Abstract:
One embodiment relates to an apparatus for adjustment of local magnetic strength in a magnetic device. A stage holds the magnetic device, and a sensor measures a magnetic field at locations above the magnetic device so as to generate magnetic field data. A computer system detects a non-uniformity in the magnetic field from the magnetic field data and determines a location and a duration for application of a pulsed laser beam to correct the non-uniformity. A laser device applies the pulsed laser beam at said location for said duration. Another embodiment relates to a method of adjusting local magnetic strength in a magnetic device. Another embodiment relates to a system for fine-tuning a magnet array with localized energy delivery. 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. 61/920,461, inventor John Gerling, filed Dec. 23, 2013, the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to magnetic structures. More particularly, the present disclosure relates to adjusting the strength of a magnetic apparatus. 
         [0004]    2. Description of the Background Art 
         [0005]    A conventional technique for adjusting the strength of a magnet involves controlling a temperature of the magnet. For example, a temperature-controlled chuck may be used to heat a magnet array in a controllable manner. By adjusting the temperature of the array, the strength of the entire magnet array may be adjusted at the same time. 
       SUMMARY 
       [0006]    One embodiment relates to an apparatus for adjustment of local magnetic strength in a magnetic device. A stage holds the magnetic device, and a sensor measures a magnetic field at locations above the magnetic device so as to generate magnetic field data. A computer system detects a non-uniformity in the magnetic field from the magnetic field data and determines a location and a duration for application of a pulsed laser beam to correct the non-uniformity. A laser device applies the pulsed laser beam at said location for said duration. 
         [0007]    Another embodiment relates to a method of adjusting local magnetic strength in a magnetic device. Steps of the method include: holding the magnetic device using a translatable stage; measuring a magnetic field using a sensor at locations above the magnetic device so as to generate magnetic field data; detecting a deviation in the magnetic field from the magnetic field data; determining a location and a duration for application of a pulsed laser beam to correct the deviation; and applying the pulsed laser beam at said location for said duration for adjustment of the local magnetic strength. 
         [0008]    Another embodiment relates to a system for fine-tuning a magnet array with localized energy delivery. The system may be automated. The system includes: a translatable stage for holding and translating the magnet array; an inspection microscope for aligning a position of the magnet array; a sensor that measures a magnetic field at locations above the magnet array so as to generate magnetic field data; a computer apparatus that detects a deviation in the magnetic field from the magnetic field data; and a laser device that applies the pulsed laser beam to correct the deviation. 
         [0009]    Other embodiments, aspects and features are also disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram showing an array of magnetic pole pieces with initial and adjusted magnetic field in accordance with an embodiment of the invention. 
           [0011]      FIG. 2  depicts an apparatus for adjustment of local magnetic strength in a magnetic device in accordance with an embodiment of the invention. 
           [0012]      FIG. 3  is a flow chart showing a method of adjusting local magnetic strength in a magnetic device in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The present disclosure provides an innovative technique that does not use temperature control for adjusting local magnetic strength in a magnetic device. Rather, the technique uses laser pulses of a specific wavelength to provide targeted energy delivery. The targeted energy delivery effectively excites and/or distorts the lattice structure of the material to quench or reduce magnetism in the local vicinity of the delivered energy. 
         [0014]    In one implementation, the wavelength may be related to absorption properties of the target magnetic material. The laser pulses may be of ultra-short duration to avoid excessive heating of the work piece that may adversely affect non-targeted magnetic domains. For example, research has found that, to adjust manganite material, the laser pulses may have a frequency above a terahertz and aduration on the order of 100 femtoseconds long. 
         [0015]    In accordance with an embodiment of the invention, the presently-disclosed technique may be used in the manufacturing process for constructing a magnetic lens array. Using this technique, the magnetic lens array may be constructed to meet specifications so as to be used as a high-performance passive electron-optic element. 
         [0016]    Meeting such specifications is difficult using conventional manufacturing processes because there are many sources that can introduce non-uniformity into the array. For example, machining tolerances may be not met, or there may be non-uniformities in the properties of the raw material, or there may be non-uniformity in the solenoid field used to charge the magnet array. In addition, it is often prohibitive to re-machine a magnetic part after magnetization of the part due to the difficult issue of removing magnetized particles. 
         [0017]    The presently-disclosed technique provides for targeted delivery of light energy at a specified wavelength so as to adjust local magnetism without heating up the overall material of the work piece. By the local and direct application of energy to tune the magnetism of the work piece, this technique avoids issues relating to machining a magnetized part. By way of this technique, the peak field strengths and general lensing properties (such as astigmatism) may be adjusted to compensate for defects in the manufacturing process that may be unavoidable. 
         [0018]    The magnetic lens array may be used, for example, as an electron-optics element in an electron beam column of an electron-beam imaging apparatus. The electron-beam imaging apparatus may be used, for example, for inspection and/or review of manufactured substrates. When peak field strength and/or astigmatism are corrected using the presently-disclosed technique, improved electron optics and imaging performance results. 
         [0019]      FIG. 1  is a schematic diagram showing an array of magnetic pole pieces  100  with initial and adjusted magnetic field in accordance with an embodiment of the invention. The array of magnetic pole pieces  100  is depicted in cross section and may include magnetic tiles  102  and pole pieces  104  and  106 . The array  100  may include multiple magnetic lenses  110  spaced periodically along a direction (the x-direction in  FIG. 1 ). In another embodiment, the array may be spaced periodically in two direction (both x and y directions). In other embodiments, the work piece may be a magnetic device that is not a periodic array. The array may generate a magnetic field. The magnetic field&#39;s z-direction component (Bz field) as a function of position in the x-dimension is shown in the graph above the array. 
