Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2000-10-04
2003-04-29
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S310000, C250S397000, C250S307000, C250S251000
Reexamination Certificate
active
06555819
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a scanning electron microscope for detecting a signal generated from a sample by irradiation of an electron beam and forming a scanned image of the sample.
A scanning electron microscope is an apparatus for scanning an electron beam which is generated from an electron source and finely limited by a focusing lens and an object lens on a sample using a deflector, detecting a signal generated from the sample by irradiation of the electron beam by a detector, and forming a sample image by processing the detection signal in synchronization with scanning of the electron beam. In order to improve the yielding rate of semiconductor devices, it is important to observe and analyze foreign substances and faults on devices. In correspondence with recent super refinement of semiconductor elements, observation and analysis of fine foreign substances and faults on semiconductor devices are required and for observation of foreign substances and faults, a scanning electron microscope is used instead of an optical microscope.
Information on irregularity such that observed foreign substances and faults are hollowed or projected is important information for analysis of foreign substances and faults. As a method for obtaining irregularity information of a sample, as described in Japanese Patent Publication 6-043885, there is a method for detecting a signal emitted in a direction at a small angle with the sample surface and forming a sample image. To observe fine foreign substances and faults, improvement of the resolution of a scanning electron microscope is desired. As a means for improvement of the resolution, by a method for bringing a sample close to an object lens or for leaking the magnetic field of an object lens on the sample side, the distance between the main surface of the object lens and the sample is made shorter.
According to the aforementioned prior art, a detector for detecting a signal emitted in a direction at a small angle with the sample surface must be arranged so as to look steadily at the electron beam irradiation position onto the sample and is inevitably arranged between the object lens and the sample. However, when the distance between the object lens and the sample is made shorter for improvement of the resolution, a problem arises that the amount of signals which can be detected by the detector is reduced and the SN ratio of sample images lowers. When an object lens of a magnetic field leakage type is used for improvement of the resolution, a problem arises that the track of a signal generated from a sample is bent by the magnetic field of the object lens and the signal cannot arrive at the detector.
SUMMARY OF THE INVENTION
The present invention was developed in consideration of the aforementioned problems of the prior arts and is intended to provide a scanning electron microscope for observing images at high resolution and obtaining irregularity information of a sample.
When an electron beam is irradiated onto the surface of a sample which is uneven due to foreign substances, the emission direction of reflected electrons emitted in a direction at a small angle with the sample surface from the uneven inclined parts is directive. Namely, most of the reflected electrons are emitted in the direction of the inclined surface and few reflected electrons are emitted in the backward direction of the inclined surface. Therefore, in a reflected electron image formed by detecting reflected electrons emitted in a specific direction from the sample, information concerning the inclination direction of inclined surface existing on the sample surface is included. By analysis of this inclination direction, irregularity information of the sample surface is obtained.
To detect reflected electrons emitted in a direction at a small angle with the sample surface by a scanning electron microscope having a short distance between the main surface of an object lens and a sample, according to the present invention, a detector is arranged on the side of an electron source of a magnetic field leakage type object lens and by controlling any one of or both of a negative voltage applied to the sample and an acceleration voltage for accelerating reflected electrons emitted from the sample, reflected electrons emitted in a direction at a small angle with the sample surface are detected. Reflected electrons emitted in a direction at a small angle with the sample surface are emitted from the sample by applying a negative voltage to the sample, and the track thereof is bent in the direction of the optical axis of the electron beam by the leakage magnetic field of the object lens, and the reflected electrons pass through the object lens by the acceleration electric field and is detected by the detector arranged on the electron source side by the object lens.
Namely, the scanning electron microscope of the present invention is characterized in that it has an electron source for generating an electron beam, a focusing lens for focusing the electron beam, a magnetic field type object lens for finely limiting the focused electron beam and irradiating it onto a sample, a deflector for two-dimensionally scanning the electron beam on the sample, a detector arranged on the electron source side of the object lens so as to detect a signal emitted from the sample by irradiation of the electron beam, a display means for displaying the signal detected by the detector as a sample image, a deceleration electric field generation means for generating an electric field for decelerating the electron beam to be irradiated onto the sample, and a voltage control means for controlling a voltage applied to the deceleration electric field generation means, and the voltage control means controls the voltage to be applied to the deceleration electric field generation means so that reflected electrons emitted in a direction at a small angle with the sample surface among the signal generated from the sample are detected by the detector, and a sample image having effects of light and shade in correspondence with the irregularity of the sample surface can be obtained.
The deceleration electric field generation means generates a deceleration electric field by applying a negative voltage to a sample. The deceleration electric field generation means may include an acceleration electric field generation means for generating an electric field for accelerating reflected electrons generated from the sample in the direction of the electron source and a voltage control means for controlling a voltage to be applied to the acceleration electric field generation means.
The scanning electron microscope of the present invention is also characterized in that it has an electron source for generating an electron beam, a focusing lens for focusing the electron beam, a magnetic field type object lens for finely limiting the focused electron beam and irradiating it onto a sample, a deflector for secondarily scanning the electron beam on the sample, a detector arranged on the electron source side of the object lens so as to detect a signal emitted from the sample by irradiation of the electron beam, a display means for displaying the signal detected by the detector as a sample image, an acceleration electric field generation means for generating an electric field for accelerating the signal generated from the sample, and a voltage control means for controlling a voltage to be applied to the acceleration electric field generation means, and the voltage control means controls the voltage to be applied to the acceleration electric field generation means so that reflected electrons emitted in a direction at a small angle with the sample surface among the signal generated from the sample are detected by the detector, and a sample image having effects of light and shade in correspondence with the irregularity of the sample surface can be obtained.
The object lens of the scanning electron microscope of the present invention may be of a type of generating a leakage magnetic field in a sample atmosphere.
Arai Noriaki
Kubo Toshiro
Ose Yoichi
Sato Mitsugu
Suzuki Naomasa
Anderson Bruce
Hashmi Zia R.
Kenyon & Kenyon
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