Method for operating a projection exposure system

Abstract

In a method for operating a microlithography projection exposure apparatus (10), a mask (40) is repeatedly exposed to exposure radiation (14) provided by an illumination system (20), and a mask structure (42) is imaged onto one of a number of fields (64) of a plurality of semiconductor substrates (52) in the process. During the period of repeated exposure of the mask, the illumination system is sequentially used with at least two different illumination settings (62a, 62b) of the illumination system, in which different illumination distributions (60a, 60b) of the exposure radiation are present in a pupil plane (31) of the illumination system, whereby the pupil surface (32-15, 32-30, 32-28, 32-22, 32-24, 32-5; 32-17, 32-30, 32-15, 32-28, 32-13, 32-3; 32-3; 32-3; 32-4; ​​32-5; 32-6; 32-7; 32-8; 32-9; 32-10; 32-11; 32-12; 32-13; 32-14; 32-15; 32-16; 32-17; 32-18; 32-19; ​​32-20; 32-21; 32-22; 32-23; 32-24; 32-25; 32-26; 32-27; 32-28; 32-29; 32-30; 32-15; 32-28; 32-13; 32-31; 32-32; 32-33; 32-34; 32-35; 32-36; 32-37; 32-38; , 32-22, 32-5, 32-24, 32-7) have no or up to 90% overlap of their respective illuminated pupil surfaces with the pupil surfaces illuminated with the second illumination setting (62b) (35-16, 35-29, 35-14, 35-23, 35-4, 35-6; 32-17, 32-16, 32-29, 32-14, 32-13, 32-3, 32-4, 32-23, 32-6, 32-7), and the mask is fully exposed at least once with each of the two different illumination settings.

Description

[Technical Field]

[0001] This application claims priority from German Patent Application No. 102022210088.3, ​​filed September 23, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method for operating a microlithographic projection exposure apparatus. [Background technology]

[0003] In recent years, lithographic projection exposure methods have been mainly used to produce semiconductor devices and other microstructured components. In the process, a pattern arranged on a mask or reticle is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the mask plane or object plane of the projection lens and illuminated with illumination radiation shaped by the illumination system. The radiation modified by the pattern passes through the projection lens, whereby the pattern is imaged onto a radiation-sensitive layer of a semiconductor substrate.

[0004] Exposure is increasingly optimized for specific chip layer structures, resulting in a fragmented and sometimes highly localized illumination distribution in the pupil plane of the illumination system compared to traditional shapes such as annulus, dipole, or quasar. The goal here is to obtain as high a contrast as possible of imaged structures, especially of structures essential to the function of semiconductor components. For example, these may have significantly smaller dimensions or significantly smaller distances from adjacent structures. The goal is to achieve high yields regardless of inaccuracies in exposure dose and focus position, as may be observed in real manufacturing processes.

[0005] To quantify this relationship, what is known as the lithography process window is often used. Given a lithography parameter inaccuracy, e.g., a given feature size inaccuracy, e.g., 10% line width, the dose error that precisely results in this deviation of the lithography parameter is determined for a specific focus error within the lithography process window. In principle, different illumination distributions result in different process windows. Furthermore, high insensitivity to focus errors often results in higher sensitivity to dose errors, and vice versa. Depending on the illumination choice, different weightings may be placed on one of the two influencing factors.

[0006] Due to local illumination peaks, in turn, high stress areas, especially with accompanying temperature peaks, are created on the optical surfaces, thus creating a conflict between illumination optimized for high contrast, on the one hand, and high thermally induced aberrations, on the other hand, which in turn can reduce yield, whether through positioning errors ("superposition") or through focus effects that result in reduced contrast. Summary of the Invention [Problem to be solved by the invention]

[0007] The problem addressed by the present invention is that of providing a method of the initially described kind, in which the above-mentioned problems are solved, and in particular in which a projection exposure apparatus can be operated for relatively long periods of time without significant loss of the lithographic process window. [Means for solving the problem]

[0008] According to the present invention, the above-mentioned problems can be solved, for example, through a method for operating a microlithography projection exposure apparatus, in which a mask is repeatedly exposed to exposure radiation provided by an illumination system, and the mask structure is each time imaged onto one of multiple fields of a plurality of semiconductor substrates in the process. During the period in which the mask is repeatedly exposed, the illumination system is sequentially used at least two different illumination settings of the illumination system, in which different illumination distributions of the exposure radiation are present in the pupil plane of the illumination system, so that the pupil surface illuminated at the first illumination setting has no overlap or a maximum of 90% overlap with the pupil surface illuminated at the second illumination setting. Furthermore, the mask is fully exposed at least once at each of the two different illumination settings. In other words, if the projection exposure apparatus is embodied as a scanner, at least one complete mask scan is performed at each illumination setting. According to one embodiment, the mask is fully exposed multiple times at each of two different illumination settings, for example, at least 10 times, at least 100 times, or at least 1000 times, without any intervening changes in illumination settings.

[0009] According to further embodiments, each different illumination setting is maintained for at least the time period required for two exposures of the mask. According to further embodiments, each different illumination setting is maintained for at least the time period required for at least ten exposures of the mask. According to further embodiments, each different illumination setting is maintained for at least the time period required for a complete exposure of a semiconductor substrate, particularly for the exposure of multiple semiconductor substrates, for example, for the exposure of at least ten semiconductor substrates. According to further embodiments, each different illumination setting is maintained for at least the time period required for the exposure of at least one batch of semiconductor substrates, particularly for the exposure of multiple batches of semiconductor substrates, for example, for the exposure of at least ten batches of semiconductor substrates. For example, a batch may include at least 20 semiconductor wafers, particularly at least 25 semiconductor wafers.

[0010] According to different embodiments, the overlap can be at most 80%, at most 50%, at most 20%, or at most 10%, which means that the pupil sections illuminated in the pupil plane, or the ensemble of multiple pupil sections illuminated in the pupil plane, i.e., the surfaces illuminated in the pupil plane, may not overlap at all, or may have an overlap of at most 90%, or one of the other aforementioned maximum values.

