WO2009039883A1

WO2009039883A1 - Optical imaging device with thermal stabilization

Info

Publication number: WO2009039883A1
Application number: PCT/EP2007/060226
Authority: WO
Inventors: Yim-Bun Patrick Kwan
Original assignee: Carl Zeiss Smt Ag
Priority date: 2007-09-26
Filing date: 2007-09-26
Publication date: 2009-04-02

Abstract

There is provided an optical imaging device comprising an optical imaging system (101) with at least one' optical imaging component (106.1,107.1), a support structure (102.1), a capturing device (111) and a control device (112). The support structure (102.1) supports the at least one optical imaging component (106.1,107.1) and comprises at least one actuator (106.3,107.3). The actuator (106.3,107.3) is adapted to adjust, in at least one degree of freedom, at least one of a geometric parameter of the optical imaging component (106.1,107.1), a relative position of the optical imaging component (106.1,107.1) with respect to at least one reference (108.1) and a relative orientation of the optical imaging component (106.1,107.1) with respect to the at least one reference (108.1) as a function of at least one control signal provided by the control device (112). The capturing device (111) is adapted to continuously capture, at at least one capturing location of the optical projection unit (102), an actual value of a variable representative of a temperature at the location and to provide the actual value to the control device (112). The control device (112) is adapted to generate the at least one control signal as a function of the actual value of the variable and to provide the at least one control signal to the at least one actuator (106.3,107.3) to at least, partially compensate thermally induced alterations in at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component (106.1,107.1). The control device (112) is adapted to use at least one previously established thermal model of the optical imaging system (101) to generate the at least one control signal,1 the thermal model describing at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component (106.1,107.1) at least as a function of the variable and the capturing location.

Description

OPTICAL IMAGING DEVICE WITH THERMAL STABILIZATION

BACKGROUND OF THE INVENTION

The invention relates to optical imaging devices used in exposure processes, in particular to optical imaging devices of microlithography systems. It further relates to a method of supporting an optical element of an optical imaging device. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.

Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. Said optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups are typically held by a corresponding support structure. In particular, with mainly refractive systems, such optical projection units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.

Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical exposure units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.

Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system. One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been made to use light in the extreme ultraviolet (EUV) range using wavelengths down to below 20 nm, typically 13 nm and even below. In this EUV range it is not possible to use common refractive optics any more. This is due to the fact that, in this EUV range, the materials commonly used for refractive optical elements show a degree of absorption that is to high for obtaining high quality exposure results. Thus, in the EUV range, reflective systems comprising reflective elements such as mirrors or the like are used in the exposure process to transfer the image of the pattern formed on the mask onto the substrate, e.g. the wafer.

The transition to the use of high numerical aperture (e.g. NA > 0.5) reflective systems in the EUV range leads to considerable challenges with respect to the design of the optical imaging arrangement.

Among others, the above leads to very strict requirements with respect to the relative position between the components participating in the exposure process. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system initially showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical imaging components, i.e. the mask, the optical elements and the wafer, for example, cooperating in the exposure process must be supported in a defined manner in order to maintain a predetermined spatial relationship between these optical imaging components as well to provide a high quality exposure process.

To maintain the predetermined spatial relationship between the optical imaging arrangement components throughout the entire exposure process, even under the influence of vibrations introduced via the ground structure supporting the arrangement and under the influence of thermally induced alterations, it is known to at least intermittently capture the spatial state of certain components of the optical imaging arrangement (i.e. the geometry of the these components and/or the spatial relationship between these components) and to adjust the spatial state of at least one of the components of the optical projection system as a function of the result of this capturing process. More precisely, typically, compensation of thermally induced deformation is provided by adjusting the spatial state (i.e. the geometry and/or the position and/or the orientation) of one or more of the optical elements within up to six degrees of freedom (DOF) via actuators being controlled as a function of the result of the capturing process mentioned above. In common systems, the metrology devices necessary to capture the spatial relationship mentioned above are substantially rigidly mounted to a so called metrology frame. Such a metrology frame, typically, is a heavy, generally plate shaped body. The metrology frame is supported on the ground structure via vibration isolating means to reduce the influences of vibrations of the ground structure usually lying in the range of about 30 Hz. Furthermore, considerable effort is necessary to avoid thermally induced deformations of the metrology frame in order to provide a high thermal stability of the position and orientation of the respective metrology components. Either the metrology frame, in a passive approach, has to be made of a generally expensive material with a very low coefficient of thermal expansion (CTE) or, in an active approach, an expensive temperature stabilization system has to be provided. Thus, in any case, the metrology frame is a very complex and, thus, expensive part of the system. Such a configuration is for example known from US 7,221 ,463 B2 (Mizuno et al.), the entire disclosure of which is incorporated herein by reference.

Another solution is known from WO 2006/128713 (Kwan), the entire disclosure of which is incorporated herein by reference. Here, the metrology devices necessary to capture the spatial relationship mentioned above are integrated within the optical projection unit such that a separate metrology frame can be dispensed with. However, here as well, the support structure either has to be made of a generally expensive material with a very low coefficient of thermal expansion (passive solution) or an expensive temperature stabilization system has to be provided (active solution) in order to provide a high thermal stability of the position and orientation of the optical elements as well as of the respective metrology components. Thus, in any case, here, the support structure is a very complex and, thus, expensive part of the system.

For solutions with passive thermal stability, typically, so called near zero or zero CTE materials, such as Zerodur, ULE glass, Kyoceram, Cordierite, etc., are used for the support structure. For example, a Zerodur support structure being 1.5 m high has a thermal sensitivity in the order of 75 nm/K. That means that the relative position between two optical elements located at both ends of the structure will shift by 1 nm for a temperature change of about 13 mK.

As had already been mentioned most of such near zero or zero CTE materials are, both, costly and difficult to machine and to handle during assembly. Furthermore, the passive stability sets a limit to the thermal performance of the support structure, and can only be improved by using (even more costly) materials with even lower coefficients of thermal expansion, e.g. selected Zerodur or ULE with coefficients of thermal expansion in the ppb/K range (i.e. on the range of 10^"9/K) range. If, on the other hand, a temperature stabilization system is used, care has to be taken that the cooling system does not introduce an undue amount of vibration into the system which would otherwise adversely affect the imaging quality. Thus, an approach with a typically highly complex temperature stabilization system of avoiding such generation of vibration also adds considerable cost.

Furthermore, in both cases, in particular in EUV systems, the problem arises that it is very complicated to capture thermally induced deformations of the optical elements and their support structure, respectively, (i.e. thermal expansion related alterations in the geometry of the respective components) at a sufficiently high accuracy (down to nanometer level) to be able to counteract such thermally induced deformation which would otherwise lead to deterioration of the imaging quality due to a drift of the relative position of the optical elements with respect to each other, with respect to their support structure as well as with respect to the metrology components. Thus, typically, a large number of rather expensive high precision metrology components have to be used in order to provide sufficiently high imaging quality.

In particular, US 7,221 ,463 B2 (Mizuno et al.) suggests to provide stabilized imaging quality during operation of the optical imaging device by initially providing at certain intervals an adjustment of the position and orientation of the optical elements using either a measurement of the imaging quality of the optical system or a measurement of an environmental variable of the atmosphere (temperature, refractivity, pressure, contamination etc.) within the optical projection unit together with a thermal model of the optical projection unit. Once this intermittent adjustment is provided, continuous fine adjustment of the position and orientation of the optical elements is provided over a longer period of time on the basis of the measurements of the position and orientation of the optical elements provided by the corresponding high precision metrology components.

However, as has been outlined above, the capture of the position and orientation of all the optical elements of such an optical imaging device using such high precision metrology components at nanometer level is, both, complicated and costly not least due to the large number high precision metrology components required to provide sufficient coverage of the entire system. SUMMARY OF THE INVENTION

It is thus an object of the invention to, at least to some extent, overcome the above disadvantages and to provide good and long term reliable imaging properties of an optical imaging device used in an exposure process.

It is a further object of the invention to reduce the effort necessary for an optical imaging device while at least maintaining the imaging accuracy of the optical imaging arrangement used in an exposure process.

These objects are achieved according to the invention which is based on the teaching that a reduction of the effort necessary for an optical imaging device and an optical imaging arrangement comprising such an optical imaging device while at least maintaining the imaging accuracy of the optical imaging arrangement may be achieved if, as a first alternative, at at least one capturing location of the optical imaging device, an actual value of a variable representative of a temperature at said capturing location is continuously captured and control signals for the actuators actuating one or more optical imaging components of the optical imaging system are generated as a function of these actual values captured using a thermal model of the optical imaging device.

