Patent Description:
The present application is related generally to x-ray windows.

A hermetic seal in an x-ray window can have demanding requirements. For example, light penetration through the hermetic seal into an interior of an x-ray detection device can result in interference with a signal from a sample. Thus, it can be helpful for the hermetic seal to be opaque. X-ray fluorescence from the hermetic seal can interfere with the signal from the sample, particularly x-ray fluorescence from high atomic number elements in the hermetic seal. Therefore, it can be helpful for the hermetic seal to be made of low atomic number elements.

Many x-ray devices have an internal vacuum and a getter for maintaining this vacuum. Such getters are typically activated more quickly at higher temperatures. Therefore, it can be helpful for the hermetic seal to be able to withstand high temperature. Outgassing of components of such x-ray devices can result in an undesirable rise of internal pressure. Thus, low outgassing of the hermetic seal can also be a useful characteristic. Even small leakage through components of the x-ray device can gradually cause an undesirable rise of internal pressure and malfunctioning of the device. Therefore, it can also be useful for the hermetic seal to have a low leak rate.

An x-ray window thin film can develop internal stress during manufacture or use due to coefficient of thermal expansion mismatch between the x-ray window thin film and a housing to which it is bonded. It can be helpful for the hermetic seal to relieve such stress.

Many different materials are used for the x-ray window thin film and the x-ray device housing. It can be useful for the hermetic seal to be capable of bonding to a large variety of materials.

It has been recognized that it would be advantageous to provide a hermetic seal for an x-ray window which is opaque, which is made of low atomic number elements, which can withstand high temperature, with low outgassing, with low leakage, which is able to relieve stress in the x-ray window thin film, and which can bond to many different materials. The present invention is directed to various embodiments of x-ray windows that satisfy these needs. Each embodiment may satisfy one, some, or all of these needs.

According to the invention, there is provided an x-ray window as set out in the appended claims.

The liquid crystal polymer can be opaque, gas-tight, made of low atomic number elements, able to withstand high temperature, low outgassing, low leakage, able to relieve stress in the x-ray window thin film, capable of bonding to many different materials, or combinations thereof.

As used herein, the term "adjoin" means direct and immediate contact.

As used herein, the term "annular" means ring-shaped, but is not limited to a circular shape. The annular shape can have other curved shapes, such as for example elliptical.

As used herein, "KPa" means kilopascals, "MPa" means megapascals, and "GPa" means gigapascals.

As used herein, in the unit "mbar*L/sec", "mbar" means millibars, each millibar equal to <NUM> pascals, "L" means liters, and "sec" means second.

As used herein, "nm" means nanometer(s) and "µm" means micrometer(s).

As illustrated in <FIG>, and <FIG>, x-ray windows <NUM>, <NUM>, and <NUM>, respectively, are shown comprising an adhesive layer <NUM> sandwiched between and providing a hermetic seal between a thin film <NUM> and a housing <NUM>. The thin film <NUM> can extend across an aperture <NUM>A of the housing <NUM>. The thin film <NUM> and the housing <NUM> can adjoin the adhesive layer <NUM>.

The adhesive layer <NUM> is ≥ <NUM>% thermotropic liquid crystal polymer. In other examples, outside the scope of the claims, the adhesive layer <NUM> can be or can include epoxy, or other type of adhesive.

The adhesive layer <NUM> can have certain characteristics for improved performance as will be described in the following few paragraphs.

As illustrated in <FIG>, x-ray window <NUM> can further comprise a ribbed support structure <NUM>, providing structural support to the thin film <NUM>, and sandwiched between the thin film <NUM> and the adhesive layer <NUM>. The ribbed support structure <NUM> can extend across the aperture <NUM>A of the housing <NUM>, and can provide support for the thin film <NUM> across the aperture <NUM>A of the housing <NUM>. The thin film <NUM>, the adhesive layer <NUM>, or both can adjoin the ribbed support structure <NUM>. Alternatively, there can be other or additional materials between the thin film <NUM> and the adhesive layer <NUM>. There can also be other materials between the housing <NUM> and the adhesive layer <NUM>. The ribbed support structure <NUM> can comprise silicon.

