Method for heating an optical element in a microlithography projection exposure device and an optical system

Abstract

The present invention relates to a method for heating an optical element in a microlithography projection exposure device and an optical system. In the method for heating an optical element in a microlithography projection exposure device, a heating output is introduced to the optical element using a heating array (25), said heating output is controlled based on a set value, said set value changes over time during the operation of the projection exposure device (1), and the change in the set value for the heating output includes a simulation of the effect of the change in the heating output with respect to the actual value based on a model of the thermal behavior of the optical element.

Description

Technology Field

[0001] This application claims priority to German patent application DE 10 2021 201 258.2 filed on February 10, 2021. The contents of this DE application are incorporated by reference into this specification.

[0002] The present invention relates to a method for heating an optical element in a microlithography projection exposure device and an optical system. Background Technology

[0003] Microlithography is used to produce microstructure components, such as integrated circuits or LCDs. The microlithography process is performed in a known projection exposure device having an illumination device and a projection lens. An image of a mask (=reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (e.g., a silicon wafer) that is aligned in the image plane of the projection lens and coated with a photosensitive layer (photoresist) in order to transfer the mask structure to a photosensitive coating of the substrate.

[0004] In projection lenses designed for the EUV range, that is, for example, at a wavelength of approximately 13 nm or approximately 7 nm, mirrors are used as optical components of the imaging process due to the lack of availability of suitable light-transmitting refractive materials.

[0005] One problem that actually occurs is that, among other reasons, as a result of the absorption of radiation emitted by the EUV light source, the EUV mirror heats up and undergoes associated thermal expansion or deformation, which can eventually have an adverse effect on the imaging characteristics of the optical system.

[0006] Various approaches are known to avoid surface deformation caused by thermal input into EUV mirrors and associated optical aberrations.

[0007] In particular, ultra-low thermal expansion materials ("Ultra Low Expansion Material"), for example, ULE TM It is known that titanium silicate glass marketed by Corning Inc. under the name [Name] is used as a mirror substrate material, and that a temperature known as the zero-crossing temperature is set in the region near the optically effective surface. For example, ULE TM In the case of this zero-crossing temperature where J = 30°C, the coefficient of thermal expansion has zero crossing in its temperature dependence, and near this point, there is no thermal expansion of the mirror substrate material or only a negligible amount of thermal expansion occurs.

[0008] Possible additional approaches to avoid surface deformation caused by thermal input to EUV mirrors include, for example, the use of a heating arrangement based on infrared radiation. With such a heating arrangement, active mirror heating can occur at relatively low absorption levels of EUV radiation and decreases correspondingly as the absorption of EUV radiation increases. In this case, one or more temperature sensors may be attached to the EUV mirrors, and the heating power introduced to each EUV mirror by the heating arrangement is controlled based on a set value for the temperature achieved at the location of the relevant temperature sensor.

[0009] However, in practice, an additional problem arises regarding the effect of the temperature measured at each temperature sensor location, which differs from the ultimately relevant temperature of the optically effective surface of the EUV mirror, in that the aforementioned heating output control ultimately fails to sufficiently avoid unwanted heat input and the associated surface deformation or optical aberrations due to the use of lighting settings with varying intensities for the optically effective surface of the mirror and an environment where the temperature measured at each sensor location "trails" the actual surface temperature.

[0010] With respect to the prior art, refer only by example to DE 10 2017 207 862 A1, DE 10 2013 204 427 A1 and DE 10 2017 205 405 A1.

[0011] The object of the present invention is to provide a method and an optical system for heating an optical element in a microlithography projection exposure apparatus, thereby enabling effective avoidance of surface deformation caused by heat input to the optical element and the associated optical aberrations.

[0012] This objective is achieved by a method and an optical system according to the features of each alternative independent patent claim.

[0013] In a method for heating an optical element in a microlithography projection exposure device, a heating output is introduced to the optical element using a heating array, the heating output is controlled based on a set value, the set value changes over time during the operation of the projection exposure device, and the change in the set value for the heating output includes a simulation of the effect of each change in the heating output relative to the actual value based on a model of the thermal behavior of the optical element.

[0014] The present invention is based on the concept of realizing dynamic closed-loop control of a heating output introduced into an optical element of a microlithography projection exposure device by a heating array based on a set value that is variably set over time during the operation of the relevant projection exposure device, thereby describing different operating states of the optical system or projection exposure device or considering information available in relation thereto during the operation of the optical system.

