Patent Application
20130164457
The present invention relates to the manufacture of patterned optical elements for use in the optical frequency range of x-rays.
Patterned optical elements for x-ray wavelengths, including Fresnel lenses, zone plates, gratings and resolution charts, differ from typical optical gratings for ultraviolet (UV), visible (VIS), and infrared (IR) wavelength ranges. Processes for producing optical gratings in these longer wavelength ranges cannot be used for and transferred to the production of the patterned optics for the x-ray wavelength range because of differences in the working principles of the processes, in the materials of the optical elements, in the critical dimensions and geometries, and in other aspects. A patterned optic for x-rays changes an x-ray wavefront either by modifying the amplitude or phase or both. The patterned optical element does so through spatial modulation of the electron density of the structure. It is often made of a pattern of varying transmission thickness, or a pattern of different materials, or a combination of both.
One of the simplest patterned optics is a transmission grating. One type of x-ray transmission gratings has a structure of stripes of alternative materials with different electron densities and hence different absorption coefficients and different optical indexes. The intensity and the phase of transmission x-rays are therefore modulated by this structure.
An x-ray transmission grating can be made of one material as well. Instead of alternative materials which contribute to the modulation of the intensity and phase, the grating may have an alternating thickness of the material so that the intensity and the phase are modulated through the transmission.
There are two critical geometrical parameters to describe a transmission grating: the period of the grating and the aspect ratio, which is defined as the ratio between the thickness of the structure and the period. High resolution gratings typically have a period from sub-micrometers to micrometers.
The aspect ratio, i.e. the ratio between the characteristic period and the thickness of the x-ray transmission path is a universal parameter for patterned x-ray optics. A Fresnel lens is a zone plate with concentric rings of different optical paths. The transmitted x-rays constructively interfere with each other at the focal point. The typical dimension of the “ring width” ranges from tens of micrometers to a few tens of nanometers in the x-ray region with energy of a few keV to a few 10 keV. The resolution of a Fresnel lens is determined by the outmost ring, i.e. the ring with the narrowest ring width, by 1.22·ΔRn, where ΔRn is the width of the outmost ring.
Another example of patterned x-ray optical elements is a resolution chart. A resolution chart is a pattern with variable density. The pattern may include numbers and letters of different sizes, lines of different widths and at different distances, and other different geometric patterns. When positioned in the path of an x-ray beam, the shadow image, or absorption contrast image, shows the imaging resolution of the system. Resolution charts are widely used for characterizing the resolution of x-ray detectors and x-ray imaging systems.
Electron-beam lithography (e-beam lithography) has been used to fabricate these x-ray optics, in which a periodic pattern is engraved by a focused e-beam on a thin film of absorbing material. However, for high-resolution optics, Fresnel lenses and gratings, fabricated for relatively high energy, such as 8 keV and above, the required aspect ratio is too large for e-beam lithography.
In overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides an improved method of fabricating pattered x-ray optical elements.
This method addresses issues associated with the fabrication of an optical element for producing intensity and phase modulation to an x-ray wave front. Such optical elements usually have patterned density modulation structure. The method includes utilizing a pulsed laser beam to engrave a pattern on a base plate of material which is generally transparent or less absorbing to x-rays (low-density), and then filling the grooves of the pattern with material which is less transparent to x-rays (high-density). The density modulation using a pattern of grooves filled with high-density material in the less absorbing base plate forms the basic structure of various optical elements. The shape of the pattern depends on the final application. The grooves may be, for example, parallel straight lines or concentric circles or take any other periodical pattern. These optical elements may include x-ray resolution charts for system characterization, zone plates for x-ray microscopy, and x-ray transmission gratings suitable for x-ray interferometry and for phase-enhanced x-ray imaging.
The above described method applies to the phase modulation as well. The difference of the optical indexes of the materials will modify the phase of the wavefront.
In particular, the method involves using a focused femtosecond laser beam to engrave a patterned structure on a substrate of material relatively transparent to the fundamental wavelength of the laser. The fundamental wavelength is the main wavelength of the laser that may also be accompanied by harmonics of shorter wavelengths. Generally, in the following, the term “wavelength” refers to the fundamental wavelength of the laser, unless otherwise noted.
Further, the method according to the invention involves several ways of filling the engraved microscopical structure with a different material. The density contrast between the base material and the filler material forms a density modulated pattern. The contrast of optical index between the base material and the filler material allows phase modulation to an x-ray wavefront.
Further features and advantages will become readily apparent from the following description and from the claims.
The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the views. In the drawings:
Referring now to
The laser beam source 12 is turned on with the focusing arrangement 14 having a distance from the substrate 18 that is substantially equal to the focal length FL. Accordingly, the laser beam 16 starts the ablation process at a proximate surface of the substrate 18, also called the first surface. Subsequently, the focusing arrangement 14 is moved closer to the substrate 18 in a controlled manner to ablate material at greater depths until the desired depth of grooves 20 is reached. The material of substrate 18 may be partially transparent to the laser beam wavelength. It must, however absorb the laser beam wavelength to a degree that results in a localized ablation of the substrate material in the area of the beam waist 26.
