Die-related Failure Mechanisms

Die-related Failure Mechanisms and Attributes

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Contact Migration

Contact migration refers to the diffusion of the metal atoms of a contact (usually Al or an alloy thereof) into the silicon substrate. This phenomenon is due to the natural occurrence of interdiffusion between two different interdiffusible materials in contact with each other, which are Al and Si in this case. This phenomenon of interdiffusion occurs in both ways, i.e., Al diffuses into Si and Si diffuses into Al. This is not current-related and must not be confused with electromigration, which is a different mechanism.

Junction spiking occurs when the amount of Al migration into the silicon substrate has reached the point wherein the Al has penetrated deep enough so as to short a p-n junction in its path. By that time an Al spike is said to have shorted the junction, damaging the device permanently.

The reverse, wherein the Si atoms have entirely penetrated the Al layer above, may also happen and can result in an open circuit as a result of voids in the metal contact. Silicon aggregates that have diffused through the Al layer and reached the surface are known as silicon nodules. Silicon nodules are often observed over the bond pads as small but numerous hillocks, and are known to cause wirebonding problems as well.

Al migration is usually reduced by doping the Al with Si or Cu or both, forming an alloy that is more resistant to Al-Si interdiffusion. A barrier metal such as TiW or Pt-Si may also be deposited between the Al layer and the silicon substrate.

Die Corrosion

See separate article on die corrosion.

Die Scratches

Die scratch is the presence of abrasion, scraping, or laceration damage on the surface of the die.

See separate article on die scratches.

Dielectric Breakdown

Dielectric breakdown refers to the destruction of a dielectric layer, usually as a result of excessive potential difference or voltage across it. It is usually manifested as a short or leakage at the point of breakdown.

There are many types of dielectric in a typical die circuit, varying not only in purpose but in chemical composition as well. The most commonly used dielectric is SiO2, which is an oxide of silicon. The permanent breakdown of an oxide dielectric is also usually referred to as 'oxide rupture' or 'oxide breakdown.' The most common cause of dielectric breakdown in devices with no wafer fab problem is EOS/ESD, since this can expose the dielectric layer to high voltages.

Non-EOS/ESD-related dielectric breakdowns may be classified into either an early life dielectric breakdown (ELDB) or a time-dependent dielectric breakdown (TDDB), depending on when in the device lifetime it occurs. Early life dielectric breakdown, usually occurring within the device's first year of operation, is just a special case of early life failure (ELF) involving a dielectric layer. A dielectric breakdown is usually classified as a TDDB if the device has been in operation for at least two years already. These are just guidelines, because the point at which a dielectric breakdown occurs is not just related to time, but to other factors as well. ELDB and TDDB failures are usually caused by a defect in the dielectric layer, such as stray particles which decrease the effective thickness of the dielectric making it prone to breakdown.

Since SiO2 is a very common dielectric material, its breakdown mechanism has been understood over the years. SiO2 breakdown is believed to be due to charge injection, and may be broken down into 2 stages. During the first stage, current starts to flow through the oxide as a result of the voltage applied across it. High field/high current regions are then formed as charges are trapped in the oxide. Eventually, these abnormal regions reach stage 2, a critical point wherein the oxide heats up and allows a greater current flow. This results in an electrical and thermal runaway that quickly leads to the physical destruction of the oxide.

See also Oxide Breakdown and Gate Oxide Breakdown.

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