Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2001-03-13
2003-12-02
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100, C250S492220, C250S492230
Reexamination Certificate
active
06657207
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle or mask, to a “sensitive” substrate using an energy beam). Microlithography is a key technology employed in the fabrication of microelectronic devices such as integrated circuits, displays, magnetic pickup heads, and micromachines. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as the energy beam. Even more specifically, the invention pertains to altering the focusing conditions to realize with good efficiency the correction of shape defects in transfer images caused by differences in the pattern distribution within respective subfields and manufacturing error at the time of mask preparation, and thus to realize high-resolution, high-precision pattern transfer and exposure, in a charged-particle-beam exposure method using a so-called divided-reticle transfer system.
BACKGROUND OF THE INVENTION
The prior art is discussed in the context of using an electron beam to perform microlithography of a complex pattern, such as for a layer of an integrated circuit, onto the surface of a suitable substrate such as a semiconductor wafer. Electron-beam microlithography offers prospects of very high accuracy and pattern resolution, but to date the throughput realized using electron-beam microlithography has been disappointingly low. Various approaches have been investigated to solve the low-throughput problem.
Example approaches currently being utilized to a limited extent include “cell projection,” “character projection,” and “block exposure” (collectively termed “partial-block” pattern transfer). Partial-block pattern transfer is used especially whenever the pattern to be transferred to the substrate comprises a region in which a basic pattern unit (or several basic pattern units) is repeated many times. For example, partial-block pattern transfer generally is used for patterns having large memory circuits, such as DRAMs. In such patterns, the basic pattern units are very small, having measurements on the substrate of, for example (5 &mgr;m)
2
(i.e., 5 &mgr;m×5 &mgr;m). The basic pattern units are defined on one or several exposure units on the reticle and the exposure units are exposed repeatedly many times onto the substrate to form the pattern on the substrate. Unfortunately, partial-block pattern transfer tends to be employed only for repeated portions of the pattern. Portions of the pattern that are not repeated are transferred onto the substrate using a different method, such as the variable-shaped beam method. Therefore, partial-block pattern transfer has a throughput that is too low, especially for large-scale production of integrated circuits and other microelectronic devices.
Another approach is electron-beam “direct writing” in which the pattern is drawn on the substrate line by line. Whereas this approach has application in preparing reticles and masks, the throughput obtained with this technique is much too low to be practical for large-scale production of integrated circuits and other microelectronic devices. During such direct writing, the shape of the beam can be changed (termed “variable-shaped beam” pattern transfer).
Yet another conventional approach that has been investigated in an effort to achieve a higher throughput than partial-block pattern-transfer methods is a projection microlithography method in which the entire reticle pattern for a complete die (or even multiple dies) is projection-exposed onto the substrate in a single “shot.” In such a method, the reticle defines a complete pattern, such as for a particular layer in an entire integrated circuit. The image of the reticle pattern as formed on the substrate is “demagnified” by which is meant that the image is smaller than the pattern on the reticle by a “demagnification factor” (typically 4:1 or 5:1). To form the image on the substrate, a projection lens is situated between the reticle and the substrate. Whereas this approach offers prospects of excellent throughput, it to date has exhibited excessive aberrations and the like, especially of peripheral regions of the projected pattern. In addition, it is extremely difficult to manufacture a reticle defining an entire pattern that can be exposed in one shot.
Yet another approach that is receiving much current attention is the “divided-reticle” projection-exposure approach that utilizes a “divided,” “partitioned,” or “segmented” reticle. On such a reticle, the overall reticle pattern is subdivided into portions termed herein “exposure units.” The exposure units can be of any of various types termed “subfields,” “stripes,” etc., as known in the art. For exposure, the reticle is mounted on a reticle stage and the substrate is mounted on a substrate stage. Each exposure unit is exposed individually and sequentially in a separate “shot” or scan. The image of each exposure unit is projection-exposed (typically at a demagnification ratio such as 4:1 or 5:1) using a projection-optical system situated between the reticle and the substrate. Even though the projection-optical system typically has a large field, distortions, focal-point errors, and other aberrations, and other faults in projected images of the exposure units are generally well-controlled. Although divided-reticle projection-exposure systems provide lower throughput than systems that expose the entire reticle in one shot, divided-reticle projection-exposure systems exhibit better exposure accuracy and image resolution over the entire pattern as projected.
Further regarding divided-reticle projection-exposure systems, exposure of each exposure unit generally is performed with the focal point of the projected image on the surface of the substrate. Also, as each exposure unit is projected onto the substrate, aberrations such as distortion are corrected. The respective images of the exposure units are formed at corresponding locations on the substrate such that the images are “stitched” together in a contiguous manner. Such stitching usually is performed by a combination of stage movements and beam deflection.
In divided-reticle projection-transfer using a charged particle beam, despite the control that conventionally can be exerted during projection of each exposure unit, certain problems nevertheless can arise. For example, if the beam current of the charged particle beam used to form the image is excessively large, then imaging can be affected adversely due to mutual repulsion of the charged particles in the beam. This phenomenon is termed the “Coulomb effect.” Also, the quality of imaging from one exposure unit to the next can be inconsistent, based upon changes in “feature density” or “feature distribution” from one exposure unit to another. Conventionally, changes can be made in real time to any of various imaging parameters to correct most of these changes. To such end, a modern charged-particle-beam (CPB) microlithography apparatus has a complex system for making subtle corrections to the optical performance of the system as exposure progresses. For example, in a modem variable-shaped beam system, focus correction can be predicted from the transverse area of the shaped beam and from other apparatus parameters such as acceleration voltage, beam-current density, beam-divergence angle, and axial length of the CPB-optical system.
In a conventional divided-reticle CPB-microlithography system, the dimensions of each exposure unit on the reticle range from approximately 10-&mgr;m square to 1000-&mgr;m square. (This area is extremely large compared to the area exposed per shot in any of the partial-block or variable-shaped beam approaches.) It has been reported that, in cases in which the area of the image is in this range, variations in imaging properties due to the Coulomb effect is small. See, Berger et al., “Particle-particle Interaction in Image Projection Lithography,”
J Vac. Sci. Technol
. B11(6):2294, November/December 1993. According to conventional thinking, this allows the upper limit
Hirayanagi Noriyuki
Kawata Shintaro
Okino Teruaki
Suzuki Kazuaki
Gurzo Paul M.
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
LandOfFree
Charged-particle-beam microlithography apparatus and methods... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Charged-particle-beam microlithography apparatus and methods..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Charged-particle-beam microlithography apparatus and methods... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3150463