A seed laser optical isolator, seed isolator module, EUV radiation source, lithographic apparatus and optical isolator operating method

Abstract

This invention relates to an optical isolator for a sub-laser, comprising: - an acoustic-optical modulator crystal configured to manipulate incident laser light thereon, and - at least one cooling system configured to regulate a temperature of the crystal, the cooling system comprising: ○ a cooling element including one or more channels for a fluid cooling medium, ○ a heat transfer assembly disposed between the crystal and the cooling element to transfer heat from the crystal to the cooling element, wherein the heat transfer assembly includes an active heat transfer element.

Description

[Technical Field]

[0001] This invention relates to an optical isolator for a seed laser, a seed isolator module for an EUV source, and an EUV radiation source comprising a seed laser and a seed isolator module. Further, this invention relates to a lithography apparatus comprising an EUV radiation source and a method for operating an optical isolator. [Previous Technology]

[0002] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. A lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). For example, the lithography apparatus can project a pattern at a patterned device (e.g., a photomask) onto a radiation-sensitive material (resist) layer disposed on a substrate.

[0003] To project a pattern onto a substrate, a lithography apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Compared to a lithography apparatus using radiation with a wavelength, for example, 193 nm, a lithography apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0004] EUV radiation can be generated by ionizing tin atoms in a molten tin droplet with radiation from a seed laser, thereby producing a strong plasma that radiates EUV radiation in all directions. Although the molten tin droplet absorbs a large amount of radiation from the seed laser, the radiation is also reflected back to the seed laser. To protect the seed laser, a seed isolator module with an optical isolator is used to absorb the reflected laser light from the molten tin droplet.

[0005] An acousto-optic modulator (AOM) is used to absorb reflected laser light by directing it to a so-called beam catcher. The AOM contains a crystal heated by the absorbed laser light. Above a certain temperature threshold, irreversible damage can occur in the crystal. In the event of crystal damage, the entire seed isolator module containing the AOM may need to be replaced. To avoid damaging the crystal, it needs to be cooled, currently provided by enclosing the crystal between copper blocks cooled by cooling water. Given the current optical power of the seed laser, cooling only provides a relatively small margin to that temperature threshold. Therefore, the above solution is not optimal for high-power laser applications, resulting in an excessive risk of crystal damage. As mentioned above, when the crystal fails, a complete AOM may need to be replaced at the corresponding cost and downtime. [Summary of the Invention]

[0006] In view of the above, one objective of the present invention is to provide an optical isolator that reduces the risk of laser-induced damage to the crystal of an acousto-optic modulator.

[0007] According to one embodiment of the present invention, an optical isolator for a sub-laser is provided, comprising: - an acoustic-optical modulator crystal configured to manipulate laser light incident thereon, and - at least one cooling system configured to regulate a temperature of the crystal, the cooling system comprising: ○ a cooling element including one or more channels for a fluid cooling medium, and ○ a heat transfer assembly disposed between the crystal and the cooling element to transfer heat from the crystal to the cooling element, wherein the heat transfer assembly includes an active heat transfer element.

[0008] According to another embodiment of the present invention, a seed isolator module for an EUV source is provided, which includes an optical isolator according to the present invention.

[0009] According to another embodiment of the present invention, an EUV radiation source is provided, which includes a seed laser according to the present invention and a sub-isolator module to absorb unwanted reflected radiation originating from the seed laser.

[0010] According to yet another embodiment of the present invention, a lithography device is provided, which includes an EUV radiation source according to one of the present invention.

[0011] According to another embodiment of the present invention, a method for operating an optical isolator according to the present invention is provided, the method comprising the steps of: a. providing a cooling medium flow at a predetermined temperature via a cooling element, b. using a heat transfer assembly having an active heat transfer element to transfer heat from a crystal to the cooling medium in the cooling element, such that the temperature of the outer surface of the crystal facing the heat transfer element is lower than the predetermined temperature of the cooling medium in the cooling element.

Implementation Method

[0013] Figure 1 illustrates a lithography system including a radiation source SO and a lithography device LA. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography device LA. The lithography device LA includes: an illumination system IL; a support structure MT configured to support a patterning device MA (e.g., a photomask); a projection system PS; and a substrate stage WT configured to support a substrate W.

