Patent Publication Number: US-8976933-B2

Title: Method for spatially modulating X-ray pulses using MEMS-based X-ray optics

Description:
The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the temporal modulation of X-rays, and more particularly, relates to a method and apparatus for spatially modulating X-rays or X-ray pulses using MicroElectroMechanical or microelectromechanical systems (MEMS) based X-ray optics including oscillating MEMS micromirrors. 
     DESCRIPTION OF THE RELATED ART 
     MEMS refer to very small mechanical devices driven by electricity. For example, MEMS are made up of components between 1 and 100 micrometers in size or between 0.001 mm and 0.1 mm, and MEMS devices typically range in size from 20 micrometers to a millimeter. 
     A need exists for an X-ray modulating optics mechanism for spatially modulating X-rays pulses, for example with X-ray pulses of microsecond (μs) to picosecond (ps) duration. It is desirable to provide such an X-ray modulating optics mechanism that enables modulation of X-ray pulses with a high-degree of controllability. 
     SUMMARY OF THE INVENTION 
     Principal aspects of the present invention are to provide a method and apparatus for spatially modulating X-rays or X-ray pulses using MEMS based X-ray optics. Other important aspects of the present invention are to provide such method and apparatus substantially without negative effect and that overcome some of the disadvantages of prior art arrangements. 
     In brief, a method and apparatus are provided for spatially modulating X-rays or X-ray pulses using microelectromechanical systems (MEMS) based X-ray optics. A micromirror including a torsionally-oscillating MEMS micromirror and a method of leveraging the grazing angle and reflection property of the MEMS micromirror are provided to modulate X-ray pulses with a high-degree of controllability. 
     In accordance with features of the invention, a combination of grazing angle reflection and controllable mirror-oscillation provides a method for modulating the incident X-ray beam. This modulation includes, for example, isolating a particular pulse, spatially separating individual pulses, and spreading a single pulse from an X-ray pulse-train. 
     In accordance with features of the invention, an incident X-ray beam is provided on the MEMS micromirror surface at a set angle of incidence or grazing angle, θ. The set grazing angle, θ of the incident X-ray beam is provided at a selected angle less than a critical angle, θ c , for a given X-ray wavelength and MEMS micromirror material, the incident X-ray beam is reflected off the micromirror surface with close to 100% optical efficiency. 
     In accordance with features of the invention, a MEMS micromirror includes a torsional minor. The MEMS micromirror is fabricated on a single-crystal-silicon (SCS) device layer of a Silicon-On-Insulator (SOI) wafer, using conventional semiconductor fabrication technique. 
     In accordance with features of the invention, a MEMS micromirror includes a set mirror frequency or minor oscillation frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein: 
         FIGS. 1A and 1B  schematically illustrate example MEMS X-ray optics apparatus for implementing spatially modulating X-rays or X-ray pulses respectively with example temporally dispersed X-ray pulses and with short pulse dispersion in accordance with preferred embodiments; 
         FIGS. 1C and 1D  are respective example timing sequences or waveforms illustrating the pulse train dispersion operation with example temporally dispersed X-ray pulses of the apparatus of  FIG. 1A , and short pulse dispersion operation with example temporally dispersed X-ray pulses of the apparatus of  FIG. 1B  in accordance with preferred embodiments; 
         FIGS. 2A and 2B  schematically illustrate example MEMS X-ray optics apparatus for implementing spatially modulating X-rays respectively with example incidence angles less than and greater than a critical angle in accordance with a preferred embodiment; 
         FIGS. 3 and 4  schematically illustrate a respective example MEMS micromirror of the example MEMS X-ray optics apparatus of  FIGS. 1A and 1B  and  FIGS. 2A and 2B  in accordance with preferred embodiments; 
         FIGS. 5A ,  5 B, and  5 C illustrate respective SEM micrograph of example MEMS micromirrors, and  FIG. 5D  illustrates a SEM micrograph of example MEMS comb-drive micromirrors of the example MEMS X-ray optics apparatus of  FIGS. 1A and 1B  and  FIGS. 2A and 2B  in accordance with preferred embodiments; 
         FIG. 6  illustrates change in amplitude of minor-oscillation with change in driving frequency with half-angle rotation in degrees shown relative the vertical axis and mechanical oscillation frequency shown relative the horizontal axis in accordance with preferred embodiments; 
         FIGS. 7A and 7B  respectively illustrate reflected beam and incident beam examples with incident angle shown relative the horizontal axis and reflection in degrees shown relative the vertical axis in  FIG. 7A , and reflectivity, R shown relative the vertical axis in  FIG. 7B  in accordance with preferred embodiments; 
         FIG. 8  illustrates derivative of the measured reflectivity curve of  FIG. 7B  with incident angle shown relative the horizontal axis and derivative of reflectivity shown relative the vertical axis in accordance with preferred embodiments; 
         FIG. 9  illustrates example mirror operation with time in microseconds (μs) shown relative the horizontal axis and integrated X-ray pulses shown relative the vertical axis in accordance with preferred embodiments; and 
         FIG. 10  illustrates example 75.624 KHz minor operation with time in microseconds (μs) shown relative the horizontal axis and intensity (V) shown relative the vertical axis in accordance with preferred embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of embodiments of the invention, reference is made to the accompanying drawings, which illustrate example embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     In accordance with features of the invention, a method and apparatus are provided for implementing spatially modulating X-rays. MEMS X-ray optics apparatus module X-rays by deflecting or dispersing incident X-ray beams using oscillating MEMS micromirrors. The novel MEMS X-ray optics apparatus of the invention delivers X-ray pulses with a picosecond (ps) temporal resolution with broad energy tunability, and a high pulse repetition-rate with high flux per pulse. 
