CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 from German Application No. 10 2021 203 961.8, filed, on Apr. 21, 2021, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a pupil stop for an illumination optical unit of a metrology system for determining, as a result of illumination and imaging under illumination and imaging conditions corresponding to those of an optical production system, an aerial image of an object to be measured. Further, the invention relates to an illumination optical unit for such a metrology system comprising such a pupil stop, and to such a metrology system comprising such an illumination optical unit.

BACKGROUND

A metrology system is known from US 2017/0131528 A1 (parallel document WO 2016/012425 A2), from WO 2016/012426 A1 and from US 2017/0132782 A1.

SUMMARY

It is an aspect of the present invention to improve an accuracy of a convergence of illumination and imaging conditions of an optical production system to illumination and imaging conditions of an optical measurement system, which can be part of a metrology system, in particular, particularly when using a multipole illumination setting.

According to the invention, this aspect is achieved by a convergence method having the features specified in Claim1.

A multipole illumination setting is frequently used in the context of mask exposure in projection lithography. Corresponding multipole illumination situations therefore require implementation of a metrology of correspondingly exposed objects with a high accuracy, that is to say, in particular, with a good convergence of the metrology imaging and illumination conditions to the production imaging and illumination conditions.

Surprisingly, a pupil stop with in each case at least one stop web in the respective pole passage opening was identified as improving a convergence of a metrology illumination setting to a correspondingly multi-pole production illumination setting, even though pole shadowing corresponding to such webs need not be present in the production illumination setting. A transverse dimension of the stop web across its web extension can be small as compared to a typical transverse dimension of a stop area between adjacent pole passage openings of the pupil stop. For example, such stop web transverse dimension can be no more than 50% of such typical stop area transverse dimension.

In some implementations, the stop web can have a constant transverse dimension across its web extension.

Parameters decisive for metrology, for instance especially the parameter values of NILS (normalized image log slope, derivative of an aerial image intensity curve at an edge position of an imaged structure) and CD (critical dimension), can be adapted in metrology to match the corresponding parameters of the production system well.

The two pole passage openings of the pupil stop may be spaced apart from one another along a pole direction. This pole direction may extend parallel to a direction perpendicular to an object displacement direction of a production projection exposure apparatus, to the illumination setting of which the metrology illumination setting is intended to converge. If an object plane of the production projection exposure apparatus is spanned by coordinates x and y and an object displacement direction extends along the y-coordinate, the pupil stop according to the invention may thus be realized for example as an x-dipole pupil stop or else as a C-quad pupil stop (superposition of an x-dipole and a y-dipole). Such a quadrupole superposition of two dipoles in the x- and y-directions is also known as a C-quad (cf. WO 2012/028303 A1; in particular FIG. 3a).

Pupil stops specifying a different number of poles to two (e.g., x-dipole) or four (e.g., C-quad) are also possible, for example three poles, five poles or more than five poles, for example six poles or eight poles. At least two of these poles, which in turn are each subdivided into a plurality of partial pole openings, can be spaced apart from one another along the x-coordinate.

Alternatively or in addition, the pupil stop can be arranged such that two poles of the illumination setting, as specified by the pupil stop, specify a dipole with a dipole direction, that is to say a direction along a spacing between the two poles of the dipole, which extends perpendicular to the object structures of the object to be measured.

A symmetric stop web arrangement according to Claim2was found to be particularly advantageous for a good convergence to the illumination and imaging conditions. The center of the pupil stop can correspond to a pupil center of an illumination optical unit when the pupil stop is arranged in said illumination optical unit.

Exactly one stop web per pole passage opening according to Claim3has proven its worth in practice.

Directions of extent of the stop webs according to Claims4and5may ensure, in particular, a good convergence to measurement parameters relating to the imaging of vertical object structures, that is to say structures with a structure alignment perpendicular to a dipole direction of a dipole, which is specified by two poles of the multi-pole illumination specified by way of the pupil stop. In this case, the dipole direction is the direction along a distance between the two poles of the dipole.

The advantages of an illumination optical unit according to Claim6and a metrology system according to Claim7correspond to those which were already explained above with reference to the pupil stop.

These advantages take effect, in particular, in the case of a metrology system comprising an imaging optical unit with a large image-side numerical aperture according to Claim8.

The metrology system can be used to measure a lithography mask provided for projection exposure for producing semiconductor components with very high structure resolution, which is better than 30 nm, for example, and which can be better than 10 nm, in particular.

The metrology system can comprise a quick-change tool holder for replacing the respective dipole pupil stop with a change stop for specifying a selectable measurement illumination setting. This increases the flexibility of the metrology system.

