Preparation and characterization of silver self-metallization on polyimide

Introduction

Polymer metallization has attracted attention in recent years due to its wide use in microelectronics, electronics technology, and the automotive industries.^1–3 Polyimide (PI) has excellent thermal and chemical stability, and it has good adhesion and electrical properties long-term operation ability.^2,4 On the other hand, silver (Ag) is the most conductive noble metal. It has unique electronic and optical features, and these features make it ideal for use in coating polymer in electronic material. ⁵

Metallization is the best method for Ag plating on polymer films when aiming to improve properties that are equivalent to those of Ag powder. ⁶ However, the adhesion of passive metals, such as Ag, Au, and Cu, to polymers is notoriously poor.^6,7

Metallization, or the autocatalytic metal plating of polymers, can be achieved using the following methods: solid and molten metal-based deposition, gaseous phase-based metal deposition, and solution-based metal deposition. ⁷ Because nonconductive materials—primarily synthetic polymers—cannot be metalized through simple electrolytic deposition, the surface of these materials must be activated with a metal-reducing catalyst through a process called autocatalytic plating.^8,9

Nickel, gold, Ag, platinum, palladium, and copper are frequently deposited through this process to produce industrial products, such as printed circuit boards, hard memory disks, automotive parts, and decorative elements.^10,11 Additionally, many researchers have prepared metallized polymer films by heating positive metal ions and polymer solutions to stimulate the reduction of metal ions. ¹² However, the polymers were already deformed or cracked before the reduction of metal ions at a high temperature, and adhesion problems still existed.^13,14 In view of the shortcomings of the above methods, developing a more efficient method to prepare metallizing polymer films is of great significance.^15,16

In recent years, researchers have been focused on polymer surface modification and metalized it by using different metals. Ji et al. prepared the metal-plated polyimide and treated (etching) it by base solution and then activated by silver. The modified polyimide film was immersed into the electroless copper plating solution, which has different molar ratios of nickel in the solution. The thickness and surface morphology of the copper layer on the polyimide films were characterized with scanning electron microscopy (SEM). ¹⁷ Wu et al. metalized polyimide (PI) by using nickel (Ni) and formed it using a solution-based process including imide-ring opening reactions, the implanting of Ni ions, the reduction of catalytic Ni nanoparticles, and the electroless deposition of a Ni film. They report that the KOH concentration significantly affects the implanting, reduction, and deposition behavior of Ni. A uniform Ni layer can be grown on a PI film with full coverage through electroless deposition with a KOH concentration of 0.5 M and higher. ¹⁸

In this research, Ag-metalized PI films were prepared using the autocatalytic deposition method. The quality of Ag coatings improved after etching by KOH solution, SnCl₂ sensitization, and PdCl₂ activation of the PI surface. The metalized films were characterized as a function of different deposition times.

Experimental

Results and discussion

Figure 3 illustrates the EDX analysis, which showed a strong signal in the Ag region and confirmed the formation of Ag nanoparticles. The Ag nanoparticles typically showed an optical absorption peak at approximately 3 keV, and Yuen et al. presented a similar absorption peak. ¹⁹ Other peaks were observed in the C, O, and Cl biomolecules involved in capping the surfaces of these nanoparticles.OPEN IN VIEWER

The XRD patterns of the Ag deposited at different deposition times are presented in Figure 4. These XRD patterns exhibited only the peaks of metallic silver (JCPDS 04–0783) without any other signals, indicating that the synthesized samples are highly purified and face-centered cubic (fcc)-phased Ag NPs, which is in agreement with Zhang et al. ²⁰ OPEN IN VIEWER

The diffraction peaks at 2θ= 38.2^°, 44.5^°, 64.6^°, and 78.2° represent the 111, 200, 220 and 311 planes of silver, respectively. The silver oxide phase was not detected in the deposits. Figure 4 also shows that the corresponding peaks became sharp and the peak width became slightly narrower, indicating that the peaks for the crystalline structure of the silver deposits at different times became more vivid.^20,21

The position of the peaks obtained from the diffraction pattern also provides information about the translational size and shape of the crystal. The crystallite size of the silver nanoparticles was calculated using Scherrer’s equation ( D = 0.9 λ β c o s θ ) , where λ is the wavelength of X-ray, β is the width at half maximum height and θ is the Bragg’s angle.

The results are shown in Table 1. The results show that the crystallite size increased slightly, and this tendency shows agreement with the change in grain size (illustrated in Figure 8).

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Table 1.

