EP3899663A1

EP3899663A1 - Lithographic apparatus with thermal conditioning system for conditioning the wafer

Info

Publication number: EP3899663A1
Application number: EP19809088.8A
Authority: EP
Inventors: Marinus Aart Van Den Brink
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2018-12-19
Filing date: 2019-11-28
Publication date: 2021-10-27
Also published as: CN113330369A; WO2020126389A1; KR20210104853A; NL2024322A; TW202041974A

Abstract

A lithographic apparatus receives radiation for imaging a pattern via projection optics onto a plurality of target areas on a substrate. Each target area receives a heat load through absorption of the radiation during imaging. The apparatus has thermal conditioning system to maintain the substrate at a spatially uniform, constant first temperature during the imaging. The thermal conditioning system has a heat sink operative to extract heat from the substrate; and a first heater system that supplies during the imaging, a first additional heat load to a part of the substrate. This part is the complement of the specific target area onto which the pattern is being imaged. The first additional heat load per unit area equals or exceeds a magnitude of the heat load per unit area.

Description

LITHOGRAPHIC APPARATUS WITH THERMAL CONDITIONING SYSTEM FOR

CONDITIONING THE WAFER

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 18213862.8, which was filed on 19 December 2018 and which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The invention relates to a lithographic apparatus.

BACKGROUND ART

Thermally induced effects

Properties of physical matter tend to change in response to a change in temperature. Thermally induced changes in properties of a system’s physical component may be critical to the system’s performance. Generally, it is therefore preferable to thermally stabilize the system via controlled heating and/or controlled cooling of the component. Such stabilizing reduces the variations of the component’s temperature-dependent properties that are caused by a heat load received from the component’s environment in operational use of the system. For example, the component may be thermally connected to a heat-sink that extracts the heat from the component. As a result, the component may reach a thermal equilibrium and assume a constant, spatially uniform temperature, thus retaining a thermally induced deformation that remains constant and spatially uniform as well.

A lithographic system is an example of a system, wherein limiting the thermally induced deformation is critical to the system’s performance. A lithographic system is configured to image a pattern onto a photo-sensitive resist (“resist”) that covers a semiconductor wafer (or: semiconductor substrate). Upon imaging, the thus exposed resist is developed to form a mold for a processing step, wherein a next layer is created in a stack of multiple layers that eventually form an array of integrated electronic circuits. A lithographic system includes a light source and a scanner (also referred to as: a lithographic apparatus). The electromagnetic radiation produced by the light source is used in the scanner to project the pattern, present on a mask (also referred to as: a patterning device, or: a reticle) via projection optics onto the layer of resist on the wafer. Actinic and non-actinic radiation

In practice, the electromagnetic radiation produced by the light source may comprise actinic radiation and non-actinic radiation. As known, actinic radiation catalyzes changes in the photoresist via, for example, breaking molecular bonds so as to be able to print the mask’s pattern into the photoresist. Non-actinic radiation does not bring about such changes and accordingly cannot be used for printing the pattern into the photoresist. Non-actinic radiation is also being referred to as out-of-band (OOB) radiation. Especially the non-actinic radiation may give rise to undesired thermal loads on the wafer and, hence, to undesirable thermally induced deformation of the wafer during the imaging process.

Thermally induced effects on overlay & focus

A lithographic scanner has physical components whose functioning is sensitive to thermal loads and whose thermal states, e.g., thermally induced deformations, are critical to the scanner’s performance. The scanner’s performance may be expressed in terms of overlay and focus. The term“overlay” is shorthand for“overlay error” and indicates the extent to which successive layers in an integrated circuit chip are laterally displaced relative to one another. The overlay therefore indicates how well a pattern for forming one layer on the wafer is aligned with a layer previously formed in the wafer. The overlay is a measure of horizontal distance, i.e., in the plane of the wafer, between the patterns and may be described as a location-dependent two-dimensional (vector-) field. The term“focus” is shorthand for“focus error” and indicates a vertical deviation of the actual focal plane, during imaging, from an ideal plane of focus. An important aspect in lithography is maintaining uniformity in the size of the features created on the surface of substrates. Variations in feature size are required to be less than a tiny fraction of a pre determined nominal value. Critical to achieving these levels of performance is the tight control of the focus error.

Control of overlay and focus becomes increasingly more important

With every new generation of integrated circuitry, the sizes of the features imaged onto the wafer are further decreased by means of using actinic radiation of shorter wavelength. Currently, extreme ultraviolet (EUV) lithography is at the forefront using actinic radiation with a wavelength of 13.5 nm or even lower. As a result, tighter control of the thermally induced effects in the wafer becomes increasingly more important with every new generation of integrated circuitry.

