DE102018216628A1 - Method and device for determining the heating state of an optical element in an optical system for microlithography - Google Patents

Abstract

The invention relates to a method and a device for determining the heating state of an optical element in an optical system for microlithography, the method comprising the following steps: generating, using a light source (303), one by the optical element (200, 300) transmitted first sub-beam (310) and a second sub-beam (320) not transmitted through the optical element, and determining the heating state of the optical element on the basis of a measurement of the transit time difference between the first sub-beam and the second sub-beam.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a method and a device for determining the heating state of an optical element in an optical system for microlithography.

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 (for example 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 arises in practice is that EUV levels include as a result of absorption of the radiation emitted by the EUV light source, it experiences heating and an associated thermal expansion or deformation, which in turn can have an adverse effect on the imaging properties of the optical system.

To take this effect into account, it is known as a mirror substrate material is a material with ultra-low thermal expansion ("ultra-low expansion material"), e.g. to use a titanium silicate glass marketed under the name ULE ™ by the company Corning Inc. and to set the so-called zero crossing temperature (= "zero crossing temperature") in an area close to the optical effective surface. At this zero crossing temperature, which e.g. for ULE ™ is approximately ϑ = 30 ° C, the thermal expansion coefficient has a zero crossing in its temperature dependence, in the vicinity of which there is no or only negligible thermal expansion of the mirror substrate material.

In practice, however, the problem arises here that an EUV mirror is exposed to changing intensities of the incident electromagnetic radiation during operation of the microlithographic projection exposure system, both locally, e.g. due to the use of lighting settings with intensity varying over the optical effective area of the respective EUV game gel, as well as in terms of time, the EUV mirror in question typically heating up from its comparatively lower temperature to its operating temperature reached in the lithography process, especially at the beginning of the microlithographic exposure process .

One approach to overcoming the problem described above and in particular to avoid surface deformations caused by varying heat inputs into an EUV mirror and the optical aberrations associated therewith involves the use of preheaters e.g. based on infrared radiation. With such preheaters, active mirror heating can take place in phases of comparatively low absorption of EUV useful radiation, this active mirror heating being reduced accordingly with increasing absorption of the EUV useful radiation.

Controlling the operation of such preheaters with the aim of maintaining a mirror temperature that is as constant as possible (typically the above-mentioned zero crossing temperature) requires knowledge of the radiation power that is incident on the mirror in question, so that the preheating power can be adjusted accordingly. For this purpose (in addition to infrared cameras, which are not always practical for reasons of installation space), temperature sensors, for example in the form of thermocouples or temperature sensors based on electrical resistance (e.g. NTC), are used, which can typically be attached to different positions of the respective mirror in a force-locking or material-locking manner.

By attaching such thermocouples, on the one hand, undesirable mechanical stresses can be induced in the mirror substrate, and on the other hand - especially if a large number of temperature sensors are required to determine a spatially varying temperature distribution within the mirror - the manufacturing effort is significantly increased and, if necessary, the mechanical stability of the mirror is impaired.

The state of the art is only given as an example DE 36 05 737 A1 , DE 10 2005 004 460 A1 and WO 2012/069351 A1 referred.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and a device for determining the heating state of an optical element in an optical system for microlithography, which, while avoiding the problems described above, enable the heating state to be known as precisely as possible.

This object is achieved by the method according to the features of independent patent claim 1 and the device according to the features of independent claim 15.

• - Erzeugen, unter Verwendung einer Lichtquelle, eines durch das optische Element transmittierten ersten Teilstrahls und eines nicht durch das optische Element transmittierten zweiten Teilstrahls; und
• - Bestimmen des Erwärmungszustandes des optischen Elements auf Basis einer Messung der Laufzeitdifferenz zwischen dem ersten Teilstrahl und dem zweiten Teilstrahl.

A method according to the invention for determining the heating state of an optical element in an optical system for microlithography has the following steps:

• Generating, using a light source, a first partial beam transmitted through the optical element and a second partial beam not transmitted through the optical element; and
• - Determining the heating state of the optical element based on a measurement of the transit time difference between the first sub-beam and the second sub-beam.

