Atomic layer deposition allows layer-by-layer growth of conformal and virtually defect free thin films made of e.g. Al2O3, TiO2 or other materials. Industry targets improvement of ALD productivity through spatial ALD setups and through reducing gas purge times without losing the favourable film properties to penetrate new cost sensitive markets and larger surface areas. Thereby, the acceleration of the process is limited by the fact that no contact of the two involved precursor gases (e.g. trimethylaluminium (TMA) and water for Al2O3 layers) in gas phase may occur and further the gases must be given sufficient time to settle on the surface as monolayer. If that is not the case, the layer thickness might be inhomogeneous with a higher thickness near the gas nozzles in the ALD machine. Figure 13 shows a thickness distribution of an Al2O3 layer where the gas distribution was not optimized. The image has been acquired by sequentially measuring the thickness on 360 spots along the visible grid with Spectrocopic Ellipsometry.