Method for calibrating a heating element of a projection exposure system

Abstract

Method for calibrating a heating element (31, 32, 33, 34) designed to heat a component (M4, 42, 43, 44) of a projection exposure system. Heating the component (M4, 42, 43, 44) changes at least one wavefront parameter of a projection system (22) of the projection exposure system. The procedure comprises the following steps: tempering the component (M4, 42, 43, 44) to a first setpoint, activating the heating element (31, 32, 33, 34) starting from the first setpoint, so that the component (M4, 42, 43, 44) is heated starting from the first setpoint, recording a first wavefront parameter profile during the heating starting from the first setpoint, tempering the component (M4, 42, 43, 44) to a second setpoint different from the first, activating the heating element (31, 32, 33, 34) starting from the second setpoint, so that the component (M4, 42, 43, 44) is heated starting from the second setpoint.44) takes place, recording a second wavefront parameter profile during the heating process starting from the second setpoint, determining a difference profile between the first and second wavefront parameter profiles, simulating the difference profile using a wavefront parameter simulation model, and calibrating the heating element (31, 32, 33, 34) by comparing the determined difference profile with the simulated difference profile.

Description

[0001] The present invention relates to a method for calibrating a heating element.

[0002] For the fabrication of microstructured components, microlithographic projection exposure systems are used, comprising an illumination system and a projection system. These systems image a photomask located in the object plane of the projection system onto a wafer situated in an imaging plane. Structures on the photomask can thus be transferred to the wafer. Components not directly involved in the optical imaging, such as mounts, holders, or housing parts, as well as the optical elements themselves, such as lenses or, in the case of EUV lithography, mirrors, change their expansion or surface shape when heated or cooled, which directly affects the image quality of the projection system.It is therefore generally desirable to keep the imaging-relevant components, such as the optical elements themselves, in a steady thermal state during operation in order to avoid a deterioration of the image quality.

[0003] The heating of imaging-relevant components during operation can be caused, for example, by electrical components that heat up during operation, or by the absorption of some of the useful radiation used to image the photomask onto the wafer. Due to various operating states of the projection exposure system, which are associated with different thermal loads, the imaging-relevant components can be subject to undesirable temperature changes over time. It is generally known to provide heating elements with which the imaging-relevant components are actively heated at certain times to counteract these undesirable temperature changes (see DE 10 2017 207 862 A1, DE 10 2022 131 353 A1, DE 10 2022 205 814 A1).Particularly during a downtime of the system, for example when changing the wafer or photomask, and the associated cooling of the components, it can be advantageous to actively heat the system or the components in order to create a state in which the projection exposure system and its components are each set to temperatures that are close to the values ​​achieved in production operation.

[0004] To ensure that the relevant component for imaging is heated precisely, the heating elements used must be calibrated before use. Previously, temperature sensors were used to directly detect the heating of a component by a heating element and thus calibrate the heating element. The object of the present invention is to provide an improved method for calibrating a heating element. This object is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0005] Accordingly, the invention relates to a method for calibrating a heating element designed to heat a component of a projection exposure system. Heating the component modifies at least one wavefront parameter of a projection system within the projection exposure system. The method comprises the following steps: - Tempering the component to a first setpoint, - Activating the heating element starting from the first setpoint, so that the component is heated starting from the first setpoint, - Capturing an initial wavefront parameter profile during the heating process starting from the first setpoint, - Tempering the component to a second setpoint different from the first, - Activating the heating element starting from the second setpoint, so that the component is heated starting from the second setpoint, - Capturing a second wavefront parameter profile during the heating process starting from the second setpoint, - Determining a difference profile between the first wavefront parameter profile and the second wavefront parameter profile, - Simulating the difference profile using a wavefront parameter simulation model; - Calibrating the heating element by comparing the determined difference profile with the simulated difference profile.

[0006] Within the scope of the invention, it was discovered that highly precise calibration is possible with minimal effort by measuring at least one wavefront parameter. Upon activation of the heating element, the component heats up from the previously set point. This heating leads to a change in the wavefront parameters, allowing a wavefront parameter profile to be recorded. The measurement data acquired in this way directly provides information about how the wavefront parameter changes due to the component's heating. Furthermore, by recording the wavefront parameter over time from at least two different setpoints to which the component was previously heated, and by calculating a difference profile from this data, it is possible to further improve the calibration quality.In particular, it was recognized that inhomogeneities and non-linear dependencies in the difference curve of the wavefront parameter are eliminated during the difference calculation and therefore cannot have a disturbing influence on the calibration.

