Atomic Layer Deposition of CaB₂O₄ Films Using Bis(tris(pyrazolyl)borate)calcium as a Highly Thermally Stable Boron and Calcium Source

The atomic layer deposition of CaB₂O₄ was carried out using bis(tris(pyrazolyl)borate)calcium (CaTp₂) and water as precursors. CaTp₂ melts at 280 °C, undergoes solid state thermal decomposition at 385 °C, and sublimed on a preparative scale at 180 °C/0.05 Torr in about 3 hours with 99.7% recovery and 0.2% nonvolatile residue. Self-limited ALD growth was established at 350 °C with CaTp₂ pulse lengths of ≥ 2.0 s. An ALD window was observed from 300 to 375 °C, in which the growth rate was between 0.34 and 0.36 Å/cycle. The thin film compositions were assessed by elastic recoil detection analysis (ERDA) and X-ray photoelectron spectroscopy (XPS). The B/Ca ratios for CaB₂O₄ films deposited at 275, 325, 350, and 400 °C were 1.84(11), 1.85(11), 1.89(13), and 1.42(10), respectively, as determined by ERDA. Within the ALD window, hydrogen concentrations ranged from 0.22(2) to 0.35(4) atom % and the carbon and nitrogen concentrations were below the detection limits. XPS analyses on representative CaB₂O₄ thin films showed all expected ionizations. X-ray diffraction experiments revealed that the as-deposited films were amorphous. The surface morphology was assessed by atomic force microscopy and scanning electron microscopy. The rms surface roughness of a typical 2 μm x 2 μm area for films deposited at 325 and 350 °C was 0.3 nm. Scanning electron micrographs of these films showed no cracks or pinholes.