WO2004012236A2

WO2004012236A2 - High reflectivity and high flux x-ray optic element and method of making same using ald

Info

Publication number: WO2004012236A2
Application number: PCT/US2003/023753
Authority: WO
Inventors: Stephen John Henderson; Steven M. George
Original assignee: Stephen John Henderson; George Steven M
Priority date: 2002-07-30
Filing date: 2003-07-30
Publication date: 2004-02-05
Also published as: WO2004012236A3; CA2492890A1; AU2003272209A8; AU2003272209A1

Abstract

This invention relates to a method of forming multiple bilayers on a substrate using an atomic layer deposition process. The reflecting layer of the bilayer pair can be a high electron density metal and the spacing layer of the bilayer pair can be a low electron density material. Because atomic layer deposition can deposit atomic layer controlled and conformal films, multiple bilayers can be deposited on the internal surfaces of monocapillary tubes. By applying a graded multiple bilayer, much higher reflectivity and higher flux optical elements can be obtained than those based on total external reflection. Deposition of a graded multiple bilayer on an elliptic or parabolic tapered tube leads to focused or collimated output from a point source input. Various atomic, layer deposition techniques are described to produce the graded multiple bilayer high quality X-ray focusing devices, especially for 'hard' X-rays.

Description

HIGH REFLECTIVITY AND HIGH FLUX X-RAY OPTIC ELEMENT AND METHOD OF MAKING SAME USING ATOMIC LAYER DEPOSITION

TECHNIQUES

This invention was made with government support under a grant awarded by the _'United States Air Force Office of Scientific Research. The government has certain rights in the invention.

The present invention relates to x-ray optic elements that provide high reflectivity and therefore high flux.

X-rays are used in various analytical methods, such as SAXS (small angle x- ray scattering), WAXS (wide angle x-ray scattering) and XRF (X-ray fluorescent analysis). Laboratory X-ray sources are point or line sources that emit X-rays in diverging directions, so the intensity naturally decreases away from the source according to the familiar inverse square law. Narrow, intense beams are needed for these analytical methods. It is very useful to provide some optic by which the X-ray beam is focused. By focusing the beam, the X-ray flux reaching the target can be increased by several orders of magnitude.

The currently most effective X-ray focusing devices are crossed or confocal (so- called) Gobel mirrors. The reflective surfaces of Gobel mirrors are specially designed to increase X-ray flux to the sample by minimizing destructive interference between the reflected X-rays for specific incident angles. The reflective surface of a Gobel mirror is made up of multiple bilayer pairs of materials. The bilayer pairs are very thin, typically on the order of about 10-200 Angstroms. Each bilayer pair consists of a reflective layer and a spacing layer, which are of two different materials. The reflective layer is a relatively high electron density material that reflects a small proportion of incident X-rays. The high electron density material is typically a relatively heavy metal or a material containing a relatively heavy metal.

Because each bilayer reflects only a small proportion of the incident X-rays, multiple bilayer pairs are used so that more X-rays are reflected and the overall x- ray flux reaching the sample is increased. However, in order to avoid destructive interference between the X-rays reflected by the many reflective layers, the spacing of the reflective layers should satisfy the Bragg equation, in which (ignoring small modifications due to finite absorption by the layers) twice the spacing between reflective layers times the sine of the angle of incidence of the X-rays is some whole number multiple of the wavelength of the X-rays (i.e., nλ=2d sin θ, where n is an integer, λ is the X-ray wavelength, d is the bilayer spacing between reflective layers and θ is the angle of incidence of the X-rays onto the reflective surface). This d- spacing (sometimes referred to as the "Bragg" spacing) is the sum of one spacing layer and one reflective layer. This spacing layer is of a lower electron density material that is relatively transparent to the X-rays and acts mainly to space the high electron density layers at the proper distance apart so as to satisfy the Bragg equation. The two most common wavelengths produced in laboratory x-ray sources are 1.54 Angstrom from copper sources and 0.71 Angstrom from molybdenum sources, which are referred to as "characteristic" wavelengths for laboratory sources. Synchrotron X-ray sources are "white" sources, meaning that a broad range of wavelengths are present in similar amount, so any wavelength value can be used in the design of the optic of this invention.

To enable a relatively high, monochromatic X-ray flux to reach the target (which in practice is either the sample that follows the optic or the detector that follows the sample), the Bragg equation should be satisfied for X-rays reflecting from the entire reflective surface of the optic. Because the angle of incidence (θ) varies at varying distances from the X-ray source, the d-spacing should vary as well to preserve the Bragg equation. For most mirror geometries, 0 decreases as one moves further from the X-ray source, so the required d-spacing correspondingly increases with increasing distance from the source. The spacing of the bilayers should in these cases increase continuously along its length. Bilayer pairs having spacings that vary in this manner are said to be "graded". The design of graded bilayer pairs is discussed in detail in U. S. Patent No. 6,226,349, incorporated herein by reference.

Currently, multiple (stacked) bilayer pairs are usually deposited onto the surface of Gobel mirrors using sputtering or electron beam deposition techniques. These methods are so-called "line of sight" methods, in that the material being deposited travels in a defined, straight-line path until the material is intercepted by the substrate. As such, these methods are only useful for applying bilayer pairs to planar substrates or substrates having very open geometry. When these methods are applied to more complex substrates (such as tubes), they are unable to deposit layers that are sufficiently uniform to provide an efficient mirror. Atomic layer deposition (ALD) is another method of deposition based on sequential self-hmiting surface reactions. Atomic layer deposition methods are based on the individual and sequential introduction of gas-phase reactants. Atomic layer deposition yields atomic layer controlled thin film growth and produces extremely conformal thin films. An atomic layer deposition method for preparing the bilayer pairs on a flat substrate (i.e., an open geometry) is described in U. S. Patent No. 5,945,204 and in H. Kumagai et al., App. Phys. Lett. 70, 2338 (1997) and M. Ishϋ et al., J. Crystal Growth 180, 15 (1997).

