Packaging Optoelectronic Components
An Emerging Model
BY BRUCE W. HUENERS
Optoelectronic components carry data from the optical network to and from the end user. Source lasers generate signals and pump lasers amplify them. Electro-optical modulators create a fiber-coupled data stream by converting high-frequency electrical signals into optical pulses, while hybrid and passive devices shape the signal.
Proprietary methods and high intellectual property barriers prevail in the manufacture of active devices. These methods include custom wafer processing, thin film processing, device and subassembly packaging, fiber handling and alignment, and the finish steps of tuning, adjusting, and testing. Multiple fabrication techniques and processes are common, coupled with a lack of packaging and material handling standards.
Transmitter and receiver optical subassemblies (TOSA and ROSA) are the most costly and precisely manufactured parts of a transceiver. Contract optoelectronic component manufacturers with materials management supply chain and precision manufacturing competencies realize high volume, cost-effective manufacturing of these complex components. The expertise that these contract manufacturers bring to equipment companies will make fiber-to-the-home (FTTH) or other forms of FTT-X an affordable option for the consumer.
Device Functionality and Package Formats
Typically, device functionality dictates package format. High-performance devices, such as source lasers, pump lasers, and modulators, generally are assembled in butterfly packages. Lower-performance, shorter-range, and cost-sensitive devices are assembled in lower-cost package formats, such as transistor outline packages TO-46 and T0-56 (Figure 1).
Figure 1. Automatic assembly of transistor outline packages.
Optical OEMs increasingly outsource manufacture of their terminal-active optical components to CEMs. Consequently these CEMs have become a major manufacturer of transceivers, including sourcing and manufacture of the TOSA and ROSA, which are the most costly elements of a transceiver.
Since the optoelectronic package is a hybrid processor of both electronic and photonic signals, device fabrication requires specialized materials, including silicon, quartz, doped silica, LiNBO3, GaAs, and InP. This distinguishes optoelectronic device assembly from conventional microelectronic manufacturing, and requires a high level of process and manufacturing expertise. Consequently, vertical integration in manufacturing shortens the supply chain — saving procurement, packing, and shipping costs.
Fiberoptic transmission systems require a laser source with low noise and an extremely narrow frequency spectrum. The distributed feedback (DFB) laser meets this need, using a feedback mechanism to stabilize the output frequency. DFB lasers remain virtually single frequency under modulation and variations in drive current or operating temperature, making them robust for dense wave division multiplexing (DWDM) applications.
Laser Diode Module Manufacturing Process Flow
Chip Fabrication. At the wafer level, a substrate crystal orientation of (001) is essential for fabricating lasers since parallel mirrors should be made by cleaving the wafer in the (110) plane that is normal to the (001) crystal plane. Fifty-µm-deep grooves are fabricated photolithographically with spacing of about 200 to 300 µm in the wafer to provide scoring lines for chip singulation. Wafers are normally polished from the back side down to about 100-µm thickness.
Cleave Facets. Next, the wafer is cleaved along the direction normal to the grooves in the wafer. Special care is required to prevent defects such as dislocations from the chip edges to the inner crystal due to surface damage during cleaving. In this stage, each diode is tested under pulsed operation, and good devices are selected.
Facet Coating. After cleaving the wafer into diode arrays or bars, it is necessary to protect mirrors or facets on the waveguide with a dielectric film such as SiO2, Al2O3, and Si3N4. These insulating films are deposited by RF sputtering or chemical vapor deposition (CVD). Protecting the mirror surface from exposure to the atmosphere suppresses oxidation of the facet, which causes long-term degradation. It also reduces surface recombination velocity, an important parameter for catastrophic failure.
Package Assembly. For lower data rate and shorter distance communications applications, laser diodes and photo diodes are commonly packaged in TO-cans. The laser diode, or photo diode, is first mounted onto a submount, and then onto a TO-can header. At lower data rates (e.g. 100 Mbps), the die attach medium is typically silver-filled epoxy. At higher data rates (e.g. 1 Gbps), AuSn solder predominates for optimal thermal management.
