Atmospheric pressure plasma jets have great potential for the surface modification of polymers. In this work, the authors report on polystyrene etching by a radio frequency driven atmospheric pressure plasma jet with a focus on the role of H, O, and OH radicals in this process. The absolute flux of H, O, and OH radicals reaching the surface of the polymer was determined by a comsol multiphysics reacting fluid dynamics model incorporating detailed transport phenomena in the boundary layer near the substrate. The simulated results of H and OH densities in the jet effluent were experimentally verified by two-photon absorption laser induced fluorescence and laser induced fluorescence, respectively. The carbon atom removal flux from the polystyrene surface was taken from previously reported measurements using the same plasma source. The authors show that the boundary layer effects in the interfacial region above the substrate can have a significant impact on the calculated etching probabilities. The reaction probability (β) has a significant uncertainty although a variation of 2 orders of magnitude in β leads to uncertainties of approximately 1 order of magnitude variation in the determined etching probability. The etching probability of polystyrene by OH radicals was confirmed to be at least an order of magnitude larger than the polystyrene etching probability by O radicals. The authors also confirmed the weak polystyrene etching probability by H radicals. The model suggests that the presence of a 30 ppm O2 impurity can lead to the production of OH radicals in the far effluent of the Ar+1%H2 plasma jet close to the substrate at sufficient densities to enable effective etching.

The modification of the surface properties of polymers is essential for improving adhesion, printing, or biocompatibility.1,2 The application of low-pressure plasmas to modify the surface of polymers has been widely studied.1,3–5 Wertheimer et al. elucidated the role of vacuum ultraviolet (VUV) photons, reactive species, and ions in the surface modification of polymers by a low-pressure plasma.4 Chan et al. reported the differences between the modification of polymers using plasma and UV produced by a high power mercury lamp.6 Sapieha et al. and Shenton and Stevens reported on the differences in the characteristics of polymers treated by an atmospheric pressure and a low-pressure air plasma.7,8 A key difference between atmospheric pressure and low-pressure plasmas is that the ion energy is near thermal for steady-state atmospheric pressure plasma near the interface while the ion energy can be highly energetic in low-pressure plasmas due to collisionless ion acceleration in sheaths near the interfaces.9 This enables effective sputtering in low-pressure plasmas while interactions at atmospheric pressure will have more pronounced contributions of neutral reactive species.

A key advantage of atmospheric pressure plasmas over low-pressure plasmas is that they obviate the need for the vacuum systems and also offer roll-to-roll processing capability compared to the inherent batch processing of low-pressure plasmas. Corona treatment of polymers operating in ambient air is extensively used in industry. They enhance the surface wettability, surface roughness, surface energy, and adhesion properties of polymers.10–13 Atmospheric pressure plasma jets (APPJs) have emerged as an additional promising tool for processing of various materials. APPJs are often operated in a noble gas such as helium or argon. The plasma is typically generated with a radio frequency (RF), kHz, or high repetition rate nanosecond pulses.14 RF modulation and low repetition rate pulses help in having high chemical reactivity while maintaining the gas temperature close to room temperature. The amount of the chemical species generated by the plasma can be enhanced by adding different molecular admixtures to the noble feed gas such as O2, H2, H2O, or air.15–17 The reactive species from an APPJ are mainly generated between the electrodes and transported out of the APPJ by convection of the gas flow.18 This enhanced chemical reactivity and the remote operation of APPJs make them attractive for heat sensitive applications such as surface modification of polymers. Plasma jet treatment of polymers is performed for the surface activation for higher adhesion,19 surface cleaning,20 improvement of hydrophobic or hydrophilic properties,21 and etching of polymers.22,23

While plasmas are extensively used for surface modification, the process of how these modifications occur is challenging to control and is not well understood. Several researchers have conducted experimental measurements and numerical simulations to understand the plasma polymer interaction mechanisms. Dorai and Kushner24 reported a computational investigation of the gas phase and surface reactions during the modification of polypropylene (PP) in a humid-air corona discharge. They reported the change of functional groups present at the polymer surface after treatment by the plasma with variations in deposition of energy, relative humidity, gas temperature, and web speed. OH and O radicals were suggested to play a dominant role in the initiation, propagation, and termination of PP chains resulting in the formation of alcohols and acids on the surface of PP.

Shaw et al. reported an experimental study of the mechanisms underpinning the surface energy increase of PP using the afterglow of a helium APPJ with oxygen admixture.19 A strong correlation between the production of atomic oxygen in the APPJ and the change in the water contact angle of the plasma treated PP surface was observed. Knoll et al. studied the effect of VUV photons from APPJs driven by RF and kHz excitation on the etching of polystyrene (PS).25 The presence of O2 results in the suppression of VUV photon-induced effects on the polymer. A higher etching rate was observed for an ArO2 RF driven APPJ than for the same jet in pure argon. Luan et al. reported the PS etching probability of atomic oxygen radicals to be of the order of 104 by directly correlating the experimentally measured bulk atomic oxygen density in a free APPJ to the etching depth of PS in an oxygen containing APPJ.22 Similarly, the PS etching probability of the order of 102 was estimated for OH radicals.23 However, recombination, diffusion, and convection are important in the interfacial boundary layer above the polymer surface at atmospheric pressure and can cause significant species gradients. Hence, the density of reactive species at the surface can be significantly different from the bulk density. In addition, the radical species density profile at the substrate is expected to be much broader than the initial plasma jet diameter. Further, the density of the reactive species at the PS surface will depend on the surface reaction probability (β). Our previously reported estimates of the etching probability of O and OH did not consider these important interfacial effects and the possible impact of interfacial boundary layer processes on the estimated etching probability remains a key question.

In this manuscript, we investigate the near interfacial gradients and radical species distributions near the substrate and use the gas phase study to assess their effects on the previously reported etching probability. The gas phase H, O, and OH radical fluxes reaching the PS surface were obtained by a combination of a 2D axisymmetric reacting fluid dynamics model and experimentally measured H, O, and OH densities for Ar+1%H2, Ar+1%O2, and Ar+1%H2O plasmas. The modeling in addition to the experiments is required because it is challenging to spatially resolve the species density gradients in the boundary layer near the substrate at atmospheric pressure by laser induced fluorescence (LIF). The work presented in this manuscript is predominantly focused on experiments and the model is not intended to be self-consistent and does not describe the entire studied process. The model is only used to deduce fluxes of radicals to the polymer substrate. Using this detailed investigation, we assess the impact of these previously neglected effects on the polystyrene etching probability of O, H, and OH radicals. We obtained the etching probability by correlating the area integrated flux of the gas phase reactive species at the surface to the area integrated flux of C atoms removed from the material surface by an APPJ. The etching profiles of the material surface were taken from previously published results.22,23 As H radicals are not expected to lead to significant etching, we investigate the origin of the observed etching using Ar+1%H2 plasma. The effect of the reaction probability (a parameter not accurately known for the experimental conditions being investigated but that can impact significantly the absolute radical flux) on the etching probability of PS by O, H, and OH radicals is also reported.

A RF driven modulated APPJ, as shown in Fig. 1, was used in this work. A detailed description of the APPJ can be found in Refs. 26–29. In short, a 1.6mm(ID)×3mm(OD) cylindrical quartz tube surrounds a 1 mm (ϕ) tungsten needle electrode. A 20 kHz modulated RF signal (13.6 MHz) with a duty cycle of 20% was generated by a function generator (Tektronix AFG 2021) amplified by an RF amplifier (Amplifier Research AF75A250A) and applied through a matching box to the tungsten needle electrode. Argon with 1% admixtures of O2, H2, or H2O at a total flow rate of 1.5 standard liters per minute (slm) flows through the quartz tube and acts as a feed gas for generating the plasma.