[0014] The irradiation system IL is configured to adjust the EUV radiation beam B before it is incident on the patterning device MA. Additionally, the irradiation system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide an EUV radiation beam B having the desired cross-sectional shape and desired intensity distribution. The irradiation system IL may include other mirrors or devices besides or replacing the faceted field mirror device 10 and the faceted pupil mirror device 11.

[0015] After such adjustment, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate stage WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thus forming an image with features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is described as having only the two mirrors 13, 14 shown in FIG. 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0016] The substrate W may include a previously formed pattern. In this case, the lithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0017] A relative vacuum may be provided in the radiation source SO, in the irradiation system IL and / or in the projection system PS, that is, a small amount of gas (e.g., hydrogen) at a pressure sufficiently lower than atmospheric pressure.

[0018] The radiation source SO shown in Figure 1 is of the type that may be referred to as a laser-generated plasma (LPP) source. A laser system 1, which may include, for example, a CO2 laser, is configured to deposit energy via a laser beam 2 onto a fuel (such as tin (Sn)) supplied by, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel may be used. The fuel may be, for example, in liquid form and may be, for example, a metal or an alloy. The fuel emitter 3 may include a nozzle configured to guide, for example, tin in droplet form along a trajectory to a plasma forming region 4. The laser beam 2 is incident on the tin at the plasma forming region 4. The deposition of laser energy into the tin generates tin plasma 7 at the plasma forming region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during the de-excitation and recombination of electrons and ions in the plasma.

[0019] EUV radiation from the plasma is collected and focused by collector 5. Collector 5 includes, for example, a near-normal incident radiation collector 5 (sometimes more generally referred to as a normal incident radiation collector). Collector 5 may have a multi-layered mirror structure configured to reflect EUV radiation (e.g., EUV radiation with a desired wavelength such as 13.5 nm). Collector 5 may have an ellipsoidal configuration having two foci. The first of the foci may be located at the plasma formation region 4, and the second of the foci may be located at the intermediate focal point 6, as discussed below.

[0020] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be delivered from the laser system 1 to the radiation source SO by means of a beam delivery system (not shown) comprising, for example, a suitable guide mirror and / or a beam expander and / or other optical elements. The laser system 1, the radiation source SO, and the beam delivery system may be considered together as a radiation system.

[0021] The radiation reflected by collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at intermediate focal point 6 to form an image at intermediate focal point 6 of the plasma present in plasma formation region 4. The image at intermediate focal point 6 serves as a virtual radiation source for irradiating system IL. Radiation source SO is configured such that intermediate focal point 6 is located at or near opening 8 in enclosure structure 9 of radiation source SO.

[0022] Although Figure 1 depicts the radiation source SO as a laser-generated plasma (LPP) source, any suitable source such as a discharge-generated plasma (DPP) source or a free electron laser (FEL) can be used to generate EUV radiation.

[0023] Although the laser beam 2 is intended to be completely absorbed by the fuel (i.e., the molten tin droplets at the plasma formation region 4) to form plasma 7, in practice, a portion of the laser beam 2 will be reflected from the molten tin droplets, and the reflected laser light can travel back to the laser system 1. This can damage the laser system 1 or interfere with its operation.

[0024] To prevent damage to the laser system 1, the laser system 1 or the radiation source SO may include a seed isolator module SIM, partially depicted in FIG2, to absorb reflected laser light traveling back to the laser system 1.

[0025] The seed isolator module SIM includes an optical isolator OI and a control unit CU for controlling the optical isolator OI.

