Method for operating a projection exposure system
Patent Information
- Application Number
- EP2023773281
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-09-23
- Filing Date
- 2023-09-20
- Publication Date
- 2025-07-30
AI Technical Summary
Lithographic projection exposure systems face challenges in maintaining high yield and contrast while minimizing thermally induced aberrations, which lead to reduced process windows due to localized illumination and temperature peaks on optical surfaces.
The method involves operating a projection exposure system using multiple lighting settings with minimal overlap in the pupil plane, switching between these settings periodically to prevent excessive wavefront aberrations, and maintaining each setting for extended periods to ensure consistent exposure across semiconductor substrates.
This approach maintains a larger lithographic process window by reducing thermal wavefront aberrations, ensuring high yield and contrast without significant losses, even with inaccuracies in exposure dose and focus position.
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Figure 1.1
Abstract
Description
[0001] Method for operating a projection exposure system
[0002] This application claims priority from German patent application 10 2022 210 088.3 filed on September 23, 2022. The entire disclosure of this patent application is incorporated by reference into this application.
[0003] Background of the invention
[0004] The invention relates to a method for operating a projection exposure system for microlithography.
[0005] Today, lithographic projection exposure processes are predominantly used to manufacture semiconductor devices and other finely structured components. In this process, a pattern arranged on a mask or reticle is positioned in a projection exposure system between an illumination system and a projection lens in the area of a mask 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, thereby imaging the pattern onto a radiation-sensitive layer of a semiconductor substrate.
[0006] Exposure is increasingly optimized specifically for specific chip-layer structures, resulting in fragmented and sometimes highly localized illumination distributions in the pupil plane of the illumination system compared to traditional forms such as annular, dipole, and quasar. The goal is to achieve the highest possible contrast of the imaged structures, especially those critical to the function of the semiconductor component. These can, for example, have particularly small dimensions or are located very close to neighboring structures. The goal is to achieve high yield regardless of inaccuracies in the exposure dose and focus position, as observed in the actual manufacturing process.
[0007] To quantify such relationships, a plot of a so-called lithographic process window is often used. In this process, the dose error that leads to the deviation of a lithographic parameter, such as a given feature size inaccuracy (e.g., 10% line width), is determined for a certain focus error. Different illumination distributions generally lead to varying process windows. Furthermore, high insensitivity to focus errors is often traded off with greater sensitivity to dose errors, and vice versa. Depending on the choice of illumination, the emphasis can be placed differently on one of the two influencing factors.
[0008] Due to the localized illumination peaks, highly stressed areas with temperature peaks are now created on optical surfaces, resulting in a conflict between illumination optimized for high contrast on the one hand and high thermally induced aberrations, which in turn can reduce the yield, either through positioning errors (“overlay”) or through focus effects, which in turn reduce the contrast.
[0009] Underlying task
[0010] It is an object of the invention to provide a method of the type mentioned above, which solves the aforementioned problems and, in particular, allows the projection exposure system to be operated over a long period of time without significant losses in the lithographic process window. Solution according to the invention
[0011] The above-mentioned object can be achieved according to the invention, for example, with a method for operating a projection exposure apparatus for microlithography, in which a mask is repeatedly exposed to exposure radiation provided by an illumination system and mask structures are imaged on one of a plurality of fields of a plurality of semiconductor substrates.During the period in which the repeated exposure of the mask takes place, the illumination system is used successively in at least two different illumination settings of the illumination system, in which the illumination distributions of the exposure radiation in a pupil plane of the illumination system are so different that a pupil area illuminated in the first illumination setting has no overlap or an overlap of a maximum of 90% of the illuminated pupil area in each case. Furthermore, the mask is fully exposed at least once in each of the two different illumination settings. In other words, if the projection exposure system is designed as a scanner, at least one complete mask scan is performed in each of the illumination settings.According to one embodiment, the mask is fully exposed in each of the two different illumination settings several times, for example at least ten times, at least 100 times or at least 1000 times, without changing the illumination setting in between.
[0012] According to a further embodiment, the different illumination settings are each maintained for at least a period of time required to expose the mask twice. According to a further embodiment, the different illumination settings are each maintained for at least a period of time required to expose the mask at least ten times. According to a further embodiment, the different illumination settings are each maintained for at least a period of time required to completely expose a semiconductor substrate, in particular to expose a plurality of semiconductor substrates, for example to expose at least ten semiconductor substrates.According to a further embodiment, the different illumination settings are each maintained for at least a period of time required to expose at least one batch of semiconductor substrates, in particular to expose a plurality of batches of semiconductor substrates, for example to expose at least ten batches of semiconductor substrates. A batch can, for example, comprise at least twenty semiconductor wafers, in particular at least twenty-five semiconductor wafers. According to different embodiments, the overlap can be a maximum of 80%, a maximum of 50%, a maximum of 20% or a maximum of 10%. This means that a pupil section illuminated in the pupil plane or a total of a plurality of pupil sections illuminated in the pupil plane, i.e. the areas illuminated in the pupil plane, do not overlap at all or have an overlap of a maximum of 90% or one of the other stated maximum values.
[0013] In other words, the pupil area illuminated in the first illumination setting has an overlap of a maximum of 90% with the pupil area illuminated in the second illumination setting or one of the other specified values of the illuminated pupil area, whereby the overlap can also be 0%.
[0014] Since the mask is illuminated with the corresponding illumination at a specific lighting setting, overlap does not mean that the relevant surface section(s) are illuminated simultaneously. Rather, overlap refers to one or more surface sections being illuminated at different times with the relevant lighting setting.
