Oxide Breakdown

Oxide Breakdown refers to the destruction of an oxide layer (usually silicon dioxide or SiO2) in a semiconductor device. Oxide layers are used in many parts of the device: as gate oxide between the metal and the semiconductor in MOS transistors, as dielectric layer in capacitors, as inter-layer dielectric to isolate conductors from each other, etc. Oxide breakdown is also referred to as 'oxide rupture' or 'oxide punch-through.'

Oxide breakdown has always been of serious reliability concern in the semiconductor industry because of the continuous trek towards smaller and smaller devices. As other features of the device are scaled down, so must oxide thickness be reduced. Oxides become more vulnerable to the voltages fed into the device as they get thinner. The thinnest oxide layers today are already less than 50 angstroms thick. An oxide layer can break down instantaneously at 8-11 MV per cm of thickness, or 0.08 - 0.11 V per angstrom of thickness.

Oxide breakdowns may be classified as one of the following: 1) EOS/ESD-induced dielectric breakdown; 2) early-life dielectric breakdown; or 3) time-dependent dielectric breakdown (TDDB).

Oxide rupture due to EOS/ESD events generally involves a high voltage being applied across the oxide layer, causing a 'weak' spot within it to exhibit dielectric breakdown and allow current to flow. This current flow, which is basically due to loss of dielectric isolation at that spot, causes localized heating, which induces the flow of a larger current. A vicious cycle of increasing current flow and localized heating ensues, eventually causing a meltdown of the silicon, dielectric, and other materials at the 'hot spot'. This meltdown creates a short circuit between the layers supposedly isolated by the oxide. See also: EOS/ESD Failures.

Figure 1. Photo of an ESD-induced Oxide Breakdown

Early-life and time-dependent oxide breakdowns will result in the same failure attributes, but the former involves a breakdown that occurs early in the life of the device (say, within the first 2 years of normal operation), while the latter involves a breakdown that occurs after a much longer time of use (mainly in the 'wear-out' stage). Both categories involve destruction of the oxide while under normal bias or operation.

Early life and time-dependent dielectric breakdowns are primarily due to the presence of weak spots within the oxide layer arising from its poor processing or uneven growth. These weak spots or dielectric defects may be caused by: 1) the presence of mobile sodium (Na) ions in the oxide; 2) radiation damage; 3) contamination, wherein particles or impurities are trapped on the silicon prior to oxidation; and 4) crystalline defects in the silicon such as stacking faults and dislocations.

The risk of dielectric breakdown generally increases with the area of the oxide layer, since a larger area means the presence of more defects and greater exposure to contaminants. The worse cases of oxide defects are the ones that result in early life dielectric breakdowns. It must be pointed out, however, that even very high quality oxides can suffer breakdown with time, especially in the 'wear-out' period of its lifetime. This latter case is the classic 'TDDB' mechanism.

The SiO2 TDDB Process

Previous studies have shown that SiO2 Time-Dependent Dielectric Breakdown (TDDB) is a charge injection mechanism, the process of which may be divided into 2 stages - the build-up stage and the runaway stage.

During the build-up stage, charges invariably get trapped in various parts of the oxide as current flows in the oxide. The trapped charges increase in number with time, forming high electric fields (electric field = voltage/oxide thickness) and high current regions along the way. This process of electric field build-up continues until the runaway stage is reached.

During the runaway stage, the sum of the electric field built up by charge injection and the electric fields applied to the device exceeds the dielectric breakdown threshold in some of the weakest points of the dielectric. These points start conducting large currents that further heat up the dielectric, which further increases the current flow. This positive feedback loop eventually results in electrical and thermal runaway, destroying the oxide in the end. The runaway stage happens in a very short period of time.

The presence of defects in the dielectric greatly reduces the time needed to transition from the build-up to the runaway stage. These defects actually have the effect of 'thinning' down the oxide where they are located, since they are occupying space that should have been occupied by the dielectric. The effective electric field is higher in these thinned-out areas compared to defect-free areas for any given voltage. This is why it takes a lower voltage and shorter time to break down the dielectric at its defect points.

There are many lifetime equations used in the industry today to model the reliability of an oxide layer. One of the simplest, however, can be seen in www.semicon.toshiba.co.jp. According to this site, TDDB may be modelled by:

T_f = Ae^(-BV)

where:

Tf = the time to failure;

A = a constant;

V = the voltage applied across the dielectric layer; and

B = a voltage acceleration constant that depends on the properties of the oxide.

Numerous studies have shown that oxide breakdown is accelerated not just by the voltage applied across the oxide, but by elevated temperature as well. Thus, the tendency of a lot to fail by oxide breakdown is usually assessed by burn-in, which subjects the samples to both electrical and thermal stresses.

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