Evaluation of the permeability of ultrathin parylene AF4 films to determine minimum closed thickness for nanoscale packaging

The miniaturization and portability trend regarding the design of electronic devices has driven the development of numerous flexible photovoltaic devices, including flexible liquid crystal displays and flexible solar cells.¹ For these devices, waterproof materials are crucial to prevent malfunctions caused by the influence of external water vapor. However, achieving both sufficient waterproofing and flexibility while maintaining high light transmission poses a major challenge.^1–3 Therefore, nanoscale waterproof layers are required to ensure optimal performance in these next-generation electronics.

In the selection of flexible waterproof materials for electronics, polymers offer superior flexibility than inorganic materials (e.g., ceramics and glass).¹ Among polymers, parylene stands out for its low liquid and gas permeability^3,4 (a comparison of the water vapor transmission rate with other polymer materials is shown in Table I),^7–10 in addition to its ability to form colorless and transparent films.^4,5 However, the deposition of nanoscale parylene films is associated with the risk of insufficient coverage, which can lead to nonuniformities, cracks, and voids on the substrate surface; these voids facilitate the aggregation of water molecules, which can accelerate the infiltration of the water molecules and can thus compromise the film’s barrier properties.^6,7 Therefore, for nanoscale parylene films, addressing the problem of void formation is crucial; potential solutions to this problem include the application of surface modification techniques, such as inorganic film coating to fill defects, or the use of materials with inherently high hydrophobicity to inhibit water diffusion.^3,6,7

In summary, parylene demonstrates excellent waterproof performance, with parylene N, C, and D being the most widely used variants.^4,5 However, for outdoor applications, concerns regarding ultraviolet (UV)-induced degradation of polymer materials arise. In this context, parylene AF4 stands out owing to its superior UV resistance.^11–13 In addition, its inherent hydrophobicity (as demonstrated by its surface contact angle of 120° ± 4°) enhances its water repellency.⁴ These combined properties render parylene AF4 an effective choice for waterproof layers designed for outdoor use.

This study investigated the minimum thickness (15, 20, or 25 nm) required for a parylene AF4 film to serve as an effective nanoscale waterproof layer. The study employed closeness analysis and electrochemical impedance spectroscopy (EIS) to determine the minimum thickness required for a closed parylene AF4 waterproof layer. The closeness analysis method was adopted from Rapp et al.,⁶ and it entailed measuring a film’s resistance; a resistance value of >20 MΩ could be considered to indicate a closed film. Moreover, EIS was employed to assess solution penetration through a film by analyzing the impedance change at the solution–film interface.¹⁴ Accordingly, EIS was determined to be suitable for monitoring film permeation and to complement the closeness analysis method.

The detection principle of an interdigital electrode (IDE) involves applying an AC variable electric field to both ends of the electrode.^15,16 As the solution penetrates the surface electrode and the film, the capacitive reactance changes between the IDE fingers. Therefore, an IDE test piece can effectively monitor concentration changes resulting from solution penetration. In this study, to investigate the waterproof properties of parylene AF4, an IDE-coated test piece was fabricated and immersed in phosphate-buffered saline (PBS) solution (0.01M, pH = 7.4). The IDE test piece’s interdigital linewidth, line spacing, and interdigital length were 2300, 800, and 4500 µm, respectively; moreover, the test piece comprised four interdigital pairs.

Figure 1 shows the manufacturing process of the IDE test piece, which involved the following steps: (a) Glass slides were used as substrates and were sequentially cleaned with acetone, methanol, isopropanol, and deionized water. (b) A sputtering machine deposited a 200 Å chromium layer followed by a 2000 Å gold layer on the glass surface. (c) Photoresist S1813 was spin-coated on the surface of the gold layer. (d) IDE patterns were defined through exposure and development. (e) The gold and chromium layers were etched using gold and chromium etchants, respectively. (f) The photoresist layer was removed using acetone, followed by cleaning the surface of the IDE test piece with methanol and isopropanol.