In particular, it has been found that such a thermal model of the optical imaging device may not only be used for an intermittent rough initial adjustment of the optical imaging components but may be used for continuous real-time fine adjustment of the spatial state (i.e. the geometry and/or the position and/or the orientation) of the respective optical imaging component (e.g. between image calibration processes which, typically, occur every five minutes). Thus, at any point in time during operation a sufficiently accurate information on the actual spatial state of the respective optical imaging component is available allowing proper compensation of thermally induced alterations in the actual spatial state of the respective optical imaging component is possible. This is particularly beneficial between start of operation of the optical imaging device (e.g. the actual start of wafer exposure) and the subsequent first image calibration process since, during this initial phase of exposure operation, a considerable change of the temperature situation is to be expected which may lead to unacceptable exposure results (i.e. a considerable amount of scrap products) unless properly compensated.

More precisely, it has been found that such a thermal model may provide a very high prediction accuracy of the actual spatial state of the optical element as a function of the captured temperature related variable while sufficiently accurate capture of such temperature related variables may be achieved at considerably less expense (in relation to the known high precision distance measurements) such that a considerable reduction in the overall costs of such a system may be achieved. In particular, the position of the at least one capturing location may be relatively easily optimized adding to the overall accuracy of the adjustment of the spatial state of the optical imaging component.

Such a thermal model of the optical imaging device may have been previously established in a theoretical manner (e.g. by computer modeling and simulation of the optical imaging device), in an empirical manner (e.g. by manufacturing and testing the physical optical imaging device) or a combination thereof. For example, the respective optical imaging device, in a thermal model setup phase, may be subject to one or several well defined thermal situations (typically captured by the thermal sensors used later during operation) while at the same time capturing the spatial state (i.e. geometry and/or position and/or orientation) of the optical imaging component(s) of the optical imaging system (e.g. by simultaneously measuring the imaging quality of the optical imaging system). Depending on the thermal resolution obtained in this phase and the thermal resolution necessary for sufficiently accurately describing the thermal behavior of the optical projection unit the data obtained in this setup phase (i.e. the relation between temperature situation and optical element state) itself may represent the thermal model of the optical projection unit. However, is also possible that the thermal model is further refined in a theoretical manner by adding further data e.g. obtained by suitable interpolation algorithm(s) etc.

It will be appreciated that the thermal model may describe, for example, the thermal behavior of an optical projection unit as a central optical imaging device in an optical imaging arrangement (the optical elements of the respective unit then forming the respective optical imaging components in the sense of the present invention). However, components of an optical imaging arrangement (e.g. a microlithography device) other than the illumination unit or the optical projection unit may be described by or included in such a thermal model. In particular, either one of a thermal model of an illumination unit, a mask unit, and a substrate unit may be a part of a thermal model also comprising the thermal model of the optical projection unit. In general, such a thermal model may be set up for any optical imaging device (e.g. illumination unit, mask unit, optical projection unit, substrate unit) optically participating in the optical imaging process.

It will be appreciated that, with such a thermal model based control as outlined above, at least a control of the image position in two degrees of freedom (typically referred to as x and y) in the plane of the substrate at a resolution of better than 0.1 nm may be achieved. However, it will be appreciated that, with other embodiments of the invention, any other desired variable or parameter affecting the imaging quality and being itself affected by the temperature situation of the optical imaging device may be corrected (in up to six degrees of freedom).

It will be further appreciated that the thermal model based control may not be limited only to taking into account the actual temperature situation captured via the capturing device. In particular, other parameters or variables captured via corresponding capturing devices may be used as well as data derived from the temperature situation data. For example, historical data on the temperature situation may be taken into account. Predictive algorithms may be used to predict a future temperature situation and the result of such predictions may be compared to the actual development of the temperature situation in order to verify the quality of the thermal model and to refine and correct the thermal model, if necessary (i.e. in case of unacceptable deviations between the predicted and the actual situation). Thus, continuous optimization of the thermal model may be achieved.

Thus, according to a first aspect of the invention there is provided an optical imaging device comprising an optical imaging system with at least one optical imaging component, a support structure, a capturing device and a control device. The support structure supports the at least one optical imaging component and comprises at least one actuator. The actuator is adapted to adjust, in at least one degree of freedom, at least one of a geometric parameter of the optical imaging component, a relative position of the optical imaging component with respect to at least one reference and a relative orientation of the optical imaging component with respect to the at least one reference as a function of at least one control signal provided by the control device. The capturing device is adapted to continuously capture, at at least one capturing location of the optical projection unit, an actual value of a variable representative of a temperature at the location and to provide the actual value to the control device. The control device is adapted to generate the at least one control signal as a function of the actual value of the variable and to provide the at least one control signal to the at least one actuator to at least partially compensate thermally induced alterations in at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component. The control device is adapted to use at least one previously established thermal model of the optical imaging system to generate the at least one control signal, the thermal model describing at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component at least as a function of the variable and the capturing location.

According to a second aspect of the invention there is provided an optical imaging arrangement comprising an illumination unit, a mask unit adapted to receive a mask having a pattern, a substrate unit adapted to receive a substrate an optical projection unit comprising an optical element system with at least one optical element, a support structure, a capturing device and a control device. The illumination unit is adapted to illuminate the mask while the optical element system is adapted to transfer an image of the pattern onto the substrate. The support structure supports an optical imaging component and comprises at least one actuator, the optical imaging component being at least one of an optical component of the illumination unit, the at least one optical element, the mask and the substrate. The actuator is adapted to adjust, in at least one degree of freedom, at least one of a geometric parameter of the optical imaging component, a relative position of the optical imaging component with respect to at least one reference and a relative orientation of the optical imaging component with respect to the at least one reference as a function of at least one control signal provided by the control device. The capturing device is adapted to continuously capture, at at least one capturing location of the optical imaging device, an actual value of a variable representative of a temperature at the location and to provide the actual value to the control device. The control device is adapted to generate the at least one control signal as a function of the actual value of the variable and to provide the at least one control signal to the at least one actuator to at least partially compensate thermally induced alterations in at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component. The control device is adapted to use at least one previously established thermal model of the optical imaging arrangement to generate the at least one control signal, the thermal model describing at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component at least as a function of the variable and the capturing location.

According to a third aspect of the invention there is provided a method of supporting at least one optical imaging component of an optical imaging system of an optical imaging device comprising supporting the at least one optical imaging component via at least one actuator, continuously capturing, at at least one capturing location of the optical imaging device, an actual value of a variable representative of a temperature at the location, generating at least one control signal as a function of the actual value of the variable and providing the at least one control signal to the at least one actuator to adjust via the actuator, in at least one degree of freedom, at least one of a geometric parameter of the optical imaging component, a relative position of the optical imaging component with respect to at least one reference and a relative orientation of the optical imaging component with respect to the at least one reference to at least partially compensate thermally induced alterations in at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component. The at least one control signal is generated using at least one previously established thermal model of the optical imaging device, the thermal model describing at least one of the geometric parameter, the relative position and the relative orientation of the optical imaging component at least as a function of the variable and the capturing location.

Furthermore, the above objects are achieved according to the invention which is based on the teaching that a reduction of the effort necessary for an optical imaging device while at least maintaining the imaging accuracy of the optical imaging arrangement may be achieved if, as a second alternative, strongly localized introduction of heat into either the optical imaging components or their support structure is prevented by shielding these parts from heat sources of the optical imaging device in such a manner that heat transfer to these components is homogenized.

This homogenization of the heat transfer has the advantage that localized heating up of the shielded component is prevented or at least counteracted which otherwise would lead to the introduction of considerable local stresses and, thus, local deformations into the structure. Such local deformation otherwise would lead to a considerable deterioration of the imaging quality. In particular, if combined with the thermal model based control as outlined above, strongly localized heating and the associated strongly localized deformation (due to high local temperature gradients) would either deteriorate prediction quality of the thermal model (at a given spatial resolution of the capturing locations) or require an increased number of capturing locations (i.e. an increase in the spatial resolution of the capturing locations). With the homogenizing shielding according to the invention the transferred heat is distributed over a wider area leading to a reduced variation in the local deformation of the affected components which is much easier to handle (and compensate). Thus, the effort for maintaining the required high imaging quality may be considerably reduced.

The homogenization of the heat transfer may be obtained by any suitable shielding means. For example, the shielding device may have a very high thermal conductivity which has the effect that the heat coming from a heat source is distributed within the shielding device prior to further transferring it to any other component. Furthermore, the heat may be distributed over the shielding device by other means, for example by circulating a corresponding heat carrier medium or cooling medium within the shielding device.

