In situ growth stresses in iron-platinum and iron-copper alloy
Intrinsic thin film stress is evitable with the thin film deposition process and plays an important role in tuning the physical properties of thin films. In this thesis, the in situ and post growth stress evolution of the Fe-Pt and Fe-Cu alloy system was studied and correlated to the microstructure evolutions. At ambient temperature and constant deposition pressure, the growth stresses of both the Fe-Pt and Fe-Cu alloy were found to be dependent on the compositions and affected by their growth rates. The final intrinsic stress states after growth could be tuned to be either tensile, zero or compressive depending upon composition and deposition rate for similar grain sizes. This is due to the preferential segregation of one species (the more mobile element) to the grain boundaries. At elevated growth temperatures, the Fe-Pt alloy forms ordered phase while the Fe-Cu alloy forms phase separation. The magnitude of the compressive stress state is reduced as the Fe54Pt46 thin film orders in situ during growth. The compressive stress relaxation rate is increased with increasing substrate temperature or order parameter. This compressive stress reduction has been rationalized as a reduction of adatom mobility on the surface as Fe and Pt occupy specific lattice sites for L10 on each grain. The ordered nature of the grains contributes to additional chemical energy at the boundary which, upon ceasing deposition, significantly increases the stress relaxation rate. In contrary, the growth compressive stress of the Fe51Cu49 alloys in the continuous growth regime is increased with substrate temperature. This has been rationalized as the migration of adatoms to thermodynamically preferred surfaces during growth.