Electrochemical migration of Ag nanoink patterns controlled by atmospheric-pressure plasma

https://doi.org/10.1016/j.mee.2013.01.041Get rights and content

Abstract

Highly contrasting surface energies were induced on polyimide (PI) substrates using atmospheric-pressure plasma (APP) to allow precise printing of Ag electrodes that showed mitigated electrochemical migration (ECM). The substrate surface was made uniformly hydrophobic via APP tetraethyl orthosilicate (TEOS) polymerization. Selected areas were then made hydrophilic via oxygen APP applied through a patterned metal mask. Ag nanoink was then inkjet-printed onto the hydrophilic portions and sintered for 30 min at various temperatures ranging from 100 to 250 °C. The resulting Ag patterned electrodes were of the desired dimensions and showed sharp edges. The Ag ECM dendrites deposited at the cathode took ca. 39% longer than in similar patterns printed on pristine substrates. The contrasting surface-energies induced by the plasma allowed precise control of the Ag electrodes’ edges, which led to reduce ECM.

Highlights

APP is efficient for precise metallization by varying surface energy of a substrate. ► The edge shape of Ag nanoink electrode was controlled to improve the ECM resistance. ► The ECM time of Ag electrode was increased by an average of 39% after APP treatment.

Introduction

High-integrated, miniaturized electronic devices require fabrication process that can achieve high resolutions on various substrates [1], [2], [3]. Unlike conventional metallization by photolithography and vacuum deposition, direct printing by such as screen, gravure and inkjet printing allows additive manufacturing placing conductive materials at designated positions [4]. Of the established direct printing techniques suitable for fine-pitch metallization, inkjet printing is the most widely researched [5], [6], [7]. It involves the ejection through a nozzle onto a substrate of picoliter-level droplets of nanoink containing conductive nanoparticles dispersed in a suitable solvent. The ejection through the nozzles of a printer head can be driven by thermal, piezoelectric or Rayleigh breakup methods [8], [9]. Inkjet printing provides versatility, drop-on-demand, real-time control and noncontact operation. Microcircuits are required to be highly integrated with finely-pitched electrodes for high-performance devices. Inkjet printing such structures requires the precise control of patterns’ shapes and dimensions.

Inkjet printing, while attractive for the production of conductive circuits due to its low cost and environmental compatibility, is hampered by inadequate fluidic control of the nanoink to avoid nozzle clogging and the coffee stain effect [10]. Electrochemical migration of printed electrodes is also a problem due to their electrochemical instability under a bias voltage at certain temperatures and humidities. Atmospheric-pressure plasma (APP) can lessen the roughness of printed lines that is induced by the flow and spreading of solution-based nanoinks. Its use is compatible with the mass production of printed electronics as it is applied in a single-step, simple, inexpensive and continuous process [11]. Surface treatment by APP can efficiently improve the quality of fine-pitch metallization by varying the surface-energy contrast of the polyimide (PI) substrate. Such selective surface treatment can easily achieve fine-pitch circuits and controlled sharp edges in printed circuits without the development of precise printer nozzles or re-engineering the properties of the nanoink.

Electrodes’ reliable operation is important and it can be undermined by the electrochemical migration (ECM) of the Ag nanoink electrodes of microelectronic devices. It is a significant problem at high temperature, humidity and applied bias and impedes the application of Ag nanoinks in various fields [12]. ECM occurs through the conductive electrodes responding to the ionization of metals under the applied bias, and the formation of conductive dendrites at the cathode which grow toward the anode, eventually leading to short-circuit failure [13], [14], [15]. ECM characteristics in Ag nanopaste electrodes has been reported to be affected by their microstructural evolution [16] and their imprecisely patterned edges, which are common in circuits fabricated by direct printing [17], [18], [19].

This work reports the influence of edge shape control on the ECM of Ag nanoink electrodes. ECM was assessed through measuring the ECM time, defined as the time required for dendrites to grow sufficiently to join two adjacent electrodes. It was found to depend on sintering temperatures, bias voltage and APP treatment.

Section snippets

APP treatment

The surface energy of the PI substrate was modified using an APP system (APP Inc., Suwon, Korea). A plasma generator head and 13.56 MHz radio frequency supply with an L-C matching unit generated atmospheric radio frequency glow-discharge plasma. The PI surface was cleaned with distilled water and isopropyl alcohol and a plasma polymer film of tetraethyl orthosilicate (TEOS) was obtained using a polymerization reactor which employed a working pathway (4 L/m) of argon (Ar) to sustain the discharge

Results and discussion

For inkjet-printed patterns to have sharp edges, high resolution with a large contrast of surface energies is required. TEOS polymerization by APP treatment reduced the surface energy of the entire PI substrate. High surface energy was then selectively induced by oxygen APP treatment through a stainless steel mask. Table 1 lists the contact angles of distilled water and CH2I2 and the surface energies of the PI substrate after each APP treatment. TEOS polymerization increased the contact angles

Conclusions

The APP treatment of PI substrate was tested to improve the shape control of inkjet-printed Ag electrodes and their ECM. A combination of TEOS APP polymerization and oxygen APP treatment resulted in large surface-energy contrasts between areas of the substrate. Printing electrodes on the patterns formed using APP resulted in the precise placement of Ag on the substrate. The printed Ag patterns were sintered at 100–250 °C; their electrical resistivity decreased greatly with increasing sintering

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0083540).

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