The invention is based in particular on the concept of carrying out a time-of-flight comparison between at least two partial beams to determine the heating state of an optical element, of which one partial beam is transmitted through the optical element and the other is not transmitted through the optical element. The invention makes use of the fact that the refractive index of the material of the optical element acting on the transmitted partial beam is temperature-dependent, with the result that there is also a temperature dependence with regard to the transit time of the partial beam passing through the optical element, which in turn depends on the refractive index.

In particular, the present invention includes the concept of carrying out a time-of-flight comparison between partial beams reflected on different surfaces of the optical element to be characterized with regard to its heating state, wherein these different surfaces can furthermore in particular be opposite surfaces of the optical element. For example, when the invention is implemented on an optical element in the form of a mirror, the transit time difference between a partial beam reflected on the optical active surface (ie the “front”) of the mirror (which is therefore not transmitted through the mirror) and one on the “rear” “Of this mirror reflected (and thus transmitted through the mirror material) partial beam can be determined, both partial beams being generated by splitting the same light beam impinging on the mirror from the light source. Such an embodiment has the advantage that there are essentially identical beam paths outside the optical element for the two partial beams used for measuring the transit time difference, with the result that effects occurring along the beam path (for example in the form of “streaks” in the respective atmosphere) also agree and ultimately an increased measurement accuracy can be achieved.

According to one embodiment, a surface treatment for increasing the reflectivity for the first partial beam (transmitted through the optical element) and / or the second partial beam (not transmitted through the optical element) is carried out on the optical element. This surface treatment can e.g. include the application of a reflective coating and / or the implementation of a polishing process and, in the above-described application of a mirror, are typically carried out on the rear of the mirror (which, unlike the optical active surface or front, is generally not yet sufficiently reflective). In further embodiments, if necessary, such a surface treatment for increasing the reflectivity can also be carried out for the second (not transmitted through the optical element) partial beam.

According to one embodiment, a frequency-modulated light source is used as the light source. In this embodiment, the so-called “LIDAR principle” can be used according to the invention, in which the beat frequency of a superposition signal generated by superimposing the two partial beams is determined and used to determine the transit time difference.

However, the invention is not restricted to such a lidar-based runtime determination and also includes embodiments in which the runtime determination is carried out in any other (e.g. electronic) manner.

The fact that a non-interferometric transit time measurement (in particular in the form of the said LIDAR principle) is carried out in the method according to the invention has among other things. the advantage of a comparatively lower susceptibility to faults with regard to vibrations, for example relative to interferometric test methods, so that a particularly robust method can be realized as a result.

According to one embodiment, the heating state is determined with the additional inclusion of a model describing the thermal behavior of the optical element. This can take into account in particular the fact that the inventive determination of the propagation time difference between the partial beam not transmitted through the optical element and the partial beam transmitted through the optical element initially only concludes the “integral effect” of the heating state and so far no spatial resolution A determination of the temperature at different positions within the optical element is achieved. With the above-mentioned inclusion of a model describing the thermal behavior of the optical element, a temperature distribution within the optical element can now be inferred from the measurement signals determined according to the invention on the basis of a typical (for example exponential) temperature profile.

According to one embodiment, the optical element for the first (not transmitted through the optical element) partial beam has a transmittance of at least 10%, in particular of at least 20%, further in particular of at least 50%. The sufficient transparency of the optical element in this sense can be achieved either by specifying a specific material (e.g. a typical mirror substrate material such as ULE or Zerodur) by appropriately selecting the wavelength of the light source (e.g. using the materials ULE or Zerodur at least 400nm, furthermore in particular at least 500nm) , or, given the wavelength of the light beam generated by the light source used according to the invention, the material of the optical element can be selected accordingly, for example by adapting the specific composition or recipe for the corresponding optimization of the transmission behavior for the above-mentioned mirror substrate materials such as ULE or Zerodur.

In embodiments of the invention, the method comprises the generation of a plurality of partial beams transmitted through the optical element in different light paths, the heating state then being determined on the basis of measurements of the transit time difference between one of these partial beams and one partial beam not transmitted through the optical element. In other words, in such embodiments, several test beam paths for characterizing the heating state with partial beams from different directions (in the sense of a tomography) are provided in order to achieve an improved spatial resolution with regard to the determination of the heating state of the optical element.

