Lithographic apparatus having a monitoring device for detecting contamination

A lithographic apparatus is disclosed. The apparatus includes a projection system configured to project a first radiation beam onto a target portion of a substrate, and at least one monitoring device for detecting contamination in a interior space. The monitoring device includes at least one dummy element having at least one contamination receiving surface. In an aspect of the invention, there is provided at least one dummy element which does not take part in transferring a radiation beam onto a target portion of a substrate, wherein it is monitored whether a contamination receiving surface of the dummy element has been contaminated.

FIELD

The present invention relates to a lithographic apparatus that includes a monitoring device for detecting contamination. The present invention also relates to an apparatus and a method for detecting contamination and a method for manufacturing a device.

BACKGROUND

In case of an interior space of the apparatus, for example a vacuum space, it is generally desired to monitor the interior space for contamination. This is the case, for example, when the interior space is used in a lithographic process, or, for instance, when the space is included in a lithographic apparatus. In that case, it is desired to detect contamination quickly—preferably within a fraction of a second—so that a lithographic process can be halted immediately, to prevent contamination sensitive optics being spoiled by the contamination. For example, the lithographic process can be halted by stopping the exposure, by stopping a radiation source and/or by closing a shutter in the path of the radiation beam. However, the interior space can also be applied in different fields, for instance general semiconductor industry, general vacuum technology industry, space technology and the-like. The present invention can therefore also explicitly be applied outside the field of lithography.

United States Patent Application Publication No. 2002/0083409 A1 relates to EUV lithography devices and processes, wherein a quartz crystal microwave is use as a measuring device.

European Patent Application Publication No. EP 1 452 851 A1 relates to a method and device for measuring contamination of a surface of a component of a lithographic apparatus. The measuring device has a radiation transmitter device for projection radiation on at least a part of said surface and a radiation receiver device for receiving radiation from the component.

SUMMARY

It is desirable to provide an improved apparatus and method for monitoring contamination, wherein the occurrence of contamination can be detected swiftly, using a relatively simple, inexpensive monitoring device.

According to an aspect of the invention, there is provided a lithographic apparatus, comprising a projection system configured to project a first radiation beam onto a target portion of a substrate, the lithographic apparatus further including at least one monitoring device for detecting at least one contamination species in an interior space of the apparatus, wherein the monitoring device includes at least one dummy element having at least one contamination receiving surface, which contamination receiving surface is in contact with said interior space, wherein the at least one monitoring device includes at least one emitter configured to emit a second radiation beam, which second radiation beam is projected at least partially onto the contamination receiving surface of said dummy element, and wherein the monitoring device includes a detector which is constructed to detect whether the at least one contamination receiving surface of the dummy element has been contaminated.

In an aspect of the invention, there is provided a lithographic apparatus, comprising a projection system configured to project a first radiation beam onto a target portion of a substrate, the lithographic apparatus further including at least one monitoring device for detecting at least one contamination species in an interior space of the apparatus, wherein the monitoring device includes at least one dummy element having at least one contamination receiving surface, which contamination receiving surface is in contact with said interior space, wherein the at least one monitoring device includes at least one emitter configured to emit a second radiation beam, which second radiation beam is projected at least partially onto the contamination receiving surface of said dummy element, and wherein said dummy element is at least partially transparent to the radiation of said at least second radiation beam. The monitoring device can include a first detector which is arranged to receive a part of said second radiation beam which is being transmitted and/or reflected by said dummy element.

In an aspect of the invention, there is provided a lithographic apparatus, comprising a projection system configured to project a first radiation beam onto a target portion of a substrate, the lithographic apparatus further including at least one monitoring device for detecting at least one contamination species in an interior space of the apparatus, wherein the monitoring device includes at least one quartz crystal monitor or surface acoustic wave detector having at least one contamination receiving surface, which contamination receiving surface is in contact with said interior space, wherein the at least one monitoring device includes at least one emitter configured to emit a second radiation beam, which second radiation beam is projected at least partially onto the contamination receiving surface. The quartz crystal monitor or surface acoustic wave detector is configured to detect whether the at least one contamination receiving surface has been contaminated.

In an aspect of the invention, there is provided a lithographic apparatus, comprising a projection system configured to project a first radiation beam onto a target portion of a substrate, the lithographic apparatus further including at least one monitoring device for detecting at least one contamination species in an interior apparatus space, wherein the monitoring device includes at least one dummy element having at least one contamination receiving surface, which contamination receiving surface is in contact with said interior space, wherein the at least one monitoring device includes at least one emitter configured to emit an electron beam or ionising beam, which electron beam or ionising beam is projected at least partially onto the contamination receiving surface of said dummy element. The monitoring device can include a detector which is constructed to detect whether the at least one contamination receiving surface of the dummy element has been contaminated.

In an aspect of the invention, there is provided a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, utilizing at least one interior space, for example a vacuum, wherein the apparatus includes at least one dummy element which does not take part in transferring said pattern onto said substrate, wherein the dummy element is contact with said interior space, wherein the apparatus includes at least one radiation emitter for irradiating said dummy element with radiation, which radiation serves to modify, crack and/or detect contamination which the dummy element can receive from said interior space.

