Scanning reflection ion microscopy in a helium ion microscope

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Reflection ion microscopy (RIM) is a technique that uses a low angle of incidence and scattered ions to form an image of the specimen surface. This paper reports on the development of the instrumentation and the analysis of the capabilities and limitations of the scanning RIM in a helium ion microscope (HIM). The reflected ions were detected by their “conversion” to secondary electrons on a platinum surface. An angle of incidence in the range 5–10° was used in the experimental setup. It was shown that the RIM image contrast was determined mostly by surface morphology but not by the atomic composition. A simple geometrical analysis of the reflection process was performed together with a Monte Carlo simulation of the angular dependence of the reflected ion yield. An interpretation of the RIM image formation and a quantification of the height of the surface steps were performed. The minimum detectable step height was found to be approximately 5 nm. RIM imaging of an insulator surface without the need for charge compensation was successfully demonstrated.

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# Scanning reflection ion microscopy in a helium ion microscope

Yuri V. Petrov *1 and Oleg F. Vyvenko 2

Full Research Paper [Open Access](http://www.beilstein-journals.org/bjnano/about/openAccess.htm)

Address:
1Interdisciplinary Resource Center for Nanotechnology,

Saint-Petersburg State University, Ulyanovskaya 1, Saint-Petersburg
198504, Russia, and 2 Faculty of Physics, Saint-Petersburg State

University, Ulyanovskaya 1, Saint-Petersburg 198504, Russia

Email:
Yuri V. Petrov * [- y.petrov@spbu.ru](mailto:y.petrov@spbu.ru)

- Corresponding author

Keywords:

helium ion microscope; low-angle ion scattering; reflection

microscopy; surface imaging; surface morphology

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

[doi:10.3762/bjnano.6.114](http://dx.doi.org/10.3762%2Fbjnano.6.114)

Received: 02 February 2015

Accepted: 15 April 2015

Published: 07 May 2015

This article is part of the Thematic Series "Helium ion microscopy".

Guest Editor: A. Gölzhäuser

License and terms: see end of document.

## Abstract

scanning RIM in a helium ion microscope (HIM). The reflected ions were detected by their “conversion” to secondary electrons on

a platinum surface. An angle of incidence in the range 5–10° was used in the experimental setup. It was shown that the RIM image

contrast was determined mostly by surface morphology but not by the atomic composition. A simple geometrical analysis of the

reflection process was performed together with a Monte Carlo simulation of the angular dependence of the reflected ion yield. An

interpretation of the RIM image formation and a quantification of the height of the surface steps were performed. The minimum

detectable step height was found to be approximately 5 nm. RIM imaging of an insulator surface without the need for charge

compensation was successfully demonstrated.

## Introduction

Reflection ion microscopy (RIM) is a technique that uses low
angle, scattered ions to form an image of the specimen surface.

This technique is similar to reflection electron microscopy

(REM) proposed by Ruska in 1933 [1] who realized REM using

an angle of 90° between the incident and reflected beams. Later,

in the 1950s, a REM technique employing lower incidence

angles [2-4] was developed to increase the sensitivity to vertical

surface irregularities. A vertical resolution of tens of nanome

ters was obtained and it was shown that the negative effect of

chromatic aberration was also reduced at low incidence angles,

yet was still more pronounced in REM as compared to TEM

[2,4]. The further development of REM in ultrahigh vacuum

conditions allowed imaging of the single atomic steps [5-8] and

monitoring of atomic layer-by-layer crystal growth by means of

reflection high energy electron diffraction (RHEED) [9]. In the

late 1960s, scanning reflection electron microscopy (SREM)

1125

was developed [10,11] for the scanning electron microscope

(SEM). Chromatic aberration does not appear in SEM because

the sample is placed outside of the electron optics. Both REM

and SREM require a sufficiently long depth of focus for high

quality imaging because of the glancing angle of incidence

geometry.

The helium ion microscope is a new scanning microscope with

a single atom ion source that was developed in 2006 [12-15].

One of the main features of this device is a large depth of focus

provided by a narrow beam divergence angle of about 0.5 mrad

[15,16], which is ten times less than the best beam divergence

angle possible in SEM. The large depth of focus makes helium

ion microscopy (HIM) a very promising tool for scanning

reflection microscopy. During the last decade the imaging capa
bilities of HIM were examined in the field of material science

[15,17-23] as well as in biology [15,16,24-26]. Different tech
niques such as secondary electron energy filtering [27], helium

ion channeling contrast [28-30], helium ion transmission

microscopy [31], secondary ion mass spectrometry [32,33], and

ionoluminescence [34,35] were developed.

