Chip Packaging for Harsh Environments Photonics Chips Are Molecularly Fused Instead of Glued

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Photonic integrated circuits should survive radioactive radiation or cryogenic temperatures undamaged. A special packaging process is designed to make the fiber-chip connection more secure.

Photonic integrated circuits (PICs) transmit data at extremely high speeds with minimal energy consumption. However, their use in demanding environments has so far been severely limited. The reason lies in the assembly and interconnection technology (AVT) and, more precisely, in the fiber-chip coupling. Researchers at the US National Institute of Standards and Technology (NIST) have now presented a packaging process that enables PICs to withstand even the most extreme conditions such as strong radioactive radiation, ultra-high vacuum or cryogenic temperatures without damage.

Organic Adhesives As A Weak Point

A good AVT protects the chip and connects it reliably to the outside world, for example with optical fibers and electrical contacts. Packaging is particularly critical for PICs, as the sensitive optical connections must remain perfectly aligned.

However, previous standard adhesives based on organic polymers tend to crack, outgas or degrade under extreme conditions. If this bond fails, the chip loses its function. This rules out conventionally packaged PICs for many future and niche markets: Quantum computers require temperatures close to absolute zero and ultra-high vacuum. Space missions or particle accelerators involve high radiation exposure, while industrial and energy applications often involve enormous heat and pressure.

Hydroxide Catalysis Bonding (HCB)

To make the fiber-chip connection more resistant, the NIST scientists used a method originally developed by NASA for the assembly of highly stable optical telescope systems: hydroxide catalysis bonding (HCB).

Instead of relying on organic adhesives, HCB creates an inorganic, glass-like chemical bond between the glass fiber and the photonic chip. The process uses a tiny amount of sodium hydroxide solution to fuse the surfaces together at a molecular level. The result is a highly stable, rigid bond. The NIST team was able to demonstrate for the first time that HCB not only enables the precise alignment and efficient light coupling required for photonic circuits, but also forms an extremely robust package.

The assembly was subjected to a series of load tests:

• cooled to cryogenic temperatures,
• exposed to massive temperature shocks,
• bombarded with intense ionizing radiation and
• under high vacuum.

The result: the HCB-bonded fiber connection remained intact and the chip functioned flawlessly. Further studies also showed that the HCB-based connection remains mechanically stable even at temperatures far above what conventional adhesives can withstand (even if commercial optical fibers were still the limiting factor for the overall test).

"This approach creates a connection that is as resistant as the glass fiber itself," explains NIST physicist Nikolai Klimov, who led the project. "This means that photonic integrated circuits can now be used in areas that were previously simply closed to them."

Outlook for Production

Currently, the HCB bonding process still takes several days. According to the researchers, however, this is merely an engineering challenge and not a physical limit. With targeted further development of the processes, the time could be drastically reduced, which would also make the technology attractive for the mass production of PICs in the future. (heh)

(Source: Based on publications from NIST / Photonics Research)

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