Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2000-08-11
2003-04-01
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S296000, C430S942000
Reexamination Certificate
active
06541169
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography, which is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, this invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam.
BACKGROUND OF THE INVENTION
The following discussion of conventional microlithography is set forth in the context of using an electron beam as a representative charged particle beam.
Pattern transfer using electron-beam drawing is highly precise, but has the fault of low throughput. Considerable research effort has been expended in investigating various technical approaches to improving throughput. These approaches include partial-pattern, single-shot exposure techniques referred to as cell projection, character projection, or block exposure.
In partial-pattern, single-shot techniques, certain repetitive portions of the circuit defined on a reticle are exposed many times onto respective regions of a die on the wafer (typically, these repetitive regions measure approximately 5 &mgr;m-square on the wafer). Each different repetitive portion is defined by at least one respective region on the reticle. This technique is used especially whenever the reticle pattern has large numbers of identical features having the same configuration, such as in a memory chip. Unfortunately, certain portions of the circuit that are not repeated must be transferred to the wafer using another technique, such as a variable-shaped-beam technique. Having to employ two or more different microlithography techniques to expose each die on the wafer results in a throughput that often is too low for use in mass production of wafers. Also, the throughput obtainable using the variable-shaped-beam technique is low.
To improve throughput, electron-beam reduction-transfer microlithography techniques have been devised. In certain apparatus employing such a technique, the reticle has a field defining the pattern for an entire die to be transferred to the wafer. The reticle field is illuminated using an electron irradiation beam, and an image of the illuminated field is “reduced” (demagnified) and transferred onto the wafer by a projection-optical system. Whereas this full-field transfer technique offers prospects of vastly higher throughput than partial-pattern single-shot techniques, aberrations over a large field currently cannot be controlled satisfactorily to achieve the desired resolution. Also, reticles suitable for transferring an entire field in one shot are extremely difficult to fabricate.
In view of the above, electron-beam microlithography techniques currently receiving the most attention are those in which the reticle pattern is divided into a large number of field portions, termed “subfields,” that are exposed individually and sequentially onto the wafer by the electron beam. Such techniques are termed “divided-reticle” microlithography methods. The subfields are exposed and transferred using a projection-optical system having a large optical field. Each subfield is exposed using a respective “shot.” To expose a subfield, the illumination beam is directed to the desired subfield so as to transfer an image of the subfield to a respective position on the wafer. As the subfield is being exposed, aberrations such as image defocusing and field distortion can be corrected in real time. On the wafer, the images of individual subfields are situated such that they are “stitched together” (placed contiguously relative to each other) to form the complete pattern. Divided-reticle exposure achieves better resolution and transfer accuracy over a wider optical field than achievable using a single-shot transfer of an entire die. Exploiting the wide exposure field in a divided-reticle exposure apparatus, high throughput can be achieved by continuously moving the respective stages on which the reticle and wafer are mounted.
In a cell-projection apparatus, the reticle is stationary during exposure, but the wafer is moved continuously. Under such conditions, the reticle image must be displaced smoothly using a deflector so that the reticle image tracks the motion of the wafer. Whenever an image is displaced using a deflector, the magnitude of beam deflection correspondingly changes, which causes continuous changes in the magnitude of deflection distortion. Heretofore, this change in deflection distortion was relatively slight and exhibited virtually no effect on exposure-position errors and image-defocusing. As a result, the amount of correction applied to correct deflection distortion was not revised (updated) continuously according to changes in the magnitude of deflection.
However, recent R&D has been aimed at expanding the deflection range and increasing the velocity of stage motion so as to further increase throughput. Deflection distortion generally is proportional to the cube of the corresponding magnitude of beam deflection. Consequently, as the magnitude of beam deflection increases, the magnitude of change in deflection distortion during a shot can reach a level that no longer can be ignored. Also, as circuit patterns continue to increase in density and complexity, the required tolerances for exposure accuracy and image defocusing become increasingly stringent. Therefore, it now is necessary to consider changes in aberrations, previously regarded as negligible, that accompany changes in the deflection magnitude during each shot.
In divided-reticle microlithography apparatus, the respective image of each illuminated subfield of the reticle typically is reduced (demagnified) as projected onto the wafer. Projection typically is performed using symmetrical magnetic doublet (SMD) electron-lens systems. Projection and exposure are performed while continuously and synchronously moving the reticle stage and the wafer stage. Demagnification is according to a “demagnification ratio,” which is a factor by which an image as formed on the wafer is smaller than the corresponding region on the reticle. During exposure, the velocity of the wafer stage to the velocity of the reticle stage nominally is equal (but see below) to the demagnification ratio of the projection-optical system. Even under such conditions, the magnitude of deflection of the reticle image relative to the projection-optical system will change during each shot. I.e., the trajectory of the imaging beam from the illuminated region of the reticle to the imaging position on the wafer changes during each shot.
In actual practice, in a divided reticle, individual subfields typically are separated from one another by struts. The struts provide substantial rigidity and mechanical strength to the reticle, and serve to conduct heat away from the reticle during illumination of the reticle. The struts normally are configured in a grid pattern, with individual subfields being located in respective spaces between adjacent struts. These aspects will be described later below with reference to FIGS.
2
(A)-
2
(C). Respective images of the struts are not projected onto the wafer. Consequently, the ratio of movement velocity of the substrate to the movement velocity of the reticle is not exactly equal to the demagnification ratio. I.e., the reticle moves slightly faster than indicated by the ratio, as described below with reference to FIG.
3
.
Because of the higher velocity of the reticle during exposure, it is necessary to change the position of the projected image as “seen” from the projection-optical system during each shot. In other words, it is necessary to continuously change the magnitude of positional change from the viewpoint of the optical-lens column (i.e., the amount of movement in the image point due to movement of the object point of the projection lens), as well as to continuously change the magnitude of beam deflection to compensate for the velocity differential.
Ideal positioning of the reticle stage, on which the reticle is mounted, is not limited to microlithographic exposure systems that transfer a continuously moving reticle. In conventional
Kojima Shin-ichi
Okino Teruaki
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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