Optoelectronics packaging advances make PNT devices more robust for harsh environments

The US military relies on reliable positioning, navigation, and timing (PNT) devices for its day-to-day operations, as well as missions around the world. And PNT devices are increasingly being used in harsh environments such as space — where they deal with high mechanical shock, vibration, and huge temperature changes — or others with high humidity or dust.

This can lead to performance issues because the way optoelectronics embedded in PNT devices are traditionally packaged and assembled is not suitable for harsh environments. It involves robotic alignment and bonding of various optical components and miniature mounts to a ceramic substrate or bench, using tacky adhesives sensitive to their environment.

To address this problem, Draper engineers in Cambridge, MA, developed a new packaging and assembly method to make optoelectronics more robust and reduce complexity, labor requirements and manufacturing costs.

The team’s progress in materials and methods for optoelectronic packaging “was driven by the continuing need for smaller size, higher performance, increased robustness and improved manufacturability for PNT solutions for a wide variety of missions and platforms”, says Gilbert Feke, senior member of Draper. of technical personnel working on photonics systems.

In many cases, “the key optical components of PNT devices are quite unique and application-specific, such as wound fiber coils used for interferometric fiber optic gyroscopes (IFOGs) and the vapor cells for atomic optical clocks,” says Feke. †

Draper and others improved the design of these components because the mission and platform must continue in harsh, highly dynamic environments. “But the design and packaging of the general-purpose components, such as lenses and beam splitters, are equally important to the advancement of these PNT devices, as they must work in tandem with the esoteric components to achieve single-digit operation of the device. parts per million or better precision in harsh environments,” he explains.

For example, it is well known that broadband multilayer dielectric beam splitters “are not perfectly spectrally flat, but have residual spectral non-uniformity that depends on the angle of reflection,” Feke adds. “Small changes in the beamsplitter’s angular alignment result in spectral shifts at the parts-per-million level, resulting in a proportional measurement error.”

To meet this challenge, the engineers turned to Draper’s in-house microfabrication facility to fabricate and optimize silicon optical benches (SiOBs). “SiOBs are wafer-level platforms composed of precisely etched surface features such as wet-etched v-grooves and pyramidal pits, and deep reactive ion-etched (THREE) trenches,” says Feke.

These surface features efficiently and directly locate and integrate discrete microscale optical components without the need for additional mountings. “Our method makes it possible to drop multiple lenses and beam splitters onto the etched silicon topography, along with pieces of solid, low-melting temperature glass frit prefabricated into 3D shapes such as rectangular blocks and circular donuts,” explains Feke. “They are designed to optimally register and anchor the optical components, then fuse with the SiOBs simultaneously within minutes during a single melting/cooling step.” (See Fig. 1.)

Furthermore, they lithographically define v-grooves located with respect to the pyramidal wells, the purpose of which is to align the optical fiber assembled in the v-grooves with respect to the spherical lenses assembled in the wells (see figure 2). “Our designs are suitable for both face-polished fibers where the v-groove is centered in relation to the pyramidal pits so that the fiber is coaxial with the ball lens, and for corner polished fibers where the v-groove is slightly off-center respecting the pit to to account for refraction on the corner-polished surface,” he says.

The new process replaces the time-consuming and high cost of ownership combination of precision robotics and liquid adhesives, where gripping, loosening and drifting of the optical components during adhesive curing is often problematic.

A die-attach challenge and optical self-assembly

Silicon optical bench mold must be pre-attached to a ceramic substrate before assembling the optical components, so a major challenge for the researchers was developing a die-attach method – thermocompression bonding – in which the materials are coated with metal films and features. “This is followed by the simultaneous application of heat and mechanical pressure to produce a metallurgical bond that can withstand the subsequent temperatures required for optical assembly,” says Feke.

One of the coolest aspects of this optoelectronic packaging work is the optical assembly method – it essentially assembles itself. “We could see under a microscope that once the starting materials were pre-positioned and then brought to temperature, the etched features in the silicon interact with the shapes and surface tension of the glass frit material to force the physical registration and then freeze the optical components. to the etched silicon features,” he says.

Optimization of these etched features is critical. “For example, our technique involves placing the reflective surface of the beam splitter against one of the four side walls of a rectangular DRIE slot,” explains Feke. “While the simplistic approach to defining such slits in silicon would be to introduce filled rectangular shapes into the photolithography layer, this results in sidewalls that are not perpendicular to the top surface of the silicon wafer, but are slightly angled, allowing the reflected beam from the plane.Instead of simple filled rectangular shapes, we etch rectangular outlines that define unetched rectangular islands that disappear at the end of the process.By optimizing the width of the etched outline, we can determine the perpendicularity of the sidewalls of the optimize resulting slots.”

These advances can be applied to rapid prototyping of robust PNT devices, where the concepts of rapid prototyping and robustness were previously contradictory. It can also reduce packaging costs and increase production scalability – both important goals for the military.

“We look forward to engaging with current and new customers to identify and execute opportunities for our optoelectronic packaging capabilities,” said Feke.

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