CROSS-REFERENCE TO RELATED APPLICATIONS

This claims priority to Provisional Patent Application No. 61/237,025, entitled, "SHUTTER AND METROLOGY ASSEMBLY," filed on August 26, 2009, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

This invention relates to laser systems, and more particularly to metrology modules for laser systems.

BACKGROUND

Electric discharge gas lasers are well-known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a laser gas to produce a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many discharge gas lasers are operated in a pulse mode. Excimer lasers are a particular type of electric discharge gas laser. Embodiments of excimer lasers useful for integrated circuit lithography are described in U.S. Pat. No. 7,567,607 issued July 28, 2009 entitled "Very narrow band, two chamber, high rep-rate gas discharge laser system."

When used for integrated circuit lithography, excimer lasers are typically continuously operated in an integrated circuit fabrication line. Down-time can be very expensive. For this reason most of the components are organized into modules which can be quickly replaced.

SUMMARY

Embodiments of metrology modules for use in a laser system are disclosed. The modular nature of the metrology modules can make them easy to install and replace within the laser system. Further, the modules can be relatively compact, allowing them to be installed in small spaces within a laser system's housing. In some embodiments, the metrology modules feature optical assemblies that can withstand the rigors of extended use in an industrial environment. For example, modules can be used in high power UV laser systems, such as laser systems used in lithography tools. The modules can include optical components that degrade relatively slowly upon repeated exposure to high power UV laser radiation.

In certain embodiments, the metrology modules feature optical elements that are bulk components without thin film optical coatings (e.g., uncoated wedge-elements, uncoated plane -parallel plates, or uncoated lenses). Under certain conditions, bulk optics can withstand extended exposure to high power laser radiation better than optical coatings, which can degrade under such conditions. Thus, the use of uncoated optical elements can provide more robust metrology modules.

In embodiments, the module includes a beam pick off that derives one or more sub-beams from a main beam without causing significant degradation to the outgoing laser radiation. The sub-beams can be produced, e.g., with relatively little wavefront distortion, with similar polarization to the main beam, and in a way that they can be easily separated from secondary reflections from the optical elements (e.g., from second surfaces) in the metrology module, making the sub-beams useful for a variety metrology measurements.

In some embodiments, the metrology module can efficiently measure information about the polarization state of the main laser beam (e.g., the ratio of orthogonal polarization states). Alternatively, or additionally, in certain embodiments, metrology modules can monitor variations in the propagation direction (pointing) of the laser beam.

Various aspects of the invention(s) are summarized below.

In general, in one aspect, the invention features a laser system, including a laser; a housing including the laser and a first chamber (e.g., a purge volume) arranged to received laser radiation from the laser during operation of the laser system; a first optical element separating the first chamber from an adjacent chamber, the first optical element being arranged in a path of the laser radiation and forming a window between the first chamber and the adjacent chamber; a beamsplitter arranged in the path of the radiation either upstream or downstream from the first optical element; and a radiation detection device. During operation of the laser system, the beamsplitter receives the laser radiation, transmits a first portion of the laser radiation as a main beam and directs a second portion of the laser radiation as a first sub-beam towards the radiation detection device.

Embodiments of the laser can include one or more of the following features and/or features of other aspects. For example, the radiation can have a wavelength less than 300 nm (e.g., a wavelength of 248 nm or 193 nm). The laser can be an excimer laser. The laser can include an argon fluoride lasing gas medium.

The beamsplitter and the radiation detection device can be arranged within the first chamber. The second portion of laser radiation forming the first sub-beam can be reflected from a surface of the beamsplitter.

The first optical element can be a second beamsplitter which during operation directs a third portion of the laser radiation as a second sub-beam along a path different from a path of the main beam. The laser system can include a second radiation detection device different from the first radiation detection device, the second radiation detection device being positioned to receive the second sub-beam.

The optical element can be a wedge-shaped element. The optical element can compensate for a deflection or offset of the main beam due to the beamsplitter so that a path of the main beam after interacting with the optical element is parallel to a path of the radiation incident on the beamsplitter.

The beamsplitter can direct a third portion of the radiation as a second sub-beam in a direction different from the direction of the first sub-beam. The beamsplitter can include a first surface and a second surface non-parallel to the first surface, where the first surface derives the first sub-beam from the radiation and the second surface derives a second sub-beam from the radiation.

The beamsplitter can be a wedge-shaped element. In some embodiments, the beamsplitter and the optical element are both wedge-shaped elements. The wedge- shaped elements can both have a first optical surface, where the first optical surface of the beamsplitter is parallel to the first optical surface of the optical element. The laser radiation can be incident on the first surface of the beamsplitter and a main beam of the laser radiation exits through the first optical surface of the optical element. Both the beamsplitter and the optical element can have second optical surfaces that face each other and are parallel to each other.

The wedge-shaped elements can both have the same wedge angle. The portions of the wedge-shaped elements in the path of the laser radiation can include uncoated surfaces (e.g., the surfaces can be free of optical coatings, such as anti-reflection coatings). One or both of the beamsplitter and the first optical element can include uncoated optical surfaces.

The first optical element can be arranged so that radiation is incident on at least one surface of the first optical element at Brewster's angle.

The chamber can be sealed from the adjacent chamber.

In general, in a further aspect, the invention features assemblies that include a chamber comprising a first wall and a second wall opposite the first wall, the first wall comprising a first aperture and the second wall comprising a second aperture; a first beam splitter arranged within the chamber in a path between the first and second apertures; an optical element positioned at the second aperture; and a radiation detection device. The beamsplitter is positioned to received radiation entering the chamber along a path through the first aperture and to direct a portion of the radiation as a main beam towards the additional optical element, and to direct a second portion of the laser radiation as a first sub-beam towards the radiation detection device.

Embodiments of the assembly can include one or more of the following features and/or features of other aspects.

In general, in another aspect, the invention features apparatus for measuring information about the polarization of a laser beam, the apparatus including a chamber; a first beamsplitter arranged in the chamber, wherein during operation the first beamsplitter receives radiation from a laser, directs a portion of the radiation as a main beam along a first beam path, and directs a second portion of the radiation as a first sub-beam along a second beam path. The apparatus further includes a radiation detection device; a second beamsplitter arranged to receive the first sub-beam, to derive a second sub-beam from the first sub-beam, and direct the second sub-beam to the radiation detection device; and an optical device arranged in the path of the first sub-beam between the first and second beamsplitters, the optical device being configured to alter a polarization state of the first sub-beam.

Embodiments of the apparatus can include one or more of the following features and/or features of other aspects. For example, the first and second beamsplitters can each have a reflective surface, wherein during operation the reflective surface of the first beamsplitter reflects the second portion of the radiation along the second beam path and the reflective surface of the second beamsplitter reflects the reflects a portion of the radiation of the first sub-beam to form the second sub-beam, and the reflective surfaces and optical device are arranged so that the second sub-beam has light with substantially the same mixture of orthogonal polarizations as the main beam.

The first and second beamsplitters can be arranged so that the second sub-beam has radiation with substantially the same mixture of orthogonal linear polarizations as the main beam.

The first and second beamsplitters can be arranged so that the second sub-beam has radiation with a transmittance for two orthogonal linear polarization directions that are identical to the main beam.

A path to the radiation detection device can be free of diattenuation.

The first and second beamsplitters can each have a reflective surface and during operation the reflective surface of the first beamsplitter reflects the second portion of the radiation along the second beam path and the reflective surface of the second

beamsplitter reflects a portion of the radiation of the first sub-beam to form the second sub-beam, the reflective surfaces being arranged so that, in the absence of the optical device in the path of the second sub-beam, the second sub-beam has radiation of substantially the same mixture of orthogonal polarizations as the main beam.

The optical device can be a polarization rotator. The polarization rotator can be configured to rotate the polarization state of the first sub-beam by 90°.

The optical device can include an optical element formed from a birefringent material. The optical device can be a retarder (e.g., a half-wave plate).

In some embodiments, the optical device includes an optical element formed from an optically-active material. The optical device can be a wedge-shaped element. The optically-active material can be crystalline quartz. The radiation detection device can detect a power or an energy of the second sub- beam.

The apparatus can include a third beamsplitter and a second radiation detection device, the third beamsplitter being positioned in the path of the first sub-beam or in a path of a transmitted sub-beam from the second beamsplitter and being arranged so that during operation the third beamsplitter derives a third sub-beam from the first or transmitted sub-beams and directs the third sub-beam to the second radiation detection device, the third beamsplitter can be arranged in the path of the first sub-beam between the first and second beamsplitters. The third beamsplitter can be arranged in the path of the first sub-beam between the first beamsplitter and the optical device. The first, second and third beamsplitters and the optical device can be arranged so that the second and third sub-beams have substantially the same polarization state as the main beam with respect to a coordinate system defined by the path of the respective sub-beams. The first, second and third beamsplitters and the optical device can be arranged so that polarization state in the third sub-beam is substantially rotated by 90° from the polarization state in the second sub-beam with respect to a coordinate system defined by the path of the respective sub- beams. The first, second and third beamsplitters can be arranged so that the second and third sub-beams have substantially the same polarization state as the main beam in the absence of the optical device in the path of the first sub-beam. The first, second and third beamsplitters and the optical device can be arranged so that so that the polarization state in the third sub-beam is substantially rotated by 90° from the polarization state in the second sub-beam in the absence of the optical device in the path of the first sub-beam.

