Abstract: Diffraction gratings for a phase-stepping measurement system for determining an aberration map for a projection system are disclosed. The gratings are two-dimensional diffraction gratings for use as wafer level gratings in an EUV lithographic apparatus. In particular, the diffraction gratings include a substrate provided with a two-dimensional array of circular through-apertures and are self-supporting. In some embodiments, a ratio of the radius of the circular apertures to the distance between the centers of adjacent apertures may be selected to minimize the gain and cross-talk errors of a wavefront reconstruction algorithm. For example, the ratio may be between 0.34 and 0.38. In some embodiments, the circular apertures are distributed such that a distance between the centers of adjacent apertures is non-uniform and varies across the diffraction grating. For example, a local pitch of the grating may vary randomly across the diffraction grating.

Abstract: A plate for use in an imaging system to determine at least two optical properties of an illumination beam of the imaging system, the imaging system configured to illuminate an illumination region with the illumination beam, the plate including a plurality of markers, wherein a first subset of the plurality of markers includes a first type of markers for determining a first optical property of the illumination beam, and a second subset of the plurality of markers includes a second type of markers for determining a second optical property of the illumination beam, wherein the plurality of markers are located within a marker region of the plate and the marker region generally corresponds to the illumination region.

Abstract: A radiation filter for reducing transmission of an unwanted wavelength of radiation. The radiation filter includes zirconium and SiN3. The radiation filter may form part of a radiation sensor including a passivation layer arranged to receive radiation transmitted by the radiation filter, a photodiode arranged to receive radiation transmitted by the passivation layer, and circuit elements connected to the photodiode. The radiation sensor may perform optical measurements as part of an extreme ultraviolet lithographic apparatus or a lithographic tool.

Abstract: A transmissive diffraction grating for a phase-stepping measurement system for determining an aberration map for a projection system comprises an absorbing layer. The diffraction grating is for use with radiation having a first wavelength (for example (EUV radiation). The absorbing layer is provided with a two-dimensional array of through-apertures. The absorbing layer is formed from a material which has a refractive index for the radiation having the first wavelength in the range 0.% to 1.04.

Abstract: A first diffusor configured to receive and transmit radiation has a plurality of layers, each layer arranged to change an angular distribution of EUV radiation passing through it differently. A second diffusor configured to receive and transmit radiation has a first layer and a second layer. The first layer is formed from a first material, the first layer including a nanostructure on at least one surface of the first layer. The second layer is formed from a second material adjacent to the at least one surface of the first layer such that the second layer also includes a nanostructure. The second material has a refractive index that is different to a refractive index of the first layer. The diffusors may be configured to receive and transmit EUV radiation.

Abstract: A radiation filter for reducing transmission of an unwanted wavelength of radiation. The radiation filter includes zirconium and SiN3. The radiation filter may form part of a radiation sensor including a passivation layer arranged to receive radiation transmitted by the radiation filter, a photodiode arranged to receive radiation transmitted by the passivation layer, and circuit elements connected to the photodiode. The radiation sensor may perform optical measurements as part of an extreme ultraviolet lithographic apparatus or a lithographic tool.

Abstract: A method of patterning lithographic substrates, the method including using a free electron laser to generate EUV radiation and delivering the EUV radiation to a lithographic apparatus which projects the EUV radiation onto lithographic substrates, wherein the method further includes reducing fluctuations in the power of EUV radiation delivered to the lithographic substrates by using a feedback-based control loop to monitor the free electron laser and adjust operation of the free electron laser accordingly.

Abstract: An optical measurement method using an optical sensor apparatus (15), the optical sensor apparatus comprising an optical element (20) comprising a mark (22) configured to selectively transmit incident radiation (24), a photodetector (26) configured to receive radiation transmitted by the mark and provide an output signal that is indicative of the received radiation, and a support (30) which supports the optical element and is in thermal contact with the optical element. A thermal conductivity of the support is greater than a thermal conductivity of the optical element and a coefficient of thermal expansion of the support is greater than a coefficient of thermal expansion of the optical element. The method comprises performing a first measurement using the optical sensor apparatus, the first measurement including illuminating the mark with radiation. The temperature of the optical element changes during the first measurement.

Abstract: A patterning device comprising a reflective marker, wherein the marker comprises: a plurality of reflective regions configured to preferentially reflect radiation having a given wavelength; and a plurality of absorbing regions configured to preferentially absorb radiation having the given wavelength; wherein the absorbing and reflective regions are arranged to form a patterned radiation beam reflected from the marker when illuminated with radiation, and wherein the reflective regions comprise a roughened reflective surface, the roughened reflective surface being configured to diffuse radiation reflected from the reflective regions, and wherein the roughened reflective surface has a root mean squared roughness of about an eighth of the given wavelength or more.

Abstract: A delivery system for use within a lithographic system. The beam delivery system comprises optical elements arranged to receive a radiation beam from a radiation source and to reflect portions of radiation along one or more directions to form a one or more branch radiation beams for provision to one or more tools.

