The crystallization of thin silicon nitride (Si3N4) films deposited on polycrystalline SiC substrates was investigated by X-ray diffractometry as a function of annealing time. The amorphous Si3N4 films were produced by means of reactive r.f. magnetron sputtering. Annealing at temperatures between 1300 and 1700 degreesC led to the formation of crystalline films composed of alpha-Si3N4 and betaSi(3)N(4). The fraction of beta-Si3N4 in the films reaches approximately 40% at temperatures above 1550 degreesC. Both polymorphic modifications were formed simultaneously during the crystallization process. A transformation of alpha-Si3N4 to beta-Si3N4 could not be observed in the time and temperature range investigated. The crystallization process of amorphous Si3N4 can be described according to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) formalism, assuming a three-dimensional, interface controlled grain growth from pre-existing nuclei. The rate constants show an Arrhenius behaviour with an activation enthalpy of approximately 5.5 eV. (C) 2003 Elsevier B.V. All rights reserved.