Mirror system, method for operating a mirror system, projection lens of a microlithographic projection exposure system

Abstract

Mirror system comprising an EUV mirror (M1-M6), a fluid reservoir (42) for a temperature control fluid, and a pump (33). The EUV mirror (M1-M6) includes a mirror body (38) and an optical surface (32) formed on the mirror body (38) with high reflectivity for EUV radiation. A fluid channel (37) for the temperature control fluid is formed in the mirror body (38), with a first supply line (35) and a second supply line (36) arranged between the fluid reservoir and the fluid channel (37). A switching device (39) is arranged between the fluid channel (37) and the fluid reservoir (42). In a first switching state (47) of the switching device (39), the pump (33) delivers the temperature control fluid from the fluid reservoir (42) through the first supply line (35) to the fluid channel (37). In a second switching state (48) of the switching device (39), the pump (33) delivers the temperature control fluid through the second supply line (36) to the fluid channel (37).The invention also relates to a method for operating a mirror system and a projection lens of a microlithographic projection exposure system.

Description

[0001] The invention relates to a mirror system, a method for operating a mirror system and a projection lens of a microlithographic projection exposure system.

[0002] Microlithographic projection exposure systems are used for the production of integrated circuits with particularly small structures. A photomask illuminated with very short-wavelength, extreme ultraviolet (EUV) radiation is imaged onto a lithographic object to transfer the mask structure to the object.

[0003] The projection exposure system comprises several EUV mirrors, each with an optical surface that reflects the EUV radiation. The EUV mirrors have a precisely defined shape and are precisely positioned to ensure sufficient image quality from the mask onto the lithographic object.

[0004] 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.

[0005] To influence the temperature of the mirror system in a desired manner, the mirror body can be equipped with a fluid channel through which a temperature control fluid is passed. As the fluid passes through the mirror body, heat transfer occurs between the mirror body and the temperature control fluid, resulting in a change in the temperature of the temperature control fluid as it passes through the fluid channel. It is therefore not entirely easy to pass a temperature control fluid through the mirror body in such a way that a uniform temperature control effect is achieved across the mirror body.

[0006] An uneven temperature control effect is particularly disadvantageous when a constant and uniform temperature distribution across the mirror body is desired, as is the case, for example, when the mirror body is made of a material with a zero-crossing temperature and the temperature is to be maintained close to the zero-crossing temperature. Asymmetric temperature distributions can then lead to asymmetric deformations of the mirror (relative to the mirror's symmetry axes), which can result in asymmetric wavefront errors with respect to the pupil coordinate and the field dependence. Such aberrations are generally difficult to compensate for by manipulation within the rigid-body degrees of freedom of the optical elements of a projection lens.

[0007] The invention is based on the object of presenting a mirror system, a method for operating a mirror system, and a projection lens for a microlithographic projection exposure system with which the aforementioned disadvantages can be mitigated. This object is achieved by the features of the independent claims. Advantageous embodiments are specified in the subclaims.

[0008] A mirror system according to the invention comprises an EUV mirror, a fluid reservoir for a temperature control fluid, and a pump. The EUV mirror comprises a mirror body and an optical surface formed on the mirror body with high reflectivity for EUV radiation. A fluid channel for the temperature control fluid is formed in the mirror body, with a first supply line and a second supply line being formed between the fluid reservoir and the fluid channel. A switching device is arranged between the fluid channel and the fluid reservoir. In a first switching state of the switching device, the pump conveys the temperature control fluid from the fluid reservoir through the first supply line to the fluid channel. In a second switching state of the switching device, the pump conveys the temperature control fluid through the second supply line to the fluid channel.

