Potential measurement device
BACKGROUND OF THE INVENTION 
     This invention relates to a potential measurement device for measuring potential while irradiating a sample with an electron beam and in particular to a potential measurement device for measuring variations in potential with respect to time (potential waveform) at a fine place (for example, circuit node) in an LSI by using a scanning electron microscope. 
     It is known that it is possible to measure the potential at a place irradiated with an electron beam by adding a device for analyzing the energy of secondary electrons to a scanning electron microscope (JP-B-47-51024). 
     FIG. 2A shows this principle. A control grid 5 is disposed between a sample 1 to be tested and a secondary electron detector 4 mounted opposite thereto. This control grid 5 forms a potential barrier for discriminating energies of secondary electrons 3 emitted by irradiation of the sample with an electron beam 2. FIG. 2B is a scheme for explaining the function of this potential barrier. In the case where there were disposed no control grid 5 above the sample, all the secondary electrons would be detected by the secondary electron detector 4. Energies of the secondary electrons emitted by the sample 1 at the earth potential are distributed as indicated by A in FIG. 2B. When the potential of the sample 1 is -5 V, the energy distribution is indicated by B. When the control grid 5 is disposed and a voltage of -5 V is applied thereto, detected secondary electrons are restricted to those having energies higher than 5 eV. Therefore the output of the secondary electron detector varies, depending on the potential of the sample. Since the output depends on the potential of the sample in this way, it is possible to know the potential of the sample 1 by using the output of the secondary electron detector contrariwise. 
     FIG. 3 illustrates the construction of a prior art voltage measurement device. In this example a hemi-spherical control grid 5 is disposed above an objective lens 6 composed of a magnetic path 12 and a coil 11. The electron beam 2 is focused by the objective lens 6 and the sample 1 is irradiated therewith. Secondary electrons 3 are attracted and accelerated upward by a positive potential supplied to an extraction grid 7 by a positive voltage source 8. Accelerated secondary electrons are subjected to the focusing action of the objective lens 6. They are collected to a focal point and diverged again after having passed therethrough, as indicated in FIG. 3. Then their energies are analyzed by the control grid 5 by a normally negative voltage source 18. The center of the sphere constituting the control grid 5 and the focal point of the secondary electrons are in agreement with each other so that the diverged secondary electrons are projected perpendicularly to the control grid 5. This adjustment is effected by varying the potential supplied to the extraction grid 7 by the positive voltage source 8. The potential measurement device of this structure has a feature that the focal length of the objective lens 6 is short. However, when the trajectory of the secondary electrons was analyzed, it was found that the diameter of the secondary electron beam is only 3-4 mm, when it has reached the control grid 5. In the control grid 5 there is formed an aperture 13, through which the irradiation electron beam passes. The size of the aperture is about 2 mmφ and it was found that this is too great with respect to the spread of the secondary electron beam. In the neighborhood of the aperture, since the electric field is disordered, secondary electrons are projected to the control grid 5 not perpendicularly, which gives rise to measurement errors. In addition the detection efficiency is lowered, because the electrons entering the aperture 13 are not detected. However, from the constructional point of view, it is difficult to reduce further the size of the aperture 13. This is because shield grids 9 should be disposed before and behind the control grid 5 and in addition a cylinder 10 should be introduced in the aperture 13, as indicated in FIG. 3. A constant positive potential is given to these shield grids 9 and the cylinder 10 by a voltage source 24 so that it is prevented that the electron beam 2 is deflected by variations in potential at the control grid 5. 
     Secondary electrons 3, which have overcome the potential barrier formed by the control grid 5, are detected by secondary electron detectors, each of which consists of a scintillator 14, a light guide 15 and a photomultiplier 16. The positive potential by voltage sources 17 plays the role of attracting secondary electrons to the scintillator 14. In FIG. 3 the portion generating the electron beam 2, scanning coils, etc. are omitted. 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide a potential measurement device, by means of which these measurement errors due to the aperture and lowering in detection efficiency are prevented and potential measurement is effected with a high precision and a high efficiency. 
     The above object can be achieved by shifting the trajectory of the secondary electrons from the aperture. However, if the primary electron beam were deflected, a problem would be produced that the electron beam cannot be focused to a fine beam. 
     Therefore, according to this invention, a deflector, which doesn&#39;t defect the primary electron beam, but deflects only the secondary electron beam, is adopted. Combining an electric field and a magnetic field so as to balance them with respect to the primary electron beam, the deflector can deflect only the secondary electrons without deflecting the primary electron beam. 
     Now the operation of this invention will be explained, referring to FIG. 4. In FIG. 4, the electric field and the magnetic field are perpendicular to each other. When voltages of ±Vp are applied to two deflecting plates having a length Z and disposed with an interval g, the primary electron beam (energy: eV 1 ) is deflected by: 
     
         θ.sub.S =tan.sup.-1 (z·Vp/V.sub.1 /g). 
    
