Supplementary material
1. Experimental Setup
Fig. 1: Schematic diagram of the dual-frequency capacitively coupled plasma chamber used for
complete-floating double probe, optical emission spectroscopy and hairpin resonant probe
measurements.
Our experimental setup used for the complete-floating double probe, optical
emission spectroscopy and hairpin resonant probe measurements is schematically
shown in Fig. 1. The upper electrode is 21 cm in diameter and is driven by a 2/60
MHz dual-frequency power source. The lower electrode is 15 cm in diameter and is
grounded together with the chamber wall. The discharge gap can be adjusted from 1.5
cm to 6 cm, by varying the height of the lower electrode. The positive ion density at
the discharge center is measured by utilizing a complete-floating double probe
technique (for more details of the probe measurements, see Ref. [19] in the main text.).
The optical emission intensity is detected by an optical fiber probe, which consists of
an optical fiber of 1.0 mm diameter placed inside the Al2O3 ceramic tubular arm. The
optical fiber was connected to a Mechelle spectrograph integrated with ICCD
detectors (Andor Technology) with a spectral resolution of about 0.1 nm in the
spectral range of 200–900 nm. The distance between the detective end of the fiber and
the aperture of the ceramic tube is 50 mm so that the plasma emission can be
measured axially with a high spatial resolution. The ceramic tubular arm together with
the fiber was then mounted on a 2D holder so that they could move in the vertical and
horizontal directions inside the chamber. The aperture (acquisition end) of the optical
probe is moved axially at a fixed radial distance r ≈ 10.5 cm, i.e., at the periphery of
the upper electrode. Note that what we measure here is actually the radial integral of
light intensity, not exactly the light intensity at the discharge center as in the case of
the Langmuir probe measurement. The Ar(I) line with wavelength 811.5 nm is taken
as a representative. The electron density at the discharge center is measured by a
hairpin resonant probe. An R&S ZVL6 network analyzer is used to operate the hairpin
probe. The frequency range of this device is up to 6 GHz with a maximum resolution
of 10 Hz. The output (reflection) port is connected via a coaxial cable in series with
the SMA connection to the probe. A 0.125 mm diameter tungsten wire is bent into an
approximately 3mm-wide and about 4 cm-long U-shape as hairpin probe tip, which is
electrically isolated from the antenna loop so that the rf disturbance can be greatly
reduced. Moreover, the double probe, optical emission and hairpin measurement are
performed separately, so that each measurement is not affected by the others.
It needs to be mentioned that one may argue on the validity of the measurement for
positive ions by the double probe in an electronegative discharge, because of the
presence of negative ions. However, even when the probe does not provide
quantitative measurements, it allows at least to detect some basic trend. With the help
of the optical probe, one can confirm the validity of the qualitative measurement of
the double probe, such as seen in Fig. 1 and Fig. 3 in the main text.
2. The effect of BRH on the electronegativity
FIG. 2 (color online). PIC MCC simulation results: the O2+ ion density (in black) and electron
density (in blue) versus electrode gap L in O2 discharges with a pressure of 2.7 Pa, and both HF
and LF voltages fixed at 75 V.
FIG. 3 (color online). EEPFs (left Y axis) from PIC MCC simulations at 2.7 Pa for L = 2 cm, 3 cm
and 4 cm, with both HF and LF voltages of 75 V, as well as the cross section (right Y axis) for
dissociative attachment e + O2 → O + O-. It should be noted here that the full space EEPFs are
adopted.
In comparison with the Ar discharge, the electron density in the oxygen discharge
has a quite weak dependence on L, as is shown in Fig. 2, although the ionization rate
is actually enhanced with decreasing L. One can easily infer that a decay mechanism
of electrons becomes more significant with the drop of L. Indeed, one can conclude
from Fig. 3 that with the decrease of L, the high-energy electrons in the tail of the
EEPF are highly populated, and consequently the electron dissociative attachment
reaction e + O2 → O + O- is enhanced, due to the fact that this reaction has a large
cross-section at  > 20 eV, which lies well in the high-energy tail of the EEPF due to
BRH (see also Fig. 3). This reaction creates negative ions (O-) and destroys electrons.
In addition, the two main decay mechanisms of O-, i.e., mutual neutralization and
molecular oxygen detachment, are almost independent on the gap length L. As a result,
we will see an enhanced electronegativity when decreasing the discharge gap L.
3. The effect of bulk electric field reversal on electron heating
FIG. 4 (color online). EEPFs from simulations: (a) both HF voltage VH and LF voltage VL are 75 V,
pressure is 1.3 Pa for argon (black solid line) and 2.7 Pa for the oxygen discharge (red dashed line),
and (b) LF voltage VL = 50 V and pressure P = 4.0 Pa for different HF voltages VH = 200 V（black
solid line）
，VH = 100 V ( red dashed line), 75 V (blue dotted line), and 50 V (Green dash-dotted
line). It should be noted here that the full space EEPFs are adopted at an electrode gap of 2 cm.
The effect of the bulk electric field on electron heating can be intuitively seen in
Fig. 4(a), which shows the EEPFs for argon and oxygen discharges. The other
discharge parameters are the same as in Fig. 2 of the main text. One can see that the
EEPFs can roughly be divided into two regions at about  ≈ 20 eV, i.e., a low-energy
region at  < 20 eV and a high-energy region at  > 20 eV, respectively. In the former,
the EEPF changes its profile from typical bi-Maxwellian-like to Druyvesteyn-like due
to the high electric field in the bulk of electronegative oxygen discharges. The BRH
would benefit from the high-energy electrons in the bulk. In the high-energy region,
we observe an extended high energy tail in the EEPF, due to the contribution from
high-energy resonant electrons. However, in the oxygen discharge, the knee point in
the extended high-energy tail almost disappears, in other words, the maximum energy
that a resonant electron can obtain from the BRH decreases, and the BRH is more or
less suppressed, which is well consistent with the single electron behavior in Fig. 2(b3)
in the main text.
The effects of bulk electric field reversal on electron heating at varying HF voltages
are illustrated in Fig. 4(b), which shows the EEPFs for HF voltage VH = 200, 75 and
50 V, at a fixed LF voltage VL = 50 W and a pressure of 4.0 Pa. One can see that the
EEPFs can also roughly be divided into two regions at about  ≈ 20 eV, i.e., a
low-energy region at  < 20 eV and a high-energy region at  > 20 eV, respectively. In
the former, the EEPF changes again its profile from typical bi-Maxwellian-like to
Druyvesteyn-like due to the enhanced electric field in the bulk when the discharge
mode transits from electropositive to electronegative with the decrease of the HF
voltage. In the high-energy region, with the decrease of VH, the knee point in the
extended high-energy tail gradually disappears and the maximum energy that a
resonant electron can obtain from the BRH decreases, indicating a suppressed BRH,
which further confirms the analysis in Fig. 4 of the main text.