Constitution and crystallization behaviour of ultrathin physical vapor deposited (PVD) Al2O3/SiO2 laminates

Ultrathin Al2O3/SiO2 multilayers were produced by physical vapor deposition (PVD) using a double source jumping beam PVD-coater. Al2O3/SiO2 multilayer formation is controlled by the electron beam jumping frequency yielding double-layer thicknesses of about 2, 5, 9, and 30 nm. The as-deposited Al2O3/SiO2 laminates are non-crystalline and display periodical contrast modulations in TEM cross-sections as long as the nominal thickness of the Al2O3/SiO2 double-layer is >5 nm. EDX line scans and 29Si–MAS–NMR spectroscopy provide evidence of nanosized pure SiO2 and pure Al2O3 layers. XRD analyses show that films consisting of 30 nm thick Al2O3 and SiO2 layers at 1000°C form transition alumina only. Transition alumina plus minor amounts of mullite appear at 1000°C in alumino silicate coatings with intermediate Al2O3 and SiO2 layer thickness (5 and 9 nm), while only mullite occurs in samples with 2 nm thick compositional modulations. The crystallization of PVD-produced alumino silicate films with double layer thicknesses >5 nm behaves similar to diphasic (type II) mullite precursors, while Al2O3/SiO2 double-layers 2 nm thick behave like single phase mullite precursors (type I). The latter is surprising because of the diphasic character of the double layers. Obviously, two conditions are required for mullite formation at 1000°C: Interdiffusion-produced chemical homogeneization between Al2O3 and SiO2 layers, and formation of homogeneization zones large enough for mullite nucleation (about 2 to 5 nm in size).