Patent Grant
10161808
The invention concerns a method and arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus.
Microlithography is used for the production of micro structured components such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated via the illumination system is projected via the projection objective on to a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range, that is to say at wavelengths of for example about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials. A problem which arises in practice is that the EUV mirrors experience a rise in temperature and linked thereto thermal expansion or deformation, as a consequence of absorption of the radiation emitted by the EUV light source but also because of cleaning in a hydrogen atmosphere, and such expansion or deformation in turn can result in worsening of the imaging properties of the optical system. To be able to assess those effects and possibly compensate for them there is a need to determine the extent of the mirror heating as accurately as possible.
An object of the present invention is to provide a method of and an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, which without substantially worsening operation of the optical system, permit reliable characterisation of the heating condition.
That object is attained by the method in accordance with the features of independent claim 1 and the arrangement in accordance with the features of further independent claim 19.
A method of determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the mirror is an EUV mirror, comprises the following steps:
The operation of determining the change in temperature, which is effected in contact-less fashion by analysis of an output measuring beam produced from the input measuring beam after interaction with the mirror can be effected directly in the EUV projection objective and also during operation thereof, wherein, depending on the respective actual structure involved, as is described in greater detail hereinafter, the measuring light can be supplied and also carried away by way of optical fibers. Consequently, no additional electronics are required within the typically evacuated housing of the projection objective, and no stray light is introduced into the optical system. The method can be carried out in particular with a measuring arrangement which is connected in the form of a separate module to the projection objective.
In an embodiment the operation of ascertaining the at least one optical parameter includes ascertaining at least one value dependent on the refractive index of the mirror material. In that case it is possible in accordance with the invention to make use of the fact that the low coefficients of expansion which typically exist in the mirror material of EUV mirrors involve a comparatively pronounced temperature dependency in respect of the refractive index, which in turn makes it possible to infer a change in temperature of the mirror from measurement of the change in refractive index, with comparatively great accuracy (as described in fuller detail hereinafter).
In an embodiment the operation of ascertaining the at least one optical parameter includes ascertaining an optical path length change.
In an embodiment the operation of ascertaining the at least one optical parameter includes ascertaining the reflectivity at a predetermined wavelength.
In a further embodiment the operation of ascertaining the at least one optical parameter includes ascertaining the beam deflection of the output measuring beam relative to the input measuring beam.
In a further embodiment the operation of ascertaining the at least one optical parameter includes ascertaining the wavelength of the output measuring beam.
In an embodiment a plurality of input measuring beams is deflected on to the mirror. Those input measuring beams can be deflected on to the mirror in particular at different azimuth angles with respect to an optical axis of the optical system in order to obtain information about the temperature distribution of the mirror and possibly also any symmetry in respect of the heating effect in conjunction with certain illumination settings.
In an embodiment the output measuring beam is produced from the input measuring beam by passing at least once through the mirror.
The inventions also relates to a method of determining the heating condition of a mirror in an optical system, in particular in a micro lithographic projection exposure apparatus, wherein the method comprises the following steps:
The mirror comprises a substrate and a reflecting surface. Accordingly, the input measuring beam passes at least once through the substrate (corresponding to the region between the reflecting surface of the mirror and the mirror rear side). In particular, the input measuring beam can pass at least once through the substrate without being reflected at the (EUV-)reflecting surface of the mirror.
In particular the output measuring beam can be produced from the input measuring beam by passing through the mirror (or the mirror substrate, respectively) a plurality of times, whereby it is possible to achieve an increase in the length of the measuring section and thus an increase in measuring accuracy.
In an embodiment the output measuring beam is produced from the input measuring beam by reflection at an optical element arranged at the mirror.
The optical element can have at least one DBR (DBR=‘Distributed Bragg Reflector’). In addition the optical element can be a surface-emitting laser (VCSEL).
The invention also relates to a method of determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the method comprises the following steps:
In particular, the mirror comprises a substrate and a reflecting surface, wherein the at least one DBR can be provided on the substrate of the mirror.