If the adhesive layer <NUM> is not viscous enough, it can wick into channels of the ribbed support structure <NUM>. The adhesive layer <NUM> in such channels can damage the ribbed support structure <NUM>. The adhesive layer <NUM> in such channels can block or fluoresce x-rays, which can interfere with an x-ray signal from a sample. Therefore, it can be helpful for the adhesive layer <NUM> to have high viscosity. Example viscosity values of the adhesive layer <NUM> include ≥ <NUM>,<NUM> cps, ≥ <NUM>,<NUM> cps, ≥ <NUM>,<NUM> cps, ≥ <NUM>,<NUM> cps, ≥ <NUM>,<NUM> cps, or ≥ <NUM>,<NUM> cps and ≤ <NUM>,<NUM> cps, ≤ <NUM>,<NUM> cps, or ≤ <NUM>,<NUM>,<NUM> cps, each at <NUM>. All viscosity values herein are static viscosity.

For improved bonding of the thin film <NUM> to the housing <NUM>, the adhesive layer <NUM> can have shear thinning properties, with non-Newtonian behavior and a reduction of viscosity when under shear stress.

As illustrated in <FIG>, the x-ray windows <NUM> or <NUM> can be part of an x-ray device, such as for example an x-ray detection device or an x-ray tube. An x-ray component <NUM>, which can be an x-ray detector or an electron emitter, can be located at least partially within an interior <NUM>I of the housing <NUM> and can face the x-ray window <NUM> or <NUM>.

These x-ray devices typically have a vacuum in the interior <NUM>I of the housing <NUM>. A getter is commonly used to achieve and maintain this vacuum. Because the getter is more quickly activated at higher temperatures, manufacturing time can be reduced if the thin film <NUM> is sealed to the housing <NUM> at a high temperature. Therefore, it can be useful for the adhesive layer <NUM> to be able to withstand high temperature without degradation. It can be useful for the adhesive layer <NUM> to have a high melting temperature, or if the adhesive layer <NUM> is a liquid crystal polymer, to have a high transition temperature TLC (temperature of transition between solid and liquid crystal states). Example transition temperature TLC values of the liquid crystal polymer include ≥ <NUM>, ≥ <NUM>, ≥ <NUM>, ≥ <NUM>, ≥ <NUM>, or ≥ <NUM> and ≤ <NUM>, ≤ <NUM>, or ≤ <NUM>.

Visible or infrared light can interfere with operation of the x-ray component <NUM>, such as an x-ray detector. Therefore, it can be helpful for the adhesive layer <NUM> to be opaque. For example, transmissivity of light, with a wavelength of <NUM>, with a wavelength of <NUM>, or both, through the adhesive layer <NUM> into the interior <NUM>I of the housing <NUM>, can be of ≤ <NUM>%, ≤ <NUM>%, or ≤ <NUM>%.

The adhesive layer <NUM> can have a low Young's Modulus, low tensile strength and high elongation to compensate for a mismatch of the coefficient of thermal expansion between the thin film <NUM> and the housing <NUM>. Such properties can allow flexure of the adhesive layer <NUM> when the thin film <NUM> and the housing <NUM> expand or contract at different rates during temperature changes, thus avoiding window fracture or distortion.

Example Young's Modulus values of the adhesive layer <NUM> include ≥ <NUM> MPa, ≥ <NUM> MPa, or ≥ <NUM> GPa and ≤ <NUM> GPa, ≤ <NUM> GPa, ≤ <NUM> GPa, or ≤ <NUM> GPa, each at <NUM>. Example tensile strength values of the adhesive layer <NUM> include ≥ <NUM> MPa, ≥ <NUM> MPa, ≥ <NUM> MPa, or ≥ <NUM> MPa and ≤ <NUM> MPa, ≤ <NUM> MPa, ≤ <NUM> MPa, or ≤ <NUM> MPa, each at <NUM>. Each of these tensile strength values can be for machine direction (roll-out direction of the liquid crystal polymer), transverse direction (transverse to the machine direction / roll-out direction), or both.

