Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-04-26
2002-04-23
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S398000
Reexamination Certificate
active
06376848
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography in which a pattern, defined on a mask or reticle, is transferred to a suitable substrate using a charged particle beam such as an electron beam. This type of microlithography has especial utility in the fabrication of semiconductor integrated circuits and displays. More particularly, the invention pertains to suppression of aberrations arising from increased beam deflection used for correcting errors in stage-position control.
BACKGROUND OF THE INVENTION
In recent years, as semiconductor integrated circuits increasingly have become miniaturized, the resolution limits of optical microlithography (i.e., microlithography performed using ultraviolet light as an energy beam) increasingly have become apparent. As a result, considerable development effort is being expended to develop microlithography apparatus and methods that employ an alternative type of energy beam offering prospects of better resolution than optical microlithography. One candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam. The charged particle beam passes through a charged-particle-beam (CPB)-optical system from a source (e.g., electron gun) through a reticle to a substrate (e.g., semiconductor wafer). Typically, the CPB-optical system includes an illumination-optical system that directs the beam from the source to the reticle, and a projection-optical system that directs the beam from the reticle to the wafer.
It currently is impossible to provide a CPB-optical system having an optical field large enough to expose an entire die pattern at one instant while achieving adequate control and minimization of aberrations. Hence, the pattern as defined on the reticle is typically divided into multiple small regions (subfields). Such a reticle is termed a “divided” or “segmented” reticle, in which each subfield defines a respective portion of the overall pattern. The subfields normally are illuminated one at a time and thus sequentially “transferred” onto the wafer. During transfer of the subfield images to the wafer, the wafer is mounted on and appropriately moved by a wafer stage to accurately place the subfield images on the wafer. As projected on the wafer, the respective images of the illuminated subfields desirably are arranged so as to be connected (“stitched”) together in the proper order and arrangement so as to form the entire pattern on the wafer after completing exposure of all the subfields of the reticle. General aspects of this “divided-reticle pattern-transfer” technology are described, for example, in U.S. Pat. No. 5,260,151 and in Japanese Kôkai patent document no. Hei 8-186070.
In divided-reticle pattern transfer, to ensure accurate transfer of the pattern portion defined by a reticle subfield to the wafer, the image of the subfield must fall accurately within the respective target region on the wafer. Conventionally, the wafer is mounted on a wafer stage, and wafer-stage position is measured using a laser interferometer that can measure stage position very accurately. In actual practice, however, errors occur in the actual wafer-stage position. For example, a positional error can be 1 &mgr;m in one or both of the X- and Y-directions. Conventionally, such errors are conventionally corrected using a deflector configured to impart a compensating deflection to the beam. This operation normally is repeated for each transfer of a subfield.
Unfortunately, the conventional remedies summarized above result in the positions on the wafer of the projected subfield images being shifted from their respective calibrated positions. Also, use of a deflector results in the beam being consistently laterally shifted from the ideal central coordinates of the projection-optical system, and an overall increase in aberration due to deflecting the beam.
SUMMARY OF THE INVENTION
In view of the shortcomings of conventional practice as summarized above, an object of the invention is to provide charged-particle-beam (CPB) projection-exposure apparatus and methods that adequately suppress aberrations arising from shifts of beam position from an “ideal” (or “target”) deflection position (at which a subfield image would be accurately transferred by the projection-optical system) due to errors in stage-position control.
To such end, and according to a first aspect of the invention, CPB microlithography apparatus are provided. In a representative embodiment of such an apparatus, an illumination-optical system is situated and configured to illuminate a charged-particle illumination beam from a source onto a reticle defining a pattern to be transferred onto a sensitive substrate. The reticle is divided into multiple subfields each defining a respective portion of the pattern and each being individually illuminated by the illumination-optical system for transfer of the respective pattern portion (the illumination beam passing through the illuminated subfield forms a patterned beam propagating downstream of the reticle). The apparatus also comprises a reticle stage on which the reticle is movably mounted to allow the illumination-optical system to illuminate a region of the reticle with the illumination beam. The position of the reticle stage is detected using a reticle-stage position detector. The apparatus also comprises a projection-optical system situated and configured to project and focus the patterned beam onto a sensitive substrate. The projection-optical system comprises an image-positioning deflector. The substrate is movably mounted on a substrate stage to allow the patterned beam, passing through the projection-optical system, to form an image of the illuminated subfield at a respective location on the sensitive substrate. The position of the substrate stage is detected using a substrate-stage position detector. The apparatus also comprises a controller connected to the illumination-optical system, the projection-optical system, the reticle stage, the reticle-stage position detector, the substrate stage, and the substrate-stage position detector.
The controller is configured to perform several functions, as follows: (1) controllably operate each of the projection-optical system, the reticle stage, the reticle-stage position detector, the substrate stage, and the substrate-stage position detector, so as to transfer the pattern subfield-by-subfield in a sequential manner from the reticle to the substrate; (2) controllably energize the image-positioning deflector so as to arrange the images of the subfields contiguously on the substrate; (3) detect errors in positioning of one or both the reticle stage and substrate stage for exposing an image of a subfield on the substrate; (4) correct the stage-positioning errors for subfields in a range of subfields by appropriately energizing the image-positioning deflector; (5) in a memory, store data concerning the stage-positioning errors detected in the range; (6) calculate an error statistic concerning stage-positioning errors detected regarding the subfields in the range, so as to yield data concerning an error trend in the range; and (v7) during exposure of a subsequent range of subfields, recalling the data from the memory and utilizing the recalled data and the data concerning the error trend in a previous range to control positioning of the stages during exposure of the subfields in the subsequent range so as to minimize average errors in stage positioning in the subsequent range. The data stored in the memory can include data on image rotation detected for the subfields in the range. The statistic can be, for example, a sum of squares of effective errors detected for the subfields in the range.
The controller desirably directs operation of the image-positioning deflector so as to achieve, in each range, a deflection of the patterned beam sufficient to reduce deflection aberrations compared to deflection aberrations obtained during exposure of subfields in a previous range.
Also desirably, the subfields on the reticle are arranged into a plu
Klarquist & Sparkman, LLP
Nguyen Kiet T.
Nikon Corporation
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