The majority of semiconductor device reliability modeling is done using the exponential probability distribution model, which assumes the semiconductor has a constant failure rate and there is no wear-out mechanism. However, there are a number of factors which affect the overall laser diode lifetime and reliability testing measures.
Semiconductor laser diodes degrade due to a variety of factors; these factors include imperfection in the wafer growth, contaminations, imperfections, and stresses from processes performed at the wafer level, or imperfections from packaging of the laser chips.
Laser diodes are prone to wear-out which means the exponential probability model is not appropriate for laser reliability testing. As such, other forms of probability modeling should be considered to achieve accurate reliability statistics.
Laser Diode Failure
The three main types of failures seen in all laser diodes are:
- infant mortality
- external hazards
- and wear-out
Defects in manufacturing and semiconductor defects result in infant mortality (premature failure). The rate at which these failures occur diminishes quickly as the devices are used. Influences coming from outside of the device, such as electrostatic discharge are external hazards failures which also contribute to laser diode failure.
Preventing Laser Diode Failure
The estimation of laser diode lifetime and reliability is important to both manufacturers and users of laser diodes. To shorten the testing process, accelerated aging tests (accelerated lifetime testing) are performed at higher than normal temperature and operating currents and sequentially ensure laser diode reliability.
Automatic Testing Equipment (ATE) systems designed for laser diode burn-in and reliability testing that perform accelerated lifetime testing and various other tests to push the laser diode to its operating limit and screen out premature failure.
One of the key factors with the aging process of laser diodes is the operating temperature. The degradation speed rises exponentially with the operating temperature. Proper cooling is crucial to extending the life of the laser. For high power lasers, the cooling fluid chemistry is needed to be compatible with the heatsink.
Other factors that affect reliability are surge currents in laser diodes that can lead to the rapid failure. If the current in a laser diode is raised and increases the output optical power, a point is reached where the output power suddenly falls away and irreversible damage occurs.
Laser Diode Reliability Modeling
Depending on the application there are various standards to ensure laser diode reliability modeling. MIL-HDBK and military standards (MIL-STD-217), are documents from the U.S. Department of Defense, outlining performance characteristics rather than design specifications.
Standards such as Telcordia (Bellcore) TR-332 are used for reliability prediction procedures in the telecommunications and electronics industry. These standards use a series of models for various categories of electronic, electrical and electro-mechanical components to predict steady-state failure rates which environmental conditions, quality levels, electrical stress conditions and various other parameters affect.