#  _In-situ_ Plasma Studies using a Direct Current Microplasma in a Scanning
Electron Microscope

Lukas Grünewald  Authors contributed equally to this work.  Dmitry Chezganov
Authors contributed equally to this work.  Robin De Meyer  Authors contributed
equally to this work.  Andrey Orekhov  EMAT, University of Antwerp,
Groenenborgerlaan 171, B-2020 Antwerp, Belgium  Sandra Van Aert  EMAT,
University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium  Annemie
Bogaerts  PLASMANT, University of Antwerp, Universiteitsplein 1, B-2610
Antwerpen-Wilrijk, Belgium  Sara Bals  EMAT, University of Antwerp,
Groenenborgerlaan 171, B-2020 Antwerp, Belgium  Jo Verbeeck  EMAT, University
of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
jo.verbeeck_at_uantwerpen.be

###### Abstract

Microplasmas can be used for a wide range of technological applications and to
improve our understanding of fundamental physics. Scanning electron
microscopy, on the other hand, provides insights into the sample morphology
and chemistry of materials from the - down to the -scale. Combining both would
provide direct insight into plasma-sample interactions in real-time and at
high spatial resolution. Up till now, very few attempts in this direction have
been made, and significant challenges remain. This work presents a stable
direct current glow discharge microplasma setup built inside a scanning
electron microscope. The experimental setup is capable of real-time _in-situ_
imaging of the sample evolution during plasma operation and it demonstrates
localized sputtering and sample oxidation. Further, the experimental
parameters such as varying gas mixtures, electrode polarity, and field
strength are explored and experimental $V$-$I$ curves under various conditions
are provided. These results demonstrate the capabilities of this setup in
potential investigations of plasma physics, plasma-surface interactions, and
materials science and its practical applications. The presented setup shows
the potential to have several technological applications, e.g., to locally
modify the sample surface (e.g., local oxidation and ion implantation for
nanotechnology applications) on the -scale.

###### keywords:

Microplasma, Plasma, SEM, ESEM, In-situ, EDS, Sputtering

EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
NANOlab Center of Excellence, University of Antwerp, Groenenborgerlaan 171,
B-2020 Antwerp, Belgium EMAT, University of Antwerp, Groenenborgerlaan 171,
B-2020 Antwerp, Belgium NANOlab Center of Excellence, University of Antwerp,
Groenenborgerlaan 171, B-2020 Antwerp, Belgium PLASMANT, University of
Antwerp, Universiteitsplein 1, B-2610 Antwerpen-Wilrijk, Belgium NANOlab
Center of Excellence, University of Antwerp, Groenenborgerlaan 171, B-2020
Antwerp, Belgium NANOlab Center of Excellence, University of Antwerp,
Groenenborgerlaan 171, B-2020 Antwerp, Belgium

##  1 Introduction

A plasma is a complex and versatile state of matter with many established
applications, e.g., in the semiconductor industry 1, as well as promising
emerging technologies. The potential of plasma technology spans a broad range,
including biomedical applications 2, materials science 3, and gas conversion
for environmental applications 4. Many different types of plasma exist 5, but
the simplest geometry consists of two electrodes separated by a gas (at low,
atmospheric, or elevated pressure), with a voltage being applied between the
electrodes 6. More recently, so-called “microplasmas” have received a rising
interest in the scientific community 7, 8, 9, 10, 11. Microplasmas have at
least one dimension in the sub- range 8. Besides the (i) practical aspect of
reduced operation cost of microplasma setups compared to large plasma reactors
for laboratory-scale experiments and (ii) a general trend toward
miniaturization of devices in plasma-application areas, microplasmas also have
interesting properties. For example, the large surface-to-volume ratio and
short gap distances between the electrodes (typically a few
$100\text{\,}\mathrm{\SIUnitSymbolMicro m}$) lead to a non-equilibrium state
where the ion/gas temperature is lower than the electron temperature 3. This
results in a “cold” plasma with gas temperatures close to room temperature 9,
8, 7, which shows great promise, e.g., in nanomaterial and nanoparticle
fabrication 3. In addition, microplasmas are not confined to vacuum operation.
Paschen’s law relates the breakdown voltage of a gas with the product $pd$ of
the pressure $p$ and the gap distance $d$ between two parallel electrode
plates. For many gases, the smallest breakdown voltages lie in the range of
about
$10\text{\,}\mathrm{Pa}\text{\,}\mathrm{cm}1000\text{\,}\mathrm{Pa}\text{\,}\mathrm{cm}$
9. Reducing $d$ to $100\text{\,}\mathrm{\SIUnitSymbolMicro m}$ or less allows
plasma operation at or near atmospheric pressure
($p=$101\text{\,}\mathrm{kPa}$$).
The plasma setup presented in this work is a direct current (DC) microplasma
where one of the electrodes is a nozzle with a small orifice through which gas
is supplied (see Figure 9a in the Experimental section). Whereas the geometry
closely resembles that of a jet, the setup isn’t technically defined as a
plasma jet since the plasma is generated in the gap between the nozzle and the
grounded electrode/sample 12. The interaction of plasmas with flat surfaces or
nanoparticles is of interest for technical applications and a better
understanding of plasma physics and chemistry. Often, _ex-situ_ structural and
chemical investigations on the milli- to nanometer scale are performed after
plasma treatment of a material.
For these length scales, scanning electron microscopy is a valuable technique
for microstructural and chemical investigations (typically using energy-
dispersive x-ray spectroscopy, EDS). Recently, the first microplasmas were
generated inside scanning electron microscopes (SEMs) 13, 14, 15. A plasma-in-
SEM setup not only reduces the time between plasma treatment and subsequent
SEM analyses compared to a separate plasma setup, but also prevents exposure
of the sample surface to ambient air. The latter aspect enables studies of
plasma-treated surfaces where subsequent contact with oxygen, humidity, or
contamination must be avoided.
Different approaches to generate plasmas in SEMs were demonstrated in earlier
studies. For example, local sputter etching was achieved by Mulders and
Trompenaars 14 by introducing a small gas nozzle into an SEM and using the
electron beam for ionization. In the setup by these authors, the electron beam
is scanned in a small slit in the nozzle near the orifice to generate ions in
the gas stream. The generated ions flow out of the orifice with the gas flow
and are then accelerated toward the sample using an applied voltage between
the nozzle and the stage. Modern SEMs often have a built-in option to apply
the required negative voltage to the sample stage, typically used for beam-
deceleration SEM imaging 16, 17. This approach does not require reaching the
breakdown voltage of the gas, hence leading to a low-energy ion bombardment of
the sample. With this setup, low-energy Ar+ ions with energies ranging from
$20\text{\,}\mathrm{e}\mathrm{V}500\text{\,}\mathrm{e}\mathrm{V}$ were used to
remove amorphous surface layers 18, 19.
Another plasma setup consists of a micro hollow cathode (or anode) DC plasma
configuration in an environmental SEM (ESEM) 15. In the latter, the chamber
pressure and gas type (in this case Ar) is directly controlled with the ESEM.
A supplied high voltage (in this case $\pm 1\text{\,}\mathrm{kV}$) generates
the plasma, and the plasma-surface interaction subsequently can be analyzed
within the ESEM. Depending on the electrode polarity, either (i) redeposition
of sputtered material from the counter electrode onto the sample surface or
(ii) direct sputtering of the sample surface with positive ions was observed.
The sputtered area had a relatively large width of about
$2\text{\,}\mathrm{mm}$ 15. A benefit of this experimental setup is that the
gas-flow controls of the ESEM are used, which reduces the requirements for the
hardware modifications to an SEM. However, a drawback is that using the low-
vacuum mode reduces the image quality due to electron-beam scattering in the
gas, resulting in a so-called electron-beam skirt 20, 21. This aspect impedes
_in-situ_ SEM imaging of the plasma-sample interactions, limiting high-quality
imaging to the normal high-vacuum mode of the ESEM. To optimize image quality
in gaseous environments, the distance between the end of the microscope’s pole
piece and the sample, i.e., the gas-path length, is typically minimized to
reduce the beam skirt. However, the gas-path length cannot be reduced too much
for plasma experiments due to the risk of unwanted arcing to the microscope
hardware. Indeed, arcing from the micro hollow cathode to the microscope
hardware over a relatively large distance of about $25\text{\,}\mathrm{mm}$
was reported for this setup using the low-vacuum mode 15.
Matra et al. 22, 13, 23 demonstrated a working jet-like microplasma setup
inside an SEM. This approach combines the properties of a jet (enabling a
comparably high pressure in the gas jet compared to its environment) with the
small dimensions of a microplasma for local plasma application (typically
within a few ten ). The gas flows from a gas nozzle with a small orifice
(nominal diameter of a few ten ) toward a (flat) sample surface, whereas the
chamber is continuously pumped to maintain a low overall pressure. A plasma is
generated by applying a voltage, here denoted as source voltage
$V_{\text{S}}$, between the nozzle and the sample, somewhat similar to a
plasma reactor with two electrode plates 6. However, the non-uniform pressure
profile between the nozzle and the sample makes this plasma configuration
unique, complicating the characterization of the plasma discharge. The gap
distance can be adjusted by using SEM imaging for alignment. The pressure
profile between the nozzle and the sample can be modified by changing the gas
flow, though it will also be heavily affected by the distance between the
orifice and the sample. A plasma is generated by applying at least the
breakdown voltage between the nozzle and the sample (although the electron
beam can be used to aid plasma ignition). Depending on the gas, material
removal by Ar+ sputtering 13 and growth of an C-rich thin film 24 on a Si
surface were observed. This proof-of-principle study 13 showed that a
microplasma jet can be generated in the evacuated SEM chamber.
However, the (desired) DC glow discharge was reported not to be fully stable,
resulting in arcing to the sample 13 and a self-pulsing plasma mode for
discharge currents in the range of about $3\text{\,}\mathrm{\SIUnitSymbolMicro
A}30\text{\,}\mathrm{\SIUnitSymbolMicro A}$ (depending on voltage, gas flow
rate, and gap distance) 22. This arcing led to strong local heating and
pronounced damaged spots on the sample 13. Furthermore, these previous studies
did not investigate the possibility of “true” _in-situ_ SEM imaging, i.e.,
live SEM imaging during plasma operation. Instead, SEM images were taken
before and after the plasma-treatment steps (also in ref. 15), which will be
denoted as “quasi” _in-situ_ operation in this work. Still, these studies
prove that a microplasma can be generated in an SEM and used for surface
treatment. This provides the opportunity to observe _in-situ_ changes of a
sample’s morphology and chemistry on the mm to nm scale during plasma
treatment using an SEM, ultimately leading to a better understanding of
plasma-surface interactions and fundamental plasma properties.
However, the availability of more studies is hampered by (i) the required non-
trivial modifications of an SEM and (2) the lack of commercial solutions. In
this work, a microplasma setup built inside a modern ESEM based on the work of
Matra et al. 13 is presented. A stable operation of a DC discharge without
arcing is realized. Further, we present real-time _in-situ_ SEM imaging during
plasma operation and show exemplary applications of our plasma-in-SEM setup
for sputtering and local surface oxidation. Finally, experimental challenges
and potential upgrades of the setup are discussed.

##  2 Results and discussion

The first part of this section shows results related to the microplasma and
_in-situ_ SEM imaging. The second part discusses some exemplary results when
applying the microplasma to materials. Finally, the third part reviews the
limitations of this setup and proposes potential solutions to overcome these
limitations.

