- Email: [email protected]
Enhancing adhesion strength of photonic sintered screen-printed Ag circuit by atmospheric pressure plasma
Microelectronic Engineering 202 (2018) 37–41
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Enhancing adhesion strength of photonic sintered screen-printed Ag circuit by atmospheric pressure plasma Bum-Geun Parka, Choong-Jae Leeb, Seung-Boo Jungb, a b
T
⁎
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 440-746, Republic of Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 440-746, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Adhesion enhancement Atmospheric-pressure plasma Surface free energy Intense pulsed light sintering Printed electronics
Intense pulsed light sintering of printed metallic circuits on polymer substrates is an attractive technique. However it still does not yield the mechanical quality required for industrial applications. This technology must improve adhesion and flexibility to produce highly reliable flexible electronic devices. The effects of atmospheric pressure oxygen plasma and acrylic acid polymerization treatments upon the adhesion and flexibility properties were investigated. Modified polyimide surfaces were characterized by surface free energy measurements, and adhesion and flexibility properties were evaluated by means of roll-type 90° peel tests and IPC sliding tests. The strength of adhesion with oxygen-treated and acrylic acid polymerized samples increased about 17- and 20-fold compared with that of untreated samples, and the flexibility was improved by about 2 fold, respectively. The evident improvements in surface free energy, adhesion and flexibility were attributed to the creation of oxygen functional groups on the polyimide surface after atmospheric pressure plasma treatment.
1. Introduction Direct printing technology has attracted interest as an alternative method to conventional metallization of flexible substrates because it can reduce processing time, manufacturing cost and material waste. Direct printing may be suitable for radio-frequency identification devices (RFID), sensors, organic light-emitting devices (OLED) and flexible printed circuit boards, and thus may enable a variety of wearable electronic devices [1–10]. For wider adoption of direct printing, however, numerous issues remain unsolved. In particular, the conventional thermal sintering process requires high process temperature and long processing time, making it a bottleneck for mass production of flexible electronic devices. To overcome the problems of conventional sintering, various alternative techniques have been proposed to achieve high throughput metallization, such as electrical sintering, microwave irradiation, atmospheric pressure plasma (APP) exposure and photonic irradiation [11–14]. In particular, intense pulsed light (IPL) sintering with a xenon lamp has the merits that it can be carried out at room temperature in a very short time, on the order of milliseconds [15–18]. This sintering technique opens up the opportunity to use flexible polymer substrates with low glass transition temperature without damaging them. In addition, one of the biggest benefits is that printing
⁎
and post-processing can be applied in a roll-to-roll process [19–21]. All over the world many research groups are actively participating in the fabrication of printed metallic circuit by IPL on flexible substrate [22–24]. However, the IPL sintering process has not yet achieved the mechanical quality needed for product applications; present problems include crack formation and delamination of the metal layer [25]. Accordingly, a number of surface modification techniques have been applied to enhance adhesion, such as UV irradiation, pulse ion irradiation and plasma treatment [26,27]. Atmospheric-pressure plasma (APP) is the most suitable surface treatment technique to apply in the fabrication of printed electronics because of its simplicity, its ability for large-area application and its ability to be carried out fairly rapidly in air [28]. In this study, plasma polymerization and oxygen plasma treatments were utilized to enhance the mechanical properties of IPL-sintered Ag circuits. The IPL-sintered printed Ag circuits with various surface treatments were characterized using several flexural endurance tests including a roll-type 90° peel test and an IPC sliding test.
Corresponding author. E-mail address: [email protected] (S.-B. Jung).
https://doi.org/10.1016/j.mee.2018.10.006 Received 1 May 2018; Received in revised form 5 September 2018; Accepted 12 October 2018 Available online 25 October 2018 0167-9317/ © 2018 Elsevier B.V. All rights reserved.
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
Fig. 1. A schematic diagram of (a) the atmospheric pressure plasma (APP) system and the specimen structure of (b) the roll-type 90° peel test and (c) IPC sliding endurance test.
Table 1 Sintering condition of intense pulsed light irradiation.
