The invention concerns the technical field of radiation window foils and radiation windows. Especially the invention concerns a radiation window structure that has very low unwanted absorption of X-rays and good tolerance of pressure differences even if the window is large, and even if the window needs to tolerate wide variations in temperature.
A radiation window is a structural element with an opening arranged for electromagnetic radiation to pass through. In most cases a radiation window foil covers the opening, separating for example the inside of e.g. a detector apparatus from its outside. The radiation window foil should absorb the desired radiation as little as possible, but it must simultaneously be strong enough and pinhole-free to withstand and maintain a pressure difference.
In the fourth step from above in
In
Despite their numerous advantageous features, radiation windows and window foils produced with the methods of
The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.
In accordance with a first aspect of the invention, there is provided a radiation window foil for an X-ray radiation window. The radiation window foil comprises:
In accordance with a second aspect of the invention, there is provided a radiation window. The radiation window comprises:
In accordance with a third aspect of the invention, there is provided a method for manufacturing a radiation window foil. The method comprises:
Various exemplifying embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of the exemplifying embodiments when read in connection with the accompanying drawings.
The exemplifying embodiments of the invention presented in this document are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this document as an open limitation that neither excludes nor requires the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
In this description we use the following vocabulary concerning quasi-two-dimensional structural elements. 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 grid is a special case of a mesh, comprising essentially parallel beams that extend across the whole area covered by the grid, so that the openings mentioned above are the elongated slits that remain between the beams.
Additionally we use the following vocabulary concerning window foils and windows. A radiation window foil is a foil that has suitable characteristics (low absorption, sufficient gastightness, sufficient mechanical strength etc.) for use in a radiation window of a measurement apparatus. A radiation window is an entity that comprises 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.
Additionally we use the following vocabulary concerning the interfacing of adjacent layers. Two layers are stacked together, or one layer is stacked on the other, when they form an integral structure and when their stacked configuration has come into existence without both layers existing previously in separate layer form. Thus, for example, when a thin film deposition method (such as chemical vapour deposition, atomic layer deposition, pulsed laser deposition or the like) is used to form or “grow” a new material layer onto a previously existing material layer, as a result the new layer becomes stacked on the previously existing material layer. Other examples of methods that produce stacked layers are ion implantation, annealing, and other surface treatments, which cause the characteristics of a treated surface up to a certain depth to change sufficiently so that the affected portion will behave differently than the material portion(s) below it. As a result, the affected portion constitutes a layer stacked together with the other layer(s) constituted by the material portion(s) below it.
Contrary to stacking, two layers are sandwiched together, or one layer is sandwiched on the other, when both layers existed in layer form before their configuration as two solid parts of a layered structure came into existence. It should be noted that sandwiching as a term does not exclude attaching, even relatively tightly, the layers to each other. As an example of sandwiching, a known manufacturing technique of SOI (Silicon On Insulator) wafers comprises bringing two highly polished pieces of silicon together, so that they become bonded by van der Waals forces. Bonded layers are thus a special case of sandwiched layers. Spreading a liquid substance onto a solid surface and subsequently allowing the liquid substance to solidify is another special case of sandwiching, because what becomes the solid top layer previously existed as a liquid layer before the configuration came into existence where two solid layers are adjacent to each other. Clearly sandwiched configurations are such where two previously manufactured foils are laminated together, or a separate reinforcement grid or mesh is placed adjacent to a radiation window foil to enhance its mechanical strength.
The second step illustrated in
The third step illustrated in
Alternative methods can be used to provide the layered structure illustrated in the third step of
A requirement placed by the SOI method explained above is that the first etchable layer 103 exists in solid, layer-like form before it comes into contact with the etch stop layer 102. This in turn sets certain minimum thickness requirements to the first etchable layer 103, although such minimum thickness requirements naturally depend on the technology that is used to produce and handle the first etchable layer 103 before bonding it to the etch-stop-layer-covered carrier. In semiconductor component manufacturing processes the thicknesses of wafers are in the order of several hundreds of micrometers: for example silicon wafers typically come in thicknesses from the 275 micrometers used for 2-inch wafers to the 925 micrometers that is expected to be a standard thickness of the future 450 millimeter wafers. Thicknesses of wafers aimed for photovoltaic components are typically in the order of 200-300 micrometers. The first etchable layer 103 may be monocrystalline, especially if it comes from a manufacturing process that was originally aimed at producing wafers for the production of semiconductor components.
After successful bonding to the etch-stop-layer-covered carrier the first etchable layer 103 does not need to be as thick anymore, because it is mechanically supported by the etch-stop-layer-covered carrier to which it is bonded.