Parylene AF4 films can be produced through the pyrolysis of liquid precursors or the method proposed by Gorham et al.^6,17,18 This study employed the method proposed by Gorham et al. Figure 2(a) illustrates the parylene AF4 deposition process. Initially, 1 g dimer was placed in a vaporizer at 130 °C for evaporation. Subsequently, the evaporated dimer was pyrolyzed into monomers at 650 °C. The monomers in the deposition chamber then facilitated thin film deposition at 20 °C. Film thickness control is typically achieved by adjusting the dimer’s weight or modifying the pressure within the deposition chamber.⁶ Considering the challenges associated with precisely controlling film thickness at the nanoscale level through dimer weight adjustments, this study opted to control the deposition chamber’s height, thereby inducing pressure changes to achieve the desired nanoscale film thickness. The method for controlling chamber height is displayed in Fig. 2(b). Details of the coating parameters are listed in Table II. The volume of the deposition chamber was changed by adjusting the position of the chamber lid, which was self-made. When the chamber height was increased, the monomer concentration decreased, facilitating the deposition of thinner films. Conversely, when the chamber height was decreased, the monomer concentration increased, enabling the deposition of thicker films. To check the surface quality of parylene AF4, an atomic force microscope (AFM) was used to observe the coating surface, as shown in Fig. 3. For the 15, 20, and 25 nm films, it was found that the average roughness (Ra) values of test pieces were 3.49–4.01 nm. The surface roughness measurements of the test pieces were similar after the deposition of parylene.

Figure 4 shows the closeness analysis process conducted on the produced parylene films.⁶ The process comprised the following steps: (a) A parylene AF4 film was deposited on the IDE piece’s surface. (b) The parylene film was removed using a knife to expose the electrode’s surface. (c) Next, 0.25 ml of potassium chloride (KCL, 1M) was applied to the IDE piece’s surface, followed by a 15 min wait period. Surface resistance was then measured using a digital multimeter (HIOKI DT4282, Hioki Corp., Japan). The resistance measurement may yield two outcomes. In the first outcome, the resistance value may be >20 MΩ if the parylene film thickness was sufficient and the film had no voids, indicating a closed film [Fig. 4(c1)]. Conversely, in the second outcome, the resistance value may be <20 MΩ if the film contained voids or gaps through which the solution penetrated and engendered electrode conduction, indicating a nonclosed film [Fig. 4(c2)].

EIS measurements were performed by immersing the parylene AF4-coated IDE test piece in a PBS solution. An electrochemical AC impedance analyzer (CHI614D, CHI Instrument, Austin, TX) was used to measure film impedance at a frequency range of 100 kHz to 0.1 Hz and an AC voltage of 1 mV. The experimental setup is illustrated in Fig. 5. A three-electrode system was used for electrochemical analysis. The IDE test piece served as the working electrode, with silver/silver chloride serving as the reference electrode. The immersion temperature was maintained at 20 °C. To prevent the evaporation of the solution from affecting the PBS concentration during the soaking test, a sealed container was used (12 samples per group, with three samples removed for testing at different time points).

To control film thickness with a fixed dimer weighing 1 g, the chamber height was adjusted, and the thickness of the deposited film was verified using a surface profilometer (Alpha Step IQ, KLA Tencor, Milpitas, CA). Deposition was repeated three times on different days to ensure the reproducibility of the relationship between the chamber height and film thickness. Figure 7 illustrates the observed variation between the chamber height and the thickness of the deposited film: An increased chamber height resulted in a thinner film. This behavior was attributed to the changing monomer concentration within the chamber. As the chamber volume increased (with an increase in the chamber height), the monomer concentration decreased, leading to a thinner film. Conversely, a decrease in the chamber height led to an increase in the monomer concentration, which resulted in a thicker film.

A 0.25 ml KCl solution was applied to the surface of the IDE test piece, which was then allowed to sit for 15 min. The resistance was then measured using a digital multimeter. The results are listed in Table III, indicating a resistance value of 1.35 MΩ (considerably lower than the 20 MΩ threshold) for the 15 nm film. This indicated the presence of voids on the film’s surface. After 15 min, the infiltration of the solution was accelerated, and the electrode surface exhibited a conductive loop, resulting in a resistance value of <20 MΩ. Therefore, the 15 nm film could be classified as nonclosed. Moreover, when the film thickness increased, the resistance value increased. The resistance values of the 20 and 25 nm films exceeded 20 MΩ (66.1 and 111.7 MΩ, respectively). This signifies the absence of voids and confirms the closed nature of these thicker films.

