Radiation window

According to an example aspect of the present invention, there is provided a radiation window manufacturing method, comprising patterning a mask on a top surface of a bulk wafer or compound wafer, etching the bulk or compound wafer from the top surface, based on the mask, either by timed etching of the bulk wafer, or until an inner insulator layer of the compound wafer, thereby generating recesses in the bulk or compound wafer, filling the recesses, at least partly, with a filling material, polishing the top surface of the bulk or compound wafer, and providing a membrane layer on the polished top surface, and etching the bulk or compound wafer from a bottom surface, opposite the top surface, to build a supporting structure for the membrane layer in accordance with a shape defined by the mask.

FIELD

The present invention relates to window constructs that are at least partially transparent to radiation, such as x-rays.

BACKGROUND

Radiation measurement devices operate by determining a reaction of a detector device to incoming radiation. For example, an x-ray camera may receive x-rays and determine their intensity as a function of location on a two-dimensional charge-coupled device, CCD, array. A spectrometer, on the other hand, may be configured to determine spectral characteristics of incoming radiation, for example to determine an astrophysical redshift or to identify characteristic emission peaks of elements to analyse elemental composition of a sample.

When measuring soft x-rays, by which it may be meant, for example, x-rays with energy below about 1 keV, providing the radiation to a detector presents with challenges. For example, air scatters soft x-rays and many materials absorb soft x-rays, wherefore the radiation most conveniently is conveyed to a detector through vacuum, wherein the detector may be placed in the vacuum. Most elements exhibit characteristic emissions above 1 keV.

When operating in atmospheric conditions, a suitable window may be arranged to admit soft x-rays into the vacuum where a detector may be arranged to analyse the radiation. Such a window would ideally be transparent to the soft x-rays and durable of construction, and impermeable to air to protect the detector.

Transparency to x-rays may be increased by reducing the thickness of the window. For example, beryllium windows have been used, wherein the thinner the window is, the larger a fraction of incoming radiation is admitted through the window. On the other hand, the thinner the window is, the likelier it is to break in real-life circumstances.

To increase durability of a window, the window may be reinforced with a supporting structure, such as a mechanical grid, or it may be sandwiched between supporting structures. Supporting structures may take the form of web-like support structures, which partially cover and partially expose the window material. In parts where the window material is exposed by supporting structures, the window is maximally transparent to incoming radiation.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a radiation window manufacturing method, comprising patterning a mask on a top surface of a bulk wafer or compound wafer, etching the bulk or compound wafer from the top surface, based on the mask, either by timed etching of the bulk wafer, or until an inner insulator layer of the compound wafer, thereby generating recesses in the bulk or compound wafer, filling the recesses, at least partly, with a filling material, polishing the top surface of the bulk or compound wafer, and providing a membrane layer on the polished top surface, and etching the bulk or compound wafer from a bottom surface, opposite the top surface, to build a supporting structure for the membrane layer in accordance with a shape defined by the mask.

According to a second aspect of the present invention, there is provided a radiation window construct, comprising a radiation window comprised of a membrane layer, and a supporting structure built of a bulk wafer or a compound wafer, wherein the radiation window construct has been manufactured using a process wherein a filling material has been provided to fill etched recesses in the bulk or compound wafer, wherein grooves are created in corners of the recesses where the membrane layer and material of the bulk or compound wafer meet when polishing the top surface of the bulk or compound wafer.

According to a third aspect of the present invention, there is provided an x-ray detector comprising a radiation window construct in accordance with the second aspect.

EMBODIMENTS

Supporting structures may be built for radiation windows using processes described herein. A filling material is employed into recesses of a wafer, to enable a polishing phase where a structure with enhanced stress resistance is obtained for a window comprised of a membrane layer on the wafer. The wafer may be a bulk wafer. The wafer may alternatively comprise a compound wafer comprising two or more sub-wafers and separate layers between the adjacent sub-wafers. The sub-wafers may be made of silicon, glass or carbon fibre, for example. The layer between the sub-wafers may be comprised of an insulator, for example silicon oxide, silicon nitride or aluminium oxide. Most generally, the layer may be of a material with etching properties which differ from etching properties of the sub-wafers. The compound wafer may comprise a silicon-on-insulator (SOI) wafer, for example. A stress-relieving groove may be obtained in a part of a window structure where the membrane layer meets the silicon wafer, in a corner of a recess.

