METHOD, APPARATUS AND COMPUTER PROGRAM FOR MEASURING AND PROCESSING A SPECTRUM OF AN XUV LIGHT SOURCE FROM SOFT X-RAYS TO INFRARED WAVELENGTH

Abstract

Method for measuring and processing by means of a broadband spectrometer (1) a spectrum of light (7) generated by an XUV source for generating light in a wavelength range from soft x-rays to infrared wavelengths, wherein the processing is based on the assessment of a wavelength range in the measured spectrum which has a negligible higher order contribution to longer-wavelengths than said range.

Claims

1. Method for measuring and processing by means of a broadband spectrometer a spectrum of light in a wavelength range from soft x-rays to infrared wavelengths, comprising the steps of: a) assessing in a measured spectrum a longest wavelength 0, such that the contribution of higher diffraction orders of wavelengths shorter than the longest wavelength 0 to the part of the spectrum for wavelengths longer than 0 is below a previously defined value.

2. The method as claimed in claim 1, wherein the broadband spectrometer includes a shutter, one of a pinhole and a slit, at least one transmission grating and a camera, wherein the processing further comprises the steps of: (b) removing for wavelengths , in the range given by 0<<20 a broadening in the intensity of the light as recorded by the camera, due to the pinhole or slit, and dividing the intensity in the resulting wavelength range by the efficiencies of the grating and the camera, thus obtaining a recovered spectrum in a first spectral range, (c) calculating contributions of all higher order diffractions in the range given by 0<<20 to the range given by 20<<40 and subtracting these contributions from the intensity of the light as recorded by the camera, thus obtaining a recovered spectral range for wavelengths in the range given by 20<<40, and (d) repeating the calculation according to steps (b) and (c) for the next adjacent wavelength range, thus obtaining a next adjacent recovered spectral range for wavelengths in a next adjacent range, until the complete spectrum as recorded by the camera has been processed and the spectrum from the source has been recovered.

3. The method as claimed in claim 2, wherein the broadband spectrometer further comprises at least one spectral filter, characterized in that the step of removing for wavelengths in the range given by 0<<20 a broadening in the intensity of the light as recorded by the camera further comprises dividing the intensity in said wavelength range by the transmission efficiency of the filter.

4. The method as claimed in claim 3, wherein the light source is EUV light, and the step of measuring the spectrum of the EUV light comprises the measuring of an out-of-band spectrum by using a spectral filter which has a low transmission characteristic for radiation with a wavelength about 13.5 nm and a high transmission characteristic for out-of-band wavelengths.

5. The method as claimed in claim 2, wherein the spectral resolution of the spectrometer is maximized by locating the pinhole or slit and the grating within the spectrometer at a maximum distance from the camera.

6. An apparatus for measuring and processing a spectrum of light in a wavelength range from soft x-rays to infrared wavelengths, comprising: a broadband spectrometer, which spectrometer comprises a shutter, one of a pinhole and a slit, at least one transmission grating and a camera, characterized in that the apparatus is provided with processing means for assessing in a measured spectrum a longest wavelength 0, such that the contribution of higher diffraction orders of the spectrum for wavelengths shorter than the longest wavelength 0 to the part of the spectrum for wavelengths longer than 0 is below a previously defined value.

7. The apparatus as claimed in claim 6, wherein the spectrometer comprises at least one spectral filter.

8. The apparatus as claimed in claim 6, wherein the shutter is held in a carrier which is mounted on a motorized translation stage for movement in a transverse direction with respect to the incoming beam.

9. The apparatus as claimed in claim 7, wherein the at least one spectral filter has a low transmission characteristic for light at an in-band wavelength and a high transmission characteristic for out-of-band wavelengths.

10. The apparatus as claimed in claim 9, wherein the spectrometer is an EUV spectrometer and the in-band represents a bandwidth of 2% around a central wavelength of 13.5 nm.

11. The apparatus as claimed in claim 7, wherein the spectral filter is one selectable out of a set, which set is held in a carrier.

12. The apparatus as claimed in claim 11, wherein the carrier holding the set of spectral filters is mounted on motorized translation stages for movement in transverse directions with respect to the incoming beam.

13. The apparatus as claimed in claim 6, wherein the pinhole or slit is held in a carrier which is mounted on motorized translation stages for movement in transverse and longitudinal directions with respect to the incoming beam.

14. The apparatus as claimed in claim 6, wherein the transmission grating is one selectable out of a set, which set is hold a carrier.

15. The apparatus as claimed in claim 14, wherein the carrier holding the set of transmission gratings is mounted on motorized translation stages for movement in transverse and longitudinal directions with respect to the incoming beam.

16. The apparatus as claimed in claim 14, wherein the set of transmission gratings is provided by a microchip showing an array containing individual transmission gratings.

17. The apparatus as claimed in claim 16, wherein the array is a 3 by 7 matrix in which the individual transmission gratings have line densities of respectively 500, 780, 1000, 1500, 1850, 2000, 2500 lines per mm and starting from 3000 up to 10000 with 1000 lines per mm increments.

18. The apparatus as claimed in claim 6, wherein the pinhole or slit and the grating are arranged at a distal position with respect to the camera.

