Window member for an x-ray device

Abstract

A window member for separating an internal environment of an x-ray device from an environment external to the x-ray device is provided. The window member comprises a substrate and a coating layer disposed upon a surface of the substrate. The substrate is formed from a polycrystalline material and is substantially transparent to low-energy x-rays. The coating layer is non-porous, covers the crystal grains at the surface of the substrate and extends into the grain boundaries therebetween, such that the coating layer forms an impermeable barrier between the substrate and the external environment.

Claims

1. A method for forming a window member for separating an internal environment of an x-ray device from an environment external to the x-ray device, the method comprising: providing a window member comprising a substrate having a first surface and a second surface opposite the first surface, and a coating layer having a first surface and a second surface opposite the first surface, wherein the substrate is closer to the internal environment than the coating layer, and the coating layer is closer to the external environment than the substrate, wherein the first surface of the substrate faces the internal environment, the second surface of the substrate faces the first surface of the coating layer, and the second surface of the coating layer faces the external environment, wherein the coating layer is disposed upon the second surface of the substrate by way of atomic layer deposition, and wherein: the substrate is formed from a polycrystalline material and is substantially transparent to low-energy x-rays; and the coating layer is non-porous, covers the crystal grains at the second surface of the substrate and extends into the grain boundaries therebetween, such that the coating layer forms an impermeable barrier between the substrate and the external environment.

2. A method according to claim 1, wherein the coating layer extends into the grain boundaries to a depth of at least 100 nm below the second surface of the substrate.

3. A method according to claim 1, wherein the coating layer extends into each grain of the boundaries to a depth at which the spacing between the grains at the boundary is the atomic scale.

4. A method according to claim 1, wherein the coating layer forms a continuous film having a uniform thickness and covering the second surface of the substrate.

5. A method according to claim 1, wherein the thickness of the coating layer is less than 200 nm.

6. A method according to claim 1, wherein the coating layer conforms to the second surface profile of the substrate as defined by the crystal grains and grain boundaries.

7. A method according to claim 1, wherein the porosity of the coating layer is less than or equal to 1%.

8. A method according to claim 1, wherein the pinhole density of the coating layer is less than 10 cm′.

9. A method according to claim 1, wherein the attenuation of low-energy x-rays caused by the coating layer is less than or equal to 5% of the attenuation of low-energy x-rays caused by the substrate.

10. A method according to claim 1, wherein the thickness and the constituent material of the coating layer are selected in combination such that the attenuation of low-energy x-rays caused by the coating layer is less than or equal to 5% of the attenuation of low-energy x-rays caused by the substrate.

11. A method according to claim 1, wherein the coating layer is formed only from materials comprising elements having atomic number between 8 and 80.

12. A method according to claim 1, wherein the coating layer comprises an adhesion layer and a protection layer, wherein the adhesion layer affixes the protection layer to the second surface of the substrate, and wherein the protection layer is non-porous.

13. A method according to claim 12, wherein the porosity of the protection layer is sufficiently low to prevent the second surface of the substrate being exposed to atmospheric gas or liquid.

14. A method according to claim 12, wherein the adhesion layer comprises Al.sub.2O.sub.3, and the protection layer comprises TiO.sub.2.

15. A method according to claim 12, wherein the thickness of each of the adhesion layer and the protection layer is less than 200 nm.

16. A method according to claim 1, wherein the coating layer comprises an electrically conductive material.

17. A method according to claim 1, wherein the coating layer comprises TiO.sub.2 doped with Nb.

18. A method according to claim 1, wherein the coating layer has a conductivity greater than 10.sup.−6 Sm.sup.−1.

19. A method according to claim 1, wherein the substrate is substantially transparent to x-rays in the 0.5-10.0 keV energy band.

20. A method according to claim 1, wherein the substrate has a transmissivity greater than 90% for x-rays in the 0.5-10.0 keV energy band.

21. A method according to claim 1, wherein the transmissivity of the substrate and the coating layer in combination is greater than 85% for x-rays in the 0.5-10.0 keV energy band.