###  2.1 Microplasma Characterization

####  2.1.1 Gas-Pressure Profile

Figure 1: Investigation of the gas density profile in the plasma gap. a SE-SEM
image of the gas flow into vacuum acquired with a primary electron energy of
$2\text{\,}\mathrm{keV}$. b A spot with a slightly increased SE signal is
visible on the sample surface (marked with a dashed arrow) when a sample is
brought into proximity, probably due to an increased gas density when the gas
jet hits the sample surface. c After plasma treatment, the bright spot
coincides with the plasma-treated region, indicating that the gas spot in b
can be used for aiming the microplasma at the desired region of interest.

The used plasma setup has a non-uniform gas pressure along the plasma gap. The
gas density profile can be visualized by SEM imaging (Figure 1a) by using a
low primary electron energy (here $2\text{\,}\mathrm{k}\mathrm{e}\mathrm{V}$)
to increase the electron-scattering probability and secondary electron (SE)
generation within the gas cloud 25. As a result, the SE-SEM image presumably
shows higher intensity in regions with higher gas densities (Figure 1a). Here,
the gas cloud in Figure 1a flows into the microscope vacuum without
obstruction. The contrast variations in the background result from out-of-
focus imaging of the sample stage a few mm below the nozzle along the
electron-beam direction. The gas density is highest close to the orifice and
gradually decreases away from it. This monotonic decrease is in accordance
with calculated gas density profiles of restricted gas flows, e.g., in
references 26, 27, 28. More explicitly, Salehi et al. 29 report an exponential
decay of the gas density away from an orifice from a simulation of gas jets
for different pressure differences between the inside of the nozzle and the
chamber. Experimental measurements of the pressure gradient away from the
nozzle by Patel et al. 30 reveal a continuous pressure decrease away from the
nozzle for a distance of about $20$ orifice diameters (in their experiment
about $20\text{\,}\mathrm{mm}$ for a $0.8\text{\,}\mathrm{mm}$ orifice
diameter), which would correspond to a continuous pressure decrease away from
the orifice of about $400\text{\,}\mathrm{\SIUnitSymbolMicro m}$ for a nominal
$20\text{\,}\mathrm{\SIUnitSymbolMicro m}$ orifice diameter. From comparison
with these results, we suspect a monotonic decrease in gas density and
pressure across the microplasma gap in our experimental setup.
However, if the gap distance is reduced by bringing the sample close to the
orifice (here about $120\text{\,}\mathrm{\SIUnitSymbolMicro m}$), an increase
in SE signal is visible on the sample surface as well (Figure 1b, dashed
arrow). The increased SE signal at the sample indicates an increased gas
density at the sample surface. From these observations, it becomes clear that
the gas density profile in the gap depends, among other parameters, also on
the gap distance. This non-uniform gas pressure impedes predictions and
comparison with conventional plasma reactors with a constant pressure between
the electrodes. As a beneficial side aspect, the visible gas spot on the
sample surface can be used to predict the plasma-spot region. This can be seen
by comparing the images before and after plasma operation in Figures 1b and c,
respectively, where the pit due to plasma sputtering forms in the region
predicted in Figure 1b.

####  2.1.2 Voltage-Current Characteristics of the Plasma

Figure 2: Voltage-current characteristics of a N2 microplasma for different
gas flow rates and gap distances. All axis limits are equal for easier
comparison. The data is shown for decreasing gap distance from left to right.
Two measurements were performed for each gas flow rate and gap distance. No
discharge was observed for $125\text{\,}\mathrm{\SIUnitSymbolMicro
m}$/$2.5\text{\,}\mathrm{sccm}$. Slight deviations between these measurements
are mainly caused by uncertainties in gap distance. The apparent drop in
current for higher voltages (marked with arrows in b and c) is a measurement
artifact caused by sample-surface sputtering. In general, larger discharge
currents are observed for higher gas flow rates and smaller gap distances. A
positive slope for all curves indicates a so-called abnormal glow discharge
behavior.

Next, the voltage-current characteristics (i.e., the dependence of discharge
voltage $V_{\text{D}}$ and discharge current $I_{\text{D}}$) of a N2
microplasma were investigated for three gap distances
($75\text{\,}\mathrm{\SIUnitSymbolMicro m}$,
$100\text{\,}\mathrm{\SIUnitSymbolMicro m}$, and
$125\text{\,}\mathrm{\SIUnitSymbolMicro m}$) and three gas flow rates
($2.5\text{\,}\mathrm{sccm}$, $5.0\text{\,}\mathrm{sccm}$, and
$7.5\text{\,}\mathrm{sccm}$). Nitrogen was chosen over Ar because it resulted
in lower chamber pressures for the same gas flow rate, allowing for higher gas
flow rates (up to $8\text{\,}\mathrm{sccm}$) into the high-vacuum microscope
chamber. The pumping speed of different gases is discussed in more detail in
the supplementary information. For each gas-flow/gap-distance pair, two
measurements were taken for repeatability (here denoted in the brackets in the
figure legends). Figures 2a–c show the values sorted with decreasing gap
distance from left to right. The same axis limits were used for easier
comparison.

In general, a positive slope is visible for all curves, indicative of a so-
called abnormal glow discharge plasma 6. This was also observed by Matra et
al. 22, but not in all of their measurements. After this initial positive
increase of discharge current with discharge voltage, nearly all curves show a
maximum current followed by a current decrease (cf. arrows in Figures 2b and
c). The last aspect is a measurement artifact, probably caused by rapid
sputtering of the electrode, and should not be interpreted as an actual
voltage-current characteristic of the microplasma. This artifact is discussed
in more detail in the supplementary information (Figure S1).
Next, the ordinate intercepts of the curves in Figure 2 are discussed. These
points correspond to the lowest discharge voltage at which a plasma discharge
can be sustained. Note that this isn’t equal to the breakdown voltage, as the
voltage required to initiate a breakdown is often (significantly) higher than
the voltage required to sustain one 5. The actual breakdown voltages were not
measured since our setup does not produce the necessary uniform gas pressure
for a given gap distance for a controlled measurement 31. Figures 2a–c show a
decreasing minimum discharge voltage for increasing gas flow rates for the
same gap distance. Since an increase in gas flow rate for a constant gap
distance is assumed to result in an increasing gas density, this decreasing
minimum discharge voltage offers an interesting insight into the plasma
discharge. As described by the Paschen curve for simple parallel-plate and
uniform-pressure DC plasma systems, an increased pressure heavily affects the
discharge properties (see also supplementary Figure S2).
On the one hand, if the gas density is higher than the optimum (i.e., the
point with the lowest minimum discharge voltage, similar to the minimum in the
Paschen curve), the electrons undergo many collisions, which limit their
possibility to gain enough energy to ionize a molecule. This ionization is
required to create an avalanche effect, which is needed to sustain a
discharge. In this case, a higher voltage is required to sustain the discharge
to ensure the electrons can gain sufficient energy to cause subsequent
ionization.
On the other hand, if the gas density is lower than the optimum, the electrons
can easily gain sufficient energy, but they may not collide frequently enough
to cause the further ionization required to sustain the discharge. Then,
again, a higher voltage is required to ensure that the collisions will cause
ionization. As the minimum voltage required to sustain a discharge decreases
with increasing gas density, it is implied that the gas density is lower than
the optimal case overall. This is analogous to being on the left side of the
minimum in the Paschen curve. It should be noted, though, that given the
strong pressure gradient in this setup, the discharge mechanisms are not as
straightforward as they are assumed by the Paschen curve, so a direct
comparison is difficult. This behavior of the minimum discharge voltage
implies that the plasma could be categorized as a so-called obstructed
abnormal glow discharge 6, 5.

When comparing the curves for the same gas flow rate and different gap
distances in Figures 2a-c, both the gas density and the gap distance are
varied since the former is affected by the latter. Assuming that the gas
density at a constant gas flow rate increases for a decreasing gap distance,
the changes in minimum discharge voltage in Figures 2a-c indicate that the gas
density is increasing non-linearly (in contrast to linearly decreasing
distance) and more substantial than the gap distance.

An additional complication affecting the interpretation of the data is the
setup geometry. The shown setup with a rounded nozzle with an orifice as one
electrode and a possibly textured sample surface as another electrode is
different from earlier publications studying various electrode geometries 31,
32, 33, 34. Microplasmas are especially sensitive to surface effects due to
the small spatial scale in the sub-mm range, as the electric field can be
strongly altered by small morphological changes in the electrode surfaces 34.
In addition, due to the high-pressure gradient, it is impossible to accurately
control the pressure in the discharge gap using this setup.

####  2.1.3 Plasma Generation and Stability in an SEM

The plasma-in-SEM setup enables studying the interplay between the electron
beam of the SEM and the plasma. Different aspects of this interaction are
discussed in the following.

Firstly, an electron beam can be used to ignite the plasma at lower voltages
than required for the self-ignition when reaching the breakdown voltage 13
(Figure 3a). For example, in one case a plasma discharge could not be
achieved, even when applying a maximum source voltage
$V_{\text{S}}=$2\text{\,}\mathrm{kV}$$ to the nozzle without an electron beam.
However, scanning with the electron beam caused a plasma discharge already at
$V_{\text{S}}=$920\text{\,}\mathrm{V}$$ for the same gap distance and gas flow
rate. This can be explained by the generation of SEs, backscattered electrons
(BSEs), and x-rays upon the interaction of the electron beam with the sample,
which then triggers the plasma ignition. Notably, the electron beam ignites
the plasma even if not directly scanning in the gap region. A webcam video
comparing plasma ignition by (i) reaching breakdown voltage (the conventional
way) or (ii) using the electron beam is found in the supplementary information
(_Breakdown-vs-SEM-Plasma.mp4_). In this video, the SEM-triggered plasma shows
a less intense plasma cloud than the self-ignited plasma. Therefore, the
electron beam can be advantageously employed to ignite a less intense plasma
at lower voltages (cf. middle and right images in Figure 3a). In addition, for
conditions where a plasma is not self-sustainable, i.e., with a large gap
distance and/or low gas flow rate, a plasma discharge was observed that was
only active during active electron-beam scanning (see supplementary
information Figure S3).

Figure 3: Aspects of microplasma operation in a scanning electron microscope.
a Webcam images of plasma operation. The electron beam can be used to ignite
the plasma at a lower applied source voltage to generate a less intense plasma
(right) compared to self-ignition by reaching the breakdown voltage (middle).
The shown plasma images correspond to the plasma conditions right after plasma
ignition. b Top-view SE-SEM images ($10\text{\,}\mathrm{keV}$) of the nozzle
and sample (left) without and (right) with applied voltage on the nozzle
($V_{\text{S}}=$920\text{\,}\mathrm{V}$$). In this example, the electrons are
attracted to the positive potential on the nozzle, which enables imaging of
the orifice area. c True _in-situ_ SE-SEM imaging during plasma operation is
possible and shows the formation of a pit in the sample due to sputtering.

