Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
1998-12-28
2001-02-27
Berman, Jack (Department: 2878)
Radiant energy
Means to align or position an object relative to a source or...
C250S492200, C250S492230
Reexamination Certificate
active
06194732
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithographic projection-exposure apparatus and methods employing a charged particle beam, such as an electron beam or ion beam, for projecting a pattern defined by a mask or reticle onto a sensitive substrate. Such apparatus and methods are primarily used in the manufacture of semiconductor devices. More specifically, the invention pertains to charged-particle-beam (CPB) projection-exposure apparatus and methods exhibiting reduced exposure-pattern defocusing and distortion compared to conventional apparatus and methods, while exhibiting improved fidelity and stitching accuracy of projected pattern portions.
BACKGROUND OF THE INVENTION
Relevant conventional apparatus and methods are exemplified by certain electron-beam projection-exposure apparatus. Projection-exposure using an electron beam is highly accurate but current methods are flawed by low throughput, and various approaches have been investigated in attempts to solve such problems. For example, various “batch”-type projection-exposure systems, termed “cell-projection,” “character projection,” or “block exposure” systems, have been developed. With such systems, a circuit pattern comprising a large number of repeating units of a particular arrangement of features is defined on a mask. Such a circuit pattern is typical of, for example, memory chips. The entire mask is not exposed in one exposure (or “shot”) but rather one unit at a time in a repeating manner onto the substrate. As projected onto the substrate, each unit measures, e.g., about 10 &mgr;m by 10 &mgr;m square. However, such systems exhibit difficulty in projecting non-repeated areas of the circuit pattern.
Current efforts in the microlithography industry have been directed to the development of electron-beam projection-exposure apparatus that perform “demagnifying” projection-exposure (i.e., exposure in which the projected image of the mask is demagnified relative to the mask) of the mask pattern at much higher throughput than the batch-type systems summarized above. Various approaches have been investigated in which the mask defines the entire circuit pattern for a chip. In one approach, the electron beam is irradiated only on a certain portion of the overall mask pattern at any one instant, and an image of the irradiated portion is demagnified and “transferred” to (i.e., projection-exposed onto) the substrate using a projection lens. If one were to attempt to transfer, using such an apparatus, the entire mask pattern to the substrate in one shot, the entire mask pattern would not be transferred with sufficient accuracy. In addition, preparation of a mask for use in such an apparatus is very difficult.
Therefore, systems that are the subject of the most recent active research are not those in which the entire die pattern (or even multiple die patterns) is exposed in one shot. Rather, the most current approaches are directed to systems in which the projection optical system has a large optical field and the mask pattern is divided into multiple “subfields” that are projection-exposed one subfield at a time onto the substrate. Such systems are termed “divided” projection-exposure systems. With a divided projection-exposure system, exposure of each mask subfield is performed while correcting projection aberrations, such as image focus or field distortion, etc. Thus, projection-exposure can be performed with better image resolution and accuracy across an optically wider field than with systems that projection-expose an entire die in one shot.
In most conventional batch-type and divided projection-exposure systems, the projection-optical system projects a demagnified image of the irradiated portion of the mask onto the substrate. At the mask surface, if all locations of the transverse area of the beam used to illuminate the mask are not incident exactly orthogonally (i.e., telecentrically) at the mask surface, the trajectory of the beam between the mask and the substrate will not be ideal and more aberrations will be manifest.
In addition, at the substrate surface, if all locations of the transverse area of the beam used to project the image onto the substrate surface are not incident exactly orthogonally (i.e., telecentrically) on the substrate surface, problems will arise such as variations in image size as projected and/or image rotation at each transferred subfield, especially if the substrate surface exhibits any variation in elevation relative to the focusing plane of the projection-optical system. Such problems can seriously undermine, for example, the accuracy with which transferred subfields are “stitched” together on the substrate surface.
With semiconductor devices that require highly accurate fabrication, such as 1-Gbit and 4-Gbit DRAMs, the accuracy with which transferred subfields must be positioned and stitched together on the substrate surface is extremely high: about 10 to 30 nm. Therefore, it is becoming increasingly crucial, in such systems, to increase the accuracy with which all portions of the transverse area of the beam are incident orthogonally to the mask and substrate, and to be able to measure such orthogonality with high accuracy.
Conventionally, incidence orthogonality of the center of a beam flux is measured as described in Sturans et al., “Optimization of Variable Axis Immersion Lens For Resolution and Normal Landing,”
J. Vac. Sci. Technol. B
8(6):1682-1685 (Nov./Dec., 1990). According to the method disclosed in that paper, a variable-shaped beam is projected onto the substrate. The size of the imaged pattern is controlled by shaping deflectors. Incidence orthogonality of the beam is measured only at the center of the beam flux. Unfortunately, in divided projection-transfer systems, such corrections cannot be realized.
SUMMARY OF THE INVENTION
The shortcomings of conventional methods as summarized above are addressed by methods according to the present invention.
A first representative embodiment of the invention is directed to certain improvements in a method for transferring a pattern from a mask to a sensitive substrate. In the method the mask defining the pattern is illuminated substantially orthogonally with a charged particle beam, and an image of the charged particle beam that has passed through the mask is projection-exposed onto the sensitive substrate. At each of multiple locations, at about “mask level” but displaced from each other in an optical-axis direction, a beam-parallelism measurement is obtained from a transverse profile of the charged particle beam; this profile is obtained from a measurement of two opposite-edge positions of the illumination beam at the mask. From data concerning these positional measurements, a respective parallelism of the charged particle beam as incident at the mask is determined. Based on these determinations, the parallelism of the charged particle beam incident to the mask is adjusted as required to reduce an error in the parallelism found as a result of the determinations. After performing the beam-parallelism adjustment, a projection-exposure can be performed of the substrate with the pattern defined by the mask.
According to a second representative embodiment, the mask defining the pattern is illuminated with a charged particle beam, and an image of the charged particle beam that has passed through the mask is projection-exposed substantially orthogonally onto the sensitive substrate. At each of multiple locations, at about “substrate level” but displaced from each other in an optical-axis direction, a beam-parallelism measurement is obtained from a transverse profile of the charged particle beam; this profile is obtained from a measurement of two opposite-edge positions of the illumination beam at the substrate. From data concerning these positional measurements, a respective parallelism of the charged particle beam incident at the substrate is determined. Based on these determinations, the parallelism of the charged particle beam incident at the substrate is adjusted as required to reduce an error in the parallelism found as a r
Berman Jack
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Nikon Corporation
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