[TITLE]  
Research Progress on Materials for MEMS and Electronics Devices of Electronics Materials Development Group

[AUTHORS]  
Hiroshi Kawarada, Toshio Kamiya

[AFFILIATIONS]  
Graduate School of Advanced Science and Engineering, Waseda University, Tokyo  
Materials & Structures Laboratory, Tokyo Institute of Technology, Tokyo

[JOURNAL]  
プロジェクト研究報告 (Project Research Report)

[DOI]  
10.2320/materia.54.232

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Restored Main Text:

1. Lead-Free Piezoelectric Ceramic Thin Films for Microelectromechanical System Devices

Regulations on the use of materials containing hazardous elements are progressing, especially in Europe. Currently, most commercialized piezoelectric ceramics are based on lead-containing oxides, specifically Pb(Zr, Ti)O₃ (PZT). Therefore, the development of lead-free piezoelectric materials is an urgent issue. Among various lead-free materials, (K,Na)NbO₃ (KNN) has attracted attention as a compound with a relatively high Curie temperature and excellent piezoelectric properties. The EcoTopia Science Institute at Nagoya University is working on the fabrication of KNN-based thin films using a chemical process with metal-organic compound precursor solutions, which allows for high homogeneity, precise composition control, and low-temperature synthesis.

To control the properties of KNN thin films and obtain the desired characteristics, different elements are substituted or doped into KNN. Doping with multivalent ions such as Mn solves the significant problem of decreased electrical insulation. The mechanism involves Mn ions, present as Mn²⁺ or Mn³⁺ in the thin film, trapping carriers in the film, thereby improving insulation (1). Currently, in collaboration with the Waseda University Nanoscience and Engineering Research Organization, we are evaluating the properties that emerge and investigating the device application of these thin films. We are analyzing various electrical properties of KNN thin films, including electric field-induced strain characteristics, and examining the effects of microfabrication on these properties by developing cantilever-type actuators that integrate materials and device processing technologies (Fig. 1). Furthermore, we are working on applying these films to convert natural energies such as vibrations into electrical energy.

2. Fine Bonding Technology Using Nanoporous Structures

To improve the efficiency of power energy and increase the ratio of renewable energy, power modules and energy modules have attracted attention. Power modules generate more heat as they control larger currents, and as modules become smaller and lighter with higher component density, temperature rises more easily. In such high-temperature environments, the performance of semiconductor devices may deteriorate, making the advancement of element technologies within devices, including bonding techniques, essential. Currently, high-melting-point solders containing hazardous substances such as Pb (e.g., Pb-5Sn) are mainly used in these joints. However, due to growing environmental awareness, establishing alternative materials and bonding processes that do not contain hazardous substances has become an urgent issue.

The Joining and Welding Research Institute at Osaka University, in collaboration with the Waseda University Nanoscience and Engineering Research Organization, is working to develop alternative materials for high-lead-content solders (such as Pb-5Sn and Pb-10Sn, used in power devices) and to establish alternative bonding processes. They are developing fine bonding technologies using new nanoporous structures. Using nanoporous Au materials fabricated from Au-Ag alloys (Fig. 2), they have demonstrated that by controlling the bonding process, it is possible to achieve bonding strengths of 20 MPa or higher, equivalent to Cu/Cu bonding, and to bond Au-plated surfaces (2)(3).

3. Electrodes for SiC Power Semiconductors Using Carbon Nanotubes

The EcoTopia Science Institute at Nagoya University has developed...

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