[0015] This means that a pupil area illuminated in a first lighting setting, also referred to as the first area, differs from a pupil area illuminated in the second lighting setting, also referred to as the second area, in that the two areas each correspond to a maximum of 90% of the total area in question, or do not correspond at all, i.e. the illuminated areas differ from one another by more than 10%. This means that the partial area of the first area which has a correspondence in the second area, i.e. corresponds to the relevant partial area of the second area, makes up 90% or less of the total area of the first area. Conversely, this also applies to the partial area of the second area; this makes up 90% or less of the total area of the second area.Similarly, when using more than two different lighting settings, those parts of the areas illuminated in the different lighting settings that match each other in all lighting settings each make up a maximum of 90% of the total area of the area in question.
[0016] A pupil plane of the illumination system is characterized in that the local intensity distribution of the illumination radiation, which converges on a specific field point on the mask, corresponds in the pupil plane to the angle-resolved intensity distribution at this field point.
[0017] By using the illumination system sequentially according to the invention in the above-described at least two different illumination settings of the illumination system, it is possible to prevent excessive wavefront aberrations from developing due to local heating of optical elements of the projection exposure system, in particular optical elements of the projection lens. By using the different illumination settings, the radiation distribution on the optical elements can be adjusted before local heating relevant to possible wavefront aberrations occurs.
[0018] According to one embodiment, during the repeated exposure of the mask, the two different illumination settings are changed at a time interval of less than 200 minutes, in particular less than 60 minutes or less than 20 minutes.
[0019] According to a further embodiment, the pupil surfaces illuminated in the different illumination settings each have a plurality of mutually delimited surface sections.
[0020] According to a further embodiment, a change is made from a first of the two different illumination settings with a first configuration of surface sections in the pupil plane to the second illumination setting with a second configuration of surface sections in the pupil plane by stepwise switching from an illumination of a surface section or a subgroup of the surface sections of the first configuration to an illumination of another surface section or another subgroup of the surface sections of the second configuration.
[0021] According to a further embodiment, the switch from a first of the two different lighting settings to the second lighting setting is made by gradually adapting the first lighting distribution associated with the first lighting setting to the second lighting distribution associated with the second lighting setting. This procedure can also be referred to as morphing.
[0022] According to a further embodiment, the illumination system comprises a pupil facet optic arranged in a pupil plane of the illumination system, comprising a plurality of individual optics, and a field facet optic arranged in a plane conjugate to the mask plane. The field facet optic has a plurality of further individual optics configured to illuminate the individual optics of the pupil facet optic to form a respective radiation channel of the beam path of the illumination radiation. During the step-by-step adaptation of the first illumination distribution to the second illumination distribution, switching takes place between different radiation channels by successively moving one or more of the individual optics of the field facet optic. This means that the exposure radiation previously guided in one radiation channel is then guided in another radiation channel.
[0023] According to a further embodiment, the illumination distributions in the pupil plane present for the different illumination settings are each assigned to at least one uniform field point in a mask plane of the projection exposure system. In other words, the first illumination distribution assigned to the first illumination setting is assigned to the same field point or the same plurality of field points as the second illumination distribution assigned to the second illumination setting.
[0024] According to a further embodiment, in the respective illumination setting, the respective illumination distribution or an illumination distribution deviating by a maximum of 5%, in particular by a maximum of 1%, is assigned to a plurality of field points in a mask plane of the projection exposure system. In other words, the illumination distribution in the pupil plane mentioned in the respective illumination setting is present for a plurality of field points in the mask plane of the projection exposure system, wherein the illumination distribution is still referred to as the same illumination distribution even if the pupil area deviates by a maximum of 5%.
[0025] According to a further embodiment, the plurality of field points forms a contiguous area in the mask plane. The pupil area illuminated when one of the illumination distributions in the pupil plane is present is not a contiguous area in this embodiment, but rather comprises several separate surface sections. According to an alternative embodiment, at least one of the pupil areas illuminated in the different illumination settings is a contiguous area.According to a further embodiment, for each of the illumination settings, an area of a lithographic process window for imaging a predetermined type of mask structures without taking into account thermal wavefront aberrations which are due to thermal heating effects in a projection lens of the projection exposure apparatus caused by the exposure radiation is at most 20% smaller, in particular at most 10% smaller, than the area of an associated optimized lithographic process window which is optimized for imaging the predetermined type of mask structures by varying the illumination setting.
[0026] Such a lithographic process window is formed, for example, by a diagram in which an area is bordered by a curve and two coordinate axes on which a dose variation of the exposure radiation and a defocus of the image are plotted, whereby at points represented by the area a lithographic parameter lies within a tolerance range around a target value. The lithographic parameter can, for example, be a variation of a critical dimension, such as a line width in the photoresist (also called CD variation) of a mask structure imaged onto the semiconductor substrate with the respective illumination setting. Alternatively, an azimuth angle of an imaged structure in the photoresist or lateral displacements of resist structures (also called "overlay") can also serve as lithographic parameters.
[0027] The aerial image generated by the projection exposure system in the image plane can be used to computationally estimate the line width, taking into account a resist threshold. A resist threshold is an intensity threshold above which exposure of the photoresist occurs. To experimentally determine a process window, a focus-dose matrix (also known as a FEM matrix) on a semiconductor substrate coated with photoresist can be evaluated. The lithographic process window optimized for imaging the specified type of mask structure is understood to be the process window resulting from modifying the illumination setting without considering wavefront aberrations, whose area is maximum and whose shape does not fall below certain minimum requirements.Such a minimum requirement could include, for example, the aspect ratio of the process window, measured, for example, by the ratio of the intercepts of the process window. The intercepts should not deviate too much from each other, meaning that the range for focus variation should not be increased too much at the expense of the range for dose variation, and vice versa.
