osa manufacture moves into the future

As semiconductor laser packages become more of a commodity, changes in their manufacturing processes become necessary.

James F. Dormer, Lucent Technologies` Microelectronics Group

For many years, the assembly of complex, cooled laser packages has been based on the use of hand-assembly operations, relying on the artisan-like skill of the fabrication personnel. In some cases, limited mechanization is used for repetitive tasks.

However, as companies around the globe try to meet market-based price-reduction pressures and ever-increasing quality expectations, they must also implement changes in manufacturing processes.

At the Optoelectronics Div. of Lucent Technologies` Microelectronics Group, a new manufacturing facility, Laser 2000, was opened in January 1997. The facility focuses on the automated assembly and testing of optical subassemblies (osas) to be used as building blocks for a variety of Lucent`s and FiNet Technologies` product offerings. (FiNet Technologies is a partnership between Lucent Technologies and Furukawa Electric.)

The Laser 2000 manufacturing facility was conceived in 1995 as an independent project to re-engineer the manufacture of semiconductor laser packages. A team of engineers and scientists from the company`s manufacturing, development, and research areas were tasked with designing a family of products and an associated low-cost manufacturing facility capable of meeting the increasing market demand for semiconductor lasers.

A building block approach

To minimize manufacturing costs, the team struck a balance between the need to customize a product offering for a specific application and the need to cost-effectively manufacture and replicate the product. For laser package manufacturing, this was achieved through design and process standardization. The team defined a set of design rules and developed an equipment infrastructure that supports the assembly and test of the osa--the fundamental building block for the new product family. This approach is similar to that which is commonly found in the automotive industry: Different engine options are available for a given production model, or the same engine is used in multiple products. In this new laser facility, the "engine" of the laser package is the osa.

The osa consists of a semiconductor laser diode, a spherical ball lens, and a monitor photodiode mounted on a silicon optical bench (siob) as depicted above. The footprint and contact locations for the osa have been standardized across multiple product families. The substitution of different semiconductor lasers in the basic osa allows its use in a range of applications, from electroabsorptive-modulated lasers for high-speed digital applications to Fabry-Perot and distributed feedback lasers for uncooled flat-pack applications.

Standardizing osa size and probe contact position allows the device`s capabilities to be shared by multiple product families. Therefore, for a given package design, the "engine" can be chosen from a variety of sources without the need to redesign the equipment platform. Since cost minimization is a driving force in any manufacturing endeavor, this approach enables the greatest amount of product flexibility while maintaining the necessary conditions for efficient use of equipment.

Design simplification and manufacturability were paramount concerns in establishing the osa design. The generation of products replaced by the new design required the use of a separate laser submount and photodiode submount. These were removed from the new design, creating a planar assembly that can be rapidly assembled. In addition, the osa is now tested and characterized as an assembly, including the first optical element (spherical lens). This provides improved correlation with the final package performance tests, thus allowing for performance parameters to be determined before expensive packaging takes place.

osa assembly and characterization

Fabrication of the osa can be divided into two parts: the mechanical assembly steps and testing. During mechanical assembly, the laser diode, photodiode, and lens are attached to the siob. This epoxyless attachment is performed by a pick-and-place gantry robot using machine vision. Accurate force and temperature controls provide excellent process repeatability. Features designed into the siob permit placement accuracy on the order of a few microns, resulting in precision alignment of components. Such accurate alignment offers added benefits when the osa is assembled into a final package by providing a known location for the optical axis. This eliminates the need to do active alignment during internal package assembly.

In the next assembly step, ribbon bonds are added to connect both the laser diode and the photodiode to the siob. This integrated approach eliminates hand-construction of this subassembly, which was necessary with previous product generations. The automated process also obviates the need to bin parts associated with the stack-up tolerances encountered when using the multiple submounts.

Once the osas have gone through the mechanical assembly steps, they move in groups of 50 in a multiposition waffle pack to the test phase. These tests include a high-temperature, high-current stress test, followed by continuous-wave performance measurements for power and wavelength. The final step is a performance test specific to the application. For example, digital products typically undergo a modulated bit-error-rate test. These tests, once very labor-intensive, are now performed in completely automated facilities in which the technicians merely load and unload the waffle packs.

Because the size of the osa and the location of its electrical contacts have been standardized, all the different test facilities can use the same handling equipment regardless of the test being performed. This greatly simplifies maintenance and necessary spare parts inventory while yielding greater flexibility in handling a changing product mix. In addition, the assembly and test equipment procedures are completely independent of one another, although they "communicate" through a building-wide intranet. A central computer system tracks the complete history of the osa, including yields, test data, and component history.

The Laser 2000 facility was built to address the expanding needs of laser-component-based products. Because its equipment is so flexible, with few modifications the same platform can be used to build other products, such as integrated waveguide products. u

James F. Dormer is manager of the Manufacturing Realization Center, Optoelectronics Div., of Lucent Technologies` Microelectronics Group, Breinigsville, PA. He can be reached at [email protected].