In this paper we report on the development of the instrumenta
tion and the analysis of the capabilities and limitations of scan
ning reflection ion microscopy (RIM) in a helium ion micro
scope. In the experimental part of our work we describe a con
figuration of a sample holder developed for RIM and demon
strate first results obtained for standard test samples. In the

discussion part a simple theoretical treatment of the image

contrast formation in RIM is provided, which is necessary to

obtain quantitative information from RIM images.

## Experimental

All investigations were performed in a Zeiss Orion scanning

helium ion microscope. The detection of reflected ions (RIs)

requires an additional detector installed below the specimen.

One of the easiest ways to detect the reflected ions is by their

“conversion” to secondary electrons (SEs). Previously, such a

RI-to-SE conversion was used for the detection of backscat
tered electrons in SEM [36] and for scanning transmission

mode in HIM [37]. A schematic of the originally developed RI

detection system is shown in Figure 1.

The sample (1) was mounted on a stage at a grazing angle rela
tive to the focused helium ion beam. The reflected helium ions

were directed to the Pt-coated plate (2) which served as the

main RI-to-SE converter. Platinum was chosen because of its

high SE yield under He-ion excitation [12]. The SE signal was

measured by a conventional Everhart–Thornley (ET) detector

(3). The sample was surrounded by grounded metal shields (4

and 5) and was kept under a positive bias of 100 V to exclude

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

Figure 1: Schematic of the detection of reflected ions in the helium ion
microscope. 1 – sample, 2 – Pt-coated surface, 3 – secondary electron detector, 4 – SE grounded shield, 5 – additional SE shield (SE3),
6 – slit diaphragm, 7 – backscattered ion detector.

the detection of SEs coming from the sample and SEs excited

by backscattered ions from the chamber walls and objective

lens (by the so-called SE3). The shield (5) is shown as trans
parent in Figure 1 in order to make the ion beam paths visible.

The detection of SEs excited from the conductive sample

occurred when the bias was switched off, as the SEs excited

from the conductive sample exceeded the signal of the RI-to-SE

converter. For insulating samples, the SE signal from the

sample could not be detected because of positive charging as

will be described further in detail below.

The angular aperture of detected reflected ions was limited by

the slit (6) installed under specular reflection conditions. The

slit, with the width of about 1 mm, was mounted parallel to the

tilt axis of the sample stage at a distance of about 14 mm, which

corresponds to an RI detection angular aperture of 4°. Backscat
tered ions (BSI) were detected by the microchannel plate

detector (7). The width of the scanned area was less than tens of

micrometers, corresponding to the variation of the detection

angle of less than 0.1°. The incidence angle of the RIs was also

practically unchanged during the scan over the small sample

area, which assured the proportionality between the number of

SEs excited from the Pt plate and the number of RIs.

All measurements were performed in the chamber with a base
pressure in the range of 10 −7 Torr. The energy of the He ions

was 35 keV with a beam current of 0.5 pA. The compensation

of the sample charge by the electron flood gun was not possible

due to the configuration of the sample holder.

1126

The following model test samples were investigated: (1) a Au

on carbon SEM test sample from Agar scientific, (2) an AFM

standard TGQ-1 from NT-MDT: a grid of SiO2 square bars of
22 nm thickness on a silicon substrate, and (3) a mica (phlogo
pite) sample prepared by mechanical cleavage.

## Results

Comparison of RI, SE and BSI imaging
modes: Au on carbon test sample

Figure 2 represents the RI and BSI images of Au on a carbon

sample taken at the grazing angle of incidence of 5° and the

same angle for RI detection.

In Figure 2, one can easily recognize that only the central part

of the images is in focus, which is due to the tilt of the sample

with respect to the incident beam. This allows the independent

estimation of the divergence angle of the He-ion beam from the

pixel size and the depth of focus. The depth of focus is defined

as the distance between the edges of the in-focused part of the

image plane divided by the tangent of the grazing angle. From

the image in Figure 2, the depth of focus can be estimated to be

approximately 10 µm. Using digital signal processing in HIM,

the image remains sharp unless the diameter of the beam is

larger than the pixel size in the image. From the width of the

image field of view (2.5 μm) and the number of acquisition

points 1000 × 1000, the pixel size was calculated to be 2.5 nm.

These values gave the angle of the beam divergence of approxi
mately 0.5 mrad, which is in a good agreement with the manu
facturer’s data.

Figure 2 represents the images of a Au on carbon sample that is

partially covered by Au nanoparticles in the central part only.

