Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1998-08-07
2002-02-19
Berman, Jack (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S307000, C250S397000
Reexamination Certificate
active
06348690
ABSTRACT:
FIELD OF THE INVENTION
In general, the present invention relates to an inspection method using an electron beam and an inspection apparatus adopting the method. More particularly, the present invention relates to an inspection method using an electron beam suitably for inspecting a pattern such as a circuit on a substrate in a process of fabricating a semiconductor device and an inspection apparatus adopting the method.
BACKGROUND OF THE INVENTION
There exists an apparatus for observing a specimen by using an electron beam which is known as a scanning electron microscope referred to hereafter simply as an SEM. In addition, as one of apparatuses for inspecting a semiconductor device, there is known a scanning electron microscope for length measurement referred to as a length measurement SEM. However, while the ordinary SEM and the length measurement SEM are suited for observation of a limited field of vision at a high magnification, they are unsuitable for finding the location of a defect on a wafer. This is because, in order to find the location of a defect on a wafer, it is necessary to search an extremely large area of the wafer or the entire surface of the wafer with a high degree of scrutiny. It takes a very long time to search such an extremely large area by using an ordinary or length measurement SEM because the current of the electron beam thereof is small, resulting in a slow scanning speed. As a result, if such SEMs are used in a process to fabricate a semiconductor device, the time it takes to complete the processing steps becomes very long, making the SEMs apparatuses of no practical use.
As an apparatus used for solving the problems described above, there is known an inspection apparatus using an electron beam for detecting a defect on a wafer by comparison of pictures. The apparatus is characterized in that: a large current electron beam is used; a specimen stage is continuously moved while the electron beam is being radiated to a specimen; a high acceleration voltage is used to accelerate the electron beam generated by an electron source; a retarding voltage is applied to the specimen to reduce the speed of the electron beam so as to prevent the specimen from being electrically-charged; and charged particles emanating from the specimen due to the radiation of the electron beam are detected after passing through an objective lens in a so-called TTL (through the lens) system.
As a result, the apparatus described above allows a mask or a pattern on a wafer serving as a specimen to be inspected for a defect with a higher degree of efficiency than the conventional SEM. It should be noted that this related technology is disclosed in documents such as Japanese Patent Laid-open No. Hei 5-258703.
With the TTL system whereby charged particles emanating from a specimen are detected after passing through an objective lens, the distance from the specimen to the objective lens can be shortened. As a result, the objective lens can be used at a short focal distance, allowing the amount of aberration of the electron beam to be reduced and, hence, a high-resolution picture to be obtained. On the other hand, the TTL system brings about unnegligible problems such as a hindrance to the improvement of the scanning speed and a big rotation change of the electron beam accompanying a variation in specimen height, causing a resulting picture to rotate as well.
FIG. 13
is a diagram showing a relation between the retarding voltage and the efficiency of detection of secondary electrons. Curve (
2
) shows a relation for the TTL system. As shown by curve (
2
), the TTL system has a problem that, as the retarding voltage is reduced, the efficiency of detection of secondary electrons also decreases to such a small value that the problem caused by a low detection efficiency can not be ignored anymore. Secondary electrons emanating from a specimen converge after passing through a magnetic field of an objective lens. The position of convergence in the axial direction changes when the retarding voltage is modified due to a variation in electron beam radiation energy. This phenomenon is the main reason why the efficiency of detection of secondary electrons decreases.
SUMMARY OF THE INVENTION
It is thus a first object of the present invention to provide an inspection method capable of increasing the speed of scanning a specimen using an electron beam and an inspection system adopting the method.
It is a second object of the present invention to provide an inspection method using an electron beam resulting in a small picture rotation and an inspection system adopting the method.
It is a third object of the present invention to provide an inspection method using an electron beam resulting in a small change in efficiency of detection of charged particles and an inspection system adopting the method.
In a configuration of the present invention, an electron beam generated by an electron source is converged on a specimen by means of an objective lens; the specimen is scanned by using the electron beam; and charged particles emanating from the specimen due to the scanning operation are detected by means of a charged particle detector provided between the specimen and the objective lens.
In another configuration of the present invention, an electron beam generated by an electron source is converged so as to generate a crossover and the electron beam is converged on a specimen by means of an objective lens provided between the crossover and the specimen; the specimen is scanned by using the electron beam; and charged particles emanating from the specimen due to the scanning operation are detected by means of a charged particle detector provided between the specimen and the objective lens.
In still another configuration of the present invention, an electron beam generated by an electron source is converged so as to generate a crossover while the electron beam is being converged on a specimen by means of an objective lens provided between the crossover and the specimen; the specimen is scanned by using the electron beam while the specimen is being moved continuously; and charged particles emanating from the specimen due to the scanning operation are detected by means of a charged particle detector provided between the specimen and the objective lens.
Then, charged particles detected by the charged particle detector are converted into an electrical signal conveying picture information and pictures are compared with each other on the basis of the picture information in order to detect a defect.
The comparison of pictures to detect a defect as described above includes comparison of a picture of an area on a specimen with a picture of another area on the same specimen and comparison of a picture of a an area on a specimen with a reference picture provided in advance.
According to a preferred embodiment of the present invention, a voltage for decelerating an electron beam is applied to a specimen. The voltage works as an acceleration voltage for charged particles emanating from the specimen, causing the charged particles to tend to form parallel beams.
According to another preferred embodiment of the present invention, charged particles emanating from the specimen are deflected by a deflection electric field and a deflection magnetic field which are substantially orthogonal to each other in the same direction. The amount of deflection of an electron beam radiated to a specimen by the deflection electric field and the amount of deflection of the electron beam by the deflection magnetic field are substantially equal to each other in magnitude but have mutually opposite directions so that one of the deflections cancels the other. As a result, a disturbance to deflection of an electron beam, that is radiated to the specimen, caused by the deflection electric field and the deflection magnetic field does not substantially exist.
According to still another preferred embodiment of the present invention, since charged particles are detected without passing through an objective lens, unlike the TTL system, even if a retardin
Fukuhara Satoru
Ichihashi Mikio
Iwabuchi Yuko
Kaneko Yutaka
Mori Hiroyoshi
Berman Jack
Hitachi , Ltd.
Kenyon & Kenyon
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