Abstract:
A scanning-type instrument is realized which utilizes a charged-particle beam and automates adjustments, if a voltage is applied to the specimen, to thereby provide excellent operational controllability. When a voltage is applied to the specimen, the electron beam would normally defocus. A signal corresponding to the voltage applied to the specimen is supplied to a CPU. An objective lens current is supplied to the coil on the objective lens from a power supply under control of the CPU to refocus the beam. As a result, the beam hitting the specimen is prevented from defocusing if a voltage is applied to the specimen. When the strength of the objective lens is varied, scanning signals to the deflection coils are adjusted in response to the variation. Variation in the magnification of the image and rotation of the image are corrected.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a scanning-type instrument which utilizes a charged particle beam and images a specimen by focusing the beam onto the specimen with an objective lens, scanning the beam across the specimen, detecting a signal arising from the specimen, such as secondary electrons, and displaying an image of the specimen based on the detected signal. The invention also relates to a method of controlling this instrument.  
           [0003]    2. Description of Related Art  
           [0004]    A scanning electron microscope, that is, a kind of scanning-type instrument utilizing a charged-particle beam, is designed to sharply focus a primary electron beam onto a specimen and to scan the beam within a given range on the specimen. The electron beam is directed at the specimen, resulting in secondary electrons. These secondary electrons are then detected. The resulting signal is sent to a CRT synchronized with scanning of the primary electron beam. In this way, a scanning image of the specimen is displayed.  
           [0005]    The fundamental structure of such a scanning electron microscope is shown in FIG. 3. This microscope has an objective lens  101 , an electron gun  102  for producing a primary electron beam EB and accelerating it, condenser lenses  103 , and other components are mounted above the objective lens  101 . The primary electron beam EB is sharply focused by the condenser lenses  103  and objective lens  101  and directed at a specimen  104 . Scan coils  105  are disposed above the objective lens  101  to scan the primary electron beam EB in two dimensions across the specimen  104 .  
           [0006]    As the electron beam EB is directed at the specimen  104 , secondary electrons are produced. These secondary electrons are collected by a mesh collector (not shown) to which a voltage of about 100 V is applied. The collected secondary electrons are guided to a secondary electron detector  106 , which is composed of a corona ring  107 , a scintillator  108 , and a photomultiplier tube  109 . A high voltage of the order of 10 kV is applied to the corona ring  107  from a corona ring voltage source (not shown). The secondary electrons are accelerated and made to strike the scintillator  108 . The photomultiplier tube  109  converts the light from the scintillator  108  into an electrical signal.  
           [0007]    The secondary electron signal detected by the secondary electron detector  106  is supplied as a brightness-modulating signal via an amplifier and known signal-processing circuitry (none of which are shown) for adjusting the brightness and contrast to a CRT (not shown) synchronized with the scanning of the primary electron beam. As a result, a scanning secondary electron image of a certain two-dimensional area on the specimen is displayed on the viewing screen of the CRT.  
           [0008]    In this scanning electron microscope, the geometry of the objective lens  101  is an important factor in determining the instrumental resolution. Therefore, in order to improve the resolution, the aberration coefficient of the objective lens  101  must be reduced. Accordingly, the aberration coefficient is suppressed to within 3 mm by adopting an “in-lens” objective lens in which the magnetic field on the specimen is intensified. Alternatively, a “semi-in-lens” objective lens (strong magnetic-field objective lens) may be adopted.  
           [0009]    With the in-lens objective lens, the specimen is placed within the magnetic field set up by the objective lens. With the semi-in-lens objective lens, a single magnetic lens field is produced below the bottom surfaces of the inner and outer polepieces. The specimen is placed within this magnetic lens field.  
           [0010]    To reduce the effects of objective lens aberration, it is customary to decelerate the electron beam immediately before the specimen. For this purpose, the primary electron beam having increased energies is passed through the objective lens. This method is known as a retarding field method. The resolution can be enhanced further at low accelerating voltages. In one retarding field method, a negative voltage is applied to the specimen as disclosed, for example, in Japanese Patent Laid-Open No. 10-334844/1998.  
           [0011]    Where a retarding field method is adopted in which a negative voltage is applied to a specimen, a voltage is applied to the specimen in the SEM imaging mode. In this case, the focus of the objective lens varies. Accordingly, in the case of a magnetic objective lens, it is necessary to vary the conditions under which the objective lens is focused by varying the objective lens current.  
           [0012]    If the conditions under which the objective lens is focused are varied, some parameters (e.g., magnification, image rotational angle, and angular aperture control lens current) vary concomitantly. In the existing scanning electron microscope, if a voltage is applied to the specimen, these parameters and conditions are maintained. Therefore, at this time, other parameters and conditions varying concomitantly with the application of the voltage to the specimen need to be adjusted manually. The required operations are quite cumbersome to perform.  
