Patent Publication Number: US-2020299833-A1

Title: Radiation window structure and a method for manufacturing the radiation window structure

Description:
TECHNICAL FIELD 
     The invention concerns in general the technical field of radiation applications. Especially the invention concerns radiation windows. 
     BACKGROUND 
     A radiation window is a part of a measurement apparatus that allows a desired part of electromagnetic radiation to pass through. In many cases the radiation window must nevertheless be gastight, in order to seal and protect an enclosure where reduced pressure and/or a particular gas contents prevail. In order to cause as little absorption as possible of the desired radiation, a major part of the radiation window should consist of a thin foil with dimensions applicable in the application area. 
     Beryllium is known as a very good material for radiation window foils especially in X-ray measurement apparata, because it has a low atomic number ( 4 ) and consequently exhibits very low absorption of X-rays. Another characteristic of beryllium that makes it very useful for radiation window foils is its exceptional flexural rigidity. The thinnest beryllium foils that are commercially available for use in radiation windows at the time of writing this description have a thickness in the order of 8 micrometres. According to prior art, the beryllium foil is manufactured from an ingot by rolling. Various coatings can be applied to the beryllium foil for example to enhance its gastightness and corrosion resistance as well as to keep undesired parts of the electromagnetic spectrum (such as visible light) from passing through the foil. An example of known radiation window foils is the DuraBeryllium foil available from Moxtek Inc., Orem, Utah, USA. It comprises an 8 micrometres thick beryllium foil coated with a DuraCoat coating. DuraBeryllium, DuraCoat, and Moxtek are registered trademarks of Moxtek Incorporated. 
     At the time of writing this description it appears that the rolling technology has met its limits in the sense that it has not been shown capable of manufacturing beryllium foils thinner than 8 micrometres so that they would still be sufficiently gastight. This phenomenon is associated with the relatively large grain size (larger than foil thickness), which results from the grain structure of the original beryllium ingot. Grain boundaries in the beryllium foil tend to cause gas leaks through the foil. Additionally beryllium has disadvantages as a material because it is toxic. This brings additional requirements for the manufacturing process. Also, the future in the utilization of beryllium is uncertain due to tightening requirements by different national authorities. 
     However, if the radiation window is very thin, the mechanical stress on different sides of the radiation window may break the thin radiation window. 
     Thus, there is a need to mitigate the aforementioned problems and develop a solution for providing a thin and gastight radiation window. 
     SUMMARY 
     An objective of the invention is to present a radiation window structure and a method for manufacturing a radiation window structure. Another objective of the invention is that the radiation window structure is gastight and the method for manufacturing a radiation window structure provides a gastight radiation window structure. 
     The objectives of the invention are reached by a radiation window structure and a method as defined by the respective independent claims. 
     According to a first aspect, radiation window structure is provided, the radiation window structure comprising a substrate made of silicon nitride and a coating layer grown from a vapor phase on outer surface of the substrate. 
     The coating layer may be made of pyrolytic carbon. 
     The substrate may be between 5 and 500 nanometers thick. 
     The coating layer may be between 20 nanometers and 5 micrometers thick. 
     Alternatively or in addition, the radiation window structure may comprise a mesh layer made of silicon between the substrate and the coating layer at least on one surface of the substrate. 
     Alternatively or in addition, the radiation window structure may comprise a silicon nitride layer between the mesh layer on at least one surface of the substrate and the coating layer. 
     The mesh layer may be between 20 nanometers and 500 micrometers thick. 
     The silicon nitride layer may be between 5 and 500 nanometers thick. 
     According to a second aspect, a method for manufacturing a radiation window structure is provided, the method comprising growing a coating layer from a vapor phase on outer surface of a substrate made of silicon nitride. 
     The growing of the coating layer may be provided simultaneously on the whole outer surface of the substrate. 
     The coating layer grown from the vapor phase may be made of pyrolytic carbon. 
     The said growing may comprise pyrolysis. 
     The gas used in said growing the coating layer from the vapor phase may be methane. 
     Alternatively or in addition, the method may comprise producing a mesh layer between the substrate and the coating layer at least on one surface of the substrate. 
     Alternatively or in addition, the method may comprise producing a silicon nitride layer between the mesh layer on at least one surface of the substrate and the coating layer. 
     The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. 
     The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objectives and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       The embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. 
         FIG. 1  illustrates a method and a radiation window according to an embodiment of the invention. 
         FIG. 2  illustrates a method and a radiation window according to another embodiment of the invention. 
         FIG. 3  illustrates a method and a radiation window according to yet another embodiment of the invention. 
         FIG. 4  illustrates a method and a radiation window according to yet another embodiment of the invention. 
