JP5581068B2 - Charged particle beam device and method for adjusting charged particle beam device - Google Patents

Description

  The present invention relates to a charged particle beam apparatus and a method for adjusting a charged particle beam apparatus, and more particularly to a charged particle beam apparatus and a charged particle beam apparatus adjusting method capable of performing beam scanning while suppressing the influence of a stray magnetic field or the like. About.

  A scanning electron microscope (SEM) scans an electron beam on a sample surface with a deflector, detects secondary electrons generated from the sample, and images or shape profiles of the sample. Is a device for generating a waveform signal called. The scanning electron microscope causes image disturbance such as image shaking when disturbances such as vibration, noise, and stray magnetic field are applied during image acquisition. As an example, in a scanning electron microscope (CD-SEM: Critical Dimension-Scannig Electron Microscope) that measures the length of a pattern of a device or the like, if the measured value of the pattern varies due to the occurrence of disturbance, Reproducibility is poor. For this reason, it is important to reduce image disturbance due to disturbance, and a number of means are used.

  Specifically, a technique for reducing the influence of vibration transmitted from the floor by installing an electron microscope on a vibration isolation table is known. For noise, as described in Patent Document 1, there is a method of reducing the influence of noise by generating sound having an opposite phase to external noise. For the stray magnetic field, as described in Patent Document 2, a scanning electron microscope is covered with a magnetic field shielding material, and as described in Patent Document 3, a magnetic field for canceling an external magnetic field is used. The technique that occurs is known.

  In addition, in order to reduce the image shake, a technique for setting the sub-scanning period of the beam as an integer multiple of the disturbance component as described in Patent Document 4, and Patent Document in order to suppress image drift. As described in FIG. 5, there is known a method of performing image integration by correcting the position between frames to be integrated.

JP 10-148235 A JP 2001-143999 A JP 58-214256 A Japanese Patent Laid-Open No. 11-154481 WO03 / 044821 publication

  The techniques described in the above cited references are effective in reducing disturbances and preventing image disturbances during image acquisition. However, since there is a limit to the magnitude of disturbances that can be removed, when disturbances exceeding the limits occur constantly or temporarily, the effects of disturbances are reduced, but they cannot be completely removed, and image disturbance Will occur. In addition, the above technique requires an external device for reducing disturbance, and the system configuration of the electron microscope may be complicated. As a result, the maintainability may be reduced.

  More specifically, when a magnetic shield is employed, it is necessary to increase the thickness in order to suppress disturbance, and there is a concern about an increase in the size of the apparatus and a decrease in maintainability. In addition, the disturbance can be suppressed to some extent by a method of canceling and suppressing the disturbance by generating a magnetic field, alignment at the time of image integration, or adjusting the period of the scanning line. It is not necessarily reduced, but only the processing that does not have the influence of the stray magnetic field on the beam due to the generation of other magnetic fields and image processing, etc. It is not a countermeasure.

  Hereinafter, a charged particle beam device and a method for adjusting the charged particle beam device for the purpose of suppressing the direct influence of the stray magnetic field on the beam by adjusting the optical conditions of the charged particle beam device will be described.

  As one aspect for achieving the above object, in a charged particle beam apparatus including a focusing lens for adjusting a focusing position of a charged particle beam between a charged particle source and an objective lens, focusing of the charged particle beam by the focusing lens is performed. Based on the charged particles detected in the process of changing the focusing position and changing the focusing position, detection signals at a plurality of different focusing positions are acquired, and the evaluation value of the image shake amount or the image shift amount is relatively small. A charged particle beam apparatus for selecting a focusing lens condition at a focusing position and a method for adjusting the charged particle beam apparatus are proposed.

  According to the above configuration, it is possible to set an optical condition capable of suppressing the influence of the stray magnetic field on the beam.

Schematic explanatory drawing of a scanning electron microscope. The flowchart which shows an optical condition adjustment process. The figure which shows the relationship between the excitation current value of a lens, and resolution. The figure which shows the relationship between the exciting current value of a lens, and the amount of image shaking. The figure explaining the difference between the line profile formed when the amount of image shake is large, and the line profile formed when the amount of image shake is small. The figure which shows the relationship between the set excitation current value and the amount of image shaking. The figure which shows the relationship between the set excitation current value and the amount of image shaking.

