The HTRB (High Temperature Reverse Bias) reliability test is an accelerated life test that accelerates the aging of device terminals through dual stress of temperature and electrical stress, thereby activating defects present in the design or manufacturing process of power semiconductor devices. This test evaluates the reliability and stability of the device under long-term high-temperature and high-voltage stress, and is an important means of assessing the voltage withstand characteristics of power devices.
       HTRB testing method: Taking MOSFET devices as an example, after testing a certain number of samples, they are placed in a constant temperature chamber, with the temperature generally set at the maximum allowable operating junction temperature (150°C). The source and gate of the device are short-circuited, and a voltage VDS is applied between the source and drain (to avoid failure modes that do not match the actual situation, the voltage usually does not exceed 80% of the rated voltage). Under this electrothermal stress condition for a certain period of time (typically 168 hours or 1000 hours), the change in drain current (IDSS) can be monitored in real time during the assessment process. After the test, the device is cooled to room temperature for dynamic and static parameter testing. If the parameters exceed the specifications, it is determined as failure.

Failure mechanism of HTRB
       Under high temperature and high voltage stress, the movable ions (taking Na+ as an example) within the device will accelerate and accumulate in the high field strength region, forming surface charges. The accumulated surface charges will change the electric field in the termination region, leading to an increase in leakage current, which can cause device failure in severe cases. There are two main mechanisms by which the movable ions Na+ cause degradation of the device's withstand voltage: induced channeling and depletion layer widening.
       Induced channel: When the device is in a reverse bias state, mobile ions (Na+) move along the electric field direction on the surface of the device, attracting electrons from the N-type silicon region to gather towards the device surface. This results in the formation of an electron accumulation layer on the N-type silicon surface. Electrons then move along the electron accumulation layer formed on the device surface towards higher potentials. The continuous directional flow of electrons forms leakage. This leakage caused by mobile ions is not normal PN junction leakage, leading to increased device leakage, weakened reverse blocking capability, and degradation of voltage withstanding capability.

      Depletion layer widening: When Na+ ions move along the electric field direction on the surface of the device to the region near the low potential, they will accumulate on the surface of the PN junction. When the concentration of mobile ions reaches a certain level, Na+ induces negative charges in the P-type region, increasing the electric field strength on the device surface, thus degrading the device's voltage withstand capability.
       The movable ions (Na+) primarily originate from chip manufacturing processes (such as ion implantation and oxidation) and packaging processes (including solder flux and epoxy resin). Their primary locations within the chip include: the surface and interior of the passivation layer, the surface and interior of the oxide layer, the interface between the metal oxide layer, and the Si-SiO2 interface.

Common types and manifestations of device failures in actual HTRB assessments:
       The charge of mobile ions alters the distribution of the terminal electric field, leading to a decrease in voltage resistance and an increase in leakage current.
       Moisture intrusion leads to internal short circuits and discoloration of the frame.
       Poor contact or voids in solder joints manifest as increased resistance.
       Gate oxide defects manifest as instability in threshold voltage and overlap in current output curves.
       The mismatch in thermal stress of the material leads to delamination, with the failure manifesting as a decrease in voltage resistance and a sharp increase in leakage current.

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