In general, in a further aspect, the invention features apparatus for measuring information about the polarization of a laser beam, the apparatus including a first beamsplitter; a second beamsplitter; and a radiation detection device. During operation the first beamsplitter receives radiation from a laser, directs a portion of the radiation as a main beam along a first beam path, and directs a second portion of the radiation as a first sub-beam along a second beam path; the second beamsplitter is arranged to receive the first sub-beam, derives a second sub-beam from the first sub-beam, and directs the second sub-beam to the radiation detection device; and the first and second beamsplitter are configured so that a plane of incidence of the laser beam with respect to the first beamsplitter is non-parallel to a plane of incidence of the laser beam with respect to the second beamsplitter.

Embodiments of the apparatus can include one or more of the following features and/or features of other aspects. For example, the apparatus can include an optical device arranged in the path of the first sub-beam between the first and second beamsplitters, the optical device being configured to alter a polarization state of the first sub-beam.

In some embodiments, the apparatus includes a third beamsplitter and a second radiation detection device, wherein the third beamsplitter is positioned in a path of a transmitted beam of the second beamsplitter, during operation the third beamsplitter derives a third sub-beam from the transmitted beam to the second radiation detection device, and the third beamsplitter is configured so that a plane of incidence of the laser beam with respect to the third beamsplitter is parallel to the plane of incidence of the laser beam with respect to the first beamsplitter. The third sub-beam can have light of a substantially different mixture of orthogonal polarization states as the second sub-beam. The apparatus can include an optical device configured to alter the polarization state of the third sub-beam. The third sub-beam and the second sub-beam can be used to calibrated the first and second radiation detection devices.

In general, in another aspect, the invention features methods for monitoring information about a polarization state of a laser beam, the method including dividing laser radiation from a laser into a main beam and a sub-beam; making a first

measurement of the power or energy of the sub-beam; making a second measurement of the power or energy of the sub-beam, wherein making the second measurement includes altering the polarization state of the sub-beam; and determining information about the polarization of the laser beam based on the first and second measurements, wherein determining the information comprises calculating a ratio of or difference between the power or energy of the first and second measurements.

Implementations of the method can include one or more of the following features and/or features of other aspects. For example, rotating the polarization state of the sub- beam can include rotating the polarization state of the sub-beam by 90°.

In general, in another aspect, the invention features methods for monitoring information about a polarization state of a laser beam, the methods including deriving a first sub-beam and a second sub-beam from the laser beam; making a first measurement of the power or energy of the first sub-beam using a first radiation detection device; making a second measurement of the power or energy of the second sub-beam using a second radiation detection device different from the first radiation detection device; and determining information about the state of polarization of the laser beam based on the first and second measurements, wherein determining information about the state of polarization comprises calculating a ratio of or difference between the power or energy of the first and second measurements.

Implementations of the method can include one or more of the following features and/or features of other aspects. For example, the first and second measurements can be performed simultaneously. The polarization state in the second sub-beam can be rotated by 90° from the polarization state in the first sub-beam.

In general, in a further aspect, the invention features methods for monitoring information about a polarization state of a laser beam, the method including deriving a first sub-beam and a second sub-beam from the laser beam; making first measurements of the power or energy of the first and second sub-beams using a first radiation detection device and a second radiation detection device, respectively; making second

measurements of the power or energy of the first and second sub-beams using the first radiation detection device and the second radiation detection device, respectively, wherein making either the first measurement or the second measurement includes rotating the polarization state of the first sub-beam; determining a calibration factor based on the first measurements, wherein determining the calibration factor comprises calculating a ratio or difference between the first measurement of the energy of the first sub-beam and the first measurement of the energy of the second sub-beam; and determining the information about the polarization state of the laser beam based on the second measurements and the calibration factor, wherein determining the information about the polarization state comprises calculating a ratio or a difference between the second measurements of the energies of the first and second sub-beams.

Implementations of the method can include one or more of the following features and/or features of other aspects. For example, the first measurements using the first and second sub-beams can be performed simultaneously. Alternatively, or additionally, the second measurements using the first and second sub-beams can be performed simultaneously.

The calibration factor can be determined when the first sub-beam and the second sub-beam have substantially the same polarization state as the main beam.

The polarization state in the second sub-beam can be rotated by 90° from the polarization state in the first sub-beam.

In general, in another aspect, the invention features apparatus for monitoring variations in a propagation direction of a laser beam, the apparatus including a beamsplitter arranged to derive a sub-beam from the laser beam; a sensor which during operation monitors variations in a location of radiation on the sensor; an optical element arranged in a path of the sub-beam between the beamsplitter and the radiation sensor, the optical element focusing the sub-beam onto the sensor; and a light conversion plate arranged between the optical element and the sensor. During operation the light conversion plate absorbs radiation of the sub-beam at a first wavelength and emits radiation at a second wavelength different from the first wavelength, the radiation at the second wavelength being detectable by the sensor.

Embodiments of the apparatus can include one or more of the following features and/or features of other aspects. For example, the optical element can be a lens or a mirror. The sensor can be a position sensitive diode or a quadrant photodiode. The sensor can include a detection surface having a maximum dimension and the light conversion plate can have a plate thickness less than the maximum dimension of the detection surface. The plate thickness can be less than half the maximum dimension of the detection surface (e.g., less than quarter the maximum dimension of the detection surface). The light conversion plate can have a maximum dimension larger than the maximum dimension of the detection surface plus twice the plate thickness.

The apparatus can include a substrate supporting the light conversion plate, the substrate being positioned between the fluorescent plate and the sensor, wherein during operation at least some radiation incident on the light conversion plate is coupled into the substrate and is wave guided to an edge of the substrate.

In another aspect, the invention features systems that include a laser and the apparatus the foregoing aspect, wherein during operation of the system the laser provides the laser beam from which the beamsplitter derives the sub-beam. The laser can be an excimer laser. The laser beam can have a wavelength of 248 nm or 193 nm.

The system can include an electronic processing device (e.g., control electronics) in communication with the sensor, the electronic processing device being programmed to receive information from the sensor and output information about variations in the direction of the laser beam.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic showing various components of an embodiment of a laser system.

FIG. 2 shows an embodiment of a laser beam pick-off assembly and a shutter assembly.

FIG. 3 shows an embodiment a laser beam pick-off assembly and a shutter assembly.

FIG. 4 shows an embodiment of a setup for polarization-neutral energy measurement.

FIG. 5 shows a plot of the ratio of reflectance of p-polarized light and s-polarized light as a function of incidence angle.

FIG. 6 shows an embodiment of a setup for polarization-neutral energy measurement of the beam exiting the laser.

FIG. 7A shows an embodiment of a setup for polarization measurement of the beam exiting the laser.

FIG. 7B is a flow chart for polarization measurement using the apparatus shown in

FIG. 7A.

FIG. 8 shows a graph of relative signal at energy detector as a function of polarization ratio.

FIG. 9 shows an embodiment of a setup for simultaneous polarization-neutral energy measurement and polarization measurement. FIG. 1 OA shows an embodiment of a setup for calibration the setup shown in FIG. 9

FIG. 1OB is a flow chart for a method of using the system of FIG. 1 OA

FIG. 1OC is a flow chart for another method of using the system of FIG. 1OA. FIG. 11 shows an embodiment of a beam pointing measuring assembly.

FIG. 12 shows a plot of calculated beam intensity at a diode.

FIG. 13 shows an embodiment of a metrology module with two separate sub- modules.

FIG. 14 shows an embodiment of the metrology module of FIG. 13.

FIG. 15 shows an embodiment of a setup for two dimension imaging of the laser beam.

FIG. 16A shows an embodiment of a detector assembly.

FIG. 16B shows another embodiment of a detector assembly

FIG. 17 shows an image measured by a CCD camera.

FIG. 18 shows an embodiment of an assembly for speckle contrast measurement.

FIG. 19 shows a folding scheme for the measurement assembly of FIG. 18.

FIG. 20 shows a camera layout for various measurements.

FIG. 21 shows an embodiment of a rotational feedthrough using shaft seal.

FIG. 22 shows an embodiment of a manual drive.

FIG. 23 A shows an embodiment of a pneumatically driven linear actuator.

FIG. 23B is a close-up view of the pneumatically driven linear actuator of FIG. 23A.

