Challenges in Optoelectronic Packaging
by Edward Palen and Golden Shu

Advanced Process Development
Teledyne Optoelectronics
Los Angeles, California
Unit of Teledyne Electronic Technologies

Presented at IMAPS-UK 2000 Conference

Bandwidth demand of the arriving information age has driven a greater need for the supply of optoelectronic components. Implementation of optical amplification using EDFAs and of DWDM has significantly increased the demand for qualified optoelectronic component suppliers. The transformation of bandwidth availability to a commodity is driving component price compression. This requires higher process yields from wafer foundaries all the way to packaging optical alignment and is rewarding suppliers with higher levels of process automation.

Optical coupling of optoelectronic devices to optical fiber remains the process that distinguishes the optoelectronics manufacturing industry from the electronics industry. Precision placement and maintenance of optical coupling through Bellcore and Telcordia requirements has these general dimensional placement tolerances:

Multimode fiber MMF: 5 micrometer Singlemode fiber SMF: 0.25 micrometer Polarization maintaining fiber PMF: 0.25 micrometer and 1 degree rotation

Methods of optical fiber attachment determine the alignment tolerance achieved. UV cured epoxy allows snap cure of components that can be mechanically strengthened using thermally cured epoxies. While highly filled epoxies are available to lower thermal expansion to 30 ppm/oC, thermal related dimensional shift severely limits product yields for SMF integration. Additionally, telecom performance requirements generally prohibits their use inside hermetically sealed packages.

Fixing alignment positions using eutectic solder attachment suffers from solder solidification position shift precluding it from high yields in SMF integration. Solder attachment also requires metallization of optical fibers, lenses, filters and other free standing optics or their holders.

The only present method to achieve optical alignment with submicron tolerances over the lifetime of optoelectronic components is by laser weld attachment followed by post-weld shift correction. This shift correction is made by bending the optical train or by a laser hammering process where an external force is applied during optical tracking optimization. Laser hammering has been commercially successful for axial optical train configurations but not for planar optical train configurations which generally use metal clips to hold metallized optical fiber soldered into a metal ferrule. Successful post-weld shift correction relies upon predictable mechanical shifts in the optical train geometries. Commercial success in laser welding alignment requires a high degree of process automation and generally lays beyond the mere capital reach of small players in the market.

Optical submount assemblies of thermal electrical coolers (TECs), optical benches, thermistors, back facet diodes, filters, and free standing optics require generally either solder or epoxy attachment. Solder attachment without flux or no clean flux processes are commonly sought to reduce organic contamination and consequent deterioration of laser die facets. Package housing cleaning, including plasma pre-wire bonding cleaning, is generally not performed once optoelectronic die have been integrated into the assembly. However, there are examples of successful solvent cleaning of packages with integrated laser and photodiode dies when appropriate process controls are used.

Required optical coupling efficiencies are application dependent. Expectations for practical optical coupling efficiencies range from 60% with lensed tip SMF to 10-20% for freestanding optical trains. Antireflection AR coatings on laser die, collimating lenses, focusing lenses and optical fiber tip surfaces can improve optical coupling efficiencies from under 20% to over 80%.

Hermetic seal requirements for telecom products are met either by solder attachment of metal housing to a metallized optical fiber or by using a metal housing with a preintegrated optical window. Product design choices whether to perform all optical alignments outside of the package prior to integration into the housing, or to perform the alignment with optical submounts inside a packaging housing can significantly affect processing yield and cost.

Channel waveguide integration with optical fiber arrays presents optical alignment challenges solved with UV cured epoxy, thermal cured epoxy and index matching epoxy attachment of optical fibers prepositioned in v-groove mounts polished prior to attachment.

As the bandwidth demand increases, the data transmission rate is increasing from 2.5 Gbps per channel (OC-48) to 10Gbps (OC-192) and very soon to 40Gbps (OC-768). New packaging configurations for electrical design and optical integration will be embraced by the telecom community for the higher data rates. To realize these high bandwidths will require accounting for impedance matching circuitry, interface matching, and understanding of parasitics. Three dimensional microwave field simulation, circuit analysis and electromagnetic radiation modeling will be necessary tools in optoelectronic packaging design in order to realize device bandwidth capability at these higher data rates. Highly integrated electronics may soon become the norm for many components such as transmitters, laser drivers, modulators, amplifiers, receivers, mux, demux, clock recovery circuitry and switches. Products of higher levels of integration in the future from integrated reconfigurable add/drop multiplexers to integrated optical switches and to transceivers will require more challenging packaging solutions.

Telecom product qualification requirements of long time storage testing, 2000 hours, and accelerated aging testing, 5000 hours, has led to extreme market demand of time compression for product development and qualification and for qualification of production lines. Risk sharing to commence manufacturing at 1000 hours is common. This qualification time constraint has led to a resistance to explore design changes that could impact manufacturing yield, automation and cost due to immediate product delivery pressures.

The future will reward manufacturers that embrace higher levels of automation to meet production capacity demands and to compete with component price compression. Also rewards will be for those with products of high levels of integration that address bandwidth performance demands for higher data transmission rates.

Edward J. Palen, Ph.D., P.E. was Director of Advanced Process Development at Teledyne Optoelectronics,
a provider of high volume optoelectronic outsource manufacturing and optoelectronic packaging design services.
Teledyne Optoelectronics provides all of the services described in this article to their customers.

Further information may be found at
www.teledyneoptoelectronics.com

Inquiries should be addressed to
Dianna German
(310) 574-2057
dianna_german@teledyne.com


Edward Palen, Ph.D.

phone: 415-850-8166

email: epalen@earthlink.net

--return to home webpage--

Copyright © 2002, 2004 E. J. Palen
all rights reserved