Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-04-13
2001-10-23
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492200, C250S492300, C250S3960ML, C430S312000, C430S296000
Reexamination Certificate
active
06307209
ABSTRACT:
FIELD OF INVENTION
The invention pertains to charged-particle-beam pattern-transfer methods and apparatus for transferring patterns from a mask to a wafer or other substrate.
BACKGROUND OF THE INVENTION
In conventional charged-particle-beam (CPB) pattern-transfer methods, an electron beam (or other charged-particle beam) is used to transfer a pattern from a mask to a wafer in order to obtain the high resolution imagery characteristic of CPB optical systems. Although charged-particle beams provide high-resolution imaging, pattern transfer using CPB pattern-transfer methods is generally slower than pattern transfer using optical radiation. Methods and apparatus have been disclosed that attempt to speed pattern transfer and increase throughput using CPB methods.
One CPB pattern-transfer method is known variously as cell-projection, character-projection, or block exposure. This method is suitable for transferring repetitive circuit patterns defined by a small area of the mask to a wafer or other substrate. In this method, several so-called “unit patterns” are defined on the mask and these unit patterns are repeatedly transferred onto the wafer using a charged-particle beam. For this method to be useful, the unit patterns should be no larger than about 5 &mgr;m by 5 &mgr;m as projected onto the wafer. In addition, the patterns to be transferred must consist primarily of repeats of the unit patterns and the method is unsuitable for patterns that are not repetitive. Using this method in the production of semiconductor integrated circuits such as DRAMs, throughput can be as much as ten times larger than using methods that do not take advantage of pattern repetitiveness.
Another method for increasing throughput in CPB pattern transfer is a demagnifying pattern-transfer method disclosed in laid-open Japanese Patent Document No. Hei 5-160012. In this technique, a mask is used to define the patterns for an integrated circuit. The mask is divided into multiple fields that are further divided into a plurality of subfields. An electron beam illuminates a subfield and a demagnified image of the subfield is projected onto the wafer with a two-stage projection lens system. The images of the patterns in the remaining subfields are successively projected onto the wafer so that the projected subfield patterns are aligned to form the patterns for the integrated circuit. Because the electron beam does not illuminate the entire mask simultaneously, alignment of the projected subfields is an important consideration. To achieve accurate subfield placement, the CPB optical system can be adjusted to improve the placement and resolution of each subfield image. This method is referred to as a “divisional” pattern- transfer method and is disclosed in, for example, U.S. Pat. No. 5,260,151. This method has not been successfully demonstrated for commercial production of integrated circuits.
The CPB pattern-transfer methods described above are useful only if the patterns in the subfields are accurately stitched together on the wafer. Laid-open Japanese Patent Application No. Sho 63-1032 discloses a method for improving the accuracy with which patterns are stitched together. In this method, a common pattern is formed in edge portions of two subfields whose pattern images are to be adjacent on the wafer. The subfields are projected onto the wafer so that the images of the common pattern in the two subfields overlap. The common pattern formed in the edge portions is exposed twice at a relatively low dose in different exposures so that the total dose (i.e., the total exposure to the electron beam) in the edge portions is approximately the same as that of other areas.
Unfortunately, the JP 63-1032 method is only useful in pattern transfer using a variably shaped beam or a focused beam, and no effective method for improving the accuracy of the pattern stitching, which is generally applicable to a mask-pattern transfer process using an electron beam, has been established yet. In particular, a method for efficient control of the dose distribution within a unit exposure area has not been proposed yet.
In shaped-beam pattern-transfer methods, the charged-particle beam is generally shaped with a shaping aperture. The boundary of a subfield transferred to a wafer is defined by the image of the shaping aperture on the mask instead of an edge or other feature on the mask. Distortion or blur of the shaping-aperture image degrades the quality of the transferred patterns. The illuminating charged-particle beam scans the entire field. If the field is large, then the image of the shaping aperture exhibits distortion, particularly at the most off-axis portions of the field. If the distortion is large, then the total exposure varies in the double-exposure area on and near the pattern-connection boundary. This can cause inaccurate stitching of the patterns in adjacent fields.
It will be apparent from the foregoing that improved methods and apparatus are needed for CPB pattern transfer.
SUMMARY OF THE INVENTION
According to one aspect of the invention, pattern-transfer methods are provided for transferring a pattern to a substrate. In one embodiment, the pattern is divided into a plurality of fields, each field having connection ends for connecting to patterns of an adjacent field as transferred to the substrate. The connection ends transferred adjacent to each other on the substrate contain a common pattern that is projected from the fields to substantially overlap on the substrate.
In another embodiment, the fields are divided into subfields and the subfields are illuminated with an image of a shaping aperture that is illuminated with a charged-particle beam (CPB). The image of the shaping aperture is stepped or scanned across the fields so that images of the subfields are projected onto the substrate. The images of the common patterns of adjacent fields are projected onto substantially the same substrate location and are thus exposed by the CPB at least twice.
In yet another embodiment, the image of the shaping aperture is vibrated as the image is scanned or stepped across the fields. In one example, the direction of vibration of the shaping aperture is perpendicular to the stepping of scanning direction. The vibration of the shaping-aperture image provides a variable CPB dose on the wafer from connection ends of the mask illuminated by the vibration of the shaping-aperture image. Connection ends are provided that extend along the stepping or scanning direction. Exposure of connection ends of adjacent fields with the vibration of the shaping-aperture image connects the adjacent fields so that the dose on the wafer is uniform. The dose on the wafer from exposure of a single connection end produces a non-uniform dose but the sum of the non-uniform doses from several connection ends is a uniform dose. The dose received by the substrate can vary linearly in the connection ends. The linearly varying dose connects patterns from adjacent fields even if the connection ends are displaced from exact overlap as projected on the substrate.
According to another aspect of the invention, pattern-transfer apparatus for transferring a pattern from a mask to a sensitized substrate are provided. The pattern is divided into a plurality of overlapping pattern segments. The pattern segments define fields on a mask and the overlapping portions of the pattern segments define connection ends of the fields. An illumination system illuminates the mask and a projection lens system images patterns from the mask onto the substrate. The illumination system includes a shaping aperture for shaping the transverse profile of the CPB. The illumination system images the shaping aperture onto the mask to illuminate subfields of the fields and includes deflectors for scanning or stepping the image of the shaping aperture on the mask. Beam vibration deflectors can be provided to vibrate the image of the shaping aperture to produce a linearly varying dose in the connection ends so illuminated. The vibration frequency is such that the vibration speed is much larger than
Nakasuji Mamoru
Okino Teruaki
Anderson Bruce
Klarquist & Sparkman, LLP
Nikon Corporation
Wells Nikita
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