Atomic layer deposition of CaB2O4 films using bis(tris(pyrazolyl)borate)calcium as a highly thermally stable boron and calcium source

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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% non-volatile residue. Self-limited ALD growth was established at 350 °C with CaTp₂ and water pulse lengths of ≥2.0 and ≥0.3 s, respectively. An ALD window was observed from 300 to 375 °C, in which the growth rate was between 0.34 and 0.36 Å per 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 × 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.