DE102019207559A1 - Assembly for cooling an optical element, in particular for a microlithographic projection exposure system - Google Patents

Abstract

The invention relates to an assembly for cooling an optical element, in particular for a microlithographic projection exposure system, with at least one cooling channel which is formed in the optical element (105, 205), a fluid in the cooling channel and which has a vapor pressure of at most 3 Pascal ( Pa), and at least one pressure compensation reservoir (130, 230), which is coupled to the at least one cooling channel in a closed, pumpless cooling circuit (110, 210).

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to an assembly for cooling an optical element, in particular for a microlithographic projection exposure system.

State of the art

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

In projection lenses designed for the EUV area, i.e. at wavelengths of e.g. about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process due to the lack of suitable translucent refractive materials.

A problem that occurs in practice is that the EUV mirrors experience heating and an associated thermal deformation, in particular as a result of the absorption of the radiation emitted by the EUV light source, which in turn can impair the imaging properties of the optical system. This is particularly the case when lighting settings with comparatively small lighting poles (e.g. in dipole or quadrupole lighting settings) are used, in which the mirror heating or deformation varies greatly across the optical surface of the mirror.

A transfer of solution approaches known for VUV lithography systems (with a working wavelength e.g. of approx. 200 nm or approx. 160 nm) to overcome the above-described problem of mirror heating to EUV systems takes place, among other things. partly as difficult in that the number of optical active surfaces available for active deformation compensation is relatively severely limited due to the comparatively smaller number of optical elements or mirrors (to avoid excessive light losses due to the necessary reflections).

Known approaches for dissipating heat loads from optical components, in particular a microlithographic projection exposure system, include the use of cooling channels, which are used to remove heat with a cooling fluid such as Water flow through and are connected to an inlet and an outlet for the cooling fluid.

A problem that arises in practice here is that the pressurization that is necessarily associated with the pumping of the cooling fluid through the cooling channels causes a deformation of the respective optical component (e.g. the mirror), these deformations in turn being carried out in further measures such as e.g. are to be taken into account during surface processing and / or characterization.

Another problem that arises when using cooling channels through which cooling fluid flows is that unavoidable hydrostatic forces occur at the cooling fluid connections, which hydrostatic forces have to be actively compensated for in order to avoid undesired deformation of the respective optical component or the mirror.

Furthermore, the above-described use of cooling channels through which a cooling fluid flows also requires additional control effort, with fluctuations in the respective ambient pressure having to be taken into account in particular.

The state of the art is only given as an example DE 10 2010 002 298 A1 referred.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an assembly for cooling an optical element, in particular for a microlithographic projection exposure system, which enables an effective avoidance or at least reduction of thermally induced deformations or associated impairments of the imaging behavior while at least partially avoiding the problems described above .

This object is achieved by the assembly according to the features of independent claim 1.

• - wenigstens einen Kühlkanal, welcher in dem optischen Element ausgebildet ist,
• - ein in dem Kühlkanal befindliches Fluid, welches einen Dampfdruck von maximal 3 Pascal (Pa) besitzt, und
• - wenigstens ein Druckausgleichsreservoir, welches an den wenigstens einen Kühlkanal in jeweils einem geschlossenen, pumpenlosen Kühlkreis gekoppelt ist.

An assembly according to the invention for cooling an optical element, in particular for a microlithographic projection exposure system, has:

• at least one cooling channel which is formed in the optical element,
• - A fluid in the cooling channel, which has a vapor pressure of maximum 3 Pascal (Pa), and
• - At least one pressure compensation reservoir, which is coupled to the at least one cooling channel in a closed, pumpless cooling circuit.

The present invention is based in particular on the concept of cooling an optical element, such as, for example, completely dispensing with active pumping or actively induced coolant circulation. of a mirror by realizing that in a closed, pumpless cooling circuit an essentially stationary fluid (and in particular not actively pressurized to produce a flow) is used for heat dissipation. Using the (intrinsic) thermal conductivity of this fluid and the physical process of thermal conduction, heat is removed from the optical element (e.g. from the optical surface of the mirror) e.g. to an external heat exchanger or an external cooling circuit.

