THIN FILM COATING LAYER COMPOSITION AND COATING METHOD

THIN FILM COATING LAYER COMPOSITION AND COATING METHOD

This invention relates to the field of solution-based film coating of substrates like polyester film, polyimide film, polyvinyl chloride film, semi-embossed film, polyvinyl chloride film and like, and is specifically concerned with coating substrates with a coating based on SU-8 and poly(4-vinyl pyridine) (P4VP).

Recently, flexible electronics are gaining increasing research interest due to their promising applications in many practical fields, such as wearable electronics, portable devices, medical implants, etc.( S. R. Forrest, Nature 2004, 428, 911 -918.; D.H. Kim, N.S. Lu, R. Ma, Y.S. Kim, R.H. Kim, S.D. Wang, J. Wu, S. Μ. Won, Η. Tao, A. Islam, K. J. Yu, T. I. Kim, R. Chowdhury, Μ. Ying, L. Ζ. Xu, Μ. Li, H. J. Chung, H. Keum, Μ. McCormick, Ρ. Liu, Y. W. Zhang, F. G.

Omenetto, Y. G. Huang, Τ. Coleman, J. A. Rogers, Science 2011,333, 838.; Υ.

G. Sun, J. A. Rogers, Adv. Mater. 2007,19, 1897-1916.) Flexible circuit, as “blood circulation system” of flexible electronic products, plays an especially important role. Attributed to flexible digital processing mode and a rapid scalable manner, nowadays printing techniques are providing a powerful tool for the fast design and fabrication of different patterns. The application of printing technique in fabricating flexible electronics will undoubtedly open a new door for the production of flexible circuits. Since the printer can make patterns in a highefficiency mode, the conversion of printed patterns into conductive circuits naturally becomes the crux of the question. Electroless metal deposition (ELD), relying on an autocatalytic redox reaction to deposit thin-layer metal on a catalystpreloaded substrate, provides a good solution to this question. (R. S. Guo, Υ. Yu, Z. Xie, X. Liu, X. Zhou, Yufan Gao, Ζ. Liu, F. Zhou, Υ. Yang, Ζ. Zheng, Adv.; Μ.

S. Miller, H. L. Filiatrault, G. J. Ε. Davidson, Μ. Luo, T. Β. Carmichael, J. Am.

Chem. Soc. 2010,132, 765-772.; Τ. Zhang, X. Wang, T. Li, Q. Guo and J. Yang, J. Mater. Chem. C, 2014, 2, 286-294.) With the assistance of printing techniques, active catalyst can be deployed on the specified area of flexible substrate, and thus induce the formation of as-required metal pattern. However, as an open problem, it is known that untreated flexible plastics cannot well grasp catalyst moieties due to lacking binding sites, and simple physical absorption usually results in the diffusion of catalyst into ELD solution and poor adhesion of as-deposited metal to the substrate, and further leads to bad coating quality especially when metal layer become thick, such as delamination and metal bump, and thus it is necessary to modify the surface of flexible substrate for effective uptake of catalyst moieties and improved adhesion of as-deposited metal to the substrate.

Currently, there are mainly two general approaches for surface modification of plastics, which can be classified into surface reforming and surface addition.

Surface reforming refers to changing surface roughness or making active functional groups on the original surface via in-situ oxidizing reaction, such as chemical etching, oxygen plasma. (A. Garcia, T. Berthelot, Ρ. Viel, A. Mesnage, P. Jégou, F. Nekelson, Sébastien Roussel, S. Palacin, ACS Appl. Mater.

Interfaces 2010, 2, 1177-1183.; J. Β. Park, J. S. Oh, Ε. L. Gil, S. J. Kyoung, J. Τ.

Lim, G. Y. Yeom, J. Electrochem. Soc., 2010, 157, D614-D619.) Surface addition means to add an extra active layer onto existing plastics surface, typically including polymer grafting, (A. Garcia, J. Polesel-Maris, P. Viel, S. Palacin, Τ.

Berthelot, Adv. Funct. Mater. 2011,21, 2096-2102.; A. Garcia, T. Berthelot, P.

