Mirror system, method for operating a mirror system, projection lens for a microlithographic projection exposure system, and computer program product

Abstract

The invention relates to a mirror system comprising an EUV mirror (M1-M6), a temperature sensor (30), a control unit (29) and a temperature-control module (26), wherein the EUV mirror (M1-M6) has a mirror body (38) and an optical surface (39) which is formed on the mirror body (38). The temperature sensor (30) determines a measured temperature value via the temperature of the mirror body (38) and transmits the measured temperature value to the control unit (29). The control unit (29) evaluates the measured temperature value to determine control commands for the temperature-control module (26). The temperature-control module (26) is actuated by the control commands so that the temperature-control module (26) influences the temperature of the mirror body. In a first operating state of the mirror system, the control unit (29) determines the control commands by processing a first temperature target value for the temperature of the mirror body (38). In a second operating state of the mirror system, the control unit (29) determines the control commands by processing a second temperature target value for the temperature of the mirror body (38). The invention also relates to a method for operating a mirror system, to a projection lens for a microlithographic projection exposure system, and to a computer program product.

Description

[0001] Mirror system, method for operating a mirror system, projection lens for a microlithographic projection exposure system, computer program product

[0002] The present patent application takes priority from the German patent application filed on 19 July 2023

[0003] DE 10 2023 206 859 . 1, to which reference is made and whose contents are incorporated herein in their entirety ( "incorporation by reference").

[0004] The invention relates to a mirror system, a method for operating a mirror system, a projection lens for a microlithographic projection exposure apparatus and a computer program product.

[0005] Microlithographic projection exposure systems are used for the production of integrated circuits with particularly small structures. A mask (= reticle) illuminated with very short-wave, extreme ultraviolet radiation (EUV radiation) is imaged onto a lithographic object to transfer the mask structure to the lithographic object.

[0006] The projection exposure system comprises several EUV mirrors, each with an optical surface that reflects the radiation. The EUV mirrors have a precisely defined shape and are precisely positioned to ensure sufficient image quality of the mask onto the lithography object.

[0007] The EUV radiation incident on the optical surface of the mirror is partially reflected and partially absorbed. The absorbed portion of the EUV radiation causes the mirror body to heat up. Temperature changes in a body are generally accompanied by thermal deformation. This is undesirable for mirrors in a microlithographic projection exposure system because a change in the geometric shape of the mirror changes the wavefront of the EUV radiation reflected from the optical surface. This usually leads to a reduction in image quality.

[0008] The problem is that the image quality is not equally good in different operating states of the projection exposure system.

[0009] The invention is based on the object of presenting a mirror system, a method for operating a mirror system, a projection lens for a microlithographic projection exposure system, and a computer program product with which these disadvantages are avoided. This object is achieved by the features of the independent claims. Advantageous embodiments are specified in the subclaims.

[0010] A mirror system according to the invention comprises an EUV mirror, a temperature sensor, a control unit and a temperature control module. The EUV mirror has a mirror body and an optical surface formed on the mirror body. The temperature sensor determines a temperature measurement value via the temperature of the mirror body and transmits the temperature measurement value to the control unit. The control unit evaluates the temperature measurement value in order to determine control commands for the temperature control module. The temperature control module is controlled with the control commands so that the temperature control module influences the temperature of the mirror body. In a first operating state of the mirror system, the control unit determines the control commands by processing a first temperature setpoint for the temperature of the mirror body.In a second operating state of the mirror system, the control unit determines the control commands by processing a second temperature setpoint for the temperature of the mirror body. The invention recognizes that the thermal deformation of the mirror body associated with a temperature change can be used to improve the image quality of the projection lens. The invention proposes providing different, specifically adapted temperature setpoints for the mirror body for different operating states of the mirror system.

[0011] The mirror body can be made of a ZCT material. "Consisting" means that, based on the mass of the mirror body, at least 70%, preferably at least 90%, of the material of the mirror body is a ZCT material. A ZCT material is a material whose thermal expansion coefficient has a zero crossing temperature.