22. A method according to claim 1, wherein the transmittance of the substrate and the coating layer in combination is greater than 95% of the transmissivity of the substrate alone.

23. A method according to claim 1, wherein the substrate is formed from beryllium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Some examples of the methods and product according to the invention are now described, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic illustration of four stages of an atomic layer deposition process;

(3) FIG. 2 shows a top view of a window member according to the invention alongside a perspective view of an assembly of an x-ray device comprising a window member according to the invention;

(4) FIG. 3 schematically shows a portion of the surface of a window member according to the invention;

(5) FIG. 4 shows the dependents of coating layer thickness upon the atomic number of coating layer constituent elements;

(6) FIG. 5 shows transmission spectra for various substrates and coating layers according to the invention;

(7) FIG. 6 schematically shows an uncoated substrate alongside a coated substrate according to the invention, both before and after exposure to simulated high humidity conditions;

(8) FIG. 7 shows output x-ray spectra for x-ray tubes with uncoated windows together with that of a tube comprising a coated window member according to the invention; and,

(9) FIG. 8 shows the dependence of transmissivity of x-ray windows on photon energy.

DESCRIPTION OF EMBODIMENTS

(10) With reference to, FIG. 1, four stages of an example method for producing a window member according to the invention are shown. Specifically, the illustrated example is an atomic layer deposition (ALD) process. This is an ultra-thin coating method based on cyclic deposition and reaction. During such coating processes, two or more chemical vapours or gaseous precursors react sequentially on a substrate surface, producing a solid thin film. In such methods, an inert carrier gas flows through a system and precursors are injected by way of very short pulses into the carrier flow that transports the precursor into a reaction chamber.

(11) At stage A a surface of a substrate 103 is exposed to a suitable gaseous reactant 121 which, upon reacting with the surface 103, forms a monolayer.

(12) At stage B, excess reactant 121 and any by-products are purged out, or evacuated, by way of passing inert gases such as argon or nitrogen into the chamber, thus displacing the reactants or by-products.

(13) At stage C (for an oxide film) an oxidant 123 such as atomic oxygen (O), hydrogen peroxide (H.sub.2O.sub.2), ozone (O.sub.3), or water (H.sub.2O), is introduced as a pulse into the chamber or tube. This has the effect of fully oxidising the previously adsorbed layer 122, thereby forming an oxide. Other reactants are used to form nitrides or carbides, as is well known in the art.

(14) At stage D the excess oxidant 123 is purged out of the chamber or tube, again by way of delivering a pulse of argon or nitrogen or another suitable inert gas. Thus the substrate 103 coated with an oxide coating layer 106 is formed. Depending upon the desired thickness of the coating layer 106, the cycle of deposition illustrated through stages A-D may be repeated accordingly.

(15) The result of the ALD process is a highly homogeneous and conformal film that extends continuously over the entire surface of the substrate 103. Coating 106 applied using this technique extends deep into trenches in the surface of the substrate, although no such trenches are shown in the portion of the substrate 103 shown in FIG. 1. As noted above, the number of times the reaction is repeated determines the thickness of the deposited coating 106.

(16) An example window member according to the invention is shown in FIG. 2 together with a portion of an x-ray device 210 in which the member is installed. The window member 201 comprises a polycrystalline beryllium substrate, wherein the micrometre sized beryllium grains are coated with an impermeable coating at a surface of the substrate. The x-ray window 201 is a circular disc with a diameter that is between 5 and 20 mm and a thickness that is between 12 and 500 μm.

(17) There are a number of criteria by which the material and thin film growth method of the coating layer may be selected in order for the window member to meet the requirements of an x-ray window.