In an aspect of the invention, there is provided a method for monitoring contamination in an interior space used in a lithographic process, the method including: providing at least one dummy element which does not take part in transferring a first radiation beam onto a target portion of a substrate; wherein a contamination receiving surface of the dummy element is in contact with said interior space for receiving said contamination, if any, therefrom, wherein it is monitored whether the contamination receiving surface has been contaminated by said contamination.

Also, according to an aspect of the invention, there is provided a lithographic apparatus, including: a number of optical elements configured to project a first radiation beam onto a target portion of a substrate; at least one radiation source which is configured to emit a second radiation beam; a radiation beam controller configured to direct the second radiation beam onto at least one surface part of at least one of said optical elements; and at least one detector which is configured to detect electrical current, which current emanates from said at least one surface part of said optical element when that surface part interacts with said second radiation beam.

According to an aspect of the invention, there is provided a method for monitoring contamination of an optical element, the method comprising: projecting a beam of radiation onto at least one surface part of said optical element, to generate an electrical monitoring current in the element; measuring said electrical monitoring current.

According to an aspect of the invention, there is provided a lithographic device manufacturing method, comprising: providing a projection system which projects a first radiation beam onto a target portion of a substrate, the projection system including at least one optical element; projecting a second beam of radiation onto at least one surface part of said optical element, to generate an electrical monitoring current in the element; and measuring said electrical monitoring current.

In an aspect of the invention, there is provided a method for monitoring contamination of at least one optical element, the method comprising: scanning a beam of radiation over a surface of said optical element for producing an electrical monitoring current in the element via the photo-electric effect; measuring said electrical monitoring current.

Further, according to an aspect of the invention, there is provided a device manufacturing method comprising projecting a patterned beam of radiation onto a substrate, utilizing a number of optical elements, wherein the device manufacturing method includes: scanning a second beam of radiation over a surface of said optical element for producing an electrical monitoring current in the element; measuring said electrical monitoring current.

According to an aspect of the invention, a system for monitoring contamination of an optical mirror element includes: at least one radiation source which is configured to emit a radiation beam; a radiation beam controller configured to direct the radiation beam towards different locations of a surface of at least one of said optical elements; and at least one detector which is configured to detect electrical current, which current emanates from said optical element when the optical element interacts with said second radiation beam.

According to an aspect of the invention, there is provided a computer program product comprising computer code portions, configured to be run on the processor of a system for monitoring contamination of an optical mirror element: to detect deviations or changes of the current, measured by said at least one detector; and/or to process and/or store measurement results, which results include electrical current measured by said detector when said second radiation beam is being projected onto said optical element, and the respective location of said surface part of said optical element which receives the radiation beam.

DETAILED DESCRIPTION

In the present description, equal or similar features can be referred to by equal or similar reference signs.

FIG. 1schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a beam of radiation (e.g. UV radiation or other radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to a—conditioned—first radiation beam PB by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”. The projection system can, for example, be located in a so called projection optics box POB.

As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask). Alternatively, the apparatus may be of a transmissive type (e.g. employing a transmissive mask).

The illuminator may be used to condition the beambeam of radiation emitted by the source SO, to have a desired uniformity and intensity distribution in its cross-section. The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the beam of radiation. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator and a condenser.

AsFIG. 1schematically shows, an interior space11of the apparatus can include at least one monitoring device10,110,210,310,410,510for monitoring contamination. The monitoring device10,110,210,310,410,510can be located in various locations of the device, for example in or near the projection system, in the projection optics box POB, in a reticle zone, in a substrate zone, the illuminator IL or in other locations of the apparatus.

The interior space11can be located, for example in a vacuum chamber. The interior space11can be, for example, a vacuum space. The vacuum of the vacuum space cap have varying vacuum pressures, which can depend, for example, on the radiation type of the first radiation beam PB used in the lithographic apparatus. Alternatively, the interior space can be an immersion liquid space which is filled with an immersion fluid during use. Also, an interior space that has been provided with at least one monitoring device10can be, for example, a space which also contains the patterning means, a space which also contains the substrate support, or an other space of the apparatus. The interior space can also have an atmospheric pressure, or a different pressure.

FIG. 2schematically shows a first embodiment of an above-mentioned contamination monitoring device10, which can be located in an interior space for monitoring contamination, for example in an interior of a lithographic apparatus, for example in an apparatus as shown inFIG. 1or in an other apparatus.

The monitoring device10can be arranged to detect at least one contamination species in the interior apparatus space11. In the embodiment ofFIG. 2, the monitoring device is installed in a lithography projection optics box POB, which box POB comprises said interior space11. This optical box POB can include at least part of said projection system PS. This projection system PS can also be in contact with the interior space11during use.

The monitoring device10can include at least one dummy element1having at least one contamination receiving surface. The dummy element1does not take part directly in transferring a radiation beam onto a target portion of a substrate during a lithographic process. For example, the above-mentioned first radiation beam PB is preferably not being projected onto the dummy element1during use. The monitoring device can comprise for example a plurality of dummy elements1(as is shown inFIG. 2), for example at least 20 dummy elements1, or a different number of dummy elements1.

