Lithographic apparatus, device manufacturing method, and device manufactured thereby

A lithographic apparatus includes an illumination system for providing a beam of radiation, a support structure for supporting a patterning device, the patterning device serving to impart the beam with a pattern in its cross-section. The apparatus further includes a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, and a collector which is arranged for transmitting radiation, received from a first radiation source, to the illumination system. The apparatus includes at least a heater for heating the collector when the collector receives substantially no radiation from the first radiation source. Further aspects of the invention relate to a device manufacturing method as well as a device manufactured thereby.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus. The invention further relates to a method of manufacturing a device, as well as a device manufactured thereby.

2. Brief Description of Related Art

The known apparatus comprises a collector for collecting radiation from a radiation source. The collector is arranged for transmitting the collected radiation to an illumination system. The illumination system is arranged for providing a projection beam of radiation, which is used for applying a desired pattern onto a target portion of a substrate. The collector comprises for example suitable radiation reflecting mirror shells. Such a collector is known from the European patent application EP 1225481, which is incorporated herein by reference. Particularly, the collector is arranged to focus incoming radiation, received from the radiation source, onto a small focusing area or focussing point.

A disadvantage of the known apparatus is, that the collector is heated up by the incoming radiation, each time the respective radiation source is switched on. As a result, the collector expands, leading to a shift in direction of the outgoing radiation and/or a translation of the focussing point of the radiation and distortion of a resulting illumination beam. Consequently, it is difficult or impossible for the illuminating system to generate a projection beam of radiation with certain desired characteristics, having for instance a suitable homogeneity, radiation distribution, beam direction, beam intensity, beam cross-section and the like. Besides, the distortion of the projection radiation, caused by the thermal expansion of the collector, hampers the manufacturing of devices with a desired high precision.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to improve the lithographic apparatus. In particular, an aspect of the invention aims to provide a lithographic apparatus for manufacturing devices with high precision.

According to an aspect of the invention, there is provided a lithographic apparatus comprising: an illumination system constructed to provided a beam of radiation; a support structure constructed to support a patterning device, the patterning device serving to impart a cross-section of the beam with a pattern to form a patterned beam; a substrate table for holding a substrate; a projection system that projects the patterned beam onto a target portion of the substrate; a collector being constructed to receive radiation from a first radiation source and transmit radiation to the illumination system; and at least one heater for heating the collector when the collector receives substantially no radiation from the first radiation source.

During use, the heater heats the collector at least when the collector receives substantially no radiation from the first radiation source. Therefore, temperature variations of the collector can be diminished or prevented. Preferably, the temperature of the collector is kept substantially constant by using the heater, or, for instance, within a relatively small range. The small temperature range may comprise small offsets from a desired operating temperature. Such temperature range may be, for example, a range of plus and minus about 50° C. from an average operating temperature. As a result, the dimensions or the shape of the collector can be kept relatively constant or within certain limits, so that the radiation may be transmitted in a desired direction and/or focussed onto a desired spot. Thus, a radiation beam with desired, substantially constant, optical characteristics can be provided. Such radiation beam can be advantageously used in the manufacturing of devices.

According to an aspect of the invention, there is provided a collector for use in a lithographic apparatus, the collector comprising: reflecting elements being constructed to receive radiation from a first radiation source, each of the reflecting elements having a reflecting surface to reflect the radiation received from the first radiation source towards a focussing point; and a heater thermally coupled to the reflecting elements and constructed to heat the reflecting elements.

During use, deformation of the collector can be prevented or diminished by additionally heating the collector using the heater. Such heating may be applied during each period when the first radiation source is switched off, wherein no radiation is received by the collector from the first radiation source. In that case, a cooling of the collector is prevented by a compensating heating, resulting from the activation of the heater. By preventing the deformation of the collector during an active period of the first radiation source, the radiation of the first radiation source can be transmitted by the collector with a desired uniformity, in a desired pattern, focussed in a desired direction and/or angle or such.

Alternatively, the heater may be used to heat up the collector continuously to such a high operating temperature, with respect to a temperature the collector would reach by a heating due to the radiation of the first radiation source only, that the overall operating temperature of the collector is relatively constant in time. In that case, the radiation of the first radiation source has substantially no influence anymore on the operating temperature of the collector, due to the already high temperature thereof.

