Design and Performance of Indium Seals for Size-Constrained Tunable Laser Spectrometers

Indium seals have been used extensively in ultra-high vacuum and cryogenic applications. Typically, these seals use indium alongside or in place of other metal gaskets in stainless-steel vacuum flanges, with some custom applications for flanges sealing directly with glass (optics or tubes). Here we present the design and performance of three pressed indium seals (99.99% In) between aluminum and 0.5” diameter sapphire optics, and aluminum and gold coated Kovar semiconductor packages (TO-66 and TO-39). Test fixtures were designed to mimic those of future tunable diode laser spectrometers for Earth, Planetary and manned spaceflight environmental monitoring applications. Successful high-hermeticity seals (< 10-10 cm3 He atm-1 s-1) were achieved for all seals formed with sufficient pressure applied to allow the indium to flow between mating surfaces. The hermeticity of the seals was maintained after temperature cycling ( 10 – 80 oC, 20 cycles), with the optical seals surviving extended duration tests (-55 – 85 °C, per MIL-STD-883). Semiconductor packages (TO-39) subjected to these extended tests saw a moderate increase in leak rate (ca. 5 × 10-9 cm3 He atm-1 s-1), however further testing showed that either the glass-metal package seals or the indium were affected (the sample size was too small to draw firm conclusions for future applications). Overall, these results suggest long-term survivability of indium seals for Kovar-aluminum and sapphire-aluminum interfaces (>10 years at 10-10 cm3 He atm-1 s-1), where the coefficient of thermal expansion differs by around ×4. The improvement of future design changes and applications for miniaturized tunable laser spectrometers are discussed.