Radiation imagery chemistry: process – composition – or product th – Registration or layout process other than color proofing
Reexamination Certificate
2002-04-24
2004-12-28
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Registration or layout process other than color proofing
C430S030000, C430S296000, C430S942000, C250S491100, C250S492100, C250S492200, C250S492220, C250S492300
Reexamination Certificate
active
06835511
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, inter alia, to charged-particle-beam (CPB) microlithography performed using a pattern-defining segmented reticle on which the pattern is divided into multiple subfields each defining a respective portion of the pattern, and to methods by which distortion of projected subfield images, as caused by reticle deformation, is corrected quickly, inexpensively, and with high accuracy.
BACKGROUND
Conventional methods and apparatus are described below in the context of using an electron beam as a representative charged particle beam.
With the relentless drive to progressively smaller feature sizes (now less than 0.10 &mgr;m) the pattern-resolution limitations of optical microlithography have become a major limitation. To solve this problem, considerable effort currently is being expended to develop a practical “next generation” microlithography technology. A major effort to such end involves using a charged particle beam (e.g., an electron beam) as the lithographic energy beam. Charged-particle-beam (CPB) microlithography is expected to produce substantially better pattern resolution for reasons similar to the reasons for which electron microscopy yields better image resolution than optical microscopy.
Current CPB microlithography technology does not yet embody a solution to the problem of projecting an entire pattern in one shot from the reticle to the substrate. Consequently, according to one conventional method, the pattern is divided into individual exposure units usually termed “subfields” each defining a respective portion of the overall pattern. The subfields are defined on a “segmented” reticle and exposed in a prescribed order subfield-by-subfield. This exposure scheme is termed “divided-reticle pattern transfer,” as described for example in U.S. Pat. No. 5,260,151 and Japan Kôkai (published) Patent Document No. Hei 8-186070. As can be surmised, the optical field of CPB optics required to transfer a single subfield is much smaller than otherwise would be required to transfer the entire pattern in one shot. During transfer of each subfield, the respective subfield image is formed on the substrate in a manner such that, when exposure is complete, the subfield images are “stitched” together in a manner by which they collectively form the entire contiguous pattern on the substrate.
The subfields typically are arrayed on the reticle in rows and columns, wherein each row has a length substantially equal to the diameter of the optical field of the CPB optical system. During exposure of a row of subfields, the charged particle beam is deflected laterally as required to transfer the subfields in the row in sequential order. In progressing from one row to the next, the reticle and substrate typically are scanned mechanically in opposite lateral directions.
From the foregoing, it will be understood that conventional divided-reticle pattern transfer exhibits substantially lower “throughput” (number of wafer substrates that can be processed lithographically per unit time) than optical microlithography in which an entire die can be exposed in one shot.
Two types of reticles generally are used in divided-reticle pattern transfer. The first type is termed a “scattering-stencil” reticle, and the second type is termed a “scattering-membrane” reticle. In a scattering-stencil reticle, pattern elements are defined by respective apertures (through-holes) in a “CPB-scattering” membrane (usually of silicon) having a thickness of approximately 1 to 5 &mgr;m. In a scattering-membrane reticle, pattern elements are defined by a corresponding patterned layer of a highly CPB-scattering material formed on a thin, relatively non-scattering membrane.
Both types of reticles summarized above are produced by first fabricating a suitable “reticle blank” (typically made from a silicon wafer) including a reticle membrane, and then forming the pattern on or in the membrane. The pattern normally is formed by electron-beam drawing followed by etching of the membrane to form a scattering-stencil reticle or of the layer of highly scattering material to form a scattering-membrane reticle. Forming the elements of the pattern in this manner on the reticle membrane can result in distortion and deformation of the respective pattern portions as defined in the subfields. Distortion and deformation also may arise when the reticle is mounted on a reticle stage of the CPB microlithography apparatus by electrostatic chucking or the like. Whenever a lithographic exposure is performed with a deformed reticle, the pattern image as projected from the reticle onto a lithographic substrate exhibits a corresponding deformation, which degrades the accuracy of pattern transfer (especially manifest as overlay errors or stitching errors). Accordingly, minimizing reticle deformation is important from the standpoint of obtaining the best possible pattern-transfer accuracy.
Methods have been proposed for measuring reticle deformation before using the reticle for microlithography. Subsequent lithographic exposure using the reticle is performed while correspondingly correcting the deformation. Corrections are made by, for example, adjusting the projection-optical system of the microlithography apparatus to make appropriate changes to image magnification, rotation, and position. The adjustments are made based on the measurement data obtained prior to commencing lithography.
In one conventional method, measurement marks are defined on the support struts of the reticle between the subfields. Before using the reticle for lithographic exposures, relative positions of the measurement marks are determined using an inspection device such as a coordinate-measurement device. Detected positional anomalies indicating reticle deformation are corrected.
In another conventional method, measurement marks are defined on the membrane portions of individual subfields of the reticle, as disclosed in Japan Kôkai Patent Document Nos. Hei 11-30850, 11-142121, and 2000-124114. The marks are illuminated using an electron beam of the microlithography apparatus. The relative positions or dimensions of the marks are measured, and positional or dimensional anomalies indicating reticle deformation are corrected.
In actual practice there are many diverse causes of reticle deformation. As a result, sufficient correction of reticle deformation usually cannot be obtained using the conventional corrective schemes summarized above. Also, the conventional deformation-correction methods summarized above require long reticle-inspection times in order to ascertain positional errors in all the subfields of the reticle. Consequently, inspection costs can be prohibitively high.
SUMMARY
In view of the shortcomings of conventional methods as summarized above, the present invention provides, inter alia, lithographic-exposure methods in which reticle deformation is measured substantially more rapidly, more inexpensively, and with greater accuracy than conventionally.
A first aspect of the invention is set forth in the context of a microlithography method, performed using a microlithography apparatus, in which a device pattern to be transferred onto a sensitive substrate is defined on a reticle that is divided into multiple subfields each defining a respective portion of the pattern. The reticle is illuminated subfield-by-subfield with an illumination beam to produce a corresponding patterned beam carrying an aerial image of the illuminated region of the reticle. The aerial image carried by the patterned beam is projected and focused as a subfield image at a respective location on the sensitive substrate, and the subfield images on the substrate are stitched together to form the device pattern on the substrate. More specifically, in the context of such a
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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