Optics: measuring and testing – Lens or reflective image former testing
Reexamination Certificate
2002-08-14
2004-11-09
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Lens or reflective image former testing
Reexamination Certificate
active
06816247
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a moiré method for measuring the distortion of an optical imaging system and a system suitable for carrying out the method.
2. Description of the Related Art
Optical imaging systems are employed in numerous fields of engineering and research that impose increasingly stringent demands on their imaging quality. An example is photolithographic fabrication of semiconductor devices and other types of microdevices, where submicrometer-range structures are created using high-performance projection lenses. In semiconductor-device fabrication, imaging optics that have intolerably high distortion may, for example, cause defects in integrated circuits that will reduce yields of good integrated circuits and thereby increase fabrication costs. Such projection lenses have elaborate optical trains having large numbers of lenses that usually make it impossible to derive their optical characteristics from theoretical computations. The optical characteristics of their imaging systems must thus be reliably measured. The stringent demands on the imaging accuracies of their optical imaging systems impose even more stringent demands on the accuracies of the testing methods employed for checking their imaging systems. In particular, high-precision measurements of their distortion are absolutely essential.
Numerous testing methods for measuring the distortion of optical systems are based on exploitation of the moiré effect. For example, analyzing moiré fringe patterns created by linear grids in order to determine lens distortion along an image direction of a two-dimensional image field, in which a so-called “object grid” that has a transparent substrate bearing a large number of parallel, opaque lines that form an object pattern is arranged in the object plane of the imaging system to be tested, is known. An image grid having an image pattern similar to the object pattern is arranged in the lens' image plane. Both grids are arranged such that they are orthogonal to the optical axis of the imaging system. The object pattern and the image pattern are adapted to suit one another such that a moiré pattern exhibiting moiré fringes is created when the object pattern is imaged onto the image pattern using the imaging system.
Creation of a moiré fringe pattern may be achieved by accurately matching the ratio of the grid constants of the image grid and object grid to the lens' prescribed magnification. Rotating one of these grids with respect to the other grid about the optical axis will then create a moiré pattern consisting of bright and dark fringes running nearly orthogonal to the grid lines on the object and image grids. The number of pairs of fringes created is equal to the number of grid lines of one grid that intersect a grid line of the other grid. A moiré fringe pattern may also be created by slightly altering the grid constants of the object grid and image grid, for example, altering them by a few percent, duly allowing for the image magnification involved. Moire methods of this type are described in, for example, U.S. Pat. No. 5,767,959, or U.S. Pat. No. 5,973,773, which has a substantially identical content.
Lens distortion, which is usually small compared to the grid constant of the image grid, may, for example, be determined by phase shifting, in which various moiré fringe patterns, each of which is created by progressively shifting grids with respect to one another parallel to their grid lines by a fraction of their grid period, are recorded by a camera. A periodic change in intensity is observed at each image location during this phase shifting. The relative phase of these signals at various measurement locations is a measure of lens distortion orthogonal to the grid lines.
The aforementioned method allows determining distortion along a single image direction, namely, that orthogonal to the grid lines, only. If distortion components along other image directions are to be determined, the lens is normally rotated about its axis through, for example, 90°, before any further measurements are performed, which requires employment of a mechanically complex measurement setup.
European Patent No. EP 0 418 054 describes another moiré method that allows initially determining the distortion components along an image direction and then determining the distortion components along an image direction orthogonal thereto in two consecutive steps, where, in the case of one embodiment, the object grid bears a two-dimensional object pattern in the form of a cross-hatched pattern. The image grid, on the other hand, is one-dimensional and has a set of parallel lines. Moreover, the projection lens whose distortion is to be measured is equipped with a translatable pupil filter that may be used to transmit either the first-order diffracted reflections in the x-direction or those in the y-direction and block all other reflections. Switching between the measurement directions involves both translating the pupil filter and rotating the image grid through 90°. This system thus allows dispensing with rotating the lens. However, it necessitates an intervention into the projection lens in order to incorporate a suitable pupil filter and construction of an elaborate mount for the image grid that will allow rotating the image grid through 90°.
SUMMARY OF THE INVENTION
It is one object of the invention to provide a moiré method and an associated measuring system that will allow rapid, reliable, two-dimensional distortion measurements, i.e., distortion measurements along several, differing, image directions, while maintaining the mechanical requirements imposed on the measurement system low. It is another object to provide a moiré method and measuring system that allow simultaneously measuring the distortion components of an imaging system along two mutually orthogonal image directions.
As a solution to these and other objects the invention provides a moiré method for measuring the distortion of an optical imaging system comprising:
arranging an object grid having a two-dimensional object pattern in an object plane of the imaging system; arranging an image grid having a two-dimensional image pattern in an image plane of the imaging system;
wherein the object pattern and image pattern are adapted to suit one another such that a two-dimensional moiré fringe pattern may be created when the object pattern is imaged onto the image pattern using the imaging system;
imaging the object pattern onto the image pattern in order to create a two-dimensional moiré fringe pattern;
detecting the two-dimensional moiré fringe pattern;
determining at least one first distortion component and at least one second distortion component from the two-dimensional moiré fringe pattern, where the first distortion component is correlated to a first image direction of an image plane and the section distortion component is correlated to a second image direction transverse to the first image direction.
An associated system for making two-dimensional measurements of both distortion components comprises an object grid having a two-dimensional object pattern arranged in an object plane, an image grid having a correspondingly adapted two-dimensional image pattern arranged in an image plane, devices for imaging the object pattern onto the image pattern and for detection the two-dimensional moiré fringe pattern created, and a device for determining the first and second distortion components from the moiré fringe pattern.
To be interpreted as a “two-dimensional pattern” in the sense of this application are both the pattern itself, which is extended in two dimensions, and the pattern's spectrum, which is also extended in two dimensions, i.e., is modulated in two dimensions. In other words, a “two-dimensional pattern” refers to both the field and the pupils.
A preferred setup provides that the object grid will be irradiated using an illumination device and that radiation modified by the object pattern will be imaged onto the image grid by the imaging system. Suitable grids may
Arnz Michael
Heppner Joachim
Kuechel Michael
Massig Juergen
Penzing Juergen
Carl Zeiss SMT AG
Font Frank G.
Merlino Amanda
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