[0015] The present invention includes a principle of determining, in particular, the effect of a change in heating output introduced to an optical element by a heating array relative to each (actual) value in a model-aided manner, and then, depending on the effect thus determined, arbitrarily specifying a new heating output set value for said closed-loop control during the operation of the optical system. In this case, the effect of the heating output change described above may be particularly related to the wavefront characteristics of the optical system (e.g., the image of the projection lens of a microlithography projection exposure device or the wavefront provided on the wafer plane). Furthermore, the model-aided determination of the effect of the change in each heating output may include the implementation of optical forward propagation for the relevant optical system.

[0016] In addition, the wavefront provided by the optical system at a specific plane (e.g., the wafer plane of the projection lens) can also be measured. In particular, the corresponding measurement results can be used for the forward propagation described above or for the correction of the model.

[0017] As a result, the closed-loop control of the heating output according to the present invention, based on a time-varying setpoint during the operation of the optical system, can be used for the dynamic adaptation of each heating process to the actual existing and fluctuating operating state of the optical system, and in particular, online simulation and future propagation of the thermal state can be performed in each case.

[0018] According to the embodiment, the setting value is changed by taking into account the lighting setting currently set in the projection exposure device.

[0019] According to the embodiment, the setting value is changed by taking into account the reticle currently used in the projection exposure device.

[0020] According to an embodiment, the set value is changed based on the measurement of the intensity distribution currently existing on a predetermined plane of the projection exposure device.

[0021] According to an embodiment, a model is generated by considering a known spatial distribution of zero-crossing temperatures in the material of an optical element.

[0022] According to an embodiment, a model is generated using an artificial intelligence method, and the model is trained using a plurality of training data in the learning phase, and the training data each include a value of heating output and a wavefront characteristic of a projection exposure device assigned to this value.

[0023] According to the embodiments, these training data are provided at least partially based on model-based simulations of expected wavefront characteristics for different operating conditions of a projection exposure device.

[0024] According to additional embodiments, these training data are provided at least partially based on wavefront characteristics previously measured in a projection exposure device for different operating conditions.

[0025] According to an embodiment, the optical element is heated in such a way that the spatial and / or temporal change in the temperature distribution in the optical element is reduced.

[0026] According to an embodiment, the optical element is heated in such a way that optical aberrations occurring elsewhere in the projection exposure device are at least partially compensated.

[0027] According to the embodiment, the optical element is a mirror.

[0028] According to the embodiment, the optical element is designed for an operating wavelength of less than 30 nm, particularly less than 15 nm.

[0029] The present invention also relates to an optical system of a microlithography projection exposure apparatus having at least one optical element, a heating array for heating the optical element, and a control unit for closed-loop control of a heating output introduced to the optical element by the heating array based on a set value, wherein the set value changes over time during the operation of the optical system and the change in the set value for the heating output includes a simulation of the effect of each change in the heating output compared to the actual value based on a model of the thermal behavior of the optical element.

[0030] In the embodiments, the optical system is configured to perform a method having the features described above. For the advantages of the optical system and additional preferred configurations, refer to the above description in relation to the method according to the present invention.

[0031] Further configurations of the present invention are apparent from the detailed description and dependent claims.

[0032] The present invention is described in more detail below based on exemplary embodiments illustrated in the accompanying drawings. Brief explanation of the drawing

[0033] FIG. 1 illustrates a schematic diagram of a possible structure of a microlithography projection exposure device designed to operate in EUV; FIG. 2 illustrates a highly simplified diagram to explain a basic possible procedure of the method according to the present invention for heating an optical element; FIG. 3 illustrates a diagram illustrating a possible procedure for a model-based online simulation to determine a new heating output setpoint, performed according to the method of FIG. 2. Specific details for implementing the invention

[0034] FIG. 1 illustrates a schematic diagram of a projection exposure apparatus (1) designed to operate in EUV and in which the present invention can be realized as an example.

[0035] According to FIG. 1, the projection exposure device (1) includes a lighting device (2) and a projection lens (10). The lighting device (2) serves to illuminate the object field (5) of the object plane (6) with radiation from a radiation source (3) through a lighting optical unit (4). Here, a reticle (7) arranged in the object field (5) is exposed. The reticle (7) is held by a reticle holder (8). The reticle holder (8) can be displaced, particularly in the scanning direction, by a reticle displacement drive (9). For illustrative purposes, a Cartesian xyz coordinate system is shown in FIG. 1. The x direction extends backward perpendicular to the plane of the drawing. The y direction extends horizontally, and the z direction extends vertically. The scanning direction extends along the y direction in FIG. 1. The z direction extends perpendicular to the object plane (6).

