Addressing VCSEL reliability in parallel optical interconnects

Vertical-cavity surface-emitting laser (VCSEL)-based parallel optical interconnects are becoming vital components of communications systems for which short distance (less than 300 meters) and high throughput (more than 10 Gbits/second) are required to connect data communications equipment in the core of the network. Their low cost, high aggregate throughput, small size, and low power make them ideal solutions to the data path bottleneck created by traditional copper backplanes. However, the reliability of these components, in condensing environments, is far less than their single channel optical transceiver counterparts. This raises questions as to their use in optical networking applications.

One way to improve the reliability of VCSEL-based parallel optical modules is to use low-moisture-absorbing underfill as an environmental seal between flipped-chip VCSEL and positive-intrinsic negative (PIN) diode arrays and a sapphire substrate. This kind of assembly technology uses standard semiconductor packaging equipment and processes to provide increased reliability in high volume manufacturing environments.

The low cost of 850-nanometer VCSEL-based optical modules makes them ideal for use in data communications systems where high speed (1.25-Gbit/s), short-distance (< 1-kilometer) links are required in the backbone of enterprise local area networks (LANs). Typically, these optical modules are single-channel transceivers that interface to multimode fiber (MMF), are compact and dissipate significantly less power than their single-mode cousins that employ single-mode edge emitters such as Fabret Perot (FP) and distributed feedback (DFB) lasers. The latter are typically used in longer-distance applications (>2 km) commonly found in telecom metropolitan area networks (MANs) and transport equipment used in wide area networks (WANs).

One commonality among these single-channel transceivers is the environment in which the laser resides. The lasers in these modules are isolated from the environment by hermetic seals. These seals prevent moisture and contaminants from entering the chambers in which the devices are housed, thus preventing condensation on the laser's surface that can produce deleterious effects on the device's performance and reliability or useful life. For reference, reliability requirements of data communications systems, specified as the mean time to failure (MTTF), is typically measured in the millions of device hours.

Recently, a new application for VCSEL-based optical modules has emerged known as parallel optical interconnects. This application finds VCSEL-based parallel modules employed in very short-distance applications, typically 2 to 300 meters, where it's desirable to keep cost, size and power dissipation as low as possible. However, the reliability requirements are equal to those of optical modules found in the longer-distance applications discussed above. One of the major issues facing manufacturers of this new class of optical module is the environment in which the VCSEL array resides.

The main issue to the integrity of the VCSEL array is oxidation and degradation of the aluminum-gallium-arsenide (AlGaAs) p-mirrors. The VCSEL is a mesa type structure that leaves the sidewalls susceptible to oxidation in condensing environments. Oxidation of the aluminum in the AlGaAs mirrors causes increased resistance and changes in the transmissivity/reflectivity of the mirrors. This is not an issue in single-channel transceiver products since the VCSELs are packaged in low-cost hermetically sealed environments.

Array-based parallel optical module products, however, do not allow for the low cost hermetic sealed packaging for several reasons. These include, but are not limited to, providing access to the VCSEL apertures for optical transfer elements, wire bonds between drive electronics and the VCSELs, as well as the nonstandard mechanical arrangements/packaging of the subassembly containing the laser diode drivers, VCSEL array and optics. Currently, VCSEL manufacturers are attempting to improve the reliability of the device by adding nitride and/or polyamide layers to sensitive areas. The issue today: How good is this protective layer and does it provide the required moisture resistance to pass MIL standard 85C /85 percent RH testing? Discussions with several manufacturers of parallel modules as well as VCSEL arrays has revealed dissatisfaction and concern over the reliability of 850-nm VCSELs in nonhermetic environments borne out through 85/85 testing.

Engineers at Peregrine Semiconductor have developed packaging technology, based on UTSi CMOS circuitry to enable the environmental sealing of the VCSEL array's sensitive surfaces using a low-moisture-absorbing material. The use of this environmental seal is made possible by the mechanical arrangement of the O-E array and the transparent nature of the sapphire substrate on which it is mounted. By providing this environmental seal for the O-E arrays, the VCSEL manufacturers' need to develop and qualify different fabrication processes for their single and multichannel products is reduced. This type of circuitry is uniquely positioned to provide module and systems OEMs the capability to operate in condensing environments today. Furthermore, providing the reliability data to support these claims can make using parallel modules more palatable to the system OEM as it reduces the internal need to perform costly and time-consuming qualifications.

A similar argument applies for the high-speed PIN diode arrays, designed for flip-chip attach, that have dual top-side contacts.