Pre-heating Sintering
Pulse number
Pulse number
Pulse time [ms]
Pulse voltage [V]
Irradiation energy [J/cm2]
30 1
1 1
0.3 0.95
1000 1300
0.65 4.48
and the argon gas (4.5 L/min) was used to sustain the discharge; each PI substrate was fixed on a reciprocating chuck and treated for 1 min at a speed of 100 mm/s. Fig. 1 (a) shows a schematic diagram of the APP system employed in this study. The system included atmospheric radio-frequency glowdischarge plasma, which was constructed from a plasma generator head and a 13.56 MHz RF supply with an inductive-capacitive matching circuit. After plasma surface treatment, contact angles of distilled water and diiodomethane (CH2I2) were measured on the APP-treated polyimide substrate surface using a contact angle analyzer (Contact Angle Portable Type, GIT Soft, Korea). Surface free energy was calculated using the Owens–Wendt method [29]. Chemical bonding on the surface was studied using X-ray photoelectron spectroscopy (ESCA 2000, VG Microtech, East Grinstead, UK); XPS spectra were calibrated based on the C1s peak.
Table 2 The calculated surface free energy and the contact angle measured on the surface of PI substrate. Oxygen flow [sccm]
Untreated
0
10
20
30
Contact angles of DI water (degree [°]) Contact angles of diiodomethan (degree [°]) Dispersion component γsd [mJ/mm2] Polar component γsp [mJ/mm2] Surface free energy γs [mJ/mm2] Argon flow in acrylic acid [L/min] Contact angles of DI water (degree [°]) Contact angles of diiodomethan (degree [°]) Dispersion component γsd [mJ/mm2] Polar component γsp [mJ/mm2] Surface free energy γs [mJ/mm2]
65.6 21.7
34.4 12.1
32.3 38.4
27.2 30.26
24.7 29.4
37.5 10.2 47.7 0 27.2 30.26
24.2 37.1 61.23 0.2 22.4 26.3
27.0 36.2 63.1 0.4 17.7 25.1
29.8 36.7 66.5 0.6 17.7 20.2
29.9 37.9 67.8 0.8 17.6 20.0
29.8 36.7 66.5
34.6 38.7 69.0
34.7 38.6 70.9
34.5 38.4 71.2
34.5 38.4 71.3
2. Experimental details
2.2. Design and test sample preparation
2.1. Plasma treatment
An Ag nanopaste was used as a metal ink; this ink combining with commercial Ag nanoparticles of mean size 40 nm were dispersed in polymeric matrix solvent (α-terpineol) comprising binder (ethyl cellulose) and dispersing agent (Polyvinylpyrrolidone) with the solid loading of 75 wt%. This ink was printed onto APP-treated polyimide substrates by a screen printing method. Specimens for peel strength and resistance to flexural fatigue test (Fig. 1(b) and (c)) were designed according to the IPC TM-650 industrial standard. The circuits for the measurement of adhesion were printed with 3 mm line in width, and then an adhesive layer was attached onto the sintered tracks. And the patterns for sliding test had a 1 mm line in width and 1 mm line in space. All printed circuits were sintered at room temperature by means of IPL energy. The IPL sintering system (PSTEK Co., Ltd., Korea) consisted of a xenon flash lamp, a lamp house, a pulse controller and a power
Kapton polyimide (PI) film substrate with 25 μm thick was purchased from DuPont. The PI surface was cleaned with ethanol in an ultrasonic bath and then rinsed in distilled water, followed by drying with a stream of nitrogen gas. Prior to metal printing, surface treatment of the PI was carried out using atmospheric pressure plasma (MyPL Auto-150, APP Inc., Korea). Two different APP treatments were investigated: an oxygen treatment (gas flow: 0, 10, 20 or 30 sccm of O2 with an argon flow of 4.5 L/min) and an acrylic acid polymerization treatment (gas flow: 0, 0.2, 0.4 or 0.6 L/min of acrylic acid carried in argon, with an oxygen flow of 20 sccm). During plasma treatment, the plasma power was set at 100 W
38
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
Fig. 2. Surface free energy of PI surface is caculated by Owen-wednt method after (a) the oxygen plasma treatment and (b) the plasma polymerization.
