Micromachined silicon housings cut size and cost for optical transceiver subassemblies

Improvements in silicon micro-electromechanical system (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, particularly for optical communications. Hymite believes that 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

The photonics industry as a whole is 10 to 20 years behind more mature industries like semiconductors in terms of standardisation, 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 fibres, accurate alignment of optical components, hermetic sealing and a greater variety of materials - each with its own properties.

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 optimising the 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 now spurring further growth in photonics for local (LAN) and storage (SAN) area networking. The industry is facing more 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 multi source agreements (MSAs) that enable them to provide better compatibility between different designs, securing second sourcing between several suppliers and lower costs [1].

The main requirements for optical components such as transceiver (TOSA) and receiver (ROSA) optical subassemblies are protection, interconnection and manufacturability. Today's data communications market has a variety of metal-based housings, such as butterfly packages and derivates and TO cans. These still dominate TOSA/ROSA packaging because there is no simple way to place a component on the substrate material, properly align it with the fibre 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 fibre, 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 avalanche photodiodes (APDs) for light detection. Such a platform can be deployed over a broad 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 packages using electrical feedthroughs with metal packages and different kinds of optical feedthroughs or windows. The electrical feedthroughs are insulated with glass or ceramics. Metal packages with windows are housings that contain a suitable lens and a flange for fibre pigtailing outside the housing. Optical feedthroughs for metal housings are achieved by using a fibre or fibre 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 - MEMS technology - makes low-cost manufacturing easily available through a foundry model. Fabrication of silicon-micromachined housings is characterised by batch processing (on the front-end), and replicating the same patterns on a 150mm silicon wafer.

With traditional manufacturing technologies, batch processing ended at the packaging (back-end) stage, where a large portion of the cost for assembly and testing was incurred. This is not the case with the approach described here. In this concept, the silicon housing provides a platform for component assembly using pick-and-place, along with die and wire bonding at the wafer level. Manual labour and handling of piece parts is no longer required. This also means that component yield is dramatically improved. The 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 subsequently 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.

Standardisation of micromachined silicon housings can also play a key role in cutting costs. Use of additional landing pads and vias to accommodate for various pad layouts for lasers and ICs enables straightforward coplanar design. If custom design is required, standard silicon can eliminate shrinkage typical of high-temperature co-fired ceramic materials - greatly simplifying package design.

Micromachined silicon packages and optical connectivity
Optical alignment is a large differentiator within the field of optical packaging. Requirements for multimode fibres and single-mode fibres differ greatly. With silicon micromachining, however, small package size and exact dimension control (offering tolerances below 1µm) add value to the demanding alignment methodologies required.

Laser light emission from within both telecom wavelength windows (1320nm and 1550nm) is characterised 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 also necessitates a beam redirection from the horizontal to the vertical plane, which is accomplished using a silicon mirror with a polished surface and an aluminium thin film metallisation. A miniature silicon housing facilitates both these requirements.

Silicon (planoconvex) or glass lenses can be part of the housing and/or integrated into receptacles. With a collimated beam design, the optical isolators required for DFB lasers in telecom can be integrated into the receptacle. For short-wavelength datacom applications, plastic receptacles are used with integrated lenses.

The optical interfaces formed by optical fibre connectors combined with the housing create TOSA or ROSA functional units. The ROSA also 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, an 850nm 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.

Silicon as a packaging material for data and telecom applications provides key benefits in electrical performance. The metallisation technology enables coplanar lines with well-controlled impedance and low propagation losses. This holds also for the combination of lines with vias, even up to frequencies of 40GHz. For example, the optical to electrical transfer function of a ROSA module shown in Figure 1 has a 3dB bandwidth at 8.5GHz dominated by the TIA and the photodiode. The transmission lines of the package do not distort the signal. Figure 2 shows the eye diagrams for a bit-rate of 10.7Gb/sec measured at two different power levels. The sensitivity of the ROSA module depicted in Figure 3 is 19.24dBm (10.7Gbit/s, PRBS23). For certain designs, it is possible also to integrate the laser driver and a bias T very 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 1nF and resistors for termination of impedance matched lines.

Thermal performance
Silicon as a substrate material is an excellent heat conductor and with a room-temperature thermal conductivity of 148W/m-K 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 60µm. Alternatively, if better heat spreading is required, the design can be reversed so that the components are mounted e.g. onto a 400µm-thick silicon section. Generally, the design freedom offered by the package allows 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 thermoelectric cooler (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 the back of the flex. The heat sink, being a part of the TOSA, can be attached to the transceiver housing.

Modern SMD technology using tight solder bump/thermal via spacing on the flex allows for efficient heat transfer through the flex as shown in thermal simulations. These show a heat load of 500mW conducted from the silicon part to the heat sink through an area of approximately 10mm2, 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 metallisation on the backside or work with a cut-out in the flex through which a heat sink is attached.

Authors:
Jochen Kuhmann, Hymite chief technology officer (CTO) and founder; Arnd Kilian, head of design; Ralf Hauffe, senior engineer; and Gordon Elger, head of the back-end group.
Contact jk@hymite.com


Reference:
1. Lightwave magazine and website, June 8, 2004. "Optical chip and module manufacturers release common specifications for 10-Gbit/sec devices based on the Miniature Device MSA for XFP TOSA and ROSA." The group is composed of Eudyna Devices, Mitsubishi Electric, Oki Electric Industry, Opnext and Sumitomo Electric Industries.