  Hereinafter, without using a complicated external device, image fluctuation caused by a stray magnetic field generated constantly or temporarily is detected during image acquisition, and each optical element is adjusted based on the detection. Now, a charged particle beam apparatus that reduces image fluctuation due to a stray magnetic field and a method for adjusting the charged particle beam apparatus will be described.

  More specifically, image fluctuations generated by the stray magnetic field are detected by utilizing the fact that the influence of the stray magnetic field on the charged particle beam differs depending on the crossover position (focusing position) of the charged particle beam. A charged particle beam apparatus having a function of reducing image shake due to a stray magnetic field by adjusting the above will be described. In the following embodiments, a scanning electron microscope, which is one type of charged particle beam apparatus, will be described as an example. However, not only the scanning electron microscope but also other charged particle beam apparatuses in which the influence of stray magnetic fields is a concern. (Application to a focused ion beam device or the like) is also possible.

  As an example of a specific effect, by applying the above-described method to a CD-SEM, image fluctuation due to a stray magnetic field can be reduced during acquisition of an image or a line profile. It is possible to prevent deterioration of the reproducibility of measured values.

  Hereinafter, a scanning electron microscope capable of detecting image fluctuation caused by a stray magnetic field during image acquisition and reducing the image fluctuation due to the stray magnetic field will be described with reference to the drawings.

  FIG. 1 is a diagram schematically showing the configuration of a scanning electron microscope. In FIG. 1, this scanning electron microscope includes an electron source 1, an anode electrode 2, a first focusing lens 4, an astigmatism correction lens 5, an aperture 6, a second focusing lens 7, a scanning deflection coil 8, an objective lens 9, and a scintillator 12. , A photomultiplier tube 13 and an electron microscope control device 14.

  Hereinafter, an optical system of a scanning electron microscope that detects image fluctuation caused by a stray magnetic field during image acquisition and reduces image fluctuation due to the stray magnetic field and its operation will be described with reference to FIG.

  When an extraction voltage is applied between the electron source 1 and the anode electrode 2, an electron beam 3 is emitted from the electron source 1 along the linear optical axis. The electron beam 3 has an electron beam 3a that spreads to several tens of mrad. After image formation by the first focusing lens 4 and the second focusing lens 7, the electron beam 3 is further reduced by the objective lens 9 and is minutely applied to the surface of the sample 10. A strong crossover.

  At this time, the opening angle of the electron beam 3 or the amount of electron beam current is limited by the diaphragm 6 installed between the first focusing lens 4 and the objective lens 9. Further, the electron beam 3 is scanned two-dimensionally in the X and Y directions on the sample 10 by the scanning deflection coil 8.

  The electrons 11 (secondary electrons and / or reflected electrons) emitted from the sample 10 rise while receiving the lens action of the objective lens 9. The raised electrons 11 collide with the scintillator 12 to which a positive high voltage is applied, emit light, and are converted into an electric signal by the photomultiplier tube 13 and amplified, and can be observed as a scanning electron microscope image (SEM image). At this time, the astigmatism correction lens 5 is used to correct astigmatism during image observation.

  In the embodiment described with reference to FIG. 1, a technique for directly detecting electrons emitted from a sample with a detector is employed. However, the present invention is not limited to this. It is good also as the method of converting into an electron and detecting it with a detector. Further, although there are two focusing lenses, the number is not limited to this, and three or more focusing lenses may be provided.

  Each optical element of the scanning electron microscope described in FIG. 1 is connected to an optical system control device 15, and the applied voltage and excitation current to each optical element are adjusted by the optical system control device 15. Further, the image processing device 16 has a function of detecting image fluctuation due to a stray magnetic field appearing in the obtained SEM image. Furthermore, the optical system control device 15 and the image processing device 16 are connected to the electron microscope control device 14, and all adjustments are automatically performed by the electron microscope control device 14.

