**1. Introduction**

In microelectromechanical system (MEMS) and packaging technologies, high reliability of Cu metallization of glass substrates is strongly required. In the field of structural materials, fusion welding such as arc welding and laser welding is usually applied for dissimilar-metal or dissimilar-material welding, but in these welding methods, extremely high energy are needed for melting of metals, and it is difficult for joining of micrometer-scale or nanometer-scale precision. Recently, solid-state welding methods such as friction stir welding and magnetic pulse welding are developed. These methods have no high energy comparing with the fusion welding because these methods are achieved for joining at solid state. However, atomic diffusion for achievement of joining needs to be accelerated at solid state. For acceleration of the atomic diffusion, friction is usually generated between the welding materials. Therefore, brittle materials such as glass and silicon wafer are broken when the solid-state welding methods are applied for the MEMS stacks. Also, these fusion welding and solid-state welding methods usually produce thick brittle intermixing layer such as intermetallic compound with a scale of micrometers or larger. The brittle layer produced at the interface lowers the mechanical strength of the joint. Therefore, atomically scaled interface bonding is demanded for fabricating a highly reliable Cu/glass structure.

One of the solutions for strengthening the interface bonding is the formation of an adhesion layer between a Cu film and a glass substrate. It is a common sense in vacuum engineering to insert a reactive metal such as Al or Ti between Cu and glass. However, this technique is not very useful in three-dimensional MEMS/packaging, because

these metals must be deposited with sequential high-vacuum deposition methods, which do not give a good step coverage. In addition, reactions between Cu and those metals can lead to a significant increase in resistance when the Cu film is thin.

Koike et al. have investigated the interfacial properties of the annealed Cu-Mn/ glass structure and reported that adhesion improvement was observed by formation of a several nm thick Mn oxide layer at the interface [1, 2]. Yi et al. reported the formation of interfacial layer by annealing in Cu-Mg/glass and showed that the adhesion strength improved by formation of a Mg oxide layer at the interface [3]. Other elements, such as Al, Ti, and Cr, added to Cu were studied previously, and they were reported to improve the adhesion strength between Cu and various substrates (not only glass) [4–6]. These studies mentioned above indicate that the effective adhesion layers contain elements that are easily oxidizable and miscible in Cu. However, it should be noted that these studies required heat treatment during/after deposition. For achieving the general trend of temperature reduction during microelectronic fabrication, room-temperature or lower-temperature adhesion improvement is required.
