Constitution and crystallization behaviour of ultrathin physical vapor deposited (PVD) Al 2 O 3 /SiO 2 laminates

Abstract

Abstract Ultrathin Al 2 O 3 /SiO 2 multilayers were produced by physical vapor deposition (PVD) using a double source jumping beam PVD-coater. Al 2 O 3 /SiO 2 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 Al 2 O 3 /SiO 2 laminates are non-crystalline and display periodical contrast modulations in TEM cross-sections as long as the nominal thickness of the Al 2 O 3 /SiO 2 double-layer is >5 nm. EDX line scans and 29 Si–MAS–NMR spectroscopy provide evidence of nanosized pure SiO 2 and pure Al 2 O 3 layers. XRD analyses show that films consisting of 30 nm thick Al 2 O 3 and SiO 2 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 Al 2 O 3 and SiO 2 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 Al 2 O 3 /SiO 2 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 Al 2 O 3 and SiO 2 layers, and formation of homogeneization zones large enough for mullite nucleation (about 2 to 5 nm in size).