Mechanical properties of optical films

There are three areas to consider when designing/implementing wire bonding to advanced ULSI damascene-copper chips having copper metallization and low dielectric-constant polymers embedded beneath them (Cu/LoK). These are: 1) the copper-pad top-surface oxidation inhibitor coating - metal/organic/inorganic. (Current work involves evaluating the metal and inorganic options); 2) the low dielectric constant materials available; 3) under-pad metal/polymer stacks and support structures necessary for bondability and reliability. There are also various polymer/metallurgical interactions, resulting in long term packaged-device reliability problems, that can occur as the result of the wire bonding process over low modulus, LoK materials with barriers. These include cracked diffusion barriers, copper diffusion into the LoK polymers, cracking/spalling/crazing of the LoK materials, and bond pad indentation ("cupping"). Low-K polymer materials, with high expansion coefficients and low thermal conductivities, can also increase the stress and further extend any existing damage to barriers. Well designed LoK and the underpad structures should have no negative effect on bonding parameters and be invisible to the bonding process.

/pdf/wire-bonding-to-advanced-copper-low-k-integrated-circuits-1jgcpcxbhl.pdf

Wire bonding to advanced copper, low-K integrated circuits, the metal/dielectric stacks, and materials considerations

The process of diffusion in the solid state is one aspect of behaviour in gold thin film systems which has received increased attention as gold layers have had to be made significantly thinner without sacrificing their reliability. This article reviews the characteristics of this process for gold over the various one- and two-layer base metal systems which are in common use by the electronics industry. A number of interesting phenomena are described and related to the service problems which they may cause in various devices.

/pdf/diffusion-related-behaviour-of-gold-in-thin-film-systems-3v0neptm1p.pdf

Diffusion-related behaviour of gold in thin film systems

Diffusion problems in microelectronic packaging

Interdiffusion in the CuAu thin film system at 25°C to 250°C

The strengths of thermocompression bonds made between gold plated copper lead frames and gold metallized thin film circuits decrease in time when aged at 200-300°C in air or vacuum unless there is a diffusion barrier (such as nickel) between the copper and gold. After pulling bonds to destruction, failure modes and failed surfaces were characterized by scanning electron microscopy, Auger spectroscopy, electron microprobe, stylus probe, and X-ray diffraction. The failed surface on the substrate side of the bond was very rough, appearing to consist of "mountains" of almost pure Cu resting on a relatively smooth layer of grains, identified as Cu 3 Au, where the Cu 3 Au regions were Visible in the valleys between the copper mountains. The mating failed surface on the lead side of the bond was similarly rough, but with only a few scattered grains of Cu 3 Au evidently pulled out from the opposite surface. Oxidation, pore formation (Kirkendall effect), and ordered phase formation were considered as possible mechanisms for the degradation. Oxidation was ruled out by the observation that bonds degraded as fast in vacuum as in air. Ordered phase formation was judged more likely than pore formation as the primary mechanism. A parameter, \tau , to describe the time required for measurable degradation was defined as the time for the average 90° peel strength of a 0.25 X 0.75 mm lead to decrease from six or seven pounds to four pounds. This parameter was found to be temperature activated with an activation energy of 0.8 eV. Since ordering and pore formation are both dominated by interdiffusion, \tau was extrapolated to lower temperatures using the diffusion coefficient measurements of Pinnel and Bennett, resulting in a value of ten years at 50°C. However, with a 5000A ± 2500A nickel diffusion barrier between the copper lead and the gold plate, the value of \tau extrapolated to 50°C was considerably greater than 40 years, as desired in Bell System equipment.

Strength of Gold-Plated Copper Leads on Thin Film Circuits Under Accelerated Aging

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Strength of Gold-Plated Copper Leads on Thin Film Circuits Under Accelerated Aging