Patent Application
20080296518
The present invention relates generally to radiation detection systems and associated high strength radiation detection windows.
Radiation detection systems are used in connection with detecting and sensing emitted radiation. Such systems can be used in connection with electron microscopy, X-ray telescopy, and X-ray spectroscopy. Radiation detection systems typically include in their structure a radiation detection window, which can pass radiation emitted from the radiation source to a radiation detector or sensor, and can also filter or block undesired radiation.
Standard radiation detection windows typically comprise a sheet of material, which is placed over an opening or entrance to the detector. As a general rule, the thickness of the sheet of material corresponds directly to the ability of the material to pass radiation. Accordingly, it is desirable to provide a sheet of material that is as thin as possible, yet capable of withstanding pressure resulting from gravity, normal wear and tear, and differential pressure.
Since it is desirable to minimize thickness in the sheets of material used to pass radiation, it is often necessary to support the thin sheet of material with a support structure. Known support structures include frames, screens, meshes, ribs, and grids. While useful for providing support to an often thin and fragile sheet of material, many support structures can interfere with the passage of radiation through the sheet of material due to the structure's geometry, thickness and/or composition. The interference can be the result of the composition of the material itself and/or the geometry of the support structure. In addition, many known support structures have drawbacks. For example, screens and meshes can be rough and coarse, and thus the overlaid thin film can stretch, weaken and burst at locations where it contacts the screen or mesh. A drawback associated with unidirectional ribs is that the ribs can twist when pressure is applied. This twisting can also cause the overlaid film to stretch weaken and burst. Unidirectional ribs are set forth U.S. Pat. No. 4,933,557, which is incorporated herein by reference. Additionally, there can be substantial difficulty in manufacturing many known support structures, thus resulting in increased expense of the support structures and associated windows.
Accordingly, it has been recognized that it would be advantageous to develop a radiation detection system having a high strength, yet thin radiation detection window that is economical to manufacture, and further has the desirable characteristics of being minimally absorptive and minimizing interference with the passage of radiation therethrough. It is also desirable to provide a radiation window having a support structure that will maintain intact thin films that overlay the support structure.
Accordingly, the present invention provides a high strength window for a radiation detection system. A window for a radiation detection system includes a plurality of intersecting ribs defining a grid having openings therein, with tops of the ribs terminating substantially in a common plane. The intersecting ribs are oriented non-perpendicularly with respect to each other and define non-rectangular openings. The window also includes a support frame around a perimeter of the plurality of intersecting ribs, and a film disposed over and spanning the plurality of intersecting ribs and openings. The film is configured to pass radiation therethrough.
An associated radiation detection system includes a high strength window as described above and a sensor. The sensor is configured to detect radiation passing through the high strength window.
There has thus been outlined, rather broadly, various features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken together with the accompanying claims, or may be learned by the practice of the invention.
a is a top view of a support grid of the high strength window of
b is a photograph of the support grid of
Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.
The present invention provides embodiments pertinent to a high strength window for a radiation detection system, an associated radiation detection system, and an associated method of manufacturing a high strength grid for a window in a radiation detection system. In accordance with these embodiments, various details are provided herein which are applicable to all three of the window, system and method.
As illustrated in
As described above, the window 10 can be subjected to a variety of operating and environmental conditions, including for example, reduced or elevated pressures, a substantial vacuum, contamination, etc. Such conditions tend to motivate thicker, more robust windows. Such radiation detection systems, however, can potentially be utilized to sense or detect limited or weak sources. In addition, certain applications require or demand precise measurements. Such systems or applications tend to motivate thinner windows. Support ribs can span the window to provide support to thinner windows. These supports, however, can introduce stress concentrations into the window due to their structure (such as wire meshes), have different thermal conductivity than the window and introduce thermal stress, and can itself interfere with the radiation directly or even irradiate and introduce noise or errors. In addition, difficulty can arise in the manufacture of these supports, thus making these support structures costly and expensive. Therefore, it has been recognized that it would be advantageous to develop an economical window that is thin as possible and as strong as possible and resist introducing noise or interfering with the radiation.