[0026] The optical isolator (OI) includes an acousto-optic modulator crystal (CR) to manipulate light incident upon it. In this example, the crystal CR is oriented such that the optical axis of the laser light used to reflect back to the laser system is perpendicular to the plane of the diagram in FIG2, i.e., perpendicular to the X and Y axes depicted in FIG2. This invention does not concern the working principle of the crystal CR itself, and therefore will not include a detailed explanation, as those skilled in the art are already familiar with the working principle of the acousto-optic modulator crystal CR. The crystal CR may be (e.g., as a separate component or as an integral component) part of an acousto-optic modulator that introduces acoustic vibrations into the crystal CR. However, broadly speaking, by subjecting the crystal CR to acoustic vibrations, the crystal CR is able to guide laser light reflected from molten tin droplets to an absorption element AE, which is also part of the optical isolator (OI) or seed isolator module (SIM) and is depicted schematically in FIG2. The absorption element AE may alternatively be referred to as a beam catcher. Laser beam 2 and the reflected laser light will also be absorbed at least partially by the crystal CR itself, thereby causing the crystal CR to heat up. If no measures are taken, the temperature of the crystal CR may increase above a certain temperature limit, which may cause damage to the crystal CR, resulting in at least partial loss of function, but usually complete loss of function within a short period of time.

[0027] The seed isolator module SIM may further include other components such as polarization protection optics, for example, which are not depicted here to keep the diagram simple.

[0028] To maintain the temperature of the crystal CR below a predetermined temperature, preferably a temperature threshold minus a safety margin of at least a certain number of degrees Celsius, the optical isolator OI includes at least one cooling system. The embodiment of Figure 2 is more extensive because the optical isolator OI includes: a first cooling system CS1 located at a first crystal surface FC of the crystal CR; and a second cooling system CS2 located at a second crystal surface SC of the crystal CR, wherein the first and second crystal surfaces FC and SC are parallel opposing surfaces of the crystal CR in this example, allowing the crystal CR to be cooled on both sides.

[0029] The first and second cooling systems CS1 and CS2 are constructed similarly in this case, and are described in conjunction with the reference cooling system, although it is also contemplated that the construction of the first cooling system CS1 differs from that of the second cooling system depending on the cooling requirements of the crystal CR.

[0030] In this context, "similar" means that the first and second cooling systems have the same components that have the same function. "Similar" includes the possibility that the first and second cooling systems are identical, but also includes cases where the dimensions differ, for example, due to the second crystal surface SC being larger than the first crystal surface, as is the case in the embodiment of FIG2. Differences in size or construction can be caused by differences in available space or asymmetric heating of the crystal CR.

[0031] The cooling systems CS1 and CS2 in Figure 2 include a cooling element CE, which includes one or more channels CH for a fluid cooling medium such as water. The cooling element CE, including one or more channels CH, is schematically depicted here as a cooling block having a single channel CH, an inlet IN to the channel CH, and an outlet OU of the channel CH. The channel CH can be connected to a cooling circuit that provides a cooling medium at a predetermined temperature to the inlet IN. The cooling medium in the channel CH can then absorb heat as it flows toward the outlet OU. An advantage of the cooling medium is that heat can be easily removed and disposed of in environments where temperature requirements are less stringent.

[0032] The cooling medium may be a liquid such as water or any other suitable liquid, or a cooling gas such as air or any other suitable gas. One or more channels may be closed channels, but may also be channels open on one side, as in the case of a typical fin structure where gas is blown through the space between the fins, wherein the fins provide sufficient cooling surface for heat transfer to the gas. The space between the fins can be considered as an open channel.

[0033] The cooling systems CS1 and CS2 in Figure 2 further include a heat transfer assembly HT disposed between the crystal CR and the corresponding cooling element CE to transfer heat from the crystal CR to the cooling element CE and ultimately to the cooling medium. The heat transfer assembly HT includes an active heat transfer element, such as a Peltier element, which will be described in more detail below. The active heat transfer element transfers heat from the crystal side of the active heat transfer element to the cooling element side of the active heat transfer element. The crystal side of the active heat transfer element will be alternatively referred to as the cold side of the active heat transfer element, and the cooling element side of the active heat transfer element will be alternatively referred to as the hot side. During operation, a temperature difference may exist between the cold side and the hot side. The thermal resistance between the hot side of the cooling element CE and the cooling medium is preferably sufficiently low. Furthermore, compared to the case where there is no active heat transfer element and heat is mainly transferred from the crystal CR to the cooling element CE by (passive) heat conduction, the active heat transfer element can set the temperature of the cold side to be lower than the temperature of the fluid cooling medium, thereby reducing the temperature of the crystal CR.