According to one embodiment, the optical element is a mirror.

According to one embodiment, the optical element is designed for a working wavelength of less than 30 nm, in particular less than 15 nm.

According to one embodiment, the optical element is preheated on the basis of the determination of the heating state in order to at least partially compensate for changes in the heating state of the optical element that occur during operation of the optical system. In further embodiments, optical aberrations caused by the heating condition in the optical system can also be compensated for by suitable manipulators (e.g. adaptive mirrors). As an alternative or in addition, correspondingly compensating changes in the gas pressure, the radiation intensity, the radiation wavelength and / or the lighting settings can also be carried out in the respective optical system.

According to one embodiment, the determination of the heating state is carried out during the operation of the optical system (e.g. a microlithographic projection exposure system).

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

The invention is explained below with reference to an embodiment shown in the accompanying figures.

Figure list

• 1 eine schematische Darstellung des möglichen Aufbaus einer für den Betrieb im EUV ausgelegten mikrolithographischen Projektionsbelichtungsanlage; und
• 2-3 schematische Darstellungen zur Erläuterung möglicher Ausführungsformen eines erfindungsgemäßen Verfahrens.

Show it:

• 1 a schematic representation of the possible structure of a microlithographic projection exposure system designed for operation in the EUV; and
• 2-3 schematic representations to explain possible embodiments of a method according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

1 shows a schematic representation of a designed for operation in the EUV Projection exposure system 100 , in which the invention can be implemented, for example.

According to 1 has an illumination device of the projection exposure system 100 a field facet mirror 103 and a pupil facet mirror 104 on. On the field facet mirror 103 the light of a light source unit, which in the example is an EUV light source (plasma light source) 101 and a collector mirror 102 includes, directed. In the light path after the pupil facet mirror 104 are a first telescope mirror 105 and a second telescopic mirror 106 arranged. In the light path below is a deflecting mirror 107 arranged that the radiation striking it on an object field in the object plane of a six mirror 121-126 comprehensive projection lens. There is a reflective structure-bearing mask at the location of the object field 131 on a mask table 130 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) 141 on a wafer table 140 located.

In operation of the microlithographic projection exposure system 100 the electromagnetic radiation striking the optical active surface or the impinging surface of the existing mirrors is partially absorbed and, as explained at the outset, leads to heating and an associated thermal expansion or deformation, which in turn can have an adverse effect on the imaging properties.

The method according to the invention and the device according to the invention for determining the heating state of an optical element can in particular, for example, on any mirror of the microlithographic projection exposure system 100 from 1 be applied.

The structure and mode of operation of a method according to the invention are described in an exemplary embodiment with reference to the schematic representations in FIG 2-3 described.

According to the invention, the heating state of an optical element such as a mirror of the microlithographic projection exposure system is determined by 1 based on a measurement of the transit time difference between two partial beams, only one of which is transmitted through the optical element or the mirror in question. The given dependence of the said transit time difference on the refractive index of the material of the optical element has the consequence, due to the temperature dependence of this refractive index, that the transit time difference measured according to the invention can be used as a measure of the temperature in the material of the optical element.

2nd initially shows an optical element in a schematic and highly simplified representation 200 in the form of a mirror, the optical active surface or front side with " 201 "And its back with" 202 "Is designated. With " 201a " respectively. " 202a “Are partial areas from the front 201 or back 202 Designated, which should provide the reflecting interfaces for the above-mentioned, used for determining the time difference of the partial beams. While in the case of a mirror the optical active surface is already sufficiently reflective, sufficient reflectivity can also be provided in the area 202a at the back 202 a suitable surface treatment is carried out, for example, by applying a coating and / or carrying out a polishing process, in addition as in 2a indicated by a corresponding processing of the back 202 that area can also be achieved 202a essentially with the same slope and offset parallel to the one on the front 201 located area 201a runs. With this configuration, essentially matching beam paths can be created for the areas in question 201a , 202a reflected partial beams are guaranteed with the result that effects occurring along the respective beam path cancel each other out when determining the time difference.

