CN117203488A

CN117203488A - Laser beam metrology system, laser beam system, EUV radiation source and lithographic apparatus

Info

Publication number: CN117203488A
Application number: CN202280030882.9A
Authority: CN
Inventors: W·P·贝克; K·P·阿南丹; V·P·甘古利
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2021-04-29
Filing date: 2022-03-29
Publication date: 2023-12-08

Abstract

A laser beam metrology system (500) configured to cooperate with a laser beam system configured to sequentially direct first and second laser beam pulses (430) along two independent optical paths to a target, the laser beam metrology system comprising a beam steering device (470) and a detection system (510). A laser beam system comprising a laser beam metrology system and an EUV source is also described.

Description

Laser beam metrology system, laser beam system, EUV radiation source and lithographic apparatus

Cross-reference to related applicationBy using

The present application claims priority from EP application 21171106.4 filed on month 29 of 2021 and priority from EP application 21177836.0 filed on month 4 of 2021, which are incorporated herein by reference in their entireties.

Technical Field

The present application relates to a laser beam metrology system that may be applied in a laser beam system for an EUV radiation source. The laser beam metrology system may evaluate characteristics of a plurality of laser beams used to irradiate a target, such as a target in an EUV radiation source.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). The lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of the radiation determines the smallest dimension of the features that can be formed on the substrate. A lithographic apparatus that uses Extreme Ultraviolet (EUV) radiation having a wavelength in the range of 4-20nm (e.g., 6.7nm or 13.5 nm) may be used to form smaller features on a substrate than a lithographic apparatus that may, for example, use radiation having a wavelength of 193 nm.

To obtain the desired EUV radiation, EUV lithographic apparatus use EUV radiation sources. Such a radiation source may generate pulses of EUV radiation by targeting tin droplets with laser pulses. In known radiation sources, EUV radiation is produced by irradiating a tin droplet with a first laser pulse (which may be referred to as a pre-pulse) and then irradiating the tin droplet with a second laser pulse (which may be referred to as a main pulse). In order to ensure efficient conversion of the laser pulse energy into EUV radiation, accurate alignment between the laser beam pulse and the target is required. Known systems for determining such alignment may be prone to measurement errors introduced by complex routing of the laser beam signal to the metrology system.

Disclosure of Invention

It is an object of the present invention to provide a laser beam metrology system that enables more accurate alignment of laser beam pulses to a target (such as fuel for an EUV radiation source). Thus, according to one aspect of the present invention, a laser beam metrology system is provided. The laser beam metrology system is configured to cooperate with a laser beam system configured to sequentially direct a first laser beam pulse and a second laser beam pulse along two independent optical paths to a target. The system laser beam system may be part of an EUV source and the target may be any type of fuel for producing EUV radiation, such as tin droplets. Thus, the laser beam metrology system may be used on or may be adapted for use with an EUV source. The laser beam measuring system includes:

A beam steering device configured to:

redirecting a portion of a reflection of the first laser beam pulse along a first direction, the reflection being from the target,

a portion of the second laser beam pulse is reflected along a first direction,

a detection system configured to

A reflected portion of the second laser beam is received,

receiving a redirected portion of the first laser beam, an

A relative orientation between the reflection of the first laser beam pulse and the second laser beam pulse is determined. The determination may be made based on the received beam.

According to another aspect of the present invention, there is provided a laser beam metrology system configured to cooperate with a laser beam system configured to sequentially direct first and second laser beam pulses along two independent optical paths to a target, the laser beam metrology system comprising:

a first optical element configured to:

reflecting a portion of the second laser beam pulse along a first direction;

receiving a reflection of the first laser beam pulse, the reflection being from the target;

Reflecting a reflected portion of the first laser beam pulse in a direction substantially opposite the first direction;

a retro-reflector configured to:

receiving a reflected portion of the first laser beam pulse, and

a reflected portion of the first laser beam pulse is reflected back substantially along the first direction;

wherein the first optical element is further configured to:

a reflective back-reflecting portion receiving the reflection of the first laser beam pulse, and

a portion of the reflected back-reflected portion of the first laser beam pulse is transmitted substantially along the first direction.

According to another aspect of the present invention, there is provided a laser beam system including:

a first laser beam source configured to generate a plurality of first laser beam pulses;

a second laser beam source configured to generate a plurality of second laser beam pulses;

an optical assembly configured to direct the plurality of first laser beam pulses and the plurality of second laser beam pulses to a respective plurality of targets;

a control system configured to control the first laser beam source, the second laser beam source, and the optical assembly to sequentially direct a first laser beam pulse of the plurality of first laser beam pulses and a second laser beam pulse of the plurality of second laser beam pulses to a target of the plurality of targets, and

The laser beam measuring system according to the present invention.

According to a further aspect of the invention, there is provided an EUV radiation source comprising:

a laser beam system according to the invention, and

a fuel emitter configured to produce a plurality of targets.

According to a further aspect of the invention, there is provided a lithographic system comprising:

EUV radiation source according to the invention, and

a lithographic apparatus.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic system according to the invention comprising a lithographic apparatus and a radiation source;

FIG. 2 depicts a laser beam system according to an embodiment of the invention;

fig. 3 a-3 c depict the operation of a laser beam system according to an embodiment of the present invention.

Fig. 4 and 5 schematically depict a laser beam metrology system in accordance with an embodiment of the present invention.

Fig. 6 schematically depicts an embodiment of the invention in which the first optical element and the counter reflector may be comprised in one single component.

Detailed Description

FIG. 1 depicts a lithographic system according to the invention comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO (e.g. an EUV radiation source according to the invention) is configured to generate an EUV radiation beam B and to provide the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA includes an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Thus, illumination system IL may include facet field mirror device 10 and facet pupil mirror device 11. Together, facet field mirror device 10 and facet pupil mirror device 11 provide a desired cross-sectional shape and a desired intensity distribution for EUV radiation beam B. Illumination system IL may include other mirrors or devices in addition to or in place of facet field mirror device 10 and facet pupil mirror device 11.