FIG. 24 shows an embodiment of a microlithography apparatus. Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, a laser system 100 includes a housing 101 enclosing a laser module 110, a metrology module 120, an electronics module 130, and a gas supply 140. Laser module 110 and metrology module 120 are further enclosed within a purge volume 151, which is connected to gas supply 140. Gas supply 140 and purge 150 provide a controlled atmosphere in which laser module and metrology module can operate.

Electronics module 120 is in communication with laser module 110 and metrology module 120 and controls the operation of the system. During operation of laser system

100, laser module 110 generates laser radiation that is directed into metrology module 120. Metrology module 120 measures various properties of the laser radiation before the radiation exits system 100 as an output beam 160.

Laser module 110 includes an optical cavity, a gain medium, a pump (e.g., an optical pump) and associated laser optics. In some embodiments, laser module 110 is an excimer laser module.

Metrology module 120 includes various optical elements and one or more radiation detectors for characterizing radiation received from laser module 110. Various embodiments of optical assemblies that can be included in metrology module 120 are described below.

Electronics module 130 includes control electronics and a power supply. Output from the radiation detectors can be interfaced with electronics 130 that in turn controls an electrical signal sent to laser module 110 in order to influence or modify the operation of the laser cavity.

Gas supply 140 includes gas handling apparatus and supplies purging gas to the purge volume 151. In some embodiments, the purge volume corresponds to the portions of laser system 100 through which the laser radiation propagates. Gas supply 140 supplies this volume with a gas suitable for the propagation of the radiation, e.g., a gas that is inert with respect to the radiation. In certain embodiments, the purge volume can be purged with nitrogen.

While purge volume 151 is depicted in FIG. 1 as a single volume, in general, the structure of the purge volume can vary. For example, purge volume 151 can include multiple chambers, each sealed with respect to the other. In general, purge volume 151 provides a controlled atmosphere in which the laser radiation can propagate before exiting laser system 100.

Furthermore, while laser module 110 and metrology module 120 are depicted as being contained entirely within purge volume 151 , in general, components of either of these modules can be housed outside of the purge volume. For example, in some embodiments, detectors in metrology module 120 are placed outside of purge volume 151. Such an arrangement may involve placing the input face of the detector in close proximity to the purge volume. This arrangement may have the advantage that only the input face of the detector needs to be sealed within the purge volume. The exterior of the detector can be placed outside of the purge volume so that the use of electrical feedthroughs into the purge volume can be minimized. In alternative embodiments, instead of one signal purge volume for the laser module and metrology module, multiple purge volumes may be partitioned with each of these volumes having its respective housing and sealing.

In general, the wavelength of output beam 160 can vary depending on the type of laser module used. In some embodiments, laser system 100 emits radiation with a wavelength of less than 300 nm. For example, laser system 100 may be an excimer laser that produces an output radiation having a wavelength of 248 nm or 193 nm. Laser module 110 may include a lasing gas medium such as molecular argon fluoride to produce output radiation at 193 nm.

As discussed previously, in general, a variety of different assemblies can be used in metrology module 120. In some embodiments, metrology module 120 includes a pick- off assembly to derive one of more sub-beams from laser radiation. An embodiment of a pick-off assembly is shown in FIG. 2. Here, a main beam pick-off assembly 200 includes a first beamsplitter 210 and a second beamsplitter 220. Pick-off assembly 200 also includes a shutter mirror 230 and a power sensor 240.

Both beamsplitters 210 and 220 are wedge-shaped elements and each of the beamsplitter surfaces in the path of a main beam 201 entering the metrology module pick off a respective sub-beam. The sub-beams are labeled 202, 203, 204 and 205 in FIG. 2.

Main beam 201 exiting laser module 110 is incident on a beamsplitter 210 and a portion of the main beam is picked off as a sub-beam 202 for metrology. One or more sub-beams (e.g., sub-beams 202, 203) can be generated by beamsplitter 210. The portion of main beam 201 that is transmitted through beamsplitter 210 is then incident on a beamsplitter 220. One or more sub-beams (e.g., sub-beams 204, 205) can be generated by reflecting from the beamsplitter. The portion of main beam 201 that is transmitted through beamsplitter 220 can then be directed towards a shutter mirror 230 that is selectively inserted or retracted from the beam path of main beam 201. When shutter mirror 230 is retracted from the beam path of main beam 201, the portion of main beam 201 that is transmitted through beamsplitter 220 is incident on power sensor 240. Power sensor 240 can be of the type described in U.S. 7,567,607, for example, the contents of which is herein incorporated in its entirety. When power sensor 240 is retracted from the path of main beam 201, main beam 201 exits metrology module 220 and is directed for use in further applications downstream of laser system 100.

Due to the orientation of the surfaces of beamsplitter 210 with respect to main beam 201 , and the refraction that occurs at those surfaces, main beam 201 is spatially offset and deflected with respect to its initial propagation direction once transmitted by beamsplitter 210. A second wedge-shaped beamsplitter (i.e., 220) can correct the beam offset and deflection of the main beam introduced by the first wedge-shaped beamsplitter (i.e., 210). In some embodiments, it is advantageous for an entrance surface 212 of wedge-shaped beamsplitter 210 and an exit surface 224 of beamsplitter 220 to be parallel (or nearly parallel) .

Alternatively, or additionally, an exit surface 214 of beamsplitter 210 and an entrance surface 222 of wedge-shaped beamsplitter 220 can also be parallel (or nearly parallel). In such a configuration, beam deflections, chromatic effects and beam magnification or demagnification due to its symmetry will be relatively small, if present at all. The thicknesses of the wedge-shaped beamsplitters and the distance between them may be chosen to reduce beam offset by the wedge-shaped beamsplitters. Beam offset can be zero in some embodiments, or it can be set to any other desired value.

In some embodiments, none of the optically-active surfaces of either beamsplitter 210 or 220 include an optical coating.

Wedge-shaped beamsplitters may be advantageous over thin, substantially plane- parallel beamsplitters because they can allow for relatively easy separation of the beams reflected from different surfaces of the beamsplitter, enabling easy isolation of a single sub-beam for metrology purposes. For example, the reflection of an incident beam from the first (entrance) surface and the second (exit) surface of a thin plane -parallel beamsplitter may be too spaced and propagate along parallel paths, making it difficult to isolate one sub-beam from the other. The use of wedge-shaped beamsplitters without the use of a coated surface may also be advantageous over beamsplitters that utilize optical coatings (e.g., an anti- reflective (AR) coating to reduce unwanted second surface reflections from the beamsplitter) since the uncoated surface can be arranged to naturally have relatively low reflectivity (e.g., in the range of about 1%) and does not suffer the degradation experienced by optical coatings when exposed to laser radiation (e.g., in continuous high power exposure).

Although beam-splitters 210 and 220 are wedge-shaped in the assembly 200, in general, other types of beamsplitter can also be used. For example, plane -parallel plate beamsplitters (e.g., with or without optical coatings) can be used.

As mentioned previously, the two wedge-shaped beamsplitters shown in FIG. 2 deliver altogether four separate reflected sub-beams. In general, any number of these sub-beams can be used for metrology. The beamsplitters can be designed so that each beam has different properties. For example, different beams can contain different relative powers or polarization properties, depending on how the system will use the beam. For example, in some embodiments, two of the sub-beams can be used for metrology and two can simply be discarded (e.g., directed to a beam stop). Accordingly, the beamsplitters can be configured so that the two discarded sub-beams contain very little or no energy depending on the degree of polarization of the laser.

In some embodiments, a surface of a beamsplitter can be arranged at Brewster's angle with respect to the incident beam. This arrangement can reduce (e.g., minimize) transmission loss at that surface. Brewster's angle Θ _B can be computed by Θ _B = tan ^" (nt/rii) where n _t is the refractive index of the transmitting medium and U ₁ is the refractive index of the medium where the incident beam is in. At Brewster's angle, only a component of incident light polarized normal to the incident plane (s-polarization) will be reflected, while all light with a component polarized parallel to the incident plane (p- polarization) will be transmitted at the interface.

In some embodiments, inner surfaces 214 and 224 of the wedge-shaped beamsplitters are both arranged at Brewster's angle with respect to the beam incident on those surfaces in order to minimize transmission losses in main beam 201. For example, if the laser beam shown in FIG. 2 is assumed to be polarized mainly parallel to the plane of the paper, then as a result of the Brewster's angle arrangement, sub-beams 203 and 204 may contain very little or no energy. In certain embodiments, however, the optical assembly can be arranged to generate four reflected beams each with sufficient energy to supply different radiation detectors or other instruments.