Abstract: An electron source, e.g. for a free electron laser used for EUV lithography comprises: • a cathode (203) configured to be connected to a negative potential (100, 101); • a laser (110) configured to direct pulses of radiation onto the cathode so as to cause the cathode to emit bunches of electrons; • an RF booster (180) connected to an RF source and configured to accelerate the bunches of electrons; and • a timing corrector (303, 313, 400, 401) configured to correct the time of arrival of bunches of electrons at the RF booster relative to the RF voltage provided by the RF source. The timing corrector may comprise a correction electrode (303, 313) surrounding a path of the bunches of electrons from the cathode to the RF booster and a correction voltage source (400, 401) configured to apply a correction voltage to the correction electrode.

Abstract: A patterning device comprising a reflective marker, wherein the marker comprises: a plurality of reflective regions configured to preferentially reflect radiation having a given wavelength; and a plurality of absorbing regions configured to preferentially absorb radiation having the given wavelength; wherein the absorbing and reflective regions are arranged to form a patterned radiation beam reflected from the marker when illuminated with radiation, and wherein the reflective regions comprise a roughened reflective surface, the roughened reflective surface being configured to diffuse radiation reflected from the reflective regions, and wherein the roughened reflective surface has a root mean squared roughness of about an eighth of the given wavelength or more.

Abstract: A method of patterning lithographic substrates, the method comprising using a free electron laser to generate EUV radiation and delivering the EUV radiation to a lithographic apparatus which projects the EUV radiation onto lithographic substrates, wherein the method further comprises reducing fluctuations in the power of EUV radiation delivered to the lithographic substrates by using a feedback-based control loop to monitor the free electron laser and adjust operation of the free electron laser accordingly.

Abstract: A method of patterning lithographic substrates, the method including using a free electron laser to generate EUV radiation and delivering the EUV radiation to a lithographic apparatus which projects the EUV radiation onto lithographic substrates, wherein the method further includes reducing fluctuations in the power of EUV radiation delivered to the lithographic substrates by using a feedback-based control loop to monitor the free electron laser and adjust operation of the free electron laser accordingly.

Abstract: A patterning device comprising a reflective marker, wherein the marker comprises: a plurality of reflective regions configured to preferentially reflect radiation having a given wavelength; and a plurality of absorbing regions configured to preferentially absorb radiation having the given wavelength; wherein the absorbing and reflective regions are arranged to form a patterned radiation beam reflected from the marker when illuminated with radiation, and wherein the reflective regions comprise a roughened reflective surface, the roughened reflective surface being configured to diffuse radiation reflected from the reflective regions, and wherein the roughened reflective surface has a root mean squared roughness of about an eighth of the given wavelength or more.

Abstract: A patterning device comprising a reflective marker, wherein the marker comprises: a plurality of reflective regions configured to preferentially reflect radiation having a given wavelength; and a plurality of absorbing regions configured to preferentially absorb radiation having the given wavelength; wherein the absorbing and reflective regions are arranged to form a patterned radiation beam reflected from the marker when illuminated with radiation, and wherein the reflective regions comprise a roughened reflective surface, the roughened reflective surface being configured to diffuse radiation reflected from the reflective regions, and wherein the roughened reflective surface has a root mean squared roughness of about an eighth of the given wavelength or more.

Abstract: An electron source, e.g. for a free electron laser used for EUV lithography comprises: a cathode (203) configured to be connected to a negative potential (100, 101); a laser (110) configured to direct pulses of radiation onto the cathode so as to cause the cathode to emit bunches of electrons; an RF booster (180) connected to an RF source and configured to accelerate the bunches of electrons; and a timing corrector (303, 313, 400, 401) configured to correct the time of arrival of bunches of electrons at the RF booster relative to the RF voltage provided by the RF source. The timing corrector may comprise a correction electrode (303, 313) surrounding a path of the bunches of electrons from the cathode to the RF booster and a correction voltage source (400, 401) configured to apply a correction voltage to the correction electrode.

Abstract: A photocathode comprises a substrate in which a cavity is formed and a film of material disposed on the substrate. The film of material comprises an electron emitting surface configured to emit electrons when illuminated by a beam of radiation. The electron emitting surface is on an opposite side of the film of material from the cavity.

Abstract: A method of patterning lithographic substrates, the method comprising using a free electron laser to generate EUV radiation and delivering the EUV radiation to a lithographic apparatus which projects the EUV radiation onto lithographic substrates, wherein the method further comprises reducing fluctuations in the power of EUV radiation delivered to the lithographic substrates by using a feedback-based control loop to monitor the free electron laser and adjust operation of the free electron laser accordingly.

Abstract: Passage through LINACs of electron bunches in their acceleration phase is coordinated with passage through the LINACs of electron bunches in their deceleration phase. Each successive pair of electron bunches are spaced in time by a respective bunch spacing, in accordance with a repeating electron bunch sequence. The electron source provides clearing gaps in the electron bunch sequence to allow clearing of ions at the undulator. The electron source provides the clearing gaps in accordance with a clearing gap sequence such that, for each of the plurality of energy recovery LINACS, and for substantially all of the clearing gaps: for each passage of the clearing gap through the LINAC in an acceleration phase or deceleration phase the clearing gap is coordinated with a further one of the clearing gaps passing through the LINAC in a deceleration phase or acceleration phase thereby to maintain energy recovery operation of the LINAC.