During use, heat may be removed from the collector in different ways, for conditioning the collector temperature and/or to prevent the collector from overheating. Heat may be removed from the collector by heat radiation, heat convection and/or heat conduction. Several heat transporters may be provided for conditioning the temperature of the collector, for instance heat pipes, cooling structures, thermal shrouds, heat conditioning fluids, liquids and/or gasses and the like.

According to a further aspect of the invention, a method of manufacturing a device is provided, comprising: providing a substrate; providing a first radiation source; transmitting radiation from the first radiation source to an illumination system through a collector; providing a beam of radiation using the illumination system; imparting the beam with a cross-sectional pattern to form a patterned beam of radiation; and projecting the patterned beam of radiation onto a target portion of the substrate; and maintaining the collector at a substantially constant operating temperature by selectively heating the collector.

According to a further embodiment of the invention, a lithographic apparatus is provided comprising: receiving and transmitting means for receiving radiation from a first radiation source and transmitting radiation to an illumination system; and means for heating the receiving and transmitting means.

According to a further embodiment of the invention, a method of manufacturing a device is provided comprising: transmitting radiation from a first radiation source to an illumination system through a collector; maintaining the collector at a substantially constant operating temperature by selectively heating the collector; and projecting a beam of radiation onto a target portion of a substrate.

This provides the above-mentioned advantages. Also, devices, for instance semiconductor devices or other types of devices, can be made with high precision by the method according to an aspect of the present invention.

Patterning devices may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned. In each example of patterning device, the support structure may be a frame or table, for example, which may be fixed or movable and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device”.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

FIG. 1schematically depicts a lithographic apparatus. The apparatus comprises: an illumination system (illuminator) IL for providing a projection beam PB of radiation (e.g. UV or EUV radiation); a first support structure (e.g. a mask table) MT for supporting a patterning device (e.g. a mask) MA and connected to first positioning structure PM for accurately positioning the patterning device with respect to item PL; a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist coated wafer) W and connected to second positioning structure PW for accurately positioning the substrate with respect to item PL; and a projection system (e.g. a reflective projection lens) PL for imaging a pattern imparted to the projection beam PB by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask or a programmable mirror array of a type as referred to above). Alternatively, the apparatus may be of a transmissive type (e.g. employing a transmissive mask).

A first radiation source SO is provided for providing radiation R. The radiation R is collected by a collector K. The collector K transmits the collected radiation R in the form of a radiation beam to the illuminator IL. The first source SO and the lithographic apparatus may be separate entities, for example when the source is a plasma discharge source. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is generally passed from the first source SO to the illuminator IL with the aid of the radiation collector K comprising for example suitable collecting mirrors and/or a spectral purity filter. In other cases the source may be integral part of the apparatus, for example when the source is a mercury lamp. The source SO, collector K and the illuminator IL, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster that adjusts the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. The illuminator provides a conditioned beam of radiation, referred to as the projection beam PB, having a desired uniformity and intensity distribution in its cross section.

The projection beam PB is incident on a patterning device, illustrated in the form of the mask MA, which is held on the mask table MT. Being reflected by the mask MA, the projection beam PB passes through the lens PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning structure PW and position sensor IF2(e.g. an interferometric device), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning structure PM and position sensor IF1can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning structures PM and PW. However, in the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes.

In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the projection beam is projected onto a target portion C in one go (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

FIGS. 2–4show, in more detail, a known collector KPA, which can be used in a lithographic apparatus similar to that shown inFIG. 1instead of the collector of the embodiments of the present invention. The collector KPAcomprises mirrors11for focussing the radiation R, which radiation the collector KPAreceives from a first, intermittent, radiation source SO. The first radiation source SO is depicted inFIG. 1, but not inFIGS. 2–8.

The collector KPAmay be arranged at least partly according to the collector as known from EP 1 225 481. For instance, the collector may be suitable for illumination systems with a wavelength of <=193 nm, preferably <=126 mn, particularly preferred, EUV wavelengths. The collector preferably comprises a plurality of reflecting elements11in the form of rotationally symmetrical mirror shells, which are arranged in one another around a common axis of rotation. The axis of rotation is depicted by line Y inFIG. 2. The collector shells11are attached to each other for instance by coupling members12, for example spokes or spoke-like elements, as has been depicted inFIG. 4. These coupling members12are not depicted inFIGS. 2 and 3. Each reflecting element11comprises a reflecting inner surface13for reflecting the radiation R, as well as an outer surface14or back side which is faced away from the reflecting surface13, seeFIGS. 2 and 4.

