Abstract:
A window membrane is permeable to electromagnetic radiation, especially soft X-rays. It comprises a film ( 201 ) and a metallic reinforcement mesh ( 202 ) attached to the film ( 201 ). A preferable way of attaching the metallic reinforcement mesh ( 202 ) to the film is to use a positive-working photosensitive glue ( 204 ) and allow the reinforcement mesh ( 202 ) to act as the exposure mask.

Description:
TECHNICAL FIELD  
       [0001]     The invention concerns generally the technology of reinforced membranes that have certain desired transmission characteristics of electromagnetic radiation. Especially the invention concerns a membrane that can be used as a window in X-ray detector and analyzer devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     The inside of an X-ray detector and/or analyzer appliance, or at least the inside of the component in which X-rays propagate, is often evacuated to a degree at which for practical purposes it constitutes a vacuum. A window in the wall of the vacuum container, through which the X-rays should pass, must fulfill contradictory requirements. On one hand it should attenuate the soft X-rays as little as possible, in order not to interfere with the measurement. On the other hand it must be mechanically strong enough to withstand the pressure difference.  
         [0003]     In this description we use the term “film” to mean a thin material layer of uniform thickness, and the term “membrane” to mean generally a structure that is relatively thin, i.e. has a very small overall dimension in one direction compared to its dimensions in the other, perpendicular dimensions. A membrane may consist of several materials and may have significant local variations in its thickness, and may exhibit structural topology, such as reinforcement ridges.  
         [0004]      FIG. 1  illustrates the cross section of a membrane structure for X-ray detector and analyzer devices known from the patent publication U.S. Pat. No. 5,039,203. The solid, continuous window film  101  is made of e.g. diamond, beryllium or a plastic like polyimide, which can be easily grown or spun into desired thickness on the flat surface of a specifically prepared substrate. The substrate may be e.g. a silicon wafer. During the manufacturing process the other surface of the substrate is patterned with a photoresist, and the gaps in the pattern are etched away to leave a grid of reinforcement bars that appear in the cross-section of  FIG. 1  as blocks  102 . In other words, the same material that appeared as the substrate during the manufacturing also appears as a reinforcement in the completed structure. Wider continuous sections  103  of the combined substrate and reinforcement material frequently remain at the edges of the window to make it easier to attach it into an attachment frame.  
         [0005]     Another membrane structure is known from patent publication U.S. Pat. No. 5,578,360. In a cross section drawing it resembles that of  FIG. 1 , even if the manufacturing method and the whole structural approach are completely different. The starting point is again a window film  101  made of plastic like polyimide. However, the reinforcement grid is not made of the substrate material of the manufacturing time, but of a photosensitive polymer that is spread on top of the window film. Those parts of the photosensitive polymer that should remain as reinforcement bars are exposed to ultraviolet radiation, which causes them to polymerize and solidify, while the gap portions can be removed. Finally the combination of the window film and the reinforcement pattern is detached from the substrate material.  
         [0006]     Other prior art publications that consider membrane structures and radiation-permeable windows are U.S. Pat. No. 4,119,234, U.S. Pat. No. 4,061,944, U.S. Pat. No. 3,319,064, U.S. Pat. No. 3,262,002, and U.S. Pat. No. 2,241,432.  
         [0007]     A thin polyimide film as such lets through gas molecules too easily to be used as the sole constituent of the window film. A barrier treatment of e.g. ceramic nature is often used to decrease the unwanted diffusion of gases through the window membrane. Barrier deposition may also be used to block out unwanted visible light or other interfering bandwidths of the electromagnetic spectrum. However, the barrier treatments have only a negligible effect in the structural considerations that are involved in this description, and can therefore be mainly omitted by mentioning that a person skilled in the art would know to add the barrier(s).  
         [0008]     There are certain drawbacks in the membrane structures that follow the principle of  FIG. 1 . Using silicon as the combined substrate and reinforcement material results in modest tolerance of changes in temperature. The thermal expansion coefficients of the materials of the window film  101  and the silicon reinforcement grid are typically so different that the lateral force resulting from different amounts of thermal expansion easily causes the window film to be peeled off, especially if polymer window films are used that otherwise would have many advantages over diamond.  