Secondly, applying an electric potential to the nozzle will create an electric
field that deflects the incoming electron beam, e.g., toward the positive
potential on the nozzle (Figure 3b). The deflection depends on the electron
energy (less deflection for higher ) and probably also on the extent of the
exposed metal part of the steel nozzle. In our setup, insulating tape was used
to cover most of the steel nozzle, excluding the tip (see black tape in Figure
9). The deflection may be minimized by (i) shielding the open metallic surface
of the nozzle tip and (ii) using a higher primary electron energy. However,
the deflection can also be used advantageously. For example, the deflection
can be strong enough so that the SE-SEM image is formed from the nozzle-tip
surface, e.g., at $V_{\text{S}}=$920\text{\,}\mathrm{V}$$ for a primary beam
energy of $10\text{\,}\mathrm{keV}$ (Figure 3b, right). In this way, the tip
region of the nozzle can be imaged with the SEM even though it is aligned
parallel to the electron beam, i.e., without a direct line of sight. This
effect is more pronounced at lower electron energies. A supplementary movie
(_SEM_Plasma_Ignition.mp4_) shows correlative imaging of the webcam and SEM
images during a gradual increase in the source voltage $V_{\text{S}}$ and
subsequent SEM-induced plasma ignition. The SEM image is increasingly “tilted”
toward the nozzle with increasing $V_{\text{S}}$.

Thirdly, it was observed that _in-situ_ SEM imaging during plasma operation is
indeed possible, opening up the opportunity for time-resolved studies. In SE-
SEM imaging, a working plasma leads to an increase in signal (brightness)
using the Everhart-Thornley detector (ETD). For imaging, this effect can be
compensated by reducing the ETD bias setting. For a CO2 plasma, this method
proved effective for discharge currents up to about
$7\text{\,}\mathrm{\SIUnitSymbolMicro A}$, after which the ETD was saturated
(i.e., no further reduction in bias possible), and no SE-SEM imaging was
possible. It is remarkable that _in-situ_ SE-SEM imaging during plasma
operation is feasible, despite several challenges: (i) the electron-beam
current used (few ) is about a thousand times lower than the measured
discharge current (few ), (ii) many spurious SEs are likely generated in the
plasma region 6, 5, and (iii) the positive suction voltage on the ETD of
$250\text{\,}\mathrm{V}$ to attract SEs is comparatively low compared to the
nozzle voltage (typically $>1\text{\,}\mathrm{kV}$). For example, three SE-SEM
images taken during continuous microplasma operation are shown in Figure 3c.
The plasma duration increases from left to right, leading to increasing pit
diameter and depth due to surface sputtering. The most notable distortion in
the SE-SEM image is caused by the applied nozzle voltage, resulting in an
electron-beam deflection (Figure 3b).
Similarly, BSE-SEM imaging was tested by negatively biasing the ETD with
$-150\text{\,}\mathrm{V}$ to suppress (mainly) SEs from the image signal. In
contrast to SE-SEM imaging, the BSE-SEM image brightness is not affected by
the discharge current during plasma operation, meaning that BSE-SEM imaging is
still possible even when the SE signal becomes saturated at high discharge
currents (e.g., $>7\text{\,}\mathrm{\SIUnitSymbolMicro A}$ for CO2). A video
comparing BSE- and SE-SEM imaging is found in the supplementary information
(_In-Situ-SEM_SE-vs-BSE.mp4_). Since the ETD covers only a relatively small
solid angle, it is inefficient for BSE detection. This results in a lower
signal yield than for SE-SEM imaging. However, the low BSE signal may be
increased by using a more efficient and low-vacuum compatible BSE detector 35,
36, but this was not tested in this work. Since both SE- and BSE-SEM imaging
is possible and similar to conventional SEM imaging, the signals can be chosen
depending on the experiment, or both signals can be collected with two
different detectors. This enables more surface-sensitive imaging with SEs and
$Z$-dependent imaging with BSEs 21.

An application-relevant observation from the demonstrated setup is the absence
of undesired high-current and high-frequency arc discharges, which were
reported by Matra et al. 22 as a self-pulsating plasma mode. Instead, we
observed stable DC glow discharges with discharge currents ranging from about
$0.1\text{\,}\mathrm{\SIUnitSymbolMicro
A}175\text{\,}\mathrm{\SIUnitSymbolMicro A}$, which can be controlled by
adjusting $V_{\text{S}}$. This corresponds to current densities ranging from
$5\text{\,}\mathrm{m}\mathrm{A}\,\mathrm{c}\mathrm{m}^{-2}$ to
$9\text{\,}\mathrm{A}\,\mathrm{c}\mathrm{m}^{-2}$ for an assumed plasma-spot
diameter of $50\text{\,}\mathrm{\SIUnitSymbolMicro m}$. The latter can vary
depending on the gap distance. We did not investigate higher currents than
$175\text{\,}\mathrm{\SIUnitSymbolMicro A}$ since the
$30\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thick Cu target is sputtered away
in a few (ten) seconds at the plasma spot. Conversely, the plasma could not be
sustained below the lower limit of about
$0.1\text{\,}\mathrm{\SIUnitSymbolMicro A}$.
The absence of arcing may be explained by the lower chamber pressure in our
used SEM (about $2\text{\times}{10}^{-2}\text{\,}\mathrm{Pa}$) compared to the
reported values “below $1\text{\,}\mathrm{Pa}$” 22. Notably, a self-pulsing
plasma was observed for the shown setup when powering it in ambient air during
prototyping. The high-frequency arcing in this self-pulsing mode (a few ten )
causes significant electromagnetic interference to surrounding electronic
devices, including the SEM. In addition, powering the setup in the low-vacuum
mode of the SEM at a chamber pressure of $40\text{\,}\mathrm{Pa}$ leads to
undesired discharges in the SEM chamber, similarly as observed by Pardinas 15.
This restricts the plasma operation to the high-vacuum mode (below
$3.3\text{\times}{10}^{-2}\text{\,}\mathrm{Pa}$ for the used SEM). Here, only
occasional arcs during plasma operation were observed when non-flat samples
with surface protrusions were used. It may be possible to fully mitigate the
self-pulsing plasma mode by an optimal choice of electronic components in the
circuit. Still, in our case, the reduced chamber pressure (about
$2\text{\times}{10}^{-2}\text{\,}\mathrm{Pa}$) is the most likely reason for a
stable DC plasma operation compared to Matra et al. 22.

###  2.2 Microplasma Applications

####  2.2.1 Sputtering and Cone Formation

Sputtering is the process of removing atoms of the target material by
impinging ions. Sample material was removed by this process in all
experiments, where the sample was used as the cathode. The positively charged
ions are accelerated toward the cathode and cause sputtering, as is common in
glow discharges. This results in changes in surface morphology in the plasma-
spot regions, with diameters ranging from about
$50\text{\,}\mathrm{\SIUnitSymbolMicro
m}150\text{\,}\mathrm{\SIUnitSymbolMicro m}$ (depending on the gap distance,
pressure, discharge voltage/current, and plasma duration). In the following,
results for sputtering on (i) a polished or (ii) a Ni nanoparticle-covered Cu
surface are shown.

Figure 4: Cone formation after Ar^+-ion sputtering for different
concentrations of surface particles. The lower row shows higher magnification
SEM images of the upper row. a Cone formation is not visible in the shown
region of a polished Cu surface. A small cone is visible on the edge (b),
probably due to a small contaminating particle on the sample surface. c and d
Debris on the Cu surface forms cones under plasma treatment. e Ni particles
deposited on a Cu substrate show clear cone formation in the plasma-treated
region. In the early stages of sputtering, the Ni particles locally
agglomerate to form a cone (see the example in f marked with a dashed arrow).

The formation of a pit under CO2 and Ar-containing plasma was observed for a
polished Cu surface. An example is shown in Figure 4a, which was created with
Ar plasma. Experimentally, this pit formed after $1.2\text{\,}\mathrm{keV}$
Ar^+ exposure with a discharge current of about
$15\text{\,}\mathrm{\SIUnitSymbolMicro A}$ (current density of
$1.2\text{\,}\mathrm{A}\,\mathrm{c}\mathrm{m}^{-2}$ for a plasma-spot pit
diameter of $40\text{\,}\mathrm{\SIUnitSymbolMicro m}$) for about
$10\text{\,}\mathrm{s}$. The rapid pit formation is indicative of the high
sputter rates of the setup. The pit surfaces are rougher than the original
polished surface. A comparably small conical structure is visible at the edge
of the pit, which is magnified in Figure 4b. This may have been an impurity or
other contamination present in or on the Cu surface, which deformed to the
shown conical structure during sputtering. Its bright appearance in the SE-SEM
images may be explained by the penetration depth of primary electrons, here at
an energy of $15\text{\,}\mathrm{keV}$. For relatively thin structures such as
the shown impurity in Figure 4b, SEs are emitted not only on the entrance
surface of the beam but also on the exit surface of the cone (and also the
sample material behind the cone). The additional SE emission from the exit
surface (relative to the incoming electron-beam direction) leads to higher SE-
SEM image intensity for thinner sample regions.

Cone formation is observed for random debris (Figures 4c and d) or full
coverage with Ni nanoparticles (Figures 4e and f). For the latter, the
nanoparticles seem first to cluster together (see region marked with an arrow
in Figure 4f) and then tend toward a conical shape during prolonged
sputtering. The latter aspect was studied in more detail by monitoring the
same area after a certain plasma duration with SEM imaging (Figure 5). Between
each plasma treatment, the sample area was moved onto the optical axis of the
SEM to allow for high-magnification imaging. A few exemplary areas are
annotated with arrowheads, which are discussed in the following.
Region (1) shows the shape evolution of Ni particles under Ar plasma. Between
$0\text{\,}\mathrm{s}30\text{\,}\mathrm{s}$, the shape gradually changes from
round nanoparticles (Figure 5a) toward a conical shape (Figure 5d). After
reaching the final conical shape, the cone is milled away during further
sputtering (cf. region (1) for Figures 5d and e). Similarly, region (3) shows
the removal of smaller cones between
$30\text{\,}\mathrm{s}60\text{\,}\mathrm{s}$ of sputtering (cf. region (3) for
Figures 5d and e). For region (2), a few Ni nanoparticles agglomerate between
$10\text{\,}\mathrm{s}20\text{\,}\mathrm{s}$. It is unclear from the images if
these particular particles result from the present particles in the shown
region or were redeposited from remote sample areas. Further plasma exposure
leads to a merging of the individual nanoparticles and the formation of a
larger cone with smooth surfaces (cf. region (2) for Figures 5d and e).
Overall, after $60\text{\,}\mathrm{s}$, the underlying Cu surface is partly
exposed (Figure 5e).

Figure 5: Quasi _in-situ_ observation of Ni nanoparticle agglomeration and
subsequent cone formation during Ar^+-ion sputtering
($5\text{\,}\mathrm{\SIUnitSymbolMicro A}$,
$1.32\text{\,}\mathrm{k}\mathrm{e}\mathrm{V}$) for the given duration shown
above the SE-SEM images. The latter were taken with a large-field detector
(LFD) in the low-vacuum mode ($40\text{\,}\mathrm{Pa}$) after each plasma
operation in high-vacuum mode. A few interesting regions are marked with
arrows. Region (1) shows gradual change from (a) a round particle morphology
to an increasing distortion toward a conical shape. After reaching the latter
in d, the cone is then starting to be removed by sputtering, as visible in e.
Region (2) shows the sudden agglomeration of a few nanoparticles in c. A
larger cone is forming from this agglomeration (d and e). Region (3)
exemplifies that, after initial formation, the cones are sputtered away under
further Ar^+-ion bombardment (d and e).

