Electrodeposition and characterisation of thin films for the fabrication of microinductors
Stress in electrochemically deposited (ECD) magnetic films is an important parameter that can have a critical effect on the performance of MEMS devices such as microinductors. This is especially the case when thick layers of materials are required and where it is important to monitor and hence control stress to prevent cracking and delamination. The reliability of devices, therefore, deeply depends on process parameters and conditions used in depositing these materials on silicon wafers. A MEMS technique for measuring stress spatially around such a wafer has been developed and used to characterise the materials involved in the fabrication of a microinductor. This thesis discusses the design and fabrication of test structures, along with a custom built automatic measurement technique to wafer map the spatial variation of strain, on any sized wafers. The effect of agitation on the grain structure of NiFe has been observed to affect strain which were spatially mapped and correlated with the film composition and thickness. Film uniformity were also shown to improve in the absence of agitation in the bath. To further understand the fundamentals of ECD small scale beaker level galvano-static experiments have been employed to use the same test structures fabricated on small Si chips. The effects of hydrogen evolution on film stress and efficiency with the inclusion of boric acid and saccharin, have been discussed. It was concluded that the tensile stress developed in Ni and NiFe films have an inversely proportional relationship with the plating efficiency. The characterisation of electrodeposited copper films is also of importance as copper films are integrated with magnetic materials in the form of windings for microinductors. The variations in recrystallization and evolution of grains of ECD copper, is for the first time demonstrated spatially using the test structures. The effect of additives in bath on film uniformity was investigated and it was observed that with carrier and additive together the three phases of self annealing were more pronounced. Finally the use of these strain test structures have been demonstrated on thick polymer SU-8 films, which is employed as a structural material in microinductors. The effect of UV exposure dose on the cross linking properties of SU-8 has also been studied. It was observed that non-uniformity in the coated film thickness over the wafer can cause variations in the UV exposure during photolithography that effects the cross linking of the polymer hence, inducing different levels of tensile stress in the material. This unique methodology has therefore opened up many possibilities and can be used for characterising newer materials employed in MEMS, fine-tuning the manufacturing processes to achieve set goals in terms of material properties as well as uniformity and gaining a better understanding of the influence of processing conditions on the produced films.