Investigations on the removal mechanisms of diverse alumina based polishing slurries for chemical mechanical polishing of electro-plated NiFe 45/55

Abstract

Magnetic micro electro–mechanical systems (MEMS), such as magnetic sensors or actuators, are used in various areas of application, including bio-medical, wireless communication or power supplies. Many functional elements of these systems are made of soft magnetic materials. The increasing demands on miniaturization and higher efficiency of these functional elements require the application of thin-film structuring (photolithographic patterning) and electroplated magnetic materials. As a result, micro patterns filled with relatively thick magnetic materials (∼10–40 μm) are desired. However, the deposition of thicker soft magnetic layers (NiFe, CoFe) is restricted by a strong increase in roughness (edge overflow) as well as the formation of internal stress resulting in warping or delamination of electroplated structures. An effective technique that reduces surface roughness and thus allows the manufacture of multi-layered, multi-material systems is the planarization of each layer by chemical mechanical polishing (CMP). CMP of common semiconductor materials, such as silicon and its oxides, copper and tungsten is well-investigated, but there is almost no published research on CMP of soft magnetic materials, especially regarding iron based alloys (NiFe, CoFe). Therefore, the development of a reliable and efficient CMP process for these materials can be a crucial drive for improving functionality, decreasing dimensions and extending the application field of magnetic MEMS. In this paper, three commercial alumina based polishing slurries with different chemical compositions are tested. Electroplated NiFe 45/55 thin-film structures on silicon are polished under constant experimental conditions. During the experiments, surface roughness and material removal rate (MRR) are determined. In order to expose the underlying effects of material removal for each of the slurries, randomly selected sections of the NiFe surface are analyzed in detail using atomic force microscopy (AFM) before and after the experiments. The impact of the chemical components of the slurries on the mechanical properties of exposed surfaces is investigated using nanoindentation techniques. The results permit to identify the impact of the composition of the used slurry on the determinant removal mechanism in CMP of NiFe 45/55. These results can be applied as well to most technically used NiFe alloys.