The observed formation of cones is a commonly observed modification of metal
surfaces under ion bombardment 37, 38, 39. The cone shape is commonly thought
of as a combined result of varying sputter yield depending on the (i) ion-
incidence angle and (ii) material. The sputter yield typically increases with
increasing ion-incidence angle up until a maximum value $\Theta_{\text{m}}$,
and then decreases rapidly toward grazing incidence (i.e., the ion direction
being parallel to the sample surface) 38. This results in a cone shape of
impurities and surface particles before complete removal by sputtering. The
seeds for the cones can be intrinsic elemental impurities in an otherwise flat
surface or particles on the surface with lower sputter yield. The latter
correlates with the melting temperature of a material. Wehner 39 has tested
numerous surface/seed combinations of metals with different melting
temperatures and found that cone formation requires seed materials with higher
melting temperatures than the surface material. This is the case for Ni
particles ($T_{\text{melt}}=$1728\text{\,}\mathrm{K}$$) on a Cu substrate
($T_{\text{melt}}=$1358\text{\,}\mathrm{K}$$) observed in Figure 5. Note that
the used nanoparticles are large enough (around $100\text{\,}\mathrm{nm}$) so
that a reduction in melting points is assumed to be negligible 40, 41.
A mean apex angle of
$\Theta=$61.4(11.1)\text{\,}\mathrm{\SIUnitSymbolDegree}$$ (the error being
the standard deviation) was measured for $40$ cones. According to Stewart and
Thompson 42, $\Theta$ is related to the ion-incidence angle for maximum
sputter yield $\Theta_{\text{m}}$, as
$\Theta_{\text{m}}=\left($180\text{\,}\mathrm{\SIUnitSymbolDegree}$-\Theta\right)/2=$59.3(5.6)\text{\,}\mathrm{\SIUnitSymbolDegree}$$.
The experimental value $\Theta$ is in good agreement with the maximum
$\Theta_{\text{m,sim}}\approx$65\text{\,}\mathrm{\SIUnitSymbolDegree}$$ of a
simulation of the angle dependence of the sputter yield of Ar on Ni using SRIM
(supplementary information Figure S4). The differences between measured and
simulated values can be explained by (i) limited statistics based on only $40$
measured cones, (ii) systematic errors in the angle measurement from SEM
images, and (iii) uncertainties in the simulation 43.

####  2.2.2 Local Oxidation

Plasma finds applications in both the oxidation and reduction of materials 44,
45. Here, we investigate the possibilities of local plasma-induced sample
oxidation in the SEM. As a first example, a polished Cu surface was exposed to
a CO2 plasma (Figure 6). The gap distance was approximately
$130\text{\,}\mathrm{\SIUnitSymbolMicro m}$ (Figure 6a). In Figure 6a, a
sputtered hole from a previous experiment is visible in the top right corner,
and the nozzle is visible in the bottom right corner. The applied source
voltage was $V_{\text{S}}=$2\text{\,}\mathrm{kV}$$ and discharge currents
between $70\text{\,}\mathrm{\SIUnitSymbolMicro
A}120\text{\,}\mathrm{\SIUnitSymbolMicro A}$ were measured. After
$10\text{\,}\mathrm{s}$ of plasma operation, Figure 6b, a pit starts forming
with a diameter of about $70\text{\,}\mathrm{\SIUnitSymbolMicro m}$. Chemical
analysis by EDS shows increased Cu and decreased O signals in the pit region,
indicating a removal of the native Cu oxide by sputtering. This exposes the
underlying Cu metal, leading to a higher Cu L$\alpha$ signal. After
$50\text{\,}\mathrm{s}$, the pit is widened to about
$100\text{\,}\mathrm{\SIUnitSymbolMicro m}$ diameter (Figure 6c). The
sputtered pit area still shows a higher Cu signal than the unaffected Cu
surface around it, similar to Figure 6b. The reduction in Cu L$\alpha$ signal
in the top part of the Cu elemental map in Figure 6c results from shadowing of
the generated Cu L$\alpha$ signal x-rays from the inside of the pit toward the
EDS detector. An increase in O K$\alpha$ signal is visible at the pit’s edge
(Figure 6c). This observation indicates the oxidation of Cu in this region.
The O signal increases under prolonged CO2-plasma exposure (not shown here),
which we attribute to the continuous growth of this Cu-oxide layer.

Figure 6: Sputtering and oxidation of a polished Cu surface under CO2 plasma.
a SE-SEM image showing the sample surface opposite to the nozzle with a
$130\text{\,}\mathrm{\SIUnitSymbolMicro m}$ gap. The hole in the top-right
corner is from an earlier experiment. b and c Images and O/Cu elemental maps
after $10\text{\,}\mathrm{s}$ and $50\text{\,}\mathrm{s}$ plasma treatment. A
pit forms due to sputtering. A higher Cu signal in the pit indicates the
removal of the native oxide in the plasma spot. c Enhanced O signal is visible
at the pit’s edge (marked with a solid arrow). The depletion of Cu signal is
due to the shadowing of the x-ray signal toward the detector. d Top-view BSE-
SEM image of various pits and holes in the Cu foil after plasma treatment. e
Side-view SE-SEM image of a hole showing vertical side walls. f and g The
elemental maps reveal enhanced oxidation around the plasma spots and higher Cu
signal in the pits similar to b and c. h Light-microscopy image showing
interference effects in the oxidized regions around the plasma spots.

The sample was investigated again in the SEM and using light microscopy after
the _in-situ_ experiments (Figures 6d–h). The top-view BSE-SEM image acquired
at $20\text{\,}\mathrm{keV}$ shows different experimental sites of local CO2
plasma treatment (Figure 6d). The black areas show regions where the total
thickness (about $30\text{\,}\mathrm{\SIUnitSymbolMicro m}$) of the Cu support
was sputtered away, leaving holes behind. One of the holes is also displayed
in the SE-SEM image in Figure 6e. The tilted view reveals the high aspect
ratio of the sputtering process, resulting in vertical sidewalls. The
elemental map of O shows an increased O signal around the plasma spots,
similar to Figure 6c, which is decreasing in radial direction away from the
spots. For the pits, the removal of the native oxide layer of Cu leads to an
increased Cu L$\alpha$ signal. The increased O concentration around the holes
reduces the effective atomic number relative to metallic Cu. This results in a
reduced BSE intensity in Figure 6d in the oxidized regions due to the BSE
signal’s atomic number $Z$ dependence 21. Interestingly, the oxidation of the
Cu surface reaches a few hundred away from the initial plasma spots. This
phenomenon is more clearly visible in the light-microscopy image (Figure 6h),
which shows interference effects related to the gradually changing thickness
of the grown Cu-oxide film (Newton rings). In the top left corner of the
image, there is an unaffected (i.e., without plasma-induced oxidation) area of
the sample (marked with an arrow in Figure 6h). Overall, the polished Cu
surface is sputtered away under CO2 plasma. A local CO2 plasma causes
oxidation around the plasma spot, probably forming a Cu-oxide film with
decreasing thickness away from the plasma spot. This oxidation is most likely
caused by oxygen species (such as atomic or ionized O) generated in the
plasma. These species can be transported out of the plasma (so-called
afterglow) by the gas flow, explaining why the oxidation of the Cu is observed
away from the plasma spot as well.

Next, similar experiments with CO2 plasma on Ni nanoparticles were performed
(Figure 7a, left column). The Ni particles were deposited on a Cu support film
and formed a layer with a (varying) thickness of a few (Figure 4e). The gap
distance was $250\text{\,}\mathrm{\SIUnitSymbolMicro m}$, and the discharge
current was $5\text{\,}\mathrm{\SIUnitSymbolMicro A}$. Local oxidation was
observed _inside_ the plasma spot, as marked by the arrow in the elemental map
acquired after $10\text{\,}\mathrm{s}$ plasma exposure. The O signal increases
with increasing plasma duration from
$0\text{\,}\mathrm{s}60\text{\,}\mathrm{s}$. This aspect is not as evident in
the noisy elemental maps but more clearly visible in the summed-up and
normalized EDS spectra (see Figure S5 for details) from the plasma-spot region
as an increasing O K$\alpha$ peak (Figure 7b, left). This observation is
different from the oxidation _outside_ the plasma spots observed for a flat Cu
sample (Figure 6). This may be caused by a more pronounced sputtering of Cu
compared to Ni, where any oxidized Cu in the central plasma spot is directly
removed by ion bombardment. In addition, the ion dose applied to the Ni
nanoparticles (Figure 7, $5\text{\,}\mathrm{\SIUnitSymbolMicro A}$) was lower
than for bare Cu (Figure 6, about $70\text{\,}\mathrm{\SIUnitSymbolMicro
A}120\text{\,}\mathrm{\SIUnitSymbolMicro A}$), resulting in more sputtering
for the latter. Besides oxidation, the sputtering during CO2 plasma changed
the morphology of the Ni particles inside the plasma spot from round shapes
toward conical shapes, as discussed earlier (Figure 4). Overall, the EDS
signals for Ni and Cu (from the underlying substrate) are nearly unchanged for
CO2 plasma for this ion dose (Figure 7b, right).

Besides using CO2, oxidation and sputtering of Ni nanoparticles was also
studied for a $25\text{\,}\mathrm{\char 37\relax}$
O2-$75\text{\,}\mathrm{\char 37\relax}$ Ar gas mixture (denoted as Ar/O2 in
the following). The plasma parameters were kept the same as for CO2 (gap
distance of $250\text{\,}\mathrm{\SIUnitSymbolMicro m}$ and an approximate
discharge current of $5\text{\,}\mathrm{\SIUnitSymbolMicro A}$). The oxidation
of the Ni particles by Ar/O2 plasma is similar to CO2 plasma; the oxidation is
localized to the plasma region (Figure 7a, right column), and the oxidation
gradually increases with plasma duration (see O K$\alpha$ signal in Figure 7c,
left). It is noteworthy, that the oxygen-rich spot at $0\text{\,}\mathrm{s}$
in Figure 7a for Ar/O2 (marked with a dashed arrow) results from a previous
experiment. Overall, the sputter rate of Ni particles for Ar/O2 plasma is
higher than for CO2. The enhanced sputter yield for Ar/O2 plasma is evident
from the change in Ni and Cu K$\alpha$ signals in the right plot in Figure 7c,
where the Ni/Cu signal decreases/increases due to the continuous removal of Ni
particles and subsequent exposure of the underlying Cu support. This aspect is
also slightly visible as a reduction of O signal in the central part of the
plasma spot in the O elemental map after $60\text{\,}\mathrm{s}$ (Figure 7a).
After the removal of the oxidized Ni particles in this area, the underlying Cu
support is not oxidized _inside_ the plasma-spot region, leading to the
observed O depletion (cf. with O maps in Figure 6f). This observation
qualitatively agrees with simulated sputter yields using SRIM (Table S1 in the
supplementary information), where Ar has higher sputter yields $Y$ than O.
However, CO is another typical molecule in CO2 plasmas 46 that could not be
simulated and compared with Ar using SRIM.