The collector KPAis arranged to focus the incoming radiation R onto a certain focus point FP. The focus point may be, for instance, a small focussing area. The focus point FP may be situated, for instance, before the illuminator IL of the lithographic apparatus. During use, the collector KPAwill be heated by the incoming radiation R, resulting in an expansion of the collector plates11.FIGS. 2 and 3show, that the resulting expansion of the collector shells11leads to the shifting of the focussing point FP. The focussing point may, for instance, shift towards the collector KPAor away from the collector KPA, depending amongst others on the arrangement and structure of the collector KPA. For an illuminator IL, it may be hard or impossible to provide a desired projection beam of radiation PB during such shifting of the radiation focus point. Particularly, the illuminator IL will not be able to provide a projection beam PB having desired constant optical properties for a high precision illumination of a substrate W. To make matters worse, the first radiation source SO of the lithographic apparatus usually only operates during short amounts of time. The first radiation source SO will be switched off, for instance, during substrate W loading and unloading, as well as during substrate illumination in between consecutive substrate dies C. To this aim, the first radiation source SO may be, for instance, a pulsed source. Therefore, the collector KPAreceives radiation from the first radiation source SO intermittently, leading to an iterative heating and cooling of the mirror shells11, resulting in a collector expansion and contraction, as well as a shifting of the focus point FP each time the first radiation source SO is switched on. InFIG. 2, this shift is shown by an arrow X. Further, upstream parts of the collector KPA, located near or opposite the first radiation source SO, for instance near a first collector side A, receive higher radiation fluxes than the downstream collector parts which are located further away from the first radiation source SO, for instance near the second collector side B. Besides, each collector shell11may receive a different amount of radiation from the first radiation source SO. This leads to an inhomogeneous expansion and contraction of the collector KPA. As a result, the shift of foci of the different collector shells11is inhomogeneous, leading to a further undesired variation of the generated projection beam of radiation PB (seeFIGS. 2 and 3).

FIG. 5shows a first embodiment of a collector1according to the present invention, which can be used in the lithographic apparatus shown inFIG. 1. A number of heaters2are provided for heating the collector1. In the present embodiment, each heater2comprises a second radiation source2a. The heaters2are arranged to supply heat to the outer surfaces14of the reflecting elements11by heat radiation. The heaters2are preferably located substantially out of the path of the radiation R which is to be focussed by the collector1onto the focussing point FP, to prevent the heaters2from blocking the transmission of that radiating R. The heaters2may comprise different radiation sources, for instance X-ray sources, infrared light sources, lamps and/or other radiation sources. To absorb heat from the heaters2, the outer surfaces14of the collector shells11preferably comprise a heat absorbing material, a heat absorbing coating, for instance a dark or substantially black coating, a heat absorbing structure and/or the like.

The heaters2are arranged to heat the collector1when the collector1receives substantially no radiation from the first radiation source SO. The heaters2may also be arranged for heating the collector1when the collector1receives radiation R from the first radiation source SO. Preferably, the heaters2are arranged for maintaining the collector1at a certain substantially constant operating temperature, particularly before, during as well as after the use of the first radiation source SO. The heaters may be arranged, for example, to maintain the operating temperature within a range of plus and minus about 50° C. from an average operating temperature, and preferably within a range of plus and minus about 25° C. from an average operating temperature. Depending on the arrangement, composition and/or structure of the collector1, the heaters2may be arranged to maintain the operating temperature within different temperature ranges, for instance smaller or larger temperature ranges.

The heaters2are provided with a controller4, for example a computer, for controlling the heaters2. The controller4may be arranged in several ways. For instance, the controller4may be arranged to activate one or more heaters2when the first radiation source SO is deactivated. Also, the controller4may be arranged to activate one or more heaters2when a drop of the temperature of the collector1, or of one or more collector shells11, is detected. The controller4may also be arranged to control the heaters2individually, in groups and/or or simultaneously. Further, the controller4may be arranged to switch one or more heaters2on and off, wherein each heater2provides a substantially constant amount of radiation for heating collector parts11. On the other hand, the controller4may be arranged to control and vary the amount of radiation which is generated by one or more of the heaters4, as desired.

In the present embodiment, the controller4is coupled to the heaters2by wiring3. The coupling between the controller4and the heaters may be achieved in various ways, for instance using wireless communication structures, such as one or more transmitters and receivers. One or more controllers4may also be integrated in the collector1and/or in or near one or more heaters2. To the skilled person, it is clear how to provide, program and/or arrange such a controller4for a desired functioning thereof.

