Selective Surface Treatment Using Atmospheric Ar Plasma Jet for Aluminum‐doped Zinc Oxide Based Transparent and Flexible Electronics

Transparent and flexible electronics are emerging technologies with the potential to enable new applications. However, to ensure high‐performance transparent electronics, post‐processing such as thermal annealing and vacuum plasma treatment is necessary, which are difficult to apply to polymer‐based flexible substrates. This study analyzed the feasibility of applying selective Ar plasma jet treatment at atmospheric pressure to transparent flexible electronics. When atmospheric Ar plasma treatment is applied to transparent flexible aluminum‐doped zinc oxide (AZO), it showed a maximum 83.1% improvement in sheet resistance while maintaining a high transmittance performance, of over 70%. To verify the mechanism behind the surface treatment effect using atmospheric Ar plasma, comprehensive analyses are performed using atomic force microscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy, which confirmed that the effect is due to oxygen vacancy formation caused by ion bombardment and thermal diffusion. The application of atmospheric plasma treatment to a patterned transparent flexible AZO device resulted in a reduction in contact resistance, and it is confirmed that the performance improvement effects can be retained for >500 h by applying additional passivation.

Transparent flexible electrodes are electrodes formed on flexible substrates that have both electrical conductivity and excellent optical transparency (visible light region: 380-780 nm).The most important features of transparent and flexible electrodes are their high transmittance (>70%) and low electrical resistivity (<10 −3 Ω•cm). [8,9]Materials commonly used for transparent electrodes include aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), and zinc oxide (ZnO), which are all transparent conductive oxides (TCOs), and 2D materials such as MXenes and graphene. [10,11]Among these, ITO electrodes are the most widely used because of their low sheet resistance and high transmittance. [12]However, Indium is expensive, and ITO is difficult to produce at low temperatures and requires additional thermal treatment processes, which make it difficult to use with polymer-based flexible substrate electrodes. [13,14]ZnO electrodes are widely used, but they have a relatively high sheet resistance. [15]For these reasons, AZO electrodes, which are doped with Al to create an n-type doping level and have a lower sheet resistance, are being utilized. [16][19][20] However, the polymer substrate used to fabricate transparent and flexible electrodes can be easily deformed by thermal stress, which makes it difficult to use the thermal annealing method. [21]Plasma treatment, which can be used at relatively lower temperatures, is considered to be a good alternative to improve sheet resistance.
Plasma is a state that occurs when gas is heated to an extremely high temperature, causing it to separate into ions with both electrons and positive charges. [22,23]In most cases, plasma is formed by ionization.Ions can also be separated using an electric field, resulting in the emission of light.The sustainability of a plasma discharge is determined by factors such as pressure, electric field, and material characteristics. [24]Plasmas are classified as low-pressure plasma or atmospheric pressure plasma, based on the pressure in which they are created. [25]Although low-pressure plasma is mainly used in semiconductor processes, it requires a vacuum environment and pressure adjustment, which leads to high cost, and also makes it difficult to apply plasma treatment selectively in specific locations.
On the other hand, atmospheric pressure plasma is created by applying high voltage to electrodes to generate an arc and passing gas through them to create plasma. [26]Atmospheric pressure plasma is mainly used to affect the surface energy of a material to improve adhesion, hydrophilicity, and hydrophobicity through molecular-level changes and chemical reactions.One of its advantages is that it can be selectively applied to specific locations for cleaning and surface treatment. [27]Atmospheric plasma jets have also been widely applied for the surface modification of metals, [28] polymer substrates, [29,30] carbon cloth, [31] and organic materials. [32]It can also be used as a treatment to improve the sheet resistance of transparent and flexible electrodes because it does not cause thermal deformation or require a highly limited type of process environment.
This study analyzed the effects and mechanism of atmospheric pressure plasma treatment to improve the sheet resistance of transparent and flexible AZO electrodes using a plasma pipette.The potential for selective surface treatment was confirmed, and the effect of AZO plasma treatment on three types of substrates (glass, PET, PI) was analyzed.XRD, AFM, and XPS analyses were conducted to investigate the mechanism behind the atmospheric pressure plasma treatment effects.Improvement in sheet resistance due to the formation of oxygen vacancies was confirmed.Finally, an analysis of treatment applicability at the device level was performed by analyzing the contact resistance after AZO patterning and plasma treatment and verifying the effect and reliability.

