Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-10-26
2002-09-03
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S311000, C250S306000
Reexamination Certificate
active
06444981
ABSTRACT:
BACKGROUND OF THE INVENTION
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 “Some approaches to low-voltage scanning electron microscopy” by Müllerová 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 800 V, 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 difficu
Ezumi Makoto
Takami Sho
Todokoro Hideo
Berman Jack
Fernandez K
Hitachi , Ltd.
Kenyon & Kenyon
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