Monocapillary optics, consisting of a tube or capillary that is internally profiled in either an elliptical or parabolic shape, are an alternative to Gobel mirrors: Such optical systems are available from, for example, Australian X-ray Crystallography Optics PTY, Ltd. (AXCO) and the Cornell High Energy Synchrotron Sources (CHESS) facility at Cornell University. These systems have the theoretical advantage that they can be located closely to the X-ray source, thereby intercepting a larger solid angle of the source beam and increasing the X-ray flux to the target. These monocapillary tubes are prepared by drawing or pulling heated glass capillaries. However, bilayer pairs have not previously been formed on the internal surfaces of these capillaries. The usual line-of-sight methods (such as sputtering or electron beam methods) are inapplicable. Thus, current commercial monocapillary optics usually omit the bilayer pairs and operate on a different principle, i.e., critical external reflection. The commercial monocapillaries are designed such that the angle of incidence of the X-rays onto the reflective surface is ver small (<3 milliradians for a glass monocapillary), since only X-rays below this shallow angle of incidence are reflected. This means that in practice the solid angle intercepted by the monocapillary is quite small. The lack of bilayer pairs leads to poo monochromation of the beam compared to a multilayer optic. It would thus be desirable to further improve the performance of these monocapillary optics.

In another method for producing monocapillary tubes, films, including bilayer pairs, can be deposited on a tapered mandrel and the mandrel can subsequently be removed to leave a free-standing tube. A process where the bilayer pairs are deposited onto a mandrel, followed by deposition of a supporting external shell, is described in U. S. Patent No. 6,278,754. The process is said to provide capillary optics having bilayer pairs on the internal reflecting surface. However, this process is very expensive and requires the removal of the mandrel from the completed capillary. This tends to damage the bilayer pairs and impairs performance.

Figure 1 is a side view, partly in section, of an X-ray focusing device of the invention.

Figure 2 is a schematic illustration of an embodiment of the process of the invention.

Figure 3 is a cross-section showing bilayer pairs suitable for forming an X-ray reflecting device of the invention.

Figure 4 is a graph showing the measured x-ray reflectivity for a 64 bilayer pair of a W/AI2O3 multilayer with a total thickness of ~2000 Angstroms.

In one aspect, this invention is a method of making an X-ray focusing device having multiple bilayer pairs having a predetermined spacing and each bilayer pair including a reflecting layer and a spacing layer, comprising depositing alternating reflecting layers of a relatively high electron density metal and spacing layers of a relatively low electron density material onto a reflective surface of a substrate via an atomic layer deposition process. This process permits very fine control over the thicknesses of both the metal layers and the spacing layers. This aspect of the invention permits one to produce X-ray optics with metal reflecting layers that provide good X-ray flux to the target as well as a monochromatic beam.

In a second aspect, this invention is a process of making an X-ray focusing device having multiple bilayer pairs having a predetermined spacing and each bilayer pair including a reflecting layer and a spacing layer, comprising depositing alternating reflecting layers of a relatively high electron density metal and spacing layers of a relatively low electron density material onto a reflective surface of a substrate via an atomic layer deposition process, such that the spacing of the bilayer pairs is graded. In this embodiment, the spacing of the bilayer pairs varies from location to location on at least a portion of the reflecting surface of the focusing device in relation to the angle of incidence of the X-rays at each such location, such that the bilayer pair spacing at each such location satisfies the Bragg equation nλ=2d sin θ for at least one X-ray having a specific wavelength λ, where θ is the angle of incidence of the X-rays at a point on the reflective surface of the focusing device, d is the bilayer spacing at such point, and n is an integer. This method permits the formation of focusing devices having bilayer pairs in which each layer of the pair has a controllable, predetermined thickness at substantially all points on the reflecting surface. The individual layers have excellent surface smoothness and sharp layer boundaries.

In a third aspect, this invention is a method of making a tubular X-ray focusing device, comprising depositing multiple bilayers onto an internal surface of a tubular substrate via an atomic layer deposition process, wherein the substrate has an eULiptically or parabolically tapered longitudinal bore with a circular cross-section, the bore being shaped such that a beam entering an entrance end of the bore is reflected to form an elliptically or parabohcally focused beam exiting an exit end of the bore. As before, the spacing of the bilayers is chosen such that the bilayer pair spacing satisfies the Bragg equation nλ=2dsin θ for at least. one X-ray having a specific wavelengthλ, where θ is the angle of incidence of the X-rays at a point on the reflective surface of the focusing device, d is the bilayer spacing at such point, and n is an integer. The bilayers are preferably graded as in the previous aspect.

In a fourth aspect, this invention is an X-ray focusing device, comprising a substrate tube having an elliptically or parabohcally tapered longitudinal bore with a substantially circular cross-section, the bore being shaped such that a beam entering an entrance end of the bore from a point source is reflected to form an elliptically- or parabolically-focused beam exiting an exit end of the bore, wherein the longitudinal bore has on its internal surfaces multiple bilayer pairs of ALD-applied materials. The bilayer pair spacing is chosen to satisfy the Bragg equation, as. before, with respect to at least one X-ray having a specific wavelength λ, and is graded as discussed with respect to other aspects above. The tubular focusing device of the invention allows for excellent beam focusing and thus high flux of X-ray beams. The tubular configuration allows the device to be located close to the source of the beam. The tubular device can intercept a large solid angle of the source beam, thereby providing high flux to the target. The focusing device does not use very shallow angles of incidence, as is the case with conventional capillary devices based on critical external reflection, so flux is much higher. In addition, with proper control of the thickness of the multiple bilayer pairs (and their constituent layers), highly monochromatic beams can be produced. In preferred embodiments, the spacing of the bilayer pairs is graded along the internal surface of the tube. In this invention, bilayer pairs are deposited onto a substrate via an atomic layer deposition (ALD) process (also known as atomic layer epitaxy, or "ALE"). The process allows for excellent control of the thickness of each bilayer pair and each constituent layer within the bilayer pairs, and provides smooth and sharp boundaries between the various layers. The ALD process permits graded bilayer pairs to be formed on a wide variety of substrate geometries, in each case with excellent control over the spacing of the layers.

