Surface treatment for Cu metallization on polyimide film by atmospheric pressure dielectric barrier discharge plasma system

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Abstract

Atmospheric pressure plasma (APP) surface treatment method was applied to surface modification of polyimide (PI) films to enhance the adhesion between copper layer and PI surface by electroless plating. Also, APP was ignited by radio frequency (RF; 13.56 MHz) plasma power supply. In this study, nontoxic gases (nitrogen, oxygen, and NO (nitrogen:oxygen = 1:1)) plasma effects were mainly investigated instead of toxic gas such as NH3. The influences of APP treatment on chemical composition of the PI surface were investigated by using Fourier transform infrared spectroscopy (FT-IR). The wettability was also investigated by water contact angle measurement. The changes of binding energy of PI substrate were investigated by X-ray photoelectron spectroscopy (XPS). The contact angle results show that was dramatically decreased to below 30° from 58° (bare PI). Electroless copper depositions were carried out with the free-formaldehyde method using glyoxylic acid as the reducing reagent and mixture palladium chloride, tin chloride as activation solution. Adhesion property between polyimide surface and copper layer was investigated by tape test. Also, scanning electron microscope (SEM) images show the morphologies of electroless plated Cu layers with/without APP pre-treatment. None pre-treated Cu layer and Cu layer with O2 and N2 + O2 (NO) plasma pre-treatment shows voids and cracks in Cu foil on PI substrate. Only N2 plasma pre-treated Cu foil shows smooth and dense surface.

Introduction

A printed circuit board (PCB) is the platform upon which electronic components such as integrated circuit chips and capacitors are mounted. It can be classified into rigid and flexible types. A flexible PCB (FPCB) has many advantages compared to rigid boards because of their flexibility, especially in applications demanding more integrated electronic devices. FPCB, which is used for cellular phones, consists of flexible copper clad laminate (FCCL) and cover layer. Electric signals are conveyed via FCCL of FPCB between keypad and display panel. Polyimide (PI) is used in the electronics industry as a material for flexible chip carriers because of its low cost and outstanding properties such as flame resistance, high upper working temperature (250–320 °C), high tensile strength (70–150 MPa), and high dielectric strength (22 kV/mm). However, in spite of the extensive usage as well as the detailed characterization of the PIs, applications of the materials are limited by their hydrophobic surfaces, which result in poor wettability and adhesion. The adhesion of a metal to the PI film is generally poor, unless the PI surface is pre-treated prior to metallization. For this application, the PI must be metallized with copper to form the chip inter-connects, however, the poor adhesion properties of PI must be first overcome by wet-chemical or plasma-chemical processing [1], [2], [3], [4], [5], [6], [7].

Pre-treatment of polymers with the purpose to change adhesion properties can be classified in two main methods: chemical surface modifications and physical surface modifications. Wet chemical processes are highly efficient but result in the disadvantage of a very strong substrate roughening. Among those modification methods, plasma and ion-beam treatments are being used to an increasing extent. The effects caused on the polymer surface by these techniques are the incorporation of functional groups, changes in the surface morphology, and alteration in the chain structure.

Atmospheric pressure plasma (APP) is a new technology for plasma surface treatments in ambient atmospheric, non-vacuum chambers. Nonthermal APP having temperatures of 50–200 °C are used to enhance hydrophilicity and adhesiveness, and clean surfaces. High-energy electrons in the plasma produce ions and reactive species that interact with surfaces to modify their characteristics. Nonthermal APP systems have structures simpler than those of vacuum systems and are capable of continuous treatments in single step and simple processes. The APP system is potentially one of the most suitable ways to improve wettability and as such, is very attractive given their high productivity and low costs [1].

In this work, APP was applied to the surface of PI films as a treatment to enhance the adhesion of electroless copper deposition layers. The influences of nitrogen APP treatment on characteristics of electroless copper coatings as well as on the chemical composition of the PI film surface were investigated at various operating parameters such as time, gas flow rate, and reactive gases. Furthermore, effect of the APP treatment on adhesion of Cu layer and PI films was examined.

Section snippets

Experimental

All experiments were performed on 125 μm thickness of polyimide (PI; C22H10N2O5) films (Kapton®, DuPont™). The commercial atmospheric pressure plasma (APP) surface treatment system (MINIPLASMA-station, PLASMART) was used for surface modification of PI films by N2, O2, and N2 + O2 plasmas. APP system has simple dielectric barrier discharge (DBD) unit for plasma ignition. Sample stage was moved from forward to backward with settable moving distance (mm) from center position, speed (mm/s), and

Results and discussion

Fig. 1 shows the IR spectra of PI substrate with/without plasma surface modification. The almost same spectrum was shown in Fig. 1(a). The range of 1750–600 cm−1 is the finger print region. If chemical structure of PI was changed by plasma treatment, we could observe lots of changes of absorption bands in this region. However, any change of absorption band was not observed. On the other hand, intensity of absorption at 3450 cm−1 was increased by N2 plasma treatment, only. Fig. 1(b) shows the

Conclusions

In this paper, N doping by APP method was mainly investigated for electroless Cu plating on PI by using N2 plasma. From the IR spectra, increasing of N–H stretching mode was observed after N2 plasma treatment without any structural change of PI bulk. C–N, N–O, and C–NH4+ binding were increased after plasma treatment. Especially, blue shifts of N 1s and O 1s were observed with N2 plasma treated PI. APP surface treatment led to increasing the surface energy. Especially, O2 plasma (including NO

Acknowlegdements

This work was supported by grants NRF-2010-0029699 (Priority Research Centers Program), NRF-2010-0029417 (Plasma Bioscience Research Center, SRC Program), and NRF-2010-0025481 (Basic Science Research Program). Also, this paper was supported by Samsung Research Fund, Sungkyunkwan University, 2010.

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