FIG. 1.

Schematic of RF driven atmospheric pressure plasma jet used for gas phase diagnostics together with the LIF and TaLIF setup.

FIG. 1.

Schematic of RF driven atmospheric pressure plasma jet used for gas phase diagnostics together with the LIF and TaLIF setup.

Close modal

The polymer treatment by the APPJ was performed by placing a PS substrate below the APPJ nozzle in a sealed chamber with controlled gas atmosphere.23 The substrate was placed at distances 4, 8, 12, 16, and 20 mm from the APPJ nozzle. The plasma power was adjusted such that the tip of the visible plasma plume was 0.5 mm above the substrate when the substrate was 4 mm below the APPJ nozzle. The corresponding plasma dissipated power in the Ar+1%O2, Ar+1%H2, and Ar+1%H2O plasmas for such APPJ-substrate distances was 2 W, 1.26 W, and 2 W, respectively.

The treatment of the polymer surface was performed on a time scale of minutes. However, the time scale of the measurement of gas phase densities by the LIF took hours to finish because of the required measurement of the quenching time constants (τQ) of the laser excited state and gas temperatures. In a sealed chamber, the composition of the gas surrounding the APPJ is expected to change during the long duration of the measurement. In order to keep the gas composition in the measurement region constant for the duration of all the LIF measurements and enable O2 free measurements, a shielding flow of N2 (10 slm) was applied through an acrylic tube [12.7mm(ID)×19mm(OD)] surrounding the plasma jet (Fig. 1). Etching of PS by the APPJ changes the surface morphology and the distance between the APPJ nozzle and the substrate on long time scales. To avoid such excessive etching of the polymer during the long duration of the gas phase density measurements, an inert alumina plate (2.5×2.5cm2) with a thickness of 1 mm was used as a substrate. A grounded aluminum block of the same size as the alumina plate was positioned below the alumina plate. The gas phase density measurements are performed by placing the substrate 4, 8, and 12 mm from the APPJ nozzle. The chemical composition of alumina (Al2O3) and PS [(C8H8)n] are widely different. The density of the reactive species at the alumina and polymer surfaces could be different due to the chemical reactions of the reactive species with the substrates. However, the measurement has insufficient spatial resolution to resolve the steep near boundary species gradients to directly measure the density of radical species at the substrate.

1. OH density

The gas phase density of hydroxyl radicals (OH) was performed by LIF identical to the method described in Ref. 30. A nanosecond pulsed Nd:YAG laser [Spectra-Physics LAB-170-10H, full width at half maximum (FWHM)5.8ns] with a second harmonic output at 532 nm was used to pump a dye laser (Sirah Precision Scan) to generate a laser beam after second harmonic generation at the appropriate wavelength to excite the ground state OH radicals. An intensified charge-coupled device (ICCD) camera (Andor IStar 340) with a UV Nikor lens placed perpendicular to the laser beam was used to capture the fluorescence. The ICCD gate was set at 50 ns, which is larger than the fluorescence lifetime of the OH(A) state measured. Rhodamine 6G dye was used to generate a laser beam with a wavelength of 565nm, which was subsequently converted by a second harmonics generation (SHG) crystal into a laser beam with a wavelength of 282.59 nm. A combination of a spherical lens (f=25cm or f=12.5cm) and a cylindrical lens (f=50cm) shape the laser beam into a sheet with a FWHM of 206μm and height of either 3.8 or 7.8 mm, respectively. The excitation scheme and the four levels used for obtaining the absolute OH density is shown in Fig. 2(a). Fluorescence at 309 nm was recorded through a bandpass filter (307 nm,FWHM=10nm) placed in front of the ICCD camera. The absolute calibration of the OH density was performed by Rayleigh scattering of air and a four-level model that considers collisional transfer and quenching from ambient gas molecules. The reader is referred to Ref. 30 for details of the used method. The lifetime of the OH(A) state was calculated by using the gas concentration profiles obtained by a computational fluid dynamics model described in Sec. II C. The decay time constants of the fluorescence intensity were measured by measuring the fluorescence at different times after the end of the laser pulse. These time constants were verified to be consistent with the fluorescence decay time calculated using the gas concentration profiles from the computational fluid dynamics model. As the lifetime of the excited state was similar or smaller than the laser beam pulse width, the accuracy of the gas composition obtained by the model was higher.

FIG. 2.

(a) LIF excitation scheme for measuring the OH radical density with the four vibrational levels and transitions considered in the model. A: Einstein emission coefficient, B: Einstein absorption coefficient, Q: collisional quenching rate, and V: vibrational energy transfer rate. (b) TaLIF excitation schemes for H radicals and krypton.

FIG. 2.

(a) LIF excitation scheme for measuring the OH radical density with the four vibrational levels and transitions considered in the model. A: Einstein emission coefficient, B: Einstein absorption coefficient, Q: collisional quenching rate, and V: vibrational energy transfer rate. (b) TaLIF excitation schemes for H radicals and krypton.

Close modal

2. H· density

The gas phase density of atomic hydrogen radicals (H) was performed using two-photon absorption laser induced fluorescence (TaLIF).31,32 The laser system used for TaLIF is the same as described in Sec. II B 1. A 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) special dye was used to generate fluorescence at 615 nm. The resulting laser beam was frequency doubled by using an SHG β-barium borate (BBO) crystal to generate a laser beam at a wavelength of 307.5 nm. The required radiation of approximately 205 nm for TaLIF was generated by mixing the original 615 nm with the 307.5 nm beam through a second BBO crystal. The resulting laser beam was focused inside the effluent of the APPJ by a plano–convex lens with a focal length of 25 cm. The cylindrical laser beam spot had an FWHM of 157μm at the location of the plasma. The schematic of the two-photon excitation of H is shown in Fig. 2(b). The fluorescence was recorded through a bandpass filter (656 nm, FWHM=10nm) placed in front of the ICCD camera. The two-photon excitation scheme of krypton used for calibration is shown in Fig. 2(b). The fluorescence at 587.1 nm was recorded through a bandpass filter (590 nm, FWHM=10nm) placed in front of the ICCD camera. The absolute density of H was obtained by the following equation:31 

nH=γTKrTHηKrηHσKr(2)σH(2)aKraH(hνHhνKr)2gKrgHSHSKrnKr,
(1)

where the subscripts H and Kr represent the quantities for atomic H and krypton, respectively, n is the density, T is the gas temperature, η is the photon transmission efficiency of the setup, σ(2) is the two-photon absorption cross section, hν is the photon energy, a is the branching ratio, g is the two-photon overlap integral, and S is the fluorescence intensity. The ratio of σ(2) for Kr and H is σKr(2)σH(2)=0.62 as reported in Ref. 32. The two-photon overlap integral was calculated from the laser beam wavelength profile (FWHM=0.60pm) and the measured absorption profile.31,33

The branching ratio a is defined as

a=AijAi+qkqnq,
(2)

where kq is the quenching rate coefficient for quencher labeled “q” of the excited state, n is the density of the quencher (Ar, Kr, O2, N2, H2O, and H2), Aij is the spontaneous emission coefficient of the measured transition, and Ai=1/τ is the total spontaneous emission rate from the excited state. The branching ratio for H is determined using the constants in Table I and the gas concentration density obtained from the computational fluid dynamics (CFD) model described in Sec. II C. The branching ratio for Kr for the calibration condition used in this work was previously measured.31 However, the recommended Aij and Ai in the NIST database was changed since the publication of Ref. 31. Using the recommended and updated coefficients from the NIST database,36 the branching ratio of Kr for Ar+0.58% Kr at room temperature (295 K) and atmospheric pressure becomes aKr=2.05×104.

