AbstractIn this work we investigated the influence of the AlN material quality on the etching rate in KOH‐based solutions. Thus, AlN layers were deposited by three different methods on sapphire and silicon substrates (i) by metal organic chemical vapor deposition (MOCVD), (ii) by molecular beam epitaxy (MBE), and (iii) by reactive sputter deposition. The etch rate is strongly dependent on crystal quality and etch temperatures. The high quality MBE‐AlN could be etch anisotropic with a preferred lateral component in [11$ \bar 2$0] direction at 60 °C while the polycrystalline AlN layers we etched isotropic and homogenously already at room temperature. The wet chemical etching in KOH solution is mainly an etching along defects and grain boundaries. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)