Analysis of Atmospheric Plasma Treatment Effect on Transparent and Flexible AZO Electrode
To obtain high-performance transparent flexible AZO electrodes, post-treatment processes are required after fabricating to improve the sheet resistance of the AZO layer.However, since most flexible substrates use polymer materials with low glass transition temperatures, conventional post-treatment methods such as thermal annealing are excluded. [33]Plasma treatment, which is conventionally used as an alternative, requires a complex process that involves the creation and maintenance of a vacuum environment, and does not allowing a specific annealing area to be selected.Therefore, in this study, we employed a plasma pipette with atmospheric argon plasma to conduct selective plasma treatment and then analyzed the improvement in the transparent flexible AZO electrodes, as shown in Figure 1a.Using this method, free electrons from the argon (Ar) ion bombardment improved the sheet resistance in the AZO films by inducing the formation of oxygen vacancies, and thermal diffusion of the AZO film, in a room temperature atmosphere environment. [34]In addition, the plasma treatment was selectively applied to the desired area, as shown in Figure 1a.
The plasma pipette device used in this study with the plasma jet method is schematically illustrated in Figure 1b.Due to the characteristics of the atmospheric pressure plasma device, the plasma treatment effect can be controlled through power, distance, and time.Atmospheric pressure plasma is generated by applying high voltage to the electrodes to generate an arc, and gas passes through the nozzle during the process to create plasma (Figure 1b).This Ar atmospheric pressure plasma jet was applied at a distance of 1 cm from the AZO film based on a 4 W output standard.To clearly analyze the effects of the atmospheric pressure plasma jet after producing transparent and flexible AZO electrodes, the plasma treatment was applied under detailed conditions such as those shown in Table S1 (Supporting Information).
Transparent and flexible AZO electrodes were produced using the in-line sputtering method with the structure shown in Figure S1 (Supporting Information) and the sputtering conditions shown in Table S2 (Supporting Information).AZO electrodes were fabricated on glass, PET, and PI substrates to compare the effects of plasma treatment.When the sputtering power was set to 1.5 kW, and the oxygen flow rate was set to 0, 0.1, and 0.2 sccm, the thicknesses of the resulting AZO films were 454.67, 479.67, and 530.33 Å, respectively (Table S3, Supporting Information).The AZO electrodes fabricated on the glass, PET, and PI substrates were subjected to plasma treatment using a plasma jet at a distance of 1 cm with an output of 4 W (Figure 2a).For the AZO electrodes fabricated on a glass substrate with a sputtering power of 1.5 kW and oxygen flow rates of 0, 0.1, and 0.2 sccm, it was observed that the sheet resistance increased with increasing oxygen flow rate (Figure 2b,c).When plasma treatment was applied for up to 5 min in each case, the sheet resistance improved proportionally with the plasma application time (Figure 2b,c).In particular, when the AZO electrodes were fabricated at an oxygen flow rate of 0.2 sccm, the sheet resistance was greatly improved, by up to 43.8%, and the effect of selective surface treatment was observed only on the area where the plasma jet was applied (Figure 2c and Table 1).The improvement in sheet resistance is expected to be determined by the initial sheet resistance.Specifically, a much greater improvement in the sheet resistance of AZO on flexible substrate was obtained when there was a much higher initial sheet resistance, that is, for the samples formed under O 2 0.2 sccm compared with the 0 and 0.1 sccm.
When applying plasma treatment under the same conditions to AZO produced on flexible substrates such as PET and PI for up to 5 min, the sheet resistance improved proportionally, as with the glass substrates (Figure 2d,e).In particular, AZO produced under a 0.2 sccm oxygen flow rate showed a maximum improvement in sheet resistance of 83.1% on PET substrates and 80.9% on PI substrates (Table 1).These results demonstrate that the improvement in sheet resistance is greater on flexible substrates such as PET and PI, than on glass substrates.
To investigate the effect of plasma treatment on thicker AZO films, AZO films were prepared under the condition of 2.5 kW power and an oxygen flow rate of 0.2 sccm, which had shown the most improvement, and then plasma treatment evaluation was performed.It was confirmed that under this condition the resulting AZO film was twice as thick as when it was made under the condition of 1.5 kW (Table S3, Supporting Information).Even with such a thick AZO film, it was observed that the sheet resistance was improved by plasma treatment in proportion to the treatment time, up to 5 min, for all flexible substrates (Figure 3a,b).For the PET and PI substrates, the sheet resistance of the AZO electrode improved by 31.6% and 22.2%, respectively (Table 2).This trend was similar to the AZO film produced at 1.5 kW power and an oxygen flow rate of 0.2 sccm.The reason for this trend is discussed in detail in Section 2.2.The transparent flexible AZO electrode exhibited an improvement in sheet resistance following the plasma treatment, but an additional analysis was conducted to confirm whether this effect was retained over time.As seen in Figure 3c,d, following 5 min of plasma treatment on all substrates, after 5 days had passed, over 90% of the sheet resistance had recovered.This sheet resistance recovery effect is because the oxygen vacancies formed by the plasma were restored by recombination with oxygen.To prevent this recovery effect and ensure the reliability of sheet resistance, a passivation process is necessary after plasma treatment.This possibility is discussed in detail in Section 2.3, which discusses the applicability of the component's contact resistance reduction.
One of the most important features of transparent flexible AZO electrodes are their high transmittance. [35]Therefore, in addition to the improvement in sheet resistance, we also analyzed the transmittance trend after plasma treatment (Figure S2, Supporting Information).As shown in Figure 3e,f, there was almost no change in transmittance before and after plasma treatment for each PET, and PI substrate.Before plasma treatment, the PET and PI showed transmittances of 75.41% (at 555 nm) and 47.65% (at 650 nm), respectively, and after plasma treatment, they slightly decreased to 75.26% (at 555 nm) and 47.27% (at 650 nm), respectively.If the post-processing is conducted at a temperature higher than the glass transition temperature of the flexible polymer substrate, it may worsen the surface roughness, leading to a reduction in transmittance. [21]However, in this study, we applied a low-temperature plasma jet treatment, which resulted in minimal changes in surface roughness, thus the material retained its high transmittance.
The recovery of transmittance effect was analyzed using the same criteria as the AZO sheet resistance, and the recovery effect was confirmed 5 days after plasma treatment.For PET, the transmittance recovered to a value similar to that before plasma treatment, at 75.31% (at 555 nm).However, the PI retained a value of 47.18% (at 650 nm), which was almost the same as immediately after plasma treatment.In conclusion, the plasma treatment did not have a significant impact on the transmittance of AZO after treatment.These results confirm that plasma treatment can improve the sheet resistance of AZO while preserving its transmittance.

