Compact wideband VUV spectrometer

Abstract

The invention relates to a compact wideband vacuum ultraviolet (VUV) and soft X-ray grazing incidence spectrometer based on a plane amplitude diffraction grating. The spectrometer enables simultaneous detection of a VUV spectrum in a positive first order of diffraction and a negative first order of diffraction. The technical result of the invention is that of recording a spectrum in a wide spectral range (3-200 nm) with a moderate spectral resolution (/15-30) and with a significantly higher spectral resolution (/100-200) in a narrow soft X-ray or extreme ultraviolet range with the possibility of measuring the absolute radiation output in these regions of the spectrum.

Claims

1. A wide-range vacuum ultraviolet (VUV) spectrometer, comprising: an entrance slit located in a housing illuminated by a remote radiation source and emitting a radiation beam which illuminates a diffraction grating having a constant period d under a grazing angle , and a detector, the diffraction grating is designed with flat reflective working faces which lie in a grating plane, and with non-reflective grooves between the working faces, whereas the detector is capable of recording vacuum ultraviolet (VUV) spectra in a negative first (1.sup.st) diffraction order, wherein a detector edge defines a long-wavelength limit .sub.1 of the negative first (1.sup.st) diffraction order working spectral region.

2. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein a working spectral region in the negative first (1.sup.st) diffraction order is in a range of 5 to 200 nm.

3. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein a width of the diffraction grating working faces equals d/2, a diffraction grating half-period.

4. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein the depth of the grooves between the grating working faces is greater than d/4 sin .

5. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein the diffraction grating is designed with either rectangular or trapezoidal profile of grooves between working faces.

6. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein a radiation source angular size, defined by a ratio of its characteristic dimension a and its distance from the entrance slit A, is not greater than 10.sup.3 rad: a/A10.sup.3 rad.

7. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein the grazing angle is in a range of 4 to 6 degrees.

8. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein the detector is a multi-element one.

9. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 featuring spectral sensitivity calibration.

10. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein the detector is a two-dimensional one, and the entrance slit is illuminated through an additional slit installed between the entrance slit and the radiation source and parallel to a dispersion plane.

11. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 wherein the housing is configured to have a quick-release part which allows to connect the spectrometer to a vacuum chamber containing a vacuum ultraviolet (VUV) radiation source.

12. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 1 with additional VUV spectrum recording in the positive first (+1.sup.st) diffraction order, whereas the long-wavelength limit of the positive first (+1.sup.st) order working spectral region .sub.+1 is many times less than the long-wavelength limit of the negative first (1.sup.st) order working spectral region .sub.1:
.sub.+1<<.sub.1.

13. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 12 wherein the detector is installed in such a manner that the diffraction angle in a direction of the detector edge is not greater than 90.

14. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 12 wherein the detector edge defines the long-wavelength limit .sub.+1 of the positive first (+1.sup.st) diffraction order working spectral region.

15. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 12 wherein the grazing angle is selected in such a manner that a cut-off wavelength .sub.co corresponding to a diffraction angle =90 is greater than an EUV lithography wavelength of 13.5 nm: .sub.co>13.5 nm, and the positive first (+1.sup.st) order working spectral region includes the wavelength of 13.5 nm: .sub.+1>13.5 nm.

16. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 12 wherein the positive first (+1.sup.st) order working spectral region includes a wavelength of 13.5 nm and the long-wavelength limit .sub.+1 of the working spectral region is close to the wavelength of 13.5 nm:
0<(.sub.+113.5 nm)<<13.5 nm.

17. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 12 wherein a gate is installed before the detector which inhibits radiation in a 0 diffraction order.

18. The wide-range vacuum ultraviolet (VUV) spectrometer as claimed in claim 12 wherein the spectrometer is provided with software designed for acquisition, processing, display and storage of spectral measurement data in the positive first (+1.sup.st) and negative first (1.sup.st) diffraction orders.

