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 angstrom/cycle at substrate temperatures ranging from 175 to 250 degrees 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. Published under an exclusive license by the AVS.