Thus, according to a fourth aspect of the invention there is provided an optical imaging device comprising an optical imaging system with at least one optical imaging component, a support structure supporting said at least one optical imaging component, and at least one shielding device with a shielding unit. The shielding unit is spatially associated to at least one shielded unit, the shielded unit being at least one of a part of the support structure and a part of the optical imaging component. The shielding unit shields the shielded unit from a heat source of the optical imaging device in a manner homogenizing heat transfer to the shielded unit.

According to a fifth aspect of the invention there is provided an optical imaging device comprising an illumination unit, a mask unit adapted to receive a mask with a pattern, a substrate unit adapted to receive a substrate, an optical projection unit comprising an optical system with at least one optical element, a support structure and at least one shielding device with a shielding unit. The illumination unit is adapted to illuminate the mask while the optical system is adapted to transfer an image of the pattern onto the substrate. The support structure supports at least one of an optical component of the illumination unit, the mask, the at least one optical element and the substrate. The shielding unit is spatially associated to at least one shielded unit, the shielded unit being at least one of a part of the support structure and a part of at least one of the optical component, the mask, the optical element and the substrate. The shielding unit shields the shielded unit from a heat source of the optical imaging device in a manner homogenizing heat transfer to the shielded unit.

According to a sixth aspect of invention there is provided a method of supporting at least one optical imaging component of an optical imaging system of an optical imaging device comprising supporting the at least one optical imaging component via a support structure and, via a shielding unit, shielding a shielded unit from a heat source of the optical projection unit in a manner homogenizing heat transfer to the shielded unit. The shielded unit is at least one of a part of the support structure and a part of the optical imaging component

Further aspects and embodiments of the invention will become apparent from the dependent claims and the following description of preferred embodiments which refers to the appended figures. All combinations of the features disclosed, whether explicitly recited in the claims or not, are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a preferred embodiment of an optical imaging device according to the invention with which preferred embodiments of methods according to the invention may be executed; Figure 2 is a block diagram of a preferred embodiment of a method of supporting an optical component according to the invention which may be executed with the optical imaging device of Figure 1 ;

Figure 3 is a schematic representation of a further preferred embodiment of an optical imaging device according to the invention with which preferred embodiments of the method according to the invention may be executed.

DETAILED DESCRIPTION OF THE INVENTION

First embodiment

In the following, a first preferred embodiment of an optical imaging arrangement 101 according to the invention with which preferred embodiments of methods according to the invention may be executed will be described with reference to Figures 1 and 2.

Figure 1 is a schematic and not-to-scale representation of the optical imaging device in the form of an optical exposure apparatus 101 operating in the EUV range at a wavelength of 13 nm. The optical exposure apparatus 101 comprises an optical imaging system with an optical projection unit 102 adapted to transfer an image of a pattern formed on a mask 103.1 of a mask unit 103 onto a substrate 104.1 of a substrate unit 104. To this end, the optical exposure apparatus 101 comprises an illumination system 105 illuminating the reflective mask 103 (via a suitable light guide not shown in Figure 1 ). The illumination system 105 in turn comprises a light source and one or more corresponding optical element(s) forming the beam shaping optics of the illumination system 105 (not shown in further detail in Figure 1 )

The optical projection unit 102 receives the light reflected from the mask 103.1 and projects the image of the pattern formed on the mask 103.1 onto the substrate 104.1 , e.g. a wafer or the like. To this end, the optical projection unit 102 holds an optical element unit group 106 of optical element units. This optical element unit group 106 is held within a housing 102.1 of the optical projection unit 102, often also referred to as the projection optics box (POB) 102.1 , which forms a first support structure unit. The optical element unit group 106 comprises a number of optical elements in the form of mirrors, of which only the mirrors 106.1 , 107.1 , 108.1 are shown. These optical elements 106.1 , 107.1 , 108.1 are positioned with respect to one another along an axis 102.2 of the optical projection unit 102 in up to all six degrees of freedom (DOF). The optical projection unit 102 receives the part of the light path between the mask 103.1 and the substrate 104.1. The projection surfaces 106.2, 107.2, 108.2 of its optical elements 106.1 , 107.1 , 108.1 cooperate to transfer the image of the pattern formed on the mask 103.1 onto the substrate 104 located at the end of the light path.

The mask 103.1 is received on a mask table 103.2 of the mask unit 103, the mask table 103.2 being supported by a suitable second support structure unit 110.1 (also supporting the illumination unit 105) on a ground structure 109. In a similar way, the substrate 104.1 is received on a substrate table 104.2 of the substrate unit 104, the substrate table 104.2 as well being supported by a suitable third support structure unit 1 10.2 on the ground structure 109.

The embodiment shown is a system without a conventional metrology frame. Thus, the optical projection unit 102 is directly supported on the ground structure 109 via vibration isolating units 109.1. Similarly, the second support structure unit 110.1 as well as the third support structure unit 1 10.2 is supported on the ground structure 109 via vibration isolating units 109.2 and 109.3, respectively.

Typically, the resonant frequency of such vibration isolating units 109.1 to 109.3 will be chosen between 0.01 and 10 Hz. Preferably, the resonant frequency of the support provided by the vibration isolating units 109.1 to 109.3 is at about 0.1 Hz or below. Such a vibration isolating unit 109.1 to 109.3 may be provided by well known so called magnetic gravity compensators as they are disclosed in WO 2005/028601 A2 (Mϋhlbeyer et al.), the entire disclosure of which is incorporated herein by reference, having lateral drift control which may also operate according to an electromagnetic principle (e.g. using voice coil motors etc.) and may also provide the drift control at a resonant frequency of about the same order, i.e. preferably of about 0.1 Hz or below. However, it will be appreciated that, with other embodiments of the invention, other working principles, e.g. purely mechanical working principles with spring arrangements or pneumatic arrangements of suitably low resonant frequency, may be chosen for the vibration isolating units.

The image of the pattern formed on the mask 103.1 is usually reduced in size and transferred to several target areas of the substrate 104.1. The image of the pattern formed on the mask 103.1 may be transferred to the respective target area on the substrate 104.1 in two different ways depending on the design of the optical exposure apparatus 101. If the optical exposure apparatus 101 is designed as a so called wafer stepper apparatus, the entire image of the pattern is transferred to the respective target area on the substrate 104.1 in one single step by irradiating the entire pattern formed on the mask 103.1. If the optical exposure apparatus 101 is designed as a so called step-and-scan apparatus, the image of the pattern is transferred to the respective target area on the substrate 104.1 by progressively scanning the mask table 103.2 and thus the pattern formed on the mask 103.1 under the projection beam while performing a corresponding scanning movement of the substrate table 104.2 and, thus, of the substrate 104.1 at the same time.

In both cases, the relative position of the optical elements of the optical element unit group 106, i.e. the mirrors 106.1 , 107.1 , 108.1 , with respect to each other as well as with respect to the mask 103.1 and with respect to the substrate 104.1 has to be maintained within predetermined limits to obtain a high quality imaging result.

During operation of the optical exposure apparatus 101 , the relative position and orientation of the mirrors 106.1 , 107.1 , 108.1 with respect to each other as well as with respect to the mask 103.1 and the substrate 104.1 is subject to alterations resulting from, both, intrinsic and extrinsic, disturbances introduced into the system. The same applies to the geometry of the mirrors 106.1 , 107.1 , 108.1 as well as to the geometry of the mask 103.1 and the geometry of the substrate 104.1. Such disturbances may be mechanical disturbances, e.g. in the form vibrations resulting from forces generated within the system itself but also introduced via the surroundings of the system, e.g. the ground structure 109. The disturbances may however also be thermally induced disturbances, e.g. position alterations and/or orientation alterations and/or geometry alterations due to thermal expansion of the parts of the system.

In order to keep the above predetermined limits of the relative position of the mirrors 106.1 , 107.1 , 108.1 with respect to each other as well as with respect to the mask 103.1 and the substrate 104.1 , the mirrors 106.1 and 107.1 may be actively positioned and oriented in space via actuator units 106.3 and 107.3, respectively. Similarly, the mask table 103.2 and the substrate table 104.2 may be actively positioned and oriented in space via suitable actuators (not shown in further detail) arranged between the mask table 103.2 and the substrate table 104.2, respectively, and the associated support structure 1 10.1 and 1 10.2, respectively. In addition or as an alternative, corresponding actuators may be arranged between the mask 103.1 and the mask table 103.2 as well as between the substrate table 104.2 and the substrate 104.1. Furthermore, suitable actuators may be provided to adjust the geometry of the mirrors 106.1 , 107.1 , 108.1 the mask 103.1 and the substrate 104.1.