(18) Firstly, the coating should be uniform and should cover the entire window surface, or the entire surface of the substrate that is to be exposed to the environment external to the x-ray device 210. The coating is grown on the substrate surface such that the profile of the coating repeats the surface profile of the substrate. A cross section of a grain boundary 308 between two adjacent beryllium crystal grains 307 in the surface of a substrate is shown schematically in FIG. 3. The coating layer 306 has been applied by way of atomic layer deposition and, as such, conforms to the surface profile of the grains 307. The coating layer therefore extends into the grain boundary 308 to a depth of approximately 100 nm below the outward most parts of the grains 307 that define the plane of the substrate surface. This is due to the ALD process uniformly coating all parts of the exposed grain surfaces down to a depth where the gap between adjacent beryllium crystalline reaches the atomic scale. Depths of over 100 nm are typically the point within surface grain boundaries at which the grain boundary gap in beryllium substrates narrows to this size. The isotropic growth caused by ALD therefore leaves no part of the substrate exposed, that is, not covered by the coating layer. In this way, the substrate of an installed window member 201 such as that in device 210 is fully isolated from the ambient atmosphere that is external to the device 210.

(19) Another criterion for the coating layer is that it is in the form of a low-porosity film. That is, the coating contains no or negligible interstices through which any liquid or gas may pass. Thus, the coating serves to isolate the substrate upon which the film is disposed completely from the atmosphere, environment, or fluids on the other side of the coating layer to the substrate.

(20) A further criterion is that the coating layer has a high transmissivity, of greater than 90%, to low energy x-ray radiation, that is in the 0.5-10.0 keV energy band.

(21) A further consideration is that the coating layer has a high chemical stability, meaning that it is not reactive to the substrate or to the atmosphere or environment to which it may be exposed. Such a chemically stable coating layer will therefore retain its impermeable structure and total coverage of the window so as to continue to perform as a barrier that isolates the substrate from the external atmosphere.

(22) The uniformity and the porosity of the coating layer depend in part upon the growth methods used to apply it. Table 1 below lists some commonly used growth methods (sputtering, pulsed laser deposition (PLD), chemical vapour deposition (CVD), atomic layer deposition (ALD) and sol-gel (solution-based)). A measured porosity and three-dimensional uniformity that is achievable using each method is set out in Table 1. It can be seen that a good degree of impermeability can be achieved using all methods except for sol-gel. However, as can be seen from the table, isotropic layer growth, which results in the formed layer conforming to the surface profile of the substrate and the grains therein and is indicated by 3D uniformity, can be achieved only by way of ALD and CVD. A reason for this is that isotropic growth should take place with a surface-limited reaction’, and so sputtering and PLD-based growth is directional and occurs predominately or solely on surfaces or regions of the grains which are angled towards or facing the precursor material. Since beryllium and other polycrystalline materials comprise grains at their surface, which results in a non-smooth surface wherein the normal factor varies according to the grain structure intersected by grain boundaries, the grain boundaries may remain uncoated when using such methods. In these cases, the boundaries can provide leakage paths by which the external environment may contact the substrate.

(23) A further consideration for the method by which the coating layer is formed is the ability to precisely control the thickness of the coating layer so that a side thickness may be achieved. This is advantageous since any excess thickness will reduce x-ray transmissivity owing to the presence of any greater depth of coating material through which x-rays must pass. Atomic layer deposition allows the greatest degree of layer thickness control, since it involves depositing material one atomic per cycle. Various techniques for performing CVD are available, varying in the pressure of the gas environment present during growth. Most commonly, CVD is performed at relatively high pressures ranging from atmospheric pressure to the order of several mTorr. In a variation on the previously described example method, the coating layer is applied using CVD at low pressure, approximately 10.sup.−3 Torr. This method allows a fine level of control of the coating layer thickness. The CVD method of applying the coating layer to the polycrystalline window is suitable for examples using very low atomic number materials to form the coating layer. Examples of such materials are diamond-like carbon, aluminium nitride, and boron carbide. Owing to their low atomic number, the coating layers produced from such materials give rise to x-ray attenuation in the desired energy band of 5% and under, in spite of the high growth rate and thick layers that typically result from CVD-based coating processes.