In the embodiment ofFIG. 2, each dummy element1extends substantially within the interior apparatus space11. Therefore, the contamination receiving surface of each dummy element1is in contact with said interior space11during use. Thus, during use each dummy element1can be contaminated by the same contamination as the projection system PS, for the case that contamination is present in the interior space11.

The monitoring device10further can include at least one emitter2which is configured to emit a second radiation beam9. This second radiation beam9is projected at least partially onto the contamination receiving surface of said dummy element1during use. For example, this second radiation beam9is preferably not being projected onto the projection system PS during use (seeFIG. 2).

The radiation of said at least second radiation beam9can be selected, for example, from the group consisting of the following radiation types: visible light, infra red light, ultra violet light, Deep UV light, extreme ultra-violet light and microwave radiation. The radiation can be chosen, for example, to modify or crack at least one contamination species into one or more compounds having improved absorbability to the second radiation beam9. The radiation type of the second radiation beam9can be chosen, for example, to cause carbonization of said dummy element1in case the apparatus interior space11contains hydrocarbon contamination. For example, an Ultra Violet (UV) second radiation beam9can be used, which can crack certain hydrocarbon contamination into better detectable matter, which matter leads to the carbonization of the dummy elements1. Good carbonization can also be achieved by using a second radiation beam9of microwave radiation or electron beam radiation.

The cracking of the at least one contamination species can also resemble the process, for example when it takes place in the projection beam PB during illumination. It can also to have a continuing effect, for example: adsorption can stop while the contamination can continue.

Also, the radiation beam can be a beam of visible light, or infrared light. Visible light or infrared light is relatively easy producible, with inexpensive means.

Besides, there could be used two beams at once, for example a first beam to cause a reaction and/or modification of contamination, and a second beam to measure the effect of the reacted and/or modified contamination.

The radiation emitter2can be designed in various ways. For example, the radiation emitter can be a laser, an electron beam emitter, or a different type of radiation emitter.

In the embodiment ofFIG. 2, each dummy element1is at least partially transparent to the radiation of said at least second radiation beam9. For example, each dummy element1can be substantially transparent to that radiation. For example, the at least one dummy element1can be made of glass, a transparent plastic, quartz or calcium fluoride for IR, et cetera. Alternatively, said contamination receiving surfaces of said dummy elements1can reflect substantially all radiation of said second radiation beam during use, which is described below with reference toFIG. 3.

Each dummy element1can have various shapes. For instance, each dummy element can be a rod-like or plate-like element, or a differently shaped element. For example, two surfaces of each said dummy element1, which two surfaces are substantially faced away from each other, can serve as contamination receiving surfaces during use, as is shown in the embodiment ofFIG. 2.

In an aspect of the invention, said monitoring surface of each dummy element1includes the same material as a radiation receiving surface, or an outer material, of said projection system PS. In that case, each contamination receiving surface of each dummy element1can have about the same sensitivity to certain contamination types as surfaces of the projection system PS.

The at least one dummy element1can be constructed to provide an array of contamination receiving surfaces, wherein at least part of said at least second radiation beam passes all of the contamination receiving surfaces of said array at least once during use. The monitoring device can also include a plurality of said dummy elements1which provide a plurality of contamination receiving surfaces that are spaced-apart from each other. Contamination receiving surfaces of neighbouring dummy elements can be located opposite one another.

In the embodiment ofFIG. 2, the contamination monitor10includes an above-mentioned array of dummy elements1. This array provides an array of contamination receiving surfaces, each of which is in contact with said interior apparatus space11. At least part9bof said at least second radiation beam9passes all of the contamination receiving surfaces of said array at least once during use. In the embodiment ofFIG. 2, the contamination receiving surfaces are formed by the outer surfaces of the dummy elements1which extend substantially parallel to each other. Alternatively, for example, at least a number of the contamination receiving surfaces can extend in a non-parallel fashion with respect to each other.

During use of the embodiment ofFIG. 2, part9bof the second radiation beam passes each radiation receiving surface substantially perpendicularly, to provide a short path-length of that radiation passing through a respective dummy element1. This can provide a high transmission ratio of transmission of the radiation through the dummy element1. Alternatively, part9bof the second radiation beam can pass one or more radiation receiving surfaces in a non-perpendicular manner. This can result in a higher sensitivity to the contamination layer.

By providing a plurality of subsequent contamination receiving surfaces (seeFIG. 2or3), the contamination sensitivity of the contamination monitor can be greatly enhanced.

At least one radiation reflector can be provided, which reflector is arranged to reflect radiation of said at least second radiation beam9at least partially towards said contamination receiving surface of said dummy element1. As an example only, said dummy element1can be located between at least two radiation reflectors, wherein the reflectors are arranged to reflect radiation of said second radiation beam towards each other, through said dummy element. Then, at least part of the radiation beam can be passed a plurality of times towards the contamination receiving surface or surfaces of said at least one dummy element, for enhancing contamination sensitivity. In the embodiment ofFIG. 2, the array of dummy elements1is located between two radiation reflectors3,5. The reflectors3,5are arranged to reflect radiation of said second radiation beam towards each other, through said dummy elements1. The radiation reflectors3,5can be arranged in various ways, and can be made of various suitable materials, as will be clear to the skilled person.