[0036] The projection lens (10) serves to form an image of the object field (5) onto the image field (11) of the image plane (12). The structure on the reticle (7) is formed onto the photosensitive layer of the wafer (13) arranged in the image field (11) area of ​​the image plane (12). The wafer (13) is held by a wafer holder (14). The wafer holder (14) can be displaced longitudinally, particularly in the y-direction, by a wafer displacement drive (15). The displacement of the reticle (7) by the reticle displacement drive (9) first and the displacement of the wafer (13) by the wafer displacement drive (15) second can be implemented to be mutually synchronized.

[0037] The radiation source (3) is an EUV radiation source. The radiation source (3) emits EUV radiation in particular, which is also referred to below as the use radiation or illumination radiation. In particular, the use radiation has a wavelength in the range of 5 nm to 30 nm. The radiation source (3) may be, for example, a plasma source, a synchrotron-based radiation source, or a free electron laser (FEL). The illumination radiation (16) emitted from the radiation source (3) is focused by a collector (17) and propagated to an illumination optical unit (4) through the intermediate focus of an intermediate focal plane (18). The illumination optical unit (4) includes a deflection mirror (19), and a first faceted mirror (20) (having a schematically shown facet (21)) and a second faceted mirror (22) (having a schematically shown facet (23)) arranged downstream therefrom in the beam path.

[0038] The projection lens (10) comprises a plurality of mirrors (Mi (i=1, 2, ...)) numbered sequentially according to their arrangement in the beam path of the projection exposure device (1). In the example illustrated in FIG. 1, the projection lens (10) comprises six mirrors (M1 to M6). Alternatives having four, eight, ten, twelve, or any other number of mirrors (Mi) are also possible. The second-to-last mirror (M5) and the last mirror (M6) each have a through-opening for the illumination radiation (16). The projection lens (10) is a double-shielded optical unit. The projection lens (10) has an image-side aperture number that can be greater than 0.3, and also particularly greater than 0.5, and even more particularly greater than 0.6.

[0039] During the operation of the microlithography projection exposure device (1), electromagnetic radiation incident on the optically effective surface of the mirror is partially absorbed and, as described in the introduction, causes heating and associated thermal expansion or deformation, which can eventually lead to damage to the imaging characteristics of the optical system. Each active mirror heating using a preheater can occur at a relatively low absorption stage of the EUV radiation used, as described in the introduction, and the active mirror heating is correspondingly reduced as the absorption of the EUV radiation used increases.

[0040] According to the present invention, the heating output introduced to at least one mirror by a corresponding heating array is now controlled based on a set value that changes over time during the operation of an optical system or a projection exposure device. Different operating states of the optical system are described in this manner, and information available during the operation of the optical system (e.g., an illumination setting currently set in the projection exposure device or a reticle currently used in the projection exposure device) is taken into account.

[0041] A heating array is schematically illustrated in FIG. 1 and labeled "25," and this heating array (25) is used to introduce a heating output to the mirror (M3) in the example. In this case, the present invention is not further limited with respect to the manner in which the heating output is introduced or the configuration of the heating array used for this purpose. Purely as an example, the heating output may be introduced by an infrared radiator or by an electrode arranged on an optical element or mirror to which a voltage may be applied and heated in a manner known in itself.

[0042] Furthermore, the present invention is not further limited with respect to the number of optical elements or mirrors to be heated, and as a result, the closed-loop control according to the present invention may be applied only to the heating of a single optical element or to the heating of multiple optical elements.

[0043] FIG. 2 initially illustrates a highly simplified drawing to explain an exemplary embodiment of the method according to the present invention.

[0044] According to FIG. 2, the effect of a change in heating output relative to each actual value of the wavefront characteristics provided by the optical system is determined in the function block (220) by simulation “online” or in a model-based manner during the operation of the optical system (i.e., based on a “thermal model” that simulates the thermal state of the optical element or mirror). This determination by simulation of the effect of each change in heating output is performed according to FIG. 2 based on input data provided to the function block (210), said input data includes both the light distribution existing downstream of the reticle in relation to the optical beam path and the current heating output P(t) radiated by the heating array. In particular, a thermal model can be generated by taking into account the known spatial distribution of the zero crossing temperature in the material of the optical element.