Elongation is strain before failure in tensile testing. Elongation is equal to (final length - initial length) / initial length. Example elongation values of the adhesive layer <NUM> include ≥ <NUM>%, ≥ <NUM>%, or ≥ <NUM>% and ≤ <NUM>%, ≤ <NUM>%, or ≤ <NUM>%, each at <NUM>.

The adhesive layer <NUM> can have isotropic properties, depending on its method of manufacture. Anisotropy of the adhesive layer <NUM> close or equal to zero might be preferable for uniform bond characteristics. For example, anisotropy of the adhesive layer <NUM> can be ≤ <NUM> MPa, ≤ <NUM> MPa, ≤ <NUM> MPa, ≤ <NUM> MPa, ≤ <NUM> MPa, ≤ <NUM> MPa, ≤ <NUM> MPa, or ≤ <NUM> MPa, each at <NUM>. Anisotropy is calculated by subtracting tensile strength in the transverse direction from tensile strength in the machine direction.

The thin film <NUM> and the housing <NUM> can have low coefficient of thermal expansion values. It can be helpful for the adhesive layer <NUM> to have a coefficient of thermal expansion close to that of the thin film <NUM>, the housing <NUM>, or both. Example coefficient of thermal expansion values of the adhesive layer <NUM> include ≥ <NUM>/(m*K), ≥ <NUM>-<NUM> m/(m*K), or ≥ 2x10-<NUM> m/(m*K) and ≤ 5x10-<NUM> m/(m*K), ≤ 10x10-<NUM> m/(m*K), ≤ 20x10-<NUM> m/(m*K), ≤ 30x10-<NUM> m/(m*K), ≤ 35x10-<NUM> m/(m*K), ≤ 40x10-<NUM> m/(m*K), or ≤ 45x10-<NUM> m/(m*K), each at <NUM>.

It can be helpful for the adhesive layer <NUM> to have low outgassing under vacuum after bonding. This property can avoid reduction of an internal vacuum of the final device. Weight loss of the heated adhesive layer <NUM> can be used to quantify outgassing. For example, weight loss of the adhesive layer <NUM>, prior to placing the adhesive layer <NUM> between the thin film <NUM> and the housing <NUM>, can be ≤ <NUM>%, ≤ <NUM>%, ≤ <NUM>%, ≤ <NUM>%, ≤ <NUM>%, or ≤ <NUM>%, at a temperature of <NUM> during a <NUM> hour period.

Use of a liquid crystal polymer as the adhesive layer <NUM> can provide a strong hermetic seal between many different x-ray window thin film <NUM> materials and many different housing materials, thus reducing gas leakage through the hermetic seal. For example, liquid crystal polymers can bond effectively to silicon, boron, aluminum, stainless steel, nickel, copper, or combinations thereof. The thin film <NUM>, the housing <NUM>, or both can include one or more of these materials. Examples of the leak tightness of the x-ray windows <NUM>, <NUM>, and <NUM> described herein include average helium leak rate of ≤ <NUM>-<NUM> mbar*L/sec, ≤ <NUM>-<NUM> mbar*L/sec, or ≤ 3x10-<NUM> mbar*L/sec, at a temperature of <NUM>, for at least the first <NUM> hours after forming the hermetic seal.