Fig. 3. C1 s XPS spectra of the PI surface : (a) before the APP treatment, (b) after oxygen plasma treatment, (c) after acrylic acid plasma polymerization and (d) peak area fraction.
were subjected to sliding deformation.
supply. Table 1 lists the conditions used to carry out IPL sintering. Using a single pulse of 4.48 J/cm2, IPL energy was irradiated for 0.95 ms. Adhesion between screen-printed Ag and PI substrates after various plasma treatments was evaluated by means of roll-type 90° peel tests, in which Ag circuits were peeled vertically from the substrate at the fixed speed of 50.8 mm/min. To evaluate the flexibility of screen-printed Ag circuits, their changes in electrical resistance were measured while they 39
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
Fig. 4. After IPL sintering at 1300 V for 0.95ms, peel strength of screen-printed Ag circuit and APP finished Polyimide substrates; (a) oxygen plasma and (b) acrylic acid polymerization.
3. Results and discussion 3.1. Effect of the atmospheric pressure plasma treatment on the surface characteristics The contact angles were measured for the various APP treatment conditions to evaluate the changes in physical interaction. The results of contact angle and surface free energy are listed in Table 2. Fig. 2 shows the surface free energies of the APP-treated PI surface. The surface free energy of the PI substrate was calculated using the Owens–Wendt equation. The contact angles of distilled water and CH2I2 were used to determine the polar (γsp) and dispersion (γsd) components in surface free energy, respectively.
γs = γ ds + γsp Fig. 5. After IPL sintering, electrical resistance changes with various APP treatments during IPC sliding endurance test.
γsd
(1)
γd (cosθd + 1) − = ⎛ 2 ⎜ γdd − ⎝
γdp
γ (cos θw γwp w γd ⎞ γdp ⎛ γwp ⎞ ⎟ ⎝ w⎠⎠
+ 1)
(2)
Fig. 6. Fracture surface of Ag circuit by optical (a~d) after sliding test : (a) before the APP treatment, (b) after Ar plasma, (c) oxygen plasma, (d) acrylic acid plasma polymerization treatment and (e) crack size distribution. 40
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
γsp =
4. Summary
γw (cosθw + 1) − 2 γ ds γ dw 2
γwp
(3)
The effects of APP treatment upon the adhesion and flexibility of screen-printed Ag on PI were studied. Oxygen plasma and plasma polymerization (acrylic acid) treatments were applied to improve adhesion between Ag circuit and the PI substrate. Surface free energy was increased with increasing oxygen and Ar/ acrylic acid gas flow rates. The increase of oxygen functional groups such as CeO, C]O (carbonyl) and HO–C=O (carboxyl) was evaluated by XPS spectra. The peel strength of samples treated with oxygen plasma and Ar/acrylic acid plasma increased approximately 17- and 20fold compared to that of untreated samples. Also, the samples' flexibility was dramatically increased after APP treatment. These results showed that the increase of oxygen functional groups was the main factor in the improved adhesion between screen-printed Ag and the PI substrate. Therefore, APP surface modification treatment is suitable for the enhancement of mechanical properties in the fabrication of flexible electronic devices.
The surface free energy increased with increasing oxygen flow rate (Fig. 2(a)) and then became saturated over the oxygen gas flow of 20 sccm. However, the surface free energy was saturated because of oxygen gas used in base gas (Fig. 2(b)). It was observed that plasma treatment considerably increased the surface free energy: The surface free energy of the treated substrate was approximately 1.5 times that of the untreated PI substrate. The APP treated PI surface was more hydrophilic and higher surface free energy by changing the surface characteristics. The effects of plasma treatment on the