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

As expected, the Au particles exhibit a bright contrast in the

BSI image on the dark background of the carbon substrate due

to its significantly larger atomic number. On the contrary, the

RI image does not show any noticeable signal difference

between two different materials, giving information about

surface morphology only. Additionally, the ratio of the signals

from the top and side surface of the Au particles in the BSI and

RI images are inverted. The top surface of the particles is

brighter than the side wall in the RI image, whereas in BSI

image, the situation is reversed.

Figure 3a,b represents a comparison of the SE and RI imaging

modes of Au on carbon. One can see that in both imaging

modes there is no noticeable signal level difference between the

Au nanoparticles in the central part of the image and the carbon

substrate in the upper and the lower parts of the image. This

implies that the RI yield at low ion incidence angles does not

depend on the element properties and the RI contrast in the

images originates from the surface morphology only in contrast

to the case of normal incidence. One should also note that SE

and RI images focusing on the same structural peculiarities

differ from each other. A more detailed description of the

difference will be treated in the next section.

Reflection ion microscopy of silicon dioxide
steps on silicon
Figure 4 shows images of SiO2 bars on silicon obtained by the
detection of SEs (Figure 4a) or RIs (Figure 4b) under the

grazing and detection angles of 10°. One can easily recognize

two main differences between the RI and SE images. Firstly, the

contrast between silicon and silicon dioxide is clearly distin
guishable in SE image, but not in the RI image. Note that the

Figure 2: Images of Au on carbon by detection of reflected He ions (a) and backscattered ions (b).

1127

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

Figure 3: Images of Au on carbon obtained by detection of secondary electrons (a) and reflected He ions (b).

Figure 4: Images of silicon dioxide bars on silicon (TGQ-1 sample) obtained by detection of (a) secondary electrons, (b) reflected ions. (c) Sketch of
ion beam path: green arrows correspond to the direction of the primary beam and reflected beam, the red axis corresponds to the vertical axis of the
images in (a) and (b). “Upward” and “downward” steps are marked with “u” and “d”, respectively.

silicon dioxide bars are darker than the silicon substrate in the

SE image (see Figure 4a), while there was no noticeable SE

signal difference between Au and carbon (see Figure 3a). Addi

tionally, the dark contrast of the upward steps (marked “u” in

Figure 4c) seems to be broader in the RI image than in the SE

image.

1128

The profiles of the SE signal and RI signal measured across the

silicon dioxide bar are presented in Figure 5a. The profiles are

measured along the beam projection, and the profiling path is

shown by the red line in Figure 5b. The edge of upward step

and the edge of downward step are marked with “u” and “d”,

respectively. The comparison of the SE profile (dashed line in

Figure 5a) and RI profile (solid line in Figure 5a) revealed that

the dark area in the RI image of the upward step was broader

than the dark area in the SE image for the same step. The width

of the bright area in the RI image of the downward step was

found to be equal to the width of this area in the SE image.

Reflection ion microscopy of cleaved mica

Figure 6 shows an RI image of a cleaved mica surface obtained

at a magnification of 100,000× under the grazing angle of 5°

without charge compensation or conductive coatings. One can

see that all details of the step-like surface are well reproduced

without any noticeable image distortion that might be expected

due to surface charging. Some distortion of the image caused by

the charging appeared only at significantly higher magnifica
tion. It should be noted that the overall detector signal of regis
tered electrons was not changed by the variation of the sample

bias, indicating that SEs excited immediately near the sample

could not reach the detector because of sample surface

charging.

The step of a minimum height in the field of view of the image

in Figure 6 is marked with arrows. The value of the height, as

retrieved from the image processing, was found to be 7 ± 2 nm.

The details of the calculation procedure will be described in the

next section.

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

Figure 6: RIM image of cleaved mica. White arrows shows mark the
smallest step in this image. Accelerating voltage: 30 keV, beam
current: 0.3 pA.

## Discussion

Contrast formation in scanning reflection ion
microscopy

The results presented in the previous sections showed that

imaging using an incident ion beam at low grazing angles

exhibits following properties:

- RI detection is insensitive to the atomic number or

density of the material;

Figure 5: (a) Signal profiles: secondary electrons (dashed line) and reflected ions (solid line), red arrows shows the position of upward (u) and downward (d) steps. (b) Schematic view of ion reflection, green arrows correspond to the direction of the primary beam and reflected beam, the red line
corresponds to the profile path, and the red arrows show the position of upward (u) and downward (d) steps.

1129

- there is a difference between SE and RI imaging of

upward steps but not for the downward case;

- no distortion caused by charging effects is observed in an

RI image for insulators.