After such adjustment, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is produced. The projection system PS is configured to project the patterned EUV radiation beam B' onto a substrate W. To this end, the projection system PS may comprise a plurality of mirrors 13, 14, the plurality of mirrors 13, 14 being configured to project the patterned EUV radiation beam B' onto a substrate W held by the substrate table WT. The projection system PS can apply a reduction factor to the radiation beam B' of the patterned EUV, forming an image having features 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 shown in fig. 1 as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns an image formed by the patterned EUV radiation beam B' with a pattern previously formed on the substrate W.

A relatively vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, the illumination system IL and/or the projection system PS.

For example, the radiation source SO shown in FIG. 1 is of a type that may be referred to as a Laser Produced Plasma (LPP) source. The laser system 1 (which may for example comprise CO, for example) ₂ A laser) is arranged to deposit energy into a fuel, such as tin (Sn) provided by, for example, a fuel emitter 3, via a laser beam 2. The laser system 1 may for example comprise a laser beam system according to the invention, comprising a laser beam metrology system according to the invention. Although described below with reference to tin, any suitable fuel may be used. The fuel may be, for example, in liquid form, and may be, for example, a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin in the form of, for example, droplets along a trajectory towards the plasma formation region 4. Excitation deviceThe light beam 2 is incident on tin in the plasma formation region 4. The deposition of laser energy into the tin causes a tin plasma 7 to be generated in the plasma formation region 4. During de-excitation and recombination of electrons with ions of the plasma, radiation, including EUV radiation, is emitted from the plasma 7.

EUV radiation from the plasma is collected and focused by a collector 5. For example, the collector 5 includes a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may have a multilayer mirror structure arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may be of an elliptical configuration with two foci. As described below, the first focus may be at the plasma formation region 4 and the second focus may be at the intermediate focus 6.

The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may be transferred from the laser system 1 to the radiation source SO by means of a beam transfer system (not shown) comprising, for example, suitable directing mirrors and/or beam expanders, and/or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered as a radiation system.

The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present in the plasma formation region 4. The image at the intermediate focus 6 serves as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or adjacent to an opening 8 in a closed structure 9 of the radiation source SO.

Although fig. 1 depicts the radiation source SO as a Laser Produced Plasma (LPP) source, any suitable source, such as a Discharge Produced Plasma (DPP) source or a Free Electron Laser (FEL), may be used to produce EUV radiation.

According to an aspect of the present invention, a laser beam metrology system is provided. In an embodiment, the laser beam metrology system is included in a laser beam system that may be used in an EUV radiation source (such as an LPP source). For clarity, the laser beam metrology system is explained in the context of a laser beam system, but all elements associated with the laser beam metrology system may operate outside the context of the laser beam system. Fig. 2 schematically illustrates a laser beam system 200 according to an embodiment of the invention. According to the invention, the laser beam system 200 is configured to generate a plurality of first laser beam pulses 210.1 by means of a first laser beam source 210. The first laser beam pulses are sequentially generated and emitted by the first laser beam source 210. The laser beam system 200 further comprises a second laser beam source 220, said second laser beam source 220 being configured to generate a plurality of second laser beam pulses. The second laser beam pulses are sequentially generated and emitted by the second laser beam source 220.

The laser beam system 200 further comprises an optical assembly 230, the optical assembly 230 being configured to direct the plurality of first laser beam pulses and the plurality of second laser beam pulses to a respective plurality of targets. In the case of the laser beam system 200 being applied to an EUV radiation source, the plurality of targets may be, for example, a series of tin droplets 240.1, the series of tin droplets 240.1 being sequentially emitted by a fuel emitter 240, as shown in fig. 1. According to the present invention, the laser beam system 200 further comprises a control system 250, the control system 250 being configured to control the first laser beam source, the second laser beam source and the optical assembly to sequentially direct the first laser beam pulse of the plurality of first laser beam pulses 210.1 and the second laser beam pulse of the plurality of second laser beam pulses 220.1 onto a target of the plurality of targets. Thus, when an object 240.1 of the plurality of objects is irradiated by a first laser beam pulse of the plurality of first laser beam pulses 210.1, the object will be irradiated at a later time by a second laser beam pulse of the plurality of second laser beam pulses 220.1. This may be achieved, for example, by providing appropriate control signals 250.1 to the laser beam sources 210 and 220 and the optical assembly 230. The optical assembly 230 of the laser beam system 200 according to the present invention may include various components 230.11 and 230.21 (such as mirrors or lenses) to direct the laser beam pulses 210.1 and 220.1 toward a series of targets 240.1. The laser beam system 200 also includes a laser beam metrology system 260 according to the present invention. Such a laser beam metrology system will be described in more detail below.

In the illustrated embodiment, the optical assembly 230 is configured to provide a first optical path 230.1 for the plurality of first laser beam pulses 210.1 between the first laser beam source 210 and the target 240.1. One or more components 230.11 (also referred to as directing elements) may be used along the first optical path 230.1 to properly direct and shape the laser beam pulses 210.1. The optical assembly 230 is further configured to provide a second optical path 230.2 for the plurality of second laser beam pulses 220.1 between the second laser beam source 220 and the target 240.1. Various components 230.21 (also referred to as guide elements) can be used along the second optical path 230.2.