Each bilayer pair consists of a reflective layer and a spacing layer. The reflective and spacing layers thus alternate, with spacing of the reflective layers being determined by the thickness of the reflective layers themselves and the thickness of the intervening spacing layer. The number of bilayer pairs can be applied in any number, but in practice, standard multilayer theory sets this number for optimum flux based on the x-ray absorption in the spacing and reflecting layers, typically at 50-200 bilayer pairs.

The thickness of the layers is a function of the geometry of the system and the wavelength of the incoming beam. This relationship is described by the Bragg equation, nλ=2d sin θ, or in rearranged form, d=nλ/2sin θ, where n is an integer, λ is the characteristic or specific wavelength of the X-ray, d is the d-spacing between reflective layers and θ is the angle of incidence of the beam onto the reflective surface. Because the wavelength is a variable in the Bragg equation, the required d- spacing of the bilayer pairs will depend on the particular beam source (which dictates the characteristic wavelength of the X-rays, for laboratory sources).

The d-spacing, and therefore the thickness of the bilayer pair, must be determined uniquely for each device, taking into account the source and the geometry of the system, including the size and geometrical type of the mirror, the distance from the mirror to the source, and the distance from the mirror to the target (either detector or sample). Typically, the d-spacing will be of the order of about 10 to about 200 Angstroms. The thickness of the reflecting layer is normally about 15- 40% of the d-spacing, and the thickness of the spacing layer is normally about 60- 85% of the d-spacing, as determined by standard multilayer theory.

Similarly, the geometry of the mirror is usually such that the angle of incidence (θ) is not constant for all locations on the reflective surface of the mirror. Accordingly, the required d-spacing is not the same for all locations on the reflective surface of the mirror. For example, a tubular focusing device four inches long located at a distance of four inches from an X-ray point source and having an inner diameter of 0.476 inches at the inlet increasing elliptically to 0.619 inches at the outlet would require a d-spacing of approximately 18 Angstroms at the inlet and 36 Angstroms at the outlet, for a 1.54 Angstrom CuKα X-ray source. The d-spacings at intermediate positions in the tube will have intermediate values.

Grading of this sort is described in greater detail in U. S. Patent No. 6,226,349, incorporated herein by reference.

The precise curvature and dimensions of the focusing device are determined by the choice of elliptical or parabolic styles, the position of the optic, and the locations of the source and target. The geometry of the mirror is not particularly critical to the invention, if the reflective surface of the substrate will focus and monochromate incoming beams as desired. Typically, the reflective surface of the device is formed to focus the beams from a point source to either another specific point in space (i.e., elliptical focusing), or to form parallel beams (i.e., parabolic focusing). Tubular substrates that form parallel beams are parabolic in longitudinal cross-section (truncated at both ends), and circular in transverse cross-section. A point source located at the focus of the parabola is reflected to form a parallel, monochromatic beam. Tubular substrates that form point-focused beams are elliptical in longitudinal cross-section (truncated at both ends), and circular in transverse cross-section. A point source located at one focus of the ellipse is reflected to form a monochromatic beam directed to the other focus of the ellipse.

A schematic of a tubular focusing device capable of elliptical (point) focusing is illustrated in Figure 1. X-ray beams 24, 26 and 28 are generated at point source 21 and directed into entrance 30 of tube 22 (shown in section). Within tube 22, beams 24, 26 and 28 reflect from interior surface 27 of tube 22 and are directed through exit 29 to a target or detector at point 23. The angle of incidence of a reflected beam is equal to the angle of reflection. These angles are indicated as θi and θ₂ for beams 24 and 26, respectively. The longitudinal cross-sectional shape of interior surface 27 is that of an ellipse that is truncated at both ends, with the foci of the ellipse being located at point source 21 and target 23. Interior surface 27 is circular in transverse cross-section at every point along its longitudinal axis, so that x-rays hitting any portion of interior reflective surface 27 (or at least a substantial portion of such X- rays) are monochromated and reflected toward point 23. Interior surface 27 has a plurahty of bilayer pairs as described herein. Plug 31 occupies the center of opening 28. It is positioned and sized such that X-rays that enter tube 22 at an angle such that they will not reflect from interior surface 27 are blocked. This prevents unreflected X-rays from interfering with the desired monochromated, focused X-rays that reflect from interior surface 27 toward target 23. The effect of the geometry illustrated by Figure 1 is to produce an el ptically focused (i.e., point-to-point) beam. A parallel beam is produced if the interior reflective surface of the tube is instead longitudinally parabolic in shape.

Plug 31 is a means of preventing unfocused, non-monochromated beams from reaching the target. It can be replaced, for example, with a smaller diameter optic as described herein, that is nested within the larger tube and which in turn has a much smaller plug. Such a smaller diameter optic can intersect X-rays that would otherwise not pass through the larger tube due to being blocked by plug 31, monochromating them and focusing them at the intended target, in the same manner as does the larger tube. The effect is to further increase the flux of X-rays to the target. The smaller inner optic would have larger d-spacings on its bilayer pairs in order to compensate for the smaller incident angles on its surface.

Tubular focusing devices made in accordance with the invention have a diameter ranging from very small to about 10-40 millimeters.

The focusing device of the invention is particularly suitable for focusing so- called "hard" X-rays having an energy of >2.0 keV (corresponding to wavelengths of <6 Angstrom). These hard X-rays can be produced by electron beam interaction with metal targets. The two most common wavelengths produced in laboratory X-ray sources are 1.54 Angstroms (8 keV) from copper sources and 0.71 Angstrom (16 keV) from molybdenum sources. Accordingly, the composition of the reflective layer is such that (1) the reflective layer has a relatively high electron density and (2) the reflective layer can be deposited via an ALD process to form a smooth layer adherent to an adjacent spacing layer. The chemical precursors used to make the reflective layer should wet the surface of the spacing layer so that a smooth, adherent reflective layer can be formed. Relatively high electron density materials are typically metals having an atomic number of 25 or greater, particularly 40 or greater, or compounds of such metals, preferably an oxide, nitride or carbide thereof, as discussed in more detail below.

