Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Electron beam imaging
Reexamination Certificate
2002-06-11
2004-06-29
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Electron beam imaging
C430S030000, C430S942000
Reexamination Certificate
active
06756182
ABSTRACT:
FIELD
This disclosure pertains to microlithography (transfer of a pattern to a sensitive substrate), especially as performed using a charged particle beam. Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, and micromachines. More specifically, the disclosure pertains to charged-particle-beam (CPB) microlithography of reticle patterns in a manner that reduces the so-called Coulomb effect.
BACKGROUND
Substantial development effort is being expended currently to develop a practical “next generation” microlithography technology capable of transferring fine patterns having minimum feature sizes of less than 0.1 &mgr;m (i.e., below the 0.1 &mgr;m rule). Among several microlithography techniques that have been considered, charged-particle-beam (CPB) microlithography (especially electron-beam microlithography) having a throughput sufficiently high for use in the mass production of semiconductor chips (especially memory chips) is an attractive candidate. This technique, termed “divided-reticle-reduction” electron-beam (EB) microlithography, uses an “EB stepper.” In preliminary tests of a prototype EB stepper, divided-reticle-reduction EB microlithography has exhibited a capability of transferring patterns having critical dimensions of 0.1 &mgr;m or smaller.
Conventional EB microlithography techniques include the so-called “cell-projection” technique that differs from so-called “direct-write” techniques. In the cell-projection technique a desired pattern is formed on the substrate (“wafer”) by projecting and connecting together basic pattern-element shapes termed “cells.” A variety of such basic shapes are defined on an aperture plate upstream of the substrate, and the cells are selected and projected in a “mix-and-match” manner to reconstruct the pattern cell-by-cell on the substrate. Each cell typically is very small, e.g., about 5-&mgr;m square, requiring a very small-diameter beam.
In contrast to the “cell-projection” apparatus, an EB stepper configured to perform divided-reticle-reduction microlithography utilizes a reticle that defines pattern elements configured as the elements are to be projected onto the substrate. (Hence, “mix-and-match” reconstruction of pattern elements on the substrate, which is slow, is not required.) The EB stepper utilizes a reticle on which the pattern is divided into a large number of exposure units, usually termed “subfields,” each defining a respective portion of the pattern. Each subfield typically is much larger (up to about 1-mm square on the reticle) than a single “cell,” requiring a correspondingly larger-diameter beam. Hence, since more of the pattern is transferred in each exposure “shot,” divided-reticle-reduction microlithography typically achieves a much higher throughput than cell-projection.
One type of reticle suitable for use in an EB stepper is a so-called “scattering-stencil” reticle. In a scattering-stencil reticle each subfield includes a respective portion of the reticle membrane, wherein the subfields are separated from one another by non-patterned regions occupied by support struts. The reticle membrane constitutes an electron-scattering body. In each subfield, the respective pattern elements are defined by a corresponding arrangement of apertures that freely transmit electrons of an incident beam. In other words, regions of the subfield in which electron transmission without scattering is desired are defined as respective apertures in the membrane, while the remaining regions of the membrane are respective regions in which electron transmission is accompanied by large forward-scattering. Most of the electrons that are forward-scattered during passage through the membrane are blocked by a downstream “scattering” aperture. Consequently, the electrons reaching the surface of the substrate are more or less exclusively the electrons that have passed through the apertures only.
Another reticle type used in divided-reticle-reduction EB microlithography is the so-called “scattering-membrane” reticle. A scattering-membrane reticle is divided into subfields in a manner similar to the scattering-stencil reticle. However, the membrane in the scattering-membrane reticle does not define pattern elements by corresponding apertures. Rather, the pattern elements are defined by corresponding regions of a highly scattering film formed on a relatively thin (thickness of 0.1 &mgr;m or less) membrane through which incident electrons pass with substantially no scattering.
Generally, the percentage of incident electrons passing through the membrane of a scattering-membrane reticle is about 40%. This seemingly low number does not prevent attainment of sufficient contrast for good imaging of the pattern on the substrate. However, such reticles do pose a risk of chromatic aberration caused by forward-scattering of electrons during passage through the membrane. Chromatic aberrations can affect pattern resolution adversely.
A key operational goal of EB steppers is an ability to perform mass-production of semiconductor wafers, especially high-density memory chips (e.g., DRAMs having a memory capacity of at least 16 Gbits). To achieve such performance, the EB stepper must exhibit a correspondingly high level of pattern resolution. Examples of factors that contribute to such resolution achieved by an EB stepper include: (1) high acceleration of the electron beam, (2) low geometrical aberrations, and (3) high suppression of Coulomb effects and other resolution-destroying effects by the EB optical system of the stepper. Reduction of Coulomb effects is very important. In an EB stepper, whereas it is desirable to increase the beam current as much as possible in order to maximize productivity, increasing the beam current correspondingly increases the density of electrons in the beam. In conditions of high electron density, the repulsive forces between adjacent electrons in the beam are stronger than in lower-density beams. The resulting mutual repulsion of electrons away from each other in the beam is termed the “Coulomb effect.” During microlithography performed under such conditions, electrons reaching the surface of the substrate produce an image exhibiting a characteristic blur that degrades the resolution of the projected image.
SUMMARY
In view of the problems summarized above, the present invention provides, inter alia, charged-particle-beam (CPB) microlithography apparatus and methods that exhibit better control of Coulomb effects as manifest in a pattern as transferred from a segmented reticle to a sensitive substrate.
In one embodiment of the method, the segmented reticle can be either a scattering-stencil reticle or a scattering-membrane reticle. Selected regions of the reticle are individually illuminated with a CPB illumination beam to produce a corresponding patterned beam. Most of the charged particles in the patterned beam that are highly scattered during passage through the reticle are blocked by a contrast aperture from reaching the sensitive substrate. Meanwhile, charged particles that are not scattered and/or weakly scattered during passage through the reticle pass through the contrast aperture and are focused as a projected image on the sensitive substrate. For exposing the pattern, the beam current of the patterned beam reaching the sensitive substrate is reduced, relative to the beam current actually passing through the reticle, to 50% or less. This reduction is performed by: (a) establishing the pattern, as defined on the reticle, as a normal-tone pattern or as an inverted-tone pattern, and (b) establishing the resist on the substrate as a positive or negative resist.
If the reticle is a scattering-stencil reticle (in which the pattern elements are defined by respective non-scattering, CPB-transmissive apertures in a highly CPB-scattering reticle membrane), then, in step (a), above, the beam current reaching the sensitive substrate is reduced by establishing an opening ratio of 50% or less for the pattern as a whole as defined on the reticle. The opening ratio is ex
Shimizu Sumito
Suzuki Kazuaki
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
LandOfFree
Charged-particle-beam microlithography methods exhibiting... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Charged-particle-beam microlithography methods exhibiting..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Charged-particle-beam microlithography methods exhibiting... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3310222