         [0020]    In this illustrative example, the Bz field is shown as may be measured before adjustment  120 . As depicted, the Bz field before adjustment  120  is shown to have a non-uniform peak  125  due to one lens  110  that is substantially higher than the peaks  127  due to other lenses  110  in the array. This non-uniform peak  125  indicates a non-uniformity in the corresponding lens  110  that may be corrected in accordance with an embodiment of the invention. 
         [0021]    As further depicted, the non-uniform peak  125  may be shifted to one side of the expected location of the peak. A pulsed laser beam  130  of wavelength λ may be directed to a section of the corresponding lens  110  so as to make an adjustment  135  to the non-uniformity of the magnetic (B) fields. The Bz field after the adjustment  140  so as to correct the non-uniformity is illustrated. 
         [0022]    In the illustrated example, the non-uniform peak  125  is shifted to the left side compared with the expected position of the peak relative to the corresponding lens  110 . In this case, the pulsed laser beam  130  of wavelength λ may be directed to a section on the right side of the corresponding lens  110  so as to correct the non-uniformity. 
         [0023]    An exemplary apparatus for detecting and correcting non-uniformities of a magnetic device is described below in relation to  FIG. 2 . An exemplary procedure for detecting and correcting non-uniformities of a magnetic device is described below in relation to  FIG. 3 . 
         [0024]      FIG. 2  depicts an apparatus  200  for adjustment of local magnetic strength in a magnetic device  202  in accordance with an embodiment of the invention. The magnetic device  202  may be, for example, a magnet array  100  such as described above in relation to  FIG. 1 . 
         [0025]    As depicted, the apparatus  200  may include an XYZ stage  204  that holds the magnetic device  202 . The XYZ stage  204  may be used to move the magnetic device  202  in the x, y, or z directions under control of a computer apparatus  212 . An inspection microscope  210 , which may be an optical microscope, may be used to image the magnetic device  202  for alignment and visual inspection. The microscope may have various illumination capabilities, such as for bright field and dark field imaging, for example. 
         [0026]    A Hall sensor  206  may be positioned above the magnetic device  202 . The stage  204  may be translated in x, y, and/or z directions under the Hall sensor  206  so as to measure the magnetic field at different locations above the magnetic device  202 . The inspection microscope  210  may be used for aligning the magnetic device  202  relative to the hall sensor  206 . Measurement data from the Hall sensor  206  may be provided to the computer apparatus  212 . In other embodiments, other types of magnetic sensors may be used, such as, for example, magnetorestrictive sensors, giant magnetorestrictive sensors, and magneto optical Kerr effect sensors. 
         [0027]    A laser device  208  may be positioned such that a pulsed laser beam may be directed at the magnetic device  202 . In one implementation, the laser device  208  may have a controllable orientation so as to controllably change an incident angle of the pulsed laser beam that is directed to the magnetic device  202 . The stage  204  may be translated in x, y, and/or z directions under the laser device  208  so that the pulsed laser beam may be directed to a desired location on the magnetic device  202 . The inspection microscope  210  may be used for aligning the magnetic device  202  relative to the laser device  208  and for targeting the pulsed laser beam onto a desired location on the magnetic device  202 . 
         [0028]    The targeted location at which the pulsed laser beam impinges upon the magnetic device  202  may be imaged and observed using the inspection microscope  210 . The inspection microscope  210  may also be used to visually inspect the magnetic device  202  for observable defects. 
         [0029]      FIG. 3  is a flow chart showing a method  300  of adjusting local magnetic strength in a magnetic device  202  in accordance with an embodiment of the invention. The method  300  may be implementing, for example, using the apparatus  200  described above in relation to  FIG. 2 . The computer  212  in the apparatus  200  may be provided with a control module that is programmed to automate the method  300  so as to fine-tune the magnetic field generated by the magnetic device  202 . 
         [0030]    Per block  302 , the magnetic device is held on a translatable stage. The magnetic device may be, for example, a magnet array  100  as described above. The stage may be, for example, an XYZ stage  204  as described above. 
         [0031]    Per block  304 , magnetic fields generated by the magnetic device may be measured. This step may involve using a Hall sensor (a sensor that uses the Hall effect) to measure the magnetic field at locations above the magnetic device. The magnetic device may be translated under the Hall sensor using the translatable stage, and the measurement data may be received, stored, and analyzed by a computer system. 
         [0032]    Per block  306 , a determination may be made of non-uniformities (i.e. 
         [0033]    deviations) in the magnetic field to be corrected, if any. This determination may be made by the computer system or by an operator using the computer system. An example of a non-uniformity needing correction is discussed above in relation to  FIG. 1 . For example, if the magnetic field deviates from the expected (uniform) field by more than a threshold field strength, then that deviation (non-uniformity) may be deemed as needing correction. 
         [0034]    Per block  308 , if it is determined that no non-uniformities need correction, then the method  300  may be done (complete). Otherwise, if it is determined that one or more non-uniformities need correction, then, per block  310 , the method  300  may determine locations and durations for application of corrective laser pulses. This determination may be made by the computer system or by an operator using the computer system. For example, the location for application of corrective laser pulses may be determined from the location of the field non-uniformity, and the duration for application of corrective laser pulses at a particular location may be determined from the magnitude of the deviation needing correction. 
         [0035]    Per block  312 , corrective laser pulses may then be applied at the locations and for the durations determined in block  310 . Thereafter, the method  300  may, optionally, loop back to block  304  and a further field check inspection may be performed so as to verify that the non-uniformity has been corrected or determine if any remaining non-uniformities still exist that need correction.