Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2000-01-03
2003-10-21
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S311000, C250S390060, C250S397000, C250S398000, C250S442110, C250S492100, C250S492200
Reexamination Certificate
active
06635873
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a scanning electron microscope for scanning an electron beam on the surface of an observation sample such as an IC, detecting a secondary signal generated from the sample, whereby obtaining a two-dimensional scanned image indicating the shape or composition of the sample surface.
The scanning electron microscope accelerates electrons emitted from a heating or field emission type electron source, forms a fine electron beam (primary electron beam) using an electrostatic field lens or a magnetic field lens, scans the primary electron beam two-dimensionally on a sample to be observed, detects a secondary signal, such as secondary electrons or reflected electrons, generated secondarily from the sample by irradiation of the primary electron beam, and converts the intensity of the detected signal to an brightness modulation input of the CRT scanned in synchronization with scanning of the primary electron beam, whereby obtains a two-dimensional scanned image.
A general scanning electron microscope accelerates electrons emitted from an electron source with a negative voltage applied between the electron source and the anode at the grounding voltage and scans the primary electron beam on a test sample at the grounding voltage.
When observing the processing shape of a wafer in the semiconductor process by the scanning electron microscope, to prevent the insulator within the wafer from charging by electron scanning, the shape is observed at a low acceleration voltage of 2 kV or less. This relates to the secondary electron generation efficiency &dgr; generated when electrons are irradiated to a substance. In this case, the secondary electron generation efficiency &dgr; is defined by [(secondary electron amount)/(primary electron amount)].
FIG. 1
shows the relationship between the secondary electron generation efficiency &dgr; and the acceleration voltage. When the acceleration voltage (1 kV to 2 kV) when the secondary electron generation efficiency &dgr; becomes almost 1 is selected, electrons entering a sample (incident electrons) and electrons (secondary electrons) coming out from the sample are equal in number and hence generation of charging can be prevented. The acceleration voltage when the secondary electron generation efficiency &dgr; becomes 1.0 is almost 1 to 2 kV though it varies with a substance. At the acceleration voltage when the secondary electron generation efficiency &dgr; is more than 1, discharge of secondary electrons is stronger than incidence of primary electrons, so that the surface of the insulator is positively charged. This positively charged voltage is several volts at most and stable, so that observation of scanned images provides no trouble. However, within the range from 1 kV to 2 kV, the secondary electron generation efficiency &dgr; may not be 1 or more depending on a sample. As a result, an unstable negative charge is generated. Therefore, in the case of observation of a wafer including an insulator by a conventional scanning electron microscope, an acceleration voltage within the range from 500 V to 1000 V which allows the secondary electron generation efficiency &dgr; to exceed 1.0 and sufficiently accelerates the electron beam is selected.
A semiconductor wafer is observed under such a condition, though a big problem in terms of practical use is observation of a deep contact hole.
A contact hole
102
is used to electrically connect a conductive board
103
to wires (not shown in the drawing) formed on the top of an insulator
101
. The object of observation of the contact hole is to check the opening of the hole
102
for etching the insulator
101
. Unless the conductive board
103
is exposed surely on the bottom of the contact hole
102
, even if the contact hole
102
is filled with a metal (deposition), it is a bad conductor which cannot connect with the conductive board
103
.
The contact hole is observed by displaying the condition of the contact hole
102
formed in the insulator
101
provided on the board
103
on the display screen of the scanning electron microscope. As shown in
FIG. 14A
, when the contact hole
102
perfectly reaches the board
103
, the bottom of the contact hole
102
clearly shows the shape of a bottom B and it is observed that the good contact hole
102
is formed. However, as shown in
FIG. 14B
, when the contact hole
102
does not perfectly reach the board
103
and there are residues on the bottom, the bottom of the contact hole
102
is observed as a shadow
102
B.
In such an observation, as shown in
FIG. 2
, a large part of secondary electrons
104
generated at the bottom of the contact hole
102
collides with the wall of the hole
102
and disappears and only a part of secondary electrons
104
a
emitted upward gets out of the hole. When the contact hole is shallow (the aspect ratio <1 to 2), although signals are reduced, a considerable part of secondary electrons gets out of the hole
102
, so that the hole can be observed. However, when refinement advances like recent semiconductor devices and the aspect ratio is more than 3, it is impossible to observe the bottom of the contact hole. As an example of difficult observation by such a conventional scanning electron microscope as shown in U.S. Pat. No. 5,412, 209, for example, a sample example shown in
FIG. 3
may be cited.
When a sample that metal wires
105
, for example, aluminum wires embedded in the insulator
101
as shown in
FIGS. 3A and 3B
are observed at a low acceleration voltage causing no charging, as mentioned already, the surface of the simulator is charged positively and stably by balancing of secondary electrons. Therefore, even if the internal wires
105
are provided, the scanning electron microscope cannot observe the existence thereof.
SUMMARY OF THE INVENTION
The present invention has been developed to eliminate the difficulties of the prior arts mentioned above and is intended to provide a scanning electron microscope for observing the bottom of a contact hole formed on an observation sample such as an IC and internal wires.
The object of the present invention can be accomplished by scanning a sample at the predetermined acceleration voltage before starting observation of the sample, giving the surface charge desirable for observation to the insulator surface of the sample, and then scanning and observing the charged surface of the sample at an acceleration voltage different from the aforementioned acceleration voltage.
Furthermore, the object of the present invention is accomplished by positively charging the insulator surface of the sample which is desirable for observation and then scanning and observing the surface of the positively charged sample at an acceleration voltage different from the aforementioned acceleration voltage. Concretely, the object of the present invention is accomplished when the first acceleration voltage is within the range from 500 V to 1 kV and the second acceleration voltage at the time of image observation is within the range from 1 to 2 kV.
Furthermore, the object of the present invention is accomplished by negatively charging the insulator surface of the sample which is desirable for observation and then scanning and observing the surface of the negatively charged sample at an acceleration voltage different from the aforementioned acceleration voltage. Concretely, the object of the present invention is accomplished when the first acceleration voltage is 2 kV or more and the second acceleration voltage at the time of image observation is within the range from 1 to 2 kV.
Furthermore, the object of the present invention is accomplished when the first acceleration voltage is within the range from 500 V to 1 kV and the second acceleration voltage at the time of irradiation of an electron beam is within the range from 20 to 30 V.
REFERENCES:
patent: 5412209 (1995-05-01), Otaka et al.
patent: 5491339 (1996-02-01), Mitsui et al.
patent: 5757409 (1998-05-01), Okamoto et al.
Ezumi Makoto
Todokoro Hideo
Usami Yasutsugu
Kenyon & Kenyon
Vanore David A.
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