         [0009]     We may also consider the characteristic dimensions designated as A, B, C and D in  FIG. 1  and their effect to the applicability of the window. The thickness A of the window film is typically little less or little more than one micrometer, like 0.3-0.5 micrometers for polyimide and 4 micrometers for diamond. In U.S. Pat. No. 5,039,203 the thickness of the silicon substrate, which in the completed product appears as the thickness D of the reinforcement grid, is 200 micrometers. In the polymer-reinforced structure of U.S. Pat. No. 5,578,360 the polyimide grid is about 300 micrometers thick. The width B of the reinforcement bars varies from the 40-50 micrometer scale of the polymer reinforcement to the 600 micrometer width of the silicon laths in U.S. Pat. No. 5,039,203, and the gap width C is about 150 micrometers in the polymer-reinforced structures and several millimeters in U.S. Pat. No. 5,039,203.  
         [0010]     If the gap width C becomes smaller than the reinforcement thickness D, the collimating effect of the reinforcement grid begins to grow disturbingly large. In other words, since the gaps between adjacent reinforcement bars begin to resemble an array of tiny, mutually parallel tubes, the window has better permeability to radiation coming at a right angle than to radiation that comes at an oblique angle. This is often an undesired characteristic. Making the gap width larger would diminish the collimating effect, but this requires also increasing the thickness of the window film, which in turn increases unwanted attenuation. Additionally a larger structural module of the reinforcement mesh makes the thermal expansion problems worse.  
         [0011]     It is possible to decrease the reinforcement grid thickness if a separate mechanical support mesh made of a mechanically strong material like tungsten is placed in stack with the window membrane so that the last-mentioned may lean against the support mesh. However, such an arrangement has the inherent drawback that the support mesh only helps against a pressure difference in one direction. Should the direction of the pressure difference change e.g. due to the window being placed incorrectly or due to a pressure fluctuation during a manufacturing or servicing step, the window will burst immediately onto that side that does not have a support mesh. Using two support meshes, one on each side, would introduce too much attenuation, especially if the meshes were not perfectly aligned, which is difficult.  
       SUMMARY OF THE INVENTION  
       [0012]     An objective of the present invention is to present a window membrane and a window member that has advantageous mechanical characteristics and isotropic permeability. Another objective of the invention is to present a window membrane and a window member that is widely applicable to different kinds of detector and analyzer devices. A yet another objective of the invention is to present a method for manufacturing the window membrane and the window member mentioned above in a way that has low unit cost and good yield.  
         [0013]     The objectives of the invention are achieved by glueing a reinforcement mesh onto a window film using a positive-working photosensitive glue.  
         [0014]     A window membrane according to the invention is characterized in that it comprises a film and a metallic reinforcement mesh attached onto one surface of the film.  
         [0015]     A window member according to the invention is characterized in that it comprises a membrane in which a film and a metallic reinforcement mesh attached to the film form a composite structure, and an edge of said membrane for installing the window member gastightly to an opening in an X-ray detector or X-ray analyser device.  
         [0016]     A method for manufacturing a window membrane according to the invention is characterized in that it comprises producing a film and attaching a metallic reinforcement mesh onto one surface of the film.  
         [0017]     Materials such as tungsten that have good tensile strength do not need to be thick to make a mesh that can withstand considerable pressure in the direction perpendicular to the mesh. This property has been previously utilized in solutions where a complete window consists of a stack of a reinforced window membrane and a separate support mesh. The present invention introduces a composite structure, in which a reinforcement mesh is permanently attached to one surface of the window film. An advantageous material for attaching is a positive-working photosensitive glue, where “positive-working” means that unexposed parts solidify whereas exposed parts can be easily removed later in the process. Using a positive-working photosensitive glue is especially advantageous, because the reinforcement mesh can itself act also as an exposure mask.  
         [0018]     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 unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.  
