Optical Packaging
09/01/2004
SILICON HOUSINGS CHANGE MANUFACTURE OF OPTICAL TRANSCEIVER SUBASSEMBLIES
BY JOCHEN KUHMANN, RALF HAUFFE, ARND KILIAN AND GORDON ELGER
Improvements in silicon MEMS technology, wafer level assembly and test enable both the reduction of current packaging costs and reductions in size. This will create valuable bandwidth for new applications.
The photonics industry as a whole is 10 to 20 years behind more mature industries like the semiconductor industry in terms of standardization, cost structure and assembly techniques. The lag is in part due to key differences between semiconductor and photonic packaging. Photonics components require accurate alignment of glass optical fibers, accurate alignment of optical components, hermetic sealing and a greater variety of materials — each with its own properties.
Shifting Market Priorities
During the optical surge of the 1990s, market pull for optical components was so massive that once the active chip was made, little effort was put into optimizing production flow of the completed optical component. As a result, low-volume packaging procedures were used, and the only way to scale-up production was to hire more skilled employees. For 15 years, there has been little change in the basic technology. The result is a gap between perceived value and actual cost.
New demand in the data communications sector is spurring further growth in photonics components for the LAN (local area network) and SAN (storage area network) markets. The industry is facing pressing cost and bandwidth issues, and it must quickly find solutions to avoid losing momentum. In fact, optical chip and module manufacturers recently banded together to create multisource agreements (MSAs) that enable them to provide better compatibility between different designs, securing second sourcing between several suppliers and lower costs.
The main requirements for optical components such as transceiver optical subassemblies (TOSA) and receiver optical subassemblies (ROSA) are protection, interconnection and manufacturability. Today, in data communications, the various forms of metal-based housings, such as butterfly packages and derivates and TO cans, still dominate TOSA/ROSA packaging because there is no simple way to place a component on the substrate material, properly align it with the fiber and hermetically seal it with the necessary electrical I/O connections.
Building on the Silicon Optical Bench
Silicon as a packaging material was introduced in photonics more than 10 years ago, and was described as a "silicon optical bench" (SiOB). Various companies began to apply the concept, either as a submount for edge-emitting lasers containing V-grooves that fixed in place a single mode optical fiber, or in a laser/turning mirror and lens combination.
This article takes the SiOB platform and demonstrates how it can be developed as an effective packaging platform for lasers, including vertical cavity surface-emitting lasers (VCSELs), Fabry Perot (FP) lasers, distributed feedback (DFB) diode lasers and PIN or APD diodes for light detection. Such a platform can be deployed over a range of applications from low-cost data communications systems to high-end telecom systems.
The basic functions of packaging are to connect the components to the outside world by providing suitable interfaces for the electrical and optical signals, enabling them to transgress undisturbed through package boundaries that protect the components from environmental influences like moisture.
Typically, such interfacing is achieved with traditional packages using electrical feedthroughs with metal packages and different kinds of optical feedthroughs or windows. The electrical feedthroughs are insulated with glass or ceramics. The metal packages with windows are housings that contain a suitable lens and a flange for fiber pigtailing outside the housing. Optical feedthroughs for metal housings are achieved by using a fiber or fiber stub threaded through the sidewalls of the package followed by solder sealing (with or without the help of a ferrule).
The concepts described in this article use the "window" approach, where light leaves the package at a right angle to the top-surface of the housing.
Silicon Micromachining
The advantage of using silicon as a package is clear: the existing infrastructure for silicon micromachining, or MEMS technology, makes low-cost manufacturing easily available through a foundry model. Fabrication of silicon-micromachined housings is characterized by batch processing, and replicating the same patterns on a 6-in. silicon wafer. With traditional manufacturing, batch processing ended at the packaging stage, where a large portion of cost for assembly and testing was incurred.
This is not the case with the approach described here. The silicon housing provides a platform for component assembly using pick-and-place, die bonding and wire bonding at wafer level. Manual labor and handling of piece parts is no longer required. This also means that component yield is dramatically improved. Post-assembly functional tests required even for low-cost, high-volume products (such as a simple DC test to measure the laser threshold) can now be executed at the wafer level, allowing more cost reductions.
The functional devices are then solder-sealed with a glass or silicon lid, while the housings are still in their original wafer format. An optical gross and fine leak test is performed, followed by the burn-in of the components (powering the components at elevated temperatures in order to identify faulty devices). After burn-in, the components are inked and diced.
Standardization of micromachined silicon housing also plays a role in cutting costs. Use of additional landing pads and vias to accommodate for various pad layouts for laser and IC enables straightforward coplanar design. If custom design is required, standard silicon can eliminate shrinkage typical of high-temperature cofired ceramic materials — simplifying package design.
Micromachined Silicon Packages and Optical Connectivity
Optical alignment is a large differentiator within the field of optical packaging. Requirements for multimode fibers and single-mode fibers differ greatly. With silicon micromachining, however, the small size of the package and the exact dimension control (offering tolerances below 1µm) add value to the demanding alignment methodologies required.