19. A wide-range vacuum ultraviolet (VUV) spectrometer, comprising: an entrance slit located in a housing illuminated by a remote radiation source and emitting a radiation beam which illuminates a diffraction grating having a constant period d under a grazing angle , and a radiation detector, the diffraction grating is designed with flat reflective working faces which lie in a grating plane, and with non-reflective grooves between the working faces; the radiation detector allows for vacuum ultraviolet (VUV) spectrum recording in a positive first (+1.sup.st) diffraction order and a negative first (1.sup.st) diffraction order, whereby a long-wavelength limit .sub.+1 of the positive first (+1.sup.st) order working spectral region is many times less than a long-wavelength limit .sub.1 of a negative first (1.sup.st) order working spectral region: .sub.+1<<.sub.1, and a spectral resolution (/).sub.+1 in the negative first (1.sup.st) order working spectral region is many times greater than the spectral resolution (/).sub.1 in the negative first (1.sup.st) order working spectral region: (/).sub.+1>>(/).sub.1, wherein a detector edge defines a long-wavelength limit .sub.1 of the negative first (1.sup.st) diffraction order working spectral region.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The technical substance and working principle of the proposed device are demonstrated in the drawings, namely:

(2) FIG. 1wide band VUV spectrometer layout view according to this invention,

(3) FIG. 2spectrum of laser produced Sn plasma in the 6-200 nm VUV range,

(4) FIG. 3spectra of laser produced Sn plasma in the +1st and 1st diffraction orders with suppression of the 0 diffraction order,

(5) FIG. 4measurement design with additional slit in the diffraction plane for source size visualization,

(6) FIG. 5spectra images with spatial resolution in the direction perpendicular to the diffraction direction,

(7) FIG. 6an image of spectrometer engineering prototype.

(8) These drawings do not cover and certainly do not restrict the complete scope of embodiments of this technical concept; they are provided only as supporting materials to demonstrate specific instances of its implementation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

(9) This description is provided to illustrate how the invention can be implemented and in no way to demonstrate the scope of this invention.

(10) According to an example of the invention implementation illustrated in FIG. 1, the compact wide-range VUV grazing incidence spectrometer comprises the following parts located in housing 1: an entrance slit 2 illuminated by a remote radiation source 3 and emitting a radiation beam 4 which illuminates a diffraction grating 5 having a constant period d under the grazing angle , and a detector 6. The spectrometer is characterized by the diffraction grating 5 being designed to be flat, with a relief, with flat reflective working faces 7 which lie in the plane of the grating 5, and with non-reflective grooves 8 between working faces 7, whereas the detector 6 is capable of recording VUV spectra in the 1st diffraction order.

(11) The diffraction order (1) corresponds to diffraction angles <, where is the angle of incidence, and the diffraction order (+1) corresponds to diffraction angles >. In FIG. 1, beams 9, 10, and 11 are reflected from diffraction grating 5 in the 1st, 0 and +1st diffraction orders, correspondingly.

(12) To prevent excessive exposure of detector part 6, a gate 12 can be installed in front of it to block radiation in the 0 diffraction order. At the same time, the housing 1 can be equipped with an outside lever which allows to move the gate into the operating position.

(13) Detector 6 is preferably located inside detector housing 13 which can be designed as a removable part of spectrometer evacuated housing 1. The detector is connected to control unit 16 by cable 15 via sealed cable gland 14 with a connector. In device embodiments, the control unit 16 may be located inside detector housing 13. Further, control unit 16 is connected by means of a USB-cable to a personal computer (PC) 17. Spectrometer power is preferably supplied via the computer USB port; it is operated using a control program which is also a tool for performing various evaluations of recorded spectra.

(14) Preferably, detector 6 is a multi-element one. A digital backlit CCD camera may be used as a detector for recording VUV spectra. This ensures high sensitivity and spectrometer calibration capability for taking quantitative measurements.

(15) In preferred embodiments of the invention, the width of working faces 7 of the diffraction grating is d/2, i.e. half the diffraction grating period; it eliminates all even diffraction orders and ensures deep suppression of higher odd orders.

(16) Diffraction grating installation under a grazing angle eliminates light scattering and reflection by grooves 8 of the grating. For this purpose, the depth of grooves 8 between working faces 7 of the grating is greater than d/4 sin , which ensures a simple geometry of diffraction grating with reflection only from working faces 7.

(17) All the above-mentioned features ensure measurement with a high signal level and a low noise level.

(18) For simplification purposes, the diffraction grating is designed with either rectangular or trapezoidal profile of grooves between working faces.