The optical components of the illumination system 105, the mask 103.1 , the substrate 104.1 as well as the optical elements 106.1 , 107.1 and 108.1 of the optical projection unit 102, in the sense of the invention, all form optical imaging components of the optical exposure apparatus 101. They are supported by the support structure formed by the housing 102.1 (first support structure unit), the second support structure unit 1 10.1 and the third support structure unit 110.2.

In a conventional exposure apparatus the continuous active positioning of such optical imaging components during operation (e.g. during exposure of the wafer) is performed on the basis of the measurement results of a plurality of metrology arrangements capturing the spatial relationship between such optical imaging components with respect to each other as well as with respect to a reference (typically, a so-called metrology frame) using high precision distance measurement. Such an approach is known, for example, from initially mentioned US 7,221 ,463 B2 (Mizuno et al.).

However, the known solutions suffer from the problem that, during operation, thermal disturbances may also affect the metrology arrangements themselves and alter their relative position and orientation. Furthermore, the alterations in the relative position and orientation as well as in the geometry of the optical imaging components may be of an amount which is hard to capture even using such high precision distance measurements. This may lead to a non negligible thermal drift within the measurement results and, consequently, to an increasing misalignment of the optical imaging components which in turn leads to a deterioration of the imaging quality obtained.

In order to reduce the extent of such thermal drifts, typically, the number and accuracy of the high precision distance measurements may be increased in order to deal with this problem. Furthermore, typically, after a certain amount of operation time (typically about five minutes) a calibration step is performed in order to recalibrate the entire system. To this end, typically, a test exposure is performed and the image quality of an exposed testing pattern is captured and used to readjust the position of the optical imaging components and is known from US 7,221 ,463 B2 (Mizuno et al.). As an alternative, this intermittent calibration may be performed on the basis of a thermal model of the exposure apparatus as it is also known from US 7,221 ,463 B2 (Mizuno et al.). However, this intermittent calibration is a time- consuming procedure which considerably reduces the throughput of the exposure apparatus such that the intervals of the calibration may not be arbitrarily reduced.

In contrast to this, with the present invention, after providing the components of the exposure apparatus 101 in a step 114.1 and supporting the optical imaging components via the respective support structure in a step 114.2, the active positioning of the adjustable ones of the optical imaging components (i.e. the adjustable optical components of the illumination system 105, the mask 103.1 , the substrate 104.1 as well as the adjustable optical elements 106.1 , 107.1 ) is performed continuously during exposure of the substrate 104.1 in an exposure step 1 14.3 on the basis of a thermal model of the exposure apparatus 101.

To this end, the exposure apparatus 101 comprises a capturing device 1 11 which concurrently with the exposure process, in a capturing step 114.4, captures at a plurality of capturing locations actual values of a variable representative of the actual temperature at the respective capturing location. To this end, the capturing device comprises a capturing unit

1 11.1 and a plurality of sensor elements 1 11.2 to 11 1.10. Each one of the sensor elements

1 11.2 to 1 11.10 is located at the respective capturing location and connected to the capturing unit 11 1.1. Each sensor element 1 11.2 to 11 1.10 continuously provides corresponding temperature signals representative of the actual temperature at the respective capturing location.

The type of the variable captured varies depending on the type of sensor elements used. For example, if common thermocouples are used, a voltage is captured as the variable representative of the actual temperature at the capturing location. However, it will be appreciated that, with other embodiments of the invention, any other suitable type of temperature sensor may be used.

The capturing unit 1 11.1 is connected to a control device 112 of the exposure apparatus 101 and provides corresponding temperature signals representative of the respective actual temperature at the respective capturing location to the control device 112. In a determination step 114.5, the control device uses a thermal model of the exposure apparatus 101 (stored in a memory 112.1 of the control device 1 12) in order to determine the spatial state (i.e. the actual position, the actual orientation with respect to a given reference and the actual geometry) of the optical imaging components (i.e. the optical components of the illumination system 105, the mask 103.1 , the substrate 104.1 as well as the optical elements 106.1 , 107.1 and 108.1 ) of the exposure apparatus 101 in at least one degree of freedom (in the embodiment shown: in all six degrees of freedom) on the basis of these actual values provided by the capturing unit 1 11.1 to the control device 112.

In principle, any suitable part of the exposure apparatus 101 may be selected as the given reference. For example, a part of the support structure 102.1 , 110.1 , 1 10.2 may be selected as the given reference. However, in the embodiment shown, the large mirror 108.1 located closest to the substrate is selected as the reference of the thermal model. This is nonetheless due to the fact that this mirror 108.1 , typically, is a rather heavy, (actively and/or passively) thermally stabilized component which is highly suitable for such a reference. However, it will be appreciated that, with other embodiments of the invention, any other optical imaging component in the sense of the invention may be used as the reference for the thermal model.

The thermal model of the exposure apparatus 101 may have been previously established in a theoretical manner (e.g. by computer modeling and simulation of the exposure apparatus 101 ), in an empirical manner (e.g. by testing the exposure apparatus 101 ) or a combination thereof. For example, the exposure apparatus 101 , in a thermal model setup phase, may have been subject to one or several well defined thermal situations (captured by the thermal sensors 1 11.2 to 11 1.10) while at the same time capturing the spatial state (i.e. the geometry and position and orientation) of the optical imaging components of the optical imaging system (e.g. by simultaneously measuring the imaging quality of the optical imaging system).

Depending on the thermal resolution (i.e. the number of different thermal situations) obtained in this phase and the thermal resolution necessary for sufficiently accurately describing the thermal behavior of the exposure apparatus 101 the data obtained in this setup phase (i.e. the relation between temperature situation and optical element state) itself may represent the thermal model of the exposure apparatus 101. However, is also possible that the thermal model is further refined in a theoretical manner by adding further data e.g. obtained by suitable interpolation algorithms etc. Furthermore, it is possible that such refinement of the thermal model is performed later during actual calculation of the spatial state out the optical imaging components. To this end, the corresponding refinement algorithms may be stored in the memory 112.1 and accessed by the control device 112 as needed.

In the embodiment shown the thermal model describes the thermal behavior of the entire exposure apparatus 101. However, it will be appreciated that, with other embodiments of the invention, the thermal model used may only describe the thermal behavior of a part of the optical exposure apparatus. For example, the thermal model may only describe the optical projection unit or the elimination unit as a central optical imaging device in the optical imaging system (the optical elements of the respective unit then forming the respective optical imaging components in the sense of the present invention).

However, other components other than the illumination unit or the optical projection unit may be described by or included in such a thermal model. In particular, either one of a thermal model of an illumination unit, a mask unit, and a substrate unit may be a part of a thermal model also comprising the thermal model of the optical projection unit. In other words, the thermal model of the exposure apparatus may be composed of a plurality of different partial thermal models of components. In general, such a thermal model may be set up for any optical imaging device (e.g. illumination unit, mask unit, optical projection unit, substrate unit) optically participating in the optical imaging process.

The control device 1 12 uses the spatial state of the optical imaging components of the exposure apparatus 101 obtained in the determination step for generating control signals for the actuators of the respective actuated optical imaging component (actuated components of the illumination unit 105, mirrors 106.1 , 107.1 , 108.1 , mask 103.1 and substrate 104.1 ) in order to compensate (in real-time) thermally induced deviations of the actual spatial state of the optical imaging components from a given setpoint spatial state of these optical imaging components. In a control step 1 14.6, the control device 1 12 provides these control signals to the actuated optical imaging components to compensate the thermally induced alterations in order to maintain a high imaging quality.

The actuators of the respective actuated optical imaging component may be of any suitable type in order to provide the desired actuation accuracy. Suitable actuators are a well-known and, thus, will not be described here in further detail. Typically such actuators are able to provide spatial state adjustment in up to six degrees of freedom, primarily meant for improving image quality by ultra-fine (down to picometer level) position adjustments.

In the embodiment shown, the thermal model is used to determine the spatial state of the optical imaging components in all six degrees of freedom. However, it will be appreciated that, with other embodiments of the invention, it may be sufficient that the thermal model is only used to determine the spatial state of one or several optical imaging components in less than six degrees of freedom. It may even be sufficient that the thermal model based determination of the spatial state of one or several optical imaging components is only provided in one degree of freedom. In particular, this may be the case if the thermal disturbances to be expected only have a noticeable influence along the respective degree of freedom such that determination of the spatial state in the remaining degrees of freedom may be dispensed with.

Furthermore, it will be appreciated that, with other embodiments of the invention, in addition or as an alternative to thermal model based determination of the spatial state of the optical imaging components, at least in some degrees of freedom a conventional determination (e.g. using conventional distance measurements) of the spatial state of the optical imaging components may be used (e.g. capturing and using the actual values of a different, second variable representative of the respective distance for determining the spatial state). If such conventional determination of the spatial state is used in parallel to be thermal model based determination of the spatial state it is possible to provide a plausibility or sanity check of the thermal model based control.