[0009] The invention is based on the assumption that the mirror system has different operating states, with the heat loads acting on the mirror body being distributed differently depending on the operating state. In the case of asymmetric heat distribution, the flow direction of the temperature control fluid can be selected such that the asymmetry resulting from the temperature change of the temperature control fluid as it passes through the fluid channel counteracts the asymmetry of the heat distribution.In other words, the flow direction of the temperature control fluid can be selected such that the temperature difference between the temperature control fluid and the mirror body is greater in those areas of the mirror body where high heat transfer between the mirror body and the temperature control fluid is desired, and such that the temperature difference between the temperature control fluid and the mirror body is smaller in those areas of the mirror body where lower heat transfer between the mirror body and the temperature control fluid is desired. By conveying the temperature control fluid between the fluid channel and the fluid reservoir either along the first supply line or along the second supply line, it is possible to reverse the flow direction of the temperature control fluid. The flow direction of the temperature control fluid can be adjusted in each case to counteract the asymmetry of the temperature distribution across the mirror body.

[0010] The mirror system according to the invention can comprise a fluid circuit that is closed between the fluid channel of the mirror body and a fluid reservoir via the first supply line and the second supply line. Viewed along the flow direction of the temperature control fluid, the pump can be arranged between the fluid reservoir and the fluid channel of the mirror body. It is also possible for the pump to be arranged between the fluid channel of the mirror body and the fluid reservoir. In other words, the pump can be arranged upstream of the fluid channel or downstream of the fluid channel with respect to the flow direction of the temperature control fluid.

[0011] The fluid reservoir can be designed as a unidirectional fluid reservoir. With a unidirectional fluid reservoir, there is a predetermined direction in which the fluid reservoir is integrated into a fluid circuit. In particular, the fluid reservoir can have precisely one outlet opening through which the temperature control fluid intended for the fluid channel of the mirror body leaves the fluid reservoir. The term "fluid reservoir" does not imply any restriction with regard to a specific structural configuration of the fluid reservoir. The fluid reservoir can be designed as a fluid supply from which the temperature control fluid conveyed to the mirror body is drawn. The fluid reservoir can comprise components for adjusting the pressure of the temperature control fluid to a specific value or maintaining it constant. However, these are not necessary features of a fluid reservoir.

[0012] The fluid reservoir can be equipped with a temperature control module so that the temperature of the temperature control fluid upon exiting the fluid reservoir lies within a specified range. The difference between the upper and lower end of the range can be less than 1 K, preferably less than 0.5 K, and more preferably less than 0.2 K.

[0013] The switching device can be designed such that, in the first switching state, it couples the outlet opening of the fluid reservoir to the first supply line. The mirror system can be designed such that, in the first switching state, a fluid flow coming from the fluid reservoir can flow via the switching device and via the first supply line to the fluid channel in the mirror body. The switching device can be designed such that, in the second switching state, it couples the outlet opening of the fluid reservoir to the second supply line. The mirror system can be designed such that, in the second switching state, a fluid flow coming from the fluid reservoir can flow via the switching direction and via the second supply line to the fluid channel in the mirror body.

[0014] The switching device can be designed such that, in the first switching state, it couples a return line leading to the fluid reservoir to the second supply line. The mirror system can be configured such that, in the first switching state, a fluid flow coming from the mirror body can flow back to the fluid reservoir via the second supply line. The switching device can be designed such that, in the second switching state, it couples the return line to the first supply line. The mirror system can be configured such that, in the second switching state, a fluid flow coming from the mirror body can flow back to the fluid reservoir via the first supply line.

[0015] The switching device can be designed as a switching block in which both the first supply line and the second supply line are coupled in the desired manner with a single switching operation. It is also possible to implement the function of the switching device through a combination of several switching valves. To switch between the first switching state and the second switching state, the plurality of valves can be actuated simultaneously or in a time sequence.

[0016] The pump can be arranged in a section of the fluid circuit located between the fluid reservoir and the switching device. The pump can be arranged in a feed line between the outlet opening of the fluid reservoir and the switching device. It is also possible for the pump to be arranged in the return line between the switching device and the fluid reservoir. A pump arranged between the fluid reservoir and the switching device can maintain its flow direction regardless of the switching state of the switching device in order to convey the fluid flow toward the mirror body.

[0017] Alternatively, the pump can be arranged in a section of the fluid circuit located between the switching device and the mirror body. The pump can then be controlled in such a way that the pump's flow direction is reversed along with the switching state of the switching device.