Similarly, the composition of suitable spacing layers for X-ray mirrors is such that (1) the spacing layer is relatively transparent to the "hard" X-rays (of the wavelength for which the mirror is to be used) and (2) the spacing layer can be deposited via an ALD process. Suitable spacing layers include compounds of metals and/or nonmetals having an atomic number below 25, preferably 14 or below. Better performance is usually achieved as the electron density difference of the reflective and spacing layers increases. A difference of atomic number of 15 or more, preferably at least 30 or more, between the metal component of the reflective layer and the highest atomic number element of the spacing layer is particularly suitable. For purposes of comparing atomic numbers, impurities or trace materials consisting of less than 5% by weight of the layer are ignored. Metal/AbOa reflective/spacing layer pairs are particularly useful for reflecting "hard" X-rays. A system of particular interest is a W/AI2O3 system of bilayer pairs.

The bilayer pairs are deposited via an ALD technique. Atomic layer controlled growth techniques permit the deposition layers of up to about 3 Angstroms in thickness per reaction cycle, and thus provide a means of extremely fine control over layer thickness. In the ALD process, each reflective layer and each spacing layer is formed in a series of two or more self-limited reactions, which in most instances can be repeated to sequentially deposit additional material until the layer achieves a desired thickness. A general description of the ALD process is given in S.M. George et al., "Surface Chemistry for Atomic Layer Growth", J. Phys. Chem. 100, 13121 (1996), incorporated herein by reference. Each set of reactions will generally deposit a film having a thickness approximately equal to a monolayer of the material. Once the series of reactions to form a (spacing or reflective) layer of desired thickness is completed, a second series of reactions is conducted to deposit the next layer. As before, this second series of reactions may be repeated as necessary until the layer achieves the desired thickness. In this way, alternating spacing and reflective layers, each of a predetermined thickness (as specified by application of the Bragg equation), are produced.

In the ALD process, the reactants are introduced sequentially in the gas phase. As the reactants are often solids or liquids at room temperature and O 2004/012236

atmospheric pressure, elevated temperatures and/or reduced pressures are ordinarily used to provide conditions in which the reactants will vaporize. Thus, the reactants are materials either that are gasses under standard conditions, or which can be volatilized under moderate temperature conditions (preferably at 1000K or below). Preferred reactants have vapor pressures of at least 10 torr or greater at a temperature of 300K.

The reactions are generally performed at elevated temperatures, preferably from about 400-1000K, except in cases where the reaction is catalyzed, in which case lower temperatures are sometimes useful so long as the reactants remain in the form of a gas. The substrate is generally held in a chamber that can be evacuated to low pressures. Each reactant is introduced sequentially into the reaction zone, typically together with an inert carrier gas. The reactant reacts at the surface of the substrate to form a thin surface film, as discussed before. Before the next reactant is introduced, the reaction by-products and unreacted reagents are removed before conducting the next reaction. This can be done, for example, by subjecting the substrate to a high vacuum, such as about 10⁵ torr or lower, after each reaction step. Another method of accomphshing this, which is more readily applicable for industrial application, is to sweep the substrate with an inert purge gas between the reaction steps. This purge gas can also act as a carrier for the reagents. Then, the next reactant is introduced, where it reacts at the surface of the substrate. After removing excess reagents and reaction by-products, as. before, the reaction sequence can be repeated as needed to build layers of the desired thickness.

General methods for conducting ALD processes are described, for example, in 3. W. Klaus et al, "Atomic Layer Controlled Growth of Siθ2 Films Using Binary Reaction Sequence Chemistry", Appl. Phys. Lett. 70, 1092 (1997) and O. Sneh et al., "Atomic Layer Growth of Siθ2 on Si(100) and H2O using a Binary Reaction Sequence", Surface Science 334, 135 (1995), both incorporated herein by reference. .

Because the reaction precursors are all in the gas phase, the ALD process is not a "line-of-sight" method of depositing the multiple bilayers. Instead, the reactants diffuse isotropically to fill the available space. The reactants cover all surfaces of the substrate, even those surfaces which are not in the direct path of the precursors as they are brought into the reaction chamber and removed. Further, as the reactions are seK-limiting, and the precursors form only monolayer films on the substrate surface during each exposure, the resulting film that is deposited per reaction cycle is highly uniform in thickness. This permits the formation of high quality bilayers on surfaces of substrates having a wide range of geometries.

The ALD technique readily facilitates variations in the thickness of the bilayers (and/or their constituent layers) by selectively controlling the number of repetitions of the reaction that occur at various locations on the surface of the substrate. Areas in which thicker bilayers are needed are exposed to a greater number of repetitions of the reaction(s).

Layers of variable thickness can be created inside the bore of a tube, for example, by introducing the reactants through a smaller capillary that is inserted into the bore. The capillary can be inserted into either the entrance or exit opening of the bore. By progressively inserting the capillary farther into the tube as reaction sequences are repeated, areas downstream of the capillary opening are subjected to a larger number of reaction sequences to form a progressively thicker layer. Because the gaseous reacts isotropically fill their containing volume, measures are required to prevent the reactants from flowing backwards. This can be accomplished by damming the tube upstream of the capillary opening, but more preferably is done with a positive flow of a viscous flow carrier gas. The direction of flow of the carrier gas is the direction in which the capillary is progressively inserted. The carrier preferably is introduced under laminar flow conditions to avoid turbulence and eddy currents.

Such a method is illustrated in Figure 2. In Figure 2, bilayer pair 37, consisting of reflective layer 36 and spacing layer 35 is deposited, onto, the bore of tube 31. The thickness of layers 35 and 36 and the geometry of the bore of tube 31 are exaggerated, for purposes of illustration. The reactants that are used to form the reflective and spacing layers are introduced into tube 31 through capillary 32, which is inserted into tube 31 through opening 33. In this embodiment, a viscous flow carrier gas is flowed through tube 31 as reactants are discharged from capillary 32, in the direction indicated by arrows 39. The flow rate of the viscous flow carrier gas is sufficient to sweep the reactants exiting capillary 32 in the general directions indicated by arrows 38, thereby preventing the reactants from moving upstream of capillary tip 40 (i.e., back toward opening 33). The reactant gases emitted from capillary 32 have to diffuse laterally to reach the walls of tube 31. This diffusion process will lead to some variation of the reactant exposure versus distance from capillary tip 40. Conditions are generally selected (in particular flow rates of the viscous flow carrier gas) so that the lateral diffusion is rapid enough that the variation of the reactant exposures versus distance from capillary tip 40 is minimal. By progressively moving capillary tip 40 farther into tube 31 as the reaction sequences are completed, areas of the bore downstream of the capillary tip are exposed to a greater number of reaction cycles than areas upstream. The thickness of layers 35 and 36 therefore become progressively thicker as one moves from left to right.