     
    
    
     DESCRIPTION OF SOME EMBODIMENTS 
     In this description we use the following vocabulary. A layer means a quantity of essentially homogeneous material that by its form has much larger dimensions in two mutually orthogonal directions than in the third orthogonal direction. In most cases of interest to the present invention, the dimension of a layer in said third orthogonal direction (also referred to as the thickness of the layer) should be constant, meaning that the layer has uniform thickness. A foil is a structure, the form of which may be characterised in the same way as that of a layer (i.e. much larger dimensions in two mutually orthogonal directions than in the third orthogonal direction) but which is not necessarily homogeneous: for example, a foil may consist of two or more layers placed and/or attached together. A mesh is a special case of a layer or foil, in which the constituents do not make up a continuous piece of material but define an array of (typically regular, and regularly spaced) openings. A radiation window foil is a foil that has suitable characteristics (such as low absorption, sufficient gas tightness, sufficient mechanical strength etc.) for use in a radiation window of a measurement apparatus. A radiation window is an entity that may comprise a piece of radiation window foil attached to a (typically annular) support structure so that electromagnetic radiation may pass through an opening defined by the support structure without having to penetrate anything else than said piece of radiation window foil and the (typically gaseous) medium that otherwise exists within said opening. In this application a term radiation window structure corresponds to the term radiation window foil. 
       FIG. 1  illustrates a workpiece in various steps of a method for manufacturing a radiation window structure according to an embodiment of the invention. The topmost step illustrates a substrate  102  made of silicon nitride. The substrate  102  may be provided in a silicon frame in order to ease the handling of the radiation window structure. The substrate  102  may be between 5 and 500 nanometers thick. Preferably, the substrate  102  is between 40 to 50 nanometers thick. If the substrate  102  was thinner, its mechanical strength would be so low that the need for additional support solutions could be required. If the substrate  102  was thicker, its absorption might come too high concerning very sensitive X-ray fluorescence measurements, like the detection of sodium. The use of a substrate  102  made of silicon nitride enables manufacturing a thin and gastight radiation window. 
     In the next step of the method illustrated in  FIG. 1  a coating layer  104  is grown from a vapor phase on outer surface of the substrate  102 . In the other words the substrate  102  is coated with the coating layer  104 . The gas may be methane, for example. The coating layer  104  may be grown on the whole surface of the substrate  102  simultaneously and at the same speed. In other words all the outer surfaces of the substrate  102  are coated with the coating layer  104  simultaneously. By growing the coating layer  104  simultaneously and at the same speed on the whole surface of the substrate  102 , as illustrated in  FIG. 1 , deformation of the coating layer  104  and/or defects in the coating layer  104  may be at least partly reduced. Thus, also the quality of the coating layer  104  may be improved at least partly. The material for the coating layer  104  may preferably be selected so that the thermal expansion coefficient of the coating layer  104  material is close to the thermal expansion coefficient of the substrate  102  material. The thermal expansion coefficient of the silicon nitride is 3.27·10 −6 ° C. −1 . Preferably, the thermal expansion coefficient of the coating layer  104  material may differ from the thermal expansion coefficient of the silicon nitride less than 3·10 −6 ° C. −1 . The coating layer  104  may be made of pyrolytic carbon. The thermal expansion coefficient of the pyrolytic carbon is 0.5·10 −6 ° C. −1 . The use of pyrolytic carbon in the coating layer  104  is preferable, because of at least the following reasons: the pyrolytic carbon is not toxic, the atomic number of the pyrolytic carbon is small, helium leakage through the pyrolytic carbon is low. However, the substrate made of silicon nitride is a good diffusion barrier that assures that the radiation window is gastight. The thickness of the coating layer  104  may be between 20 nanometers and 5 micrometers. If the coating layer  104  was thinner, the manufacturing of the coating layer might become unreliable and more challenging. If the coating layer  104  was thicker, its absorption might come too high. Preferably, the coating layer  104  may be between 300 and 500 nanometers. 
     Pyrolysis may be used to grow the coating layer  104  from a vapor phase. Pyrolysis is a reaction used to coat a preformed substrate with a layer of pyrolytic carbon. Pyrolysis occurs at high temperature, such as 1000 to 2000 degrees Celsius. The gas used in pyrolysis may be methane, for example. 
       FIG. 2  illustrates part of a radiation window manufacturing method according to yet another embodiment of the invention. The first step is comparable to corresponding step in  FIG. 1  earlier. Thus, the topmost step illustrates a substrate  102  made of silicon nitride. At the second step of  FIG. 2  a mesh layer  202  is produced on at least on one surface of the substrate  102 . In 
       FIG. 2  the mesh layer  202  is produced on the second surface of the substrate  102 . Alternatively or in addition, the mesh layer  202  may be produced on the first surface of the substrate  102 .  FIG. 3  illustrates part of a radiation window manufacturing method according to yet another embodiment of the invention, wherein the mesh layer  202  is produced on the first surface of the substrate  102  and on the second surface of the substrate  102 . Otherwise, the steps in  FIG. 3  are comparable to corresponding steps in  FIG. 2 . The mesh layer  202  may be manufactured using a thin film deposition technique. Suitable methods for depositing the mesh layer  202  include, but are not limited to, chemical vapour deposition and pulsed laser deposition. At first, the mesh layer  202  is uniform. The mesh layer  202  may be made of silicon, for example. The criteria to be applied in choosing the material for the mesh layer  202  include reasonable mechanical strength, good binding to the substrate  102 , suitability to etching with an etching method and/or agent that does not affect the substrate  102  to a significant extent, low absorption of radiation, absence of disturbing fluorescence peaks and/or other awkwardly handled anomalities at the wavelengths of electromagnetic radiation at which the radiation window is to be used, as well as easy handling in a thin film deposition process. Preferably, the mesh layer  202  may be between 20 nanometers and 500 micrometers thick. 