Viel, P. Jégou, S. Palacin, ChemPhysChem 2011, 12, 2973 -2978.) surface silanization (S. Sawada, Y. Masuda, P. Zhu, K. Koumoto, Langmuir 2006, 22, 332-337.; Y. Chang, C. Yang, X.-Y. Zheng, D.-Y. Wang, Z.-G. Yang, ACS Appl.

Mater. Interfaces 2014, 6, 768-772.) and layer-by-layer deposition of polyelectrolytes, (K. Cheng, M.-H. Yang, W. W. W. Chiu, C.-Y. Huang, J. Chang, T.-F. Ying, Y. Yang, Macromol. Rapid Commun. 2005, 26, 247-264.; T. C. Wang, B. Chen, M. F. Rubner, R. E. Cohen Langmuir 2001,17, 6610-6615.) etc.

As is described, herein there are mainly two purposes for surface modification of flexible substrates, namely realizing selective and efficient uptake of catalyst moieties, and improving the adhesion between the substrate and metal. Consequently, surface modification of plastic substrate should at least take care of these two aspects. On the one hand, modified surface must contain the functional groups that can effectively grasp catalyst moieties; on the other hand, modified surface should be chemically resistant to electroless plating bath and can further play a buffer layer between original substrate and metal for better adhesion. A lot of reports have indicated that modified surface by different methods can enhance the compatibility of metal and organic plastics, but most of them are still far away from scalable low-cost application, either due to complex or environment-unfriendly technological process, or because of the difficulty in scaling up. For instance, typical chromium-containing etching agent for surface modification of printed circuit board have been prohibited in many countries due to its harm to the environment; ligand-containing silane modified film is not acid or alkali resistant, and thus cannot withstand long-time electroless metal deposition because most of metal plating bath is relatively alkali; the grafting of polymer brush usually involves complex steps and harsh requirements for experimental conditions; layer-by-layer polyelectrolyte deposition is extremely slow and lowefficiency and will cost too much time due to tens of repeated coating operation.

Therefore, these methods are not suitable for surface modification of large-area flexible plastics on a large scale.

P4VP molecules can also be directly coated on the surface of substrate, but simple physical absorption usually results in poor adhesion of modified layer.

Thus there is a must to develop a more cost effective method for enhanced adhesion of P4VP molecules on the substrates. As early as 1980s, it had been found that pyridine molecules can help to cure epoxy, (Xue, G.; Ishida, Η.; Konig, J. L. Makromol. Chem., Rapid Commun. 7 (1986) 37; Idem. , Angew. Makromol.

Chem. 142 (1986) 17) and subsequently P4VP also shows the ability to cross link epoxy. (Meng, F.; Zhang, W.; Zheng, S. J. Mater. Sci. 40 (2005) 6367-6373) Based on this mechanism, in this invention, we employ epoxy to cross link P4VP molecules. On the one hand, epoxy has strong reactivity and can form good chemical and mechanical adhesion with polymer substrate; on the other hand, epoxy molecules can also react with each other and P4VP molecules to buildup cross-linked polymer network on the substrate.

It is an object of the invention to provide a simple one-step solution-based coating method for scalable surface modification of the films with different sizes, which will largely decrease the cost of film treatment while meeting the requirements for high-quality metal deposition.

Another object of the invention is to provide an efficient coating to make sure that the deposited copper layer with the thickness more than 7 microns can be easily made on the surface of flexible substrate without any delamination, which is difficult to achieve in other modified surface may attributed to our thicker modified layer produced by dip coating for better adhesion promotion.

Another object of the invention is to provide a printing friendly film coating which enables laser printing, inkjet printing, screen printing, gravure techniques and like to make the mask or to directly deposit functional catalyst on the film surface, in order to induce the formation of metal pattern.

These and other objects are accomplished by our invention which is described below.

We have invented a solution-based method for the fast surface modification of flexible plastics. The coating process can be completely executed under atmosphere at a relatively low temperature, which renders this method suitable for large-scale surface modification of large-area flexible substrates. As-employed surface modifier was composed of polymer ligands and reactive adhesive, and they cannot only react with each other to form cross-linked polymer network, and also reactively bond with the substrates to produce a highly adhesive alkali resistant ligand layer on the surface of substrates for the selective and effective uptake of catalyst moieties.