[0011] In other words, the pupil surface illuminated with the first illumination setting has an overlap with the pupil surface illuminated with the second illumination setting of up to 90%, or any other of the above values ​​for the respective illuminated pupil surfaces, and the overlap can also be 0%.

[0012] Since the mask at a particular illumination setting is each time exposed by the corresponding illumination radiation, overlap should not be understood to mean that the associated surface portion or portions are illuminated simultaneously, but instead, overlap relates to one or more surface sections illuminated by the associated illumination settings at different times.

[0013] That is, the pupil surface illuminated with a first illumination setting, also referred to as the first surface, differs from the pupil surface illuminated with a second illumination setting, also referred to as the second surface, to the extent that the two surfaces correspond to each other in each case over a portion of at most 90% of their respective total areas, or do not correspond at all. That is, the illuminated surfaces differ from each other by more than 10%. This should be understood to mean that the portion of the first surface that has a correspondence in the second surface, i.e., that corresponds to the respective portion of the second surface, occupies 90% or less of the total area of ​​the first surface. The same applies conversely to the portion of the second surface, which occupies 90% or less of the total area of ​​the second surface. Similarly, when more than two different illumination settings are used, the portions of the surfaces illuminated with the various illumination settings that correspond to each other in all illumination settings occupy at most 90% of the overall area of ​​the respective surfaces.

[0014] The pupil plane of the illumination system is characterized in that the local intensity distribution of the illumination radiation that converges to a particular field point on the mask corresponds in the pupil plane to an angularly resolved intensity distribution at this field point.

[0015] The inventive sequential use of an illumination system with the above-mentioned at least two different illumination settings of the illumination system makes it possible to prevent excessive formation of wavefront aberrations due to local heating in the optical elements of the projection exposure apparatus, in particular in the optical elements of the projection lens. As a result of using different illumination settings, it is possible to change the radiation distribution on the optical elements before the occurrence of local heating associated with possible wavefront errors.

[0016] During repeated exposures of the mask, according to one embodiment there is a time interval between switching between two different illumination settings of less than 200 minutes, in particular less than 60 minutes or less than 20 minutes.

[0017] According to a further embodiment, the pupil surfaces illuminated with the different illumination settings each have a plurality of surface portions separated from one another.

[0018] According to a further embodiment, switching from a first of two different illumination settings with a first configuration of surface portions in the pupil plane to a second illumination setting with a second configuration of surface portions in the pupil plane is performed by gradually switching each time from illumination of a surface portion or a subgroup of surface portions in a first configuration to illumination of another surface portion or another subgroup of surface portions in a second configuration.

[0019] According to a further embodiment, switching from a first of two different lighting settings to a second lighting setting is performed by gradually adapting a first lighting distribution applied to the first lighting setting to a second lighting distribution applied to the second lighting setting, a procedure that may also be referred to as morphing.

[0020] According to a further embodiment, the illumination system comprises a pupil facet optical unit having a plurality of individual optical units arranged in a pupil plane of the illumination system, and a field facet optical unit arranged in a plane conjugate to the mask plane, wherein the field facet optical unit comprises a plurality of further individual optical units configured to illuminate the individual optical units of the pupil facet optical unit for forming respective radiation channels of the beam path of the illumination radiation, wherein switching between the different radiation channels is performed by sequentially moving one or more of the individual optical units of the field facet optical unit during the gradual adaptation from the first illumination distribution to the second illumination distribution, i.e. exposure radiation previously guided in one radiation channel is subsequently guided in another radiation channel.

[0021] According to a further embodiment, the illumination distributions in the pupil plane that exist for the different illumination settings are each applied to at least one constant field point in the mask plane of the projection exposure apparatus, in other words, a first illumination distribution applied to a first illumination setting is applied to the same field point or to the same field points as a second illumination distribution applied to a second illumination setting.

[0022] According to a further embodiment, in each illumination setting, a related illumination distribution, or an illumination distribution having a deviation of 5% or less, in particular 1% or less, is applied to a plurality of field points in the mask plane of the projection exposure apparatus. In other words, the illumination distribution stated in each illumination setting exists in the pupil plane for a plurality of field points in the mask plane of the projection exposure apparatus, and the illumination distribution is still referred to as the same illumination distribution even if the pupil surface deviates by a maximum of 5%.

[0023] According to a further embodiment, the plurality of field points form a contiguous region in the mask plane. The pupil surface illuminated in the pupil plane in the presence of one of the illumination distributions is not a contiguous surface in this embodiment, but has a plurality of distinct surface portions. According to an alternative embodiment, at least one of the pupil surfaces illuminated at the various illumination settings is a contiguous surface.

[0024] According to a further embodiment, at each illumination setting, the area of ​​the lithography process window for imaging a given type of mask structure, without taking into account thermal wavefront aberrations due to thermal heating effects in the projection lens of the projection exposure apparatus caused by the exposure radiation, is no more than 20% smaller, in particular no more than 10% smaller, than the area of ​​the assigned optimized lithography process window, optimized for imaging a given type of mask structure by changing the illumination setting.

[0025] For example, such a lithography process window is formed by a diagram in which the dose variation of exposure radiation and the defocus of imaging are plotted, with an area surrounded by a curve and two coordinate axes, and the lithography parameter at the point represented by the area lies within an acceptable range with respect to the target value. For example, the lithography parameter can be a critical dimension of a mask structure imaged onto a semiconductor substrate using an associated illumination setting, such as a linewidth variation in the photoresist (also referred to as CD variation). Alternatively, for example, the side angle of an imaged structure in the photoresist, or the lateral displacement of a resist structure (also referred to as "overlay") can also serve as a lithography parameter.

