YMnO3 thin films have been sputtered on Si(1 0 0) substrates under different ambient conditions. Microstructures of the YMnO3 thin films have been investigated with transmission electron microscopy (TEM) after a rapid thermal annealing process at 850degreesC in N-2 ambient. The YMnO3 film deposited in Ar ambient had random orientations. However. two layers were apparently formed in the YMnO3 film deposited in Ar + O-2 ambient. One was a c-axis oriented YMnO3 layer in the top region and the other was a random oriented YMnO3 layer in the bottom region, which was clearly elucidated by a dark-field TEM image. As the c-axis oriented YMnO3 layer was formed on the poly-YMnO3 layer. stress by thermal expansion difference was relieved and no crack was formed, The memory window was improved due to the partial c-axis oriented YMnO3 layer. (C) 2002 Elsevier Science B.V. All rights reserved.