Furthermore, a number of temperature sensors6is provided for measuring at least a temperature of at a least part of the collector1. In the present embodiment, the sensors6are arranged to measure temperatures of the reflecting elements11of the collector1. The temperature sensors6may be arranged in various ways, for instance comprising thermocouples, radiation detectors or the like. The sensors6may for instance be integrated in the collector1, in the collector shells11and/or be located in or near the heaters2. The sensors6may also be connected to the controller4by electrical wiring, by wireless communication structures and the like.

The first embodiment, as shown inFIG. 5, is used in the apparatus according toFIG. 1in a device manufacturing method, comprising: providing a substrate W; providing a projection beam of radiation PB using an illumination system IL; using a patterning device MA to impart the projection beam with a pattern in its cross-section; and projecting the patterned beam of radiation PB onto a target portion of the substrate W.

For providing the projection beam PB, the first radiation source is a intermittent source which, for instance, is arranged to produces radiation pulses. The radiation R, generated by the first intermittent radiation source SO, is collected by the collector1, to be transmitted to the illumination system IL. The collector1is maintained at a substantially constant operating temperature by heating the collector1, using the heaters2. The collector1may be heated substantially only by the heaters2when the collector1receives substantially no radiation from the first radiation source SO, such that the operating temperature is maintained substantially constant. The temperature of the collector1may be maintained, for instance, within a range of plus and minus about 50° C. from an average operating temperature, and preferably within a range of plus and minus about 25° C. from an average operating temperature, or within a smaller or larger range.

On the other hand, the heaters2may heat the collector1continuously to such a high operating temperature, that a further heating, which may be caused by absorption of radiation from the first radiation source SO, leads to substantially no or only a small change of the temperature of the collector1. In this case, the additional heat load, resulting from the radiation R of the first radiation source SO, leads to substantially no or only a small rise of the temperature of the collector1with respect to a temperature rise which would occur when the heaters2were not implemented. This can also be explained as follows. Radiative thermal transport scales with T4(wherein T is the temperature in Kelvin). The temperature rise per unit of absorbed power thus scales with T3. Consequently, it is for example found that the temperature rise of a collector1at 500 K is 27/125=0.2 times smaller than the temperature rise of a collector1at 300 K, for a similar heat input from the source. Thus, it has been found that is advantageous to heat the collector1continuously for obtaining a rather constant operating temperature thereof. Also in this case, the temperature of the collector1may be maintained, for instance, within a range from an average operating temperature.

Besides, the term “a substantial constant operating temperature” may at least be understood as a temperature within a range of plus and minus about 50° C. from an average collector operating temperature, and preferably within a range of plus and minus about 25° C. from an average operating temperature, or within a smaller range. Herein, the term “average” should be understood as being average concerning the overall collector structure. For instance, different collector parts may have different substantially constant operating temperatures.

Preferably, the amount of heat produced locally by the different heaters2is correlated to the amount of radiation R received locally by the respective collector parts, to provide a desired average collector operating temperature. To this aim, the heaters2may be arranged to produce different amounts of heat for heating different collector parts, wherein the different amounts of heat are correlated to the amounts of radiation R received by the different collector parts. For instance, the heaters2may be arranged to produce more heat at the upstream collector parts, which are located near the first radiation source SO and extend for instance from the first collector side A, compared to the collector parts which are located further away from the first radiation source SO, extending for example towards or near the downstream collector side B. Also, different collector shells11may be kept at different operating temperatures. Besides, the heaters2may be arranged, for example, to compensate for non-isotropic incoming radiation R.

The controlling of the heaters2may be provided by the controller4. To this aim, the controller4may be coupled to the first radiation source SO, for instance such that the controller4activates the heaters2when the first radiation source SO is switched off, wherein the controller4deactivates the heaters2when the first radiation source SO is activated again. In that case, the heaters2are preferably arranged to supply such an amount of heat to the outer sides14of the collector shells11, that each collector shell11experiences substantially no temperature drop due to each switching off of the first radiation source SO. Besides, the controller4may be arranged to control the heaters2based on the actual temperatures of the collector shells11, which temperatures may be measured by the temperature sensors6. On the other hand, the controller4may be arranged for heating the collector parts11continuously to the high operating temperature.

Since the collector1is maintained at or about a certain operating temperature, substantially independently of the operating condition of the first radiation source SO, the location of the focusing point or focussing area FP of the transmitted radiation R remains substantially unaltered with respect to the illuminator IL. Therefore, a projection beam of radiation PB can be provided by the illuminator IL, having a desired substantially constant uniformity and intensity distribution in its cross section, so that devices can be made with high precision.