X-ray windows are commonly used with x-ray detection devices. X-ray fluorescence from material other than the sample measured / detected can interfere with the x-ray signal from the sample. This problem is usually worse if such interfering x-ray fluorescence is from a material with high atomic number. Therefore, it can be helpful if the adhesive layer <NUM> is made of material(s) with low atomic numbers. For example, ≥ <NUM>%, ≥ <NUM>%, ≥ <NUM>%, or ≥ <NUM>% of the atoms in the adhesive layer <NUM> can have an atomic number ≤ <NUM>. As another example, ≥ <NUM>%, ≥ <NUM>%, ≥ <NUM>%, or <NUM>% of the atoms in the adhesive layer <NUM> can have an atomic number ≤ <NUM>. Therefore, the adhesive layer <NUM> can be made primarily of carbon (Z=<NUM>), hydrogen (Z=<NUM>), and oxygen (Z=<NUM>).

Various types of liquid crystal polymer can be used as the adhesive layer <NUM> in the embodiments described herein. For example, the liquid crystal polymer can be main chain, side chain, linear, cyclic, branched, crosslinked, or combinations thereof.

The liquid crystal polymer is thermotropic. In other examples, outside the scope of the claims, the liquid crystal polymer can be lyotropic. The liquid crystal polymer can be an aromatic polyester. The polymer of the liquid crystal polymer can be formed from the following monomers: <NUM>-hydroxybenzaldehyde, <NUM>-hydroxy-<NUM>-naphthaldehyde, <NUM>,<NUM>'-biphenol, terephthalaldehyde, ethane-<NUM>,<NUM>-diol, or combinations thereof. The mesogen used in formation of the liquid crystal polymer can be disc-like, rod-like, amphiphilic, or combinations thereof. The liquid crystal polymers <NUM> can form regions of highly ordered structure while in the liquid phase.

Proper selection of a width of a mounting surface can improve bonding, reduce leakage, and reduce cost. As illustrated in <FIG> on x-ray window <NUM>, the housing <NUM> can have a flange <NUM>F encircling an aperture <NUM>A. The adhesive layer <NUM> can be located on the flange <NUM>F and can have an annular shape encircling the aperture <NUM>A of the housing <NUM>. Although x-ray window <NUM> in <FIG> shows mounting on an exterior of the flange <NUM>F, this invention is equally applicable to a mount on an inside of the flange <NUM>F, at an interior <NUM>I of the housing <NUM>. Example widths W<NUM> of the adhesive layer <NUM> on the flange <NUM>F include ≥ <NUM>, ≥ <NUM>, or ≥ <NUM> and ≤ <NUM>, ≤ <NUM>, ≤ <NUM>, or ≤ <NUM>.

It can be useful for the thin film <NUM> to be strong (especially strong enough to withstand a differential pressure of <NUM> atm) and allow a high percent transmission of x-rays. The thin film <NUM> can have sufficient thickness for strength, but not have a thickness that will cause excessive attenuation of x-rays. For example, the thin film <NUM> can have a thickness of ≥ <NUM> micrometers, ≥ <NUM> micrometers, ≥ <NUM> micrometers, or ≥ <NUM> micrometers; and ≤ <NUM> micrometers, ≤ <NUM> millimeter, or ≤ <NUM> millimeters. The thickness can depend on material of construction, span-width, differential-pressure, and application. Material of construction for the thin film <NUM> can include or consist of materials with an atomic number ≤ <NUM>, ≤ <NUM>, or ≤ <NUM>; and can include beryllium, hydrogen, oxygen, carbon, silicon, and nitrogen.

A differential pressure across the thin film <NUM> (e.g. a vacuum on one side and air or vacuum on an opposite side) can cause it to bow or deflect excessivley, damaging the x-ray window, and also possibly causing a short circuit by creating an unintended electrical-current path, or, for an x-ray tube, a change in electron-beam focusing. Thus, it can be useful to minimize the deflection distance. The thin film <NUM> described herein can be made sufficiently strong and thus can have a relatively small deflection distance. For example, the thin film <NUM> can have a deflection distance of ≤ <NUM> micrometers, ≤ <NUM> micrometers, ≤ <NUM> micrometers, or ≤ <NUM> micrometers, with one atmosphere differential pressure across the thin film <NUM>.