         SUMMARY OF THE INVENTION  
         [0013]    It is an object of the present invention to provide a scanning-type instrument which utilizes a charged-particle beam and automates adjustments, even if a voltage is applied to a specimen, to thereby provide excellent operational controllability. It is another object of the present invention to provide a method of controlling this instrument.  
           [0014]    A scanning-type instrument which utilizes a charged-particle beam and built in accordance with the present invention comprises: an objective lens for focusing the charged-particle beam onto a specimen; deflection means for scanning the beam within a desired area on the specimen; and detection means for detecting a signal produced as the specimen is irradiated with the charged-particle beam. The instrument obtains an image of the specimen based on the signal detected by the detection means. The instrument is characterized in that it further includes a voltage application circuit for applying a voltage to the specimen and first control circuit for controlling the strength of the objective lens according to the voltage applied to the specimen. Defocus of the beam caused by the application of the voltage to the specimen is corrected under control of the first control circuit.  
           [0015]    In a method of controlling a scanning-type instrument utilizing a charged-particle beam in accordance with the present invention, the scanning-type instrument has an objective lens for focusing the charged-particle beam onto a specimen; deflection means for scanning the beam within a desired area on the specimen; and detection means for detecting a signal produced as the specimen is irradiated with the charged-particle beam. The instrument obtains an image of the specimen based on the signal detected by the detection means. The method consists of applying a voltage to the specimen and controlling the strength of the objective lens according to the voltage applied to the specimen to thereby correct defocus of the beam caused by the application of the voltage to the specimen.  
           [0016]    Other objects and features of the invention will appear in the course of the description thereof, which follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a diagram schematically illustrating a scanning electron microscope according to the present invention;  
         [0018]    [0018]FIG. 2 is a diagram schematically illustrating a modified CPU used in the scanning electron microscope according to the present invention; and  
         [0019]    [0019]FIG. 3 is a diagram of a conventional scanning electron microscope.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Referring to FIG. 1, there is shown a scanning electron microscope according to the present invention. This instrument has an electron gun  21  producing an electron beam EB sharply focused onto a specimen  2  by condenser lenses  22  and an objective lens  1 . The beam EB is deflected by deflection coils  3  and  4  to scan the position on the specimen  2  hit by the beam EB. Two-dimensional scanning signals are supplied to the deflection coils  3  and  4  from a scanning signal-generating power supply  5 . An angular aperture control lens  6  is located above the deflection coils  3  and  4 . An aperture control lens current is supplied to the angular aperture control lens  6  from a lens current power supply  7 .  
         [0021]    The objective lens  1  is composed of a yoke  8 , an inner polepiece  9 , an outer polepiece  10 , and a coil  11 . Apertures  12  are formed in the inner polepiece  9  near its bottom end surface. The number of the apertures  12  is two or four to make the structure of the objective lens  1  axisymmetric. A secondary electron detector  13  is mounted outside an aperture  12  in the inner polepiece  9  and is located opposite to the outer polepiece  10 . An objective lens current is supplied to the coil  11  on the objective lens  1  by an objective lens current power supply  14 . The scanning signal-generating power supply  5 , power supply  7  for the angular aperture control lens  6 , and current power supply  14  for the objective lens  1  are controlled by a CPU  15  through a schematically illustrated bus line  23 .  
         [0022]    The secondary electron detector  13  is similar in structure with the secondary electron detector  106  already described in connection with FIG. 3 and consists of a combination of a scintillator and a photomultiplier tube. An annular electrode (corona ring) is mounted around the circular scintillator. A positive voltage is applied to the electrode to attract secondary electrons. The voltage is set to a value of about 10 kV. The output signal from the secondary electron detector  13  is amplified by an amplifier  24  and then fed to a CRT  25  synchronized with the scanning of the electron beam EB.  
         [0023]    A variable voltage source  16  is connected with the specimen  2  to permit one to apply any desired voltage to the specimen  2 . The operation of the instrument constructed in this way is as follows.  
         [0024]    When a scanning secondary electron image is observed, given scanning signals are supplied from the scanning signal-generating power supply  5  to the deflection coils  3  and  4 . A desired two-dimensional region on the specimen  2  is raster-scanned by the electron beam EB.  
         [0025]    The objective lens  1  is so designed that a single magnetic lens field is produced below the bottom surfaces of the inner polepiece  9  and outer polepiece  10 . The specimen  2  is positioned within this magnetic lens field. As the specimen  2  is irradiated with the electron beam EB, secondary electrons, and so on, are produced. Those of the secondary electrons which have lower energies (e.g., less than 3 eV) are confined by the magnetic lens field and directed upward helically.  