Figure 7: Local oxidation of Ni particles under CO2 and O2/Ar plasma
treatment. a Elemental maps showing the O K$\alpha$ intensity for increasing
plasma duration between $0\text{\,}\mathrm{s}60\text{\,}\mathrm{s}$ (top to
bottom) for CO2 plasma (left column) and O2/Ar plasma (right column) for
similar discharge current (about $5\text{\,}\mathrm{\SIUnitSymbolMicro A}$)
and gap distance (about $250\text{\,}\mathrm{\SIUnitSymbolMicro m}$). A spot
of local oxidation is visible after $10\text{\,}\mathrm{s}$ (marked with
horizontal arrows). The O-rich spot at $0\text{\,}\mathrm{s}$ for O2/Ar is
from a previous experiment (dashed vertical arrow). (b and c) Comparison of
extracted EDS signals in selected energy region for the O (left), and Ni and
Cu energy regions (right). The increase in O signal for increasing plasma
duration is visible. c For O2/Ar plasma, sputtering of Ni particles and
subsequent exposition of the underlying Cu support reduces the Ni K$\alpha$
signal and increases the Cu K$\alpha$ signal. This effect is absent in b,
indicating a significantly reduced sputter yield for CO2 plasma. For
comparison, the EDS spectra in b and c were normalized to the integrated
signals in the energy intervals
$[$2\text{\,}\mathrm{keV}$,$5\text{\,}\mathrm{keV}$]$ and
$[$10\text{\,}\mathrm{keV}$,$14\text{\,}\mathrm{keV}$]$ containing only
bremsstrahlung background signal.

Since the sputtering is primarily caused by the bombardment of the grounded
sample surface (relative to a positively biased nozzle) with positively
charged ions, switching the polarity between the nozzle and the sample can
mitigate sputtering. This aspect was verified experimentally by switching the
polarity upon using another DC-DC converter (XP Power, CA12N) than the
previously used one (XP Power, CA20P). The experiment was then repeated using
again CO2 gas and a Cu target. The experimental setup is shown in Figure 8a
with the EDS acquisition area marked with a dashed line. The polarity between
the nozzle and the sample is reversed compared to all other conducted
measurements in this work.

Figure 8: Local oxidation of a polished Cu surface under CO2 plasma treatment
with reversed electrode polarity. a Overview SEM image of the plasma gap with
the EDS region for (b) marked with a dashed rectangle. Note the reversed
nozzle/sample polarities. b Quasi _in-situ_ EDS measurements before (upper
row) and after (lower row) CO2 plasma treatment. The increased O signal is
caused by oxidation and re-deposition of oxidized Cu from the nozzle. c BSE-
SEM image ($5\text{\,}\mathrm{keV}$) of the plasma treated after plasma
experiments. d Higher magnification SE-SEM image of the central plasma spot. e
Pits formed in the central plasma spot, probably caused by ion- sputtering.

Comparison of the O elemental maps before and after plasma treatment (Figure
8b) reveals a pronounced oxidation of the surface in a comparatively wide area
(about $400\text{\,}\mathrm{\SIUnitSymbolMicro m}$ diameter), i.e., larger
than the actual plasma spot. The latter is not clearly visible in the highly
tilted view onto the Cu target’s surface in Figure 8b, but it is visible in
the top-view BSE-SEM image in Figure 8c. This BSE-SEM image was captured
during the investigation of the same sample after the plasma experiments using
standard SEM imaging parameters. The top-view BSE-SEM image in Figure 8c
reveals the plasma spot with a higher image intensity relative to the
surrounding dark area related to the oxidized Cu surface. Note that a low
primary electron energy of $5\text{\,}\mathrm{keV}$ was used for BSE imaging
to increase surface sensitivity. The increased BSE-image intensity of the
bright plasma spot (Figure 8c) can be explained by mild sputtering in this
region by negatively charged ions bombarding the positively charged Cu
surface. This removes the oxide layer and reveals metallic Cu, ultimately
leading to higher BSE image intensity due to a higher average $Z$ than the
surrounding oxidized Cu surface. Even though mild sputtering is present, no
large pit or hole is visible in the plasma-spot region (Figures 8d and e)
compared to the initially used negative sample polarity (Figure 6d). The
plasma spot area has a diameter of about
$25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ (marked with a dashed circle in
Figure 8d) and shows the formation of small pits with
$200\text{\,}\mathrm{nm}300\text{\,}\mathrm{nm}$ (surface) diameter (Figure
8e). These pits are likely caused by the sputtering process and may show its
initial stage. Overall, the sputtering of the sample surface is highly reduced
when the sample surface is positively biased relative to the nozzle.
In the configuration shown in Figure 8a, the mainly positively charged ions
are accelerated toward the negatively biased nozzle, resulting in sputtering
of the nozzle surface. Indeed, the orifice diameter increased after these
experiments and sputtered material was re-deposited inside the orifice (Figure
S6). The sputtered nozzle material is likely also re-deposited onto the
opposing sample surface. Since the same nozzle was used throughout all
experiments here, previously deposited sample material (mostly Cu) _onto_ the
nozzle from earlier experiments is now sputtered and re-deposited _from_ the
nozzle onto the sample (see the schematic in Figure S6j). In our case, the
orifice area is mostly covered with (oxidized) Cu (Figure S6b) before the
experiments shown in Figure 8, meaning that part of the oxidized region is
likely caused by redeposited Cu oxide from the nozzle.
To test this hypothesis, the gas was switched from CO2 to N2, and the plasma-
treated area showed a N and O signal (see Figure S7 in the supplementary
information). For a N2 plasma on a Cu surface, an O signal is unexpected and
should not be present without considering the aforementioned re-deposition
effects. Our results suggest that part of the O signal in Figure 8b is caused
by re-deposited oxidized Cu from the nozzle from previous experiments. Even
though this effect is undesired for pure oxidation with plasma-generated
radicals, it may be interesting to study film growth during sputtering.

In summary, oxidation of Ni nanoparticles was observed for CO2 and Ar/O2.
Oxidation is limited to the central plasma-spot region. For the same ion dose,
Ar/O2 sputtering of Ni nanoparticles is more pronounced than for CO2. In
contrast, oxidation of a flat Cu surface occurs around the central plasma-spot
region, which is mostly sputtered rather than oxidized. Sputtering with
positively charged ions causes rapid removal of sample material when the
nozzle is used as an anode (positive polarity). This results in pits and holes
in the central plasma region.
Sputtering of the sample can be strongly reduced by reversing the polarity
between the sample and the nozzle, leading to less damage during oxidation.
However, sputtering of the nozzle material in this configuration causes damage
to the tip of the nozzle and redeposition of this material onto the sample
surface. Pure sample oxidation without sputtering or redeposition of material
requires other plasma configurations.

###  2.3 Limitations and outlook

The current setup presented here demonstrates significant advances compared to
the state-of-the-art, including a stable DC discharge and true _in-situ_ SEM
imaging. This enables further research regarding plasma-surface interactions,
plasma physics, sputtering, and more. However, certain limitations remain,
particularly in terms of expanding the scope of potential research areas. For
example, fields such as plasma catalysis or biomedical applications of plasma
are growing rapidly, increasing the need for more advanced experimental
techniques to study, e.g., plasma-catalyst or plasma-cell interactions 47, 48.
For such research topics, this setup is currently unsuited since sputtering of
the sample (or redeposition of material from the nozzle) is undesirable and
prevents studying the samples under relevant conditions. In order to study
such samples, the sputtering behavior of the plasma should be eliminated. In
principle, the current setup could be optimized further to reduce the
discharge voltage to decrease the ion energy, lowering the sputtering rates.
One potential approach would be to increase the ballast resistor in the
system, to limit the current and lower the discharge voltage. Another approach
would be to further increase the pressure, as it is expected that the current
setup operates below the optimum value. However, increasing the gas flow rate
would require an upgrade to the pumping system of the SEM since the current
experiments were performed at the limit of the microscope when operating in
high-vacuum mode. The pressure could also be increased by decreasing the gap
distance, but this would then also increase the probability of unwanted arcing
behavior, as was also observed in our experiments.
Rather, we believe that in order to expand the research potential of (quasi)
_in-situ_ plasma in SEM experiments without sputtering or redeposition
effects, a fundamentally different plasma type may be required. However, this
would require a significant alteration of the plasma setup and a complete
redesign of the electronics. A number of plasma types could be of interest,
each with their potential applications and limitations, as well as practical
drawbacks.
A common plasma discharge is the dielectric barrier discharge (DBD) 5. This
alternating current (AC, or pulsed) discharge is characterized by a dielectric
layer covering one or both electrodes, limiting the current and thus
preventing arc formation. This is a non-thermal plasma which is often used in
plasma catalysis and biomedical research. However, DBD plasmas are generally
filamentary, where the filaments consist of microdischarges (short duration,
high current discharges). These filaments make the plasma treatment of the
sample heterogeneous, complicating the analysis, and cause issues with
electromagnetic interference. In principle, DBDs can be operated in a uniform
mode 49, but this requires precise tuning of all relevant parameters
(including the dielectric material, voltage, frequency, discharge gas, and
pressure) further impeding rapid development of such an experimental setup.
An alternative discharge based on the DBD is the so-called surface discharge.
This plasma is similar to the DBD, but one of the electrodes is embedded or
below the dielectric, whereas the other electrode is placed on the surface of
the dielectric. With this, the discharge will be generated at the surface of
the dielectric. This plasma still requires AC or pulsed power, but is
generally more convenient to operate in a uniform mode 5.
Another approach could be using a plasma jet. Many geometries exist, either
powered by DC, pulsed, or AC power, but they all have in common that the
plasma is generated within a device, after which it flows outwards, e.g., to a
sample 12. The main difference with the setup presented here is that in the
current setup, the plasma is generated in the gap between the nozzle and the
sample rather than in the nozzle and sent to the sample. A main advantage of
such a plasma jet could be the elimination of the sputtering behavior, as
charged plasma species are not predominant (or even absent in the so-called
afterglow). Based on this geometry, an electron beam plasma can be generated
5, of which a variation was previously introduced in an SEM 14. In such
plasmas, a high-energy electron beam is sent through a neutral gas, where the
electrons ionize gas molecules. The plasma can then be sent to a sample
through a gas flow, or the ions/electrons could be selectively attracted by
biasing the sample. An external AC or DC circuit can also be added to further
sustain and alter the plasma discharge, depending on the desired properties.
Having access to a high-energy electron beam makes an SEM promising to further
explore such plasmas.
Note that all AC or pulsed-powered plasmas are very likely to interfere with
the true _in-situ_ imaging of the SEM since the electron beam will be
deflected periodically during scanning, drastically decreasing the image
resolution. Depending on the desired experiment, this issue could be overcome
by turning off the plasma during image acquisition, though this does limit the
_in-situ_ capabilities of the setup.
Further, introducing a microplasma may enable very different experiments and
applications. On the one hand, the _in-situ_ plasma may lead to new analytical
techniques in an SEM, such as glow discharge optical emission spectroscopy
(GDOES) 50, 51, where the emission from sputtered material in a plasma is
studied while ablating the sample material for depth profiling (similar to
secondary ion mass spectroscopy in focused ion beam instruments 52). On the
other hand, established (e.g., EDS or wavelength dispersive x-ray
spectroscopy, WDS 53) or more recently available (e.g., electron energy loss
spectroscopy, EELS 54) analytical methods in SEMs may have the potential to
probe the ionic species in the plasma cloud. This would provide essential and
direct _in-situ_ feedback for plasma simulation codes and holds promise for
improved control over plasma setups.