In the embodiment shown, the first optical path 230.1 and the second optical path 230.2 are separate from each other, i.e. they do not coincide. Thus, the first optical path 230.1 and the second optical path 230.2 may be referred to as separate optical paths. Thus, in an embodiment, the first optical path and the second optical path are physically at different locations or places from the laser source to the target. In such embodiments, the plurality of first laser beam pulses and the second laser beam pulses do not share any optical path from the source to the target. In the illustrated embodiment, the first laser beam pulse and the second laser beam pulse are generated by different laser beam sources 210 and 220. This facilitates the application of different pulse characteristics, such as power and/or radiation wavelength, to the plurality of first laser beam pulses and the plurality of second laser beam pulses. In an embodiment, the laser beam source may be configured to generate pulses having different radiation frequencies. As an example, the first laser source 210 may, for example, be configured to generate laser beam pulses having a radiation wavelength of about 1 μm or around 1 μm (e.g., 1028 nm), while the second laser source 220 may, for example, be configured to generate laser beam pulses having a radiation wavelength of about 10 μm (e.g., 10.6 μm). In an embodiment, the radiation wavelength of the plurality of second laser beam pulses is 5 to 25 times the radiation wavelength of the plurality of first laser beam pulses. In an embodiment, the radiation wavelength of the plurality of second laser beam pulses is 10 times the radiation wavelength of the plurality of first laser beam pulses.

In the case that the laser beam system 200 is used to generate EUV radiation by aiming the tin droplet 240.1 sequentially with the first laser beam pulse 210.1 and the second laser beam pulse 220.2, it is preferable to ensure that the laser beam pulses 210.1 and 220.2 are properly aimed (aimed) at the target 240.1 and hit the target at a predetermined location, thereby ensuring the most efficient conversion to EUV radiation. In the case of laser beam system 200 applied to an LPP EUV source, the laser beam pulses may be configured to irradiate a tin target at a plasma formation region (such as plasma formation region 4 shown in fig. 1).

Laser energy is deposited into the tin droplet (e.g., target 240.1), for example at plasma formation region 4, generating a tin plasma. Radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of electrons with ions of the plasma. It has been observed that a more efficient conversion of EUV radiation can be obtained when deposition of laser energy is performed with two or more laser pulses. The laser beam system 200 according to the invention can advantageously be applied to do so. In particular, the laser beam system 200 may be configured to illuminate the target 240.1 with a first laser beam pulse 210.1 emitted by the first laser source 210 at a first instant in time and to illuminate the target 240.1 with a second laser beam pulse 220.1 emitted by the second laser beam source 220 at a second, later instant in time. In such an arrangement, the first laser beam pulse 210.1 will cause the target 240.1 (e.g., a tin droplet) to change the shape of the droplet. During the period between irradiation of the target 240.1 by the first laser beam pulse 210.1 and irradiation of the target 240.2 by the second laser beam pulse, the target 240.1 may be deformed from a droplet (e.g., having a substantially spherical shape) to a flatter object, such as a disc or pie-shaped object, for example. During this period of time, the target (tin droplet or flat object) will also continue to propagate along the trajectory 240.2 of the target 240.1.

According to the invention, the first laser beam pulse 210.1 emitted at the first instant may for example be referred to as a pre-pulse. The second laser beam pulse 220.1 emitted at a second later instant in time may for example be referred to as the main pulse. It may also be noted that in embodiments of the present invention, the main pulse (e.g., laser beam pulse 220.1) may be transmitted before the plurality of pre-pulses are transmitted. When a target (e.g. a flat tin droplet) is irradiated by the second laser beam pulse 220.1, the target is converted into a plasma that can emit EUV radiation. In a preferred embodiment, the cross-sectional dimension of the second laser beam pulse 220 is substantially larger than the dimension of the flat tin droplet. Preferably, the trajectory of the second laser beam pulse 220.1 is such that the second laser beam pulse 220.1 overlaps the flat tin droplet.

To ensure the most efficient generation of EUV radiation, it will be appreciated that the applied laser beam pulses should irradiate the droplet or target at a preferred location. Thus, both laser pulses, i.e. the first laser beam pulse and the second laser beam pulse, should be aimed accurately at a target, e.g. target 240.1. Note that in the present invention, the target may refer to an element or feature to be irradiated by any laser beam pulse generated. Thus, in an embodiment, the target may refer to a tin droplet, for example, produced by an emitter (such as emitter 240), or may refer to a deformed droplet, for example, deformed by interaction with a previously emitted laser beam pulse. To evaluate whether a pulse is accurately aimed at its target, the laser beam system 200 according to an embodiment of the present invention further comprises a laser beam metrology system 260 according to the present invention. Using a laser beam metrology system 260 (explained in more detail below), it is assessed whether the pulse is accurately aimed at the target.

To do this, the laser beam metrology system 260 according to the present invention is configured to receive at least part of the second laser beam pulse 220.1 aimed at the target 240, but also receive at least part of the reflection of the first laser beam pulse 210.1 that has been previously emitted to the target 240.1. This is explained in more detail in fig. 3a-3 c.

Fig. 3a schematically shows the laser beam system 200 of fig. 2 at a time when the first laser beam source 210 emits a first laser beam pulse 210.1 along a first optical path 230.1 towards the target 240.1. The dashed line 230.2 represents the second optical path of the laser beam system, i.e. the optical path along which the second laser beam source 220 emits laser pulses. However, it may be noted that the first optical path 230.1 and the second optical path 230.2 do not actually meet at the same target location, as will be appreciated by those skilled in the art. In contrast, due to the movement of the target 240.1, the position of the target 240.1 illuminated by the second laser beam pulse 220.1 is located somewhat downstream of the position of the target 240.1 illuminated by the first laser beam pulse 210.1.