Crystallite size of Ag metalized PI films with various deposition times (5, 10, and 15) min.

Time of deposition (min)	Crystal size (nm)
	(111)	(200)	(220)	(311)
5	20.2	25.4	29.8	35.7
10	22.8	27.6	31.6	39.8
15	24.5	29.2	33.8	40.3

FTIR analysis for polyimide and metalized polyimide at different deposition times are shown in Figure 5. For the pure PI spectrum, the main characteristic bands of PI at 1952 and 1602 cm⁻¹ unsymmetrical and symmetrical for C=O stretching of imide groups, whereas the band at 1488 cm⁻¹ is due to stretching of the C–C bond of the imide ring representing the successful chemical imidization process of PAA conversion to polyimide.^22,23OPEN IN VIEWER

The FT-IR analysis for Ag metalized PI at different deposition times (5, 10, 15) minute is shown in Figure 5 (B, C, and D). We can see that the position of =C-O-C= stretching absorption bands at 1735 cm⁻¹ had no changes in position, only a significant difference in peak size at the deposition time increase. The formation of new bands (N–H stretching absorption band and N–C stretching band) assigned at 2325 and 955 cm⁻¹ indicated strong coordination between N atoms and silver nanoparticles. ¹⁸ The C–C stretching and = C–H plane vibration band at 1488 and 1117 cm⁻¹ are shifted to 1413, and 1112 cm⁻¹ indicates the interaction of the silver nanoparticle with the polymer backbone.^24,25

The thermal stability of PI and Ag metalized PI films with various deposition times (5, 10, 15) minute was examined using TGA analysis. Table 2 shows the initial decomposition temperature (T₀), temperature for 5% weight loss (T₅), temperature for 10% weight loss (T₁₀), and maximum decomposition temperature (T_max).

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Table 2.

Thermal analysis of PI and Ag metalized PI films with various deposition times (5, 10, and 15) min.

Samples	T0 [°C]	T5 [°C]	T10 [°C]	Tmax [°C]
PI	388	402	414	515
Ag metalized PI at deposition time 5 min	392	412	419	553
Ag metalized PI at deposition time 10 min	405	415	423	576
Ag metalized PI at deposition time 15 min	415	422	429	588

As shown in Figure 6, PI completely decomposes at temperatures above 550°C, and this behavior agrees with ref. [26,27]. PI, T₀, T₅, T₁₀, and T_max were observed at 388, 402, 414, and 515°C. As the deposition time increased, the thermal stability increased, and Ag deposited on polyimide at 5 min resulted in T₀=392, T₅=412, T₁₀=419, and T_max=553°C. Ag-metalized PI at a deposition time of 10 min increased the thermal properties compared to PI and Ag-metalized PI at a deposition time of 5 min.OPEN IN VIEWER

Consequently, Ag-metalized PI at a deposition time of 15 min further increased the thermal stability, which would benefit electronic devices.^28,29

Figure 7 shows cross-sectional SEM micrographs of the deposits as a function of deposition time. The surface morphology studies imply that the deposited films are uniform, smooth, homogeneous, and nearly densely packed and free from voids, cracks, or pinholes.^19,24 However, the surface of the PI substrate was comparatively rough due to chemical etching. The rough surface could have been caused by the strong interfacial interaction between Ag and the PI substrate.^18,30OPEN IN VIEWER

The surface morphology of the films shows that the Ag NPs are closely packed and form continuous films along with the PI substrates, and the thickness was highly dependent on the deposition time. We can see that the thickness increased from (5–9.5) μm as the deposition time increased from 5 to 15 min.

The surface morphology and the average grain size of the films are shown in Figure 8 from the SEM images.OPEN IN VIEWER

The obtained specific grains are the fastest, favoured growing with no preferential growth direction.^20,25 We can see crystal growth at increasing deposition time via a slight increase in grain size.^31–33 The development and formation of granular structures can be observed, and the grains become tight (closely connected) and regularly shaped, but Ag deposited at 15 min, the grain size increases rapidly.^19,34

Conclusion

In this study, Ag metalized an autolytic plating technique fabricated PI film. The quality of Ag coatings improved after etching by KOH solution, SnCl₂ sensitization, and PdCl₂ activation of the PI surface. FT-IR analysis showed that the modification opened the ring structure of the imide of the film and formed a new ring between Ag and polyimide. The results show that the thermal stability increases as the deposition time increases.

Surface characterization of silver metalized PI showed that the coating surface morphology became more compact, regular, and intense, and optimized as the plating time reached 15 min.