Radiation windows may benefit from layers deposited thereon, to enhance their desired characteristics, which may include gas impermeability, optical properties or spectral selectivity, for example. To facilitate provision of such layers, radiation windows in accordance with at least some embodiments of the present invention are provided with supporting structures enhancing their structural robustness on one side and the layer or layers on the other side. The side of the radiation window with the layer or layers may be left without a robustness-enhancing supporting structure to facilitate creation of a continuous, high quality layer. Examples of such layers include aluminium, graphene, aluminium oxide, silicon oxide, silicon carbide, nitride films such as aluminium nitride, silicon nitride, boron nitride, titanium nitride, metal-carbo-nitrides such as TiAlCN, pyrolytic carbon, and polymers such as polyimide.

FIG.1illustrates an example system capable of being operated with at least some embodiments of the present invention. The illustrated system relates to x-ray fluorescence, to which the present invention is not limited, rather, windows built in accordance with the present invention may find application also more broadly.

FIG.1illustrates an analytic device110, which comprises an x-ray detector120. X-ray detector120is in this example configured to determine spectral characteristics of x-rays incident on itself, for example to enable elemental composition analysis based on characteristic emissions.

In use, the arrangement ofFIG.1irradiates sample130with primary radiation102from primary x-ray or particle source140, stimulating matter comprised in sample130to emit, via fluorescence, secondary x-ray radiation103, spectral characteristics of which are determined, at least partly, in x-ray detector120.

X-ray detector120comprises a window region115, which is arranged to admit x-rays into X-ray detector120. Window region115is illustrated in an enlarged view115E at the bottom ofFIG.1, wherein a gap in the outer housing of analytic device110is shown. Arranged in the gap is an opening wherein a window layer117is disposed, preventing inflow of air from outside analytic device110to inside analytic device110while allowing x-rays, such as, for example, soft x-rays, to enter analytic device110, so that these x-rays may be analysed in x-ray detector120. Window layer117may be comprised of silicon nitride, for example. Further examples of materials the window layer117may be comprised of include aluminium oxide, aluminium nitride, silicon oxide, silicon carbide, titanium oxide, silicon nitride, titanium nitride, metallo-carbo-nitrides such as TiAlCN, boron nitride, boron carbide, boron, beryllium, beryllium oxide, graphene, pyrolytic carbon and polymers, such as polyimide. In some embodiments, window region115may be disposed in the housing of analytic device110, rather than at X-ray detector120.

Window layer117is supported by supporting structure119on one side. While illustrated on the inner side facing the inside of X-ray detector120, supporting structure119may, in other embodiments, alternatively be on the outward facing side. Supporting structure119may, in some embodiments, be present on one side but not the other side, in other words, supporting structure119may be limited to one side of window layer117. Supporting structure119may be comprised of silicon, for example.

While window layer117and supporting structure119are illustrated inFIG.1as slightly separate, with a gap in between, this is for clarity of illustration purposes. In actual embodiments of the invention, window layer117may be attached to supporting structure119, for example by being deposited on a wafer from which supporting structure119is constructed. Supporting structure119may be constructed by etching, for example.

Supporting structure119may take a form and shape that is suitable for supporting window layer117thereon, to withstand atmospheric pressure, for example, in case the inside of x-ray detector120is maintained at low pressure, or, indeed, vacuum or near-vacuum. For example, supporting structure119may comprise a square or rectangular layout, or a spider-web shape, to provide support for window layer117while not obscuring too much of window layer117.

In general, supporting structure119, attached to window layer117, will partially obscure and partially expose window layer117. In detail, a part of window layer117touching support structure119will be obscured by it, by which it is meant that x-rays passing through window layer117will at these places be partially prevented, by support structure119, from reaching x-ray detector120. In parts of window layer117not touching support structure119, x-rays that penetrate window layer117may proceed directly to x-ray detector120. The larger the part of window layer117touching, and obscured by, supporting structure119, the stronger is the support provided to window layer117and the larger the effect supporting structure119has on x-rays incoming through window layer117. The strength of supporting structure119may thus be seen as a trade-off between transmittance through window layer117and strength of the radiation window structure which comprises window layer117and supporting structure119. In general, window layer117may be completely exposed on a first side and partly exposed on a second side, the supporting structure being on the second side. By completely exposed, or continuously exposed, it is meant window layer117is exposed in a manner that an area of window layer117in active use is not obstructed by a support structure on the continuously exposed side.

Window layer117may be continuous in nature, by which it is meant the layer is not interrupted, for example, in accordance with the support structure. A continuous layer may be planar in the sense that it lies in a single plane.

Window layer117may be thin, in the nanometer range, while extending over an opening which is in the order of a few millimetres, or centimeters, in size.