With the invented coating composition and through electroless copper plating, high-quality copper layer with controllable thickness can be deposited on the flexible substrates. Ultra-thick copper layer (> 7 pm) can be achieved by increasing the plating time, which well overcome the existing problem of thick copper deposition on flexible substrate and open a new way for real industrial production and application of flexible circuits.

Furthermore, double-side flexible circuits with higher integration can be

fabricated fast on modified plastics, which will save more cost and space for flexible electronic devices. In addition, this method for surface modification of flexible plastics can further be extended to other substances, such as 3D objects, paper, cloth, wood, and so on, which will provide a powerful tool for the metallization of isolated materials.

Figure 1a P4VP using SU-8 and PET film coated with a mixture of a schematic flow diagram;

Figure 1b is a photograph of a pure transparent PET film is thin;

Figure 1c is a PET film using SU-8 and the P4VP modified;

Figure 1 d 1 h is covered by a PET film with a copper plating copper layer.

Figure 2a P4VP respectively, and SU-8 P4VP composite coating, and SU-8 P4VP composite coating without NaOH treatment, after treatment P4VP 1Μ NaOH 1 hour after curing SU-8 and the composite coating FT-IR Spectrum;

Figure 2b is a diagram of the contact angle of pure water and the PET film;

Figure 2c is a schematic view of a contact angle of water with the modified PET film;

Figure 2d is a schematic view of water treatment and post-curing modified PET film contact angle of sodium hydroxide;

Figure 3a is a laser printer to produce a flexible circuit schematic printed on the surface modification of toner base reticle;

Figures 3b and 3c are two circuit patterns on the two different sides of the same piece of PET film;

Figures 4a and 4b are SEM images of the surface of a copper layer of copper over 10min;

Figures 4c and 4d are 30min, and 1 h after the copper plating layer on the surface SEM image;

Figure 4℮ and 4f respectively through 1 h, 12h copper deposition layer of copper cross-sectional SEM image.

Figure 5 shows the surface resistivity of the copper layer versus plating time and a copper plating layer thickness increases with plating time.

Figures 6 shows cross-sectional SEM images of layers with different thickness of the copper plating of time.

The following examples illustrate the present invention and its use in the manufacture of printed electronics.

In this invention, based on the thermally initiated cross-linked reaction between epoxy and pyridine rings, we employ SU-8 molecules and poly (4-vinyl pyridine) (P4VP) as the main components of film-making solution, in which SU-8 behaves as curing agent and adhesives, and P4VP acts as metal ligand, and then dip coat on the surface of plastic substrate followed by low-temperature curing.

Preferably the inventive film coating composition includes one or more of the following components: poly (4-vinyl pyridine), SU-8,1,4-dioxane, 2-propanol and ethanol.

In accordance with the invention, a method of coating substrates, such as polyester film, polyimide film, polyvinyl chloride film, semi-embossed film, polyvinyl chloride film, and like with a film coating, comprises the steps of dissolving 1) poly(4-vinyl pyridine), 2) SU-8 into 1,4-dioxane and 2-propanol mixture to form an uniform coating solution, applying an effective amount of the coating solution onto the substrates using dip-coating, spin-coating, blade coating, inkjet printing, screen printing and like to form an uniform film coating on the substrates, and baking the film coating on the substrates in an oven, optionally, but preferably, one or more of the following components is/are mixed into the coating solution with the poly(4-vinyl pyridine), SU-8,1,4-dioxane and 2-propanol to achieve the desired properties such as surface tension, viscosity etc.

for different coating techniques mentioned above: glycerol, ethanol, polyvinyl pyrrolidone, polyethylene glycol, surfactant and like.

Poly (4-vinyl pyridine) (P4VP) has been a good candidate of surface modifiers used for uptake of transitional metal ions attributed to its good alcohol solubility, chelating ability, and pyridine ligands-bearing. 4-vinyl pyridine, as a kind of reactive monomer, can be used to modify substrate surfaces by in-situ polymerization under UV or plasma.