Analysis of Physical and Chemical Properties of the AZO Layer After Atmospheric Plasma Treatment
The physical and chemical properties of the transparent flexible AZO electrodes were analyzed to understand the mechanism behind the atmospheric pressure plasma treatment using plasma jet for each substrate.The AZO characteristics of each substrate prepared at 1.5 kW power and an oxygen flow rate of 0.2 sccm were analyzed, to determine which showed the best plasma treatment effect.
First, X-ray diffraction (XRD) analyses were conducted to investigate changes in the crystallinity of the AZO electrode after atmospheric pressure plasma treatment.The 2 value was obtained through XRD measurement, and the distance between the crystals and the crystal size were analyzed by inverse calculation.The XRD analysis indicated there was no significant difference between the Ar plasma-treated and untreated samples (Figure 4a-c).This indicates that the plasma treatment did not affect the crystal structure of the AZO or the substrate, and the decrease in the sheet resistance of the AZO film was not due to a change in crystal structure.
Next, the roughness of the AZO surfaces on each substrate was analyzed using an Atomic Force Microscope (AFM) before and after atmospheric pressure plasma treatment (Figure 4d-f).When examined by AFM, the root-mean-square roughness (R q ) of the AZO on the glass, PET, and PI substrates before plasma treatment was 2.0, 17.60, and 1.31 nm, respectively.However, after plasma treatment, R q increased to 8.028, 18.86, and 2.533 nm, respectively, indicating that the roughness increased slightly.These results were consistent with the trend in decreased transmittance of the AZO after atmospheric pressure plasma treatment (Figure 3g-i).This confirmed that the plasma treatment had a subtle effect on the surface roughness of the AZO, inducing changes in transmittance.
X-ray Photoelectron Spectroscopy (XPS) was used to analyze the chemical bonding structure of the AZO electrodes after atmospheric pressure plasma treatment.The Ar plasma treatment can form oxygen vacancies in n-type materials and metal vacan-cies in p-type materials, which increase the electrical conductivity of the oxide thin film by generating free electrons and holes, respectively. [36]This study aimed to induce a reduction in sheet resistance by using atmospheric pressure Ar plasma bombardment and thermal diffusion to induce the formation of oxygen vacancies (Figure 5a).For the AZO fabricated on PET substrates, an increase in oxygen vacancies and O 2− was observed after plasma treatment (Figure 5b,c).The AZO on PI substrates showed almost the same trend after plasma treatment (Figure 5d,e).XPS analysis of the AZO fabricated on Si wafers also showed the same trend after plasma treatment (Figure S3, Supporting Information).
The XPS results analyzing the oxygen vacancy ratio in the AZO samples before and after plasma treatment showed that it in- creased from 23.00% to 28.82% for the PET substrates, from 24.45% to 29.24% for the PI substrates, and from 32.54% to 34.13% for Si wafers (Figure S4, Supporting Information).These results confirmed that the increase in oxygen vacancies after plasma treatment was even greater for AZO fabricated on polymer substrates.These results are also consistent with the trend of improved sheet resistance and confirm that the plasma treatment induces changes in oxygen vacancies, leading to improved sheet resistance (Table 1, Table 2).
In this study, Ar ion bombardment was performed, and oxygen vacancies were induced by the atmospheric pressure plasma and thermal diffusion, as shown in Figure 5a.To analyze the si-multaneous effects of the two factors, Ar ion bombardment and thermal diffusion, the plasma treatment was applied in two ways, continuously and stepwise (Figure S5, Supporting Information).The continuous method continuously applied plasma treatment at a fixed point while increasing the time, allowing simultaneous confirmation of the effects of Ar ion bombardment and thermal diffusion.On the other hand, the stepwise method moved the point according to the applied time of each plasma treatment, making it possible to confirm the relative effect of the Ar ion bombardment.
The change in sheet resistance was examined after treatment using the continuous and stepwise methods according to oxygen flow rate, and it was found that the sheet resistance reduction effect was relatively greater for the continuous method after treatment of 2 min or less (Figure S5, Supporting Information).This indicates that oxygen vacancies are being simultaneously generated by thermal diffusion.These results show the same tendency as those observed in previous studies, where the sheet resistance of ITO decreased due to the increase in oxygen vacancies after Ar plasma treatment. [34]onventional thermal treatment methods can damage flexible substrates.However, this study confirmed the plasma jet was an effective method of improving sheet resistance, as the ion bombardment generated oxygen vacancies during atmospheric pressure plasma treatment, while minimizing substrate damage. [34]In various previous studies, when thermal annealing and plasma treatment methods were applied to AZO/Ag/AZO electrodes they were primarily conducted in high-temperature or low-pressure environments (Table 3).However, this study demonstrates that plasma treatment can enhance the performance of AZO/Ag/AZO electrodes even at room temperature and atmospheric pressure.