Window layer117may have, for example on a side not facing support structure119, at least one supplementary layer. Examples of supplementary layers include a thin aluminium layer and a graphene layer. An aluminium layer may block, at least partly, visible light from entering through window layer117. Graphene, on the other hand, may enhance an ability of window layer117, for example when made of silicon nitride, to prevent gas molecules, such as air, from penetrating through window layer117. When one side of window layer117is clear from supporting structures, such supplementary layers may be applied easier and the resulting layers have fewer defects. This provides the beneficial technical effect that the layers function better in their respective purposes. Supplementary layers may alternatively be referred to as surface layers.

In general, a compound silicon wafer may comprise a construct wherein two or more silicon wafers are attached one on top of one another. There may be a layer or layers arranged in between the silicon wafers comprised in the compound silicon wafer.

FIG.2A-2Fillustrates a manufacturing method in accordance with at least some embodiments of the present invention. The manufacturing method is one which employs a surface filling phase. The process begins at the situation ofFIG.2A, where a compound wafer, such as a silicon on insulator, SOI, wafer is obtained. The compound wafer comprises a first silicon wafer201and a second silicon wafer202, with an insulator layer212therein between, as illustrated. The insulator layer may comprise silicon oxide, silicon carbide or boron nitride, for example. The compound wafer is processed to cause mask layers214and217to form on wafers202and201, respectively, as illustrated. While the layers are herein referred to as silicon oxide layers, in general other materials are usable, as well. In other words, silicon oxide is herein employed as an example material used in a mask layer.

In general, the wafers201,202may comprise silicon, carbon fibre or glass wafers, for example, although silicon may be referred to in the present disclosure as an example. Mask layers214and217may, in general, comprise silicon oxide, aluminium oxide or silicon nitride, for example.

Mask layer214is patterned to impart thereon a shape of a supporting structure that is to be constructed for the window layer. Further, silicon wafer202has, in the situation illustrated inFIG.2A, been patterned in accordance with the mask of mask layer214. This patterning may comprise etching, for example. The patterning extends until the insulator layer212. As a result of the patterning, recesses are formed into the silicon of silicon wafer202, as illustrated.

Moving to the phase illustrated inFIG.2B, the recesses in wafer202have been filled with a filling material216. Examples of suitable filling materials216, in general and not only relating toFIGS.2A-2F, include silicon oxide, polysilicon, silicon nitride and spin-coating materials. In general, the filling material may be high temperature resistant or low temperature resistant and capable of being etched away from silicon. Filling material216fills the recesses and forms a layer on wafer202and mask layer214. In some embodiments, mask layer214is removed before applying the filling material.

Advancing to the phase illustrated inFIG.2C, a polishing phase is performed, for example chemical mechanical polishing, to remove the filling material layer extending on wafer202and mask layer214, if it is still present. Thus the top side of the compound wafer will have the recesses, which are filled with the filling material. The top surface of wafer202will be polished.

Advancing to the phase illustrated in2D, a membrane layer218, forming the x-ray window layer, is applied on the polished surface of wafer202. The bottom surface side of the compound wafer is patterned by imparting a backside pattern on oxide layer217. Membrane layer218may, in general and not limited toFIGS.2A-2F, comprise silicon nitride, boron nitride or silicon carbide, for example.

Advancing to the phase illustrated inFIG.2E, the bottom surface is etched in accordance with the backside pattern, as illustrated, until the insulator layer212is reached. Advancing then to the phase illustrated in2F, the etching from the bottom direction is continued, removing the exposed part of insulator layer212and the filling material216from the recesses, exposing membrane layer218, in part, from the bottom side. A supporting structure is thus formed of silicon of wafer202on the bottom side of membrane layer218. The shape and form of the supporting structure is defined by mask layer214.

As a result of the polishing phase, combined with the removal of the filling material by etching from the backside, small grooves are formed in corners of the recesses connecting with membrane layer218. One of these corners is illustrated as x0inFIG.2F. These grooves provide a beneficial effect in terms of enhancing stress resistance of the resulting window construct, as they enable a small physical deformation of the window constructs.

A variant of the method ofFIGS.2A-2Fis one which used a bulk wafer, rather than a compound wafer. A bulk wafer does not have the insulator layer212between sub-wafers, and consists therefore of a single wafer. In such an embodiment, the insulator layer212is not used as an etch stop, since layer212is not present. Rather, the etches may be controlled by controlling the etch time (time controlled etching). The resulting supporting structure may be comprised of the filling material, which is suitably chemically resistant against silicon. Such a bulk wafer process is illustrated inFIGS.7A-7E. The polishing phase results in similar small grooves as occurs in the compound wafer processes.

