Patent Application
20080266654
The invention relates to an extreme ultraviolet (EUV) microscope for analysing a sample.
An embodiment of an ultraviolet microscope is known from U.S. Pat. No. 5,450,463, hereby incorporated by reference in its entirety. The microscope is arranged with a source emitting ultraviolet radiation in a range of 43.7 to 65 angstroms. The X-ray microscope is arranged for providing an X-ray transmission image, whereby in order to enable a suitable image contrast a non-linear optical medium is provided in a vacuum chamber in which an X-ray optical system of the X-ray microscope is installed. In this embodiment, X-ray radiation rays having a wavelength longer than that of the ultraviolet rays are made incident upon the non-linear optical medium to convert said radiation rays into ultraviolet rays, and the converted ultraviolet rays are made incident upon a sample to be examined.
Another embodiment of an X-ray microscope is known from U.S. Pat. No. 5,107,526, hereby incorporated by reference in its entirety. The microscope is arranged to generate X-rays in a wide spectrum. The illuminating system of the microscope comprises a highly polished primary mirror and a highly polished secondary mirror, both mirrors being coated with a specific multilayer structure. For the multilayer structure, a Tungsten/Silicon multilayer having pre-selected K- and L-absorption edges is used. This has an effect of a substantial transmission of X-rays through the bandpass of the water window (2-6 nm) and of a substantial rejection of ultraviolet and visible radiation wavelengths outside the bandpass of the water window.
The present invention relates to an EUV microscope that provides various advantages over prior art microscopes, such as X-ray microscopes.
It is an aspect of the present invention to provide an EUV microscope with a simple architecture, yet enable high quality images of the sample.
In an embodiment, an EUV microscope is provided. The EUV microscope includes an optical system constructed and arranged with at least one mirror comprising a multilayer structure for in-phase reflection of at least a portion of the radiation in the range of about 2-6 nm.
In an embodiment, an EUV microscope configured to analyze a sample is provided. The EUV microscope includes a source of EUV radiation constructed and arranged to generate the EUV radiation with a wavelength in a range of about 2-6 nm, and an optical system constructed and arranged to illuminate the sample with the EUV radiation and to collect a radiation emanating from the sample. The optical system is arranged with at least one mirror comprising a multilayer structure for in-phase reflection of at least a portion of the radiation in the range of about 2-6 nm.
By providing a suitably formed multilayer structure arranged for in-phase reflection of the portion of the EUV radiation, the typically rigidly formulated specifications for a suitable source of extreme ultra-violet radiation may be relaxed, thereby substantially simplifying the architecture of the microscope and substantially reducing its production costs.
The source specifications for the EUV microscope may be relaxed with respect to bandwidth, because the optics arranged for the EUV microscopy may accept a much larger bandwidth then the commonly used zone plate. This may allow the effective (used) output of the source to be larger. Furthermore, the output of the new sources is 100 times larger then the sources used so-far. Moreover, the transmission of an optical system based on multilayer coated mirrors is much larger then one based on a zone plate due to the higher reflectivity of the mirrors and the larger accepted bandwidth.
Suitable materials for production of the multilayer structure for in-phase reflection of the EUV radiation may include any one of the following combinations of materials: Mo/B; La/B4C; Mo/B4C; Ru/B4C; FeCrNi/B4C; W/B4C; Al2O3/C; Co/C; Ni/C; CrB2/C; RhRu/C; Ru/C; W/C; V/C; NiCr/C; Fe/C; Ru/C; CO2C3/C; Ge/C; FeCrNi/C; W/Sc; Cr/Sc; Al2O3/V; Cr/V; Ni/V; Cr/Ti; C/Ti; W/Ti; and Ni/Ti. These multilayers may be relatively easily obtained, and may provide a superior multilayer mirror for EUV microscopy. A suitable EUV source for the EUV microscope according an embodiment may comprise either a discharge plasma source or a laser induced plasma source. The multilayer structure may be arranged with a plurality of alternating first layers and second layers, whereby the first layer comprises a first material and a second layer comprises a second material. The plurality may be chosen in a range of about 200-500 alternating layers. The multilayer structure may be formed by a repetition of a unit structure having the first layer and the second layer. The unit structure may have a thickness in a range of about 1-2 nm. The unit structure may have a thickness of about 1.5 nm. This may enable in-phase reflection of the EUV radiation having a wavelength in the range of about 3.10-3.13 nm. A thickness of the first layer may be in a range of about 40%-60% of the thickness of the unit structure. The thickness of the first layer may be about 0.6-1.5 times the thickness of the second layer.
These and other aspects of the invention will be discussed in more detail with reference to drawings.
The EUV radiation emanating from the source 2, is schematically represented by a ray 2a, and reflects from a suitable mirror comprising a multilayer structure 4 arranged in an illuminator system 3, which may also be referred to as an optical system. The properties of the multilayer structure are set forth in the foregoing. The multilayer structure 4 is arranged to reflect in-phase radiation in accordance with Bragg law of refraction, each individual layer being a reflective surface. The reflected radiation 2b impinges on a suitable sample 5, for example, a biological sample. The sample 5 disperses the beam 2b, thereby yielding a dispersed beam 6, which is collected by a suitable projection module 7. The projection module 7 may comprise a plurality of optical elements, for example, a plurality of mirrors comprising a multilayer structure 7a. The mirrors comprising the multilayer structure 7a may be aspheric mirrors. The projection optical box may comprise 6 multilayer mirrors. A collected beam 8 exits the projection module 7 and passes to a detector 9. The detector 9 may comprise a CCD camera constructed and arranged to produce an electronic image. Alternatively, the detector 9 may comprise an EUV sensitive film.
As discussed above, the multilayer structure may be made from suitable materials for in-phase reflection of the EUV radiation may include any one of the following combinations of materials: Mo/B; La/B4C; Mo/B4C; Ru/B4C; FeCrNi/B4C; W/B4C; Al2O3/C; Co/C; Ni/C; CrB2/C; RhRu/C; Ru/C; W/C; V/C; NiCr/C; Fe/C; Ru/C; CO2C3/C; Ge/C; FeCrNi/C; W/Sc; Cr/Sc; Al2O3/V; Cr/V; Ni/V; Cr/Ti; C/Ti; W/Ti; and Ni/Ti. In an embodiment, the multilayer structure comprises Cr/Sc.
The EUV microscope 10 may be arranged as a table-top microscope to enable investigation of suitable biological samples. The X-ray range between about 2 and 6 nanometers, corresponding to a region between the Kα absorption edge of carbon and the Kα absorption edge of oxygen, is found to be particularly suitable for investigation of biological matter, because in this range, the absorption of carbon and nitrogen is large, while absorption of oxygen and hydrogen is low. Therefore, by using the range between about 2 and 6 nm, it is possible to observe biological specimens mainly composed of proteins (living tissue) with high resolution in water.
While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