For example, the first reflector3, which is located between one side of the dummy elements1and the radiation emitter2, can be arranged to reflect a major part of incoming radiation, received from the dummy elements1, back towards the dummy elements1. The second reflector5, which is arranged between the opposite side of dummy elements1and a second detector6, can be arranged to reflect only part of incoming radiation back towards the dummy elements1, and to transmit the remaining beam part9ctowards the second detector6. In that case, the second reflector5is a partial reflector.

The monitoring device can further include a detector which is constructed to detect whether the at least one contamination receiving surface of the dummy element has been contaminated.

The embodiment ofFIG. 2is provided with a first radiation detector4and a second radiation detector6. The first detector4is arranged to receive a part9aof said second radiation beam9directly, without that beam part9abeing transmitted and/or reflected by said dummy element1. The second detector6is arranged to receive a part of said second radiation beam after that radiation beam has been transmitted by said dummy element1during use. For example, the second detector6is located downstream with respect to the array of dummy elements, to receive said second radiation beam9cafter the second radiation beam has been transmitted once or several times through the contamination receiving surfaces of said dummy elements1. From radiation detection results, that can be provided by the second detector6during use, an absorption amount concerning absorption of radiation9bof the second radiation beam9during the passing of the dummy elements1, can be determined. Herein, for example, the absorption of radiation9bcan be determined, from which determination the amount of contamination can be determined. Alternatively, from radiation detection results, of the second detector6, it can be determined, whether any change in radiation absorption has occurred during the passing of the dummy elements1.

The detection results of the first detector4and second detector6can be compared with each other to determine, in what amount radiation9bof the second radiation beam9has been absorbed during the passing of the dummy elements1, or whether a change in incident radiation has occurred. The results of the first detector6can be used, for example, as a reference concerning detection results of the second detector4. Also, the first detector4can be used to calibrate the second detector6, for example when no contamination is present yet in the apparatus interior space11.

The first and second detector can be arranged in various ways, which depends on the type of radiation to be detected. For example, one or both detectors can include at least one photo diode. Also, one or both detectors can be constructed to detect electrons. Also, one or both detectors can be designed to detect one or more of the above-mentioned radiation types.

The apparatus can also include a beam splitter3, which is arranged to split the second radiation beam9into a first beam part9aand a second beam part9b. In the present embodiment, the beam splitter3and the first radiation reflector are integrated with each other. For example, the beam splitter can direct the incoming first beam part9atowards the first radiation detector4, and the second beam part9btowards the possibly contaminated radiation receiving surface of the nearest of the dummy elements1(seeFIG. 2).

Further, the monitoring device10can include a thermal controller13which controls the temperature of one or more dummy elements1during use. The thermal controller13can be arranged to cycle the temperature of one or more dummy elements1between a plurality of monitoring temperatures. To the skilled person, it will be clear that the thermal controller13can be constructed in various ways. The thermal controller can comprise, for example, one or more electric heaters, a heat exchanging fluid, a heat pipe and/or an other type of temperature controlling device. Also, for example, different dummy elements can be held at different constant temperatures, for example by the thermal controller.

In an aspect of the invention, a method for monitoring contamination in a lithographic process includes: providing at least one dummy element which does not take part in transferring a radiation beam onto a target portion of a substrate.

This method will now be explained, with reference to the embodiment ofFIG. 2, as an example of the invention.

During a use of the embodiment ofFIG. 2, the contamination receiving surfaces of dummy elements1are in contact with the interior space11for receiving contamination, if any, therefrom. Then, it is monitored whether the contamination receiving surfaces have been contaminated by one or more contamination species. To this aim, at least one second radiation beam9is being emitted by the radiation emitter2. The second radiation beam9can be emitted continuously, in short radiation pulses and/or in any other way. Downstream of the emitter2, the emitted radiation beam9is being split by beam splitter3into a first beam part9a, which is projected onto the first detector4, and into a second beam part9b, which is being projected onto the array of dummy elements1. The first beam part9ais being detected by the first detector4.

The second beam part9bis being transmitter through the array of dummy elements1, towards the second reflector device5. The second reflector5reflects part of the incoming radiation back towards the first reflector3, and transmits a remaining radiation part9cto the second radiation detector6, which detects the radiation. The latter beam part9ccontains radiation that has traversed the dummy elements1only once, and also radiation that has been reflected between the two opposite reflectors3,5and that has therefore traversed the dummy elements1more than once. Therefore, part of the second radiation beam9can be projected a plurality of times onto contamination receiving surfaces of the dummy elements1, after which that beam part is being detected by the second detector6.

As long as the contamination receiving surfaces of the dummy elements1are substantially not contaminated during use, the second radiation detector6will detect a certain first amount of radiation. Once a number of the surfaces of the dummy elements are being contaminated, a change of radiation will be detected by the second radiation detector6, which may trigger an alarm or a different action. Also, for example, a certain ratio between a signal of the first detector4and a signal of the second detector6can be used to trigger the alarm and/or to trigger an other action.