TABLE I.

Quenching rate constants (kq), Einstein coefficient (Aij), and radiative lifetime (τ) for the excited states of H· and Kr and the photon energy corresponding to the monitored transition [see Fig. 2(b) for details] (Refs. 34, 32, and 35).

kq (10−10 cm−3/s)
SpeciesArN2O2H2OH2Aij (×106 s−1)τ (ns) (eV)
H· 3.93 20.1 32.6 110 19.9 44.1 17.6 6.05 
Kr 1.29 3.35 6.34 — — 0.71 ± 0.014 28.59 ± 0.07 6.08 
kq (10−10 cm−3/s)
SpeciesArN2O2H2OH2Aij (×106 s−1)τ (ns) (eV)
H· 3.93 20.1 32.6 110 19.9 44.1 17.6 6.05 
Kr 1.29 3.35 6.34 — — 0.71 ± 0.014 28.59 ± 0.07 6.08 

The APPJ was modulated at 20 kHz with a duty cycle of 20%. The plasma was on for 10μs and off for 40μs. The OH density close to the jet changes during the duty cycle while the H density remains constant (Fig. 3). However, the OH density near the substrate was in good approximation constant during the RF modulation cycle. While the H and OH densities at the substrate do not vary more than 10% during the RF cycle(Fig. 3), the density of H and OH reported in this work is the mean of the density 2μs after the plasma is switched off and 2μs before the start of the RF pulse.

FIG. 3.

Variation in the density of H and OH radicals during one RF modulation cycle for Ar+1%H2O plasma with the substrate 4 mm below the tip of the APPJ nozzle.

FIG. 3.

Variation in the density of H and OH radicals during one RF modulation cycle for Ar+1%H2O plasma with the substrate 4 mm below the tip of the APPJ nozzle.

Close modal

3. Gas temperature

The gas temperature of the plasma effluent in the region between the APPJ nozzle and the substrate was measured by Rayleigh scattering.37 The Nd:YAG laser beam at 532 nm was focused inside the plasma plume and the scattered photons perpendicular to the laser beam were captured by an ICCD camera with a UV Nikkor lens.38 The intensity of the scattered photons is proportional to the density of the gas.37 The recorded Rayleigh scattering signal when the plasma was off was used as a reference signal. The gas temperature was deduced by using the ideal gas law as follows:

TPlasmaOn=IPlasmaOffIPlasmaOnTPlasmaOff,
(3)

where T is the gas temperature and I is the Rayleigh scattering intensity. TPlasmaOff was the ambient room temperature of 295 K. The possible enhanced mixing of the surrounding N2 in the argon effluent due to the plasma does not significantly impact the Rayleigh scattering signal as the differential cross section for argon and N2 are very similar.39,40

The goal of the model described in this section is to obtain the flux of H, O, and OH species to the polymer substrate and only accounts for the reactions in the plasma effluent as for a radio frequency driven plasma jet, and the radical species are mostly produced inside the quartz tube and blown out of the nozzle by the gas flow.41 The etching measurements were performed for conditions where the visible plasma plume was not in contact with the substrate even for the closest treatment distance of 4 mm. The numerical simulation of the plasma–polymer interaction was performed in a two-dimensional axisymmetric cylindrical geometry in comsol multiphysics. The geometry used in the model reflecting the experimental configuration is shown in Fig. 4. The feed gas with the reactive species produced by the plasma exit the nozzle and interact with the shielding gas and ambient air from the surroundings. A total of 12 species [Ar, O2, H2, N2, H2O, O, O2(a1Δg),O3, H, OH, HO2, and H2O2] were considered in the model. The reactions considered in the model are listed in Table II. Reactions involving N2 were not considered as they do not significantly impact the O, H, and OH densities for the investigated experimental conditions. This was also the reason for using N2 as the shielding gas. However, reactions of H and O with O2 were included (Table II: R20 and R21) as they dominantly contribute to the depletion of H and O. Charged species are expected to be predominantly present inside the APPJ tube and the visible plasma plume region. We ignore the effect of the additional production of the radical species due to charged species in the visible plume region. The good correspondence of the radical densities from the experiment and the model suggests that this assumption is valid. This approach has been successfully implemented by several groups.53,54 Hence, electron and ion-induced reactions were neglected in the model. For Ar+O2 plasma, Van Gaens et al. showed that the production/destruction reactions of O were dominantly neutral reactions in the afterglow.55 Verlackt et al. used the same approach to model the afterglow of a plasma.53 Excited short-lived species such as atomic oxygen O(1S) and O(1D) have not been considered in the current model.

FIG. 4.

Geometry used in the simulation.

FIG. 4.

Geometry used in the simulation.

Close modal
TABLE II.

Reactions considered in the model with the corresponding reaction rate coefficients.

ReactionReaction rate coefficient (s−1, cm3 s−1, cm6 s−1)Reference
·OH + ·OH + M → H2O2 + Ma (R1) 6.9 × 10−31/(T/300)0.8b 42  
·OH + ·OH → H2O + O· (R2) 4.2 × 10−11exp ( − 240/T42  
·OH + H· + M → H2O + Ma (R3) 2.59 × 10−31/(T/300)2 43  
·OH + H· → H2 + O· (R4) 7 × 10−14(T/300)2.8exp ( − 1950/T44  
·OH + O· → O2 + H· (R5) 2 × 10−11exp (112/T45  
·OH + H2 → H2O + H· (R6) 9.54 × 10−13(T/300)2exp ( − 1490/T44  
OH+HO2H2O+O2 (R7) 4.8 × 10−11exp (250/T46  
OH+H2O2H2O+HO2 (R8) 4.53 × 10−12exp ( − 288.9/T15  
H· + H· + M → H2 + Ma (R9) 1.8 × 10−30/T 43  
H+HO2H2+O2 (R10) 2.06 × 10−11(T/300)0.84exp ( − 277/T47  
H+HO2OH+OH (R11) 4.68 × 10−11exp ( − 122/T47  
H+HO2H2O+O (R12) 5 × 10−11exp ( − 866/T42  
H· + H2O2 → H2O + ·OH (R13) 1.7 × 10−11exp ( − 1800/T43  
H+H2O2HO2+H2 (R14) 2.8 × 10−12exp ( − 1890/T43  
H· + O· + M → ·OH + Ma (R15) 1.5 × 10−32/(T/300) 15  
O+HO2OH+O2 (R16) 2.71 × 10−11exp (224/T46  
O· + H2 → ·OH + H· (R17) 1.6 × 10−11exp ( − 4570/T48  
O+H2O2OH+HO2 (R18) 1.1 × 10−12exp ( − 2000/T43  
HO2+HO2+MH2O2+O2+Ma (R19) 1.9 × 10−33exp (980/T49  
H+O2+MHO2+Ma (R20) 1.7 × 10−30/T0.8 43  
O· + O2 + M → O3 + Ma (R21) 6.4 × 10−35exp (663/T50  
O· + O· + M → O2 + Ma (R22) 5.21 × 10−35exp (900/T46  
O· + O3 → 2O2 (R23) 1.5 × 10−11exp ( − 2250/T51  
O2(a1Δg) + O3 → 2O2 + O· (R24) 6.01 × 10−11exp ( − 2853/T52  
O2(a1Δg) + M → O2 + Ma (R25) 3 × 10−18exp ( − 200/T46  
O3 + M → O· + O2 + Ma (R26) 7.3 × 10−10exp ( − 11400/T50  
ReactionReaction rate coefficient (s−1, cm3 s−1, cm6 s−1)Reference
·OH + ·OH + M → H2O2 + Ma (R1) 6.9 × 10−31/(T/300)0.8b 42  
·OH + ·OH → H2O + O· (R2) 4.2 × 10−11exp ( − 240/T42  
·OH + H· + M → H2O + Ma (R3) 2.59 × 10−31/(T/300)2 43  
·OH + H· → H2 + O· (R4) 7 × 10−14(T/300)2.8exp ( − 1950/T44  
·OH + O· → O2 + H· (R5) 2 × 10−11exp (112/T45  
·OH + H2 → H2O + H· (R6) 9.54 × 10−13(T/300)2exp ( − 1490/T44  
OH+HO2H2O+O2 (R7) 4.8 × 10−11exp (250/T46  
OH+H2O2H2O+HO2 (R8) 4.53 × 10−12exp ( − 288.9/T15  
H· + H· + M → H2 + Ma (R9) 1.8 × 10−30/T 43  
H+HO2H2+O2 (R10) 2.06 × 10−11(T/300)0.84exp ( − 277/T47  
H+HO2OH+OH (R11) 4.68 × 10−11exp ( − 122/T47  
H+HO2H2O+O (R12) 5 × 10−11exp ( − 866/T42  
H· + H2O2 → H2O + ·OH (R13) 1.7 × 10−11exp ( − 1800/T43  
H+H2O2HO2+H2 (R14) 2.8 × 10−12exp ( − 1890/T43  
H· + O· + M → ·OH + Ma (R15) 1.5 × 10−32/(T/300) 15  
O+HO2OH+O2 (R16) 2.71 × 10−11exp (224/T46  
O· + H2 → ·OH + H· (R17) 1.6 × 10−11exp ( − 4570/T48  
O+H2O2OH+HO2 (R18) 1.1 × 10−12exp ( − 2000/T43  
HO2+HO2+MH2O2+O2+Ma (R19) 1.9 × 10−33exp (980/T49  
H+O2+MHO2+Ma (R20) 1.7 × 10−30/T0.8 43  
O· + O2 + M → O3 + Ma (R21) 6.4 × 10−35exp (663/T50  
O· + O· + M → O2 + Ma (R22) 5.21 × 10−35exp (900/T46  
O· + O3 → 2O2 (R23) 1.5 × 10−11exp ( − 2250/T51  
O2(a1Δg) + O3 → 2O2 + O· (R24) 6.01 × 10−11exp ( − 2853/T52  
O2(a1Δg) + M → O2 + Ma (R25) 3 × 10−18exp ( − 200/T46  
O3 + M → O· + O2 + Ma (R26) 7.3 × 10−10exp ( − 11400/T50  
a

