"X-RAY MICROSCOPE HAVING AN X-RAY SOURCE FOR SOFT X-RAYS

Abstract: Soft X-rays are very suitable for the examination of biological samples by means of an X-ray microscope. It is known to generate such soft X-rays by means of a laser excited plasma in a fluid jet. According to the invention the X-rays are generated by focusing an electron beam 6 onto a fluid jet 2, thus producing a very small electron focus on the jet and hence a very small monochromatic X-ray spot 8. The electron spot 8 can be obtained by means of a standard electron microscope (a SEM) or by means of a standard electron gun for a cathode ray tube (a CRT gun). The imaging optical elements 18, 34, 40 in the X-ray microscope may be Fresnel zone plates.

Description: X-ray microscope naving an X-ray source for soft X-ravs The invention relates to an X-rav micioscoDe which includes a device toi generating X-rays, which device is provided with * means for producing a fluid jet * means tor forming a focused radiation beam whose focus is situated on the fluid jet A device for generating soft X-ravs is know n from the published patent application WO 97/40650 (PCT/SE 97/00697) The means tor producing a fluid jet the known device are formed by a nozzle wherefrom a fluid such as water is ejected under a high pressure The means for producing a focused radiation beam are formed by a combination of a pulsating laser and a focusing lens which focuses the pulsating radiation beam produced b> the laser in such a manner that the focus is situated on the fluid jet Because of the high power density of the laser pulses, the laser light thus induces a plasma in the fluid jet, thus generating said soft X-rays. The cited patent application descπbes how these X-rays, notably those of a wavelength of 2.3-4.4 nm, can be used for X-ray microscopy. Generating X-rays by way of pulsed laser plasma emission has a number of drawbacks A first drawback in this respect is due to the fact that it is necessary to operate the laser in the pulsating mode in order to achie \ e an adequate power densιt\ of the laser The cited patent application mentions a power density of from 10 -10 W/cm " , if this powei is to be generated by means of a laser in continuous operation, an extremely large laser would be required As a result, this known X-ray source produces only X-rays of a pulsating nature A further drawback of lasei -induced plasma emission consists in the phenomenon that many particles (molecules, radicals, atoms (ionized or not), which usuall y have a high kinetic energy and may be \ ery reactive chemically are present m the vicinity o. the location where the X-rays are formed (the X-iay spot) The formation of these particles can be explained as follows when energy is applied to the target (so the fluid jet) by means of laser light, as the intensity increases first the electrons of the outer shell of the target mateπal will be ionized whereas the electrons of the inner shells, producing the X-rays, are excited only after that The particles then formed could damage the sample to be examined by means of the X-ray microscope In order to mitigate or prevent such damage it is feasible to arrange an optical intermediate element (for example, a condenser lens in the form of a Fresnel zone plate) between the physical X-ray spot and the actually desired location of the X-ray spot, thus creating an adequate distance between the X-ray spot and the sample without seπously affecting the imaging properties of the X-ra \ microscope Because condenser lense*. are not very effective in the X-ray field howe powe*- generated for the imaging m the X-ray microscope is thus lost Moreovei, some other types of condensers (foi example multilayer mirrors oi grazing incidence mirrors ) are verv susceptible to damage by said high energetic particles It is an object of the invention to a \ oid said drawbacks by providing an X-ra \ source for comparatively soft X-rays which can operate continuously while forming no oi hardly any detπmental particles in the X-ray target This ob)ect is achieved according to the invention in that the focused radiation beam consists of a beam of electrically charged particles The above-mentioned drawbacks are avoided by irradiating the fluid jet by means of said particles Because of the much shorter wavelength of said particles, moreover, an advantage is obtained in that the focus formed by means of said particles can be much smaller than the focus of the beam of laser light The invention offers an additional advantage in that the energy of the electrically charged particles can be continuously controlled in a wide range by variation of the acceleration voltage of said particles, such control is realized by vaπation of the acceleration voltage of these particles The beam of electrically charged particles is formed by