Atomic Layer Deposition of Metal Fluorides Using HF–Pyridine as the Fluorine Precursor

The atomic layer deposition (ALD) of a variety of metal fluorides including ZrF4, MnF2, HfF4, MgF2, and ZnF2 was demonstrated using HF from a HF–pyridine solution. In situ quartz crystal microbalance (QCM) studies were utilized to examine the growth of these metal fluorides. ZrF4 ALD using tetrakis(ethylmethylamido) zirconium and HF as the reactants was studied as a model system. The QCM measurements determined a mass gain per cycle (MGPC) of 35.5 ng/(cm2 cycle) for ZrF4 ALD at 150 °C. This MGPC was equivalent to a growth rate of 0.9 Å/cycle at 150 °C. MnF2, HfF4, MgF2, ZnF2, and additional ZrF4 were also grown using bis(ethylcyclopentadienyl) manganese, tetrakis(dimethylamido) hafnium, bis(ethylcyclopentadienyl) magnesium, diethylzinc, and zirconium tetra-tert-butoxide as the metal precursors, respectively. The growth rates for MnF2, HfF4, MgF2, ZnF2, and ZrF4 ALD were 0.4, 0.8, 0.4, 0.7, and 0.6 Å/cycle, respectively, at 150 °C. All of these metal fluoride ALD systems displayed self-limiting reactions. Ex situ measurements of the growth rates using X-ray reflectivity and spectroscopic ellipsometry analysis agreed with the in situ QCM measurements. Analysis of the QCM mass changes after the individual metal precursor and HF exposures quantified HF adsorption after the HF reaction. The ZrF4 and HfF4 films acted as strong Lewis acids and adsorbed an average of two HF per deposited MFy species after the HF reaction. In contrast, the MnF2, MgF2, and ZnF2 films all behaved as weak Lewis acids and did not adsorb HF after the HF reaction. The refractive indices of the metal fluoride films were in agreement with previous optical measurements. Most of the metal fluoride films were crystalline as measured by X-ray diffraction studies. The majority of the metal fluoride films also had high purity as established by X-ray photoelectron spectroscopy studies. This pathway for metal fluoride ALD using HF–pyridine as the fluorine precursor will be useful for many applications of metal fluoride films such as optical coatings in the ultraviolet wavelength region.