Abstract
(Englisch)
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Investigations on VCSEL Reliability
Reliability is of highest importance for market acceptance of new products. Current Telecom or Datacom applications require component lifetimes of at least 50 000 hours continuous operation under the most disadvantegous conditions allowed by the specifications. These are typically a temperature of 85°C and maximum operating current.
Due to the fact, that until now no devices from the GSQ-project have been available, we investigated various structures of GaAs based VCSELs emitting between 760nm and 850nm fabricated with different processing. This allows to define the guidelines for the reliability experiments on the QD-VCSELs from the GSQ project.
For lifetime tests, Avalon uses air-circulation furnaces operated at 3 different temperatures (T=75°C, 100°C and 125°C). The TO-46 packaged VCSELs are mounted in burn-in sockets on printed circuit boards (PCBs). The edge connectors on the boards allow inserting the PCBs into the lifetime furnaces, and a series of current sources are used to operate the lasers. For measurements, the VCSELs are periodically removed from the furnace and characterized at room temperature. These measurements are typically carried out once a week.
A typical result of such investigations is shown in figure 1. This diagram shows the output power at a current of 5mA for devices from one wafer with oxide confined 850nm VCSELs stressed at 125° and 12 mA. As we can see, the wearout of the lasers start well above 600h.
Figure 2 shows the lognormal failure distribution where the solid line represents a fit, from which the median failure time (MFT = exp(µ))for this wafer is determined.
The wearout lifetime strongly depends on drive current and temperature. To describe these dependencies, we use an acceleration model, where the median failure time (MFT) scales with current and temperature as
MFT proportional. to : I-n . exp(Ea/kb . Tj) (1)
The MFT is the time when 50% of the devices have failed. In equation 1, I is the drive current, Ea the activation energy, kb the Boltzmann constant, n the current dependence and Tj the junction temperature of the device (in degrees Kelvin).
This equation allows estimating the acceleration factor for certain stress conditions, which is mainly a function of the activation energy Ea and the current dependence n.
From experiments at various currents and temperatures we derived Ea and n. We could clearly see, that Ea and n are about the same for all wafers of a certain structure with identical processing, even when the lifetime of the wafers is significantly different. This enables process control by accelerated aging of a small number of devices under high stress conditions and allows to predict the wearout lifetime under operating conditions of devices from this wafer. On the other hand, we found enormous differences in Ea and n for different structures: For oxide confined 850nm devices we determined Ea = 0.7eV and n = 2, wheres non-oxidized 760nm VCSELs showed Ea =0.2eV and n = 2.5.
From this it is clear, that for the investigation of new structures like the QD-VCSELs from the project it is necessary to carry out lifetime experiments at different temperatures and currents to determine the a valid lifetime model, which requires at least about 60 devices from one wafer.
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