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Micro/Nano Lithography

Process Is Everything

Fabricating wafer-based micro-optics enables high bandwidth in a small package.

From oemagazine January 2005
31 January 2005, SPIE Newsroom. DOI: 10.1117/2.5200501.0009

The 12-channel MTP connector has become a widely accepted, low-cost standard for parallel data communications. The molded ferrules at the center of the connector align 12 fibers in a row on 250-µm spacings with high precision and repeatability. The connector is well suited to the 4+4 format for 10 Gb/s data communications; for example, four center fibers remain dark, while four transmit channels and four receive channels run in parallel over a low-cost ribbon cable.

That's the theory, anyway. In practice, integrating transmit and receive functions into a form factor approximately 3-mm wide presents significant challenges. The MTP receiver and transmitter both use wafer-based micro-optics to achieve an optimal solution, with vortex launch on transmit and high-numerical-aperture relay on receive fibers. Our group achieved this design through wafer-based manufacturing of micro-optical elements.

The first fabrication step involves patterning the eight-level diffractive vortex elements (top left). Defocussed images of lenses on the second surface appear at left. The second step involves producing photoresist lenses (top right). Photoresist pedestals protect diffractive elements. The finished 3.2 mm-element (bottom), diced and coated, features fused-silica lenses adjacent to diffractive elements.

Vortex launch with diffractive optical elements reduces back reflection into the laser source and improves the bandwidth over multimode fiber links. In vortex launch, a diffractive element introduces a phase spiral, which creates an axial singularity in the beam. The propagating beam does not encounter an axial defect in multimode fiber or reflect back into the laser source. A typical vortex-launch design uses wafer-based manufacturing to fabricate several hundred 12-channel diffractive-vortex arrays on one side of a silica wafer, and corresponding refractive lenses on the other. Aligning these arrays with laser arrays in an MTP ferrule creates tiny, 12-channel transmitter assemblies.

Producing the 4+4 architecture in an integrated module requires fabrication of the transmit elements (diffractive vortex array) and receive elements (refractive microlens array) together on a single wafer. These design constraints present a unique micro-machining challenge because diffractive elements require thin-resist processes, and refractive elements require thick-resist re-flow. If the diffractive elements are made before the lenses, they must be protected during subsequent processing. Refractive microlenses formed by re-flow require extremely uniform thick-resist processing and controlled-selectivity etching. The thick resist can be uniformly applied over the 1 to 2 µm topography of the diffractive elements, but any protective structure would either perturb the resist uniformity or poison the selectivity control for the refractive transfer etch.

The process we devised involves first fabricating the eight-level diffractive elements, then patterning and re-flowing a second thick resist for the refractive elements. The resist lenses are chemically stabilized to make them resistant to subsequent developing. A third, very-thick resist is then coated over the resist lenses and patterned to protect the diffractive elements. The result is a surface with uniform-resist lenses adjacent to the very-thick-resist pedestals that protect the diffractive optical elements. Transfer etching completes the process to yield fused silica next to diffractive vortex elements (see figure). The monolithic 4+4 element features additional refractive lenses on the back surface. Because all center-to-center and back-to-front lens alignments are done lithographically, accuracy is a few microns. oe


Development of the 4+4 micro-optics was a team effort, led by Patricia Lee with substantial technical contribution from Tom Suleski, William Delaney, Harris Miller, Tres Brooks, and Jennifer Pagan.

Jennifer Plyler, Jim Morris, Alan Kathman
Jennifer Plyler is a senior manufacturing process engineer, Jim Morris is a principal optical engineer, and Alan Kathman is design manager at Digital Optics Corp., Charlotte, NC.