A discussion of the challenges for high-accuracy bonding.
By GILBERT LECARPENTIER AND LIVIA RACZ
The fabrication of optoelectronic (OE) devices requires highly precise processing, as well as strategies for coping with 3-D structures unknown in conventional electronics packaging. To manufacture these optical modules at reasonable cost, automated production has to replace the widespread manual manufacturing currently done now in the lab. Firms producing OE modules are working closely with equipment companies to provide process solutions that enable high-quality volume production of these modules.
The Drive for Cost Reduction
To push the market further for fiber optic network technologies, the cost of optical network components must be reduced. An important factor in reducing costs for OE devices is a much higher level of automation in the process flow for manufacturing hybrid integrated modules. Until now, photonic packaging has been the primary inhibitor to innovation and cost reduction in optical systems deployment. Automation will usher optical modules into the era of mass production, allowing improvements in manufacturing repeatability, reliability and yield, which will result in lower cost per device, as well as reduced footprint and time-to-market. However, it remains common for technicians to assemble optical components by hand in cleanrooms that more closely resemble laboratory conditions, rather than in an automated environment with volume production.
Cost is the most important factor in making emerging broadband services attractive. For complex OE modules, assembly and packaging costs can account for a majority of the total product cost. Recently developed assembly platforms can make it technically and economically feasible to align any combination of active and passive devices within a single component-sized footprint. Such approaches will allow telecom system manufacturers to get high-performance and high-quality devices at a reasonable price.
 Figure 1. Populated OCM optical bench. |
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A Case Study
One example of a product assembled with a new assembly platform is an optical channel monitor (OCM), which consists of about 15 discrete (active and passive) OE chips (Figure 1). The task of monitoring the performance of a large number of optical channels becomes increasingly difficult as networks expand in capacity and complexity. Fault management of networks at the optical layer is essential in reducing network downtime and aiding in identifying impending failures and performance trends (e.g., number of wavelengths and mix of transmission bit rates, protocols and network topologies). The AXSUN hybrid integration packaging platform creates a family of optical monitors that are at least 10 times smaller than solutions based on bulk optics. The resulting cost and size benefits enable true embedded monitoring. The entire optical subsystem of the OCM (25 discrete elements) is contained in a low-profile 14-pin butterfly package (Figure 2). This optical module resides on a circuit assembly containing thermoelectric cooler (TEC) control, a digital signal processing unit, and RS232 and dual-port RAM interfaces (Figure 3).
 Figure 2. AXSUN OCM subsystem module in a 14-pin butterfly package. |
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The Process
The process platform consists of two basic process steps. First, the passive placement and attachment of devices onto the substrate is performed. (In this work, FC150 and FC250 device bonders from SUSS MicroTec were used.) For the final active alignment, a proprietary robotic alignment tool was developed for device alignment to an order of magnitude higher precision level.
For devices that require alignment precision of better than 0.1 µm, micro-mechanical alignment structures have been developed to serve as chip carriers. These carriers are manufactured using LIGA process technology and can be actively aligned (Figure 4). (LIGA is an acronym from German words for lithography, electroplating and molding and was developed in the early 1980s at the Karlsruhe Nuclear Research Center.) Based on this process, tiny, lithographically precise metal structures are manufactured with feature dimensions and precision exceeding the capabilities of traditional machining. These mechanical alignment structures serve as the intermediate carriers for the optical devices that require placement on the substrate. These include active devices, mirrors, lenses, filters and fibers.
This final active alignment is performed by the robotic alignment technology developed by AXSUN Technologies, in which the micro-robot refines the placement of the LIGA structure and serves as the key chip carrier.
Another challenge in the assembly of complex OE modules is that so much of the real estate of the substrate is filled with components, typically with a large variety of component geometries and sizes. The ability to pick up very tiny devices — a laser diode is on the order of 200 µm — as well as large components is key. Therefore, a critical feature of the bonding tool that performs the passive alignment is the ability to correct the alignment many different times and in many different positions before placing the chip, or to bypass those steps if they are not required. The current process shows a global passive placement precision for all parts relative to the others of ±3 µm at 3 s.