Consider, for example, wedge-shaped beamsplitters with a refractive index of

1.50, (as is the case for a beamsplitter made of calcium fluoride at a laser radiation wavelength of 193 nm), when the outer surface 212 is oriented at an angle of 45° with respect to the beam path of the main beam 201 (to provide a 90° reflection for sub-beam 202 that is directed to radiation detector), Brewster's condition at the inner surface 214 of the wedge-shaped beamsplitter is obtained if the beamsplitter has a wedge angle of

5.569°. The reflectivity for the two different orthogonal linear polarizations at each of the wedge-shaped surfaces are as follows:

outer surfaces (e.g., 212, 224) at 45° with respect to main beam 201 : 0.85% (p) / 9.24% (s)

inner surfaces (e.g., 214, 222) at Brewster's angle: 0.00% (p) / 14.86% (s)

Thus, for this configuration, the total transmission loss for the main beam due to main beam pick-off assembly 200 is thus 1.7% for p-polarization and 40.3% for s- polarization. If the laser beam is nearly fully p-polarized, the loss is < 2%. Thus, due to the difference in transmittance between the s and p polarizations, main beam pick-off assembly 200 also helps to clean up polarization of main beam 201.

Alternatively, if light is incident on surfaces 212 and 224 at an angle different from 45°, the wedge angle can be adjusted accordingly to match the Brewster's angle at the inner surfaces.

In some embodiments, beamsplitter 220 is integrated with housing partitions 225 as depicted in FIG. 2. Housing partitions 225 can be used to define a purge volume. In the present embodiment, beamsplitter 210 is located in the purge volume defined on the left hand side of housing partitions 225 while a shutter mirror 230 maybe be located in a different purge volume defined on the right hand side of housing partitions 225. In some embodiments, beamsplitter 220 may serve as the exit window of metrology module 120 and in some cases, as the exit window of laser system 100. In some embodiments, housing partitions 225 include indium films that contact beamsplitter 220. Depending on the sealing schemes used at housing partition 225, mechanical stress may be introduced on beamsplitter 220. Mechanical stress on beamsplitter 220 can introduce wavefront deformations into sub-beams 204 and 205. Accordingly, these sub-beams may be unsuitable for measurements that require low wavefront distortion (e.g., beam profile measurements). As a result, in some

embodiments, sub-beams 204 and/or 205 are used for metrology measurements that are relatively insensitive to wavefront deformations, such as, e.g., for spectral or energy measurements.

Beamsplitter 210, on the other hand, can be mounted within assembly 200 in a way that is relatively stress-free compared to beamsplitter 220 since it is not integrated with a housing partition . Accordingly, in certain embodiments, sub-beams 202 and/or 203 are used for wavefront sensitive measurements such as imaging.

In general, a variety of properties of main beam 201 can be measured via sub- beams 202-205. For example, metrology module 120 can be used to measure one or more of beam profile, beam position, beam divergence, beam pointing, wavefront, pulse duration, pulse energy, polarization properties, spectrum, bandwidth, wavelength, speckle contrast, lateral or temporal coherence length of the main beam.

While beamsplitter 210 is positioned upstream of beamsplitter 220 in the embodiment depicted in FIG. 2, alternative arrangements are possible. For example, in some embodiments, beamsplitter 210 is positioned downstream from beamsplitter 220. In other words, beam 201 is incident on beamsplitter 220 prior to beamsplitter 210.

Shutter mirror 230 controls output of beam 201 from laser system 100 and is coupled to an actuator (e.g., an electro -mechanical actuator) that optionally inserts the mirror into the path of main beam 201 . When shutter mirror 230 is inserted into the path of main beam 201 , there is no (or negligibly little) radiation output exiting laser system 100. Instead, the main beam is directed from shutter mirror 230 to a beam dump (e.g., a water-cooled beam dump).

In some embodiments, shutter mirror 230 can be arranged to direct the main beam back through the beamsplitter 220 to a beam dump located at a suitable location next to the main beam path. Such a configuration may help to alleviate space constraints. In certain embodiments, shutter mirror 230 may replace one of the beamsplitters when the shutter mirror is inserted into a beam path of main beam 201 , such that no additional space is required by shutter mirror 230.

In certain embodiments, a prism shutter can be used instead of a mirror. For example, referring to FIG. 3, a right-angled prism 330 is used as a shutter. When shutter prism 330 is not inserted into the beam path of a main beam 301, beamsplitters 310 and 320 generate one or sub-beams in the manner described above and direct the sub-beams to one or more radiation detectors. Shutter prism 330 is inserted into the beam path of main beam 301 between beamsplitter 310 and beamsplitter 320 to prevent main beam 301 from exiting laser system 100. Main beam 301 enters the input face of shutter prism 330 and undergoes total internal reflection at the hypotenuse surface of the prism, exiting shutter prism 330 substantially normal to the initial propagation path of main beam 301 and is then directed towards a beam dump.

The optical surfaces of prism 330 may be uncoated surfaces. An uncoated surface may withstand greater exposure to laser radiation compared to a coated surface, since the laser radiation may cause degradation of the coating. For example, excimer laser radiation of less than 300 nm can degrade multi-layer coatings used to form dielectric mirrors. Accordingly, an uncoated prism shutter may have an enhanced lifetime compared to a coated shutter mirror. In addition, the arrangement shown in FIG. 3 may also allow less space to be used to direct the main beam to a beam dump in cases when no light should exit laser system 100.

As discussed above, the energy of the sub-beams picked off from beamsplitter 210 is usually sensitive to the polarization state of main beam 210 if uncoated surfaces in beamsplitter 210 are used. In order to obtain an energy measurement which does not depend on the polarization state of the laser, the metrology assembly may be arranged so that the polarization state of the beam at an energy measurement apparatus substantially matches the polarization state of the main beam. When the polarization state of two beams match, the measured intensity of light in two orthogonal polarization directions in the two beams is substantially identical. These intensities can be obtained by measuring the amount of light transmitted after a polarizer aligned to each of the two orthogonal polarization directions is placed in the beam. The polarization state of a beam can also be described as the mixture of orthogonal polarizations contained within the beam. For example, the percentage of s-polarized light and the percentage of p-polarized light contained in the beam with respect to the plane of incidence of the surface from which the beam last reflected off are the same in both beams when two beams have the same polarization state. In other words, the two beams are free of diattenuation, which is the differential attenuation of light amplitudes polarized parallel and perpendicular to two orthogonal polarization directions (eigenpolarizations). The idea of a matching polarization state can also be expressed in terms of Stokes parameters. When the polarization state of the sub-beam is the same as the main beam, the ratio of normalized Stokes vector components S ₁ ZS ₀ and S2ZS0 in both the sub-beam and the main beam are identical.

In some embodiments, the two orthogonal polarization directions are two orthogonal linear polarizations.

In order to accurately measure the energy of the main beam, a polarization-neutral pick-off scheme that produces a sub-beam having the same polarization state as the main beam may be used. For example, beam-splitting surfaces that use two Fresnel reflections from uncoated surfaces that have their respective planes of incidence oriented in a non- parallel fashion with respect to each other can provide polarization-neutral pick-off. A plane of incidence of a surface is the plane containing both the surface normal and the propagation vector of the radiation incident on the surface.

Referring to FIG. 4, a polarization-neutral pick-off can be achieved using beamsplitters 410 and 420 where a main beam 401 strikes beamsplitter 410 at an incidence angle of 45°. In order to illustrate how a polarization-neutral pick-off is obtained, consider, for example, that main beam 401 has an equal polarization mixture of p-polarized light and s-polarized light (50% each), where the p- and s- polarized light are defined with respect to the plane of incidence of light at beamsplitter 410. Due to the incidence angle (45°) of main beam 401 at beam splitter 410, R _p of the p-polarized light and R _s of the s-polarized light in main beam 401 are reflected off beamsplitter 410 as sub- beam 402 and is directed towards beamsplitter 420. R _p and R _s are the percentage reflectivity of p- and s- polarized light at a surface. Beamsplitter 420 is oriented such that its plane of incidence is non-parallel with respect to the plane of incidence of beamsplitter 410. Beamsplitter 420 is similarly oriented at 45° with respect to sub-beam 402. As a result of the orientation of beamsplitter 420, R _p of the p-polarized light contained in sub- beam 402 picked off from main beam 401 is now s-polarized with respect to the plane of incidence of beamsplitter 420 andi? _^ of this s-polarized light is reflected off beamsplitter 420 as sub-beam 403 to be directed towards an energy measurement apparatus 430.

After reflecting off both beamsplitters 410 and 420, an overall R (R _p x R _s ) of the p-polarized light contained in main beam 401, defined with respect to beamsplitter 410, is directed to energy measuring apparatus 430. Similarly, the s-polarized light defined with respect to beamsplitter 410 becomes p-polarized light with respect to beamsplitter 420 due to the orientation of beamsplitter 420. Beamsplitter 420 therefore reflects and directs R _p of this light towards energy measurement apparatus 430. After reflecting off both beamsplitters 410 and 420, R (R _s x R _p ) of the s-polarized light contained in main beam 401, defined with respect to beamsplitter 410, is directed to energy measuring apparatus 430, a percentage that is identical to that for p-polarized light.