When a plurality of dummy elements1is being used for transmitting the second radiation beam, contamination can be detected swiftly, because of the cumulative effect on radiation absorption at the radiation receiving surfaces of the dummy elements1. In that case, for example, the contamination can be detected at an early stage, before the contamination can do major harm to the projection system PS. When the contamination monitor10has detected the presence of contamination, it can trigger an alarm, an automatic halt of a lithographic process and/or an automatic stop of the emission of the first radiation beam PB. Then, damage to the projection system due to the first radiation beam PB illuminating contaminated parts of the projection system PS, can be prevented.

For example, when at least 20 dummy elements1are provided, at least twice that amount, i.e. at least 40 contamination receiving surfaces will be available, leading to a cumulative sensitivity improvement of 40× with respect to the use of one contamination receiving surface only.

The detection of changes in radiation transmission through the array of dummy elements1can be achieved with or without using the detection results of the first radiation detector4. By using the results of the first radiation detector4, a reliable contamination monitoring can be provided.

Hydrocarbon contamination is relatively harmful to optics of the projection system PS. Therefore, it is desirable to detect such hydrocarbon contamination fast, before it can do harm to the projection system PS by carbonizing parts of that system (for example under the influence of the first radiation beam PB).

In an aspect of the invention, said second radiation beam9modifies and/or cracks contamination that is present on or near said contamination receiving surface. For example, hydrocarbons can be cracked by the radiation beam9, provided that a suitable cracking radiation type is used. Also, said monitoring surface of each dummy element1can include the same material as a radiation receiving surface of said projection system PS. Then, by providing a plurality of contamination receiving surfaces of dummy element(s)1, these surfaces can be carbonized due to hydrocarbons present in the apparatus interior space11. The cumulative effect of the carbonization of the plurality of the contamination receiving surfaces can be detected relatively early, for example before any surface of the projection system PS has been carbonized too much to necessitate projection system cleansing or repair. In an embodiment of the invention, the second radiation beam can be turned on before the radiation source SO of the projection system PS is switched on. Then, only when no change in radiation transmission through the dummy elements1is being detected, the radiation source SO of the projection system PS can be switched on safely.

The contamination monitor10can also be used to detect other contamination types, for example to detect water.

For example, the temperature of said dummy element1can be controlled to a certain temperature. The temperature of said dummy element can be cycled between a plurality of monitoring temperatures, for instance at least to a monitoring temperature at which certain contamination condenses onto the contamination receiving surface(s). In an aspect of the invention, a lithographic process includes the use of a projection system which transfers a patterned radiation beam onto a target portion of a substrate, wherein said monitoring surface of said dummy element includes the same material as a radiation receiving surface of said projection system. In that case, any undesirable effects of contamination to the projection system can be substantially the same as contamination effects that can be detected via the dummy element. This makes the dummy element a useful indicator for monitoring the presence of undesired species in the apparatus during the lithographic process.

FIG. 3depicts an embodiment of the invention, which differs from the embodiment ofFIG. 2in that the at least one dummy element101is arranged to reflect substantially all radiation of the second radiation beam109. Also, said contamination receiving surface of said dummy element reflects substantially all radiation of a second radiation beam109during use. A detector106is arranged to receive said second radiation beam109after the second radiation beam109has been reflected at least once by the contamination receiving surface of said dummy element.

In the embodiment ofFIG. 3, the dummy element101comprises two opposite dummy reflecting surfaces, each of which provides a contamination receiving surface that is in contact with the apparatus interior space11during use. During use, the second radiation beam109is reflected a plurality of times between the two dummy reflecting surfaces, as is indicated inFIG. 3. For example, the second radiation beam109can be reflected at least 20 times by each of the two reflecting surfaces, before that beam109is being detected by the radiation detector106. In this way, the radiation beam109has been reflected at least 40 times by a contamination receiving surface, so that the second radiation beam can be cumulatively affected by contamination that has been received on those surfaces. This leads to a desirable enhancement of the sensitivity of the contamination monitor110.

In an aspect of the invention, the apparatus includes at least a second emitter configured to emit a third radiation beam. The wavelength of the third radiation beam can differ from the wavelength of the second radiation beam. The third radiation beam is also projected at least partially onto the contamination receiving surface of said dummy element during use, particularly for modifying or cracking said contamination species into one or more compounds having improved absorbability of radiation of said second radiation beam.

For example, the second radiation beam can be projected onto the contamination receiving surface for the detection of contamination. The radiation type of the second radiation beam can be specifically chosen for allowing a relatively uncomplicated monitoring of the contamination receiving surface. For example, the second radiation beam can be a light beam of visible light, a laser beam, or an other suitable radiation beam.

FIG. 4shows an embodiment, wherein a further radiation emitter212is provided for emitting an above-mentioned third radiation beam219. The third radiation beam219is projected onto the contamination receiving surface of at least one dummy element201(only one such element is shown inFIG. 4). The type of radiation of the third beam219can be specifically chosen to modify or crack at least one species into one or more compounds having improved absorbability of radiation of said second radiation beam. For example, the third radiation beam can be of a radiation type which can crack hydrocarbons.