         [0026]    The voltage applied to the front face of the detector  13  forms an electric field near the apertures  12  in the inner polepiece  9 . This electric field directs the lower-energy electrons toward the detector  13 . The electrons collide against the scintillator of the detector  13 , thus producing scintillation that is detected by the photomultiplier. The output signal from this photomultiplier is supplied via the amplifier  24  to the CRT  25  synchronized with the scanning signals. As a result, a secondary electron image of a desired region on the specimen  2  is displayed on the CRT  25 .  
         [0027]    When no voltage is applied to the specimen  2 , a given objective lens current is supplied from the power supply  14  to the objective lens  1  so that the electron beam EB is focused onto the surface of the specimen  2  according to the energy of the electron beam EB (i.e., the accelerating voltage).  
         [0028]    A signal for scanning the electron beam EB is supplied from the power supply  5  to the deflection coils  3  and  4  to achieve the set magnification. This magnification is determined by the magnitude of the amplitude of the scanning signals. To maintain the magnification constant even if the energy of the electron beam EB varies, it is necessary to adjust the amplitude of the scanning signals according to the accelerating voltage on the electron beam EB. An aperture lens current is supplied from the power supply  7  to the angular aperture control lens  6  to maintain the angular aperture of the electron beam EB hitting the specimen  2  in an optimum state according to the energy of the electron beam EB.  
         [0029]    Under this condition, if a voltage is applied to the specimen  2  from the voltage source  16 , the electron beam EB is decelerated immediately ahead of the specimen  2 . If it is assumed that the accelerating voltage on the electron beam EB is 5 kV and that a voltage of 4 kV is applied to the specimen  2  from the voltage source  16 , the accelerating voltage varies to 1 kV immediately ahead of the specimen  2 . In this way, the electron beam EB is focused by the objective lens  1  at a high accelerating voltage and so the aberration can be reduced. On the other hand, the electron beam EB now less accelerated is made to hit the specimen  2 . Where the specimen  2  is a semiconductor or insulator, charging of the surface of the specimen  2  can be prevented or greatly suppressed. Of course, the energy of the electron beam EB directed at the specimen  2  is reduced. Consequently, the specimen  2  is much less damaged by the irradiation.  
         [0030]    When a voltage is applied to the specimen  2 , the electron beam EB is defocused compared with the case where the voltage is not yet applied. Therefore, as a voltage is applied to the specimen  2 , the power supply  14  needs to be adjusted to focus the beam. This focusing operation has been heretofore performed by adjusting the power supply  14  manually. In the present invention, a signal corresponding to the voltage applied to the specimen  2  is supplied to the arithmetic part  15   b  of the CPU  15  (see FIG. 2). An objective lens current is automatically supplied to the coil  11  on the objective lens  1  from the power supply  14  to refocus the electron beam EB by a first controller comprising the control part  15   a  of the CPU  15  and associated circuitry.  
         [0031]    This new objective lens current corresponding to the voltage applied to the specimen  2  is varied using the following function. Let ƒ(α) be the objective lens current through the objective lens when a voltage of α V is applied to the specimen  2 . The objective lens current ƒ(α) supplied into the objective lens  1  is found in the following manner:  
         f        (   a   )       =       f        (   0   )       ×   N             N   =         E   0     -   a       E   0     1   α                               
 
         [0032]    where E 0  is the energy of the primary electron beam EB and α is a coefficient.  
         [0033]    When the voltage applied to the specimen  2  is varied, the arithmetic part  15   b  of the CPU  15  finds the new lens current using the equations above, and the control part  15   a  controls the power supply  14  according to the found current. In this way, the voltage applied to the objective lens is set to the found value. If a voltage is applied to the specimen  2 , the electron beam EB directed at the specimen  2  does not defocus.  
         [0034]    In the above-described automated control of the objective lens  1 , a given calculation is performed to find a new objective lens current whenever the voltage applied to the specimen  2  is varied. Another method is also possible. As shown in FIG. 2, a storage part  15   c  for storing a look-up table of combinations of optimum objective lens current values, values of the accelerating voltage on the primary electron beam EB, and values of the voltage applied to the specimen  2  is prepared in the CPU  15 . These values of the objective lens current are calculated using the above formulas. When a voltage is applied to the specimen  2 , data about the optimum objective lens current is read from the look-up table stored in the storage part  15   c.  The control part  15   c  of the CPU  15  controls the power supply  14  based on the data.  
         [0035]    Where a voltage is applied to the specimen  2 , the lens current through the objective lens  1  is varied in this way. The primary electron beam EB is focused onto the surface of the specimen  2 . However, varying the objective lens current changes the lens strength. This, in turn, varies the deflection angle of the primary electron beam EB. In consequence, the region scanned with the electron beam EB varies, thus changing the magnification.  