chemical bonding at the substrate surface (Fig. 3) were evaluated with XPS analysis. After the oxygen plasma treatment, oxygen-containing functional groups CeO (288.5 eV) and C]O (290 eV) (increased from 16% to 26% and from 0% to 2.7%, respectively. It was also observed that carboxyl groups (O=C–OH, 287 eV)) were formed after the acrylic acid polymerization treatment. Fig. 3(d) shows the C1s peak area fraction for the substrate surfaces subjected to various treatments. After plasma treatment, oxygen-containing functional groups (CeO, carbonyl and carboxyl) on the PI surface increased. These chemical changes have been reported to contribute to the formation of Ag-O-C bonds between the Ag circuit and the polymer surface. Ag-O-C species are one of the most important factor to improve adhesion between metal and polymer surface [30]. These result indicated that the formed Ag-O-C bond contribute to enhance the adhesion and flexibility of the Ag circuit by reducing the delamination of Ag circuit on PI substrate after IPL sintering.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03035587). This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20174030201800). References
3.2. Adhesion strength between screen-printed Ag circuit and polyimide substrate
[1] [2] [3] [4] [5]
The results of roll-type 90° peel testing for screen-printed and IPLsintered Ag circuits on untreated, oxygen plasma–treated and plasmapolymerized PI samples are plotted in Fig. 4. The untreated PI sample showed the peel strength of about 17 N/m. The plasma-treated sample showed approximately 10 times greater peel strength, of up to 172 N/ m. The peel strength increased up to 290 N/m with increasing oxygen flow rate. The peel strength results of the plasma-polymerized sample are shown in Fig. 4(b). The maximum peel strength was recorded to be about 345.4 N/m, for the sample treated with acrylic acid solution in an Ar gas flow of 0.4 L/min. Increasing the Ar gas flow to 0.6 L/min or greater during plasma polymerization treatment decreased the plasma stability, thereby worsening the peel strength of the resulting sample. These result indicated that the formed Ag-O-C bond contribute to enhance the adhesion between Ag circuit and PI substrate, and reduce the delamination of Ag circuit on PI substrate after IPL sintering.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
3.3. Flexibility of screen printed Ag circuit sintered by using IPL energy
[17] [18] [19]
Fig. 5 shows electrical resistance changes of the Ag circuits during repeated sliding endurance testing. The electrical resistance increased with the cycle number due to initiation and propagation of cracks in the Ag circuit. The flexibility of the Ag circuit sintered by IPL increased about 2 fold after the plasma treatment. These results indicated that enhancements to adhesion between the screen-printed Ag circuit and the PI substrate influenced the circuit's flexibility. After the sliding test of 100,000 cycles, the microstructure of cracks, generated on each Ag films, was observed by optical microscopy (Fig. 6(a-d)). As shown in Fig. 6(e), the crack size distribution was obtained by OM and SEM images using the image analyzer software. The screen-printed Ag circuit without plasma treatment had a larger amount of and more critical cracks than that of the plasma treated Ag circuits. Especially, the cracks of samples treated with oxygen and Ar/acrylic acid plasma shows the narrowest and shortest cracks among the Ag circuits.
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
41