(24) TABLE-US-00001 TABLE 1 Impermeability 3D uniformity Atomic layer deposition good excellent Chemical vapor deposition good good Sputtering (DC/RF) good poor Pulsed laser deposition good poor Sol-gel poor good

(25) As indicated previously, the transmissivity of a coating layer to low energy x-rays is influenced by a combination of the atomic number (Z) of constituent elements of the coating layer, and the thickness of the layer. The lower the atomic number, the greater the thickness of the layer that can be employed in order to produce a coating of a given desired transmissivity. Experimental data demonstrating this relationship is shown in FIG. 4. Here the Y axis corresponds to the thickness of the coating layer. The of x-rays after passing through the beryllium substrate and coating layer should preferably not be less than 90% of the incident x-ray beam. The graph therefore illustrates the dependence of thickness of the coating layer upon the atomic number of the coating layer constituents that can still give rise to a coated window having an overall transmissivity above a 90% threshold. It can be seen that as the atomic number of the coating layer materials increases, the maximum thickness of coating layer which results in a transmissivity greater than a particular threshold value presenting a desired level of transparency decreases.

(26) In the present example, the coating layer is formed from tungsten oxide (WO.sub.3). The atomic number for tungsten is Z=74. It has been found that this example coating layer is effectively transparent to low energy x-rays in the aforementioned energy band for film thicknesses up to 100 nm. Measured transmissivity data for this example is shown in FIG. 5D. It has been found that coating thicknesses of 100 nm are sufficient to provide a degree of protective environmental isolation for a substrate to produce a five-fold increase in the useful lifetime of a window member.

(27) FIGS. 5A, 5B, and 5C show the measured transmission spectra for 1 μm thick Al.sub.2O.sub.3, 500 nm thick, ZrO.sub.2, and 300 nm thick SnO.sub.3 coating layers respectively. Each of these is plotted alongside the transmission spectrum of a 127 μm thick beryllium substrate window, in combination with that of a beryllium substrate 127 μm thick, forming a coated window member. From these comparisons it can be seen that the transmission through the coating layers is comparable to the transmission through the beryllium window. Thus it can be seen that the additional x-ray attenuation attributable to the coating layers is insignificant.

(28) A wide range of materials have been identified as being suitable, in accordance with the criteria set out above, for being used as corrosion-protective coating layers. Table 2 below lists a number of example coating layer materials alongside the growth methods of those mentioned previously which may be used to apply the respective coating layer materials to a substrate.

(29) TABLE-US-00002 TABLE 2 Materials Growth method TiO.sub.2 ALD, CVD Al.sub.2O.sub.3 ALD, CVD ZrO.sub.2 ALD, CVD SiN ALD SiO.sub.2 ALD TiB.sub.2 CVD SnO.sub.2 ALD, CVD SnO.sub.2:Sb ALD, CVD AlN ALD, CVD GaN ALD, CVD InN ALD, CVD TiN ALD, CVD WO.sub.3 ALD, CVD HfO.sub.2, ALD, CVD In.sub.2O.sub.3 ALD, CVD Ga.sub.2O.sub.3 ALD, CVD Ta.sub.2O.sub.5 ALD, CVD SrTiO.sub.3 ALD, CVD CeO ALD, CVD BaTiO.sub.3 ALD, CVD ZnO ALD, CVD ZnO:Al ALD, CVD MgO ALD, CVD Ba.sub.X(Y.sub.1−X)ZrO.sub.3 ALD, CVD LaCoO.sub.3 ALD, CVD Nb.sub.2O.sub.5 ALD, CVD NiO ALD, CVD Y.sub.2O.sub.3 ALD, CVD LaNiO.sub.3 ALD, CVD La.sub.2O.sub.3 ALD, CVD YB.sub.2Cu.sub.3O.sub.7−X ALD, CVD CaO ALD, CVD CuO ALD, CVD SiC ALD, CVD TiC ALD, CVD BC ALD Ti ALD Zr ALD Ni ALD

(30) Comparative studies of beryllium substrates according to the first example window member, for the purpose of illustrating the resistance of the coating layer to corrosion, have been performed with and without a coating layer. Coated and uncoated beryllium substrates were exposed to high levels of ambient humidity. High humidity was simulated by way of forming water drops on the centres of the beryllium substrates, and the substrates were placed on a hot plate. The experiments showed that uncoated beryllium substrates eroded over time, and formed a pit, while the surface of a beryllium substrate with a coating layer remained unchanged after being contacted with moisture. These results are illustrated in FIG. 6. The schematic drawing of Figure A shows an uncoated beryllium substrate 603A with a thickness of 127 μm. After being contacted with moisture in the conditions described above, a pit 619 is formed in the substrate 603A owing to the corrosion caused by the simulated high humidity. A similar substrate 603B, to which a 100 nm thick coating layer 606 was applied formed no pit in the same conditions. The unchanged state of the window member 601 comprising the coated substrate can be seen in the before and after drawings at B.

