Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-07-19
2004-12-14
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C430S296000, C430S942000
Reexamination Certificate
active
06830852
ABSTRACT:
FIELD
This disclosure pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Microlithography is a key technology used in the production of microelectronic devices such as semiconductor integrated circuits, writing and pickup heads used in magnetic memory devices, displays, and micromachines, for example. More specifically, the disclosure pertains to reticles as used in charged-particle-beam (CPB) microlithography, especially to so-called “stencil” reticles and to methods for defining a pattern on one or more such reticles.
BACKGROUND
As microelectronic devices have reached ever-higher levels of integration, it has become increasingly difficult to use optical microlithography for forming circuits and other patterns on the surface of a substrate. Consequently, “next generation” lithography (NGL) technology currently is under intensive development. A promising NGL technology is the so-called “charged-particle-beam” (CPB) lithography technology that utilizes a charged particle beam such as an electron beam for making pattern exposures. CPB lithography offers prospects of substantially greater pattern resolution for reasons similar to the reasons for which electron microscopy yields better pattern resolution than optical microscopy.
Aberrations, distortion, and the like in a CPB optical system make it impossible for a wide pattern area (e.g., corresponding to the area of a single “chip”) to be transfer-exposed in a single exposure “shot.” Consequently, patterns must be a transfer-exposed in multiple steps each pertaining to a respective portion of the pattern. More specifically, a pattern for a layer of a chip is divided into multiple exposure units (typically called “subfields”) each defining a respective portion of the pattern. The pattern portions defined in the subfields are transferred individually to a lithographic substrate, on which respective images of the pattern portions are formed adjacent each other in a manner that achieves “stitching” of the pattern portions together to form the complete pattern for a chip.
This manner of exposure is diagrammed in
FIGS. 12 and 13
.
FIG. 12
depicts a lithographic substrate (“wafer”) on which multiple chip patterns have been formed. The pattern for a single chip is divided into multiple stripes (four stripes are shown), and each stripe contains multiple subfields arranged in a rectilinear array of rows and columns. The corresponding pattern as defined on the reticle (not shown in
FIG. 12
) is similarly divided.
FIG. 13
depicts an actual exposure performed using a charged particle beam. The reticle, situated upstream in the figure, is mounted on a reticle stage (not shown). The substrate, situated downstream in the figure, is mounted on a wafer stage (not shown). The reticle stage and a wafer stage move the reticle and substrate in the directions shown at respective constant velocities. Note that the stage-movement directions are opposite each other. In the figure, an “illumination beam” is shown in position to begin exposure of a row of subfields in a stripe on the reticle. Similarly, the “patterned beam,” propagating downstream of the reticle, is shown in position to begin imaging of the subfields in a corresponding row in a corresponding stripe on the substrate. The illumination beam is directed to the desired subfield on the reticle by an “illumination-optical system” and the patterned beam is directed to the desired subfield on the substrate by a “projection-optical system.”
As exposure of the stripe progresses, the respective stages move the reticle and substrate in the directions shown while deflectors in the illumination-optical system and projection-optical system deflect the illumination beam and patterned beam, respectively, laterally as shown. Note that the directions of beam deflection are substantially perpendicular to the respective directions of stage movement. Thus, the subfields in each row and the rows of subfields in each stripe are exposed in sequential order. After exposure of a row of subfields is complete, exposure of the subsequent row begins, but with a direction of beam deflection that is opposite the direction of beam deflection used in the just-completed row. In other words, the subfields in the rows are exposed in a raster manner, which is time-efficient and thus maximizes throughput.
Since the respective pattern portion in each subfield is exposed in a respective “shot,” throughput is greater using the depicted technique than was obtained previously using conventional “direct writing,” “cell projection,” and “character projection” techniques of performing CPB microlithography.
Further with respect to a reticle for use in CPB microlithography as shown in
FIGS. 12 and 13
, the subfields are defined in respective membrane portions of the reticle. The membrane portions are not contiguous with each other but rather are divided from one another by support struts. The struts provide substantial mechanical strength to the membrane portions. In a representative membrane portion, the respective subfield (which is the portion of the membrane portion actually defining the respective portion of the pattern) is surrounded by a non-patterned “skirt.” The skirt is the region of the membrane portion in which the edges of the illumination beam fall whenever the respective subfield is being illuminated by the illumination beam for exposure. Thus, as each subfield is exposed, only the respective pattern portion is transferred to the substrate.
Before CPB lithographic exposure can be performed, the pattern to be exposed on the substrate must be defined on the reticle. CPB microlithography reticles can be classified broadly into two types. A first type is the so-called “stencil” reticle, in which pattern elements are defined as respective voids (through-holes) in a reticle membrane that is 1 to 5 &mgr;m thick. The voids are highly transmissive to the incident illumination beam with substantially no forward scattering, whereas intervening portions of the reticle membrane cause substantial forward scattering of charged particles in the illumination beam. A second type of reticle is the so-called “continuous-membrane” reticle, in which pattern elements are defined as respective openings in a highly scattering layer formed on a continuous, relatively low-scattering membrane layer. In other words, the openings have relatively high transmissivity to the incident beam, and regions occupied by the highly scattering layer have relatively low transmissivity to the incident beam. “Transmissivity” as referred to here takes into account the degree of forward-scattering. A relatively low transmissivity means not only that a portion of the beam may be absorbed but also that portions of the incident beam transmitted through the respective region of the reticle are forward scattered sufficiently so as not to reach the substrate.
With either type of reticle, a “contrast aperture” is situated at a beam crossover of the projection-optical system. As the patterned beam (containing both scattered and relatively non-scattered charged particles) propagates from the reticle to the substrate, the contrast aperture blocks the relatively highly scattered charged particles of the beam, thereby preventing such particles from reaching the substrate. Relatively low-scattered and non-scattered charged particle are transmitted by the scattering aperture and focused on the substrate. Thus, the image formed on the substrate is provided with contrast.
A continuous membrane reticle as summarized above exhibits problems with thermal absorption of incident charged particles of the illumination beam. As a result, a continuous membrane reticle tends to exhibit chromatic aberration. A stencil reticle, on the other hand, does not exhibit these problems, which is regarded as a desirable aspect of stencil reticles because better pattern resolution usually can be obtained with them.
However, stencil reticles are prone to certain problems if the reticle defines “donut”-shaped (annular) pattern features or linear pa
Kamijo Koichi
Kawata Shintaro
Takahashi Shin-ichi
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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