The low vapor pressure of a maximum of 3 Pascals (Pa) of the fluid used in accordance with the invention ensures that the fluid in question remains liquid at all times and does not change into the gaseous state.

At the same time, the pressure equalization reservoir present in the assembly according to the invention ensures that the fluid used according to the invention e.g. can expand or contract when temperature changes occur and the absolute pressure of the fluid thus has no influence on the surface topology of the optical element or mirror: The connection of the (at least one) cooling channel comprising the fluid in a closed, pumpless cooling circuit to the pressure compensation reservoir is particularly important As a result, the fluid pressure is automatically adjusted to the respective ambient pressure, so that regardless of the respective ambient pressure - which, for example, of an EUV projection lens in vacuum operation 3 Pascal (Pa), when ventilating during maintenance work but also e.g. 1 bar - there are no mechanical stresses due to pressure differences and the associated deformations on the optical element or mirror.

A further advantage of dispensing with fluid circulation in the cooling circuit according to the invention is that there is a substantially increased freedom or flexibility with regard to the specific design (dd of the “layout”) of the cooling circuit, with any desired branching or lattice structures of channels without the need of Flow resistances or the like are possible.

According to a further advantageous aspect, the placement of the at least one cooling channel in the assembly according to the invention can take place at a comparatively small distance (for example of the order of 1 mm) from the optical active surface of the optical element, the invention taking advantage of the fact that the invention is used Fluid causes no pressure-related deformations of the optical active surface.

According to one embodiment, the fluid has a thermal conductivity of at least 5 Wm ^-1 K ^-1 .

According to one embodiment, the fluid has a liquid metal or a liquid metal alloy.

According to one embodiment, the fluid has mercury (Hg), a gallium-indium-tin alloy or a sodium-potassium alloy.

According to one embodiment, the assembly also has at least one temperature control device for temperature control of the fluid.

According to one embodiment, the at least one temperature control device is designed for active regulation of the temperature of the fluid.

According to one embodiment, the at least one temperature control device has a heat exchanger. In other embodiments, any other suitable heating or cooling device, e.g. electrical (resistance) heating elements and / or Peltier elements can be used for temperature control.

According to one embodiment, a dynamic decoupling is also provided between the optical element and the at least one temperature control device. In this way, a transfer of excitations or vibrations to the optical element on the part of the temperature control device or an external support structure, which may receive it, can be avoided.

According to one embodiment, the cooling circuit has a plurality of cooling channels. These cooling channels can preferably run in parallel, but alternatively can also run in series or in series.

According to one embodiment, the assembly has a plurality of pumpless cooling circuits, a pressure compensation reservoir being coupled to at least one cooling channel in each of these cooling circuits. By way of example only, several hundred cooling circuits can be provided for cooling an optical element in the form of a comparatively large mirror with dimensions of the order of one or more meters.

According to one embodiment, each of the cooling circuits is assigned a temperature control device for temperature control of the fluid, in particular for active regulation of the temperature of the fluid. In this way, the temperature can be regulated individually for individual zones of the optical element.

The invention further relates to an optical system of a microlithographic projection exposure system, in particular an illumination device or a projection objective, with at least one assembly with the features described above.

Further refinements of the invention can be found in the description and the subclaims.

The invention is explained in more detail below on the basis of exemplary embodiments illustrated in the attached figures.

Figure list

• 1 eine schematische Darstellung zur Erläuterung des Aufbaus einer Baugruppe gemäß einer Ausführungsform der Erfindung;
• 2 eine schematische Darstellung zur Erläuterung des Aufbaus einer Baugruppe gemäß einer weiteren Ausführungsform der Erfindung; und
• 3 eine schematische Darstellung zur Erläuterung des möglichen Aufbaus einer für den Betrieb im EUV ausgelegten mikrolithographischen Projektionsbelichtungsanlage.