[0012] It is common practice to bring the mirrors of a projection exposure system to a temperature close to the ZCT during operation. This has the advantage that slight temperature fluctuations do not affect the geometric shape of the mirror body. Thus, slight temperature fluctuations can be accepted without impairing the image quality of the projection lens. The invention can be implemented such that the first temperature setpoint corresponds to the ZCT temperature and the second temperature setpoint deviates from the ZCT temperature.

[0013] In one embodiment, both the first temperature setpoint and the second temperature setpoint deviate from the zero-crossing temperature of the ZCT material. The difference between the ZCT temperature and the nearest temperature setpoint is preferably at least 0.2 K, more preferably at least 1 K, more preferably at least 2 K. If both temperature setpoints deviate from the ZCT temperature, this has the advantage of greater sensitivity, so that even small temperature changes result in noticeable thermal deformation.

[0014] The difference between the first temperature setpoint and the second temperature setpoint can be greater than 0.1 K, preferably greater than 0.2 K, more preferably greater than 0.5 K. As a rule, the difference between the first temperature setpoint and the second temperature setpoint is not greater than 3 K, preferably not greater than 2 K. The first temperature setpoint and the second temperature setpoint can relate to the average temperature that the mirror body has on the optical surface.

[0015] The operation of a mirror system according to the invention can comprise several phases. In a rest state, in which the projection exposure system is not in operation, the mirror body can have a rest temperature that can correspond to the ambient temperature. The ambient temperature can, for example, be at room temperature, i.e., on the order of 20°C.

[0016] When the projection exposure system is put into operation, a preheating phase may follow, during which the mirror body is heated to a temperature close to the intended operating temperature. It is not mandatory that the mirror body be heated to a temperature during the preheating phase that is identical to the target operating temperature.

[0017] The mirror system can comprise a heating device in the form of a so-called preheater, which supplies heat energy to the mirror body during the preheating phase. The preheater can, for example, be designed to direct suitable electromagnetic radiation, in particular infrared radiation, to the mirror body. It is also possible to supply the heat energy using a heat transfer fluid. The heat transfer fluid can, for example, be directed through channels formed in the mirror body.

[0018] The first temperature setpoint can be a setpoint for the operating temperature, and the second temperature setpoint can be a setpoint for the operating temperature. The operating temperature is the temperature that is desired as a steady state during operation of the projection exposure system. Since thermal energy is continuously supplied to the mirror body during operation of the projection exposure system, the operating temperature is higher than the ambient temperature. The operating temperature can, for example, be between 1 K and 20 K, preferably between 3 K and 15 K, higher than the ambient temperature.

[0019] The operating temperature depends on the amount of heat supplied to the mirror body during operation of the projection exposure system. This includes heat energy from absorption of the EUV radiation. This may further include heat energy supplied to the mirror body by a heating device. The control unit may be configured to control the heating device such that the heating power supplied to the mirror body is in a suitable ratio to the EUV radiation directed onto the mirror.

[0020] The operating temperature further depends on the amount of heat dissipated by the mirror body during operation of the production exposure system. This can include thermal energy radiated into the environment. This can also include thermal energy dissipated by the mirror body using suitable cooling devices. For example, the mirror body can include cooling channels through which a cooling fluid is conducted during operation of the production exposure system. The control unit can be designed to control the heating device such that the heating power supplied to the mirror body is in a suitable ratio to the thermal energy dissipated by the mirror body.

[0021] The heating device can be designed to emit thermal radiation onto the mirror body. The thermal radiation can impinge on the optical surface of the mirror, i.e., the same surface impinged by the EUV radiation. The heating device can be designed as a sector heater, so that different areas of the optical surface can be exposed to thermal radiation independently of one another.

[0022] The mirror system can be operated with a first illumination setting in the first operating state and with a second illumination setting in the second operating state. The illumination setting describes the distribution of the EUV radiation across the cross-section of the beam path. In general, the illumination setting of a microlithographic projection exposure system is set such that the radiation intensity is constant across the illuminated area in the image plane, i.e. in particular on the surface of the lithography object. In contrast, the angular distribution of the radiation incident in the image plane can vary from illumination setting to illumination setting. The different angular distributions in the image plane translate into different intensity distributions in the pupil plane.