[0028] According to a further embodiment, in one of the illumination settings, the area of the lithographic process window is optimized for imaging a given type of mask structures without taking into account the thermal wavefront aberrations.
[0029] According to a further embodiment, in at least one of the illumination settings, the area of the lithographic process window for imaging a predetermined type of mask structures without taking into account the thermal wavefront aberrations is at least 5% smaller than the area of the associated optimized process window.
[0030] According to a further embodiment, when configuring the mask structures and the illumination distribution of at least one of the illumination settings, thermal heating effects in a projection lens of the projection exposure system are taken into account. These heating effects occur within a period in which the mask is exposed with the respective illumination setting, i.e., the period prior to a change between the exposure settings. According to a further embodiment, the projection exposure system is designed for an operating wavelength in the EUV wavelength range. Operating a projection exposure system in the EUV wavelength range means dispensing with refractive media, which are no longer useful at this wavelength, and switching to pure mirror systems that operate either with nearly perpendicular incidence or with grazing.At normal incidence, approximately one-third of the incident light is absorbed by each mirror (depending on the specific angle of incidence spectrum); at grazing incidence, typical absorption values are one-quarter or one-fifth. In refracting media with an anti-reflective coating, the absorbed intensity is in the per mille range, by comparison. This explains the significantly greater temperature changes in EUV optics compared to systems operating with UV light.
[0031] Since temperature gradients translate into surface defects due to the thermal expansion coefficient, they lead to significant optical aberrations, especially in mirrors, which degrade the image in relation to the useful wavelength. Accordingly, EUV mirrors are manufactured from materials with a particularly low thermal expansion coefficient, e.g., Zerodur or ULE ("ultra low expansion") material. These materials react nonlinearly to temperature changes. In the vicinity of a zero-crossing temperature, which often coincides with the expected mean mirror temperature, they exhibit very little thermally induced volume change. However, if the temperature deviates noticeably from this optimal zero-crossing temperature locally, the volume change and the resulting surface deformation and wavefront disturbance increase disproportionately. Local peaks in illumination intensity, which lead to hot spots, are therefore particularly critical.
[0032] The features specified with regard to the above-mentioned embodiments, exemplary embodiments, or embodiment variants, etc. of the method according to the invention are explained in the description of the figures and the claims. The individual features can be implemented either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments that are independently protectable and whose protection may be claimed only during or after the application is filed.
[0033] Brief description of the drawings
[0034] The above and other advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments of the invention with reference to the accompanying schematic drawings. It shows:
[0035] Fig. 1 is a sectional view of an embodiment of a projection exposure apparatus for microlithography with an illumination system comprising a field facet mirror and a pupil facet mirror,
[0036] Fig. 2 a top view of the field facet mirror and the pupil facet mirror,
[0037] Fig. 3 shows the pupil facet mirror in plan view with two different illumination distributions in a first embodiment,
[0038] Fig. 4 shows the pupil facet mirror in plan view with the two different illumination distributions according to Fig. 3 as well as further intermediate illumination distributions,
[0039] Fig. 5 the pupil facet mirror in plan view with two different illumination distributions in a further embodiment, as well as
[0040] Fig. 6 Lithographic process windows for a given mask structure at different illumination settings. Detailed description of embodiments according to the invention
[0041] In the exemplary embodiments or embodiments or variants described below, functionally or structurally similar elements are provided with the same or similar reference numerals wherever possible. Therefore, to understand the features of the individual elements of a specific embodiment, reference should be made to the description of other exemplary embodiments or the general description of the invention.
[0042] To facilitate the description, a Cartesian xyz coordinate system is shown in the drawing, from which the respective positional relationships of the components shown in the figures are derived. In Fig. 1, the y-direction runs perpendicular to the plane of the drawing, the x-direction to the right, and the z-direction upward.
[0043] Fig. 1 shows a schematic view of an embodiment of a projection exposure system 10 for microlithography, which is configured for producing microstructured components, e.g., components containing integrated circuits. The projection exposure system 10 serves to transfer mask structures 42 arranged on a mask 40 in the form of a reticle, by means of a projection objective 50, onto a photosensitive layer in the form of a lithographic resist of a semiconductor substrate 52 arranged in an image plane 53 of the projection exposure system 10. Examples of such mask structures 42 include dense lines and spaces designated by reference numeral 42a and isolated lines designated by reference numeral 42b. The exposed points on the surface of the mask 40 are referred to as field points 43 in the mask plane 44.So-called wafers made of silicon or another semiconductor material are generally used as the semiconductor substrate 52. During the transfer of the mask structures 42 to the photosensitive layer, the mask structures 42 arranged in an imaging field on the mask 40 are imaged onto a field 64 of the semiconductor substrate 52. In successive exposures of the mask 40, the semiconductor substrate 52 is shifted in its xy position so that the mask structures 42 are imaged onto different fields 64 of the semiconductor substrate 52. After all fields 64 of the semiconductor substrate 52 arranged on the substrate table 54 have been exposed, the semiconductor substrate 52 is replaced with a new semiconductor substrate 52, whereupon all fields 64 are again exposed on this new semiconductor substrate 52. This is done for a plurality of semiconductor substrates 52, as illustrated in the lower right section of Fig. 1.In other words, the mask 40 is repeatedly exposed to the exposure radiation, wherein the mask structures 42 of the mask 40 are each imaged onto one of a plurality of fields 64 of a plurality of semiconductor substrates 52.