The window 10 of the present invention has a plurality of intersecting ribs 12 defining a grid 18 having openings 20 therein, and a support frame 14 around a perimeter of the plurality of intersecting ribs. The support frame carries and supports the ribs. The window also has a thin film 16 disposed over and spanning the plurality of intersecting ribs and openings. This film is configured to pass radiation therethrough.
The support frame 14 can be made of the same material as the plurality of ribs 12 defining the grid 18. Accordingly, both the ribs and support frame can be or include a silicon material, although this is not required. According to one aspect, the support frame can be integral with the grid. In this case, both the support frame and grid can be formed from a single piece of material by removing or etching the openings 20 in the grid to leave the ribs joined at their ends to the support frame. Alternatively, the support frame can form a separate piece that can be coupled to the grid by an adhesive for example. In another embodiment, the support frame can be made of a material that is different from the material comprising the ribs. In addition to providing support for the grid and the layer of thin polymer film 16, the support frame can be configured to secure the window 10 to the appropriate location on a radiation detection system. Each rib comprising the plurality of intersecting ribs can be less than 100 μm wide.
The thin film 16 is disposed over and spans the plurality of ribs 12 and openings 20. The film can be selected to be highly transmissive of X-rays, for example, and of X-rays having energies greater than 100 electron volts, while blocking visible light energy and other unwanted radiation. In addition, the film can be selected to withstand fluid pressures of up to one atmosphere (caused by fluids into which the structure may be immersed) without breaking so that fluid may not penetrate the window.
The thin film can include a layer of polymer material, such as poly-vinyl formal (FORMVAR), butvar, parylene, kevlar, polypropylene, lexan or polyimide. Nonpolymer materials such as boron, carbon (including cubic amorphous and forms containing hydrogen), silicon, silicon nitride, silicon carbide, boron nitride, aluminum and beryllium could also be used. In one aspect, the film can include doped silicon, Desirably, the film should be configured to avoid punctures, uneven stretching and localized weakening. To further reduce the chance of these undesirable characteristics, the tops of the ribs 12 can be rounded and/or polished to eliminate sharp corners and rough surfaces.
The thin film should be thick enough to withstand pressures to which it will be exposed, such as gravity, normal wear and tear and the like. However, as thickness of the layer increases so does undesirable absorption of radiation. If radiation is absorbed by the layer of thin material, it will not reach the sensor or detector. This is particularly true with respect to soft X-rays, which are likely to be absorbed by a thicker film. Therefore, it is desirable to provide a thin film that is as thin as possible but sufficiently thick to withstand the pressures explained above. In one aspect, the film will be able to withstand at least one atmosphere of pressure, and thus the film can have a thickness substantially equal to or less than about 1 μm (1000 nm).
In addition, a gas barrier film layer can be disposed over the thin film.
The material comprising the thin film 16 can be different than the material comprising the intersecting ribs 12 and/or support frame 14. Alternatively, all three of the thin film material, ribs and support frame can be or include the same material. According to one embodiment, the thin film, the support frame and the intersecting ribs can be integrally formed of the same material. By way of example, and not by way of limitation, silicon may be used for this purpose. In another embodiment, the plurality of intersecting ribs can comprise silicon and the thin film material can comprise a polymeric film.
To reduce the chance of damage that can result to the thin film 16 overlaying the grid 18, the top edges of the intersecting ribs 12 can be rounded and/or polished to eliminate sharp corners and rough surfaces which might otherwise cause damage. In one aspect, forming the ribs from a single crystal of silicon by etching results in the rounding and polishing action desired. Alternatively, if other materials and method of construction are used, the tops of the ribs may require rounding and/or polishing via known mechanical and/or chemical processes.
