Stability and effect of annealing on the optical properties of plasma-deposited Ta2O5 and Nb2O5 films

Abstract Tantalum and niobium oxide optical thin films were prepared at room temperature by plasma-enhanced chemical vapor deposition using tantalum and niobium pentaethoxide (M(OC 2 H 5 ) 5 ) precursors. We studied the evolution of their optical and microstructural properties as a result of annealing over a broad temperature range from room temperature up to 900 °C. The as-deposited films were amorphous; their refractive index, n , and extinction coefficient, k , at 550 nm were n = 2.13 and k − 4 for Ta 2 O 5 , and n = 2.24 and k − 4 for Nb 2 O 5 . The films contained a small amount of residual carbon (∼ 2–6 at.%) bonded mostly to oxygen. During annealing, the onset of crystallization was observed at approximately T C1 = 650 °C for Ta 2 O 5 and at T C1 = 450 °C for Nb 2 O 5 . Upon annealing close to T 1 (300 °C for Nb 2 O 5 and 400 °C for Ta 2 O 5 ), n at 550 nm decreased by less than 1%. This was correlated with the decrease of carbon content, as suggested by Fourier transform infrared spectroscopy, elastic recoil detection and static secondary ion mass spectroscopy (SIMS) results. During annealing, we observed phase transition from the δ- (hexagonal) phase to the L - (orthorhombic) phase between 800 °C and 900 °C for Ta 2 O 5 , and between 600 °C and 700 °C for Nb 2 O 5 . The structural changes were also marked by silicon diffusion from the substrate into the oxide layer at annealing temperatures above 500 °C for Ta 2 O 5 and above 400 °C for Nb 2 O 5 . As a consequence of oxygen, silicon and metal interdiffusion, the interface between the Si substrate and the metal oxide (Ta 2 O 5 or Nb 2 O 5 ) is characterized by its broadening, well documented by spectroscopic ellipsometry and SIMS data.