[0044] In Fig. 1, the imaging of the mask 40 is shown solely by imaging an exemplary point 42P of the mask surface to be imaged onto one of the semiconductor substrates 52. For this purpose, an imaging beam path 48 is drawn through the projection lens 50 with respect to the point 42P. Other points on the mask surface are imaged onto the semiconductor substrate 52 using corresponding imaging beam paths. For example, points arranged in the left-hand section of the mask 42 are also imaged onto the semiconductor substrate 52.
[0045] For this purpose, the projection exposure system 10 according to Fig. 1 contains an exposure radiation source 12, an illumination system 20, a reticle table (not shown in the drawing) for holding and positioning the mask 40, the aforementioned projection lens 50 in the form of an imaging optical system with a plurality of optical elements for imaging the mask structures 42 onto the semiconductor substrate 52 during an exposure operation of the projection exposure system 10, and a substrate table 54 for holding and positioning the semiconductor substrate 52. The imaging of the mask structures 42 onto the semiconductor substrate 52 takes place via the aforementioned imaging beam path 48 passing through the projection lens 50.
[0046] During exposure operation, the illumination system 20 serves to radiate exposure radiation 14 with a suitable angular distribution onto an object field of the mask 40 arranged in a mask plane 44. In other words, the illumination system 20 generates an illumination field 46 on the object field in the form of an intensity distribution of the exposure radiation 14 in the mask plane 44. The exposure radiation 14 is illustrated in Fig. 1 using a plurality of individual beams 39. These include individual beams 39-1, 39-2, and 39-3 radiated onto a mirror element 24-4, individual beams 39-4, 39-5, and 39-6 radiated onto a mirror element 24-5, and individual beams 39-7, 39-8, and 39-9 radiated onto a mirror element 24-6.
[0047] To generate the illumination field 46, the illumination system 20 in the embodiment shown comprises three optical modules. The optical modules comprise a field facet optic in the form of a field facet mirror 22 comprising the aforementioned mirror elements 24, a pupil facet optic in the form of a pupil facet mirror 30, and a so-called G-mirror 36. The field facet mirror 22 is arranged essentially parallel to or along a system surface conjugated to the mask plane 44 in the form of a field plane 23. The pupil facet mirror 30 is arranged essentially parallel to or along a system surface in the form of a pupil plane 31 of the illumination system 20. The G-mirror 36 has a mirror surface 36a, which is arranged parallel to or along a system surface 41.
[0048] In this text, a system surface of the illumination system 20 is understood to mean a 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. In the case where an optical module, such as the present optical modules, is a reflective optical module with multiple mirror elements, the system surface runs essentially parallel to or along the reflective surfaces of the mirror elements. In embodiments not shown in the drawings, the optical modules can also contain lenses; in this case, the system surfaces run essentially parallel to or along respective front or back sides of the lenses. Furthermore, the optical modules can also each contain only one mirror element or only one lens.
[0049] The exposure radiation 14 is generated by the aforementioned exposure radiation source 12, which is designed as a point radiation source, and is radiated 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 illumination direction 58. Depending on the design of the projection exposure system 10, the wavelength of the exposure radiation 14 can be in the UV wavelength range, e.g., at approximately 365 nm, approximately 248 nm, or approximately 193 nm, or in the EUV wavelength range, i.e., in a wavelength range of less than 100 nm, in particular at a wavelength of approximately 13.5 or approximately 6.8 nm. In the present case, the illumination radiation 14 is EUV radiation, so all optical elements of the exposure beam path of the projection exposure system 10 are designed as mirrors.
[0050] The field facet mirror 22 comprises a two-dimensional grid of individual optics in the form of mirror elements 24. In alternative embodiments for illumination radiation in the UV wavelength range, the individual optics can also be designed as lenses. In the left-hand area of Fig. 2, the field facet mirror 22 according to Fig. 1 is shown in plan view with an exemplary embodiment of the grid with three by three mirror elements 24. These are numbered 1 to 9. In further embodiments, the field facet mirror 22 can comprise fewer or more mirror elements 24. The respective shape of the mirror elements 24 is adapted to the shape of the illumination field 46 in the mask plane 44 and is therefore rectangular or crescent-shaped.
[0051] In the case of a projection exposure system 10 designed as a step and scan exposure system, the illumination field 46 is understood to be the area on the mask 40 illuminated by the scanner slit at a given time. The two-dimensional grid of the mirror elements 24 is orthogonal in the embodiment shown. In Fig. 1, the field facet mirror 22 is shown in a sectional view along a section line 26 from Fig. 2. The aforementioned mirror elements 24-4 to 24-6 are arranged along this section line 26. Each of the mirror elements 24 of the field facet mirror 22 is mounted for individual adjustment by means of a respective manipulator 28-4 in the form of an actuator. In particular, individual tilting of the respective mirror element 24 about two mutually orthogonal tilt axes is possible. The manipulators 28-4, designed as actuators, can be controlled centrally.
[0052] The pupil facet mirror 30 also comprises a two-dimensional arrangement of individual optics in the form of mirror elements, which are designated by the reference numeral 32. In the right-hand area of Fig. 2, the pupil facet mirror 30 is shown in plan view with an exemplary embodiment of an arrangement of thirty-six mirror elements 32. These are numbered 1 to 36. In further embodiments, the pupil facet mirror 30 can comprise fewer or more, in particular several hundred or several thousand, mirror elements 32.