If the viscous flow carrier gas has a very rapid velocity, the entrainment process together with the diffusion of the reactant gases can produce a large variation of the reactant exposure versus distance from tip 40 of capillary 32. This effect can also be used to form graded layers. In that case, the individual reactions would reach completion where the reactant exposures are the largest and would not reach completion where the reactant exposures are smaller. Since smaller reactant exposures would lead to lower film growth rates, a continuous tapering of the d- spacing can be achieved by controlling the viscous flow carrier gas velocity in the monocapillary tube. Control of the layer thicknesses is more difficult using this method alone. This method can be used in conjunction with the movement of the capillary tube described above to help form smoother gradations in the thickness of the deposited layers. This smoothing helps reduce or eliminate discontinuities in the d-spacing that can be detrimental to the performance of the mirror.

In addition to the method illustrated by Figure 2, the converse procedure works to the same effect. The capillary can be progressively retracted from the tube as the reaction sequences are repeated. As before, means such as a viscous flow carrier gas (in this case flowing in a direction opposite to the capillary movement) are used to prevent the reactants from flowing back upstream of the capillary tip. The movement of the capillary allows different number of ALD reactant cycles to be applied to the tube at different locations. In this case, thinner layers are formed as one progresses down the bore in the direction of the retracting smaller tube.

Mirror surfaces of other geometries can be coated with graded multiple bilayers using variations of the same principles described above. Different functional forms for the variation in the d-spacing can be created depending on the exact movement of the capillary versus time and the viscous flow carrier gas velocity.

In most instances, the ALD process begins by introducing some functional group on the exposed surface, such as an M-H, M-O-H or M-N-H group, where M represents an atom of a metal or semi-metal. The substrate should be treated before initiating the reaction sequence to remove volatile materials that may be absorbed onto the surface. This is readily done by exposing the substrate to elevated temperatures and/or vacuum. In some instances, a precursor reaction may be performed to introduce desirable functional groups onto the surface of the substrate.

Examples of binary reaction sequences for producing metal layers (which are preferred reflective layers for hard X-ray mirrors) are described in copending application no. 09/523,491 entitled "A Solid Material Comprising a Thin Metal Film on its Surface and Methods for Producing the Same", which is incorporated herein by reference. A specific reaction scheme described therein involves sequential reactions of a substrate surface with a metal halide followed by a metal halide reducing agent. The metal of the metal halide is preferably a transition metal or a semimetallic element, including tungsten, rhenium, molybdenum, antimony, selenium, tellurium, platinum, ruthenium and iridium. The halide is preferably fluoride. The reducing agent is suitably a silylating agent such as silane, disilane, trisilane and mixtures thereof. Other suitable reducing agents are boron hydrides such as diborane.

For depositing a tungsten coating, for instance^ the overall reaction, WFβ + Si2Hβ->W + 2SiF3H + 2H2, can be split into a sequence of reactions represented as:

M-OH* (substrate surface) + SisHe → M-O-SiH₃* + SiH₄ M-O-S1H3* + WFe -> M-O-WF5* + SiFH₃ (precursor reactions)

M-0-WF5*.+ Si2H6→ M-O-W-SiF2H* + SiF₃H + 2H2 (Al)

M-0-W-SiF₂H* + WF₆ → M-O-W-WF5* + SiFaH (Bl)

M is a surface metal or semimetal. The asterisk (*) indicates the species that resides at the surface of the substrate or deposited film. Once the precursor reaction is completed, reactions Al and Bl are alternatively performed until a tungsten layer of desired thickness is formed.

Another binary reaction scheme suitable for depositing a metal (M²) film on a substrate having surface hydroxyl or amine groups can be represented as:

M*-Z-H (substrate surface) + M²Xn - M-Z-M²*Xn-ι + HX

(precursor reaction) M-Z-M²X* + H_y - M-Z-M²-H* + HX (A2)

M-Z-M²-H* + M R - M-Z-M*-M**R + HR (B2)

R refers to an organic Mgand such as alkyl, alkylamino or acetylacetonate species, Z represents oxygen or nitrogen, and X is a displaceable nucleophilic group. H_y is either a hydrogen atom (y=l) or a hydrogen molecule (y=2). Also as before, the asterisk (*) refers to the species residing at the surface. By heating to a sufficient temperature, hydrogen bonded to the surface as M²-H will thermally desorb from the surface as H2, thereby generating a final surface composed of M² atoms. For hard x-ray reflecting layers, high electron density metals such as tungsten molybdenum, ruthenium, tantalum, rhenium, iridium and platinum are preferred metals for coating according to reaction sequence A2 B2.

The spacing layers for X-ray mirrors are suitably oxides, carbides or nitrides of metals or semimetals, preferably metals or semimetals having a valence of 3 or 4 and an atomic number of less than 25, preferably 14 or below. Other suitable spacing lawyers include single-element species with low atomic number such as silicon or carbon. Examples of suitable materials for spacing layers, which can be deposited via an ALD technique, include alumina (AI2O3), silica (Siθ2), titanium oxide (Tiθ2), boron nitride (BN), aluminum nitride (AIN) and silicon nitride (Si3N ). Single element spacing layers such as silicon can also be deposited by an A T) technique using hydrogen radicals. Particular spacing layers are selected not only for their transparency to X-rays, but in addition for their ability to adhere to the reflective layer and form smooth, sharp layer boundaries. Spacing layers made from amorphous materials tend to form smoother layer boundaries. Thus, in any particular case, the composition of the spacing layer is typically selected in conjunction with that of the reflective layer to obtain the optimum reflectivity.