[0026] The aerial image of the projection exposure apparatus produced in the image plane, taking into account the resist threshold, can serve to calculate the line width. Resist threshold should be understood to mean the intensity threshold above which the photoresist is exposed. To experimentally determine the process window, it is possible to evaluate the focus-dose matrix (also known as the FEM matrix - focus exposure matrix) on a semiconductor substrate coated with photoresist.

[0027] Specifically, a lithography process window optimized for imaging a given type of mask structure should be understood to mean a process window resulting from changing illumination settings without taking wavefront aberrations into account, where the area of ​​the process window is maximized and the shape of the process window does not undershoot certain minimum requirements. For example, such minimum requirements should be understood to mean the aspect ratio of the process window, measured, for example, using the ratio of the axial portions of the process window. The axial portions should not deviate too greatly from each other. That is, the play in focus variation should not be increased too much to the detriment of the play in dose variation, and vice versa.

[0028] According to a further embodiment, at one of the illumination settings, an area of ​​the lithographic process window is optimized for imaging a predetermined type of mask structure without taking into account thermal wavefront aberrations.

[0029] According to a further embodiment, in at least one of the illumination settings, the area of ​​the lithography process window for imaging a given type of mask structure without taking into account thermal wavefront aberrations is at least 5% smaller than the area of ​​the applied optimized process window.

[0030] According to a further embodiment, the mask structure and the configuration of the illumination distribution of at least one of the illumination settings take into account thermal heating effects in the projection lens of the projection exposure apparatus that occur during the time period in which the mask is exposed with the respective illumination setting, i.e., the time period that elapses until the change between exposure settings.

[0031] According to a further embodiment, the projection exposure apparatus is designed for an operating wavelength in the EUV wavelength range. Operating a projection exposure apparatus in the EUV wavelength range means doing away with refractive media, which can no longer be used meaningfully at this wavelength, and transitioning to a pure mirror system operating at essentially normal incidence or grazing incidence. At normal incidence, approximately one-third of the incident light is absorbed on each mirror (depending on the specific incidence angle spectrum), while at grazing incidence, typical absorption values ​​are one-fourth or one-fifth. In refractive media with anti-reflection layers, the absorbed intensity is, for comparison, on the order of one thousandth. This explains the significantly greater temperature changes in EUV optical units compared to systems operated with UV light.

[0032] Because temperature gradients lead to surface defects due to the thermal expansion coefficient, the result, precisely in the mirror, is substantial optical aberrations that cause image degradation related to the wavelength used. Therefore, EUV mirrors are manufactured from materials with extremely low thermal expansion coefficients, such as Zerodur or ULE ("ultra-low expansion" materials). These materials respond nonlinearly to temperature changes. Near the zero-crossing temperature, which often corresponds to the expected average mirror temperature, the materials exhibit very small thermally induced changes in volume. However, if the local temperature deviates significantly from this optimal zero-crossing temperature, the volume change and, consequently, the surface deformation and wavefront disturbances increase disproportionately. Therefore, local peaks in illumination intensity, which cause hot spots, are extremely significant.

[0033] The features specified with respect to the above-described embodiments, exemplary embodiments, and embodiment variations of the method according to the present invention are set forth in the description of the drawings and in the claims. The individual features may be implemented separately or in combination as embodiments of the present invention. Furthermore, they may describe advantageous embodiments that are independently protectable, and in some cases, protection for those embodiments may only be claimed during or after the prosecution of this application.

[0034] The above-mentioned and further advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments or of embodiments according to the invention, with reference to the accompanying schematic drawings. [Brief explanation of the drawings]

[0035] [Figure 1] 1 is a cross-sectional view of an embodiment of a microlithographic projection exposure apparatus having an illumination system with a field facet mirror and a pupil facet mirror; [Figure 2] FIG. 2 is a plan view of a field facet mirror and a pupil facet mirror. [Figure 3] FIG. 2 is a plan view of a first embodiment of a pupil facet mirror with two different illumination distributions; [Figure 4]FIG. 4 shows a plan view of a pupil facet mirror with two different illumination distributions according to FIG. 3 and a further intermediate illumination distribution. [Figure 5] FIG. 10 is a plan view of a further embodiment of a pupil facet mirror with two different illumination distributions; [Figure 6] FIG. 1 illustrates the lithographic process window for a given mask structure under different illumination settings. DETAILED DESCRIPTION OF THE INVENTION

[0036] In the exemplary embodiments, or embodiments, or embodiment variants described below, elements that are functionally or structurally similar to each other are given the same or similar reference numerals as much as possible. Therefore, to understand the characteristics of individual elements of a specific exemplary embodiment, reference should be made to the descriptions of other exemplary embodiments or the general description of the present invention.

[0037] For ease of illustration, a Cartesian xyz coordinate system is indicated in the figures, from which the relative positions of the components shown in the figures become apparent. In Figure 1, the y direction extends perpendicular to and into the plane of the drawing, the x direction extends to the right, and the z direction extends upwards.

[0038] FIG. 1 shows a schematic diagram of an embodiment of a microlithography projection exposure apparatus 10 configured to produce microstructured components, e.g., components containing integrated circuits. The projection exposure apparatus 10 serves to transfer, by means of a projection lens 50, a mask structure 42 arranged on a mask 40 in the form of a reticle onto a photosensitive layer in the form of a lithography resist of a semiconductor substrate 52 arranged in an image plane 53 of the projection exposure apparatus 10. Examples of such mask structures 42 include dense lines indicated by reference numeral 42a, as well as gaps and isolated lines indicated by reference numeral 42b. Exposure points on the surface of the mask 40 are referred to as field points 43 in a mask plane 44. In principle, what is known as a wafer made of silicon or any other semiconductor material can be used as the semiconductor substrate 52.