An explanation for these RIM properties and an analysis of the

advantages and limitations of RIM require a theoretical descrip
tion of the ion reflection and detection processes. In this part of

the paper such a description will be provided combining a

simple geometrical modeling and ion–matter interaction simula
tions.

The following designations are used to describe the incident and

reflection paths as shown in Figure 7: Θ0 - grazing angle
between the incident beam and the specimen plane, Θ1 - angle
between the incident beam and a local detail of the specimen

surface, Θ2 - angle between the reflected beam and the specimen plane, ΔΘ - angular aperture of RI detection, δΘ
halfwidth of the angular divergence of RI, φ1, φ2 - polar angles
of the incident and of the reflected beams, respectively, in the

specimen plane (not shown in Figure 7). To describe the surface

morphology we will use the angle α between the specimen

plane and the detail of the specimen surface roughness under

consideration. From this geometry, Θ1 = Θ0 + α. We will
neglect the incident beam divergence angle since it is much

smaller than both the angular aperture of the RI detector

diaphragm and the halfwidth of the angular distribution of the

RIs. Thus, we will further consider the incident beam as being

infinitely narrow.

Figure 7: A schematic diagram of the incident and reflected ion paths
with designations given in the text.

The number of reflected ions per incident ion is called the

reflection coefficient or reflected ion yield. In the simple

geometrical model, the reflection coefficient,η, depends

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

on the angle of incidence and the reflection only, that is,

η = η(Θ1,φ1,Θ2,φ2).

Firstly, we assume that the reflection coefficient is independent

of the polar incident angle, φ1, that is, η = η(Θ1,φ1,Θ2,φ2) =
η(Θ1,Θ2,φ2). This assumption is valid for amorphous materials
and in the absence of ion channeling in crystalline materials.

Ion channeling can cause the reflection coefficient to become

strongly dependent on the angle in the vicinity of the chan
neling critical angles [28,30] and can be eliminated by a proper

choice of the angle between the incident beam and crystal

atomic planes [38].

The RI signal is the number of SEs per second measured by the

ET detector as the flux of SEs from the Pt RI–SE converter (see

(2) in Figure 1). The total number of reflected ions is propor
tional to the number of primary ions, _N_ I, and the proportionality constant is the reflection coefficient, η(Θ1,Θ2,φ2). In fact,
only those reflected ions that are transmitted through the slit

aperture can reach the Pt-coated surface and excite the SEs. The

portion of the RIs that reach the Pt converter is a function of the

ion reflection angles _f_ (Θ2,φ2). The number of SEs excited from
th Pt surface is proportional to the number of ions reflected

within the aperture, and the proportionality constant is the sec
ondary electron yield for the Pt-coated surface, γ0. The SE yield
depends on the angle of incidence and is determined by Θ2,
thus, γ0 = γ0(Θ2). Thus, the detected RI signal should be calculated as the product of the number of reflected ions and the effi
ciency of SE detection, while integrating over all angles as

shown in Equation 1.

In the case of the slit-like aperture installed at a fixed angle, Θ2,
one can assume that the portion of RIs that reaches the Pt

converter is independent of the polar angle, that is, _f_ (Θ2,φ2) =
_f_ (Θ2). This assumption gives rise to the explicit form of the
_f_ (Θ2) dependence: _f_ (Θ2) = 1 if Θ0−ΔΘ/2 < Θ2 < Θ0+ΔΘ/2,
otherwise _f_ (Θ2) = 0.

The SE yield from the Pt surface depends on the ion angle of

incidence with this surface, β, and can be approximated by the

inverse cosine law [19]: γ(β) = γ0/cosβ. The typical ion grazing
angle of incidence in our work was 5° and the slit angular aper
ture was 4°. In this case, β can vary between 6° and 14°. The

cosine function varies only within 2% for this angular range and

we can neglect dependence of the SE yield on the ion reflection

angle, thus, γ0(Θ2) = γ0. We factor out the integral sign for the
number of primary ions and SE yield and integrate Equation 1

over Θ1 and φ2 to arrive at the expression:

(1)

1130

(2)

According to this expression, the RI signal is determined by the

ion reflection coefficient and its angular dependence, the

angular distribution of the reflected ions and the angular aper
ture of the slit diaphragm. We will now consider the effect of all

these factors separately.

Angular dependence of the ion reflection
coefficient

The dependence of the reflection coefficient for singly charged

He ions on the grazing angle as obtained by a Monte Carlo

simulation with SRIM software [39] for different materials is

depicted in Figure 8. Note that the SRIM Monte Carlo simula
tion assumes that the ions are reflected from an amorphous

target and all of them are collected by a detector.