Examples of ASML Background Art

In order to sketch some examples that illustrate efforts made in the field of lithography within the context of thermal control of the wafer, reference is made to the following documents. International application publication WO 2018041491, filed for Cox et al., assigned to ASML and incorporated herein by reference, discloses an EUV lithographic apparatus with a projection system which is configured to project via a slit a radiation beam, patterned by means of a mask, onto an exposure area on a substrate held on a substrate table. The lithographic apparatus operates in a scanning mode, wherein the mask and the substrate are scanned synchronously during the projection. The lithographic apparatus comprises a cooling apparatus located between the projection system and the substrate. The cooling apparatus provides localized cooling of the substrate in the vicinity of the area where the patterned radiation beam is incident on the substrate via the slit. The amount of cooling provided by the cooling apparatus is generally constant in a direction along the slit, i.e., in a direction substantially perpendicular to a direction of the scanning. It may be desirable to provide different amounts of cooling at locations in the direction along the slit. This is because heating of the substrate caused by the patterned radiation beam may be different at different positions of the substrate, in the direction along the slit, within the exposure area E. The amount of heating of the substrate caused by the patterned radiation beam depends upon the intensity of the radiation beam, and this may vary across the exposure area along the direction of the slit. Different parts of the mask may have different reflectivity, as the spatial variation of reflectivity is determined by properties of pattern features on the mask. Therefore, the lithographic apparatus also comprises a heating apparatus with one or more radiation sources, e.g., infrared lasers, configured to provide one or more additional radiation beams which illuminate and heat part of the substrate The one or more additional radiation beams may illuminate and heat at least part of an exposure area, i.e. the area of the substrate which receives the patterned radiation beam. The heating apparatus further comprises one or more sensors configured to sense infra-red radiation from the substrate. A controller controls the infrared lasers to adjust the power of their radiation beams as required in order to selectively provide desired amounts of heating at different sections within the exposure area. As a result of the operation of the infrared lasers the net heating of the substrate across the exposure area is maintained substantially constant Consequently, distortion of the substrate which would otherwise have been caused by different amounts of heating at different positions underneath the slit on the substrate is reduced.

US patent 9,983,489, issued to Berendsen et al., assigned to ASML and incorporated herein by reference, discloses a method for compensating for an exposure error in an exposure process of a lithographic apparatus. The method comprising: obtaining a dose measurement indicative of a dose of IR radiation that reaches substrate level, wherein the dose measurement is used to calculate an amount of IR radiation absorbed by an object during an exposure process; and using the dose measurement to control the exposure process so as to compensate for an exposure error associated with the IR radiation absorbed by the object during the exposure process. Examples of the object include: the substrate, the substrate table, a mirror of the projection optics. Examples of the control of the exposure process include:

controlling a heater to heat the mirror; controlling a thermal conditioning system to control a temperature of the substrate support of the lithographic apparatus.

US patent 7,630,060, issued to Ottens et al., assigned to ASML and incorporated herein by reference, discloses a lithographic apparatus that includes an illumination system for providing a beam of radiation, and a support structure for supporting a patterning device. The patterning device serves to impart the beam of radiation with a pattern in its cross-section. The apparatus also includes a substrate support for holding a substrate, and a projection system for projecting the patterned beam onto a target portion of the substrate. The apparatus is provided with an additional heating system for providing additional heating of the substrate. The substrate is additionally heated, such that the substrate has a relatively constant heat load. Herein, the term "constant" means constant in time and/or position. The term "additionally" means additionally to the illumination of the projection beam. The complementary heating is preferably provided by complementary illuminating the substrate using an additional illumination system. The additional heating, for example illumination, is supposed to heat the substrate without influencing, for example, resist patterning. Therefore, it may be advantageous in the embodiment where additional illumination is used to use radiation for which the resist is insensitive, for example, infrared radiation, to additionally illuminate the substrate. A feed-forward system may control the dose of the complementary illumination system, based on the (known) heat that is produced by the patterning beam. The complementary illumination system may include, for example, a laser and/or a 'classical' radiation source. The complementary illumination system may, for example, irradiate the whole substrate surface uniformly, the slit uniformly, the slit in a patterned fashion or regions around the slit. Herein, the slit is the actual line of light projected onto the substrate. Other complementary illumination strategies are also possible. An advantage of the complementary heating is that the temperature of the substrate may be kept relatively stable during the lithographic process. Therefore, the thermal expansion of the substrate may be kept constant in time and/or uniformly in position. Another advantage is that the substrate temperature may be controlled relatively easy. As a result, overlay errors may be minimized, because all substrates may be exposed at the same temperature.