[0053] According to one embodiment, the field facet mirror 22 and the pupil facet mirror 30 together comprise several tens of thousands of mirror elements or more than one hundred thousand mirror elements. In this case, the facet mirrors 22 and 30 can each be designed as MEMS mirror grids, wherein, according to one embodiment, the individual mirror elements can be grouped into functional units. The mirror elements combined in a functional unit can, for example, each assume the function of one of the mirror elements 24 and 32, respectively, according to Fig. 2.
[0054] In the embodiment shown in Fig. 2, the number of mirror elements 32 of the pupil facet mirror 30 is four times the number of mirror elements 24 of the field facet mirror 22. In other embodiments, the number of mirror elements 32 of the pupil facet mirror 30 can also be comparatively larger or smaller. In particular, the number of mirror elements 32 of the pupil facet mirror 30 can be less than or more than four times the number of mirror elements 24 of the field facet mirror 22. In the embodiment shown, the mirror elements 32 are hexagonal and arranged along concentric circles, so that the overall arrangement resembles a honeycomb structure.
[0055] In Fig. 1, the pupil facet mirror 30 is shown in a sectional view along a section line 33 from Fig. 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 the facet mirrors 22 and 30 can also be designed, in particular, according to one of the variants described in US 201 1 / 0001947 A1.
[0056] Each of the thirty-six 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 element 24-1 to 24-9 of the field facet mirror 22 assigned to the corresponding mirror element 32, via the corresponding mirror element 32 and via the G-mirror 36 explained in more detail below, up to the mask plane 44.
[0057] To activate corresponding radiation channels 35-1 to 35-36, the corresponding mirror elements 32-1 to 32-36 of the pupil facet mirror 30 are illuminated by appropriately tilting the mirror elements 24-1 to 24-9 of the field facet mirror 22. Since the mirror elements 24-1 to 24-9 can each illuminate only one of the mirror elements 32-1 to 32-36, a maximum of nine of the radiation channels 35-1 to 35-36 can be activated simultaneously.
[0058] In the illumination setting 62a of the illumination system 20 shown in Fig. 1, the mask 40 is illuminated with an angular distribution corresponding to the illumination distribution 60a in the pupil plane 31 shown on the left in Fig. 3. For this purpose, the exposure radiation 14 is radiated onto the illumination field 46 only by the mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5, ie, only the radiation channels 35-15, 35-30, 35-28, 35-22, 32-24, and 35-5 are active. In the sectional view of Fig. 1, only the mirror elements 32-15 and 32-5 of the pupil facet mirror 30 are active, ie only these mirror elements are illuminated by the mirror elements 24 of the field facet mirror 22 assigned to them in order to form the radiation channels 35-15 and 35-5 which respectively illuminate the entire illumination field 46.
[0059] The illumination distribution 60a in the pupil plane is also referred to herein as the illuminated pupil area. This consists of the totality 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 section. In the illustrated embodiment, these surface sections have a hexagonal shape and are each separated from one another by a distance.
[0060] To form the illumination distribution 60a, the mirror element 32-30 of the pupil facet mirror 30 can be illuminated by the mirror element 24-1 of the field facet mirror 22, the mirror element 32-15 by the mirror element 24-4, the mirror element 32-28 by the mirror element 24-7, the mirror element 32-22 by the mirror element 24-3, the mirror element 32-5 by the mirror element 24-6, and the mirror element 32-24 by the mirror element 24-9. The mirror elements 24-2, 24-5, and 24-8 of the field facet mirror 22 are tilted such that the radiation component of the input wave 16 impinging on them does not impinge on the pupil facet mirror 30.
[0061] 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 leading via the pupil facet mirror 30 and the G-mirror 36 to the mask plane 44 form, in the illustrated embodiment, an illumination beam path 34 in the illumination system 20. The G-mirror 36 is a grazing incidence mirror, also referred to as a "grazing incidence mirror." The wave radiated from the G-mirror 36 onto the mask plane 44 is also referred to as the output wave 38 of the illumination system 20.
[0062] The illumination beam path 34 comprises a plurality of individual beams 39. In this context, an individual beam is understood to be a light path within the beam path, represented by a line. Each of the radiation channels active in the setting illustrated in Fig. 1 comprises a bundle of individual beams.
[0063] Only a few individual beams of this beam are shown in Fig. 1 as examples. These are the individual beams 39-1 and 39-3 delimiting the radiation channel 35-15 in the drawing plane, as well as the individual beam 39-2 running centrally in the radiation channel 35-15. Also shown are the individual beams 39-4 and 39-6 delimiting the radiation beam radiated onto the mirror element 24-5, as well as the individual beam 39-5 running centrally in this radiation beam. Furthermore, the individual beams 39-7 and 39-9 delimiting the radiation channel 35-5 in the drawing plane, as well as the individual beam 39-8 running centrally in the radiation channel 35-5, are shown. The central individual beams 39-2, 39-5 and 39-8, which extend from the illumination radiation source 12 to the mask plane 44, are shown in Fig. 1 only in the area between the illumination radiation source 12 and the field facet mirror 22 to simplify the illustration.The field facet mirror 22 is mounted as a whole in a positionally adjustable manner relative to a frame element 29-1 of the illumination system 20 by means of a manipulator 28-1. The manipulator 28-1 is configured as an adjustment device with which several rigid body degrees of freedom, in particular all six rigid body degrees of freedom, i.e., translations and rotations, can be adjusted with respect to all three orthogonal spatial directions, as indicated by arrows in Fig. 1. The adjustment device can be manually adjustable or can also have electrically controllable actuators. The individual mirror elements 24-1 to 24-9 of the field facet mirror 22 are further mounted in an individually adjustable manner by means of the manipulators 28-4. According to the described embodiment, each of the mirror elements 24 can be tilted about two mutually orthogonal tilt axes.