FIG. 6shows a second embodiment101of the present invention, which differs from the embodiment shown inFIG. 5, that heating plates107are provided for heating the collector shells111. In the embodiment shown inFIG. 6, heating plates107are mounted on the back sides of the reflecting collector shells111. The heating plates107comprise electrical heaters102for heating the heating plates107. The heating plates107further comprise, for example, a material having a high coefficient of thermal conductivity, so that the heating plates107can be heated relatively homogeneously by the heaters102. The electrical heaters102may comprise, for instance, electrical resistances, electrical wiring, and the like. Also inductive and capacitive heating and/or suchlike heating may be applied. The heaters102may, for instance, be in direct and/or indirect contact with the collector shells111. With a heating arrangement as shown inFIG. 6, the collector101can be heated relatively fast and homogeneously to be maintained at or about a desired operating temperature. The functioning of the second embodiment101is similar to the described workings of the first embodiment 1. In the present second embodiment, the collector101is heated electrically, by the heaters102of the heating plates107. The heating plates107can be provided in various ways and shapes. Alternatively, the collector shells111may, for instance, integrally comprise electrical heaters102. Besides, one or more collector shells111may be provided with one or more heating layers which serve as electrical heaters, as will be explained below with regard to the fourth embodiment.

FIGS. 7 and 8show a third embodiment of the invention. The third embodiment differs from the embodiments shown inFIGS. 5 and 6, that an external electrical heating shield212is provided for heating the collector201. The heating shield212is spaced-apart from the outer collector shell211. The heating shield212comprises electrical heaters202for heating the heating shield212. Preferably, the heating shield212is mechanically connected to the collector shells211by a coupling frame218, comprising for instance spokes, part of which frame is shown inFIG. 8. Preferably, the coupling frame218attaches the heating shield212at different locations to each collector shell211. The heating shield212and the coupling frame218are heat conducting elements, which are arranged to conduct heat from the heaters202to the collector shells211. To this aim, the heating shield212and coupling frame218preferably comprise one or more materials having a high heat conduction coefficient. Besides, preferably, the heating shield212is arranged to heat the collector201by heat radiation, for instance infrared radiation. The functioning of this third embodiment201also is substantially similar to the functioning of the first embodiment. In the present case, the collector201is heated using the heaters202of the heating shield212, for instance by heat radiation and/or heat conduction.

FIG. 9shows part of a fourth embodiment of the invention. Particularly,FIG. 9depicts a cross-section of one of the collector shells311of a collector301. The fourth embodiment differs from the second embodiment, shown inFIG. 6, that the collector shell311has been provided with a heating layer302, which serves as an electrical heater, instead of a heating plate. In the present embodiment, the heating layer302is located on top of an intermediate layer320which has been provided on the back side S of the respective collector shell311, similar to positions of the heating plates shown inFIG. 6. The intermediate layer320is arranged to electrically isolate the heating layer302from the collector shell plate311. The intermediate layer320may consist, for instance, of one or more suitable isolator materials, for example silicon-oxide, glass and/or other materials. Alternatively, the heating layer302may be located directly on top of the collector shell311, depending amongst others on the electrical resistance of the collector shell311. Furthermore, more than one heating layer may be provided on and/or in the collector shell311. Besides, the outer surface of the heating layer302may be provided with a covering, for instance comprising one or more protective layers or the like. Furthermore, the collector shell311as such may be used for conducting a heating current for heating the collector shell311.

The heating layer302is an electrically conductive layer, for conducting certain electrical heating current. InFIG. 9, this heating current has been depicted by an arrow I. Besides, the heating layer302has a certain electrical resistance, so that the heating layer302heats up when conducting the heating current I. This can be achieved, for instance, by providing the heating layer302with one or more materials having a suitable electrical conductivity. Preferably, the heating layer302contains one or more vacuum compliant compounds, for instance nickel and/or other suitable substances. Application of the layer arrangement, shown inFIG. 9, leads to a collector301which is particularly suitable for use in a vacuum environment.

Furthermore, the heating layer302may have certain suitable dimensions for providing a desired heating layer resistance. For instance, the thickness of the heating layer302, which thickness is depicted inFIG. 9by arrow Z, may be relatively small to obtain a desired layer resistance. The use of a thin heating layer302also prevents optical losses during use of the collector301. The thickness Z of the heating layer302may be, for instance, smaller than about 100 μm, or for instance smaller than about 10 μm. In that case, the heating layer may substantially consist, for instance, of nickel or a similar material. To the skilled person it is clear, that, depending on the desired heating capabilities of the heating layer320and/or on the composition of the heating layer, other heating layer dimensions may also be used.