US patent application publication 2018/0173116, filed for Koevoets et al., assigned to ASML and incorporated herein by reference, discloses a scanning lithographic apparatus with a projection system configured to project a patterned radiation beam to form an exposure area on a substrate held on a substrate table. The lithographic apparatus further has a heating apparatus for heating the substrate. The heating apparatus comprises first and second heating elements configured to heat substrate areas located at opposite ends of the exposure area in a non-scanning direction of the lithographic apparatus. The heating apparatus may heat an area which overlaps with the exposure area. The heating apparatus is advantageous because it prevents or reduces distortion of the substrate at the ends of the exposure area in the non-scanning direction. This allows the overlay performance of the lithographic apparatus to be improved. More specifically, the heating elements deliver localized heating to the substrate which acts to heat the portion of the substrate which is immediately outside of the edge of the exposure area illuminated by the radiation beam. As a result, the temperature of the substrate does not drop off rapidly at the edge of the exposure area, but instead reduces more slowly. This is advantageous because distortion of the substrate which would otherwise be caused by such a temperature drop is reduced. This enables an improvement of the accuracy with which a pattern may be projected onto the substrate, i.e., an improvement of the overlay performance of the lithographic apparatus. The first and second heating elements may be located above the substrate support and located at opposite ends of the exposure area in a non-scanning direction of the lithographic apparatus. The heating elements may each comprise an array of LEDs that emit infrared radiation, or two or more lasers that emit radiation of an non-actinic wavelength.

SUMMARY

The invention relates to, among other things, further improving the control of the exposure process in a lithographic apparatus.

The invention relates to a lithographic apparatus configured to receive radiation for imaging a pattern via projection optics onto a plurality of target areas on a substrate. Each specific one of the target areas is operative to receive a heat load through absorption of at least part of the radiation during imaging onto the specific target area. The lithographic apparatus comprises a thermal conditioning system. The thermal conditioning system is configured to maintain the substrate at a spatially uniform, constant first temperature during the imaging. The thermal conditioning system comprises a heat sink operative to extract heat from the substrate, and a first heater system operative to supply, during the imaging, a spatially uniform first additional heat load to a part of the substrate. The part is the complement of the specific target area onto which the pattern is being imaged. A magnitude of the first additional heat load per unit area equals, or exceeds, a magnitude of the heat load per unit area.

At a spatial uniform temperature of the substrate, the imaging of the pattern onto a plurality of target areas is made independent of variations in thermally induced deformation across the substrate. In an embodiment, the thermal conditioning system is configured to supply the first heat load through irradiating the complement. To this end, the thermal conditioning system may comprise at least one of an LED and a scanning laser, arranged for supplying the first additional heat load.

In a further embodiment, the thermal conditioning system comprises a second heater system operative to supply a second additional heat load to the specific target area. Such second additional heat load may be used in case the heat load from the imaging radiation is lower, per unit of area than the first additional heat load provided per unit of area. The second heater system may be configured to supply the second additional heat load via irradiating the specific target area. To this end, the second heater system may comprises at least one of: a second LED and a second scanning laser, arranged for supplying the second additional heat load.

In a further embodiment, the lithographic apparatus comprises a pre-heating system configured to pre-heat the substrate to substantially a spatially uniform second temperature prior to starting the imaging. The first temperature may equal the second temperature. The pre -heating stem may be configured to pre heat the substrate through irradiating the substrate. To this end, the pre -heating system comprises at least one of: a third LED and a third scanning laser.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further explained, by way of example and with reference to the accompanying drawings, wherein:

Fig.1 is a diagram of an EUV lithographic system, comprising a radiation source and a scanner;

Fig.2 is a diagram of an expose location in the scanner;

Figs.3 and 4 are diagrams of different states of the substrate being moved to an exposure position at the expose location;

Fig.5 is a diagram illustrating the use of a scanning laser to control producing an additional heat load on the substrate;

Fig.6 is a diagram of a measure location of the scanner; and

Fig.7 is a diagram of part of the substrate support carrying the substrate.

Same reference numerals or same reference acronyms in the diagrams indicate similar or corresponding parts.

DETAILED DESCRIPTION

Fig.1 is a diagram of an Extreme Ultraviolet (EUV) lithographic system, comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO includes an EUV radiation source, e.g., a laser-produced plasma (LLP) EUV source, or a free-electron laser (FEL). The lithographic apparatus LA includes an I d IV scanner. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system (or: projection optics) PS and a substrate support WT configured to support a substrate W.

