By Bruce W. Hueners, Palomar Technologies
Optoelectronic devices add functionality to many electronic products, primarily because they enable significantly higher data transmission rates. Initially used in telecommunications applications, OE photonic devices are now being integrated with ICs in a growing number of products. Since most OE devices are wafer fabricated and are, in fact, chips that perform optical and/or mechanical functions, the assembly and packaging of these devices builds upon a solid IC packaging base. Background Photonic devices enable today's global optical networks to handle massive data transmission volumes. Source lasers convert ubiquitous high-frequency electronic signals into high-data-rate optical pulses to send down optical fibers. Pump lasers amplify the source laser signal to extend its reach in long haul and metro optical transmission networks. In only two decades, the workhorse semiconductor laser diode has evolved into a family of robust, reliable devices, with individual conversion efficiencies of better than 50 percent, continuous output powers of several kW, modulation rates of several tens of GHz and wavelengths from 450nm to beyond 2000nm. Manufacturing and packaging optical devices is done in an environment rampant with proprietary methods, intellectual property barriers and trade secrets.
Methods include custom wafer processing, thin film processing, device and subassembly packaging (including eutectic and epoxy component attach and wire bonding), fiber handling and alignment and the final steps of tuning, adjusting and testing. Furthermore, multiple fabrication techniques and processes are common, coupled with a lack of packaging and material handling standards. Industrial engineering approaches common in other batch-oriented manufacturing industries are largely absent from today's "handcrafted" photonics environment. Other industries use process engineering, design for manufacturing, design for test, standardization, outsourcing and automation. These methodologies are highly interrelated and any discussion of automation must include these practices together with the specific types of materials employed in OE components. Optoelectronic Components OE component modules are assembled using semiconductor devices, surface-mount passives, substrates, alignment aids, housings, adhesives, solders and thermal management elements, along with one or more optical elements.
These elements may include aligned and bonded optical fibers, lenses, mirrors, isolators, wavelength locking crystals and refractive index matched adhesives. Much of the assembly technology employed is similar to that used in IC assembly. A high-yield, low-cost packaging focus is needed to reduce OE component costs. Package Formats Device functionality dictates package format. High-performance devices such as source lasers, pump lasers and modulators are generally assembled in rugged butterfly packages. Lower-performance, cost-sensitive devices are assembled in less expensive transistor outline package formats such as TO-46 and TO-56. Compound semiconductors are normally used for fabricating light-emitting and detection tasks and are more temperature sensitive than silicon devices. GaAs-based lasers are formed from alloys of Ga, Al, In, As and P and grown in crystal compositions that are lattice-matched to GaAs. These lasers can emit at any wavelength from about 630nm to about 1100nm. The most common lasers operate at 635, 650, 680 and 780nm, and are used in optical storage and displays. Lasers operating at 785, 808, 830, 920 and 940nm are employed for various pumping and printing applications. In telecom, lasers operating at 980nm are used for pumping fiber amplifiers. Transmission Systems Fiber optic transmission systems require a laser source with low noise and an extremely narrow frequency spectrum, a need met by the distributed feedback laser (DFB). Instead of relying only on mirrors to provide feedback within the laser resonator, the DFB uses a grating pattern similar to a precision corrugated structure embedded within the laser structure itself in close proximity to the waveguide. By far the largest application for DFB lasers is in fiber-optic transmission systems. The first commercial devices were at 1300nm wavelength. Today most source lasers output around 1550nm to match the minimum attenuation window of silica optical fiber and the gain bandwidth of erbium-doped fiber amplifiers (EDFA). Commercially, most are sold as fiber-coupled devices, with typical output powers ranging from 1-35mW in the fiber.
Photonic Materials and Interactions The optoelectronic package is a hybrid processor of both electronic and photonic signals, and employs many unique materials used in device fabrication. This variety of materials is one of the factors that distinguishes OE device assembly from conventional microelectronics assembly. Chip Fabrication 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-micron-deep grooves are fabricated photolithographically with spacing of about 200-300 microns in the wafer to provide scoring lines for chip singulation. Wafers are normally polished from the backside down to about 100µm in thickness. Cleaving Facets Next, the wafer is cleaved along the direction normal to the grooves in the wafer. At this point, special care must be taken avoid introducing defects such as dislocations from the chip edges to the inner crystal due to surface damage during the cleaving. In this stage, each diode is tested under pulsed operation, and good devices are selected. After cleaving the wafer into diode arrays or bars (a more convenient and practical method than handling individual diodes), both mirrors of facets on the waveguide are protected by a dielectric film such as SiO2, Al2O3 and Si3N4. These insulating films are deposited by RF sputtering or chemical vapor deposition (CVD). Assembly Processes During device fabrication, external stress may be applied to the diodes. In the cleaving of the processed wafers into diode arrays, mechanical damage can be induced from the cleaved edge of the crystal.
In some cases, crystal orientation dislocations are 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. To minimize stress, this assembly process should be automated and recipe-driven for extremely repeatable and consistent results. The importance of recipe-driven process control for optoelectronic assembly can be illustrated by the attachment of a laser diode within a source or pump laser. This extremely temperature-sensitive device requires careful process control during assembly. Precision eutectic component attach includes:
The solder temperature reflow profile during in-situ eutectic die attach is engineered to provide consistent melting and a void-free attach interface. This results in consistent heat transfer from the laser diode and contributes significantly to temperature stabilization during laser operation (see Figure 4). |
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