α to γ phase transformation in electrodeposited Invar film by short pulse laser treatment

Bulk Invar (64 wt% Fe:36 wt% Ni) is known for its extraordinarily low coefficient of thermal expansion (CTE). The low CTE makes the material especially attractive for applications, which require minimum deformation caused by thermal mismatch such as Invar films on silicon for micro electro-mechanical systems (MEMS). However, Invar films produced by sputtering and electrodeposition at room temperature do not consist of the desired fcc γ FeNi phase but mainly of the α phase with bcc structure. An additional heat treatment is necessary in order to obtain only the γ phase which may be incompatible with the thermal stability of the MEMS structures. This paper reports preliminary experimental results of Nd:YAG Q-switch pulsed-laser surface treatment of α Invar thin films deposited on silicon substrates which avoids this problem. The short laser pulses limit the heated zone to a thickness of several hundreds of nm so that the underlying structures are not affected. Electrodeposited α Invar films of different thickness have been irradiated under protective atmosphere with radiant exposures of 1 J/cm2 and higher, applying different numbers of laser pulses. Subsequent XRD measurements showed that with increasing number of pulses, the α to γ transformation increases progressively. For the same pulse number, the calculated average degree of transformation decreases with increasing film thickness. This indicates that the transformation reaches a certain maximum depth, which is determined by the total pulse number. XPS analysis revealed no significant oxidation of the film. Texture analysis of as-deposited and laser-treated films suggests that the transformation takes place in the solid state. To better understand the transformation mechanism, the progress of the transformation has been evaluated quantitatively. The results indicate that the transformation is diffusion controlled but also shows an orientation relationship between parent and transformed phase like a martensitic transformation.