Plasma-polymerized thiophene films for corrosion protection in microelectronic devices

Abstract

The corrosion failure of electronic devices has long been a major reliability concern. This failure is an ongoing concern because of miniaturation of integrated circuits and the increased use of polymers in packaging [1–3]. Recently, copper is replacing aluminum in advanced microelectronic devices as the interconnecting metal due to its lower bulk resistivity. However, copper does not form a self-passivating oxide compared with aluminum and the resistance to corrosion decreases in humid and oxygen-containing environments [4]. On the other hand, in advanced logic devices, the number of interlayer dielectrics has increased to four or five, so that wiring delay is beginning to dominate the total signal delay in ultra-large-scale-integrated circuits (ULSIs). To lower the signal delay, plasma-polymerized thin films with low dielectric constant have been developed by varying the deposition parameters [5–7]. However, these plasma-polymerized thin films as interlayer dielectrics have a high tendency to adsorb water in the presence of moisture and can act as an electrolyte between two dissimilar metals, forming a galvanic cell in the microelectronic devices [8–10]. Therefore, it is necessary to investigate the effect of corrosion induced by interlayer dielectrics in the microelectronic devices. In this paper, thiopene was considered as a possible candidate for the interlayer dielectric for multilevel metallization of semiconductor devices [11]. Generally, the electrical conductivity of thiopene was found to be very low (10−15–10−10 S/cm) [12]. The protective abilities of thiopene in 3.5 wt% NaCl solution were examined by electrochemical measurements and wettability tests. Plasma polymerization was carried out in a vacuum chamber made of stainless steel. Polymer-like thin films were deposited on disk-shaped steel specimens by plasma-enhanced chemical vapor deposition (PECVD). After cleaning the sample using acetone, isopropyl alcohol and distilled water, the substrates were in-situ pre-treated with Ar plasma to give an oxygen-free surface and a buffer layer for enhancing film adhesion. The deposition lasted up to 2 h, depending on the RF power and deposition temperature. The general deposition pressure and temperature were 2–4 × 10−1 Torr and 473 K, respectively. The typical conditions of the PECVD process applied in this study are 30–100 W of RF power, 20 sccm of Ar carrier gas, and 20 sccm of H2 bubbler gas. Thiopene (C4H4S) was used as the organic precursor. For a better understanding of the effect of thiopene films on corrosion behavior, impedance (Z) measurements for the steel electrodes coated and uncoated with the films were performed at Ecorr after immersion for 2 h in 3.5 wt% NaCl solution open to air. A perturbation AC potential of amplitude 10 mV was applied over the frequency range 100 kHz to 10 mHz. Only 5 frequencies per decade were measured. All potentials were referred to the saturated calomel electrode (SCE), and the counter electrode was a highpurity graphite rod. The plot was analyzed with Z-view softwave. Polarization measurements of the electrodes, both bare and covered with thiopene film were carried out potentiodynamically in 3.5 wt% NaCl solution at room temperature. After immersion of the electrode in the solution for 2 h, the potential of the electrode was swept at a rate of 0.166 mV/s from the initial potential of −250 mV vs. Ecorr to the final potential of 1000 mV vs. Ecorr. In order to evaluate the wettability of the surface film, contact angle measurements were carried out on the coated surfaces by the sessile drop method with a microscopic goniometer. The measurement was performed using a drop of 2 μl water at room temperature.