Computer-aided design and analysis of circuits and semiconductor – Nanotechnology related integrated circuit design
Reexamination Certificate
2001-11-30
2003-07-08
Pert, Evan (Department: 2829)
Computer-aided design and analysis of circuits and semiconductor
Nanotechnology related integrated circuit design
C716S030000, C438S795000
Reexamination Certificate
active
06591412
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 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 respective images of the subfields are transferred from the reticle to the substrate.
BACKGROUND
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. Rather, the pattern is divided into individual exposure units usually termed “subfields” that 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 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 collectively form the entire contiguous pattern. 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 mechanically scanned in opposite lateral directions.
Whenever a pattern is being transferred from a segmented reticle, it is desirable to achieve the greatest possible pattern-transfer accuracy. To such end, the subfield images are formed at respective locations on the substrate that desirably result in proper “stitching” together of the individual subfield images. Such stitching (i.e., positioning of respective subfield images) must be performed extremely accurately. However, in a practical sense, situations do arise in which adjacent subfield images are slightly misaligned with each other, resulting in a corresponding “stitching” error. Stitching errors can be manifest, for example, as shorts between adjacent wiring traces caused by overlap of one wiring trace over another, and as breaks in wiring traces that should be contiguous with each other. With increases in the number of intersections of adjacent subfields as imaged on the substrate and/or increases in the number of patterns elements per subfield that are joined together, the number of such faults tends to increase commensurately. In addition, in conventional segmented reticles, subfield boundaries sometimes are situated between the source and gate or between the gate and drain of a transistor of the pattern. In such a situation, a stitching error at the subfield boundary can cause the respective transistor simply not to function or to function improperly.
Japan Kôkai Patent Document No. Hei 9-97759 discloses a conventional method by which a pattern as defined on the reticle is divided into subfields. A plotter diagram of the pattern is evaluated so as to place subfield-division lines in respective locations that reduce intersections of the lines with pattern elements as much as possible. Unfortunately, in this method, pattern evaluation requires that an operator handle very large amounts of data; hence, data processing requires a long time to complete.
Whenever a pattern is divided to form a segmented reticle, situations frequently arise in which certain pattern elements cannot be defined entirely in a single subfield. For example, certain pattern elements inevitably result in the presence of “island,” “donut,” “peninsular,” or other reticle features that are not self-supporting. In such instances, at least two complementary subfields must be used to define the respective pattern portion. U.S. Pat. No. 5,166,888 discloses a conventional method for dividing a pattern portion into complementary subfields. In the disclosed method, a stability value is assigned to each inside corner of the pattern element. The stability value is a function of the length of the perimeter of all edges between adjacent outside corners of the pattern element and the length of the shortest distance between the adjacent outside corners. Based on such data, pattern elements having a sub-threshold stability value are divided into multiple rectangular sections. The resulting sections are distributed between two complementary subfields. Unfortunately, this method tends to result in the element being divided up more than actually necessary, which results in a higher than acceptable incidence of stitching errors between adjacent sections of the element as transferred to the substrate.
Also, the methods summarized above are directed to, and thus have been applied only to, abstract artificial patterns rather than actual LSI patterns. Hence, there is a need for pattern-element division methods that are more applicable to actual LSI patterns.
SUMMARY
In view of the shortcomings of conventional methods as summarized above, this invention provides, inter alia, transfer-exposure methods for performing division of pattern elements that are more similar to actual LSI patterns, and that provide improved pattern-layering accuracy and reduced stitching errors.
To such end, and according to a first aspect of the invention, methods are provided for dividing a pattern to be defined on a segmented reticle for use in charged-particle-beam (CPB) microlithography. The pattern is divided into multiple subfields each defining a respective portion of the pattern. In an embodiment of such a method, the reticle pattern is initially divided using initial subfield-boundary lines that are spaced apart from one another according to a subfield dimension determined in advance. Then, the initial subfield-boundary lines are corrected to produce corrected subfield-boundary lines so as to correct pattern-element stitching when the pattern is transfer-exposed to the sensitive substrate.
If the pattern elements as defined on the reticle are arranged at certain respective pitches in the X direction and Y direction, then the initial subfield-boundary lines can be established at respective integer multiples of the respective pitches without, thereby, defining subfields that are larger than a maximum subfield size that can be exposed without excessive aberrations. In addition to dividing the reticle into subfields each defining a respective portion of the pattern, certain subfields can be divided as required into respective complementary subfields so as to avoid the “donut” problem and/or the “peninsula” problem. For example, in the reticle subfields that are divided into respective complementary subfields, each respective reticle subfield can be divided by at least one line extending in the X or Y direction at an interval of ¼ of the respective pitch or an integer multiple of ¼ of the respective pitch.
Typically, the reticle patt
Klarquist & Sparkman, LLP
Nikon Corporation
Pert Evan
LandOfFree
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