To this end, a plurality of conventional contactless distance measurement devices (as they are indicated in Figure 1 by the dashed contours 1 13) may be provided and connected to the control device 112. Suitable contactless distance measurement devices and their implementation are known for example from initially mentioned US 7,221 ,463 B2 (Mizuno et al.). A further suitable arrangement of such distance measurement devices is known, for example, from initially mentioned WO 2006/128713 (Kwan) and, thus, will not be described here in further detail. Such distance measurement devices may work according to any desired principle (e.g. interferometric principle, encoder principle, capacitive principle etc.).

It will be appreciated that these conventional distance measurement devices may provide measurement with respect to a different second reference than the first reference used for the thermal model. However, preferably, the reference for these distance measurements is the same as the reference for the thermal model since this allows an easy plausibility or sanity check of the thermal model based control. Thus, in the present example, the large mirror 108.1 preferably used as a reference for the thermal model based control also serves as the reference for the distance measurements.

It will be further appreciated that the control of at least some of the actuators in some degrees of freedom may only be based on the measurement results of these conventional distance measurement devices. In particular, this may be the case if sufficiently accurate capturing of the spatial state of the respective optical imaging component via these conventional distance measurement devices is to be expected.

In the embodiment shown, the steps 114.4 to 114.6 are continuously repeated (over a certain amount of operating time) concurrently with the exposure process. It will be appreciated that this continuous repetition includes variants where a certain short delay occurs between subsequent repetitions of these steps 1 14.4 to 1 14.6. The delay may be selected as a function of the shortest thermal time constant of the separate components of the imaging apparatus 101. More precisely, the delay may be adapted to the shortest interval to be expected during operation in which a noticeable change in the temperature situation leading to a noticeable change in the spatial state of the optical imaging components may occur. In other words, if one of the components and a high thermal sensitivity (i.e. a short thermal time constant) a short delay between subsequent repetitions of the steps 1 14.4 to 1 14.6 (adapted to this thermal time constant) is to be chosen in order to avoid unduly high alterations in the spatial state between these repetitions. Preferably, the control bandwidth for the thermal compensation loop (steps 1 14.4 to 114.6) is selected to range from 0.01 Hz to 1 Hz in order to provide proper compensation while still maintaining the control noise at an acceptable level and respecting the limits due to the time constant of the temperature sensors.

After a certain amount of operating time (typically about 5 to 10 minutes), the exposure step 114.3 is interrupted and a conventional calibration step 114.7 is performed using a test exposure of a defined test pattern performed via the optical exposure system of the exposure apparatus 101 and a corresponding image quality capturing unit 1 11.1 1 of the capturing device 11 1. The result of this calibration step is then not only used by the control device 1 12 to adjust the spatial state of the optical imaging components. The result is also used to adapt or refine the thermal model stored in the memory 112.1. This adaptation or refinement is done depending on the deviation between the spatial state resulting from the preceding thermal model based control and the setpoint spatial state captured in the calibration step.

In a step 1 14.8 is then determined if a further exposure step is to be performed. If this is the case, the method jumps back to step 1 14.3. Otherwise, execution the method is stopped in a step 114.9.

It will be appreciated that the number and location of the capturing locations depends on the design of the exposure apparatus 101. The capturing locations may be located at virtually any location within the exposure apparatus 101. For example, it is possible that at a certain capturing location the temperature of an atmosphere within the exposure apparatus 101 or surrounding the exposure apparatus 101 is captured. Preferably, the capturing locations are located at the components of the support structure and/or at the optical imaging components since these are the components influencing or directly related to the spatial state of the optical imaging components. Thus, reliable and sufficiently accurate capturing results may be obtained adding to the overall accuracy of the system.

It will be particularly appreciated that, with preferred embodiments of the invention, apart from a suitable number of capturing locations provided at the support structure (predominantly defining the position and orientation of the respective optical imaging component), a suitable number of capturing locations may be provided directly at the optical imaging components in order to be able to easily determine the actual geometry of the respective optical imaging component via the thermal model. In particular, this may be the case if no active temperature control (e.g. cooling and/or heating) of the respective optical imaging component (e.g. the respective optical element) is provided. In particular, the number and location of the capturing locations (i.e. the temperature sensors) is selected as a function of the thermal conductivity of the material of the support structure 102.1 , 1 10.1 , 110.2 and the material of the optical imaging components. More precisely, if the respective material has a low thermal conductivity a larger number of capturing locations is necessary to provide a sufficiently high sensitivity of the control.

This is due to the fact that in a material having a poor thermal conductivity a considerable thermal disturbance may not yet have propagated to a capturing location while it may have already caused a considerable alteration in the spatial state of the respective component. Thus, in order to properly reflect the actual thermal state of the respective components, the number and location of the capturing locations is selected as a function of the thermal conductivity of the respective component. Furthermore, in addition, the number and location of the capturing locations may also be selected as a function of the likelihood of occurrence and/or severity of a thermal disturbance to be expected at the respective location.

In case the sources of thermal disturbances or the temperature distribution to be expected during normal operation of the exposure apparatus 101 are not known precisely, a large number of capturing locations has to be provided to obtain sufficiently accurate information for the thermal model based corrections outlined above.

It will be appreciated that for the support structure 102.1 , 1 10.1 , 1 10.2 as well as for some parts of the optical imaging components (e.g. the bodies of the mirrors 106.1 , 107.1 , 108.1 ) zero or near zero thermal expansion materials may be used such as, for example, Zerodur, ULE glass, Kyoceram, Cordierite etc. However, as outlined above, these materials not only have a low coefficient of thermal expansion (CTE) but also a low thermal conductivity. Thus, a rather high number of capturing locations would be necessary.

For example, if the support structure was made of a Zerodur structure being about 1.5 m high it would have a thermal sensitivity in the order of 75 nm/K. That means that the relative position between the two mirrors 106.1 and 108.1 will shift by about 1 nm for a temperature change of 13 mK within the support structure. Under typical operating conditions of the exposure apparatus 101 the temperature change rate is expected to be in the order of 0.1 to 1 K per five minutes without active temperature control (with a typical POB structure weighing some 500 to 1000 kg, this corresponds to somewhere between 300 W to 3 kW of thermal power to be added to or extracted from the structure).

In a preferred variant of the present invention, at least for the support structure 102.1 , 1 10.1 , 1 10.2 materials with a somewhat higher CTE (preferably below 4 ppm/K) but with a much higher thermal conductivity are used. For example, aluminum nitride (AIN) may be used for the support structure 102.1 , 110.1 , 110.2. Such an aluminum nitride (AIN) typically has a CTE of 4 ppm/K (i.e. approximately 100 times the CTE of Class 1 Zerodur) but a thermal conductivity of 180 W/(m K) (i.e. approximately also 100 times the thermal conductivity of Class 1 Zerodur). Thus, for a housing 102.1 being about 1.5 m high, at a temperature accuracy of the capturing device 1 11 of 1 mK, an effective thermal stability after correction of 6 nm, irrespective of rate of temperature change may be obtained.

This is equivalent to being able to maintain the temperature at an accuracy of 80 mK for a Zerodur structure with an active temperature control (active cooling/heating) or a passive Zerodur structure (with no active temperature control) but with a maximum temperature rise of 80 mK between image calibration. In other words, the advantage of an aluminum nitride (AIN) support structure lies not within the absolute thermal deformation performance, but lies in the fact that its high thermal conductivity ensures that the temperature captured at a limited number of (eventually randomly distributed) capturing locations is more representative of the actual temperature distribution over the structure, and hence the accuracy of the thermal model based correction is substantially improved.

It will be appreciated that, with other embodiments of the invention, other materials may be used for the support structure 102.1 , 1 10.1 , 110.2 as well as other parts of the optical imaging components. Preferably, materials are used which have a coefficient of thermal expansion being greater than 1.5 ppm/K and/or a thermal conductivity being greater than 50 W/(m K). Apart from aluminum nitride (AIN), preferred materials are silicon (Si), silicon carbide (SiC), tungsten (W), boron carbide (B₄C), Invar and steel. Furthermore, a carbon fibre reinforced silicon carbide (SiC) may be used. The latter typically has a thermal conductivity of more than 20 W/(m K) (which may, however, be increased by a factor of 2 to 5)and a coefficient of thermal expansion (CTE) of 0.1 ppm/K. Preferably, materials are used having a coefficient of thermal expansion which is lower than 5 ppm/K and/or a thermal conductivity which is higher than 100 W/(m K). It will be appreciated that arbitrary combinations of the above materials may also be used.