an electron beam in a preferred embodiment of the invention This embodiment offers the advantage that use can D. made of existing apparatus such as a scanning electron microscope Such apparatus is arranged notably to obtain a very small electron focus that is, a focus with a diameter as small as a few nanometers The cross-section of the fluid jet in the direction of the focused beam m a further embodiment of the invention is smaller than that in the direction transversely thereof This embodiment is of importance in all cases wnere the particle beam has a width which 1*. larger than approximately the penetration depth into the fluid jet If a fluid jet having a circular cross-section were used in such circumstances, the X-rays generated in a comparatively thin region at the surface of the jet would be absorbed in the inteπor of the fluid jet again, so that a useful yield of the X-rays would be lost This adverse effect is strongly mitigated or even avoided when a "flattened" fluid jet is used _ The fluid jet in another embodiment of the invention consists mainly of hquiα oxygen or nitrogen. In addition to the advantage that a fluid jet of a liquefied gas has excellent cooling properties, and hence can be exposed to heavy thermal loading, such a fluid jet also has a high degree of spectral puπty, notably in the range of soft X-rays, that is, in the so-called water window (wavelength λ =2 3-4 4 nm) This wavelength range is particularh suitable for the examination of biological samples by means of an X-ray microscope, becaust the absorption contrast between water and carbon is maximum in this range The means for producing a focused beam of electrically charged particles m another embodiment of the invention are formed by a standard electron gun for a cathode rav tube, the X-ray microscope also being provided with a condenser lens which is arranged between the fluid jet and the object to be imaged by means of the X-ray microscope According to the invention a first advantage of the use of a standard electron gun of a cathode ray tube resides in the fact that such elements already are manufactured in bulk and have already proven their effectiveness for many years Another advantage resides in fact that such electron sources are capable of delivering a comparatively large current (of the order of magnitude of 1 mA) The electron spot, however, has a dimension of the order of magnitude of 50 μm, being of the same order of magnitude as the dimensions of the object to be imaged, so that in this case a condenser lens is required which concentrates the radiation from the X- ray spot onto the sample. Even though X-ray intensity is lost due to the use of the condenser, the current in the electron beam is so large that this loss is more than compensated for. The properties that can be offered by an existing electron microscope so as to implement the invention can be used to good advantage An electron microscope produces _. focused electron beam and may be provided with a device for generating X-rays which is characterized according to the invention in that it is provided with means for producing a fluid jet and means for directing the focus of the electron beam onto the fluid jet An X-ray microscope can thus be incorporated in the electron microscope, the device for generating X- rays then acting as an X-ray source for the X-ray microscope Notably a scanning electron microscope is suitable for carrying out the present invention, because such a microscope can readily operate with acceleration voltages of the electron beam which are of the order of magnitude of from 1 to 10 kV, these values correspond to values necessary so as to generate soft X-rays in the water window The invention will be descπbed in detail hereinafter with reference to the Figures; corresponding elements therein are denoted by corresponding reference numerals Therein Fig. 1 shows diagrammatically some configurations of an electron beam with a fluid jet for the purpose of comparison; Fig. 2 shows diagrammatically the beam path in a transmission X-ray microscope according to the invention; Fig 3 shows diagrammatically the beam path m a scanning transmission microscope according to the invention, and Fig. 4 shows diagrammatically the beam path m a transmission X-ray microscope provided with a standard electron gun for a cathode ray tube in accordance with the invention. The Figs, la to lc show a number of configurations in which a fluid jet which is assumed to extend perpendicularly to the plane of drawing is irradiated by an electron beam. In Fig. la this beam oπginates from a spot forming objective of a scanning electron microscope (SEM); in the Figs. 1 and b the electron beam oπginates from a standard electron gun for a cathode ray tube (CRT gun). In Fig. la the fluid jet 2, for example a jet of