Packaging and integrating optoelectronic (OE) devices presents a significantly greater challenge to developers than to their counterparts in the mainstream semiconductor sector. Packaging is also far more integral to the success of the final product and accounts for up to 80% of the cost.
Several factors impact the packaging of OE components, including thermal management, attachment techniques, fiber alignment requirements, long-term reliability, and switching frequency. Optimizing the performance of OE components or modules requires a thorough understanding of the challenges and tradeoffs present at different stages in package design (see figure 1).
Figure 1. Packing issues crop up in ever step of optoelectronic component manufacturing, including adhesives used at the sub-component mounting stage, the actual packaging stage itself, and the connectorization stage.
Figure 2. In a typical optoelectronic component, individual elements are mounted on an optical bench that is bonded to a thermoelectric cooler. This combination gets housed in the package and then connectorized with fiber.
A fundamental difference between standard semiconductors and OE devices is the need to maintain a high accuracy of alignment between the constituent components of an OE component such as microlenses and diode laser die (see figure 2). Given the goal of maintaining alignment over the lifetime of the module, how best to attach these components to the optical bench or substrate becomes an important question.
In the assembly of thermoelectric coolers and thermistors, solder attach provides good thermal contact. Epoxy or acrylic adhesives are established for use with optical benches, filters, and freestanding optics. Epoxies and adhesives don't provide suitable bonding for parts that contain laser die, such as laser diodes, however. Certain unwanted organic compound residues in the adhesives can lead to catastrophic optical damage (COD) of the laser die.1
Eutectic materials or vacuum solder attachments provide another way to attach optical components to substrates. Solders that use flux can lead to the same COD problems as organic-based adhesives, however, and should be avoided for components containing laser die. Careful thermal matching of the substrate and solder materials minimizes optical misalignment and mechanical stress over a range of operating temperatures.
Epoxies tend to absorb moisture and may outgas in hermetically sealed packages, producing small quantities of contaminants that may pose a threat to long-term reliability of diode-laser-based products. Photo-disassociation of organic compounds in the presence of the laser can trigger deposition of carbon on the laser facet. Since carbon suffers high optical absorption, these deposits can raise the facet temperature, eventually melting the semiconductor and resulting in COD. sealing the deal
The susceptibility of OE components to contamination and moisture is one reason why hermetic sealing of these packages has become standard practice. The interior of the device must provide free space through which light can travel. Various techniques available for hermetic sealing include resistance welding and solder reflow. Laser welding, now gaining in popularity, introduces the least heat distortion to the package and lid. Laser welding is compatible with the widest variety of materials but requires the most expensive equipment. To ensure the required level of contamination control and minimize the resulting possibility of COD, the sealing process typically takes place in a controlled atmosphere after a vacuum-stabilization bake.
Hermetic packages do more than just protect the internal components from contamination. Optical devices are greatly dependent on thermal variations. Some dense wavelength-division multiplexing distributed-feedback laser die require temperature control of ±0.1°C to maintain the specified lasing threshold and output wavelength. Such requirements can be met through the use of thermoelectric coolers and temperature-control integrated circuits.
In other situations, the hermetic package needs to simply transfer the heat away from the internal components. The selection of suitable interface/attach materials performs a critical role of transferring heat from the photonic devices to the package. The final choice of heat-sink materials depends on several material properties such as cost, electrical conductivity, thermal expansion coefficient, and outgassing. Matching the material with the application also requires consideration of the operating temperature range and the mounting pressure (see table).
Thermal modeling using CAD tools, including finite element analysis, offers an increasingly practical option. These tools allow users to assess a number of candidate materials prior to the building of prototypes. Such an approach can also provide confidence about the long-term performance and reliability of a potential product. getting it straight
Providing a reliable seal at the point at which the optical fiber meets the package is a particular challenge with hermetic devices. A standard approach consists of attaching the optical fiber to a cylindrical metal ferrule. The process involves stripping the cladding from a section at the end of the fiber and replacing it with an area of metallization. The metallization allows the fiber to be soldered to the ferrule, which in turn can be brazed to the side of the package for a hermetic seal.