[0007] Calibration is performed by simulating the effect of component heating on the wavefront parameter using a generally known wavefront parameter simulation model. This simulation generates a simulated temperature difference profile, which is then compared to the measured temperature difference profile. The comparison may involve a mathematical fitting process, in which one or more calibration parameters are varied until a good or even optimal match between the simulated temperature difference profile and the measured temperature difference profile is achieved.

[0008] This method allows for the calibration of multiple heating elements by performing the procedure for each element. In contrast, with previously known calibration methods that rely on temperature sensors, it was often very time-consuming to provide a sufficient number of temperature sensors to calibrate all heating elements.

[0009] In one embodiment, the method serves to calibrate a first heating element, wherein the component is tempered to the first and / or the second setpoint using a second heating element different from the first. Alternatively, it is also possible for the method to serve to calibrate a heating element, wherein the component is tempered to the first and / or the second setpoint using the same heating element.

[0010] The component can be, in particular, an optical element. In this case, the heating element can be a so-called preheater, which serves to heat the entire optical element uniformly. Alternatively, the heating element can also be a sector heater, which serves to heat a local sub-area, a so-called sector, of the optical element. For example, the method can be carried out on an optical element that has both a preheater and one or more sector heaters for heating corresponding sectors. In this case, the preheater can be used to heat a sector of the optical element to the first setpoint, with the sector heater assigned to that sector then being activated from the setpoint to acquire the first wavefront parameter profile.The second setpoint can then be set using the preheater, after which the sector heater can be reactivated and the second wavefront parameter profile recorded. Alternatively, a setpoint can be set using a first sector heater, with a different sector heater then used to heat the system from the setpoint.

[0011] It can be provided that the activation of the heating element from the first setpoint and the activation of the heating element from the second setpoint are performed with the same heating power. It is possible that the activation of the heating element from the first and second setpoints is carried out in a first measurement series with a first heating power, and that the activation of the heating element from the first and second setpoints is carried out in a subsequent second measurement series with a second heating power different from the first. By using two different heating powers in two consecutive measurement series, the accuracy of the method can be further increased.

[0012] In one embodiment, the temperature difference between the first setpoint and the second setpoint is greater than the temperature difference traversed when activating the heating element from one of the setpoints during the acquisition of the wavefront parameter profile.

[0013] It may be planned that the acquisition of the first wavefront parameter profile and the acquisition of the second wavefront parameter profile each involve the acquisition of a plurality of wavefront parameters. The wavefront parameters to be measured can, for example, be described by Zernike coefficients of the projection system.

[0014] Advantageous embodiments of the invention are explained below by way of example with reference to the accompanying drawings. These show: Fig. 1: a schematic representation of a projection exposure system to illustrate the method according to the invention; Fig. 2: a partial excerpt from the Fig. 1 in an enlarged view; Fig. 3: a schematic representation of an optical element of the projection exposure system of the Fig. 1; Fig. 4: Exemplary wavefront parameter profiles recorded within the framework of the method according to the invention.

[0015] In Fig. Figure 1 shows a schematic representation of a microlithographic EUV projection exposure system. The projection exposure system comprises an exposure beam source 14, an illumination system 10, and a projection system 22, which are operated together in a vacuum chamber 23.

[0016] The exposure source 14 generates electromagnetic radiation in the EUV range, specifically with a wavelength between 5 nm and 30 nm. The exposure radiation emitted by the exposure source 14 is focused by a collector 15 into an intermediate focal plane 16. Exposure radiation passing from the intermediate focal plane 16 is directed by the illumination system 10 into an object plane 12, so that an object field in the object plane 12 is illuminated with uniform radiation intensity.

[0017] The illumination system 10 comprises a deflecting mirror 17, which deflects the illumination radiation onto a first faceted mirror 18. A second faceted mirror 19 is arranged downstream of the first faceted mirror 18. The facets of the first faceted mirror 18 are imaged onto the object plane 12 by the second faceted mirror 19. It is possible to set various illumination configurations with the illumination system 10, each of which can exhibit different intensity distributions in the object plane.

