Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-06-18
2002-08-13
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492200, C250S492230, C250S398000, C250S3960ML, C250S3960ML
Reexamination Certificate
active
06433347
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography methods and apparatus employing a charged particle beam (e.g., electron beam) to perform projection-exposure of, e.g., a circuit pattern defined by a reticle or the like onto a suitable substrate (e.g., semiconductor wafer). More specifically, the invention pertains to such methods and apparatus permitting projection-exposure of desired exposure units on the reticle while achieving improved throughput and transfer accuracy.
BACKGROUND OF THE INVENTION
Projection microlithography is used extensively in the manufacture of semiconductor integrated circuits, displays, and the like. Virtually all contemporary microlithography methods utilize a beam of light (typically ultraviolet light) to perform pattern transfer (microlithography using a light beam is termed “optical” microlithography). However, due to the current inability of light to achieve resolution of feature sizes required for the next generation of integrated circuits, various microlithography methods using a beam other than light are now being considered. Among such new methods, microlithography using a charged particle beam (e.g., an electron beam) is currently the subject of intensive investigation (microlithography using a charged particle beam is termed “CPB” microlithography).
In either optical or CPB microlithography, the pattern to be transferred to the substrate is defined by a reticle. All or a portion of the reticle is illuminated by an “illumination beam” passing through an “illumination-optical system” located upstream of the reticle. Portions of the illumination beam passing through the illuminated region of the reticle the “patterned beam” or “imaging beam”) are projected using a projection-optical system (located downstream of the reticle) onto a semiconductor wafer or other suitable substrate. The substrate is coated with a resist that, when exposed to the patterned beam, is imprintable with the pattern.
The reticle can define the pattern for a single chip (i.e., for a single “die”) or for multiple dies. Alternatively, the reticle can define a single inspection pattern or multiple inspection patterns. The entire reticle need not be exposed in a single exposure or “shot.” For example, the reticle can comprise multiple regions (“exposure units”) that are individually exposed. To achieve such selected exposure, certain conventional optical microlithography systems have a movable mechanical field aperture (“reticle blind”) that trims the illumination beam to illuminate only a desired portion of the reticle while not illuminating other portions of the reticle. Certain other conventional optical microlithography apparatus perform exposure by scanning selected exposure fields of the reticle using a slit-shaped portion of the illumination beam. In either method of exposure, the illumination beam is maintained at a constant intensity and focal position for the selected exposure field. I.e., within the selected exposure field, the beam intensity and focal position are not variable.
In conventional optical microlithography apparatus employing a reticle blind, it is desirable to have the reticle blind located immediately adjacent (just upstream of) the reticle to minimize defocusing of the image of the reticle blind on the reticle. Also, with such apparatus that perform scanning exposure of the reticle, it is necessary to scan the field aperture as the reticle stage is being scanned, as disclosed in Japanese Kôkai Patent Publication No. Hei 6-232031.
With a field aperture, the magnitude of defocusing of the image of the field aperture at the image plane is proportional to the axial distance between the field aperture and the reticle pattern. To solve this problem, the illumination-optical system in some conventional optical microlithography systems defines a plane conjugate with the reticle, and the field aperture is situated at that plane. However, such a configuration results in excessive complexity of the illumination-optical system. Furthermore, even with such a configuration, as the reticle is scanned during exposure, it is necessary also to scan the field aperture, as disclosed in Japan Kôkai Patent Publication No. Hei 7-94387.
Whereas field apertures have been employed as described above with conventional optical microlithography systems, no such employment of field apertures has been proposed for CPB microlithography apparatus and methods. For example, placing a reticle blind immediately upstream of the reticle is not practical with conventional CPB microlithography apparatus in which multiple coils, deflectors, and the like are typically situated just upstream of the reticle to minimize disturbance fields at the reticle. Also, in situations in which the field aperture should be scanned along with scanning the reticle stage, having to include the necessary scanning mechanism for the field aperture precludes placement of the field aperture adjacent to and immediately upstream of the reticle.
Many semiconductor “system” integrated circuits (termed “system LSI” circuits) have been produced recently. System LSI circuits combine logic circuits and memory circuits in a single chip. During manufacture, the layer-to-layer step differences within each system LSI chip tend to be relatively high, making them susceptible to differences in the best-focus position during microlithography steps. Also, during manufacture of such chips, the optimal exposure dose varies depending upon positional factors such as differences in resist thickness due to variations in step height and/or differences in beam reflectivity of material underlying the resist. Consequently, it has not been possible with conventional CPB microlithography apparatus employing single-shot and/or slit-illumination scanning exposure to control the exposure dose and focal position adequately in each exposure unit.
Furthermore, in the face of such problems, conventional CPB microlithography methods and apparatus cannot achieve sufficiently high throughput to be of practical utility in a contemporary wafer-fabrication facility, especially for mass-producing semiconductor devices such as DRAMs and the like. Low throughput with conventional CPB systems is also the result of limitations on the size of the optical field that will provide good correction of aberrations.
SUMMARY OF THE INVENTION
The present invention addresses the types of problems summarized above with the prior art. An object of the invention is to provide methods and apparatus that projection-expose a pattern, defined on a reticle or analogous device, onto a sensitive substrate using a charged particle beam (e.g., electron beam). Another object is to provide such methods and apparatus that achieve such ends with an acceptable level of throughput and pattern-transfer accuracy.
According to a first aspect of the invention, methods are provided for performing projection microlithography using a charged particle beam. According to a first representative embodiment of such a method, a pattern is defined on a reticle for transfer of the pattern to a sensitive substrate (e.g., semiconductor wafer). The pattern is divided on the reticle into separate exposure units each defining a respective portion of the pattern. The individual exposure units are sequentially illuminated using a charged-particle illumination beam. As particles in the illumination beam pass through each exposure unit, a charged-particle patterned beam is formed propagating downstream of the reticle. For each illuminated exposure unit, the respective patterned beam is projected onto a respective region of the sensitive substrate. Thus, an image of the illuminated exposure unit is formed on the respective region of the substrate. The respective regions are located so as to cause the images of the illuminated exposure units to be stitched together. Also according to the method, ON/OFF control data are provided for the illumination beam for each of the exposure units on the reticle. Each exposure unit is exposed based on the ON/OFF data.
In the foregoing method, the pattern can
Anderson Bruce
Klarquist & Sparkman, LLP
Nikon Corporation
Wells Nikita
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