water, has a diameter of approximately 10 μm. The electron beam 6 focused onto the fluid jet by the objective 4 of the SEM is subject to an acceleration voltage of, for example, 10 kV and transports a current of, for example, 5 μA. An electron spot having a cross-section of 1 μm generates an X-ray spot having a dimension of approximately 2 μm with soft X-rays and a wavelength of α = 2.4 nm with a weak background of Bremsstrahlung in a region 8. The suπounding water still has a monochromatizing effect and will suitably transmit the line with the wavelength of 2.4 nm, but will strongly absorb the Bremsstrahlung of a higher energy. The soft X-rays thus obtained can be used so as to lπadiate an object to be imaged in an X-ray microscope. In Fig. lb the fluid jet 2 is irradiated by an electron beam 6 which oπginates from a standard CRT gun (not shown). In this case the fluid jet 2 has an elliptical cross- section with a height of, for example, 20 μm and a width of, for example, 100 μm. The electron beam 6 focused onto the fluid jet by the CRT gun produces an electron spot 8 having a cross-section of approximately 50 μm. The electron beam is subject to an acceleration voltage of. for example, 30 kV and transports a current of, for example, 1 mA. As is the case in Fig. la, the surrounding water has a monochromatizing effect on the soft X-rays generated. When an elliptical fluid jet of the above (comparatively large) dimensions of 20x100 μm is used, it may occur that the vacuum system cannot adequately discharge the vapor produced by the jet, so that the pressure in the system could become too high for the use of an electron εun. In such cases use can be made of the configuration shown in Fiε. lc in which the fluid et 2 is also irradiated by an electron beam 6 which oπginates from a standard CRT gun (not shown) The cross-section of the electron beam again amounts to 50 μm, but in this case the fluid jet 2 has a circular cross-section of the order of magnitude of, for example, 10 μm As a result of this configuration, the X-ray spot 10 has a dimension which is not larger than the cross-section of the fluid jet, that is 10 μm in this case Fig 2 shows diagrammatically the beam path m a transmission X-ra \ microscope according to the invention In a transmission X-ray microscope the image is formed by irradiating the object to be imaged (the sample) moie or less uniformly by means of X-rays, the object thus lπadiated being imaged by means of a projecting objective lens which is in this case formed by a Fresnel zone plate A Fresnel zone plate is a dispersive element This could give πse to imaging defects w hich limit the resolution and are, of course undesirable Thus, it is necessary for the irradiating X-iav source to be as monochromatic as possible, this requirement is more than adequately satisfied by the X-ray source according to the invention In the configuration shown in Fig 2 it is assumed that the X-ray source is formed by an X-ray spot 8 which itself is formed in a fluid jet 2 by an electron beam 6 which oπginates from a SEM system, the flow direction of said fluid jet 2 extending perpendicularly to the plane of drawing. In this case the electron spot, and hence the X-ray spot, is (much) smaller than the cross-section of the fluid jet. The X-ray beam 12 oπginating from the X-ray spot 8 more or less uniformly irradiates the object 14 to be imaged by means of the X-ray microscope The object 14 is situated at a distance 26 of, for example, 150 μm from the X-ray spot X-rays are scattered by the obiect 14 as represented by a sub-beam 16 o r scattered X-rays Each irradiated point-shaped area of the obiect produces such a sub-beam The sub-beams thus formed are incident on the objective 18 which has a typical focal distance of 1 mm and a typical diameter of 100 μm The objective images the relevant point on the image plane 22 via the sub-beam 20 When the obiect distance 28 is then equal to 1.001 mm and the image distance equals 1000 mm. the magnification is 1000 \ tor the giver focal distance of 1 mm In order to pievent the X-rav spot 8 which irradiates through the object 14 from being imaged by the objective 18 m the space between the objective and the image plane 22, thus overexposing the image in the image plane, an X-ray adsorbing shielding plate 24 is aπanged at the center of the objective A detector which is sensitive to the X-rays of the relevant wavelength is aπanged in the image plane 22 For this purpose use can be made of an X-ray-sensitive CCD camera whose detector surface is coincident with the image plane 22 An example of such a CCD camera is a CCD camera of the so-called ' back illuminated" type such as the camera type NTE/CCD-1300 EB from "Pπnceton Instruments", a "Roper Scientific" company Fig. 3 is a diagrammatic representation of the beam path