Abstract:
A method to stabilize planar nanostructures, for example grating and zone plate lenses that are typically used for directing or focusing x-ray radiation, includes the deposition of a top, stabilizing layer. The structures are typically made on a flat substrate, and therefore are only fixed at the bottom. At high aspect ratio, the stability can be poor since small forces such as electrostatic forces and van de Waals forces that are often present can alter the structure. The top coating of a metallic material such as titanium constrains the nanostructures at the top and at the same time eliminates electrostatic forces and reduces any thermal gradient that may be present across the device.

Description:
BACKGROUND OF THE INVENTION 
     High-aspect ratio nanostructures with widths between 10 nanometers (nm) and several micrometers and a height-to-width ratio in the range of 5-100 are becoming more widely used in many fields such as micro-electronics, nanotechnology, and diffractive optics. Like tiny sky-scrapers (e.g. Sear&#39;s Tower has an aspect ratio of roughly 15), stabilization of these thin and tall structures is a major engineering hurdle. An important challenge in producing these nanostructures is developing methods to enhance their stability without compromising their performance. This can sometimes be done easily, particularly in systems with no moving parts, such as semiconductor integrated circuits, but is often difficult in most other applications. 
     Because of the wavelengths involved, diffractive x-ray optics presents some of the greatest challenges. Two examples are gratings and zone plate lenses. These optical elements include repeating structures that block or phase-shift x-ray radiation. As with visible light optics, x-ray gratings are typically used to deflect an x-ray beam and spectrally separate polychromatic beams. A zone plate can be thought of as a circular grating, but with decreasing grating period towards the rim according to the relation r n   2 =nl f Z +an 2 l 2 . Such a zone plate behaves like a lens with focal length f Z =2rdr/l, where dr is the width of the outer-most and also the finest zone, and l is the wavelength. The diffraction limited resolution, according the Rayleigh criterion is simply d=1.22 dr, slightly larger than the outer zone width and independent of the wavelength. 
     Because of the short wavelength of x-ray radiation, feature sizes of x-ray optics must be very fine in order to diffract the beam to a large enough angle, but on the other hand the large penetration depth of x-ray radiation requires thick optics. As a consequence of these two properties, x-ray gratings and zone plates typically require high aspect ratios and the ratios increase when the x-ray energy is increased as the wavelength shortens and the penetration depth increases. For example, zone plates for focusing “soft” x rays with energy between 250 electron-Volts (eV) and 1000 eV typically have finest zone widths of 15-50 nm and aspect ratios of 3-10. For “hard” x rays with energies between 5 keV and 10 keV, the finest zone widths are 30-100 nm and aspect ratios are 15-30. 
     SUMMARY OF THE INVENTION 
     Although there are typically no moving parts in x-ray optical elements, stabilizing the nano-structures is more difficult than in integrated circuits. For example, filling the spaces between the structures with organic material is not possible because the material would not be stable under prolonged x-ray exposure and filling the spaces with metallic material will affect the differential x-ray absorption properties and consequently reduce the performance. 
     The present invention concerns stabilizing planar structures, including nanostructures, such as x-ray grating-like diffractive optical elements including gratings and zone plate lenses. These structures are typically made on a flat substrate, and therefore are only fixed at the bottom. At high aspect ratios, the stability can be poor since even electrostatic and van de Waals forces can alter the structure. Embodiments of the present invention involve the use of a top coating, possibly including metallic materials such as titanium, aluminum, or molybdenum, to constrain the nanostructures at the top and at the same time eliminate electrostatic forces, and also reduce any thermal gradients that may be present across the device. 
     In general, according to one aspect, the invention features a diffractive x-ray optic. Examples of such an optic include gratings and zone plate lenses, and also hybrid devices that use a combination of diffraction and refraction. The optic comprises a substrate; a diffractive layer on the substrate having structures forming a diffractive optic. A top layer is further provided. It is disposed on the opposite side of the diffractive layer from the substrate. This top layer bridges between at least some of structures. 
     In specific embodiments, the structures form a periodic grating diffractive optic and/or a zone plate lens diffractive optic. Preferred materials for the diffractive layer include gold, tungsten, and/or silicon. 
     In the current embodiment, the top layer is used to mechanically stabilize the structures, even when they have aspect ratios in the range of 10-50, and higher. Preferred materials for the top layer include titanium, molybdenum, and/or aluminum. The top layer should be thick enough to provide mechanical stability but thin enough so as to not substantially undermine the diffractive performance; currently the top layer has a thickness of 50-500 nm. 
     In general, according to another aspect, the invention features method for fabricating a diffractive x-ray optic. This method comprises forming a diffractive layer on a substrate with structures of a diffractive optic and depositing a top layer on an opposite side of the diffractive layer from the substrate, the top layer bridging between at least some of structures. For example, in the case of the zone plate, only the outer zones with the highest aspect ratios will be bridged, in some embodiments. 
