Meaning
Reliability testing methodology used to evaluate the long-term stability of gate oxides in metal-oxide-semiconductor field-effect transistors. By applying a constant voltage to the gate at elevated temperatures, bias temperature stress accelerates the generation of defects at the silicon-dioxide interface. The resulting shift in threshold voltage indicates the degradation of the device.
This technique operates within specific temperature limits to avoid thermal runaway.
Degradation Mechanism
Charge trapping in the oxide layer and the creation of interface states represent the primary causes of the observed threshold voltage shift. When the test is applied at high temperatures, the energy barrier for hydrogen release is lowered, facilitating the diffusion of hydrogen species away from the silicon interface. This reaction leaves behind dangling silicon bonds that trap electrons or holes.
The accumulation of these trapped charges alters the voltage required to turn the transistor on.
Test Configuration
Automated test equipment executes the measurement by placing the silicon wafers on a heated chuck while probing the gate electrodes. Typical stress runs involve maintaining temperatures between 100 and 250 degrees Celsius while applying gate fields of several megavolts per centimeter. The system interrupts the stress period at logarithmic time intervals to measure the electrical characteristics of the device at the stress temperature or after a rapid cool-down.
These intervals must be brief to minimize the recovery of the trapped charges.
Regulatory Audit
Domestic foundries in China must document the voltage shift curves to comply with factory audits conducted by the Ministry of Industry and Information Technology. If a foreign fabless customer initiates a quality dispute, the results of bias temperature stress act as the standard test record to resolve liability issues. The audit teams examine the calibration records of the heated chucks and the precision of the measurement units to ensure compliance with national standards.
These verified reports are essential for getting the fabrication process certified under international automotive quality regimes.