[0018] In the object plane 12, a photomask 13 is arranged, which is imaged onto an image plane 21 by means of a plurality of mirrors M1-M6 of the projection system 22. A structure formed on the photomask 13 is transferred by means of the mirrors M1-M6 onto a radiation-sensitive layer of a wafer 20 arranged in the image plane 21. The photomask 13 is suspended from a first scanning device 24, and the wafer 20 rests on a second scanning device 25, so that the wafer 20 can be exposed in a scanning operation in which the photomask 13 and the wafer 20 are moved synchronously. The photomask 13 can have an aspect ratio between 1:1 and 1:3, preferably between 1:1 and 1:2, and particularly preferably 1:1 or 1:2. The photomask 13 can be substantially rectangular in shape. The photomask 13 can preferably be 5 to 7 inches long and wide, especially preferably 6 inches long and wide.Alternatively, the photomask can be 5 to 7 inches long and 10 to 14 inches wide, preferably 6 inches long and 12 inches wide.

[0019] In the Fig. In the example shown, projection system 22 comprises six mirrors M1-M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage for the illumination radiation, making projection system 22 a double-obscured optical system. Projection system 22 has an image-side numerical aperture greater than 0.3, and which can also be greater than 0.6, for example, 0.7 or 0.75. The mirrors M1-M6 are fixed to a support frame. The projection exposure system also includes a sensor frame with sensors arranged on it for determining the actual position of the optical elements. For clarity, the support frame and the sensor frame are shown in Fig. Figure 1 is not shown. The sensors can operate, in particular, without contact. The sensor frame is mechanically and thermally almost completely decoupled from the support frame, so that the sensor frame provides a fundamentally unchanging reference system for determining the position of the optical elements. The use of such support and sensor frames is generally known from the prior art (see DE 10 2011 075 393 A1), so that a detailed explanation is unnecessary here.

[0020] The projection system 22 can in particular be anamorphic, i.e. it has in particular different image scales β x , β y in the x and y directions. The two image scales β x , β y of the projection system 22 are preferably located at (β x , β y) = (+ / - 0.25, / +- 0.125). A magnification β of 0.25 corresponds to a reduction in the ratio of 4:1, while a magnification β of 0.125 results in a reduction in the ratio of 8:1. A positive sign for the magnification β indicates a magnification without image inversion, a negative sign indicates a magnification with image inversion.

[0021] The reflective surfaces of mirrors M1-M6 can be designed as freeform surfaces without an axis of rotational symmetry. Like the mirrors of the illumination system 10, mirrors M1-M6 include reflective coatings for the illumination radiation. These reflective coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon. Despite the highly reflective coatings of the mirrors, a considerable proportion (e.g., 30%) of the illumination radiation is absorbed by the mirrors and converted into heat. This results in a high heat input into the mirrors M1-M6 of the projection system 22.

[0022] The heat input into the mirrors M1-M6 caused by the exposure radiation can vary during operation. In particular, the heat input is absent during an exposure break, which is required, for example, when changing the wafer 20 or the photomask 13. Furthermore, the heat input can change if the illumination setting generated by the illumination system 10 is altered. To maintain the optical elements M1-M6 in a near-steady thermal state despite the changing heat input, the projection exposure system includes heating elements with which the optical elements can be actively heated. Fig. Figure 1 shows an example of a heating element 31, which in this case serves to heat the optical element M4. The projection exposure system includes further heating elements, which are shown in Fig. Figure 1 is not shown for clarity. The heating element 31 is a preheater comprising electrically actuated electrodes and in thermal contact with a rear side of the optical element M4. The invention is not limited with regard to the manner of introducing heating power or the design of the heating elements used for this purpose. For example, the heating power can be introduced in a generally known manner using infrared emitters.

[0023] Fig. Figure 2 shows a partial section of the projection system of the Fig. Figure 1 shows an enlarged representation, in which, in addition to the heating element 31, three heating elements 32, 33, 34 formed by infrared emitters are shown. The heating elements 32-34 are sector heaters and serve to irradiate and thereby heat a sector 42, 43, 44 of the optical element M4 assigned to the respective sector heater with infrared radiation.

[0024] In Fig. Figure 3 shows a schematic front view of the optical element M4. This illustration shows the division of the optical element M4 into three sectors 42, 43, and 44. This division is merely exemplary, and other embodiments may have a different number of separately heatable sectors. The three sectors 42-44 can be heated independently of one another by the sector heaters 32-34.

[0025] In contrast to the state of Fig. 1, in which the wafer 20 is located in the image plane of the projection system, is the projection system in the Fig. Figure 2 is shown in a calibration state in which a wavefront sensor 29 is located in the image plane instead of the wafer 20. Furthermore, the view of the Fig. Figure 2 shows a control unit 36, which is designed to control the heating elements 31-34 and to acquire wavefront data from the wavefront sensor 29. In this calibration state, the method according to the invention can be carried out in the manner described below.