D.A. Pardo, G.E. Jabbour, N. Peyghambarian, Adv. Mater. 12 (2000) 1249. Y. Kim, B. Lee, S. Yang, I. Byun, I. Jeong, S.M. Cho, Curr. Appl. Phys. 12 (2012) 473. B.G. Park, K.H. Jung, S.B. Jung, J. All. Comp. 699 (2017) 1186. C. Liu, Adv. Mater. 19 (2007) 3783. A.C. Siegel, S.T. Philips, M.D. Dickey, N. Lu, Z. Suo, G.M. Whitesides, Adv. Funct. Mater. (2010) 28. M.L. Scarpello, I. Kazani, C. Hertleer, H. Rogier, D. Vande Ginste, IEEE. Antenn. Wirel. PR. 11 (2012) 838. J.R. Windmiller, J. Wang, Electroanalysis 25 (2013) 29. S.J. Hong, J.W. Kim, C.J. Han, Y.S. Kim, T.W. Hong, J. Microelectron. Packag. Soc. 17 (2010). E. Song, R.P. Tortorich, T.H. da Costa, J.W. Choi, Microelectron. Eng. 145 (2015). R.P. Tortorich, J.W. Choi, Nano 3 (2015) 3. K.S. Kim, B.G. Park, K.H. Jung, J.W. Kim, M.Y. Jeong, S.B. Jung, J. Nanosci. Nanotech. 15 (2015) 2333. M. Allen, A. Alastalo, M. Suhonen, T. Mattila, IEEE Trans. Microwave Theory Tech. 97 (2011) 791. J. Perelaer, B.J. de Gans, S.S. Ulrich, Adv. Mater. (18) (2006). S. Wünscher, S. Stumpf, J. Perelaer, U.S. Schubert, J. Mater. Chem. C 2 (2014) 1642. C.J. Han, B.G.Park.M.S. Oh, S.B. Jung, J.W. Kim, J. Mater. Chem. C 5 (2017) 9986. H.S. Kim, S.R. Dhage, D.E. Shim, H.T. Hahn, Appl. Phys. A Mater. Sci. Process. 97 (2009) 791. J. Niittynen, E. Sowade, H. Kang, R.R. Baumann, M. Mäntysalo, Sci. Rep. 5 (2015). H.J. Hwang, W.H. Chung, H.S. Kim, Nanotechnol. 23 (2012) 485205. J.H. Lee, Y.M. Lee, S.J. Lim, D.K. Choi, J.W. Yu, Sol. Energy Mater. Sol. Cells 143 (2013) 517. S.H. Ko, Semicond. Sci. Technol. 31 (2016). K. Eun, M.W. Chon, T.H. Yoo, Y.W. Song, S.H. Choa, Microelecton. Reliab. 55 (2015) 838. J.W. Youn, S.J. Lee, K.S. Kim, D.U. Kim, Appl. Surf. Sci. 441 (2018) 508. W. Li, Y. Yang, B. Zhang, C.F. Li, J. Jiu, K. Suganuma, Adv. Mater. Interfaces (2018) 1800798. J.H. Kwak, S.J. Chun, C.H. Shon, S. Jung, Appl. Phys. Lett. 112 (2018) 153103. D.J. Lee, S.H. Park, S. Jang, H.S. Kim, J.H. Oh, Y.W. Song, J. Micromech. Microeng. 21 (2011) 125023. S.C. Gong, I.S. Shin, S.H. Bang, H.C. Kim, S.O. Ryu, H.T. Jeon, H.H. Park, C.H. Yu, H.J. Chang, J. Microelectron. Packag. Soc. 16 (2009) 21. K.S. Kim, W.R. Myung, S.B. Jung, J. Adhes. 88 (2012) 337. K.S. Seo, J.H. Cha, M.K. Han, C.S. Ha, D.H. Kim, H.J. Lee, H.J. Lee, Jpn. J. Appl. Phys. 54 (2015). D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. L.J. Gerenser, J. Vac. Sci. Technol. A 6 (1988) 2897.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Enhancing adhesion strength of photonic sintered screen-printed Ag circuit by atmospheric pressure plasma Bum-Geun Parka, Choong-Jae Leeb, Seung-Boo Jungb, a b
T
⁎
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 440-746, Republic of Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 440-746, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Adhesion enhancement Atmospheric-pressure plasma Surface free energy Intense pulsed light sintering Printed electronics
Intense pulsed light sintering of printed metallic circuits on polymer substrates is an attractive technique. However it still does not yield the mechanical quality required for industrial applications. This technology must improve adhesion and flexibility to produce highly reliable flexible electronic devices. The effects of atmospheric pressure oxygen plasma and acrylic acid polymerization treatments upon the adhesion and flexibility properties were investigated. Modified polyimide surfaces were characterized by surface free energy measurements, and adhesion and flexibility properties were evaluated by means of roll-type 90° peel tests and IPC sliding tests. The strength of adhesion with oxygen-treated and acrylic acid polymerized samples increased about 17- and 20-fold compared with that of untreated samples, and the flexibility was improved by about 2 fold, respectively. The evident improvements in surface free energy, adhesion and flexibility were attributed to the creation of oxygen functional groups on the polyimide surface after atmospheric pressure plasma treatment.