A further advantage of the invention lies within the fact that such a structure has a large thermal time constant, whether provided in a zero or near zero thermal expansion material (e.g. Zerodur etc.) or in another other preferred material having a higher thermal expansion (e.g. AIN etc.). Thus, the capturing of the temperature only needs to be provided at a low bandwidth (preferably less than 1 Hz) which allows effective filtering of measurement noise. A further improvement of the thermal stability of the exposure apparatus 101 is achieved in the embodiment shown via a shielding device 115 comprising a plurality of shielding units 1 15.1. Each shielding unit 115.1 shields at least a part of the support structure supporting the mirror 106.1 and 107.1 (and forming a shielded unit in the sense of the invention) from a heat source of the optical projection unit 102. Such a heat source may for example be the actuators 106.3 and 107.3 actuating the respective mirror. The heat source may however also be the respective mirror 106.1 and 107.1 which heats up during the exposure process due to the partial absorption of exposure light incident on the respective mirror. In particular, this may be the case if no active temperature control (e.g. cooling and/or heating) of the respective optical element is provided.

The respective shielding unit 115.1 preferably is supported in a manner decoupled from the respective optical imaging component (here: mirrors 106.1 and 107.1 ) as well as the respective support structure (here: housing 102.1 ) in terms of mechanical and thermal conduction disturbances. Thus, on the one hand, the introduction of dynamic and/or static mechanical disturbances into the respective optical imaging component as well as the respective support structure is prevented as far as possible. Furthermore, disturbances resulting from heat introduced into the respective optical imaging component as well as the respective support structure by a thermal conduction is prevented as far as possible.

The respective shielding unit 115.1 may, for example, be connected to a separate shield support structure (not shown in Figure 3) that does neither contact the respective optical imaging component (here: mirrors 106.1 and 107.1 ) nor the associated support structure (here: housing 102.1 ) of the respective optical imaging component. Such a separate shield support structure may be directly supported on the ground structure 209. For example, a commonly known external cooling jacket (not shown in Figure 3) for the housing 102.1 may serve as such a separate shield support structure.

Furthermore, according to the invention, the respective shielding unit 115.1 shields the respective shielded unit in a manner homogenizing the transfer of heat from the respective heat source to the shielded unit. In the present example this homogenization of the heat transfer is achieved via a highly thermally conductive material of the respective shielding unit 1 15.1.

This high thermal conductivity of the material of the respective shielding unit 115.1 has the effect that the heat coming from the respective heat source is distributed or spread, respectively, within the respective shielding unit 1 15.1 prior to transferring it further to the shielded unit. Thus, in other words, a heat load hitting the respective shielding unit 115.1 at its side facing the heat source in a first area is spread within the shielding unit 115.1 such that the subsequent transfer of this heat load from the shielding unit 115.1 to the shielded unit occurs over a second area which is larger than the first area. Thus, in other words, compared to a first heat flow density impinging on the shielding unit 1 15.1 (from the respective heat source), a reduction of the heat second flow density impinging on the shielded unit (from the shielding unit 1 15.1 ) is obtained via the shielding unit 1 15.1.

The reduction of the heat flow density obtained via the shielding unit 1 15.1 in an advantageous manner results in a homogenization of the temperature profile within the shielded unit since thermal disturbances resulting from the respective heat source are distributed over a wider area leading to a reduction of the local temperature differences within the shielded unit. In other words, the shielding unit 1 15.1 acts as a sort of spatial low- pass filter to even out any local heating from either the inside or the outside the structure, by simply spreading the heat out over a larger area.

It will be appreciated that the homogenizing shielding device 1 15 represents a separate inventive idea which may be applied independently from the thermal model based control outlined above. The use of such a homogenizing shielding device 1 15 is particularly beneficial in combination with structures having a low thermal conductivity (such as the ones made of near zero or zero thermal expansion materials mentioned above, e.g. Zerodur etc.) since it reduces a locally concentrated thermally induced distortion of the entire exposure apparatus.

It will be appreciated that such homogenizing heat shielding units 1 15.1 are preferably applied on all sides of the shielded unit which are exposed to (internal or external) heat sources. The shielding unit 1 15.1 only needs to be of a material with high thermal conductivity. It will be appreciated that the shielding unit 115.1 has not necessarily to be kept at a constant temperature. Thus, an active temperature control 115.1 may be dispensed with.

Materials such as copper (Cu), aluminum (Al) , silver (Ag), aluminum nitride (AIN), beryllium oxide (BeO), silicon (Si) silicon carbide (SiC), arbitrary combinations thereof etc. are suitable for the respective shielding unit 1 15.1. It will be appreciated that the coefficient of thermal expansion (CTE) of the material used for the shielding unit 1 15.1 is of virtually no importance since the shielding unit preferably is thermally and mechanically (i.e. dynamically and statically) decoupled from the support structure as well as the optical imaging component (i.e. preferably has no direct physical contact). Preferably, the shielding unit 1 15.1 is made at least in part of a material having a thermal conductivity of more than 120 W/(m K). It will be appreciated that, with the thermal model based control as outlined above, at least a control of the image position in two degrees of freedom (typically referred to as x and y) in the plane of the wafer 104.1 at a resolution of better than 0.1 nm may be achieved. However, it will be appreciated that, with other embodiments of the invention, any other desired variable or parameter affecting the imaging quality and being itself affected by the temperature situation of the imaging device may be corrected (in up to six degrees of freedom).

Second embodiment

In the following, a second preferred embodiment of an optical imaging device in the form of an exposure apparatus 201 according to the invention with which preferred embodiments of methods according to the invention may be executed will be described with reference to Figure 3.

The optical exposure apparatus 201 in its general design and functionality largely corresponds to the exposure apparatus 101 of Figure 1. Thus, it is here mainly referred to the differences only. In particular, identical parts have been given the same reference numbers raised by the amount 100 and with respect to the properties of these parts reference is made to the explanations given above in the context of the first embodiment.

One difference between the exposure apparatus 101 and the exposure apparatus 201 lies within the fact that the exposure apparatus 201 as a conventional metrology frame 216 supporting the optical projection unit 202 and a conventional contactless metrology arrangement 213 capturing the spatial relationship between the illumination unit 205, the mask unit 203, the optical elements 206.1 , 207.1 , 208.1 of the optical projection unit 202, the substrate unit 204 and a reference element 216.1 of the metrology frame 216.

The metrology frame 216 is designed in a conventional manner as a (actively and/or passively) thermally stabilized component. The same applies to the reference element 216.1 which, in the embodiment shown, is a block shaped component made of a zero thermal expansion material such as Zerodur etc. (providing passive thermal stability).

The reference element 216.1 is supported on the metrology frame 216 and serves as the reference for all measurements provided by the metrology arrangement 213. However, It will however be appreciated that, with other embodiment of the invention, the reference element 216.1 may also be directly supported on the ground structure 209, if desired, as is indicated by the dashed contour 217. Furthermore, it will be appreciated that any other suitable contactless metrology arrangement may be selected. A plurality of such contactless metrology arrangements which may be used in combination with thermal model based control according to the invention and as outlined above are known from initially mentioned US 7,221 ,463 B2 (Mizuno et al.), for example.

In particular, a cascaded contactless metrology arrangement may be selected as is known, for example, from US 7,221 ,463 B2 (Mizuno et al.). Such a cascaded metrology arrangement may have a metrology arrangement part sitting on the metrology frame and providing distance measurements for some of the components of the apparatus with respect to a first reference and a second metrology arrangement part supported on the ground structure and providing further distance measurements for some of the components and/or for the first reference with respect to a second reference.

A further difference with respect to the exposure apparatus 101 lies within the fact that the shielding device 215 of the exposure apparatus 201 comprises a heat carrier fluid circulating device 215.2 circulating a heat carrier fluid (i.e. a liquid or a gas) within the respective shielding unit 215.1. This circulation of the fluid also has a homogenizing effect on the heat transfer from the respective heat source to the shielded unit. Due to the circulation of the heat carrier fluid in a similar manner the heat introduced into the shielding unit 215.1 is spread within the shielding unit 215.1 leading to a reduction in the heat flow density impinging on the shielded unit.

It will be appreciated that material of the shielding unit 215.1 again may be selected to have a high thermal conductivity. However, given the homogenizing effect on the circulating heat carrier fluid, any other material may be used for the shielding unit 215.1.

