Multiplex communications – Wide area network – Packet switching
Reexamination Certificate
2002-09-19
2004-01-13
Stafira, Michael P. (Department: 2877)
Multiplex communications
Wide area network
Packet switching
C356S239200
Reexamination Certificate
active
06678240
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for optimizing the image properties of at least two optical elements, in which at least one of the optical elements is moved relative to at least one stationary optical element.
The invention also relates to methods for optimizing the image properties of at least three optical elements, in which the relative position of the optical elements with respect to one another is adjusted.
Optical assemblies comprising at least one movable optical element are known on the market, and include projection objective lens systems for microlithography. With these, but also with other optical assemblies, a high image quality is required in order to produce a picture of a structure that is as free of defects as possible. The movability of at least one optical element within such a projection objective lens system serves to vary the image properties of the projection objective lens system with the aim of reducing the occurring image defects.
The choice of the position to which a moveable optical element should be adjusted so that the image defect of the optical assembly is thereby minimized, has hitherto often been made by individual measurement of the image properties of the optical elements before they are assembled. Since many image defects are produced only during assembly, for example as a result of pressure influences of the holders for the optical elements, such an approach to achieving a high image quality is too inaccurate.
Other approaches, in which the positioning of the optical elements is optimized on the basis of readily visualisable target quantities reproducing, though only incompletely, the image quality and that have been obtained from the interaction of the optical elements, rely on the experience of the technician entrusted with the adjustment of the assembly to find the most favorable rotational position. Such optimization methods are insufficiently deterministic.
A method that necessarily leads to the optimum relative position between the moveable and the stationary optical element takes measurements of the image defects of the optical assemblies, including both the moveable and the stationary optical elements, at all achievable positions of the movable optical element. This procedure is too tedious and complicated since as a rule a plurality of achievable positions exists for the accurate positioning of the movable optical element.
In the search for possible ways of improving projection objective lens systems in order to satisfy increasingly stringent requirements as regards image quality, projection objective lens systems have been proposed in which movable optical components can be arranged at various positions within the projection objective lens system. In this connection the number of movable optical components is not limited to one; instead there may often be several movable optical elements within the projection objective lens system.
With such projection objective lens systems the question arises, at which position should a movable optical element be provided within the projection objective lens system in order to be able to correct a specific image defect, and how many optical elements may optionally have to be moved for this purpose. In addition there is the question, what degree of freedom of movement can be employed in order to correct a specific image defect. Such degrees of freedom of movement include the rotation of optical elements within the projection objective lens system, the displacement of optical elements along the optical axis of the projection objective lens system (focusing) and vertical thereto (centering), and the tilting of optical elements within the projection objective lens system.
Overall there exists a plurality of degrees of freedom that are in principle available for correcting image defects within a projection objective lens system.
With the previously known optical assemblies a choice of the degrees of freedom that were employed for correcting image defects was made on the basis of trial-and-error methods. In the same way as when finding the most favorable rotational position, here too the experience of the respective technician was decisive in finding useful degrees of freedom, which however led to adjustment results that were not deterministically reproducible. Often the choice of the lenses to be moved as well as the choice of the degrees of freedom of movement were very time-consuming and also did not always achieve predefined specifications.
Also in those cases in which it is in principle known which lenses within a projection objective lens system have to be moved in order to correct specific image defects, as a rule a multidimensional problem still always exists with a mobility of several lenses within a projection objective lens system, with the result that an optimal position configuration of all movable lenses in which the overall image defect falls below predetermined specifications and/or reaches an absolute minimum often cannot be found with reasonable effort and expenditure.
A first object of the present invention is accordingly to provide a method for optimizing the image properties of at least two optical elements, in which at least one of the optical elements is moved relative to at least one stationary optical element, by means of which the overall image defect of the at least two optical elements can be specifically reduced with comparably little effort.
This object is achieved according to the invention by a method involving the following procedural steps:
a) measurement of the overall image defect of all optical elements, consisting of the image defect of the movable optical element and the image defect of the stationary optical element, in which all optical elements are traversed by measuring light;
b) representation of the measured overall image defect as a linear combination of the base functions of an orthogonal function set;
c) movement of the movable optical element relative to the stationary optical element to a new measurement position;
d) renewed measurement of the new overall image defect of all optical elements, consisting of the image defect of the movable element and the image defect of the stationary optical element, in which all optical elements are traversed by measuring light;
e) representation of the new overall image defect as a linear combination of the base functions of an orthogonal function set;
f) calculation of the image defect of the movable optical element and calculation of the image defect of the stationary optical element, using the representations obtained in steps b) and e);
g) calculation of a target position of the movable optical element from its image defect and from the image defect of the stationary optical element, in which the overall image defect is minimized;
h) movement of the movable optical element to the target position.
The method according to the invention first of all determines in situ the image defect contributions of the movable and of the stationary optical elements. This determination utilizes the fact that, in the representation of the overall image defect as a linear combination of the base functions of an orthogonal function set, the overall image defect of the movable and of the stationary optical elements both before as well as after the movement of the movable optical element consists, in a well defined manner, of the individual contributions of the image defects of the movable and of the stationary optical elements. Accordingly conclusions can be drawn as regards the separate image defects of the movable and of the stationary optical elements from two measurements of the overall image defect, namely before and after a movement of the movable optical element. The separate image defects determined in this way may then be used to calculate a target position, i.e. a position of the movable optical element relative to the stationary optical element in which the overall image defect of the optical elements is minimized.
By means of this method the position configuration with the least overa
Geh Bernd
Gräupner Paul
Stammler Thomas
Stenkamp Dirk
Stühler Jochen
Carl Zeiss SMT AG
Factor & Partners
Stafira Michael P.
Valentin II Juan D
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