The schematic representations of 3a and 3b serve to illustrate the functional principle of the method according to the invention, whereby to 2nd analog or essentially functionally identical components are designated with reference numbers increased by “100”.

According to 3a-3b is from one of a light source 303 radiated light beam 305 a first partial beam 310 through the optical element 300 or the mirror transmitted and at the area 301a the front of the optical element 300 reflected, whereas also from the light beam 305 generated second partial beam 320 already at the area 302a the back 302 reflected and therefore not by the optical element 300 or the mirror is transmitted.

wobei c die Lichtgeschwindigkeit, n(x) die ortsabhängige Brechzahl, n die gemittelte Brechzahl, D die geometrische Dicke des optischen Elements 300 am betreffenden Ort und α den Einfallswinkel auf die Grenzflächen 301a, 302a bezeichnet. In guter Näherung hängt lokal die Brechzahl n gemäß $$n\left(T+\Delta T\right)=n\left(T\right)+\frac{\partial n}{\partial T}\Delta T$$

von einer gegenüber einer Referenztemperatur abweichenden Temperatur ab. Die mit einer Änderung der Temperatur entlang des jeweils durchlaufenden Bereichs im optischen Element 300 bei Übergang von 3a zu 3b demzufolge einhergehende Brechzahländerung führt wiederum zu einer Laufzeitdifferenz zwischen den beiden Teilstrahlen 310, 320, welche sich zu $$\Delta t_2-\Delta t_1=2\ast \left(n\text{'}-n\right)\ast D/\left(c\ast cos\alpha \right)$$

berechnet.With "340" is in 3b one from the first beam 310 swept area of the optical element 300 with compared to 3a deviating temperature is shown schematically. Going through this area 340 leads for the first beam 310 due to the temperature dependence of the refractive index to a change in the optical path length and consequently also to a change in the transit time difference between the partial beams 310 , 320 when transitioning from the scenario 3a to that Scenario according to 3b . The measurement of this transit time difference can therefore be used according to the invention to characterize the heating state of the optical element 300 or mirror can be used. The time difference Δt between the two partial beams 310 , 320 is calculated $$\Delta t=\mathrm{2nd}{\displaystyle {\int }_{Unterseite}^{Oberseite}n\left(x\right)dx\frac{1}{\text{c}\ast \text{cos}\alpha }}=\mathrm{2nd}\overline{n}D\frac{1}{\text{c}\ast \text{cos}\alpha }$$

where c is the speed of light, n (x) is the location-dependent refractive index, n the average refractive index, D the geometric thickness of the optical element 300 at the location and α the angle of incidence on the interfaces 301a , 302a designated. In a good approximation, the refractive index n depends locally $$n\left(T+\Delta T\right)=n\left(T\right)+\frac{\partial n}{\partial T}\Delta T$$

from a temperature that differs from a reference temperature. The one with a change in temperature along the respective continuous area in the optical element 300 at the transition from 3a to 3b consequent change in refractive index in turn leads to a transit time difference between the two partial beams 310 , 320 , which become too $$\Delta t_{\mathrm{2nd}}-\Delta t_1=\mathrm{2nd}\ast \left(n\text{'}-n\right)\ast D/\left(c\ast cOs\alpha \right)$$

calculated.

In further embodiments, the beam path through the optical element 300 or the mirror of the transmitted partial beam depending on the specific application situation (in particular on the specific geometry of the optical element and the space available) can be selected in another suitable manner, in particular also parallel to the optical active surface or obliquely through the optical element, or several partial beams can be selected pass through the optical element in different directions in order to achieve a greater local resolution.