[0018] A change in the flow state in the fluid channel of the mirror body can trigger unwanted mechanical vibrations (FIV, Flow Induced Vibrations). To minimize the risk of such vibrations, the volume flow in the fluid channel can be stopped before the switching state of the switching device is changed. A control unit of the mirror system can be designed to control the pump and / or the switching device accordingly.

[0019] Additionally or alternatively, the pump can be controlled such that each change in the flow state is stretched over a transition phase. The transition phase can be at least a factor of 2, preferably at least a factor of 5, more preferably at least a factor of 10 longer than the duration of a conventional adjustment, in which the pump's input power is immediately set to the new setpoint. In absolute terms, the transition phase can extend, for example, over a period of at least 30 seconds, preferably at least 1 minute, more preferably at least 2 minutes.

[0020] A predefined maximum value can apply to the rate of change of the volume flow during the transition phase. The volume flow can be changed so slowly that the predefined maximum value is not exceeded. In one embodiment, the change in the volume flow is guided along a ramp. The predefined maximum value for the rate of change of the volume flow can, for example, be less than 30% / min, preferably less than 20% / min, and more preferably less than 10% / min.

[0021] In all variants, the temperature control fluid supplied to the fluid channel in the mirror body can be used to bring the mirror body to a desired temperature. Depending on the ratio between the temperature of the temperature control fluid and the temperature of the mirror body, the structure can be either heated or cooled. The temperature control fluid can be a temperature control liquid, particularly water. To minimize disturbances caused by turbulence, it is advantageous if the volume flow of the temperature control fluid is low. The volume flow can in particular be less than 2 l / min, preferably less than 1 l / min.

[0022] The fluid channel can be fanned out so that several fluid channel sections extend parallel to one another within the structure. An inlet distributor can be formed between the supply line and the fluid sections, via which the temperature control fluid coming from the supply line is distributed to the fluid channel sections. The inlet distributor can be formed within the mirror body so that the temperature control fluid enters the mirror body via a single inlet opening.

[0023] The mirror system can be configured such that the switching device is placed in the first switching state when the mirror system is operated with a first illumination setting, and such that the switching device is placed in the second switching state when the mirror system is operated with a second illumination setting. In general, the heat distribution across the mirror differs depending on the illumination setting. By switching the flow direction of the temperature control fluid, a given asymmetry in the heat distribution can be counteracted in each of the illumination settings.

[0024] The illumination setting refers to the distribution of EUV radiation across the cross-section of the beam path. Generally, the illumination setting of a microlithographic projection exposure system is adjusted so that the radiation intensity is constant across the illuminated area in the image plane, i.e., specifically on the surface of the lithographic object. However, the angular distribution of the radiation arriving at 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. In this context, the mirror system can be a near-pupil mirror system of a projection lens of a microlithographic projection exposure system.

[0025] The mirror system can include a control unit designed to control the interaction of components of the mirror system. Under the control of the control unit, one or more of the following processes can be controlled. The position of the mirror body relative to a support frame can be changed. A heating device can be controlled to change the amount of heat supplied to the mirror system. The pump can be controlled to adjust the volume flow of the supplied tempering fluid.

[0026] The control unit of the mirror system according to the invention can comprise a memory element in which data for operating the mirror system with different lighting settings is stored. In particular, an assignment between the first switching state of the switching device and a first lighting setting, as well as an assignment between the second switching state of the switching device and a second lighting setting, can be stored in the memory element. The control unit can be configured such that, during operation, the correct assignment between the lighting settings and the states of the switching device is automatically made. In other words, the control unit can be designed to control the switching device based on a predetermined assignment between switching states and lighting settings.

[0027] The invention further relates to a projection lens with a plurality of EUV mirrors for imaging a photomask into an image plane. The projection lens comprises one or more mirror systems according to the invention. The invention further relates to a microlithographic projection exposure system with such a projection lens.