[0023] In one embodiment, the EUV mirror of the mirror system according to the invention is a near-field EUV mirror in the beam path of the projection lens. A mirror is referred to as near-field if the distance to the nearest field plane measured along the beam path is smaller, preferably less than a factor of 2, more preferably at least a factor of 3, than the distance to the nearest pupil plane. Using different illumination settings, the microlithographic projection exposure system can be adapted to different applications, in particular to different types of lithography objects. The illumination setting is derived individually from the application in each case. The illumination setting of the first operating state can be, for example, an annular, a dipole, or a quadrupole illumination setting.The illumination setting of the second operating state can, for example, be an annular, dipole, or quadrupole illumination setting, whereby the illumination setting of the second operating state is different from the illumination setting of the first operating state. The various possible illumination settings can be combined with one another as desired under the operating states.

[0024] The first temperature setpoint can be determined by describing the EUV mirror as a model. The model can be used to describe the geometric shape of the mirror body as a function of temperature. The geometric shape can be determined as a function of a temperature distribution across the mirror body. Additionally or alternatively, the geometric shape can be determined as a function of the average temperature on the optical surface of the mirror.

[0025] To determine the temperature target value, an optimization process can be carried out with which the geometric shape of the mirror body is optimized by varying the temperature. Various parameters of the imaging beam path can be considered as quality criteria for the optimization process. In particular, parameters of the imaging beam path that are not amenable to correction within the rigid body degrees of freedom of the mirror body are suitable as quality criteria. In one embodiment, the optimization criterion is a minimization of an error NCEzz / zs that cannot be corrected within the rigid body degrees of freedom of the mirror body and is defined as follows using the Zernike coefficients Z2, Z3.

[0026] It is also possible to pursue the minimization of a quality criterion known as RMS5 as an optimization goal, which is defined as follows.

[0027] Here, E± are the RMS weights for the Zernike coefficients Zi with a radial order n . The factor k has the value 2 for spherical terms and 1 otherwise.

[0028] Another possible variant would be to optimize the field distribution coefficient kia in the Zernike decomposition of the wavefront aberration. If the aberration considered in the cross-section of the EUV beam path is subjected to a Zernike decomposition, kia denotes the coefficient of the third power in the second Zernike polynomial Z2(x).

[0029] The coefficient kia is the first field coefficient in the Zernike polynomial Z2(x) which cannot be corrected within the rigid body degrees of freedom of the EUV mirrors of the projection lens.

[0030] Other optimization criteria are possible. For example, certain performance parameters can be set to a specified value. This can be useful, for example, to minimize deviations between different microlithographic projection exposure systems. In other application examples, optimization can be performed for specific use cases specified by the user.

[0031] The second temperature setpoint can be determined in the same way.

[0032] Optimization can be limited to a single mirror. Optimization across multiple mirrors is also possible. This allows the possibility of individual mirrors compensating for each other's errors. The result of such an optimization process across multiple mirrors is optimized temperature setpoints for each of the mirrors involved.

[0033] The control unit of the mirror system according to the invention can comprise a memory element in which data for the operation of the mirror system in various operating states are stored. In particular, the memory element can store an assignment between the first operating state of the mirror system and the first temperature setpoint, as well as an assignment between the second operating state of the mirror system and the second temperature setpoint. The control unit can be configured so that the correct assignment between operating states and setpoint temperatures is automatically made during operation.

[0034] The mirror system can have more than two operating states, in particular at least three operating states, preferably at least five operating states, each operating state being assigned a temperature setpoint. The features mentioned in connection with the first or second operating state also apply to the further operating states. The invention further relates to a projection objective having a plurality of EUV mirrors with which a reticle is imaged into an image plane. The projection objective comprises such a mirror system. The invention further relates to a microlithographic projection exposure system having such a projection objective.

[0035] The invention also relates to a method for operating a mirror system, wherein the mirror system comprises an EUV mirror which has a mirror body and an optical surface formed on the mirror body. A temperature measurement value relating to the temperature of the mirror body is determined, wherein the temperature measurement value is evaluated in order to control a temperature control module so that the temperature control module influences the temperature of the mirror body. In a first operating state of the mirror system, the control commands are determined by processing a first temperature setpoint for the temperature of the mirror body. In a second operating state of the mirror system, the control commands are determined by processing a second temperature setpoint for the temperature of the mirror body.