##  3 Conclusions

A custom-built microplasma setup was realized inside an SEM based on the
design by Matra et al. 13. A nozzle with a small orifice feeds a gas into the
evacuated SEM chamber, from which a plasma can be generated by applying a
certain electrical potential. Stable DC glow discharge plasmas with Ar, Ar/O2,
CO2, and N2 gases were successfully generated in the SEM’s vacuum chamber. In
general, larger discharge currents were measured for higher gas flow rates and
smaller gap distances. A non-uniform gas-pressure profile was observed in the
plasma gap, which — in combination with a non-uniform electric field of the
electrode geometry — complicates a direct comparison of the shown setup with
conventional plasma systems. Simultaneous SEM imaging with SEs and BSEs during
plasma operation was demonstrated, enabling _in-situ_ studies of sample-plasma
interactions in the SEM.
A few exemplary plasma-sample interactions were studied. Sputtering of Cu
surfaces and Ni nanoparticles under different gases was observed. The lower
sputter yield of the Ni particles compared to the Cu support, as well as the
incidence-angle dependence of the sputter yield, results in the local
formation of cones in the plasma-treated area. The same phenomenon was studied
with conventional plasma reactors, which shows that our setup can replicate
such experimental conditions on the local scale of several tens of . Local
oxidation of Cu and Ni was observed for CO2 gas and an Ar/O2 gas mixture. At
the same time, however, the sample was either simultaneously sputtered away by
ion bombardment on the sample, or nozzle material was redeposited on the
sample by sputtering of the nozzle. These limitations might be overcome by
further optimizations of the setup, though for applications where sputtering
is detrimental, other types of plasma are to be considered.
In conclusion, we have demonstrated that _in-situ_ studies of plasma-sample
interactions in a modern SEM are possible. This approach provides direct
insight into morphological and chemical changes (via EDS) of the sample during
and after plasma treatment. Overall, this may lead to a better understanding
of plasma physics and plasma-surface interactions.

##  4 Experimental

###  4.1 SEM Operation with the Plasma Setup

Plasma experiments were performed using an FEI Quanta 250 ESEM equipped with
an Oxford Instruments X-Max EDS detector
($80\text{\,}\mathrm{m}\mathrm{m}^{2}$ sensor area). Figure 9a schematically
shows the main parts of the plasma setup that was built in-house. A
horizontally aligned steel nozzle with a small orifice (SS-1/8-TUBE-CAL-20,
$20\text{\,}\mathrm{\SIUnitSymbolMicro m}$ nominal orifice diameter, Lenox
Laser) is fixed opposite to a nearly vertically aligned sample surface. The
sample surface is slightly tilted with an angle
$\alpha\approx$10\text{\,}\mathrm{\SIUnitSymbolDegree}$$ toward the electron
beam for better SEM imaging conditions. The sample-nozzle distance (“Gap” in
Figures 9a-c) determines the plasma gap distance and can be adjusted by moving
the sample with SEM microscope stage controls. A gas flows from the nozzle
into the gap toward the sample surface. The nozzle can be biased with a DC
voltage $V_{\text{S}}$ in the range of
$-1.25\text{\,}\mathrm{kV}2\text{\,}\mathrm{kV}$, i.e., with a positive or
negative polarity relative to the sample. A ballast resistance
$R_{\text{B}}=$4.3\text{\,}\mathrm{M\SIUnitSymbolOhm}$$ is used to limit the
discharge current. The discharge current
$I_{\text{D}}=V_{\text{M}}/R_{\text{M}}$ is measured by the voltage drop
$V_{\text{M}}$ across a $R_{\text{M}}=$1\text{\,}\mathrm{k\SIUnitSymbolOhm}$$
resistor.

Figure 9: Schematics and images of the plasma-in-SEM setup. a Schematic
showing the experimental setup and the most important components. Gas flows
from a nozzle orifice over an adjustable gap distance toward a sample surface.
A high voltage $V_{\text{S}}$ is applied to ignite the plasma. The sample
surface is slightly tilted at an angle $\alpha$ toward the SEM incidence,
allowing for _in-situ_ SEM imaging. b Image of the setup taken with the built-
in infrared camera of the SEM showing the setup. A few additional components
are shown compared to a, such as a webcam and the electron detectors, ETD and
LFD. c Higher-magnification side-view of the plasma region using the webcam
without plasma (upper) and with ignited plasma (lower, the microplasma is
marked with an arrow)

Figure 9b displays the experimental setup with an image taken with the
microscope’s built-in infrared (IR) camera. A few additional components
compared to the schematic in Figure 9a are visible, which are explained from
top to bottom in the following. The ETD and the large-field detector (LFD) are
used for SEM imaging in high-vacuum and low-vacuum modes, respectively. The
shown images in this work are mainly SE-SEM images. Selected BSE-SEM images
are mentioned explicitly in the text. A pressure-limiting aperture (PLA) with
a $500\text{\,}\mathrm{\SIUnitSymbolMicro m}$ diameter is mounted on the SEM
pole piece to restrict gas flow into the microscope column. An IR-USB webcam
(Arducam B0205) is mounted in addition to the microscope’s built-in IR camera
to improve imaging conditions of the plasma and control the gap distance. The
sample stage consists of a threaded metal rod that is rigidly fixed with two
nuts to a Teflon piece. The Teflon piece isolates the sample from the
microscope stage to prevent current flow through the latter and possible
damage to the microscope. Instead, the current flows via a cable to the
measurement resistor $R_{\text{M}}$. The sample stage with the threaded metal
rod and the Teflon block are fixed on an SEM stub, which itself is fixed on
the moveable SEM stage. Two micrometer stages (Thorlabs MS3/M) are used to
laterally position the nozzle close to the optical axis (below the SEM pole
piece) before closing the SEM chamber. The nozzle and the webcam are mounted
on a Al platform that is fixed above the moving microscope stage. The height
of the Al platform can be adjusted to change the working distance between the
SEM column and the sample (typically $15\text{\,}\mathrm{mm}$). The gas line
and electrical connections are routed through a custom home-made feedthrough
flange.
A detailed image of the plasma gap is shown in the webcam view (Figure 9c).
Commercially available grids or apertures made for transmission electron
microscopy (TEM) with $3\text{\,}\mathrm{mm}$ diameter (Gilder Grids GA50 Cu
apertures) were typically used as sample or sample support for nanoparticles.
The sample is mounted on an Al wedge with conductive Ag paste (EM-Tec AG15).
The Al wedge was ground at an angle $\alpha$ and fixed to the threaded metal
rod’s end with conductive Ag paste. The lower image in Figure 9c shows the
working setup with a glowing DC microplasma. More details about the
experimental setup can be found in the supplementary information (Figure S8).

###  4.2 Plasma Operation

Plasma experiments were performed in the high-vacuum mode of the microscope
since undesired discharges in the SEM chamber in low-vacuum mode were observed
when applying high voltage between the nozzle and the sample. The high-vacuum
mode reached a stable chamber pressure of around
$2\text{\times}{10}^{-2}\text{\,}\mathrm{Pa}$ while providing a gas flow of
about $2\text{\,}\mathrm{sccm}8\text{\,}\mathrm{sccm}$ through the nozzle
($20\text{\,}\mathrm{\SIUnitSymbolMicro m}$ nominal orifice diameter as per
the manufacturer) into the microscope chamber. The gas flow was monitored
using an Alicat flow meter (M-200SCCM-D/5M). We used CO2 (purity
$99.995\text{\,}\mathrm{\char 37\relax}$), Ar ($99.9999\text{\,}\mathrm{\char
37\relax}$), and N2 ($99.9999\text{\,}\mathrm{\char 37\relax}$) gases, and a
$75\text{\,}\mathrm{\char 37\relax}$Ar/$25\text{\,}\mathrm{\char 37\relax}$O2
gas mixture (measured: $74.88\text{\,}\mathrm{\char
37\relax}$/$25.12\text{\,}\mathrm{\char 37\relax}$) in this work (bought from
Air Products).
The plasma was operated by applying and controlling the voltage difference on
the nozzle relative to the sample. A DC-DC converter with a
$1\text{\,}\mathrm{M\SIUnitSymbolOhm}$ output resistor (CA20P or CA12N
depending on polarity, XP Power) was powered by an RS PRO IPS-3303 power
supply. The $1\text{\,}\mathrm{M\SIUnitSymbolOhm}$ output resistor limits the
output current of the DC-DC converter in standalone usage for user safety. The
output resistor is in series with a $3.3\text{\,}\mathrm{M\SIUnitSymbolOhm}$
resistor, resulting in a total ballast resistance
$R_{\text{B}}=$4.3\text{\,}\mathrm{M\SIUnitSymbolOhm}$$. The output high
voltage $V_{\text{S}}$ of the DC-DC converter was adjusted with a control
voltage between $0\text{\,}\mathrm{V}5\text{\,}\mathrm{V}$ using a Keysight
E36106B power supply. After plasma ignition, the discharge current was
regulated by adjusting $V_{\text{S}}$ with the control voltage. Voltage-
current characteristics of the plasma were measured with a Keithley 2400
source measurement unit. The highest source voltage of $2\text{\,}\mathrm{kV}$
was applied, after which the source voltage was gradually reduced while
registering the current until no discharge current was measurable. The
discharge voltage of the DC plasma $V_{\text{D}}$ is calculated as
$V_{\text{D}}=V_{\text{S}}-I_{\text{D}}\left(R_{\text{B}}+R_{\text{M}}\right)$
6.

###  4.3 Sample Preparation

A Cu TEM aperture ($50\text{\,}\mathrm{\SIUnitSymbolMicro m}$, Gilder Grids
GA50) with a diameter of $3\text{\,}\mathrm{mm}$ and a thickness of about
$30\text{\,}\mathrm{\SIUnitSymbolMicro m}$ was used in most experiments to
ensure a well-defined, flat electrode opposing the nozzle. For experiments
with nanoparticles, commercial Ni particles (nanopowder,
$<100\text{\,}\mathrm{nm}$ nominal average particle size,
$>99\text{\,}\mathrm{\char 37\relax}$ purity, Sigma-Aldrich, CAS number
7440-02-0) were mixed with acetone and then drop cast on the Cu disc. After
solvent evaporation, a thin film of Ni particles is left on the Cu surface.
Drop casting was repeated multiple times until the TEM aperture was fully
covered with Ni particles.

###  4.4 Data Processing

_Fiji_ 55 was used for general image processing. Images were stitched together
using the “Grid/collection stitching” plugin 56. Image series were registered
using the “Descriptor-based series registration (2d/3d + t)” plugin 57. The
background-corrected x-ray peak intensities (net intensities) for the EDS maps
were extracted using the “TruMap” function in the Oxford Instruments _AZtec_
software (version 2.1). Additional analyses of extracted (summed-up) EDS
spectra from specific regions were processed with the _HyperSpy_ Python
package 58.

##  5 Data Availability Statement

Raw data files and data-treatment scripts are available at Zenodo 59
(https://doi.org/10.5281/zenodo.8042029).

##  6 Author Contributions

LG: Conceptualization, Methodology, Investigation, Software, Validation,
Formal Analysis, Data Curation, Visualization, Writing – Original Draft. DC:
Conceptualization, Methodology, Investigation, Writing –- Review & Editing.
RDM: Conceptualization, Methodology, Investigation, Validation, Writing –
Original Draft. AO: Conceptualization, Methodology, Writing –- Review &
Editing. SVA: Conceptualization, Supervision, Project Administration, Funding
Acquisition, Writing –- Review & Editing. AB: Conceptualization, Supervision,
Project Administration, Funding Acquisition, Writing –- Review & Editing. SB:
Conceptualization, Supervision, Project Administration, Funding Acquisition,
Writing –- Review & Editing. JV: Conceptualization, Methodology, Supervision,
Project Administration, Funding Acquisition, Writing –- Review & Editing

##  7 Conflicts of Interest

There are no conflicts to declare.