[0039] When the mask structures 42 are transferred to the photosensitive layer, the mask structures 42 arranged within the imaging fields on the mask 40 are imaged onto the fields 64 of the semiconductor substrate 52. During the sequential exposures of the mask 40, the semiconductor substrate 52 is displaced each time with respect to its xy position so that the mask structures 42 are imaged onto different fields 64 of the semiconductor substrate 52. When all fields 64 of a semiconductor substrate 52 arranged on the substrate table 54 have been exposed, the semiconductor substrate 52 is replaced with a new semiconductor substrate 52, and all fields 64 on the latter are now exposed. This is performed for multiple semiconductor substrates 52, as shown in the lower right part of FIG. 1. In other words, the mask 40 is repeatedly exposed with exposure radiation, and the mask structures 42 of the mask 40 are each imaged onto one of the multiple fields 64 of the multiple semiconductor substrates 52.

[0040] 1, imaging of the mask 40 is shown solely based on the imaging of an exemplary point 42P of the mask surface that is to be imaged onto one of the semiconductor substrates 52. To this end, an imaging beam path 48 associated with point 42P is plotted through the projection lens 50. Other points on the mask surface are also imaged onto the semiconductor substrate 52 via corresponding imaging beam paths. Thus, for example, points located within the left portion of the mask 42 are also imaged onto the semiconductor substrate 52.

[0041] 1 comprises an exposure radiation source 12, an illumination system 20, a reticle stage (not depicted in the drawings) for mounting and positioning a mask 40, the above-mentioned projection lens 50 in the form of an imaging optical system having a number of optical elements for imaging a mask structure 42 onto a semiconductor substrate 52 during an exposure operation of the projection exposure apparatus 10, and a substrate stage 54 for mounting and positioning the semiconductor substrate 52. Imaging of the mask structure 42 onto the semiconductor substrate 52 is performed via the above-mentioned imaging beam path 48 which passes through the projection lens 50.

[0042] During the exposure operation, the illumination system 20 serves to emit exposure radiation 14 having a suitable angular distribution onto the object field of a mask 40 arranged in a mask plane 44. In other words, on the object field, the illumination system 20 creates an illumination field 46 in the form of the intensity distribution of the exposure radiation 14 in the mask plane 44. In Figure 1, the exposure radiation 14 is shown based on a number of individual rays 39. These include individual rays 39-1, 39-2, and 39-3 emitted onto mirror element 24-4, individual rays 39-4, 39-5, and 39-6 emitted onto mirror element 24-5, and individual rays 39-7, 39-8, and 39-9 emitted onto mirror element 24-6.

[0043] To create the illuminated field 46, the illumination system 20 in the illustrated embodiment comprises three optical modules. The optical modules include a field facet optical unit in the form of a field facet mirror 22, which includes the above-mentioned mirror element 24, a pupil facet optical unit in the form of a pupil facet mirror 30, and what is known as a G mirror 36. The field facet mirror 22 is arranged substantially parallel to or along a system surface in the form of a field plane 23, which is conjugate with the mask plane 44. The pupil facet mirror 30 is arranged substantially parallel to or along a system surface in the form of a pupil plane 31 of the illumination system 20. The G mirror 36 includes a mirror surface 36a arranged parallel to or along a system surface 41.

[0044] In this document, the system surface of the illumination system 20 should be understood to mean the surface parallel to or along which an optical module, such as the field facet mirror 22, the pupil facet mirror 30, or the G mirror 36, is arranged. If the optical module is a reflective optical module having multiple mirror elements, as in the present optical module, the system surface extends substantially parallel to or along the reflective surfaces of the mirror elements. In embodiments not shown in the drawings, the optical module may also include lens elements, in which case the system surface extends substantially parallel to or along the front or back side of each of the lens elements. Furthermore, the optical module may also include in each case only one mirror element or only one lens element.

[0045] The exposure radiation 14 is produced by the above-mentioned exposure radiation source 12, embodied as a point radiation source, and is emitted onto the field facet mirror 22 in the form of a diverging input wave 16 emanating from a source point 18 and propagating in an incoming radiation direction 58. Depending on the design of the projection exposure apparatus 10, the wavelength of the exposure radiation 14 can be in the UV wavelength range, for example around 365 nm, around 248 nm, or around 193 nm, or in the EUV wavelength range, i.e. in a wavelength range below 100 nm, in particular at wavelengths of around 13.5 or around 6.8 nm. In the case shown here, the illumination radiation 14 is EUV radiation, and therefore all optical elements in the exposure beam path of the projection exposure apparatus 10 are embodied as mirrors.

[0046] The field facet mirror 22 comprises a two-dimensional grid of individual optical units in the form of mirror elements 24. In an alternative embodiment for illumination radiation in the UV wavelength range, the individual optical units may also be designed as lens elements. The left-hand region of FIG. 2 shows the field facet mirror 22 according to FIG. 1 in a plan view using an exemplary embodiment of a grid with 3 by 3 mirror elements 24, which are numbered consecutively by the numerals 1 to 9. In further embodiments, the field facet mirror 22 may comprise a fewer or alternatively a larger number of mirror elements 24. The shape of each of the mirror elements 24 is adapted to the shape of the illuminated field 46 in the mask plane 44 and is therefore rectangular or sickle-shaped.

[0047] In the case of a projection exposure apparatus 10 embodied as a step-and-scan exposure apparatus, the illumination field 46 is understood to mean that area on the mask 40 that is illuminated by the scanner slot at a given time. The two-dimensional grid of mirror elements 24 is orthogonal in the illustrated embodiment. FIG. 1 shows the field facet mirror 22 in a cross-section along the cross-section line 26 from FIG. 2. The aforementioned mirror elements 24-4 to 24-6 are arranged along this cross-section line 26. Each of the mirror elements 24 of the field facet mirror 22 is mounted so as to be individually adjustable by means of a respective manipulator 28-4 in the form of an actuator. In particular, an individual tilt of each mirror element 24 about two mutually orthogonal tilt axes is possible. The manipulators 28-4 embodied as actuators can be centrally controlled.

