News Column

Patent Issued for Charged Particle Beam System and Method of Axial Alignment of Charged Particle Beam

June 18, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- A patent by the inventors Yamada, Mitsuru (Tokyo, JP); Nakamura, Motohiro (Tokyo, JP), filed on June 3, 2013, was published online on June 3, 2014, according to news reporting originating from Alexandria, Virginia, by VerticalNews correspondents.

Patent number 8742343 is assigned to Jeol Ltd. (Tokyo, JP).

The following quote was obtained by the news editors from the background information supplied by the inventors: "The present invention relates to a method of axially aligning a charged particle beam and also to a charged particle beam system.

"In recent years, scanning electron microscopes for making observations and measurements on fine structures of living organisms, materials, semiconductors, and so on and charged particle beam systems such as metrology SEMs used for measurements on semiconductor device circuit patterns have become known.

"In such a charged particle beam system, the axis of a charged particle beam is aligned with the optical axis of an optical member such as an objective lens.

"For example, JP-A-6-176721 discloses an aligner for aligning the electron beam of a scanning electron microscope. The aligner comprises means for obtaining two filament images by controlling a condenser lens/scan coil under two sets of conditions, means for obtaining two-valued images by binarizing the two original images, a histogram creating means for obtaining a histrogram by histrogramming the two-valued images in the X- and Y-directions, means for finding two coordinates indicating a maximum value from the histogram, and means for controlling the excitation current through the alignment coil such that the distance between the two coordinates is brought to within a tolerable range.

"JP-A-2005-174812 discloses a scanning electron microscope in which the axis of a charged particle beam is adjusted based on the result of a comparison between first and second images after detecting the first image by scanning the aperture of an objective lens with an electron beam in a first direction and detecting the second image by scanning the aperture of the objective lens with the beam in a second direction opposite to the first direction.

"In the aligner of JP-A-6-176721, however, the excitation currents through the gun alignment coils need to be corrected repetitively until the distance between the coordinates that maximizes the two-valued images derived from filament images fall within a tolerable range.

"Furthermore, in the scanning electron microscope of JP-A-2005-174812, a sequence of manipulative operations needs to be carried out repetitively until the excitation currents through the beam alignment coils reach their optimum value."

In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "In view of the foregoing problems, the present invention has been made. One object associated with some aspects of the present invention is to offer a method of easily making axial alignment of a charged particle beam.

"Another object associated with some aspects of the invention is to offer a charged particle beam system capable of easily making axial alignment of a charged particle beam.

"(1) A method of making axial alignment of a charged particle beam in accordance with the present invention is implemented by a charged particle beam system having a first alignment coil for deflecting the beam in a first direction, a second alignment coil for deflecting the beam in a second direction crossing the first direction and an objective lens for focusing the beam onto an object, and operating to adjust the axis of the beam by means of the first and second alignment coils, to detect a signal emanating from the object and to obtain image data. The method starts with obtaining at least first to third sets of image data by scanning the beam on a scanning area including a shielded region and repeating the scanning while varying conditions of excitation currents flowing through the first and second alignment coils, respectively. The values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the beam are calculated based on the obtained first to third sets of image data. During the step of obtaining the at least first to third sets of image data, the first set of image data is obtained under conditions where the values of the excitation current through the first alignment coil is a first current value and where the value of the excitation current through the second alignment coil is a second current value. The second set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is the first current value and where the value of the excitation current through the second alignment coil is a current value obtained by varying the second current value by a second incremental amount of current. The third set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is a current value obtained by varying the first current value by a first incremental amount of current and where the value of the excitation current through the second alignment coil is the second current value.

"In this method of axial alignment of a charged particle beam, the values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the charged particle beam can be found from the three sets of image data. Since only a limited number of sets of image data need to be obtained for axial alignment of the beam in this way, it is easy to axially align the beam. This reduces the burden on the operator of the system. The time taken to axially align the beam can be shortened.