Evaluation of AZO Device Applicability and Reliability After Atmospheric Plasma Treatment
The potential benefits and mechanism of applying atmospheric pressure plasma jet treatment to transparent and flexible AZO were investigated, and its applicability and reliability were evaluated in actual devices.The process of verifying contact resistance was carried out by patterning electrodes on the AZO surface treated with plasma (Figure 6a).The resistance recovery phenomenon was observed in previous tests after plasma treatment,  [42]   but in this device application, the AZO electrodes were covered for protection after plasma treatment, before testing their reliability.The calculation of contact resistance was performed using Equations ( 1) and ( 2), and the difference between the two results yields Equation (3).
The experimental results for each substrate are shown in Figure 6b-d when the AZO electrodes were plasma-treated under the specified conditions.The results confirmed that the contact resistance of the AZO decreased in all substrates, and the improvement in contact resistance was greater in the PET and PI substrates than for glass substrates.
To confirm the reliability of the AZO contact resistance at the device level after plasma treatment, after 500 h the contact resistance of samples prepared with and without plasma treatment were measured again and compared (Figure 6e-g).When the AZO electrode was exposed to air, the sheet resistance recovered to its initial value ≈5 days after plasma treatment.
However, when the AZO electrode was covered with AZO again after plasma treatment, even after 500 h, the sheet resistance remained at an improved level.These results confirmed that plasma treatment by atmospheric pressure plasma jet could be sufficiently applied at the device level, and reliability could be maintained for a long-time using passivation.