[0034] In one embodiment, it is possible to cool the cold side of the active heat transfer element and possibly other components below the dew point of the ambient air that could cause condensation. To avoid or minimize condensation, the optical isolator may include a gas chamber or space containing conditions different from normal air, such as a dry purge gas environment, like N2 purging or extremely clean dry air.

[0035] The active heat transfer element (i.e., Peltier element) may be open-loop controlled, but preferably, a control unit CU is used to provide closed-loop control. For this purpose, the heat transfer assembly, as will be described in more detail below, preferably includes a temperature sensor to measure a temperature that can be used as the basis for driving the active heat transfer element. For example, when a cooling medium at a predetermined temperature is provided via a cooling element, the temperature sensor can measure a temperature representing the temperature of the outer surface of the active heat transfer element facing the crystal, and the control unit CU can be configured to drive the active heat transfer element so that heat is transferred from the crystal CR to the cooling medium in the cooling element CE using the heat transfer assembly having the active heat transfer element, such that the temperature of the outer surface of the crystal facing the active heat transfer element is lower than the predetermined temperature of the cooling medium in the cooling element CE.

[0036] It should be explicitly stated here that the embodiment of FIG2 is a broad example and can be simplified. For example, only one cooling system may be provided instead of two, or an active heat transfer element such as a Peltier element may be provided in only one of the two cooling systems.

[0037] The active heat transfer element may be a single component or an assembly of two or more individual sub-elements configured in parallel and / or serially. This means, for example, two active heat transfer sub-elements (e.g., two Peltier elements) may be configured close together to operate in parallel to transfer heat from the crystal CR to the cooling element CE. When both the first cooling system CS1 and the second cooling system CS2 have the same number of active heat transfer sub-elements, the crystal CR is sandwiched between two pairs of active heat transfer sub-elements.

[0038] Figure 3 schematically depicts the heat transfer assembly HT of Figure 2 in more detail. It indicates the cooling elements CE and the crystal CR on both sides of the heat transfer assembly HT. In this example, the heat transfer assembly HT is a stack of the following layers starting at the crystal side of the heat transfer assembly HT: - Thermal contact layer TP1 - Heat sink layer HSP - Temperature sensor TSE - Thermal contact layer TP2 - Peltier element PEL - Thermal contact layer TP3

[0039] Thermal contact layers TP1 to TP3 are provided to improve the thermal contact between two adjacent layers by reducing the thermal resistance between adjacent layers. Thermal contact layer TP1 is therefore configured to reduce the thermal resistance between the crystal CR and the heat sink layer HSP. It should be understood that the heat sink layer HSP is a heat sink layer, wherein the heat sink layer is configured to distribute heat in the plane of the heat sink layer. Thermal contact layer TP2 is configured to reduce the thermal resistance between the temperature sensor TSE and the Peltier element PEL, and thermal contact layer TP3 is configured to reduce the thermal resistance between the Peltier element PEL and the cooling element CE of FIG2.

[0040] The thermal contact layers TP1 to TP3 may comprise, for example, an elastic or deformable material in a paste-like form, which can adjust its shape according to the irregularities of the material surfaces of adjacent layers, thereby increasing the effective surface area available for direct thermal conduction between the two adjacent layers. A thermal contact layer cannot be provided when there is a sufficient effective surface area available for direct thermal conduction, for example because the material surfaces of adjacent layers are smooth and well-matched, or because one of these layers is deformable enough to adapt its shape to the other layer.

[0041] Preferably, a heat sink layer HSP made of a highly conductive material (e.g., a metal such as copper or aluminum) is provided to distribute heat from the crystal CR across the entire cross-section of the Peltier element PEL, thereby improving the efficiency of the heat transfer assembly. This is advantageous when the crystal CR itself has a significantly lower thermal conductivity than the heat sink HSP. When the thermal conductivity of the crystal is sufficiently high, the heat sink layer HSP can be omitted. For example, the thermal contact layers TP1 or TP2 may also provide similar functionality to the heat sink layer HSP.