Oxide spacing layers can be prepared on an underlying substrate or layer having surface hydroxyl or amine groups using a binary (AB) reaction sequence as follows. The asterisk (*) indicates the species that resides at the surface, and Z represents oxygen or nitrogen. M¹ is an atom of the metal (or semimetal such as silicon), particularly one having a valence of 3 or 4, and X is a displaceable nucleophihc group. The reactions shown below are not balanced, and are only intended to show the reactions at the surface (i.e., not inter- or intralayer reactions).

M-Z-MPX* + H₂O → M-Z-M¹ OH* + HX (B3)

In reaction A3, reagent M^xXn reacts with the M*-Z— H groups on the surface to create a new surface group having the form — M*-Xn-ι. M¹ is bonded through one or more Z (nitrogen or oxygen) atoms. The -M¹— Xn-i group represents a site that can react with water in reaction B3 to regenerate one or more hydroxyl groups. The hydroxyl groups formed in reaction B3 can serve as functional groups through which reactions A3 and B3 can be repeated, each time adding a new layer of M¹ atoms. Note that in some cases (such as, e.g., when M¹ is silicon, titanium, boron or aluminum) hydroxyl groups can be eliminated as water, forming M D-M¹ bonds within or between layers. This condensation reaction can be promoted if desired by, for example, annealing at elevated temperatures and/or reduced pressures.

Binary reactions of the general type described by equations A3 and B3, where M¹ is silicon, are described more fully in J. W. Klaus et al, "Atomic Layer Controlled Growth of SiO₂ Films Using Binary Reaction Sequence Chemistry"; Appl. Phys. Lett. 70, 1092 (1997) and O. Sneh et al., "Atomic Layer Growth of SiO₂ on Si(100) and H2O using a Binary Reaction Sequence", Surface Science 334, 135 (1995), both incorporated herein by reference. Binary reactions of the general type described by equations A3 and B3, where M¹ is aluminum, are described in A. C. Dillon et al, "Surface Chemistry of AI2O3 Deposition using A1(CH3)3 and H2O in a Binary reaction Sequence", Surface Science 322, 230 (1995) and A. W. Ott et al., "AI2O3 Thin Film Growth on Si(100) Using Binary Reaction Sequence Chemistry", Thin Solid Films 292, 135 (1997). Both of these references are incorporated herein by reference. General conditions for these reactions as described therein can be adapted to construct Siθ2 and AI2O3 coatings on particulate materials in accordance with this invention. Analogous reactions for the deposition of other metal oxides, such as TLO2, are described in M. Ritata et al, Thin Solid Films 225, 288 (1993), incorporated herein by reference.

A preferred spacing layer material for X-ray mirrors in the hard X-ray region is alumina. An example of an overall reaction for depositing an alumina coating is 2A1(CH3)3 + 3 H2O -> AI2O3 + 6 CH₄. This overall reaction can be spht into a sequence of reactions represented as:

A1OH* + A1(CH₃)₃ → Al-O-Al(CH₃)₂* + CH₄ (A4)

Al-CGHβ)* + H2O -» Al-OH* + CH₄ (B4)

Analogous reaction sequences can be performed to produce nitride and sulfide coatings. An illustrative reaction sequence for producing a nitride coating is:

M-Z-H* +

→ M-Z-MⁱXn-i* + HX (A5)

M-Z-M^JX* + NH₃→ M-Z-M¹ NH₂* + HX (B5)

Ammonia can be ehminated to form M tf-M¹ bonds within or between layers. This reaction can be promoted if desired by, for example, annealing at elevated temperatures and/or reduced pressures. A suitable binary reaction scheme for depositing an inorganic nitride coating, such as Siβ i is described in J. W. Klaus et al, Surf. Sci, 418, L14 (1998), incorporated herein by reference.

An illustrative reaction sequence for producing a sulfide coating is: . .

M-Z-M*X* + H₂S→ M-Z-M¹ SH* + HX (B6) .

Hydrogen sulfide can be ehminated to form M¹-S-M¹ bonds within or between layers. As before, this reaction can be promoted by annealing at elevated temperatures and/or reduced, pressures. A suitable biliary . reaction scheme for depositing an inorganic sulfide coating, such as CdS, is described in. M. Han et al Surf. Sci. 415, 251 (1998), incorporated herein by reference.

A suitable binary reaction scheme for depositing an inorganic phosphide coating, such as AIP, is described in M. Ishϋ et al, Crystal. Growth 180, 15 (1997), incorporated herein by reference. In the foregoing reaction sequences, suitable replaceable nucleophihc groups will vary somewhat with M¹, but include, for example, fluoride, chloride, bromide, alkoxy, alkyl, acetylacetonate, and the like. Specific compounds having the structure M^ that are of particular interest are sihcon tetrachloride, tetramethylorthosUicate (Si(OCH3)₄), tetraethyl-orthosilicate (Si(OC2Hs)4), trimethyl aluminum (A1(CH3)3), triethyl aluminum (A1(C H5)3), other trialkyl aluminum compounds, and the like.

In addition, catalyzed binary reaction techniques such as described in U. S. Patent No. 6,090,442 incorporated by reference, are suitable for producing coatings, especially oxide, nitride or sulfide coatings, most preferably oxide coatings. Reactions of this type can be represented as follows:

M-OH" Cι + R-M -R→ M-O-Mⁱ-R + R-H + Ci (A7b)

M-O-M¹-R + C2 - M-O-M¹-R C₂ (B7a)

M-O-M--R- "C2 + H₂O → M-O-MMDH + R-H + C₂ (B7b)

Ci and C2 represent catalysts for the A7b and B7b reactions, and may be the same or different. Each R represents a functional group (which may be the same or different), and M and M¹ are as defined before, and can be the same or different. Reactions A7a and A7b together constitute the first part of a binary reaction sequence, and reactions B7a and B7b together constitute the second half of the binary reaction sequence. An example of such a catalyzed binary reaction sequence is:

Si-OH* + C5H5N → Si-OH -C5H5N* (A8a)

Si-OH- "C5H5N* + SiCl₄ → Si-O-SiCls* + C5H5N + HC1 (A8b)

Si-O-SiCl₃* + CδHsN→ Si-O-SiClr-CsHδN* (B8a)

Si-O-SiCls -CsHsN* + H₂O→ Si-O-SiOH* + C_εH₅N + HC1 (B8b)

where the asterisks (*) again denote species at the surface. This general method is applicable to forming various other coatings, including zirconia or titania.