The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution in the cross-section. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto a series of target areas of the substrate W, one target area at a time. After a specific target area has been exposed to the radiation beam B’, the substrate support WT is repositioned relative to the path of the radiation beam B’ so as to position a next target area in the path of the beam B’. The projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate support WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in the diagram of Fig. 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors). For clarity, it is remarked here that the lithographic apparatus LA typically operates in a scanning mode, wherein the support structure MT and the substrate support WT are scanned synchronously while a pattern imparted to the radiation beam B’ is projected onto a target area (i.e., a single dynamic exposure). The velocity and direction of the substrate support WT relative to the support structure MT may be determined by the (de- )magnification and image reversal characteristics of the projection system PS.

The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns for each target area the image, formed by the patterned EUV radiation beam B’ , with a pattern previously formed on the target area.

After the substrate W has entered the scanner LA and has been positioned on the substrate support WT, the substrate W is subjected to a measurement operation. The measurement operation is carried out in order to obtain data representative of the locations of the target areas in a plane perpendicular to an axis of the radiation beam B’ exiting the projection system PS, and representative of the variation in elevation of the substrate W relative to that plane. The substrate W gets exposed to the radiation beam B’ under control of the data thus obtained, in order to accurately print the pattern of the mask MA onto the target areas on the substrate W. The data may be used to control the position and orientation of the substrate support in six degrees of freedom, and/or to control the projection optics, etc.

Fig.2 is a diagram of a vertical cross-section of an expose location 202 in the EUV scanner LA. The expose location 202 is a region within the scanner LA, where the wafer W gets exposed to the radiation beam B’ in order to print the pattern of the mask MA onto a target area 204 on the substrate W.

The scanner LA has a thermal conditioning system configured to thermally condition the substrate W.

The thermal conditioning system includes a heat sink 206 that is configured to extract heat from the substrate W. In the example shown, the heat sink 206 is accommodated in the substrate support WT. The heat sink 206 comprises, e.g., a cooling system through which a cooling fluid is made to flow that extracts heat from the substrate W via the thermal contact between the substrate W and the substrate support WT.

The thermal conditioning system includes a first heater system 208 operative to supply a first additional heat load to that part of the substrate W that is the complement of the specific target 204 area onto which the pattern is being imaged via radiation beam B’. That is, the first heater system 208 is supplying the first additional heat load to all of the substrate W except to the target area 204 that is currently being exposed to the radiation beam B’ .

The first heater system 208 in the example shown includes an array of LEDs that is configured to irradiate the substrate W with infrared light, whose absorption by the substrate W gives rise to heat and, therefore, to thermally induced deformation. Operation of the first heater system 208 is explained with reference to the diagrams of Figs. 3 and 4.

Figs. 3 and 4 are diagram showing part of the expose location 202 in plan view at different stages of moving the substrate W to an expose position. The feature indicated by reference numeral 302 schematically represents a slit through which the radiation beam B’ propagates from the projection system PS to the relevant target area on the substrate W when the target area of the substrate W is present underneath the slit. Assume that the substrate W has a spatially uniform temperature before the substrate W enters, supported by the substrate support WT, the expose location 202. Assume further that the LEDs of the first heater system 208 are initially all turned off. While the substrate support WT is moving the substrate W into the expose location 202, the substrate W is coming gradually within range of the first heater system 208. Once the substrate W has come entirely within range of a subset of the LEDs, the LEDs of this subset are turned on. This results in the substrate W being irradiated by infrared light in a spatially uniform manner. This stage of the substrate W being entirely within range is illustrated in the diagram of Fig. 3. The LEDs turned-on are indicated in white, such as LEDs 208.1 and 208.2. The LEDs turned-off are indicated in black, such as LEDs 208.3, 208.4, 208.5 and 208.6.

While the substrate W is being moved further so as to bring the substrate W towards a position for a target area to be exposed (in a scanning fashion) to the radiation beam B’ , some LEDs that were on are turned off, and other LEDs that were turned off are turned on, so as to maintain a uniform irradiation of the substrate W and, hence, a uniform heat load on the substrate W. In the diagram of Fig.4, the LEDs 208.1 and 208.2 that were on in the diagram of Fig.3 are now turned off, whereas the LEDs 208.5 and 208.6 that were off in the diagram of Fig.3 are turned on in the diagram of Fig.4.

In the diagram of Fig.3, the substrate W has not yet reached a position wherein the substrate W is underneath the slit 302, whereas the substrate W is present underneath the slit 302 in the diagram of Fig.

4.