[0026] In a first step, the optical element M4 is heated to an initial temperature setpoint of 25 °C using the preheater 31. The control unit 36 ​​sends a corresponding control command to the preheater 31, which is then activated and supplies the optical element M4 with the appropriate heating power until the desired setpoint is reached. A temperature sensor can be located on the preheater 31 itself and / or on the optical element, which sends feedback about the current temperature of the optical element M4 to the control unit.

[0027] After reaching the temperature setpoint of 25 °C, a wavefront measurement is initiated at time t=t1. During this measurement, a number of wavefront parameters are recorded over time by the wavefront sensor 29. The corresponding wavefront data are then forwarded to the control unit 36. Furthermore, at time t1, the first sector heater 32 is activated for a predetermined period and subsequently deactivated. During the activation of the sector heater 32, the optical element in the area of ​​sector 42 heats up, which can lead to a change in the imaging properties of the optical element M4 and a corresponding change in the measured wavefront parameters. This change is reflected in the recorded wavefront data. At time t2 > t1, the process is repeated for the second sector heater 33, and at time t3 > t2 for the third sector heater.The measurement series recorded from the first setpoint as described above is also referred to as the first wavefront parameter profile in the context of this description.

[0028] In the next step, the optical element M4 is heated to a second temperature setpoint of 30 °C using the preheater 31. After reaching the second temperature setpoint, the wavefront measurement is repeated as described above, and the sector heaters 32, 33, and 34 are activated sequentially at successive time points, also as described above. The wavefront data acquired in this way are also transmitted to the control unit 36. The heating power set on the sector heaters 32–34 for the first measurement series starting from the first setpoint corresponds to the heating power set during the measurement series starting from the second setpoint. Furthermore, the time intervals and the relative time intervals between the activation of the sector heaters for the first and second measurement series are identical.The measurement series recorded from the second setpoint as described above is also referred to as the second wavefront parameter profile in the context of this description.

[0029] In a further step, two more series of measurements can be carried out, in which a heating power is set on the sector heaters that differs from the heating power used in the first two series of measurements.

[0030] Fig. Figure 4 shows an exemplary wavefront parameter Z as a function of time t for a first measurement series 39 (first wavefront parameter profile) starting from a first setpoint, and for a second measurement series 40 (second wavefront parameter profile) starting from a second setpoint. In this example, in addition to a preheater used to set the first and second setpoints, there are a total of six heating elements, which are activated sequentially at times t1 to t6 during a measurement series. Time t1 is, for both measurement series 39 and 40, the time at which the first sector heater is activated after reaching the respective setpoint, which is shown in Fig. 4 is illustrated by the rectangular graph 45. The activation of the other sector heaters is shown in Fig. 4 is illustrated by the further rectangular graphs 46 to 50.

[0031] Based on the Fig.As can be seen in Figure 4, the wavefront parameter Z can undergo more or less significant changes during the activation of the heating elements, with these changes varying in magnitude for the two different setpoints. It has been shown that the change in the wavefront parameter as a function of temperature can exhibit non-linear components and inhomogeneities, which complicate or even prevent the calibration of the respective heating element.

[0032] To eliminate the non-linear component and inhomogeneities, a differential wavefront parameter profile is determined for each heating element, as explained below. The first setpoint is referred to as temperature T1 and the second as temperature T2. Activating a heating element can result in a temperature profile T(t) = T1 + TH(t) starting from the first setpoint and a temperature profile T'(t) = T2 + TH(t) starting from the second setpoint. In this case, the wavefront parameter profile is given by Z(T) and the second by Z(T'). Determining the differential profile can involve first calculating the difference ΔZ(t) between the wavefront parameter profiles Z(T(t)) and Z(T'(t)): ΔZ(t) = Z(T'(t)) - Z(T(t)). Furthermore, the difference ΔZ(t) can be related to a reference time t0: ΔZ(t)-ΔZ(t0).The difference profile determined in this way by measuring the wavefront parameter is subsequently also referred to as (ΔZ(t) - ΔZ(t0)). exp denoted by the subscript "exp" indicating that the data were obtained by measuring the wavefront parameter. A corresponding simulated difference profile (ΔZ(t) - ΔZ(t0)) sim It can also be calculated using a generally known simulation model. By comparing the difference curves, a calibration parameter g can be determined. H The heating element's value can be determined as follows: (ΔZ(t) - ΔZ(t0)) exp = g H G V (ΔZ(t) - ΔZ(t0)) sim' where g V This refers to the calibration parameter of the preheater. The calibration parameter of the preheater must therefore be known in advance.