The invention also relates to a method of determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the method comprises the following steps:
In particular, the mirror comprises a substrate and a reflecting surface, wherein the VCSEL can be provided on the substrate of the mirror.
The invention also comprises embodiments in which the input measuring beam (from the light source) passes through the mirror substrate in order to reach the DBR or the VCSEL and/or in order to arrive (from the DBR or the VCSEL) at a detector.
In a further aspect the invention concerns an arrangement for determining the heating condition of a mirror in an optical system, in particular in a microlithographic projection exposure apparatus, wherein the mirror is an EUV mirror, the arrangement comprising:
In an embodiment the measuring light source unit has a plurality of measuring light sources. In addition the detector unit can have a plurality of detectors. In particular a plurality of measuring light sources and/or a plurality of detectors can be arranged around the mirror at different azimuth angles with respect to an optical axis of the optical system.
In an embodiment a plurality of measuring light sources and/or a plurality of detectors are arranged matrix-like.
In a further aspect, the invention also relates to an arrangement for determining the heating condition of a mirror in an optical system, in particular in a micro lithographic projection exposure apparatus, the arrangement comprising:
In a further aspect, the invention also relates to an arrangement for determining the heating condition of a mirror in an optical system, in particular in a micro lithographic projection exposure apparatus, the arrangement comprising:
In a further aspect, the invention also relates to an arrangement for determining the heating condition of a mirror in an optical system, in particular in a micro lithographic projection exposure apparatus, the arrangement comprising:
Further configurations of the invention are set forth in the description and the appendant claims.
The invention is described in greater detail hereinafter with embodiments by way of example illustrated in the accompanying drawings.
In the drawings:
Hereinafter embodiments of the invention are described with reference to
A measuring arrangement 100 by way of example for temperature measurement at a mirror 101 is shown in
Referring to
The light (input measuring beam 102) transmitted by the beam splitter 110 passes through the mirror 101 to be measured, is reflected at a deflection prism 120 and after again passing through the mirror 101 passes through a lambda/4 plate 125 which converts the light which is substantially linearly polarised (as a consequence of the negligible polarisation influence of the deflection prism 120) into circularly polarised light (output measuring beam 103). Here and in the following embodiments (and as can be gathered from the drawings) the mirror 101 comprises a substrate 101B and a reflecting surface 101A. The substrate 101B can e.g. be made from a material, i.e. can comprise a material or consist of a material, said material having a zero-cross temperature (zero-crossing temperature), i.e. also a temperature-dependent thermal expansion coefficient (=coefficient of thermal expansion), such as e.g. zerodur® or ULE®.
As can be gathered from
Polarisation beam splitters 130 and 140 serve for measurement of the component (measuring output beam 103) which is then transmitted and reflected respectively by the beam splitter 110. Downstream of the second polarisation beam splitter 140 there are angles in respect of the polarisation direction of 0° and 90° for the component transmitted by the beam splitter 110, the associated light components being measured by way of photodiodes 150 and 170 respectively. Downstream of the first polarisation beam splitter 130 there are angles in respect of the polarisation direction of 180° and 270° for the component reflected in the beam splitter 110, the associated light components being measured by way of photodiodes 180 and 160 respectively.
Accordingly comparatively disturbance-insensitive evaluation of four push-pull signals is effected via the measuring arrangement of
In practice after the light source of the optical system has been brought into operation or switched on the start temperature or switch-on temperature (for example T0 =22° C.) is determined via an absolute temperature sensor and used as a reference temperature by a counter at that start time being set to zero and then the interference stripes which occur upon heating of the measured mirror 101 in interferometric measurement being counted. In that way, the change in temperature of the mirror can be ascertained via equation (1), for example with degrees of accuracy in the millikelvin (mK) range, a degree of resolution by way of example which can be achieved according to the invention being 0.4 mK. The relationship between the optical path length difference OPD which is interferometrically determined via the measuring arrangement and the change in temperature causing that path length difference OPD is given by:
OPD(δ)=dn/dδ*Δδ*L (1)
wherein δ denotes the temperature (in units of Kelvin, K), dn/dδ denotes the temperature-dependent change in refractive index (in units of 1/K) and L denotes the measuring distance. The value of dn/dδ is known for the respective mirror material used and for example in the case of glasses with a low degree of thermal expansion is dn/dδ>10*10−6 K−1.