As indicated, the ribs define a grid 18 having openings 20 therein. The ribs terminate substantially in a common plane. The ribs 12 can include or can be formed entirely of a silicon material in order to provide a high strength support for the thin film while being as thin as possible. For example, the height of the ribs can range from about 100 μm to about 385 μm, and the width of each rib can be about 60 μm. The ribs are oriented non-perpendicularly with respect to each other and define non-rectangular openings. Non-rectangular openings can assume a variety of different shapes so long as the ribs defining the openings intersect one another at other than 90 degree angles. The ribs can include a first set of parallel ribs that intersect and are oriented non-orthogonally to a second set of parallel ribs.
According to one embodiment, the openings 20 can be shaped substantially like a hexagon. The openings can also be shaped in the form of a trapezoid, such as a parallelogram. This shape can prevent twisting problems that are commonly associated with unidirectional line ribs, which experience maximum stress at the two opposing ends of the longest rib when the window receives a pressure load. When a window incorporating the unidirectional line ribs fails it is usually due to breakage at one or both ends of the longest rib. Mechanical analysis also indicates that many structures incorporating support ribs will twist when a load is applied to the window. This twisting action weakens the rib support structure and the window in general.
The arrangement of ribs 12 and openings 20 in the grid 18 of the present invention can minimize or even prevent the twisting problems experienced in prior teachings. According to one embodiment, at least one corner of each opening includes a fillet that is partially filled with a material, such as the same material as the ribs. By filling the corners, twisting action of the ribs can be further minimized or eliminated altogether. Filling the corners also results in an overall increase in strength of the support grid.
The material used to fill the corners of the openings 20 and the material used to form the ribs 12 can be the same. In one embodiment, this material can be or can include silicon, although the present invention is not limited to the use of silicon. The intersecting ribs can be integrally formed from a single piece of material. Silicon can also be incorporated into this embodiment. Likewise, the ribs and the filled corners can be formed from a single piece of silicon material by removing or etching the openings or cavities to form the interwoven grid 18. The manufacture of the ribs and filling of corners can occur substantially simultaneously. Alternatively, the ribs can be formed first and the corners filled thereafter. In this case, the ribs may comprise a material that is not the same as the material used to fill the corners of the openings.
The result of the geometry of the intersecting ribs 12 in combination with the filled corners of the openings 20 is that the tolerant strength of the window 10 is increased. By increasing the tolerant strength, it is possible to also increase the percentage of open area within the support frame 14 and/or reduce the overall height of the ribs, both of which are desirable characteristics since this they increase the ability of the window to pass radiation.
Specifically, in accordance with the present invention, the openings 20 preferably occupy more area within the perimeter of the support frame 14 than the plurality of ribs 12 or grid. This is due to the fact that the openings will typically absorb less radiation than the surrounding ribs and radiation can more freely pass through the openings than through the ribs. In one aspect, the openings take up between about 75% to about 90% of the total area within the perimeter of the support frame. For example, in one embodiment the openings in the grid comprise at least about 75% of the total area within the perimeter of the support frame and the plurality of ribs comprise no more than about 25% of the total area within the perimeter support frame. Alternatively, the openings can comprise at least about 90% of the total area within the support frame, and the plurality of ribs can comprise no more than about 10% of the total area within the frame.
In addition to increasing the open area within the support frame 14, the arrangement of ribs 12 and openings 20 makes it possible to reduce the height and/or thickness of the ribs, and thus the collimation required for passing radiation through the window 10 can be reduced to some degree. By reducing the amount of collimation required it is possible to increase the amount of radiation that can pass though the window since the amount of collimation required is proportional to the amount of radiation that is absorbed, and therefore not passed through the window.
Referring to
A method of manufacturing a high strength grid for a window in a radiation detection system includes growing a first oxide layer on a bare silicon wafer by thermal oxidation. The oxide layer can then be patterned by traditional lithography techniques. The plurality of intersecting ribs can be formed by anisotropic etching of a silicon wafer. Since the silicon etching rate along some particular planes of single silicon is much faster than other directions, those silicon beams have super flat side walls. As a result of the etching, the corners near the ends of those ribs and the edges between the top and bottom surfaces and side walls of the ribs can be very sharp and rough. The corners can be rounded and smoothed.
It is to be understood that the above-referenced arrangements are only illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention. While the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth herein.
