Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2000-10-13
2002-08-13
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S296000, C430S942000, C250S492200
Reexamination Certificate
active
06432594
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, to a suitable substrate using an energy beam). Microlithography is a key technology used in the manufacture of microelectronic devices (e.g., semiconductor integrated circuits), displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, the invention pertains to charged-particle-beam (CPB) microlithography apparatus and methods exhibiting reduced deflection aberrations, and to methods for adjusting deflection aberrations to reduce them.
BACKGROUND OF THE INVENTION
Charged-particle-beam (CPB) microlithography apparatus as developed over the past several years generally are grouped as follows: (1) spot-beam exposure systems, (2) variable-shaped beam exposure systems, and (3) block-exposure systems. In terms of resolution, each of these types of CPB exposure systems offers prospects of vastly superior performance than the previous batch-transfer optical microlithography systems (using visible or ultraviolet light). Unfortunately, each of these three types of exposure systems has poor throughput. The spot-beam exposure systems and variable-shaped beam systems exhibit especially poor throughput because they perform exposures by tracing the pattern using an extremely narrow beam typically having a square transverse profile (spot diameter). Block-exposure systems were developed to improve throughput by grouping groups of pattern features into uniformly shaped blocks each containing a partial pattern, and batch-exposing the blocks. However, because the number of pattern elements that can be placed on a reticle is limited, throughput was not improved as much as expected.
The development of “divided-reticle” CPB microlithography methods and apparatus offered prospects of improved throughput. In these methods and apparatus, the reticle is divided or segmented into a large number of pattern portions that individually and sequentially are projection-exposed onto the substrate. A conventional divided-reticle apparatus is depicted in
FIGS. 6 and 7
.
FIG. 6
depicts a wafer that has been exposed with multiple “dies” each corresponding to an individual “chip.” As can be seen, each die is divided into multiple “stripes,” and each stripe is divided into multiple “subfields.” The reticle (not shown) defining the pattern transferred to each die is divided similarly. Each subfield on the reticle is exposed and thus transferred to the wafer individually.
A typical divided-reticle microlithographic exposure of a die pattern is depicted in FIG.
7
. For exposure, the reticle is mounted on a movable reticle stage and the wafer is mounted on a movable wafer stage (neither stage is shown). From
FIG. 7
, it can be seen that the pattern as defined on the reticle is larger (by a predetermined magnification ratio) than the pattern as transferred onto the wafer. In other words, as the pattern is transferred from the reticle to the wafer, the pattern is “demagnified” by a “demagnification ratio” which is the reciprocal of the magnification ratio. The demagnification ratio is a characteristic of the projection lens that is used to make the exposure. In view of the demagnification, during exposure the reticle stage and wafer stage are moved so that the centers of corresponding stripes on the reticle and wafer, respectively, travel at respective constant velocities that are related to each other by the demagnification ratio. As each subfield is exposed, it is illuminated by a CPB “illumination beam.” Portions of the illumination beam passing through the illuminated subfield become an “imaging beam” that is projected onto a wafer coated with a suitable resist. Projection is performed by passing the imaging beam through a projection-optical system.
Exposure normally is performed stripe-by-stripe and, within a stripe, subfield-by-subfield. During sequential exposure of the subfields in a stripe, the CPB illumination beam is deflected in a direction roughly perpendicular to the direction of travel of the reticle stage, thereby sequentially illuminating the subfields in a row within the stripe. The corresponding imaging beam is deflected similarly to place the subfield images properly on the wafer surface. As exposure of each row is completed, exposure progresses to the next row in the stripe with a concurrent reversal in the sweep direction of the beam. Exposure continues in this “raster” (switch-back) manner until all rows of subfields in the stripe are exposed. Exposure then progresses to the next stripe in the pattern. By sweeping the beam sequentially back and forth in a raster manner as shown in
FIG. 7
, throughput is improved compared to a scheme in which the beam is swept only in one direction across the rows of subfields.
In divided-reticle microlithography, since all the pattern elements in each of the subfield regions are exposed in a respective shot, and all the pattern elements to be transferred are defined on a single reticle, throughput can be improved markedly compared with other conventional CPB microlithography apparatus and methods.
In most reticles as used for divided-reticle CPB microlithography each subfield is surrounded by struts that strengthen the reticle (and separate the subfields one from another). Reticles used in optical microlithography typically do not have struts. Partly as a result of the struts, the illumination beam must be deflected and sized accurately to illuminate only the desired subfield at a given instant in time.
To increase throughput further in a divided-reticle microlithography apparatus, it is necessary to decrease the time during which the wafer and reticle are moving (to expose the next subfield) and to reduce the number of switch-backs. It also is necessary to minimize the overhead time consumed in starting and stopping the stages (to expose each subfield). One way in which this can be achieved is by expanding the beam-deflection range as much as possible. Unfortunately, as the magnitude of lateral deflection of a charged particle beam increases, deflection aberrations increase or arise. This is because, in being deflected a greater distance laterally, the beam passes through regions of the projection-optical system that are farther off-axis. Deflection aberrations are problematic because they cause blur and distortion of the image being exposed onto the wafer surface. Deflection aberrations can be reduced by adjusting the induction current of the deflectors used to deflect the beam and by configuring the deflection trajectory in a manner that reduces deflection aberrations. Although these remedies are useful to a limited extent, actual cancellation of deflection aberrations is desired through the use of corrective optical components such as stigmators (astigmatism compensators) and the like.
A stigmator conventionally is constructed by superimposing two quadrupole magnetic poles that are shifted 45° from one another about the optical axis. The respective magnitudes of electrical current supplied to each quadrupole can be adjusted separately. Hence, respective magnetic-field components can be generated that are proportional to cos[2&thgr;], where the field distribution is expressed in a cylindrical coordinate system (z,r,&thgr;) in which &thgr; is the rotational angle around the optical axis. With such a scheme, aberrations proportional to the aperture angle of the illumination beam and aberrations proportional to the size of the illumination-beam subfield can be reduced or eliminated. Aberrations proportional to the illumination-beam aperture angle include, e.g., deflection astigmatism. Aberrations proportional to the size of the illumination-beam subfield include, e.g., deflection astigmatic distortion.
However, whenever the lateral beam-deflection distance is great, non-linear aberrations relative to the aperture angle and subfield size that are generated by even higher-order magnetic-field dis
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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