Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2001-02-20
2004-11-09
Lee, John R. (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492200, C250S492220, C250S492300, C250S3960ML, C250S3960ML, C250S398000
Reexamination Certificate
active
06815693
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography (imaging of a pattern, defined by a reticle or mask, onto a sensitive substrate). Microlithography is a key technique in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as an energy beam. Even more specifically, the invention pertains to devices and methods for correcting proximity effects in charged-particle-beam (CPB) microlithography.
BACKGROUND OF THE INVENTION
The most common conventional microlithography technology used for fabricating integrated circuits is the so-called “optical stepper” employing ultraviolet light as an energy beam. Microlithography technology employing a charged particle beam is still limited, in a practical (and hence commercial) sense, to CPB “writing” systems mainly used for producing reticles used as pattern masters in optical steppers. However, in view of the resolution limits of optical microlithography, CPB microlithography has received considerable attention as a possible successor technology to optical microlithography for reasons similar to the argument that electron microscopy achieves greater resolution than optical microscopy.
One reason for the delay in establishing CPB microlithography as a principal lithographic technology used for mass-production of patterned wafers is the low throughput currently obtainable with this technology. One group of techniques currently used for performing CPB microlithography includes the “partial-pattern block exposure” techniques (e.g., cell projection, character projection, and block exposure). Partial-pattern block exposure is used mainly for transferring a pattern including an array containing a large number (typically thousands) of repeated individual pattern units, such as the memory cells on a memory chip. The repeated units normally are very small, typically about 5 &mgr;m square on the substrate. To form the array, the repeated units are exposed over and over again within a region on the wafer corresponding to the chip. As readily understood, considerable time is required to expose the array in each chip, which results in low throughput. Also, this technique is not used to transfer non-repeated portions of the chip pattern. Instead, the non-repeated portions usually are exposed by direct writing using a variable-shaped beam. This need to exploit multiple different techniques to expose each chip further compromises throughput. As a result, the partial-pattern block exposure techniques currently do not provide the throughput required for mass-production wafer fabrication.
A technique offering prospects of substantially greater throughput than the partial-pattern block exposure techniques involves exposing, in a single “shot,” a reticle defining the entire pattern to be transferred to a chip or defining a pattern for multiple chips. The reticle is exposed with “demagnification,” by which is meant that the reticle image as formed on the wafer is smaller (usually by an integer factor termed a “demagnification ratio”) than the corresponding pattern as defined on the reticle. Whereas the throughput potentially achievable using this technique is at least as good as currently achievable using optical microlithography, this technique unfortunately has several serious problems. One problem is the current impossibility of fabricating a reticle configured to be exposed in a single shot of a charged particle beam. Another problem is the current impossibility of adequately correcting off-axis aberrations, especially in peripheral regions of the large image produced by the charged particle beam.
A more recently considered approach is termed “divided-reticle” pattern transfer, which involves dividing the pattern, as defined on the reticle, into multiple individual exposure units usually termed “subfields.” Each subfield is exposed individually onto a respective region on the wafer. The subfield images are transferred to the wafer so that, after exposing all the subfields, the subfield images are “stitched” together in a contiguous manner to form the entire chip pattern. As each subfield is exposed, corrections are made to achieve proper focus and reduction of aberrations (e.g., distortion) for the particular subfield. Divided-reticle pattern transfer allows exposures to be made over an optically wide field with much better resolution and accuracy than could be obtained by exposing the entire reticle in one shot. Although divided-reticle pattern transfer does not yet achieve the same throughput as optical microlithography, the throughput nevertheless is much better than obtainable using the partial-pattern block exposure technique.
Certain aspects of divided-reticle pattern transfer are shown in
FIGS. 23 and 24
.
FIG. 23
depicts a wafer on which multiple chips have been exposed. As exposed, each chip comprises multiple “stripes,” and each stripe comprises multiple subfields arranged in rows. This same divided arrangement of stripes and subfields is used to define the pattern on the reticle.
FIG. 24
depicts an actual exposure. For exposure, the reticle and wafer are mounted on respective stages (not shown but well understood in the art) configured to move the reticle and wafer horizontally (in the figure) as required for exposure. During exposure of a stripe (a portion of which is shown), the reticle stage and wafer stage both move along the longitudinal center line of the respective stripes. Movements of the reticle and wafer are at constant respective velocities (but in opposite directions) in accordance with the demagnification ratio. Meanwhile, the charged particle beam incident on the reticle (the beam upstream of the reticle is termed the “illumination beam” and passes through an “illumination-optical system” to the reticle) illuminates the subfields on the reticle row-by-row and subfield-by-subfield within each row (the rows extend perpendicularly to the movement directions of the reticle and wafer). As each subfield is illuminated in this manner, the portion of the illumination beam passing through the respective subfield (now termed the “patterned beam” or “imaging beam”) passes through a projection-optical system to the wafer.
During exposure of a stripe, to expose the rows and subfields within each row of the stripe in a sequential manner, the illumination beam is deflected at right angles to the movement direction of the reticle stage and the patterned beam is deflected at right angles to the movement direction of the wafer stage. After completing exposure of each row, the illumination beam is deflected in the opposite direction, as shown in
FIG. 24
, to expose the subfields in the next row of the stripe. This exposure technique reduces extraneous deflections of the beam and improves throughput.
Whenever a “sensitive substrate” (e.g., resist-coated wafer) is irradiated with a charged particle beam, backscattering of charged particles from the resist and substrate causes the actual exposure dose to vary according to the distribution of pattern elements in the proximity of the beam. This phenomenon commonly is known as a “proximity effect.” The proximity effect also arises from forward-scattering of incident charged particles into the resist. Forward-scattering and backscattering results in a net outward propagation (spreading propagation) of charged particles from the respective points of incidence through the resist. This spreading out of charged particles from the respective points of incidence applies exposure energy to areas of the resist (adjacent to points of incidence) where exposure is not desired. Current methods for solving this problem include adjusting the radiation dose to obtain the desired amount of accumulated energy on the substrate, as described in Japan Kôkai Patent Document No. Hei 11-31658, and modifying the profiles of individual pattern elements as defined on the reticle (“local resizing”) so as to achieve d
Kamijo Koichi
Okamoto Kazuya
Okino Teruaki
KIarquist Sparkman, LLP
Lee John R.
Nikon Corporation
Souw Bernard E.
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