Method for determining wavefront aberrations caused by an optical system

Abstract

The invention relates to a method and a device for determining wavefront aberrations caused by an optical system. In a method according to the invention, an object field (101) located in an object plane and illuminated by an illumination system is projected by a projection lens (100) onto an image field (105) located in an image plane, wherein for at least one field point in the image plane, a plurality of measurements of the wavefront aberration generated at that field point are carried out, these measurements differing from one another with respect to the respective effective exit pupil of the beam path.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The invention relates to a method for determining wavefront aberrations caused by an optical system with the possibility of calculating corrections. State of the art

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a projection exposure system, which includes an illumination unit and a projection lens. The mask (= reticule) illuminated by the illumination unit is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens. This transfers the mask structure onto the photosensitive coating of the substrate.

[0003] In the lithography process, there is a need to test the mask quickly and easily, ideally under conditions similar to those found in the projection exposure system. Mask inspection systems are known for this purpose; these systems include an illumination system and a projection lens, whereby the illuminated area of ​​the mask is projected onto a sensor array, such as a CCD camera, by means of the projection lens.

[0004] As lithography systems with ever-increasing resolution and the associated rise in accuracy requirements develop, the execution of the respective adjustment procedure—during which the optical system is brought "into specification" by utilizing available degrees of freedom (such as adjusting manipulators, mirror shape, mechanical modifications, etc.)—presents an increasingly demanding challenge. This adjustment allows for the reduction or minimization of wavefront aberrations, in particular, through positional manipulation of one or more optical elements of the respective optical system (especially the projection exposure unit or the mask inspection system).

[0005] It is generally desirable to perform the respective (positional) manipulation on the component(s) responsible for the occurring wavefront aberration. This necessitates not only the quantitative determination of the wavefront aberrations generated during imaging in the respective optical system, but also their targeted assignment to the specific optical elements responsible for them within the optical system. A well-known approach is to utilize the field dependence of these wavefront aberrations by assigning aberrations with a comparatively low field dependence (where virtually the same aberration is generated at all field points) to optical elements within the system that are relatively close to the pupil, whereas aberrations with a comparatively high field dependence are more likely to be assigned to optical elements located closer to the field.

[0006] However, a problem that arises in practice is that optical systems with comparatively small object field sizes (especially mask inspection systems and microscopes) exhibit correspondingly small field profiles of the respective aberration. Consequently, assigning a measured wavefront aberration to the specific optical element responsible for it (especially the one closest to the pupil or the field) becomes difficult or even impossible. This ultimately leads to inadequate alignment and thus reduced optical performance of the optical system in question.

[0007] For examples of the state of the art, reference is made only to WO 2010 / 034 674 A1, DE 10 2012 205 096 B3, DE 10 2015 206 448 B4, DE 10 2018 219 127 A1, DE 602 23 102 T2, DE 10 2011 080 437 A1 and DE 691 32 120 T2. SUMMARY OF THE INVENTION

[0008] Against the above background, it is an object of the present invention to provide a method for determining wavefront aberrations caused by an optical system, which enables a targeted assignment of occurring wavefront aberrations to the respective causative optical elements and thus also a correspondingly targeted adjustment even with a comparatively small object field size, while at least partially avoiding the problems described above.

[0009] This problem is solved by the method according to the features of independent claim 1.

[0010] The invention relates to a method for determining wavefront aberrations caused by an optical system, - wherein in the optical system an object field located in an object plane and illuminated by a lighting system is imaged by a projection lens onto an image field located in an image plane; and - wherein for at least one field point in the image plane a plurality of measurements of the wavefront aberration generated at that field point are carried out, wherein these measurements differ from each other with respect to the respective effective exit pupil of the beam path.

[0011] The invention is based in particular on the concept of separating the respective aberration contributions of different optical elements when determining wavefront aberrations caused by an optical system such as a projection lens of a mask inspection system by determining the wavefront aberration generated in the optical system several times for one or more field points - by varying the respective effective exit pupil.

[0012] In this and the following, the term "effective exit pupil" is understood, in accordance with the usual terminology, to be the respective angular distribution with which the respective light strikes the image plane or a field point located therein during imaging.

[0013] Depending on the specific application scenario, the optical elements mentioned may be, for example, lenses or mirrors.

[0014] The variation of the effective exit pupil according to the invention can be achieved, as described in more detail below, by means of a cropping of the optical beam path via one or more aperture diaphragms, in particular also by using a manipulable aperture diaphragm or by changing the shape and / or size (relative to the angular space used) of the effective exit pupil.

