Abstract:
A scanning type charged particle beam microscope has a scanning signal generator for generating a scanning signal, a scanning device for scanning a surface of a sample to generate a scanned image of the surface of the sample in accordance with the scanning signal, and correcting device for correcting a shift of the scanned image. An image storing apparatus stores the scanned image after the shift of the scanned image has been corrected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates a scanning type charged particle beam microscope, such as a scanning electron microscope or a scanning ion microscope, and to a method of sampling a scanned image to a storing apparatus of the scanning type charged particle beam microscope. 
     2. Description of the Related Art 
     In a scanning type charged particle beam microscope, such as a scanning electron microscope or a scanning ion microscope, a secondary electron signal provided from a sample by a scanning signal is sampled to a storing apparatus in synchronism with the scanning signal to thereby constitute image data. 
     However, there is a delay of a signal in an electronic circuit and therefore, a scanning position of a charged particle beam and a storing position thereof to a storing apparatus are shifted from each other by an amount of delay time of the electronic circuit. Further, when the scanning speed is changed, there is a problem that at a fast scanning speed a storing position of a storing image is delayed in comparison to that at a slow scanning speed and a positional shift is produced in the image data. These problems exist because in the related art technology start of scanning and start of storing an image are carried out simultaneously with each other and, therefore, the scanning position and the storing position are shifted from each other by an amount of delay time of the electronic circuit. Further, the delay time of the electronic circuit is provided with a dispersion produced by a characteristic of an apparatus and, therefore, the delay time cannot be corrected uniquely. 
     SUMMARY OF THE INVENTION 
     In order to resolve the above-described problem, according to an aspect of the invention, when a timing signal such as a start signal of scanning is utilized as an image recording control signal, by delaying the signal by an amount of delay of an electronic circuit by using a shift register having a variable stage number, the delay of the electronic circuit is corrected to thereby prevent a positional shift of an image. 
     According to a scanning type charged particle beam microscope constituted as described above, a storing position is shifted from a scanning position by the amount of a delay time period of the electronic circuit and therefore, an operational position and the storing position coincide with each other. Further, when a stage number of the shift register having the variable stage number is adjusted while observing the image, delay time inherent to an apparatus can simply be adjusted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simple constitution view of a scanning type charged particle beam microscope; 
     FIG. 2 is a signal timing chart showing operation of FIG. 1; 
     FIG. 3 is a signal timing chart in adjusting an electronic circuit; and 
     FIG. 4 is a constitution diagram of a shift register having a variable stage number defined by the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An explanation will be given of embodiments of the invention in reference to the drawings as follows. 
     FIG. 1 shows an embodiment of a scanning type charged particle beam microscope according to the invention. An explanation will be given of the operation thereof with reference to a signal timing chart shown in FIG.  2 . 
     A scanning signal generator  1  produces a start pulse  12  based on a referenced clock  11 , generates a scanning signal  13  with the pulse as a reference, amplifies the signal by a magnitude necessary for a deflection amplifier  2 , deflects a charged particle beam  4  by a deflector  3  and scans a surface of a sample  5 . Secondary electron  6  having an amount in accordance with a shape or properties thereof is generated from the surface of the sample  5  and therefore, the secondary electron  6  is detected by a secondary electron detector  7  to thereby provide a secondary electron signal  14 . As is apparent from FIG. 2, the secondary electron signal  14  is delayed from the scanning signal  13  owing to delay of an electronic circuit thereof. 
     The start pulse  12  produced by the scanning signal generator  1  is delayed based on the reference clock  11  by a stage number designated by a stage number switching switch  9  by a variable stage number shift register  8  and supplied to a storage start input of an image storing apparatus  10 . 
     The image storing apparatus  10  generates a write signal  16  according to the invention when the image storing apparatus  10  is provided with a storage start signal which is delayed based on the reference clock  11  by the stage number designated by the stage number switching switch  9  by the variable stage number shift register  8  and stores the secondary electron signal  14  based thereon at a storing apparatus at inside thereof. The stored data is displayed by a display apparatus or utilized in image processing by a computer. 
     According to the related art apparatus, the secondary electron signal  14  is stored at a storing apparatus at inside thereof based on a write signal  15  of the related art apparatus. 
     When the apparatus according to the invention is compared with the conventional apparatus, it is the feature of the invention that with regard to data stored to the storing apparatus, data at start time point of the scanning signal  13  and data at storage start point, substantially coincide with each other. Further, the delay time of the electronic circuit shows a characteristic inherent to the apparatus and therefore, when the delay time is corrected, the delay time can simply be corrected by switching the stage number of the variable stage number shift register  8 . 
     FIG. 3 shows a method of adjusting correction of delay time of an electronic circuit according to the invention. Scanning signals  19  and  20 , secondary electron signals  21  and  22  and write signals  23  and  24  in the case of high-speed scanning and in the case of low-speed scanning, are shown to align for comparison. A difference of speed in the both cases is caused by a multiplication factor of four in this example. According to the variable stage number shift register, the stage number is switched by 0, that is, without delay. Generally, the delay of the electronic circuit shows a substantially constant value without being dependent on the scanning speed and therefore, according to the example, data stored to the image storing apparatus becomes effective from a fifth pixel in the case of high-speed scanning and from a second pixel in the case of low-speed scanning. 
     When the delay is adjusted, adjustment can simply be carried out by correcting a position of an image provided in high-speed scanning while switching the stage number of the variable stage number shift register in the high-speed scanning to coincide with an image provided in low-speed scanning when the correction is 0. In this example, the delay may be set to four stages (4 clocks). 
     FIG. 4 is a constitution example of the variable stage number shift register defined by the invention. A plurality of D flip flop  26  connecting outputs and inputs thereof are aligned in series and driven by a reference clock  29 . An output of the respective flip flop is connected to respective tap of a stage number switching switch  27  and the stage number can be selected. A signal provided to a signal input  28  appears at a signal output  30  by being delayed by a period of the reference clock  29  multiplied by the stage number of selected ones of the D flip flop  26 . 
     Although according to the embodiment, stage switching is represented by simple switches, an element such as a digital multiplexor can naturally be used. Further, although according to the invention, the storage start signal is delayed, the delay can similarly be realized by delaying a signal such as the write signal. 
     The scanning type charged particle beam microscope according to the invention is constructed by the constitution in which shift of a scanning image caused by a delay of an electronic circuit is corrected by delaying a timing of sampling a signal to a storing apparatus by an amount of delay of the electronic circuit by using a shift register having a variable stage number and therefore, an effect described below can be achieved. 
     (1) The secondary electron information is stored to the storing apparatus after correcting the delay of the electronic circuit and therefore, the scanning position and the storing position of a storing apparatus of the charged particle beam coincide with each other. 
     (2) The delay time can simply be corrected by observing an actual image.