[0048] The pupil facet mirror 30 also comprises a two-dimensional array of individual optical units in the form of mirror elements, indicated by the reference numeral 32. The right-hand area of ​​Fig. 2 shows the pupil facet mirror 30 in plan view with an exemplary embodiment of an array of 36 mirror elements 32, which are numbered consecutively by the numerals 1 to 36. In further embodiments, the pupil facet mirror 30 may comprise a smaller or alternatively larger number of mirror elements 32, in particular even hundreds or thousands of mirror elements 32.

[0049] According to one embodiment, the field facet mirror 22 and the pupil facet mirror 30 together comprise tens of thousands of mirror elements or more than hundreds of thousands of mirror elements. In this case, the facet mirrors 22 and 30 may each take the form of a MEMS mirror lattice, the individual mirror elements being grouped into functional units according to one embodiment variant. In this case, the mirror elements combined into a functional unit may, for example, in each case assume the function of one of the mirror elements 24 or 32 according to Figure 2.

[0050] 2, the number of mirror elements 32 in the pupil facet mirror 30 is four times the number of mirror elements 24 in the field facet mirror 22. In other embodiments, the number of mirror elements 32 in the pupil facet mirror 30 may also be larger or smaller in comparison. In particular, the number of mirror elements 32 in the pupil facet mirror 30 may be larger or smaller than four times the number of mirror elements 24 in the field facet mirror 22. In the illustrated embodiment, the mirror elements 32 are hexagonal and arranged along concentric circles, so that the overall arrangement resembles that of a honeycomb.

[0051] Figure 1 shows pupil facet mirror 30 in a cross-sectional view along section line 33 from Figure 2. Six mirror elements 32-15, 32-29, 32-35, 32-33, 32-23 and 32-5 are arranged along this section line 33. The structure of facet mirrors 22 and 30 may in particular take the form of one of the variants described in US Patent Application Publication No. 2011 / 0001947(A1).

[0052] Each of the 36 mirror elements 32-1 to 32-36 of the pupil facet mirror 30 is assigned a respective radiation channel 35-1 to 35-36 which extends from the mirror elements 24-1 to 24-9, which are assigned to the corresponding mirror element 32 of the field facet mirror 22, via the corresponding mirror element 32 and via the G mirror 36, which will be described in detail below, to the mask plane 44.

[0053] To activate a corresponding radiation channel 35-1 to 35-36, the corresponding mirror element 32-1 to 32-36 of the pupil facet mirror 30 is illuminated through a suitable tilt of the mirror elements 24-1 to 24-9 of the field facet mirror 22. Since each of the mirror elements 24-1 to 24-9 can illuminate only one of the mirror elements 32-1 to 32-36, it is possible to activate up to nine of the radiation channels 35-1 to 35-36 simultaneously.

[0054] In the illumination setting 62a of the illumination system 20 shown in Fig. 1, the mask 40 is illuminated with an angular distribution that corresponds to the illumination distribution 60a in the pupil plane 31 shown on the left in Fig. 3. To this end, the exposure radiation 14 only illuminates the illuminated field 46 via the mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5. That is, only the radiation channels 35-15, 35-30, 35-28, 35-22, 32-24, and 35-5 are active. Therefore, in the cross-sectional view of Fig. 1, only the mirror elements 32-15 and 32-5 of the pupil facet mirror 30 are active. That is, only these mirror elements are illuminated by the respectively assigned mirror elements 24 of the field facet mirror 22 to form the radiation channels 35-15 and 35-5 that in each case illuminate the entire illuminated field 46.

[0055] In this document, the illumination distribution 60a in the pupil plane is also referred to as the illumination pupil surface. The latter consists of the sum of the surfaces 32o of the mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5. Each of the surfaces 32o represents a defined surface portion. In the illustrated exemplary embodiment, these surface portions have a hexagonal shape and are in each case separated from one another by a certain distance.

[0056] To form illumination distribution 60a, mirror element 32-30 of pupil facet mirror 30 may be illuminated by mirror element 24-1 of field facet mirror 22, and mirror element 32-15, mirror element 32-28, mirror element 32-22, mirror element 32-5, and mirror element 32-24 may be illuminated by mirror element 24-4, mirror element 24-7, mirror element 24-3, mirror element 24-6, and mirror element 24-9, respectively. Mirror elements 24-2, 24-5, and 24-8 of field facet mirror 22 are tilted so that the radiation portion of input wave 16 incident on them does not enter pupil facet mirror 30.

[0057] In the embodiment shown, the radiation channels 35-30, 35-22, 35-15, 35-5, 35-28 and 35-24 emanating from the mirror elements 24-1, 24-3, 24-4, 24-6, 24-7 and 24-9 of the field facet mirror 22 and passing through the pupil facet mirror 30 and the G mirror 36 to the mask plane 44 form the illumination beam path 34 in the illumination system 20. The G mirror 36 is a mirror operated with grazing incidence and is also referred to as a grazing incidence mirror. The wave from the G mirror 36 that illuminates the mask plane 44 is also referred to as the output wave 38 of the illumination system 20.

[0058] The illumination beam path 34 comprises a number of individual light rays 39. In the present context, an individual light ray is understood to be a light path that is arranged within the beam path and is represented based on a line. Each of the active radiation channels in the setup shown in Figure 1 comprises a bundle of individual light rays.

[0059] Only a few individual rays of this bundle are shown in Figure 1 by way of example. These are the individual rays 39-1 and 39-3 that delimit the radiation channel 35-15 in the plane of the drawing, and the individual ray 39-2 that extends centrally within the radiation channel 35-15. Also shown are the individual rays 39-4 and 39-6 that delimit the radiation beam that illuminates the mirror element 24-5, as well as the individual ray 39-5 that extends centrally within this radiation bundle. Also shown are the individual rays 39-7 and 39-9 that delimit the radiation channel 35-5 in the plane of the drawing, as well as the individual ray 39-8 that extends centrally within the radiation channel 35-5. For simplicity of illustration, the central individual rays 39-2, 39-5, and 39-8 that extend from the illumination radiation source 12 to the mask plane 44 are shown in Figure 1 only in the region between the illumination radiation source 12 and the field facet mirror 22.

