Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-03-18
2001-03-13
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000
Reexamination Certificate
active
06200710
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography using a charged particle beam (e.g., electron beam). More specifically, the invention pertains to segmented reticles and masks (generally termed herein “reticles”) for such microlithography, to methods for making such reticles, and to methods for using such a reticle for making a microlithographic exposure.
Background of the Invention
Conventional methods for performing microlithography using a charged particle beam are essentially categorized into the three following methods: (1) “spot beam drawing”, (2) “variable-shaped beam exposure”, and (3) “cell-projection exposure.” Compared with single-shot offset projection-exposure methods using conventional light, the charged-particle-beam (CPB) exposure methods listed above offer tantalizing prospects of vastly improved resolution but tend to exhibit disappointingly very low throughput. In particular, exposure methods (1) and (2) involve tracing the pattern feature-by-feature using an electron beam having an extremely small spot diameter or square-shaped transverse profile; consequently, throughput is extremely low with these methods. Exposure method (3), the “cell-projection exposure” method, was developed in an effort to improve throughput. In method (3), the reticle defines one or more portions of a die pattern. Typically, each portion is repeated multiple times in the overall pattern, and each portion is defined by a respective “cell” on the reticle. For example, a cell as projected onto the substrate measures 5 micrometers by 5 micrometers, and the same cell can be projected many times on the substrate (each projection of the cell being at a different respective location on the substrate) to form a complete die. Throughput is improved by performing a single-shot projection-exposure of each cell rather than exposing the pattern feature-by-feature.
Unfortunately, the “cell-projection exposure” method has limited applicability. Typically, a reticle defining a pattern in which a certain pattern portion is repeated a large number of times (e.g., memory cells) is most amenable to cell-projection exposure. Portions of such a pattern that appear only once (and virtually every pattern has such portions) cannot be exposed using cell-projection exposure (and hence are usually not defined on the reticle). To accommodate the need to expose such uniquely configured portions along with the highly repeated portions of the pattern, cell-projection exposure must be combined with another exposure method, typically the “variable-shaped beam exposure” method. Having to combine methods in this manner is a key reason why cell-projection exposure generally does not produce a throughput that is improved as much as might be expected or desired.
In order to further improve the throughput of conventional CPB microlithography methods, “divided-pattern exposure” apparatus and methods have been devised that currently are the subject of intensive development. In such apparatus and methods, the reticle is divided (typically into “fields” and “subfields”) and only a portion of the reticle pattern (e.g., a subfield) is projected onto the substrate per “shot” at any one instant. For example, in
FIG. 3
, the substrate is depicted as a semiconductor wafer. Typically, the substrate is exposed with multiple dies (each destined to become a separate “chip”). Each die comprises multiple “stripes” and each stripe comprises multiple subfields. The corresponding reticle is “segmented” by which is meant that the reticle is divided into corresponding stripes and subfields.
Divided-pattern exposure apparatus normally perform exposures in a manner as shown generally in FIG.
4
. The substrate and reticle are mounted on respective stages. The wafer stage and the reticle stage move at respective fixed velocities in opposite directions to each other. The relative velocities of the stages conform roughly to the demagnification ratio of the microlithography apparatus. The charged particle beam illuminates the subfields on the reticle individually and in an ordered manner (e.g., sequentially). The charged particle beam passing through the illuminated subfield forms an image of the illuminated subfield on a corresponding subfield on the substrate surface. Progression of the charged particle beam from one subfield on the reticle to the next is typically performed by sweeping the beam at right angles to the scanning motion of the stages.
With divided-projection exposure, the subfields are arranged in sequential order on the reticle and each subfield is exposed in one pass and with one respective “shot” of the beam. Therefore, divided-pattern exposure apparatus can exhibit a greatly improved throughput (compared to conventional CPB exposure apparatus).
Unlike a reticle used in optical microlithography, the reticle used for divided-pattern exposure is divided (“segmented”) into subregions (e.g., subfields) each defining a respective portion of the overall pattern defined by the reticle. The subregions are separated from one another by peripheral struts. The struts provide mechanical strength to the reticle and help ensure that each reticle is illuminated separately from its neighbors by the charged particle beam.
A segmented reticle can be of a “stencil” type. However, stencil reticles pose problems for features having, e.g., doughnut-shaped profiles. If a stencil reticle is required and the pattern includes such features, a single reticle usually cannot be used to project the entire pattern and two “complementary reticles” are used instead. Each complementary reticle is exposed separately, and subfields containing doughnut features are exposed twice, once with each complementary reticle. I.e., the respective pattern portion defined by each subfield is divided between the two complementary reticles and two separate exposures are performed for each subfield to achieve a complete transfer of each subfield.
Respective examples of complementary reticles are shown in FIG.
5
(
a
) and FIG.
5
(
b
). In FIG.
5
(
a
), two immediately adjacent stripes form a complementary pair of stripes; in FIG.
5
(
b
), two immediately adjacent subfields form a complementary pair.
Therefore, if a single reticle is insufficient for covering an entire pattern for a die, multiple reticles can be used. In any event, during transfer of the pattern from each reticle to the substrate, the subfields are projected onto the substrate in respective locations that effectively “stitch” the subfields together on the substrate surface to form the complete pattern on each die.
Because the divided-pattern exposure method may require use of complementary reticles, conventional data-conversion methods cannot be used. Therefore, there is a need for data-conversion methods for use with divided-pattern exposure methods including exposure of complementary reticles.
SUMMARY OF THE INVENTION
This invention was developed in response to the situation summarized above. An object of the invention is to provide methods for generating data for use in forming a reticle, defining an LSI pattern, useful for performing a microlithographic exposure of the LSI pattern onto a substrate using a divided-pattern exposure method. Another object is to provide such methods that can be performed at a suitably high speed.
According to a first aspect of the invention, a representative such method comprises the following steps: From LSI design data concerning the particular pattern to be transferred to the substrate, data are generated that are useful for producing a segmented reticle defining the pattern as divided into multiple subfields, and for making a microlithographic exposure using the segmented reticle. The generated data are supplied to a reticle-writing apparatus to produce the segmented reticle. Also, the generated data are supplied to a microlithographic exposure apparatus for making a microlithographic exposure of the pattern, defined by the segmented reticle, onto the substrate. The microlithographic exposure is typically performed using a charged particle beam. The gener
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Nikon Corporation
Young Christopher G.
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