Fig. 3b schematically shows the effect of the laser beam pulse 210.1 on the target 240.1. Due to the laser beam pulse 210.1, reflection of the laser beam pulse 210 will occur in the various directions indicated by arrow 270. Among these reflections 270, reflections 270.1 may occur that are substantially or approximately directed along the second optical path 230.2 of the laser beam system 200. In accordance with the present invention, this reflection 270.1 of the first laser beam pulse 210.1 emitted by the first laser beam source 210 may propagate along an optical path similar to, or approximately or substantially corresponding to, the second optical path in a direction opposite to the second laser beam pulse 220.1 and may be received by the laser beam metrology system 260 arranged in said second optical path 230.2. Considering a typical displacement of the target 240.1 between the time the target 240.1 is irradiated by the first laser beam pulse 210.1 and the time the target 240.1 (i.e. the slightly deformed target) is irradiated by the second laser beam pulse 220.1, the angular difference between the reflection 270.1 of the first laser beam pulse 210.1 and the second laser beam pulse may be, for example, a few mrad.

Fig. 3c schematically shows the moment at which the second laser beam source 220 emits a second laser beam pulse 220.1 along the second optical path 230.2 towards the target 240.1. According to the invention, the second laser beam pulse 220.1 is also received by a laser beam metrology system 260 arranged in the second optical path.

In an embodiment, the laser beam metrology system 260 as schematically shown in fig. 2, 3a-3c may be configured to determine characteristics of the received pulses, in particular characteristics of the reflection 270.1 of the first laser beam pulse 210.1 and the second laser beam pulse 220.1. In an embodiment, the laser beam metrology system 260 may be configured, for example, to determine the relative orientation of two received pulses. In this regard, it may be noted that the first laser beam pulse 210.1 emitted by the first laser beam source 210 is not received by the laser beam metrology system 260 due to the use of two different optical paths 230.1 and 230.2. As will be further explained, this is advantageously applied in the design of the laser beam metrology system according to the present invention.

Fig. 4 schematically illustrates an embodiment of a laser beam metrology system 400 according to the present invention. Such a laser beam metrology system 400 may be used, for example, as the laser beam metrology system 260 in the laser beam system 200 shown in fig. 2, 3a-3 c.

The laser beam metrology system 400 according to an embodiment of the present invention may be advantageously used in a laser beam system (e.g., laser beam system 200) configured to sequentially direct a first laser beam pulse (e.g., pre-pulse (PP)) and a second laser beam pulse (e.g., main Pulse (MP)) to a target.

In the illustrated embodiment, the laser beam metrology system 400 includes a first optical element 410 disposed in an optical path 420. Referring to fig. 2, 3a-3c, the optical path 420 may be the optical path 230.2, i.e. the optical path that emits the second laser beam pulse towards the target and propagates the reflection of the first laser beam pulse, but in the opposite direction.

In the illustrated embodiment, arrow 430 indicates a second laser beam pulse propagating along optical path 420. In the illustrated embodiment, arrow 440 indicates the reflection of the first laser beam pulse propagating along optical path 420. The reflection 440 may be caused, for example, by the first laser beam pulse illuminating the target. In the illustrated embodiment, the first optical element 410 disposed in the optical path 420 is configured to:

-reflecting a portion 430.1 of the second laser beam pulse 430 in a first direction indicated by arrow 450;

-receiving a reflection of the first laser beam pulse 440, the reflection being from a target;

-reflecting the reflected portion 440.1 of the first laser beam pulse in a direction 460 substantially opposite to the first direction 450.

With respect to the reflection at the first optical element 410, it can be noted that, in general, the reflection will occur at the front and rear surfaces of the element. In an embodiment, the first optical element 410 may be designed such that only one of the reflections is used for measurement. As an example, the optical element 410 may for example be designed such that the reflection of the reflected portion 440.1 of the first laser beam pulse occurs at the rear surface of the first optical element 410 and the reflection of the portion 430.1 of the second laser beam pulse 430 also occurs at the rear surface of the first optical element. Alternatively, the two reflected portions 440.1 and 430.1 may occur at the front surface of the first optical element 410, or the two reflections may occur at different surfaces. In this regard, the back surface of the first optical element may, for example, refer to the surface 410.1 closest to the target, while the front surface of the first optical element may, for example, refer to the surface 410.2 facing the incident second laser beam pulse 430. In the embodiment shown in fig. 4, it can be seen that a portion 430.1 of the second laser beam pulse 430 is indicated as being reflected at the front surface 410.2, while a reflected portion 440.1 of the first laser beam pulse 440 is indicated as being reflected at the rear surface 410.1 of the first optical element 410.

In the illustrated embodiment, the laser beam metrology system 400 further includes a retro-reflector 460, the retro-reflector 460 configured to:

a reflected portion 440.1 receiving the first laser beam pulse 440, and

a reflected portion 440.1 of the first laser beam pulse being reflected substantially in a reverse direction of the first direction 450 as indicated by arrow 440.2.

As an example of such a retro-reflector 460, a corner cube may be used. However, other types of retro-reflectors are also contemplated. The retro-reflector 460 may, for example, comprise a glass cube, such as a glass cube made of fused silica, whereby the outer surface is coated, such as with a layer of metal or other reflective material. In the illustrated embodiment, the first optical element 410 is further configured to:

a reflected back reflection portion 440.2 receiving the reflection of the first laser beam pulse 440, and

a portion 440.3 of the reflected back-reflected portion of the first laser beam pulse is emitted (or transmitted) substantially along the first direction 450.

Thus, the laser beam metrology system 400 according to an embodiment of the present invention is configured to emit a signal 430.1 representing the second laser beam pulse emitted to the target and a signal 440.3 representing the reflection of the first laser beam pulse emitted to the target in substantially the same direction, i.e. the indicated first direction 450. The signals 430.1 and 440.3 may further propagate along substantially the same optical path by the particular optical components 410, 460 used. In practice, the path followed by the portion 440.3 of the reflected back-reflected portion of the first laser beam pulse (also referred to as signal 440.3) and the reflected portion 430.1 of the second laser beam pulse 430 (also referred to as signal 430.1) may substantially coincide. Thus, any other optical components required to direct the signals 430.1 and 440.3 may be common to both signals, thereby avoiding interference with the spatial relationship between the two signals.