While the substrate W is within range of the first heater system 208 but not yet underneath the slit 302, the substrate W receives a constant and spatially uniform heat load. The heat load received from the first heater system 208 per unit of time and the heat extracted from the substrate W via the heat sink 206 per unit of time are controlled to balance each other so that the substrate W is maintained at constant, spatially uniform temperature.

Now, when a target area such as target area 204 is under the slit 302 for being exposed to the radiation beam B’, the target area is not covered anymore by the infrared radiation from one or more LEDs of the first heater system 208. Instead, the target area 204 receives a heat load during exposure through absorption of part of the radiation from beam B’ . If the spatial density of the heat load received from the first heater system 208 per unit of time equals, for all practical purposes, the spatial density of the heat load received via absorption of part of the radiation of beam B’ per unit of time, the substrate W will experience no change in a net heat load received and, therefore, remain at a spatially uniform and temporally constant temperature during exposure.

The thermal conditioning system further includes a second heater system 210 operative to supply a second additional heat load to the target area 204 that is under the slit 302 for being exposed to the radiation beam B’. The rationale for this is the following. The first heater system 208 provides a spatial density of a heat load to the substrate W per unit of time that should match the spatial density of the heat load provided by the radiation beam B’ to the substrate W per unit of time. The spatial density of the heat load supplied by the beam B’ per unit of time may vary over time, and at different time scales. For example, the magnitude of parameters of the beam B’, which eventually determine the heat generation at the target area 204 through absorption, may drift. As another example, different masks MA may reflect different amounts of the radiation received from the source SO. As yet another example of varying heat loads, different illumination modes (i.e., different intensity distributions in the cross-section of the beam B) may be used to illuminate different masks MA. Examples of such illumination modes are: annular, dipole, quadrupole, hexapole, customer-specific, polarized, unpolarized, etc. For some background information on illumination modes, please see, e.g., US patent application publication US 20180224715, filed for Voogd et a , assigned to ASML and assigned to ASML, or US patent application publication 20170293229, filed for Godfried et a , assigned to ASML and incorporated herein by reference. As yet another example, the radiation from the source SO may include, in addition to EUV, OOB radiation, such as infrared (IR) and deep-ultraviolet (DUV). Part of the OOB radiation may get absorbed by the substrate W, and may also give rise to undesired heat generation in the substrate W. Such OOB radiation generation may depend on, for example, the set-point of the performance of the source SO. Accordingly, if the heat load supplied to the substrate W by the radiation beam B’ varies over time, the spatial densities of the heat loads supplied by the first heater system 208 and by the radiation beam B’ may not match accurately enough so as to keep the substrate W at a spatially uniform, constant temperature. The first heater system 208 may then be controlled to adjust the additional heat load so as to restore the balance. A further control option is provided by the second heater system 210 to be controlled to restore the balance of the spatial densities of the heat loads by supplying a second additional heat load to the target area 204 to make up for the difference. In the diagrams of Figs.3 and 4, the second heater system 210 comprises a further array of LEDs positioned near the slit 302. Of the LEDs in second heater system 210, LEDs 210.1 and 210.2 are explicitly indicated. In the diagram of Fig.3 the LEDs of the second heater system 210 are turned off, whereas in the diagram of Fig.4 they are turned on.

The thermal conditioning system also has a controller 212 that serves to control the first heater system 208 and the second heater system 210 and, optionally the heat sink 206 so as to maintain the substrate W at a constant and spatially uniform temperature. To this end, the thermal conditioning system includes one or more sensors, e.g., temperature sensors and/or stress sensors at the substrate support WT, whose sensor signals can be used as feedback input to the controller 212. This control aspect of the thermal conditioning system will be discussed in further detail with reference to Fig.7.

The controller 212 may independently control individual ones of the LEDs (or scanning lasers) of the first heater system and/or of the second heater system 210 so as to even out local temperature differences at the substrate W. The controller 212 may also control of the LEDs and/or scanning lasers so as to adjust the global heat generated at the substrate W. That is, the controller 212 may control the LEDs and/or scanning lasers in a common mode and superpose a modulation per individual one of the LEDs and/or scanning lasers. The diagram of Fig.2 shows the inventor 214 contemplating the control options of the thermal conditioning system. As indicated with reference to the diagrams of Figs 3 and 4, the first heater system 208 may be controlled in dependence on: a position of the substrate W relative to the slit 302, and a speed of the substrate W relative to the slit 302. The position and speed determine which LEDs in the first heater system 208 to turn on and which ones to turn off. As an aside: keeping all LEDs on all the time would give rise to several problems, such as: an undesirable extra heat load supplied to the substrate support WT ; a gradient in the temperature of the substrate W owing to the fact that the portion of the substrate W first entering the range of the first heater system 208 would receive more heat than the portion entering the range last. Furthermore, as mentioned above, the first heater system 208 may be controlled in dependence on the spatial density of the heat load supplied by the beam B’ to the relevant target area during exposure thereof. In this respect, the first heater system 208 and/or the second heater system 210 may also be controlled in dependence on feedforward information about upcoming exposure -process variations. For example, the second heater system 210 may be switched off or its heat output may be turned down while skipping the illumination of a specific target area on the substrate W.