     Having reference now to the drawings, in  FIGS. 1A-1D , there is shown an example MEMS X-ray optics apparatus for implementing spatially modulating X-rays or X-ray pulse generally designated by the reference character  100  in accordance with the preferred embodiment. 
     MEMS X-ray optics apparatus  100  includes a MEMS micromirror generally designated by the reference character  102  shown supported by an electrode  104  and an area detector  106 . X-rays reflect off micromirror  102  at incidence angles, θ&lt;θc, critical angle as shown in  FIGS. 1A-1D . MEMS X-ray optics apparatus module X-rays by deflecting or dispersing incident X-ray beams using oscillating MEMS micromirrors. 
     In accordance with features of the invention, as shown in  FIGS. 1A-1B , an incident X-ray beam of an incident X-ray beam from a synchrotron source, such as the Advanced Photon Source (APS) at Argonne National Laboratory, is placed on the MEMS micromirror  102  at a very low, grazing angle, θ. When the grazing angle, θ is less than the critical angle, θ c , for a given X-ray wavelength and MEMS-mirror-material, the incident X-ray beam is reflected off the micromirror surface with close to 100% optical efficiency. However, at angles greater than the critical angle the optical efficiency drops sharply. 
     In  FIG. 1A , temporally dispersed X-ray pulses  110  are placed on surface of the MEMS micromirror  102  at the low grazing angle, θ are spatially dispersed at positions  112  onto the area detector  106 . Referring to  FIG. 1C , the temporally dispersed X-ray pulses dispersion is illustrated including respective waveforms labeled CANTILEVER DEFLECTION  114 , MEMS REFLECTIVITY  116 , HYBRID BUNCH TRAINS  118 , and POSITIONS  112  at detector  106 . 
     In  FIG. 1B , a short X-ray pulse  120 , is placed on surface of the MEMS micromirror  102  at the low grazing angle, θ is spatially dispersed at position  122  onto the area detector  106 . Referring to  FIG. 1D , the short X-ray pulse dispersion is illustrated including respective waveforms labeled CANTILEVER DEFLECTION  124 , MEMS REFLECTIVITY  126 , SINGLE  100 ps PULSE  128 , and position  122  at detector  106 . 
     Referring to  FIGS. 2A and 2B  there is shown an example MEMS X-ray optics apparatus for implementing spatially modulating X-rays designated by the reference character  200  respectively with and greater than the critical angle θ&gt;θc in accordance with the preferred embodiment. 
     In  FIG. 2A , example incoming X-rays  210  with incidence angles less than the critical angle θ&lt;θc are reflected off the micromirror  102  providing reflected X-rays  212  to a sample  214 . As illustrated in  FIG. 2B , example incoming X-rays  220  are transmitted through the micromirror  102  with incidence angles greater than the critical angle θ&gt;θc providing transmitted X-rays  222  spaced from the sample  214 . 
     In accordance with features of the invention, the micromirror  102  is implemented by a torsionally-oscillating micro-electro-mechanical system (MEMS) micromirror together with a method of leveraging the grazing-angle reflection property, to modulate X-ray pulses with a high-degree of controllability. 