Several techniques are useful for monitoring surface reactions during the ALD process. For example, vibrational spectroscopic studies can be performed using Fourier transform infrared techniques. The deposited coatings can be examined using spectroscopic elhpsometry or X-ray reflectivity. Atomic force microscopy studies can be used to characterize the roughness of the coating relative to that of the surface of the substrate. Depth-profiling X-ray photoelectron spectroscopy can be used to determine the elemental composition and chemical state of the atoms in the film. X-ray diffraction can ascertain the crystallographic structure of the coating.

Growth rates of the ALD-deposited layers tends to be relatively uniform per AB reaction cycle, so that layer thickness is usually predictable from the number of cycles that are repeated. For example, AI2O3 is typically deposited via ALD at a growth rate of 1.1-1.2 Angstroms per reaction cycle at 180°C. In comparison, tungsten is deposited at a growth rate of about 2.5 Angstroms per reaction cycle at 180°C. These growth rates can be affected by changing the reaction temperature. In cases where one member of the bilayer pair has a smaller growth rate, the member of the bilayer pair that has the smaller growth rate provides the best control over the d- spacing.

In some instances, as with metal layers such as tungsten, the deposited spacing or reflective layer may be polycrystaUine. This polycrystalline structure may cause some interfacial roughness between adjacent layers. If desired, an impurity may be introduced during the formation of polycrystalline layers in order to suppress the crystalhnity somewhat and produce a smoother interface. For example, carbon can be added in -the form of a gaseous hydrocarbon (such as ethylene) during the ALD process for forming metals such as tungsten. The hydrocarbon will decompose under the conditions of the ALD process to add carbon impurities. .

The following examples are provided to illustrate the invention, but are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated. Example 1

In this example, multilayer pairs are deposited onto a sihcon substrate using ALD techniques. Figure 3 is a cross-sectional transmission electron microscope (TEM) image of the resulting structure prepared in accordance with this example. Alternating reflective layers 61 (W, tungsten) and spacing layers 62 (AI2O3, alumina) are shown as deposited onto a silicon [Si(100)} substrate 63. This superlattice is composed of four AI2O3 W bilayers 64. Each individual layer is deposited using a sufficient number of AB cycles to form individual AI2O3 and W layers approximately 125 A thick. The layers in this example are somewhat thicker than would generally be useful to focus X-rays. The greater layer thicknesses are selected in this Example to allow for good TEM imaging of the multilayer structure, so that layer uniformity and thickness control are more easily illustrated.

The AI2O3 layers 62 are grown using a tι-t2-t3-t₄ sequence of 1-5-1-5 where ti is the TMA reactant pulse time, t₂ is the subsequent purge time, t3 is the H2O reactant pulse time and t₄ is the subsequent purge time. The times are all in seconds. The AI2O3 layers 62 are each deposited using 111 such TMA/H2O AB cycles at 177 °C. In

Figure 3, there is an interfacial oxide layer 66 of ~15 A at the interface between the Si(100) substrate 63 and the first AI2O3 layer 62. This Siθ2 layer results from the partial oxidation of the sihcon substrate.

The tungsten layers 61 are deposited using a tι-t2-t3-t₄ sequence of 10-5-1-5 where ti is the Si2Hβ reactant pulse time, t2 is the subsequent purge time, t3 is the WFβ reactant pulse time and t₄ is the subsequent purge time. The tungsten layers 61 are each deposited using 32 Si2H6/ F6 AB cycles at 177°C. Fewer AB cycles are required to deposit the tungsten layers because each W ALD AB cycle has a much larger growth rate (about 2.5 Angstroms/cycle) than the AI2O3 ALD AB cycle (1.1-1.2 Angstroms/cycle) .

Figure 3 illustrates how the AI2O3 layers 62 and W layers 61 can be produced controllably and reproducibly. The interface between the top of each AI2O3 layer and the next tungsten layer is extremely smooth in each instance because the AI2O3 layers are amorphous and nearly atomically smooth. The interface between the top of each tungsten layer and the succeeding AI2O3 layer displays some greater roughness because the tungsten layers are polycrystalline. Depositing the AI2O3 layer atop the underlying tungsten layer results in some smoothing of this roughness, as shown in Figure 3.

In Figure 3, the bilayer spacing is the sum of the thickness of each bilayer pair, i.e., the sum of the thickness of a tungsten layer 61 and an adjacent alumina layer 62.

X-ray diffraction measurements indicate that the tungsten layers are polycrystalline and alumina layers are amorphous. The polycrystalline grains are consistent with an α-W which has a body-centered cubic structure. The x-ray diffraction peaks are very broad as expected for extremely small polycrystalline grains. Depth-profiling secondary ion mass spectrometry (SIMS) measurements further confirm the chemical identity of these W and AI2O3 layers. In agreement with the TEM image, an extremely sharp interface is observed at each W/AI2O3 interface. Example 2

ALD methods as generally described in Example 1 are used to deposit 64 AI2O3/W bilayer pairs onto a sihcon substrate. The 64 pairs have a total film thickness of approximately 2000 Angstroms. The d-spacing for each bilayer is approximately 30 Angstroms; Each W and AI2O3 layer has a thickness of approximately 15 Angstroms.