If M = N2, O2, Ar → coefficient × 1.0, if M = H2, O3, O· → coefficient × 2.5, if M = H2O → coefficient × 5.0 (Ref. 42).

b

T is the gas temperature in units K.

Experimentally measured reactive species densities at the nozzle such as H, O, and OH were used as the boundary conditions for the flow model. The other species densities such as HO2 and H2O2 at the nozzle were taken from the plug flow model results in Ref. 56. While these species are introduced through boundary 1, no reactions are considered within the quartz tube. The inlet concentrations at boundary 1 are shown in Table III. For Ar+1%H2O plasma, the density of O atoms about 0.4 times the density of H atoms at the jet nozzle exit to allow for species conservation as all other O-containing species have a much smaller concentration.56 

TABLE III.

Species density used in the reactive flow model as boundary conditions at boundary one (see Fig. 4) (Refs. 34, 32, and 35).

Species density (×1021 m−3)
PlasmaO·O2(a1Δg)O3H··OHH2O2HO2
Ar+1% O2 20 — — — — 
Ar+1%H2 — — — 25 — — — 
Ar+1%H2O 20 — — 50 0.3 0.4 0.05 
Species density (×1021 m−3)
PlasmaO·O2(a1Δg)O3H··OHH2O2HO2
Ar+1% O2 20 — — — — 
Ar+1%H2 — — — 25 — — — 
Ar+1%H2O 20 — — 50 0.3 0.4 0.05 

The governing equations for the neutral gas model consist of the momentum conservation equation, i.e., Reynolds-averaged Navier–Stokes equation, the mass continuity equation, species balance equations, and the heat transfer equation:

ρ(u)u=p+τ,
(4)
(ρu)=0,
(5)
(ρDiCi+ρCiu)=Ri,
(6)
ρCpuT=(λT),
(7)

where u is the fluid velocity, Ci is the concentration of reactive species i, and τ is the stress tensor, calculated from components with, respectively, molecular and viscosity. The kε turbulence model was used to enable simulation of the mixing near the substrate. Di is the effective diffusion coefficient of the ith species with the mixture. The turbulent mixing is accounted for through additional diffusivity given by νt/Sct, where νt is the turbulent kinematic viscosity and Sct is the turbulent Schmidt number, set to 0.7. The mixture viscosity is μ = μT + μ, where μ is based on Wilke’s mixture rule57 and μT is the turbulent viscosity. The gas density of the mixture is calculated as ρ=ρixi, where the ith gas density ρi is the function of gas temperature T, and xi is the mole fraction of the ith species. Cp is the specific heat capacity and λ is the thermal conductivity of the gas mixture. The pressure is assumed to obey the ideal gas law. Ri describes the production and loss of reactive species i due to reactions in the plasma afterglow.

The boundary conditions are summarized in Table IV. For fluid flow, the velocity at the inlets are given by

(nu)dS=Q,
(8)

where Q is the flow rate, n is the normal vector to the inlet surface, and S is the inlet surface. At the remote boundary 4, the Dirichlet-type boundary condition for pressure is imposed: p=1atm. The temperature at the needle electrode surface and the feed gas at the inlet were assumed to be equal to the measured temperature at the nozzle exit. This approach ensures that the simulated gas temperature at the nozzle is equal to the measured gas temperature and provides the appropriate initial condition to model the reactions in the plasma effluent. The boundary conditions at the substrate surface depend on the reactivity of the species. When an atom/molecule impinges on to a surface, it can reflect from the surface or stick to the surface and potentially undergo further reactions. The relative probability of such occurrences is often not known and hence a model that considers all the surface losses with a single loss term was used to describe the behavior of the reactive species in the vicinity of the surface. The loss in the flux of a reactive species at the surface can be described by58,59

Γloss=β1β/214nvth,
(9)

where n and vth are the specific species density and thermal velocity, respectively. β is the surface loss probability. The reported surface loss probability of the different reactive species for PS and other materials are tabulated in Table V. Due to the long lifetime and low internal energy of O2(a1Δg) and O3, they were assumed to reflect from the substrate (β=0). The effect of the large variation in the reported values of β on the species density gradients near the substrate is discussed in Sec. III C.

TABLE IV.

Boundary conditions used in the model. The numbers used to indicate the boundaries are defined in Fig. 4.