[0015] This clipping of the optical beam path can occur, in particular, with respect to the optical beam path before a last optical element of the optical system on the image plane side. In particular, the clipping of the optical beam path can also occur with respect to the optical beam path before a first optical element of the optical system on the object plane side.

[0016] Furthermore, the clipping of the optical beam path can alternatively take place directly in the exit pupil (relative to the optical beam path after a pupil plane of the optical system) or on the side of the entrance pupil (corresponding to the angular distribution of the light emanating from a field point of the object plane and relative to the optical beam path in front of a pupil plane of the optical system).

[0017] Based on the aforementioned principle of varying the effective exit pupil, the invention now utilizes the fact that the extent to which a change in the measured wavefront aberration is observed during said variation allows a conclusion to be drawn as to whether the optical element responsible for the wavefront aberration is located near the pupil or near the field. Wavefront aberrations that remain nearly constant when the exit pupil is varied can be attributed to optical elements located near the field, whereas comparatively strongly varying aberrations (which change relatively significantly when the exit pupil is changed) can be attributed to optical elements located near the pupil.

[0018] As a result, even in optical systems with comparatively small object field sizes, a targeted assignment of occurring wavefront aberrations to the respective causative optical elements and thus a correspondingly targeted adjustment is made possible.

[0019] The invention thus incorporates the principle of deliberately introducing an asymmetry into the optical system by clipping the optical beam path during imaging, in order to enable the desired correlation of an existing wavefront aberration to the optical element responsible for it – and thus also targeted adjustment. In particular, the maximum optically achievable exit pupil is limited to only a portion (i.e., a "partial pupil") in each measurement, and this partial pupil can be modified, in particular shifted, for different measurements.In other words, in each of the aforementioned measurements, only a reduced area of ​​the maximum available exit pupil is used for imaging, which in turn can be placed at different positions in the optical system in order to illuminate the optical system with separate portions of the aforementioned maximum possible exit pupil.

[0020] According to one embodiment, the measurements differ from each other with regard to the shape and / or size of the exit pupil.

[0021] According to one embodiment, the optical beam path is clipped for different measurements using at least one aperture diaphragm.

[0022] According to one embodiment, the optical beam path is clipped in front of the last optical element of the optical system on the image plane side. This design has the advantage of reducing the thermal load on said last optical element on the image plane side.

[0023] According to one embodiment, the optical beam path is clipped in front of the first optical element of the optical system on the object plane side. This design has the advantage of achieving a maximum reduction of the thermal load on the optical elements present in the optical system.

[0024] According to one embodiment, the optical beam path is clipped relative to the optical beam path in front of a pupil plane of the optical system.

[0025] According to another embodiment, the optical beam path is clipped with respect to the optical beam path according to a pupil plane of the optical system.

[0026] According to one embodiment, for different measurements, a change in the effective exit pupil is carried out using at least one manipulable aperture diaphragm.

[0027] According to one embodiment, depending on a variation of the wavefront aberration generated in the different measurements, the wavefront aberration is assigned to a causative optical element of the projection lens.

[0028] According to one embodiment, based on this assignment and the measurements of the wavefront aberration, a manipulation of the causative optical element, in particular a position manipulation, is carried out.

[0029] According to one embodiment, this manipulation is carried out in such a way that a wavefront aberration generated in the optical system in the image plane is reduced compared to a configuration without the manipulation.

[0030] According to one embodiment, the method according to the invention is carried out on a projection lens of a mask inspection system.

[0031] The disclosure further relates to a device designed to carry out a method with the features described above. For advantages and advantageous embodiments of the sensor arrangement or the device, reference is made to the above embodiments in connection with the method according to the invention.

[0032] Further embodiments of the invention can be found in the description and the dependent claims.

[0033] The invention is explained in more detail below using an exemplary embodiment with reference to the accompanying illustrations. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] They show: Fig. 1 a schematic representation to explain the principle underlying the method according to the invention using an exemplary embodiment; Fig. 2a-2b schematic representations to illustrate one of the problems underlying the present invention by comparing possible scenarios in a conventional determination of wavefront aberrations in a projection lens with a comparatively large object field ( Fig. 2a) and a mask inspection system with a comparatively small object field ( Fig. 2b); and Fig. 3 a schematic representation of the basic possible structure of a mask inspection system as an exemplary application of the present invention. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0035] The following discussion will refer to the schematic representation of Fig. 1. The principle underlying the present procedure is explained using an exemplary embodiment. However, it is first made with reference to… Fig. 2a-2b a brief explanation of the problem underlying the invention, which has already been mentioned in the introduction.

