanning systems, since the beam diameter is very small at less than 1 &mgr;m, beam axial alignment can be performed using methods as used in scanning electron microscopy. For instance, the beam is scanned over a reference plane (represented by the reticle surface or the substrate surface) and the location of the beam axis is accurately determined by detecting and analyzing signals created by electron
Hashmi Zia R.
Klarquist & Sparkman, LLP
Nikon Corporation
LandOfFree
Beam-adjustment methods and apparatus for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Beam-adjustment methods and apparatus for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Beam-adjustment methods and apparatus for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2960457