Therefore the method may comprise thinning the first etchable layer 103 into a predetermined thickness. For example, after bonding to the etch-stop-layer-covered carrier, the first etchable layer 103 can be thinned to a thickness in the order of some tens of micrometers, like 15 micrometers. For thinning, known methods exist and are used for example in manufacturing SOI wafers. These methods may include at least one of grinding, etching, and polishing.
It should be noted that after the bonding of the first etchable layer 103 to the etch-stop-layer-covered carrier 101, and before any thinning is made, the structure may exhibit significant symmetry (depending on the original thicknesses of the carrier 101 and the first etchable layer 103). Therefore a possibility exists to switch the roles of the carrier and the first etchable layer in the continuation; for example the layer to be subsequently thinned may be the one that first received the etch stop layer on its surface. The designations “carrier” and “first etchable layer” are just names that are used in this description to illustrate the role of certain layers in a particular embodiment of the invention.
Yet another possibility for providing the layered structure of the third step in
The lowest step in
The beams or ribs of the first mesh or grid layer define openings that have certain shape and size. In case of a grid, the openings are elongated; in case of a mesh, the openings may be e.g. hexagonal, triangular, or rectangular, or they may have the shape of a diamond or a trapezoid defined by straight beams that intersect at oblique angles. The characteristic dimensions of the mesh may be for example in the order of 20 to 500 micrometers across each opening, with a width of the ribs in the mesh in the order of 5 to 20 micrometers.
The topmost step in
The second step in
In principle it would be possible to etch away the portions of the first etchable layer and the second etchable layer in the same etching step. However, since the difference in thickness between the two layers is so large, the etching time required by the two of them is very different, and consequently a better result is in many cases achieved by using two separate etching steps. Since the first-made mesh or grid layer is there already when the second etching begins, appropriate measures must be taken to protect the already completed mesh or grid layer during the second etching step. Basically it is possible to do the etching steps in any order, but since the carrier (i.e. the second etchable layer) has a major supporting function while it is still intact, it may be more advantageous to make the etching steps in the order in which they have been described above.
The beams or ribs of the second mesh or grid layer define openings that have certain shape and size. In case of a grid, the openings are elongated and the beams of the grid are preferably spaced at intervals between 2 millimeters and 10 millimeters; in case of a mesh, the openings may be e.g. hexagonal, triangular, or rectangular, or they may have the shape of a diamond or a trapezoid defined by straight beams that intersect at oblique angles. The characteristic dimensions of the mesh may be for example in the order of 3 to 10 millimeters across each opening. A width of the ribs in the mesh (or beams in the grid) may be in the order of 10 to 1000 micrometers.
The width of the ribs may depend, at least to a certain extent, on the thickness of the second etchable layer before etching, as well as on factors like the crystal orientation of the material of the second etchable layer. Similar considerations must be taken into account in all process steps where material is removed by etching. For example, certain crystal orientations are more prone to so-called underetching than others, for which reason they may set limits to the width-to-height ratio of patterns that are expected to remain after the etching. If the material to be etched is monocrystalline silicon, it is known that KOH etches up to 400 times faster in the 100 direction than in the 111 direction (the three-digit codes are the widely used Miller indices). TMAH exhibits similar anisotropy by etching almost 40 times faster in the 100 direction than in the 111 direction.
In the structure discussed above, if only mechanical optimisation was considered, the beams or ribs of the second mesh or grid layer should have their height to width ratio as large as possible. However, the etching method(s) to be used may prompt to make them wider, in order to avoid the beams or ribs to be eaten too thin or even destroyed by the underetching phenomenon. Also when the mask is designed for the etching, certain directions (in relation to the crystal orientation) may be deliberately favoured or avoided, in order to control the amount of underetching and the amount, quality, and edge formation of open area that is to be exposed.
Different etching methods can be combined to optimize processing time and accuracy. It has been found that a particularly advantageous combination for producing the second mesh or grid layer is to first use dry etching (for example: plasma etching) to eat away a majority (like 90%) of the silicon to be removed, and to then accomplish the final opening of the grid or mesh with wet etching in KOH or TMAH. Such a combination of etching methods is relatively fast in overall processing time, and it helps to control the crystal-orientation-dependent phenomena, because the wet etching time remains relatively short.
As with above in association with the first mesh or grid layer, the dimensions illustrated in
The second step in
A second mesh or grid layer is stacked on or bonded to the second side of the continuous window layer 502. Also here the stacked/bonded nature of the configuration comes from the method that was used to originally produce the etch stop layer that became the continuous window layer: methods involving thin film deposition technologies as well as those resembling the SIMOX process result in a stacked configuration. A bonded configuration could come from a process resembling the manufacturing of SOI wafers from two component wafers, if the oxide layer was first produced on what became the first etchable layer instead of on the second etchable layer.