In telecom applications, pigtailing (aligning and attaching a fiber to an OE component) needs to be considered when designing a component package. A singlemode fiber can require placement to within ±0.3 µm of the optical axis; for polarization-maintaining fiber, the requirements include a rotation change of less than 1° during alignment. Multimode fibers, with around five times the core diameter of singlemode fibers, can accept larger tolerances of approximately ±5 µm. Although most manufacturers still use manual alignment for singlemode and polarization-maintaining fiber, computer-controlled motion stages can cut down on processing time and increase yields.
Pigtailing typically begins with aligning the fiber and then temporarily tacking it in place with a UV-cured epoxy followed by a second, thermally cured epoxy to strengthen the joint. UV-cured epoxies can lead to alignment problems during the life of the product, however, because of thermal expansion mismatches between the epoxy and the component or substrate, especially in singlemode or polarization-sensitive applications. Thermal expansion presents less of an issue for multimode fiber applications. In the case of products with less strenuous (not sub-micron) alignment requirements and that do not incorporate laser die, adhesive attach systems can provide a simple, practical, economical alternative.
Eutectic solder provides a second alternative for fiber attachment. The solder requires that the individual components being attachedwhether the fiber or devices such as filtersare metallized prior to the assembly process, which increases cost. Solidification of the solder can also shift the relative position between fiber and component. To minimize this, packaging manufacturers need to tightly control the initial alignment position based on a full understanding of the solder, including eutectic point and shrinkage versus temperature. Solder-based alignment and attachment yields with singlemode fiber can be relatively low and may also require significant reworking of individual joints, increasing overall process cost.
The third alternative, producing the highest level of accuracy and reliability of the three, is a laser weld attach. Here the component is welded in place. Any variation in coupling efficiency resulting from post-weld cooling is corrected by a mechanical bending of the optical path, which can be done manually or by a laser hammering process; laser hammering adjusts the relative position of component and fiber through additional, calculated spot welds. Mechanical repositioning is possible within the solder process, but using a laser hammering process to produce predictable levels of movement with a solder system can prove to be extremely difficult.
The laser weld technique can be a manual operation, but process automation is improving throughput and consistency. As with the solder process, a full characterization of the material properties, laser power, and laser pulse duration are critical to success. This technique provides the best process for alignment while also ensuring that potential contaminants are not introduced to the package interior. planning for the future
In addition to attachment, package, seal, alignment, and integration issues, the device will also have to withstand environmental, mechanical, and vibration testing, meeting requirements such as those specified by Telcordia standards such as GR-1209-CORE and GR-1221-CORE for passive components and GR-468-CORE, which covers active components.2 Of course, new components with increased performance characteristics are also changing how designers package OE components.
The issue of design for high frequency is playing a larger role in OE packaging, just as it has with microwave components. With higher bandwidths comes an increased need to manage package-related parasitic capacitance and inductance, as well as a greater reliance on techniques such as impedance matching, microwave design in 3-D, and electromagnetic radiation modeling. The move toward higher frequencies is also expected to prompt a migration from wirebond to flip-chip mounting.
With the shift toward higher-frequency components being just one of several technology trends, it is clear that the packaging and integration challenges of OE components will continue to change. Ongoing advances such as increased levels of integration in both monolithic and hybrid modules promise to radically change the packaging requirements of these components. oe
1. www.sony.net/Products/SC-HP/QR/PDF/Chapter2.pdf (see section 2.4.10 Laser Diode (LD) Reliability).
2. See www.telcordia.com.
David Ruxton, Bill Ashby
David Ruxton is the CEO and Bill Ashby is a project manager for Optocap Ltd., Livingston, UK.