Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2003-01-06
2004-01-27
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S306000, C250S307000, C250S311000, C250S3960ML, C250S492300
Reexamination Certificate
active
06683308
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for measuring a thickness of a thin film, using an electron beam.
2. Description of the Related Art
Hitherto, for example, JP-A-H06-273297 discloses a method wherein, upon forming a sample into a thin film by ion beam irradiation, an electron beam is applied to the sample simultaneously with an ion beam so as to prevent excessive etching thereof by detecting the electron beam transmitted through the sample with a Faraday cup.
Similarly, JP-A-H08-5528 discloses a method wherein, when an ion beam processing apparatus is used for preparing a sample for a transmission electron microscope, the ion beam processing amount is controlled by applying an electron beam to a processing portion to detect a current amount of the electron beam transmitted through the processing portion.
According to the methods described in JP-A-H06-273297 and JP-A-H08-5528, however, inasmuch as the amount of the electron beam transmitted through the sample is measured, it is necessary that the sample subjected to measurement be processed to be thin for allowing the electron beam to pass therethrough. Therefore, it has been difficult to measure a thickness of a thin film formed on a support substrate like in case of a normal semiconductor device.
Further, as described in, for example, JP-A-S63-9807, there has been known a method wherein an electron beam is applied to a thin film to collect secondary electrons emitted from the inside of the thin film and, based on a correlation between an amount of collected secondary electrons and a thickness of a thin film, the thickness of the thin film on a substrate is measured. In this method, however, it has been difficult to precisely collect secondary electrons emitted from a thin film formed at the bottom of a hole with a high aspect ratio.
For solving the foregoing problems of the prior art, the present inventor has succeeded in developing a technique wherein the value of substrate current that flows in a substrate upon applying an electron beam to a thin film formed on the substrate is measured, and the thickness of the thin film is calculated based on reference data, and has filed a patent application (JP-A-2000-180143). In this method, since the substrate current value is measured directly from the substrate, i.e. not measuring the amount of the electron beam transmitted through the sample, it is possible to measure even the thickness of a thin film formed on the substrate.
FIG. 1
is a block diagram showing a film thickness measuring apparatus disclosed in JP-A-2000-180143. This apparatus comprises an electron gun
3
for radiating an electron beam to a thin film
2
on a substrate
1
, an electrode
4
disposed in contact with the underside of the substrate
1
, and a current measuring section
5
for measuring the value of substrate current collected to the electrode
4
. The current measured at the current measuring section
5
is adjusted through a current amplifier
6
and a differential amplifier
7
and converted into a digital signal by an A/D converter
9
. The film thickness measuring apparatus further comprises a measured current storing section
10
for storing a measured current value converted into a digital signal, an analytical curve data storing section
11
for storing analytical curve data measured using available standard samples, and an analytical curve data comparing section
12
for comparing the analytical curve data and the measured current value.
According to the film thickness measuring apparatus thus configured, there is an effect that the thickness of a thin film, particularly an extremely thin film, can be accurately measured.
The invention of JP-A-2000-180143 employs the following principle. When an electron beam with low energy ranging approximately from several hundred eV to several keV is applied to a sample, secondary electrons are emitted from the neighborhood of the surface of the sample. In general, the secondary electron emission capability of conductors or semiconductors is small, while that of insulators is large. For example, the secondary electron emission capability of silicon being a semiconductor is approximately 0.9, while that of a silicon oxide film being an insulator is approximately 2.
Accordingly, when an electron beam is applied to a semiconductor device with a silicon oxide thin film formed on the surface of a silicon substrate, more secondary electrons are emitted from the silicon oxide film. In this event, electrons flow out from the silicon substrate into the silicon oxide film for compensating for the secondary electrons emitted from the silicon oxide film. That is, the substrate current that is the sum of a current generated by the applied electron beam and a compensation current in the direction opposite to the direction of the generated current flows in the silicon substrate.
FIGS. 2A and 2B
are an exemplary diagram showing this principle. As shown in
FIG. 2A
, in case a silicon oxide thin film is formed on a silicon substrate, when one electron is applied thereto by means of an electron beam, two electrons are emitted from the silicon oxide film as secondary electrons. This results in that one electron is lost from the silicon oxide film, so that one electron flows out from the silicon substrate into the silicon oxide film for compensating for the lost electron. In this event, a substrate current in the direction opposite to the direction of a current generated by the electron beam flows in the silicon substrate.
On the other hand, as shown in
FIG. 2B
, in case no silicon oxide film is formed on a silicon substrate, when one electron is applied thereto by means of an electron beam, 0.9 electron is emitted from the silicon substrate as secondary electron. As a result, a substrate current corresponding to an amount obtained by subtracting an emitted electron amount from an applied electron amount flows in the silicon substrate in the direction of a current generated by the electron beam.
As described above, when the silicon oxide film is not formed on the silicon substrate, the secondary electron emission amount is small and thus the current generated by the electron beam is dominant, while, as the thickness of the silicon oxide film increases, the compensation current increases. Therefore, by deriving in advance reference data showing a correlation between film thicknesses and substrate current values with respect to standard samples and comparing a measured substrate current value with the reference data, the thickness of a thin film can be calculated.
However, if the film thickness measuring method described in JP-A-2000-180143 is applied to the measurement of a thickness of a thin film formed at the bottom of a hole with a high aspect ratio, a portion of secondary electrons emitted from the thin film is accumulated on the wall of the hole as shown in FIG.
3
. As a result, an electric field is generated in the hole due to the secondary electrons accumulated on the wall thereof. It has been found out that further emission of secondary electrons from the surface of the thin film is suppressed due to an influence of this electric field, so that even if the thicknesses are equal to each other, substrate current values differ from each other between a thin film formed on the flat surface and a thin film formed at the bottom of the hole. Specifically, the amount of the secondary electrons emitted from the thin film formed at the bottom of the hole is reduced and thus the compensation current is resultantly reduced, therefore, there arises a problem wherein assuming that the direction of the current generated by the electron beam is a positive direction, the substrate current value is deviated in the positive direction as compared with the flat surface.
The foregoing problem is peculiar to the case where the method described in JP-A-2000-180143 is employed in measuring the thickness of not only a thin film formed on a flat surface, but also a thin film formed at the bottom of a hole with a high
Itagaki Yosuke
Ushiki Takeo
Yamada Keizo
Fab Solutions, Inc.
Foley & Lardner
Lee John R.
Souw Bernard
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