"(2) Another method of making axial alignment of a charged particle beam is based on the method (1) above and further characterized in that, during said step of calculating values of the excitation currents, a first image displacement vector indicative of an amount of positional deviation between a first image formed by the shielded region and indicated by the first set of image data and an image obtained by rotating the first image through 180 degrees about an image center, a second image displacement vector indicative of an amount of positional deviation between a second image formed by the shielded region and indicated by the second set of image data and an image obtained by rotating the second image through 180 degrees about an image center, and a third image displacement vector indicative of an amount of positional deviation between a third image formed by the shielded region and indicated by the third set of image data and an image obtained by rotating the third image through 180 degrees about an image center. The values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the charged particle beam may be calculated based on the first to third image displacement vectors.

"(3) A further method of making axial alignment of a charged particle beam is based on the method (1) above and further characterized in that the charged particle beam system has scan coils for scanning the charged particle beam when the image data are obtained and that fourth to sixth sets of image data are obtained during the step of obtaining the at least first to third sets of image data. The fourth set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is the first current value and where the value of the excitation current through the second alignment coil is the second current value. The fifth set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is the first current value and where the value of the excitation current through the second alignment coil is a current value obtained by varying the second current value by the second incremental amount of current. The sixth set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is a current value obtained by varying the first current value by the first incremental amount of current and where the value of the excitation current through the second alignment coil is the second current value. The first to third sets of image data are obtained by scanning the charged particle beam in a first scanning direction by means of the scan coils. The fourth to sixth sets of image data are obtained by scanning the beam in a second scanning direction opposite to the first scanning direction by means of the scan coils. During the step of calculating the values of the excitation currents, the values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the beam may be calculated based on the first to sixth sets of image data.

"(4) An additional method of axial alignment of a charged particle beam is based on any one of (1)-(3) above and further characterized in that the objective lens has an aperture for permitting passage of the charged particle beam and that the shielded region may be formed by the objective lens.

"(5) A still further method of axial alignment of a charged particle beam is based on any one of (1)-(3) above and further characterized in that the shielded region may be formed by an orifice for creating a pressure difference between a space where the objective lens is placed and a space where a sample is placed.

"(6) A yet other method of axial alignment of a charged particle beam is based on any one of (1)-(3) above and further characterized in that the shielded region may be formed by an objective aperture for restricting the charged particle beam impinging on the objective lens.

"(7) A yet additional method of axial alignment of a charged particle beam is based on any one of (1)-(3) above and further characterized in that the shielded region may be formed by a charged particle beam detection aperture for detecting variations in the charged particle beam.

"(8) A still further method of axial alignment of a charged particle beam is based on any one of (1)-(7) above and further characterized in that the first and second alignment coils may be placed at an upstream-side from an objective aperture for restricting the charged particle beam traveling for the objective lens.

"(9) A further additional method of axial alignment of a charged particle beam is based on any one of (1)-(5) above and further characterized in that the first and second alignment coils may be placed at a downstream-side from an objective aperture for restricting the charged particle beam traveling for the objective lens.

"(10) A charged particle beam system associated with the present invention has a first alignment coil for deflecting a charged particle beam in a first direction, a second alignment coil for deflecting the beam in a second direction crossing the first direction and an objective lens for focusing the beam onto an object, and operates to adjust the axis of the beam by means of the first and second alignment coils, to detect a signal emanating from the object and to obtain image data. The charged particle beam system comprises: image data acquisition means for obtaining at least first to third sets of image data by scanning the beam on a scanning area including a shielded region and repeating the scanning while varying conditions of excitation currents through the first and second alignment coils, respectively; and calculation means for calculating the values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the beam based on the first to third sets of image data obtained by the image data acquisition means. The first set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is a first current value and where the value of the excitation current through the second alignment coil is a second current value. The second set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is the first current value and where the value of the excitation current through the second alignment coil is a current value obtained by varying the second current value by a second incremental amount of current. The third set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is a current value obtained by varying the first current value by a first incremental amount of current and where the value of the excitation current through the second alignment coil is the second current value.