It may further be noted that the laser beam metrology system 400 is thus capable of generating a signal 430.1 representing the second laser beam pulse and a signal 440.3 representing the reflection of the first laser beam pulse in a substantially aligned or collinear manner using only a few optical components. Based on these signals, the detection system may evaluate whether the laser beam pulse is aimed at the target accurately, as will be explained further. In this respect it may further be pointed out that the laser beam metrology system according to the present invention advantageously makes use of a separation of the optical paths of the first laser beam pulse and the second laser beam pulse. Because the first laser beam pulse illuminating the target reaches the target along an optical path different from optical path 420, the first laser beam pulse does not interfere with laser beam metrology system 200 or interact with laser beam metrology system 200. In particular, the first laser beam pulse does not interfere with the first optical element 410 or interact with the first optical element 410 and does not cause any reflection in the first direction.

In one embodiment, the first optical element 410 is configured to transmit at least 90% of the second laser beam pulse. Since the first optical element 410 of the laser beam metrology system 400 is arranged in the optical path 420 of the second laser beam pulse 430 (e.g., the main pulse for irradiating the target), it may be advantageous to ensure that only a small portion of the second laser beam pulse 430 is reflected. Preferably, greater than 95% or greater than 99% of the second laser beam pulse 430 is transmitted through the first optical element 410. In one embodiment, a transmission of about 99.7% is achieved.

In an embodiment, the first optical element 410 is configured to transmit 30% -70% of the first laser beam pulse (e.g., the pre-pulse for irradiating the target), preferably 40% -60%. As can be seen from fig. 4, a portion or portion 440.3 of the retro-reflective portion 440.2 of the reflection 440 of the emitted or transmitted first laser beam pulse first undergoes reflection at the first optical element 410 and, after being retro-reflected in the optical element 460, undergoes transmission through the first optical element 410. Thus, the optimum value for both transmittance and reflectance will be 50% so that 25% of the reflection 440 arrives as signal 440.3. In this case, about 50% of the reflection of the first laser beam pulse 440 will then be transmitted through the first optical element 410, said component being denoted by reference numeral 440.4. It may be noted that the characteristics of the first optical element with respect to the transmittance and reflectance of radiation having a wavelength corresponding to the wavelength of the first laser beam pulse need not be optimized. Conversely, it is more relevant that the radiation has a high transmittance of radiation having a wavelength corresponding to the wavelength of the second laser beam pulse. The second or main laser beam pulse typically has a greater power than the first laser beam pulse. Thus, it is relevant to ensure that the dissipation or absorption of the power of the second laser beam pulse is kept low to avoid adverse heating effects in the first optical element 410. Thus, the transmittance of the first optical element may also be in the range of 10% -90%, or even in the range of 1% -99%. When using a laser beam metrology system in a laser beam system of an EUV radiation source, the power of the main pulse may be quite large. The first optical element 410 should be able to withstand such loads. As an example, the first optical element may comprise a diamond substrate. The diamond substrate may for example be coated with an anti-reflective coating, for example, to achieve the high transmission required by the laser beam pulses, in particular the second laser beam pulses. In one embodiment, both sides of the diamond substrate may be coated. One side may for example be coated with a slightly different coating to obtain a non-zero reflectivity of the second laser beam pulse at one side of the first optical element.

In order to obtain desired values of the reflectivity and the transmissivity of the first optical element, the first optical element may be designed by taking into account the radiation wavelengths of the first laser beam pulse and the second laser beam pulse. In an embodiment of the invention, the first laser beam pulse may for example have a wavelength of-1 μm, while the second laser beam pulse may for example have a wavelength of-10 μm.

In one embodiment, the laser beam measuring system according to the present invention further comprises a detection system. Such a laser beam metrology system is schematically illustrated in fig. 5.

Fig. 5 schematically illustrates a laser beam metrology system 500 according to the present invention, the metrology system 500 comprising a detection system 510 in addition to the laser beam metrology system 400 illustrated in fig. 4. In the illustrated embodiment, the detection system 510 is configured to:

a reflective part 430.1 receiving the second laser beam pulse 430, and

a portion 440.3 of the reflected back-reflecting portion receiving the first laser beam,

-determining the relative orientation between the reflection of the first laser beam pulse and the second laser beam pulse.

In the illustrated embodiment, portions 430.1 and 440.3 from the second laser beam 430 and the reflected first laser beam 440, respectively, are first received by an optical element 530, the optical element 530 being configured to at least partially reflect the portions 430.1 and 440.3. The reflected portions or portions reflected from the second optical element 530 are indicated as 440.5 and 430.2, the reference numeral 440.5 indicates the portion of the portion 440.3 reflected from the element 530, and the reference numeral 430.2 indicates the portion of the portion 430.1 reflected from the element 530.