Next, based on the flowchart shown in FIG. 2, a method for detecting an image fluctuation due to a stray magnetic field and generating an SEM image with reduced image fluctuation due to the stray magnetic field will be described. First, in step 100, in the configuration of this scanning electron microscope, the excitation current value (I ₀ ) set in advance as an initial value is applied to the focusing lens by the optical system control device 15, and the electron beam is applied to the scanning deflection coil. Then, one line in the one-dimensional direction (here, the X direction) is scanned to obtain a detection signal for one line. Next, in step 101, the excitation current value (electron beam crossover position) of the focusing lens is set to I ₁ and I _{2 under the} control of the optical system control device 15, and the detection signal of the same line is acquired again. The detection signal may be a waveform signal such as a line profile, or may be two-dimensional information such as an image signal.

The values of I ₁ and I ₂ are values obtained as follows. When the excitation current value is changed, as shown in FIG. 3, the resolution changes. Therefore, the excitation current values (I _max and I _min ) in the range (focal depth range) within which the SEM image can be acquired are obtained in advance. ) The focal depth range is determined by the relationship between the optical magnification and the pixel resolution, but may be arbitrarily set. However, it is smaller than the pixel resolution. As shown in FIG. 3, I ₁ is an intermediate value between the initial value (I ₀ ) and I _max . Similarly, I ₂ is an intermediate value between the initial value (I ₀ ) and I _min . The range of the excitation current value that can be changed is within the range of the focal depth.

When there is a stray magnetic field, the amount of image fluctuation varies depending on the positional relationship between the crossover position and the added stray magnetic field, for example, as shown in FIG. Since it is often unknown from which position of the scanning electron microscope the stray magnetic field is applied, one or both of the first focusing lens and the second focusing lens are changed to automatically acquire an SEM image. At this time, the amount of electron beam current is adjusted by the anode electrode 2 so as to be constant. Further, the scanning direction of the electron beam when acquiring the SEM image at the same location as in (S100) may be the same direction as the scanning direction in (S100) or the opposite direction. In addition, the current value of the focusing lens may be changed again between I ₁ and I ₂ to acquire detection signals under some conditions.

  In this embodiment, the example in which the excitation current is adjusted in the predetermined range has been described. However, the present invention is not limited to this. For example, in the case of an electrostatic lens, the applied voltage applied to the electrostatic lens is It becomes an index representing the lens strength. In addition to the excitation current and applied voltage, an index indicating the lens strength may be provided, and adjustment may be performed based on the value. Next, in step 102, the image processing device 16 performs matching with a pre-registered template or creates a line profile of the obtained SEM image as shown in FIG. The image shake amount is obtained by comparing the above and waveform matching. The image shake amount is, for example, the amount of positional deviation between the reference template and the detection signal, and the amount of positional deviation can be detected by a known pattern matching technique. Note that the image shift amount may be expressed by the number of pixels or the shift of the peak position of the line profile, and the number of pixels or the shift amount may be expressed by other evaluation values. As an example of the evaluation value, ranking may be performed according to the degree of deviation, and the rank may be used as a variable on the vertical axis in FIG.

  The relationship between the image shake amount thus obtained and the excitation current value is obtained, and the relationship between the plurality of excitation current values and the image shake amount is approximated using a functional equation.

  Next, in step 103, based on the relational expression obtained in step 102, it is determined whether there is a local minimum point at which the image shake amount is minimum. In this case, an excitation current value that is a relatively small image shift amount compared with other image shift amounts is selected from among a plurality of image shift amounts. For example, a predetermined threshold is provided for the image shift amount. In addition, an excitation current value that is lower than the threshold value may be selected.

As shown in FIG. 6, when there is a minimum point, the process proceeds to step 105. As shown in FIG. 7, when there is no minimum point and the value is minimum in I ₁ or I ₂ , the process proceeds to step 104.

In step 104, the excitation current value when the image shake amount is minimized with I ₁ to I _max, Likewise, when it becomes minimum at I ₂ is set to I _min, it acquires the detection signal In the same manner as in step 102, an excitation current value that minimizes the image shake amount is calculated. Excitation current value for obtaining a detection signal, I _max, not limited to the I _min, from I ₁ I _max, may be a value between I _min of I _2.

