Time-dependent gate oxide breakdown (or time-dependent dielectric breakdown, TDDB) is a kind of
transistor aging
Transistor aging (sometimes called silicon aging) is the process of silicon transistors developing flaws over time as they are used, degrading performance and reliability, and eventually failing altogether. Despite the name, similar mechanisms ma ...
, a failure mechanism in
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s, when the
gate oxide
The gate oxide is the dielectric layer that separates the gate terminal of a MOSFET (metal-oxide-semiconductor field-effect transistor) from the underlying source and drain terminals as well as the conductive channel that connects source and drain ...
breaks down as a result of long-time application of relatively low electric field (as opposed to immediate breakdown, which is caused by strong electric field). The breakdown is caused by formation of a conducting path through the gate oxide to
substrate due to electron
tunneling current, when MOSFETs are operated close to or beyond their specified operating voltages.
Models
The defect generation in the dielectric is a
stochastic process
In probability theory and related fields, a stochastic () or random process is a mathematical object usually defined as a family of random variables. Stochastic processes are widely used as mathematical models of systems and phenomena that appea ...
. There are two modes of breakdown, intrinsic and extrinsic. Intrinsic breakdown is caused by electrical stress induced defect generation. Extrinsic breakdown is caused by defects induced by the manufacturing process. For Integrated circuits, the time to breakdown is dependent on the thickness of the dielectric (gate oxide) and also on the material type, which is dependent on the manufacturing
process node
Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are pres ...
. Older generation products with gate oxide thickness > 4nm are based on SiO2 and the advanced process nodes with gate oxide < 4nm are based on
high-k dielectric materials. There are different breakdown models and thickness of the gate oxide determines the validity of the model. E model, 1/E model and
power law
In statistics, a power law is a Function (mathematics), functional relationship between two quantities, where a Relative change and difference, relative change in one quantity results in a proportional relative change in the other quantity, inde ...
exponential model are common models which depict the breakdown behavior.
The failure types for integrated circuit (IC) components follow the classic
bath tub curve. There is infant mortality, which is decreasing failure rate typically due to manufacturing defects. A low constant failure rate which is random in nature. Wear out failures are increasing failures due to aging semiconductor degradation mechanisms. TDDB is one of the intrinsic wear out failure mechanisms. Performance of the IC components can be evaluated for semiconductor wear out mechanisms including TDDB for any given operating conditions. The breakdown models mentioned above could be used to predict the time to fail for the component due to time dependent dielectric breakdown (TDDB).
Test method
The most commonly used test for the investigation of TDDB behavior is "constant stress".
Constant stress tests can be applied in form of constant voltage stress (CVS) or constant current stress. In the former, a voltage (that is often lower than the breakdown voltage of the oxide) is applied to the gate, while its leakage current is being monitored. The time it will take for the oxide to break under this constant applied voltage is called the time-to-failure. The test is then repeated several times to obtain a distribution of time-to-failure.
These distributions are used to create reliability plots and to predict the TDDB behavior of oxide at other voltages.
See also
*
QBD
*
High-temperature operating life
High-temperature operating life (HTOL) is a reliability test applied to integrated circuits (ICs) to determine their intrinsic reliability. This test stresses the IC at an elevated temperature, high voltage and dynamic operation for a predefined pe ...
References
Semiconductor device defects
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