Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-09-20
2002-11-26
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000
Reexamination Certificate
active
06485870
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (transfer of a pattern, defined by a reticle or mask, to a suitable substrate). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam as an energy beam. Yet more specifically, the invention pertains to reticles and masks as used in charged-particle-beam microlithography and to methods for inspecting such reticles and masks to ascertain whether they have been formed properly.
BACKGROUND OF THE INVENTION
Conventionally, microelectronic devices (e.g., semiconductor integrated circuits, displays, and the like) are manufactured by a complex series of steps. Many of the steps involve microlithography, by which a pattern, defined by a reticle or mask (collectively termed a “mask” herein) is transferred via an energy beam to a suitable substrate such as a semiconductor wafer. In microlithography apparatus as currently available and in use, the energy beam is a light beam (usually ultraviolet light); an image of the mask pattern is transferred by projection, in which the light beam passes from the mask through a projection-optical system to the substrate.
Despite the ability of current technology to achieve microlithographic resolution of about 0.25 &mgr;m, much conventional “optical” microlithography (in which a light beam is used) still is performed at a resolution (minimum pattern-element width, or “critical dimension”) of about 1 &mgr;m. Microlithography in which the critical dimension is about 1 &mgr;m or higher typically is performed at 1:1 (“full size”), by which is meant that the image as formed on the substrate is the same size as the image defined on the mask. However, as the required (and achievable) critical dimension has been reduced to below 1 &mgr;m, the benefits of “demagnifying” the image as formed on the substrate have increased. For example, some current microlithography is performed at a “demagnification ratio” of ¼, by which is meant that the image as formed on the substrate is ¼ the size of the pattern as defined on the mask. Microlithography performed at a demagnification ratio of less than unity is termed “reduction” microlithography. In reduction microlithography, the fact that all the pattern elements defined on the mask are bigger (by the reciprocal of the demagnification ratio) than the corresponding pattern elements as transferred to the substrate, the necessary accuracy with which the pattern elements are defined on the mask is eased correspondingly.
Many conventional masks are produced using electron-beam or laser-beam patterning, in which the pattern on the mask is “drawn” line-by-line and element-by-element. Unfortunately, these “direct-writing” mask-making techniques consume large amounts of time. Other mask-making techniques have been developed in which the masks themselves are produced using a microlithography technique, wherein the mask used to produce the mask used in microlithography of a substrate is termed the “parent” mask and the mask produced using the parent mask (and used in microlithography of a substrate) is termed the “progeny” mask. Reduction microlithography allows parent and progeny masks to be made with high accuracy. For example, the critical dimension in a progeny mask often can be reduced according to the demagnification ratio, relative to the critical dimension in the parent mask.
In producing a progeny mask from a parent mask, it is necessary to ascertain whether the pattern (as defined on the parent mask) has been transferred to the progeny mask properly and accurately. Conventionally, inspections are performed using an optical microscope, with which an enlarged image of the pattern on the parent mask is inspected and compared with a reference pattern. To achieve satisfactory inspection, the imaging resolution of the optical microscope should be approximately ten times finer than the minimum element size of the pattern being inspected. Because the imaging resolution of a current mask-inspection device employing an optical microscope is on the order of 0.1 &mgr;m, the minimum pattern-element size on a mask that can be inspected in this manner is approximately 1 &mgr;m.
The time required to perform a mask inspection depends upon the minimum pattern-element width of the pattern defined by the mask and on the target measurement accuracy. For example, a 5-inch mask can be inspected in approximately two hours.
With the increased integration of circuit elements in semiconductor integrated circuits (“chips”), the critical dimension of circuit elements in such circuits rapidly is approaching 100 nm, which is the expected critical dimension in the next generation of integrated circuits. It currently is impossible to transfer patterns having such small circuit elements onto a wafer using conventional optical microlithography equipment. As a result, so-called “divided-reticle” microlithography methods and apparatus utilizing a charged particle beam (e.g., electron beam) have been the subjects of intensive development effort. By “divided-reticle” is meant that the pattern as defined on the reticle is divided into multiple pattern portions that are exposed individually, rather than having the entire reticle pattern for a chip transferred in a single exposure “shot.”
Divided microlithography typically is performed in a “reduced” manner, with a demagnification ratio of, for example, ¼ or ⅕. At a demagnification ratio of ¼, attainment of a resolution of 100 nm on the wafer requires that the minimum dimension of the circuit pattern formed on the mask be approximately 400 nm. To adequately inspect such a mask, the inspection device must have a resolution of 40 nm. Consequently, an optical microscope cannot be used. The use of an SEM (scanning electron microscope) instead of an optical microscope has been considered, but an SEM is impractical because of the massive amount of time required to analyze and compare the pattern as formed on the mask with the circuit-design data used to create the pattern on the mask.
Hence, with respect to masks as used in charged-particle-beam microlithography of a pattern at a resolution of about 100 nm, there is a need for an effective and practical method for inspecting the circuit pattern formed in the mask.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide practical methods for inspecting charged-particle-beam (CPB) microlithography masks, especially for such methods that can be used in conjunction with methods for fabricating such masks.
According to a first aspect of the invention, methods are provided for fabricating a charged-particle-beam (CPB) mask. In a representative embodiment of such a method, a parent mask is provided that defines elements of a desired circuit pattern. Using the parent mask as a lithography mask, the pattern defined on the parent mask is transferred (desirably by optical lithography) to a progeny mask by reduction lithography. The progeny mask (destined to become the CPB mask) defines elements of the desired circuit pattern that correspond to but are smaller than corresponding elements on the progeny mask. The pattern as formed on the progeny mask is compared with the pattern formed in the parent mask to determine the accuracy with which the progeny mask defines the circuit pattern.
In the foregoing method, the step of providing the parent mask desirably includes the steps of preparing circuit-design data for the desired circuit pattern, converting the data into circuit-pattern elements to be defined by the parent mask, and preparing the parent mask from the converted data.
After providing the parent mask, the foregoing method desirably can include the step of comparing and contrasting the parent mask with data concerning the desired circuit pattern. If the parent mask corresponds with the data concerning the desired c
Klarquist & Sparkman, LLP
Nikon Corporation
Rosasco S.
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