Figure 8 shows the total impedance changes at different frequencies for IDE test pieces with films of varying thickness that were immersed in a PBS solution. The total impedance can be expressed as the sum of the film impedance and the solution impedance. Since it takes several minutes to perform EIS measurement, the first EIS measurement after immersion is completed around the tenth minute. Figure 8(a) shows the Bode plot for the 15 nm thick film. At the beginning of immersion, the total impedance was at its highest level. However, the impedance decreased after 10 min of immersion owing to solution penetration, as demonstrated by the decreasing imaginary and real impedances on the Nyquist plot. The total impedance values measured at various frequencies after 10, 20, and 30 min of immersion were similar and decreased slowly; these results indicate a minimal concentration difference between the film and the solution, implying a diminished mass transfer phenomenon. Therefore, one could infer that the solution concentration in the film remained stable from 10 to 30 min. The diffusion of the solution throughout the film occurred within the initial 10 min of immersion, rendering the film’s protection ineffective. Figure 8(b) displays the Bode plot of the 20 nm thick film. The film’s total impedance was initially highest upon immersion. However, after 10 min of immersion, the total impedance of the film decreased, and the total impedance values measured at various frequencies after 20 and 30 min of immersion were similar, indicating the uniform solution concentration in the film between 20 and 30 min. Figure 8(c) illustrates the Bode plot of the 25 nm thick film. The film’s total impedance was initially highest upon immersion. However, after 10 min of immersion, the total impedance of the film decreased, the total impedance values measured at various frequencies after 50 and 90 min of immersion were also similar. In addition, after 10 min of immersion, the total impedance of the 20 nm thick film dropped more significantly than that of the 15 nm thick film. We speculate that there may be dispersed and uneven defects within the 20 nm film, which could have been formed during the film fabrication process. However, these defects do not significantly affect the determination of whether the film is a closed film. The assessment of whether the film is closed primarily depends on observing changes in the film’s impedance over time as the PBS permeates into the film. In the experiment with a 15 nm film, the impedance of the film did not change significantly over time, indicating that the solution could directly penetrate the entire film. In contrast, in the experiment with a 20 nm film, the film’s impedance continuously decreased over time, suggesting that the PBS could not immediately penetrate the entire film thickness.

Figure 9 presents the Nyquist plots derived for IDE test pieces with films of varying thickness that were immersed in a PBS solution. Figures 9(a1)–9(a3) display the plots derived for the 15 nm AF4 film. Figures 9(a2) and 9(a3) show the enlarged views of the red and blue frame areas of Fig. 9(a1), respectively. Once the film was immersed, arc-shaped and oblique straight impedance patterns appeared on the complex plane, indicating that the solution proceeded with mass transfer and penetrated the film [Fig. 9(a3)]. Notably, the plots obtained after 10, 20, and 30 min of immersion closely overlapped [Fig. 9(a2)], suggesting the minimal concentration difference between the film’s internal and external solutions. This indicated a weakening mass transfer phenomenon and a gradual decrease in impedance. Therefore, the solution had almost diffused throughout the film, rendering its protective function ineffective. Given the time required for impedance measurements, one can infer that the solution penetrated the 15 nm film within 10 min.

Figures 9(b1)–9(b3) illustrate the Nyquist plots derived for the 20 nm AF4 film, with Figs. 9(b2) and 9(b3) showing the enlarged views of the red and blue frame areas of Fig. 9(b1), respectively. From the initiation of immersion to the 30 min mark, the low-frequency oblique line on the Nyquist plot was influenced by Z_w, with the tail line of Z_w exhibiting a decreasing trend. Notably, the plots obtained after 20–30 min of immersion appear nearly similar. This indicates a mass transfer phenomenon and a gradual decrease in impedance.

Figures 9(c1)–9(c3) display the Nyquist plots derived for the 25 nm AF4 film, where Figs. 9(c2) and 9(c3) present the enlarged views of the red and blue frame areas of Fig. 9(c1), respectively. From the initiation of immersion to the 90 min mark, the low-frequency oblique line on the Nyquist plot was influenced by Z_w, with the tail line of Z_w exhibiting a decreasing trend. Notably, the plot obtained after 50–90 min of immersion look nearly similar. This indicates a mass transfer phenomenon and a gradual decrease in impedance.

According to the equivalent circuit model displayed in Fig. 6, when the solution penetrated the film thickness, reached the electrode surface, and began to accumulate, a second arc typically manifested on the Nyquist plot [Fig. 6(c)]. However, a soaking test involving a nanoscale film did not reveal a second arc. This phenomenon could be attributed to the extremely thin film, which allowed the solution to permeate through the electrodes.