The functioning of this fourth embodiment301is substantially similar to the functioning of the above-described embodiments. During use of the fourth embodiment, the heating layer302is used for heating the collector shell311, such that thermally induced collector shell311deformations are substantially prevented. To this aim, a suitable heating current I is applied through the heating layer302. Suitable structure for providing the current I, for instance a current source, have not been depicted inFIG. 9. To the skilled person, it is clear how to provide and connect a current source to the heating layer302for providing a desired heating current. Heating layers302of different collector shells311of the collector301may be provided with different heating currents I, for the case that the different collector shells311receive different amounts of radiation from the first radiation source SO. As in the above, a controller4may be used for controlling the heating layer302. The controller4may be arranged, for instance, for controlling the power which is electrically dissipated in the heating layer302. As an example, the electrically induced power dissipation may be controlled such, that a total power absorption by the respective collector shell311is substantially constant in time, for instance after switching on the first radiation source SO, so that the temperature of the collector shell311remains substantially constant. Herein, the total power absorption includes the power absorption resulting from the radiation of the first radiation source SO.

FIG. 10is similar toFIG. 9. However, inFIG. 10, the thickness Z of the heating layer302′ has been varied. InFIG. 10, the heating layer302′ comprises broader parts322, as well as a grooved part321The grooved heating layer part321locally provides a higher electrical resistance with respect to remaining broader parts322and, consequently, a higher heat dissipation during use. Such grooved part321may be manufactured in various ways, for instance using suitable etching techniques, laser etching or the like. The heating layer302′ may comprise a suitable number of such grooved parts321for optimising heat dissipation. Besides, the grooved part321may be a interruption of the heating layer302′, for instance for forming a local, hot area. Then, the local thickness Z of the heating layer is zero or substantially zero. Furthermore, by interrupting the heating layer302′, the heating layer302′ may be divided into different heating layer parts. During use, one heating current or different heating currents may be applied to different heating layer parts of each collector shell311.

Preferably, the amount of heat produced locally by the heater302is correlated to the amount of radiation R received locally by the respective collector shell311. This further reduces collector deformation and may prevent, for instance, additional temperature gradients and additional temperature related stress in the collector shell311during use of the collector301. For instance, the local resistance of the heating layer302may be correlated to the amount of radiation R received locally by the respective collector shell311, to further prevent thermal deformations of the collector shell311during use. The heating layer302may be arranged to produce more heat at upstream collector shell parts, which are located near the first radiation source SO, compared to the collector shell parts which are located further away from the first radiation source SO. To this aim, the current density gradient of the heating current I the heating layer302may be higher near upstream collector parts than at the downstream parts thereof. For instance, the heating layer302may be relatively thin at upstream collector shell parts, whereas the heating layer302may be wider at downstream collector shell parts. The heating layer302may have a certain thickness profile, viewed from the first collector side A to the second collector side B. Such thickness profile may be, for instance, substantially inversely proportional to the amount of radiation R received locally by the respective collector shell311during use. Besides, the heating layer may comprise a suitable number of grooves layer parts321at suitable locations, for providing the desired variation of the local heat production during use. The heating layer thickness may also be varied, for instance, in other directions, for example in circumferential direction with respect to the collector axis Y (seeFIG. 2). The heating layer302may be dimensioned to compensate for non-isotropic incoming radiation R.

While specific embodiments of the invention have been described above, it will be appreciated that aspects of the invention may be practiced otherwise than as described. The description is not intended to limit aspects of the invention.

For instance, the collector1may be arranged in various ways and/or forms and comprise several collector elements11. The collector1may be generally arranged according one or more collectors as described in EP 1225481, and/or otherwise.

Besides, the term “a substantial constant operating temperature” may at least be understood as a temperature within a range of plus and minus about 50° C. from a certain average collector operating temperature, and preferably within a range of plus and minus about 25° C. from a certain average operating temperature, or within a smaller range. Herein, the term “average” should be understood as being average concerning the overall collector structure. For instance, different collector parts may have different substantially constant operating temperatures.

Besides one or more heaters2,102,202,302, heating elements, heating shields, heating layers, heating structures and the like may be provided for heating the collector.