It can be useful for x-ray windows to have a high transmissivity of x-rays, including a high transmission of low-energy x-rays. The thin film <NUM> described herein can have a high transmissivity of x-rays. For example, the thin film <NUM> can have a transmissivity of ≥ <NUM>%, ≥ <NUM>%, ≥ <NUM>%, ≥ <NUM>%, or ≥ <NUM>% for x-rays having an energy of <NUM> keV.

For some applications, it can be useful for x-ray windows to block visible and infrared light transmission in order to avoid creating undesirable noise in sensitive instruments. For example, the thin film <NUM> described herein can have a transmissivity of ≤ <NUM>%, ≤ <NUM>%, or ≤ <NUM>% for visible light at a wavelength of <NUM> nanometers and/or a transmissivity of ≤ <NUM>%, ≤ <NUM>%, or ≤ <NUM>% for infrared light at a wavelength of <NUM> nanometers.

The thin film <NUM> can include some or all of the properties (e.g. low deflection, high x-ray transmissivity, low visible and infrared light transmissivity) of the x-ray windows described in U. Patent Number <CIT>.

A method of making an x-ray window <NUM> can comprise some or all of the following steps, which are illustrated in <FIG>. These steps can be performed in the following order or other order if so specified. There may be additional steps not described below. These additional steps may be before, between, or after those described. The x-ray window <NUM> and its components can have properties as described above for the x-ray windows <NUM> or <NUM>.

The method can include placing an adhesive layer <NUM> between a thin film <NUM> and a housing <NUM>. Another step in the method can be applying a pressure P to press the thin film <NUM> and the housing <NUM> towards each other and towards the adhesive layer <NUM> sandwiched between them. An additional step in the method can be heating the x-ray window, such as for example in oven <NUM>. Applying the pressure P and heating can be done simultaneously or sequentially. For example, a small weight can be placed on the thin film <NUM> and the housing <NUM>.

Pressure, duration under such pressure, and temperature can be adjusted for optimal bonding and throughput. Increased pressure can reduce processing time and increase bond strength, but can result in damage to sensitive components or undesirable thinning of the adhesive layer <NUM>. Examples of pressure ranges, particularly applicable if the adhesive layer <NUM> is liquid crystal polymer, include ≥ <NUM> KPa, ≥ <NUM> KPa, ≥ <NUM> KPa, ≥ <NUM> KPa, ≥ <NUM> KPa, ≥ <NUM> KPa, or ≥ <NUM> KPa and ≤ <NUM> KPa, ≤ <NUM> KPa, ≤ <NUM> KPa, ≤ <NUM> KPa, or ≤ <NUM> KPa.

Increased temperature can reduce processing time and better fill gaps and holes in the bonded materials, but can result in thermal damage to the adhesive layer <NUM> or other x-ray window components. The optimal temperature can be selected based on applied pressure P, processing time, temperature sensitivity of components, and desired bonding strength. For example, the x-ray window <NUM>, with liquid crystal polymer as the adhesive layer <NUM>, can be heated to a temperature above the transition temperature TLC of the liquid crystal polymer. For example, the x-ray window <NUM> can be heated ≥ <NUM>, ≥ <NUM>, or ≥ <NUM> above the transition temperature TLC of the liquid crystal polymer. Further, the x-ray window <NUM> can be heated ≤ <NUM>, ≤ <NUM>, or ≤ <NUM> above the transition temperature TLC of the liquid crystal polymer. As another example, the x-ray window <NUM> can be heated to a temperature of ≥ <NUM>, ≥ <NUM>, ≥ <NUM>, ≥ <NUM>, or ≥ <NUM> and ≤ <NUM>, ≤ <NUM>, ≤ <NUM>, or ≤ <NUM>.