Show it:

• 1 is a schematic representation for explaining the structure of an assembly according to an embodiment of the invention;
• 2nd a schematic representation to explain the structure of an assembly according to a further embodiment of the invention; and
• 3rd a schematic representation to explain the possible structure of a microlithographic projection exposure system designed for operation in the EUV.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, exemplary embodiments of an assembly according to the invention for cooling an optical element are described with reference to the schematic representations of 1 and 2nd described.

The optical element can, for example (but without the invention being restricted to this) be a mirror of a microlithographic projection exposure system designed for operation in the EUV. In principle, the invention can advantageously be used in any other application outside of lithography, in which an effective removal of heat from an optical element is to be realized while avoiding the problems described at the beginning.

1 shows an optical element in only a schematic representation 105 in the form of a mirror, the optical active surface of which is exposed to electromagnetic radiation during operation is labeled "105a".

As in 1 is also only indicated schematically, is in the optical element 105 formed at least one cooling channel and in a closed, pumpless cooling circuit 110 to a pressure compensation reservoir 130 , which in the exemplary embodiment is designed as a metallic bellows, is connected. In the at least one cooling channel of this closed, pumpless cooling circuit 110 there is a fluid with a low vapor pressure of at most 3 Pascal (Pa) and good thermal conductivity of preferably at least 5 Wm ^-1 K ^-1 .

In exemplary embodiments of the invention, this fluid can in particular be a liquid metal, such as e.g. Mercury, or a liquid metal alloy, e.g. a gallium-indium-tin alloy or a sodium-potassium alloy.

An essential feature of the invention is that in the closed cooling circuit 110 fluid is not circulated via a pump device to heat from the optical element 105 dissipate. Rather, the heat is dissipated through the physical process of the heat conduction taking place within the fluid, the fluid itself being essentially at rest, since no movement of the fluid is required for said heat conduction.

Since there is no actively induced flow in the fluid or is required for heat dissipation, the ambient pressure acts on the fluid alone, so that no deformations are exerted on the respective cooling channel or on the optical active surface 105a can be transferred. At the same time, the pressure equalization reservoir 130 An automatic adaptation of the fluid pressure to the ambient pressure is guaranteed even in the event of temperature differences.

With " 140 “Is in 1 one (in 1 denotes a heat exchanger) tempering device for tempering the fluid, via which (for example in connection with an additional external cooling water circuit 150 ) influences the temperature of the fluid and can be actively controlled in embodiments of the invention. With " 120 “Is in 1 denotes a dynamic decoupling which occurs between the optical element 105 and the temperature control device 140 and optionally an outer support structure is provided.

In this context, it should be pointed out that the existence of the heat-conducting fluid within the closed cooling circuit is essential 110 an essentially homogeneous temperature distribution within the optical element 105 is made possible because local temperature fluctuations are converted into a merely global and comparatively easy to correct temperature variation. Against this background, no active temperature control is required in principle in the inventive concept.

In embodiments of the invention, however, active temperature control can be used to regulate the temperature of the optical element 105 to any suitable temperature (ie not necessarily the ambient temperature of, for example, 22 ° C.). Such active temperature control can, for example, use one or more local temperature sensors in the immediate vicinity of the optical active surface 105a or also indirectly by regulating the temperature of the fluid.

The invention is not based on the presence of only a single cooling circuit 110 limited in the assembly according to the invention. In further embodiments, an arrangement of a plurality of (in each case closed) cooling circuits can also be used (in particular for cooling an optical element or mirror with larger dimensions of, for example, several meters) 110 be used. In particular, each of these cooling circuits 110 be assigned its own active temperature control. With such a configuration, locally varying heat loads can be taken into account if necessary, or a mirror that can be deformed in a plurality of degrees of freedom can be realized.

Although in 1 the pressure compensation reservoir 130 separately from the optical element 105 the invention is not limited to this. 2nd shows accordingly another embodiment, being relative to 1 analog or essentially functionally identical components are designated with reference numbers increased by “100”.