Figure 8: Dependence of the reflection coefficient of 35 keV He + on
the grazing angle calculated with SRIM software for different materials.

As it can be seen in Figure 8, the ion reflection coefficient is a

monotonically decreasing function that tends to the yield of

backscattered ions when the grazing angle is greater than 90°.

As Θ1 approaches zero, the reflection coefficient tends towards
unity, implying that for reflection at very low angles nearly all

ions will pass over the specimen surface. The highest relative

difference in the reflection coefficient for the different ma
terials (i.e., material contrast) is observed at normal incidence,

whereas this difference reduces for low grazing angles and the

material contrast vanishes. At the same time, the slope of

angular dependence on the reflection coefficient is high at low

grazing angles and low for normal incidence. This explains why

the morphology contrast in the RI images is determined mostly

by the surface morphology but not by the surface composition

in our experiments.

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

In fact, the RI contrast is determined by the angular depend
ence of the reflection coefficient only when all of the reflected

ions are detected. In turn, this assumption is valid when the

halfwidth of the angular aperture is larger than the halfwidth of

the RI angular distribution, that is, ΔΘ > δΘ, and the sample

surface is sufficiently smooth so that |Θ1−Θ0| = α < ΔΘ. Under
these conditions the expression for the RI signal given by

Equation 2 can be simplified as:

(3)

(4)

where η0(Θ1) is the RI reflection coefficient as a function of the
grazing angle.

Effect of angular distribution of reflected ions

As indicated above, the signal of the reflected ions is deter
mined by the angle between surface and primary ion beam if all

reflected ions are detected. In practice, some part of the

reflected ions may be stopped by the diaphragm or by the

elements of the specimen surface (shadowing effect). Firstly,

we suppose that the angular distribution of the reflected ions
η(Θ1,Θ2) has a maximum at an angle of Θ2 0 that corresponds to
specular reflection: Θ2 0 = Θ2 0 + 2α and the shape of the angular
distribution of reflected ions is independent of the Θ1 angle.

The reflection coefficient can be presented as two multiplicands: η(Θ1,Θ2) = η0(Θ1) _g_ (Θ2−Θ2 0 ), where η(Θ1) is described
by Equation 3 and _g_ (Θ2–Θ2 0 ) is the normalized angular distribution of the reflected ions, centered at Θ2 0 . Substituting this
reflection coefficient into Equation 2:

(5)

Using the notation _t_ = Θ2−Θ2 0 we obtain:

(6)

and variations of α can be described now as a shift of the inte
grating window. In particular, the comparison of Equation 4 and

Equation 6 shows that the RI signal will decrease when α > ΔΘ.

Further, a suggestion for an explicit analytical shape of _g_ ( _t_ ) is

required for the calculation of the RI signal profile with the help

1131

of Equation 6. If α > δΘ/2 + ΔΘ/4, then we can assume that the

signal tends to zero because we integrate Equation 6 far from a

maximum of _g_ ( _t_ ), that is, all ions are stopped by the diaphragm.

Hence, such regions of the sample will appear in dark contrast

in the RI image. Additionally, double (or multiple) reflections

can take place giving rise to a “shadow effect”. This so-called

shadow effect means that after the first reflection from the

surface with low α, the ions are iteratively reflected from some

pronounced details of the specimen and do not contribute to the

signal. The simplest example of the shadow effect is the rectan
gular upward step that was investigated for the SiO2 sample
bars on silicon (Figure 4).

To further simplify the description, we assume that the surface

between the steps is flat and parallel to the specimen plane (i.e.,

α = 0) and the step sidewall is perpendicular to the surface (i.e.,

α = 90°). The area at the step can be then divided into three

regions, as shown in Figure 9.

Figure 9: Designation of the regions of the upward step (a), and their
projection onto the image plane (b): 1 – sidewall region, 2 – shadow
region, 3 – penumbra region.

In region 1 in Figure 9 all ions are reflected by the step side
wall away from the aperture and do not contribute to the

detected signal, _S_ (α) = 0. In this case, a dark area of width

_d_ 1 = _h_ cosΘ0 appears in the image, where _h_ is the step height.

In region 2, all ions reflected from a plane bottom sample

surface will be scattered by the sidewall, and an additional dark

area (shadow) appears in the image (see Figure 9). If we denote

the distance from step along the substrate as _x_, then the ions can

pass through aperture when: Θ2 = arctan( _h_ / _x_ ). In this case, the
detected signal is described as follows:

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

(7)

The width of the shadowed region 2, as calculated from simple

geometry, is given by _d_ 2 = _h_ ∙sinΘ0/tan(Θ0 + ΔΘ).