(19) The amplitude diffraction grating designed as described above is more robust and reliable as compared to a transmissive grating, as well as more readily accessible and easier to produce. Geometry of such a diffraction grating allows to reliably calculate its reflection coefficient for various wavelengths; along with using an absolutely calibrated detector, this allows to take quantitative measurements in a wide spectral range.

(20) Spectral resolution 82 is determined by the number N of diffraction grating marks engaged in diffraction, the width of monochromatic line image in the registration plane for point and extended sources and by the width of the instrument function of the detector used. According to the invention, these factors which determine the resulting value of spectral resolution, are optimized as follows.

(21) To optimize the number N of marks engaged in diffraction, the diffraction grating is installed in such a manner that the grazing angle is preferably in the range of 4 to 6 degrees. This also allows to improve reflection of diffraction grating 5 in the VUV range. Furthermore, the grazing angle of 4 to 6 degrees corresponds to a small cut-off wavelength on the short-wavelength side of the working spectral region: 4 to 6 nm.

(22) To optimize spectrometer resolving power / by means of reducing the width of monochromatic line image in the registration plane of detector 6, the angular size of radiation source 3, determined by the ratio of its characteristic dimension a and its distance from entrance slit A, is preferably not greater than 10.sup.3 rad: a/A10.sup.3 rad.

(23) For the same purpose, the diffraction grating is positioned as close to the entrance slit as possible, in particular, the distance 1 from diffraction grating 5 to entrance slit 2 is much less than the distance L from the diffraction grating to the detector: l<<L. This also helps minimize spectrometer size.

(24) For optimum detector operation and agreement of its spatial resolution with the real spectral structure, the diffraction direction to detector edge determines the long-wavelength limit .sub.1 of the working spectral region in the 1st diffraction order.

(25) Detector edge is understood as the edge of the detector working area. For a multi-element linear detector, this is the outermost detector element, while for a two-dimensional detector, this is the outermost column of detector elements oriented perpendicular to the diffraction plane.

(26) Preferably, the working spectral region in the 1st diffraction order is in the range of 5 to 200 nm. Spectral measurements in this range allow to diagnose plasma radiation sources, including those designed for projection EUV lithography.

(27) Typically, radiation sources for projection EUV lithography are based on using laser produced tin plasma (Sn). FIG. 2 shows a spectrum of laser produced Sn plasma in the VUV range of 6-200 nm obtained in the 1st diffraction order using a spectrometer designed according to this invention. In line with spectrometer software capabilities, the spectrum is presented in the semi-logarithmic scale.

(28) In the preferred embodiment of the invention illustrated in FIG. 1, the spectrometer is characterized by additional VUV spectrum recording in the +1st diffraction order. Furthermore, the long-wavelength limit .sub.+1 of the +1st order working spectral region is many times less than the long-wavelength limit .sub.1 of the 1st order working spectral region, .sub.+1<<.sub.1, and the spectral resolution (/).sub.+1 in the 1st diffraction order is many times greater than the spectral resolution (/).sub.1 in the 1st diffraction order: (/).sub.+1>>(/).sub.1.

(29) To efficiently use detector 6, the angle of diffraction in the direction towards detector edge is not greater than 90, whereas the direction of diffracted beam towards the edge of detector 6 corresponds to direction towards the limit wavelength of the +1st order working spectral region.

(30) In preferred embodiments of the invention, to ensure optimum use of the detector, both its edges determine the long-wavelength limits .sub.1 and .sub.+1 of the working spectral regions in the 1st and +1st order.

(31) In the spectrometer designed according to the invention, the diffraction pattern in the (+1) and (1) diffraction orders is strongly asymmetrical. The +1st order has the cutoff wavelength of .sub.co which corresponds to the diffraction angle =90. In proximity of .sub.co, spectrometer dispersion D [nm/mm] is very low, while spectral resolution (/).sub.+1 is high. Grazing angle can be selected in such a manner that the wavelength range of interest, in particular, =13.5 nm, is as close to .sub.co as possible. In this case, a medium spectral resolution (/).sub.+1 of up to (100-200) can be achieved at =13.5 nm in the (+1) diffraction order, and a very wide spectral range can be observed in the (1) order with a low spectral resolution: (/).sub.1 up to (15-30).