Based on the determined relative orientations or directions of the reflection of the first laser beam pulse and the second laser beam pulse, a control system of a laser beam system according to the present invention (e.g., the laser beam system 200 shown in fig. 2, 3a-3 c) may be configured to control the laser beam source 210 or 220 or the optical assembly 230. In this regard, it may be noted that the relative orientation of interest is the relative orientation between the reflection of the first laser beam pulse reflected from the target 240.1 and the direction of the second laser beam pulse toward the target 240.1. The relative orientation is maintained throughout the measurement system since both the reflection of the first laser beam pulse and the second laser beam pulse are received by the same first optical element. In other words, the relative orientation between the reflection 430.1 of the second laser beam pulse 430 and the portion 440.3 of the reflected retro-reflective portion 440.2 of the first laser beam 440 is substantially the same as the relative orientation between the reflection of the first laser beam pulse reflected from the target 240.1 and the direction of the second laser beam pulse toward the target 240.1. Based on the determined relative orientation, an evaluation of the accuracy of the aiming of the laser beam pulses at the target may be made. The determined relative orientation (which may also be referred to as the angle between the reflected chief ray of the received first laser beam pulse and the chief ray of the second laser beam pulse) thus provides feedback to the control system of the laser beam system according to the invention. The feedback may be used, for example, by a control system to control operation of the laser beam source (e.g., to control timing or orientation of the generated laser beam pulses), and/or to control operation of an optical component of the laser beam system (e.g., to control position or orientation of one or more elements of an optical component of the laser beam system (e.g., optical component 230 of laser beam system 200)). The reflection of the first laser beam pulse and the relative orientation of the second laser beam pulse may also be referred to as a beam angle offset. It is desirable to maintain this relative orientation or beam angle offset below a predetermined value to ensure proper or desired aiming of the laser beam pulse with the target. As described above, irradiation of the target 240.1 by the first laser beam pulse will result in deformation of the target, for example to a flat shape. To allow for such deformation, a specific period of time is required during which the target will move further. The distance the target moves within said time period (corresponding to the time period between the application of the first laser beam pulse and the second laser beam pulse) may be, for example, -200 μm. Based on this distance and information about the optical layout of the laser beam system, a desired angular difference or relative orientation between the second laser beam pulse and the reflection of the first laser beam pulse reflected from the target 240.1 can be determined. Typically, the required angle difference may be, for example, 1-2mrad.

In the embodiment shown in fig. 5, the detection system 510 is configured to receive the reflected portion 430.1 of the second laser beam pulse and the portion 440.3 of the reflected, retro-reflected portion of the first laser beam pulse via the common entrance window 510.1. In the illustrated embodiment, the detection system 510 is configured to:

directing a portion 440.3 of the reflected, retro-reflected portion of the first laser beam pulse to a first sensor 512 of the detection system 510, and

the reflected part 430.1 of the second laser beam pulse is directed to a second sensor 514 of the detection system 510.

It may be noted that the detection system 510 receives only part or a portion of the signals 430.1 and 440.3, as will be shown below. In other words, the intensities of the signals 430.1 and 440.3 reflected, transmitted by the first optical element 410, respectively, may be further reduced along the optical path that directs these signals to the detection system 510.

In the illustrated embodiment, the detection system 510 includes two reflectors 516, 518 to direct the received portions 430.1 and 440.3 to the first sensor 512 and the second sensor 514. As will be appreciated by those skilled in the art, the reflectors 516, 518 need to have the required reflectivity and transmissivity to enable the signals 430.1 and 440.3 to be directed toward the sensors 512, 514. It may also be noted that by changing the layout of the sensors 512, 514, it is sufficient to use only one reflector.

In an embodiment it is sufficient to use only one reflector, as long as the sensor is sufficiently sensitive to the radiation wavelength of both the first laser beam pulse and the second laser beam pulse. In an embodiment of the invention, the first laser beam pulse may for example have a wavelength of-1 μm, while the second laser beam pulse may for example have a wavelength of-10 μm. In such an embodiment, two different sensors may preferably be applied.

In the illustrated embodiment, the first sensor 512 of the detection system 510 is configured to provide a first signal 512.1 representative of the position of the portion 440.3 of the reflected, retro-reflected portion of the first laser beam pulse on the first sensor 512, and the second sensor 514 is configured to provide a second signal 514.1 representative of the position of the reflected portion 430.1 of the second laser beam pulse on the second sensor 514. The detection system 510 further comprises a processing unit 520, the processing unit 520 being configured to determine a relative orientation between the reflection of the first laser beam pulse and the second laser beam pulse based on the first signal 512.1 and the second signal 514.1. In an embodiment, the processing unit 520 may also output an output signal indicative of the relative orientation of the reflection of the first laser beam pulse and the second laser beam pulse. Such output signals may then be used by a control system of a laser beam system including a laser beam metrology system. It may be noted that the processing unit 520 of the laser beam metrology system 500 may alternatively be included in the control system of the laser beam system in which the metrology system is applied.

In the illustrated embodiment, the laser beam metrology system 500 further includes a second optical element 530, the second optical element 530 configured to:

redirecting a portion 430.2 of the reflected portion 430.1 of the second laser beam pulse 430 to the detection system 510, and

redirecting either the portion 440.5 or the portion 440.3 of the reflected back-reflected portion 440.3 of the first laser beam pulse to the detection system 510.

In the embodiment shown, the second optical element 530 is designed in such a way that a portion 430.2 of the signal 430.1, i.e. the reflected portion of the second laser beam pulse 430, is transmitted through the element 530 and finally enters the beam dump 540, where the excess energy in the signal 430.1 is absorbed. Depending on the strength of the signals 430.1 and 440.3 and the requirements of the detection system 510 (in particular the desired signal strength to the sensors 512, 514), the reflectors 516 and 518 and the second optical element 530 may be designed such that the required strength is received at the sensors. As an example, the reflector 518 may also be designed to partially transmit the portion 430.2 of the received signal 430.1 such that only a further reduced portion of the signal 430.1 reaches the sensor 514.

In the illustrated embodiment, the first optical element 410 includes a first surface 410.1 for receiving the reflection 440 of the first laser beam pulse and a second surface 410.2 for receiving the second laser beam pulse 430. In one embodiment, the surfaces are substantially parallel to each other. In an embodiment, the first surface 410.1 and the second surface 420.2 are arranged in a non-parallel manner. Thus, the first optical element 410 may be wedge-shaped. By doing so, any unwanted reflections may be discarded or filtered out. Thus, with reference to FIG. 4, by properly shaping the first optical element 410, it can be ensured that the metrology system 400 or 500 uses only the reflection that occurs at the back surface 410.1 of the first optical element 410, while filtering out the reflection that occurs at the front surface 410.2.