  Each optical element is adjusted so that the image shake amount obtained in steps 103 to 105 is minimized, and the same line is scanned again to generate an SEM image with reduced image shake. In this case, autofocus may be performed again.

  In step 106, the line is moved by one line in the Y direction, and then steps 101 to 105 are executed. At the same time, this process is repeated to generate a two-dimensional plane SEM image with reduced image shaking. Anyway. Thereby, it is also possible to correct image fluctuation due to a stray magnetic field temporarily generated. Note that the deviation may be evaluated in units of a plurality of lines.

  In the present embodiment, the example in which the SEM image is generated after obtaining the excitation current value with reduced image fluctuation has been mainly described. However, in step 101, the excitation current value is changed at a certain interval, and the SEM image is generated. After acquiring the image, an SEM image in which the influence of the stray magnetic field is reduced for each line may be selected to generate a two-dimensional plane SEM image.

  Thus, the automatic generation of the SEM image in which the image shake due to the stray magnetic field is reduced is completed.

DESCRIPTION OF SYMBOLS 1 Electron source 2 Anode electrode 3 Electron beam 3a Spread electron beam 4 First focusing lens 5 Astigmatism correction lens 6 Aperture 7 Second focusing lens 8 Scanning deflection coil 9 Objective lens 10 Sample 11 Secondary electron 12 Scintillator 13 Photomultiplier Tube 14 Electron microscope control device 15 Optical system control device 16 Image processing device

Claims (8)

A charging device comprising a scanning deflector for deflecting a charged particle beam emitted from a charged particle source in the X direction and the Y direction, and a focusing lens for adjusting a focusing position of the charged particle beam between the charged particle source and the objective lens. In the adjustment method of the particle beam device,
A detection signal at a different focusing position is acquired based on the charged particles detected in the process of changing the focusing position while the focusing position of the charged particle beam by the focusing lens is changed , and formed based on the detection signal. A focal position where the evaluation value of the image shake amount or the image shift amount is relatively small , and the image shake amount or the image shift amount in the X direction or the X direction and the Y direction of the image is calculated. A method for adjusting a charged particle beam apparatus, wherein the focusing lens condition is selected. In claim 1,
A method for adjusting a charged particle beam apparatus, wherein a range of change of a focusing position of the charged particle beam by the focusing lens is set to a range in which a predetermined resolution of the charged particle beam is maintained. In claim 1,
A method for adjusting a charged particle beam apparatus, wherein an evaluation value of the image shake amount or the image shift amount is obtained in a predetermined scanning line unit. In claim 3,
The method for adjusting a charged particle beam apparatus, wherein the focusing lens condition is selected in units of the predetermined scanning line. A charged particle source, a scanning deflector that deflects a charged particle beam emitted from the charged particle source in the X and Y directions, an objective lens that focuses the charged particle beam emitted from the charged particle source, and the charged In a charged particle beam apparatus comprising a focusing lens that is disposed between a particle source and an objective lens and focuses the charged particle beam, and a control device that controls the focusing lens,
The controller, together with the changes the focusing position of the charged particle beam by the focusing lens, based on the charged particles detected in the process of changing the focusing position, obtains a detection signal at a plurality of different focusing positions, the A waveform signal formed based on the detection signal, or an image shake amount or an image shift amount in the X direction or the X direction and the Y direction of the image is calculated, and an evaluation value of the image shake amount or the image shift amount A charged particle beam apparatus is characterized in that a focusing lens condition at a focusing position with a relatively small is selected. In claim 5,
The charged particle beam apparatus is characterized in that the range of change of the focused position of the charged particle beam by the focusing lens is set to a range that maintains a predetermined resolution of the charged particle beam. In claim 5,
The charged particle beam apparatus characterized in that the control device obtains an evaluation value of the image shake amount or the image shift amount in a predetermined scanning line unit. In claim 7,
The charged particle beam device, wherein the control device selects the focusing lens condition in units of the predetermined scanning line.

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