Patent Application
20070111617
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.
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.
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.
Another membrane structure is known from patent publication U.S. Pat. No. 5,578,360. In a cross section drawing it resembles that of
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.
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).
There are certain drawbacks in the membrane structures that follow the principle of
We may also consider the characteristic dimensions designated as A, B, C and D in
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.
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.
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.
The objectives of the invention are achieved by glueing a reinforcement mesh onto a window film using a positive-working photosensitive glue.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
We may consider certain aspects of dimensioning the parts shown in
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.
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.
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.
A manufacturing method that has corresponding steps that were described above is illustrated stepwise in
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.
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.