1. Introduction Direct printing technology has attracted interest as an alternative method to conventional metallization of flexible substrates because it can reduce processing time, manufacturing cost and material waste. Direct printing may be suitable for radio-frequency identification devices (RFID), sensors, organic light-emitting devices (OLED) and flexible printed circuit boards, and thus may enable a variety of wearable electronic devices [1–10]. For wider adoption of direct printing, however, numerous issues remain unsolved. In particular, the conventional thermal sintering process requires high process temperature and long processing time, making it a bottleneck for mass production of flexible electronic devices. To overcome the problems of conventional sintering, various alternative techniques have been proposed to achieve high throughput metallization, such as electrical sintering, microwave irradiation, atmospheric pressure plasma (APP) exposure and photonic irradiation [11–14]. In particular, intense pulsed light (IPL) sintering with a xenon lamp has the merits that it can be carried out at room temperature in a very short time, on the order of milliseconds [15–18]. This sintering technique opens up the opportunity to use flexible polymer substrates with low glass transition temperature without damaging them. In addition, one of the biggest benefits is that printing
⁎
and post-processing can be applied in a roll-to-roll process [19–21]. All over the world many research groups are actively participating in the fabrication of printed metallic circuit by IPL on flexible substrate [22–24]. However, the IPL sintering process has not yet achieved the mechanical quality needed for product applications; present problems include crack formation and delamination of the metal layer [25]. Accordingly, a number of surface modification techniques have been applied to enhance adhesion, such as UV irradiation, pulse ion irradiation and plasma treatment [26,27]. Atmospheric-pressure plasma (APP) is the most suitable surface treatment technique to apply in the fabrication of printed electronics because of its simplicity, its ability for large-area application and its ability to be carried out fairly rapidly in air [28]. In this study, plasma polymerization and oxygen plasma treatments were utilized to enhance the mechanical properties of IPL-sintered Ag circuits. The IPL-sintered printed Ag circuits with various surface treatments were characterized using several flexural endurance tests including a roll-type 90° peel test and an IPC sliding test.
Corresponding author. E-mail address: [email protected] (S.-B. Jung).
https://doi.org/10.1016/j.mee.2018.10.006 Received 1 May 2018; Received in revised form 5 September 2018; Accepted 12 October 2018 Available online 25 October 2018 0167-9317/ © 2018 Elsevier B.V. All rights reserved.
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
Fig. 1. A schematic diagram of (a) the atmospheric pressure plasma (APP) system and the specimen structure of (b) the roll-type 90° peel test and (c) IPC sliding endurance test.
Table 1 Sintering condition of intense pulsed light irradiation.
Pre-heating Sintering
Pulse number
Pulse number
Pulse time [ms]
Pulse voltage [V]
Irradiation energy [J/cm2]
30 1
1 1
0.3 0.95
1000 1300
0.65 4.48
and the argon gas (4.5 L/min) was used to sustain the discharge; each PI substrate was fixed on a reciprocating chuck and treated for 1 min at a speed of 100 mm/s. Fig. 1 (a) shows a schematic diagram of the APP system employed in this study. The system included atmospheric radio-frequency glowdischarge plasma, which was constructed from a plasma generator head and a 13.56 MHz RF supply with an inductive-capacitive matching circuit. After plasma surface treatment, contact angles of distilled water and diiodomethane (CH2I2) were measured on the APP-treated polyimide substrate surface using a contact angle analyzer (Contact Angle Portable Type, GIT Soft, Korea). Surface free energy was calculated using the Owens–Wendt method [29]. Chemical bonding on the surface was studied using X-ray photoelectron spectroscopy (ESCA 2000, VG Microtech, East Grinstead, UK); XPS spectra were calibrated based on the C1s peak.
Table 2 The calculated surface free energy and the contact angle measured on the surface of PI substrate. Oxygen flow [sccm]
Untreated
0
10
20
30