SU-8 plays a bridging agent to anchor P4VP molecules on the substrate surface. Attributed to strong covalent bonding, as-formed coating layer will have a good adhesion to the substrate. Furthermore, as a result of ring opening reaction of epoxide groups, carbon-oxygen bonds will be the dominant bonding type. In contrast to silicon-oxygen bond and ester groups in other polymer grafting, carbon-oxygen ether bonds are more alkali resistant. It is absolutely beneficial for subsequent electroless copper deposition in basic bath.

Preferably, the poly (4-vinyl pyridine) (P4VP) is dissolved in 2-propanol to form a uniform solution, the preferred concentration is 1 w/v% ~ 8 w/v%, more preferred 3 w/v% ~ 6 w/v%. Preferably, the SU-8 is dissolved in 1,4-dioxane to obtain a uniform solution as well, the preferred concentration is 0.1 w/v% ~ 2 w/v%, more preferred 0.3 w/v% ~ 1 w/v%. Preferably the two solutions are mixed to get a transparent coating solution. The preferred solution contains 0.5 w/v% ~ 4 w/v% P4VP and 0.05 w/v% ~ 1 w/v% SU-8, more preferred 1.5 w/v% ~ 3 w/v% P4VP and 0.15 w/v% ~ 0.5 w/v% SU-8.

The ranges of each components of the coating composition of the invention are as follows, by weigh/volume:

The following examples illustrate the invention and its use in the fabrication of printed electronics.

The poly (4-vinyl pyridine) is dissolved in 2-propanol to form 4 w/v% solution, and SU-8 is dissolved in 1,4-dioxane to obtain 0.4 w/v% solution. Then the two solutions were mixed at 1:1 ratio to get a transparent solution. The final solution contains 2 w/v% P4VP and 0.2 w/v% SU-8.

Transparent PET film is cleaned by the mixed solution of 1:1 ethanol and acetone, and then is treated with oxygen plasma followed by dip coating or directly immersed into the film-making solution for dip-coating without oxygen plasma introduced. After 30 seconds, the film is drawn out of the solution slowly and dried in air. In the next, the coated film is put into oven of 120°C for 20mins for in-situ cross-linking reaction of P4VP and SU-8. The thickness of coated layer can be controlled by adjusting the concentration of P4VP and SU-8 in mixed solvent of 2-propanol and 1,4-dioxane.

Upon completion of the coating process, the PET film shows a smooth surface with excellent surface uniformity. The film coating on the PET substrate possesses an excellent long-lasting uniformity, minimal tackiness, good film adhesion.

To demonstrate the functionality of the invented coating, AgNCb was dissolved into deionized water to get 1w/v% AgN03 solution, and the coated PET film is soaked into the AgNÜ3 solution for 10 seconds for the uptake of silver ions. Then the film is washed several times by water to remove free silver ions without bonding with pyridine ligands. The film is dried and put into electroless copper plating bath for different time. Electroless copper plating bath consists of CuS04*5H20 (14 g/L), NaOH (12 g/L), potassium sodium tartrate (16 g/L), EDTA*2Na (20 g/L), HCHO (16.5 mL/L), 2, 2’-dipyridyl (20 mg/L), and potassium ferrocyanide (10 mg/L).

Figure 1a shows the schematic flow of coating PET film by P4VP&SU-8 composites. Oxygen plasma was employed for surface activation to introduce oxygen-containing groups and free radicals on the surface. In principle, these active groups excited by plasma can react with the epoxide groups of SU-8 to form covalent bonding. Figure 1b and 1c present the digital photos of pristine transparent PET film and P4VP and SU-8 modified PET film respectively. It can be seen that, although coated by a layer of P4VP and SU-8 composites, the film is still flexible and highly transparent. The introduction of thin-layer of P4VP and SU-8 composites did not affect the appearance and mechanical properties of PET film a lot.

To further demonstrate the inner principle of the invented coating composition, FT-IR analysis is performed using FT-IR NICOLET 6700 (Thermo Scientific Co.). The contact angle of water with different substrates was measured by Ramé-Hart Contact Angle Goniometer.