 Figure 3. Complete OCM subsystem with control electronics and heat sink. |
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Bonding Technology
A useful capability in an OE device bonder is the ability to perform standard component attachment as well as flip chip bonding. The various components in an OE product often require both processes. Gentle material handling also is important because the components often are made of brittle materials, such as compound semiconductors, and they can be very small.
Other challenges in equipment design include high-temperature processing and a wide range of forces needed for bonding various components. Temperatures up to 450°C often are required, for example. The process described here also takes advantage of the low force (10 g) possible with the bonder used in the study. Bonding forces up to 200 kg also are possible when needed for thermo-compression processes used for bonding large numbers of indium or gold bumps, or for using anisotropic conductive adhesives.
The tool's imaging system, bonding arm and granite structures result in a post-bonding accuracy of less than 1 µm at production throughput rates. The optical system delivers a 400X magnification to the automatic alignment system and allows simultaneous chip and substrate viewing.
 Figure 4. LIGA alignment structures provide active alignment for critical elements such as metallized fibers and lenses. |
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Flux and Epoxy Coating Processes
Solder flux and epoxy coating are important processes in OE module assembly. The bonder used in this study uses the offset-printing principle to deposit a uniform layer of coating material such as flux, solder paste or conductive adhesive on the chip. Partially immersed in the fluid, the roller rotates at a constant speed and is covered with a uniform fluid layer. A blade placed at a controlled distance from the roller controls the film thickness. The roller then is moved over the component surface.
In traditional systems, the components stamping is performed on a rotating plate covered with the coating material. The module usually is offset from the alignment position, resulting in placement accuracy loss. The alternative is to stamp before alignment, but in this case the vision system can be disturbed by the flux or the epoxy. With a new module on the tool, the components are covered with the coating material immediately following the alignment step in an in-situ process without any disturbances to the operation of the automatic alignment system. This results in higher accuracy and throughput.
Fiber Bonding Technology
Other capabilities in an OE assembly platform should include fiber handling and an ultraviolet (UV) curing system. The fiber-handling tool in this equipment is built of quartz and, due to its transparency, UV light can reach the process area for the curing process. The quartz tool consists of a quartz base plate to which a v-shaped quartz support and quartz pusher is attached. The v-shaped support picks up the fiber ferrule with vacuum and the pusher applies a low force to the fiber tip to push it into the v-groove until curing is finished.
Handling multiple fiber ribbons can be a challenge. While a single fiber can be aligned perfectly to a v-groove during the alignment sequence, it is more difficult to align a multi-fiber ribbon because of the pitch's inaccuracy between the fibers. To align a fiber ribbon, a scrubbing function can facilitate insertion of the fiber tips into the v-groove.
The UV curing system includes a UV light guide (fiber) and an adjustable collimating lens. Because of the variety of component shapes and sizes, it is useful for OE assembly to have the position of the fiber and the collimating lens be programmable.
Conclusion
The combined capabilities of equipment companies and manufacturers of integrated OE modules will pave the way into the era of automated packaging of OE hybrid integrated subsystems. In spite of the decline of the global OE component market from $8.9 billion in 2000 to $7.6 billion in 2001, most analysts predict a rebound in the near future, and automation will be the key driver of the recovery.
ACKNOWLEDGEMENTS
The authors would like to thank Jim Lewis, senior VP of marketing, of AXSUN Technologies and Bill Ahern, product line manager, of AXSUN Technologies for contributing information to the article.
Illustration by Gregor Bernard
Gilbert Lecarpentier may be contacted at SUSS MicroTec S.A., Avenue des Colombières, F-74490 Saint-Jeoire; 33 (0)4 50 35 83 92; Fax: 33 (0)4 50 35 88 01; E-mail: [email protected]. Livia M. Racz may be contacted at AXSUN Technologies, 1 Fortune Dr., Billerica, MA 01821; (978) 262-0049 ext. 111; E-mail: [email protected].