By Terrence E. Thompson, Editor-at-Large
Assembly and packaging gear for optoelectronics, MEMS and MOEMS devices typically consist of machines designed for silicon ICs and modified-after the fact-either by the manufacturer or user. Some vendors have been providing specific A & P equipment and systems for over a decade to address these "new-to-the-mainstream" device packaging needs. One often hears widely divergent and sometimes contradictory comments about what equipment and processes will, or will not, work on the opto assembly line. Don't Panic! With all the legitimate interest, demand and, unfortunately, some hype, surrounding optoelectronic assembly and packaging issues, let's look at the practical aspects of the tasks at hand. Don't panic when your designers insist that you must begin incorporating opto parts into your modules and products. Many vendors that you now work with can offer experience and suggestions on how to make the transition. The operative word here is transition. The high-tech world will be electronics-centric for decades to come, and opto technology will simply help improve products that we already know and depend on. Why is Chip Scale Review taking a look at two important equipment categories for optoelectronic device and module assembly: die attach and wire/ribbon bonding systems? Simply because they form a familiar set of processes. (In future articles, we'll look at dispensing systems for opto packages as well as specialized handling and alignment equipment that makes equipment and process integration possible.) Wafer-Fab Techniques Most optoelectronic (photonic) devices are made with semiconductor wafer fabrication techniques, as are many MEMS and MOEMS. Some MEMS and MOEMS are micromachined with laser- or lithographically defined etching processes. Photonic devices are invariably compound semiconductors. Most common applications include light-emitting and light-detecting (detectors) photonic devices for converting electrons to photons and vice versa. Due to their physical composition, photonic devices carry a number of caveats, including fragility, temperature intolerance and relatively high cost. They are much more expensive than silicon. MEMS/MOEMS devices are widely used to switch, attenuate or reroute light. Optics, very small lenses and other components on optical benches within the module, must be precisely aligned and fixed in place. What Do We Know for Certain? Compound semiconductor opto chips simply are more temperature-sensitive and fragile than silicon ICs or discretes and more expensive. MEMS/MOEMS are sensitive for different reasons, but must be handled very carefully, too. The availability of affordable opto devices has been limited, although that situation is changing as some companies begin to provide more rugged and affordable alternatives (see "Motorola Discloses Plans for III-V-on-Silicon Technology," Chip Scale Review, January/February 2002). Unfortunately, most opto and MEMS/ MOEMS assembly and packaging operations have been very inefficient by IC packaging standards to date. Opto, which in this discussion will include MEMS/MOEMS devices, typically requires electrical power and electronic signals to function and interconnect. Moreover, the opto assembly process must emphasize cleanliness in packaging. Opto device emitter/detector/lens surfaces must be clean to prevent light intensity losses. MEMS and MOEMS, often providing deflectable micro mirrors for light beam switching or simple shutters for on/off functions, are equally intolerant of less-than-clean conditions. Critical Alignment Alignment is critical. The task of assembling an IC to matching substrate pads can be somewhat forgiving since perfect alignment is rarely needed. The only requirement is to have "just enough" solder-overlap area to permit current flow without excessive voltage drops. An inconsistent blob of solder or adhesive, however, will not work when attaching devices that require optical-path beam alignment and efficient heat transfer out of the package. Depending on device and package design, some assemblies are more tolerant of variations, but this tolerance is usually achieved only by companies with considerable opto expertise.
'Of Course We Make Opto Packaging Equipment' Virtually every company that makes IC die attach/bonding or wire/ribbon bonding equipment says that its systems have been performing these operations for some time. That's true, but few of these machines have been employed at even low-volume applications by IC standards. Is it fair to compare high-volume ICs with relatively low opto-device volumes, productivity and manufacturing efficiencies? Absolutely. There will never be the realized dream of affordable global broadband communications convergence without IC-like efficiencies for opto devices and packages with comparable low-cost board and OEM product manufacturing. What equipment features can opto device makers expect from the cadre of machine producers? From ESEC's point-of-view [esec.com], the current downturn certainly had negative effects. At the same time, however, it also provided introspective opportunities for the company, according to Walter Gisler, technical manager. ESEC, like many others, performed a thorough market segmentation analysis and redefined the application range for opto, and addressed a new customer base (Figure 1). Gisler adds that the variety of assembly steps, along with the progress made in automating equipment, will probably result in attempts to integrate multiple assembly operations on one machine to enable component manufacture in a single-pass mode.
Supplier Partnerships Gisler noted that some specialized equipment suppliers are actively looking for partnerships with other suppliers. More telling, however, is his observation that too many conditions in the industry continue to change to allow one company to dominate the market. Looking ahead, the time-to-market in introducing new opto-electronic assembly devices for higher bandwidth applications will be a critical success factor. On the other hand, driven by the need for cheaper low-end components, time-to-volume will be equally important for the industry.
At Kulicke & Soffa (Figure 2) [kns.com], Laurie Roth, market research director says, "The issue with photonics packaging continues to be a lack of standards and low volumes. "Now that the initial exuberance-or excesses might be a better term-have cooled, more companies are taking a serious look at cost reduction. There are more multisource agreements and some discussions on revising the Telcordia standards to cover applications that are not subject to harsh environments. These are first steps in the right direction," she says. "It is important," Roth adds, "when talking about packaging issues to understand which devices we are targeting. The challenges for a 40 Gbps long haul, hermetically sealed butterfly package are different from those of a very-short-reach (VSR) transceiver. "MEMS and MOEMS have their own requirements. The VSR and Metropolitan Area Networks (MANs), where the higher volumes are today, pose less-challenging packaging issues that can generally be solved with more conventional semiconductor assembly equipment." Moving into the more complex packages, however, the requirements become more specialized. For example, gold ribbon wire wedge bonding provides better signal performance at high frequencies and deep access capability is needed to reach down into those high-walled butterfly packages, according to Roth. 'Semiconductor-Like' Productivity Although bond-pad pitch is usually not an issue, small bond-pad size is, so machines need to be extremely accurate. To keep the costs down as volumes increase, machines need to have 'semiconductor-like' productivity. She adds that the industry is moving toward improving automation for laser-to-fiber alignment. And while there has been progress in this area, there is still a way to go to speed up the process while reducing the sticker price of the equipment. If device designers could focus on a more elegant way of directing the light output to the fiber, the cost of the assembly equipment would drop dramatically. As Roth observes, "One of the principle contributors to poor yield is epoxy. Con-tamination can ruin optical coatings and many companies do not properly understand the curing cycle or the properties of the materials they are using." |
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