As an alternative to LEDs, the first heater system 208 and/or the second heater system 210 may use one or more scanning lasers or a hybrid combination of one or more scanning laser and LEDs. For some background on scanning lasers, see, e.g., Wikipedia under the entries“Laser scanning’’ and“Laser lighting display’’. For completeness, it is remarked here that the first heater system 208 and the second heater system 210 may be provided with their own heat sinks (not shown in the diagrams) or a common heat sink (not shown in the diagrams) so as to prevent undesired variations of the supplementary heat loads from these heater systems 208 and 210 on the substrate W.

For completeness, it is remarked here that a set of turned-on LEDs of the first heater system 208 may produce an illumination pattern in the plane of the substrate W that is spatially non-uniform to some extent. This may be due to the shape of, and the intensity distribution in, the area of the plane illuminated by a turned-on LED. However, in the scanner LA, each target portion is irradiated by scanning the pattern of the mask MA through the projection radiation beam B (please see Fig.l) in a given direction (the "scanning" -direction) while synchronously scanning the substrate W parallel or anti-parallel to this given direction. That is, the substrate W is moving underneath the LEDs of the first heater system 208 and underneath the slit 302 at such a pace, that the illumination pattern produced by the LED is blurred in a coordinate frame that is stationary to the substrate W. The blurring thus tends to even out the non uniformities in this illumination pattern and in the resulting heat generation in the substrate W. Similar considerations may apply to the use of scanning lasers in the first heater system 208. Also, similar considerations may apply to the second heater system 210.

In the diagrams of Figs 2, 3 and 4, the first heater system 208 and the second heater system 210 are drawn as arrays of multiple entities (LEDs and/or scanning lasers) that irradiate the substrate W with infrared beams that have relatively small angles of incidence on the substrate W relative to a direction perpendicular to the substrate W. For clarity it is remarked here that the magnitudes of the angles of incidence are a design choice that may depend on several factors. One such a factor is, for example, the volume available in the scanner LA. Another such factor is, for example, the ways wherein the first heater system 208 and the second heater system 210 themselves may need to be thermally conditioned so as to minimize their thermal load on components in the scanner LA , other than the substrate W.

Fig.5 is a diagram illustrating the use of a scanning laser 502 within the context of the invention and in a plane perpendicular to the substrate W. The scanning laser 502 is a laser device that produces a laser beam 504 whose direction is controllable. In the diagram, the laser beam 504 is shown twice, once when being incident perpendicularly on the substrate W, and once when being incident on the substrate W under a non-zero angle Q in the plane shown. By varying the direction of the laser beam 504 in this plane, different portions of the substrate W can be irradiated. The magnitude of the irradiated surface area of the substrate W depends on the angle Q. The width of the surface area being irradiated by the perpendicularly incident beam 504 is indicated by“Xi”. The width of the surface area being irradiated by the beam 504 striking the substrate W under an angle Q is indicated by“X2”. The width X2 and the width Xi comply with: X2 = Xi / cos Q. That is, the area irradiated depends on the angle at which the laser beam 504 strikes the substrate W. By controlling the intensity of the laser beam 504 in dependence on the angle Q, i.e., by varying the intensity as the reciprocal of cos Q, a spatially uniform heat load can be produced regardless of the direction of the beam 504.

The above discusses the angle -dependent control of the intensity of the beam 504 in a particular plane perpendicular to the substrate W. If the direction of the beam 504 is controlled in two planes perpendicular to the substrate W and to each other, the intensity is controlled to depend on two angles, one in each plane, so as to provide a spatially uniform heat load.

If the scanning laser 502 supplies a pulsed beam 504, i.e., it delivers the laser energy in discrete pulses instead of continuously, then the duty cycle and/or the repetition rate of the pulses may be controlled in dependence on the angles so as to produce a spatially uniform heat load. In this case, the intensity may remain constant. A combination of angle-dependent control of the intensity, as well as of the repetition rate and of duty cycle may also be feasible.