The heat carrier fluid and, thus, the shielding unit 215.1 , may optionally be temperature controlled by an active temperature control. For example, the heat carrier fluid may be kept at a constant temperature, e.g. a fixed reference temperature (e.g. identical to that of the exposure apparatus 201 ), via the heat carrier fluid circulating device 215.2 (then comprising a heat exchanger or the like). However, in a manner similar to the passive shielding unit 1 15.1 , a rise in the temperature of the heat carrier fluid due to the impinging heat load may also be accepted as long as the distribution of the heat within the shielding unit 215.1 is guaranteed to do this circulation of the heat carrier fluid. In this case, the at carrier fluid circulating device 215.2 may be a simple pumping device.

In general, with such a shielding device, any localized heating by radiation (or convection, if applicable depending on the atmosphere prevailing) is firstly attenuated by the respective shielding unit. Any residual thermal print-through is evened out to a much more uniform temperature profile due to the high conductivity of the material of the shielding unit and/or the circulating heat carrier fluid. The remaining temperature change in the shielded structure is then largely spatially uniformly, and the thermal model based correction method as described above can be applied much more effectively, even if a relatively poor thermal conductor is used as the material for the respective structure.

Furthermore, the different measures (thermal model based control, selection of the material of the respective structure, selection of the number and location of the capturing locations, contactless distance measurements, presence and design of shielding devices) as they have been outlined above may be applied in an arbitrary combination. Preferably, if a low temporal temperature gradient may be achieved e.g. due to using the shielding devices, zero or near zero thermal expansion materials (such as Zerodur etc.) may be used for the respective structure and a temperature related correction via the thermal model based control may be dispensed with. If localized heating with a high thermal load is to be expected, a material with a high thermal conductivity (such as AIN)is selected for the respective structure in combination with a thermal model based control to compensate deviations in the spatial state of the optical imaging components. If highest performance is required for the exposure apparatus and a high heat load is to be expected, it is preferred to use a combination of shielding devices with zero or near zero thermal expansion materials (such as Zerodur etc.) and thermal model based control to compensate deviations in the spatial state of the optical imaging components.

Although, in the foregoing, embodiments of the invention have been described where the optical elements are exclusively reflective elements, it will be appreciated that, with other embodiments of the invention, reflective, refractive or diffractive elements or any combinations thereof may be used for the optical elements of the optical element units. In particular, the invention may be implemented in the context of any other optical processes using light at a different wavelength (e.g. conventional semiconductor lithography operating at 193 nm etα).

_{* * * * *}

Claims

What is claimed is:

1. An optical imaging device comprising:

- an optical imaging system with at least one optical imaging component,

- a support structure, - a capturing device and

- a control device;

- said support structure supporting said at least one optical imaging component and comprising at least one actuator;

- said actuator being adapted to adjust, in at least one degree of freedom, at least one of a geometric parameter of said optical imaging component, a relative position of said optical imaging component with respect to at least one reference and a relative orientation of said optical imaging component with respect to said at least one reference as a function of at least one control signal provided by said control device; - said capturing device being adapted to continuously capture, at at least one capturing location of said optical projection unit, an actual value of a variable representative of a temperature at said location and to provide said actual value to said control device;

- said control device being adapted to generate said at least one control signal as a function of said actual value of said variable and to provide said at least one control signal to said at least one actuator to at least partially compensate thermally induced alterations in at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component;

- said control device being adapted to use at least one previously established thermal model of said optical imaging system to generate said at least one control signal, said thermal model describing at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component at least as a function of said variable and said capturing location.

2. The optical imaging device according to claim 1 , wherein at least one of

- said capturing device is adapted to continuously capture an actual value of said variable at each one of a plurality of capturing locations of said optical imaging device and - said at least one capturing location is located at one of said support structure and said optical imaging component.

3. The optical imaging device according to claim 1 or 2, wherein

- said support structure comprises at least one passive support element being made of a material having a coefficient of thermal expansion and a thermal conductivity; - at least one of said coefficient of thermal expansion being greater than 1.5 ppm/K and said thermal conductivity being greater than 50 W/(m K).

4. The optical imaging device according to claim 3, wherein said material of said passive support element is selected from a material group consisting of aluminum nitride (AIN), silicon (Si), silicon carbide (SiC), carbon fiber reinforced silicon carbide (SiC), tungsten (W), boron carbide (B₄C), Invar, steel and combinations thereof.

5. The optical imaging device according to claim 3 or 4, wherein at least one of said coefficient of thermal expansion is lower than 5 ppm/K and said thermal conductivity is higher than 100 W/(m K).

6. The optical imaging device according to any one of claims 1 to 5, wherein said reference is one of a reference structure separate from said support structure, a part of said support structure and a part of a further optical imaging component of said optical imaging system.

7. The optical imaging device unit according to any one of claims 1 to 6 wherein

- said variable is a first variable and said reference is a first reference and - said capturing device is adapted to capture and provide to said control device an actual value of at least one second variable representative of one of a geometric parameter of said optical imaging component, a relative position of said optical imaging component with respect to at least one second reference and a relative orientation of said optical imaging component with respect to said at least one second reference in at least one degree of freedom.

8. The optical imaging device according to claim 7 wherein said control device is adapted to generate said at least one control signal as a function of said actual value of said second variable.

9. The optical imaging device according to claim 7 or 8, wherein said second reference is one of a reference structure separate from said support structure, a part of said support structure and a part of a further optical imaging component of said optical imaging system.

10. The optical imaging device according to any one of claims 7 to 9, wherein

- said optical imaging component is one of an optical element and

- said capturing device comprises an image quality capturing device,

- said image quality capturing device being adapted to capture, as said at least one actual value of at least one second variable, a variable representative of an image quality of a reference image generated at least via said optical element.

1 1. The optical imaging device according to any one of claims 7 to 10, wherein said control device is adapted to modify said thermal model as a function of at least one of said actual values of said second variable.

12. The optical imaging device according to any one of claims 1 to 1 1 , wherein - said optical imaging component is one of an optical element, a mask and a substrate;

- said optical element being part of an optical element system operative in transferring a pattern formed on said mask onto said substrate.

13. The optical imaging device according to any one of claims 1 to 12, wherein - at least one shielding device with a shielding unit is provided;

- said shielding unit being spatially associated to at least one shielded unit, said shielded unit being at least one of a part of said support structure and a part of said optical imaging component; - said shielding unit shielding said shielded unit from a heat source of said optical imaging device in a manner homogenizing heat transfer to said shielded unit.

14. The optical imaging device according to claim 13, wherein said shielding unit is adapted to internally spread an amount of heat transferred from said heat source to said shielding unit prior to transferring heat to said shielded device.

15. The optical imaging device according to claim 13 or 14, wherein at least one of

- said shielding unit is made at least in part of a highly thermally conductive material,

- said shielding unit has a high heat capacity and

- said shielding device comprises a cooling fluid circuit with a cooling fluid source circulating a cooling fluid in a cooling fluid duct within said shielding unit;

- said shielding unit is at least one of thermally and mechanically decoupled from said shielded unit.

16. The optical imaging device according to any one of claims 13 to 15, wherein said shielding unit is at least one of - made at least in part of a material selected from a material group consisting of copper (Cu), aluminum (Al), silver (Ag), aluminum nitride (AINi) and beryllium oxide (BeO), Silicon (Si), silicon carbide (SiC) and combinations thereof; and

- made at least in part of a material having a thermal conductivity of more than 120 W/(m K).

17. An optical imaging arrangement comprising:

- an illumination unit,

- a mask unit adapted to receive a mask having a pattern,

- a substrate unit adapted to receive a substrate;

- an optical projection unit comprising an optical element system with at least one optical element,

- a support structure,

- a capturing device and

- a control device; - said illumination unit being adapted to illuminate said mask;

- said optical element system being adapted to transfer an image of said pattern onto said substrate;

- said support structure supporting an optical imaging component and comprising at least one actuator, said optical imaging component being at least one of an optical component of said illumination unit, said at least one optical element, said mask and said substrate;

- said actuator being adapted to adjust, in at least one degree of freedom, at least one of a geometric parameter of said optical imaging component, a relative position of said optical imaging component with respect to at least one reference and a relative orientation of said optical imaging component with respect to said at least one reference as a function of at least one control signal provided by said control device;

- said capturing device being adapted to continuously capture, at at least one capturing location of said optical imaging device, an actual value of a variable representative of a temperature at said location and to provide said actual value to said control device;

- said control device being adapted to use at least one previously established thermal model of said optical imaging arrangement to generate said at least one control signal, said thermal model describing at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component at least as a function of said variable and said capturing location.

