Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2001-10-22
2003-01-28
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
Reexamination Certificate
active
06512227
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an inspection method using an electron beam and an inspection apparatus therefor, and particularly to an inspection method using an electron beam and an inspection apparatus therefor, which are suitable for inspecting patterns of circuits, etc. on wafers in the course of fabricating semiconductor devices.
BACKGROUND OF THE INVENTION
As an apparatus for inspecting circuit patterns used for a process of fabricating semiconductor devices, lithography masks, reticles, or the like, there has been known an optical inspection apparatus for detecting a defect on a circuit pattern by irradiating the circuit pattern with light and detecting the reflected light with a CCD or the like. The optical inspection apparatus, however, has a limitation in its resolution, and therefore, as the width of a circuit pattern becomes fine, it is difficult to detect a defect on the pattern by the optical inspection apparatus. Accordingly, an inspection apparatus using an electron beam, which has a high resolution, has come to be used for inspecting a defect on a fine pattern.
As one of apparatuses for observing a sample with an electron beam, there is known a scanning electron microscope (hereinafter, referred to as a “SEM”). Also, as one of apparatuses for inspecting a semiconductor device with an electron beam, there is known a critical dimension scanning electron microscope (hereinafter, referred to as a “CDSEM”). The SEM or CDSEM is suitable for observing a by restricted field of vision at a high magnification; however, it is unsuitable for searching a defect position on a semiconductor wafer. To be more specific, to search a defect position on the semiconductor wafer, it is required to inspect a very wide region, that is, the entire surface region of the semiconductor wafer ranging from 200 mm to 300 mm in diameter, and it takes a lot of time to inspect such a wide region by using the SEM or CDSEM because the electron beam current is low and thereby the scanning speed is low in the SEM or CDSEM. Accordingly, if the SEM or CDSEM is used for inspecting patterns at midway steps of a process of fabricating a semiconductor device, it taken an excessively longer time from the practical viewpoint to inspect the patterns. An inspection apparatus used for inspecting patterns at midway steps of a process of fabricating a semiconductor device is required to speed up the inspection time for increasing the throughput.
An inspection apparatus to solve the above problem has been disclosed, for example, in Japanese Patent Laid-open No. Hei 5-258703, which is configured to detect a defect on a wafer by making use of comparison between images. The inspection apparatus is characterized in (a) using a large electron beam current; (b) continuously moving a sample stage while irradiating a sample or a substrate with an electron beam; (c) using a high acceleration voltage to accelerate an electron beam generated from an electron source; (d) applying a retarding voltage to a sample to decelerate an electron beam, thereby preventing the charging of the sample; and (e) detecting charged particles generated from a sample by irradiation of an electron beam after the charged particles pass through an objective lens, which technique is called a TTL (Through The Lens) type method. The above inspection apparatus makes it possible to more efficiently inspect a defect on a mask or a wafer at a higher speed as compared with the conventional SEM.
In the TTL method, since charged particles generated from a sample are detected after passing through an objective lens, the distance between the objective lens and the sample can be shortened; and also the focal point of the objective lens can be shortened, to reduce aberration of an electron beam, thereby obtaining an image with a high resolution. The TTL method, however, has a non-negligible problem that the rotation of an electron beam largely varies depending on a change in height of a sample, to rotate the obtained image. Accordingly, the TTL method must ensure the accuracy in height of a sample, and therefore, it has a limitation in improvement of the inspecting speed.
The inspection apparatus described in the above document, Japanese patent Laid-open No. Hei 5-258703 adopts a collimated beam for avoiding dimness of the focal point due to Coulomb repulsive interaction of electrons in an electron beam. The adoption of such a collimated beam, however, causes a problem. When a collimated beam is blanked during movement of a sample on a sample stage, part of the collimated beam is not shielded by a stop disposed in a midway point of the trajectory of the electron beam during blanking, whereby a region not required to be irradiated, which is adjacent to a region required to be irradiated, is irradiated with the part of the collimated beam not shield. This results in a possibility that the obtained image is different from the actual one.
FIG. 14
shows a relationship between a retarding voltage and an efficiency of detecting secondary electrons, which is obtained by using a wafer as a sample in a process of fabricating a semiconductor device. In the TTL method shown by (
2
) in
FIG. 14
, there occurs a problem that as the retarding voltage is reduced, the efficiency of detecting secondary electrons becomes as low as not to be non-negligible. In the TTL method, secondary electrons generated from a sample are converged through a magnetic field in an objective lens, and the main reason why the efficiency of detecting secondary electrons becomes low as the retarding voltage is reduced is that when the retarding voltage is changed, the irradiation energy given from the electron beam to the sample is changed, with a result that the converged positions of secondary electrons in the axial direction are changed.
To prevent the reduction in efficiency of detecting secondary electrons, it may be considered to increase the retarding voltage; however, if the retarding voltage is increased, since the retarding voltage is applied to a sample stage and a shield frame which surround the end portion of the sample stage is earthed, a discharge occurs between the end portion of the sample stage and the shield frame. This causes an inconvenience in reducing the effect of applying the retarding voltage, or making unstable the electron beam due to occurrence of noise.
Further, since the ease of charge of the sample is dependent on the material of the sample, the magnitude of the retarding voltage must be changed depending on the ease of charge of the kind of sample.
With respect to the irradiation position of an electron beam, the position of a sample stage on which a sample is mounted is accurately measured, and the irradiation position of the electron beam is determined on the basis of the position of the sample stage. An interferometer using a laser beam is provided to measure the position of the sample stage, wherein a laser beam is made incident on mirrors mounted on the sample stage and a minutely changed amount of the position of the sample stage is measured on the basis of the interference of the reflected laser beam. On the other hand, the retarding voltage is applied to the sample via the sample stage on which the sample is mounted, and accordingly, the retarding voltage is also applied to the mirrors mounted to the end portions on two sides of the sample stage. In this case, since the mirror is made from glass, an electric field is concentrated at the end portion of the mirror. As a result, there is a possibility that a discharge occurs between the mirror and another member such as a shield frame provided in proximity to the mirror and earthed. If the mirror is not made from glass, there may occur a discharge between an edge of the mirror made from metal and said another member.
Accordingly, it is required to take into account not only a discharge between the end portion of the sample stage to which the retarding voltage is applied and the shield frame surrounding the sample stage but also the concentration of an electric field at the end portion
Fukuhara Satoru
Ichihashi Mikio
Iwabuchi Yuko
Kaneko Yutaka
Mori Hiroyoshi
Lee John R.
Smith II Johnnie L
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