Conclusion
In summary, we demonstrated the effects of the selective surface treatment of high-performance transparent flexible AZO us-ing atmospheric Ar plasma.The improvement in sheet resistance following the atmospheric plasma treatment of transparent flexible AZO electrodes was greater in electrodes of ≈50 nm thickness, produced at 1.5 kW, compared to those of ≈100 nm thickness produced at 2.5 kW.The improvement effect was also greater in AZO electrodes produced under high oxygen flow rate (0.2 sccm).Compared to the glass substrate, the transparent flexible AZO produced on PET and PI substrates showed a greater improvement in sheet resistance, with a maximum improvement of 83.1%.The atmospheric plasma treatment produced a significant improvement in sheet resistance without affecting transmittance.
To investigate the principle mechanism behind this atmospheric Ar plasma treatment, XRD and AFM analyses were conducted, which showed that there was little effect on the crystallinity and roughness of the AZO.However, XPS analysis confirmed that the surface resistance was improved due to the formation of oxygen vacancies caused by ion bombardment and thermal diffusion.The ratio of oxygen vacancy formation was consistent with the trend of surface resistance improvement tested earlier.Based on these results, the transparent flexible AZO was applied and evaluated at device level.The results confirmed the improvement in contact resistance and the reliability of the performance improvement, which lasted for >500 h, was also confirmed following passivation of the plasma treatment area.

Experimental Section
Transparent Flexible AZO Electrode Fabrication: To investigate the effect of power and O 2 variation on the characteristics of the thin films and plasma reactivity, AZO electrodes were fabricated by sputtering under two power conditions, 1.5 and 2.5 kW, and three O 2 conditions, 0, 0.1, and 0.2 sccm.The target used to deposit the AZO film was a 98% AZO alloy (2 wt% Al 2 O 3 -doped ZnO) to ensure better adhesion of the film.In order to reduce the error in deposition rate, glass substrates of 1 mm thickness (soda-lime glass), PI substrates of 125 μm thickness, and PET substrates of 200 μm thickness were all prepared in the same size of 75 mm x 16 mm.Organic and inorganic particles were also removed from the substrate through a Figure 6.Plasma treatment effects and reliability analysis on the contact resistance of AZO electrodes at the device level: a) Method for measuring AZO patterning and contact resistance according to selective plasma treatment application.b) Current/voltage characteristics analysis according to plasma treatment of AZO devices fabricated on glass substrate.c) Current/voltage characteristics analysis according to plasma treatment of AZO devices fabricated on PET substrate.d) Current/voltage characteristics analysis according to plasma treatment of AZO devices fabricated on PI substrate.e) Performance reliability evaluation after 500 h following plasma treatment of AZO devices fabricated on glass substrate.f) Performance reliability evaluation after 500 h following plasma treatment of AZO devices fabricated on PET substrate.g) Performance reliability evaluation after 500 h following plasma treatment of AZO devices fabricated on PI substrate.cleaning process.After cleaning, the substrate was attached to a 2G size glass plate (main glass) with a size of 37.0 cm x 47.0 cm using kapton tape and fixed it to a jig, then transferred it from the load chamber of the in-line magnetron sputter to the main process chamber.To ensure a smooth deposition process, the base vacuum of the main process chamber should be secured to 6 × 10 −6 torr.Additionally, the vacuum atmosphere was stabilized by pre-sputtering and removed impurities on the target surface to prevent arc discharge.In particular, since the power and O2 conditions were different for each sample in this study, the AZO film was deposited under the same conditions, where the voltage (V) and current (I) could yield the same value for each sample.
Plasma Treatment and Characterization of the AZO Electrodes: Plasma treatment was performed on the AZO electrodes using a plasma pipette (Femto Science Co.) of the plasma jet type.The distance between the AZO electrode and the plasma pipette was set to 1 cm, and a power output of 4 W was applied.The thickness and surface roughness of the AZO film deposited on the glass substrate were measured using an -step surface profiler at different powers and O 2 flow rates, and the change in AZO surface resistance after plasma treatment was measured using a fourpoint probe.In addition, the transmittance of the AZO electrode was analyzed using a UV-vis spectrometer.To analyze the mechanism of the plasma treatment effect on the AZO electrode, Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) were used.