Figure 2a shows FT-IR spectrum of P4VP and its composites coated on the substrates. Different spectra present some discrepancies in peak position and intensity. In reference of standard infrared absorption of different functional groups, we can get much information from the spectra. The peaks located in 871 crrr¹ well matches with the absorption of benzene ring, which indicates the introduction of SU-8 in composite coating layer. We can also see that, after curing, the epoxide groups at 915 cnrr¹ almost completely disappear, which demonstrate that strong reactive epoxide groups were nearly consumed up at relatively high curing temperature. Plus, the vibration absorption at 1664cm^-1 that belongs to amide groups was enhanced, which further indicated that cross-linked reaction occurred between pyridine groups and epoxide groups, and new amide groups-bearing products were formed, which is consistent with other research reports. In addition, there are two strong absorption peaks between 1500 cm^-1

and 1600 cm^-1 for all the coating layers, which belong to pyridine rings of P4VP molecules. Before and after curing, the strength and position of the two peaks did not almost change a lot. It indicates that, during curing process, only a small amount of pyridine ligands are consumed by epoxide groups due to much higher content of P4VP in the composites, and a lot of residual pyridine ligands will be available for the uptake of catalyst moieties in the following steps. Figure 2a also presents FT-IR spectrum of cured P4VP and SU-8 composite layer treated by 1 Μ NaOH for 1h. The spectrum is nearly the same with the sample untreated by NaOH, which means that the initial coating layer was still well maintained on the surface of the substrate, and can withstand the erosion of basic solution to some extent. Figure 2b show the contact angle of water with pristine PET film, and it is about 46 degree. After surface modification, the contact angle increase to about 77 degree maybe due to the introduction of hydrophobic SU-8. After being treated by NaOH, the contact angle decreases slightly but is still much larger than that on pristine PET.

Evidently, the invented coating changes the surface energy of PET, and makes PET more hydrophobic. Perhaps enhanced hydrophobicity is not favorable for the wettability of film, but can prevent excessive spreading of aqueous ink, and will be helpful for improving the resolution of printed ink on the substrate once the modified film was used as the substrate of inkjet printing.

In the following examples 2, a functional circuits are fabricated based on the invented coating composition. SEM investigation are conducted to further demonstrate the functionality of this invention.

The coating composition and coating methods are exactly the same as that in example 2. The coated PET film is activated by 1w/v% AgNOe solution by soaking the film into the solution for 10 seconds, and then dried for printing.

Commercial HP laser printer 6700 is used for the printing of toner mask. After printing, the film is put into the oven of 90°C for 1 min for the stabilization of toner mask, and then soaked into electroless copper plating bath for different time. The exposed area will be coated by copper, and copper cannot be formed in the place covered by mask due to the deactivation of the catalyst. After obtaining certain thickness of copper pattern, the mask layer can be washed in acetone by sonication or washed directly by dichloromethane or tetrahydrofuran.

Figure 3 shows the detailed schematic diagram for the production of flexible circuits by employing laser printer to print toner mask on the modified substrates. Figure 3b and 3c show two circuit patterns presented on two different sides of one piece of PET film. The green area is the printed toner.

The SEM images and energy-dispersive X-ray (EDX) spectrum are taken by a Hitachi S-4500 field-emission scanning electron microscope (FE-SEM) at a 5 kV accelerating voltage. Figure 4 presents SEM images of as-deposited copper layers. The surface morphology of copper layer with 10 mins of copper plating was displayed in figure 4a and 4b. We can see a lot of small pits on the surface of copper layers that may be attributed to soft template effects of hydrogen bubbles generated during electroless copper plating.

Further, the change of the thickness of copper layer with plating time is investigated and the relevant images and curves were showed in Figure 5 and Figure 6. Meantime the corresponding conductivity at different thicknesses is also presented. It can be seen that within 2 hours, the copper layer grew up continuously, and in the first hour, the copper layer had the faster growth rate attributed to high initial concentration of copper ions and PH value of copper plating bath. With the continuous consumption of copper ions and hydroxide ions during electroless plating process, the growth of copper became slower and slower until all the copper ions were consumed.

We have found that after 12 hours of electroless plating, the thickness of copper layer can achieve to 7 pm. Then the sheet resistance of copper layer was investigated.