The speed at which the direction of the beam 504 can be changed, and therefore, the speed at which different surface areas of the substrate W can get irradiated, is much higher than the propagation speed of the heat throughout the substrate W. Therefore, and for all practical purposes, imposing a heat load on the substrate W using the scanning laser 502 has the same thermal effect as imposing a heat load via the array of stationary LEDs discussed above with reference to Figs. 2 and 3. Fig.6 is a diagram of another location 602 at the scanner LA, external to the expose location 202. The other location 602 of the scanner is, for example, a measure location within the scanner. At the measure location, the substrate W is subjected to measurement operations, as discussed earlier, in order to obtain data for control of the exposure of the target areas on the substrate W at the expose location. Alternatively, or in addition, the other location may be a substrate handler (or: wafer handler) or a load- lock. A substrate handler is a mechatronic module designed to move substrates via a load-lock in and out of the (vacuum) heart of the scanner LA. A load-lock is an apparatus for transferring substrates between two or more environments wherein different conditions prevail, for example different pressures, different temperatures, different gas compositions, etc. In an EUV scanner, the load-lock serves to transfer the substrate from an environment with ambient pressure to another environment of a much lower pressure that, for all practical purposes, is referred to as“vacuum”. For more background information on a substrate handler, please see, e.g., US patent 9,885,964 issued to Westerlaken et a , assigned to ASML and incorporated herein by reference. For more background on a load-lock, please see, e.g., US patent 7,878,755 issued to Klomp et a , assigned to ASML and incorporated herein by reference.

At the other location 602, the substrate W is pre -heated to a spatially uniform, constant pre heating temperature using a pre-heating system 604. The pre-heating system 604 may be implemented in a way, similar to the first heater system 210 or the second heater system 210, e.g., using an array of LEDs or one or more scanning lasers or a combination of one or more LEDs and one or more scanning lasers.

The constancy and the uniformity of the pre-heating temperature facilitates control of the first and second heater systems 208 and 210. It is remarked here that, in practice, the spatial uniformity of the substrate’s pre-heating temperature is more important than its constancy as non-uniformity is harder to compensate than an offset in the temperature’s magnitude.

Assume that the other location 602 is the measure location within the scanner LA. During and after measuring the properties of the substrate W, the dimensions of the substrate W preferably do not change anymore. Therefore, the spatially uniform pre-heating temperature of the substrate W is preferably the same as the spatially uniform temperature, at which the substrate W is being kept at the expose location. The spatially uniform temperature of the substrate W at the expose side is preferably equal or larger than the local temperature the substrate W would locally assume in the target area if the substrate W were only subjected to the heat load received through absorption of the radiation from the source SO. Accordingly, the temperature of the substrate W can be kept constant and uniform, independent of the exposure’s heat load.

The pre-heating of the substrate W may be performed before the measurement operations are carried out so as to exclude the effect of spatial variations in thermally induced deformations on the data that are extracted from the results of the measurement operations for control of the expose process. If the constant, spatially uniform pre-heating temperature is different from the constant, spatially uniform temperature of the substrate W that is produced by the thermal conditioning system at the expose location 202, this difference may be taken into account by the expose process. As both the pre-heating temperature of the substrate W and the temperature of the substrate W brought about by the thermal conditioning system are constant and spatially uniform, the shape of the substrate W at the pre-heating temperature and the shape of the substrate W at the temperature at the expose location 202 are the same, apart from a uniform scaling factor. This scaling factor can then be used in determining the locations of the target areas for the expose process, give the locations of these target areas produced by the measurement operations. Optionally, the uniform pre-heating temperature of the substrate W may be set a bit higher than the uniform temperature of the substrate W during exposure so as to account for heat loss during the substrate W traveling from the location of pre -heating to the expose location. However, this implies that the heat loss and, therefore, the resulting change in thermally induced deformation of the substrate W needs to be calibrated and taken into account in the control of the thermal conditioning.

As another option, the pre -heating system 604 above the substrate W may move along with the substrate support WT from the measure side 602 to the expose side 202. The first heater system 208 may take over when the substrate W is getting within range of the first heater system 208.

In short, therefore, the temperature of the substrate W is preferably kept uniform and constant all the time during its presence in the scanner LA.

The control of the thermal conditioning system via the controller 212 is explained with reference to Fig.7. The diagram in Fig.7 represents the portion 702 of the substrate support WT carrying the substrate W in the diagram of Fig.2. Please see the reference numeral 702 in the diagram of Fig.2.