During use, the second radiation beam209is emitted by the respective radiation emitter202, and projected onto the contamination receiving surface of the dummy element201as well. In the present embodiment, the second radiation beam is being transmitted by the dummy element201, towards a suitable radiation detector306. Alternatively, the second radiation beam can be reflected by the dummy element towards a suitable radiation detector (not shown). From the detection of the second radiation beam, it can be evaluated whether the radiation receiving surface of the dummy element201has been contaminated, for instance by compounds that have been fixed to that surface under the influence of the second radiation beam

In an aspect of the invention, said dummy element is a quartz crystal monitor or a surface acoustic wave detector. Such a crystal monitor or wave detector can also monitor, whether its contamination receiving surface has been contaminated.

FIG. 5schematically shows an embodiment of a contamination monitoring device310, comprising a quartz crystal monitor301. Alternatively, the device can include a surface acoustic wave detector instead of the quartz crystal monitor.

The quartz crystal monitor301of the embodiment ofFIG. 5has at least one contamination receiving surface, which contamination receiving surface is in contact with the interior apparatus space11—to be monitored—during use. The monitoring device310includes at least one emitter302configured to emit a second radiation beam309. The second radiation beam309is projected at least partially onto the contamination receiving surface of the quartz crystal monitor301during use. The quartz crystal monitor301can detect whether the at least one contamination receiving surface has been contaminated. In the embodiment ofFIG. 5, said dummy element is the quartz crystal monitor, wherein the dummy element/quartz crystal monitor also serves as said detector during use. The second radiation beam309can, for example, be specifically chosen to modify or crack at least one species into one or more compounds. For example, the second radiation beam309can be of a radiation type which can crack hydrocarbons, for example to carbonize contamination onto the quartz crystal monitor301. In one embodiment of the invention, for accurate measurement using the quartz crystal monitor301, the radiation second beam309can be switched off at certain measurement periods.

FIG. 6schematically shows a fifth embodiment of a contamination monitoring device. In the embodiment ofFIG. 6, the lithographic apparatus includes at least one monitoring device410which is configured to detect at least one contamination species. The monitoring device includes at least one dummy element401having at least one contamination receiving surface. The contamination receiving surface is in contact with said apparatus interior space11during use. The at least one monitoring device includes at least one emitter402configured to emit an electron beam409, which electron beam409is projected at least partially onto the contamination receiving surface of said dummy element401during use. The electron beam409can, for example, crack hydrocarbon contamination onto the surface of the dummy element401.

The monitoring device also includes a electron detector406which is constructed to detect whether the at least one contamination receiving surface of the dummy element has been contaminated. The detector406can, for example, be constructed to detect secondary electrons420, which secondary electrons420can be emitted and/or reflected by said contamination receiving surface as a result of projecting said electron beam onto that surface. The amount of secondary electrons420, being detected by the electron detector406, can be used as an indication of the presence of contamination on the dummy element401.

Alternatively, for example instead of the electron beam emitter402and electron detector406, the emitter can be configured to emit an ionising beam, whereas the detector can be an ion detector. For example, the emitter can be an EUV beam emitter. Also, the detector can be an ion detector, which can detect ions, which ions can be emitted and/or reflected by said contamination receiving surface as a result of projecting said ionising beam onto that surface.

Also, the apparatus can include at least a second emitter412configured to emit a fourth radiation beam419, wherein the wavelength of the fourth radiation beam differs from the wavelength of the second radiation beam, wherein the fourth radiation beam is also projected at least partially onto the contamination receiving surface of said dummy element during use, wherein a further detector416is arranged to receive a part of said fourth radiation beam419which is being transmitted and/or reflected by said dummy element401during use. Radiation of said at least fourth radiation beam419can be selected from the group consisting of the following radiation types: visible light, infra red light, ultra violet light, Deep UV light, extreme ultra-violet light and microwave radiation, or from other radiation types. The further detector416can detect the fourth radiation beam419, to measure changes of radiation absorption. The amount of radiation of the fourth radiation beam, being detected by the further detector416, can also be used as an indication of the presence of contamination on the dummy element401.

Besides, according to an aspect of the invention, the sensitivity of the monitoring device10,110,210,310,410can be enhanced, by using a completely different duty cycle for the said second, third and/or fourth radiation beam that leads to contamination conversion, for example carbonization, for example of absorbed CxHymolecules. For example, when a continuous radiation source is used as a second, third or fourth radiation beam, the reaction and/or modification process for some contamination species (volatile species) will be enhanced compared to a pulsed first radiation beam.

In another aspect of the invention, a lithographic apparatus can include at least one monitoring system510for monitoring contamination, as is shown schematically inFIGS. 1 and 7. The monitoring system510can be configured to monitor contamination of at least one optical element PSM of the projection system PS, which optical element PSM is located in or near an interior space11of the lithographic apparatus. The optical element PSM can be, for example, a mirror which receives said first radiation beam PB during a lithographic process. For example, said mirror PSM includes at least one mirror surface to reflect said first radiation beam PB. Parts of this monitoring system510can be located in various locations of the apparatus, for example in or near the projection system, in the projection optics box POB, in the illuminator IL or in other locations of the apparatus.

