Atomic layer deposition of calcium fluoride for barrier coating applications

We report on the development of an atomic layer deposition (ALD) process for calcium fluoride thin films using bis(N,N-di-i-propylformamidinato)calcium(II) and anhydrous HF as reactants. Deposition rates were observed to be linear versus the number of ALD cycles performed, varying between 0.4 and 0.3 Å/cycle at substrate temperatures ranging from 175 to 250 °C, respectively. The optical properties of the resulting nanocrystalline CaF2 films were characterized by spectroscopic ellipsometry and show good transparency down to a UV wavelength of 193 nm, and good agreement with reference refractive index parameters. One motivation for the development of this ALD process is for chemical barrier applications in fluorine-containing chemistries. The robustness of the coating was tested by reactive ion etching of CaF2 films deposited on silicon in an SF6 plasma, yielding an Si:CaF2 etch ratio of greater than 2000:1 at a plasma power of 300 W.