Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
1998-10-27
2002-05-07
Grant, William (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C700S097000, C703S007000
Reexamination Certificate
active
06385498
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography apparatus and methods, particularly for use in manufacturing semiconductor devices, liquid-crystal display devices, and the like. More specifically, the invention pertains to design and manufacture methods as applied to scanning-exposure microlithography equipment.
BACKGROUND OF THE INVENTION
Recently, as the feature sizes of semiconductor devices (e.g., memories, processors, custom integrated circuits, etc., as well as displays such in TUFT displays, etc.) have become progressively smaller, the devices themselves have generally increased in size. Generally, such devices are manufactured by processes that include at least one microlithography step.
In “projection” microlithograpy, a circuit or other feature pattern as defined on a reticle (mask) is projected, using a projection lens, onto the surface of a substrate such as a semiconductor wafer. Microlithography apparatus that perform such multiple exposures are termed “steppers” because, after each exposure at a particular site (“die”) on the wafer, the apparatus “steps” to the adjacent die on the wafer (by moving the wafer relative to the reticle) for the subsequent exposure. Usually, the entire reticle pattern is formed on each die on the substrate surface. With “step-and-repeat” steppers, the entire reticle pattern is exposed at the same instant at each die; with “step-and-scan” steppers, the reticle pattern is scanned to expose each die.
In steppers, the projection lens is usually “reducing,” by which is meant that the image of the reticle pattern formed on the surface of the wafer is smaller (usually by some integer factor such as four or five) than the actual reticle pattern. The projection lens can have reflective elements, a combination of reflective and refractive elements, or all refractive elements.
In many prior-art step-and-repeat steppers, the projection lens has a circular field. The size of each die thus formed on the water surface is limited by the field diameter of the projection lens. As a result, every time a change is required in the size and/or degree of feature resolution of the dies to be formed on the wafer, a new stepper is required. For example, an increase in die size with an accompanying decrease in feature size requires a stepper equipped with a projection lens having a larger projection field (field diameter) and improved resolution (greater numerical aperture).
At least with refractive-type projection lenses, an increase in field size and numerical aperture usually requires an increase in the number and diameter of the optical elements (lens elements) comprising the projection system. This causes much difficulty in the mass production of the projection lenses, especially such lenses that are operable with ultraviolet light sources. For example, projection lenses operable with excimer laser light sources such as 248-nm or 193-nm sources, with a numerical aperture (N.A.) on the substrate side of approximately 0.6 and a projection field diameter of approximately 30 mm, typically comprise at least twenty lens elements. The lens elements can include quartz lenses with diameters of about 130 to 240 mm and fluorite lenses with diameters of about 130 to 170 mm. Such elements are extremely expensive. Moreover, the mass production of large-diameter quartz and fluorite lenses is much more difficult than the manufacture of similarly sized glass lens elements. Thus, the need to design and provide a new projection lens every time there is an incremental change in device size, density, or feature size poses both prohibitive expense and difficulty for both purchasers and manufacturers of steppers.
Step-and-scan steppers as briefly described above recently have been increasingly favored because they are more flexible in accommodating changes in device size, density, or feature size without having to change the projection lens. The principle of step-and-scan systems is discussed in, e.g.,
J. Vac. Sci. Technol
. 17:1147-1155, September/October 1980, in which a reducing projection lens is used with a ring-field (arc-shaped) slit. Step-and-scan can also be employed with a linear slit (part of a rectangular field) as described in, e.g.,
SPIE
, vol. 922 (Optical/Laser Microlithography), pp. 256-268 (1988). A step-and-scan projection exposure device is also disclosed in Japan Kôkai Patent Publication No. HEI 4-277612, wherein the effective projection field is restricted to a linear slit extending along the diameter inside a circular field.
In the foregoing types of step-and-scan apparatus employing a reducing projection lens, the reticle (mounted on a “reticle stage”) and wafer (mounted on a “wafer stage”) face each other on opposing axial ends of the projection lens. The reticle and wafer must move synchronously at relative velocities that differ from each other by the projection reduction-magnification factor (e.g., ⅕ or ¼). Such coordinated movement of the stages must be extremely smooth and accurate at least during scanning and exposure.
Hence, in step-and-scan steppers (as in step-and-repeat steppers), the positioning accuracy and the stepping precision of the wafer and reticle stages are critically important for achieving “transfer precision” (i.e., faithful reproduction of the reticle pattern on each exposure area with good positional registration and feature resolution). In step-and-scan steppers, unlike step-and-repeat steppers, it is critical that the wafer and reticle stages synchronously move with extreme precision during scanning. Otherwise, transfer precision is unacceptably compromised, resulting in deterioration of image quality from, for example, line-width errors, image distortion, registration errors, and magnification errors.
Certain prior-art step-and-scan steppers as disclosed in, e.g.,
SPIE
, vol. 1088 (Optical/Laser Microlithography), pp. 424-433 (1989) achieve smooth synchronous velocity control of the wafer and reticle stages by driving them with linear motors while using laser interferometers to measure the stage positions. Such control has to be achieved in an environment in which stresses and strains encountered by the stages and their drive mechanisms are always changing.
As a result, each of the mechanisms used for supporting and moving the substrate and reticle stages, as well as the column structures on which the stage mechanisms and projection optical system are mounted, must have an optimal structural design. Representative methods for performing structural analysis simulations of individual assemblies such as the reticle and substrate stages are discussed in, e.g., “Development of a High-Speed, High-Precision Positioning Stage,”
Proceedings of the
69
th Regular Conference of the Japan Society of Mechanical Engineering
, vol. C, pp. 11-13 (Apr. 1-3, 1992). Such methods enable one to evaluate the hypothetical properties of a proposed mechanism, such as for a stage, in isolation from other structures.
Unfortunately, a step-and-scan apparatus does not necessarily exhibit a desired transfer precision, even if the various mechanical systems for moving the stages have been optimized. This is because transfer precision is affected not only by the characteristics of the various mechanical assemblies (such as the stages), but also by other factors such as the characteristics of the various control components (e.g., drive motors and laser interferometers) that move and control motion of the stages, characteristics of columns and other supporting structures, air quality and flow inside the chamber in which these subassemblies are contained, the degree to which floor vibrations are isolated from the apparatus, and other factors. Consequently, attempts at optimization of specific assemblies and mechanisms by isolated structural-analysis simulations for each specific mechanism (e.g., vibration-mode optimization), as in the prior art, have been unsatisfactory for accurately estimating the overall transfer precision of a microlithography exposure apparatus from those simulation results.
According to prior-art desi
Garland Steven R.
Grant William
Klarquist & Sparkman, LLP
Nikon Corporation
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