The mixture of light having orthogonal polarization directions contained in sub- beam 403 is identical to that contained in main beam 410 as a result of the optical configuration shown in FIG. 4. For example, when both beamsplitters are oriented at 45° with respect to each incoming beam, R _p is 0.085%, R _s is 9.24%, andi? is 0.0785%. In contrast, sub-beam 402 which is generated from one reflected off beamsplitter 410 has a ratio of s-polarized to p-polarized light of 10.8 (9.24% / 0.85%).

As a further example, consider a main beam A and a main beam B both having the same energy, but with different mixtures of s- and p- polarized light as follows:

The last column in Table 1 indicates the percentage of main beam energy contained within a sub-beam after a single reflection off a beamsplitter. Thus, two beams having the same energy but with different polarization states would give a different energy reading at energy measurement apparatus 430 if the energy measurement picked- off is not made in a polarization-neutral way. This example illustrates the importance of polarization-neutral energy measurements for the main beam.

Identical incident angles of 45° at both beamsplitters 410 and 420 are one of many configurations possible for use in the setup shown in FIG. 4. For example, the setup can be configured for other identical incident angles (besides 45°) at beamsplitters 410 and 420. Alternatively, the setup can be configured for different incident angles at beamsplitters 410 and 420. For example, one beamsplitter can be arranged to provide a surface at an incidence angle above Brewster's angle and one below as long as the reflectance ratio for p-polarized light and s-polarized light (p/s ratio) is the same for both incident angles.

FIG. 5 shows the p/s reflectivity as a function of the incident angle. In addition, a few options for incident angles are listed in Table 2. In general, reflectance of p- polarized light can be calculated using the equation:

_R _ tan ² (θ _; -ΘJ

p tan ² (θ, +θ,)

and the reflectance of s-polarized light can be calculated by:

_R _ sin ² (θ _; -θ,)

5 sin ² (θ, +θ,)

where θ _t is the angle made between the surface normal and the transmitted light in both equations.

A pair of incident angles that is different from two 45° reflections off

beamsplitters 410 and 420 allows the energy of sub-beam 403 to be adjusted to a level higher or lower than that obtained from two 45° reflections. In this way, the energy of the sub-beam 403 can be tailored to be within the measurement range of energy measurement apparatus 430.

The configuration shown in FIG. 4 enables a polarization-neutral energy measurement with respect to the mixture of polarization states contained in main beam 401 at the energy measurement apparatus 430. However, this configuration is not necessarily polarization-neutral with respect to an outgoing beam 404. A polarization- neutral energy measurement with respect to outgoing beam 404 should take into account changes in the polarization state experienced by main beam 401. Such changes may occur when main beam 401 is transmitted through beamsplitter 410 as outgoing beam 404. In some embodiments, further optics between the main beam pick-off assembly and the laser exit may cause additional changes in the polarization state of main beam 401 as it is transmitted through as outgoing beam 404. Furthermore, changes in the polarization state caused by reflection off additional surfaces may need to be considered if, for example, a reflection off a rear surface is used for directing outgoing beam 404 to the laser exit. As the sub-beams picked off from main beam 401 are sent to energy detectors for laser energy measurements, the polarization state of the beam at each energy measurement apparatus preferably matches the polarization state of the beam at the laser exit.

For example, the polarization state of outgoing beam 404 transmitted through a wedge-shaped beamsplitter with the rear surface of the beamsplitter arranged at

Brewster's angle, and the front surface of the beamsplitter arranged at 45° with respect to incoming main beam 401 can be calculated as follows. The p-polarized component of main beam 401 undergoes a 45° reflection at the front surface, and 99.15% of p-polarized light is transmitted through the front surface. The p-polarized light is fully transmitted at the rear surface, which is arranged at the Brewster's angle. As a result, 99.15% of p- polarized light contained in incoming main beam 404 is present in outgoing beam 404. On the other hand, 90.76% of the s-polarized light contained in main beam 401 is transmitted through the front surface, and 86.14% of the s-polarized light that is transmitted at the rear surface oriented at Brewster's angle. As a result, 78.18% of s- polarized light contained in incoming main beam 404 is present in outgoing beam 404. Thus, the polarization state of outgoing beam 404 is different from the polarization of incoming main beam 404 as a result of the reflections and transmissions at the wedge- shaped beamsplitter.

In some embodiments, the pick-off assembly is used to produce a sub-beam having a polarization state that is matched to the polarization state of the outgoing beam exiting the laser. Such a sub-beam can then be directed to an energy measurement apparatus for making polarization-neutral measurements with respect to the outgoing beam. For example, referring to FIG. 6, a polarization-neutral pick-off assembly 600 includes a beamsplitter 610 and a beamsplitter 620. Beamsplitter 610 is a wedge-shaped beamsplitter having its front surface oriented at 45° to an incoming main beam 601 to produce a sub-beam 602, similar to the embodiment shown in FIG. 4 However, in contrast to the embodiment shown in FIG. 4, instead of orienting beamsplitter 620 at 45° with respect to sub-beam 602, beamsplitter 620 is adjusted to produce a sub-beam 603 at energy measurement apparatus 630 that exactly matches the polarization state of transmitted main beam 604. In some embodiments, beamsplitter 620 is oriented at an incident angle of 66.05° with respect to sub-beam 602 to produce sub-beam 603 that matches the polarization state of transmitted main beam 604 at the laser exit.

In general, for two plane -parallel beamsplitters in which the first is arranged at an angle of 45° with respect to the beam propagation direction, the tilt angle of the second beamsplitter is determined based on which reflections are used on each of the beamsplitters. As an example, consider the following two possible sets of configurations, both of which yield a sub-beam having the same s/p transmittance ratio at the energy measurement apparatus as that present in transmitted main beam 604 (the listed numerical ratios are the transmittance ratios s/p with respect to the first beamsplitter):

transmitted main beam 604: 0.838 a) first beamsplitter front surface at 45° 10.83

second beamsplitter front surface at 45.98° 0.0774

total to energy measurement: 0.838 b) first beamsplitter both surfaces at 45° 9.96

second beamsplitter both surfaces at 46.01° 0.0841 total to energy measurement 0.838

In example a), the second surface of the first beamsplitter is configured at Brewster's angle, such that the ratio 10.83 is obtained in the beam which undergoes just a single reflection. On the other hand, the second surface of first beamsplitter in b) also contributes to the reflected beam due to the small separation between the first and the second surfaces. That is, in example b), the ratio 9.96 is obtain after (i) reflection of main beam at the first surface of the first splitter, (ii) reflection at the second surface of the first splitter, and (iii) transmission at the first surface of the first splitter. Similar

considerations apply if the tilt angle of the first surface is different from 45°, if the plate is not plane -parallel, or if tilt angles on opposite sides of Brewster's angle are chosen,

(e.g., combinations as shown in FIG. 5 can be selected). In general, the first and second beamsplitter can each be adjusted to provide the desired incidence angle.

In addition to the configurations for polarization-neutral energy measurements with respect to the incoming beam described in FIG 4 and the polarization-neutral energy measurements with respect to the outgoing beam described in FIG. 6, embodiments that provide polarization measurements of the laser beam are shown in FIG. 7 A. Referring to FIG. 7A, an assembly 700, includes a polarization rotator 730 in addition to the components shown in FIG. 6 and described previously. Polarization rotator 730 allows the metrology module to combine energy measurements described above with a polarization measurement. Polarization rotator 730 can be inserted between beamsplitters 610 and 620, in the beam path of sub-beam 602 during polarization measurements.

Beamsplitters 610 and 620 are both oriented in the same fashion as shown in FIG. 6 when used for polarization-neutral energy measurements. Polarization rotator 730 alters the mixture of orthogonally polarized light in sub-beam 602, producing a sub-beam 603 having a different mixture of orthogonally polarized light. As a result, the mixture of orthogonally polarized light in sub-beam 603 which is directed to energy measurement apparatus 630 is no longer polarization-neutral with respect to the mixture of

orthogonally polarized light in main beam 601. Instead, energy measurement apparatus 630 will record a signal that depends on the mixture of orthogonally polarized light of main beam 601, which in turns provides a polarization measurement of main beam 601. To illustrate the effect of the wave plate on the mixture of orthogonally polarized light measured at the energy measurement apparatus, consider a polarization rotator that rotates the polarization of sub-beam 602 by 90°. Further consider the component of light polarized parallel to the plane of incidence of beamsplitter 610 (i.e., p-polarized light) directed into sub-beam 602 from main beam 601. This p-polarized light component becomes perpendicular to the plane of incidence of beamsplitter 610 after passing through rotator 730, which rotates the p-polarized light by 90°. In other words, the p- polarized light becomes s-polarized. As described above, the plane of incidence of beamsplitter 620 is non-parallel to the plane of incidence of beamsplitter 610. As a result, the s-polarized light produced by the wave plate when incident on beamsplitter is now p- polarized with respect to the plane of incidence of beamsplitter 620. Therefore, the p- polarized component of main beam 601 undergoes reflections at beamsplitters 610 and 620 both as p-polarized light, relative to the respective beamsplitters 610 and 620, giving rise to a sub-beam 603 that is not polarization-neutral. In contrast, when rotator 730 is retracted from the path of sub-beam 602, the p-polarized light (with respect to

beamsplitter 610) undergoes a reflection at beamsplitter 620 as a s-polarized beam with respect to the plane of incidence of beamsplitter 620. In this case, the p-polarized component of main beam 601 undergoes reflections at beamsplitters 610 and 620 as p- polarized, and s-polarized light, respectively. The lower reflectance of p-polarized light at beamsplitter 610 is compensated by the higher reflectance of the same component of light that is now s-polarized with respect to beamsplitter 620 beam. Sub-beam 603 is therefore polarization-neutral. The same analysis can be applied for the s-polarized component of main beam 601 in an analogous fashion.