Determining the change in temperature in accordance with the invention can be effected directly in the EUV projection objective and also during operation thereof, for which purpose for example the measuring light can be both supplied and also taken away by way of optical fibers whereby the measuring arrangement can be connected in the form of a separate module to the projection objective. That configuration has the advantage that no additional electronics are required within the typically evacuated housing of the projection objective and also no stray light is introduced into the optical system.
It will be appreciated that the measuring arrangement described with reference to
Referring to
The number and azimuthal orientation of the measuring arrangements can be selected variably in each case in accordance with the specific requirements. For example, in a further embodiment, eight measuring arrangements of the structure described with reference to
As soon as the heating condition of the mirror 101 or 201 has been determined suitable correction measures can be initiated. Reference is made to
That approach is based on the fact that the effect which dominates for heat transport in the atmosphere typically used (for example krypton, argon etc) is that of heat conduction. For that purpose the structure shown in
Associated with the Peltier elements 310a, . . . is a measuring sensor 330 which in the illustrated embodiment is disposed on a ceramic substrate 325 and directly measures the temperature with which the respective Peltier element 310a, . . . radiates into the mirror 301. Suitable cooling or heating bores can be arranged on the rear side of the mirror, which prevent the Peltier elements 310a, . . . from touching each other and prevent an unwanted ‘cross-talk’ occurring between them.
Temperature regulation can be effected via the structure shown in
Although the structure in
As shown in
Embodiments of the invention are described hereinafter with reference to
Accordingly, there is a refractive index gradient from the light entrance surface 401a (which is heated most strongly) to the mirror rear side 401b.
Now, as shown in
As can be gathered from
The light beam 411 which passes through the mirror 401 perpendicularly to the refractive index gradient produced by virtue of the heating condition of the mirror 401 ‘sees’ different refractive indices as a consequence of the refractive index gradient over the beam cross-section. In particular expressed in graphic terms, the left-hand edge of the light beam 411 (whose diameter can be in the range of 1-5 mm just by way of example and without the invention being restricted thereto) experiences a higher or lower refractive index than the right-hand edge of the light beam 411, which results in different speeds of propagation over the cross-section of the light beam 411 and thus deflection of the latter. Accordingly therefore the refractive index gradient which is continuously set over the mirror 401 to be measured is converted into a change in position of the light beam 411, that can be measured via the position detector 420.
It will be appreciated that the invention is not limited to the geometry shown in
In further embodiments a plurality of light beams can also be deflected by the mirror 401 insofar as a corresponding arrangement (linear, in matrix form or any other) of laser diodes (for example at a spacing of a few millimeters from each other) is arranged along the mirror and the change in position is ascertained by way of a suitable position detector arrangement (for example in the form of a quadrant photodiode or another, in particular matrix-form detector arrangement). In further embodiments therefore a plurality of light beams can also be deflected by the mirror 401 at different beam angles, whereby in particular account can also be taken of the configuration (being concave in the example) of the curved optically effective surface of the mirror 401 and the variation involved therewith in the change in temperature of the mirror in the x- and y-directions respectively.
A plurality of light sources or laser diodes 401 can also be arranged in azimuthally displaced relationship in the peripheral direction around the mirror 401 to provide information about the temperature distribution of the mirror 401 and any symmetry in heating in conjunction with given illumination settings. In that way it is possible to take account of or measure a non-homogenous heating effect of the mirror 401, that is involved with the use of given illumination settings. The passage of a plurality of light beams through the mirror 401 at different beam angles can be effected both via an arrangement of a plurality of light sources or laser diodes 410 at different angles or also by a suitable configuration of reflecting mirror side surfaces on the mirror 401.