BoundaryVelocity/flow rateSpeciesTemperature
Q = 1.5 slm xH2, xO2, or xH2O=0.01. The concentrations of reactive species are taken from LIF, TaLIF, and global model (Ref. 56Tn=0 
Q = 10 slm xN2=1 Tn=0 
uz = 0.1 m/s xN2=0.79, xO2=0.21 Tn=0 
Outlet p = 1 atm Cin=0 Tn=0 
Substrate surface Wall Surface loss flux λT=λd(TT0)Lda 
Needle surface Wall Cin=0 T = 370 K 
Quartz tube surface Wall Cin=0 Tn=0 
Shielding tube surface Wall Cin=0 T = 293.15 K 
BoundaryVelocity/flow rateSpeciesTemperature
Q = 1.5 slm xH2, xO2, or xH2O=0.01. The concentrations of reactive species are taken from LIF, TaLIF, and global model (Ref. 56Tn=0 
Q = 10 slm xN2=1 Tn=0 
uz = 0.1 m/s xN2=0.79, xO2=0.21 Tn=0 
Outlet p = 1 atm Cin=0 Tn=0 
Substrate surface Wall Surface loss flux λT=λd(TT0)Lda 
Needle surface Wall Cin=0 T = 370 K 
Quartz tube surface Wall Cin=0 Tn=0 
Shielding tube surface Wall Cin=0 T = 293.15 K 
a

λd (0.03 W m−1 K−1) and Ld (5 mm) are the thermal conductivity and the thickness of the polystyrene substrate, respectively (Ref. 60).

TABLE V.

Reported surface loss probability (β) of the reactive species O·, ·OH, H·, HO2, and H2O2 on different substrates. PET: Polyethylene terephthalate, PTFE: Polytetrafluoroethylene, PS: Polystyrene.

SpeciesβMaterialReference
O· 10−5–10−3 Fused silica 61  
 10−3 Pyrex 62  
 0.01 − 1 Polypropylene 63  
 10−3 PET, PTFE, PS 64  
·OH 5 × 10−3 Aluminum 65  
 0.2–0.6 Silica 66–68  
H· 2 × 10−5 Pyrex 69  
 10−5 Silica 70  
 10−3 PET, PTFE, PS 64  
HO2 (1–2) × 10−2 Solid NaCl 71  
H2O2 7.8 × 10−4 Sulfuric acid 72  
SpeciesβMaterialReference
O· 10−5–10−3 Fused silica 61  
 10−3 Pyrex 62  
 0.01 − 1 Polypropylene 63  
 10−3 PET, PTFE, PS 64  
·OH 5 × 10−3 Aluminum 65  
 0.2–0.6 Silica 66–68  
H· 2 × 10−5 Pyrex 69  
 10−5 Silica 70  
 10−3 PET, PTFE, PS 64  
HO2 (1–2) × 10−2 Solid NaCl 71  
H2O2 7.8 × 10−4 Sulfuric acid 72  

Schneider et al. correlated the Γloss term to the flux of C atoms removed from the surface to obtain the etching probability.59 The surface loss probability (β) of materials for different reactive species has not been extensively studied and their values are often not reported. For example, β of OH radicals has not been reported for polystyrene. Γloss depends on β and correlating Γloss to the flux of C atoms removed from the surface impacts the species flux to the substrate and hence adds uncertainty to the calculation of the etching probability. In this work, we define etching probability of a reactive species as the ratio of the area integrated flux of etched C atoms (NC) from the polymer surface to the area integrated flux of reactive species reaching the polymer surface. This definition is closely analogous to the definition of etching probability used in the low-pressure plasmas. The amount of etched C atoms was measured by scanning the APPJ over an area of 9.6×9.6mm2 and the amount of etched C atoms was obtained from previously reported measurements.22,23 The absolute fluxes of reactive gas phase species to the polymer surface were estimated by the reacting flow model described in Sec. II C. Specific details for each radical are discussed in Sec. III. Ar+1%O2 and Ar+1%H2 plasmas dominantly generate O and H atoms, respectively, although we will show that for Ar+1%H2 plasma impurities dominate the etching process. The PS etching probability of O and H was obtained by the slope of the linear relation between the amount of etched C atoms per unit time with the area integrated flux of O and H atoms from Ar+1%O2 and Ar+1%H2 plasmas, respectively (Secs. III D 1 and III D 2). Ar+1%H2O plasma dominantly generates a combination of O, H, and OH radicals. The rate of C atoms removed due to OH radicals in this case was obtained by subtracting the effect of the rate of C atoms etched by O and H atoms from the rate of C atoms removed by Ar+1%H2O plasma, neglecting the possible synergy between different species. The PS etching probability of OH radicals was obtained by the slope of the linear relation between the rate of C atoms removal and the area integrated flux of OH radicals from and to the polymer surface, respectively (Sec. III D 3).

The gas flow from the APPJ for the same APPJ operating in ambient air without the presence of a substrate was found to not have vortices up to five nozzle diameters (10 mm) at a flow rate of 1.5 slm.73 However, the presence of a substrate induces vortices in the flow. Figure 5 shows a comparison of the gas temperature along the axis of symmetry obtained from the experiment, from a laminar flow model and a low turbulence flow model (Schmidtnumber=0.7). The laminar model did not capture the enhanced mixing in the plasma effluent and the Ar,H2O, O2, and N2 concentrations from the laminar model did not lead to an OH(A) quenching time that corresponds well with the experimentally measured quenching time of OH(A) (Fig. 5). The Ar, H2O, O2, and N2 concentrations from the low intensity turbulent model lead to a calculated quenching time of the OH(A) state to show a better correspondence with the experimentally measured OH(A) fluorescence lifetimes. Hence, the low intensity turbulent model was used to obtain the flux of reactive species reaching the substrate. This choice was made to accurately describe the conditions of the near substrate region in spite that the flow inside the tube will be laminar (Reynoldsnumber710).

FIG. 5.

Axial variation in the measured and simulated (for laminar and turbulent model) OH(A) quenching time constants (τ) and gas temperature (Tg) along the axis of symmetry for Ar+1%H2O plasma and a substrate location of 8 mm from the nozzle.

FIG. 5.

Axial variation in the measured and simulated (for laminar and turbulent model) OH(A) quenching time constants (τ) and gas temperature (Tg) along the axis of symmetry for Ar+1%H2O plasma and a substrate location of 8 mm from the nozzle.

Close modal

The steady-state 2D velocity field obtained from the model is shown in Fig. 6. The presence of a substrate below the APPJ obstructs the free flow of the gas into the ambient atmosphere. As the gas flow approaches the surface of the substrate, the axial gas velocity reduces resulting in a stagnation point of zero axial velocity close to the substrate. The reduction in the axial velocity results in the increase of the radial component of the velocity and the flow becomes parallel to the surface in close proximity to the substrate. Figure 7 shows the axial velocity between the nozzle exit and the surface of the substrate on the axis of symmetry (r=0 mm) and on a line parallel to this axis midway between the needle surface and the inner surface of the quartz tube (r=0.65 mm). The conical tip of the needle electrode results in a stagnation point in the vicinity of the needle tip. This stagnation point causes an initial increase in the axial velocity along the axis of symmetry as the flow exits the tube of the nozzle, while such an increase does not occur at the radial position corresponding to r=0.65 mm. The axial velocity at both the radial locations reduces with the increase in the distance from the APPJ nozzle reaching a zero axial velocity in close proximity of the substrate. The rapid decrease in the axial velocity magnitude near the surface of the substrate results in a change in the dominant transport mechanism of chemical species from convection in the bulk region to diffusion in this interfacial region in the vicinity of the substrate. This increases the time required by the reactive species to reach the substrate and is the key reason why the boundary layer at the substrate can have a huge impact on the flux of the reactive species to the substrate.

FIG. 6.

Magnitude of the velocity and the flow direction from Ar+1%H2O plasma with a substrate at 8 mm. The direction of the arrows represent the velocity direction and the size of the arrows represent the logarithmic magnitude of the flow velocity.

FIG. 6.

Magnitude of the velocity and the flow direction from Ar+1%H2O plasma with a substrate at 8 mm. The direction of the arrows represent the velocity direction and the size of the arrows represent the logarithmic magnitude of the flow velocity.