[0036] The invention also relates to a computer program product or a set of computer program products comprising program parts which, when loaded into a computer or into interconnected computers connected to a device according to the invention, are designed to carry out the method according to the invention.

[0037] The disclosure includes further developments of the mirror system with features that are described in connection with the method according to the invention. The disclosure includes further developments of the method with features that are described in connection with the mirror system according to the invention. The invention is described below by way of example with reference to the attached drawings using advantageous embodiments. They show:

[0038] Fig. 1: an embodiment of a projection exposure system according to the invention;

[0039] Fig. 2: a schematic representation of a mirror system according to the invention;

[0040] Fig. 3: a schematic representation of the control unit of the mirror system from Fig. 2;

[0041] Fig. 4: a first embodiment,

[0042] Fig. 5: a second embodiment.

[0043] Fig. 1 schematically illustrates a microlithographic EUV projection exposure system. The projection exposure system comprises an exposure beam source 14, an illumination system 10, and a projection lens 22, which are operated together in a vacuum chamber 23. A vacuum is applied in the vacuum chamber 23 during operation of the EUV projection exposure system.

[0044] The exposure beam source 14 generates electromagnetic radiation in the EUV range, i.e. in particular with a wavelength between 5 nm and 30 nm. The exposure radiation emanating from the exposure beam source 14 is bundled into an intermediate focal plane 16 by a collector 15. Exposure radiation passing from the intermediate focal plane 16 is guided into an object plane 12 by the illumination system 10, such that an object field in the object plane 12 is illuminated with uniform radiation intensity. The illumination system 10 comprises a deflection mirror 17, with which the exposure radiation is deflected onto a first facet mirror 18. A second facet mirror 19 is arranged downstream of the first facet mirror 18. The facets of the first facet mirror 18 are imaged into the object plane 12 by the second facet mirror 19.

[0045] A mask referred to as reticle 13 is arranged in the object field and is imaged into an image plane 21 via a plurality of mirrors M1-M6 of the projection objective 22. A structure formed on the reticle 13 is transferred to a radiation-sensitive layer of a wafer 20 arranged in the image plane 21. The reticle 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 process in which the reticle 13 and the wafer 20 are moved synchronously with one another.

[0046] The various mirrors of the projection exposure system, at which the exposure radiation is reflected, are designed as EUV mirrors. The EUV mirrors comprise, as shown in the mirror system shown in Fig. 2 using the example of mirror M2, a mirror body 38 and an optical surface 39 formed on the mirror body 38, which is formed by a highly reflective coating. This can be a multilayer coating, in particular a multilayer coating with alternating layers of molybdenum and silicon. The EUV mirrors reflect approximately 70% of the incident EUV radiation. The remaining approximately 30% is absorbed and leads to heating of the EUV mirrors.

[0047] The EUV mirrors M1-M6 of the projection lens 22 can be made of a material that has ultra-low thermal expansion (ultra-low expansion material), e.g. a titanium silicate glass sold under the name ULE™ by Corning Inc. During operation of the projection lens, the temperature of the mirror body can be set to a value close to the so-called zero-crossing temperature (= zero-crossing temperature, ZCT). At this zero-crossing temperature, which can be specified individually for each mirror, the thermal expansion coefficient has a zero crossing, in the vicinity of which changes in temperature cause no or only negligible thermal expansion of the material of the mirror body.

[0048] To maintain the temperature of the mirror body at the specified value, active cooling is provided, in which a cooling liquid is passed through the mirror body. This is indicated schematically in Fig. 2 by cooling channels 37 formed in the mirror body 38.

[0049] The projection exposure system comprises a heating device 26 designed to direct infrared radiation 27 onto the optical surface 39 of the EUV mirror M2 in order to locally heat the EUV mirror M2. The wavelength of the infrared radiation can be, for example, between 1050 nm and 1600 nm, and the heating power can be, for example, between 2 W and 100 W. The local heating causes local thermal expansion, which can be specifically designed so that the temperature of the mirror body 38 assumes a desired value.