FIG.3B-FIG.3Fillustrate an example manufacturing process in accordance with at least some embodiments of the present invention. In this process, surface filling is used together with a separation layer. Like numbering denotes like structure as inFIGS.2A-2E.

The process ofFIGS.3B-3Fbegins with a phase identical to that ofFIG.2A, wherefore, for the sake of clarity, there is noFIG.3A. Rather,FIG.2Amay be seen as a first phase of the process ofFIGS.3B-3F. InFIG.3B, a conformal deposition or growth process, for example a thermal oxidation process, has been performed, resulting in a separation layer301coating the recesses in wafer202. This layer may comprise silicon oxide or aluminium oxide, for example.

Advancing to the phase illustrated asFIG.3C, the recesses have been filled with the filling material216, and a polishing phase, for example similar to the one described in connection withFIG.2C, has been performed to remove mask layer214and the layer of filling material216overlying the non-recessed parts of wafer202. The recesses, coated with separation layer301, remain filled with filling material216. The filling material, as described above, may comprise silicon oxide, polysilicon silicon nitride or spin-coating materials, for example.

Advancing to the phase illustrated asFIG.3D, a membrane layer218, for example of silicon nitride, boron nitride or silicon carbide, has been applied on the polished top surface of the compound wafer. Membrane layer218is the window layer, as is described herein above. Further, the bottom surface has been patterned by imparting a bottom pattern to mask layer217.

Advancing to the phase illustrated asFIG.3E, the compound wafer is etched from the bottom side, in accordance with the bottom pattern, to reach the insulator layer212, and to remove exposed parts of insulator layer212. Finally, advancing to the phase illustrated asFIG.3F, etching has continued to remove the exposed matter of wafer202, until membrane layer218is partially exposed also from the bottom side. The supporting structure, in this embodiment, is formed of the filling material216, coated with separating layer301. In some embodiments the separating layer is further etched away, wherefore in general the supporting structure is in this embodiment formed of the filling material216, coated or uncoated by the separating layer301.

A variant of the method ofFIGS.3B-3Fis one which uses a bulk wafer, rather than a compound wafer. A bulk wafer does not have the insulator layer212between sub-wafers, and consists therefore of a single wafer. In such an embodiment, the insulator layer212is not used as an etch stop, since layer212is not present. Rather, the etches may be controlled by controlling the etch time (time controlled etching).

FIG.4B-FIG.4Fillustrate an example manufacturing process in accordance with at least some embodiments of the present invention. The process uses a buried mask212with surface filling.

The process ofFIGS.4B-4Fbegins with a phase identical to that ofFIG.2A, wherefore, for the sake of clarity, there is noFIG.4A. InFIG.4B, the top and bottom surface mask layers214,217have been removed, and the buried insulator layer212has been patterned by partially etching it away such that it is thinner in its exposed parts.

Advancing to the phase illustrated asFIG.4C, the filling material, described above, is deposited to fill the recesses in wafer202of the compound wafer. Advancing to the phase illustrated asFIG.4D, the polishing operation described above is carried out and the membrane layer218, for example silicon nitride, boron nitride or silicon carbide, is placed on the polished surface of wafer202. The recesses remain filled by the filling material216. A bottom mask416is employed to create a bottom pattern. Bottom mask416may, in general, be silicon nitride, aluminium oxide or silicon oxide, for example.

Advancing to the phase illustrated asFIG.4E, the compound wafer is etched from the bottom side to reach the buried insulator layer212, and to remove the thinner parts thereof. Thus the filling material filling the recesses is exposed from the bottom side, as are remaining parts of the buried insulator layer212, coating the parts of wafer202which separate the recesses from each other.

Advancing to the phase illustrated asFIG.4F, the etching from the bottom side is continued, until membrane layer218is partially exposed from the bottom side. The remaining parts of insulator layer212may be used as an etch stop in this phase of the etching.

The aforementioned grooves are generated also in the embodiments ofFIGS.4B-4F, in corners of the recesses where membrane layer218meets wafer202. One such corner is indicated as x0inFIG.4F.

FIGS.5B-5Eillustrate manufacturing methods resembling those ofFIGS.4B-4F, as will be described now. The method employs a buried mask with surface filling.FIG.5Ais absent, being illustrated asFIG.2A. As the process advances to the phase illustrated inFIG.5B, the top and bottom surface mask layers214,217have been removed, and the buried insulator layer212has been patterned by etching it away from its exposed parts, such that wafer201is exposed through the recesses in wafer202.