[0042] The temperature sensor TSE provides the advantage of being able to measure temperature to allow closed-loop control of the PEL element. The temperature sensor TSE can be a separate layer as indicated in Figure 3 and can include, for example, wires whose impedance depends on the temperature. However, the temperature sensor TSE can also be integrated into another layer, for example, the heat sink HSP.

[0043] As shown in Figures 2 and 3, the heat transfer assembly HT preferably matches the corresponding crystal surface it faces, but this is not necessarily required. Therefore, the heat transfer assembly HT may be smaller or larger than the corresponding crystal surface. Furthermore, although Figure 3 indicates that the Peltier element extends across the entire cross-section of the heat transfer assembly HT, it is also contemplated that the Peltier element or its sub-elements extend only across a portion of the cross-section of the heat transfer assembly HT. The Peltier element PEL may be formed by a single Peltier device, except alternatively by two or more sub-elements SE1, SE2 indicated by the dashed lines, each sub-element comprising a single Peltier device.

[0044] Figure 4 schematically depicts a control scheme for an optical isolator (OI) or seed isolator module (SIM) according to another embodiment of the present invention. The schematically depicted stack of components includes: - an acousto-optic modulator crystal (CR). As indicated above, this component can absorb radiation and heat up during use; - a temperature sensor (TSE); - an active heat transfer element, in this embodiment a Peltier element (PEL); and - a cooling element (CE).

[0045] The acousto-optic modulator crystal CR is part of an acousto-optic modulator used to direct radiation in a desired direction for absorption by a so-called beam catcher.

[0046] In a simplified embodiment, the temperature sensor TSE and the active heat transfer element PEL are part of a heat transfer assembly. A thermal contact layer or heat sink layer is not provided, but may be present if necessary to improve heat transfer.

[0047] The cooling element CE is part of the cooling system to remove heat from the optical isolator OI. Therefore, the heat transfer assembly is configured to use an active heat transfer element PEL to transfer heat from the crystal CR to the cooling element CE. In this embodiment, the active heat transfer element PEL may be a Peltier element. Given this heat transfer direction, the crystal side of the active heat transfer element PEL is alternatively referred to as the cold side CSI of the Peltier element PEL, and the cooling element side of the active heat transfer element PEL is alternatively referred to as the hot side HIS of the Peltier element PEL.

[0048] A control unit CU is provided to drive the Peltier element PEL based on the output of a temperature sensor TSE. The temperature sensor TSE is located at the cold side CSI of the Peltier element, i.e., between the Peltier element PEL and the crystal CR, thereby allowing effective control of the temperature of the crystal CR. The Peltier element PEL has two electrodes indicated by a "-" and a "+" sign at the Peltier element PEL. By applying an appropriate voltage to the Peltier element PEL, the Peltier element will transfer heat from the cold side CSI of the Peltier element PEL to the hot side HSI. The control unit CU typically drives the Peltier element PEL such that the temperature at the cold side CSI of the Peltier element PEL, as measured by the temperature sensor TSE, is maintained at a desired temperature, alternatively referred to as a stationary temperature.

[0049] Although the above embodiments relate to EUV radiation sources for lithography devices, the present invention can also be readily applied to other high-power radiation sources where the acousto-optic modulator can be used to absorb radiation and requires sufficient cooling. Another example of this application is a radiation source for laser cutting. Therefore, the optical isolator according to the present invention can be suitable for other high-laser-power environments, such as EUV or laser cutting. High-laser-power environments can be considered to be from 10 W, preferably from 100 W up to at most 1 kW, 10 kW, 100 kW, or even higher.

[0050] Another embodiment of the present invention describes a method of operating an optical isolator according to any of the embodiments of the present invention, wherein the method comprises the steps of: a. providing a cooling medium flow at a predetermined temperature via a cooling element, and b. using a heat transfer assembly having the active heat transfer element to transfer heat from the crystal to the cooling medium in the cooling element, such that the temperature of the outer surface of the crystal facing the heat transfer element is lower than the predetermined temperature of the cooling medium in the cooling element.

