Method for operating an optical system

Abstract

Method for operating an optical system, wherein the method comprises the following steps: a) Sensor-based measurement of values ​​of at least one physical quantity at a plurality of different sensor positions in the optical system; b) Diagnosing an existing or expected malfunction of the optical system based on this measurement; characterized in that, based on the values ​​measured in step a), a model-based determination of at least one parameter is carried out at further positions, each not corresponding to any of the sensor positions, wherein the diagnosis in step b) is further carried out on the basis of this model-based determination.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The invention relates to a method for operating an optical system. State of the art

[0002] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure system, which has an illumination device and a projection lens. The image of a mask (= reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.

[0003] In projection lenses designed for the EUV range, ie at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process due to the lack of availability of suitable light-transmitting refractive materials.

[0004] In a known construction, the projection lens can have both a load-bearing support structure in the form of a support frame and an independently provided measuring structure in the form of a sensor frame, wherein both the support structure and the measuring structure are mechanically connected to a base of the optical system independently of one another via mechanical connections acting as dynamic decoupling.

[0005] A problem that occurs during the operation of a projection exposure system is that thermally induced deformations of the sensor frame can occur due to thermal influences (which include both the electromagnetic radiation acting during operation and heat dissipation on components such as actuators or heating devices), which ultimately cause optical aberrations during the operation of the projection exposure system.

[0006] A continuing problem in practice is that the temperature sensors used to determine the thermal state of the optical system or projection lens of the projection exposure system are only available in limited numbers and, in particular, are often not located at the respective positions of the components to be monitored for reliable operation. This circumstance, together with the existing complexity of the optical system composed of various modules, means that troubleshooting and the initiation of appropriate countermeasures, such as the replacement or maintenance of certain components, are only initiated with a delay (e.g., only after an unscheduled failure of the optical system has occurred), which undesirably limits the availability of the projection exposure system.

[0007] DE 10 2013 203 338 A1, WO 2009 / 039883 A1, and US 2012 / 0293781 A1 disclose methods for correcting the positions of optical elements of a projection exposure system. Temperatures are measured and numerical models are used to evaluate / predict / control system behavior. SUMMARY OF THE INVENTION

[0008] Against the above background, it is an object of the present invention to provide a method for operating an optical system which enables error detection as reliably and as early as possible and the planning of suitable countermeasures.

[0009] This object is achieved by the method according to the features of independent patent claim 1.

[0010] A method according to the invention for operating an optical system comprises the following steps: a) Sensor-based measurement of values ​​of at least one physical quantity at a plurality of different sensor positions in the optical system; and b) diagnosing an existing or expected malfunction of the optical system on the basis of this measurement;wherein, on the basis of the values ​​measured in step a), a model-based determination of at least one parameter is carried out at further positions, each of which does not correspond to any of the sensor positions, wherein the diagnosis in step b) is further carried out on the basis of this model-based determination.

[0011] The invention is based in particular on the concept of realizing the diagnosis of a malfunction (in particular by locating the corresponding causes of the error) during operation of an optical system with increased information density in that, in addition to the sensor-supported measurement of one or more physical variables (such as temperature), a suitable model is included in the diagnosis in order to additionally determine a parameter relevant for this diagnosis (e.g. heat load) at further positions that cannot be directly "observed" via the existing sensors.

[0012] As a result, the method according to the invention can be used to detect errors much more reliably and, in particular, earlier, as well as to plan appropriate countermeasures.

[0013] The at least one physical quantity measured in step a) may in particular comprise the temperature, but in further embodiments additionally or alternatively also comprise, for example, the wavefront provided by the optical system in a predetermined plane.

[0014] The at least one model-based determined parameter may in particular include the heat load.

[0015] According to one embodiment, the said further positions, each of which does not correspond to any of the sensor positions, are each located on a component of the optical system to be monitored with regard to its operation.

[0016] According to one embodiment, based on the model-based determination of at least one parameter, a countermeasure to eliminate or prevent the malfunction is planned at additional positions that do not correspond to any of the sensor positions. In particular, a warning or the like can also be issued, which may also include an indication of a suspected defective component.

[0017] According to one embodiment, this scheduling is additionally based on an assessment of the relevance of the malfunction. In particular, it can be taken into account if an impending failure of a component, for example, does not justify shutting down the entire optical system, so that in this case, the next, already scheduled maintenance break can also be used for a necessary replacement of the component in question.

[0018] According to one embodiment, the optical system is an optical system for microlithography, in particular a projection lens of a microlithographic projection exposure apparatus.

[0019] According to one embodiment, the sensors are arranged on a sensor frame of the projection exposure system. The additional positions, each of which does not correspond to any of the sensor positions, can be located, in particular, on a support frame of the projection exposure system.

