JP2712184B2 - Axis adjustment device for charged particle beam - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charged particle beam axis adjusting device in a scanning electron microscope, a charged particle beam exposure device, and the like. [Prior Art] The axis adjustment of an electron beam for aligning the axis of an electron beam with the axis of an electron lens in an electron optical system is performed while observing an image. , A phenomenon of moving while changing the magnification or the like. Therefore, in the related art, when adjusting the axis of the electron beam, an AC current is added to a DC current for DC excitation of the electron lens, but it is asynchronous with the frame scanning of the electron beam. [Problems to be Solved by the Invention] In the prior art, since the AC current added to the DC current for DC excitation of the electron lens and the frame scanning of the electron beam are not synchronized, the sharpness of the image is not improved. Every time,
There is a problem in that different portions such as a magnification and a moving amount appear on the same screen and the image is distorted. Therefore, an object of the present invention is to optimize an image when adjusting the axis of a charged particle beam such as an electron beam. [Means for Solving the Problems] In order to solve the above problems, according to the present invention, an AC signal (including current and voltage) to be added to a DC signal (including current and voltage) for DC excitation of the electron lens. ) Was synchronized with the frame scanning of the charged particle beam. Also, in the embodiment of the present invention, when the axis of the charged particle axis is adjusted, the display means binarizes the image signal and displays the latest binarized screen and the previous binarized screen in a superimposed manner. did. The above charged particle beam includes an electron beam, an ion beam and the like, and the electron lens has a function of focusing or diffusing them. That is, the electronic lens refers to an objective lens, a condenser lens, and the like. [Operation] In the present invention, since the AC signal added to the DC signal for DC excitation of the electron lens and the frame scanning of the charged particle beam are synchronized, distortion of an image in the same screen is eliminated.
Further, according to the embodiment of the present invention, the image signal is binarized and displayed at the time of adjusting the axis of the charged particle beam, so that the contour of the sample pattern becomes clear. Embodiment FIG. 1 is a schematic configuration example of an electron optical system and peripheral devices of the present invention. Secondary electrons or reflected electrons obtained by scanning the surface of the sample 7 with the electron beam are detected by the detector 13 and become image signals. The image signal is supplied to the preamplifier 14 and the image signal processing device 15
The sample image of the scanning area of the electron beam is displayed on the image display 16 via the. The image signal processing device 15
Has a function of setting a threshold value for binarizing an image signal and a frame memory capable of storing image signal data for one screen. The electronic lens control unit 17 includes a DC current source 10, an AC current source 11, an amplifier 8, and a switch 9. The switch 9 is turned on by setting from the operation unit 12, and the excitation coil of the electronic lens 6 that functions as an objective lens is turned on. The strength of the electron lens 6 is controlled by adding the supplied current to the DC current from the DC current source 10 and the AC current from the AC current source 11. Except at the time of adjusting the axis of the electron beam, the electron lens 6 is normally excited only by the DC current of the DC current source 10. FIG. 2 shows an example of how to synchronize the vertical synchronizing signal (a) synchronized with the frame scanning of the electron beam and the alternating currents (b) and (c) added to the direct current for direct current excitation of the electron lens 6. It is a time chart. During the H level period of the vertical synchronizing signal (a), the electron beam scans the sample area for one image screen, and the L level period is the retrace time of the electron beam. The level of the alternating current (b) does not change during the period A or B of the vertical synchronization signal (a). At the time of adjusting the axis of the electron beam, the switch 9 is turned on by the setting of the operation unit 12, and the AC current shown in the example of FIG. 2B is added to the DC current for DC excitation of the electron lens 6. Therefore, the operation of increasing the intensity of the electron lens 6 during the period A 'in FIG. 2B and decreasing it during the period B' is repeated. As a result, the observed image repeatedly moves with the sharpness and the magnification changed alternately one by one. At the same time, the image signal processing device 15 prepares two frame memories A and B, and alternately writes and reads the binary image signal data alternately for each screen. For example, the vertical synchronization signal shown in FIG.
One screen of the binarized image signal data is written to the frame memory A in the period A of the second time, and is displayed on the image display 16. In the next first period B, similarly, one screen of the image signal data binarized is written to the frame memory B,
The image is superimposed and displayed on the binarized image in the first period A. In the second period A, the image signal data of the frame memory A in the first period A is erased while the binarized image of the first period B is displayed, and the newly binarized image is displayed. Write one screen of signal data to frame memory A,
The binarized image is displayed. Similarly, after the second B period, the display of the binarized image of the next A or B period is repeated while the binarized image of the previous B or A period is displayed. Therefore, the image display 16 displays the binary images 31 and 32 of the sample pattern as shown in the examples of FIGS. 3A and 3B, respectively, as the vertical synchronization signal A of FIG. 2A.