Close modal
FIG. 7.

Axial velocity for the Ar+1%H2O plasma jet. r=0 mm corresponds to the axis of symmetry and r=0.65 mm is a line parallel to this axis shifted by 0.65 mm. The cases for a nozzle-substrate distance of 4, 8, and 12 mm are shown.

FIG. 7.

Axial velocity for the Ar+1%H2O plasma jet. r=0 mm corresponds to the axis of symmetry and r=0.65 mm is a line parallel to this axis shifted by 0.65 mm. The cases for a nozzle-substrate distance of 4, 8, and 12 mm are shown.

Close modal

The inlet boundary conditions for H and OH densities used in the model are determined based on the experimental results near the nozzle. The densities of H, OH, O, H2O2, and HO2 at the nozzle used in the model for Ar+1%H2O plasma are shown in Table III. A comparison of the measured and modeled H and OH densities for Ar+1%H2O plasma with a substrate 8 mm below the APPJ nozzle is shown in Fig. 8. The measured and simulation results agree within the uncertainty of the measured results for distances in excess of 2.5 mm from the nozzle. The difference between the model and the experiment does not exceed a factor of 2.25. The H density decreases monotonically with the distance from the jet nozzle. However, the measured OH density peaks at 1 mm from the APPJ nozzle and decreases for larger distances. This maximum in the density of OH was, however, not captured in the simulation, where the OH density was observed to peak at the nozzle and monotonously decreased with increasing distance from the APPJ nozzle. This discrepancy could be a consequence of not considering the time dependent variation in the OH density near the nozzle in the model. The H andOH densities show a steep gradient at the substrate surface. Laser scattering, vignetting of the observed fluorescence near the substrate, and the uncertainty in the concentrations of the quenchers of the excited state of H and OH (O2, N2, and H2O) that diffuse into the reactive species zone hamper the accurate experimental determination of the density of species in the close vicinity of the substrate. Vignetting was corrected by using the reference Rayleigh scattering signal. With this correction, we were able to measure up to 50μm above the substrate; however, the correction for vignetting is a source of significant uncertainties in the obtained radical densities close to the substrate. Hence, the simulation results are used to obtain the radical densities reaching the surface of the polymer.

FIG. 8.

Comparison between the measured and simulated H and OH density for an Ar+1%H2O plasma impinging on a substrate at 8 mm distance. (a) Measured H density, (b) simulated H density, (c) measured OH density, and (d) simulated OH density.

FIG. 8.

Comparison between the measured and simulated H and OH density for an Ar+1%H2O plasma impinging on a substrate at 8 mm distance. (a) Measured H density, (b) simulated H density, (c) measured OH density, and (d) simulated OH density.

Close modal

The etching probability depends on the flux of reactive species reaching the surface of the polymer, the surface reaction probability, and the flux of C atoms removed from the polymer surface. The flux of reactive species reaching the surface will depend on the composition of the feed gas and the surroundings. In Sec. III C, we will discuss the density distribution of the gas phase reactive species generated by Ar+1%O2, Ar+1%H2, and Ar+1%H2O plasmas with a focus on the boundary layer effects. The effect of the surface reaction probability and the flux of reactive species at the surface on the etching probability will be discussed in Sec. III D.

1. Ar + 1% O2 plasma

Atomic O, O2(a1Δg), and O3 are the dominant reactive species in the afterglow region for RF driven O2 containing plasma jets.56,74–77 It was previously shown that (V)UV photons do not play a role in polymer etching upon the addition of O2 to argon.25 With the increase in the distance from the APPJ nozzle, the etching depth of the PS drops exponentially similar to the decrease in O density.22 The density of O3 increases with the increasing distance from the nozzle while the density of O2(a1Δg) decreases at a much smaller rate than O.22,56,76 This suggests an unlikely dominant contribution of O3 and O2(a1Δg) to the etching of PS.

Figure 9 shows the modeled atomic O density along the central axis of the plasma jet for Ar+1%O2 plasma for substrate locations of 4, 8, and 12 mm. The figure also shows the measuredO density measured at the nozzle by TaLIF for one condition close to the nozzle when the APPJ was operated in open air atmosphere and all other parameters are identical.78 Atomic oxygen is produced inside the quartz tube and transported toward the target by convection. The atomic O density decreases along this path mainly due to the reaction of O with O2 forming ozone.76,79 This decrease is enhanced when the APPJ is operated in ambient air surroundings due to the enhanced influx of O2 into the plasma effluent (Fig. 9).

FIG. 9.

Calculated axial atomic O density for Ar+1%O2 plasma as a function of the distance from the nozzle exit with the surface loss probability, βO=103 for treatment distances of 4, 8, and 12 mm. The O density measured at the nozzle as reported in Ref. 78 is also shown.

FIG. 9.

Calculated axial atomic O density for Ar+1%O2 plasma as a function of the distance from the nozzle exit with the surface loss probability, βO=103 for treatment distances of 4, 8, and 12 mm. The O density measured at the nozzle as reported in Ref. 78 is also shown.

Close modal

Figure 10 shows the radial dependence of the O flux at the substrate for 4, 8, and 12 mm substrate locations. The area of the radial distribution of the atomic O flux at the surface for which the O flux is equal or more than half the maximum O flux is at least 11 times larger than the area encompassing the initial APPJ nozzle diameter. The radial O density distribution at the surface of the substrate is much narrow for an air surrounding compared to the radial O density distribution in N2 surroundings due to the fast recombination of O in the presence of in the higher density of O2 (Fig. 10).

FIG. 10.

Radial profile of atomic O fluxes for Ar+1%O2 plasma directed toward the surface for a treatment distance of 4, 8, and 12 mm with βO=103 for air and N2 surroundings. r=0.8 mm corresponds to the radius of the APPJ nozzle.

FIG. 10.

Radial profile of atomic O fluxes for Ar+1%O2 plasma directed toward the surface for a treatment distance of 4, 8, and 12 mm with βO=103 for air and N2 surroundings. r=0.8 mm corresponds to the radius of the APPJ nozzle.

Close modal

The density of O atoms at the substrate depends on the value of β. Figure 11 shows the variation of O density close to the substrate for three different values of βO. With the increase in the value of β, the fraction of O atoms reacting with the substrate increase and the density of O atoms at the surface reduces. The O density and the flux at the substrate were approximately 3.4 and 12.4 times lower forβO=102 compared to βO=104, respectively. These results underline that the modeling of the diffusion layer above the substrate is essential for an accurate determination of species fluxes at the surface.

FIG. 11.

Axial variation of O density close to the substrate for Ar+1%O2 plasma for a treatment distance of 12 mm with βO=105, 104, and 105 in N2 surroundings.

FIG. 11.

Axial variation of O density close to the substrate for Ar+1%O2 plasma for a treatment distance of 12 mm with βO=105, 104, and 105 in N2 surroundings.

Close modal

2. Ar + 1% H2 plasma

The dominant species generated by Ar+1%H2 plasma are H radicals. Figure 12 shows the modeled and experimentally measured H density as a function of the distance from the APPJ nozzle for substrate locations of 4, 8, and 12 mm. The atomic H density is Approximately 2.5×1022/m3 at the nozzle of the plasma jet and reduces by 2 orders of magnitude from the nozzle to the substrate at 12 mm.

FIG. 12.

Axial atomic H density for Ar+1%H2 plasma as a function of distance from the nozzle with βH=103 for treatment distances of 4, 8, and 12 mm.

FIG. 12.

Axial atomic H density for Ar+1%H2 plasma as a function of distance from the nozzle with βH=103 for treatment distances of 4, 8, and 12 mm.