The thickness (i.e. the characteristic dimension in the direction perpendicular to the plane defined by the radiation window foil) of the second mesh or grid layer is 20 to 120 times the thickness of the first mesh or grid layer. As an example, the thickness of the continuous window layer may be between 10 nanometers and 200 nanometers; the thickness of the first mesh or grid layer may be between 5 micrometers and 15 micrometers; and the thickness of the second mesh or grid layer may be between 300 micrometers and 600 micrometers.
The last step in
The mutual order of the last two steps of
In the embodiment described above we have assumed that only the second mesh or grid layer has a mesh or grid portion encircled by a frame portion. However, it is possible to make also the first mesh or grid layer have a mesh or grid portion encircled by a frame portion, preferably aligned with those of the second mesh or grid layer.
Above we have also assumed that the layer that was originally produced as the etch stop layer would alone constitute the continuous window layer 502. However, additional layers can be used.
A class of embodiments of the invention has such an additional layer as the main layer of the foil portions that span openings in the first mesh or grid layer. The term “main layer” means that such a layer would be the principal carrier of loads that result from the surrounding gaseous substance trying to even out the pressure difference across the radiation window by flowing through one opening in the first mesh or grid layer. The previous patent publication WO2011/151506, which is incorporated herein by reference, describes in detail how such a main layer is produced after etching away the appropriate portions to make the first mesh or grid layer, but in a process step where the second etchable layer on the other side of the etch stop layer is still continuous. A feature of this class of embodiments of the invention is that the etch stop layer, which appeared as layer 102 in
A radiation window foil according to an embodiment of the invention has truly exceptional tolerance of temperature differences, compared to known radiation window foils. A commercially available radiation window foil that is well-known and widely used at the date of writing this description can hardly tolerate an increase of temperature in the order of 40 degrees centigrade. Concerning the present invention, tests were made to evaluate the temperature tolerance of the radiation window foil by maintaining a pressure difference of one atmosphere across the foil and subjecting it to repeated temperature cycles between liquid nitrogen (−196 degrees centigrade) and a heated oven at +250 degrees centigrade. The temperature difference of almost 450 degrees centigrade did not have any noticeable effect on the gastightness or structural strength of the radiation window foil.
The exceptional tolerance of wide variations in temperature appears to be a consequence of the fact that all principal materials of the radiation window foil have their coefficients of thermal expansion very close to each other, as well as of the fact that the various layers have been integrated through processing, i.e. stacked or bonded, without any glues or other additional attaching means. In one embodiment of the invention, there are only the silicon nitride of the continuous window layer and the silicon of the first and second mesh or grid layers. The coefficients of thermal expansion of silicon nitride and silicon at room temperature are 3.2 ppm/K and 2.6 ppm/K respectively. As a comparison, the coefficients of thermal expansion of beryllium is 11.3 ppm/K, copper 16.5 ppm/K, tin-lead solder in the order of 27-30 ppm/K and epoxy about 55 ppm/K. Of pure metals, only tungsten (4.5 ppm/K, although some sources report values ranging between 5.7 and 8.3 ppm/K) comes even relatively close to silicon and silicon nitride by its coefficient of thermal expansion.
If the radiation window is very large and/or if it must stand very large pressure differences, the radiation window foil can be further reinforced. In the embodiment illustrated in
Several precautions may be taken to avoid problems that could otherwise occur due to the different coefficients of thermal expansion of the materials. The material of the radiation window frame 801 may be selected so that its coefficient of thermal expansion is a suitable compromise between that of the radiation window foil and that of the “can” 804. Also design features of the radiation window frame 801 may be employed. In the embodiment of
The view in
Variations to the embodiments described above are possible without departing from the scope of protection defined by the appended claims. For example, a mesh or grid does not need to repeat itself in exactly similar form across the whole of the radiation window foil, but there may be mesh or grid portions where the form of openings, pitch of beams or ribs, or other structural parameter of the mesh or grid changes either abruptly or little by little. As another example, the attachment of the radiation window foil to the radiation window frame may take place on any side of the radiation window foil, or even on both sides if the radiation window foil is squeezed between a matching pair of radiation window frame halves or if a securing ring is attached on top of the joint between the radiation window foil and an annular radiation window frame. As another example, a common radiation window foil may seal two or more adjacent openings in the radiation window frame.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FI2012/050804 | 8/22/2012 | WO | 00 | 6/30/2015 |
Publishing Document | Publishing Date | Country | Kind |
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WO2014/029900 | 2/27/2014 | WO | A |
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Number | Date | Country | |
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20150357150 A1 | Dec 2015 | US |