     Referring to  FIGS. 3 and 4  there are shown a respective example MEMS micromirror generally designated by the respective reference character  300  and reference character  400  in accordance with preferred embodiments. MEMS micromirrors  300  and  400  include a respective micromirror  302 ,  402  provided together with a respective pair of torsional hinge  304 ,  404  and a respective pair of comb-drive actuator  306 ,  406  disposed on opposed sides of the respective micromirror  302 ,  402 . Oscillation of the micromirrors  300  and  400  is provided by the respective in-plane comb-drive actuator  306 ,  406 . 
     In accordance with features of the invention, the MEMS micromirrors  102 ,  300  and  400  are fabricated, for example, on the single-crystal-silicon (SCS) device-layer of a Silicon-On-Insulator (SOI) wafer, using standard semiconductor fabrication processes. 
     Referring also to  FIGS. 5A ,  5 B, and  5 C a respective SEM micrograph of example MEMS micromirrors are shown, and  FIG. 5D  illustrates a SEM micrograph of example MEMS comb-drive actuator for the micromirrors of the example MEMS X-ray optics apparatus  100  and  200  in accordance with preferred embodiments. 
     In  FIG. 5A , an example MEMS micromirror generally designated by the respective reference character  502 A is shown. The MEMS micromirror  502 A has a generally rectangular shape. 
     In  FIG. 5B , an example MEMS micromirror generally designated by the respective reference character  502 B is shown. The MEMS micromirror  502 B has an improved rectangular shape with rounded corners. 
     In  FIG. 5C , an example MEMS micromirror generally designated by the respective reference character  502 C is shown. The MEMS micromirror  502 C has an improved generally oblong shape with rounded corners. 
     The MEMS micromirrors  502 B,  502 C have improved or optimized torsional springs and anchors. The MEMS micromirrors  502 A,  502 B have resonant frequencies, for example, of 2 KHz to 16.5 KHz, and have been X-ray tested. The MEMS micromirror  502 C has resonant frequencies, for example, of approximately 75 KHz. 
     In  FIG. 5D , an example MEMS comb-drive actuator generally designated by the respective reference character  506  is shown for the micromirrors  502 A,  502 B,  502 C 
     In accordance with features of the invention, the MEMS micromirrors  300  and  400  are controllably oscillated, about the respective two torsional-beams  304 ,  404 , at varying amplitudes and frequencies, using the respective integrated comb-drive actuators  306 ,  406 . 
       FIG. 6  illustrates an example frequency response of X-ray MEMS micromirrors with the change in amplitude of minor-oscillation and in driving frequency, with half-angle rotation in degrees shown relative the vertical axis and mechanical oscillation frequency shown relative the horizontal axis. 
     In accordance with features of the invention, the combination of grazing angle reflection and controllable mirror-oscillation results in a method for modulating the incident X-ray beam. This modulation includes, but is not limited to, isolating a particular pulse, spatially separating individual pulses, and spreading a single pulse from an X-ray pulse-train. 
       FIGS. 7A and 7B  respectively illustrate reflected beam and incident beam examples with incident angle shown relative the horizontal axis and reflection in degrees shown relative the vertical axis in  FIG. 7A , and reflectivity, R shown relative the vertical axis in  FIG. 7B  in accordance with preferred embodiments. In  FIG. 7B  measured data values are shown relative to calculation values. 
       FIG. 8  illustrates derivative of the measured reflectivity curve of  FIG. 7B  with incident angle shown relative the horizontal axis and derivative of reflectivity shown relative the vertical axis in accordance with preferred embodiments. The derivative of measured reflectivity curve shows, for example, the mirror curvature of less than 0.02°. 
       FIG. 9  illustrates example mirror operation with time in microseconds (μs) shown relative the horizontal axis and integrated X-ray pulses shown relative the vertical axis in accordance with preferred embodiments. With a first minor frequency, such as 75.624 KHz and the first incident X-ray angle or grazing angle, θ, of 0.053°; and a second minor frequency, such as 75.635 KHz and the second incident X-ray angle or grazing angle, θ, of 0.054° the pulse intensity and pulse duration is changed. 
       FIG. 10  illustrates example 75.624 KHz minor operation with time in microseconds (μs) shown relative the horizontal axis and intensity (V) shown relative the vertical axis in accordance with preferred embodiments. With a fixed minor frequency, such as 75.624 KHz, varying the incident X-ray angle or grazing angle, θ, for examples between 0.197°; 0.1495° and 0.1°, the pulse intensity and pulse duration is changed. 
     While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.