Abstract:
One embodiment disclosed relates a method of detecting a patterned electron beam. The patterned electron beam is focused onto a grating with a pattern that has a same pitch as the patterned electron beam. Electrons of the patterned electron beam that pass through the grating un-scattered are detected. Another embodiment relates to focusing the patterned electron beam onto a grating with a pattern that has a second pitch that is different than a first pitch of the patterned electron beam. Electrons of the patterned electron beam that pass through the grating form a Moiré pattern that is detected using a position-sensitive detector. Other embodiments, aspects and features are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims the benefit of U.S. provisional patent application No. 61/479,023, filed Apr. 26, 2011, by inventors Shinichi Kojima et al., the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to electron beam detection apparatus and methods of using same. 
         [0004]    2. Description of the Background Art 
         [0005]    Electron beam instruments include tools used in automated inspection and review of manufactured substrates, electron beam lithography systems, and other instruments that use electron beam technology. Electron beam instruments often utilize an apparatus to detect the position of an electron beam and also to measure various characteristics of the electron beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a cross-sectional diagram of an apparatus to detect the position of a patterned electron beam in accordance with a first embodiment of the invention. 
           [0007]      FIG. 2  is a cross-sectional diagram of an apparatus to detect the position of a patterned electron beam in accordance with a second embodiment of the invention. 
           [0008]      FIG. 3  is a cross-sectional diagram of an apparatus to detect the position of a patterned electron beam in accordance with a third embodiment of the invention. 
           [0009]      FIG. 4  is a cross-sectional diagram of an apparatus to detect the position of a patterned electron beam in accordance with a fourth embodiment of the invention. 
           [0010]      FIG. 5  depicts a first example of a Moiré pattern which may be detected by a position-sensitive detector in accordance with the fourth embodiment of the invention. 
           [0011]      FIG. 6  depicts a second example of a Moiré pattern which may be detected by a position-sensitive detector in accordance with the fourth embodiment of the invention. 
       
    
    
     SUMMARY 
       [0012]    One embodiment disclosed relates a method of detecting a patterned electron beam. The patterned electron beam is focused onto a grating with a pattern that has a same pitch as the patterned electron beam. Electrons of the patterned electron beam that pass through the grating un-scattered are detected. 
         [0013]    In another embodiment, the patterned electron beam is focused onto a grating with a pattern that has a second pitch that is different than a first pitch of the patterned electron beam. Electrons of the patterned electron beam that pass through the grating form a Moiré pattern that is detected using a position-sensitive detector. 
         [0014]    Other embodiments, aspects and features are also disclosed. 
       DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a cross-sectional diagram of an apparatus  100  to detect the position of a patterned electron beam  101  in accordance with a first embodiment of the invention. The patterned electron beam  101  may be, for example, an electron beam patterned into lines or dots. 
         [0016]    In relation to the example shown in  FIG. 1 , consider that the patterned electron beam  101  is patterned into lines that are normal to the plane of the page of  FIG. 1  (i.e. the lines of the patterned electron beam  101  lie along the y-direction). In other words, the patterned electron beam  101  forms a striped pattern with lines lying along the y-direction (and spaced apart in the x-direction) when it impinges upon a flat surface in the x-y plane. The lines, and the spacing between them, for the patterned electron beam  101  may be one micron in width, for example. Other line and spacing widths may be used depending on the application. 
         [0017]    As shown in  FIG. 1 , the apparatus  100  may be arranged such that the patterned electron beam  101  impinges upon a metal stencil  102  which is supported by a thin membrane  103 . The membrane  103  may be made of silicon (Si), or other low Z material, and may be of a thickness of two microns, for example, so as to provide structural support for the stencil  102  while being largely transparent or semi-transparent to the electron beam  101 . 
         [0018]    The stencil  102  may be composed of a high Z material, such as molybdenum or platinum, for example, and may be a couple of microns thick, for instance. The thickness of the stencil  102  may depend, for example, on the energy of the electron beam and should be of a thickness to provide sufficient scattering of the beam. 
         [0019]    In accordance with an embodiment of the invention, the stencil  102  may be patterned into lines that are normal to the plane of the page of  FIG. 1  (i.e. the lines of the stencil  102  lie along the y-direction). In other words, similar to the patterned electron beam  101 , the stencil  102  may have a striped pattern with lines lying along the y-direction (and spaced apart in the x-direction). The lines and the spacing between them for the stencil  102  may be one micron in width, for example. Other line and spacing widths may be used depending, for example, on the line and spacing widths of the patterned electron beam  101 . 
         [0020]    In one implementation, the striped pattern of the stencil  102  may have the same pitch as the patterned electron beam  101 . As such, if the patterned electron beam  101  is properly aligned in the x-direction to the detector apparatus  100 , then the lines of the patterned electron beam  101  will pass through (or mostly pass through) the spacing between the lines of the stencil  102 . The patterned electron beam  101  will then pass through the opening of the spray aperture  104  so as to impinge upon the scintillator  106 . 