As is depicted inFIG. 7, in an aspect of the invention, the contamination monitoring system510can include: at least one second radiation source540which is configured to emit a second radiation beam543; a radiation beam controller542configured to direct the second radiation beam543towards different surface parts545of a surface of said optical element PSM; and at least one current detector547which is configured to detect electrical current, which current emanates from said surface parts545of said optical element PSM when that surface part545receives said second radiation beam543. For example, the electrical current can arise in the optical element PSM via the photo-electric effect.

The second radiation source540can be designed in various ways. For example, the second radiation source540can be configured to emit a photon beam, electromagnetic beam, or electron beam. In an aspect of the invention, said second radiation source540is configured to emit a second radiation beam having a radiation energy that is at least 2 eV, or larger than 2 eV. This radiation energy is the energy of the individual photon or electron respectively. For example, said radiation source can be configured to emit a second radiation beam having a radiation energy in the range of about 2 eV−30 eV.

Also, for example, the second radiation beam can be of such an energy, that the second radiation beam does not lead to removal of material from the optical element PSM (for example, as in sputtering). Then, a relatively low-energy radiation can be produced with a relatively inexpensive, easy to generate and easy controllable radiation source540. For example, the second radiation source540can be a laser, one or more lamps, or an other suitable type of radiation source. The above-mentioned energy of about 2 eV corresponds to a photon wavelength of about 600 nm. Thus, in the sixth embodiment, shown inFIG. 7, a second radiation beam with a wavelength of about 600 nm or a shorter wavelength can be used. For example, the second radiation beam543has a higher wavelength than EUV light. The second radiation beam543can then be controlled, for example, using relatively uncomplicated beam steering and/or beam transporting devices, such as one or more optical fibres541,544and other devices.

In theFIG. 7embodiment of the invention, a first radiation directing device541is provided for directing the second radiation beam from the second radiation source540to the radiation beam controller542. This first radiation directing device541can be designed in various ways. For example, the radiation directing device541can be simply an optical fibre, or an other suitable device, depending on the type of radiation to be directed thereby. Alternatively, the radiation beam controller542and the radiation source540are integrated with one another, wherein no first radiation directing device is provided.

The radiation beam controller542can be configured to direct said second radiation beam at least to the surface area of said optical element, which surface area receives said first radiation beam PB during use. Thus, contamination of the surface area, which receives the first radiation beam PB during a lithographic process, can be monitored during use of the monitoring system510. For example, said radiation beam controller542can be configured to direct the second radiation beam543onto a mirror surface of the optical element PSM.

The radiation beam controller542can include a scanning device548, configured to scan said second radiation beam over a plurality of surface parts of said optical element PSM. InFIG. 7, scanning directions are schematically indicated by a double arrow x. Various scan directions can be provided, for example by scanning the second beam543along two orthogonal axes over the surface of the optical element PSM, such that the plurality of surface parts lie in a 2-dimensional scanning pattern, as will be clear to the skilled person.

The scanning device can be configured in various ways. The scanning device can include, for example, one or more controllable mirrors, a movable beam director, or an other suitable device. In the embodiment ofFIG. 7, the scanning device comprises a second radiation directing device541, for example an optical fibre, which is movable to scan the second radiation beam543over the desired surface part of the optical element PSM. For example, in case the first radiation beam PB illuminates a first surface area of said optical element during lithography, said radiation beam controller542can be configured to scan the second radiation beam543at least over said first surface area of the optical element PSM.

The at least one detector547can be configured in various ways. For example, the detector547can be electrically connected to said optical element PSM, to receive electrical current therefrom. For example, the electrical connection between the current detector547and the optical element PSM can comprise suitable wiring, a current multiplier and/or other suitable electrical connection means. Alternatively, a current detector549can be configured to receive secondary electrons from said optical element PSM, which secondary electrons can emanate from each of said plurality of surface parts545of said optical element when that surface part545receives said second radiation beam. Also, in an embodiment of the invention, an electrical field can be created above the optical element PSM in order to make the measurement less sensitive to fields created in the measurement process, and to make the emission of secondary electrons more effective.

The monitoring system510can include at least one processor546which is configured to detect deviations or changes of the current, measured by said at least one detector. The processor546can also be configured to process and/or store measurement results, which results include: electrical current measured by said detector when said second radiation beam is being directed at said at least one surface part of said optical element; and the respective location of said surface part of said optical element.

In the embodiment ofFIG. 7, said various functions can be incorporated in a single processor546. To the skilled person, it will be clear that more than one processor can be included to perform different operations and/or provide different functions. Each such processor456can be configured in various ways. A computer program product can be provided, comprising computer code portions, configured to be run on the processor546to provide certain operations and/or functions. Besides, as an example, said current detector547and the processor546may be integrated with one another.

During use, in a method for monitoring contamination of the optical element PSM, the second radiation beam543can be projected subsequently onto the plurality of surface parts545of said optical element PSM, to generate an electrical monitoring current in the element PSM.

The second radiation beam543can be scanned over at least part of said optical element by said beam controller542, to monitor contamination of the plurality of surface parts. Also, in the present embodiment, the second beam543is not being projected onto said substrate W.

For example, the method for monitoring contamination can be part of a lithographic device manufacturing method which comprises: providing a projection system which projects a first radiation beam PB onto a target portion of a substrate, the projection system including at least one optical element PSM.