X-ray reflectivity measurements are performed with samples that are overfilled by the x-ray beam at close to grazing incidence. Very pronounced Brag peaks, or so-called "satellite" peaks are observed from the AI2O3 W multilayer. These pronounced Bragg peaks or so-called "satellite" peaks are displayed in Figure 4. In Figure 4, the reflected X-ray intensity is plotted against incident angle for a 1.54 Angstrom CuKα X-ray beam. At θ=0, the sample is aligned parallel to the incident x- ray beam and the sample blocks one-half of the incident intensity. The x-ray intensity initially drops because of geometric factors related to the beam overfilling the sample. At Θ=0.1-0.3°, the x-ray intensity increases and becomes nearly equal to the intensity at Θ=0 because the sample is still intercepting only one-half of the beam at these very grazing angles. The intensity levels off and reaches a "plateau" at Θ=0.3-0.4°. In this region, the beam is undergoing total external reflection and is not penetrating into the sample. At the critical angle at Θ~0.4°, the x-rays begin to penetrate the sample and the x-ray intensity then decreases inversely with Θ⁴.

The Bragg peak or so-called "satellite" peak is observed at Θ~ 1.5°. The x-ray intensity for this peak is large and nearly approaches the intensity for total external reflection. Since the entire beam is reflected for total external reflection, this reflectivity is on the order, but somewhat less, than unity. This peak results from the constructive interference described by the Bragg equation, nλ = 2d sinΘ, where n=l, λ = 1.54 Angstrom, d~30 Angstroms and Θ ~1.5°. Because of the geometric factors, the x-ray reflectivity is estimated to be in the range of 70-80%. This high x- ray reflectivity indicates that the interfacial smoothness and conformahty of the W and AI2O3 nanolayers is superb.

The high reflectivity of 70-80% in the hard X-ray region at λ =1.54 Angstroms for the AI2O3/W multilayer is excellent and comparable with X-ray reflectivity from hard X-ray mirrors prepared using sputtering techniques, even though this structure has not been optimized for the highest X-ray reflectivity. It is expected that X-ray reflectivity for this system will increase by increasing the number of bilayer pairs, at least up to about 124 pairs. In addition, higher reflectivities are predicted at similar numbers of bilayer pairs if the W:Al2θ3 thickness ratio is changed from about 1:1 to about 1:2 to 1:3,

Claims

WHAT IS CLAIMED IS:

1. A method of making an X-ray focusing device having multiple bilayer pairs having a predetermined spacing and each bilayer pair including a reflecting layer and a spacing layer, comprising depositing alternating reflecting layers of a relatively high electron density metal and spacing layers of a relatively low electron density material onto a reflective surface of a substrate via an atomic layer deposition process.

2. The method of claim 1 wherein the relatively high electron density metal is tungsten.

3. The method of claim 2 wherein the relatively low electron density material is aluminum oxide or sihcon dioxide.

4. The method of claim 1 wherein the reflective surface is not amenable to the deposition of bilayer pairs using a line-of-sight method.

5. The method of claim 4 wherein the reflective surface is the interior surface of a tube.

6. A process of making an X-ray focusing device having multiple bilayer pairs having a predetermined spacing and each bilayer pair including a reflecting layer and a spacing layer, comprising depositing alternating reflecting layers of a relatively high electron density metal and spacing layers of a relatively low electron density material onto a reflective surface of a substrate via an atomic layer deposition process, such that the spacing of the bilayer pairs is graded.

7. The process of claim 6, wherein the bilayer pairs have a spacing that varies from location to location on at least a portion of the reflecting surface of the focusing device in relation to the angle of incidence of the X-rays at each such location, such that the bilayer pair spacing at each such location satisfies the Bragg equation nλ=2dsin θ for at least one X-ray having a wavelength λ, where θ is the angle of incidence of the X-rays at a point on the reflective surface of the focusing device, d is the bflayer spacing at such point, and n is an integer.

8. The process of claim 7, wherein the high electron density material is a metal.

9. The process of claim 8, wherein the metal is tungsten and the relatively low electron density material is aluminum oxide or silicon dioxide.

10. A method of making a tubular X-ray focusing device, comprising depositing multiple bilayer pairs onto an internal surface of a tubular substrate via an atomic layer deposition process, wherein the substrate has an eUiptically or parabohcaUy tapered longitudinal bore with a substantiaUy circular cross-section, the bore being shaped such that a beam entering an entrance end of the bore from a point source is reflected to form a focused or coUimated beam exiting an exit end of the bore.

11. The method of claim 10, wherein the bUayer pairs have a spacing that satisfies the Bragg equation, nλ=2dsin θ, for at least one X-ray having a wavelength λ, where θ is the angle of incidence of the X-rays at a point on the reflective surface of the focusing device, d is the bUayer spacing at such point, and n is an integer.

12. The method of claim 11, wherein the spacing of the bUayer pairs is graded.

13. The method of claim 12, wherein each bUayer pair consists of a layer of a relatively high electron density material and a layer of a relatively low electron density material.

14. The method of claim 13, wherein the relatively high electron density material is a metal.

15. The method of claim 14, wherein the metal is tungsten and the relatively low electron density material is either aluminum oxide or silicon dioxide.

16. The method of claim 10, wherein the atomic layer deposition process is conducted by introducing reactive materials sequentiaUy into the bore of the tubular or monocapUlary substrate through a smaUer tube that is inserted into the bore of the tubular or monocapUlary substrate.

17. The method of claim 12, wherein the atomic layer deposition process is conducted by introducing reactive materials sequentiaUy into the bore of the tubular or monocapUlary substrate through a smaUer tube that is inserted into the bore of the tubular or monocapUlary substrate.

18. The method of claim 17, wherein the graded multiple bUayer is fabricated by progressively inserting a smaUer tube progressively farther into the tubular or monocapUlary substrate as the reaction sequences are repeated and a positive flow of carrier gas is introduced through the tube in the same direction as the smaUer tube movement.

19. The method of claim 17, wherein the graded multiple bUayer is fabricated by progressively retracting a smaUer tube from the tubular or monocapUlary substrate as the reaction sequences are repeated and a positive flow of carrier gas is introduced through the tube in the opposite direction as the smaUer tube movement.

20. An X-ray focusing device, comprising a substrate tube having an eUipticaUy or parabohcaUy tapered longitudinal bore with a substantiaUy circular cross-section, the bore being shaped such that a beam entering an entrance end of the bore from a point source is reflected to form an eUipticaUy- or parabohcaUy-focused beam exiting an exit end of the bore, wherein the longitudinal bore has on its internal surfaces multiple bUayer pairs of ALD-applied materials.