(32) Accordingly, in the preferred embodiment of the invention the grazing angle is selected in such a manner that the cut-off wavelength .sub.co corresponding to the diffraction angle =90 is greater than the EUV lithography wavelength of 13.5 nm: .sub.co>13.5, and the +1st order working spectral region includes the wavelength of 13.5 nm: .sub.+1>13.5 nm. At the same time, it is preferable that the long-wavelength limit .sub.+1 of the working spectral region is close to the wavelength of 13.5 nm: 0<(.sub.+113.5 nm)<<13.5 nm.

(33) The spectrometer was tested using laser produced plasma as radiation source 3. To generate plasma, a CO.sub.2 laser was used, with the radiation wavelength of 10.6 m, the energy of 0.5 J in a pulse of 100 ns duration and the focal spot 300 m in diameter. Tin and other materials were used as the target material. Hamamatsu S7030-1006N SPL detector was employed, with the number of pixels 1,02458, and the working area of 24.61.36 mm.

(34) Testing was performed under the following parameters:

(35) entrance slit width 84 m,

(36) distance radiation source entrance slit 400 mm.

(37) grazing angle =5.5,

(38) distance entrance slitdiffraction grating 3.5 mm,

(39) diffraction grating: period d=3 m, size 33 mm, material SiO.sub.2, rectangular profile grooves with the width of d/2=1.5 m,

(40) distance diffraction grating detector 56 mm,

(41) In FIG. 3, measured spectra of laser produced Sn plasma are shown. The spectrometer allows to observe a very wide spectral range of 6-200 nm in the (1) order with a low spectral resolution (/).sub.115-30, and a narrow spectral range close to the EUV lithography wavelength of =13.5 with a medium spectral resolution of (/).sub.+1100-200. In FIG. 3, a spectrum with 0 diffraction order suppression is shown. This protects the detector from excessive exposure and ensures spectrum recording with a high signal level and a low noise level.

(42) According to the embodiment of the invention, schematically illustrated in FIG. 4, the spectrometer features a two-dimensional detector 6, and the entrance slit 2 is illuminated through an additional slit 18 installed between the entrance slit and the radiation source 3 and parallel to the dispersion plane. This allows to record the size of VUV source emitting region at various wavelengths of the VUV spectrum with a high spatial resolution.

(43) Images of spectra (without 0 order diffraction suppression) of laser produced silicon (Si) and lithium fluoride (LiF) plasma obtained with the Andor digital VUV camera as two-dimensional detector, are shown in FIG. 5.

(44) Preferably, the spectrometer is provided with software that allows acquisition, processing, display and storage of spectral measurement data in the +1st and 1st diffraction orders. This ensures that the spectrometer is user-friendly while recording a unique set of parameters.

(45) FIG. 6 shows an image of spectrometer engineering prototype demonstrating that it is extremely small in size; spectrometer largest dimension does not exceed 30 cm.

(46) For ease of operation spectrometer housing is designed as the quick-release part of a pressure-tight joint. For this purpose, as shown in FIG. 6, spectrometer housing 1 may comprise the KF-40 inlet flange, which allows to quickly connect the spectrometer to the vacuum chamber containing the VUV radiation source, and disconnect it.

(47) Thus, the spectrometer allows to simultaneously record spectra on the edge of the (+1) reflection order with a sufficiently high spectral resolution (/100150) and in the (1) order with a moderate resolution (/15-30), but in a very wide spectral range (5-200 nm). Prior calculation of the reflection coefficient of the amplitude reflective diffraction grating and using an absolutely calibrated detector, in particular, a CCD detector, allow to take absolute and quantitative measurements of intensities in these spectral ranges. The spectrometer is efficient for checking radiation sources with a small angular aperture. Spectrometer embodiments provide for recording dimensions of the radiation source emitting area in various ranges of the VUV spectrum. The spectrometer is characterized by a very small size and ease of operation.

INDUSTRIAL APPLICABILITY

(48) The proposed invention is intended for use in instrument engineering, in particular, for building VUV range spectrometers, including for checking plasma radiation sources designed for projection EUV lithography.

(49) Description of the preferred invention embodiment is provided for illustration and description purposes. It is not intended to be exhaustive or to limit the invention by the specific forms disclosed. Obviously, many modifications and versions will be evident to professionals in this technical field. It is assumed that the scope of the invention is determined by the following claims and their equivalents.