Laser light emission from within the two telecommunication windows (1320 and 1550) is characterized by a significant beam divergence that is perpendicular to the laser surface. This characteristic requires lenses situated as close as possible to the laser. The configuration necessitates a beam redirection from the horizontal to the vertical plane, using a silicon mirror with a polished surface and an aluminum thin film metallization. A miniature silicon housing facilitates these requirements.
Silicon lenses (planoconvex) or glass lenses can be part of the houseing and/or glass lenses can be integrated into receptacles. With a collimated beam design, optical isolators that are required for DFB lasers in telecom can be integrated into the receptacle. For short-wavelength datacom applications, plastic receptacles are used with integrated lenses.
Advantages of Silicon in Electrical Performance
 Figure 1. ROSA module based on silicon, with glass lid, ??-vias and SDM contacts to a flex board. |
The optical interfaces formed by optical fiber connectors combined with the housing create a functional unit known as a TOSA (transmitter optical subassembly) or a ROSA (receiver optical subassembly). The ROSA contains a transimpedance amplifier (TIA) that must be closely assembled with the PIN or APD diode in order to achieve optimal signal quality.
For some inexpensive applications, the 850-nm VCSEL is the sole part of the TOSA housing, but most components require stringent optical power level monitoring. For VCSEL applications in particular, silicon as a housing material offers an advantage: the pin diode can be integrated into the package itself using ion implanting. This reduces piece parts and assembly steps and contributes to cost reduction in the very cost-sensitive data communications market.
 Figure 2. Eye diagrams of silicon-based ROSA module, operated at a bit rate of 10.7 Gbits/s at 0 dB input power (above) and -20 dB (below). |
Silicon as a packaging material for data and telecommunications provides key benefits in electrical performance. The metallization technology provides means for realizing coplanar lines with well-controlled impedance and low propagation losses have been measured. This holds also for the combination of lines with vias, even up to frequencies of 40 GHz. For example, the optical to electrical transfer function of a ROSA module shown in Figure 1 has a 3-dB bandwidth at 8.5 GHz, dominated by the TIA.and the photodiode. The transmission lines of the package do not distort the signal.
 Figure 3. ROSA subassembly measured. |
Figure 2 shows the eye diagrams for a bit rate of 10.7 Gb/s measured at two different power levels. The sensitivity of the ROSA module depicted in Figure 3 is 19.24 dBm (10.7 Gbit/s, PRBS23). It is possible to integrate the laser driver and a bias T closely with the optical subassembly, using a stacked approach.
Integration of passives into the package is currently under development; for example, capacitors with up to 1 nF and resistors for termination of impedance matched lines.
Thermal Performance
Silicon as a substrate material is an excellent heat conductor and with 148W/mK is approximately six times better than Al2O3-based substrates. Using silicon micromachining for manufacture, heat generated by laser components and resistors inside the TOSA package can be even more effectively spread and conducted, since the components are etched and integrated into the bottom of a cavity. The thickness of the bottom can be designed to specific requirements, but for standard layouts is 60 µm. If better heat spreading is required, the design can be reversed so that the components are mounted e.g. onto a 400-µm-thick silicon section. Generally, the design freedom offered by the package allows building housings that have electrical connections only in a small area, for example on one side, and use the remainder of the backside area for heat sinking.
For some applications in long haul transmission and in dense wavelength division multiplexing (DWDM) applications, DFB lasers require cooling. For these applications a project is under way, in which a small TEC will be integrated into the hermetic silicon package.
The silicon package is attached to a flexible circuit board using surface mount device (SMD) technologies. For many transceiver designs it is advantageous if the heat can be transferred from the silicon housing through the flex to a heat sink attached to back of the flex. The heat sink, being a part of the TOSA, can be attached to the transceiver housing.
 Figure 4. Heat transfer from Si-based TOSA to flex circuit. |
Modern SMD technology using tight solder bump/thermal via spacing on the flex allows for efficient heat transfer through the flex, as illustrated in Figure 4. It shows the result of a thermal simulation, in which a heat load of 500 mW is conducted from the silicon part to the heat sink through an area of approximately 10 mm2, resulting in a temperature difference of only 1.2°C between the silicon part and the heat sink. Lower-cost approaches could make use of a single layer flex with metallization on the backside or work with a cut-out in the flex, through which a heat sink is attached.
Conclusion
The silicon platform holds the promise of significant advances in transceiver assembly automation that can increase first pass yield, decrease costs and offer multiple benefits for high-volume, inexpensive applications in data communications within the telecommunications sector. As photonics technology adapts to modern assembly methods, and as packaging helps make this transition possible, there will be future advancement of the technology.
JOCHEN KUHMANN, CTO and founder, ARND KILIAN, head of the design group, RALF HAUFFE, senior engineer, and GORDON ELGER, head of the back-end group, may be contacted at Hymite GmbH, Carl-Scheele Straße 12, D-12489 Berlin, Germany; 49 0 30 678 260 0; e-mail: [email protected].