In some embodiments, polarization rotator 703 can be a birefringent wave plate (λ/2 -plate) having a fast axis oriented at 45° to a plane of oscillation of either the s- or p- light polarized with respect to beamsplitter 610. Furthermore, in order to avoid etalon- like interference effect in the wave plate, polarization rotator 703 can be a slightly wedged wave plate.

In certain embodiments, polarization rotator 703 may be formed from an optically active material, such as crystalline quartz. When a linearly polarized light ray traverses an optically active crystal (such as a quartz crystal) along an optical axis, its state of polarization remains the same but the orientation of the plane of oscillation of the electrical field vector rotates by an angle that depends on the distance traveled. The optical axis of a crystal, also referred to as axis of isotropy, is defined by the property that there is only one velocity of light propagation associated with the direction of the optical axis. In some embodiments, the thickness of the optically active crystal is chosen to rotate the polarization of sub-beam 602 by an odd multiple of 90°. By way of example, the optical activity of crystalline quartz at a wavelength of 193 nm is 324°/mm. The thickness of the polarization rotation made of crystalline quartz may thus be an odd multiple of 0.278 mm, which can be conveniently processed during manufacturing. In addition, a polarization rotator based on optical activity in crystalline quartz is sufficiently insensitive to the thickness of the rotator plate such that it can be slightly wedged without significantly affecting the amount of polarization rotation of the transmitted beam.

In some embodiments, polarization rotator 730 can be inserted into the beam path of sub-beam 602 manually. Alternatively, polarization rotator 730 can be introduced into the beam path using an actuation mechanism (e.g., an electro-mechanical or pneumatic actuator), described below.

The sensitivity of the polarization measurement depends on the polarization imbalance in sub-beam 603 when polarization rotator 730 is inserted in the beam path of sub-beam 602, which in turn depends on the incident angles at beamsplitters 610 and 620 and the polarization rotation imparted by polarization rotator 730.

FIG. 8 illustrates how the mixture of polarization states in main beam 601 can be determined based on a measurement having polarization rotator 730 inserted into the beam path of sub-beam 602 and a measurement having polarization rotator 730 retracted from sub-beam 602. Referring to FIG. 8, lines 810 and 820 shows a plot of relative signal at an energy measuring apparatus 630 as a function of the polarization ratio at the laser exit, with and without a polarization rotation, respectively. A polarization ratio of 1.00 indicates that the beam is fully p-polarized. Line 810 is the relative signal when that polarization rotator 730 shown in FIG. 7 A is not inserted into the beam path of sub-beam 602, while line 820 is the relative signal measured when rotator 730 is inserted into the beam. When polarization rotator 730 is inserted into the beam path of sub-beam 602 to rotate the beam polarization by 90°, it causes an imbalance in the energy measurement such that the s-component of the beam (with respect to the first splitter) is 127 times stronger than the p-component. As a result, the s-component of the beam can be directly measured. The overall transmission is also high enough that small deviations from full p- polarization can be measured, as shown in FIG. 8. A change in polarization ratio from

100% to 97% (a decrease in the amount of p-polarized light) corresponds to a signal rise from 9% to 26% at the detector when polarization rotator 730 is inserted. Polarization state of the main beam can therefore be measured by assembly 700.

In general, a polarization measurement can be made using assembly 700 using the following steps:

1. Measure the energy of sub-beam 603 directed to energy measurement apparatus 630 without polarization rotator 730 in the beam path of sub-beam 602.

2. Insert polarization rotator 730 in the beam path of sub-beam 602.

3. Measure the energy of sub-beam 603 directed to energy measurement apparatus 630 with polarization rotator 730 in the beam path of sub-beam 602.

4. Determine the energy ratio of the signal recorded at energy measurement apparatus 630 with and without polarization rotator 730

5. Calculate the mixture of the polarization states in main beam 601 based on the differences in the relative signal measured at the energy measuring apparatus 630 when polarization rotator 730 is inserted or removed from the beam path and compare the measurement results again a calibration graph like that shown in FIG. 8 to determine the polarization ratio in main beam 601.

FIG. 7B is a flow chart showing the method described above.

In the preceding discussion, embodiments that measure either the polarization- neutral energy of the main beam, or the polarization state of the main beam are described. In some embodiments, however both of these beam properties can be measured at the same time. For example, referring to FIG. 9, a metrology system 900 includes a main beamsplitter 910, a second beamsplitter 920, an energy measurement apparatus 930, a third beamsplitter 940 and a polarization measurement apparatus 950. The portion of metrology system 900 that includes beamsplitters 910 and 920 is similarly configured as shown in FIG. 6 for polarization-neutral energy measurements at energy measurement apparatus 930 of an outgoing beam 904. Metrology system 900 uses beamsplitter 940 to direct a sub-beam 905 that is transmitted from beamsplitter 920 for polarization measurements at polarization measurement apparatus 950. Beamsplitter 940 is oriented with its plane of incidence parallel to the plane of incidence of main beamsplitter 910 but non-parallel to the plane of incidence of beamsplitter 902.

As described above in FIG. 7 A, introducing rotator 730 into the beam path of 602 may disrupt an initially polarization-neutral measurement of sub-beam 603. In some embodiments, beamsplitter 940 can function similarly to polarization rotator 730 and be used to produce sub-beam 906, which has a different mixture of polarization states measured at the polarization measurement compared to the mixture of polarization states in sub-beam 903 measured at energy measurement apparatus.

To illustrate how such a measurement can be made, consider the component of light polarized parallel to the plane of incidence of beamsplitter 920 (p-polarized light). In the absence of a polarization rotator in the beam path of sub-beam 902, this p- polarized component of light is the s-polarized light with respect to the plane of incidence of beamsplitter 910 as reflected from main beam 901. The p-polarized component of sub- beam 905 that is transmitted through beamsplitter 920, is incident on beamsplitter 940 as s-polarized light relative to the plane of incidence of beamsplitter 940. This s-polarized light that is reflected off beamsplitter 940 is also the s-polarized component of main beam 901. As described above, because the s-polarized component of sub-beam 906 is the result of two reflections that are both s-polarized light relative to each of beamsplitter 910 and beamsplitter 940, light sent to polarization measurement apparatus 950 is not polarization-neutral, and therefore, information about the polarization state of main beam 901 can be obtained at polarization measurement device 950. The same analysis can be applied for the p-polarized component of main beam 901 in an analogous fashion.

In other words, as a result of the arrangement of metrology system 900, sub-beam 906 contains a similar mixture of orthogonally polarized light as would be obtained in sub-beam 903 should a 90° rotator be inserted in the beam path of sub-beam 902. As a result, in contrast to the setups shown in FIGS. 6 and 7 with insertable polarization rotator 730 where polarization-neutral energy measurements and polarization

measurements have to be made sequentially, both polarization-neutral energy measurements and polarization measurements can be made simultaneously using metrology system 900.

In general, two different sub-beams from two different beamsplitters off the main beam can be used for this type of simultaneous measurements of polarization-neutral energy and polarization state of the main beam.

In addition to making simultaneous measurements of the polarization-neutral energy and polarization state of the main beam using two measurement devices (e.g., energy measurement apparatus 930 and polarization measurement apparatus 940), in certain embodiments, a calibration between the two measurement devices is possible. Referring to FIG. 1OA, a metrology system 1000 contains an additional insertable polarization rotator 1060 in the path of sub-beam 905 compared to metrology system 900. The other elements of metrology system 1000 are identical to those shown in metrology system 900 of FIG. 9. Metrology system 1000 can be used to further calibrate the polarization measurement apparatus 940 with the energy measurement apparatus 930.