Figure 1 .
Figure 1.Concept of selective surface treatment using atmospheric pressure plasma jet: a) Surface treatment of transparent and flexible AZO using a plasma pipette.b) Structure and operating principle of an atmospheric pressure plasma pipette.

Figure 2 .
Figure 2. Improvement in sheet resistance after application of plasma jet to transparent and flexible AZO fabricated under 1.5 kW condition [Graphs (b-e) represent the mean value ± standard deviation (n = 5)]: a) Plasma jet application methods for glass, PET, and PI substrates.b) Sheet resistance of AZO fabricated under O 2 0 and 0.1 sccm conditions on a glass substrate as a function of surface treatment time.c) Sheet resistance of AZO fabricated under O 2 0.2 sccm conditions on a glass substrate as a function of selective surface treatment time and location.d) Sheet resistance of AZO fabricated under O 2 0 and 0.1 sccm conditions on a PET substrate as a function of surface treatment time.e) Sheet resistance of AZO fabricated under O 2 0 and 0.1 sccm conditions on a PI substrate as a function of surface treatment time.

Figure 3 .
Figure 3. Analysis of the effect of plasma jet on the sheet resistance and improvement in transmittance of transparent flexible AZO electrodes fabricated under 2.5 kW conditions, and the recovery effect [Graphs (a-e) represent the mean value ± standard deviation (n = 5).]:a) Sheet resistance of AZO produced on a PET substrate as a function of surface treatment time under 2.5 kW, 0.2 sccm conditions.b) Sheet resistance of AZO produced on a PI substrate as a function of surface treatment time under 2.5 kW, 0.2 sccm conditions.c) Sheet resistance recovery effect after surface treatment of AZO electrode produced on a PET substrate.d) Sheet resistance recovery effect after surface treatment of AZO electrode produced on a PI substrate.e) Analysis of transmittance change of AZO electrode fabricated on PET substrate after surface treatment and its recovery effect.f) Analysis of transmittance change of AZO electrode fabricated on PI substrate after surface treatment and its recovery effect.

Figure 4 .
Figure 4. XRD and AFM analysis of AZO according to plasma jet application: a) XRD analysis of AZO electrodes on glass substrates with and without plasma treatment.b) XRD analysis of AZO electrodes on PET substrates with and without plasma treatment.c) XRD analysis of AZO electrodes on PI substrates with and without plasma treatment.d) Analysis of surface roughness of AZO electrodes on glass substrates with and without plasma treatment.e) Analysis of surface roughness of AZO electrodes on PET substrates with and without plasma treatment.f) Analysis of surface roughness of AZO electrodes on PI substrates with and without plasma treatment.

Figure 5 .
Figure 5. Analysis of atmospheric plasma jet treatment principles by XPS: a) Oxygen vacancy formation principle due to Ar plasma bombardment and thermal diffusion.b) XPS results of AZO film on PET substrate before plasma treatment.c) XPS results of AZO film on PET substrate after plasma treatment.d) XPS results of AZO film on PI substrate before plasma treatment.e) XPS results of AZO film on PI substrate after plasma treatment.

Table 1 .
Improvement in sheet resistance after 5 min of plasma treatment for AZO electrodes manufactured under 1.5 kW conditions.

Table 2 .
Sheet resistance improvement according to plasma treatment time on AZO electrodes produced under 2.5 kW condition.

Table 3 .
Comparison with post-process methods and conditions of previous studies on AZO/Ag/AZO electrodes.