Stability and effect of annealing on the optical properties of plasma-deposited Ta_2O_5 and Nb_2O_5 films

摘要

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_2H_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℃. The as-deposited films were amorphous; their refractive index, n, and extinction coefficient, k, at 550 nm were n = 2.13 and k < 10~(-4) for Ta_2O_5, and n=2.24 and k < 10~(-4) forNb_2O_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℃ for Ta_2O_5 and at T_(C1)=450℃ for Nb_2O_5. Upon annealing close to T_1 (300℃ for Nb_2O_5 and 400℃ for Ta_2O_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℃ and 900℃ for Ta_2O_5, and between 600℃ and 700℃ for Nb_2O_5. The structural changes were also marked by silicon diffusion from the substrate into the oxide layer at annealing temperatures above 500℃ for Ta_2O_5 and above 400℃ for Nb_2O_5. As a consequence of oxygen, silicon and metal interdiffusion, the interface between the Si substrate and the metal oxide (Ta_2O_5 or Nb_2O_5) is characterized by its broadening, well documented by spectroscopic ellipsometry and SIMS data.