Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-09-26
2004-05-18
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
Reexamination Certificate
active
06737658
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a pattern observation method for performing a pattern observation using a charged particle beam.
FIG. 1
illustrates a technical concept of a conventional alignment exposure using an electron beam.
As is shown in
FIG. 1
, a sample to be aligned is formed such that an insulator film
102
and a resist
103
are provided on a silicon substrate
101
. An underlying mark
104
is formed in the insulator film
102
on a surface of the silicon substrate
101
.
An electron beam
105
is made to scan the underlying mark
104
located at a deep position from the surface of the resist
103
. Reflected electrons
106
from the underlying mark
104
is detected by a detector
107
. Based on a detection signal, alignment exposure is carried out. However, when the energy of the electron beam
105
is low, the range of electrons is short. Consequently, the electrons cannot reach the underlying mark located at a deep position from the surface of the resist
103
.
In order to solve this problem, an alignment exposure method as illustrated in
FIGS. 2A
to
2
D has been proposed.
FIG. 2A
illustrates a technical concept of the alignment exposure. A sample to be aligned is the same as shown in FIG.
1
.
An electron beam
105
having a predetermined acceleration voltage is radiated on an underlying mark
104
. Thereby, a charged portion
112
created by the electron beam
105
appears on the surface of the sample. A difference between electrostatic capacitances
113
and
114
occurs between a region where the mark
104
is formed at a deep position from the surface of the sample and a region where it is not formed, due to an unevenness or a nonuniformity in material of a pattern. The difference in electrostatic capacitance causes a surface potential difference in the charged portion
112
on the surface of the sample.
The surface potential difference appears as a voltage contrast image of secondary electrons
115
at the time of the radiation of the electron beam
105
. The contrast image is detected by the detector
107
. Thus, the position of the underlying mark
104
can be detected and the alignment in the electron beam exposure can be effected.
FIG. 2B
shows a surface potential of the sample in a case where the sample is charged with positive electricity. A region where the surface potential is high corresponds to the portion at which the underlying mark
104
is formed.
FIG. 2C
shows a secondary electron waveform based on the surface potential difference of the sample in a case where the sample is charged with positive electricity. A region where the amount of secondary electrons greatly decreases corresponds to the portion at which the underlying mark
104
is formed.
If the same charging phenomenon is utilized, a misalignment measurement in a lithography step of a semiconductor fabrication process can be carried out using a scanning electron microscope (SEM).
FIGS. 3A and 3B
illustrate a technical concept of a misalignment measurement utilizing the charging phenomenon.
FIG. 3A
is a plan view of misalignment measuring marks, and
FIG. 3B
is a cross-sectional view of the misalignment measuring marks. In
FIGS. 3A and 3B
, reference numeral
151
denotes a silicon substrate,
152
a silicon nitride film,
153
a silicon oxide film,
154
an anti-reflection film,
156
a first mark formed on the underlying silicon substrate, and
157
a second mark formed of photoresist. The first mark
156
is formed by removing portions of the silicon substrate and silicon nitride film. The silicon oxide film
153
is formed over the entire surface of the substrate such that the first marks
156
are buried. A top surface of the silicon oxide film
153
is flattened by chemical mechanical polishing (CMP). A misalignment inspection is performed by scanning an electron beam across the first mark
156
and second mark
157
. A scanning path is indicated by an arrow
158
in FIG.
3
A. Thus, a signal waveform of secondary electrons having peaks near the first mark
256
and second mark
157
can be obtained.
The above method, however, has the following problem.
FIG. 2D
is a view for explaining the problem with the conventional alignment method utilizing the charging phenomenon. Specifically,
FIG. 2D
illustrates a relationship between a radiation time and a surface potential of a sample. Assume that the sample is charged with positive electricity. A solid line indicates a surface potential of a region where the underlying mark
104
is not formed, and a broken line indicates a surface potential of a region where the underlying mark
104
is formed. In order to enhance a signal-to-noise ratio (S/N) at a position of the underlying mark
104
for alignment, it is necessary to scan the beam over the underlying mark
104
several times and to average and add the detection signals.
However, the aforementioned phenomenon utilizing the charging is a temporally transient one, as shown in FIG.
2
D. When the radiation time is divided into time periods t
1
to t
3
, a sufficient surface potential difference is obtained in radiation time period t
2
and a mark image with full contrast can be observed in this radiation time period.
In radiation time period t
3
, the charge is excessively high. As a result, only a small surface potential difference is obtained, and a mark image becomes difficult to observe. By contrast, in radiation time period t
1
, the amount of the radiation beam is small and the charging phenomenon itself will hardly occur. In this time period, it is difficult to observe the mark image. On the other hand, if the beam current for observing the mark image is too high, excessive charging occurs in a short time and the length of the time period t
2
in which the mark image can be observed is decreased. If the beam current for observing the mark image is too low, the length of the time period t
1
in which the mark image cannot be observed is increased and quick observation of the mark image cannot be carried out.
The optimal condition for mark detection varies depending on the thickness and kind of the insulator film formed over the underlying mark
104
. However, as is understood from the above-described problem, it is difficult, in fact, to set the condition for image observation. The same problem as with the alignment exposure also arises in the misalignment measurement as illustrated in
FIGS. 3A and 3B
.
Another problem with the alignment exposure will now be described.
In usual cases, when alignment exposure is performed, an electron beam is scanned in a single direction to detect the mark on the sample. When the electron beam is scanned, all secondary electrons produced by the radiated electron beam do not enter the detector. In addition, where the surface of the sample has been charged with the electron beam radiated immediately before, such a phenomenon occurs that the secondary electrons re-enter, in particular, the surface of the sample. If the detected secondary-electron image is observed, a dark portion appears on a peripheral portion of the pattern. This is due to the re-entrance of secondary electrons.
If this problem is studied in greater detail, it is understood that the secondary electrons re-entering the surface of the sample travel asymmetrically. Specifically, if an electron beam is scanned in a single direction, the electron beam radiated immediately before charges the surface of the sample negatively, on which the electron beam has been radiated immediately before. On the other hand, the surface of the sample, which has not yet been scanned by the electron beam, is less charged. If this phenomenon is left as it is, the detected secondary-electron signal waveform becomes asymmetric, and a read error of the mark position will occur. Of course, this problem applies to the misalignment measurement.
As has been described above, in the conventional alignment method for the electron beam exposure, the mark located at a deep position from the surface of the resist can be made detectable by utilizing the charging phenome
Koike Toru
Nakasugi Tetsuro
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Johnston Phillip A
Lee John R.
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