Crack formation due to annealing of Al 2 O 3 films grown on Si(100) by MOCVD
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Crack formation due to annealing of Al2O3 films grown on Si(100) by MOCVD M. P. Singh and S.A. Shivashankar Materials Research Centre, Indian Institute of Science, Bangalore –560 012, India Email: [email protected], [email protected] ABSTRACT We report crack formation in alumina films grown on Si(100), caused by annealing in a controlled oxidizing ambient. The films were grown in a low-pressure CVD reactor, using aluminium acetylacetonate as precursor. High purity argon and nitrous oxide were employed as carrier and oxidizing gas, respectively. The films were characterized by optical microscopy and SEM/EDAX. The proportion and chemical nature of the heteroatoms, namely C and H, incorporated into the films from the precursor, were characterized by XPS, and FTIR. Asdeposited films do not exhibit any cracks, while post-deposition annealing results in cracks. Apart from the delamination of the films, annealing in nitrous oxide ambient leads to an unusual crack geometry, which we term the ‘‘railway-track’’. These twin cracks are very straight and run parallel to each other for as much as several millimeters. Often, two such linear tracks meet at exactly 90°. Between some of these tracks lie bullet-like structures with very sharp tips, oriented in a specific direction. As cracks are generally activated by residual stress, both thermal and intrinsic, the origins of the stresses that generate these linear cracks are discussed. The redistribution of stress, arising from the removal of C and H during annealing, will also be discussed. An attempt has been made to correlate the formation of cracks with the crystal structure of the film. INTRODUCTION Coatings and thin films of alumina, Al2O3, are extensively used in various fields of technology, such as in optical devices and cutting tools. Moreover, Al2O3 is a promising candidate to replace SiO2 as the gate dielectric in integrated electronics of the next generation. Therefore study of thin films of alumina on Si has received much attention recently. The properties of coatings and thin films depend on the structure (i.e., crystallographic phases, microstructure, defects, interface quality, and chemical stability) and the concentration of heteroatoms present in the bulk of the film. Hence, a study of morphological and compositional evolution of thin films containing such heteroatoms becomes important in identifying and understanding features that influence material properties which, in turn, influence the quality of devices made therefrom. Therefore, a study of the microstructure variation of MOCVD-grown Al2O3 films with processing parameters is warranted. Crack formation and its relationship with growth processes have been studied in detail [1-4]. There are reports suggesting that films grown by low-pressure chemical vapour deposition (LPCVD) have a higher residual stress than films grown by other techniques [1-3]. Reports indicate that thin films of alumina on stainless steel, deposited by MOCVD using aluminium trisec-butaoxide as precursor, suffer from
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