Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1998-11-13
2001-07-10
Anderson, Bruce C. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S397000
Reexamination Certificate
active
06259094
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an inspection method and apparatus using an electron beam and, more particularly, to an inspection method and apparatus suitable for inspection of a pattern formed on the surface of a semiconductor wafer.
Conventional scanning electron microscopes such as the one shown in
FIG. 1
observe and inspect samples such as a semiconductor wafer or the like by irradiating an electron beam onto the sample surface. An electron beam is emitted by a cathode
35
, is focused by a focusing lens
36
and objective lens
38
, and is scanned over the surface of the grounded sample
39
by a scanning electrode
37
.
A point light source is normally used for the cathode
35
. In order to obtain a resolution that allows pattern observation, the electron beam emitted by the point light source is temporarily focused by, e.g., a focusing lens
36
, and is focused again by an objective lens
38
to reduce the size of the spot. The scanning electrode
37
scans an electron beam over the surface of the sample
39
. Secondary and reflected electrons are emitted from the surface of the sample
39
, and are detected and converted into a detection signal by a detector
40
. The detection signal is amplified by an amplifier
41
, and is then supplied to an image processor
43
. A sync. signal for synchronizing the detection signal with the scans of the electron beam is supplied from a scanning circuit
42
to the image processor
43
. The luminance of the monitor
44
is determined by the amount of information indicated by the detection signal. This signal is synchronized by the sync. signal and output from the amplifier
41
. In order to increase the S/N ratio of the image, the image signals for each frame output from the amplifier
41
are combined by the image processor
43
, and the resultant image is displayed on the monitor
44
.
An example of a conventional electron beam inspection apparatus is shown in FIG.
2
. This apparatus was proposed by Japanese Patent Laid-Open No. 5-109381 and is balled a direct reflected electron microscope. A primary electron beam emitted by an electron gun
1
is focused by an irradiation lens system
6
, and is then deflected by a first Wien filter
3
(that applies an electric field and a magnetic field to the electron beam), before hitting the surface of the sample
4
at perpendicular angle. Secondary and reflected electrons emitted from the sample
4
are accelerated by an emission lens
5
. The electron beam is then projected onto the first screen
7
by the first projection lens system
2
, thus allowing the operator to directly observe the projected image.
Energy analysis can also be made. To do this, the first screen
7
is removed from the optical axis and the second Wien filter
8
is set to only allow secondary and reflected electrons of a certain energy level to pass straight through. The secondary and reflected electrons that pass straight through the second Wien filter
8
are enlarged to a predetermined size by a second imaging lens system
9
, and are displayed on the second screen
10
.
Such conventional electron beam inspection apparatuses, however, suffer from the following problems. The scanning electron microscope shown in
FIG. 1
uses a scanning electrode, scanning coil to allow the operator to observe a wide range of the sample surfaces while attaining suitable resolution. However, the speed of scanning is limited in this method since the linearity of scanning must be maintained. Furthermore, since the electron beam must be focused, the current amount decreases, resulting in a drop in the S/N ratio.
Conventionally, in order to solve such problem, images formed by secondary and reflected electrons are processed and stored in a memory, and are combined in single frame unit. However, in this method, the image display speed decreases.
Furthermore, upon deflecting the electron beam to scan a broader surface area, the electron beam shifts from the center of the lens optical axis, thereby producing lens aberrations, and resulting in poor resolution.
In the direct reflected electron microscope shown in
FIG. 2
, a primary electron beam is generated by a standard electron gun
1
and irradiation lens system
2
. In this arrangement, however, the electron beam cannot cover a sample region larger than 200 &mgr;m
2
at once. When the magnification must be changed in order to inspect a new region, the size of the primary electron beam must also be changed. However, in a conventional electron microscope, the beam shape cannot be changed without decreasing the current density. Furthermore, secondary and reflected electrons passing through the emission lens
5
are affected by the aberrations and transmittance of the emission lens
5
. Such influences, however, are not taken into consideration.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an electron beam inspection method and apparatus, which can solve the problems of conventional scanning electron microscopes (i.e., low scanning speed and low image display speed), and can also solve the problems of conventional direct electron microscopes (i.e., the inability to change the beam shape in accordance with the inspection region).
According to the present invention, there is provided an electron beam inspection apparatus comprising an electron beam irradiation unit which emits an electron beam having a rectangular cross section from a linear cathode, and irradiats the inspection region of the sample with the electron beam; a projection optical unit which forms an image (formed of secondary and reflected electrons emitted from the inspection region irradiated by the electron beam) onto an electron beam detection unit at a certain magnification; and an electron beam detection unit which generates and outputs a detection signal in accordance with the image formed by the projection optical unit, wherein the electron beam formed by the election beam irradiation unit covers substantially the same area as the inspection region on the sample surface, and irradiates the entire inspection region at once with the formed electron beam.
In the present invention the ratio of the area irradiated by the electron beam to the inspection region should fall within the range of 0.9 to 2.0.
The electron beam irradiation unit may be located obliquely above the sample surface, or may have a rotation-asymmetric electrostatic lens.
The electron emitting surface of the linear cathode may be rectangular in shape and have an aspect ratio within the range of 10:1 to 50:1.
The projection optical unit may comprise a cathode lens for accelerating and focusing the secondary and reflected electrons produced from the inspection region; an aperture for determining a divergent angle of the secondary and reflected electrons accelerated and focused by the cathode lens; a field aperture for determining the field of view of the secondary and reflected electrons, the divergent angle of which is determined by the aperture; and a projection lens for forming an image (defined by the secondary and reflected electrons, the field of view of which is determined by the field aperture) on the electron beam detection unit at a predetermined magnification.
The cathode lens may comprise an electrostatic cathode lens, and may have metal mesh positioned near the sample, through which the secondary and reflected electrons produced from the inspection region pass.
The cathode lens may comprise an electrostatic cathode lens, and first and second electrodes, through which the secondary and reflected electrons produced from the inspection region pass, and may be connected via a high-resistance material having a resistance of not less than 10
6
&OHgr;.
The cathode lens may comprise a magnetic lens, and may have a metal mesh positioned near the sample, through which the secondary and reflected electrons produced from the inspection region pass.
An electron beam inspection method of the present invention comprises the steps of: emitting an electron beam having a rectangular
Miyoshi Motosuke
Nagai Takamitsu
Yamazaki Yuichiro
Anderson Bruce C.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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