[0020] Further embodiments of the invention can be found in the description and the dependent claims.

[0021] The invention is explained in more detail below with reference to embodiments shown in the attached figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] They show: Fig. 1 a schematic representation to explain an exemplary architecture in which a method according to the invention can be implemented; Fig. 2 is a diagram for explaining a principle underlying the present invention; and Fig. 3 a schematic representation of the possible structure of a microlithographic projection exposure system designed for operation in the EUV. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] Fig. 1 shows a merely schematic and highly simplified representation of a possible architecture within a microlithographic projection exposure system in which the method according to the invention can be implemented.

[0024] According to Fig. 1, a plurality of mirrors 101 are mounted on a load-bearing support structure in the form of a support frame 110, with actuators designated by "102" for positioning the mirrors 101. Furthermore, a measuring structure in the form of a sensor frame 120 is provided, which is dynamically decoupled from the support frame 110. Also shown in Fig. 1 shows cooling devices for the support frame 110, the sensor frame 120, and also for a heat shield located between the support frame 110 and the sensor frame 120, through which a cooling fluid flows. Specifically, "130" designates a thermal shield of the optical path, and "131" designates a heat shield (e.g., water-cooled) between the support frame 110 and the sensor frame 120, through which a cooling fluid flows.

[0025] According to the Fig. In the thermal architecture shown in Figure 1, a plurality of sensors 125 are used to measure the temperature at different positions.

[0026] A key feature of the method according to the invention is that, based on the sensor-based measured values ​​(in the example, temperature), a relevant parameter (in the example, the heat flow) is calculated model-based at other positions that do not correspond to the sensor positions and used as a basis for diagnosing an existing or expected malfunction of the optical system. In the specific example of Fig. 1, in particular the respective heat flow in the area of ​​the actuators 102 can be calculated on a model-based basis, so that any existing or impending malfunction of the actuators 102 can be diagnosed without having to resort to temperature sensors in the area of ​​the actuators 102, which are in the structure according to Fig. 1 are not present there. The model- and measurement-based method according to the invention can also be used to draw conclusions about other heat loads, such as heating devices or parasitic loads from electrical leads or cables, or even interface loads to the remaining part of the optical system (e.g., the projection exposure system).

[0027] As a result, the invention provides a significantly increased information density based on a model-based approach compared to the exclusive use of temperature-based measured values, which in turn enables fault detection and the initiation of appropriate countermeasures with greater reliability and, in particular, at a significantly earlier stage.

[0028] Fig. 2 shows a diagram with exemplary time-dependent temperature profiles at different positions of the optical system, where the solid curves correspond to the measured data recorded at different sensor positions. The dashed curves in Fig. 2, however, correspond to data that are calculated model-based at other positions (not corresponding to the sensor positions) as described above. It should be noted that the diagram of Fig. 2 is merely exemplary, whereby in particular the number of (dashed) curves or data which are determined model-based for the further positions (which do not correspond to the sensor positions) can also significantly exceed the number of sensor-based measured curves.

[0029] Based on a model, a relationship can be determined between the heat loads at different positions within the optical system or the projection lens and the measured temperatures: T=B⋅Q where T[K] is the measured temperature at different sensor positions and Q[W] is the dissipated heat flow of individual components. B[K / W] denotes a sensitivity matrix that can be determined using a thermal model for the optical system or projection lens and updated using measurements. Equation (1) can be represented in matrix notation as [T1t1T2t1T3t1⋯Tkt1T1t2T2t2T3t2⋯Tkt2T1t3T2t3T3t3⋯Tkt3⋯⋯⋯⋯⋯T1tnT2tnT3tn⋯Tktn]=[B1t1B2t1B3t1⋯Blt1B1t2B2t2B3t 2⋯Blt2B1t3B2t3B3t3⋯Blt3⋯⋯⋯⋯⋯B1tnB2tnB3tn⋯Bltn]⋅[Q11Q12Q13⋯Q1mQ21Q22Q23⋯Q2mQ31Q32Q33⋯Q3m⋯⋯⋯⋯⋯Ql1Ql2Ql3⋯Qlm]

[0030] The heat load can be defined model-based at any number of locations in the optical system or projection lens. The effects of this heat load on a specific temperature sensor are determined based on the entries in the sensitivity matrix B.

[0031] With a known relationship according to equation (1), the heat flow at various other positions in the optical system (each of which does not correspond to a sensor position) can be determined using a model and sensor-based temperature measurements to locate any existing thermal overload. Furthermore, measurements of other physical quantities (e.g., measurements of electrical voltage or electrical current) can be used to determine a change in actuator performance. This information can then be used to determine whether an existing excessive thermal load most likely originates in one or more of the actuators or at other positions in the optical system.