[0064] Both the pupil facet mirror 30 as a whole and the G-mirror 36 are mounted so as to be adjustable in position relative to a frame element 29-2 or 29-3 of the illumination system 20 by means of a corresponding manipulator 28-2 or 28-3. Analogous to the manipulator 28-1 assigned to the field facet mirror 22, the manipulators 28-2 and 28-3 are each configured as an adjustment device with which several rigid body degrees of freedom, in particular all six rigid body degrees of freedom, i.e., translations and rotations with respect to all three orthogonal spatial directions, can be adjusted. The adjustment devices can be manually adjustable or also have electrically controllable actuators.
[0065] As mentioned above, the mask 40 is exposed multiple times, with the mask structures 42 being imaged onto the fields 64 of a plurality of semiconductor substrates 52. During the time required for this, the illumination system 20 is operated in at least two different illumination settings—in the exemplary embodiment according to Fig. 3, a first illumination setting 62a and a second illumination setting 62b. The exposure begins approximately with the first illumination setting 62a, in which, as described above, only the radiation channels 35-30, 35-15, 35-28, 35-22, 35-24, and 35-5 are used.
[0066] Due to this highly inhomogeneous illumination of the illumination beam path 34 and thus also of the imaging beam path 48 in the projection lens 50, wavefront errors can develop in the projection lens 50 due to thermal heating of the relevant optical elements in the projection lens 50 and resulting surface changes to these. To prevent this, after a certain period of exposure operation, e.g., after the exposure of one or more batches of semiconductor substrates 52, or even after exposure of some semiconductor substrates 52 of a batch or after exposure of some fields 64 of a semiconductor substrate 52, the illumination is switched to the second illumination setting 62b.After a certain additional period of exposure operation, before wavefront errors can also develop due to the new radiation signature in the imaging beam path 48 caused by heating of the relevant optical elements, the illumination setting is reset to the first illumination setting 62a. This cycle can be repeated multiple times.
[0067] In the second illumination setting 62b, the pupil plane 31 is illuminated with a second illumination distribution 60b. The exposure radiation 14 is no longer radiated onto the mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5, as in the illumination distribution 60a, but instead onto the mirror elements 32-16, 32-29, 32-14, 32-23, 32-4, and 32-6. The pupil area illuminated in the first illumination setting 62a, which corresponds to the totality of the surfaces 32o of the mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5, has no overlap with the pupil area illuminated in the second illumination setting 62b, which corresponds to the totality of the surfaces of the mirror elements 32-15, 32-30, 32-28, 32-22, 32-24, and 32-5.Thus, in the second illumination setting 62b, only the radiation channels 35-16, 35-29, 35-14, 35-23, 35-4, and 35-6 are used, which are completely different radiation channels than in the first illumination setting 62a. The signature of thermal heating occurring in the respective optical elements in the projection lens 50 in the second illumination setting 62b is thus so different from the signature occurring in the first illumination setting 62a that surface changes on the optical elements of the projection lens 50, which may develop in the initial stages due to the signature of the first illumination setting 62a and cause wavefront errors, do not increase further, but may even regress.
[0068] The signature of the second illumination setting 62b can, of course, cause other surface changes on the optical elements of the projection lens 50, which can also lead to wavefront errors. Therefore, operation in the second illumination setting 62b is preferably switched back to operation in the first illumination setting 62a after the aforementioned certain additional period of time, which is short enough that these wavefront errors also cannot develop to a disturbing extent.
[0069] According to an embodiment illustrated in Fig. 4, switching from the first illumination setting 62a to the second illumination setting 62b can be performed step by step. In each step, the illumination of a mirror element 32 or a group of mirror elements 32 is switched to another mirror element 32 or another group of mirror elements 32.
[0070] For this purpose, 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 between different radiation channels 35 one after the other.
[0071] In other words, the illumination of a surface section or a subgroup of the surface sections of a first configuration of surface sections in the pupil plane 31, defined by the first illumination setting 62a, is switched step by step to the illumination of another surface section or another subgroup of the surface sections of a second configuration of surface sections in the pupil plane 31, defined by the second illumination setting 62b. This step-by-step switching can 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 takes place in three sections.
[0072] In a first step, a transition from the illumination distribution 60a to a first intermediate illumination distribution 60z1 occurs by switching the respective illumination of the mirror elements 32-30 and 32-22 to the mirror elements 32-16 and 32-4. This occurs by appropriately tilting the mirror elements 24-1 and 24-3. In a second step, a transition from the first intermediate illumination distribution 60z1 to a second intermediate illumination distribution 60z2 occurs by switching the respective illumination of the mirror elements 32-15 and 32-5 to the mirror elements 32-29 and 32-23. This occurs by appropriately tilting the mirror elements 24-4 and 24-6. In a third step, a transition from the second intermediate illumination distribution 60z2 to the second illumination distribution 60b takes place by switching the respective illumination of the mirror elements 32-28 and 32-24 to the mirror elements 32-14 and 32-6.This is done by tilting the mirror elements 24-7 and 24-9 accordingly.
[0073] Fig. 5 shows a further embodiment in which the pupil area illuminated in the first illumination setting 62a has a certain overlap with the pupil area illuminated in the second illumination distribution 62b. The pupil area illuminated in the first illumination setting 62a corresponds to the total surfaces of the 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 area illuminated in the second illumination setting 62b corresponds to the total of the surfaces 32o of the 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 the illumination distributions 62a and 62b affects the surfaces of the mirror elements 32-17, 32-13, 32-3, and 32-7, and thus four of the ten illuminated mirror elements. The overlap thus amounts to approximately 40% of the illuminated pupil area.According to further embodiments, the overlap can be a smaller or a larger percentage, but a maximum of 90% of the illuminated pupil area. Since at least a portion of the pupil area 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 lead to wavefront errors not forming or forming to a lesser extent.