For example, said first radiation beam PB can illuminate a first surface area of said optical element PSM during the lithographic process, wherein a second surface area of said optical element PSM is illuminated by said second radiation beam543during the contamination monitoring. In an embodiment, said second surface area can be a small section of said first surface area, for example to pinpoint a location of surface contamination.

Further, said second radiation beam543can be scanned at least over the same surface area of the optical element PSM, as an surface area that receives said first radiation beam PB during said lithographic process.

The scanning of the optical element PSM by the second radiation beam can be performed at various instances, for example before and/or after a lithographic projection process is being performed using the optical element PSM. For example, when the radiation source SO is a pulsed source that emits a pulsed first radiation beam, the scanning of the optical element PSM by the second radiation beam can be performed between the periods when the first radiation beam pulses are being projected thereon. First and second radiation beam pulses can be, for example, out-of-sync with respect of one another during use. Also, the scanning of the optical element PSM by the second radiation beam543can be performed before, during and/or after a cleaning process to clean the optical element PSM, for example a cleaning process to remove carbon contamination from that element PSM.

During a contamination monitoring method, the generation of current in the optical element PSM can simply take place via the photo electric effect. The invention makes use of the fact that materials can emit electrons when being illuminated by photons. To the skilled person it will be clear, that this can depend on quantum yields, absorbance of the materials, and the electrical field near the material. For example, electrons can originate from the top few nanometers of the surface parts545of the optical element PSM, when being irridiated. The number of generated electrons can depend on the amount of photon absorption, which depends on the type of material present at the surface of the optical element PSM. The surface of the optical element can include, for example, Ruthenium, Iridium, Silicon, Molybdenum, Rhodium, Paladium, Gold, Zirconium, Niobium or other suitable materials.

During use of the embodiment ofFIG. 7, said electrical monitoring current is measured by the current detector547. The monitoring current can be the total current that runs through the optical element PSM. The monitoring current can include a current of secondary electrons, which secondary electrons can emanate from each of said plurality of surface parts545of said optical element when that surface part545receives said second radiation beam. Then, at least the following measurement results can being processed and/or stored by said processor546: the monitoring current measured when said second radiation beam543is being directed at said surface of said optical element; and the respective surface part of said optical element.

Also, deviations or changes of the monitoring current can be detected by said processor546. Such deviations are depicted inFIGS. 8A and 8B.FIGS. 8A and 8Bshow the detected monitoring current I as function of the scanning location x at which the second radiation beam543intersects the surface of the optical element PSM.

As an example, during use, the second radiation beam543is a photon beam having an energy of about 30 eV, whereas the surface material of the optical element PSM consists substantially of Ruthenium.FIG. 9shows the dependence of photon absorption as a function of the energy, for the materials Si, C, Ru, RuO2 and SiO2. FromFIG. 9it follows that, in case the surface of the optical element PSM substantially contains Ru, and the photon energy is about 30 eV, a carbon contamination of that surface will lead to less photon absorption and therefore to less electron emission. Any oxidation of the Ru-surface (to RuO2) will lead to more photon absorption and therefore to more electron emission (at the same photon energy).

In the detection result shown inFIG. 8A, the monitoring current I increases at a certain surface location q1of the optical element PSM. Following from the above andFIG. 9, this current increase can be explained by a local oxidation of the surface of the optical element PSM.

In the result ofFIG. 8B, the monitoring current I decreases at a certain surface location q2of the optical element PSM. Following from the above andFIG. 9, this current decrease can be explained by a local carbonation of the surface of the optical element PSM.

In an aspect of the invention, after a certain type of degradation/contamination of the optical element PSM has been detected, for example degradation due to either carbonization or oxidation of the optics surfaces, appropriate action can be taken. For example, the locally detected contamination can be submitted to a local cleaning treatment, and/or the contaminated optical element PSM can be removed or replaced. Therefore, undesired transmission losses or reflection losses of overall projection system can be prevented, or detected at an early stage.

For example, degradation of each individual optical element of the projection system can be monitored, for example to decide, which optical element needs to be replaced or cleaned. To this aim, the lithographic apparatus can include one or more of the contamination monitoring apparatus.

In an aspect of the invention, local information can be obtained by illumination of a small part of the optical element, and measuring a resulting current. For example, as follows from the above, a scanning of the surface and a measuring of the current provides information whether the degradation of the surface is due to carbon or oxidation. In an embodiment, scanning of the surface of the optical element can be done with light of any wavelength, provided that the photon energy is large enough to provide electrons with enough energy to overcome the work function, thus leave the surface. An example: suppose that the mirror is locally contaminated, and one scans over the contaminated area with light of 30 eV. By going from a clean area of the surface to a contaminated area, the resulting current changes. When the current drops, this can mean that the surface is carbonated. When the current increases, this can indicate oxidation. This method can be applied as well during cleaning of carbon.

The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength lower than 20 nm), as well as particle beams, such as ion beams or electron beams.

Besides, various combinations of different embodiments described above and in the figures and/or the claims can be made. For example, one or more embodiments according to or similar to any ofFIGS. 2-6can be used also in combination with an embodiment according or similar to that ofFIG. 7.