In further embodiments the measuring section can also be increased in length by the respective mirror side surfaces being of a reflecting nature via a suitable coating (for example of aluminum) so that this provides an increase in the resulting beam deflection and thus also an increase in the accuracy of measurement. When using only one measuring section or laser diode, it may be advantageous if, as shown in
The operation of determining the change in refractive index, which is effected as shown in
A calibration operation can be performed prior to the above-described measurement procedure, in the course of which calibration the relationship between the change in position of the light beam 411 and the change in temperature of the mirror 401 is ascertained for predetermined and specifically set values of the change in temperature.
Although the measurement principle described with reference to
In accordance with further embodiments, to suppress interfering measurement noise and to increase measuring accuracy via the measuring arrangement of
Further embodiments of the invention are described hereinafter with reference to
As shown in
A light source 620 (for example a laser diode) whose wavelength is preferably in the infrared range (for example at λ≈1 μm) deflects the measurement light 602 on to the DBR 610 and a suitable detector 630 (for example a CCD camera) measures the light 603 reflected at the DBR 610. The invention also comprises embodiments in which the input measuring beam 602 from the light source 620 passes through the mirror substrate in order to reach the DBR 610 and/or in order to arrive (from the DBR 610) at the detector 630. The use of infrared light is particularly suitable for the reason that the influence of temperature on the reflectivity curve in the infrared range is substantially greater than for example with EUV light. The measurement arrangement 600 of
As a consequence of mounting the DBR 610 as shown in
In accordance with a further measurement principle described with reference to
In further embodiments, instead of the VCSEL, it is also possible to use a quantum cascade laser which can be excited by way of microwaves (by using a microwave source instead of the light source 710).
As the change in emission wavelength of the surface-emitting laser, that is utilised with the measurement principle described hereinbefore with reference to
As regards the working wavelength of the measurement light source 620 or 720 respectively, used in the measuring arrangements of
The light of the laser diode 820 is passed via a glass fiber 805 and a beam splitter 830 as well as an UHV-compatible glass fiber transmission means 840 into the vacuum chamber of the projection objective of the projection exposure apparatus. There the light can be further passed by way of UHV-compatible glass fibers to the respective measurement points at one or more mirrors 801 of the projection objective. A suitable collimator optical system (not shown) at the end of the glass fibers focuses the light on to the surface-emitting laser 810 on the mirror 801 and again couples the light emitted by the surface-emitting laser 810 into the glass fiber 805. The collimator optical system can be of such a design configuration that the surface-emitting laser 810 or VCSEL chip is of a size of 0.5 * 0.5 mm2. The emitted light is passed back on to the beam splitter 830 by way of the glass fiber 805 and from there coupled into a detector 860 by way of an edge filter 850 which fades out the pump wavelength.
Coupling the pump light into and out of the surface-emitting laser 810, which is adopted in the
A thermal contact of the component which is respectively used (DBR or VCSEL) in relation to the mirror to be measured is necessary in each case in the embodiments described with reference to
Even if the invention has been described by reference to specific embodiments numerous variations and alternative embodiments will be apparent to the man skilled in the art, for example by combination and/or exchange of features of individual embodiments. Accordingly it will be appreciated by the man skilled in the art that such variations and alternative embodiments are also embraced by the present invention and the scope of the invention is limited only in the sense of the accompanying claims and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 061 950 | Nov 2010 | DE | national |
This application claims is a continuation of, and claims priority under 35 U.S.C. § 120 to, international patent application PCT/EP2011/070320, filed Nov. 17, 2011, which claims priority under 35 U.S.C. § 119 to German Patent Application DE 10 2010 061 950.7 filed on Nov. 25, 2010. International patent application PCT/EP2011/070320 also claims priority under 35 U.S.C. § 119(e)(1) to US patent application 61/417,552, filed on Nov. 29, 2010. The content of these applications is hereby incorporated by reference.
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| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2011/070320 | Nov 2011 | US |
| Child | 13863595 | US |