A slow cure at a relatively lower temperature and pressure can result in an improved bond, but can increase cost due to reduced throughput. Examples of processing time (time during which the x-ray window <NUM> is maintained under pressure P and heat) include ≥ <NUM> minutes, ≥ <NUM> minutes, ≥ <NUM> minutes, ≥ <NUM> minutes, or ≥ <NUM> hour and ≤ <NUM> hours, ≤ <NUM> hours, ≤ <NUM> hours, ≤ <NUM> hours, or ≤ <NUM> hours.

The liquid crystal polymer can be initially dissolved in a solvent. The solution (liquid crystal polymer and solvent) can be applied to the thin film <NUM>, the housing <NUM>, or both, then baked to remove the solvent and bond the thin film <NUM> to the housing <NUM>. Alternatively, the liquid crystal polymer can be in pellet form, which can be pressed and placed onto the thin film <NUM>, the housing <NUM>, or both, then baked. The liquid crystal polymer can be formed into a sheet / film. This sheet / film can then be cut to shape and placed on the thin film <NUM> or the housing <NUM>. The thin film <NUM> and the housing <NUM> can be pressed together with the liquid crystal polymer between, then baked. A choice between these methods can be based on manufacture cost of the liquid crystal polymer, final bond strength, manufacture cost of the x-ray window, and isotropic properties of the liquid crystal polymer.

As illustrated in <FIG>, <FIG>, x-ray windows <NUM>, <NUM>, and <NUM>, respectively, are shown comprising a housing <NUM> including a flange <NUM>F encircling an aperture <NUM>A, a thin film <NUM>, and a pair of adhesive layers <NUM>. The thin film <NUM> can be sandwiched between the pair of adhesive layers <NUM>, and can be hermetically sealed to the housing <NUM> by one or both of the adhesive layers <NUM>. The pair of adhesive layers <NUM> can be pressed and cured together as described above in the Method section. Use of two adhesive layers <NUM> can balance compressive stress in the thin film <NUM>, and can improve leak tightness of the x-ray window.

As illustrated in <FIG>, one or both of the adhesive layers <NUM> can be a sheet extending across the aperture <NUM>A of the housing <NUM>. As illustrated in <FIG>, one or both of the adhesive layers <NUM> can have an annular shape with an aperture. The aperture of the adhesive layers <NUM> can encircle the aperture <NUM>A of the housing <NUM>. The aperture of the adhesive layers <NUM> can be aligned with the aperture <NUM>A of the housing <NUM>. A decision between these different embodiments can be made based on cost, manufacturability, x-ray attenuation by the adhesive layers <NUM>, and leak-tightness.

As illustrated in <FIG> and <FIG>, the thin film <NUM> and the pair of adhesive layers <NUM> can be mounted inside of the flange <NUM>F of the housing <NUM>. As illustrated in <FIG>, the thin film <NUM> and the pair of adhesive layers <NUM> can be mounted outside of the flange <NUM>F of the housing <NUM>. See US Patent Publication Numbers <CIT> and <CIT> for additional information about the advantages of mounting inside or outside of the flange.

The adhesive layer <NUM> described above can be one layer of the pair of adhesive layers <NUM>, specifically a proximal adhesive layer <NUM>p, closer to the housing <NUM> than the other layer, the distal adhesive layer <NUM>d. The proximal adhesive layer <NUM>, the distal adhesive layer <NUM>d, or both, can have properties as described above for the adhesive layer <NUM>. The pair of adhesive layers <NUM> can have a same material composition with respect to each other. The housing <NUM> and the thin film <NUM> can also have properties as described above.

Claim 1:
An x-ray window (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprising:
a thin film (<NUM>) configured for transmission of x-rays;
a housing (<NUM>); and
an adhesive layer (<NUM>) sandwiched between and providing a hermetic seal between the thin film (<NUM>) and the housing (<NUM>), characterised in that the adhesive layer (<NUM>) is ≥ <NUM>% thermotropic liquid crystal polymer.