Without the invention being restricted to this, the abovementioned determination of the transit time difference can be carried out in particular using the “LIDAR principle” known per se. In this case it is used as a light source 303 to generate the light beam 305 uses a frequency-modulated light source. The first partial beam 310 or the second sub-beam 320 Corresponding measurement signals are sent to a detector (possibly via a signal coupler) 330 supplied, the beat frequency detected on the detector side of the superimposition of these signals characteristic of the transit time difference between the partial beams 310 , 320 is: When signals with the time delay Δt are superimposed, the following relationship exists between the beat frequency Ω and the chirp rate ξ: $$\Omega :=\xi \cdot \Delta t$$

von 10^-5K^-1 für eine Änderung der mittleren Temperatur um 5K bei einer beispielhaften Dicke des optischen Elements von 200mm eine Änderung der optischen Weglänge um 20µm entsprechend einem Laufzeitunterschied von 7*10^-14 Sekunden.A quantitative analysis is based on a typical order of magnitude of the coefficient $\frac{\partial n}{\partial T}$

from 10 ^-5 K ^-1 for a change in the mean temperature by 5K with an exemplary thickness of the optical element of 200mm, a change in the optical path length by 20µm corresponding to a transit time difference of 7 * 10 ^-14 seconds.

The optical element can be preheated on the basis of the determination of the heating state according to the invention 300 or mirror for at least partial compensation of the changes over time in the heating state. Furthermore, optical aberrations caused by this heating state in the optical system can be compensated for by suitable manipulators (eg adaptive mirrors). As a result, a design of the optical system that is robust against thermal influences can be achieved and a consistently high image quality can be ensured.

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 limited only 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 3605737 A1 [0009]
• DE 102005004460 A1 [0009]
• WO 2012/069351 A1 [0009]

Claims (15)

Method for determining the heating state of an optical element in an optical system for microlithography, the method comprising the following steps: a) generating, using a light source (303), a first partial beam (310) transmitted by the optical element (200, 300) and a second partial beam (320) not transmitted by the optical element (200, 300); and b) determining the heating state of the optical element (200, 300) on the basis of a measurement of the transit time difference between the first partial beam (310) and the second partial beam (320). Procedure according to Claim 1 , characterized in that the first partial beam (310) and the second partial beam (320) are reflected on different surfaces of the optical element (200, 300). Procedure according to Claim 1 or 2nd , characterized in that a frequency-modulated light source is used as the light source (303). Procedure according to one of the Claims 1 to 3rd , characterized in that step b) comprises a superposition of the first sub-beam (310) and the second sub-beam (320), the determination of the heating state of the optical element (200, 300) taking place on the basis of the beat frequency of this superimposition. Method according to one of the preceding claims, characterized in that the optical element (200, 300) for the first partial beam (310) has a transmittance of at least 10%, in particular of at least 20%, furthermore in particular of at least 50%. Method according to one of the preceding claims, characterized in that the light source (303) is designed to generate light with a wavelength of at least 400 nm, in particular of at least 500 nm. Procedure according to one of the Claims 2 to 6 , characterized in that a surface treatment for increasing the reflectivity for the first partial beam (310) and / or for the second partial beam (320) is carried out on the optical element (200, 300). Method according to one of the preceding claims, characterized in that the heating state is determined with the additional inclusion of a model describing the thermal behavior of the optical element (200, 320). Method according to one of the preceding claims, characterized in that step a) comprises generating a plurality of partial beams transmitted by the optical element on different light paths, the determination of the heating state in step b) further based on measurements of the transit time difference between each this partial beams and a partial beam not transmitted through the optical element (200, 300). Method according to one of the preceding claims, characterized in that the optical element (200, 300) is a mirror. Method according to one of the preceding claims, characterized in that the optical element (200, 300) is designed for a working wavelength of less than 30 nm, in particular less than 15 nm. Method according to one of the preceding claims, characterized in that, on the basis of the determination of the heating state, a preheating of the optical element (200, 300) for at least partial compensation of changes in the heating state of the optical element (200, 300) occurring during operation of the optical system. and / or compensation of optical aberrations caused by the heating state in the optical system is carried out. Method according to one of the preceding claims, characterized in that the determination of the heating state is carried out during the operation of the optical system. Method according to one of the preceding claims, characterized in that the optical system is a microlithographic projection exposure system. Device for determining the heating state of an optical element in an optical system for microlithography, characterized in that it is configured to carry out a method according to one of the preceding claims.

Images