[0051] Although reference may be specifically made herein to the use of lithography devices in IC manufacturing, it should be understood that the lithography devices described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guiding and detecting patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0052] Although specific reference may be made herein to embodiments of the invention within the context of lithography apparatus, embodiments of the invention can be used in other apparatuses. Embodiments of the invention may form part of a photomask inspection apparatus, a measuring apparatus, or any apparatus for measuring or processing objects such as wafers (or other substrates) or photomasks (or other patterning devices). Such apparatuses may generally be referred to as lithography tools. Such lithography tools may be used under vacuum conditions or ambient (non-vacuum) conditions.

[0053] Although reference may be made specifically to the use of embodiments of the invention in the context of optical lithography, it should be understood that the invention is not limited to optical lithography where the context permits, and can be used in other applications such as imprinting.

[0054] Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. Machine-readable media may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, machine-readable media may include: read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagation signals (e.g., carrier waves, infrared signals, digital signals, etc.) and others. Furthermore, firmware, software, conventions, and instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only, and such actions are actually caused by a computing device, processor, controller, or other device executing firmware, software, conventions, instructions, etc., and in performing such actions may cause actuators or other devices to interact with the physical world.

[0055] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in other ways different from those described. The above description is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set forth below. [Simplified Explanation of the Diagram]

[0012] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic diagrams, in which: - FIG1 depicts a lithography system including a lithography device and a radiation source; - FIG2 depicts a seed isolator module according to an embodiment of the present invention; - FIG3 depicts in more detail the heat transfer assembly of the optical isolator of the seed isolator module of FIG2; and - FIG4 depicts a schematic control scheme for an optical isolator or seed isolator module according to an embodiment of the present invention.

Claims

1. An optical isolator for a sub-laser, comprising: an acoustic-optic modulator crystal configured to manipulate incident laser light; a first cooling system disposed on a first crystal surface of the acoustic-optic modulator crystal; and a second cooling system disposed on a second crystal surface of the acoustic-optic modulator crystal, wherein each of the first and second cooling systems comprises: a cooling element including one or more channels for a fluid cooling medium; and a heat transfer assembly disposed between the crystal and the cooling element to transfer heat from the crystal to the cooling element, wherein the heat transfer assembly includes an active heat transfer element.

2. The optical isolator of claim 1, wherein the active heat transfer element includes a Peltier element.

3. The optical isolator of claim 1 or 2, wherein the second crystal surface is opposite to the first crystal surface.

4. An optical isolator as claimed in claim 1 or 2, wherein the first crystal surface and the second crystal surface are parallel to each other.

5. The optical isolator as claimed in claim 1 or 2, wherein the heat transfer assembly includes a thermal contact layer disposed between the active heat transfer element and the cooling element, wherein the thermal contact layer is configured to reduce a thermal resistance between the active heat transfer element and the cooling element.

6. The optical isolator as claimed in claim 1 or 2, wherein the heat transfer assembly includes a thermal contact layer disposed between the crystal and the active heat transfer element, wherein the thermal contact layer is configured to reduce a thermal resistance between the crystal and the active heat transfer element.

7. The optical isolator as claimed in claim 1 or 2, wherein the heat transfer assembly includes a thermal spreader layer disposed between the crystal and the active heat transfer element, wherein the thermal spreader layer is configured to distribute heat in the plane of the thermal spreader layer.

8. The optical isolator as claimed in claim 1 or 2, wherein the heat transfer assembly includes a temperature sensor disposed between the crystal and the cooling element to measure a temperature.

9. The optical isolator of claim 8, wherein the temperature sensor is disposed between the crystal and the active heat transfer element.

10. A seed isolator module for an EUV source, comprising an optical isolator as claimed in any one of claims 1 to 9.

11. The seed isolator module of claim 10, further comprising a control unit for driving the active heat transfer element of the heat transfer assembly.

12. The seed isolator module of claim 11, wherein the optical isolator is one of the optical isolators of claim 8 or 9, and wherein the control unit is configured to drive the active heat transfer element in relation to the output of one of the temperature sensors.

13. An EUV radiation source comprising a sub-laser and a seed isolator module as claimed in any one of claims 10 to 12 to absorb unwanted reflected radiation originating from the seed laser.

14. A lithography system comprising an EUV radiation source as claimed in claim 13.

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