The method of using a polarization rotator to disrupt an initially polarization- neutral measurement of a sub-beam by inserting the rotator into the sub-beam has been illustrated in reference to FIG. 7A. In metrology system 1000, however, polarization rotator 1060 performs the converse function by rotating the polarization of sub-beam 905 to make sub-beam 906 polarization-neutral. Consider the case of a polarization rotator 1060 rotating the polarization of the s-polarized component (relative to beamsplitter 920) of sub-beam 905 by 90°. This s-polarized component can be attributed to the p-polarized component of light (when viewed relative to the plane of incidence at beamsplitter 910) in main beam 901. The s-polarized component light in sub-beam 905 relative to beamsplitter 920 will become p-polarized relative to the same plane of incidence of 920 after transmitting through polarization rotator 1060. This p-polarized light will be reflected as s-polarized light off beamsplitter 940, relative to the plane of incidence of beamsplitter 940. In other words, sub-beam 906 is the result of two reflections off beamsplitters 910 and 940. Considering the s-polarized component of light contained in sub-beam 906 relative to beamsplitter 940, these two reflections involves first a p- polarized light relative to beamsplitter 910 and s-polarized light beamsplitter 940. By selecting two suitable incident angles at each of beamsplitters 910 and 940, sub-beam 906 can be made polarization-neutral. As described earlier, the two incident angles, one at beamsplitter 910 and one at beamsplitter 940 need not be identical, different angles above and below Brewster's angle can be chosen as along as the reflectance ratio of s-polarized light to p-polarized light (s/p) are the same for the two different angles.

As a result of polarization 1060 in the beam path of sub-beam 905, both sub- beams 903 and 905 are polarization-neutral. The signals recorded on energy measurement apparatus 930 and polarization measurement apparatus 940 can then be calibrated relative to each other to give a polarization measurement calibration. Such a calibration is preferable to alternatives that involves calibrating polarization measurement apparatus 940 with an external polarization meter. Calibrations may be necessary due to the degradation of photodiodes (even for UV-hardened/ enhanced photodiodes) caused by laser radiation at 193 nm. In some embodiments, photodiodes are recalibrated after about 1 billion laser shots. Separately, power sensor 240 as described in reference to FIG. 2 can be used to calibrate both energy measurement apparatus 930 and polarization

measurement apparatus 940.

Making polarization measurements and calibrations using metrology system 1000 involves the following steps:

1. Insert polarization rotator 1070 in beam path of sub-beam 905.

2. Measure the signal recorded at energy measurement apparatus 930 and the signal recorded at polarization measurement apparatus 940.

3. Calculate the energy ratio measured at energy measurement apparatus 930 and polarization measurement apparatus 940.

4. Determine a calibration factor between the two measurement apparatus based on the energy ratios measured in Step 3.

5. Retract polarization rotator 1070 from the beam path of sub-beam 905.

6. Measure the signal recorded at energy measurement apparatus 930 and the signal recorded at polarization measurement apparatus 940.

7. Calculate the energy ratio or difference measured between energy measurement apparatus 930 and polarization measurement apparatus 940.

8. Calculate the mixture of the polarization states in main beam 901 based on the differences in the relative signal measured at the polarization measurement apparatus 940 when polarization rotator 1060 is inserted or removed from the beam path and compare the measurement results again a calibration graph like that shown in FIG. 8 to determine the polarization ratio in main beam 901 , and the calibration factor determined in Step 4.

FIG. 1 OB shows the method described above in a flow chart form.

In some embodiments, sub-beam 906 can be configured to be polarization-neutral in the absence of polarization rotator 1060 in the beam path of sub-beam 905. In such embodiments, calibrations can be made directly without inserting polarization rotator 1060 into the beam path. Instead, polarization rotator 1060 is inserted into the beam path of sub-beam 905 when polarization measurements are made.

Making polarization measurements and calibrations using the system described above involves the following steps:

1. Retract polarization rotator 1070 from the beam path of sub-beam 905.

2. Measure the signal recorded at energy measurement apparatus 930 and the signal recorded at polarization measurement apparatus 940.

3. Calculate the energy ratio measured at energy measurement apparatus 930 and polarization measurement apparatus 940.

4. Determine a calibration factor between the two measurement apparatus based on the energy ratio measured in Step 3.

5. Insert polarization rotator 1070 from the beam path of sub-beam 905.

6. Measure the signal recorded at energy measurement apparatus 930 and the signal recorded at polarization measurement apparatus 940.

7. Calculate the energy ratio or difference measured between energy measurement apparatus 930 and polarization measurement apparatus 940.

8. Calculate the mixture of the polarization states in main beam 901 based on the differences in the relative signal measured at the polarization measurement apparatus 940 when polarization rotator 1060 is inserted or removed from the beam path and compare the measurement results again a calibration graph like that shown in FIG. 8 to determine the polarization ratio in main beam 901 , and the calibration factor determined in Step 4.

FIG. 1OC is a flow chart showing the method described above.

While foregoing embodiments describe various methods of obtaining

polarization-neutral energy measurements and polarization measurements, metrology module 120 can also be used to measure other characteristics of the laser beam exiting laser module 110. For example, the propagation direction of a laser beam can be measured using pointing measurements described in the following embodiment. An exemplary embodiment of an apparatus for monitoring the propagation direction of the laser beam is shown in FIG. 11. Here, a pointing measurement apparatus 1000 includes lens 1110, a photodiode 1120, a light conversion plate 1130, and a housing 1140.

Apparatus 1000 monitors the propagation direction of the laser beam as follows. Lens 1110 focuses laser beam 1101 (e.g., a sub-beam of the main beam of laser system 100) onto a light conversion plate 1130. The light conversion plate receives laser light and converts the received radiation to radiation at different (e.g., longer) wavelengths. In some embodiments, light conversion plate 1130 converts incident ultraviolet (UV) light to longer wavelength visible light. For example, the visible light may have a wavelength between 500 nm to 600 nm. In some embodiments, 193 nm radiation may be converted to visible light have a wavelength of about 550 nm. The light conversion can be performed by, e.g., phosphorescence or fluorescence.

As an example, in response to the incident laser light, light conversion plate may fluoresce where the fluorescent light is transmitted towards photodiode 1120, which is positioned within a gap in housing 1140. The fluorescence may then be detected by photodiode 1120.

Photodiode 1120 can be a position sensitive detector (PSD) based on lateral resistivity in a photodiode (e.g., a commercially-available PSD, such as from

Hamamatsu). The combination of lens 1110 and PSD enables a fast readout of the signal as it automatically measures the center-of-mass of the beam spot incident on the photodiode and the signal is independent of the actual beam divergence. PSD calculates the position of the beam spot based on the differences of photo currents from 4 electrodes along the edges. PSDs offer a resolution in the range of 1 μm when a low-noise amplifying circuit is used. In some embodiments, lens 1110 can have a focal length of approximately 0.5 m for pointing measurements with a resolution of 20 μrad or less.

In some embodiments, converging laser beam 1101 propagates in a purge volume, and light conversion plate 1130 seals the purge volume against housing 1140. In some embodiments, the photodiode is placed outside of the purge volume and behind a light conversion plate in order to minimize contamination risks and lifetime issues.

The spatial distribution of the signal measured on a PSD can be calculated, for example, as depicted by the plot shown in FIG. 12. This plot shows the calculated intensity spot on a PSD placed behind a light conversion plate with the x- and y-scale in mm. Placing the PSD surface 2 mm away from the light conversion plate (converter) increases the spot size detected at the PSD to approximately 4 mm, providing good averaging of signal on the PSD surface. This enlarged spot can also be measured using a quadrant photodiode (QPD) instead of a PSD. Light conversion plate 1130 preferably has a plate thickness that is less than for example, a quarter of a maximum dimension of the PSD, in order to keep the fluorescent light spot size small on the PSD.

In addition to the embodiments described above, pointing measurements can be made by imaging the beam spot from the fluorescent plate to the diode, the diode can be coated with a fluorescence converter, or a UV-hardened photodiode can be used without a light converting plate .

Stray light background on PSD can reduce sensitivity of the detector because background signal does not change when the beam spot moves on the PSD. Possible stray light sources include stray light from inside the metrology module and stray light generated inside fluorescent plate . To reduce the stray light background, in some embodiments, a maximum dimension of the fluorescent plate is larger than the sum of twice the thickness of the fluorescent plate and a maximum dimension of the PSD. By maintaining a small air gap between light conversion plate 1130 and the PSD, stray light experiences total internal reflection in the light conversion plate and is trapped within the light conversion plate and prevented from reaching the PSD. Increasing the diameter of light conversion plate 1130 and indium seal around housing 1150 also helps to ensure that stray light does not reach PSD. In addition, a layer of glue or an absorbing layer can be applied to light conversion plate 1130.