[0045] The determination of the effect of changing each heating output performed in the function block (220) may include model-based optical forward propagation in particular in an optical system, and for this purpose, online correction may be performed according to the function block (230) based on temperature sensor data (e.g., from at least one temperature sensor located on each optical element or mirror) and wavefront data (e.g., from a wavefront sensor located in a region of the wafer plane).

[0046] FIG. 3 illustrates a diagram for a more detailed explanation of a model-based determination of each new heating output set value during the operation of an optical system, implemented within the method according to the present invention.

[0047] According to FIG. 3, each temperature distribution, resulting deformation, resulting wavefront effect, and resulting phase formation characteristic is determined based on the input data described above according to the functional block (210) using a thermal model used as a reference according to the functional block (220). According to FIG. 3, this is implemented in both the reference path (240) (upper path in FIG. 3) and the comparison path (250) (lower path in FIG. 3). Existing parameter (M i Optical forward propagation for ,x,y,z,t0) is performed in the reference path (240), but with correspondingly changed parameters (M i ,x,y,z,t k Each forward propagation for ) is performed in a comparison path (250) according to a simulation of the changed heating output setpoint. As a result of the optimization performed, a new heating output setpoint is output and used as a standard for controlling the heating array.

[0048] Alternatively, optimization can also be performed solely by selecting the best parameters in relation to the merit function in the comparison path, that is, without explicit simulation of the reference path. The merit function can minimize specific characteristics of the wavefront that are particularly relevant to each application, such as wavefront aberrations (W), coma, or astigmatism. If the wavefront itself is optimized, the final step illustrated in Fig. 3, which calculates imaging characteristics based on the wavefront, is not necessary. However, the wavefront itself is often not the most relevant application variable, but rather the imaging characteristics occurring in the wavefront. For example, this may be an effect on the structural size of the relevant structure formed on the wafer (often denoted as "CD" = "critical dimension") or an effect on the position of the structure (i.e., an effect on "overlay"). This effect can be calculated from the wavefront using a simulation model, or specifically using a predetermined sensitivity to wavefront aberrations. These aberrations are typically minimized using the merit function.

[0049] In the embodiments, the thermal model used as a standard for determining the effect of each change in heating output may be generated using an artificial intelligence method, and the model is trained using multiple training data during the training phase. This training data may include values ​​for the heating output and wavefront characteristics of the projection lithography device associated with each of these values, and may be provided based on model-based simulations of expected wavefront characteristics for different operating states of the projection lithography device and / or based on wavefront characteristics measured by the projection lithography device for different past operating states. To generate appropriate training data through measurements of the projection lithography device, targeted "calibration measurements" may be performed, for example, during the startup of the device, during which combinations of measurement parameters considered representative for later applications are changed. In particular, training may be used with the help of measurements when sufficiently accurate simulations are not available. For example, this may be due to the available simulation model itself or due to the limited accuracy of the included conditions (e.g., actual material properties of the mirror or knowledge of thermal ambient conditions). Conversely, training with simulation data has the particular advantage of not requiring measurement time on the device and being able to model conditions that cannot be generated on the device during training. In particular, a combination of initial training with data generated from simulation and subsequent training with measurement data from the device itself during the learning phase can combine the advantages of both training methods. The use of artificial intelligence is attractive, particularly because the optimization of the heating output of the present invention preferably occurs during the operation of the device with high throughput ("online"). As a result, requirements regarding the time required for optimization become stringent, which can be met by the typical acceleration capabilities during the application phase of artificial intelligence (e.g., compared to complex physical simulations).

[0050] At least one optical element or mirror is heated in one application of the present invention to reduce spatial and / or temporal variation of the temperature distribution of the optical element, or to reduce thermally induced deformation associated therewith, but the present invention is not limited thereto. Accordingly, in further embodiments, at least one optical element or mirror may also be heated to at least partially compensate for optical aberrations occurring elsewhere in the projection exposure device.

[0051] Although the present invention has also been described based on specific embodiments, various modifications and alternative embodiments will be apparent to those skilled in the art, for example, by combinations and / or exchanges of features of individual embodiments. Accordingly, it goes without saying to those skilled in the art that such modifications and alternative embodiments are incidentally incorporated by the present invention, and that the scope of the present invention is limited only to the meaning of the appended claims and their equivalents.