Advancing to the phase illustrated asFIG.5C, the filling material described above,216, is applied to fill the recesses and to coat wafer202, at least partially.

Advancing to the phase illustrated asFIG.5D, the polishing process is carried out to remove the part of filling material216which is not in the recesses and to prepare the top surface of wafer202for the membrane layer218, which is also applied. The membrane layer218may comprise silicon nitride, boron nitride or silicon carbide, for example. A bottom mask layer516is applied and patterned with a bottom pattern. The bottom mask layer516may comprise silicon oxide, aluminium oxide or silicon nitride, for example.

Advancing to the phase illustrated asFIG.5E, the compound wafer is etched from the bottom side to expose the bottom side of membrane layer218. Insulator layer212may partially be used as an etch stop layer in defining the supporting structure, which is thus constructed of the silicon of wafer202.

The aforementioned grooves are generated also in the embodiments ofFIGS.5B-5E, in corners of the recesses where membrane layer218meets wafer202. One such corner is indicated as x0inFIG.5E.

FIG.6B-6Fillustrate an example manufacturing process in accordance with at least some embodiments of the present invention. This method uses a buried mask with surface filling and a separation layer.FIG.6Ais absent, being illustrated asFIG.2A. As the process advances to the phase illustrated inFIG.6B, the top and bottom surface masks have been removed, the buried insulator layer212has been etched away where exposed by the recesses, exposing wafer201through the recesses, and a conformal deposition or growth process, for example a thermal oxidation process, has been applied to generate a separation layer301to coat the insides of the recesses in wafer202and on wafer201.

As the process advances to the phase illustrated inFIG.6C, the filling material, described above, has been deposited on the top surface, filling the recesses and covering the top surface of wafer202.

As the process advances to the phase illustrated inFIG.6D, the top surface is polished, as described above, to remove the filling material which is not in the recesses, and to prepare the surface for membrane layer218. The membrane layer218is, further, deposited on the top surface of the compound wafer, as illustrated. The membrane layer, forming the window layer as in the other embodiments, may comprise silicon nitride, boron nitride or silicon carbide, for example.

As the process advances to the phase illustrated inFIG.6E, the bottom surface is provided with a mask layer, which is patterned with a bottom pattern, and the compound wafer is etched from the bottom side, based on the bottom pattern, to expose the buried insulator layer212and the filling material216, which fills the recesses in wafer202.

As the process advances to the phase illustrated inFIG.6F, the etching is continued to remove the filling material216and to expose the membrane layer218from the bottom side. The supporting structure is thus formed, of the matter of wafer202, where that material is present between the recesses.

The aforementioned grooves are generated also in the embodiments ofFIGS.6B-6F, in corners of the recesses where membrane layer218meets silicon wafer202. One such corner is indicated as x0inFIG.6F.

FIG.8is a flow graph of a method in accordance with at least some embodiments of the present invention.

Phase810comprises patterning a mask on a top surface of a bulk wafer or a compound wafer. After phase810, processing advances to phase820A in case the wafer is a bulk wafer. In phase820A, the bulk wafer is etched from the top surface, based on the mask, by timed etching of the bulk wafer, thereby generating recesses in the bulk wafer. In the case of a compound wafer, processing advances from phase810to phase820B, where the compound wafer is etched from the top surface, based on the mask, until an inner insulator layer of the compound wafer, thereby generating recesses in the compound wafer. Etching until the insulator layer may comprise etching until a first surface of the insulator layer is reached, etching until the insulator layer is partly etched away or stopping the etching once the insulator layer had been completely penetrated. Phase830, following either phase820A or phase820B, comprises filling the recesses, at least partly, with a filling material, polishing the top surface of the bulk or compound wafer, and providing a membrane layer on the polished top surface. Finally, phase840comprises etching the bulk or compound wafer from a bottom surface, opposite the top surface, to build a supporting structure for the membrane layer in accordance with a shape defined by the mask.

In general, the filling material may be a high temperature resistant material. In low temperature processes, the filling material may comprise a photo resist or other spin-coating materials, for example.

As described herein above, the etching may comprise, for example, a timed etch, an etch stopped at dopant-based etch stop layer, or an etch stopped at a mask layer disposed inside the silicon wafer.

INDUSTRIAL APPLICABILITY

At least some embodiments of the present invention find industrial application in measurement devices, such as soft x-ray measurement devices, for example.

ACRONYMS LIST

REFERENCE SIGNS LIST