We have also found that the corresponding sheet resistance decreased dramatically with increasing the copper thickness. After 1 h of plating, the sheet resistance of copper layer can reach 0.021 Q/sq. According to the equation ρ = Rs*t, in which ρ is the bulk resistivity, Rs is the sheet resistance, and t is the thickness of metal layer, we can calculate the bulk resistivity of as-deposited copper ρ. Based on the data of thickness and corresponding sheet resistance, we get to know that the bulk resistivity of as-deposited copper layer at 10 mins is ca.

4.8 χ 10'⁸ Ω·η∩, which is 2.7 times of normal bulk copper. With the thickness of copper increased, the bulk resistivity decreased dramatically and get closer and closer to bulk copper. When the plating time increased to 1 h, the bulk resistivity of copper layer turned into ca. 2.8 χ 10'⁸ Ωτη, which is 1.6 times of normal bulk copper.

Furthermore, when the thickness of copper achieve to 7 pm, the conductivity of copper layer can achieve to nearly 70% of normal bulk copper.

Consequently, the thickened copper layer cannot only increase the conduction of copper layer, and also improve the conductivity. High conduction will obviously decrease the wastage of electrical energy and strongly favor the loading of highpower electronic components in flexible electronics.

In the following examples 3-10, the components of each formulation are mixed together, formed into a coating solution, and applied to PET films, as in Example 1 and Example 2, to obtain film coatings possessing a smooth surface, an excellent long-last alkaline solution endurance, minimal tackiness and ultrastrong metal adhesion.

EXAMPLE 10

EXAMPLE 11

EXAMPLE 12

EXAMPLE 13

Regarding preparation of the inventive coating solution, it also may be prepared by adding the individual components of the inventive coating composition directly into solvent and then mixing to form the coating solution.

Preferably, the separate prepared solution is mixed together at a ratio of 1:1.

We have found that the surface of modified PET film carries a lot of pyridine ligands attributed to the bonding of a lot of P4VP molecules, which can effectively capture various transitional metal ions from the solution. As we know, Pd²⁺ and Ag⁺ ions are two typical catalysts for electroless copper plating. They can be attacked by lone pair electrons of nitrogen atom of pyridine ligands to form strong coordination bonds. For example, once the modified PET film was soaked into AgNÜ3 solution, the silver ions will be chemically absorbed onto the surface of PET. Different from simple physical absorption, chemical bonding is much stronger and the absorbed silver ions hardly escape from the surface. Figure 1d shows the copper clad PET film after 1 h of electoless copper plating. As is seen, copper can be well coated on the whole PET substrate and show good flexibility,

We have found that the distribution of the pits is homogeneous but the arrangement is irregular. With continuous copper plating, the copper layer become thicker and thicker, and the pits were filled gradually. Figure 4c and 4d show the surface morphologies of copper layers with 30mins and 1 h of copper plating respectively. Obviously with increasing the copper plating time, the copper grain grows up, and the copper layer becomes denser. Figure 4℮ and 4f show the cross section of copper layer with 1 h and 12h of copper deposition respectively.

We also have found that the thickness of copper layer is about 1,3~1.4 pm after 1 h of copper plating. Meanwhile copper layer was attached onto the substrates tightly and no delamination was found when the invented coating was applied. Scotch tape test was used to check the adhesion of copper layer, and it was found that copper layer can be tore out of PET surface. Even with the thickness of 7 microns, copper layer still has a good adhesion to the substrate (Figure 4f). However, in some other cases, such as for oxygen plasma or concentrated Na0H/H2S04 treated surfaces, or silane/other small molecules grafted surfaces, once the copper layer become thicker, copper tends to delaminate or bubble up from the substrate, which will seriously affect the quality of copper deposition and the reliability of printed circuits. Also, it can be seen that, with the plating time elongated, the under layer of copper began to turn into continuous phase, and the grainy structure disappeared gradually, which will be conducive for the improvement of the conductivity.

Further, based on this invention, we can obtain ultra-thick copper layer on PET substrate. Moreover, as is above-mentioned, the surface modification did not affect the transparency and flexibility of PET film at all. Thus the modified film was very suitable to function as flexible substrate for the printing of flexible circuits.