Microstructural patterning of thin films using extrinsic seed crystals

Arizona State University, Scottsdale AZ

Investigators

Abstract

Non-Technical Summary Thin films have a wide variety of applications ranging from computer chips and solar cells to cutting tools, optical lenses and biomedical devices. Depending on the application, the thin films need to exhibit specific mechanical and physical properties. For example, thin metallic films used in computer chips must have high electrical conductivity and be able to operate at elevated temperatures without failure. These properties are determined to a large extent by the microstructure (size, shape and orientation of crystals) of the thin films. Therefore, if the microstructure of the thin films can be controlled, their properties can be altered as desired, which will lead to considerable improvement in their performance. The ultimate result would be lower power consumption in electronic devices, greater efficiency in solar panels, higher lifetimes for cutting tools, to name a few. This award supports fundamental research that enables the fabrication of metallic alloy films with precisely controlled microstructures and addresses critical questions related to how crystals form and grow in metallic alloys. The research activities are integrated with education and outreach efforts that include a mentoring program for undergraduates, hands-on activities and demonstrations for school students to encourage them to pursue careers in materials science and engineering, and training of graduate students in materials synthesis and characterization. Technical Summary Designing thin films with unique physical and mechanical properties requires explicit control over a wide range of microstructural parameters including the mean grain size, grain size dispersion, texture and phase composition. But despite substantial advances in thin film processing techniques, our ability to independently tune multiple microstructural parameters is still limited. This fundamental problem is addressed by the development of a novel synthesis method based on the following hypothesis: Amorphous precursor films embedded with appropriate seed crystals can be controllably crystallized to obtain precisely tailored microstructures. To validate the proposed hypothesis, amorphous NiTi films embedded with carefully chosen seed crystals are synthesized by magnetron sputtering and crystallized by controlled thermal annealing. A combination of advanced transmission electron microscopy techniques are used to reveal how the spatial distribution and orientation of seed crystals in the amorphous films influence the final microstructure the crystallized films. Apart from tailoring the overall grain size distribution, texture and phase composition of thin films, the method can be used for location specific variation of these parameters, which enables local control of the mechanical and physical properties. Education and outreach activities include the Materials Research Ambassadors (MRAs) program through which a diverse set of undergraduate students are recruited to work on this project and perform outreach activities. The MRAs engage school students visiting Arizona State University (ASU) with hands-on activities, and visit local middle schools to perform demonstrations and introduce students to materials research. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.