Seam Sealing While MEMS and MOEMS do best in hermetic packages, MOEMS devices are particularly challenging-both from particulate and moisture standpoints. A key requirement for hermetic packages is improvement in hermetic sealing equipment, Roth maintains. Traditional seam-sealers are usually glove-box type and are expensive to automate. Laser welding seam-sealers require special fixturing to keep the lids even. Solder and epoxy sealing do not always offer sufficient reliability. Instead of solving the problem with better equipment, perhaps more development in packaging materials would reap better rewards. There are several companies looking at "virtually" hermetic packages.
'Automation Is the Challenge' Christoph Scheiring, manager of advanced technology at Datacon, Austria, is convinced that automating optoelectronics packaging is the challenge (lead photo, Figure 3). Scheiring says that "the growing demand for high-power communications requires broadband connections via fiberoptic networks. The economical installation and operation of these requires cost-effective, high-performance, reliable optoelectronic components. "At present," he adds, "some 60 to 80 percent of production costs are incurred in packaging, presenting a great opportunity for cost reduction. Improve-ment will come with new ways of automating the manufacture of optoelectronic components. "A glance at the typical structure of an optical transmitter for fiber-optic transmission shows very diverse components where standard automated assembly is not up to the task. Automating the packaging of optical and optoelectronic components is not easy. It far exceeds the demands made by the hybrid assembly of microwave MCMs," says Scheiring. A Variety of Components Magnifying the problem is the sheer variety of components in opto, says Scheiring. All kinds of different elements, some non-electrical, must be positioned with the highest precision (relative placement), installed and finally tested. Many of these components are particularly sensitive, e.g., InP or GaAs ICs, or very small (≤ 200 µm) and must be handled carefully. Substrates and packages are just as varied, and may be procured in laminate strips, boards, ceramic holders or assembly frames. Even wafers as substrates, as in stacked-die applications, are no rarity in the optoelectronic field. Futhermore, the peculiarities of optoelectronic manufacture include not only thermal processes, but also the optional rapid curing of epoxies by means of ultraviolet light. Quick curing is needed because many of the components in optoelectronic structures can be magnetic and attract other metal parts, thus causing components to change their relative placement after positioning. Before this can occur, they are permanently fixed in place through rapid UV curing of their epoxy. This is easier said than done, Scheiring reports, however, because most materials, including glass, are not directly transparent for ultraviolet radiation.
'Ongoing Challenge' "The assembly of optoelectronic modules has been an ongoing challenge for manufacturing equipment," says Gilbert Lecarpentier, strategic marketing manager for flip-chip bonders at SUSS MicroTEC [suss.com]. "The alignment of a laser diode to a single mode fiber or an optical waveguide requires post-bonding alignments better than 1 µm. If accuracy is not within this limit, module performance decreases dramatically." Lecarpentier says that some current industry bonding technologies may deliver either poor or insufficient accuracy. SUSS recently developed a new methodology, known as passive alignment, for assembling optoelectronic modules. This technology is the basis of its FC250 product (Figure 4). "Processes for advanced fiberoptic assembly include eutectic component attach (in-situ pulse heat with AuSn metallization), epoxy component attach, wire bond (typically with 25 µm diameter Au wire and optical fiber align and attach," says Bruce W. Hueners, director of sales, Palomar Technologies (Figure 5) [palomar.com].
"In automated equipment, these processes are implemented with precise multi-axes positioning systems for spatial device alignment, high-resolution machine vision (gray scale pattern recognition with pre- and post-bond inspection), precision force control which is critical for consistent handling of fragile materials such as InP diode laser chips (force control for programmable descent velocity and acceleration profiles, low force touch and programmable ascent profiles)," Hueneres says. For optoelectronic modules, automatic attachment of InP laser diodes uses Au-Sn eutectic solder with a programmable time and temperature controlled process. The p-side up InP laser diode should be attached to the submount with 10-µm precision. Automatic handling for submounts and modules can augment throughput and reduce operator handling. Next-Generation Products "What is really needed to produce their next generation products?" asks Dan Crowley, vice president of sales at MRSI, a Newport Corp. Company [mrsigroup.com]. "Five micron accuracy is necessary for optimal transmission of light between components. For proper device function on the substrate, laser diodes and VCSELs must be placed with sufficient accuracy to allow for faster alignment of the fiber in later processes," Crowley observes. Additionally, laser diodes must be placed within a 5-µm tolerance of the edge of a submount for best performance, and planarity in die placement and bonding is critical. Die tilt can affect the direction in which light is reflected by an optical component, and bond integrity is partially determined by the consistency of the bond line between the die and substrate material. Crowley adds, "Delicate indium phosphide and gallium arsenide die can be damaged if the temperatures at which they are eutectically bonded are not carefully controlled. Ramp up rates and cooling rates must be programmable on automated equipment to ensure that each die can be treated as required." Conclusion While it may appear that the surprising growth of the opto packaging market caught some equipment makers by surprise, the real story is not quite that easy to explain. During the semiconductor industry's boom periods, equipment makers are typically stretched to bursting trying to meet delivery demands. The industry's latest recession, however, gave many IC equipment makers a chance to regroup and to address the needs of the expanding opto market.
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