in a scanning transmission X-ray microscope according to the invention In a scanning transmission X-ray microscope the image is formed by scanning the object to be imaged in conformity with a given scanning pattern, that is, with a reduced image of the detecting the X-rays scattered by the object as a function of the location on the object lπadiated by the image of the X-ray spot. The image of the X-ray spot is then obtained b\ means of an objective lens When this lens is formed as Fresnel zone plate, the lπadiating X- ray source should again be as monochromatic as possible For the configuration shown in Fig 3 it is assumed again that the X-ray source is formed by an X-ray spot 8 which is formed in a fluid jet 2 by an electron beam 6 oπgmating from a SEM system, the flow direction of said jet extending perpendicularly to the plane of drawing. The electron spot, and hence the X-ray spot, is (much) smaller than the cross-section of the fluid jet. In this case the width of the fluid jet in the direction perpendicular to the electron beam is much greater than that in the direction of the electron beam, for example, it has a width of 100 μm and a height of 20 μm. The electron beam 6 is scanned across the fluid jet in the longitudinal direction 32a, for example, by means of the standard scan coils m a SEM. As a result, the X-ray spot thus produced moves in the same way. The objective lens 34 formed by the Fresnel zone plate is aπanged in such a manner that it images the X-ray spot 8 formed in the fluid jet on the object 14. Due to said displacement of the X-ray spot m the direction 32a, the image 36 thereof which is formed or the object is also displaced, that is, the direction of the aπow 33b which opposes the direction 32a due to the lens effect of the objective 34 The X-rays 38 scattered by the objec are detected again by the detector 22 and, like in the configuration shown in Fig. 2, an X-raλ absorbing shielding plate 24 is aπanged m the objective so as to prevent the X-ray spot 8 from coming into sight of the detector 22 Fig 4 shows diagrammatically the beam path in a transmission X-ra> microscope in which the electron source generating the X-rays is formed by a standard electron gun (not shown) for a cathode ray tube which is capable of dehveπng a beam cuπent of the order of magnitude of 1 mA. The configuration shown in Fig. 4 is mainly identical to that shown in Fig. 2, except for the already mentioned difference concerning the electron source and the presence of a condenser lens 40 Fig 4. Because the X-ray spot 8 in this configuration has dimensions of the same order of magnitude as the object 14 (for example from 50 to 100 μm), the condenser lens 40 is provided in the form of a Fresnel zone plate 40 The condenser lens 40 images the X-ray spot 8 on the object 14 in reduced form; the entire further imaging process is the same as already descπbed with reference to Fig. 2.

Claims: CLAIMS 1 An X-ray microscope which includes a device for generating X-rays which device is provided with * means for producing a fluid jet (2), * means for forming a focused radiation beam whose focus is situated on the fluid jet. characteπzed m that the focused radiation beam consists of a beam (6) of electπcally charged particles 2 An X-ray microscope as claimed in Claim 1, wherein the beam of electncall) charged particles is formed by an electron beam 3. An X-ray microscope as claimed in Claim 1, wherein the cross-section of the fluid jet (2) in the direction of the focused beam is smaller than that in the direction transversely thereof. 4. An X-ray microscope as claimed in Claim 1, wherein the fluid jet consists mainly of liquid oxygen or nitrogen 5. An X-ray microscope as claimed in Claim 1, wherein the means for producing a focused beam of electπcally charged particles are formed by a standard electron gun for a cathode ray tube, the X-ray microscope also being provided with a condenser lens (40) which is aπanged between the fluid jet (2) and the object (14) to be imaged by means of the X-ra) microscope 6 An electron microscope which produces a focused electron beam (6) and is provided with a device for generating X-rays, characteπzed in that the device includes: * means for producing a fluid jet (2), * means for directing the focus of the electron beam (6) onto the fluid jet 7 An electron microscope as claimed in Claim 6 which is provided with an X- ray microscope and wherein the device for generating X-rays acts as the X-ray source for the X-ray microscope 8 An electron microscope as claimed in Claim 6 or 7, the electron microscope being a scanning electron microscope

Inventors: BUIJSSE BART, BUIJSSE, BART
Applicants: PHILIPS ELECTRON OPTICS BV, PHILIPS ELECTRON OPTICS B.V.
Application year: 2000
Main classification: G21K7
IPC codes: G21K1, G21K5, G21K7, H05G2