Several other measurements are described below to determine other pertinent characteristics of the laser beam exiting laser module 110. For example, FIG. 13 shows a shutter and metrology assembly 1300 that includes a metrology module 1310 and a core module 1320. Assembly 1300 is divided in at least two parts: core module 1310 includes main beam optics, shutter mirror or shutter prism as shown in FIGS. 2 and 3, and metrology module 1310 the includes metrology optics and detectors. In some

embodiments, metrology module 1310 and core module 1320 are two separately purged volumes separated by a window which transmits the beam from the a first beamsplitter in path of the main beam path to metrology module 1310.

In further embodiments, several measurement devices are housed in the same metrology module. For example, referring to FIG. 14, metrology module 1320 includes a CCD camera 1330, and metrology systems for beam pointing, speckle contrast and 2D imaging of near and far field of the laser. In some embodiments, the beam is attenuated to a specified level within core module 1320 so that the minimum amount of light enters metrology module 1320, helping to reduce stray light. A separate metrology module is easy to replace, repair or upgrade without having to remove the full module that includes core optics and components, so there is no impact on the main beam. A separate metrology module may also be easier to transport and install if it is separated from the core module. Within metrology module 1320, three measurement paths are generated by reflections at a front side and a rear side of a wedge and by a beam passing through the wedge. In addition, other splitting / distribution schemes are possible.

Two dimensional imaging of the laser beam provides a method for characterizing a beam quality of the laser beam exiting laser module 110. For example, referring to FIG. 15, two dimensional (2D) imaging assembly 1500 includes a convex lens 1510, a concave lens 1520 and a 2D camera 1530 are used to image the laser beam. 2D imaging of near and far field can be done with the very compact imaging assembly 1500. Near field imaging is done by transmission of a measurement sub-beam through a convex and concave lens for imaging (e.g. with magnification 0.3) and far field imaging is done by transmission of the measurement sub-beam through a convex lens and a double reflection inside the concave lens (e.g. with a focal length of 400 mm). Imaging assembly 1500 is described in detail in DE 10 2006 018 804 Al, the contents of which is herein incorporated in its entirety.

In general, metrology detectors can be arranged either inside a purge volume or outside a purge volume of the metrology module. For example, referring to FIG. 16A, a detector assembly 1600 is housed within a purge volume. Detector assembly 1600 includes an internal CCD camera 1610 in a metrology volume 1650 that is purged.

Internal CCD camera 1610 can be, for example, a phosphor-coated CCD camera.

In contrast, referring to FIG. 16B, a detector assembly 1605 includes a camera 1620 that is external of a purge volume 1660. Here, detector assembly 1605 includes an an objective 1630, and a fluorescent plate 1640 in addition to camera 1620. Fluorescent plate 1640 may be made of a doped glass or a crystal and forms a window for radiation to exit the purge volume. Detector assembly 1605 may have the advantage that no electronics are inside purge volume 1660.

In some embodiments, a single camera can be used to make one or more distinct measurement of the laser beam. For example, FIG. 17 shows an image 1700 captured by a single camera. Image 1700 includes a near field (profile) image 1710 and a far field (divergence) image 1720 of an excimer laser beam.

In some embodiments, speckle contrast of the laser beam can be measured. FIG. 18 shows a speckle measurement assembly 1800 that includes a spinning diffuser 1810, a focusing lens 1820, a convex mirror 1830 and a camera 1840. The spinning diffuser 1810 allows the measurement to be independent of camera inhomogeneity. The focal length of focusing lens 1820 has to be long enough to resolve speckle grains with camera pixels. In some embodiments, focusing lens 1820 has a focal length of approximately 1 meter for a camera with a pixel size of 6.4 μm. Atelephoto arrangement includes a convex lens and a convex mirror setup. In some embodiments, a telephoto arrangement is used and may help to save space.

In some embodiments, additional features can be added to compensate for space limitations in some metrology modules. For example, FIG. 19 shows additional folding mirrors 1910 for reducing the space used to allow speckle measurement assembly 1920 to fit within the metrology module.

Speckle measurement assembly 1800 can share camera 1330 with 2D imaging to save cost, as shown in FIG. 14. FIG. 20 shows a schematic layout of a profile

measurement 2010, a divergence measurement 2020 and a speckle measurement 2030 on a camera 1330. The 2D imaging and speckle measurement beam can be placed next to each other on camera 1330 (or a fluorescent plate) using a split mirror, a beamsplitter or other optical elements. As illustrated in the foregoing embodiments, shutter mirror 230, shutter prism 330, polarization rotator 730, polarization rotator 1300, and other optics or radiation detection devices are temporarily inserted into a beam path of a main beam or a sub- beam. These elements can be actuated manually, by an electrical DC motor, pneumatic cylinder or other mechanisms. When a motor or pneumatic cylinder are used to actuate the elements, the motor or pneumatic cylinder can be placed outside of the purge volume to avoid contamination of the purge volume. A mechanical feedthrough is then used to connect the electrically or pneumatically actuator to move the element (e.g., shutter mirror 230, polarization rotator 730, power sensor 140 etc. ) inside the purge volume.

As an example, with reference to FIG. 21 and FIG. 22, a positioning assembly

2100 includes a rotational shaft seal 2110, a lever arm 2120, an rotational actuator 2130, and an element 2140 (which may be an optical component, such as a mirror) to be moved. Rotational shaft seal 2110 is used to seal a mechanical feedthrough. Alternatively, vacuum sealed feed through methods can be applied, such as magnetic coupling devices. Actuator 2130 can be driven manually or it can be driven, e.g., pneumatically or by an electric DC motor. Actuator 2130 can perform rotations with defined angles, e.g., 45° or 90° movement with steps that are integrated in the actuator. Actuator 2130 rotates lever arm 2120 such that element 2140 is moved.

A linear actuator can be used to drive optical or mechanical parts, e.g. the shutter mirror or the power meter. The linear actuators can be driven, e.g. pneumatically or magnetically. Referring to FIG. 23 A, an actuator device 2300 includes a supply line 2130 for gas (e.g., nitrogen), a socket 2320, a bellow 2330, a pneumatic cylinder 2340, and a mount 2350 for housing an actuated element 2360 (e.g., an optical element, such as a mirror). Bellow 2310 seals pneumatic cylinder 2340 which is used to drive actuated element 2360 up and down. Bellow 2310 should be able to withstand many cycles of operation. Bellow 2310 separates a purge volume within which element 2140 is mounted from the actuator located outside the purge volume. A magnetic coupling device can be used for both rotational and linear actuations.

In general, laser systems (e.g., laser system 100)can be used in a variety of applications. In some embodiments, laser systems are used as a source in a lithography exposure apparatus. Referring to FIG. 24, for example, laser system 100 is the source of a lithography apparatus 10. The projection exposure apparatus 10 includes an

illumination system 12 for generating projection light 13, which comprises a light source 14, illumination optics indicated at 16 and an aperture 18. In the embodiment illustrated the projection light 13 has a wavelength λ of 193 nm. The projection exposure apparatus 10 also includes a projection lens 20 containing a multiplicity of lenses, only some of which are indicated as examples in FIG. 24 for reasons of clarity, and which are denoted by Ll to L5.

The projection lens 20 serves to image a mask 24 arranged in an object plane 22 of the projection lens 20 on a reduced scale on a photosensitive layer 26. The layer 26, which may consist, for example, of a photoresist, is arranged in an image plane 28 of the projection lens 20 and is applied to a carrier 30.

The carrier 30 is fixed to the bottom of a basin-like, upwardly open container 32 which is movable parallel to the image plane 28 (in a manner not illustrated in detail) by means of a traversing device. The container 32 is filled with an immersion liquid 34 to a level at which the last lens L5 of the projection lens 20 on the image side is immersed in the immersion liquid 34 during operation of the projection exposure apparatus 10. Instead of a lens, the last optical element of the projection lens 20 on the image side may be, for example, a plane -parallel terminal plate. The refractive index of the immersion liquid 34 approximately coincides with the refractive index of the photosensitive layer 26. In the case of projection light having a wavelength of 193 nm or 248 nm, high-purity deionized water, for example, is possible as the immersion liquid 34.

The container 32 is connected via an inlet pipe 36 and an outlet pipe 38 to a conditioning unit 40 in which elements including a circulation pump and a filter for cleaning the immersion liquid 34 are contained. The conditioning unit 40, the inlet pipe 36, the outlet pipe 38 and the container 32 together form an immersion device designated 42 in which the immersion liquid 34 circulates while being cleaned and maintained at a constant temperature. The absolute temperature of the immersion liquid 34 should be set as accurately as possible since imaging by the projection lens 20 can be impaired by focusing errors and image shell defects in the case of deviations from the reference temperature. Such imaging defects may in turn lead to a reduction in size of the process window available for an exposure. Other uses of laser system 100 include annealing of flat-panel components, surgical uses, such as eye surgeries, and for dermatological treatments, for example.

Other embodiments are in the claims.