[0033] The process described above can be carried out for a plurality of wavefront parameters, for example for the Zernike coefficients Z1 to Z1. 25of the projection system. Since different wavefront parameters typically react differently to the heating of a component of the projection exposure system, the accuracy of the calibration can be significantly increased by using a multiple of wavefront parameters.

[0034] The above-described procedure can also be carried out separately for each heating element, so that all heaters of the projection exposure system can be calibrated.

[0035] If the calibration parameter of the preheater is unknown, it can also be determined using the method according to the invention by using the preheater to set the setpoints and simultaneously selecting it as the heating element to be calibrated. Starting from the respective setpoint, the preheater is activated to further heat the optical element. In this case, the calibration parameter of the preheater can be determined by comparing the difference profile determined by measurement with the simulated difference profile based on the following relationship: (ΔZ(t)−ΔZ(t0))exp=gV2(ΔZ(t)−ΔZ(t0))sim.

Claims

[1] Method for calibrating a heating element (31, 32, 33, 34) configured for heating a component (M4, 42, 43, 44) of a projection exposure system, wherein at least one wavefront parameter of a projection system (22) of the projection exposure system is changed by heating the component (M4, 42, 43, 44), comprising the steps: - Tempering the component (M4, 42, 43, 44) to a first setpoint, - Activation of the heating element (31, 32, 33, 34) starting from the first setpoint, so that heating of the component (M4, 42, 43, 44) takes place starting from the first setpoint, - Capturing an initial wavefront parameter profile during the heating process starting from the first setpoint, - Tempering the component (M4, 42, 43, 44) to a second setpoint different from the first, - Activation of the heating element (31, 32, 33, 34) starting from the second setpoint, so that heating of the component (M4, 42, 43, 44) takes place starting from the second setpoint, - Capturing a second wavefront parameter profile during the heating process starting from the second setpoint, - Determining a difference profile between the first and second wavefront parameter profiles, - Simulating the difference profile using a wavefront parameter simulation model, - Calibrating the heating element (31, 32, 33, 34) by comparing the determined difference profile with the simulated difference profile. [2] Method according to claim 1, wherein a first and second wavefront parameter profile is recorded for a plurality of different wavefront parameters, wherein a difference profile is determined for each of the plurality of wavefront parameters, wherein a difference profile is simulated for each of the plurality of wavefront parameters, wherein the heating element (31, 32, 33, 34) is calibrated by means of a mathematical adaptation of the plurality of determined difference profiles to the respective simulated difference profiles. [3] Method according to claim 1 or 2, which serves to calibrate a first heating element (32, 33, 34), wherein the component (M4, 42, 43, 44) is tempered to the first and / or the second setpoint with a second heating element (31) different from the first. [4] Method according to claim 1 or 2, which serves to calibrate a first heating element (31), wherein the component (M4, 42, 43, 44) is tempered with the first heating element (31) to the first and / or the second setpoint. [5] Method according to any one of claims 1 to 4, wherein the component (M4) is an optical element. [6] Method according to claim 5, wherein the heating element (31) is a preheater of the optical element. [7] Method according to any one of claims 1 to 4, wherein the component (42, 43, 44) is a sector of an optical element. [8] Method according to claim 7, wherein the heating element is a sector heater (32, 33, 34) of the optical element. [9] Method according to any one of claims 1 to 8, wherein the activation of the heating element (31, 32, 33, 34) is carried out starting from the first setpoint with a first heating power, wherein the activation of the heating element (31, 32, 33, 34) is carried out starting from the second setpoint with the first heating power. [10] Method according to any one of claims 1 to 9, wherein a temperature difference between the first setpoint and the second setpoint is greater than the temperature difference traversed when activating the heating element starting from one of the setpoints during the acquisition of the wavefront parameter profile. [11] Method according to one of claims 1 to 10, wherein the activation of the heating element (31, 32, 33, 34) is carried out starting from the first and second setpoint in a first measurement series with the first heating power, wherein the activation of the heating element starting from the first and second setpoint is carried out in a subsequent second measurement series with a second heating power different from the first. [12] Method according to any one of claims 1 to 11, which is carried out for a plurality of heating elements (31, 32, 33, 34) of the projection exposure system.

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