[0028] The invention also relates to a method for operating a mirror system, wherein the mirror system comprises an EUV mirror having a mirror body and an optical surface formed on the mirror body with high reflectivity for EUV radiation. A fluid channel for a temperature control fluid is formed in the mirror body. A first supply line and a second supply line are formed between a fluid reservoir and the fluid channel. In a first switching state of the mirror system, the temperature control fluid is conveyed through the first supply line from the fluid reservoir to the fluid channel. In a second switching state of the mirror system, the temperature control fluid is conveyed through the second supply line from the fluid reservoir to the fluid channel.

[0029] The disclosure includes further developments of the mirror system with features described in connection with the method according to the invention. The disclosure includes further developments of the method with features described in connection with the mirror system according to the invention.

[0030] The invention is described below by way of example with reference to advantageous embodiments in the accompanying drawings. They show: Fig. 1: a schematic representation of a microlithographic projection exposure system; Fig. 2: a schematic representation of a first embodiment of a mirror system according to the invention; Fig. 3: the switching device of the mirror system Fig. 2 in a first switching state; Fig. 4: the view according to Fig. 3 in a second switching state of the switching device; Fig. 5: a schematic representation of a second embodiment of a mirror system according to the invention.

[0031] In Fig. Figure 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.

[0032] 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 beam emanating from the exposure beam source 14 is focused into an intermediate focal plane 16 by a collector 15. Exposure beam emanating from the intermediate focal plane 16 is guided into an object plane 12 by the illumination system 10, so that an object field in the object plane 12 is illuminated with uniform radiation intensity.

[0033] The illumination system 10 comprises a deflecting 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 second facet mirror 19 images the facets of the first facet mirror 18 onto the object plane 12.

[0034] A photomask 13 is arranged in the object plane 12 and is imaged into an image plane 21 via a plurality of mirrors M1-M6 of the projection lens 22. A structure formed on the photomask 13 is transferred to 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 process in which the photomask 13 and the wafer 20 are moved synchronously with one another.

[0035] Each of the EUV mirrors M1-M6 comprises a mirror body made of a mirror substrate material with ultra-low thermal expansion ("ultra-low expansion material"). One example of such a material is a titanium silicate glass marketed by Corning Inc. under the name ULE™, which exhibits a so-called zero-crossing temperature. At this zero-crossing temperature, which for ULE™, for example, is approximately 30°C, the thermal expansion coefficient exhibits a zero crossing in its temperature dependence, around which no or only negligible thermal expansion of the mirror substrate material occurs.

[0036] A multilayer coating system with alternating layers of molybdenum and silicon is applied to the mirror body, forming an optical surface of the EUV mirror. The optical surface is optimized for the reflection of EUV radiation in the extreme ultraviolet spectral range with wavelengths between 5 nm and 30 nm. With such a coating, approximately 70% of the incident EUV radiation can be reflected. The remaining approximately 30% is absorbed and leads to heating of the EUV mirrors. The heat distribution across the optical surface 32 depends on the illumination setting and can vary significantly depending on the illumination setting.

[0037] In Fig. 2 shows a mirror device in which a mirror body 38 of a mirror M1-M6 is held on a frame structure 31 via actuators 30. The position of the mirror body 38 relative to the frame structure 31 can be changed via the actuators 30 in order to align and position the mirror body 38. An optical surface 32 is formed on the mirror body 38, at which EUV radiation is reflected. In the projection exposure system of Fig. 1, each of the mirrors M1-M6 of the projection lens 22 is designed as a mirror device according to Fig. 2. It is also possible to equip only some of the mirrors M1-M6 in this way.

[0038] Formed inside the mirror body 38 is a fluid channel in the form of a cooling channel 37, which extends along a serpentine path through the mirror body 38. The cooling channel 37 is part of a cooling system in which a pump 33 conveys a temperature control fluid along a closed cooling circuit. The coolant absorbs heat generated by the absorbed EUV radiation from the environment of the cooling channel 37 and dissipates it from the mirror body 38. In the present embodiment, the temperature control fluid is a cooling liquid, in particular water.

[0039] The cooling circuit extends between a liquid reservoir 42 and the cooling channel 37 formed in the mirror body 38. The liquid reservoir 42 forms a fluid reservoir within the meaning of the invention. A supply line 40 is connected to an outlet opening 45 of the liquid reservoir 42, which extends from the liquid reservoir 42 to a switching device 39. A return line 41 runs from the switching device 39 back to the liquid reservoir 42. The cooling channel 37 of the mirror body 38 is connected to the switching device 39 via a first supply line 35 and a second supply line 36.

