Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1998-10-30
2001-07-24
Anderson, Bruce C. (Department: 2878)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S397000
Reexamination Certificate
active
06265719
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 inspecting patterns on semiconductor wafers, photomasks and the like.
As semiconductor devices continue to shrink in size, greater sensitivity is required for detecting defects, foreign matter, and the like on semiconductor wafers and photomasks. In general, since a detection sensitivity of {fraction (1/2+L )} that of the wiring width of a pattern or less is required, the sensitivity of optical column defect inspection will reach its limit in the near future. In place of optical apparatus, an inspection apparatus using an electron beam has been developed and proposed in, e.g., Japanese Patent Laid-Open Nos. 5-258703, 7-249393.
Japanese Patent Laid-Open No. 7-249393 discloses a wafer pattern defect detection apparatus. This apparatus comprises an electron optical column
81
which has a rectangular electron emission cathode, and irradiates a surface
85
of a sample
82
with an electron beam, a secondary electron detection system
84
having a line sensor type secondary electron detector
86
for detecting secondary electrons
83
produced from the sample
82
irradiated with the electron beam, and a circuit
87
for processing the detection signal, as shown in FIG.
1
. This defect detection apparatus is characterized by an ability to conduct high-speed pattern defect inspections on semiconductor wafers by setting the aspect ratio of a rectangular beam (which irradiates the sample surface), and executing parallel signal processing in a secondary electron detection system
84
.
In relation to the resolving performance of a projection optical system for imaging a one- or two-dimensional image of secondary/reflected electrons produced from the sample, the electric field strength generated between the first electrode of the projection optical system and the sample can be increased and uniformity can be improved by placing the sample in the vicinity of the projection optical system. Hence, the projection optical means is placed so that its optical axis extends perpendicular to the sample surface.
However, conventionally, in order to realize such arrangement, the sample surface must be obliquely irradiated with a rectangular electron beam formed by a primary optical system due to the layout of the primary optical system and projection optical system in the vicinity of the sample surface.
Oblique incidence of the electron beam poses the following problems:
(1) When a pattern with a three-dimensional shape on the sample surface is obliquely irradiated with the irradiated beam, a region which is not irradiated with an electron beam is formed on the side opposite to the incident direction. For this reason, a portion opposite to the incident direction of the pattern appears as a shadow produced by secondary and reflected electrons. For this reason, it has been impossible to observe and inspect defects, foreign matter, and the like present on a pattern side wall, between adjacent patterns, and the like.
(2) Upon irradiation with the electron beam, a negative voltage is applied to the sample. Because of this, when the sample surface is obliquely irradiated with the irradiation beam, the incident position of the electron beam onto the sample shifts from the original axis due to the influence of the electric field present between the sample and projection optical system. It is very hard to attain optical axis adjustment among the irradiated beam system, sample, and projection optical system due to the presence of the electric field.
(3) The electric field is present between the sample and projection optical system, as mentioned above. When this electric field changes, the position on the sample surface irradiated with the electron beam moves, resulting optical axis adjustment errors.
(4) When the electron beam is obliquely incident, electrons reflected by the sample have a distribution in the total opposite direction of the electron beam irradiated. For this reason, the transmittance of reflected electrons to the projection optical system whose axis is perpendicular to the sample surface had been reduced.
As described above, in the conventional method, since the sample surface is obliquely irradiated with an electron beam, it is impossible to inspect defects present on the pattern side wall, and is hard to adjust the optical axis, and so forth.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an inspection method and apparatus using an electron beam, which can solve various problems posed due to oblique incidence of the electron beam.
According to the present invention, there is provided an electron beam inspection apparatus comprising an electron beam irradiation unit for irradiating a sample with an electron beam, a projection optical unit for forming, on an electron beam detection unit, a one- or two-dimensional image of secondary and reflected electrons produced in accordance with the sample surface upon irradiating the sample with the electron beams by the electron beam irradiation unit, the electron beam detection unit for outputting a detection signal on the basis of the one- or two-dimensional image of the secondary and reflected electrons formed by the projection optical unit, an image display unit for receiving the detection signal output from the electron beam detection unit, and displaying the one- or two-dimensional image of the sample surface, and an electron beam deflection unit for changing an incident angle of the electron beam received from the electron beam irradiation unit onto the sample, and making the projection optical unit capture the secondary and reflected electrons received from the sample.
In this way, according to the inspection method and apparatus using an electron beam according to the present invention, the incident angle upon irradiating the sample surface with the electron beam is changed by an electron beam deflection unit, and the projection optical unit captures secondary and reflected electrons produced at the sample surface via the electron beam deflection unit. Hence, any defects or foreign material present in the vicinity of the side wall of the pattern on the sample surface can be inspected, the optical axis and the layout among the electron beam irradiation unit, sample, and projection optical unit can be easily adjusted, and the optical performance of the projection optical unit can also be improved.
The electron beam deflection unit may receive the electron beam from the electron beam irradiation unit between an angle of 10° to 40° from the axis running perpendicular to the sample, and may change the angle of the electron beam to make the electron beam be incident on the sample at 90°±5°.
The cross section of the electron beam from the electron beam irradiation unit may be linear, rectangular, or elliptic in shape. Electron beams of such shapes can improve inspection precision since they can obtain a high current density.
The difference between the incident angle of the electron beam irradiated by the electron beam irradiation unit and deflected by the electron beam deflection unit, and the capturing angle of the secondary and reflected electrons produced from the sample by the mapping projection optical unit via the electron beam deflection unit preferably falls within the range of −5° to +5°.
The electron beam deflection unit may comprise a means for deflecting the electron beam by forming a field in which an electric field and a magnetic field intersect each other on a plane perpendicular to an optical axis of the projection optical unit.
The electron beam irradiation unit may have an electron optical lens system including one or two or more multi-pole lenses.
The electron optical lens system of the electron beam irradiation unit may include quadrupole lenses.
Alternatively, the electron beam irradiation unit may be placed at a position obliquely above the sample surface.
An inspection method
Miyoshi Motosuke
Nagai Takamitsu
Yamazaki Yuichiro
Anderson Bruce C.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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