In the illustrated embodiment, element 580 is schematically represented as a guiding element for an optical component of a laser beam system using a laser beam metrology system. The guiding element 580 may be, for example, part of a focusing unit of a laser beam system configured to focus the second laser beam pulse 430 onto a target. In the arrangement shown, the guiding element 580 is also configured to receive a reflection of the first laser beam pulse aimed at the target and to guide the reflection along the optical path 420.

In another embodiment, the first optical element 410 and the retro-reflector 460 may be included in one component, such as the beam steering device 470 shown in FIG. 6. Thus, this embodiment is compatible with the embodiments of fig. 2-5, and all of the features described in these figures may be included in this embodiment in any combination. FIG. 6 depicts a laser beam metrology system suitable for use in an EUV source of an EUV lithographic apparatus. The laser beam metrology system is configured to cooperate with a laser beam system configured to sequentially direct the first 210.1 and second 220.1, 430 laser beam pulses to the target. In another embodiment, the first laser beam pulse and the second laser beam pulse 430 are directed along two separate optical paths to the target.

The laser beam metrology system includes a beam steering device 470. Beam steering device 470 is configured to modify the direction of a portion of the first laser beam pulse and the second laser beam pulse. For the first beam pulse, beam steering device 470 is configured to reflect a portion of the first laser beam pulse along first direction 450. For the second beam pulse, the beam steering device is configured to reflect a portion of the second laser beam pulse along the first direction 450. The laser beam metrology includes a detection system configured to

Receiving a redirected portion of the first laser beam,

receiving a reflected portion of the second laser beam,

To direct a portion of the first beam pulse into the first direction 450, the beam steering device 470 may include the first optical element 410 and the retro-reflector 460 described in previous embodiments. Thus, due to this use, the first optical element 410 and the retro-reflector 460 of the beam steering device are configured to perform the same actions as described above in fig. 2-5. For example:

the first optical element 410 is configured to

o reflects a portion 430.1 of the second laser beam pulse 430 along the first direction 450;

o receiving a reflection of the first laser beam pulse 440, the reflection being from the target;

o reflects a reflected portion 440.1 of the first laser beam pulse 440 in a direction 460 substantially opposite to the first direction 450;

o a reflected back reflection portion 440.2 receiving the reflection of the first laser beam pulse 440, and

o emits or transmits a portion 440.3 of the reflected back-reflected portion 440.1 of the first laser beam pulse 440 substantially along the first direction 450.

The retro-reflector 460 is configured to receive the reflected portion 440.1 of the first laser beam pulse 440 and retro-reflect the reflected portion 440.1 of the first laser beam pulse 440 substantially along the first direction 450 (indicated by arrow 440.2).

Accordingly, the detection system 510 is further configured to receive the reflected portion 430.1 of the second laser beam pulse, receive the portion 440.3 of the reflected, retro-reflected portion of the first laser beam pulse, and determine the relative orientation between the reflected first laser beam pulse and the second laser beam pulse.

It will be appreciated that the embodiment of fig. 6 is compatible with all of the previous figures. Thus, the above-described embodiments may include them. For simplicity, the embodiment of fig. 1-5 is not disclosed again in the context of fig. 6, but the embodiment of fig. 1-5 is partially included in combination with the embodiment of fig. 6 described above.

The laser beam system according to the invention, which comprises the laser beam metrology system according to the invention, can be advantageously applied in an EUV radiation source according to the invention. By applying the laser beam system according to the invention in an EUV radiation source, the target alignment of the laser beam pulses can be accurately assessed, so that EUV radiation can be efficiently generated.

The EUV radiation source according to the invention may advantageously be used in a lithographic system according to the invention, which system comprises an EUV radiation source according to the invention and a lithographic apparatus. In such a system, the lithographic apparatus may be configured to use the generated EUV radiation during exposure.

Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Other possible applications include the fabrication of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid Crystal Displays (LCDs), thin film magnetic heads, etc.

Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These devices may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

While specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography, and may be used in other applications, for example imprint lithography, where the context allows.

Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented by instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include Read Only Memory (ROM); random Access Memory (RAM); a magnetic storage medium; an optical memory medium; a flash memory device; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc., and that performing such operations may cause actuators or other devices to interact with the physical world.

While specific embodiments of the invention have been described above, it should be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative, and not restrictive. It will thus be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

1. A laser beam metrology system configured to cooperate with a laser beam system configured to sequentially direct first and second laser beam pulses along two independent optical paths to a target, the laser beam metrology system comprising:

a first optical element configured to:

reflecting a portion of the second laser beam pulse along a first direction;

a retro-reflector configured to:

receiving a reflected portion of the first laser beam pulse, and

wherein the first optical element is further configured to:

2. The laser beam metrology system of clause 1, further comprising a detection system configured to:

a reflection portion for receiving the second laser beam, and

a portion of the reflected back-reflected portion of the first laser beam is received,

a relative orientation between the reflection of the first laser beam pulse and the second laser beam pulse is determined.

3. The laser beam metrology system of clause 2, wherein the detection system is configured to receive the reflected portion of the second laser beam pulse and the portion of the reflected retro-reflected portion of the first laser beam pulse via a common entrance window.

4. The laser beam metrology system of clause 3, wherein the detection system is configured to:

directing a portion of the reflected, retro-reflective portion of the first laser beam pulse to a first sensor of the detection system, and

A reflected portion of the second laser beam pulse is directed to a second sensor of the detection system.

5. The laser beam metrology system of clause 4, wherein the first sensor is configured to provide a first signal representative of a position of a portion of the reflected, retro-reflected portion of the first laser beam pulse on the first sensor, and wherein the second sensor is configured to provide a second signal representative of a position of the reflected portion of the second laser beam pulse on the second sensor.

