The present invention relates to a scanning electron microscope for obtaining a two-dimensional scanned image to representing the shape or the composition of the surface of a sample by scanning the surface of the sample with an electron beam and detecting secondary-signal electrons generated from the sample, and especially to a scanning electron microscope suitable for observing a semiconductor sample in a low acceleration region with a high-resolution. 
A scanning electron microscope is an apparatus in which electrons emitted from a heating-type or field-emission-type electron source are accelerated, and formed into a thin electron beam (the primary electron beam) with an electrostatic-field or magnetic-field lens, the sample to be observed is two-dimensionally scanned with the primary electron beam, the secondary signal of secondary or reflected electrons generated from the sample irradiated with the primary electron beam is detected, and a two-dimensional scanned-image is obtained by inputting the strength of the detected secondary signal to a luminance modulation device of a Braun tube which is scanned synchronizing with the scanning of the primary electron beam. 
The scanning electron microscope accelerates electrons emitted from the electron source to which negative voltage is applied, toward an anode whose voltage is the ground voltage, and scans an observed sample with the primary electron beam.  Since the micro-processing has been greatly improved in the semiconductor industry, scanning electron microscopes have been widely used for examining the processing of semiconductor elements or processed semiconductor elements (for example, size measurement or electrical operations using an electron beam) in place of an optical microscope.  In order to observe a sample (a wafer) made of insulating material, it is necessary to accelerate the primary electron beam with a voltage less than 1 kV, which makes it possible to examine the sample during semiconductor processing without electrifying the sample.  In the above conventional scanning electron microscope (referred to as a SEM), the resolution attained under the condition of the 1 kV acceleration voltage has been 10 nm.  In accordance with the development of finer processing of semiconductor elements, a scanning electron microscope whose resolution is finer than 10 nm under a low acceleration voltage has been in greater demand. 
The retarding method is well known as a method of solving the above subject.  In this method, the diameter of an electron beam accelerated under an acceleration voltage higher than 1 kV is decreased, and the negative voltage is applied in advance to a sample to be irradiated with the electron beam. 
Accordingly, the acceleration voltage applied to the emitted primary electron beam is decreased to a required value due to the negative voltage applied to the sample just before the electron beam is injected in the sample.  By using this retarding method, the aberration of an object lens can be reduced, which improves the resolution of the scanning electron microscope. 
The fundamental composition of a scanning electron microscope using the retarding method is shown in FIG.  3 on page 402 in the paper xe2x80x9cSome approaches to low-voltage scanning electron microscopyxe2x80x9d by Mxc3xcllerova et al.,  Ultramicroscopy 41 (1992), pp.  399-410, North-Holland. 
Further, in Japanese Patent Application Laid-Open No.  Hei.  9-171791, a scanning electron microscope using the retarding method is disclosed.  In this scanning electron microscope, the boosting method of further accelerating the primary electron beam in an object lens is adopted in addition to the retarding method of applying a negative voltage to a sample.  The boosting method also contributes to the improvement of the resolution. 
Furthermore, in this SEM, an electrode arranged between a sample holder and the object lens, to which the same negative voltage as that applied to the sample holder is applied is disclosed.  According to the above composition, the conductive members to which the same negative voltage is applied are arranged over and under the sample, respectively.  Moreover, even if the sample is made of insulating material, it becomes possible to apply a desired amount of negative voltage (hereafter referred to as the retarding voltage) to the sample. 
For example, if the sample is a silicon wafer whose top and bottom surfaces are covered with oxide film, when the negative voltage is applied to the sample holder, the value of the voltage applied to the sample is a value determined according to the ratio of the electrostatic capacitance formed between the object lens and the sample to the electrostatic capacitance formed between the sample and the sample holder, and the desired retarding voltage cannot be precisely applied. 
The technique disclosed in Japanese Patent Application Laid-Open No.  Hei.  9-171791 is devised to solve the above problem. 
That is, even if the sample is made of insulating material, by arranging the sample in a region in which the potential is equal to the negative retarding voltage, which is formed by the two conductive members (the electrode and the sample holder), it is possible to apply any desired retarding voltage. 
To attain the above object, an aperture to pass the primary electron beam is provided in the electrode (hereafter referred to as the shield electrode).  The diameter of the aperture is determined as the size such that the electric field generated by the potential difference between the point irradiated with the primary electron irradiation point and elements outside the aperture (the object lens or the boosting electrode) reaches the irradiated point.  This is because if the aperture is so narrow that the generated electric field does not reach the irradiated point, the secondary-signal electrons (especially secondary electrons) cannot be transmitted to the side of the detectors. 
In the boosting method disclosed in Japanese Patent Application No.  Hei.  9-171791, the acceleration tube to which the high positive voltage is applied is located inside the electron beam passing hole of the objective lens.  In this composition, since a strong electric field is formed between the sample and the acceleration tube, if the sample is a semiconductor element, the sample may be broken or deteriorate according to the kind of material the sample is composed of. 