The laser diode bonding process requires placement accuracy between 5 and 20 µm (true position radial), depending on the application (transmission distance and data rate). The photo diode die bonding accuracy requirement is about 20 µm. TO-cans are the most common laser diode and photo diode package today for applications below 2.5 Gbps, and will continue to be the highest volume package configuration. TO-cans are often used for low to mid-end telecom laser diodes as well, incorporating a silicon microbench with lenses and mirrors (Figure 2).
Figure 2. The laser diode bonding process of transmitter and receiver optical subassembly require accuracies between 5 to 20 µm.
Precision eutectic component attach includes:
- Pick-and-place of Si, GaAs or InP chips;
- In-situ reflow of preform or pre-tinned devices, with programmable x-, y- or z-axis agitation;
- Programmable pulse heating or steady-state temperature.
During device fabrication, external stress may be applied to the diodes. Cleaving the processed wafers into diode arrays can induce mechanical damage from the crystal's cleaved edge. Dislocations are sometimes generated from such regions. During die attach or wire bonding, thermal or mechanical stress is applied to the diode chips, which may accumulate elastic strain in the diode or generate mechanical damage or scratches in the crystal. Automating this process minimizes stress, producing repeatable and consistent results.
The value of recipe-driven process control for optoelectronic assembly is illustrated by considering laser diode attachment within a source or pump laser. This is an extremely temperature-sensitive device that requires careful process control during assembly. The reflow profile during an in situ eutectic die attach process is engineered to provide consistent melting and a void-free attach interface. This is necessary for consistent heat transfer from the laser diode and contributes significantly to temperature stabilization during laser operation.
Fiber Align and Attach. The final assembly step of a laser diode is aligning and attaching an optical fiber. To optimize coupling efficiencies, final alignment must be within 0.1 µm of the actual peak. Due to manufacturing variances in the diode, diode assembly and optical fiber core center, the fiber has to be actively aligned in all six axes. This is where the positioning of an optical component is based on feedback from, or induced by, the device being placed.
Methodologies used for permanent attach of the fiber include laser weld, solder or epoxy. The most critical parameter is the final, post-attach optical power. The attach process may induce stress or mechanical shift which must be accounted for. Even a submicron shift is enough to render a device useless, so control of the attach process is critical (Figure 3).
Figure 3. Configurable hardware and software enable automated alignment and attach to accommodate a wide range of devices.
Device Test. One method of gauging chip attach integrity is a LIV (light, current, voltage) test. The drive current through the laser is ramped from zero to the laser's maximum operating current and the output power of the laser and the forward voltage across the diode junction are recorded.
The comparison between continuous wave (cw) and pulsed current LIV curves also indicates chip attach integrity. If heat flow through the die bond is high, the two curves will be similar. A flaw in the bond lowers heat flow, the laser chip will heat during cw operation, and the two curves will be substantially different.
In the above method, the conclusion drawn is pass/fail. The chip attach process is either good enough or the test indicates failure. A more quantitative test of die bond efficacy is junction temperature, on which output power, forward voltage, wavelength, and threshold current depend. The most straightforward method measures the laser wavelength and calculates thermal impedance (Rth), which is a direct measure of chip attach quality.
Functional requirements of a typical LIV test system include:
- Current sourcing with high precision to drive the laser diode;
- Measurement of sub-picoamp currents (1 pA=10-12 A) with femtoamp-level resolution (1 fA=0.001 pA); and
- Control of the thermoelectric cooler (TEC) that regulates the operating temperature of the module. Temperature control to ±0.01°C is necessary to ensure consistent wavelength output from the module and protect the device from damage caused by overheating.
Automating the Optoelectronic Assembly Process
Automating a process disciplines it. Material and process variations that are tolerated in a manually assisted process create difficulties when that process is automated. Consider, for example, the case of automatic die bonding vs. manual die bonding. Machine vision replaces the operator's 'organic image processor,' high-speed precision mechanisms and end-effectors supplant human tactile sensing, and fine motor skills and programmable machine logic replace the reasoning powers of a skilled operator.