[0060] By means of a manipulator 28-1, the field facet mirror 22 as a whole is mounted so as to be adjustable with respect to its position relative to a frame element 29-1 of the illumination system 20. In this case, the manipulator 28-1 is configured as an adjustment device that allows multiple rigid degrees of freedom, in particular all six rigid degrees of freedom, i.e., translation and rotation in each case in all three orthogonal spatial directions, as indicated by the arrows in FIG. 1 . In this context, the adjustment device may be manually adjustable or else may comprise electrically controllable actuators. Furthermore, the individual mirror elements 24-1 to 24-9 of the field facet mirror 22 are mounted so as to be individually adjustable via a manipulator 28-4. According to the above-described embodiment, each of the mirror elements 24 is tiltable about two mutually orthogonal tilt axes.

[0061] By means of corresponding manipulators 28-2 or 28-3, both the pupil facet mirror 30 as a whole and the G mirror 36 are mounted so as to be adjustable in terms of their position relative to frame elements 29-2 or 29-3 of the illumination system 20. In a manner similar to the manipulator 28-1 applied to the field facet mirror 22, the manipulators 28-2 and 28-3 are also configured as adjustment devices which in each case allow multiple rigid degrees of freedom, in particular all six rigid degrees of freedom, i.e. in each case translation and rotation in all three orthogonal spatial directions, to be set. In the present context, the adjustment devices may be manually adjustable or else comprise electrically controllable actuators.

[0062] As described above, the mask 40 is exposed multiple times, each time imaged onto the field of view 64 of a number of semiconductor substrates 52. During the time period required to achieve this, the illumination system 20 is operated at at least two different illumination settings, a first illumination setting 62a and a second illumination setting 62b according to the exemplary embodiment according to Fig. 3. In this case, the exposure starts, for example, with the first illumination setting 62a, in which only radiation channels 35-30, 35-15, 35-28, 35-22, 35-24, and 35-5 are used, as described above.

[0063] Due to this highly non-uniform illumination of the illumination beam path 34, and therefore also of the imaging beam path 48 in the projection lens 50, thermal heating of the relevant optical elements in the projection lens 50 and the resulting surface deformations on said optical elements can cause the formation of wavefront errors in the projection lens 50. To prevent this, after a certain duration of the exposure operation, for example after the exposure of one or more batches of semiconductor substrates 52, or even after the exposure of several semiconductor substrates 52 in a batch or after the exposure of several fields 64 on the semiconductor substrates 52, the illumination can be switched back to the first illumination setting 62a again after a certain further duration of the exposure operation, before wavefront errors can likewise occur due to heating of the relevant optical elements caused by new radiation traces in the imaging beam path 48. This cycle can be performed multiple times.

[0064] In the second illumination setting 62b, the pupil plane 31 is illuminated with a second illumination distribution 60b, in which case the exposure radiation 14 no longer irradiates mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5 as in the illumination distribution 60a, but instead irradiates mirror elements 32-16, 32-29, 32-14, 32-23, 32-4, and 32-6. The pupil surface illuminated by the first illumination setting 62a, which corresponds to the ensemble of surfaces 32o of mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5, has no overlap with the pupil surface illuminated by the second illumination setting 62b, which corresponds to the ensemble of surfaces of mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5.

[0065] Therefore, only radiation channels 35-16, 35-29, 35-14, 35-23, 35-4, and 35-6 are used in the second illumination setting 62b, and therefore completely different radiation channels from those used in the first illumination setting 62a. The thermal heating signature that occurs in the relevant optical elements in the projection lens 50 in the second illumination setting 62b is therefore so different from the signature that occurs in the first illumination setting 62a that surface modifications in the optical elements of the projection lens 50 that began to form due to the signature of the first illumination setting 62a and that could cause wavefront errors do not grow any larger, and in some cases even decrease in size.

[0066] Imprints of the second illumination setting 62b may of course cause other surface modifications in the optical elements of the projection lens 50, which may likewise give rise to wavefront errors. Therefore, operation with the second illumination setting 62b is preferably converted back to operation with the first illumination setting 62a after a certain further duration as mentioned above, which is short enough that these wavefront errors likewise cannot form to the extent that they become bothersome.

[0067] According to an embodiment shown in Figure 4, the switching from the first illumination setting 62a to the second illumination setting 62b may be performed gradually. In the process, the illumination of a mirror element 32 or group of mirror elements 32 is switched in each step to another mirror element 32 or another group of mirror elements 32. To this end, two of the mirror elements 24-1, 24-3, 24-4, 24-6, 24-7 and 24-9 of the field facet mirror 22 are switched in turn between different radiation channels 35 in each case.

[0068] In other words, the illumination of each surface portion or subgroup of surface portions in the pupil plane 31 according to a first configuration, defined by a first illumination setting 62a, is gradually switched to the illumination of a different surface portion or a different subgroup of surface portions in the pupil plane 31 according to a second configuration, defined by a second illumination setting 62b. This gradual switching may also be referred to as morphing. Specifically, in the embodiment according to Fig. 4, the transition from the first illumination distribution 60a according to Fig. 3 to the second illumination distribution 60b according to Fig. 3 is performed in three steps:

[0069] In this case, in a first step, a transition from illumination distribution 60a to first intermediate illumination distribution 60z1 is made by switching the illumination of mirror elements 32-30 and 32-22 to mirror elements 32-16 and 32-4, respectively. This is achieved by appropriate tilting of mirror elements 24-1 and 24-3. In a second step, a transition from first intermediate illumination distribution 60z1 to second intermediate illumination distribution 60z2 is made by switching the illumination of mirror elements 32-15 and 32-5 to mirror elements 32-29 and 32-23, respectively. This is achieved by appropriate tilting of mirror elements 24-4 and 24-6. In a third step, a transition from second intermediate illumination distribution 60z2 to second illumination distribution 60b is made by switching the illumination of mirror elements 32-28 and 32-24 to mirror elements 32-14 and 32-6, respectively. This is achieved by appropriate tilting of mirror elements 24-7 and 24-9.