[0040] The liquid reservoir 42 includes a temperature control module 44, which is arranged in front of the outlet opening 45 of the liquid reservoir 42. The cooling liquid from the liquid reservoir 42 is passed through the temperature control module 44 before leaving the liquid reservoir 42 through the outlet opening 45. In the temperature control module 44, the cooling liquid is heated to a precisely defined temperature, which can, for example, be on the order of 20°C. A precisely defined temperature of the cooling liquid is required so that the cooling power acting on the mirror body 38 can be adjusted with the desired accuracy. The liquid reservoir 42 is therefore designed for unidirectional operation.If the liquid supply 42 were operated in the opposite direction and coolant were sucked in via the return line 41, the coolant would not have a defined temperature and an exact adjustment of the cooling capacity would not be possible.

[0041] In Fig. 3, the switching device 39 is shown in a first switching state 47, in which the switching device 39 forms a connection between the flow line 40 and the first supply line 35 and a connection between the second supply line 36 and the return line 41. This results in a cooling circuit that extends from the outlet opening 45 of the liquid reservoir 42 via the flow line 40 and the first supply line 35 to the fluid channel 37 and from there back via the second supply line 36 and the return line 41 to the liquid reservoir 42. The pump 33 sucks cooling liquid from the liquid reservoir 42 and conveys it along the cooling circuit to the cooling channel 37.

[0042] In a second switching state 48 ( Fig. 4) The switching device 39 forms a connection between the supply line 40 and the second supply line 36, as well as a connection between the first supply line 35 and the return line 41. This results in a cooling circuit extending from the outlet opening 45 of the liquid reservoir 42 via the supply line 40 and the second supply line 36 to the fluid channel 37. The return of the cooling liquid from the cooling channel 37 to the liquid reservoir 42 occurs via the first supply line 35 and the return line 41.

[0043] The flow direction of the cooling liquid through the cooling channel 37 formed in the mirror body 38 is therefore opposite in the second switching state 48 of the switching device 39 to the flow direction in the first switching state 47 of the switching device 39.

[0044] The operation of the mirror system is controlled by a control unit 34. In addition to other Fig. 2 For functions not shown, in particular the pump 33 and the switching device 39 are subject to the specifications of the control unit 34. During operation of the mirror system, the pump 33 is controlled in such a way that it conveys a uniform and slow volume flow through the cooling channel 37 so that no mechanical vibrations are excited.

[0045] The switching state of the switching device 39 is derived from input information that informs the control unit 34 of the lighting setting with which the projection lens 22 is operated. An assignment is stored in a memory module 43, from which an assigned switching state of the switching device 39 results for each lighting setting. Before commissioning the mirror system with a new lighting setting, the control unit 34 reads the assignment from the memory module 43 and sends a control signal to the switching device 39, with which the switching device 39 is set to the switching state matching the lighting setting.

[0046] The switch to a different illumination setting occurs during an operational pause of the microlithographic projection exposure system. At the end of the previous operating phase, the control unit 34 sends control commands to the pump 33 so that the volume flow of the cooling fluid is slowly reduced over a period of several minutes until the cooling fluid has come to a standstill. The control unit 34 queries the switching state for the next operating phase from the memory module 43 for the switching device 39 and sends a control signal to the switching device 39 so that the switching device 39 assumes the corresponding switching state 47, 48. At the beginning of the next operating phase, the pump 33 is started up to slowly restart the volume flow through the cooling channel 37.

[0047] In Fig.Figure 5 shows an alternative embodiment of a mirror system according to the invention, in which the pump 33 is arranged in the first supply line 35, which extends between the switching device 39 and the mirror body 38. The pump 33 is designed as a bidirectional pump that can both convey a volume flow coming from the switching device 39 toward the mirror body 38 and a volume flow coming from the mirror body 38 toward the switching device 39. The conveying direction is set by a control signal from the control unit 34.