19. The apparatus as claimed in claim 6, wherein the camera comprises a CCD chip, and the spectrometer includes a blackened plate having an aperture corresponding to the surface dimensions of the CCD chip, placed between the grating and the camera in perpendicular position with respect to the path of the light beam.

20. The apparatus as claimed in claim 6, wherein the control means are adapted for controlling an XUV light source in order to optimize a spectrum of such light source.

21. A non-transitory computer-readable medium that stores a computer program for performing the Computer program for performing a method as claimed in claim 2 when the computer program runs on a computer.

Description

[0041] The invention will now be elucidated hereinbelow on the basis of exemplary embodiments, with reference to the drawings.

[0042] In the drawings

[0043] FIG. 1 shows a flow chart of an embodiment of the method according to the invention,

[0044] FIG. 2 shows a spectrum of a beam of EUV light as emitted by an EUV source, incident to an EUV spectrometer,

[0045] FIG. 3 shows the spectrum shown in FIG. 2 as recorded by the EUV spectrometer,

[0046] FIG. 4a-FIG. 4h show the spectrum of FIG. 3 after respective intermediate steps of the processing according to the invention,

[0047] FIG. 5 shows the spectrum of FIG. 2 as it has been recovered by the processing according to the invention,

[0048] FIG. 6 shows a schematic view of an EUV spectrometer, and

[0049] FIG. 7 shows a block diagram of the EUV spectrometer shown in FIG. 6, in combination with an EUV source and a controller according to the invention.

[0050] Corresponding components are designated in the figures with the same reference numerals.

[0051] FIG. 1 shows a flow chart of an embodiment of the method according to the invention, with steps (i) to (xiii) as can be implemented as a computer program,

[0052] FIG. 2 shows a spectrum of a beam of EUV light as emitted by an EUV source, incident to an EUV spectrometer 1 (schematically shown in FIGS. 6-7). This spectrum is the one to be recovered, according to the method of the invention.

[0053] FIG. 3 shows the spectrum of in FIG. 2 as recorded by a CCD camera 6 of the EUV spectrometer 1 (schematically shown in FIGS. 6-7). The spectrum shows several higher order contributions, due to the grating 5 in the EUV spectrometer 1, and broadening due to pinhole 4. The spectrum as recorded (represented by line 17 in FIG. 7) by the CCD camera 6 is inputted into a controller, CPU (central processing unit) 18, thus providing the data for the first step (i) START for the processing as illustrated in the flow chart of FIG. 1.

[0054] FIG. 4a to FIG. 4h show several intermediate steps (v) and (viii) with parameter k increasing from k=1 to k=8 and .sub.0=5 nm, in the processing according to the flow chart of FIG. 1, illustrating the processing of the spectrum as recorded.

[0055] FIG. 5 shows the recovered incident spectrum, as obtained after a sufficient amount of iterations.

[0056] FIG. 6 shows an EUV spectrometer 1, which comprises a shutter 2 at its entrance, a filter array 3 for selecting specific wavelength bands from the source spectrum, a slit or a pinhole 4, a transmission grating chip 5 for dispersing the light 7 and a detector 6 which is a back-illuminated CCD camera for detection of the spectrum. The shutter 2 is hold in a carrier 22 which is mounted on a motorized translation stage 32 for movement in transverse direction (indicated by arrow 8) with respect to the incoming beam 7. The light 7 from the EUV source is directed to the grating 5 which diffracts each wavelength at a different angle towards the CCD camera 6. Light with a long wavelength is diffracted at larger angles. Consequently the spectral content of the incoming beam 7 can be calculated back from the image recorded by the CCD camera 6. All the components of the spectrometer are contained in a vacuum chamber (not shown). The filter 3 is one selectable out of a set, which set hold in a carrier 23, which is mounted on motorized translation stages 33, 43 for movement in transverse directions (indicated by arrows 8, 9) with respect to the incoming beam 7. The pinhole 4 or slit is hold in a carrier 24 which is mounted on a motorized translation stage 34 for movement in transverse direction 8 and longitudinal direction (indicated by arrow 11) with respect to the incoming beam 7. The transmission grating 5 is one selectable out of a set, which set is hold a carrier 25, which is mounted on motorized translation stages 35, 45 for movement in transverse directions 8, 9 and longitudinal direction 11 with respect to the incoming beam 7. The movements of said translation stages 32, 33, 43, 34, 35, 45, 55 are vacuum compatible motorized, and can be controlled with a computer using a graphical user interface (schematically shown in FIG. 7). The control system allows automated and in situ alignment.

[0057] FIG. 7 shows the EUV spectrometer 1 (dashed lines), in combination with an EUV source 20 and a controller 18, which both generates control signals 12, 13, 14, 15, 16 for controlling respectively the shutter 2, the filter array 3, the pinhole 4, the grating 5 and the CCD camera 6, as well as calculates from the output signal 17 of the CCD camera 6 a recovered spectrum according the method of the invention (represented as output signal 19). Moreover, the controller 18 generates control signals 21 for controlling the light source 20 in order to optimize the spectrum of the light emitted by that source.