In region 3 a smooth transition (penumbra) from zero RI signal

to a constant RI signal from the substrate takes place. The form

of the transition curve is determined by the RI angular distribu
tion according to:

(8)

The total width of the dark area in the image of the step will

then be a sum of the widths of regions 1 and 2:

(9)

Thus, the step height can be calculated from the dark contrast

width in the RI image and the halfwidth of the slit aperture as

follows:

(10)

Using Equation 10 we calculated the step height from the

experimental data presented in Figure 4 (Θ0 = 10°, ΔΘ = 2°,
_d_ = 40 ± 10 nm), where a value of _h_ = 22 ± 6 nm was obtained,

in good agreement with the AFM results.

In conclusion, the shadow effect and a finite aperture size result

in dark areas in the image. These dark areas appears in the

regions of a sample where α > δΘ/2 + ΔΘ/4. If a surface feature

can be described with the rectangular step model, then

Equation 10 can be used for the calculation of the step height.

Effect of ion transmission

The contrast formation mechanisms presented above describe

the RI signal from the parts of the sample surface facing the

incident ion beam. On the opposite side, the primary beam does

not reach the surface and, accordingly, no RI signal can be

obtained. In the vicinity of a sharp edge of a feature the ion

transmission may contribute to the image contrast formation. As

was demonstrated in Figure 4a, a bright contrast was observed

in the downward step region. The peak position of the RI

contrast coincided with one of the SE bright contrasts and with

1132

the sharp edge of the step. The bright RI contrast originates

from the transmission of ions through the step edge and their

subsequent reflection [40]. This divides the sample surface into

two regions of reflection: the upper surface of the bar and the

substrate surface beyond the step edge (see Figure 10a). To

quantify the bright contrast profile we will use following

simplified model.

Figure 10: Ion transmission through the edge of a downward step (a)
and its projection to the image plane (b).

The reflection coefficient of the primary beam from the upper
surface (beam 1 in Figure 10a) is denoted as η * (Θ1,Θ2). The
asterisk is used to emphasize that the reflection coefficient near

the step edge differs from the reflection coefficient of the bulk

sample surface when the distance to the step edge is compa
rable with the ion penetration depth. Near the step edge some

part of the incident beam penetrates through it with the proba
bility ρ(Θ1,Θ3) and hits the substrate at an angle Θ3, which we
assume to be close to the angle of incidence, that is, Θ3 ≈ Θ1.
The transmitted ions are reflected from the substrate with the

reflection coefficient η(Θ3,Θ2) (beam 2 in Figure 10a). Using
these designations the total reflection coefficient can be written

as:

(11)

Figure 11 shows the dependence of the reflection and transmis
sion coefficients on the distance from rectangular step edge

along the top surface. All values were calculated with SRIM

Monte Carlo simulation software for a silicon dioxide step on

silicon and 35 keV He ions. The ion transmission probability

(dotted line in Figure 11) decreases from unity when the ion

beam moves away from the step edge. On the other hand, the

number of ions reflected from the upper surface (dashed line in

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

Figure 11) increases with the distance from the step edge. The

total reflection coefficient calculated using the Equation 11

(solid line in Figure 11) exhibits a maximum at a distance of

about 50 nm from the edge of the step, which is related to the

bright line in the RI image.

Figure 11: Dependence of the coefficients on the distance from the
edge of the step: reflection coefficient from upper surface (dashed
line), ion transmission probability (dotted line) and the total reflection
coefficient calculated with Equation 11 (solid line) (angle of incidence
was 10°).

The solid line in Figure 12 represents the profile of the RIM

contrast calculated according to Equation 8 at a rectangular

downward step of a SiO2 bar on Si. Far away from the step edge
the reflection coefficient tends towards the constant value of the

bulk material. The contrast is presented in Figure 12 with

respect to the reflection coefficient value for a SiO2 bar.

Figure 12: Profile of the relative contrast of a downward step: dots –
experimental data, solid line – data calculated with SRIM software.
Zero corresponds to the reflection from bulk SiO2.

1133

From Figure 12 one can see that the simulated curve is in good

agreement with experimental data, confirming the validity of

the suggested simple theoretical model. A difference between

the experimental points and the calculated results can be seen in

the tails of the profile, which is caused by the finite size of the

ion beam and the angular distribution of transmitted ions

(assumed to be zero in the calculations).

