Low emissivity high-temperature tantalum thin film coatings for silicon devices

The authors study the use of thin (⁠$∼230 nm$⁠) tantalum (Ta) layers on silicon (Si) as a low emissivity (high reflectivity) coating for high-temperature Si devices. Such coatings are critical to reduce parasitic radiation loss, which is one of the dominant loss mechanisms at high temperatures (above $700 °C$⁠). The key factors to achieve such a coating are low emissivity in the near infrared and superior thermal stability at high operating temperatures. The authors investigated the emissivity of Ta coatings deposited on Si with respect to deposition parameters, and annealing conditions, and temperature. The authors found that after annealing at temperatures $≥900 °C$ the emissivity in the near infrared (⁠$1−3 μm$⁠) was reduced by a factor of 2 as compared to bare Si. In addition, the authors measured thermal emission at temperatures from $700$ to $1000 °C$⁠, which is stable up to a heater temperature equal to the annealing temperature. Furthermore, Auger electron spectroscopy profiles of the coatings before and after annealing were taken to evaluate thermal stability. A thin (about 70 nm) $Ta2O5$ layer was found to act as an efficient diffusion barrier between the Si substrate and the Ta layer to prevent Si diffusion.