     In a current embodiment, the deposition step includes depositing at least part of the top layer at an angle of angle of 30-75 degrees to a plane the substrate. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIGS. 1A-1H  are schematic drawings illustrating an electroplating fabrication process for a typical x-ray grating or zone plate lens; 
         FIGS. 2A-2H  are schematic drawings illustrating of a lift-off fabrication process for a typical x-ray grating or zone plate lens; 
         FIG. 3  is an image of a zone plate lens with aspect ratio of 15 acquired with a scanning electron microscope; 
         FIGS. 4A and 4B  are schematic views showing a top coating process according to embodiments of the present invention; 
         FIG. 5  is a plot of the 1/e attenuation length and 2 p phase shift length for gold and titanium; and 
         FIGS. 6A and 6B  are images of a zone plate during and after, respectively, the top coating process according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Gratings and zone plates are diffractive elements that manipulate light by the principle of diffraction. With x-ray radiation in the range of 100 eV and 10 keV, most materials provide little refractive effect but become very absorptive. As a consequence, diffractive optical elements have become the most effective means of changing the direction of and focusing x-ray beams. These diffractive optical elements are typically fabricated lithographically, with processes similar to that used in the fabrication of microelectromechanical systems (MEMS) and semiconductor integrated circuits. Two types of processes are often used today: 1. electroplating process shown in  FIGS. 1A-1H ; and a lift-off method shown in  FIGS. 2A-2H . 
       FIGS. 1A-1H  show an exemplary electroplating process for diffractive x-ray optics fabrication. 
     A substrate  100  is first coated with a conducting material that functions as an electroplate base  110 , as illustrated in  FIGS. 1A and 1B . Typically the conducting electroplate base  110  is gold or titanium (Ti), being several nanometers in thickness. 
     A first layer of photo-resist  112  is deposited on the electroplate base  110 . The thickness of the first photo-resist  112  should equal the thickness of the desired nanostructures and is usually controlled with a spinning technique by adjusting the photo-resist viscosity and spinning speed. 
     Then a metal barrier layer  114 , such as titanium, of several nanometers thickness is deposited on the first photoresist  112  followed by a thin layer of a binary photo-resist  116 , as shown in  FIG. 2B . 
     The desired pattern is then generated by selectively exposing the top layer binary photo-resist  116  with patterned radiation or energetic particles  153 , as shown in  FIG. 1C . In some examples, UV, EUV, or x-ray radiation is used in combination with a mask  152 . In other examples, an electron beam writer is used to directly expose the desired radiation pattern into the binary resist layer  116 . 
     After the exposure process, a wet chemical etch process is used to remove the irradiated regions and thereby create a patterned binary photo-resist  116 ′. This selectively exposes the titanium barrier layer  114 , as shown in  FIG. 1D . A further wet chemical etching process  154  removes the exposed titanium regions to form a patterned titanium layer  114 ′, as shown in  FIG. 1E . 
     Then using the patterned titanium layer  114 ′ as a mask, a directional plasma etching process  156  is used to etch deep through the main photo-resist layer to produce a “negative” of the desired diffractive optic pattern such as a zone plate pattern in a patterned first photoresist layer  112 ′. This step will typically remove the top binary photo-resist  116 ′ and titanium layer  114 ′, as shown in  FIG. 1F . 
     The assembly including the patterned first photoresist layer  112 ′ is then placed in an electroplating bath and the metallic zones  118  are formed through the open areas defined by the “negatives” of the patterned photoresist  112 ′, as shown in  FIG. 1G . A common material for the zone plate layer is gold. In other examples, the diffractive or zone plate layer is tungsten or silver or copper. Still other metals are cobalt, copper, platinum, or lead. Finally, the remaining portions of the first photoresist  112 ′ are removed by a further dry etching process, leaving on the desired structures, such as a zone pattern,  118 , as shown in  FIG. 1H . 
       FIGS. 2A-2H  show an exemplary lift-off process for diffractive x-ray optics fabrication, which is used according to another fabrication process. 
     A substrate  100  is first coated with first layer of photo-resist  112  as shown in  FIGS. 2A and 2B . The thickness of the first photo-resist  112  should equal the thickness of the desired nanostructures and is usually controlled with a spinning technique by adjusting the photo-resist viscosity and spinning speed. 
     Then the metal barrier layer  114 , such as titanium, of several nanometers thickness is deposited on the first photoresist  112  followed by a thin layer of a binary photo-resist  116  as shown in  FIG. 2B . 
     The desired pattern is then generated by selectively exposing the top layer binary photoresist  116  with patterned radiation or energetic particles  153  in step  2 C. In some examples, UV, EUV, or x-ray radiation is used in combination with the mask  152 . In other examples, an electron beam writer is used to directly write the desired pattern into the binary resist  116 . 
     After the exposure process, a wet chemical etch process is used to remove the irradiated regions to produce a patterned binary photo-resist  116 ′ and therefore selectively expose the titanium barrier layer  114 , as shown in  FIG. 2D . A further wet chemical etching process  154  removes the exposed titanium regions to form a patterned titanium layer  114 ′ as shown in  FIG. 2E . 
