Residual stress in nitride thin films: integrated experiments and development of a predictive model

Brown University, Providence RI

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: Nitride thin films are used in a wide variety of applications. An example is the hard coating that is put on a tool bit to extend its lifetime. Such coatings are made by depositing atoms on the tool's surface from a vapor, referred to as physical vapor deposition. The resulting films often have large stress because the process of deposition does not allow the atoms time to get into their ideal positions. Consequently, this stress can lead to failure of the film either by cracking or delaminating. The goal of this research is to reduce the stress through understanding how the stress is related to the underlying physical processes. Through the use of a mathematical model of the processes affecting stress, it is possible to predict it for different processing conditions. TECHNICAL DETAILS: The focus of this work is to understand and control the stress in transition metal nitride thin films. Stress can be strongly affected by the processing conditions such as the energy of the deposited species or rate of film growth, but there is not a fundamental understanding of how stress reduction occurs. In this project, measurements of the stress are made during the nitride deposition using a real-time technique (based on the curvature induced in the wafer substrate). These measurements enable the stress to be quantitatively related to the processing parameters such as the growth rate and gas pressure. After growth, the film's microstructure is characterized by making a cross-section with a focused ion beam and measuring the grain structure with electron microscopy. This characterization enables the dependence of the stress on the film's grain size and microstructural evolution to be determined. The experimental results are used to refine and validate a model that relates the stress to the underlying processes in the film. This new model incorporates the kinetics of the deposited atoms and the defects that have not been included in previous models. This approach enables a deeper understanding of the origins of stress that can be used by thin film growers to predict the stress under various conditions for different materials systems. The model is made available to interested users on the researcher's website. A short course for industrial and academic professionals describes the mechanisms that control stress evolution. Students (at both the undergraduate and graduate level) participate in this research to develop skills in state-of-the-art experimental and modeling techniques. Students from underrepresented groups are recruited to participate in this research and promote their ability to develop careers in science and engineering.