From the point of view of applications of this technique, it is

interesting to know the minimum value of the step height that

can be distinguished in RIM contrast. Figure 11 shows that the

edge contrast becomes negligible when the distance from the

step edge is less than _x_ min = 10 nm. At the same time, the edge
contrast can appear only when the primary beam projection

passes above the bottom boundary between the step and the

substrate. Consequently, the step height must be greater than

_h_ - _x_ mintanΘ1 to observe edge contrast. In our case Θ ≈ 10°,
and the edge contrast can be observed if the step height is

greater than 2 nm.

In summary, the ion transmission through small features on the

surface and their subsequent reflection can result in bright

contrast in the RIM image. One should note that despite of the

similarity of the bright contrasts in the SE and in RI images, the

origin of their mechanisms are quite different.

Effect of a surface charge

The RI image formation mechanisms described above originate

from the ion scattering by the geometrical specimen relief and

do not take into account the possible impact of surface charging

produced by the positively charged ions. Our experiments with

the mica surface demonstrated very sharp RI images without

any noticeable effect of charging on the image quality, despite a

strong positive surface charging that was revealed as the

absence of the SE in the detected signal due to their attraction to

the sample. Although the charging makes SE detection for

imaging both at the normal and glancing ion incidence impos
sible to use, RI detection can still be successfully implemented.

Despite the fact that the energy of the SEs is three orders of

magnitude less than the energy of the primary ions, the influ
ence of surface charge on their trajectories might not be negli
gible, and the question regarding possible image distortion

arises.

Under the action of the electric field of the positively charged

sample surface, the trajectories of the primary ions become

curved as schematically shown in Figure 13. The primary ion

beam is decelerated before the reflection from the surface, but

the reflected beam is accelerated back to its initial energy since

the electric field is almost uniform.

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

Figure 13: Reflection of ions from the charged surface. Trajectories of
ions reflected from a charged surface (solid blue line) and trajectories
of ions reflected from a neutral surface (dashed black line).

In general, the grazing angle changes due to surface charging

should be taken into account for the metrology of the surface

relief. Theoretically, the changes of the angle can proceed until

the beam is no longer parallel to the surface. Since that poten
tial increases inversely proportionally to the size of the scan

area with the same charge, the model predicts an increase of the

charging effect on the beam trajectory with the increasing in the

magnification. Accordingly, the simplest test to check for the

presence of the effect of beam charge bending is a comparison

of results of two scans preformed with different magnifications.

Similar results will provide evidence of the accuracy of the

lateral size definition. As for the accurate calculation of the step

height from the shadow contrasts, it should be noted that the

dependence of the width of the shadow on the angle of inci
dence (according to Equation 10) is rather weak for the experi
mental parameters used in RIM. In fact, a variation of the angle

of incidence from 0° to 10° results in the relative variation of

the shadow width of a few percent and does not affect the accu
racy of the measurements.

In summary, reflection ion microscopy is useful for imaging of

the surface of dielectric materials without the need for charge

compensation, and the accumulated surface charge plays te role

of positive sample bias. The impact of the charging on the

height of the surface steps is negligible.

## Conclusion

In summary, we investigated the capabilities of a scanning

reflection helium ion microscopy technique that was realized in

a helium ion microscope for the first time. No additional RI

detector was installed in the He IM chamber. Instead, reflected

ions were detected with a conventional SE detector by using RI

conversion to secondary electrons on a Pt-coated surface.

It was demonstrated that in contrast to the case of a backscat
tered geometry, the reflection coefficient of the ions, which are

incident and reflected at low grazing angles, is not sensitive to

1134

the atomic number of the sample material, and the contrast in RI

images is determined mostly by the surface morphology. This

experimental fact is in agreement with the results of the Monte

Carlo simulations of the angular dependence of the ion reflec
tion coefficient. Accordingly, RIM can be used for the imaging

of surface morphology but not for the imaging of surface

composition.

The obtained experimental results show that there are some

quantitative differences in the surface morphology imaging

between SE and RI imaging modes. The quantitative calcula
tion of the shadow contrast at a rectangular step was performed

based on a simple geometrical model. The calculation results

show that the width of the dark contrast area in the RI image of

an upward step is approximately twice broader than the same

step width in the SE image. At the same time, the width of a

bright area is practically the same in the RI and SE image of a

downwards step. It should be also noted that in contrast to RIM,

the width of the dark contrast in the SE image can vary

depending on the SE detector collector grid voltage. Accord
ingly, the precision of the RIM definition of the height of an

individual surface rectangular step is noticeably better than with

the SE detector. With regards to the subject of surface

morphology, we should compare RIM with other methods of

surface investigation such as REM and AFM.