         [0021]    On the other hand, if the patterned electron beam  101  is not properly aligned in the x-direction to the detector apparatus  100 , then the lines of the patterned electron beam  101  will be at least partially scattered by the lines of the stencil  102 . The scattered electrons will then be deflected away from the z-axis such that it is blocked (or largely blocked) by the spray aperture  104  (instead of passing through the opening of the spray aperture  104 ). The spray aperture  104  may be formed of a high Z material, such as molybdenum or platinum, for example, and may be a couple of hundred microns thick, for instance. 
         [0022]    The combination of the stencil  102  and the spray aperture  104  advantageously generates an effective contrast between aligned and misaligned signals. As the patterned electron beam  101  becomes closer to the proper alignment in the x-direction, it is expected that a larger fraction of the electrons in the beam  101  will impinge upon the scintillator  106 . On the other hand, as the patterned electron beam  101  becomes further out of alignment in the x-direction, it is expected that a smaller fraction of the electrons in the beam  101  will impinge upon the scintillator  106 . 
         [0023]    The scintillator  106  may be made, for example, of a single crystal Yttrium-aluminum-garnet (YAG) plate. Other materials may also be used for the scintillator  106 . The scintillator  106  converts the electrons that impinge upon it into photons  110 . By converting the electron signal to photons  110 , the detected signal will be of a form that is unaffected by magnetic and electric fields or changes in those fields. 
         [0024]    An optical lens  108  may focus the photons  110  onto a phosphor screen  112  that is attached to a photomultiplier tube (PMT), photon counter, or other light detector  114 . The lens  108  will preferably be of a material that has a high transmittance at the wavelength of photons  110  generated by the scintillator  106 . The phosphor screen  112  receives the photons  110  and luminesces to generate a light signal that is received by the light detector  114 . The type of light detector  114  used may depend on the electron-beam current, the anticipated intensity of the light signal from the phosphor screen  112 , and the dynamic range required for the detection. The light detector  114  converts the light signal from the phosphor screen  112  into an electronic signal. A connection socket  116  and cable  117  may be configured to provide the electronic signal to detection electronics  118 . 
         [0025]      FIG. 2  is a cross-sectional diagram of an apparatus  200  to detect the position of a patterned electron beam in accordance with a second embodiment of the invention. The components and their arrangement in the detection apparatus  200  of  FIG. 2  are similar to the components and their arrangement in the detection apparatus  100  of  FIG. 1 . 
         [0026]    However, in the detection apparatus  200  of  FIG. 2 , there is no need for the stencil  102 , membrane  103 , or spray aperture  104 . Instead, a metal grating  202  may be fabricated directly on top of the substrate of the scintillator  106  using, for example, semiconductor manufacturing process technologies. The metal grating  202  may be made of tungsten, molybdenum, or platinum, for example, and may be of a thickness of two microns to block electrons of a 50 keV electron beam  101 , for example. Other thicknesses may be used for the grating  202  in other implementations. The thickness of the grating  202  may depend, for example, on the grating material and the energy of the electron beam. 
         [0027]    In one implementation, the striped pattern of the grating  202  may have the same pitch as the patterned electron beam  101 . The lines, and the spacing between them, for the patterned beam  101  and for the grating  202  may be one micron in width, for example. Other line and spacing widths may be used depending on the application. 
         [0028]      FIG. 3  is a cross-sectional diagram of an apparatus to detect the position of a patterned electron beam in accordance with a third embodiment of the invention. The components and their arrangement in the detection apparatus  300  of  FIG. 3  are similar to the components and their arrangement in the detection apparatus  100  of  FIG. 1 . 
         [0029]    However, instead of a stencil  102  formed above the membrane  103  in the apparatus of  100   FIG. 1 , a metal grating  302  is mounted on the underside of the membrane  103  in the apparatus  300  of  FIG. 3 . The metal grating  302  may be made of molybdenum or platinum, for example, and may be of a thickness of greater than twenty microns, for example, to block or scatter higher energy electrons. The thickness of the grating  302  may depend, for example, on the grating material and the energy of the electron beam. In one implementation, the striped pattern of the grating  302  may have the same pitch as the patterned electron beam  101 . The lines and the spacing between them for the patterned beam  101  and for the grating  302  may be one micron in width, for example. Other line and spacing widths may be used depending on the application. 
         [0030]    In addition, instead of a spray aperture  104  arranged between the stencil  102  and the scintillator  106  in the apparatus  100  of  FIG. 1 , a metal aperture  304  is formed directly on the scintillator  106  in the apparatus  300  of  FIG. 3 . Furthermore, the apparatus  300  of  FIG. 3  may include a band-pass (BP) filter  311  in front of the phosphor screen  112  and may have its socket  116  attached to a bottom side of the PMT  114 . 