         [0019]     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 objects 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 DRAWINGS  
       [0020]      FIG. 1  illustrates a prior art membrane structure,  
         [0021]      FIG. 2  illustrates a structural principle according to an embodiment of the invention,  
         [0022]      FIG. 3  illustrates some method steps according to an embodiment of the invention,  
         [0023]      FIG. 4  illustrates manufacturing a window membrane according to an embodiment of the invention, and  
         [0024]      FIG. 5  illustrates a manufacturing method according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 2  illustrates schematically a composite membrane structure according to a principal embodiment of the invention. The basic structural parts of the membrane are a continuous window film  201  and a reinforcement mesh  202 . Since  FIG. 2  is a cross-section drawing, only some portions of the reinforcement mesh are visible in the form of hatched rectangles. The reinforcement mesh may continue as extended solid portions  203  towards the edges of the window to facilitate more reliable fitting to a frame (not shown in  FIG. 2 ). Between the reinforcement mesh  202  and the window film  201  there is a layer of solid material  204 , which acts like a glue and attaches the reinforcement mesh  202  to the surface of the window film  201 .  
         [0026]      FIG. 3  illustrates an excerpt of a manufacturing process, in which at some previous manufacturing steps  301  and  302  there are formed a window film and a reinforcement mesh respectively. At some later step  303  in the process, the window film and the reinforcement mesh are attached together to form a composite structure. The manufacturing method may include other steps before steps  301  and  302  and after step  303 , as well as between steps  301  and  302  and step  303 .  
         [0027]      FIG. 4  is a step-by-step schematic illustration of the manufacturing of a window membrane according to an embodiment of the invention. We will omit any edge considerations and only consider what happens in a certain central section of a window membrane that is produced. A flat surface of a substrate  401 , such as a silicon wafer, is first prepared for the manufacturing of polymer thin films on top of it just like in any known thin film manufacturing techniques. A polymer film  402  is produced on said surface for example by spinning a polymer solution into a desired uniform thickness and curing the polymer into solid form.  
         [0028]     On top of the polymer film  402  there is formed another polymer layer  403  of a positive-working photosensitive polymer. Positive-working photosensitive polymers are materials that solidify slower if exposed to particular kind of radiation, typically ultraviolet radiation. Examples of positive-working photosensitive polyimide materials are the brands RN-901 and RN-902 of Nissan Chemical Industries Ltd; other varieties have been widely treated in standard literature of photochemistry. The polymer layer  403  is soft baked in order to facilitate easier handling in the subsequent step, but not fully cured.  
         [0029]     Next, there is formed yet another polymer layer  404  on top of the previous polymer layer  403 . Layer  404  consists also of a positive-working photosensitive polymer, and may well be of the same substance as layer  403 . While the topmost layer  404  is still wet, a reinforcement mesh  405  is placed on top of it, with the obvious effect that the reinforcement mesh  405  at least partly sinks into the wet polymer solution of layer  404  or at least sticks to its surface. The reinforcement mesh  405  is made of material having high tensile strength; the question of dimensioning the mesh is considered in further detail later.  
         [0030]     Since the lower positive-working polymer layer  403  was still not fully cured, we may assume that in practice it forms, together with the upper positive-working polymer layer  404 , a combined layer  406 . In the following step the layered structure is exposed to ultraviolet radiation coming from a normal direction of the plane of the substrate  401 , and from that side on which the polymer layers and the reinforcement mesh have been placed. The radiation keeps the positive-working polymer of the combined layer  406  from solidifying on exposed areas, which are those coincident with holes in the reinforcement mesh  405 . Directly under the wires of the reinforcement is shadow, so the cross-hatched regions  407  will solidify.  
         [0031]     In a developing stage the exposed, unsolidified photosensitive polymer is removed, leaving just the reinforcement mesh  405  that is glued to the polymer film  402  by the solidified polymer regions  407 . In order to complete the curing of these regions, the structure is subjected to hard baking. The composite membrane, which consists of the polymer film  402 , solidified polymer regions  407  and the reinforcement mesh  405 , is removed from the substrate  401  for example by wet etching. Before the application of any barrier treatments, the polymer film  402  is still relatively permeable to the molecules of the etching substance, which means that it is not necessary to etch out the whole substrate  401 . It is sufficient to let some of the etching substance diffuse through the polymer film  402  to detach the composite membrane from the surface of the substrate  401 . The lowest part of  FIG. 4  shows the membrane detached from the substrate.  
         [0032]     We may consider certain aspects of dimensioning the parts shown in  FIG. 4 . The thickness of the substrate  401  is of no importance. For example, if a silicon wafer is used as a substrate, it suffices to select a wafer that is readily available at reasonable cost, is applicable to the production of polymer thin films on its surface and lends itself well to handling in the process. The thickness of the cured polymer film  402  is typically in the order of some hundreds of nanometers, for example 300 nm. It should be as thin as possible to minimize attenuation, but thick enough to stand the pressure difference across the regions that coincidence with holes in the reinforcement mesh.  