As mentioned above, it is required that an electric field of a certain strength be generated between the sample and the elements outside the aperture of the shield electrode.  On the other hand, it is also required that an excessively strong electric field should not act on the sample. 
Both the above retarding method and the boosting method are used to improve the resolution.  That is, both these methods are used to set the energy (acceleration energy) of the primary electron beam at a level higher than that of the electron beam injected in the sample.  For example, by accelerating the primary electron beam with 7 kV when the electron beam passes through the object lens and by setting the final acceleration voltage of the electron beam as 800V, the resolution of 10 nm obtained at the acceleration voltage of 1 kV can be improved to the resolution of 3 nm. 
As one of the means for materializing the above acceleration-voltage arrangement, the following boosting means is possible, that is, a boosting means in which an electron beam with energy of 800V is emitted, and the electron beam is accelerated with about 7 kV when the electron beam passes through an object lens by applying the positive voltage of 6.2 kV to an acceleration tube provided at an electron-beam passing aperture in the object lens.  This boosting means cause a problem that since the electron beam with low acceleration voltage of 800V tends to be affected by an electric or magnetic field, the electron beam receives effects of electrification due to stains on the inside surfaces of the microscope, or the outside magnetic field, which makes it difficult to obtain the theoretical resolution.  Further, a comparatively difficult design of the acceleration tube is required to stably applying the voltage of 6.2 kV to the acceleration tube without discharge from the acceleration tube. 
As another one of the means for materializing the above acceleration-voltage arrangement, the following retarding means is possible, that is, a retarding means in which an electron beam with energy of 7 kV is emitted, and the final acceleration voltage of the electron beam is adjusted as 800V by applying the voltage of 6.2 kV to the sample after the electron beam has passed through the object lens.  In this retarding means, although the electron beam is hardly affected by electrification of the inside surface of the microscope or the outside magnetic field because the electron beam possesses the high energy of 7 kV, it is required to apply the voltage of 7 kV and 6.2 kV to the electric gun and the sample, respectively.  Applying such a high voltage to the electric gun needs a greatly careful design of the electric gun.  It is generally said that the difficulty of a design of the electrical gun is proportional to the applied voltage.  This can be also said for applying the retarding voltage of 6.2 kV to the sample. 
The present invention has been devised to address the above contradictions, and is aimed at providing a scanning electron microscope using the retarding and boosting methods, which is capable of generating an electric field sufficient and necessary to transmit the secondary-signal electrons to detectors without applying an unnecessarily strong electric field to a sample. 
To attain the above object, the present invention provides a scanning electron microscope with high resolution realized by using both the retarding method and the boosting method and combining a control electrode provided between an acceleration tube and a sample while well balancing the retarding, boosting, control electrode voltage values from economic and technical standpoints.  For example, the electron-gun voltage is set to about 2 kV, and the voltage of about 5 kV is applied to the acceleration tube.  It is not technically difficult to apply the voltage of 5 kV to the electron gun, although the structure of the electron gun is complicated, and the electron gun includes a complicated electrical circuit.  Moreover, it is comparatively easy to compose the acceleration tube possessing a withstand voltage of 5kV. Naturally, it is also possible to set the voltage of the electron gun to 3 kV and to apply the voltage of 4 kV to the acceleration tube.  One of main features is to provide a scanning electron microscope composed so that the voltage distribution among the electron gun, the acceleration tube, and the sample can be optimally set from the technical and economic standpoints.  The composition of each scanning electron microscope according to the present invention are described below. 
That is, the present invention provides a scanning electron microscope including an electron source, an object lens for converging a primary electron beam emitted from the electron source, and at least one detector for detecting electrons generated from a sample irradiated with the primary electron beam converged by the object lens, the scanning electron microscope comprising:
a sample holder for holding the sample on the sample holder;
a shield electrode arranged between the object lens and the sample, in which an aperture for passing the primary electron beam is formed;
negative voltage applying means for applying a negative voltage to the sample holder and the shield electrode;
an acceleration tube located in an electron-beam passing hole in the object lens, provided to pass the primary electron beam, for further accelerating the primary electron beam; and
a control electrode arranged between the acceleration tube and the sample, in which an aperture whose size is smaller than the aperture formed in the shield electrode is provided to pass the primary electron beam, a positive voltage in the positive direction to the negative voltage being applied to the control electrode, superimposed on the negative voltage. 
Furthermore, in the above scanning electron microscope, a value of voltage applied to the electron gun is in a range of about 2 kV to 5 kV, a positive voltage less than 7 kV is applied to the acceleration tube, and an acceleration voltage applied to the primary electron beam injected in the sample is adjusted by changing the negative voltage applied to the sample holder and the shield electrode. 
According to the above composition, the control electrode can reduce the strong electric field generated between the acceleration tube and the sample, and since the voltage obtained by superimposing a definite amount of positive voltage to the negative voltage applied to the sample is applied to the sample, it is possible to transmit the secondary-signal electrons generated at the point of the sample, irradiated with the primary electron beams above the aperture of the control electrode.