[0050] 2, the heating device 26 comprises an optical module 40 to which a plurality of light guides 41 are connected. An infrared radiation source 28 comprises a plurality of infrared radiators, each of which feeds infrared radiation into one of the light guides 41. The optical module 26 comprises an optical element for each of the light guides 41, for example in the form of a lens, with which optical element the infrared radiation emerging from the light guide 41 is formed into an infrared beam with which a specific surface area 34, 35, 36 on the optical surface 39 of one of the mirrors M1-M6 is illuminated. The heating device 26 forms a temperature control module of the mirror system according to the invention. In the simplified illustration in FIG. 2, the heating device 26 is shown with three heating channels. In practice, the number of heating channels of the heating device 26 may be larger.

[0051] The mirror system comprises a control unit 29, which controls the infrared radiation sources 28 depending on various input variables. Fig. 2 shows an example of a sensor 30, which detects a measured value of a state variable of the EUV mirror 20, such as the temperature, and feeds it to the control unit 29. Using suitable control signals from the control unit 29, the power of the infrared radiation sources 28 can be adjusted as desired.

[0052] A mirror system according to the invention comprises a control unit 29, to which measured values ​​are supplied from a temperature sensor 30. The temperature sensor 30, which is only shown schematically in Fig. 2, can comprise a plurality of sensor elements distributed over the optical surface 39 of the respective mirror M1-M6, from which an average value of the temperature of the mirror body 38 on the optical surface 39 can be determined. The sensor elements can be integrated into the mirror body 38, arranged on the surface of the mirror body 38, or measure without contact.

[0053] The measured value of the average temperature of the mirror body 38 on the optical surface 39 is fed to a central computer 42 of the control unit 29. The central computer 42 compares the measured temperature with a temperature setpoint stored in a memory element 43. Based on the deviation, a control command is determined, which is sent to a command transmitter 44. The command transmitter 44 controls the radiation source 28, which adjusts the heating power directed to the mirror body 38 according to the control command.

[0054] Via an input interface 45, the mirror system can be switched between different operating states, which correspond to different lighting settings of the projection exposure system. Depending on the operating states, different temperature setpoints are stored in the memory element 43. The central computer 42 is designed to automatically read the temperature setpoint corresponding to the selected operating state from the memory element 43. Depending on the temperature setpoint, with the same measured value from the sensor 30, different control commands result, with which the infrared radiation source 28 is controlled.

[0055] The temperature setpoints stored in the memory element 43 can be determined using an optimization method with which a quality criterion is optimized. The table in Fig. 4 is based on an optimization method according to the invention which was carried out using an optical system with which a photomask is imaged into an object plane via a plurality of mirrors. In the conventional procedure, a temperature setpoint T is determined for each of the mirrors and remains constant regardless of the respective operating state of the optical system. This is indicated in line 1 of Table 4 for the temperature setpoint TI, the value of which remains unchanged regardless of whether the optical system is operated with an illumination setting in the form of an x-dipole or with an illumination setting in the form of a y-dipole.

[0056] In a first attempt, the optimization procedure was performed using the RMS5 quality criterion. For the x-dipole, a new temperature setpoint was determined that was 0.2 K lower than the previous temperature setpoint TI, resulting in a reduction in the RMS5 aberration by about one-third. For the y-dipole, however, the temperature setpoint increased by 0.34 K, resulting in a reduction in the RMS5 aberration by about 40%.

[0057] If the optimization is instead performed using the performance criterion NCEZ2 / Z3, the x-dipole temperature setpoint is increased by 0.89 K and the aberration of the NCEzz / zs is reduced by more than 60%. For the y-dipole, the temperature setpoint is increased by 1.98 K and the aberration is reduced by more than 70%. The corresponding temperature values ​​are given in Fig. 4, line 3.

[0058] In Fig. 4, line 4, the optimization procedure was performed to achieve a joint optimization of the RMS5 and NCE Z2 / Z3 performance criteria. For the x-dipole, the temperature setpoint is reduced by 0.2 K. For the y-dipole, the temperature setpoint is increased by 1.43 K.