Conventional REM demonstrates the capability of imaging of a

single atomic step, and it is used mostly for this purpose. Single

atomic steps were not observed by means of RIM yet, but there

are several features of the developed detection system that limit

the imaging capabilities of RIM. The REM imaging of surface

steps exploits the specific diffraction conditions, and the

obtained contrast is a superposition of phase contrast and

diffraction contrast [9]. All described mechanisms of RIM

contrast formation neglect the dependence of a reflection coeffi
cient on the polar angle because a slit diaphragm was used in

the experiment. In reality, this dependence contains additional

information on the surface details [41], and the channeling

effect and the deviation from channeling conditions can be used

for RIM contrast formation analogous to diffraction conditions

in REM. A 2D aperture instead of slit is required to limit both

angles of the reflected beam. Another distinction of the REM

setup is a high vacuum specimen chamber that is required for

the observation of the surface steps. The high vacuum chamber

is also required to reduce the effect of contamination as noted

below. Thus, observation of atomic steps in RIM seems to be

possible, but requires modification of the specimen chamber

and detection conditions.

Atomic force microscopy as well as RIM allows for precise,

quantitative measurements of the height of surface details;

_Beilstein J. Nanotechnol._ 2015, _6,_ 1125–1137.

however, the scanning process is very slow. From this point of

view, RIM has the same advantage as SEM: fast scanning and a

large maximum field of view. On the other hand, scanning and

measurements in AFM can be performed for all sides of the

sample features, whereas in RIM (as well as in REM) only the

side facing the beam is suited for investigation.

In this work, RIM was demonstrated to obtain sharp images of

an insulator surface without the need for charge compensation

or a conductive coating. A positive surface charge produced by

ions attracts SEs but does not significantly change the RI trajec
tory and their detection. The changes in trajectory caused by the

electric field of the charged surface can reduce the ion incident

and reflection angles, but our calculation shows that their influ
ence on the step height determination is negligible and can be

monitored by changing the imaging magnification. The alter
native method of the imaging of an insulating surface in HIM is

charge compensation by an electron flood gun. Imaging with a

flood gun requires accurate adjustment of the flood gun parame
ters and ion beam parameters to neutralize the surface charge.

The scan speed decreases when line-by-line charge compensa
tion is used. The reflected ions are less sensitive to surface

charge than SEs, thus RIM of insulating surfaces is more

straightforward. At the same time, RIM requires a special

sample holder. One of the main disadvantages of RIM is the

inability to image the whole surface of a sample. Part of a

sample is always hidden by sample features. Thus, the potential

advantage of RIM over the use of a flood gun is determined by

the sample surface details. Unfortunately, the construction of

the RIM specimen holder does not allow for charge compensa
tion, and a direct comparison of these methods is impossible.

Beside the factors mentioned above, the sensitivity and the

precision of the RIM measurements are obviously defined by

the signal-to-noise ratio that increases with the image acquisi
tion duration. There are two factors that limit measurement time

in RIM: the ion beam induced deposition (IBID) of hydrocar
bons and the sputtering of the surface material.

The first problem exists both in a case of RIM and in conven
tional HIM. Fortunately, IBID that is mainly stimulated by sec
ondary electrons [21] can be eliminated in RIM by applying a

positive bias to the sample and reducing the contamination

growth rate, and it can be completely avoided by usage of high

vacuum conditions.

Ion beam-induced sputtering always takes place and, unfortu
nately, the sputtering yield increases at low angles, and even for

ions as light as He this can exceed one atom per ion. Thus, this

effect is the main factor limiting the acquisition time. Most of
the images were obtained with a dose of about 10 14 cm −2, and

1135

the largest ion dose used for the acquisition of RI images of Au
on carbon was about 10 15 cm −2 . In this case we expect the sput
tering of a single atomic layer of the sample. The resolution of

the obtained images was insufficient to observe such a sput
tering effect, but it can be a limiting factor for the observation

of the atomic steps. Thus, the further enhancement of the detec
tion system is desirable to reduce the measurement time and to

avoid sputtering of a sample.

In conclusion, reflection helium ion microscopy is a promising

tool for the investigation of surface morphology and is espe
cially useful when applied to non-conductive materials. Further

development of this technique would require a special, more

sensitive RI detector, higher vacuum conditions, and a polar

angle limiting aperture.

## Acknowledgements

The experiment was performed at the Interdisciplinary

Resource Center for Nanotechnology of Saint-Petersburg State

University. We thank Taras Sharov for providing the AFM stan
dard and mica samples.

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