(31) Thus it was found that the rate of erosion of the beryllium substrate is decreased significantly by the presence of the coating layer. No clear signs of erosion at all were found in the coated beryllium window.

(32) Additionally, experiments with brazed window assemblies were performed. In these experiments it was found that assemblies with coated substrates exhibited more than five times the useful longevity of brazed window assemblies with uncoated windows.

(33) Two methods of coating a substrate with corrosion resistive layers are coating a brazed window assembly, or brazing a coated window disc. The preferred method will depend upon the properties of the chosen coating materials. Materials capable of withstanding brazing temperatures of around 800° C. may be coated on their window discs, that is polycrystalline substrate discs, before brazing. Examples of coatings that are suitable to be applied in this way include boron nitride, boron carbide, and titanium nitride, which have melting temperatures greater than 2,500° C.

(34) Materials with lower melting points are more suitable for being deposited upon window assemblies after they have been brazed. Examples of such materials include titanium dioxide, aluminium(iii) oxide, and zirconium dioxide.

(35) Simulations and experimental studies have been performed in order to establish optimal or suitable coating layer thicknesses for window members. These have shown that materials with constituent atomic numbers up to Z=80 can be used. The optimum thickness depends upon the mass attenuation coefficient of the materials. This coefficient depends on the atomic number and the density of the material. The greater the values of these parameters, the smaller the coating layer thickness must be in order to achieve a sufficiently high transmissivity. Both elemental and compound materials may be used to form the coating layer.

(36) In a second example window member, the coating layer comprises multiple sub-layers. In particular, the present example includes, as part of the coating layer, an adhesion layer and a protection layer. These different sub-layers, with different chemical compositions, are deposited on top of one another so as to form a stack upon the substrate. This arrangement comprises Al.sub.2O.sub.3 formed upon the substrate surface as an adhesion layer, with TiO.sub.2 grown on top of the adhesion layer, as a protection layer.

(37) As noted above, the maximum allowable thickness of a coating layer varies according to the mass attenuation coefficient of the coating layer. The criterion for this maximum allowable thickness may be determined by the requirement that the spectrum of x-ray transmission through the coating layer in the relevant x-ray energy band should be comparable to the transmission spectrum of the beryllium substrate itself. The spectra are plotted, as mentioned above, in FIGS. 5A-5D.

(38) Variations of the example window members described thus far include substrates made from aluminium, titanium, diamond, and silicon nitride. Substrates formed from these various materials will have different degrees of surface roughness. Depending upon the surface roughness of each substrate, the minimum thickness for the coating layer that allows the coating layer to act as a protective, impermeable barrier, may be reduced to as little as 10 nm.

(39) A further consideration in selecting constituent materials for a window member is that, as well as not significantly reducing the transmitted x-ray intensity, the window member materials must not contaminate output x-ray spectra by way of x-ray fluorescence caused by elements present in the window member itself. Experimental results from tests performed using an x-ray tube comprising an x-ray window coated with a TiO.sub.2 coating layer are shown in FIG. 7. This data compares the x-ray spectra of tubes with coated and uncoated windows. It can be seen that no fluorescence lines from the coating material are present in the spectrum. Emission peaks at 2.7 keV, 20.21 keV and 22.72 keV correspond to rhodium L.sub.α, K.sub.α, and K.sub.β lines, respectively. No titanium fluorescence lines being detected in the spectrum means that this is a suitable material for transmitting uncontaminated x-rays through the window member coated with this protective coating.