"With this charged particle beam system, the values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the charged particle beam can be found from the three sets of image data. Since only a limited number of sets of image data are required to be obtained for axial alignment of the beam in this way, it is easy to axially align the beam. This reduces the burden on the operator of the system. The time taken to make axial alignment of the beam can be shortened.

"(11) Another charged particle beam system associated with the present invention is based on the system of (10) above and further characterized in that the calculation means calculates a first image displacement vector indicative of an amount of positional deviation between a first image formed by the shielded region and indicated by the first set of image data and an image obtained by rotating the first image through 180 degrees about an image center, a second image displacement vector indicative of an amount of positional deviation between a second image formed by the shielded region and indicated by the second set of image data and an image obtained by rotating the second image through 180 degrees about an image center, and a third image displacement vector indicative of an amount of positional deviation between a third image formed by the shielded region and indicated by the third set of image data and an image obtained by rotating the third image through 180 degrees about an image center. The values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the charged particle beam may be calculated based on the first to third image displacement vectors.

"(12) A further charged particle beam system associated with the present invention is based on the system (10) above and further characterized in that the system further includes scan coils for scanning the charged particle beam when the image data are obtained and that the image data acquisition means obtains fourth to sixth sets of image data. The fourth set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is the first current value and where the value of the excitation current through the second alignment coil is the second current value. The fifth set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is the first current value and where the value of the excitation current through the second alignment coil is a current value obtained by varying the second current value by the second incremental amount of current. The sixth set of image data is obtained under conditions where the value of the excitation current through the first alignment coil is a current value obtained by varying the first current value by the first incremental amount of current and where the value of the excitation current through the second alignment coil is the second current value. The first to third sets of image data are obtained by scanning the charged particle beam in a first scanning direction using the scan coils. The fourth to sixth sets of image data are obtained by scanning the beam in a second scanning direction opposite to the first scanning direction using the scan coils. The calculation means may calculate the values of the excitation currents through the first and second alignment coils, respectively, for axial alignment of the beam based on the first to sixth sets of image data.

"(13) A further charged particle beam system associated with the present invention is based on any one of (10)-(12) above and further characterized in that the objective lens has an aperture for permitting passage of the charged particle beam and that the shielded region may be formed by the objective lens.

"(14) An additional charged particle beam system associated with the present invention is based on any one of (10)-(12) above and further characterized in that the shielded region may be formed by an orifice for creating a pressure difference between a space where the objective lens is placed and a space where a sample is placed.

"(15) A yet other charged particle beam system associated with the present invention is based on any one of (10)-(12) above and further characterized in that the shielded region may be formed by an objective aperture for restricting the charged particle beam impinging on the objective lens.

"(16) A still other charged particle beam system associated with the present invention is based on any one of (10)-(12) above and further characterized in that the shielded region may be formed by a charged particle beam detection aperture for detecting variations in the charged particle beam.

"(17) An additional charged particle beam system associated with the present invention is based on any one of (10)-(16) above and further characterized in that the first alignment coil and the second alignment coil may be disposed at an upstream-side from an objective aperture for restricting the charged particle beam impinging on the objective lens.

"(18) An additional charged particle beam system associated with the present invention is based on any one of (10)-(14) above and further characterized in that the first alignment coil the second alignment coil may be disposed at a downstream-side from objective aperture for restricting the charged particle beam impinging on the objective lens."

URL and more information on this patent, see: Yamada, Mitsuru; Nakamura, Motohiro. Charged Particle Beam System and Method of Axial Alignment of Charged Particle Beam. U.S. Patent Number 8742343, filed June 3, 2013, and published online on June 3, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8742343.PN.&OS=PN/8742343RS=PN/8742343

Keywords for this news article include: Jeol Ltd.

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Source: Journal of Engineering


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