Table IV lists the EIS fitting data derived for the 15, 20, and 25 nm films; in this table, Y₀ denotes the lowest value when the test piece was initially immersed. However, as the immersion time increased, Y₀ increased because the solution gradually penetrated between the atoms of the film, increasing the overall capacitance value.^3,23,24

During the initial immersion stage of the 15 nm film, a high-impedance oblique line appeared on the end line segments, indicating the onset of diffusion; y₀ represents the minimum value observed at this point (with σ indicating its significance), which corresponds to the appearance of a high-impedance 45° oblique straight line at the end of Z_w. Because of the substantial difference in the interfacial concentration, the mass transfer phenomenon became evident, leading to a high Warburg impedance level. Furthermore, the y₀ values were highly similar after 10–30 min of immersion, indicating the minimal concentration difference between the solutions inside and outside the film. Consequently, the mass transfer phenomenon weakened, and impedance gradually declined. These results indicate that the solution diffused throughout the film, resulting in the failure of film protection.

Similar to the 15 nm film, the 20 nm film was involved in solution diffusion during the initial stages of immersion. After 20 min of immersion, the value of y₀ increased (indicating a decrease in σ), and a low-impedance 45° oblique line appeared at the end of Z_w; this indicates that the solution concentration in the film increased, the concentration difference decreased, the mass transfer phenomenon decreased, and also, the Warburg impedance decreased. The y₀ values observed after 20–30 min of immersion were similar, further supporting the notion of nearly complete solution diffusion in the film after 20 min. The 25 nm film was also involved in solution diffusion during the initial immersion stage, and the y₀ values became stable only after 50 min of immersion, implying complete solution diffusion by that time point. On the basis of the soaking test results (Figs. 8 and 9 and Table IV), this study conservatively estimated the film failure times as follows: <10 min for the 15 nm film, 20 min for the 20 nm film, and 50 min for the 25 nm film.

Notably, the film penetration time did not exhibit a linear relationship with film thickness. In particular, the 15 nm film exhibited the highest permeability level, which is consistent with the observations of Ortigoza-Diaz et al.⁸ Although surface voids were present in the AF4 film, they likely accelerated solution penetration,⁷ explaining the faster permeation of the 15 nm film compared with its thicker counterparts. Conversely, with an increase in film thickness, the solution encountered a longer diffusion path, effectively strengthening the barrier and extending the penetration time.

Initially, the parylene AF4 film acted as a barrier to the external PBS solution, as displayed by the equivalent circuit model in Fig. 6(a). However, prolonged immersion triggered solution penetration through the film due to mass transfer processes, as illustrated in Fig. 6(b). This ongoing penetration led to solution accumulation at the film–electrode interface, eventually causing film delamination. Excessive solution accumulation further induced surface cracks on the film. In this study, the parylene AF4 films with thicknesses of 15, 20, and 25 nm all exhibited cracks; thus, the images of the 25 nm thick film displaying cracks was selected as a representative photograph. The scanning electron microscopy (SEM) images of the cracked films are depicted in Fig. 10.

This study investigated the permeability of parylene AF4 films and determined the minimum thickness required for a closed film by using EIS and closeness analysis. The closeness analysis results reveal that the resistance value of the 15 nm film was 1.35 MΩ (less than the threshold value of 20 MΩ). The resistance values of the 20 and 25 nm films were 66.1 and 111.7 MΩ, respectively (more than the threshold value of 20 MΩ). The EIS results indicate that when the IDE test piece was immersed in a PBS solution at 20 °C, the failure times of the 15, 20, and 25 nm films were <10, 20, and 50 min. Therefore, on the basis of these results, this study classified the 15 nm film as nonclosed and the 20 and 25 nm films as closed. Furthermore, the 20 nm film demonstrated the smallest thickness for a closed film. These findings provide valuable reference points for nanoscale waterproofing and packaging coatings. The nanoscale waterproof film in this study can effectively solve the waterproof problem of paper speakers.²⁵ The advantage of nanoscale film thickness is that it will not affect the frequency response of the ultra-thin electret diaphragm, thereby maintaining high-quality sound performance. In addition, because parylene AF4 has excellent UV resistance, this technology is also suitable for outdoor speaker diaphragms, providing long-term UV protection without affecting the performance of sensors or actuators. In addition, it can also be used in other applications, such as wearable devices and outdoor electronic devices, to provide effective waterproof and UV protection solutions without adding weight or bulk to the device.

The authors acknowledge the National Science Council of Taiwan for providing financial support (Grant No. MOST 106-2622-E-327-015-CC3).

The authors declare no conflicts of interest.

Chien-Hao Chung: Conceptualization (equal); Data curation (equal); Methodology (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Hsiang-Yu Wu: Conceptualization (equal); Data curation (equal); Methodology (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Wen-Cheng Kuo: Conceptualization (equal); Data curation (equal); Methodology (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.