[0059] Fig. 5 shows the temperature setpoints determined when a joint optimization is performed for two other mirrors SI, S2 of the same optical system. When optimized using the performance criterion RMS5, the temperature setpoint TI for the first mirror S1 increases by 2.49 K and the temperature setpoint T2 for the second mirror S2 decreases by 0.91 K, see Fig. 5, column 2. Applying the performance criterion NCEz2 / z3 leads to an increase in the temperature setpoint TI by 0.99 K and an increase in the temperature setpoint T2 by 1.55 K. Optimizing the two mirrors SI, S2 using the combined performance criteria RMS5 and NCEz2 / z3 results in an increase in the temperature setpoint TI by 2.49 K and an increase in the temperature setpoint T2 by 0.73 K.

Claims

Patent claims 1. Mirror system, comprising an EUV mirror (M1-M6), a temperature sensor (30), a control unit (29) and a temperature control module (26), wherein the EUV mirror (M1-M6) has a mirror body (38) and an optical surface (39) formed on the mirror body (38), wherein the temperature sensor (30) determines a temperature measurement value via the temperature of the mirror body (38) and transmits the temperature measurement value to the control unit (29), wherein the control unit (29) evaluates the temperature measurement value in order to determine control commands for the temperature control module (26), and wherein the temperature control module (26) is controlled by the control commands so that the temperature control module (26) influences the temperature of the mirror body, wherein in a first operating state of the mirror system, the control unit (29) executes the control commands by processing a first temperature setpoint for the temperature of the mirror body (38) and wherein in a second operating state of the mirror system the control unit (29) determines the control commands by processing a second temperature setpoint for the temperature of the mirror body (38).

2. Mirror system according to claim 1, wherein the mirror body (38) consists of a ZCT material.

3. The mirror system of claim 2, wherein the first temperature setpoint and the second temperature setpoint deviate from the zero-crossing temperature of the ZCT material.

4. Mirror system according to one of claims 1 to 3, wherein the difference between the first temperature setpoint and the second temperature setpoint is greater than 0.1 K, preferably greater than 0.2 K, more preferably greater than 0.5 K.

5. Mirror system according to one of claims 1 to 4, wherein the first temperature setpoint is a setpoint for the operating temperature of the mirror body (38) and wherein the second temperature setpoint is a setpoint for the operating temperature of the mirror body (38).

6. Mirror system according to one of claims 1 to 5, wherein the mirror system is operated in the first operating state with a first illumination setting and in the second operating state with a second illumination setting.

7. Mirror system according to one of claims 1 to 6, wherein the EUV mirror is a near-field EUV mirror.

8. Mirror system according to one of claims 1 to 7, wherein the first temperature setpoint and / or the second temperature setpoint are determined using an optimization method.

9. Mirror system according to claim 8, wherein a minimization of the field coefficient kia in the Zernike decomposition of the wavefront aberration is used as the optimization criterion.

10. Mirror system according to claim 8 or 9, wherein a joint optimization is carried out over several EUV mirrors (M1-M6).

11. Mirror system according to one of claims 1 to 10, wherein the mirror body (38) is provided with cooling channels (30).

12. Mirror system according to one of claims 1 to 11, wherein the temperature control module (26) is designed as a sector heater.

13. Projection lens of a microlithographic projection exposure apparatus, comprising a mirror system according to one of claims 1 to 12 with a plurality of EUV mirrors (M1-M6) with which a reticle (13) is imaged into an image plane (21).

14. Method for operating a mirror system with an EUV mirror (M1-M6) which has a mirror body (38) and an optical surface (39) formed on the mirror body (38), wherein a temperature measurement value is determined via the temperature of the mirror body (38), wherein the temperature measurement value is evaluated in order to control a temperature control module (26) so that the temperature control module (26) influences the temperature of the mirror body, wherein in a first operating state of the mirror system the control commands are determined by processing a first temperature setpoint for the temperature of the mirror body (38) and wherein in a second operating state of the mirror system the control commands are determined by processing a second temperature setpoint for the temperature of the mirror body (38).

15. Computer program product or set of computer program products comprising program parts which, when loaded into a computer or into interconnected computers connected to a mirror system according to the invention, are designed to carry out the method according to the invention.

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