[0036] In Fig. 2a and Fig. Figure 2b shows a conventional scenario for determining wavefront aberrations or assigning them to the optical elements responsible for them in the respective system, wherein Fig. 2a in a highly simplified representation a projection lens 200 with a comparatively large object field 201 (e.g. a projection lens of a microlithographic projection exposure system) and Fig. 2b represents a projection lens 210 of a mask inspection system with a comparatively small object field 211. The respective optical elements of the optical system that generates the image from the object field 201 or 211 into the image field 205 or 215 are shown in Fig. 2a with “202, 203 or 204” and in Fig. 2b is designated as "212, 213 and 214 respectively" and is symbolized as lenses for simplicity. Optical elements 202 and 212 are located near the field, while optical elements 203 and 213 are located near the pupil.

[0037] While in the scenario of Fig. 2a. While a conventional aberration measurement procedure for different field points achieves a comparatively good separation between optical elements near the field and near the pupil with regard to their aberration contribution, i.e., a relatively good assignment of the aberration to the respective causative optical element, this approach fails in the scenario of Fig. 2b. Because according to Fig. 2b. Due to the relatively small object field, largely identical aberrations are measured for different field points in the image field, so that a targeted assignment of a specifically measured wavefront aberration to the causative optical element (in the sense of deciding whether it is a field-near or pupil-near element) is not possible.

[0038] To overcome this problem, the following measures are now being taken according to Fig. 1 for one or more field points in the image field designated “105”, a multiple measurement of the wavefront aberration occurring in each case, whereby these measurements differ from each other with respect to the respective exit pupil.

[0039] Specifically, according to Fig. 1 For this purpose, in a projection lens 100, the optical beam path is clipped on the entrance pupil side in different ways for the aforementioned measurements, which in turn is achieved by using aperture diaphragms 106 and 107. The optical beam path produced by using aperture diaphragm 106 (but without aperture diaphragm 107) is shown with a long dashed line, and the optical beam path produced by using aperture diaphragm 107 (but without aperture diaphragm 106) is shown with a short dashed line. As can be seen from Fig. 1. As can be seen, the two beam paths differ significantly with regard to the respective exit pupil (i.e., the angular distribution of the light striking the relevant field point in the image field 105) as well as with regard to the subaperture generated in the pupil plane or at an optical element 103 near the pupil.

[0040] If a large variation in wavefront aberration measured at a specific point in the image field 105 is observed across different exit pupils set according to the invention, this suggests that the optical element responsible for this aberration is located near the pupil. Conversely, a comparatively small change in the aberration measured at a point in the image field 105 for different exit pupils suggests that the optical element responsible for this aberration is located near the field.

[0041] Based on the measurement data obtained for different exit pupils, the influences of various optical elements can be separated, provided the corresponding signatures are available from design simulations. Optical elements located near the pupil exhibit a more pronounced aberration profile with varying exit pupils, whereas optical elements located near the field show a less pronounced profile. Adjustment is then performed simultaneously, taking into account both the measured and design aberrations for all exit pupil variations.

[0042] The variation of the exit pupil according to the invention can be achieved using a manipulable aperture diaphragm or by changing the shape or size of the exit pupil. If the shape or size of the numerical aperture (NA) is changed, a corresponding change in a (Zernike) development system of the measured wavefront may need to be taken into account during the adjustment described below.

[0043] For the adjustment calculation itself, both an optimization of all pupil positions for individual field points (as described in the following example) and a simultaneous optimization of several field points and pupil positions can be performed.

[0044] Mathematically, the following problem is solved, or at least approximated, when minimizing aberrations. minx∈X(b−Mx)

[0045] Here, b is the vector of all measured aberrations, x is the vector of all degrees of freedom to be adjusted, X is the set of all permissible travel paths (e.g., limited by the maximum travel ranges of manipulators), and M is a matrix that describes how the individual degrees of freedom affect the aberrations (where the corresponding entries of the matrix M are also referred to as "sensitivities"). In the conventional adjustment concept, the vector b has the following entries:

[0046] For each field point (indexed as 1 to l), the aberrations are represented in a basis system with n coefficients (e.g., Zernike coefficients). The vector b thus has length l·n. The vector x has the form (x1x2xk), where the lens has k degrees of freedom for adjustment.

[0047] Accordingly, the matrix M ∈ ℝ l·n×k the form (m1,1,1⋯m1,1,k⋮⋱⋮ml,n,1⋯ml,n,k) where m i1,i2,i3This describes the change in the aberration coefficient i2 at field point i1 when the degree of freedom i3 is changed by one unit. To solve this problem, algorithms known in themselves are used, for example, whereby reference is made to DE 10 2012 205 096 B3 and the further references cited therein as examples of the prior art.