     In the third step in  FIG. 2 , regularly spaced portions of the originally uniform mesh layer  202  are removed to turn said uniform layer into a mesh. In  FIG. 2  regularly spaced mesh is used as an example, but also irregular mesh may be used. The characteristic dimensions of the mesh may include for example a diameter of openings in the order of 1 to 5000 micrometres, and a width of the ribs in the mesh in the order of 50 to 5000 micrometres. Assuming that the process of turning the originally uniform layer into a mesh does not significantly affect those parts of the layer that are to form the ribs, the height (the dimension in the vertical direction in  FIG. 2 ) of the ribs constituting the mesh will be essentially the same as the original thickness of the uniform layer. These dimensions are examples only, and the eventual selection of characteristic dimensions should be made in accordance with the mechanical strength that the completed radiation window should have. Several methods for turning a previously uniform, thin-film-deposited layer into a mesh layer are known, including but not being limited to photolithography, wet etching, dry etching, plasma etching, electron beam lithography, and ion beam lithography. The method should naturally be selected so that it still leaves the window continuous. The mesh layer  202  may be used to improve the mechanical strength of the window structure. 
     At the last step in  FIG. 2 , a coating layer  104  is grown from a vapor phase on outer surface of the combined structure comprising substrate  102  and at least one mesh layer  202  as described above. Thus, the mesh layer  202  is provided between the substrate  102  and the coating layer  104  at least on one surface of the substrate  102 . The last step in  FIG. 2  is comparable to corresponding last step in  FIG. 1  earlier. The mesh layer  202  between the substrate  102  and the coating layer  104  enables that the coating layer  104  may be thinner than without the mesh layer  202 . Preferably, the thickness of the coating layer  104  may be similar to the thickness of the substrate  102  if the mesh layer  202  is provided between the substrate  102  and the coating layer  104 . 
       FIG. 4  illustrates part of a radiation window manufacturing method according to yet another embodiment of the invention. The first three steps are comparable to corresponding steps in  FIG. 2  earlier. Thus, the topmost step illustrates a substrate  102  made of silicon nitride. At the second step of  FIG. 4  a mesh layer  202  is produced on at least on one surface of the substrate  102 . In the third step in  FIG. 4 , regularly spaced portions of the originally uniform mesh layer  202  are removed to turn said uniform layer into a mesh. At the next step in  FIG. 4  a silicon nitride layer  402  is produced on the at least one mesh layer  202 . The silicon nitride layer  402  may be manufactured using a thin film deposition technique. Suitable methods for depositing the silicon nitride layer  402  include, but are not limited to, chemical vapour deposition and pulsed laser deposition. Preferably, the silicon nitride layer  402  is between 5 and 500 nanometers thick. The silicon nitride layer  402  on top of the mesh layer  202  may be used to improve at least partly the mechanical strength of the window structure. 
     At the last step in  FIG. 4 , a coating layer  104  is grown from a vapor phase on outer surface of the combined structure comprising substrate  102 , at least one mesh layer  202 , and silicon nitride layer  402  as described above. Thus, the silicon nitride layer  402  is provided between the mesh layer  202  on at least one surface of the substrate  102  and the coating layer  104 . The last step in  FIG. 4  is comparable to corresponding last step in  FIGS. 1 and 2  earlier. 
     The method for manufacturing a radiation window structure described above may be implemented to manufacture a single radiation window structure that may be attached to a support structure. Alternatively, the method may be implemented to manufacture a plurality of radiation window structures on a one wafer. The plurality of the radiation window structures may be cut from the wafer. Furthermore, the plurality of the cut radiation window structures may be attached to corresponding support structures. 
     Advantages of the invention include the possibility of manufacturing radiation windows where the radiation window foil is very thin and yet gastight, and causes very little unwanted absorption or spurious responses in a measurement involving X-rays. Furthermore, the invention enables manufacturing radiation windows where the mechanical stress caused for example by thermal expansion may be at least partly reduced. Additionally, the pyrolytic carbon used in the coating layer of the radiation window structure is not toxic and it is environmentally sustainable also in the long term. 
     The specific examples provided in the description given above should not be construed as limiting the applicability and/or the interpretation of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.