         [0033]     The role of the first positive-working photosensitive polymer layer  403  is to protect the polymer film  402  during the manufacturing process, so that the edges of the mesh wires will not come into contact with the polymer film  402 , and to add flexibility to the complete structure by ensuring that in each part of the structure there will be at least some additional polymer as a buffer between the polymer film  402  and the reinforcement mesh  405 . In experiments it has been found that a suitable thickness of the first positive-working photosensitive polymer layer  403  could be in the order of a few micrometers, like 5 micrometers for example.  
         [0034]     The role of the second positive-working photosensitive polymer layer  404  is to act as a glue. The layer should be thick enough to ensure complete wetting of the reinforcement mesh  405 . Similarly with the first positive-working photosensitive polymer layer  403 , the second positive-working photosensitive polymer layer  404  could be a few micrometers thick, like 5 micrometers for example. We assume that for a workable solution, the combined thickness of the first and second positive-working photosensitive polymer layers should be more than one micrometer and less than 25 micrometers.  
         [0035]     The dimensioning and material of the reinforcement mesh  405  are selected to ensure sufficient tensile strength to withstand the pressure difference between atmospheric pressure and the very low pressure inside an X-ray detector or analyzer device. Another thing to consider is suitability for strong adhesive bonds with the photosensitive polymer in its cured form. If tungsten is used as the material of the reinforcement mesh, holes in the mesh constitute something like 70% of its surface area, and the overall window diameter is in the order of about one centimeter, the thickness of the reinforcement mesh  405  in the direction perpendicular to the plane of the mesh could be between 10 and 50 micrometers, typically 25 micrometers. The shape of the holes in the mesh does not have much importance to the invention, but conventionally they are circular, triangular or hexagonal. Hole diameter is typically in the order of a few micrometers. Known techniques exist for producing this kind of a mesh for example by electron beam lithography.  
         [0036]     A manufacturing method that has corresponding steps that were described above is illustrated stepwise in  FIG. 5 . Step  501  means preparing the substrate and step  502  means spinning the initial polymer layer that will constitute the window film onto the substrate. Step  503  is needed to cure the film so that its thickness, evenness and continuity will not be affected by the subsequent steps. Steps  504  and  505  mean applying and preliminarily solidifying the first positive-working photosensitive polymer layer. In step  506  the second positive-working photosensitive polymer layer is applied, and at step  507  the reinforcement mesh is dipped into it; a preparation step  508  of the reinforcement mesh is shown separately.  
         [0037]     The soft baking step  509  makes the structure stabile enough for taking it to the exposure step  510 , after which there follows developing at step  511  where the exposed portions of the positive-working photosensitive polymer are removed. Hard baking is made at step  512  and the membrane is etched off the substrate at step  513 . Dry baking at step  514  dries off the etching substance. Gas and light barrier layers are applied according to known practice at step  515 . Typically towards the end of the manufacturing process there are also steps like cutting the individual windows loose from a batch in which they were manufactured together, and attaching the window membrane to an installing frame.  
         [0038]     The examples described above should not be construed as exclusive limitations. For example, other polymers than polyimide can be used, and the whole membrane does not need to consist of layers of the same basic polymer. Basically even the film material does not need to be a polymer, although polymers have significant advantages concerning e.g. easy handling in the manufacturing process. Tungsten is not the only possible material of the reinforcement mesh, but other materials, especially other metals, that have suitable tensile strength and other advantageous properties could be used as well. The mesh does not need to consist of one material only, but it may comprise e.g. an alloy of different metals or it may in turn consist of layers attached together previously in the manufacturing sub-process of the mesh. Instead of applying the protective layer and glue layer onto the polymer film and placing a dry mesh into the stack it may prove to be possible to pre-wet the mesh and apply it as such directly onto the film. It may also prove to be possible to omit the protective layer and use a glue layer only (i.e. to apply a glue layer onto a clean film and placing the mesh on top), if a suitable thickness, constitution and other parameters can be found for such a “standalone” glue layer.