6. The laser beam metrology system of clause 5, wherein the detection system comprises a processing unit configured to determine a relative orientation between the reflection of the first laser beam pulse and the second laser beam pulse based on the first signal and the second signal.

7. The laser beam metrology system of any of the preceding clauses, wherein the first optical element is configured to transmit at least 90% of the second laser beam pulse.

8. The laser beam metrology system of any of the preceding clauses, wherein the first optical element is configured to transmit 10% -90% of the first laser beam pulses, preferably 40% -60% of the first laser beam pulses.

9. The laser beam metrology system of any of the preceding clauses, further comprising a second optical element configured to:

redirecting a portion of the reflected portion of the second laser beam pulse to the detection system, and

a portion of the reflected back-reflected portion of the first laser beam pulse is substantially redirected to the detection system.

10. The laser beam metrology system of clause 9, wherein a portion of the reflected portion of the second laser beam pulse and a portion of the reflected, retro-reflected portion of the first laser beam pulse are redirected into substantially the same direction.

11. The laser beam metrology system of any one of the preceding clauses, wherein the first optical element comprises a first surface that receives a reflection of the first laser beam pulse and a second surface that receives the second laser beam pulse.

12. A laser beam system, the laser beam system comprising:

the laser beam metrology system of any of the preceding clauses.

13. The laser beam system of clause 12, wherein the optical assembly is configured to:

providing a first optical path for the plurality of first laser beam pulses between the first laser beam source and the target, an

A second optical path for the plurality of second laser beam pulses is provided between the second laser beam source and the target.

14. The laser beam system of clause 13, wherein the first optical element of the laser beam metrology system is disposed in the second optical path.

15. The laser beam system of clause 14, wherein the optical assembly comprises a first guiding element disposed in the second optical path, the first guiding element configured to:

Directing the second laser beam pulse along the second optical path toward the target, and

directing a reflection of the first laser beam pulse substantially along the second optical path toward the first optical element.

16. The laser beam system according to any one of clauses 12 to 15, wherein the radiation wavelength of the plurality of first laser beam pulses is different from the radiation wavelength of the plurality of second laser beam pulses.

17. The laser beam system of clause 16, wherein the radiation wavelength of the plurality of second laser beam pulses is about 10 times the radiation wavelength of the plurality of first laser beam pulses.

18. An EUV radiation source, the EUV radiation source comprising:

the laser beam system according to any one of the items 12 to 17, and

a fuel emitter configured to produce a plurality of targets.

19. A lithography system, the lithography system comprising:

the EUV radiation source according to clause 18, and

a lithographic apparatus.

Claims

A beam steering device configured to:

a portion of the second laser beam pulse is reflected along a first direction,

a detection system configured to receive the reflected portion of the second laser beam,

receiving a redirected portion of the first laser beam, an

2. The laser beam metrology system of claim 1, wherein the beam steering device comprises:

a first optical element configured to:

reflecting a portion of the second laser beam pulse along the first direction;

a retro-reflector configured to:

receiving a reflected portion of the first laser beam pulse, and

Wherein the first optical element is further configured to:

a portion of the reflected back-reflected portion of the first laser beam pulse is transmitted substantially along the first direction,

wherein the detection system is further configured to receive a portion of the reflected back-reflected portion of the first laser beam.

3. The laser beam metrology system of any one of the preceding claims, wherein the detection system is configured to:

directing a redirected portion of the first laser beam or a portion of a reflected, retro-reflected portion of the first laser beam pulse to a first sensor of the detection system, and

4. The laser beam metrology system of claim 3, wherein the first sensor is configured to provide a first signal representative of a position of a redirected portion of the first laser beam on the first sensor or a position of a portion of a reflected, retro-reflected portion of the first laser beam pulse on the first sensor, and

Wherein the second sensor is configured to provide a second signal representative of the position of the reflected portion of the second laser beam pulse on the second sensor, and

wherein the detection system comprises a processing unit configured to determine a relative orientation between the reflection of the first laser beam pulse and the second laser beam pulse based on the first signal and the second signal.

5. The laser beam metrology system of any of the preceding claims, wherein the first optical element is configured to transmit at least 90% of the second laser beam pulses, and wherein the first optical element is configured to transmit 10% -90% of the first laser beam pulses, preferably 40% -60% of the first laser beam pulses.

6. The laser beam measurement system of any one of the preceding claims, further comprising a second optical element configured to:

a portion of the redirected portion of the first laser beam or the reflected, retro-reflected portion of the first laser beam pulse is substantially redirected to the detection system.

7. The laser beam metrology system of claim 6, wherein a portion of the reflected portion of the second laser beam pulse and a portion of the redirected portion of the first laser beam or the reflected, retro-reflected portion of the first laser beam pulse are redirected in substantially the same direction.

8. The laser beam metrology system of any one of the preceding claims, wherein the first optical element comprises a first surface that receives a reflection of the first laser beam pulse and a second surface that receives the second laser beam pulse.

9. A laser beam system, comprising:

The laser beam measurement system according to any one of the preceding claims.

10. The laser beam system of claim 9, wherein the optical assembly is configured to:

11. The laser beam system of claim 10, wherein the first optical element of the laser beam metrology system is disposed in the second optical path.

12. The laser beam system of claim 11, wherein the optical assembly comprises a first guiding element disposed in the second optical path, the first guiding element configured to:

13. The laser beam system according to any one of claims 9 to 12, wherein the radiation wavelength of the plurality of first laser beam pulses is different from the radiation wavelength of the plurality of second laser beam pulses.

14. The laser beam system of claim 13, wherein the radiation wavelength of the plurality of second laser beam pulses is about 10 times the radiation wavelength of the plurality of first laser beam pulses.

15. An EUV radiation source, the EUV radiation source comprising:

the laser beam system according to any one of claims 9 to 14, and

a fuel emitter configured to produce a plurality of targets.