In one form, the laser beam 16 is an ultra-short pulse laser beam that creates the required pattern of grooves 20 in the substrate 18 contained in the patterned optics. A typical laser for this process has a pulse length of 100 femtoseconds and consists of a regenerative amplifier with a laser center wavelength of approximately 800 nm. The beam is transversally monomode and has a beam propagation parameter of M2 of ˜1. The pulse energy is typically in the range of several 10 nJ to several 100 nJ or higher in the Micro-Joule range. Due to the short pulse length, there is no significant heat transfer to the residual bulk material of substrate 18 so that a sharp boundary between removed material and still intact material is attainable. Where the laser pulses hit the material of substrate 18, the laser beam energy is absorbed by the bulk material. In locations where the flux density of the laser beam 16 is sufficient to cause a material break-down, the bulk material is ablated and leaves a pattern of grooves 20 with clean and precise edges. The laser beam 16 can engrave structures with high aspect ratios and grooves 20 having a width that may be smaller than the diffraction limit of the wavelength of the laser beam source 12 as described in more detail in connection with
In various implementations, the ultra-short pulsed laser beam 16 can be used in combination with a stage or handling platform 15. The laser beam 16 can be scanned relative to the handling platform 15 to ablate material in the pattern of the grooves 20.
As discussed below in connection with
Under normal operation conditions, the smallest achievable structure width of the patterned optic to be produced is given by the diffraction limit of the laser at the given laser wavelength and single transversal mode operation. Normal operating conditions exist where the flux density of the laser beam 16 anywhere across its defined diameter specifications on the substrate 18 interface exceeds the break-down threshold specific to the material of substrate 18. Material removal occurs across that diameter. Due to the short pulse length, there is no significant heat transfer to the residual bulk material of substrate 18 outside the diameter of laser beam 16 so that there is virtually no heat-affected zone and the boundary between removed material and intact material remains very well defined.
If a substrate 18A is sufficiently transparent to the laser beam wavelength, a configuration as shown in
As illustrated in
For achieving a pattern of high feature density and high aspect ratio, the laser scan, or the ablation of the material, has to be three-dimensional. One approach is scan the laser beam 16 in two dimensions to achieve the pattern with the depth of the structure determined by the laser volume above the break-down threshold. Then the laser beam 16 is repositioned perpendicular to the surface of the substrate 18, and the two-dimensional scan is repeated. Multiple iterations may be needed to achieve the desired aspect ratio.
However, one could devise a different beam shape with a characteristic, engineered flux distribution. The respective sub-area 27 of the beam 16 with a flux density exceeding the break-down threshold 28 of the flux density causes the material to be ablated. Preferably, the laser focus position is chosen to create material break-down in the vicinity of a substrate surface to enable a controlled expansion of the removal material which creates a high local pressure. This may be at the first surface of substrate 18 in
Additional techniques such as super-resolving apertures can be used in the optical setup to reduce the center area of the beam.
Additionally, the bulk structure of substrate 18A may be immersed in liquid 29 to control the process better. A typical liquid is water, water with a surfactant to increase wetting, alcohol, or another solvent with good wetting properties to penetrate into the small ablated features and others. The liquid 29 damps an expansion of the removed material and thus enhances the controllability of the process. The liquid also works in conjunction with an immersion objective used as the focusing arrangement 14.
The finished machined patterned substrate 18 of
After the pattered structure in substrate 18 or 18A is formed, the next step involves filling the grooves 20 of the patterned structure with a filling material 24, typically consisting of a heavy element or a mix of heavy elements. The term “heavy element” in this context designates an element with a high electron density, for instance a metal. The choice of one or more elements depends on the desired x-ray absorption, phase change, and the physical properties of the materials. Some examples include metals, preferably, with a high atomic z-number and with low surface tension and a low melting point such as tin and low melting metal alloys such as Field's metal (32.5% Bismuth, 16.5% Tin, and 51.0% Indium) with a very low melting point of 149° F. or an alloy of 5 parts Bismuth, 3 parts Tin with a melting point of 202° F. The physical properties determine the process of filling the grooves 20. Because the characteristic width of the patterned structure of substrate 18 (or 18A) is very small, it is difficult to achieve a wetting of the grating surface by a liquid filling material and to make the filling material penetrate the grooves 20.
a through 5d illustrate the further process of manufacturing an x-ray grating with spatial density modulation by filling the grooves 20 with a liquid or deformable filling material 24. The process starts according to
For this approach, the elements for filling material 24 with low melting point and low viscosity and low surface tension are preferred. Different elements may be mixed to provide a mixture having low melting temperature or low viscosity or low surface tension, or any combination of these properties to facilitate injecting the mixture into the voids of grooves 20 of the patterned structure in substrate 18.
In the final steps, the residual filling material 24 is removed from the top surface of the substrate 18 or 18A as shown in
Other methods are conceivable to fill in the voids 20 of the patterned structure. One example is filling in the voids with nanoparticles of high electron density material, and then fixed the structure by melting the filler material 24 or by a top coat. It is, for example possible to fill the voids of the patterned structure of substrate 18 with high-density nanomaterials. Some heavy materials in the form of nanoparticles have been developed with a typical dimension of less than 100 nm. These materials might be suitable for filling in the voids of the patterned structure. Heat melting the filler material or a coating securing the nanoparticles in the grooves 20 can be applied to make the filled structure permanent.
The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
