Thermally induced damages of PECVD SiN x thin films
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John Dell School of Electrical, Electronic and Computer Engineering, The University of Western Australia, Crawley, Western Australia 6009, Australia (Received 12 April 2011; accepted 18 July 2011)
This study investigates thermally induced structural damages to amorphous plasma-enhanced chemical vapor deposition (PECVD) SiNx thin films at elevated temperatures, including chemical structure, microstructure, and physical integrity. The films were synthesized by means of PECVD method. Heating to elevated temperatures in air was found to cause multiple forms of chemical, structural, and physical damages. Chemically the films were found to oxidize and lose their nitrogen and hydrogen contents. Structurally the amorphous SiNx matrix was found to convert partially into SiO2 as a result of oxidation and to crystallize into Si3N4 crystallites. The physical damages include pinholes, circular “penny” cracks, random “dry mud” cracks, and spalling. The types of the damages were observed in different temperature regimes. The formation of the penny cracks is attributed to excessive compressive stresses created in the film by oxidation, which is associated with a large volume expansion. The formation of the random cracks is attributed to tensile stresses caused by crystallization, which is associated with a large volume contraction. Such damages limit the suitable application conditions for devices made of these films.
I. INTRODUCTION
Silicon nitride thin films are commonly used as a structural material for microelectromechanical systems (MEMS).1–3 Among the commonly used chemical vapor deposition techniques, plasma-enhanced chemical vapor deposition (PECVD) is preferred over low-pressure chemical vapor deposition in the fabrication of MEMS devices that utilize temperature-sensitive materials, such as those integrated on group III–V and II–VI semiconductors.4,5 The extra energy supplied by the plasma in the PECVD process allows for low-temperature deposition of thin films. The process of PECVD synthesis of thin films, in particular SiNx thin films, involves complex chemical reactions in the plasma system. These reactions often involve extensive bond scrambling and are often incomplete, particularly at low deposition temperatures. Consequently, these films are often found to be amorphous, chemically nonstoichiometric to bulk Si3N4, and contain substantial amounts of bonded hydrogen.6–8 As a result, these films have been found to be chemically and physically unstable in certain environmental conditions.9–12 Changes in chemical and physical structures will have direct impact on the properties, subsequent performances, and long-term reliability of the MEMS devices that use these films. Therefore,
it is of critical importance to study the environmental stability of these films and to understand their physical integrity. This article reports on the thermal damages of PECVD SiNx films caused by heating to elevated temperatures in air. II. EXPERIMENTAL METHODS
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.201
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