ISSN 10637397, Russian Microelectronics, 2014, Vol. 43, No. 5, pp. 361–370. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © Yu.A. Novikov, 2014, published in Mikroelektronika, 2014, Vol. 43, No. 5, pp. 373–383.
A scanning electron microscope (SEM) is widely
used in science and engineering. Recently, it has found
increasing application in nanotechnology for visualiz
ing the surface relief of solids and measuring linear
sizes in the nanometer (1–1000 nm) range [1–4].
Diameters of electron probes of modern serial SEMs
are usually over 10 nm [5, 6]: they overlap the ranges
10–30 nm for new microscopes and 30–100 nm for
those used for more than three years. Thereby, SEM
probe diameters are comparable with the sizes of
nanostructure elements or exceed them within almost
the entire nanotechnology range 1–100 nm. There
fore, to visualize the relief and measure the linear
sizes, it is important to know the shape of an electron
density distribution in a SEM probe. However, the
electron distribution density of modern SEMs is
unknown; moreover, the SEM probe diameter is not
specified. This is due to the lack of internationally rec
ognized techniques for measuring the parameters of
SEM electron probes.
Scanning electron microscopes can be divided into
the three following groups by the value of a primary
electron energy [7, 8]: high voltage (electron probe
keV), low voltage (
keV), and inter
mediateenergy (2 keV <
< 10 keV). The low voltage
SEMs hold a special place. For these SEMs, special
devices, CD SEMs, for measuring the critical (mini
mum) microcircuit sizes, have been developed.
For high and low voltage SEMs, the methods for
measuring the electron probe diameter were developed
[5, 6], in which test objects with a trapezoidal profile
and large slopes of side walls of relief elements are used
[7–12]. These methods were brought up to the formu
lation of the Russian State Standards [13, 14] (GOST R)
for test objects and application in scanning electron
microscopy. This allows studying more complex char
acteristics of electron probes using similar methods and
test objects. Among these characteristics, the key one is
the electron distribution density in a SEM probe.
The knowledge of this distribution is necessary to
apply SEMs in nanoelectronics for measuring linear
sizes of microcircuit elements comparable with the
electron probe diameter . In addition, the probe
defocusing technique for measuring ultrasmall sizes
in scanning electron microscopy was developed .
Obviously, the electron distribution density in the probe
changes upon defocusing. Therefore, understanding
the defocusing effect on measurements of nanostruc
ture sizes requires the knowledge of the true electron
distribution in a probe at any focusing. In addition, the
knowledge of the electron distribution in a probe is nec
essary for creation of the virtual SEM .
2. THEORY OF THE METHOD
The electron distribution density in a SEM probe is
easy to understand by an example of the low voltage
SEM. The low voltage SEMs involve microscopes
with the probe electron energy
keV [7, 8]. When
entering a solid, electrons undergo multiple scattering
. As a result, an initial probe diameter of 10–30 nm
broadens in the substance to several micrometers.
Therefore, the SEM signal is formed not only by the
primary beam but also by multiple scattered electrons.
These contributions are very difficult to separate and
have not been separated so far.
2.1. Scattering of Low Voltage SEM Probe Electrons
in the Sample
The electron free path in a substance is expressed as
λ= ρ σ
Electron Distribution Density in a Low Voltage SEM Probe
Yu. A. Novikov
A. M. Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991 Russia
Received April 10, 2013
—The method for measuring the electron distribution density in a probe of a low voltage scanning
electron microscope is proposed. It is shown that a focused probe has the Gaussian shape. A defocused probe
can be presented as several Gaussian probes of different intensities shifted relative to each other.