Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-08-21
2003-07-15
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C359S727000
Reexamination Certificate
active
06593998
ABSTRACT:
The invention relates to a projection exposure system, in particular for microlithography, comprising a catadioptric projection objective and a light source.
In such projection exposure systems, illumination-induced imaging errors occur that are due to the thermal deformation of the optical components present-in the projection objective. The thermal deformation results in this case from the heating of the optical components occurring as a result of the residual absorption of the projection light.
A further illumination-induced effect that results in the imaging errors is the refractive index change due to the illumination light in the transilluminated material of the optical components. Such refractive index change effects may occur reversibly, with the result that the refractive index of the optical components is the same after the irradiation as before, or they may also be irreversible.
Said illumination-induced imaging errors impair the imaging quality of the projection exposure system and cannot be accepted, in particular, at those points where the resolution of very fine structures is desired.
The object of the present invention is therefore to develop a projection exposure system of the type mentioned at the outset in such a way that illumination-induced imaging errors are reduced.
According to the invention this object is achieved in that
a) at least one mirror and at least one lens assigned to it of the projection objective are composed of materials chosen in such a way as a function of a given light intensity distribution in the projection objective and
b) the position of the mirror and the position of the lens are similar inside the projection objective in such a way
c) that the imaging changes in the projection objective that are due to an illumination-induced imaging change in the reflection surface of the mirror counteract illumination-induced imaging changes in the lenses.
The invention is based on the observation that changes in the radius of curvature of optical components, that is to say, for example, a reduction in the radius of curvature of a concave optical surface, have a different effect in the case of a reflecting surface on the optical imaging properties of said surface than in the case of a refracting surface.
In particular, a lens (or a lens group) and a mirror that are disposed in a similar position inside a projection objective and either both have a collecting or both have a divergent effect counteract one another because of the imaging changes with regard to their imaging properties that occur in them as a result of the illumination-induced heating. This results in the possibility of bringing about compensation for the illumination-induced imaging changes. “Similar position” is understood as meaning such a positional assignment of the reflecting surface with respect to the refracting surface, that is to say of the mirror with respect to the at least one lens that the subaperture ratios in the reflecting and in the refracting surface do not differ considerably. In this connection, the subaperture ratio is the ratio between the distance between diametrically opposite impingement points of peripheral rays that proceed from a field point, that is to say from a point in the object to be projected, on the optical surface and the unobstructed aperture of said optical surface inside the projection objective.
For a given lens design of a projection objective, a lens or a lens group whose subaperture ratio does not differ considerably from that of the mirror can as a rule be specified for a mirror inside the projection objective. A selection of the materials that can be supported by a calculation and from which the mirror and the lens or the lens group are to be made results in total in imaging properties of the projection objective that depend only to a small extent or not at all on illumination-induced changes in the imaging properties of the individual optical components.
In this connection, the material is selected so as to take account, for example, of the thermal conductivity, of the coefficient of thermal expansion and of the refractive index behaviour during a temperature change in the respective material. In addition, the refractive index behaviour of the respective material can be taken into account as a function of the illumination intensity.
Such compensating projection objectives are to a large extent independent of illumination-induced drift of the imaging properties of the individual components. This increases the achievable throughput of the projection exposure system.
Preferably, subaperture ratios of the mirror and of the lens differ at their respective positions inside the projection objective by less than 25%. This ensures that the imaging beams that are assigned to the individual object points are influenced in the same way by the assigned optical surfaces, with the result that as great a compensation as possible can take place of imaging errors that are illumination-induced in the individual components.
More than one lens may be assigned to the mirror. Owing to the optical conditions during reflection, dimensional changes in a reflecting surface result in greater imaging changes than identical dimensional changes in a refracting surface. It is therefore advantageous to use a plurality of lenses to compensate for the illumination-induced imaging changes in a mirror. Since, as a rule, the number of lenses in known catadioptric projection objectives is substantially greater than the number of reflecting surfaces, such an assignment can be carried out, as a rule, without fairly large changes to the optical design.
Preferably, precisely two or precisely three lenses are assigned to the mirror. In known designs of a projection objective, a lens group that comprises two individual lenses in one design and three individual lenses in another design is disposed inside a projection objective in such a way that it differs in its subaperture ratio only very slightly from that of the mirror.
Said lens group is therefore particularly well suited for compensating for illumination-induced imaging changes that are due to the heating of the mirror surface. In addition, the lens group in these known projection objectives is disposed directly adjacently to the mirror so that they can form a unified assembly with the latter. This simplifies the retrofitting of the known projection objectives with an optical assembly that compensates in the above sense.
The lens may have a lens body composed of CaF
2
. Calculations have shown that the use of lenses composed of said lens material together with conventional materials that are used for mirrors in catadioptric projection objectives result in a good compensation for imaging errors.
The mirror may have a mirror base composed of one of the glass materials BK3, BK6 or BK7. These materials are robust, can readily be machined and have thermal properties that make them appear well suited for use as a compensating element.
Alternatively, the mirror base may be constructed of the glass material SK1. The coefficient of thermal expansion of SK1 is between BK3 and BK7, with the result that this material recommends itself for certain applications.
In a further alternative embodiment, the mirror may have a mirror base composed of one of the glass materials FK51 or FK54. These materials have relatively high coefficients of thermal expansion compared with the abovementioned glasses and therefore offer the potential of a large optical compensation effect for illumination-induced imaging errors at least of a mirror.
Alternatively, the mirror may have a glass support composed of the transparent glass ceramic material marketed under the trademark name Zerodur®. Zerodur® is a glass that has an extremely low coefficient of thermal expansion since it is composed of crystalline and amorphous constituents. Illumination-induced effects that result from the heating of Zerodur® are therefore very small. In the case of such a Zerodur® mirror, the illumination-induced imaging changes are therefore small and provide, for example, the possibi
Gaber Erwin
Gruner Toralf
Von Bünau Rudolf
Carl Zeiss SMT AG
Factor & Partners
Sever Andrew
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