Temperature resistant anti-reflective coating on Si-wafer for long-wave infra-red imaging

Area of Science:

• Materials Science
• Optical Engineering
• Semiconductor Devices

Background:

• Hermetic sealing of microbolometers using silicon lids and CuSn bonding is crucial for low-cost thermal cameras.
• Uncoated silicon surfaces exhibit significant reflection (~30%) in the long-wave infrared (LWIR) spectrum, necessitating anti-reflective treatments.
• Conventional multi-layer anti-reflective coatings are unsuitable due to thermal expansion mismatches during the high-temperature (270°C) CuSn bonding process.

Purpose of the Study:

• To develop a novel anti-reflective coating for silicon microbolometer lids.
• To ensure the coating maintains anti-reflective properties after thermal cycling up to 300°C.
• To enable effective hermetic sealing for microbolometer applications.

Main Methods:

• A two-layer anti-reflective coating was designed using Zinc Sulfide (ZnS) and Yttrium Fluoride (YF3).
• The coating layers were deposited onto silicon substrates at a relatively low temperature of 100°C.
• The coating's performance and thermal stability were evaluated through transmission measurements and thermal cycling tests.

Main Results:

• The developed ZnS/YF3 coating demonstrated excellent thermal stability, retaining its properties after heat cycling to 300°C.
• The coated silicon samples exhibited an average increase in transmission of 30% in the 8-12 μm wavelength range compared to uncoated silicon.
• The low-temperature deposition process is compatible with the requirements of CuSn solid-liquid interdiffusion bonding.

Conclusions:

• A robust, two-layer anti-reflective coating (ZnS/YF3) has been successfully developed for silicon microbolometer applications.
• This coating overcomes the limitations of traditional coatings, enabling high-temperature hermetic sealing processes.
• The improved transmission characteristics enhance the performance of low-cost thermal cameras utilizing microbolometer technology.