[0070] 5 shows a further embodiment in which the pupil surface illuminated by the first illumination setting 62a has a certain amount of overlap with the pupil surface illuminated by the second illumination distribution 62b. The pupil surface illuminated by the first illumination setting 62a corresponds to the ensemble of surfaces of mirror elements 32-17, 32-30, 32-15, 32-28, 32-13, 32-3, 32-22, 32-5, 32-24, and 32-7. The pupil surface illuminated by the second illumination setting 62b corresponds to the ensemble of surfaces 32o of mirror elements 32-17, 32-16, 32-29, 32-14, 32-13, 32-3, 32-4, 32-23, 32-6, and 32-7. The overlap between illumination distributions 62a and 62b relates to the surfaces of mirror elements 32-17, 32-13, 32-3, and 32-7, and therefore to four of the ten mirror elements illuminated in each case. The overlap is therefore approximately 40% of the pupil surface illuminated in each case. According to further embodiments, the overlap may account for a smaller or larger percentage, but may amount to up to 90% of the pupil surface illuminated in each case. Because at least a proportion of the pupil surface is no longer illuminated when switching between the two illumination settings 62a and 62b, the effects described above with reference to the embodiment according to FIG. 3 occur, which may result in a situation in which no wavefront errors, or only less noticeable ones, may occur.

[0071] The illumination distributions 60a, 60b, and optionally 60z1 and 60z2, in the pupil plane 31 shown in Figures 3, 4 and 5 correspond, according to one embodiment variant, to the respective angular distributions of the exposure radiation 14 irradiating the various illumination field points 43 on the mask 40. According to another embodiment variant, the angular distribution varies slightly between the field points 43, in which case the illumination distributions 60a, 60b, and optionally 60z1 and 60z2 shown in Figures 3, 4 and 5 refer at least in each case to one constant field point 43, i.e. to the same field point or points.

[0072] According to a further embodiment variant, the respective illumination distributions are applied in the respective illumination settings 62a and 62b to a plurality of field points 43, which in particular form a continuous region 45 in the mask plane 44 (see FIG. 1), and said illumination distributions correspond to those shown in FIGS. 3, 4 and 5 or deviate therefrom in each case by at most 5%, in particular by at most 1%. In other words, according to this embodiment variant, the illumination distributions 60a and 60b described for the respective illumination settings 62a and 62b in the pupil plane exist for field points 43 arranged in the continuous region 45, and the illumination distributions 60a and 60b still refer to the same illumination distribution even if the pupil surface deviates by at most 5%.

[0073] FIG. 6 shows lithography process windows for a given mask structure 42 under different illumination settings. These process windows are represented in each case by an area in a diagram in which the defocus Δf of the image of the mask structure 42 in the image plane 53 is plotted against the dose change ΔD of the exposure radiation 14 produced by the exposure radiation source 12. The aforementioned area is formed by the region in the Δf-ΔD diagram bounded by the process window curve 66 and the Δf and ΔD coordinate axes in the diagram. At all points in the process window represented by the area, the lithography parameter lies within an acceptable range relative to the target value. For example, the lithography parameter may be a critical dimension of the mask structure 42 imaged onto the semiconductor substrate 52 using the associated illumination setting, such as a linewidth change in the photoresist (also referred to as CD change).

[0074] FIG. 6 illustrates an optimized process window 68o for the above-described predetermined mask structure 42 using a process window curve 66o in the form of a dashed-dotted line. That is, the process window 68o is based on an illumination distribution in the pupil plane 31 that maximizes the area of ​​the process window 68o while maintaining the ratio of the process window 68o. However, this process window 68o only has this size at the beginning of the exposure process, as long as thermal heating effects are not yet substantially active within the projection lens 50. However, after a certain exposure operating time, thermal heating effects cause wavefront aberrations, also referred to herein as thermal wavefront aberrations. These wavefront aberrations result in the process window curve 66o being shifted to smaller ΔD and Δf values ​​(see process window curve 66ot), and therefore the process window 68o is reduced to a process window 68ot of a correspondingly reduced size.

[0075] In one exemplary embodiment, the illumination distributions 60a and 60b in the illumination settings 62a and 62b are selected in each case in such a way that the area of ​​the associated process windows 68a and 68b, respectively, resulting without taking into account thermal wavefront aberrations, is in each case not more than 10%, in particular not more than 20%, smaller than the area of ​​the optimized process window 68o. The process windows 68a and 68b are in each case defined by the process window curves 66a and 66b. According to an embodiment variant, the area of ​​the process windows 68a and 68b, respectively, is in each case at least 5% smaller than the area of ​​the optimized process window 68o.

[0076] The process windows 68a and 68b are in each case smaller than the optimized process window 68o, but they are larger than the process window 68ot that begins a certain amount of time later due to thermal wavefront aberrations. If the exposure operation of the projection exposure apparatus 10 in one of the illumination distributions 60a and 60b is relatively long, the corresponding process window will also shrink in size. To be precise, the process window 68a will shrink to the process window 68at defined by the process window curve 66at, and the process window 68b will shrink to the process window 68bt defined by the process window curve 66bt. However, as explained above, this is prevented by switching back and forth between the illumination settings 60a and 60b before thermal wavefront aberrations can occur.

[0077] Therefore, use of illumination settings 62a and 62b with process windows 60a and 60b may permanently ensure a larger process window than would be the case with continued use of illumination settings applied to optimized process window 68o.

[0078] According to a further exemplary embodiment, one of the two illumination settings 60a and 60b may be configured to produce an optimized process window 68o, and the other illumination setting may correspond to a process window smaller than process window 68a or process window 68b.