The substrate support WT comprises a module 704 that carries actuators (not shown) that are controlled to position the substrate W for exposure and for measuring. The module 704 accommodates a substrate table 706 for carrying the substrate W. The substrate table 706 interfaces with the substrate W through a plurality of burls, of which two are explicitly indicated by reference numerals 708 and 710. The substrate table 706 interfaces with the module 704 through a plurality of other burls, of which two are explicitly indicated by reference numerals 712 and 714. The substrate W is clamped to the substrate table 706 via an electrostatic clamp (not shown). The substrate table 706 is clamped to the module 704 via another electrostatic clamp (not shown). For some background on electrostatic clamps, please see, e.g.,

US patent 9,366,973, issued to Ockwell et a , assigned to ASML and incorporated herein by reference.

The substrate table 706 of the substrate support WT has one or more temperature sensors, e.g., a temperature sensors 716, 718 and 720, positioned between the burls at the side of the substrate table 706 that faces the substrate W in operational use. The one or more temperature sensors 716-720 can be used to control the first heater system 208 and/or the second heater system 210 and/or the pre-heating system 604. The temperature sensors 716-720 serve to sense the temperature of the substrate W. Therefore, the influence of the temperature of the substrate table 706 on the sensing is preferably limited by means of thermally isolating the temperature sensors 716-720 from the substrate table 706.

A sensor signal from one of the temperature sensors 716-720 is representative of the local temperature as sensed by this temperature sensor. The control is implemented by maintaining the sensor signals from the temperature sensors 716-720 all constant and all representative of the same temperature, also referred to as temperature set-point. The first heater system 208, the second heater system 210 and the pre-heating system 602 are each controlled to generate locally more heat in the substrate W when the local temperature as sensed falls below the temperature setpoint and to generate locally less heat when the local temperature as sensed rises above the temperature setpoint.

The spatial density of the temperature sensors at the substrate table 706 is chosen such that the uncertainty of the magnitude of deformation of the substrate W due to an uncertainty in the sensed temperature is acceptable for the accuracy of the exposure process, i.e., the accuracy of the imaging proper of the pattern onto the substrate W.

The substrate table has one or more stress sensors, e.g., stress sensors 722 and 724, positioned between the burls at the side of the substrate table 706 that faces the module 704 in operational use. The stress sensors 722 and 724 include, e.g., strain gauges, and can be used to determine if the substrate table 706 is undergoing a global deformation (as opposed to only local deformation). If a global deformation is being sensed the controller 212 adjusts, in response thereto, the global irradiation by the first heater system 208 and/or by the second heater system 210 so as to bring about a stable thermal condition of the substrate W.

Claims

1.A lithographic apparatus configured to receive radiation for imaging a pattern via projection optics onto a plurality of target areas on a substrate, wherein:

each specific one of the target areas is operative to receive a heat load through absorption of at least part of the radiation during imaging onto the specific target area;

the lithographic apparatus comprises a thermal conditioning system;

the thermal conditioning system is configured to maintain the substrate at a spatially uniform, constant first temperature during the imaging;

the thermal conditioning system comprises:

a heat sink operative to extract heat from the substrate; and

a first heater system operative to supply, during the imaging, a first additional heat load to a part of the substrate,

the part is the complement of the specific target area onto which the pattern is being imaged; and a magnitude of the first additional heat load per unit area equals or exceeds a magnitude of the heat load per unit area.

2. The lithographic apparatus of claim 1 , wherein the thermal conditioning system is configured to supply the first heat load through irradiating the complement.

3. The lithographic apparatus of claim 2, wherein the thermal conditioning system comprises at least one of: an LED and a scanning laser, arranged for supplying the first additional heat load.

4. The lithographic apparatus of claim 1, 2 or 3, wherein the thermal conditioning system comprises a second heater system operative to supply a second additional heat load to the specific target area.

5. The lithographic apparatus of claim 4, wherein the second heater system is configured to supply the second additional heat load via irradiating the specific target area.

6. The lithographic apparatus of claim 5, wherein the second heater system comprises at least one of: a second LED and a second scanning laser, arranged for supplying the second additional heat load.

7. The lithographic apparatus of claim 1, 2, 3, 4, 5 or 6, comprising a pre-heating system configured to pre-heat the substrate to substantially a spatially uniform second temperature prior to starting the imaging.

8. The lithographic apparatus of claim 7, wherein the first temperature equals the second temperature.

9. The lithographic apparatus of claim 7, wherein the pre -heating stem is configured to pre-heat the substrate through irradiating the substrate.

10. The lithographic apparatus of claim 9, wherein the pre-heating system comprises at least one of: a third LED and a third scanning laser.