[0048] To calculate an adjustment recipe in the inventive method, the same mathematical problem can be used. minx∈X(b−Mx) This can be solved, whereby the variables must be adjusted accordingly. The vector b is now represented with the following entries:

[0049] The vector b now has the length l·n·p. The vector x remains identical as in the conventional adjustment concept, but the matrix M must also be extended to include the effects for the different pupil variations 1 to p. M=(m1,1,1,1⋯m1,1,1,k⋮⋱⋮mp,l,n,1⋯mp,l,n,k) where now m i1,i2,i3,i4 describes the change in the aberration coefficient i3 at field point i2 in pupil setting i1 when the degree of freedom i4 is adjusted by one unit.

[0050] To solve the minimization problem, optimization algorithms known per se can be used, with reference again only being made to DE 10 2012 205 096 B3 and the further references cited therein, by way of example. In the method according to the invention, the size of the vector b and the matrix M is multiplied by the number of introduced pupil variations. Since, in the application scenario of a mask inspection system, the number l is very small due to the comparatively small number of measurable field points, the increase in the size of the matrix M iA does not lead to a significant increase in the computation time or the scope of the algorithm to be used.

[0051] As in Fig. Figure 3 is shown only schematically. A mask inspection system 300 comprises a lighting system 310 and a projection lens 320, wherein light from a (in Fig. 3 light source (not shown) enters the illumination system 310 and strikes a mask 330 arranged in the object plane of the projection lens 320, and the illuminated area of ​​the mask 330 is imaged onto a sensor arrangement 340 via the projection lens. In order to predict the imaging result obtained with a mask when carrying out the lithography process in a projection exposure system, a measurement of the mask is first performed in the mask inspection system. Fig. 3 or the intensity distribution obtained with the sensor arrangement. Preferably, the same wavelength is used in the mask inspection system as is used in the projection exposure system during the lithography process.

[0052] Even though the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments are apparent to the person skilled in the art, for example, through the combination and / or exchange of features of individual embodiments. Accordingly, it is understood to the person skilled in the art that such variations and alternative embodiments are included in the present invention, and that the scope of the invention is limited only to the extent of the appended claims and their equivalents.

Claims

[1] Method for determining wavefront aberrations caused by an optical system, - wherein in the optical system an object field (101) located in an object plane and illuminated by a lighting system is imaged by a projection lens (100) onto an image field (105) located in an image plane; - wherein for at least one field point in the image plane a plurality of measurements of the wavefront aberration generated at that field point are carried out, wherein these measurements differ from each other with respect to the respective effective exit pupil of the beam path. [2] Method according to claim 1, characterized by that the measurements differ from each other with regard to the shape and / or size of the effective exit pupil. [3] Method according to claim 1 or 2, characterized by, that for different measurements the optical beam path is clipped using at least one aperture stop (106, 107). [4] Method according to claim 3, characterized by , that the clipping of the optical beam path occurs in relation to the optical beam path before a last optical element of the optical system on the image plane side. [5] Method according to claim 3 or 4, characterized by , that the clipping of the optical beam path occurs in relation to the optical beam path in front of a first optical element of the optical system on the object plane side. [6] Method according to any one of claims 3 to 5, characterized by , that the clipping of the optical beam path occurs in relation to the optical beam path in front of a pupil plane of the optical system. [7] Method according to any one of claims 3 to 5, characterized by, that the clipping of the optical beam path occurs with respect to the optical beam path according to a pupil plane of the optical system. [8] Method according to any one of the preceding claims, characterized by , that for different measurements a change of the effective exit pupil is carried out using at least one manipulable aperture diaphragm. [9] Method according to any one of the preceding claims, characterized by , that depending on a variation of the wavefront aberration generated in the different measurements, an assignment of the wavefront aberration to a causal optical element (102, 103, 104) of the projection lens (100) is made. [10] Method according to claim 9, characterized by, that based on this assignment and the measurements of the wavefront aberration, a manipulation of the causative optical element (102, 103, 104), in particular a position manipulation, is carried out. [11] Method according to claim 10, characterized by that this manipulation is carried out in such a way that a wavefront aberration generated in the optical system in the image plane is reduced compared to a configuration without the manipulation. [12] Method according to any one of the preceding claims, characterized by , that this is carried out on a projection lens (100, 320) of a mask inspection system (300).

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