Laser Diode Production Burn-In

High temperature burn-in screening is used in laser diode manufacturing to screen out devices that are likely to have unacceptably short lives and to ensure that the remaining population of lasers will have a statistically acceptable level of reliability. Laser diode reliability may be broadly defined as the ability to operate the device satisfactorily in a defined environment for a specified time. Many of the issues related to laser diode reliability are revealed by the hazard rate characteristic curve for a population of lasers, also known as a "Bath Tub Curve" (Figure 1). Hazard rate is defined as the probability of failure per unit time given that the device has survived until that time. The various hazard rates associated with laser diodes fall into three categories. First, infant mortality failures are caused by defects that were introduced during the manufacturing process or by intrinsic semiconductor defects. Second, external factors such as current surges and electrostatic discharge events create a constant hazard rate over the lifetime of a device. Finally, wear out failures in lasers occur that are usually caused by the growth of non-radiative, optically absorbing defects within the active region of the laser.
Hazard rate characteristic curve for unscreened laser diodes
Figure 1. Hazard rate characteristic curve for unscreened laser diodes.
In the screening process, devices are evaluated for changes in one or more key operating parameters that are measured before and after high temperature burn-in. Commonly-measured operating parameters and screening criteria are shown in Table 1.

Operating Parameter  Symbol   Typical Screening Criteria
 Threshold current  Ith  Change > 5 to 30%
 Optical output power at specified operating current  Pop @ Iop  Change > 5 to 30%
 Current required to achieve specified optical output power  Iop @ Pop  Change > 5 to 30%
 Slope efficiency η  Change > 5 to 30%

Table 1. Commonly measured operating parameters and screening criteria.

The challenge for production burn-in testing is to achieve high throughput and failure identification at very low cost. Important considerations when choosing a production burn-in system include:

  • Device Protection - As with reliability and life test systems, device protection is a critical element of a production burn-in test system. System designs should ensure that devices are protected from damaging surges/transients, overcurrents, reverse currents, etc.
  • Current Control - To effectively test and stress devices, it is imperative that a precise and repeatable current is provided to the devices under test. This can be done by driving devices individually or in series. Some systems use a series drive configuration to reduce cost, increase capacity, and improve testing throughput. Other systems utilize parallel drive to control devices individually, which simplifies failure identification to the individual device level.
  • Temperature Control - Temperature control, temperature uniformity, and temperature stability are all critical during device burn-in. In many instances, water cooling is needed.
  • Modularity - The many variables and modifications associated with production burn-in make it advantageous to have separate modules for unique functions. Modular flexibility in the design facilitates troubleshooting and repair and ensures prolonged utility of the test system.
  • Scalability - As with life test systems, a production burn-in test system that can scale with customer growth is desirable. As production ramps, a production burn-in test system that can easily add more capacity can save significant time and expense.

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