         [0036]    Since the strength of the objective lens  1  varies, the primary electron beam EB within the magnetic field set up by the objective lens  1  rotates. This rotates the region scanned with the electron beam EB. Consequently, the obtained secondary electron image turns. Furthermore, the angular aperture of the electron beam EB hitting the specimen  2  varies in response to variations in the strength of the objective lens  1 .  
         [0037]    Therefore, in the present embodiment, when a voltage is applied to the specimen  2 , the power supply  5  for supplying scanning signals to the deflection coils  3  and  4  is controlled. The amplitude of the scanning signals is adjusted to correct the variation in the magnification. When a voltage is applied to the specimen  2 , the strength of the objective lens  1  varies, and the rotational angle of the primary electron beam EB within the magnetic field varies. The obtained image also rotates. The variation in the rotational angle is corrected by varying the direction of raster scan supplied from the power supply  5  to the deflection coils  3  and  4  by an amount corresponding to the variation in the rotational angle. Consequently, magnification variation and image rotation are prevented if the strength of the objective lens  1  is varied as a voltage is applied to the specimen  2  by a second controller comprised of CPU  15  and the associated circuitry.  
         [0038]    Furthermore, when the strength of the objective lens  1  is varied, the angular aperture of the primary electron beam EB hitting the specimen  2  varies. The power supply  7  is controlled by the CPU  15  such that a lens current for correcting the variation in the angular aperture is supplied from the power supply  7  to the angular aperture control lens  6 .  
         [0039]    When the strength of the objective lens  1  varies, the scanning signals supplied to the deflection coils  3  and  4  are optimized and the strength of the angular aperture control lens  6  is varied under control of the CPU  15 . In particular, the CPU  15  further includes a storage part  15   d  holding a look-up table of variations in the scanning signal supplied to the deflection coils  3  and  4  when the strength of the objective lens  1  is varied at various values of the accelerating voltage on the primary electron beam EB. When a voltage is applied to the specimen  2  and the strength of the objective lens  1  is varied concomitantly, the control part  15   a  of the CPU  15  reads data about variations in the scanning signals from the look-up table stored in the storage part  15   d  and controls the power supply  5  that produces the scanning signals.  
         [0040]    The strength of the angular aperture control lens  6  is varied similarly. That is, a look-up table of variations in the lens current supplied to the angular aperture control lens  6  in response to variations in the strength of the objective lens  1  is stored in the storage part  15   e  of the CPU  15 . When a voltage is applied to the specimen  2  and the strength of the objective lens  1  is varied concomitantly, a third controller comprising the control part  15   a  of the CPU  15  and associated circuitry reads data about the variations in the lens current through the angular aperture control lens  6  from the look-up table stored in the storage part  15   e  and controls the power supply  7  for the control lens  6 .  
         [0041]    In the above description, as a voltage is applied to the specimen  2  and the strength of the objective lens  1  is varied concomitantly, and the scanning signals supplied to the deflection coils  3  and  4  and the angular aperture control lens  6  are also controlled. Deflection systems used for astigmatic correction and axial alignment need to be adjusted when the strength of the objective lens  1  varies. These deflection systems can also be automatically optimized under control of the CPU  15 .  
         [0042]    While a preferred embodiment of the present invention has been described thus far, the invention is not limited thereto but rather various changes and modifications are possible. For example, a scanning electron microscope is taken as an example in the foregoing description. The invention can also be applied to an ion beam apparatus, such as a scanning ion microscope, where a specimen surface is scanned with an ion beam. In the above embodiment, a magnetic lens field is used as the objective lens  1 . The invention is also applicable to the case where an electrostatic lens is used as the objective lens. In this case, as a voltage is applied to the specimen, the strength of the electric field produced by the electrostatic objective lens is varied. Where a lens in which a magnetic field and an electric field are superimposed is used as the objective lens, as a voltage is applied to the specimen, one or both of the magnetic and electric fields are varied in strength.  
         [0043]    As described thus far, in the instrument and method utilizing and controlling a charged-particle beam in accordance with the present invention, as a voltage is applied to a specimen, the strength of the objective lens is varied to correct defocus of the charged-particle beam caused by the application of the voltage. Consequently, the operational controllability can be improved immensely.  
         [0044]    Furthermore, the deflection means is controlled according to the voltage applied to the specimen, and variation in the magnification of the specimen image and rotation of the image due to the application of the voltage are corrected. Hence, the operational controllability can be enhanced further.  
         [0045]    In addition, an angular aperture control lens is mounted to control the angular aperture of the primary charged-particle beam impinging on the specimen. This lens is controlled according to the voltage applied to the specimen. A variation in the angular aperture of the beam due to the voltage applied to the specimen is corrected. In consequence, the operational controllability can be improved further.  
         [0046]    Having thus described the invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.