[0074] The illumination distributions 60a, 60b and possibly 60z1 and 60z2 in the pupil plane 31 illustrated in Figures 3, 4 and 5 correspond, according to one embodiment variant, to the respective angular distributions of the exposure radiation 14 radiated onto the various illuminated field points 43 on the mask 40. According to another embodiment variant, the angular distribution varies somewhat from field point 43 to field point 43, in this case the illumination distributions 60a, 60b and possibly 60z1 and 60z2 illustrated in Figures 3, 4 and 5 relate at least to a uniform field point 43, ie in each case to the same field point or several of the same field points.
[0075] According to a further embodiment, a plurality of field points 43, which in particular form a contiguous region 45 in the mask plane 44 (cf. Fig. 1), are assigned respective illumination distributions in the individual illumination settings 62a and 62b, which either correspond to the illumination distributions shown in Figures 3, 4 and 5 or deviate therefrom by a maximum of 5%, in particular by a maximum of 1%. In other words, according to this embodiment, the illumination distributions 60a and 60b described for the respective illumination settings 62a and 62b are present in the pupil plane for the field points 43 arranged in the contiguous region 45, wherein the illumination distributions 60a and 60b are still referred to as the same illumination distribution even if the pupil area deviates by a maximum of 5%.
[0076] Fig. 6 illustrates lithographic process windows for a given mask structure 42 at different illumination settings. These process windows are each represented by an area in a diagram in which a defocus Af of the image of the mask structure 42 in the image plane 53 is plotted against a dose variation AD of the exposure radiation 14 generated by the exposure radiation source 12. The aforementioned area is formed by the region in the Af-AD diagram that is bordered by a process window curve 66 and the Af and AD coordinate axes of the diagram. At all points of the process window represented by the area, a lithographic parameter lies within a tolerance range around a target value.The lithographic parameter may, for example, be a variation of a critical dimension, such as a line width in the photoresist (also referred to as CD variation) of the mask structure 42 imaged onto the semiconductor substrate 52 with the respective illumination setting.
[0077] In Fig. 6, an optimized process window 68o for the aforementioned predefined mask structure 42 is shown by a process window curve 66o executed as a dashed line. This means that the process window 68o is based on an illumination distribution in the pupil plane 31 in which the area of the process window 68o is maximized while maintaining the proportions of the process window 68o. However, this process window 68o is only present in this size at the beginning of an exposure process, as long as thermal heating effects in the projection lens 50 do not yet play a significant role. After a certain period of exposure operation, however, thermal heating effects lead to wavefront aberrations, which are also referred to as thermal wavefront aberrations in this text.These wavefront aberrations cause the process window curve 66o to shift to smaller AD and AF values (see process window curve 66ot), thus reducing the process window 68o to a correspondingly smaller process window 68ot. In one embodiment, the illumination distributions 60a and 60b are each selected in the illumination settings 62a and 62b such that the respective area of the associated process windows 68a and 68b, which result without taking thermal wavefront aberrations into account, is each at most 10%, in particular at most 20%, smaller than the area of the optimized process window 68o. The process windows 68a and 68b are each defined by the process window curves 66a and 66b. According to one embodiment, the respective area of the process windows 68a and 68b is each at least 5% smaller than the area of the optimized process window 68o.
[0078] Although the process windows 68a and 68b are each smaller than the optimized process window 68o, they are larger than the process window 68ot that develops after some time due to thermal wavefront aberrations. During prolonged exposure operation of the projection exposure system 10 in one of the illumination distributions 60a and 60b, the corresponding process window would also shrink, namely the process window 68a to a process window 68at defined by a process window curve 66at, or the process window 68b to a process window 68bt defined by a process window curve 66bt. This is prevented, however, by switching back and forth between the illumination settings 60a and 60b, as explained above, before thermal wavefront aberrations can develop.
[0079] Thus, by using the lighting settings 62a and 62b with the process windows 60a and 60b, a larger process window can be permanently ensured than with the continued use of a lighting setting assigned to the optimized process window 68o.
[0080] According to a further embodiment, one of the two illumination settings 60a and 60b may be configured to generate the optimized process window 68o and the other illumination setting may correspond to a process window that is smaller than the process window 68a or the process window 68b.
[0081] The above description of exemplary embodiments, forms of embodiment, or variant embodiments is to be understood as exemplary. The disclosure thus made enables one skilled in the art, on the one hand, to understand the present invention and the associated advantages, and on the other hand, also encompasses obvious variations and modifications of the described structures and methods to the understanding of one skilled in the art. Therefore, all such variations and modifications, insofar as they fall within the scope of the invention as defined in the appended claims, as well as equivalents, are intended to be covered by the protection of the claims.