[0079] The above description of exemplary embodiments, embodiments, or embodiment variations should be understood as examples. The disclosure achieved thereby first enables those skilled in the art to understand the present invention and its attendant advantages, and second encompasses modifications and variations of the above-described structures and methods that are similarly obvious to those skilled in the art. Accordingly, all such modifications and variations are intended to be within the scope of the present invention as defined in the appended claims, and equivalents are intended to be protected by the claims. [Explanation of symbols]

[0080] 10 Projection exposure equipment 12 Exposure radiation source 14 Exposure to radiation 16 Input Wave 18 Source point 20 Lighting System 22 Field Facet Mirror 23 Viewing plane 24, 24-1 to 24-9 mirror elements 26 Section line 28-1 Field facet mirror manipulator 28-2 Pupil Facet Mirror Manipulator 28-3 G-mirror manipulator 28-4 Manipulator for mirror elements of field facet mirror 29-1~29-3 Frame elements 30 Eye Facet Mirror 31 Pupil plane 32, 32-1 to 32-32 Mirror elements 32o mirror element surface 33 Section line 34 Illumination beam path 35-1~35-32 Radiation Channel 36 G mirror 36a Mirror surface 38 Output Wave 39 Individual Rays 40 Mask 41 System Surface 42 Mask Structure 42P Points on the mask surface 43 field points 44 Mask Plane 45 Contiguous area in the mask plane 46 Illumination Field 48 Imaging beam path 50 Projection Lens 52 Semiconductor substrate 53 Image plane 54 Circuit board stand 58 Incoming radiation direction 60a First illumination distribution in pupil plane 60b Second illumination distribution in the pupil plane 60z1 First intermediate lighting distribution 60z2 Second intermediate lighting distribution 62a First Lighting Setting 62b Second Lighting Setting 64 Field of view on semiconductor substrate 66a Process Window Curve Process window curve with 66at thermal aberration 66b Process Window Curve 66bt Process window curve with thermal aberration 66o Optimized Process Window Curve 66ot Optimized process window curve with thermal aberration 68a Process Window Process window with 68at thermal aberration 68b Process Window 68bt Process window with thermal aberration 68o Optimization Process Window Optimized process window with 68ot thermal aberration

Claims

1. 1. A method for operating a microlithographic projection exposure apparatus, comprising: the mask is repeatedly exposed to exposure radiation provided by the illumination system, and the mask structure is imaged each time onto one of the multiple fields of the multiple semiconductor substrates in the process; during the period during which the repeated exposures of the mask are performed, the illumination system is used sequentially with at least two different illumination settings of the illumination system, with different illumination distributions of the exposure radiation present in a pupil plane of the illumination system, whereby a pupil surface illuminated with the first illumination setting has no or at most 90% overlap of the respective illuminated pupil surfaces with a pupil surface illuminated with the second illumination setting; The method wherein the mask is fully exposed at least once at each of the two different illumination settings.

2. 10. The method of claim 1, wherein during the repeated exposures of the mask, there is a time interval of less than 200 minutes between switching between the two different illumination settings.

3. The method of claim 1 or 2, wherein the pupil surfaces illuminated with the different illumination settings each have a plurality of surface portions separated from one another.

4. 4. The method of claim 3, wherein switching from a first of the two different illumination settings with a first configuration of surface portions in the pupil plane to the second illumination setting with a second configuration of surface portions in the pupil plane is performed by gradually switching each time from illumination of a surface portion or a subgroup of the surface portions in the first configuration to illumination of another surface portion or another subgroup of the surface portions in the second configuration.

5. 5. The method according to claim 1, wherein switching from a first of the two different lighting settings to the second lighting setting is performed by progressively adapting the first lighting distribution applied to the first lighting setting to the second lighting distribution applied to the second lighting setting.

6. 6. The method of claim 5, wherein the illumination system comprises a pupil facet optical unit having a plurality of individual optical units arranged in a pupil plane of the illumination system, and a field facet optical unit arranged in a plane conjugate with the mask plane, the field facet optical unit including a plurality of further individual optical units configured to illuminate the individual optical units of the pupil facet optical unit for forming respective radiation channels of the beam path of the illumination radiation, and switching between different radiation channels is performed by sequentially moving one or more of the individual optical units of the field facet optical unit during the gradual adaptation from the first illumination distribution to the second illumination distribution.

7. 7. The method according to claim 1, wherein the illumination distributions in the pupil plane that exist for the different illumination settings are each assigned to at least one constant field point in a mask plane of the projection exposure apparatus.

8. 8. The method according to claim 1, wherein at each illumination setting, the associated illumination distribution, or an illumination distribution having a deviation of 5% or less, is applied to a plurality of field points in a mask plane of the projection exposure apparatus.

9. The method of claim 8 , wherein the plurality of field points form a contiguous area in the mask plane.

10. 10. The method according to claim 1, wherein, at each of the illumination settings, an area of ​​a lithographic process window for imaging a predetermined type of mask structure, not taking into account thermal wavefront aberrations due to thermal heating effects in a projection lens of the projection exposure apparatus caused by the exposure radiation, is no more than 20% smaller than the area of ​​an assigned optimized lithographic process window, optimized for imaging the given type of mask structure by changing the illumination settings.

11. 11. The method of claim 10, wherein at one of the illumination settings, the area of ​​the lithography process window is optimized to image a predetermined type of mask structure without considering the thermal wavefront aberrations.

12. 12. The method of claim 10 or 11, wherein, in at least one of the illumination settings, the area of ​​the lithography process window for imaging a predetermined type of mask structure without considering the thermal wavefront aberrations is at least 5% smaller than the area of ​​the assigned optimized process window.

13. 13. The method according to claim 1, wherein the configuration of the illumination distribution of the mask structure and at least one of the illumination settings takes into account thermal heating effects in a projection lens of the projection exposure apparatus that occur during a time period in which the mask is exposed with the respective illumination setting.

14. The method according to any one of the preceding claims, wherein the projection exposure apparatus is designed for an operating wavelength in the EUV wavelength range.

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