18. A method of supporting at least one optical imaging component of an optical imaging system of an optical imaging device comprising:

- supporting said at least one optical imaging component via at least one actuator;

- continuously capturing, at at least one capturing location of said optical imaging device, an actual value of a variable representative of a temperature at said location; - generating at least one control signal as a function of said actual value of said variable and providing said at least one control signal to said at least one actuator to adjust via said actuator, in at least one degree of freedom, at least one of a geometric parameter of said optical imaging component, a relative position of said optical imaging component with respect to at least one reference and a relative orientation of said optical imaging component with respect to said at least one reference to at least partially compensate thermally induced alterations in at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component; - said at least one control signal being generated using at least one previously established thermal model of said optical imaging device, said thermal model describing at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component at least as a function of said variable and said capturing location.

19. The method according to claim 18, wherein said actual value of said variable is captured at each one of a plurality of capturing locations of said optical projection unit.

20. The method according to claim 18 or 19, wherein said at least one capturing location is located at one of said support structure and said optical imaging component.

21. The method according to any one of claims 18 to 20, wherein

- said variable is a first variable and said reference is a first reference and

- an actual value of at least one second variable representative of one of a geometric parameter of said optical imaging component, a relative position of said optical imaging component with respect to at least one second reference and a relative orientation of said optical imaging component with respect to said at least one second reference in at least one degree of freedom is captured and

- one of said at least one control signal is generated as a function of said actual value of said second variable and said thermal model is modified as a function of at least one of said actual values of said second variable.

22. The method according to claim 21 , wherein

- said optical imaging component is an optical element and, - as said at least one actual value of at least one second variable, a variable representative of an image quality of a reference image generated at least via said optical element is captured.

23. The method according to any one of claims 18 to 22, wherein, - via a shielding unit, a shielded unit is shielded from a heat source of said optical imaging device in a manner homogenizing heat transfer to said shielded unit;

- said shielded unit being at least one of a part of said support structure and a part of said optical imaging component;

24. The method according to claim 23, wherein said shielding unit internally spreads an amount of heat transferred from said heat source to said shielding unit prior to transferring heat to said shielded device.

25. The method according to claim 23 or 24, wherein at least one of

- as said shielding unit a unit is used that is made at least in part of a highly thermally conductive material, - as said shielding unit a unit is used that has a high heat capacity;

- a cooling fluid is circulated in a cooling fluid duct within said shielding unit;

26. An optical imaging device comprising: - an optical imaging system with at least one optical imaging component,

- a support structure supporting said at least one optical imaging component, and

- at least one shielding device with a shielding unit;

- said shielding unit being spatially associated to at least one shielded unit, said shielded unit being at least one of a part of said support structure and a part of said optical imaging component;

- said shielding unit shielding said shielded unit from a heat source of said optical imaging device in a manner homogenizing heat transfer to said shielded unit.

27. The optical imaging device according to claim 26, wherein said shielding unit is adapted to internally spread an amount of heat transferred from said heat source to said shielding unit prior to transferring heat to said shielded device.

28. The optical imaging device according to claim 26 or 27, wherein at least one of - said shielding unit is made at least in part of a highly thermally conductive material,

- said shielding unit has a high heat capacity and

29. The optical imaging device according to any one of claims 26 to 28, wherein said shielding unit is at least one of

- made at least in part of a material selected from a material group consisting of copper (Cu), aluminum (Al), silver (Ag), aluminum nitride (AIN), beryllium oxide (BeO), Silicon (Si), silicon carbide (SiC) and combinations thereof; and

30. The optical imaging device according to any one of claims 26 to 29, wherein

- a capturing device and a control device is provided; - said support structure comprising at least one actuator;

- said capturing device being adapted to continuously capture, at at least one capturing location of said optical imaging device, an actual value of a variable representative of a temperature at said location and to provide said actual value to said control device; - said control device being adapted to generate said at least one control signal as a function of said actual value of said variable and to provide said at least one control signal to said at least one actuator to at least partially compensate thermally induced alterations in at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component;

- said control device being adapted to use at least one previously established thermal model of said optical imaging device to generate said at least one control signal, said thermal model describing at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component at least as a function of said variable and said capturing location.

31. The optical imaging device according to claim 30, wherein at least one of

32. The optical imaging device according to claim 30 or 31 , wherein

33. The optical imaging device according to claim 32, wherein said material of said passive support element is selected from a material group consisting of aluminum nitride (AIN), silicon (Si), silicon carbide (SiC), carbon fiber reinforced silicon carbide (SiC), tungsten (W), boron carbide (B₄C), Invar, steel and combinations thereof.

34. The optical imaging device according to claim 32 or 33, wherein at least one of said coefficient of thermal expansion is lower than 5 ppm/K and said thermal conductivity is higher than 100 W/(m K).

35. The optical imaging device according to any one of claims 30 to 34, wherein said reference is one of a reference structure separate from said support structure, a part of said support structure and a part of a further optical imaging component of said optical imaging system.

36. The optical imaging device according to any one of claims 30 to 35, wherein

37. The optical imaging device according to claim 36, wherein said control device is adapted to generate said at least one control signal as a function of said actual value of said second variable.

38. The optical imaging device according to claim 36 or 37, wherein said second reference is one of a reference structure separate from said support structure, a part of said support structure and a part of a further optical imaging component of said optical imaging system.

39. The optical imaging device according to any one of claims 36 to 38, wherein

- said optical imaging component is an optical element and - said capturing device comprises an image quality capturing device,

- said image quality capturing device being adapted to capture as said at least one actual value of at least one second variable a variable representative of an image quality of a reference image generated at least via said optical element.

40. The optical imaging device according to any one of claims 36 to 39, wherein said control device is adapted to modify said thermal model as a function of at least one of said actual values of said second variable.

41. The optical imaging device according to any one of claims 26 to 40, wherein - said optical imaging component is one of an optical element, a mask and a substrate;

42. An optical imaging device comprising:

- an illumination unit,

- a mask unit adapted to receive a mask with a pattern,

- a substrate unit adapted to receive a substrate;

- an optical projection unit comprising an optical system with at least one optical element,

- a support structure and

- at least one shielding device with a shielding unit;

- said illumination unit being adapted to illuminate said mask;

- said optical system being adapted to transfer an image of said pattern onto said substrate;

- said support structure supporting at least one of an optical component of said illumination unit, said mask, said at least one optical element and said substrate;

- said shielding unit being spatially associated to at least one shielded unit, said shielded unit being at least one of a part of said support structure and a part of at least one of said optical component, said mask, said optical element and said substrate;

43. A method of supporting at least one optical imaging component of an optical imaging system of an optical imaging device comprising:

- supporting said at least one optical imaging component via a support structure and,

- via a shielding unit, shielding a shielded unit from a heat source of said optical projection unit in a manner homogenizing heat transfer to said shielded unit; - said shielded unit being at least one of a part of said support structure and a part of said optical imaging component;

44. The method according to claim 43, wherein said shielding unit internally spreads an amount of heat transferred from said heat source to said shielding unit prior to transferring heat to said shielded device.

45. The method according to claim 43 or 44, wherein at least one of

- as said shielding unit a unit is used that is made at least in part of a highly thermally conductive material,

- as said shielding unit a unit is used that has a high heat capacity; - a cooling fluid is circulated in a cooling fluid duct within said shielding unit;

46. The method according to any one of claims claim 43 to 45, wherein,

- said optical imaging component is supported on said support structure of via at least one actuator;

- at at least one capturing location of said optical projection unit, an actual value of a variable representative of a temperature at said location is continuously captured;

- at least one control signal is generated as a function of said actual value of said variable and providing said at least one control signal to said at least one actuator to adjust via said actuator, in at least one degree of freedom, at least one of a geometric parameter of said optical imaging component, a relative position of said optical imaging component with respect to at least one reference and a relative orientation of said optical imaging component with respect to said at least one reference to at least partially compensate thermally induced alterations in at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component;

- said at least one control signal being generated using at least one previously established thermal model of said optical imaging device, said thermal model describing at least one of said geometric parameter, said relative position and said relative orientation of said optical imaging component at least as a function of said variable and said capturing location.

47. The method according to claim 46, wherein said actual value of said variable is captured at each one of a plurality of capturing locations of said optical imaging device.

48. The method according to claim 46 or 47, wherein said at least one capturing location is located at one of said support structure and said optical imaging component.

49. The method according to any one of claims 46 to 48, wherein

- said variable is a first variable and said reference is a first reference and

- an actual value of at least one second variable representative of one of a geometric parameter of said optical element, a relative position of said optical imaging component with respect to at least one second reference and a relative orientation of said optical imaging component with respect to said at least one second reference in at least one degree of freedom is captured and

50. The method according to claim 49, wherein as said at least one actual value of at least one second variable a variable representative of an image quality of a reference image generated at least via said optical imaging component is captured.

_{* * * * *}