[0082] List of reference symbols
[0083] 10 projection exposure system
[0084] 12 Exposure radiation source
[0085] 14 Exposure radiation
[0086] 16 Input shaft
[0087] 18 Source point
[0088] 20 Lighting system
[0089] 22 field facet mirrors
[0090] 23 Field level
[0091] 24, 24-1 to 24-9 mirror elements
[0092] 26 Cutting line
[0093] 28-1 Manipulator of the field facet mirror
[0094] 28-2 Manipulator of the pupil facet mirror
[0095] 28-3 G-mirror manipulator
[0096] 28-4 Manipulator of the mirror elements of the field facet mirror
[0097] 29-1 to 29-3 frame elements
[0098] 30 pupil facet mirrors
[0099] 31 Pupillary plane
[0100] 32, 32-1 to 32-32 mirror elements
[0101] 32o Surface of a mirror element
[0102] 33 Cutting line
[0103] 34 Illumination beam path
[0104] 35-1 to 35-32 radiation channels
[0105] 36 G mirrors
[0106] 36a Mirror surface
[0107] 38 Output shaft
[0108] 39 single beam
[0109] 40 Mask
[0110] 41 system area
[0111] 42 mask structures
[0112] 42P point on the mask surface 43 field points
[0113] 44 Mask layer
[0114] 45 contiguous area in the mask layer
[0115] 46 lighting field
[0116] 48 imaging beam path
[0117] 50 projection lens
[0118] 52 semiconductor substrate
[0119] 53 Image plane
[0120] 54 Substrate table
[0121] 58 Beam direction
[0122] 60a first illumination distribution in the pupil plane
[0123] 60b second illumination distribution in the pupil plane
[0124] 60z1 first intermediate lighting distribution
[0125] 60z2 second intermediate lighting distribution
[0126] 62a first lighting setting
[0127] 62b second lighting setting
[0128] 64 field on semiconductor substrate
[0129] 66a Process window curve
[0130] 66at process window curve with thermal aberrations
[0131] 66b Process window curve
[0132] 66bt process window curve with thermal aberrations
[0133] 66o optimized process window curve
[0134] 66ot optimized process window curve with thermal aberrations
[0135] 68a Process window
[0136] 68at process window with thermal aberrations
[0137] 68b Process window
[0138] 68bt process window with thermal aberrations
[0139] 68o optimized process window
[0140] 68ot optimized process window with thermal aberrations
Claims
Claims 1. A method for operating a projection exposure apparatus for microlithography, in which: a mask is repeatedly exposed to exposure radiation provided by an illumination system, and mask structures are thereby imaged on each of a plurality of fields of a plurality of semiconductor substrates, wherein, in a period in which the repeated exposure of the mask takes place, the illumination system is used successively in at least two different illumination settings of the illumination system, in which such different illumination distributions of the exposure radiation are present in a pupil plane of the illumination system,that a pupil area illuminated in the first illumination setting has no overlap with a pupil area illuminated in the second illumination setting or an overlap of a maximum of 90% of the respectively illuminated pupil area, and wherein the mask is fully exposed at least once in each of the two different illumination settings.
2. Method according to claim 1, wherein during the repeated exposure of the mask, the two different illumination settings are alternated at intervals of less than 200 minutes.
3. Method according to claim 1 or 2, wherein the pupil surfaces illuminated in the different illumination settings each have a plurality of mutually delimited surface sections.
4. Method according to claim 3, in which a change is made from a first of the two different illumination settings with a first configuration of surface sections in the pupil plane to the second illumination setting with a second configuration of surface sections in the pupil plane by stepwise switching from an illumination of a surface section or a subgroup of the surface sections of the first configuration to an illumination of another surface section or another subgroup of the surface sections of the second configuration.
5. Method according to one of the preceding claims, in which a change is made from a first of the two different lighting settings to the second lighting setting in such a way that the first lighting distribution assigned to the first lighting setting is gradually adapted to the second lighting distribution assigned to the second lighting setting.
6. The method according to claim 5, wherein the illumination system comprises a pupil facet optic arranged in a pupil plane of the illumination system with a plurality of individual optics and a field facet optic arranged in a plane conjugate to the mask plane, wherein the field facet optic has a plurality of further individual optics which are configured to illuminate the individual optics of the pupil facet optic to respectively form a radiation channel of the beam path of the illumination radiation, wherein during the step-by-step adaptation of the first illumination distribution to the second illumination distribution, switching takes place between different radiation channels by successively moving one or more of the individual optics of the field facet optic.
7. Method according to one of the preceding claims, in which the illumination distributions in the pupil plane present for the different illumination settings are each assigned to at least one uniform field point in a mask plane of the projection exposure system.
8. Method according to one of the preceding claims, in which, in the respective illumination setting, the relevant illumination distribution or an illumination distribution deviating by a maximum of 5% is assigned to a plurality of field points in a mask plane of the projection exposure system.
9. The method of claim 8, wherein the plurality of field points form a contiguous region in the mask plane.
10. Method according to one of the preceding claims, wherein, for each of the illumination settings, an area of a lithographic process window for imaging a predetermined type of mask structures without taking into account thermal wavefront aberrations which are due to thermal heating effects in a projection lens of the projection exposure apparatus caused by the exposure radiation is at most 20% smaller than the area of an associated optimized lithographic process window which is optimized for imaging the predetermined type of mask structures by varying the illumination setting.
11. The method of claim 10, wherein at one of the illumination settings the area of the lithographic process window is optimized for imaging a given type of mask structures without taking thermal wavefront aberrations into account.
12. Method according to claim 10 or 11, in which, for at least one of the illumination settings, the area of the lithographic process window for imaging a given type of mask structures without taking thermal wavefront aberrations into account is at least 5% smaller than the area of the associated optimized process window.
13. Method according to one of the preceding claims, wherein, in the configuration of the mask structures and the illumination distribution of at least one of the illumination settings, thermal heating effects in a projection lens of the projection exposure apparatus are taken into account, which occur within a period in which the mask is exposed with the respective illumination setting.
14. Method according to one of the preceding claims, wherein the projection exposure apparatus is designed for an operating wavelength in the EUV wavelength range.