[0048] The control unit 34 is configured to adjust the flow direction of the pump 33 to match the switching state 47, 48 of the switching device 39. If the switching device 39 is in the first switching state 47, the pump 33 pumps the coolant toward the mirror body 38. The cooling circuit extends from the outlet opening 45 of the fluid reservoir 42 via the feed line 40, the first supply line 35 to the cooling channel 37, and from there back to the fluid reservoir 42. If the switching device 39 is in the second switching state 48, the pump 33 pumps the coolant toward the switching device 39. This results in a cooling circuit from the fluid reservoir 42 via the feed line 40 and the second supply line 36 to the cooling channel 37.

Claims

[1] Mirror system, with an EUV mirror (M1-M6), with a fluid reservoir (42) for a temperature control fluid and with a pump (33), wherein the EUV mirror (M1-M6) comprises a mirror body (38) and an optical surface (32) formed on the mirror body (38) with high reflectivity for EUV radiation, wherein a fluid channel (37) for the temperature control fluid is formed in the mirror body (38), wherein a first supply line (35) and a second supply line (36) are formed between the fluid reservoir and the fluid channel (37), wherein a switching device (39) is arranged between the fluid channel (37) and the fluid reservoir (42), wherein in a first switching state (47) of the switching device (39), the pump (33) supplies the temperature control fluid from the fluid reservoir (42) through the first supply line (35) to the fluid channel (37) promotesand wherein in a second switching state (48) of the switching device (39), the pump (33) conveys the tempering fluid through the second supply line (36) to the fluid channel (37). [2] Mirror system according to claim 1, comprising a fluid circuit which is closed between the fluid channel (37) and the fluid reservoir (42) via the first supply line (35) and the second supply line (36). [3] Mirror system according to claim 1 or 2, wherein the fluid reservoir (42) is designed as a unidirectional fluid reservoir (42). [4] Mirror system according to claim 3, wherein in the first switching state (47) of the switching device (39) an outlet opening (45) of the fluid reservoir (42) is coupled to the first supply line (35) and wherein in the second switching state (48) of the switching device (39) the outlet opening (45) of the fluid reservoir is coupled to the second supply line (36). [5] Mirror system according to one of claims 1 to 3, wherein the fluid reservoir (42) comprises a temperature control module (44) such that the temperature of the temperature control fluid is within a predetermined range when exiting the fluid reservoir (42). [6] Mirror system according to one of claims 1 to 5, wherein the fluid flow along the fluid channel (37) is brought to a halt before the switching state (47, 48) of the switching device (39) is changed. [7] Mirror system according to claim 6, wherein the stopping of the fluid flow is extended over a transition phase and wherein the transition phase has a length of at least 30 s, preferably at least 1 minute, more preferably at least 2 minutes. [8] Mirror system according to claim 6 or 7, wherein a maximum value for the rate of change of the fluid flow is predetermined during the transition phase. [9] Mirror system according to one of claims 1 to 8, comprising a control unit (34) which is designed to control the switching device (39) on the basis of a predetermined assignment between switching states (47, 48) and lighting settings. [10] Projection objective of a microlithographic projection exposure apparatus, with a plurality of EUV mirrors (M1-M6) with which a photomask (13) is imaged into an image plane (21), wherein at least one of the EUV mirrors (M1-M6) is an EUV mirror (M1-M6) of a mirror system according to one of claims 1 to 9. [11] Method for operating a mirror system, in which an EUV mirror (M1-M6) comprises a mirror body (38) and an optical surface (32) formed on the mirror body (38) with high reflectivity for EUV radiation, wherein a fluid channel (37) for a temperature control fluid is formed in the mirror body (38), wherein a first supply line (35) and a second supply line (36) are formed between a fluid reservoir and the fluid channel (37), wherein in a first switching state (48) of the mirror system the temperature control fluid is conveyed through the first supply line (35) from the fluid reservoir (42) to the fluid channel (37), and wherein in a second switching state (48) of the mirror system the temperature control fluid is conveyed through the second supply line (36) from the fluid reservoir (42) to the fluid channel (37).

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