Various Substrates

It was observed that the discharge and the deposited film appearance show dependence on the types of the substrates used for deposition. Figure 38(a) [same picture as Figure 22(a)] and Figure 38(b) show the He/MMA DBD jet images at 1.0 W when a microscope slide and a silicon wafer were used as the substrates, respectively. The

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plasma spot diameter (~6 mm) on the microscope slide appears larger than that (~2 mm) on the silicon wafer. Similar spot size to that on the silicon wafer was also observed when an aluminum sheet was employed as the deposition substrate. A relatively large plasma spot was observed on the microscope slide as the dielectric material (microscope slide) distributes the discharge over the entire surface (i.e., surface discharge) [7]. This is a common phenomenon in the DBD plasmas. In contrast, the conductive/semi-

conductive substrates (e.g., aluminum and silicon wafer) allow the discharge current to flow through, leading to the plasma spot size similar to the dimension of the plasma jet. The plasma spot size further affects the growth area of the deposited film. Figure 38(c) [same picture as Figure 25(a)] and Figure 38(d) show the as-deposited PMMA films on the microscope slide and silicon wafer, respectively, for comparison. The PMMA film on the microscope slide is around 8 mm, whereas that on the silicon wafer is about 2 mm. Note that here we refer to the relatively continuous and uniform pattern formed directly downstream of the DBD jet as the “film” in the figure. These images indicate that the plasma spot size on the substrate determines the film dimension. The larger the discharge spot, the greater the continuous deposited film. This further confirms the results in Section 4.3.3, in which we showed that an increase in the discharge power (i.e., an increase in the plasma spot size) leads to the expansion of the film diameter. It can be observed from Figure 38(c) and Figure 38(d) that both deposited results contain the droplet-like patterns in the outer region. As shown in Figure 38(d), there exists an outer thin, continuous film between the film edge and the droplet-like patterns on the silicon wafer. Film thickness measurement using the profilometer shows that the thickness

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(several micrometers) of the central film deposited directly in the plasma spot is significantly greater than that (several tens of nanometers to several hundreds of nanometers) of the outer thin film. The formation of the outer thin film is likely due to the presence of those longer-lived species in the plasma afterglow to activate the continuous film formation.

Figure 38. Comparison of images of (a) discharge on microscope slide, (b) discharge on silicon wafer, (c) as-deposited PMMA film on microscope slide, and (d) as-deposited PMMA film on silicon wafer at 1.0

W.

In order to demonstrate that this technique is suitable for temperature-sensitive substrates, various substrate materials, such as plastic, rubber, onion, and even fingernail, were employed in the PMMA deposition. Figure 39 shows the photos of the deposition results on these different substrates. In each case, a power of about 2–3 W is utilized to

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generate the DBD jet within the transition regime, and as before the gap between the tube end and the substrate is set to 10 mm. As shown in Figure 39(a), a half-circle PMMA film can be observed on a sheet of transparent polyethylene plastic. The lower edge of the film is formed by covering a portion of the substrate with a tape mask during deposition. This makes the observation of the film clearer. Figure 39(b) presents the PMMA deposition result on a piece of EPDM rubber. No mask is used in this case since the difference between the film and the substrate can be easily distinguished.

Figure 39. Photographs of PMMA deposition results on different substrates: (a) plastic; (b) rubber; (c) onion epidermis; (d) fingernail.

To gain an insight into the influence of the low-temperature DBD jet on the organic material, a piece of onion epidermis (a monolayer of onion cells) is also peeled

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off and fixed to a microscope slide for PMMA deposition. Figure 39(c) displays the image of the onion epidermis after deposition. Masking using a cover slip was employed to facilitate visualization of the PMMA film edge indicated by the dash line on Figure 39(c). The region above the line is coated with PMMA, while the region below the line is uncoated. The clear difference between the coated and the uncoated surfaces can be seen by the insets, which are the images obtained by optical microscopy. Compared to the image of the uncoated part with distinct plant cells (bottom inset), the top inset clearly shows that there are PMMA films covering on top of the cells. Certain of the cell wall structures appear intact beneath the film layer. In deposition onto the cells certain yet unexplained non-uniformities are observed in the coating. Additional issues are the potential gas emission from the organic substrate which may cause porosity in the films. Furthermore, PMMA deposition is achieved on a fingernail, as shown in the inset of Figure 39(d). Tape was used as a mask to create the edge of the film for easy observation (not shown in the figure). A clear PMMA pattern on the fingernail can be seen in Figure 39(d) after 10 minutes of deposition.

4.4 Section Conclusions

A polymer deposition system by using a floating-electrode DBD jet was developed, which enables active plasma and energetic species to be generated in the vicinity of the substrate. This floating-electrode DBD jet deposition technique can be operated not only at atmospheric pressure but also in ambient air with a large gap distance from the tube end to the treated objects. Two distinct modes, which are diffuse

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mode and concentrated mode, were observed in the DBD jet with the rising applied power. MMA was employed experimentally as the monomer for depositing PMMA films. The operation modes of discharge were found to result in different influences on the film deposition rates, while the deposition temperature increases linearly as the discharge power rises. The results showed that quite high deposition rate (22 nm·sec−1_), which is 3-10 times higher than those in many prior works using atmospheric plasma jets as their depositing tools, can be attained with discharge power of 3.5 W and 39 °C deposition temperature. In addition to typical transparent PMMA films, opaque films with wrinkled microstructures can be obtained by using the concentrated-mode DBD jet with relatively high-power operation likely due to a buckling effect. High quality (RMS roughness is 0.4±0.1 nm) of the deposited transparent films was also demonstrated by SEM and AFM imaging techniques. Similar functional groups were observed comparing pure PMMA and the films deposited at different powers by using FTIR, though there are variations in the concentrations of different bonds in the films surfaces as the discharge power is changed. It was also shown that higher power operation leads to higher C:O ratio in the deposited films. Besides, the higher the power of DBD jet is applied, the less retention of ester groups and the higher concentration of the –CHn groups are observed. Additional C=O/O–C–O bonds were seen from the XPS analysis. This could be

attributed to the scission of the ester groups in MMA monomers. The existence of ether/alcohol groups in the deposited films were also proven by both XPS and FTIR results. OES results showed the light emitted from the He DBD jet is mainly due to the NO, OH, N , 2 N+2, He, and O transitions. It was also observed that the higher the

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2

N , He, and O lines relative to the N lines. The MMA ₂ addition to the He DBD jet inhibited the generation of NO and OH excited species. By using this proposed ambient polymer deposition technique with floating-electrode DBD jet, rapid polymer film growth can be achieved on various types of temperature-sensitive substrates (e.g., plastic, rubber, onion, and fingernail) with similar characteristic features to the conventional polymer films.

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