     On the other hand the deflection angle due to the magnetic field B can be given by: 
     
         θ.sub.B tan.sup.-1 (k·V.sub.1.sup.-1/2 ·B·z) 
    
     
         k=(e/m/2).sup.1/2, 
    
     where m represents the electron mass. Since the deflection thereof produced by the electric field and that produced by the magnetic field are opposite to each other, if θ S  =θ B , the primary electron beam is not deflected. The condition therefor is given by: 
     
         Vp/B=k·g·V.sub.1.sup.1/2 
    
     Since the secondary and primary electrons fly in directions opposite to each other, the direction of the deflection for the formers is inverted to that for the latters. Therefore the formers are never balanced, but they are deflected strongly by an angle, which is the sum of the two deflections. The acceleration energy of the secondary electrons being eV 2 , the deflection angle θ thereof is given by; 
     
         θ.sub.2 =tan.sup.-1 (z/V.sub.2 ·Vp/g) 
    
     
         +tan.sup.-1 (k·V.sub.2.sup.-1/2 ·B·z). 
    
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a scheme illustrating the construction of an embodiment of this invention; 
     FIG. 2A is a scheme for explaining the principle of the potential measurement by using an electron beam; 
     FIG. 2B shows the energy distribution of secondary electrons; 
     FIG. 3 is a scheme for explaining a prior art voltage measurement device; 
     FIG. 4 is a scheme for explaining the principle of a deflector used for realizing this invention; 
     FIG. 5 is a cross-sectional view along the line A--A&#39; in FIG. 1 for explaining the deflecting portion in the embodiment of this invention more in detail; and 
     FIG. 6 is a scheme for explaining the advantage obtained by the fact that the deflection pivot and the centers of the sphere constituting the control grid. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an embodiment of this invention. In this embodiment an electromagnetic coil 20 generating a magnetic field and deflecting plates 21 and 22 generating an electric field are disposed above the objective lens 6. These electric and magnetic fields are balanced so that the primary electron beam 2 is not influenced by them, but secondary electrons 3 are subjected to the deflecting action thereof. The electromagnetic coil 20 and the magnetic path 19 are sealed in a deflecting case 25 so that the atmospheric pressure is held within the deflecting case 25, when the path of the electrons is evacuated, in order to avoid that vacuum is worsened because of outgas from the coil 20. The lead lines of the coil 20 are taken out from the deflecting case 25 through feedthroughs 23. A coil 11 of the objective lens 6 is sealed in an objective lens case 26 in a similar manner and the lead lines thereof are taken out from the case 26 through feedthrough 27. Power sources for driving the coils 11, 20 and the deflecting plate 22 are not indicated in the figure. The other construction is identical to the prior art example indicated in FIG. 3. FIG. 5 is a scheme of this part in the cross-section (A--A&#39;). The coils 20 and 20&#39; are wound around the magnetic path 19 and magnetic flux φ passes as indicated by arrows, which produces magnetic field B at the central portion. On the other hand a voltage is given to the deflecting plates 21 and 22, which produces electric field E. The magnetic field B and the electric field E are so regulated that the forces imparted to primary electrons by them are balanced and therefore the primary electrons are not deflected. Although the electromagnetic coil is divided into two, left and right, 20 and 20&#39;, in this embodiment, it is also conceivable that each of the two partial coils 20 and 20&#39; is further divided into two parts, up and down (in FIG. 5), and that the ratio of currents passing through four partial coils is varied so as to improve the orthogonality of the magnetic field B and the electric field E. 
     FIG. 6 shows the aspect of deflection of secondary electrons more in detail. The secondary electrons 3 emitted by a sample are focused by the lens portion L, pass through a point C and are diverged. After that, they are deflected by the deflecting portion D. As the result of this deflection it can be thought that the point of divergence C of the secondary electrons is shifted to another point C&#39; by l·θ. In general the center of the sphere constituting the control grid 5 and the point C are located so as to be in agreement with each other so that the secondary electrons are projected perpendicularly to the grid 5. However, in the case where the deflection angle θ is great, since the incident angle of the secondary electrons to the grid 5 is no more perpendicular, measurement errors increase. A method for preventing this consists in that the control grid is so located that the center thereof is at the new point C&#39;, to which the divergence point is shifted by the deflection. However this method has a disadvantage that, since the focusing point of the secondary electrons is fixed, regulation is difficult. In order that the secondary electrons are projected always perpendicularly to the grid, independently of the deflection angle, it may be sufficient that the center of the sphere constituting the control grid 5 and the deflection pivot (D) are in agreement with each other and further the deflection pivot (D) and the focusing point C are in agreement with each other. Since l=0, C and C&#39; are always in agreement. As described previously, it is easy to adjust the focusing point by regulating the potential supplied to the extracting electrode 7 by the positive voltage source 8 or by regulating the current intensity flowing through the coils in the lens. 
     According to this invention, it becomes possible to eliminate measurement errors, which were produced heretofore at the aperture part for making the electron beam (primary electrons) pass through, and to increase the measurement precision of the voltage measurement device using an electron beam. In particular, in a device, in which the deflection pivot and the center of the spherical control grid are in agreement with each other, the effect become further remarkable.