Effect of electron dose density on silicon nitride compared between two different atomic column elemental maps by
2D
moiré and conventional methods
Authors: Kondo Yukihito , Okunishi Eiji 
Published: December 2016

A challenge to expand a variety of samples in atomic column elemental mapping [1] is to overcome the sample damage caused by electron beam irradiation, since the signal of this method is smaller than that of imaging, due to small probability to generate the signals for elemental identification, such as energy loss electrons or X‐ray emission.On the other hands, scanning transmission microscopy (STEM) moiré method is useful for strain analysis in semiconductor industry [2], since the method allows us to observe larger field view than conventional geometrical phase analysis utilizing real lattice fringes. Concurrently, the electron dose density is greatly reduced in STEM moiré method, since a pixel interval to form the pseudo lattice fringe is sparser than that to form a direct lattice.

We have tried to apply the 2D STEM moiré method to obtain a pseudo atomic column map [3]. In the 2D STEM moiré method, the electron density is reduced to be < 1 % of one used for the conventional method. With a flat area of a sample, the pseudo image map show identical intensity distribution and chemical information compare to the conventional elemental map, because the electron probe profiles identical positions from each atomic sites in both methods, The pixel are intervals

d
rmoire

=

d
lattice

+

Δ

moire

for the moiré method and

d
rdirect

=

Δ

direct

for the conventional method when
Δ

moiré

is similar to
Δ

direct

, where

d
rmoire

,

d
rdirect

and

d
lattice

are a pixel interval in moiré method, a pixel interval in conventional method and the width of unit cell of a sample. It is noted that
Δ

moire

is identical to
Δ

direct

in the two methods. Figure 1 show the scheme of the relation. In this paper, we compare the effect of average electron dose density on atomic elemental column maps obtained by the two methods.

For the experiment, we used an aberration corrected microscope (JEOL, JEM‐ARM200F) equipped with a cold field emission gun and dual SDD X‐ray detection system, whose total solid angle is 1.73 sr. The results in this paper were obtained under the conditions: acc. Volt. = 120 kV, probe current = 38 pA, and probe size = 0.2 nm. The sample we selected to be Si
3
N
4
, which is more of a robust sample for electron beam irradiation, since the sample was more likely to survive under beam irradiation. The crystallographic parameters for the sample are listed as

a

=

b

= 0.7617 nm and
c
= 0.291 nm. The sample thickness was estimated to be 45 nm judging from the X‐ray emission rate.

Figure 2 shows the results taken by the 2D moiré and conventional methods. The results were accumulated over 508 cyclic acquisitions. The dwell time for a pixel in both methods was 1 ms. The peaks in the Si map coincident with the atomic sites of the sample crystal structure in both methods. However, the peaks in the N map do not show the atomic site configuration of the crystal structure in the map by direct method, especially on the N‐Si hexagonal ring. Figure 3(a) and (b) show intensity profiles of Si, N and HAADF image signals on the maps by the two methods. While the peaks in the profile on the map by the 2D moiré method shown in Fig. 3(a) correlate well with atomic sites of N and Si. However, those in profile on the map by conventional method do not correlate with atomic sites of N. The difference between these elemental maps comes from the dose density. The dose densities for maps by moiré method is derived to be 4.79 × 10
11
electrons/nm
2
from the measured

n
a

and

n
b

, where

n
a

and

n
b

are numbers of pixels inside the mignified unit cell in the 2D moiré HAADF image. The dose density for the map by the direct method is simply calculated to be 1.62 × 10
15
electrons /nm
2
from the experimental conditions. The reduced rate of the dose density by 2D moiré method was one three‐thousandth to the direct method.

In conclusion, the results clearly show the effect of dose density on sample in atomic resolution elemental mapping with reduced dose density realized by the 2D moiré method. The method can be used for analysis of beam‐sensitive samples, , which does not need high resolution but need to see the atomic sites in a unit cell such as ions in battery materials. And the method can be used to check the critical dose density for unanalyzed samples.