According to 2nd is the pressure compensation reservoir 230 via a closed cavity on the optical element 205 realized itself, this cavity via a flexible and eg with a bellows structure 231 provided membrane (preferably in the area of the back of the optical element 205 or in the outer edge area) to provide the flexibility required for pressure compensation. This flexible membrane can be fixed by gluing or soldering. Since there are typically no significant pressure differences between the fluid and the environment, no particular strength of the said adhesive or soldered connection is required.

According to 2nd is also a flexible thermal coupling (FTL = "flexible thermal link") 220 provided over which the cooling circuit 210 with simultaneous realization of a dynamic decoupling to a temperature control device 240 or temperature control is thermally coupled via the physical process of heat conduction.

3rd shows a schematic representation of an exemplary projection exposure system designed for operation in the EUV, in which the present invention can be implemented, for example.

According to 3rd has a lighting device in a projection exposure system designed for EUV 300 a field facet mirror 303 and a pupil facet mirror 304 on. On the field facet mirror 303 becomes the light of a light source unit, which is a plasma light source 301 and a collector mirror 302 includes, directed. In the light path after the pupil facet mirror 304 are a first telescope mirror 305 and a second telescopic mirror 306 arranged. In the light path below is a deflecting mirror 307 arranged, which the radiation striking it on an object field in the object plane of a in this case, for example, six mirrors 351-356 comprehensive projection lens. There is a reflective structure-bearing mask at the location of the object field 321 on a mask table 320 arranged, which is imaged with the aid of the projection lens in an image plane in which there is a substrate coated with a light-sensitive layer (photoresist) 361 on a wafer table 360 located.

Although the invention has been described in terms of specific embodiments, numerous variations and alternative embodiments, e.g. by combining and / or exchanging features of individual embodiments. Accordingly, it is understood by those skilled in the art that such variations and alternative embodiments are included in the present invention, and the scope of the invention is only limited within the meaning of the appended claims and their equivalents.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102010002298 A1 [0010]

Claims (14)

Assembly for cooling an optical element, in particular for a microlithographic projection exposure system, with: • at least one cooling channel which is formed in the optical element (105, 205); • a fluid in the cooling channel, which has a vapor pressure of maximum 3 Pascal (Pa); and • at least one pressure compensation reservoir (130, 230), which is coupled to the at least one cooling channel in a closed, pumpless cooling circuit (110, 210). Assembly after Claim 1 , characterized in that the fluid has a thermal conductivity of at least 5 Wm ^-1 K ^-1 . Assembly after Claim 1 or 2nd , characterized in that the fluid comprises a liquid metal or a liquid metal alloy. Assembly according to one of the Claims 1 to 3rd , characterized in that the fluid comprises mercury (Hg), a gallium-indium-tin alloy or a sodium-potassium alloy. Assembly according to one of the preceding claims, characterized in that it also has at least one temperature control device (140, 240) for temperature control of the fluid. Assembly after Claim 5 , characterized in that this at least one temperature control device (140, 240) is designed for active regulation of the temperature of the fluid. Assembly after Claim 5 or 6 , characterized in that it has at least one temperature control device (140, 240) a heat exchanger. Module according to one of the preceding claims, characterized in that a dynamic decoupling (120, 220) between the optical element (105, 205) and the at least one temperature control device (140, 240) is also provided. Module according to one of the preceding claims, characterized in that the cooling circuit (110, 210) has a plurality of cooling channels. Module according to one of the preceding claims, characterized in that it has a plurality of pumpless cooling circuits (110, 210), wherein in each of these cooling circuits (110, 210) a pressure compensation reservoir (130, 230) is coupled to at least one cooling channel. Assembly after Claim 10 , characterized in that each of these cooling circuits (110, 210) is assigned a temperature control device (140, 240) for temperature control of the fluid, in particular for active regulation of the temperature of the fluid. Assembly according to one of the preceding claims, characterized in that the optical element (105, 205) is a mirror. Module according to one of the preceding claims, characterized in that the optical element (105, 205) is designed for a working wavelength of less than 30 nm, in particular less than 15 nm. Optical system of a microlithographic projection exposure system, in particular lighting device or projection objective, with at least one assembly according to one of the Claims 1 to 13 .

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