Contact angles of DI water (degree [°]) Contact angles of diiodomethan (degree [°]) Dispersion component γsd [mJ/mm2] Polar component γsp [mJ/mm2] Surface free energy γs [mJ/mm2] Argon flow in acrylic acid [L/min] Contact angles of DI water (degree [°]) Contact angles of diiodomethan (degree [°]) Dispersion component γsd [mJ/mm2] Polar component γsp [mJ/mm2] Surface free energy γs [mJ/mm2]
65.6 21.7
34.4 12.1
32.3 38.4
27.2 30.26
24.7 29.4
37.5 10.2 47.7 0 27.2 30.26
24.2 37.1 61.23 0.2 22.4 26.3
27.0 36.2 63.1 0.4 17.7 25.1
29.8 36.7 66.5 0.6 17.7 20.2
29.9 37.9 67.8 0.8 17.6 20.0
29.8 36.7 66.5
34.6 38.7 69.0
34.7 38.6 70.9
34.5 38.4 71.2
34.5 38.4 71.3
2. Experimental details
2.2. Design and test sample preparation
2.1. Plasma treatment
An Ag nanopaste was used as a metal ink; this ink combining with commercial Ag nanoparticles of mean size 40 nm were dispersed in polymeric matrix solvent (α-terpineol) comprising binder (ethyl cellulose) and dispersing agent (Polyvinylpyrrolidone) with the solid loading of 75 wt%. This ink was printed onto APP-treated polyimide substrates by a screen printing method. Specimens for peel strength and resistance to flexural fatigue test (Fig. 1(b) and (c)) were designed according to the IPC TM-650 industrial standard. The circuits for the measurement of adhesion were printed with 3 mm line in width, and then an adhesive layer was attached onto the sintered tracks. And the patterns for sliding test had a 1 mm line in width and 1 mm line in space. All printed circuits were sintered at room temperature by means of IPL energy. The IPL sintering system (PSTEK Co., Ltd., Korea) consisted of a xenon flash lamp, a lamp house, a pulse controller and a power
Kapton polyimide (PI) film substrate with 25 μm thick was purchased from DuPont. The PI surface was cleaned with ethanol in an ultrasonic bath and then rinsed in distilled water, followed by drying with a stream of nitrogen gas. Prior to metal printing, surface treatment of the PI was carried out using atmospheric pressure plasma (MyPL Auto-150, APP Inc., Korea). Two different APP treatments were investigated: an oxygen treatment (gas flow: 0, 10, 20 or 30 sccm of O2 with an argon flow of 4.5 L/min) and an acrylic acid polymerization treatment (gas flow: 0, 0.2, 0.4 or 0.6 L/min of acrylic acid carried in argon, with an oxygen flow of 20 sccm). During plasma treatment, the plasma power was set at 100 W
38
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
Fig. 2. Surface free energy of PI surface is caculated by Owen-wednt method after (a) the oxygen plasma treatment and (b) the plasma polymerization.
Fig. 3. C1 s XPS spectra of the PI surface : (a) before the APP treatment, (b) after oxygen plasma treatment, (c) after acrylic acid plasma polymerization and (d) peak area fraction.
were subjected to sliding deformation.
supply. Table 1 lists the conditions used to carry out IPL sintering. Using a single pulse of 4.48 J/cm2, IPL energy was irradiated for 0.95 ms. Adhesion between screen-printed Ag and PI substrates after various plasma treatments was evaluated by means of roll-type 90° peel tests, in which Ag circuits were peeled vertically from the substrate at the fixed speed of 50.8 mm/min. To evaluate the flexibility of screen-printed Ag circuits, their changes in electrical resistance were measured while they 39
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
Fig. 4. After IPL sintering at 1300 V for 0.95ms, peel strength of screen-printed Ag circuit and APP finished Polyimide substrates; (a) oxygen plasma and (b) acrylic acid polymerization.
3. Results and discussion 3.1. Effect of the atmospheric pressure plasma treatment on the surface characteristics The contact angles were measured for the various APP treatment conditions to evaluate the changes in physical interaction. The results of contact angle and surface free energy are listed in Table 2. Fig. 2 shows the surface free energies of the APP-treated PI surface. The surface free energy of the PI substrate was calculated using the Owens–Wendt equation. The contact angles of distilled water and CH2I2 were used to determine the polar (γsp) and dispersion (γsd) components in surface free energy, respectively.
γs = γ ds + γsp Fig. 5. After IPL sintering, electrical resistance changes with various APP treatments during IPC sliding endurance test.
γsd
(1)
γd (cosθd + 1) − = ⎛ 2 ⎜ γdd − ⎝
γdp
γ (cos θw γwp w γd ⎞ γdp ⎛ γwp ⎞ ⎟ ⎝ w⎠⎠
+ 1)
(2)
Fig. 6. Fracture surface of Ag circuit by optical (a~d) after sliding test : (a) before the APP treatment, (b) after Ar plasma, (c) oxygen plasma, (d) acrylic acid plasma polymerization treatment and (e) crack size distribution. 40
Microelectronic Engineering 202 (2018) 37–41
B.-G. Park et al.
