Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2001-03-23
2004-11-09
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S306000, C250S307000, C250S311000
Reexamination Certificate
active
06815677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a scanning electron microscope (SEM) device, and, more particularly, to a critical dimension scanning electron microscope (CD-SEM) device and a method of measuring a pattern size obtained by the CD-SEM device.
2. Description of the Related Art
For today's ultra-fine semiconductor devices, a design rule of 0.1 &mgr;m or smaller is being used. To produce such ultra-fine semiconductor devices, it is necessary to form a pattern having a line width of 0.1 &mgr;m or smaller.
By a conventional method of measuring a pattern width by a CD-SEM, an electron scanning region (or a field of view) is scanned with electrons, and secondary electrons released from the sample to be measured are subjected to brightness conversion by the scintillator. The converted amount of the secondary electrons is displayed. In a CD-SEM, the brightness level is used for obtaining image data and line-width data.
FIG. 1
is a flowchart showing a conventional method of changing the magnification and measuring the pattern size by a CD-SEM, and
FIGS. 2A
to
2
C illustrate the method shown in FIG.
1
.
As shown in
FIG. 1
, a magnification of 50K (K is 10
3
) is selected in step S
1
. Here, the scanning range of an electron beam is indicated by a region
1
shown in FIG.
2
A. This region
1
includes a pitch line pattern
2
formed on a device chip (a sample). In step S
2
, the scanning range is irradiated with the electron beam. In step S
3
, secondary electrons released from the sample are sent to a scintillator.
In step S
4
, based on a signal generated by brightness conversion performed by the scintillator, an image that reproduces the region
1
obtained by the CD-SEM device at the 50K magnification is outputted to a CRT. In step S
5
, the line width (the measured value) of the pitch line pattern
2
measured by the CD-SEM device at the 50K magnification is outputted to the CRT.
If desired information cannot be obtained from the image outputted to the CRT in step S
4
, the magnification is increased. For instance, a magnification of 100K is selected in step S
6
. As shown in
FIGS. 2A and 2B
, the electron beam scanning range is indicated by a region
3
. In step S
7
, the scanning range constituted by the region
3
is irradiated with an electron beam. In step S
8
, secondary electrons released from the pitch line pattern
2
are captured by the scintillator.
In step S
9
, based on a signal generated by brightness conversion performed by the scintillator, an image that reproduces the region
3
obtained by the CD-SEM device at the 100K magnification is outputted to a CRT. In step S
10
, the line width (the measured value) of the pitch line pattern
2
measured by the CD-SEM device at the 100K magnification is outputted to the CRT.
If desired information is not obtained from the image outputted to the CRT in step S
9
, the magnification is further increased. For instance, a magnification of 150K is selected in step S
11
. As shown in
FIGS. 2B and 2C
, the electron beam scanning range is indicated by a region
5
. In step S
12
, the scanning range constituted by the region
5
is irradiated with an electron beam. In step S
13
, secondary electrons released from the pitch line pattern
2
are supplied to the scintillator. In step S
14
, based on a signal generated by brightness conversion performed by the scintillator, an image that reproduces the region
5
obtained by the CD-SEM device at the 150K magnification is outputted to the CRT. In step S
15
, the line width (the measured value) of the pitch line pattern
2
measured by the CD-SEM device at the 150K magnification is outputted, and the operation then comes to an end.
In the above operation, the timing of the scintillator capturing the secondary electrons forms uniform intervals. For instance, the capturing intervals may be 10
−7
(sec). With the electron irradiation amount per unit area in the region
1
at the 50K magnification shown in
FIG. 2A
being normalized to 1.0, the electron irradiation amount per unit area in the region
3
at the 100K magnification shown in
FIG. 2B
is 4.0, and the electron irradiation amount per unit area in the region
5
at the 150K magnification shown in
FIG. 2C
is 9.0.
As described above, as semiconductor chips have rapidly become smaller, there is an increasing need to produce a CD-SEM having a high magnification so as to measure the line widths of an ultra-fine pattern. With a higher magnification, a resolution per pixel becomes higher, and a high-precision SEM image and pattern measurement can be carried out.
However, the above method causes problems that hinder high-precision measurement of pattern widths. More specifically, to increase a measuring magnification, the area of an electron beam irradiation region or a field of view (FOV) is reduced. If the FOV is small while the magnification is high, the quantity of electron irradiation per unit area on the surface of the sample increases, and a large amount of electrons is applied to a small area. As a result, the influence from charges and contamination becomes greater at a high magnification. For instance, there will be problems that the contrast varies to make the object (a measured pattern) look darker, and that the line widths in a reproduced image are different from the original.
The influence from charging can be eliminated by reducing the current density at a high modification. With a reduced current density, however, there will be other problems, such as focus deviation of a scanning electron beam and a decrease in S/N ratio of an obtained brightness signal. As for the contamination, a material such as amorphous carbon in the chamber is polymerized with incident grains, and accumulates on the sample. To reduce the adverse influence from the contamination, the vacuum degree in the chamber is increased, and a technique, such as a cold trap technique, for collecting amorphous carbon, has been employed, but no sufficient results have been reported to this date.
As described so far, with the conventional techniques, it is difficult to obtain high-precision SEM images and to perform accurate line width measurement, because of the strong influence from the charging and contamination at a high magnification.
Japanese Laid-Open Patent Application No. 10-213427 discloses a method of measuring a size of a circuit pattern for a short period of time by reducing damage and charges applied on the circuit pattern on the wafer. However, this method is utilized for scanning a smallest possible area for measuring the edges of a circuit pattern. As the magnification becomes higher, the quantity of electron irradiation per unit area becomes larger. As a result, the problem of deterioration in measurement accuracy remains. Also, the entire area is subjected to electron irradiation so as to determine a scanning range, resulting in contamination (or carbonization) on the scanned area.
SUMMARY OF THE INVENTION
A general object of the present invention is to provide scanning electron microscopes in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a scanning electron microscope that can constantly obtain a high-precision SEM image and a pattern width measurement value, without damaging an object to be measured even at a high magnification, and a method of measuring a pattern size using the scanning electron microscope.
The above objects of the present invention are achieved by a method of controlling a scanning electron microscope, the method comprising the steps of: irradiating an object with an electron beam; and detecting electrons generated from the object due to the irradiation, at a frequency depending on a magnification for observing the object.
By this method, a desired image can be obtained regardless of the detection magnification.
The above objects of the present invention are also achieved by a method of controlling a scanning electron microscope, the method comprising the steps of:
irradiating a surfac
Ikeda Takahiro
Nagai Kouichi
Armstrong Kratz Quintos Hanson & Brooks, LLP
Fujitsu Limited
Gurzo Paul M.
Lee John R.
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