##  8 Acknowledgments

LG, SB, and JV acknowledge support from the iBOF-21-085 PERsist research fund.
DC, SVA, and JV acknowledge funding from a TOPBOF project of the University of
Antwerp (FFB 170366). RDM, AB, and JV acknowledge funding from the Methusalem
project of the University of Antwerp (FFB 15001A, FFB 15001C). AO and JV
acknowledge funding from the Research Foundation Flanders (FWO, Belgium)
project SBO S000121N.

Details of $V$-$I$ measurements, exemplary Paschen curve for N2, sputter yield
simulations, details about EDS spectrum comparison, SEM/EDS characterization
of the orifice and redepostion effects, and more details about the
experimental setup are found in the supplementary information.

##  9 Supplementary Information for “ _In-situ_ Plasma Studies using a Direct
Current Microplasma in a Scanning Electron Microscope”

###  9.1 Details about Voltage-Current-Characteristic Measurements

The main text shows voltage-current characteristics of the generated
microplasma in the scanning electron microscope’s (SEM’s) chamber. A problem
during measurements was the continuous sputtering of the sample surface when
using the nozzle as an anode with positive bias. Typical measurements of the
voltage drop across the measurement resistor $R_{\text{M}}$ versus the
measurement duration are shown in Figure S1. All measurements were started by
first applying the highest possible source voltage $V_{\text{S}}$ with the DC-
DC converter ($2\text{\,}\mathrm{k}\mathrm{V}$, visible as strong onset in the
plots) and then gradually decreasing the source voltage with
$40\text{\,}\mathrm{V}$ steps until no voltage across the measurement resistor
$R_{\text{M}}$ was measurable anymore. A small parasitic offset voltage was
measured and subtracted from a reference region (e.g., the shaded area in
Figure S1a). Ideally, the voltage steps in the measured curve should be
horizontal plateaus, whereby each step corresponds to a defined step in the
applied source voltage (here in steps of $40\text{\,}\mathrm{V}$). However, as
visible in the inset in Figure S1a, the discharge current was not stable but
instead steadily increasing, especially in the first few seconds of plasma
operation and at high currents. We attribute this to surface sputtering and
rapid change of the electrode geometry, which significantly decreased the
resistance of the gap and thus increased the discharge current (given the
constant applied voltage). Even though it is, in principle, possible to
correct the slope of the $V_{\text{M}}$-time curve, we opted to simply
calculate the average value of each voltage step by manual extraction (see
Jupyter notebook on Zenodo 59). Figure S1b shows a more extreme example of
higher discharge currents, resulting in faster surface sputtering and an even
more pronounced discharge-current increase over time. A strong slope is
visible in the inset figure for the first few seconds of plasma operation. The
voltage steps become horizontal at around $20\text{\,}\mathrm{s}$ in the plot.

Figure S1: Measurement problems of voltage-current characteristics, shown for
two different measurements. a The shaded area marks a reference region without
actual microplasma operation, which was used to determine and subtract the
voltage offset. The sharp onset shows the plasma ignition for the highest
source voltage of $2\text{\,}\mathrm{kV}$, followed by subsequent reduction in
$40\text{\,}\mathrm{V}$ steps. The ideally-horizontal steps show a positive
slope due to a continuous discharge current increase, especially in the first
few seconds of plasma operation. This is attributed to rapid sputtering of the
sample surface. b Another example of higher gas flow rate, resulting in higher
discharge currents. The sputtering is faster, and the slope is more pronounced
than in (a).

A theoretical Paschen curve calculated for N2 is shown in Figure S2. The
vertical line marks the minimum breakdown voltage
$V_{\text{B,min}}=$346\text{\,}\mathrm{V}$$ of the curve at
$(pd)_{\text{min}}=$142\text{\,}\mathrm{Pa}\,\mathrm{c}\mathrm{m}$$. The
breakdown voltages were calculated according to the equation 6

$\displaystyle{\displaystyle
V_{\text{B}}={\frac{Bpd}{\ln(Apd)-\ln\left[\ln\left(1+{\frac{1}{\gamma_{\text{see}}}}\right)\right]}}}\quad\text{,}$
(1)

with the parameters
$A=$11.8\text{\,}\mathrm{c}\mathrm{m}^{-1}\,\mathrm{Torr}^{-1}$=$8.85\text{\times}{10}^{-2}\text{\,}\mathrm{c}\mathrm{m}^{-1}\,\mathrm{Pa}^{-1}$$,
$B=$325\text{\,}\mathrm{V}\,\mathrm{c}\mathrm{m}^{-1}\,\mathrm{Torr}^{-1}$=$2.44\text{\,}\mathrm{V}\,\mathrm{c}\mathrm{m}^{-1}\,\mathrm{Pa}^{-1}$$,
and $\gamma_{\text{see}}=0.01$ (secondary-electron-emission coefficient) for
N2 60. The Python code of the Wikipedia user “Krishnavedala”
(https://commons.wikimedia.org/wiki/File:Paschen_curves.svg) was used and
modified for Figure S2.

Figure S2: A calculated Paschen curve for N2 to show the two typical plasma
regimes for DC glow discharges on the left and right branch relative to the
minimum of the curve (referred to as “optimum” in the main text).

No continuous plasma discharges were observed for specific combinations of
(large) gap distances, (low) gas flow rates, and (low) source voltages. In
such cases, the electron beam may be able to ignite the plasma and initiate a
continuous discharge. In other cases, a discharge current was only measured
when the electron beam was on and immediately vanished after the beam was
switched off (Figure S3). Two examples for the latter are shown in Figures S3a
and b, where each step in the signal corresponds to the electron beam being
switched on or off. A smoothed signal is plotted as well for better visibility
of the steps. The signal was smoothed using locally weighted regression
(LOWESS) with _HyperSpy_ 58 with _smoothing_parameter=0.03_ and
_number_of_iterations=1_. The calculated current is in the nA-range, which is
typical for SEM measurements, but was not explicitly measured here. This
implies that the generated plasma current directly relates to the electron-
beam current. The beam conditions were $15\text{\,}\mathrm{keV}$,
$30\text{\,}\mathrm{\SIUnitSymbolMicro m}$ objective aperture, and spot size
$5$. Interestingly, some steps show an initial current spike in the non-smooth
signal (about $40\text{\,}\mathrm{nA}100\text{\,}\mathrm{nA}$ for Figure S3a
or $20\text{\,}\mathrm{nA}50\text{\,}\mathrm{nA}$ Figure S3b) and then the
reduction to the actual electron-beam current (presumably, but not measured).

Figure S3: Measured voltage across a $1\text{\,}\mathrm{k\SIUnitSymbolOhm}$
resistor (left abscissa, note the $1\text{\times}{10}^{-5}$ factor) and
corresponding discharge current (right abscissa) for experimental conditions
that did not result in a continuous plasma for two different nozzle-sample
polarities. Raw and smoothed signals are displayed. Each step corresponds to
the time the electron beam was on and scanning. The measured current of a few
nA is within the range of typical electron-beam currents for the used beam
parameters. a Nozzle as the anode, and (b) nozzle as the cathode.

###  9.2 Sputter Yield Simulation

Monte Carlo simulations using SRIM 2013 (http://www.srim.org/) were run to
investigate sputter yields for different ions and materials (Table S1) and
ion-incidence angles (Figure S4).
Overall, heavier ions (here Ar) show a higher sputter yield $Y$ than lighter
ions (C and O) on Cu and Ni targets. Note, that CO2 plasma creates mostly CO
and O 46 instead of elemental C, meaning that the latter is given here only
for completeness. CO sputtering could not be simulated with SRIM. The higher
sputter yield of Cu compared to Ni may explain the formation of Ni cones on a
Cu substrate under Ar^+ bombardment.
Regarding the angle dependence of the sputter yield, Figure S4, Ar+ ions with
an energy of $1.5\text{\,}\mathrm{keV}$ hitting a Ni target show that the
maximum sputter yield is found for an incidence angle of about
$65\text{\,}\mathrm{\SIUnitSymbolDegree}$. This angle is related to the cone
shape, and the measured value and its standard deviation for the maximum-
sputter-yield angle are shown with a vertical line and the shaded area. The
differences between the experiment and the simulation may be explained by
limited statistics for the experimental value and the simplified flat geometry
used in the SRIM simulations (e.g., neglecting nanoparticle morphology) and
their limited accuracy 43.

Table S1: Sputter yields $Y$ for different $1.5\text{\,}\mathrm{keV}$ ions for normal incidence on Cu and Ni surfaces obtained from Monte Carlo simulations using SRIM-2013. Ion | $Z$ | Target |  $Y$ (atoms/ion)
---|---|---|---
C | 6 | Cu | $2.21$
|  | Ni | $1.83$
O | 8 | Cu | $2.99$
|  | Ni | $2.47$
Ar | 18 | Cu | $4.58$
|  | Ni | $3.60$
Figure S4: Simulated sputter yield $Y$ versus ion-incidence angle for Ar+ ions
on a Ni target. $Y$ increases with angle up to a maximum around
$65\text{\,}\mathrm{\SIUnitSymbolDegree}$ and then falls off again. The
experimental value $59.3(5.6)\text{\,}\mathrm{\SIUnitSymbolDegree}$
(plus/minus its standard deviation) calculated from the cone angles is marked
with a vertical line (shaded area).

###  9.3 Spectrum Normalization in EDS

Figure S5: Comparison of EDS spectra without (left) and with (right)
normalization with respect to the summed intensity in a given energy interval
(shaded in gray). The upper row shows the full energy range from
$0\text{\,}\mathrm{keV}15\text{\,}\mathrm{keV}$, and the lower row shows
exemplary energy windows containing the C and O signal, and the Ni and Cu
signals. The normalization accounts for varying total x-rays in the EDS
spectra, impeding a direct comparison.

Figures S5a–c show energy-dispersive x-ray spectroscopy (EDS) spectra without
normalization and varying total electron dose, resulting in different total
x-ray counts. The upper plot shows the full energy range from
$0\text{\,}\mathrm{keV}15\text{\,}\mathrm{keV}$ (Figure S5a). The insets in
the lower row (Figures S5b and c) show selected energy ranges for energy
windows containing the C and O signals (Figure S5b), and the Ni and Cu signals
(Figure S5c). For these spectra, a direct comparison is impeded by the
difference in total x-ray counts, resulting in varying peak heights even
without relative changes between spectra. After normalization (Figures S5d–f),
the increasing O signal is revealed (Figure S5e), and the signals for Ni and
Cu are unchanged (Figure S5f). The shaded areas in Figure S5d mark the regions
used for spectrum normalization. These contain no elemental peaks and only
bremsstrahlung background. The sum of the x-ray counts in these two areas was
used for normalization of the spectra.

###  9.4 Characterization of the Nozzle Orifice

Figure S6: Low magnification (upper row) and high magnification (middle row)
SEM images of the fresh (left column, below a) nozzle and after it was used as
anode (middle column, below b) and cathode (right column, below c). EDS maps
of the orifice are shown in the lower middle row starting from (g). Note that
the absence of x-ray signals from the lower left corner in the maps is caused
by the shadowing of x-rays by the nozzle. Fe signal stems from the steel
nozzle, Cu and N signals from material redeposition/sputtering. The last row
depicts the situation of the nozzle when used as an anode or a cathode.