     Then using the patterned titanium layer  114 ′ as a mask, a directional plasma etching process  156  is used to etch deep through the main photoresist layer  112  to produce a “negative” of the desired diffractive optic pattern such as a zone plate pattern in the patterned first photoresist layer  112 ′. This step will typically remove the top binary photo-resist  116 ′ and titanium layers  114 ′, as shown in  FIG. 2F . 
     The assembly including the patterned first photoresist layer  112 ′ is then exposed to a directional deposition process  158  such as evaporation, as shown in  FIG. 2G  or a non-direction deposition process to deposit the diffractive layer. This deposition forms the diffractive optic, such as metallic zones  118 , through the open areas defined by the “negatives” of the patterned photoresist  112 ′. A common material for the zone plate layer is gold, silicon or tungsten. Still other metals are cobalt, copper, platinum, or lead. Finally, the remaining portions of the first photoresist  112 ′ are removed by a further dry etching process or other lift-off process  159 , leaving on the desired structures or zone pattern  118 , as shown in  FIG. 1H . 
     Note that the zone structures  118  of the diffractive layer of the zone plate  200 , which are formed with these methods shown in  FIGS. 1A-1H  and  2 A- 2 H, are connected only at the bottom  170  to the substrate  100 , and possibly via the electroplate base  110 , other intervening layer, or directly to the substrate  100 , and can thus be unstable at high aspect ratios. For example, zone plates can suffer from deterioration of performance when used in intense x-ray beams for an extended period of time, as shown in  FIG. 3 . The cause of the lose of performance, mainly in term of reduction of x-ray diffraction efficiency, is due to the outer most zones shifting, tilting and falling over to each other. The tilting may also be caused by stress in the small amount of remaining polymer that was left over from zone plate fabrication process, or may be due to the stress at interface between zones and base at or near the interface at the structure bottoms  170 . 
     In a current embodiment, this tilting of the zones is prevented by depositing a continuous or non-continuous layer of material, usually a metal and preferably a low density metal. The material layer is deposed on the top of the structure or zones  118 , on an opposite side of the structures  118  from the substrate  100 . The deposited top layer anchors top of the structures or zones together, bridging between the structures, while foot  170  of the zones  118  are attached to the plating base  110  or substrate. An additional benefit of this hardening process is that since zone plate is covered by a layer of thin film, it is less susceptible to mechanical shock, vibration and moisture, and it can even be brushed and cleaned. 
     The thin film material of the top coating is preferably a low density and low Z number material, so that x-rays easily pass through this material and the amount of phase shift it produces is significantly less than what the zone plate material  118  itself produces. Ideally, the material should also be a metal material that can be easily deposited by evaporation. 
       FIGS. 4A and 4B  show the deposition process used in one embodiment. The thin film material used is low-Z and strong metal, e.g. titanium, molybdenum, and/or aluminum. In other examples, chromium or nickel is used. 
     In an exemplary process, the zone plate  200  was first coated with a thin layer  406  of titanium, molybdenum, and/or aluminum by electron-beam evaporation  410  with the substrate  100  positioned at an oblique angle α to the path of the directional deposition process. In one example, the first thin top layer  406  is coated to a thickness of 10 to 200 nm. In a current embodiment, the thickness is about 40 to 120 nanometers, or about 80 nm thick. In other examples, the metal coating is chromium or nickel. 
     In some examples, this initial deposition is enough for the top layer to begin the process of bridging between at least some of structures, see references  420 ,  422 , but possibly not between all of the structures, see reference  416 . The angle α is between 30-75 degree off normal. Preferably the substrate  100  is also rotated  412  by a rotation stage at 50-200 revolutions per minute (RPM) during the evaporation process. Because of shadowing effect from neighboring zones, angle deposition only covers the top edge of the zones rather than filling all of the gaps completely between zones  416 , in some examples. This incomplete filling is shown in  FIG. 6A . 
     Then a second deposition  414  of 20 to 300 nm thick titanium, molybdenum, chromium, nickel, and/or aluminum, preferably 50 to 200 nm, or about 100 nm thick titanium is deposited at normal angle to produce the final top cap layer  408 , which is needed to cap the zone plate, as shown in  FIG. 4B , and fill the gaps  416  joining the zones together. The completed top cap layer  408  is shown in  FIG. 6B . 
     It should be noted, however, that the top cap layer  408  does not need to bridge every gap between the structures  118 . Only the structures with the highest aspect ratios are in the greatest danger of damage and toppling over. Thus in some embodiments, the top layer  408  is thin to only bridge between the smaller structures, such as the outer zone structures of a zone plate lens. 
     Since zone plates have many different sizes for the out most zones, the gap between zones varies with zone plate specs. Therefore, the thickness of the top cap layer  408  layer should be scaled according to the outer most zone width of the zone plate. 
       FIG. 5  shows that 1/e attenuation length and 2 p phase shift length for two typical zone plate material, gold, and top coating material titanium. This shows that titanium minimizes absorption in the structural top coating layer 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.