Close modal

Without impurities (O2 and H2O), the modeled H density does not match the experimentally measured H density beyond 5 mm from the APPJ nozzle (Fig. 12). When 30 ppm O2 is added to the N2 shielding gas, the simulated H density agrees well with the experiment (Fig. 12). Figure 13 shows a comparison of the measuredH and OH densities with the simulated H and OH densities along the axis of symmetry in the effluent. This comparison includes the effect of O2 impurity gas in the Ar+1%H2 feed gas and in N2 shielding gas. In contrast to the experimentally measured OH density, the addition of 30 ppm O2 impurity in the feed gas results in a maximum of the OH density at the nozzle. Nonetheless, the model is also consistent with an enhanced production of OH radicals near the surface of the substrate. The comparison between measured and simulated OH densities suggests that it is difficult to model the unknown amount of impurities in the gas flow. In addition, the effect of the trace amounts of the impurities could be different between the etching measurements and the radical density measurements as they were performed in two different laboratories. Hence, no fluxes of radicals species have been used for this case to derive etching rates. Nonetheless, it can be concluded that impurities have a key impact on the radial species concentration particularly in the far effluent even if the impurities are 3 orders of magnitude smaller than the H2 density.

FIG. 13.

Simulated and experimentally measured H and OH densities along the axis of symmetry for Ar+1%H2 plasma as a function of distance from the nozzle for a surface loss probability of βH=103 and a treatment distance of 12 mm.

FIG. 13.

Simulated and experimentally measured H and OH densities along the axis of symmetry for Ar+1%H2 plasma as a function of distance from the nozzle for a surface loss probability of βH=103 and a treatment distance of 12 mm.

Close modal

3. Ar + 1% H2O plasma

Figure 14 shows the measured and simulated densities of H andOH along the axis of symmetry for Ar+1%H2O plasma. Both the H and the OH densities decrease with the increasing distance from the APPJ nozzle. The simulated densities of the major reactive species are shown in Fig. 15. The H and O densities are about 2 orders of magnitude higher than the OH density at the nozzle. This orders of magnitude difference between .OH and H densities is consistent with .OH and H densities reported for a high power density nanosecond pulsed spark discharge as reported by Luo et al.80 The observed higher O and H densities in a high power density RF plasma jet might be similarly due to the electron impact dissociation of OH radicals in the core of the jet for conditions, where significant amount of H2O is dissociated. The dominant consumption reactions of H are three body recombination reactions with H and O (R9 and R15 in Table II). The dominant OH recombination reaction is reaction R3 in Table II. OH is continuously produced by reaction R15 along the axial direction while H is being consumed. This results in the faster decrease of H density with increasing distance from the nozzle compared to the decrease in OH density.

FIG. 14.

H and OH densities along the axis of symmetry for Ar+1%H2O plasma as a function of distance from the nozzle and βH=βO=βOH=103.

FIG. 14.

H and OH densities along the axis of symmetry for Ar+1%H2O plasma as a function of distance from the nozzle and βH=βO=βOH=103.

Close modal
FIG. 15.

Densities of various reactive species along the axis of symmetry for Ar+1%H2O plasma with a treatment distance of 12 mm and βH=βO=βOH=103.

FIG. 15.

Densities of various reactive species along the axis of symmetry for Ar+1%H2O plasma with a treatment distance of 12 mm and βH=βO=βOH=103.

Close modal

Figure 16 shows the radial dependence of the OH flux at the substrate for a treatment distance of 4, 8, and 12 mm. The increase in the radial OH flux between 7 and 10 mm radial positions is due to the production of OH radicals as a result of the influx of a small amount of oxygen from the surrounding air (in spite of the N2 shielding gas) via reactions R11 and R17 (Table II). This increase has the same origin as the observed increase of OH in the afterglow in H2OO2 mixtures.81 The area of the radial distribution of the OH flux at the surface for which the OH flux is equal or more than half the maximum OH flux is at least 10 times larger than the area encompassing the initial APPJ nozzle diameter.

FIG. 16.

Radial profile of atomic OH fluxes for Ar+1%H2O plasma directed toward the surface for a treatment distance of 4, 8, and 12 mm with βOH=103. r=0.8 mm corresponds to the radius of the APPJ nozzle.

FIG. 16.

Radial profile of atomic OH fluxes for Ar+1%H2O plasma directed toward the surface for a treatment distance of 4, 8, and 12 mm with βOH=103. r=0.8 mm corresponds to the radius of the APPJ nozzle.

Close modal

Similar to the case of O density discussed in Sec. III C 1, the OH density at the substrate for 12 mm distance was a factor of approximately 13 and 2 larger for βOH=103 and βOH=102, respectively, compared to βOH=101 (data not shown). The resulting flux of OH at the substrate was approximately 3.4 and 27.1 times lower for βOH=101 and βOH=102 compared to βOH=103.

1. Ar + 1% O2 plasma: atomic oxygen (O·)

A linear correlation was obtained when the area integrated flux of atomic O to the substrate for distances of 4, 8, and 12 mm was plotted against the observed rate of removal of C atoms from the PS surface (Fig. 17). The etching probability of O atoms was estimated from the slope of the plots in Fig. 17. As the measured surface loss probability of O atoms is approximately 103 for PS,64 the etching probability of O atoms is (3.2±6.8)×105. The uncertainty in the etching probability of O was calculated by assuming an uncertainty in the O flux to the substrate of a factor of 2. Nonetheless, the β coefficient was measured for different conditions than the current work and might lead to significant uncertainties in the O. flux. To this end, we assess the effect of β on the determination of the etching probability by varying the surface loss probability of O atoms from 104 to 102 (Fig. 17). The corresponding etching probability of atomic O changes by an order of magnitude from 0.2×104 to1.4×104. This suggests that one C atom is removed for 104O radicals reaching the surface of polystyrene. Table VI summarizes the variation in the etching probability of O radicals for a range of surface reaction probability values. Assuming that the largest uncertainty is for the surface loss probability, the obtained etching probability of (0.21.4)×104 is nearly 1 order of magnitude smaller than the value reported by Luan et al.22 The main reason is that the O flux in the previous work was assumed to have a constant radial profile along the polymer surface confined to the cross sectional area of the jet nozzle. As shown in Fig. 10, the radial profile of O is much broader than the initial nozzle diameter. In addition, the atomic O flux at the substrate is significantly different for air and N2 surroundings and the average atomic O flux at the central point of the surface in the previous study was underestimated by using the O atoms density measured/calculated for an Ar/O2 APPJ operating in ambient air instead of N2 (Fig. 9).

FIG. 17.

Etching probability of O as a function of the measured O flux for surface loss probabilities (βO) 104, 103, and 102.

FIG. 17.

Etching probability of O as a function of the measured O flux for surface loss probabilities (βO) 104, 103, and 102.

Close modal
TABLE VI.

Obtained etching probability (γ) for different surface loss coefficients (β).