SEM imaging and chemical analysis with EDS were used to inspect the nozzle’s
orifice between different plasma experiments (Figure S6). The unused, fresh
nozzle in the left column of Figure S6 shows some contamination near the
orifice (Figure S6d), but the inner walls of the laser-cut orifice are well-
defined. The outer diameter is about $87\text{\,}\mathrm{\SIUnitSymbolMicro
m}$, which is substantially larger than the nominal
$20\text{\,}\mathrm{\SIUnitSymbolMicro m}$. Since the measured gas flow rate
was close to the nominal value, we suspect that the orifice diameter gets
gradually smaller toward the inside of the nozzle. The EDS elemental map of Fe
K$\alpha$ resulting from the steel nozzle is used to show the absence of
debris (Figure S6g). The lack of signal from the lower left corner of the EDS
map is caused by shadowing effects toward the EDS detector (no direct line of
sight for emerging x-rays).
When the nozzle is used as an anode with a positive bias, most of the
positively charged ions are hitting the sample surface, and the sputtered
material is redeposited on the nozzle tip. Since mostly Cu apertures were used
as sample material, a pronounced Cu K$\alpha$ signal is visible around the
orifice (Figure S6h). The deposited material reduces the outer diameter of the
orifice (here to about $80\text{\,}\mathrm{\SIUnitSymbolMicro m}$).
Afterward, the nozzle was used as a cathode and the ions are now mostly
bombarding the nozzle instead of the sample. Sputtering of the orifice region
leads to the removal of the previously-deposited Cu. The sputtered Cu is now
deposited on the sample instead. The orifice is widened after sputtering (here
about $130\text{\,}\mathrm{\SIUnitSymbolMicro m}$ diameter) and filled with
redeposited material. The EDS maps reveal a higher signal for Cu at the
orifice edges, probably due to pronounced sputtering, which may have removed
the native or CO2-plasma-induced Cu oxide. Since N2 was mostly used as a gas
with the nozzle as a cathode, N is implanted at the orifice edge. The lower
row (Figure S6j) schematically shows the described deposition/removal of
material on the nozzle depending on its polarity.

###  9.5 Redeposition Effects with Nozzle as Cathode ($-$)

Figure S7: Comparison of N2 and CO2 plasma interactions on a Cu surface with
reversed polarity (nozzle as a cathode). a Top-view BSE-SEM image of the Cu
foil used in the experiments. The left/right spot(s) result from N2/CO2 plasma
treatment. Magnified SE-SEM images of the central plasma regions for N2 (b and
c) and CO2 (e and f) reveal grain stemming from redeposited nozzle material
(here Cu oxide). d Grain size measurement from the SEM image in (c). g EDS
elemental maps reveal an O signal besides N after N2 plasma treatment,
probably caused by Cu oxide redeposition. h EDS elemental maps reveal the
oxidized spots diameters of about $400\text{\,}\mathrm{\SIUnitSymbolMicro m}$
on the Cu foil for CO2 plasma.

Material from the sample is redeposited onto the nozzle by sputtering when the
latter is used as anode. _Vice versa_ , if the nozzle is used as a cathode,
material from the nozzle is redeposited onto the sample (Figure S6j). Figure
S7 shows an analysis of this aspect after exposing a Cu sample to N2 and CO2
plasmas with the nozzle used as a cathode.

The overview backscattered electron (BSE)-SEM image (taken at
$10\text{\,}\mathrm{keV}$), Figure S7a, shows reduced image intensity in the
plasma-treated regions due to a locally reduced average atomic number $Z$. Two
spots on the right result from CO2 plasma treatment (marked in blue in Figure
S7a), and the left one is from N2 plasma treatment (marked in yellow). Higher
magnification secondary electron (SE)-SEM images of the central plasma spots
(marked with dashed circles in Figure S7b for N2 and Figure S7e for CO2)
reveal a finely-grained surface structure. An approximate particle size of
$49.9(23.9)\text{\,}\mathrm{nm}$ (arithmetic mean and standard deviation) was
determined from Figure S7c by analyzing the minimum Feret diameters of the
grains (Figure S7d). A similar surface structure is found for N2 (Figure S7c)
and CO2 (Figure S7f). The latter shows the transition region between the
central plasma spot (flat) and the surrounding area (grains). For CO2, the
central area is relatively flat with small pits, which is probably a result of
ion- sputtering and longer plasma treatment duration than for N2.
Inspection of the elemental maps for CO2 reveals apparent oxidation of the
plasma-treated region (Figure S7h). Similarly, the N2 plasma causes a local N
signal in EDS (Figure S7g), indicating N implantation into the Cu sample.
However, Figure S7g also shows an unexpected O signal for N2 plasma treatment.
We suspect that this is caused by the redeposition of previously-oxidized Cu
that is present on the nozzle from earlier experiments (Figure S6j). The layer
of redeposited (oxidized) Cu is likely also the cause of the grain structure
visible in Figure S7c and Figure S7f. Independent of the gas used for plasma
treatment, some material is sputtered from the nozzle when the latter is used
as the cathode (negative polarity). Material near the nozzle orifice is then
primarily sputtered and redeposited onto the opposing sample.

###  9.6 Details about the Experimental Setup

Figure S8: Photos of the experimental setup with opened SEM chamber. See the text for a detailed discussion. Table S2: Table of used components with their part number/stock-keeping unit (SKU). Component | Manufacturer | Part number / SKU
---|---|---
Gas nozzle | LenoxLaser | S-1/8-TUBE-CAL-20
Micrometer stages | Thorlabs | MS3/M
DC-DC converter (+2 kV) | XP Power | CA20P
DC-DC converter (-1.25 kV) | XP Power | CA12N
Power supply 1 | Keysight | E36106B
Power supply 2 | RS PRO | IPS-3303
USB Webcam | Arducam | B0205
Flow meter | Alicat | M-200SCCM-D/5M
Voltage measurement | Keithley | 2400
Cu-aperture targets | Gilder Grids | GA50

Figure S8 shows photos of the assembled microplasma setup with opened SEM
chamber. The subfigures are explained in detail in the following, and the
parts are listed in Table S2.

  * •

Figure S8a (top view): Some components (gas nozzle, webcam, DC-DC converter)
are mounted on an Al platform to isolate them from the movements of the sample
stage of the SEM. The gas nozzle is fixed with a binder clip to two micrometer
stages. The high-voltage cable (starting at the BNC connection of the DC-DC
converter, here visible below the gas line) is fixed close to the end of the
nozzle and here hidden below the black insulating tape. The DC-DC converter
was mounted on the side of the Al platform to save space on the top. The
sample is positioned in the center of the image opposite to the nozzle
(Figures S8b and d). The printed circuit board (PCB, made according to the
data sheet of the DC-DC converter,
https://www.xppower.com/portals/0/pdfs/SF_CA_Series.pdf, with an output
resistor of $1\text{\,}\mathrm{M\SIUnitSymbolOhm}$) with the measurement
resistor $R_{\text{M}}$ is fixed directly on the moveable part of the SEM
stage. Its SMA connector and the cable go directly to the vacuum flange
(Figure S8c).

  * •

Figure S8b (top view): Close-up view of the nozzle-sample configuration. An
adapter made of Teflon is used to isolate the sample from the SEM stage. This
choice was made to protect the SEM electronics from possible current bursts
(e.g., arcing). The discharge current runs through the red cable, then through
$R_{\text{M}}$ (see Figure S8a), and finally through the flange to the
electronics outside the SEM chamber (not shown here). The Cu foil on the top
part of this Teflon piece is used to minimize charging. Since a
$500\text{\,}\mathrm{\SIUnitSymbolMicro m}$ diameter pressure-limiting
aperture (PLA) was used, the field-of-view (FOV) for SEM imaging is
significantly reduced. The actual visible area is around
$1\text{\,}\mathrm{mm}$ and is exemplarily marked in the image with a circle.
The center of the circle is given by the optical axis of the SEM as
manufacturer-calibrated to the $x=y=0$ position of the SEM stage. After
setting the stage to $x=y=0$ without a mounted sample, the orifice of the
nozzle is positioned as close as possible to the optical axis using the two
micrometer stages. The nozzle position is fixed after closing the SEM chamber.
In case the nozzle is out of the SEMs FOV after pumping the chamber, its
position has to be realigned after venting the chamber. When the nozzle is
positioned within the SEMs FOV, the sample can be brought closer to/moved away
from the nozzle using the SEM stage controls to change the gap distance.

  * •

Figure S8c (side view): The PLA is visible on the bottom of the SEM pole
piece. It reduces gas flow into the SEM column to keep it at higher vacuum
levels compared to the plasma-gap region. A self-made flange with gas and
electronic feedthroughs is mounted on one of the free chamber ports (here
visible in the back). Notably, the gas line does a
$90\text{\,}\mathrm{\SIUnitSymbolDegree}$ bend in the feedthrough to account
for x-ray safety. The cable of the DC-DC converter goes on the flange on the
opposite side (not visible here, but in the top part of Figure S8d), where a
DB9 connector is present. This connector and the corresponding feedthrough are
typically used for cooling/heating SEM stages provided by the microscope
manufacturer. The pin layout was measured, and the shown custom cable with a
DB9 connector was made to control the DC-DC converter.

  * •

Figure S8d (side view): The image from this angle reveals the sample stage
made of an “angle adapter” to tilt the sample surface slightly toward the
incident electron beam for analyses. Different angle adapters were ground with
angles from
$5\text{\,}\mathrm{\SIUnitSymbolDegree}20\text{\,}\mathrm{\SIUnitSymbolDegree}$.
These are fixed to the threaded metal rod with conductive Ag paste. The sample
used here is a $3\text{\,}\mathrm{mm}$ Cu disc with a small aperture in the
center ($50\text{\,}\mathrm{\SIUnitSymbolMicro m}$ diameter, Gilder Grids
GA50), which is glued onto the angle adapter with conductive Ag paste.

The default vacuum system of the SEM (FEI Quanta 250 FEG) was used, consisting
of a pre-vacuum rotary pump, a turbo molecular pump, and ion getter pumps
(IGPs) for the electron-gun area. Without gas flow, the SEM-chamber pressure
was able to reach the $4\text{\times}{10}^{-4}\text{\,}\mathrm{Pa}$ range
after a few hours of pumping. Especially, the residual air inside the gas line
takes this time to get pumped through the nozzle orifice. The gas lines were
flushed with the process gas before experiments to reduce contamination with
air. With gas flow, the chamber pressure for a given gas flow rate depends on
the gas type. The pressure is typically around
$2\text{\times}{10}^{-2}\text{\,}\mathrm{Pa}$ for gas flow rates of about
$5\text{\,}\mathrm{sccm}$, which is just below the threshold value of the
microscope software for the high-vacuum mode (about
$3.3\text{\times}{10}^{-2}\text{\,}\mathrm{Pa}$). If the chamber pressure
exceeds the threshold value, the gas flow rate must be reduced to allow for
microscope and plasma operation in high-vacuum mode.
It was observed that the use of Ar leads to higher chamber pressures than for
N2 or CO2 and, thus, Ar-containing gas mixtures must be used more carefully.
The SEM chamber is mainly pumped by the turbo molecular pump, so a higher
pumping speed for gases with smaller molecular weight is expected 61, i.e.,
higher pumping speed for N2 ($28\text{\,}\mathrm{Da}$), followed by Ar
($40\text{\,}\mathrm{Da}$), and finally CO2 ($44\text{\,}\mathrm{Da}$). In
practice, Ar is probably less efficiently pumped than CO2 because the IGP of
the electron column might contribute to the total pumping speed as well. As
indicated by the microscope manufacturer in the microscope’s manual, “the
argon use should be minimized to a short time, because the IGPs are not
optimized for pumping of it at all.”, meaning that N2 and CO2 are likely to be
more efficiently pumped by the IGP.

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