Precision automatic die bonders are extremely powerful, productivity-enhancing tools that require equipment suppliers to understand their customers' processes. This is especially true in the case of complex process automation, such as automating complex dispense and die attach processes, high I/O wire bonding and active optical alignment.
Physical, thermal, electrical, mechanical, and manufacturability considerations, in addition to price and time-to-market concerns, challenge the package designer. These issues face the equipment designers who must not only satisfy current requirements, but anticipate the capabilities that will be required in equipment platforms 5 years into the future. Large-area air bearings, accurate linear motors and encoders, voice coil drives, powerful machine software and rich graphical-user-interfaces have enabled machine architecture and functional advances. Equipment capable of placing p-side-down laser diodes with an accuracy of 1.5 µm, 3 sigma at production volume, in a footprint less than 1 m2, is available.
Augmenting production capabilities with automated, high-precision assembly systems enables high yield at high-volume production levels, revenue growth and improved margins. New, higher volume optoelectronic components requiring scalable manufacturing capacity dictate a cost-effective manufacturing strategy tailored to meet the unique demands of precision optical component assembly.
The optoelectronic component manufacturing industry has been under considerable downward cost pressure for the last 3 years. Most manufacturers have recognized that a paradigm shift in assembly methods must occur for them to drive down component costs and accelerate time-to-volume. That shift is emerging as a move from the manual assembly processes that have characterized the industry for decades to high-accuracy, high- yield automated manufacture and test.
Putting It All Together
It is a daunting challenge to enable ease of communication within a company among design, process, and manufacturing engineers, supply chain managers, documentation providers, shipping, and all parties involved in bringing a product to market on time, on budget, and with the highest quality. When this must be accomplished between the OEM and the EMS, and possibly the equipment manufacturer, the challenges are even greater. Adding the dimension of the complexity of the parts and the processes, it can be overwhelming. Nonetheless, since the burst of the telecom bubble, new equipment, processes, collaborations, and models have emerged for manufacturing complex components and packages.
Automation eventually will provide complete process solutions for the optical component assembly industry. Currently, however, there are significant challenges to automation. The combination of extreme accuracy and process requirements, significant variations in height, unusual form factors, unusual materials, and the inherent difficulties arising from the requirement to guide light have prevented most equipment available today from being used in optical component packaging.
However, a few systems are available that have the flexibility to support these difficult tolerances and process requirements. Specifically, die attachment and wire bonding techniques developed in the hybrid semiconductor and high frequency wireless industries are currently being incorporated into the optical component manufacturing industry. Production-grade, precision fiber alignment solutions are now available to automate what traditionally has been a manual process.
Early results from the implementation of this equipment are encouraging. In actual case histories, companies have experienced a 50% increase in yielded throughput and a 67% reduction in required clean room floor space by replacing four manual die bonding stations with a single automatic die bonder, enabling a 3-month payback on the capital investment.
The increase in throughput and yield is even greater for fiber attach. Upgrading manual fiber alignment processes to a precision automated process has resulted in ROIs measured in weeks or months and often results in greater product performance as well. Furthermore, automated wire bonders are facilitating the move to higher-frequency devices, as they have superior control of wire length and loop profiles compared to manual processes. Automated fiber alignment tools are enabling the move to smaller optical channels through their ability to accurately position a fiber or array of fibers in vector space.
This equipment, along with the heightened skills, processes and tools of the EMS, will increasingly enable OEMs to outsource the manufacture of complex components, assemblies and packages. The EMS, in turn, will need to continue to hone supply chain skills for further cost reductions. The equipment and process expertise that these suppliers are developing today will not only facilitate the efficient manufacture of optical components, but will enable the component designers to continue to push the envelope of communications technology into the foreseeable future.
BRUCE W. HUENERS, VP, Marketing and Business Development, may be contacted at Palomar Technologies Inc., 2230 Oak Ridge Way, Vista, CA 92083; (760) 931-3600.