         [0031]    As shown in  FIGS. 1-3 , the detection apparatus ( 100 ,  200  and  300 ) may be mounted on a rotary platter  120 . The rotary platter  120  may be magnetically floated or levitated above the X-Stage  125  and may be configured to spin about the z-axis. The X-Stage  125  is a mechanical stage that is configured to be controllably translated in the x-direction. 
         [0032]    Conventional detection apparatus, such as a semiconductor electron detector or a Faraday cup to detect the electron signal directly, would be disadvantageous to mount on such a magnetically-floated spinning rotary platter  120 . This is because the spinning rotary platter  120  generates magnetic fields that may interfere in a detrimental manner with the trajectory of the electron beam with the conventional detection apparatus. In contrast, the detection apparatus ( 100 ,  200  and  300 ) described above are each designed to operate in a robust manner in spite of the magnetic fields generated by the spinning rotary platter  120 . 
         [0033]      FIG. 4  is a cross-sectional diagram of an apparatus  400  to detect the position of a patterned electron beam  401  in accordance with a fourth embodiment of the invention. The apparatus  400  includes a metal grating  402 , a position-sensitive detector  404 , and a data processing apparatus  406 . The data processing apparatus  406  is configured to process the detected data from the position-sensitive detector  404 . 
         [0034]    In the illustrated embodiment, the patterned electron beam  401  and the metal grating  402  may comprise a pattern of lines along the y-direction that are spaced in the x-direction. In this embodiment, the position-sensitive detector  404  is position sensitive in the x-direction (and optionally also in the y-direction). 
         [0035]    The metal grating  402  may be made of molybdenum or platinum, for example, and may be of a thickness of greater than twenty microns, for example, to block or scatter higher energy electrons. The thickness of the grating  402  may depend, for example, on the grating material and the energy of the electron beam. 
         [0036]    The striped pattern of the grating  402  may have a pitch p 2  in the x-direction that is fractionally shorter than the pitch p 1  in the x-direction of the patterned electron beam  101 . In other words, p 2 =p 1 −Δp, where Δp is the difference between the pitches. As a result of this difference in pitch, a type of Moiré pattern is formed on the position-sensitive detector  404 . 
         [0037]      FIG. 5  depicts a first example of a Moiré pattern which may be detected on the detecting surface of the position-sensitive detector  404  in accordance with the fourth embodiment of the invention. Shown in  FIG. 5  within the dashed rectangles are blocked areas  502 . The blocked areas  502  correspond to areas and the detecting surface which are blocked from being illuminated by the electrons of the beam  401  because of the metal lines of the grating  402 . 
         [0038]    Also shown in  FIG. 5  are solid black rectangles that show the projection  504  of the patterned electron beam  401  onto the detecting surface. Portions the patterned electron beam  401  that overlap with the blocked areas  502  are blocked from reaching the detecting surface. These blocked portions may be considered to be “hidden behind” the blocked areas  502 . 
         [0039]    The resultant projection  504  that does reach the detecting surface is in the form of a Moiré pattern. The Moiré pattern has a beat wave with a wavelength λ that is given below in Equation (1). 
         [0000]      λ=2 p   1   p   2 /( p   1   −p   2 )≈2 p   1   2   /Δp    (1)
 
         [0000]    In Equation (1), λ is the wavelength of the “beat wave”, p 1  and p 2  are the pitches of the e-beam pattern and the grating, and Δp=p 1 −p 2 . 
         [0040]    While the resolution of the position-sensitive detector may make it difficult to resolve the individual lines of the e-beam projection  504 , the beat wave of wavelength λ will be more readily visible in the detected data. In particular, a location in the x-direction of a maximum intensity  510  of the beat wave may be determined from the detected data. This location  510  corresponds to a phase location of the beat wave and indicates an alignment of the patterned electron beam  401  relative to the grating  402 . An alignment or misalignment of the patterned electron beam  401  relative to the grating  402  may be determined from this phase information. 
         [0041]    If the alignment of the patterned electron beam  401  relative to the grating  402  changes, then the phase location of the beat wave will shift. The beat wave after such a shift is depicted in  FIG. 6 . As seen, the beat wave maximum  610  in  FIG. 6  has shifted relative to the beat wave maximum  510  of  FIG. 5 . 
         [0042]    The apparatus ( 100 ,  200 , and  300 ) described above in relation to  FIGS. 1-3  are designed to have the same pitches for the e-beam  101  and the stencil and/or grating. However, a small difference between these pitches would cause the brightness of the phosphor screen to vary as the e-beam is swept across the grating. This may cause inaccuracies in the measurement. In contrast, using the apparatus  400  of  FIG. 4  to detect the positioning of Moiré patterns as described above takes purposeful advantage of a small difference in pitch to determine misalignment. 
         [0043]    Applicants contemplate that a fixed microscope may be used as a reference point. Alternatively, two Moiré patterns, one with the longer pitch on the bottom, and the other with the longer pitch on the top, may be used to cross-reference the misalignment. The phase difference between the beat wavelengths of the two patterns may be proportional to the misalignment. 
         [0044]    The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
         [0045]    These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.