And B or B and A in two periods. FIG. 3A shows an example of a binarized image observed when the axis of the electron lens 6 and the axis of the electron beam coincide with each other. The binarized images 31 and 32 of the sample pattern make an oscillating motion with ()) as the center of rotation. FIG. 3 (b) shows the case where the axes are not aligned.
In the example of the digitized image, since the electron beam rotates around the axis of the electron lens 6, the center of the electron beam moves, and the binarized images 31, 32 of the sample pattern make an irregular rotational movement. When the axis of the electron lens 6 does not match the axis of the electron beam, the electron beam is adjusted by moving the electron beam by the electron beam deflectors 3 and 4 in FIG. The period of A 'or B' of the alternating current shown in FIG.
In accordance with the frame scanning speed of the electron beam, it can be set to an integral multiple such as twice (FIG. 2C) or three times (not shown) the period of the vertical synchronizing signal (a). Further, in the image signal processing device 15, if the electron beam frame scanning can be performed at high speed and the afterimage effect of the image display 16 can be used, one or no frame memory can be used. In the procedure of binarizing the image signal by the image signal processing device 15 and displaying the binarized image on the image display 16, the binarization of the image signal may be performed at the stage of writing to the frame memory. May be performed at the stage of reading from the. According to the embodiment described above, the image observed at the time of adjusting the axis of the electron beam is a binarized screen, and the AC current added to the DC current for DC excitation of the electron lens has a rectangular waveform. Since the movement of the sample pattern is not observed during the movement of the sample pattern because it is synchronized with the frame scanning of the electron beam, the movement of the pattern can be clearly confirmed.
This makes it easier to adjust the axis of the electron beam. [Effects of the Invention] As described above, according to the present invention, the AC signal added to the DC signal for DC excitation of the objective lens is synchronized with the frame scanning of the charged particle beam, so that the image is not distorted and the electronic The beam axis can be easily adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a schematic configuration example of an electron optical system and peripheral devices according to one embodiment of the present invention, and FIG. 2 is a vertical synchronization signal (a) synchronized with frame scanning of an electron beam. ) And the AC current (b) (c) added to the DC current for DC excitation of the electronic lens 6.
FIG. 3 is an example of a binarized image of a sample pattern observed at the time of adjusting the axis of the electron beam according to the present embodiment. FIG. FIG. 7B is a diagram illustrating a case where the axes match, and FIG. 7B illustrates a case where the axes do not match. [Description of Signs of Main Parts] 1 ... Electron gun, 3, 4, 5 ... Electron beam deflector, 6 ...
... Electron lens, 7 ... Sample, 8 ... Amplifier, 9 ... Switch, 10 ... DC current source, 11 ... AC current source, 12 ... Operation unit, 13 ... Detector, 14 ... Prefix Amplifier 15 Image signal processing device 16 Image display.

Claims (1)

(57) [Claims] In a charged particle beam axis adjustment device for adjusting the axis of the charged particle beam to align the axis of the charged particle beam with the axis of the electron lens, the period is synchronized with the frame scanning of the charged particle beam, and the period is positive in the scanning period. AC signal generating means for generating an AC signal having an even number of times and an even duty, and changing the cycle of the AC signal in accordance with the speed of frame scanning, and the AC signal for DC excitation of the electronic lens. An axis adjusting device for a charged particle beam, comprising: an adding unit for adding to a DC signal; and a unit for displaying a sample image. 2. The display means binarizes an image signal obtained from a signal from a sample, and superimposes and displays one screen of the latest binarized image on a binarized image one screen before. An axis adjusting device for a charged particle beam according to claim (1). 3. In a charged particle beam axis adjusting device for adjusting the axis of the charged particle beam to align the axis of the charged particle beam with the axis of the electron lens, an AC signal generation for generating an AC signal synchronized with a frame scan of the charged particle beam Means, an adding means for adding the AC signal to a DC signal for DC excitation of the electronic lens, and binarizing an image signal obtained from a signal from a sample, and displaying one screen of the latest binary image, A charged particle beam axis adjusting device, further comprising: a sample image display means for displaying a sample image superimposed on a binarized image one screen before.