Speciesβγ
O· 10−4 (1.9 ± 0.1) × 10−5 
 10−3 (3.2 ± 0.2) × 10−5 
 10−2 (1.4 ± 0.2) × 10−4 
H· 10−5 <8.3 × 10−6 
 10−3 <1.8 × 10−5 
·OH 10−3 (2.8 ± 0.1) × 10−3 
 10−2 (5.750 ± 0.001) × 10−3 
 10−1 (3.5 ± 0.1) × 10−2 
Speciesβγ
O· 10−4 (1.9 ± 0.1) × 10−5 
 10−3 (3.2 ± 0.2) × 10−5 
 10−2 (1.4 ± 0.2) × 10−4 
H· 10−5 <8.3 × 10−6 
 10−3 <1.8 × 10−5 
·OH 10−3 (2.8 ± 0.1) × 10−3 
 10−2 (5.750 ± 0.001) × 10−3 
 10−1 (3.5 ± 0.1) × 10−2 

2. Ar + 1% H2 plasma: atomic hydrogen (H·)

The etching of PS treated by Ar+1%H2 plasma was used to assess the etching probability of atomic H. Luan et al.23 reported that the etching depth of PS by Ar+1%H2 plasma did not change significantly with increasing treatment distance up to 16 mm. Hence, the etching of PS must involve contributions of reactive species other than H generated in the effluent of Ar+1%H2 plasma as the H density reduces more than 2 orders of magnitude over that distance (Fig. 12). H2 containing plasmas produce H2 dissociation continuum UV radiation.82 However, the UV radiation also significantly reduces with the increasing distance from the nozzle.26 

The significant etching of PS by Ar+1%H2 plasma at higher distances is likely due to the formation of other reactive species such as OH radicals in the presence of the impurities. The measured OH density in the Ar+1%H2 plasma is of the same order as the H density in the far effluent and is in the first approximation independent of the distance from the nozzle and has thus the same trend as the observed etching rate (Fig. 13).

The H density at a treatment distance of 4 mm was used for estimating the maximum etching probability of H radicals. Zaplotnik et al. reported the PS surface loss coefficient of H to be (1.5±0.4)×103. Table VI summarizes the variation in the etching probability of H for β=105103. The obtained upper limit in the polystyrene etching probability of atomic H is 1.8×105 with a loss coefficient with 103 of H. The H etching probability is expected to be much less than that for O as a significantly larger etching depth was observed with Ar+1%O2 plasma compared to Ar+1%H2 plasma for similar H and O fluxes to the substrate.22,23 The obtained etching probability of OH reported below is consistent with an OH induced etching process in Ar+1%H2 plasma for the measured OH densities. Luan et al. observed an increase in the etching depth of polystyrene by a factor of 3 when the Ar+1%H2 plasma jet was operated in a 99% N2 + 1% O2 environment compared to 100% N2 environment.23 This observed larger etching depth in the presence of a small concentration of O2 further supports that OH, rather than H, may be the dominant etchant for Ar+1%H2 plasma.

3. Ar + 1% H2O plasma: hydroxyl radical (OH)

Figure 18 shows the etching probability ofOH obtained by considering a range of surface loss probabilities reported in the literature and removing the contribution of O and H to the etched C atoms. The upper limit of the etching probability of H atoms was 1.8×105 (Sec. III D 2). However, a linear fit of the amount of C atoms/s versus OH radicals/s has a smaller coefficient of determination (R2<0.95) when the upper limit of γH was used (dotted lines in Fig. 18). An almost perfect linear correlation (R2>0.99) was obtained for γH=0, suggesting that the contribution of H to etching probability is negligible compared to OH in Ar+1%H2O plasma and that γH is significantly smaller than the obtained upper limit.

FIG. 18.

Etching probability of OH as a function of the integrated OH flux for surface loss probability (βOH) of OH from 103 to 101. Both the cases γH=0 and γH=1.78×105 are shown.

FIG. 18.

Etching probability of OH as a function of the integrated OH flux for surface loss probability (βOH) of OH from 103 to 101. Both the cases γH=0 and γH=1.78×105 are shown.

Close modal

The polystyrene surface loss probability of OH has not been reported. Table VI summarizes the variation in the etching probability of OH radicals for a range of surface reaction probability values. The etching probability needs to be smaller than the surface loss probability as multiple reactions with PS are required to remove a C atom. The obtained etching probability of OH becomes larger than the surface loss probability for β=103 suggesting that the surface loss probability of OH is larger than 103. The obtained etching probability of OH radicals is γOH=(0.283.5)×102. This suggests that approximately 100OH radicals are needed to remove a C atom from the polystyrene surface.

The obtained OH etching probability of PS is similar to the value reported by Luan et al.,23 while the OH flux in the previous work was assumed to be confined to the cross sectional area of the jet nozzle. In addition, the contribution of O. to etching due to the orders of magnitude higher concentration of O radicals was not considered. The combined effect of underestimation of the OH flux due to the geometrical assumption and overestimation of the etching probability of .OH by neglecting the contribution of O. radicals compensate a similar etching probability for .OH was found.

The etching probability of polystyrene follows the trend OH>O>H. This is consistent with the suggestion of higher reactivity of OH with polymers compared to O by Dorai and Kushner24 based on reported analogous gas phase reaction rates with large hydrocarbon molecules. The large etching probability of OH suggests that the plasma-induced etching rate of polystyrene can be enhanced by increasing the OH flux to the substrate. Yue et al. showed that the OH density at the nozzle from a DC pulsed APPJ increases with increasing plasma dissipated power.83 However, for an RF high power discharge, increasing the power density of the plasma jet not necessarily leads to a higher OH density at the nozzle exit due to the increasing dissociation of the formed OH to O and H (see also Ref. 80). Nonetheless, the high power density and dissociation degree leads to an increased flux of OH radicals in the far afterglow plasma effluent close to the substrate due to the delayed production of OH through the recombination of H and O radicals uniquely enabling large etching rates in the far effluent of the plasma jet.

The simulated H and OH densities considering only neutral chemical reactions of Ar+1%H2 and Ar+1%H2O RF driven atmospheric pressure plasma jet show excellent agreement with the bulk gas phase measured LIF and TaLIF results validating the accuracy of the modeling approach. The radial profile of the radical species at the substrate in the effluent was found to cover an area more than 10 times larger than the area of the nozzle. The radial flux of O radicals from Ar+1%O2 plasma was significantly different for air and N2 surroundings. Depending on the value of β, a drop in the reactive species density by a factor of 2–3.5 was observed in the diffusion layer, corresponding to a change in the radical flux to the substrate between 3.4 and 12.4. The modeling of this diffusion layer above the substrate was essential for an accurate determination of species fluxes at the surface.

The effect of the PS etching probability by O, H, and OH radicals due to the presence of the interfacial boundary layer above the PS substrate, the radial distribution in the density of the radical species at the surface, and the surface reaction probability was evaluated. The etching probability of PS by H, O, and OH was obtained by correlating the measured etching depth with the simulated radical species fluxes at the surface of the substrate.

Boundary layer spreading leads to an order of magnitude lower O radical etching probability than previously reported. However, an uncertainty in the O etching probability of an order of magnitude was found due to not accurately knowing β coefficient, which was varied by 2 orders of magnitude. The previously reported OH etching probability of PS is within the range of the value reported in this work. While the near boundary layer effects play a role, the effect of O etching that was previously neglected compensates for this effect. Again, the uncertainty in β leads to an intrinsic uncertainty of the etching probability of an order of magnitude.

The PS polymer etching by Ar+1%H2 plasma is not due to H but due to the production of radicals, most likely .OH generated by the presence of O2 impurities in the feed gas. Although the densities of H and O for Ar+1%H2O plasma for the current experimental conditions are about 2 orders of magnitude higher than the OH density at the nozzle exit, the OH radicals still play a dominant role in polystyrene polymer etching. The results presented in this manuscript suggest that an optimization of polymer etching should focus on increasing the OH radical flux to the polymer.

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences General Plasma Science program under Award Number DE-SC001939, DE-SC0016053, and the University of Minnesota. Yashuang Zheng acknowledges the support of the Chinese Scholarship Council that enabled her research stay at the University of Minnesota. The authors acknowledge the Minnesota Supercomputing Institute at the University of Minnesota for providing resources that contributed to the results reported in this manuscript.

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