γsp =
4. Summary
γw (cosθw + 1) − 2 γ ds γ dw 2
γwp
(3)
The effects of APP treatment upon the adhesion and flexibility of screen-printed Ag on PI were studied. Oxygen plasma and plasma polymerization (acrylic acid) treatments were applied to improve adhesion between Ag circuit and the PI substrate. Surface free energy was increased with increasing oxygen and Ar/ acrylic acid gas flow rates. The increase of oxygen functional groups such as CeO, C]O (carbonyl) and HO–C=O (carboxyl) was evaluated by XPS spectra. The peel strength of samples treated with oxygen plasma and Ar/acrylic acid plasma increased approximately 17- and 20fold compared to that of untreated samples. Also, the samples' flexibility was dramatically increased after APP treatment. These results showed that the increase of oxygen functional groups was the main factor in the improved adhesion between screen-printed Ag and the PI substrate. Therefore, APP surface modification treatment is suitable for the enhancement of mechanical properties in the fabrication of flexible electronic devices.
The surface free energy increased with increasing oxygen flow rate (Fig. 2(a)) and then became saturated over the oxygen gas flow of 20 sccm. However, the surface free energy was saturated because of oxygen gas used in base gas (Fig. 2(b)). It was observed that plasma treatment considerably increased the surface free energy: The surface free energy of the treated substrate was approximately 1.5 times that of the untreated PI substrate. The APP treated PI surface was more hydrophilic and higher surface free energy by changing the surface characteristics. The effects of plasma treatment on the chemical bonding at the substrate surface (Fig. 3) were evaluated with XPS analysis. After the oxygen plasma treatment, oxygen-containing functional groups CeO (288.5 eV) and C]O (290 eV) (increased from 16% to 26% and from 0% to 2.7%, respectively. It was also observed that carboxyl groups (O=C–OH, 287 eV)) were formed after the acrylic acid polymerization treatment. Fig. 3(d) shows the C1s peak area fraction for the substrate surfaces subjected to various treatments. After plasma treatment, oxygen-containing functional groups (CeO, carbonyl and carboxyl) on the PI surface increased. These chemical changes have been reported to contribute to the formation of Ag-O-C bonds between the Ag circuit and the polymer surface. Ag-O-C species are one of the most important factor to improve adhesion between metal and polymer surface [30]. These result indicated that the formed Ag-O-C bond contribute to enhance the adhesion and flexibility of the Ag circuit by reducing the delamination of Ag circuit on PI substrate after IPL sintering.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03035587). This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20174030201800). References
3.2. Adhesion strength between screen-printed Ag circuit and polyimide substrate
[1] [2] [3] [4] [5]
The results of roll-type 90° peel testing for screen-printed and IPLsintered Ag circuits on untreated, oxygen plasma–treated and plasmapolymerized PI samples are plotted in Fig. 4. The untreated PI sample showed the peel strength of about 17 N/m. The plasma-treated sample showed approximately 10 times greater peel strength, of up to 172 N/ m. The peel strength increased up to 290 N/m with increasing oxygen flow rate. The peel strength results of the plasma-polymerized sample are shown in Fig. 4(b). The maximum peel strength was recorded to be about 345.4 N/m, for the sample treated with acrylic acid solution in an Ar gas flow of 0.4 L/min. Increasing the Ar gas flow to 0.6 L/min or greater during plasma polymerization treatment decreased the plasma stability, thereby worsening the peel strength of the resulting sample. These result indicated that the formed Ag-O-C bond contribute to enhance the adhesion between Ag circuit and PI substrate, and reduce the delamination of Ag circuit on PI substrate after IPL sintering.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
3.3. Flexibility of screen printed Ag circuit sintered by using IPL energy
[17] [18] [19]
Fig. 5 shows electrical resistance changes of the Ag circuits during repeated sliding endurance testing. The electrical resistance increased with the cycle number due to initiation and propagation of cracks in the Ag circuit. The flexibility of the Ag circuit sintered by IPL increased about 2 fold after the plasma treatment. These results indicated that enhancements to adhesion between the screen-printed Ag circuit and the PI substrate influenced the circuit's flexibility. After the sliding test of 100,000 cycles, the microstructure of cracks, generated on each Ag films, was observed by optical microscopy (Fig. 6(a-d)). As shown in Fig. 6(e), the crack size distribution was obtained by OM and SEM images using the image analyzer software. The screen-printed Ag circuit without plasma treatment had a larger amount of and more critical cracks than that of the plasma treated Ag circuits. Especially, the cracks of samples treated with oxygen and Ar/acrylic acid plasma shows the narrowest and shortest cracks among the Ag circuits.
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
41
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