Claims

Claim 1 A method comprising the steps of: using a heating array to introduce a heating output to an optical element within a microlithography projection exposure device, wherein the optical element includes a temperature sensor and the heating output is controlled based on a set value for a temperature achieved at the location of the temperature sensor; measuring wavefront data of the microlithography projection exposure device; modeling wavefront characteristics of the microlithography projection exposure device; comparing the measured wavefront data with the modeled wavefront characteristics; and changing the heating output by changing the set value for a temperature achieved at the location of the temperature sensor during operation of the microlithography projection exposure device based on the comparison of the measured wavefront data and the modeled wavefront characteristics. Claim 2 A method according to claim 1, comprising changing a set value by taking into account the lighting setting currently set in the microlithography projection exposure device. Claim 3 A method according to claim 1 or 2, comprising changing a set value by taking into account the reticle currently used in a microlithography projection exposure device. Claim 4 A method according to claim 1 or 2, comprising changing a set value based on the measurement of an intensity distribution currently existing on a predetermined plane of a microlithography projection exposure device. Claim 5 A method according to claim 1 or 2, comprising generating a model by considering a known spatial distribution of zero-crossing temperatures in the material of the optical element. Claim 6 A method according to claim 1 or 2, wherein a model is generated using an artificial intelligence method, the model is trained using a plurality of training data in a learning phase, and each training data includes a value of heating output and a wavefront characteristic of a microlithography projection exposure device assigned to such value. Claim 7 In claim 6, the training data is based at least partially on a model-based simulation of expected wavefront characteristics for different operating conditions of a microlithography projection exposure device. Claim 8 A method according to claim 6, characterized in that the training data is based at least partially on wavefront characteristics measured in a microlithography projection exposure device for different operating conditions. Claim 9 A method according to claim 1 or 2, comprising heating an optical element such that at least one of the spatial variation and the temporal variation of the temperature distribution in the optical element is reduced. Claim 10 A method according to claim 1 or 2, comprising heating an optical element to at least partially compensate for optical aberrations occurring elsewhere in a microlithography projection exposure device. Claim 11 A method according to claim 1 or 2, wherein the optical element comprises a mirror. Claim 12 A method according to claim 1 or 2, comprising operating a microlithography projection exposure device using an operating wavelength of less than 30 nm. Claim 13 A method according to claim 1, wherein changing the set value for the temperature achieved at the location of the temperature sensor includes simulating the respective effect of changing the heating output to the wavefront provided by the microlithography projection exposure device in the image of the projection lens of the microlithography projection exposure device or on the wafer plane. Claim 14 A method according to claim 1, comprising implementing optical forward propagation to a microlithography projection exposure device for determining model support for the effect of change in heating output. Claim 15 A method according to claim 14, comprising using a measurement of a wavefront provided on the plane of a microlithography projection exposure device to correct optical forward propagation or a model. Claim 16 A method for controlling the temperature of an optical element within a microlithography projection exposure device, wherein the optical element includes a temperature sensor, and the method comprises the steps of: changing a set value of the temperature achieved at the location of the temperature sensor during the operation of the microlithography projection exposure device based on a model of the thermal behavior of the optical element and a comparison of the simulation and actual values ​​of the respective effects of changing the heating output regarding the wavefront characteristics provided by the microlithography projection exposure device; and controlling the heating output introduced to the optical element using the set value. Claim 17 In claim 16, the step of changing the set value of the temperature achieved at the location of the temperature sensor comprises simulating the respective effect of changing the heating output on the image of the projection lens of the microlithography projection exposure device or the wavefront provided by the microlithography projection exposure device on the wafer plane. Claim 18 A method according to claim 16, comprising implementing optical forward propagation to a microlithography projection exposure device for determining model support for the effect of change in heating output. Claim 19 A method according to claim 18, comprising using a measurement of a wavefront provided on the plane of a microlithography projection exposure device to correct optical forward propagation or a model. Claim 20 An optical system comprising an optical element including a temperature sensor, a heating array configured to heat the optical element, and a control unit configured to provide closed-loop control of the heating output introduced to the optical element by the heating array based on a set value for the temperature at the location of the temperature sensor, wherein the set value may change over time during the operation of the optical system, and the change in the set value includes a simulation of the respective effect of the change in heating output compared to an actual value regarding the wavefront characteristics provided by the optical system based on a model for the thermal behavior of the optical element, and wherein the optical system is an optical system of a microlithography projection exposure device.

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