[0032] In further embodiments, sensor-based optical aberrations can also be used to determine the origin of a thermal overload. Thermal effects leave a specific signature of the overlay error, which can be used to localize thermal overloads in the optical system or projection lens. Similar to equation (1), the following relationship can be stated: LoS=T⋅M yt1yt2yt3⋯ytn=[T1t1T2t1T3t1⋯T28t1T1t2T2t2T3t2⋯T28t2T1t3T2tT3t3⋯T28t3⋯⋯⋯⋯⋯T1tnT2tnT3tn⋯T28tn]⋅[M1M2M3⋯Mn]

[0033] In one example, the optical measurement may show an increased overlay contribution, but the temperature measurement results suggest a thermal problem. Using the measured temperatures, this suspicion can be confirmed or refuted using Equation (3) based on a model. If confirmed, the system of equations (1) is then used with all available measured information to locate the source of the problem.

[0034] Fig. Figure 3 shows a schematic representation of a projection exposure system 1 designed for operation in the EUV range, in which the invention can be implemented, for example. The description of the basic structure of the projection exposure system 1 and its components is not intended to be limiting.

[0035] One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the illumination system. In this case, the illumination system does not include the light source 3.

[0036] 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 via a reticle displacement drive 9, in particular in a scanning direction. Fig. For illustrative purposes, a Cartesian xyz coordinate system is shown in Figure 1. The x-direction is perpendicular to the plane of the drawing. The y-direction is horizontal, and the z-direction is vertical. The scanning direction is in Fig. 1 along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0037] The projection lens 10 is used to project the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is projected onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the displacement of the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with each other.

[0038] Radiation source 3 is an EUV radiation source. Radiation source 3 emits, in particular, EUV radiation, which is also referred to below as useful radiation or illumination radiation. The useful radiation has, in particular, a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be, for example, a plasma source, a synchrotron-based radiation source, or a free-electron laser (FEL). The illumination radiation 16 emanating from the radiation source 3 is bundled by a collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optics 4. The illumination optics 4 has a deflection mirror 19 and, downstream of this in the beam path, a first facet mirror 20 (with schematically indicated facets 21) and a second facet mirror 22 (with schematically indicated facets 23).

[0039] The projection lens 10 has a plurality of mirrors Mi (i= 1, 2, ...), which are numbered according to their arrangement in the beam path of the projection exposure system 1. In the Fig. In the example shown in Figure 1, the projection lens 10 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection lens 10 is a doubly obscured optical system. The projection lens 10 has a numerical aperture on the image side that is greater than 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0040] Although the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments will become apparent to those skilled in the art, e.g., by combining and / or interchanging features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are encompassed by the present invention, and that the scope of the invention is limited only by the appended claims and their equivalents.

Claims

[1] A method for operating an optical system, the method comprising the following steps: a) Sensor-based measurement of values ​​of at least one physical quantity at a plurality of different sensor positions in the optical system; b) Diagnosing an existing or expected malfunction of the optical system on the basis of this measurement; characterized by that, on the basis of the values ​​measured in step a), a model-based determination of at least one parameter is carried out at further positions which do not correspond to any of the sensor positions, wherein the diagnosis in step b) is further carried out on the basis of this model-based determination. [2] Method according to claim 1, characterized by that the at least one physical quantity measured in step a) comprises temperature. [3] Method according to claim 1 or 2, characterized bythat the at least one physical quantity measured in step a) comprises the wavefront provided by the optical system in a given plane. [4] Method according to one of claims 1 to 3, characterized by that the at least one model-based parameter includes the heat load. [5] Method according to one of the preceding claims, characterized by that the other positions, each of which does not correspond to any of the sensor positions, are each located on a component of the optical system whose operation is to be monitored. [6] Method according to one of the preceding claims, characterized by that, based on the model-based determination of at least one parameter at other positions which do not correspond to any of the sensor positions, a countermeasure to eliminate or avoid the malfunction is automatically planned. [7] Method according to claim 6, characterized bythat this automatic scheduling is also based on an assessment of the relevance of the malfunction. [8] Method according to one of the preceding claims, characterized by that the optical system is an optical system for microlithography, in particular a projection lens of a microlithographic projection exposure system. [9] Method according to claim 8, characterized by that the sensors are arranged on a sensor frame (120) of the projection exposure system. [10] Method according to claim 8 or 9, characterized by that the further positions, each of which does not correspond to any of the sensor positions, are located on a support frame (110) of the projection exposure system.

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